Introduction

In this work, “rare metals” refers to metals with a low natural availability. It is divided into four groups: fissionable elements (U and Th), rare earth elements (La–Lu), and trace elements (Nb, Ta, Mo, Ni, Co, Cu, Sn, and W) [1,2,3,4,5,6,7,8,9]. According to [10] granite-hosted rare metal deposits are created either in the formed granite or accompanied with the hydrothermal alteration of magmatic rocks.

Weathering and hydrothermal alteration are two types of alteration that occur in rocks and affect their original characteristics. The classification relies on where the modifying fluids come from. Even though these phrases are frequently used interchangeably, the conditions under which these processes develop are different. In contrast to hydrothermal alteration, which occurs when hydrothermal fluids channel through parent rocks, weathering occurs in surface environments, according to [11, 12].

With the development of advanced satellite missions with higher spatial, spectral, radiometric and temporal resolutions, the remote sensing techniques are widely employed for mineral exploitation and geologic mapping, and their capabilities have increased [13,14,15]. Data of the satellite remote sensing (Landsat-8 OLI) was analyzed to trace the hydrothermal alterations in the form of iron-oxides, OH-bearing minerals and carbonate through the usage of the band ratio technique. Band ratio was broadly applied in geological and mineral alteration mapping in the central parts of the Eastern Desert of Egypt [16,17,18,19,20,21].

The GEOISO-Windows™ programme [22] was used in this study to calculate and visualize the mass balance/volume change by determining the absolute mobility of the components using [23] equations and [24, 25] isocon diagrams.

REE electron configurations cause irregular shapes of the normalized patterns, which can be attributed to the tetrad effect [26]. Mineral deposits, particularly those of precious and base metals, are frequently present along with hydrothermal alteration processes in acidic rocks [27,28,29,30,31].

The present study aims to differentiate and map the rock units using remote sensing techniques and characterize the highly and slightly altered monzogranites geochemically and mineralogically at Wadi Sikait, in the southern parts of the Eastern Desert.

Remote sensing data and geologic setting

Landsat-8 scene data for alteration mapping for Wadi Sikait area

To distinguish ferrugination, ferromagnesian, and OH-bearing minerals, three Landsat-8 band ratios were developed [18] (b4/b2, b5/b6, & b6/b7, Fig. 1a, b, c). The brighter regions signify the location of the target materials' dense concentration. Statistics and threshold values applied on the above-mentioned band ratios are summarized in (Table 1). Landsat-8 for delineating the hydrothermal alteration zones, colored composite images were for the aforementioned three image ratios (Fig. 1d), respectively. The image revealed that the Iron oxides (Ferrugination) appear in red and magenta color and are mainly concentrated in the monzogranites.

Fig. 1
figure 1figure 1

Landsat-8 ratios of the Wadi Sikait area in the Eastern Desert of Egypt: a Iron oxy-hydroxides minerals (Landsat-8 ratio b4/b2) red color shows anomalies after threshold is applied. b Ferromagnesian minerals B5/B6 ratio image, green color shows anomalies after threshold is applied. c OH-bearing and carbonate minerals (micas and clays) (Landsat-8 ratio b6/b7), blue color shows anomalies after threshold is applied. d Color composite of Landsat-8 ratio (b4/b2), (b5/b6), (b6/b7) in RGB

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Table 1 Statistics, threshold values, and confidence of the common Landsate-8 band ratios that used for the Wadi Sikait area
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Geologic setting

Red Sea’s shorelines covers more than 2 million km2 of the Arabian Nubian Shield (ANS), which has been formed in the time interval 540–1100 Ma [32,33,34]. The Arabian Nubian Shield is thought to be formed because of the collision of East and West Gondwana plates during the Pan-African Orogeny; it is considered a northern extension of the Mozambique belt [8, 35,36,37,38]. The Red Sea rifting separates the Nubian Shield from the Arabian Shield, which is composed primarily of Pre-Cambrian basement complex in Egypt, Sudan, and Ethiopia [34, 39]. On the other bank, in Yemen, Saudi Arabia, and Jordan, some crystalline outcrops of the foundation rocks make up the Arabian Shield.

The Sikait-Nugrus region is exposed in the southern Eastern Desert and resembles the body of a fish with its head pointing towards the northwest and its tail extending towards the southeast [38, 40,41,42]. It is located on the Red Sea coast at about 70 km SW of Marsa Alam town.

The study area represents the eastern part of Sikait-Nugrus area and lies between lat. 24° 38ʹ 36ʺ to 24° 39ʹ 21ʺ N and long. 34° 47ʹ 03ʺ to 34° 48ʹ 06ʺ E (Fig. 2a, b). From youngest to oldest the tectono-stratigraphic sequence is represented by the lamprophyre dykes, monzogranites, ophiolitic mélange, and psammitic gneisses [43]. The rocks are sheared, jointed, banded, hematized, and mylonitized; they also exhibit silicification and dipping by 50°–69° trending NNW-W.

Fig. 2
figure 2

a Landsat image showing the location of Wadi Sikait, and b a geological map of the study area, South Eastern Desert (SED), Egypt (modified after [42])

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The ophiolitic mélange is primarily found in the eastern portion of the map region and is made up of blocks and fragments of ultramafic rocks that have been tectonically mixed together within a matrix with varying degrees of fineness. On the western side of Wadi Sikait, the mineralized zones of the psammitic gneiss consist of mélange matrix (composed of schists) containing considerable metagabbros, metaperidotites, and metapyroxenites fragments of various sizes [43,44,45]. These layered, highly foliated, featured, and folded matrixes are characterized by many rock schist types including tourmaline-garnetiferous-biotite, quartzo-feldspathic, sillimanite, and talc, schist [38].

