WATERLOO AND AGINCOURT PROSPECTS, NORTHERN QUEENSLAND - CONTRASTING STYLES OF MINERALIZATION WITHIN THE SAME VOLCANOGENIC HYDROTHERMAL SYSTEM

The Waterloo and Agincourt prospects are two small volcanic-hosted mas sive sulphide deposits that occur close to the stratigraphic top of a large alteration zone within the Trooper Creek Formation of the Cambro -Ordovician Seventy Mile Range Group. Although only 1 km apart, the tw o deposits show differences in the style and mineralogy of mineralized zones in terms of metal contents, ratios and correlations, and sulphu r isotope systematics. The Waterloo deposit is a small, high-grade zin c-copper-rich massive sulphide lens, whereas the Agincourt deposit con sists of low- to moderate-grade disseminated zinc and lead through muc h of the alteration zone with only minor baritic sulphide lenses. Thes e differences resulted from higher temperature fluids in the Waterloo deposit, and the swamping of the Agincourt hydrothermal system by fels ic volcaniclastic flows. Major ore minerals present at both deposits i nclude pyrite, sphalerite, galena, chalcopyrite and barite. Waterloo h as significant quantities of tellurides and bornite, minerals not obse rved at Agincourt. The Waterloo deposit is characterized by high coppe r ratios (100Cu/[Cu+Zn+Pb]) and high zinc ratios (100Zn/[Zn+Pb]). Agin court has low copper ratios and more typical zinc ratios. At Waterloo gold correlates with copper, whereas at Agincourt gold correlates with zinc. Alteration of footwall andesitic units at Waterloo is character ized by two principal mineralogical assemblages: (i) weak carbonate-py rite-sericite; and (ii) a strong sericite-quartz-pyrite+carbonate alte ration. Major and trace element data for relatively unaltered and vari ably altered andesitic volcanic units indicate that Al, Ti, P, Zr, Nb and the rare earth elements remained essentially immobile during hydro thermal alteration, whereas the most intense sericite-quartz-pyrite al teration resulted in strong relative enrichment in K, Rb and Bi, and m ared relative depletion in Ca, Na, Sr, Mn and Cu. delta(34)S values of sulphide minerals from Waterloo and Agincourt are lower than other Ca mbro-Ordovician volcanic-hosted massive sulphide deposits, and the val ues increase from the footwall alteration zone into the massive sulphi de/barite lenses. These characteristics are most likely caused by sulp hide deposition from oxidized hydrothermal fluids.