Granite-hosted Uranium Deposits Research Articles

Whole-rock and biotite geochemistry of granites from the Miao’ershan batholith, South China: Implications for the sources of granite-hosted uranium ores

Granite-hosted uranium deposits are the most important types of uranium deposits in South China, but the metal sources of these deposits are still poorly constrained. The Miao’ershan batholith (South China) is a natural laboratory to address this issue, as well as to highlight the use of biotite composition as an indicator for the petrogenesis and uranium fertility of granites. Here, we present whole-rock and biotite geochemical data for U-bearing (Zhangjia and Douzhashan) and barren (Xiangcaoping and Paleozoic) granites of the Miao’ershan batholith. Contents of Li, Be, Mn, Sn, In, Nb, Ta, Ga, W, and U in the studied biotites increase from barren to U-bearing granites, while Mg, Ni, Co, V, and Rb display the opposite trend, which may indicate higher degrees of fractionation of the U-bearing granites. An Fe/(Fe + Mg) ratio of 0.65 in biotite appears to be an efficient threshold to discriminate between U-bearing and barren granites. New ternary discrimination diagrams are introduced, based on Li–V × 20–Mn and Sn–Co × 5–Nb, respectively, to aid the recognition of the U-bearing and barren granites. The relatively high whole-rock U concentrations (8.0–38.6 ppm) and low whole-rock ratios of Th/U (0.5–3.2) and REE/U (2.1–13.5) of the Zhangjia and Douzhashan granites favored the crystallization of uraninite. The samples experiencing fluid-wallrock interaction from the Zhangjia granite are characterized by low whole-rock U concentrations of 3.0 to 4.2 ppm and high Th/U ratios of 8.2 to 10.1, suggesting that a significant part of the U has been leached out by oxidizing meteoric waters. This confirms that the granite represents the primary source of U at the Zhangjia uranium deposit.

Geochemistry and Acid Hydrometallurgy Accessibility of Uraninite from Mianhuakeng Granite-Hosted Uranium Deposit, South China

Systematic study of the surface chemical properties of uranium minerals is necessary to improve the uranium ore extracting process. The presented work aims to argue geochemistry and acid hydrometallurgy accessibility of uraninite from the Mianhuakeng (MHK) granite-hosted uranium deposit, South China, which provides insight on this ore extracting domain. Mineralogy, geochemical composition, U–Th–Pb chemical age, and uranium deportment of the uraninite were systematically analyzed by using scanning electron microscope with energy dispersion spectrum (SEM-EDS), an electron probe microanalyzer (EPMA), and x-ray photoelectron spectroscopy (XPS). The results showed that uraninite was intergrowth with coffinite, probably due to uraninite being partly metasomatized into coffinite along the fissures. The major element content of uraninite such as for UO2, SiO2, and CaO were 79.46 ± 2.03 wt%, 6.19 ± 1.36 wt%, and 5.09 ± 0.80 wt%, respectively. Single-point U–Th–Pb chemical ages for uraninite grains were calculated with the EPMA data, and the results showed ages ranging from a few million to dozens of million years, indicating Pb loss after uraninite formed. Uranium deportment in uraninite generally existed in the forms of UO2, U3O8, and UO3, and mostly showed high valence states suggested by XPS. Uranium on the surface of the uraninite grain was partially oxidized by sulfuric acid leaching, which led to tetravalent uranium converting to hexavalent uranium, suggesting uraninite in the MHK uranium deposit is accessible to be leached by sulfuric acid.

Zircon U-Pb geochronology and geochemistry of granites in the Zhuguangshan complex, South China: Implications for uranium mineralization

