THE BRITISH COLUMBIA SEDIMENT-HOSTED GOLD · PDF fileIn 1997 the British Columbia ... initiated a project to identify prospective areas for sediment-hosted gold min- ... Belt rocks

KEYWORDS: Economic geology, mineraldeposits, Cordilleran geology, gold, Carlin-typedeposits, sediment-hosted disseminated golddeposits.

INTRODUCTION

In 1997 the British Columbia GeologicalSurvey (BCGS) initiated a project to identifyprospective areas for sediment-hosted gold min-eralization. The inspiration for the project camefrom presentations and articles by HowardPoulsen of the Geological Survey of Canada(1996a, 1996b). He pointed out that there ispotential to find hypogene, sediment-hostedgold mineralization in Canada akin to depositsfound in Nevada. He mentioned that if an intru-sive association is important to generate the min-eralization, then two different geological envi-ronments might host this style of mineraliza-tion. Accreted terranes with a basement contain-ing carbonate lithologies intruded by Mesozoicor Cenozic plutonism would be a prospectivegeological setting, specifically the Stikine andQuesnel terranes. The second favourable envi-ronment is within the sediments deposited alongthe continental margin of ancestral NorthAmerica which have been cut by Mesozoic mag-mas, such as are found in the Kootenay Arc andSelwyn Basin (Figure 1).

Others have considered British Columbia asprospective territory for sediment-hosted gold.Early work by companies focused on the InsularBelt rocks exposed on Vancouver Island and theQueen Charlottes. These exploration programswere based largely on an epithermal-style modelfor the mineralization which was in favour at thetime. The discovery of the Babe deposit (nowcalled the Specogna or Cinola) in the QueenCharlotte Islands in 1970 increased the interestin this model because it was initially identifiedas a Carlin-type deposit (Richards et al., 1976;Champigny and Sinclair, 1982). Subsequent

work showed that the Babe is a classic hot springdeposit and although some jasperoid occur-rences were discovered on the Queen Charlottes,these programs did not result in any major sedi-ment-hosted gold discoveries (Lefebure, 1998).Also in the early 1970s, Harry Warren of the

Geological Fieldwork 1998, Paper 1999-1 165

THE BRITISH COLUMBIA SEDIMENT-HOSTED GOLD PROJECT

By David V. Lefebure, Derek A. Brown and Gerald E. Ray,British Columbia Geological Survey

Figure 1. Map of western North America showing thesedimentary rocks deposited along the ancestral con-tinental margin and the accreted terranes to the west(derived from Poulsen, 1996b). The Slide Mountainterrain consists of miogeoclinal sediments.

University of British Columbia became interest-ed in exploring for Carlin-Cortez type depositsin the province (Warren and Hajek, 1973). Hemade the prophetic statement that the initiationof gold production at the Carlin deposit mightprove to be as important a milestone for goldproduction as the first exploitation of a copperporphyry deposit was for copper.

A number of companies have carried outreconnaissance programs for sediment-hostedgold in British Columbia; but these are not partof the public record. In the early 1980s, Chevronsearched for Carlin-type deposits in the Stikineterrane in the northwestern part of the provinceand found the Golden Bear deposit. Althoughinitially discovered using exploration techniquesapplicable to Carlin-type deposits, the refractorystyle of mineralization and predominance of vol-canic host-rocks delayed confirmation as thisdeposit type (Oliver, 1996).

During the first year (1997) of the BCGSproject, Gerry Ray visited a number of goldoccurrences, including Golden Bear, Watson Barand Summit in British Columbia and Brewery

Creek in the Yukon. The following year, DerekBrown took over leadership of the project andcarried out detailed studies of the Golden Bearand Howell Creek areas.

