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Article

Hydrothermal Monazite Geochemistry and Petrochronology Signatures: Metallogenic Age and Tectonic Evolution Model of the Koka Gold Deposit, Eritrea

by
Song Ouyang
1,
Xiaojia Jiang
2,*,
Xianquan Lei
1,*,
Baoquan Wan
3,
Zhenlong Quan
3 and
Yizhao Li
4
1
CINF Engineering Co., Ltd., Changsha 410019, China
2
School of Earth Science and Resources, China University of Geosciences, Beijing 100083, China
3
China Shanghai (Group) Corporation for Foreign Economic & Technological Cooperation, Shanghai 200032, China
4
Sinoma Geological Engineering Exploration and Research Institute Co., Ltd., Beijing 100102, China
*
Authors to whom correspondence should be addressed.
Minerals 2025, 15(8), 851; https://doi.org/10.3390/min15080851
Submission received: 11 July 2025 / Revised: 9 August 2025 / Accepted: 9 August 2025 / Published: 11 August 2025
(This article belongs to the Special Issue Role of Granitic Magmas in Porphyry, Epithermal, and Skarn Deposits)
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Abstract

The metallogenic process of gold deposits is typically characterized by multi-stage mineralization and complex tectonic evolution. Precise determination of metallogenic age is thus critical yet challenging for establishing ore-forming models and tectonic evolutionary frameworks. The Koka gold deposit in Eritrea represents the largest gold discovery to date in the area, though its metallogenic age and tectonic evolution remain debated. This study employs in situ micro-analysis techniques to investigate major/trace elements and U-Pb geochronology of hydrothermal monazite coexisting with gold mineralization, providing new constraints on the metallogenic timeline and tectonic setting. Petrographic observations reveal well-crystallized monazite with structural associations to pyrite and native gold, indicating near-contemporaneous formation. Trace element geochemistry shows peak formation temperatures of 270–340 °C for monazite, consistent with fluid inclusion data. Genetic diagrams confirm a hydrothermal origin, enabling metallogenic age determination. Monazite Tera–Wasserburg lower intercept ages and weighted mean 208Pb/233Th ages yield 586 ± 8.7 Ma and 589 ± 2.3 Ma, respectively, overlapping error ranges with published sericite 40Ar/39Ar ages. This confirms Ediacaran gold mineralization, unrelated to the Koka granite (851 ± 2 Ma). Statistical analysis of reliable age data reveals a three-stage tectonic evolution model: (1) 1000–875 Ma, Rodinia supercontinental rifting, with depleted mantle-derived mafic oceanic crust formation and Mozambique Ocean spreading; (2) 875–630 Ma, subduction-driven crustal accretion and Koka granite emplacement; and (3) 630–570 Ma, post-collision crustal/lithospheric remelting, with mixed metamorphic–magmatic fluids and meteoric water input driving gold precipitation.

1. Introduction

The precise determination of the metallogenic epoch of gold deposits has long been a technical challenge in the field of ore deposit geology worldwide. Due to the relatively open system characteristics commonly observed in gold deposits and the complex fluid–rock interaction processes, the Rb-Sr isochron age determination of sphalerite exhibits significant limitations, making it difficult to obtain reliable data [1,2]. Although pyrite, chalcopyrite, and pyrrhotite can provide reliable Re-Os isotope dating data [3,4], the dating results may be affected by low radioactive isotope content and impurity interference in mineral fractures [5]. The Re-Os dating method for molybdenite, while relatively mature and precise [6], is constrained by the rarity of molybdenite in gold deposits and its limited coexistence with gold-bearing minerals, further complicating the accurate determination of the geochronology of gold deposits. With the development of testing and analysis techniques, an increasing number of in situ micro-area testing techniques have been applied in ore deposit geology research, such as rutile U-Pb geochronology [7], titanite U-Pb geochronology [8], calcite U-Pb geochronology [9,10], and garnet U-Pb geochronology [11]. However, the inability to conduct gold metallogenic geochronology studies often stems from a lack of suitable dating minerals or failure to meet testing requirements. Therefore, selecting an appropriate mineral to determine the metallogenic epoch of gold deposits is urgent.
Monazite, a monoclinic crystal system phosphate rich in light rare earth elements (LREE) [12], is characterized by enrichment in Th and U and low common Pb content [13]. It is widely distributed in hydrothermal deposits, magmatic rocks, and low- to high-grade metamorphic rocks [14,15,16]. Therefore, estimating the physicochemical conditions of monazite formation can quantitatively indicate its formation environment and discern its genesis [17,18,19]. Hydrothermal monazite, owing to its high lead diffusion closure temperature and greater susceptibility to chemical reactions under hydrothermal conditions [20,21,22,23,24,25], can even dissolve or recrystallize at temperatures below 450 °C, controlled by hydrothermal fluid composition, water content, and temperature [26,27]. During dissolution or recrystallization, monazite can alter its Th/U ratio through reactions such as [(Th, U)4+ + Si4+ = (REE, Y)3+ + P5+] or [(Th, U)4+ + Ca2+ = 2(REE, Y)3+], with this ratio change serving as an important indicator of hydrothermal activity [28]. Numerous studies indicate that hydrothermal monazite not only constrains the formation ages of gold deposits (including orogenic, Carlin-type, etc.) but also delineates the evolutionary histories of complex tectonic and hydrothermal superimposition [29,30,31,32]. Consequently, hydrothermal monazite can effectively record key hydrothermal event time information during high-, medium-, and low-temperature fluid activity processes [33,34,35], making it an ideal geochronometer for metallogenic hydrothermal activity [36,37,38].
The Nakfa region in Eritrea hosts a series of gold deposits, serving as a natural laboratory for studying gold mineralization. The Koka gold deposit, the largest discovered within the belt (with total Au reserves of 46 tons), remains controversial regarding its mineralization type and age of formation. This study focuses on monazite coexisting with gold mineralization, conducting in situ micro-area major and trace element analyses and U-Pb geochronology research. Combined with previous research on diagenetic and metallogenic epochs, we explore the formation environment of monazite, determine the formation time of the gold deposit, reveal the relationship between magmatism and gold mineralization, and establish a tectonic evolution model for the Koka gold deposit.

