Geology and Mineral Systems of the Mike Deposit

7/27/2019 Geology and Mineral Systems of the Mike Deposit

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Mike Deposit

GEOLOGY AND MINERAL SYSTEMS OF THE MIKE DEPOSITJohn W. Norby1 and Michael J.T. Orobona2

ABSTRACT

The Mike gold-copper-zinc deposit is located in the Maggie

Creek mining district of the central Carlin trend, Eureka County,Nevada. Mike is at the northwest end of a 3-mile (5-km) longbelt of Carlin-type gold deposits aligned along and footwall tothe northeast-dipping Good Hope fault. Mike is subdividedinto the West Mike deposit in the footwall of this apparentreverse fault and the Main Mike deposit along the fault and inthe hanging wall. Contact-metamorphic, Carlin-type, andsecondarily enriched mineral systems are hosted in variablehornfels after Silurian to Devonian carbonate and siliciclasticrocks of the Roberts Mountains Formation, PopovichFormation, and Rodeo Creek unit, and in mafic to intermediatedikes of at least 107 Ma age. The host section is overlain by400 to 800 feet (120240 m) of postmineral, 15.1 to 14.4 Ma

volcaniclastic rock of the Carlin Formation.Contact-metamorphic mineralization is coincident with

potassium metasomatism dated at 111107 Ma, hornfels, andlocal skarn. Mineralization typically consists of quartz-sulfideveins dominated by coarse-grained pyrite and iron-richsphalerite with minor galena, chalcopyrite, and molybdenite.Quartz-carbonate veins hosting an arsenic-bismuth-lead-silversulfosalt also occur throughout the deposit. At northwest WestMike, sphalerite-dominated, replacement-style base-metalmineralization is concentrated along the contact between theRodeo Creek unit and the Popovich Formation. Diopside-quartz-garnet skarn, locally accompanied by molybdenite andstibnite, also occurs in this area of the deposit. Scheelite and

powellite occur near the base of the hornfels section.Carlin-type gold mineralization is concentrated along the

northwest-dipping Soap Creek fault, the Good Hope fault, andthe west-dipping Valley fault. West Mike gold mineralizationis roughly flat lying and stratiform, and segregated into upperand lower zones. The upper zone is 200 to 450 feet (60135m) thick, decarbonatized, oxidized, and grades 0.025 opt (0.86g/t) gold. The lower zone has similar thickness, is partiallyoxidized, grades 0.080 opt (2.7 g/t), and is coincident with a70- to 200-foot (2160 m) thick dolomitic front at the base ofdecarbonatization. Gold at Main Mike grades an average 0.037opt (1.2 g/t) and occurs in an oxidized and decarbonatized zoneat the intersection of the Soap Creek and Good Hope faults.

Mike deposit sulfide-zone gold occurs in micron-size, arsenianpyrite rims coating euhedral, coarser-grained pyrite. Alterationproducts include sooty pyrite/marcasite, variable silicification,kaolinite, sulfide-silica-matrix breccia, dissolution-collapsebreccia, and quartz-orpiment veins. Gold is consistentlyaccompanied by silver (1:1 ratio) and locally by zincconcentrations of 0.021.00 wt.%.

Secondary copper, zinc, silver, and gold concentrationsoverprint contact-metamorphic and Carlin-type mineralization.

Alunite dated at 19.7 Ma crosscuts secondary chalcocite andcovellite, providing a minimum age of secondarymineralization. Supergene copper grading 0.20.6 wt.% is

concentrated in two lobes along the northwest-striking GoodHope and Corridor faults, and is further enriched (0.41.0 wt.%)where these structures intersect the Soap Creek fault zone. Insection, two 100- to 250-foot (3075 m) thick, parallel copperlayers occur 150 feet (45 m) and 450 feet (135 m) above thebase of oxidation in each lobe, and dip gently towards the centerof the deposit. Oxide-zone copper occurs in copper silicates,clays, arsenates, phosphates, oxides, and carbonates. Copperis sited in chalcocite and locally covellite in the top-of-sulfidezone and in sulfide lenses in overlying oxidized rock. Copper-bearing zones are typically decarbonatized, clay altered, aluniteveined/replaced, and iron-oxide stained. Secondary zincgrading 1.04.0 wt.% is concentrated in the top 200 feet (60

m) of the sulfide zone. Strongest concentrations are at centralWest Mike, in the footwall of the Corridor fault, along the GoodHope fault, and along the Soap Creek fault zone. Top-of-sulfidezinc is hosted in micron-size, brown-yellow sphaleriteoccurring with manganosiderite and arsenopyrite in clay-alteredsections. Secondary silver is concentrated in 90-foot (27-m)thick layers, which locally straddle the base of oxidation alongthe Soap Creek fault zone and grade up to 1.2 opt (41 g/t).Supergene silver occurs with lead at an average 1:60 ratio, butnot with gold. Supergene gold remobilization is suggested byconcentrations in chalcocite layers, and by the uniformdistribution of gold in the zone of oxidation relative to that inthe sulfide zone. Oxide gold deposits at both Main Mike and

the analogous Tusc, 4,000 feet (1,200 m) to the southeast,contain higher-grade (>0.05 opt [1.7 g/t]), flat-lying, bedding-discordant corespossible supergene upgrades.

A copper-molybdenum-gold porphyry northwest of Mikeis inferred by the distributions of hornfels, potassium feldspar,tungsten, and molybdenum. This porphyry and associatedmesothermal mineral system are inferred to be Cretaceousbased on the age of apparently related replacement-stylepotassium feldspar at Mike. Sphalerite-dominated quartz veinsat Mike also suggest a location peripheral to a zoned porphyrysystem. Secondary copper, zinc, and silver concentrationsdiminish to the southeast, indicating a source of these metalsto the northwest. The covered southeast margin of the

Richmond stock could be the porphyry-style source. It is inthe vectored location (based on its magnetic signature); whereit crops out it is the same age as the replacement-style potassiumfeldspar at Mike; and it has a similar associated element suiteto that present in the mesothermal system at Mike. Significantoriginal components of the secondary zinc and silver depositsat Mike may have derived locally from oxidized sphalerite andsilver-sulfosalt-bearing veins in the upper part of the deposit.

A vertically oriented, cylindrical stock is inferred at 6,000-foot (1,830-m) depth at north Mike, based on a deeper-sourcedmagnetic anomaly (high) there. This stock is interpreted to beEocene because its magnetic signature is virtually identical to

1Geologic consultant, Spring Creek, Nevada2Newmont Mining Corporation


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the nearby, outcropping 3837 Ma Welches Canyon stock. TheNorth Mike stock may have served as the heat engine, and

possibly the gold source, for the apparently Eocene Carlin-

type gold deposits along the Good Hope fault trend, including

Mike and Gold Quarry. The preferred interpretation, however,

is that this Carlin-type gold system originated in relation to

regional Eocene magmatism, but is not directly related to

specific intrusions inferred from near-surface geology or

geophysical signatures in the vicinity of Mike.

LOCATION AND GEOLOGIC SETTINGOF THE DISTRICT

The Mike gold-copper-zinc deposit is located in northern Eureka

County, Nevada (Sec. 28, T34N, R51E; fig. J-1), approximately

9 miles (14 km) northwest of the town of Carlin, in the historical

Maggie Creek mining district. Newmont Mining Corporation

has a controlling interest in the property. Located on the eastern

flank of the Tuscarora Mountains, the Maggie Creek district is

regionally situated in the central Great Basin physiographic

province. This district comprises the central portion of the Carlintrend, a belt of predominantly sedimentary rock-hosted,

disseminated, Carlin-type gold deposits aligned along a N35W

azimuth. Gold concentrations in the Maggie Creek district

include the Gold Quarry, Tusc, and Mac Mines; and the Mike

and Little Hope deposits (fig. J-2, plate 2). These are primarily

hosted by lower-plate carbonate and clastic rocks of Devonian

and Silurian age exposed in a domed window through the

Roberts Mountains allochthon. The Carlin window (Roberts,

1957, 1960) is an erosionally breached, northwest-trending

anticlinorium, exposing an autochthonous core beneath upper-

plate, siliceous and carbonate-clastic rocks of Devonian to

Ordovician age in the Roberts Mountains overthrust sheet (Cole,

1995). Lower-plate exposures are bound by high-angle, normalfaults on the southeastern, southwestern, and northwestern

flanks of the Schroeder Mountain uplift (fig. J-2, plate 2). Those

margins of the window are down-faulted, and disconformably

overlain by volcaniclastic sedimentary rock and gravel of the

Tertiary Carlin Formation. Most of the deposits are in the

southwestern part of the Carlin Window, in a mile-wide corridor

(Tusc Corridor) along the moderately northeast-dipping Good

Hope reverse fault. They are localized at intersections with high-

angle, northeast-striking cross faults. Larger gold deposits (10

30 million oz [310930 t]) occur as stratabound replacement

bodies in the footwall of the Good Hope fault. These deposits

are hosted by limy siltstone and siliceous mudstone of the

Devonian Rodeo Creek unit, and silty limestone and calcarenite

of the underlying Devonian Popovich Formation. Smaller gold

deposits (1.01.5 million oz [3147t]) occur along and in the

immediate hanging wall of the Good Hope fault, in silty

limestone of the Devonian-Silurian Roberts Mountains

Formation. At Mike, mineralization along and in the hanging

wall of the Good Hope fault is referred to as the Main Mike

deposit; mineralization in the footwall is referred to as the West

Mike deposit. Mineralization is completely covered by

postmineral volcaniclastic sediment of the Tertiary Carlin

Formation.

