Introduction

Thorite is thorium silicate (ThSiO4) that crystallizes in the tetragonal system. It is a common member of the Th minerals which also include huttonite (monoclinic ThSiO4), thorogummite [(Th(SiO4)1−x(OH)4x)], uranothorite [(Th, U)SiO4)], ferrothorite [(Th, Fe)SiO4)], orangite [(ThSiO4)H2O)], thorianite (ThO2), thorutite [(Th, U,Ca)Ti2(O, OH)6], and brockite [(Ca, Th, Ce)(PO4)(H2O)]. The tetragonal thorite forms a lower P-T than its monoclinic dimorph huttonite (Finch et al. 1964) that is considerably less common. Thorite, unlike huttonite, occurs mostly in the metamict state (Farges and Calas 1991). Metamictization and hydration of thorite lead to the loss of radiogenic Pb, yielding younger apparent ages (von Blanckenburg 1992). In natural samples, thorite (ThSiO4) forms extensive solid solutions with other tetragonal minerals including zircon (ZrSiO4), xenotime (YPO4), and coffinite (USiO4) (Förster 2006). However, the extent range of solid solution between the hydrous varieties of thorite + zircon and thorite + coffinite is less than 10 mol% (Mumpton and Roy 1961) and, hence, raises doubts on the stability of the intermediate members of the tetragonal system (Förster 2006).

Natural occurrences of thorite have been described from a wide range of geological environments including, but they are not limited to, (1) felsic igneous rocks (Hetherington and Harlov 2008; Hirtopanui et al. 2013; Abu Zeid 2014; Vasilatos and Papoutsa 2023), (2) metasediments (Speer 1982; Bingen et al. 1996; Franz et al. 1996), (3) ore deposits (Chen et al. 2022; Sun et al. 2024), and (4) fault zones in granitic plutons (Kamineni and Lemaire 1991). Thorite may count for up to 50 wt% of the Th, 20 wt% of the U, and 10 wt% of the bulk-rock heavy rare earth elements (HREE) and Y in weakly peraluminous granites (Förster 2006). In peralkaline, metaluminous-, and peraluminous-A type granites, up to 40 wt% of the Th and 20 wt% of the U contents may reside within thorite (Bea 1996). Although thorite is the most common Th mineral in nature, it is not the primary ore of thorium. The phosphate mineral monazite is mined for thorium and REE. If monazite deposits become scare or the demand for Th increases, then thorite and to a lesser extent, the other Th minerals will gain great importance. Currently, uranothorite is an important ore for U production (Hirtopanu et al. 2013).

Thorite forms inclusions in many minerals including zircon (Pointer et al. 1988; Hassan and Brunarska 2023), xenotime and monazite (Hetherington and Harlov 2008), allanite-group minerals (Buda and Nagy 1995; Guha et al. 2020; Zhang et al. 2023), and bastnäsite (Gieré et al. 1998). It also occurs as primary grains in hard rock (Förster 2006). Thorite enrichment and its occurrences commonly found in granites that are highly fractionated (Ragland et al. 1967; Pagel 1982; Bea 1996). The author here reports on the unique occurrence of thorite in a granodiotite from the basement complex, southwestern Egypt. Transmitted-light and BSE microscopy and EDS analyses were performed on the thorite and its host rock. My goal is to characterize thorite in these samples and to assess its sources. Thorite sources seem to be controlled by secondary, rather than primary, processes. These findings should provide clarification on why thorite may occur in less fractionated felsic rocks rather than in highly evolved granites.

Geological background

The Nubian granodiorite forms part of ~ 20 km2 basement complex that is exposed at two adjacent sites in southwestern Egypt (Fig. 1) and also includes granites, tonalities, and monzodiorites. Tectonically, it is related to the tonalite-granodiorite magma, emplaced in succeeding phases, containing amphiboles (hornblende), micas (biotite) or both during the Proterozoic (Sabet 1972; List et al. 1989). The basement complex rocks occur in the forms of small knobs and residual boulders partially covered with sand that forms the country rocks in the entire region of southwestern Egypt. The knobs and boulders are the remnants of a mountain succumbed to desert erosion which is evident from the large accumulations of rock debris and granite-derived soil in the studied two sites. The soil does not have a developed profile and is largely medium to coarse-grained. Mineralogically, it comprises quartz, albite, illite, aluminous hematite, vermiculite, chlorite, and biotite (Hassan and Gunnlaugsson 2013). Present-day climatic conditions are hot summers and rains occurring on rare occasions.

