GEOLOGY AND GOLD MINERALIZATION OF THE TURF DEPOSIT
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Transcript of GEOLOGY AND GOLD MINERALIZATION OF THE TURF DEPOSIT
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8/8/2019 GEOLOGY AND GOLD MINERALIZATION OF THE TURF DEPOSIT
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Turf Deposit
GEOLOGY AND GOLD MINERALIZATION OF THE TURF DEPOSITJerry W. Mohling1
ABSTRACT
The Turf deposit is another significant discovery of deep, high-
grade, refractory gold ore on the Carlin trend. The deposit iscentered 2 miles (3 km) north of the Carlin Mine and about
a2
1 a
21
a2
1 a
21
a2
1
mile (0.8 km) north of the West Leeville deposit in Newmonts
North Area of Operations. Turf was discovered in 1994 by
Newmont Exploration Ltd. during the first drill test of the Four
Corners structural corridor. The deposit lies at a depth of 1,800 to
2,600 feet (540780 m) beneath the western flank of the main
Tuscarora Mountains at the eastern margin of Little Boulder Basin.
Turf is hosted by impure limestones and dolostones of the
Devonian Popovich limestone and the Devonian-Silurian Roberts
Mountains Formation in the lower plate of the Roberts Mountains
thrust. Gold mineralization in the Turf deposit is controlled
primarily by the north-striking, 5060W-dipping Turf fault and
by ancillary parallel structures in the footwall of the Turf fault.The ore-grade gold within the Turf fault ranges from 5 to 40 feet
(1.512 m) in true thickness (averaging approximately 30 feet
or 9 m). The ore zone extends 300 to 600 feet (90180 m) down
dip and rakes gradually to the north. Stratabound, high-grade
gold (>0.20 opt [troy ounces per short ton] or 6.86 g/t) extends
as much as 300 feet (90 m) laterally from the fault along favorable
carbonate beds, and ranges from 10 to 75 feet (323 m) in
thickness. In plan, the deposit is a north-trending body 3,800
feet (1,140 m) long by 300 to 800 feet (90240 m) wide.
The micron-sized, high-grade disseminated gold is
associated with decalcification, silicification, late-stage sooty
pyrite, and kaolinite. Calcite, barite, and dolomite occur as
fracture-filling veins that postdate the main episode of gold
deposition. Gangue minerals, such as montmorillonite, illite,
alunite, and K-feldspar, are minor and do not have a direct
correlation with gold ore. The bioturbated or wispy-
laminated units of the upper Roberts Mountains Formation and
the lower parts of the Popovich limestone were most susceptible
to decalcification and subsequent stratabound replacement
mineralization.
The geometry and litho-structural controls of the Turf
deposit have been interpreted from exploration diamond drill
holes and by limited infill definition drilling. No historical or
recent mine workings have been developed on the deposit.
Based on intercepts from 13 core holes that were analyzed
by fire assay, the main part of the Turf deposit was modeled by
computer and a gold resource was calculated to be 2.7 million
short tons (2.4 Mt [million metric tons]) with a 15% diluted
grade of 0.37 opt (12.7 g/t) gold (993,000 oz or 30.9 t gold).
Subsequent polygonal estimates applied to a 1,200-foot (360-
m) long continuation of the Turf deposit, north of the main
zone, suggest a further resource of 229,000 oz (7.1 t) gold with
an undiluted grade of 0.43 opt (14.7 g/t). Extensions remain
open to the north and northeast.
INTRODUCTION
Regional Geology
The Turf deposit lies within the North Lynn subdistrict of the
Carlin trend in Newmonts North Area of Operations and is
centered 2 miles (3 km) north of the Carlin minesite (fig. G-1).
The deposit is 100% controlled by Newmont Mining
Corporation. Turf straddles the eastern margin of Little Boulder
Basin adjoining the western range front of the main Tuscarora
Mountains. The name Turf is an acronym for Tuscarora Range
Front (Jackson and Bell, 1995).
The Paleozoic sedimentary sequence and the tectonic
evolution of the Carlin trend have been studied and documented
by numerous workers. The North Area of the Carlin trend was
mapped by the U.S. Geological Survey (Evans, 1980), and
recent overviews of the geology and structural framework of
the Carlin trend have been compiled by Christensen and others
(1988) and by Teal and Jackson (1997a). A brief overview of
the regional setting is given here as it pertains to the host rocks
and structural development.
During the Paleozoic, the area that is now northeastern
Nevada occupied part of the western margin of the North
American craton. Sedimentary rock units grade westward from
miogeoclinal shelf carbonates (eastern assemblage) to deeper
marine carbonates and siliciclastic slope facies of the western
assemblage (Christensen, 1993). The eastern assemblage strata
record a rapid bathymetric change in sedimentary environment
during Late Devonian time when eastern Nevada was evolvingfrom a passive to an active (compressive) continental margin.
This compressive event resulted in the Devonian-Mississippian
Antler orogeny wherein siliceous eugeoclinal rocks of the
western assemblage were thrust as much as 90 miles (145 km)
eastward, along the Roberts Mountains thrust system, over the
autochthonous eastern assemblage shelf facies carbonate rocks.
Subsequent regional compression during the Middle
Pennsylvanian Humboldt orogeny and the Triassic Sonoma
orogeny contributed to the development of the broad northwest
trending, north plunging Tuscarora antiform in the main
Tuscarora Mountains. Within the northern portion of the Carlin
trend, two dominant magmatic episodes of Late Jurassic and
late Eocene age are recognized. The Late Jurassic suite consists
mostly of equigranular diorite related to the Goldstrike stock
and Little Boulder Basin stock. Additionally there are
porphyritic lamprophyre dikes and porphyritic diorite sills and
dikes. The late Eocene intrusive rocks include the Betze dacite,
the Post rhyodacite, and the Beast dacite (Ressel, 1998).
