The Role of OM in the Formation of Sandstone-Type Uranium Ore—A Review

Abstract

Sandstone-hosted uranium deposits represent one of the most critical global uranium resources suitable for in situ recovery, with their formation closely associated with organic matter (OM). We conducted a systematic literature review to synthesize over 100 published studies sourced from authoritative databases (Elsevier, Google Scholar, Web of Science, Scopus, CNKI, etc.). This study systematically summarizes the types and geological characteristics of OM in sandstone reservoirs and thoroughly analyzes the geochemical mechanisms by which OM regulates the transport and precipitation of aqueous uranium. By integrating case studies of representative sandstone uranium deposits globally, three major OM-related metallogenic models are proposed with distinct core characteristics: the humic-dominated model is driven by the complexation and direct reduction of uranium by humic substances/coal-derived OM; the roll-front model relies on reactions between oxidized uranium-bearing fluids and scattered OM, as well as microbially generated sulfides at the migration front; and the seepage-related model is fueled by upward-migrating deep hydrocarbon fluids (petroleum, methane) that act as both uranium carriers and reductants. Furthermore, this review explores the spatial coupling relationships between OM distribution and uranium mineralization in typical geological settings, evaluates the guiding significance of OM for uranium exploration, and outlines key unresolved scientific issues. The findings refine the genetic theoretical framework of sandstone-hosted uranium deposits and provide important technical support and theoretical guidance for future uranium exploration deployment and resource potential evaluation.

1. Introduction

Sandstone-hosted uranium deposits are epigenetic–diagenetic systems developed in clastic sedimentary rocks such as sandstone and conglomerate [1]. Globally, about 39.6% of identified uranium deposits fall into this category [2]. Owing to their shallow burial depth, large tonnage, and suitability for in situ recovery, they are widely regarded as an economically attractive and comparatively low-impact source of uranium, accounting for nearly 30% of global uranium resources [3,4].

Organic matter (OM) has long been recognized as a key regulator of the mobilization, transport, and precipitation of uranium in these systems [5,6,7,8]. The interactions between uranium and OM are multifaceted, involving adsorption, complexation, and redox processes [9]. These interactions are modulated by the type and functional group chemistry of OM, as well as environmental factors, including pH, redox potential (Eh), and associated ions [10]. Importantly, the manner in which OM participates in uranium mineralization varies across regions [11,12]: North American deposits often feature abundant humic plant detritus; European examples commonly record the influx of petroleum-derived hydrocarbons; Asian deposits reflect the combined influence of indigenous sedimentary organics and deeper hydrocarbon seepage; and Australian and African cases provide classic instances of long-term radiation-driven OM modification [9]. These differences in the type, source, and occurrence of OM across regions directly lead to regional variations in uranium metallogenic mechanisms, thereby requiring targeted adaptation of exploration strategies [13]. Focusing on characteristic OM markers in different geological contexts can effectively facilitate the precise delineation of favorable mineralization zones.

This study examines the role of OM in the genesis of sandstone-hosted uranium deposits. Specifically, we address four key objectives: (1) identify common types of OM in uranium-bearing sandstones; (2) elucidate the geochemical mechanisms of OM that affect uranium migration and precipitation; (3) summarize ore-forming models and typical examples related to OM; (4) and define key directions for future research and their prospecting implications.

2. Methodology

This review employed a systematic bibliographic selection methodology. Key databases, including Elsevier, Google Scholar, Web of Science, Springer, Scopus, and CNKI, were utilized. The literature published between 2000 and 2024 was prioritized, with a focus on peer-reviewed journals, geological survey reports, and high-impact conference proceedings. Irrelevant studies lacking direct links to OM–uranium interactions were excluded. Data on OM characteristics, geochemical mechanisms, and deposit case studies were extracted and cross-validated to ensure comprehensiveness and authority [14].

3. Types and Geological Characteristics of OM in Strata

From a mobility perspective, OM in sandstone-hosted uranium reservoirs can be divided into two broad categories: sedimentary OM and mobile OM. Sedimentary OM refers to primary residues preserved in the sediments, such as plant detritus, coal fragments, and humic substances. Mobile OM denotes hydrocarbon fluids generated by the degradation of OM that can migrate within the basin.

3.1. Sedimentary OM

In sandstone-hosted uranium reservoirs, sedimentary OM typically occurs as carbonaceous debris, thin coal streaks, or humic material. Its abundance varies significantly with paleoclimate. Humid paleoclimates promote vigorous vegetation growth and enhanced OM preservation, resulting in relatively high total organic carbon (TOC) contents in sandstones. For example, the gray fluvial–lacustrine sandstones of the Early Jurassic Zhiluo Formation in the Ordos Basin can reach TOC values of 0.3–1.1 wt% [15,16]. In contrast, sandstones deposited under arid, oxidizing conditions usually contain little OM (<0.1 wt%), which is insufficient to maintain a persistent reducing barrier [17].

