Summary:
Deposit Type
Three Hills Mine is a low-sulfidation, epithermal gold deposit, and occurs in a zone of pervasive silicification within the outcropping Siebert Formation immediately above and along the contact with the underlying Fraction Tuff. Mineralization occurs in discontinuous, irregular 0.05in to 0.5in wide veinlets, vein stockworks, and erratic breccia veins of chalcedony and quartz. Oxidation has destroyed sulfide minerals within the deposit. The currently drilldefined extent of mineralization is approximately 1,000ft east–west by 2,700ft north–south with a maximum depth of 500ft. Mineralization remains open at depth, down-dip to the east along the Siebert/Fraction Tuff contact.
Hasbrouck Mine is a low-sulfidation, epithermal gold–silver deposit located in the western portion of the Divide Mining District. Host rocks are primarily tuffs and sediments of the Siebert Formation with limited mineralization within the underlying Fraction Tuff. An erosional remnant of silica sinter, deposited during hot spring activity, has been mapped near the top of the mountain. Gold and silver mineralization consists principally of 0.1in to 1.0in wide, discontinuous silica-pyrite veinlets, sheeted veinlets and stockworks, all closely associated with larger, but erratic bodies of hydrothermal breccia. Sulfide minerals have been largely oxidized. Mineralization is accompanied by strong pervasive silicification, with associated adularia and pyrite, and has a known extent of 2,800ft east–west by 2,400ft north–south, with a maximum depth of 900ft. Mineralization is open at depth and to a limited extent to the northwest and east.
Hill of Gold, located in the Tonopah Mining district, is a low-sulfidation, epithermal gold deposit, and occurs in two settings, the first within a northeast trending structural zone along long narrow “lenses”, and second is at or near the Oddie-Siebert and Oddie-Fraction contact.
Mineralization
Gold and silver mineralization at Hasbrouck appears to have been emplaced at very shallow depths, and is associated with young hot spring-related systems with siliceous sinter deposits. In addition, the mineralized zones are typically associated with silica ±adularia replacement of volcaniclastic host rocks. These types of deposits contain characteristically low concentrations of both total sulfide minerals and base metals.
Mineralization in the near-surface, epithermal environment takes place in and beneath hot springs, and the slightly deeper underlying hydrothermal conduits. Normal faults, margins of grabens, coarse clastic caldera moat-fill units, radial and ring dike fracture sets, and hydrothermal and tectonic breccias can act as hydrothermal fluid channeling structures. Throughgoing, branching, bifurcating, anastomosing and intersecting fracture systems are commonly mineralized. Mineralization commonly forms where dilatational openings and cymoid loops develop, typically where the strike or dip of veins change.
Mineralization characteristically comprises pyrite, electrum, gold, silver, and argentite. Other minerals can include silver sulphosalt and/or selenide minerals, chalcopyrite, sphalerite, and galena. Tellurides, roscoelite and fluorite are abundant in alkalic-rock hosted systems, which may include significant molybdenite as an accessory mineral.
Gold mineralization at Three Hills is commonly associated with areas of higher permeability lithologies, rock unit contacts, and structural features. Previous authors have described the mineralization as “disseminated”though examination of outcrops and drill core shows the higher gold grades associated with discontinuous, irregular 0.05- to 0.5inch-wide veinlets, vein stockworks, and erratic breccia veins of chalcedony and quartz. Lower gold grades in the top of the south hill are found in zones of grey to brown chalcedony, and hydrothermal breccia veins and pipes.
Hasbrouck mineralization has a 2,800ft east–west by 2,400ft north–south areal extent, with a maximum depth of 900ft. Mineralization is open at depth and to a limited extent to the north and east. Precious metals mineralization at the Hasbrouck Mine is concentrated within the Siebert Formation, stratigraphically below the chalcedonic sinter horizons that outcrop near the peak of Hasbrouck Mountain. The overwhelming bulk of mineralization lies within the Main zone. Weakly mineralized, sub-parallel structures occur between the Main and South zones.
Hasbrouck mineralization is primarily characterized as structurally controlled, with secondary lithologic control. Westnorthwest and northeast fault orientations localize higher-grade mineralization (+0.05 oz Au/ton) with lower grade material occurring as halos and straddling the clastic and tuffaceous package contact within the Siebert Formation. North-south faults appear to be late-stage or offset mineralization. Mineralization remains open at depth along the intersection of the crosscutting structural fabrics.