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20 pages, 5778 KiB

Open AccessArticle

Characters and Metallogenetic Significance of Organic Matter in Coal from the Daying Sandstone-Hosted Uranium Deposit in the Northern Ordos Basin, China

by Qi Li, Bailin Wu, Jingjing Luo, Songlin Yang, Miao Wang, Mingyi Liu, Zhouyang Lin, Xiaorui Zhang, Long Zhang, Jiangqiang Wang and Mengdi Yang

Minerals 2023, 13(8), 1002; https://doi.org/10.3390/min13081002 - 28 Jul 2023

Cited by 1 | Viewed by 1109

Abstract

Coal is usually found in many sandstone-hosted uranium deposits in western China. However, it remains unclear whether coal organic matter is related to the mineralization of uranium in sandstone. In this study, the organic matter of coal containing lenticular carbon plant strips and [...] Read more.

Coal is usually found in many sandstone-hosted uranium deposits in western China. However, it remains unclear whether coal organic matter is related to the mineralization of uranium in sandstone. In this study, the organic matter of coal containing lenticular carbon plant strips and carbon clastics was analyzed in different alteration zones in the Daying sandstone-hosted uranium deposit located in the northern Ordos Basin. According to geological and geochemical characters, Daying sandstone can be classified into three alteration zones: the oxidation zone, the transition zone, and the reduction zone. The results are as follows. Firstly, the maceral organic matter of the coal at Daying is mostly vitrinite, indicating the characters of humic coal. The maturity of the organic matter is low, which is in the transitional evolution stage of immature lignite or lignite-long-flame coal. Secondly, the kerogen is mainly type III (humic type), which can be easily transformed into humic acid with a strong adsorption capacity for uranium. Thirdly, the vitrinite reflectance (Ro) of the coal organic matter in the transition zone is higher than that of the other alteration zones. Meanwhile, there are bright white bands in the microscopic components of the organic matter of the coal in the transition zone (mineralized zone) and a certain composition of the sapropelite has strong fluorescence. All three aforementioned phenomena are related to the radioactivity of uranium, and each of them possesses the potential for application in mineral exploration. Fourthly, the extraction and separation of humic substances indicates that humic acid plays a key role in organic matter-related uranium mineralization. In the transition zone, uranium can co-precipitate as a humate, and the transition zone’s organic carbon content increases. Therefore, the organic matter in coal contributes to sandstone-hosted uranium mineralization, providing a further guide to prospecting methodologies. Full article

(This article belongs to the Special Issue Geochemistry, Mineral Chemistry and Geochronology of Uranium Deposits)

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28 pages, 8838 KiB

Open AccessFeature PaperArticle

Uranium Mineralization in the MacInnis Lake Area, Nonacho Basin, Northwest Territories: Potential Linkages to Metasomatic Iron Alkali-Calcic Systems

by Kerstin Landry, Erin Adlakha, Andree Roy-Garand, Anna Terekhova, Jacob Hanley, Hendrik Falck and Edith Martel

Minerals 2022, 12(12), 1609; https://doi.org/10.3390/min12121609 - 14 Dec 2022

Cited by 1 | Viewed by 2621

Abstract

The intracratonic Paleoproterozoic Nonacho Basin, deposited on the western margin of the Rae craton, contains historic polymetallic (i.e., U, Cu, Fe, Pb, Zn, Ag) occurrences spatially associated with its unconformable contact with underlying crystalline basement rocks and regionally occurring faults. This study presents [...] Read more.