At the pore scale, sedimentary OM occurs as carbonized plant fragments or charcoal, lens-like streaks, and humic films or gels that line intergranular pores, grain rims, and cement contacts [18]. It commonly coexists with fine-grained pyrite and minor clay minerals, forming a microscale “OM–sulfide–clay” composite reducing system [8]. Maceral composition is dominated by vitrinite and inertinite and is rich in oxygen-bearing functional groups—carboxyl (-COOH/-COO-), phenolic (Ar-OH), and quinone/semiquinone groups—which supply complexation/adsorption sites and can act as electron donors or acceptors [19,20].

For example, Peng et al. (2022) documented multiple occurrences of pitchblende (uraninite) within carbonaceous debris in the uranium reservoirs of the Fuxin Formation on the southeastern margin of the Songliao Basin—clustered aggregates, cavity fillings, granular–gelatinous forms, and radiating forms—frequently intergrown with pyrite, providing direct microscale evidence that carbonaceous debris serves as principal host for uranium minerals (Figure 1) [21,22,23]. In the Fuxin reservoirs, carbonaceous debris and pyrite are pervasive, whereas external reductants (e.g., thick coal seams or hydrocarbon accumulations) are scarce. Under these conditions, oxidized uranium-bearing waters migrating from the basin margin advanced only a short distance before being intercepted by abundant in situ reductants. Consequently, the redox front—and thus, uranium mineralization—remained localized near the basin margin, strictly tracking the redox transition zone (Figure 2) [24].

3.2. Mobile OM

Compared with sedimentary OM, mobile OM can migrate over long distances through the stratigraphic sequence, introducing deep-sourced materials and reductants into uranium mineralizing systems. It is primarily derived from hydrocarbon expulsion from deep, organic-rich source rocks and is enriched in saturates, aromatics, and resins/bitumen. In the rock record, it commonly appears as oil seeps, bitumen veins, oil stains, and as hydrocarbons in adsorbed gases or fluid inclusions [25,26].

Across Europe, many sandstone-hosted uranium deposits are closely associated with the migration of petroleum [9,27]. Classic examples include the Lodève district in France, where large volumes of solid bitumen occur in sandstones and are intergrown with uranium minerals—providing evidence that ancient oil migrated along faults into the sandstones and induced the precipitation of uranium [28]. Likewise, in Precambrian unconformity-type systems (e.g., Canada’s Athabasca Basin and the Francevillian Basin in Gabon), uranium minerals are widely associated with black bitumen. This bitumen is generally interpreted as insoluble OM formed by the polymerization of ancient oil in a high-radiation environment [29]. Near the Oklo natural fission reactors, bitumen within uranium ore contains abundant radiogenic fission products, supporting a radiolysis-driven polymerization origin [30]. Together, these observations indicate that deep liquid hydrocarbons not only act as carriers and reductants for uranium, but that their solid residues (e.g., bitumen veins) are valuable indicators of otherwise concealed deposits.

Beyond liquids, hydrocarbon gases (such as methane) can also influence sandstone-hosted uranium systems [31]. Because of seepage and leakage, methane is widespread in petroliferous basins. Although relatively inert under anoxic conditions, methane can be “activated” by microbial consortia to produce strongly reducing products that promote uranium precipitation. Under sulfate-bearing, anaerobic groundwater conditions, certain archaeal–bacteria consortia mediate anaerobic oxidation of methane (AOM); sulfate-reducing bacteria (SRB) utilize methane as the electron donor and sulfate as the acceptor, producing CO₂ and H₂S [32,33]. The resulting H₂S is a strong reductant that converts dissolved U(VI) (as UO₂²⁺) to insoluble U(IV) (as UO₂), commonly with co-precipitation of sulfides, such as U-bearing pyrite [34]. In many roll-front systems, uranium precipitates alongside sulfides at the advancing front. Sulfur isotope data from these pyrites often show strong light-sulfur enrichment (δ³⁴S less than < −20‰), which indicates sulfate-reducing bacterial activity [7,35].

This implies that in organic-poor sandstone aquifers, deeply sourced methane and its microbially mediated transformation to H₂S can provide the effective reductants that “indirectly” drive uranium reduction and deposition. In arid basins hosting sandstone-type uranium deposits, such as parts of Kazakhstan and Mongolia, this mechanism may be particularly common because indigenous solid organic carbon is scarce, and deep natural gas plus AOM-linked microbial processes become the dominant reductant sources [13].

As one example, Jaireth et al. argued that uranium precipitation in the Beverley uranium deposit was triggered by petroleum-driven reduction [36]. They proposed two hydrocarbon sources: (1) accumulations in the Eromanga and Arrowie basins and (2) thermal degradation of OM within the sediments. Reactivation of faults in the Lake Frome region likely facilitated reductants from the underlying basins to migrate upward into the host sandstones (Figure 3) [37].

In Precambrian unconformity-related uranium systems (e.g., Canada’s Athabasca Basin and Gabon’s Francevillian Basin) and in Ukraine’s uraniferous bitumen-type deposits (such as Adamovskoye, Berekskoye, and Krasnookolskoye), uranium minerals are pervasively intergrown with black bitumen [38]. Migration pathways controlled by salt diapirism, paleochannel architecture, faulting, and zones of post-depositional reduction allowed deep petroleum to migrate upward along fractures. These reductants secondarily reduced the overlying primary redbeds and, simultaneously, reduced infiltrating oxidized U-bearing fluids, thereby precipitating uranium ore (Figure 4).

