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Article

Key Ore-Controlling Factors and Genetic Model of the Tamusu Super-Large Sandstone-Type Uranium Deposit, Bayingobi Basin

by
Chao Lu
1,
Zhongyue Zhang
2,
Yangquan Jiao
3,
Zhao Li
4,
Xiaoyi Yuwen
5,
Yinan Zhuang
6,
Chengyuan Jin
6,
Chengcheng Zhang
1,
Weihui Zhong
4 and
Qilin Wang
6,*
1
No. 208 Geology Party, China National Nuclear Corporation, Baotou 014010, China
2
Liaoning Earthquake Agency, Shenyang 110031, China
3
School of Earth Resources, China University of Geosciences, Wuhan 430074, China
4
Research Institute of Exploration and Development, PetroChina Changqing Oilfield, Xi’an 710018, China
5
Liaoning Nuclear Industry Geology 242 Co., Ltd., Xingcheng 125100, China
6
School of Earth Sciences, Northeast Petroleum University, Daqing 163318, China
*
Author to whom correspondence should be addressed.
Minerals 2026, 16(4), 357; https://doi.org/10.3390/min16040357
Submission received: 14 January 2026 / Revised: 22 March 2026 / Accepted: 24 March 2026 / Published: 27 March 2026 / Corrected: 30 July 2026
(This article belongs to the Special Issue Genesis of Uranium Deposit: Geology, Geochemistry, and Geochronology)
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Abstract

Tamusu, the only identified super-large sandstone-hosted uranium deposit in the Bayingobi Basin, provides an important natural laboratory for evaluating ore-controlling factors and genetic models of sandstone-type uranium mineralization. Based on core descriptions from more than 200 boreholes, log facies analysis and geochemical environmental proxies, this study constrains the sedimentary–mineralization architecture and key controlling factors of the deposit. Uranium orebodies are mainly hosted in the upper member of the Lower Cretaceous Bayingobi Formation (Sq2) within a gravity flow-dominated fan-delta–lacustrine system. Braided distributary channel sands on the fan-delta plain and subaqueous distributary channel sands on the delta front constitute the principal uranium reservoirs, controlling both the migration pathways and storage space for U-bearing fluids. Mineralization is jointly governed by fan-delta architecture, interlayer oxidation zonation and reducing agents. The interlayer oxidation zone displays a north-thick–south-thin geometry, and uranium orebodies are concentrated at redox transition positions, with grades of 0.01–0.33 wt%. The metallogenic evolution can be summarized in three stages: syndepositional uranium pre-enrichment, interlayer oxidation mineralization, and a late hydrothermal/diagenetic overprint that mainly modified reservoir properties, favored ore preservation, and did not contribute to the primary uranium budget. Accordingly, a genetic model of “fan-delta architecture + interlayer oxidation control + late overprint and preservation” is proposed to guide exploration in the Bayingobi Basin and analogous sandstone-type uranium systems.

1. Introduction

Sandstone-hosted uranium deposits occur within sedimentary basins and are products of water–rock interaction [1,2,3]. Their genesis and evolution are governed primarily by uranium source/provenance, reservoir sand bodies and redox conditions [3,4,5,6,7,8,9,10,11,12,13]. Drawing on more than 360 representative deposits across China, Chinese researchers have established metallogenic models for over 100 typical deposits [14,15,16]. These models emphasize different ore-controlling factors—structure, sedimentation, and fluid regimes—resulting in a diverse and complex suite of genetic types [17,18,19,20,21,22,23]. Although numerous factors influence uranium mineralization, reservoir sand bodies are a necessary precondition for sandstone-type uranium formation: they provide migration pathways for mineralizing fluids and constitute the storage space for ore accumulation [1,24,25]. Changes in the valence state of uranium are a fundamental principle universally observed during mineralization and are controlled predominantly by oxidation–reduction reactions [26,27,28,29,30,31,32,33,34,35].
Since the 1990s, multiple uranium deposits and occurrences have been identified in the Lower Cretaceous Bayingobi Formation of the Bayingobi (Bayingebi) Basin, among which the super-large Tamusu (with uranium resource reserves exceeding 50,000 tons) is the most representative sandstone-hosted uranium deposit [36,37,38,39,40,41,42]. It also provides the prime natural laboratory for investigating the metallogenic conditions and genetic models of sandstone-type uranium mineralization in the basin. Although substantial progress has been made on the Tamusu deposit, it comprises more than 50 discrete orebodies of several types, including sandstone-hosted, syndepositional mudstone-hosted, sand–mud mixed (heterolithic), and epigenetic mudstone-hosted, rendering its metallogenic conditions and genetic interpretation complex. Current debates focus on two key scientific questions: the depositional system architecture and its control of uranium reservoirs, and the influence of hydrothermal fluids on uranium mineralization [37,41,43,44]. Depositional interpretations diverge between a braided-river delta and a fan-delta system. The involvement of hydrothermal fluids is likewise contentious: some studies [45,46], based on mineral assemblages such as fluorite and ankerite (ferroan dolomite) within the deposit, propose participation of deep-seated hydrothermal fluids in uranium enrichment [37]; whereas others contend that hydrothermal fluids do not further concentrate uranium and are manifested only as diagenetic strengthening of the mineralized strata [41,42].
In view of this, the present study focuses on the super-large Tamusu uranium deposit. Based on core logging of more than 200 boreholes together with log facies analysis, we delineate depositional system types, spatial architecture, and favorable criteria of the ore-bearing intervals and uranium reservoir sand bodies. We further characterize the zonation of the interlayer oxidation zone and the mechanisms of uranium precipitation. By systematically integrating coupling among tectonics, sedimentation, fluids, and uranium mineralization, we propose a genetic model that enriches the metallogenic framework for sandstone-type uranium deposits in strike-slip pull-apart basins and provide a scientific basis for exploration targeting in the Bayingobi Basin and for analogous deposits across the Central Asian metallogenic domain.

