Pyrite trace element behavior in magmatic-hydrothermal environments: an LA-ICPMS imaging study

Trace elements in pyrite are increasingly being used in ore deposit exploration as a geochemical pathfinder to ore. This is because the trace element systematics of pyrite are sensitive to subtle changes in hydrothermal fluid composition and temperature. In this paper, we present LA-ICPMS trace element maps of pyrite from three styles of ore deposits related to magmatic-hydrothermal processes (epithermal Au-Ag, IOCG, and porphyry Cu-Au-Mo), showing the common geochemical characteristics of pyrite in each regime. Pyrite from porphyry Cu-Au-Mo and IOCG systems has high to very high amounts of Co (commonly ≥1000 ppm) but generally lower and variable Ni, As, and Se (<0.01 to > 1000 ppm). These four elements typically define oscillatory growth zonation patterns, reflecting fluctuations in fluid temperature and chemistry during crystal growth. The economic metals Cu, Au, and Mo are almost exclusively present as inclusions of Cu sulfides, native Au/electrum, Au-Ag tellurides, and molybdenite, respectively, which line or fill fractures in the pyrite crystals. In contrast, pyrite from epithermal Au-Ag systems is characterized by variable but generally low Co, Ni, and Se (i.e., <0.01 to 100 ppm), as well as high As (>1000 ppm). Gold and Ag are also generally high in pyrite from these systems, but unlike IOCG and porphyry-style pyrite they are predominantly deported in solid solution. Tellurium can either be low (0.01-10 ppm) or high (>100 ppm, not counting Au-Ag-Bi telluride inclusions), and other elements such as Sb can also be present in solid solution. This study highlights the importance of imaging mineral grains for elucidating growth history, defining mineral parageneses in hydrothermal systems, and providing key geological information for ore deposit genesis.