The Zhuguangshan complex, composed of Caledonian, Indosinian, and Yanshanian granites, and Cretaceous mafic dykes, is one of the most important granite-hosted uranium producers in South China. Here we present LA-ICP-MS zircon U-Pb and hornblende 40Ar/39Ar geochronology and whole-rock and biotite geochemistry for the granites in this complex to evaluate the magmatism and its constraints on uranium mineralization. Samples collected from the Fuxi, Youdong, Longhuashan, Chikeng, Qiling, and Sanjiangkou intrusions yield zircon weighted 206Pb/238U ages of 426.7 ± 5.4 Ma, 226.4 ± 3.5 Ma, 225.0 ± 2.7 Ma, 152.2 ± 3.0 Ma, 153.9 ± 2.1 Ma, and 155.2 ± 2.1 Ma, respectively. A new Ar-Ar dating of the hornblende of the diabase from the Changjiang uranium ore field yields a plateau age of 145.1 ± 1.5 Ma. These results coupled with published geochronological data indicate that six major magmatic events occurred in the study area at 420−435 Ma, 225−240 Ma, 150−165 Ma, ~140 Ma, ~105 Ma, and ~90 Ma. Both U-bearing and barren granites occur in this complex, and they display differences in whole-rock and biotite geochemistry. The barren granites show higher Al2O3, CaO, TFMM, Rb, Zr, Ba, SI, Mg#, (La/Yb)N, and Eu/Eu*, but lower SiO2, ALK, Rb, DI, Rb/Sr, and TiO2/MgO than those of the U-bearing granites. Biotites in the U-bearing granites are close to the Fe-rich siderophyllite-annite end member with Fe/(Fe + Mg) ratios higher than 0.66, whereas those in the barren granites are relatively close to the Mg-rich eastonite-phlogopite end member with Fe/(Fe + Mg) ratios <0.66. The U-bearing granites were mainly derived from the partial melting of pelitic sedimentary source, whereas the psammitic source generated the barren granites. In addition, the barren granites show higher TFMM, Ba, and Eu/Eu* but lower SiO2, Rb/Sr and Al2O3/TiO2 ratios with higher zircon saturation temperatures relative to the U-bearing granites. These results indicate that the geochemical compositions of the U-bearing and barren granites are dictated not only by the compositions of source rocks but also the physicochemical conditions of partial melting. Our study suggests that these two factors are also the major factors that control uranium ore potential of the granites in the Zhuguangshan complex. The geochemical variations of U-bearing and barren granites can serve as a potential detector for granite-hosted uranium deposits.

Isotope geochronology, geochemistry, and mineral chemistry of the U-bearing and barren granites from the Zhuguangshan complex, South China: Implications for petrogenesis and uranium mineralization

The Zhuguangshan complex carries some of the most important granite-hosted uranium deposits in South China. Here we investigate the Changjiang and Jiufeng granites which represent typical U-bearing and barren granites in the complex, using zircon U-Pb ages, whole-rock geochemistry, Sr-Nd isotopic and zircon Hf isotopic data, and mineral chemistry, to constrain the petrogenesis and uranium mineralization. LA-ICP-MS zircon U-Pb dating shows that both the Changjiang and Jiufeng granites were emplaced ca. 160Ma. These rocks show high silica, weakly to strongly peraluminous compositions, enrichment in Rb, Th, and U, and depletion in Ba, Nb, Sr, P, and Ti. These features coupled with the high initial 87Sr/86Sr ratios, negative εNd(t) values and εHf(t) values, and the Paleoproterozoic two stage model ages of these two granites suggest that the two granites belong to S-type granites, and the parental magmas of the two granites were derived from the Paleoproterozoic metasedimentary rocks. However, the granitoids show different mineralogical characteristics. The biotite in the Changjiang granite belongs to siderophyllite, marking higher degree of chloritization, whereas the biotite in the Jiufeng granite is ferribiotite, characterized by only slight chloritization. Compared with the Jiufeng granite, the biotite in the Changjiang granite has lower crystallization temperature and oxygen fugacity, but higher F content, and the uraninite has higher UO2 content but lower ThO2 content, and stronger corrosion. The chemical ages of uraninites from both granites are (within error) consistent with the zircon U-Pb ages and are considered to represent the emplacement ages of granites. Chemical ages of pitchblende in the Changjiang granite yield 118±8Ma, 87±4Ma, and 68±6Ma, representing multiple episodes of hydrothermal events that are responsible for the precipitation of U ores in the Changjiang uranium ore field. Our study suggests that the degree of magma differentiation and physicochemical conditions of the magmatic-hydrothermal system are the key factors that control the different U contents of these two granites. The mineralogical characteristics of uraninite and biotite can be used to distinguish between U-bearing and barren granites, and serve as a potential tool for prospecting granite-hosted uranium deposits.

