Stratigraphy and Host Rock Controls of Gold Deposits of the Northern Carlin Trend

7/27/2019 Stratigraphy and Host Rock Controls of Gold Deposits of the Northern Carlin Trend

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STRATIGRAPHY AND HOST ROCK CONTROLS OF GOLD

DEPOSITS OF THE NORTHERN CARLIN TRENDJohn Jory1

ABSTRACT

Pre-mine gold resources totaling 100 million troy ounces (3,110

t [metric tons]) in 42 deposits in the northern Carlin trend

account for the most productive gold province in North

America, including the giant >5-million-ounce (156 t) deposits

at Betze-Post, Screamer, Meikle, Genesis, Carlin, and West

Leeville-Turf. The northern Carlin trend is a dominantly N15W

alignment of sedimentary rock-hosted gold deposits. Low-cost

production exceeding 4 million ounces (124 t) annually since

1994 has been achieved from these large, disseminated gold

deposits hosted by carbonate strata of the Devonian-Silurian

Roberts Mountains Formation and the Devonian Popovich

Formation. Host rocks are composed of silty limestone with

intercalated biogenic debris that were deposited along thewestern margin of ancestral North America. Late Silurian to

mid-Devonian biohermal to carbonate shoal accumulation

(Bootstrap limestone) occurred over a period of 50 to 100

million years in the northernmost Carlin trend. Subaerial

exposure and erosion shed bioclastic debris with fan-like

geometries producing thick sequences of proximal, upper slope

fossiliferous debris flows that are mineralized in the Meikle,

Goldbug, and Betze-Post deposits, and mid-slope silty facies

at the Blue Star-Genesis and Carlin deposits. Bioturbated wavy

(wispy) laminated silty limestone of the upper Roberts

Mountains Formation provides the most favorable host rock

on the northern Carlin trend, containing 65% of the gold

resource in about 400 feet (120 m) of the stratigraphic section.Gold deposits of the northern Carlin trend are

predominantly exposed near the fault-bounded margins of the

uplifted Lynn and Bootstrap structural windows. Flanking these

Paleozoic windows are deep, northerly trending basins filled

by postmineral Tertiary volcaniclastic sediments and gravels.

Tectonic uplift and erosion have exposed autochthonous

carbonate strata beneath the Roberts Mountains thrust. The

central portion of the northern Carlin trend is intruded by a

large, late Jurassic granodiorite intrusion (Goldstrike stock)

and numerous Eocene rhyolite to dacite dikes. Two of the

highest grade gold deposits on the Carlin trend, Deep Star and

Deep Post, occur along the sheared margins of the Goldstrike

stock, and most of the gold deposits occur within 2 miles (3km) of the stock along the dike-filled Post-Gen fault zone. It is

inferred that Eocene magmatism is the major process that drove

hydrothermal circulation forming Carlin gold deposits.

Gold deposits represent a continuum of styles from

stratiform, permeability-controlled end members (e.g., Carlin,

West Leeville, Pete, and Tara) to shear zone-hosted end

members (e.g., Deep Star, Deep Post, and Meikle). The

conventional Carlin-type gold deposits of the stratigraphically

controlled end member are therefore only a portion of a broader

spectrum, which includes large collapse breccia-hosteddeposits, formed by extensive decarbonatization of favorable

host rocks adjacent to structural conduits (e.g., Betze-Post,

Goldbug, and Genesis). This spectrum of deposit types

produces remarkable contrasts in orebody geometry, tonnage,

grade, host rock, structural orientation, hydrothermal alteration,

geochemistry, metallurgy, hydrology and ground conditions,

and has wide-reaching implications for exploration, mining,

and processing.

INTRODUCTION

The Carlin trend is a 40-mile (65 km) long north-northwest

alignment of sedimentary rock-hosted disseminated gold depositsin northeast Nevada. Total gold production from the trendexceeded 50 million ounces (156 t) in 2002. Most of the reservesand gold production are located in the northern one-third of theCarlin trend within a belt measuring 10 miles (16 km) north-south by 2 miles (3 km) east-west, and extending from Dee andCapstone in the north to Carlin and Pete in the south. The northernCarlin trend pre-mine resource is approximately 80 millionounces of gold; 50% of the resource is contained in fourorebodies: Betze, Post, Screamer, and Meikle. Northern Carlintrend reserves were 65 million ounces (2,020 t) of gold in 1998,or about 80% of total Carlin trend reserves.

The purpose of this paper is to provide a stratigraphic

framework and description of Devonian-Silurian strata of thenorthern Carlin trend, and to compare and contrast relationships

among stratigraphy, structure, and intrusive bodies that are

important controls to gold mineralization (plate 1, fig. B-1).

The paper includes a review of 42 gold deposits and their

preferred host rocks in the northern Carlin trend. Formal

stratigraphic units are reviewed, and informally named subunits

are described in detail and related to deposit types. The

commonly used terms Bootstrap limestone and Rodeo Creek

unit are not recognized formations. Ounces and grade in this

paper generally refer to gold resources and do not have reserve

or economic implications.

Studies by Roberts and others (1958), Evans (1980), and

Ettner (1989) formed the early stratigraphic framework usedby Carlin trend geologists. The need to better understanddetailed stratigraphic controls for gold mineralization at theCarlin Mine led to the recognition of informal subunits byNewmont Mining Corporation geologists in 19931994, notablyS. Moore and R. Harris. These subunits were subsequentlyrecognized and correlated throughout Newmonts northernCarlin trend properties by 1997 (Teal, 1996a; Clode, 1997; Joryand others, 1997). Deposit host rocks, stratigraphic units, anddepositional environments were first linked via a fence diagramtied to a Rodeo Creek/Popovich Formation datum line in 1995

1Newmont Mining Corporation


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Stratigraphy

Pete

WestBetze

WestLeeville

Turf

Island

Winston

Ren

Tara

Bootstrap

Capstone

GoldenZia

NW Genesis

WestRen

Banshee

SouthMeikleMeikle

Barrel

WestGriffin

East Griffin

NorthBetze

W BazzaLongLac

N. Skarn Hill

ShaloskyBetze

Pancana5-0

West #9

Genesis

Bobcat

Payraise

BlueStar

Widge

BlueStarRidge

Beast

NW Extension

S. Lantern

N.Lantern

CarlinWest

Hardie FW

Carlin East

?

Dee

Storm (Rossi)

FenceCarlin

BigSix

AntimonyHill

Exodus

LEEVILLEFAUL

T

LYNN

FAU

LT

HARDIE

FAULT

Gold deposits

Granodiorite intrusions

Fault, solid where observed, dashed where inferred

DEE

FAUL

T

Rodeo

Screamer

BazzaPt

Skarn Hill

Bazza

VIVIANGULCH

FAULT

Pancana

GOLDSTRIKESTOCK

LITTLEBOULDER

BASINSTOCK

(concealed)

FourCorners

FOUR

CORNE

RSFAULT

CASTLE

REEF

FAULT

GEN

FAULT

CASTLEREEF

FAULT

Deep Star

Elko County

Eureka County

Perry

0 5,000 10,000 feet

0 1,500 3,000 meters

Goldbug

DeepPost

Lower Post

POST

FAULT

Post

North Star

Winnemucca

Elko

Carlin

Ely

Reno

Lovelock

Las Vegas

80

80

H U M B O L D T E L K O

N Y EM I N E R A L

ESMERALDA

LYON

L I N C O L N

C L A R K

P E R S H I N G

C H U R C H I L L W H I T E

P I N EE

U

R

E

K

A

LA

N

D

E

R

WA

SHOE

80

80

Figure B-1. Gold deposits, northern Carlin trend, Nevada. Modified from Teal and Jackson, 1997b.


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by L. Schutz and C. Williams. This product, extending fromStorm (Rossi) in the north to Pete in the south, was updated in1997 by J. Jory, D. Harris, H. Unger, and R. Koefed. BarrickGoldstrike Mines Inc. geologists working in the Betze,Screamer, Rodeo, and Meikle areas are familiar with informalNewmont stratigraphic subunits, but have not adopted them.

Carlin-type gold deposits have commonly been described

as being exemplified by the stratigraphically controlled gold

mineralization in the original Carlin orebody, discovered in1961 (Radke, 1985). However, the Carlin trend includes a

diverse assemblage of gold deposits with wide variations in

size, grade, geometry, host rock, structural fabric, hydrothermal

alteration, metallurgy, geochemistry, hydrology, and

geotechnical conditions. The successful gold explorationist on

the Carlin trend needs to constantly assess these variable deposit

characteristics and diverse gold models in the pursuit of new

discoveries. This paper is primarily based on the observations

and interpretations of numerous geologists employed by

Newmont and Barrick on the Carlin trend.

