Genetic implications of Th/U, Th/K, and U/K ratios for U mineralizations: A case study from El-Missikat and El-Erediya shear zones, Eastern Desert, Egypt

The current work is an attempt to reveal the possible utilization of the radiometric measurements to build-up a complete genetic scenario for magmatic, hydrothermal, and supergene uranium mineralization. For this purpose, ground gamma-ray survey was performed through the exploratory tunnels dug perpendicular to El-Missikat and El-Erediya shear zones, the Central Eastern Desert of Egypt. Contents of U, Th, and K were measured for the host pink granite (e.g., avg.15.94 U ppm, 35.62 Th ppm, and 6.63% K), alteration zones (brecciation, silicification, greisenization, kaolinization and hematitization) (e.g., avg. 124.01 U ppm, 63.67 Th ppm, and 3.13% K), and mineralized silica veins (e.g., avg. 312.65 U ppm, 92.22 Th ppm, and 2.62% K). All of these data were graphically represented as correlation plots of Th vs. U, Th/U vs. U, Th vs. K, and U/K vs. Th/K. The overall results indicate magmatic, hydrothermal, and supergene sources of El-Missikat and El-Erediya U mineralization. The magma-derived U contents are enclosed mainly in the pink granite that is mostly characterized by normal Th/U (2.5–5) and Th/K ratios (3–5*10–4). The hydrothermal processes through the alteration zones and mineralized silica veins are reflected by the weak correlation of Th with U (e.g. r = 0.13 and − 0.39), the strong negative correlation of Th/U ratio with U (e.g. r = − 0.82), 2.5˃Th/U˃0.1, Th/K˃5*10–4, Th/K < 3*10–4, and the strong positive correlation of U/K with Th/K (e.g. r = 0.91) as well as the occurrence of thorite, columbite, xenotime and hydrothermal zircon (0.5 > Th/U ≤ 0.1). Afterwards, the hydrothermal mineralization underwent some degrees of chemical weathering that resulted in supergene U mineralization whose fingerprints can be traced by the occurrence of secondary U minerals (e.g. kasolite and uranophane), Th/U ratios ≤ 0.1, and the weak correlation between Th/K and U/K (e.g. r = 0.39 and − 0.11).


Introduction
U, Th, and K are considered to be large ion lithophile elements well-known for their radioactivity and heat production as well as the common occurrence, at relative abundances, in granites compared to the other rock types [8,62].They have a characteristic geochemical behavior (e.g. the incompatible magmatic fractionation of U and Th in relative to K and the post-magmatic mobilization of U and K compared to Th) under magmatic, hydrothermal, and supergene conditions (e.g [16,18,23,24,37,47,55,59,60], making them significant tracers in the geochemical exploration and mapping the hydrothermal and supergene alteration zones (e.g; [1,13,20,26,27,36,46,48,49,51,58,64,66,67].Apart from the other geochemical approaches such as fluid inclusions and whole rock geochemistry, the current work tries to build-up a radioelement ratio-based genetic scenario for vein-type uranium mineralizations by measuring Th/U, Th/K, and U/K ratios through host rocks, alteration zones, and U mineralized-veins.For this purpose, polymetallic, vein-type U mineralizations hosted by El-Missikat and El-Erediya pink granites, Eastern Desert of Egypt, were undertaken as a case study.The radioactivity of such localities was first discovered, under supervision of the Egyptian Authority of Nuclear Materials, using airborne gammaray survey during a project of uranium exploration between latitudes 25-27ºN, through the Central Eastern Desert of Egypt [7].This was followed by ground inspection of these anomalous sites, leading to dig a number of exploratory mining tunnels through the northwestern part of El-Missikat pluton and the southern part of El-Erediya pluton [3,4,10,31,35]).