BACKGROUND

The State of Nevada has become the world�sfourth largest gold producer after South Africa,Australia and the CIS. Much of this gold is beingproduced from sediment-hosted disseminatedgold deposits. Many geologists call thesedeposits Carlin-type (Berger and Bagby, 1991)after the mine of the same name in Nevada. TheCarlin mine was discovered in 1961 byNewmont Exploration Limited (Roberts, 1986)and subsequently was used to characterize a newclass of deposits that are now identified as sed-iment-hosted deposits with microscopic gold.Subsequent discoveries around Carlin are nowbeing mined and have made it the most prolificgold belt in North America with a currentresource of more than 100 million ounces (Tealand Jackson, 1997). This belt, known as the

166 British Columbia Geological Survey Branch

Figure 2. Location of Carlin-type gold deposits in Nevada showing the major trends.

Carlin Trend

Getchell Trend

Battle Mountain-Eureka Trend

GetchellTwinCreeks

Carlin

Pinson

Cortez

Rain

Winters Ck.

Jerritt Cyn.

AlligatorRidge

Betze / Post & MeikleIvanhoe

Deep StarTurf

CoveMcCoy

PipelineLone Tree

Gold Bar

Taylor

WindfallArchimedes

GoldQuarry

0 10050

kilometres

Carlin Trend, is just one of the areas in Nevadawith these types of deposits (Figure 2).

Since the gold mineralization is microscopicand associated features are often visuallyunspectacular at outcrop scale, these deposits arepoorly understood and the mineralization is dif-ficult to recognize. Fortunately, more than thirtyyears of exploration and mining in Nevada haveresulted in a number of well described mines anddocumented the wide range of styles of mineral-ization. Some general information about Carlin-type deposits is presented in this article. For amore complete description of these deposits thereader is referred to the numerous articles pub-lished over the last ten years, includingoverviews by Arehart (1996), Teal and Jackson(1997) and Vikre et al. (1997) and numerouspublications by the Nevada Geological Society(for example, Green and Struhsacker, 1996).

SEDIMENT-HOSTED DISSEMINATEDGOLD DEPOSITS (CARLIN-TYPE)

Although initially called Carlin-type, manygeologists have adopted other terms to facilitatedeveloping a deposit model that is more generic,such as disseminated gold (Roberts, Radtke andCoats, 1971), sediment-hosted disseminatedgold, sediment-hosted micron gold, siliceouslimestone replacement gold and invisible gold(Schroeter and Poulsen, 1996). In this paper weuse the terms sediment-hosted disseminated goldand Carlin-type interchangeably. With the dis-covery of many more mines over the last 20years, a broader definition of Carlin-typedeposits has emerged. They are now identifiedas �sediment-hosted, disseminated, strataboundyet structurally controlled precious metal miner-alization� (Ralph J. Roberts Research forResearch in Economic Geology, web page,1998).

Key Features

Carlin-type deposits contain micron-sizedgold within very fine-grained disseminated sul-phides in stratabound zones and in discordantbreccias. The most common host rocks areimpure carbonates, however, other sedimentaryand igneous lithologies can host mineralization.Christensen (1993, 1996) points out that the goldmineralization appears to be disseminated at the

deposit scale, however, gold is structurally con-trolled at all scales. For a deposit to be considereda sediment-hosted disseminated gold deposit itusually has many of the following features:

s non-visible, micron gold within arseni-calpyrite or pyrite (refractory ore)

s non-visible, extremely fine native gold with-in iron oxide or attached to clay (oxide ore)

s anomalous values of silver, arsenic,mercury,and antimony and low base metals

s associated realgar, orpiment and/or stibnitesedimentary host sequences containing siltycarbonate or calcareous siltstone

s intensely silicified zones, commonly calledjasperoid

s carbonate dissolution (decalcification)

s associated brittle structures

Carlin-type deposits are difficult to find fora number of reasons. The gold is microscopic;visible gold is only very rarely reported from thehighest grade zones. The lower grade depositshave low sulphide contents and even in highergrade hypogene deposits the gold is typicallyassociated with a very fine-grained, black, sootypyrite that can be difficult to identify macro-scopically. Furthermore, surface weathering canconvert all sulphides to iron oxides, such ashematite and limonite, and rarely produces sig-nificant gossans. Exploration for these depositshas tended to draw on empirical relationshipsobserved in Nevada, such as the presence ofthick sedimentary sequences along a paleo-con-tinental margin, the presence of significant bedsof impure carbonate, identification of jasperoids,and the presence of realgar, orpiment and stib-nite. Yet even with the identification of thesefavourable features, it has taken numerous, sys-tematic geochemical and drilling programsalong known mineralized trends in Nevada tomake new discoveries.

