Introduction
Helvine-group minerals are anhydrous sulfosilicates, cubic and isostructural with the space group P $\bar{4}$
3n and are sodalite structural types. They have the general formula A8Be6(SiO4)6S2, in which the species are defined by the cation in the A crystallographic site. The species helvine (Mn4Be3Si3O12S), danalite (Fe4Be3Si3O12S) and genthelvite (Zn4Be3Si3O12S) form a solid solution, whose proportions are defined by the states of reduction, sulfidation and alkalinity of the system (Burt, Reference Burt1980). Complete miscibility should exist between the three end-members (Hassan and Grundy, Reference Hassan and Grundy1985), however, compositional gaps exist between the end-members Zn–Fe, Zn–Mn and Mn–Fe and pure danalite has not been observed in Nature (Oftedal and Saebø, Reference Oftedal and Saebø1963; Clark and Fejer, Reference Clark and Fejer1976; Dunn, Reference Dunn1976; Larsen, Reference Larsen1988; Perez et al., Reference Perez, Dusausov, Babkine and Pagel1990; Langhof et al., Reference Langhof, Holtstam and Gustafsson2000; Bilal, Reference Bilal2013).
The helvine-group minerals are only found in peralkaline and alkaline granites, syenites, rare metal pegmatites, albitites, greisens, skarns and contact zones (Deer et al., Reference Deer, Howie, Wise and Zussman2004). In this work, we have investigated the occurrence of genthelvite in pegmatite veins hosted by the albite-enriched granite facies of the Madeira granite. This facies corresponds to the Madeira deposit, which is characterised by an association of Sn with cryolite, Nb, Ta, Y, rare earth elements (REE), Li, Zr, U and Th in the same albite-enriched granite that hosts a massive cryolite deposit. The genthelvite crystals occur in pegmatite veins found in the most differentiated portion in the centre of the pluton. These pegmatites fit in the CMS (Dill, Reference Dill2016) classification as the 24dE type because they are hosted in peralkaline igneous rocks and are carriers of REE–Y ores. According to the classification in Černý and Ercit (Reference Černý and Ercit2005), they belong to the Rare Elements class and the NYF family, as they are rich in REE, Nb, Y and F, and are associated with A-type granites in environments with low pressures and temperatures. The study of genthelvite brings new constraints on the conditions of the magmatic-hydrothermal transition in the albite-enriched granite system.
Previous work
Geological setting
The Pitinga Province is located (Fig. 1) in the southern portion of the Guyana Shield (Almeida et al., Reference Almeida, Hasui, Brito Neves and Fuck1981), in the Tapajos-Parima Tectonic Province (Santos et al., Reference Santos, Hartmann, Gaudete, Groves, McNaughton and Fletcher2000). The volcanic rocks of the Iricoume Group (Veiga et al., Reference Veiga, Nunes, Fernandes, Amaral, Pessoa and Cruz1979) predominate in the Pitinga Province (Fig. 1) and have 207Pb/206Pb zircon ages between 1881 ± 2 and 1890 ± 2 Ma (Ferron et al., Reference Ferron, Bastos Neto, Lima, Costi, Moura, Prado and Galarza2006). They consist mostly of effusive and hypabyssal rhyolites, highly welded ignimbrites, ignimbritic tuffs, and surge deposits formed in a subaerial environment with cyclic effusive and explosive activities (Pierosan et al., Reference Pierosan, Lima, Nardi, Bastos Neto, Campos, Jarvis, Ferron and Prado2011; Simões et al., Reference Simões, Almeida, Souza, Silva and Rocha2014). The Iricoume Group host the Madeira Granite (Fig. 1).
Figure 1. (a) Location map and (b) geological map of the Madeira Granite, Pitinga, Brazil (modified from Costi, Reference Costi2000).
The Madeira granite (Figs 1, 2) contains four facies (Horbe et al., Reference Horbe, Horbe, Costi and Teixeira1991; Lenharo et al., Reference Lenharo, Pollard and Born2003; Costi et al., Reference Costi, Borges, Dall'agnoll, Marini, Queiroz and Ramos2005, Reference Costi, Dall'Agnol, Pichavant and Ramo2009; Bastos Neto et al., Reference Bastos Neto, Pereira, Ronchi, Lima and Frantz2009). The oldest, mostly metaluminous porphyritic amphibole–biotite granite (1824 ± 2 Ma, Costi et al., Reference Costi, Dall'agnoll and Moura2000), contains plagioclase-mantled K-feldspar megacrysts, sometimes have reverse-zoned K-feldspar-mantled plagioclase ovoids, and is usually referred to as the ‘rapakivi’ subfacies. The amphibole–biotite granite was followed by the metaluminous biotite granite (1822 ± 2 Ma, Costi et al., Reference Costi, Dall'agnoll and Moura2000). The younger facies are the hypersolvus porphyritic alkali feldspar granite (1818 ± 2 Ma, Costi et al., Reference Costi, Dall'agnoll and Moura2000) and the albite-enriched granite (Fig. 2). The latter is the host to the pegmatites investigated in this work. The age of the albite-enriched granite is only very roughly constrained at 1822 ± 22 Ma (Bastos Neto et al., Reference Bastos Neto, Ferron, Chauvet, Chemale, Lima, Barbanson and Costa2014) due to the metasomatic alteration of zircons. According to Costi (Reference Costi2000), these younger facies were emplaced simultaneously. The hypersolvus granite has K-feldspar phenocrysts in a fine- to medium-grained matrix composed dominantly of K-feldspar and quartz.
Figure 2. Geological map of the albite-enriched granite (modified from Minuzzi, Reference Minuzzi2005).
The albite-enriched granite
The albite-enriched granite is an oval-shaped body with a surface outcrop of ~2×1.3 km. It is divided into subfacies of a core albite-enriched granite and a border albite-enriched granite. The core albite-enriched granite is a peralkaline subsolvus granite, porphyriticto-seriate in texture, fine-to-medium grained, and composed of quartz, albite and K-feldspar in approximately equal proportions (25–30%). The accessory minerals are cryolite (5%), polylithionite (4%), green–brown mica (3%), zircon (2%) and riebeckite (2%). Pyrochlore, cassiterite, xenotime, columbite, thorite, magnetite and galena occur in minor proportions. The border albite-enriched granite is peraluminous and exhibits types of texture and essential mineralogy similar to that of the core albite-enriched granite, except for being richer in zircon, with fluorite instead of cryolite. Iron-rich silicate minerals are absent, having been completely removed by autometasomatic processes (Costi et al., Reference Costi, Dall'agnoll and Moura2000, Reference Costi, Dall'Agnol, Pichavant and Ramo2009).