2. Regional Geological Background

Eritrea is located on the northern margin of the East African Orogenic Belt (Figure 1a), with approximately 60% of its land area composed of Precambrian basement rocks, forming a part of the southern Arabian–Nubian Shield (ANS) (Figure 1b). The shield was formed during the Neoproterozoic Pan-African orogenic cycle (approximately 900–550 Ma) when the closure of the Mozambique Ocean led to the collision of the East and West Gondwana continents [39]. It consists of Neoproterozoic juvenile island arcs, volcanic–sedimentary basins, syn- to post-tectonic mafic to felsic intrusions, and pre-Neoproterozoic crustal enclaves [40,41,42].
Based on lithological and tectonic characteristics, the Neoproterozoic basement in Eritrea is divided into five tectono-stratigraphic units (Figure 2a; [43]): (1) the Barka Terrane (BT) in the west, primarily composed of amphibolite to granulite facies metasedimentary and mafic granulite complexes; (2) the Hagar Terrane (HT) in the north, mainly consisting of mafic and felsic volcanic rocks, interpreted as a supra-subduction or back-arc tectonic setting [44]; (3) the Adobha Abi Terrane (ADT) in the central-west, a narrow zone composed of ophiolites and accreted back-arc basin sediments, highly deformed due to regional shearing and interpreted as the collision suture between the Nakfa block and the western block [45]; (4) the Nakfa Terrane (NT) in the center, accounting for more than half of the Precambrian basement, including greenschist facies volcanic–sedimentary rocks and syn- to post-collision granites [46,47,48]; and (5) the Arag Terrane (AT) in the east, a narrow zone along the Red Sea lowlands, composed of gneisses and syn- to late-tectonic granites.
In the Precambrian granitoid–greenstone belts, NNW-NNE trending brittle–ductile shear zones and strike–slip faults, thrust faults, fold structures, and locally visible en echelon quartz veins and feather fractures are prevalent, with an overall tectonic trend of NNE-NNW [49]. Among them, ductile strike–slip shear zones are the most prominent tectonic features in the region, with the Augaro–Adobha Belt (AAB) in the west and the Asmara–Nakfa Belt (ANB) in the east being the two main transpressive strike–slip shear zones in the area (Figure 2a). Syn- to late-post-collision intrusive rocks are scattered within the terranes between the AAB and ANB. These intrusions primarily consist of granites, granodiorites, and diorites, occurring as stocks or plutons, with fine-grained rocks, diabases, and quartz porphyries present as dykes [43,50]. These tectonic movements and magmatic activities provided favorable conditions for gold mineralization in the region, with gold mineralization in Eritrea concentrated along the AAB and ANB zones and the intervening shield area, with Koka being a typical representative of the gold mineralization district [51].

3. Geological Characteristics of the Deposit

The Koka deposit, the largest and longest-mined gold deposit in Eritrea (with total gold reserves of 46 tons and an average grade of 3.5 g/t), is located on the western margin of the Nakfa Terrane and is controlled by the Augaro–Adobha compressional strike–slip belt. Lineament direction in the mining area is nearly north–south, characterized by a series of asymmetric overturned isoclines and reverse faults, with high-angle reverse faults being the main rock and ore-controlling structures [49]. The lithology in the mining area is divided into two main categories (Figure 2b): metasedimentary and metamorphosed basalt sequences in the west, and magmatic rock sequences in the east, composed of granite intrusions, rhyolites, felsic tuffs, volcaniclastic sedimentary rocks, and limited presence of gabbro dykes. Affected by intense regional compressional deformation, tectonic-related structures are well-developed in both sedimentary and volcanic rocks, with widespread development of strong cleavage and foliation (Figure 3).
Mineralization occurs within a north–south trending lenticular Koka granite body (Figure 3a), with a zircon LA-ICP-MS U-Pb age of 851.2 ± 1.9 Ma, believed to have formed in an island arc environment induced by subduction [52]. Subsequent tectonic compression–strike–slip processes caused brittle fracturing of the Koka granite body, providing adequate space for gold-bearing fluids to circulate and ultimately form a quartz vein mineralization network (Figure 3b–d). Mineralization types can be divided into quartz vein and altered rock types, with quartz vein types being predominant. Structurally, within the mineralized zone, gold-bearing quartz veins intrude into altered rocks. Mineralization initially occurs along tectonic fractures (i.e., quartz vein formation) and progressively impregnates into the country rocks (Figure 3e), resulting in significantly stronger mineralization intensity in quartz veins compared to the surrounding country rocks (Figure 3f–j).
Gold primarily exists in the form of native gold, mostly exhibiting anhedral granular structures, distributed between pyrite and quartz grains or included within pyrite crystals (Figure 4a–c). Small amounts of fine-grained gold can also be found locally in sulfide minerals such as galena and chalcopyrite (Figure 4d–f). Additionally, sulfides such as tetrahedrite and sphalerite are developed (Figure 4g–j), along with accessory minerals like apatite and monazite (Figure 4k,l), widely present in quartz–sulfide veins and altered mineral assemblages. Monazite also exists as inclusions in pyrite or coexists with pyrite and apatite, closely associated with gold-bearing sulfides (Figure 4k,l). The alteration types related to gold mineralization mainly include silicification, pyritization, carbonatization, and sericitization.
Based on field outcrops, microscopic observations, and mineral associations, three mineralization stages can be identified in the Koka gold deposit (Figure 5): the first stage is characterized by pyrite–quartz, with pyrite mostly exhibiting fragmented textures (Figure 4d), and later sulfides such as galena and sphalerite filling fractures; this stage is characterized by weak gold mineralization. The second stage is the gold–quartz–polymetallic sulfide stage, the main mineralization stage, where gold-bearing fluids intrude into rock fractures, forming gold-bearing quartz–polymetallic sulfide veins (Figure 4a,f). The third stage is the quartz–calcite stage, where fluids are essentially barren, developing milky-white sulfide-poor quartz veins along post-main mineralization stage fractures; these quartz veins develop limited mineralization, but fluid action during this stage causes secondary local enrichment of the main mineralization stage veins, often resulting in higher mineralization intensity at the intersections of later quartz veins and early quartz sulfide veins (Figure 3b).

4. Sample Description and Analytical Methods

4.1. Sample Location and Selection

Gold-bearing quartz veins were sampled from the principal ore-forming zone of the Koka deposit (Figure 2b), with monazite selection prioritizing the most petrographically representative specimens (Figure 3d). According to the petrographic characteristics of monazite, the most representative samples or grains/crystals were selected for backscattered electron imaging (BSE) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis.