Faults that underlie this region provided conduits for
circulation of hydrothermal fluids; however, the progenitor
thermomagmatic source(s) of the gold-bearing fluids remains
a topic of debate.1Geologic consultant, Tucson, Arizona
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Pete
We stBetze
WestLeeville
Turf
Winston
Ren
Tara
Bootst rap
Capstone
GoldenZia
NW Gene sis
WestRen
Banshee
South Meikle
Meikle
Barrel
WestGr iffi n
East Griffin
NorthBetze
WBazzaLongLac
N . Skarn Hil l
ShaloskyBetze
Pancana 5-0
West # 9
Gen esis
Bobcat
Payraise
BlueStar
Wi dge
BlueStarRidge
Beast
N W Extension
S. Lantern
N .Lantern
Hardie FW
Carlin East
?
D ee
FenceCar lin
Big Six
Antimon y Hill
Exodus
LEEVILLEFAUL
T
LYNN
FAU
LT
HARDIE
FAULT
Gold deposits
Granodiorite intrusions
Fault, solid where observed, dashed where inferred
DEE
FAULT
Rod eo
Screamer
Bazza Pt
Skarn Hill
Bazza
VIVIANGULCH
FAULT
Pancana
GOLDSTRIKESTOCK
LITTLEBOULDER
BASINSTOCK
(concealed)
FourCo rners
FOUR
CORNERS
FAULT
GEN
FAULT
CASTLEREEF
FAULT
Deep Star
Elko County
Eureka County
Perry
CASTLE
REEF
FAULT
0 5,000 10,000 feet
0 1,500 3,000 meters
TURF
FAULT
Island
N orth Star
Carlin West
Storm(Rossi)
Goldbug
D eep Post
Lower Post
POST
FAULT
Post
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
L
A
N
D
E
R
WA
SHOE
80
80
Figure G-1. Turf deposit location map, northern Carlin trend, Nevada. Modified from Teal and Jackson, 1997b.
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Turf Deposit
Late Tertiary tectonic extension of the North Lynn
subdistrict is manifested by the graben of Little Boulder Basin
(fig. G-1). Throw along the graben-bounding faults is estimated
to be on the order of 3,000 to 5,000 feet (9001,500 m) relative
to the adjacent rocks to the east and west of the graben. This
episode of tectonism crosscut and reactivated some of the earlier
structures. Along the Carlin trend, several structural and
erosional windows through the allochthonous rocks reveal the
lower plate, eastern assemblage carbonate rocks that host near-surface disseminated gold deposits. However, within the Turf
project area, the lower plate host rocks remain concealed by
1,500 to 4,000 feet (4501,200 m) of the upper plate, western
assemblage allochthonous rocks.
Previous Work
There has been no mine production in the immediate vicinity
of the Turf deposit; only a few scattered quartz-barite veins
and gossanous breccias have been prospected. Newmont has
episodically explored the North Lynn subdistrict since the 1970s.
Prior to 1992, exploration near the Turf deposit consisted of
geologic mapping, surface sampling, and intermittent drilling.Approximately fifty shallow rotary holes (mostly 10 feet [3 m] of0.20 opt [6.86 g/t] gold) that define
the gold deposit.
GEOLOGIC SETTINGOF THE TURF DEPOSIT
Overview
The Turf deposit spans at least 3,800 feet (1,140 m) in strike
length and lies at a depth of 1,700 to 2,400 feet (510720 m)
below the surface. It lies below the western flank of the main
Tuscarora Mountains rangefront to the east of Little Boulder
Basin (figs. G-1 and G-2). The surface lithology along this
western range front is dominated by rocks of the upper plate
of the Roberts Mountains thrust. These imbricately thrusted
rocks are flaser-textured, siliceous mudstones, chertymudstones, and interbedded limy siltstones of the Ordovician
Vinini Formation. Drill data from the project area indicate that,
here, the Vinini Formation ranges from 1,700 to 3,500 feet
(5101,050 m) thick.
In the north and northeast part of the project area, the Vinini
Formation contains semi-concordant granodiorite sills that
extend from the Jurassic Little Boulder Basin (LBB) stock that
underlies Little Boulder Basin. There are two principal outcrops
of the sub-horizontal sills; they are elongate northwest, and
occupy approximately b
13
b
13 b
13
b
13
b
13 square mile (~1 km2) in area (figs. G-
1 and G-2). The sills display weak propylitic and argillic
alteration assemblages. At the surface, an irregular
metamorphic aureole extends for about 2,000 feet (600 m) awayfrom the stock and sills and has converted the rocks of the
Vinini Formation to quartz hornfels and calc-silicate hornfels.
The Turf deposit is hosted entirely by carbonate rocks in
the lower plate of the Roberts Mountains thrust. Upper strata
of the Devonian-Silurian Roberts Mountains Formation and
upper parts of the Devonian Popovich limestone (directly below
the Devonian Rodeo Creek unit) are the principal host rocks
for replacement gold mineralization (fig. G-3).
Structural data derived from core logs and geologic models
indicate that the lower plate rocks at Turf strike northerly and
dip moderately to the east. This geometry is interpreted to result
from effects of pre- to post-mineral extension along north-
striking, west-dipping faults with attendant easterly rotation
of the segmented blocks (fig. G-2). In the northern part of the
project area, several northeast striking, steeply northwest-
dipping conjugate(?) normal faults have successively down-
dropped the strata to the north-northwest several hundred feet
in aggregate.
The metamorphic aureole from the LBB stock also extends
into lower plate rocks some 4,000 feet (1,200 m) laterally from
the intrusive margin. The approximate front of metasomatism
passes through the central portion of the Turf deposit and is
manifested by marbleization and lesser calc-silicate hornfels.
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Figure G-2. Block diagram of the Turf deposit, looking N30W, -30 degrees.
