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Tindle, A. G. and Webb, P. C.
(1989).
DOI: https://doi.org/10.1016/0016-7037(89)90313-X
Abstract
Electron microprobe data are presented from zoned niobian wolframite crystals in a quartz-wolframite-(cassiterite) vein and wall-rock from a zinnwaldite-bearing granite pluton that forms part of the late-Caledonian Glen Gairn granitic complex in the Eastern Highlands of Scotland. Compositions range from Nb- and Ta-rich wolframite, (Fe,Mn)(W,Nb,Ta)O 4 , forming strongly zoned, difficult-to-polish regions of crystals, to almost pure hübnerite MnWO 4 , forming relatively unzoned, easily polished regions. Nb and Ta substitution is very restricted in Mn-rich (>80 mol% hübnerite) wolframites but increases regularly in more Fe-rich compositions through a coupled substitution mechanism between Mn 2+ W 6+ and Fe 3+ (Nb,Ta) 5+ . Most of the wolframite crystals contain two forms of zonation: a physical zonation, observed optically and by electron imaging, and a compositional zonation revealed by X-ray traverses. Compositional zoning is progressive, involving gradual changes in mineral chemistry from core to rim, on which is superimposed a form of oscillatory zoning involving repeated resetting of compositions. Physical zones are layers of uniform width separated by distinct boundaries; they are arranged parallel to crystal faces and reflect intermittent crystal growth. These zones vary in width from less than 5 m in the niobium-rich wolframite, to over 200 m in regions of niobium-poor hübneritic wolframite. Hydrothermal alteration, particularly of the intensely zoned niobium-rich regions, has resulted in assemblages of altered Nb- and Ta-rich wolframite (tungstite?), secondary hübneritic wolframite and scheelite. Nb-rich inclusions 5-10 m across are found sporadically in the larger wolframite crystals. Progressive variation in wolframite chemistry is readily explained in terms of gradually changing physiochemical conditions, such as pH, f O 2 , temperature, pressure and fluid composition, during crystallization. The oscillatory zonation reflects cyclicity of those conditions. Plausible models which could explain such effects include: 1. (a) intermittent mixing of chemically distinct magmatic and meteoric fluids 2. (b) repeated hydraulic fracturing and injection of fluids leading to fluctuating pressure and temperature of the hydrothermal fluid. Volatile species (F, Cl, CO 2 ) may have been effective as complexing agents thus affecting the supply of Nb from the fluid. Deposition of Nb- and Ta-rich compositions were probably favoured by high f O 2 , low pH conditions.
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