Carlin Garwin

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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; Garwin, 2002).

A second population of gold occurs as rare, disseminated and fracture-controlled native grains (>99%pure), commonly 100 to 500 microns and locally up to 2 mm in mean diameter, which comprise a

minor component of the ore at Rodeo-Gold Bug and Gold Quarry. Emsbo (2002) believes this nativegold to be of SEDEX origin, having formed during the deposition of the upper portion of theDevonian Popovich Formation, and estimates this mode of gold occurrence to contribute as much as

60 tonnes of gold in the deposits north of the Goldstrike intrusion (Emsbo, oral. commun., 2002). Ifthis is true, then native gold constitutes about 3% of the total gold resource in this area.

S th i d Di i


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fluid (pH~4) by carbonate wall rock at a temperature of about 200 to 220o

C (Hofstra and Cline, 2000).However, other potential mechanisms exist for gold deposition, including fluid mixing.

The major controls to mineralisation for the deposits of the Carlin trend are summarized below:

1) Fluid focussing by a structural culmination created by an anticline, thrust duplex or horst;2) Steeply dipping faults, fracture zones and intrusions create conduits for hydrothermal fluids;3) Gently to moderately dipping lithologic contacts and thrusts create traps for fluid and, in the

case of the thrusts, can thicken prospective stratigraphic section;

4) Receptive and reactive host rocks encourage fluid infiltration and dissolution;5) Contrasting rheology and porosity and permeability of host rocks create competency contrast

and localize fluid flow; and6) Decalcification, dissolution, brecciation and silicification contribute to volume-loss and

provide preparation for sulphidation and gold deposition.

Carlin Trend - Looking Forward

The Carlin trend holds the potential for the discovery of an additional 1000 tonnes of gold (30 millionounces) to the 3000 tonnes already found. The future of the trend lies in the efficient targeting of high-grade underground sulphide ore bodies, such as Deep Post and Deep Star. Such deposits can be found

through rendering the upper plate to the Roberts Mountains thrust transparent by better understandingthe stratigraphy of the allochthon and delineating zones of structural complexity that point towardstargets in the lower plate carbonate rocks. In addition, geological and geochemical vectors to ore need

to be enhanced by applying methods that utilise fracture mapping, alteration zoning and 3D models.Non-traditional host rocks, such as intrusions, the upper plate siliciclastic sequence and theautotochthon, below the Roberts Mountains Formation, should be tested rigorously. Innovatived illi t h i ill b i d t l f d d it f d d l tf L tl


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Coope, J.A., 1991, Carlin Trend exploration history: Discovery of the Carlin deposit: Nevada Bureauof Mines and Geology Special Publication 13, 16 p.

Dunbar, W., 2001, A structural model mineralization at Deep Star, Carlin trend, Nevada, in Shaddrick,

D.R., Zbinden, E.A., Mathewson, D.C., and Prenn, C., eds., Regional tectonics and structuralcontrol of ore: The major gold trends of northern Nevada: Reno, Nevada, Geological Society of

Nevada, Special Publication no. 33, 19 p.

Emsbo, P., 1999, Origin of Meikle high grade gold deposit from the superposition of Late DevonianSEDEX and mid-Tertiary Carlin-type gold mineralisation: unpublished Ph.D. thesis, Golden,Colorado, Colorado School of Mines, 394 p.

Emsbo, P., 2002, Geology, geochemistry and genesis of sedex Au ores on the Carlin trend, Nevada:Abstracts with Programs, Annual Meeting and Exposition, Denver 2002, Geological Society of

America, abstract 63-4, p. 140.Evans, J.G., 1908, Geology of the Rodeo Creek and Welches Canyon quadrangles, Eureka County,

Nevada: US Geological Survey Bulletin 1473, 81 p.Garwin, S.L., 2002, Geologic overview of the Carlin trend, Nevada: oral presentation in: Thorman, C.,

compiler, Carlin-type gold deposits of northern Nevada and their regional setting field trip,

Global Exploration 2002: Integrated methods for discovery, Society of Economic Geologists,

Littleton, CO, USA, 10 p.Hofstra, A.H. & Cline, J.S., 2000, Characteristics and models for Carlin-type gold deposits: Reviews

in Economic Geology, v. 13, p. 163-220.

