Tectonic Setting, Magmatic Sequence And Fluid Of Gold Metallogenic System Of The Sanshandao-Cangshang Fault In Jiaodong, China

The Jiaodong peninsula is the most important gold camp and gold industrial base of China. Gold deposits were formed in early Cretaceous and strictly controlled by regional faults. Though intensively studied, there are controversies on gold tectonic settings, dynamic systems, source of ore forming fluid, etc. In the direction of metallogenic system theory, analysis were done to tectonic settings of regional Mesozoic granitoid, geochemistry of ore forming fluid, geochemistry of hydrothermal alteration, and mineral composition and particle size distribution of fault gouge from the Sanshandao-Cangshang fault. Based on these, I try to reconstruct the time sequence of regional magmatic-tectonic evolution, decipher hydrothermal alteration and mineralization process, and re-appear the dynamics of regional gold metallogenic system and the resultant mineralization networks.Studies show that regional granitoid of late Triassic, late Jurassic, middle period of early Cretaceous and late period of early Cretaceous were formed successively in different stages of interactions of the Yangtze Plate (YP), the North China Craton (NCC) and the Pacific plate. The gold deposits were formed in regional extension settings, and in consistent with the Pacific plate subduction to the NCC and the great lithospheric thinning of the NCC. Fluid inclusion thermometric and composition determinations indicate a low salinity, low to medium temperature, NaCl-H2O-CO2 fluid with low fO2 and slightly acidic to slightly basic pH. Pyrite contains more reliable ore forming fluid than quartz. Stable isotopic data indicate a Mesozoic meteoric origin of water and mantle source of CO2. The ore forming fluid is rich in HREE and barren in LREE, with anomalous rich in Eu and Sm. The fluid pH transition from slightly acidic to slightly basic due to decompression boiling is responsible for gold precipitation from Au-S complex in fluid. After hydrothermal alteration, trace elements were imported except for Ba and Sr; LREE were imported, while the HREE were exported. Metallic elements show fractal distributions, which may be caused by the multiple hydrothermal events and related element occurrences.Fault gouges are composed of quartz, feldspar, illite, illite/semctite formation, kaolinite, minor carbonate, etc. Particle size distributions (PSDs) show single, broad single or double peaks in log-volume plots. Most of PSD fractal dimensions are above 2.70, which indicate the fault movement is in creep rather than in stick.Analyzed by metallogenic system theory, ore materials were sourced from neutral-basic magma derived from high temperature melting of mafic minerals of the Jiaodong group after low temperature partial melting and separation of felsic minerals. Ore elements were transported by fluid composed of deep cycled Mesozoic meteoric water and mantle CO2 from lower crust to the upper crust to form gold deposits. The hot mantle provided heat to the thinned crust of NCC, and the rapid subduction of Pacific plate to the NCC provided the tectonic dynamics. The gold deposits, mineral occurrences and hydrothermal alteration zones compose the whole structure of regional gold mineralization network.