2. Geological Background and Deposit Geology

The Bayingobi Basin developed in a strike-slip pull-apart tectonic setting and contains single-fault and double-fault sags, with pronounced Early Cretaceous magmatism [8,47]. The sedimentary cover comprises Jurassic, Cretaceous, and Quaternary strata [48,49,50]. Jurassic strata are limited in distribution and are mainly of Lower to Middle Jurassic age. The Cretaceous succession forms the principal basin fill, but it is locally incomplete owing to uplift and erosion: within the Lower Cretaceous, the Bayingobi Formation (K1b) is preserved, whereas the Suhongtu (K1s) and Yinggen (K1y) formations are either absent or have been eroded. The basin experienced an evolution characterized by two episodes of compressional uplift and denudation, two episodes of extensional rifting, and a subsequent phase of thermal subsidence sagging [47]. The Tamusu uranium deposit is situated along the Zongnaishan slope belt on the northern margin of the Yinggejing Depression (Figure 1a). In the southern Bayingobi Basin, structural inversion-related fold assemblages are common, and the Tamusu deposit occurs between two major fault zones [37,41]. The basin-bounding fault on the northern margin belongs to the southern segment of the Zongnaishan–Shalazhashan fault system and was reactivated with reverse (inversion) motion during the Paleogene–Neogene (Figure 1b). Uplift and denudation along this inversion fault have exposed the upper member of the Bayingobi Formation at the surface to the northwest of the deposit, a condition favorable for the development of interlayer oxidation zones (Figure 1b). In the Tamusu area, the traditional stratigraphic scheme subdivides the Lower Cretaceous Bayingobi Formation into a lower member (K1b1) and an upper member (K1b2). The lower member is dominated by lacustrine deposits, whereas uranium mineralization is mainly hosted in the upper member.

3. Sampling and Analytical Methods

Samples for sandstone petrography and for characterizing the occurrence modes of uranium minerals were collected from the Bayingobi Formation of the Tamusu uranium deposit. The collected rocks primarily consist of gray and yellow sandstones from the boreholes. Twenty sandstone samples were examined by polarizing optical microscopy and scanning electron microscopy (SEM). The polarizing optical microscopy was conducted using a Leica DM4 P microscope (Leica Microsystems, Wetzlar, Germany) at the National Key Laboratory of Multi-Resource Collaborative Green Exploitation for Continental Shale Oil, Northeast Petroleum University. The SEM and Energy Dispersive Spectroscopy (EDS) were carried out at the Key Laboratory of Tectonics and Petroleum, China University of Geosciences (Wuhan), using a Carl Zeiss EVO LS 15 SEM equipped with an AZtec X-Max 20 energy-dispersive X-ray spectrometer (Carl Zeiss, Oberkochen, Germany). Operating conditions were as follows: accelerating voltage 5–25 kV, filament current 2.4–2.8 A, probe (beam) current 100–500 µA, and working distance 8–9 mm.
Sandstone samples for geochemical environmental proxies were collected from Bayingobi Formation sandstones representing different geochemical types within the Tamusu deposit. A total of 269 samples were analyzed, including 195 gray sandstones, 44 yellow sandstones, and 30 red sandstones. Analyses were performed at the Analytical Testing Center of Nuclear Industry Geological Brigade No. 208, using an HCS-140 high-frequency infrared carbon–sulfur analyzer (Dekai Instrument Co. Ltd., Shanghai, China)and a VIS-723N visible spectrophotometer (Beifen-Ruili Analytical Instrument Co. Ltd., Beijing, China); Fe3+/Fe2+, S2−, and ΔEh were determined by titrimetric (volumetric) methods following standard laboratory protocols.

4. Results

4.1. Petrological Characteristics

In the upper member of the Bayingobi Formation, sandstone types are dominated by arkosic sandstone (Figure 2a), accounting for 86.4%; lithic arkose (Figure 2b) makes up 10.7%; and feldspathic quartz sandstone (subarkose) constitutes only 2.9%. Detrital framework grains are chiefly quartz and feldspar, with lithic fragments being subordinate (Figure 2c,d). Lithic fragments are mainly granitic, with minor volcanic and metamorphic lithics; accessory constituents include mica and heavy-mineral grains. The sandstones comprise framework grains and interstitial material. Framework content is 72%–93% (avg. 86.3%), whereas interstitial material is 7%–28% (avg. 13.7%) and consists of matrix and cement. Matrix occurs only in a few matrix-supported specimens and is dominated by illite (up to 25%), followed by kaolinite and sericite (hydromica). Abundant calcite cement is present (Figure 2c), and limonite and related Fe-oxyhydroxides are disseminated within the matrix (Figure 2d).
The braided distributary channel sands on the delta plain and the subaqueous distributary channel sands on the delta front constitute the principal uranium-bearing reservoirs, dominated by sandstone and pebbly sandstone with relatively good sorting and roundness (Figure 2).
Oxidized sandstones can be subdivided into red-oxidized and yellow-oxidized varieties. They commonly occur in coarser-grained rocks—principally sandy conglomerates and sandstones across a range of grain sizes—with low matrix contents, poor to moderate sorting, and slightly higher textural and compositional maturity. Their colors are chiefly purplish-red (Figure 3a), reddish-brown (Figure 3b), and yellow (Figure 3c). Mineralogically, cements are pervasively hematitized and limonitized; in incompletely oxidized sandstones, relict primary gray patches may still be observed.
Reduced sandstones are dominated by (pebble-bearing) coarse sandstone, medium- and fine-grained sandstone, and conglomerate. They contain slightly less cement and are poorly to moderately sorted. Colors are mainly gray, dark gray, and pale grayish-green (Figure 3d). Diagnostic mineralogical features include carbonized plant debris (Figure 3e) and cubic pyrite (Figure 3f).
Reservoir quality parameters (e.g., sand body thickness, net-to-gross, porosity and permeability) were compiled from integrated core descriptions, routine core-plug measurements and calibrated well-log interpretation. These parameters are summarized in Figure 4 and subsequent stratigraphic panels.

