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1.
The Younger Granites of Yahmid-Um Adawi area, located in the southeastern part of Sinai Peninsula, comprise two coeval Late Neoproterozoic post-collisional alkaline (hypersolvous alkali-feldspar granites; 608–580?Ma) and calc-alkaline (transsolvous monzo- and syenogranites; 635–590?Ma) suites. The calc-alkaline suite granitoids are magnesian and peraluminous to metaluminous, whereas the alkaline ones are magnesian to ferroan alkaline to slightly metaluminous. Both granitoid suites exhibit many of the typical geochemical features of A-type granites such as enrichment in Nb (>20?ppm), Zr (>250?ppm), Zn (>100?ppm) and Ce (>100?ppm) and high 10000*Ga/Al2O3 ratios (>2.6) and Zr?+?Nb?+?Y?+?Ce (>350?ppm). Accessory mineral saturation thermometers demonstrated former crystallization of apatite at high temperatures prior to zircon and monazite separation from the magma for both granitoid suites. The mild zircon saturation temperatures of the studied Younger Granites (around 800?°C) imply low-temperature crustal fusion and incomplete melting of the largely refractory zircon. The two Younger Granite suites were semi-synchronously evolved during the post-collisional stage of the Arabian-Nubian Shield subsequent to the collision between the juvenile shield crust and the older pre-Neoproterozoic continental blocks of west Gondwana. Their parental magmas has been generated by melting of crustal source rocks with minor involvement from mantle, which might participated chiefly as a source of heat necessary for fusion of the crustal precursor. Extensive in-situ gamma-ray spectrometry revealed anomalously high radioactivity of some Younger Granite exposures along Wadi Um Adawi (eU; 388–746?ppm and eTh; 1857–2527?ppm) and pegmatitic pockets pertaining to the calc-alkaline suite (equivalent U and Th; 212–252?ppm and 750–1757?ppm, respectively). The radioactivity of the syngenetic pegmatites arises from the primary radioactive minerals uranothorite and thorite together with the U- and/or Th-bearing minerals zircon, columbite, samarskite and monazite. The anomalously high radioactivity of some Younger Granite exposures in Wadi Um Adawi stem from their appreciable enclosure of the epigenetic uranium minerals metatorbenite and uranophane.  相似文献   

2.
High levels of U (up to 5570 μg/L) have been discovered in well waters near Simpsonville, South Carolina, USA. In order to characterize the mineralogical source of the U and possible structural controls on its presence, a deep (214 m) well was cored adjacent to one of the enriched wells. The highest gamma-ray emissions in the recovered core occur in coarse biotite granite at a depth just below 52 m. A slickenlined fault plane at 48.6 m and narrow pegmatite layers at depths of 113, 203 and 207 m also yield high gamma-ray counts. Thin sections were made from the above materials and along several subvertical healed fractures. Uraninite and coffinite are the principal U-rich minerals in the core. Other U-bearing minerals include thorite and thorogummite, monazite, zircon and allanite. Primary uraninite occurs in the biotite granite and in pegmatite layers. Secondary coffinite is present as tiny (<5 μm) crystals dispersed along fractures in the granite and pegmatites. Coffinite also occurs along the slickenlined fault plane, where it is associated with calcite and calcic zeolite and also replaces allanite. Coffinite lacks radiogenic Pb, hence is considerably younger than the uraninite.Dissolution of partially oxidized Ca-rich uraninite occurring in the surficial biotite granite (or secondary coffinite in fracture zones) is likely the main source for the current high levels of U in nearby area wells. The high-U well waters have a carbonate signature, consistent with pervasive calcite vein mineralization in the core. Aqueous speciation calculations suggest U transport as an uranyl (U6+) hydroxyl-carbonate complex. Later reduction resulted in secondary precipitation along fractures as a U4+ mineral (i.e., coffinite).  相似文献   

3.
《International Geology Review》2012,54(11):1067-1077
Mineral chemistry and typomorphic characteristics are used to monitor the physicochemical evolution of late-magmatic to postmagmatic alteration processes that resulted in the development of a radioactive and albite-enriched microgranite stock. The mineral paragenesis indicates that postmagmatic fluids were enriched in Nb, Zn, Mn, U., Th, Zr, and Y, in addition to Rb, Li, and F Manganocolumbite with extremely high Nb/(Nb+Ta) (0.99), Mn/(Mn+Fe) (0.82), and zircon with high Zr/(Zr+Hf) (0.97) indicate crystallization under alkaline, relatively high-temperature conditions (>425° C). The close association of manganocolumbite, Nb-Mn-Zn- rich ilmenite (with 1.2 to 14.5 wt% ZnO), spessartine garnet (with 68.2-89.4 mol% spessartine), zircon, xenotime, zinnwaldite mica (up to 5.98 wt% F), and fluorite indicates the strong affinity of the elements of Nb, Y., Zr, Mn, and Zn for stable complexing by K+, Na+, Li+, and F? rich supercritical fluids during the course of extraction and transportation.

