Cobalt-rich porphyry deposits derived from multiple mafic magma injections

In addition to copper, gold, and molybdenum, porphyry deposits are important reservoirs of critical metals such as rhenium, selenium, tellurium, and platinum group elements (PGEs). However, enrichment of cobalt (Co) has received little attention. Several studies have shown that Co enrichment does occur in porphyry deposits, however, the source(s) of Co and the mechanism(s) responsible for its enrichment in the high-temperature hydrothermal systems that ultimately form Co-rich porphyry deposits, are poorly understood. In order to address this knowledge gap, we investigated the Jinchang porphyry deposit in Northeast China which is one of the most Co-enriched porphyry deposits worldwide. In-situ elemental and Fe-S isotopic analysis, as well as electron backscatter diffraction, have been conducted on two types of pyrite (Py1 and Py2). Py1 exhibits a core-mantle-rim structure, with Co enrichment in the core (Avg. 4.5 wt%) and rim (Avg. 7.5 wt%). Py2 displays a distinct core-rim structure, with Co enrichment only in the rim (Avg. 8.4 wt%). The early Co-rich fluid led to the formation of the Co-rich Py1 core. As pyrite continued to grow, Co in the fluid was depleted, leading to the formation of the Co-poor Py1 mantle and Py2 core. The most significant changes in δ56Fe values and Co contents were observed between the Py2 core and Py2 rim (δ56Fe: Δ0.94 ‰, Co: Δ10.67 wt%). This significant variation was generated by the re-injection of Co-rich fluids, which led to the coupled dissolution-reprecipitation of pyrite, leading to the formation of the Co-rich Py1 rim and Py2 rim. Each injection of Co-rich fluid not only formed a Co-rich zone in pyrite, but also precipitated Co-bearing minerals, such as siegenite and cobaltite. The magmatic δ34S isotope signature of pyrite and chalcopyrite (1.5–5.3 ‰) rules out the possibility that Co originated from a sedimentary source. Due to the low Co content in felsic magmas, the repeated injections of Co-rich mafic magma are the only plausible source for the formation of such Co-rich fluids. Besides other possible causes, the heavy δ56Fefluid values derived from mafic magmas suggest the addition of serpentinized oceanic crust slab during subduction, which directly contributed to the formation of mafic magmas.