| Citation: | Mohamed Abd El Monsef, Mabrouk Sami, Fatma Toksoy-Köksal, Rainer Abart, Martin Ondrejka, Khaled M. Abdelfadil. Role of Magmatism and Related-Exsolved Fluids during Ta-Nb-Sn Concentration in the Central Eastern Desert of Egypt: Evidences from Mineral Chemistry and Fluid Inclusions. Journal of Earth Science, 2023, 34(3): 674-689. doi: 10.1007/s12583-022-1778-y
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The rare metals of Abu Dabbab area in the Central Eastern Desert of Egypt have been investigated for their mineralogy and conditions of precipitation using combination of EMPA and fluid inclusions studies, in order to delineate the source, mechanism of formation and evolutionary model for these economic metals. The (Ta-Nb-Sn)-bearing minerals at the Abu Dabbab area include columbite group minerals (CGMs), wodginite and cassiterite. In both granitic intrusion and its enclosed quartz veins, most of zoned CGMs and cassiterite grains are commonly characterized by a well-developed two-stage texture. Hence, columbite-(Mn) (CGM-Ⅰ) represents the early formed phase of CGMs that is characterized by high Mn# values (0.64–0.92) with quite low Ta# values (0.13–0.49). It was invaded by Ta-rich phases including tantalite-(Mn) (CGM-Ⅱ; Ta# = 0.13–0.49) and wodginite, which contain high Ta2O5 and SnO2 (17.91 wt.%). In regard to cassiterite, there are distinct compositional differences between the early-phase cassiterite (Cst-Ⅰ) and the late-phase one (Cst-Ⅱ), where the latter is enriched in Ta2O5, Nb2O5 and FeO. The chemistry and textural criteria of the early stage CGM-Ⅰ and Cst-Ⅰ, all are indicative of magmatic origin. While, the latter CGM-Ⅱ, wodginite and Cst-Ⅱ were influenced by the late magmatic Ta-rich fluids. Fluid inclusions microthermometry shows criteria of phase separation represented by both boiling and fluid immiscibility. The initial fluid was supposed to be of magmatic origin (magmatic CH4), that was consequently influenced by fluid mixing/dilution with post-hydrothermal/meteoric water with respect to the decompression process during uplift. Isochore construction gave rise to an estimate
In many localities all over the world, Ta, Nb and Sn oxides (e.g., columbite, tantalite, wodginite and cassiterite) commonly occur in the rare metal granites, pegmatites, and their related mineralized quartz veins (Sami et al., 2020; Zhu et al., 2015; Yin et al., 2013; Cuney et al., 1992). The genesis of Ta-Nb-Sn oxides of quartz veins in relation to their hosting granitic plutons is still unresolved. Previous studies reported that the genesis of rare metals hosted in granites is either of metasomatic or magmatic origin. Some authors suggested that the Ta-Nb-Sn ore minerals could be formed directly from the crystallization of a residual magmatic melt (Schwartz, 1992; Pollard, 1989), while others argued that these ore minerals were formed by late-stage hydrothermal fluids caused by the interaction of fertilized fluids at higher-level of consolidated granites (Dostal et al., 2015; Zhu et al., 2015; Wu et al., 2011). On the other hand, some other authors suggested a combination of both scenarios of origin, in which the metasomatism could strengthen the former existing magmatic rare-metal enrichment (Zoheir et al., 2020; Heikal et al., 2019; Helba et al., 1997).
Although Ta-Nb-Sn-bearing quartz veins are widely distributed in most Egyptian rare metals granitic plutons (e.g., Abu Dabbab, Nuweibi, Ineigi, Waggat and Abu Rushied) as a part of the Arabian-Nubian Shield (ANS, Fig. 1), less attention has been paid to the Ta-Nb-Sn oxide minerals in these veins than the host granitic plutons (e.g., Sami et al., 2022, 2020; Fawzy et al., 2020; Mahdy et al., 2020; Zoheir et al., 2020; Azer et al., 2019).
The Abu Dabbab area was chosen for the present study because it is considered as one of the most prospective localities for rare metals (especially; Ta and Sn) in Egypt. Moreover, the Abu Dabbab intrusion hosts economic concentrations of Ta-Sn mineralization with grades of Ta2O5 (243 g/t) and Sn (0.09%) with a total estimation of 40 Mt ores from measured and inferred resources, which is expected to become a major supplier to the world demand for Ta/Sn in industry (Küster, 2009). Although, comprehensive descriptions of the Abu Dabbab pluton and enclosed Ta-Nb-Sn mineralization have been documented in many previous literatures (e.g., Lehmann et al., 2020; Zoheir et al., 2020; Heikal et al., 2019; Abdalla et al., 2008; Renno, 1997; Mohamed, 1993). However, the evolution and effect of the magmatic to hydrothermal process for the generation of the Abu Dabbab mineralization are still insufficiently understood.
This paper presents the texture and chemical composition of Ta-Nb and Sn minerals in Abu Dabbab mineralized quartz veins and their hosting granitic intrusion in order to determine the influence of magmatic-hydrothermal and subsolidus processes during the ore formation. Moreover, detailed fluid inclusions (F.I.) studies were applied to the Nb-Ta-Sn bearing quartz veins to characterize the composition, properties and nature of the mineralizing fluid as well as the pressure-temperature conditions for entrapment. A combination of mineral chemistry and fluid inclusion thermobarometry serves as an effective means of tracking the hydrothermal system responsible for polymetallic (Ta-Nb-Sn) mineralization as a preliminary step for possible future exploration implications. Although the current study was carried out on specific area (i.e., Abu Dabbab), the rare metals granite in the Abu Dabbab resembles common style of mineralization in Egypt that could be linked and compared with other similar localities in Egypt and all over the world.
