Geochemistry and zircon U-Pb age of the Tongchanggou porphyry Cu-Mo deposit on the southern margin of Geza arc, northwest Yunnan Province, and its geological significance

The Geza tectonic-magmatic belt is an important part of Yidun arc and also an important ore concentration area of copper polymetallic deposits in the Sanjiang region of southwest China. Based on the study of petrologic and geochemical characteristics and zircon U-Pb geochronology of the Tongchanggou oreforming porphyry on the southern margin of Geza arc, the authors systematically analyzed the petrogenesis, tectonic environment, diagenesis and metallogenic geodynamic background of the granite. The rock type of Tongchanggou metallogenic porphyry is granodiorite porphyry, the research results of the zircon U-Pb geochronology show that the age of emplacement and crystallization of the magma is 87.62±0.59Ma (MSWD=0.52) by LA-ICP-MS method. Petrographic and lithogeochemical characteristics indicate that Tongchanggou granodiorite porphyry has characteristics of high silicon (63.02%~74.06%), high alkali (K₂O+Na₂O=6.97%~8.79%), and rich K (K₂O/Na₂O=0.71~2.13). The rocks belong to high-K calc-alkaline rock series enriched in light rare earth elements. The fractionation between LREE and HREE is obvious. δEu values are 0.89~0.97 with a weak negative anomaly. The analytical results of trace elements show that the rocks are enriched in large ion lithophile elements of K, U, Th, Rb, Ba, Sr, and depleted in high field strength elements of Nb, Ta, P, Ti, suggesting that the formation of magma might have been derived from the partial melting of crust materials. The granite was formed in the conversion stage of post-collision and extensional tectonics, belonging to the product of the collision orogenic process. The collision induced upwelling of the lithospheric mantle which led to partial melting of the crust and formed the potassium-rich and orebearing initial magma. Magma rising and intrusion along the deep faults eventually triggered the late Yanshanian tectonic-magmatic and metallogenic events in Geza area.