Geochemistry and exploration significance of ironstones and barite-rich rocks in the Proterozoic Willyama Supergroup, Olary Block, South Australia

Abstract

The Proterozoic Willyama Supergroup in the Olary Block, South Australia, contains a number of occurrences of ironstones and barite-rich rocks. The barite-rich rocks are present as lenticular horizons, which grade laterally and vertically into quartz-magnetite ± hematite and magnetite-hematite rich iron-formations with enclosing quartz-albite rocks. The stacking of these iron-formations and barite-rich rocks are interpreted as facies changes in the original sedimentary environment, competing volcanogenic and chemical sedimentation and multiple hydrothermal fluid events resulting in the deposition of different hydrothermal products. Epigenetic ironstones may have formed by replacement of iron-formations and associated rocks due to oxidizing, saline hydrothermal fluids causing Fe 3+, Cu and Au remobilization during and/or after prograde metamorphism. Barite-rich rocks, ironstones and iron-formations are characterized by exceptionally low Pb and Zn values, however, scattered elevated Cu and Au concentrations point to a potential for stratabound Cu-Au mineralization. Localized manganiferous iron-formations are characterized by the presence of Fe-Mn garnet, F-apatite, Mn-grunerite and Mn-fayalite and elevated concentrations of As, Au, Mn, P, U and Zn. They represent pure chemical precipitates from high-temperature hydrothermal fluids and thus have mineralogical and geochemical characteristics similar to those of banded iron-formations associated with stratiform Pb-Zn-Ag sulfides in the adjacent Broken Hill Block.