Metasomatic tourmalinite formation along basement-cover decollements, Orobic Alps, Italy

Abstract

Cryptocrystalline tourmalinites that occur discontinuously for ???30 km along basement-cover de??collements of the Orohic Alps (Italy) formed by the metasomatism of aluminous cataclasites derived from Permian conglomerates and/or feldspathic sandstones. Using Al as an immobile element monitor, calculations show that the majority of tourmalinites in the region formed through the addition of moderate to significant amounts of B, Mg, Na, Sr, and Be, and the loss of moderate to significant Mn, Ca, K, P, Rb, Ba, and Cr; minor Si, Ti, V, light REE, and Eu also were lost. Data tor relatively immobile Al, Zr, Th, Sc, Nb, and heavy REE indicate that, on average, these tourmalinites formed through ???12% net mass loss assuming an original conglomerate protolith, or through ???7% net mass loss assuming a sandstone protolith. The B and other introduced constituents in the tourmalinites were deposited by hydrothermal fluids focused along and near basement-cover de??collements. These fluids, believed to be associated with late Hercynian felsic magmatism, probably are related to fluids that formed the tourmaline-rich U-Mo-Zn deposits at the nearby Novazza mine and the U-Zn deposits at the nearby Val Vedello mine.