Summary:
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.
Gunnison Deposit Geology
The Gunnison deposit is covered by un-mineralized basin fill, varying between 300 and 800 feet in thickness. The mineralized Paleozoic host rocks below the basin fill strike approximately north-northwest and dip 20° to 45° eastnortheast. Baker (1953) recognized three sets of faults in the Johnson Camp area and similar faults have been interpreted in the Gunnison deposit area. These faults include the “Northeaster” (N10° to 30°E striking; 70° to 75° dip to the SE), “Easter” (N60° E to S60° E striking; 30° to 50° S and higher angle reverse faults dipping 75° S) and “Northwestern” orientations (N15° W strike; steep E or W dip). Only minor displacements are thought to have occurred in the Gunnison deposit area; however, numerous sheared and brecciated faults, generally filled with copper-oxide mineralization, cut through the deposit.
The Paleozoic host rocks have been intruded by the Texas Canyon quartz monzonite along the western margin of the deposit. The intrusion has formed wide zones of calc-silicate and hornfels alteration, as well as extensive low-grade copper sulfide mineralization within the Paleozoic rocks. Metamorphic alteration grading outward from the stock includes garnet-wollastonite-idocrase, diopside, tremolite and chlorite-talc (Kantor, 1977) (Figure 7-3). More specifically, the Martin Formation grades from a wollastonite-diopside-rich rock near the porphyry, to a distal diopside-tremolite-actinolite assemblage, and finally to dolomite. The Abrigo has garnet-actinolite-epidote-diopside alteration with some biotite hornfels near the porphyry, and this grades to a distal tremolite alteration leading into un-metamorphosed limey shale. Quartz-orthoclase-carbonate ± magnetite and chalcopyrite veins are characteristic of the lower Abrigo where it is mineralized.
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.
Texturally, pyrite and magnetite are later than, and replace, the skarn minerals, and chalcopyrite formed last. The magnetite occurs as disseminated 0.2 to 0.5 mm euhedral to anhedral grains and is closely associated with pyrite. Ninety percent of the magnetite is in the skarns and may compose up to five percent by volume of the rock. The disseminated magnetite and magnetite bearing veins are most likely what is giving the magnetic response for the deposit (Colburn and Perry, 1976).
Primary chalcopyrite-molybdenite disseminations and veins also 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.
Both oxide and sulfide mineralization exhibit strong fracture control. This fracturing and faulting are best developed in terms of width and close spacing in a zone around the intrusive contact, and this decreases away from the intrusive contact in the less altered rocks to the east. The initial formation of the skarn created denser minerals and liberated CO2 resulting in volume reduction, which created significant fracturing, and a consequent increase of porosity and permeability, allowing penetration by the later copper-bearing fluids. Weitz (1976) calculated a 30% volume reduction in the skarn-altered portions of the Abrigo and Martin formations at the Gunnison Deposit.
Oxide copper also exists within the transition zone. It mainly occurs along fractures and in quartz vein selvages as chrysocolla. Secondary supergene copper sulfide minerals such as chalcocite are often associated with the oxide mineralization in the transition zone. The transition zone is typically 100 feet to 200 feet in thickness and is strongly fractured and broken, similar to the oxide zone.