Tourmaline and quartz in the igneous and metamorphic rocks of the Tashisayi granitic batholith, Altyn Tagh, northwestern China: Geochemical variability constraints on metallogenesis

Abstract This study examines five rock units related to the Tashisayi chrysoberyl-bearing granitic batholith, Altyn Tagh, NW China: biotite granite, two-mica granite, muscovite granite, granitic pegmatite, and the surrounding biotite plagiogneiss. Three types of tourmalines and quartz are described, occur in the biotite plagiogneiss (Tur-1, Qtz-1), granitic rocks (two-mica granite [Tur-2a, Qtz-2a] and muscovite granite [Tur-2b, Qtz-2b]), and granitic pegmatite (Tur-3 and Qtz-3; tourmaline and quartz coexisting with chrysoberyl). Zircon U Pb dating of the muscovite granite and granitic pegmatite yielded ages of 490–486 Ma for the Tashisayi batholith. The major-(by EPMA) and trace-element contents (by LA-ICP-MS) of the tourmaline and trace-elements (by LA-ICP-MS) and O-isotope (SIMS) of quartz indicates that: 1) The tourmalines are alkali schorl (Fe-rich tourmaline) and foitite; 2) tourmalines and quartz from the pegmatite have similar elemental concentrations and substitution mechanisms as tourmalines and quartz in granitic units of the Tashisayi batholith, but different from those of tourmalines and quartz in the biotite plagiogneiss, especially for Li, Be; 3) LA-ICP-MS analyses show that Qtz-1 in the biotite plagiogneiss has lower Ge/Ti values, lower Li, Be, Rb concentrations than Qtz-2a, Qtz-2b, and Qtz-3. Moreover, Qtz-2a, Qtz-2b, and Qtz-3 have a good positive correlation, consistent with whole-rock compositions, indicating crystal fractionation process in the formation of pegmatite, the chrysoberyl-bearing pegmatite is derived from a more evolved fluid than granitic rocks, thus, Li, Be are enriched by magmatism; 4) The O-isotopic compositions of Qtz-1 (+0.03 to +1.90‰) are consistent with a metamorphic water source, whereas the quartz in the two-mica granite (Qtz-2a; +8.85 to +9.58‰), in the muscovite granite (Qtz-2b; +9.27 to +9.93‰), and in the granitic pegmatite (Qtz-3; +14.8 to +17.8‰) are derived from the magmatic water or fluids from granite.