On the western side of the plotted area, there are more monzogranite rocks. They are made mostly of quartz, biotite, feldspars, and plagioclase; they have grey to white pinkish in colour and is considered coarse to very coarse-grained. The rocks are jointed, blocky, weathered (Fig. 3a), sheared, mylonitic (Fig. 3b), and hematized (Fig. 3c) in some parts, while they are deformed and have a diagnostic gneissose structure (Fig. 3d) along their contacts with the oldest rocks. Sometimes, they are highly altered and become whitish with a considerable radioactive content (Fig. 3e, f). The quartz veins are striking E-W cutting all the aforementioned rocks and vary from 0.2 m to 1 m thick (Fig. 3g).

Fig. 3
figure 3

Field photographs in Wadi Sikait area indicating a General view of monzogranite showing blocky weathering, Looking NE, b Mylonitized in monzogranites at the contact with psammitic gneisses, looking S., c Hematization in monzogranites, looking W., d Gneissose structure in altered monzogranites, e Gradational contact between altered mineralized (whitish) and barren monzogranite (dark), Looking N., f Contact between altered mineralized (Whitish) and barren monzogranite (dark), Looking W., and g Quartz vein cutting monzogranite, Looking W

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Materials and methods

On November 3, 2019, a terrain corrected and cloud-free single Landsat-8 L1T (terrain corrected) scene of UTM projection (path173/row 43), WGS-84 datum, zone 36 N was taken (Table 2). Band rationing, a form of image transformation, referring to processing multiple images from different bands into one single image. Theoretically, band rationing works because the effect of terrain and shadow is divided out in the calculations. Dividing the brightness values (DNs) at peaks/maxima and troughs/minima in a reflectance curve will elevate the DN values of specific geologic materials and may enhance the contrast between them. The choice of bands is based on their spectral reflectance and the absorption features of the mapped minerals. The data is processed using ENVI 5.3 software.

Table 2 Identification of the Landsat 8 Scene that used for Wadi Sikait area, South Eastern Desert (SED), Egypt
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Several field trips were taken in order to gather rock samples that would reflect the various geological units and mineralized varieties. Ten thin-sections and polished slabs were petrographically investigated and eleven representative samples were mineralogically studied. The petrographic examination was carried out using Olympus polarizing microscope. For estimating the heavy minerals content (fraction size 0.125–0.250 mm) using the bromoform, some representative samples were crushed and grinded then sieved and quartered. For a precise mono-mineralogical fractionation and identification, heavy minerals were collected under a binocular microscope. A scanning electron microscope ESEM model Prisma E, attached with an energy dispersive X-ray spectrometer (EDS) spot chemical analysis of minerals was used in the Nuclear Materials Authority (NMA) of Egypt. The analytical conditions were 25–30 kV accelerating voltages, 1–3 nm beam resolution and 60–120 s counting time. The thin sections were studied using plane polarized microscope (model OLYMPUS BX53) attached with digital camera. The major oxides and trace elements in a total of 11 samples were analyzed using coupled plasma mass spectrometry (ICP-MS) in Acme Lab, Vancouver, Canada. Detection limits for trace elements and major oxides were 0.01–0.5 ppm and 0.001 wt%–0.04 wt%, respectively. The analytical precision, as calculated from replicate analyses, was 0.5% for major oxides and varied from 2 to 20% for trace elements. The total silica (SiO2) was determined using a spectrophotometric method in NMA (Nuclear Materials Authority) labs.

Normative mineral abundances were estimated based on the main elemental analysis and a set plots were created using the igneous petrology software® GCDkit (S1). The isocon diagrams equations of [23,24,25], the GEOISO-Windows™ programme [22], and EASYGRESGRANT spreadsheet of [46] were used to carry out mass-balance modeling and calculate the volume changes in metasomatic systems.

Petrography and mineralogy

Petrography

Megascopically, the granite under consideration is medium–grained (about 2 mm in length) and characterized by pink to greyish-pink colors with equigranular texture and gneisossity. Microscopic examination classified this granite as slightly altered (Fig. 4) and highly altered (Fig. 5) monzogranites. The slightly altered one is composed mainly of quartz (~ 20–35%), alkali feldspar (~ 25–30%), plagioclase (~ 25–30%), and mica (~ 5–15%) (Fig. 4a). Quartz represents the main constituent occurring as anhedral elongated crystals, mostly showing undulose extinction (Fig. 4b). Perthite (K-feldspar) is present as patched of subhedral crystals and scattered microcline crystals of cross-hatched twinning (Fig. 4a). Plagioclase is found as euhedral to subhedral tabular oligoclase (An18) crystals that display its distinctive lamellar twining. Some crystals are zoned and characterized by selective alteration for the more sodic core, where sodium is replaced by potassium provided by hydrothermal solutions [47, 48]. Mica is represented by biotite and muscovite; biotite is the main mica mineral occurring as platy flakes exhibiting its characteristic interference colors (Fig. 4c). Accessory minerals are represented by zircon and monazite (Fig. 4d).

Fig. 4
figure 4

Photomicrographs of Wadi Sikait slightly altered monzogranite showing a Minute crystals of zircon (Zr) associating the essential minerals; quartz (Qz), oligoclase (Olig) and microcline (Mic), b Elongation and undulose extinction of quartz, c Foliated flakes of biotite (Bi) associating elongated quartz, and d Minute crystal of monazite (M) in quartz surrounded by glassy zone

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Fig. 5
figure 5

Photomicrographs of Wadi Sikait highly altered monzogranite showing a Biotite and muscovite with inclusion of zircon, CN (crossed nicols), b Fine crystals of epidote associating biotite, CN, c Euhedral crystal of metamictized zircon dismissing its interference colors, d Anhedral crystal of pale brown tetragonal uranothorite, e Well-formed crystals of allanite (All) exhibiting its characteristic interference colors (3rd order), CN, f Euhedral crystal of violet or purple color fluorite (Fl) associated with Nb mineral, PPL, g Oval crystal of highly radioactive Nb-mineral (ishikawaite) associated with Zircon (Zr), CN, and h EDX and Back-scattered image for the Nb-mineral (ishikawaite)

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On the other hand, the portion of this granite close to the shearing zone is intensively altered and characterized by yellowish color, where plagioclase is saussiritized and biotite is kinked. Some flakes of biotite are depleted in iron oxides and transformed to muscovite (muscovitization), and others are enriched by iron oxides and transformed to annite containing pleochroic halos enclosing very fine crystals of radioelements-bearing monazite and zircon (Fig. 5a). Occasionally, it is associated with fine crystals of secondary epidote (Fig. 5b).

