Sulphide mineralisation in the Olary Block, South Australia

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The South Australian portion of the Willyama Inliers hosts a diversity of small sulphide and uranium deposits and numerous outcropping gossans. This fact, together with geological similarities to the adjacent Broken Hill Block has led to extensive exploration. A broad classification distinguishes two main types of sulphide mineralisation: 1) stratiform iron sulphide-dominated (±Cu, Zn, Co) deposits which occur widespread within specific stratigraphic intervals, and stratabound occurrences of syn-depositional to diagenetic origin which show some structural control; 2) syn-tectonic to post-peak metamorphic replacement and vein-type deposits (Fe-Cu-Au and Cu-Zn-Pb), which are hosted by fractures and within faults and shear zones. These occurrences show no stratigraphic control and are not spatially related to type 1 mineralisation. Late-stage deposits also differ from stratiform/stratabound mineralisation in their texture, mineral assemblage and geochemical composition. Much of the sulphide mineralisation in the Olary Block has been interpreted as resulting from rift-associated syn- to diagenetic processes, such as hot spring exhalations and base metal precipitation along reduction-oxidation interfaces. Subsequent granitic intrusive, high grade metamorphic and multiphase deformation events would have induced remobilisation and redeposition of sulphides in a variety of epigenetic modes. However, a detailed petrographic and geochemical study of sulphide mineralisation in the Olary Block demonstrates that due to the lack of abundant pervasive fluids, translocation and modification of preexisting sulphides were restricted to less than a few centimetres. Instead, widespread syn-tectonic to epigenetic (i.e., post-peak metamorphic) mobilisation of ore constituents occurred to form retrograde sulphide mineralisation as well as multiple generations of late-stage vein deposits. These epigenetic deposits are genetically unrelated to synsedimentary and diagenetic occurrences, an aspect of significance for exploration in the Olary Block. Temporal separation of peak metamorphism in deeper crustal levels from its occurrence in shallow levels, periodic tectonic disturbances and repeated seismic pumping are processes believed to have resulted in intermittent mobilisation of ore constituents from a deep-seated metasedimentary reservoir.