Chlorite alteration in pegmatite from the Madan Pb-Zn deposits: mineral association, chemical composition and temperature of formation

Chlorites are common constituent of the secondary mineral assemblage formed as alteration products after aluminosilicate host rocks in the Pb-Zn deposits in Madan district. In concordant pegmatite body from the Petrovitsa deposit, they are formed after mica flakes. Such transformation often results in abundant rutile grains. The dominant chlorite compositions fall in the clinochlore-chamosite series. Minor and trace elements incorporation of Li, B, V, Co, Ni, Zn, Ga, Rb, Sr, Cs, and Ba is detected. Calculated To of formation ranges within 298–306 °C.

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

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.

Bohseit z beryl-columbitového pegmatitu D6e v Maršíkově (silezikum, Česká republika)

Bohseite was found in a lenticular body of D6e beryl-columbite granitic pegmatite near Maršíkov, which is hosted by amphibole gneisses of the Sobotín Amphibolite Massif (Silesicum, northeastern part of Czech Republic). Bohseite forms chalky white aggregates up to 1 cm in size, which are hosted by small vugs in the coarse-grained pegmatite. It is associated with small crystals of quartz, adularia, albite, muscovite and epidote. Bohseite is orthorhombic, space group Cmcm with following unit-cell parameters refined from X-ray powder diffraction data: a 23.210(2), b 4.955(2), c 19.428(3) Å and V 2234.5(1.0) Å3. The electron microprobe compositional data of bohseite are presented. Bohseite from Maršíkov contains 14.3 - 42.5 mol. % of bavenite component, up to 0.06 apfu Na and 0.13 - 0.36 apfu F. The association with other beryllium-rich phases (milarite, bertrandite) points to variable activities of Be and Al during hydrothermal stage of evolution of the pegmatite body. The likely source of Be was beryl, which is sometimes completely dissolved and vugs after its crystals are lined by small crystals of above mentioned hydrothermal phases.

Minerály řady bavenit-bohseit z pegmatitu Schinderhübel I v Maršíkově (silezikum, Česká republika)

Bavenite and bohseite were found in an archive sample from Schinderhübel I granitic pegmatite, situated ca. 50 m NE from the famous chrysoberyl-bearing pegmatite body Schinderhübel III near Maršíkov (Silesicum, Czech Republic). Minerals of the bavenite-bohseite series together with minor quartz, muscovite and albite form chalky white radial aggregates up to 3.5 cm in size within a fissure cutting the pegmatite. The electron microprobe data revealed 29.0 - 65.4 mol. % of bavenite component, 0.03 - 0.12 apfu Na and 0.05 - 0.20 apfu F. Bavenite seems to be older than bohseite in the studied aggregate. The collected data suggest significant increase of Be/Al during growth of the studied aggregate, which could be explained in two ways. First, one can assume that different primary minerals with contrasting Be/Al ratios were dissolved during different stages of alteration (i.e., chrysoberyl in the early stage giving rise to bavenite-rich compositions and beryl during late stage giving rise to bohseite-rich members). Second, chemical fractionation of Be and Al due to complexation by fluoride anions is suggested from negative correlation between Al and F in the studied members of the bavenite-bohseite series. Identical behaviour is observed also in bavenite-bohseite from Piława Górna and Maršíkov D6e pegmatites, suggesting potential importance of fluoride complexation during hydrothermal stage of evolution of granitic pegmatites.

Komplexní magmaticko-hydrotermální vývoj columbitu, mikrolitu a fersmitu z beryl-columbitového pegmatitu D6e u Maršíkova, Česká republika

The recently rediscovered small D6e granitic pegmatite body, enclosed in amphibole gneiss of the Sobotín amfibolite massif (Jeseníky Mountains, Czech Republic), is characterized by numerous accessory minerals, including common columbite group minerals (CGM) and minor microlite and fersmite related to blocky K-feldspar unit. The CGM show complex internal zoning. Primary magmatic columbite-(Mn) occurs as corroded domains of prevailing homogeneous pattern, followed by less evolved oscillatory zonation. Primary CGM were overprinted by extensive recrystallization controlled by late-magmatic to post-magmatic fluids and leading to a formation of complex patchy and convolute oscillatory domains of secondary (hydrothermal) CGM. Primary columbite-(Mn) shows significantly limited Ta/(Ta+Nb) and Mn/(Mn+Fe) ratios, whereas secondary columbite-(Fe) to -(Mn) show slightly wider Fe-Mn and Nb-Ta compositional variations. Complex textures and the element fluctuations indicate a partial dissolution-reprecipitation of primary CGM caused by late- to post-magmatic fluids. Moreover, late calciomicrolite I, II and fersmite precipitated on the cracks of columbite crystals. Rare U-rich calciomicrolite I was extensively replaced by fersmite and oscillatorily zoned U-poor calciomicrolite II, slightly enriched in F. Their formation sequestrated part of hydrotermally released Na, Ca, U and represents the final subsolidus fluid-driven stage of the pegmatite evolution. Textural and compositional variations of Nb-Ta mineralization point to a complex magmatic to hydrothermal evolution of the D6e beryl-columbite pegmatite similar to other pegmatites in this region.

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.

