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Carlin-Type Gold Deposits in Nevada:Critical Geologic Characteristics and Viable Models

JEAN S. CLINE,†

University of Nevada, Las Vegas, 4505 Maryland Parkway, Box 454010, Las Vegas, Nevada 89154-4010

ALBERT H. HOFSTRA,

Mineral Resources Program, U.S. Geological Survey, Mail Stop 973, Box 25046, Denver, Colorado 80225

JOHN L. MUNTEAN,

Nevada Bureau of Mines and Geology, Mail Stop 178, University of Nevada, Reno, Nevada 89557-0088

RICHARD M. TOSDAL, AND KENNETH A. HICKEY

Mineral Deposit Research Unit, University of British Columbia, 6339 Stores Road, Vancouver, British Columbia, Canada V6T 1Z4

Abstract

Carlin-type Au deposits in Nevada have huge Au endowments that have made the state, and the UnitedStates, one of the leading Au producers in the world. Forty years of mining and numerous studies have provided a detailed geologic picture of the deposits, yet a comprehensive and widely accepted genetic model re-mains elusive. The genesis of the deposits has been difficult to determine owing to difficulties in identifyingand analyzing the fine-grained, volumetrically minor, and common ore and gangue minerals, and because of postore weathering and oxidation. In addition, other approximately contemporaneous precious metal depositshave overprinted, or are overprinted by, Carlin-type mineralization.

Recent geochronological studies have led to a consensus that the Nevada deposits formed ~42 to 36 m.y. ago,and the deposits can now be evaluated in the context of their tectonic setting. Continental rifting and deposi-tion of a passive margin sequence followed by compressional orogenies established a premineral architectureof steeply dipping fluid conduits, shallow, low dipping “traps” and reactive calcareous host rocks. Sedimentary rock sequences that formed following continental margin rifting or in a foreland basin ahead of an advancingthrust front contain reactive pyritic and carbonaceous silty limestones, the primary host rocks in almost every deposit. The largest deposits now lie in the lower plate to the Devonian to Mississippian Roberts Mountainthrust, which placed nonreactive, fine-grained siliciclastic rocks with less inherent rock permeability, above

more permeable carbonate stratigraphy, forming a regional aquitard. North-northwest– and west-northwest–striking basement and Paleozoic normal faults were inverted during postrifting compressionalevents and formed structural culminations (anticlines and domes) that served as depositional sites for aurifer-ous fluids in the Eocene. These culminations are now exposed as erosional windows through the siliciclasticrocks of the Antler allochthon.

During the Eocene, northwesterly to westerly extension reopened favorably oriented older structures asstrike-slip, oblique-slip, and normal-slips faults. Fluid flow and mineral deposition appear to have been fairly passive as there is minimal evidence for overpressured hydrothermal fluids, complicated multistage vein dila-tancy, or significant synmineralization slip. Geologic reconstructions and fluid inclusions indicate that depositsformed within a few kilometers of the surface. Ore fluids were moderate temperature (~180°–240°C), low salinity (~2–3 wt % NaCl equiv), CO2 bearing (


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Introduction

CARLIN-TYPE Au deposits are largely replacement bodies with visually subtle alteration dominated by decarbonatization of silty carbonate host rocks, and they contain Au in solid solu-tion or as submicron particles in disseminated pyrite or mar-casite. The deposits occur in clusters or along trends (Fig. 1)

and exhibit both structural and stratigraphic controls. They were first recognized as a new deposit type after discovery of the Carlin deposit in 1961, even though some deposits hadbeen mined since the early 1900s.

Current production from Carlin-type deposits dominatesAu production in the country and constitutes ~9 percent of

world production, making the United States one of the

452 CLINE ET AL.

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containing Au. Metamorphism at midcrustal levels may have contributed fluids, all of which were collected intobasement-penetrating rift faults, where they continued to rise and scavenge various components, evolving incomposition to become ore fluids. North-northwest–trending paleo-normal faults and northeast-trendingpaleo-transform faults, preferentially dilated during Eocene extension, controlled the regional position, orien-tation, and alignment of the deposits. Eventually the ore fluids accumulated in areas of reduced mean effec-tive stress, particularly boundaries of older Jurassic and Cretaceous stocks and structural culminations. The orefluids were diluted by exchanged meteoric water as extension increased fault permeability in the upper crust. Within a few kilometers of the surface, fluids were diverted by structural and stratigraphic aquitards into reac-

tive host rocks, where they sulfidized host rock iron and deposited Au.Sedimentary rock-hosted disseminated Au deposits in other parts of the world exhibit many similarities to

Nevada Carlin-type Au deposits, yet no district has been discovered anywhere else that approaches Nevada’sAu productivity. The deposits found in other parts of the world are products of diverse, well-recognized, hy-drothermal systems (e.g., low-sulfidation epithermal, porphyry Cu-Mo-Au, reduced intrusion-related epizonalorogenic, and sedimentary exhalative or sedex). Of these, the deposits in southern China are remarkably simi-lar to Nevada Carlin-type deposits and are interpreted to have formed where metamorphic fluids reacted with wall rocks and local meteoric water.

FIG. 1. Digital elevation model of northern Nevada, showing locations of major mineral belts and districts, Carlin-typedeposits (circles; Peters et al., 2003), other significant Au, Ag, Pb, Zn, or Cu deposits (crosses; Long et al., 2000), easternlimit of the Roberts Mountain allochthon (bold white line; Crafford and Grauch, 2002), cities (small circles), and highways(black lines). Inset (white box) shows the distribution of Carlin-type deposits in the northern Carlin trend (Thompson et al.,2002). Figure provided by D. Sweetkind, U.S. Geological Survey, and modified from Hofstra et al. (2003).


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leading Au-producing countries in the world (Nevada Bu-reau of Mines and Geology, 2004). Although several Carlin-type Au deposits in Nevada are world class (>100 t Au) andgiant (>250 t Au), total contained Au and ore grades vary

widely across districts and within deposits (Fig. 2). Explo-ration drilling through cover has greatly expanded the sizeof several districts and, along with developed mines, shows

that deposits occur in camps or linear groups (trends) andthat small districts may have significant resources at depth.Ten deposits in the Carlin, Getchell, and Battle Mountain-Eureka trends contain more than 5 million ounces (Moz) of Au and four deposits contain more than 10 Moz (Fig. 2).The Carlin trend contains the largest endowment of Auidentified to date for such deposits in Nevada, and produc-tion has now exceeded 50 Moz (Nevada Bureau of Minesand Geology, 2004).

Despite the huge production and more than forty years of mining and research, a generally accepted genetic model forthe deposits has yet to emerge (e.g., Joralemon, 1951; Hausenand Kerr, 1968; Bagby and Berger, 1985, Radtke, 1985; See-dorff, 1991; Kuehn and Rose, 1995; Ilchik and Barton, 1997;Hofstra and Cline, 2000; Emsbo et al., 2003). Development of a comprehensive genetic model has been hampered by widely conflicting data and, until recently, lack of agreement on theage of the deposits. Field relationships and conventional

geochronological studies (App. Tables A1, A2) now indicatethat the deposits formed during a narrow time interval dur-ing the mid to late Eocene, between about 42 and 36 Ma(App. Table A3; Hofstra et al., 1999; Tretbar et al., 2000; Are-hart et al., 2003). Establishing this timing was critically im-portant as the tectonic regime during deposit formation cannow be incorporated into a genetic model. Sometime during

the middle Eocene, northern Nevada began to undergo atransition from contractional to extensional deformation of the upper crust with associated calc alkaline magmatism(App. Fig. A1). This transition is considered instrumental todeposit formation.

Current models of deposit formation incorporate (1) mete-oric water circulation (Ilchik and Barton, 1997; Emsbo et al.,2003), (2) epizonal plutons that contributed heat and possibly fluids and metals (Sillitoe and Bonham, 1990; Henry and Res-sel, 2000; Johnston and Ressel, 2004), and (3) deep meta-morphic fluids ± magmatic fluids (Seedorff, 1991; Hofstraand Cline, 2000). Much of the controversy regarding the gen-esis of Carlin-type Au deposits results from the fact that

within some of the districts and trends and at other locationsin Nevada, there are sedimentary rock-hosted disseminatedAu deposits that have some attributes in common with, andother features that are distinct from, Carlin-type deposits(e.g., Hofstra and Cline, 2000; Hofstra, 2002).

