Geology, mineralogy, geochemistry, and petrogenesis of Ni-Cu-(PGE) and PGE-(Cu)-(Ni) deposits in China

Abstract

Nickel, copper, and platinum-group minerals are regarded as “Critical Minerals” that are crucial to the national economy and sustainable development. Magmatic sulfide deposits account for approximately 93% of Chinese nickel resources, 7.3% of Chinese copper resources, and more than 90% of Chinese platinum-group element resources. This study focuses on the mineralogy, geochemistry, and isotope geochemistry of magmatic sulfide deposits in China, including detailed studies of the Jinbaoshan and Bamazhai deposits in the Emeishan Large Igneous Province, leading to a metallogenic model for the spectrum of magmatic sulfide deposits in China. The 260 Ma Jinbaoshan platinum-group element (PGE) deposit in SW China is a sulfide-poor magmatic PGE deposit that experienced multiple phases of post-magmatic modification. It is characterized by: 1) high-temperature violarite-pyrite1-millerite-chalcopyrite and 2) low-temperature violarite-(polydymite)- pyrite2-chalcopyrite assemblages with more than 16 varieties of platinum-group minerals. Postmagmatic hydrothermal fluids enriched the mineralization in lead, silver, cadmium, and zinc. Cobalt and platinum were also added into violarite, and expelled palladium to the margins of high-temperature violarite and millerite, which also caused the formation of pallidum-enriched minerals. Late-temperature violarite inherited palladium, rhodium, iridium, and osmium from primary pentlandite. Overall, the atypical sulfide assemblages in Jinbaoshan deposit result from multiple overprinted post-magmatic processes, but they did not significantly change the platinum-group element contents of the mineralization, which is interpreted to have formed at high magma:sulfide ratios (500~50000) through interaction of crustal sulfide and a hybrid high-magnesium magma derived by melting of a modified region of the Earth’s mantle. The ~259 Ma Baimazhai nickel-copper-(platinum-group element) deposit is a typical magmatic sulfide deposit in Emeishan Large Igneous Province. The economic No. 3 intrusion is lenticular and concentrically-zoned from finely dispersed through “net-textured” to massive sulfides (margin to core). The sulfide assemblage comprises pyrrhotite, chalcopyrite, and pentlandite, with lesser magnetite, cobaltite, violarite, and galena. The mineralization is enriched in nickel, copper, and cobalt relative to platinum-group elements. Combined with the geochemical features of Baimzhai host rocks, the sulfides appear to have formed from a PGE-depleted magma derived from mantle source that was modified by crustal contamination and formed at moderate magma:sulfide ratios (100–1000). Post-magmatic alteration modified the primary sulfide assemblage, resulting in secondary sulfides enriched in nickelcobalt and antimony-lead-silver-gold. The tectonic and petrogenetic settings of Baimazhai and other deposits in China highlight the potential of nickel-copper deposits to occur in post-subduction settings and exploration potential remains for the Ailaoshan orogenic belt to host additional magmatic sulfide deposits. Unlike other magmatic sulfide deposits in the world, many of which are older and formed primarily in extensional settings, all known Chinese deposits are younger and many are inferred to have formed in compressional settings. Mineral chemical, whole-rock geochemical, ore geochemical, and isotopic data for 18 typical deposits have been used to aid in the assessment of their genesis and prospectively. Most deposits in mountain belts appear to have been derived from magmas formed by partial melting of a modified but originally PGE-depleted mantle source with minor crustal contamination. Most deposits in the Eemeishan Large Igneous Province appear to be hosted by rocks derived from magmas generated from originally more enriched mantle sources with variable degrees of crustal contamination. Deposits related to the breakup of Rodinia exhibit transitional geochemical characteristics. Taken together with the geochemical and isotopic evidence, it is suggested many Chinese magmatic sulfide deposits were derived by melting modified mantle, most likely produced by interaction of recycled oceanic crust with depleted asthenospheric mantle.