Genesis of the Tianbaoshan Pb-Zn-Cu-Mo polymetallic deposit in eastern Jilin, NE China: Constraints from fluid inclusions and C-H-O-S-Pb isotope systematics

摘要

The Tianbaoshan Pb-Zn-Cu-Mo deposit is located in the eastern part of Jilin-Heilongfiang region, NE China which is considered to be the eastern segment of the Central Asian Orogenic Belt. Field and geochronological evidences indicate that this deposit experienced three types of mineralization including Hercynian skarn, cryptoexplosion breccia pipe, and Yanshanian quartz vein. The early stages of the Pb-Zn-Cu mineral systems in skarn and cryptoexplosion breccia pipe are characterized by a high-temperature, high-salinity H2O-CO2-NaCI system of hydrothermal fluids that were possibly exsolved from the Hercyriian wall-rock granodiorite. These fluids show H-O isotopic compositions similar to those of typical magmatic fluids. By contrast, the low temperature hydrothermal fluids of the later stages are represented by low-salinity NaCI-H2O systems with low H-O isotopic values. The skarn and cryptoexplosive breccia pipe types of Pb-Zn-Cu mineralization tend to have wealdy negative delta S-34 values of -4.0 parts per thousand to -0.8 parts per thousand (mean values of 2.31 parts per thousand and 2.16 parts per thousand, respectively), indicating that the sulfur was sourced from the Hercynian magma. Therefore, the early stage ore-forming fluids of the skarn and cryptoexplosive breccia pipe were most likely sourced from high-temperature and high-salinity fluids closely related to the cooling and fractional crystallization of the Hercynian granodiorite, while the later stages changed to NaCI-H2O meteoric water influx. Whereas the ore-forming fluids of the quartz vein type of Mo mineralization were high-temperature, high-salinity NaCI-H2O systems that differed from those of the skarn and cryptoexplosion breccia pipe, but their H-O isotopic compositions also indicate a magmatic fluid. The weakly enriched delta S-34 values of molybdenite from the quartz vein type Mo mineralization (delta S-34 = 0.2-2.8 parts per thousand, average of 1.65 parts per thousand) are comparable with those of other Mesozoic Yanshanian Mo deposits (delta S-34 = 0.4 parts per thousand-4.1 parts per thousand, with an average of 1.39 parts per thousand-3.15 parts per thousand), but differ significantly from those of the Hercynian Pb-Zn-Cu skarn and cryptoexplosion breccia pipe. This indicates that the sulfur of quartz vein type of Mo originated from Mesozoic Yanshanian magmatism and that the ore-forming fluids were derived from Yanshanian magmatic rocks rather than being a product of the Hercynian activity. The delta C-13 values of the fluid inclusions in quartz from the skarn, cryptoexplosion breccia pipe and quartz vein types are in a narrow range of -19.5 parts per thousand to -93 parts per thousand, similar to those of the Shanxiuling Group, which indicates that the carbon of the three types of mineralization had the same primary origin in the Shanxiuling Group. The lead isotope compositions of ores from the skarn and cryptoexplosion breccia pipe types of mineralization ((206)pb/(204)pb = 18.0725-18.3627, (207)pb/(204)pb = 15.3721-15.5694 and (208)pb/(204)pb = 37.5542-38.8208) overlap with those of the Hercynian granodiorite and Shanxiuling Group marble, suggesting that the lead was probably derived from a mix of two different sources, the Shanxiuling Group and the Hercynian granodiorite. Whereas the lead isotope compositions of ores from the quartz vein type of Mo mineralization ((206)pb/(204)pb = 18.3837-18.6949, Pb-207/Pb-204 = 15.6824-15.