西秦岭造山带东段西成矿田内发育一系列泥盆纪的喷流沉积型（SEDEX）铅锌矿床，甘肃徽县洛坝铅锌矿床就是其中的代表型矿床之一。它产于中泥盆统安家岔组焦沟层上部，矿体分布受洛坝背斜和南、北两翼断裂控制，与地层产状基本一致，矿石主要为纹层状、层状、浸染状、块状构造。经过详细的显微构造和 SEM 扫描电镜分析，鉴定出与喷流沉积有关的通道构造、淬火构造等。矿床地球化学分析显示，该矿床赋矿围岩的 Ba、As、Sb、B、Ag、Hg 等喷流沉积指示元素值均较高，且两者均具有 LREE 相对富集、明显的 Eu 正异常和中等 Ce 正异常特征，对比表明，成矿流体中的 REE 主要继承赋矿围岩（硅质岩）。矿床包裹体及硫同位素分析表明：成矿温度为168～350℃，以中、高温为主；早期矿化硫化物的δ34 S 值（平均9．8‰）明显大于晚期矿化硫化物的δ34 S 值（平均5．6‰），说明后期改造作用中有地层硫的参与。综合研究表明，该矿床为热水喷流沉积型铅锌矿床，后期又受到海西期印支期的叠加改造，并确定该矿床为秦岭后造山期板内伸展热水喷流的产物，最后建立了洛坝铅锌矿床热水喷流沉积成矿模式。%A series of Devonian SEDEX deposits occur at the Xicheng ore field in east section of the Western Qinling orogenic belt,and the Luoba Pb Zn ore deposit in Hui County of Gansu Province is a typical one.The Luoba deposit is hosted in the upper section of the Jiaogou Layer in the Anjiacha Formation (Middle Devonian period)and controlled by Luoba anticline and fractures developing in its north and south wings.The ore bodies are accordant with the hosting strata in occurrence.The predominately ore structures include lamellar,layered,disseminated and massive structures.Channel and quenching structures,commonly related to SEDEX,are identified by the analysis of microstructure and SEM.Ba,As,Sb,B,Ag and Hg contents in the ore and wall-rock are relatively high and LREE are enriched.Obvious positiveδEu and medium positiveδCe imply that the REE in the ores inherited from the wall-rocks (chert).Fluid inclusions studies show that the mineralizing temperature ranged from 168 ℃ to 350 ℃.Theδ34 S values of the early sulfides (averaged at 9.8‰)are significantly higher than those of the late ones (averaged at 5.6‰),which indicates that the strata sulfur was involved in the late reconstruction processes.Authors suggest that the Luoba Pb Zn ore deposit belong to the SEDEX type,and be reformed by Late Hercynian-Indosinian metamorphism.It was derived from the hydrothermal fluid at the later Qinling orogenic intraplate extension stage.A metallogenic model for the Luoba SEDEX lead-zinc deposit is also given in this paper.
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