The enrichment of the interacting fluid in U and Th is depicted by the presence of up to 1.6% UO2 in manganocolumbite and Hf-bearing zircon, and up to 10.5% ThO2 in monazite, in addition to locally abundant thorite and uranophane. It is suggested that the uranium mineralization, mainly as fracture fillings, formed during the waning stage of hydrothermal activity.  相似文献   

4.
A natural, altered zircon crystal from an alkaline pegmatite from the Zomba–Malosa Complex of the Chilwa Alkaline Province in Malawi has been studied by a wide range of analytical techniques to understand the alteration process. The investigated zircon shows two texturally and chemically different domains. Whereas the central parts of the grain (zircon I) appear homogeneous in backscattered electron images and are characterised by high concentrations of trace elements, particularly Th, U, and Y, the outer regions (zircon II) contain significantly less trace elements, numerous pores, and inclusions of thorite, ytttrialite, and fergusonite. Zircon II contains very low or undetectable concentrations of non-formula elements such as Ca, Al, and Fe, which are commonly observed in high concentrations in altered radiation-damaged zircon. U–Pb dating of both zircon domains by LA-ICPMS and SHRIMP yielded statistically indistinguishable U–Pb weighted average ages of 119.3 ± 2.1 (2σ) and 118 ± 1.2 (2σ) Ma, respectively, demonstrating that the zircon had not accumulated a significant amount of self-irradiation damage at the time of the alteration event. Electron microprobe dating of thorite inclusions in zircon II yielded a Th–U-total Pb model age of 122 ± 5 (2σ) Ma, supporting the age relationship between both zircon domains. The hydrothermal solution responsible for the alteration of the investigated zircon was alkaline and rich in CO3 2−, as suggested by the occurrence of REE carbonates and CO2-bearing fluid inclusions. The alteration of the crystalline, trace element-rich zircon is explained by an interface-coupled dissolution-reprecipitation mechanism. During such a process, the congruent dissolution of the trace element-rich parent zircon I was spatially and temporally coupled to the precipitation of the trace element-poor zircon II at an inward moving dissolution-precipitation front. The driving force for such a process was merely the difference between the solubility of the trace element-rich and -poor zircon in the hydrothermal solution. The replacement process and the occurrence of mineral inclusions and porosity in the product zircon II is explained by the thermodynamics of solid solution-aqueous solution systems.  相似文献   

5.
U-bearing solid phases from Forsmark, Sweden, a proposed host for radioactive waste repositories, have been identified and characterized. Elevated dissolved U was found in some groundwater samples during the site investigations, prompting a need to study the local U geochemistry. Previous hydrochemical and whole-rock geochemical studies indicated that U was derived from local pegmatites, and mobilized and re-deposited during several geological events. In this study, down-hole gamma logs guided sampling of local pegmatites, cataclasites, and fracture fillings. Back-scattered electron-imaging, petrographic microscopy, and electron microprobe analyses were used to find and analyze U phases in thin sections. The results show that the principal U sources at Forsmark include pegmatitic uraninite (PbO up to ∼22 wt%) and metamict uranothorite. These primary minerals show variable degrees of alteration such as enrichment in Ca and Al and/or replacement by secondary Ca–U(VI)-silicates, haiweeite and uranophane. The haiweeite contains up to ∼5 wt% Al2O3, a chemical signature reflecting early (Proterozoic) events of hydrothermal fluid migration. Coffinitized, secondary uraninite is found in association with FeAl-silicates or Palaeozoic sulfide/sulfate minerals, indicating remobilization-precipitation and/or a secondary, sedimentary source of U. It is inferred that U was oxidized during geologically early periods. Later, U(IV) phases formed in fractures open to fluid circulation during the Palaeozoic. This study establishes the phases available as local U sources and/or sinks, and which will be considered in future isotopic and hydrochemical studies aimed to constrain the mechanisms and timing of water–U phase interaction.  相似文献   

