Abstract

Archean shallow-level lode-gold deposits in Western Australia are located in rocks of prehnite-pumpellyite to lower greenschist facies and are characterized by (1) predominantly brittle shear zones that formed well after volcanism and late in the structural history, (2) breccia-hosted lodes, (3) low-temperature ore and gangue minerals such as silver sulfosalts, stibnite, and precursor chalcedony, and (4) vertical metal zonation of the orebodies. Ore-fluid temperatures ranged from 150 to 325 °C, and a depth of em-placement of <5 km is inferred. Isotopic analyses of alteration minerals such as quartz, sericite, and chlorite indicate higher δDfluid (-6‰ ± 9‰ relative to SMOW [1σ; n = 19]) and lower δ18Ofluid (4.4‰ ± 2.3‰ relative to SMOW [1σ; n = 167]) values compared to such values for hydrothermal fluids that deposited Au in other Archean lode-gold deposits with similar structural timing. The fluid-inclusion and stable isotope data and the presence of chalcedony in this deposit style are best explained by the influx of surface water at shallow levels and interaction of that water with advecting metamorphic-magmatic fluids.

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