The gold-bearing Metalore shear zone and Golden Highway quartz-carbonate vein in the Beardmore-Geraldton Archean Greenstone Belt occur along the Paint Lake splay faults at the contact between metavolcanic rocks and metaconglomerates intruded by pre-ore diorite. The Metalore and Golden Highway deposits were emplaced during a late tectonic event. They consist primarily of quartz, clinochlore, ankerite, potassium feldspars, sericite, pyrite, argentite and chalcopyrite with native gold. The minerals have been deformed and are cut by at least three healed fractures by fluid inclusions. Gold typically occurs in recrystallized quartz. Microthermometric and Raman spectroscopy techniques were used to study fluid inclusions in quartz, calcite, ankerite, chlorites and potassium feldspars.
Six types of fluid inclusions were found to occur in three separate generations of hydrothermal fluid activity in the Metalore and Golden Highway. The three generations of hydrothermal fluids represented by fluid inclusions are: (1) pre-ore, low-salinity (<2wt.%) NaCl-CaCl2 aqueous inclusions with small amounts of daughter mineral and H2O-CO2 inclusions with 10 and 40 mole percent CO2 occurring in the Golden Highway and Metalore respectively; (2) syn-ore, MgCl2 - H2O-CO2 aqueous and vapour rich inclusions with small amounts of daughter minerals with 50 and 80 mole percent in the Golden Highway and Metalore, respectively; (3) post-ore, CO2-H2O liquid-vapour and CO2 -rich liquid-vapour inclusion with variable CO2 contents ranging from 10 to 50 mole percent CO2. Homogenization temperatures of pre-ore H2O-CO2 inclusions during are 220°-230°C and 356°C; syn-ore 266°C; and post-ore 21°C to 66°C.
The aqueous and CO2-rich inclusions are interpreted to have been trapped as two immiscible phases in three separate generations. Precipitation of gold may have been induced by pressure, temperature fluctuations and chemical changes from CO2 effervescence. Metamorphic fluids are the likely source for the onset of precious-metal deposition from reduced sulphur complexes along the Metalore splay fault and precious- and base-metal deposition from both reduced sulphur and chloride complexes along the Golden Highway splay fault.
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