Mineral Resource Assessment of the Iditarod Quadrangle ... › mf › 2005 › 2219b › mf2219b_pamphlet.pdfThe mineral resource assessment of the Iditarod quad-rangle is the finalproduct

2005

U.S. Department of the InteriorU.S. Geological Survey

Prepared in cooperation with the Alaska Division of Geological and Geophysical Surveys

Mineral Resource Assessment of the Iditarod Quadrangle,West-Central Alaska

By Marti L. Miller, Thomas K. Bundtzen, and John E. Gray

Pamphlet to accompanyMiscellaneous Field Studies Map MF–2219–B

The Idaho claim, a classic residual gold placer, lies near the top of Chicken Mountain.

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Contents

PageAbstract .................................................................................... 1Introduction ................................................................................. 1Geology .................................................................................... 2 Previousinvestigations ..................................................................... 2 Geologicsetting .......................................................................... 3 Descriptionoflithologicunits ............................................................... 4 Pre-accretionary,pre-Cretaceousrockbelts ................................................. 4 Post-accretionary,CretaceousandTertiaryoverlapassemblages ................................. 4 Surficialdeposits ...................................................................... 5 Geologicstructureandhistory ............................................................... 6Geochemicaldatasets ......................................................................... 7 Rockgeochemistry ........................................................................ 7 Reconnaissancedrainagebasingeochemicalsurvey .............................................. 7 NUREgeochemicalsurvey ................................................................. 8Geophysics.................................................................................. 9Mines,prospects,mineraloccurrences,androckanomalies............................................ 9Procedureforassessingmetallicmineralresources .................................................. 10 Methodsofassessment ..................................................................... 10 Estimatesofundiscoveredresources .......................................................... 11 Assessmentpresentation.................................................................... 12 Resourceareas—themodel-basedassessment ............................................... 12 Landtracts—theland-basedassessment .................................................... 12Mineraldepositmodelsandareadescriptions....................................................... 12 Peraluminousgranite-porphyry-hostedgold-polymetallicdeposits(model1) .......................... 13 Resourcearea1a,YankeeCreek-GanesCreekdikeswarm ..................................... 14 Resourcearea1b,GraniteCreekarea ...................................................... 15 Resourcearea1c,JulianCreekarea ....................................................... 15 Resourcearea1d,DonlinCreektrend ..................................................... 15 Plutonic-hostedcopper-gold-polymetallicstockworkandveindeposits(model2) ...................... 16 Resourcearea2a,MountJoaquin ......................................................... 17 Resourcearea2b,MooreCreek .......................................................... 17 Resourcearea2c,FourthofJulyCreekarea................................................. 18 Resourcearea2d,Iditarod-Flatarea ....................................................... 18 Resourcearea2e,MosquitoMountain ..................................................... 19 Plutonic-related,boron-enriched,silver-tinpolymetallicdeposits(model3) ........................... 20 Resourcearea3a,BeaverMountains ...................................................... 21 Resourcearea3b,TakotnaMountain ...................................................... 22 Resourcearea3c,VABMTatalinaarea..................................................... 22 Resourcearea3d,CamelbackMountain.................................................... 22 Resourcearea3e,northeastofFlat ........................................................ 23 Resourcearea3f,SwingingDome ........................................................ 23 Resourcearea3g,BismarckCreek ........................................................ 24 Resourcearea3h,southeastofGraniteCreek ............................................... 24 Resourcearea3i,GraniteMountain ....................................................... 25 Epithermalmercury-antimony(gold)deposits(model4) .......................................... 26 Resourcearea4a,HickeyCreek .......................................................... 26 Resourcearea4b,MooreCreekHghalo ................................................... 27 Resourcearea4c,BonnieCreekarea ...................................................... 27 Resourcearea4d,VABMTatalinaHghalo ................................................. 28 Resourcearea4e,Iditarod-FlatHghalo .................................................... 28 Resourcearea4f,CamelbackMountainHghalo ............................................. 29 Resourcearea4g,SugarloafMountainarea ................................................. 30

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Resourcearea4h,theDohertyCreekarea .................................................. 30 Resourcearea4i,northeastofBismarckCreek............................................... 31 Resourcearea4j,DeCourcyMountainarea ................................................. 31 Resourcearea4k,TwinButtesarea ....................................................... 32 Resourcearea4l,VABMGeorgearea ..................................................... 32 Resourcearea4m,KuskokwimMountains.................................................. 32 Resourcearea4n,southeastcornerofIditarodquadrangle ..................................... 33 Volcanic-hostedpreciousandothermetalsdeposits(model5)...................................... 33 Resourcearea5a,westofIditarodRiver.................................................... 34 Resourcearea5b,YetnaRiverarea........................................................ 34 Mafic-ultramaficrelatedCr-Ni-Codeposits(model6) ............................................ 35 Resourcearea6,DishnaRiverarea........................................................ 35 Heavy-mineralplacerdeposits(model7)....................................................... 35 Resourcearea7a,GanesCreek-YankeeCreek ............................................... 36 Resourcearea7b,CarlCreek ............................................................ 37 Resourcearea7c,MooreCreekarea....................................................... 37 Resourcearea7d,Iditarod-Flatarea ....................................................... 38 Resourcearea7e,LittleWaldrenFork ..................................................... 39 Resourcearea7f,GraniteCreekandMuntherCreek .......................................... 39 Resourcearea7g,JulianCreekarea ....................................................... 39 Resourcearea7h,AmericanCreek ........................................................ 40 Resourcearea7i,LittleCreek ............................................................ 40 Resourcearea7j,ReturnCreek........................................................... 40 Resourcearea7k,DonlinCreekarea ...................................................... 40Assessmentofnon-metallicresources ............................................................ 40 Hydrocarbonresources ..................................................................... 40 Peat .................................................................................... 42 Sandandgravel .......................................................................... 42 Riprapandbedrockaggregate ............................................................... 43 Otherindustrialminerals ................................................................... 43Summaryandconclusions ...................................................................... 43Acknowledgments ............................................................................ 44Referencescited.............................................................................. 44

FIGURes

1. LocationoftheIditarodquadranglestudyarea .................................................. 512. MapshowinglandjurisdictionfortheIditarodquadrangle ......................................... 523. Mapshowingsummarylandtracts............................................................ 534. Mapshowingplacerminingdistricts .......................................................... 545. MapshowinglocationofvitriniteandTAIsamples .............................................. 55

tABLes

1. Mines,prospects,occurrences,androckgeochemicalanomalies .................................... 562. Listofdeposittypes/modelsusedtodescribemetallicmineralresources .............................. 873. Definitionsofresourceassessmentterms....................................................... 874. Estimateofnumberofundiscovereddeposits ................................................... 885. Characteristicsoflodedepositresourceareas ................................................... 896. Characteristicsofheavy-mineralplacerdepositresourceareas...................................... 927. Summaryoflandtracts..................................................................... 938. Correclationofthermalmaturityindicatorstohydrocarbongeneration ............................... 949. VitrinitereflectanceinKuskokwimGrouprocks ................................................. 9510. Thermalalterationindex(TAI)valuesinKuskokwimGrouprocks .................................. 96

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ABstRACt

By Congressional mandate, the U.S. GeologicalSurveyischargedwithassessingtheNation’senergyandmineral resources.This report provides information onthemineral resourcepotential of oneof the least stud-iedareasof theNationand thussignificantly improvestheBureau’sability tofulfill themandate.TheIditarodquadrangle contains Alaska’s third largest placer goldminingdistrictandtherecentlyidentifiedDonlindeposit,thelargestknownlodegolddepositinAlaska.Ourstudyprovides a modern geological and metallogenic frame-workforthearea.Thisassessmentsynthesizesnewandpreviously existing geological, geochemical, and geo-physical data to identify areas of high, moderate, andlowpotentialfortheexistenceofundiscoveredmetallicresources and also provides an estimate of the numberofundiscovereddeposits.Toaidland-useplanners,thisinformationissummarizedtoshowwheremineraldevel-opment would most likely take place. Past productionfrom thequadrangle includes approximately65,100kg(2,093,000oz)ofgold,aminimumof7,300kg(235,000oz) of silver, 51,700kg (1,500flasks) ofmercury, andbyproductsofantimonyandtungsten;thevastmajorityofgoldandsilverproductionhascomefromplacerdepos-its.The probability is high that additional resources ofbothlodeandplacergoldexist.Thepotentialforsilver-tinlodedepositsisalsohigh;however,thisdeposittypehasnotbeenminedinthisquadrangletodate.Significantadditional resources of mercury and antimony are alsolikelypresent.Although this reportprimarilyaddressesmetallicresources,italsoincludesasummaryandbriefassessmentofnon-metallic resources(hydrocarbonandindustrialminerals).

ThemineralresourceassessmentoftheIditarodquad-rangleisthefinalproductofamulti-yeareffortbytheU.S.GeologicalSurveyincooperationwiththeAlaskaDivi-sionofGeologicalandGeophysicalSurveys.Thereportisdividedintosectionsbeginningwithsummariesofthegeology,geochemicaldatabases,andavailablegeophysi-calinformation.ThegeologyissummarizedfromMillerandBundtzen(1994),anearlierproductofthiscoopera-tive study.The primary geochemical databases consid-eredforthisassessmentareanalysesofrock(McGimseyand others, 1988) and drainage basin samples (streamsediment,heavy-mineralconcentrate,andaquaticmoss)collectedinthecourseofthisstudy.Thedrainagebasindataaresummarizedinthreemapsthatshowthedistri-butionofsamplescontaininganomalousconcentrationsofvariousmetallicelements(Grayandothers,1997a,c,andd).Nonewgeophysicaldatawerecollectedduringthecourseofthiscooperativestudy;however,asummaryofavailabledataisprovided.

Acomprehensive,descriptivesummaryofthemines,prospects, occurrences, and significant rock geochemi-

calanomaliesoftheIditarodquadrangleformsacriticalcomponentofthisreport.Thesedescriptionsarebasedonhistoricalinformationprovidedinpublishedgovernmentreports,unpublishedmintrecords,anddocumentedoralhistoricalaccountsgivenbylocalminers,supplementedbynewgeologicandgeochemicaldatafromthisstudy.Thedescriptionsareprovidedinatableformat(table1);locationsaregivenonmapA.Also summarized in thetablearethenewmineraloccurrencesdiscoveredduringthecourseofthiswork.

Themetallicresourceassessment,whichisbasedonalltheabovedata,formsthemainbodyofthisreport.Itdepartsfromotherresourceassessmentsbypresentingthedataintwodifferentways.OnmapB,forty-sixmineralresourceareasareoutlinedonthebasisofsevendepositmodelsandassignedhigh,moderate,orlowpotentialfordiscovery of additional deposits. This map is the basictoolforevaluatingtheundiscoveredresourcesoftheareaandshowswhichareasareprospectiveforspecificcom-modities.Adetaileddescriptionofeachof the forty-sixareasisprovidedinthetext.Anotherwaytosummarizetheassessmentisintermsofthetotalmineralendowment,thatis,allmetalliccommoditiescombined(fig.3).Thisviewwouldbemoreusefulinland-planningdecisions,asitbetterpredictswherefutureminingmighttakeplace.

Thefinalsectionofthereportisadiscussionofthenon-metallic resources of the quadrangle including oilandgas,coal,sandandgravel,andriprap;however,nospecificresourceareasortractsareoutlined.Thissectionincludesasummaryofnewvitrinitedata,whichconfirmthatitisunlikelytheIditarodquadrangleholdsanyoilorgasreserves.

IntRodUCtIon

The Iditarod quadrangle lies about 350 km west-northwest of Anchorage in west-central Alaska (fig.1). It encompasses17,300km2and is characterizedbyroundeduplandareasandmarshylowlands.TheKusko-kwimMountainscoverabroadswathacrossthemiddleandsoutheastpartsofthequadrangle;ridgesareroundedandgenerallyriseto400–600mfromvalleyfloorsthataverage150minelevation.Maximumrelief(1,235m)isfoundintheBeaverMountains,arugged,glaciated,plu-tonic-coredmountainrangethatliesinthenortheastpartof the study area.The western third of the quadrangleconsists ofwetlands and rollinghills of low relief thataverage150minelevation.Exposuresaregenerallypoordue to a thick cover of colluvium and vegetation.TheKuskokwim River, the second longest river inAlaska,flows through the southeast corner of the quadrangle.Surfaceaccesstothestudyareaislimited,hencenearlyallfieldworkwashelicoptersupported.Approximately60kmofsecondaryroadsaremaintainedintheFlatareaforaccesstoseveralplacerminestreams.Anadditional

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35kmofsecondaryroadscrossthenortheastpartofthequadrangleandprovideaccessviatheKuskokwimRiverto the abandoned town of Ophir and its surroundingplacermines(includingsomethatliewithintheIditarodquadrangle).

LandjurisdictionoftheIditarodquadrangleisdivis-ibleintofourmaincategories(fig.2).Nearlyhalfofthelandarea(47percent)isStateland.Roughly11percent,mostofthenorthwestcorner,ispartoftheInnokoNationalWildlifeRefuge.Approximately5percentof thequad-rangle consists ofNative lands.Most of the remainingarea(37percent)isBureauofLandManagement-admin-isteredFederal land, someofwhichwill eventuallybeconveyedtoNativecorporationsandtheStateofAlaska(AlaskaDepartmentofNaturalResources,1995).

Thisreportandaccompanyingtablesandmapssum-marizetheknowndepositsandevaluatethepotentialforundiscoveredmineralresourcesoftheIditarodquadrangle.Thisworkrepresentsamulti-yearcooperativeeffortbytheU.S.GeologicalSurvey(USGS)andAlaskaDivisionofGeologicalandGeophysicalSurveys(ADGGS)undertheauspicesoftheAlaskaMineralResourceAssessmentProgram(AMRAP).Jointfieldworkbeganin1983andterminated in1989,butmostof theeffortwasconcen-tratedduring1984to1986.AdditionalmineralresourcestudieswerecarriedoutnearFlatin1988and1989.Thestudies included geologic mapping, geochemical sam-pling,andprospectevaluation;nonewgeophysicaldatawere acquired. Information obtained from previouslypublishedreportsbybothstateandfederalagencieshasalsoaidedtheresourceassessmenteffort.Thecoopera-tiveagreementallowedbothagenciestomaximizetheirresourcesbyfullysharingdataandfieldexpenses.Eachagencyfocusedontheirpreferredproducts—theUSGSwasresponsibleforregional(1:250,000-scale)geologicandgeochemicalstudies,andtheADGGSwasresponsi-bleforthemoredetailed(1:63,360-scale)geologicmap-pingandprospect-levelstudies.

The Iditarod quadrangle contains the third larg-est placer gold district in Alaska and prospecting hasoccurredsporadicallysincethelate1800’s.Thequadran-glewasaprimeareaforstudybecausethegeologyandlode sources were poorly known. Since 1989, a majorgoldresourcehasbeenidentifiedbyprivate industryatDonlin Creek (Dodd, 1996), and exploration work hasbeencarriedoutatFlatandintheGeorgeRiverdrainage(BundtzenandMiller,1997).

The first step in evaluating the metallic resourcesofthisareawastoprovideabettergeologicframeworkwithinwhichtobothunderstandthesettingoftheknownmines,prospects,mineraloccurrences,androckgeochem-ical anomalies and to better predict the unknown min-eralendowment.Theresultinggeologicmap(MillerandBundtzen,1994)istheprimarydatabaseforthismineralresourceevaluation.Thesecondstepwasaninventoryof

knownmines,prospects,mineraloccurrences,androckgeochemicalanomaliesfromtheliteratureandourowndatabases.These206sitesaresummarizedintable1andshownonmapA.Theregionaldatabaseofgeochemi-calanomaliesdefinedbystream-sediment,nonmagneticheavy-mineral concentrate, and aquatic moss samples(Gray andothers, 1988a, 1997a, c, andd;Bennett andothers, 1988;Motooka andothers, 1988;Arbogast andothers,1991;Hopkinsandothers,1991)providedimpor-tantinformationforoutliningareasofpossibleeconomicmineralconcentration.Detailedexaminationsofselectedprospects(Bundtzenandothers,1986,1988,and1992)and analytical data from 1,601 rock samples (McGim-seyandothers,1988)alsoassistedinunderstandingtheknowndepositsanddefinedregionalbackgroundvalues.The last stepprior todelineating resourceareaswas toclassify the known mineral deposits (and occurrences)intogeneticmodelsinordertobetterunderstandtheoregenerationsettingandhence,tobetterpredictundiscov-eredresources.Weintegratedtheinformationintosevenmetallicmineraldepositmodels that arebasedonana-loguessummarizedinCoxandSinger(1986),RytubaandCox(1991),Noklebergandothers(1993),andBundtzenandMiller(1997).

Thisresourceevaluationissummarizedintwowaysto satisfy different purposes. Resource areas that havehigh,moderate, or lowpotential for the sevenmetallicmineraldeposittypesaredelineatedonmapB;theareasofhighpotentialcouldeasilyserveasexplorationtargets.A more useful summary for land-planning purposes isprovidedinfigure3.Onthisfigure,model-derivedareasare combined to form non-overlapping land tracts thatarerankedasveryhigh,high,moderate,low,orunknownaccordingtotheiroverallmetallicmineralresourcepoten-tial.Non-metallicresourcesarealsodiscussedinthetext,butnoresourceareaswerespecificallydelineated.

GeoLoGy

PREVIOUSINVESTIGATIONS

Duringthefirsthalfoftheeighteenthcentury,Rus-sianexplorersbegantoinvestigatesouthwestAlaskaviathe Kuskokwim River. They traveled from the BeringSeaupriverapproximately240km,discoveredcinnabarat the deposit now known as the Kolmakof Mine, andconstructed a fort just a few kilometers away (fig. 1).TheRussianreportswerethefirsttoaddressthemineralresourcesofsouthwestAlaska;however,thegeologyoftheregionwasrelativelyunknownuntil1898whenJ.E.SpurroftheU.S.GeologicalSurveyleadapartydowntheKuskokwimRiverfromitssourceintheAlaskaRange.ThisexpeditiontraversedpartoftheIditarodquadrangleandprovidedthefirstdescriptionofrocks(Spurr,1900)thatwouldlaterbenamedtheKuskokwimGroup(Cadyandothers,1955).

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ThediscoveryofpayingquantitiesofgoldonGanesCreek(Innokodistrict,fig.4)in1906andonOtterCreek(Iditaroddistrict,fig.4)in1908promptedfurtherinvesti-gationsbytheU.S.GeologicalSurveywithintheIditarodquadrangle.In1908,A.G.Maddren(1910)brieflylookedat placers of the Innoko district, and in 1910 he madeageologic reconnaissancesurveyof thegeneral regionwhile investigating placer deposits in both the InnokoandIditaroddistricts(Maddren,1911).In1912and1913,onalimitedbudget,H.M.EakinmanagedtovisitmostoftheminesbetweenRubyandIditarod,furthercontributingtothebaseofknowledge(Eakin,1913,1914).In1914,A.G.MaddrenreturnedtostudythegoldplacerdepositsofthelowerKuskokwimareafromIditarodsouthwest-wardtotheTuluksakRiver(RussianMissionandBethelquadrangles)andwasthefirsttodescribeplacerclaimsstakedin1909nearCrookedCreek(Maddren,1915)inthe southern Iditarod quadrangle. P.S. Smith traversedfromLakeClarktoIditarodin1914;hisdescriptionsofthegeneralgeologyand resources (Smith,1915,1917)remainedthemostcompleterecordoftheregionforthenext 10–15 years. Mertie and Harrington (1924) andMertie (1936) further expanded the knowledge of thestudyarea—especiallytherelationshipofplacergoldtotheCretaceousplutons.

W.M. Cady of the U.S. Geological Survey headedamajorgeological studyof theKuskokwimarea.Thisworkbegan in1941asanoutgrowthofdetailed inves-tigationsofthemanymercurydepositsintheSleetmutequadrangle.The published summary (Cady and others,1955) remains a definitive work for the region to thepresentday.In1942,E.J.WebberandW.M.CadyoftheU.S.GeologicalSurveyvisitedtheDeCourcyMountainarea,siteoftheonlymercurymineintheIditarodquad-rangle,andmappedthemineworkingsandthesurround-ingcountry.E.J.WebberandJ.M.HoareinvestigatedtheDeCourcy area further in 1943, and Hoare visited theareaagainin1946,1947,and1948.B.S.WebberoftheU.S.BureauofMinesconductedanextensivetrenchingandsamplingprogramatDeCourcyin1943(Webberandothers,1947).TheDeCourcyMountainMinewasfurtherexploredunderaDMEA(DefenseMineralsExplorationAdministration)programbetween1953and1957.Sup-plementalfieldworkwasconductedbyE.M.MacKev-ett,Jr.andR.S.ValikanjeoftheU.S.GeologicalSurveyintheyears1953and1957.AsummaryofthepropertywasincludedinastudyofthequicksilverdepositsoftheKuskokwim region (Sainsbury andMacKevett, 1965).Duringthelatterpartofthisexploratoryperiod,Kimball(1969)andotherU.S.BureauofMinespersonnelcom-pletedadetailedsurfacesamplinginvestigationofgold-quartzveinsonChickenMountainnearFlat.

In the late 1970’s, the ADGGS began a detailedgeologic mapping program in the northeast part of theIditarodquadrangle.Fieldworkbetween1978and1984

resultedinthree1:63,360-scalegeologicmaps(IditarodD–2 plus eastern D–3, D–1, and C–3) (Bundtzen andLaird, 1982, 1983;Bundtzen andothers, 1988; respec-tively)aswellasanolderpreliminarygeologicmapofthe Innoko district (Bundtzen and Laird, 1980). Map-ping performed jointly by theADGGS and the USGSbetween1984and1989hasresultedinthepublicationofa1:63,360-scalemapoftheIditarodB–4andeasternB–5quadrangles(Bundtzenandothers,1992).Inadditiontothemapping,knownmineralizedareaswerestudiedandseveralnewcopper-silver,tin-arsenic-zinc,andmercury-antimony-goldoccurrenceswerereported.Acompilationof existing field data for theA–5 quadranglewas alsopublishedbyADGGS(Deckerandothers,1984).Thesestudiesrepresentthemost-detailedpublishedworkintheIditarodquadrangletodate.

GEOLOGICSETTING

BedrockmappingintheIditarodquadrangleisham-peredbylowrelief,poorexposure,andathickcoverofcolluviumandvegetation.ThegeneralizedgeologicmapofMillerandBundtzen(1994)reliedheavilyuponinfor-mationgatheredonhelicopter-supportedfoottraverses,amethodrequiredtoobtainanaccuratedepictionofsuchpoorlyexposedterrain.AscreenedversionoftheMillerand Bundtzen (1994) map appears as an underlay onbothmapsAandBofthisreport.AbriefsummaryofthegeologyoftheIditarodquadrangle,includingcommentsabouttheassociatedmetallicresources,followsbelow.

Rocks of the Iditarod quadrangle are broadly sub-divided into two groups by age and tectonic history:(1) fault-bounded slivers of pre-Cretaceous rocks, and(2) Cretaceous and younger overlapping assemblagesof sedimentary, volcanic, and plutonic rocks (MillerandBundtzen,1994).Theknownmines,prospects,andoccurrences(table1)aremainlyassociatedwiththelattergroupofrocks.

The pre-Cretaceous rocks are concentrated in anarrow northeast-trending structural belt (the DishnaRiverFaultzone)andaredividedintofourunitsthatrep-resentawiderangeinprotolithsandages.Thebeltcon-tainsstructuralsliversofJurassicmafic-ultramaficrocks,MississippianandTriassicchertandvolcanicrocks,andtwounitsofregionallymetamorphosedcontinentalcrustalrocksofEarlyProterozoictoPaleozoicage(MillerandBundtzen,1994).Theallochthonoussliversareprobablyrelatedtosimilarunitsknownelsewhereinwest-centralandsouthwestAlaska suchas themaficandultramaficcomplexesoftheTozitna-Innokobelt,chertandvolcanicrocksof theInnokoterrane, thegreenschist faciescon-tinental crust of the Ruby terrane, and the amphibolitefacies continental crust of the Early Proterozoic IdonoComplex(MillerandBundtzen,1994).

BeginninginearlyLateCretaceous time, theolderterraneswereerodedandpartlycoveredbyterrigenous

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clasticdepositsthatbecametheKuskokwimGroup.Thisregionally extensive unit was deposited primarily byturbidity currents into an elongate, probably strike-slipbasin(BundtzenandGilbert,1983;MillerandBundtzen,1992).Locallyinterbeddedtuffsandvolcanicflowsmayindicate initiationofvolcanismthat laterculminated inwidespreadLateCretaceous andearlyTertiary igneousactivity. Volcanic-plutonic complexes of Late Creta-ceousandearlyTertiaryagelocallyoverlieandintrudetheKuskokwimGroup.OtherLateCretaceousandearlyTertiaryigneousrocksintheIditarodquadrangleincludeextensive subaerial volcanic rocks, peraluminous hyp-abyssal granite porphyry, altered intermediate tomaficdikes,andfelsicplugsandplutons(MillerandBundtzen,1994).LateTertiary(?) toQuaternary surficial depositscoveroverhalfoftheIditarodquadrangle.

The dominant deformation of the region began inLate Cretaceous time, although earlier deformationaleventsarelocallypreservedinthepre-Cretaceousrocks(Miller andBundtzen,1994).Theoverlapassemblageswere deformed in a right-lateral wrench fault tectonicenvironment that yielded north- northeast-trending enéchelon folds andhigh-angle faults (Miller andBundt-zen,1992).Evidenceindicatesatleasttwogenerationsoffolds.The well expressed, generally northeast-trendingtightfolds(F1amplitudesrangefrom0.5to2km)weretheearliesttoform,followedbylater,north-trendingopenfolds(F2amplitudesrangefrom3to5km).Numeroushigh-anglefaultsparallelbothsetsoffolds.Thewrenchfault tectonic environment probably controlled (1) theKuskokwimbasinformation,(2)thefoldingandfaultingoftheregion,and(3)LateCretaceousandearlyTertiaryvolcanismandplutonismandassociatedpolymetallicoreformationofseveraldeposittypes(MillerandBundtzen,1992,1994;BundtzenandMiller,1997).

DESCRIPTIONOFLITHOLOGICUNITS

Pre-accretionary,pre-Cretaceousrockbelts

Fault-bounded slivers of four distinct units occuralong the Dishna River Fault zone in the west-centralpartof theIditarodquadrangle.Correlativeunits to thenorth are interpreted as having been assembled duringpre-midCretaceous thrust faulting thatemplacedmaficandultramaficophiolitic rocksover chert andvolcanicrocks, that in turn were thrust over polymetamorphicrocks(PattonandBox,1989;Pattonandothers,1994a,b). Their current juxtaposition along high-angle shearzones likely reflects post-mid-Cretaceous deformation(MillerandBundtzen,1994).

Theoldestunit(unitXi)isafragmentofEarlyPro-terozoiccontinentalcrust,theIdonoComplexofMillerand others (1991). This assemblage of rocks includesamphibolite-grade granitic to dioritic orthogneiss,amphibolite, and metasedimentary rocks. Isotopic data

suggesttheIdonoComplexisgeneticallyrelatedtotheKilbuckterrane(Boxandothers,1990),whichliesabout250kmtothesouthwest.Thesetwoassemblagesconsti-tutetheoldestknownrocksinAlaska(MillerandBundt-zen,1994).

Paleozoic and possibly Proterozoic greenschist-facies meta-igneous and metasedimentary rocks (mapunitPz<g) thatarecorrelatedwiththeRubyterraneofJones andothers (1987) also representmetamorphosedcontinental crustal rocks. In the Iditarod quadrangle,maficgreenschist,lesserpeliticschist,phyllite,calcare-ousschist,quartzite,andminorgraniticorthogneisschar-acterizethisunit,buttheexposuresareextremelypoor.

AweaklymetamorphosedassemblageofMississip-pian to Triassic age rocks (unit ^Mc), which includesradiolarian chert, basalt to basaltic andesite, lithic tuff,metasiltstone, and minor limestone, correlates with theInnokoterrane(Jonesandothers,1987).Themainout-cropbeltliesinthewesternhalfofthequadrangle,butsmall slivers of this unit occur along high-angle faultsin thenortheastmap area, suggesting that rocks of theInnokoterranemayformthebasementbeneaththeCre-taceousflysch(MillerandBundtzen,1994).

Jurassicmafic-ultramaficrocks(unitJdr),whichareprobablyophioliticinorigin(Miller,1990),comprisetheyoungestofthepre-accretionaryterranes.TheycorrelatewithalargebeltofsimilarrocksdescribedbyPattonandothers(1994a)astheTozitna-Innokobeltofmafic-ultra-maficcomplexes.

Although a few rock geochemical anomalies areassociatedwiththepre-Cretaceousrocks,nosignificantmineraloccurrencesareknown.Threesamplesofmeta-siltstoneand/orchertfromtheMississippiantoTriassicassemblageofchertandvolcanicrocks(Innokoterrane)contain 1–2 parts per million (ppm) silver (nos. 3, 24,and38, table1);however, theseAgvaluesarenotsig-nificant enough to warrant consideration as potentialmineraldeposits.OnesampleofserpentinizedultramaficrockfromthesouthendofthebeltofJurassicmaficandultramaficrocksyieldeda30ppmvalueforsilver (no.51,table1).Althoughthisisarelativelyhighamountofsilver,neitherthesamplenorotherrocksatthelocalitylookmineralized.Theserpentinitesdocontainhighchro-mium,nickel,andcobaltvalues(McGimseyandothers,1988),butthesemetalsareenclosedinrefractoryminer-alslikeolivineandclinopyroxeneandthusmirrorultra-maficgeochemistry.

Post-accretionary,CretaceousandTertiaryoverlapassemblages

The oldest overlap assemblage is a poorly knownsandstoneandsiltstoneunit(unitKs)thatoccursonlyinthewesternpartofthestudyarea.Itiscompositionallydistinct fromthewidespreadKuskokwimGroupand isEarlyCretaceousinageonthebasisofpalynomorphsin

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onesample(MillerandBundtzen,1994).Nogeochemi-calanomaliesareassociatedwiththisunit.

TheUpperCretaceousKuskokwimGroupisaregion-allyextensivebasin-fillsequencethatunderliesoverhalfof the Iditarod quadrangle. This sedimentary sequenceis interpreted as predominantly deep marine turbidites(flysch),but italsoincludesshallow-marineandfluvialstratathatweredepositedalongthemarginsofthebasin(BundtzenandGilbert,1983;PachtandWallace,1984;MillerandBundtzen,1994).Minorvolcanicrockscon-sistingoftuffsandflowsarelocallyinterbeddedwiththeflysch(BundtzenandLaird,1982;MillerandBundtzen,1994).TheKuskokwimGroupintheIditarodquadrangleisdividedintofivemapunits—Kks,Kkq,Kkt,Kka,andKkv(MillerandBundtzen,1994).Mostoftherockscon-sistof interbeddedshale,siltstone,and lithicsandstone(unit Kks), which are interpreted as basin-floor turbi-dites.Theturbiditesaresuccessivelyoverlappedonthewesternmarginofthebasinbyquartzosesandstone,silt-stone,conglomerate,andlocalcoal-richlayers(unitKkq)thatare interpretedas shoreface, foreshore,and locallynon-marine deposits. Although volumetrically minor,theremainingthreemapunits(Kkt,Kka,andKkv)con-tainvolcanic tuffsandflows, indicating theoccurrenceofintermittentvolcanismduringdepositionoftheKus-kokwimGroup.AllmercuryoccurrencesintheIditarodquadrangleareeitherhostedintheKuskokwimGrouporinrocksthatintrudeit.

Volcanic-plutoniccomplexesofLateCretaceousandearlyTertiary age (Miller and Bundtzen, 1994) consistof intermediateandmaficvolcanicrocks(unitsKitandTKil) of the Iditarod Volcanics (Miller and Bundtzen,1988) and comagmatic monzonite to quartz monzoniteplutons(unitTKm).ThetuffsandflowsoftheIditarodVolcanicsgenerallyoverlie and locally interfingerwithsedimentary rock of the Kuskokwim Group. Hornfelsaureolesupto2kmwidesurroundmostofthelargerplu-tonsanddeveloped inboth theclasticsedimentaryandoverlyingvolcanicrocks.Polymetallicveinscontaininggold,silver,andvariousbasemetalsarelocallyhostedintheplutonicrocksandassociatedhornfels.

Hypabyssal-textured felsic to intermediate igneousrocksofLateCretaceousandearlyTertiaryagecropoutdiscontinuouslyacrossthequadrangle.Althoughofsimi-larage,theyareprobablynotdirectlyrelatedtothevol-canic-plutonic complexes and instead represent meltedcrust(MillerandBundtzen,1994).Thehypabyssalrocksaredividedintotwounits(basedprimarilyonsilicacon-tent) that may be genetically related. Granite porphyrydikes,sills,andplugs(unitTKgp)showadistinctlyper-aluminouschemistryandcommonlycontaingarnetphe-nocrysts.Thesebodiesaregenerallysmallbutarewidelydistributed inKuskokwimGroupflysch and frequentlyformmappablelinearbeltsofdikesandsmallhypabys-sal plutons. They are spatially related to gold-bearing

veinandshear-zonedepositsandtoseveralplacergoldoccurrences,makingthemanimportantpotential targetfor gold exploration. Three plugs of pilotaxitic dacite-andesite (unitTKp) are similar to thegraniteporphyryrocksinmineralogyandmodeofoccurrencebutarenotapparentlyassociatedwithanomalousgold.

Other intrusive rocks of the Iditarod quadrangleincludeTertiary andCretaceous altered intermediate tomafic dikes (unitTKd), Tertiary and Cretaceous alkaligranite(unitTKg),andaTertiaryporphyriticgranodio-riteplug(unitTp);therelationbetweentheseandotherintrusiveunitsisuncertain.Thedikes,whichintrudeKus-kokwimGroupflysch,arewidespreadandubiquitouslyalteredtochlorite,calcite,andsilica(orsilica-carbonatealtered). Mercury mineral occurrences are occasionallyhostedinaltereddikesaswellasintheintrudedflysch,butthepresenceofdikesdoesnotseemtoberequired.Severalexposuresofalkaligraniteoccurinthewesternpartofthequadrangle,andoneporphyriticgranodioriteplugliesnorthofFlat,butneitherunitisassociatedwithgeochemicalanomalies.

LateCretaceousandearlyTertiarysubaerialvolca-nicrocksreferredtobyMillerandBundtzen(1994)asvolcanicrocksoftheYetnaRiverarea(unitTKy)under-liemuchofthewesternpartoftheIditarodquadrangle.Andesite,dacite,andrhyolitelavaflowsaremostabun-dant,butrhyolitictoandesiticash-flowtuffsarepresentlocally,particularly in thenorthernpartof thevolcanicfield. This area is low lying and very poorly exposed,but scattered rock samples yield low-level geochemi-cal anomalies in lead, tin, and evengold.Significantlyyounger (about 10 Ma) basaltic andesite (unit Tb) issparsely scattered overlying the Late Cretaceous andearlyTertiaryvolcanicrocksnearthewesternboundaryofthequadrangle.Nogeochemicalanomaliesareassoci-atedwiththeseyoungerrocks.

Cretaceousandyoungerrockshostalloftheknownmines, prospects, and occurrences, and a vast majorityof themetallicgeochemicalanomalies.Mercuryoccur-rences are primarily associated with Upper Cretaceousflysch.LateCretaceous-earlyTertiaryintrusiverocksareparticularly important for gold, silver, and base metaloccurrences. Late Cretaceous-early Tertiary subaerialvolcanicrocksarelocallyanomalousingold,lead,andtin.PlacergoldishostedbylateTertiary(?)andQuater-naryfluvialandbenchdeposits.

Surficialdeposits

Surficial deposits, which cover at least 50 percentof thequadrangle, consist largelyof colluvial, alluvial,andeoliandepositsbutlocallyincludetalus,siltandpeat,terrace deposits, glacial deposits, and minor landslidedeposits.TheyareprimarilyQuaternaryinagebutmayalsocontainlocalPlioceneterracedeposits.

6

Isolated highland massif areas such as the BeaverMountainsandGraniteMountainhavebeenglaciatedatleastfourtimesinQuaternarytime.DepositionoftillandoutwashinseveralstreamsthatdraintheBeaverMoun-tainsresultedinstreampiracyanddrainagemodification.ThisinturnhadamajoreffectontheformationofgoldplacerdepositsintheadjacentInnokodistrict(Bundtzen,1980).PlacergoldoftheIditarodquadrangleishostedbythreemorphological typesofsurficialdeposits:modernstreamalluvium,ancestralterracealluvium,andresidualandeluvial(hilltop)placers.Cinnabarandscheelitearelocally important components ofmodern streamplacerdeposits.

GEOLOGICSTRUCTUREANDHISTORY

Convergence of the lithotectonic terranes of west-ernAlaskaendedinEarlyCretaceoustime(Box,1985;Pattonandothers,1994b;Deckerandothers,1994)andbyLateCretaceoustime,thetectonicenvironmentoftheregionwasdominatedby transcurrentorwrench faultswithsignificantright-lateraloffsets(WallaceandEnge-bretson,1984;Millerandothers,2002). In theIditarodquadrangle, the primary structural features are north-east-trending en échelon folds and subparallel high-angle faults that formed in a right-lateral wrench faulttectonicenvironment(MillerandBundtzen,1992,1994;Millerandothers,2002).Evidenceofolderdeformationispreservedinsomeofthepre-Cretaceousrocksofthequadrangle,but thecurrent juxtapositionof thesefault-bounded pre-Cretaceous units along high-angle shearzones is likely the resultof theLateCretaceousstrike-slipenvironment.

MuchofthepostEarlyCretaceous(orpost-tectonic-assemblage)geologichistoryoftheIditarodquadrangleisrelatedtothiswrench-faultenvironment.Inmid-andLateCretaceoustime, thepre-CretaceousterraneswereerodedandpartlycoveredbyterrigenousclasticrocksoftheKuskokwimGroup.Thesedimentsweredepositedinanelongate,probablystrike-slipbasinthatevidentlycon-tinuedtodeepenwhilefilling(MillerandBundtzen,1992,1994).Wrenchfaultsystemsseldomexhibitstrictlyparal-lelmovementandcontractionalandextensionalregimescommonlyalternate.TheLateCretaceousandearlyTer-tiaryvolcanicandplutonicrocksthatoverlieandintrudetheKuskokwimGroupwereprobablyemplacedduringintermittentperiodsofextension(MillerandBundtzen,1992,1994).Theoverlapassemblageswerebothfoldedandfaultedin thewrenchfault tectonicsettingtoyieldgenerally northeast-trending en échelon folds and sub-parallelfaults.

Themainwrench-faultzoneintheIditarodquadran-gle ismarkedby thenortheast-trending Iditarod-NixonForkFault,whichbisects the Iditarodquadrangle.Thisfault has had a minimum of 90 km right-lateral offsetsinceLateCretaceoustime(MillerandBundtzen,1988);

thefaultwasactiveintoatleastlateTertiaryorQuater-narytime(MillerandBundtzen,1994)andsegmentsofthefaultmaystillbeactive.Numerousotherhigh-anglefaults parallel the general northeast- to north-trendingstructural grain that characterizes the map area. TheYankee-GanesCreekFault,whichliesabout8kmnorth-west of the Iditarod-Nixon Fork Fault, appears to be asplayofthemainfaultzonethathasbeeninactivesinceabout77Ma(MillerandBundtzen,1994).TheDishnaRiver Fault zone in the west-central part of the quad-ranglecontainstectonicsliversofrocksthatrangefromProterozoictoCretaceousinage.Evidenceindicatesthatthis fault, which represents the southwest extension ofa 270-km long zone, was active sometime between 77and69Ma,butmayalsohavebeenactiveearlier,duringdepositionoftheKuskokwimGroup(betweenabout90and77Ma)(MillerandBundtzen,1988,1994).

The overlap assemblages underwent at least twoperiods of open to isoclinal folding beginning in lateCretaceousandcontinuingintoTertiarytime.Theearlier,mainfoldingevent ismarkedbythenortheast-trendingen échelon folds mentioned above. Evidence for laterfoldingisfoundinthesoutheasternpartofthestudyareawheretheearlierfoldsarebroadlyrefoldedalonganorth-erly-trending fold axis. The Late Cretaceous and earlyTertiaryplutonsappeartocrosscuttheregionalstructureandprobablypost-datetheearlierfoldingevent(MillerandBundtzen,1992).TheIditarodVolcanicshavebeenbroadly folded into synclines and anticlines, probablyduringthelaterfoldingevent.

All lodemineral occurrences in the Iditarodquad-rangle are hosted in the Cretaceous and younger over-lapassemblages.Thewrench-faulttectonicenvironmentheavilyinfluenced,ifnotcontrolled,thepost-Cretaceousgeologichistoryoftheregionandthereforealsoaffectedoregeneration.Mostofthemetallicresources—includ-ingmercuryandantimony—aregeneticallyrelatedtotheLateCretaceous-earlyTertiaryplutonicrocks.Grayandothers(1997b)suggestedthathigherheatflowassociatedwithmagmatismmayhavefacilitatedreleaseofmercuryfrom the sedimentary rocks into hydrothermal fluids.Regardlessoftheultimatesourceofthemercuryeitherfrom the plutonic or sedimentary rocks, deposition ofcinnabar-stibnite-quartzveinsappears tobestructurallycontrolled along high-angle faults. Some mineralizedrocksareoffsetalongstrike-slipfaults.Thegranitepor-phyrydikesoftheGanesCreekarea(eastoftheBeaverMountains)areprobablyoffsetalongtheIditarod-NixonFork Fault from a similar swarm of granite porphyrydikesandplugsintheDonlinCreekareaforadistanceof90km;similarplacergoldandgold-polymetalliclodedepositsoccurinbothbeltsofrocks.Also,anorth-north-east-trending high-angle fault may have right laterallyoffsetamineralizedgraniteporphyrydikeswarmintheJulianCreek-MontanaCreekareafromasimilarswarm

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intheWillowCreek-GraniteCreekarea,some10kmtothenortheast.

GeoChemICAL dAtA sets

Geochemical data played a major role in delinea-tion of resource areas and definition of deposit typesfor the Iditarod quadrangle.A reconnaissance drainagebasin survey consisting of systematic stream-sediment,heavy-mineral concentrate, and aquatic moss samplingwasperformedforthisstudy.Theresultssupport,andinsomecasesareusedtodefine,resourceareas.Geochemi-caldatafromrocksamplescollectedduringthisprojecthavehelpedtodefinemineraldeposittypesandindicatefavorablerockunits.Previouslyexistingdatafromsedi-ment and water samples collected in 1981 by the U.S.Department of Energywere of limited use in definingresource areas, but provided supportive information.Eachofthesedatasetsisbrieflydiscussedbelow.Theirroleindefiningresourceareasisgivenlaterintheindi-vidualareadescriptions.

ROCKGEOCHEMISTRY

As part of the reconnaissance geochemical surveyoftheIditarodquadrangle,primarilyinconjunctionwithgeologicmapping,1,601rocksampleswerecollectedandanalyzed(McGimseyandothers,1988).Manyof thesewereunmineralizedgrabsamplescollected toestablishbackgroundvaluesfordifferentrocktypesintheregion.However,toaidincharacterizingthedeposits,suitesofmineralized samples were collected from known pros-pects and from new occurrences discovered during thecourse of the fieldwork. Both background andminer-alizedsampleswereanalyzedfor31elementsbysemi-quantitative emission spectrography, and for additionalselectedelementsbyatomicabsorptionspectrophotom-etry,specificion,modifiedinstrumental,andcolorimet-ric methods. The locations, rock types, and completeanalyses are reported in McGimsey and others (1988).Significant rock geochemical anomalies and newmin-eraloccurrencesdefinedbythatdatasetaresummarizedin table 1. Rock geochemical data previously reportedbyBundtzenandLaird(1982,1983)andBundtzenandothers(1988,1992)werealsoutilizedduringthismineralresourceassessment.

RECONNAISSANCEDRAINAGEBASINGEOCHEMICALSURVEY

Stream-sediment,heavy-mineral-concentrate,stream-water,andaquatic-moss(bryophyte)sampleswerecollectedfromactivechannelsofperennialfirst-andsecond-orderstreamsoftheIditarodquadrangle.Theareaofmostdrainagebasinsrangedfrom2.6km2(1mi2) toabout13km2(5mi2).Thestream-sedimentsamplingden-sitywasapproximately1samplesiteper23km2(9mi2).

The stream-sediment samples were sieved tominus-80-mesh (0.17 mm) and chemically analyzed.The panned-concentrate samples were sieved to minus30-mesh (0.60mm), separated inbromoform,and thenfurtherseparatedmagneticallytoobtainanonmagnetic,heavy-mineral-concentrate sample. These concentrateswere split for separate mineralogical and geochemicalanalyses. Aquatic moss samples were preferably col-lectedfrombouldersordead-fallvegetationnearorjustabovethecurrentwaterlevelintheactivestreamchannel.However,whenmosscouldnotbefoundwithintheactivestreamchannel,itwascollectedfromthechannelwallsoroverbank.Themossandsedimenttrappedwithinthemosswerecollectedtogether,andthesedimentwaslaterremovedbyagitatingthesamplesinwater(Arbogastandothers,1991).Themosssampleswerethendried,groundinamill,ashed,andchemicallyanalyzed.

Duringthecourseof thisstudy,1,151stream-sedi-mentsampleswerecollected.Atotalof799heavy-min-eral concentrates were collected and microscopicallyexamined for their mineralogical content; 662 of thesesamples were chemically analyzed, the remainder con-tainedinsufficientmaterialtoperformthelaboratoryanal-yses. Stream-sediment and heavy-mineral-concentratesampleswereanalyzedformulti-elementsuitesbysemi-quantitative emission spectrography (Gray and others,1988a).Stream-sedimentsampleswerealsoanalyzedforselectedelementsbyinductivelycoupledplasmaanaly-sis (Motooka and others, 1988) and for gold, mercury,tellurium,andthalliumbyatomicabsorptionspectropho-tometry(Hopkinsandothers,1991).Themineralogicalcontentsofthenonmagnetic,heavy-mineral-concentratesamplesweredeterminedmicroscopically(Bennettandothers,1988).

Althoughsearchedforateverysite,aquaticmosseswerelocatedandcollectedatonly863ofthesites.Thesesamples were analyzed for multi-element suites bysemiquantitativeemissionspectrography(Arbogastandothers, 1991). Some 1,050 stream-water samples werecollected and analyzed (Gray and others, 1988b), butthesesamplesweredeterminedtobeanineffectivegeo-chemical exploration medium (Gray and others, 1992)andwillnotbediscussedfurther.

Threemapsshowingthedistributionofanomalousconcentrationsofselectedelementsinstreamsediment,heavy-mineral concentrate, and aquatic moss samples(Grayandothers,1997a,c,andd)wereusedextensivelyfor this resource assessment.The value of the concen-trationsselectedasanomalousgenerallyliebetweenthe90thtothe95thpercentile.Forthemapthatshowsthedistributionofmercury,antimony,andarsenic,Grayandothers (1997a) plotted locations of samples having thefollowingconcentrations:0.80ppmHgand15ppmSbinstreamsediments;200ppmSband1,000ppmAsinheavy-mineral concentrates; 1–5 percent visible cinna-

8

barinconcentrates;and50ppmSband200ppmAsinmoss samples.For themap that shows thedistributionofgoldandsilver,Grayandothers(1997c)plottedloca-tions of samples having the following concentrations:0.01ppmAuandless than0.5ppmAginstreamsedi-ments;20ppmAuand1ppmAginheavy-mineralcon-centrates; any visible gold in concentrates; and 3 ppmAginmosssamples.Forthemapthatshowsthedistri-butionofcopper,lead,zinc,tin,andtungsten,Grayandothers (1997d) plotted locations of samples having thefollowingconcentrations:100ppmCu,70ppmPb,300ppmZn,10ppmSninstreamsediments;300ppmCu,300 ppmPb, 1,500 ppmZn,more than 2,000 ppmSnin heavy-mineral concentrates; and 1–5 percent visiblescheeliteinconcentrates.

NUREGEOCHEMICALSURVEY

In1981 theU.S.DepartmentofEnergycompleteda reconnaissance geochemical study of the Iditarodquadrangle, as part of the National Uranium ResourceEvaluation (NURE) program. During the study, 888stream-sediment, 505 lake-sediment, and 1,410 hydro-geochemicalsampleswerecollected.Compositestream-sediment sampleswerecollected fromsmallfirst-orderstreamsatadensityofaboutonesampleper23km2(9mi2). Inswampy lowlandareaswherestreamsedimentisscarce,lake-sedimentsampleswerepreferentiallycol-lected.Hydrogeochemicalsampleswerecollectedfromasmanystreamsandlakesaspossibleinthequadrangle.Allsampleswereanalyzedat theLosAlamosNationalLaboratory by various methods including X-ray fluo-rescence, fluorescence spectroscopy, plasma-sourceemission spectrometry, instrumental neutron-activation,arc-sourceemissionspectrography,anddelayed-neutroncounting (NationalUraniumResourceEvaluationProj-ect,1983).

TheNUREdatawereevaluatedbyGrayandothers(1988c)usingR-modefactoranalysistodefinethemajorgeochemical associations in the stream-sediment andlake-sediment data sets. This technique identifies ele-ment groups (called factors) that tend to be associatedwithmineraldeposittypesorwithrocklithologies.TheNUREstream-sedimentdatabestfitasix-factormodel,while the lake-sediment data were best described by afive-factormodel. Inbothcases, twoof themodel fac-torswereinterpretedtoindicatemineraldeposittypes:aprecious-metalfactorandabase-metalfactor(Grayandothers,1988c).However,asfurtherdescribedbelow,thepredictivepowerofthesefactorsisuncertain.

FortheNUREstream-sedimentdata,anFe-Mn-Asfactor ismost consistentwithgeochemical favorabilityforprecious-metalmineraloccurrences.However,sincenoAuvaluesabovethedetectionlimitarefoundinthedataset,theassociationofAuwiththeFe-Mn-Asfactorisnotdocumented.AlthoughSbandWwereeliminated

fromthefactoranalysisbecausemostsampleswerebelowthe detection limit for these elements, stream-sedimentsamplescontaininganomalousconcentrationsofoneorbothoftheseelementsareoftenassociatedwiththeFe-Mn-Asfactor(Grayandothers,1988c).Stream-sedimentsamplesshowingthisfactorarefoundin11areas(Grayandothers,1988c,fig.1).Fourof theseareascoincidewithknownprecious-metal lodeorplacerdeposits(theFlat,Moore,Ganes,andDonlinCreekareas;nos.107,83,10,and195,respectively,mapA).Oftheremainingsevenareas,one liesdownstream froma resourcearea(Granite Mountain) that is outlined as potentially con-tainingprecious-metaldepositsbasedonotherdata(seeresource area 3i description in later section).The finalsix areas that show the Fe-Mn-As factor are not sup-portedbyotherdataasbeingparticularlyfavorableforprecious-metaloccurrences,andprobablydonotconsti-tuteresourceareas.ThesesixareaslieintheeasternandsouthernpartsoftheIditarodquadrangle;theirlocationsmaybefoundinGrayandothers,(1988c,fig.1).

Asecondelementassociation(Cu-Cs-Ni)determinedfromtheNUREstream-sedimentdataset is interpretedto be favorable for base-metal mineral occurrences.Stream-sedimentsamplesthatshowtheCu-Cs-NifactorarefoundineightsmallareasintheIditarodquadrangle(Grayandothers,1988c,fig.1).Twoof thesecoincidewithknownAg-Sn-polymetallicmineralizedpartsoftheBeaver Mountains (see resource area 3a description inlater section).A third small area lies in the vicinity ofanother resource area outlined as potentially contain-ing Ag-Sn-polymetallic deposits based on other data(seeresourcearea3gdescriptioninalatersection).Theremainingfivesmallareasaredifficulttoexplainandarenotconsideredsignificant.Theylie in thesouthernandsoutheasternpartsoftheIditarodquadrangle;theirloca-tionsmaybefoundinGrayandothers(1988c,fig.1).