Fig. 1
figure 1

Geological map of southwestern Egypt (revised from GSMA 1981)

Full size image

Petrography of the Nubian granodiorite and its associated granites has demonstrated that these samples have characteristic REE accessory minerals. These are: (1) allanite-(Ce) in the Nubian granodiorite (Hassan 2008) and britholite-(Ce) in the granites (Hassan 2023). Chemical (major) analyses of two representative samples of the Nubian granodiorite are reported by Hassan (2008) and only a brief summary of the results is given here. The samples, namely S1 and S2 have a high acidic signature (SiO2 = 72.5 wt%). They are peraluminous, with values of peralkalinity index [PAI = mol Na2O + K2O/Al2O3] = 0.77, Al saturation index [(ASI = mol Al2O3/(CaO – (3.33×P2O5) + Na2O + K2O)] = 1.1, and corundumnormative = 0.89. The peraluminous character suggests possible S-type source similar in this respect to some granitoids of the Eastern Desert, Egypt (Abu El-Leil 1980). S-type granitiods originate from a sedimentary source involving the removal of sodium into sea or evaporites and calcium into carbonates, and therefore, relative enrichment of the source in Al occurs (Mc Birney 1993). However, origin of the Nubian granodiorite by a sedimentary source is not consistent with the contents of Na2O and K2O of the samples falling within the igneous-type source field of the Chappell and White (1974) granite classification (not shown). It is also inconsistent with the presence of magmatic allanite-(Ce) as well as the absence of both garnet and endogenic xenoliths in these samples.

Sampling and analysis

Samples S1 and S2 of the Nubian granodiorite, previously studied by Hassan (2008) were used for the present research. They are fine to medium-grained, light grey, and exhibit slightly gneissose texture, where lighter minerals are thicker than the mafic ones that constitute thin streaks. Powder X-ray diffraction (XRD) analysis of the two samples was made using a Philips PW 1710 diffractometer. In these samples, besides major quartz and feldspar, ferrohornblende was identified by XRD as the main iron component: it is associated with minor amounts of clinochlore. Thin section of the samples S1 and S2 were prepared and studied by conventional optical methods in transmitted and reflected light using a polarizing microscope Olympus BX41 equipped with a digital camera. In addition, selected minerals in carbon-coated thin sections were analyzed using a field emission-scanning electron microscope (FE-SEM, Hitachi S-4700) attached to Norman Vantage EDS. FE-SEM operation conditions are: accelerating voltage = 20 kV, working distance = 1.2 cm, beam current = 10 µA and counting time = 100 s. The correction procedure Phi-Rho-Z was employed for raw data. The accuracy of the results was checked through analyzing duplicate mineral spots. The analytical errors of SEM-EDS method (standard deviation from duplicate samples) were < 5% for major elements and up to 40% for trace elements.

For whole-rock minor chemical analysis, aliquots of the Nubian granodiorite samples S1 and S2 (0.20 g) were mixed with a flux of lithium tetraborate (LiBO2)/metaborate (Li2B4O7), and then fused in a furnace at 1050°C. The cooled bead was dissolved in a 5% nitric acid (HNO3) solution. The dissolved mixture was taken up in 1% HNO3 and then analyzed by a Perkin Elan inductively coupled plasma-mass spectrometer (ICP-MS). The analyzed elements were Rb, Sr, Ba, Y, Zr, Th, U, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. The accuracy of these analyses was monitored by the use of standard materials (OREAS45E, OREAS24P, SO-18, DOLOMITE-2), analysis of reagent blanks, and replicate analyses. The analytical errors (standard deviation determined from different sample duplicates) were 5–9%. For comparison, samples of the leucocratic and biotite granites associated with the Nubian granodiorite were analyzed for 21 minor elements (Rb, Sr, Ba, Y, Zr, Th, U, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) using the ICP-MS method described above.