34
3
1
2
35
36
T36N,R50ET35N
TurfDeposit
W.Leeville
CG-110
CG-114
CG-113
CG-110
CG-114
CG-113
JULYFAULT
CG-109CG-90
CG-87
LBB-68
CG-111TURF
FAULT
JULYFAULT
CG-109
CG-90CG-87
CG-111TU
RF
FAULT
Carlin Formation
LBB stock, Vivian sill
Popovich Formation
Roberts Mountains Formation
wispy laminated units silty limestone 13
> 0.200 opt (6.86 g/t) Au, drill indicated
Fault, dashed where inferred
Roberts Mountains thrust fault
N
1000' 1000'
1000'
7000'
6000'
5000'
4000'
3000'
2000'
5000'
4000'
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Turf Deposit
The dominant structural feature in the project area is the
Four Corners structural corridor. This northeast trending, 500-
to 1,000-foot-wide zone of tectonic disturbance exhibits bothpre- and post-mineral faults and breccia. To the northeast the
Four Corners structural corridor flexes northward where it
coincides with the Vivian horst. The western edge of the Vivian
horst is demarcated by the Turf fault (fig. G-4, plate 1). At depth,
the Turf fault is the dominant ore-controlling structure in the
project area. The location and geometry of the fault have been
determined from mapping and drilling. The fault strikes nearly
due north, dips 5060 to the west and has accommodated 150
feet (45 m) or more of normal, pre-mineral displacement.
Tectonic breccias 5 to 40 feet (1.512 m) thick are everywhere
associated with the fault. Portions of the fault, most notably
within the southern part of the deposit, are intruded by a pre-
mineral monzonite porphyry dike (McComb, 1996a) that can
attain as much as 70 feet (21 m) of true thickness.
High-grade refractory gold (>0.20 opt [6.86 g/t]) was
deposited along the Turf fault and also as gently dipping
stratabound replacements where the fault cuts receptive units
within the Popovich limestone and Roberts Mountains
Formation. The ore zones, both stratabound and structural,
range up to 75 feet (23 m) in true thickness. Approximately
half the gold resource in the project is contained within the
Turf fault.
JKint
Viviansill
VininiFormation
Rodeo
Creek
Unit
PopovichFm.
RobertsMts.
Fm.
Popovich Formation
Roberts Mountains Formation
Dp1
Dp2
DSr1
DSr2
DSr3
DSr4
Debris flow marker bed; fine to coarse angular fragments oflimestone supported in limey sandstone matrix 5 to 30 feetthick
Upper Popovich limestone; micrite, commonly silty, variablycarbonaceous. Often displays contorted soft sediment de-formation. Average 120 feet thick
Lower Popovich limestone; thick to thin bedded siltymicrite, planar laminated. Distinct interbeds to 1/4 inch ofcalc-arenite and calcsiltite which may comprise 5-15% of therock. Variably carbonaceous. Average 90 to 140 feet thick
Silty limestone; planar laminated; upper 150 feet containrepetitive coarse, bioclastic interbeds to 2 feet thick. Fullthickness >700 feet thick
Silty limestone; wispy laminated; has distinctive fine-grainedbioclastic and calc-arenite interbeds up to 2 feet thickconstituting up to 25% of the rock. Average 85 feet thick
Silty limestone; with nearly uniform wispy bioturbatedlamination. Very minor detrital and bioclastic interbeds.Average 50 feet thick
Silty limestone; Gradational with Dp2 contains 10-20% siltinterbeds notable for wispy bioturbated lamination. Average40 to 50 feet thick
RelativeGoldMineralizationatTurf
Intercalated greenstone (up to 40 feet thick)
Roberts Mountains thrust
Popovich limestone
Roberts Mountains Formation
Ovi Vinini Formation (1,400 to 3,500 feetthick) Siliceous mudstone; distinctive flasershear fabric; may contain paraconcordant
dikes-sills of granodiorite (Goldstrike stockequivalent) up to 1,100 feet thickMetamorphic aureole surrounding sill hasconverted the siliciclastic rocks tocarbonaceous hornfels and quartz hornfels.
Vinini Lime Member (250 to 800 feetthick) Impure silty limestone and limysiltstone with intercalated mudstone.commonly metamorphosed to marble andcalcsilicate hornfels. Relict flaser texture
Rodeo Creek unit (10 to 300 feet thick)siliceous mudstone, cherty mudstone, thinbedded, planar laminated, rhythmicallybanded
Figure G-3. Generalized tectonostratigraphic column, Turf deposit, North Lynn subdistrict.
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FigureG-4.GeneralizedgeologicmapofpartoftheNorthLynnsub
districtshowingtheplanshapeoftheTurfgolddeposit.
T35N
T36N
R50E
36
35
2
1
Tur
f
W.Leeville
xxxx
xx
xxx
xxx
xxxxx
xxx
xxxxx
xxx xxxxx
xx
xx
xx
Trench
97-2
Trenc
h
97-3
Trench
97-1
Trench
97-4
Trench
98-1
Trench
98-2
Trench
98-3
Trench
98-4
Trench
98-5
Trench
98-6
ChevasPeak
AntimonyHill
Vivian
Sill
N.VivianHill
LBBStock
concealed
Jig
Jig
Jig
Jig
Jig
Jig
Ovi
Ovi
OvI
OvI
Vivian
Horst
Vivian
Sill
7
5
48
60
40
-50
45
Ovi
GULC
H
FAULT
VIVIAN
JULYFA
ULT
TURFFAULT
BASIN
FAUL
T
BO
UNDIN
G
LEEVIL
LE
FAULT
ZON
E
FOUR
COR
NERS
STRU
CTUR
ALCO
RRID
OR
SILB
AR
FAULT
LYNN
FAUL
T
48
CG
-87
A
A'
(Fig.
G-5)
DrillholestoLowerPlaterocks
Drillholesdescribedintext
Trench
Faultssh
owingdip,barandballwhere
inferred,arrow
whereobserved
Dikesofuncertainage,latiteto
lamproph
yrecomposition
Jig:grano
dioritedikesandsills
comagmaticwithLittleBoulderBasin
stock
Ovi:Vinin
iFormation,siliceousto
chertymudstone
Planshapeofgoldzone>10opt-foot
Lineofcr
osssectioninfigureG-5
0
1500feet
0
500meters
su
bcrop
outcrop
su
bcrop
outcrop
A
A'
CG-83
CG-87
CG-82 C
G-86
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Turf Deposit
Lithology
There are three principal lithologic regimes within the North
Lynn subdistrict: (1) allochthonous siliciclastic rocks in the
upper plate of the Roberts Mountains thrust; (2) impure
carbonate rocks in the lower plate of the Roberts Mountains
thrust and (3) the Little Boulder Basin (LBB) granodiorite stock
and sills. In addition, minor dikes of mafic to felsic composition
cut both the upper and lower plate. A generalized tectono-stratigraphic column for the project area (fig. G-3) depicts the
lithology and structural relationships. A brief summary of the
main lithologic units is given below.
