老挝巴莱县班康姆铜金矿床流体包裹体特征及其对成矿作用的约束

CHARACTERISTICS OF FLUID INCLUSIONS AND THEIR CONSTRAINS ON METALLOGENESIS OF THE PANGKUAM Cu-Au DEPOSIT IN PARKLAI COUNTY,LAOS

老挝巴莱县班康姆矿床是近年来在琅勃拉邦—黎府成矿带新发现的一个大型矽卡岩型铜金矿床。其成矿过程可划分为5个阶段:(Ⅰ)矽卡岩阶段,(Ⅱ)退化蚀变阶段,(Ⅲ)中温铜金矿化蚀变阶段,(Ⅳ)低温金矿化阶段,(Ⅴ)碳酸盐阶段。该矿床主要发育三类流体包裹体,分别是富液气液两相、富气相两相及含子晶的三相包裹体。矽卡岩阶段透辉石富液包裹体均一温度为394~435℃,盐度w(NaCleq)为5.26%~5.41%,密度为0.49~0.59g/cm^3。退化蚀变阶段与磁铁矿共生的石英含子晶富液包裹体均一温度为327℃,盐度w(NaCleq)为34.07%。中温铜金矿化蚀变阶段脉石矿物富液相包裹体均一温度范围为138~298℃,盐度范围w(NaCleq)为4.49%~30.92%,在温度-盐度图解中,部分包裹体投点落在NaCl饱和曲线上。低温金矿化阶段脉石矿物富液包裹体均一温度为99~156℃,盐度w(NaCleq)为1.74%~12.68%。碳酸盐阶段方解石富液包裹体均一温度为62~95℃,盐度w(NaCleq)为0.53%~12.81%。根据沸腾包裹体的NaCl-H2O体系三相平衡公式计算得班康姆矿床Ⅱ号矿体成矿深度为685m。流体包裹体研究表明,班康姆矿床成矿流体来自岩浆出溶的低密度含矿流体,在矽卡岩阶段未发生明显的相分离,但在退化蚀变阶段晚期及中温矿化阶段发生了沸腾。流体沸腾、流体混合及水岩反应是导致班康姆矿床铜金矿石沉淀的重要机制,这些机制在成矿过程中单独或一起作用于成矿流体,形成了班康姆矿床总体“上Au下Cu”的矿化分带。

The newly discovered Pangkuam deposit in ParkLai County of Laos is a large scale skarn type copper-gold deposit in the Luang Prabang-Loei metallogenic belt.Its ore-forming process can be divided into five stages:(1)Skarn Stage;(2)Degradation Aalteration Stage;(3)Medium-temperature Hydrothermal Alteration and Copper-gold Mineralization Stage;(4)Low Temperature Gold Mineralization Stage and(5)Carbonate Stage.Primary fluid inclusions of liquid-rich gas-liquid two phase inclusions,vapor-rich two phase inclusions and halite-bearing liquid-rich inclusions are developed in this deposit.The homogenization temperature of diopside rich inclusion in skarn stage is 394-435℃,the salinity w(NaCleq)is 5.26%-5.41%,and the density is 0.49^-0.59g/cm^3.The homogenization temperature of quartz(with daughter crystal and magnetite rich inclusions)in the stage of degenerated alteration is 327℃and the salinity w(NaCleq)is 34.07%.In intermediate temperature mineralization stage,the homogenization temperature and salinity of liquid-rich inclusions in gauge minerals range from 138to 298℃and w(NaCleq)4.49%to w(NaCleq)30.92%respectively,and part of the inclusions plot on or near the NaCl solubility curve of H2O-NaCl system in Th-w(NaCleq)/%diagram.In low temperature gold mineralization stage,the ranges of homogenization temperature of liquid-rich inclusions in gauge minerals are from 99to 156℃,and salinities from w(NaCleq)1.74%to w(NaCleq)12.68%.The homogenization temperature and salinity of the calcite rich liquid inclusion in the carbonate stage are 62-95℃and w(NaCleq)0.53%-12.81%.According to the calculation of PTVx relation for halite-liquid-vapor equilibrium,theⅡ#ore body in the deposit was formed at a depth of 685meters.The study of fluid inclusions indicates that the ore-forming fluid of the Pangkuam deposit was originated from the low density magmatic ore-bearing fluid has no obvious phase separation in the skarn stage,but boiling occurred in the late degenerated alteration stage and the intermediate temperature mineralization stage.It is considered that the fluid boiling,fluid mixing and water-rock reaction are important mechanisms that lead to the precipitation of copper and gold ore in Pangkuam deposit.These mechanisms respectively and simultaneously act on the ore-forming fluids in the mineralization process,forming zoning of the upper part Au mineralization and lower part Cu mineralization in Pangkuam Cu-Au deposit.