The Arabian-Nubian Shield (ANS) is an exposure of crystalline rocks on both the flanks of the Red Sea. This shield extends from Jordan to the north and Eritrea with Ethiopia to the south and includes Egypt to the west (Nubian Shield) and Saudi Arabia with Oman to the east (Arabian Shield) (Fig. 1). It mainly consists of Precambrian igneous and metamorphic rocks including gneisses, ophiolites, metavolcanics and metase-diments in island-arc system, granitoids, calc-alkaline volcanics, molasse-type sediments as well as mantle-derived fresh gabbros and sheeted dykes. The basement rocks in the Eastern Desert of Egypt were formed as a result of different thermo- tectonic events (e.g., subduction, collision, thickening, delamination and extension) of Pan‐African orogeny between 900 and 550 Ma (Abdelfadil et al., 2022; Abdel-Karim et al., 2021; Ali et al., 2021). The Eastern Desert of Egypt contains more than 17 rare metal granitic plutons which emplaced during the Late Neoproterozoic (590–540 Ma) and intruded into the older Early Neoproterozoic (640–715 Ma) gneisses and migmatites, ophiolite, gabbro-diorite-granodiorite rocks (Sami et al., 2018, 2017; Helba et al., 1997).
The Abu Dabbab is an area within the Central Eastern Desert of Egypt which is considered as a part of Nubian Shield belonging to the ANS. The Abu Dabbab albite granite mass is a quite small pluton (~2 km2), surrounded by several rock units as follows: metasedimentary-metavolcanic assemblage, serpentinites and their associated rock varieties (hornfels, metagabbro, amphibolites and talc-carbonates) that have been described previously as "ophiolitic mélange" (Fig. 2a). The albite granitic intrusion is generally ellipsoidal and commonly surrounded by various metamorphic rocks with knife-sharp contact. In the northwestern part of the granitic pluton, there is a narrow offshoot (goose neck), approximately 150 m in length. Abdalla et al. (2008) described petrographical zonal patterns in Abu Dabbab granite in form of lower and medium grained zones overlined by fine-grained roof. They reported (Op. cit) that the zoning profiles may be related to magmatic evolution. The contact of albite granite at the western part with metasedimentary rocks is shallow-dipping and characterized by marginal pegmatite zones "stockscheider" comprised of gigantic quartz crystals and quartz segregations "quartz cap". Near the contact with the metavolcanic rocks at the eastern part, an intrusion breccia zone was recognized where the albite granite is rich with small xenoliths composed of fragments of previously formed metavolcanics that become enveloped during the emplacement and solidification of the hosting albite mass. Occasionally, the Abu Dabbab pluton has been subjected to several alteration processes including amazonitization, greisenization, and the impregnation of secondary Mn-mineral patches (Küster, 2009; Renno, 1997). The albite granite body is cut across by series of NNW-SSE to NE-SW greisen and quartz veins (10–50 cm width), that commonly carry cassiterite, wolframite, topaz, beryl, fluorite, malachite and azurite (Fig. 2b). The stockwork of quartz/greisen veins is white to yellowish white in color, which varies in thickness from a few millimeters' veinlets to few of centimeters and may reach 40 cm in width. Green amazonite veins and fluorite veinlets were also detected in the intersection with other quartz/greisen veins and cutting through the albite granites. Tantalum-niobium-tin mineralization is restricted to the albite granite mass and related greisen/quartz veins. The Ta-Nb-Sn minerals are found as disseminated grains in pure white samples of albite granite that exhibit higher mineralization rather than those samples admixed with Fe-Mn oxides patches. Cassiterite and subordinate wolframite are also visually observed in the quartz/greisen stockwork veins and veinlets.
The quantitative electron-microprobe analyses of Ta-Nb-Sn oxides were performed using twelve representative samples from the Abu Dabbab granitic pluton and its enclosed quartz/greisen veins. Minerals were analyzed in thin polished sections to obtain information about the mineral texture, zonation and paragenetic assemblages of the samples. Back-scattered electron images (BSE) were acquired to study the compositional variation of individual mineral grains. The chemical compositions of minerals were measured using a CAMECA SX5 electron microprobe operated in wavelength-dispersive mode at the Department of Lithospheric Research, University of Vienna, Austria; some samples were analyzed further using JEOL JXA-8230 at the Electron Microscope Lab. of Central Laboratory, Middle East Technical University, Ankara, Turkey. The accelerating voltage was 15 kV and applied beam currents were 20 or 40 nA, with a beam diameter ranging between 1 and 5 μm. Natural and synthetic standards were used for calibration, and the PAP correction (Pouchou and Pichoir, 1991) was applied to the raw data. The following standard elements and minerals were used: columbite (Nb), Ta2O5 (Ta), SnO2 (Sn), metallic W (W), andradite (Fe), wollastonite (Ca), spessartine (Mn), synthetic TiO2 (Ti), PbO (Pb), metallic U (U), thorianite (Th) and zircon (Zr).
Samples from the mineralized quartz veins were selected and prepared as doubly-polished sections approximately 100 µm thick. Microthermometric measurements were performed on a LINKAM THMS 600 heating/freezing stage with a temperature range between -200 and +600 ℃ at the General Directorate of Mineral Research and Exploration (MTA), Ankara, Turkey. The heating rates of 5 ℃/min were applied to record most of temperatures related to phase changes; including the first melting temperature of ice or eutectic temperature (Teu), the final ice melting temperature (Tm ice), the homogenization temperature of CO2 (Th CO2), the homogenization temperature of methane (Th CH4), the total homogenization temperature (Th total) and decrepitation temperature (Td). An extreme care between 1 and 3 ℃/min was applied by temperature cycling and heating rate near the final ice melting temperature (Tm ice), carbonic phase melting (Tm CO2) and final clathrate melting (Tm clath.). Eighty (80) fluid inclusions were analyzed by cycles of freezing down and heating up. Fluid properties, mole fractions, compositions, and isochores were calculated using the software package of "FLUIDS 1" programs (Bakker, 2003) and online specific programs of Zhenhao Duan Research Group. Salinity, density and amount of substance fractions for each phase were calculated using ICE program (version 12/02; Bakker, 2003) based on the equation of state for Zhang and Frantz (1987) and Duan et al. (1992). Total salinity and bulk density were calculated with the help of BULK program (version 01/03; Bakker, 2003) using equations of state by Duan et al. (1996) and Bakker (1999). Salinities were expressed as wt.% NaCl eq., fluid components of various species (XH2O, XCO2, XCH4, XNa+ and XCl-) by mole fractions and densities (d) by g/cm3.
