Molybdenum isotope fractionation mechanism of metal sulfides in magmatic hydrothermal systems: Constraints from the Pulang porphyry Cu deposit, Yunnan, Southwest China

Exploration of the application of Mo isotope in the field of mineralogy with precise knowledge of the geochemical behaviors of Mo in the PCDs is of considerable concerns. This study puts critical constraints on the Mo isotope geochemistry in magmatic-hydrothermal systems based on analysis of Mo isotope of the metal sulfides from different mineralization stages (i.e., epidote-chlorite, chlorite-illite, and quartz-illite stages) in Pulang porphyry Cu deposit, Yunnan, Southwest China. We observed that pyrites, chalcopyrites and pyrrhotites with highly variableMo abundances and heavier δ⁹⁸Mo (-0.34 ± 0.04‰ to 3.01 ± 0.03‰) in comparison to molybdenites (δ⁹⁸Mo: -0.90 ± 0.04‰ to 0.08 ± 0.04‰), whole-rock ore-forming porphyries (δ⁹⁸Mo: -0.18 ± 0.03‰ to -0.08 ± 0.03‰) and surrounding rocks (-0.41± 0.05‰ to -0.14± 0.03‰).The variability of δ⁹⁸Mo signatures of these metal sulfides is independent of lithology of originally magma sources, which instead constrained by the evolutions of ore-forming fluids, changes of ore-forming temperature or metallogenic settings, as well as differential geochemical behaviors of Mo species.

In the magmatic hydrothermal metallogenic system, there is a progressive partitioning of Mo into exsolved metallic minerals, with molybdenites preferentially partitioning lighter δ⁹⁸Mo while pyrites, chalcopyrites and pyrrhotites are enriched in heavier δ⁹⁸Mo. Additionally, these metallic sulfides have a preference for relatively heavier δ⁹⁸Mo enrichment in the early metallogenic stage, thereby leaving residual metallogenic fluids with progressively homogenized lighter δ⁹⁸Mo, and consequently relatively homogeneous but lighter isotopes of the metallic minerals in the later stages. The extensively maintained Mo isotope fractionation in PCDs is indicative of a maldistribution of ore-forming components in the sulfides, of which it is this disequilibrium effect that points to the enrichment of metal elements as well as the significant process of magmatic hydrothermal metallogenesis. Mo isotope systematics provide a robust geochemical proxy to interrogate PCDs genetic mechanisms and metal precipitation stages, allowing to consider it as a subservience indicator for the complex metal enrichment that can be extrapolated to other porphyry-type deposits.