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Article

Trace Element Compositions of Galena and Cerussite from the Bou Dahar MVT District, Morocco: Insights from LA-ICP-MS Analyses

by
Kai Zhao
1,*,
Fafu Wu
1,*,
Xiang Cheng
1,
Shunbo Cheng
1,
Jinchao Wu
2,
Yaoyan He
1,
Chenggang Wang
1,
Noura Lkebir
1,
Sen Cui
1,
Peng Hu
1,
Jianxiong Wang
1,
Peng Xiang
1 and
Jiangtao Liu
1
1
Wuhan Center, China Geological Survey (Geosciences Innovation Center of Central South China), Wuhan 430205, China
2
School of Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China
*
Authors to whom correspondence should be addressed.
Minerals 2024, 14(8), 748; https://doi.org/10.3390/min14080748
Submission received: 18 June 2024 / Revised: 20 July 2024 / Accepted: 22 July 2024 / Published: 25 July 2024
(This article belongs to the Special Issue Ag-Pb-Zn Deposits: Geology and Geochemistry)
Browse Figures
Versions Notes

Abstract

:
The Bou Dahar Pb-Zn district, located in the Moroccan High Atlas, is a typical carbonate-hosted Pb-Zn ore district (>30 Mt at 4 wt.% Pb, 4 wt.% Zn). In situ trace element analysis was performed using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) on galena and cerussite from different ore types. The galena is generally enriched in Ag and Sb, secondarily enriched in Cu, with a trace amount of Cd and As, but extremely depleted in Bi and Tl. The main substitution mechanism in galena is (Ag, Cu)+ + Sb3+ ↔ 2Pb2+, and at high Sb concentrations, the further substitution of 2Sb3+ + □ ↔ 3Pb2+ (where □ represents a vacancy) took place. Micro-inclusions of Cu-Sb-bearing minerals (such as tetrahedrite) and Ag-bearing minerals (such as acanthite) may exist in some situations. The features of trace elements in galena show the existence of different coupled substitutions in vein-related ore, breccia-related ore, and strata-bound ore. This suggests that the Bou Dahar district experienced multistage mineralization. The MVT model alone cannot fully explain the ore-forming process. The cerussite replacing strata-bound galena is enriched in Sr, Ba, Ag, and Cu, with minor Sb, As, and Tl. Strontium and Ba are directly substituted with Pb in the cerussite lattice. Copper and Ag are likely present in cerussite as nano-inclusions, which differs from the coupled substitution mechanism of the original galena. High concentrations of Ag may occur due to minor electrum inclusions. The enrichment of Ag, Cu, and Au in cerussite during the oxidation process may guide the optimization of ore processing, especially in extracting valuable trace/minor elements.

1. Introduction

In recent years, the analysis of trace element compositions in sulfides has become a common practice for scrutinizing the distribution of elements and mechanisms of substitution, understanding the physicochemical characteristics of fluids involved in ore formation, and unraveling the ore-forming genesis [1,2,3,4,5]. Several studies have confirmed the incorporation of trace elements into the crystal structure of galena and related sulfides or occurred as microscopic inclusions within lead-zinc deposits [1,6,7]. Typically, elements such as Te, Tl, Se, Sb, Bi, and Ag preferentially integrate into galena, while Ga, Ge, In, Cd, Fe, Mn, and Cu are concentrated in coexisting sphalerite [3,5,6,7,8,9]. The trace element characteristics of sphalerite are widely used for determining the genesis of ore deposits [1,2,3,4,9]. Nevertheless, discriminating ore-forming types based on galena geochemistry is relatively new [4,5,6].
The intracontinental Atlas system of Northwest Africa was primarily formed from the Mesozoic to the Cenozoic, making it one of the most significant Tethyan–Atlantic metallogenic provinces. It hosts over 1000 Mississippi Valley-type (MVT) occurrences and deposits [10,11,12,13]. Situated within this domain, the Moroccan High Atlas region encompasses the large and historically significant Bou Dahar district, with reserves exceeding 30 million tons at 4 wt.% Pb and 4 wt.% Zn [12]. Since its discovery, Bou Dahar has attracted widespread attention [12,14]. Previous research on the Bou Dahar district has mainly focused on its geological features, mineralogy, ore-forming fluid inclusions, and stable isotope geochemistry (C-O-S-Pb) [14,15,16,17,18,19,20,21]. However, the mechanisms of trace element incorporation and substitution in Pb-bearing minerals have rarely been discussed.
Previous studies have shown differing viewpoints regarding the genesis of the Bou Dahar Pb-Zn deposits. The syngenetic origin hypothesis was first proposed for the strata-bound and numerous lens-shaped Pb-Zn ore bodies hosted in reef limestone [15,17,22]. Subsequently, based on mapping and extensive mineralogical studies, the genesis of the Bou Dahar deposits was considered to be linked to the movement of deep magmatic or brine fluids during the post-Cretaceous tectonic phase [23,24,25]. In contrast to earlier findings, more evidence for ore-forming fluid and metal sources indicates the ore emplacement took place during the mid-to-late Tertiary period in response to large-scale crustal processes triggered by the Atlas orogeny. The lead-zinc mineralization in the Bou Dahar district was considered to be of the MVT ore deposits [10,12,13,14,18,26,27]. Hence, further studies are needed to enhance the understanding of the genetic model.
The Bou Dahar district is characterized by both sulfide and non-sulfide ores, with a high content of galena and Pb oxides. This district presents an ideal natural example for studying the incorporation mechanism of trace elements in Pb-bearing minerals. Additionally, it provides insight into the migration pattern of elements during sulfide oxidation processes. However, there has been little discussion on the mechanisms of trace element incorporation and substitution in Pb-bearing minerals, with a limited number of studies documenting the trace element composition of sphalerite [21]. This present paper aims to identify the distribution features and substitution mechanisms of trace elements, determine the variation pattern of trace elements during oxidation processes, and explore the potential for utilizing the trace element signature of galena to discriminate ore genetic types.