In both quartz veins and hosting granite, most of CGMs occur as euhedral to subhedral crystals that are commonly hosted by quartz (Figs. 3a–3c and 4a–4d). The BSE imaging revealed a two-generation texture in most of CGMs grains (early CGM-Ⅰ and late CGM-Ⅱ phases). The core of the CGMs is essentially composed of CGM-Ⅰ with an external zone of CGM-Ⅱ with gradual boundary between them.
Representative electron-microprobe results for CGMs from both quartz veins and their host granites are given in Tables S1 and S2. The gradual change of CGMs compositions is obviously shown by color variations of the BSE images from core to rim (Figs. 3a–3c and 4a–4d). All analytical results plotted in the columbite quadrilateral diagram (Fig. 5a) display the compositional features of CGMs. The CGMs of the Abu Dabbab fall in the right part of the Mn/(Fe + Mn) (Mn#) vs. Ta/(Nb + Ta) (Ta#) diagram (Fig. 5a). CGM-Ⅰ and CGM-Ⅱ form two separate groups, where the CGM-Ⅰ plots fall in the lower right quarter of the diagram, with high Mn# values (0.64–0.98) and quite low Ta# values (0.11–0.26), and thus classified as columbite-(Mn). While CGM-Ⅱ has restricted Mn# values (0.91–0.95) ratio with a gradual increase in Ta# values (0.35–0.68) ratio. Therefore, it shows a composition variation from columbite-(Mn) to tantalite-(Mn) (Fig. 5a). Quantitative analyses also reveal a restricted content of Ti and Sn, as only a few crystals of CGMs contain up to 1.58 wt.% TiO2 and up to 2.27 wt.% SnO2. Moreover, slightly low concentrations of U (up to 0.33 wt.% UO2) were only recorded in few CGMs crystals. The increase of Mn/(Mn + Fe) ratio of the CGMs crystals from core to rim could be attributed to the crystal fractionation process (Belkasmi et al., 2000). Most CGMs show homogeneous Nb-rich cores followed by Ta-rich rims with a progressive increase in Ta concentration (Figs. 3a–3c, 4a–4d and 5a), such typical zoning may be attributed to a normal fractionation pattern of decreasing Nb/Ta ratios from core to rim of crystals (Černý and Ercit, 1985).
Wodginite is found widely in Li-Cs-Ta pegmatites, but it rarely occurs in rare metals granites and associated quartz veins (Huang et al., 2002). However, wodginite is a kind of common rare mineral in the Abu Dabbab quartz veins and their host granite. It is closely associated with CGM-Ⅱ (Figs. 4b–4c) and could be formed as a secondary mineral after cassiterite (Figs. 3e–3f, 4f). Representative compositions of wodginite collected from both quartz veins and their host granites are presented in Table S3. Based on the crystal chemical assumption, Ta is always dominant over Nb with an atomic ratio of Ta/(Ta + Nb) > 0.73, and MnO (7.61 wt.%–11.66 wt.%) is mostly dominant over FeO (1.08 wt.%–4.07 wt.%). The data set of wodginite occupies the upper-right quarter of the Mn#-Ta# diagram (Fig. 5b). In addition, wodginite displays complicated compositions with high contents of SnO2 (up to 20.07 wt.%; Table S3). Galliski et al. (2008) suggested that the change from CGMs to wodginite could be attributed to the high activity of Sn and Ti in the melt or the increased f(O2) during crystallization.
Cassiterite is present predominantly as large (> 300 μm) subhedral to euhedral crystals in quartz veins (Figs. 3d–3e) and as small (< 80 μm) crystals in the host granites (Figs. 4d–4f). Moreover, it sometimes occurs in association with CGMs (Figs. 3a–3c and 4e). Cassiterite is usually homogenous (Figs. 3c–3d and 4d) and, in some cases, exhibits normal zoning patterns (Figs. 3e, 4e). The Abu Dabbab cassiterite can be categorized into two phases. The early-stage phase of cassiterite (Cst-Ⅰ) had been partially altered along rims or cracks by the later-stage one (Cst-Ⅱ) and/or wodginite (Figs. 3e–3f, 4e–4f).
The representative EMPA results of cassiterite from both quartz veins and host granites are presented in Tables S4 and S5. The two stages of cassiterite (Cst-Ⅰ and Cst-Ⅱ) exhibit different chemical compositions. The Cst-Ⅰ contains a notably low Nb2O5 (0.01 wt.%–1.12 wt.%) and Ta2O5 concentrations (0.11 wt.%–4.09 wt.%) compared to Cst-Ⅱ, which has high Ta2O5 concentrations (8.68 wt.%–11.94 wt.%). A difference between the two phases of cassiterite is observed in the quadrilateral diagram (Fig. 6a), which indicates limited variations of the Fe-Mn contents, with high variability of Ta-Nb ratios. In addition, Cst-Ⅰ crystals of quartz veins have a low Mn# ratio (0.01–0.20) compared to those of the host granites (Mn# = 0.38–0.48). The two cassiterite phases are plotted along a distinct 1 : 2 correlation line between Fe + Mn and Nb + Ta (Fig. 6b), corresponding to the ideal coupled substitution 3Sn4+ ↔ (Fe, Mn)2+ + 2(Nb, Ta)5+ in the cassiterite structure (Černý and Ercit, 1989). The Ta# ratios of most cassiterite crystals are greater than 0.6 (Fig. 6a, Tables S4 and S5), which is expected from the partition coefficients for Ta and Nb in this mineral (Černý and Ercit, 1989).
