Typomorphic Characterization and Geological Significance of Megacrystalline Uraninite in the Haita Area, Kangdian Region, Southwestern China

Abstract

Megacrystalline uraninite within Neoproterozoic migmatites in the Haita area of the Kangdian region of China provides a unique condition for the investigation of uraninite typomorphism under high-temperature conditions. The present study represents the first systematic characterization of the typomorphic signatures and genetic significance of megacrystalline uraninite via optical microscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XRS), and electron probe microanalysis (EPMA). The results show that uranium mineralization occurs as euhedral megacrystalline uraninite (black grains ≤ 10 mm) hosted in quartz veins, exhibiting frequent rhombic dodecahedral and subordinate cubic–octahedral morphologies. The paragenetic assemblage is quartz–uraninite–titanite–apatite–molybdenite. The investigated uraninite is characterized by elevated unit-cell parameters and a reduced oxygen index, with complex chemical compositions enriched in ThO₂ and Y₂O₃. These typomorphic characteristics indicate crystallization under high-temperature reducing conditions with gradual cooling. Post-crystallization tectonic fragmentation and uplift-facilitated oxidation occur, generating secondary uranium minerals with concentric color zonation (orange–red to yellow–green halos). Mineralization was jointly controlled by migmatization and late-stage tectonism, with the breakup of the Rodinia supercontinent serving as the key driver of fluid mobilization and ore deposition. The data materialized in the present study improve our knowledge about uranium mineralization during continental breakup events.

1. Introduction

As the primary industrial source of uranium, uraninite has been the focus of extensive research in recent decades, particularly regarding its (1) geochronology and tectonic significance [1,2,3,4,5]; (2) geochemical evolution, including compositional variability, alteration processes, and trace-element distributions [6,7,8,9,10,11]; and (3) geochronometry (e.g., thermal ionization mass spectrometry (TIMS), secondary ion mass spectrometry (SIMS), laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U–Th–Pb dating) and U–Pb substitution mechanisms [12,13,14,15,16,17]. However, typomorphic analysis of natural uraninite remains challenging because of its typical submillimeter grain size (<1 mm), dispersed occurrence, and imperfect crystal development. Consequently, our current understanding of uraninite typomorphism relies heavily on synthetic analogs [18,19,20,21].

The recent discovery of megacrystalline uraninite in the Neoproterozoic migmatites of the Haita area of the Kangdian region (southwestern China) has provided a unique natural laboratory. These crystals exhibit characteristics that have rarely been documented anywhere in the world, including exceptional crystallinity and large grain sizes (black grains ≤ 10 mm) [22,23,24,25,26,27]. Therefore, the present study presents the first systematic investigation of uraninite typomorphism under natural high-temperature conditions. The typomorphic signatures of Haita megacrystalline uraninite are characterized through integrated microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and microprobe analysis to elucidate their implications for uranium mineralization during continental breakup events.

2. Geological Background

The Kangdian region in southwestern China (Figure 1a) represents a polydeformed Precambrian terrane with a complex structural architecture, diverse lithological assemblages, and world-class polymetallic mineralization [28,29,30]. As a critical segment of the Yangtze Craton, it exposes extensive Meso-Neoproterozoic (1.6–0.8 Ga) strata recording multi-stage orogenesis. Polyphase tectono-thermal events have generated composite structural domains hosting economically significant Cu–Fe–U metallogenic belts [30]. Recent exploration has revealed ultra-rich uranium occurrences dominated by megacrystalline uraninite along a 300 km NNW–SSE corridor stretching from Haita (Miyi County) to Shujie (Mouding County) via the Datian area of Panzhihua (Figure 1b). These uranium occurrences contain exceptionally coarse uraninite crystals (≤10 mm)—surpassing global average sizes by an order of magnitude—highlighting the favorable uranium mineralization conditions of the Kangdian region [22].

