Britholite-(Ce) from the metaluminous granite of SW Egypt

Samples GR1 and GR2 are light grey, fine to medium grained, and exhibit slightly gneissose texture, where felsic layers are thicker than the mafic ones that constitute thin streaks. The samples are composed of oligoclase + K-feldspars + quartz + biotite, the latter is slightly altered to clinochore. The oligoclase occurs as subidiomorphic to xenomorphic crystals that are partially altered to sericite. The K-feldspar is represented by orthoclase and microcline, some of which exhibit perthitic texture and are partially kaolinitized. Quartz was found as strained crystals showing undulose extinction. Small quartz grains are enclosed in larger plagioclase crystals or form myrimkitic intergrowths around plagioclase boundaries. The accessory minerals found in the two studied samples, in decreasing order of abundance, are: britholite-(Ce) > titanite > zircon > magnetite.

Britholite-(Ce) (Fig. 2) typically occurs as independent crystals in the forms of fine-euhedral rhombic prismatic and platy shapes (200-600 µm in size), with straight to suture edges. In plane polarized light, it exhibits a tan pinkish to light brown color; some grains have a darker orange color. Under cross-polarized light, britholite-(Ce) crystals are heterogeneous and exhibit high relief, dark greenish to brown greenish interference colors. Primary britholite-(Ce) includes magnetite, apatite and zircon and is sometimes included in biotite, leading to the conclusion that it began crystallizing at broadly the same time as biotite, but significantly after feldspars. Several crystals of britholite-(Ce) have slightly concentric zoning and, sometimes, are swelled with spongy texture and resorbed border. The swelling of the crystals indicates that the samples underwent hydration and exchange reactions. Other crystals are intensely fractured, probably due an alteration effect by oxidizing solutions.

Figure 2.

Examination of britholite-(Ce) of the samples GR1 and GR2 was made using backscattered electron (BSE) imaging. Typical examples are shown in Figure 3. In BSE images, margins and many parts of the britholite-(Ce) crystals are obscured and, occasionally, are absent, indicating that the crystals are highly altered and decomposed (Fig. 3A, B). Further examination of BSE images reveals that the britholite-(Ce) is made up of smaller grains and fibers with a range of gray shades from white to black (Fig. 3C, D) or it is metamict. Since backscattering increases with increasing atomic number, BSE contrast within a grain is closely related to the distribution of chemical elements (Hall, Lloyd 1981). The change of britholite to another REE phase is seen in BSE images as variations from dark gray to light gray (Fig. 3E, F).

Energy dispersive spectroscopy analyses demonstrate that Ce is predominant over other REE in all of the britholite samples analyzed for this work [Supplementary Electronic Material (SEM) Table S1]. Major elements in the samples (in wt %) are:

Figure 3.

∑REE₂O = 44.8-66.0, Si₂O = 8.9-19.8, CaO = 5.0-13.2, ThO₂ = 4.3-10.1, FeO = 1.1-9.6, Al₂O₃ = 3.0-8.2, MgO = 0.9-5.1, F = 1.2-6.1, Na₂O = 0.6-1.8. The FeO, Al₂O₃, and MgO are generally high compared to reported values for britholite-group minerals from other localities (e.g. Oberti et al. 2001; Pasero et al. 2010; Zozulya et al. 2019). The presence of Fe, Al, and Mg at higher concentions is likely associated with occurrence of microcrystals of clinochlore and/or biotite dispersed within britholite- (Ce) crystals. The britholite-(Ce) analyzed for this study contains significant amounts of Th and U which causes metamictization.

Silica of britholite-(Ce) crystals of samples GR1 and GR2 is negatively correlated with the ∑REE₂O (R² = -0.81). As SiO₂ decreases and ∑REE₂O increases the primary magmatic britholite-(Ce) changes to another REE-(Ce) phase. The SiO₂ and Al₂O₃ released during the alteration process form a clay mineral that leached out of the britholite or remains in cracks within the parent mineral. Although the alteration may liberate Th and REE from britholite-(Ce), the presence of fluorine in the oxidizing fluids will prevent the elements from long distance transport and result in their precipitation.