Based on the chemical and textural features, various zoning patterns that have been recognized in the Abu Dabbab CGMs due to Nb-Ta and Fe-Mn substitutions, even though some massive crystals without zoning were also present. The normal zoning is relatively common in the Abu Dabbab deposits, where many CGMs show rather homogeneous Nb-rich cores, increasing progressively Ta contents towards the rims (Figs. 3b, 4a–4b), according to a normal fractionation pattern of crystals with decreasing Nb/Ta ratios from core to rim (Černý and Ercit, 1985). The reverse zoning was recorded only in a few crystals displaying partial reverse zoning. In these cases, the rim of the CGM crystals is rich in Nb relative to the inner zones of the crystal, although it commonly responds to a later replacement event that truncates previous oscillatory or normal zoning. Some CGMs crystals display oscillatory zoning characterized by episodic Nb- and Ta-rich zones alternating with each other, commonly forming thin and well-defined layers from core to rim, parallel to the crystal edges (Fig. 4a). Complex zoning of CGMs is also recorded, which consists of irregular patches of Ta-rich phases (CGM-Ⅱ) superimposed on early columbite-rich phases (CGM-Ⅰ), late infilling of micro veinlets and local inclusions of other Nb-Ta oxides such as wodginite, containing up to 17.91 wt.% of Sn (Fig. 4c). The majority of cassiterite crystals are homogenous (Fig. 3d). In some cases, it exhibits normal and patchy zonation in which the Cst-Ⅰ band is overgrown or penetrated by Ta-rich Cst-Ⅱ (Figs. 4e–4f).
Fluid inclusions (F.I.) microthermometric measurements were performed on selected samples from the mineralized (Ta, Sn-enriched) quartz veins cutting through the Abu Dabbab pluton.
Primary, pseudosecondary and secondary F.I. were observed in the analyzed samples. The majority of the F.I. appears to be of a primary nature, distributed randomly throughout the host quartz crystals, as isolated or grouped in clusters or colonies. In some crystals with growth zonation, F.I. were also observed in the form of trials following the growth planes of quartz crystals, these trails are not related to cracks or fractures and do not extend completely to the edge of the crystals (pseudosecondary F.I.). In the latter generation, micro-cracks and fractures reflect deformation after mineral deposition; they are always filled with small sized F.I. that were mostly trapped after the formation of host quartz crystals (secondary F.I.), but they were not investigated because of their small size and their irrelevance to the primary mineralization process. Mono-phase liquid inclusions are always represented as secondary inclusions. Most of the F.I. are of the two phase-inclusions type, although three phase fluid inclusions are also recorded but in minor abundance relative to the former type. The F.I. exhibit a variety of shapes that are mostly round, elongate, elliptical or irregular and less commonly as a euhedral 'negative crystal'. Generally, the inclusions are small and range between 7 and 30 µm. Three compositional types of F.I. were identified based on their optical characteristics at room temperature and microthermometric measurements: Type Ⅰ (two-phase F.I.), Type Ⅱ (two-immiscible liquids F.I.), and Type Ⅲ (three-phase F.I.).
Type Ⅰ (liquidH2O + vapor): They represent about 60%–70% of the total number of inclusions at room temperature. They are commonly found in the form of colonies, trails of primary and pseudosecondary nature or as secondary inclusions. These inclusions range in size between 7 and 30 µm. Their shape is mostly irregular or sometimes oval, rounded and rarely elongated. They exhibit a wide range of filling degree (F) varying from 0.6 to 0.9 (Figs. 7a, 7b, 7c).
Type Ⅱ (liquidH2O + liquidCO2): The CO2 liquid phase always has typically dark boundaries (meniscus) against the liquid H2O fraction. They count about 25%–30% of the total number of inclusions. They are distributed randomly as isolated inclusions of primary origin; the sizes range from 15 to 30 µm. They have mostly elongated (simple or tabular) or lensed shape and oriented randomly within the quartz grains. They are characterized by relatively high-carbonic content; the proportion of the homogeneous aqueous phase with respect to the carbonic phase (H2O/CO2 volume percentage) ranges from 10% to 40% (Fig. 7c).
Type Ⅲ (liquidH2O + liquidCO2 + vapor): They represent around 5%–10% of the total number of inclusions. The shape of inclusions is variable from sub-rounded, irregular to negative crystal. Generally, the inclusions are relatively small ranging between 10 and 15 µm. They are characterized by a small vapor phase, the degree of fill (F) ranges from 0.8 to 0.9, H2O/CO2 volume percentage is mainly in the range of 50%–70% in all measured inclusions (Fig. 7d).