EXPLORATION HISTORY

Earliest prospecting in the Maggie Creek district was in the

1870s, and several hundred tons of gold, silver, copper, and

lead ores were produced through 1952 (Roberts and others,

1967). In the 1880s, oxide-copper mineralization was

discovered on the Copper King claims, one mile (1.6 km)

southeast of the later-defined Mike deposit and directly

southwest of what became the Tusc gold mine (Doyle-Kunkel,1993; fig. J-2). Development of the Copper King Mine began

in 1952, and through 1958 the Copper King Company produced

approximately 14,800 short tons (13,400 t) of oxide ore that

averaged 3.4 wt.% copper from underground workings and

small open cuts (Doyle-Kunkel, 1993). Occidental Minerals

Corporation explored for a northwest extension of this ore

during the early 1970s, drilling three holes through the later-

defined Good Hope copper lobe of the Mike deposit (Akright,

1974). Occidental identified a blanket of secondary copper

oxides and chalcocite in the Paleozoic section plunging beneath

increasingly thicker Tertiary cover towards the northwest, but

did not define economic copper mineralization. The drill

cuttings were erratically assayed for gold.Newmont Mining Corporation discovered the Main Mike

gold deposit in 1989, following a series of economic discoveries

at Gold Quarry, Mac, and Tusc during the late 1970s and the

1980s. Charles Ekburg and Robert Ryneer are credited for the

discovery. The exploration strategy consisted of tracking gold

mineralization along the northwest extension of the Good Hope

fault, from Gold Quarry and Tusc, under postmineral cover to

the postulated intersection with the Soap Creek fault, a zone of

northeast-trending drainages and gravity gradients that bounds

the northwest margin of the Schroeder Mountain uplift (Arkell,

1991a). The late Michael Wilson, for whom the deposit is named,

and Brian Arkell performed follow-up exploration. Definition

of the deposit through infill drilling and further exploration

continued through 1994. This work included discovery of the

West Mike gold deposit and the Corridor copper lobe in 1992

(Arkell, 1993), and expansion of the gold and copper resources

at both West and Main Mike in 1993 and 1994 (Arkell, 1994;

Branham, 1995a). Widespread occurrence of oxide copper

minerals in core prompted estimates of copper inventories for

Main Mike (Teal and others, 1994), and for both Main and West

Mike (Branham, 1995a). Following a 3-year hiatus, exploration

resumed 19972000 and resulted in delineation of the West Mike

lower gold zone, expansion of the West Mike copper resource,

and discovery of a deposit-wide blanket of zinc.

Contributions to the understanding of the deposit includeannual reports (Arkell, 1991a, 1993, 1994; Branham, 1995a),

an audit of the geology and mineral inventory at Main Mike

(Teal and others, 1994), and previous review publications

(Branham and Arkell, 1995; Teal and Branham, 1997). Two

summaries provide more current understandings of the Mike

deposit contact metamorphic, epithermal, and supergene

mineral systems (Norby and Orobona, 2000) and structural

setting (Orobona and Norby, 2001). Concurrent with the final

revision of this paper, Bawden (2002) completed a Masters of

Science thesis on the supergene enrichment of copper at Mike.

This work advanced the understandings of the source of the


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Mike Deposit

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NORTH AREA

MAGGIE

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Figure J-1. Location of the Mike and other gold deposits of the Carlin trend.


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Map limits in UTM meters, zone 11 North are approximately

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Gold deposit

Copper King Mine

20

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Carlin Formation (Miocene); volcaniclastic rock and gravel

Intrusive rocks (Eocene); dacite and diorite of WelchesCanyon intrusive complex; dacite dikes at Marys Mountain

Western siliceous assemblage; chert, mudstone

Marys Mountain sequence; limy siltstone/calcarenite,siliceous, mudstone, pebble conglomerate

Rodeo Creek unit; siltstone (locally limy), siliceousmudstone (quartz hornfels)

Popovich Formation; micrite, calcarenite, silty limestone(calc-silicate hornfels, marble)

Roberts Mountains Formation; silty limestone, calcarenite(calc-silicate hornfels, marble)

Hanson Creek Formation; dolomite

Unconformity

Thrust fault

Roberts Mountains Thrust

8565

Figure J-2. Maggie Creek district geology.


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Mike Deposit

Commodity Cutoff Tonnage (millions) Grade Total

Gold 0.006 opt 408 short tons 0.021 opt 8,568,000 oz

0.2 g/t 370 t 0.72 g/t 266 t

Copper 0.10 wt.% 151 short tons 0.34 wt.% 1,027 million lbs

137 t 466,000 t

Zinc 1.00 wt.% 19 short tons 2.13 wt.% 809 million lbs

17 t 367,000 t

Table J-1.MMain and West Mike 1999 drill-indicated mineral inventory

copper, the geological and geochemical controls governing the

supergene copper enrichment process, and the distribution of

secondary copper minerals in the deposit.

MINERAL INVENTORY

Gold and base-metal inventories are defined at Main Mike by

41 drill holes spaced at 150 to 300 feet (4590 m), and in West

Mike by 24 drill holes spaced 300 to 1,000 feet (90300 m)apart. Only nine of the holes at West Mike were drilled

completely through the lower gold zone, which remains open

to the northwest. Copper and zinc intercepts also remain open

in that direction. Mike deposit resource models (DaSilva and

Orobona, 1998; Norby, 1999a) estimate a drill-indicated

mineral inventory in a hypothetical cone constructed at $400/

ounce (US$12.86/g) gold and $0.80/pound (US$1.36/kg)

copper (table J-1). Zinc mineralization is given no value in the

cone estimation. Drill-indicated gold, copper, and zinc metal

deposits in the Maggie Creek district are outlined on plate 2.

Development of the Mike deposit is challenged by 400 to 800

feet (120240 m) of postmineral volcaniclastic sedimentary

rock cover. This deposit is one of the larger undevelopedmineral resources in North America.

GEOLOGY OF THE TUSC CORRIDOR

The Tusc Corridor is a 1-mile (1.6-km) wide, northwest-

elongated belt of subdued topography along and in the footwall

of the Good Hope reverse fault between Marys and Schroeder

Mountains (fig. J-2). It is largely covered by a 100- to 300-

foot (3090 m) thick veneer of Carlin Formation, which

thickens considerably (>500 feet [150 m]) toward its down-

faulted northwest and southeast ends. Beneath this veneer in

the Paleozoic section, the southwest margin of the Carlin lower-

plate window, defined by the Roberts Mountains thrust fault,

is delineated by drilling along the length of this corridor (fig.

J-2, plate 2). Gold deposits in the Maggie Creek district align

along the Tusc Corridor.

A prominent geologic feature of the corridor is a gentle,

asymmetric domical fold, footwall to the Good Hope fault and

coaxial with the Carlin window anticlinorium (fig. J-2, plate

2). The fold hinge trends essentially parallel to the strike of the

Good Hope fault and plunges gently to the northwest and

southeast, away from its crest at the Mac deposit. High-angle,

apparent-normal, northeast-striking cross faults typically dip

away from the crest and enhance the apparent double plunge

(fig. J-3, plate 3). Southeast of the K-W fault, the fold is

antiformal and bounded to the southwest by the Hewettite-Ice

fault. The folds at Mac and the Alta anticline mapped at Gold

Quarry (Sagar and others, 1997) appear to be continuous

segments of the same structure. Northwest of the K-W fault

zone, toward the West Mike deposit, the fold is less pronounced

and monoclinal, with stratigraphy rolling northeastward in theimmediate footwall of the Good Hope fault. Another fold of

similar trend, the Snowbird anticline, occurs in the southwest

part of the Gold Quarry deposit. This fold is down-dropped to

the southeast along southeast-dipping faults, resulting in an

apparent southeast plunge.

