Geochronological and geochemical characteristics of Qiansui quartz dioriteporphyrite in the Geza Arc, Yunnan Province, and constrairts on diagenesis and mineralization

The Indosinian Qianui copper polymetallic deposit discovered in recent years is located in the central and southern segment of the eastern Geza porphyry belt of southern Yidun arc in Southwest China. A large number of indo-Chinese large-size ore deposits were formed in the metallogenic area, which is hence an important polymetallic belt in China. In this paper, the authors studied the ages of the Qiansui quartz diorite porphyry with zircon LA-ICP-MS U-Pb dating method and obtained the formation age of the copper mineralized quartz diorite porphyry of 220.3±0.66 Ma (MSWD=0.99). Geochemical characteristics indicate that Qiansui ore-bearing quartz diorite porphyry is characterized by rich Na (K₂O/Na₂O=0.05-1.42, averagely 0.86), metaluminous nature (0.82-1.19, averagely 1.07), enrichment of light rare earth elements (LREE, Ba, Th, U, Sr), obvious REE fractionation (La_N/Yb_N=19.6-28.8), enrichment of large ion lithophile elements (LILE, Ba, Th, U, Sr), and depletion of high field strength elements (HFSF, Ta, Nb, Ce, P, Ti). The rock belongs to typical Ⅰ-type granite. The ore-forming process of Qiansui metallogenic rock body occurred in the late Indosinian, and the trace element characteristics, tectonic background and isotopic characteristics of the orebearing porphyry suggest that Qiansui rock was formed in an island arc environment, and that its formation was probably related to the subduction of oceanic crust in Indosinian.The study of geochemical characteristics and petrogenetic age of Qiansui reveals the fixed formation age of Qiansui and the time limit of Ganzi-Litang oceanic crust subduction orogeny, suggesting that Pulang and Hongshan copper deposits were the products of the evolution of magma structure in the same period. The results obtained by the authors have important significance for the study of Gezan island arc tectonic magmatic evolution and petrogenetic mineralization.