The study area, located 8.7 km southwest of Miyi County (Figure 1c), experienced extensive metamorphism and migration. The lithology is dominated by medium- to high-grade metamorphic rocks of the Proterozoic Wumaqing strata, with widespread exposure of these metamorphic series, along with carbonate and clastic rocks of the Sinian–Cambrian Dengying Formation. Substantial multi-stage magmatism was widely distributed in the region, characterized by intermediate to felsic compositions from the Jinningian period as well as alkaline intrusives from the Hercynian–Indochina period. The area displays complex folds, ductile shear zones, brittle faults, and ductile faults, primarily controlled by north–south-trending deep faults (e.g., the Mopan Mountain and Anning River faults), with dominant north–south and northeast–southwest tectonic trends. Significant deformation and metamorphism have influenced the rock formations, although surface weathering obscures crystal-related tectonic traces. The Wumaqing Group metamorphic rocks and migmatites host three uranium occurrences (A10, A19, and 2811), with ore-hosting rocks classified according to their degree of migmatization as follows: (1) vein material, (2) migmatized metamorphic rock, (3) migmatite, and (4) vein body, where two-mica quartz schists serve as protoliths for the migmatitic vein material.

A uraninite-rich quartz vein (Haita 2811 Occurrence), which lies beneath the Quaternary cover, formed within dense fissure zones during the migmatization of extensively weathered two-mica quartz schists. The orientation of this vein, with a width of 35–50 cm, aligns with the schistosity of the surrounding rock. This vein contains more than 100 giant uraninite grains with well-faceted euhedral morphologies and large particle sizes; such uraninite-rich quartz veins are rare worldwide. Within the vein and adjacent fissure zones, the maximum γ-ray intensity reaches 6800 Sv, decreasing laterally (Figure 2).

3. Samples and Analytical Methods

Five samples were collected from the uraninite-rich quartz vein for megacrystalline uraninite extraction. Thin sections, polished sections, and electron probe samples were prepared at the Geochemistry Department of Chengdu University of Technology.

Rock/mineral identification, reflectance measurements, and microhardness tests were conducted at the Comprehensive Rock and Mineral Identification Laboratory of Chengdu University of Technology using the following instruments: an Eclipse LV100 POL polarizing microscope (Nikon Corporation, Tokyo, Japan); an AxioScope A1 Pol reflective polarized light microscope (Carl Zeiss AG, Oberkochen, Germany); and an HV-1000NDT Micro Hardness tester (Truer Corporation, Shanghai, China).

XRD analysis was performed using a Bruker D8 Advance diffractometer (Bruker Corporation, Billerica, MA, USA) at Chengdu University of Technology. The instrument was operated with Cu Kα radiation (λ = 1.5406 Å) at 40 kV and 40 mA. Data were collected over a 2θ range of 5–80° with a step size of 0.02° and a scanning speed of 2°/min, using a LynxEye array detector (also by Bruker Corporation). The uraninite (UO₂) sample was finely ground and homogeneously loaded into a glass sample holder, with continuous sample rotation during measurement to ensure data representativeness. Phase identification was conducted using MDI Jade 6.5 software with the ICDD-PDF2 database, achieving a measurement precision of ±0.01° 2θ as verified by the NIST SRM 640c silicon standard.

Electron Probe Microanalysis (EPMA) was conducted at the State Key Laboratory of Nuclear Resources and Environment at East China University of Technology using a JXA-8230 microanalyzer (JEOL Corporation, Tokyo, Japan). All measurements were performed under high vacuum conditions with an accelerating voltage of 15.0 kV, beam current of 20.0 nA, and probe diameter of 1–2 µm. For elemental analysis, peak counting times were set at 10 s for major elements (Al, Mg, Ca, Na, K, Si, Fe) with 5 s background measurements, while trace elements (P, Ti, U, Th, REEs, Y, Pb) employed 20 s peak counting and 10 s background measurements. Calibration standards included natural minerals (garnet for Si, rutile for Ti, dolomite for Ca, hematite for Fe, biotite for Al, galena for Pb, jadeite for Na, orthoclase for K, and monazite for La/Ce/P) and synthetic U-Th-Pb oxides. Analytical uncertainties were <1% for major elements and <10% for trace elements. Data were processed using the ZAF correction procedures following the Chinese National Standard GB/T 15617-2025. Elemental mapping was performed using a EPMA-1600 (Shimadzu Corporation, Kyoto, Japan)instrument at the Tianjin Center of China Geological Survey.

