INVESTIGATION OF SUPERGENE PROCESSES AT THE RESOLUTION COPPER DEPOSIT

The Late Cretaceous-age Resolution Cu-Mo porphyry system, Superior District, Arizona, is hosted in Precambrian- and Paleozoic-age sedimentary and intrusive rocks. This study investigates the supergene profile developed as a consequence of apparent early Tertiary uplift and exhumation of the Resolution hydrothermal system. The deposit is characterized by a thick (up to 250 meters), well-developed, hematite-dominant leach capping and comprise variably-preserved leached capping intervals represented by iron oxides with well-developed cellular textures, and apparent hypogene sulfide mineralization. These hypogene sulfides comprise a high-sulfidation assemblage of pyrite with chalcopyrite, chalcocite-digenite, and bornite; mineral intergrowths of these sulfides are indicative of hypogene textures, and we interpret reaction rims around sulfide minerals to represent hypogene copper enrichment. Although the well-developed leached capping represents significant metals oxidation and consequent mobility, the subjacent sulfide zone assemblages display textures characteristic of hypogene enrichment, with no indication of supergene enrichment. Integrated ⁴⁰Ar/³⁹Ar alunite ages of 48.5 ± 0.3 Ma and 40.0 ± 0.2 Ma suggest prolonged (?) or multiple-cycle (?) oxidation of the Resolution porphyry system; additional alunite ages are expected to resolve the timing and duration of Resolution oxidation and metals transport. Sulfur isotope δ34S values for pyrite vary from -3.8‰ to -0.4‰, while coexisting anhydrite δ34S vary from 7.0‰ to 13.7‰; these values are consistent with a magmatic sulfur source. Alunites, however, are characterized by δ34S values from 1.7‰ to 4.0‰; these values are not consistent with those of anhydrite for a hypogene origin, nor consistent with (the) pyritic sulfur for a supergene origin. As additional alunite analyses become available we hope to resolve the issue of alunite origin(-s).We conclude initially that there exists no evidence for supergene sulfide textures or enrichment in rock volumes subjacent to the well-developed leached capping. As such, we suggest that the abundance of hematite with only minor goethite and trace jarosite in the leached capping, especially within abundant cellular-boxwork structures, suggests multiple cycles of leaching and transport of copper.