Tourmaline from Quartz Veins Associated with Rare Metal Granites of Severny Pluton, Chukotka, Northeastern Russia

This paper is focused on tourmaline from quartz veins associated with rare-metal granites of the Severny (Northern) Pluton, Chukotka, within which the Erguveem, Kekurnoe, Stremitelnoe, and Glubokoe tin deposits have been discovered, with the giant Pyrkakai tin deposit located close to the south border of the pluton. Tourmalines from four host types have been described: replacement lenses (RL) in pegmatoid pockets and three types of quartz veins cutting through granite rocks of the pluton. Tourmalines have been studied with electron microprobe, laser ablation inductively coupled plasma mass-spectrometry (LA-ICP-MS), Mössbauer spectroscopy, and Raman spectroscopy. The tourmalines are classified as fluor-dravite, dravite, oxy-dravite, fluor-schorl, schorl, oxy-schorl, foitite, oxy-foitite, magnesio-foitite, and ferro-bosiite. Ferro-bosiite from the type III veins is Sn-bearing. Leading substitutions in the tourmalines are Al + WO2– ↔ Fe2+ + WOH–(F–), X-site vacancy + Al ↔ Na + Fe2+, and Fe3+ ↔ Al. An increase in the Fe3+/Fetot value from 10% in the RL tourmalines to 14% in the type III vein tourmalines along with abundant ferro-bosiite in the latter, indicate an increase in oxidation potential. This increased oxidation potential possibly contributed to a cassiterite deposition. Raman data are consistent with composition of the tourmalines.