Lead isotope constraints on the age and source of gold mineralization in the Archean Yilgarn Block, Western Australia

Abstract

Model mineralization ages based on whole earth lead isotope growth curves are used to date sulfides (mainly galena with minor Fe sulfides and Pb tellurides) related on one hand to stratiform base metal and Ni-Cu mineralization and on the other to predominantly lode-type gold mineralization in greenstone belts of the Archean Yilgarn block, Western Australia. Galenas related to undoubted syngenetic, volcanogenic base metal mineralization and Fe sulfides to komatiite-associated Ni-Cu deposits yield Pb model ages that are broadly consistent with Sm-Nd, Pb-Pb, and U-Pb zircon greenstone belt geochronology; such deposits from the Murchison province give model ages of ca. 3.0 b.y. or greater, whereas those of the Eastern Goldfields province give model ages of ca. 2.85 to 2.80 b.y. for the northeastern part of the province and ca. 2.75 to 2.70 b.y. for the highly mineralized Norseman-Wiluna belt. The majority of galenas related to gold mineralization record model ages of ca. 2.85 to 2.75 b.y. for the Murchison province and ca. 2.75 to 2.70 b.y. for the Southern Cross province, with most deposits from the Norseman-Wiluna belt having model ages around or below ca. 2.7 b.y. A few examples of gold mineralization, all but one economically insignificant, have galenas with more complex multistage lead isotope histories with mineralization ages lying in the Proterozoic.Lead model ages of galenas from gold deposits in the Murchison province are consistently younger then those related to syngenetic mineralization; those from gold deposits in the Southern Cross province broadly coincide with the best estimates of the age of peak metamorphism; and those from the Norseman-Wiluna belt are indistinguishable or slightly younger than the ages of syngenetic base metal and Ni-Cu mineralization in the same belt. The lead isotope compositions of most gold-related galenas yield 238 U/ 204 Pb (mu ) values that are consistent with a previously proposed epigenetic model for economically important gold mineralization; in this model, lead (and by inference, gold) was derived from basalts and komatiites by major hydrothermal systems which developed due to devolatilization of the greenstone pile during regional metamorphism from between 250 m.y. to less than 50 m.y. after eruption.The small deposits that give younger mineralization ages may be related to, or affected by, minor thermal events which occurred later in the history of the Yilgarn block and which were also responsible for resetting rock and mineral Rb-Sr systems. The contrast in isotopic histories between galenas (and possibly Fe sulfides) from major and minor gold deposits may have application as an exploration tool.Significantly higher 238 U/ 204 Pb values for sulfides from the Norseman gold deposits provide support for the suggestion that crust older than 3.0 b.y. existed prior to formation of the greenstone belts of the Yilgarn block.