黔西北福来厂铅锌矿床Pb同位素研究及地质意义

摘要

单阶段演化正常铅用H-H法可获得高准确性的模式年龄,在黔西北福来厂铅锌矿床中采集16件矿脉和围岩样品,测定它们的Pb同位素组成,206 pb/204 Pb为18.5346 ～ 18.7294(均值18.5935),207pb/204 Pb为15.7408～15.7603(均值15.7519),208 pb/204 Pb为38.9718 ～39.1766(均值39.0731).分析结果表明该矿区铅是正常铅,利用Pb-Pb同位素测年方法,获得了矿脉铅的主模式年龄为240 ～ 229 Ma,围岩铅的主模式年龄为254～230 Ma.研究表明该矿脉Pb模式年龄能代表成矿年龄,反映铅主成矿期在印支期,印支期区域隆升导致NW向区域正断层活动,这些断裂控制本区铅锌矿床的分布.矿脉与围岩的Pb同位素组成相似,年龄相近,结合Pb同位素构造模式和△β-△γ图解,我们认为上地壳围岩是福来厂铅锌矿床成矿物质的主要来源,基底能提供部分物质.峨眉山玄武岩后期热液对成矿物质有一定的影响.%Precise model age of single-stage evolution ordinary lead can be calculated by Holmes-Houtormans method. Sixteen samples of ore veins and wall rocks from the Fulaichang lead-zinc ore deposit were analyzed for their Pb isotopic compositions. Both the ore veins and wall rocks present similar geochemical characteristics; 206Pb/204Pb = 18.5346 ~ 18. 7294 (average 18.5935), 207 Pb/204 Pb = 15.7408-15.7603 (average 15.7519), 208Pb/204Pb =38. 9718 ~ 39. 1766 (average 39. 0731 ). Accordingly, the lead in the lead-zinc ore deposit belongs to ordinary lead. Using Pb-Pb dating method, we get Pb model age of the ore veins (240 ~ 229 Ma) and the wall rock (254 -230 Ma). Pb model ages of the ore veins are consistent with the ages of the wall rocks. Thus, the Pb model ages of the veins represent the major Pb mineralization period and indicate that the lead-zinc ore deposit was formed during Indosinian. The regional NW normal faults are products of regional uplift in Indosinian. These faults controlled the distribution of lead-zinc ore deposits in this area. According to Pb isotope tectonic model diagram and △β-△γ diagram, we conclude that the metallogenic materials were derived from the upper crust. The upper crustal capping rocks (wall rocks) provided the major source of ore metals. Additionally, the metamorphic basement may also provide part of Pb. The Emeishan basalts are unlikely the source of the metals.