The Middle–Late Permian to Late Cretaceous Mediterranean-Type Karst Bauxites of Western Iran: Authigenic Mineral Forming Conditions and Critical Raw Materials Potential

Abstract

The Sanandaj–Sirjan Zone and Zagros Fold–Thrust Belt in Iran host numerous Mediterranean-type karst bauxite deposits; however, their formation mechanisms and critical raw material potential remain ambiguous. This study combines mineralogical and geochemical analyses to explore (1) the formation of authigenic minerals, (2) the role of microbial organic processes in Fe cycling, and (3) the assessment of their critical raw materials potential. Mineralogical analyses of the Late Cretaceous Daresard and Middle–Late Permian Yakshawa bauxites reveal distinct horizons reflecting their genetic conditions: Yakshawa exhibits a vertical weathering sequence (clay-rich base → ferruginous oolites → nodular massive bauxite → bleached cap), while Daresard shows karst-controlled profiles (breccia → oolitic-pisolitic ore → deferrified boehmite). Authigenic illite forms via isochemical reactions involving kaolinite and K-feldspar dissolution. Scanning electron microscopy evidence demonstrates illite replacing kaolinite with burial depth enhancing crystallinity. Diaspore forms through both gibbsite transformation and direct precipitation from aluminum-rich solutions under surface conditions in reducing microbial karst environments, typically associated with pyrite, anatase, and fluorocarbonates under neutral–weakly alkaline conditions. Redox-controlled Fe-Al fractionation governs bauxite horizon development: (1) microbial sulfate reduction facilitates Fe³⁺ → Fe²⁺ reduction under anoxic conditions, forming Fe-rich horizons, while (2) oxidative weathering (↑Eh, ↓moisture) promotes Al-hydroxide/clay enrichment in upper profiles, evidenced by progressive total organic carbon depletion (0.57 → 0.08%). This biotic–abiotic coupling ultimately generates stratified, high-grade bauxite. Finally, both the Yakshawa and Daresard karst bauxite ores are enriched in critical raw materials. It is worth noting that the overall enrichment appears to be mostly driven by the processes that led to the formation of the ores and not by the chemical features of the parent rocks. Divergent bauxitization pathways and early diagenetic processes—controlled by paleoclimatic fluctuations, redox shifts, and organic matter decay—govern critical raw material distributions, unlike typical Mediterranean-type deposits where parent rock composition dominates critical raw material partitioning.

1. Introduction

Critical raw materials (CRM) are economically vital but face risks of scarcity and supply disruptions driven by political or economic factors [1]. In recent decades, the global demand for critical raw materials, particularly scandium (Sc), gallium (Ga), strontium (Sr), vanadium (V), cobalt (Co), hafnium (Hf), tantalum (Ta), niobium (Nb), tungsten (W), and rare earth elements and yttrium (REEY), has surged due to their essential role in high-tech industries such as renewable energy, aerospace, smartphones, telecommunications, electric vehicles, and semiconductor [1,2,3,4]. This growing demand has raised concerns about their long-term availability and supply security. Modern technology relies on a broad spectrum of metals, ranging from common elements like iron (Fe) and aluminum (Al) to nearly all stable elements in the Periodic Table, making the sustainable supply of these materials a global priority [5].

Bauxite deposits can be classified into three main types based on tectonic setting, bedrock lithology, and genesis [6,7]: (1) lateritic-type bauxites, formed through in situ lateritization of underlying aluminosilicate rocks, predominantly occur in stable platform regions [7,8,9]; (2) tikhvin-type bauxites, detrital deposits overlying eroded aluminosilicate bedrock, represent erosional remnants of lateritic bauxites [10]; (3) and karstic-type bauxites, developed on carbonate rock erosional surfaces, are primarily associated with orogenic belts [11]. The present study focuses on a deposit exhibiting characteristics consistent with the third group, namely the karst-type bauxite. These deposits are residual accumulations on paleo-karstic topography that arise from the chemical weathering of aluminosilicate precursor rocks, like granite, basalt, or sedimentary rocks, in warm, humid climatic conditions typical of tropical to subtropical regions [11,12,13,14,15,16]. In addition to being a primary source of aluminum, bauxites are a substantial reservoir for critical elements, including Ga, Co, Ta, Nb, Hf, Sc, V, and REEY, as highlighted in the latest European Report on Critical Raw Materials [17,18,19]. However, the formation of these critical metals in karst-type bauxites can be affected by factors such as climate change, the composition of the protolith(s), soil pH, redox conditions, drainage systems, and the paleogeographic setting [1].

Iranian karst-type bauxites, part of the expansive Irano-Himalayan karst bauxite belt, exhibit striking similarities to Mediterranean-type bauxites [20,21]. These deposits predominantly formed during pivotal geological periods, including the Permian, Permo-Triassic, Triassic, Triassic–Jurassic, and Cretaceous, under favorable tectonic and climatic conditions that facilitated their development [12]. Their origins trace back to mafic precursor rocks spanning the Middle–Late Permian to the Late Triassic–Early Jurassic, as well as Late Cretaceous argillaceous limestone, reflecting complex and dynamic geological conditions [19,22]. During the evolution of Iran’s geotectonic units, numerous small-scale karstic bauxite deposits were formed within the Zagros orogenic-metallogenic belt (Figure 1). These deposits are primarily located in the northwestern part of the Sanandaj–Sirjan Zone (SSZ) and the central part of the Zagros Fold–Thrust Belt (ZFTB), including the Middle–Late Permian Yakshawa and Late Cretaceous Daresard bauxite deposits, respectively (Figure 2 and Figure 3). Previous studies have mainly focused on the genesis of REEY (rare earth elements and yttrium)-bearing minerals, paleoenvironmental settings, chemical fractionation, and provenance [19]. Interestingly, unique geological phenomena have been documented in these deposits. For instance, the Yakshawa deposit is characterized by the presence of clinochlore-rich ferruginous layers, while the Daresard deposit features Ce-rich bauxite ore. Additionally, these deposits display significant differences in their mineralogical composition, particularly in clay minerals and the distribution of critical elements. These distinctive features underscore the complex geochemical and metallogenic processes that have influenced the formation and evolution of bauxite deposits in the study area. Key processes include (1) diagenetic to low-grade metamorphic modifications, (2) in situ fluid-assisted dissolution–reprecipitation reactions [19], (3) synergistic biotic–abiotic interactions, and (4) organic matter-controlled iron mobilization through microbial activity. In light of this issue, we aimed to explore the authigenic mineral forming conditions, the deposition mechanisms of Fe-rich horizons, and the potential of the Yakshawa and Daresard deposits as reservoirs for critical raw materials (CRMs). This study utilizes comprehensive mineralogical and geochemical data to elucidate the processes governing these deposits’ formation and evaluate their economic significance as potential sources of strategic elements.

