Characteristics,Genesis,and Geodynamic Setting Of Representative Gold Deposits In The Xiong’ershan District,Southern Margin Of The North China Craton

The southern margin of the North China craton (NCC) represents one of the most important metallogenic belts in China, hosting numerous Au, Mo, and Ag-Pb-Zn deposits. These deposits are clustered in three mineralizing districts, including Xiaoqinling, Xiaoshan, and Xiong’ershan. Of these, the Xiong’ershan districts ranks the third largest gold concentration in China, following the Jiaodong and Xiaoqinling districts. Most gold deposits in the Xiong’ershan district have been extensively described and discussed in the last two decades, issues on their formation ages, source of mineralizing fluids and other components, and tectonic setting under which gold mineralization took place remain not well understood or hotly debated.. In this thesis, I present a comprehensive geological, mineralogical, geochemical, and geochronological study on four representative gold deposits from the Xiong’ershan district, including Qianhe, Dianfang, Shanggong and Gongyu. The objective of this study was to provide tight constraints on the timing and genesis of the district-wide gold mineralization and shed lights on the tectonic setting responsible for the pervasive gold mineralization. Based on detailed field work and petrographic examinations, a comprehensive geochronology employing40Ar/39Ar, Re-Os, and Sm-Nd isochron methods was conducted. In addition, zircon U-Pb dating of granitoid dikes spatially related to gold veins in the Qianhe and Gongyu gold deposits was carried out. The geochronological results are used to constrain the ages of gold emplacement and provide a better understanding in the temporal and spatial relationship between gold mineralization and magmatism in the district. We further use mineralogical, stable isotope (C-H-O-S) and noble gas isotope (He-Ar) data to provide information about the sources of ore fluids and genesis of these deposits. These data, in combination with regional geological, geochemical, and geochronological evidence, are used to discuss a possible link between the Mesozoic gold mineralization and the evolution of Qinling orogenic belt and the destruction of the North China craton. The main results and conclusions are summarized as the follows:The deposits under investigation are mostly hosted in the unmetamorphosed volcanic successions of intermediate to acidic compositions of the Paleoproterozoic Xiong’er Group and the amphibolite facies metamorphic rocks of the Neoarchean to early Paleoproterozoic Taihua Group. Gold mineralization consists mainly of auriferous quartz veins with minor amounts of disseminated gold in alteration zones in proximity of gold lodes. Most gold veins are structurally controlled by northeast-striking faults, and are particularly hosted in various discontinuities along brittle-ductile to brittle structures, such as dilational jogs or bends, changes in strike, bifurcations, splays, and extensional fracture arrays. Field and petrographic data indicate that gold mineralization consists of three stages, including quartz-pyrite stage (Ⅰ), quartz-polymetallic sulfide stage (Ⅱ) and carbonate-quartz stage (Ⅲ). Stages Ⅱ and Ⅲ are the most important in terms of gold deposition. Pyrite is the predominant sulfide mineral in all mineralization stages in gold deposits, with minor amounts of galena, sphalerite, chalcopyrite, quartz, sericite, K-feldspar, chlorite and epidote. In addition, a large number of hydrothermal granet and ankerite are developed in the Dianfang and Shanggong gold deposits. Hydrothermal alteration is well developed, mostly marked by silicification, sericitization, sulfidation, biotitization, carbonatiza-tion and the combination. Sericitization is the most spectacular and pervasive alteration style, invariably coexisting with pyrite and locally other sulfide minerals and being closely related to gold mineralization. Reflected light microscopy and scanning electron microscopy combined with energy dispersive spectrum (SEM-EDS) were utilized to present the occurrences and compositions of gold minerals. Visible gold is distributed in the fractured pyrite and is recognized showing isolated, veinlets and particularly related to microfractures, which mostly composed of native gold and electrum.Hydrothermal sericite40Ar/39Ar, molybdenite Re-Os and garnet Sm-Nd dating of Qianhe, Shanggong, Gongyu, Xiaoshan and Dianfang gold deposits indicates two separate episodes of gold genesis at156.1to123.8Ma and236.5to202.0Ma respectively. The mineralization ages overlap zircon U-Pb ages of156.4to132.6Ma and236.2±1.7Ma for the granitic and alkaline dikes in and close to the Qianhe and Gongyu gold mine. Together with the geochronological data for intrusions and ore deposits from adjacent region (e.g. Xiaoqinling and West Qinling) in recent years, it is suggested that the southern margin of NCC underwent two independent magma-mineralization events in the early and late Mesozoic, respectively.Stable isotopes, including C-H-O-S isotopes, and noble gas isotopes (He-Ar) were used to provide information about the sources of ore fluids and and other components. The stable isotope data including C-H-O isotopes show that, the samples from Qianhe and Gongyu gold deposits are plotted in close proximity to the magmatic water field in the δDH2o vs. δ18OH2O diagram. However, the samples from Shangong gold deposit are closed to the metamorphic water field. The C-O isotopic features show difference from the Qiyugou and many gold deposits in the Jiaodong gold district, but consistent with typical orogenic gold deposits from Victorian, Australia, indicating significant differences in the sources of ore-forming fluid between Xiong’ershan and Jiaodong gold district.534S values of sulfide minerals including pyrite, chalcopyrite and galena, mostly range between-21.1%o and+6.84%o. On one hand, the positive sulfur isotopic compositions indicate a deep-seated sulfur source most likely of magmatic or mantle origin. On the other hand, the negative δ34S values probably related to the intense water-rock reaction and the transformation from the reducing to the oxidation environment. Noble gas isotopes of sulfide minerals provide additional sisights into the source of mineralizing fluids.3He/4He ratios of pyrite from the Gongyu, Dianfang, Shanggong and Xiaoshan gold deposits ranging from0.20～1.78Ra (mean value of1.16Ra)、0.18～1.09Ra (mean value of0.62Ra)、0.03～0.95Ra (mean value of0.35Ra) and0.23～1.11Ra (mean value of0.60Ra) respectively, suggest contribution of mantle-derived components to gold formation.The late Mesozoic gold deposits in the Xiong’ershan district share many similar features with typical orogenic gold deposit. These include ore-controlling structures, vein geometry, alteration assemblages, and geochemical association. However, they are distinctly different on some key factors including the relationship between mineralization and metamorphism, fluid inclusion characteristics, and origins of the ore fluid and material. Nevertheless, the late Triassic Shanggong gold deposit was contemporaneous or slightly later with the final uplift of the Qinling orogen formed by the Triassic continental collision and is similar with the orogenic gold deposit on occurrences of ore body, assemblages of ore minerals, characteristics of ore-forming fluids, and compositions of C-H-O-S isotopes, both indicating that the Shanggong deposit is a typical orogenic gold deposit in the area.Collectively, the Xiong’ershan district has witnessed two separate periods of tectono-magmatism and gold mineralization in the early and late Mesozoic. This is the case for the Xiaoqiling district immediately to the west of the Xiong’ershan. The early Mesozoic gold mineralization and magmatism was temporally consistent with the formation and evolution process of the Qinling orogen. This suggests that the early Mesozoic gold deposits have a genetic relationship with the continental collision between the Yangtze and North China cratons. The late Mesozoic gold deposits coincide in time and space with the numerous metamorphic core complexes, intracontinental rift-basins, felsic and mafic intrusions, intermediate-basic volcanic rock, and mafic dykes widely developed in the margins of the North China craton, indicating that the lithospheric thinning of the eastern half of the North China craton could be the geodynamic background for the late Mesozoic large-scale magmatism and mineralization in the southern North China Craton.