Application Of Thermochronology To Hydrothermal Gold Deposits,Northwestern Jiaodong Peninsula, China

The Jiaodong gold province is mainly composed of Precambrian basement and ultrahigh pressure metamorphic rocks with the intrusion of large scale Mesozoic granites, concentration of large amount of hydrothermal gold deposits, and accumulation of huge gold resources. It has undergone the continental collision between North China Craton and Yantze Craton, subduction of the paleo-Pacific plate, and related complex transformation of tectonic regime. More than 90% of the proven gold resources are located in the northwestern part of the province. Based on the research on the distribution of the ores, alteration zones, mineral association, and deformation, low- and medium-temperature thermochronology, such as 40Ar/39 Ar dating of muscovite and sericite, zircon and apatite fission track dating as well as zircon(U-Th)/He test, combing with zircon U-Pb, hornblende, muscivte and biotite 40Ar/39 Ar ages, have been applied to constrain the evolution of the regional tectonics, and formation, preservation and exhumation of the gold deposits in the northwestern Jiaodong Peninsula.With the formation of the Linglong Metamorphic Core Complex(MCC), the Linglong granite intruded into the Precambrian basement rocks at ~163-155 Ma on the footwall of the Linglong detachment fault. Subsequently, rapid cooling and exhumation occurred along the Linglong detachment fault at 155-134 Ma forming the ultramylonite-mylonite zones. The exhumation and uplift of the Linglong MCC weaken down since ~134 Ma. Simultaneously, the area underwent significant ductile-brittle transition. The syntectonic Guojialing-Type granitoid, which had undergone obvious ductilte deformation, emplaced at 132-123 Ma. Thereafter, the ductile deformation ceased with the overprint of further intense brittle deformation at 123-120 Ma.With the development of the ductile deformation and metamorphism under upper greenschist condition, the early gold mineralization occurred at ca. 162 Ma in the Precambrian metamorphic rocks. It is simultaneously with the partial melting of tonalite-trondhjemite-granodiorite(TTG) gneiss of Neoarchean Jiaodong Group, migmatization, and the crystallization of Linglong granite. All these occurred at the early stage of reginonal extension.Two periods of Early Cretaceous gold mineralization were distinguished in the northwestern Jiaodong Peninsula. Controlled by the brittle-ductile deformation, the earlier period of gold mineralization occurred at 130±4 Ma in the Linglong detachment fault zone, which is corresponding to the rapid cooling and exhumation of the footwall along the Linglong detachment fault as well as the intrusion of the Guojialing granitoid(132-123 Ma). It was always overprinted by the gold mineralization and related intense brittle deformation at ca. 120 Ma. The disseminated and veinlet pyrite-sericite-quartz altered ores formed just on the footwall of the ore-controlling faults near the major fault plane, while the auriferous quartz-sulfide veins filled in the high angel brittle faults far away from the major faults.Most parts of the Late Jurassic ores have been denuded due to the intense uplift and exhumation of the Linglong MCC. After the Early Cretaceous mineralization, tectonic activities weakened down. But several post-mineralization structures formed cross-cutting the orebodies. The post-mineralization normal movements of the Linglong detachment and Jiaojia faults resulted in the different uplift and exhumation between their footwall and hangingwall. The post-mineralization exhumation of the orebodies is estimated at 3-7 km for deposits located in the block between the Linglong detachment and Jiaojia faults. Given that the ore-forming depth of 5-10 km, to some extent, the orebodies have been eroded. However, the major orebodies have been preserved well, which is good news for further exploration.