Fluid inclusions in mineralized quartz veins from the ca. 2890 Ma McPhees gold deposit, Pilbara Craton, Western Australia, were examined to determine the composition of the mineralizing fluid and to constrain the physical conditions of gold deposition. The composition of inclusions was determined by standard microthermometric and laser Raman analyses of selected assemblages of fluid inclusions. The veins contain: (1) abundant, Type-Ia monophase, CO₂-rich inclusions and coeval Type-Ib two-phase, mixed H₂O + CO₂ inclusions; (2) secondary, two-phase, Types II and III, respectively, low- and high-salinity aqueous inclusions, and (3) secondary Type-IV monophase, mixed CO₂–CH₄–N₂ inclusions. Type-Ib inclusions have highly variable bulk-compositions (10–100 vol.% CO₂–CH₄) and variable salinity (3–12 eq. wt.% NaCl). Types II and III inclusions have similar morphologies and modes of occurrence, but have distinctly different salinities (0.3–12.6 eq. wt.% and >21.0 eq. wt.%, respectively). Heating experiments indicate minimum temperatures of trapping of 350 641°C for Type-I inclusions, 207 660°C for Type-II inclusions, and 143 <637°C for Type-III inclusions. Oxygen isotope compositions of quartz–actinolite and albite–actinolite pairs indicate a temperature of gold-associated alteration of ~350°C, consistent with arsenopyrite thermometry, which indicates gold mineralization at <480°C. The early CO₂-rich fluid inclusions have densities that range from 0.6 to 1.05 g/cm3 which, at 350°C, correspond to 1–2 kbar pressure, consistent with geological relations indicating that the McPhees deposit formed at <7 km. Type-I inclusions are interpreted to contain early vein-related fluids that carried gold, but this assemblage (nearly pure CO₂ and subordinate, coexisting H₂O-rich fluid inclusions) is unusual for orogenic gold deposits. It is most likely a result of fluid mixing that may have played a role in gold deposition in veins; however, host-rock lithology seems to have been a first-order control in localizing the gold.
The Canadian Mineralogist Vol. 42, Issue 5, p. 1405-1424