CARLIN-TYPE Au DEPOSITS, NEVADA 453

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0.1 1.0 10 100 1000

.1

1

10

100

0 . 0 1 t A

u

0 . 1 t A u

1 t A u

1 0 t A u

1 0 0 t A u

1 0 0 0 0 t A u

1 0 0 0 t A u

Meikle

CarlinGetchell

Betze

Cortez

Hills

Screamer JC NCT

GTSCT

GB

AR

EU

CTZ

Rabbit

Creek

Pipeline

Gold Quarry

Mike CCT

Metric Tons Ore

g / t A

u

FIG. 2. Ore grade (g/t Au) vs. metric tons of ore for Carlin-type deposits (small diamonds), deposits containing greaterthan 5 Moz (155.5 t) gold (squares), and major districts (triangles). AR = Alligator Ridge district, CCT = central Carlin trend;CTZ = Cortez district in Battle Mountain-Eureka trend (BMET), EU = Eureka district in BMET, GB = Gold Bar district inBMET, GT = Getchell trend, JC = Jerritt Canyon district, NCT = north Carlin trend, SCT = south Carlin trend.


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We believe that these sedimentary rock-hosted dissemi-nated Au deposits are products of several well-recognized anddistinctly different types of hydrothermal systems (Table 1; A.Hofstra and P. Emsbo, unpub. synthesis, 2004). Of these, dis-tal disseminated deposits associated with porphyry Cu-Mo-Au systems (Cox and Singer, 1990, 1992) have frequently been called upon to explain Carlin-type deposits (e.g., Sillitoe

and Bonham, 1990; Johnston and Ressel, 2004). These intru-sion-related deposits exhibit many of the characteristics of Carlin-type Au deposits, but they have higher Ag and basemetal concentrations, form from higher temperature andhigher salinity fluids, and have clear spatial and genetic rela-tionships with porphyry systems (Cunningham et al., 2004).Outside of Nevada, sedimentary rock-hosted disseminatedAu deposits are concentrated in southern China (Li and Pe-ters, 1998, Peters, 2002) but also occur at scattered locationsaround the world. Although the tectonic settings are differ-ent, the sedimentary rock-hosted disseminated Au deposits inthe West Qinling belt and Dian-Qian-Gui area of southernChina are most similar to Nevada Carlin-type deposits. Muchof the controversy over whether or not Carlin-type deposits,distal disseminated deposits, and other sedimentary rock-hosted disseminated Au are essentially the same rather thandiscrete deposit types results from the assumption that there isonly one way to produce a deposit with Carlin-type attributes,

which we believe to be incorrect. This debate is not merely aca-demic, as the various deposit types (Table 1) have different en-dowments of Au and will lead exploration geologists into dif-ferent geologic environments in search of new discoveries.

This paper builds on earlier studies by incorporating new data and considering critical geologic features of Carlin-type

deposits identified during detailed comparisons of the fivemajor trends and districts in Nevada (Carlin, Getchell, BattleMountain-Eureka, Jerritt Canyon, Alligator Ridge; Tables 2,A1, A2). A review (Hofstra and Cline, 2000) benefiting fromknowledge of the age of the deposits evaluated available datain search of a best-fit genetic model but concluded that nosingle model could accommodate all credible data. Our cur-

rent goal is to derive, through evaluation of new data andcomparison of districts, a genetic model that can be used as abaseline for comparison with other sediment-hosted dissemi-nated Au deposits around the world.

Another goal of this paper is to relate details of deposit ge-ology to the tectonic evolution of northern Nevada. Why aremajor economic deposits of this type restricted to this area?Has a lengthy and unique geologic history or particular geo-logic event contributed to their formation? In comparing thegeology of districts and trends we are struck by the consis-tency of geologic features at scales from regional to submi-croscopic. These consistent characteristics, the relatively small size of the region that contains the deposits, and thenow widely accepted narrow time frame for Au depositionthat coincides with incipient extension and widespread mag-matism, convince us that deposits in all districts are geneti-cally linked and formed in response to crustal-scale orogenicprocesses that operated across northern Nevada.

Our study leads us to a genetic model that incorporates fea-tures proposed in earlier models and calls upon deep crustalprocesses to generate some components of a primitive orefluid. Some ore-fluid components may have been sourced inthe lower crust, where they initially accumulated and begantheir lengthy ascent toward the surface. Metamorphic ± mag-

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TABLE 1. Deposits with Geologic Features Similar to the Nevada Carlin-Type Gold Deposits

Deposit type Examples References

Distal disseminated deposits associated Lone Tree and Marigold, Nevada Theodore (2000), with porphyry Cu-Mo-Au systems (also McKibben and Theodore (2002)Carlin-like deposits) Seedorff (1991)

Melco and Barneys Canyon, Utah Presnell and Parry (1996), Gunter and Austin (1997),Cunningham et al. (2004)

Star Pointer, Nevada Smith et al. (1988)Jeronimo and Silica del Hueso, Chile Colley et al. (1989), Gale (2000),

Simian and Hitzman (2000)Bau, Malaysia Percival et al. (1990), Sillitoe and Bonham (1990,

Schuh (1993), Percival and Hofstra (2002)Mesel, Indonesia Turner et al. (1994), Garwin et al. (1995),

Corbett and Leach (1998), Turner et al. (2002)Zarshuran and Agadarreh, Iran Daliran et al. (1999, 2002), Mehrabi et al. (1999),

Asadi et al. (2000), Lescuyer et al. (2003)

Distal disseminated deposits associated Bald Mountain, Nevada Hitchborn et al. (1996), Nutt et al. (2000), with reduced intrusion-related systems Nutt and Hofstra (2002)

True North, Alaska and Thompson and Newberry (2000),Brewery Creek, Yukon Flanigan et al. (2000), Poulsen (1996)

Epizonal and disseminated orogenic deposits Fosterville, Victoria and Reefton, New Zealand Beirlein and Maher (2001), Bierlein et al. (2002)

Sedex deposits Rodeo, Nevada Emsbo et al. (1999, 2002), Emsbo (2000)

Hybrid deposits with sedex and Meikle (footwall dolomitization) and Emsbo et al. (2003)stratiform ore partially overprinted by Rodeo (stratiform), NevadaCarlin-type mineralization


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matic processes may have generated or contributed to theoriginal deep ore fluid, which likely evolved significantly as

various components, including Au, were added and removedalong fluid pathways.

Pre-Eocene Tectonic Framework

The superposition of tectonic events extending back to theProterozoic in northeastern Nevada produced geologic fea-tures that are common to all Nevada Carlin-type deposits.These pre-Eocene events, namely continental rifting followedby several compressional orogenies, built a premineral archi-tecture of steeply dipping fault and fracture meshes thatacted as fluid conduits. The resulting low-angle faults com-bined with impermeable rock units formed aquitards to in-hibit fluid ascent, causing the steep faults to channel fluidsinto permeable and reactive calcareous host rocks (App. Fig.A2).

Geologic history

During the assembly of the Laurentia supercontinent, Pa-leoproterozoic terranes were accreted to the Archean

Wyoming craton. The suture zone, which is well defined inthe Cheyenne belt of Wyoming, has a westward trend intonorthern Nevada where it becomes a broad zone of inter-mixed Paleoproterozoic and Archean rocks (Lush et al.,1988; Tosdal et al., 2000; App. Fig. A3). In the Meso- andNeoproterozoic, rifting between 1.0 and 1.3 and 0.6 and 0.9Ga separated Laurentia from an adjoining crustal block(Karlstrom et al., 1999; Timmons et al., 2001), thereby form-ing a progressively westward-thinning margin of continentalcrust (Tosdal et al., 2000). A westward-thickening wedge of

Neoproterozoic and early Cambrian clastic rocks buried thethinned crystalline basement during the rift phase of exten-sion (Stewart, 1972, 1980; Poole et al., 1992). Following ac-tive rifting of the continent, a miogeoclinal sequence devel-oped on the trailing continental margin with deposition of

interbedded carbonate and shale on the shelf. To the west,silty carbonate rocks were deposited along the continentalslope.

During the Late Devonian and early Mississippian Antlerorogeny, the Roberts Mountain thrust (Roberts et al., 1958;Stewart, 1980) formed as eugeoclinal siliciclastic and basalticrocks were thrust eastward over the shelf-slope sequencealong a fold and thrust belt (Fig. A2). Loading by the al-lochthon, which was up to several kilometers thick (E.L.Miller et al., 1992), onto the continental margin produced aforedeep basin in eastern Nevada that migrated eastward infront of the thrust belt. Early Mississippian synorogenic andPennsylvanian postorogenic sedimentary rocks filled the fore-

deep (Poole et al., 1992). Intermittent shortening and exten-sion in the Pennsylvanian and through the Permian (e.g., thesouth-directed Humboldt orogeny of Ketner, 1977; Theodoreet al., 2004) was followed by eastward emplacement of theGolconda allochthon during the Sonoma orogeny in the Early Triassic.