The Madeira rare-metal deposit
The Pitinga Province is the largest Sn producer in Brazil. The alluvial ore deposits were discovered in 1979 (Veiga et al., Reference Veiga, Nunes, Fernandes, Amaral, Pessoa and Cruz1979) and are almost exhausted. The primary ores are associated mainly with the Madeira deposit, which has been exploited since 1989. The Madeira deposit corresponds to the albite-enriched granite facies of the Madeira Granite and presents disseminated ore with grades of 0.17 wt.% Sn (cassiterite), 0.20 wt.% Nb2O5) and 0.024 wt.% Ta2O5 (both in pyrochlore and columbite). The potential by-products of the disseminated ore are F (4.2 wt.% cryolite), Y and heavy rare earth elements (HREE) (xenotime), Zr and Hf (zircon), Th (0.07 wt.% ThO2, thorite), and U (pyrochlore). Regardless of the disseminated character of the albite-enriched granite mineralisation, there are small zones of enrichment associated with the granite in which particular minerals are considerably abundant. These are: (1) thick pods and bands of the pegmatitic albite-enriched granite (~50 cm, rarely up to 10 m thick; Stolnik, Reference Stolnik2015) that show gradational contacts with the albite-enriched granite; These contain the same minerals as the core albite-enriched granite, although polylithionite, riebeckite, xenotime and thorite are more abundant and of large size; (2) border pegmatites at the contact between the border albite-enriched granite and the older facies (Fig. 2) characterised by increased size and amounts of quartz and zircon, advanced alterations of K-feldspar and biotite and by local enrichments in fluorite, polylithionite, thorite and secondary hematite (Lengler, Reference Lengler2016); (3) several bodies of massive cryolite intercalated with core albite-enriched granite and hypersolvus granite, which are sub-horizontal, up to 300 m long and 30 m thick and composed of cryolite (~87 vol.%), quartz, zircon and feldspar (Minuzzi et al., Reference Minuzzi, Bastos Neto, Pereira and Flores2006); and (4) pegmatite veins (Paludo et al., Reference Paludo, Bastos Neto, Pereira and Botelho2018; Ronchi et al., Reference Ronchi, Althoff, Bastos Neto and Dill2019) which are not mappable, occur more commonly in the central, northern and northwest parts of the core albite-enriched granite, and have thicknesses ranging from a few centimetres up to 2 m. Genthelvite occurs only in these pegmatite veins and was first identified by Ronchi et al. (Reference Ronchi, Bastos Neto, Gedoz, Weber, Pereira, Andrek, Frantz, Charão and Jost2011). There are two types of pegmatite veins visible in the whole mine front (140 to 210 meters above sea level) hosted by the core albite-enriched granite. The prevalent type is that of tabular bodies emplaced in horizontal extension fractures. The other group is formed by tabular bodies emplaced in subvertical reverse fault planes. These fractures and faults served as a conduit for the fluids, with transportation from southwest to northeast, in a compressive system, with horizontal tension and at low solidus temperature (Ronchi et al., Reference Ronchi, Althoff, Bastos Neto and Dill2019). These have been separated into three groups in Paludo et al. (Reference Paludo, Bastos Neto, Pereira and Botelho2018) on the basis of composition and modal values as: (1) those rich in amphibole (riebeckite, fluoro-arfvedsonite and fluoro-eckermannite), with intermediate contents of K and Na; (2) rich in polylithionite, with high K contents; and (3) cryolite-rich, with high Na contents.
Evolution of the albite-enriched granite system
Bastos Neto et al. (Reference Bastos Neto, Pereira, Ronchi, Lima and Frantz2009, Reference Bastos Neto, Ferron, Chauvet, Chemale, Lima, Barbanson and Costa2014) consider that the A-type magmatism in Pitinga evolved in a post-collisional extensional setting, probably in a within-plate scenario in which extensional and trans-tensional tectonic regimes dominated. In this context, the albite-enriched granite magma would have been related to the isotherm rise, which occurred when the mantle fluid ascended further into the crust promoting fenitisation-type reactions (Martin, Reference Martin2006) in rocks previously enriched in Sn, and introduced elements such as F, Nb, Y, REE and Th in anomalous concentrations. The input of a F-rich fluid generated metasomatism causing the rock to become fusible.
Lenharo (Reference Lenharo1998) and Costi (Reference Costi2000) considered that the albite-enriched granite magma evolved towards an extremely Na-, F-enriched residual melt. Costi (Reference Costi2000) interpreted that, at the point of H2O saturation, the extremely F-enriched residual fluid separated into an aqueous, relatively F-poor portion and a low-H2O, Na–Al–F-rich portion. The latter fraction resulted in the formation of massive cryolite bodies, whereas the H2O-rich fraction formed the associated quartz-, feldspar- and mica-bearing pegmatitic rocks. In accordance with Bastos Neto et al. (Reference Bastos Neto, Pereira, Ronchi, Lima and Frantz2009), extreme fluorine enrichment in the residual melt is unlikely to have been attained because the F content was buffered by crystallisation of magmatic cryolite (Dolejs and Baker, Reference Dolejs and Baker2007). Furthermore, fluid-inclusion data (Bastos Neto et al., Reference Bastos Neto, Pereira, Ronchi, Lima and Frantz2009; Ronchi et al., Reference Ronchi, Bastos Neto, Gedoz, Weber, Pereira, Andrek, Frantz, Charão and Jost2011) show that the massive cryolite deposit was formed from an aqueous, saline hydrothermal fluid. The higher homogenisation temperature of 400°C, measured in massive cryolite, determines the minimum starting temperature for the hydrothermal process.