More than 60 inclusions were analyzed for phase's changes through cooling and heating schemes to determine the various fluid inclusion types and compositions. These cycles were generally repeated several times to ensure accurate measurements and avoid problems with nucleation during freezing runs. The measurements were performed only on primary and pseudosecondary inclusions. During the microthermometric work, several runs were undertaken down to the lowest limit of temperature of the heating-freezing stage (~200 ℃), to detect the possible presence of CH4 and/or N2. The microthermometric data and fluid properties are summarized in Table 1.
| Type | Sub-type | Composition | Th total (℃) | Total salinity | Bulk density | Bulk fluid composition (mole fractions) | ||||
| (wt.% NaCl eq.) | (g/cm3) | XH2O | XCO2 | XCH4 | XNa+ | XCl+ | ||||
| Type Ⅰ | A | H2O-NaCl | 155–290 | 3.8–8.9 | 0.8–0.96 | 0.94–0.97 | - | - | 0.012–0.028 | 0.012–0.028 |
| B | H2O-NaCl ± FeCl2 ± MgCl2 | 125–134 | 14.3–16.1 | 1.02–1.03 | 0.89–0.90 | - | - | 0.04–0.05 | 0.04–0.05 | |
| C | H2O-CH4-NaCl | 388–415 | 7.2–8.5 | 0.74–0.75 | 0.85–0.86 | - | 0.093–0.094 | 0.020–0.024 | 0.020–0.024 | |
| Type Ⅱ | - | H2O-CO2-CH4-NaCl | 250–405 | 0.03–2.5 | 0.34–0.37 | 0.84–0.89 | 0.03–0.09 | 0.009–0.12 | 0.003–0.007 | 0.003–0.007 |
| Type Ⅲ | - | H2O-CO2-CH4-NaCl | 280–350 | 3.7–3.8 | 0.55–0.56 | ≈0.92 | 0.03–0.04 | 0.007–0.014 | 0.010–0.012 | 0.010–0.012 |
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Type Ⅰ (two-phase fluid inclusions): The F.I. of this type is found as two-phase inclusions at room temperature. However, three different subtypes (A, B and C) of inclusions have been recognized during microthermometric measurements: in subtype (A), the Teu was observed at temperatures of -21 to -22.5 ℃, indicating the presence of NaCl molecules dissolved in the fluid (Shepherd et al., 1985). Hence the fluid composition can be modelled in the (H2O-NaCl system). The Tm ice was observed at temperatures from -5.8 to -2.3 ℃, corresponding to salinity ranging from 3.8 wt.% to 8.9 wt.% NaCl eq. (Bodnar, 1993). The salinity was estimated from the final melting temperature of ice and total temperature of homogenization. Densities were calculated at 0.8–0.96 g/cm3 with a median of 0.91 g/cm3. The Th total was achieved at temperatures from 155 to 290 ℃ to liquid state with peak cluster around 220 ℃. In subtype (B), the Teu was observed at temperature range of -32 and -33 ℃, indicating the presence of FeCl2 and/or MgCl2 salts in addition to NaCl salt dissolved in the fluid (H2O – NaCl ± FeCl2 ± MgCl2 system). The Tm ice has temperature range of -12.2 and -10.4 ℃, corresponding to high salinities ranging from 14.3 wt.% to 16.1 wt.% NaCl eq. (Dubois and Marignac, 1997) and densities ranging from 1.02 to 1.03 g/cm3. The Th total to liquid state was recorded at 125 to 134 ℃. In subtype (C), the Teu could not be detected due to the small size of the F.I., a homogenization temperature was barely detected at around -110 ℃, referring to the homogenization temperature of methane (Th CH4). The fluid composition can be expressed as aqueous-hydrocarbon F.I. (H2O-CH4-NaCl system). The molar proportions of methane (mol.% CH4) in the gas phase can be estimated using equation of state of Rettich et al. (1981), the results show that CH4 values are around 9 mol.%. The Tm ice was measured at -5.4 to -4.5 ℃, corresponding to salinity ranging from 7.2 wt.% to 8.5 wt.% NaCl eq. (Duan et al., 1992), assuming pure NaCl salt, the bulk densities have a narrow range of 0.74–0.75 g/cm3. The Th total was always to vapor state, and had a relatively high temperature range of 388 to 415 ℃. The dissolution temperatures of all inclusions of (Type Ⅰ) were never achieved due to decrepitation. Histograms of Type Ⅰ F.I. for the total homogenization temperatures differentiating the various subtypes (A, B and C) are shown in Fig. 8a.
Type Ⅱ (two-immiscible liquid fluid inclusions): The inclusions of this type appear as two immiscible liquid phases at room temperature ≈20 ℃, a nucleate vapor bubble is created by cooling that transforms them into three-phase inclusions as two-phase liquid and one vapor phase. The final melting temperature of solid CO2 (Tm CO2) was measured at temperatures between -58 and -73 ℃, the triple point for pure CO2 inclusions is -56.6 ℃ (Swanenberg, 1979; Hollister and Burruss, 1976). The depression of the Tm CO2 reflects the presence of additional gases (CH4, H2S, SO2 and N2) inside the F.I. The presence of CH4 molecules in the gas phases was manifested by the final homogenization temperature of CH4 (Th CH4) in a range of -88 to -107 ℃. Thus, the fluid inclusions of (Type Ⅱ) can be expressed in the model of H2O-CO2-CH4-NaCl system. The Tm ice was measured at temperatures of -4 to -7 ℃. The Th CO2 shows a wide range of temperatures due to density variations as well as the diverse amount of CH4 associated with CO2 molecules. The molar proportions of methane (mol.% CH4) in the non-aqueous phase vary from 8 mol.% to 80 mol.% CH4, the amounts of CH4 (CH4 mol.%) relative to the molar proportions of CO2 (CO2 mol.%) were calculated using the binary CO2-CH4 inclusion as a function of Tm (CO2) (van den Kerkhof, 1988) (Fig. 8c). The F.I. display two different temperature ranges for homogenization of CO2 (Th CO2 > 0) and (Th CO2 < 0). Hence most inclusions belong to the first mode behavior. In the first group of inclusions (Th CO2 > 0), the homogenization was observed at temperatures range of 6.1 to 19.9 ℃, corresponding to density of non-aqueous phase (d CO2 + CH4) of 0.06 to 0.08 g/cm3. The second group of inclusions (Th CO2 < 0) shows the homogenization at temperatures of -1 to -5 ℃, corresponding to density of non-aqueous phase of (0.07–0.09 g/cm3). In most cases, they were homogenized to liquid state and rarely to critical state (through fading of the meniscus). The Tm clath. can provide an estimate of salinity (Fall et al., 2011; Diamond, 1992). However, the presence of significant amounts of CH4 can profoundly affect the salinity (Collins, 1979). Clathrate melting was measured between 7.5 and 12.2 ℃, giving an estimated salinity of 0.003 wt.% and 2.5 wt.% NaCl eq. Salinity was determined based on the final melting temperature of ice, clathrate and the volume percentage of aqueous liquid solutions. The bulk densities for fluid inclusions of Type Ⅱ were estimated in the range of 0.34–0.37 g/cm3 with a median of 0.36 g/cm3. The total homogenizations (Th total) were achieved at temperatures range of 250 to 405 ℃ (into liquid phase), with the majority around 320 ℃ (Fig. 8b). Most of Type Ⅱ fluid inclusions were homogenized to liquid phase before decrepitation. However, some were decrepitated before the total homogenization due to the high internal pressure inside these inclusions (Zhang et al., 1989). In such cases, the decrepitation temperature (Td) can be considered as the lower limit of the homogenization temperature (Burruss, 1981).