Since the gold is micron-sized, it cannot beidentified by panning and Carlin-type depositsare normally not associated with placer goldworkings. The alteration associated with thesedeposits is often inconspicuous (Christensen,1994). It consists of carbonate dissolution(decalcification), pervasive silica replacementand deposition, alteration of aluminosilicates to


clay and sulphidation of iron to form pyrite.Gold is usually the only metal recovered fromCarlin deposits. While minor sphalerite andgalena are noted in some zones, the depositsrarely have any copper minerals. A more com-plete listing of the geological characteristics ofCarlin-type deposits is given in Table 1.

The sediment-hosted microscopic golddeposits in Nevada are well known for theintense supergene alteration that occurs near sur-face. This feature is directly responsible formaking heap leaching a viable process for treat-ing low grade ores. It is generally attributed tothe prolonged arid weathering of rocks inNevada. It has been noted that oxidation featuresassociated with the supergene alteration extendfurther below the surface in many deposits thanin the surrounding areas (Christensen, 1994).Christensen postulates that this is because thehost rock for these deposits has been structural-ly prepared and altered.

In the last ten years a number of deepdeposits with higher grades have been discov-ered along the Carlin Trend. These high gradeorebodies exhibit breccia features and vein-likeores (Figure 3) as well as low grade, strataboundreplacement mineralization like the Carlin mine(Christensen et al., 1996; Groves, 1996). Theinitial underground mining of these ore zonesstarted with declines from open pit workings,such as are used for the Deep Star and Carlin

East deposits. However, in 1996 the Meiklemine started production utilizing a conventionalshaft and has become the largest undergroundgold producer in the United States. These deepdeposits are below the Miocene surface weather-ing and the ore is commonly refractory. Analysisof the orebodies along the Carlin Trend showsthat stratabound replacement orebodies are lesscommon than the vein-like and breccia mineral-ization styles.

Stratabound replacement mineralization isexemplified by the Carlin deposit. Large vol-umes of altered rock contain relatively low gradegold zones which have had minor structural dis-ruption. Stratabound replacement deposits foundat, or near surface, in Nevada are particularlyamenable to heap leaching because the primaryrefractory ore has been oxidized by the arid cli-mate that has existed since the Cretaceous.Using bulk mining techniques, relatively lowgrade surface material can be mined. Otherexamples of the stratabound replacement style ofmineralization are the Pete, Deep West portionof Gold Quarry, West Leeville, Hardie Footwall,Goldbug and Screamer (Christensen et al., 1996;Teal and Jackson, 1997).

The second style of mineralization is calledstockwork (Christensen, 1993) or breccia ore(Groves, 1996) and is found near structuralintersections which can produce complex rela-tionships. An example of this deposit type is the


Figure 3. Different styles of Carlin-type mineralization (modified from Christensen, 1993, 1996). The heavysolid lines are faults.

Stratiform Type

Vein Type Breccia Type

Lower Grade Ore( 1 to 4g/t Au)

Higher Grade Ore( 5 to 10 g/t Au or >)

Dykes


Table 1. Geological Characteristics of Carlin-type Deposits.

TECTONIC SETTINGS: Passive continental margins with subsequent deformation and intrusive activityand island arc terranes.

DEPOSITIONAL ENVIRONMENT / GEOLOGICAL SETTING: Host rocks to the Nevadan depositswere deposited in shelf-basin transitional (somewhat anoxic) environments, formed mainly ascarbonate turbidites (up to 150 m thick), characterized by slow sedimentation. These rocks arepresently allochthonous in thrust fault slices and have been overprinted by Miocene basin and rangeextension. There are Mesozoic to Tertiary felsic plutons near many deposits.