Newly discovered uranium mineralization at ~2.0 Ma in the Menggongjie granite-hosted uranium deposit, South China

The southeastern part of the Nanling metallogenic province, South China contains numerous economically important granite-hosted, hydrothermal vein-type uranium deposits. The Miao’ershan (MES) uranium ore field is one of the most important uranium sources in China, hosts the largest Chanziping carbonaceous-siliceous-pelitic rock-type uranium deposit and several representative granite-hosted uranium deposits. The geology and geochemistry of these deposits have been extensively studied. However, accurate and precise ages for the uranium mineralization are scarce because uranium minerals in these deposits are usually fine-grained, and may have formed in several stages, thus hindering the understanding of the uranium metallogenesis of this province.The Menggongjie (MGJ) uranium deposit is one of the largest granite-hosted uranium deposits in the MES ore field. Uranium mineralization in this deposit occurs at the central part of the MES granitic complex, accompanied with silicification, fluorination, K-metasomatism and hematitization. The ore minerals are dominated by uraninite, occurring in quartz or fluorite veinlets along fractures in altered granite. In-situ SIMS U-Pb dating on the uraninite yields the U–Pb isotopic age of 1.9±0.7Ma, which is comparable to the chemical U-Th-Pbtol uraninite age of 2.3±0.1Ma. Such ages agree well with the eruption ages of the extension-related Quaternary volcanics (2.1–1.2Ma) in South China, suggesting that the uranium mineralization have formed at an extensional setting, possibly related to the Quaternary volcanic activities. Therefore, our robust, new dating results of the MGJ uranium deposit make it the youngest granite-hosted uranium deposit reported so far in South China and the mineralization event represents a newly identified mineralization epoch.

Timing of uranium mineralization and geological implications of Shazijiang Granite-Hosted uranium deposit in Guangxi, South China: New constraint from chemical U-Pb age

Miaoershan (MES) uranium ore field is one of the most important uranium sources in China, hosts the largest Chanziping carbonaceous-siliceous-pelitic rock type uranium deposit in South China together with many other granite-hosted uranium deposits. The Shazijiang (SZJ) uranium deposit is one of the representative granite-hosted uranium deposits in the MES uranium ore field, situated in the Ziyuan, Guangxi Province, South China. Uranium mineralization in the SZJ deposit mainly occurs as uraninite with quartz and calcite veins that is spatially associated with mafic dykes in the region. The hydrothermal alteration includes silicification, carbonation and hematitization. New uraninite chemical U-Pb geochronology and petrographic evidences provide the timing constraints and new insights into the formation of the SZJ uranium deposit. The results show that the first stage of uranium mineralization formed at 97.5±4.0 Ma, whereas another stage of uranium mineralization occurred at 70.2±1.6 Ma. Two stages of uranium mineralization are fairly consistent with two episodic crustal extensions that occurred at ~100 and ~70 Ma throughout South China. This study indicates that there are two uranium mineralization events in SZJ uranium ore field controlled by mafic dyke, supporting that mafic dykes play an important topochemical role in uranium concentration and/or mobilization. Therefore, geochemical U-Pb age firstly reinforces that ore-forming age of the SZJ uranium deposit mainly yields at 97.5±4.0 and 70.2±1.6 Ma. Additionally, geochemical age method is particularly useful for interest samples which record information on multi-stage uranium mineralizations in South China.

Interpretation of Magnetic Data and Locate Granite Hosted Uranium Deposit Targets in Namibia

Interpretation of Magnetic Data and Locate Granite Hosted Uranium Deposit Targets in Namibia

In-situ SIMS uraninite U–Pb dating and genesis of the Xianshi granite-hosted uranium deposit, South China

