Fluid and mass transfer during metabasalt alteration and copper mineralization at Mount Isa, Australia

This study aims to understand the large-scale hydrothermal processes that determined the chemical composition of syn to postmetamorphic ore brines, prior to being focused into chemically anomalous metasediments that acted as a chemical trap for localized copper precipitation. Field mapping of the metabasalts provided the framework to determine the relative timing of district-scale alteration events, which were then studied by mineralogic observations and bulk mass balance measurements. The timing and fluid-tracer observations in conjunction with thermodynamic mass transfer modeling permit correlation between district-scale alteration processes and the mine-scale hydrothermal events associated with copper deposition at Mount Isa. The regional feature of greatest significance to ore formation at Mount Isa, and to exploration for similar high-grade copper deposits elsewhere, is not the somewhat elevated initial copper content of the metabasalts nor their locally significant copper depletions, but their moderatley high oxidation state evident from the ubiquitous occurrence of Fe-rich epidote and the large-scale Fe oxide-altered fracture zones. -from Authors