The geochemical results from lake-sediments col-lectedduringtheNUREprojectwerealsoexaminedbyGrayandothers (1988c)usingR-mode factoranalysis,andtwofactors(As-Mn-FeandPb-Cs-Cu-Ni)wereinter-pretedaspossiblemineraldepositassociations.Thelackof proximity of the lake-sediment samples to exposedbedrockmakes interpretation of these factors difficult.TheAs-Mn-Fe factor was judged most consistent witha precious-metal association (Gray and others, 1988c),althoughonlyoneanomalousgoldvalue(520ppb)wasreported in the NURE lake-sediment data. Gray andothers (1988c, fig. 2) indicated lake-sediment samplesshowingtheAs-Mn-Fefactorarefoundinsevenareas,twoofwhichareneartheknowngoldprospectsnearFlat(no. 107, map A). Three other areas roughly coincidewithresourceareasoutlinedinthisreportaspotentiallycontaining precious metal-bearing deposits based onotherdata(seeresourcearea2e,3f,and5bdescriptionsin later section). The anomalies in the remaining two

9

areasareprobablynotsignificant.TheseareaslieinthesoutheastpartoftheIditarodquadrangle;theirlocationsmay be found inGray and others (1988c, fig. 2). Thelake-sedimentPb-Cs-Cu-Nifactorwasjudgedmostcon-sistent with a base-metal association (Gray and others,1988c).Theonlyareacontainingasignificantnumberoflake-sedimentsamplesthatshowthisfactoristheBeaverMountains, where severalAg-Sn-polymetallic depositsareknown(BundtzenandLaird,1982;thisreport).

Water samples were collected from streams andlakesintheIditarodquadrangleduringtheNUREstudy,but the geochemical data from these samples are diffi-culttointerpret.Forexample,stream-watersamplesthathaveanomalousconcentrationsofAg(≥6ppb)arescat-teredwidelyacrossthestudyarea.Lessthan40percentof these samples cluster into specificareas andnumer-ous single-site anomalies are distant from delineatedresource areas. Also, stream-water samples containinganomalousconcentrationsofCu,Zn,andFdonotclus-terwellandareequallydifficulttointerpret.TheNUREstream-water samples appear to be poor indicators ofcopper-bearing deposits. Of four areas known to con-tainsignificantcopperinmineraloccurrences,onlytwoshowedanomalousCuvaluesintheNUREstream-waterdata.Asimilarproblemwasobserved in thedata fromstream-water samples collectedduring this study.Grayandothers(1992)foundthatstream-watersampleswerelesseffective than stream-sedimentandheavy-mineral-concentratesamplesindetectingunderlyingmineralizedrockintheIditarodquadrangle.Lake-watersamplescol-lectedduring theNUREstudyare sounevenlydistrib-utedandheavilybiasedtowardthewesternlowlandsthattheydonotprovidedatausefulforthisstudy.

Insummary, theNUREstream-sedimentand lake-sedimentdata setsprovideduseful supportive informa-tion for assessing themineral resourcesof the Iditarodquadrangle.Inaboutadozencases,geochemicalanoma-liesdefinedbytheNUREdataclusterinareasthathavebeendelineatedasresourceareasinthisstudy.However,anumberof resourceareas thatwere foundduring theAMRAPinvestigationswouldnothavebeenfoundwiththeNUREdatasets.Inafewcases(discussedabove),theNUREdata indicateanomalies thathavenotbeencon-firmedbytheAMRAPgeochemicaldata.Theseareasaredifficulttoreconcile,butthegeologicalandgeochemicalevidencecollectedduringthisstudydonotsupportdesig-nationoftheseareasasfavorableformineralresources.

GeophysICs

Nonewgeophysicaldatawerecollectedduringthisinvestigation.Previouslypublisheddataincluderegionalgravity and reconnaissance aeromagnetic maps. ThegravitydataweresummarizedbyBarnes(1977),Barnesandothers(1994),andMeyerandothers(1996),butthe

coverage in the Iditarod quadrangle is sparse and thepublished maps did not provide information useful inevaluatingeitherthemetallicornonmetallicendowmentof theregion.Areconnaissancemagneticsurveyof theIditarodquadranglewasflownin1979fortheNationalUranium Resource Evaluation (NURE) using a broad,6-mile, flight-line spacing. A machine-contoured totalmagnetic intensity anomaly map summarized this data(NURE,1982).Therawdata(AeroServiceCorporation,1980)waslaterreprocessedandmergedwithdatafromneighboringquadrangles(MeyerandSaltus,1995).

Althoughnotdetailedenoughtoassist inoutliningfavorable areas for mineral deposits, the aeromagneticdata locally reinforcegeologic interpretationsbasedonfieldmapping and suggest the subsurface continuationofbothavolcanicrockunitandafaultzone.Themostprominentfeatureofthemergedaeromagneticmap(seeMeyerandSaltus,1995,sheet1,mapB)isachangeinthemagneticcharacterfromthewestsidetotheeastsideoftheIditarodRiverinthewesternpartofthequadran-gle.Apatternoflocalizedmagnetichighsandlowschar-acterizes the regionwestof the IditarodRiver, an areaprimarily underlain by subaerial volcanic rocks (unitTKy).Totheeast,inanareaprimarilyunderlainbyKus-kokwimGroupflysch,themagneticpatternisrelativelyfeatureless.Theaeromagneticdatasuggestthatthevol-canicrocks(andtheirunderlyingbasementrocks)con-tinueundertheYetnaRiverswampwestofDikeman,aconclusionconsistentwiththemappedgeology.Amajorsouthwest-trending fault system (Miller and Bundtzen,1994)inthenorth-centralpartofthequadranglefollowsthe boundary suggested by the magnetic data for halfof itsdistance.Theaeromagneticdatasuggest that thisfaultzonecontinues in thesubsurface to thesouthwestcornerofthemaparea.Justeastofthisfaultzone,nearthenorthernboundaryoftheIditarodquadrangle,amag-netichigh(seeMeyerandSaltus,1995,sheet1,mapB)reflectsthepresenceofmaficandultramaficrocks(unitJdr)oftheDishnaRiverarea(Miller,1990;MillerandBundtzen, 1994). This anomaly dies out to the south-west,consistentwiththegeologicmapping.

Theavailablegeophysicaldataisnotdetailedenoughtoprovidedefinitiveanswers,butdoesofferlimitedsup-portofgeologicinterpretationoftheregion.Acquisitionofmoredetailedgeophysicaldata,particularlymagneticdata,wouldclarifytheobservedanomaliesandcontrib-utesignificantlytotheresourceevaluation.

mInes, pRospeCts, mIneRAL oCCUR-RenCes, And RoCk AnomALIes

Muchoftheknowledgeofthemineralresourcesofthe Iditarod quadrangle is derived from the search for,and the development of, gold and mercury prospects.About51,700kg(1,500flasks)ofmercurywererecov-

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eredfromtheDeCourcyMountainMine(no.184,table1).Anestimated99.1kg(3,185oz)ofgoldwasproducedfrom two lodemines (IndependenceandGoldenHorn,no.16andno.110,respectively,table1).Thevastmajor-ity of the gold produced in the quadrangle was recov-eredfromplacerdeposits.ThetotalplacerproductionfortheIditarodquadrangleisapproximately65,000kg(2.1millionoz)ofgoldandbyproductsofsilver,antimony,andtungsten.Thisfigureforgoldproductionis25per-centgreaterthanthevalueobtainedbyaddingindividualpropertyproductionnumberslistedintable1(whichtotalaminimumof1,682,257ozofgold).Thediscrepancyisprimarilyduetothenatureoffederal,state,andterritorialproductionfigures,whicharegenerallycompletefordis-trictsandsubdistricts,butincompleteforspecificstreambasinsorindividualmines.

Thestudyareacontainspartsoffourplacerminingdistricts defined by Ransome and Kerns (1954)—theIditarod, Innoko, McGrath, and Aniak districts (fig.4). Placerdepositswerefirstdiscovered in the Innokodistrict in1906,andIditaroddistrict in1908.By1917,almostallotherknownplacerdepositshadbeen foundor were developed. Exceptions of note were the Gran-itePup(ofWillowCreek)andDeadwoodCreekplacerdeposits, which were found in 1924 and 1932, respec-tively(nos.142and56, table1andmapA).Lodedis-coveriesfollowedtheplacerminingdevelopments.TheIndependence gold mine in the Innoko district was inproductionaround1912.TheDeCourcyMountainmer-curyminewasdiscoveredaround1910,stakedin1919,andproduced intermittently from1921 to1949.At theGolden Horn lode, in the heart of the Iditarod district,underground mining began in 1922; the last ore wasshippedin1937.

The earliest published mineral studies of the areaincluded thoseofMaddren(1910,1915),whoreportedon gold placer developments in the newly discoveredcampsintheInnokodistrictandtheDonlinareaoftheAniak district, and Mertie (1936) and Mertie and Har-rington (1924),whodescribedactivitiesof the IditarodandInnokodistrictstentotwentyyearsaftertheirrespec-tive discovery dates. Brooks (1916), Cady and others(1955),Kimball(1969),BundtzenandLaird(1980),andBundtzenandothers (1988,1992)havedescribedvari-ousactivitiespertainingtothedevelopmentoflodegold.A detailed summary of historical mine developmentsnearFlatwaspublishedbyBundtzenandothers(1992).Topical resource work by theADGGS, in cooperationwith this study, was focused on heavy-mineral prov-enancestudiesofplacerdeposits(Bundtzenandothers,1987)andindividuallodeandplacerprospectexamina-tions (Bundtzen and Laird, 1982, 1983; Bundtzen andothers,1985,1986).Maloney(1962)andSainsburyandMacKevett(1965)summarizedthegeologyandresourcepotential of mercury deposits in the region, informa-

tionthatwasimportanttothenation’sstrategicmineralassessmentsofpreviousyears.WhiteandKilleen(1953)investigated the area for radioactive deposits and alsoevaluatedthestrategicmaterialsduringthe1950’s.Sum-mariesof preciousmetal-bearing lodes associatedwithLate Cretaceous and early Tertiary igneous rocks, andantimony-mercury lodes have recently been published(BundtzenandMiller,1997;Grayandothers,1997b).

The locations and descriptions of known mineraldepositsandoccurrences,whichareimportantfordelin-eatingresourceareas,arepresentedintable1andonmapA. For this report, the following definitions are used:Mine—anysitethathasrecordedproduction,nomatterhowsmalltheamount;Prospect—anysitewherethereisevidenceofpastorcurrentexploratorywork,suchasaprospectpit;Occurrence—oremineralsarepresentandthe site ismore significant thana single-sampleanom-aly; Rock anomaly—rock containing anomalous metalvalue(s);oremineralsmaybepresent,butarenotcon-firmed.Table 1 includes a comprehensive summary ofpublishedandunpublisheddataonthepreviouslyknownminesandprospects.Italsoincludesinformationonpre-viously unreported prospects, new mineral occurrencesfound during this investigation, and rock grab samplescontaining anomalous metal values; also included aresiteswheregoldwasvisuallyidentifiedinheavy-mineralconcentratesamples.Themineraldeposittypethatbestfitseachlocalityisalsolisted,usingthesevenmetallicmineraldepositmodelsdefinedforthisstudy,whichareoutlinedintable2.

pRoCedURe FoR AssessInG metALLIC mIneRAL ResoURCes

METHODSOFASSESSMENT

The assessment of the metallic mineral resourcesoftheIditarodquadranglefollowsthetechniqueusedinmanyAlaskamineralresourceassessments(forexampleRichterandothers,1975;PattonandMoll,1983;Nelsonandothers,1984;andMillerandothers,1989),whereinresourceareasaredefinedprimarilybygeologicandgeo-chemicaldata,butrefinedbymine,prospect,andmineraloccurrenceinformation.Geophysicaldataarenormallyalsoconsidered,butthereconnaissance-scaleNUREdataavailablefortheIditarodquadrangleplayedonlyaminorroleindefinitionofresourceareas.

Thisreportdepartssomewhatfromtheprobabilistic“three-part quantitative assessment” that has been uti-lizedbytheUSGSinvaryingformssince1975.ThethreepartsoutlinedbySinger (1993)maybesummarizedasfollows:(1)resourceareasaredelineatedonthebasisofdeposittypepermittedbythegeology,(2)gradeandton-nagemodelsareusedtoestimatethepotentialamountofcontainedmetal,and(3)whereappropriate,probabilistic

11

estimatesaremadeofthenumberofundiscovereddepos-itsofeach type.The Iditarodassessment follows thesethreestepsinageneralway,butitistailoredtoaccountforparticularsthatrelatetotheIditarodquadrangle,suchasthelevelofknowledge,thelackofoutcropexposures,thelocallydetaileddataavailable,andtheneedforinfor-mation related tobothmineral resource evaluationandland planning purposes. An “unnumbered” final step(HarrisandRieber,1993)of the three-partquantitativeassessmentmethodutilizesacomputerprogramtocom-bine theprobabilisticestimates (part3)withgradeandtonnagedata(part2)toobtainaprobabilitydistributionfortheamountofcontainedmetalorthe“gross-in-placevalue”(GIPV)ofthemineralresource.GIPVestimatesareusefulforland-planningpurposes(forexampleBrewandothers,1992),particularlywhencomparingandcom-piling thevalueofavarietyof resourcesforanygivenlandparcel.However,GIPVestimatescanbemisleadingand,giventhatnospecificlandusedecisionsarecurrentlypendinginIditarodquadrangle,nonearereportedhere.Thismineralassessmentdoesprovideprobabilisticesti-matesofnumbersofundiscovereddeposits,fromwhichGIPVestimatesmaybecalculated.Presentingprobabi-listicvalues(withoutGIPVestimates)emphasizestheirinherentsubjectivenatureanduncertainty,bothofwhichareeasilyignoredwhenaGIPVdollarvalueisstated.

The resource areas of the Iditarod quadrangle aredefined using seven mineral deposit models deemedappropriate based on the geology and geochemistry.These deposit models generally follow definitions andconcepts presented in Cox and Singer (1986), but alsoincorporate recently published mineral deposit modelconceptspresentedduringcomparisonsbetweenAlaskanandnortheastRussianmetallogenicbelts(Noklebergandothers,1993,1997).Applyingthefirststepinthethree-partassessmentmethod,thesevendepositmodelsguideddesignationofresourceareas.Theseareasoffavorabilityare further classified into the three-tiered hierarchy ofhigh, moderate, or low mineral resource potential fol-lowingthedefinitionsthatwerefirstsuggestedbyTaylorand Steven (1983), adopted by Goudarzi (1984), andlatersummarizedbyHansen(1991)(seetable3)foruseinjointUSGSandU.S.BureauofMinesmineralsurveysofFederallands.Inthisreport,46prospectiveareasaredelineatedandeachareaisrankedashavinghigh,mod-erate,orlowresourcepotentialfortheoccurrenceofoneormoreofthesevenmodeltypes.Areaspermissibleformorethanonedeposittypemaycarrymultiplerankings,thatis,theareamayhavehighpotentialforonedeposittypeandmoderatepotentialforaseconddeposittype.

The degree of confidence placed in ranking aresourcearea(ashigh,moderate,orlow)dependsontheadequacyofdataavailableandthelevelofunderstandingofthedeposittype.Thisreportusesthelevelofcertaintyrating(definedintable3)proposedbyVoelkerandothers

(1979) and adopted by Goudarzi (1984) and Hansen(1991)tohelpconveythisvariation.Acquisitionofaddi-tional information would positively affect the assignedconfidence level (become more certain), but wouldn’tnecessarily change the area rank of high, moderate, orlowmineralresourcepotential.

ESTIMATESOFUNDISCOVEREDRESOURCES

Thefinalstepintheassessmentprocesswastoesti-mate the number of undiscovered deposits of selecteddeposit types for the Iditarod quadrangle. These esti-matesweremadeby consensus amonga committeeoffourconsistingofthethreeauthors(geologist,economicgeologist,andgeochemist)joinedbyanadditionaleco-nomic geologist, who is knowledgeable about the areaandresources.Thismethodisavariationofthe“densityofdepositmethod”discussedbyCox (1993), inwhichthe number of undiscovered deposits is inferred for apoorlyknownareabycomparingittoageologicallysim-ilar,well-exploredareaforwhichthedensityofdepositsis known. Some of the deposit models formulated fordescription of metallogenic belts in the Iditarod quad-rangledonotstrictlyadheretomodelspublishedbyCoxandSinger(1986).Inthesecases,tonnage/gradecurveswererevisedtoaccommodatethedifferences.

Thecommitteeconcludedthatestimatingthenumberofundiscovereddepositswouldbeappropriateforfourofthesevendepositmodels(table4).Forthesefourmodels,sufficientgeologiccontrolisavailableandthemodeltypeis sufficiently understood (including supportive data ofmines, prospects, and occurrences) to make reasonableestimates.Themeansizeconsideredforeachdeposittypecomesfromtheappropriatetonnage/gradecurvesforthatmodel(eitherCoxandSinger,1986,ordefinedherein).Estimatesweremadeatthe90,50,and10percentcon-fidence levels. The estimates given in table 4 roughlyapproximate probability distributions for deposits ofmodeltypes1through4.Probabilisticestimatesofcon-tainedtonsofmetalwerenotgeneratedforthisreport.

Quantitativeestimateswerenotmadefortheremain-ingthreedepositmodels(models5,6,and7,table2).Thecommitteeconcludedthatmodel5(volcanic-hostedepi-thermal)wasunsuited toquantitativeestimation in Idi-tarodquadrangle,becauseofsparsedataandthemodelisonlypoorlyunderstoodasappliedtotheYetnavolca-nicfield(unitTKy).Thecommitteealsodidnotestimateundiscovereddepositsforbedrockmodel6(mafic-ultra-mafic related Cr-Ni-Co). The geology of the DishnaRivermafic-ultramaficbeltiswelldefinedandthemodel(Cox and Singer, 1986) is well understood; however,in the Iditarodquadrangle, there are nooccurrencesorgeochemicalanomaliesthatsuggestthisdeposit typeispresent;hence,theestimatednumberofdepositswouldhavebeenverylow.Perhapsthemostdifficultdecision

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among committee members was to not quantitativelyestimateresourcesfortheheavy-mineralplacerdeposits(model7).Hereisawell-definedmodelforwhichthereis abundant data and hence great confidence in desig-nationofareasofhighpotential.However,theIditarodquadrangle has been explored for placer deposits by adedicatedgroupofsamplers—theplacerprospectorsaswellasourgeochemicalinvestigations.Ifundiscoveredplacerdepositsexist,theywouldlikelyliewithinoneoftheresourceareas,mostlikelyinoneofthefiveareasthathavehighpotentialbutlittleornoknownplacerdevelop-ment.Given thehigh-levelofplacerexploration in thequadrangle, it is not likely that any of thesefive areaswouldyield anewdeposit equal toorgreater than themedianfortheregion.Instead,itismorelikelythatundis-coveredresourceswillcomeintheformofextensionsofknowndepositsorminingofverysmalldeposits.

ASSESSMENTPRESENTATION

The main focus of this study was to evaluate theundiscovered mineral resources of the Iditarod quad-rangle.However,giventhatboththeassessmentandthecontributing data have multiple applications, the dataare summarized in several ways. The descriptive textand detailed mine, prospect, and mineral occurrencetable provide a comprehensive summary of the avail-abledataforgeneralinterestpurposes.Delineationofthemost favorable resourceareasbydeposit typeprovideskey information for evaluatingundiscovered resources,butmayalsoguidemineralexploration.ResourceareasderivedonthebasisofdepositmodelsarepresentedonmapB.Landplannersontheotherhandarenotusuallyconcernedwithdeposittype,butratherwiththerelativevalueofregionsandthelikelylocationofpossiblefuturemining.Thedataaresummarizedinaformatmoreusefulfor land planning purposes in figure 3 where depositmodelsarecombinedtoemphasizerelativetotalmineralendowment;thesecompositeareasaredesignated“landtracts.”

Resourceareas—themodel-basedassessment

One or more resource areas are outlined for eachof thesevenselectedmineraldepositmodels.Forty-sixresourceareas(someoverlapping)aredefinedandshownonmapB.Eachresourceareacarriesanumber(1to7)toreflectthemineraldepositmodelthatprimarilydefinesit,followedbyalettertoidentifyindividualareas.Hence,thefourareasdescribedbymodel1arelabeled“1a”to“1d”;thefiveareasdefinedbymodel2arelabeled“2a”to “2e”; and soon (mapB).Some resource areasmayalso containdeposits definedbyothermodels.Tohelpillustratethis,eachdepositmodelisrepresentedbyapat-tern (or in the caseofmodel 7, nopattern); areas rep-resentedbymorethanonemodelwillhaveintersectingpatterns.

Informationabouttherank(high,moderate,orlow)of each resource area with respect to deposit model isalsoconveyedonmapBbycolor.Redpatternsdenotehighmineralresourcepotentialforthatparticularmodel;a pink pattern denotes moderate potential; and a bluepattern denotes low mineral resource potential. Areasdefinedbymodel7(placer)arenotpatterned,butinsteadoutlinedbyboldblacklines;alldesignatedplacerareasareconsideredtohavehighpotential.

Eachofthe46resourceareasisdescribedinsomedetail in the following pages. The main characteristicsofthe35loderesourceareasaresummarizedintable5;theirassignedrank(high,moderate,orlow)isalsolistedalongwiththelevelofcertainty.Table6presentsasum-maryofthemaincharacteristics,rank,andlevelofcer-taintyforthe11placerresourceareas.

Landtracts—theland-basedassessment

Resource areas that are delineated on the basis ofmineral deposit models are grouped together into 23land tracts on figure 3. The purpose of this summarydiagram is to define and rank the tracts of landon thebasis of total potentially contained metallic resourcesor mineral endowment. The tracts are designated by aletterfollowedbyanumber.Thelettersdenoteveryhigh(VH), high (H), moderate (M), and low (L) total min-eralendowmentpotential.Thenumbersindicaterelativerankingwithineachofthesecategories.Thelandtractswere rankedbasedonnumeric scoresassignedaccord-ingtomodel(andcommodity)represented,weightedbytheirresourcepotential(thatishigh,moderate,orlow).Atractshowinghighpotentialforseveraldeposittypesscored higher than a tract showing moderate potentialforonedeposittype;atractshowingonlylowpotentialscoredzero.Theacreagecoveredbyatractwasnotcon-sideredindeterminingitsrelativevalue.Table7listsforeach land tract themodel-derived resource areas (frommapB)itcontainsandliststhetractsinorderofdecreas-ingoverallrelativeendowmentvalue.Nodollarfiguresareattachedtothesetracts.

Land tracts having moderate, high, or very highmineral endowment potential are readily distinguishedbecausetheyareshaded;landtractsoflowpotentialareunshaded,butlabeled(L1orL2);theregionofunknownresource potential is unshaded and not labeled. Thelandtractsshowingmoderate,high,andveryhighmin-eral endowmentpotential are indicatedby increasinglydarker shading. Future mining would most likely takeplacewithinthetractsofdarkestshading.

mIneRAL deposIt modeLs And AReAdesCRIptIons

The mineral resources of Iditarod quadrangle aredescribed in terms of seven different mineral deposit

13

types.Analysis of individual prospects and districts inthestudyarea,includingexaminationofthegeochemicalcontentofores,thestructuralandbedrockgeology,andelemental patterns in soil, rock, stream sediments, andheavy-mineral concentrates, and isotopic data contrib-utedtotheselectionofthesesevendeposittypes(table2).SixoftheseventypesaredescribedinsomedegreebymodelspublishedinCoxandSinger(1986);theremain-ing deposit type (the peraluminous granite-porphyry-hosted gold-polymetallic) is a new deposit model typedescribedforsouthwestAlaskainBundtzenandMiller(1997).

Peraluminous granite-porphyry-hosted gold-polymetallicdeposits(model1;table2)havegeological,geochemical, traceelement,andisotopiccharacteristicsthatsignificantlydifferfromstandardpublishedmineraldeposit models.Although these deposits have some ofthe attributes of porphyry Cu-Au (Cox, 1986b, model20c)orporphyrygold(RytubaandCox,1991)deposits,they lackalterationandtraceelementcharacteristicsofthe published mineral deposit types.The peraluminousgranite-porphyry-hosted gold-polymetallic deposit typemaybemostsimilartothegranitoid-relatedgoldsystemsofSidorovandRozenblum(1989)orAksenova(1990),assummarizedinNoklebergandothers(1993).Bundtzenand Miller (1997) compared the peraluminous-granite-porphyrygold-polymetallicdepositsoftheKuskokwimmineral belt with granite and alaskite-hosted aurifer-ous deposits of the Mineral Ridge and Divide districtsof southwestNevada (Spurr,1906;Bercawandothers,1987; Prudden and Jucevic, 1988), and the Maiskoyegold deposit in northeast Russia (Sidorov and Eremin,1995; Nokleberg and others, 1993, 1997). Laboratoryandfielddatacollectedduringtheir(andthis)studysug-gestthatmineraldepositmodel1,assummarizedinthisanalysis,containsattributesofbothhigh-andlow-tem-perature metallogenesis. Bundtzen and Miller (1997)proposed that the peraluminous-granite-porphyry gold-polymetallicdepositsprobablyconstituteanewsubclassofgranitoid-relatedgolddeposits.

IntheIditarodquadrangle,twodeposittypesoccurin association with volcanic-plutonic complexes: plu-tonic-hosted copper-gold-polymetallic stockwork andveindeposits(model2,table2)andtheplutonic-related,boron-enriched, silver-tin-polymetallic deposits (model3,table2).Thehostplutonsforthelatterdeposittypearegenerallylessdifferentiatedthantheintrusionsthathostthe former deposit type (Bundtzen and Miller, 1997).Boronmetasomatismusuallycharacterizesthesilver-tin-polymetallic systems,but is sometimesassociatedwiththe copper-gold polymetallic deposits. Since these twodeposit types may be spatially related, resource areasdescribedforonemodelareusuallyalsoassignedamod-eratepotentialfortheothermodel.Theplutonic-hostedcopper-gold-polymetallic stockwork and vein deposits

somewhatresemble theporphyryCu-AumodelofCox(1986b, model 20c) and the polymetallic vein modelofCox (1986a,model 22c), but also are similar to thealkaliccopper-goldporphyrymodelofVilaandSillitoe(1991).Theplutonic-related,boronenriched,silver-tin-polymetallic deposits best conform to the porphyry tinmodelofReed(1986,model20a)andtotheSn-polyme-tallicveinmodelofTogashi(1986,model20b).Model3isalsosimilartotheRussiantinsilicate-sulfidedeposittypedescribedinNoklebergandothers(1993).

Epithermal deposits are widespread in the studyarea. Epithermal mercury-antimony (gold) deposits(model 4, table 2) hosted in rocks of the KuskokwimGroup, resemble thehot-springmercurydepositmodelofRytuba(1986,model27a),exceptthatthehostrockissedimentary,notvolcanic.Thevolcanic-hostedpreciousandothermetalsdeposits(model5,table2)resemblethehotspringsAu-AgmodelofBerger(1986,model25a),butourinformationisnotdetailedenoughtodistinguishspecificmodeltypes.

Mafic-ultramafic rocks of the quadrangle are per-missiblehostrocksforpodiformchromite(Albers,1986,model8a)orLimassolForestCo-Ni(Page,1986,model8c) deposits. No mafic-ultramafic associated mineraloccurrencesareknownforthestudyarea,soanypoten-tialcommoditiesrelatedtotheserocktypesareincludedintheCr-Ni-Codeposittype(model6,table2).

Theheavymineralplacerdepositsofthestudyarea(model7,table2)conformroughlytotheplacerAu-PGEdepositmodelofYeend(1986,model39a).

General descriptions of the seven mineral deposittypes, accompanied by descriptions of the supportivegeology,geochemistry,andknownmineraloccurrencesfor each resource area defined by thatmodel, are pro-videdbelow.

PERALUMINOUSGRANITE-PORPHYRY-HOSTEDGOLD-POLYMETALLICDEPOSITS(MODEL1)

Goldresourcesassociatedwithperaluminousgran-iteporphyry,alaskite,andminorgranodioritesills,dikes,and small stocks have only recently been identified inthe Iditarod quadrangle although their association withplacergolddepositshavebeenknownforyears(Mertie,1936; Bundtzen and Miller, 1997).These gold-bearingintrusions mainly consist of porphro-aphanitic to fine-grainedphaneriticvarietiesofgranite,alaskite,andminorgranodiorite that intrude theKuskokwimGroup flyschthroughout the Iditarod quadrangle in northeast-trend-ingbeltsalmostcertainlycontrolledbyhigh-anglefaultzones.Usuallythesedikeswarmscontaintwoormore,clearlydefined,intrusiveevents.K-Arisotopicagesfromthegraniteporphyryandalaskite intrusionsrangefrom71.5to63.5Ma(MillerandBundtzen,1994).Geochemi-cally,thedikesandsillsexhibitheavyREEdepletion,are

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peraluminousandcorundumnormative,andcontainhighRb/Sr ratios,allofwhichareconsistentwith theirsus-pectedoriginascrustalmeltsinaregionalzoneofhighheatflowwithinanextensionalenvironment(BundtzenandSwanson,1984;Bundtzenandothers,1992;MillerandBundtzen,1994;BundtzenandMiller,1997).

Oremineralogyconsistsofarsenopyrite,pyrite,stib-nite, complex arsenate-sulfosalts, free gold, minor cin-nabarandrarecassiterite.Sulfidesandsulfosaltsappearto be disseminated in the intrusive rocks and structur-ally localized (with quartz) in shear zones and faults,either in the intrusiverocksor in immediatelyadjacentKuskokwim Group sedimentary rocks. Cinnabar, gold-arsenate,andstibnitearecommonlynotedas late-stageintroductionsinvugsorbrecciazonesinveins.Weather-ingcharacteristicsincludeminorgossanandbleachinginoxidizedzones.Microprobeanalysesindicatethatmostofthegoldoccursinlatticestructuresofpyriteandarse-nopyrite, hence “refractory.” Representative elementalcontentfromfourdepositsintheKuskokwimmineralbelt(BundtzenandMiller,1997)indicatesstrongcorrelationbetween gold, silver, arsenic, antimony, and mercury.Thegranite-porphyry-hostedgold-polymetallicdepositsaredepletedincopper,lead,zinc,andmolybdenum.

Weakargillic,phyllic,silicic,andpotassicalterationand dickite have been recognized in granite porphyrydikes in the Donlin deposits (no. 192, mapA) and atSpauldingCreek(no.11,mapA).Dolomiteandankeritereplacementofpotassiumfeldsparphenocrystshasbeennoted atDonlin and in theYankeeCreek-GanesCreekdikeswarm(attheIndependenceMine,no.16,mapA).Biotite and othermaficminerals commonly show pro-pyliticalterationingraniteporphyriesofthestudyarea;butitisunclearhowmuchofthisisduetonearsurfaceweathering or to hydrothermal alteration. Siderite-cal-citeveinalterationiscommoninthealtered,mineralizeddikes.

Trace elements derived from rock, moss, stream-sediment, andheavy-mineral-concentrate samples fromeachdepositordepositareaalmostalwaysshowelevatedlevelsofAu,Ag,As,Sb,andHgandlocallyshowele-vatedlevelsofB,Cu,Pb,Sn,W,andZn(table5).

Cumulative evidence summarized by BundtzenandMiller(1997)suggests that theperaluminousgran-ite-porphyry-hosted gold-polymetallic deposits formedunderbothmesothermalandepithermalconditions.Fourmineralresourceareascontainingthegraniteporphyry-gold polymetallic model have been delineated.All arerelatedtoconcentrationsoffelsicdikesorsillsintrudingKuskokwimGroupsedimentaryrocks.

Resourcearea1a,YankeeCreek-GanesCreekdikeswarm

Resourcearea1a(mapB)hasahighpotential(cer-tainty levelD)forcontainingperaluminousgranite-por-

phyry-hostedgold-polymetallicdeposits.Area1aincludesmineralizedgraniteporphyrydikesandsills(unitTKgp)that cut Kuskokwim Group sedimentary rocks (primar-ily unit Kks) and slivers of Innoko terrane basementrocks(unit^Mc)alonganarrow,50-kmlong,curvilin-earbeltfromFourthofJulyCreeknortheastwardtotheupperdrainagebasinsofYankeeandGanesCreeksinthenorthernpartofthequadrangle.Thedikeandsillswarmliesalongamajorfaultsystem,theYankee-GanesCreekFault(MillerandBundtzen,1994),whichhasbroughtuppre-Cretaceous basement rocks. This northeast-trendingstructuralbelt,referredtobyBundtzenandMiller(1997)astheGanes-YankeeCreekdikeswarm,controlsthedis-tributionofatleast35separatebodiesofgraniteporphyrydikes, smallmonzonite stocks, and alteredmafic dikes.TheYankeeCreek-GanesCreekdikeswarmisapparentlytruncatedonthesouthbytheIditarod-NixonForkFault,amajorright-lateraltranscurrentfaultinwesternAlaska(MillerandBundtzen,1988,1994).

Resourcearea1ahasyieldedatleast5,042kg(about162,100oz)ofgoldand699kg(about22,500oz)ofsilverfrom six placer mines in the Ganes Creek andYankeeCreek drainage basins (nos. 1, 10, 11, 12, 13, and 19,table1andmapA).Thehighestconcentrationofmetallicmineralizedrockoccursinazone4kmby0.5kmnearthe divide separating Yankee and Ganes Creeks. LodemineraldepositsknownintheresourceareaincludetheIndependencegoldmine(no.16,table1),whichproduced14.9 kg (478 oz) of gold around 1912, and the nearbyKatzantimonyprospect, (no.15, table1).Bothdepos-itsarecontainedin,orborderon,structurallycontrolledgranite porphyry dikes. At the Independence deposit,disseminatedarsenopyrite,pyrite,cinnabar,stibnite,andstephaniteoccurindikerockandalteredsandstoneoftheKuskokwimGroup.Carbonatealteration,whichisquitepersistent in the deposit, is controlled by hairline frac-tures inwall rockandhostdikes.Fiveotherprospects,occurrences,orrockgeochemicalanomaliesknowntoberelatedtograniteporphyryintrusionsoraltereddikesarealsoaretobefoundinthisresourcearea(nos.2,14,17,18,and43,table1).Visiblegoldwasobservedinseveralquartzcarbonateveinsingraniteporphyryandinveinsadjacenttodikes(nos.17and18,table1).BundtzenandLaird(1980)believedthatvirtuallyallplacergoldintheYankee Creek and Ganes Creek drainages originatedfromtheseandsimilarundiscoveredlodesources.

Anomalies instream-sediment,heavy-mineral-con-centrate,andaquaticmosssampleshelpdefineresourcearea1a.Oneheavy-mineral-concentratesamplecollectedfromSpauldingCreekcontainsvisiblegold.Onestream-sediment sample collected from a tributary of GanesCreekcontains0.5ppmAg,100ppmCu,and70ppmPb.Fromthecentralpartofresourcearea1a,onestream-sedimentsamplecontains23ppmAsandanothercon-tains 12 ppm Hg.A heavy-mineral-concentrate sample

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from the latter site has 5–20 percent microscopicallyvisiblecinnabar;aconcentratesamplefromanothersitein central1ahas300ppmPb.Central1a alsohasonemosssamplecontaining2,000ppmAsandanothercon-taining150ppmSb.Thesouthwesternpartofresourcearea1aincludesonestream-sedimentsamplecontaininglessthan0.5ppmAgandoneheavy-mineral-concentratesamplecontaining2,000ppmSnand1–5percentcinna-bar;anaquaticmosssamplecontains100ppmSb.

Resourcearea1b,GraniteCreekarea

Resourcearea1b(mapB)hasahighpotential(cer-taintylevelD)forcontainingperaluminousgranite-por-phyry-hosted gold-polymetallic deposits. The area isdefinedbyperaluminousgraniteporphyrydikesandsills(unitTKgp)andasmallmonzonitestock(unitTKm)thatcutKuskokwimGroupsedimentary rocksandoccur inanelongatebelt that extends20kmfrom just southofWillow Creek, a tributary of the George River, north-eastward to theupperdrainagebasinofLittleWaldrenForkoftheTakotnaRiver.Duringthisstudy,auriferousstibnitegashveinswere found ingraniteporphyryandshalehostrockattheWyricklode(Bundtzenandothers,1986;thisstudyno.143,table1)andintwoadditionalmineralized zones associated with small granite por-phyrybodies(nos.123and144,table1).Withinresourcearea1b,WillowCreekandGranitePup(no.142,table1)haveintermittentlyproducedmodestquantitiesofplacergold(101kgor3,250oz)since the1920’s; thegraniteporphyryswarmisthesuspectedlodesourceofthecom-mercialplacerdeposits.

Resourcearea1bissupportedbylocalanomaliesinstream-sediment, heavy-mineral-concentrate, and mosssamples. One stream-sediment sample contains 0.046ppmAu,anothercontains31ppmAs,anda thirdcon-tains0.86ppmHg.Aheavy-mineral-concentratesamplefromthethirdsitealsocontains1–5percentmicroscopi-cally visible cinnabar. A heavy-mineral-concentratesample from the gold-bearing site contains 7,000 ppmB.Another heavy-mineral-concentrate sample contains5,000 ppm zinc, and microscopically visible cinnabar(1–5percent), stibnite (<1percent),cassiterite (<1per-cent),andscheelite(<1percent).Anaquaticmosssample(fromthesitethatyielded31ppmAsinthestream-sedi-mentsample)contains3,000ppmAs.

Resourcearea1c,JulianCreekarea

Resourcearea1c(mapB)hasahighpotential(cer-tainty level C) for containing peraluminous granite-porphyry-hosted gold-polymetallic deposits. The areaincludes granite porphyry dikes and small stocks (unitTKgp)thatintrudeKuskokwimGroupsedimentaryrocksalonganarrow,35-kmlong,east-northeast-trendingbeltthat extends from the North Fork of the George RiverthroughJulianCreektoabout10kmeastoftheGeorge

River.Threelodemineraloccurrenceswerefoundinthisresourceareaduringtheinvestigations.Theyinclude:(1)amineralizedquartzveinstockworkingraniteporphyryattheheadofJulianCreek(no.159,table1),whichcon-tainsupto10g/tonneAg,greaterthan0.2percentAs,11ppmBi,3.3ppmCd,and100ppmSn;(2)limonitebrec-cias infracturedhornfelsalongthecontactofagraniteporphyryintrusion(no.162,table1),wheregrabsamplescontainupto250ppbAu,2,000ppmSb,610ppmAs,and200ppmW,andmorethan14ppmHg;and(3)mas-sivestibniteveinsinbrecciatedhornfelsnearcreekleveleastofno.162above,wheregrabsamplescontainupto1percentSb,200ppmSn,and60ppmAs(no.163,table1).OneadditionalHganomaly(1.6ppm,no.164,table1)isassociatedwithagraniteporphyrydikewithinthisarea.Resourcearea1chasyieldedatleast373kg(12,000oz)ofgoldand51.6kg(1,660oz)ofsilverfromthreeplacerdeposits (nos.160,161, and172, table1).Goldandotherheavymineralsfoundintheseplacerdepositsareprobablyderivedfromgraniteporphyrybodiesattheheadwardportionofeachrespectivedrainage.

Boththesouthwestlimitofresourcearea1b(GraniteCreekarea)and thenortheast limitof resourcearea1c(JulianCreekarea)aretruncatedalonganorth-northeast-trendingtranscurrentfault(MillerandBundtzen,1994).Bothareaswereprobablypartofasinglebeltofmineral-izedgraniteporphyrydikesandsillsthatwasrightlater-allyoffset12kmalongthisfault.

Anomalous stream-sediment, heavy-mineral con-centrate, and moss samples help define resource area1c.Onestream-sedimentsamplecontains0.10ppmAuandanothercontainslessthan0.25ppmAu.Threeotherstream-sediment samples contain 89 ppmAs, 1.2 ppmHg,and5.0ppmHgand0.5ppmAg.Oneheavy-min-eral-concentrate sample contains more than 2,000 ppmSnandthatsample,plusoneadditionalsample,containless than 1 percent microscopically visible cassiterite.Anotherheavy-mineral-concentratesamplecontains1–5percent microscopically visible scheelite. One aquaticmosssamplecontains200ppmSb.

Resourcearea1d,DonlinCreektrend

Resourcearea1d(mapB)hasahighpotential(cer-taintylevelD)forcontainingperaluminousgranite-por-phyry-hosted gold-polymetallic deposits. The area is a50-kmlongnortheast-trendingbeltdefinedbyaseriesofgraniteporphyrysillsanddikes(unitTKgp)thatintrudeKuskokwimGroupsedimentaryrocks.ThebeltextendsfromOmegaGulch in thesouthernpartof the IditarodquadranglenortheasttoBonanzaCreek,whereitistrun-catedbytheIditarod-NixonForkFault.Thisstructurallycontrolled dike and sill swarm contains the importantDonlin gold polymetallic deposits (nos. 192 and 196,table 1), which have been the subject of intense min-eralindustryexplorationinrecentyears(Retherfordand

16

McAtee,1994;Dodd,1996;Szumigalaandothers,2000).ExtensionoftheresourceareatothenortheastofDonlinissupportedbyadditionalmineraloccurrencesandrockanomalies.Atleast918kg(29,500oz)ofplacergoldhasbeenminedfromdeposits(nos.188,191,193–195,and205,table1)inthisresourcebelt.

TheDonlindepositsliewithina2kmby6kmareain the southwestern part of resource area 1d. Spatiallyassociated with the deposits are at least three separategenerations of granite porphyry dikes (Retherford andMcAtee, 1994); although composed of distinct, cross-cutting intrusive events, the period of intrusion likelyspannednomorethanfivemillionyearstime(MillerandBundtzen,1994).Eightdiscrete,gold-antimonyarsenic-bearing mineralized zones including the Lewis/Roche-lieu, Queen, Snow, 400, ACMA, Duqum, Dome, andFarSide (nos. 192 and196, table1) havebeen identi-fiedintheDonlinprospectarea(Dodd,1996;Szumigalaand others, 2000). Four separate exploration programsconducted by four different mining companies during1987–2001haveassessedtheDonlinCreeklodesystem.On the basis of about 15,000m of drilling conductedduring 1987–1990 by Western Gold Exploration andMiningCompany(supplementedbyadditionalworkbyTeckExplorationLtd.in1993),theDonlinmineralizedareawasestimatedtocontain3.87milliontonnesgradingabout3.15g/tonnegoldorabout12,190kg(379,110oz)gold(RetherfordandMcAtee,1994).Explorationworkanddrilling conductedbyPlacerDomeU.S. Inc. from1995 to 1998 substantially added to the gold resourcebaseatDonlin.ThisworkincreasedtheDonlinresourceestimateto61milliontonnesof3.4g/tgoldor207,400kg (6.7 M oz) gold from all reserve categories (Dodd,1996;PlacerDomeInc.,4/27/98pressrelease).In2001,NovaGoldResourcestookovertheexplorationprogramat Donlin and based on further definition drilling andreassessmentofformerdrilldata, theyannouncedevenhigher resourceestimates.AsofApril2003 theDonlindepositisreportedtocontain259milliontonnesgrading3.10g/tonnegoldor792tonnes(25.45Moz.)gold(Szu-migalaandothers,2004).

Two additional occurrences were identified in thegeneral Donlin area during the course of our studies.Bothoccurrences (nos.189and197, table1)areasso-ciatedwithgraniteporphyrydikes—grabsamplesfromoneoccurrencecontainupto0.9ppmAu,1.5ppmAg,6.4ppmHg,and0.2percentAs;grabsamplesfromtheothercontainupto440ppmZn,200ppmAs,2.3ppmHg,andtraceAg.PlacergoldproductionfromtheDonlinareaamountstoabout730kg(23,430oz)goldand31kg(990oz)silverfromsevencreeksthatdrainthemineral-izedarea.

The northern-most peraluminous granite porphyryhostedgold-polymetallicoccurrencesinresourcearea1dlienearPrinceCreek(nos.135and136, table1)—one

contains2.9ppmAuand1.7ppmHg,theothercontainsgreaterthan1percentSb,1ppmAg,300ppmPb,120ppmAs,andtraceAu.Inthecentralpartofresourcearea1dgrabsamplesofgraniteporphyrydikesfromtwosites(nos.157and170,table1)contain50ppbAuandupto1.8ppmHg.

Withinresourcearea1d,foursamplesitesneartheDonlinprospectsyieldedanomalousmetalvalues fromstream-sediment, heavy-mineral-concentrate, and (or)aquatic moss samples.A stream-sediment sample fromAmericanCreek(justsouthoftheDonlinprospects)con-tains0.011ppmAuand110ppmAs.Astream-sedimentsamplefromQueenGulchcontains0.035ppmAu,100ppmAs,0.5ppmAg,and1.6ppmHg;theheavy-min-eral-concentratesamplefromthissitecontainsmorethan2,000ppmSnandlessthan1percenteachofmicroscopi-callyvisiblesphalerite,scheelite,andcinnabar.Astream-sedimentsamplefromSnowGulchcontains41ppmAsand the heavy-mineral concentrate contains 2,000 ppmSn.Aheavy-mineralconcentratefromDomeCreek(nearthe northern border of the Donlin prospects) contains1,000ppmWandlessthan1percentmicroscopicallyvis-iblecassiterite.Aquaticmosssamplesfromthreeofthesefoursitescontainasmuchas200ppmSband5,000ppmAs.Inthenorthernpartofresourcearea1donestream-sedimentsamplecontains0.010ppmAuandoneheavy-mineralconcentratefromadifferentsitehaslessthan1percentmicroscopicallyvisiblecassiterite.

Although outlined primarily as a gold-bearingresource area, 1d also carries a moderate potential forepithermal Hg-Sb deposits (underlying resource area4m).At seven localitieswithinarea1d (nos.134,168,169,171,185,186, and187, table1),grab samplesofgraniteporphyrydike,orofcountryrockspatiallyassoci-atedwithfelsicdikes,containanomalousHg(1.2to7.1ppm)andlocallyupto700ppmAs.

Resourcearea1dmayoriginallyhavebeencontigu-ouswithresourcearea1a,butwasoffsetfromitsnorth-ernhalfalongtheIditarod-NixonForkFault.MillerandBundtzen(1988)proposedaright-lateraloffsetsolutionalong the Iditarod-Nixon Fork Fault of about 90 km,basedmainlyontheapparentmatchingofnearlyidenti-calvolcanicsectionsintheDeCourcyMountainareaonthesouthandtheBeaverMountains-MooreCreekregiononthenorth.Thissameoffsetsolutionwouldeffectivelymatch the trend of the Donlin Creek granite porphyrydikeandsillswarm(resourcearea1d)withtheYankeeCreek-GanesCreekdikeswarm(resourcearea1a).

PLUTONIC-HOSTEDCOPPER-GOLD-POLYME-TALLICSTOCKWORKANDVEINDEPOSITS

(MODEL2)

Plutonic-hosted, copper-gold-polymetallic stock-workandveindepositsare locatedin thevolcanic-plu-toniccomplexesoftheIditarod-Flat,MooreCreek,and

17

Mount Joaquinareas.Thevolcanic-plutoniccomplexesareprobably remnantsof stratovolcanoes emplaced inaback-arcsettingduringLateCretaceousandearlyTer-tiarytime.Plutonsandvolcanicfieldshavebeenlocalizedalongzonesof trans-tensioninstrike-slipfaultsystemssuchastheIditarod-NixonForkFault.

The monzonitic intrusions of the Iditarod-Flat areahost the best examples of the mineral deposit model 2type(Bull,1988;Bundtzenandothers,1992).Theintru-sionsare ilmenite-series,magnetite-poor (I type),meta-aluminous, alkali calcic to quartz alkalic suites thatpetrographicallyexhibitawelldevelopedolivine-ortho-pyroxene-diopside-biotitedifferentiationtrend.Feldspargeothermometrydataindicateshallow,epizonalemplace-ment depths for the intrusions in the Iditarod-Flat area(Bull,1988).Mineralogyofthedepositsincludespyrite,free gold, arsenopyrite, scheelite, stibnite, silver sulfo-salts, and minor cinnabar.Also sphalerite, chalcopyriteandmolybdenitearefoundlocally,andtantalum-enrichedilmenorutileandcassiterite,althoughrare,havealsobeennotedinlodesoftheIditaroddistrict(Bundtzenandothers,1992;BundtzenandMiller,1997).Principlemetallicele-ments are copper, gold, silver, arsenic, and antimony.Elevatedlevelsoflead,tungsten,bismuth,uranium,andmolybdenumalsooccurinthisdeposittype.Silver/goldratiosfromfiveplutonic-hostedcopper-gold-polymetallicstockworkandveindepositsintheKuskokwimmineralbeltaverage3.3:1(BundtzenandMiller,1997).

Aprogressivesequenceofmineralizationspanningthe mesothermal and epithermal temperature ranges isindicated by detailed studies in the Iditarod-Flat area(Bundtzenandothers,1992;Bull,1988).Cinnabar-stib-nite veins occur in the highest levels of the deposits,whereashighertemperaturebase-metal-dominantassem-blages (mainly chalcopyrite/molybdenite-bearing veinstockwork)occurdeeperandearlierinthehydrothermalsystems.Mostorebodiesaretabulartosteeplydipping,duetotheinfluenceofhigh-anglestructuralcontrols.

Alterationtypes,whererecognized,include biotite+potassiumfeldspar+quartz+dolomite+ankeriteinplutoniccenters and propylitic (chlorite-iron oxide) alterationdistalfromintrusivecenters.Silicic,phyllic,gypsum,andalunite-kaolin zones typical of “gold porphyry” depos-its (Vila andSillitoe, 1991;Rytuba andCox,1991) areabsent. Secondary biotite and sericite alteration ubiqui-touslyoccurinthehornfelssedimentaryrockinthecon-tactzoneandalso inmostplutonicphases.Bull (1988)identifieddolomitebrecciasassociatedwithlatephasesofmineralizationattheGoldenHorndeposit(no.110,table1andmapA).Overall,lodesintheIditarod-Flatareaaregold-basemetalstockworkssuperficiallysimilartothosefound in alkalic copper-gold stockwork porphyry andvein systems described byVila and Sillitoe (1991) andbyCox(1986b,model20c).Associatedmineralizedveinsandshearzones,suchastheGoldenGround(no.87,table

1andmapA)andGoldenHorndeposits correspond todepositmodel22c(polymetallicveins)ofCox(1986a).

Resourcearea2a,MountJoaquin

Resourcearea2a(mapB)hasahighpotential(cer-tainty level C) for containing plutonic-hosted copper-gold-polymetallic deposits; it also has a moderatepotential (certainty level C) for the plutonic-related,boron-enrichedsilver-tin-polymetallicdeposittype.Theareaincludesthe12-km2monzonite-monzodioritecom-positepluton(unitTKm)onMountJoaquinandthesur-roundinghornfelssedimentaryrock.Oneprospectnearthesummitofthemountain(no.32,table1)consistsofthin,gold-bearingveinsthatcontaincinnabarandarse-nopyrite.TheveinletscutbothintrusiverockandsmallroofpendantsofcalcareoussandstoneoftheKuskokwimGroupalongaN.10ºW.toN.10ºE.fracturesystem.Theveinletsrangefrom10cmto100cminwidthandextendalong strike for about150m,basedon limited surfacetrenching.Disseminatedchalcopyriteoccursina200-m2areaofsilicifiedmonzoniteabout1kmwestofthearse-nopyrite-cinnabarquartzveinletsdescribedabove.Weaktomoderatesecondarybiotitealterationenvelopesbothmineralizedareas.

