四川乌斯河铅锌矿床成矿物质来源及矿床成因:来自原位S-Pb同位素证据

Metal sources and ore genesis of the Wusihe Pb-Zn deposit in Sichuan,China:New evidence from in-situ S and Pb isotopes

乌斯河铅锌矿床位于扬子板块西南缘,是川滇黔铅锌矿集区代表性大型铅锌矿床之一,估算资源量超过370万吨,Pb+Zn平均品位~15.7%。该矿床铅锌矿体主要呈层状、似层状、透镜状产于震旦系灯影组的白云岩地层中,其围岩蚀变较弱,以白云石化和方解石化为主。矿石类型主要包括块状、条带状、角砾状、脉状和浸染状等,其中矿物组成相对简单,以闪锌矿、方铅矿、白云石和方解石为主,含少量沥青和黄铁矿。该矿床地质地球化学研究程度相对较低,其成矿物质来源不清,致使该矿床存在热水沉积成因、喷流沉积-后期热液叠加改造以及MVT型等多种成因争议,难以建立统一成矿模式。本文对乌斯河铅锌矿床不同成矿阶段的硫化物(包括黄铁矿、闪锌矿和方铅矿),开展原位硫和铅同位素地球化学研究,以查明该矿床的成矿物质来源、还原硫的形成机制和示踪成矿过程,为认识该类矿床铅锌成矿作用提供新地球化学依据。原位S同位素分析结果显示,乌斯河铅锌矿床硫化物的还原硫存在不同硫来源,成矿早阶段硫化物δ34S值较低,介于+1.3‰^+14.2‰之间,暗示可能有来自于赋矿地层圈闭古油气系统中的H2S;主成矿阶段硫化物相对富集重硫同位素,δ34S值在+11.0‰^+23.3‰之间,表明其为赋矿地层的蒸发岩的热化学还原作用的产物。此外,硫化物的LA-MC-ICPMS原位Pb同位素组成分析显示,该矿床成矿金属元素主要来源于变质基底地层,水岩反应可能使赋矿地层贡献少量的成矿物质。综合矿区地质特征和已有的地球化学研究成果,本文认为乌斯河铅锌矿床属于MVT铅锌矿床,富Pb、Zn等成矿元素的流体与富H2S的流体混合是该矿床金属硫化物沉淀的主要机制。

The Wusihe Pb-Zn deposit,located at the southeastern margin of Yangtze Block,is one of the typical large-scale Pb-Zn deposits in the Sichuan-Yunnan-Guizhou(SYG)metallogenic province with an estimated Pb+Zn reverse>3.70 Mt at an average grade of^15.7%.Sulfide orebodies were hosted in the dolostone of Sinian Dengying Formation,occurring as stratiform,stratoid and lenses with weak wall rock alteration.Sulfide ores are mainly massive,banded,breccias,veins and disseminated ores.The mineral assemblage of the Wusihe deposit is relatively simple,including sphalerite,galena,hydrothermal dolomite and calcite with minor pyrite,bitumen and quartz.The ore genesis of Wusihe remains controversial due to the lack of geological and geochemical studies,and related views about its mineralization include hydrothermal sedimentation,later hydrothermal superimposition on an exhalative sedimentation and MVT.In this study,in-situ S and Pb isotopes of sulfides(sphalerite,pyrite and galena)from Wusihe at different stages were investigated,aiming to:(1)constrain the sources of metals and the formation mechanism of reduced S;(2)to trace its ore-forming process;and further(3)to provide new evidence for understanding the ore genesis.In-situ S isotopic compositions in the Wusihe deposit show different S sources during ore-forming process.Sulfides at early ore-stage had significant depletion of34 S and theδ34 S values(ranging from+1.3‰to+14.2‰),which may suggest that the H2 S was derived from a paleo-petroleum system in host rock;while theδ34 S values of sulfides at late ore-stage vary from+11.0‰to+23.3‰,i.e.,it is enriched in heavy S isotopes,indicating that it has undergone a marine sulfate thermochemical reduction of evaporate in ore-hosted strata.The LA-MC-ICPMS in-situ Pb isotopic ratios of sulfides(sphalerite and galena)show that the ore-forming metals of the Wusihe deposit are mainly derived from basement rocks with a small contribution from host rock via water-rock interaction.Geology and in-situ S and Pb isotope data,together with early published studies,suggest that the Wusihe deposit belongs to an MVT Pb-Zn deposit,and the metal-bearing fluid mixing with reduced S-enriched fluid is the key factor for metal sulfide precipitation.