Geophysical Signature of the Mons Cupri VMS Deposit, Western Australia

Abstract

Mons Cupri is an Archaean proximal, volcanogenic massive-sulphide deposit in the west Pilbara of Western Australia. Mineralisation occurs in the Mons Cupri Volcanics of the Whim Creek Belt. Stockwork copper-sulphides in altered Mount Brown Rhyolite are overlain by shallowly dipping, massive, copper-lead-zinc sulphides in volcaniclastic sedimentary rocks. Two airborne magnetic surveys detected a 70 nT anomaly in broad correlation with the outline of the mineralisation. The likely source of this anomaly is magnetite within a chlorite alteration pipe. One trial and two surveys with airborne electromagnetics have been conducted. None of the systems, including GEOTEM II, yielded anomalies over Mons Cupri itself. Seven trials and one survey with ground electromagnetics were also completed. Again, none produced anomalous results attributable to the mineralisation. Two induced polarisation surveys detected slightly lower resistivities over the stockwork mineralisation; a magnetic induced polarisation survey obtained the opposite result. However, higher percentage frequency effects, chargeabilities, and relative phase shifts clearly coincided with the stockwork mineralisation. None of the systems unequivocally detected the massive sulphides at depth. Mons Cupri is a difficult geophysical target. Low total-sulphide content, deep massive sulphides and a lack of electrical continuity combine to produce an enigmatic response and to defeat detection by electromagnetic systems.