2. Regional Geology

The Bou Dahar district is situated within the Moroccan High Atlas (Figure 1), a constituent of the fertile Pb-Zn metallogenic belt within the intracontinental Atlas system [13,27,28]. This system evolved primarily from the Mesozoic to the Cenozoic, spanning approximately 2000 km from the passive continental margin of the Moroccan Atlantic to the eastern Tunisian Mediterranean coasts [12]. The Moroccan segment of the Atlas comprises the Middle Atlas in the north and the High Atlas in the central region. The Variscan Meseta, accreted to the African plate during the Hercynian orogeny, delineates the northern part of the High Atlas, while the Anti-Atlas massif, consisting of Precambrian and Paleozoic cover, marks the southern boundary [29].
The entire High Atlas primarily consists of a Paleozoic basement overlain by Mesozoic to Cenozoic sedimentary rocks and has undergone deformation associated with the Hercynian orogeny and subsequent modification during the Alpine orogeny in Cenozoic [27]. The inherited tectonics of the Paleozoic are closely linked to the geometric characteristics associated with the opening of the central Atlantic Ocean and the formation of the Tethys Ocean [30,31]. The formation of Mesozoic basins is controlled by ENE–WSW trending normal faults, believed to have formed during the Paleozoic [32,33,34].
The Early Jurassic saw the formation of extensive carbonate platforms overlapping the Triassic rift basins due to the rifting associated with the opening of the central Atlantic Ocean. These platforms were later graded upward into marl, limestone, and calcareous turbidites [26]. By the end of the Middle Jurassic, regional uplift took place, resulting in a marine regression and the subsequent deposition of continental red beds [35,36,37]. The region experienced a brief sedimentary hiatus extending into the Cretaceous, and red sandstones and limestones were developed upon sedimentary resumption. The establishment of carbonate platforms in the Upper Cretaceous marks a brief but significant late Cenomanian–Turonian transgressive event, followed by continental sedimentation during the late Cretaceous, Paleogene, and until the mid-Neogene in the High Atlas region [29,32,38].
Tectonic inversion resulting from the convergence of the African and European plates began in the late Cretaceous, followed by successive compression events during the Cenozoic and Quaternary periods [13,33]. Two major compressive events were recorded, the first occurring in the Middle–Late Eocene, followed by a quiescent period in the Oligo–Miocene [13]. The second significant tectonic event, leading to the uplift and formation of the High Atlas, occurred during the Late Miocene–Pliocene. Thick-skinned structures dominate in the High Atlas region, indicating deformation of the deep crust, explained as the tectonic framework for the formation of the Moroccan High Atlas, shaping the current geomorphological features [13,38].

3. Deposit Geology

The Bou Dahar district lies in the eastern vicinity of Beni Tadjite City, situated approximately 100 km east of Rich City, which hosts numerous Zn-Pb ore deposits (e.g., Beni Tadjite, Riss, Sebbab, and Taboudaharte deposits) [15]. These ore deposits are predominantly distributed along the margins of the Lower Jurassic carbonate platform (Figure 1), which extends as an elongated ENE-trending flat-topped mountain spanning over 40 km × 15 km [15,29].
Paleozoic metasedimentary and volcaniclastic rocks are at the core of the carbonate platform, referred to as the Sebbab–Kébir inlier. Unconformably overlying these Paleozoic formations is a Triassic sedimentary sequence consisting of deformed red-bed layers and CAMP basalts. The Jurassic carbonate, which forms the bulk of the Bou Dahar reef, is characterized by six distinct units displaying a range of facies transitions, from offshore basinal shale and limestone to back-reef facies exhibiting significant porosity [15]. Platform development commenced in the Sinemurian and was concluded by the Upper Pliensbachian–Middle Toarcian [21,39,40]. The E-W-trending normal faults activities resulted in the delineation of three distinct paleogeographic regions arranged from north to south: (i) the Sebbab Kebir paleohigh, (ii) the Jbel Bou Dahar Plateau, and (iii) the Beni Bassia basin (Figure 2) [13]. Alternating marl and argillaceous limestone beds make up the Bou Dahar platform’s margins, suggesting a consistently deep marine environment from the Toarcian to the Bajocian [13]. These normal faults experienced reactivation during the late Cretaceous and Miocene due to the Alpine orogeny, which caused the uplift of the contemporary Bou Dahar plateau.
The Zn-Pb ore bodies are restricted to the Lower Jurassic carbonate. They appear as strata-bound mineralization in the silicified oolitic limestone beds from the Middle–Late Sinemurian (Figure 3f). Additionally, they manifest as vein-type mineralization within the massive bioclastic and reef limestone of the Pliensbachian carbonate platform (Figure 3c). The strata-bound orebodies, identified as 20 to 30 m thick [15], are partly exposed in the Sebbab Kebir paleohigh domain, primarily composed of disseminated galena as the main ore mineral (Figure 3c). Sulfide occurs in various-sized cubic crystals, replacing bioclasts and oolites (Figure 3l). The majority of the sulfide of Zn-Pb mineralization hosted in Pliensbachian is characterized by the occurrence of multiple parallel veins associated with faults trending E-W to ENE-WSW (Figure 3b), occasionally confined to the brecciated and karst filling (Figure 3a). The combined length of the identified veins was estimated to be 130 km, with most of the single veins measuring 300 m in length and 1 m in width [15]. Oxidation processes are widespread in the Bou Dahar district, with the oxidized Zn-Pb orebodies extending to depths of at least 100 m below the present surface, with grades ranging from 31%–50% Zn [12], while sulfides occupy the deepest zones.
Based on the macroscopic and microscopic mineral assemblages, two mineralizing periods were identified (Figure 4). The hydrothermal mineralizing period formed the primary sulfide ore body, comprising fluctuating quantities of sphalerite, galena, chalcopyrite, and pyrite, accompanied by calcite, barite, and minor dolomite as the primary gangue minerals. The supergene mineralizing period formed the oxidized ore body, comprising smithsonite, hemimorphite (Zn clays), cerussite, malachite, and sulfate minerals. The appearance of these oxides was also described in detail by [21].

