Hydrothermal Uranium Deposits Research Articles

Quantifying element mass transfer in the Jiling Na-metasomatic hydrothermal uranium deposit, Northwest China

The Na-metasomatic hydrothermal uranium deposits are relatively widespread, low in grade (less than 1 % U3O8) but high in tonnage. Although it has been considered that this type of deposit was formed due to hydrothermal alteration unrelated to magmatic activity, the detailed evolution of fluids and ore-forming process are still not well understood. Through element-mass-balance calculation and geochemical mapping of regional rocks, we investigated the Jiling uranium deposit in northwestern China and evaluated the composition and source of fluids and element-transfer behavior through Na-metasomatism and uranium mineralization. The findings show that, in the early Na-metasomatism stage, the Na-, HFSE- and REE-rich late-magmatic hydrothermal fluids caused Na-metasomatism of wall rocks, enriching Na2O (>53 %) while removing K2O (<-78 %), depleting SiO2 (30 % in granite and 3 % in diorite), and massively mass-transferring Fe, Ti, P and some incompatible elements. With increased rock permeability and the formation of partial Fe2+-bearing minerals, the Na-metasomatic alteration produced reducing agents and migration channels for ore-forming fluids, as well as the creation of ∼ 15 vol% porosity in the altered granite for metallogenic space. In the late uranium mineralization stage, CO2-rich fluids extracted uranium and HREEs, converted Fe2+ to Fe3+, and subsequently precipitated uranium to form pitchblende with apatite, calcite and chlorite. Thus, the Na-metasomatic alteration caused by late-magmatic hydrothermal fluids is critical for the production of large Na-metasomatic hydrothermal uranium deposits. Our new geochemical mapping reveals that the mass-concentration changes of Na, K and Si are more credible to defining Na-metasomatic alteration, while Fe, Ti, P, ∑(Zr-Hf-Nb-Ta) and ∑LREE/∑HREE vary strikingly during the uranium mineralization process.

Tracing Sources of Geochemical Anomalies in a Deeply Buried Volcanic-Related Hydrothermal Uranium Deposit: the Daguanchang Deposit, Northern Hebei Province, North China Craton

Tracing Sources of Geochemical Anomalies in a Deeply Buried Volcanic-Related Hydrothermal Uranium Deposit: the Daguanchang Deposit, Northern Hebei Province, North China Craton

Mineralization age of the Xiangshan uranium ore field, South China redefined by hydrothermal apatite U-Pb geochronology

The Xiangshan volcanic-related uranium ore field, located in the Ganzhou-Hangzhou Uranium-Polymetallic Metallogenic Belt (GHMB) in South China, has been the country’s largest historic uranium producer. Although this ore field has been intensely studied for decades, the absolute age of uranium ore formation was still debated. Conventional U-Pb dates on U-minerals (i.e., pitchblende, brannerite) and Ar-Ar dates on associated alteration minerals (i.e., illite) from different uranium deposits yield a wide range of ages (122 to 85 Ma). The uncertainty regarding the exact timing of uranium mineralization at Xiangshan has limited the development of accurate ore genesis models and precluded a better understanding of the links between metallogenesis and regional-scale geodynamics during the Cretaceous-Tertiary period in South China.In this work, we focus on applying in-situ LA-ICP-MS U-Pb geochronology to magmatic and hydrothermal apatite from two typical uranium ore deposits at Xiangshan, intending to indirectly constrain the exact timing of uranium mineralization and reconstruct the magmatic-hydrothermal history. U-Pb age determinations on magmatic zircon and apatite from the three types of ore-hosting volcanic rocks yielded precise crystallization ages of 133 to 136 Ma and 135 to 140 Ma, respectively. These results are identical within uncertainty to previously reported U-Pb dates obtained in the study area, and thereby tightly constrain the emplacement age of the Xiangshan volcanic complex that hosts uranium deposits. Moreover, the potassium feldspar-biotite Ar-Ar dating for the major volcanic rocks demonstrates that these host rocks were cooled and completely solidified through the biotite/potassium feldspar closure temperature (below 300 °C) at 133–131 Ma. Subhedral or anhedral aggregates of hydrothermal apatite ubiquitously formed in the high-grade uranium ores at the Shannan and Zoujiashan deposits and are commonly intergrown with fluorite, illite and uranium minerals. Relative to magmatic apatite from the host rocks, the hydrothermal apatite is typically characterized by much higher U and Th contents, depletion of LREEs, enrichment in HREEs and a much more pronounced negative Eu anomaly. As such, the hydrothermal apatite is unequivocal of hydrothermal origin and formed synchronously with uranium mineralization. LA-ICP-MS U-Pb dating of these hydrothermal apatite crystals yielded an absolute 206Pb/238U age of 126.7 ± 0.6 Ma for the Shannan deposit and 131.3 ± 7.2 Ma for the Zoujiashan deposit. The new ages are much older (>10 Ma) than those previously reported ages on ore or gangue minerals such as pitchblende and illite. Thus, one unidentified mineralization event likely occurred at ca. 131–127 Ma, soon after solidification and subsequent brittle deformation or faulting within the volcanic caldera. We, therefore, proposed that volcanic magmatism was highly likely to be the ‘heat engine’ driving hydrothermal fluid convection and possibly provide the essential conditions (i.e., magmatic fluids, U sources) for the formation of the U mineralization. Another implication of this study is that dating hydrothermal apatite with high U concentrations associated with uranium mineralization could be considered a complementary geochronological tool in the study of analogous hydrothermal uranium deposits worldwide.

