ORIGIN OF ORE TEXTURES: PORPHYRY COPPER DEPOSITS

By last count, there are more than 100 porphyry copper deposits in North America, but there is scant information on textures of ore minerals in this type of deposit, or in the other types of porphyry deposits as well. In this study, 76 polished thin sections were examined in reflected light from 8 porphyry copper deposits in Montana, Utah, Arizona, and New Mexico. In the giant Bingham Canyon deposit, Utah, primary copper sulfides of chalcopyrite and bornite responsible for the copper ore grade of 0.75%, are disseminated throughout the host quartz monzonite. In the primary ore, these textures are dominated by lattice intergrowths of chalcopyrite-bornite in the upper part of the deposit. They are also modified by small patches of chalcocite and covellite, and many are mantled by rims of chalcopyrite. In the deep parts of the deposit that are currently being mined, lattice intergrowths of bornite-chalcocite are most common. Intergrowths such as these have long been interpreted as the result of exsolution processes, and were first studied by George Schwartz in the 1930s. A much less common intergrowth texture in porphyry deposits, known as “chalcopyrite disease” (Barton, 1970; Eldridge, Barton, and Ohmoto, 1983; Craig and Vaughan, 1981) is also observed in some skarns and at the Continental mine in the Butte deposit, Montana. It is still uncertain whether the disseminated chalcopyrite in sphalerite is due to processes of replacement, exsolution, or penecontemporaneous growth. In the Arizona porphyry deposits, including those at Morenci, Ray, and Copper Cites, samples were collected mostly from the oxide and supergene enrichment zones. Ore textures in these samples are also classified as intergrowths, but consist mostly of replacements of pyrite and less commonly of chalcopyrite. In the replacement process, the secondary covellite and chalcocite commonly form rim replacements of the pyrite, which penetrate the grains along permeable fractures, or the cleavage planes of chalcopyrite (eg., Morenci). The same is true for the porphyry deposit at Santa Rita, New Mexico, where well developed veins of pure supergene chalcocite, as much as 0.6 cm wide, are observed.