4. Sampling and Analytical Methods

4.1. Sample Description

In this study, six representative ore samples were collected from the Bou Dahar district to conduct mineralogical research. BJ10 and BJ13, representing the brecciated ore (Figure 3d) and veined ore (Figure 3e), sampled from the underground workings in the Beni Tadjite deposit (Figure 1), respectively, occurring within the fault zone. TA3, TA4, TA5, and TA6 represent the strata-bound ore (Figure 3f) characterized by its strata-bound features sampled from the surface outcrops in the Sebbab deposit (Figure 1). All samples exhibit varying degrees of oxidation. Due to strong dissolution and alteration, sphalerite was rarely observed in the primary sulfide ore, whereas galena was more commonly found. In highly oxidized samples (Figure 3g,h), sphalerite was completely oxidized to Zn-clay, smithsonite, and hydrozincite, while galena tends to remain in place. Cerussite commonly replaces galena along the mineral edges (Figure 3k,l). The panorama of these thin sections was provided by the SD-3000A Fully Automatic Microscopic Image Scanning System. Occasionally, cerussite completely replaces galena in vein-type ores (Figure 3i). After examining the thin sections in the reflected and transmitted light of these samples, the representative galena and cerussite from different ore types were selected for in situ geochemical analyses.

4.2. In Situ LA-ICP-MS Analysis

The LA-ICP-MS spot analyses in galena and cerussite were carried out at Guangzhou Tuoyan Analytical Technology Company Limited., using a Thermo Scientific iCap-RQ quadrupole inductively coupled plasma mass spectrometer (ICP-MS) coupled with a New Wave Research 193 nm ArF excimer laser ablation system. To reduce the rate of oxide formation during the laser ablation process, the ICP-MS was optimized using NIST 610 standard glass. During the laser ablation process, 0.7 L/min of He was used as the carrier gas flow, and 0.89 L/min of Ar was used as the make-up gas to adjust sensitivity. The laser operated at a fluence of 3.5 J/cm2, a repetition rate of 6 Hz, and a 30 μm spot size. Single spot measurements commenced after a 25 s background measurement period, providing approximately 15 s of background data before signal detection, followed by a 45 s analysis period and an additional 25 s of background measurement.
Trace element quantification was calibrated against reference materials (NIST SRM 610, NIST SRM 612, and MASS-1) using Pb as an internal standard, with Pb content values of 86.5% for galena and 76.94% for cerussite [41]. Glass standards GE8 and BCR-2Ga were also analyzed to assess data quality [42,43]. Raw isotope data were processed using the 3D Trace Element data reduction scheme (DRS, [44]) in IOLITE 4.9.3 software, where user-defined time intervals were established for baseline correction to calculate session-wide baseline-corrected values for each isotope. Detailed analytical procedures are provided in [41].

4.3. Principal Component Analysis (PCA)

The principal component analysis (PCA) module in Origin 2011 was utilized to analyze trace element datasets in galena and cerussite. Selected trace elements included Zn, Cu, Au, Ag, Sb, Cd, Ge, Tl, Bi, and As, as other elements were generally below the detection limit. The PCA results, depicted in Figure 5, comprise the loading plot, score plot, and variance explanation of the principal components.

5. Results

5.1. Trace Element Contents in Galena

Galena from three types of sulfide ore samples with different occurrences was used for in situ trace element analysis. A total of forty-eight spot analyses were performed on six galena samples, and trace element composition data are summarized in Table 1. The complete LA-ICP-MS trace element compositions of galena and cerussite are given in Supplemental Materials. Figure 6 shows the absolute concentration ranges of selected trace elements in galena from the different ore types, while Figure 7 presents representative time-resolved profiles of trace elements in galena. Similar trace element characteristics, as indicated by PCA, are shown in Figure 6.
Overall, the contents of trace elements in galena, such as Ag and Sb, are relatively high, while Cd and As are relatively low, with only trace amounts of Tl and Bi. The concentrations of Se, Zn, Mn, Sn, Ga, Ge, In, and Au are almost below the detection limit (B.D.L.). Cu is only detected in galena from the strata-bound ore.
The content of Ag and Sb is highest in galena from brecciated ore (Gn-b), veined ore (Gn-v), and strata-bound ore (Gn-s) (Figure 6). Gn-s displays a wider range of Ag and Sb content compared to Gn-b and Gn-v. The signals of Ag and Sb in Gn-b and Gn-v show parallel fluctuations, while Gn-s shows peaks of Cu and Sb, indicating micro-inclusions (Figure 7c,d). The distribution of these two elements is similar in PCA (Figure 5c,d). The Cu content in galena varies significantly among the three types of sulfide ores (Table 1, Figure 6). Gn-s has a significantly higher and wider range of Cu content compared to Gn-b and Gn-v. The signals of Cu show similar fluctuating features to those of Sb (Figure 7c,d). Trace elements such as Cd, As, Bi, and Tl are present in meager amounts in galena from Gn-b, Gn-v, and Gn-s. The Cd content in Gn-s is slightly higher compared to Gn-b and Gn-v, while the Bi content is lower in Gn-s (Figure 6).
The trace element datasets, encompassing Zn, Cu, Ag, Sb, Cd, Tl, Bi, and As in galena from the three ore types, were further analyzed using PCA. The results are depicted in Figure 5, show the trace element concentrations projected onto the PC1–PC2 plane, explaining 52% of the variability in Gn-v and Gn-b (Figure 5a,b,f) and 49.7% of the variability in Gn-s (Figure 5c,d,g). The distributions of trace elements show the similitude between PCl and PC2, which could represent the behaviors of most elements in galena. Three correlation clusters of elements can be observed obviously load on PC1 (25.4%), namely, Ag, Zn-Cu-Bi, and As-Sb-Tl in Gn-v and Gn-b (Figure 5b). Silver is the representative element in Gn-v considering Figure 5a. As-Sb-Tl represents a feature of Gn-b. The correlation clusters of elements in Gn-s are not significant (Figure 5c,d); Ag and Cu exhibit a negative correlation (Figure 5d).

