In-situ U–Pb geochronology of Ti-bearing andradite as a practical tool for linking skarn alteration and Pb–Zn mineralization: A case study of the Mengya’a deposit, tibet

Abstract

Ti-bearing andradite (Ti-Grt) occurs in a variety of geological environments, such as Si-unsaturated igneous rocks, high-temperature metamorphic rocks, and contact-metasomatized skarns. This type of garnet remains stable throughout a wide range of pressure-temperature conditions. Abundant skarn Pb–Zn deposits, where the ore-related pluton is not exposed, have been discovered along the eastern Nyainqentanglha metallogenic belt, western China. However, the mineralization age of these skarn Pb–Zn deposits remains controversial. Therefore, in this study, petrography, geochemistry, and in-situ U–Pb geochronology of the Ti-Grt are investigated together with the Re–Os geochronology of the molybdenite in the large Mengya’a Pb–Zn deposit. The potential of Ti-Grt dating is explored and the age relationships between the skarn alteration and the Pb–Zn mineralization are constrained. The results show Ti-Grt samples from the Mengya’a deposit are brown-black in color with TiO2-rich (mean of 3.47 wt%) features. Laser ablation inductively coupled plasma mass spectrometry dating showed that these Ti-Grts have a U–Pb age range of 54.9–53.8 Ma, which is consistent with the ages of two adjacent molybdenite Re–Os models (53.36–52.86 Ma). This indicates that the skarn alteration is temporally associated with a Pb–Zn mineralization event during the Indian-Asian main-collisional orogeny. The positive linear Ti vs. U correlation of the Ti-Grts and other compiled data indicate that Ti-bearing garnets may have high contents of U. The incorporation of U and Ti in garnet is largely controlled by the crystal chemistry of garnet and the components of hydrothermal fluids in a nearly closed system. Thus, this study confirms the reliability and accuracy of in-situ U–Pb dating of Ti-Grt and emphasizes that Ti-bearing garnets can become an invaluable geochronological tool for magmatic, metamorphic, and hydrothermal mineral assemblages.