Evidence for a mafic connection to the roots of caldera-related epithermal vein mineralization in north Queensland

Abstract

North Queensland contains numerous, large Carboniferous to Permian caldera complexes that are associated with economically significant magmatichydrothermal mineralisation. The recently discovered Piccaninny Epithermal System (PES), 200km west of Cairns, occurs on the southeastern margin of the poorly exposed North Scardons Igneous Complex which is interpreted to be a large caldera (~300 km2). The roots of the epithermal system are exposed in the Proterozoic McDevitt Metamorphics and the veins are characterised by dominantly comb to locally banded crustiformcolloform quartz that host Ag-Pb-Zn-Au-As mineralisation. The vein array is spatially associated with basalt dykes that have exploited caldera-rim parallel NESW faults, and E-W and NW-SE structures. The PES lies on a gravity gradient defined by a gravity high related to the basalt dykes and buried mafic intrusions and gravity lows related to the caldera and felsic intrusions. A basaltic magma emplaced into the upper crust may have been responsible for crustal melting that produced felsic rocks associated with the caldera. Furthermore, the strong spatial and temporal association of basalt dykes and veins may also suggest a mafic source for metals and sulphur in the PES.