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Abstract

Carbon and oxygen isotope studies of carbonate minerals from the two carbonate alteration styles that predate regional metamorphism and gold mineralization in the Norseman-Wiluna greenstone belt of Western Australia have significance for the sources of carbon and oxygen in ore fluids that produce Archean mesothermal gold deposits. The mantle-derived carbon reservoir in regional carbonation zones is the most likely source for auriferous ore fluids rather than the seawater-derived carbon reservoir in altered basalts. The wide range of δ13C values for gold-related carbonates reflects dissolution of different carbonate species and/or a variable input of organic and seawater-derived carbon.

The oxygen isotope compositions of auriferous ore fluids in the Norseman-Wiluna belt (δ18O = 4-9‰) indicate that these fluids (1) could have been in equilibrium with sea floor-altered basalts at 500° to 600°C, (2) derived from regional carbonation zones or interacted with them, or (3) derived from felsic porphyries or granitoids only if modified by subsequent fluid-rock interaction or changing P-T-X conditions.

There is a positive correlation between δ13C and δ18O for carbonates from gold-related alteration (both district and mine scale), with data for the two largest Archean mesothermal gold deposits in the world (Kalgoorlie, Western Australia; and Timmins, Ontario) plotting at the most positive end of this trend. Correlated δ13C and δ18O variations in gold-related carbonates on a district scale probably relate to source-conduit heterogeneities and host-rock compositional controls, whereas on a mine scale a model involving finite reservoir effects for carbon, temperature-controlled oxygen isotope fractionation, and possibly phase separation can account for the observed trends.

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