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STRATIGRAPHY AND HOST ROCK CONTROLS OF GOLD DEPOSITS OF THE NORTHERN CARLIN TREND John Jory1 ABSTRACT controlled end member are therefore only a portion of a broader spectrum, which includes large collapse breccia-hosted deposits, formed by extensive decarbonatization of favorable host rocks adjacent to structural conduits (e.g., Betze-Post, Goldbug, and Genesis). This spectrum of deposit types produces remarkable contrasts in orebody geometry, tonnage, grade, host rock, structural orientation, hydrothermal alteration, geochemistry, metallurgy, hydrology and ground conditions, and has wide-reaching implications for exploration, mining, and processing. Pre-mine gold resources totaling 100 million troy ounces (3,110 t [metric tons]) in 42 deposits in the northern Carlin trend account for the most productive gold province in North America, including the giant >5-million-ounce (156 t) deposits at Betze-Post, Screamer, Meikle, Genesis, Carlin, and West Leeville-Turf. The northern Carlin trend is a dominantly N15ºW alignment of sedimentary rock-hosted gold deposits. Low-cost production exceeding 4 million ounces (124 t) annually since 1994 has been achieved from these large, disseminated gold deposits hosted by carbonate strata of the Devonian-Silurian Roberts Mountains Formation and the Devonian Popovich Formation. Host rocks are composed of silty limestone with intercalated biogenic debris that were deposited along the western margin of ancestral North America. Late Silurian to mid-Devonian biohermal to carbonate shoal accumulation (Bootstrap limestone) occurred over a period of 50 to 100 million years in the northernmost Carlin trend. Subaerial exposure and erosion shed bioclastic debris with fan-like geometries producing thick sequences of proximal, upper slope fossiliferous debris flows that are mineralized in the Meikle, Goldbug, and Betze-Post deposits, and mid-slope silty facies at the Blue Star-Genesis and Carlin deposits. Bioturbated wavy (“wispy”) laminated silty limestone of the upper Roberts Mountains Formation provides the most favorable host rock on the northern Carlin trend, containing 65% of the gold resource in about 400 feet (120 m) of the stratigraphic section. Gold deposits of the northern Carlin trend are predominantly exposed near the fault-bounded margins of the uplifted Lynn and Bootstrap structural windows. Flanking these Paleozoic windows are deep, northerly trending basins filled by postmineral Tertiary volcaniclastic sediments and gravels. Tectonic uplift and erosion have exposed autochthonous carbonate strata beneath the Roberts Mountains thrust. The central portion of the northern Carlin trend is intruded by a large, late Jurassic granodiorite intrusion (Goldstrike stock) and numerous Eocene rhyolite to dacite dikes. Two of the highest grade gold deposits on the Carlin trend, Deep Star and Deep Post, occur along the sheared margins of the Goldstrike stock, and most of the gold deposits occur within 2 miles (3 km) of the stock along the dike-filled Post-Gen fault zone. It is inferred that Eocene magmatism is the major process that drove hydrothermal circulation forming Carlin gold deposits. Gold deposits represent a continuum of styles from stratiform, permeability-controlled end members (e.g., Carlin, West Leeville, Pete, and Tara) to shear zone-hosted end members (e.g., Deep Star, Deep Post, and Meikle). The conventional Carlin-type gold deposits of the stratigraphically 1 INTRODUCTION The Carlin trend is a 40-mile (65 km) long north-northwest alignment of sedimentary rock-hosted disseminated gold deposits in northeast Nevada. Total gold production from the trend exceeded 50 million ounces (156 t) in 2002. Most of the reserves and gold production are located in the northern one-third of the Carlin trend within a belt measuring 10 miles (16 km) northsouth by 2 miles (3 km) east-west, and extending from Dee and Capstone in the north to Carlin and Pete in the south. The northern Carlin trend pre-mine resource is approximately 80 million ounces of gold; 50% of the resource is contained in four orebodies: Betze, Post, Screamer, and Meikle. Northern Carlin trend reserves were 65 million ounces (2,020 t) of gold in 1998, or about 80% of total Carlin trend reserves. The purpose of this paper is to provide a stratigraphic framework and description of Devonian-Silurian strata of the northern Carlin trend, and to compare and contrast relationships among stratigraphy, structure, and intrusive bodies that are important controls to gold mineralization (plate 1, fig. B-1). The paper includes a review of 42 gold deposits and their preferred host rocks in the northern Carlin trend. Formal stratigraphic units are reviewed, and informally named subunits are described in detail and related to deposit types. The commonly used terms “Bootstrap limestone” and “Rodeo Creek unit” are not recognized formations. Ounces and grade in this paper generally refer to gold resources and do not have reserve or economic implications. Studies by Roberts and others (1958), Evans (1980), and Ettner (1989) formed the early stratigraphic framework used by Carlin trend geologists. The need to better understand detailed stratigraphic controls for gold mineralization at the Carlin Mine led to the recognition of informal subunits by Newmont Mining Corporation geologists in 1993–1994, notably S. Moore and R. Harris. These subunits were subsequently recognized and correlated throughout Newmont’s northern Carlin trend properties by 1997 (Teal, 1996a; Clode, 1997; Jory and others, 1997). Deposit host rocks, stratigraphic units, and depositional environments were first linked via a fence diagram tied to a Rodeo Creek/Popovich Formation datum line in 1995 Newmont Mining Corporation 20 Stratigraphy ELKO HUMBOLDT WASHOE Storm (Rossi) Winnemucca Carlin Lovelock 80 Reno CHURCHILL Ren WHITE PINE Ely LYO N West Ren Tara EUREKA PERS HING Capstone Bootstrap 80 Elko 80 LANDER DEE FAUL T Dee MINERAL Banshee NYE Meikle South Meikle Elko County ESMERALDA Eureka County East Griffin West Griffin Barrel Rodeo Goldbug T P OS North Betze West Betze T UL Long W Bazza Lac Bazza Pt Shalosky Winston CLARK Las Vegas Post Lower Post FA Screamer Bazza LINCOLN Betze Deep Post Pancana Pancana 5-0 N. Skarn Hill V IV West #9 Skarn Hill ? IAN GU LC H Deep Star FA UL T North Star Genesis NW Genesis Antimony Hill Beast Turf 3,000 meters EE F FA LT Carlin LT Perry Pete CA ST LE Fault, solid where observed, dashed where inferred Figure B-1. Gold deposits, northern Carlin trend, Nevada. Modified from Teal and Jackson, 1997b. 21 Fence AU Carlin West Island Granodiorite intrusions Carlin East EF T UL Gold deposits FA U TL E Hardie FW ILL EV LE 1,500 R 0 AS IE S. Lantern C West Leeville FA NW Extension HA 10,000 feet NN 5,000 LY 0 FO UR C N. Lantern Big Six OR NE RS T Four Corners Exodus T FAUL Blue Star Ridge LITTLE BOULDER BASIN STOCK (concealed) RD Widge UL G EN Bobcat Blue Star Payraise FAU LT GOLDSTRIKE STOCK Golden Zia RE EF FA U LT 80 by L. Schutz and C. Williams. This product, extending from Storm (Rossi) in the north to Pete in the south, was updated in 1997 by J. Jory, D. Harris, H. Unger, and R. Koefed. Barrick Goldstrike Mines Inc. geologists working in the Betze, Screamer, Rodeo, and Meikle areas are familiar with informal Newmont stratigraphic subunits, but have not adopted them. Carlin-type gold deposits have commonly been described as being exemplified by the stratigraphically controlled gold mineralization in the original Carlin orebody, discovered in 1961 (Radke, 1985). However, the Carlin trend includes a diverse assemblage of gold deposits with wide variations in size, grade, geometry, host rock, structural fabric, hydrothermal alteration, metallurgy, geochemistry, hydrology, and geotechnical conditions. The successful gold explorationist on the Carlin trend needs to constantly assess these variable deposit characteristics and diverse