Methods
Genthelvite crystals from several pegmatitic veins were described and identified by combining optical properties, using compositional analyses and powder X-ray diffraction data. The pegmatite veins are spread throughout the core albite-enriched granite and are not mappable due to their small size (up to 2 m thick). Sampling was carried out mainly in the central area of the core albite-enriched granite, on the surface of the open pit. Over 50 thin sections of the pegmatites were analysed and, among those which contained genthelvite, 10 were examined by back-scattered electron microscopy (BSE), with qualitative analysis using an energy-dispersive X-ray detector (Zeiss, model EVO MA10) at the Centre for Microscopy and Microanalysis in Universidade Federal do Rio Grande do Sul (UFRGS).
Mineral compositions of genthelvite were obtained using electron probe micro-analysis (EPMA) (JEOL JXA-8230) at the EPMA Laboratory of the Universidade de Brasília (UnB). The operation conditions were: 15 kV accelerating voltage and 10 nA beam current (F, Mg, Zn, Al, Si, Hf, Nb, P, Cl, S, Bi, Ti, Mn, Y, Ta, Sn, Ca, Zr, Fe, V and Rb), and 20 kV and 50 nA (Na, Er, Tm, Yb, Ho, Lu, K, Pb, Dy, Tb, Sm, Gd, Eu, Sr, Th, Pr, Nd, Ce, La, Ba and U), 1 μm beam diameter, and interference corrections were applied in all cases of peak overlap. The wavelength dispersive X-ray spectrometer (WDS) crystals used were: TAP (Si, Zn, Na, Al); PETJ (Nb, P, Hf, Cl, S, K, Bi, Sr, Y, Ta, Sn, Th, Pb); PETH (Rb, Zr, U); LIF (Ti, Mn, Sm, Eu, Gd, Dy, Er, Ho, Tb, Tm, Yb, Lu), LIFH (Ca, Fe, Ba, V, La, Ce, Pr, Nd); and LDE1 (F). The counting times on the peaks were 10 s for all elements, and half that time for background counts on both sides of the peaks. The following natural and synthetic standards were used: microcline (Si, K, Al); albite (Na); apatite (P; Ca); andradite (Fe); topaz (F); forsterite (Mg); vanadinite (V, Pb, Cl); pyrite (S); MnTiO3 (Mn); YFe2O12 (Y); LiNbO3 (Nb); LiTaO3 (Ta); MnTiO3 (Ti, Mn); ZnS (Zn); Bi2O3 (Bi); RbSi (Rb); BaSO4 (Ba); baddeleyite (Zr); PbS (Pb); HfO2; SrSO4 (Sr); SnO2; ThO2; UO2; and synthetic REE-bearing glasses.
Crystallographic investigations were performed in the X-ray diffraction laboratory at UFRGS using a Siemens D5000 X-ray Diffractometer (XRD) with a scanning step of 0.05°2θ, a time of 1 s, between 5 and 100°2θ, CuKα radiation (1.5418 Å) and a Ni filter. Crystallographic parameters were determined using the UnitCell program (Holland and Redfern, Reference Holland and Redfern1997), and the diffractions of 19 (reflections) faces were processed. The error in the processed values was 0.00017, with 95% reliability. The crystallographic information files have been deposited with the Principal Editor of Mineralogical Magazine and are available as Supplementary material (see below).
Results
Mineralogy and petrography
The contact of the pegmatite veins with the host rock is abrupt, marked by a thin, well-defined border, typically on the scale of centimetres. From the border towards the centre of the bodies, there is a systematic increase in the size of the minerals, though without the establishment of a clear zoning pattern. The interior of the bodies is homogeneous, characterised by anhedral to subhedral minerals. In most veins, the pegmatitic texture is marked by crystals (up to 10 cm) of polylithionite, quartz, cryolite, microcline and albite, by crystals (up to 7 cm) of riebeckite, xenotime and genthelvite, and crystals (up to 3 cm) of thorite, galena and, more rarely, zircon, cassiterite and gagarinite.
Both the horizontal and the subvertical pegmatite veins exhibit the same mineralogical composition. The paragenetic succession is illustrated in Fig. 3. The major mineral assemblage consists of quartz, microcline, cryolite, polylithionite and riebeckite. The minor minerals are xenotime, thorite, gagarinite-(Y), and genthelvite. Accessory minerals are zircon, cassiterite, pyrochlore, columbite, galena, sphalerite, native lead, native bismuth, pyrite, hematite and chlorite. The matrix primarily consists of albite, quartz and grains are commonly anhedral, colourless in natural 0.3–1 cm. Additionally, polylithionite, cryolite and riebeckite are present. The entire magmatic paragenesis has undergone significant alteration due to the influence of F-rich hydrothermal fluids, ultimately leading to the formation of the secondary phases depicted in Fig. 3.
Figure 3. Paragenetic evolution of the genthelvite-bearing pegmatite veins. The line thickness is indicative of the abundance of the mineral. The precursor minerals for the most important replacement reactions are indicated by arrows.
Genthelvite crystals from the pegmatite veins hosted by the core albite-enriched granite have sizes from 1.0 mm to 4.7 cm and present a light pink colour in macroscopic samples (Fig. 4). Optically, the grains are commonly anhedral, colourless in natural light and isotropic in polarised light (Fig. 5a,b). In the pegmatites genthelvite occurs predominantly as massive aggregates of crystals surrounding polylithionite and magmatic quartz and includes the accessory minerals pyrochlore, thorite and zircon (Fig. 5a–d). Subordinately, genthelvite occurs filling voids and microfractures in magmatic quartz (Fig. 5e) and polylithionite phenocrysts or arranged interstitially in the matrix with magmatic quartz and orthoclase (Fig. 5f).
Figure 4. Macroscopic sample of genthelvite from the pegmatite veins hosted by the core albite-enriched granite. Genthelvite (Ghv) occurs surrounding polylithionite (Pln) crystals.
Figure 5. Microphotographs and BSE image of genthelvite from the pegmatite veins hosted by the core albite-enriched granite: (a) typical genthelvite from the pegmatites, with triangular cleavage, associated with polylithionite, plane polarised light (PPL); (b) same as in (a), crossed polarised light (XPL); (c) genthelvite filling the space between polylithionite crystals, PPL; (d) genthelvite associated with magmatic quartz, pyrochlore, thorite and zircon, PPL; (e) genthelvite filling voids in magmatic quartz, XPL; (f) genthelvite in the matrix with magmatic quartz and microcline, XPL; (g) BSE image of genthelvite with microfractures filled by hydrothermal cryolite; (h) hydrothermal quartz along genthelvite growth lines, XPL. Abbreviations: Crl II = hydrothermal cryolite, Ghv = genthelvite, Mcc = microcline, Pcl = pyrochlore, Pln = polylithionite, Qz I = magmatic quartz, Qz II = hydrothermal quartz, Thr = thorite and Zrn = zircon. Mineral symbols after Warr (Reference Warr2021).
