青海玉树东莫扎抓铅锌矿床围岩蚀变和黄铁矿-闪锌矿矿物学特征及意义

摘要

The Dongmozhazhua Pb-Zn ore deposit in Yushu area of Qinghai Province is one of the important ore deposits located in the "Sanjiang" sediment-hosted base metal ore belt.In the ore deposit, several types of alteration and hydrothermal minerals were recognized in previous studies.However, the spatial distribution of the alterations, the characteristics of element content in the minerals, and the spatial and temporal relationships of different minerals in the orefield remain ambiguous, thus the data available couldn't provide clear ore-searching direction.In view of such a situation, the authors made systematic thin section observations of ore and host rocks, systematic electron-probe micro-analysis (EPMA) of the most important sulfides pyrite and sphalerite, and ICP-MS analysis of important metallic compositions of typical ore rocks, with the purpose of finding detailed scientific information on the above problems.Four hydrothermal alteration types were recognized in the ore deposit, namely dolomitization, pyritization, silicification and baritization.Dolomitization is almost distributed in the whole orefield and occurred from S1 to S4 ore-forming stages, whereas pyritization occurred from S3 to S4 ore-forming stages; in both alterations considerable lead and zinc minerals precipitated, and the pyritization was closely accompanied by Pb-Zn mineralization.Silicification occurred in the same time with pyritization and is mainly distributed to the south of the main thrust fault in the ore deposit.Barite precipitated before and after S3 polymetallic sulfide ore-forming stage, and baritization is mainly developed along the faults south of the main thrust fault.Relatively high content of Cd was observed from the typical ore rocks by ICP-MS.According to EPMA, during S3 ore-forming stage, S and Fe content and atomic ratios of S/Fe in pyrite were higher than their theoretical values, some As and Cu were observed, and the ratio of Co/Ni was usually higher than 1; in sphalerite the S content and the atomic ratios of S/Zn were higher than their theoretical values while the Zn content was lower than their theoretical values, and some Cd and Fe were observed.The atomic ratios of S/Fe and S/Zn in pyrite and sphalerite crystallized from most active hydrothermal fluids were obviously higher than their theoretical values; Micro-sections were studied for spherical pyrite and strawberry and crustal sphalerite in this ore-forming stage and some changes of As, Fe, S and atomic ratios of S/Fe in pyrite and Cd, S and Zn content and S/Zn atomic ratios in sphalerite were observed.During S4 ore-forming stage, the sphalerite had similar element content to that in S3 ore-forming stage, but it had lighter color, lower Zn content and higher atomic ratios of S/Zn.Combining the petrographic characteristics and the EPMA results of pyrite and sphalerite, the authors have reached three conclusions: ① in the S3 ore-forming stage, relative to metal ions, S was enough for sulfide crystallization, and the hydrothermal environment was stable in the early time of this stage; with the precipitation of sphalerite, however, the temperature and activity of S and Ph of the ore-forming fluid decreased while the Cd/Zn ratio rose; ② there were two active centers of hydrothermal fluids in the above ore-forming stage, which were located near Line 0 in the MI ore belt and in the north of MIV belt, respectively; ③ the properties of fluids at S3 and S4 ore-forming stages were different, and the latter stage was characterized by lower temperature and higher sulfur fugacity.Based on all these data, the authors give detailed suggestions on the prospecting for similar Pb-Zn mineralization: ① searching for dolomitization and silicification regionally; ②searching for pyritization in the dolomitization and silicification areas; ③ searching for sphalerite and galena at the positions with most pyrite; ④ searching for small-size but high-grade Pb-Zn ore bodies in the baritization areas along faults, and ⑤ searching for Zn-Pb mineralization in limestones with high content of Cd.Four Pb-Zn mineralization locations worthy of detailed work for further prospecting in the Dongmozhazhua Pb-Zn ore deposit are delineated.%青海玉树地区的东莫扎抓铅锌矿床是"三江"沉积岩容矿贱金属成矿带中的重要矿床之一.矿区过去研究鉴定出多种蚀变类型和矿物种类,但不同蚀变在矿区的空间分布特征、不同矿物中元素含量特点、矿物相互之间的时间和空间关系均不清楚,从而导致所指示的找矿意义也不明确.为此,作者进行了系统岩矿石光薄片的显微镜下鉴定、矿区主要硫化物--黄铁矿和闪锌矿的电子探针成分分析、典型矿石中重要金属元素含量的ICP-MS测定等工作,以期为上述问题的认识提供详实的基础资料.研究发现,与矿化有关的蚀变主要有白云石化、黄铁矿化、硅化和重晶石化4种类型,其中白云石化贯穿于成矿的前4个阶段,分布基本遍及矿区;黄铁矿化和硅化在S3和S4两个硫化物沉淀阶段发育,前者和铅锌矿化紧密伴生,后者主要发生在矿区主逆冲断裂以南;重晶石化在S2和S4两个阶段出现,但仅在矿区主逆冲断裂以南的多条断裂附近局限性发育.ICP-MS分析发现典型矿石中存在含量较高的Cd.电子探针成分分析发现:在S3阶段,黄铁矿中S、Fe含量和S/Fe原子比普遍高于理论值,有一定量的As、Cu存在,Co/Ni比值一般大于1;闪锌矿中S含量和S/Zn原子比较理论值偏高,Zn含量较理论值稍低,有一定量的Cd、Fe存在;黄铁矿S/Fe和闪锌矿S/Zn原子比在流体活跃部位明显高于理论值;球形黄铁矿、草莓状和皮壳状闪锌矿从边部到核心某些元素含量存在规律性变化.在S4阶段,闪锌矿较S3阶段颜色浅,Zn含量低,S/Fe原子比高.综合岩相学特征和电子探针分析结果,指出:在S3阶段,S源相对金属离子充足,黄铁矿沉淀之初热液环境稳定,随着闪锌矿的沉淀,含矿流体的温度、S离子活度和pH值降低,但Cd/Zn比增高,此阶段矿区存在两处流体活动中心;S4阶段流体较S3阶段温度低,硫逸度高.综合以上讨论,提出了在矿区和区域上寻找同类铅锌矿化的具体建议,并在矿区指出了4处具有找矿前景的位置.