Results

Microscopy and textural characteristics

Host rock

The Nubian granodiorite, under optical microscope, is consisted of quartz, plagioclase, and potash feldspar, with a minor amount of ferrohornblende (Fig. 2). The ferrohornblende forms cleavable streaks arranged in parallel directions along the gneissosity planes and is partially altered to clinochlore. Quartz occurs as small grains filling the interstitial spaces between other minerals, and also as medium to large crystals showing wavy extinction and few suture edges. The plagioclase is represented by albite and oligoclase, forming phyenocrysts partially altered to epidote and to a greater extent sericite. The K-feldspar, mainly orthoclase has a cloudy appearance and is partially replaced by clay. The accessory minerals found in the studied samples are: thorite < zircon < allanite-(Ce). Scare barite found in these samples forms crystallites up to 20 μm in length.

Fig. 2
figure 2

(a–b) Cross-polarized transmitted-light images show the textural and mineral composition of samples collected from the Nubian granodiorite. Mineral abbreviations: Ab—albite, Ccl—clinochlore, F-hbl—ferrohornblende, Olig—oligoclase, Or—orthoclase, Qz—quartz, Zrc—zircon

Full size image

Thorite

Thorite occurs as small grains in a fine-grained matrix consisting of altered plagioclase (mainly oligoclase) and ferrohornblende, varying in size from ~ 20 to ~ 100 μm (Fig. 3a and b). Optically, it is nearly opaque and, in plane polarized light, appears yellow-orange. On the BSE image, thorite grains exhibit characteristics of undeformed crystals surrounded by corona-type structures comprising clinochlore and hematite, with some barite (Fig. 3c and d).

Fig. 3
figure 3

Plane-polarized transmitted-light (a, c) and BSE images (b, d) of thorite grains from the Nubian granodiorite. a Euhedral (lower left) and kidney-shaped grains (center) in altered oligoclase and ferrohornblende. b Detail enlargement showing the kidney shaped grain (center) depicted in (a). c Subrounded grain in altered oligioclase. d Same grain as in (c), detail enlargement. Mineral abbreviations not yet identified are: Brt—barite, Hem—hematite, Thr—thorite

Full size image

Representative EDS elemental data of thorite grains in Fig. 3 are provided in Table 1. Thorite major constituents are Th, Si, and to a lesser extent U. The minor constituents, on the other hand, are: Al2O3, Y2O3, Ce2O3, Nd2O3, Fe2O3, CaO, MgO, P2O5, Na2O, and Cl.

Table 1 Representative EDS quantitative chemical data (wt%) from the Nubian granodiorite, southwestern Egypt
Full size table

Allanite-(Ce)

Allanite-(Ce) occurs as euhedral to subhedral crystals in a fine-grained plagioclase matrix in the samples (Fig. 4a and b). The size of allanite-(Ce) grains can be up to 1100 μm as observed in thin sections. Most of the allanite-(Ce) crystals examined are highly affected by post-magmatic transformations. Petrographic observations indicate that allanite-(Ce) show dissolution-reprecipitation textures. The altered regions of allanite-(Ce) appear darker in BSE images than the unaltered ones (Fig. 4c–f). Furthermore, alteration of allanite-(Ce) is characterized by the partial replacement by thorite, clay, epidote, britholite-(Ce), and others. The thorite occurs in or near altered regions of allanite-(Ce), forming crystallite agglomerations ranging from ~ 0.1 to ~ 10 μm. Thorite crystallites, on BSE images, appear as bright spots (Fig. 4c–f). The crystallites are obvious in these images: because the mean atomic number of thorite is high, it appears bright. Their sizes and distributions vary across the hosting allanite-(Ce); areas of alterations contain more but smaller thorites.

Fig. 4
figure 4

Optical and BSE images of allanite-(Ce) grains from the Nubian granodiorite. a Cross-polarized transmitted-light image of euhedral grain showing some sign of alteration. b Plane-polarized transmitted-light image prismatic grain showing excessive alteration. The grains of both (a) and (b) images are associated with oligoclase and are characterized by radial cracks extending out into the surrounding minerals. c-f BSE images of altered allanite-(Ce) grains partially replaced by secondary thorite (bright spots) and other secondary minerals. Points in images: 1—britholite-(Ce); 2—LREE-silicate; 3—clay. Aln-(Ce)—allanite-(Ce), Ser—sericite

Full size image

Representative EDS elemental data of thorites shown in Fig. 3 are listed in Table 1. Thorite major constituents are Th, Si, and to a lesser extent U, with minor amounts of Al2O3, La2O3, Ce2O3, Nd2O3, Fe2O3, CaO, and Na2O.