The contact with polylithionite is rectilinear and slightly reactive. The contact with magmatic quartz is undulated and shows reactive rims. The contact with pyrochlore, zircon and thorite is undulated and reactive, and these minerals have a partially dissolved aspect. The samples show evidence of hydrothermal alteration. Genthelvite is characterised by corrosion features such as cavities and microfractures which are commonly filled by hydrothermal cryolite (Fig. 5g). Hydrothermal quartz also occurs associated with genthelvite, specifically filling the channels opened along genthelvite growth lines (Fig. 5h). On the basis of these characteristics, genthelvite is considered a mineral of late crystallisation, preceded by the crystallisation of polylithionite and early magmatic quartz and formed before the hydrothermal cryolite.
Genthelvite composition
The helvine-group minerals can be represented by the general formula A2+8Be6Si6O24S2. Structural formulae calculated on the basis of 26 O and S atoms and Be = 6 atoms per formula unit (apfu) (Zito and Hanson, Reference Zito and Hanson2017) show cations systematically in deficit on the A site and in excess on the Si site. Therefore, in this study calculations were performed with the following assumptions: the crystal structure is charge balanced; the anion site is fully occupied (i.e. O + S = 26); the Si site is fully occupied by P5+, Si4+, Ti4+ and Al3+ (preferential order of occupation is P > Si > Ti > Al according to ionic potential); the excess in the Si site (i.e. IVSi > 6 apfu) is allocated in the IVBe site with the preferential order of occupation Al > Ti > Si > P with Be2+ calculated to yield a total of Be + Si = 12 apfu (i.e. site IVBe = 6) (Dunn, Reference Dunn1976; Finch, Reference Finch1990); and the A site has a vacancy (□) and is occupied by U4+, A3+ (Ce, Pr, Nd, Sm, Eu, Gd, Ho and Er), A2+ (Zn, Fe, Mn, Mg, Pb and Ba) and A+ (Na and K) (i.e. site IVA = 8 – □).
Representative compositions for genthelvite are given in Table 1. The low analytical totals are probably due to the existence of vacancies within the genthelvite structure, hydration as a result of hydrothermal alteration, and the inherent errors associated with Be calculation. The elements P, Ti, Al, REE, Ba, Na and K have concentrations near the detection limits of the EPMA. Consequently, discussions regarding the incorporation of these elements into the crystal structure of genthelvite must be approached with caution. Additionally, low concentrations (hundreds to thousands of ppm) of V, Th, Cr, La, Dy, Tm, Yb, Lu, Bi, Ca, Ni, Sr and Cl were detected. These elements were not considered in the totals of the analyses and in structural calculations.
Table 1. Representative compositions from EPMA (wt.%) for Mn–Fe-rich genthelvite in the pegmatite veins hosted by the albite-enriched granite.
*BeO calculated considering a total of Be + Si = 12 apfu; n.d. = not detected.
Genthelvite has a homogeneous composition within individual grains and is characterised as a Mn–Fe-rich genthelvite. It represents a solid solution within the genthelvite–danalite–helvine system, with relatively limited substitutions occurring between Zn2+, Fe2+ and Mn2+. However, genthelvite does exhibit moderate overall compositional variability, falling within the range of 36.96 to 49.45 wt.% ZnO, 2.10 to 10.94 wt.% FeO, and 0.61 to 3.03 wt.% MnO. The composition of genthelvite plotted in terms of the relative proportions of Zn, Fe and Mn (expressed as percentages of [Zn + Fe + Mn] atoms) (Fig. 6) reflect the predominance of compositions along the upper part of the Zn–Fe join, although invariably the presence of a small component of helvine occurs.
Figure 6. Compositions of genthelvite, danalite and helvine expressed as percentages of Zn + Fe + Mn atoms.
In genthelvite, other cations that occupy the structural site of Zn, in addition to Mn and Fe, are uncommon, and trace concentrations of K, Ca and Mg are the most commonly reported. In genthelvite from Pitinga the maximum values were 0.062 wt.% MgO, 0.061 wt.% PbO, 0.14 wt.% BaO, 0.33 wt.% Na2O and 0.05 wt.% K2O. Uranium concentrations have not been reported in genthelvite from other localities, but in Pitinga it occurs in all genthelvite samples with 0.13 to 0.25 wt.% UO2. Additionally, the genthelvite samples have high contents of REE (maximum 0.40 wt.% REE2O3) relative to that from Cheyenne Canyon (USA, 4.1 ppm REE2O3; Zito and Hanson, Reference Zito and Hanson2017) and other helvine-group minerals such as the Mn–Zn-rich danalite from Sucuri (Brazil, maximum 363 ppm REE2O3; Raimbault and Bilal, Reference Raimbault and Bilal1993) and the Zn–Fe-rich helvine from Dajishan (China, maximum 13 ppm REE2O3; Raimbault and Bilal, Reference Raimbault and Bilal1993). The average concentration of light rare earth elements (LREE) (723 ppm) is slightly higher than that of HREE (565 ppm) in genthelvite from Pitinga.
Genthelvite from this work exhibits a REE normalised distribution pattern (Fig. 7) that closely resembles that of the host pegmatite, except for a positive anomaly in Pr and Eu, and the absence of La, Tb, Dy, Tm, Yb and Lu. The mineral incorporated LREE preferentially, although helvine-group minerals have high affinity with HREE (Raimbault and Bilal, Reference Raimbault and Bilal1993; Deer et al., Reference Deer, Howie, Wise and Zussman2004).
Figure 7. Chondrite-normalised REE patterns (chondrite of Anders and Grevesse, Reference Anders and Grevesse1989).