4.2. Geochemical Characteristics of Rocks

A total of 269 samples were analyzed (gray sandstone group, n = 195; yellow sandstone group, n = 44; red sandstone group, n = 30). Because some parameters were not measured for all samples and an outlier screening step was applied, the effective sample count for each proxy is reported in Table 1.
Redox-environmental proxies for oxidized sandstones show that red oxidized samples contain up to Fe3+ = 3.84% and Fe2+ = 2.47%, whereas yellow oxidized samples contain up to Fe3+ = 5.99% and Fe2+ = 1.71%. Here, Fe3+ and Fe2+ denote the contents of Fe in different valence states (reported as wt% Fe, rather than oxide wt%). In addition, red oxidized sandstones have mean organic carbon of 0.97%, mean S2− of 0.17%, mean CO2 of 3.51%, and mean ΔEh of 30.83; the corresponding mean values for yellow oxidized sandstones are 1.38%, 0.30%, 5.12%, and 42, respectively (Table 1). CO2 represents carbonate-derived CO2 measured as a bulk proxy, and ΔEh is a composite redox index derived from the measured Fe and sulfur species.
Reduced sandstones are characterized by Fe3+ contents up to 2.91% (mean 0.99%) and Fe2+ contents up to 2.73% (mean 0.92%), both far lower than in red and yellow oxidized sandstones. In addition, organic carbon reaches 6.32% (mean 0.57%); total sulfur reaches 4.98% (mean 1.24%); sulfide sulfur (S2−) reaches 2.16% (mean 0.56%); CO2 reaches 19.09% (mean 4.13%); and ΔEh reaches 90 (mean 41.11) (Table 1). These signatures are closely linked to the abundance of reducing phases—notably carbonized plant debris and pyrite—within the reduced sandstones.

4.3. Occurrence Characteristics of Uranium Minerals

SEM observations indicate that uranium minerals occur mainly within detrital feldspar grains (especially plagioclase) and along feldspar grain margins. The dominant uranium minerals are pitchblende and uraninite (Figure 5). These minerals appear as dot- to line-like precipitates along cleavage traces at grain edges, as film-like coatings carpeting entire cleavage planes (Figure 5a), and as infills of dissolution/etch pits on feldspar surfaces (Figure 5b).
A portion of the uranium minerals resides within intergranular pores, commonly in association with cement. Where pore sizes are relatively uniform, U minerals are uniformly disseminated as fine to very fine grains within the cement; where pores are larger, U minerals occur primarily as pore-filling aggregates (Figure 5c).
Uranium minerals also occur within plant detritus. After gel-like (colloform) pyrite replacement of plant cell walls, the original cellular fabric is preserved, and uranium precipitates are observed around the margins of the colloform pyrite or within the plant cell lumina (Figure 5d).