A representative Chinese example is the Jingchuan deposit (southwestern Ordos Basin), formed in eolian sandstone belts with petroleum as the main reductant [40]. Its uranium reservoir is dominated by desert facies sandstones and conglomerates, characteristically low in carbonaceous debris, pyrite, and even clay minerals [41,42], with residual oil stains [43]. Microscopic observations confirm hydrocarbon activity as banded hydrocarbon inclusions (elliptical, yellow-green fluorescence, 4–15 μm) are observed along secondary fractures adjacent to quartz (Qtz) (Figure 5a,b). Since the Late Cretaceous, compressional tectonics in the Jingchuan area have triggered folding and fault reactivation, forming a recharge–flow–discharge ore–fluid system [44]. These faults connected deep hydrocarbon reservoirs, promoting the upward migration of reducing fluids that mixed with oxidized uranium-bearing meteoric waters. Redox reactions at hydrocarbon pool margins and within favorable sand bodies led to uranium enrichment, with ore zones predominantly distributed near faults or small-scale uplifts (Figure 6).

4. Influencing Mechanisms of OM on Sandstone-Hosted Uranium Mineralization

During the formation of sandstone-hosted uranium deposits, OM affects uranium occurrence and enrichment primarily through two pathways: (1) enhancing the solubility and transport of uranium in groundwater and (2) promoting uranium precipitation from solution under suitable conditions.

4.1. Promotion of U Transport by Dissolved Organic Complexation

In oxidizing groundwater, hexavalent uranium occurs as the uranyl ion (UO₂²⁺) and predominantly forms carbonate complexes, such as UO₂(CO₃)₂²⁻ and UO₂(CO₃)₃⁴⁻. When humic acids or dissolved OM (DOM) rich in carboxyl and phenolic groups are present, UO₂²⁺ can form even more stable organo-complexes [45,46]. Representative complexation reactions can be schematically expressed as:

UO₂²⁺ + 2RCOO- → UO₂(RCOO)₂

(1)

where RCOO- denotes carboxylate groups on humic acids or humic substances.

Early experiments by Nakashima and co-workers showed that aromatic humic acids can bind UO₂²⁺ through carboxylate oxygens to form multidentate chelates under mildly acidic to mildly alkaline conditions, thereby holding uranium stably in solution [9]. Around pH = 7, U(VI) forms complexes such as UO₂(OH)HA(I) (logβ = 14.7–15.3) and UO₂HA(II) (logβ = 5.8–6.3). In neutral to slightly alkaline and carbonate-bearing waters, it can also form x UO₂(Cternary compleO₃)₂HA(II)⁴⁻, with a cumulative stability constant of logβ₀ = 24.5. These stability constants indicate that organo–carbonate ternary complexes can become the dominant aqueous species in natural carbonate-rich groundwaters, significantly increasing U solubility and mobility.

In the oxidized zones of sandstone-type uranium systems, coal-derived humic substances within the sand bodies weather to release humic acids [46]. Once these acids encounter percolating U(VI)-bearing recharge water, mobile U–humic complexes form and are transported down-gradient toward the roll front [43]. For example, studies in the Dongsheng district (Ordos Basin) show that Jurassic Zhiluo Formation strata adjacent to coal seams released abundant humic matter during oxidation, which complexed uranyl and enabled the long-distance groundwater transport of uranium [47]. At the ore-field scale, lacustrine mudstone–sandstone couplets commonly record humic acids being flushed by compaction waters or laterally through aquifers into adjacent or underlying sandstones, where uranium is subsequently concentrated (e.g., the Grants mineral belt). These observations support long-distance, interformational, and along-strata transport of uranium via DOM complexes and colloidal carriers.

4.2. Enrichment of Uranium by Sedimentary OM

When migrating uranium encounters strata containing solid OM (e.g., coal fragments, humus, woody debris), the first step involves a capture-and-concentrate phase dominated by complexation or adsorption. Under sufficiently reducing conditions, a subsequent valence-change step can convert adsorbed U(VI) to reduced U(IV)-bearing solids. The key is the surface chemistry of OM: carboxyl (-COOH/-COO-), phenolic (Ar-OH), and quinone/semiquinone moieties not only coordinate UO₂²⁺ at the surface but also, in some cases, provide electrons [48,49]. A schematic surface-complexation representation is:

RCOO- + UO₂²⁺ → RCOO-UO²⁺

(2)

where RCOO- denotes surface functional groups on OM. Multidentate complexes formed under mildly acidic to neutral conditions are relatively stable, extending local residence time and enhancing the concentration of U(VI). This is consistent with classic humic–uranyl experiments showing that aromatic humic acids significantly stabilize and mobilize U(VI) in mildly acidic to mildly alkaline waters while simultaneously enabling its recapture onto organic phases when conditions favor sorption.

A large body of work indicates that the porous texture of humic substances and their abundant reactive groups provide highly effective adsorption sites for uranium [8,48]. Canonical experiments demonstrate that peat, lignite, or plant humus exhibit high capacity and selectivity for U(VI), with U(VI) uptake increasing steeply with pH between 3 and 7, even at low dissolved humic concentrations (µg·L⁻¹–mg·L⁻¹).

