Age and evolution of the Nanyangshan rare-metal mineralized pegmatite revealed by cassiterite U-Pb geochronology and tourmaline chemistry

Abstract

Determing the age and precise bulk composition of pegmatites is challenging due to 1) the metamictization of zircon grains and 2) their coarse grain size and heterogeneity. Cassiterite and tourmaline are common in pegmatites and in the case of the former can be used to date pegmatites and while the latter can be used to monitor petrogenesis. This study presents in situ cassiterite U-Pb dating and tourmaline geochemical and B isotopic data from the Nanyangshan Li-Be-Nb-Ta-Rb-Cs-Sn mineralized pegmatite in the North Qinling terrane of China to investigate their age and petrogenesis. The Nanyangshan pegmatite host abundant spodumene, lepidotite, beryl, cassiterite, columbite and tourmaline and can be roughly divided into black tourmaline (Tur B)-cassiterite and pink tourmaline (Tur P)-spodumene bearing phases. Most of the cassiterite are homogenous in appearance although some (<10 %) have core-rim structures. The cassiterite crystals mainly yielded U-Pb ages of ∼ 410 Ma, although the the rims yielded ages of ∼ 370 Ma. Both Tur B and Tur P are rich in Li and are elbaites, with Tur B closer to shorl and Tur P to elbaite. Clear correlations exist between some trace elements (Li and Sn) with major elements (Na/(Na + X□) in the tourmalines, indicating the potential crystal chemical effects on their incorporation. The boron isotopes of the tourmalines vary from −19.36 ‰ to −15.91‰ and decrease from Tur B to Tur P. This implies the boron in the Nanyangshan pegmatite was mainly derived from partial melting of metasedimentary souce rocks with the variation likely caused by fractionation. Tur P is closely associated with spodumene and has higher concentratios of Li, Be, Nb, Ta whereas the Tur B is closely associated with cassiterite and has higher concentrations of Sn, suggesting that tourmaline chemistry could be a reliable geological record of the environment and useful exploration tool.