Comparison of fluid inclusion data and mineralization processes for Australian orogenic gold and intrusion-related gold systems

We have examined the fluid inclusion data and fluid chemistry of Australian orogenic and intrusion-related gold deposits to determine if similar mineralization processes apply to both styles of deposits. The fluid inclusion data from the Yilgarn craton, the western subprovince of the Lachlan orogen, the Tanami, Tennant Creek and Pine Creek regions, and the Telfer gold mine show that mineralization involved fluids with broadly similar major chemical components (i. e. H2O + NaCl + CO2, ± CH4 ± N2,). These deposits formed over a wide range of temperature-pressure conditions (< 200 to > 500°C, < 100 - 400MPa). Low salinity, CO2-bearing inclusions and low salinity aqueous inclusions occur in both systems but the main difference between these two types of deposits is that most intrusion-related gold deposits also contain at least one population of high-salinity aqueous brine. Oxygen and hydrogen isotope data for both styles of deposit usually cannot distinguish between a magmatic or metamorphic source for the ore-bearing fluids. However, sulfur and lead isotope data for the intrusion-related gold deposits generally indicate either a magmatic source or mixing between magmatic and sedimentary sources of fluid. The metamorphic geothermal gradients associated with intrusion-related gold deposits are characterized by low pressure, high temperature metamorphism and high crustal geothermal gradients of > 30/km. Where amphibole breakdown occurs in a granite source region, the spatially related deposits are more commonly associated with Cu-Au deposits rather than Au-only deposits that are associated with lower temperature granites. The dominant processes thought to cause gold precipitation in both types of deposits are fluid-rock interaction (e. g. desulfidation) or phase separation. Consideration of the physical and chemical properties of the H2O-NaCl-CO2 system on the nature of gold precipitation mechanisms at different crustal levels infers different roles of chemical (fluid-rock interaction) versus rheological (phase separation and/or fluid mixing) host-rock controls on gold deposition. This also implies that at the site of deposition, similar precipitation mechanisms operate at similar crustal levels for both orogenic and intrusion-related gold deposits.