Summary:
Deposit Type
The Gunnison Deposit is a classic copper skarn (Einaudi et al, 1980 and Meinert et al, 2005). Skarn deposits range in size from a few million to 500 million tonnes and are globally significant, particularly in the American Cordillera. They can be stand-alone copper skarns, which are generally small, or can be associated with porphyry copper deposits and tend to be very large. The Gunnison Deposit is large, at the upper end of the range of size for skarn deposits and is likely associated with a mineralized porphyry copper system that has not been discovered.
Copper skarns generally form in calcareous shales, dolomites, and limestones peripheral or adjacent to the mineralized porphyry. Copper mineralizing hydrothermal fluids are focused along structurally complex and fractured rocks and convert the calcareous shales and limestones to andradite rich garnet assemblages near the intrusive body, and to pyroxene and wollastonite rich assemblages at areas more distal to the stock. Retrograde hydrothermal fluids produce actinolite-tremolite-talc-silica-epidote-chlorite assemblages that overprint earlier garnet and pyroxene. The mineralization is typically pyrite-chalcopyrite-magnetite proximal to the mineralizing porphyry and chalcopyrite-bornite more distally from the body.
Copper-zinc skarns are important in the region and have been historically mined from the Republic, Copper Chief, Moore, and Mammoth mines from underground operations (Baker, 1953). These copper and zinc rich skarns are probably more distal to the mineralized porphyry, whereas the Gunnison and South Star skarns contain only Cu and are proximal to the mineralizing porphyry system. Tungsten and minor lead-silver-gold have also been produced in the district (Cooper and Silver, 1964).
Mineralization
Within the Project area the important mineralized host rocks include the Abrigo and Martin Formations and, to a lesser extent, the Horquilla Limestone, and the lower parts of the Escabrosa Limestone. Mineralization is also found in the Bolsa Quartzite and Precambrian basement rocks. Copper mineralization is related to calc-silicate skarns that have replaced these carbonate rocks adjacent to the Texas Canyon quartz monzonite (TQM).
Oxidation has occurred to a depth of approximately 1,600 feet and has resulted in the formation of dominantly chrysocolla with minor tenorite, copper oxides, and secondary chalcocite. Copper-oxide mineralization is present in the calc-silicate skarns as fracture coatings and vein fillings mainly in the form of chrysocolla. The remainder of the oxide mineralization occurs as replacement patches and disseminations. Copper-oxide mineralization extends over a strike length of 11,100 feet, has an aerial extent across strike of up to 3,000 feet and is more than 900 feet thick in places.
Copper sulfide mineralization has formed preferentially in the proximal (higher metamorphic grade) skarn facies, particularly within stratigraphic units such as the Abrigo and Martin Formations, and within structurally complex zones. There are three types of sulfide mineralization within the skarns. In decreasing order of abundance, these are fracture coatings and vein fillings, distinct quartz-orthoclase-carbonate ± magnetite and chalcopyrite veins 0.2 to 10 cm wide (Weitz, 1976), and disseminations. The veins have retrogressive haloes of chlorite, actinolite and epidote. Primary mineralization also occurs as stringers and veinlets of chalcopyrite and bornite.
Primary chalcopyrite-molybdenite disseminations and veins occur in the mineralized porphyry below and to the west of the skarn mineralization at the Gunnison Deposit. Only nine drillholes intersected the quartz monzonite over significant lengths (lengths > 100 feet). Most were mineralized with a best interval of 289 feet averaging 0.31% Cu and 0.028% Mo, including 30 feet at a grade of 1.35% Cu. This mineralization has never been fully assessed.