An east-dipping subduction zone was established along the western margin of North America by the Middle Triassic.North-central Nevada lay east of the main magmatic arc,

which is represented by the Mesozoic granitic batholiths of the Sierra Nevada Range. Magmatism in north-centralNevada began with emplacement of Middle Jurassic, back-arc volcanic-plutonic complexes and lesser lamprophyre


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TABLE 2. Features Characteristic of Nevada Carlin-Type Gold Deposits

N Carlin Central Carlin S Carlin Battle Mtn-Eureka Getchell Jerritt AlligatorDeposit features trend trend trend trend trend Canyon Ridge

Pre-Eocene structural and stratigraphic architecture XX XX XX XX XX XX XX Underlain by Archean or thinned and mixed

Paleoproterozoic and Archean transitional crust XX XX XX XX XX XX XX Underlain by thick Neoproterozoic to Early Cambrian

rift-related clastic rocks X X X X X X X Location east of or near continental margin XX XX XX XX XX XX XX Proximal to regional thrust fault XX XX XX XX XX XX -Proximal to reactivated rift structures X X X X X X ?High-angle northwest and northeast structures XX XX XX XX XX XX XX

control oreLow-angle structures control ore XX XX - XX XX XX -Rheologic contrast around older stock controls ore XX XX - XX XX - -C- and pyrite-rich silty limestone or limey siltstone XX XX XX XX XX XX XX

host rocksProximal coeval igneous rocks X X X X - X -Characteristic alteration present XX XX XX XX XX XX XX Characteristic ore and late ore minerals present XX XX XX XX XX XX XX Au in arsenian trace element-rich pyrite or marcasite XX XX XX XX XX XX XX Deep magmatic ± metamorphic source identified X nd nd nd XX nd nd

for He, Pb, Nd, ±Sr

Magmatic ± metamorphic water identified in ore fluid X - - - XX - -Meteoric water identified in ore fluid XX XX nd nd - XX ndMagmatic ore S source identified X - - - X - -Sedimentary ore S source identified XX XX XX XX - XX XX Postore oxidation XX XX XX XX XX XX XX

Notes: XX = important deposit feature, X = observed feature, - = not present, nd = no data, ? = unknown


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dikes. Coeval shortening produced a north-trending belt of east-verging folds and thrusts in eastern Nevada (Elkoorogeny of Thorman et al., 1991). Plutons evolved from I-type granitoids in the Early Cretaceous to S-type peralumi-nous granites in the Late Cretaceous, as the crust was pro-gressively thickened (Barton, 1990) during the LateCretaceous Sevier and Laramide orogenies (Burchfiel et al.,

1992). At about 65 Ma magmatism shifted eastward into Col-orado and did not resume in Nevada until about 42 Ma (Lip-man et al., 1972; Hickey et al., 2003b). The area of the north-ern Carlin trend underwent the last significant period of exhumation in the Late Cretaceous and had cooled to


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F I G . 3 . S t r a t i g r a p h i c c o l u m n s f o r d i s t r i c t s c o n t a i n i n g C a r l i n - t y p e g

o l d d e p o s i t s . B a r s i n d i c a t e t h e l o c a t i o n o f C a r l i n - t y p e g o l d m i n e r a l i z a t i o n i n e a c h s e c t i o n .

F o r m a -

t i o n k e y : C d = C D u n d e r b u r g , C e = C E l d o r a d o , C g = C G e d d e s , C h

= C H a m b u r g , C o m = C O s g o o d M o u n t a i n ,

C O s c = C - O r d S n o w C a n y o n , C p = C P r e b l e , C s c =

C S e c r e t C a n y o n , C p m = C P r o s p e c t M o u n t a i n , C w = C W i n d f a l l , D

b = D e v B o o t s t r a p , D d = D e v D e n a y , D d g =

D e v . D e v i l s G a t e , D e f = D e v E a s t e r n F a c i e

s , D g =

D e v G u i l m e t t e , D m = D

e v M a r y s M o u n t a i n , D m c = D e v M c C o l l e y

C a n y o n , D M p = D e v - M i s s P i l o t , D o c = D e

v O x y o k e C a n y o n , D p = D e v P o p o v i c h , D r c = D e v

R o d e o C r e e k , D s = D e v S i m o n s o n , D s l = D e v S e v y L a k e t o w n , D t c = D

e v T e l e g r a p h C a n y o n , D w h = D e v W e n b a n H o r s e C a n y o n , D w = D e v W e n b a n ( P i p e l i n e - C o r t e z ) ,

D w = D e v W o o d r u f f ( S

C a r l i n t r e n d ) , M c = M i s s C h a i n m a n ( E u r e k a , A l l i g a t o r R i d g e ) , M c = M i o C a r l i n ( N o r t h

a n d C e n t r a l C a r l i n t r e n d ) , M j = M i s s J o a n a ,

M P h =

M i s s - P e r m H a v a l l a h , M w

= M i s s W e b b , O c = O r d C o m u s , O e = O r d

E u r e k a , O p = O r d P o g o n i p , O S h c = O r d - S i l H a n s o n C r e e k , O v a = O r d V a l m y , O v i = O r d

V i n i n i ,

P P e = P e n n - P e r m E t c h a

r t , Q a l = Q u a t A l l u v i u m , S D r m = S i l - D e v R o

b e r t s M o u n t a i n , S l m = S i l L o n e M o u n t a i n , S

r m = S i l R o b e r t s M o u n t a i n . D a t a f r o m F r e n c h e t a l .

( 1 9 9 6 ) , T e a l a n d J a c k s o n , ( 1 9 9 7 ) , V i k r e a n d M a h e r ( 1 9 9 6 ) , G . A . G h i d o t t i a n d M . D . B a r t o n , w r i t . c o m m u n . ( 1 9 9 9 ) ,

M c L a c h l i n e t a l . ( 2 0 0 0 ) , Y i g i t ( 2 0 0 1 ) , B e t t l e s

( 2 0 0 2 ) ,

H a r l a n e t a l . ( 2 0 0 2 ) , J o r y

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0 3 ) , D . R e i d , U . S . G o l d C o r p , w r i t . c o m m u n . ( 2 0 0 3 ) ,

P l a c e r D o m e , u n p u b . d a

t a ( 2 0 0 4 ) .


8/34

The fold belts and windows have been attributed to Meso-zoic shortening (Madrid and Roberts, 1991; Peters, 2000) origneous doming (Roberts, 1960). Other evidence suggests aPaleozoic age. In the Carlin trend, northwest-trending foldspredate the Jurassic Goldstrike stock (Moore, 2001). In thePiñon Range around the Rain deposit, northwest-trendingfolds and reverse faults are localized along inverted Early Mississippian normal faults that disrupted the north-strikingAntler fold and thrust belt prior to the deposition of late Mis-sissippian postorogenic strata (Fig. 5; Tosdal, 2001). Theseobservations suggest that the narrow northwest-trendingfold belts are at least in part the result of inversion of the

north-northwest– and west-northwest–striking basement andPaleozoic normal faults, most likely in the late Paleozoic.Other examples of inverted Paleozoic normal faults include (1)the north-northwest–striking Getchell fault and the hanging-

wall, asymmetric, west-verging anticlines of folded growth se-quences as interpreted from seismic sections; (2) the north-northwest–striking Post fault system in the Carlin trend and

west-verging Ernie anticline (Baschuk, 2000); and (3) the west-northwest–striking “flower structures” in the WestBazza pit at Betze-Post (Lauha, 1998) and Rain (Williams etal., 2000). Significantly, these fabrics were also reactivated inthe Mesozoic and the Early Tertiary (see below).

458 CLINE ET AL.

0361-0128/98/000/000-00 $6.00 458

Carbonate reef

(area of calcareoussediment production)

R e e f s l

o p eBasin

(anoxia environment)

d e b r i

s f l o w

synsedimentaryfault(?)

Bootstrap LimestonePopovichFm.

Rodeo Creekunit

Progressive drowning of Bootstrap reef

b.

ELKO COUNTY

EUREKA COUNTY

Stock of Little Boulder BasinGoldstrikeStockC A S T L E R E E F F A U L T

C A S T L E R E E F F A U L T

P OS T

L E E V

I L L E

F A U L T

POST

ANTICLINORIUM

0 1,200 METERS

N

Dee-Rossi

Capstone

Tara

Ren

Meikle

Beast

Betze-Post

Carlin

Bootstrap

shelf margin

F A U L T

EOCENE

EXTENSION

c.