gold models in the pursuit of new discoveries. This paper is primarily based on the observations and interpretations of numerous geologists employed by Newmont and Barrick on the Carlin trend. Ordovician to Devonian upper plate rocks of the allochthonous western siliceous assemblage are among the oldest rocks on the Carlin trend (figs. B-2 and B-3, plate 1). Thickness is uncertain owing to structural thickening along numerous low-angle faults, but is estimated to exceed 2,000 feet (600 m). The Ordovician Vinini Formation is composed dominantly of siltstone and cherty mudstone with a penetrative shear fabric imposed by Antler thrusting. The regionally extensive Roberts Mountains thrust (Roberts and others, 1958) places western assemblage siliciclastic rocks on top of autochthonous Ordovician through Devonian limestone, dolomite, quartzite, siltstone, and mudstone. Lower plate carbonate rocks vary considerably in facies, composition, thickness, and host rock potential. Contacts are mostly conformable and gradational; however, conodont zones indicate depositional hiatuses in some areas (G. Griffin, personal commun.). The oldest lower plate rocks exposed on the northern Carlin trend are dolomite and shaly limestone of the Ordovician Pogonip Group (Evans, 1980). The Pogonip Group is about 1,000 feet (300 m) thick and generally includes gray limestone in its upper portion and dense, dark gray dolomite in the lower two-thirds. A 1,000-foot (300-m) thick homogenous package of white Ordovician Eureka Quartzite conformably overlies the Pogonip Group. The Eureka Quartzite and overlying dark gray Hanson Creek Formation are prominent ridge formers. The 600-foot (180 m) thick Hanson Creek dolomite contains several cherty and sandy interbeds in the uppermost 100 feet (30 m). The light gray Devonian-Silurian Roberts Mountains Formation is approximately 1,200 feet (360 m) thick, consisting of a lower 800 feet (240 m) of planar laminated silty limestone grading upward into wavy (“wispy”) laminated silty limestone with abundant bioclastic debris (fig. B-3). The Roberts Mountain Formation generally shows an upward fining of grain size, progressive increase in bioclastic interbeds and debris, and increase in organic carbon. Fossiliferous debris flows and 1- to 6-inch (3–15 cm) thick calcarenite beds are common in the uppermost 400 feet (120 m) of the Roberts Mountains Formation. The overlying Devonian Popovich Formation ranges in thickness from 500 to 800 feet (150–240 m), and is composed of dark gray, carbonaceous silty to muddy limestone grading upward into calcarenite and limestone conglomerate with planar laminated silty limestone (Evans, 1980). The stratigraphic position of the Roberts Mountains/Popovich Formation contact is debated on the Carlin trend; however, the correlative and mappable rock units are well agreed upon. Some workers place the contact about 400 feet (120 m) lower than described in this paper, at the top of the planar laminated silty limestone of the Roberts Mountains Formation and base of debris flows (Evans, 1980; Armstrong and others, 1997). The informally named Bootstrap limestone is as much as 1,300 feet (390 m) thick at the north end of the Carlin trend (figs. B-2 and B-3). This unit defines a dramatic facies change from late Silurian through mid Devonian times, consisting of a massive light gray oolitic to sparry limestone with silty limestone interbeds in its lower REGIONAL GEOLOGY Carlin trend regional geology is well documented. The following description is summarized from Stewart (1980). Lower Paleozoic: Northeast Nevada was located along the stable western margin of the North American craton. Western facies eugeoclinal and eastern facies miogeoclinal sediments were deposited on the continental shelf /slope. Late Devonian-Early Mississippian: The Antler orogeny placed western facies siliceous rocks over eastern facies carbonate rocks along the Roberts Mountains thrust. Mesozoic: Northeast-directed compressive tectonism of the Sonoma and Elko orogenies produced uplift, folding and faulting, highlighted by north-northwest fold axes and faults. Late Jurassic: Late/post-Elko orogeny plutonism included 158 Ma emplacement of the granodiorite Goldstrike stock, Little Boulder Basin and Vivian stocks/dikes, and contact metamorphism. Eocene: Extension and magmatism with coeval main stage 36–40 Ma gold mineralization and Tertiary dikes. Miocene: 14–20 Ma basin-and-range extension occurred with north-south faulting, deposition of Carlin Formation volcaniclastic sediments in basins, and exposure of lower Paleozoic rocks along the Tuscarora Range and Tuscarora Spur. Sedimentary Host Rocks STRATIGRAPHY Key factors controlling large bulk-mineable gold deposits in the northern Carlin trend were structural preparation and favorable primary and secondary host rock porosity and permeability. Stratigraphic and facies relationships are discussed in this section. 22 Stratigraphy Bootstrap Mine Ovi Qal Quaternary cover Jig Jurassic Goldstrike diorite Drc Rodeo Creek unit Dp Popovich Formation DSb Bootstrap limestone DSr Roberts Mountains Formation Olp Ordovician lower plate rocks Ovi DSr Qal DSb Meikle Mine Ovi Dp Ovi T UL FA ST PO Post Anticline Drc Ovi Qal Ovi Betze-Post Mine Boulder Valley DSr Betze Anticline Ordovician Vinini Formation (upper plate rocks) Ovi Ovi Dp Qal Little Boulder Basin Drc Jig Major fault Anticline Qal Ovi Ovi e Ovi Drc aR a Tu T EF RE L FAU r LB B LE IL FA U EV pu ra S LT LE Tuscarora Anticline caro LE ST Dp U FA Drc sca LT Tus CA Ovi ror FAU Drc GEN Dp ng Genesis Mine Ovi DSr Ovi LT Ovi Qal Dp Drc DSr Olp CA ST LE RE DSr EF Carlin Mine FA UL T Drc DSr Dp Drc Dp Olp DSr 0 0 5,000 feet RICHMOND Ovi MOUNTAIN 1,500 meters Qal CARLIN VALLEY Drc Figure B-2. Simplified geologic map of the northern Carlin trend, Nevada. 23 300 feet 100 meters Tmc Ovi Tmc: Tertiary Carlin Formation Tuffaceous sedimentary rocks (10 to 600 feet thick). Poorly consolidated, friable, interbedded light gray sandy air fall tuffs (aft) and tan lacustrine silt, minor 10- to 50foot-thick matrix-supported gravels. Basal 5 to 40 feet silty clay with 40 to 60% smectite, halloysite, and illite. Basal gravels (30-200 feet thick) Poorly to moderately consolidated, matrix- and clast-supported gravels lying unconformably on Goldstrike granodiorite and Vinini Formation. Clast-supported gravels include 70-100% Ovi and JKi clasts in tan silt matrix. Clast types are: (1) angular 1 to 6 inches gray Ovi cherty mudstone and siltstone, (2) light green to brown clay-weathered granodiorite grus. ROBERTS MTS. Upper Post THRUST Drc gradational Upper Goldbug Dp0 Lower Post Deep Post Lower Goldbug Dp1 Dp2 DSr1 DSr2 DSr3 DSr4 Ovi: Ordovician Vinini Formation Chert, mudstone. Black and gray cherty mudstone and siltstone. Planar to wavy banded with alternating dark gray siltstone beds and 1- to 5-mm-thick light gray dolomitic calcarenite interbeds. Becomes more siliceous with depth. Metamorphosed to quartz hornfels adjacent to Goldstrike intrusive. 20- to 50-footthick light gray to black limestone in middle portion, metamorphosed to calc-silicate hornfels adjacent to Goldstrike intrusive. Allochthonous unit with mylonitic flaser structure in thrust contact with underlying Drc unit. Strongly fractured and oxidized to produce widespread limonitic interlocked "ice-cube" tectonic breccia. >2,000 feet thick. Drc: Devonian Rodeo Creek unit Dark gray inch-thick rhythmically bedded, banded siliceous mudstone, with gray to tan planar laminated siltstone and minor fine-grained sandstone. Mixed siliceous mudstone and siltstone in upper 200 to 300 feet. Siliceous mudstone with shaly partings and quartz-filled tension gashes dominant in lower 300 feet. Basal 50 feet gradational with Dp silty limestone. Discontinuous 5- to 20-foot-thick basal silicified calcarenite/sandstone. Metamorphosed to quartz hornfels adjacent to Goldstrike intrusion. 