The strong negative correlation (R 2 = 0.963) between Zn and the sum of the cations Fe, Mn, Mg, Pb, Ba, Na, K, U and REE attest to their location at the A site (Fig. 8a). The positive correlation of Mn and Fe (R 2 = 0.672, Fig. 8b) is evidence of their concomitant entrance when substituting for Zn. Concerning the elements in the IVBe and IVSi structural sites, the values are similar in all the samples, with little variations in the concentrations of Si (29.30 to 32.19 wt.% SiO2), S (5.07 to 5.52 wt.% S), and calculated Be (11.73 to 12.49 wt.% BeO). The BeO concentration is similar to those found by inductively coupled plasma atomic emission spectroscopy (ICP–AES) analyses in danalite, which had an average content of 13.1% BeO (Raimbault and Bilal, Reference Raimbault and Bilal1993). In addition, maximum values of 0.38 wt.% P2O5, 0.30 wt.% TiO2 and 0.25 wt.% Al2O3 were observed. In contrast to all other minerals found in the pegmatites, genthelvite does not contain fluorine. This absence is probably due to the fact that F has an ionic radius that is too small for sodalite-type structures. The ΣA site gave a negative correlation with Si + Al + P + Ti (R 2 = 0.900, Fig. 8c), meaning that the Si substituting for Be is charge balanced by the presence of vacancies at the A site.
Figure 8. Binary diagrams for genthelvite from the pegmatites associated with the albite-enriched granite: (a) ΣA site (– Zn) versus Zn; (b) Mn versus Fe; and (c) ΣA site versus Be and ΣA site versus Si + Al + P + Ti. Concentrations are expressed in apfu.
Lattice parameters of genthelvite
Genthelvite from the pegmatites associated with the albite-enriched granite have an average a parameter of 8.127 Å, varying between 8.117 Å and 8.134 Å (error of 0.00017 Å), which is in accordance with the expected values for this mineral (Table 2). In the helvine-group minerals the constancy of the structural dimensions of the BeO4 and SiO4 tetrahedra (Hassan and Grundy, Reference Hassan and Grundy1985) imply that variations in the unit cell parameter are related to the Mn–Zn–Fe proportions (Oftedal and Saebø, Reference Oftedal and Saebø1963), as well as the proportions of other elements in the A site. For genthelvite from Pitinga and other localities (Table 2), the correlation between the average a parameter and the average Mn content is strongly positive (R 2 = 0.84, Fig. 9a), but with the average Zn + Fe content this parameter presents a strong negative trend (R 2 = 0.89, Fig. 9b). Though there is a moderate overlap of unit cells, as indicated by the error bars, the correlation can be associated tentatively with the ionic radii of the elements. The ionic radius is larger for Mn (0.66 Å) and smaller for Zn (0.60 Å) and Fe (0.63 Å) (Shannon, Reference Shannon1976).
Table 2. Crystallographic parameters of genthelvite from the pegmatites (Madeira deposit, Pitinga Mine) and other localities, in descending order of the average of parameter a.
Abbreviations: n.a. = not available.
Figure 9. Correlation of the unit cell parameter a (Å) versus the Mn (a) and Zn + Fe (b) concentrations of genthelvite from Pitinga (this work, filled circle) and other localities (open circles, Table 2). Mn, Zn and Fe are expressed as wt.%. The bars indicate the error for the parameter a.
Discussion
Geological environment of genthelvite occurrences
Beryllium is a rare element both in meteorites and on Earth, but it is a crustal element par excellence, with an average of 2.1 ppm BeO in rocks of the upper continental crust, in contrast to 1.4 ppm BeO in the lower crust and 0.07 ppm BeO in the mantle (Rudnick and Gao, Reference Rudnick, Gao and Rudnick2005). The first paragenesis of magmas are formed by minerals whose structure inhibits the capture of Be in melting. Therefore, Be enrichment occurs in the final stages of magmatic crystallisation, mainly in granitic pegmatites and alkaline rocks (Grew, Reference Grew and Grew2002). In the later stages of differentiation, there is a supersaturation of SiO2 and accumulation of alkalis and volatiles, allowing the formation of Be minerals and quartz (Pulz et al., Reference Pulz, Cunha and Formoso1998).
A significant majority of occurrences of genthelvite are in pegmatites and in late-formation rocks as hydrothermal veins, greisens and skarns, associated with alkaline to peralkaline granites and syenites (Table 3). These occurrences have in common a highly evolved magma enriched in high-field-strength elements and associated hydrothermal events and/or metasomatism. The genthelvite investigated occurs in pegmatite veins that represent the most evolved fluid of a peralkaline magmatic system, but contrast with other genthelvite occurrences of the world showing an unusual mineralogical association and geochemical trend.
Table 3. The genthelvite investigated from Madeira deposit, Pitinga Mine, Brazil and comparison with other occurrences.
References: (1) This work; (2) Glass et al. (Reference Glass, Jahns and Stevens1944); (3) Zito and Hanson (Reference Zito and Hanson2017); (4) Langhof et al. (Reference Langhof, Holtstam and Gustafsson2000); (5) Vasil'ev (Reference Vasil'ev1961); (6) Von Knorring and Dyson (Reference Von Knorring and Dyson1959); (7) Raimbault and Bilal (Reference Raimbault and Bilal1993); (8) Bilal (Reference Bilal2013); (9) Miranda (Reference Miranda2018); (10) Dunn (Reference Dunn1976); (11) Haapala and Ojanperã (Reference Haapala and Ojanperã1972); (12) Haapala and Lukkari (Reference Haapala and Lukkari2005); (13) Botelho (Reference Botelho1992); (14) Freitas (Reference Freitas2000); (15) Lunts and Saldau (Reference Lunts and Saldau1963); (16) Morgan (Reference Morgan1967); (17) Clark and Fejer (Reference Clark and Fejer1976); (18) Es'kova (Reference Es'kova1957); (19) Oftedal and Saebø (Reference Oftedal and Saebø1963); (20) Larsen (Reference Larsen1988); (21) Bollinberg and Petersen (Reference Bollinberg and Petersen1967); (22) Dushin et al. (Reference Dushin, Prokopchuk, Koz'min, Zhuklin and Trutnev2018); (23) Kingsbury (Reference Kingsbury1961); (24) Bulakh and Frank-Kamenetsky (Reference Bulakh and Frank-Kamenetsky1961); (25) Cianciulli and Verbeek (Reference Cianciulli and Verbeek2003); (26) Leavens et al. (Reference Leavens, Zullo and Verbeek2009)
*Findings by Kingsbury are considered dubious see e.g. Ryback et al. (Reference Ryback, Clark and Stanley1998)
Mineral assemblages of genthelvite worldwide
Considering the stability fields of minerals, genthelvite can be formed from willemite and phenakite; willemite can be altered to sphalerite; genthelvite can be altered to sphalerite and phenakite or bertrandite; and all of them together with quartz (Burt, Reference Burt1988). Therefore, commonly associated minerals with genthelvite are quartz, feldspar, micas and other Zn-bearing phases such as sphalerite, willemite and gahnite, as well as other Be-containing phases such as phenakite and bertrandite (Burt, Reference Burt1988). There is also willemite in these associations, especially in peralkaline rocks. Genthelvite with willemite, phenakite, Na-fluorides as gagarinite, weberite and pachnolite have been reported in metasomatic peralkaline rocks from Russia (Kudrin, Reference Kudrin and Korzhinskii1978). In the Ilimaussaq Complex, Greenland, willemite with chkalovite was reported as the only Be-bearing mineral, whereas genthelvite was found at a different location within the Complex (Metcalf-Johnson, Reference Metcalf-Johnson1977). In Mont St. Hilaire, Canada, willemite and genthelvite were reported, together with sphalerite and galena (Bank, Reference Bank1975; Dunn, Reference Dunn1976).
