Alteration, mineralization, and fluid evolution at the Eloise Cu-Au deposit, Cloncurry District, Northwest Queensland, Australia

Abstract

The Eloise deposit (3.1 million metric tons, Mt,5.5% Cu, 1.4 g/t Au and 16 g/t Ag) is hosted by Proterozoic rocks of the Soldiers Cap Group in the Eastern fold belt of the Mount Isa inlier. The deposit is characterized by very high grade, chalcopyrite-pyrrhotite-rich mineralization hosted by mafic silicate alteration. Alteration and mineralization lie within and adjacent to a major shear zone. The main metasomatism comprised three stages. Early pervasive albite alteration (stage I) was overprinted by stage II hornblende-biotite-quartz veins and alteration; stage III chalcopyrite, pyrrhotite, magnetite, pyrite, calcite, quartz, chlorite, and actinolite replaced and brecciated the earlier mafic silicates. The lodes occupy a narrow portion of a zoned, mineralized system that is at least 2 km in length. Magnetite-pyrite-rich mineralization occurs in the south, with chalcopyrite-pyrrhotite-rich mineralization in the central lode zone, and pyrrhotite-rich mineralization in the north. The system has a distinct chemistry, with enriched Cu, Au, Ag, Co, Ni, and Zn.Fluid inclusions in pre-, syn- and late- to postmineralization veins indicate an evolving fluid history. Primary inclusions and associated hornblende-biotite assemblages in premineralization veins indicate a high-temperature fluid (450 degrees -600 degrees C), and salinity estimates suggest it was an ultrasaline brine (32-68 wt % total salts). Fluid inclusions in stage III quartz in mineralized veins display a wide range of homogenization temperatures ( approximately 100 degrees -500 degrees C), and gangue assemblages (actinolite-chlorite-muscovite) suggest mineralization was a cooler event (200 degrees -450 degrees C). Inclusions associated with the mineralization have a lower salinity (30-47 wt % total salts) than do premineralization inclusions, and variable fluid inclusion types occur within the same clusters, which may reflect fluid mixing. The ultrasaline nature of the fluids and preliminary sulfur isotope results (0.4-2.0ppm) suggest the ore fluid was predominantly magmatic. Metals were carried as chlorocomplexes and H 2 S was the dominant sulfur species. Important factors in ore deposition were a decrease in temperature and salinity, and sulfidation of Fe-silicates. Eloise is interpreted to be a structurally controlled, distal, magmatic, hydrothermal vein deposit.