西藏帮布勒矽卡岩型Pb-Zn-Cu-Fe矿床流体来源及演化:来自流体包裹体及稳定同位素的制约

Fluid Source and Evolution of the Bangbule Skarn Pb-Zn-Cu-Fe Deposit in Tibet: Constraints from Fluid Inclusions and Stable Isotopes

位于冈底斯-念青唐古拉Pb-Zn-Cu-Fe-Ag成矿带最西端的帮布勒矿床为一新发现的大型Pb-Zn-Cu-Fe矿床。该矿床是一个典型的矽卡岩型矿床,发育早期和晚期矽卡岩阶段、早期和晚期硫化物阶段以及表生氧化物阶段5个成矿阶段。除表生氧化物阶段外,其他成矿阶段的流体包裹体均以富液两相(VL型)为主,可见少量含子矿物三相(SVL型)和富气两相(LV型)包裹体。各阶段的流体包裹体均一温度范围分别为:529～580℃、365～498℃、173～386℃和113～366℃。早期矽卡岩阶段的流体包裹体盐度为4.5%NaCleqv;晚期矽卡岩阶段盐度平均为43.4%NaCleqv(SVL型)和4.16%NaCleqv(VL型);而早期和晚期硫化物阶段盐度较低,主要集中于4.0%～10.0%NaCleqv和0～11.0%NaCleqv。矽卡岩期的流体密度较高,石英-硫化物期的流体密度较小,从早期矽卡岩阶段到晚期石英-硫化物阶段依次为1.40g/cm^3、1.39～1.41 g/cm^3、1.01～1.19 g/cm^3及1.00～1.18 g/cm^3。流体捕获压力整体亦显示降低的趋势,分别为45.26 MPa、32.74 MPa、26.30 MPa及18.83 MPa。推算成矿深度较浅,在1.5～2.5 km之间,与矿区内石英斑岩体所显示的浅成岩深度一致。C-H-O同位素结果显示矽卡岩期具有岩浆流体的特征,石英-硫化物晚期逐渐过渡为以大气降水为主。综上所述,帮布勒早期成矿流体具有高温、中高盐度的岩浆水特征,而在石英-硫化物期由于大量大气降水的混入,使得成矿流体的温度、盐度等明显降低,最后在有利的构造位置上,由于流体混合、局部减压沸腾、温度降低等共同作用使金属硫化物沉淀成矿。

The Bangbule Pb-Zn-Cu-Fe deposit is a newly discovered deposit in the western Gangdese-Nyainqentanglha Metallogenic Belt. Orebodies mainly occur in the contacts between the quartz porphyry and the marble, limestone and sandstone, and the interlayer structures of the marble and limestone, interfaces of different lithology, and fractures as well. The ore forming processes can be divided into three epochs(skarn, quartz-sulfide, and supergene) and five stages(early skarn, late skarn, early sulfide, late sulfide, and supergene stage). Fluid inclusions in the different ore-forming stages are mainly liquid-rich and gas-liquid inclusions(type VL), with minor daughter mineral containing inclusions(type SVL) and gas-rich phase gas-liquid inclusions(type LV). The homogenization temperatures of the early skarn, late skarn, early sulfide and late sulfide stages are in ranges of 529 – 580 ℃, 365 – 498 ℃, 173 – 386 ℃, and 113 – 366 ℃, respectively. The salinity of the early skarn stage is 4.50% NaCleqv. In the late skarn stage, the average salinities of SVL-type and VL-type fluid inclusion are 43.4% NaCleqv and 4.16% NaCleqv, respectively. The peak values of salinity in the early and late sulfide are 4.0% – 10.0% NaCleqv and 0 – 11.0% NaCleqv, respectively. The average densities of fluids are 1.40 g/cm^3, 1.39 – 1.41 g/cm^3, 1.01 – 1.19 g/cm^3, and 1.00 – 1.18 g/cm^3, respectively. The average pressures are 45.26 MPa, 32.74 MPa, 26.30 MPa, and 18.83 MPa, respectively. The ore-forming depth ranges from 0.82 km to 4.34 km(average 1.97 km), with the peak values between 1.5 km and 2.5 km, which belongs to shallow mineralization depth and is consistent with the depth of the quartz porphyry. The C-H-O isotopes show that magmatic fluid probably played an important role in the skarn stage. In the quartz-sulfide stage, the fluid mixed with large quantity of meteoric water, and gradually shifted to meteoric water during the late quartz-sulfide stage. In conclusion, during the early stages, the ore-forming fluid was dominated by magmatic water, which was characterized by low acidity, high temperature, high salinity, and high oxygen fugacity. During the quartz-sulfide phase, the temperature and salinity of ore-forming fluid decreased obviously owing to mixing with a large amount of meteoric water. Finally, in the favorable structure position, the pressure reducing and cooling of the ore-forming fluid gave rise to massive metal sulfide precipitation.