Summary:
The mine operates two open pits in two distinct areas; the Round Mountain Pit (RMP) and the Gold Hill Pit (GHP).
Round Mountain deposit
The Round Mountain Gold deposit is a very large, epithermal, low-sulfidation, volcanic-hosted, hot-springs type, precious metal deposit, interpreted to be located along the margin of a buried volcanic caldera. The deposit genesis is intimately associated with the Tertiary volcanism and caldera formation. Intracaldera collapse features and sympathetic faulting in the metasedimentary rocks provided the major structural conduits for gold-bearing hydrothermal fluids. In the volcanic units, these ascending fluids deposited gold along a broad westnorthwest trend.
Gold mineralization within the Round Mountain deposit occurs as electrum in association with quartz, adularia, pyrite and iron oxides. Shear zone fractures, veins and disseminations within the more permeable units (typically open pumice sites) host the mineralization. Primary sulfide mineralization consists of electrum associated with or internal to pyrite grains. In oxidized zones, gold occurs as electrum associated with iron oxides, or as disseminations along fractures.
Alteration of the volcanic units within the Round Mountain deposit can be characterized as a continuum from fresh rock progressing through chlorite, clay, sericitic+quartz, adularia+quartz+sericite, and quartz+adularia alteration assemblages. The alteration is zoned outward from potassic at the center to propylitic on the margin. There is a reasonable correlation between increasing gold grades and increasing degrees of alteration. The central ore zone is characterized by pervasive K-feldspar found replacing the rock groundmass, replacing primary sanidine, or as crystal growths in open-space.
Ore zones within the metasediments are more subtle, largely defined by secondary quartz overgrowths, pyrite and adularia associated with narrow northwest-trending structures.
Ore-grade mineralization is also present in the Stebbins Hill unit as silicified breccias with strong argillic alteration.
The Round Mountain deposit includes the Los Gazabo Vein, Keane Vein, Mariposa Vein sheeted zone, Great Western sheeted zone, Black Hawk sheeted zone, 921 Section, 471 Vein, and Fault Fissure vein. The veins were so closely spaced as to form sheeted zones; the dips of veins steepen and change with depth. Gold grade of ore has a positive correlation with the abundance of NW-striking joints. The ore-hosting tuff consists of a 400 to 500 foot thick non-welded base, a densely welded central portion about 800 feet thick, and a less densely welded vapor-phase altered top that is 75-100 feet thick. Where narrow fractures cut the lower, non-welded portion, mineralization occurs as thin quartz veinlets and as disseminations throughout the pumiceous tuff. In the welded portion, mineralization occurs wholly as veins along high angle fractures or along hydrothermally dilated low angle joints. Dip of the ain ore body is 15SW at cap, 85SW at base.
Gold is intergrown with vein quartz associated with limonite and minor manganese oxide in small fissures. Visible gold often occurs on projecting quartz crystals in drusy cavities and is distinctly crystalline, usually in fairly well-defined octahedral and more complex forms. Gold on adularia was observed in specimens from the rich veinlets on the top of Round Mountain.
Gold Hill deposit
The Gold Hill mine is a small deposit located near the Round Mountain mine. Gold Hill is approximately 3,000 feet long in the east-west direction and up to 2,600 feet in the north-south direction.
The Gold Hill deposit is located on the western margin of the Toquima Caldera Complex and straddles the fault boundary between the outcrop-dominated range to the east and the alluvium-covered pediment to the west. Approximately 70 percent of the Gold Hill area is covered by alluvium and pediment gravel which blankets the bedrock surface in the Gold Hill area. Paleo-channels and topographic breaks are evident in outcrop, trenches, drilling sumps, and in drill sections. The alluvium thickness ranges between 50 and 250 ft east of the Gold Hill site and reaches a thickness of 650-ft at the western-most extent of the project area. The alluvial sequence is characterized as poorly stratified with channel deposits, well-rounded cobble and boulder layers, sand and clay beds.
Bedrock outcrop in the Gold Hill area consists of poorly-to-moderately welded tuff referred to as the Gold Hill member. At depth, the welded tuff grades into a densely welded tuff referred to as the Surprise member, which crops out east of the Toquima Shaft. Mineralization in the Gold Hill area follows a generally east-west striking, steeply dipping fault-fracture set. This trend is cross-cut by the north-south trending range bounding structures.
Major rock types encountered within the Gold Hill Area include Tertiary igneous rocks of the southern Toquima Range. A rhyolite intrusion occurs at depth and below the Gold Hill main zone and crops out north and south of the Gold Hill Area along the margin of the Toquima Caldera Complex. This unit consists of devitrified, flow-banded rhyolite porphyry with feldspar and quartz phenocrysts and disseminated fine-grained pyrite.