Type Ⅲ (three-phase fluid inclusions): Microthermometric data revealed that, the composition of the fluid inclusions of Type Ⅲ belongs to the model of H2O-CO2-CH4-NaCl system. The CO2-phase in the coexisting F.I. were melted (Tm CO2) at temperatures between -59 and -61.5 ℃, confirming the presence of methane inside the gas phase. The final homogenization temperature of methane (Th CH4) was recorded between -120 and -125 ℃. The temperature of homogenization for CO2 (Th CO2) ranges between 21 and 22.3 ℃, the homogenization state of CO2 was always to liquid phase. The density of non-aqueous phase (d CO2 + CH4) was estimated at 0.07 to 0.08 g/cm3. The molar proportions of methane (mole% CH4) in the non-aqueous phase range from 18 mol.% to 35 mol.% CH4 (Fig. 8c). The melting temperature of ice (Tm ice) ranges from -4.9 to -6.2 ℃, the melting temperature of clathrate (Tm clath.) ranges from 7.7 to 8.4 ℃. They always exhibit temperature of total homogenization between 280 and 350 ℃ to the liquid phase, with a distinct peak at 320 ℃ (Fig. 8b). In regard to the fluid properties; salinity has a narrow range of 3.7 wt.%–3.8 wt.% NaCl eq. and bulk density ranges from 0.55 to 0.56 g/cm3 with a median of 0.553 g/cm3.
The textural features of CGMs such as their occurrence as accessory crystals disseminated among the main rock-forming minerals of the Abu Dabbab granite, as well as normal and oscillatory zoning in most crystals, all support their magmatic origin (Tindle and Breaks, 2000). This is enhanced further by the main trend observed in Fig. 5a, which was proven in several CGMs from other worldwide localities (Melcher et al., 2015; René and Škoda, 2011). Moreover, the substitution mechanism (Fe, Mn)2+ + 2(Nb, Ta)5+ ↔ 3(Ti, Sn)4+ (Černý and Ercit, 1985) supports the magmatic affinity of the CGMs from the Abu Dabbab area. The deviations of some CGMs points away from this trend could be related to the local supersaturation effects during mineral crystallization (London, 2014). Moreover, the oscillatory zoning (i.e., different Nb/Ta and Fe/Mn ratios in each zone) of some CGMs could be formed as a direct response to magma mixing, rapid cooling and/or degassing/decompression of magma which causes the local fluctuations in the composition of the mineral-forming melt or fluid under near-equilibrium conditions (Holten et al., 1997). In the case of complex compositional zoning of CGMs, it is difficult to differentiate between magmatic and late hydrothermal effects. However, this zoning pattern suggests that the primary CGMs were subjected to late-magmatic to subsolidus dissolution-reprecipitation processes, as well as late low-temperature alterations (Černý et al., 1986).
This is further supported by the typical texture originating from fluids in the CGMs and cassiterite where CGM-Ⅱ and wodginite surround the original primary CGM-Ⅰ (Figs. 3a–3c and 4a–4d). In addition, the Cst-Ⅱ and wodginite overprint or fill the Cst-Ⅰ cracks. These textures indicate that the primary CGM-Ⅰ and Cst-Ⅰ were later influenced by late-magmatic/ hydrothermal fluids facilitating the dissolution and recrystallization processes and formation of late phases CGM-Ⅱ, wodginite and Cst-Ⅱ.
It is important to mention that the early phase Cst-Ⅰ of the Abu Dabbab granite has similar Ta2O5 contents of cassiterite from several localities worldwide such as the Cinovec granitic cupola (Rub et al., 1998), Podlesí (Breiter et al., 2007) and Ponte Segade, in Spain (Canosa et al., 2012). While the late phase Cst-Ⅱ contains similar Ta2O5 amounts recorded in Songshugang, China (Zhu et al., 2015). Moreover, the Abu Dabbab cassiterite has a tendency to incorporate more FeO than MnO, particularly in the later cassiterite phase Cst-Ⅱ (Tables S4, S5), which could be related to the competence with other magmatic minerals regarding Mn (e.g., columbite (Mn) and tantalite (Mn)) and suggests the magmatic origin of cassiterite. The Fe, Mn, Ta and Nb have the ability to enter the cassiterite structure by means of ideal coupled substitution 3Sn4+ ↔ (Fe, Mn)2+ + 2(Nb, Ta)5+ (Fig. 6b), which is a common feature in similar ore deposits associated with highly fractionated peraluminous granites (Xie et al., 2018, 2016; Llorens and Moro, 2012; Abdalla et al., 2008).
