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Montana Tech LibraryDigital Commons @ Montana Tech
Graduate Theses & Non-Theses Student Scholarship
7-2014
Geochemistry and Stable Isotopes of Surface Waterand Groundwater in the Continental Pit in Butte,Montana, USAAmber McGivernMontana Tech of the University of Montana
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Recommended CitationMcGivern, Amber, "Geochemistry and Stable Isotopes of Surface Water and Groundwater in the Continental Pit in Butte, Montana,USA" (2014). Graduate Theses & Non-Theses. Paper 3.
Geochemistry and Stable Isotopes of Surface Water and Groundwater in the Continental Pit in
Butte, Montana, USA
AmberL.McGivern
ANon‐thesisResearchPapersubmittedaspartialrequirementsfor:
MasterofScienceDegreeGeosciences:HydrogeologicalEngineeringOption
DepartmentofGeologicalEngineeringMontanaTechoftheUniversityofMontana
Butte,Montana
July17,2014
ResearchCommittee:
Dr.ChrisGammons(Chair)
Dr.GlennShaw(MontanaTech,Dept.ofGeologicalEngineering)
Dr.GaryIcopini(MontanaBureauofMinesandGeology)
1
Abstract
TheContinentalporphyryCu‐Momine,located2kmeastofthefamousBerkeleyPitlakeof Butte, Montana, contains two small lakes that vary in size depending on miningactivity.IncontrasttotheacidicBerkeleyPitlake,theContinentalPitwatershavenear‐neutral pH and relatively low metal concentrations. The main reason is geological:whereastheBerkeleyPitminedhighly‐alteredgraniterichinpyritewithnoneutralizingpotential, the Continental Pit is mining weakly‐altered granite with lower pyriteconcentrationsandupto1‐2%hydrothermalcalcite.
The purpose of this studywas to gather and interpret information that bears on thechemistryofsurfacewaterandgroundwaterintheactiveContinentalPit. Pre‐existingchemistrydata fromsamplingof theContinentalPitwerecompiled fromtheMontanaBureau of Mines and Geology and Montana Department of Environmental Qualityrecords. In addition, in March of 2013, newwater samples were collected from themine’smaindewateringwell,theSarsfieldwell,andanearbyacidicseep(PavilionSeep)and analyzed for trace metals and several stable isotopes, including D and 18O ofwater,13C of dissolved inorganic carbon, and34S of dissolved sulfate. InDecember2013, several soil samples were collected from the shore of the frozen pit lake andsurroundingarea.ThesoilsampleswereanalyzedusingX‐raydiffractiontodeterminemineralcontent.
BasedonVisualMinteqmodeling,waterintheContinentalPit lakeisnearequilibriumwithanumberofcarbonate,sulfate,andmolybdateminerals,includingcalcite,dolomite,rhodochrosite (MnCO3), brochantite (CuSO4·3Cu(OH)2), malachite (Cu2CO3(OH)2),hydrozincite (Zn5(CO3)2(OH)6), gypsum, and powellite (CaMoO4). The fact that thesemineralsareclosetoequilibriumsuggeststhattheyarepresentontheweatheredminewalls and/or in the sediment of the surface water ponds. X‐Ray Diffraction (XRD)analysisof thepond“beach”sample failed toshowanydiscretemetal‐bearingphases.Oneofthesoilsamplescollectedhigherinthemine,nearanareaofactiveweatheringofchalcocite‐richore,containedover50%chalcanthite(CuSO4·5H2O).Thiswater‐solublecoppersaltiseasilydissolvedinwater,andisprobablyamajorsourceofcoppertothepond and underlying groundwater system. However, concentrations of copper in thelatter are probably controlled by other, less‐soluble minerals, such as brochantite ormalachite.
Although the acidity of thePavilion Seep is high (~11meq/L), the flow ismuch lessthan the SarsfieldWell at the current time. Thus, the pH, major andminor elementchemistry in the Continental Pit lakes are buffered by calcite and other carbonateminerals. FortheContinentalPitwaterstobecomeacidic, the influxofacidicseepage(e.g.,PavilionSeep)wouldneedtoincreasesubstantiallyoveritspresentvolume.
Keywords:neutralminedrainage,tracemetals,pitlake,geochemistry,mineralsolubility
2
Introduction
JusteastofthefamousBerkeleyPitlakeinButte,Montana(DavisandAshenberg,1989),
there is a lesser known active open pitmine, the Continental Pit. The Continental Pit
containsa largebut low‐gradeCu‐Moporphyrydeposit (Czehura,2006). Twosurface
waterbodiesexistontheproperty,herecalledtheNorthPondandtheSouthPond(Fig.
1).Presently,theelevationsoftheselakesaremaintainedatalowlevelbytheSarsfield
pumpingwell,locatednearthenorthshoreoftheNorthPond.Pumpingceasedin2001‐
2003 after the mine temporarily shut down in 2000 due to unfavorable economics.
Dewatering resumed in 2003when themine reopened, and continues to the present
day,ata typical rateof800 to1200 litersperminute (S.Czehura,pers. comm.March
2013).
