| The geotectonic position of Gaosong ore field in Gejiu tin deposit is located in the western margin of Youjiang geosynclinal fold zone in South China,and at the junction of Pacific plate and subduction and collision of Indian plate with Eurasian plate.It is one of the richest tin ore deposits in the giant tin ore belt around the Pacific Ocean.In this paper,the representative fracture structures in Gaosong ore field of Gejiu tin deposit are selected to analyze the fracture structure from the geometry,mechanics,kinematics and spatial variation characteristics,and determine the mechanical properties of the fracture structural plane and its transformation process by studying the developed conjugate shear joints on the side of the fault.On the basis of above research,the structural system of the ore field and the tectonic movement formed in different periods are studied,and the tectonic evolution history of the ore field is established,thus we provide a basis for the study of the formation process of fault structure of the ore field and the peripheral ore prospecting direction.The main achievements of this paper are as follows:(1)There are lots of structural phenomena in Gaosong ore field.Faults,joint and vary fractures are well developed.Through field investigation of faults in the area,it is determined that there are four groups of fault structures with different directions in the ore field,namely,EW,NE,NW and SN trending fault group.(2)The joints along the faults in the ore field are well developed,and the joints are mainly shear joints with steep dip angles.The joint trends are firstly concentrated in the NW(290°-330°),EW(90°-100°),NE(50°-70°)and SN(350°-360° and 0°-10°)directions,and then concentrated in the NWW(280°-290°),NEE(70°-80°),NNW(330°-350°)and NNE(10°-30°)directions.By analyzing joint structure,it is concluded that the ore field is affected by four stages stress,which are SN direction,NE-SW direction,NW-SE direction and EW direction respectively.Among them,SN direction stress is the most obvious,while EW direction stress is relatively weak.(3)There is a certain relationship between the morphology change of fault and the formation of ore bodies.The EW trending Qilinshan fault and the NW trending Daqing fault show more abundant characteristics of fault tectonic geology and mineralization in the areas with higher coefficient of variation of fault width.However,in the NE Lutangba fault,only geological phenomena are more abundant in the region with higher coefficient of variation of fault width,and the correlation between the variation degree of fault shape and the ore body spatial distribution in the lateral fault zone is weak.(4)Based on the analysis of fault structure,it is determined that Gaosong ore field has experienced at least four periods of tectonic movement since Mesozoic,which is roughly south-north trending,north-west trending,north-south trending,east-west trending,thus forming four different tectonic systems.(5)Inversion of the tectonic evolution history of the ore field shows that the four stages of Gaosong ore field play different roles:(1)Late Indosinian to early Yanshanian: the EW trending tectonic belt was formed,and the initial rock and ore controlling pattern was established in the ore field.This period was mainly pre-metallogenic tectonic movement.(2)The middle and late Yanshanian: the NE trending tectonic belt was formed,and the important metallogenic tectonic system in the ore field was also born in this period.This tectonic period is an important metallogenic period in the ore field.(3)Early Himalayan: NW trending tectonic belt was formed,but this period only played a role in the destruction of ore bodies,and it was a post-metallogenic tectonic movement.(4)Late Himalayan: a poorly developed north-south structural belt was formed in the region,which further destroyed the ore body and resulted in the formation of the sand-tin deposit,which was also a post-mineralization movement. |
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