Characteristics of the ore-forming fluids and its constraints on the genesis of Abra Pb-Cu deposit, western Australia

The Abra Pb-Cu deposit was discovered in 1981 in the Bangemall Basin, north-western of Western Australia. The ore body is hosted in the Meso-proterozoic Edmund Group shallow marine clastic sequences. The layered ores present in the upper section, and the veined ores occur in the lower section. Because of its complicated geology and lack of detailed investigations, the genesis of the Abra deposit is debatable. In this paper, the characteristics, origin and evolution of the ore-forming fluid are investigated. Based on the results of micro-thermometry, the fluid-inclusions in the quartz can be divided into three groups: two phases H2O fluid-inclusions, fluid inclusions with CO2 and CH4, and fluid inclusions with mineral crystals. The homogenization temperatures of these fluid inclusions in the upper hematite jasperite and magnetic quartzite vary from 162.0℃ to 194.7℃, and the salinities vary in the range of 5.8%~13.0% wNaCl; while the homogenization temperatures of the inclusions in the lower chloritized quartz sandstone vary from 172.7℃ to 250.7℃, and the salinities vary within the range of 7.0%~17.0% wNaCl. The wide range of homogenization temperatures and salinities in different sections demonstrated that there was more than one phase of ore-forming fluid activities. The data of S and Sr isotopes show that the ore-forming fluid was the mixture of sea water and metamorphic water, and the ore-forming elements were leached from the host strata. And the host rocks have contributed to the ore deposit formation. The calculated densities of the fluid inclusions are from 0.885 g/cm3 to 1.012 g/cm3 based on the homogenization temperatures and salinities. The theoretical ore forming depth is around 0.22 to 1.39 km below the surface, and the Abra deposit can be categorized as a meso- and epithermal deposit.