Magmatic hydrothermal fluids at the sedimentary rock-hosted, intrusion-related Telfer gold-copper deposit, Paterson Orogen, Western Australia: Pressure-temperature-composition constraints on the ore-forming fluids

The Neoproterozoic Telfer deposit, one of Australia's largest gold-copper deposits, is located in the Paterson orogen. Several highly differentiated calc-alkaline to alkali-calcic peraluminous granites intruded the metasedimentary rocks near (5-20 km) Telfer contemporaneously with structurally controlled gold-copper mineralization. Fluid inclusion assemblages with different fluid inclusion types were identified in samples from a range of different vein types. These inclusion types range from three-phase aqueous L_aq+ V_aq+ S_halite, high-salinity (≤50 wt % NaCl equiv), high-temperature (≤460°C) inclusions to two-phase aqueous or two-phase aqueous carbonic, low- to moderate-salinity (2-22 wt % NaCl equiv), moderate- to high-temperature (≤480°C) fuid inclusions. Fluid inclusion trapping mechanisms and interpreted precipitation mechanisms for gold and copper include (1) adiabatic cooling between 450° and 200°C in all veins and, (2) locally, fluid phase separation at about 300°C. The trapping pressure of fluid inclusion assemblages trapped during phase immiscibility was calculated to be approximately 1.5 kbar. For fluid inclusion assemblages that lack evidence for phase immiscibility, a pressure at the temperature of final homogenization of up to 3 kbar was calculated. This high pressure value is interpreted to be related to local fluid overpressure, as a consequence of fault zone movement, in faults and fractures that localized gold at Telfer. Phase immiscibility and gold precipitation were induced during sharp pressure decrease accompanying fault zone movement. In situ laser inductively coupled plasma-mass spectrometry (ICP-MS) analyses of fluid inclusions revealed high trace element contents in all fluid inclusion assemblages. Manganese/Fe ratios of <0.24 in all vein types suggest that reduced fluids dominated the system, but, locally, a switch to more oxidized conditions with Mn/Fe ratios >0.24 is observed. Given the high temperatures and salinities of up to 480°C and 42 wt % NaCl equiv, Au and Cu were likely transported as chloride complexes. This interpretation is supported by the observation that the highest base metal contents occur in the highest-salinity fluid inclusion. Potassium/Ca ratios of >1 in most assemblages, the high homogenization temperatures (<480°C) in many fluid inclusion assemblages, and the high trace element contents (e.g., Fe, Mg, K, Na) in most of the fluid inclusion assemblages are compatible with involvement of a magmatic hydrothermal fluid during gold-copper mineralization. This fluid was probably derived from the coeval granites in the Telfer area and, thus, Telfer is interpreted to be a distal, intrusion-related, metasedimentary rock-hosted, gold-copper deposit type.