The study of the sheared portion is enhanced by ESEM and EDX analyses to clarify the effect of tectonism in transformation and alteration and the role of hydrothermal solution in enriching the accessory minerals by the radioelements. In addition to zircon and monazite, this sheared portion is characterized by uranothorite, allanite, fluorite and Nb-minerals (ishikawaite) where the radioelements are incorporated in their structures by the circulation of the hydrothermal solutions. Zircon is intensively metamictized dismissing its interference colors (Fig. 5c). Uranothorite present as pale brown tetragonal crystals with masked interference colors associating zircon (Fig. 5d). The most prevalent accessory mineral, allanite comes in a variety of forms and is distinguished by its distinctive faint brown colour. It manifests as well-developed euhedral crystals that display their distinctive interference colours, (Fig. 5e). Presence of the radioelements in the crystal lattices is shown by radial cracks surrounding the crystals with disguised interference colors. Fluorite crystallizes at low temperatures (about 500 °C) and is found as a late-crystallizing mineral in felsic igneous rocks as a result of hydrothermal activity [49, 50]. Sometimes it is characterized by a violet or purple color and associated with Nb mineral (Fig. 5f). Nb-minerals are highly radioactive with the variable composition of Nb, U, and Th, and are surrounded by pleochroic hallos (Fig. 5g). They identified in the thin sections by opacity and kidney-shape and could be recognized by scanning electron microscopy (SEM) supported by semi-quantitative energy dispersive spectrometry (EDS) as ishikawaite (Fig. 5h).

Mineralogy

After applying systematic heavy mineral separation steps, the obtained handpicked mineral grains were then subjected to mineralogical identification under the reflected light microscope. Using a quantitative energy depressive X-ray (EDX) unite and a scanning electron microscope, the concerned grains were examined. Detailed mineralogical studies have been performed upon the slightly altered and highly altered monzogranite indicating the presence of: (1) Radioactive minerals, (2) Nb–Ta minerals, (3) REE minerals, and )4) Accessory minerals.

Radioactive minerals

Uranium minerals (uranyl silicates)

Most uranyl complexes that are in contact with the free silicate ions may precipitate uranyl silicate due to their solubility in water. The ESEM data of the uranyl minerals showed the presence of soddyite, uranophane, and kasolite (uranyl silicate minerals) in the highly altered monzogranite. Uranyl ion U6+ (which is easily combined with cations such as Ca2+, Cu2+, and Pb2+ to create kasolite and uranophane as secondary uranium minerals) is formed when U4+ is oxidized in the oxidation zone [51].

Soddyite (UO2)2SiO4(H2O)2 is frequently mixed with curite in the oxidized uranium ores. It is confirmed by ESEM that it has 68% U by wt. and 28.7% Si by wt. (Fig. 6a).

Fig. 6
figure 6

EDX and BSE images showing radioactive minerals in the altered monzogranites. a Soddyite, b Uranophane, c Kasolite, d–e Thorite, f Uranothorite, and g–j ThSiO4 associated with Zr, Sn and pyrite

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Uranophane [Ca(UO2)2SiO3(OH)2.5(H2O)], is known as uranotile; it is the most abundant uranyl silicate. Uranophane occurs as idiomorphic crystals that are prismatic or acicular, sometimes reticulate, but more often in divergent clusters or tufts (200 μm in length). In the present study, uranophane occurs as clusters and has 76.4% U by wt., 9.8% Ca by wt., and 13.8% Si by wt. (Fig. 6b).

Kasolite [Pb(UO2) (SiO3) (OH)2], has been tentatively identified as fine-grained aggregates. It is found as patches on the mica surface in the highly altered monzogranite. ESEM and semi-quantitative EDX analyses show U concentration in kasolite (46 wt. %) and Pb (33 wt. %) (Fig. 6c).

Thorium mineral

Thorite (ThSiO4) is crystalline in the tetragonal form and isomorphous with Zr and hafnium. It is stated by the ESEM techniques in the highly and slightly altered monzogranite samples. The obtained ESEM data and its EDX reflect the chemical composition of the thorite (Th 69.7 wt. %) and Y (4.5 wt. %), (Fig. 6d) in the highly altered samples; while in the slightly altered samples the obtained data reflect the chemical composition of the thorite (Th 41 wt. %), U (8.7 wt. %), with traces of LREEs (Fig. 6e).

Uranothorite [(U, Th) SiO4] may carry by the substitution for Th by U. The EDX analyses show that uranothorite has Th (59 wt. %), and U (19.5 wt. %) (Fig. 6f).

Thorite behavior during low-temperature alteration is important for element mobility and redistribution. Here we describe the alteration of ThSiO4 by hydrothermal fluids: as primary ThSiO4 associated with Zr, Sn, and pyrite (Fig. 6g-j). Most crystals have undergone significant compositional alteration. The change is variously attributed to fluid infiltration along fissures and reprecipitation (Fig.S1).

Nb–Ta minerals

Betafite [(Ca,U)2(Ti,Nb,Ta)2O6(OH)], has rare earth elements in its chemical composition; it is one of the Rare Earth Oxides [52]. The analyses show that it is enriched in U (43.9–53 wt. %), Th (13.3–0 wt. %) Nb (17–16.2 wt. %), with traces of REEs (Fig. 7a, b) in the highly and slightly altered monzogranite respectively.

Fig. 7
figure 7

Back scattered images and EDX data for the recorded minerals in the altered monzogranite:—a–b Betafite, c Plumbobetafite d–e Columbite, f–g Fergusonite, and h Aeschynite

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Plumbobetafite [(Pb,U,Ca)(Nb,Ti)2O6(OH)] exists as small aggregates on the surfaces of quartz in the highly altered monzogranite. Its EDX indicated the presence of 51% U by wt., 16.9% Nb by wt., and 16.9% Pb by wt. (Fig. 7c).