6.
Two brecciated shear zones (NNW-SSE) are found crosscutting cataclastic rocks. The cataclastic rocks (3.0 km2) occupy the core of the granitic pluton and enclose a roof pendant of mafic-ultramafic rocks. The NNW-SSE-extending lamprophyre dykes vary in thickness from 0.5 m to 1 m and up to 800 m long, cutting the cataclastic rocks and are composed mainly of plagioclases, amphiboles, relics of pyroxenes and K-feldspar phenocrysts embedded in fine-grained groundmass. They are characterized as being peraluminous, calc-alkaline in composition (chemical trap) and enriched in calcite, sulfide and P2O5. The lamprophyres were affected by hydrothermal alteration (chlorite-carbonate alteration) while the cataclastic rocks were affected by diagenetic alteration (K-feldspar-albite alteration). Uranium mineralization is the product of hydrothermal events and has been investigated by X-ray diffraction (XRD) and environmental scanning electron microscopy (ESEM), involving primary uranium minerals (U3O8) and secondary uranium minerals (uranophane and beta-uranophane, kasolite, torbernite, autonite and meta-autonite) in addition to U- bearing minerals (astrocyanite, betafite and fergusonite). The presence of different mineral parageneses associated with clay minerals indicates that the lamprophyres were subjected to acidic and alkaline mineralizing solutions. Moreover, the U-Zr/U, U-Ce/U values show negative correlations, confirming U-enrichment in both cataclastic rocks and shear zones while the Th-eU/eTh, Th-Zr/Th and Th-Ce/Th values show negative correlations, indicating that the U-bearing solutions are rich in Th in the cataclastic rocks only.  相似文献   

7.
《Gondwana Research》2009,15(4):569-586
This paper outlines the CHIME (chemical Th–U-total Pb isochron method) dating method, which is based on precise electron microprobe analyses of Th, U and Pb in Th- and U-bearing accessory minerals such as monazite, xenotime, zircon and polycrase. The age-mapping technique that is applicable to young monazite and zircon is also described. CHIME dating consists of analyzing multiple spots within homogeneous age domains that show sufficient compositional variation, and then these data are used to construct a “pseudo-isochron” from which an age can be obtained via regression. This method, when coupled with discrimination of possibly concordant age data by chemical criteria such as the (Ca + Si)/(Th + U + Pb + S) ratio for monazite and Ca and S contents for zircon, has the potential advantage of significant precision, and the ability to work with minerals that have a significant initial common Pb component. This technique can identify two or more homogeneous domains that are separated by age gaps smaller than the error on individual spot age analysis. Many features that are insignificant in major element analysis can have major impact in the acquisition of trace element data. Critical factors include the roles of collimator slit, detector gas, background estimation, accelerating voltage, probe current, X-ray interferences and count rate in affecting the accuracy, and a way to apply the Th and U interference correction without pure Th- and U-oxides or synthesized pure ThSiO4. The age-mapping procedure for young monazite and zircon includes acquiring PbMα (or PbMβ) intensity of individual pixels with multiple spectrometers, correcting background with background maps computed from a measured background intensity by the intensity relationships determined in advance of the measurement, calibrating of intensity with standards and calculating of ages from the Th, U and Pb concentrations. This technique provides age maps that show differences in age domains on the order of 20 Ma with in monazite as young as 100 Ma. The effect of sample damage by irradiation of intense and prolonged probe measurement is also described.  相似文献   

8.
This paper outlines the CHIME (chemical Th–U-total Pb isochron method) dating method, which is based on precise electron microprobe analyses of Th, U and Pb in Th- and U-bearing accessory minerals such as monazite, xenotime, zircon and polycrase. The age-mapping technique that is applicable to young monazite and zircon is also described. CHIME dating consists of analyzing multiple spots within homogeneous age domains that show sufficient compositional variation, and then these data are used to construct a “pseudo-isochron” from which an age can be obtained via regression. This method, when coupled with discrimination of possibly concordant age data by chemical criteria such as the (Ca + Si)/(Th + U + Pb + S) ratio for monazite and Ca and S contents for zircon, has the potential advantage of significant precision, and the ability to work with minerals that have a significant initial common Pb component. This technique can identify two or more homogeneous domains that are separated by age gaps smaller than the error on individual spot age analysis. Many features that are insignificant in major element analysis can have major impact in the acquisition of trace element data. Critical factors include the roles of collimator slit, detector gas, background estimation, accelerating voltage, probe current, X-ray interferences and count rate in affecting the accuracy, and a way to apply the Th and U interference correction without pure Th- and U-oxides or synthesized pure ThSiO4. The age-mapping procedure for young monazite and zircon includes acquiring PbMα (or PbMβ) intensity of individual pixels with multiple spectrometers, correcting background with background maps computed from a measured background intensity by the intensity relationships determined in advance of the measurement, calibrating of intensity with standards and calculating of ages from the Th, U and Pb concentrations. This technique provides age maps that show differences in age domains on the order of 20 Ma with in monazite as young as 100 Ma. The effect of sample damage by irradiation of intense and prolonged probe measurement is also described.  相似文献   

