Characteristics And Metallogenic Model Of Superimposed Metallogenic System In Lincang Old Plant Polymetallic Deposit

The Laochang poly-metallic deposit is one of the most important deposits in the south of Sanjiang metallogenic belt, whose mineral exploration and development have a long period of history. The predecessors explored this deposit mineralization mainly from a single aspect or a single metallogenic system. They have different opinions about mineralization process, genesis and metallogenic model. In recent years, mining geological research and geological prospecting have made important breakthrough, and it has been difficult for the mineralization models to explain the new ore types and new geological phenomena. It is theoretically essential to rediscover the relationship between mine construction and metallogenic evolution, the main types of metallogenic geological, the metallogenic geological characteristics, the space-time structure and process of mineralization, the superposition relationships and mode of the different mineralized systems. On the basis of existing research and according to the idea of metallogenic system, this thesis analyzes the above subjects by studying the system of geology and geochemistry and acquiring new data.Through regional geological setting and comprehensive analysis of tectonic evolution, Lancang mining belongs to the southern rift of Changning-Menglian which is located in the west fault of western Yunnan. The continental basements in this area were formed from Mid and Late Proterozoic to the Early Precambrian Era. It separated from the plates like Baoshan and formed superimposed Changning-Menglian microplate in the Early Paleozoic Era. In the Late Paleozoic Devonian Era, it started cracking, and gradually formed Changning-Menglian Rift. Until the Late Carboniferous the rift extended deeply to the mantle and aroused massive volcanic activities by causing extensive magma of alkaline basalt upwelling. SEDEX was formed. In the period of Mesozoic-Cenozoic, a regional expansion-extrusion process happened once again with the development of the Neo-Tethys in this area. In the former time, a fault basin, where the Mesozoic red bed was developed, was formed because the borders of the closed rift valley fractured and expanded. During the time of collisional orogeny in the Mesozoic, intermediate-acidic hypabyssal magma intruded along the spreading-centers of the Tethys rift and Neo-Tethys rift with the massive mineralization especially hydrothermal porphyry. The movement brought not only a large skarn zone but also the mineralization of the very prospective molybdenum and copper related to porphyry. Since the Late Paleozoic, the variety of geological environments and the alternate effects of regional dynamic conditions have leaded to the superimposition of the multi-phased, multi-functional and multi-elemental metallogenic geography in the mining which has laid a good foundation for the formation of superimposed metallogenic system.This paper deals with the main metallogenic geological bodies of metallogenic systems in different mineralization systems by the analysis of mining petrology, petrochemistry, trace elements and rare earth elements, and reveals the attributes and origins of the main rocks. The main metallogenic geological bodies of SEDEX metallogenic system is the intermediate basic volcanic formation. Calc-alkalic series are predominant in the early phase and alkaline series play a chief role in the late phase during the cycles of effusive rocks which originate from the constantly differentiation and evolution of comagmatic. This related to the continental rift extending more and more deeply and the depth of the source region increasing gradually in the late Paleozoic. The main geological body of hydrothermal porphyry metallogenic system is a typical granite-porphyry (SiO2=71.26%, Rittman index σ=1.93-2.65, K20/Na20=7.73, aluminum saturation index A/CNK=1.03<1.1, SI=4.05～8.04, DI=83.23～89.52). It belongs to aluminum and high-K calc-alkaline series and has strong acidic magma with the complete evolution and differentiation. In summary, the Laochang porphyry is the result of crust-mantle mixed alkali-rich magma’s shallow-ultra shallow invasion in the dynamic intracontinental orogenic environment in the Cenozoic.By a detailed study on the ore body shape, material composition, structure, genesis features, it confirms that the SEDEX mineralization and porphyry hydrothermal mineralization systems exists a typical phenomenon of position superimposition in the mining. Space-time structure of these two kinds of metallogenic systems is specific and complex. In terms of time, SEDEX mineralization system was formed in the