The behaviour of halogens in the magmatic-hydrothermal evolution of Sn-W systems and their potential as fluid source indicators

A regional assessment of the halogens distribution across the Sn-W mineralised Cornubian batholith is presented in this study, including a comprehensive dataset of the halogen chemistry of whole rocks, minerals and fluids.

Overall, the F content in most halogen-bearing minerals increases with progressive differentiation of the granite. On the other hand, Cl only increases during early stages of magmatic fractionation, in correspondence to the second regional pulse of magma across the region (forming G3 granite), and the successive exsolution of fluid from the melt resulted in a rapid drop in Cl content, observed in both whole rock content and halogen-bearing minerals (in particular micas and apatite).

The fluid inclusion record, covering the entire evolution from magmatic to hydrothermal conditions, shows no evident differences in terms of halogen ratios between intermediate density fluids, brines and diluted fluids. However, a significant decrease in Br/Cl and I/Cl for late magmatic and late hydrothermal samples is observed. This shift can be attributed to mixing with an external fluid relatively poor in Br and I, or to the effect of progressive fractionation, causing a decrease in both ratios according to existing experimental studies, with the latter hypothesis being preferred.

The overlap in Br/I across all samples suggest that Br and I do not significantly fractionate from each other during the magmatic-hydrothermal evolution of such granites, and behave similarly to Cl. This represents further evidence that the external fluids mixing with magmatic fluids giving rise to the Sn-W and polymetallic mineralisation across the Cornubian batholith are halogen-free and therefore have a meteoric origin.

Whole-rock leaching experiments demonstrate that Cl, Br and possibly I are for the most part not tightly-bound to minerals, but rather hosted in fluid inclusions or forming weak bonds on mineral surfaces, whereas F is obviously strongly bound in micas, apatite and tourmaline, and in very evolved granites (G5 and Gx) in fluorite and topaz.

This study shows the potential of triple halogens analysis (Cl-Br-I) in recognising fluid sources and as tracer of fluid mixing even at large scale, independently from other tracers more commonly used in the literature to discern these processes.