Columbite [(Fe,Mn)(Nb,Ta)O4] is composed mainly of Nb (56.2–58 wt. %), Ta (9.1–11 wt. %), and Fe (26.9–23.4 wt. %) (Fig. 7d, e) in highly altered and slightly altered samples respectively.

Fergusonite has chemical formula [(Y,REEs)NbO4], where REE are the rare-earth elements in a solid solution with Y. Defining this mineral has been stated using the ESEM and has Nb (34.3–33.3 wt. %), Ta (5.4–2.6 wt. %), Y (29.7–25 wt. %), and REEs (19–29.5 wt. %) with U and Th traces (Fig. 7f, g) in highly altered and slightly altered samples respectively.

Aeschynite its general formula is [(Y,Ca,Fe)(Ti,Nb)2(O,OH)6]; its chemical composition, in the highly altered monzogranite, is Ti (13 wt. %), Nb (27 wt. %), Y (16 wt. %), Ta (4 wt. %), and U + Th (3 wt. %) (Fig. 7h).

REEs minerals

Monazite [(Ce,La,Th)PO4] is isostructural with Huttonite (ThSiO4), and Cheralite [(LREE,Th, Ca,U)(P,Si)O4] [53]. It is typically replaced by thorium in monazite. As a result of the complicated chemical compositions of the REEs minerals, isostructural minerals exhibit a variety of linked replacements in nature based on the pressure, temperature, and chemical compositions of the hydrothermal solutions and magma [54].

* The mechanisms of thorium substitution in monazite is as follows:

Thorium enters monazite via two paired substitutions: the cheralite (1) replacement and the huttonite exchange (2) [55].

$${\text{2LREE}}^{{{3} + }} \leftrightarrow {\text{Th}}^{{{4} + }} + {\text{Ca}}^{{{2} + }}$$
(1)
$${\text{P}}^{{{5} + }} + {\text{REE}}^{{{3} + }} \leftrightarrow {\text{Th}}^{{{4} + }} + {\text{Si}}^{{{4} + }}$$
(2)

Cheralite is usually the more prevalent of these two end members in natural monazite. In the highly altered samples, the EDX spectrum shows that these crystals range in composition from monazite to cheralite (Fig. 8a–c) due to the hydrothermal alteration effect.

Fig. 8
figure 8

EDX and BSE images showing rare earth minerals in the highly altered monzogranites. a–c Monazite to Cheralite, d–e Xenotime, f–g Zircon and h Fluorite

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Xenotime (HREE, Y, PO4) is a heavy rare earth element (HREE) phosphate, the principal host to economic HREE mineralization and a common accessory mineral in igneous, metamorphic and sedimentary rocks [56]. The existence of xenotime may be related to the abundance of the main component yttrium [57]. The EDX data shows that xenotime is composed of Y (35–17.6 wt. %), P (22–14 wt. %), HRREs (15–17 wt. %), Th + U (4–26.3 wt. %) and Fe (2.1–4.7 wt. %) (Fig. 8d, e) in highly altered and slightly altered samples respectively.

Accessory minerals

Zircon (ZrSiO4) was found as a solid solution with thorite as confirmed by EDX analyses Zr (50 wt. %), Hf (3 wt. %) with traces of U + Th (7 wt. %) (Fig. 8f). Some crystals contain inclusions of xenotime in the slightly altered monzogranite (Fig. 8g).

Fluorite (CaF2) exists in the form of fracture filling (Fig. 8h). The intimate association of purple fluorite with uranium minerals has been recognized in different deposits; this association is typical of epigenetic hydrothermal systems. The dark-blue/violet color of fluorite has been attributed to the effects of radioactivity, as postulated by many authors [49,50,51,52,53,54,55,56,57,58], which generates a disturbance in its crystal lattice rather than as a result of its chemical composition. Pi-Puig [59] concluded that the uranium-rich fluorite precipitated in the initial phases of mineralization from a reducing fluid, with low salinity (< 8% NaCl eq.) and an intermediate temperature (110–230 °C), and that the presence of organic compounds and sulfides (mainly pyrite) favored the simultaneous precipitation of uraninite (pitchblende variety) and fluorite.

Geochemistry

Major oxides

Eleven samples of altered monzogranites from Wadi Sikait were chemically analyzed to estimate major oxides, trace, and REEs using ICP-MS (Table 3). The altered samples are subdivided into slightly and highly altered rock samples. Four samples were chosen representatively from the slightly altered monzogranite, and seven samples from the highly altered ones. The altered monzogranite samples contained 73.4 –74.6 wt.% SiO2, 12.14–12.36 wt.% Al2O3, 3.3–2.12 wt.% Fe2O3, and 7.61–8.9 wt.% K2O + Na2O for the highly and slightly altered samples, respectively.

Table 3 Chemical analysis of major oxides (Wt. %) and some trace elements and REEs (ppm) for Wadi Sikait monzogranite mineralized zone, SED, Egypt
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Trace elements

They were normalized to the primitive mantle [60]. It is indicated from the trace element patterns that the altered granites (Fig. 9) are effectively enriched in large ion lithospheric elements (Rb, Th, & U) and REEs (ƩREE + Y of 345 & 287.6 ppm) and depleted in Cu, Ni, Cr, and La. In addition, all samples had high Ga (38–31 ppm) contents for the highly and slightly altered samples.

Fig. 9
figure 9

Primitive mantle–normalized trace element for Wadi Sikait altered monzogranite using the primitive mantle values of [60]

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Different alteration processes have affected the altered monzogranites under study along the fault plane. Their chemical analyses exhibit an increase in Th, U, Zr, Nb, and MREEs and depletion in L & HREEs for the highly altered samples; while in the slight one, there is an increase in Nb, U, Th, and LREEs and depletion in MREEs and HREEs (Fig. 10a, b).