2. Geological Setting

The Iranian Plateau records a complex geodynamic history influenced by the sequential opening and closure of the Paleo-Tethys and Neo-Tethys Ocean basins [23,24,25,26]. This protracted tectonic evolution has resulted in the development of distinct structural domains, each characterized by unique stratigraphic successions (Figure 1). During the Permian through Cretaceous interval, Iranian carbonate platforms occupied a paleoequatorial position, dominated by shallow marine deposition with limited clastic input [27]. Two distinct tectonic–climatic episodes generated Iran’s major bauxite deposits during the Permian–Cretaceous: (1) Late Permian Paleo-Tethys closure induced platform emergence and karstification, forming Ruteh Formation bauxites, and (2) Late Cretaceous reactivation of Neo-Tethyan structures (post-rift phase) triggered block faulting and paleokarst development, enabling Sarvak Formation bauxitization [16,20,22]. The tropical paleoclimate (warm/humid) intensified chemical weathering during subaerial exposure intervals. This combination of tectonic preconditioning (paleotopography creation) and climatic weathering led to economically significant bauxite deposits at these stratigraphic levels.

2.1. The Middle–Late Permian Ruteh Formation and Yakshawa Deposit Geology

The Ruteh Formation represents one of the most extensive stratigraphic units in the northwestern Sanandaj–Sirjan Zone, where this study focuses specifically on its outcrop exposure in the Yakshawa region (Figure 2a–d). The Sanandaj–Sirjan Zone (SSZ), host to the Yakshawa bauxite deposit, is a NW-SE trending tectonic belt extending approximately 1500 km in length and 150–250 km in width along the northeastern margin of the Zagros Main Thrust (representing the Neo-Tethys paleo-suture; Figure 1). This zone consists of metamorphic basement rocks, shallow marine sedimentary sequences, and arc-related plutonic complexes [28,29]. The studied section of the Ruteh Formation exhibits a total thickness of 201 m (Figure 2e), displaying disconformable contacts with both underlying and overlying units—specifically, the Lower Permian Dorud Formation at its base and the Lower Triassic Elika Formation at its top [30]. The Ruteh Formation is an upper Permian sedimentary cycle deposited in the Paleo-Tethys Ocean. It began with transgressive detrital facies, followed by fusulinid-bearing carbonates, and concluded with a long-lasting disconformity at the top [31]. This unconformity is associated with significant tectonic movements during the Late Permian period that profoundly impacted the Iranian sedimentary basins. During the Middle–Late Permian, a combination of mid-oceanic ridge expansion and glacial retreat triggered a major marine transgression that reestablished carbonate platform deposition, represented by the Ruteh and Nessen Formations [31]. This phase of carbonate production was periodically interrupted by epeirogenic uplift events, which caused repeated hiatuses in both carbonate and shale sedimentation across the region. These tectonic disturbances facilitated basaltic volcanism, regional uplift episodes, and, most notably, the development of karst-related bauxite deposits within the upper part of the Ruteh Formation [20].

The Yakshawa bauxite deposit, located 13 km SE of Bukan (NW Iran), forms NE- SW-trending lens-shaped bodies in karstic depressions of the Permian Ruteh Formation, with uniform thicknesses of 11–18 m [19], overlain by the Lower Triassic Elika (Figure 2f). Integrated textural, mineralogical, and geochemical analyses define four vertically stacked units in the Yakshawa bauxite deposit: (I) a basal clay-rich horizon (1 m thick) displaying yellowish-white coloration that grades downward into laterite-cemented breccia, reflecting progressive in situ weathering (Figure 2c,f); (II) an iron-bearing bauxite ore (approximately 2.5 m thick) characterized by dispersed ferruginous oolites and colloformic textures, sharply overlying the underlying clayey bauxite (Figure 2b,c,f); (III) a massive bauxite ore (5–6 m thick) exhibits mineralogical heterogeneity with nodular textures (1–5 cm diameter), pervasive fracture networks, rare clay veinlets (<5% vol.), and significantly higher hardness relative to adjacent layers (Figure 2b,f); and (IV) the upper bleached horizon, roughly 1–2 m thick, is marked by soft, sheet-structured bauxite with sparse diasporic ooids (Figure 2d,f).

2.2. The Late Cretaceous Sarvak Formation and Daresard Deposit Geology

The Late Cretaceous marked a period of major paleoenvironmental transitions in the Zagros Basin, coinciding with global eustatic fluctuations. During this interval, the Sarvak Formation was deposited under highstand sea-level conditions, reflecting one of the most extensive marine transgressions of the Cretaceous [21]. The Sarvak Formation, which hosts the Daresard deposit, represents an Upper Cretaceous (Cenomanian–Turonian) carbonate succession within the Zagros Fold–Thrust Belt (ZFTB) of southwestern Iran, deposited in a shallow-marine carbonate ramp environment along the Neo-Tethyan margin [32]. The Late Cretaceous tectonic evolution of the Zagros Basin, driven by the incipient closure of the Neo-Tethys Ocean and the onset of the Zagros orogeny, induced profound paleogeographic modifications during the late Cenomanian to Santonian interval. These tectonic processes resulted in regional-scale uplift events and the development of significant stratigraphic hiatuses (erosional surfaces and non-depositional intervals [27,33,34]). The mid-Turonian sea-level drop and associated tectonic uplift in the ZFTB produced a regional-scale unconformity, evidenced by conglomerates, breccias, iron-rich sediments, paleosols, hematite nodules, karstification, and lateritization in the upper Sarvak Formation (Figure 3a–d), collectively documenting subaerial exposure and weathering [12,35,36,37]. The Sarvak Formation, with a total thickness of 821 m, mainly consists of argillaceous micritic limestone, chalky limestone with iron-rich siliceous layers, rudist-foraminifera bioherms, and ferruginous limestones capped by bauxite deposits (Figure 3e). The Sarvak Formation conformably overlies organic-rich black shales of the Kazhdumi Formation and is disconformably overlain by marly limestones of the Gurpi Formation [4].

The bauxite deposits in the study area occur as 18–24 m thick, lens-shaped ores within karstic depressions of the Sarvak Formation (Cenomanian–Turonian), primarily along the limbs of NW–SE-trending anticlines parallel to the Zagros Fold–Thrust Belt [19], notably the Kuhe-Nil anticline (65 km long, 4 km wide), which hosts the Daresard deposit 30 km east of Dehdasht, SW Iran (Figure 3f). The Daresard bauxite profile comprises: (I) a basal 1–3 m breccia bauxite with bauxitic cement and rounded fragments overlying argillaceous limestone (Figure 3c,f); (II) an 11 m thick oolitic-pisolitic bauxite ore with yellow pods in a red matrix, sparse ooids, and 0.5–1 m spheroidal boulders from onion-skin weathering (Figure 3d,f); and (III) a 5–7 m bleached horizon of light-colored boehmite oolites showing deferrification, abruptly capping the sequence (Figure 3b,f).

