Carbon and oxygen isotope constraints on fluid sources and fluid-wallrock interaction in regional alteration and iron-oxide-copper-gold mineralisation, eastern Mt Isa Block, Australia

Abstract

The source of metasomatic fluids in iron-oxide-copper-gold districts is contentious with models for magmatic and other fluid sources having been proposed. For this study, delta 18 o and delta 13 C ratios were measured from carbonate mineral separates in the Proterozoic eastern Mt Isa Block of Northwest Queensland, Australia. Isotopic analyses are supported by petrography, mineral chemistry and cathodoluminescence imagery. Marine meta-carbonate rocks (ca. 20.5 per mil delta 18 o and 0.5 per mil delta 13 C calcite) and graphitic meta-sedimentary rocks (ca. 14 per mil delta 18 o and -18 per mil delta 13 C calcite) are the main supracrustal reservoirs of carbon and oxygen in the district. The isotopic ratios for calcite from the cores of Na-(Ca) alteration systems strongly cluster around 11 per mil delta 18 o and -7 per mil delta 13 C, with shifts towards higher delta 18 o values and higher and lower delta 13 C values, reflecting interaction with different hostrocks. Na-(Ca)-rich assemblages are out of isotopic equilibrium with their metamorphic hostrocks, and isotopic values are consistent with fluids derived from or equilibrated with igneous rocks. However, igneous rocks in the eastern Mt Isa Block contain negligible carbon and are incapable of buffering the delta 13 C signatures of CO2-rich metasomatic fluids associated with Na-(Ca) alteration. In contrast, plutons in the eastern Mt Isa Block have been documented as having exsolved saline CO2-rich fluids and represent the most probable fluid source for Na-(Ca) alteration. Intrusion-proximal, skarn-like Cu-Au orebodies that lack significant K and Fe enrichment (e.g. Mt Elliott) display isotopic ratios that cluster around values of 11 per mil delta 18 o and -7per mil delta 13 C (calcite), indicating an isotopically similar fluid source as for Na-(Ca) alteration and that significant fluid-wallrock interaction was not required in the genesis of these deposits. In contrast, K- and Fe-rich, intrusion-distal deposits (e.g. Ernest Henry) record significant shifts in delta 18 O and delta 13 C towards values characteristic of the broader hostrocks to the deposits, reflecting fluid-wallrock equilibration before mineralisation. Low temperature, low salinity, low delta 18 O (<10 per mil calcite) and CO2-poor fluids are documented in retrograde metasomatic assemblages, but these fluids are paragenetically late and have not contributed significantly to the mass budgets of Cu-Au mineralisation.