Summary:
Copper Flat is a porphyry copper-molybdenum deposit that is approximately 1,400 by 2,100 ft in plan occurring within a small structurally controlled quartz monzonite stock that has intruded a circular block of andesite approximately 4 miles in diameter. The porphyry includes a variably mineralized west-northwest-trending hydrothermal “breccia pipe,” that lies immediately south of the Patten fault, that is about 1,400 ft long and 500 ft wide.
Copper Flat has been categorized as an alkalic copper-gold bearing breccia pipe, surrounded by and genetically linked to an alkalic porphyry system. The deposit is situated along the eastern edge of the Cretaceous Arizona-Sonora-New Mexico porphyry copper belt and along with Tyrone, New Mexico, forms a linear mineralized feature known as the Santa Rita lineament (SRK, 2010; McLemore et al., 2000). Analogous deposits include Terrane Metal’s Mount Milligan, British Colombia deposit and the Continental breccia pipe located in the Central Mining district of New Mexico (SRK, 2010).
Although copper occurs almost exclusively as chalcopyrite locally accompanied by trace amounts of bornite, minor amounts of chalcocite and copper oxide minerals are locally present near the surface and along fractures. The supergene enrichment typical of many porphyry copper deposits in the Southwest is virtually non-existent at Copper Flat. During the early mining days, a 20- to 50-ft leached oxide zone existed over the ore body, but this material was stripped during the mining activities that occurred in the early 1980s. Most of the remaining ore is unoxidized and consists primarily of chalcopyrite and pyrite with some molybdenite and locally traces of bornite, galena and sphalerite. Recently completed mineralogical studies indicate that fine grained disseminated chalcopyrite is often intergrown with pyrite and occurs interstitial to silicate minerals. Bornite and digenite coat and locally replace chalcopyrite but also occur interstitially to chalcopyrite and the silicates. Although deposition of chalcopyrite and molybdenite (76.2 Ma) was within the same mineralizing event with minor pyrite, bornite-digenite appears to represent a weaker and slightly later event of continuous copper mineralization.
Although low concentrations of gold (less than 100 parts per billion [ppb]) and silver (<2.0 ppm) occur throughout the deposit, intervals of higher gold concentrations (150 to more than 2,000 ppb) may be structurally controlled and are more commonly present in the eastern half of the deposit.
The sulfide mineralization first formed in narrow veinlets and as disseminations in the quartz monzonite with weakly developed sericitic alteration. This stage of mineralization was followed by the formation of the breccia pipe with the introduction of coarse, clotty pyrite, chalcopyrite, as well as molybdenite with strong potassic alteration either biotite or k-spar dominant.
The “breccia pipe”, which can best be described as a crackle breccia, consists largely of subangular fragments of mineralized CFQM, with locally abundant mineralized latite where dikes exposed in the CFQM projected into the brecciated zone that range in size from an inch to several inches in diameter. Andesite occurs only as mixed fragments partially in contact with intrusive CFQM and appears to represent the brecciation of relatively unaltered andesite xenoliths in the CFQM. The matrix contains varying proportions of quartz, biotite (phlogopite), potassium feldspar, pyrite, and chalcopyrite, with magnetite, molybdenite, fluorite, anhydrite, and calcite locally common. Apatite is a common accessory mineral. Breccia fragments are rimmed with either biotite or potassium feldspar, and the quartz and sulfide minerals have generally formed in the center of the matrix. It should be noted that the biotite breccia is gradational into both the quartz-feldspar breccia as well as the CFQM.
The total sulfide content ranges from 1 percent (by volume) in the eastern part of the breccia pipe and the surrounding CFQM to 5 percent in the CFQM to the south, north, and west. Sulfide content is highly variable within the breccia, with portions in the western part of the breccia containing as much as 20 percent sulfide minerals. Based on drilling, the strongest copper mineralization is concentrated in the western half of the breccia pipe and in the adjoining stockwork veined CFQM in the vicinity of the intersection of the Patten fault and the Hunter fault zone. Sulfide mineralization is concentrated in the CFQM and breccia pipe and drops significantly at the andesite contact. Minor pyrite mineralization extends into the andesite along the pre-mineral dikes and in quartz-pyrite-bearing structures, some of which were historically prospected for gold.
Molybdenite occurs in some steeply dipping quartz veins or as thin coatings on fractures. Minor sphalerite and galena are present in both carbonate and quartz veinlets in the CFQM stock. Evaluations of the mineralization at Copper Flat indicate that copper mineralization concentrates and trends along the N50°W structural influences, whereas the molybdenum, gold and silver appear to favor a N10°-20°E trend.