REGIONAL GEOLOGYCarlin trend regional geology is well documented. The

following description is summarized from Stewart (1980).

Lower Paleozoic: Northeast Nevada was located along the

stable western margin of the North American craton.

Western facies eugeoclinal and eastern facies miogeoclinal

sediments were deposited on the continental shelf /slope.

Late Devonian-Early Mississippian: The Antler orogeny

placed western facies siliceous rocks over eastern facies

carbonate rocks along the Roberts Mountains thrust.

Mesozoic: Northeast-directed compressive tectonism of the

Sonoma and Elko orogenies produced uplift, folding andfaulting, highlighted by north-northwest fold axes and faults.

Late Jurassic: Late/post-Elko orogeny plutonism included 158

Ma emplacement of the granodiorite Goldstrike stock, Little

Boulder Basin and Vivian stocks/dikes, and contact

metamorphism.

Eocene: Extension and magmatism with coeval main stage

3640 Ma gold mineralization and Tertiary dikes.

Miocene: 1420 Ma basin-and-range extension occurred with

north-south faulting, deposition of Carlin Formation

volcaniclastic sediments in basins, and exposure of lower

Paleozoic rocks along the Tuscarora Range and Tuscarora

Spur.

Sedimentary Host Rocks

STRATIGRAPHY

Key factors controlling large bulk-mineable gold deposits in

the northern Carlin trend were structural preparation and

favorable primary and secondary host rock porosity and

permeability. Stratigraphic and facies relationships are

discussed in this section.

Ordovician to Devonian upper plate rocks of the

allochthonous western siliceous assemblage are among the

oldest rocks on the Carlin trend (figs. B-2 and B-3, plate 1).

Thickness is uncertain owing to structural thickening along

numerous low-angle faults, but is estimated to exceed 2,000

feet (600 m). The Ordovician Vinini Formation is composed

dominantly of siltstone and cherty mudstone with a penetrative

shear fabric imposed by Antler thrusting. The regionally

extensive Roberts Mountains thrust (Roberts and others, 1958)places western assemblage siliciclastic rocks on top of

autochthonous Ordovician through Devonian limestone,

dolomite, quartzite, siltstone, and mudstone. Lower plate

carbonate rocks vary considerably in facies, composition,

thickness, and host rock potential. Contacts are mostly

conformable and gradational; however, conodont zones indicate

depositional hiatuses in some areas (G. Griffin, personal

commun.).

The oldest lower plate rocks exposed on the northern

Carlin trend are dolomite and shaly limestone of the

Ordovician Pogonip Group (Evans, 1980). The Pogonip

Group is about 1,000 feet (300 m) thick and generally includes

gray limestone in its upper portion and dense, dark gray

dolomite in the lower two-thirds. A 1,000-foot (300-m) thick

homogenous package of white Ordovician Eureka Quartzite

conformably overlies the Pogonip Group. The Eureka

Quartzite and overlying dark gray Hanson Creek Formation

are prominent ridge formers. The 600-foot (180 m) thick

Hanson Creek dolomite contains several cherty and sandy

interbeds in the uppermost 100 feet (30 m).

The light gray Devonian-Silurian Roberts Mountains

Formation is approximately 1,200 feet (360 m) thick, consisting

of a lower 800 feet (240 m) of planar laminated silty limestone

grading upward into wavy (wispy) laminated silty limestone

with abundant bioclastic debris (fig. B-3). The RobertsMountain Formation generally shows an upward fining of grain

size, progressive increase in bioclastic interbeds and debris,

and increase in organic carbon. Fossiliferous debris flows and

1- to 6-inch (315 cm) thick calcarenite beds are common in

the uppermost 400 feet (120 m) of the Roberts Mountains

Formation.

The overlying Devonian Popovich Formation ranges in

thickness from 500 to 800 feet (150240 m), and is composed

of dark gray, carbonaceous silty to muddy limestone grading

upward into calcarenite and limestone conglomerate with planar

laminated silty limestone (Evans, 1980). The stratigraphic

position of the Roberts Mountains/Popovich Formation contact

is debated on the Carlin trend; however, the correlative andmappable rock units are well agreed upon. Some workers place

the contact about 400 feet (120 m) lower than described in this

paper, at the top of the planar laminated silty limestone of the

Roberts Mountains Formation and base of debris flows (Evans,

1980; Armstrong and others, 1997). The informally named

Bootstrap limestone is as much as 1,300 feet (390 m) thick at

the north end of the Carlin trend (figs. B-2 and B-3). This unit

defines a dramatic facies change from late Silurian through

mid Devonian times, consisting of a massive light gray oolitic

to sparry limestone with silty limestone interbeds in its lower


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Stratigraphy

Ovi

Drc

DSr

DpOlp

CARLIN

VALLEY

DSr

DSr

Drc

Dp

Drc

Ovi

CarlinMine

Olp

DSr

Drc

Dp

DSr

Dp

Drc

Ovi

Qal

Drc

Ovi

OviOvi

Drc

Dp

DSr Ovi

Ovi

Ovi

Ovi

Ovi

Ovi

Drc

Ovi

Dp

DSb

DSr

Ovi

GenesisMine

RICHMOND

MOUNTAIN

Drc

Qal

Little

Boulder

Basin

GENFAULT

TuscaroraSp

ur

POSTFAULT

Drc

DSr

Ovi

DSb

Jig

Qal

Jig

Tuscarora

Range

BootstrapMine

MeikleMine

Post Anticline

TuscaroraAnticline

Betze Anticline

Boulder

Valley

Betze-PostMine

Dp

DpOvi

Qal

Qal

Qal

Qal

Quaternary cover

Jurassic Goldstrike diorite

Rodeo Creek unit

Popovich Formation

Bootstrap limestone

Roberts MountainsFormation

Ordovician lower platerocks

Ordovician VininiFormation (upper platerocks)

Major fault

Anticline

CASTLER

EEFFAULT

LBBFA

ULT

LEEVIL

LE

FAU

LT

CASTLEREEFFAULT

0

1,500 meters0

5,000 feet

Olp

Figure B-2. Simplified geologic map of the northern Carlin trend, Nevada.


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DeepPost

UpperPost

LowerPost

LowerGoldbug

UpperGoldbug

ROBERTS MTS.

THRUST

Drc

Tmc

Ovi

Dp0

Dp1

Dp2

DSr1

DSr2

DSr3

DSr4

Tmc: Tertiary Carlin Formation

Tuffaceous sedimentary rocks (10 to 600 feet thick). Poorly consolidated, friable,interbedded light gray sandy air fall tuffs (aft) and tan lacustrine silt, minor 10- to 50-foot-thick matrix-supported gravels. Basal 5 to 40 feet silty clay with 40 to 60%smectite, halloysite, and illite.

Basal gravels (30-200 feet thick) Poorly to moderately consolidated, matrix- andclast-supported gravels lying unconformably on Goldstrike granodiorite and VininiFormation. Clast-supported gravels include 70-100% Ovi and JKi clasts in tan siltmatrix. Clast types are: (1) angular 1 to 6 inches gray Ovi cherty mudstone andsiltstone, (2) light green to brown clay-weathered granodiorite grus.

Ovi: Ordovician Vinini Formation

Chert, mudstone. Black and gray cherty mudstone and siltstone. Planar to wavybanded with alternating dark gray siltstone beds and 1- to 5-mm-thick light graydolomitic calcarenite interbeds. Becomes more siliceous with depth.Metamorphosed to quartz hornfels adjacent to Goldstrike intrusive. 20- to 50-foot-thick light gray to black limestone in middle portion, metamorphosed to calc-silicatehornfels adjacent to Goldstrike intrusive. Allochthonous unit with mylonitic flaserstructure in thrust contact with underlying Drc unit. Strongly fractured and oxidized toproduce widespread limonitic interlocked "ice-cube" tectonic breccia. >2,000 feetthick.

Drc: Devonian Rodeo Creek unit

Dark gray inch-thick rhythmically bedded, banded siliceous mudstone, with gray totan planar laminated siltstone and minor fine-grained sandstone. Mixed siliceousmudstone and siltstone in upper 200 to 300 feet. Siliceous mudstone with shalypartings and quartz-filled tension gashes dominant in lower 300 feet. Basal 50 feetgradational with Dp silty limestone. Discontinuous 5- to 20-foot-thick basal silicifiedcalcarenite/sandstone. Metamorphosed to quartz hornfels adjacent to Goldstrike

intrusion. 600 to 700 feet thick.