5.2. Trace Elements Contents in Cerussite

The trace elements in the cerussite replacing galena from the strata-bound ore were determined by 14 LA-ICP-MS spot analyses from three samples, and the results are presented in Table 1. Cerussite from the strata-bound ore is characterized by an enrichment of Ag, Cu, Sr, and Ba. Strontium is the most enriched trace element in cerussite, with concentrations ranging from 454 to 1273 ppm (median 473 ppm). The time-resolved signals of Sr are typically smooth and flat (Figure 7e,f). The barium content ranges from 7.59 to 5296 ppm (median 112 ppm), displaying signal profiles similar to those of Sr (Figure 7e,f). Copper is relatively enriched in cerussite, with concentrations ranging from 4.87 to 37,435 ppm (median 444 ppm), and its signal profiles are smooth and flat (Figure 7e,f). Silver is also enriched in cerussite, with concentrations ranging from 2.34 to 2527 ppm. The time-resolved signals of Ag exhibit two features: peaks and flat profiles (Figure 7e).
The contents of Sb are slightly lower among these elements. The signals of Sb exhibit parallel fluctuations in the time-resolved profiles to those of Zn (Figure 7e), indicating micro-inclusions in cerussite. The contents of Zn, Cd, Au, Tl, and Bi are relatively low in cerussite. Most of the analysis points for these elements were below the detection limits. The distributions of Cd, Au, Bi, and Tl show similarity as indicated in PCA (Figure 5e).
The datasets of trace elements in cerussite, including Ba, Sr, Ag, Au, Cu, Sb, Zn, Cd, Bi, and Tl, were further analyzed using PCA, with the results presented in Figure 5e,h. In summary, the element contents are projected onto the PC1-PC2 plane, accounting for 56.8% of the total variability. The distribution patterns of trace elements indicate that PC1 predominantly influences the behavior of most elements in cerussite. Two distinct correlation clusters of elements are evident: Au-Cd-Bi-Zn-Tl and Sr-Ag-Cu-Sb-Ba (Figure 5e).

6. Discussion

6.1. The Substitution Mechanism of Trace Elements in Galena

Galena, the most abundant and significant lead ore mineral, exhibits a simple chemical formula (Pb2+, S2−) [45]. Extensive research on galena has revealed that numerous minor and trace elements can substitute into its crystal lattice at varying concentrations [7,46,47,48,49,50,51]. The primary factors influencing substitution into galena include the ionic radius and the availability of other ions for coupled substitutions [8]. LA-ICP-MS trace element analysis is commonly used to distinguish between nano-inclusions and solid solutions trace elements (which show flat in the time-resolved signals) and micro-inclusions (which show peaks) [1,5,7,52,53,54,55].
Silver and Sb exhibit the highest concentrations in galena from the Bou Dahar district (Table 1, Figure 6). The time-resolved acquisition profiles of Ag and Sb are nearly smooth and generally flat, and they are also basically parallel with the signal of Pb (Figure 7a,b), suggesting that Ag and Sb are likely present in Gn-b and Gn-v either as solid solutions or nano-inclusions. Previous studies on galena have shown that trace elements such as Ag and Sb are present in high concentrations due to the coupled substitution mechanism Ag+ + Sb3+ ↔ 2Pb2+ [7,48,49,50,51]. The positive correlation between Sb and Ag (Gn-b, r2 = 0.85; Gn-v, r2 = 0.96), which is close to l on the binary plot (Figure 8a), indicates that this coupled substitution also occurred in Gn-b and Gn-v. In an ideal coupled substitution, the mol.% of Ag would match that of Sb [6]. Obviously, the mole percentage ratio of Ag to Sb is greater than 1 (Table 1). Silver is a characteristic element in Gn-v and appears in the discrete distribution with Sb in the loading plot (Figure 5b), indicating Ag is independent of the coupled substitution of Sb. Some studies have demonstrated that submicron inclusions of Ag-bearing minerals such as acanthite could explain the excessive Ag in galena [6,56,57]. In Gn-v, different processes are thought to be at work, including substitution mechanisms involving empty spaces and submicron inclusions of Ag-minerals. In contrast, in Gn-b, the mole percentage ratio of Ag to Sb is less than 1 (Table 1), suggesting that Ag+ is likely insufficient for Sb3+ to enter the galena lattice through the mentioned substitution mechanism, and these elements may be entering the lattice through other means. Additionally, Sb and Pb have parallel flat time-resolved profiles (Figure 7b), indicating an additional substitution mechanism, 2Sb3+ + □ ↔ 3Pb2+ (where □ represents a vacancy), occurring in Gn-b, as proposed by [6].
High Cu concentrations in Gn-s distinguish it from Gn-v and Gn-b, with time-resolved profiles of Ag, Sb, and Cu being approximately flat and parallel to Pb, indicating solid solutions or nano-inclusions of these elements entering the Gn-s (Figure 7c). PCA shows Ag and Sb have an analogous distribution section, with a discrete distribution between Ag and Cu (Figure 5d). Considering the relatively weak correlation between Ag and Sb (Figure 8b) and the positive correlation in (Ag + Cu) vs. Sb (Figure 8c), we propose that these elements are incorporated into Gn-s through the coupled substitution of (Ag, Cu)+ + Sb3+ ↔ 2Pb2+ [6]. Most of the mole percentage ratios of (Ag + Cu) to Sb are greater than 1 (minimum close to 1) (Table 1), which may imply submicron inclusions of Cu-bearing or Ag-bearing minerals. Some spot analyses on Gn-s show parallel peaks of Cu and Sb on the time-resolved profiles, also reflecting the existence of micro-inclusions of Cu-Sb-bearing minerals such as tetrahedrite (Figure 7c,d).
In conclusion, we propose that the primary mechanism for the enrichment of Ag, Cu, and Sb in Bou Dahar galena is the coupled substitution of (Ag,Cu)+ + Sb3+ ↔ 2Pb2+. At higher Sb concentrations, there is the additional substitution mechanism of 2Sb3+ + □ ↔ 3Pb2+. The higher concentrations of Ag in Gn-v and higher concentrations of Cu and Sb in Gn-s are attributed to the presence of micro-inclusions of Ag-bearing minerals (such as acanthite) and Cu-Sb-bearing minerals (such as tetrahedrite). The presence of minor amounts of Tl, As, and Bi in Bou Dahar galena can be explained by an extended coupled substitution involving trivalent and monovalent cations, (Ag,Cu,Tl)+ + (Sb,Bi,As)3+ ↔ 2Pb2+, which is supported by [7,58].
Generally, galena is likely to host Ag, Tl, Bi, Sb, and Se, whereas Cd, In, Hg, and Mn are systematically enriched in co-existing sphalerite [6]. Some spot analyses on Gn-b and Gn-s show the presence of Zn (maximum 43.7 ppm). Individual Zn peaks have also been observed on the time-resolved profiles (Figure 7b,d), indicating the inclusions of Zn-bearing minerals, which may infer that Cd is concentrated in Cd-rich sphalerite [5,6].

