Geochemical characteristics of biotite from composite pluton and its implications for diagenesis and mineralization in Hongniu-Hongshan copper deposit,northwestern Yunnan
The Hongniu-Hongshan porphyry-skarn large-scale copper deposit in northwestern Yunnan is located in the central part of the Gezan copper polymetallic ore concentration area.The mineralization of this deposit is closely related to the Late Cretaceous granitic magmatic activity,and the Late Yanshanian granite porphyry-quartz mon-zonitic porphyry composite pluton is developed in the area.The ore bodies are mainly disseminated and veinlet in the porphyry and porphyry-related skarn.The spatial distribution of copper-molybdenum mineralization is obvi-ously different.In the deep part,granite porphyry with weak chalcopyrite and molybdenite mineralization is ex-posed,while strong copper-molybdenum mineralization is developed in the shallow quartz monzonite porphyry.The characteristics of whole rock mineralization are generally characterized by high degree of mineralization in the shallow and contact zone,and gradually decreasing to the deep.The reasons for this difference are not yet clear.On the basis of detailed petrographic observation,Electron Microprobe Analysis(EMPA)and Laser Abla-tion Inductively-Coupled Plasma Mass Spectrometry(LA-ICP-MS)techniques were used to investigate the major and minor element compositions of biotite in the composite rock mass,so as to identify the physicochemical con-ditions of the granite porphyry-quartz monzonitic porphyry composite pluton in this area and the restrictive fac-tors leading to the difference of ore-bearing between the two rocks.The results show that the two types of biotite have the characteristics of low Al,high Mg and Ti,and both belong to magnesian biotite.The biotite composition shows that the Hongniu-Hongshan Late Yanshanian composite rock mass is an I-type granite with crust-mantle mixed source characteristics,which belongs to high temperature,shallow-ultrashallow phase and high oxygen fu-gacity rock mass.The crystallization temperature of granite porphyry is 737～751℃,the pressure is 46～72 MPa,corresponding to the depth of 1.74～2.73 km,with an average of 2.17 km.The crystallization temperature of quartz monzonite porphyry is 722～754℃,the pressure is 37～58 MPa,and corresponding to the depth of 1.42～2.20 km,with an average of 1.91 km,the oxygen fugacity is slightly higher than that of granite porphyry.TheⅣ(F)values of biotite in granite porphyry are 0.91～1.06,and the Ⅳ(Cl)values are-4.67～-4.48.The Ⅳ(F)values of biotite in quartz monzonite porphyry are 0.56～0.77,and the Ⅳ(Cl)values are-5.16～-4.81.The log(fH2O/fHF)fluid values of hydrothermal fluid coexisting with granite porphyry are 1.31～1.47,and the log(fH2O/fHF)fluid val-ues of hydrothermal fluid coexisting with quartz monzonite porphyry are 0.96～1.17.It shows that the two litholo-gies of the complex rock mass show high fluorine and chlorine characteristics,and the fluid of quartz monzonite porphyry is more rich in HCl and HF.These characteristics indicate that the Hongniu-Hongshan Late Yanshanian composite pluton has the potential for simultaneous copper and molybdenum mineralization.Moreover,copper and molybdenum elements tend to be enriched in quartz monzonite porphyry.Higher oxygen fugacity and halo-gen content may be the main reason for the difference in ore-bearing between quartz monzonite porphyry and granite porphyry.
biotiteelement geochemistrycrystallization temperaturepetrogenesis and mineralizationHongniu-hongniu copper depositsouthwestern Sanjiang
国家科技期刊平台
NSTL
万方数据
