Magmatic sulfide formation and oxidative dissolution in the SW Okinawa Trough: A precursor to metal-bearing magmatic fluid

Abstract

Abstract Magmatic sulfide formation and oxidative dissolution are effective in metal pre-enrichment and remobilization during magmatic processes, and are considered to be a prerequisite for the formation of magmatic–hydrothermal ore deposits in subduction zones. However, this sequential process and sulfide oxidation mechanism are poorly understood. In this study, we report fine-grained sulfides with variable degrees of oxidation distributed within mafic cumulates, brought into felsic magma as mafic magma intrusions in the southwestern Okinawa Trough. Thermodynamic modeling suggests that the oxygen fugacity (fO2) of primitive mafic magma is ∼FMQ + 2.5, reducing to ∼FMQ + 1 in evolved mafic magma and to ∼FMQ + 0.5 in felsic magma. This variation results from the continuous assimilation of reduced country rocks, as reflected by negative δ34SVCTD values (–0.62‰ ± 0.23‰) of magmatic sulfides and highly radiogenic Sr–Nd isotopic compositions of host dacite. Magmatic reduction and addition of crust-derived sulfur result in early formation of crystalline sulfides, dominantly pyrrhotite, enriched in Cu (0.93 ± 0.38 wt.%). A new injection of oxidized mafic magma triggers partial to complete oxidation of the earlier magmatic sulfides, ultimately transforming them into hematite–magnetite (HM) intergrowths. This means that sulfide oxidation is not directly associated with the mafic magma itself, but more likely with high-fO2 magmatic fluid exsolved from the primitive mafic magma. The oxidative dissolution process releases >90% of Cu from the primary sulfides, with this Cu preferentially moving into magmatic fluid. This indicates that metal remobilization and transfer is very efficient under HM buffer conditions, in which trisulfur S3− ions with high affinities for Au and Cu are probably the dominant sulfur species. We suggest that the periodic injection of oxidized mafic magma into continental crust containing reduced lithologies is essential for the sequential process of early sulfide formation and later oxidative dissolution in the Okinawa Trough and in other subduction zone settings with similar tectono-magmatic conditions. This process could be a precursor to the generation of metal-bearing magmatic fluid, contributing to mineralization in subduction zones.