5. Discussion

5.1. Mineralization-Controlling Mechanism of the Sedimentary System

(1)
Types of sedimentary systems
Based on core logging and log facies analysis from 248 boreholes in the Tamusu deposit, the upper member of the Lower Cretaceous Bayingobi Formation (K1b2) in the Bayingobi Basin is interpreted as a fan-delta–lacustrine system, rather than the traditionally inferred braided-river delta system. This conclusion is supported by core-observed sedimentary characteristics, petrological features, and established sedimentological principles [52,53,54,55,56].
The study interval formed in a fault-controlled lacustrine basin, a setting typical of fan-deltas, characterized by steep slopes and proximal sediment sources [52,53]. In contrast, braided-river deltas generally develop in tectonically stable settings with gentle slopes and distant sediment sources [52,53]. Consistent with this, the depositional system is divided into fan-delta plain and fan-delta front subfacies, with prodelta that is only locally developed. The fan-delta plain is dominated by coarse clastics such as conglomerate and pebbly sandstone, and exhibits prominent exposure indicators, including calcareous concretions, root traces (rhizoliths), and invertebrate fossils [57], attesting to episodic subaerial exposure [52]. These features, together with gypsum cementation, are all diagnostic of proximal fan-delta environments [52]. Petrologically, K1b2 sandstones are dominated by arkosic sandstone (86.4%) with granite-dominated lithic fragments (Figure 2), indicating a short transport distance and proximal sediment source [52], consistent with fan-delta characteristics. Braided-river deltas rarely exhibit widespread gypsum cementation and typically have higher sandstone maturity due to their distal setting [53]. Additionally, abundant calcite cement and pervasive limonitization in the sandstones further reflect the arid sedimentary environment of the fan delta [52].
Widespread sandy–gravelly gravity flow beds, synsedimentary slump structures, and inverse grading (observed in both fan-delta plain and front) further confirm a fan-delta, as these are typical of high-energy, proximal fault-controlled systems [53,54]. The principal uranium-bearing reservoirs (braided distributary channel and subaqueous distributary channel sands) are dominated by sandstone and pebbly sandstone with relatively good sorting and roundness, while oxidized and reduced sandstones are mainly coarse-grained with poor to moderate sorting—consistent with rapid proximal accumulation of fan-deltas [52]. In contrast, braided-river deltas are dominated by traction currents, where gravity flow deposits and slumps are rare, and their sandstones generally have more stable sorting due to longer transport [54]. Additionally, fan-delta sand bodies are enclosed by lacustrine mudstones, showing a sharp terrestrial–lacustrine transition consistent with fan-delta geometry, unlike the gradual transition of braided-river deltas [53].
In summary, the fault-controlled setting, fan-delta subfacies division, proximal coarse lithofacies, petrological characteristics (arkosic sandstone dominance, calcite cement, limonitization), and widespread gravity flow features confirm a fan-delta–lacustrine system. This revises traditional understanding and provides a sedimentological basis for subsequent reservoir and ore distribution analysis.
(2)
Favorable zone for uranium reservoirs
As described above, these lacustrine deposits form overlying and underlying aquitards to the fan-delta sand bodies, establishing a favorable “mud–sand–mud” reservoir–seal assemblage. This architecture is highly conducive to uranium mineralization, as it facilitates the focused migration of mineralizing fluids and promotes uranium accumulation [3,24,58].
On the basis of a regional marker bed between K1b2-1 and K1b2-2 and six lacustrine flooding surfaces, the upper member of the Bayingobi Formation is subdivided into eight parasequence sets: K1b2-1, K1b2-2, K1b2-3, K1b2-4, K1b2-5, K1b2-6, K1b2-7, and K1b2-8 (Figure 6). Considering the stratigraphic positions of uranium orebodies, four principal ore-bearing horizons are identified (K1b2-2, K1b2-3, K1b2-4, and K1b2-5), with marked vertical differences in metallogenic potential (Figure 6).
In the Tamusu area, sand body thickness, conglomerate thickness and sand content (net-to-gross) within the upper Bayingobi Formation parasequence sets K1b2-2, K1b2-3, K1b2-4 and K1b2-5 exhibit a consistent NW–SE trend (Figure 6). Sandstone-type uranium orebodies are concentrated near the distal termini of sand body highs, particularly where sand body thickness decreases abruptly (pinch-out positions), suggesting stabilization of redox fronts at permeability contrasts. The scale of mineralization is closely coupled to reservoir development: K1b2-2 and K1b2-3 host the most extensive and laterally connected sand body systems and correspondingly the largest mineralization footprints; up-section, sand body scale decreases and mineralization becomes weaker and more discontinuous. Overall, mineralization is chiefly hosted in K1b2-2 to K1b2-5, with K1b2-2 and K1b2-3 being more intensely mineralized, with broader ore distribution and more tabular industrial orebodies, whereas K1b2-4 and K1b2-5 contain smaller and more scattered economic orebodies. Stratigraphically, repeated mud–sand–mud stacking associated with base-level cycles provides alternating conduits and aquitards, favoring both focused flow and preservation of the interlayer oxidation zonation.
(3)
Mineralization-controlling mechanism of the fan-delta
The development of sandstone-type uranium mineralization in the Tamusu deposit is controlled by the fan-delta depositional system: the vast majority of orebodies occur within the fan-delta front, with fewer number of small-scale orebodies in the delta plain, and minor syndepositional mudstone-hosted uranium orebodies in the lakeshore to shallow lacustrine facies (Figure 4).
Within the upper Bayingobi Formation, uranium mineralization is restricted to the middle lithologic interval where the fan-delta depositional system is relatively well developed, reflecting the presence of large-scale reservoir sand bodies. The lower and upper lithologic intervals are barren, attributable to a greater lacustrine extent, smaller fan-delta scale, and consequently limited reservoir development (Figure 6 and Figure 7). In parasequence sets K1b2-2 and K1b2-3, braided distributary channel sands on the fan-delta plain and subaqueous distributary channel sands on the delta front are prominently vertically stacked; sand body thickness is 150–260 m, net-to-gross is 0.55–0.65, porosity is 15%–22%, and permeability is 10–50 mD. These constitute the most favorable uranium reservoirs; tabular industrial orebodies account for ~78% of all economic orebodies in the deposit. In contrast, in K1b2-4 and K1b2-5, the fan-delta contracts, sand body thickness decreases to 80–150 m, net-to-gross declines to 0.45–0.55, petrophysical quality deteriorates (permeability 5–15 mD), and industrial orebodies are sparse, with the overall mineralization scale being only about one-third of that in K1b2-2 to K1b2-3. Thickness and net-to-gross values are derived from integrated well-log interpretation and stratigraphic correlation, whereas porosity and permeability are compiled from routine core-plug measurements (summarized in Figure 4).
From the standpoint of fan-delta spatial architecture, sedimentation also governs the morphology of interlayer oxidation, and the fan-delta sand bodies and uranium orebodies exhibit a clear spatial coupling. From the delta plain through the delta front to the prodelta and lacustrine settings, the degree of sand body development and the intensity of oxidation decrease successively; burial depth and oxidation penetration depth increase; heterogeneity strengthens; and the abundance of reducing media progressively increases. The redox zonation is systematic: the oxidized zone is developed mainly on the delta plain, the redox transition zone (front) is concentrated in the distal delta plain and proximal delta front, and the reduced zone predominates in the distal delta front. Sandstone-hosted uranium is chiefly enriched in the transitional belt between the fan-delta plain and fan-delta front, reflecting the control of distributary bifurcation on the routing of mineralizing fluids, whereas mudstone-hosted uranium occurs mainly within the fine-grained distal-delta deposits (Figure 7).
In addition, the proposed “dual-source supply” of uranium is closely linked to fan-delta sedimentation. During the syndepositional stage, weathering products of uranium-bearing granitic rocks from the Zongnaishan uplift were transported basinward by the fan-delta system, producing mildly pre-enriched U-bearing sandstones in the upper Bayingobi Formation (mean U ≈ 5.30 μg/g [59]). This background enrichment provides a necessary uranium inventory for subsequent mobilization and redistribution in sandstone-type systems [6,10,11,60]. In the later ore-forming stage, oxygenated U-bearing fluids introduced through erosional windows migrated along permeable fan-delta sand bodies and precipitated at redox transition zones through interaction with reducing media.