From the texture and chemical composition criteria of the studied Sn-Ta bearing minerals, it is clear that the later stage CGM-Ⅱ, Cst-Ⅱ and wodginite are texturally and compositionally heterogeneous compared to the early primary CGM-Ⅰ and Cst-Ⅰ, indicating that the magmatic system was enriched with residual fluids. However, the effects of these fluids on the mineralization of rare metals are unclear (Linnen et al., 2014). It has been proved experimentally that the rare metals such as Nb, Ta and Sn are partitioned preferentially into the granitic melt rather than into the fluid (Timofeev and Williams-Jones, 2015). However, the crystallization of tantalite (Mn) and their occurrence as veinlets penetrating and/or surrounding the early formed columbite (Mn), probably indicate that the original magmatic system was influenced by a fluid-rich environment. We suggest the later mineralization stage of rare metals (e.g., CGM-Ⅱ, wodginite and Cst-Ⅱ) may have been generated by an interstitial fluid-rich melt with Ta enrichment at the magmatic-hydrothermal transitional stage. Zaraisky et al. (2010) suggested that Nb-Ta may be remobilized locally during hydrothermal alteration, in which the residual hydrothermal fluids overprint the magmatic system were responsible for Ta-rich CGMs. Therefore, we also suggest that increased fluid activity may have enhanced the co-precipitation and intergrowth of cassiterite and columbite.
Several hypotheses for the synthesis of CH4 in F.I. has been proposed: (1) primary magmatic source (Krumrei et al., 2007); (2) late magmatic source by reduction of carbonic-rich primary fluid through cooling in closed system (Ryabchikov and Kogarko, 2006); and (3) late to post-magmatic Fischer-Tropsch-type reaction (Potter et al., 2004; Salvi and Williams-Jones, 1997). In all cases, the reaction was assumed to be CO2 + 2H2O → CH4 + 2O2.
Based on petrography and microthermometric data for F.I., the primary magmatic origin of methane has been introduced as a source for CH4. The microthermometric data revealed the presence of at least three fluid generations: It is suggested that, the CH4-rich F.I. of (subtypes I-C, Type Ⅱ and Type Ⅲ) could be the earliest formed-fluid inclusions coexisting with the formation of hosting quartz grains; they exhibit higher temperatures of homogenization than other inclusions with smaller amount of methane, or CH4-free inclusions (subtypes I-A, I-B). These fluids are always represented as primary inclusions that have been trapped due to fluid immiscibility and/or phase separations. Fluid mixing is commonly accompanied by a progressive decrease in trapping temperatures under variable pressure conditions (Roedder, 1984; Ramboz et al., 1982). During uplift and crystallization, the temperature decreased and the amount of CH4 in the F.I. also decreased (CH4 degassing), resulting in the trapping of CH4-poor inclusions, the gas loss could be caused by hydraulic fracturing and associated pressure fluctuations (Roedder, 1984). A recognizable feature in all measured inclusions in types Ⅱ & Ⅲ that, the temperature of total homogenization increases as the amount of CH4 increases. A significant positive correlation was observed between increasing Th total (℃) and increasing molar proportions of methane (mol.% CH4) in the non-aqueous fluid phase (Fig. 8d). The plot of salinity (wt.% NaCl eq.) versus Th total (℃) for the aqueous-carbonic F.I. (types Ⅱ & Ⅲ) represents isothermal mixing for the originally CO2/CH4-rich aqueous fluids (Wilkinson, 2001) (Fig. 8e). The late aqueous inclusions (subtypes I-A, I-B) appear to be the last generation for fluid evolution that have been trapped at a higher level near the surface, characterized by a wide range of homogenization temperatures of 125–290 ℃. The range of (Th total) is too large to be simply the result of a pressure change. This could indicate a dilution of a hot, saline magmatic fluid with lower salinity surface- derived fluid (meteoric fluid?). The trend line for the (Type Ⅰ) F.I. shows the surface dilution trend of the original fluid with meteoric water (Wilkinson, 2001) (Fig. 8f). Both mixing and dilution processes were probably very active during fluid evolution, which led to large fluctuations in salinity and homogenization temperature. The coexisting molecules of FeCl2 ± MgCl2 salts in (subtype I-B) F.I. could be explained by contamination process with meteoric water and the surrounding rocks (mafic type). Scheme diagram for the evolution model of the mineralized fluid at the Abu Dabbab is shown in Fig. 9.
The trapped F.I. under the conditions of boiling and/or immiscibility have been regarded as direct P-T indicators that do not require for pressure correction (Brown, 1998; Roedder and Bodnar, 1980). The P-T entrapment conditions can be estimated by intersection method of isochores for the coexisting aqueous and carbonic F.I. (Hollister et al., 1981; Roedder and Bodnar, 1980). Because of a recognizable variation in aqueous/ carbonic ratio within F.I., which can be emphasized by phase separation and/or liquid immiscibility, both primary aqueous and aqueous-carbonic F.I. can be used together to delineate the trapping condition. Bettencourt et al. (2005) and the references therein reported that the effervescence of CO2 in each medium plays an important role during the deposition of rare metals through magmatic-hydrothermal activities. Isochores of the early formed H2O-rich F.I. (subtype I-C) versus CO2-rich F.I. (types Ⅱ and Ⅲ), were constructed using the state equations (Bakker, 1999; Duan et al., 1996, 1992). The calculations of isochoric points were based essentially on the fluid properties (composition and density) and the total homogenization temperatures (Th total) by the help of ISOC program (version 01/03) by Bakker (2003). Using the full range of isochores for considering multiple types of F.I. (highest and lowest densities) can provide evidences for the temperature and pressure conditions of trapping. The P-T boundary conditions outlined are from 330 to 370 ℃ and between 22 and 50 MPa (Fig. 10). Hence the fluid pressure was supposed to be lithostatic, where the pressure between 0.22 and 0.5 kbar gives an estimated formation depth (h) ranging from 0.8 to 2 km (Avg. 1.4 km); assuming the average density of granitic rocks (ρ = 2.7 g/cm3) and specific gravity of the Earth (Sp.G. = 9.8). The estimated depth for purely lithostatic pressure is regarded as the minimum depth of formation. Such P-T conditions are proportional to the geothermal gradients around 200 ℃/km. This high geothermal gradient might be accounted for a nearby magmatic event (intrusion emplacement) (Zoheir et al., 2008).