Zircon

In the Nubian granodiorite, zircon is abundant as euhedral gains ranging in size from ~ 60 to ~ 100 μm, showing embayment corrosion, and containing inclusions of light rare earth elements (LREE). Small zircon grains are often included within plagioclase and quartz, suggesting a wide range of zircon crystallization during the solidification of the parent magma. The elemental compositions of zircon are close to pure (Zr, Hf) silicate (Table 1), suggesting the EDS method employed in this study is satisfactory.

ICP-MS results

The ICP-MS analytical results are provided in the online electronic supplementary material (ESM; Table S1). The Nubian granodiorite samples S1 and S2 have less U and more Y, Th, and REE than the leucocratic or biotite granite. They are characterized by a strong negative europium anomaly (Eu/Eu* = 0.22–0.25). This may reflect high oxidation conditions (Rudnik and Gao 2003) or could also be inherited from magma sources (Khoshnoodi et al. 2017).

The Nubian granodiorite Th/U ratios exceed the range 3–4 expected for primary igneous rocks (Keevil 1944; Wedepohl 1995). Higher Th/U ratios reflect the retention of Th and release of U, which likely occurred as a result of oxidizing conditions. Under oxidation influence U forms the uranyl ion (UO2 + 2) in which U has a valence of + 6. The UO2 + 2 form compounds that are soluble in solution. Therefore, U is mobile element under oxidizing conditions and is separated from Th which exists as tetravalent ion and whose compounds are generally insoluble in solution. Mobilization of UO2 + 2 is associated with fluorine complexing at high temperature conditions (Kimberley 1978; Keppler and Wyllie 1990).

Discussion

Thorites may form at an early magma stage (Vasilatos and Papoutsa 2023) or from late- to post-magmatic (pegmatite and hydrothermal) fluids (Hirtopanui et al. 2013; Sun et al. 2024). Magmatic thorites should have well-crystallized shapes like prismatic zircons or have distinct straight boundaries. These characteristics are absent in thorites of the Nubian granodiorite. Thus, the Nubian granodiorite thorites would have grown from secondary aqueous solutions rather than magmatic melts. In Fig. 3, thorite grains are enclaved in the oligoclase that was polycrystalline and experienced obvious and heavy alteration. The enclavation of thorite grains in these oligoclase domains would suggest that these grains were precipitated during an alteration stage. Thorite grains show characteristics of underformed crystals that are surrounded by well-preserved, corona-type structures comprising clinochlore and hematite, with some barite. The corona structures (excluding clinochlore) suggest that the thorite crystallized under relatively high conditions of oxygen fugacity (fO2) and sulfate (SO42−) activity (Kamineni and Lemire 1991. The inferred fO2 and SO42− conditions point to hydrothermal solutions. The SO42− ions might be involved in the transportation of Th. Si, U, and other elements necessary for the nucleation and crystallization of thorite (Sun et al. 2024). The clinochlore found around thorite (Fig. 3) was a typically alteration mineral which formed from ferrohornblende, as indicated from the petrography. Its presence supports the contention that the thorite was precipitated in cavities during alteration processes.

The thorite (Fig. 3) appears remarkably unalterd—a feature that has been reported for thorite in granites (Kamineni and Lemire 1991; Förster 2006). Generally, thorite is more stable in geological environments than other actinide minerals. This is simply because thorium (as thorite) exists in only one oxidation state (Th4+) that is essentially immobile in solution. The tetravalent Th4+ cation does not undergo either oxidative dissolution as occurs for uranium (U4+) or reductive dissolution as could occur for plutonium (Pu4+) (Cleveland 1979). Furthermore, the intensity of radioactivity of Pu4+, for example, could cause damages to the host material. However, for most conditions in a nuclear fuel waste burial chamber, the tetravalent actinide oxides are relatively stable (Lemire and Garisto 1989). Thorium, which is about three times as common as uranium in the earth’s crust (Rudnik and Gao 2003), could be used as a replacement for uranium in nuclear power generation. Th-based nuclear wastes are characterized by fewer radioactive elements than other conventional radioactive elements such as uranium (van Gosen et al. 2009), which led to worldwide attentions on Th resource exploration.