6.2. Distribution and Substitutions of Trace Elements in Cerussite

Previous studies have demonstrated that changes in geochemical properties and physicochemical conditions (such as pH, temperature, etc.) can lead to the enrichment of different trace elements in various non-sulfide minerals during the supergene oxidation of zinc-lead sulfides [8,59,60]. The oxidation process releases trace elements from sulfides into the oxidizing fluids, resulting in their formation as independent minerals and decreasing trace element content in oxides compared to the primary sulfide. Conversely, trace elements in the oxidizing fluids can infiltrate oxide structures or form micro-inclusions sorbed onto the surface of oxides, thereby increasing the element content [7,8,61,62].
The Bou Dahar district underwent intense supergene oxidation during the Mid-Tertiary period (Cretaceous to Miocene), coinciding with the closing stages of the Alpine orogeny in the Atlasic orogenic belt [12]. Cerussite is widespread in all deposits of the Bou Dahar district and is commonly associated with lesser amounts of anglesite. Lead sulfide can be readily oxidized directly to cerussite in carbonate environments [62]. The simplified reactions can be represented as follows: PbS (Galena)+ H2O + 2O2 + CO2 → PbCO3 (cerussite) + SO42− + 2H+ [61,62,63].
The concentrations of Sr, Ba, Ag, and Cu are relatively high in cerussite (Table 1, Figure 6). Minerals such as strontianite (SrCO3), cerussite (PbCO3), and witherite (BaCO3) are orthorhombic carbonates with similar crystal structures [63]. The Sr2+, Pb2+, and Ba2+ have similar ionic radii, and the charges of these divalent cations make them able to substitute for each other in the crystal lattice [63,64]. The time-resolved profiles of Sr and Ba, which are approximately flat and parallel to Pb (Figure 7e,f), indicate that these elements enter the cerussite crystal as solid solutions.
Compared to Gn-s, the concentrations of Ag and Cu are significantly higher in cerussite but lower for Sb, As, and Tl (Table 1, Figure 6). Silver, Cu, and Sb exhibit analogous distribution patterns in the loading plot (Figure 5e), and the plot of (Ag + Cu) vs. Sb shows positive correlations (Figure 8d), implying that they may share similar characteristics and be incorporated into cerussite. The acquisition profiles of Ag, Cu, and Sb in the time-resolved profiles of some spot analyses on cerussite are smooth, flat, and generally parallel with the signal of Pb (Figure 7e,f), indicating that these elements seem to undergo the same coupled substitution of (Ag,Cu)+ + Sb3+ ↔ 2Pb2+, which occurred in Gn-s as well. However, the concentration of Sb is much lower (median 0.65 ppm) than Gn-s (median 128 ppm), which suggests that the aforementioned coupling mechanism is not primarily responsible for the cerussite. The disparities in ionic radii and charges between silver (Ag+), copper (Cu+ or Cu2+), and lead (Pb2+) restrict the possibility of Ag and Cu incorporation into the cerussite lattice through coupled substitution in environments deficient in Bi or Sb. The smooth and flat acquisition profiles of Ag and Cu (Figure 7f) probably indicate the nano-inclusions of Ag-bearing minerals and Cu-bearing minerals (such as malachite or azurite) existed in cerussite. Some spot analyses with highly concentrated Ag in the time-resolved profiles show peaks of Ag, reflecting the presence of Ag microminerals (Figure 7e). These data points also indicate the presence of trace amounts of Au (Table 1). If gold is also present, electrum inclusions will be more prevalent [65,66]. We proposed that a small amount of electrum inclusions adsorb onto the surface of cerussite, increasing the Ag concentration.
In summary, the dominant substitution mechanism of Ag and Cu is likely present in cerussite as nano-inclusions, which differs from the coupled substitution mechanism of Gn-s. During the oxidation process of galena to cerussite, Sb was released from galena and formed micro-inclusions of Sb and Zn, as supported by the parallel peak of Sb and Zn observed on the time-resolved profiles (Figure 7e). A small amount of electrum inclusions plays a significant role in increasing the Ag concentration in cerussite.