QUATERNARY ALLUVIUM

MIOCENE CARLIN FORMATION

MIOCENE VOLCANIC ROCKS

SILURIAN - DEVONIAN

ELDER FORMATION AND

ORDOVICIAN VININI FORMATION

DEVONIAN RODEO CREEK UNIT

DEVONIAN - POPOVICH FORMATION

SILURIAN - DEVONIAN

BOOTSTRAP LIMESTONE

SILURIAN - DEVONIAN

ROBERTS MOUNTAINS FORMATION

ORDOVICIAN - SILURIAN

HANSON CREEK FORMATION

Volcaniclastic fluvial-lacustriansiltstone, sandstone

Rhyolite flows

Mudstone, siltstone, sandstone, limestone, chert,micaceous siltstone, limey siltstone and chert

ROBERTS MOUNTAINS THRUST

ARGILLITE MEMBER - AA Limey siltstone, mudstone and chert

BAZZA SAND MEMBER - BSLimey siltstone, sandstone and mudstone

ARGILLITE-MUDSTONE MEMBER - AMLimey mudstone and chert (argillite)

Disconformity

UPPER MUD MEMBER - UMLaminated limey to dolomitic mudstone

SOFT SEDIMENT DEFORMATION MEMBER - SDThin bedded limey to dolomitic mudstone andmicritic limestone with Bootstrap-derived boulders

PLANAR MEMBERS - PLLaminated limey to dolomitic mudstone

WISPY MEMBER - WSThick bioclastic debris flows with clasts derivedfrom the Bootstrap interbedded with limey todolomitic mudstone with wispy laminations

Fossiliferous limestone anddolomite shown on left side ofcolumn

Laminated dolomitic, limey mudstone andsiltstone

Disconformity

Sandy Dolomite 1 0 0 + m

3 0 0 - 5 0 0 m

6 0 - 9 0 m

6 0 m

6 0 0 + m

3 0 - 6 0 m

6 0 m

6 0 - 1 2 0 m

3 0 - 9 0 m

3 0 m

0 - 1 0 0 0 m

0 - 3 0 0 m

Selectedmineralized

zones

B e t z

e

L o w e r P o s t

M e i k l e

S c r e a m e

r

R o d e o

P o s t O x i d e

W e s t B a z z a

L o n g L a c

B a z z a

a.

FIG. 4. a. Stratigraphic column through the Goldstrike mine area, showing the localization of the small structural con-trolled orebodies within the Rodeo Creek unit and the concentration of high gold grades and large structural and strati-graphic controlled orebodies in the Popovich Formation (from Volk et al., 1997; Bettles, 2002). b. Paleoenvironment of thehost rocks in the northern Carlin trend, showing distribution of carbonate facies and their formations. The Devonian rocksin the northern Carlin trend represent the reef foreslope facies and the progressive drowning of the Bootstrap reef by theRodeo Creek unit. c. Location of Bootstrap shelf margin. Also shown is the predicted slip pattern on faults that would be ac-tive during Eocene extension.


9/34

Inversion resulted in structural culminations (doubly plunging anticlines, domes), some of which subsequently acted as depositional sites for auriferous fluids. Once ex-humed these culminations are now the erosional windowsthrough the siliciclastic rocks of the Antler allochthon. TheLynn window in the Carlin trend is the best-known example.

Preore (Mesozoic) stocks: The three largest Carlin-type dis-tricts (northern Carlin trend, Getchell, Pipeline-Cortez) arespatially associated with Jurassic or Cretaceous plutons thatare preore. Small satellite stocks and numerous dikes are alsocommon. These bodies were emplaced along the existingfault systems in each district, which subsequently controlled


0361-0128/98/000/000-00 $6.00 459

Normal or inverted normal fault (Miss.)Thrust or reverse fault ofAntler age (Early Miss.)

Approximate mid-Mississippian land surface immediately prior to deposition of onlap (Late Mississippian Fm.)

RAIN HORST

Down-dropped block

Down-dropped block

Alluvium (Quat.)

Tonka Fm. (Late Miss.)Melancro Fm. (Early Miss.)Webb Fm. (Early Miss.)

Woodruff Fm. (Dev.)

Devils Gate Limestone (Dev.)and older carbonate rocks

Normal fault (Tert.)

7000

6000

5000 f e e t a b o v e s e a l e v e l

SSW NNE

RainRAIN HORST

Underlying

rocks

unknown

unknown

rocks

R a i n f

a u l t

R a i n

f a u l tDevils Gate Limestone

Webb Fm.5.1 g/t Au shell

Breccia

0 200

feet

a.

b.

syngenetic

bedded

barite

discordant dolomite alteration

along Paleozoic normal faults

1500

2000

m e t e r s a b o v e s e a l e v e l

500 0meters

FIG. 5. Simplified (a) cross section and (b) restored cross sections looking west-northwest (~300°) and normal to the Rainfault, northern Piñon Range, Nevada. The Late Mississippian and Early Pennsylvanian Tonka Formation (Trexler et al.,2003), which is part of the post-Antler overlap sequence, was deposited on a paleosurface that beveled different levels of theEarly Mississippian Antler fold and thrust belt as well as northwesterly striking Early Mississippian normal faults that cut thefold and thrust belt. The Mississippian normal faults shown in panel b were inverted during late Paleozoic (?) southward-di-rected shortening as steeply to moderately dipping reverse faults. The Rain horst (Mathewson, 2001) is an elevated block of Devonian Devils Gate Limestone, and older subjacent rocks, that forms either the autochthonous footwall to the fold andthrust belt or represents a structurally deeper thrust sheet as is seen in the Railroad area in the central Piñon Range (Tosdal,2001). The margins of the Rain horst are constrained by gravity data (Mathewson and Beetler, 1998). Inset diagram showsdiverging inverted normal faults and subsidiary accommodation reverse faults at a dip change in the Rain fault, which for themost part dips steeply (Longo et al., 2002). Premineral solution collapse and tectonic breccia lies in the hanging wall of the

Rain fault; premineral dolomitized limestone lies in the Devils Gate Limestone below the orebodies. Syngenetic barite lensesare shown schematically in the Webb Formation. The Rain deposit as shown by the 5.1-g/t Au ore shell lies at the top of thebreccia and below the calcareous siltstones that compose the early Mississippian Webb Formation. The Devonian Woodruff Formation, which is correlative to the Rodeo Creek unit in the northern Carlin trend (see Fig. 3), consists of organic-rich(>3% C) metalliferous black shale.


10/34


11/34

fault to another (e.g., Deep Star deposit, Heitt et al., 2003;Post fault in Betze-Post, Moore 2001). The net effect was acomplex fracture mesh adjacent to the plutons that includessmall-scale faults with vertical and horizontal slip vectors, allof which have been recognized in structural studies (e.g.,Deep Star; Heitt et al., 2003).

The heterogeneous strain around the stocks essentially

formed extensive damage zones marked by increased faultand fracture density. These sites of enhanced vertical perme-ability are located asymmetrically around the margins of theintrusions (Fig. 6). The local reduced mean stress associated

with the enhanced fracture density favored Au deposition inthese sites. Intrusions locally resulted in the formation of re-active ferroan carbonates (Twin Creeks, Fortuna et al., 2003;Deep Star, Heitt et al., 2003), which likely facilitated ore de-position by sulfidation, as discussed below. In addition, sill-like apophyses extending from the Mesozoic stocks served aslocal aquitards that helped to develop high grades in theirfootwalls (e.g., Meikle, Volk et al., 1995; Getchell, Tretbar,2004).

Eocene Setting for Carlin-Type Au deposits

The Cretaceous-Paleogene tectonic framework

The western margin of North America underwent near-continuous easterly convergence from the early Mesozoic tothe end of the Oligocene (Engebretson et al., 1985). The Cre-taceous Sevier-Laramide orogeny was a product of that con-

vergence and northeastern Nevada is generally thought tohave occupied the thickened core of the Sevier hinterland(Coney and Harms, 1984; D.M. Miller et al., 1992; Camilleriet al., 1997). From the Late Cretaceous to middle Eocene,the oceanic Farallon and Kula plates were spreading apart

while subducting beneath North America (App. Fig. A4). The

spreading ridge intersected the North American plate some- where between British Columbia and Mexico (Engebretsonet al., 1985), with the slab window produced by the subduct-ing ridge passing northward through Nevada at the beginningof the Eocene, at ~54 Ma (Breitsprecher et al., 2003). TheFarallon plate is thought to have had a subhorizontal geome-try resting on the base of the western North American lithos-phere (Dickinson and Snyder, 1978; Christiansen and Yeats,1992).

