In situ U-Pb isotopic dating of columbite-tantalite by LA-ICP-MS

Columbite-tantalite (Coltan) is the most important niobium (Nb)- and tantalum (Ta)-bearing economic mineral, commonly occurring in rare metal granite and pegmatite, alkaline granite, syenite and carbonatite. Its high U but low common Pb contents make it an ideal mineral for U-Pb isotopic dating of Nb-Ta mineralization. In order to establish a feasible coltan dating method by in situ laser-ablation (LA) ICP-MS, we determined the U-Pb ages of five coltan samples from different pegmatites and rare-metal granites in China. In order to evaluate the potential matrix effect between different minerals, a 91500 zircon was used as external standard during analyses. The results show that, compared to the recommended ages, approximately 7-15% younger ages were yielded for the analyzed coltan samples in both single spot and line raster scan analytical methods, indicating a significant matrix effect between coltan and zircon. However, by using a coltan standard from Namibia (Coltan139), the coltan sample from Dahe pegmatite (SNNT) has a weighted mean ²⁰⁶Pb/²³⁸U age of 363±4Ma (2σ, n=25) and 357±5Ma (2σ, n=20) in single spot and line raster scan analytical methods, respectively; the coltan samples from Altai No.3 pegmatite (713-79), Yichun topaz-lepidolite granite (Yi-1) and Huangshan albite granite (LS-15) have weighted mean ²⁰⁶Pb/²³⁸U ages of 218±2Ma (2σ, n=20), 160±1Ma (2σ, n=20) and 130±1Ma (2σ, n=20), respectively, in single spot mode. These ages agree well with the previously published data, and hence support the reliability of our analytical method. Although the analyzed coltan minerals show a large variation of chemical compositions, no significant matrix effect was observed, which suggests that a coltan material should be used as an external standard for U-Pb dating of coltan by LA-ICP-MS. Using the established analytical protocol, we date the Nanping pegmatite (NP155), a main Nb-Ta deposit in China without known age, and obtain a weighted mean ²⁰⁶Pb/²³⁸U age of 391±4Ma (2σ, n=20), which is considered as the best estimation of Nb-Ta mineralization time in the area.