6.3. Implications for Ore Genesis and Exploration

The trace element characteristics of sulfides, such as pyrite, sphalerite, and galena, are influenced by the sources of ore-forming fluid and metal, physicochemical parameters during the mineralization process, and coexisting mineral assemblages [53,67,68,69,70]. These characteristics are commonly used to indicate the genetic types of Pb-Zn ore deposits [3,6,7,8,70,71].
Previous studies have used trace element data of galena to distinguish different genetic types [3,6,7,70,71]. Box and whisker plots of six trace elements in galena from the Bou Dahar district, combining data from representative genetic types such as skarn, CD, epithermal, and MVT, illustrate differences in the concentration of Ag, Cu, Sn, Cd, Bi, and Sb in galena (Figure 9).
The trace element composition of galena varies among different genetic types of ore deposits. Galena from skarn deposits is enriched in Bi but depleted in Sb due to high mineralization temperatures [72]. Bi-transport is linked to high-temperature fluids, whereas Sb is more mobile at low temperatures [58,73]. In CD deposits, galena exhibits moderate Bi and high Sn content [5]. Low-temperature deposits such as epithermal and MVT feature galena with low Bi but high Sb levels [1,5]. The Cu content of epithermal deposit galena is higher than that of MVT, likely due to the presence of different hydrothermal fluids magmatic-hydrothermal fluid for epithermal deposits and basinal brine for MVT deposits [71]. Galena from the Bou Dahar district is relatively enriched in Ag and Sb and significantly depleted in Bi, Sn, and Cd. Gn-s primarily enriches copper, whereas Gn-v and Gn-b do not contain it. The high content of Cu in Gn-s from the Bou Dahar district is likely due to the influence of the presence of micro-inclusions such as tetrahedrite. Comparing the features of trace elements in the aforementioned galena with those of genetic types, the Bou Dahar district galena is similar to MVT and epithermal deposits.
With ongoing research into the genesis of the Bou Dahar Pb-Zn deposits, understanding of the origin has gradually shifted from syngenetic to epigenetic, mainly based on the regional deep-seated E-W and NE-SW thrust faults controlling the circulation of ore-forming fluids and the emplacement of ore bodies [13,15,18,19,20,21,39,40]. According to geochemical studies of the deposits, the ore-forming fluid in the Bou Dahar deposits mostly came from basin brines. The metals came from the upper crust and orogenic reservoirs, possibly from the Paleozoic basement [13,15,19,20]. These pieces of evidence support the idea that the ore genesis model bears similarities to numerous MVT deposits, especially in the Tethys metallogenic domain [13,14,18]. This has led to a gradual disregard for earlier syngenetic models based on field investigations. There is no denying that the precise timing of regional Pb-Zn mineralization remains undetermined due to the lack of suitable minerals for radiometric age dating [12]. Currently, research on trace elements in sulfides from the Bou Dahar district is extremely scarce. In this study, we identified different coupled substitutions between Gn-v and Gn-b, and the high concentration of Cu in Gn-s distinguished it from Gn-v and Gn-b in the Bou Dahar district. These phenomena may imply that multistage mineralization occurred in the Bou Dahar district. The ore-forming process in the Bou Dahar district cannot be fully explained by the MVT model alone. More detailed and comprehensive studies are required to characterize the mineralization process accurately.
Trace element concentrations can serve as indicators of ore formation processes and provide insights into deposits that have undergone superimposed metamorphism and deformation [6]. The concentration of Ag, Cu, and minor amounts of Au existing in cerussite by nano-inclusions during the oxidation process may guide the optimization of ore processing, especially in extracting valuable trace/minor elements [6]. Moreover, we hope there is potential for using the trace element geochemistry of Pb-bearing minerals in mineral exploration.

7. Conclusions

This study is the first big step toward understanding how trace elements are distributed and how they can be replaced, as well as how the ore in the Bou Dahar Pb-Zn district was formed. This was achieved through in situ trace element analysis of galena and cerussite from various ore types within the district.
Galena from the Bou Dahar district is enriched in Ag and Sb, secondarily enriched in Cu, with a trace amount of Cd and As, but extremely depleted in Bi and Tl. The main substitution mechanism in galena is (Ag, Cu)+ + Sb3+ ↔ 2Pb2+, and at high Sb concentrations, the further substitution of 2Sb3+ + □ ↔ 3Pb2+ (where □ represents a vacancy) took place. Micro-inclusions of Cu-Sb-bearing minerals (such as tetrahedrite) and Ag-bearing minerals (such as acanthite) may exist in some situations.
Cerussite from the Bou Dahar district is enriched in Sr, Ba, Ag, and Cu, with minor Sb, As, and Tl. Strontium and Ba are directly substituted with Pb in the cerussite lattice. Copper and Ag are likely present in cerussite as nano-inclusions, which differs from the coupled substitution mechanism of the original galena. High concentrations of Ag may occur due to minor electrum inclusions. The concentration of Ag, Cu, and Au during the oxidation process may guide the optimization of ore processing, especially in extracting valuable trace/minor elements.
The genesis of the Bou Dahar Pb-Zn deposits is mainly considered to be of the MVT type but remains controversial. While research has shifted the interpretation from syngenetic to epigenetic origins, influenced by regional thrust faults, the precise timing and full explanation of mineralization are still undetermined. The characteristics of trace elements in galena from this study indicate the existence of different coupled substitutions in vein-related ore, breccia-related ore, and strata-bound ore, implying that multistage mineralization occurred in the Bou Dahar district. This ore genesis cannot be fully explained by the MVT model alone. The mineralization process requires more detailed and comprehensive studies to be accurately characterized.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/min14080748/s1, Table S1: LA-ICP-MS Trace Element Compositions of Galena and Cerussite.

