Molybdenum isotope heterogeneity of metal sulfides from magmatic hydrothermal systems

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.

精准明晰斑岩铜矿床（porphyry copper deposits, PCDs）内钼的地球化学行为，并探索钼同位素在矿物学领域的应用价值，是当前备受关注的研究课题。本研究以中国西南云南普朗斑岩铜矿床不同成矿阶段（即绿帘石-绿泥石阶段、绿泥石-伊利石阶段以及石英-伊利石阶段）的金属硫化物为研究对象，通过其钼同位素组成分析，为岩浆热液系统中的钼同位素地球化学特征提供了关键约束。研究发现，黄铁矿、黄铜矿与磁黄铁矿的钼含量变化范围极大，且其δ98Mo值（-0.34 ± 0.04‰ 至 3.01 ± 0.03‰）显著重于辉钼矿（δ98Mo：-0.90 ± 0.04‰ 至 0.08 ± 0.04‰）、全岩成矿斑岩（δ98Mo：-0.18 ± 0.03‰ 至 -0.08 ± 0.03‰）以及围岩（-0.41± 0.05‰ 至 -0.14± 0.03‰）。上述金属硫化物的δ98Mo同位素组成变化与原始岩浆源区的岩性无关，反而受控于成矿流体演化、成矿温度变化或成矿背景差异，以及不同钼物种的地球化学行为分异。在岩浆热液成矿系统中，钼会逐步分配至析出的金属矿物中：辉钼矿优先富集轻同位素δ98Mo，而黄铁矿、黄铜矿与磁黄铁矿则富集重同位素δ98Mo。此外，在成矿早期阶段，这类金属硫化物更倾向于富集相对更重的δ98Mo，致使残余成矿流体的δ98Mo值逐渐均一化且趋于偏轻，最终导致后期形成的金属矿物同位素组成相对均一并偏轻。斑岩铜矿床中广泛存在的钼同位素分馏现象，表明硫化物中成矿组分的分布并不均衡；正是这种非平衡效应，印证了金属元素的富集过程以及岩浆热液成矿作用的显著特征。钼同位素体系可为揭示斑岩铜矿床的成矿机制与金属沉淀阶段提供可靠的地球化学指标，同时可作为复杂金属富集过程的辅助示踪指标，这一结论可推广至其他斑岩型矿床。