Fig. 10
figure 10

Plots of some major oxides, trace, and rare earth elements for a the highly altered monzogranites, and b the slightly altered monzogranite

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REEs geochemistry

The two types of rocks (highly and slightly altered monzogranite) can be also distinguished by their chondrite normalized REE patterns. The altered granites show ‘seagull’ forms in chondrite-normalized REEs diagrams (Fig. 11). In the slightly altered samples, LREE and HREE are highly fractionated (LREE/HREE) (9.3); while in the highly altered there is less REE fractionation (LREE/HREE) (1.28). The total REE concentrations of 241.8 & 249.7 ppm, (La/Yb)N of 0.46–8.9, negative Ce anomaly (δCe) 0.98–0.96, and (negative Eu anomaly) δEu of 0.16–0.24 (in average) for the highly and slightly altered samples (Table 4). A weak REE fractionation, a strong negative Eu anomaly, and a weak negative Ce anomaly are all indicators of a weak REE fractionation [62,63,64]. Chondrite-normalized REE patterns demonstrate that the study samples were distinguished by LREE enrichment in the slightly altered samples and HREE in the highly altered, with strong negative Eu anomalies.

Fig. 11
figure 11

Chondrite-normalized REEs diagram of [60] for the altered monzogranites. For denotations see Fig. 9

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Table 4 Calculated tetrad effect and geochemical parameters of the Wadi Sikait altered monzogranites, SED, Egypt
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REE tetrad effects phenomenon in the altered monzogranites

The tetrad effect in highly evolved granitic systems is possibly caused by (1) fractional crystallization during igneous crystallization [71], (2) fluid–melt interaction during crystallization of the silicate melt [72, 73], (3) hydrothermal alteration during hydrothermal fluid–rock interaction [74], and (4) weathering after granite formation [75].

Masuda et al. [76] proposed two distinct categories of tetrad impacts. Both types are derived from each other and mirror themselves (M-type in solid samples as residue and W-type in the interacting fluids as extract). The labels “M” and “W” refer to REE patterns with upwards or downwards-curved tetrads respectively.

As suggested by [77] the development of the lanthanide tetrad effect (M-type) with pronounced negative Eu* in highly evolved granitic rocks implies the removal of a mirroring REE-pattern (W-type) with positive Eu*, corresponding to a coexisting high-temperature aqueous fluid. As a result, the majority of the granite facies under consideration exhibit steadily larger degrees of diversity in the REE fingerprints of silicate melts [78, 79].

The tetrad implication can be quantified using the techniques that proposed by [74, 77]. The TE1,3 values range from 1.17 to 1.02 in the highly and slightly altered monzogranite (Table 4).

According to the quantification approach suggested by [77] for the measurement of the 3rd and 4th tetrads which can be estimated as follows.

$${\text{t1}} = \left( {{\text{Ce}}/{\text{Ce}}^{*} \times {\text{Pr}}/{\text{Pr}}^{*}} \right)$$
$${\text{t3}} = \left( {{\text{Tb}}/{\text{Tb}}^{*} \times {\text{Dy}}/{\text{Dy}}^{*}} \right)$$
$${\text{t4}} = \left( {{\text{Tm}}/{\text{Tm}}^{*} \times {\text{Yb}}/{\text{Yb}}^{*}} \right)$$

Then the tetrad implication TE1,3 = SQRT(t1 × t3).

Highly altered samples show convex tetrad implications of M-type in the 3rd and 4th tetrads, and the slightly altered ones have a flat HREE pattern based on the normalized REEs distribution patterns (Fig. 11). Rare earth elements in the highly altered granites are related to hydrothermal alteration during hydrothermal fluid–rock interaction.

Tetrad implications mechanism

Though the presence of tetrad impacts in geochemical operations, the primary procedure of this trait remain unknown [80]. Procedures like the melt/fluid interaction that is generally contributed to the hydrothermal activities and magmatic are the main aim of the present study. Yurimoto et al. [81] stated that the tetrad impacts of the normalized REEs distribution patterns may be attributed to the mineralization. REE3+ complexation by different ligands in various geological environments is considered a possible procedure for the tetrad implications. The outputs of this study show that in the hydrothermal F-rich and magmatic systems, REE-fluoride complex formation is an essential factor. Convex-type tetrad impacts are primarily caused by REE-Cl combination, whereas concave-type effects are caused by REE-F combination, according to thermodynamic modeling of Cl and F combination [82]. On the other hand, the activity of F-rich fluids in geological operations has been attributed to the M-type tetrad effects.

The late magmatic differentiates with deuteric alteration or substantial hydrothermal interactions have the greatest REEs tetrad impact. Additionally, the tetrad effect frequently occurs in conjunction with other altered geochemical action of numerous trace elements, which has been named by [83] as non-CHARAC (Charge & Radius Controlled) behavior. This activity is generally found in highly advanced magmatic systems that are abundant in elements like F, B, Li, and Cl, as well as in H2O, CO2, and other components [84]. This shows that an aqueous fluid system becomes increasingly important in the latter phases of acidic rocks crystallization.

Some researchers noticed the Co-existence of the W-shaped concave and M-shaped convex tetrad implications in many sites around the world such as Ti–rich bauxite in Iran [85], El Sela Eastern Desert, [86, 87], Qahr-Abad fluorite deposits in Iran [88], and the Eastern and Western Deserts of Egypt [10, 29]. The M-type tetrad impact is frequently noticed in the rare metal hydrothermal systems that produce granites [84]. The production of REE tetrad effects in granites is primarily governed by melt-fluid interaction in the late fractional crystallization phases [89]. The REE tetrad systems of these rocks were made by fractionation of the residual melt from a high-temperature aqueous fluid in the late phases of magma differentiation; just like the negative Eu anomalies were [90, 91].

In Fig. 12a the degree of Eu negative anomaly increases in proportion to the degree of tetrad impact (TE1,3). Based on the depletion of elements including Ba, Sr, P, Eu, and Ti, [92, 93] hypothesized that the collected granite had undergone considerable plagioclase and K-feldspar fractionation and small fractionation of Fe-Ti, and oxides. In plagioclase and alkali feldspar matrices, strontium is a compatible element, but rubidium is incompatible with plagioclase and very mildly incompatible/compatible with alkali feldspar [94]. The Zr/Hf, TiO2, and CaO ratios likewise show that they are directly proportional to the level of the Eu anomaly (Fig. 12b, c).