The southeastern part of the Nanling metallogenic province, China is host to numerous granite-hosted vein-type hydrothermal uranium deposits. The geology and geochemistry of these deposits have been extensively studied. However, accurate and precise ages for the uranium mineralization are scarce because the uranium minerals in these deposits are usually fine grained, and may have formed in several stages. Therefore, the ages previously obtained by the bulk dating techniques are possibly a mixed age.The Xianshi uranium deposit, located in the southeastern part of the Guidong granite complex, is a major uranium deposit in South China. The uranium mineralization from this deposit is mainly fine grained uraninite in quartz or calcite veins which are spatially associated with the Cretaceous mantle-derived mafic dykes. Micro-Raman spectroscopy and X-ray diffraction analyses indicate that the dominant uranium mineral occurs as a rare form of uraninite (U3O7). Three distinct generations of uranium minerals have been identified based on petrographic and field relations. Stage 1 uraninite has the lowest UO2 and highest PbO contents whereas Stage 3 uraninite has the highest UO2 and lowest PbO contents.Uraninite from the Xianshi deposit was dated using an in-situ SIMS U–Pb dating technique. The results show three distinct age groups: 135±4Ma, 113±2Ma and 104±2Ma, which are in excellent agreement with the ages of three episodes of mantle-derived mafic dykes. Therefore, the Xianshi uranium deposit has experienced at least three hydrothermal events that are responsible for the deposition of uranium ores, which are genetically related to the emplacement of three sets of mafic dykes.

Petrography and genetic history of coffinite and uraninite from the Liueryiqi granite-hosted uranium deposit, SE China

Coffinite and uraninite banded aggregates from the Liueryiqi granite-hosted uranium deposit, SE China, form a unique “bull's-eye” texture. These aggregates consist of concentric bands of uraninite and coffinite that range in thickness from 1 to 10 μm. Electron microprobe analyses indicate that coffinite and uraninite contain Ca and Al impurities. Altered uraninite, feldspar, quartz or pyrite grains form the center of these aggregates. Other minerals associated with the “bull's-eye” aggregates are marcasite, sphalerite, galena, chalcopyrite, bornite, clausthalite, microcrystalline quartz, fluorite and calcite. The aggregates occur within highly altered and brecciated zones of a two-mica leucogranite. The mineral assemblage associated with these aggregates is consistent with low-temperature fluids (∼125 to 200 °C), pH=6.0 to 8.5, Eh=−0.640 to −0.704 V, and dissolved silica contents of 10 −3.5 to 10 −2.7 mol/l (19 to 120 ppm as SiO 2). At depth, CO 2 concentrations of mineralizing fluids percolating through the breccia and fractured zones of the altered host granites likely ranged from 3.5 to 2.6 mol% at temperatures between 126 and 178 °C and a lithostatic pressure of 500 to 800 bars. Uranium was likely transported as a uranyl dicarbonate ion, which is the dominant dissolved uranium carbonate species in the pH range 6.0 to 8.5. Periodic changes in dissolved silica content in the mineralizing fluids resulted in the formation of the coffinite–uraninite intergrowth-banded aggregates. The precipitating mechanism resulting in the precipitation of the reduced uranium phases was likely the oxidation of reduced S 2− and Se 2−, which were supplied by the mineralizing fluids and were in equilibrium with uranyl ions.

The Rössing Uranium Deposit: a product of late-kinematic localization of uraniferous granites in the Central Zone of the Damara Orogen, Namibia

The Rössing granite-hosted uranium deposit in the Central Zone of the Pan-African Damara Orogen, Namibia, is situated in the “SJ area” to the south of the Rössing Dome. The coincidence of a number of features in this area suggests that mineralization is closely linked to late-kinematic evolution of the Rössing Dome. These features include: (1) the rotation of the dome's long axis (trend of 017°), relative to the regional F 3 trend of 042°; (2) southward dome impingement, concomitant with dome rotation, producing a wedge-shaped zone of alkali-leucogranites, within which uranium mineralization is transgressive with respect to granites and their host lithologies; uranium mineralization and a high fluid flux are also confined to this arcuate zone to the south and south-east of the dome core and (3) fault modeling that indicates that the SJ area underwent late-D 3 to D 4 brittle–ductile deformation, producing a dense fault network that was exploited by leucogranites. Dome rotation and southward impingement occurred after a protracted period of transtensional tectonism in the Central Zone, from ca. 542 to 526 Ma, during which I- and S-type granites were initiated in a metamorphic core complex. Late-kinematic deformation involved a rejuvenation of the stresses that acted from ca. 600 to 550 Ma. This deformation overlapped with uranium-enriched granite intrusion in the Central Zone at 510 ± 3 Ma. Such late-kinematic, north–south transpression, which persisted into the post-kinematic cooling phase until at least 478 ± 4 Ma, was synchronous with left-lateral displacement along NNE-trending (“Welwitschia Trend”) shears in the vicinity of Rössing. Late-kinematic deformation, causing block rotation, overlying dome rotation and interaction of the more competent units of the Khan Formation with the Rössing Formation in the dome rim was pivotal in the localization of uranium-enriched granites within a highly fractured, high-strain zone that was also the site of prolonged/high fluid flux.

