黔东大河边铅锌矿床Zn-Cd同位素特征及其成因指示意义

The southeast margin of Yangtze Block is an important lead-zinc metallogenic belt. Numerous sediment-hosted Pb–Zn deposits (showings) were grouped or zoned, and mainly hosted in the Qinxudong Formation of Cambrian system. Besides the Qinxudong Formation, stratabound Zn-Pb sulfide mineralization hosted by the Doushantuo Formation of Sinian system was universally discovered recently. Also, these Zn-Pb sulfide mineralization and barite mineralization have a close spatial relationship, which can be summarized as, the top is stratiform barite ores, and the bottom is zinc-lead sulfide ores. These special characters have aroused economic-geologist’s extensive attention. The current research has the controversy among the Mississippi Valley type (MVT), “Yangtze-type”, and exhalative-sedimentary type (SEDEX), and is also difficult to determine whether a genetic linkage between the zinc-lead deposit and the stratiform barite deposit. The crucial factor is that lack of effective method can be used to accurately limit the Pb-Zn mineralization metal source and genetic process. Therefore, this proposal intends to choose the zinc-lead deposits and these overlaid stratiform barite deposits which settled in the Dahebian region, eastern Guizhou, for the first time to carry out the in suit analysis of Zn-Cd isotopic geochemistry to constrain the ore-forming processes and metal source of Pb-Zn deposit, to identify the mineralization regularity, to reveal the genetic linkage between the zinc-lead deposit and the stratiform barite deposit, and finally to discuss the regional Pb-Zn mineralization potential along the southeast margin of the Yangtze Block.

扬子板块东南缘是我国重要的铅锌成矿带，带内铅锌矿床（矿点）成带和成群产出。除主要的赋矿层位—寒武系清虚洞组外，近年来在该区震旦系陡山沱组地层内也发现大量规模大、层位产出稳定的铅锌矿床（点），并表现出与寒武纪早期形成的巨量喷流沉积型重晶石矿床具有“上部为层状重晶石矿床，下部为铅锌硫化物矿床”的紧密空间联系的特点，而引起广泛的关注。其成因目前存在“密西西比河谷型（MVT）”、“扬子型”、以及“喷流沉积型（SEDEX）”之争，铅锌矿床与重晶石矿床是否具有成因上的联系亦难以判定。归根结底在于缺乏有效的手段来分辨成矿物质来源及成矿过程。本项目拟以带内典型的大河边铅锌矿床及上覆重晶石矿床为研究对象，对铅锌矿石、重晶石矿石及蚀变岩进行系统的Zn-Cd同位素分析，约束成矿物质来源及成矿过程，厘定此类铅锌矿床的成矿机理，揭示铅锌矿化与重晶石矿化之间的成因联系，丰富和完善扬子板块东南缘铅锌成矿理论。

近年来在扬子板块东南缘震旦系地层内发现大量规模大、层位产出稳定的铅锌矿床，往往与寒武纪早期形成的巨量喷流沉积型重晶石矿床具有“上部为层状重晶石矿床，下部为铅锌硫化物矿床”的紧密空间联系，但其成因存在较大争议，铅锌矿体与重晶石矿体是否具有成因上的联系尚未揭示。因此，本项目以带内典型的大河边铅锌矿床及上覆重晶石矿床为研究对象，开展了详细的矿床地质、矿物学、以及Zn-Cd同位素地球化学研究。研究显示：（1）扬子板块东南缘此类型的铅锌矿床分布于南华裂谷盆地内，空间上与早寒武世层状重晶石矿床具有紧密相邻，在矿物学上呈现出硫化物及重晶石“你中有我，我中有你”的紧密联系；（2）系统的Zn-Cd同位素研究表明大河边铅锌床属于喷流沉积型铅锌矿床，与早寒武世巨量重晶石喷流成矿属于同期成矿。Zn同位素揭示出大河边铅锌矿床及重晶石矿床成矿热液的δ66ZnJMC LYON为0.44‰左右，铅锌矿段的闪锌矿、重晶石之中的硫化物、区域上的V矿层及Ni-Mo-PGE层属于同期热液活动在不同的热液演化阶段发生矿化，整体的Zn同位素分馏遵循瑞利分馏模型。Cd同位素揭示出初始成矿热液的δ114CdNIST SRM 3108在0附近，按照瑞利分馏模型计算出不同的金属富集所处流体演化阶段与Zn同位素计算出的结果高度吻合；（3）Zn-Cd同位素表明板溪群赋存的多金属脉型属于岩浆热液型铅锌矿床，对稀散金属In发生富集；（4）对扬子板块东南缘多金属脉型、沉积喷流型及MVT型铅锌矿床的成矿时代、物质来源、动力学背景进行了系统分析、比对。通过系统的闪锌矿微量元素及S-Pb同位素研究，揭示出多金属脉型、沉积喷流型及MVT型铅锌矿床的硫分别来自广西运动导致的岩浆作用、早寒武世海水以及早寒武世地层中孔隙水，成矿金属分别主要源自广西运动导致的岩浆作用、基底以及清虚洞组。本项目的研究为揭示扬子板块东南缘震旦系赋存铅锌矿床的成矿机制，丰富和完善扬子板块东南缘铅锌成矿理论具有重要意义。