9.
Development of unconventional shale gas wells can generate significant quantities of drilling waste, including trace metal-rich black shale from the lateral portion of the drillhole. We carried out sequential extractions on 15 samples of dry-drilled cuttings and core material from the gas-producing Middle Devonian Marcellus Shale and surrounding units to identify the host phases and evaluate the mobility of selected trace elements during cuttings disposal. Maximum whole rock concentrations of uranium (U), arsenic (As), and barium (Ba) were 47, 90, and 3333 mg kg−1, respectively. Sequential chemical extractions suggest that although silicate minerals are the primary host for U, as much as 20% can be present in carbonate minerals. Up to 74% of the Ba in shale was extracted from exchangeable sites in the shale, while As is primarily associated with organic matter and sulfide minerals that could be mobilized by oxidation. For comparison, U and As concentrations were also measured in 43 produced water samples returned from Marcellus Shale gas wells. Low U concentrations in produced water (<0.084–3.26 μg L−1) are consistent with low-oxygen conditions in the wellbore, in which U would be in its reduced, immobile form. Arsenic was below detection in all produced water samples, which is also consistent with reducing conditions in the wellbore minimizing oxidation of As-bearing sulfide minerals.Geochemical modeling to determine mobility under surface storage and disposal conditions indicates that oxidation and/or dissolution of U-bearing minerals in drill cuttings would likely be followed by immobilization of U in secondary minerals such as schoepite, uranophane, and soddyite, or uraninite as conditions become more reducing. Oxidative dissolution of arsenic containing sulfides could release soluble As in arsenate form under oxic acidic conditions. The degree to which the As is subsequently immobilized depends on the redox conditions along the landfill flow path. The results suggest that proper management of drill cuttings can minimize mobilization of these metals by monitoring and controlling Eh, pH and dissolved constituents in landfill leachates.  相似文献   

10.
Black sands in the southern Eastern Desert (SED) of Egypt contain substantial reserves of heavy minerals (up to 5 %), and are found mainly in three basins namely: Hodein, Ibib and Diit between Shalateen and Halayeeb cities. The heavy minerals in these black sands include ilmenite-leucoxene (31 %–44 %), magnetite (15–18 %), zircon (11–21 %), garnet and green silicates (11–15 %), rutile (6–12 %) and monazite (2–4 %). Cassiterite, thorite, uranothorite, gold, xenotime and chromian spinel are minor quantities (<1 %). Magnetite (FeO: 75–93.5 wt%) and ilmenite (TiO2: 42.7–56.9 wt%), hosting high Mn, V, Zr, Zn, Cr, Nb and Co, were probably derived from gabbroic rocks. The detrital chromian spinel composition (Cr#, 0.51–0.61; Mg#, 0.5–0.63; TiO2 < 1.0 wt%) and its morphology are similar to those of spinels in fore-arc peridotites from the SED of Egypt, suggesting dominance of fore-arc basins for peridotite emplacement. These basins were formed during arc-arc or arc-oceanic crust collision and encolsed ophiolites, gabbroic rocks and I-type granites as sources of the SED black sands. The studied garnets are mostly almandine in composition with few grossularite and spessartine; they might have been derived from I-type granites and gneisses sources. The rutile and monazite show enriched LREE relative to HREE, and display marked defeciency in Eu, suggesting highly fractionated granitic rocks as a main source. Two distinct types of zircon are recorded: radioactive (Hf: 1578–8770, Y: 319–1335, U: 36–114 and Th: 40–64 ppm) and non radioactive (Hf: 427, Y: 44, U: 2 and Th: 2 ppm); they were probably derived from different granitic sources. Compositions and P-T conditions (T: 655–970 °C, P: 1.18–9.53 kbar) of magmatic amphiboles are similar to those derived from I-type granitoids. Bulk analyses of the economic heavy mineral assemblages show significant concentrations of Fe (393 kg/ton), Zr (183 kg/ton) and Ti (129 kg/ton) with minor Cr (14 kg/ton), Ba (7 kg/ton), Hf (4.9 kg/ton), Th (up to 3.34 kg/ton) and U (0.29 kg/ton). The elevated contents of Th and U could be related to the occurrence of monazite and zircon with subordinate thorite, uranothorite and xenotime. The total REE contents of these bulk analyses range from 1 to 4 kg/ton, where LREEs form 80–90 % of total REEs. Monazite (ΣREEs: 443604 ppm on average), garnet, zicon (ΣREEs: 421 ppm) and rutile (ΣREEs: 309 ppm) are the main host of REEs in the investigated black sands. Tonnages of raw sands, to a depth of one meter, are estimated per 10 km2 in each basin, giving 18 million tons for Ibib basin and 19 million tons for both Diit and Hodein basins. Economic heavy minerals constitute 6–26 % of the total heavy minerals and around 1.0 % of total raw sands. Calculated reserves of these economic minerals, per 10 km2 of black sands, range from 0.1 million ton in Ibib and Hodein basins to 0.2 million ton in the Diit basin.  相似文献   

