The chrysoberyl- and phosphate-bearing pegmatite of Malga Garbella, Val di Rabbi, Trento province, Italy

This paper describes the mineralogy and petrology of an albite pegmatite dike cropping out close to Malga Garbella, Rabbi Valley, Trento province, Italy. The pegmatite is hosted by high-grade metamorphic rocks of the crystalline basement of the Tonale-Ultimo Unit, Austroalpine domain. The dike, now almost completely exploited, was 10 m long and 1 m thick. The internal structure of the pegmatite cannot be reconstructed, but from the available specimens it is possible to infer that a blocky albite unit and masses of quartz (quartz cores) formed the main units of the dike. The pegmatite, composed mainly of primary albite (Ab96-Ab100), also contains primary chrysoberyl, almandine, schorl, and muscovite as common accessory minerals, while beryl, zircon, monazite-(Ce), and uraninite represent rare minerals. Secondary uranium-bearing minerals were also detected. The primary phosphate association is given by graftonite þ sarcopside þ (minor) triphylite masses, and wolfeite was found alone at the contact between albite and quartz. The secondary phosphate association was formed in both a non-oxidized paragenesis and an oxidized and metasomatized one. The petrological processes leading to the crystallization of the secondary phosphates are complex and include: (1) hydration and Ca, Mg, and Mn leaching, mainly of graftonite; (2) hydration and oxidation of Fe; (3) destabilization of albite. The age of magmatic crystallization of the Malga Garbella pegmatite, measured using monazite-(Ce), is 257 6 3 Ma, consistent with the Brissago (Canton Ticino, Switzerland) phosphate-bearing pegmatite.