Trace elements in magnetite from porphyry Cu–Mo–Au deposits in British Columbia, Canada

Abstract

This study examines trace elements in hydrothermal magnetite from five porphyry Cu–Mo–Au deposits and two skarns in British Columbia, Canada. Trace element concentrations vary several orders of magnitude both within and between magnetite from skarn and porphyry deposit settings. The heterogeneous composition of hydrothermal magnetite may in part be due to the short duration, low temperature and multiple-fluid events that attend the formation of porphyry ore deposits. Principal component analysis shows two dominant patterns of trace element abundances in hydrothermal magnetite. Firstly, positive correlations of Ti, Al and V, which account for nearly 40% of the total variation in magnetite, are inferred to depend on temperature and oxygen fugacity. Secondly, antithetic abundances of lower valence cations (Co, Mn) with higher valence cations (Sn and Mo) may reflect variations in the redox potential, acidity and metal speciation of hydrothermal fluids. The Cu/Fe and Mn/Fe ratios calculated for fluids in equilibrium with the hydrothermal magnetite using experimental partitioning data are similar to those measured directly in brines trapped in quartz-hosted fluid inclusions from porphyry Cu–Mo–Au deposits.