UPPER PLATE ROCKS
At surface, the western range front of the main Tuscarora
Mountains is dominated by outcrops of the Vinini Formation.
These western assemblage siliciclastic rocks are part of the
Roberts Mountains allochthon. These rocks consist of
imbricately thrusted, flaser-textured siliceous mudstones and
cherty mudstones with interbeds of limy siltstones.
The depth to the Roberts Mountains thrust, and hence the
apparent thickness of the Vinini Formation, increases
progressively northward from the structural window at the
Carlin minesite. Drill data show that in the vicinity of Turf the
Vinini Formation ranges from 1,700 to 3,500 feet (5101,050
m) thick. Within the adjacent Little Boulder Basin graben,
drilled thickness of the Vinini Formation ranges from 3,600 to
6,100 feet (1,0801,830 m) (Clarke and others, 1998).
LOWER PLATE ROCKS
The stratigraphy and nomenclature of the lower plate host
rocks at Turf are the same as at the nearby deposits of Carlin
Underground, West Leeville, and Four Corners. The thickness
of some lower plate units, as measured in drill intercepts, is
highly variable due to extensional faults and to attenuation
by hydrothermal alteration and collapse. The autochthonous
rock assemblages are described below in descending
stratigraphic order.
Rodeo Creek unit (Drc) is highly variable in thickness because
the upper contact is determined by basal imbricate faults of
the Roberts Mountains thrust (fig. G-3). Drilled thickness at
Turf ranges from 5 to 280 feet (1.584 m), generally thinning
to the north. The unit consists of thin-bedded, planar-laminated
silty to siliceous or cherty mudstone. Repetitive thin interbeds
of siliceous muds and chert have prompted its categorization
as rhythmically banded argillite. It is not recognized as a host
rock at Turf.
Popovich limestone (Dp) has a drilled thickness that ranges
from 110 to 330 feet (33100 m) and averages approximately
230 feet (70 m) thick. It is typified by thin- to thick-bedded
micrite and silty micrite that are divided into two subunits:
Dp-1 and Dp-2 (fig. G-3).
Dp-1 has an average thickness of about 120 feet (36 m). The
upper 5 to 20 feet (1.56 m), directly below the Rodeo Creek
unit, are defined by fine to coarse clastic debris flows,
sandstone, and calcarenite. The remainder is distinctly micritic,
thin laminated, and commonly displays contorted soft-sediment
deformation.
Dp-2 averages 90 to 140 feet (2742 m) in thickness and is
gradational with the overlying Dp-1. This lower member is
typically a silty micrite that contains distinctive interbeds up
to c
41
c
41 c
41
c
41 c
41 inch (6 mm) thick of calc-siltite and calcarenite that may
locally comprise 5 to 15% of the rock. It is dark gray to black,variably carbonaceous, and typically has planar bedding.
Roberts Mountains Formation (DSrm) is the principal host
rock to replacement-style mineralization. The formation is
dominantly thin-bedded silty limestone that is divided into four
mappable units based on degree of bioturbation and quantity
of bioclastic and silt interbeds (Moore, 1995a). Total thickness
of the formation has not been drill tested because only the upper
three units are recognized as host rocks for high-grade
replacement ore. These individual units are shown on figure
G-3 but for visual clarity, units DSr-1, 2, 3 are combined in
figures G-2 and G-5.
DSr-1 averages 40 to 50 feet (1215 m) thick. The contact
with the overlying unit Dp-2 is gradational and marked by the
first consistent appearance of wispy lamination caused by
worm burrow bioturbation. The impure limestone is finely
laminated, dark gray to black, and carbonaceous. The wispy
texture occurs throughout and the unit may contain 10 to 20%
silt interbeds that range from 0.2 to 2 inches (550 mm) thick.
DSr-2 is a silty limestone that displays nearly uniform wispy
lamination with very minor bioclastic and detrital interbeds.
Where fresh, the limestone is medium to dark gray. Drill-
indicated thickness ranges from 25 to 105 feet (7.532 m), and
averages about 50 feet (15 m).
DSr-3. Most of this unit consists of wispy-laminated limestone;
however, fine to medium-grained bioclastic and calcarenite
interbeds up to 2 feet (0.6 m) thick that constitute up to 25% of
the section are diagnostic. It is dark gray, calcareous, and
variably carbonaceous. Drilled thickness ranges from 30 to
120 feet (936 m), averaging approximately 85 feet (26 m).
DSr-4 is more than 700 feet (210 m) thick but the total thickness
in the area of the Turf deposit is not known due to incomplete
drill testing of this unfavorable unit. It is dominantly silty
limestone that lacks bioturbation and has diagnostic thin-
bedded planar lamination. The upper 150 feet (46 m) contain
repetitive bioclastic interbeds 1 to 20 inches (2.550 cm) thickwith coarsely fragmented crinoids and rugose corals.
Throughout most of the Turf project area, this member is
variably metamorphosed to marble and calc-silicate hornfels.
The bioclastic interbeds contained within units DSr-3 and
DSr-4 are most prominent in the northern portion of the project
area and diminish to the south in the vicinity of the West Leeville
deposit. These interbeds are interpreted to be debris flows derived
from a Devonian bioherm whose northwest extension is exposed
at the Bootstrap Mine some 6 miles (10 km) to the northwest
(Teal and Jackson, 1997a).