A wedge of hornfels in the Rodeo Creek unit extends

southeast along the Tusc Corridor from the Mike deposit through

the Mac deposit (fig. J-2). The hornfels is typically bounded by

the northwest-striking Corridor and Good Hope faults. At Mike,

it extends lower in the section into the middle Popovich

Formation (fig. J-3), and expands to the northeast across the

Good Hope fault into the Roberts Mountains Formation.

GEOLOGY OF THE MIKE DEPOSIT

Stratigraphy

Previous stratigraphic descriptions have been compiled for the

Mike deposit (Branham and Arkell, 1995; Teal and Branham,

1997), and for the Maggie Creek district (Evans, 1980; Rota,

1993; Cole, 1995). As the Mike deposit Paleozoic host section

is completely covered, bedrock geology (fig. J-4) is interpreted

entirely from 65 drill holes collared between 150 and 1,000

feet (45 and 300 m) apart. Besides the lack of outcrop,

stratigraphic interpretation of the Paleozoic section is further

complicated by contact metamorphism and hydrothermalalteration. Therefore, a relogging program of holes in the Tusc

Corridor, begun outside of the Mike area and continued toward

increasingly cryptic stratigraphy in the Mike deposit, was

critical to understanding that area. During the period 1996-

1999, the relogging of 300,000 feet (90,000 m) of drill cuttings

and core by the authors and L. Teal, in conjunction with pit

mapping by the Mines Geology Group at Gold Quarry and

Tusc, enabled a unified stratigraphic interpretation of the Tusc

Corridor. The data indicate stratigraphy at West Mike is


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4000

3000

2000

5000

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Elevation(feet)

A"A'Elevation

(feet)

>0.01 opt Au

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Base of hornfels

Base of decarbonatization

Zn

Cu

DSF

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DSr

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No vertical exaggeration

>1.00 wt.% Zn

>0.10 wt.% Cu

Base of oxidation

W

W

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ROBERTS

THRUST

MOUNTAINS

MAC THRUSTS

Alunite zone

Tmc Carlin Formation; volcaniclastic rock, gravel

DmMarys Mountain sequence; limy siltstone,calcarenite, siliceous mudstone

Drc Rodeo Creek unit; siltstone, siliceousmudstone (quartz hornfels)

DpPopovich Formation; silty limestone,calcarenite, micrite (calc-silicate hornfels)

DSrRoberts Mountains Formation; silty limestone,calcarenite (calc-silicate hornfels, marble)

l l l l l

Figure J-3. West Mike to Gold Quarry 135 section AAA (looking northeast), split into two sections.Line of section shown on figure J-2.


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Mike Deposit

5500

5450

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MikeJasperoid

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Marys Mountain sequence; flaser-textured limysiltstone, calcarenite, siliceous mudstone

Dike; fine grained, intermediate to mafic, brecciated

DrcRodeo Creek unit; siltstone, siliceousmudstone (quartz hornfels)

DSrRoberts Mountains Formation; silty limestone,calcarenite (calc-silicate hornfels)

Contours (in feet) of surface at top of Paleozoicsection (below Carlin Formation)


Map limits in UTM meters, zone 11 North are approximatelyLL 562400E, 4516500N; UR 564400E, 4518600N

Normal fault showing dip, locallygravity gradient

Fracture zone showing dip, locally

gravity gradient

Reverse fault

Thrust fault

70

70

45

20

Outer limits of hornfels textures

4750

0 1,000 feet

0 300 meters

C

Figure J-4. Interpretive bedrock geology of the Mike deposit.


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consistent with that logged and mapped elsewhere in the district

(plate 2). Figure J-5 is a generalized tectono-stratigraphic

column for the Mike deposit area. Individual units are described

below in ascending order. See plate 2 for detailed paleontologic

age information and citations.

HANSON CREEK FORMATION

The Hanson Creek Formation (SOh) in the Maggie Creek

district is a black, massive dolostone to dolomitic limestone

with white quartz veins (Rota, 1993). Black, knobby to lensatic

chert content increases up-section. A tan-brown, sandy

dolostone or sandstone commonly marks the top of the

formation, which is in conformable contact with the overlying

Roberts Mountains Formation. The bottom of the Hanson Creek

section has not been mapped or drilled in the district. The upper

parts of the unit are exposed on Schroeder Mountain (fig. J-2,

plate 2). The measured thickness in the nearby Lynn lower-

plate window is 1,070 feet (321 m) (Evans, 1980), and fossils

indicate an age range of late Middle Ordovician to Early

Silurian. A single, deep drill hole in the Mike area may have

penetrated the Hanson Creek Formation where it drilled black,medium-grained, rounded-grain (possibly olitic) dolostone

that locally contains calc-silicate minerals. To date, the Hanson

Creek Formation has not proven a significant ore host on the

Carlin trend. Geochemically anomalous gold and silver values

occur locally (Rota, 1995).

ROBERTS MOUNTAINS FORMATION

The Roberts Mountains Formation (DSr) in the Maggie Creek

district is gray, carbonaceous, planar-laminated silty limestone

with a coarse-grained texture. Calcarenite beds and wavy to

discontinuous wispy laminations occur near the top of the

unit. Wispy-laminated intervals in the upper RobertsMountains constitute an important gold host throughout the

Carlin trend. Evans (1980) described the formation as variably

dolomitic. The Roberts Mountains Formation is exposed on

the northern wall of the Gold Quarry pit and crops out on

Schroeder Mountain (fig. J-2, plate 2), where it weathers to

distinctive, purple-tan plates. The unit is 1,200 to 1,500 feet

(360450 m) thick in the district (Rota, 1993). Fossils

collected at Gold Quarry Schroeder Mountain, and northeast

of Maggie Creek Canyon indicate an age range of Early

Silurian to Early Devonian. The Roberts Mountains Formation

is a gold and secondary-copper host at the Main Mike deposit,

a local gold host at the Gold Quarry deposit (Chukar Footwall),

and the main gold host at the Tusc deposit.

At Main Mike, the Roberts Mountains Formation is a

planar-laminated siltstone with subordinate lenses of maroon

or white, bleached (no carbonaceous material), silty-textured,

calc-silicate hornfels and fine-grained marble. Relict amoeboid-

shaped porphyroblasts developed on bedding laminae (spotted

hornfels texture) indicate the siltstone was an incompletely calc-

silicated silty limestone prior to decarbonatization. The siltstone

occurs in thin, contorted beds and as fragments in unhealed

and sheared, gossanous, clay-matrix-supported dissolution-

collapse breccia at the core of the Main Mike deposit. This

mineralized breccia occurs beneath the Mike jasperoida

dense quartz- and/or potassium feldspar-flooded section

(Arkell, 1994; Teal and others, 1994). In the footwall of the

Good Hope fault at West Mike, sparse deep drill information

indicates the formation is mostly unmetamorphosed, with

exoskarn to marble locally developed on dike margins and at

the upper gradational contact with the Popovich Formation.

POPOVICH FORMATION

The Popovich Formation (Dp), first described by Roen (1961)

and named by Hardie (1966), conformably overlies the Roberts

Mountains Formation. The type section is described by Evans

(1980) on Popovich Hill, in the northern Carlin trend. Along

the Tusc Corridor, the Popovich Formation is subdivided into

three informal members (fig. J-5). The lower member (Dp3)

comprises 400 to 700 feet (120210 m) of black, massive

micrite with subordinate silty limestone, light gray calcarenite,

and debris-flow limestone. The bottom of the unit is defined

as the base of the lowermost micrite, which is in gradational

contact with underlying silty limestone of the Roberts

Mountains Formation. The middle Popovich member (Dp2)consists of medium-bedded calcarenite and carbonaceous silty

limestone. Bioclastic sections up to 50 feet (15 m) thick,

containing crinoids, brachiopods, coral, and fossil trash, occur

in calcarenite-dominated layers. The middle Popovich member

is between 300 and 400 feet (90 and 120 m) thick at Mike but

is only 250 feet (75 m) thick southeast of Gold Quarry,

indicating gradual thinning to the southeast. The upper

Popovich member (Dp1) consists of 200 to 500 feet (60150

m) of dark, medium- to thick-bedded, carbonaceous silty

limestone and sparse beds of light gray calcarenite. The

Popovich Formation crops out on the northeast wall of Maggie

Creek canyon and at the crest of the Tusc Corridor, and is

exposed on the northwest wall of the Gold Quarry pit (fig. J-2,plate 2). In the Maggie Creek district, the three Popovich

members total 1,200 to 1,300 feet (360390 m) in combined

thicknesstwo to three times the thickness of the section in

the northern Lynn Window (Teal and Jackson, 1997b). Fossil

data indicate an age range for the Popovich Formation of

Middle to Late Devonian. The Popovich Formation is an

important gold host at Gold Quarry and is the dominant host

of gold, copper, and zinc at the West Mike deposit.