AGE OF MINERALIZATION: Mainly Tertiary, but can be any age.

HOST/ASSOCIATED ROCK TYPES: Host rocks are most commonly thin-bedded silty or argillaceouscarbonaceous limestone or dolomite, commonly with carbonaceous shale. Although less productive,non-carbonate siliciclastic and rare metavolcanic rocks are local hosts. Felsic plutons and dikes arealso mineralized at some deposits.

DEPOSIT FORM: Generally tabular, stratabound bodies localized at contacts between contrastinglithologies. Bodies are irregular in shape, but commonly straddle lithological contacts which, in somecases, are thrust faults. Some ore zones (often higher grade) are discordant and consist of brecciasdeveloped in steep fault zones. Sulphides (mainly pyrite) and gold are disseminated in both cases.

TEXTURE/STRUCTURE: Silica replacement of carbonate is accompanied by volume loss so thatbrecciation of host rocks is common. Tectonic brecciation adjacent to steep normal faults is alsocommon. Generally less than 1% fine-grained sulphides are disseminated throughout the host rock.

ORE MINERALOGY [Principal and subordinate]: Native gold (micron-sized), pyrite with arsenian rims,arsenopyrite, stibnite, realgar, orpiment, cinnabar, fluorite, barite, rare thallium minerals.

GANGUE MINERALOGY [Principal and subordinate]: Fine-grained quartz, barite, clay minerals,carbonaceous matter and late-stage calcite veins.

ALTERATION MINERALOGY: Strongly controlled by local stratigraphic and structural features. Centralcore of strong silicification close to mineralization with silica veins and jasperoid; peripheral argillicalteration and decarbonation (�sanding�) of carbonate rocks common in ore. Carbonaceous material ispresent in some deposits.

WEATHERING: Nevada deposits have undergone deep supergene alteration due to Miocene weathering.Supergene alunite and kaolinite are widely developed and sulphides converted to hematite. Suchweathering has made many deposits amenable to heap-leach processing.

ORE CONTROLS: 1. Selective replacement of carbonaceous carbonate rocks adjacent to and along high-angle faults, regional thrust faults or bedding. 2. Presence of small felsic plutons (dikes) that mayhave caused geothermal activity and intruded a shallow hydrocarbon reservoir or area of hydrocarbon-enriched rocks, imposing a convecting geothermal system on the local groundwater. 3. Deepstructural controls are believed responsible for regional trends and may be related to Precambriancrystalline basement structures and/or accreted terrane boundaries.

from Schroeter and Poulsen, 1996

Main Gold Quarry deposit which has a variety oflithologies, including thin-bedded siltstone,shale, chert and limestone (Christensen et al.,1996). Teal and Jackson (1997) also identifyMeikle and Deep Star as deposits with signifi-cant breccia-style mineralization.

Vein-like mineralization occurs within andadjacent to high-angle faults. The structures con-tain intensely altered, igneous dikes and rela-tively high grade gold. The gold is largelyrestricted to the fault. Examples of this style ofmineralization are the Turf, Sleeper, Boot Strapand Capstone (Teal and Jackson, 1997).

Median tonnages and grades for forty-threelow-grade oxide and higher grade hypogenedeposits in Nevada are 20 Mt grading 1.2 g/t Au and6 Mt containing 4.5 g/t Au, respectively (Schroeterand Poulsen, 1996). Supergene deposits amenableto heap leaching typically grade 1-2 g/t Au; where-as, production grades for deposits with hypogeneore typically grade 5 to 10 g/t or greater.

Global Distribution

There have been remarkably few Carlin-type deposits found outside the western United

States. This could reflect the existence of specialgeological conditions in Nevada not found else-where or insufficient exploration in other partsof the world. Given the microscopic nature ofthe gold and the nondescript alteration features,it is likely that undiscovered deposits occur inareas not previously considered prospective forthis type of deposit. This was the case in Nevadabefore mapping by the United States GeologicalSurvey led Newmont Exploration Limited tocarry out the exploration program which led tothe discovery of the Carlin deposit in an areawhich had a long previous history of prospectingand mining.

