Chemical variation and significance of tourmaline from Southwest England

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doi: 10.2113/gsecongeo.90.3.495
Authors:London, David; Manning, David A. C.
Author Affiliations:Primary:
University of Oklahoma, School of Geology and Geophysics, Norman, OK, United States
Other:
University of Manchester, United Kingdom
Volume Title:special issue devoted to the geology of rare metal deposits
Volume Authors:Pollard, Peter J., editor
Source:A special issue devoted to the geology of rare metal deposits, edited by Peter J. Pollard. Economic Geology and the Bulletin of the Society of Economic Geologists, 90(3), p.495-519. Publisher: Economic Geology Publishing Company, Lancaster, PA, United States. ISSN: 0361-0128
Publication Date:1995
Note:In English. 71 refs.; illus., incl. 4 tables, geol. sketch maps
Summary:Tourmaline is a common and locally abundant mineral in all products of the granite magmatism and associated hydrothermal activity in southwest England, particularly in the county of Cornwall. Tourmaline of magmatic origin is homogeneous and is marked by high Fe/Mg, high F, and high Al in the place of divalent cations (R2 site). The substitution of Al for divalent cations such as Mg and Fe2+ is charge-compensated by a deficit of alkalies and protons in other structural sites. Tourmaline compositions from the granites clearly reflect the sequence of increased differentiation among the magma types, with biotite granites as the least evolved, and topaz granites as the most evolved. In contrast, tourmaline of hydrothermal origin within granites displays fine-scale compositional zonation with a general tendency toward more magnesian compositions nearer the schorl-dravite solid solution (i.e., little or no Al in the R2 site). Metasomatic tourmaline precipitated in the surrounding pelitic and mafic rocks also possesses fine-scale zonation, and generally reflects the compositions of the host rocks. In addition, tourmaline formed in the country rocks has a higher proportion of Fe3+ to Fe2+, which presumably indicates a higher oxidation state of fluids in the metamorphic rocks than in the granites. This variation may correlate with the mobilization of tin from the granites and its consequent deposition as cassiterite in the host rocks. The abundance of magmatic tourmaline is limited by the initial Fe-Mg content of the magmas to not more than a few modal or weight percent. Although they contain tourmaline, the biotite granites are not the most likely sources of boron for voluminous, late-stage tourmalinization. Magmas that contained only tourmaline or no Fe-Mg minerals at all, such as the those that formed the topaz granites, may have been sources of large quantities of boron. The most intense areas of tourmalinization do surround the small stocks and sheets of topaz microgranite. High concentrations of tourmaline, for example in hydrothermal veins and breccias, appear to require mixing of two different chemical reservoirs: one a source of boron (the magmas or fluids..
Subjects:Cations; Chemical composition; Composition; Country rocks; Electron probe data; Granites; Hydrothermal alteration; IGCP; Igneous rocks; Magmas; Metal ores; Metasomatism; Mineral deposits, genesis; Mixing; Plutonic rocks; Ring silicates; Silicates; Spectra; Tourmaline group; Variations; Veins; Zoning; Cornwall England; England; Europe; Great Britain; South-West England; United Kingdom; Western Europe; Hydrothermal processes; Igneous activity
Abstract Numbers:96M/1586
Record ID:1995056363
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute.
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