X-ray photoelectron spectroscopy analysis was performed by the Chengdu Scientific Compass Analysis and Testing Company using a Thermo Scientific K-Alpha+ spectrometer with the following parameters: chamber vacuum, ~5 × 10⁻⁹ mbar; monochromated Al Kα source (1486.6 eV); voltage, 15 kV; beam current, 15 mA; CAE analyzer mode; work function, 4.2; and beam diameter, 100 µm. Avantage software 6.9 was used to process the data and determine the uranium valence ratios.

4. Typomorphic Characteristics of Uraninite

4.1. Morphological Characteristics

The naturally formed uraninite exhibits well-faceted euhedral morphologies and large grain sizes (≤1 cm), occurring as discrete crystals within quartz veins (Figure 3a–d). Megacrystalline uraninite displays predominantly euhedral morphologies; rhombic dodecahedra and cubic–octahedral combinations dominate, with occasional pure cubic forms. Crystals are isometric and visibly discernible in the hand specimen (Figure 3a–d). The host quartz appeared smoky gray. Uraninite occurred as isometric granules distributed throughout the veins, with most grains measuring ~0.5 cm and the largest grains reaching ~1 cm. The uranium content amounts typically to 1–5 vol.% but occasionally exceeds 20 vol.% (the uranium content within the quartz vein). The limited sampling (to ~1 m depth) revealed an increasing abundance of uraninite grains with depth, suggesting higher uranium concentrations in deeper zones.

4.2. Physical Properties

The physical typomorphic properties of uraninite reflect its formation conditions through attributes such as color, hardness, and reflectance. These manifestations are fundamentally derived from the chemical composition and crystal structure. Field observations show black megacrystalline uraninite with a metallic luster, steel-gray streaks, and black striations. Strong oxidation results in secondary uranium minerals on the crystal surfaces (Figure 3e), including uranyl hydroxide, uranyl silicate, and uranyl phosphate. These minerals were distributed in concentric bands around the uraninite edges, displaying the characteristic color zonation of orange–red → orange → orange–yellow → yellow → yellow–green from the inner to the outer region, respectively. This sequence reflects progressive oxidation from uranyl hydroxide to uranyl silicates/phosphate, which is consistent with U⁴⁺ → U⁶⁺ transformation. Under intense oxidation, complete conversion to U⁶⁺ minerals preserved the original morphology (Figure 3f). The reflectance measurements (

Table 1. Reflectance of megacrystalline uraninite samples.

No.	Reflectance								Mean
Ht-1	14.455	14.494	13.520	13.823	14.489	14.186	15.019	14.492	14.309
Ht-2	14.756	13.958	13.338	13.802	15.000	14.065	/	/	14.153
Ht-3	14.665	14.328	14.761	13.267	15.647	14.597	15.145	14.167	14.572

“/” indicates no data due to measurement limitations or absence of detectable signal.

) yielded maximum, minimum, and average values of 14.572, 14.309, and 14.300, respectively. Microhardness tests indicated a relatively low hardness of ~330 HV, which was below the typical reference values.

4.3. Crystal Structure

Powder XRD identified the uranium mineral as uraninite (Figure 4). The diffraction intensity is strongest for {111}, followed by {220}, {200}, and {311}, with {222} showing the lowest intensity (

Table 2. Unit-cell parameters and oxygen coefficients of megacrystalline uraninite samples.