At Mike, the Popovich Formation occurs in the footwall

of the Good Hope fault (figs. J-5 and J-6). The bottom of

dominant oxidation in the Mike area typically occurs in the

upper Popovich but locally extends down into the middle

member (fig. J-5). The upper three-fourths of the section isthermally metamorphosed (fig. J-5). A transition from bleached

metamorphic rocks to fresh carbonaceous rocks is typically

within 200 feet (60 m) of the contact with the underlying

Roberts Mountains Formation. The middle Popovich member

is metamorphosed to hard and dense, light- to dark-gray calc-

silicate hornfels and lesser fine-grained marble. Marble is

preferentially developed in calcarenite intervals. The upper

member is metamorphosed to dense, white to olive-brown or

gray calc-silicate hornfels, lesser finely crystalline marble, and

rare skarn. Teal and Branham (1997) reported a calc-silicate


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151

Mike Deposit

Alluvium (0-15 feet)

Good HopeFault Zone

REDOX

unconformity

gradational

Hornfels boundary

W EST M IKE M AIN M IKE

Good Hope hanging-wall section

GOLD

SECONDARYCOPPER

GOLD

GOLD

SECONDARYCOPPER

SECONDARY

SILVER

W(

SCHEELITE)

Good Hope fault footwall section

Intrusive rocks (TJi)

SECONDARYZIN

C

(Fe-Mn)

Quartz hornfels, siliceous mudstone(300-700 feet)

Brown to purple, planar-laminated quartzhornfels in gradational contact with grainysiltstone. Subordinate, rhythmically thin- tomedium-bedded black siliceous mudstone,particularly towards base of unit. Abundant

quartz-limonite veins. Minor thin intervals ofcalc-silicate hornfels.

Siltstone (0-150 feet)

Planar-laminated to massive, decarbonatized,grainy siltstone with a characteristic marooncolor. Local thin-bedded lenses of relict limysiltstone. Minor chert and quartz hornfels.

Calc-silicate hornfels, marble (300-500 feet)(silty limestone relict texture)

Medium- to thick-bedded, carbonaceous, siltylimestone protolith. Thermally decarbonizedand metamorphosed to dense, white to olive-brown or gray calc-silicate hornfels, finelycrystalline marble, and rare skarn. Typicallydecarbonatized and argillized, and locallysilicified. Abundant folds and beddingdistortions. Porphyroblastic mottling alongrelict bedding planes. Thin lenses of silicifiedor marbleized calcarenite.

Calc-silicate hornfels, marble (300-400 feet)

(calcarenite and silty limestone relict texture)Medium-bedded, contorted interbeds of calcar-enite and silty limestone protolith; thermallydecarbonized and metamorphosed to veryhard and dense, light to dark gray calc-silicatehornfels and lesser fine-grained marble. Typicallydecarbonatized and strongly silicified, locallybrecciated. Porphyroblasts and sulfide blebsdeveloped on bedding planes highlight abun-dant folds and bedding distortions. Locallyabundant quartz and/or dolomite veinlets.

Micrite (400-500 feet)

Variably hornfelsed, thick-bedded to massive,black micrite. Lesser black silty limestone andminor light gray calcarenite intervals. Locallyabundant folds and distortions. Bedding-controlled transition between upper decarbon-ized metamorphic rocks, and lower carbon-aceous rocks - typically within 200 feet oflower contact. Locally decarbonatized and/orsilicified; increasingly calcareous with depth.

Silty limestone (800+ feet)

Dark gray, planar-laminated, carbonaceoussilty limestone with coarse, grainy texture, andlocal wavy to discontinuous "wispy"laminations (turbiditic to bioturbated) . Localskarn to marble developed on dike margins.

Volcaniclastic sediment, gravel(400-800 feet)

Buff to white (oxidized) or light green(reduced), weakly indurated volcaniclastic

siltstone to sandstone. Locally pumiceousand biotite bearing or devitrified and clayey.Subordinate lenses of multilithic channelgravel composed of rounded, siliceous peb-bles in a volcaniclastic matrix. Basal 0-200feet is pale green, waxy, tuffaceous clay;oxidized to a cherry red color along base.

Basal gravel (0-50 feet)

Strongly oxidized gravel composed of pebble-to boulder-sized, angular clasts of quartzhornfels and jasperoid in a volcaniclasticmatrix.

Mike jasperoid (0-150+ feet)Dense, "boney"-textured quartz and/or potas-sium feldspar-flooded "silica cap" crosscutby a network of quartz-limonite-clay vein-lets. Bedrock high at the intersection ofthe Good Hope and Soap Creek faults.

Grainy siltstone, calc-silicate hornfels,marble (200-800 feet)

Planar-laminated, carbonaceous silty lime-stone protolith with a coarse-grained, grainytexture. Predominantly thermally metamor-phosed to decarbonized calc-silicate horn-fels and marble; altered to decarbonatized,silicified, and clayey grainy siltstone withrellict porphyroblastic texture. Siltstone occursin contorted beds or as fragments in sheared,gossanous, clay-matrix-supported collapsebreccia at the core of the deposit, beneaththe Mike jasperoid. The unhealed (post-metamorphic) breccia crosscuts a variablydense network of quartz-limonite veins.

Calc-silicate hornfels, marble (0-150+ feet)

Relict lens of calc-silicate hornfels and finelycrystalline marble in gradational contact

with grainy siltstone and limey siltstone.Distinctive "mottled" texture resulting fromsulfide blebs and porphyroblast develop-ment along thin bedding laminae. Wedgethickens northeastward, away from theGood Hope fault.

Good Hope fault zone (50-150 feet)

Multilithic clay-matrix fault breccia devel-oped along several shears. Commonly dikefilled. Jasperoidal towards top at bedrocksurface.

Mafic to intermediate dikes and sills (up to50 feet wide) Clay-altered, finely porphyritic,lathy or felted texture.

DEVONIAN-SILURIAN

RobertsMountains

Formation(DSr)

DEVONIAN

LowerPopovich

Formation(Dp3)

DEVONIAN

MiddlePopovich

Formation(Dp2)

DEVONIAN

UpperPopovich

Formation(Dp1)

DEVONIAN

RodeoCreekunit

(Drc)

TERTIARY

CarlinFormation

(Tmc)

QUATERNARYAlluvium (Qal)

DEVONIAN-SILURIAN

Rober

tsMountains

Form

ation(DSr)

l l l l l l l l l l lll l l ll

l l l l l l l l l l

0feet

100

200

300

0meters

50

100

Figure J-5. Mike deposit tectonostratigraphic column.


10/25

152

assemblage of calcite, dolomite, quartz, orthoclase,

clinopyroxene, amphibole, and phlogopite. Amoeboid

porphyroblasts and sulfide blebs on bedding planes emphasize

abundant folds and bedding distortions in the hornfels section.

The contact between the Popovich Formation and the overlying

Rodeo Creek unit locally appears to be unconformable, but is

largely coincident with low-angle-to-bedding, gouge-filled

shears and unhealed, clast-supported clay-matrix breccias.

Similar breccia textures in the uppermost Popovich section atGold Quarry are interpreted to represent thrust faults

overprinted by dissolution collapse (Rota, 1995; Gold Quarry

Expansion core-logging team, 1999, personal commun.)

RODEO CREEK UNIT

The Rodeo Creek unit (Drc) lies stratigraphically above the

Popovich Formation. The type section was described by Ettner

(1989) near Rodeo Creek in the northern Carlin trend. Along the

Tusc Corridor, the Rodeo Creek unit consists of medium to dark

gray, planar-laminated limy siltstone with a grainy texture. This

lithology is interbedded with subordinate, rhythmically thin- to

medium-bedded siliceous mudstone concentrated in the basalpart of the section. Subordinate interbeds of black, cherty siltstone

are also present at Gold Quarry, but not at Mike. The Rodeo

Creek crops out south of Tusc and northeast of Maggie Creek,

and is exposed in the center of the Gold Quarry pit (fig. J-2,

plate 2). The drill-indicated thickness of the Rodeo Creek unit is

approximately 1,050 feet (320 m) in the Tusc Corridor. Fossil

data collected at Gold Quarry indicate an age range of Middle to

Late Devonian. At West Mike, the upper 300 to 700 feet (90

210 m) of the Rodeo Creek section is eroded. There, most of the

section typically consists of brown to purplish-tan, planar-

laminated quartz hornfels, and rare calc-silicate hornfels (both

with abundant quartz-limonite veins). The preserved upper 150

feet (45 m) typically consists of planar-laminated to massive,maroon, grainy siltstone and minor siliceous mudstone, quartz

hornfels, and limy siltstone. The Rodeo Creek unit is the dominant

gold host at Gold Quarry and Mac, a significant gold host at

Main Mike and Tusc, and a major host of both gold and secondary

copper at West Mike.