High K calc-alkaline magmatism within northern Nevadabegan ~42 Ma and swept southward with time, culminating inOligocene-Miocene volcanic activity in central-southernNevada (Fig. A4; Armstrong and Ward, 1991; Seedorff, 1991;Henry and Boden, 1998) in response to the progressive re-moval or rollback of the Farallon plate and the reintroductionof hot asthenospheric mantle to the base of the North Amer-

ican lithosphere (e.g., Humphreys, 1995; Humphreys et al.,2003). Eocene volcanism was linked to short-lived periods of upper crustal extension and development of broad depres-sions filled with fluvial-alluvial and lacustrine sediments, vol-caniclastic rocks, ash-flow tuffs, and lavas (Solomon et al.,1979; Axen et al., 1993; Potter et al., 1995; Gans et al., 2001;Rahl et al., 2002). Over much of the western cordillera, uppercrustal extensional faulting was accompanied by midcrustalflow and the localized development of Oligocene andMiocene metamorphic core complexes. Extension became

younger to the south, from the late Paleocene in British Co-lumbia to Oligocene and Miocene in Nevada (Parrish et al.,1988; Gans et al., 1989; Foster and Fanning, 1997; Mueller etal., 1999; McGrew et al., 2000; Rahl et al., 2002). The onsetof Eocene extension in northern Nevada has been linked tothe removal of the Farallon plate from the base of NorthAmerican lithosphere, either through gravity-driven collapseof the thickened Sevier hinterland (Jones et al., 1998; Liu andShen, 1998; Rahl et al., 2002) or transient shear stresses at thebase of the brittle crust as lower crustal flow was reestab-lished (Westaway, 1999).

The spatial and temporal overlap of Carlin-type deposits with the onset of Cenozoic volcanism and extension in north-ern Nevada suggests a fundamental link between these phe-nomena (Seedorff, 1991; Hofstra, 1995; Ilchik and Barton,1997; Henry and Boden, 1998; Hofstra et al., 1999). A full un-derstanding of the patterns of crustal fluid flow responsible

for Au mineralization in northern Nevada between ~42 and36 Ma is dependent upon more complete knowledge of thespatial and temporal relationships between regional extensionand volcanism.

Eocene paleogeograpic and tectonic developmentof northern Nevada

Rocks exposed along the modern erosion surface in theCarlin-Jerritt Canyon region were exposed to a major phase


0361-0128/98/000/000-00 $6.00 461

FIG. 6. Simplified geologic map of northern Osgood Mountains adapted from Hotz and Willden (1964), showing asym-metric distribution of Carlin type Au deposits around the Cretaceous Osgood Mountains stock. Also shown are interpreted

Paleozoic normal faults. Inset diagrams in upper right shows the Reidel shear fracture and fault mesh that would fail underextension or extensional shear failure and thus be dilatant under the expected northeasterly to easterly extension in theEocene. Extension directions are based on geologic information from northeastern Nevada (Tosdal and Nutt, 1999) and theBattle Mountain area (Theodore, 2000); no geologic constraints for the strain orientation in the Eocene is known for the Os-good Mountains and thus the range of orientations derived from geologic maps elsewhere in Nevada are shown. In detail,north- to northeast-striking faults would preferentially fail under extension or extensional shear in either orientation and thusbe the favored sites of enhanced fluid flow. Inset diagrams in lower right are two-dimensional models of stress trajectoriesaround rigid bodies in softer rocks (Ridley, 1993). Such rigid bodies, which in the case of the Carlin-type deposits are Meso-zoic plutons, would perturb the stress field (heavy dashed lines), thereby giving rise to areas of high and low effective meanstress. Two cases are shown: one is where the rigid body lies in homogeneous rocks and no extant faults and the second wherean older fault is pinned along the margins of the rigid body. Flow of hydrothermal fluids is enhanced in the areas of low ef-fective mean stress whose relationship to the rigid body varies depending upon the structural architecture. Both models pre-dict that the fluid flow would be distributed at discrete zones on the margins of the rigid body and not uniformly around thebody. The model is supported by the distribution of gold deposits in the Osgood Mountains and also the northern Carlintrend (see Fig. 4c).


12/34

of exhumation and cooling to


13/34


0361-0128/98/000/000-00 $6.00 463

?

?

?

?

?

? ?

??

?

?

0 40km

C a r l

i n t

r e n d

~46.1 - 40.4Ma

Elkolko

Carlinarlin

Jerritt Canyon

R M

E H

C o r t e

z R a n

g e

Emigrant

Pass

Pinion

Range

~42Ma42Ma

~46Ma46Ma

THH

THH

THH

TH?H?

1

?

??

? ?

?

?

?

~40.4 - 39.7Ma

A d o b e

R a n g e

0 40km

a

Elko Mt

western Elkoestern Elko

basinasin

eastern Elkoastern Elko

basinasin

THH

THH

THH

THH

THH

R M

E H

f

e

g

h

i

j

~39.7 - 36.0Ma

0 40km

~37.5Ma37 5Ma

~38.9Ma38 9Ma

?

?

?

??

?

?

?

?

?

b

c

d

2

3 5

4

6

Elko Mt

R M E H

THH

Elkolko

Elko Formation - lacustrine sediments (shale,limestone)

Elko Formation - fluvial-alluvial sedimentsgrading up to lacustrine sediments

Elko Formation - fluvial-alluvial sediments(sandstone, conglomerate)

tuff of Big Cottonwood Canyon (39.7Ma)

tuff of Nelson Creek (40.4-39.8Ma)

Eocene intrusive centers

Topographic high

Direction of fluvialpaleoflow

tuff of Big CottonwoodCanyon - flow direction

Pervasive rotation extensional faulting (mainly~39.7-38Ma)

Pervasive rotation extensionalfaulting (extensive post ~37Ma)

Syn-deposition faults

Crescent Valley-

Independence lineament

Indian Well Formation - air fall tuff, ash flow tuffand volcaniclastic sediments above ElkoFormation in Pinion Range (~37.5Ma)

White air fall tuff, ash flow tuff and volcaniclasticsediments resting conformably on Elko Formation

sediments near Elko Mountain (post-38.9Ma)

Eocene andesite-dacite

flow-dome complexes

1

a

(a)

(c)

(b)

TH


14/34

directly overlain by andesite lavas near Swales Mountain (Fig.8). Eocene ash-flow tuff sequences also thin northward fromthe paleovalley toward Jerritt Canyon, suggesting there may have been an Eocene topographic high in the IndependenceMountains (Figs. 8 and 9a).

The maximum amount of relief generated by Eocene ex-

tension can be estimated from the thickness of synextensionalsediments that accumulated in the extensional basins. In gen-eral, the absolute extent of footwall uplift that accompanieshalf-graben–style extension is similar to or less than theamount of absolute hanging-wall subsidence. (e.g., Schlische,1991). Assuming that sediment supply did not significantly lagbehind the growing capacity of the basin, the maximum amountof tectonically driven local relief should have been less thanthe maximum thickness of synextensional Eocene strata,

which in the Elko basin is


15/34

general orientation of Eocene dikes (Presnal, 1997; Ressel etal., 2000; Henry et al., 2001; Theodore, 2002), and by the di-rection of midcrustal flow that is demonstrably of Eocene agein the Ruby Mountains-East Humboldt Range metamorphiccore complexes (Howard, 2003, and references therein). Theunderlying Proterozoic rifted plate margin and northwest-ori-ented Paleozoic fault fabrics thus lay subparallel to the exten-sion direction, and were favorably oriented to be reactivatedas low-magnitude strike-slip (WNW-striking faults; Tosdaland Nutt, 1999; Nutt and Hofstra, 2003) or oblique-slip faults(NNW-striking faults; Heitt et al., 2003). Northeast-oriented

pre-Jurassic fault fabrics that lie at high angles to the ten-sional stress (e.g., the Crescent Valley-Independence linea-ment of Peters, 1999) were reactivated in extension (Fig. 7)and may have locally controlled the geometry of Eocenebasins. Shallow-dipping pre-Jurassic thrust faults were favor-ably oriented to reactivate as low-angle extensional faults andmay have locally accommodated much of the pervasive rota-tional extension evident in the overlying Eocene basins(Moore, 2001; Muntean et al., 2001).

