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GEOCHEMICAL CHARACTERISTICS OF FLUID AND MECHA NISM FOR ORE FORMATION IN THE BAIYANGPINGCOPPER-SILVER DEPOSIT,YUNNAN

GONG Wen-jun, TAN Kai-xuan, LI Xiao-ming, GONG Ge-lian(Changsha Institute of Geotectonics, Academia Sinica, Changsha 410013, China)  
The Baiyangping copper-silver deposit occurs in the lower Cretaceous Jingxing Formation quartz sandstone and siltstone.The deposit is controlled by a batch of northeast-trending fractures and occurs as bedded and veined orebodies.Geochemical studies of fluid inclusions indicate that the ore-forming fluid is a basin brine with a lower temperature(124~141℃ on an average) and lower mineralization degree.The composition of hydrogen-oxygen isotopes is nearthe SMOW line,suggesting that the degree of water/rock interaction is quite low,its time limit is short and the speed of fluid flowage and circulation is quitefast.The results of computer modeling indicate that during the period 13~16 Ma after the deposition of the Jingxing Formation,faulting made the fracture permeability up to the most,at the same time the temperature of fluid evolution reached 125~140℃.The high fracture permeability brought a pump-absorb action to the fluid in the around strata and formed a tunnel for fluid flowage.Thus,the high fracture permeability causes large-scale fluid flowing, circulating and mineralization in the fracture zone.
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