新疆西天山吐拉苏铅锌矿床的成因--年代学、流体包裹体和硫铅同位素的约束

Ore genesis of the Tulasu Pb-Zn deposit in the Western Tianshan,Xinjiang:Constraints from the geochronology,fluid inclusions and S-Pb isotopes

吐拉苏铅锌矿床是新疆西天山吐拉苏盆地中的代表性贱金属矿床之一。矿体赋存于晚古生代大哈拉军山组凝灰岩中,受南北向张性断裂控制。矿石中主要矿物有方铅矿、闪锌矿、黄铜矿、黄铁矿、石英和方解石,发育晶洞状、晶簇状、梳状、角砾状等代表开放空间的充填构造。赋矿凝灰岩LA-ICP-MS锆石U-Pb年龄为372.5±4.3 Ma,形成于北天山洋向伊犁-中天山板块之下俯冲的背景。含矿石英中发育大量富液相水两相包裹体[均一温度和盐度w(NaCleq)分别集中于130~210℃和8.1%~16.9%之间],偶见富气相水两相包裹体、纯气相水包裹体和含固相子晶三相包裹体。流体包裹体和矿石组构特征综合表明,流体沸腾及其伴随的温度降低是Pb、Zn沉淀的重要机制。方铅矿δ34S范围为-2.2‰~-0.5‰,显示出岩浆硫的特征。方铅矿的206Pb/204Pb、207Pb/204Pb和208Pb/204Pb值分别为18.208~18.264、15.588~15.617和37.954~38.088,与区域上金矿床和赋矿火山岩相似,指示成矿物质来源于赋矿火山岩。综合矿床地质、流体包裹体和同位素组成特征,认为吐拉苏矿床属于中硫型浅成低温热液贱金属矿床。吐拉苏盆地今后找矿勘查既要注意在已知的铅锌矿床附近寻找低硫型浅成低温热液金矿床,同时又要注意低硫型浅成低温热液金矿床深部及其周缘也可能发育中硫型的贱金属矿化。

The Tulasu Pb-Zn deposit is one of the typical base metal deposits in the Tulasu basin,Western Tianshan,Xinjiang.The outcropped orebody is hosted in the Late Paleozoic Dahalajunshan Formation tuff and controlled by the N-S trending tensional fault.The minerals in the ores mainly consist of galena,sphalerite,chalcopyrite,pyrite,quartz and calcite.The ores are characterized by the occurrence of filling textures in open space,including cave,drusy,comb and breccia textures.LA-ICP-MS U-Pb dating on zircons in ore-hosting tuff gave weighted average age of 372.5±4.3 Ma,indicating that the tuff resulted from the subduction of North Tianshan Ocean beneath the Yili-Central Tianshan Plate.A large amount of liquid-rich two-phase aqueous inclusions are developed in the ore-bearing quartz,with homogenization temperatures and salinities in the ranges of 130~210℃and 8.1%~16.9%NaCl_(eq),respectively.In addition,vapor-rich two-phase aqueous inclusions,vapor mono-phase inclusions and daughter mineral-bearing three-phase inclusions are locally observed.Both the fluid inclusions and ore textures show that fluid boiling and subsequent temperature decrease could be the important mechanism for the Pb-Zn precipitation.Galena has narrowδ~(34)S range of-2.2‰~0.5‰,indicating a contribution of magmatic-sourced sulfur.The~(206)Pb/~(204)Pb,~(207)Pb/~(204)Pb and~(208)Pb/~(204)Pb ratios of galena range from 18.208 to 18.264,15.588 to 15.617and 37.954 to 38.088,respectively,similar to those of gold deposits and ore-hosting volcanic rocks in the region,indicating that the ore-forming metals were sourced from the Dahalajunshan Formation volcanic rocks.Based on the information from geology,fluid inclusions and isotopes,the Tulasu Pb-Zn deposit is considered to be an intermediate-sulfidation epithermal deposit.For the further prospecting in the Tulasu basin,in addition to the low-sulfidation Au deposits that may occur near the identified Pb-Zn deposits,intermediate-sulfidation base metal mineralization should also be paid attention underneath the discovered low-sulfidation Au deposit and its vicinity.