The role of uranyl complex decomposition and redox conditions in the precipitation of hydrothermal uranium deposits: Insights from chlorite mineralogy and geochemistry in the Shazhou uranium deposit, Xiangshan, SE China

Uranium precipitation involves the decomposition of uranyl complexes and the reduction of hexavalent uranium, which can occur sequentially or simultaneously within one redox reaction. The redox condition of hydrothermal fluid plays a vital role in controlling the migration and precipitation of uranium in hydrothermal uranium deposits. However, little attention has been paid to the role of uranyl complex decomposition in uranium precipitation. In this study, chlorite mineralogy and geochemistry were examined to clarify the process of uranium precipitation in the Shazhou deposit, Xiangshan uranium orefield, Southeast China. Based on comprehensive petrographic and mineral chemistry studies of chlorites obtained from altered granite porphyries and uranium ores, five types of chlorites were identified: (1) chlorite present in the form of pseudomorphous biotite, which was produced by the hydrothermal alteration of biotite in rocks that underwent hematitization and chloritization (Chl-I); (2) chlorite filling the cleavage cracks in biotite in rocks that underwent hematitization and chloritization (Chl-II); (3) chlorite occurring in pyrite veins (Chl-III); (4) chlorite intergrown with pitchblende in ore veins (Chl-IV); and (5) chlorite occurring in calcite veins (Chl-V). Chlorite geothermometry revealed that the formation temperatures of the five types of chlorites ranged from 219 °C to 254 °C. Mineral chemistry analyses revealed that the five types of chlorites formed in a reductive fluid environment, where the oxygen fugacity at different stages is similar, with log fO2 values ranging from −41.6 to −39.1. Uranium precipitation started only in stage Chl-IV. The examination of the mineral assemblage revealed that the ore-forming fluid was rich in F−, HPO32−, and CO32−. Comprehensive investigation of chemistry and physicochemical conditions of the ore-forming fluid revealed that oxidized uranium (UO22+) could be complexed with HPO42− and F−, and uranyl phosphate and the uranyl fluoride complexes were the main uranium species when uranium precipitation and the decomposition occurred at stage Chl-IV. However, the assessment of oxygen fugacity of the solution equilibria between the UO2(s) and the uranyl phosphate and uranyl fluoride complexes revealed that the reducibility of the fluid favoring the reduction of uranyl ions (UO22+) to U4+ was insufficient to reduce the uranyl phosphate and uranyl fluoride complexes. This indicates that the breakup of uranyl phosphate and the uranyl fluoride complexes to release uranyl ions should occur first at stage Chl-IV, reducing uranyl ions to U4+, and leading to uranium precipitation. Hence, the decomposition of uranyl complexes played an important role in uranium precipitation. With the increase in pH, uranyl phosphate and the uranyl fluoride complexes gradually decomposed and became reducible. Moreover, the decrease in ligand concentration was conducive to the decomposition of uranyl phosphate and the uranyl fluoride complexes. During the formation of the Shazhou deposit, the fluid boiling process induced the loss of volatiles, such as CO2, CH4, HF, and H2S, leading to an increase in pH and decrease in HPO42−, F−, and CO32− concentrations in the fluid. These factors also led to the decomposition of uranyl fluoride and the uranyl phosphate complexes and the precipitation of fluorapatite and calcite. Uranium was then reduced by the action of Fe2+ and S− (pyrite) from a hexavalent to a tetravalent, and finally, uranium was precipitated to form uranium ores.

Re-evaluation of equilibrium relationships involving U6+/U4+ and Fe3+/Fe2+ in hydrothermal fluids and their implications for U mineralization

