U–Pb isotope and trace element analysis of columbite-(Mn) and zircon by laser ablation ICP–MS: Implications for geochronology of pegmatite and associated ore deposits

Abstract

U–Pb isotopes and trace elements of columbite-(Mn) and zircon from an early Cretaceous pegmatite dike in the Xiaoqinling district, North China Craton, were analyzed using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to better understand the suitability of these minerals for U–Pb geochronology when they contain unusually high U and Th. Columbite-(Mn) grains have high W, Ti, U, Th, and REE contents and yield concordant U–Pb age of 143±1Ma (2σ, n=10) that is interpreted as the emplacement age of the pegmatite dike. In contrast, zircon grains from the same dike show three distinct U–Pb age populations. Nine out of seventeen zircon grains analyzed have textural features typical of magmatic zircons and yield a weighted mean 206Pb/238U age of 143±1Ma (2σ, n=9), identical to that of columbite-(Mn) and thus constrain the timing of pegmatitic magmatism. The second population of zircon is characterized by corroded and zoned textures with the geochemical affinities of magmatic zircon. These zircon grains have a weighted mean 207Pb/206Pb age of 1879±19Ma (2σ, n=5) and are considered to be inherited components derived from Paleoproterozoic basement rocks that are widely distributed in the Xiaoqinling district. A third zircon population is characterized by high porosity and abundant Th–U-rich mineral inclusions (e.g. thorite, uranium oxides), and yield a younger U–Pb age of 127±3Ma (2σ, n=3). These younger zircon grains have elevated Hf, Ca, P, Nb, Ta, and Ti contents and much higher Th/U, LREE/MREE, and LREE/HREE ratios compared to the 143Ma zircon. The textural and geochemical data for the 127Ma zircon grains indicate that they were formed by the hydrothermal alteration of precursor zircon crystallized from the pegmatitic magmas, presumably associated with pervasive hydrothermal flow that led to the formation of numerous early Cretaceous gold deposits in the Xiaoqinling district. Results from this study demonstrate that columbite-(Mn) from the pegmatite dike in the Xiaoqinling district has been resistant to post-magmatic hydrothermal alteration that, however, have disturbed the U–Pb isotopes in some zircon grains from the same dike. Consequently, we propose that columbite-(Mn) is an ideal target for U–Pb dating of pegmatite and associated ore deposits.