Figure1.GoogleEarthimageshowingthelocationoftheNorthandSouthPondsintheContinentalPit,withthenearbyBerkeleyPitlakeforcomparison.ThePavilionSeep(notshown)islocatedonthesoutheastwall
oftheContinentalPit.ImagetakenJuly16,2002.
During the mine‐closure period of 2000‐2003, samples of groundwater from the
Sarsfield Well and/or samples of surface water from the North Pond (Fig. 2A) were
collectedmonthlybytheMontanaBureauofMinesandGeology(MBMG). Since2006,
theactiveminehasbeencollectingwatersamplestwiceayearfromseverallocationson
theirproperty,includingtheNorthandSouthPonds,theSarsfieldWell(sampledinstead
of theNorthPondwhen the latter isdryor frozen), and thePavilionSeep.Thewater
qualityresultsaresent totheMontanaDepartmentofEnvironmentalQuality(MDEQ),
andarepublicallyavailableuponrequest.ThePavilionSeepisarelativelysmallvolume
of acidic groundwater that enters the Continental Pit from a bench on the southeast
Sarsfield Well
3
highwall. Atthetimeofthiswriting,PavilionSeepwateriscollectedandpumpedtoa
siteoutsideofthepitwhereitismixedwithSarsfieldWellwaterandusedinthemill.
Becausetheactiveminingcompanyintendstocontinueopen‐pitminingfordecades,no
formalplansforclosureoftheContinentalPitexist.Thefinalclosurescenariomaywell
notincludeapitlake.Nonetheless,itisinterestingtolookatthedatainhandtopredict
thewaterqualityofahypotheticallakeformedfromfloodingoftheContinentalPit,and
tocomparethisresulttothenearbyBerkeleyPitlake(Fig.2B).
Figure2.Close‐upphotographscomparingtheappearanceofwaterintheNorthPondoftheContinentalPit(left,withasubmersiblepump)andtheBerkeleyPitlake(right).PhotoscourtesyofNickTucci,MBMG.
BerkeleyPitBackground
TheBerkeleyPitbeganfillingwithwaterin1983,whenminingoperationsceased,and
shortly thereafter became a Superfund site (Gammons and Duaime, 2006; Metesh,
2006). The pit lake is a part of the Clark ForkBasin Superfund Complex, the largest
Superfund complex in theUnited States (Moore and Luoma, 1990; US‐EPA, 1994). Its
location at theheadwaters of SilverBowCreek,whicheventually flows into theClark
Fork River, makes it an important water body to monitor. The geochemistry of the
Berkeley Pit lake has been investigated by many agencies and academic groups for
decades(DavisandAshenberg,1989;Pellicorietal.,2005;GammonsandDuaime,2006;
GammonsandTucci,2013). SamplesarecurrentlycollectedbiannuallybytheMBMG,
and water quality results posted on the internet at the Bureau’s Groundwater
InformationCentersite(GWIC,2013). BasedontheUS‐EPA’sRecordofDecision(US‐
EPA, 1994), a water treatment plant will need to begin treating Berkeley Pit water
before the elevation of the lake reaches 5410’ above sea level, the so‐called “critical
level”. Currently, the critical level is projected to be reached sometime near 2023
(Pitwatch, 2014).With the exception of the bottom of the active Continental Pit, the
critical levelof5410’represents the lowestelevation intheButteSummitValley. The
4
groundwaterdividethatseparatesthepitlakeandsurroundingfloodedmineworkings
fromSilverBowCreekisshowninFigure3.
Figure3.Groundwaterflowmapwithundergroundworkingsofvariouslevelsshown.Thedashedanddottedlinerepresentsagroundwaterdivide(Duaimeetal.,2004;Gammons,etal.,2009).
Geology
The deposits in Butte are Cu‐Moporphyry style deposits cut by rich Cu‐Pb‐Zn‐Mn‐Ag
lode veins (Houston and Dilles, 2013). These deposits are hosted in Butte Quartz
Monzonite,partof theBoulderBatholith,which is approximately76millionyearsold
(Rusketal.,2008).TheBerkeleyPitwasminedforchalcocite‐richsupergeneoresand
zonesofdeeper,closely‐spacedveins thatwere leftbehindby theundergroundmines
(Czehura, 2006). The main ore minerals of the Continental Pit are chalcopyrite and
molybdenite(Czehura,2006).ThisdepositislocatedinaportionofthePittsmontDome
thathasbeenupliftedbytheContinentalFault(Fig.4).TheContinentalFaultisanormal
faultwithover1000mofdisplacement(Rusketal.,2008).
5
Figure4.GeologiccrosssectionoftheBerkeleyandContinentalPits,showinglocationoftheAnacondaandPittsmontDomes(Czehura,2006).
Methods
WaterchemistrydatafortheNorthPondfrom2001to2003weredownloadedfromthe
MBMG’s Groundwater Information Center website (GWIC, 2013). In June of 2003,
shortly beforedewatering resumed, a vertical profile of fieldparameters in theNorth
PondwascollectedbyMBMGfromaboat,andwatersamplesweretakenatnear‐surface
andat a depthof 11m. Additional data for theNorthPond, theSouthPond, and the
Pavilion Seep collected between 2006 and 2012were provided byMDEQ. All of the
water quality analyses were conducted by certified labs following strict quality
assurance protocols. In March 2013, the author sampled the discharges from the
SarsfieldWellandthePavilionSeep.Samplesweresubmittedforchemicalanalysis,as
well as determination of 18O and D of water, 34S of dissolved sulfate, and 13C of
dissolved inorganic carbon. In December 2013, soil sampleswere collected from the
shores of the North Pond and bottom of the Continental Pit. These samples, which
contained secondary copper salts, were ground to a fine powder using amortar and
pestleandisopropylalcoholandthenanalyzedbyX‐RayDiffraction(XRD)attheDept.
ofMetallurgicalEngineering,MontanaTech.