Field examples mirror these mechanisms. In the Daying sandstone uranium deposit, coal-derived detritus is closely associated with uranium; in some samples, uranium occurs as amorphous organo-U complexes, a textbook signature of adsorption-driven enrichment [28]. Lignite-hosted uranium deposits in North America show analogous behavior: groundwater U(VI) is efficiently trapped by coal OM, yielding average uranium contents of tens to hundreds of ppm, with uranium present chiefly in adsorbed rather than crystalline mineral forms [50]. At Western Australia’s Mulga Rock deposit, uranium is likewise fixed within Paleocene peat–lignite horizons, largely as bitumen-bound organo-complexes [50].

4.3. Reduction of Uranium by OM

Economic uranium grades ultimately require the reduction of U(VI) to U(IV). OM can serve as the electron donor directly or indirectly by fueling microbial processes that generate reductants [51].

A growing body of work shows that within diagenetic, low-temperature windows (100–200 °C, and, in some cases, down to ambient conditions), hydrocarbons and sedimentary OM can abiotically reduce dissolved U(VI) to insoluble U(IV) (predominantly uraninite). For example, methane, hydrogen, and graphite efficiently drive the U(VI) to U(IV) reduction in acidic chloride brines at 100–200 °C, with −1 mM U(VI) reduced to uraninite by CH⁻ in 3 days at 200 °C [52]; U-rich lignite or gyttja can yield uraninite via dehydrogenation at 150–200 °C [9]; in a 200 °C, 500 bar water–uranium–n-alkane system, the reaction initiates synergistically with uranium reduction and hydrocarbons oxidation [53]; and at room temperature, humic acids on mineral surfaces can also abiotically reduce U(VI), forming thermodynamically more stable U(IV) products [54].

UO₂²⁺ + 2e⁻ → UO₂

(3)

In China’s Ordos and Junggar basins, uraniferous fossil wood is widespread; its formation is most plausibly explained by the direct reduction of uranyl by buried woody debris [55,56,57,58,59,60,61].

More commonly, reduction proceeds indirectly: microbes (such as sulfate-reducing bacteria, methanogens, iron-reducing bacteria, etc.) metabolize organic substrates to produce reductants that induce uranium precipitation [49]. In reduced zones, the dominant biogenic reductants are hydrogen sulfide (H₂S) and ferrous iron (Fe²⁺). H₂S is derived from the sulfate-reducing metabolism of OM: under anaerobic conditions with sulfate-rich groundwater, certain microorganisms can mediate the anaerobic oxidation of OM, using organic compounds (including hydrocarbons, lactic acid, acetic acid, etc.) as electron donors and sulfate as electron acceptors to oxidize OM into CO₂ while producing H₂S [62]. The generated H₂S reduces free UO₂²⁺ to insoluble UO₂. Simultaneously, SO₄²⁻ is reduced to S²⁻, which combines with Fe²⁺ to form pyrite [63,64]

SO₄²⁻ + 2 CH₂O → H₂S + 2 CO₂ + 2 H₂O

(4)

UO₂²⁺ + H₂S → UO₂ + S + 2H⁺

(5)

Fe²⁺ + 2 H₂S → FeS₂ + 2 H₂

(6)

Fe²⁺ originates from the microbial reduction of Fe(III) oxides and/or the reaction of H₂S with Fe²⁺ to form iron sulfides. In either case, these products react with aqueous UO₂²⁺ to yield insoluble U(IV) phases [65].

UO₂²⁺ + H₂S → UO₂ + S + H⁺

(7)

UO₂²⁺ + 2Fe²⁺ + 2H₂O → UO₂ + 2Fe³⁺ + 4H⁺

(8)

UO₂²⁺ + H₂S + 2Fe²⁺ → UO₂ + 2FeS₂ + 2H⁺

(9)

In the Chu-Sarysu Basin (southern Kazakhstan), a classic roll-front setting, pyrite grains are enveloped and replaced by uranium minerals. This texture supports a scenario in which H₂S generated by OM and sulfate-reducing bacteria (SRB) reduced U(VI), with the co-precipitation of pyrite, and parts of the pyrite were subsequently replaced by uranium minerals [66]. Similarly, at the Shihongtan deposit (Turpan–Hami Basin, China), isotopic and mineralogical evidence indicate a dual microbial role: direct microbial reduction of U(VI) and the generation of abundant H₂S that promoted sulfide-associated uranium precipitation [67].

Recent in situ/micro-spectroscopic studies further show that in anoxic, organic-rich sediments, OM-bound U(IV (via adsorption or inner-sphere complexation) can be a predominant host phase [63]. This “organically anchored” U(IV) suppresses the nucleation and growth of crystalline UO₂, allowing adsorbed U(VI) to transform partially and progressively into amorphous U(IV) that remains tethered to the organic matrix rather than precipitating as crystalline uraninite (pitchblende). This framework helps explain why coal- and humus-rich intervals can be uranium-rich yet poor in crystalline uranium minerals [68].

5. Mineralization Models Involving OM and Their Exploration Significance

5.1. Representative Models

The presence of OM not only governs the way uranium is transported and precipitated, but it also shapes the spatial architecture of sandstone-hosted ores and their associated mineral assemblages, giving rise to distinct mineralization styles (

Table 1. Types of organic matter, metallogenic models, and exploration criteria in typical sandstone-hosted uranium deposits.

