| Tongmugou and the Bizigou Cu-deposits are located in Yuanqu county, Shanxi province. Tectonically, they occur in the intersection area of the Zhongtiao trigeminal rift on southern margin of the North China craton. Zhongtiaoshan Cu-orefield is one of the most notable ore-concentrated areas in China with various deposit types due to complicated mineralization environment, which together constituting Hu-Bi type deposits, characterized by copper enrichment and paragenesis with Au, Ag, Co and other valuable components.This paper presents tectonic settings, physical and chemical conditions, mineralization patterns and models of these deposits, based on field works and testing analysis including rock ore appraisal, ICP-MS analysis and fluid inclusion analysis. Strata in mining area are dominated by Paleoproterozoic Zhongtiao formation, from lower to upper, including Jiepailiang group (quartzite), Longyu group (slate), Yuyuanxia group (marble), Bizigou group (schist) and Yujiashan group (marble), among which, Yuyuanxia, Bizigou and Yujiashan group are involved with mineralization.The studying area has undergone volatile metamorphism and deformation during Precambrian, forming nearly S-N striking Hujiayu-Shangyupo superimposed fold and fracture system. The core components of the superimposed fold are dominated by metabasic volcanic rocks, mafic intrusive rocks of Jiangxian group and Beiyu granite. Its southeast flank is marked by Zhongtiao group while the northwest flank marked by the lower strata of Zhongtiao group. The axis of the anticline is NNE-trending in middle section, NEE-trending in north section and SW-trending in south section, presenting"S"shape as a whole. The Hubi type Cu-deposits occur in eastern and southern Shangyupo-Hujiayu anticline. Bizigou deposit occurs within the east flank of the anticline and Tongmugou deposit is distributed at the end of southern anticline.Ore mineral assemblage consists of chalcopyrite, pyrite, pyrrhotite and molybdenite. gangue assemblage includes calcite, quartz, biotite, albite, sericite and chlorite. Hydrothermal alteration in Tongmugou Cu-deposit occurs as silicification, sericitization, biotitization and carbonatization, and in the Bizigou Cu-deposit presents as intense biotitization with typical silicification, sericitization, carbonatization and actinolitization.Mineralization in Tongmugou deposit is closely associated with albite siliceous rocks, biotite marble and black carbon mud schist intergrowth with marble in Bizigou group, while orebodies in Bizigou deposit are hosted by Yuyuanxia group (marble) and Longyu group (calcareous biotite schist). These strata exhibit SEDEX features. Study on fluid inclusion was undertaken to constrain physical and chemical conditions of ore-forming fluids.Hydrothermal fluids in Tongmugou deposit is recorded by vapor-liquid two phase type of fluid inclusions with 4～14um in size and 0.94~1.11 g/cm3 in density, trapped under conditions between 6% and 28% wt%NaCl. eqv, between 12 Mpa and 32Mpa with an average of 21 Mpa, in depth of 2.1km, homogenized between 120°C and 210°C, mostly concentrating in 140~150°C and 160~200°C.Two types of fluid inclusions with 4 to 14 um in size are recognized in the Bizigou deposit: vapor-liquid two-phase type and daughter minerals-bearing three- phase type. Fluid inclusions are characterized by 10 - 28 wt. % NaCl equiv in salinity in two-phase type and 28%~32%wt%NaCl in three-phase type, they were trapped under conditions of 0.96~1.16 g/cm3 in density, 10~30Mpa in pressure with an average of 22 Mpa and 2.3km in depth. Vapor-liquid fluid inclusions homogenized between 120°C and 210°C, mostly concentrated in 150~200°C, representing the temperature of mineralization.The components of fluid inclusions are dominated by cation of Na+, K+, Ca+ and Mg+ with minor Li+ and anion of F-, Cl-, SO42- with minor amount of HCO3-.The features of exhalite and high salinity of fluid inclusions indicate the involvement of the hydrothermal fluid, probably brine, enrichment in silicon, sodium, boron, magnesium, carbonate and sulphide.Based on the above analysis, we propose an alternative metallogenic model as follows:In Zhongtiao area, magmatic activity facilitated to form Hu-Bi thermal field, driving the seawater to cycle and heating it to about 200°C. The heating fluid interacted with surroundings, ejected from the submarine and formed the exhalative rock. Under the continuing effect of thermal field, the hydrothermal fluid changed its nature components due to involvement of vast seawater and progressively higher temperature, leading to the decline of PH value, which in favor of leaching copper from the surrounding rock. Copper-bearing hydrothermal fluid ejected from seabed, and the copper precipitated. With the crystallization of magma, fluid temperature dropped associated with reduced leaching capacity due to the gradual decline of thermal field. Biological activity increased in the spout region due to high temperature, carbon-silicon argillite formed by exhalation carbon deposition covered the ore-bearing rock series, indicating the end of the exhalative process. |
PDF Full Text Request