Robust dating of Pb–Zn skarn systems by LA–ICP–MS garnet U–Pb geochronology

Abstract

Lawu and Yaguila are two Pb–Zn skarn deposits in the eastern Nyainqêntanglha metallogenic belt, central Lhasa subterrane, Tibet. The genesis of Pb–Zn deposits in this belt and the crustal processes leading to their formation remain ill-constrained, mostly because of the lack of precise mineralization ages. We integrate textural information, geochemistry, and in-situ garnet U–Pb geochronology to constrain the timing and genesis of Pb–Zn mineralization. Garnets from the two deposits display oscillatory compositional zoning (And14Gr86 to And100) and contain variable U contents (0.07–5.3 ppm). Aluminum-rich garnet displays a chondrite-normalized rare earth element (REE) fractionation pattern in which LREEs are depleted relative to flattish HREE segments. In contrast, Fe-rich garnet shows LREE-enriched, relatively HREE-poor patterns with positive Eu-anomalies. Uranium concentration is correlated with total REE and Fe components in garnets, implying that U incorporation is largely controlled by coupled substitution mechanisms. The same garnets also contain measurable contents of other metals (up to hundreds of ppm), such as Sn, W, and In. The distribution and fractionation of major and trace elements in zoned garnets record periodic fluid pluses with different compositions during hydraulic fracturing. The new garnet U–Pb data show that the Yaguila deposit formed between 68.5 ± 3.4 and 65.0 ± 4.7 Ma, and the Lawu deposit formed at 54.6 ± 2.9 Ma. Within the geochronological framework of igneous rocks and Pb–Zn mineralization in the Nyainqêntanglha metallogenic belt, the new garnet ages suggest a magmatic-hydrothermal origin for related mineralization, with the causative magma likely derived from partial melting of ancient continental material due to rollback and subsequent breakoff of the Neo-Tethys oceanic slab during India–Asia continental collision. This study highlights the opportunities offered by garnet U–Pb dating for elucidating the formation age and ore genesis of base metal skarn systems.