11.
王洪作  王丹  张云龙  张鸿  舒徐洁  程成 《地质学报》2021,95(12):3828-3841
目前,沙特阿拉伯西北部Jabal Twalah地区铀钍资源勘查程度较低,对该地区的铀成矿机制研究相对薄弱.本文主要对该地区新发现的伟晶岩型和花岗岩热液型铀矿化带的矿化特征和成矿机制开展研究.区内与铀钍矿化相关的伟晶岩和围岩花岗岩中锆石LA-ICP-MS U-Pb年龄分别为638.6±3.2 Ma和632.5±1.5 Ma,两者时代基本一致.综合岩相学、地球化学以及扫描电镜能谱分析等研究结果,发现矿化伟晶岩强烈富集U、Th、稀土及稀有金属元素,复杂的高温蚀变矿物组合特征暗示可能经历了岩浆期后热液的改造,改造前后矿化伟晶岩中的铀和钍未发生分离,以副矿物形式存在而无独立铀矿物,具岩浆矿物组合的特征,如金红石、锆石、氟碳铈矿、磷钇矿、钍石等.花岗岩热液型单铀矿化带的地表样品中铀矿物主要为硅铅铀矿和硅钙铀矿,脉石矿物主要为赤铁矿、萤石、石英以及少量方解石,铀矿化受控于高铀含量的碱性花岗岩、强烈硅化构造破碎带以及晚期酸性基性脉岩活动等因素.强烈硅化的构造破碎带及其转折部位或者与脉岩交汇部位是今后重要的找矿方向.  相似文献   

12.
The Miao'ershan uranium ore district is one of the most important granite-hosted uranium producers in South China. There are several Triassic granite plutons in the Miao'ershan batholith, but uranium ore deposits mainly occur within the Douzhashan granitic body. Precise zircon U–Pb dating indicated that these Triassic granite plutons were emplaced during 204 to 215 Ma. The Douzhashan U-bearing granite lies in the central part of the Miao'ershan batholith, and has higher U contents (8.0 to 26.1 ppm, average 17.0 ppm) than the nearby Xiangcaoping granite (5.0 to 9.3 ppm, average 7.0 ppm) and the Yangqiaoling granite (6.4 to 18.3 ppm, average 11.5 ppm) in the south part of the batholith. The Douzhashan granite is composed of medium-grained two-mica granite, whereas the Xiangcaoping and Yangqiaoling granites are composed of porphyritic biotite granite. Both the Xiangcaoping and Douzhashan granites have high A/CNK ratios (> 1.10), high (87Sr/86Sr)i ratios (> 0.720) and low εNd(t) values (− 11.3 to − 10.4), suggesting that they belong to strongly peraluminous S-type granites. The Douzhashan granite has low CaO/Na2O ratios, high Rb/Sr and Rb/Ba ratios, indicating a partial melting origin of clay-rich pelitic rocks. In contrast, the Xiangcaoping granite formed from clay-poor psammite-derived melt. The Yangqiaoling granite shows different geochemical characteristics with the Douzhashan and Xiangcaoping granites, indicating a different magma source. The Yangqiaoling granite has higher εNd(t) of − 9.4 to − 8.3 and variable A/CNK values from 0.98 to 1.19, suggesting a mixture source of meta-sedimentary rocks and meta-igneous rocks. Crystallization fractionation is not the main mechanism for U enrichment in the Douzhashan granite. We suggest that U-rich pelitic rock sources may be the key factor to generate peraluminous U-bearing granites in South China. Searching for those granites which are reduced, strongly peraluminous and were derived from U-rich pelitic rocks, is the most effective way for exploring granite-hosted U deposits.  相似文献   

