Geochronology and geochemistry of ore-related intrusive rocks and skarn minerals of the Cishan gold deposit in the Tongling ore-cluster region, eastern China: Implications for petrogenesis and skarn mineralization

Abstract

The Tongling ore-cluster region is a major copper–gold polymetallic mining district situated within the Middle-Lower Yangtze Metallogenic Belt of eastern China. Recent explorations have identified the Cishan gold deposit as a porphyry-skarn mineralization within the Shizishan ore field in the Tongling region. This study presents a comprehensive investigation of the Cishan gold deposit, encompassing geochronological, mineralogical, geochemical, fluid inclusion, and Sr-Nd-Hf isotopic studies of metallogenetic intrusive rocks and ore-bearing skarn samples. Additionally, geological modeling informed by drill-hole and gravity data were conducted. Multi-mineral U-Pb dating indicates that skarn mineralization in the Cishan deposits occurred at 138–134 Ma, succeeding the associated magmatism (139–138 Ma). Both the pyroxene diorite and quartz monzonite exhibit metaluminous characteristics and belong to the high-K calc-alkaline-to-shoshonitic series. Pyroxene diorite samples yield similar whole-rock ε_Nd(t) values (−8.2 to −8.8) and zircon ε_Hf(t) values (−8.6 to −12.1) to those of the quartz monzonite samples (−7.2 to −7.3 and −7.7 to −15.4, respectively), suggesting a common magmatic source. Geochemical analysis indicated that the pyroxene diorite originated from an enriched lithospheric mantle source, whereas the quartz monzonite evolved from a dioritic magma chamber. Skarn formed via hydrothermal activity at magma-wallrock contacts, and fluid inclusion analysis implied earlier high to medium temperature and high-salinity fluids formed silicates, and later cooling precipitated sulfides/carbonates. During the Early Cretaceous, the lithospheric extensional setting could induce partial melting of the lithospheric mantle, which initiated by the rollback of the subducted Paleo-Pacific slab. This process led to the formation of a deep magma chamber within the lower crust via magmatic underplating. Subsequent magma differentiation within a shallower chamber culminated in the emplacement of shallow-level stock, apophysis and dike intrusions. These intrusive bodies were instrumental in the genesis of intrusive sequences and development of gold-bearing skarn deposits.