5.2. Interlayer and Uranium Mineralization

At the Tamusu deposit, the spatial distribution of the interlayer oxidation zone exhibits pronounced vertical differentiation and systematic plan-view patterns. Its morphology, thickness, and internal zonation are closely tied to the distribution and permeability of fan-delta sand bodies. Uranium mineralization is concentrated in the redox transition zone, reflecting that sand bodies control the oxidation zone, and the transition zone controls the ore.
(1)
Distribution characteristics of the interlayer oxidation zone
Vertically, the geometry of the interlayer oxidation zone evolves gradationally with changes in fan-delta subfacies and overall displays a trend of thickening northward and thinning southward (Figure 8). Morphologies commonly transition from banded to fingered and sinuous forms (Figure 8). The interlayer oxidation zone extends from north to south, with its degree of development governed by sand body thickness and rock permeability. Lithologies are dominated by brownish-yellow to reddish-brown coarse, medium, and fine grains, characterized by speckled limonitization and locally microcrystalline calcite infill.
In the northern part of the deposit, epigenetically oxidized sandstones occur chiefly within fan-delta plain distributary channel sand bodies that stack vertically, reaching thicknesses of up to 220 m; here, the interlayer oxidation zone is thick and tabular. Toward the central area, the interlayer oxidation zone is developed mainly within fan-delta front subaqueous distributary channels and mouth-bar sand bodies, forming sand–mud interbeds vertically; oxidized sandstone thickness generally ranges 50–180 m. The interlayer oxidation zone remains layered overall, but branches along its upper and lower flanks, and reduced sandstone lenses may occur within it, producing variable geometries and an overall thinning trend (Figure 8). In the south-central sector, the interlayer oxidation zone is principally hosted by mouth-bar and sheet-sand microfacies of the fan-delta front, or by thin sand bodies of the prodelta subfacies; vertically, thick mudstone interbedded with thin sandstone predominates. Here, oxidized sandstone is typically several to tens of meters thick, and the interlayer oxidation zone commonly transitions abruptly to fingered or sinuous forms or pinches out against gray to dark-gray mudstone (Figure 8).
In plan-view, the interlayer oxidation zone follows the fan-delta trend (NW–SE) and exhibits a belt-like concentric zonation from the oxidized zone though the transition zone to the reduced zone, with uranium mineralization strictly confined to the redox transition zone. The oxidized zone is concentrated along the frontal part of the Zongnaishan slope belt in the northwest of the deposit, corresponding to braided distributary channel sands of the fan-delta plain. Oxidation is thorough, sandstone uranium contents are below background (<2 × 10−6 by mass), and mineralization is absent. The oxidized zone is ~1.5–2.0 km wide; the oxidation front trends are approximately E–W, locally showing irregular sinuous traces influenced by distributary bifurcation. The redox transition zone, the core mineralized belt, occupies the central part of the deposit and corresponds to sand bodies from the distal fan-delta plain to the proximal fan-delta front, exhibiting highly variable widths. In the north, the transition zone exceeds 2.0 km in width, and its front trace extends more than 7.0 km; in the south, the contraction of sand body scale narrows the belt to ~800 m, forming a NW-bulging, dumbbell-like geometry (Figure 9). Sandstones within the transition zone exhibit mixed oxidized–reduced features, with hematitization coexisting with pyrite; uranium grades reach 0.01–0.33 wt%, with an industrial cutoff of 0.01 wt%. The reduced zone lies in the southeast of the deposit, corresponding to the distal fan-delta front to prodelta mudstones. Sandstones in this zone are unoxidized, gray to dark-gray, and rich in organic matter and pyrite.
(2)
Constraints of the interlayer oxidation zone on uranium mineralization
Within the gray lithologies of the upper Bayingobi Formation, two epigenetic oxidation types are recognized—red oxidation and yellow oxidation—and hematitization, limonitization, goethite, and jarositization (K-jarosite) are common; these are characteristic indicators of interlayer oxidation [6,25,61,62,63]. Within K1b2-2, K1b2-3, K1b2-4, and K1b2-5 of the upper Bayingobi Formation, the interlayer oxidation zones exhibit pronounced internal zonation, allowing clear differentiation of oxidized, transition, and reduced zones; uranium orebodies are controlled by the transition zone and the position/trace of the oxidation front (Figure 8, Figure 9 and Figure 10). Uranium enrichment in the Tamusu deposit is closely associated with organic matter and pyrite. In fine-grained sandstones, pyrite and plant detritus (organic matter) exert a strong influence on uranium precipitation, consistent with an interlayer oxidation-controlled redox mineralization model [5,21,64,65,66,67,68,69].
In the upper Bayingobi Formation at Tamusu, three lithologic intervals stack vertically to form a stable mud–sand–mud architecture. The interlayer oxidation zone is developed chiefly within the second interval, and its extent is controlled primarily by the permeability of the sand bodies in that interval. Subaqueous distributary channel sandstones of the fan-delta front exhibit relatively high porosity and permeability; as oxygenated, U-bearing waters flow through these permeable horizons, dissolved O2 is progressively consumed, establishing an interlayer redox transition zone. The interlayer oxidation zone propagates from north to south, with decreasing thickness and progressively shallower burial. In the northern part of the deposit, the oxidized sandstone thickness is broadly coextensive with the aquifer thickness; southward, the oxidized sand bodies thin, become thinner than the aquifer interval, and ultimately pinch out.
Within fluvial channel belts, the oxidized zone develops as multilayered, belt-like bodies, composed mainly of brownish-yellow to reddish-brown coarse-, medium-, and fine-grained sandstones that show disseminated limonitization or limonitic speckling; sandstones in the redox transition zone display variable degrees of epigenetic alteration. Comparative analyses of Fe3+, Fe2+, organic carbon (TOC), CO2, sulfide sulfur (S2−), total sulfur (S_total), ΔEh, and ore U grade among strongly red oxidized sandstones, yellow oxidized sandstones, mineralized sandstones in the transition zone, reduced sandstones, mineralized mudstones, and barren mudstones indicate that the oxidized zone is characterized by elevated Fe3+ and CO2 (Figure 10). Yellow oxidized sandstones exhibit slightly higher Fe3+, S2−, and U than red oxidized sandstones, implying that during groundwater infiltration and basinward migration from the basin margin, pyrite in gray sandstones is oxidized, Fe2+ in the sand bodies is progressively converted to Fe3+ (rock colors redden to red/reddish-brown), and partial Fe loss occurs—supplying iron to deeper basin sand bodies, where pyrite forms within the redox transition zone and sandstone-type uranium deposits precipitate (Figure 8). Sulfur derived from surface waters and from pyrite oxidation in gray sandstones (reddening) is transformed into SO42−, which migrates basinward and is reduced to pyrite in the redox transition zone [70,71,72,73]. By contrast, the redox transition zone (ore zone) is distinguished by higher Fe2+, Fe2+/Fe3+, TOC, S_total, ΔEh, and U, together with lower Fe3+ and CO2 and moderately elevated S2−, indicating that pyrite and carbonized plant debris served as key reductants for uranium mineralization in the sandstones [32].
By regulating the flow paths of oxygenated, U-bearing waters, the redox reaction interface (oxidation front), and the precipitating media (reducing agents), the interlayer oxidation zone dictates the spatial distribution and intensity of uranium mineralization. Its controlling influence is expressed in three aspects: the geochemical signatures of oxidation zonation, the mechanisms of U precipitation, and the coupling between interlayer oxidation processes and the depositional architecture [17,18,37,74,75]. Previous studies indicate that, after deposition of the upper Bayingobi Formation, the paleoclimate remained persistently arid and hot. Under relatively closed, ponded-water conditions, intense water–rock interaction and decarbonation (CO2 loss/consumption) destabilized and dissociated uranyl–carbonate complexes—notably [UO2(CO3)3]4− and [UO2(CO3)3]2−—as well as double salts such as MgCO3·NaUO2(CO3)2, thereby promoting uranium precipitation. Under diffusion-dominated transport, dissolved U tended to migrate toward zones of stronger water–rock interaction, leading to focused accumulation at specific stratigraphic horizons [36,76,77,78,79,80].