DataforeachchemicalanalysisoftheNorthPondbetween2001and2003(n=17)were
input into the program Visual Minteq, v. 3.0 (a recent adaptation of the original
MINTEQA2programofAllisonetal.,1991),andsaturationindices(S.I.)wereestimated
foranumberofmineralsandamorphoussolidphases.Inthispaper,theS.I.valuesfor
6
eachsolidwereadjustedtoonemetal ionperformulaunit. Forexample,amongstthe
commonsecondaryCuminerals,theS.I.valueformalachite(Cu2CO3(OH)2)wasdivided
by twoandtheS.I.value forazurite(Cu3(CO3)2(OH)2)wasdividedby three. Thiswas
donesothatS.I.valuescouldbemoreequallycomparedbetweenasetofmineralsthat
contain a common element of interest. Formodeling purposes, dissolved Cu andMn
wereassumed tobe in the+2valence, andMoandUwereassumed tobe+6.Fewas
assumedtobe+2intheSarsfieldWellandPavilionSeep,and+3intheContinentalPit
lakewaters. These redox assignments are consistentwith the presence of > 5mg/L
dissolved oxygen throughout the water column of the north lake in June 2003 (see
below).
Results
Waterqualityresultsfrom2013samplingoftheSarsfieldWellwererelativelysimilarto
pastdata;mostofthemajorionswerepresentinquantitiesslightlylowerthaninpast
samples (Table 1). Cu, Mn, and Zn concentrations were slightly higher than past
averages, while the concentration of Fe was significantly higher than the average
concentration during mine closure and moderately higher than the average
concentration during mining activity (7.47 mg/L compared to 0.05 and 4.0 mg/L
respectively).
WhilesamplingtheSarsfieldWellat itsdischargepoint, thepHwasmeasuredat5.83.
Thesamplesat in theopenfora fewminutesandduring this time thepHrose to6.8.
ThiswaslikelyduetodegassingofdissolvedCO2.ByinputtingthefieldpHof5.83,the
alkalinity, and the temperature intoVisualMinteq, a CO2partial pressure of 0.16 atm
wasobtained. This ismuchhigher than theequilibriumpartialpressureofCO2 inair
(around0.0003atmat theelevationofButte),andexplainswhyCO2(g)wouldrapidly
diffuseoutofthesampleafterpumpingtothesurface. Forthepurposeofcalculating
saturationindices(below),thepHof5.83wasused.
ThechemicalcompositionsoftheNorthandSouthPondsintheContinentalPitarevery
similar(seeTable1).BothwaterbodieshaveapHnear7,withasignificantamountof
bicarbonate alkalinity (> 100mg/L as CaCO3). The concentrations ofmostmetals of
concern,suchasCuandZn,arequitelowwhencomparedtotheBerkeleyPitlake,but
are nonetheless well above regulatory standards for surface water. However, it is
emphasizedthatthesestandardsarenotrelevantaslongasthemineisinoperation.
7
Table1.ChemicalcompositionofselectedwaterbodiesofinterestinthevicinityoftheContinentalPit.Allsoluteconcentrationsareinmg/L
Location pH SC1 Ca Mg Na K SO42‐ Cl‐ F‐ HCO3‐ NO3‐N PO4‐PDatafromthisstudy
SarsfieldWell 5.83 1940 396 32 36 5.3 1100 9.0 2.8 146 <0.01 <0.02
PavilionSeep 2.87 1853 249 73 28 6.0 1200 n/a n/a <1 n/a n/a
PreviousResults
CP‐Nsurface2 7.34 2050 433 37 38 10 1241 9.1 3.3 146 1.3 <0.5
CP‐Nsurface3 7.05 n/a 502 43 36 8 1310 8.9 4.2 123 0.6 0.04
CP‐N11m4 6.55 2140 477 46 41 11 1440 8.6 4.3 92 1.7 <0.5
CP‐Ssurface5 7.33 1860 397 38 32 6.1 1070 8.9 4.1 115 0.23 0.08
Pavilion6 3.31 2360 225 86 24 4.6 2060 5.3 4.7 <1 0.57 0.14
BPitsurface7 2.75 7340 475 534 74 9.6 8170 <50 30 <1 <5 <5
BPit210m7 2.53 8160 442 515 74 7.6 9090 <50 30 <1 <5 <5
Al As Cd Cu Fe Mn Mo Ni Si Sr U ZnDatafromthisstudy
SarsfieldWell 0.44 0.001 0.01 0.66 7.74 3.78 0.45 0.018 10.5 2.61 0.08 3.89
PavilionSeep 14.8 <.015 0.21 33.7 14.2 15 n/a 0.09 22 1.24 n/a 20.5
Previousresults
