Sources of Ore-forming Metals in Porphyry Deposits

      The ultimate sources of ore-forming materials (including ore-forming metals such as Cu, Mo, and Au, as well as volatiles such as S, Cl, and water) in porphyry mineralization systems include three geological endmembers: subducted oceanic crust, upper mantle, and crust.

      Studies of mantle xenoliths beneath magmatic arcs show that metallic pyroxene dikes formed by the metasomatism of the mantle wedge by subducting slab fluids are 2 to 800 times richer in Cu, Au, and PGE than the surrounding unmetrospatial peridotite. Os and O isotope studies of the subduction zone reveal that the contribution of subducted oceanic crust to chalcophile elements is less than 10%, indicating that Cu and Au likely originate from the mantle rather than subducting slab melts. Slab fluid metasomatism promotes the redistribution and re-enrichment of ore-forming metals within the mantle wedge. Therefore, in the magmatic arc environment of a subducting zone, ore-forming metals mainly originate from the metasomatism of the mantle wedge by subducting slab fluids. The partial melting of the mantle wedge releases metals into the arc magmatic system, forming porphyry deposits.

      For non-arc porphyry deposits, the sources of ore-forming metals vary depending on the tectonic environment: ① In collisional orogenic environments (such as the Gangdese Belt), the Early to Middle Mesozoic Gangdese mantle-derived arc magma underwent large-scale underplating and consolidation at the bottom of the crust, with local cumulonimbus formation (such as cumulonimbus amphibolite, Cu: ~1000×10-6), forming a newly formed mafic lower crust rich in metallic sulfides (gabbro and garnet amphibolite, with magmatic primary Cu sulfides). During the collision period, this newly formed lower crust remelted or decomposed, providing a large amount of metallic Cu to the magma. The mineralized porphyries (δ65Cu = 0.18‰~0.87‰) and hydrothermal chalcopyrites (δ65Cu = 0.08‰~1.01‰) within the zone are significantly enriched in heavy δ65Cu isotopes compared to the low-mineral porphyries (δ65Cu = -0.04‰~+0.18‰), further demonstrating that the mineralized magma and metal originated from sulfide-enriched newly formed lower crust rather than the mantle (δ65Cu = 0.03‰±0.24‰); ② In intracontinental orogenic belts or intraplate environments, melts produced by the melting of the delaminating lower crust (usually metal-poor due to eclogification) react with the metal-enriched lithospheric mantle (such as wedge-shaped mantle replaced by slab fluids), extracting Cu (Au) from the latter to form mineralized porphyry magma; ③ Mo mainly originates from felsic magma from the melting of continental crust with high Mo abundance.

      Studies have shown that, regardless of whether it is in a magmatic arc or non-arc environment, ore-forming magmas are usually relatively enriched in ore-forming metals (Cu, Au, Mo). However, the formation of porphyry deposits does not require that the ore-forming magma be abnormally enriched in metal components in the initial stage, but it does require that the metal sulfide phases have not undergone large-scale saturation and separation before the magma fluids exsolute. This process is controlled by factors such as magma oxygen fugacity and water content.