In this investigation, genthelvite is the only Be-bearing phase, and it is associated with polylithionite, riebeckite, quartz, microcline, albite, xenotime, cryolite and the accessory phases pyrochlore, columbite, zircon, gagarinite, galena, sphalerite and hematite. Beryl, willemite and phenakite or bertrandite were not observed. This occurrence is notably unusual because it lacks the helvine-group minerals danalite and helvine, which are commonly associated with genthelvite in other deposits. This absence is probably linked to the physical-chemical conditions discussed below. In peralkaline conditions, genthelvite associated with aluminous minerals such as beryl and topaz are restricted, but genthelvite with Na-fluorides are typical (Burt, Reference Burt1988), as observed in the genthelvite–cryolite association in the Pitinga pegmatites.
Controls on genthelvite composition
Crystallographic and structural data, the ionic radius of the A-site cations and the structural geometric model, indicate that complete miscibility should exist between the three end-members of the helvine–genthelvite–danalite solid solution (Hassan and Grundy, Reference Hassan and Grundy1985), however, there are apparent compositional gaps in the Zn–Fe, Zn–Mn and Mn–Fe trends, and pure danalite has not been observed in Nature (Oftedal and Saebø, Reference Oftedal and Saebø1963; Clark and Fejer, Reference Clark and Fejer1976; Dunn, Reference Dunn1976; Larsen, Reference Larsen1988; Perez et al., Reference Perez, Dusausov, Babkine and Pagel1990; Langhof et al., Reference Langhof, Holtstam and Gustafsson2000; Bilal, Reference Bilal2013). According to Antao and Hassan (Reference Antao and Hassan2010) the absence of pure danalite in Nature might simply indicate that another phase is more stable than danalite.
Genthelvite in this investigation has high concentrations of Zn and small concentrations of Fe in comparison with genthelvite from the Rockport Deposit, Massachusetts, USA (Dunn, Reference Dunn1976) and from the Cairngorm Mountains, UK (Clark and Fejer, Reference Clark and Fejer1976). It also has a higher Zn and lower Fe and Mn content than the more Mn–Fe-enriched genthelvite reported from Cumberland, Rhode Island, USA (Dunn, Reference Dunn1976) and from the Sucuri Granite in Brazil (Miranda, Reference Miranda2018). It is also different from the typical Fe–Mn-rich genthelvite reported in Norway (Oftedal and Saebø, Reference Oftedal and Saebø1963; Larsen, Reference Larsen1988), the Air Mountains of Niger (Perez et al., Reference Perez, Dusausov, Babkine and Pagel1990) and Uto, Sweden (Langhof et al., Reference Langhof, Holtstam and Gustafsson2000). Finally, with this work we demonstrate the existence of natural genthelvite in the upper Zn–Fe trend of the Zn–Fe–Mn ternary diagram (Fig. 6), filling a compositional gap.
Genthelvite is one of only two known silicates in which Zn and Be occur together. Regardless that Zn (ionic radius 0.60 Å; Shannon, Reference Shannon1976) and the much smaller Be (ionic radius 0.27 Å; Shannon, Reference Shannon1976) represent different chemical affinities, they both have the tendency to be concentrated by fractionated crystallisation and to adopt four-fold coordination (Burt, Reference Burt1988). Iron and Mn (IVMn = 0.66 Å, IVFe = 0.63 Å; Shannon, Reference Shannon1976) also adopt this coordination. Regardless of these coordination controls compositional variations in helvine-group minerals have been shown to have a direct association with temperature and S and O fugacity, rather than with the availability of Zn, Fe and Mn in the fluid. An investigation of several genthelvite crystals from a granitic massif (Antao and Hassan, Reference Antao and Hassan2010) showed that Mn-rich genthelvite formed at lower temperatures than Mn-poor genthelvite. In the Taghouaji Alkaline Complex, Niger, Mn-poor genthelvite occurs with sphalerite and galena in a low $f_{{\rm O}_2}$
and high $f_{{\rm S}_2}$
environment and a crystallisation temperature higher than 375°C, whereas the Mn-rich genthelvite occurs with hematite at temperatures of ~288°C (Perez et al., Reference Perez, Dusausov, Babkine and Pagel1990).
The Mn-poor composition of the genthelvite investigated here is probably influenced by its formation at relatively high temperatures, in a low $f_{{\rm O}_2}$
and high $f_{{\rm S}_2}$
environment. However, the significant variation in Zn–Fe content between individual grains (36.96 to 49.45 wt.% ZnO, 2.10 to 10.94 wt.% FeO) is probably controlled by the local mineral assemblage. In the magmatic stage of the pegmatite veins, Fe and Zn were buffered by the presence of riebeckite (~30 wt.% FeO + Fe2O3; ~2 wt.% ZnO) and polylithionite (~6 wt.% FeO; ~1 wt.% ZnO). In these minerals, the Mn content is typically <1 wt.% MnO (Paludo et al., Reference Paludo, Bastos Neto, Pereira and Botelho2018). Additionally, the available Pb and S combined to form magmatic galena. During the early hydrothermal stage, Fe was incorporated into altered thorite and altered pyrochlore, whereas both Fe and Mn were widely incorporated into secondary Mn–Fe-rich columbite. Zinc and S were incorporated into secondary sphalerite, and the remaining available S formed galena and pyrite. In the late hydrothermal stage, Fe contributed to the formation of hematite, surrounding all the previous formed minerals (Minuzzi et al., Reference Minuzzi2005; Bastos Neto et al., Reference Bastos Neto, Pereira, Ronchi, Lima and Frantz2009; Hadlich et al., Reference Hadlich, Bastos Neto, Botelho and Pereira2019). Genthelvite crystallised during the transition between the magmatic and hydrothermal stages of pegmatite evolution, following the crystallisation of riebeckite and polylithionite. Thus, the compositional variation in the genthelvite is closely related to the prior formation of these minerals, which played a significant role in buffering the Fe content relative to Zn in their immediate vicinity.