600 to 700 feet thick. Dp: Popovich Formation Three mappable units, mostly muddy limestones (600 feet thick). Matrix grain size and skeletal-rich limestone/calcarenite interbeds (DSb) increase in frequency and thickness from Dp1 to DSr3. Dp0: Dark gray carbonaceous, planar laminated silty limestone grades downward into silty micrite with diagenetic pin-striped pyrite on bedding. Uppermost 50 feet gradational with basal Drc siliceous mudstone. 3- to 10-mmthick black chert lenses comprise 5% of lower 100 feet of Dp0. 200 to 250 feet thick; thickens northward. Dp1: Dark gray carbonaceous, massive to thin-bedded lime mudstone (micrite). Diagnostic soft sediment deformation with slump bedding and flame structure. 240 to 320 feet thick; thickens northward. Dp2: Dark gray carbonaceous, planar laminated silty micrite. Localized hockeypuck cleavage and millimeter-thick DSb interbeds. 40 to 80 feet thick. DSr: Silurian/Devonian Roberts Mountains Formation DSr1: Transitional dark gray carbonaceous, planar to wispy laminated silty limestone. Coarser grained and less carbonaceous than Dp0, Dp1, and Dp2, 10 to 40 feet thick. DSr2: Medium-dark gray variably carbonaceous wispy laminated silty limestone. Fossil-rich debris flows and centimeter-thick bcls interbeds increase downsection. 80 to 200 feet thick, thins northward. DSr3: Medium to dark gray fossiliferous wispy laminated silty limestone. Abundant debris flows with 5 to 30% angular lithic and skeletal debris, biosparite and fine-grained bioclastic limestone (DSb). Basal 30 to 50 feet gradational from wispy to planar laminated silty limestone. 150 to 300 feet thick (biosparite 300 feet thick at Barrel). Commonly silicified and collapse brecciated. DSr4: Medium to light gray planar banded to laminated calcareous siltstone (lmst) Alternating 2- to 4-mm-thick light and dark gray planar laminae. Minor 1inch- to 3-feet-thick DSb interbeds and fossil debris flows. Coarser grained and less carbonaceous than DSr1 to DSr3. >800 feet thick. Igneous rocks Jig: Jurassic diorite to granodiorite of the Goldstrike sill-like intrusion (158 Ma). 1 to 3 mm plagioclase, hornblende and biotite phenocrysts with interstitial quartz. Blocky, well-jointed and propylitized with calcite veins and disseminated pyrite. Up to 1,800 feet thick; uppermost 100 feet is weathered to friable sandy grus. Qmp/Lamp: Quartz monzonite porphyry and hornblende lamprophyre dikes and sills (158 Ma) Three types: (1) 2 to 6 mm blocky or lathy plagioclase phenocrysts in aphanitic plagioclase quartz groundmass, (2) 2 to 4mm resorbed quartz eyes in aphanitic plagioclase quartz groundmass, (3) biotite + hornblende-rich lamprophyre. Bfp: Biotite feldspar porphyry (39 Ma) Tertiary dikes in the Post fault zone. 2to 4-mm biotite and plagioclase phenocrysts in aphanitic plagioclase groundmass. Figure B-3. North Post-Goldbug tectonostratigraphic column. 24 Stratigraphy LOCAL STRATIGRAPHIC SUBUNITS portion. Fossiliferous debris flows occur proximal to the Bootstrap limestone. Silty slope facies of the Roberts Mountains and Popovich Formations lapped against the Bootstrap limestone shelf, which finally drowned in the mid to late Devonian (Griffin, 1999). The Bootstrap limestone is contemporaneous with the upper part of the Roberts Mountains Formation and much of the Popovich Formation. The Popovich Formation is conformably overlain by the Devonian Rodeo Creek unit, which consists of dark gray siliceous mudstone with interbedded light gray siltstone and silty limestone (Ettner, 1989). Thickness of the Rodeo Creek unit is about 700 feet (210 m); however, its upper portion is commonly removed by the Roberts Mountains thrust or erosion (fig. B-3). In some areas, notably at the Genesis deposit, the Roberts Mountains thrust removed the entire Rodeo Creek unit and placed Vinini mudstone on top of Popovich limestone (Schutz and Williams, 1995). The youngest sedimentary rocks of the Carlin trend are clast-supported gravel, siltstone, and ash beds of the Miocene Carlin Formation. These poorly consolidated rocks are up to 2,000 feet (600 m) thick and fill deep, north-trending basins. Informal mappable rock units in the Roberts Mountains and Popovich Formations and the Rodeo Creek unit are described below, from oldest to youngest (fig. B-3). These units are the most commonly mineralized host rocks on the Carlin trend. DSr4 Lower Roberts Mountains Formation (Upper Silurian). Light gray, planar laminated silty limestone with 10–30% inch- to foot-thick calcarenite interbeds. Mid-slope facies. Thickness is 800 to 900 feet (240–270 m). Contact with the underlying Hanson Creek dolomite is marked by a 30- to 50-foot (9–15 m) thick cherty dolomite horizon. Weathers as lavender platy or flaggy limestone. DSb Bootstrap limestone (Devonian-Silurian). Massive light gray, bioclastic limestone/biosparite. Öoid packstone to grainstone with lithoclasts of fossiliferous wackestone and packstone (Armstrong and others, 1997). Basal portion at the Bootstrap Mine contains 10–50% planar laminated silty limestone interbeds. Shoal environment. Deposition and emergence span Upper Silurian (DSr4) to Middle Devonian (Dp1) time (G. Griffin personal commun.). The upper contact with Popovich Formation Dp0 strata is unconformable. Upper shelf areas at Goldbug contain 1- to 5-foot (30–150 cm) wide storm rip-up clasts. Karsted zones with cavities are filled by varved hematitic clay. Coarse calcite crystals are common at Bootstrap and Capstone. Bootstrap limestone along silicified faults at Capstone forms massive, resistant jasperoid. Thickness >1,300 feet (>390 m) at RenBanshee and Meikle. Bootstrap limestone thins southward toward Tara and Goldbug (figs. B-2, B-4, and B-5). DSr1, DSr2, DSr3 Upper Roberts Mountain Formation (Devonian-Silurian). Medium gray, carbonaceous, bioturbated wavy (“wispy”) laminated silty limestone with 5–50% fossil debris flows (lithic and skeletal debris) and centimeter-thick calcarenite interbeds. DSr3 debris flow-rich unit is locally thickened to >300 feet (>90 m) at Goldbug along the south edge of the Bootstarp bioherm. Bioclastic coral, crinoid, brachiopod and bryozoan debris decreases up-section and distally away from the Bootstrap limestone bioherm. Mid-slope to upper shelf deposition. The uppermost 30 to 50 feet (9–15 m) is transitional planar to wispy laminated in gradational contact with the overlying Popovich Formation. The top of the Roberts Mountains Formation (DSr1) is recognized by the first occurrence of wispy laminated beds. Wispy laminae have wavelengths of 2 to 6 mm, and are interpreted to represent faunal burrowing in an oxygenated substrate in a shallow, quiescent shelf environment. Thickness 0 to 400 feet (0–120 m), thins northward and onlaps against Bootstrap limestone. Commonly decarbonatized, silicified, and collapse brecciated in mineralized areas. Dp2 Lower Popovich Formation (Lower Devonian). Dark gray, carbonaceous, thin-bedded to planar laminated silty micrite with 5–10% millimeter-thick calcarenite to calcisiltite interbeds. Localized graphitic bedding plane partings produce disc-shaped “hockey puck” pieces in core. Mid-slope deposition; 70 to 100 feet (21–30 m) thick; onlaps against Bootstrap limestone. Commonly silicified in mineralized areas. Uppermost 2 feet (60 cm) contains regionally extensive graptolites of Monograptus species indicative of a starved basin (Armstrong and others, 1997). DISTRIBUTION OF ROCK UNITS The highest mountain range in the northern Carlin trend is the N15ºE-trending Tuscarora Range, with elevations above 8,000 feet (2,400 m). A lower elevation N15ºW-trending spur in the Tuscarora Range, called the Tuscarora Spur, includes the most of northern Carlin trend gold resources. The Tuscarora Spur is separated from the Tuscarora Range by the 1- to 2-mile (1.6– 3.2 km) wide Little Boulder Basin (fig. B-2). Ordovician rocks crop out extensively south of the Carlin pit in the Richmond Mountain area along the spine of the Tuscarora Range, where Pogonip limestone, Eureka Quartzite and Hanson Creek