MARYS MOUNTAIN SEQUENCE

The Marys Mountain sequence (Dm), first described by Evans

(1980), is the lowermost allochthonous sequence recognized

in the Maggie Creek district. It is an interlayered, deformed

section dominated by silty limestone to calcarenite, siliceous

mudstone, and limy mudstone. The sequence also containsglassy chert, carbonate-matrix sandstone with rounded glassy

quartz grains, sparry limestone, and local limy pebble

conglomerate (Branham, 1995b; Teal, 1996b). Adjacent to the

rhyolite, granodiorite, and diorite intrusions at Welches Canyon,

silty limestone is metamorphosed to cream-white, laminated

garnetite, green-gray calc-silicate hornfels, and coarse white

marble (fig. J-2; plate 2; Evans, 1980; Branham, 1995b).

Deformation textures are well developed adjacent to thrust

planes and are characterized by boudins and micro-fractured

beds of chert, siliceous mudstone, and limestone in a sheared

and foliated, shaly mudstone matrix. These rock characteristics

are collectively referred to as flasure texture in mine

terminology. An estimated 5,000-foot (1,500-m) thick section

is exposed on the eastern flank of Marys Mountain (Teal,

1996b), with its base atop the Roberts Mountains thrust exposed

on the south wall of the Gold Quarry pit (plates 2 and 3). A

lithologically similar section crops out northeast of Maggie

Creek Canyon on the northeastern side of the Carlin window

(Cole, 1992). The Marys Mountain sequence may be correlativeto an undated, relatively thin (


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153

Mike Deposit

gold throughout the Maggie Creek district, and they host

secondary copper and zinc at West and Main Mike.

CARLIN FORMATION

The Miocene Carlin Formation (Tmc; Regnier, 1960)

unconformably overlies both the Paleozoic section and the dikes

(fig. J-2). Fleck and others (1998) reported 15.1 to 14.4 Ma40Ar/39Ar dates on glass shards from Carlin Formation tuffs in

the Santa Renia Fields Quadrangle north-northwest of the Carlin

trend. They also suggest that the age, location, and subalkaline

geochemistry of the tuffs are consistent with derivation from a

source in the Owyhee Plateau of Idaho. The Carlin Formation

is characterized by buff to white (oxidized) or light green

(reduced), weakly indurated volcaniclastic siltstone to

sandstone, and subordinate gravel (figs. J-3, J-5, J-6, and J-7).

This section is more than 2,000 feet (600 m) thick in Carlin

Valley (fig. J-2, plate 2). Tuffaceous sediment is pumiceous

and biotite bearing, to devitrified and clayey. At southwest Mike,

the basal 350 feet (105 m) of the tuff is indurated. Multilithic

channel-gravel lenses are composed of rounded to subangular,

siliceous pebbles in a volcaniclastic matrix. A basal section of

variable thickness (0 to 200 feet [060 m]) is typically a pale

green, waxy, tuffaceous clay, locally oxidized to a cherry red

color at the base. Local occurrences of strongly oxidized basal

gravel, as much as 50 feet (15 m) thick, are composed of pebble-

to boulder-sized, angular clasts of quartz hornfels or jasperoid

in a volcaniclastic or calcite-cement matrix. Basal gravels

locally contain gold-bearing clasts.

Faults

Postmineral, volcaniclastic sediment and gravel cover the

entire Mike deposit area (fig. J-2, plate 2), so structural features

cannot be measured directly. The fault framework is interpreted

using a combination of drill-hole cross sections, lateral

projections from bedrock exposures, gravity and magnetic

gradients, and topographic lineaments. Six different fault sets

are identified in the Mike area: the Roberts Mountains thrust,

the Good Hope fault zone, the Corridor fault, northeast-striking

faults, north-striking faults, and the Tuscarora fault. Bedrock

exposures of similar fault domains to the southeast indicate

these faults probably do not occur as discrete planes, but rather

as narrow zones.

The Roberts Mountains thrust classically separates

autochthonous rocks of the Carlin window, the Hanson Creek

Formation through Rodeo Creek unit section, from overlying

allochthonous rocks of the upper-plate, the Marys Mountain

sequence and Western siliceous assemblage. There are,

however, thrust faults in the Carlin window section, indicating

it is not entirely autochthonous (plate 3). The Roberts

Mountains thrust is not exposed in the footwall of the Good

Hope fault outside of the Gold Quarry pit. This low-angle

structure has little geophysical or topographic expression and

is traced northwest through the Tusc Corridor, below Carlin

Formation cover, with drill information (fig. J-2, plate 2). Drill-

hole fences and upper-plate dips in outcrops at Marys Mountain

indicate the thrust dips gently southwest towards the Tuscarora

range front. Shear planes in the lower-plate section may be

evidence of related, smaller-scale thrusts. Shear fabric is evident

at West Mike in the middle member of the Popovich Formation,

and is locally coincident with the higher-grade base of gold

mineralization. The Roberts Mountains thrust traces southwest

of the West Mike deposit. However, prior to erosion, the thrust

may have been present over the domed Carlin window, and

the sheared and clayey allochthon could have functioned as an

impermeable cap atop an antiformal structure that focussed

gold-bearing, hydrothermal fluid flow.The Good Hope fault is a N4050W-striking, 3545

northeast-dipping, apparent-reverse fault at Mike (fig. J-4), as

indicated by drill-hole intersections and the horizontal gradient

of airborne magnetics. Multilithic, typically unhealed, clay-

matrix fault breccia is developed along several shear planes in

a 50- to 150-foot (1545 m) wide zone. At Mike, apparent-

reverse motion on this fault juxtaposed the hanging-wall Roberts

Mountains Formation against the footwall Rodeo Creek unit.

Thicknesses of these two units drilled on opposite sides of the

fault, in addition to that of the intervening Popovich Formation,

indicate a minimum of 2,500 feet (750 m) of local stratigraphic

throw. No upper or lower contact of the Roberts Mountains

Formation is intersected by a drill hole in the hanging wall ofthe fault, making it difficult to establish maximum throw.

Contours of Paleozoic bedrock elevation determined from drill-

hole data show a strike-parallel, 100- to 200-foot (3060 m)

thick trough of deeper Carlin Formation along the hanging wall

of the Good Hope fault, possibly indicating late, apparent-

normal reactivation. The fault is silicified at and below the Mike

jasperoid where it is intersected by the Soap Creek fault zone.

Mafic to intermediate dikes, up to 50 feet (15 m) wide, occur in

the Good Hope fault zone (fig. J-4). The northeastern limit of

hornfels along the Tusc Corridor is coincident with the Good

Hope fault, except at Mike where hornfels textures extend across

the fault into the Roberts Mountains Formation in the hanging

wall. Most gold mineralization in the Tusc Corridor occurs along

the trend of the Good Hope fault. Individual deposits are located

in the immediate hanging wall, or in a 2,000- to 5,000-foot

(6001,500 m) wide zone footwall to this structure. Gold

concentrations are localized at intersections with high-angle,

northeast-striking cross faults. The Good Hope fault is a primary

ore-controlling structure at Main Mike.

The Corridor fault (figs. J-2 and J-4) is interpreted from

drill-hole cross sections. Only a few holes were completed

through the 20- to 50-foot (615 m) wide, clay-matrix breccia

along this fault, which strikes N4060W and dips

approximately 60 to the southwest. Drill-hole data indicate

between 850 and 1,000 feet (255300 m) of apparent-normaldisplacement of the Paleozoic section (fig. J-6). However, there

is little displacement of the overlying Carlin Formation. The

Corridor fault has not been traced southeast of the Mac deposit

and is not exposed in outcrop. The fault has no geophysical

and only minor topographic expression, but drill logs show it

is coincident with the southwestern limit of hornfels along the

Tusc Corridor. The Corridor fault is an important control for

secondary copper and zinc mineralization at West Mike.