CP‐Nsurface2 0.98 0.001 0.05 0.43 0.05 3.1 0.69 0.031 5.5 4.07 0.29 3.14
CP‐Nsurface3 1.01 0.005 0.04 1.61 4.0 5.5 n/a 0.043 9.3 2.94 n/a 6.46
CP‐N11m4 0.18 <.005 0.17 3.71 0.04 5.5 0.55 0.039 5.4 5.27 0.22 8.92
CP‐Ssurface5 0.24 0.003 0.06 1.05 0.23 8.6 n/a 0.047 8.7 1.75 n/a 6.76
Pavilion6 41 0.02 0.43 58.6 76.7 22 n/a 0.15 28 0.66 n/a 46.2
BPitsurface7 275 0.135 2.26 79 564 261 <0.02 2.4 38 1.55 1.97 627
BPit210m7 268 0.097 2.27 153 1007 248 <0.02 1.6 36 1.43 1.45 6381SpecificconductanceinµS/cm;n/a=notanalyzed2ContinentalPitNorthPond,averageof17samplescollectedin2001‐2003(GWIC,2013)3Continental Pit North Pond or Sarsfield pumpingwell, average of 11 samples collected in 2005‐2012(MDEQ,2013)4Lakesamplecollectedatdepthof11m(GWIC,2013)5Continental Pit South Pond, average of 10 samples collected between 2005 and 2012 (MDEQ, 2013)6PavilionSeep,averageof11samplescollectedbetween2005and2012(MDEQ,2013)7BerkeleyPitlakesamplescollectedonNov.6,2007(GWIC,2013).
UnlikesomepitlakeswithneutraloralkalinepH,theconcentrationofarsenicinbothof
theContinentallakesisnotparticularlyhigh(<10g/L).ThisisnotduetoalackofAs
in theorebody, since theButtedeposits are locally rich in theCu‐As‐sulfideminerals
enargiteandtennantite(Meyeretal.,1968).ItispossiblethatAsisbeingadsorbedonto
secondaryFe‐oxy‐hydroxidemineralsonweatheredbedrocksurfaces.Incontrasttothe
Berkeley Pit, which has undetectable quantities of dissolved Mo (Table 1),
concentrationsofMointheNorthPondareelevated(inthe0.5to0.7mg/Lrange).This
differenceisgeological,astheBerkeleydepositproducednoMo,whereasmolybdenite
isanimportantoremineralattheContinentaldeposit(Czehura,2006).Between2006
and2012,thePavilionseephadanaveragepHof3.31,andconcentrationsofdissolved
8
metals andmetalloidsweremuchhigher than in theNorth and Southponds, but still
considerably lower than theBerkeleyPit lake (Table1). Compared tohistoricaldata
provided by the MDEQ, the Pavilion Seep sample collected in this study had lower
concentrationsofmostsoluteswiththeexceptionofCa,Na,andK.Verylittleisknown
about the hydrology of the Pavilion Seep, and it is possible thatwater being pumped
fromthatlocationmaybeamixtureofacidminedrainageandbackgroundgroundwater
withtherelativeproportionchangingatdifferenttimesoftheyear.
Figure 5 summarizes long‐term trends in the concentrations of Cd, Cu and Zn in the
NorthPondand/orSarsfieldWellbetween2001and2013.Duringtheperiodofmine
closure(2001‐2003),concentrationsofthesemetalssteadilyincreased.TheNorthPond
wasseasonallystratifiedduringthistime(Fig.5),withachemoclinesituatedatadepth
of4.2minJune2003.Asinglewatersamplecollectedat11mdepthhadslightlylower
pHandsignificantlyhigherconcentrationsofCd,CuandZncomparedtoanear‐surface
sample(Table1).Dissolvedoxygenconcentrationswere>5mg/Latalldepths,whichis
incontrasttotheBerkeleyPitlake,whichtypicallyhasundetectableDOconcentrations
belowthetop1‐3m(Pellicorietal.,2005).Althoughthemonitoringdatabecamemore
scatteredafterresumptionofminingoperationsin2003,averagevaluesformostwater
qualityparametersintheNorthPondfrom2001‐2003aresimilartothosefrom2006‐
2013,suggestingthatwaterqualitytrendshavebeenfairlystableinthelastdecade.
AsshowninFigure6,verticalchangesinphysicalparametersintheNorthPondinJune,
2003wererelativelysmall.Aweakchemoclineexistedatadepthof4.2m,withslightly
higherSCandlowerpHbelowthislayer.Temperatureremainedbetween8and9°Cat
alldepths.Dissolvedoxygenconcentrationswere>9mg/Labovethechemocline,but
droppedbelow6mg/Ltowardsthebottomofthelake(Fig.6).