3. Materials and Methods

3.1. Fieldwork and Sampling

Fifty-seven samples were systematically collected from vertical profiles (0.5–1 m intervals) in the Yakshawa (thirty-three samples: UBH1-UBH4 from bleached horizon, BOH1-BOH11 from bauxite ore, IBH1-IBH5 from iron-bearing bauxite ore, UCH1-UCH2 from underlying clayey bauxite, SHY1-SHY3 from Permian shales, LMY1-LMY3 from Permian limestones, and BAY1-BAY5 from mafic rocks) and Daresard deposits (twenty-four samples: UBD1-UBD5 from bleached horizon, BOD1-BOD11 from bauxite ore, BBD1-BBD2 from breccia bauxite, SRD1-SRD2 from Upper Cretaceous carbonates, and ARD1-ARD3 from argillaceous limestones), providing comprehensive material for comparative geochemical analysis of the studied bauxite deposits.

3.2. Mineralogical Analysis

The samples underwent sequential preparation, beginning with drying, followed by crushing to <4 mm using a jaw crusher, and final pulverization in an agate mortar to 75 μm (200 mesh). Prepared powders were pressed into 16 × 2.5 mm cylindrical holders using a glass plate to ensure smooth surfaces for analysis. Textural and mineralogical characterization was studied through integrated scanning electron microscopy–energy dispersive X-ray spectroscopy (SEM-EDS) and X-ray powder diffraction (XRPD) techniques. Semi-quantitative XRD analysis utilized a Siemens X’Pert diffractometer with Cu-Kα radiation (λ = 1.5418 Å, 30 mA, 40 kV) at Zarazma Mineral Studies Company, Robat Karim, Iran, employing a scanning range of 4–60° 2θ with 0.02° step size and 8°/min scan speed to resolve mineral phases. This multi-method approach enabled the comprehensive characterization of bauxite mineralogy and textures.

Microchemical analyses of rare earth element (REE) bearing minerals and selected accessory minerals were performed using a TESCAN MIRA3 field emission scanning electron microscope coupled with a Thermo Scientific UltraDry energy-dispersive X-ray spectroscopy (EDS) system at the University of Kurdistan, Sanandaj, Iran. Analyses were performed under controlled conditions (20 kV acceleration voltage, 10 nA beam current, 1 μm beam diameter, 30% humidity, 1 bar pressure, 21 °C). Quantitative EDS utilized specialized reference standards (EM-Tec RXS-40MC for 40 mineral phases and EM-Tec RXS-21RE for 21 REE standards), enabling precise elemental quantification in twenty polished sections. This advanced microanalytical approach provided high-resolution chemical data critical for understanding REE mineralization processes in the bauxite samples.

Quantitative analyses utilized the following certified standards and emission lines:

• Oxides/silicates: rutile (Ti Kα), albite (Al Kα, Si Kα), magnetite (Fe Kα), and apatite (Ca Kα, P Kα);
• Fluorides: Fluorite (F Kα), LaF₃ (La Lα), EuF₃ (Eu Lα), and ThF₄ (Th Mα);
• REEs: CeO₂ (Ce Lα), PrSi₂ (Pr Lα), and NdSi₂ (Nd Lα);
• Synthetic standards: YAG (Y₃Al₅O₁₂; Y Lα) and Gd₃Ga₅O₁₂ (Gd Lα);
• Accessory minerals: Baddeleyite (Zr Lα)

3.3. Geochemical Analysis

The bulk chemical composition of 57 representative bauxite and host rock samples (Sarvak and Ruteh Formations) was determined at ACME Laboratories (Vancouver, BC, Canada) using ICP-OES and ICP-MS techniques. Samples (~2 g) were fused with LiBO₂/Li₂B₄O₇ flux (1.5 g) at 980 °C for 30 min, followed by rapid digestion in 5% HNO₃. The method achieved detection limits of 0.002–0.04 wt% for major oxides, 0.1–8 ppm for trace elements, and 0.01–0.3 ppm for REEs, with analytical uncertainties of 0.04–0.1%, 0.1–0.5%, and 0.01–0.5 ppm, respectively. This high-precision analysis provided comprehensive geochemical data for characterizing elemental distributions and enrichment patterns. Loss on ignition (LOI) was determined gravimetrically following overnight heating at 950 °C. Total organic carbon (TOC) content was measured in selected samples using the wet oxidation method, wherein organic carbon was oxidized by potassium dichromate (K₂Cr₂O₇) in the presence of concentrated sulfuric acid (H₂SO₄). It should be noted that all charts and graphs were initially created in Microsoft Excel and subsequently redesigned using CorelDRAW Graphics Suite 2020 for enhanced visual presentation.

4. Results

4.1. Mineral Compositions

The mineralogical composition of the collected samples was thoroughly analyzed using microscopy, XRD, and SEM-EDS. In the Daresard deposit, the underlying breccia bauxite predominantly comprises microcrystalline grains of boehmite, hematite, kaolinite, and anatase. It also contains coarse-grained, rounded calcite, along with minor amounts of diaspore and pyrite (Figure 4 and Figure 5;

Table 1. XRD semi-quantitative analyses of the Yakshawa and Daresard bauxite ores. Abbreviation: Bhm = boehmite; Dsp = diaspore; Hem = hematite; Ant = anatase; Cal = calcite; Ilt = illite; Kln = kaolinite; Qtz = quartz; Gbs = gibbsite; Rt = rutile; Gth = goethite; Crs = cristobalite; Trd = tridymite; Py = pyrite; Chm = chamosite; Dck = dickite; Hal = halloysite; Clc = clinochlore; Mc = microcline; Ckt = cookeite.

