内蒙古甲乌拉大型Pb-Zn-Ag矿床稳定同位素地球化学研究

内蒙古甲乌拉银多金属矿床位于大兴安岭成矿带北段，为近年来发现的大型银铅锌多金属矿床。矿床矿体分布完全受到断裂构造的控制，金属矿物组成主要为方铅矿、闪锌矿、黄铜矿、黄铁矿、磁黄铁矿、毒砂、辉钼矿及磁铁矿等。文中重点分析了矿床的硫、氢、氧、碳和铅稳定同位素地球化学特征。研究结果表明：金属硫化物δ34S集中为1.37‰~4.10‰，平均为3.10‰(n=13)，极差为2.73‰；石英和方解石δ18Owater的变化范围较大(-18.96‰~+1.08‰) (n=9)，均值为-11.36‰；δDV SMOW的变化范围比较集中(-133.6‰~-103.4‰) (n=9)；27件样品的铅同位素组成为：206Pb/204Pb=18.228 3~18.758 7、207Pb/204Pb=15.457~15.880和208Pb/204Pb=37.841~39.049，矿床的铅组成基本为正常的放射性成因铅；方解石δ13CV PDB变化范围为-5.2‰~-8.4‰，平均为-6.8‰(n=2)。矿石硫化物的硫同位素及方解石的碳同位素均指示成矿物质可能来源于深部的岩浆活动；石英和方解石的氢氧同位素组成表明成矿流体早期以岩浆流体为主，成矿晚期加入了大量加热补给的大气降水；铅同位素组成表明成矿流体中铅的来源主要为幔源，矿床形成过程中混入少量的壳源铅。矿床稳定同位素组成显示成矿流体主要来源于深部的岩浆热液，特别与燕山晚期的火山次火山热液有较为密切的联系，在流体演化过程中大气降水的加入对矿床成矿元素的聚集和沉淀也起到有利作用。成矿作用的发生是在一种总硫浓度比较低、中等氧化环境、相对开放的非平衡体系中进行的。矿床形成的地球动力学背景为一种岩石圈大规模快速减薄的过程。甲乌拉大型Pb Zn Ag矿床的成因类型属于火山次火山热液脉状银多金属矿床。

A study of stable isotope geochemistry of the Jiawula large Pb-Zn-Ag ore deposit, Inner Mongolia

Located in the northern part of the Da Hinggan Mountain metallogenic belt,the Jiawula deposit is a large Pb-Zn-Ag polymetallic ore deposit which was discovered in recent years.Ore body is totally controlled by fault structure and the main metallic minerals are galena,sphalerite,chalcopyrite,pyrite,pyrrhotite,arsenopyrite,molybdenite,magnetite,etc.In this paper,the author analyzed geochemistry of sulfur,hydrogen,oxygen,carbon and lead isotopes of the deposit.The study indicated that the δ34S(‰) values range from 1.37 to 4.10‰ with an average of 3.10‰(n=13);that the δ18Owater(‰) values range from—18.96 to +1.08‰(n=9) with an average of—11.36‰;that the δDV-SMOW(‰) values range from—133.6 to—103.4‰(n=9);that the 206Pb/204Pb,207Pb/204Pb and 208Pb/204Pb values of 27 samples range 18.228 3—18.758 7,15.457—15.880 and 37.841—39.049,respectively,and the lead isotope compositions showed that all the lead belong to normal radiogenic lead;that the δ13CV-PDB(‰) values range from—5.2 to—8.4‰ with an average of—6.8‰(n=2).The sulfur and carbon isotope data suggested that the ore forming fluids were mainly derived from the deep magmatic fluids and the hydrogen and oxygen isotopic compositions indicated that the ore forming fluids were composed of magmatic fluids in the early stage and of meteoric water in the late stage.The lead isotopic compositions confirmed that the origin of lead in the deposit was mainly from mantle source as well as little from crust source.All the isotopic compositions demonstrated that the formation of the Jiawula large Pb-Zn-Ag deposit was associated with the volcanic-subvolcanic hydrothermal activities in the late Yanshanian period and the addition of meteoric water within the magmatic fluids created a favorable condition for the metallogenic elements to accumulate and precipitate to form such a large deposit.The occurrence of mineralization was under a low δ34S∑S,medium oxidation state,relatively open and non-equilibrium system.Geodynamic settings of the mineralization were under a process of huge lithosphere thinning.The genetic type of the Jiawula deposit can be regarded as the volcanic-subvolcanic hydrothermal vein Pb-Zn-Ag ore deposit.