In-situ U Pb geochronology of vesuvianite in skarn deposits

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 degrees C), and consequently may be suitable for dating for skarn deposits. Vesuvianite collected from nine Chinese skarn deposits, including the Jinchuantang Sn-Bi, Shizhuyuan W-Sn-Mo-Bi polymetallic, Jiepai Cu-W, 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 U-Pb 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 U-Pb concordia age (20, 153.9 +/- 0.2 Ma, MSWD = 1.6, n = 11), demonstrating that it can be taken as a potential LA-ICP-MS U-Pb reference material, which can be freely distributed to other laboratories. Using the JCT vesuvianite as a U-Pb reference material, the Kaerqueka, Saishitang, Jiepai, Dashunlong, Xianghualing, Furong, and Shizhuyuan vesuvianite yielded well-defined LA-ICP-MS U-Pb ages (239.0-152.1 Ma, 20), consistent with previously published age constraints of mineralization and cogenetic igneous rocks. Vesuvianite from the Huangshaping W-Sn-Mo-Cu polymetallic deposit, yielded U-Pb ages of 157.3 +/- 1.4 Ma (20, MSWD = 1.8) and 161.0 +/- 1.3 Ma (20, 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.