Monazite-(Ce) and xenotime-(Y) microinclusions in fluorapatite of the pegmatites from the Volta Grande mine, Minas Gerais state, southeast Brazil, as witnesses of the dissolution–reprecipitation process

Fluorapatite with monazite-(Ce) and xenotime-(Y) microinclusions occurs in the lithium–caesium–tantalum pegmatite body A of the Volta Grande mine, Minas Gerais state, Southeast Brazil. The fluorapatite displays faint zoning, detected mainly by cathodoluminescence. Electron probe and laser ablation analyses indicate that zoning in the fluorapatite corresponds to variation in Mn and rare-earth element (REE) content. Such compositional variation is attributed to partial removal of the REE from the fluorapatite structure during a dissolution–reprecipitation process, forming monazite-(Ce) and xenotime-(Y) microinclusions in the REE-depleted zones of the fluorapatite. These inclusions exhibit an inherited geochemical signature, manifested by low Th and U concentrations when compared to monazite and xenotime crystallised from melts. Rhodochrosite and calcite inclusions are also associated with monazite-(Ce) and xenotime-(Y) and are probably products of the same process, recycling Ca, Mn, and CO32− from the fluorapatite through the following reaction: [Ca(5–2a–b–½x),Naa,(Y + REE)a,Mnb][(PO4)3–x(CO3)x(F)] + Fluid[a(2Ca2+ + P5+) + (x–b)(Ca2+) + H2O)] → [Ca5(PO4)3(F,OH)] + a[(Y + REE)PO4] + b[Mn(CO3)] + (x–b)[Ca(CO3)] + Fluid a[Na+].On the basis of new fluid-inclusion analyses, we propose that a hot (T > 204.5°C), salty (16 wt.% eq. NaCl, attributed to LiCl), hydrous fluid mediated the dissolution–reprecipitation of the fluorapatite. This fluid corresponds to similarly described Li-rich fluids which were suggested to have re-equilibrated the mineralogical assemblage at the Volta Grande mine.

Mapeamento geológico do Pegmatito Alto do Tibiri: aspectos estruturais e mineralógicos

<p>The Alto Tibiri is a pegmatite body located in the southern region of the Rio Grande do Norte, in Parelhas city, geologically inserted in the Borborema Pegmatite Province and occur intruding the mica schists of the Seridó Formation. Currently, both mica and feldspar are commercially exploited in this pegmatite body. In the field missions, 39 outcrops were described, in each one the main texture and mineralogical aspects of the outcropping rock were described. The geological mapping has allowed us to distinguish two types of schists, based on content and size of the mineral phases. There is a garnet rich schists and other with a relevant cordierite content. The studied region shows narrow pegmatite dykes, composed of quartz, feldspar, muscovite and/or biotite and black tourmaline, these dikes were classified as homogeneous. It’s possible to observe in Alto Tibiri a well- defined mineral zoning, as follows: i) border area, marked by the abundance of muscovite; ii) Zone II, composed of quartz, feldspar and mica, in which the main mineral resources occur (tantalite, spodumene, apatite, etc.) and iii) quartz pockets. The observed mineral zoning is similar to that described as mixed traditional pegmatite bodies.</p>

Fluor-tsilaisite, NaMn3Al6(Si6O18)(BO3)3(OH)3F, a new tourmaline from San Piero in Campo (Elba, Italy) and new data on tsilaisitic tourmaline from the holotype specimen locality

Fluor-tsilaisite, NaMn3Al6(Si6O18)(BO3)3(OH)3F, is a new mineral of the tourmaline supergroup. It occurs in an aplitic dyke of a LCT-type pegmatite body from Grotta d'Oggi, San Piero in Campo, Elba Island, Italy, in association with quartz, K-feldspar, plagioclase, elbaite, schorl, fluor-elbaite and tsilaisite. Crystals are greenish yellow with a vitreous lustre, sub-conchoidal fracture and white streak. Fluor-tsilaisite has a Mohs hardness of ∼7 and a calculated density of 3.134 g/cm3. In plane-polarized light, fluor-tsilaisite is pleochroic (O = pale greenish yellow and E = very pale greenish yellow), uniaxial negative. Fluor-tsilaisite is rhombohedral, space group R3m, a = 15.9398(6), c = 7.1363(3) Å, V = 1570.25(11) Å3, Z = 3. The crystal structure of fluor-tsilaisite was refined to R1 = 3.36% using 3496 unique reflections collected with MoKα X-ray intensity data. Crystal-chemical analysis resulted in the empirical formula: X(Na0.69〈0.29Ca0.02)Σ1.00Y(Mn2+1.29Al1.21Li0.56Ti0.03)Σ6.00ZAl6T(Si5.98Al0.03)Σ6.00B2.92O27V(OH)3W[F0.39(OH)0.25O0.36]Σ1.00.Comparisons were performed between fluor-tsilaisite and a tsilaisitic tourmaline from the same locality as the holotype specimen. This latter tourmaline sample was selected for this study due to its remarkable composition (MnO = 11.63 wt.%), the largest Mn content found in tourmaline so far.Fluor-tsilaisite is related to tsilaisite through the substitution WF ↔ W(OH) and with fluor-elbaite through the substitution Y(Al + Li) ↔ 2YMn2+, and appears to be a stepwise intermediate during tourmaline evolution from tsilaisite to fluor-elbaite.