Deposit	Yakshawa
Minerals	Dsp	Hem	Ilt	Ant	Rt	Kln	Dck	Hal	Qz	Cal	Ckt	Clc	Mc	Py
UBH1	40.2	2.2	46.5	5.6	2.9	-	-	-	-	-	2.6	-	-	-
UBH2	35.8	2.8	48.3	4	2.6	-	-	-	-	-	6.5	-	-	-
UBH3	33.6	4.1	45.4	5.9	3.2	-	-	-	-	-	3.6	4.2	-	-
UBH4	38.1	17.4	20.1	6.4	3.5	3	2.8	4.1	-	-	-	4.6	-	-
BOH-1	36.1	14.2	32.9	5.6	2.3	-	2.5	6.4	-	-	-	-	-	-
BOH-2	34.5	16.8	40.3	4.1	2.2	-	-	2.1	-	-	-	-	-	-
BOH-3	34.2	16.5	37.1	6.3	-	-	-	3.5	2.4	-	-	-	-	-
BOH-4	32.8	19.2	38.8	5.9	-	-	-	3.3	-	-	-	-	-	-
BOH-5	32.3	19.5	33.6	5.4	3.2	2.3	3.7	-	-	-	-	-	-	-
BOH-6	30.1	22.3	31.8	5.2	2.1	-	5.4	-	3.1	-	-	-	-	-
BOH-7	31.8	25.1	34.4	6.3	-	-	-	-	2.4	-	-	-	-	-
BOH-8	30.6	20.7	35.5	5.1	3.9	-	4.2	-	-	-	-	-	-	-
BOH-9	32.2	27.4	25.9	7.3	-	-	4.8	-	-	-	-	2.4	-	-
BOH-10	35.7	21.3	28.6	5.8	2.5	6.1	-	-	-	-	-	-	-	-
BOH-11	31.7	28.8	24.5	4.5	2.1	4.8	-	-	-	-	-	-	-	3.6
IBH-1	28.1	31.6	-	4.7	2.8	9.3	-	-	-	-	-	19.2	-	4.3
IBH-2	28.4	37.9	-	5.5	-	7.2	-	-	-	-	-	17.8	-	3.2
IBH-3	26.3	35.2	-	4.4	-	12.2	-	-	3.4	-	-	15.9	-	2.6
IBH-4	23.6	40.5	-	4.8	2.3	7.4	-	-	-	-	-	19.3	-	2.1
IBH-5	25.8	41.3	-	4.1	-	8.9	-	-	-	-	-	17.4	-	2.5
UCH-1	13.8	1.6	34.5	4.6	-	-	-	-	-	35.9	-	-	9.6	-
UCH-2	7.6	1.3	40.1	5.2	-	-	-	-	-	31.2	-	-	14.6	-
Deposit	Daresard
Minerals	Bhm	Kln	Hem	Ant	Cal	Dsp	Gth	Gbs	Rt	Crs	Trd	Py	Chm
UBD-1	78.5	-	-	8.4	-	5.7	3.8	-	3.6	-	-	-	-
UBD-2	72.6	-	5.7	6.5	2.3	5.4	-	4.8	2.7	-	-	-	-
UBD-3	62.2	5.4	12.6	5.2	5.5	5.9	-	-	3.2	-	-	-	-
UBD-4	69.4	-	10.2	4.1	-	5.1	-	-	3.5	-	4.4	-	3.3
UBD-5	66.7	2.1	16.2	5.5	-	4.8	-	2.3	2.4	-	-	-	-
UBD-6	66.3	7.7	8.5	5.1	-	-	-	4.1	3.1	-	5.2	-	-
BOD-1	56.2	13.8	-	5.8	-	-	24.2	-	-	-	-	-	-
BOD-2	66.9	-	-	8.9	-	-	16.7	-	-	7.5	-	-	-
BOD-3	57.8	11.6	21.7	6.7	2.2	-	-	-	-	-	-	-	-
BOD-4	63.9	5.3	17.5	5.1	3	-	-	-	-	5.2	-	-	-
BOD-5	68.1	-	13.2	7.4	2.6	5.4	-	3.3	-	-	-	-	-
BOD-6	59.3	7.2	22.4	4.5	-	6.6	-	-	-	-	-	-	-
BOD-7	53.7	11.4	23.1	5.8	-	-	3.1	-	-	-	-	2.9	-
BOD-8	54.1	10.6	17.6	5.3	3.4	-	3.5	2.7	2.8	-	-	-	-
BOD-9	49.5	12.2	22.4	4.2	2.7	-	4.6	-	2.3	-	-	2.1	-
BOD-10	57.6	9.1	15.2	3.7	2.3	2.4	2.9	-	3.7	-	-	-	3.1
BOD-11	50.2	11.7	21	4.4	-	3.7	4.3	-	2.5	-	-	2.2	-
BBD-1	45.4	13.1	17.8	2.1	18.1	-	-	-	-	-	-	3.5	-
BBD-2	42.9	14.6	18.3	2.2	16.8	5.2	-	-	-	-	-	-	-

). The bauxite ore mainly consists of boehmite, anatase, hematite, kaolinite, goethite, and occasionally traces of diaspore, calcite, gibbsite, rutile, cristobalite, chamosite, and pyrite. The upper bleached horizon is primarily composed of boehmite, hematite, anatase, rutile, and diaspore, with minor amounts of kaolinite, calcite, tridymite, goethite, gibbsite, and chamosite. In the Yakshawa deposit, illite and calcite are the main components of underlying clayey bauxite, with small quantities of microcline, anatase, diaspore, and hematite (Figure 4 and Figure 5; Table 1). The iron-bearing bauxite ore is primarily composed of hematite, diaspore, and clinochlore, along with smaller amounts of kaolinite, anatase, and pyrite, as well as trace quantities of rutile and quartz. Semi-quantitative mineralogical analysis indicates that diaspore, illite, hematite, and occasionally anatase are bauxite ore’s most prevalent mineral phases (Table 1). Rutile is the most abundant accessory mineral, while kaolinite, dickite, quartz, and halloysite are also commonly detected. Illite and diaspore are the dominant minerals in the upper bleached horizon, accompanied by minor amounts of hematite, anatase, rutile, cookeite, kaolinite, clinochlore, halloysite, and dickite (Figure 4 and Figure 5; Table 1).

Illite is the predominant clay mineral throughout the Yakshawa bauxite profile (24.5–48.3 wt. %), except the iron-bearing bauxite ore. The backscattered electron image analysis shows four occurrences of illite, including (i) filamentous or fibrous pore-filling, (ii) lath-shaped, (iii) pseudohexagonal-shaped particles, and iv) fine scaly aggregates that imply diverse origins (Figure 6a–d). Kaolinite and clinochlore are mainly developed in the iron-bearing bauxite ore and coexist with hematite as fine scaly aggregates. Other clay minerals (e.g., dickite and halloysite) are found in small quantities in the bauxite ore, comprising up to 8.9 wt% (Figure 4 and Figure 5; Table 1). Kaolinite in the Daresard bauxite ore shows various morphologies, primarily as booklet structures [19] or fine scaly aggregates (Figure 6e). Additionally, it occasionally displays pseudohexagonal morphologies in the underlying breccia bauxite (Figure 6f).

Diaspore, the main Al hydroxide, predominantly occurs as amorphous microcrystalline aggregates, contributing to a heterogeneous aphanitic matrix alongside clay and titanium minerals in the upper bleached horizon (Figure 7a). Occasionally, it is also observed as euhedral columnar and tabular crystals (Figure 7a). In bauxite ore, diaspore is typically found with illite, hematite, and anatase, and it sometimes appears as coarse sub-rounded grains within hematite veins (Figure 7b). In iron-bearing bauxite ore, diaspore also occurs as small subhedral inclusions within pyrite crystals (Figure 7c). Together with clay and iron minerals, it constitutes the primary matrix of the Fe-bearing bauxite ore. Pyrite has only been observed in iron-bearing bauxite ores and principally occurs as cubic or irregular crystals and cryptocrystalline aggregates (Figure 7d).