Dp: Popovich Formation

Three mappable units, mostly muddy limestones (600 feet thick). Matrix grain sizeand skeletal-rich limestone/calcarenite interbeds (DSb) increase in frequency andthickness from Dp1 to DSr3.

Dp0: Dark gray carbonaceous, planar laminated silty limestone gradesdownward into silty micrite with diagenetic pin-striped pyrite on bedding.Uppermost 50 feet gradational with basal Drc siliceous mudstone. 3- to 10-mm-thick black chert lenses comprise 5% of lower 100 feet of Dp0. 200 to 250 feetthick; thickens northward.

Dp1: Dark gray carbonaceous, massive to thin-bedded lime mudstone (micrite).Diagnostic soft sediment deformation with slump bedding and flame structure.240 to 320 feet thick; thickens northward.

Dp2: Dark gray carbonaceous, planar laminated silty micrite. Localized hockey-puck cleavage and millimeter-thick DSb interbeds. 40 to 80 feet thick.

DSr: Silurian/Devonian Roberts Mountains Formation

DSr1: Transitional dark gray carbonaceous, planar to wispy laminated siltylimestone. Coarser grained and less carbonaceous than Dp0, Dp1, and Dp2, 10to 40 feet thick.

DSr2: Medium-dark gray variably carbonaceous wispy laminated silty limestone.Fossil-rich debris flows and centimeter-thick bcls interbeds increase down-section. 80 to 200 feet thick, thins northward.

DSr3: Medium to dark gray fossiliferous wispy laminated silty limestone.Abundant debris flows with 5 to 30% angular lithic and skeletal debris, biospariteand fine-grained bioclastic limestone (DSb). Basal 30 to 50 feet gradational fromwispy to planar laminated silty limestone. 150 to 300 feet thick (biosparite 300feet thick at Barrel). Commonly silicified and collapse brecciated.

DSr4: Medium to light gray planar banded to laminated calcareous siltstone(lmst) Alternating 2- to 4-mm-thick light and dark gray planar laminae. Minor 1-inch- to 3-feet-thick DSb interbeds and fossil debris flows. Coarser grained andless carbonaceous than DSr1 to DSr3. >800 feet thick.

Igneous rocks

Jig: Jurassic diorite to granodiorite of the Goldstrike sill-like intrusion (158

Ma). 1 to 3 mm plagioclase, hornblende and biotite phenocrysts withinterstitial quartz. Blocky, well-jointed and propylitized with calcite veins anddisseminated pyrite. Up to 1,800 feet thick; uppermost 100 feet is weatheredto friable sandy grus.

Qmp/Lamp: Quartz monzonite porphyry and hornblende lamprophyre dikesand sills (158 Ma) Three types: (1) 2 to 6 mm blocky or lathy plagioclasephenocrysts in aphanitic plagioclase quartz groundmass, (2) 2 to 4mmresorbed quartz eyes in aphanitic plagioclase quartz groundmass, (3) biotite +hornblende-rich lamprophyre.

Bfp: Biotite feldspar porphyry (39 Ma) Tertiary dikes in the Post fault zone. 2-to 4-mm biotite and plagioclase phenocrysts in aphanitic plagioclasegroundmass.

gradational

300feet

100meters

Figure B-3. North Post-Goldbug tectonostratigraphic column.


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Stratigraphy

portion. Fossiliferous debris flows occur proximal to the

Bootstrap limestone. Silty slope facies of the Roberts

Mountains and Popovich Formations lapped against the

Bootstrap limestone shelf, which finally drowned in the mid

to late Devonian (Griffin, 1999). The Bootstrap limestone is

contemporaneous with the upper part of the Roberts Mountains

Formation and much of the Popovich Formation.

The Popovich Formation is conformably overlain by the

Devonian Rodeo Creek unit, which consists of dark graysiliceous mudstone with interbedded light gray siltstone and

silty limestone (Ettner, 1989). Thickness of the Rodeo Creek

unit is about 700 feet (210 m); however, its upper portion is

commonly removed by the Roberts Mountains thrust or erosion

(fig. B-3). In some areas, notably at the Genesis deposit, the

Roberts Mountains thrust removed the entire Rodeo Creek unit

and placed Vinini mudstone on top of Popovich limestone

(Schutz and Williams, 1995).

The youngest sedimentary rocks of the Carlin trend are

clast-supported gravel, siltstone, and ash beds of the Miocene

Carlin Formation. These poorly consolidated rocks are up to

2,000 feet (600 m) thick and fill deep, north-trending basins.

DISTRIBUTION OF ROCK UNITS

The highest mountain range in the northern Carlin trend is the

N15E-trending Tuscarora Range, with elevations above 8,000

feet (2,400 m). A lower elevation N15W-trending spur in the

Tuscarora Range, called the Tuscarora Spur, includes the most

of northern Carlin trend gold resources. The Tuscarora Spur is

separated from the Tuscarora Range by the 1- to 2-mile (1.6

3.2 km) wide Little Boulder Basin (fig. B-2). Ordovician rocks

crop out extensively south of the Carlin pit in the Richmond

Mountain area along the spine of the Tuscarora Range, where

Pogonip limestone, Eureka Quartzite and Hanson Creek

dolomite are impressive ridge formers. In the Carlin andLantern Mine areas, the Roberts Mountains Formation is

broadly exposed, especially in the structurally uplifted block

southwest of the Castle Reef fault. Further north within the

Tuscarora Range in the West Leeville area, allochthonous Vinini

Formation forms high ridges east of Little Boulder Basin.

Mesozoic and Tertiary folding and faulting in the northern

Carlin trend expose progressively younger rocks northward in

the core of the northerly plunging Tuscarora anticline (fig. B-

2). In the Blue Star-Genesis Mine, Popovich and Rodeo Creek

rocks are exposed on the west limb of the asymmetric Tuscarora

anticline; the steepened east limb is truncated by the Gen fault

and places Tertiary sedimentary rocks against Paleozoic rocks.

The Popovich Formation averages about 500 feet (150 m) thick

in the northern Carlin trend, but is as much as 800 feet (240 m)

thick at Blue Star Ridge where near-vertical bedding is

structurally undisturbed. North of the 1-mile (1.6-km) wide

Goldstrike stock, in the Betze-Post area, Rodeo Creek mudstone

is the dominant exposed unit. Further north at Rodeo-Griffin-

Meikle, upper plate Ordovician Vinini Formation is preserved

above autochthonous units. Stepping 2 miles (3 km)

northwestward into the Bootstrap subdistrict, block faulting in

the Bootstrap window has exposed mostly Popovich and Rodeo

Creek rocks.

LOCAL STRATIGRAPHIC SUBUNITS

Informal mappable rock units in the Roberts Mountains and

Popovich Formations and the Rodeo Creek unit are described

below, from oldest to youngest (fig. B-3). These units are the

most commonly mineralized host rocks on the Carlin trend.

DSr4 Lower Roberts Mountains Formation (Upper

Silurian). Light gray, planar laminated silty limestone with

1030% inch- to foot-thick calcarenite interbeds. Mid-slope

facies. Thickness is 800 to 900 feet (240270 m). Contact with

the underlying Hanson Creek dolomite is marked by a 30- to

50-foot (915 m) thick cherty dolomite horizon. Weathers as

lavender platy or flaggy limestone.

DSb Bootstrap limestone (Devonian-Silurian). Massivelight gray, bioclastic limestone/biosparite. oid packstone tograinstone with lithoclasts of fossiliferous wackestone andpackstone (Armstrong and others, 1997). Basal portion at theBootstrap Mine contains 1050% planar laminated silty limestoneinterbeds. Shoal environment. Deposition and emergence spanUpper Silurian (DSr4) to Middle Devonian (Dp1) time (G. Griffinpersonal commun.). The upper contact with Popovich Formation

Dp0 strata is unconformable. Upper shelf areas at Goldbug contain1- to 5-foot (30150 cm) wide storm rip-up clasts. Karsted zoneswith cavities are filled by varved hematitic clay. Coarse calcitecrystals are common at Bootstrap and Capstone. Bootstraplimestone along silicified faults at Capstone forms massive,resistant jasperoid. Thickness >1,300 feet (>390 m) at Ren-Banshee and Meikle. Bootstrap limestone thins southward towardTara and Goldbug (figs. B-2, B-4, and B-5).

