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

    Steve Garwin

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

    Centre for Global Metallogeny, Department of Geology and Geophysics, 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 forma 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 lowerPaleozoic 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 wastransported by easterly-directed thrusting during the Late Devonian to Middle Mississippian Antlerorogeny (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, in press). This period of gold mineralisation coincides with the onset of approximately east-westoriented extensional tectonism and related calc alkaline magmatism in north central Nevada (Stewart

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    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 theRoberts Mountains thrust are more than 1500 m thick, including repetition of stratigraphic section dueto internal low-angle faults and thrusts (Fig. 2; Teal and Jackson, 1997; Jory, in press). TheOrdovician 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). Theautothochthonous 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 rocksconsist of limestone, dolomite, mudstone, siltstone and quartzite. Lithologic contacts are commonlyconformable, 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 PogonipGroup limestone and dolomite, which is about 300 m thick (Evans, 1980; Jory, in press). This unit is

    overlain by Ordovician Eureka Quartzite (~300 m thick) and Ordovician to Silurian Hanson CreekFormation dolomite (> 200 m thick). The Silurian to Devonian Roberts Mountains Formation is about400 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 debrisflows, 1 to 15 mm thick (Teal and Jackson, 1997; Jory, in press). This upper unit ranges up to 150 mthick and hosts the majority of the gold ore along the Carlin trend (Teal and Jackson, 1997; Bettles,2002) Th l i D i P i h F ti i t 400 thi k d i t f ilt

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    Eocene (40-37 Ma; Ressel et al., 2000) medium- to high-K calc-alkaline, porphyritic dacite andrhyolite dykes trend north-northwest to north-northeast through out the northern Carlin trend. Thesedykes 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 aconcealed Eocene pluton and a potential source of heat for auriferous fluid flow in the area. Cross-

    cutting field relationships and40

    Ar/39

    Ar radiometric dating indicate the emplacement of dacite andrhyolite dykes to be coeval with gold mineralisation in the northern Carlin trend. The perlitic texture inthe glassy margins to the rhyolite dykes in Deep Star are inferred by Ressel et al. (2000) to indicateemplacement 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 theTuscarora Spur in the vicinity of Blue Star, indicating minor amounts of post-mineralisation rotation

    due to Miocene extensional tectonsim.

    Structural Geology

    The Carlin trend represents a northwesterly trending first-order crustal basement structure, as indicatedby regional radiogenic isotope and geophysical data (Tosdal et al., 2000). A prominent structural highcoincides with the Carlin trend and is characterized by a regional anticlinorium developed during the

    late Paleozoic to early Mesozoic that has been modified by mid-Tertiary extension and horstdevelopment. This anticlinorium deforms the Roberts Mountains thrust and related folds formedduring the Antler orogeny (Roberts et al., 1958). Second- and third-order faults are steeply dipping

    and trend northeasterly and north-northwesterly / west-northwesterly, respectively (Fig. 5; Garwin,2002). The geometric relationships of these faults to the distribution of facies in lower plate rock units,late Jurassic and Eocene intrusions, and Miocene volcanic and volcaniclastic rocks indicate that bothf lt t h b ti f th D i th h th Mi H i h it d t t

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    6) Eocene dykes within or adjacent to major structures, particularly those associated withstructural intersections.Many of the gold deposits in the northern Carlin trend coincide with structural intersections formed bysteeply dipping fault and fracture zones, which are dyke-filled locally (Teal and Jackson, 1997;Madrid, 2001; Bettles, 2002; Madrid and Garwin, 2002). Through systematic structural mapping near

    the Carlin mine, Madrid and Garwin (2002) demonstrate that west-northwesterly-, northerly- andnorthwesterly-striking, syn-mineralisation gold-bearing fault and fracture systems have acted as activeconduits for mineralising fluids. Pre-mineralisation north-northeasterly-striking fault and fracture setsand pre-existing fold hinges were also sites of gold deposition where fluids leaked from the active

    fracture systems into these more passive structures. Post-mineralisation and reactivated structures,

    such as the northeasterly striking fault zones adjacent to the Carlin deposit, do not significantly offsetmain-stage gold-bearing structural systems.

    Hydrothermal Alteration

    Three major hypogene alteration types are documented in the gold deposits of the Carlin trend:

    i) decalcification / decarbonatisation and dolomitisation of the carbonate components of host rocks,ii) pervasive replacement silicification (chalcedony and fine-grained quartz) of carbonate rocks, andiii) argillic alteration of siliciclastic components of sedimentary host rocks and intrusions. Weak to

    moderate propylitic alteration (chlorite-smectite-carbonate) occurs in the Goldstrike intrusion distal tomineralised carbonate units. All these styles of hydrothermal alteration are attributed to the Eocenegold event, in contrast to the quartz-sericite-pyrite alteration and skarn assemblages that formed in

    sedimentary rocks adjacent to the Goldstrike intrusion during the late Jurassic.

    Early decalcification and decarbonatisation are characterized by the dissolution of rock-formingb t k d l t b h d th l d l it d F M b t i l A f

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    Teal and Jackson, 1997; Jory, in press; Garwin, 2002). The large deposits, such as Betze-Post andGenesis, contain elements of each of these three styles and are termed intermediate in this paper.

    Gold commonly occurs as submicron particles within 1 to 5 micron wide rims of arsenical pyrite thatform around early pyrite or marcasite of diagenic and hydrothermal origin (Arehart et al., 1993b;Emsbo, 1999; Garwin, unpublished data). Main ore-stage pyrite from Meikle and Betze-Post yield

    mean concentrations of 1800 ppm Au, 180 ppm Ag, 3.4% As, 0.3% Sb and 0.86 ppm Hg (Emsbo,1999). Individual gold concentrations may reach as high as 1.1 atomic percent in arsenical pyriteanalyzed from ore at Carlin (Garwin, unpublished electron microprobe results). Textural relationshipsindicate that the trace-element rich pyrite and marcasite precipitated after decarbonatisation and

    volume-loss, probably during, or after, replacement silicification. Ore zone gangue minerals include

    quartz, kaolinite +/- arsenopyrite and minor illite and illite-smectite (Leach, 2000; Garwin, 2002).

    Late open space- and fracture-fillings consist of at least two phases: i) early drusy quartz, realgar,orpiment, stibnite, galkhaite [(Cs,Tl)(Hg,Cu,Zn)6(As,Sb)4S12], fluorite, pyrite (brassy) and sphalerite;and ii) late kaolinite, smectite, carbonate, chalcedony, barite and alunite (Hofstra and Cline, 2000;

    Leach, 2000