METASOMATIC TOURMALINITE FORMATION ALONG BASEMENT-COVER DECOLLEMENTS,OROBIC ALPS, ITALY

Cryptocrystalline tourmalinites that occur discontinuously for similar to 30 km along basement-cover decollements of the Orobic Alps (Italy) formed by the metasomatism of aluminous cataclasites derived from Per mian conglomerates and/or feldspathic sandstones. Using Al as an immob ile element monitor, calculations show that the majority of tourmalini tes in the region formed through the addition of moderate to significa nt amounts of B, Mg, Na, Sr, and Be, and the loss of moderate to signi ficant Mn, Ca, K, P, Rb, Ba, and Cr; minor Si, Ti, V, light REE, and E u also were lost. Data for relatively immobile Al, Zr, Th, Sc, Nb, and heavy REE indicate that, on average, these tourmalinites formed throu gh similar to 12% net mass loss assuming an original conglomerate prot olith, or through similar to 7% net mass loss assuming a sandstone pro tolith. The B and other introduced constituents in the tourmalinites w ere deposited by hydrothermal fluids focused along and near basement-c over decollements. These fluids, believed to be associated with late H ercynian felsic magmatism, probably are related to fluids that formed the tourmaline-rich U-Mo-Zn deposits at the nearby Novazza mine and th e U-Zn deposits at the nearby Val Vedello mine.