Geochemical evolution and alteration styles within the Gawler Range Volcanics, South Australia

Abstract

The Gawler Range Volcanics (GRV) form part of a Mesoproterozoic Silicic Large Igneous Province (Gawler SLIP) within South Australia. The SLIP includes intrusive and extrusive rocks within the Gawler Craton and Curnamona Province that are dominantly felsic. Recent high precision dating of several GRV units has shown that they erupted between 1587 Ma and 1595 Ma allowing for geochemical comparisons with respect to a precise timeline. Trace element geochemistry has shown anomalies to be consistent through the lower and upper GRV demonstrating the main source in the GRV likely did not change. A mafic component is shown to have contributed to both the lower and upper GRV system. Eu anomalies and trace element geochemistry shows that there was a large change in magmatic evolution between the upper and lower GRV within a short time (<1 m.yr). This change is hypothesised to have occurred due to the tectonic regime during the SLIP emplacement. Hydrothermal alteration associated with the emplacement of the Gawler SLIP is known to have contributed to the formation of Iron-Oxide Copper-Gold (IOCG) and shear-hosted deposits in South Australia. More recent discoveries within the Southern Gawler Ranges display epithermal-porphyry characteristics associated with alteration in the lower GRV. Alteration within the GRV is hereby characterised in order to identify alteration associated with mineralisation. Alteration is shown to encompass a sericite – hematite dominated assemblage which has affected most of the GRV. Several other anomalous alteration assemblages exist in localised areas. Using direct evidence, it is suggested that epithermal-porphyry systems may be preserved within the upper GRV, which encompasses a larger outcrop area than the lower GRV which is underexplored.