4.2. Analytical Methods

4.2.1. Monazite BSE Imaging

Backscattered electron (BSE) imaging analysis was conducted at the Electron Probe Microanalyzer (EPMA) laboratory of Sample Solution Analysis Technology Co., Ltd., Wuhan, China. Before the experiment, the polished thin sections were coated with a thin carbon film to become conductive. BSE imaging analysis was performed using a JXA-8230 instrument equipped with a four-channel wavelength-dispersive X-ray spectrometer (WDS, JEOL, Tokyo, Japan). The analytical conditions for BSE were as follows: accelerating voltage of 20 kV, accelerating current of 20 nA, image size of 1280 × 960, and scan time of 19 s.

4.2.2. Major and Trace Element Analysis of Monazite

The major and trace element contents of monazite were analyzed at Sample Solution Analysis Technology Co., Ltd., Wuhan, China. In situ micro-area analysis was conducted using an Agilent 7900 ICP-MS (Agilent Technologies Inc., Santa Clara, CA, USA) equipped with a 193 nm ArF excimer Resonetics Resolution GeoLas HD laser ablation system (Resonetics LLC, Amherst, NH, USA). The laser operated at a frequency of 5 Hz with a spot size of 32 μm. To calibrate the trace element contents of monazite, different standard materials (BHVO-2G, BIR-1G, BCR-2G, and NIST-610) were used, with Ce as the internal standard [53,54]. Each single-point analysis took approximately 80 s, including 50 s of continuous laser ablation followed by 20 s of purging and cleaning the sampling system. Time drift correction and quantitative calibration of trace element analysis were performed using ICPMSDataCal 10.9 software [53]. The detection limits for most trace elements ranged from 0.01 to 0.5 ppm.

4.2.3. In Situ U-Pb Dating of Monazite

Monazite U-Pb isotope dating was conducted at Sample Solution Analysis Technology Co., Ltd., Wuhan, China. The ion signal intensity of monazite was obtained using an Agilent 7900 ICP-MS instrument. In this study, a laser spot size of 16 μm, a repetition rate of 2 Hz, and an energy density of 5 J/cm2 were used for monazite analysis. For larger monazite grains, multiple data points were selected within a single grain; for smaller monazite grains, only one data point per grain was chosen. Monazite standard materials 44069 (ID-TIMS age of 424.9 ± 0.4 Ma; [55]) and TRE were used as external standards for rare earth mineral U-Pb dating. A small amount of water vapor (4.1 mg min−1) was introduced into the ablation cell to improve analytical accuracy and precision [56]. For each analysis, background acquisition was performed for approximately 20–30 s, followed by 50 s of sample data acquisition. Offline selection and integration of background and analyte signals, as well as time drift correction and quantitative calibration of trace element analysis and U-Pb dating, were performed using ICPMSDataCal 10.9 and iolite 4.0 software [57,58]. Concordia diagrams were plotted, and weighted average calculations were performed using Isoplot R and iolite 4.0 [58,59].

5. Analytical Results

5.1. Major and Trace Elements

The major and trace element results for monazite from the Koka gold deposit are presented in Table 1 and illustrated in Figure 6. Among the major elements, the SiO2 content of monazite from the Koka gold deposit varies significantly, ranging from 0.01% to 6.31%, with an average content of 2.56%. The P2O5 content ranges from 23.9% to 29.7%, with an average value of 27.4%. The TiO2 content in monazite is relatively low, with negligible variation, ranging between below detection and 0.05%. Among the rare earth elements, the total rare earth element (REE) content of monazite from the Koka gold deposit ranges from 554,431 ppm to 623,286 ppm, with an average value of 587,740 ppm. The total light rare earth element (LREE) content ranges from 530,341 ppm to 597,496 ppm, with an average value of 562,851 ppm. The total heavy rare earth element (HREE) content ranges from 21,433 ppm to 26,966 ppm, with an average value of 24,588 ppm. The ratio of LREE to HREE content ranges from 21.9 to 25.5, with an average value of 22.9. The chondrite-normalized REE pattern of monazite exhibits a “right-inclined” feature (Figure 6b), with weak negative δEu anomalies (0.42–0.48) and almost no δCe anomalies (0.98–1.00).

5.2. Monazite U-Pb Dating

The U-Pb dating results for monazite from the Koka gold deposit are presented in Table 2 and illustrated in Figure 7. The error ellipses of the monazite samples are plotted on a Tera–Wasserburg diagram [63], and the distribution of these data points allows for the regression of a common lead mixing line, yielding a relatively precise lower intersection age (Figure 7a). The Koka monazite contains 2993 to 6164 ppm of Th (mean = 4068 ppm) and 49.1 to 114.5 ppm of U (mean = 71.8 ppm), with Th/U ratios ranging from 27.7 to 81.3 (Table 2). The lower intersection age and weighted average age obtained for monazite are 586 ± 8.7 Ma and 589 ± 2.3 Ma, respectively, with the age data being consistent within error ranges (Figure 7).

6. Discussion

6.1. Formation Environment and Genesis of Monazite

6.1.1. Formation Environment of Monazite

Monazite and xenotime form a limited solid solution, represented as (LREE)PO4-(Y, HREE)PO4, where LREE represents light rare earth elements La-Gd, and HREE represents heavy rare earth elements Tb-Lu. Experimental and empirical studies indicate that the equilibrium distribution of REE and Y between monazite and xenotime depends on both pressure and temperature [64,65,66,67,68]. In particular, Gratz and Heinrich [64] calibrated the distribution of Y through experiments and proposed the CePO4-YPO4 thermometer, which can be expressed as:
T = ln 100 X Y m z 1.459 + 0.0852 P 2.2745 × 10 − 3
where T is the equilibrium temperature (°C), XYmz is the mole fraction of Y in monazite, and P is the pressure (kbar). The pressure value of 1.0 kbar used in our calculations was derived from fluid inclusion microthermometry data for the Koka deposit [49]. Estimating pressure independently is necessary for temperature calculation using the above formula, but the reaction has a weak dependence on pressure; changing the estimated pressure by 1 kbar affects the temperature by only about 10 °C [68].
The calculated XYmz differences were small (0.03–0.04), resulting in small estimated temperature variations (225–372 °C, mean = 302 °C, and 442.34 °C are outliers, which have been removed during statistics, Table 3 and Figure 8a), consistent with previous fluid inclusion microthermometry results (peak values of 270–340 °C, Figure 8b; [69]). In hydrothermal deposits, the formation of monazite may be closely related to fluids [18,19]. Therefore, temperature calculations can reflect the fluid physicochemical conditions of the Koka gold deposit.

