Fluid inclusion and stable isotope constraints on the heavy rare earth element mineralisation in the Browns Range Dome, Tanami Region, Western Australia

This study reports on fluid inclusion and oxygen isotope compositions of mineralised and barren hydrothermal quartz veins and hosting metasedimentary rocks associated with the heavy rare earth element (HREE) mineralisation in the Browns Range Dome of the Tanami Region, Western Australia. The HREE mineralisation consists of quartz and xenotime-bearing hydrothermal veins and breccias that occurs along sub-vertical faults within Archean to Paleoproterozoic metasedimentary rocks. Based on analysis of nearly 550 quartz-hosted primary fluid inclusions, three fluid inclusion types were identified in the mineralised samples: type I low-salinity H2O-NaCl (largely <5 wt.% NaCl; consistent with meteoric water), type II medium-salinity H2O-NaCl (12-18 wt.% NaCl), and type III low- to high-salinity H2O-CaCl2-NaCl (1 to ca. 24 wt.% NaCl+CaCl2). Homogenisation temperatures of all fluid inclusion types vary over a relatively wide range from 100 to 250 °C. Barren quartz veins contain only type I low-salinity H2O-NaCl fluid inclusions, with homogenisation temperatures extending from 170 to 350 °C. Raman analyses of all three fluid inclusion types confirmed their aqueous nature with no carbon-bearing fluid species identified. The three fluid inclusion types indicate mixing of three hydrothermal fluids: a low-salinity H2O-NaCl meteoric fluid (< 5 wt.% NaCl), a medium-salinity H2O-NaCl (12-18 wt.% NaCl) fluid, and a high-salinity H2O-CaCl2-NaCl (ca. 24 wt.% NaCl+CaCl2) fluid. Limited LA-ICP-MS analysis found detectable Y, Ce, U and Cl only in the type III fluid inclusions, which indicates that transport of ore metals was (at least partly) by Cl complexes in the type III fluid. The δ18Ofluid values calculated from quartz from mineralised samples are in the range defined by the Archean metasedimentary host rocks of the Browns Range Metamorphics (δ18Ofluid = +1.8 to +5.2‰) and the unconformably-overlying Paleoproterozoic Birrindudu Group sandstones (δ18Ofluid = +8‰). Collectively, our fluid inclusion and oxygen isotope data, together with other field, mineralogical and geochemical data, support an ore genesis model involving mixing of the three hydrothermal fluids in fault zones and along unconformity surfaces in, and around, the Browns Range Dome. The meteoric low-salinity H2O-NaCl fluid potentially carried P from the Birrindudu Group sandstones, and the high-salinity H2O-CaCl2-NaCl fluid leached HREE+Y from metasedimentary rocks of the Browns Range Metamorphics. Ore deposition occurred following mixing of the P-bearing and HREE+Y-bearing fluids, and was associated with a widespread white mica alteration. The temperature and pressure during the fluid-fluid mixing and mineralisation was between 100 and 250 °C, and 0.4 and 1.6 kbar, respectively