Tosdal and Nutt (1998, 1999) proposed that the Carlintrend acted as an accommodation zone during Eocene exten-sion. Slip along the zone was a manifestation of minor lateraldisplacement caused by the crustal blocks on either side

undergoing differing magnitudes of Eocene extension. Evi-dence for such lateral displacement includes (1) en echelonalignment of ore-controlling faults and deposits (Dee-Rossi,Capstone-Bootstrap-Tara, and Ren-Meikle-Beast Group; Fig.4c); (2) change in strikes of major ore-controlling faults from

west-northwest to more northerly along a trend from south-east to northwest (Teal and Jackson, 1997); (3) consistentnortheast strikes of secondary faults controlling Au deposition(summarized by Teal and Jackson, 1997); (4) sinistral deflec-tion of north-trending Mesozoic folds and thrusts in the Alli-gator Ridge area (Nutt and Good, 1998; Tosdal and Nutt,

1999; Nutt and Hofstra, 2003); and (5) the broad-scale de-flection of older structural fabrics. Opposing senses of uppercrustal slip in crustal blocks on either side of the Carlin trendreflect differential extension within each block (Tosdal andNutt, 1998, 1999). The Battle Mountain-Eureka trend may also correspond to an accommodation zone across which theEocene to Miocene extensional history differs in bulk exten-sion, style, or orientation (Muntean et al., 2001).

Thermochronological evidence forEocene hydrothermal flow

Apatite fission tracks in samples collected from the northernCarlin trend were partially to fully annealed by heterogeneous


0361-0128/98/000/000-00 $6.00 465

P a l eo z o i c

2500m

1 2 5 0 m

H = Vx 2

tuff of Big Cottonwood Canyon 39.6 8±0 .15Ma

41.9±0.3Ma basal air fall tuff

40.21±0 .20Ma ash flow tuff near base

C D

(a)

3000m

2500m

2000m

1500m

e l e v a t i o n

A B

Carlin mine

Carlin Pks

Carlin Valley

R i c h

m o n

d f a

u l t

5000m

1 0 0 0 m

H = Vx538-37Ma andesite

flow dome complex

Pre-38Ma Eocene

clastic sediments

restored ~42-38Ma

Eocene deposits

Paleozoic

~4 2M a su r f ac e

~38M a su r f ace

~ 4 2M a su r f a c

e ?

~ 3 8M a su r f ac e

(b)

FIG. 9. a. Restored section across a west-northwest–trending Eocene paleovalley between the northern Carlin trend andJerritt Canyon (2× vertical exaggeration). The tuff of Big Cottonwood Canyon is a regional marker unit and was used as adatum to restore post-39.7 Ma extensional faulting. The depth to the base of the Eocene sequence at five points along thesection is shown by the vertical black bars. Isotopic ages from tuff units at, or near, the base of the sequence are also shown(K.A. Hickey, unpub. data). b. Restored Eocene landscape projected along the crest of the east Tuscarora Mountains (sec-tion AB,×5 vertical exaggeration). The restored elevations of the 42 and 38 Ma erosion surfaces were derived from the ther-mal modeling of apatite fission tracks, assuming a surface temperature of 10°C and a near-surface geothermal gradient of 30°C/km (modeling was undertaken using AFTSolve; Ketchum et al., 2000). The vertical bars represent the range of calcu-lated maximum elevations at 42 Ma that contain 95% of the modeled data with a >5% probability of matching the observeddata. The weighted mean elevation of the 100 best (highest probability) models is shown by the symbol near the center of each bar. The position of the ~38 Ma surface is constrained by the restored thickness of Eocene units in the paleovalley atthe north end of the section, and a pre-38 Ma erosion surface beneath 38 to 37 Ma andesite flows at the south end of thesection. Landscape restoration suggests that the present erosion surface through the Carlin mine lies, at the most, ~1 to 2km below the Eocene erosional surface at 42 to 38 Ma. These estimates increase to


16/34

thermal pulses from ~40 to 15 Ma. A zone of pervasive an-nealing preserves evidence for an episode of rapid coolingfrom >100°C at ~40 Ma (Fig. 10; Chakurian et al., 2003), withlocalized annealing of fission tracks as late as ~20 Ma. Thezone of annealing is broadly parallel to the Carlin trend andbecomes more heterogeneously distributed and less pervasivefarther to the northwest, toward the Post-Betze-Screamer

and Meikle deposits, the largest deposits in the district(Hickey et al., 2003b). The spatial and temporal overlap of fis-sion-track annealing and Carlin-type Au mineralization(Table A3) has been interpreted to reflect a genetic relation-ship (Chakurian et al., 2003; Hickey et al., 2003a). This asso-ciation is, however, not perfect (Fig. 10), as significant de-posits lie outside the area of pervasive annealing and, in the

466 CLINE ET AL.

0361-0128/98/000/000-00 $6.00 466

10

kms

0

Jurassic and Cretaceous stocks

dominantly siliciclastic, lower Paleozoiceugeoclinal and upper Paleozoic overlapsequences

dominantly calcareous, lower Paleozoicmiogeoclinal sequence

Eocene intrusions

Eocene sedimentary and

volcanic rocksMiocene - Recent basin fill

east Tuscarora Mts

Marys Mountain

0

1

2

3

4

5

6

7

8

f r e q

10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 200 250

n=37

Carlin trend

0

2

4

6

8

10

12

f r e q

10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 200 250

pooled AFT age

n=43

eastern assemblage Paleozoic rocks

western assemblage Paleozoic rocks

Jurassic and Cretaceous granitic stocks

04080120160200

0

30

60

90

120

Acceptable

Good

Fit

Fit

Fit

Best

Time (Ma)

T em p er at ur e ( ? C )

04080120160200

0

30

60

90

120

Acceptable

Good

Fit

Fit

Fit

Best

Time (Ma)

T em p er at ur e ( ? C )

Cretaceous

Richmond stock

lower Paleozoic

quartzite

+

++

+++++++++++++

++++ +++++ ++ ++ ++

++

+ ++ ++

++++

++ +

+

+

++

++

++

++

+

+

+

+

+++ +++ ++++

++++ + ++ ++ +++ +

37.4±3.7

119.0±7.0

110.0±7.0

146.0±13.0

83.2±7.3

95.2±7.3

93.1±13.6

45.9±7.7

51.8±5.9

37.9±4.1

34.6±2.9

45.4±6.1

199.0±21.0

70.1±10.1

85.1±8.7

125.4±14.3

248.0±31.0

37.2±8.3 18.8±11.3

41.5±10.7

34.7±2.9

38.0±4.8

112.6±8.2

14.3±1.7

56.9±3.3

27.7±6.8 36.7±5.7

18.6±4.5

63.4±8.1

97.4±13.045.6±4.1

80.7±10.3

97.5±6.7

40.9±4.0

28.1±4.8 37.8±2.1

20.6±2.3

32.5±3.3

78.8±3.1

120.0±7.0

88.4±5.8

95.2±6.3

98.7±5.7

117.0±7.0109.4±8.0

70.3±7.1

141.0±7.0

92.4±6.7

108.0±6.0

80.0±4.9

76.5±7.2

51.1±4.2

80.1±5.474.1±6.3

101.0±8.0

110.0±8.0

74.9±5.2

53.1±8.4

83.0±5.8

42.6±3.9

129.0±7.0

69.7±5.2

75.1±6.2

119.0±6.8

80.4±5.8

56.3±3.5

58.1±3.6

58.1±4.5

70.4±5.4

d r i l l h o l e

- - >

d r i l l h o l e

- - >

110.0±8.0

108.0±7.0

23.8±6.2

45.5±4.7

33.0±2.9

35.5±3.7

22.7±4.6

60.4±6.8

79.9±5.5

32.4±5.2

105±7.0

82.2±5.3

72.6±6.9

east Tuscarora

Mountains

(c)

(b)(a)

C a r l i n

t r e n

d Goldstrike stock

Lit tle Boulder stock


17/34


18/34

Ma. Most of this erosion is likely to have occurred prior to themiddle Miocene because a pre-Basin and Range low-relief landscape characterized by broad, flat summits caps themountain ranges in the immediate vicinity of the deposits.Summit-flat erosion is controlled by hillslope processes in-

volving bedrock weathering and surface runoff. Estimates of long-term denudation rates on low-relief summit flats in tem-

perate to semiarid, or arid, environments (like that in NENevada from the Miocene onward) are


19/34

Wall-Rock Alteration

Host rocks are typically decarbonatized, argillized, and variably silicified in addition to being sulfidized and enriched with Au (App. Fig. A5; e.g., Hausen and Kerr, 1968; Bakkenand Einaudi, 1986; Kuehn and Rose, 1992; Stenger et al.,

1998; Hofstra and Cline, 2000). As the reactions that formedAu-bearing pyrite are generally not the same reactions thatformed alteration minerals (Hofstra et al., 1991), ore and al-teration zoning within the deposits is irregular. This lack of atemporal, and thus spatial relationship, along with the fine-grained nature and sparse abundance of ore and alterationminerals, limits the use of alteration to identify ore.