Uranium (U) and iron (Fe) are generally considered to have contrasting aqueous geochemical behavior, with U dissolution being favored in oxidizing fluids as oxidized U (U6+) and Fe in reducing fluids as reduced Fe (Fe2+) species. However, high concentrations of both U and Fe have been reported for the ore fluids of uranium deposits, suggesting that the two elements can be co-transported in the same fluid. In this study, geochemical modeling, based on recent thermodynamic and experimental data for U and Fe species and fluid inclusion compositions from two types of hydrothermal U deposits (volcanic-related and unconformity-related), was conducted to investigate the nature and conditions of UFe co-transportation. The results of the modeling indicate that high concentrations of U (>10, up to >100 ppm) and Fe (>1000 ppm) can be dissolved in acidic aqueous fluids (pH < ∼3.3) of moderate to high chlorinity (∼ 55 to 250 g/L Cl), for a wide range of logfO2 above and below the magnetite-hematite (MH) buffer (from ∼ MH-5 to MH + 30), at temperatures from 150 to 250 °C. There are four different combinations of dominant dissolved U and Fe species depending on the pH and redox conditions: U6+ and Fe3+ species (UO2Cl20 and FeCl30) at relatively high fO2, U4+ and Fe2+ species (UCl40 and FeCl42−) at relatively low fO2, and U6+ and Fe2+ species (UO2Cl20 and FeCl42−) or U4+ and Fe3+ species (UCl40 and FeCl30) at intermediate fO2 conditions. Because the most important U mineral in hydrothermal uranium deposits is uraninite (UO2), in which U is present as U4+, a necessary condition for U mineralization is that U is either transported as U4+ species or, if transported as U6+ species, precipitation of uraninite is induced by a reducing agent. The recognition that U6+ and Fe2+ species can be co-transported in the same fluid indicates that uraninite can be precipitated without the need to invoke a reducing agent external to the ore fluid. The driver of uranium mineralization in this case is an increase in pH due to fluid-rock interaction that leads to the alteration of the rocks by clay minerals and associated coupled sorption-reduction. These findings provide for a more extensive range of conditions for U transport and deposition than previously imagined and require that interpretation of the conditions of uranium mineralization consider both oxygen fugacity and pH. This study underscores the potential for uranium deposits to occur in geological settings where reducing agents are either absent or insufficiently abundant to account for the observed mineralization, and the possibility for the transport of U in environments that might otherwise be considered unfavorable.

Contrasting alteration textures and geochemistry of allanite from uranium-fertile and barren granites: Insights into granite-related U and ion-adsorption REE mineralization

AbstractAllanite is an important rare earth element (REE)-U-bearing mineral in granites, and it can act as a metal source for the formation of some hydrothermal uranium deposits and ion-adsorption REE deposits. To investigate the potential of allanite as a mineral probe of granite-related uranium mineralization processes and the formation of ion-adsorption REE deposits, we present textures, geochemistry, and in situ U-Pb isotope data for allanite from the fertile Changjiang granite associated with the Changjiang uranium ore field and barren Jiufeng granite in the Zhuguangshan batholith, South China. Alteration of allanite in the Changjiang granite is characterized by the altered domains with lower backscattered electron (BSE) intensities than the unaltered domains and replacement by other secondary minerals such as REE fluorocarbonates, calcite, fluorite, thorite, clay minerals, quartz, chlorite, and epidote. Crystals from the Jiufeng granite were partly replaced by the altered domains appearing darker in BSE images and minor REE fluorocarbonates. The darker domains of the Changjiang and Jiufeng allanite grains have higher Fe3+/(Fe3++Fe2+) ratios and U concentrations than those of the brighter domains, indicating that the alteration of allanite was probably related to more oxidized fluids. This study suggests that the Changjiang granite might have been subjected to the influx of F- and CO2-bearing fluids.The brighter domains of the Changjiang and Jiufeng allanite grains have weighted mean 207Pb-corrected 206Pb/238U ages of 156.7 ± 4.3 Ma and 161.6 ± 5.3 Ma, respectively, consistent with the corresponding zircon 206Pb/238U ages of 156.1 ± 1.4 Ma and 159.8 ± 1.8 Ma. The darker domains of the Changjiang allanite grains yield a weighted mean 206Pb/238U age of 141.4 ± 5.6 Ma, which overlaps within error the timing of a uranium mineralization event (~140 Ma) in the Changjiang uranium ore field and the age of a crustal extension event (140–135 Ma) in South China. The BSE images and elemental maps reveal that rare earth elements such as La and Ce have been released from the Changjiang allanites during alteration and were precipitated as REE-fluorocarbonates that are susceptible to chemical weathering, which sets the stage for the formation of an ion-adsorption REE deposit. Our study suggests that the regional crustal extension might have played an important role in the formation of both granite-related uranium and ion-adsorption REE deposits in South China, as it could have triggered alteration or breakdown of REE-U-bearing minerals in source rocks.

Application of optically stimulated luminescence in volcanic hydrothermal uranium deposits in the Guyuan Basin, North China

Volcanic basins in North China have great potential for the discovery of volcanic-related uranium deposits. However, most of the traditional geochemical exploration that have the inadequacies of the unexpected errors, weak anti-interference ability and exception information have become untenable in exploring uranium deposits overlain by volcanic rocks and sediments. To identify and track the abnormal geochemical source, feldspar infrared stimulated luminescence (IRSL) and quartz optically stimulated luminescence (OSL) were utilized for the first time to explore deeply concealed volcanic-related uranium deposits in North China. Our results confirm that feldspar IRSL-based drill-core samples are oversaturated and therefore fail to trace the deep uranium orebodies. In contrast, quartz OSL in sedimentary covers over the drill holes hosting economic and subeconomic uranium mineralization yields older apparent ages than their actual depositional ages. The high apparent ages of quartz OSL in the sediments and low helium isotope ratios (3He/4He) of the drill core indicated that the soil radon gas (Rn) anomaly was caused by Rn migration from deeply buried uranium ore decay.