Pyrite is commonly found as a single crystal within the diaspore-rich matrix, where it typically coexists with diaspore and anatase (Figure 7e). Pyrite crystals may occasionally host numerous diaspore inclusions and anatase particles (Figure 7c). Hematite aggregates with oval morphologies are found within cavities or micro-cracks, often surrounded by a matrix composed of diaspore or anatase (Figure 7f). Additionally, hematite is observed as round nanometer-scale crystals [19] and microveins (Figure 7b) distributed throughout the Yakshawa bauxite profile, with its abundance gradually increasing toward the iron-bearing bauxite ore.

In the Daresard deposit, iron-bearing phases are found as cubic goethite pseudomorphs after euhedral pyrite [19] or as microcrystalline particles within a pelitomorphic matrix. Furthermore, iron oxyhydroxides may nearly completely replace the boehmite in the pisoid core (Figure 8a). Boehmite is observed throughout the Daresard bauxite profile, and its abundance gradually decreases toward the underlying breccia bauxite (Figure 4). In the upper bleached horizon, boehmite exhibits a distinctly irregular granular texture characterized by varied particle sizes and shapes (Figure 8b). Traces of detrital minerals are also evident, predominantly appearing as fine angular fragments dispersed throughout the matrix (Figure 8b). Additionally, diaspore lumps of various shapes, along with detrital minerals (e.g., rutile), are dispersed within the kaolinite matrix, suggesting that the diaspore lumps and rutile formed simultaneously (Figure 8c). In bauxite ores, boehmite is also present as cryptocrystalline aggregates within the cores of pisoids, typically surrounded by a heterogeneous aphanitic matrix consisting of kaolinite and hematite (Figure 8d). Anatase is distributed almost uniformly across all bauxite horizons (Figure 4) and often coexists closely with pyrite and diaspore within a fine-grained matrix, indicating their simultaneous formation (Figure 7e). In some instances, anatase fragments with subhedral morphologies are found within a nanometric illite matrix (Figure 7a). Rutile and zircon are important detrital minerals that mainly accumulate in the upper bleached horizon. They exhibit subhedral to euhedral morphologies and are widely distributed within the aphanitic matrix composed of illite and diaspore as micrometer-scale crystals, typically ranging from 5 to 20 μm in size (Figure 8e). In addition, titanium-rich minerals occasionally appear as lattice and needle-like crystals (Figure 8f).

Additionally, previous studies have demonstrated the presence of significant amounts of detrital and authigenic REEY minerals in the studied bauxite deposits [19]. In the present study, we have identified LREE fluorocarbonates of the bastnaesite group in the Yakshawa deposit for the first time. Parisite generally occurs as irregular grains, up to 150 μm in size, and is closely associated with pyrite and diaspore (Figure 9a). Notably, abundant fine inclusions of diaspore can be observed within the parisite grains (Figure 9b). Locally, it also occurs as fine irregular aggregates within the groundmass, which is composed of diaspore and anatase (Figure 9C and Figure 10a). These parisite aggregates are notably smaller (>15 µm) compared to other types. Bastnaesite typically occurs either as neoformed amorphous particles on the surface of diaspore grains or as individual irregular crystals embedded within a pelitomorphic matrix (Figure 9d and Figure 10b).

4.2. The Occurrence of Microorganisms

A notable feature of the studied deposits is the presence of active bacterial communities, observable through a diverse array of microorganisms exhibiting various shapes and sizes. These microorganisms play a crucial role in the biogeochemical processes within the deposits, contributing to nutrient cycling and the degradation of organic materials. Their morphological diversity highlights the complexity of the microbial ecosystem and indicates the potential for interaction with the surrounding environment. The most salient feature of both bauxite ore and iron-bearing bauxite ore is the intricate relationship between the bacterial consortium and specific minerals, including diaspore, goethite, hematite, and pyrite (Figure 9e,f). Microorganisms involved in this process exhibit various forms, predominantly filamentous and spherical shapes. They thrive within the porous cavities of the ore, where they can either be found as fossilized remnants, preserving ancient microbial life, or as currently active filamentous structures engaging in metabolic processes (Figure 9e,f). This interaction between bacteria and minerals not only contributes to the formation and alteration of the ore but also highlights the significant role of microbial activity in geological and mineralogical transformations.

4.3. Ore Geochemistry

4.3.1. Major Element Geochemistry

The geochemical data for representative samples from the Yakshawa and Daresard bauxite deposits are presented in Supplementary Table S1. The upper bleached horizon in the Yakshawa and Daresard bauxite deposits has the highest Al₂O₃ (median = 34.58 wt% and 62.45 wt%, respectively) and TiO₂ (median = 6.61 wt% and 3.00 wt%, respectively) contents. Iron behaves differently from aluminum and titanium, gradually increasing towards the lower parts of the bauxite profiles. In the Yakshawa and Daresard deposits, the highest Fe₂O₃ contents are found in iron-bearing bauxite ore (median = 33.50 wt%) and bauxite ore (median = 17.84 wt%), respectively. SiO₂ exhibits distinct distribution patterns within the studied deposits. In the Daresard deposit, the highest SiO₂ contents are observed in the underlying breccia bauxite (median = 11.41 wt%), while in the Yakshawa deposit, the upper bleached horizon displays significant silica enrichment (median = 41.17 wt%). The mineralization of illite results in significant K₂O enrichment across all bauxite layers in the Yakshawa deposit, except for the iron-bearing bauxite ore. The bauxite ore from the Yakshawa and Daresard deposits contain the highest P₂O₅ contents, with median values of 0.63 wt% and 0.27 wt%, respectively. The CaO contents are generally low across the different datasets, except for the underlying clayey bauxite (median = 29.74 wt%) and the underlying breccia bauxite (median = 13.24 wt%). The concentration of other major oxides is significantly low, which can be attributed to their high mobility and subsequent leaching that occurs during the weathering process.

The chemical composition of the Yakshawa and Daresard bauxites has been compared with other karst bauxites worldwide, revealing some fascinating features (Figure 11, Figure 12 and Figure 13). The Al₂O₃ and SiO₂ contents of the Daresard bauxite are similar to other considered districts, whereas the Yakshawa bauxite forms a cluster of values relatively depleted in Al₂O₃ and enriched in SiO₂ (Figure 11). The only exceptions are the Spain and China deposits, which deviate from the other districts, presenting a wider range of values with SiO₂ between 13.48 and 50.7 wt % and 0.45 and 47.3 wt %, respectively. The Yakshawa bauxite has higher median TiO₂ and Fe₂O₃ contents, as compared with other bauxites worldwide (Figure 11). In contrast, the median content of these elements in the Daresard deposit is partly similar to the values measured in other districts, especially Zagros. The only remarkable exception is the SSZ bauxite deposits, which are characterized by significantly higher TiO₂ content (median = 5.44 wt%).

