Geographic dependency of epidote and chlorite chemistry in distal porphyry alteration and its impact on developing global exploration tools

摘要

The utility of epidote and chlorite as an exploration tool for hydrothermal ore deposits, with a specific focus on porphyry Cu systems, has been the theme of several AMIRA research projects since 2004. This research has led to significant advances in the understanding of the propylitic alteration facies and also to the development of exploration tools readily used by industry. Rio Tinto has been a sponsor of all AMIRA Projects on distal porphyry alteration and routinely uses mineral chemistry globally as a robust exploration tool. Since 2010 we have collected major and trace elements of epidote and chlorite along with MLA quantified mineralogy at the Bundoora Laboratory facility in Melbourne. The high quality of Rio Tinto internal data is assured through rigorous QAQC and routine inter-laboratory comparisons with the participant laboratories of the AMIRA Projects and the USGS round-robin of LA-ICPMS external standards. The Rio Tinto trace element database currently contains over 100,000 epidote and chlorite trace element analyses. Samples comprise a diverse suite from global exploration projects, as well as from background alteration domains and orientation surveys around known porphyries. In addition to this in-house data, we reference the out-of-confidentiality AMIRA projects database, a further valuable global data compilation of over 13,000 epidote and chlorite trace element analyses. This compilation allows a robust and updated assessment of the fertility of hydrothermal systems and vectoring to porphyry centres, as well as providing an advanced understanding of the alteration mineral assemblages.