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.

精准掌握斑岩铜矿床（porphyry copper deposits, PCDs）中钼的地球化学行为，并探索钼同位素（Mo isotope）在矿物学领域的应用价值，一直是学界广泛关注的重点课题。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，通过分析其钼同位素组成，对岩浆热液系统中的钼同位素地球化学特征进行了严格约束。研究发现，黄铁矿、黄铜矿和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‰）、全岩成矿斑岩体（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同位素组成变异特征与原始岩浆源区的岩性无关，而是受控于成矿流体演化、成矿温度变化或成矿环境差异，以及钼物种的不同地球化学行为。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：辉钼矿优先富集较轻的δ98Mo，而黄铁矿、黄铜矿与磁黄铁矿则富集较重的δ98Mo。此外，在成矿早期阶段，这些金属硫化物更倾向于富集相对较重的δ98Mo，使得残余成矿流体中的δ98Mo逐渐趋于均一化且逐步变轻，进而导致后期形成的金属矿物具有相对均一但更轻的同位素组成。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匀；正是这种非平衡效应，揭示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。钼同位素体系可为解析斑岩铜矿床的成因机制与金属沉淀阶段提供可靠的地球化学代用指标，可将其作为复杂金属富集过程的辅助示踪指标，并可推广应用至其他斑岩型矿床。