镉同位素对铅锌矿床的地球化学示踪研究—以川滇黔低温成矿域为例

Recently, “non-traditional stable isotopes” has been wildly used in different research field in earth science and becomes a hotspot of isotope geochemistry. Cadmium has received increasing attention for its specific geochemistry and isotopic fractionation. At present, cadmium isotopes have been successfully applied in tracing nutrient elements circulation in marine system, identifying source of heavy metals and so on. These studies have shown that cadmium can provide a wealth of information on geological processes. However, there are few researches focused on cadmium isotope studies in Pb-Zn ore deposits, which are considered as one of the most important Cd reservoirs in the earth. Prior studies showed that different genetic types of Zn-Pb deposits contain different cadmium contents and cadmium isotopic compositions, suggesting that cadmium isotope has a great potential to be a powerful proxy to trace the behavior of the ore-forming fluids in Zn-Pb deposits. In this proposal, Chuan-Dian-Qian epithermal mineralization area has been chosen as a research subject and the Cd isotopes are considered as the main geochemical tracer, coupling with some traditional study methods of ore-deposit. We will focus on the characters of cadmium isotopic compositions and cadmium isotopic fractionation mechanisms, and trace the source of the ore-forming materials in typical Zn-Pb deposits. Basing on the motioned above work, we try to construct the cadmium isotope tracer model in the Chuan-Dian-Qian epithermal mineralization area.

“非传统稳定同位素”是同位素地球化学研究的热点之一，镉因其特殊的地球化学性质及同位素分馏机制等而引起了广泛的重视。已有的研究表明，Cd同位素在示踪海洋营养元素循环和重金属污染源等方面显示出较好的示踪效果和巨大的应用潜力。然而，作为地球上镉重要储库之一的铅锌矿床，其镉同位素研究还处于起步阶段。已有的初步研究显示，不同成因的铅锌矿床，具有不同的Cd含量及其同位素组成，暗示Cd同位素具有示踪铅锌矿床中成矿物质来源的巨大潜力。本项申请拟以川滇黔低温成矿域为研究平台，以镉同位素为主要研究手段，并结合传统的矿床学研究方法，揭示Cd同位素在铅锌矿床中的分馏特点和分馏机制，探讨该区分散元素的富集机制和成矿物质来源，并建立Cd同位素的地球化学示踪模型。

川滇黔地区是我国重要的铅锌资源区，也是我国重要的分散元素富集区。本项目以Cd同位素为主要研究手段，对川滇黔地区典型的铅锌矿床中同位素进行系统研究，发现矿物沉淀是控制镉同位素分馏的主要机制。同时，镉同位素在闪锌矿中呈现规律性的变化，即底部镉同位素值较低，随着高度增加，镉同位素值增加。因此，镉同位素可能用作铅锌矿床的深部找矿等应用。另外，通过对不同时代地层和矿床的研究，根据质量守恒定律，我们评估了川滇黔地区不同矿床中镉的来源，其具有一定的差异，其中富乐矿床中镉绝大部分来自于地层，少量来自峨眉山玄武岩；而会泽铅锌矿床可能约一半的镉来自于峨眉山玄武岩，一半来自于地层。通过对不同硫化物的氧化产物的镉同位素测定，发现风化过程能造成0.33‰的镉同位素分馏，因此，在利用镉同位素示踪土壤中的污染源时，需注意风化过程对土壤中同位素的分馏。同时，本项研究为了解全球的镉同位素分布特征提供了重要参考。

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