Structural Deformation And Palaeostress Evolution Of The Jinshan Gold Deposit In The Dexing Area, Northeastern Jiangxi Province

The Jinshan gold deposit is one of the main ore deposits in the Dexing large Au-Cu ore concentrating area. It is located at the collisional zone of the Jiuling terrane and HuaiYu terrane in the eastern segment of the Jiangnan Orogen. This deposit is developed in the Jinshan Ductile Shear Zone, which is a secondary shear zone of the Northeastern Jiangxi Province Fault Zone. It is a super-large gold deposit related to the dynamic metamorphim of these ductile shear belts.The gold deposit is developed in the weakly metamorphosed Neoproterozoic Shuangqiao Group. The ore-bearing wall rocks have experienced strong dynamic metamorphism and were transformed into mylonite series. Two different types of gold ore-bodies are preserved in different positions in the deposit area:the NW-trending ultramylonite-type gold ore-bodies are preserved in the Wujiawu-Dawu section, while the NE-trending quartz veins-type gold ore-bodies are preserved in the Yangshan section. Based on field investigation and thin-section observation, two periods of ductile shearing deformation are discovered in this deposit area:the earlier ductile thrust shearing resulted in the NW-trending ultramylonite-type gold ore-bodies, which mainly occurred in the Wujiawu-Dawu section; while the later sinistral strike-slip ductile shearing produced the NE-trending quartz veins-type gold ore-bodies, which mainly occurred in the Yangshan section. By analysis of the gold deposit formation age and the shear zone development age, it is suggested that both deformational events occurred during the Neoproterozoic, corresponding to two tectonic-magmatic-metamorphic events during the syn-and post-orogenic stages in the Jiangnan Orogen, respectively.After the ore formation, the Jinshan deposite underwent a NW-SE compressional event, producing NE-trending thrust faults, NW-trending normal faults and NS-trending sinistral transpressional faults. Subsequently, a NW-SE extension occurred, resulting in NE-trending normal faults.The regional palaeostress inversion could be realized through fault-slip analysis. This investigation measured the slip data of62faults in the Neoproterozoic strata and28faults in the Jurassic rocks, and geometrical illustration and numerical methods are adopted to obtain palaeostress. Based on the geometrical method, three directions of the maximal principal stress were obtained:NW-SE trending, NE-SW trending and near-vertical compression. Using the multiple inversion method (MIM), six periods of palaeostress were obtained, of which the direction of principal stress is basically consistent with the results obtained from geometrical method. Combined with field geological observations and regional tectonic events, the palaeostress evolution of investigated area since the Neoproterozoic era was deduced as follows:(1) in the Neoproterozoic era, this area underwent NNW-to NW-trending compression, which subsequently changed to strike-slip, then back to contraction.(2) in the Early Paleozoic era, this area underwent a NE-SW trending compression and a NW-SE trending extension under the strike-slip stress background.(3) in the Triassic to early Jurassic era, a S-N trending compressional event occurred in this area in the transpressional stress background.(4) in the Middle Jurassic era, the stress background of the region turned to a NW-SE trending extension.(5) in the Late Jurassic era, the inversion result shows a NW-SE trending contraction and a NE-SW trending extension in the strike-slip background.(6) in the Early cretaceous era, this area underwent a NW-SE trending compressional event in the transpression stress background.(7) in the Late Cretaceous to Paleogene, the directions of extension changed from NE to NW due to a change from transtension to uniaxial extension of the stress background.