Bismuth Minerals from the Intragranitic La Elsa NYF Pegmatite, Potrerillos Granite, Argentina: Monitors of Fluid Evolution from Magmatic to Hydrothermal Stage

ABSTRACT The NYF La Elsa pegmatite forms a subcircular, ∼30 m long, ∼20 m wide body enclosed in the parental Potrerillos granite, Las Chacras-Potrerillos batholith, Argentina. The pegmatite has a typical zonal internal structure with a volumetrically significant quartz core and pods of magmatic fluorite. Along with biotite, mostly in the outer units, tourmaline (schorl, fluor-schorl) is common to abundant in most pegmatite units. Accessory minerals include common strongly hematitized ilmenite and rare euhedral crystals of bismuthinite, up to 2 cm long, occurring at the transition between the blocky zone and the quartz core. The bismuthinite was significantly replaced by bismutite I according to the reaction Bi2S3(s) + CO2(aq) + 6O2(aq) + 3H2O(l) = Bi2CO3O2(s) + 3H2SO4(aq). Subsequently, bismutite I was replaced by bismutite II and kettnerite. The former requires an influx of Ca and F and its formation can be characterized by the reaction Bi2CO3O2(s) + 2Ca2+(aq) + 2F–(aq) + CO32–(aq) = 2CaBiCO3OF(s). At the late hydrothermal stages bismutite was replaced by clinobisvanite/pucherite during influx of V according to the reaction Bi2CO3O2s + 2H3VO4(aq) = 2BiVO4(s)+ CO2(aq) + 3H2O(l). All Bi minerals are close to the ideal formulae with only minor Pb and ±Cu in bismuthinite and its secondary products. The crystallization sequence of Bi minerals is magmatic bismuthinite (S2–) → early hydrothermal bismutite I (CO32–) → medium stage bismutite II + kettnerite (CO32–, F–) → late stage clinobisvanite, pucherite (VO43–). Pegmatite-derived early subsolidus fluids were enriched in CO2, which was confirmed by confocal Raman microspectroscopy of fluid inclusions in quartz and caused breakdown of bismuthinite to bismutite. Calcium and F, necessary for kettnerite formation, were released during alteration of magmatic fluorite at acidic conditions. Vanadium was supplied by meteoritic H2O enriched in elements from altered magmatic minerals (biotite, ilmenite), either from the pegmatite or from the host granite.

The Distribution of Rare Metals in the LCT Pegmatites from the Giraúl Field, Angola

The Giraúl granitic pegmatite field in Angola is composed of five pegmatite types, the most evolved belong to the beryl-columbite, beryl-columbite-phosphate and spodumene types. Pegmatites are concentrically zoned with increased grain size toward a quartz core; the most evolved pegmatites have well-developed replacement units. These pegmatites are rich in Nb-Ta oxide minerals and the field has a moderate interest for critical elements such as Ta and Hf. Tourmaline, garnet and micas occur as accessory minerals. The abundance of Zr and Nb-Ta minerals increases with the evolution of the pegmatites, as well as the proportions of beryl and Li-rich minerals. The Ta/(Ta + Nb) ratio in Nb-Ta oxide minerals and the Hf/(Hf + Zr) ratio in zircon also increase with the evolution of the pegmatites and within each pegmatite body from border to inner zones, and especially in the late veins and subsolidus replacements. Textural patterns and occurrence of late veins with Ta-rich minerals suggest that Nb and especially Ta can be enriched in late hydrothermal fluids exsolved from the magma, along with Hf and other incompatible elements as Sn, U, Pb, Sb and Bi.

Mapeamento Geológico do Pegmatito Alto do Urubu, município de Picuí (PB), Faixa Seridó, Província Pegmatítica da Borborema

The Alto do Urubu Pegmatite is located in the municipality of Picuí, northern of Paraíba, in Borborema Pegmatitic Province (PPB), the area that has the greatest concentration of pegmatites in the Brazilian Northeast, corresponding to the eastern part of the Seridó Belt. PPB is known worldwide for the abundance of pegmatites, with some incidence of mineralized bodies. In the region we studied, 42 outcrops were described, in each of them the main textures and mineralogical aspects of rock were described. The geological mapping made it possible to observe the predominance of the rocks of the Seridó Formation. Schists and pegmatites intruded in the form of dikes and sills are very common in the place. The schists of the region present a dark, grayish coloration, fine to medium granulation, and are composed mainly of quartz, biotite, muscovite and granade porphiroblasts, in some cases, there are cordierite centimeter nodules. It is possible to observe in these rocks foliation Sn and Sn+1, as well as crenulations. The studied pegmatite has an elongated shape, it is about 106 meters long and 18 meters wide and N-S direction, in agreement with the foliation of the schist. It shows a well defined mineral zoning, namely: i) border zone (zone I); ii) zone III and iii) quartz core. Zone I corresponds to the outermost part of the body, measuring about 30 cm in width and consisting essentially of muscovite. Zone III is composed of quartz, feldspar and mica and it is where the majority of the minerals of economic interest (especially tantalite and beryl) are concentrated, in a smaller quantity there is the occurrence of arrojadita and espersatita. In the innermost part of the pegmatite, there is the quartz nucleus, composed of milky and rosacea quartz. The observed mineral zoning enables us to classify it as a heterogeneous pegmatite type.

Petrogenetic significance of LA-ICP-MS trace-element data on quartz from the Borborema Pegmatite Province, northeast Brazil

Quartz from different zones within five granitic pegmatites of the rare-element class from the Borborema Pegmatite Province in northeast Brazil were analysed for fourteen trace elements using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The concentrations of Li (6—150 ppm), B (1—9 ppm) and Ge (1—23 ppm) in quartz show a positive correlation with Al (30—770 ppm). The concentrations of these elements increase from the border zone to the quartz core of pegmatites of the spodumene or lepidolite subtypes. The Ge concentrations in the quartz core are the highest so far reported in igneous quartz. In the less evolved pegmatites of the beryl-columbite subtype, the Al, Li, B, and Ge concentrations in quartz from all zones remain at the same level as the border and wall zones. The Ti concentrations in quartz from the core of the more evolved pegmatites are below 3 ppm (with Al >250 ppm), contrasting with 7—25 ppm (with Al <280 ppm) in samples from the border and wall zones of the less evolved and more evolved pegmatites. The concentrations of Al. Li, B, Ge, and Ti in quartz are therefore confirmed as good indicators of the degree of magma fractionation and analyses of pegmatite quartz cores can be used for exploration purposes to distinguish pegmatites with high metallogenic potential. Atoms of Li and Al are incorporated into quartz such that Li/Al ranges between 0.75 and 1.0. This suggests a coupled substitution of the form Si4+ ↔ (Li+ + Al3+). The other elements analysed either showed an erratic distribution (e.g. Be and P) or were below the respective limits of detection (Na, K, Rb, Ca, Sr, Mn, Fe) in most samples.