Lead and Nd isotopic evidence for a crustal Pb source of the giant Broken Hill Pb–Zn–Ag deposit, New South Wales, Australia

Abstract

Major element, trace element, Nd isotopic data and U–Pb zircon ages of quartz±Fe-oxide±garnet±accessory mineral-bearing rocks (hereafter referred to as QFeGA), as well as Pb isotope data from ca. 1685Ma high Fe–Ti meta-tholeiites and the Rasp Ridge Gneiss are presented. These rocks are found in the Proterozoic Willyama Supergroup, Broken Hill Domain, southeastern Australia. U–Pb zircon dating of the QFeGA rocks yielded ages of ca. 1600Ma, comparable to the accepted age of the Olarian Orogeny, with no evidence of inherited zircons from a detrital component. Despite the U/Pb age, the lack of detrital zircons in combination of bulk rock major and trace element geochemistry, suggests that the QFeGA rocks formed during the early stages of the geological evolution of the Broken Hill Domain (ca. 1710–1685Ma) in a progressively deepening ensialic rift environment, due to seafloor hydrothermal activity similar to hydrothermal sediments at present day spreading centres, rather than during the Olarian Orogeny. The Nd isotope compositions of the QFeGA rocks (εNd=−2.2 to −6.6) are comparable to those of other garnet-rich rocks associated with the Broken Hill deposit and with the metasedimentary rocks of the Willyama Supergroup. This suggests that the hydrothermal fluids responsible for the deposition of the QFeGA protoliths were in equilibrium with the Willyama supracrustal sequence.Lead isotope data from the high Fe–Ti meta-tholeiites and the Rasp Ridge Gneiss lie within error on the same 1685Ma 207Pb/206Pb reference isochron, with an initial Pb isotope composition similar to that of the Broken Hill orebody (galena). This is inconsistent with independent major and trace element and Nd isotopic evidence showing that the mafic and felsic melts were derived from end-member mantle and crustal sources respectively. Hence a common source for the Pb of the mafic and felsic rocks is likely. It is inferred that this Pb source reflects a period of regional Pb isotope homogenisation due to pervasive supracrustal hydrothermal flux at the time of emplacement of the mafic and felsic rocks (ca. 1685Ma) and coincident with formation of the QFeGA rocks and the Broken Hill deposit. It is suggested that the Pb (and other metals) scavenged by this hydrothermal system from the Willyama Supergroup metasedimentary sequences provided the metals for the Broken Hill ore body.