The Ta-Nb-Sn mineralization in the Abu Dabbab deposits is represented by disseminated/scattered grains or patches found within the albite granite and its enclosed quartz/greisen veins. The CGMs, wodginite and cassiterite are the major minerals hosting these rare metals. The chemical and textural data proved that there are two different phases of crystallization for both CGMs (CGM-Ⅰ and CGM-Ⅱ) and cassiterite (Cst-Ⅰ and Cst-Ⅱ). These phases have different chemical compositions, since the latter phases clearly contain an appreciable amount of Ta and Sn, compared to the early phases. We suggest that the early phases columbite (CGM-Ⅰ) and cassiterite (Cst-Ⅰ) are of magmatic origin based on the accurate chemical data. Afterwards, they were affected by the late- to post-magmatic hydrothermal fluids that play a significant role during the formation of the latter Ta-rich phases (CGM-Ⅱ, wodginite and Cst-Ⅱ). The fluid inclusions microthermometry showed that fluid evolution could not be described by a single stage, it was introduced by a sequence of events during evolution, indicating the combination of different processes (i.e., boiling, immiscibility and dilution trends) as responsible factors for the mineralization. The microthermometric data revealed that, the parental fluids were of magmatic origin (i.e., magmatic CH4). Moreover, the isochore intersections revealed conditions of depositions as (T = 330–370 ℃, P = 22–50 MPa) using combination of different types of F.I., corresponding to an average formation depth of approximately 1.4 km. These conditions establish an innate link between the magmatic fractionation of the albite granite and subsequently hydrothermal fluids activities, in which the residual volatile-rich melt and its related-exsolved fluids enhance the mechanism for the Abu Dabbab Ta-Nb-Sn enrichment.
ACKNOWLEDGMENTS: The authors would like to express their appreciations to the staff and technicians of the Mineral Research and Exploration (MTA), Ankara, Turkey for hosting the first author during fluid inclusions study. They thank Prof. Dr. Theodoros Ntaflos and Franz Kiraly for assistance with the electron microprobe and the staff and technicians of the Department of Lithospheric Research, University of Vienna and others at Middle East Technical University, Ankara, Turkey. Mohamed Abd El Monsef expresses his gratitude to TÜBİTAK 2221 fellowship for continuous help. Mabrouk Sami would like to thank the Ministry of Higher Education of Egypt for supporting his research stay at University of Vienna (Austria) as a postdoctoral fellow. Special thanks also go to the Egyptian Knowledge Bank (EKB) for helping of performing the language proofreading and editing for this paper. Constructive comments and suggestions by the editors and anonymous reviewers are sincerely acknowledged. The final publication is available at Springer via https://doi.org/10.1007/s12583-022-1778-y.| Abdalla, H. M., Matsueda, H., Obeid, M. A., et al., 2008. Chemistry of Cassiterite in Rare Metal Granitoids and the Associated Rocks in the Eastern Desert, Egypt. Journal of Mineralogical and Petrological Sciences, 103(5): 318–326. https://doi.org/10.2465/jmps.070528a |
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Figures(10) / Tables(1)
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Mohamed Abd El Monsef, Mabrouk Sami, Fatma Toksoy-Köksal, Rainer Abart, Martin Ondrejka, Khaled M. Abdelfadil. Role of Magmatism and Related-Exsolved Fluids during Ta-Nb-Sn Concentration in the Central Eastern Desert of Egypt: Evidences from Mineral Chemistry and Fluid Inclusions. Journal of Earth Science, 2023, 34(3): 674-689. doi: 10.1007/s12583-022-1778-y
| Type | Sub-type | Composition | Th total (℃) | Total salinity | Bulk density | Bulk fluid composition (mole fractions) | ||||
| (wt.% NaCl eq.) | (g/cm3) | XH2O | XCO2 | XCH4 | XNa+ | XCl+ | ||||
| Type Ⅰ | A | H2O-NaCl | 155–290 | 3.8–8.9 | 0.8–0.96 | 0.94–0.97 | - | - | 0.012–0.028 | 0.012–0.028 |
| B | H2O-NaCl ± FeCl2 ± MgCl2 | 125–134 | 14.3–16.1 | 1.02–1.03 | 0.89–0.90 | - | - | 0.04–0.05 | 0.04–0.05 | |
| C | H2O-CH4-NaCl | 388–415 | 7.2–8.5 | 0.74–0.75 | 0.85–0.86 | - | 0.093–0.094 | 0.020–0.024 | 0.020–0.024 | |
| Type Ⅱ | - | H2O-CO2-CH4-NaCl | 250–405 | 0.03–2.5 | 0.34–0.37 | 0.84–0.89 | 0.03–0.09 | 0.009–0.12 | 0.003–0.007 | 0.003–0.007 |
| Type Ⅲ | - | H2O-CO2-CH4-NaCl | 280–350 | 3.7–3.8 | 0.55–0.56 | ≈0.92 | 0.03–0.04 | 0.007–0.014 | 0.010–0.012 | 0.010–0.012 |
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