No.	Grain Size (nm)	Unit-Cell Parameters a0 (Å)	d	hkl	Rwp	Oxygen Coefficient
HT-01	15.5	5.458	3.1294	{111}	9.844	2.09
			2.7202	{200}
			1.9318	{220}
			1.6435	{311}
			1.5716	{222}
HT-02	16.9	5.457	3.1511	{111}	7.566	2.0975
			2.7202	{200}
			1.9280	{220}
			1.6408	{311}
			1.5716	{222}
HT-03	15.4	5.462	3.1620	{111}	5.387	2.06
			2.7122	{200}
			1.9241	{220}
			1.6381	{311}
			1.5691	{222}
HT-04	15.6	5.461	3.1565	{111}	7.683	2.0675
			2.7162	{200}
			1.9280	{220}
			1.6408	{311}
			1.5716	{222}
HT-05	15.0	5.460	3.1510	{111}	7.928	2.075
			2.7201	{200}
			1.9318	{220}
			1.6407	{311}
			1.5691	{222}

). The unit-cell parameters for five single grains range from 5.457 to 5.462 Å.

In the crystallochemical formula (U⁴⁺, U⁶⁺, Th, rare earth elements (REEs), Pb)O_x, the typical oxygen index (x) is 2.17–2.92 [31,32,33]. According to the relationship between the unit-cell parameters and the oxidation degree (Figure 5), the oxygen index for the megacrystalline uraninite was 2.0600–2.0975.

4.4. Chemical Composition

The compositional ranges of UO₂, PbO, ThO₂, and Y₂O₃ in five megacrystalline uraninite particles were 67.63–83.67% (mean = 77.99%), 0.12–4.42% (mean = 2.64%), 2.50–17.93% (mean = 8.77%), and 0.03–1.72% (mean = 0.43%), respectively (Table S1). The contents of the other elements were below the detection limit of 0.01%. Correlation and linear fitting analyses (Figure 6) indicated negative correlations between UO₂ and SiO₂ (R² =−0.53) and between UO₂ and ThO₂ (R² =−0.87), but positive correlations for PbO (R² =0.32) and Y₂O₃ (R² =0.41). EPMA mapping (Figure 7) shows smaller fragmented particles but a homogeneous U–Th distribution.

4.5. Mineral Assemblage

Integrated field, microscopic, and electron probe analyses revealed a simple mineral assemblage in the uraninite-rich quartz vein. Beyond uraninite and its oxidation-derived secondary minerals, the composition primarily comprised titanite and apatite, with minor amounts of molybdenite and xenotime (Figure 8). Titanite occurred as clustered euhedral stubby prisms (predominantly crystalline, rarely subhedral), with the largest crystals (reaching several millimeters) coexisting with the megacrystalline uraninite. Apatite displays distinctive equant hexagonal crystals (crystalline to subhedral), whereas acicular or elongated forms are absent. Low birefringence resulted in dark interference colors under cross-polarized light, with some grains exhibiting near extinction. Given the scarcity of molybdenite and xenotime, the paragenetic sequence was defined as quartz–uraninite–titanite–apatite–molybdenite [26].

4.6. Valence State Characteristics

Table 3. Valence state characteristics of two representative uraninite samples.

No.	101		102
Valence	U4+	U6+	U4+	U6+
Middle peak	380.46	382.12	380.42	381.99
Peak width	2.03	1.80	1.99	2.05
Atomic ratio	91.57	8.43	90.32	9.68

presents the XPS-derived valence state characteristics of two representative uraninite samples (101 and 102). The data show consistent U⁴⁺ dominance (90.32–91.57 atomic%) with minor but measurable U⁶⁺ components (8.43–9.68 atomic%). The U⁴⁺ binding energy peaks at 380.42–380.46 eV with peak widths of 1.99–2.03 eV, while the U⁶⁺ peaks occur at 381.99–382.12 eV (widths 1.80–2.05 eV). This ~2 eV chemical shift between oxidation states agrees well with the established XPS standards for uranium oxides. The consistent ~10% U⁶⁺ content across samples suggests this minor oxidized component represents an intrinsic feature of the primary uraninite crystallization rather than a post-formational alteration.