Author Contributions

K.Z. and F.W. Conceived and designed the experiments; Data curation K.Z. and J.W. (Jinchao Wu); Funding acquisition, F.W. and P.H.; Software, J.L. and P.X.; Methodology, K.Z. and X.C.; Investigation, K.Z., F.W., X.C., S.C. (Shunbo Cheng), C.W., Y.H., J.W. (Jianxiong Wang) and S.C. (Sen Cui); Writing-original draft, K.Z.; Writing-review and editing. K.Z., F.W. and N.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by [The Ministry of Commerce of the People’s Republic of China under project] grant number [202107], [201426] and [China Geological Survey, International cooperation in Northern Africa] grant number [DD20230575].

Data Availability Statement

Data are contained within the article and Supplementary Materials.

Acknowledgments

We thank BENLAKHDIM AHMED and KHALID EL HMIDI from the Direction of Geology, Mines, and Hydrocarbons of the Ministry of Energy Transition and Sustainable Development of the Kingdom of Morocco to give us the Unreserved support for the fieldwork. Reviews and edits by anonymous Minerals referees are greatly appreciated.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Geological map of Bou Dahar district showing early Mesozoic lithologies, emphasizing regional geology, main tectonic structures, and stratigraphic distribution of major MVT. Line A–B indicates the cross section shown in Figure 2. Modified after [21].
Figure 1. Geological map of Bou Dahar district showing early Mesozoic lithologies, emphasizing regional geology, main tectonic structures, and stratigraphic distribution of major MVT. Line A–B indicates the cross section shown in Figure 2. Modified after [21].
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Figure 2. N-S schematic geological section along strike of Bou Dahar early Mesozoic carbonate platform showing the spatial distribution of main lithologies associations and MVT deposits; see Figure 1. for location of section. Modified after [21].
Figure 2. N-S schematic geological section along strike of Bou Dahar early Mesozoic carbonate platform showing the spatial distribution of main lithologies associations and MVT deposits; see Figure 1. for location of section. Modified after [21].
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Figure 3. Outcrops of different orebody types, selected hand samples, and microscopic characteristics of different ore types from the Bou Dahar district deposits that illustrate representative styles of sulfide and non-sulfide mineralization and textural variations in different ore types. (a) Outcrop of brecciated orebody hosted in Pliensbachian limestone which was controlled by the EW trending fault (F). (b) Outcrop of veined orebody hosted in Pliensbachian limestone. Notice the normal 20 cm width of a single vein. (c) Outcrop of strata-bound orebody hosted in Sinemurian oolitic limestone. (d) Hand sample of brecciated ore showing brecciated dolomite (Dol) and sphalerite (Sp), galena (Gn-b), and gangue minerals such as barite (Brt). (e) Hand sample of veined ore showing coarse-grained euhedral galena (Gn-v) and calcite (Cal). (f) Hand sample of strata-bound ore showing fine-grained disseminated galena (Gn-s), which replaces the oolite (Oo). (g) Hand sample of Zn-oxide ore showing the disseminated Zn-clay associated with smithsonite (Sm). (h) A hand sample of Zn-oxide ore showing the banded smithsonite (Sm) replacing the dolomite. (i) Hand sample of Pb-oxide ore showing galena (Gn-v) associated with calcite (Cal) is oxidized to cerussite (Cer). (j) Typical colloform sphalerite encloses fine-grained galena (Gn-b), pyrite (Py), and malachite (Mal), partly oxide to smithsonite (Sm) (reflected light). (k) Coarse-grained euhedral galena (Gn-v) associated with calcite (Cal), cerussite (Cer) replacing Gn-v along its edges (reflected light). (l) Galena (Gn-s) replacing oolitic limestone, which was oxidized to cerussite (Cer) from the outer side to the inner (reflected light). Some representative analytical positions on Gn-v (k) and Gn-s (l) are marked by red dots, and the yellow dots indicate cerussite.
Figure 3. Outcrops of different orebody types, selected hand samples, and microscopic characteristics of different ore types from the Bou Dahar district deposits that illustrate representative styles of sulfide and non-sulfide mineralization and textural variations in different ore types. (a) Outcrop of brecciated orebody hosted in Pliensbachian limestone which was controlled by the EW trending fault (F). (b) Outcrop of veined orebody hosted in Pliensbachian limestone. Notice the normal 20 cm width of a single vein. (c) Outcrop of strata-bound orebody hosted in Sinemurian oolitic limestone. (d) Hand sample of brecciated ore showing brecciated dolomite (Dol) and sphalerite (Sp), galena (Gn-b), and gangue minerals such as barite (Brt). (e) Hand sample of veined ore showing coarse-grained euhedral galena (Gn-v) and calcite (Cal). (f) Hand sample of strata-bound ore showing fine-grained disseminated galena (Gn-s), which replaces the oolite (Oo). (g) Hand sample of Zn-oxide ore showing the disseminated Zn-clay associated with smithsonite (Sm). (h) A hand sample of Zn-oxide ore showing the banded smithsonite (Sm) replacing the dolomite. (i) Hand sample of Pb-oxide ore showing galena (Gn-v) associated with calcite (Cal) is oxidized to cerussite (Cer). (j) Typical colloform sphalerite encloses fine-grained galena (Gn-b), pyrite (Py), and malachite (Mal), partly oxide to smithsonite (Sm) (reflected light). (k) Coarse-grained euhedral galena (Gn-v) associated with calcite (Cal), cerussite (Cer) replacing Gn-v along its edges (reflected light). (l) Galena (Gn-s) replacing oolitic limestone, which was oxidized to cerussite (Cer) from the outer side to the inner (reflected light). Some representative analytical positions on Gn-v (k) and Gn-s (l) are marked by red dots, and the yellow dots indicate cerussite.