4.3.2. Trace Element Geochemistry

The upper bleached horizon in the Yakshawa and Daresard bauxite deposits contains the highest concentrations of the following elements (Supplementary Table S1): gallium (median = 56.0 wt% and 66.7 wt%, respectively), hafnium (median = 7.84 wt% and 18.35 wt%, respectively), niobium (median = 77.45 wt% and 59.75 wt%, respectively), tantalum (median = 10.91 wt% and 4.7 wt%, respectively), and zirconium (median = 449.50 wt% and 840.25 wt%, respectively). In contrast, the bauxite ores from the Yakshawa and Daresard deposits reveal distinct geochemical characteristics, with the highest median contents of thorium at 14.8 ppm and 60.5 ppm, nickel at 159 ppm and 226 ppm, yttrium at 56.2 ppm and 86.8 ppm, and cesium at 3.2 ppm and 1.3 ppm, respectively (Supplementary Table S1). These findings underscore the unique elemental distribution patterns in both the upper bleached horizon and the bauxite ores of these deposits. The distribution patterns of other trace elements differ between the Yakshawa and Daresard profiles. In the Yakshawa deposit, vanadium (median = 906 ppm), scandium (median = 31 ppm), and uranium (median = 7.7 ppm) are primarily concentrated in the iron-rich bauxite ore. Conversely, the Daresard deposit exhibits the highest levels of these elements in the bauxite ore, with medians of 282 ppm, 54 ppm, and 9.2 ppm, respectively. The highest contents of barium (median = 159 ppm), rubidium (median = 156 ppm), and tungsten (median = 2.2 ppm) are found in the bauxite ore of the Yakshawa deposit, whereas at the Daresard deposit, these elements are primarily concentrated in the upper bleached horizon, where the median values are 638 ppm, 1.1 ppm, and 9.95 ppm, respectively. In the Yakshawa deposit, the highest chromium contents are found in the upper bleached horizon (median = 448 ppm), while the Daresard deposit exhibits notable chromium enrichment in the bauxite ore, which has a median concentration of 403 ppm.

Although rare earth elements (REEs) in both the Yakshawa and Daresard deposits exhibit broadly comparable geochemical behavior and are predominantly concentrated within the bauxite ore horizon (median = 917.9 ppm and 1081.8 ppm, respectively), the spatial distribution of their maximum enrichment differs significantly between them (Figure 14a–d; Supplementary Table S1). In the Daresard deposit, the highest concentrations of REEs are found in the lower part of the bauxite profile, while in the Yakshawa deposit, peak REE enrichment occurs in the middle part of the bauxite profile (Figure 14a,c). Two distinct trends in the distribution of the Ce/Ce* ratio with depth have been recognized in the studied deposits. In the Yakshawa deposit, the upper bleached horizon displays the highest positive Ce anomaly (Ce/Ce* = 2.58 − 2.17), with the Ce/Ce* ratio progressively decreasing downward; however, localized fluctuations are observed in samples BOH10 and BOH11 (Figure 14c). In contrast, the upper bleached horizon of the Daresard deposit displays Ce/Ce* ratios typically close to the unit, while the highest positive Ce anomaly in the Daresard deposit occurs in the lower part of the bauxite ore horizon, where the Ce/Ce* ratio reaches a maximum value of 5.24 (Figure 14d).

Gallium, Cr, Nb, Ni, U, Y, and HREE contents of the studied bauxite deposits are similar to the other considered districts (Figure 12 and Figure 13). The studied bauxites have higher median V, Ba, Sr, and Ta contents in the Yakshawa bauxite and W, Th, and Zr contents in the Daresard deposits, as compared with other karst bauxites worldwide (Figure 12 and Figure 13). The only exception is the Greece and Dominican bauxite deposits, characterized by higher W and Sr contents, respectively. The median content of Co, Hf, and Sc in the Daresard bauxite ores is similar to other bauxite deposits, whereas the Yakshawa bauxite forms a relatively depleted set of these elements (Figure 12 and Figure 13). The median LREE content in both studied deposits is significantly higher than that of the Iranian karst-type bauxites and observed in a few levels of Dominican, Montenegro, Chinese, Greek, and Italian karst bauxites.

5. Discussion

5.1. Main Authigenic Mineral Forming Conditions

Since the amount of clay minerals in the studied deposits is often non-negligible, especially in the Yakshawa ore, evaluating their origin may furnish relevant clues about the processes affecting the bauxite ores genesis.

Illite, as a secondary clay mineral in weathered rocks, could have derived from several mechanisms, including the transformation of primary rock-forming minerals such as mica [38,39], the hydrothermal alteration after feldspar [40], and the neoformation under meteoric conditions from K-feldspar crystals due to partial dissolution [38,41]. Illite may also occasionally occur as an authigenic mineral in many sedimentary rocks [39]. During bauxitization, illite is generally a transition phase evolving toward kaolinite [42]. In the Yakshawa deposit, authigenic illite often occurs as fine scaly aggregates or filamentous/fibrous pore-filling minerals (Figure 6a,d). Previous studies show that authigenic illite is mostly associated with the dissolution of an unstable aluminous mineral phase [39,43], which in the Yakshawa Permian deposit can be kaolin, muscovite, or microcline. Authigenic filamentous/fibrous illite may form by the following isochemical reaction in which Al³⁺, K⁺, and silica are transferred from the surface of dissolving kaolinite and K-feldspar to the site of illite growth:

Al₂SiO₅(OH)_4(kaol) + KAlSi₃O_8(Kspar) = KAl₃Si₃O₁₀(OH)_2(ilt) + 2SiO_2(qz) + H₂O

The potassium required for the formation of illite appears to be supplied by K-feldspar on the underlying clayey horizon. Furthermore, under typical weathering conditions, feldspar can hydrolyze directly to illite when K⁺ remains available in the system, preventing further degradation of kaolinite. This reaction occurs in neutral to slightly alkaline pH with limited leaching, where K⁺ stabilizes the 2:1-layer structure of illite (KAl₃Si₃O₁₀(OH)₂) [42,44]. Unlike kaolinite (formed in acidic, K⁺-depleted environments) or smectite (Mg²⁺/Ca²⁺-rich systems), illite indicates moderate chemical weathering and is common in shales, temperate soils, and the Permian bauxites [45].

