The Moonta-Wallaroo mining district, South Australia: A geophysical perspective

Abstract

Located on the Yorke Peninsula in South Australia, the Moonta-Wallaroo area is an important area of historical copper mining. Hydrothermal copper-gold-silver sulphide mineralisation occurs in Palaeoproterozoic and Mesoproterozoic metamorphic and igneous rocks, but there is no outcrop of these basement rocks due to subdued relief and extensive sedimentary cover. The lack of outcrop, plus the complexity of the local geology, means that basement geological maps of the area are largely speculative. An interpretation of semi-regional aeromagnetic data from the area suggests the basement comprises meta-igneous rocks overlain to the east by an increasing thickness of sedimentary and/or meta-sedimentary cover. Anomalies interpreted to be of stratigraphic origin are quite well developed in areas of meta-sedimentary rocks, and regional strike is interpreted as trending northeast-southwest. Linear anomalies to the north and west of the Wallaroo Mines are interpreted as resulting from magnetite deposited in faults during a regional iron-rich metasomatic event. Based on this interpretation, a major east-west/northeast-southwest trending deformation zone is recognised, interpreted as a locus of reverse and dextral faulting. The entire study area is cross-cut by late brittle faults that trend mainly northwest-southeast. This interpretation/structural scenario is consistent with mine-scale structural studies in both the Moonta and Wallaroo Mines, although, the resolution of the aeromagnetic data is insufficient to map deposit-scale structures. Ground-based geophysical exploration in the Moonta-Wallaroo area is hindered by the generally electrically conductive environment and the presence of cultural features, such as wire fences. However, surface and downhole time-domain EM surveys have successfully located massive-sulphide mineralisation comprising the Poona and Wheal Hughes deposits. The Poona deposit has also been shown to give rise to an IP response, and there is evidence that the lodes elsewhere in the Moonta area are also associated with IP anomalies.