As a simple uranium oxide, uraninite primarily occurs as tetravalent uranium (U⁴⁺) with minor U⁶⁺. Valence state variations and their proportions serve as critical proxies for oxygen fugacity (fO₂) in ore-forming melts/fluids. Our data confirmed the exclusive presence of U⁴⁺ within the Haita 2811 occurrence.

5. Discussion

5.1. Geological Significance of Uraninite Typomorphic Characteristics

Crystal morphology and mineral grain size are valuable indicators for elucidating the genetic environment of uraninite [18]. For example, uraninite crystallized in pegmatites predominantly displays a combination of cubic and rhombic dodecahedral crystal forms; uraninite formed in hypothermal deposits primarily exhibits cubic and octahedral crystals, whereas uraninite formed in mesothermal deposits typically yields cubic crystals. The particle size distribution of minerals reflects the stability of the physicochemical conditions during their formation. The fundamental distinction between uraninite and pitchblende lies in the physicochemical gradient during mineralization, with the rate of temperature decline critically controlling mineral morphology. When the temperature decreases slowly, the crystallization rates decrease, favoring uraninite development, whereas rapid cooling promotes pitchblende formation [19]. The synthesis experiments producing octahedral uraninite (2.5 × 3.5 mm) demonstrate that optimal growth requires strongly reducing conditions, a temperature > 450 °C, pressure > 120 MPa, high acidity (pH < 4), and diluted solutions (U concentration < 23.8 mg/mL) [20]. The ratio of tetravalent (U⁴⁺)-to-hexavalent (U⁶⁺) uranium in uraninite (ideal formula UO₂) carries significant geological implications, particularly as a robust indicator of redox conditions. A predominance of U⁴⁺ typically reflects reducing environments, such as those found in deep-seated granites or the primary zones of uranium deposits. This valence state is commonly associated with low oxygen fugacity and high-temperature magmatic or hydrothermal processes [35]. The valence analysis of uraninite from the Haita 2811 occurrence confirms the predominant presence of tetravalent uranium, indicating formation under reducing conditions. The well-developed crystal forms suggest stable physicochemical conditions during growth, which is consistent with crystallization from high-temperature, slightly acidic fluids under slow cooling, implying formation in a deep-seated, high-temperature reducing environment.

Uraninite typically appears black or brownish black. As the formation temperature and pressure decrease, the U⁴⁺/U⁶⁺ ratio progressively declines, causing gradual lightening of the mineral color. For example, uranyl minerals in supergene zones exhibit paler hues than those in redox transition zones. Uranyl hydroxide signals proximal to primary uranium mineralization spatially correlate with uraninite/pitchblende as products of in situ oxidation under weakly basic to alkaline conditions. These uranyl hydroxide minerals form only in high-grade ores with sharp concentration gradients, primarily within hydrothermal or pegmatitic uranium deposits [18,31,32,33]. The non-weathered surfaces of the Haita megacrystalline uraninite retained a uniform black coloration, indicating stable formation conditions. Color zonation (orange–red → orange → orange–yellow → yellow → yellow–green) in the peripheral uranyl hydroxides reflects post-crystallization uplift and oxidation in near-surface environments. Furthermore, the presence of uranyl hydroxide suggests an increase in oxidation intensity at shallower depths. Notably, the measured reflectance and hardness values for megacrystalline uraninite were lower than those of the standard references. Potential causes include the following: (1) analytical errors or sample surface imperfections, (2) reflectance reduction by secondary uranyl minerals from surface oxidation, and (3) late-stage tectonic fracturing, which is consistent with the EPMA mapping observations.