Early Carboniferous and its mineralized age is between355and295Ma. Porphyry hydrothermal mineralization system was formed in the Eocene and its age of mineralization is between45and43.78Ma. As to space, SEDEX mineralization system’s orebodies consist of Ⅴ, Ⅰ, Ⅱ orebody groups with the structure of stable triple lamellar orebodies from the bottom to the top. Porphyry hydrothermal mineralization system consists of Ⅵ, Ⅵ, Ⅲ orebody groups which genetically relate to blind porphyry ore body. They separately belong to porphyry-skarn disseminated-stockwork molybdenum (copper) orebody group, a large veins lead-zinc-silver orebody group and an irregular lead-zinc-silver orebody group, which constitute a complete "three zones" type porphyry hydrothermal mineralization system. From the deep soil zone to the surface, types of ores are as followed:high temperature hydrothermal filling porphyry Mo (Cu) orebodies (Ⅵ)→volcanic exhalation-sedimentary copper pyrite bodies (Ⅴ)→volcanic exhalation-sedimentary pyrite zinc silver bodies (Ⅰ-Ⅱ)→hydrothermal filling-type lead--zinc-silver bodies (Ⅲ,Ⅳ).It’s a rare ore polygenetic composition system that is composed of different genetic types of ores with different dynamic conditions in different eras. Six ore clusters constituted a special "three layers and three zones " mineralizing texture in spatial distribution.Through the analysis of the typical minerals and rocks in different mineralization systems from the aspects of S, Pb, H, O, Sm-Nd, Rb-Sr isotopes, the essay compares the differences and similarities of deposit’s geochemical characteristics in the different metallogenic systems. It shows that mineral sources of two metallogenic systems are hereditary, regenerated and multiple. In the SEDEX mineralization system S is mainly from volcanic hydrothermal; Pb is mainly from ancient continental crust basement and volcanic rocks; H-O characteristics indicate that ore-forming fluids mainly derive from magmatic water and brine mixing section. In the Porphyry-hydrothermal mineralization system, S mainly comes from the mantle or crust-mantle magma homogenization; Pb is mainly from the crust-mantle mixing; H-O characteristics indicate that ore-forming fluids mainly derive from magmatic water and meteoric water, and ore fluid from the late phase shows more characteristics of meteoric fluid.87Sr/86Sr of granite porphyry are between the average of crust and mantle rocks; isotope data of Sm-Nd is close to the EM II, and the magma source is in the crust-mantle transition zone.Based on the exhaustive division of porphyry hydrothermal mineralization stages and a representative sample of samples from the different stages of mineralization, this article researches the fluid inclusions of porphyry hydrothermal systems. The main types of inclusions are fluid. Two-phase, three-phase inclusions with CO2are very few in number. In early quartz stage mineralizing fluid temperature was260～350℃while salinity was6.2～12.9NaCleqv; in molybdenite-(chalcopyrite)-quartz stage (molybdenum mineralization stage), mineralizing fluid temperature was230～340℃while salinity was6.1～13.6NaCleqv; in pyrite-sphalerite-galena-calcite-quartz stage (stage lead-zinc-silver sulphide), ore-forming fluid temperature was220-300℃while salinity was6.1～12.1NaCleqv; in realgar-quartz-carbonate phase, ore-forming fluid temperature was190～280℃while salinity was4.34-10.86NaCleqv. Ore-forming fluid is mainly composed of H2O. most anions are Cl and F (Cl> F-) and most cations are K+and Na+. With increasing depth to the rock direction, K+content has a gradually increased trend. Mineralizing fluid pH is5.6～6.4and Eh is0.094～0.177. It is the oxidative fluid evolving from the neutral to alkaline. Molybdenum mineralization occurred mainly in neutral→weakly alkaline fluid transition phase. This article, through the forming fluid data comparison, also insists that two ore-forming fluids in the system temperature, salinity, depth, composition and production areas exist significant differences and have relatively independent evolution, which indicate the multi-source and newborn properties of hydrothermal fluids.According to comprehensive study of the geological and geochemical deposits, the conditions, genesis and formation and transformation process in the Laochang in Lancang mineralization have been identified and tectonic structure, geochemical and typical sign of the times in the SEDEX mineralization and porphyry hydrothermal mineralization have been confirmed.This paper has improved the syntopogenic-superposition metallogenic model according to the products (ore groups) in the form of time with large time span characteristics of two different regional dynamic environment metallogenic systems.