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12'/0.23
62'/0.86
10'/0.27
71'/0.47
16'/0.23
15.5'/0.43
88'/0.48
84.5'/0.17
CG-82
CG-73
CG-83
CG-86
CG-73
CG-83 CG-82
CG-86
el 3500'
el 4000'
el 4500'
E1
7,0
00'
E1
6,0
00'
E1
5,0
00'
E1
5,0
00'
E1
6,0
00'
el 3500'
el 4000'
el 4500'
E1
7,0
00' ALTERATION
Gold mineralized zone,
>0.200 opt (6.86 g/t)
Silicification
Decalcification
Breccias, hydrothermal
and collapse
Faults
Roberts Mountains thrust
Drill holes with goldintercepts, thickness
(feet)/ opt gold
LITHOLOGY
Felsic dikes
Vinini Formation
Rodeo Creek unit
Popovich limestone
Roberts Mountains Formation
silty limestone units 1-3
silty limestone unit 4
0 500 feet
0 150 meters
0 500 feet
0 150 meters
A A'
A'A
RM THRUST
RM THRUST
TURF
FAU
LT
RM THRUST
RM THRUST
TURF
FAULT Alteration
Lithology
10'/0.27
Drc
Drc
Dp
Dp
DSr1-3
DSr1-3
DSr4
DSr4
Drc
Dp
Dp
DSr1-3
DSr4
Ovi
Ovi
Ovi
TintTint
intT
CG-73
Ovi
Figure G-5. Turf deposit section 33200N, looking north (section AA on fig. G-4).
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FigureG-6.Turfdeposit,schematicdiagram
showinggrade-thick
nessand11sectionswithgoldzones.
3500
ft
4000
ft
4500f
t
5000
ft
E 1
5 5 0 0
E 1
6 5 0 0
E 1 6 0 0 0
E 1
5 0 0 0
E 1
7 0 0 0
E 1
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E 1
8 0 0 0
E 1
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E 1
9 0 0 0
N32
000
N32
400
N32
800
N33
200
N33
600
N34
000
N34
400
N34
800
N35
200
N31
600
N35
600
N36
400
N36
000
VIVIAN
GULCH
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SILB
AR
FAULT
FOUR
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RSFAULT
CORRID
OR
JULY
FA
ULT
TUR
FFAULT
Faults
Goldzones,
>0.200opt(6.87g/t)
Planshapeofgrade-thickness:
shadow
ofgoldzonefrom
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400'
400'
500'
10
opt-footshape
20
opt-footshape
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Turf Deposit
TURF FAULT
At depth within the lower plate rocks the Turf fault is listric,
strikes northerly, and dips 50 to 60 to the west. The northerly
strike and extensional movement of the fault may be interpreted
as a dilational jog or step-over fault within the overall northeast
trend of the Four Corners structural corridor. As such, the Turf
fault became the principal conduit for the circulation of
hydrothermal fluids and for ore deposition along a mineralizedstrike of at least 3,800 feet (1,140 m). Details of the ore zones
within and adjacent to the Turf fault are discussed in following
sections.
VIVIAN GULCH FAULT
This fault marks the contact and northeastern limit of the main
body of the Vivian sill. This poorly exposed 5- to 10-foot (1.5
3 m) wide, sub-vertical structure extends 4,000 feet (1,200 m)
northwest from Antimony Hill to the Four Corners structural
corridor. The fault appears to predate the sill and limits its
emplacement. Drill hole CG-114, collared to the south of thefault, intersected 1,100 feet (330 m) of the exposed granodiorite
sill whereas CG-113, located 300 feet (90 m) north of the fault,
was drilled 3,439 feet (1,032 m) through upper and lower plate
rocks without intersecting intrusive granodiorite (fig. G-2).
Comparative depths to the lower plate rocks intercepted by
holes CG-113 and CG-114 suggest that vertical offset of the
Vivian Gulch fault is less than 200 feet (60 m). Strike-slip
motion along the fault is speculative, and the fault is not a
demonstrated conduit for mineralizing fluids.
SILBAR FAULT
This structure is parallel to and lies 3,000 feet (900 m) southwest
of the Vivian Gulch fault (fig. G-4). It is inferred from ground
magnetic data to extend northwest from the southern end of
Chevas Peak to the southern end of the July fault at the Four
Corners fault corridor. The sub-vertical Silbar fault marks the
northeast margin of a southern segment of the Vivian sill and
is named for silica-barite alteration in fault breccias seen in
prospects along its strike. The direction and magnitude of offset
along this fault are not known. No holes have been drilled to
test for mineral occurrences within or adjacent to the fault in
lower plate rocks.
LYNN FAULT
This is the easternmost structure within the area of the Turf
deposit (fig. G-4). This northeast-striking, northwest-dipping
fault is an important mineral control at the historic Big 6
workings and the jasperoid at Antimony Hill (fig. G-1). At
Antimony Hill, a small gold-bearing jasperoid developed in a
zone of dilation where the Lynn fault horsetails or deflects
into a dominant north-northwest striking, west-dipping fabric
with dip-slip and dextral strike-slip kinematic indicators (Teal,
1993).
HYDROTHERMAL BRECCIAS AND ORE
Solution collapse breccias are common in the Turf deposit.
Generally they are found in, and adjacent to, the strongest zones
of high-grade gold ore. The Turf fault was the principal conduit
along which breccias developed. Initially the rock was
brecciated tectonically; such tectonic breccia is clast-supported
and comprised of fine to coarse, rotated fragments up to 3 inches
(8 cm) in diameter. Tectonic brecciation was followed bydecalcification as the fault became a conduit for hydrothermal
fluids. Decalcification effected progressive volume loss in both
the tectonic breccias and adjacent carbonate host rocks; this
resulted in collapse with attendant brecciation. The textures of
collapse breccias range from jigsaw breccia having no
rotation or transport of the clasts, to strongly disaggregated
masses of decalcified silty limestone. These textures, absent
distinct fault planes, gouge, or kinematic indicators, are
interpreted to indicate collapse. The breccia clasts are usually
angular and corroded with stylolitic boundaries against an
interstitial filling of microcrystalline quartz.
Tectonic and collapse breccias are commonly crosscut by
ramifying hydrobreccia veins as much as several inches thick.
