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.

Microtextural, spectroscopic, and chemical characterization of amazonite from the Serra Branca Pegmatite, Northeastern Brazil

“…The Vieirópolis Pegmatite Field has several pegmatite dykes, which are hosted by the Serra Branca Granite, including the Serra Branca amazonite pegmatite, being the main body mineralized in megacrystic amazonite and focus of this article (Lira Santos et al 2020, Souza et al 2020b. The Serra Branca amazonite pegmatite consists of two similar NW-trending tabular bodies ~3 m thick (Fig.…”

“…The Serra Branca amazonite pegmatite is located in the Vieirópolis Pegmatite Field (Lira Santos et al 2020), in the context of the Rio Piranhas-Seridó Terrane of the Borborema Province (Fig. 2A).…”

“…The Serra Branca pegmatite presents two magmatic generations of amazonite, the former occurs within the main body, whereas the second is part of minor intrusive rocks, which usually presents a lighter color, besides the characteristic association with cleavelandite. Such distinction is strongly based on field relationship and chemical evidence (for details, see Lira Santos et al 2020).…”

“…The albite intergrowth from both magmatic generations has elevated contents of Fe, Pb, Sr, and Ga, with 500-719 ppm for Fe, 76-161 ppm for Sr, 36-151 ppm for Pb, and 50-78 ppm for Ga, yet the minor intrusive unit albite exhibits a high content Rb (40.5 ppm). For detailed petrogenetic chemistry, see Lira Santos et al (2020).…”

“…The main examples are the Serra do Pinheiro (Pernambuco; Lira Santos et al 2017), Serra da Maturagem (Ceará; Prado et al 1980), and Tenente Ananias pegmatitic field (Rio Grande do Norte State; Barreto 1991). Recent discoveries of remarkable amazonite mineralization in pegmatites from the Borborema Province in NE Brazil brought attention to the first occurrence of NYF pegmatites in the region, located at the Vieirópolis Pegmatite Field (Lira Santos et al 2020). The main occurrence is the megacrystic amazonite Serra Branca pegmatite, in the Amazon Mine from the Granistone Company, which differs from the Alto Serra Branca pegmatite from Pedra Lavrada where the serrabrancaite was first described (Witzke et al 2000).…”