Electrical properties of porphyry mineralisation at the Cadia Ridgeway gold-copper deposit, NSW ? implications for exploration

Abstract

The Cadia Ridgeway Au-Cu deposit is a deep (>500 m) gold-rich porphyry copper system located in central NSW. A chargeability anomaly, recognised within a prospective NW-SE mineralised corridor motivated the drilling program which led to the discovery of the deposit. In-situ and laboratory electrical tests were conducted to characterise the petrophysical properties of the deposit and host sequences. The ore is both chargeable and relatively conductive, although not sufficiently conductive for EM methods. Significant scale variation of apparent resistivity was observed. In-situ measurements, utilising electrode spacings of one to four m, are up to two orders of magnitude lower than values determined from laboratory measurements. Forward modelling of surface induced polarisation and apparent resistivity field data was conducted using petrophysical properties determined primarily from in-situ testing. The chargeability anomaly identified over the Ridgeway ore body is due largely to discontinuous halos of pyrite alteration above the economic mineralisation, but a highly chargeable source component associated with the economic mineralisation at Ridgeway is also inferred. The measured contrast in conductivity between ore and host rocks suggests that Ridgeway-style mineralisation may represent a target for magnetometric resistivity (MMR) surveying. 3D numerical modelling predicts that the MMR response of the Ridgeway deposit would be detectable by both surface and down hole MMR surveys.