These siliceous veins, with matrix-supported, fine-fragmental
clasts of silicified wallrock, commonly contain several percent
cryptocrystalline sooty pyrite. Late-stage barite and calcite
locally fill open fractures and vugs (McComb 1996c,d).
Drilling shows that bodies of ore-grade gold (0.20 opt
[6.86 g/t]) in hydrothermal collapse breccias can exceed 40
feet (12 m) in true thickness contained within greater volumes
of breccia as much as 200 feet (60 m) thick. The mineralized
portions of the breccias are frequently enveloped by zones of
barren marble or by abundant calcite veinlets.
Paragenesis of AlterationThe intrusive events, alteration assemblages, and inferred
paragenesis at the Turf deposit bear many similarities to those
at the Carlin Mine (Bakken, 1990; Kuehn and Rose, 1992) and
those described at the Betze-Post Mine (Leonardson and Rahn,
1996). Based on crosscutting relationships at Turf, the
paragenetic sequence may be interpreted as (1) hornfelsing and
marbleization that resulted from intrusion of the Little Boulder
Basin granodiorite stock and the comagmatic Vivian sills (153
Ma); (2) intrusion of monzonite porphyry dikes (40 Ma?) along
structures such as the Turf fault; (3) decalcification of the
Popovich limestone and Roberts Mountains Formation to form
collapse breccia; (4) silicification of collapse breccia anddeposition of early barren pyrite; (5) main-stage gold deposition
with an alteration assemblage of kaolinite, illite, sooty pyrite
replacements and minor orpiment and realgar; (6) late-stage
calcite veins, drusy quartz, and barite that fill vugs and open
fractures; (7) basin-and-range extension along north and
northeast striking faults accompanied by minor hydrothermal
argillic alteration. Newmont Mining Corporation has not yet
dated the alteration minerals related to the mineralizing event;
however, Ressel (1998) has dated late Eocene mineralized dikes
at the Beast deposit one mile (1.6 km) to the west of Turf.
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Drilled interval Gold(feet) (opt) Qtz Ill Kao Mont Cal Dol Pyr Bar
20702088.5 49 0 49 0 0 0 2 02088.52111 68 0 23 0 0 0 9 1
21112126 47 10 28 0 0 0 9 321262134 0.47 52 10 16 8 0 2 11 021342144 0.47 66 15 5 5 0 0 8 0
21442154 0.47 80 7 8 0 0 1 3 021542164 0.47 70 12 15 0 0 0 3 0
21642172 0.47 79 7 10 0 0 0 3 121722182 0.47 75 5 8 5 0 2 4 0
21822192 0.47 55 13 3 5 0 18 4 021922202 57 9 7 0 6 17 4 022022219 49 0 6 0 40 3 1 0
22192237 55 0 11 0 28 5 1 0
Table G-1.MTurf drill hole # CG-82 X-ray diffraction analysis (data from McComb, 1996b)
Qtz = quartz Kao = kaolinite Mont = montmorillonite Ill = illiteCal = calcite Dol = dolomite Pyr = pyrite Bar = barite
Semiquantitative weight %
Ore Zones
The characteristics of gold depositional styles are interpreted
from select drill intercepts and are discussed below with respect
to ore deposit setting (i.e., hanging wall, footwall, and Turf
fault-hosted mineralization). Locations of these drill holes are
shown on the generalized geologic map (fig. G-4). A cross
section through the center of the deposit (fig. G-5) illustrates
the litho-structural controls, dominant alteration minerals, andgold grades as determined from drill intercepts.
Suites of alteration minerals were compiled from XRD-
XRF studies performed by Newmont Metallurgical Services
(McComb, 1996a,b,c,d; Baker, 1997c). These analyses were
from select core samples of prominent breccias, ore zones, and
intervals 40 feet (12 m) above and below the ore zones.
Semiquantitative X-ray diffraction analyses for portions of the
drill holes described below (CG-82, 83, 86, 87) are given in
tables G-1 to G-4.
TURF FAULT MINERALIZATION
Hole CG-82, centrally located in the Turf deposit, cut the most
intensely developed zone of breccia above and within the Turf
fault (figs. G-4 and G-5). The breccias include a variety of
textures that range from tectonic, to hydrothermal, to collapse.
Near the top of lower plate rocks at a drilled depth of 1,830
feet (558 m), hydrothermal collapse breccias are strongly
developed in the Popovich limestone and lower parts of the
Rodeo Creek unit. The breccias are both matrix and clast-
supported with intensely decalcified and moderately silicic- to
argillic-altered fragments up to 3 inches (7.6 cm) in diameter.
The main interval of high-grade gold (71 feet of 0.47 opt
[1.6 m of 14.6 g/t]) begins at a depth of 2,126 feet (648 m) in
the lower Popovich limestone. Here, the silty micrite is strongly
decalcified, moderately carbonaceous and sulfidic, with
variably silicified and argillized collapse breccias. This high-
grade interval directly overlies the Turf fault, below which gold
grades diminish to less than 0.05 opt (1.7 g/t) in marble and
calc-silicate hornfels of the Roberts Mountains Formation (unit
DSr-4). Table G-1 displays the suite of alteration minerals
within and adjacent to the ore zone.
Hole CG-87, located along the northern segment of the
Turf deposit (figs. G-2 and G-4), cut the Turf fault in unit DSr-
3 at a drilled depth of 2,500 feet (750 m). A significant ore-grade intercept (60 feet of 0.42 opt [18.8 m of 14.4 g/t]) occurs
in a zone of breccia. In this intercept, the breccia displays
tectonic and hydrothermal collapse textures with coarse angular
fragments that are both clast and matrix-supported. Clasts are
strongly decalcified, carbonaceous, and siliceous with weak
(12%) sooty disseminated pyrite. XRD-XRF analyses show
that the ore zone is strongly siliceous (4077% quartz) with
lesser kaolinite, illite, and montmorillonite (table G-2). The
calcite, barite, and dolomite represent late-stage veinlets and
druses within the breccia matrix.