Based on the computed saturation indices (Fig. 7),water in theNorth Pond in 2001‐
2003wasclosetoequilibriumwithgypsum(CaSO4·2H2O)andfluorite(CaF2),aswellas
a number of carbonate minerals, including calcite, dolomite, smithsonite (ZnCO3),
hydrozincite(Zn5(CO3)2(OH)6),andrhodochrosite(MnCO3).Onaverage,thelakewaters
wereundersaturatedwithotavite(CdCO3).ItispossiblethatCdresidesasanimpurity
incalciteoranothercarbonatephase.AlargenumberofCu(II)mineralswerealsonear
equilibrium, including malachite (Cu2CO3(OH)2), azurite (Cu3(CO3)2(OH)2), antlerite
(Cu3(SO4)(OH)4), brochantite (Cu4SO4(OH)6), langite (Cu4SO4(OH)6·2H2O), andpossibly
atacamite(Cu2Cl(OH)3).TheSarsfieldWellsamplecollectedinMarch2013hadsimilar
9
results for the saturation indices of most minerals except carbonate minerals, which
wereconsistentlyundersaturatedrelativetotheNorthPondsamples(Fig.7).Themain
reasonforthisdifferenceisthattheSarsfieldWellhasalowerpHwhenfreshlypumped
duetoahighpartialpressureofdissolvedCO2.Ifthesaturationindexcalculationsare
repeated using the pH value of 6.8 obtained after the sample had sat in the open for
severalminutes,theS.I.valuesforcarbonatemineralsaremoresimilartothehistorical
valuesfortheNorthPond.
Figure5.ConcentrationsofCu,Cd,andZnintheNorthPondfrom2001through2003(GWIC,2013),from2006through2012(MDEQ,2013)and2013(thisstudy).SomeofthelatterdataarefromtheSarsfieldWell.TheelevatedconcentrationsfromMay13,2003werefromasamplecollectedatadepthof11metersinthe
NorthPond.
Figure6.Verticalprofilesinwatertemperature,specificconductance(SC),pH,anddissolvedoxygen(DO)
fortheNorthPondoftheContinentalPit,collectedonJune12,2003.
Temperature, °C
6 7 8 9 10
Dep
th,
m
0
2
4
6
8
10
12
SC, S/cm
1600 1800 2000 2200 2400
DO, mg/L
5 6 7 8 9 10
Dep
th,
m
0
2
4
6
8
10
12
pH
5 6 7 8 9 10
pH
DO
SC T°C
A. B.
10
Figure7.Saturationindicesofvariousmineralsbasedon17chemicalanalysesoftheNorthPondoftheContinentalPit.Rectanglesrepresentonestandarddeviation.Linesextendtomaximumandminimum
values.RedpointsrepresentdatafromtheSarsfieldWellcollectedin2013.
BoththeNorthandSouthPondshaveadistinctivegreenishbluecolor(Fig.1,Fig.2A),
whichcouldbepartlycausedbyprecipitationofoneormoreofthesesparinglysoluble
secondarycopperminerals.SamplesofprecipitatesfromaroundtheNorthPondandon
the floor and wall of the Continental Pit were obtained and analyzed using X‐ray
diffraction.Atthetimeofsampling,theNorthPondwasfrozenandsamplesfrombelow
the water line could not be obtained. A sample of a blue mineral obtained from a
chalcocite‐richpartoftheoreonthesideoftheContinentalPitwasdeterminedbyXRD
analysistobechalcanthite(Cu5SO4·5H2O).Itisunlikelythatchalcanthiteisstableinthe
pondwateritself,sinceitistoosoluble(saturationindicesarestronglynegative,andare
notshowninFig.7).Afterrainevents,thechalcanthiteonthegroundsurfaceprobably
dissolves and is flushed into the groundwater and/or themine ponds. Much of this
copperprobablyre‐precipitatesasother, lesssolubleCuminerals, suchasbrochantite
ormalachite. However, theexistenceof these lesssolubleminerals in thepondcould
notbetestedinthisstudy.
It issignificantthatmetalconcentrationsintheNorthPondappeartobecontrolledby
mineralsolubility limits,asthismeansthattheconcentrationsofelementssuchasCd,
Cu, Mn, and Zn cannot increase without a simultaneous decrease in pH and/or
11
bicarbonate concentration. Amongst the various silica polymorphs, the lake waters
were closest to equilibrium with chalcedony (micro‐crystalline quartz). Finally, it is
interesting that the lakewaterswere found to be near equilibrium or supersaturated
with several molybdate minerals, including CaMoO4, CdMoO4, CuMoO4, and ZnMoO4.
This is not surprising considering the abundance of molybdenite, an important ore
mineral,inthedeposit.
Acidity and Alkalinity
In2013,theSarsfieldWellhadanalkalinityof146mg/LCaCO3,whichisequivalentto
2.39meq/Lofalkalinity.TheacidityofthePavilionSeepcollectedonthesamedaywas
calculatedtobe5.19meq/L,asshowninthetablebelow.Thesecalculationsassume
thatalldissolvedFeinthePavilionSeepwasferrous(Fe2+),andthateachmoleof
dissolvedAlcontains3molesofmetalacidityandthateachmoleofdissolvedFe,Cuor
Zncontains2molesofmetalacidity.UsingaveragesofdatafromMDEQ,theacidityof
thePavilionSeepis11.03meq/L,significantlyhigherthanthealkalinityoftheSarsfield
Well.AlthoughtheflowfromthePavilionSeepissmallrelativetotheflowfromthe
SarsfieldWell,itisnoteworthythattheacidityofthePavilionwateris2to5times
higherthanthealkalinityofthedewateringwell.Ifthevolumeofacidicwaterinthe
ContinentalPitweretoincreaseinthefuture,thetotalaciditycouldpossiblyexceedthe
acid‐bufferingcapacityofthenon‐acidicgroundwaterandsurfacewateronthemine
property.Thispossibilitycannotbeevaluatedfurtherwiththedataofthisstudyalone.
