REPORTED SUPERGENE SPHALERITE RIMS AT THE CHUQUICAMATA PORPHYRY DEPOSIT (NORTHERN CHILE) REVISITED: EVIDENCE FOR A HYPOGENE ORIGIN

Previous studies attributed a supergene origin to sphalerite rimming copper minerals occurring in Chuquicamata in a mineral assemblage including typically chalcocite-digenite (Cu1.85S1.12 and Cu1.99S1.01), covellite (CuS and Cu1.08S0.92), and sphalerite (up to 1.2 wt % Fe). Microscopic observations on samples from a central and a southern section, completed by scanning electronic microscope (SEM) backscattering electron imaging and electron microprobe analyses, suggest that all sphalerite in Chuquicamata is hypogene. A scenario, backed by observations in each step, that explains the formation of the peculiar sphalerite rims is the following: (1) precipitation of chalcopyrite, typical of the early and main hydrothermal stages; (2) precipitation of sphalerite rimming chalcopyrite and in voids and in weakness sites during the late hydrothermal stage; (3) partial or total replacement of chalcopyrite by chalcocite-digenite during the late hydrothermal stage; and (4) formation of lamellar covellite, principally at the expense of chalcopyrite, suggesting increasingly oxidizing and/or acidic conditions. This covellite may be linked to supergene processes and/or to the late hydrothermal stage. The findings of the present work have implications for the position of the lower limit of the supergene enrichment in the eastern-southern part of the deposit, as the sphalerite rims, now interpreted as hypogene, were the only potential supergene sulfide in the eastern vertical section of the southern section.