Chemical Geology, 17 {1976) 179--193 179 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

THE GEOCHEMISTRY OF CELESTITES FROM THE AREA NEAR BRISTOL (U.K.)

M.W. WOOD* and H.F. SHAW Department of Geology and Mineralogy, University of Oxford, Oxford (Great Britain) Department of Geology, Imperial College of Science and Technology, University of London, London (Great Britain) {Received May 21, 1975; revised and accepted October 9, 1975)

ABSTRACT

Wood, M.W. and Shaw, H.F., 1976. The geochemistry of celestites from the Yate area near Bristol (U.K.). Chem. Geol., 17: 179--193.

Celestite occurs at Yate, near Bristol, as massive and geodic nodules and as beds or veins in the Keuper Marl, and in the Coal Measures and Silurian rocks which are overlain un- conformably by the Keuper Marl. Detailed mineralogical, geochemical and isotopic data have been collected on the celestites and their associated sediments. Ca and Ba values are tabulated for the celestites and their associated sediments, and for the celestites an estimate is made of the non-carbonate Ca content from their measured CO 2 values. For the celestites the mean values of Ca, Ba and the estimated non-carbonate Ca are 0.46, 1.00 and 0.27%, respectively. For the sediments the mean values of the Ca and Ba contents are 0.81 and 0.58 %, respectively. 87 Sr/86 Sr ratios are given for fourteen celestite samples and for two samples of associated sediments. Although there is some evidence that in a few localities the celestite is secondary after anhydrite, much of the celestite is formed by primary precipitation. The formation of the celestite occurred in an environment enriched in Sr compared to Ba and Ca. The enrichment of Sr over Ca is 10s--104 times greater than that found in most rocks and natural waters, and the Sr/Ba ratio of the celestites is 1,500 times greater than that of standard seawater. It is therefore unlikely that the celestite precipitated out from solutions directly derived from seawater. The most likely source of the Sr would be via the dolomitisation of the Carboniferous Limestone releasing the Sr into solution. The Sr--isotope data indicate that this source of Sr was subsequently modified by percolation through the Triassic and weathered Coal Measure sediments before incorporation in the celestite. Formation of the celestite most likely oc- curred in arid desert basins of inland drainage. There were several different episodes of for- mation, each with slightly different groundwater compositions, which are now reflected in the composition of the celestites.

INTRODUCTION

The celestite deposits in the Yate area (Fig.l) are very extensive and the mineral has been commercially extracted from shallow pits in the area since the beginning of the century up to the present.

*Present address: Pilkington Bros. Ltd., Research and Development, Lathom, Ormskirk, Lancashire, Great Britain. 180

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o Tirtwor th

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.....~__J J • ~-~YATE AREA (Fig. 2)

• BR,STOL

Fig.1. General location of the Yate area and sample localities outside the Yate area.

The celestite occurs as massive and geodic nodules and as beds or veins in the Keuper Marl, and in the Coal Measures and Silurian rocks which are overlain unconformably by the Keuper Marl. In the Keuper Marl the celestite is usually concentrated about 10--15 m below the Tea Green Marls. Celestite has however been reported from the Tea Green Marls (Kellaway and Welch, 1948) and the Rhaetic (Hamilton, 1962) in areas neighbouring Yate. Deposition of the Keuper Marl in the area took place in arid desert basins of inland drainage in a landmass consisting mainly of Carboniferous rocks. These inland basins were the sites for the deposition of saliferous sediments. Audley- Charles (1970) postulated that the groundwaters draining into these basins may have originated from Triassic seawater or from reworked older evapo