Geochemical evolution of tourmaline in the Darasun gold district, Transbaikal region, Russia: evidence from chemical and boron isotopic compositions

The Darasun gold district, Transbaikal region, eastern Russia comprises three deposits: Teremkyn, Talatui, and Darasun, where gold-bearing quartz veins are hosted in metagabbro and granitoids. Tourmaline is a common gangue mineral in these deposits and a useful indicator of fluid source. The tourmaline compositions are oxy-dravite–povondraite, dravite, and schorl. We report here in situ B-isotope analyses by secondary ion mass spectrometry (SIMS) on tourmaline from veins in metagabbro and K-rich granodiorite, as well as from a breccia pipe at the margin of granodiorite porphyry. The B-isotope composition of tourmalines from the Darasun gold district as a whole covers a very wide range from −15.7 to +11.2 ‰, with distinctive differences among the three deposits. The δ11B values in the Teremkyn tourmalines are the most diverse, from −15.7 to +2.5 ‰. Tourmaline core compositions yield an inferred δ11B value of the initial fluid of ca. −12 ‰, suggesting granitic rocks as the B source, whereas the heavier rims and late-stage grains reflect Rayleigh fractionation. The δ11B values of tourmaline from Talatuiare −5.2 to −0.9 ‰. Taking into account fluid inclusion temperatures from vein quartz (ca. 400 °C), the inferred δ11B value of fluid is heavy (−2.5 to +2.2 ‰) suggesting a B source from the host metagabbro. At the Darasun deposit, tourmaline from the breccia pipe is isotopically uniform (δ11B −6 to −5 ‰) and suggested to have precipitated from a 10B-depleted, residual fluid derived from granitic rocks. The Darasun vein-hosted tourmalines I and II (δ11B from −4.4 to +1.5 ‰) may have crystallized from strongly fractionated residual granitic fluid although mixing with heavy boron from the metagabbro rocks probably occurred as well; the boron isotopic composition of tourmaline III (−11.2 ‰) is attributed to a less-fractionated fluid, possibly a recharge from the same source.