The intracratonic Paleoproterozoic Nonacho Basin, deposited on the western margin of the Rae craton, contains historic polymetallic (i.e., U, Cu, Fe, Pb, Zn, Ag) occurrences spatially associated with its unconformable contact with underlying crystalline basement rocks and regionally occurring faults. This study presents the paragenesis, mineral chemistry and geochemistry of uranium mineralized rocks and minerals of the MacInnis Lake sub-basin of the Nonacho Basin, to evaluate the style and relative timing of uranium mineralization. Mineralization is restricted to regionally occurring deformation zones, and post-dates widely spread and pervasive albitization and more local Ba-rich K-feldspar alteration of host rocks. Uranium mineralized rocks show elevated concentration of Cu, Ag and Au relative to variably altered host rocks. Microscopic and compositionally heterogeneous altered uraninite occurs (i) as overgrowths on partially dissolved Cu-sulphides with magnetite in chlorite ± quartz, calcite veins, and (ii) with minor uranophane in hematite-sericite-chlorite ± quartz breccia and stockwork. Both uraninite types are Th poor (<0.09 wt.% ThO₂) and variably rich in SO₄ (up to 2.26 wt.%), suggesting a low-temperature hydrothermal origin in a relatively oxidized environment. Rare-earth element (+Y) concentrations in type-i uraninite are high, up to 9.5 wt.% Σ(REE+Y)₂O₃ with Ce_N/Y_N values > 1, similar to REE compositions of uraninite in metasomatic iron and alkali-calcic systems (MIAC), including low-temperature hematite-type IOCG-deposits (e.g., Olympic Dam, Gawler Craton, Australia) and albitite-hosted uranium deposits (e.g., Southern Breccia, Great Bear Magmatic Zone, Canada, and Gunnar Deposit, Beaverlodge District, Canada). Both uraninite types are variably rich in Ba (up to 3 wt.% BaO), a geochemical marker for MIAC systems, provided by the dissolution of earlier secondary Ba-rich K-feldspar. Chemical U-Th-Pb dating yields resetting ages of <875 ± 35 Ma for type-ii uraninite-uranophane, younger than strike-slip movement along regional structures of the basin that are spatially associated with the uranium occurrences. We suggest that MacInnis Lake uranium occurrences formed from oxidized hydrothermal fluids along previously altered (albitized, potassically altered) regional-scale faults. Uranium minerals precipitated on earlier Fe-rich sulfides (chalcopyrite, bornite), which acted as a redox trap for mineralization, in low-temperature (~310–330 °C, based on Al-in-chlorite thermometry) breccias and stockwork zones, late in a metasomatic iron and alkali-calcic alteration system. Full article

(This article belongs to the Special Issue Geochemistry, Mineral Chemistry and Geochronology of Uranium Deposits)

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25 pages, 9560 KiB

Open AccessArticle

Constraints on Sandstone-Type Uranium Deposits by the Tectonic Uplift and Denudation Process in the Eastern Junggar Basin, Northwest China: Evidence from Apatite Fission Track and Detrital Zircon U-Pb Ages

by Zhangyue Liu, Suping Peng, Mingkuan Qin, Shaohua Huang, Yingying Geng and Zhongbo He

Minerals 2022, 12(7), 905; https://doi.org/10.3390/min12070905 - 19 Jul 2022

Cited by 5 | Viewed by 2159

Abstract

The uplift and denudation history of the orogenic belt and the basin–mountain coupling process have directly or indirectly affected the generation, scale, and preservation of sandstone-type uranium deposits in the eastern Junggar Basin by controlling the uranium source, lithology, facies, hydrogeology, post-generation modification, [...] Read more.

The uplift and denudation history of the orogenic belt and the basin–mountain coupling process have directly or indirectly affected the generation, scale, and preservation of sandstone-type uranium deposits in the eastern Junggar Basin by controlling the uranium source, lithology, facies, hydrogeology, post-generation modification, and other mineralization conditions. Taking the eastern Junggar Basin as the research area, this study proposes the constraints on sandstone-type uranium deposits by the tectonic uplift and denudation history of the orogenic belt in the basin using the apatite fission track (AFT), detrital zircon geochronology, and other methods. The results of the AFT age test and thermal path simulation indicate that the orogenic belt in the eastern Junggar Basin underwent four rapid uplifts; (from approximately 300 Ma to approximately 250 Ma, from approximately 130 Ma to approximately 90 Ma, from approximately 65 Ma to approximately 30 Ma, and from approximately 20 Ma to 0 Ma). Moreover, the timing of the uplift has a spatial trend of gradually becoming younger from south to north. The detrital zircon U-Pb age test showed that the sediment source area of the basin is mainly distributed in three age intervals, i.e., 460–390, 360–270, and 190–170 Ma. The comprehensive evaluation of the clastic sediment composition, stratigraphic distribution of the erosion source area, and thermal history showed that a large amount of exposed Carboniferous–Permian granites in the Qinglidi and Karameri Mountain erosion source areas contributed dominant sediment material and uranium sources for the Triassic and Middle and Lower Jurassic strata in the basin. The Ordovician–Early Devonian granites only provided sediment sources for the Upper Triassic and Lower Jurassic strata in the basin. Altay Mountain contributed some sediment sources for the Middle and Upper Jurassic strata after the magmatic activity and rapid uplift occurred in the Middle Jurassic. Based on the comprehensive analysis of the influence of the tectonic uplift process of the orogenic belt and the transformation of material source areas on uranium mineralization, the granites in the erosion source areas are proposed to contribute both external and internal uranium sources for uranium mineralization. Uranium mineralization mainly occurred in the tectonic retreat period after the rapid uplifts of the Cretaceous and Paleogene. It was terminated by the intensive uplift-induced stratigraphic deformation in the Miocene. Full article