DSr1, DSr2, DSr3 Upper Roberts Mountain Formation

(Devonian-Silurian). Medium gray, carbonaceous, bioturbatedwavy (wispy) laminated silty limestone with 550% fossildebris flows (lithic and skeletal debris) and centimeter-thickcalcarenite interbeds. DSr3 debris flow-rich unit is locally

thickened to >300 feet (>90 m) at Goldbug along the south edgeof the Bootstarp bioherm. Bioclastic coral, crinoid, brachiopodand bryozoan debris decreases up-section and distally away fromthe Bootstrap limestone bioherm. Mid-slope to upper shelfdeposition. The uppermost 30 to 50 feet (915 m) is transitionalplanar to wispy laminated in gradational contact with theoverlying Popovich Formation. The top of the Roberts MountainsFormation (DSr1) is recognized by the first occurrence of wispylaminated beds. Wispy laminae have wavelengths of 2 to 6 mm,and are interpreted to represent faunal burrowing in anoxygenated substrate in a shallow, quiescent shelf environment.Thickness 0 to 400 feet (0120 m), thins northward and onlapsagainst Bootstrap limestone. Commonly decarbonatized,

silicified, and collapse brecciated in mineralized areas.Dp2 Lower Popovich Formation (Lower Devonian).

Dark gray, carbonaceous, thin-bedded to planar laminated silty

micrite with 510% millimeter-thick calcarenite to calcisiltite

interbeds. Localized graphitic bedding plane partings produce

disc-shaped hockey puck pieces in core. Mid-slope

deposition; 70 to 100 feet (2130 m) thick; onlaps against

Bootstrap limestone. Commonly silicified in mineralized areas.

Uppermost 2 feet (60 cm) contains regionally extensive

graptolites ofMonograptus species indicative of a starved basin

(Armstrong and others, 1997).


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Dp1 Middle Popovich Formation (Lower to Middle

Devonian). Dark gray, carbonaceous, medium-bedded micrite/

muddy limestone with diagnostic soft sediment deformation

slump and flame textures, minor bioclastic debris flows and

interbedded calcarenites, notably in Carlin Mine area. Upper

slope facies. Thickness is 200 to 400 feet (60120 m), thinsnorthward, and onlaps against Bootstrap limestone. Locally

thickened and metamorphosed to diopsidic calc-silicate

hornfels/marble south of the Goldstrike stock in Genesis area.

Dp0 Upper Popovich Formation (Middle Devonian).

Dark gray, carbonaceous, planar laminated silty 1imestone,

muddy 1imestone, calcarenite and limestone conglomerate.

Generally upward coarsening, mid- to upper slope deposition.

Contains diagenetic pin-striped pyrite (12 mm brassy pyrite

flecks) along bedding. Thickness is 100 to 250 feet (3075 m).

Thins northward and is regionally extensive above the

Bootstrap limestone. Includes two subunits at Rodeo

(Goldbug): lower 120 feet (36 m) is muddy limestone with 1

5%, 1- to 2-inch (35 cm) long, black chert lenses; upper 120

feet (36 m) is thin-bedded silty limestone. Limestone

conglomerate with 110%, 1- to 4-inch (310 cm) diameter

mudstone clasts form an excellent 30- to 40-foot (912 m)

thick marker horizon along the Dp0/Dp1 contact at Tara.

Calcarenite with limestone boulders up to 10 feet (3 m) in

diameter crops out conspicuously between the Blue Star Ridge

and Lantern deposits.

Drc Rodeo Creek basal calcarenite (Middle Devonian).

Gray carbonate-cemented quartz sandstone, commonly

silicified. Mid to upper slope, high-energy deposition.

Thickness 0 to 70 feet (020 m); thins northward, and is mostly

absent north of Betze-Post area.

Drc Rodeo Creek unit (Middle to Upper Devonian).

Interbedded dark gray siliceous mudstone and light gray planar

laminated silty limestone; lowermost 200 to 300 feet (6090

m) is dominantly siliceous mudstone (rhythmically beddedargillite). Deep, anoxic, marine deposition. Thickness is 100

to 700 feet (30210 m); the upper portion is locally removed

by the Roberts Mountains thrust.

MIDDLE PALEOZOIC CARBONATE DEPOSITIONMODEL

Devonian-Silurian carbonate deposition in northeast Nevada

was characterized by progressive marine transgression with

emergent periods and prolonged subaerial exposure of the

Bootstrap limestone. Paleogeographic environments are

subdivided as follows (figs. B-3 to B-6):

Silurian transgression (DSr4). DSr4 deposition is

characterized by slope and basin deposition of laminated silty

limestone with calcarenite turbidites above shelf carbonates.

DSr4 contains >50% terriginous silt content and is highly

permeable.

Devonian-Silurian emergence (DSb). Bootstrap limestone

represents shoal to shallow shelf deposition of olitic sands to

limestone muds with marginal debris flow sedimentation on

an irregular slope (Teal, 1996; Unger, 1997; Armstrong and

others, 1997). Submarine canyons along ancient fault lines

A A'

Gold zone

Bootstrap limestone

Drc/Dp datum line

Storm(Rossi)

CapstoneBootstrap Tara

Ovi

Drc

Dp0

Dp1 Dp2

DSr3

DSr4

DSr1-2

0

200

400 feet

0

100 meters

0 5,000 feet

0 1,500 meters

Figure B-4. Bootstrap area north-south stratigraphic fence diagram looking east.


8/15

27

Stratigraphy

focused thick accumulations of fossil debris flows, slump

blocks and turbidites (Jory and others; 1997). Debris flows

occur as lenticular beds, inches to tens of feet thick, containingpoorly sorted carbonate and biogenic clasts interbedded with

planar to wispy laminated silty limestone. Thickness and clast

size show an inverse relation to distance from the biohermal

source area, decreasing distally to inch-thick calcarenite beds.

Progradation occurred in local shelf areas. Basin-ward

contemporaneous deposition of planar laminated silty limestone

is interfingered with 1- to 3-foot (3090 cm) thick calcarenite

beds. This dramatic facies change is exposed in the west wall

of the Tara pit, where silty facies onlap onto the massive

Bootstrap limestone over a distance of 500 feet (150 m). The

top of the Bootstrap limestone is marked by a mid-Devonian

unconformity with extensive silicification in the Bootstrap and

Capstone pits.

Devonian-Silurian slope sedimentation (DSr1, DSr2, DSr3,

Dp0, Dp1, Dp2). Slope sedimentation was characterized byupward transition from oxygenated, terriginous, silt-richcarbonate to anoxic lime muds during shelf submergence. Mid-Devonian micrite (Dp1) sedimentation with soft sedimentdeformation occurred during prolonged, quiescent, shallow-water conditions. Middle to Late Devonian marine shallowingwith increased terriginous silt content due to frequent storms isevidenced by rip-up clasts and limestone conglomerates (Dp0).

B B'

Gold zone

Bootstrap limestone

Drc/Dp datum line

Outer limit of metamorphism

Banshee

Rodeo ( Goldbug) Betze-PostDeep Star

Blue Star

Lantern

Genesis

Drc

Ovi

Dp0

Dp1Dp2

DSr1-2

DSr3

GOLDSTRIKESTOCK

Drc

Dp0

Dp1

DSr1-2

DSr3

DSr4DSr4

Ovi

Dp2

0

200

400 feet

0

100 meters

0 5,000 feet

0 1,500 meters

Figure B-5. Betze-Post-Blue Star area north-south stratigraphic fence diagram looking east.

Mid-Devonian transgression and deep marine deposition

(Drc). The Rodeo Creek unit deposition was marked by anoxic

basin deposition of rhythmically bedded siliceous muds. Theconformable and gradational contact with underlying lime

muds and silts of the upper Popovich Formation is well exposed

at several localities, notably in the east wall of the Tara pit.

