GEOCHEMISTRY AND EXPLORATION SIGNIFICANCE OF IRONSTONES AND BARITE-RICH ROCKS IN THE PROTEROZOIC WILLYAMA SUPERGROUP, OLARY BLOCK, SOUTH AUSTRALIA

The Proterozoic Willyama Supergroup in the Olary Block, South Australi a, contains a number of occurrences of ironstones and barite-rich rock s. The barite-rich rocks are present as lenticular horizons, which gra de laterally and vertically into quartz-magnetite +/- hematite and mag netite-hematite rich iron-formations with enclosing quartz-albite rock s. The stacking of these iron-formations and barite-rich rocks are int erpreted as facies changes in the original sedimentary environment, co mpeting volcanogenic and chemical sedimentation and multiple hydrother mal fluid events resulting in the deposition of different hydrothermal products. Epigenetic ironstones may have formed by replacement of iro n-formations and associated rocks due to oxidizing, saline hydrotherma l fluids causing Fe3+, Cu and Au remobilization during and/or after pr ograde metamorphism. Barite-rich rocks, ironstones and iron-formations are characterized by exceptionally low Pb and Zn values, however, sca ttered elevated Cu and Au concentrations point to a potential for stra tabound Cu-Au mineralization. Localized manganiferous iron-formations are characterized by the presence of Fe-Mn garnet, F-apatite, Mn-grune rite and Mn-fayalite and elevated concentrations of As, Au, Mn, P, U a nd Zn. They represent pure chemical precipitates from high-temperature hydrothermal fluids and thus have mineralogical and geochemical chara cteristics similar to those of banded iron-formations associated with stratiform Pb-Zn-Ag sulfides in the adjacent Broken Hill Block.