闪锌矿Re-Os同位素体系封闭性研究及其对铅锌矿床定年的指示

Sphalerite Re-Os isotope system is a direct and efficient method to constrain the mineralization age and trace the metal source. But this isotope system is very unstable for the different types of deposits or different ore stages in the same deposit. The reason for this phenomenon remains unknown. Preliminary studies have indicated that the closure of sphalerite Re-Os isotope system may be possibly related to its ore stage, color, texture, formation temperature and chemical composition. Whether this relation is similar in different types of deposits is unknown. In this study, three types of deposits with various ore-forming temperatures, including magmatic-hydrothermal, volcanic-hosted massive sulfide and Mississippi Valley type Pb-Zn deposits, are selected. The study on mineralogical and geochemical characteristics of sphalerite will be carried out. The link between the ore stage, color, texture, ore-forming temperature, chemical composition and the Re-Os isotope composition will be constrained. Combined with the study of closure temperature, these mineralogical and geochemical characteristics will be used to discuss the factors controlling the closure of the Re-Os isotope system. The specific mineralogical and geochemical indicators will be proposed, which will provide criteria for the selection of sphalerite suitable for Re-Os dating.

闪锌矿Re-Os同位素体系是限定铅锌矿床成矿时代和示踪成矿物质来源直接有效的方法。但是该同位素体系在不同类型铅锌矿床或者同一矿床不同成矿阶段表现出不同程度的开放行为，具体的原因并不清楚。初步研究表明，闪锌矿Re-Os同位素体系封闭性或组成可能与闪锌矿的成矿阶段、颜色、结构、形成温度和化学成分有关。这种规律性关系在不同类型矿床中是否普遍存在，需要大量数据验证。本申请拟选择不同成矿温度的三种类型铅锌矿床（岩浆热液型、火山成因块状硫化物型和密西西比河谷型）为研究对象，系统地开展闪锌矿的矿物学和地球化学组成研究，建立闪锌矿的成矿阶段、颜色、结构、成矿温度、化学成分与Re-Os同位素组成的关系，结合闪锌矿的封闭温度研究，确定Re-Os同位素体系封闭性的制约因素，并提出具体的矿物学和地球化学指标，为适合于Re-Os同位素定年的闪锌矿的选择提供参考。

铅锌矿床的精确定年一直是矿床学研究的一大难题，由于缺少精确的成矿年龄，很难将成矿事件与地质事件联系起来，严重制约了这类矿床的成因研究及成矿模式的建立。硫化物如黄铁矿和闪锌矿曾用于Re-Os同位素定年，但并未得到很好的等时线年龄，多数低温铅锌矿床硫化物Re-Os同位素体系表现出开放行为。为了确定影响Re-Os同位素体系封闭的原因，本项目选择了云南会泽、富乐和老厂三个铅锌矿床进行了系统的Re-Os同位素研究，结合矿相学、矿物原位微量元素组成、硫同位素组成以及流体包裹体研究等，确定了控制低温铅锌矿床中硫化物Re-Os同位素体系封闭性的几个主要因素。研究表明，矿物形成后的流体改造，尤其是氧化性流体与矿物的相互作用，将导致矿物发生溶解再沉淀，在这个过程中Re会发生活化和重新分配，导致了Re-Os同位素体系的重置。除了后期流体改造，低温铅锌矿床普遍存在的混合流体来源也是造成Re-Os同位素体系开放的另一个原因。因此，为了获得可靠的硫化物Re-Os同位素年龄，必须进行详细的矿相学研究、划分清楚硫化物的期次，并保证无后期的流体改造。该项目取得的成果将对低温铅锌矿床硫化物Re-Os同位素定年方法的使用提供重要依据。