Table2.PavilionSeepaciditycalculations.Concentration(mmol/L)=concentration(mg/L)/gramformulaweight of element, except H+, which was calculated using the average pH of the Pavilion Seep.meq/Lobtainedbydividingtheconcentration(mmol/L)bythechargeoftheelement.
Element
AverageConcentration
(mg/L)
2013Concentration
(mg/L)
AverageConcentration(mmol/L)
2013Concentration(mmol/L)
AverageAcidity(meq/L)
2013Acidity(meq/L)
Fe2+ 77 14.2 1.38 0.25 2.76 0.51Al3+ 41 14.8 1.52 0.55 4.56 1.65Cu2+ 59 33.7 0.93 0.53 1.86 1.06Zn2+ 46 20.5 0.70 0.31 1.41 0.63H+ ‐ ‐ 0.45 1.35 0.45 1.35
TotalAcidity= 11.03 5.19
12
Water, Sulfur, and Carbon Isotopes
Resultsforstableisotopicanalysisofwater,dissolvedinorganiccarbon(DIC),and
sulfateareinTable3.
TABLE3.ResultsfromisotopicanalysisofsamplesfromtheSarsfieldWellandthePavilionSeep.
Location δ13C‐DIC δ34S‐Sulfate δ18O‐H2O δD‐H2O
SarsfieldWell ‐6.8 2.5 ‐17.8 ‐141
PavilionSeep ‐ 3.0 ‐17.7 ‐137.5
Theisotopiccompositionsofδ18OandδDintheSarsfieldWellandPavilionSeepwaters
aresimilartowaterinthefloodedmineshaftsofButte,andplotneartheintersectionof
theButteMeteoricWaterLine(MWL)andLocalEvaporationLine(LEL)(seeFig.8).As
discussedbyGammonsetal.(2006),theintersectionoftheMWLandLELmostlikely
representsaveragerechargewatersinButtethathavenotbeenaffectedbyevaporation.
Incontrast,theBerkeleyPitlakeandtheYankeeDoodletailingspondweremoderately
tohighlyevaporatedwhenthesamplesplottedinFigure8werecollected(seePellicori
etal.,2005).ThefurtheroutalongtheLELagivensampleplots,themorewaterwas
losttoevaporation.
Figure8.Isotopicdataforsamplescollectedinthisstudy(SarsfieldWell,PavilionSeep)plottedagainsttheButteMeteoricWaterLineandLocalEvaporationLine.DatafortheBerkeleyPit,thefloodedmineshafts,
andthetailingspondarefromGammonsetal.(2006)andGammonsetal.(2009)
‐170
‐150
‐130
‐110
‐90
‐70
‐50
‐20 ‐15 ‐10 ‐5 0
D‐w
ater, ‰
18O‐water, ‰
Berkeley Pit
Flooded Mine Shafts
Tailings Pond
Sarsfield Well
Pavilion Seep
13
ThefactthatthewaterintheSarsfieldWellissimilartothatofgroundwaterinthe
floodedmineshaftsandnottotheBerkeleyPitlakeortailingspondwaterisevidence
thatneithertheBerkeleyPitlakenorthetailingspondisleakingtoanysignificant
degreeintotheContinentalPitatthistime.Thewater‐isotopedataclearlyshowthatthe
waterfromtheSarsfieldWell(andfromthePavilionSeep)issimilartoaveragerecharge
watersinButte,andhasnotbeenevaporated.Monitoringthestableisotope
compositionofwaterintheSarsfieldWellovertimecouldbealow‐costmethodtoseeif
thissituationchangesinthefutureif,forexample,theactivemineexpandstowardsthe
BerkeleyPit.
The isotopic composition of sulfate in the Butte underground mine waters and the
BerkeleyPitlakeistheresultofthreeprocesses:oxidationofpyriteandothersulfides,
dissolutionofsulfateminerals,andbacterialsulfatereduction(Gammonsetal.,2009).
In Figure 9, S‐isotopic compositions of dissolvedH2S aremuchmore depleted in34S
compared to sulfate in the same mine waters due to the large fractionation factor
associated with bacterial sulfate reduction (Seal, 2003; Gammons et al., 2009).
Dissolved sulfate in the SarsfieldWell and Pavilion Seep has S‐isotopic compositions
similar to that of pyrite in the Butte deposit (Field et al., 2005). It is also similar to
sulfate in the Berkeley Pit lake (Pellicori et al., 2005). The Continental Pit shows no
evidenceofbacterialsulfatereduction,ashasbeenseeninthefloodedmineshaftsofthe
West Camp andOuterCamp (Gammons et al., 2009), so it is likely that the S‐isotopic
compositionsarefromoxidationofpyrite.