5.3. Ore Deposit Genetic Model

In synthesis, the Tamusu uranium deposit is controlled by the interlayer oxidation zone and a typical interlayer oxidation-type sandstone uranium deposit. Although subsequent hydrothermal overprinting modified the deposit, it did not produce further uranium enrichment. Accordingly, the metallogenic evolution can be divided into three stages: synsedimentary pre-enrichment, interlayer oxidation mineralization, and epigenetic hydrothermal overprinting [35,81] (Figure 11).
(1)
Syngenetic sedimentary pre-enrichment stage
Previous studies indicate that the clastic detritus of the upper Bayingobi Formation at the deposit is derived chiefly from U-rich granites of the Zongnaishan source terrane to the north. Granites of different ages in this source area contain elevated U: Late Caledonian intrusions average (2.3–3.3) × 10−6, and Variscan intrusions average (3.0–4.5) × 10−6 [40]. These sources supplied U-enriched detritus during basin sedimentation. The fan-delta sand bodies of the upper Bayingobi Formation constitute favorable uranium reservoir space, providing the uranium source foundation for primary enrichment and syndepositional uranium mineralization (Figure 7 and Figure 11a).
Gray sediments formed under different depositional settings in the upper Bayingobi Formation constitute the material foundation for U-enriched strata. During deposition of distributary channels on the fan-delta plain and front, U-rich detritus derived from the source area built the channel sand bodies. The gray channel sandstones contain abundant carbonized plant debris, pyrite, and humic horizons or organic laminae that act as reducing media, exerting adsorptive, complexing, and reductive effects on uranium. This favors pre-enrichment of uranium and the formation of U-enriched strata [3,6,19,43,82,83,84]. Reducing media are more abundant in fan-delta-front distributary channel sands than in plain distributary channel sands, leading to a higher degree of initial uranium enrichment.
(2)
Interlayer oxidation stage
Sandstone-type uranium deposits generally undergo two principal stages: a depositional–burial stage of U-bearing successions and an uplift-driven ore-forming stage of the uranium reservoir. From the depositional preconditioning stage to the ore-forming enrichment stage, the system is invariably modified by a major tectonic reorganization event [2]. Tectonic activity exerts a profound, multi-dimensional control on U enrichment and mineralization, with its influence permeating uranium sourcing and fluid migration [3,6,43,82,83,84,85].
Following deposition of the Bayingobi Formation in the Tamusu area, regional uplift led to prolonged subaerial exposure and intense weathering–denudation of the northern source terrane and the upper member of the Bayingobi Formation. Concurrently, the paleoclimate shifted from humid to arid–semiarid, accelerating uranium mobilization in the source area [38,39,86]. Oxygenated, U-bearing waters infiltrated southward along fan-delta sand bodies, establishing multiple subparallel interlayer oxidation zones that propagated from north to south within the upper Bayingobi sandstones (Figure 8, Figure 9 and Figure 11b). During transport, these waters carried externally supplied U from the source area and also leached pre-enriched U from the strata, providing a dual uranium supply that accelerated mineralization. Reducing media—chiefly organic matter and pyrite—enriched during deposition in the upper Bayingobi Formation constructed effective geochemical reduction barriers (redox traps), favoring U precipitation and accumulation (Figure 3e,f, Figure 7 and Figure 10). In addition, ingress of reducing fluids from the basin center and deeper levels further enhanced the reducing capacity, generating high-contrast redox interfaces. With progressive interlayer oxidation, uranium continued to precipitate and concentrate, ultimately culminating in the formation of the super-large Tamusu sandstone-hosted uranium deposit (Figure 11b).
(3)
Late hydrothermal overprint and preservation stage
Hydrothermal fluids often promote the dissolution of primary minerals and their subsequent precipitation and enrichment [87,88,89,90]. The abundance of hydrothermal minerals, including selenides, has made the role of hydrothermal activity in uranium mineralization a key topic of debate in the Tamusu deposit [41,45,46]. Shallow seismic interpretation indicates that, although these faults do not directly cut the ore-bearing sand bodies, they extend upward into shallow mudstones of the upper Bayingobi Formation and downward to deep fluid reservoirs, facilitating mixing between deep thermal brines and shallow oxygenated waters [76]. It should be noted that no throughgoing master fault is developed within the deposit area; hydrothermal overprinting is restricted to fault-controlled localities and did not establish a basin-scale convective circulation system [91].
Petrographic observations show no paragenetic association between hydrothermal minerals and uranium minerals: uraninite and coffinite occur mainly within the interlayer oxidation transition zone, whereas hydrothermal minerals predominantly infill unmineralized pores, confirming that hydrothermal fluids did not contribute to uranium enrichment [91]. In terms of timing, U mineralization ages indicate a main ore-forming stage in the late Early Cretaceous (115.5 ± 1.5 Ma to 109.7 ± 1.5 Ma), whereas hydrothermal overprinting is concentrated in the Late Cretaceous–Paleogene (70.9 ± 1.0 Ma to 45.4 ± 0.6 Ma). This pronounced temporal offset further demonstrates that hydrothermal fluids did not participate in the principal mineralization and represent only a late superimposed alteration [41,65]. These fluids mainly precipitated carbonate/fluorite/gypsum cements, altered redox-sensitive minerals, and tightened the pore network, thereby enhancing local sealing and preservation of pre-existing uranium minerals [65].
In summary, the Tamusu uranium deposit underwent interlayer oxidation mineralization followed by superimposed hydrothermal overprinting, with the hydro-thermal fluids not affecting any further uranium enrichment. Late hydrothermal/diagenetic circulation likely post-dated the main interlayer oxidation mineralization and was promoted by regional thermal events and fault reactivation. Where fractures provided pathways, limited local remobilization and reprecipitation may have occurred, but the dominant effects are overprinting and preservation of the established redox architecture (Figure 11c).

