Geophysical responses over the Mount Ararate Prospect

Abstract

Mount Ararat Prospect is hosted in Cambrian greenstones in western Victoria. Early exploration showed excellent correlation between the soil geochemical copper peaks and electromagnetic anomalies over a gossan. Follow-up drilling resulted in discovery of the Mount Ararat Prospect by Pennzoil in 1975. Massive banded and disseminated chalcopyrite-pyrrhotite-sphalerite mineralisation occurs in mafic and pelitic schists. The deposit is uneconomic, with an inferred resource of approximately 1 million tonnes with grades of 2.7% copper, 10 g/t silver and 0.6 g/t Chalcocite is common in a zone of supergene enrichment. Geophysical surveys, including airborne electromagnetic (INPUT) survey, vertical-loop electromagnetics, ground magnetic surveys, magnetic induced polarization, induced polarization and rapid reconnaissance magnetic induced polarization RRMIP, were used to delineate the extent of mineralisation. The INPUT survey and gradient-array induced polarization results were disappointing due to a very conductive surface layer. Magnetic induced polarization surveys were effective in delineating an anomalous conductive zone corresponding to the mineralisation. Downhole resistivity and induced polarization logs show the mineralised zone has coincident chargeability and resisitivity responses but the chargeability response is broader and more extensive indicating a possible disseminated pyrite halo in a broader zone around the massive sulphides. Experimental time domain electromagnetic surveys using the UTEM and SIROTEM systems recorded responses over the mineralisation but these techniques have not been routinely used for exploration. The early-time electromagnetic data is affected by a highly conductive near surface layer. The magnetic responses associated with the deposit are caused by the host rocks, and are not a direct response from the mineralisation. These units remain prospective along strike for base metals and gold.