Decarbonatization

Carbonate rocks were dissolved in nearly all deposits andmost ore zones and locally replaced with quartz to form

jasperoid. Fluid acidity, the amount of cooling, and the de-gree of fluid-rock interaction controlled the extent of ore

zone decarbonatization (Hofstra and Cline, 2000), which varies from minimal (e.g., Gold Bug, Screamer) to intense(e.g., West Leeville, Carlin, Meikle). Intense decarbonatiza-tion produced collapse breccias in some deposits (Bakken,1990), significantly enhancing porosity, permeability, andfluid-rock reaction, leading to formation of high-grade ore(Emsbo et al., 2003).

Zones of complete carbonate dissolution near fluid con-duits commonly are zoned outward, first to dolomite- andthen to calcite-stable zones, and farther out to zones withabundant calcite veins. The presence of various carbonateminerals—calcite, siderite, ankerite, or dolomite—reflectsthe preore composition of wall rocks, preferential dissolution

of calcite, sulfidation of ferroan carbonate minerals, or car-bonatization of silicate minerals beyond the sulfidation halo(Hofstra and Cline, 2000). Calcite veins in the outer zonegenerally have δ13C values that are similar to the carbonatehost rocks, suggesting they formed from CO2 released by de-

carbonatization or collapse of cool CO2-bearing meteoric water into the hydrothermal system; however, at least somecalcite veins are postore (Hofstra and Cline, 2000; Emsbo etal., 2003; Emsbo and Hofstra, 2003; Lubben, 2004).

Argillization

Wall rocks are argillized where moderately acidic ore fluidsreacted with older alumino-silicate minerals and formed as-semblages of kaolinite ± dickite ± illite (Folger et al., 1998;Hofstra and Cline, 2000). Basalts, lamprophyres, and otherigneous rocks are commonly intensely argillized, whereasargillization is minimal in relatively pure carbonate rocks.

Although it is difficult to confidently distinguish clay minerals

formed at the time of Au deposition from clays related to earlierhydrothermal and later supergene events, 1Md kaolinite and2M1 dickite were determined to be coincident with mineraliza-tion in some deposits (Alligator Ridge, Ilchik, 1990; Vista ore-body at Twin Creeks, Osterberg and Guilbert, 1991; Carlin,Kuehn and Rose, 1992; Getchell, unpub. Placer Dome data,1999; Deep Star, Clode et al., 1997; Altamirano-Morales, 1999;Heitt et al., 2003). Kaolinite and dickite proximal to ore-fluidconduits are zoned outward to intergrown fine muscovite-illite(2M1 or 1M) or smectite, and farther out to K-feldspar–stablezones (Carlin, Kuehn and Rose, 1992; Betze-Post, Arehart et al.,1993b; Deep Star, Heitt et al., 2003), in response to reducedfluid-rock reaction and consumption of H ions (Fig. A5).


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calcitevein quartzsericitepyrite / pyrrhotitechalcopyrite

sphaleritearsenopyritegalena jasperoiddrusy quartzmarcasitepyrite (Au and As)illite / kaolinitemarcasite (Au and As)orpimentfluoritegalkhaitestibniterealgarcalcite (clear, coarse)

calcite (coarse crystals)Fe-oxides

Pre Ore Carlin EventMain Late

Post Ore

? ?

? ?

? ?

? ?

? ?

? ?

FIG. 11. Paragenetic sequence of pre-ore, Carlin and postore-stage minerals determined for the Getchell deposit. The se-quence illustrates the minerals and their relative timing typical of many Carlin-type deposits (modified from Cline andHofstra, 2000).


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A few detailed studies have identified increasingly crys-talline illite proximal to ore, suggesting a genetic association(Gold Bar, Masinter, 1991; Chimney Creek, Osterberg andGuilbert, 1991; Jerritt Canyon, Folger et al., 1998; Pipeline,Placer Dome, unpub. data, 1999; Getchell, Cail and Cline,2001). Fine muscovite and some illite, however, have pro-

vided a wide range of ages, many of which are older than and

unrelated to Au but which previously contributed to controversy over the age of the deposits (see Hofstra et al., 1999).The range of ages indicates that preore muscovite and illiteremained stable through all but the most intense alteration,also retaining their preore Ar signatures.

Silicification

Silicification accompanied some but not all Au depositionand is manifested by the presence of jasperoid and, to a lesserextent, by fine quartz druses lining vugs; ore-stage quartz

veins are relatively uncommon in these deposits. Jasperoid isspatially associated with ore at the district scale, yet jasperoidsrange from being barren to containing high-grade ore(Bakken and Einaudi, 1986; Ye et al., 2002). Subeconomicgrades reflect, in part, the decoupling of processes that pre-cipitated jasperoid from those that deposited Au. For exam-ple, some upwelling ore fluids cooled and formed jasperoidsin fluid conduits below ore zones and prior to Au deposition,

whereas other fluids precipitated jasperoid above or lateral toore zones as fluids exited sites of Au deposition (Hofstra andCline, 2000). Where relatively pure limestones were silicified,massive jasperoids were produced. In contrast, where limey siltstones, sandstones, or shales were silicified, fine calcitelaminae were replaced by “micro” jasperoids.

Quartz druses line vugs formed by decarbonatization or incollapse breccias and, rather than veins, are a characteristictextural feature of Carlin-type deposits (Cline and Hofstra,

2000; Cline, 2001; Emsbo et al., 2003; Lubben, 2004). Gold-bearing pyrite occurs both within jasperoid and incorporatedin the base of drusy quartz crystals at the jasperoid-drusy quartz interface (Cline, 2001; Lubben, 2004). Postore drusy quartz, distinguished by cathodoluminescence studies atsome deposits, has overgrown ore-stage quartz druses (e.g.,Meikle, Emsbo et al., 2003; Betze-Post, Lubben, 2004).

The generally low abundance of ore-stage quartz and lackof one-to-one correlation between quartz and Au in Carlin-type deposits may be related to the relatively low temperatureof ore formation (see Hofstra and Cline, 2000, and referencestherein) and the fact that quartz precipitation is inhibited by kinetics below ~180°C (Henley and Ellis, 1983; Fournier,

1985; Rimstidt, 1997). As temperatures declined, quartz pre-cipitation likely diminished although Au precipitation may have continued.

Jasperoid textures are related to minerals that initially re-placed carbonate minerals and, as such, indicate approximatetemperatures and depths of formation (Lovering, 1972).Jasperoid displaying reticulate or xenomorphic textures (e.g.,Getchell; Cline, 2001) results from carbonate replacement by quartz (Lovering, 1972), indicating formation at somewhat el-evated temperatures (>180°C) and greater depths, and such“high-temperature” jasperoid is more likely to have been re-lated to Au deposition in Carlin-type deposits. By contrast,

jasperoid that displays jigsaw or chalcedony textures indicates

that lower temperature forms of silica, such as amorphous oropaline silica, initially replaced calcite, though they may havebeen subsequently recrystallized to quartz (Lovering, 1972).Observed textures suggest that amorphous or opaline silicaprecipitated at some deposits (Alligator Ridge, Beast, SouthCarlin, Gold Bar, Eureka; Nutt and Hofstra, 2003; Table A2),probably at shallow depths and from depleted ore fluids or

fluids unrelated to the Carlin system. Au-bearing arsenian pyrite

Carlin-type Au deposits are perhaps best known for theconsistent occurrence of submicron, so-called invisible Au intrace element-rich pyrite and marcasite, even in samples in

which Au exceeds several ounces per ton. Gold-bearing pyriteand marcasite occur as discrete grains, generally less than afew micrometers in diameter, or as narrow rims on earlierformed pyrites (Wells and Mullens, 1973; Arehart et al.,1993b; Simon et al., 1999; Hofstra and Cline, 2000). VisibleAu has been observed locally (Wells and Mullens, 1973;Bakken et al., 1989; Emsbo et al., 2003) but is interpreted tohave been liberated and coarsened during weathering and ox-idation of ore-stage pyrite (Wells and Mullens, 1973) or de-posited by earlier Devonian sedex or later events (Emsbo etal., 1999, 2003), although some have suggested direct Au de-position from hydrothermal fluids (Rain, Shallow, 1999; GoldQuarry, Rodeo, T. Thompson, pers. commun., 2004).