6. Conclusions

  • The super-large Tamusu sandstone-hosted uranium deposit is hosted chiefly in parasequence sets K1b2-2 and K1b2-3 of the upper Bayingobi Formation. The principal depositional facies are the fan-delta front and plain; braided distributary channel sands on the plain and subaqueous distributary channel sands on the front constitute the main uranium reservoirs.
  • The fan-delta depositional system is the first-order ore-controlling factor at Tamusu: it provides the main conduits and storage space for the migration and accumulation of U-bearing fluids and controls the distribution of the interlayer oxidation zone, thereby establishing favorable permeability pathways and redox traps for uranium precipitation.
  • The metallogenic evolution can be divided into (i) syndepositional uranium pre-enrichment associated with fan-delta sedimentation, (ii) interlayer oxidation mineralization driven by oxygenated groundwater flow, and (iii) late hydrothermal or diagenetic overprint that mainly modified reservoir properties and enhanced sealing/preservation of the established ore system. Uranium mineralization occurred primarily during stages (i)–(ii).

Author Contributions

Conceptualization, C.L., Z.Z. and Y.J.; methodology, C.L.; validation, Z.L., X.Y.; formal analysis, C.L., Z.Z., Z.L. and X.Y.; data curation, Y.Z.; writing—original draft preparation, C.L., Z.Z. and Q. Wang; writing—review and editing, C.L., Y.J. and Q.W.; visualization, C.J., C.Z. and W.Z.; project administration, C.J., C.Z. and W.Z.; supervision, Y.Z.; funding acquisition, C.L., Z.L. and Q.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was financially supported by the Uranium Geological Project of China Nuclear Industry Geological Bureau (202201), the National Key Research and Development Program of China (Grant No. 2023YFC2906705), the funding project of the Northeast Geological S&T Innovation Center of China Geological Survey (No. QCJJ2024-08), the Open Fund of the Key Laboratory of Tectonics and Petroleum Resources, Ministry of Education, China University of Geosciences (Wuhan) (No. TPR-2024-04).

Data Availability Statement

Data is contained within the article.

Acknowledgments

We thank Longhui Wang (No. 208 Geological Party, China National Nuclear Corporation, CNNC) for assistance with field geological survey and sample collection, and the Analytical Testing Center of the No. 208 Geological Party (CNNC) for support in sample analysis and testing.

