Contrasting fluid types at the Nevoria gold deposit in the Southern Cross greenstone belt, Western Australia: Implications of auriferous fluids depositing ores within an Archean banded iron-formation

Two main hydrothermal mineral assemblages have been identified in the bande d iron-formation-hosted gold mine at Nevoria. The earlier quartz +/- garnet +/- clinopyroxene +/- calcite vein group formed pre- or synmetamorphic, an d the later quartz +/- pyrrhotite +/- pyrite vein group appears to be postm etamorphic and related to gold mineralization. Fluid inclusion characterist ics are obviously different in those two vein groups. Microthermometric ana lysis indicates that the fluids with metamorphic alteration are aqueous, CO 2-rich or CO2-absent solutions; no or very small amounts of CH4 were involv ed in this fluid. Mineralizing fluids were a CH, CO2-H2O solution. The init ial auriferous fluids were CH4 dominant. Heterogeneous trapping, interactio n of the hydrothermal fluid with graphite-bearing rocks, or fluid mixing ma y cause large variations of CH4/CO2 ratios or a X-CH4 of CH4-CO2-H2O inclus ions, particularly in mineralized quartz-pyrrhotite veins. Phase mixing or separating, resulting in an increase in pH and f(O2), together with loss of reduced sulfur by mineral-fluid reactions and precipitation of sulfides, l ed to the breakdown of the gold-transporting complexes.