Syn-mineralization hydrous fluid activity in giant Jinchuan magmatic Ni-Cu sulfide deposit in North China Craton

Abstract

Giant magmatic Ni-Cu sulfide deposits in the world are hosted mainly in pristine gabbro-dolerite sills (e.g., the first largest in the Noril'sk region) and norite bodies (e.g., the second largest in Sudbury, Duluth, and Voisey's Bay), implying the petrogenic relationship between the giant deposits and anhydrous parental magmas. In contrast, the third largest Jinchuan deposit is hosted in dunite- and lherzolite-dominated peridotite bodies and characterized by the presence of hydrous minerals (e.g., hornblende and phlogopite) and intensive serpentini zation of olivine. The role of water in the formation of the magmatic Ni-Cu sulfide deposit remains unclear. To reveal the potential linkage between sulfide mineralization and ‘water’ (i.e., hydrous minerals), a detailed petrographic investigation of the Jinchuan deposit and the associated mineral chemistry analyses have been conducted in this study. The distribution of serpentinization of olivine in the Jinchuan deposit is unevenly, being more pervasive in the sulfide-bearing rocks compared to sulfide-free rocks. The degree of serpentinization de creases systematically in general sequence outwardly from the core net-textured dunite to the outer disseminated lherzolite at orebody-scale. In the sulfide-bearing rocks, the serpentinization is more intensively observed in the sulfide-rich portions compared to the sulfide-poor portions at thin section-scale. We consider sulfide content dependent serpentinization as ‘selective alteration’, which is distinct from post-magmatic modifications and should have occurred coevally with sulfide liquid emplacement and sulfide crystallization. The selective alter ation with dependence on sulfide contents is caused by hydrous fluids released from sulfide crystallization, because the crystallized sulfides cannot structurally accommodate hydrous components which previous latching onto the sulfide liquid. The olivine grains in sulfide-bearing rocks are reversely zoned with respect to Ni (Ni-poor cores, Ni-rich rims) and Co (Co-rich cores, Co-poor rims) which can be interpreted by stages of sulfide liquid olivine Ni-Co exchange and Fe-Ni exchange. Comparison of intra-grain elemental distributions of olivine grains shows that hydrous fluid activity enhanced elemental exchange between olivine and sulfide liquid. Interestingly, the presence of hydrous minerals, protects olivine from selective alteration, suggesting that the hydrous minerals were formed during syn-mineralization fluid activity. These features are linkage between the inferred fluids compounded with sulfide droplets in previous experiments and their transfer and fate in natural magmatic sulfide deposits. The selective alteration proposed here, formed during the syn-mineralization stage and indicate that the fluid activity is instrumental in the formation of magmatic sulfide ores. The selective alteration may be a common characteristic in ultramafic rock-hosted magmatic sulfide deposits and a pathfinder for mineral exploration.