Northeast-striking structures include the Perseverance,

Soap Creek, Independence, Soap Creek Parallel, and

Independence Parallel fault zones. Spaced between 400 and


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154

3500

4000

4500

5000

5500

(NW)

C(SE)

C'

Feet


Dp

Dp

DSr

DSr

Drc

Drc

Tmc

Tmc

PERSEVERANCE

SOAPCREEK

N.POINTING

DOG

COPPER

KING

INDEPENDENCE

INDEPENDENCE

PARALLEL

SOAP

CREEK

PARALLEL

M ain Mike

Tusc

DECARBONAT

IZEDCALCARE

OUS WEDG

E

DECARBON

ATIZED

CALCAREOUS W

EDGE

DpDp

Dp

Dp

DpDrc

Drc

Dm

TmcTmc

DSr

DSr

DSr

Drc

Main Mike


West Mike

TUSCARORA

CORRIDOR

VALLEY

GOODHOPE

3000

3500

4000

4500

5000

5500

(SW)B

Mikejasperoid

UPPER GOLD ZONE

LOWER GOLD ZONE

(NE)B'

Feet

Margin of hornfels

Margin of decarbonatization

Base of oxidation

>0.10 opt Au

>0.05 opt Au

>0.01 opt Au

>1.0 wt.% Cu

>0.1 wt.% Cu

>3.0 wt.% Zn

>2.0 wt.% Zn

>1.0 wt.% Zn

>0.5 wt.% Zn

Tmc Carlin Formation; volcaniclastic rock, gravel

DmMarys Mountain sequence; limy siltstone,calcarenite, siliceous mudstone, conglomerate

DrcRodeo Creek unit; siltstone, siliceous mudstone(quartz hornfels)

Popovich Formation; silty limestone, calcarenite,micrite (calc-silicate hornfels, marble)

Roberts Mountains Formation; silty limestone,calcarenite (calc-silicate hornfels, marble)

ROBERTS MOUNTAINS THRUST

Dp

DSr

Figure J-6. Soap Creek 045 section BB (looking northwest), Mike deposit.Line of section shown on figure J-4.

Figure J-7. Good Hope 135 section CC (looking northeast), Mike deposit.Line of section shown on figure J-4.


13/25

155

Mike Deposit

70

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Depth slice = 0-300 meters

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Figure J-8. Map of the Mike deposit showing residual pole-reduced airborne magnetics, gold grade x thickness contours,and top-of-bedrock structure.


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156

8565

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Map limits in UTM meters, zone 11 North are approximately

LL 559500E, 4512825N; UR 567600E, 4519600N

West M ike(gold)

Main M ike

open

(gold)

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Alunitezone

SnowbirdAnticline

AltaAnticline

0 3000 feet

0 1000 meters

RICHMOND STOCKSoutheast margin

112 Ma

NORTH

MIKESTOCK

WELCHES

CANYON

STOCK37 Ma

70

l l ll l l

l ll lll

Geologic contact

Normal fault, dashed where inferrred, dotted where hidden, showing dip

Reverse fault

Thrust fault

Anticline

Hornfels limit in Paleozoic section

Gold deposit

Copper King Mine

20

45

Dm

Tmc

SOh

DOw

Tei

Carlin Formation (Miocene); volcaniclastic rock and gravel

Intrusive rocks (Eocene); dacite and diorite of WelchesCanyon intrusive complex; dacite dikes at Marys Mountain

Western siliceous assemblage; chert, mudstone

Marys Mountain sequence; limy siltstone/calcarenite,siliceous, mudstone, pebble conglomerate

Rodeo Creek unit; siltstone (locally limy), siliceousmudstone (quartz hornfels)

Popovich Formation; micrite, calcarenite, silty limestone(calc-silicate hornfels, marble)

Roberts Mountains Formation; silty limestone, calcarenite(calc-silicate hornfels, marble)

Hanson Creek Formation; dolomite

Unconformity

Thrust fault


Dp

DSr

Drc

Figure J-9. Map of the Maggie Creek district showing airborne magnetics, regional pole-reduced total field,depth sliced 0300 m.


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Mike Deposit

1,000 feet (120300 m) apart, these structures strike N4050E

and dip 7080 to the northwest (figs. J-4 and J-7). Dips areestimated from drill sections and by correlating surface

topographic lineaments with horizontal gradients of gravity

located at fault offsets of the unconformity between thePaleozoic and Tertiary sections. Drill data indicate dikes occur

along northeast-striking structures of the Soap Creek fault zone

(fig. J-4). Less than 200 feet (60 m) of apparent-normal bedrock

displacement is interpreted from drill-hole data across anyindividual fault zone. Smaller offsets of the Carlin Formation

base indicate recurrent movement on these faults. Collectively,northeast-striking faults drop the base of the bedrock Paleozoic

section more than 500 feet (150 m) in a stair-step fashion

towards the northwest in the Mike area. Elsewhere in the

district, northeast-striking faults are mapped crosscutting the

Good Hope fault. At Mike, northeast-striking faults locallycontrol gold mineralization; they also down-drop that

mineralization and the base of oxidation. The Main Mike

deposit is situated in the hanging wall of the Good Hope fault,at its intersection with the Soap Creek fault zone. The latter is

also an important gold control at West Mike.

North-striking structures in the Mike area include theNorth-Pointing Dog, Nebulous, Valley, and D-Day faults (fig.

J-4). Spaced between 500 and 1,000 feet (150300 m) apart,

these structures strike north to north-northeast and dip 60

80 to the west. Dips again were determined by correlating

topographic lineaments with horizontal gradients of gravity.

The apparent-normal displacements across these faults vary.

Drill data indicate the Nebulous structure is a densely fractured,

dike-filled zone exhibiting little if any offset; whereas, the

bedrock Paleozoic section is significantly down-dropped west

of the D-Day fault into Carlin Valley. North-striking faults

offset the base of oxidation and mineralized layers. Offsets of

the Carlin basal unconformity across faults of this north-

striking set are commonly equal to those of the bedrock.However, some faults of this set (e.g., the Valley fault) have

greater offset of the Paleozoic section than of the unconformity,

indicating reactivation. This structural fabric apparently

predates gold mineralization as north-striking structures are

coincident with the highest-grade gold trends of the Main Mike

and West Mike deposits.

The Tuscarora range-front fault zone consists of a series

of steep, northeast-dipping shears on the southwest margin of

the Tusc Corridor (figs. J-2 and J-4; plate 2). Gradients of

gravity and airborne magnetics indicate a fault zone that

gradually steps the Paleozoic section down to the northeast.

Apparent-normal displacement across the fault zone southwest

of the Mac gold deposit is minimal. However, displacement

increases to the northwest and southeast where influenced by

northeast-striking extensional faults, which dip normally away

from the Mac area (fig. J-2). Drill-hole data indicate

displacement across the Tuscarora range-front fault in the Mike

project area is between 100 and 300 feet (3090 m), increasing

to the northwest. The fault zone apparently postdates gold and

base metal mineralization, and the development of the oxide

zone. Carlin Formation cover preserved on the northeast side

of the fault poses major geotechnical and economic challenges

to development of the Mike deposit.

Intrusive Bodies Inferred fromAirborne Magnetic Responses

An airborne-magnetics high occurs at the Mike deposit (Teal

and Branham, 1997). Branham and Arkell (1995), and Teal and

Branham (1997) interpreted that hornfels and potassium-feldspar

replacement at Mike (Larsen, 1994a,b; Williams, 1994; Odekirk,

1998a,b,c) are related to an intrusive mass that sources the

airborne anomaly. Geophysical modeling indicates the airborne-magnetics anomaly reflects a combination of two magnetic

responses (Wright, 1999, personal commun.): an annular-in-plan,

shallow magnetic source surrounding the Mike gold deposit (fig.

J-8), and a lower-amplitude, circular-in-plan, deeper source

located on the north margin of the deposit (fig. J-9). The shallow

source for the annular anomaly is interpreted from its gradients

(Wright, 1999). Furthermore, magnetic susceptibility

measurements (Wright and Freeman, 1998) and observations

of magnetite and pyrrhotite in core suggest the annular anomaly

is dominantly a response to magnetic minerals in the Paleozoic

hornfels section (Wright, 1999, personal commun.). Magnetic

minerals in the Carlin Formation may contribute a minor

component to the shallow source. The low in the center of theannular anomaly is interpreted to reflect deeper oxidation of

magnetic minerals in the Paleozoic bedrock hornfels section.

