贵州铜仁卜口场铅锌矿床Rb-Sr与Sm-Nd同位素年龄及其地质意义

摘要

因铅锌矿床的定年一直比较困难,关于黔东铅锌矿带的时代和成因认识较为混乱。本文对位于该成矿带赋存于下寒武统清虚洞组藻灰岩中的贵州铜仁卜口场铅锌矿床开展了闪锌矿Rb-Sr与方解石Sm-Nd同位素定年。矿石光薄片显示方解石和不透明矿物沿网状裂隙充填交代。用于定年的方解石,部分与方铅矿沿着同一闪锌矿细脉充填,部分沿方铅矿细脉充填,表明方解石晚于闪锌矿形成,可能属于成矿期后期产物。获得闪锌矿矿物 Rb-Sr 等时线年龄466±13 Ma (MSWD=2.0)和闪锌矿矿物+弱酸提取相+硫化物相Rb-Sr等时线年龄483±9 Ma(MSWD=8.0),二者在误差范围内一致,应代表了该铅锌矿床的主成矿期。获得方解石Sm-Nd等时线年龄422±48 Ma(MSWD=0.71),可能代表了同一成矿作用的后期阶段。即卜口场铅锌矿床后期改造成矿作用可能存在早奥陶世(483~466 Ma)和早志留世(~422 Ma)两个阶段。闪锌矿和闪锌矿分相Rb-Sr等时线对应的初始87Sr/86Sr比值分别为0.70920和0.70908,方解石Sm-Nd等时线对应的初始143Nd/144Nd比值为0.511520(εNd(t)为–11.2),指示其流体来源具有Sr、Nd富集特征,为成矿流体主要是地层封存水与区域热液流体提供了佐证。%The West Hunan-East Guizhou Pb-Zn mineralization belt has great prospecting potential, however, the metallogenetic epoch and genesis are actively debated due to the difficulty in dating Pb-Zn deposits, which is the key problem affecting the further mineralization research and mineral exploration. The Bokouchang Pb-Zn deposit in Tongren, Guizhou province situates in this belt and occurs in the algal limestones of the lower-Cambrian Qingxudong Formation. Ore-formation age of the deposit is dated by sphalerite and calcite Rb-Sr and Sm-Nd isotopic isochron method. Calcite aggregates in the ore samples have net-veined, spotted and patch distributions, and have no intercalated or dissected relationship with orebody or ore vein. Micrographs of the thin sections and polished sections of the ore samples show that the calcites and opaque minerals fill and metasomatize the network fissure. The calcites used for dating are cavity fillings in the sphalerite and galena veinlets, which shows that the calcite formed after sphalerite and maybe belong to the later stage mineralization. The Rb-Sr isochrone age of sphalerite is 466±13 Ma (MSWD=2.0), while for sphalerite, diluted acid leachates and sulfide residues the age is 483±9 Ma (MSWD=8.0). The results are consistent within the error and should represent the main mineralization epoch. Calcites define a Sm-Nd age of 422±48 Ma (MSWD=0.71) with a larger error due to only four data points and the small variation ranges of 147Sm/144Nd and 143Nd/144Nd, which may indicate the later stage of mineralization according to the occurrence of the calcites. Hence, the Bokouchang Pb-Zn deposit might have undergone two stages of mineralization, the early-Ordovician and early-Silurian, which may be mainly related to the regional Caledonian movement. The initial 87Sr/86Sr ratios, corresponding to the Rb-Sr isochrons for sphalerites and their step-dissolved phases, are 0.70920 and 0.70908, respectively. They are close to or slightly higher than those of carbonate formation and the coeval seawater. The initial 143Nd/144Nd ratio, corresponding to the Sm-Nd isochron for calcites, is 0.511520 with itsεNd(t) being-11.2. These initial 87Sr/86Sr and 143Nd/144Nd ratios indicate that the fluids were mainly derived from the formation water and regional hydrothermal fluid.