These variations in composition are also verified by the REE contents. Genthelvite from Pitinga has a REE pattern enriched in LREE relative to its host pegmatite. The average LREE content is slightly higher than that of HREE content, which differs from those of other localities, which have higher HREE contents. The HREE enrichment in relation to the LREE in the helvine-group minerals from Sucuri, Brazil and Dajishan, China (Raimbault and Bilal, Reference Raimbault and Bilal1993) was attributed by these authors only to crystallographic controls. The higher concentration of HREE in danalite from Cheyenne, USA (Zito and Hanson, Reference Zito and Hanson2017), was attributed to the presence of late F-enriched fluids. At Pitinga, the REE are primarily concentrated in xenotime (with a predominance of HREE as Dy, Yb and Lu, Bastos Neto et al., Reference Bastos Neto, Pereira, Pires, Barbanson and Chauvet2012) and polylithionite. As these minerals crystallise before genthelvite, the REE content in genthelvite is lower and skewed towards the LREE, even though the late-stage fluids are rich in fluorine. The Eu positive anomaly probably reflects a reducing magma in which the Eu2+ could preferentially substitute for Zn2+ and because of the crystallisation of albite instead of anorthite in the pegmatites.
The possibility that the presence of REE and U in genthelvite might result from hydrothermal alteration cannot be ruled out. In such a scenario, the incorporated U might have originated from the alteration of U–Pb–LREE-bearing pyrochlore into columbite during the early hydrothermal stage of the pegmatite veins (Hadlich et al., unpublished work). The released Pb was incorporated predominantly into secondary galena, whereas the U found partial accommodation within secondary U-rich columbite, and solid solutions of thorite–coffinite–xenotime and zircon.
Origin of genthelvite
Few Be-bearing minerals form in the magmatic stage of pegmatite consolidation, with beryl being dominant (Černý, Reference Černý and Grew2002). Paragenetically late (supercritical-to-hydrothermal) beryllium minerals are divided in two broad categories: (1) alteration products of the early phases of Be; (2) minerals occuring in miarolitic cavities and fissures (Černý, Reference Černý and Grew2002). In the pegmatites investigated, genthelvite is observed as massive aggregates of crystals surrounding pegmatitic crystals and including accessory minerals. This observation can be interpreted as the crystallisation of a late-evolved fluid, wherein Be was preserved though complexation (e.g. with F) as no other Be mineral is present.
Compositional zoning or intergrowths between helvine-group minerals is relatively common (Haapala and Ojanperä, Reference Haapala and Ojanperã1972; Clark and Fejer, Reference Clark and Fejer1976; Perez et al., Reference Perez, Dusausov, Babkine and Pagel1990; Antao and Hassan, Reference Antao and Hassan2010), and is attributed to changes in the physical-chemical conditions during crystallisation, such as temperature and S fugacity. The preservation of this zoning or intergrowths would require low-temperature crystallisation and a rapid crystallisation process (Antao and Hassan, Reference Antao and Hassan2010). On the basis of these observations, it is plausible to infer that the lack of zoning in the genthelvite grains signifies diffusion between Zn, Fe and Mn. This diffusion process is probably facilitated by a relatively slow crystallisation within the late-evolved fluids of pegmatite formation, which occurred under relatively high temperatures.
The mineralogical and petrographic variations of the albite-enriched granite were mapped and described in detail in Bastos Neto et al. (Reference Bastos Neto, Pereira, Ronchi, Lima and Frantz2009), and the fluid-inclusion assemblies or associations in Ronchi et al. (Reference Ronchi, Bastos Neto, Gedoz, Weber, Pereira, Andrek, Frantz, Charão and Jost2011). In these investigations, it was concluded that the large hydrothermal massive cryolite deposit in the centre of albite-enriched granite is part of an evolutionary process of a magma originally rich in volatiles, which during its polyphase crystallisation process resulted in the exsolution of hydrothermal saline deuteric fluids (salinity between 0 and 25% eq. NaCl and homogenisation temperatures from 100 to 400°C). Furthermore, the authors concluded that these fluids lowered the solidus of the system, within the albite-enriched granite dykes of the pegmatitic albite-enriched granite and pegmatite veins. In this process, phases rich in microcline, polylithionite, cryolite and genthelvite were formed and hydrothermal alteration was promoted, resultiing in albitisation, silicification, clay-formation, fluoritisation and oxidation of iron-rich minerals.
In the pegmatite veins, hosted by the albite-enriched granite, the sequence of genthelvite formation following riebeckite and polylithionite suggests that genthelvite is one of the last minerals to crystallise. However, it does form prior to hydrothermal cryolite, which is observed corroding genthelvite crystals. This observation emphasises the strong association of genthelvite with the transitional period between the late magmatic and early hydrothermal stages of pegmatite evolution.
Genthelvite formation conditions
Genthelvite is a rare mineral compared to other Be-bearing minerals or even to other members of the helvine group, as a result of its small stability field (Burt, Reference Burt1988). The elements that constitute genthelvite (Zn, Mn, Fe, Be, Si and S) are commonly found as trace elements in highly fractionated granitic systems, therefore, this mineral is typical of systems at a late stage of differentiation, in which stability is due to local and transient conditions, generally atypical in the consolidation of granitic pegmatites, including low alumina activity and relatively reducing conditions that facilitate the coexistence of sulfides and silicates (Burt, Reference Burt1988; Bilal and Fonteilles, Reference Bilal and Fonteilles1988).