Ore-stage pyrite contains a range of Au concentrations thatreach ~9,000 ppm (Wells and Mullens, 1973; Henkelman,2004; Palenik et al., 2004; Reich et al., 2005), and the overallgrade of a particular ore zone is a function of the abundanceof ore-stage pyrite and its Au concentration. Arsenic is themost abundant nonstoichiometric element in the pyrite; con-centrations reach, or in a few cases exceed, ~15 wt percent(Cline, 2001, unpub. data; Reich et al., 2005), although 2 to 8

wt percent is more common and As/Au ratios are highly vari-able. Elevated As does not, however, distinguish Carlin-typepyrite, as As concentration is also high in older intrusion-re-lated, late ore-stage, and postore-stage pyrite (Arehart et al.,1993b; Hofstra and Cline, 2000; Emsbo et al., 2003; Henkel-man, 2004). Other trace elements in pyrite and marcasite in-clude as much as ~0.25 to 1.0 wt percent Sb, Hg, Tl, Te, andCu, and variable but generally lower Pb, Mo, Zn, Mn, Bi, Ni,

W, Ag, and Co (Wells and Mullens, 1973; Hofstra and Cline,2000; Cline, 2001; Emsbo et al., 2003; Henkelman, 2004).

Metal patterns in zoned pyrite rims document an evolvingore-fluid chemistry and suggest that early fluids contained thehighest Au. Early fluids at Getchell and Meikle (Cline et al.,

2003; Emsbo et al., 2003), represented by inner pyrite rims,had elevated concentrations of As, Hg, Tl, Te, Cu, ±Pb in ad-dition to Au. With time, concentrations of most metals de-clined. Outermost pyrite rims, representing later fluids, con-tain lower concentrations of Au and the ore-metal suite withsome metals below detection but elevated concentrations of other metals including Pb and W, which are interpreted tohave been leached from wall rocks (Cline et al., 2003).

In an effort to understand the mechanisms of Au precipita-tion, numerous studies have speculated on whether Au occursin pyrite as structurally bound ionic Au+1 or as submicron in-clusions of native Au0 (Bakken et al., 1989; Arehart et al.,1993b; Fleet and Mumin, 1997). More recently, Simon et al.

470 CLINE ET AL.

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(1999) identified both Au0 and Au+1 in arsenian pyrite fromTwin Creeks and concluded that structurally bound Au+1 wasadsorbed from ore fluids that were not saturated with respectto native Au. Recent high-resolution transmission electronmicroscopy (TEM) studies of samples from several Carlintrend deposits (Palenik et al., 2004; Reich et al., 2005) deter-mined that most Au occurs in solid solution in pyrite, but

nano-sized particles of Au0 are present where Au exceeds asolid-solution molar solubility limit that is a function of Asconcentration (Au/As >0.02; Reich et al., 2005). Au+1 is pre-sent in solid solution below this solubility limit. These resultsare interpreted to indicate that Au was adsorbed onto pyritegrains from ore fluids undersaturated in native Au.

Organic C

Black carbonaceous rocks occur in every district and com-monly host some of the ore (Hofstra and Cline, 2000). In-digenous C occurs as intergranular films and particles alongsedimentary layers and is concentrated in stylolites. Carbon isalso concentrated in pores, along fractures, and in quartz andcalcite veins. Rocks with high concentrations of indigenous C(~>2 wt %) were deposited in restricted anoxic basins and arepotential petroleum source rocks, whereas those with highS/C ratios are indicative of euxinic conditions during deposi-tion (Leventhal and Giordano, 2000). Some rocks with highconcentrations of C in the Carlin trend and Alligator Ridgedistrict provide evidence of petroleum generation, migration,and accumulation in structural culminations (e.g., Kuehn,1989; Hulen and Collister, 1999).

In rocks below the Roberts Mountains allochthon, both in-digenous and concentrated organic C are generally overma-ture relative to petroleum generation and typically consist of cryptocrystalline graphite (Poole and Claypool, 1984; Hofstra,1994; Leventhal and Giordano, 2000). Field relationships in-

dicate that petroleum migration occurred prior to emplace-ment of Jurassic intrusions (e.g., northern Carlin trend,Emsbo et al., 2003), and thermal modeling suggests that pe-troleum generation and subsequent catagenesis was due toemplacement of the Roberts Mountains allochthon (Gize,1999). Laser Raman spectra show that C crystallinity is thesame in ore and waste and only increases adjacent to intru-sions such as the Goldstrike stock in the northern Carlintrend (Hofstra, 1994; P. Emsbo, pers. commun., 1998).

In rocks east of the allochthon, the thermal maturity of in-digenous and concentrated C is generally lower and more

variable, with higher maturity near igneous intrusions, Carlin-type deposits, and active geothermal areas (Poole and Clay-

poole, 1984; Ilchik et al., 1986, Hitchborn et al., 1996; Hulenand Collister, 1999). At Yankee, paragenetic relationships in-dicate there were two episodes of petroleum generation, oneduring Carlin-type mineralization and one after in responseto Basin and Range tectonism (Hulen and Collister, 1999;Nutt and Hofstra, 2003).

Hofstra and Cline (2000) found no consistent relationshipbetween organic C content and ore grade. They also notedthat many orebodies occur in rocks with low C contents andconcluded that C played little or no role in Au precipitation.At upper crustal levels in these systems, organic matter may have been a direct source of H2S and a critical ingredient forH2S generation by thermochemical sulfate reduction (Hunt,

1996). Organic matter also maintained a reduced condition inthe fluid, and thus H2S as the predominant form of S in thefluid, enabling the hydrothermal solutions to scavenge andtransport Au to the sites of ore formation (Hofstra and Cline,2000). During exhumation and weathering of the deposits, Cconsumed O in supergene fluids such that enclaves of car-bonaceous ore were locally preserved.

Lithogeochemistry

Lithogeochemical studies show fairly consistent patterns of elemental flux into and out of deposits in several districts dur-ing ore formation (Jerritt Canyon, Hofstra, 1994; TwinCreeks, Stenger et al., 1998; Getchell, Cail and Cline, 2001,Gold Bar, Yigit et al., 2003; Meikle, Emsbo et al., 2003;Screamer, Kesler et al., 2003a; Table A2). Gold/silver ratios,commonly >10 in sedimentary rocks and ~10 in igneous dikes(Table A2), demonstrate consistently high transport and pre-cipitation of Au relative to Ag into the deposits and are a key signature that distinguish Carlin-type Au deposits from mostother precious metal systems.

Elements added to host rocks in all deposits include S, As,Au, Sb, Hg, and Tl. Elements that are variably added to al-tered rocks, or identified as added in some deposits but notall, include Te, Cu, W, Mo, Se, Fe, Ag, Pb, Si, Ba, Cs, and Zn.As demonstrated by ore-stage pyrite compositions, some of these elements, including Cu and Te, appear to be compo-nents of the early, high Au ore fluid whereas others may havebeen leached from wall rocks. Base metals are typically notquantitatively added to rocks. Lead appears to have multipleorigins, with deeply sourced Proterozoic rocks contributingPb to early ore fluids, and wall rocks contributing Pb to lateore fluids (Tosdal et al., 2003). Although trace metals otherthan Au are significant from a genetic standpoint, concentra-tions are low and elements generally occur only as trace im-

purities in pyrite and marcasite.Elements removed from rocks by ore fluids include carbon-

ate C, Ca, Mg, Sr, Mn, Na, and Sc. Generally immobile ele-ments include Al, Ti, Th, and Zr (Hofstra and Cline, 2000).These patterns reflect alteration, particularly decarbonatiza-tion and argillization. Potassium is variably immobile or locally added or removed in trace amounts (Cail and Cline, 2001).

The mobility of Fe is important as it reflects the degree to which sulfidation controlled pyrite and Au deposition. Iron was relatively immobile in sedimentary rocks and mafic dikesat Jerritt Canyon (Hofstra et al., 1999), in wall rocks at TwinCreeks and Gold Bar (Stenger et al., 1998, Yigit et al., 2003),in zebra dolomite and igneous rocks at Meikle and Post-Betze

(Emsbo et al., 2003), and in most ore zones at Getchell (Cailand Cline, 2001). However, Fe was added to ore at Screamer(Kesler et al., 2003a), in some ore zones at Getchell (Cail andCline, 2001), and in minor amounts at Alligator Ridge (Nuttand Hofstra, 2003). Iron transport and the addition of pyriteto the rock are locally evident by the presence of abundantore-stage pyrite in high-grade zones in sedimentary rocksbelow basalt flows (Getchell, Twin Creeks) and other Fe-bearing aquicludes and in argillized tuffs (Tonkin Springs).

Ore Fluids and Ore Deposition

Fluid inclusions in ore-stage minerals in Carlin-type depositsare small, sparse, and difficult to relate to Au mineralization.


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Nevertheless, studies that provided similar results for severaldistricts (App. Fig. A6) show that ore-stage fluids were low- tomoderate-temperature (180°–240°C), low-salinity (~2–3 wt% NaCl equiv) aqueous fluids that contained CO2 (400°C, a near-neutral pH and chloride-

poor, sulfide-bearing ore fluid would have transported Au asAu(HS)

–2; Seward

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Carlin Type