In the reconnaissancegeochemistry study, samplescollected within or downstream from resource area 2aprovidesupportivedata.Fourstream-sedimentsamplescontainAuranging from0.011 to0.018ppm.Of thesesamples,threealsocontainHg(1.3to22.2ppm).Heavy-mineralconcentratescollectedat these latter threesitesalso contain microscopically visible cinnabar (1 to 20percent),scheelite(upto5percent),andlocallygold(<1percent)and1,000ppmW.Thegold-bearingsite(no.33,table1)isdownstreamfromthelodeprospectdiscussedabove.Oneadditionalstream-sedimentsamplecontains300ppmZn.Oneaquaticmosssamplecontains200ppmAs.

Resourcearea2b,MooreCreek

Resource area 2b (map B) has a high poten-tial (certainty level D) for containing plutonic-hostedcopper-gold-polymetallicdeposits; it alsohasamoder-atepotential(certaintylevelC)fortheplutonic-related,boron-enrichedsilver-tin-polymetallicdeposittype.Theareaincludesmonzonitetoquartzmonzonitestocks(unitTKm) that intrude coeval volcanic rocks (the IditarodVolcanics,unitsTKilandKit)nearMaybe,Willow,andMooreCreeks.Thevolcanic-plutoniccomplexintrudesandoverliesKuskokwimGroupsedimentaryrocks(unitKks). Hornfels aureoles extend about 1 km from thecontactsof the exposed intrusive rocks and local areasof gossan occur in the volcanic rocks. The stocks arealignedalonganorth-trendingzonesuggestingemplace-mentwas structurally controlled (Bundtzenandothers,1988).Westoftheplutons,distinctiveboronmetasoma-

18

tismhasreplacedoriginalvolcanictexturesinhostrocks.Ineffect,theplutonic-related,boron-enrichedsilver-tin-polymetallicdeposittypeissuperimposedovertheplu-tonic-hostedcopper-gold-polymetallicdeposittypeintheMooreCreekareasuggesting theyaremetallogenicallyrelated,perhapsthroughhydrothermalzonation.

Plutonic-hostedcopper-gold-polymetallictypedepositsandplacergoldmineraloccurrencesareassoci-atedwiththestocksandlocallywithhornfelsinresourcearea2b.TheBrokenShovelprospect(no.82,table1),isamesothermal,sulfide-tourmalinegreisendevelopedinthestocknearMooreCreek.Oremineralsidentifiedincludetetrahedrite, arsenopyrite, and scheelite, which collec-tivelycomprise5–10percentofthegreisen-veinsystem(Bundtzen and others, 1988). Secondary biotite floodsfracturesandlocallygroundmassinhornfelsandplutonicrocks. The rest of the occurrences and rock anomalieswithin resource area2b are associatedwithvolcanicorsedimentaryrocksinthehornfelsaureole.Inthewesternpartoftheresourcearea,agrabsampleofhornfelsvol-canicrockcontains0.85ppmAg,200ppmPb,9.5ppmHg,and60ppmAs(no.59,table1).Alsointhewest,agrabsampleofiron-oxidestainedvolcanicrockcontainsslightlyelevatedHg(no.57,table1).Inthenorthernpartofresourcearea2b,scatteredsilver-mercury-goldanoma-liesareassociatedwithshearandfracturezonesintheIdi-tarodVolcanics.Onemineraloccurrence(no.62,table1)containsupto5.1ppmAgand5ppmHg.Grabsamplesfromtwoothersites(nos.61and63,table1)containupto1.4ppmAu,0.2ppmAgandmorethan10ppmHg.

Placer deposits containing gold, silver, chromite,andcinnabararefounddownslopeordownstreamfromtheexposedstocks inresourcearea2b.The twoplacerminesintheareahavetogetherproducedatleast1,723kg(55,420oz)goldand391kg(about12,600oz)silver.ThesourceofgoldfortheplacerdepositattheheadofFourthofJulyCreek(no.60, table1) isbelievedtobethe nearby monzonite stock.All placer gold producedfrom Moore Creek (no. 83, table 1) is believed to bederivedfromthemineralizedMooreCreekpluton.TheplacermineconcentratesfromMooreCreekalsocontainhighamountsofchromite(Bundtzenandothers,1988),but the source is uncertain. Chromite was identifiedfrommaficphasesof theBlackCreekstock in the Idi-tarod-Flatarea(Bull,1988);suchphaseshavenotbeenrecognized in theMooreCreekpluton,butexposure ispoor.TheplutonexposednearMaybeCreekisthelikelysourceofgoldpannedfromMaybeCreek(no.58,table1).ThelodesourceforanunexploitedplacerdepositonDeadwoodCreek(no.56,table1)mayalsobethissamepluton.Twoadditionalplacergoldoccurrences(St.Pat-rickCreek,no.57andWillowCreek,no.81,table1)alsoliedownstreamfrommonzonitestocksofresourcearea2b.Oneplacergoldoccurrence(no.55,table1)lieswestofresourcearea2b,butthesourceisuncertain.

Stream-sedimentsamplescollectedduringtherecon-naissancegeochemicalsurveyaddadditionalsupportfordesignationofresourcearea2b.Stream-sedimentsamplescollectedfromWillowCreeklocallycontainupto0.077ppmAuand23ppmAs;heavy-mineralconcentratesfromthesesamesitescontainup to500ppmPb,1,500ppmW,andmorethan5,000ppmB(andalsohavelessthan1percentmicroscopicallyvisiblecassiterite).Astream-sedimentsamplefromnorthofVABMWillowhas0.056ppmAu; the heavy-mineral concentrate contains 5,000ppmWandmorethan5,000ppmB.FromtheheadofFourthofJulyCreek,onestream-sedimentsamplecon-tains0.88ppmHg;theheavy-mineralconcentratefromthatsitecontains5,000ppmSb,500ppmW,andmorethan 2,000 ppm Sn (plus 1–5 percent microscopicallyvisible cinnabar); a nearby heavy-mineral-concentratesample contains 5,000 ppmB. In the northern part ofresourcearea2b,onestream-sediment samplecontains30ppmAsandaheavy-mineralconcentratefromadif-ferentsitehasmicroscopicallyvisiblecinnabar(1–5per-cent) andchalcopyrite (<1percent), andcontainsmorethan 5,000 ppmB. In thewestern part of the resourcearea,oneheavy-mineralconcentratehasmicroscopicallyvisible gold (no. 55, table 1,mentioned above), cinna-bar (1–5percent), and scheelite (<1percent), andcon-tains300ppmAgandmorethan1,000ppmAu.Anotherheavy-mineral concentrate from a site 2 km northwestcontains50ppmAg,500ppmAu,100ppmMo,and500ppmW.Twoadditionalheavy-mineralconcentratesfromthewesternpartof theresourceareacontainup to200ppmW.Scattered aquaticmoss samples from resourcearea2bcontainupto200ppmSb.

Resourcearea2c,FourthofJulyCreekarea

The small resource area 2c (map B) has a moder-atepotential(certaintylevelB)forcontainingplutonic-hosted copper-gold-polymetallic deposits; it also has amoderate potential (certainty level B) for the plutonic-related, boron-enriched silver-tin-polymetallic deposittype.The area consists of a small monzonite intrusion(unit TKm) and the surrounding hornfels sedimentaryrockneartheBonnieCreek-FourthofJulyCreekdividein the vicinity of the Iditarod-Nixon Fork Fault. Themonzonitehostsmineralizedrockcontainingbasemetalsulfides,buttheextentisuncertain.Asinglegrabsample(86BT93; no. 64, table 1) of monzonite near hornfelscontainsanomaloussilver(1ppm),zinc(190ppm),andarsenic (30ppm).Nostream-sedimentsamples,heavy-mineralconcentrates,oraquaticmosssampleswerecol-lectedinthisimmediatearea.

Resourcearea2d,Iditarod-Flatarea

Resource area 2d (map B) has a high poten-tial (certainty level D) for containing plutonic-hostedcopper-gold-polymetallicdeposits; it alsohasamoder-

19

atepotential(certaintylevelC)fortheplutonic-related,boron-enrichedsilver-tin-polymetallicdeposittype.Thearea includes three alkali gabbro to quartz monzonitecompositestocks(unitTKm),thehornfelsvolcanicandsedimentary rocks they intruded (units TKil and Kks,respectively),andnumeroussmalldikesofintermediatetomaficcomposition(unitTKd).Themonzoniticstockslineupalonganorth-northeast-trendinghigh-anglefault.Resourcearea2dcontainstheplacergolddepositsoftheIditarod-Flatarea(nos.89,104–107,109,111,112,129,131,and132,table1andmapA),whichupto1990,hadproduced about 45,200 kg (1,450,000 oz) of gold andabout 6,100 kg (197,000 oz) of silver (Bundtzen andothers,1992).Thisamountofgoldproduction isabout1,000 kg higher than the amount obtained by addingtogethertheproductionfigureslistedintable1;asindi-catedinaprevioussection,recordedproductionfiguresarecommonlyprovidedbydistrict,andthenumbersaredifficulttorestorecreekbycreek.TheGoldenHornlodemine(no.110, table1)producedanadditional84.2kg(2,707oz)ofgold,81.5kg(2,620oz)ofsilver,4,235kglead,and296kgofzinc.

Polymetallic-gold deposits hosted in the intrusiverocks of the Flat area have been described in detail byBull(1988),Bundtzenandothers(1992),andBundtzenandMiller (1997)andwillbebrieflysummarizedhere.Principle deposits include the Golden Horn, ChickenMountain,MalamutePup,andtheIdaho(nos.110,130,111, and105, respectively, table1).ThebestknownofthesearetheGoldenHornandChickenMountaindepos-its.Mostdepositsareshallow,cupola-hosted,vein-faultandquartzstockworkconcentrationsofgold,silver,arse-nic,antimony,mercury,andtungsten.MineralassemblageandgeothermometryestimatessummarizedbyBundtzenandothers(1992)andBull(1988)indicatethatallmineraldepositsintheFlatarearepresentazonedhydrothermalsystemnowexposedatseveralerosionallevels.Highesttemperature/pressure mineral assemblages (mesother-mal)weredepositedearlyinthedeepeststructurallevelswhereas lowest temperature/pressure mineral assem-blageswere subsequentlydeposited inhigh level, near-surfaceenvironments.Onthebasisoftheirownsurfacesamplingandlimitedundergroundcontrol(drilling)madeavailable by private firms, Bundtzen and others (1992)estimatedthattheGoldenHorndepositcontainsinferredreservesof2.85milliontonnesgrading1.2g/tonnegoldand3.4grams/tonnesilver,1.9percentarsenic, and2.0percent antimony. The Chicken Mountain deposit con-tainsabout14.5milliontonnesgrading1.2g/tonnegold,4.6g/tonnesilver,0.09percentcopper,and0.46percentantimony(BundtzenandMiller,1997).Bothestimatesoftheresourcearejudgedtobeconservative.

Threeotherprospectsandoneoccurrenceinresourcearea2dareassociatedwithfracturedhornfelsratherthan

directly with the exposed stocks. The Golden Groundprospectnorth(no.87,table1)andthenearbyunnamedprospect(no.88,table1)containgold-bearingquartzveinsthat cut brecciated country rock. Samples from GoldenGround contain up to 50 g/tonne gold and 2 kg/tonnesilver;grabsamplesfromthenearbyprospectcontainupto1.0ppmAuandAg;bothprospectsalsohaveanoma-lousSb,As,W,andHg.Anunnamedprospect(no.108,table1)thatliesbetweentheGoldenHornandChickenMountaindepositshasquartzveinsinsandstonehornfelsthatcontain11ppmAuand7ppmAg.OnthesouthsideofChickenMountain, anoccurrencenearPrinceCreek(no.133,table1)consistsofveinletscontaining3.4ppmAu,10ppmAg,1,380ppmW,andvisiblecinnabarthatcuthornfelsandanintermediatedike(?).

Stream-sediment, heavy-mineral-concentrate, andlocallyaquaticmosssamplescollectedduringtherecon-naissance geochemical survey add supportive data forresourcearea2d.Onesamplesite(I0374)neartheplacerworkingsofBlackCreek(no.109,table1)yielded1.55ppmAuand1.0ppmAginthestreamsediment;500ppmAg,3,000ppmSb,1,500ppmAs,2,000ppmW,and50ppmBi in the heavy-mineral concentrate; 1–5 percentmicroscopically visible gold and scheelite; and 2,000ppmAsand100ppmSbinaquaticmosssamples.Othersamples collected downstream from Chicken Moun-tainlocallycontainupto23ppmAs,0.5ppmAg,200ppmCu,andmorethan34ppmHginstream-sedimentsamples.Oneheavy-mineralconcentratecollectedonthesouthsideofChickenMountaincontains50ppmAg.AnaquaticmosssamplefromacreekdrainingtothenorthofChickenMountaincontains3,000ppmAs.Twosamplesites from the part of resource area 2d north of OtterCreekalsoyieldedanomalousvalues.Astream-sedimentsamplefromonesitecontains22.2ppmHg;theheavy-mineralconcentratefromthissitecontains70ppmAg,500 ppmAu, and 1,000 ppmW and has 5–20 percenteach microscopically visible cinnabar and scheelite.Aheavy-mineral concentrate from the other site contains500ppmSband200ppmW.

Resourcearea2e,MosquitoMountain

Resourcearea2e(mapB),intheextremesouthwestcornerofthequadrangle,hasahighpotential(certaintylevel C) for containing plutonic-hosted copper-gold-polymetallic deposits; it also has a moderate potential(certaintylevelB)fortheplutonic-related,boron-enrichedsilver-tin-polymetallicdeposittype.TheareaiscenteredaroundMosquitoMountainwhereseveralflow,tuff,andlocallysill-likebodiesofintermediatecomposition(unitKka) are interbedded with shallow marine to nonma-rinesandstone,conglomerateandshaleof theKuskok-wimGroup(unitKkq).Anumberofthin,discontinuous,altered intermediate dikes (unit TKd) and at least onegranite porphyry dike (unit TKgp) locally intrude the

20

sedimentaryrocks.BiotitefromaninterbeddedandesitetuffyieldedaK-Arageof77Ma(MillerandBundtzen,1994);however,thehypabyssalporphyrybodiesexhibitextensivehydrothermalalterationandwerenotisotopi-callydated.Thesedimentaryrockslocallyexhibiteffectsof thermal metamorphism (hornfels) near many of thesill-like intrusive bodies. Whereas the geology under-lying resourcearea2edoesnot include themonzoniticintrusive rocks associated with the other areas of highpotentialfortheplutonic-hostedcopper-gold-polymetal-licdeposittype(2ato2d,above),thegeochemicalsigna-turesexhibitedinbothrockandstream-sedimentsamplessuggestthismodeltype.

Resource area 2e contains one mineral occurrence(no. 174, table 1) that seems to fit the plutonic-hostedcopper-gold-polymetallic deposit type with the excep-tionthatitdoesnotcontaindetectablegold.Atthislocal-ity,grabsamplesofgossaninhornfelssedimentaryrockcontain up to 0.7 ppmAg, 640 ppmZn, 500 ppmCu,110ppmSb,and6ppmBi.Grabsamplesfromsixothersites lendsupport fordesignationof this resourcearea,buttheirelementalsuitesareambiguouswithrespecttomodel type.At five of these sites (nos. 176, 177, 178,179,and203, table1),gossanzonesoccur inhornfels,usually invicinityof igneous rock.Grabsamplesvari-ouslycontainupto0.7ppmAuand1.4ppmHg(atno.176,table1),upto10ppmAg(atno.203,table1),andupto200ppmZnand400ppmAs(nos.178and179,table1).Atoneadditionalsite(no.175,table1),agrabsample of intermediate volcanic rock interbedded withsandstonecontains34ppmSb.

Twoheavy-mineralconcentratesinresourcearea2eyieldedanomalousmetalvalues.OnesamplefromAmer-icanCreekcontains200ppmAg,70ppmBi,andmorethan1,000ppmAu;thissamplealsohasvisiblegold(no.204,table1).AnothersamplefromacreekdrainingthenorthwestsideofMosquitoMountaincontains20ppmAgand200ppmAu; thestream-sedimentsamplecon-tains1.1ppmHg.

TheNUREreconnaissancelake-sedimentdatapro-videsomeadditionalsupportforresourcearea2e.UsingR-modefactoranalysis,Grayandothers(1988c)outlinedanareathat lies in thesouthwestcornerof theIditarodquadrangle(seetheirfig.1)thatfitstheirprecious-metalrelatedfactor(As-Mn-Fe)outlinedforthelake-sedimentdataset.

PLUTONIC-RELATED,BORON-ENRICHED,SILVER-TINPOLYMETALLICDEPOSITS

(MODEL3)

Like the copper-gold polymetallic deposits (model2) described above, the silver-tin-polymetallic depositsare associated with volcanic-plutonic complexes. Thehostcompositeplutonsforbothmodeltypesrangefromdiorite to quartz syenite in composition and show pro-

gressive mineralogical differentiation, including earlyolivine, clinopyroxene, and later biotite. Hornblende iscommonlyfoundasamineralphaseinplutonsassociatedwithmineraldepositmodel3,whereas it is commonlyabsent in the plutons associated with mineral depositmodel 2. The stocks and plutons associated with thesilver-tinpolymetallicdeposits(model3)containinten-sive boron metasomatism, which, although sometimeslocallypresent,isnotcharacteristicoftheintrusiverocksassociated with the copper-gold-polymetallic deposits.TheK/Arisotopicagesobtainedfromplutonsassociatedwithbothdeposittypesrangefrom73to59Ma(MillerandBundtzen,1994).Basedontheselimiteddata,thereare two distinct crystallization age groups—63 to 59Maand73to68Ma.Theolderagesareassociatedwithdepositsofbothmodeltypes2and3,butwithoneexcep-tion (amafic phase from north ofChickenMountain),allofyoungeragescomefromplutonsassociatedwiththe boron-enriched, silver-tin-polymetallic type depos-its (at Swinging Dome, Granite Mountain, andVABMTatalina).ThisyoungeragerangeisthesameageastheMcKinley plutonic suite in the southernAlaska RangeandalsoSleitatMountain,northwestofIliamna,bothofwhich host stanniferous, “S” type leucogranites (ReedandLanphere,1969;Reedandothers,1978).

The plutonic-related, boron-enriched, silver-tin-polymetallicdepositsoftheIditarodquadrangleexhibitthe following morphological types: (1) en echelon,stockwork-like, tourmaline-quartz (sulfide) veining inhigh-levelplutoniccupolaphases;(2)circularordumb-bell-shaped, tourmaline-axinite breccia pipes in bothplutonicrocksoroverlyinghornfels;and(3)largetour-maline-sulfide flooding zones in hornfels and alteredvolcanic rocks showing extensive replacement by bothtourmalineandbrowntopurpleaxinite.Accompanyingthe boronmetasomatismaremultiple phases of sulfideandmetallicoxidemineralogythatincludeschalcopyrite,sphalerite,galena,arsenopyrite,pyrite,andminorcassit-erite, scheelite, Bi-sulfosalts, and ilmenorutile. Silver/goldratiosaverageabout3,000:1fromfiverepresenta-tivedepositsofthistypeintheKuskokwimmineralbelt(BundtzenandMiller,1997).Inadditiontosilverandtin,thesecomplexmineraldepositscontainvariableamountsofcopper,zinc,arsenic,antimony,niobium,lead,tanta-lum,indium,andbismuth.

BundtzenandMiller(1997)classifiedtheplutonic-related,boron-enriched,silver-tin-polymetalllicdepositsintheKuskokwimmineralbeltintoboth20a(porphyrytin; Reed, 1986) and 20b (Sn polymetallic; Togashi,1986) deposit models. Regardless of specific depositcorrelation, theplutonic-related,boron-enriched,silver-tin-polymetallicdepositsoftheIditarodquadranglecon-stituteanimportantdeposittypeonlyrecentlyrecognizedintheregionthatmayeventuallyprovetocontainsignifi-cantresourcesoftin,silver,andothercommodities.

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Resourcearea3a,BeaverMountains

Resourcearea3a(mapB),theBeaverMountains,hasahighpotential(certaintylevelD)forcontainingplutonic-related,boron-enrichedsilver-tin-polymetallicdeposits;italsohasamoderatepotential (certainty levelC) for theplutonic-hosted copper-gold-polymetallic deposit type.The area is underlain by a volcanic-plutonic complexconsistingofmonzonite-quartzmonzoniteintrusiverocks(unitTKm)andcoevalvolcanicrocks(unitsTKilandKit);hornfelsisfoundlocallyinthevolcanicrocksandinthesurroundingKuskokwimGroupflysch(unitKks)thatthecomplex intrudes and overlies. The Beaver Mountainsarecutbynumeroushigh-anglefaults.

Plutonic-related, boron-enriched silver-tin-polyme-tallicdepositsoccurinlargezonesofboronmetasoma-tismthroughouttheBeaverMountainsigneouscomplexandadjacenthornfels.Fourmorphologicaldeposittypeshave been recognized: (1) large and aerially extensivetourmaline-richbrecciapipesandreplacementzones incupolasoftheBeaverMountainsstock(Tolstoideposit,no.6,table1;BundtzenandLaird,1982;BundtzenandMiller,1997);(2)structurallycontrolledtourmaline-sul-fidezonesalongfaultsandjoints(Cirquedeposit,no.28,and nos. 27, 29, and 30, table 1; Bundtzen and Laird,1982;BundtzenandMiller,1997); (3) intrusion-hostedstockworks(nos.7and31,table1);and(4)axinite-tour-maline-richreplacementsinhornfelsbrecciasoroverly-ingalteredvolcanicrockssometimescontainingmassivesulfides(nos.8and9,table1).VisiblegoldwasfoundonTolstoiCreek,whichdrainsnorthfromtheBeaverMoun-tains(no.5,table1).Thisgoldmaybefromareworkedauriferousterminalmoraine,butwasprobablyoriginallyderivedfromtheBeaverMountains.

Intrusion-hostedmineraldeposits(1–3above)typi-callycontainvariableamountsofchalcopyrite,arsenopy-rite,pyrite,andminorgalena,cassiterite,andscheelite.Hornfels or volcanic-hosted occurrences can containabundantpyrrhotite,marcasite,andminorarsenopyrite,butrarelyotherbasemetalsulfides.TheTolstoidepositcontains the largest zones of boron-sulfide metasoma-tism.Metalgradesareerratic,butthreeprominenttour-maline breccia pipes total a minimum of 1.50 milliontonnes of mineralized rock (Cu,Ag, Pb, and Zn). TheCirquedepositcontainsthebestaveragegradedata.The159-m-longand3-m-widezoneidentifiedonthesurfacecontains an inferred reserveof175,000 tonnesgrading3.5 percent copper, 445 g/tonne silver, and containinganomaloustin,niobium,lead,andzinc.Takenasawhole,the plutonic-related, boron-enriched silver-tin-polyme-tallicdepositsandoccurrencesintheBeaverMountainscontainupto20percentcopper,1,000g/tonnesilver,2.0percentarsenic,1,000ppmtin,300ppmtungsten,0.79percentlead,1,000ppmniobium,40ppmberyllium,and2g/tonnegold.

Resource area 3a contains two additional knownsitesofmineralizedrock,butthesedonotappeartobeofthe plutonic-related, boron-enriched silver-tin-polyme-tallicdeposittype.NearLincolnCreek,anareaofalteredmaficandfelsicdikesthatcutKuskokwimGroupsedi-mentary rock adjacent to a high-angle fault has beenprospected (no. 44, table 1). Fine gold was reportedlypannedfromprospectpitsduringexploratorywork(JoeDegnan,oral commun.,1979).Grab samples from thisstudycontainupto0.15ppmAgand1.3ppmHg.Inthewestern BeaverMountains grab samples from an iron-oxidestainedzoneinavolcanicrockroofpendant(no.4,table1)containupto200ppmZnandmorethan10ppmHg.

Resource area 3a is supported by geochemicalanomaliesinnumerousstream-sediment,heavy-mineral-concentrate,andaquatic-mosssamples.Increeksdrain-ingwest,north,andeastfromtheBeaverMountains,Auvaluesarefoundin11stream-sedimentsamples(0.010–0.79 ppm) and two heavy-mineral concentrates (8 and700ppm).Thewest-drainingheadwatersofBillyGoatCreekincludetheCirqueprospect(no.28,table1)andtwootherlodelocalities(nos.27and30,table1).Sevensites in this area yielded anomalous samples: stream-sedimentsfromtwoofthesevensitescontainAu(0.010and0.079ppm),and locally the sevensitescontainupto2ppmAg,200ppmCu,150ppmPb,120ppmAs,0.94ppmHg,20ppmSn,and2,000ppmB;heavy-min-eralconcentratescontainupto100ppmAg500ppmW,more than2,000ppmBi,andmore than5,000ppmB;and aquaticmoss samples locally containup to5ppmAg,1,000ppmAs,and70ppmSb.Inthewest-drainingheadwaters of Windy Creek, six sites yield anomalousvalues: streamsediments fromfourof sixsitescontainAu(0.010to0.015ppm),andlocallythesixsitescontainupto130ppmAs,upto100ppmCu,upto7ppmMo,upto150ppmPb,andupto30ppmSn;oneheavy-min-eral concentrate contains500ppmW;and twoaquaticmoss samples contain up to 7 ppmAg and 1,000 ppmAs. From drainages in the northern part of the BeaverMountains (including theTolstoi prospect and theTol-stoiLakeoccurrence,nos.6and7,table1),sevenstreamsitesyieldanomalousvalues:streamsedimentsfromtwoofthesevensitescontainAu(0.016and0.027ppm),andlocallythesevensitescontainupto50ppmSn,500ppmZn,1.6ppmHg,and2,000ppmB;fourheavy-mineralconcentratescontainupto500ppmW,300ppmPb,1,500ppmSn,andmorethan5,000ppmB;oneheavy-mineralconcentrate contains microscopically visible gold, cin-nabar, and chalcopyrite (no. 5, table 1); and onemosssample contains200ppmAs.The east-draining creeks(Brown,Beaver,andGanesCreeks)havetenanomaloussitesbetweenthem:streamsedimentsfromthreeofthetensitescontainAu(0.010–0.056ppm),andlocallythetensitescontainupto1.5ppmAg,180ppmAs,1.5ppm

22

Hg,150ppmPb,20ppmSn,300ppmZn,and2,000ppmB; five heavy-mineral concentrates contain up to 300ppmCu,300ppmW,andlocallymorethan2,000ppmSn; aquaticmoss locally containsup to5ppmAgand1,000ppmAs.The east-facingdrainagebasin that liesjustsouthoftheheadwatersofGanesCreekyieldedthreeanomalous sample sites: a heavy-mineral concentratefromoneofthethreesitescontains700ppmAuand100ppmAg;streamsedimentslocallycontainupto1.0ppmAg,75ppmAs,70ppmPb,300ppmZn,and2,000ppmB;oneaquaticmosscontains5ppmAg.Creeksdrainingthe south side of the Beaver Mountains yielded noAuvalues,butstream-sedimentsamples locallycontainupto21ppmAs,6.6ppmHg,and70ppmPb.

The NURE reconnaissance stream-sediment dataprovide some additional support for resource area 3a.UsingR-mode factoranalysisGrayandothers (1988c)outlinedtwoareas,oneeastandonewestoftheBeaverMountains (see their fig. 1) that fits their base-metalrelated factor (Cu-Cs-Ni) outlined for the stream-sedi-mentdataset.

Resourcearea3b,TakotnaMountain

Resourcearea3b(mapB),TakotnaMountain,hasahighpotential(certaintylevelC)forcontainingplutonic-related, boron-enriched silver-tin-polymetallic deposits;italsohasamoderatepotential(certaintylevelC)fortheplutonic-hosted copper-gold-polymetallic deposit type.Theareaisunderlainbya16-km2volcanic-plutoniccom-plex(unitsTKmandTKil)andthesurroundinghornfelssedimentary rock (unit Kks) on the Iditarod-McGrathquadrangleboundary.Stockwork-liketourmaline-quartzveinsoccurinthewesternpartofthemonzodioriteplutonnearthehornfelscontactzone.Amineraloccurrenceofiron-richhornfelsandbrecciacontainingupto13.7ppmAg,280ppmCu,1,050ppmZn,and0.03ppmAuliesalongthepluton’ssouthernborder(no.23,table1).Grabsamplesofmonzonitecutbysulfide-bearingveinsandofiron-oxide stainedhornfels sandstone from thewesternpartoftheresourcearea(no.22,table1)containupto0.3ppmAg,30ppmAs,1.1ppmHg,andtraceAu.Sam-plesofalteredintrusiveandhornfelsvolcanicrockfromTakotnaMountainintheMcGrathquadranglecontainupto0.6ppmAg,222ppmCu,and100ppmPb(BundtzenandLaird,1983).

Inthereconnaissancedrainage-basinstudy,samplescollected from three sites within or downstream fromresourcearea3byieldedanomalousvalues.Onestream-sediment sample from the southern part of the areacontains 0.010 ppmAu. In the central part of the areaa stream-sediment sample contains35ppmAsand theheavy-mineral concentrate from the same site contains300ppmW,50ppmMo,andmorethan2,000ppmSn.Oneheavy-mineralconcentratefromacreekdrainingthenorthernpartoftheareacontains500ppmPb.

Resourcearea3c,VABMTatalinaarea

Resourcearea3c(mapB),nearVABMTatalinainthe eastern part of the study area, has a high potential(certaintylevelD)forcontainingplutonic-related,boron-enriched silver-tin-polymetallic deposits; it also has amoderate potential (certainty level C) for the plutonic-hosted copper-gold-polymetallicdeposit type.The areaincludes two quartz monzonite to syenite plutons (unitTKm)andgraniteporphyrydikesandstocks(unitTKgp)thatintruderocksoftheKuskokwimGroup(unitKks).A2-kmaureoleofhornfelssurroundstheplutonunderlyingVABMTatalina;theigneous-sedimentaryrockcontactoftheotherplutonisconcealed.Anextensivearea(2km2)of manganese-rich, tourmaline-quartz-biotite breccia inthishornfelsyieldsconsistentvaluesofupto5ppmAgand100ppmCu(no.49,table1).Downslopefromtheprospect,streamcobblescontainupto33ppmAg,0.10ppmAu,1,100ppmAs,and500ppmPb(no.48,table1).Inthewesternpartoftheresourcearea,grabsamplesfromashearedcontactzonebetweenagraniteporphyrystockandhornfelssandstonecontainupto1ppmHgand500ppmAs(no.47,table1).

In the reconnaissance drainage-basin study, sam-plescollectedfromsixsiteswithinordownstreamfromresourcearea3cyieldanomalousvaluesinstreamsedi-ment,heavy-mineralconcentrate,oraquaticmoss.Threeofsamplesiteswerecollectedfromstreamsthatdraintheeasternpartofthearea,inthevicinityofVABMTatalina.Atoneofthesesites,anaquaticmosssamplecontains50ppmAs.Theothertwosites,whicharedownslopefromtheprospectdiscussedabove(no.49,table1),containupto3ppmAg,120ppmAs,200ppmPb,15ppmSn,and15ppmBiinstream-sedimentsamples;upto15ppmAg,1,000ppmAs,5,000ppmPb,morethan2,000ppmBi,andmore than 5,000 ppmB in heavy-mineral concen-trates;andupto3ppmAginaquaticmoss.Aheavy-min-eralconcentratecollectedfromthesamedrainageduringanearlierstudycontains23ppmAg,5,700ppmPb,andtraceAu (no.48, table1; site79BT104,BundtzenandLaird,1983).Heavy-mineralconcentratescollectedfromthreesitesdownslopefromthequartzmonzoniteplutoninthecentralpartofresourcearea3ccontainupto2,000ppmSb;oneofthesesamplesalsocontains1–5percentmicroscopicallyvisiblecassiterite.

Resourcearea3d,CamelbackMountain

Resourcearea3d(mapB)hasahighpotential(cer-tainty level D) for containing plutonic-related, boron-enriched silver-tin-polymetallic deposits; it also has amoderate potential (certainty level C) for the plutonic-hostedcopper-gold-polymetallicdeposittype.TheareaisunderlainbytheIditarodVolcanics(unitsTKilandKit)andtoalesserextentbytheKuskokwimGroup(unitKks),allofwhichareintrudedbymonzoniticplutons(unitTKm).

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Mineralizedrock inresourcearea3doccursas: (1)sulfide-bearingzonesinhornfelsimmediatelyadjacenttoplutoniccontacts;and(2)disseminatedandveinsulfidesinvolcanicrocksdistantfromtheknownintrusiverocks.Grab samples of quartz-sulfide veins in hornfels sand-stonejustnorthoftheCamelbackplutoncontainupto12ppmAg,710ppmAs,600ppmZn,500ppmPb,70ppmSb,35ppmCd,300ppmNi,4.2ppmHg,andmorethan2,000 ppmB (no. 77, table 1). Samples of sulfide-richfaultbrecciainhornfelswestoftheplutoncontactcontainupto0.2ppmAu,2.9ppmAg,210ppmCu,349ppmZn,127ppmAs,and3ppmHg(no.76,table1).Asamplefroma25-cmwidepyrrhotitepodinhornfelssouthoftheplutoncontains2.6ppmAg,891ppmCu,304ppmZn,1.68percentMn,and28percentFe(no.98,table1).OnemineraloccurrenceandtworockgeochemicalanomaliesarefoundmoredistantfromthemonzoniticintrusivesoftheCamelbackarea.Inthenorthernpartoftheresourcearea,agrabsampleofalteredmafictuffhasdisseminatedsulfidesandcontains100ppmMo,500ppmAs,100ppmCu,30ppmSb,1.1ppmHg,and50ppbAg(no.79,table1).Attwositeselsewhereintheresourceareagrabsam-plesofvolcanicrocklocallycontainupto1.4ppmHg,50ppmAs,and200ppmSb(nos.78and80,table1).

Inthereconnaissancedrainage-basinstudy,severalsamples containing anomalous values were collecteddownstreamfromarea3d.Theyinclude:onestreamsedi-mentcontaining22ppmAsandanothercontaining140ppmAsand5ppmMo;oneheavy-mineralconcentratecontaining3,000ppmZnandanothercontaining5,000ppmB; one heavy-mineral concentrate containing 1–5percent microscopic scheelite and another containinglessthan1percentcassiterite;andoneaquaticmosscon-taining2,000ppmAs.

Resourcearea3e,northeastofFlat

Resourcearea3e(mapB)hasahighpotential(cer-tainty level C) for containing plutonic-related, boron-enriched silver-tin-polymetallic deposits; it also has amoderatepotential (certainty levelC)for theperalumi-nousgranite-porphyry-hostedgold-polymetallicdeposittype.Area3e lies to thenortheastof resource area2d,the Iditarod-Flat area,where severalmonzonitic stocksandplutons(unitTKm)intrudealongagenerallynorth-trending linear zone, andwhich is classified as havinga high potential for the plutonic-hosted copper-gold-polymetallic deposit type. Resource area 3e has noexposed monzonite, but does have a 2–3 km2hornfelszoneinKuskokwimGroupsedimentaryrock(unitKks)that indicates thepresenceofunderlying intrusive rock(perhapsmonzonite),andalsohasnumerousgranitepor-phyrydikesandsills(unitTKgp)andintermediatedikes(unit TKd) that cut the sandstone country rock. As atMooreCreek(resourcearea2b),plutonic-hostedcopper-gold-polymetallic mineralizing systems and plutonic-

related, boron-enriched silver-tin-polymetallic systemsappear tooverlapandmaybemetallogenically related.Boronmetasomatism—largelyabsentinthestocksoftheIditarod-Flat area—is extensively developed in a pro-nouncedhornfelsinarea3enorthofGraniteCreekandinthesmallgraniteporphyrydikesandsills.

Four known mineral occurrences help defineresourcearea3eandateachlocality,mineralizedrockisassociatedwithintrusiverocks(mostlydikes)andnearbysandstone hornfels. In the western part of the resourcearea, samples froma zoneofquartzvein stringers andiron-oxide stained sandstone hornfels spatially associ-atedwithagraniteporphyrydikecontainupto200ppmAg,2.0percentPb,morethan0.2percentSb,0.2percentZn,200ppmCu,50ppmSn,0.16percentAs,100ppmCd,and18ppmBi(no.90,table1).Atalocality1.5kmtothenorth,extensivetourmalinealterationisfoundinintermediatedikesandtheenclosingsandstone;samplescontainupto200ppmSn,500ppmZn,500ppmSb,480ppmAs,200ppmPb,46ppmBi,1ppmAg,traceAu,andmorethan2,000ppmB(no.91,table1).Another2kmnorth,agrabsampleofsandstonehornfelsinthevicinityofadike(?)contains2ppmAg,850ppmAs,200ppmCu,52ppmBi,30ppmSn,and50ppbAu(no.92,table1).Inthecentralpartoftheresourcearea,quartzveinletscut sandstone, sandstone breccia, and granite porphyrydike;samplescontainupto1.2ppmAg,0.2ppmAu,290ppmSb,130ppmAs,andtraceHg(no.93,table1).

In the reconnaissance drainage-basin study, sam-plescollectedfromtensiteswithinordownstreamfromresourcearea3eyieldanomalousvaluesinstreamsedi-ment, heavy-mineral concentrate, and locally aquaticmoss.Creeksthatdraintothenorthandnorthwestfromarea3eshow the followinganomalies: stream-sedimentsamplescontainupto1ppmAg,190ppmAs,and50ppmBi;twoheavy-mineralconcentratescontain200ppmSb,onefromanothersitecontains200ppmAgandmorethan1,000ppmAu,andonefromafourthsitecontains2,000ppmSn;anotherheavy-mineralconcentratecontains1–5percent microscopically visible cassiterite; one aquaticmoss sample contains 5,000ppmAs.Creeks that draintothesouthandsoutheastfromarea3eshowthefollow-inganomalies:onestream-sedimentsamplecontains30ppmSn and another contains less than 0.5 percentAg;theheavy-mineralconcentratefromthelattersitecontains1,000ppmWandtwootherconcentratescontainupto20ppmAg,10,000ppmSb,and2,000ppmW.

Resourcearea3f,SwingingDome

Resource area 3f (map B) has a moderate poten-tial (certainty level B) for containing plutonic-related,boron-enriched silver-tin-polymetallic deposits; it alsohasamoderatepotential(certaintylevelB)fortheplu-tonic-hostedcopper-gold-polymetallicdeposittype.Theresource area is underlain by biotite-pyroxene-quartz

24

monzonitetobiotite-hornblendegranodiorite(unitTKm)thatintrudesamorequartz-richfaciesoftheKuskokwimGroup(unitKkq,shorelinefaciesofMillerandBundt-zen,1994)inthevicinityofSwingingDome.Basedontwosamples,theisotopicageofthisplutonis61–60Ma,significantlyyoungerthanotherplutonicrocksintheIdi-tarodquadrangle,whichcenteraround71Ma(MillerandBundtzen,1994).However, inotheraspects, theplutonatSwingingDomeisquitesimilartootherplutonsintheregion.Itissurroundedbyalocallybrecciatedhornfelsaureoleupto1.5kmwide.Boronenrichmentintheformoftourmalineisabundantalthoughmetallicanomaliesinrocksamplesanalyzedarefewinnumber.Onlyonerockanomaly was detected within resource area 3f; a grabsampleofhornfelsfromneartheintrusivecontactcon-tains20ppmSn,20ppmSb,40ppmAs,and2,000ppmB(no.128,table1).

Fromthereconnaissancedrainage-basinstudy,onlytwosamplescollecteddownstreamfromresourcearea3fshow anomalous values. One stream-sediment samplefromacreekthatdrainssouthwestfromSwingingDomecontainslessthan0.5ppmAg.Aheavy-mineralconcen-tratefromacreekthatdrainssoutheastfromthemoun-taincontains1,500ppmSn.

TheNUREreconnaissancelake-sedimentdatapro-videsomeadditionalsupportforresourcearea3f.Grayand others (1988c) outlined an area around SwingingDome (see their fig. 1) that fits their precious-metalrelatedfactor(As-Mn-Fe)outlinedforthelake-sedimentdataset.

Resourcearea3g,BismarckCreek

Resourcearea3g(mapB)hasahighpotential(cer-tainty level D) for containing plutonic-related, boron-enriched silver-tin-polymetallic deposits; it also has amoderate potential (certainty level C) for the plutonic-hostedcopper-gold-polymetallicdeposit type.Resourcearea 3g includes an elongate, east-west oriented horn-felszoneapproximately10km2 inaprominentuplandareaofKuskokwimGroupflysch(unitKks)attheheadof Bismarck Creek, a tributary of the George River.Noplutonwasfoundbelowthehornfelscap;however,theintensehornfelsstronglysuggeststhepresenceofaburiedpluton.

Along thewesternmarginof thehornfelszone,anextensiveeast- tonortheast-trendingzoneofstockworkcontainingabundantaxinite,tourmaline,quartz,andfer-rigenous gossan can be traced for a distance of nearly300 m and for widths up to 30 m wide (see geologicsketch,fig.16inBundtzenandMiller,1997).Basedonsurfacesampling(no.118,table1)andgeologicmodel-ing,theBismarckCreekdepositcontains498,000tonnes(549,000 tons) grading 0.137 percent Sn, 47.8 g/tonneAg,0.016percentCu,0.22percentZn,andhighlyanom-alousAs,Pb,andSb(BundtzenandMiller,1997).Tung-

stenwasnotdetected.TinvaluesintheBismarckCreekdepositrangefrom150ppmto2.80percent;cassiteritehasbeenidentifiedinpolishedsections.Sulfidemineralswerefoundtobegenerallyoxidizedinsurfacesamples.OtheranomalouselementsincludeBi(upto56ppm),Cd(upto14ppm),In(118ppm),andB(>2,000ppm).

Otherisolatedpatchesofaxinite-bearingstockworkcanbefound2–3kmeastofVABM2424(nos.120–122,table1).Grabsamplesfromthesethreelocalitieslocallycontainupto50ppmAg,1,000ppmSn,7,000ppmZn,130ppmAs,72ppmCd,150ppmCu,60ppmSb,andmorethan2,000ppmB.Inthesouthwesternpartoftheresourcearea,localgossaninfine-grainedsandstoneandshalecontains500ppmCu,140ppmSb,0.5ppmAg,and20ppmAs(no.119,table1).Inthewesternpartofthe resource area, a quartz fracturefilling in sandstonecontainstraceAu,14ppmSb,and20ppmAs(no.117,table1).

In the reconnaissancedrainage-basin studynumer-oussilverandtinanomalieswereidentifiedin,andalsodownstreamfrom,resourcearea3g.Samplesfromfoursitesalongcreeksdrainingtothesouthcontainanoma-lousvalues:threestreamsedimentscontainupto1.0ppmAg, 100 ppmCu and 500 ppmSn; twoheavy-mineralconcentrateseachcontainmorethan2,000ppmSnandupto10ppmAg,200ppmW,and5,000ppmB.Samplesfromsevensitesalongcreeksdrainingtothewestcon-tainanomalousvalues:fivestreamsedimentscontainupto1.0ppmAg,45ppmAs,100ppmCu,150ppmSn,and500ppmZn; twoheavy-mineralconcentrateseachcontainmorethan2,000ppmSnandupto7ppmAgand300ppmW;anotherheavy-mineral-concentratesamplehas5–20percentmicroscopicallyvisiblescheelite.Sam-plesfromthreesitesalongcreeksdraining to thenorthcontainanomalousvalues:onestreamsedimentcontainslessthan0.5ppmAg;threeheavy-mineralconcentratescontainfrom700tomorethan2,000ppmSn.Samplesfromsevensitesalongcreeksdraining to theeastcon-tainanomalousvalues:sixstream-sedimentscontainupto1.0ppmAgand300ppmZn,andtwoofthesecontainAu(0.012and0.014ppm);oneheavy-mineralconcen-tratecontains500ppmAu,50ppmAg,200ppmW,morethan2,000ppmSn,5,000ppmB,andhas1–5percentmicroscopicallyvisiblecinnabar;anotherheavy-mineralconcentratehasmorethan2,000ppmSn.

The NURE reconnaissance stream-sediment dataprovide some additional support for resource area 3g.Gray and others (1988c) outlined a small area north-eastofarea3g(seetheirfig.1)thatfitstheirbase-metalrelated factor (Cu-Cs-Ni) outlined for the stream-sedi-mentdataset.

Resourcearea3h,southeastofGraniteCreek

Resourcearea3h(mapB)hasahighpotential(cer-tainty level B) for containing plutonic-related, boron-

25

enrichedsilver-tin-polymetallicdeposits.Itoverlapspartofresourcearea1bsoithassomepotentialforcontainingperaluminousgranite-porphyry-hostedgold-polymetallicdepositsaswell.Resourcearea3hiscenteredaroundasmallmonzonitestock(unitTKm)thatcutsKuskokwimGroupsedimentaryrocks(unitKks)andformsahorn-fels zone up to 1.5 kmwide.The known rock anoma-liesandthesinglemineraloccurrenceareallassociatedwithsandstonehornfelsnear themonzonitecontact.Attheoccurrence,samplesfromalargelimonite-tourmalinebrecciazone inhornfelscontainup tomore than1,000ppmSn,1ppmAg,500ppmCu,350ppmZn,100ppmPb,160ppmAs,54ppmSb,1.2ppmHg,andmorethan2,000 ppmB (no. 145, table 1); the nearbymonzonitehasfinequartz-tourmaline stockworkveins in the con-tactarea.Attwootherlocalitieswithintheresourcearea,grab samples fromgossan areas inhornfels containupto2ppmAg,860ppmZn,500ppmCu,460ppmAs,10ppmBi,and5.1ppmCd(nos.146and147,table1).

Duetothesmallsizeoftheresourcearea,fewofitsdrainagesweresampledduringthereconnaissancedrain-age-basin study. One stream-sediment sample contains31ppmAsandanaquaticmosssamplefromthesamesitecontains3,000ppmAs.

Resourcearea3i,GraniteMountain

Resourcearea3i(mapB)hasahighpotential(cer-tainty level D) for containing plutonic-related, boron-enriched silver-tin-polymetallic deposits; it also has amoderate potential (certainty level C) for the plutonic-hosted copper-gold-polymetallic deposit type. The arealiesneartheheadoftheEastForkoftheGeorgeRiverandiscenteredaroundGraniteMountain,anelongatemonzo-nitetogranitestock(unitTKm)thattrendsnortheastwardforadistanceofabout10km.Minoramountsofvolcanicrockare associatedwith thepluton (unitsTKil andKit)and an extensive hornfels aureole, developed mostly inKuskokwimGroupsedimentaryrock,surroundsthestock.HornblendeisfairlyabundantasaprimarymaficmineralintheGraniteMountainpluton;whereasotherLateCreta-ceousandearlyTertiaryintrusionsofthequadranglehaveclinopyroxeneandbiotiteastheirdominantmaficphases.Anisotopicageof63–62MamakestheGraniteMountainplutondistinctlyyoungerthanmostotherplutonsofthestudyarea(whichcenteraround71Ma,MillerandBundt-zen,1994),butsimilarinagetotheplutonatSwingingDome(resourcearea3f).

Boronmetasomatismisquiteextensive throughouttheGraniteMountainstockandinadjacenthornfelsandoccursassheetedtourmalineveins,tourmaline-cementedbreccias,andtourmaline-axinitereplacementsofplutonictextures.Thelargestareasofmetasomatismoccuralongthe northwest margin of the stock where one hornfels-hosted, cylindrically shaped, tourmaline-breccia zonewasestimated tobe400mlong,up to15mwide(no.

151, table1); thepipe-likebodymaycontain5milliontonnes of boron-rich altered rock. Samples from thislocalityandtwonearbysitesofsimilarboronalteration(nos.150and153,table1)containupto7ppmAg,300ppmSn,500ppmPb,54ppmSb,100ppmAs,54ppmBi,andmorethan2,000ppmB;siteno.151alsohas50ppbAu.Visiblegoldwasnotedinaheavy-mineralcon-centratefromastreamthatdrains thesoutheastsideofGraniteMountain.Inthenorthernpartofresourcearea3i,wellawayfromtheintrusivecontact,agrabsampleofhematite-cementedsandstonebrecciacontains450ppmZn,400ppmAs,morethan10ppmHg,7ppmBi,and22ppmSb(no.152,table1).Inthesouthwesternpartoftheresourcearea,inthevicinityofamonzoniticdikeswarm,agrabsampleofiron-stainedsandstonehornfelscontains2ppmAg,700ppmCu,600ppmAs,50ppmSb,150ppmPb,andmorethan2,000ppmB(no.165,table1).About1.5kmto thenortheast (no.149, table1),chipsof heavily iron-stained sandstone contain 390 ppm Sb,850ppmAs,and0.9ppmHg,indicativeofanepithermalsignatureratherthantheplutonic-related,boron-enrichedsilver-tin-polymetallicdeposittype.

Inthereconnaissancedrainage-basinstudy,samplescollected within or downstream from resource area 3iprovide abundant supportive data. Samples from fivesites that drain the southeast side of Granite Mountainyieldedanomalousvalues:fivestreamsedimentscontain0.5to1.0ppmAg,andupto60ppmAs,100ppmPb,andlocally 2,000 ppm B; two heavy-mineral concentratescontainupto2,000ppmSb,2ppmAg,500ppmPb,700ppmW,andmorethan5,000ppmB;oneoftheseheavy-mineral concentrates also has microscopically visiblegold(no.154,table1);fouraquaticmosssamplescontainupto7ppmAg,200ppmSb,andmorethan5,000ppmAs.SamplesfromthreesitesthatdraintothenorthfromGraniteMountainyieldedanomalousvalues:onestreamsedimentcontains0.030ppmAuandlessthan0.5ppmAg;oneheavy-mineralconcentratecontains100ppmBi;another heavy-mineral concentrate has less than 1 per-centmicroscopicallyvisiblecassiterite.Creeksthatdrainthe western and southwestern parts of resource area 3iyieldedanomaloussamplesfromsixsites:fourstream-sedimentsampleslocallycontainupto0.5ppmAg,73ppmAs,2,000ppmB,300ppmZn,and34ppmHg;oneheavy-mineral-concentratesamplecontains100ppmAuandthreeotherscontain5ppmAg,500ppmW,andmorethan5,000ppmBeach.Theheavy-mineral-concentratesamplethataccompaniedthe34-ppm-Hg-bearingstreamsedimentcontains5–20percentmicroscopicallyvisiblecinnabar;anaquaticmosssamplefromthissitecontains1,000ppmAs.

The NURE reconnaissance stream-sediment dataprovide some additional support for resource area 3i.UsingR-mode factoranalysisGrayandothers (1988c)outlinedasmallareasouthwestofGraniteMountain(see

26

their fig. 1) that fits their precious-metal related factor(Fe-Mn-As)outlinedforthestream-sedimentdataset.

EPITHERMALMERCURY-ANTIMONY(GOLD)DEPOSITS(MODEL4)

Epithermal cinnabar-stibnite-quartz deposits andoccurrencesintheIditarodquadranglearepartofawellknownmercury belt (Sainsbury andMacKevett, 1965)thatextendsabout500kmfromtheRedTopMinenearBristolBaynortheasttotheWyominglodeintheCrippleCreekMountainsnorthofMcGrath(fig.1).Regionally,theepithermalmercury-bearingdepositsarepartoflargemetallogenythatproducedoredepositsinanextensionalenvironmentassociatedwithback-arcmagmaticactivityinLateCretaceousandearlyTertiarytime(BundtzenandMiller,1997).Mercuryandantimonyhavebeenrecov-eredfromadozendepositsinthisbeltandapproximately90percentofthe1.38millionkilograms(39,960flasks)of mercury produced came from the Red Devil Mine,just south of the study area. The Red Devil deposit iszonedvertically showingpure cinnabar innear-surfacepositions and increasing stibnite/cinnabar ratios to 200mbelow the surface, implying increasing temperatureswithdepth—azonedhydrothermalconduitsystem.Goldanomalieshavebeenfoundat theKolmakofMine(fig.1)intheSleetmutequadranglewhereupto3.5g/tonnegoldwerefoundinchip-channelsamples(Bundtzenandothers,1998).