6.1.2. Genesis of Monazite

Monazite, a mineral with a relatively wide range of formation temperatures, generally forms in magmatic, metamorphic, and hydrothermal environments. Due to its unique trace and rare earth element characteristics, these features can be used to distinguish monazite of different genetic types [70,71]. Previous studies have shown that the Th and U contents, as well as the Th/U ratio, of monazite are important parameters for distinguishing its genetic types [72,73]. Hydrothermal monazite typically exhibits low Th content (<1%), while magmatic monazite generally has high Th content (5%–6%). Additionally, magmatic monazite is characterized by the highest Eu anomaly values among the three genetic types, with typical values generally less than 0.08. In contrast, metamorphic monazite has Eu anomaly values ranging from 0.1 to 0.6, whereas hydrothermal monazite generally does not show significant Eu anomalies or displays weak ones. Therefore, the Eu anomaly value can serve as an important indicator for classifying the genetic type of monazite. The monazite from the Koka gold deposit is generally well-preserved with a high degree of autonomy. In discriminant diagrams (Figure 6a; [74]), it falls within the range of hydrothermal monazite and exhibits weak Eu anomalies and low Th content. Its partition curve is entirely within the range of hydrothermal monazite (Figure 6b), collectively indicating that the monazite in the Koka gold deposit is of hydrothermal origin.

6.2. Metallogenic Epoch of the Koka Gold Deposit

Hydrothermal monazite, rich in radioactive elements such as Th and U and containing almost no common lead in its mineral composition, has been widely used in recent years to determine the metallogenic epoch of hydrothermal deposits [32,75,76,77,78,79,80]. Experimental studies have shown that monazite has significantly stronger resistance to radiation damage than commonly used dating minerals like zircon, and its U-Pb system remains stable at temperatures ranging from 700 to 750 °C [81,82], thus effectively constraining the timing of gold mineralization. Monazite dating results indicate that the Koka gold deposit primarily formed 589 ± 2.3 Ma, suggesting that it originated during the Ediacaran Period.
The Au mineralization of the Koka gold deposit is closely associated in time and space with the Neoproterozoic Pan-African orogenic setting. Studies have shown that the crystallization age of the Koka granite body, which hosts the deposit, is 851.2 ± 1.9 Ma [52], significantly earlier than the main phase of the Pan-African massive accretionary orogeny (600–550 Ma; [83]), indicating that the rock body formed during the subduction stage. The formation age of altered minerals such as sericite, directly associated with gold mineralization, is concentrated between 580 and 540 Ma, corresponding to the late stage of the Pan-African orogeny. 40Ar-39Ar dating data (600–580 Ma) [49] of sericite in gold-bearing quartz veins further constrains the mineralization timeline to the late Neoproterozoic. Additionally, several contemporaneous gold deposits in the Nubian Shield (e.g., Sukhaybarat, An Najadi, Ad Duwayhi, Lega Dembi) also formed during this period [49], suggesting the prevalence of regional metallogenic events. The formation ages of regional post-orogenic A-type granites and kyanite schists [84,85] collectively indicate that the study area underwent a transition from crustal compression to extensional tectonics between 600 and 580 Ma, accompanied by regional medium- to high-grade metamorphism [49]. This tectono-thermal event combination provided favorable conditions for the formation of the Koka gold deposit.

6.3. Tectonic Evolution Model of the Koka Gold Deposit

Based on current research and published data, we propose the following tectonic evolution model (Figure 9) to explain the evolution of the Koka gold deposit:
c.1000 Ma to c.875 Ma: During the breakup of the Rodinia supercontinent, mafic oceanic crust of depleted mantle origin formed, accompanied by the expansion of the Mozambique Ocean (Figure 9a; [83,86,87]). From 875 Ma to c.630 Ma: Through subduction and terrane accretion, juvenile crust formed, and calc-alkaline magmas with island arc characteristics developed in the Nakfa terrane [88]. Due to the lack of influence from shear zones or faults, the intact subduction-related island arc–backarc system was preserved (Figure 9b; [44]). The Koka granite body, located within the Elababu shear zone on the western side of the Nakfa terrane, has a similar formation age (851 Ma) to the Augaro granite, which formed in a backarc extensional environment [89]. This suggests that the Koka granite may have also formed in a backarc extensional environment triggered by subduction. c.630 Ma to c.570 Ma: Continental collision led to the remelting of the crust and lithosphere, forming collision-related acidic and mafic dikes [90,91,92], accompanied by the generation of post-magmatic hydrothermal fluids. From 586 Ma to 589 Ma: Magmatic hydrothermal fluids mixed with metamorphic fluids to form the primary ore-forming fluids, which subsequently mixed with meteoric water at lower temperatures, promoting the formation of the Koka gold deposit (Figure 9c).
Minerals 15 00851 g009
Figure 9. Tectonic evolution model diagram of the Koka gold deposit (modified after Hu et al., 2024 [93]). (a) the expansion of the Mozambique Ocean from 1000–875 Ma, (b) the subsequent development of subduction-related Koka granite bodies between 875–630 Ma, and (c) the promotion of Koka gold deposit formation from 630–570 Ma through mixing of magmatic hydrothermal fluids with metamorphic fluids, combined with later meteoric water addition.
Figure 9. Tectonic evolution model diagram of the Koka gold deposit (modified after Hu et al., 2024 [93]). (a) the expansion of the Mozambique Ocean from 1000–875 Ma, (b) the subsequent development of subduction-related Koka granite bodies between 875–630 Ma, and (c) the promotion of Koka gold deposit formation from 630–570 Ma through mixing of magmatic hydrothermal fluids with metamorphic fluids, combined with later meteoric water addition.
Minerals 15 00851 g009