3KAlSi₃O₈ + 2H⁺ + 12H₂O → KAl₃Si₃O₁₀(OH)_2(ilt) + 6H₄SiO₄ + 2K⁺

In the Yakshawa deposit, in some cases, kaolinite is surrounded by pseudohexagonal-shaped illite, indicating that illite can take origin from kaolinite at burial depth [42,43], and the SEM images indicate that the growth of authigenic illite is highly associated with or replacing dissolved kaolinite (Figure 6c). The fine scaly aggregates of illites that developed in the matrix of bauxite ore are instead probably derived from minerals with similar properties in the protoliths. Illite ‘crystallinity’ rates increase at greater burial depths and higher temperatures [39], and the proportion of isometric pseudohexagonal morphologies increases with burial depth.

As for kaolinite genesis, it is thought that the dissolution of feldspar and mica and the formation of authigenic kaolinite occur during the early stages of the weathering process under acidic and tropical conditions [46]. In the case of K-feldspar, the transformation occurs according to the following reaction:

2KAlSi₃O_8(Kspar) + 2H⁺ + 9H₂O = Al₂Si₂O₅(OH)_4(kaol) + 4H₄SiO_4(aq) + 2K⁺

However, in our study system, Al-Si-rich solutions originate from two potential sources: (1) silicification of aluminum hydroxide minerals coupled with chamosite dissolution during uplift and exposure, leading to neoformed pseudohexagonal flake kaolinite (Figure 6f) with higher crystallinity [4], or (2) intense tropical weathering conditions (characterized by high rainfall, T > 25 °C, low pH) that promote complete leaching of K⁺ and base cations, resulting in the breakdown of illite and subsequent stabilization of kaolinite as the dominant phase [43].

2KAl₃Si₃O₁₀(OH)_2(ilt) + 3H₂O + 2H⁺ → 3Al₂Si₂O₅(OH)_4(kaol) + 2K⁺

Further, dickite and kaolinite are genetically highly related to each other. Kaolinite is metastable in the thermal range related to burial diagenesis. As temperature increases, kaolin formation proceeds thermodynamically due to organic acids’ invasion [47]. As a result, vermiform kaolinite gradually turns into blocky dickite. However, it has been suggested [43] that dickite resulted not only from the kaolinite diagenetic transformation but also from the dissolution of aluminum-rich silicates and K-feldspar with increasing temperature, likely due to the presence of organic acids.

Today, despite the possibility of diagnosing boehmite from diaspore using SEM and EPMA, understanding their genetic relationship remains ambiguous and controversial. Like many other karst bauxites, gibbsite has been the original aluminum-rich mineral in the Zagros bauxite deposits. The majority of gibbsite was transformed directly from clay minerals and feldspar under humid tropical conditions [48,49,50]. However, gibbsite becomes unstable in dry zones with low water activity, greater burial depth, and higher temperatures (range of 35–50 °C) and usually transforms into boehmite and diaspore [42,51,52]. As illustrated in Figure 8c, diaspore lumps with various shapes and sizes are scattered within the kaolinite matrix. The coexistence of diaspore lumps and detrital minerals (e.g., rutile) at the pelitomorphic matrix demonstrates that both are formed during a syngenetic stage. Based on [50], such distinctive structures (e.g., diaspore lumps) in the ores confirm that the original lump-forming material was most likely gibbsite, derived from the laterite lump, consistent with experimental observation. Nevertheless, experimental studies indicate that diaspores are not only formed through the transformation of gibbsite during earlier weathering stages but may also be directly precipitated from aluminum-rich solutions under surface conditions [14,50]. Recent studies on karstic bauxites suggest a superficial origin for diaspore precipitation, supported by mineral paragenesis [15,53], and demonstrate that diaspores can precipitate under surface conditions with the involvement of microorganisms [14,54,55].

Pyrite and anatase in iron-bearing bauxite ore commonly coexist with diaspore (Figure 7e) and precipitate under reducing conditions in a karstic environment [55]. LREE–fluorocarbonates (e.g., parisite and bastnasite) are commonly associated with pyrite and diaspore (Figure 9a), which are widely precipitated in the lower part of the Yakshawa bauxite near the bedrock limestones where pH values are neutral to weakly alkaline [4]. This study highlights strong associations between diaspore, parisite, bastnäsite, pyrite, and anatase (Figure 9a–d). Specifically, numerous fine inclusions of diaspore were detected within parisite grains (Figure 9b). Microcrystalline diaspore was also observed, cementing fine cubic pyrite (Figure 7c). These observations indicate that these phases likely formed together in reducing and alkaline conditions [15,19,56,57]. Additionally, high TOC contents (0.08 to 0.57), the presence of crystal and framboidal pyrite, and the extensive proliferation of microorganisms in both bauxite ore and iron-bearing bauxite (Figure 9e,f) suggest the involvement of microorganisms in mineral formation [55]. Zircon and rutile, as the major detrital minerals, are abundant in the upper bleached horizon due to the involvement of regional metamorphic rocks in the mineralization process [14] and their strong eluviation [1].

5.2. Deposition Mechanisms of High-Grade Bauxite and Fe-Rich Horizon

The presence of high-grade multicolored bauxite ore and the iron-rich horizon is common in the ZSFB and SSZ bauxite deposits, respectively, although the processes responsible remain ambiguous. Previous studies show that the dissolution of Si and Al is often a function of pH, whereas the rate of Fe mobility is generally controlled by both pH and Eh. Si in the pH range of karst bauxite deposits (5–9) is 10–20 times more soluble than Fe and Al [58]. In contrast, aluminum is the most insoluble from pH 4 to 8, and Al³⁺ would precipitate in the structure of hydroxide–oxide and clay minerals [59,60]. Thus, during prolonged weathering, Si is leached from the bauxite profiles, but Fe and Al will remain. Under constant pH conditions, the solubility of Fe increases significantly concerning Al by decreasing Eh [61]. It is thought that part of the Eh reduction process is probably related to the presence of the high content of organic matter and microorganisms, which is a common feature in some bauxite deposits such as the Dopolan in ZSFB [58] and the Parnassos–Ghiona deposits in Greece [62,63]. Total organic content (TOC) in most bauxite deposits is 0.01–0.22%, which can act as a carbon source for microorganisms and control the redox conditions [4]. Ref. [62] believes that microorganisms are a crucial factor influencing the solubility of iron oxides and facilitate the nucleation/growth of sulfides. The TOC values of Daresard and Yakshawa deposits vary from 0.08 to 0.31 and 0.11 to 0.57, respectively, which are higher than that of most karstic bauxite deposits worldwide (Supplementary Table S1). The TOC of both deposits gradually increases towards the basal iron-rich bauxite horizons containing pyrite minerals. It seems that when bauxite–laterite horizons are re-submerged and capped by the transgression sequences [60], the activity of bacterial species such as “Desulfovibrio desulfuricans” [58] leads to a reducing environment. Bacteria oxidize organic matter and reduce sulfate (SO₄²⁻) to hydrogen sulfide through the following reaction [64].