The effects of red beds on fluid flow and implications for the formation of volcanic-type uranium deposits in the Xiangshan basin, China

The South China uranium metallogenic province is an important hydrothermal uranium metallogenic region in China, which is spatially and temporally associated with the Cretaceous-Neogene red beds. Previous studies suggest that the red beds might have played critical role in the ore-forming processes by providing oxidizing fluids. However, the effects of the red beds on fluid flow are poorly understood. In this study, numerical modeling was conducted to investigate the roles of red beds on ore-forming fluid flow in the Xiangshan basin in Jiangxi Province, hosting the largest volcanic hydrothermal uranium deposit in China. The modeling results show that the variation of the slope gradient of the red beds greatly affects the fluid flow patterns, whereas the change of thickness and permeability of the red beds has little influence on fluid flow pattern. The fluid can circulate into the volcanic and basement rocks driven by topographic relief of the red beds. Furthermore, for models with fault, although the fluid velocities in the fault were more than one order of magnitude larger than those in the surrounding strata, the overall fluid flow patterns in the strata did not show great variation. The modelling results suggest that the topographic relief of the red beds may have driven the oxidizing fluids into the underlying rocks which are necessary for uranium mineralization in the Xiangshan basin.

Geochemical Characteristics of Chlorite in Xiangshan Uranium Ore Field, South China and Its Exploration Implication

Chlorite is one of the most important hydrothermal minerals in many hydrothermal uranium deposits worldwide and is commonly closely associated with the uranium mineralization. Trace elements in chlorite have been extensively applied to fingerprinting the hydrothermal fluid evolution and indicating the concealed ore bodies in porphyry Cu (-Au) deposits and skarn-related Pb-Zn deposits. However, this approach was rarely attempted on hydrothermal uranium deposits to date. Xiangshan uranium ore field, located in the southeast part of Gan-Hang Metallogenic (or Volcanic) Belt (GHMB), is the largest volcanic-related ore field in the whole country. In this study, the focus was placed on the petrographic characteristics and trace elements in hydrothermal chlorite from two typical deposits (Zoujiashan and Yunji) at Xiangshan. Four types of chlorites were identified, i.e., Chl1-Y and Chl2 from Yunji deposit, and Chl1-Z and Chl3 from Zoujiashan deposit. The pre-ore Chl1-Y and Chl1-Z are formed through replacing the original magmatic biotite. Chl2 and Chl3 occur as veinlets or disseminated, and are closely associated with early-ore U mineralization and main-ore U mineralization, respectively. All the four types of chlorites are typically trioctahedral chlorite. Vein-type/disseminated Chl2 and Chl3 in ore veins were precipitated directly from the hydrothermal fluids through dissolution-migration-precipitation mechanism, whereas the replacement-type chlorite was formed by the dissolution–crystallization mechanism. Empirical geothermometry indicates that the chlorite from Yunji and Zoujiashan were crystallized at 179~277 °C, indicating a mesothermal-epithermal precipitation environment. EPMA and LA-ICP-MS results show that the replacement-type chlorite has relatively consistent compositions at Yunji and Zoujiashan. Both Chl2 and Chl3 are enriched in U, Th but depleted in Mn and Ti. Compared with the Chl2 related to early-ore U mineralization, Chl3 that formed at main-ore stage has higher concentrations of Fe, U, Th, REEs, Mn and Ti, as well as higer Fe/(Fe + Mg) ratios. Such compositional differences between Chl2 and Chl3 are mainly attributed to the formation temperatures and fluid compositions/natures. Combined with petrology and chemical compositions of different types of chlorite, we propose that the presence of vein-type/disseminated chlorite with high U and Fe/(Fe + Mg) ratio but relatively low Mn, Ti and Pb contents can be regarded as an effective vector toward the most economic (high U grade) mineralized zone, whereas the occurrence of Chl2 is likely to indicate the subeconomic U mineralization and less potential exploration for uranium at depth.

