Source and partitioning of B and Sn in the Cornubian batholith of southwest England

Emplacement of the Variscan S-type Cornubian Batholith initiated the formation of the historical World-class Sn and Cu deposits and extensive tourmalinisation of southwest England. The hypothesis is tested that the Sn and tourmaline mineralisation were wholly formed by magmatically-derived hydrothermal fluids, with no requirement for a contribution from the stripping of country rocks by circulating hydrothermal fluids. From simple geochemical modelling, sufficient Sn concentrations to match those in the main (> 90% of granite outcrop) biotite granites (average of 23 mg/kg) and to produce the large Sn deposits can be derived by partial melting of pre-granite southwest England crust at geologically realistic levels of partial melting (30%), fractionation (10 to 50%) and % melt-aqueous fluid partitioning (assumed to be 6 wt.% of the mass of the granite batholith). In contrast, insufficient B (< 600 mg/kg) can be modelled for the melt to reach saturation with respect to tourmaline (>>700 mg/kg), or B concentrations found in many of the granites. We conclude that tourmaline in the biotite granites was formed from magmatically-derived hydrothermal fluids which emanated as a result of first and possibly second boiling. An additional source of B for the hydrothermal fluids may have been the devolatilisation of stoped blocks of country rock mudstones. These findings provide fundamental constraints on the evolution of the Cornubian batholith which is used as a model for a large number of Sn provinces worldwide.