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Argillization of basic hornfelses from the Land's End granite aureole, Cornwall

Published online by Cambridge University Press:  09 July 2018

P. A. Floyd
P. A. Floyd*
Affiliation:
Department of Geology, University of Keele
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Abstract

Mineralogical and chemical variation in a 24 ft wide argillized zone in basic hornfelses from Gwavas quarry, Cornwall, is described and compared with variation in unaltered wall rock hornfelses.

Hydrothermal minerals developed are well-ordered kaolinite, sericite, chlorite, quartz, and limonite. Biotite has replaced hornblende as the dominant mafic constituent in the argillized zone. With increasing argillization kaolinite progressively replaces plagioclase, biotite, chloritized biotite, quartz, and finally Fe ore. No montmorillonite is present.

From the argillized zone, Si, Na, Ca, Mg, and Fe were extensively leached, although an approximate equivalent of H was added to compensate for the loss. Al and K are relatively constant throughout the zone. F was expelled from the most heavily argillized areas and is now concentrated in the biotite of the zone's margin as a marked ‘fluorine front’.

Two tenuous kaolinite-sericite zones are observed which are thought to be due to local variation in a single hydrothermal pulse with time.

Hydrothermal alteration of the hornfelses was effected in two stages (1) K+ metasomatism (biotitization) and (2) H+ metasomatism (kaolinization and sericitization).

Type
Research Article
Information
Clay Minerals , Volume 6 , Issue 1 , July 1965 , pp. 45 - 58
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1965

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