The Relationship Between Mineralization And Structure In Pengzhou-type Copper Deposits In The Central Segment Of Longmen Shan, Western Sichuan

Western margin of Yangtzi Block, as one of most important ore belt of ferrous and non-ferrous metal, tombarthite and noble metal etc, has distinct features of evolution and resource distribution, and has been the focus of geological and metallogenic study for a very long time, especially, Longmen Shan in western Sichuan with thrust-nappe structures composed of pre-Cambrian complex, Paleozoic and Triassic. In which, Pengguan Complex as the biggest pre-Cambrian complex, is very famous for the Masongling copper deposits. To guide the prospect of different type of deposits and resource assessment, we conduct the project of"The relationship between mineralization and structure in Pengzhou-type copper deposits in the central segment of Longmen Shan, Western Sichuan", as one important sub-project of National Resources Assessment implemented by Ministry of Land and Resources, which could be helpful to the understanding of structure geology in the base of western margin of Yangtzi Block.After comprehensive collection and analysis of tectono-metallogenic theory of plate-terrane with respect to the zone studied, in combination with field investigation, the paper divides the formation and evolution of Longmenshan structure into four stages: oceanic basin and island arc development period, pro-arc terrane-accretionary tectonic period, passive margin development period and intracontinental subduction-nappe and slipping structure period. The tectonic belts generated include front Longmenshan belt, back Longmenshan belt, continental shelf nappe tectonic belt and basement nappe tectonic belt. Location of different terranes such as Pengguan terrane, Jiaoziding terrane and Datan terrane in the same Longmenshan tectonic belt is due to the accretion of terrane. Firstly, various terranes were covered by Yangzi stratum of Nanhua Period (NH)-Palaeozoic (Pz) Era. Secondly, the age of new Kangding terrane is 791+10Ma; the age of Jiaoziding terrane of Nanhua Period is 800-850Ma; the age of intrusion of granite of Jinning-Chengjiang Period is 792±11Ma and 793±11Ma respectively, which means the accretion of island arc occurred in Chengjiang Period. The formation of island arc is closely related to the subduction polarity of oceanic crust. The formation of island arc of Longmenshan is due to the eastward subduction of oceanic crust probably existing in the northwest or northwestward subduction of Longmenshan-An'ning oceanic crust in the southwest. There have been disputes about it all the time. In the Proterozoic Longmenshan belt, along the strike, there are ocean island-island arc, mature arc and rifting arc, showing that①these terranes are actually not the result of the same tectonic belt under the same tectonic setting;②in the ocean to the west of original Yangzi Block, there are pro-arc terranes of different types and development history;③they were gathered in one tectonic belt during the accretion into Yangzi Continent and formed the tectono-metallogenic belt in the western edge of the whole Yangzi Block. There are Jiaoziding Fe-Zn-Cu ore deposit, Caozigou Pb-Zn-Cu ore deposit and Masongling Cu-Zn ore deposit within the belt. The tectono-metallogenic mode of island arc has been established by comparison and analysis of Masongling Cu-Zn ore series, Tongmuliang Fe-Zn-Cu ore series and Caozigou Pb-Zn-Cu ore series.Island arc volcanic activity and Longmenshan nappe structure are important geological tectonic events within the zone studied. Masongling ore area terrane inevitably underwent combined transformation of mineralization under the tectonic setting of island arc and subsequent nappe structure. The chemical composition of volcanic rock within the ore area has been determined between tholeiite series and high-aluminium basalt series, belonging to the transition type by deep analysis of characteristics of rare earth of diabase-volcanic rock with the most outcrop in Masongling ore area, variable tuff-volcanic rock containing ore, volcanic rock in crack rock core area and research of combination features of ophiolite complex. The fact that ore-containing rock is of almost calc-alkaline series proves that imtermediate-basic rock is mainly of tholeiite series of island arc and that intermediate-acid rock is mainly of calc-alkaline series, which was formed under the mature island arc setting.Comprehensive analysis of deformation features of fault structure and fold structure in Malingsong ore area are conducted from macroscopic and microscopic view by conducting tectonic measurement at a rate of 1:50,000 and identification of tectonic chip. There are a total of 27 ore bodies in Malingsong ore area, most of which are lenticular, stratoid and phacoidal. Affected by tectonization, along the strike and trend, there are expansion, contraction, reoccurrence of pinch-out and recombination of lamination, showing the control over mineralization and distribution by tectonization. According to various structural features of stratum deformation, the structure within the zone is classified as NNW echelon structure and EN thrust-nappe structure. On the whole, the echelon structure is complex fold group composed of green schist of Proterozoic Huangshuihe Group; locally, fault and fold structures of various levels in the echelon structure are large-scale ductile shear system composed of metamorphic rock of Huangshuihe Group. Preliminary research indicates that there is occurrence of ductile shear around copper body in Masongling ore area. However, the ductile shear system is not necessarily related to mineralization. The relatively new formation age of EN thrust-nappe structure reflects the enormous extrusion stress in north, east, south and west directions since Triassic period, characterized by exotic block and imbricate fault. According to the plane distribution pattern, exposure features of structure and dynamics, NNW echelon structure and EN thrust-nappe structure are classified as two complex types-containment and connection. Namely, the echelon structure is contained in EN thrust-nappe structure. The two are reversely connected in the southern part of the zone studied. Pengguan, Datan and Jiaoziding terranes in the zone are of NNW-NNE direction, while the structure in Masongling area is of SN direction. Based on the distribution of ultrabasic rock in SN direction and in combination with features of ophiolite complex of NNW direction occurring in red rock belt, it can be estimated that the structure in the early stage is resulted from NNE and NNW counterclockwise rotation in a low angle; the tectonic stress is NNE-SN in the early period, right+thrust; left+thrust in the later period.National Resources Assessment implemented by Ministry of Land and Resources, is guided by the metallogenic theory to build the assessment and prospect modern in region, based on most of structural geology, geophysical, geochemical and remote-sensing information, etc. The project of"The relationship between mineralization and structure in Pengzhou-type copper deposits in the central segment of Longmen Shan, Western Sichuan"as a sub-project, has proved the metallogenic modern and process of Penzhou-type copper deposits in Longmen Shan under the tectonic setting of island arc. According to the analogy of different geologic unit and the principle ofore-forming elements in least space, we used a typical example of Masongling copper deposits to show how to assess the different geological, geophysical, geochemical and remote-sensing information, to prospect potential copper deposits in the ore belt.