Evolution of the tectonic stress field in the Xiazhuang uranium ore-field,southern China, and its coupling with the ore-formation

The Xiazhuang uranium deposit is the largest granite-type uranium orefield in China, located in east of the Triassic to Jurassic-Cretaceous Guidong complex granite. Uranium orebodies mainly outcroped insides NNE-, NWW-, and NEE-trending quartz-breccia zones, especially intersection areas. These three groups of fault zones have approximate equidistant distribution characteristics and cut with each other, forming a kind of checker-board lattice pattern. The formation genesis of the granite, ore-forming metallogenesis and ore-controlling factors have been well documented, however the tectonic stress field evolution of Xiazhaung uranium orefield and the structural controlling effect to the mineralization have been controversial for long time.

In this study, the scratch-lineation method was mainly adopted to inversion of evolution of tectonic stress field, mostly depending on the detailed field measurements to collect the scratch-lineation data, using the Wulff's net to determine the principal stress direction and the properties of each point, and combining the scratch intersection relations and other geological evidence to reveal the scratch activity periods, and further dividing the tectonic stress evolution stage to discuss the features of the tectonic stress field and its constraints to the uranium mineralization, and finally guiding the ore-prospecting prediction.

45 group data of scratch-lineation with total 250 pieces of data have been collected in the field. Combining with the field observations of kinematics, tectonic and metallogenic relationship signs, it is suggested that the Meso-Cenozoic tectonic stress field process of the Xiazhuang uranium orefield can be divided into three periods of eight stages, including two stages before-, four stages during- and two stages after the metallogenic period, respectively.

The first stage was happened during the late Indosinian, about 230Ma to 200Ma, when the maximum principal stress in the ore-field was probably S-N direction during the late stage collision between the southern China with the northern China blocks, leading the intrusion of the  Guidong complex granite. During the second stage (~200-165Ma) with nearly EW-trending (about 80°±) compressional and NNE-SSW-trending extensional stress, the NWW-trending diabasic dikes intruded along the NWW-trending transtensional fault zone, and nearly NS-trending ductile zone inside the Xiazhaung orefiled and NW-trending（320-330°）sinistral strike-slipping mylonite zones in Late Silunan Donggualing granite in the eastern side of the Guidong complex granite and insides the Late Trassic Damofeng granite. Two groups of conjugate fracture system formed, including the NE-SW and NW-SE trending fault zones, as main tectonic structures of the ore-field

During the first stage of the metallogenic period about 160Ma to 135Ma with main compressional stress trend to NW-SE (290-300°±), led to the emplacement of parts of NWW-trending diabasic dykes and development of NNE-trending density cleavage belt as the compressional property, while the the first stage of regional large-scale mineralization happened for the NWW-trending fault zone behaving as extensional fracture.

The maximum principal stress might be approximate vertical during the fifth stage (the second stage of the metallogenic period, about 135Ma to 115Ma) with intermediate principal stress trending to NE60° and the minimum principal stress to NW330° in the horizontal plane.