nan Cooke

Alkalic porphyry Au – Cu and associated mineral deposits of the Ordovician to Early Silurian Macquarie Arc, New South Wales

Twenty-one alkalic porphyry deposits of Late Ordovician to Early Silurian age occur in two mineral districts in New South Wales. The Cadia and Northparkes districts formed in shoshonitic volcanic centres where a major basement structure (the Lachlan Transverse Zone) cut the Ordovician Macquarie Arc obliquely. Processes of mineralisation in both districts were centred in and around quartz monzonite porphyry complexes that intruded the volcanic centres. These composite intrusive complexes comprise pipes, dykes and stocks. Hydrothermal alteration in and around the intrusions produced a complex sequence of potassic, calc-potassic, sodic, propylitic and late stage, typically fault- and fracture-controlled phyllic assemblages. Hematite dusting was a common alteration product giving the intrusions and the altered volcano-sedimentary host sequences a distinctive pink-orange coloration. Several of the deposits have bornite-rich cores, chalcopyrite-dominant annuli and pyritic outer haloes. Gold is well correlated with bornite in most of the deposits, and with chalcopyrite at Cadia Hill. The mineralising intrusions have Sr and Nd isotopic compositions consistent with derivation from a depleted mantle source regime, although the Nd data are permissive of limited crustal contamination. Epidote peripheral to the porphyry deposits has Sr isotopic compositions indistinguishable from the host intrusions, precluding the involvement of external seawater in the mineralising processes. In contrast, slightly elevated initial Sr values have been detected in epidote from nearby skarn deposits, indicative of incorporation of a minor component of Sr from limestone dissolution or seawater mixing. The alkalic porphyry deposits are difficult exploration targets because intensely developed hydrothermal alteration zones are restricted to within a few hundred metres of the monzonite complexes.