Extreme F activities in late pegmatitic events as a key factor for LILE and HFSE enrichment: The Ángel pegmatite, Central Argentina

Abstract

The Ángel pegmatite forms part of the Comechingones pegmatitic field, in central Argentina, which is made up of pegmatites characterized by low to intermediate degrees of fractionation, classified as beryl-columbite-phosphate subtype pegmatites. These pegmatites are syntectonic with a regional shear zone. The Ángel pegmatite contains associations with quartz, microcline, plagioclase, a first generation of muscovite (muscovite I), beryl, members of the columbite group, triplite, and montebrasite. This association is locally affected by two stages of replacement. The first replacement stage is characterized by early albitization, followed by the development of associations of cleavelandite, quartz, Fe-rich elbaite (elbaite I), a second generation of muscovite (muscovite II), topaz, lacroixite, fluorapatite, pollucite, columbite-(Mn), and Hf-rich zircon. Muscovite II replaces montebrasite and muscovite I, and is characterized by slight enrichments in F, Rb, and Cs. The second replacement stage generated a new mineral association characterized by muscovite III, Fe-poor elbaite (elbaite II), Cs-micas, and U-rich hydroxykenomicrolite. Muscovite III replaces muscovite II and is characterized by strong enrichments in F, Cs, and, to a lesser extent, Rb. In turn, muscovite III is replaced by the Cs-micas sokolovaite and nanpingite. The high F content of the nanpingite suggests that this could be the F- analogue of nanpingite, which would be a new mineral. The sequence of replacement is indicative of an increase in the F activity in the latest pegmatitic fluids. The high F activity of these fluids favored the transport of Ta, U, Bi, Hf, Rb, Cs, and Li, and the formation of F-rich micas could be the mechanism for precipitating these LILE and HFSE elements. The syntectonic emplacement of this pegmatite in a large shear zone could be a decisive factor in the migration of these late evolved fluids rich in F, LILE, and HFSE.