OCCURRENCES OF FOITITE AND ROSSMANITE FROM THE KOKTOKAY NO. 3 GRANITIC PEGMATITE DYKE, ALTAI, NORTHWESTERN CHINA: A RECORD OF HYDROTHERMAL FLUIDS

摘要

Two species of X-site-vacant tourmalines, foitite and rossmanite, occur in the Koktokay No. 3 granitic pegmatite dyke, Altai, northwestern China, a spodumene-subtype pegmatite. Foitite–schorl crystals develop as fillings in the interstices among Fe-rich dravite crystals in the endocontact zone between the pegmatite dyke and the metagabbro country-rock. The evolution of the tourmaline crystals in the endocontact zone occurred in two stages. The first stage is typified by the formation of Fe-rich dravite with variable compositions described by substitutions XNaY+ZR(XY+ZAl)_–1, Y+ZROH(Y+ZAlO)_–1, and XCaY+ZR₂(Y+ZAl₂)_–1(where Y+ZR = Fe_tot + Mg + Mn + Zn). The second-stage foitite–schorl has compositional variations expressed by Fe(Mg)_–1, XNaY+ZR(XY+ZAl)_–1, Y+ZROH(Y+ZAlO)_–1, and less XCaY+ZR₂(Al₂)_–1. Dravite may develop as a result of pegmatite-derived fluids reacting with fluids from the country rock, whereas the formation of foitite–schorl is mainly attributed to pegmatite-derived fluids, the occurrence of foitite further reflecting the low Ca and Na concentrations in the fluids. In the "cleavelandite"–spodumene zone, rossmanite occurs as veinlets within the main mass of elbaite crystals and exhibits a preferred orientation, and its compositional variation also can be described in two stages. In the first stage, the compositional variations of elbaite can be described by the substitution XYAl[XNaY(Fe,Mn)]_–1, whereas in the second stage, the minor compositional variation of rossmanite can be expressed by the substitution YAlO₂[YLi(OH)₂]_–1. The high X-site vacancy in elbaite may be caused by the buffering effect of a Na-rich phase, whereas the formation of rossmanite is most likely controlled by the local lack of Na and Ca in the late hydrothermal fluids.