13.
The Gabal Marwa area is located in the southeastern part of Sinai,Egypt.It comprises gneisses and migmatites,granodiorites and monzogranites.Field,petrographic,mineralogic and chemical investigations indicated that the Gabal Marwa granites are classified as granodiorites and monzogranites.The monzogranites constitute the most predominant rock unit exposed in the study area.They have been subjected to hydrothermal alterations,especially along the shear zones.Sericitization,desilicification,nametasomatism and development of spotty or dendritic manganese oxides are the most pronounced alteration features.These alterations resulted in the increase of TiO2,Al2O3,FeOt,MnO,CaO,MgO,Na2O,K2O and Cr,Zr,Rb,Y and Sr and the decrease of SiO2,P2O5,Ni,Zn,Ba and Nb.Radiometric studies indicated that the altered granites belong to high thorium,high uranium granites.The U,Th,U and Th,Th/U,Th and U-K variation diagrams suggested that magmatic processes controlled the distribution of these elements but the effect of hydrothermal alteration processes were clearly distinct.The Scanning Electron Microscope and X-ray Diffraction analyses indicated that the most important radioactive minerals include uranothorite,thorite,zircon,monazite and samarskite.Cinnabar and Mn minerals were also found.From the U,Th,Ra and K activity concentrations obtained for all the studied granitic samples,radium equivalent activity(Raeq),external hazard index(Hex),and internal hazard index(Hin),were calculated to assess the radiation hazard to human beings living in dwellings made of the studied granites.Altered monzogranites have radioactivity above the proposed acceptable level and,therefore,caution must be taken when these granites are used as building materials.  相似文献   

14.
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. 160 Ma. 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 ± 8 Ma, 87 ± 4 Ma, and 68 ± 6 Ma, 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.  相似文献   

15.
王伟 《地质与勘探》2020,56(3):491-501
青井是位于龙首山成矿带西段的一个钍、铀混合型异常点,通过钻孔查证在青井盆地的花岗质砾岩中发现了较好的矿化线索,矿石具有热液型矿化的特征,发育钾长石化、赤铁矿化、碳酸盐化和绿泥石化为主的钾交代蚀变组合。通过对含矿花岗质砾岩的岩矿鉴定、电子探针测试、地球化学分析和钍矿物U-Pb同位素年龄测定进行综合研究,认为花岗质砾岩中发育的是产于挤压-俯冲构造环境下受断裂构造和辉绿岩脉共同控制的热液成因钍、铀混合型矿化,含钍、含铀矿物主要为钍石、沥青铀矿和铀石,成矿期应为新生代中晚期。热液成矿过程中带来了大量的外来组分,矿石中的Al_2O_3、Fe_2O_3和K_2O等主量元素和轻、重稀土元素以及Rb、Nb、Nd、Zr、Hf、Ta、W、Sb等微量元素均随着Th、U含量的增加而增加。  相似文献   

