Geochemistry of the Mount Windsor Volcanics; implications for the tectonic setting of Cambro-Ordovician volcanic-hosted massive sulfide mineralization in northeastern Australia

Abstract

The Mount Windsor subprovince is an important relic of Upper Cambrian to Lower Ordovician sedimentation and volcanism in the northern part of the Tasman orogenic zone. Volcanic-hosted massive sulfide mineralization occurs at several stratigraphic horizons within the volcano-sedimentary package and one major mine is operational within the belt. Major and trace element data and Nd isotope ratios are presented for the least altered coherent units from the three major volcanic-bearing formations in the Mount Windsor subprovince. The data are used to discriminate four major phases of volcanism and related intrusive activity derived from three isotopically discrete sources and to assess the geodynamic setting in which the volcanism occurred. The earliest phase of mafic volcanism has minor and trace element characteristics suggesting an alkaline intraplate or rift association and it was probably produced by partial melting of attenuated subcontinental lithospheric mantle. The overlying Mount Windsor Formation silicic volcanics have Nd isotope characteristics (epsilon (sub Nd(480 Ma)) = -4.7 to -12.8) that suggest they were produced by partial melting of underlying Precambrian crustal rocks. Mafic volcanics of the overlying Trooper Creek Formation include a low TiO 2 suite and a high TiO 2 suite with a range of distinguishing chemical characteristics but similar Nd isotope ratios (epsilon (sub Nd(480 Ma) = 3.8-2.3), which indicate derivation from relatively depleted asthenospheric mantle variably modified by subduction processes. The high TiO 2 suite is also represented by abundant intrusions within the underlying volcanic package. The more silicic volcanics in the Trooper Creek Formation appear to be cogenetic with their mafic associates but have varying Nd isotope ratios, which suggest progressive crustal interaction with increasing SiO 2 content. Comparisons with modern volcanic compositions and ore depositional environments suggest that the volcanic and sedimentary units within the Mount Windsor subprovince were deposited in a back-arc basin developed by extension of continental lithosphere along the eastern Australian margin in the Late Cambrian and Early Ordovician. Mineralization and volcanic deposits of similar age farther north in the Tasman orogenic zone suggest that this basin may have had a north-south orientation, although there is no clear evidence remaining of the original arc front deposits.