(This article belongs to the Special Issue Geochemistry, Mineral Chemistry and Geochronology of Uranium Deposits)

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17 pages, 6301 KiB

Open AccessFeature PaperArticle

LA-ICP-MS Mapping of Barren Sandstone from the Proterozoic Athabasca Basin (Canada)—Footprint of U- and REE-Rich Basinal Fluids

by Guoxiang Chi, Eric G. Potter, Duane C. Petts, Simon Jackson and Haixia Chu

Minerals 2022, 12(6), 733; https://doi.org/10.3390/min12060733 - 8 Jun 2022

Cited by 4 | Viewed by 2699

Abstract

The Proterozoic Athabasca Basin hosts a large number of high-grade, large-tonnage unconformity-related uranium (U) deposits, many of which are also enriched in rare earth elements (REE). The basin also contains hydrothermal REE mineralization unassociated with U. Previous studies postulated that U and REE [...] Read more.

The Proterozoic Athabasca Basin hosts a large number of high-grade, large-tonnage unconformity-related uranium (U) deposits, many of which are also enriched in rare earth elements (REE). The basin also contains hydrothermal REE mineralization unassociated with U. Previous studies postulated that U and REE were derived from either the basin or the basement; however, the exact source of the metals remains ambiguous. This study provides evidence of U- and REE-rich fluids throughout the Athabasca Basin through laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) mapping of barren sandstone distal to mineralized areas. The results indicate that elevated U and REE concentrations mainly occur in the matrix; there are strong positive correlations between U and REE, Th, P and Sr, and moderate positive correlations between U and Zr, Ba, Fe, Al, K and Ca, but the few spots with the highest U are unrelated to these elements. Quantitative evaluation of the element correlations, together with scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) analysis, suggests that most of the elevated U and REE are hosted in aluminum phosphate sulfate (APS) minerals rather than apatite and monazite. As the APS minerals are of diagenetic-hydrothermal origin, the results testify to the presence of U- and REE-rich fluids within the Athabasca Basin. The elevated Th/U ratio (~10) and REE pattern (strong heavy rare earth element (HREE) depletion) are consistent with a model in which large amounts of U and REE (especially HREE) were leached from the sandstone within the Athabasca Basin and contributed to U and REE mineralization near the unconformity between the sedimentary rocks in the basin and underlying basement rocks. This study demonstrates that LA-ICP-MS mapping can be effectively used to evaluate microscale distribution of elements and their mobility in sedimentary rocks to address mineralization related problems. Full article

(This article belongs to the Special Issue Geochemistry, Mineral Chemistry and Geochronology of Uranium Deposits)

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21 pages, 4721 KiB

Open AccessFeature PaperArticle

Uranyl Minerals from Abandoned Podgórze Mine (Sudetes Mountains, SW Poland) and Their REE Content

by Marcin Daniel Syczewski, Rafał Siuda, Jan Rohovec, Šárka Matoušková and Jan Parafiniuk

Minerals 2022, 12(3), 307; https://doi.org/10.3390/min12030307 - 28 Feb 2022

Cited by 1 | Viewed by 2422

Abstract

The Podgórze uranium deposit is located near Kowary in the Sudetes Mountains, SW Poland. The mine is located in the Karkonosze-Izera block, largely comprising Cambrian to Devonian metamorphic rocks intruded by the Variscan Karkonosze granite. Uranyl minerals from the Podgórze mine can be [...] Read more.