14
Figure9.SulfurisotopecompositionofdissolvedsulfateintheSarsfieldWellandPavilionSeep,comparedtootherminewatersfromButte.DataforsulfateandsulfidefromtheWestCampExtractionWell(WCEW)and
theothermineshaftsaresummarizedinGammonsetal.(2009).DatafromtheBerkeleyPitarefromPellicorietal.(2005).DataforButtepyrite,anhydrite,andbaritearefromFieldetal.(2005).
Dissolved inorganic carbon (DIC) in the SarsfieldWell is a mix of dissolved CO2 and
HCO3‐ ion that is isotopicallyheavier thanotherminewaters in theButtedistrict (see
Fig.10). Inmostof the floodedmineshaftsofButte,DIChasaC‐isotopecomposition
that is close to equilibriumwithhydrothermal rhodochrosite in theMain Stage veins,
basedondataforthelattergiveninGarlickandEpstein(1966).DICintheOuterCamp
workings(e.g.,OrphanGirlandOrphanBoy)hasmuchlighter13C‐DIC,duetoinputsof
light CO2 from bacterial sulfate reduction (Gammons et al., 2009). The DIC in the
Sarsfield Well shows no evidence of input of biogenic CO2, and probably came from
dissolutionofcalciteintheContinentalorebody.CalciteintheContinentalPitoccursas
lateveinsanddisseminationsinalteredgranite,andcomprisesasmuchas1%ormore
oftherockvolume(NewbroughandGammons,2002;Lamsma,2012).Althoughitwas
notpossibletoanalyzeanyofthiscalciteforC‐isotopesinthisstudy,datainGarlickand
Epstein(1966)showthathydrothermalcalciteintheButteDistrictisabout3‰heavier
in13C than rhodochrosite. So, the fact that the SarsfieldWell has heavierDIC could
15
partly be explained by the abundance of calcite and scarcity of rhodochrosite in the
Continental deposit. Some isotopic enrichment of DICmay also have been caused by
lossofCO2duringsamplingorpreparationof theSarsfieldWellwater forDIC isotope
analysis. AsshownbyClarkandFritz (1997),dissolvedCO2 isabout10‰lighter in
13CcomparedtoHCO3‐ion.So,lossofCO2duringsamplingwouldhavecausedthe13C
oftheremainingDICtobecomeheavier.
Figure10.CarbonisotopecompositionofdissolvedinorganiccarbonintheSarsfieldWellcomparedtoother
minewatersfromButte(mineshaftdatafromGammonsetal.,2009).
Discussion
Thedatapresentedaboveshowastarkcontrastbetween thehighlyacidicandmetal‐
richBerkeleyPit lakeandthenearbyContinentalPit lakeswhichhaveneutralpHand
relatively lowmetal concentrations. Themain reason for thisdifference is geological.
ThetwoopenpitminesareseparatedbytheContinentalFault(Fig.1),a largenormal
fault with > 1 km of vertical displacement (Czehura, 2006; Rusk et al., 2008), down‐
dropped to the west. The Berkeley deposit is rich in pyrite (> 5 wt %) with highly
alteredgraniticbedrock(allfeldsparandmaficmineralsconvertedtomuscoviteand/or
clay + quartz) that has no acid‐neutralizing potential. In contrast, the Continental
deposit,whichrepresentsadeeperandmoreperipheralmineralassemblagewithinthe
ButteDistrict,containsweakly‐alteredgranitewallrock(freshfeldsparswithabundant
primaryandsecondarybiotite)withlowerpyritecontent(1to2wt%)andasmallbut
16
significant quantity of hydrothermal calcite. The latter occurs as thin veins and
disseminations throughout the ore body (Newbrough and Gammons, 2002; Lamsma
2012).ThinsectionsinFigure11,takenduringthestudyofNewbroughandGammons
(2002), compare the mineralogy of the pits. In addition, whereas the Berkeley Pit is
surroundedandunderlainby1000’sofkmofundergroundmineworkings(Duaimeet
al., 2004; Gammons et al., 2009), very few underground workings extended into the
ContinentalPitarea.Thus,thehydrologyofgroundwaterflowisdominatedbyopenor
backfilled/collapsed voids in the vicinity of the Berkeley Pit, as opposed to fractured
bedrockinthevicinityoftheContinentalPit.
Newbrough and Gammons (2002) characterized rock samples from the Berkeley and
Continental Pits by conducting static acid‐base accounting (ABA) tests. Three samples
werecollectedfromtheContinentalPit;twofromtheeastsideoftheContinentalFault
that areweakly potassic/phyllic altered, and one from thewest side of the fault that
represented phyllic alteration like many of the samples from the Berkeley Pit. The
results of the tests are in Figure 12. The 3:1 and 1:1 slopes represent neutralization
potential ratios (NPR), or the ratio of acid neutralizing potential to acid generating
potential(NP/AP).Itwasdeterminedthatwhilebothsamplesetsfromeachpithadacid
generatingpotential,theBerkeleyPitsampleswerefarmorelikelytogenerateacid.The
sample from the far west side of the Continental Pit behaved like the Berkeley Pit
samples,asitalsoshowedhighAPandalmostnoNP.
17
Figure11.ThinsectionsfromtheBerkeleyPit(AandB)andContinentalPit(CandD).qtz=quartz,biot=biotite,Kspar=potassiumfeldspar,plag=plagioclasefeldspar.
