The Jiguanshan porphyry Mo deposit in the Xilamulun metallogenic belt, northern margin of the North China Craton, U-Pb geochronology, isotope systematics, geochemistry and fluid inclusion studies: Implications for a genetic model

摘要

The Jiguanshan porphyry Mo deposit is located in the southern part of Xilamulun metallogenic belt at the northern margin of the North China Craton (NCC). In the Jiguanshan mining district, two stages of granitoids intrusions have been recognized: a pre-ore granite porphyry with stockworks and veins of Mo mineralization, and a granite porphyry with disseminated Mo mineralization. Zircon U-Pb data and Hf isotope analyses show that the dissemination-mineralized granite porphyry yielded a weighted mean ~(206)Pb/~(238)U age of 156.0 ± 1.3 Ma, with a crustal ε_(Hf)(t) values from - 5.6 to + 0.2, and that the main group of magmatic zircons from the pre-ore granite porphyry have a weighted mean ~(206)Pb/~(238)U age of 167.7 ± 1.7 Ma with ε_(Hf)(t) values from - 3.2 to +1.0. Combined with groundmass Ar-Ar age data of the granite porphyry and molybdenite Re-Os age, it is suggested that the Mo mineralization of Jiguanshan deposit was formed in the late Jurassic (153-155 Ma) during tectonic and magmatic events that affected northeast China. The Mo mineralization was a little bit later than the host granite porphyry. Besides disseminated in the host granite porphyry, Mo mineralization also presents in middle Jurassic pre-ore granite porphyry, Jurassic fine-grained diabase, Triassic quartz porphyry, and in rhyolitic volcanic rocks as well as syenite of Devonian age. The Jiguanshan mining district was affected by the tectonic events associated with the Paleo-Asian Ocean closure, and later by far-field tectonism, related to subduction of the Paleo-Paciflc plate (Izanagi) in the Jurassic-Cretaceous. The tectonic and thermal events linked with the latter are commonly referred to as Yanshanian tectono-thermal event, and consists of a series of geodynamic, magmatic and ore-forming processes, which in the mining district area included the intrusion of the pre-ore granite porphyry, the host granite porphyry, Mo mineralization, and fine-grained diabase. Major and trace element analyses show that the host granite porphyry is characterized by high silica abundances (SiO_2 = 77.16 to 77.51%), high Rb/Sr ratios (13.57 to 14.83), high oxidation (Fe_2O_3/FeO = 34.25 to 62.00) and high alkalies (Na_2O + K_2O = 8.21 to 8.38%). Petrographic and microthermometry studies of the fluid inclusions from Mo mineralized veins, characterized by plenty of daughter mineral-bearing inclusions, showed that the predominant homogenization temperatures range from 250 to 440℃. Combined with Laser Raman analysis of the fluid inclusions, it is indicated that Mo mineralization is related to a high-temperature, hypersaline and high-oxygen fugacity H_2O-NaCl fluid system, with high F contents. Based on geology, geochronology, isotope systematics, geochemistry and fluid inclusion studies as well as regional geology, we propose, for the first time, a genetic model for the Jiguanshan porphyry Mo deposit During the Jurassic geodynamic evolution of northeast China, high silicic, high oxidized and alkaline-rich granitic magma probably derived from partial melting of the lower crust, episodically intruded along faults into the country rocks. This fluid system, fractionating from the highly differentiated granitic magma and bearing Mo with minor Cu metals, migrated upwards and interacted with the older wall rocks and associated fractures, in which the ore minerals precipitated, resulting in the development of what we refer to as the "Jiguanshan-type" porphyry Mo deposit.