Provenance and salt structures of gypsum formations in Pb-Zn ore-bearing Lanping basin,Southwest China

中国西南地区含铅锌矿化兰坪盆地中石膏层的盐结构和来源

Large-scale gypsum rocks associated with world-class Pb-Zn ore formations are widely distributed in the Lanping Basin,Sowthwest China.Geochemical studies alongside field investigations were conducted in this study to determine the source and evolutionary processes of the gypsum rocks in this area.The gypsum sequences in the Lanping Basin developed in two formations:the Triassic Sanhedong Formation and the Paleogene Yunlong Formation.The gypsum hosted in the former displays a primary thick-banded structure withδ34SV-CDT values in the range of 14.5‰−14.8‰.Combined with the 87Sr/86Sr values(0.707737−0.707783)of limestone,it can be suggested that the Sanhedong Formation is of marine origin.In contrast,the gypsum from the Paleogene Yunlong Formation is characterized by the dome,bead and diapiric salt structures,wider range of both 87Sr/86Sr(0.707695−0.708629)andδ34SV-CDT values(9.6‰−17‰),thus indicating a marine source but with the input of continental materials.The initial layered salt formations were formed by chemical deposition in a basin and were later intensely deformed by collisional orogeny during the Himalaya period.As a result,variable salt structures were formed.We hereby propose an evolutionary model to elucidate the genesis of the gypsum formations in the Lanping Basin.

兰坪盆地广泛分布着大规模的石膏层和世界级的铅锌矿床。本研究结合野外调查和地球化学分析结果以确定本区石膏的来源和演化过程。兰坪盆地石膏层序发育于三叠系三河洞组和古近系云龙组两个地层。前者为原生厚条带状结构,δ34SV-CDT值在14.5‰~14.8‰。结合灰岩87Sr/86Sr比值(0.707737~0.707783),本文认为三河洞组为海相成因。相反,古近系云龙组石膏具有穹隆、珠状和底辟盐构造特征,87Sr/86Sr(0.707695~0.708629)和δ34SV-CDT值(9.6‰~17‰)的区间较宽,表明其为海相来源,但也有陆相物质的输入。最初的层状盐层是在盆地中以化学沉积的形式形成的,后来的多种形貌不同的盐结构是在喜马拉雅期受到碰撞造山作用的影响,发生强烈变形而形成的。在此基础上,构建了一个解释兰坪盆地石膏建造成因的演化模式。