Genesis of the Wangjiazhuang Cu-(Mo) deposits, Luxi Terrane, Eastern China: Implications from magnetite trace elements

Abstract

The Wangjiazhuang Cu-(Mo) deposit is the largest porphyry deposit in the Zouping volcanic basin in the Luxi terrane, western Shandong Province. We investigated the petrology and reported the trace element compositions in magnetite, to further unravel the fluids evolution and ore genesis. 5 types of magnetite from 4 stages were identified: Mt1 is enriched in Mg, Al, Ti, V, Mn, Co, Ni, Zn, Ga, consistent with a dominantly magmatic origin. Mt 2 and Mt 3 are magmatic-hydrothermal origins, supported by petrology and discrimination diagrams of Ti vs Ni/Cr, V vs Ti, and Ni vs Cr. Mt2 is associated with alteration minerals (e.g., chlorite, sericite) and can be further divided into Mt2a and Mt2b according to their textures. Mt2 is characterized by hematite rims and low V contents, implying high oxidation state of magma is the key factor in generating the Wangjiazhuang porphyry deposit, and it is related to the partial melting of the subducted oceanic crust during progressive Paleo-Pacific slab roll-back stage. Mt3 is associated with pyrite, chalcopyrite, and ilmenite, representing the main mineralization stage. Variation trend of V, Mn, Mg, Al, and Si contents in Mt2 and Mt3 and transfer of Fe-oxide phases (from hematite to ilmenite) indicate oxygen fugacity and fluid-rock interactions decrease until sulfide precipitation. Magnetite crystallization plays a critical role in the transition from oxidized magma to reduced porphyry, which would produce H₂S and subsequently react with chalcophile elements to precipitate sulfides. Mt4 occurs as veinlets which are formed related to later hydrothermal fluid. It has distinct binary plots such as Al vs Ti, Ga vs Zn, and (Ti + V) vs (Al + Mn), indicating a new pulse of fluid influx with higher temperature and fO₂ than the main mineralized stage fluid.