大别造山带沙坪沟斑岩型钼-热液脉型铅锌矿成矿系统:流体包裹体及稳定同位素约束

Metallogenic system of Shapinggou porphyry molybdenum-hydrothermal lead-zinc deposit in Dabie orogeny belt:Constraints from fluid inclusions and stable isotope

沙坪沟钼矿床位于安徽省金寨县,大地构造位置属于大别造山带东段,是世界第二大斑岩型钼矿床。沙坪沟钼矿外围发现有银山、盖井等多处铅锌矿床,但其与斑岩型钼矿的成因联系不明。以沙坪沟斑岩型钼矿及矿区北部的银山铅锌矿为研究对象,在野外地质调研的基础上,通过流体包裹体岩相学、显微测温学、显微激光拉曼和稳定同位素等研究,系统阐明斑岩型钼矿与外围铅锌矿的成矿流体来源、成矿流体特征及演化、成矿物质来源,初步建立斑岩型钼矿与周边铅锌矿的成因联系。流体包裹体岩相学结果表明,沙坪沟斑岩型钼矿主要发育3类流体包裹体,包括AV类(气液两相包裹体)、AC类(富CO2三相包裹体)、ADV类(含子晶多相包裹体)。成矿早阶段三种类型均有发育,均一温度范围为325～420℃,AV和AC类盐度介于0.8%～15.4%NaCleqv(质量分数)之间,ADV类盐度介于33%～46%NaCleqv之间。成矿中阶段发育AV类、AC类和ADV类,均一温度范围为226～367℃,AV类与AC类盐度介于0.4%～13.2%NaCleqv之间,ADV类盐度介于33.3%～39.6%NaCleqv之间。成矿晚阶段主要发育AV类包裹体,均一温度范围为185～300℃,盐度介于0.7%～8.4%NaCleqv之间。银山铅锌矿主成矿阶段石英中只发育AV类包裹体,其均一温度范围为175～315℃,盐度介于0.6%～4.9%NaCleqv之间。流体包裹体中水的H-O同位素结果表明,沙坪沟钼矿成矿早阶段流体主要来自岩浆水,晚期铅锌矿化阶段,成矿热液为岩浆水与大气水的混合,与银山铅锌矿主成矿阶段石英中流体包裹体中水的H-O同位素相似。S同位素数据表明两矿床主成矿阶段硫化物的硫元素主要来自深源岩浆,Pb同位素数据显示矿石铅具有壳幔混源的特征。S、Pb、H、O同位素及流体包裹体特征均指示,研究区钼矿及铅锌矿床具有一致的物质来源,成矿流体具有由高温-高盐度岩浆热液流体向中低温-低盐度流体和从富CO2至贫CO2的演化趋势,沙坪沟周边铅锌矿为斑岩成矿系统的组成部分。

The Shapinggou molybdenum deposit, located in Jinzhai County, Anhui Province, China, is the second largest porphyry molybdenum deposit in the world. Several Pb-Zn deposits have been discovered in the periphery of Shapinggou Mo deposit, such as Yinshan and Gaijing deposits, but their genetic relationship with porphyry Mo deposit is misunderstood. In this work, we base on data from field work investigation combined with laboratory analyses data such as fluid inclusions petrography, microthermometry, laser Raman microprobe and stable isotopes to characterize the source and evolution of Shapingguo porphyry Mo deposit and surrounding Yinshan Pb-Zn deposits in order to establish a genetic relationship between both deposits. Petrography fluid inclusion of lithofacies from Shapinggou porphyry Mo deposit reveals 3 types of fluid inclusions including gas-liquid phase inclusions (AV), CO2-rich three phase inclusions (AC),and crystalline-bearing inclusions (ADV),developed in three stages of mineralization. In the early stage of mineralization, the homogeneous temperature of fluid inclusions ranges from 325 to 420 ℃ with relatively low salinity of AV and AC types ranging from 0.8% to 15.4% NaCleqv and moderately high salinity of ADV type ranging from 33% to 46% NaCleqv. In the middle stage of mineralization, AV, AC and ADV have uniform temperature at around 226 367 ℃ with low salinity of AV and AC types ranging from 0.4% to 13.2% NaCleqv and relatively high salinity of ADV type ranging from 33.3% to 39.6% NaCleqv. The late stage of mineralization develops AV type of fluid inclusion with homogeneous temperature ranging from 185 to 300 ℃, and the salinity ranges from 0.7% to 8.4% NaCleqv. However, in the Yinshan Pb-Zn deposits, only one stage of mineralization has been developed and displaying AV fluid inclusions. The homogeneous temperature of fluid ranges from 175 to 315 ℃ with salinity of 0.6%-4.9% NaCleqv, which correspond to the late stage of Shapinggou porphyry Mo deposit. It clearly appear that the homogeneous temperature and salinity decrease from the earlier to the late stage of mineralization. H-O isotope results of water in fluid inclusions show that the early stage fluid of Shapinggou Mo deposit is principally originated from magmatic source, while in the late stage of Pb-Zn mineralization, the ore-forming hydrothermal fluid is a mixture of magmatic water and meteoric water, similar to H-O isotopes of water in quartz fluid inclusions in the main metallogenic stage of Yinshan Pb-Zn deposit. It appears that the early hydrothermal fluid was originated from magmatic source with a small amount of meteoric water, which has, therefore, gradually increased during the middle and late stage of mineralization. S-isotope data indicated that sulfur of the two deposits derives from deep magmatic source and Pb-isotope, however, has characteristic of crust-mantle mixed source. The characteristics of S, Pb, H, O isotopes and fluid inclusions indicate that, the Mo deposits and Pb-Zn deposits in the study area have the same material sources. The ore-forming fluid has an evolution trend from high-temperature-high-salinity magmatic hydrothermal fluid to low-temperature-low-salinity magmatic-meteoric mixed fluid and from CO2-rich to CO2-poor. The Yinshan Pb-Zn deposits around Shapinggou are part of the porphyry metallogenic system.