Potential field modelling of VHMS and sequences in the Captains Flat area, NSW

Abstract

The Geological Survey of New South Wales has commenced 2nd edition geological mapping of the Captains Flat area, southeast of Canberra. This project is a southern extension to the recently completed mapping over the Braidwood 1:100 000 and Goulburn 1:250 000 scale map sheets. Silurian Devonian extensional basin sequences, within the Braidwood and Goulburn sheets, extend into the Captains Flat area. The basin sequences are known to host economic Zn, Cu and Pb VHMS mineralization (e.g. Woodlawn, Currawang, Captains Flat). The Captains Flat area has syn-rift middle- to late Silurian sequences consisting of felsic and intermediate to mafic centres. Sequences dominated by Ordovician turbidites (Adaminaby, Bendoc and Margules groups) have been faulted to the surface on the eastern limbs of the basin. The preserved sequences at Captains Flat have been interpreted by geologists to form a highly-deformed, folded, east-dipping syncline that has been heavily faulted. These sequences appear in TMI imagery as curvilinear, near-parallel anomalies. Potential field modelling of aeromagnetic data acquired under the NSW Government's New Frontiers initiative is underway. The models are geologically constrained and will aid geological interpretation. Preliminary models for magnetic sources indicate discontinuous steeply-dipping bodies. Modelled sources, interpreted as Captains Flat Formation, are generally 200 m wide with susceptibilities of 2-5 - 10-3 SI. The Kohinoor Volcanics, which host the larger VHMS deposits in the area, have a similar width and dip but have a higher susceptibility of approximately 10 - 10-3 SI. In contrast to the Silurian sequences, formations in the Ordovician sequences (within the Margules and Bendoc groups) have negative anomalies and the modelled sources are generally wider. As yet the hinge for the syncline is not evident, indicating that it could be located at a greater depth than is resolvable.