Numerical Modeling Of Paleo Tectonic Stress Field And Metallogenic Prognosis In Dajishan Tungsten Deposits, Quannan, Jiangxi

The Dajishan tungsten deposit locates in Nanling metallogenic province of tungsten and tin. It is famous in China as a large vein-type tungsten deposit. Three groups of vein belts in NWW strike distribute between Chuandiwogou fault and Dajishanfeng fault which are all in NE strike. The magmas relating with Dajishan tungsten deposit mainly include biotite-granite, muscotive-biotite granite and muscotive-granite. Based on researching the deposit structure, measuring attitudes of conjugated joints, recovering directions of tectonic principal stresses and numerical modeling the paleo tectonic stress field, this paper discusses the genetic mechanism of the host fractures, and predicts the developmental condition of fractures and the prospecting positions. The research on the deposit structures indicates that Dajishanfeng default is a reverse default before the mineralization and no phenomenon is evident to prove the strike-slip movement. On horizontal surface, the veins are NWW strike and almost parallel to each other, however, in vertical profile, these veins consist of conjugated joints among which one group of conjugated joints dipping to NNW developed enough to be commercial veins and the other group of conjugated joints dipping to SSE is not developmental.The recovery of tectonic principal stresses directions demonstrates that optimized attitudes of the two groups of conjugated joints in the mine are 9°∠74°and 181°∠66°respectively, and optimized directions of principal stresses are 252°∠79°(σ₁), 95°∠11°(σ₂) and 5°∠4°(σ₃). Actually, the upward force of the magma intrusion isσ₁ and regional major principal stress in NNW isσ₂. Combining the optimized direction of the principal stresses and the characteristics of the deposit structures, this paper summaries the genetic mechanism: the upward force of the magma intrusion in phase of Yanshan coupling with the regional stress field during the mineralization remolded the fractures zone and weak structural surface on the axial plane of the folds in east-west strike forming in phase of Yinzhi to form the conjugated joints. In the late stage of magmatic evolution, ore-forming fluid injected into the fractures and formed the vein-type tungsten deposit. The results of numerical modeling tectonic stress field show that northern predicted area has the similar differential stress values (70-80 MPa) with that in the area around existing ore bodies, which means that in northern predicted area, developmental condition of the fractures is satisfying, the vein is deepening long and thus, this area is prospecting. However, the rock in eastern predicted area is tough Devonian sandstone which restricts the development of the fractures. Once the vein belt extends from western area into eastern area, the width of the vein rapidly decreases. The results of numerical modeling tectonic stress field also show that the differential stress values (80-90 MPa) are generally higher than that in area around the existing ore bodies, which means that the developmental condition of the fractures is poor, but in the Cambrian terrane beneath the unconformity surface, the differential stress value is lower (50 MPa) and the fractures are developmental. If the unconformity surface is over the top of blind intrusion body, the deep part of the eastern predicted area is prospecting, however, because of the restrictions of the vein deepening and total width of the veins, the mineralization scale is smaller the that in western part.