Mineralization Model	Dominant/Typical OM (OM)	Key Processes	Typical Geological Setting	Exploration Indicators	Typical Deposit Examples (Authoritative Sources)
Humic-type	Humic acids/coal-derived OM; wood fragments/carbonaceous debris; commonly co-existing authigenic pyrite	OM complexes or adsorbs U to extend transport; direct reduction of U(VI) by OM and/or indirect reduction via BSR-generated H2S	OM-rich intervals within deltaic–meandering/braided fluvial sand bodies on gentle basin slopes or platform margins; locally associated with low-rank coal interbeds	(1) Light δ34S in pyrite (BSR); (2) Petrography: U–OM intergrowths	Dongsheng–Daying (Ordos Basin, China); Stráž–Hamr (North Bohemian Cretaceous Basin, Czech Rep.)
Roll-front	Dispersed OM with authigenic sulfides as synergistic reducers (OM usually secondary)	Oxidized, bicarbonate-bearing groundwater advances along permeable sand	Gently dipping paleochannel/valley-fill sand bodies (fine–medium sand) with mudstone seals; basin margins or dome flanks; common in coastal plain or intracontinental basins	(1) Three-zone color banding + directional high gamma/low resistivity	Alta Mesa/Kingsville Dome (Texas Gulf Coastal Plain, USA); Smith Ranch–Highland (Powder River Basin, USA); Beverley–Four Mile (Lake Frome Basin, South Australia)
Deep-seated seepage/upward exfiltration	Hydrocarbons (methane/wet gas/bitumen), organic-acid-bearing deep basin fluids; commonly with BSR-H2S	Low- to moderate-temperature organic fluids or reduced brines rise from U-bearing source rocks along faults/steps into shallow sandstones	Faulted basin margins/step zones plus fluvial–deltaic sands; bitumen/”tar” veins, calcite veins, and hydrocarbon shows; frequent evidence for two mineralization episodes	(1) Bitumen/solid hydrocarbon intergrown with U minerals (microscopy/Raman/GC–MS); (2) Fluid inclusions with elevated salinity, CO2/hydrocarbon signatures; (3) light δ13C in calcite, very light δ34S in pyrite (BSR)	Arlit–Akouta (Tim Mersoi Basin, Niger); DASA (Tim Mersoi Basin, Niger); Sierra Pintada/Don Otto (Argentina)

).

5.1.1. Humic-Type Model

Abundant humic OM establishes a pre-existing reducing layer—effectively an organic “mat” draped through the sand body. When oxidized, uraniferous groundwater invades, and uranium precipitates broadly and fairly uniformly along this layer, producing stratiform, tabular sheets [69]. Classic examples include the Late Cretaceous Dakota Sandstone “tabular” uranium deposits in the United States and the Dongsheng deposit in the Ordos Basin, China. In both settings, the OM occurs mainly as lignite seams or sapropelic/gyttja horizons, and ore geometry closely mirrors the organic bed (Figure 7).

The humic-dominated model applies to areas where thick accumulations of peat/coal were deposited. Key elements include surface-to-shallow subsurface peat or coal seams, periodic infiltration of oxidizing meteoric water, and a laterally continuous, humic-rich reducing layer in the subsurface [70]. The model implies that broad uranium sheets can develop in paleo-lake–marsh settings, making the mapping of coal measure distributions a primary exploration task.

5.1.2. Roll-Front Model

Oxidized, uraniferous groundwater advances down an aquifer as a migrating oxidation front. Where this front encounters local reductants—such as scattered carbonaceous debris or H₂S generated by sulfate-reducing bacteria (SRB)—uranium precipitates locally. Because groundwater flow persists, portions of previously precipitated U can be re-oxidized and transported forward, causing the mineralized zone to migrate down-gradient and develop the classic crescentic “roll” geometry [29]. In this dynamic setting, OM acts as a localized reductant: small pockets of OM (or SRB-derived H₂S) continually remove U from solution while the oxidized plume skirts around them and continues advancing. Repetition of this process builds a stable, arcuate roll-front belt.

Many deposits in Kazakhstan exemplify this type: host rocks display sharp redox color zoning, orebodies trace roll-shaped margins along the aquifer, and pyrite of clear biogenic origin is abundant with minor carbonaceous debris. A well-documented case is the Shirley Basin (Wyoming), where orebodies occur in conglomeratic arkosic sandstones of the Eocene Wind River Formation at depths of 100–500 ft. Mineralization forms tongue-like envelopes along the boundary between altered and unaltered strata, and the main ore accumulations exhibit characteristic roll geometries at the flanks and terminations of these envelopes. The system was formed by weakly acidic (pH 4–4.5), relatively reducing (Eh = +0.10 V) uraniferous groundwater that penetrated alkaline host rocks, underwent neutralization, and precipitated uranium preferentially in the neutralization zone; pitchblende is documented to form at pH 4–5 [71].

