Magmatic-Hydrothermal Process Of The Pulang Porphyry Copper Deposit,Western Yunnan:Metallogenic Indicator From The Titanite

The Yidun island arc was formed in the Late Triassic,and is related to the the westward subduction of the Ganzi-Litang oceanic plate which belongs to a branch of the Paleo-Tethys ocean.The Zhongdian arc is located in the southern of Yidun island arc where a number of porphyry?skarn?copper-molybdenum±gold polymetallic deposits were developed.The relationship between mineralization and K-silicate zone,quartz-sericite zone and propylitic zone of the late Triassic giant Pulang porphyry copper deposit in the Zhongdian arc is still unclear and needs to be further studied.Titanite?CaTiSiO₅?is a common accessory mineral in acidic,intermediate and alkaline igneous rocks and is widely developed in various types of metamorphic rocks,hydrothermal rocks and certain sedimentary rocks.Titanite is rich in molybdenum,rare earth elements and high-field strength elements.As an effective U-Pb dating chronometer,temperature-pressure-oxygen fugacity meter and metallogenic potential indicator mineral,it is an ideal mineral to reveal the magma-hydrothermal evolution process and mechanisms of metal enrichment and precipitation.In this paper,in situ EPMA and LA-ICP-MS major and trace element concentrations of titanite were analyzed.The results show the changes of physicochemical conditions and the behavior of metal elements in the process of hydrothermal alteration from K-silicate to quartz-sericite and propylitic alteration in the Pulang copper deposit.Titanite in K-silicate alteration zone usually occurred in the shape of envelope,and it is grown independently or enclosed in feldspar.It shows lower contents of CaO,Al,Sr,Zr and Hf,Nb/Ta value,high?REE+Y,U,Th,Ta,Nb,Ga contents and FeO/Al₂O₃,LREE/HREE values,which is consistent with the characteristics of magmatic titanite from fresh quartz monzonite porphyry in Pulang,and is also similar to the geochemical characteristics of titanite from other porphyry copper deposits.The negative Eu anomaly,high U content and Th/U ratio of titanite from the K-silicate alteration zone indicate that the oxygen fugacity of magmatic-hydrothermal system decreased during the transformation from K-silicate alteration to quartz-sericite alteration and propylitic alteration in Pulang.The high oxygen fugacity is conducive to the enrichment of copper,gold and other metallogenic elements,indicating that the enrichment of copper is more closely related to the K-silicate alteration.The molybdenum content in titanite from the K-silicate alteration zone is 10²¹0⁴ times richer than that from the quartz-sericite alteration zone and propylitic alteration zone.This indicates that molybdenum was uploaded into the fluid phase or deposited in the form of sulfide before the medium-low temperature hydrothermal alteration,which may lead to the further separation and mineralization of copper and molybdenum.