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.