Post-Collisional Cu-Au Porphyry and Associated Epithermal Mineralisation in the Eastern Mount Isa Block: A New Exploration Paradigm for NW Queensland

Post-collisional Cu-Au-Ni-Co-Pt-Pd-Sc porphyry, [Duck Creek porphyry system (DCPS)], with overlying Au-Te-Bi-W-HRE epithermal mineralisation, [Highway epithermal system (HES)] has been discovered in the core of the Mitakoodi anticline, southwest of Cloncurry. Xenotime and monazite geochronology indicates mineralisation occurred between ~1490 and 1530 Ma. Host rock lithologies show widespread potassic and/or propylitic to phyllic alteration. Paragenesis of porphyry sulphides indicate early crystallisation of pyrite, followed by chalcopyrite, with bornite forming by hydrothermal alteration chalcopyrite. Cu sulphides also show the effect of supergene oxidation alteration with rims of covellite, digenite and chalcocite. Redox conditions deduced from V/Sc systematics indicate that the DCPS contains both highly oxidized (typical of porphyries) and reduced lithologies, typical of plume generated tholeiitic and alkaline suites. Ni/Te and Cu/Te systematics plot within the fields defined by epithermal and porphyry deposits. Duck Creek chalcophile and highly siderophile element (Cu, MgO and Pd) systematics resemble data from porphyry mineral systems, at Cadia, Bingham Canyon, Grasberg, Skouries, Kalmakyr, Elaisite, Assarel and Medet. SAM geophysical inversion models suggest the presence of an extensive porphyry system below the HES. A progressive increase in molar Cu/Au ratios with depth from the HES to the DCPS, supports this conclusion. Three metal sources contributed to the DCPS-HES viz., tholeiitic ferrogabbro, potassic ultramafic to mafic system and a Fe and Ca-rich alkaline system. The latter two imparted non-crustal superchondritic Nb/Ta ratios that are characteristic of many deposits in the eastern Mount Isa Block. The associated tholeiite and alkaline magmatism reflect mantle plume upwelling through a palaeo-slab window that had accreted below the eastern flank of the North Australian craton following west verging collision by the Numil Terrane. Discovery of this linked mineral system provides a new paradigm for mineral exploration in the region.