Controls of Neoproterozoic magmatism on NiCu sulfide mineralization in the Jiangnan Orogenic Belt, South China: An example from the newly discovered Sibao NiCu sulfide deposit

Abstract

The newly discovered Sibao NiCu sulfide deposit, situated in the western part of the Neoproterozoic Jiangnan Orogenic Belt (JOB), may be representative of NiCu sulfide deposits that formed in convergent margins during the Neoproterozoic. The formation of this deposit may also have been influenced by activity of the mantle plume associated with the Neoproterozoic fragmentation of Rodinia. These characteristics make the Sibao deposit unique, because most convergent margin-related NiCu sulfide deposits in China are of Paleozoic to early Mesozoic age and are typically not associated with mantle plume activity. Despite its geological significance, a paucity of research has been carried out on the Sibao deposit; the emplacement setting and mineralizing processes that operated to form this deposit, therefore, remain unclear. To address these ambiguities, we characterized the geochronological, geochemical, and isotopic characteristics of this deposit, focusing on zircon UPb dating, whole-rock trace-element chemistry and SrNd isotopes, in situ S isotopes of sulfides, and trace-element chemistry of zircon and apatite. The host rocks to the Sibao deposit crystallized at ca. 837.7 ± 3.5 Ma, synchronous with ocean–continent subduction during formation of the JOB, but occurring prior to the mantle superplume event associated with the breakup of Rodinia (ca. 825–800 Ma). The systematically negative NbTa anomalies and εNd(t) values (−5.6 to −2.2) suggest that the magma parental to the Sibao intrusion originated from metasomatized, enriched sub-continental lithospheric mantle (SCLM). This suggest that the Sibao deposit was emplaced in an Andean-style continental convergent margin, without any mantle plume contributions. Modeling of REE lambda values suggest that the mantle source of the Sibao parent magma contained a considerable amount of garnet-free pyroxenite, which formed through the reaction of mantle peridotite with fluids/melts derived from subducted oceanic crust. The fO₂ of the Sibao magma was estimated to be in the range of FMQ + 0.3 to FMQ + 1.2 based on the trace-element compositions of zircon and apatite. As demonstrated by the non-MORB δ³⁴S values of sulfides (+1.62 to +3.67 ‰), the Sibao parent magma required the addition of crustal sulfur to attain sulfide saturation. Thus, partial melting of metasomatized SCLM and incorporation of crustal sulfur were critical to the formation of the Sibao deposit. Overall, the Sibao deposit exhibits mantle source characteristics and sulfide saturation processes similar to other deposits emplaced along Andean-style continental convergent margins, such as the Xiarihamu, Erbutu, Kebu, and Aguablanca deposits. Together, these factors imply that Neoproterozoic convergent margins also have the potential to host economically significant NiCu sulfide deposits.