A fluid inclusion and stable isotope study of synmetamorphic copper ore formation at Mount Isa, Australia

Earlier structural studies indicate that the dolomitic and siliceous breccias and contained copper mineralization at Mount Isa formed during regional deformation and greenschist facies metamorphism of their host mid-Proterozoic metasediments. A working hypothesis for the syntectonic and synmetamorphic fluid-rock interaction and copper ore formation at Mount Isa is proposed. In an initial stage, dolomitic alteration occurred by chemical interaction between the Urquhart Shale and moderate amounts of two fluids of different chemical and hydrological origin, represented by CaCl ₂-rich and CO ₂-rich fluid inclusions. Fluid mixing may have been aided by extension of the limb of a regional fold which temporarily lowered fluid pressures, thus leading to hydraulic brecciation and fluid influx from two fluid regimes, above and below. The initial stage of brecciation and dolomitic alteration was followed by the main stage of silicification and copper introduction by the much more copious NaCl-rich fluid, probably at near-lithostatic fluid pressure. Quartz and chalcopyrite precipitation occurred by a combination of slight cooling, a pH increase, and possibly an increase in sulfur activity, as the initially reduced, acid, and possibly S-deficient, NaCl-rich fluids reacted with the previously dolomitized pyritic and dolomitic Urquhart Shale. -from Authors