Metal(loid) Deportment in Sulfides from the High-Grade Core of the Bingham Canyon Porphyry Cu-Mo-Au Deposit, Utah

Abstract

Abstract Porphyry deposits supply the bulk of the world’s Cu and Mo and significant amounts of Au, as well as other minor and trace metal(loid)s, including Ag, Re, Te, Pd, Se, Bi, Zn, and Pb. Porphyry deposits are gaining in importance as a source of critical raw materials with the increasing global demand for these commodities. To date, minor and trace metal(loid)s are still commonly recovered as by-products from porphyry ores without prior characterization of their host mineralogy that could inform more efficient processing and improved recoveries. We report a comprehensive metal(loid) deportment study on a complete vein paragenetic series in samples from the northwestern high-grade zone of the Bingham Canyon Cu-Mo-Au porphyry deposit, Utah. The polyphase Bingham stock comprises an early premineralization equigranular monzonite phase that was intruded by a series of five successive, ore-related porphyry intrusions. Veins with hypogene Cu-(Fe) sulfide assemblages from all five porphyry intrusions were characterized for their trace metal(loid) contents by laser ablation-inductively coupled-mass spectrometry (LA-ICP-MS). It was found that bornite and digenite contain elevated Bi, Ag, Te, and Se relative to chalcopyrite, whereas the latter contains elevated concentrations of Co, Ga, and In. A stepwise decline in sulfide abundance occurs over the porphyry intrusion sequence and is more pronounced in digenite and bornite than in chalcopyrite. The related diminishing concentration per rock volume (inventory) of Bi, Ag, Te, and Se in the youngest porphyry dikes could have been caused in part by a geochemical change in the mineralizing fluid supply across successive intrusive-hydrothermal cycles. Element mapping of exsolved digenite within bornite revealed characterstic partitioning of metal(loid)s between bornite and digenite; most notably Ag, but also Te and Au are enriched in digenite relative to enclosing bornite. Bornite domains within these composite grains reveal complex zonation of Sn, In, and Bi, which are attributed to stress-induced diffusion within bornite, resulting from the digenite exsolution process. The selective partitioning of metal(loid)s between bornite and digenite is likely a common feature in many porphyry Cu deposits, given the fundamental mineralogical characteristics of these two sulfides. Our results contribute to an improved understanding of the distribution (from mineral to deposit scale) of critical trace metal(loid)s in porphyry deposits, particularly those containing exsolved digenite. This knowledge can be applied to determine more accurately the value of ore resources, to improve geometallurgical models and by-product recoveries, and to help limit the environmental effects of metal(loid) dispersion.