扬子地台西缘大梁子铅锌矿床成因:流体包裹体及同位素地球化学约束

Genesis of the Dalingzi Pb-Zn Deposit in the Western Margin of Yangtze Plate: Constraints from Fluid Inclusions and Isotopic Evidence

扬子地台西缘大梁子铅锌矿床是川滇黔铅锌矿集区具代表性的大型矿床之一,矿体主要赋存于震旦系灯影组白云岩中,矿石矿物主要为闪锌矿和方铅矿,脉石矿物主要为白云石、石英和方解石。大梁子铅锌矿床可划分闪锌矿-黄铁矿-炭质、闪锌矿-方铅矿和方铅矿-黄铜矿-碳酸盐三个成矿阶段。H-O同位素及流体包裹体研究表明,成矿流体为中–低温(均一温度在117.5～320.3℃)及中-低盐度(盐度范围为5.11%～18.96%NaCl_(eqv))的盆地卤水,有机质参与成矿。灯影组白云岩与海相沉积碳酸盐岩具有相似的C、O同位素组成,而热液方解石C、O同位素组成均稍低于灯影组白云岩,可能是海相碳酸盐岩溶解与有机质共同作用的结果。矿区硫化物δ^(34)S值为9.8‰～20.8‰,结合成矿温度,S主要形成于灯影组海相硫酸盐的热化学还原作用。矿石硫化物的Pb同位素比值在Pb演化图解上主要落在上地壳与造山带之间,表明Pb具有壳源特征,成矿物质可能主要来源于围岩,部分来自于基底地层。热液方解石及闪锌矿的(^(87)Sr/^(86)Sr)i比值均高于围岩,表明成矿流体可能流经了具有高(^(87)Sr/^(86)Sr)i比值的基底地层,并与其发生水岩反应及同位素交换。综上,大梁子铅锌矿床成矿流体具有盆地卤水特征,盆地卤水在运移过程中萃取了基底及围岩中的Pb、Zn等成矿物质,成矿流体运移至矿区"黑破带"时,在有机质作用下发生TSR作用,形成还原S,导致Pb、Zn等金属沉淀成矿。

The Daliangzi Pb-Zn deposit is a large deposit hosted in the Sinian Dengying Formation dolostone,located in the Sichuan-Yunnan-Guizhou ore concentration area.Ore minerals are mianly sphalerite,galena,and gangue minerals consist of dolomite,quartz and calcite.The metallogenic stages may be divided into sphalerite-pyrite-carbon stage,sphalerite-galena stage and galena-chalcopyrite-carbonate stage.The ore-forming fluid is basin brine,which is characterized by medium-low temperature of 117.5℃to 320.3℃and medium salinity of 5.11%NaCleqv to 18.96%NaCleqv,moreover,the abundant CH4 and pitch in the fluid inclusions indicate that the participation of organic matter in the mineralization.Theδ13CV-PDB andδ18OSMOW values of the Dengying Formation dolostone are similar to that of marine carbonate,revealing that the dolostone belongs to marine carbonate.Both theδ13CV-PDB andδ18OSMOW values of hydrothermal calcites are lower than that of the Dengying Formation dolostone,which may result from dissolution of the Dengying Formation dolostone.Theδ34S values of ore minerals are mainly in the range of 9.8‰–20.8‰,indicating the sulfur may come from thermochemical reduction of marine sulfate in the Dengying Formation.The 207Pb/204Pb versus 206Pb/204Pb diagram manifests that Pb is crustal origin,and likely comes mainly from the wall rocks and less from the basement.(87Sr/86Sr)i ratios of sphalerites and hydrothermal calcite are higher than that of the Dengying Formation dolostone,indicating that the ore-forming fluid flew through the basement.In conclusion,the ore-forming fluid was basin brine,which extracted the metallogenic materials,Pb and Zn,from the basement and wall rocks.When the ore-forming fluid reached the“black fractured zones”,carbonized tectonic breccia zone,S2?was produced by the thermochemical reduction reaction under the influence of the organic matter,and interaction between the S2?and Pb2+,Zn2+,resulted in the precipitation of ore metals.