Platinum-group elements (PGEs) have a strong affinity for sulfur and tend to accumulate in the deep continental crust, either concentrated within magmatic Cu-Ni-PGE deposits or dispersed throughout disseminated sulfides. However, PGE enrichment in shallow magmatic-hydrothermal systems implies an obscure link to deep sulfide destabilization, which releases PGEs into ore-forming fluids. To bridge this gap, our study investigates the PGE composition of magmatic sulfides with oxidative textures in dacitic rocks from the southwestern Okinawa Trough. We identified three groups of magmatic sulfides, primarily precipitated as Cu-poor monosulfide solid solution (MSS), which formed at distinct stages of magma evolution from deep to shallow crustal levels. Group A sulfides manifest as small-sized inclusions (<30 μm) within most high-Mg olivines, whereas Group B and C sulfides are larger (50–500 μm) and occur within cognate xenoliths of mafic cumulate rocks and the groundmass of host dacites. Group B and C sulfides exhibit distinct oxidative textures and newly-formed mineral assemblages, including magnetite, hematite, goethite, and magnetite ± pyrite, alongside hydrothermal silicate minerals, respectively. We attribute the oxidation process to the infiltration of orthomagmatic fluids exsolved from mafic magma that had underplated the sulfide-bearing felsic magma reservoir, which was nearly solidified. By comparing the chemical compositions between pristine sulfides and their oxidative remnants, we observed extensive mobilization of Pd, Pt, Cu, Ag, Ni, and Co from the altered sulfides of Group B, while Au enrichment occurred as nanoparticles under high oxidation states. In contrast, Au was extracted along with other mobile metals from the altered sulfides of Group C, with Pt remaining in place under more reduced conditions. These distinct scenarios may lead to the formation of PGE-rich and Au-rich fluids, respectively. The formation of deep crustal MSS and subsequent hydrothermal oxidation under varying redox conditions thus provides a viable mechanism for -crustal PGE mobilization and inter-element fractionation, typical of PGE-rich magmatic-hydrothermal deposits.

中文翻译：

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岩浆硫化物氧化驱动地壳 PGE 动员：对热液 PGE 矿化的影响


铂族元素 (PGE) 对硫有很强的亲和力，往往积聚在大陆地壳深处，要么集中在岩浆 Cu-Ni-PGE 矿床中，要么分散在浸染状硫化物中。然而，浅层岩浆-热液系统中的铂族元素富集意味着与深层硫化物不稳定的模糊联系，后者将铂族元素释放到成矿流体中。为了弥补这一差距，我们的研究调查了冲绳海槽西南部英安岩中具有氧化结构的岩浆硫化物的 PGE 组成。我们确定了三组岩浆硫化物，主要以贫铜一硫化物固溶体（MSS）形式沉淀，形成于从深地壳到浅地壳水平的岩浆演化的不同阶段。 A 组硫化物在大多数高镁橄榄石中表现为小尺寸包裹体 (<30 µm)，而 B 组和 C 组硫化物则较大 (50–500 µm)，出现在镁铁质堆积岩的同源捕虏体和主英安岩的基质中。 B 组和 C 组硫化物表现出独特的氧化结构和新形成的矿物组合，分别包括磁铁矿、赤铁矿、针铁矿和磁铁矿 ± 黄铁矿，以及热液硅酸盐矿物。我们将氧化过程归因于从镁铁质岩浆中溶出的正岩浆流体的渗透，这些岩浆使含硫化物的长英质岩浆储层底板化，该储层几乎已凝固。通过比较原始硫化物与其氧化残余物之间的化学成分，我们观察到 B 组改变的硫化物中的 Pd、Pt、Cu、Ag、Ni 和 Co 广泛迁移，而 Au 在高氧化态下以纳米颗粒形式富集。 相比之下，Au 与其他可移动金属一起从 C 族改变的硫化物中提取出来，而 Pt 在更还原的条件下保留在原位。这些不同的情况可能分别导致富含 PGE 和富含 Au 流体的形成。因此，深部地壳 MSS 的形成以及随后在不同氧化还原条件下的热液氧化为地壳 PGE 动员和元素间分馏提供了可行的机制，这是富含 PGE 的岩浆热液矿床的典型特征。