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Origin of quartz cores in tourmaline from Roche Rock, SW England

A. Müller^1,*,, B. J. Williamson¹ and M. Smith²

¹ Department of Mineralogy, The Natural History Museum, Cromwell Road, London SW7 5BD, UK
² School of the Environment, University of Brighton, Cockcroft Building, Lewes Road, Brighton BN2 4GJ, UK

^* E-mail: Axel.Muller{at}ngu.no

The nature and mode of origin of quartz-cored tourmalines (QCT) are studied from hydrothermal quartz veins within massive quartz-tourmaline (MQT) rocks at Roche, SW England. The QCT are annular, have blue maximum absorption colour and occur together with tourmalines with brown cores rimmed by blue tourmaline. Where the quartz core is not continuous throughout the length of the QCT crystals, the tourmaline core has brown maximum absorption colour, similar to tourmalines without quartz cores. Both the blue and brown tourmalines are schorl, but are compositionally distinct showing different Fe/(Mg + Ti) ratios and Ca concentrations. Fluid inclusion data indicate quartz precipitation from a moderate salinity (~20–25 wt.% NaCl eq.) brine which periodically boiled following pressure drops within the vein system. The QCT show rheomorphic and lobate textures on their inner margins indicating selective dissolution of their brown, relative Mg-, Ti- and Ca-rich tourmaline cores and replacement with quartz. This presents a problem in terms of the nature of the fluid responsible for such selective dissolution because tourmaline is generally highly resistant under the normal range of hydrothermal fluid conditions. It is proposed that the relatively high concentrations of Ti, Mg and Ca in the brown tourmaline caused significant lattice strain, which together with an increase in pH, and probably Al, in the boiling hydrothermal fluid caused the brown cores to become unstable compared with the Fe-rich blue tourmaline rims.

KEYWORDS: quartz, tourmaline, hydrothermal, Roche Rock, England