The partial chemical alteration of allanite-(Ce) and subsequent formation of thorite in the altered domains (Fig. 4) is a direct result of dissolution-reprecipitation processes. These processes must have occurred in an effectively closed-chemical system, i.e., Th, Si, U, and other elements necessary for the nucleation and growth of thorite were already present in the host allanite-(Ce). As the dissolution of the allanite-(Ce) continues, the interface between the fluid and solid migrates as it is replaced by the new mineral phase (thotite). Interconnected pore spaces allow infiltration of fluids and thus aid the transport of the Th, Si, U, and others near or within the altered regions of the allanite-(Ce). Transport of these elements by fluids requires complexing the cations. Anions commonly involved for complexing include CO32−, SO42−, HPO42−, Cl−, and F−. Although anion contents are not available in bulk analyses, the EDS analysis indicates that fluorine, chlorine, and sulfate are present in the samples (Table 1). Fluorine and SO42− are though to be important for complexing Th, U, REE, and other elements during the alteration of allanite (Zhang et al. 2023).

The alteration of allanites is likely linked to metamictization of the samples (Morin 1977; Wood and Richetts 2000). Metamictization occurs as result of α-decay-event recoil following radioactive decay of a U or Th atom contained in a crystalline material. The recoiled particles travel a distant equivalent to a few unit cells, causing damages of the host mineral and, in some cases, can destroy its crystal lattice completely while leaving the outward appearance intact. These damaged zones are susceptible to dissolution and removal by fluids, leaving behind pore spaces. Here, the secondary thorite and porosity increase in altered domains of allanite-(Ce) (Fig. 4). Therefore, recoil induced by radioactive decay should be considered as a mechanism favoring porosity formation in the Nubian granodiorite allanite-(Ce). However, in other studies, porosity was only observed in altered domains of xenotime and monazite that have lower concentrations of Th and U (Hetherington and Harlov 2008). Therefore, radiation damage by radioactive decay in accessory minerals cannot by itself be responsible for the development of porosity in these samples. In contrast, porosity tends to be a result of secondary chemical alteration, typically resulting in some loss of material.

A review of results of chemical analyses of thorite from other studies indicates the presence of as much as 40 elements (Speer 1982). In contrast, in the present study evidence for only 13 elements is obtained (Table 1); the rest are either absent or present in amounts below the detection limit of the EDS method. ThO2, SiO2, and UO2 constitute the major components in the thorites of the Nubian granodiorite. These compositions correspond to thorium uranium silicate (Th, U)SiO4. The highest reported contents of U in thorite from granites were from Nigeria (~ 29 wt% UO2; Pointer et al. 1988) and Germany (~ 23 wt% UO2; Förster 2006). The Nubian granodiorite thorites contain ca. 5–21 wt% UO2 (Table 1). The Nubian granodiorite thorites contain variable small amounts of non-formula elements including Y2O3, Al2O3, Ce2O3, Nd2O3, Fe2O3, CaO, Na2O, MgO, P2O5, and Cl. The entry of such elements into thorite is common in natural samples (Speer 1982; Pointer et al. 1988; Žáček et al. 2009; Hassan and Brunarska 2023). This is assigned to the interaction between the metamict mineral and hydrothermal fluids. Thorites from granites always contain several wt% of REE, with a strong predominance of HREE over LREE (Bea 1996; Förster 2006). The REE patterns of thorite from Czech Republic, however, show enrichments in LREE over HREE (Žáček et al. 2009). In the present study, the content of heavy REE was found to be below the detection limit of EDS chemical analysis.