Long-Term Stabilization of Uranium By U(VI) Phosphate Mineralization in Weathered Bedrock and Soils Developed Over a Granite-Hosted Uranium Deposit

ABSTRACTThe unmined Coles Hill U deposit in the Virginia Piedmont represents a unique natural laboratory for studying the long-term containment of U by phosphate minerals. The primary ore assemblage consists of coffinite and apatite and is hosted in foliated granite. Geochemical and mineralogical studies of the weathered bedrock and soils developed over the primary ore body indicate that U transport is inhibited by the precipitation of meta-autunite group minerals. The lower part of the profile, which consists of ground water saturated saprolite, contains 1400 mg/kg U (in the solid). This concentration is 1.5 times greater than the mean ore grade of the deposit, indicating that the saprolites are enriched in U relative to the underlying primary ore. Uranium within the saprolite is dominantly associated with 10 to 500 μm long, tabular crystals of (Ba, Ca, Sr) meta-autunite. Ground waters from this zone contain less than 14 μg/L dissolved U suggesting that the U(VI) phosphate minerals present are capable of controlling dissolved U concentrations at values lower than the US-EPA maximum contaminant level (30 μg/L). Mineralogical characterization of the unsaturated soil horizons indicate that geochemical conditions in these zones are not conducive to U stabilization by meta-autunite mineralization. In the vadose zone U is primarily associated with Al phosphate (crandallite) and with P adsorbed or coprecipitated with iron oxide mineral coatings. Geochemical gradients suggest that significant amounts of U have been leached from the vadose zone by infiltrating fluids and reprecipitated below the water table, where the activity ratio of dissolved phosphate to carbonate increases. Based on regional weathering rates, the processes responsible for stabilization of U within the Coles Hill system are estimated to have been active for hundreds of thousands of years.

Uranium-series radionuclide and element migration around the Sanerliu granite-hosted uranium deposit in southern China as a natural analogue for high-level radwaste repositories

The Sanerliu uranium deposit located in Hunan Province, South China, has a number of features analogous to aspects considered in the performance assessment of concepts for the disposal of high-level radwaste deep in granite. The deposit is hosted by medium- to coarse-grained porphyritic granite with a Rb–Sr isochron age of 215.2±6.3 Ma. The orebodies have a burial depth of more than 160–400 m under the surface of the ground. The major uranium mineralizations have U–Pb ages (pitchblende) of 103.2±0.6, 88.3±0.5 and 51.2±0.4 Ma and a hydrothermal origin. The isotopic data show that the Rb–Sr systems in the whole-rock samples of less- and un-fractured host granite around the orebodies have been little disturbed since the altered granite formed (132 Ma ago). Such samples have close to secular equilibrium activity ratios for 234U/ 238U, 230Th/ 234U and 226Ra/ 230Th, indicating that obvious water–rock interactions have not taken place in them within the interval of at least the last 1 Ma. The migration of uranium, thorium and most trace elements in the ores and the host granite are limited, often less than 30–35 m over 51 Ma. The alpha activity ratios 234U/ 238U, 230Th/ 234U and 226Ra/ 230Th for the samples from the fracture zones show obvious deviations from secular equilibrium, indicating the deposition of uranium during the last 1 Ma and of radium during the last 10 3 yr. The fracture zones are channels for groundwater flow. However, the migration of uranium-series radionuclides along the fracture zones is limited by adsorption onto the clays. This is confirmed by the absence of any hydrogeochemical indication on the ground surface of the orebodies. The ores have been well preserved in a natural aquifer system without any specially designed artificial barriers. The migration behavior of radionuclides in the ores of the Sanerliu deposit may be termed a worse-case scenario for the safety assessment of a high-level radwaste disposal repository hosted by granitic rocks.