THE SHOSHONITE PORPHYRY CU-AU ASSOCIATION IN THE GOONUMBLA DISTRICT, NSW, AUSTRALIA

The Goonumbla porphyry copper-gold deposit in N.S.W., Australia, is ho sted by late Ordovician (439.2 +/- 1.2 Ma) shoshonitic igneous rocks. In terms of their petrography, the rocks vary from andesitic to daciti c lavas and tuffs which are partly intruded by monzonite stocks; they are characterized by high and variable Al2O3 (13.4-19.9 wt%), very hig h K2O values (up to 6.8 wt%) and high K2O/Na2O ratios (0.58-1.48), whi ch are typical for the shoshonite association. The rocks also have enr iched LILE concentrations (Ba up to 1200 ppm, Sr up to 1350 ppm), low HFSE (TiO2, < 0.67 wt%, Zr < 125 ppm, Nb < 10 ppm, Hf < 3.4 ppm), and very low LREE (La < 22.4 ppm, Ce < 31 ppm), which are typical for pota ssic volcanic rocks formed in a late oceanic-are setting. Mineral chem istry of selected magmatic mica and apatite phenocrysts from host rock s reveals relatively high SrO and BaO contents (micas: similar to 0.15 wt% and up to 0.28 wt%, respectively; apatites: up to 0.28 wt% and 0. 19 wt%, respectively) and very high halogen concentrations. Micas are characterized by up to 3.9 wt% F and 0.14 wt% Cl, whereas apatites hav e up to 3.6 wt% F and 0.68 wt% Cl These very high halogen contents com pared to those from barren intrusions imply that the shoshonitic magma tism was the source of mineralization. Copper-gold mineralization cons ists mainly of bornite, chalcopyrite, chalcocite and minor pyrite and tetrahedrite. Native gold occurs mainly as minute grains within silica tes of the host rocks, and more rarely as fine inclusions in the sulph ides. Mineralization is accompanied by wallrock alteration comprising a spatially restricted potassic type and a regional propylitic alterat ion type.Thus, the porphyry copper-gold deposit in the Goonumbla distr ict can be viewed as an additional example of a worldwide association between potassic/shoshonitic magmatism and base- and precious-metal mi neralization. More specifically, it appears to be the oldest recorded example of a shoshonite-associated porphyry Cu-Au deposit from a late oceanic-are setting, a possible modern analogue being Ladolam at Lihir Island, Papua New Guinea.