Deposits with Carlin-type characteristicshave been reported from Utah, South Dakota,southeast Asia, Mexico, South America andCanada (Figure 4). There are also a large num-ber of these deposits in the southwest Guizhouand western Hunan regions of southeasternChina (Cunningham et al., 1988). The setting forthese mines is generally similar to Nevada. TheChinese gold mineralization is hosted by UpperPermian to Middle Triassic shelf carbonateswhich have experienced broad, open folding andsome high angle faulting (Christensen et al.,


Figure 4. Global distribution of Carlin-type deposits (modified from Christensen et al., 1996).

1996). Although the Guizhou area apparentlylacks igneous rocks, they are reported at someother Chinese deposits (Griffin, personal com-munication, 1998).

There are relatively few Carlin-type depositshosted by volcanic rocks (Lefebure and Ray,1998). The best known examples are the BauDistrict of Sarawk, Malayasia (Sillitoe andBonham, 1990) and the Mesel deposit of NorthSulawasi, Indonesia (Turner et al., 1994; Garwinet al., 1995). The Mesel deposit, discovered in1988, contains a reserve of 7.8 Mt grading 7.3g/t Au and is currently in production. The Meseldeposit exhibits many characteristics of Carlin-type deposits, including decalcification, dolomi-tization and jasperoid development, accompa-nied by gold in disseminated fine-grained arsen-ian pyrite (Sillitoe, 1995).

In British Columbia, recent work by JimOliver as part of his Ph.D. thesis (1996) and fieldinvestigations by Howard Poulsen of the GSCand the authors have shown that the Golden Bearmine is a Carlin-type deposit.

Genesis

Currently, there is no clear understanding ofthe genesis of sediment-hosted microscopic golddeposits. Three general models have been pro-posed to explain the origin of the ore-formingfluids - magmatic, metamorphic and amagmatic.All three models involve generation ofhydrothermal fluids at temperatures of 160 to250 °C with low salinities and significant CO2contents, as found in fluid inclusions in Carlin-type deposits (Arehart, 1996; Ilchik and Barton,1997). A more detailed listing of geologicalprocesses that could form sediment-hosted dis-seminated gold deposits is given by Adams andPutnam III (1992).

Carlin-type deposits were initially consid-ered to be sediment-hosted, epithermal deposits(Radtke et al., 1980; Radtke, 1985; Rye, 1985)because of a number of similar features includ-ing high gold, silver, arsenic, antimony and mer-cury values in mineralized zones, prevalence ofsilicification, presence of alunite, and spatialassociation with antimony and mercury occur-rences. Given the abundance of epithermal gold-silver deposits hosted by volcanic rocks inNevada, it seemed logical to assume that Carlin-type deposits formed at the roots of hot spring

systems related to shallow Miocene magmatismand basin and range extension (Figure 5). Thesubsequent discovery of deep orebodies and thedocumentation of overprinting of mineralizationby basin and range deformation led most geolo-gists to consider alternative models which areconsistent with a deeper and older environmentof formation (Arehart, 1996).

Magmatic systems are considered by manyexploration and research geologists to play akey role in the formation of Carlin-typedeposits (Arehart et al., 1993). This model hasbeen clearly conceptualized by Sillitoe andBonham (1990). They suggested thathydrothermal fluids related to porphyry-typemagmatic systems generated sediment-hosteddisseminated gold deposits, somewhat akin todistal skarns (Figure 6). Some deposits, such asPost-Betze, Getchell, Bald Mountain andArchimedes, do occur near Jurassic orCretaceous intrusions. Others are associatedwith, and partially hosted by, altered dikes thatare controlled by faults. However, this modelcannot explain all deposits because several dis-tricts (e.g. Jerritt Canyon) have no known relat-ed magmatism. Furthermore, in some locationsthe gold mineralization is a different age to thespatially associated intrusions.

