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Radioelement distribution in the Tertiary Lundy granite (Bristol Channel, UK)

Published online by Cambridge University Press:  01 May 2009

Richard S. Thorpe ,
Andrew G. Tindle  and
Olwen Williams-Thorpe
Richard S. Thorpe
Affiliation:
Department of Earth Sciences, The Open University, Milton Keynes, Buckinghamshire MK7 6AA, UK
Andrew G. Tindle
Affiliation:
Department of Earth Sciences, The Open University, Milton Keynes, Buckinghamshire MK7 6AA, UK
Olwen Williams-Thorpe
Affiliation:
Department of Earth Sciences, The Open University, Milton Keynes, Buckinghamshire MK7 6AA, UK
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Abstract

The radioelement distribution and content of the Lundy granite, a coarse-grained megacrystic granite of Tertiary age, has been measured using a portable gamma-ray spectrometer in order to assess fractionation and alteration processes in the granite. Results indicate a systematic variation of K, Th and U (with a few notable exceptions) that follows a partially concentric distribution to lower concentrations inland. The plateau region of the island (particularly the southern half) is relatively depleted in all radioelements. Over the island, measurements of K vary from 1.3–4.9 wt %, Th varies from 5.0–20.3 ppm and U varies from 2.0–12.5 ppm. A petrographic, electron microprobe and autoradiography examination of the granite indicates that the radioelements mainly reside in discrete major and accessory minerals, of which K-feldspar (K), biotite (K), monazite (Th), xenotime (U), tungsteniferous columbite (U) and uraninite (U) are the most important. Uraninite is rare, being preserved only in fresh samples which come mainly from abandoned quarries. Mass balance modelling indicates that up to 76.6% of uranium could reside in uraninite and where this has been leached by secondary processes such as hydrothermal alteration or weathering then the present radioelement content no longer reflects the original rock composition. Fission track evidence is presented to show the pathways along which uranium has been mobilized from or within the granite. Secondary sites of radioelements include fractures cross-cutting all major minerals (but especially quartz), grain boundaries, altered cores of plagioclase feldspar and occasionally yellowy brown mixed chlorite/smectite replacement product after biotite. Biotite itself may exhibit secondary tracks along cleavage traces. Combined effects of crystal fractionation (primary variation) and secondary alteration best explain the distribution of radioelements, with K controlled by fractionation of the major phases K-feldspar and biotite, Th by fractionation of the accessory mineral monazite (±xenotime and uraninite) and U contents by uraninite and tungsteniferous columbite. Secondary processes have removed much of the uraninite leaving behind indeterminate Fe—U material along fractures and residual U (and Th) enrichment within altered major minerals. There is some evidence to suggest that late radioelement-bearing fluids precipitated monazite and uraniferous zircon along fractures during the waning stages of magmatic activity.

Type
Articles
Information
Geological Magazine , Volume 132 , Issue 4 , July 1995 , pp. 413 - 425
Copyright
Copyright © Cambridge University Press 1995

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