The deeper-sourced anomaly is interpreted to represent a

vertically oriented, cylindrical body at 6,000-foot (1,800-m)

depth (Wright and Lide, 1998; Wright, 1999, personal

commun.). The geometric aspect and relative amplitude of theanomaly are virtually identical to those of the Welches Canyon

stock (Wright, 1999, personal commun.), located 2.3 miles (3.7km) west-southwest of Mike (fig. J-9). Granodiorite from

Welches Canyon is dated at 37.00.8 Ma (K/Ar on biotite;

Silberman, 1971, written commun. referenced by Evans, 1980).Ressel and others (2000a) reported that the stock is a composite

of porphyritic andesite (diorite) dated at 38.340.33 Ma (40Ar/39Ar on hornblende and plagioclase), which is cut by a rhyolitedome, which is in turn cut by a rhyolite dike dated at 37.190.11

Ma (40Ar/39Ar on sanidine). The Welches Canyon stock has a

hornfels halo (fig. J-2, plate 2) but no recognized associatedmetallization (Evans, 1974b; Branham, 1995b). However,

related sediment-hosted disseminated gold and mesothermal

base-metal deposits are both apparently associated with similar-

age, Eocene porphyry bodies at the Cove and McCoy deposits,

Nevada (Johnston, 2000a), and the Bingham Canyon district,

Utah (Gunter and others, 1990; Sillitoe and Bonham, 1990).

Another airborne-magnetic response in the Mike area is

related to the southeast margin of the Richmond stock (Wright,

1999, personal commun.). Its magnetic expression extends to

within 6,000 feet (1,800 m) of the northwest margin of the

deposit (fig. J-9). Quartz monzonite of the Richmond stock is

dated at 1065 Ma where it crops out 4.2 miles (6.7 km)

northwest of Mike in the Tuscarora Mountains (K/Ar date on

biotite; Silberman, 1971, written commun. referenced by Evans,

1980). This stock was more recently dated at 112.40.6 Ma

using Pb/U in abraded zircon (Mortensen and others, 2000).

Hornfels and base-metal mineralization, similar to such features

at Mike, are present marginal to the Richmond stock in the

Tuscarora Mountains (Evans, 1974b; Evans, 1980; Mallette


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158

and Potter, 1999). At Main Mike, replacement-style potassium

feldspars in siltstone and a dike are K/Ar dated at 1112 and

1072 Ma, respectively (Branham, 1994), roughly coeval with

the Richmond stock. At the Archimedes deposit in southeast

Eureka County, Margolis (1997) reported similar K/Ar dates

of 1105 Ma and 1092 Ma on secondary sericite in quartz-

feldspar porphyry and adularia in calcite veins, respectively.

Gold and base metals in the Archimedes system are both

interpreted to be part of a single, evolving, magmatic-hydrothermal system of this age (Margolis, 1997).

MINERALIZATION ASSOCIATED WITHCONTACT METAMORPHISM

Base-metal sulfide veins and replacements, sulfosalt and

bismuth-mineral veins, tungsten minerals, and molybdenum

minerals are concentrated in the contact-metamorphosed,

potassium-metasomatized section at Mike.

Base-Metal Sulfide Veins

and ReplacementsQuartz veins containing pyrite and base-metal sulfides occur

throughout the hornfels section at Mike, and they crosscut dikes

at West Mike. Veins range in width from 1/32 to 2 inches (150

mm). Sulfides are coarse grained and euhedral. Pyrite is the

dominant sulfide; iron-rich, black-brown sphalerite is the most

abundant base-metal sulfide. Chalcopyite is the next most

common base-metal sulfide, but is minor compared to sphalerite.

Galena, stibnite, and molybdenite occur locally. The sphalerite

contains exsolution blebs of chalcopyrite (Odekirk, 1998a).

On the north-northwest margin of West Mike, base-metalsulfides also occur as patchy replacements along fractures, as

disseminated masses, and in dolomite-quartz veins.Mineralization is hosted in phosphate-lens-bearing quartzhornfels after siltstone in the basal 100 feet (30 m) of the RodeoCreek unit. It also extends 20 feet (6 m) down into the upperPopovich Formation, in calc-silicate hornfels after siltylimestone. Pyrite is the most abundant sulfide. Sphalerite isthe dominant base-metal sulfide. It is purple-blacknotablydifferent in color than the black-brown sphalerite in the morewidespread quartz veins. Galena, chalcopyrite, and lime-greenclay accompany the purple-black sphalerite. Total sulfidecontent is commonly 5 vol.%, and as much as 30 vol.% over5- to 10-foot (1.53.0 m) intervals. Dolomite-quartz veins withbase metals occur in low-angle (821) and high-angle (76

80) sets, both of which crosscut bedding. The section hostingthis apparent replacement style of base-metal mineralization

is pervasively dolomitic. Carbonate alteration may or may notbe genetically related to the base-metal replacements, as thesection immediately below hosts similar dolomite alterationcoincident with Carlin-type gold mineralization (the lower goldzone). Replacement-style base-metal sulfide along the contactbetween the Rodeo Creek unit and the Popovich Formation isnot recognized elsewhere in the Mike deposit. However, coarsepyrite layers up to 1/2-inch (1.3-cm) thick, locally withchalcopyrite and sphalerite, occur in the upper part of the middle

member (Dp2) of the Popovich Formation at Gold Quarry.

Sulfosalt and Bismuth-Mineral Veins

Quartz-carbonate veins containing a highly reflective, silver-

colored mineral occur throughout the Mike hornfels section.Trace-element abundances in vein-bearing intervals suggest

this mineral is an arsenic-bismuth-lead-silver sulfosalt.

Polished thin-section work (Williams, 1994; Odekirk,1998b) identified bismuth minerals in the hornfels section on

the northwest margin of West Mike. Bismuthinite, Bi2S3, andtetradymite, Bi2Te

2S, along with tennantite, Cu

12As

4S

13, occur

in quartz veins as replacements after chalcopyrite. Grade-

thickness contours of bismuth (not shown) indicate this element

is concentrated along the Soap Creek fault zone.

Tungsten

A contoured grade-thickness map of tungsten intersected indeeper drill holes (fig. J-10) indicates this metal is concentratedalong and northwest of the Soap Creek fault. The 100,000 ppm-foot contour (10 ppm W cutoff) defines a 1,700foot (520-m)long northeast trend. Tungsten concentration increases to thenorth-northwest and is open in that direction. A second, 600,000ppm-foot zone occurs to the north at the downdip projectionof the Perseverance fault.

Tungsten is most concentrated (0.051.00 wt.% WO3) at

the contact-metamorphic front, in the lower 150 feet (45 m) ofthe calc-silicate hornfels to marble section (fig. J-3). At Mike,the contact-metamorphic front typically occurs in thecalcarenite and silty limestone middle member of the PopovichFormation (Dp2). Tungsten is also locally present, generallyat lower concentration (0.010.04 wt.% WO

3), an additional

400 feet (120 m) upward into the metamorphosed section.Shortwave ultraviolet lamping of tungsten-bearing

intervals indicates the dominant host mineral is scheelite,

occurring as blocky replacements along bedding planes.Scheelite is also concentrated in high-angle, 1/16- to 1/8-inch(23 mm) wide calcite veins. These two mineral settingscommonly occur together. Tungsten may also substitute for

molybdenum in powellite, CaMoO4.

Molybdenum

Grade-thickness contours (10 ppm Mo cutoff) of deeper drill

intercepts (fig. J-10) indicate molybdenum, like tungsten, is

concentrated along and northwest of the Soap Creek fault. A

second, northwest trend of molybdenum parallels the strike of

the Good Hope fault.

Low-grade molybdenum (50 ppm over tens of feet) occursin molybdenite in quartz-sphalerite-chalcopyrite (-tennantite)

veins along the Soap Creek fault zone. Molybdenum also occurs

in powellite in high-angle calcite veins. Both of these

molybdenum occurrences are developed in calc-silicate

hornfels after silty limestone and calcarenite of the middle

Popovich member (Dp2).

More concentrated molybdenum (400 ppm over 200 feet

[60 m]) in molybdenite occurs in association with abundant

pyrite and sparse stibnite at northwest Mike (Arkell, 1992).

This mineral assemblage occurs below the Good Hope fault in

the Rodeo Creek unit at 3,000-foot (900-m) depth. The host


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lithology is diopside-quartz-garnet-dolomite-pyrite skarn afterlimy siltstone and siliceous mudstone (McComb, 1992a). The

garnet species has not been determined.