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Figure 4. Mineral paragenesis of the hydrothermal mineralization period and the recognition of supergene oxidation throughout the Bou Dahar district. Dashed lines represent the uncertainty of the appearance of a given mineral. Line weight indicates the relative abundance of mineral species.
Figure 4. Mineral paragenesis of the hydrothermal mineralization period and the recognition of supergene oxidation throughout the Bou Dahar district. Dashed lines represent the uncertainty of the appearance of a given mineral. Line weight indicates the relative abundance of mineral species.
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Figure 5. Loading plot of PCA of galena and cerussite from the Bou Dahar district. (a,b,f) Gn-b and Gn-v; (c,d,g) Gn-s; (e,h) Cerussite.
Figure 5. Loading plot of PCA of galena and cerussite from the Bou Dahar district. (a,b,f) Gn-b and Gn-v; (c,d,g) Gn-s; (e,h) Cerussite.
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Figure 6. Box plots of 7 trace elements in the vein-related galena (Gn-v), breccia-related galena (Gn-b), strata-bound galena (Gn-s), and cerussite from the Bou Dahar district.
Figure 6. Box plots of 7 trace elements in the vein-related galena (Gn-v), breccia-related galena (Gn-b), strata-bound galena (Gn-s), and cerussite from the Bou Dahar district.
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Figure 7. Representative time-resolved profiles for the galena and cerussite in the Bou Dahar district. (a) Gn-v. (b) Gn-b. (c,d) Gn-s; and (e,f) Cerussite.
Figure 7. Representative time-resolved profiles for the galena and cerussite in the Bou Dahar district. (a) Gn-v. (b) Gn-b. (c,d) Gn-s; and (e,f) Cerussite.
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Figure 8. Correlation plots of trace elements for galena and cerussite in Bou Dahar district. (a) Ag vs. Sb in Gn-v and Gn-b. (b) Ag vs. Sb in Gn-s. (c) Sb vs. Ag + Cu in Gn-s. (d) Cu + Ag vs. Sb in Cerussite.
Figure 8. Correlation plots of trace elements for galena and cerussite in Bou Dahar district. (a) Ag vs. Sb in Gn-v and Gn-b. (b) Ag vs. Sb in Gn-s. (c) Sb vs. Ag + Cu in Gn-s. (d) Cu + Ag vs. Sb in Cerussite.
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Figure 9. Box and whisker plots exhibiting the trace element compositions of galena from the Bou Dahar district compared to other different genetic types of deposits (the data of Skarm and CD deposits, Epithermal deposit, and MVT deposits are from [6,7], [70,71] and [3], respectively). The configuration of the box and whisker is similar to that described in Figure 6.
Figure 9. Box and whisker plots exhibiting the trace element compositions of galena from the Bou Dahar district compared to other different genetic types of deposits (the data of Skarm and CD deposits, Epithermal deposit, and MVT deposits are from [6,7], [70,71] and [3], respectively). The configuration of the box and whisker is similar to that described in Figure 6.
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Table 1. Element contents in galena and cerussite from the Bou Dahar district were determined by LA-ICP-MS.
Table 1. Element contents in galena and cerussite from the Bou Dahar district were determined by LA-ICP-MS.
Mineral SbAgCuCdAsTlZnBiSnAuSrBa(Ag + Cu) (Mol%)/Sb (Mol%)
ppmppmppmppmppmppmppmppmppmppmppmppm
Gn-vMax142324<2.2514.415.80.21<3.460.080.43<0.2110.46<0.502.58
Min51.5214<1.638.037.990.11<6.02<0.01<0.29<0.02<0.109<0.064.70
Mean (8)97.3272-11.810.70.17-0.04----3.16
Median (8)104278-11.99.970.18-0.04----3.02
Gn-bMax245116<3.1713.314.250.2143.730.14<0.72<0.21<0.17<0.940.54
Min15874.3<2.118.132.310.16<3.920.01<0.48<0.03<0.07<0.080.53
Mean (8)20494.4-10.79.840.19-0.04----0.52
Median (8)21194.5-10.711.880.20-0.03----0.51
Gn-sMax83530942578.818.20.3527.40.030.430.0715.750.1935.00
Min2.0519.0<2.504.628.150.05<3.15<0.01<0.20<0.03<0.02<0.100.78
Mean (32)17811689.913.213.00.19------4.04
Median (32)12810113.38.413.00.19------2.27
CerMax403252737,43514.1<11.500.1342.00.04<0.806.2712735296
Min<0.372.34<1.572.87<7.210.02<5.720.01<0.46<0.134547.59
Mean (14)34.740131817.93-0.0711.860.02--692898
Median (14)0.651594337.36-0.068.630.02--673112
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Zhao, K.; Wu, F.; Cheng, X.; Cheng, S.; Wu, J.; He, Y.; Wang, C.; Lkebir, N.; Cui, S.; Hu, P.; et al. Trace Element Compositions of Galena and Cerussite from the Bou Dahar MVT District, Morocco: Insights from LA-ICP-MS Analyses. Minerals 2024, 14, 748. https://doi.org/10.3390/min14080748

AMA Style

Zhao K, Wu F, Cheng X, Cheng S, Wu J, He Y, Wang C, Lkebir N, Cui S, Hu P, et al. Trace Element Compositions of Galena and Cerussite from the Bou Dahar MVT District, Morocco: Insights from LA-ICP-MS Analyses. Minerals. 2024; 14(8):748. https://doi.org/10.3390/min14080748

Chicago/Turabian Style

Zhao, Kai, Fafu Wu, Xiang Cheng, Shunbo Cheng, Jinchao Wu, Yaoyan He, Chenggang Wang, Noura Lkebir, Sen Cui, Peng Hu, and et al. 2024. "Trace Element Compositions of Galena and Cerussite from the Bou Dahar MVT District, Morocco: Insights from LA-ICP-MS Analyses" Minerals 14, no. 8: 748. https://doi.org/10.3390/min14080748

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