Methodology
The distribution of U (ppm), Th (ppm), and 40 K (%) was determined for fresh granite, alteration zones, and radioactive siliceous veins within the exploratory tunnels by handheld gamma-ray spectrometer (RS-230).This device is characterized by BGO ''bismuth germinate oxide''detector by which the radioelement concentrations are directly measured without the need for calibration resources.The background was measured on the surface for the surrounding fresh granite, out of the exploratory tunnels, at 7.4 ppm U, 26.1 ppm Th, and 6% K.The radioactive measurements were systematically taken through successive profiles, 50 cm/read along each profile, that are perpendicular to shear zone and arranged at 1 m long distance, starting from the main adit until reaching the subsequent drifts (D I and D II at El-Missikat and D II and D III at El-Erediya).The total count of these measurements for fresh granite, brecciation, greisenization, silicification, kaolinization, hematitization, brecciated and massive silica veins was estimated at 27,114,390,20,43,22,226, and 231 reads.The obtained data set was then undergone statistically processing to study the varied correlations between them and drive the radio-elemental ratios, including Th/U, U/K, and Th/K.On the other hand, the radioactive measurements were coupled by separation of radioactive minerals from some alteration zones and mineralized silica veins at El-Missikat and El-Erediya.For this purpose, representative samples were ground up to − 500 μm and then bromoform-based separation along with handpicking was conducted.The separated mineral fraction was investigated by SEM-EDX.Although the current approach depends mainly on the radiometric measurements, its validity can be substantiated by literature (e.g.Heikal et al. 2022) that compared the radioelemental ratios of the monzo-syenogranites (the Central Eastern Desert of Egypt)'' obtained by the ground-gamma ray spectrometer (e.g.avg.2.53 Th/U ratio) in relative to the data obtained by ICP-MS (e.g.avg.2.9 Th/U ratio).

Distribution of radioelements through silica veins
Through El-Missikat shear zone (Table 4), U contents in brecciated silica veins (e.g.avg., 88-121 ppm and max., 487-1007.7 ppm) are remarkably higher than the massive veins (e.g.avg., 54.66-82.19ppm and max., 224.2-676 ppm), indicating other pulses of U-bearing hydrothermal solutions emplaced due to the reactivation of shear zone that in turn caused brecciation of the earlyformed silica veins.On the other side, the red silica veins at El-Erediya are characterized by the highest U anomalies (e.g.avg., 183.26-312.65 ppm and max., 536.4-2990.5 ppm), probably due to the co-occurrence of silica and iron oxyhydroxides.The former is considered as a favorable repository for U accommodation that is facilitated by the latter whose task is to adsorb and reduce the dissolved U +6 ions [50].

Genetic implications using Th/U ratio
U and Th are incompatible trace elements mostly concentrated in granitic magmas where they behave, without fractionation, as tetravalent ions related to each other.This relation is expressed as Th/U ratio that normally varies between 2.5 and 5 [6].For the felsic magmatism, Th/U ratio is considered as a good indication for the magmatic differentiation degree between peralkaline and peraluminous granitic suites.In peralkaline magma the excess of alkalies in relative to alumina and the high temperature conditions induce high solubility of Th and U-bearing resulting in an increase of U contents and Th/U ratios < 4 [23].In all cases, Th and U contents of magmatic origin reveal a strong positive correlation.Comparing with the current study, pink granites of El-Missikat (Fig. 8a) and El-Erediya (Fig. 8b) exhibit moderate and very weak relationship between Th and U at correlation coefficients of 0.5 and 0.04, respectively, indicating U enrichment.Also, U contents exhibit a strong negative correlation with Th/U ratios for El-Missikat (r = − 0.67) (Fig. 8c) and El-Erediya pink granite (r = − 0.82) (Fig. 8d).Further, the calculated Th/U ratios fall through the normal range and below 2.5.Accordingly, magmatic and postmagmatic origin can be presumed for the granite-hosted U mineralization.The post-magmatic source can be either hydrothermal or supergene solutions.The former can be discriminated from the latter using Th/U ratio whose range ''2.5 > Th/U > 0.1'' is assigned to the hydrothermal uranium mineralizations [17,40,68], while values ≤ 0.1 are associated with supergene enrichment [15,16].These values, along with the normal ratio, are collectively represented by Th-U variation diagram on which both magmatic and hydrothermal sources are assigned to the anomalous radioactivity of the studied granite at El-Missikat (Fig. 8e) and El-Erediya (Fig. 8f ).Another clue can be driven from Th/U ratios measured by Mohammed [52] for zircon crystals separated from El-Missikat ''0.24'' and El-Erediya granite ''0.30'' .Discussion of these values here according to the recent works (e.g.[45,61,70,71] that categorized zircon into magmatic ''Th/U > 0.5'' , hydrothermal "0.5 > Th/U ≤ 0.1", and metamorphic variety ''Th/U ≤ 0.1'' , supports the aforementioned hydrothermal origin. For alteration zones and mineralized silica veins, U enrichment is also reflected by the weak correlation between Th and U (Figs. 9 10, 11).On Th-U variation diagram, most of the data are concentrated through the hydrothermal field, with an exception for the brecciated silica veins at El-Missikat as well as red silica veins and hematitization at El-Erediya where some samples fall through the field of supergene enrichment (Figs.12,13,14).Besides radiometric measurements, the mineral separation for the highly anomalous, mineralized-vein samples revealed the occurrence of some hydrothermally sourced, radioactive minerals, including thorite, betafite, coffinite, and zircon, and some supergene U minerals represented mainly by uranophane and kasolite (Table 5).The aforementioned hydrothermal minerals probably refer to the magmatic source of some pluses of the invading hydrothermal solutions.This claim is suggested according to a number of facts, including the concentration of Th in the residual magmatic fluids, the occurrence of Th-rich minerals (e.g.thorite, and zircon), and the poor liberation behavior of Th from its host rocks by hydrothermal solutions.