Characteristics of Mineralization-Forming Fluid and Metallogenic Mechanism for the Mianhuakeng Uranium Deposit in South China: Constraints from In Situ Geochemical Signatures and Sulfur Isotopes of Syn-Mineralization Pyrite and Pitchblende

The Mianhuakeng deposit is the most representative granite-related hydrothermal vein-type uranium deposit in South China; however, the characteristics of the mineralization-forming fluid and metallogenic mechanism are still less constrained. To address the scientific problems above, we investigated the trace element chemistry and sulfur isotope compositions in syn-mineralization pyrite and pitchblende from the Mianhuakeng uranium deposit. The trace element chemistry shows that the mineralization in the Mianhuakeng deposit belongs to an intermediate-to-low temperature hydrothermal system, which is consistent with the homogenization temperature distribution of fluid inclusions. Redox-sensitive elements (such as Co and Se) in syn-mineralization pyrite suggest a reductive nature of the mineralization-forming fluids. The fractionation between light and heavy REE in pitchblende from the Mianhuakeng deposit is most similar to those from the Changjiang pluton. The pronounced negative Eu anomaly is coincident with mineralization-bearing granites. The δ34S values of syn-mineralization pyrite range from −10.2 to −1.4‰, which is higher than those values of pyrite from granites near the studied area and lower than the δ34S values of pyrite from diabase in the ore district. The REE signatures of pitchblende and sulfur isotope composition of syn-mineralization pyrite suggest that the major U source for the Mianhuakeng deposit is most likely the Changjiang pluton, probably accompanied by the incorporation of mantle-derived fluids. The circulations of CO2-rich hydrothermal fluids leached uranium from granite source rocks, especially from the Changjiang pluton. The change of physicochemical conditions of the mineralization-forming fluid resulted in the deposition of the uranium minerals in favorable structural traps to form the hydrothermal vein-type Mianhuakeng uranium deposit.

Age of the Vostok Hydrothermal Uranium Deposit (North Kazakhstan Ore Province) According to U–Pb (ID-TIMS), Pb–Pb, Xen–Xes, K–Ar and Rb–Sr Methods

Age of the Vostok Hydrothermal Uranium Deposit (North Kazakhstan Ore Province) According to U–Pb (ID-TIMS), Pb–Pb, Xen–Xes, K–Ar and Rb–Sr Methods

Different hydrothermal fluids inducing alteration and uranium mineralisation in the Baquan deposit of the Xiangshan uranium ore field: Constraints from geochemistry of altered rocks and ores

The Baquan uranium deposit is one of the hydrothermal uranium deposits in the Xiangshan volcanic uranium ore field and is hosted in subvolcanic granitic porphyry rocks (GPRs). To constrain sources of the hydrothermal fluids and metals for alteration and mineralisation, respectively, we analysed the chemical compositions of chlorites and pyrites formed during alteration and mineralisation processes, in-situ sulfur isotope compositions of pyrites, whole-rock compositions and sulfur and molybdenum isotopes of the least-altered and altered GPRs, and mineralised GPRs (i.e. uranium ores). The results show distinct differences in mineralogical and geochemical features of the hydrothermally altered GPRs and uranium ores, which are as follows: (1) In the altered GPRs, hydrothermal chlorites (chlorite-I) are mainly daphnite with higher Fe/(Fe + Mg) (0.70–0.75) but lower Fe3+/Fe2+ (∼0.014), and hydrothermal pyrites (pyrite-I) have low Co and Ni contents (<0.05% and <0.04%, respectively) and a narrow range of δ34S (9.11–9.87‰). In contrast, hydrothermal chlorites (chlorite-II) in the ores are ripidolite and brunsvigite with lower Fe/(Fe + Mg) (0.43–0.62) but higher Fe3+/Fe2+ (∼0.056), and hydrothermal pyrites (pyrite-II) have high Co and Ni contents (0.02–0.3% and 0.01–0.11%, respectively) and a wide range of δ34S values (2.32–13.8‰). (2) The altered GPRs have Zr and Hf contents (135–187 ppm and 3.39–4.34 ppm, respectively) similar to those of the least-altered GPRs (162–169 ppm, and 3.79–4.01 ppm, respectively), whereas the ores have elevated Zr and Hf contents (184–548 ppm and 4.17–11.6 ppm, respectively). (3) The δ98Mo values of the altered GPRs (0.39–0.99‰) and uranium ores (0.52–1.74‰) show contrasting correlations with K2O or Na2O contents. These differences suggest that hydrothermal alteration and mineralisation in the Baquan deposit may have been caused by two fluids of different origins. Based on these characteristics and comparisons with possible sources, we suggest that the fluid responsible for the alteration of GPRs most likely originated from the subducting slab, whereas the mineralising fluid forming the Baquan uranium deposit likely originated from the brine of a nearby Cretaceous basin. The uranium in the deposit mainly originated through leaching of the volcanic rocks by the brine-derived hydrothermal fluid.

