Initiation of Zn-Pb mineralization in the Pingbao Pb-Zn skarn district, South China: Constraints from U-Pb dating of grossular-rich garnet

In this study, we discuss the application of U-Pb dating on U-rich, common-Pb bearing grossular-rich garnet and the issues of common Pb correction. Grossular-rich garnet samples were collected from the Baoshan and Huangshaping areas of the Pingbao skarn district (Hunan, South China). The Baoshan garnet comprises Gr64.3 And34.0 to And99.0 Gr0.2 (Gr = Grossular ; And = Andradite), with minor spessartine (0.43–1.45%), almandine (0–0.61%) and pyrope (0.01–0.91%). The Huangshaping 304 garnet comprises Gr 41.7 And56.0 to Gr70.2 And25.9 with minor spessartine (1.42–2.10%), whereas the Huangshaping 301 garnet comprises Gr62.0 And26.6 Spe11.2 to Gr74.5 And20. 9 Pyr2.5 Spe2.1 (Spe = Spessartine, Pyr = Pyrope). The Baoshan garnet grains are subhedral to euhedral with well-developed core-rim textures and contain approximately 1–10 ppm U. The Huangshaping 304 garnet is also euhedral with grains size ranging from several to tens of millimeters. The core-rim textured garnet grains are compacted together with interstitial quartz and contain about 5–10 ppm U. The Huangshaping 301 garnet is anhedral to tabular with interstitial quartz and magnetite, and contains up to 579 ppm U. In-situ U-Pb dating of the Baoshan, Huangshaping 304 and 301 garnet samples yielded consistent anchored y-intercept ages at 162.6 ± 2.9 Ma (n = 60; MSWD = 1.5); 160.2 ± 1.5 Ma (n = 59; MSWD = 1.9) and 160.9 ± 0.3 Ma (n = 60; MSWD = 4.1), respectively. These ages are consistent with the new and published zircon and titanite U-Pb ages (162–160 Ma), indicating that the garnet U-Pb ages are reliable and the technique is robust. We suggest that the non-U-bearing mineral correction has involved in the third-party common Pb ratio, which is believed to be a better correction for the system common Pb.