Recently exposed celestite - sphalerite - galena - pyrite mineralization occurs in Carboniferous Limestone near Chipping Sodbury, some 50 kilometres to the east of the Mendip Orefield and 20 kilometres from Bristol. In similar occurrences in the Mendip orefield, baryte usually occurs in place of celestite, but here, close to the known celestite workings at Yate, the association of minerals assemblage appears to be unique.

There are three main types of mineralization:

1. A. Cross-cutting bedded mineralization, locally brecciated, probably representing cave floor chemical sediments, with parallel bands of calcite, pyrite, marcasite, sphalerite, galena and celestite.
2. B. Stalactitic and stalagmitic formations in solution cavities involving a similar mineral assemblage to A, but on a much smaller scale, with individual stalactite growths from five to ten centimetres.
3. C. Vein-fill, with brecciated fragments of limestone in a calcite matrix and green clay-rich material which might represent the insoluble products from dissolved limestone.

Preliminary conclusions

• Mineralization was due to the circulation of low temperature ground waters depositing multiple generations of celestite, sulphides and carbonates within sub-horizontal cave systems.
• Mineralization is pre-Rhaetic (Triassic - more than 205 million years old) since it does not extend into the Penarth Group exposed in the quarry. Cave development is interpreted as pre-Triassic.
• The low temperatures of the ore-forming fluids together with the mineralogy and geological setting of the mineralisation might indicate a Mississippi Valley Type origin. However, the low salinities and low calcium contents are atypical. At our current state of knowledge, a basinal brine origin is still favoured.

1. Outer surface of stalactitic development of sulphides consisting of felted masses of intergrown pyrite and marcasite.
2. Probable cave floor development of 'schalenblende' on limestone and late scalenohedral calcite with pyrite/marcasite inclusions.
3. Celestite on calcite with an inner core of mixed sphalerite and pyrite. Later overgrowths on the celestite are galena, sphalerite, calcite, marcasite and pyrite.

This project has attracted small grant and isotope funding from the Natural Environment Research Council, and is being carried out jointly between C.J. Stanley of the NHM, A.H. Rankin of Kingston University and N. Hollingworth of NERC.

This project is part of the Earth Materials, History and Processes Research Theme.

For further information contact:

CJ Stanley,
The Natural History Museum
Cromwell Road London SW7 5BD UK
Tel: +44 (0) 171 938 9361 Fax: +44 (0) 171 938 9268
Email cjs@nhm.ac.uk

Copyright (c) 1996 The Natural History Museum