Niobian wolframite from Glen Gairn in the Eastern Highlands of Scotland: A microprobe investigation

Journal Article · Tue Aug 01 00:00:00 EDT 1989 · Geochimica et Cosmochimica Acta; (USA)

DOI:https://doi.org/10.1016/0016-7037(89)90313-X· OSTI ID:7170881

Tindle, A G; Webb, P C ^[1]

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1. The Open Univ., Buckinghamshire (Scotland)

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. 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. Progressive variation in wolframite chemistry is readily explained in terms of gradually changing physiochemical conditions, such as pH, fO{sub 2}, temperature, pressure and fluid composition, during crystallization. The oscillatory zonation reflects cyclicity of those conditions. Plausible models which could explain such effects include: (a) intermittent mixing of chemically distinct magmatic and meteoric fluids, and (b) repeated hydraulic fracturing and injection of fluids leading to fluctuating pressure and temperature of the hydrothermal fluid. Volatile species (F, Cl, CO{sub 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 favored by high fO{sub 2}, low pH conditions.

• OSTI ID: 7170881
• Journal Information: Geochimica et Cosmochimica Acta; (USA), Vol. 53:8; ISSN 0016-7037
• Country of Publication: United States
• Language: English

Related Subjects

15 GEOTHERMAL ENERGY
58 GEOSCIENCES
WOLFRAMITE
CHEMICAL COMPOSITION
MICROANALYSIS
MINERALIZATION
MINERALOGY
CRYSTAL STRUCTURE
CRYSTALLIZATION
GEOCHEMISTRY
GEOTHERMAL FLUIDS
GRANITES
INCLUSIONS
ORIGIN
ROCK-FLUID INTERACTIONS
UNITED KINGDOM
ZONING
CHALCOGENIDES
CHEMISTRY
EUROPE
FLUIDS
IGNEOUS ROCKS
IRON COMPOUNDS
IRON OXIDES
MINERALS
OXIDE MINERALS
OXIDES
OXYGEN COMPOUNDS
PHASE TRANSFORMATIONS
PLUTONIC ROCKS
REFRACTORY METAL COMPOUNDS
ROCKS
TRANSITION ELEMENT COMPOUNDS
TUNGSTEN COMPOUNDS
TUNGSTEN OXIDES
WESTERN EUROPE
152002* - Geothermal Data & Theory- Properties of Minerals & Rocks
580000 - Geosciences