Intrusive Rocks

The northern Carlin trend contains numerous igneous rocks,

mostly as stocks and dikes. Magmatic activity occurred during

Late Jurassic, Eocene, and Miocene events that constrain the

ages of gold mineralization. Temporal evolution of igneous

activity in the northern Carlin trend is:

Late Jurassic 158-Ma magmatism emplaced the

Goldstrike granodiorite stock of high Mg-Fe, calc-alkaline

affinity and widespread, coeval hornblende-biotite lamprophyre

dikes. The northeast-southwest elongate Goldstrike stock

measures 4 miles (6.4 km) long in a northeast direction by 1

mile (1.6 km) wide (figs. B-2 and B-5). Maximum thickness is

unknown although laccolithic margins adjacent to the Deep

Post and Genesis orebodies exceed 1,500 feet (450 m) in

thickness. Contact metamorphism of limestone produced

diopside hornfels adjacent to the Goldstrike stock and distal

marble. Local metasomatism formed diopside-grossular

endoskarn along intrusion margins and diopside-grossular-


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28

vesuvianite exoskarn with retrograde tremolite-epidote-calcite

skarn in limestone. Propylitic alteration of diorite formed the

alteration assemblage of calcite-chlorite-pyrite. Late-stage

acidic fluids altered detrital feldspar, illite, and clay in

sedimentary rocks to illite and sericite, with kaolinite in higher

temperature, mineralized centers. The core of the Goldstrike

stock is fresh to weakly propylitized; only sericitized contacts

over widths of 10 to 100 feet (330 m) show the effects of

hydrothermal alteration. Calc alkaline, biotite-hornblendelamprophyre dikes 1 to 3 feet (3090 cm) thick are widespread

along north-northwest-trending structural corridors, notably the

Post-Gen fault zone along the eastern, fault-bounded edge of

the Tuscarora Spur.

Eocene magmatism (37 to 40 Ma) is present along the entireCarlin trend, consisting of K-rich calc-alkaline rhyolite anddacite dikes at Deep Star/Genesis and Beast, respectively. Eocenedikes of the northern Carlin trend are immediately adjacent to alarge aeromagnetic anomaly that corresponds with the nearbyEmigrant Pass volcanic field and Welches Canyon stock. It isinferred that the anomaly represents a buried Eocene plutoniccomplex, and that Eocene magmatism is the major process that

drove hydrothermal circulation forming Carlin gold deposits(Ressel and others, 2000a). Variable ages for Eocene dikesindicate that episodic gold deposition occurred over a period of3 to 4 million years (Ressel and Noble, 1999). Fine-grainedrhyolite dikes at Deep Star and Genesis (Anne Dike) are stronglyargillized and locally mineralized. Dacitic biotite feldsparporphyry dikes emplaced along the Post-Gen fault zones occurover a north-south distance of at least 4 miles (6.4 km) fromMeikle to Lantern. At Deep Post, barren biotite feldspar porphyrydikes (39 Ma) along the Post and Dormant faults bound high-grade (>1 opt [troy ounces per short ton] or 34 g/t) gold (Emsboand others, 1996). Conversely, the rhyolitic quartz eye monzoniteporphyry of the 36.3-Ma Beast dike in the Beast deposit is

moderately mineralized (Ressel and others, 1999). The Beastdike, which is up to 150 feet (45 m) wide and 1 mile (1.6 km)long, contains 3 million tons (2.7 Mt) grading 0.016 opt (0.5 g/t) gold. Gold occurs mostly in brecciated dike and quartz veinletsalong the footwall margin of the 60E-dipping dike. 3) Youngestmagmatism includes Miocene rhyolitic lavas (15 Ma) along thewest flank of the Tuscarora Spur. Air-fall tuff and ash flows inthe basal Carlin Formation may represent distal equivalents ofthis magmatism (Ressel and Noble, 1999).

INTRUSION-RELATED GOLDMINERALIZATION

The Jurassic Goldstrike stock has two important roles in its

spatial relationship to gold deposits:

(1) Embayments and sheared margins of the stock host high-

grade orebodies at Deep Star and Deep Post in

decarbonatized and hornfelsed sedimentary rocks along the

Post-Gen fault zone; and

(2) Margins of the contact metamorphic aureole focused gold

mineralization at Betze-Post on the north side and at

Genesis-Blue Star on the south side of the stock. The

northern contact aureole averages only 300 feet (90 m) in

width, whereas it is up to 1 mile (1.6 km) wide on the

south side. Gold endowment at Betze-Post (>30 million

oz [933 t]) is approximately six times greater than at

Genesis-Blue Star (5 million oz [156 t]) (Schutz and

Williams, 1995). This difference can be partly explained

by the broader metamorphic aureole at Genesis-Blue Star

producing inferior host rocks with restricted permeability,

mostly marble and calc-silicate hornfels of the Popovich

Formation. A small group of deposits is hosted by quartz-pyrite veins in the stock (e.g., North Star-West #9, Pancana).

Some of the highest gold grades on the northern Carlin trend

are hosted by Jurassic mafic dikes and along sheared dike

margins. Availability of iron from altered biotite and hornblende

is considered important for gold precipitation within mafic

dikes or along dike margins. Dikes (and sills) are commonly

argillized and/or sericitized adjacent to gold deposits, and were

emplaced along the same faults later utilized by gold-bearing,

hydrothermal fluids. North-northwest-trending Jurassic dike

swarms are often the best indicator of deep mineralized systems.

Whereas wallrocks may be geochemically barren, dikes with

anomalous As, Sb, Hg, and Tl and sericitized hornblende,

biotite, and plagioclase are important conduits for concealed

gold deposits (e.g., Betze-Post, West Leeville). For example,

surface expression of the 1,500- to 1,800-foot (460550 m)

deep West Leeville deposit is subtle; exposed Vinini mudstone

is not hydrothermally altered but there is a strong zone of north-

south faulting and lamprophyre dikes above the deposit. The

700- to 1,600-foot (210490 m) deep Betze-Post deposit

contains numerous north-northwest-striking, argillized dikes

in the 80E-dipping Post fault zone and 50W-dipping JB fault

zone; at depth these dike swarms host high-grade mineralization

where they cut the upper Roberts Mountains Formation.

STYLES OF GOLD MINERALIZATION

Gold deposits of the northern Carlin trend represent a spectrum

of deposit styles owing to stratigraphic and structural

differences (Teal and Jackson, 1997a; this volume). Each

deposit can be assigned to a deposit class; however, they all

contain components of the three major end members:

stratigraphic, collapse breccia and structural controls (figs. B-

7 and B-8). Deposit types do not cluster geographically, but

instead demonstrate remarkable variation in host rock and style

of mineralization over short distances. Individual deposits

commonly contain several styles of mineralization. Each

deposit class with examples is discussed below.

Stratigraphically Controlled Gold Deposits

The classic Carlin-type deposits show a strong stratigraphic

control to gold mineralization and contain about 15% of the

northern Carlin trend gold resource (fig. B-8). These tabular

orebodies are characterized by extensive decarbonatization and

residual dolomite with minimal structural disruption.

Stratigraphically, there are three preferred host horizons in gold

deposits of the northern Carlin trend:


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29

Stratigraphy

C C'

Turf W est Leeville

Car lin Pete

Gold zone

Drc/Dp datum line

Ovi

Drc

Dp0

Dp1

Dp2

DSr1-2

DSr3DSr3

DSr4

0

200

400 feet

0

100 meters

0 5,000 feet

0 1,500 meters

Figure B-6. Carlin area north-south stratigraphic fence diagram looking northeast.

Upper Roberts Mountains Formation (DSr1, DSr2, DSr3).

The uppermost 400 feet (120 m) of the Roberts MountainsFormation is especially susceptible to decarbonatization by acidic

hydrothermal fluids and gold precipitation. Decarbonatizationproduces a punky, low density, silty carbonaceous rock withexcellent secondary permeability and porosity. Volume loss ashigh as 40% was documented at the Carlin Mine (Bakken andEinaudi, 1990). Silicification is variable and is mostly confinedto centimeter-thick calcarenite interbeds that constitute only 110% of the upper Roberts Mountains Formation. The RobertsMountains Formation is conformably overlain by the PopovichFormation, which includes the relatively fine-grained andimpermeable, 200- to 300-foot (6090 m) thick Dp1 micrite unit.Dp1 micrite is geochemically barren and rarely mineralized,except where cut by faults and sulfidized dikes, and likely servedas a cap rock to mineralization in underlying host rocks. Similarly,

the 800-foot (240 m) thick DSr4 unit of the lower RobertsMountains Formation is a poor host in most deposits except alongthe Castle Reef fault at Lantern and Perry, and in the lower zoneat West Leeville. The DSr4 unit was likely a relatively poor hostdue to the lack of bioclastic debris flows and calcarenite interbeds,and perhaps paucity of organic carbon and available ironcompared to the overlying upper Roberts Mountains andPopovich Formations.

Examples of dominantly stratigraphically controlled

orebodies hosted by the upper Roberts Mountains Formation

include Carlin, Pete, West Leeville, and Tara, each with notable

variations in preferred host strata in the upper Roberts

Mountains Formation and distinct feeder faults (figs. B-1 and

B-6). At Carlin the primary host is the DSr1 and uppermost

DSr2 units; secondary structural control is provided by the

northeast-striking Hardie fault and northwest-striking faults.

