Thursdays Gossan Prospect

Abstract

"Thursdays Gossan Prospect (also known as Victor 1) is the first porphyry copper style mineralisation found in western Victoria. The porphyry system is in the Mount Stavely Volcanic Complex at the intersection of two large faults. Porphyry style Cu-Ag-Mo(-Au) mineralisation is hosted by undeformed, weak to strongly phyllic altered, low-potassium, dacitic, sub-volcanic porphyries and surrounding sediments and volcanics. No true ""copper-mineralised quartz stockworks"" or economic primary copper mineralisation have been found. Subeconomic mineralisation is disseminated throughout most of the intrusions; the youngest phases are barren. Supergene enrichment of copper occurs as chalcocite in sediments, volcanics and intrusions. The copper mineralisation in the Thursdays Gossan system is low grade and uneconomic. Electrical surveys (RRMIP) in the 1970s indicated moderately high chargeable and conductive responses coincident with anomalous copper geochemistry. Subsequent RAB and diamond drilling showed the high chargeability to have been from disseminated pyrite and chalcocite and the high conductivity was attributed to the presence of massive pyrite. Geophysical surveys in the 1990s included gravity and airborne electromagnetic surveys. The high conductivity detected in airborne time-domain electromagnetic surveys probably reflects high groundwater salinity within the deeply weathered zone of the alteration system, rather than disseminated sulphides at depth. The gravity low of 2 mGals (20 mm s-2) is believed to reflect the lower density of the altered/weathered/oxidised rocks. Magnetic surveys over the prospect area were less definitive. Most of the very prospective but weakly magnetic sub-volcanic dacite porphyries could not be distinguished from the nonmagnetic sedimentary rocks."