Barren magnetite breccias in the Cloncurry region, Australia: comparisons to IOCG deposits

We have examined unmineralized and weakly mineralized magnetite-matrix hydrothermal breccias from the Cloncurry region, Australia. These breccias are similar in texture and mineralogy to regional and international IOCG deposits. Comparing mineralized and barren hydrothermal breccias affords a rare opportunity to define the critical factors that must be present to generate economic Cu-Au mineralization. Such comparisons also may lead to geochemical features that fingerprint mineralization. Here we compare rock textures, mineralogy, magnetite trace element geochemistry, and fluid inclusion petrography and geochemistry among barren magnetite breccias and IOCG deposits from Australia and Brazil. Magnetite from barren breccias is enriched in V and depleted in Mn relative to ore-related magnetite from Ernest Henry Cu-Au deposit. In contrast, fluid inclusion compositions in the barren breccias are similar to compositions of fluids from the Carajas IOCG deposits, Brazil, both of which differ markedly from brines related to porphyry copper mineralization. The similarity in brine composition between IOCG fluids and fluids from barren magnetite breccias suggests similar brine sources and fluid-rock reaction pathways for both mineralized and unmineralized systems, while the difference in magnetite chemistry may result from mineral equilibrium assemblage at the site of magnetite deposition.