Genesis of Chating Cu-Au deposit in the Middle-Lower Yangtze River Metallogenic Belt, Eastern China: implications from magnetite and biotite geochemistry

Porphyry deposits developed in intracontinental settings have some geological characteristics that differ from those developed in arc-related continental margins. The Chating deposit is a newly discovered Cu-Au deposit in the Middle-Lower Yangtze River Metallogenic Belt (MLYB), eastern China. We carried out detailed petrographic observations and major and trace element analyses of different types of magnetite and biotite from the host quartz diorite porphyry and different alteration zones in Chating to elucidate the genesis of Chating deposit.

Alteration types in Chating include skarn, sodic-calcic, potassic and phyllic, and copper mineralization is closely associated with potassic alteration of quartz diorite porphyry. The different types of magnetite and biotite at Chating show distinctive geochemical characteristics. Higher Ti and Al in magmatic magnetites and biotites compared to the hydrothermal minerals indicate higher temperature and pressure in the quartz diorite porphyry. Higher V in magmatic magnetites and lower XMg in magmatic biotites indicate higher fO₂. Similar mineral geochemistry from typical worldwide porphyry deposits, for example, Al₂O₃, TiO₂, FeOT, MgO, Na₂O, IV(F) and IV(Cl) contents in biotite, and Ni/Cr, Ti, Al, Sn and Ga in magnetite, indicate a predominately magmatic fluid source. Differences in Si, Ca, Co/Ni, Mg, and Mn in different types of magnetite, and MnO, CaO, SiO₂ differences in two types of biotite from typical porphyry copper deposits show the influence of external fluids. Moreover, high IV(F) values and low IV(Cl) values in biotites demonstrate that the magmatic-hydrothermal system in Chating was enriched in Cl and lacked F, consistent with the importance of Cl in the transport of Cu. Based on different discrimination diagrams for magnetite, such as Ni/Cr versus Ti, Ni/(Cr + Mn) versus Ti + V, and Al + Mn versus Ti + V, and the geochemical similarities of biotite from Chating and other porphyry deposits, combined with the similarity of alteration assemblages, we conclude that Chating has porphyry Cu-Au affinities, but the hydrothermal system was affected by external fluids, in contrast to more typical subduction-related porphyry deposits.