The Boron Isotopic Composition of Tourmaline as a Guide to Fluid Processes and Boron Source in the South-West England Orefield: An Ion Microprobe Study

Abstract

Recent work has shown the boron isotopic composition of tourmaline to be sensitive to a number of factors including boron source, waterrock interaction and temperature and pressure of formation (Palmer and Slack, 1989; Palmer et al, 1992). The boron isotopic composition of hydrothermal fluids has also been shown to be sensitive to boiling (Leeman et al, 1992). The boron isotopic composition of individual tourmalines from S.W. England has been determined in order to investigate both fluid source and fluid processes b y attempting to detect isotopic differences within crystals and between samples. Samples were analysed from three main rock types:granites, quartz-tourmaline (tourmalinite) and quartz-topaz-tourmaline (QTT) rocks and greisen(Sn,W)-related veins. Three main sample areas were used the St.Austell granite and its associated tourmalinites and QTT rocks at Roche and St. Mewan's Beacon, the roof zone of the Land's End granite exposed at Porth Ledden and the Cligga Head granite and sheeted vein system 9 Tourmalinites and QTT rocks are important as they are considered to represent the transition from magmatic to hydrothermal conditions (Manning, 1981;Bottrell and Yardley, 1988) 9