Summary:
The Tonopah property contains a low-sulphidation epithermal gold system characterised by nearly vertical quartzadularia-Au veins within argillite of the Op (Ordovician Palmetto). Folding within the argillite can locally extend and enhance vein formation. These quartz-adularia-Au veins continue into the overlying Tertiary volcanic and volcaniclastic rocks, but a broad zone of lower grade, disseminated mineralization extends well beyond the actual veins. The rhyolitic composition of the tuffs and tuffaceous clastic rocks, along with the increased permeability of the volcaniclastic rocks, allows hydrothermal fluids to penetrate tens to hundreds of metres beyond the large veins.
The nonconformity between the Op argillite and the overlying Tertiary volcanics varies; in some areas it appears as a clear fault, while in others it resembles a paleosol. Where the paleosol is preserved, it appears as a deeply weathered rubble zone dominated by boulders of Op argillite and derived soil, grading upwards to a mixed zone of argillite and volcanic debris, potentially indicating the onset of Tertiary volcanic activity. When mineralized, this zone forms a shallowly dipping, manto-like area of mineralization similar to those found in the overlying Tertiary volcanic sequences.
High-grade Au-bearing quartz-adularia veins occur in a series of north-to-north northwest-striking en-echelon clusters along a 2.4 km northwest-trending band of mineralization. In the Op argillite, there is minimal alteration and disseminated mineralization within the wall rock along the margins of the major quartz-adularia veins, but in the Tertiary volcanic and clastic rocks, the wall rock has extensive alteration and disseminated mineralization. The primary altered and mineralized zones are overlain by alluvial gravels, sand dunes, and playa deposits.
The Op is the oldest and deepest unit encountered in drill holes at Tonopah. In the drilled area, the Op is comprised of argillite, quartzite, siltstone, and chert. Bedding dips moderately, ranging in direction from northeast to northwest in oriented drill core measurements. Pre-Tertiary deformation produced tight to isoclinal folds and a crenulation cleavage in Op rocks; overlying Tertiary volcanic rocks are unaffected.
Deposit Type
The Tonopah deposit type is characterized by overall low original sulphide content, and quartz-adularia and claysericite 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. Similar deposits in Nevada have proven to be economic, including the Round Mountain, Midas and Bullfrog deposits.
Mineralization and Alteration
Two overlapping mineralized trends have been identified in drilling. The primary trend runs parallel to the westnorthwest Rye Patch Fault System, bearing 290-300 degrees over at least 3,000 m, and 500 m width, and open along strike.
Significant alteration and mineralization are localized within a low-angle zone near the TVL/Op nonconformity, which includes and often parallels the erosion surface of the Op, 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, depositing precious metals in a favourable horizon in the base of the Tertiary volcanics.
There are five main mineralization zones. The highest-grade mineralization occurs in the Discovery and Dauntless zones. In the western part of the deposit in the Discovery and Dauntless zones, and the down-drop side of the Discovery Fault (121 zone), mineralization occurs above and below the TVL/Op nonconformity. In the Rye Patch area in the eastern portion of the deposit, mineralization primarily occurs in the TVL unit. Further to the west is a minor mineralization zone with limited drilling, named Midway Hills.
Au mineralization is concentrated in secondary extensional fractures that 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, primarily in zones of massive quartz-adularia alteration in volcanic and volcaniclastic rocks and in veins, breccias, and silicified faults. Higher Au grades are associated with a variety of siliceous veins, and veinlets, including chalcedonic, bladed or drusy quartz, and quartz +/- iron oxide cemented breccias; these textures all suggest boiling was occurring. In the Dauntless Zone, the quartz-adularia forms a funnel-shaped zone that expands upward into the Tombstone Formation above the moderately dipping nonconformity.
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 three to six m apart. Vein and mineralized structure thicknesses vary from a few cm to over six m, averaging two m. Continuity of veins, vein zones and structures are projected, but not certain, over approximate north-south strike lengths of 30 to 100 m, and with vertical dimensions that may locally exceed 100 m. Continuity of Au mineralization and Au grades coincide, approximately, with projections of the veins and structures, but becomes far less certain at progressively higher Au grade cutoffs. At lower COGs, good continuity develops between zones, veins and structures, due largely to lower grade mineralization associated with the nonconformity between the Op and the overlying TVL. There is a tendency for well-defined veins in the Op to branch and splay upward into a broader network of veins, vein zones, veinlets in the overlying TVL. Visible Au is commonly observed in and along the edges of veins, is frequently associated with hematite, and occurs locally in coarse form. Dendritic Au has been observed in core.
In the Discovery Zone, a vertical sequence of veining is observed in the extensive K-feldspar-quartz alteration zone in the Tertiary sequence. Upper parts have rare veinlets of opaline to chalcedonic quartz, sometimes with fine-grained drusy quartz-lined cavities. Below this, significant Au values are present within and above a zone containing bladed quartz veins and veinlets with lattice-like textures of quartz after calcite. This two to seven m thick, shallow northeast-dipping textural zone indicates a boiling level in the hydrothermal system. Chalcedonic quartz veinlets become more common downward toward the Op nonconformity.
Alteration outside of the quartz-adularia zones in the Tombstone Formation is characterized as strong argillic alteration. Oxidation is extensive, with local relict patches of incompletely oxidized pyrite in altered areas.
Alteration and mineralization at the Tonopah property are typical of low-sulphidation, volcanic-hosted epithermal gold deposits found elsewhere in Nevada and around the world. They are typically associated with rhyolitic to andesitic volcanic centers which have formed in the shallow crustal settings. Mineralization is often structurally controlled.