Carlin is a cluster of gold deposits including Carlin Main, Carlin

Island, Carlin East, and Hardie Footwall located in the footwall

of the Hardie fault. The Pete deposit, located 1 mile (1.6 km)

southeast of the Carlin deposit, may represent the right-laterally

offset portion of the Carlin deposit along the northwest-striking

Leeville fault. The preferred host rock at Pete is the DSr2 unit,

and about 15% of the resource is hosted in the basal 50 feet

(15 m) of the Rodeo Creek unit. North-northeast and north-

northwest structural controls are inferred at Pete. Located 1

mile (1.6 km) north of Carlin, West Leeville includes two

stacked stratiform horizons with a vertical separation of about

200 feet (60 m) in the base of the DSr2 unit and top of theDSr4 unit. The footwall of the north-striking West-Bounding

fault is strongly mineralized, especially where cut by the

northwest-striking Rodeo Creek fault. At the north end of the

Carlin trend, the Tara deposit is hosted primarily by the DSr2

unit. At northern Tara where the DSr2 unit pinches out, the

Dp2 unit is the preferred host. The passive decarbonitized style

of mineralization at Tara contrasts remarkably with the

Bootstrap orebody only 1,000 feet (300 m) to the north, where

structurally controlled gold mineralization occurs along the

silicified and dike-filled Bootstrap fault and bleeds out laterally

along the Dp0/Bootstrap limestone unconformity.


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30

GE GenesisGR GriffinLL Long LacLG Lower GoldbugLP Lower PostME MeikleNL North LanternNS North Star/West #9

PA PancanaPY PayraisePR PerryPE PeteRD RodeoSC ScreamerSO SoldSL South LanternSR Storm (Rossi)TA TaraTU TurfUG Upper GoldbugUP Upper Post

WG West GenesisWL West Leeville

AH Antimony HilBA BarrelBZ BazzaBT BeastBE BetzeBX Big 6BR Blue Star RidgeBS Blue Star

BO BootstrapBC BobcatBM Bullion MonarchCP CapstoneCA CarlinCW Carlin WestDE DeeDP Deep PostDS Deep StarEX ExodusFE FenceFC Four Corners

COLLAPSE BRECCIA

FAULTS, FOLDS, VEINSSTRATIGRAPHY

> 5 million oz Au resource

0.5-5 million oz Au resource

< 0.5 million oz Au resource

CA TA WL PE BR

FE CW WG BS UP UG NL RD BO DE BT

EX PA LL

BX

AH

BZ

MEGR

BC

SC

BE

GE

SR

DP

LP

LG

PY SO FC PR BA BM DSCP NS TUSL

Figure B-7. Ternary plot of styles of gold mineralization for the northern Carlin trend. Modified from Teal and Jackson,1997b.

Lower Popovich Formation (Dp2 unit). The lowermost 70

to 100 feet (2030 m) of the Popovich Formation is commonly

silicified and hosts portions of stratigraphically controlled

deposits at Lower Barrel, Lower Goldbug, and Tara. Owing to

its stratigraphic proximity to the underlying DSr1-2-3 units,

the entire Dp2 through DSr3 package is mineralized in thelarger deposits (e.g., Betze-Post, Genesis).

Upper Popovich Formation (Dp0 unit). The upper 200 feet

(60 m) of the Popovich Formation is also extensively

decarbonatized, and is conformably overlain by 300 feet (90

m) of relatively impermeable siliceous mudstones of the lower

Rodeo Creek unit. Rheological contrast along the Rodeo Creek

siliciclastic/Popovich limestone contact has produced

numerous low-angle faults and bedding plane shears that

enhance permeability and porosity. However, at Betze-Post

strong argillization within the uppermost 50 feet (15 m) of the

Popovich Formation rendered this horizon impermeable by

forming a stratiform carbonaceous clay layer.

Examples of dominantly stratigraphically controlled

orebodies hosted in the upper Popovich Formation include

Upper Meikle, West Griffin, Rodeo, Upper Goldbug, Upper

Barrel, West Genesis, Bobcat, Blue Star and SOLD. In each

example, gold mineralization has ponded immediately beneath

the Rodeo Creek/Popovich contact in decarbonatized silty

limestone along major north-northwest feeder faults. The

Rodeo and Upper Goldbug deposits are hosted in the Dp0

and Dp1 units in the footwall of the N1030E-striking,

lamprophyre dike-filled Hillside fault and the footwall of the

Post fault. Availability of organic carbon in the Roberts

Mountains and Popovich Formations may have been locally

important in precipitating gold at Rodeo (Goldbug), where

grains of coarse gold up to 1 mm occur.

Collapse Breccia-Hosted Gold Deposits

Collapse breccia-hosted deposits are common in the Post

subdistrict at Betze, Lower Post, and Lower Goldbug, and at

Genesis due to widespread decalcification and subsequent

volume loss and collapse overprinting bioclastic-rich portions

of the DSr3 unit (fig. B-5). Typical Popovich Formation

thicknesses of 500 to 600 feet (150180 m) at Genesis and

Betze-Post compared to 800 feet (240 m) in unaltered Popovich

on the Tuscarora Spur suggest that as much as 40% volume

loss took place due to collapse. These deposits contain 40% of

the northern Carlin trend resource. Collapse breccia dimensions

at Betze-Post-Goldbug are about 1 square mile (2.5 square km)

in plan and 400 to 700 feet (120210 m) thick. Collapse brecciasare clast-supported with angular clasts of the DSr2-3-4 units

set in a matrix of fine-grained, residual silt, organic carbon

and pyrite. The simple genetic model envisages early

decarbonatization by acidic hydrothermal fluids focused along

north-northwest-striking faults and laterally into stratiform

fossil debris flows and calcarenite interbeds, collapse of the

upper Roberts Mountains Formation, gold precipitation, and

late-stage silicification + baritization. Heterolithic collapse

breccias contain clasts of intrusive material and are generally

higher grade. These stratiform bodies are not widely developed


12/15

31

Stratigraphy

Figure B-8. Northern Carlin trend gold distribution vs. stratigraphy.

0Gold resource (million ounces)

5 10 15 20 25 30

Simp

lifie

ds

tra

tigra

phicco

lumn

(averageun

itthickness

infeet

)

Tmc

DOw

Drc

Dp0

Dp1

Dp2

DSr1-2

DSr3

DSr4

0

200

400feet

0

100

meters

outside the Betze-Post-Goldbug area owing to the lack of

bioclastic debris flows. Within the Post subdistrict they host

large, medium- to high-grade gold deposits with stratigraphic

and structural controls (Volk and others, 1995).

The giant Betze orebody (40 million oz [1,250 t]) consistsof seven vertically stacked ore zones in the upper RobertsMountains Formation within the west-northwest-trending Betzeanticline along the northern margin of the Goldstrike stock (fig.

B-2). Excluding structurally controlled deposits along thenorthwest-striking Castle Reef fault, the Betze, and the adjacentDeep Post orebodies are unique in their northwest strike. Thethickest and highest gold grades are located along the axis ofthe Betze anticline at its intersection with northeast- and north-northwest-striking faults. Lower, more erratic grades arespatially related to intense silicification along the northeast limbof the Betze anticline. Limestone dissolution betweenimpermeable layers formed stacked, stratified collapse brecciasover a vertical distance of 700 feet (210 m) with an estimatedrock thickness reduction of 14% (Leonardson and Rahn, 1996).

The base of the stratiform, silicified, collapse breccia-hostedLower Post and Lower Goldbug deposits are sharply definedby the DSr3/4 contact (Jory and others, 1997). Debris flowswith interbedded wispy-laminated silty limestone are thickest(300 feet; 90 m) and most fossiliferous at Goldbug and LowerPost. Their occurrence marks the paleogeographic upper slopefacies of the southeast margin of the Bootstrap limestonebioherm. The Post fault zone is postulated to be an ancient

structure that served as a submarine trough along which thickerpackages of fossil debris flow were shed, forming the DSr3unit (Jory and others, 1997). DSr3 is the preferred host unit atLower Post and Lower Goldbug. The underlying DSr4 planarlaminated silty limestone is rarely mineralized due to the lackof debris flows, thereby being less susceptible to fluid migration.At Genesis the preferred host unit is silicified collapse brecciaoverprinting decalcified DSr2-3 units in the footwall of the Genfault along the crest of the Tuscarora anticline. Dimensions ofthe collapse breccia are 1,500 feet (450 m) north-south by 600feet (180 m) east-west by 200 to 300 feet (6090 m) thick.