Epithermalmercury-antimonylodes in theIditarodquadrangleoccurintwosettings.Inthefirst,depositsarestructurallycontrolledandspatiallyassociatedwithmaficdikesandsills thatcutKuskokwimGroupflysch(asattheRedDevilmercurymine);inthesecond,depositsarespatially related to largerplutonicmassesand themer-cury mineralization overprints and forms halos aroundgold-polymetallic mineralization (for instance at theGoldenHorngoldmine,no.110,mapA).Thehot-springmercury deposit model (Rytuba, 1986, deposit model27a)fitssomeofthefeaturesofthefirstsetting,whereasthe clastic-sediment-hosted mercury deposit model(afterKuznetsov,1974,andBabkin,1975),whichinfersaplutonic source,betterfits thedepositsof the secondsetting.Using40Ar/39Arages,stableandradiogeniciso-topes, and geochemical and fluid inclusion data, Grayandothers(1997b)suggestedthattheepithermalHg-Sbdeposits of southwesternAlaska formed in response toLateCretaceous-earlyTertiaryigneousactivityinteract-ingwithsurroundingsedimentarywallrocks;anomalousheatflowfromtheigneousactivityinducedhydrothermalfluidflowandleachedorefluidsandmetals(mostimpor-tantlymercury)fromlocalsedimentaryrocks.

In the Iditarod quadrangle, the two best examplesofepithermalmercury-stibnitedepositsspatiallyassoci-atedwithalteredmaficdikesare theDeCourcyMoun-tainMineandtheYousureoccurrence(nos.184and156,

table 1). At DeCourcy Mountain, Kuskokwim Groupsandstoneandshalearecutbybasalticsillsanddikesthatareextensivelyalteredtotanweathered,silica-carbonaterock.Mineralizedveinsandveinbrecciasare localizedalong intrusive contacts and bedding surfaces—simi-lar to the geologic/structural setting of the Red DevilMine.Cinnabar,stibnite,pyrite,andmarcasiteoccur inlenses, stockworks, tensional fractures, and shears andare accompanied by chalcedonic silica veins and dick-ite.TheDeCourcyMountainMineproduced51,700kg(1,500flasks)ofmercuryfrom2,250tonnesoforeatanaveragegradeof3.0percentHg.Additionalanomalouselements at the DeCourcy Mountain Mine includeAg(0.65ppm),Sb(10,000ppm),As(60ppm),andAu(400ppb)(table1).

Mercury-antimony deposits in the Iditarod quad-rangle are also found in association with larger plu-tonicbodiesofLateCretaceousandearlyTertiaryage.Disseminated and massive stibnite and minor cinnabaroccur in late-stage, silica-carbonateveins inmonzoniteandsyenitethatpostdateotherbase-andprecious-metaldeposits at the Golden Horn Mine, Chicken Mountainprospect,andonMountJoaquin(nos.110,130,and32,respectively, map A). This suggests that the cinnabar-stibnitelodesweredepositedinepithermaltemperature/pressureconditionsdistaltohighertemperature/pressureenvironments in igneous cores (Bundtzen and Miller,1997). This explains the Hg-Sb geochemical halosexpressedinrock,stream-sediment,andheavy-mineral-concentratesamples(discussedinresourceareas4b,4d,4e,4f,and4i,below).

Resourcearea4a,HickeyCreek

Resourcearea4a(mapB)hasahighpotential(cer-tainty levelC) for containingmercury-antimony (gold)deposits.Theareaisunderlainbyquartz-richclasticrocksthatrepresenttheshorelinefacies(definedbyMillerandBundtzen,1994)of theKuskokwimGroup (unitKkq).Theserocksaredeformedintoasyncline,whoseaxisfol-lowsthecentralpartoftheresourceareafromnortheastto southwest. No large intrusions or hornfels aureolesweremappedduringthisinvestigation,however,numer-oussmallhornblende-and/orbiotite-bearingdikes(lessthan1mwide)andsill-likeintrusions(unitTKd)cutthesedimentaryrockslocally,especiallyinthenorthernpartof theresourcearea.LocalzonesofhematitealterationarefoundinthesouthernpartoftheresourceareafromaboutVABMHickeyto15kmtothenortheast.

Rocksamplescontainingmoderatelyelevatedmer-cury,antimony,andarsenicvalueswerecollectedattwositeswithinresourcearea4a.Inthenorthernpartoftheresourcearea,agrabsampleofiron-oxidestainedaltereddikecontains2.4ppmHg,14ppmSb,and10ppmAs(no.26,table1).Inthesouthernpartoftheresourceareaagrabsampleofbrecciated,iron-stainedsandstonecon-

27

tains0.85ppmHg,50ppmAs,and14ppmSb(no.52,table1).

The outline of resource area 4a is primarily con-trolledbyanomaliesinstream-sedimentandheavy-min-eral-concentratesamples.Goldwasdetectedinsamplesatthreesites.Onestream-sedimentsamplefromacreekdrainingtheeastsideoftheresourceareacontains0.020ppmAu.Astream-sedimentsamplefromacreekdrain-ingthewestsideof theareacontains0.030ppmAu;aheavy-mineral concentrate from the same site contains1,000ppmCu,2,000ppmPb, and1,000ppmW.Vis-iblegoldwas identified inaheavy-mineralconcentrate(no.25,table1)fromacreekdrainingthenorthsideoftheresourcearea.Thissamplealsocontains20–50per-centmicroscopicallyvisiblecinnabarandtheaccompa-nying stream-sediment sample contains 22.5 ppm Hg.Samplescollectedatnineothersiteswithinresourcearea4ayieldedanomalousvaluesinstreamsediments,heavy-mineralconcentrates,orboth:stream-sedimentsampleslocallycontainupto3.6ppmHg,300ppmZn,andlessthan 0.5 ppm Ag; heavy-mineral concentrates locallycontain up to 1,000 ppmSb, 2,000 ppmW, and 5,000ppmZn.Microscopicallyvisiblescheelite(1–5percent)wasidentifiedatfourdifferentsites.Theheavy-mineralconcentratesfromthesesitescontainupto1–5percentcinnabarandlessthan1percentsphalerite.

Resourcearea4b,MooreCreekHghalo

Resourcearea4b(mapB)hasahighpotential(cer-tainty levelD) forcontainingmercury-antimony(gold)deposits.Theareaoverlapsandformsanirregularhaloaround resource area 2b, which has a high potentialfor containing plutonic-hosted copper-gold-polymetal-licdeposits.Theunderlyingrockunitsincludethoseofresource area2b (thevolcanic-plutonic complexof theMooreCreekareaandthesurroundingsedimentaryrockhornfels,unitsTKm,TKil,Kit,andKks).Outsideofarea2b, the halo part of resource area 4b contains Kusko-kwim Group sedimentary rocks (both units Kks andKkq),which are locally intrudedby felsic (TKgp) andmafic to intermediate (TKd) dikes. Minor fault sliversofpre-CretaceousrocksoftheInnokoterrane(^Mc)liealongtheYankee-GanesCreekFault(MillerandBundt-zen,1994),whichcutstheresourcearea.

Within resource area 2b, mercury values rangingfrom5tomorethan10ppmareassociatedwithhornfelsandalteredvolcanicrockatsomeof the lode localitiesused tooutline thisplutonic-related resourcearea (nos.59,61,62,and63,table1andresourcearea2bdescrip-tion). Also, concentrates from the two gold-producingplacermines(nos.60and83,table1)andthethreeplacergoldoccurrences (nos. 55, 58, and81, table1) in, anddownstreamfrom,area2bcontainsignificantamountsofcinnabar.Outsideoftheboundaryofresourcearea2b,inthewesternpartofresourcearea4b,grabsamplesfrom

threelocalitiesyieldHganomalies:analteredmaficdikecontains3.4ppmHg(no.41,table1);iron-oxidestainedsandstoneinthevicinityofanaltereddikecontains1.5ppmHg,0.2ppmAu,and99ppmAs(no.53,table1);andiron-stainedbrecciainchertalongtheYankee-GanesCreekFaultcontains2.5ppmHg,0.2ppmAu,and0.2ppmAg(no.54,table1).Thefinaltwoanomalousrocklocalities in resource area 4b do not yield Hg values,and their relationship toepithermalmineralizedrock isuncertain.Fromonelocalityinthenorthwesternpartofarea4b,manganese,ingradesupto1.58percent,isasso-ciatedwith iron-oxide stained,quartz-veined sandstone(no.40,table1).Fromtheotherlocality,inthesoutheastpartof resourcearea4b,a sandstonegrabsamplecon-tains2ppmAgand150ppmCu(no.84,table1).

Numeroussamplesfromthereconnaissancedrainagebasinstudycontainanomaliessupportiveoftheepither-malmercury-antimony(gold)depositmodelinresourcearea4bandoftheplutonic-hostedcopper-gold-polyme-tallicdepositmodelinresourcearea2b.Theanomaliessupportingarea2bwerediscussedinaprevioussection,but in review, they include: stream-sediment samplescontainingupto0.88ppmHg,30ppmAs,andlocallyupto0.77ppmAu;heavy-mineral-concentratesamplescontainingupto5,000ppmSb,morethan2,000ppmSn,500ppmPb,5,000ppmWandlocallyupto300ppmAgandmorethan1,000ppmAu;andmicroscopicallyvisiblecinnabar(1–5percent),gold(<1percent),andothermin-eralsinheavy-mineral-concentratesampleslocally.Out-sideofarea2b,butwithinordownstreamfromresourcearea4b,fiveadditionaldrainagesitesyieldedanomaloussamples. Stream-sediment samples from two of thesesitescontain0.90to7ppmHg;stream-sedimentsfromtwo other sites contain traceAg (<0.5 ppm); a heavy-mineralconcentratefromtheremainingsitecontains1–5percent microscopically visible cinnabar.Aquatic mosssamplesfromtwoadditionalsitescontain100ppmSb.

Resourcearea4c,BonnieCreekarea

Resource area 4c (map B) has a high potential(certainty level C) for containing mercury-antimony(gold) deposits; it also has a moderate potential (alsocertaintylevelC)fortheperaluminousgranite-porphyry-hostedgold-polymetallicdeposittype.Theareaformsanarrow,elongatebeltthatliesseveralkilometerssoutheastof theIditarod-NixonForkFaultandparallels thefaultfor about 30 km. The underlying rocks are primarilyKuskokwimGroupsandstoneandshale(unitKks) thatarelocallyintrudedbygraniteporphyrydikesandsmallplugs (unit TKgp). The known deposits are spatiallyassociated with granite porphyry dikes and consists ofrock anomalies from two sites. In the northeast cornerofresourcearea4cagrabsampleofiron-oxidestainedgraniteporphyrycontainsmorethan10ppmHg,70ppmAs, and traceAu andAg (no. 46, table 1).At another

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site,3kmto thesouthwest,agrabsampleofagraniteporphyry dike shows secondary tourmaline alterationcontaining2.2ppmHg(no.45, table1).This resourcearea may be a westerly extension of resource area 4d,whichwillbediscussedbelow.

Inthereconnaissancedrainage-basinstudy,samplescollectedfromfivesitesinordownstreamfromresourcearea4cyieldedanomalousvalues.Stream-sedimentsam-ples from two sites containAu (0.050 and0.065ppm)and 0.64 ppm Hg1; a heavy-mineral concentrate fromone of these sites contains 2,000 ppm Zn. Heavy-min-eralconcentratesfromtwoothersitescontainupto1,500ppmSn,1,000ppmCu,and3,000ppmZn.Microscopi-callyvisiblecinnabar(1–5percent)wasidentifiedintheheavy-mineralconcentratefromafifthsite.

Resourcearea4d,VABMTatalinaHghalo

Resourcearea4d(mapB)hasahighpotential(cer-tainty levelC) for containingmercury-antimony (gold)deposits; it also has a moderate potential (certaintylevel B) for the peraluminous granite-porphyry-hostedgold-polymetallic deposit type. The area overlaps andformsapartialhaloaroundresourcearea3c,whichhasa high potential for containing plutonic-related, boron-enrichedsilver-tin-polymetallicdeposits.Theunderlyingrockunitsincludethoseofresourcearea3c(monzoniteto syenite plutons, granite porphyry dikes, and Kusko-kwimGroupsedimentaryrocks,unitsTKm,TKgp,andKks,respectively,plushornfelsaroundtheVABMTata-linapluton.Outsideofarea3c,thehalopartofresourcearea4dispoorlyexposed,butcontainsavarietyofrocktypes,mostlycoveredbyyounger,unconsolidatedsurfi-cialdeposits(unitQTs).ExposedinthisareaareKusko-kwimGroupsedimentaryrocks(unitKks),volcanicrock(unitTKil),numerousgraniteporphyrydikesandsmallintrusions(unitTKgp),andasmalldacite-andesiteplugatVABMLin.

Asinresourcearea4c,theknownmercuryanoma-liesofresourcearea4dareassociatedwithgranitepor-phyryrocks.Withinresourcearea3c,grabsamplesfromashearedcontactzonebetweengraniteporphyryandhorn-felssandstonecontainupto1ppmHgand500ppmAs(no.47,table1andresourcearea3cdescription).Outsideoftheboundaryofresourcearea3c,inthesouthernpartof resourcearea4c,grabsamplesof iron-stainedfelsicdikeandquartz-veineddikecontainupto1.0percentAs,morethan10ppmHg,200ppmW,100ppmPb,120ppmZn,andtraceAg(no.66,table1).Threekilometerseastofthatsite,anothergrabsampleofgraniteporphyrydikecontains2.2ppmHg(no.67,table1).Inthewesternpartofresourcearea4d,agrabsampleofagraniteporphyry

1ThisHgvalueisanomalousatthegreaterthan90thpercentile;Grayandothers(1997a)selected0.80ppmHg,whichisgreaterthanthe95thpecentile,astheiranomalousvaluecut-off.

plugcontains200ppmAs,2.2ppmHg,28ppmSb,andtraceAg(no.65,table1).

Samples from the reconnaissance drainage-basinstudy contain anomalies that support both the epither-malmercury-antimony(gold)depositmodelinresourcearea4dand theplutonic-related,boron-enrichedsilver-tin-polymetallicdepositmodelinresourcearea3c.Theanomalies inarea3cwerediscussed inaprevioussec-tion,but inreview, theyinclude:stream-sedimentsam-ples locally containing up to 3 ppmAg, 120 ppmAs,200 ppmPb, 15 ppmSn, and 15 ppmBi; heavy-min-eral concentrates locally containing up to 23 ppmAg,1,000ppmAs,5,700ppmPb,2,000ppmSb,morethan2,000 ppmBi,more than 5,000 ppmB, and traceAu;microscopically visible cassiterite (1–5 percent) in oneheavy-mineral concentrate; and aquatic moss samplescontainingupto3ppmAgand50ppmAs.Outsideofarea3c,butwithinordownstreamfromresourcearea4d,sixadditionaldrainagesitesyieldedanomaloussamples.Aheavy-mineral concentrate collectednear thequartz-veineddikeoccurrencementionedabove (no.66, table1)contains5ppmAg,7,000ppmSb,and2,000ppmW,andhasmicroscopiccinnabar(5–20percent)andschee-lite (1–5 percent). The stream-sediment sample fromthis site contains 1.5 ppmHg.Three other heavy-min-eralconcentratesfromthesouthernpartofresourcearea4dlocallycontainupto2,000ppmSb,500ppmW,andmore than 2,000 ppm Sn; a heavy-mineral concentratefrom a nearby site contains less than 1 percent micro-scopicallyvisiblecassiterite.Aquaticmosssamplesfromthreesitescontainupto1,000ppmAs.

Resourcearea4e,Iditarod-FlatHghalo

Resourcearea4e(mapB)hasahighpotential(cer-tainty levelD) forcontainingmercury-antimony(gold)deposits. The area overlaps and forms a broad haloaround two other resource areas—area 2d, which hasa high potential for containing plutonic-hosted copper-gold-polymetallic deposits, and area 3e, which has ahigh potential for containing plutonic-related, boron-enrichedsilver-tin-polymetallicdeposits.Theunderlyingrockunitsincludethoseofresourcearea2d(thevolca-nic-plutoniccomplexoftheChickenMountainarea,thesurroundingsedimentary rockhornfels, and localdikesofmafictointermediatecomposition—unitsTKm,TKil,Kks,andTKd)andthoseofresourcearea3e(sedimen-tary rock that is locallyhornfels,graniteporphyry,andintermediate dikes—units Kks, TKgp, and TKd). Out-side of areas 2d and 3e, the halo part of resource area4econtainsKuskokwimGroupsedimentaryrocks(bothunits Kks and Kkq), which are intruded by numerousmafictointermediate(TKd)dikes,andlocallybygraniteporphyrydikes(TKgp).

Themercury-antimony(gold)signatureofresourcearea4eisinterpretedaspartofabroad,lowertempera-

29

ture, epithermal overprint encompassing and surround-inghighertemperature,pluton-hostedandpluton-relatedgold-polymetallic deposits of the Iditarod-Flat area.Withinresourcearea2d,late-stageepithermalcinnabar-stibnitezonescrosscutoldermesothermalgoldmineral-izedrockattheGoldenHornMine(no.110,table1),theIdahoprospect(no.105,table1),theChickenMountainprospect (no.130, table1),andalongMalamuteGulch(no.111,table1).NorthofFlat,butstillwithinarea2d,mineralizedquartzveinsthatcuthornfelscontainmorethan10ppmHgalongwithhighvaluesofgold,silver,antimony, and other metals (nos. 87 and 88, table 1).Concentrates from eight placer mines within or down-streamfromarea2dhavesignificantamountsofcinnabar(nos.89,104,106,107,109,129,131,132,table1).

The part of resource area 4e that lies outside theboundaries of resource areas 2b and 3e has numerouslocalitieswhereHg-Sbmineralizedrockisfound.Iron-stainedsandstoneandshaleoftheKuskokwimGrouparecommon to all of these localities, and alteredmafic tointermediatedikesthathostorearepresentatsome.TheDishnaRiverprospect(no.74,table1)isthemostprom-ising of the known localities andwas first reported inBundtzenandMiller (1997).Heregold-bearingquartz-sulfide-sulfosaltzonesandveinbrecciascutshearedshaleandsandstone.Sulfide-bearingmassescontainupto25percent stibniteanddisseminatedarsenopyrite is foundlocally.Grabsamplescontainupto11ppmAu,10,000ppm Sb, more than 10,000 ppmAs, and more than 10ppmHg.Theinferredreserveestimateof92.5kg(2,970oz)isbasedoneighteenchip-channelsamplesalongthe140-m-longmineralizedzone.Threeotherknownlocali-tiesliealongthesamenortheast-trendingridgefrom3to9kmtothesouthwestofthisprospect(nos.94,95,and96,table1).Attheselocalities,locallyextensivegossanis spatially associatedwith altereddikes; grab samplesfromallthreesitescontainanomalousHg(to3.3ppm)andAs (to1,900ppm)and samples fromone site alsocontainupto0.2ppmAu,1ppmAg,116ppmSb,and300ppmZn.AnomalousHgand(or)Sbvaluesarefoundatfourothersitesinresourcearea4e.Inthenorthernpartofthearea,ahematitealtereddikecontains740ppmSb,80ppmAs,0.68ppmHg,andtraceAu(no.71,table1)andtwograbsamplesofsandstonecontain76–92ppmSb(nos.72and73,table1).Intheeasternpartofthearea,asilica-carbonatealtereddikeinthevicinityofgossaninsandstonecontains4ppmHg(no.113,table1).

Samples from the reconnaissance drainage-basinstudycontainanomaliesthatsupportboththeepithermalmercury-antimony(gold)depositmodelinresourcearea4e and the plutonic-hosted and plutonic-related meso-thermal typedeposits in resourceareas2dand3e.Theanomaliessupportingarea2dwerediscussedinaprevi-oussection,butinreview,theyinclude:stream-sedimentsampleslocallycontainingupto1.55ppmAu,1.0ppm

Ag,23ppmAs,200ppmCu,andmorethan34ppmHg;heavy-mineral-concentrate samples locally containingup to500ppmAu,500ppmAg,3,000ppmSb,1,500ppmAs,and2,000ppmW;microscopicallyvisiblegold,cinnabar, and scheelite in heavy-mineral concentrateslocally;andaquaticmosssamplescontainingupto3,000ppmAsand100ppmSb.Theanomaliessupportingarea3ewerediscussed inaprevious section,but in review,they include: stream-sediment samples locally contain-ingupto1ppmAg,190ppmAs,50ppmBi,and30ppmSn;heavy-mineral-concentratesamples locallycontain-ingupto200ppmAg,2,000ppmSn,10,000ppmSb,2,000ppmW,morethan1,000ppmAu;microscopicallyvisiblecassiteriteinoneheavy-mineralconcentrate;andatonesiteaquaticmosscontaining5,000ppmAs.Out-sideofareas2dand3e,butwithinordownstreamfromresourcearea4e,stream-sedimentand/orheavy-mineral-concentratesamplescollectedfrom14siteswereanoma-lous.Inthenortheastcorneroftheresourcearea,wherelodesitesnos.74,94,95,and96(discussedabove)arelocated,sixsitesyieldedanomaloussamples:fourstreamsedimentslocallycontainupto33ppmAsand300ppmZn; one heavy-mineral concentrate contains 500 ppmSb;microscopicallyvisiblecassiteritewas identified inanother heavy-mineral concentrate. Two stream-sedi-ment samples fromcreeksdraining thewesternpartofresource area 4e, west of Chicken Mountain, containminorAu(0.015and<0.05ppm).Twostream-sedimentsamples from creeks draining the northern part of theresourceareaalsocontainAu(0.069and0.048ppm);aheavy-mineral concentrate from the latter site contains200ppmW.Samplesshowingadditionalanomaliesarescattered through therestof resourcearea4e:astreamsedimentfromonesitecontains45ppmAs;anothersitehastraceAg(<0.5ppm)inthestreamsedimentand300ppbPbintheheavy-mineralconcentrate;anotherheavy-mineral concentratecontainsmore than2,000ppmSn;andmicroscopicallyvisiblecinnabar(5–20percent)wasidentifiedintheconcentratefromthefourthsite.

Resourcearea4f,CamelbackMountainHghalo

Resourcearea4f(mapB)hasahighpotential(cer-tainty level C) for containing epithermal mercury-anti-mony (gold) deposits. The area overlaps and forms apartial halo around resource area3d,whichhas ahighpotentialforcontainingplutonic-related,boron-enrichedsilver-tin-polymetallic deposits. The underlying rockunitsincludethoseofresourcearea3d:monzoniticplu-tonsoftheCamelbackMountainarea,theIditarodVol-canics,andKuskokwimGroupsedimentaryrocks(unitsTKm,TKil,Kit,andKks).Thehalopartofresourcearea4fisunderlainbyKuskokwimGroupsedimentaryrocks(unit Kks), which are locally intruded by intermedi-ate dikes (unit TKd). The mercury-antimony signatureobserved in thepartof resourcearea4f that liesnorth-

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westof theIditarod-NixonForkFault,mayrepresentalower temperature, epithermal overprint encompassingand surrounding the higher temperature, pluton-relatedmineralizedareaofresourcearea3d.Themercury-anti-monysignatureobservedinthepartofresourcearea4fthatliessoutheastofthefault,maynotbedirectlyrelatedtothevolcanic-plutonicrocks.

Sixmercury-and(or)antimony-bearinglodelocali-ties (mineraloccurrencesand rockanomaliesonly)arecontainedinresourcearea4f:fiveofthesearealsopartofresourcearea3d;thesixthlieswithinthehalonorthwestoftheIditarod-NixonForkFault.Asdiscussedaboveinthe description of area 3d, anomalous values of silver(and locallygold), accompaniedbyup to4.2ppmHg,occurinhornfelsadjacenttotheplutons(nos.76and77,table1and resourcearea3ddescription).Distant fromtheplutonsof resourcearea3d,up to1.4ppmHgandlocallyupto200ppmSbvaluesarefoundinalteredvol-canicrocks(nos.78,79,and80,table1andresourcearea3ddescription).Inthehalopartof4f,justwestofCamel-backMountain,agrabsampleofiron-stained,hydrother-mallyalteredshale(no.97, table1)contained5.1ppmmercuryandtraceAg.

Samples from the reconnaissance drainage-basinstudy contain anomalies that support both the epither-malmercury-antimony(gold)depositmodelinresourcearea 4f and the plutonic-related, boron-enriched silver-tin-polymetallicdepositmodelinresourcearea3d.Theanomalies inarea3dwerediscussed inaprevioussec-tion,but inreview, theyinclude:stream-sedimentsam-pleslocallycontainingupto140ppmAsand5ppmMo;heavy-mineral-concentrate samples locally containingupto3,000ppmZnand5,000ppmB;microscopicallyvisiblescheeliteandcassiteritelocallyinheavy-mineralconcentrates;andonemosssamplecontaining2,000ppmAs.Outsideofarea3d,butwithinordownstreamfromresource area 4f, four additional drainage sites yieldedanomaloussamples; thesesitesalsoareall fromsouth-eastoftheIditarod-NixonForkFault.Onestreamsedi-mentcontains32ppmAsandanothercontains0.96ppmHg.At another site, the stream sediment contains 0.82ppmHg, theheavy-mineral concentrate contains1,500ppmW,andmicroscopicallyvisiblecinnabar (1–5per-cent) and scheelite (1–5percent)were identified in theconcentrate.Oneadditionalsitehad5–20percentmicro-scopicallyvisiblecinnabarintheheavy-mineralconcen-trate.

Resourcearea4g,SugarloafMountainarea

Resourcearea4g(mapB)wasprimarilydefinedonthebasisofstreamsedimentandheavy-mineralconcen-trategeochemistry.Theareaispoorlyexposed;80percentof it iscoveredbyunconsolidateddeposits (unitQTs).Althoughonlyonerockanomalywaslocatedduringtheinvestigation, the drainage-basin geochemistry and the

presenceofgraniteporphyrydikessuggestthatresourcearea4ghasahighpotential(certaintylevelB)forcontain-ingepithermalmercury-antimony(gold)deposits.Italsohasamoderatepotential(alsocertaintylevelB)fortheperaluminousgranite-porphyry-hostedgold-polymetallicdeposit type. The area forms an elongate belt that liesabout5kmsouthoftheplacerworkingsofMooreCreekandtrendseastforalmost40km.Beneaththeunconsoli-dateddeposit cover, theunderlying rocksareprimarilyKuskokwimGroupsandstoneandshale(unitKks) thatarelocallyintrudedbygraniteporphyrydikesandsmallplugs(unitTKgp)andbytwosmalldome-likebodiesofpilotaxiticandesite(unitTKp).Localareasofhornfels,commonlycircularinshape,occurlocallyinthiswest-trendingbelt.Thedome-shapedexposureofhornfelsonSugarloafMountainislikelyunderlainbyanunexposedplug,perhapsofgraniteporphyryorpilotaxiticandesite.Theone rockanomaly found is fromagrab sampleofone of the pilotaxitic plugs, which contains 1 ppmAg(no.99,table1);Hgwasnotanalyzedinthissample.

Inthereconnaissancedrainage-basinstudy,samplesfromelevensites inordownstreamfromresourcearea4g yielded anomalous values. Three stream-sedimentsamples,oneeachfromthewestern,northern,andeast-ernpartsoftheresourcearea,contain0.010to0.011ppmAu.Aheavy-mineralconcentratefromtheeasternmostsitehad300ppmCu,butneithertheconcentratenorthestream sediment had anomalous Hg.All of the Hg- orcinnabar-bearing samples came from eight sites in thewesternhalfoftheresourcearea.Atfiveoftheseeightsites, the stream-sediment sample contains from 1.4 tomorethan34ppmHg;atalleightsites,microscopicallyvisiblecinnabarwasidentifiedintheheavy-mineralcon-centrate,locallycomprisingfrom20to50percentofthesample.Otheranomaliesidentifiedattheseeightsamplesites include:up to3ppmAg,300Cu,2,000ppmSn,1,000ppmW,and10,000ppmZnlocally;andonecon-centratesamplecontainsmicroscopicallyvisibleschee-liteandanothercontainscassiterite.Twoadditionalsitesin resourcearea4gyieldedanomalousAg:one fromawesterndrainagehadtraceAg(<0.5ppm)inthestreamsediment;anortherndrainagehad1ppmAgintheheavy-mineralconcentrate.

Resourcearea4h,theDohertyCreekarea

Resourcearea4h(mapB)hasahighpotential(cer-tainty levelD) for containing epithermalmercury-anti-mony (gold) deposits; it also has a moderate potential(certainty level C) for the peraluminous granite-por-phyry-hosted gold-polymetallic deposit type. Similartoresourceareas4cand4g,area4hisprimarilyunder-lainbyKuskokwimGroupsedimentaryrock(unitKks)that is locally intruded by granite porphyry dikes (unitTKgp),andinadditioncontainsalteredintermediatetomaficdikes (unitTKd).Seven rockanomaliesandone

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lode prospect are scattered throughout area 4h. In thenortheastpartoftheresourceareaagrabsampleofsilici-fiedbreccia insandstonecontains50ppbAuand traceHg(no.114,table1).Alsointhenortheast,anextensivezone of pyrolusite pods occur in sheared sedimentaryrock(no.115,table1).Grabsamplescontainmorethan5,000ppmMnandlocallyupto10ppmMo,andtraceamountsofHg.Inthecentralpartoftheresourcearea,grabsamplesfromaprospectpitsunkonaquartz-calciteveinedalteredmafic?dikecontain54ppmSb,100ppmAs,0.9ppmHg,and1,000ppmCr(no.139,table1).Atthreeother localities in thecentralpartof the resourcearea, grab samples of iron-stained sandstone or altereddikecontainfrom1.1to2.3ppmHg(nos.138,140,and141,table1).Inthesouthwestpartoftheresourcearea,grabsamplesofalteredgraniteporphyrydikecontain1.4and2.0ppmHg(nos.137and158,table1).

Themercuryvalues in the lode sites are relativelylow,butdesignationoftheresourceareaissupportedbystream-sedimentandheavy-mineralconcentratesamplescollected from nine sites in or downstream from theresourcearea.Three sitesyieldedeithergoldvaluesorvisible gold: stream-sediment samples from two siteshad0.011and0.058ppmAu;anothersitehad0.5ppmAg in the stream-sediment sample and visible gold inthe heavy-mineral concentrate sample (no. 116, table1). Heavy-mineral concentrate samples collected fromsixsitescontainvisiblecinnabar(from20to50percent,locally) in the heavy-mineral concentrate; one of thesesampleshad14ppmHgvalue.Microscopicallyvisiblesphaleriteandcassiteritewerealso identified locally inthesesamples.

Resourcearea4i,northeastofBismarckCreek

Resourcearea4i(mapB)liesnortheastofresourcearea1b(theGraniteCreekareathathasahighpotentialforperaluminousgranite-porphyry-hostedgold-polyme-tallic deposits). This small resource area is not welldefinedandmaybeinterpretedasanortheastextensionofresourcearea1b.Theinformationavailablesuggestsit has a moderate potential (certainty level B) for con-tainingepithermalmercury-antimony(gold)deposits.ItisunderlainbyKuskokwimGroupflysch(unitKks);oneintermediatedikewasmapped(unitTKd)withinthearea,butmoredikesmayexist.Theareawasprimarilydefinedon the basis of drainage basin geochemistry, however,rockanomalieswerefoundattwositesandoneprospectpitwasalsonoted,butnotsampled.Fromasiteneartheprospectpit,quartz-healedsandstonebrecciacontains20ppmSb,20ppmAs,andtraceHg(no.101,table1).Atanothersite,iron-stainedsandstonecontainingsecondaryhematitecontains200ppmSb,100ppmAs,180ppmZn,and100ppmNi (no. 100, table 1).A stream-sedimentsamplefromthesouthernpartoftheareacontains0.070ppmAuandtracesilver(<0.5ppm);theheavy-mineral

concentratefromthissitecontains10ppmAg,200ppmSb,300ppmPb,andmorethan2,000ppmSn;aquaticmoss contains 50 ppm Sb. One other stream-sedimentsample fromthis resourceareacontains traceAg(<0.5ppm)andoneotherheavy-mineralconcentratecontainsmorethan2,000ppmSn.

Resourcearea4j,DeCourcyMountainarea

Resource area 4j (map B), which contains theDeCourcyMountainmercurymine(no.184,table1,mapA),hasahighpotential(certaintylevelD)forcontain-ingotherepithermalmercury-antimony(gold)deposits.Much of the resource area is underlain by the IditarodVolcanics(unitsTKilandKit),whichoverlietheKusko-kwimGroupsedimentaryrocks(unitsKks).Theareaiscut diagonally by the trace of the Iditarod-Nixon ForkFault.KuskokwimGroupsedimentaryrocks(unitsKksandKkq)lienorthwestofthefault;rocksbothnorthwestandsoutheastofthefaultarelocallyintrudedbyalteredintermediate to mafic (and locally felsic?) dikes (unitTKd).ThespecificresourceareaboundariesaredefinedbytheDeCourcyMountainmercurymineandtheYou-surecinnabaroccurrence(no.156,table1andmapA),onthesoutheastandnorth,respectively,andbyfiveotherlocalitiescontaininganomalousmercury,antimony,andbase metals. Data from the reconnaissance drainage-basinstudysupportdesignationoftheresourcearea.

Epithermal Hg-Sb ore at the past-productiveDeCourcy Mountain mercury mine occurs near thecontact between the Iditarod Volcanics and underlyingKuskokwimGroupsedimentary rocks.Mostof theoreisfoundwheretheKuskokwimGroupiscutbysill-likebodies or dikes of basalt, which are usually altered totan,silicacarbonaterock.Theorebodies,whichexhibitubiquitouschalcedony-dickitealteration,containcinna-bar, stibnite, arsenopyrite, marcasite, and arsenopyrite,in stockworks, tensional shears, and sheeted zones inalteredsedimentaryanddikerocks.Some51,700kgofmercuryhasbeenrecoveredintermittentlyfrom1908to1953fromoresaveragingabout3.0percentmercury.

ThepreviouslyunknownYousureoccurrence,whichliesalongthenorthernlimitofmineralresourcearea4j,is exposed in a cut bank along the Iditarod River.Theoccurrenceconsistsofveinsandfracturecoatingsofcin-nabar and stibnite in brecciated, silicified KuskokwimGroupshale thathasbeen intrudedbymaficdikes, thelatter of which are altered to silica-carbonate rock. Inaddition to anomalous mercury and antimony values,grabsamplesfromtheYousureoccurrencehaveyieldedanomalous arsenic (>10,000 ppm), tungsten (50 ppm),barium(5,000ppm),andtracesilver.

Atfiveotherlocalitiesscatteredthroughoutresourcearea4j,rocksamplesyieldanomalousSbandHgvalues.Atthreesites(oneofwhichwaspreviouslyprospected)altered volcanic rocks, sometimes spatially associated

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withanaltereddike,containupto240ppmSb,110ppmAs,and0.92ppmHg,andlocally100Cuppmand7Moppm(nos.166,167,and183,table1).Attwoadditionalsites,alteredsandstone(no.182,table1)andaltereddikeandsandstone(no.180, table1)containupto1.2ppmHg,10ppmSb,and20ppmAs.Inthesouthwestcornerofresourcearea4j,LittleCreek(no.181,table1)yieldedsmall amounts of gold during prospecting and testing;mineconcentratescontainedsignificantcinnabar.

In the reconnaissance drainage-basin study, sam-ples collected from seven sites in or downstream fromresourcearea4jyieldedanomalousvalues.Inthesouth-ernpartoftheresourcearea,streamsedimentsfromfivesitescontainfrom0.80to5.10ppmHg.Heavy-mineralconcentratesfromfourofthesesitescontainmicroscopiccinnabarandonesamplecontainsupto20percentcin-nabarandsomescheelite.Aheavy-mineralconcentratefromLittleCreek(no.181,table1)contains150ppmAuand15ppmAg;microscopicgoldwasalsoidentifiedinthe concentrate.Aheavy-mineral concentrate fromonesiteinthecentralpartoftheresourceareacontains500ppmWand5,000ppmB.Anaquaticmosssamplefromanearbysitehas5ppmAg.

Resourcearea4k,TwinButtesarea

Resource area4k (mapB) consists of two smallersubareasinthesouthernpartoftheIditarodquadrangleandhasamoderatepotential(certaintylevelC)forcon-tainingepithermalmercury-antimony(gold)deposits.Inthispartof thequadrangle, theKuskokwimGrouphasbeen deformed into several west-trending, shallowlyplungingfolds,butno intrusiverocksorhornfelsaure-oleswererecognized.

The Twin Buttes subarea of resource area 4k isdefinedbyweakmercury,antimony,andsilveranoma-liesdetectedingrabsamplesofKuskokwimGroupsand-stone(unitKks).GrabsamplesfromsixsitesintheTwinButtessubareayieldedupto4.7ppmHg,110ppmSb,2ppmAg,20ppmAs,and100ppmCu(nos.198–202and206,table1).Onestream-sedimentsampledrainingthenorthpartofthissubareacontainstraceAg(<0.5ppm);the heavy-mineral concentrate from this site containsmorethan2,000ppmSn.

Themoresouthernsubareaisdefinedbyoneanoma-lous drainage site. Stream-sediment samples from thissitecontainfrom1.6tomorethan34ppmHg;aheavy-mineral concentrate contains 1–5 percent microscopiccinnabarandlessthan1percentscheelite.

Resourcearea4l,VABMGeorgearea

Resourcearea4l(mapB)hasamoderatepotential(certainty level C) for containing epithermal mercury-antimony(gold)deposits.TheareaisunderlainbyKus-kokwimGroupflysch(unitKks)thatislocallyintruded

by peraluminous granite porphyry dikes (unit TKgp).Resourcearea4lformsadiffuseandirregularhaloaroundtheeasternendof resourcearea1c, the latterofwhichhasapotentialforcontainingperaluminousgranite-por-phyry-hosted gold-polymetallic deposits. Grab samplesfromthreelocalitiesintheeasternpartofresourcearea1c(nos.162–164,table1anddescriptionofarea1c)havemorethan14ppmHg,10,000ppmSb,and1.6ppmHg,respectively,suggestingthepossibilityofanepithermaloverprintonmineralizedrockatthoselocalities.Atonelocality in thesouthernpartof resourcearea4l,agrabsampleofgraniteporphyrydikecontains8ppmHg(no.173,table1).

Inthereconnaissancedrainage-basinstudy,stream-sediment samples from three siteswithin resourcearea4lcontainanomalousHg;twoofthesesamplesitesalsoliewithin resource area1c.From the two sites that liewithinarea1c,onehas5.0ppmHgand0.5ppmAginthe stream sediment; the other site has 1.2 ppm in thestreamsedimentandmicroscopicallyvisiblecinnabarintheheavy-mineralconcentrate.Inthepartofarea4lthatliesoutsideofarea1c,asiteinthewesternpartcontains6.1ppmHginthestreamsediment,andmicroscopiccin-nabar(1–5percent)intheheavy-mineralconcentrate;aheavy-mineral concentrate from the northern part con-tains2,000ppmSn.

Resourcearea4m,KuskokwimMountains

Resource area 4m (map B) has a moderate poten-tial (certainty level C) for containing epithermal mer-cury-antimony (gold) deposits. It covers a very largeregion that encompasses resource areas 4a through 4l.ThisbroadregionisunderlainprimarilybyrocksoftheKuskokwimGroup(unitsKksandKkq)thatarelocallyintrudedbydikes(unitsTKgpandTKd).LocalHgandAuanomaliesarefoundinrocks,streamsedimentsandheavy-mineralconcentratesofthisresourcearea,butthedataaretoodiffusetodefineamorespecifictarget.

Rocksamplesfromsixlocalitiesyieldedanomalousvaluesthatareconsistentwithepithermalmercury-anti-mony(gold)deposits.Twoof thesesitesare located inarea4mbetweenresourceareas4aand4b.Atthemorenorthern of these sites, a silicified zone in sandstonenearanaltereddikecontains0.2ppmAuand0.28ppmHg(no.39,table1).About13kmtothesouth,nearlyakilometerdistantfromanyknownintrusiverocks,agrabsampleofalteredsandstoneandshalecontains2.9ppmHg(no.75, table1).Anothergold-bearing locality lieseastofresourcearea4bandneararea1a.Heresilicifiedlithic tuff contains0.88ppmHgand traceAu (no. 42,table1).Theremainingthreeanomalousrocklocalitieslie in theeasternpartof resourcearea4m,westof theKuskokwimRiver.Atonesite(no.102,table1)agrabsampleofsandstonecontainsslightlyelevatedvaluesofHg(0.46ppm),As(30ppm),Sb(20ppm),andtraceAg.

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Quartzveinletsiniron-stainedsandstoneatanothersite(no.124,table1)contain0.5ppmAg,900ppmZn,150ppmSn,20ppmAs.0.96ppmHg,and2,000ppmB.TheSnandBvaluesarenotexpectedtooccurwiththeepith-ermalmercury-antimony(gold)systems,buttheHgandAsvaluesareconsistentwithanepithermaloverprint.Agrabsampleofsandstonefromthelastsite(no.155,table1) contains0.5ppmAg,200ppmPb,10ppmBi,100ppmAs,and12ppmSb;Hgwasnotanalyzed.

Inthereconnaissancedrainage-basinstudy,numer-oussamplesfromresourcearea4mlocallycontainanom-alousAu,Hg,orotherelementsthatsupportdesignationof the area for epithermal mercury-antimony (gold)deposits.However,thesamplesitesarewidespreadandtheanomalousvaluesofHgandAuaredifficulttorelatetoknowngeologyandrockanomalies.

Gold anomalies occur in stream-sediment samplescollected in theeastern,northern, southern,andcentralparts of resource area 4m; Hg anomalies do not occuratthesesites.Intheeasternpartofarea4m,roughlyintheareaofanomalousrocksitesnos.102,124,and155(mentioned above), two stream-sediment samples con-tainAu (0.010and0.071ppm)andoneheavy-mineralconcentrate contains 70 ppmAu and 20 ppmAg.Twositesinthevicinityofthequartzveinletlocality(no.124,table1)yieldanomalousvalues.Atonesite,thestream-sedimentsamplecontains0.82ppmHgandtheheavy-mineralconcentratecontains7ppmAg.Aheavy-mineralconcentratefromtheothersitecontains7,000ppmSband700ppmPb.Aquaticmosssamplesfromfoursitesintheeasternpartofarea4mlocallycontainupto20ppmAg,50ppmSb,and500ppmAs.Inthenorthernpartofarea4m,onestream-sedimentsamplecontains0.022ppmAu,anotherstream-sedimentsamplecontains19ppmAs,aheavy-mineral concentrate contains 300 ppm Cu, andtwomoss samplescontain10ppmAg. In the southernpartofresourcearea4m,twostream-sedimentsamplescontainAu (0.010and0.011ppm)and theheavy-min-eralconcentratefromathirdsitecontains1,500ppmZn.Inthecentralpartofresourcearea4m,broadlycenteredaroundresourceareas1b,3g,3h,and4i,onestream-sedi-mentsamplecontains0.018ppmAu.Othersamples inthis general central area include two stream-sedimentsamples containing 0.78 and 18.3 ppm Hg; the heavy-mineral concentrates from these sites contain up to 20percentmicroscopiccinnabar.Theremaininganomaloussamplesfromthiscentralareaareonestreamsedimentcontaining300ppmZnandanothercontaining0.5ppmAg;sixheavy-mineralconcentratesampleslocallycon-tainupto300ppmPb,500ppmW,20,000ppmZn,andmorethan2,000ppmSn;aconcentratefromanothersitecontainsmicroscopicallyvisiblescheelite(5–20percent)andgalena(<1percent).

Twoothergeneralregionswithinresourcearea4mcontainlocalHganomalies:(1)betweenresourceareas

4a and 4b, and (2) in the general vicinity of resourcearea4j(botheastandwestofit).Betweenareas4aand4b, samples from four sites yielded anomalous values:oneheavy-mineralconcentratecontainsupto5percentmicroscopiccinnabar,twootherconcentrateseachcon-tain200ppmSbandmorethan2,000ppmSn,andonestreamsedimentcontains300ppmZn.Fromasitewestofresourcearea4j,astreamsedimentcontains5.6ppmHg and the heavy-mineral concentrate contains micro-scopiccinnabarandgalena.Fromasiteeastofresourcearea4j,anaquaticmosssamplecontains50ppmSb.

Resourcearea4n,southeastcornerofIditarodquadrangle

Resourcearea4n(mapB)hasalowpotential(cer-tainty level C) for containing epithermal mercury-anti-mony(gold)deposits.Similartoresourcearea4m,area4nisunderlainprimarilybyrocksoftheKuskokwimGroup(unitsKksandKkq),butunlikearea4m,veryfewdikeswerenotedduringtheregionalgeologicmappingandnomineraloccurrencesorrockanomaliesareknown.Cinna-barinsomeheavy-mineralconcentratesandAuvaluesinsomestream-sedimentsamplesindicatetheareahassomepotentialforepithermaldeposits;however,thesesitesarescatteredanddiffuse.Fourstream-sedimentsamplesfromthis areacontaingoldvalues ranging from0.01 to0.11ppmAuandoneheavy-mineralconcentratecontains300ppmAu and 50 ppmAg.One stream-sediment samplecontains1.8ppmHg;theheavy-mineralconcentratefromthissitecontainsmicroscopiccinnabar.Fourheavy-min-eral concentrates locally contain up to 1,500 ppm Zn,10,000 ppmW, and more than 2,000 ppm Sn. In addi-tiontocinnabaratfivesites,heavymineralconcentrateslocallycontainmicroscopiccassiteriteandscheelite.Anaquaticmosssamplefromonesitecontains200ppmSb.

VOLCANIC-HOSTEDPRECIOUSANDOTHERMETALSDEPOSITS(MODEL5)

MuchofthewesternpartoftheIditarodquadrangleisunderlainbytheYetnaRivervolcanicfield(unitTKy),a poorly exposed sequence of subaerial volcanic rocksconsistingprimarilyofcalc-alkalineandesiteandrhyo-litepyroclasticand lavaflows, andminorbasaltflows.Silicification and larger areas of siliceous sinter arefound locally,mostly in themore rhyoliticpartsof thesequence.EightK-Arisotopicagesrangefrom69to54Ma(MillerandBundtzen,1994)possiblymakingthesesubaerialvolcanicrocksslightlyyoungerthanthe73to59Mavolcanic-plutoniccomplexestotheeast.

The lackof exposureand the reconnaissance levelofthisinvestigationmakeitdifficulttoassessthemetal-logenic significance of theYetna River volcanic field.AnomalousvaluesofAu,Ag,Sb,andHg,inbothrockanddrainage-basin samples, are scatteredover abroad

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partofthevolcanicfield.SomeoftheAu-andAg-bear-ingrocksamplesareassociatedwithchalcedonicaltera-tion in rhyolite; others are associated with yellowishlimonite, reddishgoethite,and locallyhematite-bearingbleached zones in rhyolite and andesite. In addition tothiswidespreadepithermalelementalsuiteofAu,Ag,Sb,andHg,asmallpartofthevolcanicfield,whichispri-marilyunderlainbyrhyolitepyroclasticandlavaflows,hasconsistentlyelevatedvaluesofSn,Pb,Be,andNb.ThiselementalassemblageissimilartothatoftheSischuvolcanicfield,aLateCretaceousandearlyTertiarysub-aerialvolcanicsuite(Moll-Stalcup,1994)intheMedfraquadrangle(PattonandMoll,1983).

The geologic setting, silicic alteration, and epith-ermal elemental suite that characterize a broad part oftheYetnaRivervolcanicfieldareconsistentwithpartsof several deposit models—hot-spring Au-Ag modelof Berger (1986, model 25a), Comstock epithermalveinsmodelofMosierandothers (1986b,model25c),andSadoepithermalveinsmodelofMosierandothers(1986a, model 25d). In model 25a, a vertically zonedhydrothermal system is expressed by siliceous sinterdeposits in near-surface conditions accompanied bygold,silver,antimony,arsenic,andmercury,progressingintodeeper-level stockworkandveins accompaniedbybothbase andpreciousmetals. Inmodel 25c, volcanichost rocks overlie clastic sedimentary (and equivalentmetamorphic) rocks,butvertical zonation is somewhatsimilar.Inmodel25d,geothermalsystemsoperatewithinbimodal volcanic rocks and associated intrusive unitsthat overlie thick, older igneous sequences; elementalassociationsareAu,Ag,andsomebasemetals.Withoutmoredetailedinformationitisnotpossibletoselectoneofthesemodelstodescribethegold-silveranomaliesoftheYetnaRivervolcanicfield.Also,theelevatedvaluesofSn,Pb,Be,andNbnotedinonepartofthefieldarenot consistent with any of the volcanic-hosted epither-malmodels.Thiselementalsuiteisassociatedwithfelsicintrusionsandmayindicatethepresenceofanunexposedgranitic body. Given the uncertainties, the potentialdepositsoftheYetnaRivervolcanicfieldaredescribedin general terms—volcanic-hosted precious and othermetalsdeposittype.

Resourcearea5a,westofIditarodRiver

Resourcearea5a(mapB)hasahighpotential(cer-tainty levelB) for containingvolcanic-hosteddeposits.Theareaisunderlainprimarilybyrhyolitictoandesiticpyroclasticflowsandsilicified rhyoliticdomes thatarepartof theYetnaRivervolcanicfield (unitTKy).Grabsamplesofrhyoliticrockfromthreesitesinthenorthernpartoftheresourcearea(nos.34–36,table1)yieldcon-sistentvaluesofSn(50–70ppm),Pb(100–150ppm),Nb(50–100ppm),andBe(10–15ppm).TwoothersitesinthesouthernpartoftheresourceareayieldedAgvalues.

Agrabsampleofiron-stained,silicifiedrhyolitecontains10ppmAg,500ppmPb,and20ppmSn(no.50,table1).Agrabsampleofalteredrhyolitetuffcontains5ppmAgand200ppmPb(no.69,table1).Hgwasnotanalyzedinanyofthesegrabsamples.

Inthereconnaissancedrainage-basinstudy,samplesfromtwositesareAu-bearingandsamplesfromanaddi-tionalsixsiteslocallycontainanomalousvaluesofAg,As,orSn.Twoheavy-mineral concentratescontainAu(150ppmand300ppm)andAg(10ppmand300ppm,respectively); thestreamsediment fromthe formersitecontains1.0ppmAg.Twootherstream-sedimentsamplesfromthisresourceareacontaintracesilver(<0.5ppm),streamsedimentsfromthreeadditionalsitescontainAs(15–32ppm),andtwoheavy-mineralconcentratescon-tainmorethan2,000ppmSn.

Resourcearea5b,YetnaRiverarea

Resourcearea5b(mapB)hasamoderatepotential(certaintylevelB)forcontainingvolcanic-hostedepith-ermaldeposits.Theareaisprimarilyunderlainbyrhyo-lite,dacite,andesite,andlocalbasaltoftheYetnaRivervolcanicfield (unitTKy).OnesmallexposureofEarlyCretaceous sedimentary rock (unit Ks), which locallyunderliesthevolcanicrocks,cropsoutinthesouthwest-ernpartoftheresourcearea.

Grab samples from eight sites in resource area 5byieldanomalousmetalvalues, includingupto0.1ppmAuatfiveofthesesites.Samplesofalteredrhyolitefromthreesites(nos.68,70,and85,table1)containAuandHg (0.05 to0.10ppmAuand60ppb to1.4ppmHg),and locally traceAg, 20 ppm Sn, and 130 ppmAs.Adacitesamplefromthesouthernpartofresourcearea5bcontains50ppbAuand18ppmSb (no.126, table1).A grab sample of hematite-altered sandstone from thesouthwesternpartoftheresourceareacontains150ppmAsand traceamountsofAu,Ag,Hg,andSb.Samplesofweldedrhyolitetufffromtwosites(nos.37and125,table1)contain2.2and1.2ppmHg,respectively.Agrabsampleofandesitefromthesouthernpartoftheresourceareacontains100ppmSn.

