萤石Sm-Nd同位素体系对晴隆锑矿床成矿时代和物源的制约

Samarium-Neodymium isotope system of fluorites from the Qinglong antimony deposit, Guizhou Province: Constraints on the mineralizing age and ore-forming materials' sources.

本文首次对黔西南晴隆锑矿床的萤石进行Sm-Nd同位素研究,来探讨该矿床的形成时间和成矿物源。研究表明,该矿主成矿期的萤石构成两组等时线,其对应的等时线年龄分别为148±8 Ma和142±16 Ma,显示该矿床的成矿作用发生在晚侏罗世。本次测定的成矿年龄数据远小于峨眉山玄武岩的成岩年龄,暗示该矿床与该区二叠纪的火山作用没有直接的成因联系。计算表明,在晴隆锑矿床成矿时(142 Ma),两组萤石的ε_(Nd)(t)值分别为-5.72～-5.81和-3.81～-3.88,远小于峨眉山玄武岩的相应值(0.40～3.27);两组萤石初始Nd同位素组成的差异,暗示其Nd的来源存在不均一性,这很可能与萤石沉淀环境围岩的局部差异有关。在ε_(Sr)(t)-ε_(Nd)(t)图解中,萤石明显分布在与赋矿围岩不同的区域,暗示该矿的成矿物质主要是来自外部(可能是下伏老地层或基底),而不是赋矿的峨眉山玄武岩和茅口组灰岩,因此,该矿并非是前人认定的“原地改造成矿”。

The Qinglong antimony deposit located at Southwestern Guizhou is an important Sb-producer in South China. Despite the previous detailed investigations, the metallogenic epoch and ore-forming materials' sources of the Qinglong deposit is still poorly constrained, which seriously hampered the further understanding on its ore genesis. The objectives of this contribution are to investigate the Sm-Nd isotope systematics of fluorites coevally with stibnite from this deposit, attempt to provide some constraints on its mineralizing age and ore-forming materials' sources, and lay some foundations for the further understandings of ore deposit origin. The Sm-Nd isotope system of fluorites coevally with stibnite from the Qinglong antimony deposit is discussed. Different-color fluorites form two isochrones, corresponding to the isochron ages of 148±8 Ma and 142±16 Ma, respectively; i. e. , Sb-F mineralization at Qinglong deposit took place during late Jurassic Period rather than Permian Period as expected. The measured dating data are coincident with the known field geological evidences, and rule out the temporally genetic connection between the Qinglong antimony deposit and the Emeishan basalt. The initial εNd values of different fluorites at the mineralizing time (142Ma) fall in the ranges of -5.72^-5.81 and -3.81 ~ -3.88, respectively. The difference in the initial 143Nd/144Nd ratios of different fluorites indicates source heterogeneity, probably as a result of Nd supply from variable local sources of wallrocks. The minor contributions of the local wallrock probably contributed significantly to the hydrothermal fluid Nd budget, and resulted in the dramatic change of Nd isotopic composition in the hydrothermal fluid, and finally affected the Nd isotopic composition of the fluorites deposited from this hydrothermal system. The ore-forming fluid was rich in radiogenic 87 Sr and depleted in radiogenic 143 Nd relative to the Emeishan basalt, which rules out the possibility that the Emeishan basalt supplied most of Sr and Nd for the ore-forming hydrothermal solution. In the εsr(t)-εNd(t) diagram, all fluorite samples fall in the field away from the host Emeishan basalt and Permian Maokou limestone, indicating that the ore-forming materials were mostly originated from the exterior, probably from the underlying older strata or from basement, rather than the host wallrocks ( Emeishan basalt and Permian Maokou limestone). The Qinglong antimony deposit did not result from the in-situ remobilization as previously expected.