The chemical compositions of thorites vary widely depending on the chemistry of the environment of formation and reflect complex solid solutions with other minerals. Förster (2006) described a complete solid solution between zircon (ZrSiO4), thorite (ThSiO4) + coffinite (USiO4), and xenotime [(Y, REE)PO4)]. In the thorites of the Nubian granodiorite, the components U, Y, Ce, Fe, and Ca are hence assumed to replace Th according to the following substitutions:

$${\left( {{\rm{REE,Y}}} \right)^{{\rm{3 + }}}}{\rm{ +\, F}}{{\rm{e}}^{{\rm{3 + }}}}{\rm{ + }}\,{{\rm{U}}^{{\rm{6 + }}}} \mathbin{\lower.3ex\hbox{$\buildrel\textstyle\rightarrow\over {\smash{\leftarrow}\vphantom{_{\vbox to.5ex{\vss}}}}$}} {\rm{3T}}{{\rm{h}}^{{\rm{4 + }}}}$$
(1)
$${\rm{2C}}{{\rm{a}}^{{\rm{2 + }}}} \mathbin{\lower.3ex\hbox{$\buildrel\textstyle\rightarrow\over {\smash{\leftarrow}\vphantom{_{\vbox to.5ex{\vss}}}}$}} {\rm{T}}{{\rm{h}}^{{\rm{4 + }}}}$$
(2)

The presence of P2O5 in the Nubian granodiorite thorites may suggest the following substitution:

$${\rm{RE}}{{\rm{E}}^{{\rm{3 + }}}}{\rm{ + }}\,{{\rm{P}}^{{\rm{5 + }}}} \mathbin{\lower.3ex\hbox{$\buildrel\textstyle\rightarrow\over {\smash{\leftarrow}\vphantom{_{\vbox to.5ex{\vss}}}}$}} {\rm{T}}{{\rm{h}}^{{\rm{4 + }}}}{\rm{ +\, S}}{{\rm{i}}^{{\rm{4 + }}}}$$
(3)

Sulfur identified in thorites from other studies probably substitutes for Si (Speer 1982), and in the present case it is below the EDS detection limit.

The geochemical behavior of radioactive and rare elements in differentiated granitoids changes with aluminum saturation. Generally, the higher the ASI, the strong depletion in Th, U, REE (except Eu), and Y (Bea 1993). However, the Nubian granodiorite with higher ASI values (peraluminous) contain more Th, REE (except Eu), and Y than the associated leucocratic and biotite granites that have low ASI values (metaluminous). Had magmatic differentiation been responsible for the element contents, the Nubian granodiorite, which is peraluminous, should have less Th, REE and Y concentrations than the other two rocks that are metaluminous and more fractionated. The relative increases of these elements in the Nubian granodiorite are unrelated to some processes of differentiation such as crystallization fractionation, but they are correlated with the presence of allanite-(Ce). The occurrence of thorite in granodiorite rather than granites in this study is related to allanite-(Ce) mineralizations and secondary processes.

Conclusions

The current study presents petrographical and mineralogical investigations of thorite in the Nubian granodiorite from the basement complex, southwestern Egypt. The results provide new data concerning enrichment of thorium (as thorite) in granodiorite rather than in the more fractionated granites in the area. Thorite enrichment is related to secondary, not magmatic differentiation processes. Two distinct types of secondary thorites are recorded in this study.

Type A thorite occurs as discrete grains that are enclaved in altered plagioclases and are characterized by corona-type structures. These microfeatures comprise clinchlore, hematite and to a lesser extent barite. The close-spatial association of type A thorite with these minerals (except clinochlore) strongly suggests it grew from hydrothermal solutions having relatively high conditions of oxygen fugacity and sulfate activity. The clinochlore was a direct result of alteration of ferrohornblende. Its presence suggests that the type A thorite precipitated during an alteration stage.

Type B thorite of the Nubian granodiorite, on the other hand, forms crystallite agglomerations in altered domains of magmatic allanite-(Ce). Formation of thorite crystallites in these altered domains is demonstrated to be a result of dissolution-reprecipitation processes where fluids dissolved some parts of allanite-(Ce) grains. Dissolved elements, mainly Si, Th, and U moved via pores and then redeposited as thorite close or within the host mineral.

The two thorite types (A and B) are mainly composed of thorium uranium silicate plus minor amounts of Y, Al, Ce, Nd, Fe, Ca, Na, Mg, P and Cl. The main compositional variations found in these thorites are most likely related to solid solution substitution reactions. These are: (1) [(REE, Y) + Fe + U ⇄ 3Th], (2) [Ca ⇄ Th], and (3) [REE + P ⇄ Th + Si]. Thorite compositions are generally in agreement with those reported for uranothorites from other occurrences.