7. Conclusions

(1) The formation temperature of monazite (225–372 °C) in the Koka gold deposit, consistent with microthermometric results from fluid inclusions, indicates that the ore-forming fluids formed in a low to moderate temperature environment (peak temperature range: 270–340 °C). Combined with petrographic and trace element characteristics, it is comprehensively determined to be of hydrothermal origin.
(2) Monazite U-Pb dating results are highly consistent with unpublished sericite 40Ar/39Ar dating data, precisely constraining the metallogenic event of the Koka gold deposit to 589 ± 2.3Ma, corresponding to the late Ediacaran Period.
(3) Tectonic evolution model of the Koka gold deposit: (a) 1000–875 Ma: The breakup of the Rodinia supercontinent led to the formation of mafic oceanic crust, accompanied by the expansion of the Mozambique Ocean. (b) 875–630 Ma: Subduction and terrane accretion formed juvenile crust, with the development of subduction-related Koka granite bodies. (c) 630–570 Ma: Continental collision triggered local remelting of the crust and lithosphere, forming collision-related intrusions. The mixing of magmatic hydrothermal fluids with metamorphic fluids, combined with the later addition of meteoric water, promoted the formation of the Koka gold deposit.

Author Contributions

Conceptualization, S.O.; software, S.O.; formal analysis, X.L.; investigation, B.W. and Y.L.; resources, X.J., X.L., B.W., Z.Q. and Y.L.; data curation, Z.Q.; writing—original draft preparation, S.O. and X.J.; writing—review and editing, X.L., B.W., Z.Q. and Y.L.; funding acquisition, X.J. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by [the National Natural Science Foundation of China (NSFC)] grant number [42403061] and The APC was funded by NSFC.

Data Availability Statement

All data have been supplied in accordance with the research requirements and are fully accessible within the manuscript.

Acknowledgments

We thank the editor and potential reviewers who helped to improve this manuscript. This study was financially supported by the “Geological Prospecting and Survey for Gold Deposit in Zara Area, Eritrea” undertaken by CINF Engineering Co., Ltd. The fieldwork received significant support from He Xin, Manager of Zara Mining Share Company.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Song Ouyang and Xianquan Lei are employees of CINF Engineering Co., Ltd. Baoquan Wan and Zhenlong Quan are employees of China Shanghai (Group) Corporation for Foreign Economic & Technological Cooperation. Yizhao Li is an employee of Sinoma Geological Engineering Exploration and Research Institute Co., Ltd. The paper reflects the views of the scientists and not the company.