CARLIN-TYPE GOLD MINERALIZATION

Carlin-type gold mineralization at Mike occurs in two

concentrations, one in the footwall of the Good Hope fault,

West Mike, and the other in the hanging wall of and along this

structure, Main Mike. Gold at West Mike is segregated into

upper and lower zones.

West Mike Gold

Plan distribution of drill-indicated gold at the Mike deposit is

illustrated on a grade-thickness contour map (fig. J-11). The

largest gold concentration occurs at West Mike, where the 5

opt-foot contour (0.01 opt [0.34 g/t] cutoff) outlines an area

2,400 feet (720 m) in northeast dimension and a similar length

in north dimension. Gold is concentrated along the Soap Creek

fault zone and Valley fault, and is open to the northwest andnorth. A separate, less-defined gold occurrence is present at

Copper Soap, 2,500 feet (750 m) south-southwest of West Mike.

West Mike gold mineralization is illustrated on Soap Creek

045 section BBI (fig. J-6). This northwest-looking section is

constructed parallel to and between northeast-striking

postmineral faults, which progressively drop down the section

to the northwest. Therefore, the geometries of stratigraphy,

northwest-striking faults, and mineral zones are depicted largely

without the complication of basin-and-range step faulting. At

West Mike, gold is concentrated in two, relatively flat-lying,

parallel layers: the upper gold zone and the lower gold zone.

Upper-zone gold concentrations occur throughout the preserved

lower half of the Rodeo Creek quartz hornfels and siltstonesection, and extend 50 to 150 feet (1545 m) down into calc-

silicate hornfels of the uppermost Popovich member (Dp1).

The upper gold-bearing sections have a combined thickness

of 200 to 450 feet (60135 m) in a 1,000-foot (300-m) wide

zone footwall to the Corridor fault. Gold grade averages 0.025

opt (0.86 g/t). The upper gold zone is thinner and lower grade

where it approaches the Good Hope fault. This gold zone occurs

entirely in the oxide zone, and gold concentrations locally

correlate with relatively higher iron-oxide content. The upper

gold zone is completely decarbonatized.

The lower gold zone is markedly higher grade, averaging

0.080 opt (2.7 g/t). The lower zone is hosted in calc-silicatehornfels of the middle Popovich (Dp2), 200 to 500 feet (60

150 m) below the contact with the overlying Rodeo Creek unit

(figs. J-3 and J-6). Clay-altered dikes are also locally

mineralized (up to 0.240 opt [8.3 g/t]). Lower-zone gold

mineralization varies in thickness from 200 to 450 feet (60

135 m), and is thickest and highest grade proximal to the steeply

west-dipping Valley fault. Drilling indicates this gold layer is

continuous for 2,400 feet (720 m) in the Soap Creek fault zone.

It is also continuous for 1,200 feet (360 m) along the Valley

fault, and is open to the north. The lower gold zone dominantly

occurs in the basal part of the decarbonatized section; however,

it extends into incompletely decarbonatized rock on its

northwest margin (fig. J-3). The bottom 70 to 200 feet (2160m) of the section shown as decarbonatized on figure J-6 is

commonly dolomitic, and is more intensely gold mineralized

(>0.2 opt [6.9 g/t]). This basal section also locally contains

clay-altered and sooty-sulfide replaced, unhealed breccias

interpreted as dissolution-collapse breccias. Gold-associated

alteration products include black sooty sulfide replacements,

sooty sulfide/silica-matrix breccia, quartz-orpiment veins/

replacements, white waxy clay (kaolinite?) on fractures,

variable silicification, and local argillization. Shear fabric is

also developed locally in the lower-gold-zone section. In the

70

5

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T34N, R51E

W estMike

20 ppm cutoff

Grade - ppm

Thickness - feet

TUNGSTEN

NWMike

10 ppm cutoff

Grade - ppm

Thickness - feet

MOLYBDENUM

Structure shown at top of Paleozoic bedrock

T34N, R51E


0 1,000 feet

0 300 meters

0 1,000 feet

0 300 meters

Figure J-10. Tungsten and molybdenum grade x thicknesscontour maps, Mike deposit.


18/25

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T34N, R51E

W estMike

CopperSoap

MainMike

0.01 opt cutoff

Grade - opt

Thickness - feet

GOLD

MainMike

CentralW estMike

CorridorFootwall

0.5 percent cutoff

Grade - percent

Thickness - feet

ZINC


T34N, R51E


0 1,000 feet

0 300 meters

0 1,000 feet

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Tusc

Perseverance

shallowdrilling(oxide)

Tusc

Figure J-11. Gold and zinc grade x thickness contourmaps, Mike deposit.

southwest part of West Mike, the lower gold zone is partially

oxidized (fig. J-6); overall about one-third is oxidized.

The best trace-element correlation with gold in the reduced

part of West Mike is arsenic. This is consistent with themineralogy, as gold-bearing zones contain fine-grained arsenian

pyrite (Odekirk, 1998a) and locally orpiment. A

photomicrograph of a West Mike 0.163 opt (5.6 g/t) goldinterval shows 1-micron crystals of arsenian pyrite in 2- to 8-

micron-thick aggregate masses occurring as rims on euhedralpyrite (fig. J-12a). Similar fine-grained, auriferous, arsenian

pyrite rims on earlier euhedral coarser-grained pyrite have been

reported at the Gold Quarry deposit (Arehart and others, 1993a)and the Carlin deposit (Bakken and others, 1989). At Mike,

intervals replaced with this fine-grained arsenian pyrite are

varying shades of black-gray depending on the intensity ofreplacement. Relatively high-grade gold concentrations are

locally coincident with orpiment and rare realgar.

The gold:silver ratio in the West Mike sulfide zone is

variable, but averages about 1:1. In sulfide gold zones, zinc ispresent in 0.02 to 1.00 wt.% concentrations. Some zinc-bearing

gold zones contain macroscopic, black-brown sphalerite, butothers contain only fine-grained, dark sulfides of undetermined

mineralogy. There is also an inconsistent correlation between

bismuth and gold in the unoxidized part of West Mike.

As noted by Arkell (1994), the geologic setting of the West

Mike gold deposit is similar to that of the Gold Quarry deposit,2.5 miles (4 km) to the southeast (fig. J-2, plate 2). Both deposits

are in the footwall of the Good Hope fault and are hosted in

the Rodeo Creek unit and upper Popovich Formation (fig. J-3,plate 3). The gold system at West Mike, however, extends lower

into the Popovich section on average. Both deposits have

relatively higher-grade gold concentrations in dolomiticsections near their bases (West Mike lower zone, Gold Quarry

Alunite zone); these locally coincide with dissolution-collapse

breccias and low-angle-to-bedding shears.

Main Mike Gold

Plan distribution of gold at Main Mike (0.01 opt [0.34 g/t]

cutoff) is shown on figure J-11. This body is 2,050 feet (625

m) long in the north-northwest direction. Plan width is 500 to

1,200 feet (150360 m), and gold mineralization is open to

the north. An additional, smaller gold occurrence in the

hanging wall of the Good Hope fault, Perseverance, occurs

2,200 feet (660 m) northwest of Main Mike. The Main Mikegold deposit is depicted in section on figure J-6. It is entirely

oxidized, and hosts abundant yellow, brown, and red iron

oxides. Gold is concentrated dominantly in the hanging wall

of the Good Hope fault, but it also occurs along this structure.

Mineralization is mostly hosted by variably calc-silicated and

marbleized silty limestone of the Roberts Mountains

Formation, and extends 400 to 600 feet (120180 m) below

the top of the Paleozoic section. The gold grade averages 0.037

opt (1.27 g/t) (Teal and others, 1994); a relatively flat core

zone grades better than 0.050 opt (1.7 g/t) (fig. J-6). The Main

Mike gold deposit is entirely decarbonatized. No dolomitic

basal or marginal zone has been defined as at West Mike.

The decarbonatization boundary tightly bounds gold to the

northeast, striking parallel to the Good Hope fault, but dipping

at a higher angle than that structure.

The Main Mike gold deposit is also shown on orthogonal

Good Hope 135 Section CCI (fig. J-7). Gold is concentrated

predominantly in the Roberts Mountains Formation, in the

hanging wall of the Good Hope fault. As in figure J-6, the

higher-grade pod is relatively flat lying. This pod and a deeper,

lower-grade gold concentration below the Good Hope fault

both occur dominantly in the footwall of the Soap Creek Parallel

fault, above a depression in the base of oxidation. Gold


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Pyrite

Pyrite

Pyrite

Gangue

SphaleriteQuartz

Quartz

50 Microns

100 Micron

Documents

Geology and Mineral Systems of the Mike Deposit