The Podgórze uranium deposit is located near Kowary in the Sudetes Mountains, SW Poland. The mine is located in the Karkonosze-Izera block, largely comprising Cambrian to Devonian metamorphic rocks intruded by the Variscan Karkonosze granite. Uranyl minerals from the Podgórze mine can be divided into three assemblages. The first one is associated with heavily ventilated mining galleries. The next assemblage is related to the outflow of water from fissures in the walls of the mine galleries. The last assemblage appears in the mine dump, where there is increased activity of other weathering products. The main purpose of this paper is to determine the mineralogical characteristics of uranyl minerals from the abandoned Podgórze uranium mine and reconstruct the physicochemical crystallization conditions based on the concentrations of rare earth elements (REEs) in these minerals. The results of thermodynamic modeling show that the aqueous species of uranyl ion in the mine water are represented by UO₂HAsO₄ (aq), UO₂CO₃(OH)₃⁻, UO₂CO₃ (aq), and UO₂OH⁺. The content of REEs and their distribution patterns were used to determine the crystallization conditions of uranyl minerals. Uranyl carbonates and arsenates have generally low concentrations of REEs compared to uranyl silicates, phosphates, and hydroxides, which have higher concentrations. The differences in REE concentration patterns may be related with the oxidizing nature of water circulating in the subsurface part of the deposit. Full article

(This article belongs to the Special Issue Geochemistry, Mineral Chemistry and Geochronology of Uranium Deposits)

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18 pages, 4724 KiB

Open AccessArticle

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

by Qing Lan, Shanling Fu and Jinrong Lin

Minerals 2022, 12(2), 227; https://doi.org/10.3390/min12020227 - 10 Feb 2022

Cited by 2 | Viewed by 1804

Abstract

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 [...] Read more.

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. Full article

(This article belongs to the Special Issue Geochemistry, Mineral Chemistry and Geochronology of Uranium Deposits)

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21 pages, 4279 KiB

Open AccessArticle

Implications of Major and Trace Element Migration in Altered Granites for Hydrothermal Alteration and Granite-Related Uranium Mineralization in the Sanjiu Ore Field, South China

by Xu Chen, Chunhua Wen, Debao Meng, Bin Li, Biguang Jiang and Jinning Qin

Minerals 2022, 12(2), 144; https://doi.org/10.3390/min12020144 - 25 Jan 2022

Cited by 4 | Viewed by 2996

Abstract

The recently discovered Sanjiu ore field (SJOF) is a granite-related uranium ore field located in the middle of Zhuguangshan (South China). The relationship between hydrothermal alteration of granite and uranium mineralization in the SJOF is crucial yet understudied. In this study, the major- [...] Read more.

The recently discovered Sanjiu ore field (SJOF) is a granite-related uranium ore field located in the middle of Zhuguangshan (South China). The relationship between hydrothermal alteration of granite and uranium mineralization in the SJOF is crucial yet understudied. In this study, the major- and trace-element contents of granite samples (fresh granite, altered granite, and tectonites) with different uranium contents were analyzed by using X-ray fluorescence spectroscopy (XRF) and inductively coupled plasma–mass spectrometry (ICP–MS). The analytical results show a relative increase in Si, S, Ca, Pb, Mo, and Sb content in altered granites and tectonites, relative to fresh granites. During the mineralization stage, the increase of the aforementioned elements is related to various hydrothermal alterations (e.g., silicification, carbonation, sulfation, etc.) and newly formed minerals (e.g., microfine crystalline quartz veins; calcite agglomerates or fine veins; and metal sulfides, such as pyrite). There is a concomitant relative decrease in Na, K, Al, Fe, Mg, and other elemental contents that may be due to mineralogical alteration processes, such as biotite to chlorite, feldspar-group minerals to clay minerals, and redox of Fe-bearing minerals. The LREE/HREE ratio in altered granites decreases significantly with the increase in uranium content, suggesting that a low LREE/HREE ratio may be a prospecting indicator. The normalized trace-element patterns of mineralized granite (ore) and the relatively high U content of fresh granite imply that granitic rocks may be the primary uranium source in the SJOF. The uranium mineralization is mainly concentrated in the redox zone that occurs at a depth of 100−300 m. The redox zone is characterized by the most developed hydrothermal alterations and enrichment of trace elements, including W, Mo, Sb, Li, and the HREE. Full article

(This article belongs to the Special Issue Geochemistry, Mineral Chemistry and Geochronology of Uranium Deposits)

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21 pages, 7441 KiB

Open AccessArticle

Hydrothermal Alteration and Its Superimposed Enrichment for Qianjiadian Tabular-Type Uranium Deposit in Southwestern Songliao Basin

by Ming-Kuan Qin, Shao-Hua Huang, Jia-Lin Liu, Zhang-Yue Liu, Qiang Guo, Li-Cheng Jia and Wen-Jian Jiang

Minerals 2022, 12(1), 52; https://doi.org/10.3390/min12010052 - 30 Dec 2021

Cited by 10 | Viewed by 2435

Abstract

The evolution characteristics of hydrothermal activity and superimposed uranium mineralization in the Qianjiadian ore field in southwestern Songliao Basin are still controversial and lack direct evidence. In this comprehensive study, a detailed identification of dolerite and hydrothermally altered un-mineralized sandstone and sandstone-hosted ore [...] Read more.