TURF FOOTWALL MINERALIZATION
Hole CG-86 intersected the strongest gold mineralization found
thus far in the footwall of the Turf fault (figs. G-4 and G-5).
There are three significant gold intercepts in this hole. The
uppermost zone (16.5 feet of 0.23 opt [5.0 m of 7.9 g/t] gold)
is in the basal sandstone of the Rodeo Creek unit at the contact
with Popovich limestone. The gold mineralized zone is
siliceous and carbonaceous crackle breccia that is laced with
late barite veinlets. The second intercept is in the upper
Popovich limestone (unit Dp-1). This zone of stratabound
replacement in micritic limestone grades 15.5 feet of 0.43 opt
(4.72 m of 14.7 g/t) gold. The host rock is decalcified,
carbonaceous, moderately pyritic, and notably fractured by
incipient collapse. The third, and principal, gold intercept
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Turf Deposit
Drilled interval Gold(feet) (opt) Qtz Ill Kao Mont Cal Dol Pyr Bar
2467.52484 54 4 8 0 27 5 1 024822503 37 4 6 5 44 3 1 025032513 0.42 43 0 7 0 47 1 2 1
25132523 0.42 63 4 4 5 15 6 2 125232534 0.42 61 4 4 5 18 6 2 125342543 0.42 77 4 3 3 2 10 2 0
25432553 0.42 72 0 4 0 2 18 1 225532563 0.42 61 3 6 5 11 14 1 0
25632583 69 0 9 5 12 2 1 125832598.5 64 0 17 0 14 5 0 0
Table G-2.MTurf drill hole # CG-87 X-ray diffraction analysis (data from Baker 1997c)
Qtz = quartz Kao = kaolinite Mont = montmorillonite Ill = illiteCal = calcite Dol = dolomite Pyr = pyrite Bar = barite
Semiquantitative weight %
Drilled interval Gold(feet) (opt) Qtz Ill Kao Mont Cal Dol Pyr Bar
1867.51885 89 5 4 0 0 0 2 0
18851906 91 4 2 0 0 0 1 119061923 0.23 80 7 3 6 0 0 3 1
19231937.5 68 9 10 9 0 0 4 01937.51951.5 86 5 6 0 0 0 2 0
1951.51967 0.43 61 4 23 8 0 0 4 019671984 74 5 12 7 0 0 2 0
19841999 82 0 11 5 0 0 2 019992017 78 5 16 0 0 0 2 020172027 0.47 76 0 13 8 0 0 2 0
20272042.5 0.47 81 0 17 0 0 0 2 02042.52053 0.47 79 0 10 6 0 0 5 0
20532064 0.47 78 4 14 0 0 0 3 020642077 0.47 76 2 12 6 0 0 4 0
20772087 0.47 82 4 11 0 0 0 3 020872095 0.47 74 0 16 6 0 0 4 0
20952107.5 0.47 59 4 9 0 17 7 3 1
2107.52124 63 0 5 0 29 3 1 021242141 72 0 3 4 14 6 1 0
Table G-3.MTurf drill hole # CG-86 X-ray diffraction analysis (data from Baker, 1997c)
Qtz = quartz Kao = kaolinite Mont = montmorillonite Ill = illite
Cal = calcite Dol = dolomite Pyr = pyrite Bar = barite
Semiquantitative weight %
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Drilled interval Gold(feet) (opt) Qtz Ill Kao Mont Cal Dol Pyr Bar
20402060 58 9 4 5 3 18 3 020602079 70 9 5 5 0 8 3 0
20792091 78 10 5 0 1 1 6 0
20912100 78 9 4 4 1 0 3 021002110 0.866 79 5 5 5 2 0 5 021102122 71 10 7 5 0 0 6 2
21222132 69 8 9 4 0 3 7 021322143 68 10 5 3 4 2 8 021432152 12 10 6 5 61 3 4 0
21522162 54 13 8 3 14 2 7 021622172.5 40 21 5 4 26 2 3 0
2172.52190.5 66 13 10 5 0 1 5 02190.52210 63 12 10 5 0 4 6 0
Qtz = quartz Kao = kaolinite Mont = montmorillonite Ill = illite
Cal = calcite Dol = dolomite Pyr = pyrite Bar = barite
Semiquantitative weight %
Table G-4.MTurf drill hole # CG-83 X-ray diffraction analysis (data from McComb 1996d)
occurs in the wispy laminated part of the Roberts Mountains
Formation (units DSr-1, 2). The mineralized zone (88 feet of
0.475 opt [26.8 m of 16.3 g/t] gold, including 37 feet of 0.80
opt [11 m of 27 g/t] gold) is controlled by both stratigraphy
and by a high-angle structure that is parallel to, and in the
footwall of, the Turf fault. This ore interval is moderately to
strongly siliceous, variably kaolinitic, and has up to 5% sooty
disseminated pyrite (table G-3). Below the ore intercept, gold
grades abruptly diminish to less than 0.02 opt (0.7 g/t) in
conjunction with a decrease of alteration.
HANGING WALL MINERALIZATION
Hole CG-83 has the most robust gold mineralization of the
holes drilled to date in the Turf deposit (figs. G-4 and G-5).
The several gold-mineralized intercepts include both
stratabound-disseminated and fault-hosted types. The main ore
zone begins at a depth of 2,100 feet (630 m) below surface in
hornfelsed and hydrothermally altered silty limestone (unit
DSr-1) of the Roberts Mountains Formation (fig. G-5). The
intercept from the sub-horizontal mineralized zone has a drilled
thickness of 62 feet (19 m) that grades 0.866 opt (29.7 g/t)
gold (including 27 feet of 1.46 opt [8.2 m of 50.1 g/t] gold).
The host rock in this intercept lacks evidence of tectonism or
hydrothermal brecciation. The original wispy laminated textureis preserved, thus indicating that the style of gold deposition is
this intercept was passive (volume for volume) replacement
similar to that at the West Leeville deposit (Teal and Jackson,
1997a). The Turf fault is interpreted to be the principal conduit
for gold-bearing hydrothermal fluids that migrated laterally
along this favorable stratigraphic unit.