The typical unit-cell parameter range for major Chinese uranium minerals is 5.347–5.488 Å. Uraninite and pitchblende typically exhibit values of 5.423–5.488 Å (mean = 5.456 Å) and 5.346–5.477 Å (mean = 5.405 Å), respectively [31,32,33]. The factors influencing these parameters include temperature, chemical composition, and post-formational oxidation. Elevated temperatures promote the complete reduction of U⁶⁺ to U⁴⁺, establishing a positive correlation between unit-cell parameters and formation temperatures. These parameters also positively correlate with UO₂ and impurity contents (Th, Pb, Y, and rare-earth elements (REEs)), although this relationship remains thermally controlled [18]. Post-crystallization oxidation (U⁴⁺ → U⁶⁺) induces lattice defects and unit-cell shrinkage. The high unit-cell parameters observed in this study (5.457–5.462 Å) confirm high-temperature uraninite formation. Th⁴⁺, Y³⁺, and REE³⁺ exhibit ionic radii similar to U⁴⁺ in hydrothermal fluids, facilitating substitution and lattice expansion, as verified by EPMA. The oxygen index reflects the formation conditions, longevity, and pre- and post-formation alterations; therefore, the values vary with the formation environment and temperature. For example, deep-seated pitchblende has lower oxygen index values than its low-temperature shallow equivalents; endogenic pitchblende shows low values, whereas exogenous varieties display high values; late-stage oxidation increases the oxygen index values; and synthetic U-oxides exhibit values of 2.00–3.00, whereas natural specimens exhibit values of 2.17–2.92 [31,32,33] (Figure 5). The oxygen index values obtained in the present study (2.0600–2.0975) indicate an endogenic origin without late oxidation (Table 2).

Uraninite is highly susceptible to fluid alteration [6,36,37]. REEs, Si, Ca, Fe, Al, and K serve as critical typomorphic indicators for physicochemical conditions [13,15,38,39,40]. Elevated Th contents are observed in metasomatized uraninite because Th and U ions coexist stably at high temperatures during deposit formation, when Th⁴⁺ ion activity is lower than that of U⁴⁺. Upon cooling, Th⁴⁺–U⁴⁺ decoupling occurs, enabling Th⁴⁺ incorporation into mineral structures. Thus, high-temperature uraninite shows Th enrichment. The positive correlations among U, Th, and Y, along with their elevated contents, provide further evidence for the relatively slow growth of uraninite crystals [40]. The U/Th ratio is a key geothermometer: >1000 indicates low crystallization temperatures; <1000 implies high temperatures; and <100 signifies magmatic conditions [10,41,42]. Yttrium behavior parallels that of thorium; its content tracks crystallization/recrystallization processes and correlates positively with the formation temperature [40,43]. U–Pb decay systems enable age determination [44], making U–Pb dating the primary method of isotopic chronology. Consequently, U and Pb contents should theoretically show negative correlations. However, owing to crystal–chemical incompatibility, Pb²⁺ tends to migrate from uraninite during fluid events, and is replaced by Ca²⁺, Si⁴⁺, and Fe²⁺/³⁺ [10,40,45], although these cations do not always participate equally in substitution.

Thus, megacrystalline uraninite shows elevated unit-cell parameters and a reduced oxygen index. According to EPMA data, the high unit-cell parameters can be attributed to the isomorphic incorporation of Th⁴⁺, Pb²⁺, and REEs. U/Th ratios <100 and significant negative U–Th correlations confirm high-temperature Th enrichment. The consistently high Y concentrations further support this interpretation. The observed positive U–Pb correlation may indicate an elevated initial common Pb content and an absence of post-formational fluid alteration. Although mapping suggests late tectonic effects, the EPMA results revealed homogeneous U–Th distributions, confirming the absence of fluid-induced recrystallization.

