Summary:
The Gunnison Project (North Star) lies on the eastern edge of the Little Dragoon Mountains.
The North Star copper deposit is a classic copper skarn (Einaudi et al, 1980 and Meinert et al, 2005).
The North Star 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 North Star 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 North Star 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 diopsidetremolite-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 unmetamorphosed limey shale. Quartz-orthoclase-carbonate ± magnetite and chalcopyrite veins are characteristic of the lower Abrigo where it is mineralized.
At North Star, the mineralized formations strike approximately N10° to N40° W and dip from 30° to 45° NE. The strong regional trend of N10° to N30° E striking normal faults is overprinted by an abundance of N10° to N40° W striking reverse faults, joint sets, and normal faults which range in dip from 35° NE, sub-parallel to bedding, to 75° NE. The reverse faults strike parallel to the long axis of the deposit. Late-stage N70° E to S70° E striking vertical faults at the north end of the deposit contain local zones of high-grade copper-oxide mineralization. Porphyritic quartz monzonite intrusions occur along the western margin of the mineralization. At the southern end, the intrusion forms a sill between the Lower Abrigo Formation and the Bolsa Quartzite. At the northern end of the deposit, the intrusion commonly occurs as thin dikes and sills which cut the strata in numerous locations.
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.