Isotopic Geochemistry Of The Yinan Au-Cu-Fe Deposit In Shandong Province, China

As a typical Mesozoic subvolcanic-related Au-Cu-Fe deposit in Western Shandong Province, the Yinan skarn-hydrothermal type gold deposit is located on the west side of the Yishu fault zone (the middle segment of the Tanlu fault zone) at the southeastern margin of the North China plate. It includes the Tongjing and Jinchang ore districts which are 6 km away from each other. The ore bodies occur in the contact zones between the Yanshanian intermediate-acidic composite intrusions and their surrounding wall rocks,and the structural weakness zones (unconformity surface, inter-layer fracture zone, slip zone) in the wall rocks. The ore bodies are zonal distribution around the intrusions. Based on the metallogenic settings, this paper has analyzed the composition characters of the H, O, C, S, Pb and Rb-Sr isotopes and the indicative meaning of these isotopes to the chemistry of the ore fluids and the sources of the ore-forming materials and the intrusions. Combining with the fluid inclusion studies, the nature and evolution characteristics of ore-forming fluids are discussed. The diagenetic and metallogenic ages are determined in the use of isotope geochronology method (Rb-Sr whole-rock isochron and single-grain zircon LA-ICPMS method).The composition of H and O isotopes shows that the ore fluids were most likely composed of magmatic water mixed with limited meteoric water at the later mineralization stage. Sulfur and carbon isotopic data suggest that the ore-forming materials were mainly derived from the deep magma and subordinately from the dissolution of wall rocks. The significant Pb isotopic values of the ore might have resulted from the interfusion with a large amount of radioactive anomalous Pb ("J"type Pb), whose calculated age (2.37Ga) implies that the ore-forming materials had close genetic relations with the crystalline basement (Taishan Group). It can be deduced from the composition of Rb-Sr isotopes that the ore-forming stocks might have been the remelting products of high maturity crustal materials or have resulted from the mixture of crustal materials with limited mantle components.Detailed study on fluid inclusions indicated that the homogenization temperature and salinity of the ore-forming fluids declined from the early to late mineralization stage, and the mineralization occurred under the low-medium pressure and epithermal conditions. The ore-bearing fluids were characterized by medium-high temperature and high salinity in the main ore-forming stages. The boiling of ore-forming fluids may have caused the precipitation and concentration of Au, Cu, and Fe.The diagenetic ages measured by different methods are basically consistent and concentrated in 113 ~ 135 Ma, indicating that the main intrusions formed in the middle-late Early Cretaceous. In view of the close time relationship between diagenesis and mineralization of skarn type deposits, the metallogenic age of Yinan gold deposit should be slightly later than the diagenetic age.