Ordovician Intrusive‐related Gold‐Copper Mineralization in West‐Central New South Wales, Australia

AbstractThree major types of Ordovician intrusive‐related gold‐copper deposits are recognized in central‐west New South Wales, Australia: porphyry, skarn and high sulphidation epithermal deposits. These deposits are mainly distributed within two Ordovician volcano‐intrusive belts of the Lachlan Fold Belt: the Orange‐Wellington Belt and the Parkes‐Narromine Belt. Available isotopic age data suggest that mineralization of the three types of deposits is essentially coeval with the Ordovician intrusive rocks (480–430 Ma).Porphyry gold‐copper deposits can be further divided into two groups. The first group is associated with monzonite showing shoshonitic features, represented by Cadia and Goonumbla. The second group is associated with diorite and dacite, including the Copper Hill and Cargo gold‐copper deposits. Gold skarn is associated with Late Ordovician (430–439 Ma) monzonitic intrusive complexes in the Junction Reefs area (Sheahan‐Grants, Frenchmans, and Cornishmens), Endeavour 6, 7 and 44, Big and Little Cadia. The epithermal gold deposits with high sulphidation including Gidginbung (Temora) and Peak Hill mainly occur within Ordovician andesite and volcaniclastic rocks, and are associated with advanced argillic alteration. Available isotopic age data indicate that both alteration and mineralization of the porphyry, skarn and epithermal gold‐copper deposits are broadly coeval with the Late Ordovician shoshonitic magmatism, which is thought to result from the melting of sub‐continental lithosphere caused by Palaeozoic subduction events.The Ordovician intrusive‐related gold‐copper deposits are restricted to two longitudinal parallel volcano‐intrusive belts, rarely extending outside them. Diagonal intra‐belt trends of mineralization are common, particularly at the intersections of longitudinal and transverse (oblique) fault/fracture zones based on the authors' review of available geological data. The locations of these gold‐copper deposits are obviously influenced by transverse (oblique) fault/fracture zones that are oriented northwest, eastwest and northeast. The conjunctions of these fault/fractures zones are thought to be zones of structural weakness, and appear to be the favourable locus for the Ordovician intrusive‐related gold‐copper deposits. Differences in structural patterns, intrusive, wall rock types, and depths of ore formation may contribute to the differences among the deposits.