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Fluid inclusion study of the Bainaimiao Cu-Au deposit in Inner Mongolia,China

LI WenBo LAI Yong SUN XiWen WANG BaoGuo Laboratory of Crustal and Orogenic Evolution,Peking University,Beijing 100871,ChinaBeijing Institute of Geology for Mineral Resources,Beijing 100012,ChinaBainaimiao Copper Company Limited,Inner Mongolia,011817,China  
The Bainaimiao Cu-Au deposit is located at the north margin of the north China craton.Cu-bearing lodes and altered ores are controlled by EW-trending faults.The hydrothermal ore-forming process can be divided into early,middle and late stages, characterized by quartz-pyrite,polymetallie sulfides and carbonate-quartz,respectively.The early stage veins and minerals are structurally deformed and broken,suggesting that they had formed in a compressional or transcompressional setting.The middle stage polymetallic sulfides are intruded into the early stage veins and cracks,and show no deformation,which suggests they formed in a tensional shear setting and had not undergone compression.The late stage quartz-carbonate veinlets cut the early and middle stage veins and minerals.The homogenization temperatures of the primary H_2O-rich fluid inclusions in the early stage quartz range from 248℃ to 380℃ and their salinities range from 4.34% to 6.59% NaCl equivalent.The homogenization temperatures of the secondary H_2O-rich fluid inclusions in the early stage quartz range from 180℃ to 260℃ and their salinities range from 3.23% to 4.18% NaCl equivalent. The homogenization temperatures of the primary H_2O-rich fluid inclusions in the middle stage quartz range from 215℃ to 241℃ and their salinities range from 2.90% to 4.18% NaCl equivalent,which are similar to those of the secondary H_2O-rich fluid inclusions in the early stage quartz.The homogenization temperature of H_2O-rich fluid inclusions in late stage quartz and calcite range from 137℃ to 180℃ with salinities of 0.50 to 2.00% NaCl equivalent.Gases in fluid inclusions are CO_2,CH_4,and N_2.The ore fluid of the Bainaimiao Cu-Au deposit is characterized by low salinity,low density and enrichment in CO_2.When this kind of fluid reached shear zones,the pressure of the ore fluid became much lower immediately and the fluid boiled and CO_2 separated.The ore fluid became oversaturation and precipitated sulfide ores.Therefore,the Bainaimiao Cu-Au deposit could be taken as an example of orogenic-type Cu-Au deposits due to its similarities in ore geology and ore fluids compared with the orogenic deposits in the world.
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