Conflicts of Interest

Zhao Li is an employee of the Research Institute of Exploration and Development, PetroChina Changqing Oilfield Company, Xiaoyi Yuwen is an employee of the Liaoning Nuclear Industry Geology 242 Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. Tectonic background (a), seismic profile (b) and geological (c) maps of the Tamusu deposit. 1—Quaternary; 2—Upper Cretaceous Ulan Su Hai Formation; 3—Upper Bayingolin Formation (Lower Cretaceous); 4—Lower Bayingolin Formation (Lower Cretaceous); 5—Middle-Lower Jurassic; 6—Middle-Lower Permian; 7—Carboniferous; 8—Paleoproterozoic; 9—Archean; 10—Triassic granite; 11—Permian granite; 12—Permian granodiorite; 13—Carboniferous diorite; 14—Silurian granite; 15—Ordovician diorite; 16—geological boundary/angle unconformity boundary; 17—fault structure; 18—surface uranium mineralization points and numbering/airborne anomaly points and numbering; 19—Tamuz uranium deposit area; 20—place names.
Figure 1. Tectonic background (a), seismic profile (b) and geological (c) maps of the Tamusu deposit. 1—Quaternary; 2—Upper Cretaceous Ulan Su Hai Formation; 3—Upper Bayingolin Formation (Lower Cretaceous); 4—Lower Bayingolin Formation (Lower Cretaceous); 5—Middle-Lower Jurassic; 6—Middle-Lower Permian; 7—Carboniferous; 8—Paleoproterozoic; 9—Archean; 10—Triassic granite; 11—Permian granite; 12—Permian granodiorite; 13—Carboniferous diorite; 14—Silurian granite; 15—Ordovician diorite; 16—geological boundary/angle unconformity boundary; 17—fault structure; 18—surface uranium mineralization points and numbering/airborne anomaly points and numbering; 19—Tamuz uranium deposit area; 20—place names.
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Figure 2. Petrological characteristics of the upper member of the Bayingobi Formation (K1b2). (a) Core photograph of feldspathic sandstone; (b) core photograph of lithic feldspathic sandstone; (c) cross-polarized photomicrograph of grayish-white medium-grained sandstone showing feldspar dissolution and calcite cement; (d) yellow sandstone with pervasive limonitization infilling interstices. Abbreviations: Mi, microcline; Qz, quartz; Det, detritus; Lmt, limonitization; Cc, calcite.
Figure 2. Petrological characteristics of the upper member of the Bayingobi Formation (K1b2). (a) Core photograph of feldspathic sandstone; (b) core photograph of lithic feldspathic sandstone; (c) cross-polarized photomicrograph of grayish-white medium-grained sandstone showing feldspar dissolution and calcite cement; (d) yellow sandstone with pervasive limonitization infilling interstices. Abbreviations: Mi, microcline; Qz, quartz; Det, detritus; Lmt, limonitization; Cc, calcite.
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Figure 3. Representative sandstone samples of different geochemical types. (a) Purplish-red sandstone; (b) reddish-brown sandstone; (c) yellow sandstone; (d) grayish-black sandstone; (e) carbonaceous clastic sandstone with carbonized plant detritus; (f) pyrite-bearing sandstone. CD, carbonized plant detritus; Py, pyrite.
Figure 3. Representative sandstone samples of different geochemical types. (a) Purplish-red sandstone; (b) reddish-brown sandstone; (c) yellow sandstone; (d) grayish-black sandstone; (e) carbonaceous clastic sandstone with carbonized plant detritus; (f) pyrite-bearing sandstone. CD, carbonized plant detritus; Py, pyrite.
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Figure 4. Porosity and permeability histogram for the Bayingobi Formation sand bodies, Tamusu uranium deposit. The colors in the lithology column chart represent those of sandstone. Reprinted with permission from [51] published by China University of Geosciences Press.
Figure 4. Porosity and permeability histogram for the Bayingobi Formation sand bodies, Tamusu uranium deposit. The colors in the lithology column chart represent those of sandstone. Reprinted with permission from [51] published by China University of Geosciences Press.
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Figure 5. Occurrence characteristics of uranium minerals. (a,b) Coffinite occurs along cleavage, fractures (a) and dissolution pores (b) in detrital plagioclase; (c) pitchblende in pores and microfractures of cements; (d) pitchblende filling fractures in carbonized plant detritus, with associated pyrite. CD, carbonized plant detritus; Cof, coffinite; Pit, pitchblende; Py, pyrite.
Figure 5. Occurrence characteristics of uranium minerals. (a,b) Coffinite occurs along cleavage, fractures (a) and dissolution pores (b) in detrital plagioclase; (c) pitchblende in pores and microfractures of cements; (d) pitchblende filling fractures in carbonized plant detritus, with associated pyrite. CD, carbonized plant detritus; Cof, coffinite; Pit, pitchblende; Py, pyrite.
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Figure 6. Stratigraphic framework, sedimentary systems and distribution of orebodies in the Tamusu uranium deposit.
Figure 6. Stratigraphic framework, sedimentary systems and distribution of orebodies in the Tamusu uranium deposit.
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Figure 7. Uranium mineralization controlled by genetic facies within the fan-delta system of the Tamusu fault depression basin.
Figure 7. Uranium mineralization controlled by genetic facies within the fan-delta system of the Tamusu fault depression basin.
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Figure 8. Geological cross-section along exploration lines in the Tamusu uranium deposit. The arrow in the picture represents the borehole.
Figure 8. Geological cross-section along exploration lines in the Tamusu uranium deposit. The arrow in the picture represents the borehole.
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Figure 9. Relationships among sedimentary systems, oxidation zones and uranium mineralization in different parasequence sets of the Tamusu uranium deposit. (a–d) parasequence sets: (a)K1b2-2, (b)K1b2-3, (c)K1b2-4, (d)K1b2-5.
Figure 9. Relationships among sedimentary systems, oxidation zones and uranium mineralization in different parasequence sets of the Tamusu uranium deposit. (a–d) parasequence sets: (a)K1b2-2, (b)K1b2-3, (c)K1b2-4, (d)K1b2-5.
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Figure 10. Oxidation–reduction zonation and environmental geochemical characteristics of sandstones in the Tamusu uranium deposit. Py, pyrite; Cal, calcite; Chl, chlorite; C-org, organic carbon; Hem, hematite; Kao, kaolinite; Ill, illite. Reprinted with permission from [51] published by China University of Geosciences Press.
Figure 10. Oxidation–reduction zonation and environmental geochemical characteristics of sandstones in the Tamusu uranium deposit. Py, pyrite; Cal, calcite; Chl, chlorite; C-org, organic carbon; Hem, hematite; Kao, kaolinite; Ill, illite. Reprinted with permission from [51] published by China University of Geosciences Press.
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Figure 11. Conceptual three-stage model for uranium mineralization in the Tamusu deposit. (a) Uranium pre-enrichment and syngenetic sedimentary uranium mineralization stage; (b) Structural inversion-interlayer oxidation main uranium mineralization stage; (c) Low-temperature hydrothermal ore-preserving stage.
Figure 11. Conceptual three-stage model for uranium mineralization in the Tamusu deposit. (a) Uranium pre-enrichment and syngenetic sedimentary uranium mineralization stage; (b) Structural inversion-interlayer oxidation main uranium mineralization stage; (c) Low-temperature hydrothermal ore-preserving stage.
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Table 1. Statistical results of geochemical environmental proxies for different sandstones.
Table 1. Statistical results of geochemical environmental proxies for different sandstones.
LithologyStatistical ValuesTOC (%)S (wt%)S2− (wt%)CO2 (wt%)ΔEh (Index)Fe3+ (wt% Fe)Fe2+ (wt% Fe)
Gray Sandstone GroupAverage0.571.240.564.1341.110.990.92
Max6.324.982.1619.0990.002.912.73
Min0.010.010.010.011.000.050.06
Count182182181178116195195
Yellow Sandstone GroupAverage1.380.280.305.1242.001.890.63
Max4.042.121.8916.0247.005.991.71
Min0.120.010.010.2635.000.420.23
Count4428444484444
Note: CO2 represents carbonate-derived CO2 measured as a bulk proxy. ΔEh is a composite redox index (dimensionless) derived from measured Fe and sulfur species. The statistical results exclude samples with values outside mean ± 3 × SD.
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Lu, C.; Zhang, Z.; Jiao, Y.; Li, Z.; Yuwen, X.; Zhuang, Y.; Jin, C.; Zhang, C.; Zhong, W.; Wang, Q. Key Ore-Controlling Factors and Genetic Model of the Tamusu Super-Large Sandstone-Type Uranium Deposit, Bayingobi Basin. Minerals 2026, 16, 357. https://doi.org/10.3390/min16040357

AMA Style

Lu C, Zhang Z, Jiao Y, Li Z, Yuwen X, Zhuang Y, Jin C, Zhang C, Zhong W, Wang Q. Key Ore-Controlling Factors and Genetic Model of the Tamusu Super-Large Sandstone-Type Uranium Deposit, Bayingobi Basin. Minerals. 2026; 16(4):357. https://doi.org/10.3390/min16040357

Chicago/Turabian Style

Lu, Chao, Zhongyue Zhang, Yangquan Jiao, Zhao Li, Xiaoyi Yuwen, Yinan Zhuang, Chengyuan Jin, Chengcheng Zhang, Weihui Zhong, and Qilin Wang. 2026. "Key Ore-Controlling Factors and Genetic Model of the Tamusu Super-Large Sandstone-Type Uranium Deposit, Bayingobi Basin" Minerals 16, no. 4: 357. https://doi.org/10.3390/min16040357

APA Style

Lu, C., Zhang, Z., Jiao, Y., Li, Z., Yuwen, X., Zhuang, Y., Jin, C., Zhang, C., Zhong, W., & Wang, Q. (2026). Key Ore-Controlling Factors and Genetic Model of the Tamusu Super-Large Sandstone-Type Uranium Deposit, Bayingobi Basin. Minerals, 16(4), 357. https://doi.org/10.3390/min16040357

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