Samples collected within resource area 5b duringtheAMRAPreconnaissancedrainagebasinsurveyhelpdefinetheoutlineofthisresourcearea.Threestream-sed-imentsamplesfromthenorthwesternsideoftheresourcearea contain 0.012 to 0.027 ppmAu; a heavy-mineralconcentratefromoneofthesesitescontains1,000ppmSb and microscopic cinnabar occurs in the concentratefrom another of these sites. Elsewhere in the resourcearea,cinnabarwasidentifiedintheconcentratefromtwoothersites.Threestream-sedimentsamplescontainupto22ppmAs, andone stream-sediment samplehas traceAg(<0.5ppm).Oneheavy-mineralconcentratecontains300ppmCuandanothercontains1,000ppmW.Galena,cassiterite,chalcopyrite,andscheelitewereidentifiedby

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microscope locally in five samples. One aquatic mosssamplecontains5ppmAg.

TheNUREreconnaissancelake-sedimentdatapro-videsomeadditionalsupportforresourcearea5b.UsingR-modefactoranalysisGrayandothers(1988c)outlinedanareaneartheheadwatersofReindeerCreek(seetheirfig. 1) that fits their precious-metal related factor (As-Mn-Fe)outlinedforthelake-sedimentdataset.

MAFIC-ULTRAMAFICRELATEDCr-Ni-CoDEPOSITS(MODEL6)

Mafic and ultramafic rocks of the Dishna Riverareacropout inanelongate, irregularbelt fromMooseCreeknortheastwardtotheIditarodquadrangleboundaryimmediatelyeastoftheDishnaRiver.Maficrocks,whichlocallyshowmineralassemblagesindicativeoflow-pres-sure, hydrothermal metamorphism to greenschist facies(Miller,1990),andserpentinizedultramaficrocks,occurasisolatedfault-boundedblocksthatarealmostcertainlypartoftheTozitna-InnokoophiolitebeltasdescribedbyPatton and others (1994a). Miller and Bundtzen (1994)assignedaJurassicage to thebelt in the Iditarodquad-rangle.

Mafic-ultramafic related chromite, cobalt-nickel,andasbestosdeposits(depositmodels8aand8c,Albers,1986,andPage,1986,respectively)arehostedinrockssimilar to themaficandultramaficrocksof theDishnaRiverarea.PodiformchromiteisknowntooccurintheTozitna-Innokoophiolitebelttothenortheastofthestudyarea(Roberts,1984;LoneyandHimmelberg,1984).Noultramafic-associatedmineraloccurrencesareknownintheDishnaRivermafic-ultramaficrocks.

Resourcearea6,DishnaRiverarea

Resourcearea6 (mapB)hasa lowpotential (cer-tainty level C) for containing mafic-ultramafic relatedCr-Ni-Codeposits.Manygrabsamplesofgabbroandhar-zburgitefromtheDishnaRivermafic-ultramaficcomplex(reported inMcGimsey andothers, 1988) containhighvaluesofchromium(>5,000ppm,locally),nickel(upto3,000 ppm), and cobalt (up to 200 ppm), but no chro-miteormassivesulfideconcentrationshavebeenidenti-fiedinthefield,andmostelevatedmetalsarebelievedtobederivedfromrefractorymineralsinalteredultramaficigneouslithologies.Atonelocalityinresourcearea6,agrabsampleofserpentinizedultramaficrockcontains30ppmAg,120ppmAs,2,000ppmCr,and2,000ppmNi(no.51,table1).TheCr-Nivaluesareprobablyrelatedtothepresenceofchromespinelandalteredolivine;thesourceoftheAgandAsisnotwellunderstood.

Afewsamplescollectedduringthereconnaissancedrainage-basin study in or downstream from resourcearea6yieldedanomalousvalues.Twostream-sedimentsamples contain Cr (>5,000 ppm and 1,500 ppm). Aheavy-mineral concentrate from the latter site contains

100ppmAuand20ppmAg;smallamountsofmicro-scopic cinnabar and scheelite were identified in thissample.Twoaquaticmosssamplesfromthewestsideofresourcearea6contain5ppmAg.

HEAVY-MINERALPLACERDEPOSITS(MODEL7)

Gold fromplacerdepositshasbeen thechief com-mercially recovered mineral resource in the Iditarodquadrangle.ThequadrangleincludespartsoffourplacerminingdistrictsasdefinedbyRansomeandKerns(1954):theAniak,McGrath,Innoko,andIditaroddistricts(fig.4).From1910to1992,anestimatedminimumof65,000kg2(2.1millionoz)ofgold,7,300kg(235,000oz)ofsilver,andsometungstenandmercuryhavebeenrecoveredfromtheplacerdepositsofthequadrangle.Incomparison,onlyabout99kg(3,185oz)ofgoldwasproducedfromlodemineral deposits in the quadrangle. Heavy minerals ofpast,current,orpotentialinterestincludegold,cinnabar,scheelite, cassiterite, ilmenorutile, magnetic chromite,platinumgroupelements(PGE),andstibnite.

Virtuallyallheavy-mineralplacerdeposits thusfarrecognizedintheIditarodquadranglehavetheirsourceinmineralizedLateCretaceousandearlyTertiaryintrusionsand volcanic rocks that crop out throughout the studyarea.Threemajormorphologicaltypesofheavy-mineralplacerdeposits havebeen recognized in the studyareaandaresummarizedbelow.

(1) Residual and hillslope (eluvial) placers thatformeddirectlyonornearmineralizedsourcerockoccurintheFlatareaoftheIditaroddistrictandinpartsoftheInnokodistrict(fig.4).TheyincludedepositsonChickenMountain(no.105,mapAand table1)andpartof theGanesCreekandYankeeCreekdrainages(nos.10and19,mapAandtable1).Residualandhillslope(eluvial)placersof thestudyareaare restricted tosmallgullies,hillslopes,altiplanationterraces,anddikeswarmscross-ing creek valleys. Host lithologies are unconsolidated,poorlysortedeluvialtalus,immaturefluvialdeposits,andigneousgruss formedbydisintegrationandweatheringofunderlyingplutonicrocks.Commonlyplacerscontainlargeamountsofclayderivedfromalterationofigneousfeldspars.Pay zones aregenerally thin anddiscontinu-ous,butlocallymaybequiterich;somepayzonesattheheadofFlatCreek(no.106,mapA)intheIditaroddis-trictaveraged25to50g/tonnegoldduringexploitation.

(2) Ancestral stream channel deposits in terrace alluvium (bench placers) are elevated fluvial depos-its ancestral tomodern streamchannels.Benchplacersarewidespreadthroughoutthestudyareaandcomprisethe most important resources of placer gold remaining

2Recallthatthetotalrecordedproductionforthequadrangleishigherthanthecumulativetotalobtainedbyaddingindividualpropertypro-duction figures from table 1, because early production figures werecommonlyreportedbydistrictorsubdistrict,notbycreek.

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throughouttheIditarodquadrangle.IntheDonlinCreekarea (no.195,mapA), thesedepositswere theaurifer-oussourcesforgoldfoundinthecommerciallyexploitedmodernstream.Apossiblemechanismforformationofbench placers is that they formed by westward streammigration during asymmetrical valley formation. Soli-fluction due to daytime (afternoon) thawing activity ismorepronouncedonsouth-orwest-facingslopes,whichcausedstreamstomigratecrossvalleytowardnorth-oreast-facingbuttresses.Throughthislongprocess,whichbegan in late Tertiary time, the eastern bench levelsformed on Ganes, Spaulding, and Deadwood Creeks(nos.10,11,and56,respectively,mapA)intheInnokodistrict,onFlat,Otter,Slate,Willow,Chicken,andPrinceCreeks(nos.106,107,112,129,131,and132, respec-tively,mapA)nearFlat in the Iditaroddistrict, andonSpruceCreekandDonlinCreek(nos.172and195,mapA)intheAniakdistrict.Inafewareas,valleyasymme-try is reversed and the stream migration could also beinfluencedby regionalupliftor structural tiltingdue totheinfluenceofhigh-anglemovementalongtheIditarod-NixonForkFaultsystem.Forexample,amajorstreampiracyatDonlinCreekthatreversedstreamdirectionsub-sequenttotheformationofDonlinbenchprobablywasinfluencedbyregionaluplift.Likewise,similarreversedvalleyasymmetryatMooreCreek(no.83,mapA)couldbeinfluencedinthesameway.

(3) Modern stream alluvium placers formed inHolocenestreamgravelsthroughoutthestudyarea;theycomprisealessimportantbutstillsignificantportionofthe total placer resources. They consist of unconsoli-dated, generally shallow gravel and silt deposits rang-ingfrom1-to3-mthick.Themostsignificantdepositsare on Yankee and Carl Creeks in the Innoko district,onFourthofJulyCreek(northeastofMooreCreek),allthird- and fourth-order streams in the Flat area (listedabove),Omega,Snow,andQueenGulchesintheDonlinCreek area, Little, Montana, and Return Creeks in theDeCourcyMountainarea,andGraniteandJulianCreeksintheGeorgeRiverarea.

Goldinallthreemorphologicalplacerdeposittypesconsists of flattened, flakey, and rounded grains; somenuggets are very large. The third largest nugget everfound in Alaska was recovered by Magnuson MiningCompanyonGanesCreekin1985andweighed3,856g(124oz).MagnusonMiningCompanypreviouslyrecov-ereda1,990g (64oz)goldnuggetonGanesCreek in1964.In1987,JohnMiscovichrecovereda28oz(871g)goldnuggetinOtterCreeknearFlat.A606g(19.5oz)nuggetwasrecoveredonMooreCreekbyDonHarrisin1984.Goldnuggetsweighingupto590g(19oz)wererecoveredbymineoperatorToivoRosanderonYankeeCreekpriorto1968.In1984,SpencerandDougLymanrecovereda280g(9oz)goldnuggetinSnowGulchoftheDonlinCreekplacerarea.

Goldplacersofallthreemorphologicaldeposittypesdescribedaboveformat thebaseof thegravelor talustrappedbyvariousnaturalrifflesinbedrocksuchasdikecontacts,beddinglayers,orfractures.Suchadeposit isgenerallyreferredtoasa“bedrockplacer”.Inrarecases,heavy-mineralconcentrationsoccuronclayhorizonsingravelor talusandare referred toasa“falsebedrock”placer.

Resourcearea7a,GanesCreek-YankeeCreek

Resourcearea7a(mapB)hasahighpotential(cer-tainty level D) for containing unexploited placer goldresources. It contains two auriferous drainages thatflow north into the Innoko River, amajor tributary oftheYukon River.Thepay streakson Ganes Creek andYankeeCreek(nos.10and19,table1),andtheirtributar-ieshavebeenminednearlycontinuouslysincetheearly20thcenturyandranksecondonlybehindpaystreaksintheIditarod-Flatresourcearea(7d).Cumulativeproduc-tionwithinresourcearea7aisestimatedtobe5,042kg(162,114 oz) of gold and 699 kg (22,468 oz) of silverprimarilyfromGanesandYankeeCreeks,butalsofromMackieCreek,SpauldingCreek,GlacierGulch,andLastChanceGulch(nos.1,11,12,and13,table1).Open-cutminingoperationsarecurrentlyextractinggoldonbothGanesandYankeeCreeks.

At least two ancestral paleo channels, 15 to 30metersabovethemodernfloodplain,havebeenminedon Ganes Creek. These paleo channels predate thebeheading of upper Ganes Creek by Beaver Creek inmid-Quaternarytime(Bundtzen,1980);hencetheyarethought tobe lateTertiaryorEarlyPleistocene inage.Theyounger(lateQuaternarytoHolocene)streamgrav-elsofGanesCreekvalleyalsocontainauriferousgrav-els. Spaulding andYankee Creeks contain similar paystreaksofunknownagethatformedatthesamerelativelevel as themodern incision.Anancestralbench leveloccursonlyontheright limitofYankeeCreek, imme-diately north of the study area. Individual pay streaksonallthreestreamsrangefrom10mto60m-wideandaverageabout1.5mthick.

Thelodesourceforplacersinallthreedrainagesiscorrelated with an extensive northeast-trending graniteporphyrytogranodiorite(andlocallymafic)dikeswarmmappedbyBundtzenandLaird(1980;1982)andreferredtohere as theYankeeCreek-GanesCreekdike swarm.The Independence Mine (a peraluminous granite-por-phyryhostedgold-polymetallicdeposit(no.16,table1)isfoundwithinthisswarm.HeavymineralsreportedbyBundtzenandothers(1987)inmineconcentratesofallstreamsinthisareaincludeabundantmagnesiochromite,scheelite, ilmenite, eckermannite (a sodic amphibole),andnativesilver,inadditiontofreegold.Theabundantmagnesiochromite is traced back to chromite-bearingolivinemonzoniteintrusionswithinthedikeswarm.The

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sodic amphibole is commonly associated with alkalineigneousrocks.

Reserve estimates are difficult to ascertain due tolackofdocumenteddata.Examinationofmine records(WarrenMagnusonandRonRosander,oral communs.,1985),suggeststhatsignificantamountsofgoldremainin unmined pay streaks, mostly in the lower GanesCreekdrainage.Intheregionaldrainagebasinstudytwosamples yielded anomalous values: a stream-sedimentsample collected froma tributary toGanesCreekcon-tains 0.5 ppmAg; a heavy-mineral-concentrate samplefromSpauldingCreekhasvisiblegold.

Resourcearea7b,CarlCreek

Resourcearea7b(mapB)hasahighpotential(cer-tainty levelC) for containingplacer gold resources. Inthis area, a small, first-order stream draining the northsideofVABMTatalinaemptiesintoCarlCreek,aneast-erlyflowingtributaryoftheKuskokwimRiver.Accord-ingtounpublishedU.S.mintrecords,0.6kg(18oz)ofplacergoldwasminedfromCarlCreekintheyears1917and 1918; additional prospecting and production mayhave subsequently occurred. Placer tailings were notrecognizedoneitherCarlCreekor theVABMTatalinatributary(no.48,table1andmapA).BundtzenandLaird(1983)reportedthataheavy-mineral-concentratesamplefromthelattertributarycontains23ppmAg,5,700ppmPb,andtraceAu.Samplescollectedfromnearlythesamesite during this study also yielded anomalous values.Twostream-sedimentsamplescontained3ppmAgandup to120ppmAs.Heavy-mineral-concentrate samplesfromthesamesitescontained15ppmAg,1,000ppmAs,andupto5,000ppmPb.ThestockatVABMTatalinaisapotential lode source for theanomalousmetalvaluesinresourcearea7b.Noestimateoftheplacerresourceispossibleatthistime.

Resourcearea7c,MooreCreekarea

Resource area 7c (map B) has a high potential(certainty level D) for containing unexploited placergoldresourcesandisoneofthemostsignificantplacerresourceareasinthequadrangle.Auriferousstreamsofthe resource area drain an upland area that has a highpotential for plutonic-hosted copper-gold-polymetallicdeposits(resourcearea2b).Twopastproductiveplacermines(FourthofJulyCreekandMooreCreek,nos.60and83,table1andmapA),oneplacerprospect(Dead-woodCreek,no.56),andthreeplacergoldoccurrences(nos.55,58,and81)helpdefinethearea.

Most placer deposits on Moore Creek occur inancestral stream channel deposits in terrace alluvium.Modern stream alluvium in the lower portions of thedrainage have additional, but minor, placer concentra-tions.TotalproductionfromMooreCreek(andNevadaGulch)between1911and1985isestimatedtobe1,722

kg(55,370oz)ofgoldand391kg(12,570oz)ofsilver.The lode source of the Moore Creek heavy-mineralplacerdepositsisbelievedtobeadeeplydissectedmon-zoniteplugthatcropsoutonthehillside2.5kmnorth-westofMooreCreek(Bundtzenandothers,1988).Muchof the Moore Creek placer pay streak was worked outbyefficientdragline-dozeroperationspriortothe1970’s.However, a significantpotential exists fordiscoveryofancestralbenchplacers.Bundtzenandothers(1988)sug-gested thatTertiary toQuaternaryplacerdepositsweresuccessivelyoffsetfromthelodesourcebyright-lateraltranscurrentmovementontheIditarod-NixonForkFault,which forms the southern structural boundary of theMooreCreekpluton(thepostulatedlodesourcefortheplacergold).Hence,potentiallysignificant,older,unex-ploitedandburiedancestralchannelsmayliesouthwestofthemainworkings.

Heavy-mineral placer deposits on Fourth of JulyCreek were first discovered in 1915 and intermittentdevelopmenthastakenplacesincethen.Theheavy-min-eralplacersappeartobeconfinedtomoremodernHolo-cene alluvium; older bench deposits have not yet beenrecognized. Bedrock pay streaks overlie decomposedandesite tuff,whichhasproventobeapoorcatchmentsurfacefortheplacergold.Bundtzenandothers(1988)speculatedthatsignificantplacergoldpotentialexists2–3kmbelowthecurrentlyknownplacerresources,wherethebedrocksurface isKuskokwimGroupclastic rocksandthehydraulicgradientismoremature.Thepotentialfordiscoveryofa2-kmlong,3-mwidepaystreakiscon-sideredhigh.ThesourceoftheplacergoldinFourthofJulyCreekisprobablyamineralizedplutonthatstraddlesthe Fourth of July Creek and Maybe Creek drainages.Placergoldwas alsopannedonMaybeCreek (no. 58,table 1) during previous investigations (Bundtzen andothers,1988).

Alarge,low-grade,shallow,unexploitedgoldplacerresourceoccupiesa1.5kmby150mareanearthehead-ward zone of Deadwood Creek (no. 56, table 1). Thedeposit,whichwasdiscoveredinthe1930’s,consistsoffinegold inshallowgravels2–3mthick.According toToivoRosander(oralcommun.,1983), thedeposituni-formly assays about 0.008 oz/yd3 (or about 0.4 g/m3).Assuming continuity of pay from the previous churndrilling program, the Deadwood Creek placer depositcontainsaspeculativeresourceof375,000m3ofgravelgrading0.4g/m3or150kg(4,820oz)gold.

During the reconnaissance drainage-basin studysamples collected from four sites in resource area 7cyielded anomalous Au values. Two stream-sedimentsamplescontain0.056and0.077ppmAu.Visiblegoldwas identified in a heavy-mineral concentrate from anunnamed drainage north of St. Patrick Creek (no. 55,table1andmapA);thissamplecontainsmorethan1,000ppmAuand300ppmAg.Aheavy-mineralconcentrate

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fromanearbysitecontains500ppmAu,50ppmAg,500ppmW,and100ppmMo.

PlacerchromiteexistsintheMooreCreekandForthof Julyplacerdeposits.Mineconcentrates fromMooreandFourthof JulyCreekscontain35percentand24.8percent chromite, respectively (Bundtzen and others,1988).Bulksamplesofplacergravelsfrombothstreamscontained0.28to2.08percentchromium(Bundtzenandothers, 1988).Although this is a significant amount ofchromium,itisprobablyonlyeconomicallyviableasabyproduct.

Resourcearea7d,Iditarod-Flatarea

Resourcearea7d(mapB)hasahighpotential(cer-tainty level D) for containing unexploited placer goldresources.Heavy-mineralplacerdepositsinthisresourcearea have accounted for about 70 percent of all goldrecovered from the Iditarodquadrangle.GoldhasbeenminednearlycontinuouslysinceitsdiscoveryonChrist-mas Day in 1908; currently (1998) three mechanizedmining operations recover placer gold. Through 1990,about45,200kg(1,450,000oz)ofgold,about6,100kg(197,000 oz) of silver, and minor amounts of tungstenandmercuryhavebeenrecoveredfromtheplacerdepos-its. Over the years many workers have described theplacerresourcesofthisarea.TheyincludeSmith(1915),Brooks (1912, 1914), Eakin (1913, 1914), Mertie andHarrington(1924),Mertie(1936),Williams(1950),andBundtzenandothers(1987,1992).Mostofthefollowingbriefsummaryisderiveddirectlyfromamorecompre-hensivesummaryinBundtzenandothers(1992).

CommerciallyproductivestreamsintheareaincludeGranite,Happy,upperFlat,Flat,Otter,Black,Malamute,Slate,Willow,Chicken,andPrinceCreeks(nos.89,104–107,109,111,112,129,131,and132,table1andmapA).Two streams—Otter and Flat Creeks—have accountedfor33,500kg(1,077,000oz)goldfrom1915to1990orabout75percentoftotaldistrictproduction.Thesourcesof theplacergold are auriferousveins,disseminations,andstockworksintheintrusiverocksofChickenMoun-tain,inthehornfelsaureolenorthofOtterCreek,andinmineralized bedrock within the valley of Otter Creek.ThemajorityofproductionfromOtter,Flat,Willow,andChickenCreekshascomefromseveralancestralterracealluvialdeposits,whichareconsistently2–4timesricherthanmodernstreamgravels.Richterracelevelsofnoteinclude the “Marietta” bench of upper Flat Creek, theWillowBench(no.129,mapA)onlowerWillowCreek,prominent terraces on Prince Creek, and most of thesouthern limits of Otter Creek valley. Residual placersonChickenMountainand in theBlackCreekdrainagewereminedalmostentirelybyhandpriortoWorldWarII;thesedepositsarenowlargelyexhausted.

Extensivegoldfinenessdatashowadistrictaverageof 864 based on 112 weighted determinations (Smith,

1941;Bundtzenandothers,1987,1992).Allthreeplacerdeposittypes—residualoreluvial,ancestralterraceallu-vialorbenchplacers,andmodernstreamalluvialdepos-its—occurintheIditarod-Flatresourcearea.ThisareaisoneofAlaska›sbestexamplesofprogressiveevolutionfromresidual toeluvial tostreamheavy-mineral-placerconcentrationsfrommineralizedsourcerock.

Most of the present placer mine development andproduction activities are focused on (1) previouslyminedpaystreakswhereexaminationofrecordsshowedsignificant gold recovery losses and (2) evaluation ofdeeplyburiedancestralchannelsnotexploitedbyearliermineoperators.The followingplacer resourcedata arederived fromexaminationof unpublishedprivate com-panyfiguresanddiscussionswithJohnMiscovich,AlvinAgoff,andthelateRichardFullerton.Alow-gradeplacerdepositbelowtheFlattownsitewassystematicallyevalu-atedusingachurndrillprogrambytheNorthAmericanDredgingCompany.Theexplorationeffortindicatedthata 3-km long and 0.5-km wide pay zone contained aninferred resource of 9,844,000 m3 (12,875,000 yd3) ofpaygradingabout0.4g/m3(0.008oz/yd3)goldor3,203kg(103,000oz)gold(RichardFullerton,oralcommun.,1989).ExplorationworkontheWillowBench(no.129,mapA),anancestralchannelofWillowCreek(Bundtzenandothers,1992),delineatedapaystreakapproximately3kmand100–150mwidethatmaycontainaninferredresourceofatleast1,580,200m3(2,067,000yd3)grading0.61g/m3(0.015oz/yd3)goldorabout964kg(31,000oz)ofgold.PlacergoldresourcesremaininPrinceandlowerChickenCreeks(nos.132and131,mapA).Mostpreviousminingonbothcreekswasconfinedtoshallowbenchesandmodernstreamalluvium.Unexploitedances-tralchannelsandalluvialfandepositsburiedby10–12mof overburden are found in both areas; recent explora-tionhasshownthatpromisingresourcesremain.Unmea-sured,butprobablysignificantplacergoldresourcesalsooccurinFlatandOtterCreeks,wheremostofthearea’spast district production has taken place. Modern mineoperatorsinbothareashavefoundeconomicconcentra-tionsofplacergoldinfractionsandalongbenchlimitsincompletelyevaluatedbypastoperators.

During the reconnaissance drainage-basin study,samples collected from four sites in resource area 7dyieldedanomalousAuand(or)Agvalues.Visiblegold(1–5percent)wasnotedinaheavy-mineralconcentratefromBlackCreek(no.109,mapA)thatcontains500ppmAg,1,500ppmAs,2,000ppmW,and3,000ppmSb;thestream-sediment samplecontains1.55ppmAuand1.0ppmAg.Aheavy-mineralconcentratecollectednorthoftheFlattownsitecontains500ppmAu,70ppmAg,and1,000ppmW.Astream-sedimentsamplecollectedfromWillowCreek,abovetheplaceroperation(no.129,mapA),contains0.5ppmAg.Westoftheplaceroperation,astream-sedimentsamplecontains0.015ppmAu.

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Resourcearea7e,LittleWaldrenFork

Resourcearea7e(mapB)hasahighpotential(cer-taintylevelB)forcontainingplacergold,however,itisunlikely that thiswould be a significant resource.TheareaisoutlinedonthebasisofmetalliclodesoftheBis-marckCreekarea(resourcearea3g,mapB)andscatteredgoldanomaliesfromtheregionalgeochemicalsurvey.Inaddition,placergoldwasreportedlyfoundin theLittleWaldrenForkdrainagebasinshortlyafterWorldWarII(DougSherrer,oralcommun.,1992).

Duringthereconnaissancedrainage-basinstudy,sam-ples collected fromsix sites in resource area7eyieldedanomalousAuand(or)Agvalues.Threeofthesesitesarein the headwaters area of Little Waldren Fork; of threestream-sediment samples, one contains 0.014 ppm Au,another contains 0.012 ppmAu and less than 0.5 ppmAg,thethirdcontains1.0ppmAgand300ppmZn.Theremainingthreesitesarefromanunnamedcreekthatdrainseastfromresourcearea3gintoLittleWaldrenFork.Twostream-sedimentsamplesfromherecontain0.5ppmAg,andathirdcontainslessthan0.5ppmAg.Aheavy-min-eralconcentratefromthelastsitecontains500ppmAu,50ppmAg,200ppmW,andmorethan2,000ppmSn.

Resourcearea7f,GraniteCreekandMuntherCreek

Resourcearea7f(mapB)hasahighpotential(cer-tainty level D) for containing unexploited placer goldresources.Theareaoverlapspartofagold-bearingloderesourcearea(1b,mapB)andencompassestwoaurifer-ouscreeks—GraniteCreekandMuntherCreek(nos.142and148,respectively,table1andmapA).GraniteCreek,amajortrunkstreamoftheGeorgeRiver,hasrecordedproduction;MuntherCreek,atributarytotheEastForkof theGeorgeRiver,wasprospected in the1920’sand1930’s (Tony Gularte, oral commun., 1986) and mayhavehadsomeproduction.Bundtzenandothers(1986)evaluatedsignificantstibnite-quartz(gold) lodeshostedingraniteporphyrydikesandsillsnearthisproperty;theinitialdiscoverywasmadebymineoperatorL.E.Wyrickduringplacerminedevelopment.Theselodeoccurrencesarebelievedtobethesourceofatleastpartoftheplacergold resources in both the Granite Creek and MuntherCreekdrainages.

Placer gold was discovered on Granite Creek in1924 and intermittent underground drift developmentandproductiontookplaceuntil1935.Afteralongdor-mant period, the deposits were developed again in theearly1980’s.Currentmineoperationshavetakenplacecontinuouslysinceabout1985.Totalproductioniscon-servativelyestimatedat101kg(3,250oz)ofgoldand12kg(400oz)ofsilverfromshallowgravelsonthemodernflood plain and from a slightly elevated terrace on therightlimitofthecreek.Principleheavymineralsbesidesgoldaregarnet,zircon,stibnite,andcassiterite.

Placer pay streaks 2 m thick in the Granite Creekdrainage probably average about 1.0 g/m3 (0.025 oz/yd3).Surfacesamplingandlimitedshallowreversecir-culation drilling results indicate about 350,000 m3 ofauriferouspay.Extensionsoftheknownresourcescanbeextrapolatedfromdelineationofthegraniteporphyryandmonzonitic plutons mapped within the placer resourcearea.TheseextensionsincludelowerportionsofHome-stake,Bismarck,andMuntherCreeksandtheuppermostreachesofGraniteCreekandLittleMooseCreek.

Fromtheregionaldrainagebasinstudy,twostream-sediment sampleswithinor downstream from resourcearea7fyieldedAuvalues.Asamplefromthelowerpartof Bismarck Creek contains 0.046 ppmAu.A samplefromwest-flowingLittleMooseCreek,whichdrainsthesameheadwatersareaastheeast-flowingMuntherCreek,contains0.018ppmAu.

Resourcearea7g,JulianCreekarea

Resourcearea7g(mapB)hasahighpotential(cer-tainty level D) for containing unexploited placer goldresources.Theareaoverlapspartofagold-bearingloderesourcearea(1c,mapB)andcontainsthreepreviouslymined placer deposits.A granite porphyry dike swarmthatunderliestheareaprovidedgoldandassociatedheavymineralstotheplacerdepositsofJulian,Michigan,andSpruceCreeks(nos.160,161,172,respectively,mapA).As previously discussed in the description of resourcearea1c, thegranite-porphyrydike swarmof the JulianCreekareais inferredtoberight-laterallyoffset12kmfromthegranite-porphyrydikeswarmthatprovidedgoldtoplacersintheGraniteCreekdrainage(placerresourcearea7f,above).

Pay streaks in resource area 7g occur in short,second-order streams as both ancestral Late Tertiary?benchlevels,2–6mabovethemodernfloodplain,andin late? Quaternary alluvium. Incomplete productionrecordsshowabout361kg(11,600oz)ofgoldand51kg (1,650oz)of silverhasbeen recovered fromplacerdeposits on Julian Creek intermittently from 1911 to1993. The remaining past production for the resourceareahasbeen fromSpruceCreek (8.5kgor274ozofgold)andMichiganCreek(3.9kgor125ozofgold).

Muchofthedevelopedplacerresourcesinresourcearea7gareminedout.ReservesremainonJulianCreek,but the Spruce Creek and Michigan Creek pay streakshave been substantially depleted. Prospecting poten-tial for gold placers exist in all drainages eroding themineralizedgraniteporphyrydike swarm,which is thesourceofthegoldplacersinformerlyproductivecreeks.These include unnamed streams in the eastern part oftheresourceareathatdrainwestandnorthwestintotheGeorge River. One stream-sediment sample collectedfromthisareaduring theregionaldrainagebasinstudycontains0.5ppmAgand5.0ppmHg.

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Resourcearea7h,AmericanCreek

Resourcearea7h(mapB)hasahighpotential(cer-taintylevelC)forcontainingplacergoldresources.TheareaincludesthelowerpartofAmericanCreekandsev-eralsmall,similarlyorientedcreeks,allofwhichdrainsoutheastintotheIditarodRiver.AmericanCreekdrainsthe southeast side of lode resource area 2e, which hasa high potential for containing plutonic-hosted copper-gold-polymetallicdeposits.Duringthisstudy,placergoldwas identified inaheavy-mineralconcentratecollectedfrom the lower part ofAmerican Creek (no. 204, mapA).Thissamplecontainsmorethan1,000ppmAu,200ppmAg,and70ppmBi.Thehornfelsandalteredintru-siverocksexposedonMosquitoMountainandinotherpartsofresourcearea2eareapossiblesourceofheavy-mineralplacersinAmericanCreek.Thisrawplacergoldprospectneedsadditionalwork.

Resourcearea7i,LittleCreek

Resourcearea7i(mapB)hasahighpotential(cer-tainty level C) for containing placer gold resources.Low-gradeplacergoldresourceshavebeenrecognizedinLittleCreek,asmall,north-flowingstreamthatoriginatesintheSleetmutequadrangleandflowsintotheIditarodRiver. Trenching and limited churn drilling programswere initiated there mainly prior to 1960. Placer goldwasfoundforadistanceof5kmofvalleylength,butinlowconcentrations.Thedepositappearstobehostedinmodernfloodplainalluvium;butitisburiedbyoverbur-denandvegetation.Thereisnorecordofproduction,butsomegoldmayhavebeenproducedduringtesting.

During the reconnaissance drainage-basin study,placergoldwasidentifiedinaheavy-mineralconcentratecollectedonLittleCreek(no.181,table1andmapA).Thisconcentratecontains150ppmAu,and15ppmAg;thestreamsedimentfromthesamesitecontains1.9ppmHg.

Resourcearea7j,ReturnCreek

Resourcearea7j(mapB)hasahighpotential(cer-taintylevelD)forcontainingplacercinnabarresources.ReturnCreekliesonthesouthsideofcinnabar-stibnitelodesources in theDeCourcyMountainarea (nos.183and184,mapA)andplacercinnabarresourceshavelongbeen recognized on this creek.A limited drill programindicatedthatsomegoldalsooccurswiththeplacercin-nabar(SpencerLyman,oralcommun.,1986).Aerialpho-tographic observations suggest that the placer depositsmayoccurinbothancestralchannelsandmodernstreamalluvium.

Resourcearea7k,DonlinCreekarea

Resourcearea7k(mapB)hasahighpotential(cer-tainty level D) for containing unexploited placer gold

resources.Theareaoverlapspartofagold-bearingloderesource area (1d, map B) and contains seven previ-ouslyminedplacerdeposits.PlacersintheDonlinCreekareahavebeenexploitedintermittentlysince1910;totalrecordedproduction is918kg (29,520oz)ofgoldandabout31kg(1,000oz)ofbyproductsilver.Goldproduc-tionbytributaryis188kg(6,039oz)inLewisGulch,4.5kg(145oz)inRubyGulch,256kg(8,238oz)inSnowGulch,61kg(1,968oz)inQuartzGulch,130kg(4,170oz)inDonlinCreek,and278kg(8,962oz)inCrookedCreekandOmegaGulch together(nos.188,191,193–195,and205,respectively,table1andmapA).Principleheavy minerals in the pay streak besides gold includegarnet,cassiterite,scheelite,monazite,andstibnite.

Mineralized granite porphyry dikes and sills aremostlikelythelodesourcesfortheplacerdeposits,butdepositionandemplacementoftheplacerresourcewasinfluencedbyacomplexQuaternaryhistory.Examinationof1:63,360-scalefalse-colorinfraredaerialphotographsand sedimentary paleocurrents in old terrace alluviumof the ancestralDonlinBench show thatDonlinCreekflowednorthintotheIditarodRiverprobablyinlateTer-tiary time.To date, all commercial quantities of placergoldarefoundwheremoreyouthfulstreamsandgulchesintersectanddowncut into theancestralDonlinBench.Mostofthesemoreyouthfulgulcheshavetheirheadwardsourceinthemineralizedgraniteporphyrydike/sillcom-plex.Theplacerpayisprobablycreatedmainlybyrecy-clingandreconcentrationofgoldinthebench,butmaybeaugmentedbyadditionsofheavymineralsfrommoreyouthfulerosionofthegraniteporphyryintrusions.

Inthereconnaissancedrainage-basinstudy,samplesfrom two creeks in resource area 7k contain Au. OnQueenGulch,astream-sedimentsamplecontains0.035ppmAu,0.5ppmAg,100ppmAs,and1.6ppmHg;theheavy-mineralconcentratecontainsmorethan2,000ppmSn. A stream-sediment sample from American Creek,whichliesbetweenLewisandOmegaGulches,contains0.011ppmAuand110ppmAs.

Discoveryofadditionalplacerresourcesinresourcearea 7k will probably involve systematic testing ofgulchesthatdrainuplandareasunderlainbygranitepor-phyryandthatintersecttheancestralDonlinBench.

Assessment oF non-metALLIC ResoURCes

HYDROCARBONRESOURCES

SedimentaryrocksoftheKuskokwimGroup,whichunderlie roughly65percentof theIditarodquadrangle,are considered apoor target forhydrocarbon resourcesduetoenvironmentofdeposition,induration,andstruc-tural complications (Mull and others, 1995; Stanley,1996). The Upper Cretaceous Kuskokwim Group in

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thestudyareaispartofaregionally-extensivebasin-fillsequencethatconsistsofmarineturbidites(unitKks)thatareflankedon thewestby subordinate shallow-marineand fluvial strata (unit Kkq) (Miller and Bundtzen,1994).Theturbiditesandstonesarematrix-richandgen-erallylackporosity;theshallow-tonon-marinequartz-osesandstonesareusuallycementedbysecondarysilicaandhematite.TheKuskokwimGroupwasdeformedinawrenchfaulttectonicenvironmentresultinginastruc-turally complex system of folds and faults (Miller andBundtzen,1994).

The flysch deposits consist of medium- to thick-beddedsandstone,siltstone,andshaleandexhibitBoumasequences(Bouma,1962).Sandstonesareusuallyfinetomedium grained, light to medium gray, poorly sorted,locallyarkosic,micaceousgraywackecontainingabun-dantclayinthematrix.Calcareoussandstoneisabundantlocally.Somecoarse-grained,pebble-bearingsandstonesare present as channel-fill deposits. Bouma sequencesinclude massive or gradedTa,Tab,Tae,Tabcde,Tcde,Tce,andTde.Duetopoorsorting,anabundanceofclaymatrix, and deep post-depositional burial (resulting insignificant compaction), these sandstones have lowporosityandpermeability.

StrataoftheKuskokwimGroupinterpretedasshal-lowmarineandfluvial,consistofmoderatelywell-sortedfeldspathictoquartzosesublithicsandstone,plant-debris-bearing siltstone, and minor coal-rich layers. Locallythickenedwedgesofquartz-pebbleconglomerateoccurthroughoutthesection,someofwhichmayrepresentflu-vialchannels.Althoughlackingtheabundantclaymatrixthatcharacterizesthesandstoneinthemarineturbidites,porosityintheshallowmarineandnonmarinesandstoneshasevidentlybeenreducedbythepresenceofahighper-centage of ductile grains, mechanical compaction, andpressuresolution.Lowporosityvalues(<3percent)havebeenrecordedfromsamplesofKuskokwimGroupsand-stoneoutsidethestudyarea(Mullandothers,1995).

PetrologicstudiesfromtheIditarodquadrangleindi-catethatmarineturbiditescontaincellulosickerogenintheformoflocallyabundantcarbonaceousplantdebris.If present in sufficient quantities, this type of kerogenisconsideredtobeasourceforgasonly;liquidhydro-carbonsarenotgenerated inanyquantitybycellulosickerogen (Tissot and Welte, 1984). Although no rocksin theIditarodquadranglehavebeenanalyzedfor totalorganiccarbon(TOC),Lyleandothers(1982)reportedTOCvaluesfor19shale-richsamplesfromtheKusko-kwimGroupthatrangefromlessthan0.5percentto1.05percentTOCandaverageabout0.70percentTOC.FourKuskokwim Group composite shale samples from thewesternMcGrathquadrangle(fig.1)rangefrom0.59to0.93percentTOCandaverage0.72percentTOC(T.K.Bundtzen, W.G. Gilbert, J.T. Kline, and E.E. Harris,unpub.data,1998).Shalescontaininglessthan1percent

TOCaregenerally consideredpoorpotential hydrocar-bonsourcerocks(TissotandWelte,1984).

Higherconcentrationsoforganicdebrisoccurinthenon-marinestrataoftheKuskokwimGroupwherecarbo-naceoussiltstonesandmudstonescontainabundantleafimpressions and occasional thin coal seams are found.Duringearlygoldrushyears,coalwasminedfor localuseintheIditarod-Flatarea(no.86,table1andmapA),but total production was very modest and used exclu-sively by local blacksmiths. The principle coal minenearFlatwasbriefly investigatedand found tocontainonlythin,friable,discontinuous10–50cmthick,“bone”coalseams.Mertie(1936)reportedthiscoaltobeclosetoanthracite incompositionhavingacalorificvalueofabout8,000andBtuvalueofabout14,000,butthatit’sphysicalfeatures(suchasfriablenature)weremorelikesubbituminouslignite.Duetothelocalnatureandsmallsizeofthesecoalseams,noresourceareaisdelineated.Ifothercoaldepositsarepresent,theywilllikelyoccurin association with the shallow-marine to non-marinestrataoftheKuskokwimGroup(mapunitKkq,whichisexposedonthewesternedgeofthepaleobasin).Thecoalwouldonlybeeconomicallyviablefornear-siteuse.

Thermalmaturity isauseful tool inevaluating theliquid and gaseous hydrocarbon potential of sourcerocks. Measures of thermal maturity, such as vitrinitereflectance (Ro), thermal alteration index (TAI), andcoloralterationindex(CAI),havebeencalibratedtothethermal windows for oil and gas generation and pres-ervation (for example Heroux and others, 1979;TissotandWelte,1984;Johnssonandothers,1993).Thether-malmaturitycategoriesdevisedbyJohnssonandothers(1993)foranAlaskastatewidestudyofthermalmaturitywere adopted for this study. In their scheme (table 8),which is based loosely on the hydrocarbon maturationregimes of Poole and Claypool (1984), the followingtermsareused:undermaturerocksareunderheatedwithrespecttotheoilwindow;mature-Irocksarewithintheoil-generationwindow;mature-II rocksarehigher thanthe oil-generation window, but within the oil and wetgaspreservationwindow;overmature and supermatureare successivelyhigher levelsoutside theoilwindows.Vitrinitereflectancedata(table9)wereobtainedfor26samplesofKuskokwimGroupmudstoneandshalefromthecentralpartoftheIditarodquadrangleaspartofthestatewidestudyofthermalmaturity(Johnssonandothers,1992).Thesamplesincludeexamplesfromboththeshal-lowmarineandbasinalrockunits.TheRovaluesrangefromalowof0.61toahighof5.13percentreflectance.SixoftheaverageRovaluesarewithinthemature-Icat-egory,thatistheyliewithintheoil-generationwindow,but the majority of the 26 samples are overmature tosupermature.TAIvaluesweredeterminedfor12samplesasabyproductofbiostratigraphicanalysis(table10).TheTAIvaluesrangefromabout2.5to3.5;thelowestvalue

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is questionably undermature and the remaining valuesareinthemature-IIcategory,thatisabovetheoilgenera-tionwindow,butwithintheoilandwetgaspreservationwindow.The locations and thermal maturity levels fortheRoandTAIdatasitesareplottedonfigure5.Extremevariationsevenonalocalscaleindicatethatthethermalmaturitypatternistoocomplextosimplybeareflectionofdepthofburial.Thermalpatternsassociatedwithcon-tactmetamorphicregimesshowabroadrangeofthermallevels locally(Rejebianandothers,1987).The thermalvariationinIditarodquadrangleislikelyrelatedtopluto-nism,andmayalsoreflectstructurallycontrolledhydro-thermalalteration.

The Kuskokwim Group has a low probability ofcontainingeconomicallyrecoverableoil.IntheIditarodquadrangle, it lacksanassociatedoil source,migrationpathways,andreservoirsandstoneofsufficientporosityandpermeability.Thecontainedorganicmaterialconsistsprimarilyofcellulosickerogen(terrigenousplantdebris),whichisapooroilsourcerock.Mostofthesandstoneshave a poor reservoir potential due to the compaction,burial,andthermalhistory,andfurther,theonlypotentialchannelsforhydrocarbonmigrationarefaults,whicharealso the conduits of hydrothermal fluids.Additionally,the irregular thermalpattern and the complex structurewouldmakeitextremelydifficulttotargetanyareasofhydrocarbon potential. Some vitrinite reflectance datado indicate that restricted areas of the basin are in thethermalwindowforhydrocarbongenerationandcoarsesandstone and conglomerate of the shallow-marine tonon-marine strata of the Kuskokwim Group (map unitKkq)couldexhibitsatisfactoryporosityandpermeabilitycharacteristics,butgoodsourcerockisstilllacking.

Thereexistsa lowprobability for thediscoveryofstructurally controlled gas deposits in the KuskokwimGroup of the Iditarod quadrangle. Hypothetically, thestrike-slip and concurrent wrench-fault tectonics thatcharacterize the structural deformation would permitaccumulationofgasinhigh-anglestructuralsettings.In1961,PanAmericanPetroleumCompany(nowAmocoPetroleum) drilled through approximately 4,500 m(15,000ft)ofrelativelyundeformedKuskokwimGroupin the Napatuk Creek No. 1 well near Bethel, about250km(150mi)southwestofthestudyarea(Mullandothers, 1995). A 12-m-long (40 ft) test interval from2,856mto2,868mindepth(9,368ftto9,406ft)builtupagood“gasblow”,butthenquicklydied(Mullandothers, 1995). This test interval also contained salinewaters,whichindicatethatsomeformationfluidsflowedintothewell.However,subsequenttestintervalsyieldedslight“gasblows”thatquicklyended,indicatingthepoorporosityandpermeabilitycharacteristicsof theKusko-kwimGroupin thatarea.Theselimiteddataandinfor-mationcollectedduringthisstudyseemtosuggest thattheonly reasonablemechanismto trapandconcentrate

gasintheKuskokwimGroupwouldbewithinpermeableshearzonesandfaultstructures.

PEAT

Just east of the Iditarod quadrangle, Bundtzenand Kline (1986) studied peat near the community ofMcGrath (fig.1) for it’s suitabilityaseitherahorticul-tural or energy resource.They concluded that dry peatbogsalongtheKuskokwimRivercouldyieldsignificantenergyaswellasbesuitableforhorticulturalpurposes.Forexample,somepeatsamplescollectedfromonebogyieldedBtuvaluesrangingfrom6,500to8,000.Exten-sivepeatresourcesexistintheIditarodquadrangle.Pro-spectivepeatsettingsincludeterracealluvium,oldglacialoutwash, and wetland in the Innoko and Dishna Riverdrainages.LargepeatdepositsalsooccuradjacenttotheYetna,Iditarod,andKuskokwimRivers.Noattemptwasmadetoquantifythepeatresourcesofthestudyarea.

SANDANDGRAVEL

Sandandgravelresourcesexistinmodernstreams,terrace alluvium, glacial outwash deposits, and placermine tailings throughout the study area. Due to theextreme remoteness of the region, sand and gravelresources were not systematically assessed because ofthelackofanyapparentlocalneedfortheseresources.PlacerminetailingshavelocallybeenusedtoconstructroadsandairstripsnearFlat,nearMooreCreek,andintheGanesandYankeeCreekareas.Becauseplacerminetailings are water-washed, sorted gravels and sands,theydoconstitutehighqualitymaterialsuitableforroadand airstrip construction and even rip rap uses. In theIditarod district, 12,745,900 m3 of material were pro-cessedbythreebucketlinestackerdredgesfrom1912to1966(Bundtzenandothers,1992).Bundtzenandothers(1988) estimated at least 1,070,400 m3 of washed andstacked aggregate exist at Moore Creek.An additional6,750,000m3ofprocessedsandandgravelexistatGanesandYankeeCreeksintheInnokodistrict.Extensivebutunmeasured resources of sand and gravel exist on theDonlinCreekbench,ontheeastlimitofBonanzaCreek,nearJulianCreek, in theupperGeorgeRiverdrainage,in theupperTakotnaRiverdrainage,downstreamfromMooreCreek,alongnearlytheentirelengthofFourthofJulyCreek,andinscattereddrainagebasinsontheNun-satukRiver.

Glacial outwash deposits form a large, significant,but unmeasured resource in tributaries draining theBeaverMountains.ThemostsignificantdepositsincludethoseonTolstoi,Windy,andBeaverCreeks,whichhavebeen depicted at 1:63,360-scale geologic mapping byBundtzen(1980)andBundtzenandLaird(1982).

Thesematerialscouldbeutilizedforconstructionofroads,bridges,andairstripsifwarranted.

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RIPRAPANDBEDROCKAGGREGATE

Kuskokwim Group shale and sandstone has beenused as an effective surfacing agent for road construc-tionintheFlat,Takotna,andYankeeCreekareas.Froman engineering standpoint, the material compacts wellandhas favorableAlaskaT-13degradation testcharac-teristics,asdefinedbystandardsadoptedbytheAlaskaDepartmentofTransportationandPublicFacilities.TheKuskokwimGroupisawidespreadunitinthestudyareaandtheserockscouldbeusedasroadmetalandaggre-gatelocallythroughoutthestudyarea.

Rip rap potential is largely confined to igneouslithologies in the area. Bundtzen and others (1989)reportedthat therocksof theKuskokwimGroupalongtheKuskokwimRiverconsistentlyfailedtestsforriprapsuitability, however, unaltered and unweathered basalt,andesite,andmonzoniticplutonicrockstestedfavorably.Monzonitethatwasgrussifiedbyweatheringconsistentlyfailedripraptests.Thesetestswererunonrocklitholo-gies in theMcGrathD–6quadrangle, immediatelyeastofthestudyarea;materialphysicalcharacteristicswouldprobablyapplyequallytotherocksoftheIditarodquad-rangle.Inconclusion,localmaterialuseofthesewouldrequirelabtestingpriortofieldapplication.ThequartziteandgranitegneissfromtheProterozoicIdonoComplex(Millerandothers,1991)mightprovesatisfactoryforuseasriprap.

OTHERINDUSTRIALMINERALS

The Iditarod quadrangle is underlain by exten-sivefields of volcanic rocks thatmight contain zeoliteresources. However, reconnaissance petrographic studyfailedtorevealpresenceofzeolites.

sUmmARy And ConCLUsIons

The Iditarod quadrangle lies in a remote part ofAlaskaandiscurrentlyaccessibleonlybyair,river,andalimitedroadsystem.Despitetheremotelocation,min-eraldepositsfromtheIditarodquadranglehaveyieldedat least65,000kg (2.1millionoz)ofgold fromplacerdepositsandanother100kg(3,200oz)oflodegoldsince1906. These same mineral deposits have also yieldedbyproducts of silver, antimony, tungsten, and zinc. Inaddition,approximately51,700kg(1,500flasks)ofmer-curywereproducedfromcinnabar-stibnitelodes.Inthisreport, thepotential foradditionalmetallic resources isassessedbasedonnewgeologicmappingandsampling,a regional drainagebasin survey, and a comprehensivesummaryofthemines,prospects,occurrences,androckgeochemicalanomalies.

Despite the fact that the region has been heavilyprospectedforplacergold, loderesourcesremaintobediscovered. Several significant newmineral findswere

located during the course of this study: (1) a hornfels-hostedsilver-tin-polymetallicdepositattheheadofBis-marckCreek(no.118,table1andmapA),whichhasaninferredreserveof498,000tonnesgrading0.137percenttin,7.8g/tsilver,0.16percentcopper,and0.22percentzinc;(2)acinnabar-stibnite-goldmineralizedareaattheheadof theDishnaRiver (no.74, table1andmapA),whichhasaninferredreserveof37,600tonnesgrading2.46g/tgold(aprospectnotpreviouslyrecordedintheliterature);and(3)acinnabar-stibniteoccurrence,dubbedthe“Yousure”(no.156,table1andmapA)neartheIdi-tarodRiver.

Deposit models form the basis for this resourceassessment, but the results are presented with multipleusesinmind—formineralassessment,mineralexplora-tion,andland-useplanning.Resourceareas,definedbythesevenmetallicdeposittypes,areoutlinedonmapBand ranked as having high, moderate, or low potentialforcontainingadditionaldepositsofthattype.Areasofhigh potential indicate where undiscovered resourceslikely lie and could serve as targets for an explorationprogram. Also useful for exploration purposes are thedetailed descriptions provided for each resource areathatclearlyoutlinewhatisknownandwhatisspeculatedindesignating theseareas.For land-planningpurposes,resource tracts (fig. 3) are derivedbygroupingmodel-drivenresourceareasandareintendedtoemphasizetotalmineralendowment.Onthisfigurelandtractsarerankedashavingveryhigh,high,moderate,orlowtotalmetallicresourcepotential.Thisdiagramis intended tobeusedqualitatively; it shows where mining activity is mostlikelytotakeplaceinthefuture.

TheIditarodquadrangle,afrontierareaintermsofmineral exploration, contains additional undiscoveredorunexploitedmetallicmineral resources.Usingsevenmineral-depositmodels(table2)toguidetheassessment,weoutlined35resourceareasthathaveahigh,moderate,orlowpotentialfordiscoveryofmetalliclodedeposits.Weoutlined11resourceareasthathaveahighpotentialfordiscoveryofplacerdeposits.Thesearedescribedinthefollowingparagraphs.

