The Serra Branca amazonite pegmatite of the Vieirópolis pegmatite field, Paraíba, Brazil: A new and unusual megacrystic amazonite deposit

Abstract

The recently discovered Serra Branca amazonite pegmatite in the state of Paraíba, Brazil, is an evolved niobium-yttrium-fluorine (NYF) granitic pegmatite, which belongs to the Vieirópolis pegmatite field, the first NYF pegmatite field described from the Borborema Province. The pegmatite is an unusually large resource (>300,000 tons) of intensely colored amazonite megacrysts up to 2 m in size, exploited in an open pit since ca. 2010 as an ornamental stone and gemological material. This study provides the first mineralogical and geochemical characterization of the Serra Branca amazonite pegmatite and its mineralogy, which are further compared with the mineralogy and composition of other NYF pegmatites. The Serra Branca pegmatite forms an approximately 800 m-long and up to 3 m-thick flat-dipping sheet that consists of two distinct structural-mineralogical zones with complex relationships: (1) the massive, megacrystic amazonite zone forming commonly in the upper part of the dike and (2) the fine-grained albite zone commonly occurring at the bottom of the dike. The amazonite zone, which comprises ca. 75 vol.% of the pegmatite body, is composed of amazonite and quartz, the major constituents, with accessory biotite, helvine, galena, ilmenite, hematite, columbite-(Mn), phenakite, titanite, magnetite, and rutile. The albite zone consists of saccharoidal albite and quartz and accessory spessartine, ilmenite, zircon, columbite-(Mn), pyrochlore, and fluorite. A younger minor intrusive pegmatitic unit composed of megacrystic euhedral amazonite and quartz and platy albite (cleavelandite) postdates the formation of the amazonite and albite zones. Geochemical analysis of the bulk pegmatite, amazonite zone, and albite zone shows that the bulk pegmatite and the amazonite zone have similar compositions, with high trace-element contents of Ba, Be, Rb, Sr, and Pb, whereas the albite zone is enriched in Zn, Nb, Zr, Ga, and Hf. The amazonite and albite zones display some similarities in terms of major elements, the presence of HFSE minerals, and the distribution of incompatible and REE, which indicates that the two zones originated from the same melt. The amazonite crystals are enriched in Rb, Pb, Sr, Cs, Ba, Tl, Fe, and Ga, characterizing the Serra Branca pegmatite as an evolved NYF pegmatite. This conclusion is confirmed by the low K/Rb ratio of biotite of ∼5.7. Higher Li and Ga and lower Sr and Ba in the amazonite of the minor unit suggests that this melt was even more highly fractionated than the melt of the first emplacement stage.