| Based on the observation of veins, cementation, composition, texture of fault altered rock in the outcrop and drilling cores in Baishun uranium ore-field, the active periods of metallogenic tectonic are divided. This paper mainly presents field measurement results of joints, fault scratches or joint surface scratches in the magmatite and fualt belts at different stages to identify the stress state is retrieved, a total of 26 joint statistical points matching the 64 groups of secondary conjugate joints and more than 50 sets of vector data of joints or fracture surface scratches are got, which mainly using the methods of conjugate joints matching and scratch vector data in tectonic stress field research. The result showed that the tectonic stress field in the Baishun ore-field area can be divided into four stages, comprehensive regional tectonic evolution and geological features of the mining area, suggesting that the uranium mineralization was consistent with the evolution of the regional tectonic stress field. The main conclusions are as follows.(1) NE-trending regional ore control and ore conducting structure mainly showed as left lateral compressive faults in the early stage, and formed fine psephicity of cataclasite at the same time. There existed 2~3 stages NW-SE extensional faulting that formation of tensional breccia, with graish white, red or variegated colour chalcedony cement during the middle stage. the scratches and steps on fault planes display dextral compressoshear features, and formed the uncemented fault gouge and fault breccia during the late stage. The nearly SN-trending ore depositing structures has experienced two major fault activities of early tension torsion and late tension, Some early certain psephicity of tensional breccia show extensional features, the late extensional fault activity controls the production of uranium mineralization in the fault belt.(2) The evolution of the regional tectonic stress field in Baishun uranium ore-field area at least went through four stages: a NW-SE trending compression, a nearly SN trending compression, subsequently, a NW-SE trending extension occurred, lastly, a nearly EW trending compression occurred. The early NW-SE trending compression corresponding to the tectonic compression event during the early Yanshanian(Late Jurassic); the nearly SN trending compression corresponding to the compressional tectonic inversion event between upper Cretaceous and lower Cretaceous during the late Yanshanian(the transition stage of early Cretaceous and late Cretaceous); the NW-SE trending extension corresponding to the tectonic extension event during late Yanshanian(Late Cretaceous); the late nearly EW trending compression corresponding to the tectonic compression event during the himalayan period.(3) Under the effect of NW-SE trending compression, producing NE-trending regional faults which provide the necessary structural channel conditions for the late uranium mineralization. the nearly SN trending compressional event, producing NE-trending left lateral compressive faults, and derived a series of nearly SN-trending faulted structures, which lead to the early tension torsion activity of nearly SN-trending structures, it provided the favourable structural location for the enrichment and mineralization of uranium-bcaring hydrohcrmal soluition during the mineralization period. The NW-SE trending extension caused NE-trending fault further generated tension, and nearly SN-trending structures suffered from the late tension again, this trending extension corresponding to the most important uranium mineralization periods in the area. The uranium deposits may lightly destroyed or transformed thereby during the nearly EW trending compression period.(4) Niulan fault a regional ore control structure, and it is also ore conducting structure and ore depositing structure; secondary Yantongling fault act as a major ore conducting structure also ore depositing structure; more secondary nearly SN-trending structures acts as the main host in the erea. Nearly SN-trending extensional structure has experienced many tectonic activities, so that it becomes the most favorable tectonic site of uranium mineralization in the area. Nearly SN-trending extensional structures can be the main direction of uranium prospecting in the region as the next step. |
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