Titanite and apatite are diagnostic high-temperature coexisting minerals with uraninite that typically occur in pegmatites and high-temperature hydrothermal deposits but rarely in low-temperature hydrothermal systems. Thus, their direct crystallization from ore-forming fluids confirms high-temperature conditions. U–Mo associations typically develop in volcanic-related epithermal deposits such as pitchblende–molybdenite pairs. The rare uraninite–molybdenite assemblage observed in the present study suggests co-precipitation under slow cooling [19], where uraninite–molybdenite pairs form during gradual temperature decline, and the pitchblende–sulfide molybdenum associations indicate rapid quenching. This slow cooling environment is compatible with the other typomorphic characteristics of uraninite.

5.2. Metallogenic Background and Genesis of Uraninite

Previous studies have attributed uranium mineralization in the study area to post-migmatization hydrothermal fluids and superimposed hydrothermal events, emphasizing the key role of migmatization [46]. However, uraninite-hosting quartz veins occur within the tectonic fissures of migmatized schists, indicating dual control by migmatization and structural fracturing. The migmatite basement comprises paragneiss-derived mica quartz schists and gneisses, exhibiting U concentrations of 5.64–12.9 × 10⁻⁶ (avg. 8.89 × 10⁻⁶) and Th/U = 2.97 (U-rich protolith). The migmatization in Haita produced granitoids via anatexis with elevated U (12.9–30 × 10⁻⁶; avg. 21 × 10⁻⁶) concentrations, Th (avg. 59.13 × 10⁻⁶) concentrations, and Th/U = 3.2, indicating a regional preconcentration of uranium [47]. Regional migmatization occurred at 830–780 Ma, whereas uraninite–titanite–molybdenite crystallization occurred at ~780 Ma, marking the late stage of migmatization [48,49]. Refs. [27,50] linked Neoproterozoic uranium mineralization in the Kangdian region to the breakup of the Rodinia assembly. Thus, examining the mineralization of Haita within the Rodinia tectonic framework offers critical insights. Following the Rodinia assembly, orogenesis ceased, transitioning to extension via intracontinental subduction and supercontinent breakup, which manifests regionally as migmatization.

After collisional orogeny, the Kangdian region underwent intraplate extension. Crustal stretching generated intense faulting, which created structural traps and ore-hosting spaces. Intermediate–felsic magmas provided both thermal energy and fluids. Tectonism critically enhanced permeability and facilitated the infiltration of meteoric water. Heated meteoric solutions dissolved wall-rock sulfates, carbonates, and halogens, forming high-fO₂ fluids that leached uranium [51]. These high-fO₂ fluids oxidized U⁴⁺ to U⁶⁺, mobilizing uranium as UO₂(CO₃)₂²⁻, UO₂(CO₃)₃⁴⁻, [UO₂F₃]⁻, and [UO₂F₄]²⁻ complexes. Fluid migration is governed by fO₂ and tectonics, whereas pH controls U-complex speciation [51,52]. Fluid mixing between meteoric water and deep-sourced tectonic-thermal fluids likely occurred [53,54,55,56], with H-O isotopes confirming the meteoric contribution that maintained high fO₂ conditions. A subsequent reduction of these oxidized fluids ultimately precipitated megacrystalline uraninite in optimal structural–geochemical settings.

6. Conclusions

The typomorphic characteristics and geological significance of megacrystalline uraninite from the Haita area of the Kangdian region can be summarized as follows:

(1)

Exceptional crystallinity and size. Naturally occurring uraninite exhibits well-developed euhedral morphologies with anomalously large grain sizes (≤10 mm).

(2)

High-temperature crystallization signatures. The typomorphic features reflect stable physicochemical conditions, indicating formation in a reduced environment (>450 °C) with gradual cooling.

(3)

Dual genetic controls. Mineralization is jointly governed by migmatization (uranium preconcentration) and tectonism (fluid pathways/ore deposition).

As an important typomorphic characterization of natural uraninite, this study advances our understanding of uranium mineralization during continental breakup events.