2 CH₂O + SO₄²⁺ → H₂S + 2 HCO³⁻

Under these conditions, Fe²⁺-soluble ions can be released from Fe oxy-hydroxides in the upper layers, combined with hydrogen sulfide and/or carbonate, and re-deposited at the lower parts of the weathering profile, generating a Fe-rich bauxite horizon [42]. This process is confirmed by the presence of cubic goethite pseudomorphs after euhedral pyrite (Figure 7c,d), Fe-rich microveinlets, hematite aggregates with oval morphologies within micro-cracks (Figure 7f), and microorganisms that accelerate this reaction through their enzymes or metabolism (Figure 9e,f). The occurrence of the deferrification process in the studied deposits is evidence of microbial reduction of Fe oxy-hydroxides in the presence of organic matter, which, according to the following reaction, leads to mobility and leaching of iron from the bauxite profile [4,58,65].

4FeO(OH) + CH₂O + 8H⁺ = 4Fe²⁺ + CO₂ + 7H₂O

However, the role of abiotic variables such as aeration status, soil temperature, and moisture should not be ignored, especially in controlling organic matter decomposition rates [22,66,67]. The decrease in TOC in the upper parts of the bauxite profile seems related to the progressive increase in Eh (soil aeration) and soil temperature, accompanied by a decrease in soil moisture. This is consistent with an upward increase in the abundance of Al hydroxides (Table 1), which requires drier conditions than those necessary for Fe oxyhydroxides formation. Thus, the predominance of the reduction conditions due to the mentioned cases leads to the Fe leaching and produces the high-grade bauxite horizon in the upper parts of the bauxite profiles.

5.3. Evaluation of the Critical Raw Materials Profitability

Raw materials are fundamental worldwide for the high technology industry and, according to the “Study of Critical Raw Materials for the [68] (European Commission, Final report), some are defined as critical based on objective criteria, including their economic importance and their supply risk. Although critical raw materials (CRMs) are often produced and used in small quantities, their features are essential for products in strategic areas, and within the 34 CRMs listed in the latest European Commission Final Report, several metals appear, including Be, Sc, V, Co, Ga, Sr, Nb, LREE, HREE+Y, Hf, Ta, and W.

As previously stated, the assessment of criticality depends on risk supply and economic importance and a low ratio of the economic importance (EI) to supply risk (SR), as is the case for HREE+Y and Ga (EI/SR < 1), suggests a supply risk, whereas a larger ratio may indicate either moderate economic importance (1 < EI/SR < 2) as for Nb, Sc, LREE, and V, or significant economic importance (EI/SR > 2), as is the case of Co, Sr, Hf, Be, Ta, and W.

The bauxitization process involves the enrichment of low-solubility trace metals, including most CRMs [1]. The karst bauxite ores of the Sanandaj–Sirjan Zone and Zagros Fold–Thrust Belt studied here show some relevant differences when the distribution of some CRMs is evaluated based on the EI/SR ratio coupled with the contents normalized to the average Upper Continental Crust (ppm bauxite/ppm UCC, hereafter CRM/CRMUCC). The Yakshawa ore (Figure 15a) is characterized by large fluctuations of the CRM/CRMUCC ratios involving enrichment concerning the UCC [69] of both moderate and significant economic importance metals such as Nb, LREE, V, and especially Ta. The maximum enrichment is close to 10× for LREE and about 14x for Ta. Enrichment with respect to the UCC is also observed, to a lesser extent, for HREE+Y and Ga, i.e., the CRMs affected by supply risk. In the Daresard ore, CRMs—excluding Co and Sr—show enrichment compared to the UCC, with the notably maximum enrichment observed for Sc (about 12×, Figure 15b).

Although differences in parent rock(s) likely affect the observed difference in the CRMs distribution between the two deposits, the different textural, mineralogical, and chemical features clearly testify that differences in the factors controlling the bauxitization paths and the early diagenesis processes have an effect as well. If P-rich minerals, fluorocarbonates, and cerianite are the main reservoirs of REE+Y, the other CRMs are mostly controlled by Al-hydroxides, Fe-oxyhydroxides, and Ti oxides [19]. It is well known that paleoclimate fluctuations influence, in karst bauxite ores, the balance between Al-hydroxides and Fe oxyhydroxides, with the latter forming under wetter conditions [22,70]. Furthermore, organic matter decomposition leading to changes in the paleoredox conditions and consequent deferrification process likely impacted the distribution of the CRMs controlled by Fe-oxyhydroxides, such as Sc, V, and Co. It has been suggested that karst bauxites derived from mafic rocks barely represent a potential source of CRMs, especially REE [71], although, in our case, the Yakshawa ore, having as a precursor mafic rocks [19], can be particularly enriched in LREE as well as Ta. The Daresard ore, instead, likely deriving from a clayey limestone [19], may result particularly enriched in Sc, even though Sc accumulation in highly weathered residual rocks is associated with mafic-ultramafic parent material [72]. Overall, this suggests that a peculiar bauxitization path coupled with early diagenesis are the main factors addressing the CRM’s profitability in both the Yakshawa and Daresard ores.

6. Conclusions

• Authigenic illite in Yakshawa forms via isochemical reactions (kaolinite/K-feldspar dissolution), with SEM confirming illite replacement of kaolinite. Burial depth enhances illite crystallinity, producing pseudohexagonal morphologies, while kaolinite originates from acidic weathering of feldspars and later silicification of Al-hydroxides.
• Diaspore forms through both gibbsite transformation (evidenced by syngenetic diaspore-rutile assemblages) and direct precipitation from Al-rich solutions in microbial-mediated, reducing karst environments (pH 7–8), as supported by its paragenesis with pyrite, LREE-fluorocarbonates (e.g., parisite), and elevated TOC (0.08–0.57%).
• Sulfate-reducing bacteria (e.g., Desulfovibrio) drive Fe³⁺→Fe²⁺ reduction under high-TOC conditions, forming Fe-rich horizons via pyrite pseudomorphs (goethite) and microveins. Subsequent oxidative weathering enriches Al-hydroxides in upper profile zones, creating high-grade bauxite.
• Microbial activity (framboidal pyrite, microfossils) and organic matter (TOC up to 0.57%) critically control mineral paragenesis (diaspore, REE phases) and Fe/Al fractionation, alongside abiotic factors (pH, Eh, burial depth).
• The bauxitization process enriches critical metals (especially Ta, LREE, and Sc) in the studied karst bauxites, controlled by host minerals (P-rich/fluorocarbonates for REE+Y; Fe/Ti/Al oxides for others), paleoclimate, and organic-mediated redox processes, highlighting their potential as valuable CRM sources.