The molybdenite Re-Os isotopic ages of the Songjianghe gold-(molybdenum) deposit in Dunhua City, Jilin Province and its geological significance

The Songjianghe gold (molybdenum) deposit is a medium-sized gold deposit (with Au reserve of 9 t) discovered in the ductile deformation zone in the past decade. In recent years, a lot of molybdenite has been found in orebodies from fifth to eighth levels and even to the ninth level of the Songjianghe deposit, with an increasing tendency of the molybdenum mineralization from the fifth to the eighth levels. In order to determine the age of molybdenum mineralization, in this paper, the high-precision ICP-MS molybdenite Re-Os isotope dating technology was used to have firstly obtained the weighted average Re-Os isotopic model age of 167± 2.4 Ma and the isochron age of 158± 10 Ma for molybdenites in the Songjianghe deposit, respectively. These ages are basically consistent with ages(164 - 162 Ma) of the Huangnihe intrusion and ages(174 - 172 Ma) of the Wudaotaohe intrusion in the Songjianghe area reported by previous researchers within their standard deviation ranges. It is confirmed that the mineralization of the deposit and the diagenesis of those intrusions occurred almost simultaneously and mainly in the early Yanshanian stage of Middle and Late Jurassic periods. Combined with published chronological data of diagenetic and metallogenic events in the central and eastern regions of Jilin Province, it is believed that many molybdenum polymetallic deposits in the region were formed in the active continental margin geodynamic background by the subduction of the Paleo-Pacific plate toward the Eurasian lithosphere during the Yanshanian movement period. The molybdenites in the deposit have ω(Re) values varying from 142.74×10 –⁶ to 217.67×10 –⁶, with an average value of 196.58×10 –⁶. Combined with geological characteristics of the deposit, O-S-Pb isotopic data of ore-bearing intrusions and ores, it is believed that the ore-forming materials of the deposit were mainly sourced from the mantle and the deeply sourced ore-forming fluids could have played important role for the mineralization of the deposit.