Four resource areas (1a–1d, map B) have a highpotential for containing additional gold-bearing lodedeposits that fit the peraluminous granite-porphyry-hostedgold-polymetallicmodeltype.Itisinferredthatatthe90thpercentconfidencelevel,threeadditionaldepos-itscontainingatleast155kg(5,000oz)ofgoldliewithinthesefourresourceareas(table4).

Fiveresourceareas(2a–2e,mapB)haveahighormoderate potential for containing additional gold-bear-ing lode deposits that fit the plutonic-hosted copper-gold-polymetallicstockworkandveinsmodeltype.Itisinferred thatat the90thpercentconfidence level, threeadditionaldepositscontainingatleast155kg(5,000oz)ofgoldliewithinthesefiveresourceareas(table4).

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Nineresourceareas(3a–3i,mapB)haveahighormoderate potential for containing additional silver-tin-bearinglodedepositsthatfittheplutonic-related,boron-enriched,silver-tin-polymetallicmodeltype.Itisinferredthatat the90thpercentconfidencelevel,eightdepositscontainingatleast175,000tonnes(193,000tons)oforeliewithinthesenineresourceareas(table4).

Fourteenresourceareas(4a–4n,mapB)haveahighormoderatepotentialforcontainingadditionalmercury±goldlodedepositsthatfittheepithermalmercury-anti-mony(gold)modeltype.Itisinferredthatatthe90thper-centconfidencelevel,sixadditionaldepositscontainingatleast3,450kg(100flasks)ofmercuryliewithinthesefourteenresourceareas(table4).

Tworesourceareas(5aand5b,mapB)haveahighormoderatepotentialforcontainingvolcanic-hostedpre-ciousandothermetalslodedeposits,butnoestimateofthe number of undiscovered deposits of this type wasmade,becausethemodelisnotwelldefinedfortheareaandthedataaresparse.

One resource area (6, map B) has a low potentialfor containing mafic-ultramafic related Cr-Ni-Co lodedeposits. No estimate of undiscovered resources wasmadesincethepresenceofeconomicconcentrationsofthesecommoditiesisunlikely.

Definedbytheheavy-mineralplacermodeltype,tenresourceareas(7a–7i,and7k,mapB)haveahighpoten-tialforcontainingadditionalgoldplacerdepositsandoneresourcearea(7j,mapB)hasahighpotentialforcontain-ingcinnabarplacerdeposits.Noestimateofthenumberofundiscoveredplacerdepositswasmadeeventhoughthemodeltypeiswelldefinedforthestudyareaandthelikelihoodforadditionaldepositsishigh.Giventhehigh-levelofplacerexplorationinthequadrangle,itisunlikelythatanynewdepositequaltoorgreaterthanthemedianfortheregionhasnotalreadybeendiscovered.Instead,it ismore likely thatundiscoveredresourceswillcomein the formofextensionsofknowndepositsorminingofverysmalldeposits.Inalimitednumberofresourceareas(7c,7d,and7g)thereissufficientdatatoconserva-tivelyestimatethat4,667kg(150,070oz)moregoldiscontainedinextensionsofknowndepositstherein.

ACknowLedGments

WewishtothankalloftheminersinIditarodquad-rangle who provided detailed information about theirproperties—AlvinAgoff,JohnandthelateRichardFul-lerton, Don Harris, Spencer and Carolyn Lyman, JohnMiscovich,andL.E.Wyrick.DonaldGrybeckoftheU.S.Geological Survey provided expertise on the mercurydepositsofsouthwesternAlaskaandhelpedwithestimat-ingthenumberofundiscoveredmineraldeposits.Wil-liamJ.Keithproducedthefiguresandmapsheetsforthis

report.ThethoughtfulandconstructivecommentsofthetwoUSGSreviewers,W.J.NoklebergandD.H.Richter,improvedthemanuscript.Wealsoappreciatedthehelp-fulcommentsfromMarkMeyersoftheAlaskaDivisionofOilandGasontheoilandgassectionofthisreport.

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51

Kus kokw

imRiver

Figure 1.LocationoftheIditarodquadranglestudyarea(shaded)inwest-centralAlaska.Alsoshownareneighboringquadranglesandotherplacesreferredtointext.

Yuko

n Rive

r

Anchorage

Dillingham

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Innoko National WildlifeRefuge

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InnokoNationalWildlifeRefuge

Bureau of Land Management-administeredFederal land (may locally include topfiledlandclaims)

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Flat

EXPLANATION

63°00'

Kusk

okw

imR

iver

Iditarod

River

Riv

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Idita

rod

Figure 2.MapshowinglandjurisdictionfortheIditarodquadrangle(fromAlaskaDepartmentofNaturalResources,1995).

53

H-8

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M-2

VH-2VH-1

VH-4

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EXPLANATION

VH- L-

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Figure 3.Mapshowingsummarylandtractsderivedfrommodel-drivenresourceareasofmapB.Land-tractnumberscorrespondtotable7.

VeryHigh

High

Moderate

Low

Unknown

EXPLANATIONResourcePotentialofLandTracts

63°00'159°00' 156°00'

62°00

Flat

Kusk

okw

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iver

Iditarod

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Idita

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0 20 40 MILES


Kusk

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Iditarod

Flat

Figure 4.MapshowingtheplacerminingdistrictsoftheIditarodquadrangle(afterRansomeandKerns,1954).

63°00'159°00' 156°00'

62°00

River

Riv

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Idita

rod

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tna

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hna

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rge

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XX

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EXPLANATION

Figure 5. Map showing location of vitrinite and thermal alteration index (TAI) samplescollectedintheIditarodquadrangleandlevelsofthermalmaturityafterJohnssonandothers(1993).Numberscorrespondtotable9;letterscorrespondtotable10.

Undermature

MatureI

MatureII

Overmature

Supermature

EXPLANATION

63°00'159°00' 156°00'

62°00

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Iditarod

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56

table 1. Mines, prospects, occurrences, and rock geochemical anomalies, Iditarod quadrangle

Siteno. Township Name Significantmetals Production,grade/ Range (Category1) (minormetals) tonnage,and/or(SewardMeridian) [Stationno(s).] Deposittype2 analyticaldata3

1 MackieCreek(M) Au(Ag) Productionof29.2kg(940oz)Auand1.7kg T.34N. Placer (54oz)Agbetween1915and1938 R.38W.

sUmmARy:Placerconcentrationsofgoldonbedrocksurfacebeneath16mofgravelandoverburden.Gulchisapproximately2kmlongand200mwide;drainsareaofmineralizedgraniteporphyrydikeswarm.MostplacergoldrecoverednearintersectionofMackieCreekandGanesCreek.ReFeRenCes:Mertie,1936;Smith,1941;BundtzenandLaird,1980,1983.

2 Unnamed(A) Ag,Cr,V Valuesofupto2,000ppmCr,1,000ppmV,and T.34N. [79BT16and79BT20] Deposittypeuncertain 7ppmAg R.38W.

sUmmARy:Grabsamplesofiron-stained,carbonate-altered,maficdikesthatcropoutdiscontinuouslyoverabout1km.HighCr,V,andAgvaluescharacterizethenumerousmaficdikesthatcropoutinthe40-km-long,GanesCreek-YankeeCreekdikeswarm.ReFeRenCes:BundtzenandLaird,1980,1983.

3 Unnamed(A) Ag(Sb) Samplecontainsupto2ppmAg,30ppmAs,8 T.33N. [84AAi602] Deposittypeuncertain ppmSb,and>5,000ppmBa R.44W.

sUmmARy:Grab sampleof iron-oxide stained,brecciatedchertofMississippian toTriassic Innoko terranecontainsnumerousquartzveinlets.Hgwasnotanalyzed.ReFeRenCes:McGimseyandothers,1988;MillerandBundtzen,1994;thisreport.

4 Unnamed(A) Hg Grabsamplescontainupto200ppmZnand T.33N. [I0911andI0912] EpithermalHg-Sb(?) >10ppmHg R.41W.

sUmmARy: Iron-oxide stainedzone involcanic roofpendantoverlyingwesternportionofBeaverMountainspluton.ReFeR-enCes:McGimseyandothers,1988;MillerandBundtzen,1994;thisreport.

5 Unnamed(O) Au(Hg,Nb) Heavy-mineral-concentratesamplescontain8 T.33N. [TolstoiGT2] Placer ppmAuand218ppmNb R.41W.

sUmmARy:PlacergoldandassociatedheavymineralsconcentratedonKuskokwimGroupbedrockandinboulderinterstices.GoldmaybefromreworkedauriferousterminalmorainethoughttobeofearlyWisconsinage.GoldandassociatedmineralswereprobablyderivedfromBeaverMountains.Principalheavymineralsincludefine-grainedgold(891fine),magnetite,ilmenite,cinnabar,dravite,chalcopyrite,andilmenorutile.ReFeRenCes:Bundtzen,1980;BundtzenandLaird,1982;Bundtzenandothers,1987.

1Categoriesandabbreviations:M,mine(hasrecordedproduction,nomatterhowsmall);P,prospect(evidenceofexploratoryworksuchasaprospectpitpresent);O,occurrence(mineralizedrockpresent);A,anomaly(rockgeochemicalanomaly—minormineralizedrockmaybepresent).

2Referstothedepositmodelthatbestfitsavailabledata;deposittypeislistedasuncertainwheredataareinsufficientforclassificationintoamodel.Modelnamesusedinthetableareabbreviatedfromnamesusedinthetextasfollows:

1.Granite-porphyry-hostedAu-polymetallic=Peraluminousgranite-porphyry-hostedgold-polymetallic 2.Plutonic-hostedCu-Aupolymetallic=Plutonic-hostedcopper-gold-polymetallicstockworkandvein 3.Plutonic-relatedAg-Snpolymetallic=Plutonic-related,boron-enriched,silver-tin-polymetallic 4.EpithermalHg-Sb=Epithermalmercury-antimony(gold) 5.Volcanichosted=Volcanic-hostedpreciousandothermetals 6.Mafic-ultramafic=Mafic-ultramaficrelatedCr-Ni-Co 7.Placer=Heavy-mineralplacer3ProductionwasobtainedfromU.S.GeologicalSurveyreports(for1900–1930),unpublishedU.S.mintrecords(1912–1969),StateofAlaskarecords,andBundtzenandothers(1987).

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table 1. Mines, prospects, occurrences, and rock geochemical anomalies, Iditarod quadrangle—continued


6 Tolstoilode(P) Cu,Ag,As,Au,Sn,Pb, Grabsamplescontainupto10%Cu,500ppm T.33N. [86BT350] Sb,Zn(Cd,Bi) Ag,20,000ppmAs,1.4ppmAu,>2,000ppm R.41W. Plutonic-relatedAg-Sn 10ppmBe,39ppmBi,200ppmCd,>20,000 polymetallic ppmPb,10,000ppmSb,200ppmSn,and 2%Zn

sUmmARy:Extensive,structurallycontrolled,alteredzoneincupolaofBeaverMountainsstockshowstourmaline-axinitemetaso-matismandquartz-sulfidereplacementdeposits.Threemajorpipe-liketourmaline-axinite-sulfidebrecciazones,300mlongandupto150mwide,contain1.50milliontonnesofmineralizedrock(averagegradeswerenotdetermined).Tourmaline-rich“pipes”arecappedbyalteredandesiteroofpendants.Anextensiveareaoftourmalinerosettesenclosedinpotassicallyalteredrocksliesinthewesternmostpartofthealteredzone.Principalsulfidesarechalcopyrite,pyrite,arsenopyrite,andminorgalena.Silver-richsulfosaltsincluding stromeyerite andboulangerite are found in associationwith chalcopyrite.ReFeRenCes:Bundtzen andLaird, 1982;McGimseyandothers,1988;Szumigala,1993;BundtzenandMiller,1997.

7 TolstoiLake(O) Ag,As,Cu,Sb,Zn,Pb, Grabsamplesofdisseminatedsulfidescontain T.33N. [79BT403andI0902] Hg(Au) upto33g/t(0.96oz/ton)Ag,400ppmAs, R.40W. Plutonic-relatedAg-Sn >2,000ppmB,1,000ppmCu,400ppmSb, polymetallic 400ppmZn,200ppmPb,>10ppmHg,trace Au

sUmmARy:Tourmaline rosettes, disseminated sulfides, and veins in alteredmonzonite ofBeaverMountains pluton.ReFeR-enCes:BundtzenandLaird,1982;McGimseyandothers,1988.

8 Unnamed(O) Ag,Zn,Cu,Sb(Au) Valuesof55g/t(1.61oz/ton)Ag,1,000ppm T.33N. [79BT451] Plutonic-relatedAg-Sn Zn,500ppmCu,100ppmSb,andtraceAu R.40W. polymetallic

sUmmARy:Rubbleofiron-oxidestainedhornfelsbreccia,whichcontainsfineveinsofmanganeseoxideandtourmaline,isspatiallyassociatedwithmetasomatizedandesiteporphyrydikesthatintrudeKuskokwimGrouprocksabout2kmnorthofBeaverMountainsstock.ReFeRenCe:BundtzenandLaird,1982.

9 Unnamed(O) Fe,Cu,Sb(Ag) Valuesof20.0%Fe,200ppmCu,40ppmSb, T.33N. [85BT99] Deposittypeuncertain and1ppmAg R.40W.

sUmmARy:PodofmassivetodisseminatedpyrrhotitefoundinhornfelsofKuskokwimGroupsedimentaryrock,adjacenttoeasternlimitofBeaverMountainsstock..Largestpodofpyrrhotiteestimatedtobe2mthickand30mlong.ReFeRenCes:McGimseyandothers,1988;thisreport.

10 GanesCreekand Au(Ag) From1906to2001producedatleast3,238kg T.33–34N. tributaries(M) Placer (104,129oz)Auand456kg(14,668oz)Ag R.38–39W. fromIditarodquadrangle(additional productionfromOphirquadrangle)

sUmmARy:LargestproducerofgoldinOphir-Innokominingdistrict.GanesCreekisthemajorstreamflowingnorthfromBeaverMountainsintoInnokoRiver(Ophirquadrangle).Auriferouspaystreakstracedfor11kminIditarodquadrangle—allplacersconfinedtocanyonincisedintoKuskokwimGroupsandstoneandalteredCretaceousdikeswarm.AtleasttwoancestralpaleochannelsoccuronboththeleftandrightlimitsofGanesCreekcanyon.ThesebenchplacersarebestpreservedbelowmouthofSpauldingCreek.Paleochannelsare15–20mabovemodernfloodplainalluvium.TheymaybeasoldasPlioceneandpredatethebeheadingofupperGanesCreekbyBeaverCreek,whichtookplaceinmid-Quaternarytime.GanesCreekpaleochannelplacersarewellknownforverycoarsegold;twonuggetsrecoveredin1984and1963weigh4,012g(129oz)and1,991g(64oz),respectively.Totalgoldproduction(seeabove)includesthatfromnumeroussmallfeedergulches.PodisieCreek,aright-limittributaryofGanesCreek,hasyieldedsig-nificantquantitiesofplacergoldsince1994(DougClark,writtencommun.,1997).FinenessofGanesCreekgoldranges817to874,averaging846.Heavymineralsincludemagnesiochromite,scheelite,stibnite,andarsenopyrite.MostplacergoldinGanesCreekisthoughttobederivedorrecycledfromGanesCreek-YankeeCreekgraniteporphyrydikeswarm.Significantgoldresourcesremain.ReFeRenCes:Smith,1941;Cobb,1973;BundtzenandLaird,1980,1982;Bundtzenandothers,1987;thisreport.

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11 SpauldingCreek(M) Au(Ag) Produced246.5kg(7,925oz)Au;49.5kg T.33N. Placer (1,591oz)Agfrom1909to1941 R.38W.

sUmmARy:LargestauriferoustributaryofGanesCreek(no.10,above).Economicplacerdepositsfoundalong3kmof9-km-longvalley,mainlydownstreamfromincisionofGanesCreek-YankeeCreekdikeswarm,theprobablesourceofgoldinSpauldingCreek.Placerdepositsoriginallycoveredby1–3mofoverburden.Goldfinenessrangesfrom835to865.Heavymineralsincludemagnetite,ilmenite,andscheelite.ReFeRenCes:MertieandHarrington,1924;Smith,1941;BundtzenandLaird,1980,1982;Bundtzenandothers,1987;thisstudy.

12 GlacierGulch(M) Au(Ag) Minorproduction;includedwithGanesCreek T.33N. Placer (no.10) R.38W.

sUmmARy:Small,2-km-long,first-ordertributarystreamentersGanesCreekabout0.5kmdownstreamfrommouthofSpauldingCreek.Smallamountsofgoldrecoverednearmouthofgulchin1908.DrainageunderlainbyGanesCreek-YankeeCreekgranitepor-phyrydikeswarm.ProductionincludedwithGanesCreek(no.10,above).ReFeRenCe:Maddren,1910.

13 LastChanceGulch Au(Ag) Minorproduction;includedwithGanesCreek T.33N. (M) Placer (no.10) R.38W.

sUmmARy:Small,1-km-long,first-ordertributarystreamentersGanesCreekdownstreamfromGlacierGulch.Goldprospectinganddevelopmenttookplaceatstreammouthpriorto1910.ProductionincludedwithGanesCreek(no.10).ReFeRenCes:Mad-dren,1910;Eakin,1914;BundtzenandLaird,1980,1983.

14 Unnamed(P) Ag,Cr(Au) Valuesof5ppmAg,1,000ppmCr,andtrace T.33N. [79BT126] Deposittypeuncertain Au R.38W.

sUmmARy:N.60°E.-trending,alteredmaficdikecontainsextensivegossanfor100masexposedinthreeprospectpits.Brecciatedfelsicdikesareexposedwestofmainmaficdike.ReFeRenCes:BundtzenandLaird,1983;thisreport.

15 Katzlode(P) Sb,Au,Ag,As Grabsamplescontainupto1.1ppmAu,2ppm T.33N. [79BT122and Granite-porphyry-hosted Ag,and35%Sb R.38W. 79BT127] Au-polymetallic

sUmmARy:Stibnite-quartz-goldveins0.1to0.3mthickoccuralongfootwallofgraniteporphyrydikeabout2kmsouthwestofIndependencemine(no.16).VeinsstrikeN.30°E.anddip75°SE.andrunsubparalleltothedikeforabout300mindirectionofIndependencemine.Stibniteandminordisseminatedarsenopyritealso found inpartsofgraniteporphyrydike.ReFeRenCes:MertieandHarrington,1924;BundtzenandLaird,1983;thisreport.

16 Independencemine Au,As,Sb,Ag,Pb,Sn, Produced14.9kg(478oz)Aufrom499t(550 T.33N. (M) Hg tons)orearound1912.Grabsamplescontain R.38W. [I1424,83AM122, Granite-porphyry-hosted upto13ppmAg,2.1%As,180ppmAu,20 and86AM419] Au-polymetallic ppmBi,>10ppmHg,1,500ppmPb,0.56% Sb,and300ppmSn

sUmmARy:Auriferousquartz-carbonate-sulfideveindeposit0.5mthickonhangingwallofgraniteporphyrydike,whichstrikesN.55–70°E.anddipssteeplysoutheast.Graniteporphyrydikeaverages10mwideandcanbetracedalongstrikeforatleast300m.Disseminatedsulfidesfoundinbothquartzcarbonateveinsanddikerockandconsistof1–5%(byvolume)arsenopyrite,pyrite,stibnite,stephanite,andtracecinnabar.VeinletsofsideriteandcalcitecutbothdikeandhostsedimentaryrocksofKuskokwimGroup.Minedevelopedundergroundbytwotunnelstotalingabout150minlength(cavedin1979).PlacerDomeExplorationcompletedadiamond-drillingprogramin1997.Grabsamplescollectedduringthisstudycontainupto13g/tAgand180g/tAu.ReFeRenCes:Eakin,1914;MertieandHarrington,1924;Mertie,1936;BundtzenandLaird,1980,1983;McGimseyandothers,1988;Noklebergandothers,1993;BundtzenandMiller,1997;thisstudy.

59



17 Unnamed(P) Au,Ag Valuesupto0.73ppmAuand5ppmAg;free T.33N. [79BT102–104] Granite-porphyry-hosted goldobservedinhandsamples R.38W. Au-polymetallic

sUmmARy: Quartz breccia and vug-veins developed along both hanging wall and footwall of several granite porphyry dikes.SheetedveinsalsooccurinKuskokwimGroupsandstonenearthedikes.Freegold,jamesonite,andarsenopyritefoundintwoprospectpits.Gossanwelldeveloped;canbetracedinN.30°E.directionforatleast200m.GeologicallysimilartoIndependencemine(no.16).ReFeRenCes:BundtzenandLaird,1983;thisreport.

18 Unnamed(O) Au Assayof7g/t(0.21oz/ton)Au;freegoldin T.33N. [79GL61and Granite-porphyry-hosted handsamples R.38W. 79GL62] Au-polymetallic

sUmmARy:Iron-oxidestainedquartzveinneardikesinGanesCreek-YankeeCreekdikeswarm;freegoldfoundinquartz.ReFeR-enCe:BundtzenandLaird,1983.

19 YankeeCreek(M) Au,Ag(W,Cr) From1909to1992producedatleast1,808kg T.33–34N. Placer (58,120oz)Auand233.4kg(7,505oz)Ag R.38W. fromIditarodquadrangle(additional productionfromOphirquadrangle)

sUmmARy:MajorplacerdepositinInnoko-Ophirdistrict.Productionderivedfromthird-orderstreamsystemthatflowsnorthfromtheIditarodquadrangleintotheOphirquadrangle.About75%oftotalgoldproductionisfromIditarodquadrangle.Small,first-andsecond-orderstreamssuchasMartenandSkookumGulchesalsocontributedtoproduction,mainlyattheirintersectionswithYankeeCreek.Auriferouspaystreakoccursinsingledistributarychannel11kmlongand15–80mwide.AncestralpaleochannelbenchesonlydevelopedonleftlimitnearIditarod-Ophirquadrangleboundary.Mostgoldoccursattheintersectionofthesmallgulchesandsecond-orderstreamswithYankeeCreek.GulchesarederivinggoldfromGanesCreek-YankeeCreekdikeswarm.Goldfinenessrangesfrom850to886.Considerableplacerscheelitefoundnearheadofdrainage.Additionalheavymineralsincludeabundantmagnesiochro-mite,ilmenite,anatase,eckermannite,andtracehidalgoite(leadarsenicsulfate).Minedcontinuouslyfrom1909–1968andagainfrom1981–1992.ReFeRenCes:Eakin,1914;MertieandHarrington,1924;Mertie,1936;Smith,1941;ToivoRosander,oralcommun.,1978;BundtzenandLaird,1980,1983;Bundtzenandothers,1987;thisreport.

20 Unnamed(A) Ag(Cr,Au) Grabsamplecontains5ppmAg,1,000ppmCr, T.33N. [79BT194] Deposittypeuncertain andtraceAu R.36W.

sUmmARy:Smallnortheast-trendingalteredporphyriticmaficdikelocallycontainsquartzvugsingossan.Minorpyrrhotiterecog-nized.ReFeRenCes:BundtzenandLaird,1983;thisreport.

21 Unnamed(A) Cu,Ag(Au) Samplecontains700ppmCu,5ppmAg,and T.33N. [79BT205] Deposittypeuncertain traceAu R.36W.

sUmmARy:Grabsampleofsmall,northeast-trending,siderite-quartzveinintrudesKuskokwimGroupsandstone.ReFeRenCe:BundtzenandLaird,1983.

22 Unnamed(A) (Ag,Au,Hg) Samplescontainupto0.3ppmAg,30ppmAs, T.33N. [I1019] Deposittypeuncertain 1.1ppmHg,andtraceAu R.36W.

sUmmARy:Grabsamplesof(1)iron-oxidestainedhornfelssandstonecontainingminordisseminatedpyriteandsmallquartzveins,and(2)monzonitecutbysulfide-bearingveins.ReFeRenCes:McGimseyandothers,1988;thisreport.

23 Unnamed(O) Ag,Zn,Cu(Au) Valuesupto13.7ppmAg,280ppmCu,1,050 T.33N. [77BT249] Plutonic-relatedAg-Sn ppmZn,and0.03ppmAu R.36W. polymetallic

sUmmARy:Iron-stainedhornfelsandbrecciazonenearborderphaseofTakotnaMountainmonzonitepluton.Brecciazoneisatleast2mthickinrubble.ReFeRenCe:BundtzenandLaird,1983.

60



24 Unnamed(A) Sb(Ag) Samplecontains1.5ppmAg,100ppmSb,and T.32N. [85AM137] Deposittypeuncertain 40ppmAs R.45W.

sUmmARy:Grabsampleofsiltstone(partoftheMississippiantoTriassicInnokoterrane)inashearzone.SamplecontainselevatedvaluesofAg,Sb,andAs;Hgwasnotanalyzed.ReFeRenCes:McGimseyandothers,1988;MillerandBundtzen,1994; thisreport.

25 Unnamed(O) Au Visiblegoldinheavy-mineral-concentrate T.32N. [I0241] Placer sample R.44W.

sUmmARy:Visiblegold(<1%)andcinnabar(20–50%)foundinnonmagnetic,heavy-mineral-concentratesample.StreamdrainsshorelinefaciesofKuskokwimGroup;sectionisintrudedbymaficdikeswarmtotheeast.OccurrencealsoliesnearfaultedcontactwithDishnaRiverophioliticrocks.ReFeRenCes:Bennettandothers,1988;MillerandBundtzen,1994;thisreport.

26 Unnamed(A) Hg(Sb) Samplecontains2.4ppmHg,14ppmSb,and T.32N. [86AM128] EpithermalHg-Sb 10ppmAs R.44W.

sUmmARy:Grabsampleofiron-oxidestained,altereddike;partoflargenortheast-trendingdikeswarmthatlieseastofVABMCabin.ReFeRenCes:McGimseyandothers,1988;MillerandBundtzen,1994;thisreport.

27 Unnamed(O) Ag,Cu,Pb,Zn(Bi,Sb, Valuesupto55.3g/t(16.1oz/ton)Ag,0.5%Cu, T.32N. [79GL321] Sn) 0.7%Pb,900ppmZn,100ppmBi,200ppm R.41W. Plutonic-relatedAg-Sn Sb,and60ppmSn polymetallic

sUmmARy:DisseminatedchalcopyriteinaN.70°E.-trendingfractureinandesiticvolcanicroofpendantoverlyingcupolalevelofBeaverMountainspluton.ReFeRenCe:BundtzenandLaird,1982.

28 Cirque(P) Ag,Cu,Pb,Zn,Sb,Sn Inferredresourceof175,000t(193,000tons) T.32N. [81BT501,83AM103, (Bi,Au) grading3.5%Cuand445g/t(13.0oz/ton)Ag R.41W. and86AGe43] Plutonic-relatedAg-Sn polymetallic

sUmmARy:Paralleltourmaline-axinite-sulfidefracturefillingsinmonzoniteandquartzsyeniteofBeaverMountainsstock.MainfracturestrikesN.75°E.,dipsalmostvertically,andisdiscontinuouslyexposedfornearly3km;maytrendintooccurrenceno.30.Cirquedepositmaybepartofalarger,laterallyzoned,sulfidesystem.Mostintenselymineralizedrockliesincirqueheadwallwherenearlymassivechalcopyrite,1mwide,isexposedforalmost20m.Lesseramountsofgalena,pyrite,sphalerite,andPb-Sb-Bi(?)sul-fosaltarefoundingangueoftourmaline,fluorite,andquartz.Axiniteisalsoabundantinhangingwall.Mainzoneisabout150mlong,3mwide,andupto250mdeep.ThisisoneofthebestexposedlodemineraldepositsinIditarodquadrangle.Samplescontainupto32.3oz/tonAg,>2,000ppmAs,1.8ppmAu,>2,000ppmB,570ppmBi,60ppmCd,21%Cu,>20,000ppmPb,7,000ppmSb,200ppmSn,100ppmW,and5,000ppmZn.ReFeRenCes:BundtzenandLaird,1982;Szumigala,1993,1995;Noklebergandothers,1993;BundtzenandMiller,1997;thisreport.

29 Unnamed(O) Ag,Cu,(Au,W) Gradesofupto2.0%Cu,110g/t(3.2oz/ton) T.32N. [79GL318] Plutonic-relatedAg-Sn Ag,0.7g/t(0.02oz/ton)Au,and300ppmW R.41W. polymetallic

sUmmARy:SulfidemineralsassociatedwithtourmalinebreccianearmajorN.20°E.-trendinghigh-anglefaultinBeaverMountainsstock.ReFeRenCe:BundtzenandLaird,1982.

30 Unnamed(O) Ag,Cu(Sb,Sn) Grabsamplecontains332g/t(9.67oz/ton)Ag, T.32N. [81BT503] Plutonic-relatedAg-Sn 8.0%Cu,100ppmSb,and100ppmSn R.41W. polymetallic

sUmmARy:Tourmaline-axinite-sulfidefracturesysteminandesiticvolcanicrocksstrikesN.75–80°E.;maybeanextensionoftheCirquedeposit(no.28).ReFeRenCe:BundtzenandLaird,1982.

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31 Unnamed(A) Cu,Pb(Ag,Hg) Samplecontains1ppmAg,>2,000ppmB,300 T.32N. [I0916] Deposittypeuncertain ppmCu,100ppmPb,and0.32ppmHg R.41W.

sUmmARy:Grabsamplefromiron-oxidestainedfracturezoneinmonzonite.ReFeRenCes:McGimseyandothers,1988;thisreport.

32 Joaquin(P) Hg(Au,Ag,As) Samplefromoredumpcontains2.0%Hg,0.24 T.32N. Plutonic-hostedCu-Au ppmAu,0.8ppmAg,and2%As R.37W. polymetallic(?)

sUmmARy:ThinquartzveinletsinMountJoaquinplutoncontainarsenopyriteandcinnabar.Exposedforabout30mofstrikeoveranaverage0.5mwidth.Propertystakedin1957,butnoproductionreported.ReFeRenCes:Malone,1962;BundtzenandLaird,1983;thisreport.


sUmmARy:Visiblegold(<1%),cinnabar(1–5%),andscheelite(1–5%)innonmagnetic,heavy-mineral-concentratesamplefromstreamdrainingeastsideofplutonatMountJoaquinnearlodeprospectno.32.ReFeRenCes:Bennettandothers,1988;MillerandBundtzen,1994;thisreport.

34 Unnamed(A) Pb,Sn(Nb) Samplecontains100ppmNb,150ppmPb,50 T.31N. [84AAi503] Deposittypeuncertain ppmSn,and15ppmBe R.48W.

sUmmARy:GrabsamplesofrhyolitetufffromtheYetnaRivervolcanicfield;nosulfidesrecognized.Hgwasnotanalyzed.ReF-eRenCes:McGimseyandothers,1988;MillerandBundtzen,1994;thisreport.

35 Unnamed(A) Pb,Sn(Nb) Samplecontains100ppmNb,100ppmPb,70 T.31N. [84AAi504] Deposittypeuncertain ppmSn,and15ppmBe R.48W.

sUmmARy:GrabsampleofalteredfelsicvolcanicrockfromtheYetnaRivervolcanicfield;nosulfidesrecognized.Hgwasnotanalyzed.ReFeRenCes:McGimseyandothers,1988;MillerandBundtzen,1994;thisreport.

36 Unnamed(A) Pb(Sn) Samplecontains100ppmPb,50ppmSn,50 T.31N. [84AAi506] Deposittypeuncertain ppmNb,and10ppmBe R.48W.

sUmmARy:Grab sample ofwelded rhyolite tuff from theYetnaRiver volcanic field.Hgwas not analyzed.ReFeRenCes:McGimseyandothers,1988;MillerandBundtzen,1994;thisreport.

37 Unnamed(A) Hg Samplecontains2.2ppmHg T.31N. [85AM164] Volcanichosted R.47W.

sUmmARy:Grabsampleofaltered,weldedrhyolitefromtheYetnaRivervolcanicfield;nosulfidesrecognized.ReFeRenCes:McGimseyandothers,1988;MillerandBundtzen,1994;thisreport.

38 Unnamed(A) (Ag,Cu) Samplecontains1ppmAgand100ppmCu T.31N. [84AM283] Deposittypeuncertain R.46W.

sUmmARy:Grabsampleofslightlyiron-stainedsiltymetachertfromtheMississippiantoTriassicagedInnokoterrane;nosulfidesrecognized.ReFeRenCes:McGimseyandothers,1988;MillerandBundtzen,1994;thisreport.

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39 Unnamed(A) (Au,Hg) Grabsamplecontains0.2ppmAuand0.28ppm T.31N. [83BT9] EpithermalHg-Sb(?) Hg R.44W.

sUmmARy:SilicifiedzonenearaltereddikethatintrudesshorelinefaciesofKuskokwimGroup.ReFeRenCe:Bundtzenandothers,1988.

40 Unnamed(A) Grades1.58%Mn T.31N. [83GL3] Mn R.43W. Deposittypeuncertain

sUmmARy:Iron-oxidestained,pyrolusite-bearingquartzveincutsKuskokwimGroupsandstonenearaltereddike.ReFeRenCe:Bundtzenandothers,1988.

41 Unnamed(A) Hg Samplecontains3.4ppmHg T.31N. [83GL10] EpithermalHg-Sb R.43W.

sUmmARy:GrabsampleofalteredmaficdikethattrendsN.50°E.nearcrestofanticlineinKuskokwimGroup.ReFeRenCe:Bundtzenandothers,1988.

42 Unnamed(A) (Hg,Au) Grabsamplecontains0.88ppmHgandtrace T.31N. [86AMc43] EpithermalHg-Sb Au R.41W.

sUmmARy:Silicified lithic tuffof the IditarodVolcanics.ReFeRenCes:McGimseyandothers,1988;MillerandBundtzen,1994;thisreport.

43 Unnamed(A) (Ag,Hg) Samplecontains1ppmAgand0.12ppmHg T.31N. [83AM113] Granite-porphyry-hosted R.40W. Au-polymetallic(?)

sUmmARy:GrabsampleofgraniteporphyrythatintrudesKuskokwimGroupsedimentaryrock;hornfelsdevelopedinsedimentaryrocksnearintrusivecontacttowestoflocality.ReFeRenCes:McGimseyandothers,1988;thisreport.

44 LincolnCreek(P) Au(Ag,Hg) Finegoldpannedfrompits;grabsamples T.31N. [I0308and86AM420] Deposittypeuncertain containupto0.15ppmAgand1.3ppmHg R.40W.

sUmmARy:AlteredmaficandfelsicdikescuttingKuskokwimGroupsandstoneareexposedintrenchesforabout300m;prospectisadjacenttoasignificantfaultsystem.Goldpannedfrompitsduringexploratorywork.ReFeRenCes:JoeDegnan,oralcommun.,1979;BundtzenandLaird,1982;McGimseyandothers,1988.

45 Unnamed(A) Hg Samplecontains2.2ppmHg T.31N. [86AM422] EpithermalHg-Sb R.38W.

sUmmARy:Grabsampleofgraniteporphyryintrusion;rockhassecondarytourmaline.ThesamplelocationwasincorrectlyshowninMcGimseyandothers(1988)tobe16kmnorthoftheactualcollectionsite.ReFeRenCes:McGimseyandothers,1988;thisreport.

46 Unnamed(A) Hg(As,Au,Ag) Samplescontain>10ppmHg,70ppmAs,trace T.31N. [86AM421andI1214] Au,andtraceAgEpithermalHg-Sb R.38W.

sUmmARy: Iron-oxide stainedgrab samplesofgraniteporphyry intrusion.ReFeRenCes:McGimseyandothers, 1988; thisreport.

63



47 Unnamed(A) As(Hg) Grabsamplescontainupto1ppmHgand500 T.31N. [86AM401and EpithermalHg-Sb(?) ppmAs R.37W. 86AM403]

sUmmARy:ShearedcontactzonebetweengraniteporphyryintrusionandhornfelssandstoneoftheKuskokwimGroup;grabsam-plesofbothcontainelevatedHgvalues.Alabeledsamplewrappedinorangeflaggingfoundatlocalitywaspresumablyleftbyexplo-rationfirm.Samples86AM403werecollected1.5kmeastofindicatedlocality,butliealongsamefaultedcontact.ReFeRenCes:McGimseyandothers,1988;thisreport.

48 TributarytoCarl Ag,Au(Pb) Heavy-mineral-concentratesamplecontains23 T.31N. Creek(O) Placer ppmAg,5,700ppmPb,andtraceAu.Stream R.36W. [79BT104andI1275- cobblecontainsupto33ppmAg,0.10ppm cobble] Au,1,100ppmAs,>2,000ppmB,and500 ppmPb

sUmmARy:First-ordertributarystreamdrainssouthintoCarlCreek.U.S.mintrecordsshowCarlCreekwasprospectedin1917and1918,producing18ozgold.Heavy-mineral-concentratesamplecontainselevatedvaluesofsilver,lead,andtracegold.MineralizedstreamcobblesprobablycomefromhornfelsneartheTatalinapluton.ReFeRenCes:Wimmler,1926;BundtzenandLaird,1983;Bundtzenandothers,1987;McGimseyandothers,1988;thisreport.

49 Tatalina(P) Ag(Cu,V) Consistentvaluesofupto5ppmAg,100ppm T.31N. [79BT267] Plutonic-relatedAg-Sn Cu,and1,000V R.36W. polymetallic(?)

sUmmARy:Extensivezoneoftourmaline-manganesebrecciainhornfelsaureoleoverlyingTatalinapluton.Atleast200mby200mindimension.Oldprospectpitsalongridgetop.Mineralizedrockssimilartofloatobservedatno.48.ReFeRenCes:BundtzenandLaird,1983;thisreport.

50 Unnamed(A) Ag,Pb(Sn) Samplescontainupto10ppmAg,500ppm T.30N. [I0002] Deposittypeuncertain Pb,and20ppmSn R.48W.

sUmmARy: Iron-oxidestained,vuggysilicifiedzoneinrhyoliteofYetnaRivervolcanicfield.GrabsamplecontainsanomalousvaluesofAgandPb,andslightlyelevatedSn;Hgwasnotanalyzed.ReFeRenCes:McGimseyandothers,1988;MillerandBundt-zen,1994;thisreport.

51 Unnamed(A) Ag(Cr,Ni) Grabsamplescontainupto30ppmAg,120 T.30N. [85AM111] Mafic-ultramafic ppmAs,2,000ppmCr, R.46W.

sUmmARy: Serpentinizedultramafic rocks ofDishnaRiver ophiolite containing abundant chrome spinel andmagnetite.Cr-Nivaluesprobablyinthespinelandsilicateminerals.SourceofAgandAsisnotknown.ReFeRenCes:McGimseyandothers,1988;Miller,1990;MillerandBundtzen,1994;thisreport.

52 Unnamed(A) Hg,As,Sb Grabsamplecontains0.85ppmHg,50ppmAs, T.30N. [86AMc225] EpithermalHg-Sb and14ppmSb R.45W.

sUmmARy:Brecciated,iron-stained,andveinedKuskokwimGroupsandstonecontainsslightlyelevatedHg,As,andSbvalues.Nearbysampleofsandstone(84BT264)contains70ppmSb.ReFeRenCes:McGimseyandothers,1988;thisreport.

53 Unnamed(A) Au,Hg,As Grabsamplecontains0.2ppmAu,1.5ppmHg, T.30N. [83BT4] EpithermalHg-Sb(?) and99ppmAs R.43W.

sUmmARy:Iron-oxideveinedsandstonenearcontactwithN.50°E.-trendingalteredmafic(?)dike.ReFeRenCes:Bundtzenandothers,1988;thisreport.

64



54 Unnamed(A) Au,Hg(Ag) Grabsamplecontains0.2ppmAu,0.2ppmAg, T.30N. [83BT20] EpithermalHg-Sb(?) and2.5ppmHg R.43W.

sUmmARy:Iron-stainedbrecciainchert,possiblyafaultsliverofInnokoterrane.Extensiveshearzonecontainsminorpyrite.ReF-eRenCes:Bundtzenandothers,1988;MillerandBundtzen,1994;thisreport.

55 Unnamed(O) Au Visiblegoldinheavy-mineral-concentrate T.30N. [I1532] Placer sample R.42–43W.

sUmmARy:Visiblegold(<1%),cinnabar(1–5%),andscheelite(<1%)innonmagnetic,heavy-mineral-concentratesample.Streamdrainsareaofvolcanicandsedimentaryrocksthatarelocallyintrudedbysmallstocks.ReFeRenCes:Bennettothers,1988;thisreport.

56 DeadwoodCreek(P) Au(Ag) 1.5kmby150mareacontainslow-gradeplacer T.30N. Placer Auresource R.42–43W.

sUmmARy: Shallow,unexploitedplacergold resourcenearheadofDeadwoodCreekdrainage.Discovered in1932,butnevermined.Fine-grainedgoldoccursin1–3mofgravel,whichiscoveredby2–3mofoverburden.Placerpaystreakasdrilledunderliesa1.5kmby150marea;probablyinyouthful(lateQuaternary)streamalluvium.Anomalousgoldalsofoundinstreamdrainagesimme-diatelyeastofDeadwoodCreek.Lodesourceofgoldproblematical—possiblyfromMaybeCreekplutonabout6kmtothesoutheast.ReFeRenCes:ToivoRosander,oralcommun.,1983;DonHarris,oralcommun.,1983;Bundtzenandothers,1988;thisreport.

57 St.PatrickCreek(A) (Hg;Au) GrabsamplecontainsslightlyelevatedHg T.30N. [I0638] Deposittypeuncertain (0.50ppm);placergoldreportednear R.42W. headwaters

sUmmARy:Iron-oxidestainedmaficvolcanicrockfromtheIditarodVolcanicssouthwestoftheBeaverMountains.PlacergoldreportedlyobtainedfromacutattheheadofSt.PatrickCreek.ReFeRenCes:DonHarris,oralcommun.,1982;McGimseyandothers,1988;MillerandBundtzen,1994;thisreport.

58 MaybeCreek(O) Au Goldconsistentlypannedfromcreek T.30N. [82BT401] Placer R.42W.

sUmmARy:Finegold(10–20grains)consistentlypannedfromshallowcoarsegravelsalong1kmofMaybeCreekbetween1,500and1,800footelevations;creekdrainsmonzonitepluton.Principalheavymineralsincludechromite,magnesiochromite,cinnabar,scheelite,andpolybasite(?).ReFeRenCes:Bundtzenandothers,1987;Bundtzenandothers,1988;thisreport.

59 Unnamed(A) Pb,Hg(Ag,As) Grabsamplecontains0.85ppmAg,200ppm T.30N. [I0644] Plutonic-relatedAg-Sn Pb,9.5ppmHg,and60ppmAs R.42W. polymetallic(?)

sUmmARy:Floatsampleofhornfelsfromnearmonzonitecontactcontainsdisseminatedsulfides.ReFeRenCes:McGimseyandothers,1988;thisreport.

60 FourthofJulyCreek Au(Ag,Cr,Hg) About1.4kg(45oz)Auproducedin1982–83. T.30N. (M) Placer Amountofearlierproduction(1915–20?) R.42W. unknown.Oneheavy-mineralconcentrate contained24.8%Cr2O3

sUmmARy:Firstdiscoveredabout1911,butsawonlyintermittentdevelopment.Testminedin1982–1983.Heavy-mineralplacersoccuralong4kmofFourthofJulyCreekfrom1,500ftelevationtocanyonbreakoutat1,200ftelevation.BedrockpaystreakliesonaltereddacitetuffatupperendandonKuskokwimGroupatlowerend.Coarsefloatpredominates.Principalheavymineralsaremagnetite,chromite,nativemercury,cinnabar,scheelite,nativesilver,tetrahedrite,andpolybasite.Goldfinenessvaluesrangefrom853to899.Haspotentialfordiscoveryofnewreserves.ReFeRenCes:Brooks,1912;Bundtzenandothers,1987;Bundtzenandothers,1988;thisreport.

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61 Unnamed(A) Au,Ag,Hg Grabsamplescontainupto1.4ppmAu,0.2 T.30N. [I1230] Plutonic-hostedCu-Au ppmAg,and>10ppmHg R.42W. polymetallic(?)

sUmmARy:VeinedandalteredvolcanicrocksoftheIditarodVolcanics;novisiblesulfides.ReFeRenCes:McGimseyandothers,1988;MillerandBundtzen,1994;thisreport.

62 Unnamed(O) Ag,Pb,Hg Grabsamplescontainupto5.1ppmAg,76ppm T.30N. [82BT404] Deposittypeuncertain Pb,and5ppmHg R.42W.

sUmmARy:ExtensivegossanandchalcedonicalterationintuffaceousbasalportionoftheIditarodVolcanics,southoftheBeaverMountains.ReFeRenCes:Bundtzenandothers,1988;MillerandBundtzen,1994.

63 Unnamed(A) Hg,As,Sb(Ag,Au) Samplescontainupto0.2ppmAg,7ppmHg, T.30N. [82BT385andI1483] Deposittypeuncertain 110ppmAs,32ppmSb,andtraceAu R.42W.

sUmmARy:Grabsamplesof(1)iron-oxidestainedbrecciainvolcanicrockoftheIditarodVolcanicsand(2)quartzveinedsandstonefloat.ReFeRenCes:Bundtzenandothers,1988;McGimseyandothers,1988;MillerandBundtzen,1994;thisreport.

64 Unnamed(A) (Zn,Ag,As) GrabsamplecontainsslightlyelevatedAg(1 T.30N. [86BT93] Plutonic-hostedCu-Au ppm),190ppmZn,and30ppmAs R.40W. polymetallic(?)

sUmmARy:Grabsampleofquartzmonzonitenearhornfelscontact.ReFeRenCes:McGimseyandothers,1988;thisreport.

65 Unnamed(A) As,Hg,Sb Samplecontains200ppmAs,28ppmSb,2.2 T.30N. [84MSL137] EpithermalHg-Sb(?) ppmHg,andtraceAg R.38W.

sUmmARy:Grabsampleofgraniteporphyryintrusion;collectornotedpossiblemercuryoxides.ReFeRenCes:McGimseyandothers,1988;thisreport.

66 Unnamed(O) Hg,As(Pb,W,Zn) Samplescontainupto1.0%As,>10ppmHg, T.30N. [86AM423] Deposittypeuncertain 200ppmW,100ppmPb,1,000ppmCr,120 R.37W. ppmZn,and traceAg

sUmmARy:Grabsamplesof iron-stainedalteredfelsicdike;onesamplehassubparallelquartzveinletsandstringers.ReFeR-enCes:McGimseyandothers,1988;thisreport.

67 Unnamed(A) Hg Samplecontains2.2ppmHg T.30N. [84AM245] EpithermalHg-Sb(?) R.37W.

sUmmARy:Grabsampleofiron-stainedgraniteporphyrydikecontainselevatedHgvalue.ReFeRenCes:McGimseyandothers,1988;thisreport.

68 Unnamed(A) Au(Hg) Samplecontains0.1ppmAuand60ppbHg T.29N. [85BT60] Volcanichosted R.49W.

sUmmARy:Grab sample of bleached rhyolite from theYetnaRiver volcanicfield;may have chalcedonic alteration.ReFeR-enCes:McGimseyandothers,1988;thisreport.

69 Unnamed(A) Ag(Pb) Samplecontains5.0ppmAgand200ppmPb T.29N. [I0027] Deposittypeuncertain R.49W.

sUmmARy:GrabsampleofalteredrhyolitetufffromtheYetnaRivervolcanicfieldcontainsanomalousAgandPb;Hgwasnotanalyzed.ReFeRenCes:McGimseyandothers,1988;thisreport.

66



70 Unnamed(A) As,Hg(Au) Samplecontains130ppmAs,1.4ppmHg, T.29N. [84AM180] Volcanichosted 0.05ppmAu,20ppmSn,andtraceAg R.48W.

sUmmARy:GrabsampleofpotassicalteredrhyoliteofYetnaRivervolcanicfieldcollectednearcontactwithLowerProterozoicIdonoComplex.ReFeRenCes:McGimseyandothers,1988;MillerandBundtzen,1994;thisreport.

71 Unnamed(O) Sb,As(Hg,Au) Samplecontains740ppmSb,80ppmAs,0.68 T.29N. [85AM323] EpithermalHg-Sb(?) ppmHg,andtraceAu R.46W.

sUmmARy:Highlyaltered,iron-oxidestained,porphyriticdikecutsshorelinefaciessandstoneoftheKuskokwimGroup.Samplehasvisiblesecondaryhematite.ReFeRenCes:McGimseyandothers,1988;MillerandBundtzen,1994;thisreport.

72 Unnamed(A) Sb Samplecontains76ppmSb T.29N. [86AMc203] EpithermalHg-Sb(?) R.45W.

sUmmARy:GrabsampleofmoderatelywellsortedKuskokwimGroupsandstonecontainsanomalousSb;Hgwasnotanalyzed.ReFeRenCes:McGimseyandothers,1988;thisreport.

73 Unnamed(A) Sb Samplecontains92ppmSband20ppmAs T.29N. [86AMc207] EpithermalHg-Sb(?) R.45W.

sUmmARy:GrabsampleofKuskokwimGroupsandstonecollectedforbackground.ReFeRenCes:McGimseyandothers,1988;thisreport.

74 DishnaRiver(P) Au,Sb,As(Ag,Hg) Inferredreserveof37,600tgrading2.46g/t T.29N. [86BT365, EpithermalHg-Sb Au,0.52%Sb,and0.45%As R.44W. 86AMc201,and I0184]

sUmmARy:Mineralizedquartz-sulfide-sulfosaltzonesandveinbrecciascutshearedshaleandsandstoneoftheKuskokwimGroup.Extensiveslickensidesdevelopedinsedimentaryhostrocks.VeinstrendN.5°W.toN.5°E.anddipsteeplytovertically.Prospectconsistsofsulfide-bearingmasseslocallycontaining15–25%stibniteupto25cmthick;disseminatedarsenopyritealsofound.Pros-pectpitsindicatedevelopmentinpreviousyears.Atleastthreedistinctmineralizedveinsidentified.Eighteenchip-channelsamplescollectedatuniformintervalsalong140moflargestveinformbasisofreserveestimate.Samplescontainupto11ppmAu,1%Sb,>1%As,>10ppmHg,and1ppmAg.ReferencehasbeenmadetoplacergoldfoundinheadwardpartofDishnaRiver.ReFeR-enCes:DonHarris,oralcommun.,1983;McGimseyandothers,1988;BundtzenandMiller,1997;thisreport.

75 Unnamed(A) Hg Samplecontains2.9ppmHg T.29N. [83GL28a] EpithermalHg-Sb R.44W.

sUmmARy:SampleofalteredsandstoneandshaleoftheKuskokwimGroupcontainselevatedHg.Samplesiteis1kmsouthofasmall,northeast-trendingfelsicdike.ReFeRenCe:Bundtzenandothers,1988.

76 Unnamed(O) Ag,Au,Cu,Zn,As(Hg) Samplescontainupto0.2ppmAu,2.9ppm T.29N. [83GL53] Deposittypeuncertain Ag,210ppmCu,349ppmZn,127ppmAs, R.43W. and3ppmHg

sUmmARy:Grabsamplesofsulfide-richfaultbreccianearmonzonite-sandstonecontact.ReFeRenCe:Bundtzenandothers,1988.

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77 Unnamed(O) Ag,As,Zn,Pb,Sb,Hg, Grabsamplescontainupto12ppmAg,15% T.29N. [I1484] Cd,Ni,Fe Fe,710ppmAs,>2,000ppmB,600ppmZn, R.43W. Plutonic-relatedAg-Sn 500ppmPb,70ppmSb,4.2ppmHg,35ppm polymetallic(?) Cd,and300ppmNi

sUmmARy:Quartz-sulfideveins in iron-oxidestainedKuskokwimGroupsandstonehornfelsnearmonzonitecontact.ReFeR-enCes:McGimseyandothers,1988;thisreport.

