The Moonta Domain forms the southern part of the Olympic Cu-Au province on the eastern margin of the Gawler Craton. Historical production comprises over 330,000 tonnes of Cu from vein and shear-hosted mineralisation in the Moonta-Wallaroo district. The domain basement comprises metasediments and metavolcanics of the Palaeoproterozoic Wallaroo Group (~1760-1740 Ma) which were deformed and metamorphosed to upper greenschist-amphibolite facies during the Kimban Orogeny (~1720 Ma). These rocks were further deformed and intruded by granitoids and minor mafic intrusions of the Hiltaba Suite between about 1600 Ma and 1575 Ma.
There is a close spatial association of high temperature Fe-Na-Ca-K metasomatism of the Wallaroo Group and Hiltaba Suite intrusions. Conor (1995) termed the most strongly altered rocks the Oorlano Metasomatites, although metasomatic mineral assemblages within this rock association vary widely. Intense albite-actinolite-magnetite ± carbonate ± epidote ± pyrite alteration of metasediments is strongly associated with the contact zones of Hiltaba Suite granites, particularly the Tickera Granite. More distal albitisation of the Wallaroo Group is common but is not generally associated with significant sulphides. Biotite ± albite ± magnetite ± quartz ± apatite ± monazite ± tourmaline alteration is commonly associated with pyrite ± minor chalcopyrite, and is particularly widespread south of Moonta where numerous magnetic and non-magnetic Hiltaba Suite granitoids (previously grouped as Arthurton Granite) intrude the Wallaroo Group. Late chlorite and K-feldspar alteration is typically of restricted extent, but may also be associated with sulphides.
Biotite-rich alteration typically forms irregular magnetic anomalies, including a major 5 x 15 km alteration zone near Weetulta, and possibly a large area (~30 km x 40 km) of strongly magnetic rock beneath Spencer Gulf. Fluid inclusion data indicate that highly saline, multi-cation fluids are associated with the alteration. Preliminary U-Pb SHRIMP dating of hydrothermal monazite from biotite-rich alteration in the Weetulta and Wallaroo areas yields ages of approximately 1585 Ma and 1620 Ma respectively. The Weetulta district data indicate a close temporal relationship of the biotite alteration and Hiltaba Suite magmatism. However, the older Wallaroo district age suggests hydrothermal activity may have commenced prior to intrusion of Hiltaba Suite granites.
Regional metamorphic and alteration characteristics of the Moonta Domain are similar to those of the Fe-Cu-Au mineral province of the Mt Isa Inlier Eastern Succession, where there are strong links between magmatism, regional albitisation, and Fe-Cu-Au mineralisation (eg., Oliver et al., 2001). Biotite-magnetite metasomatism commonly occurs proximal to major Fe-Cu-Au ore deposits in the Mt Isa Eastern Succession. The shear-hosted Cu lodes and associated alteration at Wallaroo may be an analogue in the Moonta Domain. However, apart from some very minor drill intersections in prospects in the Weetulta district, no other significant Cu-Au mineralisation associated with biotite-magnetite alteration has yet been discovered in the Moonta Domain. Given that most of the Proterozoic basement of the Moonta Domain is concealed by up to 100 metres of Neoproterozoic to Cainozoic sediments and remains largely untested by drilling, the potential for discovery of Ernest Henry-style Fe-Cu-Au deposits in the Moonta Domain remains high.
