Hydrothermal alteration associated with Mesozoic Linglong-type granite-hosting gold mineralization at the Haiyu gold deposit, Jiaodong gold province

The Haiyu gold deposit, located in the north part of the Jiaodong gold province, was discovered in 2015 as a superlarge gold deposit with gold reserves 470 t @ 4.30 g/t. The gold orebodies were hosted in the Mesozoic Linglong-type granites and controlled by the Sanshandao-Cangshang Fault (SCF). Host Longlong-type granitic rocks for Au mineralization show a complex paragenetic sequence with hydrothermal alteration. Remobilization of the SCF system allowed for infiltration of hydrothermal fluids, leading to extensive K-feldspar alteration along the main fault. Subsequently, massive sericite formation occurred along both sides of the main fault. With the formation of fault gouge, the ore-bearing fluid could not migrate to the upper wall of the fault zone; therefore, the ore-forming fluid underwent intense sericite-quartz-pyrite alteration in footwall accompanied by gold precipitation. Finally, the formation of quartz-carbonate veins indicated the decrease of hydrothermal activity related to gold mineralization. The equilibrium calculation of potash and sericite rocks revealed that SiO₂, MgO and CaO were brought in, TiO₂ and K₂O were basically unchanged, while Na₂O appeared to be taken out. Most major elements were affected by strong mineral reactions. Au, Ag, Bi, As, Pb, Zn and other related ore-forming elements showed a positive correlation and were closely related to sericite-quartz-pyrite alteration. The mass balance calculation shows that different types of elements had complex geochemical behaviors in the process of water-rock reaction. The alteration combination and fluid inclusion study shows that the ore-forming fluid was characterized by medium and low temperature (126~351℃) and medium and low salinity (1.02~10.48%NaCleqv), belonging to the CO₂-H₂O-NaCl±CH₄ system. In hydrothermal fluids, gold might have migrated mainly as Au(HS)^2- complex. During the process of sericite-quartz-pyrite alteration, Au(HS)^2- complex was destabilized and decomposed by sulfofication, leading to Au precipitation and mineralization. The reactivation of the North China Craton led to the upwelling of asthenosphere and the formation of a large number of igneous rocks, and also provided sufficient thermal energy and fluid input for large-scale gold mineralization in Jiaodong gold province.