stratigraphy and host rock controls of gold deposits of the northern carlin trend
TRANSCRIPT
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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.
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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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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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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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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
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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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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
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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.