Rhenium-osmium systematics of orogenic gold deposits through geologic time

Abstract

Here we report new and previously published Re-Os and trace elemental data on gold as well as coeval sulfides and oxides from various Archean to Alphanumeric gold deposits. When possible, measured concentrations and isotopic ratios were used to determine geochronology as well as the likely source reservoir of the gold mineralization. Rhenium-osmium ages of some gold and related minerals are consistent with existing geochronology and in some cases better constrain genetic models of mineralization. The initial Os isotopic composition is also preserved within some ore minerals and reflects the importance of both crustal and mantle material in the generation of specific deposits. However, many of the gold deposits show evidence of post-crystallization Re-Os disturbance, and so age and source information are not preserved. In these cases, it is likely that hydrothermal fluids added or removed Re or Os subsequent to primary mineralization. Preservation of age favors minerals with high Re and Os concentrations and/or minerals from monocyclic gold deposits without multiple influxes of fluids. More specifically, gold and pyrite from the 2890--2710 Ma Witwatersrand basin, typically form ca. 3000 Ma isochrons with chondritic initial 187Os/188Os values. The Os concentrations of the gold range from approximately 2 to over 4000 ppb and are significantly elevated compared to other gold deposits. The older age, mantle source and high concentration of the gold support modified paleo-placer models with minor hydrothermal modification but not significant deposition of gold via hydrothermal fluids of any age. Other gold deposits have low average Os concentrations ranging from approximately 20 ppt to 1 ppb and Re/Os ratios generally similar to average continental crust. The initial 187Os/188Os of these deposits vary but seem to have more crustal signatures than the Witwatersrand deposits. Data support gold deposition from leaching of gold and Os via hydrothermal fluids from crustal lithologies. Os concentration and Re/Os data of gold show systematic variation with the age of the deposit. These trends may result from a decrease in the efficiency of partial melts to extract juvenile gold and Os from the cooling mantle and the increasing importance of recycling gold from pre-existing continental crust.