Summary:
The mine is located in the Battle Mountain-Eureka-Cortez mineral belt in a large window of lower-plate carbonate rocks surrounded by upper-plate rocks. The lower-plate carbonates consist of (from oldest to youngest) an east-dipping section of Silurian Lone Mountain Dolomite, Devonian McColley Canyon Formation, Devonian Denay Formation, Devonian Devils Gate Limestone, and Devonian Horse Canyon siltstone. Gold mineralization is hosted primarily in the Bartine Member of the McColley Canyon Formation, which consists of carbonate wackestones and packstones approximately 250 to 380 feet thick. Minor amounts of mineralization are found in the underlying dolomitic limestone Kobeh Member of the McColley Canyon Formation where it is adjacent to apparent feeder structures
Mineralization in the Gold Bar District has most of the characteristics of Carlin-type gold deposits described in Muntean and Cline (2018), including: carbonate host rocks, tectonic setting, structural and stratigraphic ore controls, hydrothermal alteration consisting of dissolution and silicification of carbonate and argillization of silicates, auriferous arsenian pyrite, a geochemical signature containing Au-As-Hg-Sb-Tl, low Ag (Ag/Au<1), and lack of clear relationship with intrusions. Gold mineralization in the Gold Bar district is localized at the intersections of a complex array of structures with permeable and reactive strata. Carbonate dissolution, argillization of silicates, sulfidation of ferroan minerals and silicification of limestone characterize the alteration assemblage related to the main stage of mineralization. Gold is found as sub-micron inclusions or solid solution in arsenian pyrite. Oxidation has liberated gold from the original pyrite, making it amenable to cyanide leaching. Common trace elements include arsenic, antimony, thallium,and mercury.
Within the Gold Bar North (GBN) area, three gold deposits have been defined: Gold Pick, Gold Ridge and Cabin Creek. Mineralization in Gold Pick has a strike length of over 4,000 ft with a width of 1,600 ft and thickness of 100 – 150 ft. Gold Ridge and Cabin Creek are ancillary deposits comprising together approximately 22% of the mineral resource. All defined mineralization lies within 500 ft of surface in oxidized carbonate host rocks.
There are two structural blocks in the district, separated by the regionally extensive RMT. Deep-water shale and chert, known as the Vinini Formation (upper plate) were pushed eastward onto the carbonates (lower plate) along the RMT. Late-Paleozoic clastic sedimentary rocks were deposited on both upper and lower plates. Younger (post-Permian) lowangle faulting locally placed lower plate rocks on top of both the upper and lower plates. Tertiary extension resulted in the complex basin and range high-angle block faulting that defines the range today.
Significant Mineralized Zones
Gold Bar North
The Gold Pick, Gold Ridge and Cabin Creek deposits are found in the McColley Canyon Formation.
Mineralization is controlled both by stratigraphy and by overprinting structures. Much of the mineralization in all three deposits is at an orientation that mimics bedding and probably represents favorable permeability in certain debris flow subunits that were selectively more receptive to gold deposition. At Gold Pick East this bedding orientation is N50°E, 23°SE, at Gold Pick West it is N07°E, 24°E, at Gold Ridge it is N20°E, 20°SE and at Cabin Creek it is N01°E, 27°E.
Mineralization also tracks a strong northeast-southwest fabric that is supported by structures identified during surface mapping. These southwest-trending structures are at moderate to steep angles (45-70°NW). At Gold Pick East, the high-angle structures focus grade along on N245°E, 45-60°NW. At Gold Ridge, the orientation is N265°E, 70°NW. The highest gold grades in the deposit are concentrated along these structures likely at the intersection with low-angle stratiform mineralization.
All of the mineralization found to date at the GBN occurs as sediment-hosted, “Carlin-type” gold deposits. The deposits are hosted in carbonate-rich sedimentary rocks of the Devonian Nevada Formation and are characterized by micronsized gold and a distinct hydrothermal alteration suite.
If the term “Carlin-type gold deposits” is used in its strictest sense, the deposits are restricted to a small part of the North American Cordillera in northern Nevada and northwest Utah. Most such deposits are located along long-lived, deep crustal structures inherited from Late Proterozoic rifting and formation of a passive margin. They are hosted in a Paleozoic miogeoclinal carbonate sequence that is either structurally overlain by a eugeoclinal siliciclastic sequence (the Roberts Mountains Allochthon), or stratigraphically overlain by a miogeoclinal siliciclastic sequence deposited in the resulting foredeep (Overlap Assemblage). Gold mineralization is localized at intersections of a complex array of structures with permeable and reactive strata. Carbonate dissolution, argillization of silicates, sulfidation of ferroan minerals and silicification of limestone characterize alteration related to the main stage of mineralization. Gold is found as micron inclusions or solid solution in arsenian pyrite. Common trace elements are antimony, thallium and mercury.
Gold Bar South
The lower 100-200 ft of the overlying Mississippian Webb Formation (siltstone, mudstone), hosts the bulk of the GBS (Gold Bar South) gold mineralization.
The most economically prospective mineralization at GBS occurs at the crest of an east-tilted doubly plunging anticline, interpreted from consistent Devils Gate – Webb Formation contacts recorded in drill logs. Overall the anticline is tilted, with a steeper 25° dipping east limb and a flat west limb. The anticline plunges north at approximately 10° and plunges steeper to the south at approximately 20° before it is ultimately covered by deep basin-filling alluvium.
Mineralization occurred at the base of the Webb Formation and was subsequently offset by a series of east-side-downstepping northeast and northwest-trending normal faults. Mineralization is rarely preserved along these high-angle faults, though undoubtedly some of them served as conduits to fluid flow during the gold event. Locally, normal faulting has created small grabens, in which the thickness of mineralization appears to be enhanced. There is evidence, by reconstructing grade trends of some left- lateral oblique slip on northeast-trending faults, especially in the southern part of the deposit. Lastly, and similar to Gold Bar North, there appears to be some “ponding” of mineralization in troughs of the Devils Gate – Webb Formation contact. These are interpreted as potential karst dissolution zones or debris flows with enhanced permeability.
Gold mineralization at GBS was deposited in brecciated siltstones of the Webb Formation, at and immediately above its contact with the Devils Gate Limestone. Lesser, but important amounts of mineralization occur in the adjacent Devils Gate Limestone as well. Mineralization in GBS consists of epithermal, disseminated, sediment-hosted gold, in zones related to hydrothermal dissolution in limestone and the resulting collapse breccia in the overlying siliciclastic unit. Gold is associated with brecciated, oxidized, silicified, and argillized mudstones, siltstones, and sandstones of the Webb Formation and is usually accompanied by silicification and strong barite veining. Jasperoid along the trace of the fault is brecciated and contains veins of barite and scattered gold mineralization.