In-situ U[sbnd]Pb geochronology of vesuvianite in skarn deposits

Abstract

The timing of hydrothermal activity is a key factor in any predictive genetic model of skarn mineralization, but direct dating is often hindered by the absence of suitable minerals to date. Vesuvianite, a common mineral in skarn deposits, can incorporate U and has a high Pb closure temperature (>650 °C), and consequently may be suitable for dating for skarn deposits. Vesuvianite collected from nine Chinese skarn deposits, including the Jinchuantang SnBi, Shizhuyuan W–Sn–Mo–Bi polymetallic, Jiepai CuW, Dashunlong Sn, Xianghualing Sn, Furong Sn, and Huangshaping W–Sn–Mo–Cu polymetallic deposits in Nanling Range (Southern China), and the Kaerqueka and Saishitang Cu deposits in East Kunlun Orogen (Northwestern China), have high U concentrations (median 66.8 ppm for 460 LA-ICP-MS analysis spots). The flat and stable time-resolved signal spectra for U during LA-ICP-MS analysis, combined with the absence of nanoscale mineral inclusions, porosities, or lattice defects in the JCT vesuvianite, with high and consistent U concentrations, revealed by the HAADF-STEM images, and the marked positive relationships between U and REE collectively suggest that the U incorporation into vesuvianite was primarily controlled by replacement of Ca2+ in eight-fold coordination, confirming that vesuvianite may be a suitable UPb geochronometer. The JCT vesuvianite is characterized by high and consistent U concentrations (260–526 ppm, median 307 ppm) and homogeneous internal texture and elemental distribution, and yields an ID-TIMS UPb concordia age (2σ, 153.9 ± 0.2 Ma, MSWD = 1.6, n = 11), demonstrating that it can be taken as a potential LA-ICP-MS UPb reference material, which can be freely distributed to other laboratories. Using the JCT vesuvianite as a UPb reference material, the Kaerqueka, Saishitang, Jiepai, Dashunlong, Xianghualing, Furong, and Shizhuyuan vesuvianite yielded well-defined LA-ICP-MS UPb ages (239.0–152.1 Ma, 2σ), consistent with previously published age constraints of mineralization and cogenetic igneous rocks. Vesuvianite from the Huangshaping W–Sn–Mo–Cu polymetallic deposit, yielded UPb ages of 157.3 ± 1.4 Ma (2σ, MSWD = 1.8) and 161.0 ± 1.3 Ma (2σ, MSWD = 1.9), providing reliable constraints on the overprinting sequence of the skarn W–Sn–Mo and Cu mineralization events at Huangshaping, which was not previously detectable using other mineralization chronometers. Combined, these ages suggest that vesuvianite is a robust and reliable geochronometer, and offers a new tool for dating skarn deposits.