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Geologic Overview of the Gold Deposits of theCarlin Trend, Nevada, USA

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    Geologic Overview of the Gold Deposits of the Carlin Trend, Nevada, USA

    Steve Garwin

    Geoinformatics Exploration, PO Box 1675, West Perth, Western Australia 6872

    Centre for Exploration Targeting, University of Western Australia, Crawley, Western Australia 6907 INTRODUCTION The Carlin trend contains about 3000 metric tonnes (97 million ounces) of gold in combined past production and reserves from about 30 sedimentary rock-hosted disseminated gold deposits that form a northwesterly trending belt, ~60 km by 7 km, in north-central Nevada, USA (Fig. 1; Roberts, 1960; Teal and Jackson, 1997; Bettles, 2002). The majority of the gold deposits are hosted by lower Paleozoic carbonate and siliciclastic rocks in the lower plate of the Roberts Mountains thrust, above which occurs an allochthonous package of lower Paleozoic siliciclastic rocks. The upper plate was transported by easterly-directed thrusting during the Late Devonian to Middle Mississippian Antler orogeny (Roberts et al., 1958). During the late Jurassic, alkaline magmatism led to the emplacement of the Goldstrike monzodioritic intrusion and lamprophyre dyke swarms in the northern Carlin trend, near the present day location of the large Betze-Post mine (~ 1000 t Au; Teal and Jackson, 1997; Bettles, 2002). In the deposits of the Carlin trend, gold occurs as sub-micron particles in arsenian pyrite-bearing ores formed during the Eocene (40-37 Ma; Ressel et al., 2000), which display structural (fault/fracture)-, dissolution (collapse) breccia-, and stratabound-controls (Teal and Jackson, 1997; Jory, 2002). This period of gold mineralisation coincides with the onset of approximately east-west oriented extensional tectonism and related calc-alkaline magmatism in north-central Nevada (Stewart, 1980; Seedorff, 1991). Miocene Basin and Range extension, facilitated by northerly striking normal faults (20 to 14 Ma; Stewart, 1980), has reactivated segments of early formed faults, however, has not significantly disrupt the ore bodies discovered to date. The earliest recorded mining of gold in the Carlin trend occurred in 1907 from placers in Lynn Creek, near the Carlin and Leeville deposits (Fig. 1; Coope, 1991). Gold was discovered in bedrock exposed at Bootstrap in 1946 by Marion Fisher and mined on a small scale from 1957 through 1960. Gold was discovered in turquoise workings at Blue Star in 1959 and subsequently produced in 1961. In 1960, Ralph Roberts described the Lynn-Railroad mineral belt in a paper he published on the alignment of mineral districts in North-central Nevada (Roberts, 1960). This paper and discussions with Roberts encouraged John Livermore and Alan Coope, Newmont geologists, to prospect in the vicinity of the original Carlin deposit, which they discovered in November of 1961. At the onset of production in 1965, Carlin contained 10 million tonnes at 10 g/t Au (Coope, 1991). Gold was discovered in the vicinity of the Goldstrike stock in 1962 by Harry Ranspot of Atlas Minerals, however, production did not commence in this area until 1978 (Bettles, 2002). Several surface oxide ore deposits with average gold grades that range from ~ 1 to 2 g/t Au were discovered from 1979 through 1984, including Gold Quarry, Post Oxide and Genesis. The consulting work of Ralph Roberts in the mid-1980s for the Pancana Western States joint-venture at Goldstrike gave Brian Meikle and Larry Kornze the impetus to drill test a lower plate carbonate sequence target north of the Goldstrike stock (R.J. Madrid, personal communication, 2005). The giant Betze open-pit deposit, with a resource of about 900 million tonnes at 7.5 g/t Au, was discovered adjacent to the Post deposit in 1987 by American Barrick (Bettles, 2002). Exploration strategy changed and began to focus on deeper sulphide ores in 1986, following the discovery of Deep Post, beneath the present Betze-Post open-pit mine. This lead to increased drilling depths to more than 2000 m from surface and the discovery of refractory deposits grading from 12 to 30 g/t Au, including Deep Star, Rodeo-Goldbug and Meikle in 1988 to 1989, and West Leeville in 1994 (Teal and Jackson, 1997; Jory, 2002). In 2000 to 2002, high grade gold values, ranging from 8 to 52 g/t Au over drill intervals of 24 to 29 m, were discovered

