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

The newly discovered Sibao Ni-Cu sulfide deposit, situated in the western part of the Neoproterozoic Jiangnan Orogenic Belt (JOB), may be representative of Ni-Cu 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 Ni-Cu 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 U-Pb dating, whole-rock trace-element chemistry and Sr-Nd 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 Nb-Ta anomalies and epsilon Nd(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 delta 34S values of sulfides (+1.62 to +3.67 parts per thousand), 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 Ni-Cu sulfide deposits.