dolomite are impressive ridge formers. In the Carlin and Lantern Mine areas, the Roberts Mountains Formation is broadly exposed, especially in the structurally uplifted block southwest of the Castle Reef fault. Further north within the Tuscarora Range in the West Leeville area, allochthonous Vinini Formation forms high ridges east of Little Boulder Basin. Mesozoic and Tertiary folding and faulting in the northern Carlin trend expose progressively younger rocks northward in the core of the northerly plunging Tuscarora anticline (fig. B2). In the Blue Star-Genesis Mine, Popovich and Rodeo Creek rocks are exposed on the west limb of the asymmetric Tuscarora anticline; the steepened east limb is truncated by the Gen fault and places Tertiary sedimentary rocks against Paleozoic rocks. The Popovich Formation averages about 500 feet (150 m) thick in the northern Carlin trend, but is as much as 800 feet (240 m) thick at Blue Star Ridge where near-vertical bedding is structurally undisturbed. North of the 1-mile (1.6-km) wide Goldstrike stock, in the Betze-Post area, Rodeo Creek mudstone is the dominant exposed unit. Further north at Rodeo-GriffinMeikle, upper plate Ordovician Vinini Formation is preserved above autochthonous units. Stepping 2 miles (3 km) northwestward into the Bootstrap subdistrict, block faulting in the Bootstrap window has exposed mostly Popovich and Rodeo Creek rocks. 25 A Storm (Rossi) Capstone Tara Bootstrap A' Ovi 0 0 Drc 200 100 meters Dp0 400 feet Dp1 Dp2 DSr1-2 0 5,000 feet 0 1,500 meters DSr3 DSr4 Gold zone Bootstrap limestone Drc/Dp datum line Figure B-4. Bootstrap area north-south stratigraphic fence diagram looking east. Dp1 Middle Popovich Formation (Lower to Middle Devonian). Dark gray, carbonaceous, medium-bedded micrite/ muddy limestone with diagnostic soft sediment deformation slump and flame textures, minor bioclastic debris flows and interbedded calcarenites, notably in Carlin Mine area. Upper slope facies. Thickness is 200 to 400 feet (60–120 m), thins northward, and onlaps against Bootstrap limestone. Locally thickened and metamorphosed to diopsidic calc-silicate hornfels/marble south of the Goldstrike stock in Genesis area. Dp0 Upper Popovich Formation (Middle Devonian). Dark gray, carbonaceous, planar laminated silty 1imestone, muddy 1imestone, calcarenite and limestone conglomerate. Generally upward coarsening, mid- to upper slope deposition. Contains diagenetic pin-striped pyrite (1–2 mm brassy pyrite flecks) along bedding. Thickness is 100 to 250 feet (30–75 m). Thins northward and is regionally extensive above the Bootstrap limestone. Includes two subunits at Rodeo (Goldbug): lower 120 feet (36 m) is muddy limestone with 1– 5%, 1- to 2-inch (3–5 cm) long, black chert lenses; upper 120 feet (36 m) is thin-bedded silty limestone. Limestone conglomerate with 1–10%, 1- to 4-inch (3–10 cm) diameter mudstone clasts form an excellent 30- to 40-foot (9–12 m) thick marker horizon along the Dp0/Dp1 contact at Tara. Calcarenite with limestone boulders up to 10 feet (3 m) in diameter crops out conspicuously between the Blue Star Ridge and Lantern deposits. Drc Rodeo Creek basal calcarenite (Middle Devonian). Gray carbonate-cemented quartz sandstone, commonly silicified. Mid to upper slope, high-energy deposition. Thickness 0 to 70 feet (0–20 m); thins northward, and is mostly absent north of Betze-Post area. Drc Rodeo Creek unit (Middle to Upper Devonian). Interbedded dark gray siliceous mudstone and light gray planar laminated silty limestone; lowermost 200 to 300 feet (60–90 m) is dominantly siliceous mudstone (rhythmically bedded argillite). Deep, anoxic, marine deposition. Thickness is 100 to 700 feet (30–210 m); the upper portion is locally removed by the Roberts Mountains thrust. MIDDLE PALEOZOIC CARBONATE DEPOSITION MODEL Devonian-Silurian carbonate deposition in northeast Nevada was characterized by progressive marine transgression with emergent periods and prolonged subaerial exposure of the Bootstrap limestone. Paleogeographic environments are subdivided as follows (figs. B-3 to B-6): Silurian transgression (DSr4). DSr4 deposition is characterized by slope and basin deposition of laminated silty limestone with calcarenite turbidites above shelf carbonates. DSr4 contains >50% terriginous silt content and is highly permeable. Devonian-Silurian emergence (DSb). Bootstrap limestone represents shoal to shallow shelf deposition of öolitic sands to limestone muds with marginal debris flow sedimentation on an irregular slope (Teal, 1996; Unger, 1997; Armstrong and others, 1997). Submarine canyons along ancient fault lines 26 Stratigraphy Rodeo (Goldbug) B Betze-Post Deep Star Banshee B' Genesis Ovi Ovi Blue Star Lantern Drc Drc GOLDSTRIKE STOCK Dp0 Dp0 Dp1 Dp2 Dp1 Dp2 DSr1-2 DSr1-2 DSr3 DSr3 DSr4 DSr4 0 0 0 5,000 feet Gold zone Drc/Dp datum line Bootstrap limestone Outer limit of metamorphism 200 0 1,500 meters 400 feet 100 meters Figure B-5. Betze-Post-Blue Star area north-south stratigraphic fence diagram looking east. focused thick accumulations of fossil debris flows, slump blocks and turbidites (Jory and others; 1997). Debris flows occur as lenticular beds, inches to tens of feet thick, containing poorly sorted carbonate and biogenic clasts interbedded with planar to wispy laminated silty limestone. Thickness and clast size show an inverse relation to distance from the biohermal source area, decreasing distally to inch-thick calcarenite beds. Progradation occurred in local shelf areas. Basin-ward contemporaneous deposition of planar laminated silty limestone is interfingered with 1- to 3-foot (30–90 cm) thick calcarenite beds. This dramatic facies change is exposed in the west wall of the Tara pit, where silty facies onlap onto the massive Bootstrap limestone over a distance of 500 feet (150 m). The top of the Bootstrap limestone is marked by a mid-Devonian unconformity with extensive silicification in the Bootstrap and Capstone pits. Mid-Devonian transgression and deep marine deposition (Drc). The Rodeo Creek unit deposition was marked by anoxic basin deposition of rhythmically bedded siliceous muds. The conformable and gradational contact with underlying lime muds and silts of the upper Popovich Formation is well exposed at several localities, notably in the east wall of the Tara pit. Intrusive Rocks The northern Carlin trend contains numerous igneous rocks, mostly as stocks and dikes. Magmatic activity occurred during Late Jurassic, Eocene, and Miocene events that constrain the ages of gold mineralization. Temporal evolution of igneous activity in the northern Carlin trend is: Late Jurassic 158-Ma magmatism emplaced the Goldstrike granodiorite stock of high Mg-Fe, calc-alkaline affinity and widespread, coeval hornblende-biotite lamprophyre dikes. The northeast-southwest elongate Goldstrike stock measures 4 miles (6.4 km) long in a northeast direction by 1 mile (1.6 km) wide (figs. B-2 and B-5). Maximum thickness is unknown although laccolithic margins adjacent to the Deep Post and Genesis orebodies exceed 1,500 feet (450 m) in thickness. Contact metamorphism of limestone produced diopside hornfels adjacent to the Goldstrike stock and distal marble. Local metasomatism formed diopside-grossular endoskarn along intrusion margins and diopside-grossular- Devonian-Silurian slope sedimentation (DSr1, DSr2, DSr3, Dp0, Dp1, Dp2). Slope sedimentation was characterized by upward transition from oxygenated, terriginous, silt-rich carbonate to anoxic lime muds during shelf submergence. MidDevonian micrite (Dp1) sedimentation with soft sediment deformation occurred during prolonged, quiescent, shallowwater conditions. Middle to Late Devonian marine shallowing with increased terriginous silt content due to frequent storms is evidenced by rip-up clasts and limestone conglomerates (Dp0). 