Molybdenum isotopic fractionation in the Tibetan Yulong Cu-Mo deposit and its implications for mechanisms of molybdenite precipitation in porphyry ore systems

Abstract

Large variations in Mo isotopes (up to ~2‰ δ98Mo) of molybdenite from many porphyry ore deposits have been reported. However, the mechanisms of Mo isotopic fractionation and their implications for mineralization processes remain poorly understood. Here we report contrasting Mo isotopes of molybdenite formed in two discrete, successive mineralizing stages of the giant Yulong porphyry Cu-Mo deposit in eastern Tibet which, when combined with available fluid inclusion data and high-precision molybdenite Re-Os ages, provide a better understanding in the mechanisms of Mo isotopic fractionation and molybdenite precipitation. The initial ore-forming fluids of both mineralizing stages at Yulong exsolved from magmas as a single-phase, intermediate-density fluid, which subsequently separated into coexisting brine and vapor phases upon ascent and decompression. The relatively uniform Mo isotopes (δ98Mo = 0.03–0.18‰) of the early Cu-Mo stage is consistent with the precipitation of molybdenite from relatively stagnant, high-density brines at shallow levels after phase separation. In contrast, the large spread of Mo isotopes (δ98Mo = −0.16–0.60‰) of the transitional Mo stage requires a process similar to instantaneous Rayleigh fractionation and suggests molybdenite precipitation from rapidly ascending single-phase fluids before phase separation in deep parts of the deposit . These arguments imply that the amount of Mo precipitation from the single-phase fluids likely determines the degree of Mo isotopic fractionation. Further quantitative modeling results suggest that the relatively large Mo isotope variation of molybdenite in many porphyry ore deposits may be simply explained by the precipitation of significant amount of Mo from the initial single-phase fluids. Therefore, apart from the precipitation of molybdenite with economic importance from metal-enriched brines that condensed from the single-phase fluids, the direct precipitation of molybdenite from the single-phase fluids may represent another significant mechanism of molybdenite precipitation in porphyry ore systems.