16.
Mesoproterozoic (~ 1050 Ma; Stenian) zircon crystals from the Saranac Prospect, Bancroft, Ontario, contain up to ~ 1 wt.% U and ~ 0.15 wt.% Th and, correspondingly, they are for the most part extensively radiation-damaged (calculated total α-doses 2.3?35.3 × 1018/g). The crystals show textures of complex, intense chemical alteration that is attributed to multiple, low-T replacement events along fluid-controlled reaction fronts. Centers of crystals appear totally replaced; the primary zoning is virtually erased and the material has high porosity and numerous inclusions. Interior regions surrounding the central reworked areas still exhibit primary igneous-type zoning; in those regions the alteration emanates from fractures and then follows the more radiation-damaged growth zones. Altered areas are typically recognized by their high porosity, low BSE intensity, and deficient analytical totals. Those regions often have lost a significant fraction of their radiogenic Pb. They are in general somewhat depleted in Zr, Si, and U, and are notably enriched in Ca and Fe. Element maps reveal elevated concentrations of Al and Y within filled fractures. Our observations indicate that the fluid-driven ion exchange is mainly controlled by the accessibility of micro-areas with elevated levels of radiation damage to transporting fluids via “fast pathways”. Most importantly, there is apparent Zr?Si?U equilibrium between initially existing and newly formed zircon. The retention of U after the chemical replacement (94 ± 14% relative to the original U content in the respective zones) does not significantly fall below the retention of two major cations Zr (95 ± 4%) and Si (95 ± 2%). In spite of the partially extreme hydrothermal alteration overprinting, the original U zoning in the crystals is well preserved. These observations suggest that preferential chemical leaching of U from zircon is clearly not a general feature of this mineral. This in turn seems to question the general validity of hydrothermal experiments to low-T, fluid-driven alteration of zircon in geological environments. The observed apparent immobility of U may affect the interpretation of U?Pb discordance in zircon, and the performance assessment of this mineral as potential waste form for actinides.  相似文献   

17.
Uranium mineralization in the El Erediya area, Egyptian Eastern Desert, has been affected by both high temperature and low temperature fluids. Mineralization is structurally controlled and is associated with jasperoid veins that are hosted by a granitic pluton. This granite exhibits extensive alteration, including silicification, argillization, sericitization, chloritization, carbonatization, and hematization. The primary uranium mineral is pitchblende, whereas uranpyrochlore, uranophane, kasolite, and an unidentified hydrated uranium niobate mineral are the most abundant secondary uranium minerals. Uranpyrochlore and the unidentified hydrated uranium niobate mineral are interpreted as alteration products of petscheckite. The chemical formula of the uranpyrochlore based upon the Electron Probe Micro Analyzer (EPMA) is . It is characterized by a relatively high Zr content (average ZrO2 = 6.6 wt%). The average composition of the unidentified hydrated uranium niobate mineral is , where U and Nb represent the dominant cations in the U and Nb site, respectively. Uranophane is the dominant U6+ silicate phase in oxidized zones of the jasperoid veins. Kasolite is less abundant than uranophane and contains major U, Pb, and Si but only minor Ca, Fe, P, and Zr. A two-stage metallogenetic model is proposed for the alteration processes and uranium mineralization at El Erediya. The primary uranium minerals were formed during the first stage of the hydrothermal activity that formed jasperoid veins in El Eradiya granite (130–160 Ma). This stage is related to the Late Jurassic–Early Cretaceous phase of the final Pan-African tectono-thermal event in Egypt. After initial formation of El Erediya jasperoid veins, a late stage of hydrothermal alteration includes argillization, dissolution of iron-bearing sulfide minerals, formation of iron-oxy hydroxides, and corrosion of primary uranium minerals, resulting in enrichment of U, Ca, Pb, Zr, and Si. During this stage, petscheckite was altered to uranpyrochlore and oxy-petscheckite. Uranium was likely transported as uranyl carbonate and uranyl fluoride complexes. With change of temperature and pH, these complexes became unstable and combined with silica, calcium, and lead to form uranophane and kasolite. Finally, at a later stage of low-temperature supergene alteration, oxy-petscheckite was altered to an unidentified hydrated uranium niobate mineral by removal of Fe.  相似文献   

18.
In situ U-Pb isotopic measurements were carried out by ion microprobe on the Zr-rich accessory minerals zirconolite [CaZrTi2O7], tranquillityite [Fe82+(ZrY)2Ti3Si3O24] and baddeleyite [ZrO2] in low-K, high-Ti mare basalt 10047 collected during the Apollo 11 mission. The analysed minerals are concentrated in pockets of late-stage mesostasis that comprises an intergrowth of silica, barian K-feldspar and Si-Al-K glass, from a phaneritic, subophitic, basalt comprising mainly pyroxene, plagioclase, ilmenite, cristobalite and troilite. Most Zr-rich minerals are unaltered, however, some tranquillityite is replaced by a complex intergrowth of zirconolite, baddeleyite, ilmenite and fayalite, suggesting that the mineral became unstable during crystallization. Several baddeleyite crystals have also undergone alteration to secondary zircon. Zirconolite was analysed in thin section 10047,11 and tranquillityite and baddeleyite in 10047,227, using a ∼6 μm primary ion beam. Both zirconolite and tranquillityite have significant U and low initial Pb contents, and are highly suitable for Pb/Pb dating. Fifteen analyses of zirconolite give a 207Pb/206Pb age of 3708 ± 7 Ma (207Pb/206Pb:204Pb/206Pb isochron; 95% confidence, including renormalisation of ratios) and twenty-five analyses of tranquillityite give 3710 ± 6 Ma. The 207Pb/206Pb dates are consistent with each other and refine results from an earlier study. Baddeleyite data were less precise, mainly due to lower secondary ionisation efficiency. Our results show that zirconolite and tranquillityite can provide precise isotopic dates and, given their presence in other samples, they represent important U-Pb chronometers for refining lunar geology.  相似文献   