The source of uranium within hydrothermal uranium deposits of the Motianling mining district, Guangxi, South China

Uranium deposits are diverse as a result of their formation in a wide variety of geological environments. However, the source of the uranium within these deposits is difficult to identify and as a result it is usually poorly understood. This study focuses on the hydrothermal Sanqiliu and Sanqisi uranium deposits, both of which formed within NE–SW striking shear zones that cross-cut the Sanfang batholith and low-grade metasedimentary rocks of the Motianling district, southern China. The deposits contain vein-hosted pitchblende–quartz–pyrite–chlorite–mica orebodies. Fluid inclusions within quartz in the orebodies in the district homogenize at temperatures between ∼205 °C and ∼222 °C, indicating the deposit formed as a result of low- to intermediate-temperature hydrothermal activity. Pitchblende-associated hydrothermal pyrite within the Sanqiliu and Sanqisi deposits has δ34S values from −24.7‰ to 19.7‰ and from −6.0‰ to 13.6‰, respectively. Quartz separates from the Sanqiliu deposit yield δ18O values of −1.4‰–11.5‰ (mean of 5.8‰) for syn-ore hydrothermal fluids. These stable isotope data indicate that the fluids that formed these deposits were derived from the Sanfang granites, sediments of the Sibao Group, and meteoric sources. In addition, uraninite from the host Sanfang granite yields U–Pb ages of ca. 732–773 Ma. Combining these data with the geology of the ore deposits in this region suggests that these deposits were generated from deeply circulating meteoric water and the uraninite within the highly fractionated Sanfang granite is the source of the uranium within the Sanqiliu and Sanqisi uranium deposits.

Genesis and Metallogenic Process of the Lujing Uranium Deposit, Southwest Jiangxi Province, China: Constraints of Micropetrography and S–C–O Isotopes

AbstractThe Lujing uranium deposit, located in the southeastern part of the Nanling metallogenic province, is one of the representative granite‐related hydrothermal uranium deposits in South China. Basic geology, geochemistry, and geochronology of the deposit have been extensively studied. However, there is still a chronic lack of systematic research on the genesis and metallogenic process of the deposit. Thus, we recently carried out an electron microprobe and stable isotopic analysis. The main research results and progresses are as follows:Uranium minerals in this deposit include coffinite, pitchblende, and uranothorite, and small amounts of uranium exist in accessory minerals in the form of isomorphism. Coffinite, which occurs predominantly as the pseudomorphs after pitchblende, also occurs as a primary mineral and is locally formed from the remobilization of uranium from adjacent uranium‐bearing minerals. The mineralizing fluid was originally composed of a magmatic fluid generated by late Yanshanian magmatism. The high As content of pyrite in ores may reflect the addition of meteoric water, or the formation water (or both), to the magmatic hydrothermal system. The δ34S values vary from −14.4‰ to 13.9‰ (mean δ34S = −3.9‰), showing a range that is similar to nearby Cambrian metamorphic strata and Indosinian granites, indicating that these host rocks represent the source of sulfur; however, the possibility of a mantle source cannot be completely ruled out. According to our new isotopic data and recent Pb isotopic data, we conclude that the uranium in ores was derived by leaching dominantly from the uranium‐rich host rocks, especially the Cambrian metamorphic strata. The δ13CPDB values (−8.75‰ to 1.40‰; mean δ13CPDB = −5.41‰) and δ18OSMOW values (5.45–18.62‰; mean δ18O = 13.02‰) of reddish calcite from the ore‐forming stage suggest that the CO2 in the mineralizing fluids was derived predominantly from the mantle, with a small component contributed by marine carbonates. Based on these new data and previous research results, this paper proposes that uranium metallogenesis in the Lujing deposit is closely associated with mafic magmatism resulting from crustal extension during the Cretaceous to Paleogene in South China.

The genesis of granite-related hydrothermal uranium deposits in the Xiazhuang and Zhuguang ore fields, North Guangdong Province, SE China: Insights from mineralogical, trace elements and U-Pb isotopes signatures of the U mineralisation

