Summary:
The Wildcat and Mountain deposits are both low-sulphidation (quartz-calcite-adularia-illite) epithermal gold deposits within the Bimodal basalt-rhyolite assemblage in the northwestern Great Basin.
Wildcat Project Mineralization
Precious metal mineralization at Wildcat occurs with low-temperature silica, chalcedony and pyrite and can be best-described as epithermal precious metal mineralization. The entire known deposit has a footprint approximately 1,500 m long, 1,500 m wide and 150 m deep with some areas containing significantly higher Au mineralization than others. Principal controls on the mineralization are lithologic, high-angle faults, and the contact between the granodiorite and lapilli tuff breccia.
Precious metal mineralization is identified in two lithologies at Wildcat, the granodiorite and lapilli tuff breccia. Mineralization in the granodiorite is typically limited to discontinuous quartz veins that strike north-northeast, dip steeply (70° to 80°), display localized and intense acid-bleaching (kaolinization) in the adjacent host rock, and appear to occupy a set of faults shown to predate the bulk of magmatichydrothermal activity in the district. Typically, these veins range in thickness from 10 cm to 2.5 m.
The aforementioned veins are most-commonly observed at the southern part of the Project, near the historical patented “Big Hero Claim” and have been dubbed “Big Hero-type (BHT)” veins.
Alteration associated with the BHT veins and Cenozoic events is abundant on the south- and westfacing slopes of the Project; the northern reaches of the Project are covered by Cenozoic volcanic rocks. Broad zones (up to 50 vertical metres in places) of acid alteration (clay (kaolinite), jarosite, goethite, and hematite) are present in the granodiorite below the contact with the overlying rhyolite/lapilli blanket.
The prominent ridge east of Big Hero ridge hosts several historic prospects located on BHT veins. Where larger BHT veins are present, zones of acid alteration in the granodiorite up to two metres wide are common. One road cut on the southwest slopes of the Main Hill in the granodiorite, referred to as “Road Cut 65” by previous property owner Allied Nevada Gold, produced gold values as high as 3.4 g/t.
Abundant cm-scale (and smaller) veins, mineralization, and alteration across this zone are proposed to be the product of circulating fluids, driven by Cenozoic magmatism and hypabyssal rhyolite intrusions and associated feeder zones at Wildcat. Grus is common in this zone as well, driven by chemical weathering of granodiorite due to alteration from the mineralizing fluids associated with feeder zones at depth.
While the granodiorite does host mineralization at the Wildcat Project, the majority of the potentially economic gold mineralization is hosted in Cenozoic rocks, specifically the 14.8 Ma lapilli tuff breccia lithology. Mineralization largely occurs in rocks that post-date or are contemporaneous with the northeast-striking fault set and associated with the emplacement of rhyolite domes, outflows, eruptive material (lapilli) and hydrothermal breccia zones at the surface and shallow depths (<500 m and approximately 275 °C).
Gold (Ag) mineralization is relatively continuous within Cenozoic rocks, is present at the surface and is found in fine-grained, dark sulphide (reported as As-rimmed pyrite and electrum inclusions by Ford (1993)) and lesser amounts of free gold in breccia zones near the historical Wildcat Mine headframe (NBMG report: 1810-0002-85-3-272, pg. 41).
Observable Au-bearing sulphides in hand sample are common at silicified vents and breccia zones, though their abundance varies widely. The vents commonly host localized breccia, vein, and disseminated sulphide mineralization. Some of the vents may be marked on the surface by resistant, silicified rocks or buried by subtle depressions representing maars created at the time of eruption. Cleary (1994) and Ford (1993) provide thorough descriptions of mineralization in the rhyolite domes, breccias (vents), and veins at Wildcat. Ford (1993) reports grains of electrum common in sulphide mineralization in samples across the Project area.
Field mapping of the deposit shows that the main alteration types in mineralized-hosting Cenozoic rocks are silicification and clay alteration. One or both of these alteration styles are observed at nearly every outcrop mapped at the Wildcat deposit.
In the southern part of the Wildcat deposit, mineralization is spatially associated with the contact between the Cretaceous (102.2 Ma) granodiorite and the Cenozoic (14.8 Ma) lapilli tuff breccia. Numerous historical adits and shafts target this contact on the property. The principal low-grade zone that essentially encompasses all of the mineralization is tabular and dips gently to the southeast. The northwest and southern ends crop out, while the eastern end appears to weaken and die out or be cut off by post-mineral faulting.
Mountain View Project Mineralization
The mineralized zone at Mountain View has a roughly tabular shape striking towards the northwest and dipping steeply to the southwest. The mineralization occurs beneath unconsolidated alluvium, between approximately 400 and 1,000 ft below surface.
Two different styles of epithermal gold mineralization are recognized as occurring on the Project:
• Sheeted quartz veins within Permo-Triassic units at the old Mountain View mine.
• Multi-stage hydrothermal breccias and veins cutting Cenozoic rhyolites at the Severance deposit area.
Both styles of mineralization are interpreted to be the same age and are products of the same mineralizing event. Potassium-argon dating indicates that the age of mineralization is approximately 14 to 15 Ma.
Both types of mineralization are geochemically similar, with high arsenic, mercury and antimony levels, low base metal levels, and high silver to gold ratios of approximately 7:1. Petrographic and microprobe work by Homestake on high grade gold samples from the Severance deposit has identified abundant silver selenides and coarse grains of electrum.
The high-grade zones at the Severance zone occur along northwest and east-northeast trending structures.
Low sulphidation epithermal mineralization at the Severance deposit has been interpreted as a somewhat planar zone of low to moderate grade gold mineralization hosted primarily by the Severance Rhyolite. The zone has a roughly tabular shape striking toward the northwest and dipping steeply toward the southwest, roughly parallel with the interpreted orientation of the range-front fault. The mineralization occurs beneath the unconsolidated alluvium at the top of bedrock. Several small highgrade zones are interpreted as strongly structurally controlled and are completely encompassed by lower grade mineralization. They are interpreted to have generally northwest trending and northeast trending cross-cutting orientations.