Figure12.ResultsofABAtestingofsamplesfromtheBerkeleyPitandContinentalPit.BQMisButteQuartz
Monzonite.FromNewbroughandGammons(2002).
qtz
Kspar
plag
Kspar
plag sericite
biot
2° biot
2° biot
calcite
qtz
A B
C D
18
Newbrough andGammons (2002) alsoused long‐termhumidity cell tests to compare
leachatefromcrushedbedrockexposedintheBerkeleyandContinentalPits,andfound
very similar water chemistry results to the data presented in this paper. However,
NewbroughandGammonsobserved that calcitewasbeingdepleted from thecrushed
Continental Pit samples faster than pyritewas being oxidized. Extrapolation of these
resultssuggestedthattheContinentalPitleachatescouldbecomeacidicatafuturetime,
whentheneutralizingpotentialwascompletelydepleted. Dueto thesmallnumberof
samples in theNewbrough andGammons study, and the inherent difficulty of scaling
humiditycellteststofieldsettings,itisdifficulttosayiforwhenthesmallpondsinthe
ContinentalPitmightbecomeacidic.ThePavilionSeepshowsthatsomeacidicdrainage
already exists in the Continental Pit. Although the volume of this acidic water is
presentlymuchlessthanthepH‐neutral,alkalinewaterthatresidesinthepondsorthat
isbeingpumpedfromtheSarsfieldWell,theacidityoftheaciddrainageis2to5times
greater than thealkalinityof thenon‐acidicwaters.Over time, therelativevolumesof
acidicandnon‐acidicwaterscouldchangewithcontinuedweatheringoftheorebody.
ConclusionsandRecommendations
TheContinental Pit contains two small lakes that havenearneutral pHand relatively
low metal concentrations compared to the Berkeley Pit lake, which is highly acidic.
Miningactivityisexpectedtocontinuefordecades,andclosureplansfortheContinental
Pitmaynot includeapit lake.BasedonVisualMinteqmodeling, theexistingponds in
the Continental Pit are close to equilibrium saturation with a number of carbonate,
sulfate, andmolybdateminerals. Formetalswhose solubility is limited by carbonate
minerals (e.g., Zn, Cu), dissolved concentrations can only increase if pH or HCO3‐
concentrationsdecrease. Nocleartrend inthisdirection isapparent fromthepast10
years of water quality monitoring. Some acid mine drainage does exist within the
boundariesoftheContinentalPit,andit ispossiblethatthevolumeand/orseverityof
thisAMDcouldincreaseinthefuturewithcontinuedweatheringoftheorebody.This
scenariocannotbeevaluatedatthepresenttime.
Othersignificantfindingsfromthisstudyincludethefollowing:
TheSarsfieldWellhasalowerpHwhenfreshlysampledcomparedtowhenthe
waterhasbeenallowedtosit in theopenforseveralminutes. This isprobably
duetodegassingofdissolvedCO2.
19
The isotopiccompositionofwater in theSarsfieldWellshowsnoevidence that
theBerkeleyPitortailingspondiscurrentlyleakingwatertotheContinentalPit.
Basedon S‐isotopes, sulfate in the SarsfieldWell, aswell as the acidicPavilion
Seep,ismostlikelyderivedfromoxidationofpyriteandothersulfidemineralson
theminewalls.
The13CofdissolvedinorganiccarbonintheSarsfieldWellisquiteabitheavier
thanDICfromthefloodedmineshaftsofButte.Someofthisdifferencecouldbe
due to the relative amounts of calcite and rhodochrosite in the district, as
previous work has shown that hydrothermal calcite contains heavier carbon
comparedtorhodochrosite.
Somerecommendationsforfurtherworkincludethefollowing:
It is important tocontinue tomonitor thewaterqualityof thedewateringwell
and surface waters in the Continental Pit, to see if conditions change with
continuedminingandwithcontinuedweatheringoftheminewalls.
ModelingthesedatainaprogramsuchasPHREEQCcouldgivemoreinformation
aboutthewaterqualityofahypotheticalContinentalPitlake,includingchanges
in water chemistry that might occur over time in response to changes in the
relativevolumesofSarsfield‐likewaterandPavilion‐likewater.
Measuring the isotopic composition of calcite in theContinental Pitmight help
answerthequestionofwhethertheDICoftheSarsfieldWellisisotopicallyheavy
duetomineral‐buffering,orduetolossofCO2duringsampling.
MeasuringtheisotopiccompositionofwaterintheSarsfieldWellcouldbealow‐
cost method to test if leakage from the Berkeley Pit or tailings pond into the
expandingContinentalPitoccursinthefuture.
Acknowledgements
Iwouldliketothankvariouspeoplefortheirsupportwiththisproject;GaryIcopiniand
Glenn Shaw for being a part of my committee; Gary Wyss for assistance with XRD
analysis;MontanaResources for access to data and also permission to sample on the
property,andTedDuaimeforhelpingtosettheprojectup. Iappreciatethehelpfrom
NickTucciofMBMGwithsamplingandJamesCastroofMontanaDEQforhelptracking
downdata.SpecialthanksforthesupportandguidanceofmyadvisorChrisGammons.
20
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