Granite-related hydrothermal U deposits from the Xiazhuang and Zhuguang ore fields are located within the Nanling Metallogenic Belt in South China. These deposits are hosted in Triassic and Jurassic granites that were emplaced during the Indosinian orogeny and the Yanshanian post-orogenic extension. Four main types of granitic host rocks were identified based on geochemical characteristics as the most favourable sources of U for the mineralisation in the district: (i-ii) Indosinian peraluminous S-type (Maofeng) and L-type leucogranite (Baishuizhai); (iii) Indosinian highly fractionated high-K calc-alkaline A2-type granite (Xiazhuang, Maofeng and Youdong) and (iv) early Yanshanian highly fractionated high-K, (Fe) calc-alkaline A2-type granite (Sundong, Siqian and Changjiang). They contain high U concentrations ranging from 15 to 28 ppm indicating the crystallisation of uraninite, which is a source of U easily leachable by oxidised fluids for the formation of hydrothermal mineralisation.This study characterises the mineralogical, chemical and isotopic signatures of the U mineralisation from five representative deposits and occurrences in the Xiazhuang (Baishuizhai, Shituling, Xiwang and Xianshi) and Zhuguang (Mianhuakeng) ore fields. The 175 ± 16 Ma Baishuizhai occurrence and 162 ± 27 Ma Shituling deposit represent an early stage of U mineralisation (175–145 Ma) that occurred during the early Yanshanian. Disseminated Th-bearing uraninite at Baishuizhai and the association of U oxides with alteration minerals such as epidote and chlorite in both Baishuizhai and Shituling indicate temperatures >250 °C for the hydrothermal system. The early Yanshanian granites (190–150 Ma) provided the heat source and magmatic fluids to the hydrothermal system while U-rich Indosinian granites provided U for the Zr-Th-Ta-bearing U mineralisation. In contrast, the mineralisation from Xiwang, Mianhuakeng and Xianshi deposits (107 ± 16, 93 ± 15 and 79 ± 11 Ma, respectively) represent several pulses of the main mineralisation stage (110–50 Ma) that occurred during the late Yanshanian crustal extension. The mineralisation occurred at relatively low temperatures (<250 °C) as W-Y-Nb-Ti-rich pitchblende and uraninite veins associated with quartz, fluorite, calcite and pyrite. During Cretaceous-early Cenozoic crustal extension, oxidising meteoric and/or basinal waters percolated downward into the granitic basement through deep faults and mixed with CO2-rich magmatic fluids. These thermal solutions circulated along opening fractures and leached U from Indosinian and early Yanshanian U-rich granites and also possibly from early Yanshanian U mineralisation. After heating, the ore-forming fluids ascended along structures leading to the decrease of the hydrostatic fluid pressure, which promoted the deposition of U in veins. Therefore, granite-related hydrothermal U mineralisation in the province, formed over a long time span (>100 Ma) in two major stages related to different genetic conditions.

Anomalous Lead Isotopic Composition of Galena and Age of Altered Uranium Minerals: a Case study of Chauli Deposits, Chatkal–Qurama District, Uzbekistan

The enrichment of lead isotopic composition of nonuranium minerals, in the first place galena in 206Pb and 207Pb, as compared to common lead is a remarkable feature of uranium deposits. The study of such lead isotopic composition anomalous in 206Pb and 207Pb in uranium minerals provides an opportunity for not only identification of superimposed processes resulting in transformation of uranium ores during deposit history but also calculation of age of these processes under certain model assumptions. Galena from the Chauli deposit in the Chatkal–Qurama district, Uzbekistan, a typical representative of hydrothermal uranium deposits associated with domains of Phanerozoic continental volcanism, has been examined with the highprecision (±0.02%) MC-ICP-MS method. Twenty microsamples of galena were taken from polished sections. Six of them are galena hosted in carbonate adjacent to pitchblende spherulites or filling thin veinlets (approximately 60 μm) cutting pitchblende. Isotopically anomalous lead with 206Pb/204Pb and 207Pb/204Pb values reaching 20.462 and 15.743, respectively, has been found in these six microsamples in contrast to another fourteen in which the Pb–Pb characteristics are consistent with common lead. On the basis of these data and with account for the 292 ± 2 Ma age for the Chauli deposit, the age of epigenetic transformation of uranium ores of this deposit has been estimated. During this process, radiogenic lead partly lost from pitchblende was captured into galena. The obtained date is 170 Ma. In the Chatkal–Qurama district, these epigenetic processes are apparently caused by the interaction of uranium minerals with activated underground water under tectonic activity and relief transformation, which took place from the post-Permian (i.e., after the Chauli formation) to the Jurassic period.

Metallogenic Regularity of Hydrothermal Uranium Deposits in the Migmatite of the Kangdian Axis

Metallogenic Regularity of Hydrothermal Uranium Deposits in the Migmatite of the Kangdian Axis

Magmatic and hydrothermal behavior of uranium in syntectonic leucogranites: The uranium mineralization associated with the Hercynian Guérande granite (Armorican Massif, France)

