Linking Selective Alteration, Mineral Compositional Zonation and Sulfide Melt Emplacement in Orogenic-Type Magmatic Ni–Cu Sulfide Deposits

Abstract The chemical characteristics of magmatic Ni–Cu sulfide deposits in orogenic settings differ from those in cratons as they are characterized by extensive alteration, presence of hydrous minerals and typical normal mantle sulfur isotopes. How these characteristics are related to the mode of formation of the Ni–Cu sulfide deposits is still unknown. Here, we present petrological and mineralogical investigations of the Hongqiling Ni–Cu sulfide deposit, a typical representative of such numerous deposits in the Central Asian Orogenic Belt. Observations at various scales of field outcrops, hand specimens, thin sections and microscopes reveal that alterations are selectively exposed in silicate minerals, particularly olivine and pyroxenes that are close to ore bodies and sulfides. Such selective alteration is also related to sulfide type and abundance, and ore texture in the rocks/ores. It is worth noting that alteration of olivine and pyroxenes becomes weak when hydrous minerals (hornblende and phlogopite) are present. Profile analyses demonstrate that the olivine grains enclosed in or in contact with sulfides have compositional zonation with Fe, Mn, Sc and Zn concentrations higher when closer to the contacts with sulfides relative to the cores and contacts with other silicates, while those grains in sulfide-barren rocks show relatively homogeneous compositions. Similarly, the hornblende grains in contact with sulfide show distinct compositional variations. Sulfides in the rocks and ores are all free of alteration and have mantle-like sulfur isotopic compositions (pyrrhotite δ34S = −1.53 to 0.38‰; pentlandite δ34S = −1.28 to 0.25‰; chalcopyrite δ34S = −1.77 to 0.24‰). These features suggest that the selective alterations occurring in early crystallized phases were induced by sulfide melt intrusion and crystallization, instead of late-stage processes as previously thought. The sulfide melts segregated at the early stage of the magma evolution were presumably hydrous and buoyant in the silicate magma and therefore were transported upward and penetrated into silicate grains. The hydrous fluids were independent of the subducted material and metamorphic origin, and contained no significant crustal component in the late Triassic. Finally, when the sulfides crystallized, fluids would be released from the sulfide melts to hydrate surrounding mineral phases and rocks, because sulfides theoretically don’t accommodate hydrous components. The fluids also enhanced compositional exchanges between sulfides and silicates and accounted for the crystallization of hydrous minerals. From this perspective, fluids play a critical role in sulfide melt transportation and silicate compositional alteration, and selective alteration can be considered as a significant indicator of mineralization and exploration in mafic-ultramafic intrusions.