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Journal of the Geological Society; 2009; v. 166; issue.3; p. 563-582;
DOI: 10.1144/0016-76492008-080
© 2009 Geological Society of London

Research Article

On the growth of colloform textures: a case study of sphalerite from the Galmoy ore body, Ireland

Craig D. Barrie1,7, Adrian J. Boyce2, Alan P. Boyle1, Patrick J. Williams3, Kevin Blake4, Jamie J. Wilkinson5, Mike Lowther6, Paul McDermott6 & David J. Prior1

1 Department of Earth and Ocean Sciences, University of Liverpool, Liverpool L69 3GP, UK
2 Scottish Universities Environmental Research Centre, Rankine Avenue, East Kilbride, Glasgow G75 0QF, UK
3 EGRU, School of Earth and Environmental Science, James Cook University, Queensland 4811, Australia
4 EGRU, Advanced Analytical Centre, James Cook University, Queensland 4811, Australia
5 Department of Earth Science and Engineering, Imperial College London, Exhibition Road, London SW7 2AZ, UK
6 Galmoy Mines Limited, Galmoy, Thurles, Co. Kilkenny, Ireland
7 Present address: School of Geography and Geosciences, University of St Andrews, St Andrews KY16 9AL, UK.

*Corresponding author (e-mail: cdb21{at}st-andrews.ac.uk)

Colloform textures have been described from many of the world's sulphide ore deposits and involve formation of distinct microcrystalline layers. These textures provide information related to sequential stages of ore formation, yet their mechanism of growth is poorly understood. This study has analysed a series of colloform samples from the Galmoy Zn + Pb mine, Ireland. Results indicate that the growth sequence is not always intuitive and layers that appear stratigraphic upon cursory observation may not be. The crystal preferred orientation (CPO) of discrete colloform layers abruptly switches between different orientation axes. Examination of the same layers using in situ laser sulphur isotope analysis reveals equally striking changes in {delta}34S signature between end-member bacteriogenic (–25{per thousand}) and hydrothermal (+10{per thousand}) sources. Although these results, combined with petrographic observation, allow the determination of a probable growth history, there is no correlation between shifting CPO and {delta}34S signature. Trace element analysis reveals changes in trace element sequestration between colloform layers. Cadmium- and chlorine-rich layers correspond to a hydrothermal sulphur source, whereas iron-rich layers correspond to a bacteriogenic sulphur source. In the absence of alternative mechanisms we suggest that the most likely factors influencing CPO changes are temperature and degree of supersaturation during two-fluid mixing.