Contrasting evolution of fluorine- and boron-rich tin systems

Abstract

Individual Sn provinces or regions within provinces are sometimes enriched in fluorine or boron, giving rise to fluorine-rich and boron-rich environments. The structural styles of mineralisation within these environments are similar except that hydrothermal intrusive breccia pipes are more common in boron-rich environments and apogranite/massive greisen systems are more common in fluorine-rich environments. The increased solubility of H2O in B-bearing magmas compared to F-bearing magmas may play a role in the structural evolution of the mineralising systems. The greater mechanical energy produced during crystallisation of B-rich magmas provides a mechanism for breccia pipe and stockwork formation, while the more passive crystallisation of F-rich magmas often results in the formation of disseminated mineralisation. The partitioning of boron toward the aqueous fluid phase and the enhanced solubility of silica in the fluid phase frequently results in tourmalinisation and silicification of the wall-rocks in B-rich environments. In contrast, feldspathic and sericitic alterations usually predominate in F-rich environments.