The well-preserved Ashele subseafloor replacement-style volcanogenic massive sulfide (VMS) deposit in the Central Asian Orogenic Belt comprises two stages of Cu mineralization, i.e., early massive sulfides dominated by colloform and euhedral pyrite intergrown with chalcopyrite and sphalerite, which were replaced by late vein-dominated chlorite-chalcopyrite assemblages. In this study, a combined systematic Fe, B, and S isotope investigation was first applied to investigate the sulfide precipitation processes and the relative proportion of fluid sources in different alteration and mineralization stages of the Ashele deposit. Boron isotopes of Mg-rich tourmaline (δ11B from −5.57‰ to −2.73‰, average (avg.) −4.23‰) indicate significant seawater (~19%) participated during the formation of massive sulfides. A two-component mixing model is used to estimate the contribution of seawater and igneous sulfur to the total sulfur budget, and the results show the increasing contribution of magmatic sulfur from the early (35%) to late (76%) stages. In addition, δ56Fe values of pyrite gradually increase from the massive ore (−0.46‰ to −0.02‰, avg. −0.24‰), quartz-pyrite (−0.09‰ to 0.07‰, avg. −0.01‰), chlorite-chalcopyrite-quartz-pyrite (0‰ to 0.21‰, avg. 0.08‰) to the quartz-sericite zone (−0.02‰ to 0.29‰, avg. 0.14‰), which is likely related to the different extent of isotopic exchange and formation temperature, and could be used in the exploration of VMS systems. The new two-stage ore-forming model shows that in the early stage, rapid mixing of hydrothermal fluid from underlying magma chamber with abundant cold seawater led to rapid deposition of pyrite and associated Cu mineralization under relatively oxidized condition, and long-term hydrothermal activities in relatively closed systems would promote the formation of upper massive ores, which resulted in an equilibrium system between pyrite, chalcopyrite, and associated fluid with wide ranges of δ56Fe in pyrite (−0.46‰ to −0.02‰) and chalcopyrite (−1.56‰ to −0.49‰). The late hydrothermal activities in relatively open system would contribute to stringer sulfides or stockworks underlying the massive ore in relatively reduced conditions with heavier Fe isotope compositions in pyrite (−0.09‰ to 0.29‰) and chalcopyrite (−0.60‰ to −0.04‰). Overall, our study demonstrates that the coupling of B, Fe, and S isotopes could be a useful tool to indicate long-term subseafloor infilling and replacement processes for subseafloor replacement-type VMS deposits, which are the prerequisite to form large-tonnage VMS deposits.

中文翻译：

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铁、硼、硫同位素对海底置换型火山块状硫化物系统成矿过程的制约

中亚造山带保存完好的阿舍勒海底接代型火山成因块状硫化物（VMS）矿床包括两期铜矿化，即早期块状硫化物以胶状和自形黄铁矿为主，与黄铜矿和闪锌矿共生，被铜矿化所取代。晚脉为主的绿泥石-黄铜矿组合。本研究首次采用系统的Fe、B、S同位素联合研究来研究阿什勒矿床不同蚀变和成矿阶段的硫化物沉淀过程和流体来源的相对比例。富镁电气石的硼同位素（δ11B从-5.57‰到-2.73‰，平均值（avg.）-4.23‰）表明大量海水（~19%）参与了块状硫化物的形成过程。采用二组分混合模型估算了海水和火成硫对总硫预算的贡献，结果表明岩浆硫的贡献从早期（35%）到晚期（76%）逐渐增加。此外，黄铁矿的δ56Fe值从块状矿石（-0.46‰～-0.02‰，平均-0.24‰）、石英黄铁矿（-0.09‰～0.07‰，平均-0.01‰）、绿泥石-黄铜矿逐渐增加。 -石英黄铁矿带（0‰～0.21‰，平均0.08‰）到石英绢云母带（−0.02‰～0.29‰，平均0.14‰），这可能与同位素交换程度和地层温度不同有关，可用于 VMS 系统的探索。新的两阶段成矿模型表明，早期，下伏岩浆房的热液与丰富的冷海水快速混合，导致黄铁矿和伴生的铜矿化在相对氧化的条件下快速沉积，并长期热液活动。相对封闭的系统中会促进上部块状矿石的形成，从而形成黄铁矿、黄铜矿和伴生流体之间的平衡系统，其中黄铁矿（-0.46‰至-0.02‰）和黄铜矿（-1.56‰至-1.56‰）δ56Fe范围较宽。 −0.49‰）。相对开放系统中的晚期热液活动将有助于在相对还原的条件下，在黄铁矿（-0.09%至0.29%）和黄铜矿（-0.60%至-0.04%）中具有较重的铁同位素组成的块状矿石下方形成细硫化物或网状结构。总体而言，我们的研究表明，B、Fe和S同位素的耦合可能是指示海底置换型VMS矿床的长期海底充填和置换过程的有用工具，这是形成大吨位VMS矿床的先决条件。