Summary:
The Tonopah property contains a low-sulfidation epithermal gold system associated with near vertical quartz-adularia-gold veins hosted by Ordovician black argillite of the Palmetto Formation and Tertiary rhyolitic volcanics and also in association with a discontinuity at the contact with the top of the Palmetto Formation and lower sequence of the Tertiary volcanics. Gold bearing veins occur in a series of enechelon clusters along a 1.5-mile northwest-trending band of mineralization. Contact related gold mineralization is also seen along this entire band. The main altered and mineralized zones are overlain by alluvial gravels, sand dunes, and playa deposits.
Structural geology significantly influences the distribution of mineralization and alteration at Tonopah. The Rye Patch fault system is a complex, oblique-slip fault system with numerous northwest trending splays, believed to be associated with north-south trending compressional stress common in the Walker Lane structural trend. Subordinate steeply dipping, north-south striking extension fractures developed between the two bounding strike slip faults.
Detailed structural studies of bedrock exposures and oriented core from 22 drill holes indicate that alteration and mineralization developed between two moderately northeast dipping faults with rightlateral strike slip movement. Veins and hydrothermal breccias developed along sub-parallel, north-south extension fractures that terminate at the northwest faults.
Two overlapping mineralized trends have been identified in drilling. The primary trend runs parallel to the west-northwest Rye Patch Fault System, bearing 290-300 degrees over at least 3000 meters, and 500 meters width, and open along strike. Mineralization within this trend is generally within the lower portion of the tertiary volcanics, and sometime in the uppermost argillites, parallel to the Opa/ Tv contact and is generally low to moderate grade, from 0.1 ppm to 5 ppm Au.
Secondary extensional fractures range from 345 to 360 degrees strike, are near-vertical in dip, and host veins and hydrothermal breccias with higher grade mineralization, ranging from 1.0 to over 30 ppm Au. These extensional fracture zones are best represented in drilling in the Discovery and Dauntless zones.
A discontinuity has been identified in drilling at the top of the Palmetto formation, where tertiary volcaniclastics and ashfall tuffs disconformably overlay the argillite. Significant alteration and mineralization is localized within a low-angle zone which includes and often parallels the erosion surface of the Palmetto, as well as several facies in the tertiary volcanics, particularly where veins and mineralized structures intersect this contact zone.
It is interpreted that ascending fluids entering the contact zone deposit precious metals in a favorable chemical and textural horizon in the base of the tertiary volcanics. Mineralization within this zone trends parallel to the Rye Patch right-lateral oblique-slip fault system, with a general azimuth of 330 degrees. Higher grade gold mineralization and associated alteration have been identified in a series of north-striking extensional structural zones within the overall mineralized trend, including the Dauntless and Discovery Zones. Gold mineralization in the Dauntless and Discovery occurs in zones of massive quartzadularia alteration in volcanic and volcaniclastic rocks of the Tombstone Formation and in veins, breccias, and silicified faults in both the Tombstone Formation and the underlying Palmetto Formation. Quartz-adularia alteration in the Discovery Zone tends to extend laterally in the Tombstone Formation immediately above and parallel to nonconformable contact with the Palmetto Formation associated low grade disseminated gold mineralization. In the Dauntless Zone, the quartz-adularia forms a funnel-shaped zone that expands upward into the Tombstone Formation above the moderately dipping nonconformity.
Alteration outside of the quartz-adularia zones in the Tombstone Formation is characterized as strong argillic alteration, which persists to the limits of drilling to date. Oxidation is extensive, and only local relict patches of incompletely oxidized pyrite remain in the many of the altered areas.
Significant gold mineralization occurs within the quartz-adularia altered zones, with higher gold grades associated with a variety of siliceous veins, and veinlets including chalcedonic, bladed or drusy quartz, and quartz +/- iron oxide cemented breccias. In the Discovery Zone, to the southwest of the mineralized zones in the Midway Hills and northwest of the Dauntless Zone, there is a strong predominance of steeply dipping north-south trends in mineralized veins and structures of the Tombstone Formation (Rhys, 2003).
The Discovery Zone is the most densely drilled zone at the Tonopah property. Drill holes have intercepted a large number of veins, breccia-veins, and mineralized structures occurring in sub- parallel clusters 3 to 6 meters apart. According to MGC, vein and mineralized structure thicknesses vary from a few centimeters to over 6 meters, averaging 2 meters; Gustavson did not sufficiently review drill core and drill hole data to confirm that estimate. Continuity of veins, vein zones and structures is projected, but not certain, over approximate north-south strike lengths of 60 to 150 meters, and with vertical dimensions that may locally exceed 100 meters. Continuity of gold mineralization and gold grades coincides, approximately, with projections of the veins and structures, but becomes far less certain at progressively higher gold grade cut- offs. At lower cutoff grades, good continuity develops between zones, veins and structures, due largely to lower grade mineralization associated with the discontinuity contact between the Palmetto Formation and the overlying Tombstone volcanics. There is a tendency for well-defined veins in the Palmetto Formation to branch and splay upward into a broader network of veins, vein zones, veinlets in the overlying Tombstone Formation volcanics. Gold mineralization is associated with the veins, breccias and structures, and lower-grade mineralization also spreads laterally in a more disseminated fashion associated with quartz-adularia alteration in the Tombstone volcanics. The system remains open at depth in the Palmetto Formation for lack of sufficient deep drilling.
Visible gold is commonly observed in and along the edges of veins, is frequently associated with hematite, and occurs locally in coarse form. Dendritic gold has been observed in core.
Siliceous structures oriented similarly to those in the Tombstone Formation occur in the underlying Palmetto Formation. Veins hosted in the Palmetto Formation form well-defined discrete veins and hydrothermal breccias up to 2 meters wide according to MGC. Alteration in the Palmetto Formation is characterized by argillic alteration extending up to a few hundreds of meters below the nonconformity with the Tombstone Formation. Intense argillic alteration is typically limited to a zone within one to eight meters of the nonconformity, with gradual weakening of bleaching and clay alteration to greater depth. Locally, the zone of intense quartz-adularia alteration in the overlying Tombstone Formation may extend into the Palmetto Formation for a few feet (Rhys, 2003).
Alteration and mineralization at the Tonopah property are typical of low-sulfidation, volcanic hosted epithermal gold deposits found elsewhere in Nevada and around the world. The deposit type is characterized by overall low original sulfide content, and quartz-adularia and clay-sericite alteration assemblages, among others. Vein textures are indicative of high level, near surface emplacement and include void fills, crustiform coatings, colloform banding, and comb structures.