19.
《Chemical Geology》2007,236(1-2):92-111
Complexly zoned zircons (19 grains, ∼ 3.3 Ga) from a porphyritic granite in the Jack Hills, Western Australia, have been investigated using electron microprobe analysis (EMPA) and transmission electron microscopy (TEM) in order to examine the effects of radiation as a function of dose, as well as the nano-scale microstructure and composition of impurities and secondary alteration phases. In back-scattered electron (BSE) images, zones with bright contrast consist of an almost unaltered zircon with limited amounts of impurity elements. In contrast, the dark zones contain higher concentrations of trace elements: U, Th, Pb, Fe, Y, Ce, Ca and Al. The cumulative doses due to alpha-decay in the dark zones are calculated to be 0.21–1.0 × 1017 (alpha-decay events/mg), equivalent to 1.0–4.7 dpa (displacements per atom). These doses are much higher than the dose required for radiation-induced amorphization, as determined by ion-beam irradiation of synthetic zircon, 0.3–1.0 dpa (0–600 °C). However, based on high-resolution TEM observations, none of the zircons are fully amorphous, to the result of annealing under ambient conditions. The concentrations of Ca and Al, which was considered to indicate alteration, increase dramatically at a cumulative dose of 1.6 × 1016 (alpha-decay events/mg). This is the dose at which the first percolation point occurs, as amorphous domains overlap and form an interconnected network. In the altered zones, nanocrystallites of zircon are present with a random orientation, and the amorphous matrix contains the impurity elements. Although the Ce-concentration is extremely high, 1–2 wt.%, the Ce anomaly, Ce/Ce⁎, does not vary significantly as a function of dose or degree of alteration, indicating that the LREE patterns were overprinted by the fluids with a similar Ce-anomaly. The valence of Ce analyzed by EELS (electron energy loss spectroscopy) is tetravalent in the altered zone, suggesting that the altering fluids were oxidizing.  相似文献   

20.
Radiometric measurements were carried out for the beach sands from East Rosetta estuary to determine the activity concentrations of 238 U, 226 Ra, 232 Th, and 40 K, using a Hyper Pure Germanium spectrometer, to estimate the dose rates and radiation hazard indices. The average specific activities are 778.20 Bq/kg for 238 U; 646.89 Bq/kg for 226 Ra; 621.92 and 627.85 Bq/kg for the 222 Rn daughters 214 Pb and 214 Bi respectively. The average specific activity of 232 Th is 1510.25 Bq/kg, while the calculated specific activity for 40 K has an average of 8.41 Bq/kg. The average specific activity of 235 U is 38.61 Bq/kg. The average absorbed dose rate is 1211.36 nGy/h, 20 times higher than the estimated average global primordial radiation of 60 nGy/h and 6 times higher than that of the world range (10-200 nGy/h). The radium equivalent (Ra eq ) values are from 6 to 9 times the recommended value. The internal and external hazard indices (H int , H ex ) indicate that their values are from 6 to 11 times the permissible values of these indices. These higher values may be due to the presence of economic heavy minerals containing radionuclides as zircon and monazite as well as some trace minerals, thorite and uranothorite. The mineralogical study indicates the beach sands contain heavy minerals, zircon, monazite, rutile, ilmenite, leucoxene, magnetite and garnet. The average abundance of zircon is 0.175 wt% ranging from 0.125 wt% to 0.239 wt%, while it is 0.004wt% ranging from 0.001 wt% to 0.007 wt% for monazite. The average abundance is 0.087 wt% for rutile; 2.029 wt% for ilmenite; 1.084 wt% for magnetite; 0.384 wt% for leucoxene and 0.295 wt% for garnet.  相似文献   

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