The Research On Porphyry Copper Metallogenic System And Post-ore Modification&Preservation Since The Indosinian In Geza Arc, Yunnan, SW China

Geza island arc belt is one of the important tectonic units in the archipelagic arc-basin system in the Sanjiang area (Jiasha River, Nujiang River, Lancang River), which located in the eastern edge of the Dege-Zhongdian continental blocks Ganzi-Litang and the southern of Yidun arc. It began at Ganzi-Litang oceanic crust westward subduction in the Late Triassic, experienced intracontinental convergence and post-orogenic extensional phase in Yanshanian, and showed a strong thrust-nappe structure and large-scale strike-slip translation activities by the effect of the collision uplift of the Qinghai-Tibet Plateau in the Himalayan. In this area, there were strong magmatic activities, significant geophysical and geochemical anomalies, superior metallogenic geological conditions, the rich mineralization and a lot of deposits. It was a newly discovered copper polymetallic ore concentration area of China in recent years, in which the Pulang porphyry copper deposits is a typical representative of Indosinian porphyry copper deposits. This article researched on the dynamics background, metallogenic mechanism and magma source of the typical deposits formation, using the Apatite Fission Track method (AFT) restored the uplift history of granitic rocks in Geza arc and got the denudation quantitative data of intrusions to research on the evolution of deposits formationâ†'post-ore modificationâ†'preservation, the research achievements of this thesis are:1. Using the geochemical method and analyzed the temporal and spatial distribution, discussed petrology characteristics and magma origin of main metallogenic porphyries in Geza arc magmatic belts. This study shown that magmatic rocks has the zonal distribution characteristics in the temporal and spatial. The similarities of porphyries and local acidic volcanic rocks in the geochemistry composition suggest that they both have the same or the similar magmatic source. The porphyry(porphyrite) and island-arc granite rocks have the same rock series (calc-alkaline) and genetic type (I-type granite). The tectonic discrimination shown that magmatic rocks mainly formed in the subduction orogenic tectonic environment, and indicated that the material also has a deeper source. It is generally believed that the source is lower crust or upper mantle and has a crust-mantle mixed characteristic.2. Based on the previous studies, we used the LA-ICP-MS U-Pb zircon method comprehensively redefiniteded the diagenetic age, mineralization age and Hf isotope analysis of Pulang mineralization porphyries, discussed the formation age of magmatic rocks in this area, the rock genesis, material source and tectonic environment. Applicated the isotope geochemical methods researched on the Pb isotope tracing and the source of the minerals of typical porphyry copper deposits in Geza arc.3. Our studies comprehensively summarized and researched on the porphyry metallogenic system of Geza arc, which can be divided into Indosinian epoch metallogenic subsystem and Yanshanian epoch metallogenic subsystem. The Indosinian epoch mainly developed intermediate-acidic crust-mantle magmatism homologous with the andesite and formed porphyry Cu metallogenic system; the Yanshanian epoch had syn-collision magmatic activities and led to product Porphyry Mo(Cu) and W(Mo) hydrothermal metallogenic system.4. The post-ore changing and preservation was controlled and influenced by various geological processes, but the regional up lift and denudation is the most important factors. Using the biotite mineral geobarometer and Apatite Fission Track (AFT) restored the uplift evolution of Geza arc granitic porphyries, obtained the quantitative data of rock erosion degree and denudation rate. AFT age and minerogenetic isotopic age given the time of intrusions and cooling process and limited the exhumation history of main porphyry Cu deposits. The comparative studied of emplacement depth and denudation degree shown that the ore-forming rocks erosion depth was less than the emplacement, which was more favorable to the post-ore changing and preservation of porphyry Cu deposits in Geza arc. According to the quantitative calculation of erosion resources to mainly porphyry deposits, the denudation was divided into three orders of magnitude:Grade I for deposit mild denudation, II was medium denudation, III for deposit severe denudation, which provided the theory basises for evaluation of the metallogenic potential and the deep prospecting of porphyry metallogenic system.