27 vesuvianite exoskarn with retrograde tremolite-epidote-calcite skarn in limestone. Propylitic alteration of diorite formed the alteration assemblage of calcite-chlorite-pyrite. Late-stage acidic fluids altered detrital feldspar, illite, and clay in sedimentary rocks to illite and sericite, with kaolinite in higher temperature, mineralized centers. The core of the Goldstrike stock is fresh to weakly propylitized; only sericitized contacts over widths of 10 to 100 feet (3–30 m) show the effects of hydrothermal alteration. Calc alkaline, biotite-hornblende lamprophyre dikes 1 to 3 feet (30–90 cm) thick are widespread along north-northwest-trending structural corridors, notably the Post-Gen fault zone along the eastern, fault-bounded edge of the Tuscarora Spur. Eocene magmatism (37 to 40 Ma) is present along the entire Carlin trend, consisting of K-rich calc-alkaline rhyolite and dacite dikes at Deep Star/Genesis and Beast, respectively. Eocene dikes of the northern Carlin trend are immediately adjacent to a large aeromagnetic anomaly that corresponds with the nearby Emigrant Pass volcanic field and Welches Canyon stock. It is inferred that the anomaly represents a buried Eocene plutonic complex, and that Eocene magmatism is the major process that drove hydrothermal circulation forming Carlin gold deposits (Ressel and others, 2000a). Variable ages for Eocene dikes indicate that episodic gold deposition occurred over a period of 3 to 4 million years (Ressel and Noble, 1999). Fine-grained rhyolite dikes at Deep Star and Genesis (Anne Dike) are strongly argillized and locally mineralized. Dacitic biotite feldspar porphyry dikes emplaced along the Post-Gen fault zones occur over a north-south distance of at least 4 miles (6.4 km) from Meikle to Lantern. At Deep Post, barren biotite feldspar porphyry dikes (39 Ma) along the Post and Dormant faults bound highgrade (>1 opt [troy ounces per short ton] or 34 g/t) gold (Emsbo and others, 1996). Conversely, the rhyolitic quartz eye monzonite porphyry of the 36.3-Ma Beast dike in the Beast deposit is moderately mineralized (Ressel and others, 1999). The Beast dike, which is up to 150 feet (45 m) wide and 1 mile (1.6 km) long, contains 3 million tons (2.7 Mt) grading 0.016 opt (0.5 g/ t) gold. Gold occurs mostly in brecciated dike and quartz veinlets along the footwall margin of the 60ºE-dipping dike. 3) Youngest magmatism includes Miocene rhyolitic lavas (15 Ma) along the west flank of the Tuscarora Spur. Air-fall tuff and ash flows in the basal Carlin Formation may represent distal equivalents of this magmatism (Ressel and Noble, 1999). width, whereas it is up to 1 mile (1.6 km) wide on the south side. Gold endowment at Betze-Post (>30 million oz [933 t]) is approximately six times greater than at Genesis-Blue Star (5 million oz [156 t]) (Schutz and Williams, 1995). This difference can be partly explained by the broader metamorphic aureole at Genesis-Blue Star producing inferior host rocks with restricted permeability, mostly marble and calc-silicate hornfels of the Popovich Formation. A small group of deposits is hosted by quartzpyrite veins in the stock (e.g., North Star-West #9, Pancana). Some of the highest gold grades on the northern Carlin trend are hosted by Jurassic mafic dikes and along sheared dike margins. Availability of iron from altered biotite and hornblende is considered important for gold precipitation within mafic dikes or along dike margins. Dikes (and sills) are commonly argillized and/or sericitized adjacent to gold deposits, and were emplaced along the same faults later utilized by gold-bearing, hydrothermal fluids. North-northwest-trending Jurassic dike swarms are often the best indicator of deep mineralized systems. Whereas wallrocks may be geochemically barren, dikes with anomalous As, Sb, Hg, and Tl and sericitized hornblende, biotite, and plagioclase are important conduits for concealed gold deposits (e.g., Betze-Post, West Leeville). For example, surface expression of the 1,500- to 1,800-foot (460–550 m) deep West Leeville deposit is subtle; exposed Vinini mudstone is not hydrothermally altered but there is a strong zone of northsouth faulting and lamprophyre dikes above the deposit. The 700- to 1,600-foot (210–490 m) deep Betze-Post deposit contains numerous north-northwest-striking, argillized dikes in the 80ºE-dipping Post fault zone and 50ºW-dipping JB fault zone; at depth these dike swarms host high-grade mineralization where they cut the upper Roberts Mountains Formation. STYLES OF GOLD MINERALIZATION Gold deposits of the northern Carlin trend represent a spectrum of deposit styles owing to stratigraphic and structural differences (Teal and Jackson, 1997a; this volume). Each deposit can be assigned to a deposit class; however, they all contain components of the three major end members: stratigraphic, collapse breccia and structural controls (figs. B7 and B-8). Deposit types do not cluster geographically, but instead demonstrate remarkable variation in host rock and style of mineralization over short distances. Individual deposits commonly contain several styles of mineralization. Each deposit class with examples is discussed below. INTRUSION-RELATED GOLD MINERALIZATION The Jurassic Goldstrike stock has two important roles in its spatial relationship to gold deposits: Stratigraphically Controlled Gold Deposits (1) Embayments and sheared margins of the stock host highgrade orebodies at Deep Star and Deep Post in decarbonatized and hornfelsed sedimentary rocks along the Post-Gen fault zone; and (2) Margins of the contact metamorphic aureole focused gold mineralization at Betze-Post on the north side and at Genesis-Blue Star on the south side of the stock. The northern contact aureole averages only 300 feet (90 m) in The classic “Carlin-type” deposits show a strong stratigraphic control to gold mineralization and contain about 15% of the northern Carlin trend gold resource (fig. B-8). These tabular orebodies are characterized by extensive decarbonatization and residual dolomite with minimal structural disruption. Stratigraphically, there are three preferred host horizons in gold deposits of the northern Carlin trend: 28 Stratigraphy Turf West Leeville C C' Carlin Pete Ovi 0 Drc 0 200 100 meters 400 feet Dp0 Dp1 0 5,000 feet 0 1,500 meters Dp2 DSr1-2 DSr3 DSr3 DSr4 Gold zone Drc/Dp datum line Figure B-6. Carlin area north-south stratigraphic fence diagram looking northeast. variations in preferred host strata in the upper Roberts Mountains Formation and distinct feeder faults (figs. B-1 and B-6). At Carlin the primary host is the DSr1 and uppermost DSr2 units; secondary structural control is provided by the northeast-striking Hardie fault and northwest-striking faults. Carlin is a cluster of gold deposits including Carlin Main, Carlin Island, Carlin East, and Hardie Footwall located in the footwall of the Hardie fault. The Pete deposit, located 1 mile (1.6 km) southeast of the Carlin deposit, may represent the right-laterally offset portion of the Carlin deposit along the northwest-striking Leeville fault. The preferred host rock at Pete is the DSr2 unit, and about 15% of the resource is hosted in the basal 50 feet (15 m) of the Rodeo Creek unit. North-northeast and northnorthwest structural controls are inferred at Pete. Located 1 mile (1.6 km) north of Carlin, West Leeville includes two stacked stratiform horizons with a vertical separation of about 200 feet (60 m) in the base of the DSr2 unit and top of the DSr4 unit. The footwall of the north-striking West-Bounding fault is strongly mineralized, especially where cut by the northwest-striking Rodeo Creek fault. At the north end of the Carlin trend, the Tara deposit is hosted primarily by the DSr2 unit. At northern Tara where the DSr2 unit pinches out, the Dp2 unit is the preferred host. The passive decarbonitized style of mineralization at Tara contrasts remarkably with the Bootstrap orebody only 1,000 feet (300 m) to the north, where structurally controlled gold mineralization occurs along the silicified and dike-filled Bootstrap fault and bleeds out laterally along the