Other workers have suggested modelsinvolving fluids generated either by deep-seatedmetamorphism (Seedorff, 1991), crustal exten-sion (Ilchik and Barton, 1997), or the ancestralYellowstone hotspot (Oppliger et al., 1997). Theregional metamorphic model derives the H2O


Figure 5. Early interpretations of Carlin-type depositssuggested that they were epithermal deposits thatformed in sedimentary sequences (from Radtke, 1985).

and CO2 from devolatilization of metasedimen-tary rocks which are also the source of metalsand sulphur. While there may be associated mag-matic activity, it is not a critical element to themodel which emphasizes some of the similari-ties between mesothermal gold and Carlin-typedeposits (Phillips and Powell, 1993).

More recently, Ilchik and Barton (1997)have proposed an amagmatic model with ananomalous thermal gradient caused by bringingmore deeply buried, hotter rocks closer to thesurface and allowing meteoric waters to pene-trate to depths of ten to twenty kilometresbefore rising along major faults during mid-Tertiary extension in the Basin and Rangeprovince (Figure 7).

Another postulated source is metals derivedfrom the core-mantle boundary and carried upinto the crust by the plume associated with theancestral Yellowstone hotspot (Oppliger et al.,1997). The hotspot may have underlain the GreatBasin of Nevada ca. 43-34 Ma and produced

hydrothermal fluids that formed the golddeposits. While these three models may explainsome of the characteristics of the Carlin-typedeposits, they also have some inconsistencieswith published data. As well, all three modelsare tied to one geological event; therefore, theycannot explain deposits with significantly differ-ent ages. This is inconsistent with the widelyheld belief of many geologists working in theGreat Basin that there are both Cretaceous (110-76 Ma) and Eocene (42-30 Ma) age deposits(Tosdal, personal communication, 1998).

Exploration Techniques

Geology is used to identify favourable strati-graphic units, related alteration zones and possi-ble structural controls. Bedded silty carbonateand calcareous siltstone to dolomitic limestoneare the most common hosts; although somedeposits occur within sandstone, conglomerate,intrusive units and even volcanic rocks. In


Figure 6. Magmatic systems are believed by some to play a major role in the formation of sediment-hostedmicroscopic gold deposits. Sillitoe and Bonham (1990) envisioned fluids emanating from porphyry-typemagmatic systems.

CarbonateReplacement

SkarnZn-Pb-Ag (Au)

SedimentHosted

Au-As-Sb

PorphyryDike

PorphyryStock

SkarnCu-Au

SkarnCu-Mo-Au

Skarn Front

Jasperoid

Bedding Trace

Nevada, bedded units, particularly thinly beddedsediments (Photo 1), have been found to be morelikely to host mineralization.

Geochemistry has been the most importantexploration technique following identificationof geologically prospective districts. Initial fieldwork often involves systematic sampling of soilor rock. Gold has been the most important ele-ment used in exploration programs, althoughanomalous values for the pathfinder elementsarsenic, antimony and mercury have led to somediscoveries (e.g. Golden Bear, L. Dick, person-al communication, 1998). Schroeter andPoulsen (1996) identify two geochemical asem-blages - (1) gold, arsenic, mercury and tungstenwith or without molybdenum and (2) arsenic,mercury, antimony and thallium or iron.Common anomalous values in rock are 100 to1000 ppm arsenic, 10 to 50 ppm antimony and1-30 ppm mercury. Other elements that occur inanomalous amounts are silver, barium and thal-lium. Gold:silver ratios are generally greaterthan 8:1 (Adams and Putnam III, 1992; Robertset al., 1971).

Three principle alteration types are recog-nized: decarbonatization or decalcification of


Figure 7. An amagmatic model to explain the genesis of sediment-hoisted microscopic gold deposits (from Ilchikand Barton, 1997).