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Figure 1. (a) Distribution of major tectonic units in the East African Orogen. (b) Tectonic subdivision of the Arabian–Nubian Shield (ANS) and major gold deposit locations.
Figure 1. (a) Distribution of major tectonic units in the East African Orogen. (b) Tectonic subdivision of the Arabian–Nubian Shield (ANS) and major gold deposit locations.
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Figure 2. (a) Simplified geotectonic map of Eritrea and adjacent regions showing gold deposit distribution. (b) Geological map of the Koka gold deposit (modified after Johnson, 2017 [42]).
Figure 2. (a) Simplified geotectonic map of Eritrea and adjacent regions showing gold deposit distribution. (b) Geological map of the Koka gold deposit (modified after Johnson, 2017 [42]).
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Figure 3. Geological features of the Koka gold deposit: (a) mineralized zone, (b–d) distribution of quartz veins in microgranite, (e) host rock, (f,g) samples from the quartz veins, (h–j) microcrystalline granite fractures filled by quartz veins and quartz–polymetallic sulfide veins.
Figure 3. Geological features of the Koka gold deposit: (a) mineralized zone, (b–d) distribution of quartz veins in microgranite, (e) host rock, (f,g) samples from the quartz veins, (h–j) microcrystalline granite fractures filled by quartz veins and quartz–polymetallic sulfide veins.
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Figure 4. Reflected light optical microscopy and scanning electron microscopy photomicrographs of ore minerals and their textural features, Koka gold deposit (detailed descriptions in the manuscript). (a–f) Granular native gold and electrum coexist with pyrite, galena, and chalcopyrite; (g,h) Chalcopyrite intergrows with tetrahedrite and sericite; (i,j) Sphalerite coexists with pyrite and calcite; (k,l) Apatite, monazite, and xenotime associate with native gold and pyrite. Mineral abbreviations: Au—native gold, Py—pyrite, Ccp—chalcopyrite, Gn—galena, Tet—tetrahedrite, Ser—sericite, Sph—sphalerite, Cal—calcite, Mnz—monazite, Ap—apatite, Xnt—xenotime.
Figure 4. Reflected light optical microscopy and scanning electron microscopy photomicrographs of ore minerals and their textural features, Koka gold deposit (detailed descriptions in the manuscript). (a–f) Granular native gold and electrum coexist with pyrite, galena, and chalcopyrite; (g,h) Chalcopyrite intergrows with tetrahedrite and sericite; (i,j) Sphalerite coexists with pyrite and calcite; (k,l) Apatite, monazite, and xenotime associate with native gold and pyrite. Mineral abbreviations: Au—native gold, Py—pyrite, Ccp—chalcopyrite, Gn—galena, Tet—tetrahedrite, Ser—sericite, Sph—sphalerite, Cal—calcite, Mnz—monazite, Ap—apatite, Xnt—xenotime.
Minerals 15 00851 g004
Minerals 15 00851 g005
Figure 5. Paragenetic sequence of the Koka gold deposit.
Figure 5. Paragenetic sequence of the Koka gold deposit.
Minerals 15 00851 g005
Minerals 15 00851 g006
Figure 6. Monazite genetic discrimination diagram (a) and REE (Rare Earth Element) distribution patterns (b). Magmatic monazite after Keita et al., 2018 [60]; hydrothermal monazite after Bergemann et al., 2018 [61]; metamorphic monazite after Xu et al., 2018 [62]; The red circles represent the boundary points between hydrothermal monazite and magmatic/metamorphic monazite.
Figure 6. Monazite genetic discrimination diagram (a) and REE (Rare Earth Element) distribution patterns (b). Magmatic monazite after Keita et al., 2018 [60]; hydrothermal monazite after Bergemann et al., 2018 [61]; metamorphic monazite after Xu et al., 2018 [62]; The red circles represent the boundary points between hydrothermal monazite and magmatic/metamorphic monazite.
Minerals 15 00851 g006
Minerals 15 00851 g007
Figure 7. Tera–Wasserburg U–Pb concordia diagrams (a) and weighted average age (b) of monazite from the Koka gold deposit. (a) is a magnified view of the inset in (a).
Figure 7. Tera–Wasserburg U–Pb concordia diagrams (a) and weighted average age (b) of monazite from the Koka gold deposit. (a) is a magnified view of the inset in (a).
Minerals 15 00851 g007
Minerals 15 00851 g008
Figure 8. Histogram of (a) monazite formation temperature vs. (b) fluid inclusion microthermometric results, Koka gold deposit (modified after Zhao et al., 2019 [49]).
Figure 8. Histogram of (a) monazite formation temperature vs. (b) fluid inclusion microthermometric results, Koka gold deposit (modified after Zhao et al., 2019 [49]).
Minerals 15 00851 g008
Table 1. (a) Major and trace element analytical results of monazite from the Koka gold deposit. (b) Major and trace element analytical results of monazite from the Koka gold deposit (Continued1). (c) Major and trace element analytical results of monazite from the Koka gold deposit (Continued2).
Table 1. (a) Major and trace element analytical results of monazite from the Koka gold deposit. (b) Major and trace element analytical results of monazite from the Koka gold deposit (Continued1). (c) Major and trace element analytical results of monazite from the Koka gold deposit (Continued2).
(a)
No.SiO2P2O5TiO2AsYNbAgLaCePrNdSmEuGdTbDyHo
wt%wt%wt%ppmppmppmppmppmppmppmppmppmppmppmppmppmppm
KK-216-011.3424.40.0149412,7060.630.00121,586260,67433,153146,38631,880381719,53415403891340
KK-216-026.3124.00.0448410,8483.260.11112,597245,18331,381138,05130,085378618,25814243529295
KK-216-040.8325.30.0049112,6320.260.29118,578258,23033,328147,34032,202394220,19615853989336
KK-216-050.4825.90.0052813,3880.030.16116,618257,66333,363147,15832,415391220,37816294107357
KK-216-061.9926.40.0248411,5951.720.00116,650252,05531,942141,78430,475360818,44914593592313
KK-216-072.0026.10.0247711,4100.870.21116,394253,97532,438142,21930,708364418,73114653649310
KK-216-084.8125.40.0547511,6473.320.08111,245245,68931,535139,53630,337365718,69414623643307