Genetic implications using Th/K ratio
Th and K are concentrated during the late stages of magmatic differentiation, resulting in a strong positive correlation and 3-5*10 −4 Th/K ratios for the unaltered rocks [38,64].Through this context, Th values were plotted against K contents for the pink granite at El-Missikat and El-Erediya (Fig. 15a and b).This binary plot shows that the two elements are not related to each other (r = 0.14 and 0.15 for El-Missikat and El-Erediya, respectively), implying that the studied granite experienced either Th enrichment or K-metasomatism (potassic alteration).By using Th-K variation diagram constructed from the aforementioned normal ratios, the plotted samples of El-Missikat (Fig. 15c) and El-Erediya (Fig. 15d) pink granite are located through the normal range and the field of Th enrichment in which Th/K ratio > 5*10 -4 , while no samples are found through the field of K-metasomatism where Th/K<3*10 -4 .For shear zone, Th-enrichment is considered to be the dominant feature through El-Missikat alteration zones, including brecciation (Fig. 16a and b), greisenization (Fig. 16c and d), silicification (Fig. 16e), and kaolinization (Fig. 16f and g).On the other side, El-Erediya shear zone is characterized by the occurrence of Th enrichment along with normal Th/K ratio through the different alteration features, comprising brecciation (Fig. 17a and b), kaolinization (Fig. 17c and d), and hematitization (Fig. 17e and f ).It is worth to mention that the occurrence of normal Th/K ratio through El-Erediya alteration zones likely indicates some pulses of the hydrothermal solutions caused K-metasomatism.Another genetic implication of Th/K ratio is revealed here by its relationship with U/K ratio.It was found that Th/K ratio has a strong positive correlation with U/K ratio through the studied alteration zones (Figs.18,19) where thorite, zircon, betafite, and coffinite are common.On the other hand, hematitization, red and brecciated silica veins exhibit weak correlations   (Fig. 20) and characterized by the abundance of secondary uranium minerals (e.g.uranophane and kasolite).K values are the same for the two ratios, so the strength of correlation indicate the extent to which U and Th mineralizations are associated with each other.
In contrast to U, the weak mobilization of Th in supergene solutions causes much lower concentrations [60], and hence there is no chance to correlate with U. On the other side, hydrothermal U mineralization can be associated with hydrothermal Th mineralization (e.g.magmatic-hydrothermal solutions).At this point, it is claimed that Th/K vs. U/K can be used to discriminate between hydrothermal (strong positive correlation) and supergene uranium mineralizations (weak correlation).

Conclusion
The detailed study of distribution of U, Th, and K as well as their ratios (Th/U, U/K, and Th/K ratios) through U mineralizations has proven its vital role in providing sufficient information to build-up a genetic scenario about magmatic, hydrothermal, and supergene U deposits.This was manifested by considering the shear zone-related U mineralizations through El-Missikat and El-Erediya exploratory tunnels, the Central Eastern Desert of Egypt, as a case study, where varied alteration zones, primary and secondary U mineralizations are well-exposed.Accordingly, our radiogenic ratios-based genetic scenario can be summarized through three main steps: (1) Tectonic shearing of the pink granite where many pathways become available for the upcoming mineralized solutions.and red silica veins and brecciated granite) where the supergene solutions readily percolate.This is reflected mainly by Th/U ratios ≤ 0.1 and the weak correlation between Th/K and U/K ratios (e.g.r = 0.34 and 0.08 for brecciated and red silica veins, respectively).