Photo 1. Bedded Roberts Mountain Formation fromthe Jerritt Canyon area, Nevada.

carbonate rocks, silicification, and argillization.These types of alteration can display differentrelationships to gold mineralization. Arehart(1996) suggests that the most common zonationis decarbonatization on the fringes through silici-fication to argillization in the core of the deposit(Figure 8). Gold ore occurs in all three alterationtypes. However, in most deposits it is betterdeveloped in argillically or intensely silicifiedzones in the core of the system, often associatedwith a structural feature. The decarbonatizedrocks are difficult to recognize unless alterationhas been intense enough to produce a light, fri-able unit. These rocks can be brecciated becausethe volume loss associated with decarbonatiza-tion results in structural collapse. The clays in theargillized rocks are typically fine-grained and notparticularly abundant, therefore, the alterationcan be difficult to identify. Near surface, thesealtered rocks may be oxidized and are common-ly brown, beige or reddish-brown with residuallimonite or hematite after weathered pyrite. In anumber of open pit mines the higher grades areassociated with reddish-brown tabular zoneswhich lie along structures (Photo 2).


Figure 8. Schematic cross-section through a sediment-hosted disseminated gold deposit showing major alterationfeatures adjacent to a fluid feeder structure (from Arehart, 1996).

Limestone

Silty Limestone

Limey Siltstone

Limestone

Decarbonatization

Silicification

Argilization

Jasperoid

Fault

Ore zone boundary

Baritestibnitequartzcalcite(pyrite)

Jackrabbit Zone

Photo 2. The Jackrabbit Zone in the south wall of theTwin Creeks megapit, Nevada. The zone consists ofreddish-brown iron oxides. It is 6 to 24 metres thick,extends laterally and vertically hundreds of metersand averages 3 g/t gold, but can grade up to 34 g/t Au(Bloomstein et al., 1991).

One of the most obvious features of sedi-ment-hosted disseminated gold deposits is anassociation with intensely silicified hostrocks. In some cases the silicification con-tains little or no gold (Photo 3), however, itoccurs proximal to a Carlin-type deposit.Frequently, the intense silicification is relatedto particular sedimentary bed or beds (Photo4) or a fault zone. These rocks are particular-ly important exploration guide because theyare a resistant lithology which is more likelyto outcrop than other types of altered or unal-tered host rocks (Photo 3). In Nevada, theintensely silicified rocks are commonly calledjasperoid which is �an epigenetic siliceousreplacement of a previously lithified hostrock� (Lovering, 1972). This terminology hasbeen rarely used in Canada, even for intense-ly silicified rocks. For this reason, there arefew published references to jasperoid in theCanadian Cordillera.

Many, if not all, sediment-hosted dissemi-nated gold deposits are related to high-anglefaults or shear zones (Adams and Putnam III,1992) which acted as depositional sites for min-eralization or provided a conduit for fluids toreach favourable lithological units. One aspectof Carlin-type deposits that receives consider-able attention in Nevada is the alignment ofknown deposits along trends (Figure 2). Thegeological significance of these trends is notfully understood, however, they are widely usedby the exploration community to promote prop-erties and to identify prospective ground.

Geophysics are used, although usually as anindirect exploration method designed to provideinformation about structure and geology. Somedeposits exhibit resistivity lows. Aeromagneticsurveys may identify associated intrusions andpossibly regional trends (Schroeter and Poulsen,1996). The high grade refractory ore zonesshould respond to electromagnetic techniques.

PROPERTIES EXAMINED

An initial investigation of the MINFILEdatabase and discussions with government andindustry geologists identified some possibleoccurrences which warranted investigation. Inthe last two years a number of mineral occur-rences in British Columbia were examined andsampled, including the Greenwood Camp,Watson Bar, the Golden Bear mine, Slam andHowell Creek structure.