Fig. 12
figure 12

Eu/Eu* vs. a tetrad effects, b CaO, c TiO2, and d Zr/Hf variation diagrams for the altered monzogranites. For denotations see Fig. 9

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Since the pronounced negative Eu anomalies include many geochemical characteristics, such as the tetrad effect, it may be assumed that the feldspar fractionation that occurred during magmatic differentiation is what caused the Eu anomaly. In contrast to fractional crystallization, (Fig. 13a, b) suggests that the REE tetrad implication may be the result of a fluid-melt interaction. So, based on our results, it would appear that the REE tetrad impact, following the production of granitic magma in a system of magmatic-hydrothermal transition, is most likely what caused the Eu anomaly of the studied altered monzogranites.

Fig. 13
figure 13

a La/Nb, and b Zr/Hf vs. tetrad effects for the altered monzogranites. For denotations see Fig. 9

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Discussion

Alteration geochemistry

Wadi Sikait altered monzogranites display a different degree of hydrothermal alteration as silicification, sericitization, and hematitization. Before discussing petrogenesis, it is important to consider how the fluid-rock and the hydrothermal alteration interaction could modify the fundamental geochemical compositions [95,96,97,98,99].

The substantial hydrothermal alteration may have an impact on the alkali elements (e.g., Na, Ba, Sr, Rb, and K), including:

  1. i.

    K alteration causes the loss of CaO and Na2O, while K2O enrichment, and the increased mobility of LILEs due to replacing the plagioclase by K-feldspar [100,101,102],

  2. ii.

    phyllic alteration indicates the enrichment of MgO, SiO2, and Fe2O3, and the depletion of K2O, Na2O and CaO [95, 103],

  3. iii.

    silicification happens not only due to adding SiO2 but also due to the loss of most other compositions such as K2O, CaO, Fe2O3, Al2O3, Ba, Sr, and MgO that result from replacing some minerals by the quartz [95], and

  4. iv.

    sericitization shows an increase in K2O, Pb, Rb, and LOI and a loss of Sr, Na2O, and CaO due to replacing the plagioclase by sericite [104].

Based on geological and petrographic research, the hydrothermally altered rocks in the study area exhibit widespread silicification, sericitization, and albitization. The chemical index of alteration (CIA = 100 × molar proportion [Al2O3/(Al2O3 + Na2O + K2O + CaO*)] of [68] is used to calculate the relative degree of alteration. CIA ratio indicates that the highly and slightly altered rocks (of average = 55–58) were impacted by a considerable change in chemical composition concerning the fresh rocks (of average = 56) [61].

According to Meyer and Hemley [105], Wadi Sikait altered monzogranites are presented on AKF ternary diagram, where K = K2O, F = Fe2O3 + MgO + MnO, and A = Al2O3–(K2O + Na2O). It shows that the study samples fall in sericite facies (due to sericitization processes) and propylitic field (Fig. 14a). The weathering trends are plotted on the (CaO + Na2O)–K2O–Al2O3 triangular diagram (Fig. 14b) [106, 107], the samples exhibit a relative enrichment in Al2O3 and K2O compositions due to the feldspars alteration into clays [108,109,110].

Fig. 14
figure 14

a AKF ternary diagram of [105]. b Al2O3-(CaO + Na2O)-(K2O) ternary diagram after [111, 112].c Na%-K% variation diagram after [106]. d Alteration factors for Wadi Sikait altered monzogranites. For denotations see Fig. 9

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The K-Na variations diagram of [106] presents 5 trends: K-metasomatism, silicification, desilicification, Na-metasomatism, and argillation (Fig. 14c). Most samples lie in the argillation trend and K-metasomatism. The most diagnostic geochemical properties of Wadi Sikait samples shown considerable variations in the Ca, Na, Fe, and Mg content (Fig. 14d). Intensely altered samples have Na2O + CaO values in weight percent due to the gradual drop in Na2O and CaO that occurs as alteration intensity increases. This pattern is a sign of the production of muscovite and sericitization as well as the destruction of plagioclase.

Ishikawa et al. [113] developed the Ishikawa alteration index (AI) to rate the severity of chlorite and sericite alterations. AI = 100 x (K2O + MgO)/(CaO + Na2O + MgO + K2O) and the chlorite carbonate pyrite index (CCPI) = 100 ×  (Fe2O3 + MgO)/(K2O + Na2O + Mgo + Fe2O3) are two alteration indices used in the alteration box plot (Fig. 15). Sericite and chlorite replacement of sodic plagioclase as the primary processes tracked by the index. Following are some responses to these modification processes [114]:

$$\begin{gathered} {\text{3NaAlSi}}_{{3}} {\text{O}}_{{8}} + {\text{2H}}^{ + } + {\text{K}}^{ + } \mathop{\longrightarrow}\limits^{{\phantom{a}}}{\text{KAl}}_{{3}} {\text{Si}}_{{3}} {\text{O}}_{{8}} \left( {{\text{OH}}} \right)_{{2}} + {\text{6SiO}}_{{2}} + {\text{3Na}}^{ + } \hfill \\ {\mathbf{Albite}}\quad \quad \quad \quad \quad \quad \quad \quad \quad {\mathbf{Sericite}}\quad \quad \quad \quad \quad {\mathbf{Quartz}} \hfill \\ \end{gathered}$$
(3)
$$\begin{gathered} {\text{2KAl}}_{{3}} {\text{Si}}_{{3}} {\text{O}}_{{{1}0}} \left( {{\text{OH}}} \right)_{{2}} + {\text{6Mg}}^{{ + {2}}} + {\text{9Fe}}^{{ + {2}}} + {\text{3H}}_{{2}} {\text{SiO}}_{{4}} + {\text{18H}}_{{2}} {\text{O}}\mathop{\longrightarrow}\limits^{{\phantom{a}}}{\text{3Mg}}_{{2}} {\text{Fe}}_{{3}} {\text{Al}}_{{2}} + {\text{28H}}^{ + } + {\text{2K}}^{ + } + {\text{Si}}_{{3}} {\text{O}}_{{{1}0}} \left( {{\text{OH}}} \right)_{{8}} \hfill \\ {\mathbf{Sericite}}\quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad {\mathbf{Chlorite}} \hfill \\ \end{gathered}$$
(4)
Fig. 15
figure 15

a Alteration box plot diagram after [119, 120]. b A’FK diagram indicating Fe or K-gain after [118] for Wadi Sikait altered monzogranites. For denotations see Fig. 9