Dp0/Bootstrap limestone unconformity. Upper Roberts Mountains Formation (DSr1, DSr2, DSr3). The uppermost 400 feet (120 m) of the Roberts Mountains Formation is especially susceptible to decarbonatization by acidic hydrothermal fluids and gold precipitation. Decarbonatization produces a punky, low density, silty carbonaceous rock with excellent secondary permeability and porosity. Volume loss as high as 40% was documented at the Carlin Mine (Bakken and Einaudi, 1990). Silicification is variable and is mostly confined to centimeter-thick calcarenite interbeds that constitute only 1– 10% of the upper Roberts Mountains Formation. The Roberts Mountains Formation is conformably overlain by the Popovich Formation, which includes the relatively fine-grained and impermeable, 200- to 300-foot (60–90 m) thick Dp1 micrite unit. Dp1 micrite is geochemically barren and rarely mineralized, except where cut by faults and sulfidized dikes, and likely served as a cap rock to mineralization in underlying host rocks. Similarly, the 800-foot (240 m) thick DSr4 unit of the lower Roberts Mountains Formation is a poor host in most deposits except along the Castle Reef fault at Lantern and Perry, and in the lower zone at West Leeville. The DSr4 unit was likely a relatively poor host due to the lack of bioclastic debris flows and calcarenite interbeds, and perhaps paucity of organic carbon and available iron compared to the overlying upper Roberts Mountains and Popovich Formations. Examples of dominantly stratigraphically controlled orebodies hosted by the upper Roberts Mountains Formation include Carlin, Pete, West Leeville, and Tara, each with notable 29 COLLAPSE BRECCIA AH BA BZ BT BE BX BR BS BO BC BM CP CA CW DE DP DS EX FE FC > 5 million oz Au resource 0.5-5 million oz Au resource < 0.5 million oz Au resource LP LG GE SR BE DP SC Antimony Hil Barrel Bazza Beast Betze Big 6 Blue Star Ridge Blue Star Bootstrap Bobcat Bullion Monarch Capstone Carlin Carlin West Dee Deep Post Deep Star Exodus Fence Four Corners ME GR BC BZ FE CW WG BS UP UG NL RD CA TA WL PE BO DE BT BX AH SL BR PY SO FC PR BA BM CP DS NS TU EX PA STRATIGRAPHY GE GR LL LG LP ME NL NS PA PY PR PE RD SC SO SL SR TA TU UG UP WG WL Genesis Griffin Long Lac Lower Goldbug Lower Post Meikle North Lantern North Star/West #9 Pancana Payraise Perry Pete Rodeo Screamer Sold South Lantern Storm (Rossi) Tara Turf Upper Goldbug Upper Post West Genesis West Leeville LL FAULTS, FOLDS, VEINS Figure B-7. Ternary plot of styles of gold mineralization for the northern Carlin trend. Modified from Teal and Jackson, 1997b. Post fault. Availability of organic carbon in the Roberts Mountains and Popovich Formations may have been locally important in precipitating gold at Rodeo (Goldbug), where grains of coarse gold up to 1 mm occur. Lower Popovich Formation (Dp2 unit). The lowermost 70 to 100 feet (20–30 m) of the Popovich Formation is commonly silicified and hosts portions of stratigraphically controlled deposits at Lower Barrel, Lower Goldbug, and Tara. Owing to its stratigraphic proximity to the underlying DSr1-2-3 units, the entire Dp2 through DSr3 package is mineralized in the larger deposits (e.g., Betze-Post, Genesis). Collapse Breccia-Hosted Gold Deposits Collapse breccia-hosted deposits are common in the Post subdistrict at Betze, Lower Post, and Lower Goldbug, and at Genesis due to widespread decalcification and subsequent volume loss and collapse overprinting bioclastic-rich portions of the DSr3 unit (fig. B-5). Typical Popovich Formation thicknesses of 500 to 600 feet (150–180 m) at Genesis and Betze-Post compared to 800 feet (240 m) in unaltered Popovich on the Tuscarora Spur suggest that as much as 40% volume loss took place due to collapse. These deposits contain 40% of the northern Carlin trend resource. Collapse breccia dimensions at Betze-Post-Goldbug are about 1 square mile (2.5 square km) in plan and 400 to 700 feet (120–210 m) thick. Collapse breccias are clast-supported with angular clasts of the DSr2-3-4 units set in a matrix of fine-grained, residual silt, organic carbon and pyrite. The simple genetic model envisages early decarbonatization by acidic hydrothermal fluids focused along north-northwest-striking faults and laterally into stratiform fossil debris flows and calcarenite interbeds, collapse of the upper Roberts Mountains Formation, gold precipitation, and late-stage silicification + baritization. Heterolithic collapse breccias contain clasts of intrusive material and are generally higher grade. These stratiform bodies are not widely developed Upper Popovich Formation (Dp0 unit). The upper 200 feet (60 m) of the Popovich Formation is also extensively decarbonatized, and is conformably overlain by 300 feet (90 m) of relatively impermeable siliceous mudstones of the lower Rodeo Creek unit. Rheological contrast along the Rodeo Creek siliciclastic/Popovich limestone contact has produced numerous low-angle faults and bedding plane shears that enhance permeability and porosity. However, at Betze-Post strong argillization within the uppermost 50 feet (15 m) of the Popovich Formation rendered this horizon impermeable by forming a stratiform carbonaceous clay layer. Examples of dominantly stratigraphically controlled orebodies hosted in the upper Popovich Formation include Upper Meikle, West Griffin, Rodeo, Upper Goldbug, Upper Barrel, West Genesis, Bobcat, Blue Star and SOLD. In each example, gold mineralization has ponded immediately beneath the Rodeo Creek/Popovich contact in decarbonatized silty limestone along major north-northwest feeder faults. The Rodeo and Upper Goldbug deposits are hosted in the Dp0 and Dp1 units in the footwall of the N10–30E-striking, lamprophyre dike-filled Hillside fault and the footwall of the 30 Stratigraphy Gold resource (million ounces) 0 5 10 15 20 25 30 Tmc 0 0 200 Simplified stratigraphic column (average unit thickness in feet) DOw Drc Dp0 Dp1 Dp2 DSr 1-2 DSr 3 DSr 4 100 400 feet meters Figure B-8. Northern Carlin trend gold distribution vs. stratigraphy. outside the Betze-Post-Goldbug area owing to the lack of bioclastic debris flows. Within the Post subdistrict they host large, medium- to high-grade gold deposits with stratigraphic and structural controls (Volk and others, 1995). The giant Betze orebody (40 million oz [1,250 t]) consists of seven vertically stacked ore zones in the upper Roberts Mountains Formation within the west-northwest-trending Betze anticline along the northern margin of the Goldstrike stock (fig. B-2). Excluding structurally controlled deposits along the northwest-striking Castle Reef fault, the Betze, and the adjacent Deep Post orebodies are unique in their northwest strike. The thickest and highest gold grades are located along the axis of the Betze anticline at its intersection with northeast- and northnorthwest-striking faults. Lower, more erratic grades are spatially related to intense silicification along the northeast limb of the Betze anticline. Limestone dissolution between impermeable layers formed stacked, stratified collapse breccias over a vertical distance of 700 feet (210 m) with an estimated rock thickness reduction of 14% (Leonardson and Rahn, 1996). The base of the stratiform, silicified, collapse breccia-hosted Lower Post and Lower Goldbug deposits are sharply defined by the DSr3/4 contact (Jory and others, 1997). Debris flows with interbedded wispy-laminated silty limestone are thickest (300 feet; 90 m) and most fossiliferous at Goldbug and Lower Post. Their occurrence marks the paleogeographic upper slope facies of the southeast margin of the Bootstrap limestone bioherm. The Post fault zone is postulated to be an ancient structure that served as a submarine trough along which thicker packages of fossil debris flow were shed, forming the DSr3 unit (Jory and others, 1997). DSr3 is the preferred host unit at Lower Post and Lower Goldbug. The underlying DSr4 planar laminated silty limestone is rarely mineralized due to the lack of debris flows, thereby being less susceptible to fluid migration. At Genesis the preferred host unit is silicified collapse breccia overprinting decalcified DSr2-3 units in the footwall of the Gen fault along the crest of the Tuscarora anticline. Dimensions of the collapse breccia are 1,500 feet (450 m) north-south by 600 feet (180 m) east-west by 200 to 300 feet (60–90 m) thick. 