High temperature,salinity and strong oxidation ore-forming fluid at Duobuza gold-rich porphyry copper deposit in the Bangonghu tectonic belt,Tibet:Evidence from fluid inclusions.
LI GuangMing LI JinXiang QIN KeZhang ZHANG TianPing XIAO Bo 1.Key Laboratory of Mineral Resources,Institute of Geology and Geophysics,Chinese Academy of Scienees,Beijing 100029,China 2.No.5 Geological Party,Tibet Bureau of Geology and Exploration,Golmu 816000,China
The Duobuza gold-rich porphyry copper deposit,located in the Duobuza tectono-magmatic arc at northern margin of the Bangonghu suture zone,is occurred in high-level,early-Cretaceous compositionally quartz diorite porphyrite to granodiorite porphyry which intruded into the strata of the middle Jurassic Yanshiping Formation volcano-sedimentary rocks and the early Cretaceous Meiriqie Group volcanic rocks.Intensively alterations in stock and wall-rocks resulted from emplacement of the intrusion have obvious zonation, as follows:potassic alteration,moderate phyllic alteration,argillization,illite-hydromuscovite and propylitization from centre outwards. Early hydrothermal alteration produced potassic assemblage(biotite-K-feldspar)in the central part of the intrusion,but in the peripheral and upper part of the intrusion occurred later phyllic alteration and argillization,overprinting the earlier potassic alteration. Illite or hydromuscovite and silicification,propylitization occurred in the hosted volcano-sedimentary rocks and volcanic rocks. Mineralization is characterized by multiple generations of stockwork and sheeted quartz,K-feldspar,magnetite,stringer chalcopyrite and gold-bearing veins.Copper and gold mineralization is accompanied by potassic and phyllic alterations.Mineralization is Cu-Au association,belonging to typical gold-rich porphyry copper deposit.Eight main types of veins have been identified,namely M,EB,A, B,C,D,S and G veins.Three major inclusion types and ten subs are distinguished from quartz phenocrysts and various quartz veins. Based on microthermometric analyses of fluid inclusions from quartz phenocrysts and early to late-stage veins,the highest temperature multiphase inclusions(up to 935℃;average salinity 45%NaCl eq,P≈200MPa),containing a number of solids,e.g.opaque, silicate melt,hematite at room temperature,are mainly recorded in quartz phenocrysts,locally in A and C veins.This result indicates that fluids exsolved from magma are high temperature,high salinity,strong oxidation and ore metal-rich ore-forming fluids.They represent the most primitive magmatic fluid recorded at the deposit.When primitive magmatic fluid ascended up to 3km and cooled down to 580℃,they were separated to multiphase fluid inclusions of salinity 60%～80%NaCleq and lower salinity vapor-rich fluid inclusions of salinity 6.9%NaCleq by boiling.This process resulted in formation early M and A veins,and deposition of sulfides mainly,as well as some gold in certain location. Later B,C and D veins were generated by the orthomagmatic fluids boiling episodically.Our research suggests that the Duobuza gold-rich porphyry deposit is representative of the typical orthomagmatic end member of the porphyry continuum.
【Fund】： 国家自然科学基金(40672068);; 国家重点基础研究发展规划“973”项目“冈底斯-藏东斑岩铜矿成矿作用课题”(2002CB412605)联合资助
【CateGory Index】： P618.41
【CateGory Index】： P618.41