Chemical formulae of britholite-(Ce) (SEM Table S1), calculated on the basis of 13 (O,F,Cl), generally deviate from those expected for typical britholite (T = ~3, M = ~5 apfu, respectively; Pasero et al. 2010). Deviations can be attributed to alteration of samples (as also indicated by petrography and BSE imaging) and/or due to analytical errors. The main composition changes observed in britholite-(Ce) in metalminous granites are due to the following substitutions of Si⁴⁺ + REE³⁺ for P⁵⁺ + M²⁺ (Macdonald et al. 2013). The compositions reported in this study and those from literature are evaluated on the basis of these substitutions. The results (Fig. 4A) suggest that the Si⁴⁺ + substitution mechanism REE³⁺ ⇆ P⁵⁺ + M²⁺ occur to the extent of 72%. In the britholite series, the incorporation of both U and Th, amongst others, may be accommodated by the substitution mechanism M²⁺ + (Th⁴⁺,U⁴⁺) ⇆ 2 REE³⁺ (Pan, Flett 2002). The data presented here in Figure 4B suggests that 52% of the variability in the data is controlled by this mechanism.

Figure 4.

Whole-rock chemical analyses of the granite samples (GR1 and GR2) are given in SEM Table S2, along with the average composition suggested for the upper continental crust (UCC) from Rudnick and Gao (2003). Samples GR1 and GR2 with peralkalinity index (PAI = mol Na₂O + K₂O/Al₂O₃) < 1 and aluminum saturation index (ASI = mol Al₂O₃/((CaO – (3.33*P₂O₅) + Na₂O + K₂O) < 1 are metaluminous. In the IUGS quartz-alkali feldspar-plagioclase diagram of the plutonic rocks, they fall in the monzogranite field (Fig. 5). Both GR1 and GR2 are characterized by elevated Th and U. The concentrations of Th vary between ~51 to 52 ppm and U between ~9 to 10 ppm. The Th and U concentrations are enriched in these samples relative to the UCC by factors of ~5 and 3.5, respectively. Hence, the investigated granite is considered fertile for thorium.

Figure 5.

Thorium and uranium are typically covariant under primary magmatic conditions, due to the charge/ionic similarities between the two elements (U⁴⁺: 1.05 A°, Th⁴⁺: 1.1 A°). Consequently, primary igneous rocks vary slightly in Th/U ratios, most of the samples filling in the range 3-4 (Keevil, 1944); thus, GR1 and GR2 with relatively high ratios (5.26-5.64) reflect the retention of thorium and the release of uranium. The fractionation between Th and U can occur at weathering surfaces or due to the effect of hydrothermal (magmatic) fluids where the reduced form of uranium (U⁴⁺) oxidized, forming soluble and mobile ion (U⁶⁺O₂)²⁺, while Th⁴⁺ is inoxidizable and, hence, remains behind.

Sample GR1 with total 14 rare earth elements of 297 ppm has a negative Eu anomaly (Eu/Eu* = 0.68; SEM Table S2). Here, the abundance of Eu is depleted relative to the other rare earth elements and this can be taken as evidence for earlier separation of plagioclase. Whole-Rock Eu anomalies are attributed to the tendency of europium to be incorporated in crystallizing plagioclase, which in turns, is controlled by the chemistry of the melt and reduction-oxidation potential. Europium is incompatible in its trivalent oxidation state (Eu³⁺—1.07 Å) in an oxidizing magma, but it is preferentially incorporated into crystallizing plagioclase in its divalent form (Eu²⁺—1.25 Å) in a reducing magma, where it replaces calcium (Ca²⁺—1.0 Å). Thus, the GR1 with the weakly negative Eu anomaly reflects moderate redox conditions and is typical of upper continental crust rocks (Eu/Eu* = 0.67-0.72, SEM Table S2; Tang et al. 2015).