31 Structurally Controlled Gold Deposits (Faults, Folds, and Veins) Griffin, located 1,000 feet (300 m) south of Meikle, occurs within DSr2-3 units along the collapse brecciated, sheared and dike-filled footwall of the Post fault zone. The Genesis Complex includes the 3-million-ounce (93 t) Genesis deposit, hosted by the Popovich and upper Roberts Mountains Formations, and numerous smaller satellite deposits that are mostly structurally controlled along the Rodeo Creek/ Popovich contact. Mineralizing solutions exploited intersections of thrusts, northwest-striking normal and reverse faults, northeast-striking normal faults, and the crest of the N15ºW-trending Tuscarora anticline (Schutz and Williams, 1995). The barren, N10–40ºE-trending, 39-Ma K Dike is well exposed in the Genesis pit, where it is up to 100 feet (30 m) wide and strongly sericitized. The dacitic K Dike is mapped over a north-south strike length of 2 miles (3 km) from Deep Star and Genesis southward through Blue Star to the Crazy Eights prospect; the sheared footwall margin commonly contains 0.1 to 0.2 opt (3–7 g/t) gold grades over widths of 10 to 50 feet (3–15 m). West Genesis occurs on the west limb of the Tuscarora anticline in upper Popovich Formation along bedding-parallel shears. Low-angle thrusts along the Rodeo Creek/Popovich contact in an uplifted structural block on the west edge of the Genesis pit are the primary ore control at Bobcat and Payraise. Flat-lying to 30ºW-dipping quartz veins cutting the southeast margin of the Goldstrike stock are the dominant host rock at the North Star and West #9 deposits. Some gold mineralization is also focused along north-northwest faults that cut Popovich limestone exoskarn and marble. At Genesis and North Star, alteration is zoned from strongly silicified and quartz-sericite-pyrite altered core >0.10 opt (3.4 g/t) gold, outward to less intense quartz-sericite-pyrite in 0.01 to 0.10 opt (0.3–3.4 g/t) gold haloes, to decalcified waste rock. Blue Star ore is dominantly silificified calcarenite of the basal Rodeo Creek unit and decalcified upper Popovich Formation. In the adjacent Blue Star Ridge deposit 1,000 feet (300 m) to the south, the preferred host unit is the 50- to 70-foot (15–21 m) thick basal calcarenite; however, narrow high-grade zones zones are restricted to high-angle, northerly striking lamprophyre dikes. The basal calcarenite attains its greatest thickness at Blue Star and Blue Star Ridge where it forms an excellent host rock. The Beast deposit is hosted by the northerly striking Beast dike and brecciated DSr4 in the footwall of the dike. The Beast orebody occurs in the east limb of the Tuscarora anticline in the footwall of the dike-filled Gen fault. The adjacent Sold deposit 1,000 feet (300 m) northwest of Beast occurs along a decalcified and argillized east-west fault cutting Popovich limestone. Other examples of structurally controlled gold deposits of the northern Carlin trend include all deposits in the Bootstrap subdistrict, excluding the stratigraphically controlled Tara deposit (fig. B-4). These deposits occur along the near-vertical, north-south Bootstrap fault zone, including Dee, Capstone, and Bootstrap. These deposits are somewhat unusual in that 20–40% of the gold is hosted by silicified Bootstrap limestone adjacent to north-south feeder faults. However, a critical component of these deposits is the intersection of these feeders with the upper Popovich Dp0 silty limestone/Bootstrap Structurally controlled deposits account for an estimated 45% of gold in the northern Carlin trend. The three highest grade gold deposits on the Carlin trend (Deep Star, Deep Post, and Meikle) are located in the immediate footwall of the north to N20ºW-striking, 80ºE-dipping Post fault zone. The fault zone served as an important conduit for mineralizing low pH hydrothermal solutions along the east edge of the Tuscarora Spur (fig. B-2). These large, profitable orebodies formed at flexures that represent dilation zones along the Post fault. The 100- to 300-foot (30–90m) wide fault zone is poorly mineralized due to high clay content and therefore restricted permeability. Sharp grade boundaries place 1.0 opt (34 g/t) gold rock within tens of feet of unmineralized rock. Gold grades average 0.7 to 1.0 opt (24–34 g/t) gold and are as high as 6 opt (206 g/t) gold. The gold occurs as sub-micron-sized, disseminated particles in arsenian pyrite rims on pyrite nuclei (Arehart and others, 1993). These structurally controlled end members are characterized by abundant shearing and tectonic brecciation, 3–10% fine-grained “sooty” pyrite and marcasite, and elevated trace element geochemistry (gold, As, Sb, Hg, and Tl). Despite their similarities, these three deposits also have important differences. Deep Post and Deep Star are steeply east-dipping orebodies within or immediately adjacent to the metamorphic aureole of the Goldstrike stock (Harlan and Heitt, 1997), with horizontal dimensions of 500 by 1,000 feet (150 by 300 m) and 500 by 700 feet (150 by 210 m), respectively (fig. B-5). Vertical dimensions are relatively large, ranging from 800 to 1,200 feet (240–360 m). Recent work at both these deposits suggests that sharply defined bases to gold mineralization may be partly stratigraphically controlled. Hydrothermal alteration consists of a high-grade quartz+kaolinite core with 1.0 opt (3.4 g/t) gold with fine-grained sooty sulfide flooding in pervasively decarbonatized and sheared host rocks. Deep Post host rocks are the DSr2 and upper DSr3 units. An estimated 15% of the deposit is hosted in sheared diorite along the northeast margin of the Goldstrike intrusion. Deep Star is hosted by calc-silicate hornfels and silty limestone of the Popovich Formation. Deep Post averages 0.7% arsenic and contains spectacular realgar and orpiment-filled shears along the intrusive/sedimentary rock contact, while Deep Star contains only minor realgar. By contrast, arsenic sulfides are uncommon at Meikle (7 million oz at 0.7 opt [200 t at 24 g/t] gold), which consists of en echelon, high-angle gold-bearing zones cutting massive Bootstrap limestone. Meikle’s dimensions are 2,500 by 800 feet (750 by 240 m). Metamorphic rocks do not occur at Meikle and silicification is widespread. Meikle and Deep Star have similar N15ºW/70–80ºE-dipping structural orientations in the footwall of the Post fault zone, whereas the N50ºW/65ºSW-dipping orientation at Deep Post is controlled by the sheared intrusive/sediment contact. Thus, each of these 900- to 1,600-foot (270–490 m) deep, high-grade deposits has contrasting host rocks, orientations, alteration, and gangue mineralogy that require different exploration strategies. East 32 Stratigraphy and Meikle. The bulk of gold mineralization occurs within 2 miles (3 km) of the Jurassic Goldstrike diorite stock and adjacent to the north-northwest-striking, dike-filled, Post-Gen fault zone. However, review of figure B-7 for gold resources larger than 0.5 million ounces (16 t) shows that there is a roughly equal number of deposits in the structure (8) versus stratigraphy + collapse breccia (8) fields, testifying to the importance of stratigraphic control. The latter statement is especially relevant when considering the Betze deposit, by far the single largest deposit on the Carlin trend with 40 million ounces (1,250 t) of gold. Figure B-8 portrays the importance of the upper Roberts Mountains Formation as the superior host rock in the northern Carlin trend. Preferred host rocks include bioturbated, wispylaminated silty limestones with bioclastic debris flows and