西藏则学地区热液脉型铅锌矿S、Pb同位素组成及其对成矿物质来源的启示

Sulfur and lead isotope composition of hydrothermal vein type lead-zinc deposits in Zexue area,Tibet and its implications for the origin of ore-forming materials

德新和轧轧龙铅锌矿是冈底斯西段银铅锌多金属成矿带典型的热液脉型铅锌矿床,关于其成矿物质来源及与纳如松多铅锌矿成因联系的研究鲜有开展。本研究利用单矿物和全岩稳定同位素方法对德新和轧轧龙铅锌矿的主要金属硫化物和含矿花岗斑岩进行了S、Pb同位素组成分析和示踪,结果显示德新铅锌矿硫化物δ34SVCDT为3.5‰~7.4‰,平均值为6.1‰;含矿花岗斑岩δ334SVCDT为4.4‰~6.2‰,平均值为5.7‰;轧轧龙铅锌矿金属硫化物δ334SVCDT为2.7‰~8.3‰,平均值为5.1‰;德新和轧轧龙铅锌矿铅同位素比值比较稳定,变化范围基本一致;铅同位素μ值为9.48~9.82,平均值为9.64;△β、△γ变化范围基本一致,均值相差较小,表明则学地区热液脉型铅锌矿金属硫化物和斑岩具有一致的S、Pb同位素组成,铅同位素具正常铅特征;暗示硫源可能主要由花岗岩类提供,铅源则可能主要来自上地壳物质。综合前人研究认为,德新、轧轧龙热液脉型铅锌矿和纳如松多铅锌矿具有一致的硫、铅同位素组成,三者应存在成因上的联系,可能属于同一成矿系统产物;花岗斑岩为则学地区热液脉状铅锌矿化提供了成矿物质。

The Dexin and Zhazhalong Pb- Zn deposits are the representative hydrothermal vein type lead- zinc deposits in the western part of the Gangdise Ag-Pb-Zn polymetallic metallogenic belt;nevertheless, the origin of ore-forming materials of the two deposits and their genetic relation with the Narusongduo Pb-Zn deposit remain unknown. A stable isotopic method for single mineral and whole rock was used to measure the S and Pb isotopic composition of the sulfide and ore-bearing granite porphyries from the two deposits and to trace their sources of ore- forming materials. It is shown that the δ34SVCDT values of sulfide and ore- bearing granite porphyry from Dexin and sulfide from Zhazhalong range from 3.5‰ to 7.4‰, from 4.4‰ to 6.2‰, and from 2.7‰ to 8.3‰, with an average being 6.1‰, 5.7% and 5.1%, respectively;the ratios of different lead isotopes in Dexin and Zhazhalong are stable, varying in similar ranges, and the μ values of lead isotope in the two deposits vary from 9.48 to 9.82 (9.64 on average) with similar △β,△γ variation ranges and average values, which suggests that the S and Pb isotopic compositions of the sulfide are consistent with those of the ore- bearing granite porphyries in the two deposits with normal lead characteristics, granitoid origin of sulfur isotope and upper crust origin of lead isotope predominately. With comprehensive analysis of geological background and previous researches, it is concluded that the Pb-Zn mineralization in Dexin, Zhazhalong and Narusongduo Pb-Zn deposits may be products of the same metallogenic system with consistent sulfur and lead isotopic characteristics, and the granite porphyries might have supplied ore-forming materials for the hydrothermal vein type lead-zinc deposits in Zexue area.