Monitoring fluid chemistry in iron oxide–copper–gold-related metasomatic processes, eastern Mt Isa Block, Australia

Abstract

Most researchers in the Proterozoic eastern Mt Isa Block, NW Queensland, Australia, favour magmatic fluid and salt sources for sodic-(calcic) alteration and iron oxide-copper-gold mineralization. Here we compare spatial, mineralogic and stable isotope data from regional alteration assemblages with magmatic and magmatic-hydrothermal interface rocks in order to track chemical and isotopic variations in fluid composition away from inferred fluid sources. Tightly clustered δ~(18)O values for magnetite, quartz, feldspar and actinolite for igneous-hosted samples reflect high temperature equilibration in the magmatic-hydrothermal environment. In contrast, these minerals record predominantly higher δ~(18)O values in regional alteration and Cu-Au mineralization. This dichotomy reflects partial equilibration with isotopically heavier wallrocks and slightly lower temperatures. Increases in Si concentrations of metasomatic amphiboles relative to igneous amphiboles in part reflect cooling of metasomatic fluids away from igneous rocks. Variations in X_(Mg) for metasomatic amphiboles indicate local wallrock controls on amphibole chemistry, while variations in X_(Cl)/X_(OH) ratios for amphiboles (at constant X_(Mg)) indicate variable aH_2O/aHCl ratios for metasomatic fluids. Biotite geochemistry also reflects cooling and both increases and decreases in aH_2O/aHCl for fluids away from plutonic rocks. Decreased aH_2O/aHCl ratios for metasomatic fluids reflect in part scavenging of chlorine out of meta-evaporite sequences, although this process requires already saline fluids. Local increases in aH_2O/aHCl ratios, as well as local decreases in δ~(18)O values for some minerals (most notably haematite and epithermal-textured quartz), may indicate ingress of low salinity, low δ~(18)O fluids of possible meteoric origin late in the hydrothermal history of the region. Taken together, our observations are most consistent with predominantly magmatic sources for metasomatic fluids in the eastern Mt Isa Block, but record chemical and isotopic variations along fluid flow paths that may be important in explaining some of the diversity in alteration and mineralization styles in the district.