calcarenite interbeds, which contain an estimated 65% of the northern Carlin trend gold resource. Extensive decarbonatization, volume loss, and subsequent collapse brecciation enhanced permeability and porosity to produce ideal host rocks for ascending mineralizing fluids. Abrupt facies changes across the Dp0/Bootstrap limestone unconformity and across the Rodeo Creek/Popovich Formation contact localized gold mineralization throughout the northern Carlin trend. Hydrothermal alteration spatially associated with gold mineralization includes decarbonatization, silicification, argillization, sulfidation, dolomitization, with barite + stibnite + realgar + orpiment introduction with geochemically anomalous As, Sb, Hg, Ag and Tl. These characteristics are recognized in many gold mining districts throughout northeast Nevada, including Gold Quarry-Mike-Tusc, Rain-Railroad, Bald Mountain-Alligator Ridge, Cortez-Pipeline, and Twin Creeks-Getchell. Superposition of the western margin of ancestral North America with slope facies sedimentary rocks, structural reactivation along north-northwest to north-northeast fabrics, several episodes of magmatic activity, and large auriferous hydrothermal cells combine to make northeast Nevada one of the richest gold provinces in the world. limestone unconformity (fig. B-4) (Coombs and Malan, 1999). Significant gold mineralization is not recognized more than 300 feet (90 m) vertically above or below this unconformity. At Capstone the lamprophyric Capstone dike is strongly mineralized where it cuts the unconformity. Also at Capstone, a northeast zone of gold mineralization containing 80,000 ounces (2.5 t) of gold is hosted by silicified and quartz stockwork-veined silty limestone of the allochthonous Vinini Formation. The Vinini Formation is generally a poor host since it contains impermeable and unreactive siliceous rocks and high clay content along Roberts Mountains thrust-induced shears. Vinini-hosted mineralization is structurally controlled and is typically only several meters wide. Vinini siltstone is mineralized along north-northeast faults at the Big Six and Antimony Hill prospects north of the Carlin Mine. To date, no significant gold mineralization has been discovered in the oldest rocks of the northern Carlin trend, including the Ordovician Pogonip limestone, Ordovician Eureka Quartzite, and Ordovician Hanson Creek dolomite. The South Lantern deposit is structurally controlled along the northwest-striking, baritic Castle Reef fault in the DSr4 unit. South Lantern was discovered in 1993 by detailed structural mapping. It is notable for its subtle alteration with weak to no decarbonatization and intact bedding. Several small deposits are hosted by the DSr4 unit along the Castle Reef fault zone, including Bullion Monarch (Universal Gas), West Carlin, Perry and Castle Reef. These deposits are spatially associated with large linear ribs of jasperoid replacing bioclastic-rich strata along the steeply northeast-dipping Castle Reef fault. CONCLUSIONS The Carlin trend is a north-northwest alignment of over 40 gold deposits with associated hydrothermal alteration along a linear zone 40 miles (65 km) long and 1 to 2 miles (1.6–3.2 km) wide. Gold deposits of the northern Carlin trend show three structural orientations, in order of decreasing importance: (1) north to N20ºW in the footwall of the Post-Gen fault zone and numerous subparallel dike swarms; (2) N30ºW to N60ºW along the Castle Reef fault zone and along the Betze anticline; and (3) N10ºE to N30ºE at Tara, Carlin, West Leeville, and Blue Star. Numerous local variations are recognized at the mine-scale, notably a northeast fabric at Betze-Post and Genesis. Two-thirds of gold deposits in the northern Carlin trend occur within the north to N20ºW strike range (table B1); however, the largest orebody (Betze) strikes N60ºW. Gold deposits of the northern Carlin trend represent a spectrum of styles of mineralization owing to stratigraphic and structural differences (Teal and Jackson, 1997a). Each deposit can be assigned to a deposit class; however, most contain components of the three major end members: stratigraphic control, structural control, and collapse breccia. Review of figure B-7 shows that most gold deposits plot within the structurally controlled field. Structural preparation is the single most important factor in controlling Carlin trend gold deposits. The richest deposits are concentrated along or adjacent to the Post-Gen fault zone, as exemplified by Deep Star, Deep Post, ACKNOWLEDGMENTS This paper represents the cumulative geologic work of dozens of geologists on the Carlin trend. Understanding and correlation of Carlin trend stratigraphy have improved considerably in the past 5 years due to better open pit and underground exposures, increased core drilling, and continued communication between geologists. Much of the current stratigraphic nomenclature was developed in 1993 at the Carlin Mine by Newmont geologists Steve Moore and Richard Harris. Frequent reference is made to the 1997 Society of Economic Geologists newsletter publication by Teal and Jackson, entitled “Geologic overview of the Carlin trend gold deposits and descriptions of recent deep discoveries.” Newmont Mining Corporation is thanked for granting permission to publish this paper. Enlightening discussions over the past ten years with the following geologists are especially appreciated: Newmont geologists Jeff Huspeni, Ron Thoreson, Ken Paul, Leroy Schutz, Lewis Teal, Henry Unger, Cindy Williams, Bruce Harlan, Leo Coombs, Paul 33 Malan, Wayne Trudel, Don Harris, and Margie Lane; and Barrick geologists Bob Leonardson , Jerry Rahn, Mike Penick, Greg Griffin, and Gary Baschuk. Jeanette Hunter is thanked for drafting the figures. Steve Moore, Lewis Teal, Henry Unger, Ken Paul, Greg Griffin, and three anonymous reviewers provided excellent comments that significantly improved the manuscript. Finally, my thanks to Lewis Teal, Henry Unger, and Tommy Thompson for providing the encouragement to write this paper. Table B-1. Northern Carlin trend resources by deposit, host rock, and primary structural orientation. Deposit Abbreviation in figure 7 Antimony Hill Barrel Bazza Beast Betze Big Six Blue Star Ridge Blue Star Bootstrap Bobcat Bullion Monarch Capstone Carlin Carlin West Dee Deep Post Deep Star Exodus Fence Four Corners Genesis Griffin Long Lac Lower Goldbug Lower Post Meikle North Lantern North Star-West #9 Pancana Payraise Perry Pete Rodeo Screamer Sold South Lantern Storm (Rossi) Tara Turf Upper Goldbug Upper Post West Genesis West Leeville AH BA BZ BT BE BX BR BS BO BC BM CP CA CW DE DP DS EX FE FC GE GR LL LG LP ME NL NS PA PY PR PE RD SC SO SL RS TA TU UG UP WG WL Pre-mine resource (oz gold) 20K 200K 80K 50K 40M 20K 180K 400K 850K 220K 40K 300K 5M 250K 1M 4M 1.7M 400K 200K 300K 3M 500K 65K 1M 2.5M 7M 100K 230K 20K 30K 50K 1M 1.5M 5M 30K 200K 1.1M 600K 1M 1.2M 3.0 M 250K 3M Grade (opt gold) 0.05 0.20 0.05 0.02 0.20 0.05 0.02 0.04 0.10 0.03 0.17 0.10 0.40 0.06 0.10 0.70 1.00 0.10 0.20 0.25 0.10 0.40 0.05 0.25 0.13 0.70 0.03 0.08 0.05 0.02 0.02 0.06 0.40 0.18 0.02 0.04 0.41 0.06 0.37 0.35 0.04 0.07 0.37 Primary host unit Ovi Dp2 Drc DSr4 DSr2-3 Ovi Drc Dp0 DSb, Dp0 Dp0 DSr4 Ovi, Dp0 DSr1-2 Dp0 Dp0 DSr2-3 Dp Dp DSr2 DSr2-3 DSr2-3 DSr2-3 Drc DSr2-3 DSr3 DSr, DSb DSr4 JKi JKi Drc DSr4 DSr2 Dp0 DSr2-3 Dp1-2 DSr4 DSb, Dp0 DSr2 DSr2-3 Dp2 Drc Dp0 DSr2 Secondary host unit Dp0 JKi Dp0 Drc Drc Drc DSb DSr4 DSb JKi JKi Drc Dp DSr4 DSr2 Dp0 Dp Dp0 Drc Dp1-2 Dp DSr2 JKi Dp2 Dp0 Dp0-1 DSr4 Strike N20E N10W N45W N10W N60W N20E N-S N-S N-S N-S N60W N-S N50E N60W N-S N50W N20W N30W N20W N15E N15W N20W N10W N30W N30W N15W N10W N10W N15W N45E N60W N20W N40E N70W N80E N30W N20W N20E N15E N40E N20W N-S N-S K = thousands; M = millions; 1 million ounces = 31.1 metric tons (t); 1 ounce/short ton (opt) = 34.3 grams/metric ton (g/t) Data sources: Teal and Jackson (1997a) 1998 Newmont annual reserve and resource statement 34
