The classification of Western Australian greenstone-hosted gold deposits according to wallrock-alteration mineral assemblages

The epigenetic, greenstone-hosted gold deposits in the Archaean Yilgarn Block of Western Australia can be classified according to their silicate-carbonate gangue, which forms the innermost wall rock-alteration zone of the structurally controlled hydrothermal systems. The proposed classification is descriptive, and emphasizes associations of prominent, macroscopically visible minerals. The gold deposits are divided into two large groups: high-temperature deposits (400°–700°C) formed at pressures of 3–5 kbar in the lower parts of the greenstone belts (10–15 km depth), and medium-temperature deposits (250°–400°C) formed at pressures of 1 to 2 kbar in the upper parts of these belts (3–7 km depth). The high-temperature deposits are usually located in metamorphic terranes of medium grade (amphibolite facies), and are characterized by either microcline-muscovite-andalusite, garnet-pyroxene-biotite or amphibole-biotite-calcite alteration in the ore zones. Deposits showing the latter two alteration styles may also be classified as skarns in the descriptive sense of Einaudi and Burt (1982). The classic medium-temperature (mesothermal) deposits, studies of which form the basis of all current models on Archaean gold metallogenesis, represent only about one half of the total population of deposits in the Yilgarn Block, although they dominate by total gold production, as they include the only giant deposit in Western Australia, i.e., the Golden Mile at Kalgoorlie. The medium-temperature deposits are usually located in metamorphic terranes of low grade (greenschist facies), and are characterized by biotite-ankerite-albite or sericite-ankerite-albite alteration in the ore zones. It is possible that some of these deposits grade into amphibole-biotite-calcite skarns at depth.