Alteration in the highest-grade part of the intercept is
dominated by quartz, illite, kaolinite, and 38% sooty pyrite.
Montmorillonite and dolomite are minor in abundance (table
G-4). This gold intercept is underlain by sub-ore grade
disseminated mineralization (126 feet of 0.070 opt [38.4 m of
2.4 g/t] gold) in decalcified, argillaceous, and pyritic-altered
silty limestone (unit DSr-2). Deeper in the hole within unit
DSr-4, a broad interval of low-grade, disseminated and fracture-
controlled mineralization (84.5 feet of 0.170 opt [25.8 m of
5.8 g/t] gold) surrounds the Turf fault zone.
METALLURGY
Triple gold assays, preg-robbing carbon tests, and carbon-
sulfur assays were performed by Newmont on select intervals
of drilled-core that contain intercepts of >0.20 opt (6.86 g/t)gold. Generally these were the same intervals (approximately
from 40 feet [12 m] above to 40 feet [12 m] below the ore
zones) analyzed by semiquantitative XRD-XRF. These analyses
demonstrate that the gold zones at Turf are unoxidized and
sulfide refractory. The preg-robbing analyses determined that
the ore contains variable organic carbon with the deleterious
capability to adsorb gold from pregnant cyanide solution. These
metallurgical characteristics make the ore unfit for simple
crushing and heap leaching. Instead, the ore will need treatment
in a roaster or autoclave to liberate the micron-sized gold from
the sulfides and deactivate the organic carbon.
GEOLOGIC GOLD RESOURCE
The Turf deposit strikes and plunges northerly for a known
length of 3,800 feet (1,140 m). At the end of 1996, the Turf
deposit data were modeled using Newmonts Geomodel
software. The model was transferred to the Mines Engineering
Group for resource calculations. Based on 13 diamond-core
holes drilled at nominal 400-foot (120-m) centers in the main
part of the deposit, the Turf resource was calculated to be 2.7
million short tons (2.4 Mt) with a 15% diluted grade of 0.37
opt (12.7 g/t) gold (993,000 oz or 31 t of gold) (Perry, 1997).
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Turf Deposit
During 1997, four infill holes were drilled in the central part
of the deposit to validate the geologic model and four more
diamond-core holes were drilled to test a northerly extension
of the Turf deposit. High-grade gold was intercepted in the
northern extension along a 1,200-foot (366 m) strike length of
the Turf fault. The drilled thickness of the intercepts ranges
from 15 to 60 feet (4.518 m) and is inferred to span a dip
length of 200 to 300 feet (6090 m). Manual polygonal
calculations of the resource in this northern extension suggestan addition of 532,000 short tons (483,000 t) with an average
undiluted grade of 0.43 opt (14.8 g/t) gold (229,000 oz or 7.1 t
of gold) (Mohling, 1998). This brings the total geologic
resource at the Turf deposit to 1.22 million ounces (38 t) of
gold. Geologic mapping and surficial geochemical sampling
indicate that extensions to the deposit are possible to the north
and northeast. Figure G-6 illustrates the plan and sectional
shapes of the deposit as inferred from the drilling programs.
DISCUSSION
The drilling programs substantiate that Turf is a significant
discovery of deep, high-grade gold qualified for inclusion inthe gold resource inventory of deposits along the Carlin trend.
Gold deposition at Turf is controlled by a combination of faults
and by strata receptive to disseminated replacement. At Turf,
the host rocks in the lower plate of the Roberts Mountains thrust
are variably marbleized and calc-silicated with the
metamorphism generally increasing toward the Little Boulder
Basin stock. Because of the range of pre-mineral metamorphism
of the host rocks, Turf is a hybrid of Carlin-type deposits such
as Deep Star (intense calc-silicate hornfels and structure-
controlled) and West Leeville (non-hornfelsed, stratabound
replacement) (Teal and Jackson, 1997a).
The Turf deposit is localized by steeply dipping faults
along the western side of the Vivian horst. The western horst
margin is part of a broader zone of extensional faults that adjoin
the eastern margin of the graben of Little Boulder Basin. The
gold mineralizing fluids were channeled along the north-
striking, west-dipping Turf fault: the fault may represent a
dilational jog internal to the 500- to 1,000-foot (150300 m)
wide, northeast-striking Four Corners structural corridor.
Recognition of the locations, orientations, and magnitude
of dominant fault zones that control low-level gold and trace
element anomalies at surface is key to targeting the related
deep orebodies. Southwest of Turf at the Four Corners deposit,
it was recognized that high-grade gold in lower plate rocks
was controlled in part by steep conduits in the Four Corners
fault corridor. This understanding guided the successful drilltesting and discovery of the Turf deposit beneath anomalous
gold and trace elements in soils along the northeast continuation
of this structural corridor (Jackson and Bell, 1995). Likewise,
successful drill testing of the northern extensions of Turf was
enabled by advanced understanding of the down-dip geometry
of mineralized and post-mineral faults (Lisle, 1996; Mohling,
1997).
The preliminary modeling by polygonal methods and by
Newmonts Geomodel system indicate that Turf, as presently
known, is a gold resource of approximately 1.22 million ounces
(38 t) with an average undiluted gold grade of 0.42 opt (14.4
g/t) and a sulfide content of 34%. This ore should be amenable
to processing in Newmonts refractory ore treatment plant.Some portions of the highly silicified gold zones may require
fine grinding. At the time of this writing, engineering studies
were underway by Newmont Mining Corporation to determine
the optimum mining plan.
ACKNOWLEDGMENTS
I would like to thank Mac Jackson and Richard Lisle for laying
the foundations of the Turf discovery and the general
framework for the interpretive model. Also, thanks to Brad
Leach and Helen Robinson for their contributions and insights
during the infill drilling stage of exploration. Much gratitude
is owed to Newmont Mining Corporation for permission to
publish this paper and to Lewis Teal, Don Hammer, Steve
Peters, Joe Tingley, Jon Price, and Steve Castor for their
constructive editorial comments from which this paper
benefited greatly.