KK-216-095.2426.20.0446510,5573.320.00111,372243,46330,611134,76329,062353317,31613613285276
KK-216-101.4727.10.0249311,8711.170.00116,359251,19232,346141,06230,833364418,90514953726324
KK-216-112.1726.90.0247412,3581.160.00112,212247,79331,799142,01430,899376319,60915433860338
KK-216-120.0128.00.0046411,1700.120.00114,571252,37432,543143,24731,298380418,92514803590305
KK-216-135.8926.60.0146795680.370.44113,234240,64430,475130,43027,489335116,49312573070254
KK-216-141.4728.80.0148312,2570.840.00113,297246,07431,764135,82730,031368618,64614893733322
KK-216-152.0728.30.0147012,0790.960.01111,771243,59231,415138,64430,510372818,79415013722325
KK-216-162.3428.40.0146311,0390.860.00111,294244,04031,168137,77630,168371918,32314293527302
KK-216-171.7728.50.0045510,3010.260.10114,241243,43830,987136,18630,987414118,95214603484285
KK-216-181.0629.10.0048211,5560.330.08113,193245,18731,480138,34830,238379518,67714593643317
KK-216-202.4328.50.0145012,8130.940.15114,453242,53930,832134,30229,494346418,83615033884346
KK-216-210.9929.50.0145911,3001.140.00113,292241,03931,013136,12830,620397519,25915353762315
KK-216-223.5629.30.0344811,8743.270.00108,480236,20230,398131,86528,789366818,31214753718325
KK-216-234.7129.10.0446413,0093.020.10107,891231,54530,013129,20428,195349417,97714623819346
KK-216-243.1829.40.0343615,7621.460.33114,551239,03629,615125,65027,184349417,64915344279405
KK-216-252.8329.70.0247511,5421.750.00110,557237,28630,175132,56628,880356817,94914093571308
(b)
No.ErTmYbLuHfTaAuPbThUΣREELREEHREELREE/HREELaN/YbNδEuδCe
ppmppmppmppmppmppmppmppmppmppmppmppmppm
KK-216-0138724.269.44.600.060.060.002.37377964.7623,286597,49625,78923.212570.430.99
KK-216-0231919.258.53.550.070.060.002.41441058.8584,988561,08223,90623.513820.460.99
KK-216-0437421.164.94.050.080.110.021.68389061.7620,188593,61926,56922.313100.440.99
KK-216-0539723.969.04.630.110.000.002.43373364.7618,095591,12926,96621.912120.431.00
KK-216-0634221.659.92.320.550.040.003.26429774.3600,753576,51424,23823.813970.430.99
KK-216-0733919.663.34.300.000.020.003.33406568.3603,959579,37824,58123.613200.431.00
KK-216-0835420.359.54.410.000.060.002.96336560.8586,543561,99824,54522.913420.441.00
KK-216-0930019.653.63.500.250.070.001.69409657.6575,418552,80422,61424.514920.441.00
KK-216-1036321.865.14.500.170.020.002.74344263.8600,341575,43624,90423.112820.430.99
KK-216-1139023.265.54.360.220.020.003.39447988.0594,312568,48025,83222.012290.441.00
KK-216-1232919.757.03.330.140.040.002.17601175.9602,547577,83724,70923.414410.441.00
KK-216-1329116.648.33.070.260.020.671.75399349.1567,056545,62221,43325.516830.440.98
KK-216-1437822.264.44.900.090.050.551.55389455.2585,337560,67824,65922.712610.440.99
KK-216-1537421.968.84.630.110.000.001.98409756.4584,472559,66124,81122.611660.440.99
KK-216-1633120.759.53.460.040.040.071.99371459.7582,160558,16623,99523.313410.451.00
KK-216-1730217.552.83.160.140.000.002.45563470.1584,538559,98124,55722.815510.480.98
KK-216-1835120.673.74.750.270.020.001.93387761.6586,787562,24124,54622.911010.450.99
KK-216-2037923.168.74.560.320.040.003.92322288.5580,129555,08525,04422.211960.420.98
KK-216-2134220.756.53.530.060.080.003.10616584.6581,359556,06625,29322.014370.470.98
KK-216-2236121.660.54.060.190.070.003.18359996.9563,679539,40324,27622.212870.460.99
KK-216-2338523.672.55.110.290.040.004.383616114554,432530,34224,09022.010670.440.98
KK-216-2450332.71037.070.260.100.496.102993108564,043539,53024,51322.08010.460.98
KK-216-2534722.057.94.280.180.050.302.68311579.0566,699543,03123,66822.913690.450.99
(c)
No.La2O3Ce2O3Pr2O3Nd2O3Sm2O3Gd2O3Y2O3P2O5SiO2Formula
Mol Prop
KK-216-010.0440.0930.0120.0510.0110.0060.0070.1720.022(La0.2Ce0.5Pr0.1Nd0.2)1(Si0.1P0.9)1O4
KK-216-020.0410.0880.0110.0480.0100.0060.0060.1690.105(La0.2Ce0.4Pr0.1Nd0.2Sm0.1)1(Si0.2P0.8)1O4
KK-216-040.0430.0920.0120.0510.0110.0060.0070.1780.014(La0.2Ce0.5Pr0.1Nd0.2)1P1O4
KK-216-050.0420.0920.0120.0510.0110.0060.0080.1820.008(La0.2Ce0.5Pr0.1Nd0.2)1P1O4
KK-216-060.0420.0900.0110.0490.0100.0060.0070.1860.033(La0.2Ce0.4Pr0.1Nd0.2Sm0.1)1(Si0.1P0.9)1O4
KK-216-070.0420.0910.0120.0490.0100.0060.0060.1840.033(La0.2Ce0.4Pr0.1Nd0.2Sm0.1)1(Si0.1P0.9)1O4
KK-216-080.0400.0880.0110.0480.0100.0060.0070.1790.080(La0.2Ce0.4Pr0.1Nd0.2Sm0.1)1(Si0.2P0.8)1O4
KK-216-090.0400.0870.0110.0470.0100.0060.0060.1850.087(La0.2Ce0.4Pr0.1Nd0.2Sm0.1)1(Si0.1P0.9)1O4
KK-216-100.0420.0900.0110.0490.0100.0060.0070.1910.025(La0.2Ce0.4Pr0.1Nd0.2Sm0.1)1(Si0.1P0.9)1O4
KK-216-110.0400.0880.0110.0490.0100.0060.0070.1900.036(La0.2Ce0.4Pr0.1Nd0.2Sm0.1)1(Si0.1P0.9)1O4
KK-216-120.0410.0900.0120.0500.0100.0060.0060.1970.000(La0.2Ce0.4Pr0.1Nd0.2Sm0.1)1P1O4
KK-216-130.0410.0860.0110.0450.0090.0050.0050.1880.098(La0.2Ce0.4Pr0.1Nd0.2Sm0.1)1(Si0.1P0.9)1O4
KK-216-140.0410.0880.0110.0470.0100.0060.0070.2030.024(La0.2Ce0.4Pr0.1Nd0.2Sm0.1)1P1O4
KK-216-150.0400.0870.0110.0480.0100.0060.0070.1990.034(La0.2Ce0.4Pr0.1Nd0.2Sm0.1)1(Si0.1P0.9)1O4
KK-216-160.0400.0870.0110.0480.0100.0060.0060.2000.039(La0.2Ce0.4Pr0.1Nd0.2Sm0.1)1(Si0.1P0.9)1O4
KK-216-170.0410.0870.0110.0470.0100.0060.0060.2010.029(La0.2Ce0.4Pr0.1Nd0.2Sm0.1)1P1O4
KK-216-180.0410.0880.0110.0480.0100.0060.0070.2050.018(La0.2Ce0.4Pr0.1Nd0.2Sm0.1)1P1O4
KK-216-200.0410.0870.0110.0460.0100.0060.0070.2000.040(La0.2Ce0.4Pr0.1Nd0.2Sm0.1)1(Si0.1P0.9)1O4
KK-216-210.0410.0860.0110.0470.0100.0060.0060.2080.017(La0.2Ce0.4Pr0.1Nd0.2Sm0.1)1P1O4
KK-216-220.0390.0840.0110.0460.0100.0060.0070.2060.059(La0.2Ce0.4Pr0.1Nd0.2Sm0.1)1(Si0.1P0.9)1O4
KK-216-230.0390.0830.0110.0450.0090.0060.0070.2050.078(La0.2Ce0.4Pr0.1Nd0.2Sm0.1)1(Si0.1P0.9)1O4
KK-216-240.0410.0850.0110.0430.0090.0060.0090.2070.053(La0.2Ce0.4Pr0.1Nd0.2Sm0.1)1(Si0.1P0.9)1O4
KK-216-250.0400.0850.0110.0460.0100.0060.0070.2090.047(La0.2Ce0.4Pr0.1Nd0.2Sm0.1)1P1O4
Table 2. Results of LA-ICP-MS monazite U-Pb dating from the Koka gold deposit.
Table 2. Results of LA-ICP-MS monazite U-Pb dating from the Koka gold deposit.