Jory, J., in press, Stratigraphy and host-rock controls of gold deposits of the northern Carlin trend: in,Thompson, T.B., Teal, L. & Meeuwing, R.O., eds., Deep deposits along the Carlin trend:

Nevada Bureau of Mines and Geology Bulletin 111.

Leach, T.M., 2001, Comments on the geology geochemistry workshop, February, 2001, Elko,Nevada: unpublished report for Newmont Mining Corporation, 15 p.

Lewis, P.D., 2001, Structural analysis of gold deposits of the Carlin trend, Elko and Eureka Counties,

N d P t I P j t Obj ti th d l d lt bli h d t t N t


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Stewart, J.H., 1980, Geology of Nevada: Nevada Bureau of Mines and Geology Special Publication 4,136p.

Streiff, R., and Powell, J., 2002, Deep Post 2001 year-end summary report: unpublished Newmont

internal report, January 2001. 5pTeal, L. & Jackson, M., 1997, Geologic Overview of the Carlin trend gold deposits and descriptions of

recent deep discoveries: Society of Economic Geologists Newsletter, no. 31.

Tosdal, R.M. & Nutt, C.J., 1999, Late Eocene and Oligocene tectonic setting of Carlin-type Audeposits, Carlin trend, Nevada, USA: in, Stanley et al., eds., Mineral deposits: Processes to

processing: Roterdam, Balkema, p. 905-908.

Tosdal, R.M., Wooden, J.L. & Kistler, R.W., 2000, Geometry of the neoproterozoic break-up, andimplications for location of Nevadan mineral belts: in Cluer, J.K., Price, J.G., Struhsacker,

E.M., Hardyman, R.F. & Morris, C.L., eds., Geology and Ore Deposits 2000: The Great Basinand Beyond: Geological Society of Nevada Symposium Proceedings, May 15-18, 2000, p. 451-

466.


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Figure Captions

Figure 1. Simplified geology map of the Carlin trend, Nevada, which shows the location of major gold deposits

(after Teal and Jackson, 1997). The locations of the town of Carlin, Welches Canyon (WC) and Emigrant

Pass (EP) are indicated.

Figure 2. Idealized stratigraphic column for the lithologic units in the Carlin trend, which shows the correlation

between sedimentary host rocks and gold deposits (after Teal and Jackson, 1997). Recent work at GoldQuarry in the central Carlin trend indicates that the Rodeo Creek unit hosts significant gold ore, more

than the figure illustrates.

Figure 3. Simplified geology map of the northern and central Carlin trend, which shows the relationshipsbetween the rock sequences in the upper- and lower-plate of the Roberts Mountains thrust, intrusions,

major faults and folds, and gold deposits. Note that the vast majority of the intrusions labelled as

Jurassic-Cretaceous are actually late Jurassic in age. The cross-section line for figure 4 is indicated.

Figure 4. Schematic cross-section for the northern Carlin trend, adjusted to a horizontal datum for the Popovich

Formation Rodeo Creek unit contact, which shows the relationship between gold distribution (>1 g/t

Au), rock units, the Goldstrike intrusion and the stocks thermal aureole. This aureole is characterized bycalc-silicate skarn, marble and hornfels. Location of section is indicated in figures 3 and 5.

Figure 5. Structural interpretation of the northern and central Carlin trend, which shows major fault zones,

competent bodies, structural culminations or domes, and gold deposits. The direction of the regionalEocene extension-direction (s 3) is from Seedorff (1991). The northwesterly trending axis of the Carlin

trend coincides with a major structural high, as indicated by the ~3500 foot (1100 m) contour of the top

of the Roberts Mountains autotochthon, as estimated from the interpretation of wavelet edge-enhancedgravity data collected by Newmont Mining Corporation. The cross-section line for figure 4 is indicated.


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Figure 1


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Figure 2


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Figure 3


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Figure 4


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Figure 5


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Figure 7

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