The evolution characteristics of hydrothermal activity and superimposed uranium mineralization in the Qianjiadian ore field in southwestern Songliao Basin are still controversial and lack direct evidence. In this comprehensive study, a detailed identification of dolerite and hydrothermally altered un-mineralized sandstone and sandstone-hosted ore in the Yaojia Formation have been performed through the use of scanning electron microscopy observation, electron probe, carbon-oxygen-sulfur isotope, and fluid inclusion analyses. The results show that the hydrothermal fluid derived from the intermediate-basic magma intrusion is a low-temperature reducing alkaline fluid and rich in CO₂, Si, Zr, Ti, Fe, Mg, Mn, and Ca, producing different types of altered mineral assemblages in the rocks, including carbonation, pyritization, sphalerite mineralization, clausthalite mineralization, silicification, and biotitization. Specifically, the carbonate minerals in sandstone are mixed products of deep hydrothermal fluid and meteoric water, with carbon and oxygen isotopes ranging from −5.2‰ to −1.7‰ and −20.4‰ to −11.1‰, respectively. Carbon source of the carbonate minerals in dolerite is mainly inorganic carbon produced at the late stage of intermediate-basic magma evolution, with carbon and oxygen isotopes from −16.1‰ to −7.2‰ and −18.2‰ to −14.5‰, respectively. Various carbonate minerals in the rocks may have been precipitated by the hydrothermal fluid after the magmatic stage, due to the change of its CO₂ fugacity, temperature, and cation concentration during the long-term evolution stage. A series of carbonate minerals were generated as calcite, dolomite, ankerite, ferromanganese dolomite, and dawsonite. The precipitation processes and different types of carbonate mineral mixtures identified in this study mainly occur as parallel, gradual transition, interlacing, or inclusion metasomatism in the same vein body, without obvious mineralogical and petrologic characteristics of penetrating relationship. Homogenization temperature of fluid inclusions in calcite is high, in the range of 203–234 °C, with a low salinity of 0.71–4.34% NaCl, and the data range is relatively concentrated. Homogenization temperature of fluid inclusions in ankerite is usually low, ranging from 100 °C to 232 °C, with a high salinity of 4.18–9.98% NaCl. The precipitation processes of carbonate minerals and the results of this study are basically in consistent. Overall, the sandstone-type uranium deposits have a temporal and genetic relationship with hydrothermal activities during Paleogene. (1) Hydrothermal activity was directly involved in uranium mineralization, result in dissolution and reprecipitation of earlier uranium minerals, forming uranium-bearing ankerite and complexes containing uranium, zirconium, silicon, and titanium. (2) Hydrothermal fluid activity provided reducing agent to promote hydrocarbon generation from pyrolysis of carbonaceous fragments and accelerate uranium precipitation rate. (3) Regional water stagnation prolongs reaction time, contributing to huge uranium enrichment. This study provides new petrologic, mineralogical, and geochemical evidence for multi-fluid coupled and superimposed mineralization of sandstone-hosted uranium deposits in the sedimentary basin. Full article

(This article belongs to the Special Issue Geochemistry, Mineral Chemistry and Geochronology of Uranium Deposits)

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14 pages, 4901 KiB

Open AccessArticle

Genesis of Megacrystalline Uraninite: A Case Study of the Haita Area of the Western Margin of the Yangtze Block, China

by Zhengqi Xu, Minghui Yin, Youliang Chen, Lu Xiang, Hao Song, Chengjiang Zhang, Jian Yao and Hu Guo

Minerals 2021, 11(11), 1173; https://doi.org/10.3390/min11111173 - 22 Oct 2021

Cited by 6 | Viewed by 2089

Abstract

Megacrystalline uraninite (up to one centimeter in size) represents one of the most important discoveries in uranium mineralogy in the western margin of the Yangtze Block and even in China in recent years. However, the genesis of megacrystalline uraninite remains controversial. In this [...] Read more.

