Evidence for magmatic hydrothermal mineralisation at Kanmantoo Copper deposit, South Australia

Abstract

The Kanmantoo Cu-Au deposit is located 55km east of Adelaide, on the eastern edge of the Mt Lofty Ranges, South Australia. It is of Delamerian age and is hosted in the Tapanappa series of the Kanmantoo Group, a pelitic turbidite sequence metamorphosed to amphibolites facies. Models for mineralisation vary from sedimentary exhalative system to epigenetic mineralisation. Despite recent work, the structural evolution of the deposit is largely unknown and this allows for the absence of a definitive model for mineralisation. Detailed face mapping of the 1190RL bench in conjunction with handheld X-Ray Fluorescence Niton gun was adopted to further investigate the relationship between key structural features and element distribution. Micro analysis by petrographic studies, Edax element maps and δ34S isotope analysis was completed to gain understanding into fluid-rock relationships and origin of mineralising fluids. The findings of this study strongly suggest timing of copper mineralisation was associated with the first phase of orogenic extension at 490 ± 3 Ma. The extensional reactivation of compressional D3 shear zones, along with the injection of partially oxidised igneous derived fluids interacting with Fe-rich sediments, allows for the formation of the Kanmantoo magmatic hydrothermal deposit. Sulphur isotope results, and the mapping of magnetite-pyrite-chalcopyrite bearing K-feldspar veins are a very strong evidence of an igneous influence. Cu precipitation is as a result of a cooling oxidised magmatic hydrothermal fluids reacting with Fe in metasediments, and partially interacting with a reducing environment, rather than being directly associated with Fe rich metasomatism. Broad unmineralised zones of chlorite alteration suggest circulation of magmatic hydrothermal fluid with copper mineralisation preferentially precipitating in veins within and adjacent to reactivated D3 shears and D3 antiformal zones.