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    at depths of 700 to 900 m at Ren in the northern Carlin trend by Cameco Gold. The fiftieth millionth ounce of gold (~1600 t Au) was produced from the Carlin trend in April 2002, making this gold belt one of the three most prolific in the world. CARLIN TREND GEOLOGY Rock Types The composition, porosity and permeability of the local host rocks play a major role in the localization of gold ore in the Carlin trend. The Ordovician to Silurian siliciclastic rocks in the upper plate of the Roberts Mountains thrust are more than 1500 m thick, including repetition of stratigraphic section due to internal low-angle faults and thrusts (Fig. 2; Teal and Jackson, 1997; Jory, 2002). The Ordovician Vinini Formation consists mostly of cherty mudstone and siltstone and minor greenstone and limestone. The Silurian Elder Formation overlies the Vinini Formation in the northernmost part of the Carlin trend and contains micaceous siltstone, limey siltstone and chert (Bettles, 2002). The autothochthonous Ordovician through Devonian rock sequence is exposed in anticlinal hinge zones that approximately coincide with the central axis of the Carlin trend (Fig. 3). The lower plate rocks consist of limestone, dolomite, mudstone, siltstone and quartzite. Lithologic contacts are commonly conformable, however, disconformities occur locally, and intraformational fold-thrusts juxtapose mudstone and siltstone units in the upper part of the autotochthon. The oldest portion of the lower plate sequence exposed in the area consists of Ordovician Pogonip Group limestone and dolomite, which is about 300 m thick (Evans, 1980; Jory, 2002). This unit is overlain by Ordovician Eureka Quartzite (~300 m thick) and Ordovician to Silurian Hanson Creek Formation dolomite (> 200 m thick). The Silurian to Devonian Roberts Mountains Formation is about 400 m thick and includes a 250 m thick lower unit of planar-laminated silty limestone that grades upwards into a wavy- or wispy-laminated silty limestone unit with intercalations of bioclastic debris flows, 1 to 15 mm thick (Teal and Jackson, 1997; Jory, 2002). This upper unit ranges up to 150 m thick and hosts the majority of the gold ore along the Carlin trend (Teal and Jackson, 1997; Bettles, 2002). The overlying Devonian Popovich Formation is up to 400 m thick and consists of silty limestone, micrite and fossiliferous limestone with calcarenite and planar-laminated limey mudstone and silty limestone in the upper part of the unit. The base of the overlying Devonian Rodeo Creek unit, which is up to 250 m thick, is marked by a disconformity north of the Goldstrike intrusion (Bettles, 2002). The variable thickness of this unit is, in part, related to structural repetition caused by fold-thrusts that deform interbedded siliceous mudstone, siltstone and limey siltstone. In addition, the Roberts Mountains thrust has removed the entirety of the Rodeo Creek unit locally. The youngest sedimentary rocks in the Carlin trend consist of poorly consolidated volcaniclastic rocks of the Miocene Carlin Formation that fill local basins with up to 600 m of material (Jory, 2002). Three major episodes of intrusion occurred along the Carlin trend, including late Jurassic alkaline, Eocene calc-alkaline and Miocene rhyolitic events. The late Jurassic, biotite monzodioritic Goldstrike intrusion (158 Ma; Arehart et al., 1993a) is of similar composition to intrusions towards the west, the Little Boulder Basin stock and Vivian sill (Fig. 3). The Goldstrike intrusion forms a northeasterly elongate 4 by 1.5 km stock, sill and dyke complex that lies between the Betze-Post and Genesis deposits. A significant portion of the gold resources in the northern Carlin trend, about 1600 tonnes of gold, occurs adjacent to the Goldstrike intrusion, including the high grade deposits (>25 g/t Au) of Deep Post and Deep Star. Portions of the Deep Post deposit are hosted by the Goldstrike intrusion (Marino, 2002; Streiff and Powell, 2002). Northwesterly trending, quartz monzonite and lamprophyre dykes, also of late Jurassic age, cut the Goldstrike intrusion (Bettles, 2002). Jurassic intrusions are common in the northern Carlin trend, however, are much less abundant in the central part of the belt, where northwesterly trending dykes fill faults in the Mike deposit and near Gold Quarry. Cretaceous intrusions of intermediate composition occur at Welches Canyon, west of the Mike deposit (Teal and Jackson, 1997).

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    Eocene (40-37 Ma; Ressel et al., 2000) medium- to high-K calc-alkaline, porphyritic dacite and rhyolite dykes trend north-northwest to north-northeast through out the northern Carlin trend. These dykes are far less abundant than the Jurassic intrusions. The magma source to the Eocene dykes is not known, however, Ressel et al. (2000) infer that a large, ~500 km2, airborne magnetic anomaly that coincides with the Welches Canyon stock and the Emigrant Pass volcanic field may represent a concealed Eocene pluton and a potential source of heat for auriferous fluid flow in the area. Cross-cutting field relationships and 40Ar/39Ar radiometric dating indicate the emplacement of dacite and rhyolite dykes to be coeval with gold mineralisation in the northern Carlin trend. The perlitic texture in the glassy margins to the rhyolite dykes in Deep Star are inferred by Ressel et al. (2000) to indicate emplacement at depths of less than 2 km from the Eocene paleosurface. Miocene rhyolitic lava flows (~15 Ma) dip up to 15o to the west along the western side of the Tuscarora Spur in the vicinity of Blue Star, indicating minor amounts of post-mineralisati