Geology of the West Leeville Deposit
M. Jackson, M. Lane, B. Leach, 1997. "Geology of the West Leeville Deposit", Carlin-Type Gold Deposits Field Conference, Peter Vikre, Tommy B. Thompson, Keith Bettles, Odin Christensen, Ron Parratt
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The West Leeville deposit is a deep, high-grade refractory gold deposit located on the Carlin Trend, 1.5 miles north of the Carlin Mine, Eureka County, Nevada. The deposit is part of a large gold system, extending northwest from the Carlin deposit. At a cutoff of0.200 oz/st, the deposit contains a drill-indicated resource of 7,287,984 tons at an average grade of 0.436 oz/st gold (3,177,561 ounces gold). The majority of the resource is located on the Newmont/Barrick HD Venture, where Newmont, as operator with a majority interest of 60%, has conducted deep exploration since 1992. The West Leeville deposit occurs at depths of 1,500 to 2,000 feet and is hosted by flat-lying, silty limestone of the upper Silurian-Devonian Roberts Mountains Formation. The Roberts Mountains Formation is subdivided into four informal subunits (SDrm 1-4). West Leeville ore occurs in two strata-bound zones near transitional contacts between these lithologic units. The upper zone contains the bulk of the mineralization and occurs within wispy-laminated (bioturbated), silty limestone (SDrm2) and bioclastic-rich silty limestone along the SDrm2/SDrm3 contact. Lower zone mineralization occurs at the contact between wispy-laminated, bioclastic-rich limestone (SDrm3) and planar-laminated silty limestone (Sdrm4).
West Leeville is located along the western margin of the Leeville Corridor, a broad (Y2 x 1 mile), northwesttrending horst, containing extensive decalcification and local 0.100 to 0.200 oz/st gold mineralization. The deposit occurs in the footwall of the West Bounding Fault, a north-northeast-striking, 60 degrees west-dipping fault zone with 150 feet of apparent normal displacement. The thickest and highest -grade portion of the deposit is located where the northwest-striking Rodeo Creek Fault intersects the footwall of the West Bounding Fault.
West Leeville ore occurs in grey to black, decalcified (calcite removed) and weakly to moderately silicified rocks composed of 60 to 70% quartz, 10 to 30% dolomite, 5 to 12% kaolinite, 2 to 4% illite and 2 to 4% pyrite. Ore can only be distinguished from waste by assay, as decalcification is far more extensive than mineralization. Bioclastic interbeds are preferentially silicified. Zones of strong silicification or strong decarbonatization (calcite and dolomite removed) are generally barren of +0.200 oz/st gold mineralization. Veins and breccias are rare within the deposit. Overlying micrite of the Popovich formation is generally silicified and largely barren of gold. The micrite or silicification may have acted as an impermeable and/or unreactive cap to the system during gold deposition. Quartz monzonite and lamprophyre dikes occupy fault zones within and marginal to the deposit. The dikes are altered but rarely contain +0.200 oz/st gold mineralization and more commonly bound gold zones within silty limestone of the Roberts Mountains Formation.
In summary, the West Leeville deposit is representative of the more strata-bound end members of Carlin Trend deposits, similar in style to the Carlin deposit. The West Leeville and Carlin deposits occur within a large (nearly 2 square miles), strata-bound gold system in the upper Roberts Mountains Formation with + 0.010 oz/st gold mineralization continuous between the deposits. Both West Leeville and Carlin occur in horsts between northeaststriking faults and the northwest -striking Leeville fault, with mineralization located in the immediate footwall of the northeast-striking fault. Like Carlin, the deposit is largely strata-bound, but high angle faults acted as feeders and boundaries to the gold system and played a critical role in localizing the deposit.
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Including past production, reserves and resources, the Carlin Trend forms the largest and most prolific accumulation of gold deposits in North America. More than 40 separate deposits have been delineated since disseminated gold mineralization in carbonate rocks was discovered in 1961. From this discovery, a classification for this style of gold mineralization has come to be referred to as “Carlin-type” deposits. To date, more than 25 million ounces of gold have been mined on the Carlin Trend from 26 separate operating, or past producing mines (Table 1 ). Open pit mining on the Carlin Trend began in 1965 at the Carlin Mine, and underground mining began in 1993 on the same deposit.
The scope of this paper is to first present a regional overview of the Carlin Trend, followed by summary descriptions of some of the more recent discoveries of deep, predominantly refractory gold deposits. As part of a concluding discussion, a spectrum of Carlin Trend deposits are categorized on a quaternary diagram to illustrate the I relative influence of structural and stratigraphic controls on each deposit.
The Carlin Trend is a 60 kilometer long north-northwest trending alignment of gold deposits located in northeastern Nevada, within the larger Great Basin physiographic province of the western United States (Figs. 1, 2). Gold deposits are hosted in a variable stratigraphic package of Ordovician through lower, Mississippian rocks. Within specific deposits, gold mineralization hosted in Cretaceous and Tertiary dike swarms and the Jurassic-Cretaceous Goldstrike granodiorite stock constitutes up to 15% of the mineralized material.
Regional Tectonic Development Regional stratigraphic and isotopic data indicate that northeastern Nevada was situated along a stable paleo-continental margin during much of the Cambrian through Early Mississippian (Stewart,1980). During this period, a westward-thickening, prism-shaped package of sediments were deposited from the outer margins of the paleo-continental shelf into an oceanic basin. Within this depositional environment, sedimentary facies graded from western eugeoclina1, to eastern miogeoclinal sequences.
During Late Devonian through Middle Mississippian time, eastward-directed compressional tectonism associated with the Antler orogeny resulted in regional scale folding and imbricate thrusting of the western eugeoclinal assemblage of predominantly siliciclastic rocks, over the eastern autochthonous assemblage of silty carbonate rocks (Roberts et al., 1958). The resultant accretionary mass formed the emergent Antler highlands which shed an eastward directed overlap assemblage of clastic rocks during Middle Mississippian to Early Pennsylvanian (Smith and Kettner,1975).
Late Paleozoic tectonism during Early to Middle Pennsylvanian time (Humboldt orogeny) was followed by deposition of shelf carbonate sequences during the Late Pennsylvanian and Permian (Smith and Kettner,1975; Kettner, 1977).