Fig. 1
Fig. 1 Location map of El-Missikat and El-Erediya plutons showing the portal of the mining works

Fig. 2
Fig. 2 Geological maps of drifts# DI&DIII a and DII b through the northwestern part of El-Missikat pluton (after Abu Dief, 1985), with locations of the studied profiles (red dashed lines)

Fig. 3
Fig. 3 Geological maps of drifts# DII&DIII through the southern part of El-Erediya pluton (after El-Tahir, 1985), with locations of the studied profiles (red dashed lines)

Table 1
Distribution of U, Th, and K in pink granite through the main adit at El-Missikat and El-Erediya exploratory tunnels

Fig. 8 3 Fig. 9
Fig. 8 Binary plot of Th and Th/U ratio vs. U for El-Missikat a and c and El-Erediya pink granite b and d, along with variation diagrams (e-El-Missikat and f-El-Erediya) of Th/U ratio showing that the granite-contained U is of both magmatic (2.5-5 Th/U ration) and hydrothermal sources (Th/U<2.5)

Fig. 10
Fig. 10 Binary plot of Th vs. U through the different alteration zones at El-Erediya drifts# DII a, c, and e assigned to brecciation, kaolinization, and hematitization) and DIII b, d, and f assigned to the same order) showing weak correlation coefficients as an indication for U-enrichment

Fig. 11 Fig. 12
Fig. 11 Binary plot of Th vs. U through varied siliceous veins at El-Missikat (a and b brecciated and massive silica veins through drift#DI and c and d brecciated and massive silica veins through drift#DII) and El-Erediya (e &f reddish silica veins through drfits# DII and DIII)

Fig. 14
Fig. 14 Th-U variation diagrams of the mineralized silica veins at El-Missikat (a and b brecciated and massive silica veins through drift#DI and c and d brecciated and massive silica veins through drift#DII) and El-Erediya (e and f reddish silica veins through drfits# DII and DIII) discriminate between hydrothermal (2.5 > Th/U > 0.1) and supergene U-mineralization (Th/U ratios ≤ 0.1)

Fig. 16
Fig. 16 Th-K variation diagrams indicate the dominance of Th-enrichment (III) through the alteration zones of El-Missikat exploratory tunnels (a and b brecciation, c and d greisenization, e silicification, f and g kaolinization)

( 2 )
Many successive pulses of U and Th-bearing hydrothermal solutions, some of which are magmatic in origin, invaded the sheared granite and inferred by the weak correlation between U and Th (e.g.r = 0.5 and 0.04 for El-Missikat and El-Erediya granites, respectively) as well as Th and K (e.g.r = 0.14), 2.5 > Th/U > 0.1, Th/K > 5*10 -4 , the occurrence of thorite, columbite, xenotime and hydrothermal zircon (0.5 > Th/U ≤ 0.1), and the strong positive correlation between Th/K and U/K ratios (e.g.r = 0.74 and 0.80 in greisenized and kaolinized granite at El-Missikat and El-Erediya, respectively).(3)The hydrothermal U mineralization was then affected by the action of chemical weathering, resulting in the occurrence of secondary U minerals, including uranophane, kasolite, and zippeite, in places nearby the shear zone (e.g.brecciated

Fig. 17 Fig. 18 Fig. 20
Fig. 17 Th-K variation diagrams show that most measurements through El-Erediya alteration zone (a and b brecciation, c and d kaolinization, e and f hematitization) fall through the normal range (I) and Th-enrichment (III)

Table 2
Distribution of U, Th, and K for the different alteration zones through drift# D I and D# II at El-Missikat exploratory tunnels

Table 3
Distribution of U, Th, and K for the different alteration zones through drift# D II and D# III at El-Erediya exploratory tunnels Comparing with pink granite (avg., 39.21 ppm), there is a noticeable Th enrichment illustrated by Th/U > 1. Depending on the low mobilization behavior of Th in both the hypogene and supergene fluids

Table 4
Distribution of U, Th, and K through U-mineralized silica veins at El-Missikat and El-Erediya exploratory tunnels