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32

Structurally Controlled Gold Deposits(Faults, Folds, and Veins)

Structurally controlled deposits account for an estimated 45%

of gold in the northern Carlin trend. The three highest grade

gold deposits on the Carlin trend (Deep Star, Deep Post, and

Meikle) are located in the immediate footwall of the north to

N20W-striking, 80E-dipping Post fault zone. The fault zone

served as an important conduit for mineralizing low pHhydrothermal solutions along the east edge of the Tuscarora

Spur (fig. B-2). These large, profitable orebodies formed at

flexures that represent dilation zones along the Post fault.

The 100- to 300-foot (3090m) wide fault zone is poorly

mineralized due to high clay content and therefore restricted

permeability. Sharp grade boundaries place 1.0 opt (34 g/t)

gold rock within tens of feet of unmineralized rock. Gold grades

average 0.7 to 1.0 opt (2434 g/t) gold and are as high as 6 opt

(206 g/t) gold. The gold occurs as sub-micron-sized,

disseminated particles in arsenian pyrite rims on pyrite nuclei

(Arehart and others, 1993). These structurally controlled end

members are characterized by abundant shearing and tectonic

brecciation, 310% fine-grained sooty pyrite and marcasite,and elevated trace element geochemistry (gold, As, Sb, Hg,

and Tl). Despite their similarities, these three deposits also have

important differences.

Deep Post and Deep Star are steeply east-dipping

orebodies within or immediately adjacent to the metamorphic

aureole of the Goldstrike stock (Harlan and Heitt, 1997), with

horizontal dimensions of 500 by 1,000 feet (150 by 300 m)

and 500 by 700 feet (150 by 210 m), respectively (fig. B-5).

Vertical dimensions are relatively large, ranging from 800 to

1,200 feet (240360 m). Recent work at both these deposits

suggests that sharply defined bases to gold mineralization may

be partly stratigraphically controlled. Hydrothermal alterationconsists of a high-grade quartz+kaolinite core with 1.0 opt

(3.4 g/t) gold with fine-grained sooty sulfide flooding in

pervasively decarbonatized and sheared host rocks. Deep Post

host rocks are the DSr2 and upper DSr3 units. An estimated

15% of the deposit is hosted in sheared diorite along the

northeast margin of the Goldstrike intrusion. Deep Star is hosted

by calc-silicate hornfels and silty limestone of the Popovich

Formation. Deep Post averages 0.7% arsenic and contains

spectacular realgar and orpiment-filled shears along the

intrusive/sedimentary rock contact, while Deep Star contains

only minor realgar. By contrast, arsenic sulfides are uncommon

at Meikle (7 million oz at 0.7 opt [200 t at 24 g/t] gold), which

consists of en echelon, high-angle gold-bearing zones cuttingmassive Bootstrap limestone. Meikles dimensions are 2,500

by 800 feet (750 by 240 m). Metamorphic rocks do not occur

at Meikle and silicification is widespread. Meikle and Deep

Star have similar N15W/7080E-dipping structural

orientations in the footwall of the Post fault zone, whereas the

N50W/65SW-dipping orientation at Deep Post is controlled

by the sheared intrusive/sediment contact. Thus, each of these

900- to 1,600-foot (270490 m) deep, high-grade deposits has

contrasting host rocks, orientations, alteration, and gangue

mineralogy that require different exploration strategies. East

Griffin, located 1,000 feet (300 m) south of Meikle, occurs

within DSr2-3 units along the collapse brecciated, sheared and

dike-filled footwall of the Post fault zone.

The Genesis Complex includes the 3-million-ounce (93 t)

Genesis deposit, hosted by the Popovich and upper Roberts

Mountains Formations, and numerous smaller satellite deposits

that are mostly structurally controlled along the Rodeo Creek/

Popovich contact. Mineralizing solutions exploited

intersections of thrusts, northwest-striking normal and reversefaults, northeast-striking normal faults, and the crest of the

N15W-trending Tuscarora anticline (Schutz and Williams,

1995). The barren, N1040E-trending, 39-Ma K Dike is well

exposed in the Genesis pit, where it is up to 100 feet (30 m)

wide and strongly sericitized. The dacitic K Dike is mapped

over a north-south strike length of 2 miles (3 km) from Deep

Star and Genesis southward through Blue Star to the Crazy

Eights prospect; the sheared footwall margin commonly

contains 0.1 to 0.2 opt (37 g/t) gold grades over widths of 10

to 50 feet (315 m). West Genesis occurs on the west limb of

the Tuscarora anticline in upper Popovich Formation along

bedding-parallel shears. Low-angle thrusts along the Rodeo

Creek/Popovich contact in an uplifted structural block on the

west edge of the Genesis pit are the primary ore control at

Bobcat and Payraise. Flat-lying to 30W-dipping quartz veins

cutting the southeast margin of the Goldstrike stock are the

dominant host rock at the North Star and West #9 deposits.

Some gold mineralization is also focused along north-northwest

faults that cut Popovich limestone exoskarn and marble. At

Genesis and North Star, alteration is zoned from strongly

silicified and quartz-sericite-pyrite altered core >0.10 opt (3.4

g/t) gold, outward to less intense quartz-sericite-pyrite in 0.01

to 0.10 opt (0.33.4 g/t) gold haloes, to decalcified waste rock.

Blue Star ore is dominantly silificified calcarenite of the basal

Rodeo Creek unit and decalcified upper Popovich Formation.In the adjacent Blue Star Ridge deposit 1,000 feet (300 m) to

the south, the preferred host unit is the 50- to 70-foot (1521

m) thick basal calcarenite; however, narrow high-grade zones

zones are restricted to high-angle, northerly striking

lamprophyre dikes. The basal calcarenite attains its greatest

thickness at Blue Star and Blue Star Ridge where it forms an

excellent host rock. The Beast deposit is hosted by the northerly

striking Beast dike and brecciated DSr4 in the footwall of the

dike. The Beast orebody occurs in the east limb of the Tuscarora

anticline in the footwall of the dike-filled Gen fault. The

adjacent Sold deposit 1,000 feet (300 m) northwest of Beast

occurs along a decalcified and argillized east-west fault cutting

Popovich limestone.Other examples of structurally controlled gold deposits

of the northern Carlin trend include all deposits in the Bootstrap

subdistrict, excluding the stratigraphically controlled Tara

deposit (fig. B-4). These deposits occur along the near-vertical,

north-south Bootstrap fault zone, including Dee, Capstone,

and Bootstrap. These deposits are somewhat unusual in that

2040% of the gold is hosted by silicified Bootstrap limestone

adjacent to north-south feeder faults. However, a critical

component of these deposits is the intersection of these feeders

with the upper Popovich Dp0 silty limestone/Bootstrap


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33

Stratigraphy

limestone unconformity (fig. B-4) (Coombs and Malan, 1999).

Significant gold mineralization is not recognized more than

300 feet (90 m) vertically above or below this unconformity.

At Capstone the lamprophyric Capstone dike is strongly

mineralized where it cuts the unconformity. Also at Capstone,

a northeast zone of gold mineralization containing 80,000

ounces (2.5 t) of gold is hosted by silicified and quartz

stockwork-veined silty limestone of the allochthonous Vinini

Formation. The Vinini Formation is generally a poor host sinceit contains impermeable and unreactive siliceous rocks and high

clay content along Roberts Mountains thrust-induced shears.

Vinini-hosted mineralization is structurally controlled and is

typically only several meters wide. Vinini siltstone is

mineralized along north-northeast faults at the Big Six and

Antimony Hill prospects north of the Carlin Mine. To date, no

significant gold mineralization has been discovered in the oldest

rocks of the northern Carlin trend, including the Ordovician

Pogonip limestone, Ordovician Eureka Quartzite, and

Ordovician Hanson Creek dolomite. The South Lantern deposit

is structurally controlled along the northwest-striking, baritic

Castle Reef fault in the DSr4 unit. South Lantern was

discovered in 1993 by detailed structural mapping. It is notable

for its subtle alteration with weak to no decarbonatization and

intact bedding. Several small deposits are hosted by the DSr4

unit along the Castle Reef fault zone, including Bullion

Monarch (Universal Gas), West Carlin, Perry and Castle Reef.

