Ferromagnesian silicates and oxides as vectors to metamorphosed sediment-hosted Pb-Zn-Ag-(Cu-Au) deposits in the Cambrian Kanmantoo Group, South Australia

Abstract

The synsedimentary Wheal Ellen, Angas, Scotts Creek, Aclare, Strathalbyn, and St. Ives Pb-Zn-Ag-(Cu-Au) deposits occur in metamorphosed (amphibolite facies) siliciclastic sedimentary rocks of the Tapanappa Formation in the Kanmantoo Group, South Australia. These deposits are hosted in a stratabound, regionally metamorphosed hydrothermal alteration zone composed of garnet-andalusite-biotite ± staurolite schist. This alteration zone and meta-exhalative rocks, consisting mainly of iron formation, and spessartine garnet-quartz, quartz-magnetite, and quartz-gahnite rocks, serve as the major pathfinders to Pb-Zn-Ag-(Cu-Au) deposits, which include sphalerite, pyrite, galena, chalcopyrite, and pyrrhotite, with minor arsenopyrite and sulfosalts. Compositional variations in ferromagnesian silicates and oxides can also be used as guides to ore on a more local scale. Although Fe-rich garnet is common in country rock mica schists in the Kanmantoo Group, the composition of garnet is more Mn-rich with proximity to Pb-Zn-Ag-(Cu-Au) mineralization. Furthermore, the Zn concentration of garnet associated with Pb-Zn-Ag-(Cu-Au) mineralization is high (100 to 535 ppm Zn) relative to the country rocks (<26 ppm Zn), and to that spatially associated with the Kanmantoo Cu-Au deposit (up to 170 ppm), which is the largest Cu-Au deposit in the Tapanappa Formation. The Zn and Mn contents of staurolite associated with Pb-Zn-Ag-(Cu-Au) mineralization are also high (up to 5.6 wt% ZnO and up to ~5000 ppm Mn) relative to those associated with staurolite in the Kanmantoo deposit (up to 1.3 wt% ZnO and 600 ppm Mn) and in quartz-mica schists. Biotite in the Pb-Zn-Ag-(Cu-Au) deposits are enriched in various elements including up to 2600 ppm Mn, 6400 ppm Cu, and 1200 ppm Zn in the St. Ives deposit, up to 4079 ppm Zn, and 6687 ppm Mn in the Wheal Ellen deposit, and up to 1200 ppm Zn, 5400 ppm Mn, and 3.9 wt% Tl in the Angas deposit. These elemental concentrations are more elevated than those in biotite associated with the Kanmantoo deposit (<200 ppm MnO and ZnO, and < 6 ppm Tl) and in unmineralized country rock quartz-mica schists. Like staurolite and gahnite, Mn-bearing ilmenite at St. Ives is Zn-rich and contains up to 23 wt% ZnO. The enrichment of certain metals in oxides and silicates in Pb-Zn-Ag-(Cu-Au) deposits relative to those found in the same minerals in unaltered country rocks constitute potential pathfinders to base metal deposits in the Kanmantoo Group, and to metamorphosed sedimentary exhalative (SEDEX) deposits elsewhere.