78 Unnamed(A) Hg,As SlightlyelevatedvaluesofHg(upto1.4ppm) T.29N. [I1485andI1535] EpithermalHg-Sb(?) andAs(upto50ppm) R.43W.

sUmmARy:Grabsamplesofvolcanicrock(oftheIditarodVolcanics)containingdisseminatedpyrite.ReFeRenCes:McGimseyandothers,1988;MillerandBundtzen,1994;thisreport.

79 Unnamed(O) Mo,As,Cu,(Hg,Ag, Grabsamplecontains100ppmMo,500ppm T.29N. [I0196] Sb) As,100ppmCu,1.1ppmHg,50ppbAg,30 R.43W. Plutonic-relatedAg-Sn ppmSb,1,000ppmCr,and1,000ppmNi polymetallic(?)

sUmmARy:Iron-oxidestained,alteredmafictufffrombasalportionofIditarodVolcanicscontainsdisseminatedsulfides.ElevatedCrandNivalueslikelyrelatedtopresenceofolivine.ReFeRenCes:Bundtzenandothers,1988;McGimseyandothers,1988;MillerandBundtzen,1994;thisreport.

80 Unnamed(A) Sb Contains200ppmSb T.29N. [84BT277] Deposittypeuncertain R.43W.

sUmmARy:SmallN.20°E.-trendingquartzveininfresholivinebasaltoftheIditarodVolcanicscontains200ppmSb;Hgwasnotanalyzed.ReFeRenCes:McGimseyandothers,1988;MillerandBundtzen,1994;thisreport.

81 WillowCreek(O) Au Placergoldinpannedconcentrates T.29N. Placer R.42W.

sUmmARy:PlacergoldfoundinstreamalluviumonWillowCreekandonhillslopesofVABMWillow.ThelattermaybearesidualplacerdirectlyonamineralizedquartzmonzoniteplutonnearVABMWillow.Principalheavymineralsincludechromite,magnetite,andcinnabar.ReFeRenCes:Bundtzenandothers,1988;thisreport.

82 BrokenShovel(P) Ag,Pb,Sb,Cu,As(Au) Estimatedinferredreserveof14,500t(16,000 T.29N. Plutonic-hostedCu-Au tons)grading149g/t(4.35oz/ton)Ag,0.15% R.42W. polymetallic(?) Pb,0.15%Sb,0.17%As,0.13%Cu;samples alsocontainelevatedAu,Bi,W,andZn

sUmmARy:Tourmaline-sulfide-quartzveinthataveragesabout1.5minthicknessandstrikesN.20°E.canbetracedforabout200mcuttingtheMooreCreekpluton.Oremineralsincludearsenopyrite,scheelite,tetrahedrite,andPb-Sbsulfosalts.TracesofAurecognizedinmicroprobework.Sericitealterationenclosesveinsystem.ReFeRenCes:Bundtzenandothers,1988;Noklebergandothers,1993;BundtzenandMiller,1997;thisreport.

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83 MooreCreek(M) Au(Ag,Cr) MooreCreekproduced1,679kg(53,990oz)Au T.29N. Placer and389.1kg(12,510oz)Agfrom R.42W. 1913–1985.NevadaGulchproduced43kg (1,383oz)Auand2.0kg(64oz)Agfrom 1911–1929

sUmmARy:Significantplacerdepositminedfrombothancestralbenchalluviumandmodernstreamalluvium.Paystreakabout3kmlongand100–400mwide.Paygravelswere2mthickandcoveredby3mofoverburden.Sourceofgoldthoughttobethemineralizedplutonthatliesnorthwestoftheplacerworkings.OriginallyminedbyhandmethodsinNevadaGulchasearlyas1911;large-scalemechanizedoperationhasexhaustedmuchofknownpaystreak.Principalheavymineralsincludemagnetite,chromite,zircon,cinnabar,scheelite,nativesilver,andtetrahedrite.Mineconcentratescontainupto35.0%Cr2O3makingtheplacerdepositalow-gradechromeresource.Goldfinenessaverages758;nuggetsto591g(19oz)recentlyrecovered.TheIditarod-NixonForkfaultforms southern structural boundaryofmineralizedMooreCreekpluton, therefore,Tertiary(?) toQuaternaryMooreCreekplacerdepositsmayhavebeensuccessivelyoffsetrightlaterallybytranscurrentfaultmovement.Iftrue,oldunexploitedburiedchannelswouldliesouthwestofmainworkings.ReFeRenCes:Brooks,1912;Smith1941;DonHarris,oralcommun.,1982;Bundtzenandothers,1987;Bundtzenandothers,1988.

84 Unnamed(A) Ag,Cu Samplecontains2ppmAgand150ppmCu T.29N. [83AM109] Deposittypeuncertain R.42W.

sUmmARy:Sandstonegrabsamplefromwell-exposedsectionofKuskokwimGroup;nosulfidesrecognized.ReFeRenCes:McGimseyandothers,1988;thisreport.

85 Unnamed(A) Au,Hg Samplescontainupto0.1ppmAu,0.14ppm T.28N. [85BT64and Volcanichosted Hg,andtraceAg R.51W. 86AMc230]

sUmmARy:Grabsamplesofdevitrified,silicified,felsicvolcanicrocksofYetnaRivervolcanicfield.ReFeRenCes:McGimseyandothers,1988;MillerandBundtzen,1994;thisreport.

86 FlatCoal(M) Coal Minedduringperiodof1910to1920 T.28N. R.47W.

sUmmARy:Subbituminoustoanthracitecoal0.3to0.8mthickstrikesN.60°E.and50°SE.Averagecalorificvaluesof7,983andaverageBTUvalueof14,369.Mostexposuresfoundinshallow-marinetononmarinefaciesofKuskokwimGroup.Minedforblack-smithingpurposesduringearlygoldrushyearsatFlat.ReFeRenCes:Brooks,1914;Mertie,1936;Bundtzenandothers,1992.

87 GoldenGroundand Au,Ag,Pb,Sb,Cu,W, Samplescontainupto50g/t(1.5oz/ton)Au, T.28N. Neilson(P) Zn,As,Cd,Bi,Hg 2kg/t(58oz/ton)Ag,2.0%Pb,1%Sb,0.5% R.47W. [84BT300,86AGe32, Plutonic-hostedCu-Au Cu,0.2%W,0.1%Zn,>1%As,300ppm and86AGe33] polymetallic Cd,40ppmBi,and>10ppmHg

sUmmARy:Twoadits incloseproximity—themorenorthernoneis theGoldenGroundprospect; themoresouthernoneis theNeilsonprospect.Depositconsistsof1–5cmthickquartz-sulfide-tourmaline(?)veinsthatcutolivinebasalt.Iron-oxidestainedquartzrubblesampled.Arsenopyrite,galena,andscheeliterecognizedinfield.Developedbyshort15–20mdrifts.ReFeRenCes:Hol-zheimer,1926;McGimseyandothers,1988;Bundtzenandothers,1992;thisreport.

88 Unnamed(P) Au,Ag,Sb,As(Hg,W) Samplescontainupto1.0ppmAu,1.0ppmAg, T.28N. [86AGe34and Plutonic-hostedCu-Au 2,000ppmB,650ppmSb,>0.2%As,50ppm R.47W. 86AGe35] polymetallic W,and>10ppmHg

sUmmARy:Upto5cmthickquartzveinsiniron-oxidestained,brecciatedsandstonehornfels;novisiblesulfidesinsamplescol-lected.Localityhasbeentrenched.ReFeRenCes:McGimseyandothers,1988;thisreport.

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89 GraniteCreek(M) Au(Ag) Produced148kg(4,750oz)Auand19.8kg T.27–28N. Placer (636oz)Agfromabout1913–1956and R.47W. 1980–1985

sUmmARy:Small,2.5-km-longstreamdrainingmineralizedvolcanicandplutonicrocksnorthofOtterCreekdrainage.Paystreaksarecoveredbyshallowoverburden.Heavymineralsrecoveredincludescheelite,cinnabar,arsenopyrite,andilmenite.Goldfinenessis854.ReFeRenCes:Mertie,1936;Smith,1941;Bundtzenandothers,1987;Bundtzenandothers,1992;thisreport.

90 Unnamed(O) Ag,Pb,Sb,Zn(As,Cu, Samplescontainupto200ppmAg,2.0%Pb, T.28N. [86AGe38and Sn,Cd,Bi) >0.2%Sb,0.2%Zn,200ppmCu,50ppm R.46W. 86AGe39] Plutonic-relatedAg-Sn Sn,0.16%As,100ppmCd,and18ppmBi polymetallic(?)

sUmmARy:Quartzveinstringersandheavilyiron-oxidestainedsandstonespatiallyassociatedwithgraniteporphyrydike.ReFeR-enCes:McGimseyandothers,1988;Bundtzenandothers,1992;thisreport.

91 Unnamed(O) Sn,Zn,Sb,As,Pb,Bi Grabsamplescontainupto200ppmSn,500 T.28N. [85AM268,269 (Ag,Au) ppmZn,500ppmSb,480ppmAs,200ppm R.46W. and86AGe41] Plutonic-relatedAg-Sn Pb,46ppmBi,>2,000ppmB,1ppmAg,and polymetallic(?) traceAu

sUmmARy:Extensivesecondarytourmalineincalcite-chloritealteredintermediatedikesandintheKuskokwimGroupsandstonetheyintrude.ReFeRenCes:McGimseyandothers,1988;thisreport.

92 Unnamed(O) Ag,As(Cu,Bi,Sn,Au) Grabsamplecontains2ppmAg,850ppmAs, T.28N. [86BT419] Plutonic-relatedAg-Sn 200ppmCu,52ppmBi,30ppmSn,and50 R.46W. polymetallic ppbAu

sUmmARy:Iron-oxidestainedhornfelsnearbiotite-bearing,fine-grainedintrusion.ReFeRenCes:McGimseyandothers,1988;thisreport.

93 Unnamed(O) Ag,Au,Sb,As Grabsamplescontainupto1.2ppmAg,0.2 T.28N. [86AM370,371and Granite-porphyry-hosted ppmAu,290ppmSb,130ppmAs,50ppm R.46W. I1495] Au-polymetallic(?) Nb,andtraceHg

sUmmARy:Iron-oxidestainedquartzveinletsinsandstone,inquartz-healedsandstonebreccia,andingraniteporphyrydike;somesulfidespresentlocally.ReFeRenCes:McGimseyandothers,1988;thisreport.

94 Unnamed(A) As(Hg) Samplecontains470ppmAsand0.82ppmHg T.28N. [86BT363] EpithermalHg-Sb R.45W.

sUmmARy:Grabsampleofaltereddikethatcutscalcareoussandstone.ReFeRenCes:McGimseyandothers,1988;thisreport.

95 Unnamed(P) (Hg,As,Ag) Samplescontainupto0.34ppmHg,60ppm T.28N. [86BT359,86AGe46, EpithermalHg-Sb(?) As,100ppmCu,andtraceAg R.45W. andI0185]

sUmmARy:Somegossanareasarespatiallyassociatedwithsilica-carbonatealtereddikesthatcutsandstone.Severalprospectpitsinarea,butvaluesweobtainedareonlyslightlyelevated.ReFeRenCes:McGimseyandothers,1988;thisreport.

96 Unnamed(O) Au,Ag,Sb,As,Hg,Zn Samplescontainupto0.2ppmAu,1.0ppmAg, T.28N. [86BT351,352, EpithermalHg-Sb(?) 0.19%As,3.3ppmHg,116ppmSb,and300 R.45W. 84AAi572,and ppmZn 84MSL136]

sUmmARy:Extensivegossanzone(estimatedtobe2km2)inKuskokwimGroupsandstone.Iron-oxidestainedquartzveinlets,sandstonebreccia,andsilica-carbonatealtereddikearepresent.ReFeRenCes:McGimseyandothers,1988;thisreport.

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97 Unnamed(A) Hg Samplecontains5.1ppmHg(and0.1ppmAg) T.28N. [83GL69] EpithermalHg-Sb R.44W.

sUmmARy:Grabsampleofiron-oxidestainedshale.ReFeRenCe:Bundtzenandothers,1988.

98 Unnamed(O) Ag,Cu,Zn,Mn,Fe Samplecontains2.6ppmAg,891ppmCu, T.28N. [83GL59] Deposittypeuncertain 1.68%Mn,304ppmZn,and28%Fe R.44W.

sUmmARy:Massivepyrrhotitevein,25cmwide,inhornfelsnearcontactwithCamelbackMountainmonzonite.ReFeRenCe:Bundtzenandothers,1988.

99 Unnamed(A) Ag Samplecontains1ppmAg T.28N. [83AM110] Deposittypeuncertain R.42W.

sUmmARy:Grabsampleofhypabyssal,pilotaxiitic,andesiteplugcontainsslightlyelevatedAg;Hgwasnotanalyzed.ReFeR-enCes:McGimseyandothers,1988;thisreport.

100 Unnamed(A) Sb,As,Fe(Zn) Samplecontains>20%Fe,200ppmSb,100 T.28N. [86AM116] Deposittypeuncertain ppmAs,180ppmZn,and100ppmNi R.41W.

sUmmARy:Grabsampleofheavilyiron-oxidestainedalteredsandstonecontainingsecondaryhematite;iron-stainedrockisveryminoratlocality.ReFeRenCes:McGimseyandothers,1988;thisreport.

101 Unnamed(A) Hg,Sb,As Samplecontains0.42ppmHg,20 T.28N. [86AM112] EpithermalHg-Sb ppmSb,and20ppmAs R.40W.

sUmmARy:Grabsampleofiron-oxidestainedsandstonebrecciathatislocallyhealedbyquartz.Asmallprospectpitonsimilarquartz-healedsandstonebrecciawasnotedbyanotherworker1kmeastofthislocation,butnosampleswerecollectedthere.ReFeR-enCes:McGimseyandothers,1988;thisreport.

102 Unnamed(A) Hg,As,Sb(Ag) Samplecontains0.46ppmHg,30ppmAs,20 T.28N. [I1529] EpithermalHg-Sb ppmSb,and0.1ppmAg R.38W.

sUmmARy:GrabsampleofKuskokwimGroupsandstone;noalterationorsulfidesnoted,butrockcontainsslightlyelevatedvalues.ReFeRenCes:McGimseyandothers,1988;thisreport.

103 Unnamed(A) As(Hg,Sb,Au,Ag) Samplescontainupto150ppmAs,4ppmSb, T.27N. [85AM216and Volcanic-hosted andlocallytraceHg,Ag,andAu R.52W. 85AAi709]

sUmmARy:Grabsamplesofiron-oxidestainedEarlyCretaceoussandstone;secondaryhematitenoted.ReFeRenCes:McGim-seyandothers,1988;MillerandBundtzen,1994;thisreport.

104 HappyCreek(and Au(Ag) Producedatleast3,965kg(127,486oz)Auand T.27N. upperWillowCr)(M) Placer 535.2kg(17,210oz)Agfrom1910–1984 R.47W.

sUmmARy:Ancestralbenchandmodernstreamalluvialplacersform6-km-longsystemofpaystreaksinHappyCreekandupperpartofWillowCreek.UppermostHappyCreekalluvialplacersaretransitionaltoresidualtypeplacersatAlpha,Upgrade,andTraildepositsonslopeofChickenMountain.Principalheavymineralsincludezircon,magnetite,ilmenite,chromite,cinnabar,andfluor-apatite.(Zirconcontainsupto0.14%equivalentU.)MineralizedstockworksonChickenMountainarethelodesource.Goldfinenessrangesfrom862–884.Placerdepositlargelyminedout.ReFeRenCes:Mertie,1936;Smith,1941;WhiteandKilleen,1953;JohnFullerton,oralcommun.,1983,1985;Bundtzenandothers,1987;Bundtzenandothers,1992;thisreport.

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105 Idaho(P);Alpha,Trail Au(Ag) Collectivelyproducedabout2,376kg(76,400 T.27N. Upgrade(M) Plutonic-hostedCu-Au oz)Auduringearlygoldrushyearsfrom R.47W. polymetallic;andPlacer 1910–1930 (Residual)

sUmmARy:Classicresidualplacerandalluvialheavymineralplacersnearoronbedrock.Paystreaksfoundinweatheredmon-zonitegrususuallybetweenbouldersandalsoinjointfractures.Heavymineralsincludezircon,blackpyroxene,scheelite,andcin-nabar.Someheavymineralsandswerecomposedofupto40%byvolumezircon.GoldfinenessatIdahorangesfrom854to901andaverages861.ReFeRenCes:Eakin,1914;MertieandHarrington,1924;Mertie,1936;Bundtzenandothers,1987;Bundtzenandothers,1992.

106 FlatCreek(M) Au(Ag) Officialrecordedproductionof14,837kg T.27N. Placer (477,039oz)Au;additional11,017kg R.47W. (354,210oz)AuproducedfromOtterandFlat Creekscombined

sUmmARy:LargestproducerofgoldinIditarodquadrangleandoneofAlaska’srichestplacerpaystreaks.Streamflows7kmfromslopesofChickenMountainnorthtoconfluencewithOtterCreek.Placerdepositsconsistof(1)richancestralpaychannelsonleftlimitofmainstreamandinMariettaBench;(2)limitedbutricheluvialplacerdepositsonsteepslopesatheadofcreek;and(3)lowergradebutmoreextensivemodernstreamalluviumthatreachesOtterCreek.The2–3mthickpaygravelsarecoveredby3–4mofoverburden.Heavymineralsincludezircon,scheelite,cinnabar,andstibnite.Goldfinenessaverages864.Averagegradeof3.67mil-lionm3ofpayworkedbyYukonGolddredge(1912–1918)was2.23g/m3Au.Largenuggetstypicallyweighedabout50g(1.6oz).LodesourceismineralizedstockworkonChickenMountain(no.130).Workedbydredges,opencuthydraulic,andmechanizedtractoroperationsfrom1910topresent.ReFeRenCes:Maddren,1911;Eakin1914;MertieandHarrington,1924;Mertie,1936;Smith,1941;Bundtzenandothers,1987;Richard,John,andTadFullerton,oralcommun.,1988;Bundtzenandothers,1992;thisreport.

107 OtterCreek(M) Au(Ag,W) Officialproductionof7,332kg(235,721oz) T.27N. Placer Auand952.6kg(30,628oz)Agfrom R.47W. 1910–1986.Additional11,017kg(354,210 oz)AuproducedfromFlatandOtter Creekscombined.GlennGulchproduced324 kg(10,421oz)Au

sUmmARy:SecondlargestproducerofgoldinIditarodquadranglebehindFlatCreek(no.106).Large,matureplacerpaystreak6–7kmlongandupto1kmwide.Bothterraceandmodernstreamalluvialplacersrecognized;somesemi-residualtypemayoccurnearmouthofBlackCreek(nearGlennandMinnieGulches).Workedextensivelybytwobucketlinestackerdredgesfrom1914–1966.Thesystemofpaystreaksisverycomplicated.Extensivemineactivitiesalsoincludeopencutminingmethods.Principalheavymineralsincludegold,scheelite,cinnabar,stibnite,arsenopyrite,magnesiochromite,andcassiterite.Goldfinenessrangesfrom822–891.Largenuggets,upto870g(28oz),foundinareaofGlennandMinnieGulches.ReFeRenCes:Maddren,1911;Eakin,1914;MertieandHarrington,1924;Mertie,1936;Smith1941;Bundtzenandothers,1987;JohnMiscovich,writtencommun.,1989;Bundtzenandothers,1992;thisreport.

108 Unnamed(P) Au,Ag(As,W) Samplecontains11ppmAu,7ppmAg,340 T.27N. [86BT422] Plutonic-hostedCu-Au ppmAs,50ppmW,and0.14ppmHg R.47W. polymetallic

sUmmARy:Iron-oxidestainedquartzveininsandstonehornfelsexploredbysmallpit;caveddrift.Quartzfloatindicatesveinisupto20cmthick;nosulfidesrecognized.Perhaps20tonsofrockindump.ReFeRenCes:McGimseyandothers,1988;Bundtzenandothers,1992;thisreport.

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109 BlackCreek(M) Au,Ag(W) Producedatleast868.5kg(27,925oz)Aufrom T.27N. Placer 1911–1981 R.47W.

sUmmARy:Drainageisa2.5-km-longsecond-orderstreamthatentersOtterCreekvalley.DissectsmineralizedBlackCreekmon-zoniticpluton,whichisthelodesourceofmuchofplacergoldinOtterCreek.Dredgeoperatedfrom1916–1918;minedbyopencutmethodsintermittentlyto1981.Principalheavymineralsincludemagnetite,ilmenite,cinnabar,chromite,scheelite,zircon,cassiterite,ilmenorutile,andargentopyrite.Platinummetalsfoundwithingoldgrains(1.3ppm).Goldfinenessis819(oneanalysis).ReFeR-enCes:Brooks,1912;Eakin,1914;JohnMiscovich,oralcommun.,1986;Bundtzenandothers,1987;Bundtzenandothers,1992.

110 GoldenHorn(M) Au,Ag,Pb,Zn(W,Sb, Produced84.2kg(2,707oz)Au,81.5kg(2,620 T.27N. Hg) oz)Ag,4,235kgPb,and296kgZnfrom R.47W. Plutonic-hostedCu-Au 1922–1937.Estimatedresourceofabout2.85 polymetallic Mt(3.15Mtons)grading1.2g/t(0.035 oz/ton)Au,3.4g/t(0.1oz/ton)Ag,and containingcreditsofWandSb

sUmmARy:Depositconsistsofquartz-tourmaline-calciteveinscontainingstibnite,cinnabar,scheelite,sphalerite,Pb-Sbsulfosalts,andchalcopyrite.Stibniteandcinnabarcrosscutarsenopyrite,scheelite,andsilversulfosalt.Veinsoccurinirregularlydistributed,quartz-filledshearzonesintheLateCretaceousOtterCreekpluton(monzonite),ornearintrusivecontacts.Veinsystemfrom3to30mwideandatleast1kminlength;occursalong3-km-longfaultzoneoneasternsideofpluton.PlutonintrudesgraywackeandshaleofCretaceousKuskokwimGroup.Minedbyundergroundworkingsbeginningin1922.Pumpswereremovedin1935;lastorewasshippedin1937.Privatediamond-drillprogramswereconductedduring1978–1981and1997–1998.ReFeRenCes:Maloney,1962;BundtzenandGilbert,1983;Bull,1988;Bundtzenandothers,1992;Noklebergandothers,1993;BundtzenandMiller,1997.

111 MalamuteGulchor Au,Ag Placeryielded59.3kg(1,907oz)Auand7.5 T.27N. Pup;lode(P),placer Plutonic-hostedCu-Au kg(241oz)Agfrom1912–1952 R.47W. (M) polymetallic,andPlacer

sUmmARy:Lodeprospectlocatedabout122m(400ft)westofthemouthofMalamuteGulch.Consistsof21m(70ft)longadit(nowcaved)reportedlymadeonquartz-calciteveinscontainingarsenopyriteandcinnabar;saidtohavecarriedconsiderablegold.Thenearbydrainage—MalamuteGulchorPup—wasminedintermittentlybetween1912and1952.Goldfinenessaverages832(7determinations). ReFeRenCes: Mertie, 1936; Smith, 1941; Maloney, 1962; Bundtzen and others, 1987; Bundtzen and others,1992;thisstudy.

112 SlateCreek(M) Au,Ag Producedatleast108.3kg(3,483oz)Auand T.27N. Placer 18.4kg(592oz)Agfrom1915–1951 R.47W.

sUmmARy:Modernstreamalluviumhasplacerpaystreak2–3kmlong.Goldirregularlydistributedandpayzonesareerratic.Miningceasedin1951duetolackofeconomicvalues.Goldfinenessis855.ReFeRenCes:Mertie,1936;MetzandHawkins,1981;Bundtzenandothers,1987;Bundtzenandothers,1992;thisreport.

113 Unnamed(A) Hg Samplescontainupto4ppmHg T.27N. [84GL14and84BT11] EpithermalHg-Sb(?) R.46W.

sUmmARy:Silica-carbonate(?)alteredmaficdikecontains4ppmHg(84GL14).Gossaninsandstonebrecciaabout0.5kmtoNEcontains1.7ppmHg(84BT11).ReFeRenCes:McGimseyandothers,1988;Bundtzenandothers,1992;thisreport.

114 Unnamed(A) (Hg,Au) Grabsamplecontains0.16ppmHgand50ppb T.27N. [84BT223] EpithermalHg-Sb(?) Au R.44W.

sUmmARy:SilicifiedbrecciainsandstonecontainsslightlyelevatedHgandAuvalues.ReFeRenCes:McGimseyandothers,1988;thisreport.

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115 Unnamed(O) Mn(Hg,Mo) Samplescontain>0.5%Mnandupto0.24ppm T.27N. [84BT218and Deposittypeuncertain Hgand10ppmMo R.43W. 84BT219]

sUmmARy:Extensivezoneofpyrolusite-bearingmassesinshearedsandstone,shale,andvolcaniclasticconglomerate.Zoneisupto1mthickandcanbetracedforatleast300malongstrike(possiblyasedimentaryMnhorizon).ReFeRenCes:McGimseyandothers,1988;thisreport.

116 Unnamed(O) Au Visiblegoldinheavy-mineralconcentrate T.27N. [I0138] Placer R.43W.

sUmmARy:Finegoldgrainsnotedinnonmagnetic,heavy-mineral-concentratesamplefromsmallnorth-flowingtributaryofBeaverCreek.ReFeRenCes:Bennettandothers,1988;thisreport.

117 Unnamed(A) Sb,As(Au) Samplecontains14ppmSb,20ppmAs,and T.27N. [85AM118] EpithermalHg-Sb(?) traceAu R.42W.

sUmmARy:QuartzfracturefillinginKuskokwimGroupsandstonecontainselevatedSb,As,andtraceAu;Hgwasnotanalyzed.ReFeRenCes:McGimseyandothers,1988;thisreport.

118 BismarckCreek(O) Sn,Cu,Ag,Zn(Pb,As, Inferredreserveof498,000tgrading0.137% T.27N. [84BT34and Bi,Sb,In) Sn,47.8g/tAg,0.16%Cu,0.22%Zn,and R.42W. 86AM104] Plutonic-relatedAg-Sn anomalousF,Bi,Cd,As,Pb,Sb,andB. polymetallic Containsupto116ppmIn(Indium)

sUmmARy: An extensive east-west- to northeast-trending zone of stockwork-like cassiterite-axinite-tourmaline-quartz-bearinggossaninhornfelsthatcanbetracedforatleast300mandforwidthsrangingfrom2upto30m.Sulfidesareheavilyoxidized;blackcassiteritehasbeenidentifiedinpolishedsection.Occurrencehasabrecciatedstyleshowingblocksofalteredsandstoneandsiltstonethatfloatinquartzgossanmatrixwithinhosthornfels.Nointrusiverocksrecognized,but10km2hornfelsaureolesurroundingVABM2424suggestspresenceofconcealedplutonbeneathmineralizedzone.Snvaluesrangefromtraceamountsto2.8%.Seventeenchipchannelsamplestakenatuniformintervalsalong290mofstrike,overanaveragewidthof0.5m,formbasisofinferred“halfsquare”reserveestimate.ReFeRenCes:McGimseyandothers,1988;Noklebergandothers,1993;BundtzenandMiller,1997;thisstudy.

119 Unnamed(A) Cu,Sb(Ag,As) Grabsamplecontains500ppmCu,140ppm T.27N. [84BT250] Plutonic-relatedAg-Sn Sb,0.5ppmAg,and20ppmAs R.42W. polymetallic(?)

sUmmARy:Localgossaninfine-grainedsandstoneassociatedwithshaleysectionofKuskokwimGroup.ReFeRenCes:McGim-seyandothers,1988;thisreport.

120 Unnamed(O) Ag,Sn,Zn,As(Sb,Cd, Sampleslocallycontainupto50ppmAg,100 T.27N. [86BT128and Cu) ppmAs,2,000ppmB,72ppmCd,150ppm R.42W. 84MSL125] Plutonic-relatedAg-Sn Cu,60ppmSb,150ppmSn,7,000ppmZn, polymetallic and2,000ppmB

sUmmARy:Rubbleof1–3cmthickquartz-axiniteveinsinhornfelssandstonegossan.ReFeRenCes:McGimseyandothers,1988;thisreport.

121 Unnamed(A) Sn,Zn,As(Ag) Sampleofveincontains300ppmSn,400ppm T.27N. [86AM105] Plutonic-relatedAg-Sn Zn,130ppmAs,2.2ppmCd,>2,000ppmB, R.42W. polymetallic andtraceAg

sUmmARy:Rubbleofthinquartz-axiniteveinsinhornfelssandstonegossan.ReFeRenCes:McGimseyandothers,1988;thisreport.

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122 Unnamed(A) Sn,Ag Samplescontainsupto5ppmAgand>2,000 T.27N. [84AAi557and Plutonic-relatedAg-Sn ppmB;onesamplecontains1,000ppmSn R.42W. 84AAi558] polymetallic

sUmmARy:Minorrubbleofquartz-axiniteveinsmixedwithrubbleofsandstoneandhornfelssandstone.ReFeRenCes:McGim-seyandothers,1988;thisreport.

123 Unnamed(A) Au(Hg) Samplecontains0.1ppmAuand0.5ppmHg T.27N. [84GL37] Granite-porphyry-hosted R.41W. Au-polymetallic

sUmmARy:Grabsampleof250-m-widefelsicdikecontainslow-gradeAuandHganomalies.Dikemaybeanortheastextensionofanauriferousgraniteporphyrydikeswarmthatcropsout4kmtothesouthwestintheGraniteCreekarea.ReFeRenCes:McGim-seyandothers,1988;thisreport.

124 Unnamed(O) Ag,Zn,Sn(Hg) Quartzveincontains0.5ppmAg,900ppmZn, T.27N. [86BT122] Deposittypeuncertain 150ppmSn,20ppmAs,0.96ppmHg,1.9 R.37W. ppmCd,and2,000ppmB

sUmmARy:Thinquartzveinlets(uncommon) inhematitic, iron-stained,sandstoneof theKuskokwimGroup.ReFeRenCes:McGimseyandothers,1988;thisreport.

125 Unnamed(A) Hg Samplecontains1.2ppmHg T.26N. [85AM183] R.52W.

sUmmARy:GrabsampleofweldedrhyolitetuffoftheYetnaRivervolcanicfieldcontainslow-gradeHganomaly.ReFeRenCes:McGimseyandothers,1988;MillerandBundtzen;1994;thisreport.

126 Unnamed(A) Au(Sb) Grabsamplescontainupto50ppbAuand18 T.26N. [85AM202] Volcanic-hosted ppmSb R.51W.

sUmmARy:DaciteflowrocksoftheYetnaRivervolcanicfieldcontainlow-gradeAuandSbanomalies;hematiteveinletsfoundlocally.ReFeRenCes:McGimseyandothers,1988;MillerandBundtzen;1994;thisreport.

127 Unnamed(A) Sn Samplecontains100ppmSn T.26N. [85AM206] Deposittypeuncertain R.51W.

sUmmARy:GrabsampleofdevitrifiedandesitefromYetnaRivervolcanicfield.ReFeRenCes:McGimseyandothers,1988;MillerandBundtzen;1994;thisreport.

128 Unnamed(A) Sn,Sb,As Grabsamplecontains20ppmSn,20ppmSb, T.26N. [85BT260] Plutonic-relatedAg-Sn 40ppmAs,and2,000ppmB R.48W. polymetallic(?)

sUmmARy:Contactareabetweenintrusivebodyandhornfelssedimentaryrock;nobrecciaorhydrothermalalterationobserved.ThesamplelocationisincorrectlyplottedinMcGimseyandothers(1988);itactuallylies2.75kmsouth-southwestofwheretheyshowit.ReFeRenCes:McGimseyandothers,1988;thisreport.

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129 WillowBench(M) Au,Ag Producedatleast1,305kg(41,948oz)Auand T.26N. Placer 156.5kg(5,033oz)Agfrom1915–1986 R.47–48W.

sUmmARy:MajorsystemofancestralchannelsofWillowCreek,whichpartlyheadsintoHappyCreekdrainage(no.104).Paycon-tainedinterracealluviumonperchedbenchlevels20–30mabovemoderndepositsofWillowCreek,thelatterofwhichcontainlowergradeplacergold.Paygravel,overlainby8to10moffrozenmuck,continuesforatleast4kmofstrikelength,50to200mofwidth,and2to3mofdepth.Goldenrichmentsapparentlyoccuratintersectionsbetweensmallgulches,whichdrainthesouthwestflankofChickenMountain,andthebenchgravels.Airphotoanalysisandexplorationdrillingsuggestsignificantresourcesmightoccurtothesouthwestofthemainbenchlevels,probablyinmultipleancestralchannels.Goldfinenessrangesfrom856to883andaverages874.Heavymineralsincludeilmenite,cinnabar,andzircon.ReFeRenCes:Mertie,1936;Smith,1941;Bundtzenandothers,1987;RichardandJohnFullerton,oralcommun.,1988;Bundtzenandothers,1992;thisreport.

130 ChickenMountain(P) Au,As,Sb,Cu,Hg,Mo Estimatedresourceofabout14.5Mt(16M T.26N. Plutonic-hostedCu-Au tons)grading1.2g/t(0.04oz/ton)Au,4.6g/t R.47W. polymetallic Ag,0.09%Cu,and0.46%Sb

sUmmARy:Depositconsistsofquartz-sulfideveinletscontainingawidevarietyoforemineralsincludingfreegold,stibnite,cin-nabar,arsenopyrite,chalcopyrite,molybdenite,silversulfosalts,andarsenianpyrite.Quartzveinscontainupto5%sulfides.Allmin-eralizedrockiscontainedincupolazonesofalteredmonzoniteandsyeniteofChickenMountainstock.Earliermonzodiorite,alkaligabbro,andwehrlitephasesalsopresent.Pervasivesericiteandankeritealterationhalospresent.Dolomitebrecciaphasesynchronouswithmajorsulfidephase.Surfaceextentofveinsoccupiesa300mby800marea;drillingindicatesatleast250mofverticalextent.Diamond-drillprogramswereconductedduring1987–1990and1997–1998.Drillresultsandmappingindicateaverticaltemperaturezonation is present; epithermal gold-mercury-antimony zones crosscut older mesothermal gold-copper-molybdenum and arsenic-copperevents.PlutonandoreveinsyieldcoevalK-Aragesof70Ma.ReFeRenCes:JasonBressler,writtencommun.,1980;Bull,1988;RichardGosse,writtencommun.,1990;VictorHollister,writtencommun.,1992;Bundtzenandothers,1992;Noklebergandothers,1993;BundtzenandMiller,1997;thisreport.

131 ChickenCreek(M) Au(Ag) Producedatleast771kg(24,800oz)Auand T.26N. Placer 98.7kg(3,174oz)Agfrom1912–1985 R.47W. (records1940–1980absent)

sUmmARy:Modernstreamalluviumandalluvialfanplacerdepositson6-km-long,south-flowingstreamthaterodesthecentralandsouthernpartsofChickenMountainpluton.Uppermostplacersarepartiallyresidualonmineralizedstockwork(no.130);lower-mostpaystreaksarepartofanalluvialfanadjacenttoBonanzaCreek.Landslidemidwaydowncreekhasburiedplacerpaystreak.Goldfinenessrangesfrom850to870andaverages861.Principalheavymineralsincludeilmenite,chromite,andcinnabar.ReFeR-enCes:Mertie,1936;Bundtzenandothers,1987;Bundtzenandothers,1992;thisreport.

132 PrinceCreek(M) Au,Ag(Hg) Producedatleast1,053kg(33,864oz)Auand T.26N. Placer 123.8kg(3,979oz)Agfrom1913–1990 R.47W.

sUmmARy:Streamsystemofauriferousmodernalluviumandseveralancestralchannelsonasymmetrical left limit that trendssoutheastandsouthfor8kmbeforeemptyingintoBonanzaCreek.StreamheadsintosoutheastflankofChickenMountain.OriginalPrinceCreekchannelbeheadedbyadvancingChickenCreekvalley;hence,placergoldandheavymineralsfrombothPrinceandChickenCreekshaveidenticalChickenMountainlodesource.Principalheavymineralsincludecinnabar,chromespinel,ilmenite,zircon,andgarnet.(Garnetfoundwherelowerpaystreaksoverliesgraniteporphyry.)Goldfinenessrangesfrom838to886.Benchplacers have higher fineness.Has potential for development of placer reserves on relatively unexploited bench levels.ReFeR-enCes:Mertie,1936;Bundtzenandothers,1987;Bundtzenandothers,1992;thisreport.

133 Unnamed(O) Au,Ag,W,Bi(Hg) Samplecontains3.4ppmAu,10ppmAg,1,380 T.26N. [84BT98] Plutonic-hostedCu-Au ppmW,78ppmBi,and0.82ppmHg R.47W. polymetallic

sUmmARy:Hornfelsandmonzonitedike(?)atPrinceCreekplacercut.VeinletsstrikeN.20°E.anddip75°E.Cinnabarnotedinvein.ReFeRenCes:Bundtzenandothers,1992;thisreport.

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134 Unnamed(A) Hg Samplecontains1.2ppmHg T.26N. [86AMc216] EpithermalHg-Sb(?) R.46W.

sUmmARy:GrabsampleofgraniteporphyrysillcontainsslightlyelevatedHgvalue.ThesamplelocationisincorrectlyplottedinMcGimseyandothers(1988).ReFeRenCes:McGimseyandothers,1988;thisreport.

135 Unnamed(O) Au,Hg Samplescontainupto2.9ppmAuand1.7ppm T.26N. [86BT390and Granite-porphyry-hosted Hg R.46W. 86BT391] Au-polymetallic

sUmmARy:Quartzstockworkveinletsintrudesmall1–2mwidezoneingraniteporphyrysillalongBonanzaCreekupstreamfrommouthofFirstChanceCreek.Largerzoneofsilicificationrecognized.ReFeRenCes:McGimseyandothers,1988;Bundtzenandothers,1992;thisreport.

136 Unnamed(O) Sb,Ag,Pb,As(Au) Grabsamplescontain>1.0%Sb,upto1ppm T.26N. [86AGe73] Granite-porphyry-hosted Ag,300ppmPb,120ppmAs,andtraceAu R.46W. Au-polymetallic

sUmmARy:Quartz-stibniteveinupto1mthickneargraniteporphyrydikeorsill.Veinheremayberelatedtoveinsatlocalityno.135.ReFeRenCes:McGimseyandothers,1988;Bundtzenandothers,1992;thisreport.

137 Unnamed(A) Hg Samplecontains1.4ppmHg T.26N. [86BT404] EpithermalHg-Sb(?) R.45W.

sUmmARy:GrabsampleofalteredgraniteporphyrydikecontainsslightlyelevatedHg.ReFeRenCes:McGimseyandothers,1988;Bundtzenandothers,1992.

138 Unnamed(A) Hg Samplecontains2.3ppmHg T.26N. [86AGe62] EpithermalHg-Sb(?) R.45W.

sUmmARy:Grabsampleofiron-oxidestainedsandstoneinvicinityofintermediatedikecontainsslightlyelevatedHg.ReFeR-enCes:McGimseyandothers,1988;thisreport.

139 Unnamed(P) Sb,Hg,As Samplecontains54ppmSb,0.9ppmHg,100 T.26N. [84MSL146] EpithermalHg-Sb(?) ppmAs,and1,000ppmCr R.45W.

sUmmARy:Prospectpitonaltereddike.Quartz-calciteveinscontainingminorsulfidesnoted.ReFeRenCes:McGimseyandothers,1988;thisreport.

140 Unnamed(A) Hg Samplecontains1.2ppmHg T.26N. [84MSL143] EpithermalHg-Sb(?) R.45W.

sUmmARy:GrabsampleoffelsicdikecontainsslightlyelevatedHgvalue.ReFeRenCes:McGimseyandothers,1988; thisreport.

141 Unnamed(A) As,Hg Samplecontains60ppmAsand1.1ppmHg T.26N. [86AGe68] EpithermalHg-Sb R.44W.

sUmmARy:SamplefromsmallareaoflimonitegossanchipsinKuskokwimGroupsandstone;containsslightlyelevatedAsandHgvalues.ReFeRenCes:McGimseyandothers,1988;thisreport.

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142 WillowCreek/Granite Au(Ag) Producedestimated101.1kg(3,250oz)Auand T.26N. Pup(M) Placer 12.4kg(400oz)Agfrom1924–1990 R.42W.

sUmmARy:PlacergolddiscoveredonGranitePup in1924andcreekhasbeenmined intermittently to thepresentusingsmallundergrounddriftandopencutminingmethods.About1.5kmofpaystreaklengthknown.Auriferouszonesinveryshallow1–2mthickgravelscoveredby1–2mofoverburden.Sourceofatleastsomeplacergoldisgraniteporphyrydikeswarm,whichpartiallyunderliesplacerdeposits.Asmallmonzonitepluglieseastofmineandmayalsobealodesource.Placerpaystreaksaverage1.0g/m3overlargearea;reservebasewillprobablyincreasewithexploration.Principalheavymineralsareilmenite,zircon,cinnabar,garnet,cassiterite,stibnite,andpyrite.Goldfinenessranges838to871.ReFeRenCes:TonyGularte,oralcommun.,1986;Bundtzenandothers,1986;Szumigala,1993;thisreport.

143 WyrickLode(P) Sb,Ag,Au(Hg) Samplescontainupto31%Sb,0.8ppmAu,10 T.26N. [85BT112and Granite-porphyry-hosted ppmAg,and3ppmHg R.42W. 85BT275] Au-polymetallic

sUmmARy:Quartz-stibnitegashveins3–30cmwideinshearedrocksalongcontactofgraniteporphyrywithKuskokwimGroupsiltstone.Stibnitegashveinsrangefrom0.55%to30.9%Sbandaverage14.5%Sb.Averagesilvergradeisapproximately4g/t(0.11oz/ton)andaveragegoldgradeisapproximately0.7g/t(0.02oz/ton).Probablelodesourceofplacergoldinno.142.ReFeRenCes:Bundtzenandothers,1986;Szumigala,1993.

144 Unnamed(P) Ag,Sb(As,Hg,Au) Samplescontainupto0.5ppmAg,48ppmSb, T.26N. [84BT41and Granite-porphyry-hosted 0.56ppmHg,40ppmAs,andtraceAu R.41W. 85BT277] Au-polymetallic

sUmmARy:Oldprospectpitsexploringgraniteporphyrydikeswerereopenedin1985.Smallcalciteandquartzveinletscutdike.Freegoldpannedfromcrushedsamples.ReFeRenCes:Bundtzenandothers,1986;McGimseyandothers,1988;thisreport.

145 Unnamed(O) Sn,Cu,Zn,Pb(Ag,Sb, Grabsamplescontainupto>1,000ppmSn,1 T.26N. [84GL36and Hg) ppmAg,>2,000ppmB,500ppmCu,350 R.41W. 84AM60] Plutonic-relatedAg-Sn ppmZn,100ppmPb,160ppmAs,54ppm polymetallic Sb,and1.2ppmHg

sUmmARy:Largelimonite-tourmalinebrecciazoneinhornfelssandstonenearmonzonitecontact;monzoniteitselfcontainsnumer-ousquartz-tourmalinestockworkveins.Thelocationfor84GL36wasincorrectlyshowninMcGimseyandothers(1988).ReFeR-enCes:McGimseyandothers,1988;thisreport.

146 Unnamed(A) Zn,Cu,Ag(Cd,Bi) Samplescontainupto860ppmZn,300ppm T.26N. [84AM186and Plutonic-relatedAg-Sn Cu,2ppmAg,5.1ppmCd,and10ppmBi R.41W. 84BT44] polymetallic

sUmmARy:Grab samplesof sandstonehornfelsnear southerncontactofmonzonite intrusioncontainanomalousmetalvalues.ReFeRenCes:McGimseyandothers,1988;thisreport.

147 Unnamed(A) Cu,As,Bi,Ag Samplecontains460ppmAs,500ppmCu,10 T.26N. [84BT38] Plutonic-relatedAg-Sn ppmBi,and1ppmAg R.41W. polymetallic(?)

sUmmARy:Gossanareas insandstonehornfels;disseminatedpyritenoted.ReFeRenCes:McGimseyandothers,1988; thisreport.

148 MuntherCreek(P) Au Mayhavehadsmallproduction T.26N. Placer R.41W.

sUmmARy:Small,3-km-long,second-orderstreamdrainssouthsideofridgeintrudedbymonzoniteplugandgraniteporphyrydikes.Fine-grainedplacergoldpannedfromprospectpitsin1920’sand1930’s.ReFeRenCes:TonyGularte,oralcommun.toT.K.Bundtzen,1986;Bundtzenandothers,1986.

78



149 Unnamed(O) Sb,As(Hg) Grabsamplecontains390ppmSb,850ppm T.26N. [86AM387] EpithermalHg-Sb(?) As,0.9ppmHg,and>20%Fe R.41W.

sUmmARy:Chipsofheavilyiron-oxidestainedsandstonearefoundlocally.ThesamplelocationwasincorrectlyshowninMcGim-seyandothers(1988)tobeapproximately1kmsouthoftheactualcollectionsite.ReFeRenCes:McGimseyandothers,1988;thisreport.

150 Unnamed(O) Ag,Sn,Pb,Sb,As,Bi Samplescontainupto7ppmAg,300ppmSn, T.26N. [84AM220] Plutonic-relatedAg-Sn 500ppmPb,54ppmSb,100ppmAs,54ppm R.40W. polymetallic Bi,and>2,000ppmB

sUmmARy:ExtensiveboronmetasomatisminGraniteMountainpluton.Tourmaline-richquartzporphyryandtourmaline-cementedbrecciasoccurlocallyinupperportionofpluton.Disseminatedcassiteriteobservedinonesamplenearsite;nosulfidesobserved.ReFeRenCes:McGimseyandothers,1988;thisreport.

151 GraniteMountain(O) Sn,Ag,Au,Bi,Sb,As Samplescontainupto100ppmSn,0.5ppmAg, T.26N. [84BT231and Plutonic-relatedAg-Sn 50ppbAu,9ppmBi,28ppmSb,80ppmAs, R.40W. 84AM218] polymetallic and>2,000ppmB

sUmmARy:Extensiveboronmetasomatisminstockalongnortherncontactwithhornfels;occursastourmalinesheetedzones,tour-malinecementedbreccias,andaxinite-tourmalinereplacementsinplutonicrocksandhornfels.Onelargeareaoftourmalinebrecciais400mlong,15mwide,cylindricalincrosssection,andmaycontainapproximately5milliontonnesofboronalteration.Averagemetalcontentofthispipenotascertained.ReFeRenCes:McGimseyandothers,1988;BundtzenandMiller,1997;thisreport.

152 Unnamed(O) Zn,HgAs(Bi,Sb) Grabsamplecontains450ppmZn,>10ppm T.26N. [86AMc219] Plutonic-relatedAg-Sn Hg,400ppmAs,7ppmBi,22ppmSb,and R.40W. polymetallic 15%Fe

sUmmARy: Grab sample of hematite-cemented, limonite-stained breccia of Kuskokwim Group sandstone. ReFeRenCes:McGimseyandothers,1988;thisreport.

153 Unnamed(A) Ag,Pb(Sn,Sb) Samplescontainupto1.5ppmAg,200ppmPb, T.26N. [84AAi512] Plutonic-relatedAg-Sn 15ppmSn,12ppmSb,and>2,000ppmB R.40W. polymetallic

sUmmARy:Grabsamplesoftourmaline-richquartzmonzoniteborderphase.ReFeRenCes:McGimseyandothers,1988;thisreport.


sUmmARy:Visiblegoldinnonmagnetic,heavy-mineral-concentratesample.StreamdrainssoutheastsideofGraniteMountain,cut-tingthroughquartzmonzoniteandhornfelsKuskokwimGroupsedimentaryrock.ReFeRenCes:Bennettandothers,1988;MillerandBundtzen,1994;thisreport.

155 Unnamed(A) Ag,Pb,As(Bi,Sb) Samplecontains0.5ppmAg,200ppmPb,10 T.26N. [86AMc7] Deposittypeuncertain ppmBi,100ppmAs,and12ppmSb R.39W.

sUmmARy:Grabsampleoffine-tomedium-grainedsandstonecontainselevatedvaluesofAg,Pb,As,Bi,andSb;Hgwasnotana-lyzed.Nosulfidesoralterationwerenoted.ReFeRenCes:McGimseyandothers,1988;thisreport.

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156 Yousure(O) Hg,Sb,W,As(Ag,Ba) Grabsamplescontainupto15%cinnabar, T.25N. [85AM172and EpithermalHg-Sb >1.0%As,0.46%Sb,50ppmW,traceAg, R.49W. I0160] and5,000ppmBa

sUmmARy:Quartz-cinnabar-stibniteveinsandveinletscutsiltstoneofKuskokwimGroupandsilica-carbonatealteredmaficdikes;scheeliteidentifiedinonesample.Small,deepred,euhedralcinnabarcrystalsarelocallyabundantinthequartzveinrubble.Grabsamplescontainupto15%cinnabarand5%stibnite.OccurrenceexposedasrubblecropinasteepcutbankonthenorthsideoftheIditarodRiver.Quartzveinsarenomorethan5cmthick,butoneveinwastracedfor30m.Themineralizedrockisconcentratedina30-m-widezone.ReFeRenCes:McGimseyandothers,1988;thisreport.

157 Unnamed(A) Au,Hg(Ag) Samplecontains50ppbAu,1.8ppmHg,and T.25N. [86AM408] Granite-porphyry-hosted traceAg R.47W. Au-polymetallic(?)

sUmmARy:Grabsampleofiron-oxide-stained,sericiticallyalteredgraniteporphyryintrusivecontains50ppbAuandtraceAg.ReFeRenCes:McGimseyandothers,1988;thisreport.

158 Unnamed(A) Hg,As Grabsamplecontains2.0ppmHgand20ppm T.25N. [84AM93] EpithermalHg-Sb(?) As R.46W.

sUmmARy:Grabsampleofiron-oxide-stained,sericiticallyalteredgraniteporphyrydikecontainsslightlyelevatedHgvalue;dikeislocalizedalongnortheast-trendinghigh-anglefault.ReFeRenCes:McGimseyandothers,1988;MillerandBundtzen,1994;thisreport.

159 Unnamed(O) Ag,Sn,As(Bi,Cd,Sb) Quartzveinrubblecontains10ppmAg,100 T.25N. [84BT73] Granite-porphyry-hosted ppmSn,>0.2%As,11ppmBi,3.3ppmCd, R.44W. Au-polymetallic(?) and12ppmSb

sUmmARy:Graniteporphyryintrusivecontainsdisseminatedarsenopyriteandquartzveinlets.Extensivegossanzone(40mby40m)exposedinrubble.ReFeRenCes:McGimseyandothers,1988;thisreport.

160 JulianCreek(M) Au(Ag) Producedatleast361kg(11,600oz)Auand T.24–25N. Placer 51.3kg(1,650oz)Agbetween1911and1993 R.44W.

sUmmARy:Small,butrelativelyrich,3-km-long,second-orderstreamdrainseastwardtoGeorgeRiver.Shallowplacerdepositoccursinmodernstreamalluviumandinlowbenchlevelsonnorthlimit.Payzonesare10–30mwideandarecoveredwith2mofcolluviumfromhillslopes.Principalheavymineralsincluderadioactivemonazite,barite,zircon,cinnabar,chromite,magnesiochro-mite,andstibnite.Goldfinenessrangesfrom657to840(n=3).Mineralizedgraniteporphyrybody(no.159)atheadofcreekisprob-ablesourceofgold.Depositwasminedsporadicallyfrom1911to1939;andthenagainfromabout1979to1993.Muchofpaystreakworkedout,butrichfractionsremain.ReFeRenCes:Smith,1941;Cobb,1973;LarryWilmarth,oralcommun.,1984;Bundtzenandothers,1987;thisreport.

