Lithogeochemistry of orogenic gold deposits in Victoria, SE Australia: a preliminary assessment for undercover exploration

Abstract

It is probable that the most fruitful exploration strategy for discovery of new orogenic gold deposits in Victoria, SE Australia, will be to focus on areas buried by post-mineralization cover rocks. There are several possible approaches in the absence of robust predictive models and detailed subsurface data with which to apply them. This paper focusses on the detection of primary hydrothermal alteration envelopes and presents new data on the mineralogical and bulk-chemical expression of alteration in weathered outcrop around the deposits of Bendigo, Fosterville and Stawell. Little useful information was gleaned from visual inspection of weathered bedrock but short-wave infrared (SWIR) spectra could be used as vectors to gold ore. Key parameters are the depth of 1920 nm SWIR peak indicating white-mica crystallinity and the ratio of kaolinite to white mica. Observed mineralogical trends are consistent with those from studies of unweathered samples from underground openings and drillcore and from surface samples elsewhere in Victoria. Weathering appears to have resulted in relatively minor changes to rock mineralogy, mainly breakdown of hydrothermal pyrite to poorly crystalline and crystalline iron oxides and oxidation of carbonaceous material. Bulk analysis after total digest reveals that proximity to gold ore is reliably indicated by extreme enrichment in Au, As and K. Several other elements are anomalous but not at every deposit. Use of a partial leach targetting poorly crystalline and crystalline iron oxides appears to offer significant advantages over total digestion as it generates diagnostic anomalies for elements such as Co, Cu, Mn, Ni, Pb and Sr, some of which are not noticeably anomalous in the total digest data. Our preliminary data suggest that drilling through unmineralized cover and sampling weathered bedrock beneath could be a viable exploration method, providing that secondary dispersion at the unconformity has not substantially modified the primary hydrothermal dispersion. This is currently under investigation.