Controls on the distribution of molybdenum and other critical trace elements at the Bingham Canyon porphyry deposit, Utah

Abstract

Molybdenite is a common mineral in porphyry deposits and is the only economic source for molybdenum metal. Approximately 99% of the global Mo production is obtained from porphyry-type deposits (e.g. Sillitoe, 2010), making it the metal with the single strongest affinity for a specific deposit type. The controls on the formation of Mo-rich porphyry deposits remain poorly constrained. By combining fieldwork, whole rock geochemistry, detailed analysis of molybdenite (i.e. trace element composition, Mo isotope composition and Re-Os dating) and a fluid inclusion study this project assesses the magmatic-hydrothermal evolution of Bingham Canyon to help constrain the processes controlling Mo mineralisation at one of the world's most prolific Cu-Au-Mo porphyry deposits. New high precision Re-Os molybdenite data suggest that molybdenum mineralisation formed between 37.62 and 38.22 Ma during a late hydrothermal event coinciding with the emplacement of the youngest porphyry phase. Petrographic and fluid inclusion and results indicate two episodes of mineralisation. Type 1 molybdenite formed at depth during the Cu-stage from single-phase, intermediate density parental fluids, prior to phase separation and main Cu deposition. Mo-rich Type 2 mineralisation formed at a later stage from high-salinity fluids and coexisting vapours that were exsolved directly from a fractionated magma source in response to mafic replenishment and T increase. The decoupled deposition of Mo and Cu at Bingham is attributed to the prior preferential extraction of Cu from the underlying, less evolved magma chamber during earlier mineralising events. Molybdenite precipitated in response to cooling and bulk reduction of the fluids. In accordance with data from other Mo-rich porphyry systems, results indicate that Mo-Cu decoupling is controlled by a combination of changes in source magma chemistry as well as by the evolving P-T-X properties of hydrothermal fluids. Molybdenite trace element compositions from Bingham and other localities are a function of their respective magmatic sources and are capable of recording hydrothermal overprints. Mineralogically complex Bi and As mineralisations formed during prograde skarn formation and in a high-sulphidation environment during the veining stages of the hydrothermal system.