No.PbThUTh/U207Pb/206Pb207Pb/235U206Pb/238U208Pb/232Thrho207Pb/235U206Pb/238U208Pb/232Th
ppmppmppmRatio1σRatio1σRatio1σRatio1σAge(Ma)1σAge(Ma)1σAge(Ma)1σ
KK-216-012.37377964.758.40.0946240.0097051.8504850.1078470.0993910.0037900.0290330.0002670.17106438611225785
KK-216-022.41441058.875.00.1175920.0178752.2909490.1410950.1144980.0041960.0298120.0003080.27120944699245946
KK-216-032.05415060.368.90.1086500.0092031.6902830.0827650.0937890.0024210.0295060.0002340.03100531578145885
KK-216-041.68389061.763.10.0707530.0084551.5832870.0721040.1020880.0032090.0292940.0002850.0796428627195846
KK-216-052.43373364.757.70.0952680.0112001.9195280.1138900.1068430.0036380.0291910.0002840.01108840654215826
KK-216-063.26429774.357.80.1030200.0120661.8804010.1660810.1169700.0047570.0288940.0003800.11107459713275767
KK-216-073.33406568.359.50.1242280.0112421.9676050.1243670.1020240.0037160.0292170.0003290.25110443626225826
KK-216-082.96336560.855.30.1236580.0166492.1865630.1428560.1108030.0039360.0298050.0003820.10117746677235947
KK-216-091.69409657.671.20.0638330.0120552.0679220.1265540.1086070.0050680.0297520.0003730.04113842665295937
KK-216-102.74344263.853.90.1132500.0120132.1617690.1246320.1022200.0032330.0298100.0002930.01116940627195946
KK-216-113.39447988.050.90.0920680.0085011.4700640.0998860.1046420.0026040.0297170.0002740.0091841642155925
KK-216-122.17601175.979.20.0743950.0084531.4892470.0970240.0966180.0032870.0295390.0002480.2392640595195885
KK-216-131.75399349.181.30.1021630.0156492.4177110.1376280.1061390.0042720.0296290.0003460.11124841650255907
KK-216-141.55389455.270.60.0720620.0117941.7660400.1350410.1020820.0035800.0296250.0003280.01103350627215906
KK-216-151.98409756.472.70.0900440.0110781.9958490.1110470.1045780.0037080.0297270.0002590.09111438641225925
KK-216-161.99371459.762.20.0943300.0131491.8824760.0998450.1046080.0030050.0298470.0002410.22107535641185945
KK-216-172.45563470.180.40.0983790.0105631.7007380.0933280.0970450.0029490.0298300.0002190.11100935597175944
KK-216-181.93387761.662.90.0853290.0112041.8373460.0960090.1026620.0034370.0295720.0002320.12105934630205895
KK-216-203.92322288.536.40.1111350.0096821.8841700.1176220.1051940.0024510.0296610.0003230.00107641645145916
KK-216-213.10616584.672.90.1008610.0114171.5817920.1235540.0975400.0029070.0298810.0002660.0196349600175955
KK-216-223.18359996.937.10.0849710.0089831.4587200.0828490.1062120.0034800.0296400.0003120.1291334651205906
KK-216-234.38361611431.60.0905820.0080711.5237420.0834450.1072530.0024060.0296920.0003130.2194034657145916
KK-216-246.10299310827.70.1312960.0104131.9927320.1148060.1102170.0032900.0297820.0003180.13111339674195936
KK-216-252.68311579.039.40.0875480.0109411.5265270.1334480.1008380.0033250.0297720.0003850.2794154619195938
Table 3. Temperature calculations of hydrothermal monazite in the Koka gold deposit (calculations after Equation (1) [64]).
Table 3. Temperature calculations of hydrothermal monazite in the Koka gold deposit (calculations after Equation (1) [64]).
No.La2O3Ce2O3Pr2O3Nd2O3Sm2O3Gd2O3Y2O3CaOThO2P2O5TotalXYmzTe (°C)
KK-216-0114.2630.543.8817.043.712.251.610.220.4324.4598.390.03316.34
KK-216-0213.2028.723.6716.073.502.101.380.270.5023.9793.400.03274.77
KK-216-0413.9130.253.9017.163.752.331.600.280.4425.3298.930.03315.59
KK-216-0513.6830.183.9017.133.772.351.700.470.4225.8999.500.03339.87
KK-216-0613.6829.533.7416.513.552.131.470.450.4926.4397.970.03289.73
KK-216-0713.6529.753.8016.563.572.161.450.640.4626.0998.130.03278.64
KK-216-0813.0528.783.6916.253.532.161.480.480.3825.3595.140.03302.19
KK-216-0913.0628.523.5815.693.382.001.340.470.4726.2194.720.03267.45
KK-216-1013.6529.433.7916.433.592.181.510.300.3927.0898.320.03302.45
KK-216-1113.1629.033.7216.543.592.261.570.110.5126.9497.430.03326.02
KK-216-1213.4429.563.8116.683.642.181.420.340.6828.0499.800.03273.29
KK-216-1313.2828.193.5715.193.201.901.210.940.4526.6594.580.03225.46
KK-216-1413.2928.833.7215.823.492.151.560.360.4428.7798.410.03326.03
KK-216-1513.1128.543.6816.143.552.171.530.370.4728.2697.810.03320.38
KK-216-1613.0528.593.6516.043.512.111.400.370.4228.3797.510.03283.54
KK-216-1713.4028.523.6315.863.602.181.310.310.6428.5097.950.03252.28
KK-216-1813.2728.723.6816.113.522.151.470.650.4429.0699.080.03296.53
KK-216-2013.4228.413.6115.643.432.171.630.660.3728.4797.800.03345.43
KK-216-2113.2928.243.6315.853.562.221.430.350.7029.5198.770.03293.71
KK-216-2212.7227.673.5615.353.352.111.510.610.4129.2696.540.03325.73
KK-216-2312.6527.123.5115.043.282.071.650.520.4129.0995.360.04372.52
KK-216-2413.4328.003.4714.633.162.032.000.900.3429.4297.390.04442.34
KK-216-2512.9627.803.5315.443.362.071.470.000.3529.7296.700.03318.64
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Ouyang, S.; Jiang, X.; Lei, X.; Wan, B.; Quan, Z.; Li, Y. Hydrothermal Monazite Geochemistry and Petrochronology Signatures: Metallogenic Age and Tectonic Evolution Model of the Koka Gold Deposit, Eritrea. Minerals 2025, 15, 851. https://doi.org/10.3390/min15080851

AMA Style

Ouyang S, Jiang X, Lei X, Wan B, Quan Z, Li Y. Hydrothermal Monazite Geochemistry and Petrochronology Signatures: Metallogenic Age and Tectonic Evolution Model of the Koka Gold Deposit, Eritrea. Minerals. 2025; 15(8):851. https://doi.org/10.3390/min15080851

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Ouyang, Song, Xiaojia Jiang, Xianquan Lei, Baoquan Wan, Zhenlong Quan, and Yizhao Li. 2025. "Hydrothermal Monazite Geochemistry and Petrochronology Signatures: Metallogenic Age and Tectonic Evolution Model of the Koka Gold Deposit, Eritrea" Minerals 15, no. 8: 851. https://doi.org/10.3390/min15080851

APA Style

Ouyang, S., Jiang, X., Lei, X., Wan, B., Quan, Z., & Li, Y. (2025). Hydrothermal Monazite Geochemistry and Petrochronology Signatures: Metallogenic Age and Tectonic Evolution Model of the Koka Gold Deposit, Eritrea. Minerals, 15(8), 851. https://doi.org/10.3390/min15080851

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