Stratiform tourmalinites are more widespread than previously recognized in many regional metamorphic terrains. They are especially common in Proterozoic and early Palaeozoic sequences dominated by clastic metasediments and consist of conformable layers made up primarily of quartz and abundant tourmaline, the latter typically exceeding 15-20% by volume. Striking sedimentary structures such as graded bedding, cross-laminations, slump and flame structures and rip-up clasts, are preserved in a few tourmalinites. These and other geological features suggest that tourmalinites form by early diagenetic modification of a primary boron-rich chemical precipitate. Tourmalinites are significant in preserving a valuable record of unusual chemical and palaeogeographic conditions in clastic sedimentary basins. Their close association with a variety of stratabound deposits of Au, W, Sn, Co and base-metals may permit a clearer understanding of ore-forming processes, as well as the definition of prospective exploration targets. -L.C.H.
Additional publication details
Stratiform tourmalinites in metamorphic terranes and their geologic significance.