Megacrystalline uraninite (up to one centimeter in size) represents one of the most important discoveries in uranium mineralogy in the western margin of the Yangtze Block and even in China in recent years. However, the genesis of megacrystalline uraninite remains controversial. In this study, the megacrystalline uraninite found in the felsic and quartz veins in the Haita area is examined for the first time. The study examined the geochemical characteristics of uraninite in the two veins and resulted in two primary findings. (1) The genesis of the uraninite was likely intrusive and was closely related to partial melting. (2) The quartz vein and feldspar vein are cogenetic and have a simple differentiation evolution relationship. Therefore, the partial melting of felsic materials during migmatization may be the most important mechanism of uranium mineralization in the study area. Furthermore, further simple fractional crystallization may be another important mechanism for the formation of megacrystalline uraninite. This study enriches the REE database of uraninite in uranium deposits worldwide, which is meaningful for studying the genesis of megacrystalline uraninite. Full article

(This article belongs to the Special Issue Geochemistry, Mineral Chemistry and Geochronology of Uranium Deposits)

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23 pages, 11295 KiB

Open AccessArticle

Uraninite from the Guangshigou Pegmatite-Type Uranium Deposit in the North Qinling Orogen, Central China: Its Occurrence, Alteration and Implications for Post-Caledonian Uranium Circulation

by Bin Wu, Christophe Bonnetti, Yue Liu, Zhan-Shi Zhang, Guo-Lin Guo, Guang-Lai Li, Yin-Qiu Hu and Zhao-Yan Yan

Minerals 2021, 11(7), 729; https://doi.org/10.3390/min11070729 - 5 Jul 2021

Cited by 7 | Viewed by 3747

Abstract

The Guangshigou deposit is the largest pegmatite-type uranium deposit in the Shangdan domain of the North Qinling Orogenic Belt, which is characterized by the enrichment of uraninite hosted in biotite granitic pegmatites. At Guangshigou, uraninite commonly occurs as mineral inclusions in quartz, K-feldspar [...] Read more.

The Guangshigou deposit is the largest pegmatite-type uranium deposit in the Shangdan domain of the North Qinling Orogenic Belt, which is characterized by the enrichment of uraninite hosted in biotite granitic pegmatites. At Guangshigou, uraninite commonly occurs as mineral inclusions in quartz, K-feldspar and biotite or in interstices of these rock-forming minerals with magmatic characteristics (e.g., U/Th < 100, high ThO₂, Y₂O₃ and REE₂O₃ contents and low concentrations of CaO, FeO and SiO₂). It crystallized at 407.6 ± 2.9 Ma from fractionated calc-alkaline high-K pegmatitic melts under conditions of 470–700 °C and 2.4–3.4 kbar as deduced by the compositions of coexisting peritectic biotite. The primary uranium mineralization took place during the Late Caledonian post-collisional extension in the North Qinling Orogen. After this magmatic event, uraninite has experienced multiple episodes of fluid-assisted metasomatism, which generated an alteration halo of mineral assemblages. The alteration halo (or radiohalo) was the result of the combined effects of metamictization and metasomatism characterized by an assemblage of goethite, coffinite and an unidentified aluminosilicate (probably clay minerals) around altered uraninite. This fluid-assisted alteration was concomitant with the albitization of K-feldspar subsequently followed by the coffinitization of uraninite during the major period of 84.9–143.6 Ma, as determined by U-Th-Pb chemical ages. Further investigations revealed that the metasomatic overprinting on uraninite initially and preferentially took place along microcracks or cavities induced by metamictization and promoted their amorphization, followed by the release of U and Pb from structure and the incorporation of K, Ca and Si from the fluids, finally resulting in various degrees of uraninite coffinitization. The released U and Pb were transported by alkali-rich, relatively oxidizing fluids and then re-precipitated locally as coffinite and an amorphous U-Pb-rich silicate under low to moderate temperature conditions (85–174 °C). The compositional changes in primary uraninite, its structure amorphization together with the paragenetic sequence of secondary phases, therefore, corroborate a combined result of intense metamictization of uraninite and an influx of alkali–metasomatic fluids during the Late Mesozoic Yanshanian magmatic event in the region. Hence, the remobilization and circulation of uranium in the North Qinling Orogen was most likely driven by post-Caledonian magmatism and hydrothermal activities related to large-scale tectonic events. In this regards, Paleozoic pegmatite-type uranium mineralization may represent a significant uranium source for Mesozoic hydrothermal mineralization identified in the Qinling Orogenic Belt. Full article

(This article belongs to the Special Issue Geochemistry, Mineral Chemistry and Geochronology of Uranium Deposits)

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