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The initial process is typically replacing the albite by sericite [115, 116]. Based on constant Al2O3, reaction (1) results in gaining K2O and losting Cao and Na2O, while reaction (2) results in gaining in MgO and FeO, and losing K2O. Based on the various diagrams (Fig. 15a) which applied the alteration indices of Ishikawa (AI), sericite–chlorite altered in the chlorite–carbonate–pyrite index (CCPI); F = (Fe2O3 + MgO + MnO), Á = Fe2O3 + Al2O3–(CaO + Na2O + K2O); and K = K2O, each alteration type has a diagnostic alteration trend [117].

On plotting the CCPI vs. (AI), it is indicated that samples of the Wadi Sikait altered monzogranite primarily follow the sericite alteration trend and to some extend the K-feldspar trend (Fig. 15a) because of their relatively high AI values. Also, they primarily indicate the K-gain for the slightly altered samples with little Fe-gain for the highly altered samples following the A’KF ternary phase diagram of [118] (Fig. 15b).

Mass-balance calculation

Alteration indices and mass balance approaches differ primarily in that they quantify chemical changes as opposed to taking into consideration mineralogical restrictions. Field observations are dependent on alteration; thus it is not always simple to apply them to mass balance findings. However, mass balance allows for the precise measurement of the quantities of Si, Fe, Mg, K, Na, and Ca that are absorbed by the fluid or leached from the rock [120]. Another distinction between indexing and mass balancing approaches is that the latter calls for the identification of a precursor as well as the analysis of a few trace immobile elements.

Understanding geological processes, particularly the mass losing or gaining elements in the hydrothermal solutions, can be done by using mass balance calculations [95, 121]. Gresens [23] Firstly proposed modeling the immobility/mobility in a solution system, while the immobile element concept was stated by [122, 123].

Chemical mass transfer calculations in the study area include comparing the geochemical compositions of (1) the unaltered monzogranite [61] (regarded as the initial parent rock composition, i.e. the fresh rock) and (2) The Wadi Sikait altered monzogranite (the highly and least altered) composition was calculated by averaging the bulk chemical compositions of 10 unaltered rocks, sampled within 11 samples from Wadi Sikait monzogranite. For estimating the mass balance, two methods were applied.

The Gresens mass transfer equation

These techniques calculate the mass changes (volume and density changes) brought on by modification [23, 124]. The hydrothermal alteration accompanied with the altered monzogranite mineralized zone in Wadi Sikait reveals the presence of chemical ion exchange among the hydrothermal fluid, and the host and wall rocks, resulting in the creation of new different alteration minerals and the formation of several zones based on the pH, Eh, temperature, and the host rock and fluid compositions [105, 125]. The hydrothermal alteration processes are usually accompanied by the gain and loss of some mineral components of the entire rock mass [126].

The geochemical composition of the hydrothermally unaltered and altered rocks differs because of the mineralogical and bulk chemical alterations. Its zonation shows a variation in the fluid composition through time due interaction between this fluid and its host rocks. The hydrothermal alteration accompanied with the altered rocks indicates that the occurrence of chemical ion exchange between the hydrothermal fluids and the wall rocks. The gain and loss of parts of the whole rock mass are often linked to the hydrothermal alteration processes [127].

Gresens [23] was the first to propose this method, in which the simultaneous decrease and increase in volume that occurs in the hydrothermal alteration process are eliminated, allowing an evaluation of the level of the elemental mass loss or gain. [24, 25] developed the equations of [23] by supposing that the immobile elements in the alteration operations have no mass transfer along the isocon trend.

Using [23] equations and [24] isocon diagrams, the GEOISO-Windows™ software of [22] is the most modern program that is applied to analyze and visualize the mass volume/balance change due to the final mobility of the components. According to [128], elements of the high field strength (HFS), particularly Zr, Ti, and Al, were proposed to be stationary or have a limited mobility in the alteration operations. The major oxides as well as the trace elements of the studied altered monzogranite at Wadi Sikait have a significant depletion and addition based on the mass balance estimations that plotted on the isocon and histograms (Fig. 16, Table 5).

Fig. 16
figure 16

Isocon diagram comparing the mean composition of slightly altered samples and the highly altered samples of the Wadi Sikait monzogranites

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Table 5 Elements/oxides mass changes concerning original whole rock mass ((Mfi-Moi)/Mo) and concerning original elements/oxides mass in original rock ((Mfi-Moi)/Moi, data resulted from GEOISO-A Windows™ program)
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According to [23], comparing the volume and composition of the altered and unaffected rocks can help scientists identify changes in elemental composition brought on by gains, losses, or dilution. Grant [24] enhanced the [23] equation and stated that the immobile elements should be presented along the isocon line with no mass transfer.

The highly altered monzogranite samples in Wadi Sikait have mainly enriched in SiO2, Al2O3, Fe2O3, MnO, K2O with U, Y, Nb, Zr, Hf and Rb and depletion in Na2O, MgO and Ni. While the samples of the least alteration type have enrichments in SiO2, Al2O3, with U, Zn, and Nb, and depletion in Fe2O3, MnO, MgO, Y, Zr, Hf and Pb (Table 5, Fig. 16).