These deposits are spatially associated with large linear ribs of

jasperoid replacing bioclastic-rich strata along the steeply

northeast-dipping Castle Reef fault.

CONCLUSIONS

The Carlin trend is a north-northwest alignment of over 40

gold deposits with associated hydrothermal alteration along alinear zone 40 miles (65 km) long and 1 to 2 miles (1.63.2

km) wide. Gold deposits of the northern Carlin trend show

three structural orientations, in order of decreasing importance:

(1) north to N20W in the footwall of the Post-Gen fault zone

and numerous subparallel dike swarms; (2) N30W to N60W

along the Castle Reef fault zone and along the Betze anticline;

and (3) N10E to N30E at Tara, Carlin, West Leeville, and

Blue Star. Numerous local variations are recognized at the

mine-scale, notably a northeast fabric at Betze-Post and

Genesis. Two-thirds of gold deposits in the northern Carlin

trend occur within the north to N20W strike range (table B-

1); however, the largest orebody (Betze) strikes N60W.

Gold deposits of the northern Carlin trend represent a

spectrum of styles of mineralization owing to stratigraphic and

structural differences (Teal and Jackson, 1997a). Each deposit

can be assigned to a deposit class; however, most contain

components of the three major end members: stratigraphic

control, structural control, and collapse breccia. Review of

figure B-7 shows that most gold deposits plot within the

structurally controlled field. Structural preparation is the single

most important factor in controlling Carlin trend gold deposits.

The richest deposits are concentrated along or adjacent to the

Post-Gen fault zone, as exemplified by Deep Star, Deep Post,

and Meikle. The bulk of gold mineralization occurs within 2

miles (3 km) of the Jurassic Goldstrike diorite stock and

adjacent to the north-northwest-striking, dike-filled, Post-Gen

fault zone. However, review of figure B-7 for gold resources

larger than 0.5 million ounces (16 t) shows that there is a

roughly equal number of deposits in the structure (8) versus

stratigraphy + collapse breccia (8) fields, testifying to the

importance of stratigraphic control. The latter statement is

especially relevant when considering the Betze deposit, by farthe single largest deposit on the Carlin trend with 40 million

ounces (1,250 t) of gold.

Figure B-8 portrays the importance of the upper Roberts

Mountains Formation as the superior host rock in the northern

Carlin trend. Preferred host rocks include bioturbated, wispy-

laminated silty limestones with bioclastic debris flows and

calcarenite interbeds, which contain an estimated 65% of

the northern Carlin trend gold resource. Extensive

decarbonatization, volume loss, and subsequent collapse

brecciation enhanced permeability and porosity to produce ideal

host rocks for ascending mineralizing fluids. Abrupt facies

changes across the Dp0/Bootstrap limestone unconformity and

across the Rodeo Creek/Popovich Formation contact localizedgold mineralization throughout the northern Carlin trend.

Hydrothermal alteration spatially associated with gold

mineralization includes decarbonatization, silicification,

argillization, sulfidation, dolomitization, with barite + stibnite

+ realgar + orpiment introduction with geochemically

anomalous As, Sb, Hg, Ag and Tl. These characteristics are

recognized in many gold mining districts throughout northeast

Nevada, including Gold Quarry-Mike-Tusc, Rain-Railroad,

Bald Mountain-Alligator Ridge, Cortez-Pipeline, and Twin

Creeks-Getchell. Superposition of the western margin of

ancestral North America with slope facies sedimentary rocks,

structural reactivation along north-northwest to north-northeast

fabrics, several episodes of magmatic activity, and large

auriferous hydrothermal cells combine to make northeast

Nevada one of the richest gold provinces in the world.

ACKNOWLEDGMENTS

This paper represents the cumulative geologic work of dozens

of geologists on the Carlin trend. Understanding and correlation

of Carlin trend stratigraphy have improved considerably in the

past 5 years due to better open pit and underground exposures,

increased core drilling, and continued communication between

geologists. Much of the current stratigraphic nomenclature was

developed in 1993 at the Carlin Mine by Newmont geologistsSteve Moore and Richard Harris. Frequent reference is made

to the 1997 Society of Economic Geologists newsletter

publication by Teal and Jackson, entitled Geologic overview

of the Carlin trend gold deposits and descriptions of recent

deep discoveries. Newmont Mining Corporation is thanked

for granting permission to publish this paper. Enlightening

discussions over the past ten years with the following geologists

are especially appreciated: Newmont geologists Jeff Huspeni,

Ron Thoreson, Ken Paul, Leroy Schutz, Lewis Teal, Henry

Unger, Cindy Williams, Bruce Harlan, Leo Coombs, Paul


15/15

Deposit Abbreviation in Pre-mine Grade Primary Secondary Strikefigure 7 resource (opt gold) host unit host unit

(oz gold)

Antimony Hill AH 20K 0.05 Ovi N20E

Barrel BA 200K 0.20 Dp2 Dp0 N10WBazza BZ 80K 0.05 Drc N45W

Beast BT 50K 0.02 DSr4 N10WBetze BE 40M 0.20 DSr2-3 JKi N60W

Big Six BX 20K 0.05 Ovi N20E

Blue Star Ridge BR 180K 0.02 Drc Dp0 N-SBlue Star BS 400K 0.04 Dp0 Drc N-S

Bootstrap BO 850K 0.10 DSb, Dp0 Drc N-S

Bobcat BC 220K 0.03 Dp0 Drc N-SBullion Monarch BM 40K 0.17 DSr4 N60W

Capstone CP 300K 0.10 Ovi, Dp0 DSb N-SCarlin CA 5M 0.40 DSr1-2 N50E

Carlin West CW 250K 0.06 Dp0 DSr4 N60WDee DE 1M 0.10 Dp0 DSb N-S

Deep Post DP 4M 0.70 DSr2-3 JKi N50W

Deep Star DS 1.7M 1.00 Dp JKi N20WExodus EX 400K 0.10 Dp N30W

Fence FE 200K 0.20 DSr2 N20WFour Corners FC 300K 0.25 DSr2-3 Drc N15E

Genesis GE 3M 0.10 DSr2-3 Dp N15WGriffin GR 500K 0.40 DSr2-3 N20W

Long Lac LL 65K 0.05 Drc N10W

Lower Goldbug LG 1M 0.25 DSr2-3 DSr4 N30WLower Post LP 2.5M 0.13 DSr3 DSr2 N30W

Meikle ME 7M 0.70 DSr, DSb Dp0 N15WNorth Lantern NL 100K 0.03 DSr4 N10W

North Star-West #9 NS 230K 0.08 JKi Dp N10W

Pancana PA 20K 0.05 JKi N15WPayraise PY 30K 0.02 Drc Dp0 N45E

Perry PR 50K 0.02 DSr4 N60WPete PE 1M 0.06 DSr2 Drc N20W

Rodeo RD 1.5M 0.40 Dp0 Dp1-2 N40E

Screamer SC 5M 0.18 DSr2-3 Dp N70WSold SO 30K 0.02 Dp1-2 DSr2 N80E

South Lantern SL 200K 0.04 DSr4 N30WStorm (Rossi) RS 1.1M 0.41 DSb, Dp0 JKi N20W

Tara TA 600K 0.06 DSr2 Dp2 N20ETurf TU 1M 0.37 DSr2-3 Dp0 N15E

Upper Goldbug UG 1.2M 0.35 Dp2 Dp0-1 N40E

Upper Post UP 3.0 M 0.04 Drc N20WWest Genesis WG 250K 0.07 Dp0 N-S

West Leeville WL 3M 0.37 DSr2 DSr4 N-S

K = thousands; M = millions; 1 million ounces = 31.1 metric tons (t); 1 ounce/short ton (opt) = 34.3 grams/metric ton (g/t)

Data sources: Teal and Jackson (1997a)

1998 Newmont annual reserve and resource statement

Table B-1. Northern Carlin trend resources by deposit, host rock, and primary structural orientation.

Malan, Wayne Trudel, Don Harris, and Margie Lane; and

Barrick geologists Bob Leonardson , Jerry Rahn, Mike Penick,

Greg Griffin, and Gary Baschuk. Jeanette Hunter is thanked

for drafting the figures. Steve Moore, Lewis Teal, Henry Unger,

Ken Paul, Greg Griffin, and three anonymous reviewers

provided excellent comments that significantly improved the

manuscript. Finally, my thanks to Lewis Teal, Henry Unger,

and Tommy Thompson for providing the encouragement to

write this paper.

Stratigraphy and Host Rock Controls of Gold Deposits of the Northern Carlin Trend

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