Genthelvite stability in a paragenesis is restricted to systems with low S activity (Burt, Reference Burt1988). Because of the differing chalcophile behaviour of Zn>>Fe>Mn, in systems with high SO–1 (the acid anhydride of H2S), the Zn2SiO4 component would have been destabilised to form an assemblage with sphalerite and quartz (Burt, Reference Burt1988). In contrast, under low SO–1 conditions danalite and helvine are not stable, and the instability of the FeS and MnS components will lead to the formation of silicates or oxides (Burt, Reference Burt1988). The low content of Fe in genthelvite also indicates high O2 fugacity during crystallisation, with the crystallisation of hematite (Burt, Reference Burt1980).
The wide compositional variation in helvine-group minerals, genthelvite in the core and danalite in the border, in the albitites associated with the Sucuri Granite (Brazil) suggest that the increased alkalinity (albitisation) in the system favoured genthelvite growth and the subsequent increase in S fugacity favoured danalite crystallisation by the reaction: Zn8Be6Si6O24S2 + 8FeS2 ↔ Fe8Be6Si6O24S2 + 8ZnS + 4S2 (Miranda, Reference Miranda2018).
Within the albite-enriched granite, the pegmatites originated through a process of continuous magma fractionation, resulting in a peralkaline composition highly enriched in high-field-strength elements. The extensive crystallisation of microcline, albite and polylithionite effectively buffered the Al content in the later fluids, reducing the alumina activity within the system. Simultaneously, the late-stage magmatic processes involved in the genesis of the pegmatites were characterised by a substantial increase in fluorine content (Paludo et al., Reference Paludo, Bastos Neto, Pereira and Botelho2018). The pegmatites that are rich in amphibole have ~3.35 wt.% F, whereas the ones rich in polylithionite have ~4.80 wt.% F, and those rich in cryolite have an average of 37.32 wt.% F. The remarkably high concentration of fluorine, coupled with the crystallisation of magmatic galena, effectively lowered the fugacity of H2S, thus stabilising genthelvite during the transition from the late magmatic to the early hydrothermal stages of pegmatite evolution. The low Fe content in genthelvite can be attributed to the prior incorporation of Fe into riebeckite and polylithionite and indicates an O activity that is too elevated to favour danalite formation. The widespread formation of Mn and Fe oxides (columbite and hematite) during the hydrothermal stage further underscores the high O activity within the system.
Conclusion
This investigation of genthelvite in pegmatite veins hosted by the Madeira albite-enriched granite has yielded the following conclusions.
Among the various pegmatites hosted by the albite-enriched granite, the genthelvite-bearing bodies are rich in riebeckite, polylithionite and xenotime, confirming the observations by Paludo et al. (Reference Paludo, Bastos Neto, Pereira and Botelho2018).
Genthelvite was formed during the transition between the late magmatic and early hydrothermal stages of pegmatite evolution. It stands out as the sole Be-bearing mineral in this context. Its formation can be attributed to Be being preserved in the late-evolved fluid, probably by complexation with fluorine. Genthelvite crystallised surrounding polylithionite, riebeckite, quartz, xenotime, pyrochlore, thorite and zircon, and it was subsequently corroded by hydrothermal cryolite.
Genthelvite exhibits a homogeneous composition within individual grains, but the overall composition displays moderate variation. It is characterised as a Mn–Fe-rich genthelvite, forming a solid solution within the genthelvite–danalite–helvine system. The mineral features relatively high levels of Zn (36.96 to 49.45 wt.% ZnO), lower Mn content (0.61 to 3.03 wt.% MnO), and variable Fe content (2.10 to 10.94 wt.% FeO), in comparison to the compositions found in other locations, and fills a compositional gap along the upper part of the Zn–Fe join in the Zn–Fe–Mn ternary diagram. Genthelvite exhibits elevated U levels (up to 0.25 wt.% UO2), and significant REE contents (up to 0.40 wt.% REE2O3). Notably, the average LREE content is higher than that of HREE, a deviation from compositions found in other locations, where HREE content tends to be the higher one. Additionally, genthelvite from this site lacks fluorine. Regardless of these compositional disparities, the crystallographic parameters (a = 8.127 Å) of genthelvite from the pegmatite veins closely resemble those determined for crystals from other localities.
The unusual composition of genthelvite is attributed to the buffering of Fe, F and HREE by minerals that crystallised before genthelvite. These include riebeckite, polylithionite, xenotime and magmatic cryolite.
Genthelvite was affected during the late hydrothermal stage, which was associated with F-rich aqueous fluids responsible for the formation of the massive cryolite deposit, and the disseminated hydrothermal cryolite within the albite-enriched granite. The highest observed fluid-inclusion homogenisation temperature of 400°C, measured in hydrothermal cryolite (Bastos Neto et al., Reference Bastos Neto, Pereira, Ronchi, Lima and Frantz2009), establishes the minimum initial temperature for the hydrothermal process. The structure of genthelvite facilitated the incorporation of Fe, Mn, Mg, Pb, Ba, Na, K, U and REE into the Zn2+ structural site. Additionally, it allowed the allocation of excess Si, Al, Ti and P to the IVSi and IVBe structural sites. The high levels of U and REE substituting for Zn, as well as Si substituting for Be, are charge balanced by the presence of vacancies at the A site.
This crystallochemical study of genthelvite has revealed that its formation occurred in an alkaline and subaluminous environment, under stable conditions within the late-evolved fluids, and at relatively high temperatures (>400°C), in a reducing environment. The substantial presence of fluorine and the crystallisation of magmatic galena effectively reduced the fugacity of H2S, ensuring the stability of genthelvite during the transition from the late magmatic to early hydrothermal stages in the evolution of the albite-enriched granite. The moderate variation in Fe content within genthelvite can be attributed primarily to the presence of riebeckite and polylithionite, which acted as buffers for Fe content in their immediate surroundings. In addition, the extensive formation of Mn and Fe oxides (columbite and hematite) during the hydrothermal stage, underscores an O activity that is too high to favour the formation of danalite.
Acknowledgements
This work was supported by Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) through the Project 405839/2013-2018 and for granting scholarship. The authors thank the reviewers and editors for their contributions to improving the manuscript.
Supplementary material
The supplementary material for this article can be found at https://doi.org/10.1180/mgm.2023.87.
Competing interests
The authors declare none.