According to petrographic research, silicification processes may be associated with SiO2 enrichment, which may also be brought on by the more developed granites' increased the quartz amount as a free Si-bearing mineral. The enrichment of K in all samples could be related to K- metasomatism of these rocks. The change in redox potential of the fluids may have caused the Mn2+ to Mn4+ conversion, and it’s fixing as insoluble oxides and hydroxides may also have contributed to the Mn mass gain [129]. The enhanced Fe2O3 content may be attributed to the presence of hematite, goethite and pyrite that have been formed during the weathering processes in suitable pH-Eh conditions. Due to the sericitization processes, Al and Rb increase could be intimated to clay and sericite minerals. The depletion of Na could have produced due to the feldspar alteration and the release of Na into the alteration fluids. The Mg loss in the studied samples may be due to the alteration and destruction of the ferromagnesian minerals (such as micas and biotite). Y, Hf, and Zr are immobile and concentrated in the accessory minerals particularly in the xenotime, thorite, zircon, and uranothorite. U and Nb increments could be attributed to the presence of kasolite, uranophane, betafite, thorite, uranothorite, columbite, fergusonite, plumbobetafite, and aeschynite minerals.

Mass balance calculation

It is performed using the template of [46] which is based on the modified formula of [23], where the fresh monzogranite of [61] was used as the standard unaltered samples (Ci0), while highly and slightly altered samples were used as (Cia). The average concentrations were applied to reduce the implication of the compositional heterogeneity (e.g., [130, 131]).

An “isocon” is another solution for these models that exhibit no relative mass loss/gain. Plotting the elemental concentrations of the altered rock versus concentrations in the intact rock shows an isocon as a trend passing through the origin point. The mobility of the various elements is indicated by their positions on the plot relative to the isocon. Plotting the elemental concentrations of the original rock on the x-axis (marked Co) versus the elemental concentration of the altered rock on the y-axis (marked Ca). For each lithologic unit, one plot is given revealing the immobile elemental content and its increase/decrease in correlation with the isocon line.

The isocon lines are identified as the best-fit lines passing through the original point where the elements having relatively constant concentrations during the alteration process based on loss/gain ± 10% in correlation with the Ci0 both in the retrograde and prograde were chosen as immobile elements.

The isocon analysis states that there is an effective mass change (MC) (22.95 & 11.11) and volume change (VC) (1.8 & − 7.99) for the highly and slightly altered samples, respectively (Fig. 16, Table 5). Generally, some major oxides were lost from the slightly altered samples and transformed into the highly altered ones.

The graphical methods are primarily characterized by the clarity of their diagrams and their fast implementation [132]. The isocon method of [24] was applied to prove the chemical changes among the altered rocks and their accompanied intact rocks. All the calculations are applied following Grant’s approach [24]:

$$\Delta C = C^{{\text{F}}} /C^{{\text{A}}} \cdot C^{{\text{A}}} - C^{{\text{F}}}$$

where CA and CF are the concentrations in the altered (A) and fresh (F) sample, respectively; and ΔC refer to the loss/gain of trace elements in ppm, or to the major elements in g/100 g of rock. Results are presented in Table 6.

Table 6 Selected elements mass change (Gain/Loss) of the highly and slightly altered monzogranites in Wadi Sikait area, SED, Egypt
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It states that a large amount of Al, Fe, Si, and some Zr, Li dissolved in the fluid system (Fig. 17, Table 6). The elements Si, Al, Mg, Ca, and Sr exhibited strong mobility (∆Ci/C0 > 1.0) into the highly altered rocks during the hydrothermal alteration operations.

Fig. 17
figure 17

The calculated mass-change of some selected elements from the Wadi Sikait altered monzogranites

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In terms of REEs and trace elements, most of the HREEs were migrated during a hydrothermal alteration into the highly altered samples, but LRREs were gained in the slightly altered ones. The highest gain in the highly altered samples was noticed in Zr (1194.91 ppm), Nb (882.86 ppm), U which is up to (331.79 ppm), Th (345.04 ppm), whereas in slightly altered one U which is up to (14.83 ppm), and Th (42.15 ppm).

Conclusion

The alteration zones in the altered monzogranites were identified by Landsat-image techniques and confirmed by field trips. The altered monzogranites were categorized into slightly and highly altered monzogranites. The petrographic investigations were comparable with these results. It is worthy to mention that severe alteration processes lead to chemical and mineralogical changes. The noticeable increments and decrements in most major oxides, trace and rare earth elements, accompanied by distinct mineral association confirmed these changes. The samples of Wadi Sikait highly altered monzogranite have mainly enrichments in SiO2, Al2O3, Fe2O3, MnO, K2O with U, Y, Nb, Zr, Hf and Rb and depletion in Na2O, MgO and Ni, while the samples of the slight alteration have enrichments in SiO2, Al2O3, with U, Zn, and Nb, and depletion in Fe2O3, MnO, MgO, Y, Zr, Hf and Pb. Highly altered samples show convex tetrad implications of M-type in the 3rd and 4th tetrads, and the slightly altered ones have a flat HREE pattern based on the normalized REEs distribution patterns. The total REE concentrations of 241.8 & 249.7 ppm, (La/Yb)N of 0.46–8.9, negative Ce anomaly (δCe) 0.98–0.96, and (negative Eu anomaly) δEu of 0.16–0.24 (in average) for the highly and slightly altered samples. The isocon analysis indicates that there was a significant mass change (MC) (22.95 & 11.11) and volume change (VC) (1.8 & − 7.99) for the highly and slightly altered samples, respectively. Generally, some major oxides were lost from the slightly altered samples and transformed into the highly altered ones, illustrating the effects of hydrothermal and water-rich alteration processes connected to vigorous physic-chemical changes. Minerlogically; the studied altered monzogranites demonstrated the presence of these mineralogical groups, (1) radioactive minerals as uranyl silicates (soddyite, uranophane, kasolite), thorium minerals (thorite, uranothorite), (2) Nb—Ta minerals (betafite, plumbobetafite, columbite, fergusonite, aeschynite), (3) REE minerals (monazite, cheralite, xenotime,) and (4) zircon and fluorite as accessory minerals (Table 7).

Table 7 Summary of the mineralogy of the recorded minerals in the studied altered monzogranite, Wadi Sikait area, South Eastern Desert, Egypt
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