The Strong and Harris project is held by Gunnison Copper Corp.(former Excelsior Mining Corp.) through its wholly owned subsidiaries Gunnison Arizona (former Excelsior Mining Arizona, Inc.) and Gunnison Holdings (former Excelsior Mining Holdings, Inc.).
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Summary:
The Strong and Harris copper-zinc-silver deposit is a sub-type of or related to a classic copper skarn (Einaudi and Burt, 1982; 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 southwestern US. They can be stand-alone copper skarns, which are generally small, or can be spatially and temporally closely associated with porphyry copper deposits, in which case they tend to be very large. The skarn at Strong and Harris and collectively in the Cochise mining district is presumably related to the Texas Canyon Quartz Monzonite, despite the intrusive itself hosting very little known economic mineralization. Mineralization in the quartz monzonite would require more specialized conditions involving the metal and volatile content of the magma, depth of emplacement, or other factors (Burt, 1977).
Copper skarns generally form in calcareous shales, dolomites and limestones peripheral or adjacent to the margins of diorite to granite intrusions that range from dikes and sills, to large stocks or phases of batholithic intrusions, and frequently are associated with mineralized intrusions. 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 intrusive. Retrograde evolution of the hydrothermal fluids produces actinolite-tremolite-talc-quartz-epidote-chlorite assemblages that overprint earlier garnet and pyroxene. Strong and Harris occurs approximately two miles north of any known occurrences of the Texas Canyon Quartz Monzonite intrusion in the Cochise mining district, which is thought to be the source of mineralizing hydrothermal fluids. Therefore, Strong and Harris can be subcategorized as distal skarn related to a porphyry copper system. This assumption is supported by the high abundance of wollastonite alteration in the mineralized zones. The anatomy of a telescoped porphyry copper system model by Sillitoe (2010) can be used as a conceptual model to understand the spatial relationship of the Strong and Harris distal skarn and associated proximal skarns in the district.
Primary copper-zinc-silver mineralization at Strong and Harris is characterized by lenses of sulfide minerals emplaced more-or-less parallel to layering in favorable lithologic units, usually along bedding planes or in disseminated masses and blebs. Some mineralization is disseminated in certain lithologies. Less frequently, the mineralization is hosted in quartz +/- calcite +/- feldspar veins. The mineralization is typically accompanied by calc-silicate alteration of the carbonate host-rock (described as “tactite” in the logs). In some local areas or sub-units, the mineralization completely replaced the host rock with massive lenses or patches of sulfide minerals, some of which are now oxidized. The sulfide minerals include pyrite, pyrrhotite, chalcopyrite, chalcocite, and sphalerite. Minor tetrahedrite group minerals have also been reported in the historical drill logs.
Sub-units of the Earp Formation, particularly those immediately below its upper contact with the Colina Limestone, were the most favorable sites for deposition of the copper, zinc and silver minerals. However, mineralization is also present in the Colina Limestone above the Earp, as well as in the Horquilla Limestone below the Earp. Historical reports often referred to mineralization in the Horquilla as the “Peabody Sill”, as such mineralization and its host rock were termed at the historical Peabody Mine southwest of the Strong and Harris deposit. The contact between the Horquilla and this sill at the Peabody Mine was reportedly favorable, at the mine although the sill itself is thin and represents only a volumetrically minor portion of that deposit. The same relationship is observed on the western side of the Strong and Harris property where the diabase sill has been logged in several holes and is often mineralized. The thickness of the sill is typically less than 10 feet. Mineralization in tactites of the Horquilla Limestone, either stratigraphically above or below the sill, is equally if not more important than the sill itself at Strong and Harris. However, the sill is a favorable host where present.
The Strong and Harris deposit has been oxidized to varying degrees that generally decrease with depth. Three oxidation zones are currently recognized in the deposit: the oxide zone, the transition (or mixed) zone, and the sulfide zone. In the oxide zone, copper is dominantly hosted in chrysocolla with minor azurite, malachite, and tenorite. Zinc minerals noted in the oxide zone include rosasite, aurichalcite, and willemite. Sulfide zone mineralogy is dominated by chalcopyrite and sphalerite with associated pyrite and pyrrhotite. In the transition (mixed zone), the mineralogy consists of secondary sulfides (namely chalcocite) mixed with a combination of the above oxide and sulfide zone mineralogy.