161 MichiganBench(M) Au(Ag) Producedabout3.9kg(125oz)Auand0.3kg T.25N. Placer (9oz)Agin1980’s R.43–44W.

sUmmARy:SmallremnantterracealluvialdepositonsouthlimitofMichiganCreek.TerracealluviumoccurswheresmallgulchintersectsvalleyofMichiganCreek;gulchdrainsalargegraniteporphyrydikeswarm—thesuspectedsourceofgold.Placerconcen-tratescontaincinnabar,garnet,andmagnetite.ReFeRenCes:GlennBass,oralcommun.,1984;Bundtzenandothers,1987;thisreport.

162 Unnamed(O) Au,Sb,Hg,As,W Samplescontainupto250ppbAu,2,000ppm T.25N. [85AM320,85AM322, Granite-porphyry-hosted Sb,>14ppmHg,610ppmAs,200ppmW, R.43W. and86AM429] Au-polymetallic(?) 5ppmMo,and20%Fe

sUmmARy:Gossanandhematite-healedbrecciainhornfelszonesurroundingsmall,alteredgraniteporphyryintrusion.Brecciasofbothsedimentaryrockandvolcanicrockhornfelshostthemineralizedrock.Encouragingmineraloccurrence—consistsofmultipleoutcropsover0.1mi2area.ReFeRenCes:McGimseyandothers,1988;thisreport.

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163 Unnamed(O) Sb,Sn,As Samplescontainupto10,000ppmSb,200ppm T.25N. [85DB284] Deposittypeuncertain Sn,and60ppmAs R.43W.

sUmmARy:Rubblecropofbrecciatedhornfelscontainingquartzandstibniteveinsfoundnearcreeklevelbyhelicopterpilot,DavidBlair.ReFeRenCes:McGimseyandothers,1988;thisreport.


sUmmARy:Rubblechipsofiron-oxide-stainedgraniteporphyrydikecontainslightlyanomalousHgvalue.ThesamplelocationisincorrectlyplottedinMcGimseyandothers(1988);itactuallyliesapproximately9kmeastofwheretheyshowit.ReFeRenCes:McGimseyandothers,1988;thisreport.

165 Unnamed(A) Cu,Ag,As,Pb,Sb(Bi) Grabsamplecontains2ppmAg,700ppmCu, T.25N. [86AM388] Plutonic-relatedAg-Sn 600ppmAs,50ppmSb,150ppmPb,3ppm R.41W. polymetallic Bi,20%Fe,and>2,000ppmB

sUmmARy:Limitedrubbleofiron-oxidestainedsandstonehornfelsinvicinityofmonzonitedikeswarm;5kmsouthofGraniteMountainpluton.ReFeRenCes:McGimseyandothers,1988;thisreport.

166 Unnamed(A) Sb Samplecontains110ppmSb T.24N. [85BT101] EpithermalHg-Sb(?) R.50W.

sUmmARy:GrabsampleofbasalticandesitefromtheIditarodVolcanicscontainsanomalousSbvalue;Hgwasnotanalyzed.ReF-eRenCes:McGimseyandothers,1988;MillerandBundtzen,1994;thisreport.

167 Unnamed(A) Sb(Cu,Mo) Grabsamplecontains240ppmSb,100ppm T.24N. [85BT106] EpithermalHg-Sb(?) Cu,and7ppmMo R.50W.

sUmmARy:Grabsampleofhydrothermallyalteredolivine-bearingbasalticandesitefromtheIditarodVolcanicscontainsanomalousSbandslightlyelevatedCuandMo;Hgwasnotanalyzed.ReFeRenCes:McGimseyandothers,1988;MillerandBundtzen,1994;thisreport.

168 Unnamed(A) Zn,Hg Samplescontainupto240ppmZnand1.8ppm T.24N. [86AM414] Deposittypeuncertain Hg R.47W.

sUmmARy: Sericitically altered granite porphyry dike is cut by small quartz veinlets that have limonitically stained alterationenvelopes;grabsamplescontainanomalousZnandslightlyelevatedHgvalues.ReFeRenCes:McGimseyandothers,1988;thisreport.

169 Unnamed(A) (As) Samplecontains700ppmAs T.24N. [84BT275] Deposittypeuncertain R.47W.

sUmmARy:GrabsampleofgraniteporphyryplugcontainsanomalousAsbyemissionspectrographicanalysis,butnotbyatomicabsorptionspectrophotometry.Sbvalueisslightlyelevated;Hgwasnotanalyzed.ReFeRenCes:McGimseyandothers,1988;thisreport.

170 Unnamed(A) Au,Hg Grabsamplecontains50ppbAuand1.3ppm T.24N. [86AM410] Granite-porphyry-hosted Hg R.47W. Au-polymetallic(?)

sUmmARy:Grab sample of iron-oxide-stained, sericitically alteredgranite porphyry intrusive contains 50ppbAu and slightlyelevatedHg.ReFeRenCes:McGimseyandothers,1988;thisreport.

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171 Unnamed(A) Hg,As Samplecontains4ppmHgand40ppmAs T.24N. [85GL20] EpithermalHg-Sb(?) R.47W.

sUmmARy:GrabsampleofgraniteporphyrydikecontainselevatedHgandAs.ReFeRenCes:McGimseyandothers,1988;thisreport.

172 SpruceCreek(M) Au(Ag) Producedatleast8.5kg(274oz)Aupriorto T.24N. Placer 1984 R.44W.

sUmmARy:Second-orderstreamflows3kmsoutheasttoGeorgeRiver;verynarrow5–15mwidepaystreakburiedby4–6mofcolluvium.Low-gradesourceofgoldthoughttobegraniteporphyryintrusionatheadofcreek.Principalheavymineralsincludemag-netite,garnet,ilmenite,andcinnabar.Goldfinenessis897(onedetermination).Goldfirstdiscoveredin1911,butonlythemostrecentproduction(1979–1984) isknown.Thought tobenearlyminedout.ReFeRenCes:Brooks,1912;L.E.Wyrick,oralcommun.,1985;Bundtzenandothers,1987;thisreport.

173 Unnamed(A) Hg Dikecontains8ppmHg;sandstonecontainsup T.24N. [85AM327] EpithermalHg-Sb(?) to150ppmNi R.43W.

sUmmARy:East-west-trending,limonite-stained,garnet-bearinggraniteporphyrydikeintrudesKuskokwimGroupsandstone.GrabsampleofdikecontainsanomalousHg.LimoniticsandstonegrabsamplescontainanomalousNi.ReFeRenCes:McGimseyandothers,1988;thisreport.

174 Unnamed(O) Ag,Zn,Cu,Sb(As,Bi) Gossancontains0.7ppmAg,640ppmZn,500 T.23N. [85AM123] Plutonic-hostedCu-Au ppmCu,110ppmSb,6ppmBi,40ppmAs, R.53W. polymetallic(?) and20%Fe

sUmmARy:Scatteredgossan rubble inhornfelssiltstone, sandstone,andconglomerate5kmsouthwestofsummitofMosquitoMountain,nearareacontainingsmallintermediatesills(?).GossangrabsamplecontainsanomalousAg,As,Bi,Cu,Sb,Zn,andFe.GrabsamplesofthehornfelssedimentaryrockshowelevatedAg,As,B,Bi,Sb,andZn.Hgwasnotanalyzed.ReFeRenCes:McGimseyandothers,1988;MillerandBundtzen,1994;thisreport.

175 Unnamed(A) Sb Samplecontains38ppmSb T.23N. [85BT124] EpithermalHg-Sb(?) R.53W.

sUmmARy:Grabsampleofintermediatevolcanic(?)rockinterbeddedwithsandstonecontainselevatedSb;Hgwasnotanalyzed.ReFeRenCes:McGimseyandothers,1988;thisreport.

176 MosquitoMountain(A) Ag,Sb,As(Hg) Grabsamplescontainupto0.7ppmAg,50ppm T.23N. [I0048and85AM74] Deposittypeuncertain As,38ppmSb,and1.4ppmHg R.53W.

sUmmARy:Heavilyiron-oxidestainedsandstonehornfelsneartopofMosquitoMountaincontainselevatedvaluesofAg,As,Sb,andHg.ReFeRenCes:McGimseyandothers,1988;thisreport.

177 Unnamed(A) (Ag) Samplecontains0.5ppmAg T.23N. [84AM239] Deposittypeuncertain R.52W.

sUmmARy:Grabsampleofiron-oxide-stainedpebbleconglomeratefromshorelinefaciesoftheKuskokwimGroupcontainsslightlyelevatedAg.ReFeRenCes:McGimseyandothers,1988;MillerandBundtzen,1994;thisreport.

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178 Unnamed(A) Hg,As,Zn Samplescontainupto1.2ppmHg,110ppm T.23N. [85AM80,85AM81, Deposittypeuncertain Zn,60ppmAs,andtraceAg R.52W. and86AM259]

sUmmARy:Limonitic,secondaryhematite-bearingsandstoneandquartz-veinedsandstonefoundlocallyinvicinityofalteredpor-phyriticfelsicandintermediatedikes.Onesecondaryhematite-bearingsandstonesamplecontainstraceAgand60ppmAs(Hgnotanalyzedinthissample).ReFeRenCes:McGimseyandothers,1988;thisreport.

179 Unnamed(A) Zn,As,Sb(Hg,Ag) Samplecontains200ppmZn,400ppmAs,42 T.23N. [I1479] Deposittypeuncertain ppmSb,0.5ppmHg,50ppbAg,and20%Fe R.52W.

sUmmARy:Grab sampleof iron-oxide-stained sandstone thathasfinequartzveinlets contains anomalousAs,Zn, andSb, andelevatedHgandAg.ReFeRenCes:McGimseyandothers,1988;thisreport.


sUmmARy:GrabsampleofalteredfelsicdikethatintrudesKuskokwimGroupsandstonecontainsslightlyelevatedHg.ReFeR-enCes:McGimseyandothers,1988;thisreport.

181 LittleCreek(M) Au(Hg) Minorproductionduringtesting,butnot T.22–23N. [I0056] Placer recorded R.51W.

sUmmARy:Fine-grainedplacergoldandcinnabarfoundalongthemiddlecourseofLittleCreek,whichoriginatesinSleetmutequadrangleandflowsnorth intoIditarodquadrangle.Goldfirst foundonLittleCreek in1910;reconnaissancedrilling tookplaceprobablypriorto1960.Depositthoughttobemodernalluvialplacer;terracealluviumnotrecognized.LittleCreekheadsintoareaunderlainbytheIditarodVolcanics,butspecificplacergoldsourceisnotknown.ReFeRenCes:Cadyandothers,1955;SpencerLyman,oralcommun.,1984;Bennettandothers,1988;MillerandBundtzen,1994;thisreport.

182 Unnamed(A) (Sb,As) Grabsamplecontains10ppmSband20ppm T.23N. [85AAi685] EpithermalHg-Sb(?) As R.51W.

sUmmARy:Sampleofiron-oxide-stainedKuskokwimGroupsandstonecontainsslightlyelevatedSbandAs;Hgwasnotanalyzed.ReFeRenCes:McGimseyandothers,1988;thisreport.

183 Unnamed(P) As,Hg(Sb) Samplescontainupto110ppmAs,0.92ppm T.23N. [85AAi682] EpithermalHg-Sb(?) Hg,and10ppmSb R.50W.

sUmmARy:Zoneofred-orange,iron-oxide-stainedcountryrock(mixedvolcanicflowandtuffunitoftheIditarodVolcanics)associ-atedwithrubbleofalteredfelsic(?)dikes.Oldclaimpostpresentnearaprospectpitmadeoniron-oxide-stainedcalcite-quartzveins;rubbleshowsveinsupto10cmwide.VeinsamplecontainsanomalousAsandSb,butHgwasnotanalyzed.SampleofalteredfelsicdikecontainselevatedvaluesofHgandAs.ReFeRenCes:McGimseyandothers,1988;MillerandBundtzen,1994;thisreport.

184 DeCourcyMountain Hg,As,Sb(Ag,Au) Produced51,700kg(1,500flasks)Hgfrom T.23N. mine(M) EpithermalHg-Sb about2,250t(2,480tons)orepriorto1949 R.50W.

sUmmARy:KuskokwimGroupsandstoneandshalecutbysill-likemaficbodies;mineralizedveinsandveinbrecciasarelocalizedalongintrusivecontactsandbeddingsurfaces.Cinnabarandminorstibniteoccurinlenses,veins,andstockworksaccompaniedbygangueofsilica,carbonate,andclayminerals.Also,ferriancinnabar,stibnite,andarsenopyriteoccurindistinctivechalcedonicbrec-ciathatfillsfracturesinbothsedimentaryandigneoushostrocks.Discoveredin1910–11;stakedin1919;operatedintermittently1921–49.InadditiontoHg,USGSgrabsamplescontainupto0.65ppmAg,200ppmAs,and1.0%Sb.BruceHickok(oralcommun.,1988)obtained400ppbAuvaluesfromDeCourcysamples.AtributarytoReturnCreekcontainsanunevaluatedamountofplacercin-nabarderivedfromDeCourcyMountainlode;claimsstakedinvariousyearsbetween1908–53.ReFeRenCes:Webberandothers,1947;Malone,1962;SainsburyandMacKevett,1965;McGimseyandothers,1988;Noklebergandothers,1993;thisreport.

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185 Unnamed(A) Hg(Sb,As,Zn) Grabsampleofdikecontains1.78ppmHg,6 T.23N. [85AAi676] EpithermalHg-Sb(?) ppmSb,20ppmAs,and100ppmZn R.49W.

sUmmARy:Hematite-alteredKuskokwimGroupsandstoneinvicinityofhematite-alteredporphyriticfelsicdikerubble.Dikecon-tainsslightlyelevatedvaluesofHg,As,andZn;thesandstonewasnotanalyzedforHg.ReFeRenCes:McGimseyandothers,1988;thisreport.

186 Unnamed(A) Hg,Sb(Zn,As) Samplescontainupto2.3ppmHg,18ppmSb, T.23N. [85AM70] EpithermalHg-Sb(?) 110ppmZn,and30ppmAs R.49W.

sUmmARy:Localityhasrubbleofiron-oxide-stainedKuskokwimGroupsandstone,lesseriron-oxidestainedlithictuff,someheav-ilyiron-oxide-stainedalteredfelsicdike,andminorsmallchipsofquartzvein.AgrabsampleofthesandstonecontainselevatedAsandSb.ThetuffcontainselevatedAs(20ppm),Hg(2.0ppm),Sb(18ppm),andZn(110ppm).Agrabsampleoffelsicdikecontains2.3ppmHg.Thequartzveinmaterialcontains0.94ppmHg.Ablue-greenmineral identifiedbyx-raydiffractiontobedickite(?)(analystElizabethBailey,U.S.GeologicalSurvey)coatssomefracturesurfaces.ReFeRenCes:McGimseyandothers,1988;thisreport.

187 Unnamed(A) Hg,As Samplescontainupto7.1ppmHgand100ppm T.23N. [85AM69] EpithermalHg-Sb As R.49W.

sUmmARy:Alteredsandstoneinvicinityofalteredfelsicdike;bothsandstoneanddikehaveabundantsecondaryhematite.Sand-stonecontains7.1ppmHg;dikecontains1.3ppmHg.BothhaveelevatedAsandtrace(2ppm)Sb.ReFeRenCes:McGimseyandothers,1988;thisreport.

188 LewisGulch(M) Au(Ag) Producedupto188kg(6,039oz)Aufrom1918 T.23N. Placer to1939 R.49W.

sUmmARy:Small,2.5-km-long,first-orderstreamthatflowswesttointersectDonlinCreek.MostplacergoldprobablyrecycledfromauriferousDonlinCreekbench,anancestralterracealluvialdeposit;someplacergoldwasprobablyoriginallyintroducedfromlodesourcesinmineralizedgraniteporphyrydikes.Principalheavymineralsarecinnabar,stibnite,andarsenopyrite.ReFeRenCes:Cadyandothers,1955;Cobb,1972;thisreport.

189 Unnamed(O) Au,Ag,As,Hg,Sb Grabsamplesofthreedikescontainupto0.2% T.23N. [85AM161] Granite-porphyry-hosted As,0.9ppmAu,1.5ppmAg,6.4ppmHg, R.49W. Au-polymetallic and14ppmSb

sUmmARy:SericiticallyalteredgraniteporphyrydikescutKuskokwimGroupsandstone.Thenumerousdikes,eachabout1mwide,formaswarmcovering200m.Arsenopyriteidentifiedinonedike.ReFeRenCes:McGimseyandothers,1988;thisreport.

190 QueenGulch(M) Au(Ag) ProductionestimateincludedwithDonlinCreek T.23N. Placer (no.195) R.49W.

sUmmARy:Small,2.5-km-long,first-orderstreamflowsnorthwesttointersectDonlinCreek.PlacergoldthoughttoberecycledfromauriferousDonlinCreekbench,anancestralterracealluvialdeposit.Goldprobablyultimatelyderivedfrommineralizedgraniteporphyrydikeswarmtotheeast.ReFeRenCes:Cadyandothers,1955;Cobb,1972;thisreport.

191 RubyGulch(M) Au(Ag) Produced4.5kg(145oz)goldinabout1911 T.23N. Placer R.49W.

sUmmARy:Small,1-km-long,first-orderstreamthatflowswesttointersectDonlinCreek.PlacergoldthoughttoberecycledfromauriferousDonlinCreekbench,anancestralterracealluvialdeposit.Sevenassaysofgoldwerereportedtorangefrom902to910fine.Inadditiontogold,concentratesyieldedmagnetite,scheelite,cassiterite,garnet,cinnabar,stibnite,andpyrite.Placergoldprobablyultimatelyderivedfrommineralizedgraniteporphyrydikeswamtotheeast.ReFeRenCes:Maddren,1915;Cadyandothers,1955;Cobb,1972;thisreport.

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192 Donlinlodesystem(P) Au,Ag,Sb,As(Sn,Cu, Reserveestimate(allreservecategories)of259 T.23N. Pb,Zn,Hg) Mtatanaveragegradeof3.1g/tgoldor792 R.49W. Granite-porphyry-hosted tonnes(25.45Moz)gold(Szumigalaand Au-polymetallic others,2004)

sUmmARy:Explorationforlodedepositsbeganinthe1980’sandpropertywasextensivelydrilledduring1987–1990,1995–1998,and2000–2002; this description is basedonpublished information andour owndata—it doesnot include complete informationfromthemostrecentexplorationprogramsconductedbyNovaGoldResources,Inc..Blades,crystalsanddisseminationsofstibnite,arsenopyrite,complexarsenicsulfosalts,andminortotracecinnabarandfreegoldinquartzveinsandshearzonesassociatedwith4-km-longsheeteddikeandsillcomplex.Mineralizedrockcommonlyoccursnearcontactsbetweendikesandmid-CretaceousKus-kokwimGroupflysch,butlocally,auriferouszonesextendupto20mawayfromthecontact,encompassingsedimentaryrockandhornfels.Dikesvarycompositionallyandtexturallyandincludequartzmonzonite,alaskite,andgraniteporphyryvarieties.Cross-cut-tingrelationshipsindicatethereareatleastthreeagesofdikes,buttheymaybewithinaseveralmillionyeartimespan.OnegraniteporphyrydikeyieldsaminimumK-Arageof65Ma;anotherdikeyields69.5Maonbiotiteand70.9Maonmuscovite.Asampleofhydrothermalsericiteassociatedwithmineralizedrockyieldeda40Ar/39Arof>69.5Ma.Grabsamplescontainupto100ppmAg,0.5%As,23ppmAu,200ppmBe,190ppmBi,3.8ppmCd,700ppmCu,>10ppmHg,1,000ppmPb,1%Sb,70ppmSn,20ppmW,and350ppmZn.Cinnabarisrecognizedintrenches.Considerablegoldfoundinlatticestructuresofarsenicminerals.ReserveestimatequotedaboveismainlyfromtheLewis/Rochelieuarea.OtherprospectsincludeFarside,Snow,400,ACMA,andDuqum(alsoseeDomeprospectno.196,thistable).ReFeRenCes:McGimseyandothers,1988;Noklebergandothers,1993;RetherfordandMcAtee,1994;Dodd,1996;BundtzenandMiller,1997;Grayandothers,1997b;Szumigalaandothers,2000;Szumigalaandothers,2004;thisreport.

193 SnowGulch(M) Au,Ag Producedatleast256kg(8,238oz)Auand18.8 T.23N. Placer kg(605oz)Agfrom1910to1992 R.49W.

sUmmARy:GoldwasdiscoveredonSnowGulchin1910andafter1914itbecametheprincipalproducerofgoldinDonlinCreek-CrookedCreekarea.Second-orderstreamflows6kmnorthwesttointersectDonlinCreek.Virtuallyalltheplacergoldhasbeenrecov-eredwhereSnowGulchintersectstheancestralDonlinCreekbench,anancestralterracealluvialdeposit;hence,economicplacersarethoughttobetheresultofrecyclingofgoldfromtheauriferousbench.Principalheavymineralsidentifiedincludegarnet,cassiterite,scheelite, stibnite, and trace radioactivemonazite.Goldfineness is927, thehighest averageof allplacerdeposits in the Iditarodquadrangle.Placergoldiscoarseandangular;aflat280g(9oz)nuggetwasminedin1984.Thesourceofgoldandheavymineralsisprobablya1-km-widegraniteporphyrydikeswarmoftheDonlinProperty(no.192).ReFeRenCes:Maddren,1915;Cadyandothers,1955;Bundtzenandothers,1987;SpencerandCarolynLyman,oralcommun.,1990,1991;thisreport.

194 QuartzGulch(M) Au(Ag) Producedatleast61.2kg(1,968oz)Auand0.4 T.23N. Placer kg(14oz)Agfrom1910to1987 R.49W.

sUmmARy: From 1910 to 1914, Quartz Gulch was the largest producer in the Donlin Creek-Crooked Creek area.This small,first-orderstreamflows3.5kmnorthwesttointersectDonlinCreek.AlmostallplacergoldrecoveredwhereQuartzGulchintersectsauriferousDonlinCreekbenchalluvialterracedeposits.OriginofplacerssimilartothatdescribedforLewis,Queen,Ruby,andSnowGulches(nos.188,190,191,and193).ReFeRenCes:Maddren,1915;Cadyandothers,1955;Bundtzenandothers,1987;thisreport.

195 DonlinCreek/ Au(Ag) Producedatleast130kg(4,170oz)Auand3.7 T.23N. CrookedCreek(M) Placer kg(119oz)Agfrom1911to1956 R.48–49W.

sUmmARy:Third-ordertrunkstreamintowhichauriferoussecond-orderplacer-gold-producingstreams(nos.188,190,191,193,and194)enter.Thismajorplacerdepositisaconspicuousancestralterracealluvialdeposit(theDonlinCreekbench)thatextendsforatleast25kmfromOmegaGulchonthesouthwesttotheheadwardreachesofDonlinCreektothenortheast.Goldproductionactivi-tiesconfinedto10-km-long,left-limitbenchalongDonlinCreekadjacenttoLewis,Queen,Snow,andQuartzGulches.Paleocurrentdatainminecuts(imbricatepebblesinfluvialgravels)andaerialphotographicinterpretationindicatethat,atonetime,DonlinCreekflowednorth to intersect theIditarodRiverdrainagebasin,probably inLateTertiary timejudgingfromantiquityof theancestralstreamdrainage.Goldproductionderivesmostlyfromterracegravels,butsomecamefrommodernalluvialdeposits,betweenQueenandLewisGulches.ReFeRenCes:Maddren,1915;Cadyandothers,1955;Cobb,1972;Bundtzenandothers,1987;thisreport.

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196 Donlinlodesystem, Ag,Hg,Sn,As(Au,Bi) Surfacesamplesofdikecontainupto1ppm T.23N. Domearea(P) Granite-porphyry-hosted Ag,5.1ppmHg,100ppmAs,70ppmSn,3 R.48W. [85BT194and Au-polymetallic ppmBi,andtraceAu 85BT195]

sUmmARy:Thisareawasexploredanddrilledin1996and1997aspartoftheDonlinlodesystem(no.192);thisdescriptionisbasedonpublishedinformationandourowndata—itdoesnotincludeup-to-dateinformationfromthedrillingprogram.Naturalexposuresaremostlyrubblecropofgraniteporphyrydikesinbiotite-bearinghornfelssandstoneofKuskokwimGroup;quartzveins1to6cmwidecutdikeslocally.ReFeRenCes:McGimseyandothers,1988;BundtzenandMiller,1997;thisreport.

197 Unnamed(O) Zn,As,Cu,Sb,Hg(Sn, Grabsamplescontainupto440ppmZn,200 T.23N. [85GL100and Ag) ppmAs,100ppmCu,62ppmSb,2.3ppm R.48W. 86BT290–292] Granite-porphyry-hosted Hg,20ppmSn,andtraceAg Au-polymetallic

sUmmARy:Areaoflocalgossaninhornfelssedimentaryrockadjacenttoquartz-veinedgraniteporphyrydikesandsmallintrusions;contactareasarelocallybrecciated.Threegrabsamplesofthehornfelscollectivelycontainupto440ppmZn,180ppmAs,100ppmCu,60ppmSb,20ppmSn,2.3ppmHg,andtraceAg.Onegrabsampleofquartzveinsindikecontains200ppmAs,62ppmSb,1.5ppmHg,and10ppmSn.ReFeRenCes:McGimseyandothers,1988;thisreport.

198 Unnamed(A) Hg Samplecontains4.7ppmHg T.23N. [85BT269] EpithermalHg-Sb(?) R.47W.

sUmmARy:GrabsampleoflocallycalcareouslithicsandstoneoftheKuskokwimGroup;noevidenceofnearbyigneousactivity.ReFeRenCes:McGimseyandothers,1988;thisreport.

199 Unnamed(A) Sb Samplecontains110ppmSbandtraceAg T.23N. [85BT139] EpithermalHg-Sb(?) R.47W.

sUmmARy:GrabsampleoflithicsandstoneoftheKuskokwimGroup;noevidenceofnearbyigneousactivity.Samplecontains110ppmSbandtraceAg;Hgwasnotanalyzed.ReFeRenCes:McGimseyandothers,1988;thisreport.

200 Unnamed(A) Ag(Sb) Samplecontains1.5ppmAgand10ppmSb T.23N. [85BT144] Deposittypeuncertain R.47W.

sUmmARy:GrabsampleofKuskokwimGroupsandstonecontainslow-levelanomaliesof1.5ppmAgand10ppmSb;Hgwasnotanalyzed.ReFeRenCes:McGimseyandothers,1988;thisreport.

201 Unnamed(A) (Sb) Samplecontains34ppmSb T.23N. [85BT130] EpithermalHg-Sb(?) R.46W.

sUmmARy:Medium-grainedlithicsandstoneofKuskokwimGroupcontains34ppmSb;Hgwasnotanalyzed.Noevidenceofnearbyigneousactivity.ReFeRenCes:McGimseyandothers,1988;thisreport.

202 Unnamed(A) Sb,As Samplecontains14ppmSband20ppmAs T.23N. [86BT198] EpithermalHg-Sb(?) R.46W.

sUmmARy:Hematite-alteredlithicsandstoneofKuskokwimGroupcontains14ppmSband20ppmAs;Hgwasnotanalyzed.ReFeRenCes:McGimseyandothers,1988;thisreport.

203 Unnamed(A) Ag(Hg) Samplecontains10ppmAgand0.12ppmHg T.22N. [86BT307] Deposittypeuncertain R.53W.

sUmmARy:Grabsampleofshallow-waterfacies,quartzosesandstoneofKuskokwimGroup;altereddikeliesabout400mtothesouth.ReFeRenCes:McGimseyandothers,1988;thisreport.

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204 AmericanCreek(O) Au Visiblegoldinheavy-mineral-concentrate T.22N. [I0052] Placer sample R.52W.

sUmmARy:Visible gold grains (<1%of sample) found in nonmagnetic, heavy-mineral-concentrate sample takenonAmericanCreek.MineraloccurrencesonMosquitoMountaincouldpotentiallyhavecontributedtothisplaceraccumulation.ReFeRenCes:Bennettandothers,1988;thisreport.

205 CrookedCreekand Au,Ag ThispartofCrookedCreekproduced89.4kg T.22N. OmegaGulch(M) Placer (2,873oz)Auand7.87kg(253oz)Agprior R.49W. to1955.OmegaGulchproduced189kg (6,089oz)Auand0.3kg(10oz)Agfrom 1911to1939

sUmmARy:Placerdeposits inmodernCrookedCreekdrainagebelowOmegaGulchandaboveAnacondaCreekexploredanddevelopedaround1910.Littleinformationavailable;U.S.mintrecordproductionquotedabovealsomightincludeproductionfromDonlinCreek(no.195).MintrecordsindicateOmegaGulchminedbetween1911and1939.ReFeRenCes:Maddren,1915;Cadyandothers,1955;Bundtzenandothers,1987;thisreport.

206 Unnamed(A) Ag(Cu) Samplecontains2ppmAgand100ppmCu T.22N. [85AAi646] Deposittypeuncertain R.47W.

sUmmARy:GrabsampleoflithicsandstoneofKuskokwimGroupcontains2ppmAgand100ppmCu;Hgwasnotanalyzed.Noigneousrocksnearby.ReFeRenCes:McGimseyandothers,1988;thisreport.

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table 2. List of deposit types/models used to describe the metallic mineralresources of Iditarod quadrangle

Number DepositModelorType Reference

Model1 Peraluminousgranite-porphyry-hostedgold-polymetallic BundtzenandMiller(1997)

Model2 Plutonic-hostedcopper-gold-polymetallicstockworkandvein Cox(1986b);VilaandSillitoe(1991); BundtzenandMiller(1997)

Model3 Plutonic-related,boron-enriched,silver-tin-polymetallic Reed(1986);Togashi(1986); BundtzenandMiller(1997)

Model4 Epithermalmercury-antimony(gold) Rytuba(1986);Kuznetsov1974); (Babkin(1975)

Model5 Volcanic-hostedpreciousandothermetals Berger(1986);Mosierandothers (1986a,b)

Model6 Mafic-ultramaficrelatedCr-Ni-Co Albers(1986);Page(1986)

Model7 Heavy-mineralplacer Yeend(1986)

table 3. Definitions of resource assessment terms used in this report (adapted fromGoudarzi, 1984; and Hansen, 1991, p. 96–97)

Mineralresourcepotential

high:

Areaswhere(1)geologic,geochemical,andgeophysicalcharacteristicsindicateafavorableenvironmentforresourceoccurrence,(2)dataindicateahighdegreeoflikelihoodforresourceaccumulation,(3)characteristicsofadefinedmineraldeposittypearepresent,and(4)dataindicatethatmineralshaveconcentratedinatleastpartofthearea

moderate:

Areaswhere(1)geologic,geochemical,andgeophysicalcharacteristicsindicateafavorableenvironmentforresourceoccurrence,(2)dataindicateareasonablelikelihoodofresourceaccumulation,and(3)characteristicsofadefinedmineraldeposittypearewellenoughexpressedforthegroundtobeconsideredfavorable

Low:

Areaswheregeologic,geochemical,andgeophysicalcharacteristicsdefineenvironmentsinwhichtheexis-tenceofresourcesisunlikely

Unknown:

Areaswhereinformationisinadequatetoassignhigh,moderate,orlowlevelofpotential

Levelsofcertainty

Level B:

Availableinformationonlysuggeststhelevelofmineralresourcepotential;usedwherethegeneralgeologicenvironmentisknown,butkeyinformationmaybemissing

Level C:

Availableinformationgivesagoodindicationofthelevelofmineralresourcepotential;usedwherethegeo-logicenvironmentisclearlydefined,butdataisinadequatetodeterminethatmineralswereforming(ornot)

Level d:

Availableinformationclearlydefinesthelevelofmineralresourcepotential;usedwherethegeologicenviron-mentisclearlydefined,itcanbeascertainedthatmineralswereforming(ornot),andthemineraldeposittypeandindicativecharacteristicsarewellunderstood

88

table 4. Estimate of number of undiscovered deposits in the Iditarod quadrangle by deposit type

Medianestimtateofundiscovered depositsatvariousconfidencelimits

DepositModel1 Minimum 90percent 50percent 10percent depositsize

Model1—Peraluminousgranite-porphyry-hosted 155kg 3 5 10hostedgold-polymetallic (5,000oz) gold

Model2—Plutonic-hostedcopper-gold- 155kg 3 5 10polymetallicstockworkandvein (5,000oz) gold

Model3—Plutonic-related,boron-enriched, 175,000t 8 15 30silver-tin-polymetallic (193,000 tons)

Model4—Epithermalmercury-antimony(gold) 3,450kg 6 15 30 (100flasks) ofmercury

1Referstomodelslistintable2

89

table 5. Characteristics of lode deposit resource areas[Depositmodelsfromtable2.Rankofmineralresourcepotential:H,high;M,moderate;L,low.Levelsofcertainty:B,C,D,arelistedintable3]

Resource Deposit Rank/ Mapunit(s) Mines Prospects Occur- Rock Rock Moss Stream Heavy- Oremineralsin area model certainty rences anomalies sediment mineral concentrate concentrate

peraluminous granite-porphyry-hosted gold-polymetallic deposit type

1a 1 H/D TKgp,Kks 1-lode 3-lode 1-lode 2-lode Ag,As,Au, As,Sb Ag,As,Cu, Pb,Sn cinnabar,gold 6-placer Sb(Hg) Hg,Pb

1b 1 H/D TKgp,Kks 1-placer 2-lode 0 1-lode Ag,Au,Sb As As,Au,Hg B,Zn cassiterite,cinnabar, (Hg) (Ag) scheelite,stibnite

1c 1 H/C TKgp,Kks 3-placer 0 3-lode 1-lode Ag,Au,As, Sb Ag,As,Au Sn cassiterite,scheelite Sb,Sn(Hg)

1d 1 H/D TKgp,Kks 7-placer 2-lode 4-lode 9-lode Ag,Au,As, As,Sb Ag,As,Au, Sn,W cassiterite,cinnabar, Hg,Sb Hg scheelite,sphalerite

plutonic-hosted copper-gold-polymetallic stockwork and vein deposit type

2a 2 H/C TKm,hornfels 0 1-lode 1-placer 0 Au,Hg(Ag, As Au,Hg,Zn W cinnabar,gold, 3 M/C As) scheelite

2b 2 H/D TKm,TKil, 2-placer 1-lode 1-lode 4-lode Ag,Au,Hg Sb As,Au,Hg Ag,Au,B, cassiterite, 3 M/C Kit,hornfels 1-placer 3-placer 1-placer (As,Cu,Pb, Mo,Pb,Sb, chalcopyrite, Sb) Sn,W cinnabar,gold, scheelite

2c 2 M/B TKm,hornfels 0 0 0 1-lode Ag(As,Zn) none none none none 3 M/B collected collected collected collected

2d 2 H/D TKm,TKil, 1-lode 6-lode 1-lode 0 Ag,As,Au, As,Sb Ag,As,Au, Ag,As,Au, cinnabar,gold, 3 M/C hornfels 9-placer B,Hg,Sb Cu,Hg Bi,Sb,W scheelite (Cu,Pb,W, Zn)

2e 2 H/C Kka,hornfels 0 0 1-lode 6-lode Ag,Zn(As, none Hg Ag,Au,Bi gold 3 M/B 1-placer Bi,Cu,Hg, detected Sb)

plutonic-related, boron-enriched silver-tin-polymetallic deposit type

3a 3 H/D TKm,TKil, 0 3-lode 6-lode 2-lode Ag,B,Cu, Ag,As,Sb Ag,As,Au, Ag,Au,B, nonedetected 2 M/C Kit,hornfels 1-placer Sb,Sn,Zn B,Cu,Hg, Bi,Cu,Pb, (As,Au, Mo,Pb,Sn, Sn,W Pb) Zn

3b 3 H/C TKm,TKil, 0 0 1-lode 1-lode Ag,Cu,Zn none As,Au Mo,Pb,Sn, nonedetected 2 M/C hornfels (Au) detected W

90

table 5. Characteristics of lode deposit resource areas—continued[Depositmodelsfromtable2.Rankofmineralresourcepotential:H,high;M,moderate;L,low.Levelsofcertainty:B,C,D,arelistedintable3]


3c 3 H/D TKm,TKil, 0 1-lode 1-placer 1-lode Ag,As(Cu, Ag,As Ag,As,Bi, Ag,As,B, cassiterite 2 M/C TKgp Hg) Pb,Sn Bi,Pb,Sb

3d 3 H/D TKm,TKil, 0 0 4-lode 2-lode Ag,As,B, As As,Mo B,Zn cassiterite,scheelite 2 M/C Kit,Kks, Cu,Hg,Sb, hornfels Zn(Au)

3e 3 H/C TKgp,hornfels 0 0 4-lode 0 Ag,As,Au, As Ag,As,Sn Ag,Au,Bi, cassiterite 1 M/C B,Pb,Sb, Sb,Sn,W Sn,Zn(Cu)

3f 3 M/B TKm,hornfels 0 0 0 1-lode As,Sb,Sn none Ag none none 2 M/B collected detected detected

3g 3 H/D hornfels,faults 0 0 2-lode 4-lode Ag,As,B, none Ag,As,Au, Ag,Au,B, cinnabar,scheelite 2 M/C Cu,Sn,Zn detected Cu,Sn,Zn Sn,W (Au,Sb) 3h 3 H/B TKm 0 0 1-lode 2-lode Ag,B,Cu, As As none none Zn(Sn) detected detected 3i 3 H/D TKm,TKil, 0 0 4-lode 2-lode Ag,As,Pb, Ag,As,Sb Ag,As,Au, Ag,Au,B, cassiterite,cinnabar, 2 M/C hornfels 1-placer Sb,Sn(Au, B,Hg,Pb Bi,Pb,Sb, gold Bi,Hg) W

epithermal mercury-antimony (gold) deposit type

4a 4 H/C Kkq,TKd 0 0 1-placer 2-lode Hg,Sb,As none Ag,Au, Cu,Pb,Sb, cinnabar,gold, detected Hg,Zn W,Zn scheelite,sphalerite 4b 4 H/D Kks,Kkq,TKil, 2-placer 0 1-lode 9-lode Hg,Au,As Sb Ag,As,Hg, Ag,Au,B, cinnabar, Kit,TKm, 3-placer (Ag) Sn Sb,Pb,W chalcopyrite,gold, TKgp,TKd, scheelite dMc 4c 4 H/C Kks,TKgp 0 0 0 2-lode Hg(As,Au, none Au Cu,Sn,Zn cinnabar 1 M/C Ag) detected

4d 4 H/C Kks,TKgp 0 0 1-lode 3-lode Hg,Sb,As As none Ag,Sb,Sn, cassiterite,cinnabar, 1 M/B detected W scheelite

4e 4 H/D Kks,Kkq, 1-lode 7-lode 2-lode 4-lode Hg,Sb,Au, As,Sb Ag,As,Au, Ag,As,Au, cassiterite,cinnabar, TKd,TKgp, 8-placer As(Ag) Hg,Zn Pb,Sb,Sn, gold,scheelite TKm,TKil, W hornfels

4f 4 H/C Kks,TKm, 0 0 3-lode 3-lode Hg,As(Sb) As As,Hg W cinnabar,scheelite

91

table 5. Characteristics of lode deposit resource areas—continued[Depositmodelsfromtable2.Rankofmineralresourcepotential:H,high;M,moderate;L,low.Levelsofcertainty:B,C,D,arelistedintable3]


TKil,Kit

4g 4 H/B Kks,TKgp 0 0 0 1-lode Ag none Ag,Au,Hg Ag,Cu,Sn, cassiterite,cinnabar, 1 M/B detected W,Zn scheelite

4h 4 H/D Kks,TKgp, 0 1-lode 0-lode 7-lode Hg,Sb,As none Ag,Au Hg cassiterite,cinnabar, 1 M/C TKd 1-placer (Au) detected gold,sphalerite

4i 4 M/B Kks 0 0 0 2-lode Sb,Hg,As Sb Ag,Au Ag,Pb,Sb, nonedetected Sn

4j 4 H/D Kks,Kkq,TKil, 1-lode 1-lode 1-lode 4-lode Hg,Sb,As Ag Hg Ag,Au,B, cinnabar,gold, Kit,TKd 1-placer (Au,W) W scheelite

4k 4 M/C Kks 0 0 0 6-lode Hg,Sb,Ag, none Ag,Hg Sn cinnabar,sphalerite As detected

4l 4 M/C Kks,TKgp 0 0 2-lode 2-lode Hg,Sb,As none Ag,Hg Sn cinnabar (Au) detected

4m 4 M/C Kks,TKd, 6-lode Hg,Sb,As Ag,As,Sb Ag,As,Au, Ag,Au,Cu, cinnabar,galena, TKgp (Au) Hg,Zn Pb,Sb,Sn, scheelite W,Zn

4n 4 L/C Kks 0 0 0 0 nonedetected Sb Au,Hg Ag,Au,Sn, cassiterite,cinnabar, W,Zn scheelite

Volcanic-hosted precious and other metals deposit type

5a 5 H/B TKy 0 0 0 5-lode Ag,Pb,Sn none Ag,As Ag,Au,Sn nonedetected (Nb) detected

5b 5 M/B TKy 0 0 0 8-lode Hg,Sb,Au, Ag Ag,As,Au Cu,Sb,W cassiterite, As(Ag) chalcopyrite, cinnabar,galena, scheelite

Mafic-ultramafic related Cr-Ni-Co deposit type

6 6 L/C Jdr 0 0 0 1-lode Ag(Cr,Ni) Ag Cr Ag,Au cinnabar,scheelite

92

table 6. Characteristics of heavy-mineral placer deposit resource areas[M,numberofmines;P,numberofprospects;O,numberofoccurrences;Rankofmineralresourcepotential:H,high..;Levelsofcertainty:B,C,D(seetable3);ss,stream-sedimentsample;hmc,heavy-mineral-concentratesample;mv,microscopicallyvisible]

Resource Rank/ Main Placer Supportiveanomalies

area certainty commodity M P O Bedrock Drainagebasins

7a—Ganes- H/D Gold 6 0 0 Ag,Au(As) ss-Ag,YankeeCreeks hmc-mugold

7b—CarlCreek H/C Gold 0 0 1 Ag ss-Ag,As; hmc-Ag,As,Pb

7c—Moore H/D Gold 2 1 3 Ag,Au(Hg) ss-Au;Creekarea hmc-Ag,Au, Mo,W,mvgold

7d—Iditarod- H/D Gold 11 0 0 Ag,Au ss-Ag,Au;Flatarea hmc-Ag,As,Au, Sb,W,mvgold

7e—Little H/B Gold 0 0 0 nonecollected ss-Ag,Au,Zn;WaldrenFork hmc-Ag,Au,Sn, W

7f—Graniteand H/D Gold 1 1 0 Ag,Au,Cu,Sb ss-AuMuntherCreek

7g—Julian H/D Gold 3 0 0 Ag,Au,As,Sb ss-Ag,HgCreekarea

7h—American H/C Gold 0 0 1 Ag,As,Hg,Sb, hmc-Ag,Au,Bi,Creek Zn mvgold

7i—LittleCreek H/C Gold 1 0 1 nonedetected ss-Hg; hmc-Ag,Au, mvgold

7j—ReturnCreek H/D Cinnabar 0 0 1 As,Hg,Sb ss-Hg; (Ag,Au) hmc-mvcinnabar

7k—Donlin H/D Gold 7 0 0 Ag,As,Au, ss-Ag,Au,As,Creekarea Hg,Sb Hg;hmc-Sn

93

table 7. Summary of land tracts including contained resource areas, commodities,and deposit types (by model)

Land Resource Commodities Deposittypescontained3,4

tract1 areascontained2 ofinterest Model1 Model2 Model3 Model4 Model5 Model6 Model7

VH-1 1b,3g,3h,3i, Au,Ag,Sn, X X X X X 4i,7e,7f Hg,SbVH-2 2d,3e,3f,4e, Au,Ag,W, X X X X X 7d Sn,HgVH-3 3c,4d,7b Au,Ag,Hg X X X X X (Sn,W)VH-4 2b,4b,7c Au,Ag,Pb, X X X X Hg,WVH-5 1d,7k Au,Ag,Sb, X X X Hg,Sn,WVH-6 1a,7a Au,Ag,Hg X X XVH-7 1c,4l,7g Au,Ag,Hg, X X X SbVH-8 2c,4c Ag,Au,Hg, X X X X ZnVH-9 2e,7h Ag,Au,Hg, X X X X SbH-1 3d,4f Ag,Au,Hg, X X X Sb,WH-2 4j,7i,7j Au,Hg,Sb, X X Ag,WH-3 3a Ag,Cu,Sn, X X X Au,Pb,Hg, WH-4 2a Au,Hg(Ag, X X X W)H-5 3b Ag,Zn,Cu X X X (Au,Sn)H-6 4h Hg,Sb(Au) X XH-7 4g Ag,Hg(Au) X XH-8 5a Pb,Sn,Ag X (Au)H-9 4a Hg,Sb(Au) XH-10 4k(partof Ag,Hg,Sb X 4m)M-1 5b Ag,Au,Hg, X SbM-2 4m Hg,Sb(Au) XL-1 4n Hg XL-2 6 Cr X

1Landtractsrefertofigure32ResourceareasrefertomapB3Depositmodels:1—Peraluminousgranite-porphyry-hostedgold-polymetallic,2—Plutonic-hostedcopper-gold-polymetallic

stockworkandvein,3—Plutonic-related,boron-enriched,silver-tin-polymetallic,4—Epithermalmercury-antimony(gold),5—Volcanic-hostedpreciousandothermetals,6—Mafic-ultramaficrelatedCr-Ni-Co,and7—Heavy-mineralplacer

4Xmeansmodeltyperepresented

94

Table 8. Correlation of thermal maturity indicators tohydrocarbon generation

[ModifiedfromHerouxandothers(1979)andJohnssonandothers(1993)]

Thermal Vitrinite Alteration Hydrocarbon ThermalReflectance Index Generation1 Maturity (Ro,%) (TAI) Categories2

Undermature

0.6 2.6 Onset of oil generation

Mature

1.3 3.0 Limit of oil generation

Limit of oil preservation 2.0 3.7 Limit of wet gas preservation

Overmature

3.5 4.5

5.0

UUndermature

M-IMatureI

M-IIMatureII

OOvernature

SSupernature

Ig-MIgneous-

Metamorphic

1PooleandClaypool(1984)2Johnssonandothers(1993)

95

table 9. Vitrinite reflectance in Kuskokwim Group rocks from the Iditarod quadrangle[Mapnumberreferstofigure5.Mapunitreferstogeologicbasemap:Kkqrocksrepresentshallowmarinetonon-marinefaciesandKksrocksrepresentmarineturbiditefacies.SamplesanalyzedbyMarkPawlewicz,U.S.GeologicalSurvey,Denver.Levelsofthermalmaturity(fromJohnssonandothers,1993),arediscussedinthetextandabbreviatedasfollows:M–I,matureI;M–II,matureII;O,overmature;S,supermature]

VitriniteReflectance (Ro)(percent)

Map Sample Mean Min Max Map Rocktype Analystcomments Levelofnumber number unit thermal maturity

1 84BT271 1.89±0.08 1.73 2.07 Kkq Mudstone Verygoodsample;abundant M–II consistentvitrinite

2 84AM249A 1.25±0.06 1.14 1.38 Kkq Mudstone Goodsample;somerecycledpieces M–I 3 83BT12 1.84±0.13 1.68 2.18 Kkq Shale Difficultsample;multiplepopulations M–II

4 83BT18 3.89±0.40 2.70 4.78 Kkq Shale Allhighrankmaterial,butnotmuch S

5 84AM256A 3.39±0.33 2.81 3.99 Kkq Mudstone Allhighreflectance O

6 83BT32 3.85±0.63 2.92 4.92 Kks Shale Poorsample;severalpopulations; S couldbecontaminated 7 83BT82 0.83±0.14 0.65 1.06 Kks Mudstone Difficultsample;multiplepopulations M–I

8 83BT86 0.94±0.08 0.84 1.12 Kks Mudstone Difficultsample;multiplepopulations M–I

9 83BT56 0.73±0.06 0.61 0.86 Kks Mudstone Difficultsample;twodistinct M–I populations;onemuchhigher thantheonemeasured

10 83BT58 2.29±0.26 1.84 2.70 Kks Mudstone Fairsample;twodistinctpopulations; O lowrankmaterialconsidered contamination

11 83GL61 1.22±0.18 0.97 1.44 Kks Mudstone Fairsample;vitrinitecommon,but M–I notconsistent

12 83BT67 4.67±0.27 4.08 5.13 Kks Siltstone Fairsample S

13 83BT65 1.27±0.13 1.10 1.54 Kks Mudstone Difficultsample;widevarietyof M–I organics;nodominantpopulation

14 83MSL44 2.82±0.25 2.46 3.33 Kks Mudstone Twopopulations;measuredtheone O showinglowestreflectance

15 83BT47 3.81±0.63 3.00 4.75 Kks Shale Fairsample;possiblecontamination S

16 83GL72 1.67±0.13 1.48 1.85 Kks Mudstone Nearlybarrensample M–II

17 84AM166E 2.49±0.17 2.30 2.77 Kks Mudstone Fairsample;variablevitrinite; O possiblecontamination

18 84AM129A 2.15±0.12 1.95 2.37 Kks Mudstone Abundantorganicsandfairly O consistent

19 84AAi532C 1.36±0.12 1.14 1.49 Kks Mudstone Multiplepopulations,butnodomi- M–II nantgroup;possiblecontamination

20 84AM5C 4.48±0.34 3.83 4.99 Kks Mudstone Fairsample;allveryhighrank S material

21 84BT99 4.29±0.39 3.63 4.95 Kks Shale Fairsample;allveryhighrank S material

22 84BT283 4.72±0.21 4.34 5.00 Kks Mudstone Goodsample;mostlyveryhighrank S material;possiblecontamination

23 84BT79 4.72±0.22 4.37 5.08 Kks Mudstone Fairsample,butmultiplepopulations S

24 84AAi529 2.31±0.17 2.01 2.65 Kks Mudstone Allhighrankmaterial O

25 84BT250 3.90±0.20 3.52 4.30 Kks Shale Allhighrankmaterial S

26 84BT248 2.86±0.23 2.51 3.48 Kks Mudstone Allhighreflectance O

96

table 10. Thermal alteration index (TAI) values in Kuskokwim Grouprocks from the Iditarod quadrangle

[Mapletterreferstofigure5.Mapunitreferstogeologicbasemap:Kkqrocksrepresentshallowmarinetonon-marinefaciesandKksrocksrepresentmarineturbiditefacies.Levelsofthermalmaturity(fromJohnssonandothers,1993),arediscussedinthetextandabbreviatedasfollows:U,undermature;M–II,matureII.SamplesanalyzedbyMicropaleoConsultants,Inc.]

Mapletter Samplenumber TAI Mapunit Rocktype Levelofthermal maturity

A 86AM38B 3.0 Kkq Siltyshale M–II B 86AM132A 3.0? Kkq Veryfinegrainedsandstone M–II?

C 86AM364A 3.0? Kkq Siltyshale M–II?

D 84BT239 3.5 Kkq Fine-grainedsandstone; M–II containsplantdebris

E 86BT12 3.5 Kkq Siltstone M–II

F 86AM351B 3.0 Kks Sandstoneandsiltstone; M–II containsplantdebris

G 86AM182A 3.0 Kks Siltstoneandshale M–II

H 86AM16B 3.0? Kks Shale M–II?

I 85BT124 3.6 Kkq Shale M–II

J 86AM257A 3.0 Kkq Fine-grainedsandstone M–II

K 85BT130 2.5? Kks Fine-grainedsandstone; U? containsplantdebris

L 86AM229A 3.0? Kks Siltstoneandshale M–II?

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