The core exploration insight for the roll-front model centers on tracking the redox transition and OM/sulfide micro-zones along modern or paleo-flow directions within large, well-connected sand bodies. First, integrate hydrochemical, mineralogical, and well-logging criteria to delineate the roll margins; then, systematically trace these margins down the hydraulic gradient. This stepwise approach enables efficient delineation of orebodies and assessment of in situ leaching potential (Figure 8).

5.1.3. Seepage-Driven Model

The seepage-driven mode—a recently proposed concept—envisions deep, organic-rich fluids (e.g., petroleum, bitumen, hydrocarbon gases) transporting uranium upward and causing it to seep into shallow redox transition zones where rapid precipitation occurs. A diagnostic feature is the presence of deep-origin OM that is unmistakably associated with uranium minerals in otherwise organic-poor sandstones. Orebodies typically form within gray, reducing sandstone lenses that are overlain by red, oxidized sands, indicating an upward migration of mineralizing fluids into a locally reducing trap (Figure 9) [73,74]. By emphasizing deep sources for both uranium and organic reductants, the seepage model challenges traditional exploration strategies that concentrate solely on near-surface redox interfaces [75,76,77,78].

The deep-seepage concept underscores the co-supply of ore components and reductants from the depths of basins. It links uranium ore formation with the evolution of petroleum systems, prompting an integrated assessment of mineralization controlled by oil, gas, and uranium. In basins rich in petroleum, particularly where uranium-bearing source rocks are well developed, this model holds exploration significance [76].

5.2. Regional Case Studies

Sandstone-hosted uranium deposits are found in numerous sedimentary basins around the world, and the specific role of OM varies depending on the geological setting and OM type. This section examines representative districts to compare how different OM types function under contrasting mineralization backgrounds.

5.2.1. Ordos Basin (China)

Representative sandstone-hosted uranium deposits in the northeastern Ordos Basin are primarily hosted in the Middle Jurassic Zhiluo Formation, which is typical of fluvial sandstones, and mainly form tabular orebodies. An unconformable contact with the underlying Yan’an Formation coal measures acts as a preferential conduit for both OM and oxidized U-bearing groundwater, serving as a critical geological condition for uranium mineralization. The OM assemblage is dominated by solid organic matter, and the host sandstones are widely intermingled with coal-derived detritus and plant fragments. Ore grade exhibits a strong positive correlation with total organic carbon (TOC) [3], while petrographic studies confirm intimate co-occurrence of carbonaceous debris with U minerals [79]. Additionally, sapropelinite is conspicuous in some high-grade samples, and coal OM exhibits strong fluorescence [45,80], highlighting the genetic link between OM and uranium mineralization.

The mineralization mechanism is dominated by a coal-sourced humic acid pathway: specifically, humic acids complex with and transport U(VI) in the oxidized zone; when the U-bearing fluid reaches the redox transition zone, U(VI) is reduced to U(IV) by organic reductants. Carbonaceous debris and sapropelinite play a dual role in adsorbing and immobilizing U(IV), effectively promoting the concentration of uranium to form orebodies.

Regarding exploration implications: (i) The coal-bearing Yan’an–basal Zhiluo contact and associated unconformities should be prioritized as integrated OM–fluid migration and ore-forming corridors and (ii) UV fluorescence anomalies in cores or thin sections (bright bands, fluorescent spores) combined with carbonaceous debris concentrations can serve as practical ore guides.

5.2.2. United States

Uranium deposits in the United States include examples that conform to both the humic-dominated (tabular) model and the classical roll-front model. The former is typified by the Grants uranium district, New Mexico, where mineralization is hosted by Upper Jurassic Morrison Formation fluvial sandstones and forms tabular to banded ore aligned with paleoflow [43].

During the Late Jurassic–Cretaceous, lacustrine–palustrine waters enriched in humic acids infiltrated permeable sandstone aquifers. Humic acid uranyl gels formed near the water table and, with compaction during diagenesis, yielded high-grade tabular ores. Uranium occurs mainly as pitchblende/uraninite and OM-bound adsorbed U(IV) [59].

By contrast, Wyoming and adjacent Cenozoic basins host widely developed roll-front deposits [29] whose crescentic fronts track groundwater flow paths. The mineralized sandstones typically have low primary TOC with only minor carbonaceous debris and clays, yet framboidal pyrite concentrates along OM-filled fracture arrays. Reduction is predominantly inorganic, driven by H₂S/FeS₂. In this case, minor OM and its associated sulfate-reducing bacteria (SRB) generate H₂S at the advancing front, triggering U(VI) to U(IV) reduction with co-precipitation of pyrite. Sulfur-isotope data display a distinct biogenic signature [35,81]. For exploration, beyond simply mapping carbonaceous horizons, emphasis should be placed on strong sulfidation–re-oxidation indicators. Examples include alteration halos of marcasite/pyrite and Fe oxide overprints, which effectively delineate the migration pathways and remobilization history of roll-front orebodies [82,83,84,85].

5.2.3. Kazakhstan

The Chu-Sarysu Basin in Kazakhstan is characterized by classic roll-front uranium deposits, the majority of which are mined using in situ recovery techniques. The Muyunkum deposit exemplifies this type, with mineralization occurring in Lower Cretaceous fluvial sandstones that generally exhibit a pale gray to gray-green color indicative of reducing conditions. Locally developed roll-front ore zones are darker gray and rich in pyrite. Stratigraphically, the orebodies are found within channel-fill sand bodies containing carbonaceous plant debris, although the overall organic carbon content is low (typically < less than 0.1%) [10].