Most of the hydrothermal uranium (U) deposits from the European Hercynian belt (EHB) are spatially associated with Carboniferous peraluminous leucogranites. In the southern part of the Armorican Massif (French part of the EHB), the Guérande peraluminous leucogranite was emplaced in an extensional deformation zone at ca. 310Ma and is spatially associated with several U deposits and occurrences. The apical zone of the intrusion is structurally located below the Pen Ar Ran U deposit, a perigranitic vein-type deposit where mineralization occurs at the contact between black shales and Ordovician acid metavolcanics. In the Métairie-Neuve intragranitic deposit, uranium oxide-quartz veins crosscut the granite and a metasedimentary enclave.Airborne radiometric data and published trace element analyses on the Guérande leucogranite suggest significant uranium leaching at the apical zone of the intrusion. The primary U enrichment in the apical zone of the granite likely occurred during both fractional crystallization and the interaction with magmatic fluids. The low Th/U values (<2) measured on the Guérande leucogranite likely favored the crystallization of magmatic uranium oxides. The oxygen isotope compositions of the Guérande leucogranite (δ18Owhole rock=9.7–11.6‰ for deformed samples and δ18Owhole rock=12.2–13.6‰ for other samples) indicate that the deformed facies of the apical zone underwent sub-solidus alteration at depth with oxidizing meteoric fluids. Fluid inclusion analyses on a quartz comb from a uranium oxide-quartz vein of the Pen Ar Ran deposit show evidence of low-salinity fluids (1–6wt.% NaCl eq.), in good agreement with the contribution of meteoric fluids. Fluid trapping temperatures in the range of 250–350°C suggest an elevated geothermal gradient, probably related to regional extension and the occurrence of magmatic activity in the environment close to the deposit at the time of its formation. U-Pb dating on uranium oxides from the Pen Ar Ran and Métairie-Neuve deposits reveals three different mineralizing events. The first event at 296.6±2.6Ma (Pen Ar Ran) is sub-synchronous with hydrothermal circulations and the emplacement of late leucogranitic dykes in the Guérande leucogranite. The two last mineralizing events occur at 286.6±1.0Ma (Métairie-Neuve) and 274.6±0.9Ma (Pen Ar Ran), respectively. Backscattered uranium oxide imaging combined with major elements and REE geochemistry suggest similar conditions of mineralization during the two Pen Ar Ran mineralizing events at ca. 300Ma and ca. 275Ma, arguing for different hydrothermal circulation phases in the granite and deposits. Apatite fission track dating reveals that the Guérande granite was still at depth and above 120°C when these mineralizing events occurred, in agreement with the results obtained on fluid inclusions at Pen Ar Ran.Based on this comprehensive data set, we propose that the Guérande leucogranite is the main source for uranium in the Pen Ar Ran and Métairie-Neuve deposits. Sub-solidus alteration via surface-derived low-salinity oxidizing fluids likely promoted uranium leaching from magmatic uranium oxides within the leucogranite. The leached out uranium may then have been precipitated in the reducing environment represented by the surrounding black shales or graphitic quartzites. As similar mineralizing events occurred subsequently until ca. 275Ma, meteoric oxidizing fluids likely percolated during the time when the Guérande leucogranite was still at depth. The age of the U mineralizing events in the Guérande region (300–275Ma) is consistent with that obtained on other U deposits in the EHB and could suggest a similar mineralization condition, with long-term upper to middle crustal infiltration of meteoric fluids likely to have mobilized U from fertile peraluminous leucogranites during the Late Carboniferous to Permian crustal extension events.

Physicochemical parameters of formation of hydrothermal deposits: Evidence from fluid inclusions. III. Uranium deposits

An original database, which was compiled by the authors, is continuously updated, and now contains data from more than 19 800 publications on fluid and melt inclusions in minerals, was utilized to review and synthesize data on the physicochemical parameters at which hydrothermal uranium deposits and occurrences were formed. The parameters discussed below are temperature, pressure, density, salinity, gas composition of the fluid, and U concentration in the hydrothermal fluid. The database contains data of fluid inclusions in minerals from 90 U deposits and occurrences worldwide. Histograms of the homogenization temperatures of fluid inclusions are presented for such typical minerals of these deposits and occurrences as quartz, calcite, and fluorite. The temperature range most favorable for the origin of U deposits is 100–200°C (67% of the 937 measured temperature values fall within this range), which makes U deposits remarkably different from higher temperature Au-Ag, W, and Sn deposits. These deposits also differ in the salinity of the fluids. The range of fluid salinity of 10–30 wt % equiv. NaCl includes 42% of fluid salinity values measured at U deposits (our database includes 937 measured values), 27% for Au-Ag deposits (10 237 measured values), 27% for W deposits (2333 measured values), and for Sn deposits (1981 measured values). The relatively low temperature of U-bearing fluids and their high salinity testify that these solutions had a high density: 94% of all measured values lie within the range of 0.8–1.2 g/cm3. Fluid pressure at U deposits broadly varied from 2500 to 300 bar and perhaps even lower values. Data on the chemical composition of the gas phase of the fluid inclusions show a significant diversity of the fluids contained in the inclusions. In certain instances, H2O-CO2 fluids give way to fluids rich in CH4 and N2 with minor amounts of hydrocarbons. Data are reported on the gas composition of fluid inclusions in the nuclear-reactor zone at three Precambrian U deposits. Analyses of individual inclusions were utilized to evaluate U concentrations in magmatic melts and mineral-forming fluids. The geometric mean U concentration in silicate melts of composition ranging from ultramafic to silicic is 0.92 ppm (8053 measured values), and the analogous values for the fluids is 1.21 ppm (271 measured values).

Tectono‐Magmatic Activization and Fractal Dynamics of Ore‐Forming of Hydrothermal Uranium Deposits in South China

Tectono‐Magmatic Activization and Fractal Dynamics of Ore‐Forming of Hydrothermal Uranium Deposits in South China