Information concerning the Watson Barproperty is summarized by Cathro et al., 1998; itis not a Carlin-type deposit. Initial reports on theHowell Creek structure and Golden Beardeposits are presented in this volume (Brownand Cameron, 1999; Brown et al., 1999). TheSlam property in northern British Columbia islocated approximately eleven kilometers east-northeast of the Golden Bear mine. Gold miner-alization is associated with a northeast-trending,silicified limestone unit which contains dissemi-nations of fine, dark gray pyrite. Several rocksamples from the silicified zone assayed over1.0 g/t gold with anomalous values for mercury,arsenic and antimony (Walton, 1987). The Slamis similar to some of the Golden Bear depositsand is most likely a Carlin-type deposit. TheGreenwood Camp does have a number of car-bonate units with associated skarns which sug-


Photo 3. Barren jasperoid boulders located approxi-mately 100 metres from Gold Bar open pit in an areawith no outcrop.

Photo 4. Jasperoid showing relict bedding, JerrittCanyon, Nevada.

gest the area is potentially prospective for sedi-ment-hosted disseminated gold, however, nosediment-hosted disseminated gold depositshave been identified in the area.

CONCLUSIONS

Carlin-type deposits are difficult to findbecause their setting and genesis are so poorlyunderstood. Therefore, it is difficult to determinethe key features to use for exploration.Furthermore, the gold is microscopic and manydeposits have low sulphide contents (<1%).Even in the higher grade deposits the gold is typ-ically associated with a very fine-grained, black,sooty pyrite that can be difficult to identifymacroscopically. Surface weathering can con-vert all sulphides to hematite and limonite, andrarely produces significant gossans. In Nevada,it took sustained exploration over 20 years alongthe major mineralization trends to identify manyof the new sediment-hosted disseminated golddeposits and discoveries continue to be made.

In the Canadian Cordillera there has beenrelatively little exploration for these depositsdespite some geological similarities withNevada. Poulsen (1996b) noted that two differ-ent geological environments in British Columbiamight host Carlin-type mineralization. Theseare: 1) accreted terranes with a basement con-taining carbonate lithologies intruded byMesozoic or Cenozic plutonism, specifically theStikine and Quesnel terranes, and 2) sedimentsdeposited along the continental margin of ances-tral North America which have been cut byMesozoic magmas, such as are found in theKootenay Arc and Selwyn Basin.

Initial results from the sediment-hosted goldproject show that the Bear, Grizzly, Kodiak A,Kodiak B and Ursa are Carlin-type deposits andthe Slam occurrence is similar. There is consid-erable potential to find other gold occurrences ofthis type in the Tatsamenie Lake region. Furtherwork will probably identify other prospectiveregions in British Columbia.

ACKNOWLEDGEMENTS

Howard Poulsen assisted with the start up ofthe project and we continue to benefit from hisexpertise. All three authors have benefited fromthe full support of Andrew Hamilton and the

other staff during our visits to the Golden Bearmine of North American Metals Corp. DerekBrown and David Lefebure visited a number ofsediment-hosted disseminated gold mines inNevada in 1998. The many geologists we metduring our travels shared information and ideasfreely and helped us to understand Carlin-typedeposits. We would like to especially thank GregGriffin of Barrick Goldstrike Mines Inc. whofirst invited us to Nevada, Jim McDonald ofWhite Knight Resources Ltd., TommyThompson of the Ralph J. Roberts Center forResearch in Economic Geology and StephenPeters of the USGS. Brian Grant of the BCGSprovided useful comments on how to improvethe article. The diagrams in this article have beendrafted by Mike Fournier and Maurice Johnson.

REFERENCES

Adams, S.S. and Putnam III, B.R. (1992):Application of mineral deposit models inexploration: a case study of sediment-hostedgold deposits, Great Basin, western UnitedStates; in Case Histories and Methods inMineral Resource Evaluation, GeologicalSociety, Special Publication Number 63, pages1-23.

Arehart, G.B. (1996): Characteristics and Origin ofSediment-hosted Disseminated Gold Deposits:a Review; Ore Geology Reviews, Volume 11,pages 383-403.

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THE BRITISH COLUMBIA SEDIMENT-HOSTED GOLD · PDF fileIn 1997 the British Columbia ... initiated a project to identify prospective areas for sediment-hosted gold min- ... Belt rocks