Stable isotope and hydrogeochemical data suggest that the ore fluid was oxidized, uranium-bearing groundwater circulating on a basin-wide scale. As it flowed, oxygen was progressively depleted, CO₂ increased, and the system evolved towards self-generated reducing conditions [9]. In the downstream reduced zone, residual dissolved organics—likely originating from the degradation of plant detritus upstream—served as substrates for anaerobic bacteria, promoting SRB activity. The resulting H₂S reduced U(VI) at the advancing front and co-precipitated pyrite [67,86]. Pyrite δ³⁴S values of around −20‰ to −30‰ support a biogenic origin [35]. Notably, due to the scarcity of primary OM, much of the uranium exists as labile, non-crystalline U(IV), which is favorable for ISR but also increases the likelihood of natural re-oxidation and leaching. Consequently, ISR operations and environmental management should adjust oxidant injection and rehabilitation strategies according to the abundance of OM and the speciation/host of U(IV) [87,88].

5.2.4. Australia

The Mulga Rock uranium deposit in Australia is a large U–polymetallic system hosted within Paleocene paleochannel lignite [13]. It consists of several orebodies, including Ambassador, where mineralization is concentrated in near-surface woody peat (lignite) that is rich in OM and co-enriched in Ni, Co, Sc—indicative of a reducing depositional environment. The lignite serves two roles: supplying fulvic acids that complex and retain U(VI) and providing a reducing environment that converts U(VI) to precipitated UO₂ [13]. As a result, uranium is present both as dark oxide minerals and, to a large extent, as colloidal “uraniferous lignite” adsorbed to the coal matrix [57]. Mineralization also extends into the sandstones beneath and lateral to the lignite, interpreted as precipitation at redox boundaries where upwelling, U-bearing groundwater interacted with the peat [58].

Mulga Rock is widely recognized as the archetype of a wood–peat (lignite)-hosted sandstone uranium system, reflecting uranium concentration in large peat redox environments. Another characteristic is the strong post-ore alteration of radioactivity on the organic host: radiation-induced aromatization and cross-linking of biopolymers produced insoluble, inert bitumen [59]. The close association between highly matured OM and uranium in the ore highlights that U–organic interactions can continue to modify the organic phase well after mineralization.

5.2.5. Niger and Argentina

In the Tim Mersoi Basin of Niger (Arlit, Akouta), sandstone-hosted uranium deposits are primarily hosted by fluvial channel-fill alluvial sandstones (e.g., the Tarat Formation) that have undergone multistage redox overprinting [89,90]. Beyond the traditionally recognized terrigenous OM (carbonaceous debris, plant fossils), notably, allochthonous petroleum-derived OM—solid bitumen—plays a pivotal role in ore formation. This bitumen fills pores and fractures in the Tarat channel sands, within which uranium minerals (uraninite and U–Ti oxides, e.g., brannerite) occur as submicrometer intergrowths [91].

The system follows a deep-seated seepage model: when ascending U-bearing fluids enter the channel sandstones, they are reduced by solid bitumen, leading to the precipitation of uraninite and U–Ti oxides. Organic–geochemical indicators, including a bimodal n-alkane distribution pattern, Pr/Ph ratios of 0.315–3.530, and C₃₅ hopane dominance, indicate a reducing, alkaline environment, consistent with the hydrocarbon migration driven by Atlantic rifting-related thermal events. Regional source-rock distributions suggest the bitumen was sourced from Silurian “hot shales” or Precambrian strata. This insight revises models that relied solely on terrigenous OM and opens new organic–geochemical targets for exploration.

In the San Jorge Gulf Basin of Argentina, the Cerro Solo uranium deposit is hosted in Lower Cretaceous Los Adobes Formation sandstones, with orebodies that are lens-shaped to tabular. The dominant OM is dispersed detrital organic matter (e.g., carbonaceous debris) occurring as fine particles or fragments within sandstone and conglomerate matrices, rather than as thick, massive layers, such as coal or oil shale. Notably, this carbonaceous debris is closely associated with sulfides (e.g., pyrite), and coffinite commonly occurs in intimate intergrowth with OM and sulfides. This assemblage provides key geological criteria for delineating mineralized horizons and identifying favorable mineralization zones [92,93].

Fundamentally, detrital OM provides abundant sorption sites and reductive nucleation centers for uranium mineral precipitation, thereby facilitating the concentration of uranium within the mineralized intervals. For exploration practices, priority should be given to overlapping zones of paleochannels and carbonaceous horizons. Additionally, mapping redox interfaces together with pyrite–OM anomalies provides practical and effective guidance for targeting mineralization, as these features directly reflect the favorable geochemical conditions for uranium accumulation [94,95,96,97,98,99,100].

5.3. Exploration Significance of OM

Firstly, OM is widely recognized as a key geochemical indicator for uranium exploration: strata and facies with moderate organic enrichment are more likely to host uranium deposits. In China, typical primary exploration targets include Jurassic coal-bearing sandstones, paleochannel peat deposits, and sandstones overlying organic-rich source rocks. Consequently, this has led to the proposal of “coal–uranium co-prospecting” in coal-bearing basins. In contrast, for petroliferous basins, the focus should shift to anomalies at the intersections of hydrocarbon migration pathways and aquifers [22], as migrating hydrocarbons can provide both reducing conditions and sorption sites for uranium accumulation [47]. Secondly, various surface geochemical anomalies derived from organic processes can act as prospecting markers. In sandstone-hosted uranium districts, local gray-to-black reducing lenses within otherwise red oxidized sandstones—particularly those containing plant debris or bitumen stains—may indicate hidden mineralization and warrant radiometric surveys and sampling. Meanwhile, intense silicification and carbonate alteration in the surrounding rocks are commonly associated with leaching and precipitation by organic acids during ore formation [23]. Furthermore, radiation from uranium decay (α-particles and radiogenic heating) can “overprint” nearby OM, increasing its maturity, enhancing vitrinite reflectance, and promoting aromatic structures that yield inert bitumen [57]. In parts of the Ordos Basin, carbonaceous clasts adjacent to ore deposits exhibit significantly higher maturities than the background and display anisotropic, graphite-like fabrics [57,58,59,60,61].

Thirdly, at the ore-field scale, the spatial distribution of OM can assist in reconstructing paleo-redox fronts, thereby enabling the prediction of the ore deposit geometry [101]. Mapping gradients in organic carbon across an aquifer can reveal the former flow paths of oxidized, U-bearing groundwater and the locations of halted reduction fronts—information that is valuable for delineating prospective enrichment zones [35].

6. Conclusions

OM exerts multiple, essential controls on sandstone-hosted uranium systems: it complexes U(VI), thereby extending transport distances; it acts as a precipitating agent through both direct and microbially mediated reduction; and it drives wall-rock alteration and bitumen formation while leaving diagnostic organic–geochemical fingerprints. Several points of consensus have emerged over the past decade: (1) Sedimentary humic substances—particularly coal-derived humic acids—are the most common and potent drivers of sandstone uranium mineralization, with their coupled complexation–reduction processes supported by extensive experimental results and numerous field examples. (2) Microbially mediated reduction is pivotal in many deposits. In roll-front systems, H₂S generated by sulfate-reducing bacteria (SRB) is widely regarded as the dominant reductant, consistent with the prevalence of biogenic pyrite in ores. (3) Liquid and gaseous hydrocarbons provide an alternative pathway in petroliferous basins, where deep hydrocarbon fluids may potentially both transport uranium and trigger its reduction and precipitation. (4) Uranium mineralization commonly coincides with pronounced transformation of OM itself, including elevated maturity, anomalous asphaltene/bitumen residues, and related textures, which indirectly attest to intense U-OM interactions.

Despite this progress, several issues remain unresolved and materially affect genetic interpretation and prediction: (1) In any given deposit, the quantitative contributions of sedimentary OM, hydrocarbon fluids, Fe²⁺, and other reductants, as well as their hierarchy of influence, demand quantification via multi-isotope tracing and reaction network modeling. (2) Whether OM concentrated first and subsequently trapped uranium, whether U precipitation induced OM polymerization, or whether both processes were broadly synchronous appears to vary by deposit type (humic-type systems often favor “OM first,” whereas roll-front occurrences of uraniferous bitumen imply U-driven polymerization). (3) The speciation by which hydrocarbons carry U, the geometry and permeability hierarchy of conduits, and the staging of enrichment remain poorly constrained and tightly linked to basin plumbing. (4) Regarding radiation overprints on coalification and hydrocarbon generation, emerging evidence indicates that uranium radiation can accelerate OM maturation, stimulate hydrocarbon generation, and yield diagnostic gaseous products, with implications for unconventional resources in uranium provinces.

Future advancements will likely hinge on three complementary directions: (1) Greater interdisciplinarity that integrates organic geochemistry, geomicrobiology, and isotope geoscience across molecular, pore-scale, and basin-scale observations to resolve U-OM mechanisms. (2) Greater reliance on in situ/operando analyses and modeling. For instance, using in situ/operando spectroscopy and spectromicroscopy and synchrotron X-ray imaging to directly observe the nanoscale co-localization of uranium and OM in ore and distinguish non-crystalline U(IV)–OM from uranium minerals (uraninite/coffinite). Alternatively, numerical modeling can quantify the role of organic complexation in uranium transport, while real-time redox kinetics can be tracked under controlled anoxic conditions with variable pH–Eh. In reactive transport modeling, a dissolved OM (DOM) speciation framework should be integrated, coupled with surface complexation and Monod-type kinetics for bacterial sulfate reduction (BSR), anaerobic oxidation of methane (AOM), and iron reduction. (3) Multi-indicator exploration models that incorporate total organic carbon (TOC), sulfur-isotope signals, microbial biosignatures, and related metrics into predictive frameworks for targeting, evaluation, and in situ recovery (ISR) design. Pursued collectively, these lines of inquiry will refine process-based genetic models and provide a stronger scientific basis for efficient exploration and production of sandstone-hosted uranium deposits.