Geology,geochemistry and mineralogy of the lignite-hosted Ambassador palaeochannel uranium and multi-element deposit,Gunbarrel Basin,Western Australia

The Ambassador U and multi-element deposit occurs on the SW margin of the Gunbarrel Basin, Western Australia. Low-grade, flat-lying U mineralization averaging about 2 m thick at 0.03% U occurs in lignites at the redox front at the base of the weathering profile within a laterally extensive palaeochannel network. Uranium is principally associated with organic matter within the lignitic matrix, although rare discrete U minerals, such as coffinite and uraninite, are also present. The lignite is also enriched in a suite of other elements, principally base metals and sulphur, with concentrations of 0.3 ≥ 1% Cu, Pb, Ni, Co, Zn and total rare earth elements (REE) in some samples. Other element enrichments include: Cr, Cs, Sc, Se, Ta, Ti, Th, V and Zr as detrital heavy minerals of Zr, Ti and REE (oxides and silicates) or authigenic minerals of Cu, Bi, Pb, Zn, Ni, Se, Hg, Ti, Cr, Tl, V, U and REE (sulphides, vanadates, selenides, oxides, chlorides and native metals) and diffuse lignite impregnations. The Ambassador deposit probably formed from the convergence of redox-active weathering processes to unique source/host rocks, constrained within the palaeochannel. A proximal source of U and trace elements of lamproite/carbonatite origin is probable, as constrained by U–Pb isotope and U–Th disequilibria studies. Uranium and other metals were precipitated syngenetically with organic matter as it was deposited during a humid phase in the Late Eocene. Remobilization subsequently concentrated the metals in the upper 2 m of the lignite. This may have occurred during one or more periods of weathering and associated diagenesis, with the latest episode in the last 300,000 years.     

An excellent fossil wood cell texture with primary uranium minerals at a sandstone-hosted roll-type uranium deposit, NW China

A fossil wood cell texture with pitchblende and coffinite found at a sandstone-hosted roll-type uranium deposit, Xinjiang, NW China, is first reported here for the country. In the mineralized sandstone, detrital grains consisting of quartz, feldspar, rock fragments, carbonaceous trashes, mica and accessory minerals were deposited in early Turassic time and were cemented by clays and minor authigenic calcite and quartz. Pitchblende and minor coffinite are principal ore minerals at the deposit, and selectively replaced carbonized fossil wood remnants or filled fossil wood cells. An excellent fossil wood cell texture with primary uranium minerals formed. Replacement of organic debris by primary uranium minerals may be due to a local reducing environment resulting from the production of CH₄, H₂S or H₂SO₃ in its decomposition, and a biochemical reaction indicated by the common presence of framboidal pyrite.     

纳米比亚欢乐谷地区白岗岩型铀矿矿物特征研究

本文通过系统的岩矿鉴定和电子探针分析,对纳米比亚欢乐谷地区白岗岩型铀矿的矿物特征进行了详细的研究.该地区铀的赋存形式以独立铀矿物为主,少量以类质同像形式存在于钍矿物中.铀矿物的主要种类有:晶质铀矿、钍铀矿、铀石、铀钍石、钛铀矿、沥青铀矿、硅钙铀矿和钒钾铀矿等,其中,晶质铀矿、钍铀矿和钛铀矿等原生铀矿物约占69％,而反应边状铀石、铀钍石、沥青铀矿、钒钾铀矿和硅钙铀矿等次生铀矿物约占31％.由此可见,该区铀矿化主要表现为原始岩浆的分异作用与后期热液改造作用的相互叠加,其热液改造程度不大,仅使铀发生内部再分配.     

Multi-stage Ag–Bi–Co–Ni–U and Cu–Bi vein mineralization at Wittichen,Schwarzwald, SW Germany: geological setting,ore mineralogy,and fluid evolution

The Wittichen Co–Ag–Bi–U mining area (Schwarzwald ore district, SW Germany) hosts several unconformity-related vein-type mineralizations within Variscan leucogranite and Permian to Triassic redbeds. The multistage mineralization formed at the intersection of two fault systems in the last 250 Ma. A Permo-Triassic ore stage I with minor U–Bi–quartz–fluorite mineralization is followed by a Jurassic to Cretaceous ore stage II with the main Ag and Co mineralization consisting of several generations of gangue minerals that host the sub-stages of U–Bi, Bi–Ag, Ni–As–Bi and Co–As–Bi. Important ore minerals are native elements, Co and Ni arsenides, and pitchblende; sulphides are absent. The Miocene ore stage III comprises barite with the Cu–Bi sulfosalts emplectite, wittichenite and aikinite, and the sulphides anilite and djurleite besides native Bi, chalcopyrite, sphalerite, galena and tennantite. The mineral-forming fluid system changed from low salinity (<5 wt.% NaCl) at high temperature (around 300°C) in Permian to highly saline (around 25 wt.% NaCl + CaCl₂) at lower temperatures (50–150°C) in Triassic to Cretaceous times. Thermodynamic calculations and comparison with similar mineralizations worldwide show that the Mesozoic ore-forming fluid was alkaline with redox conditions above the hematite–magnetite buffer. We suggest that the precipitation mechanism for native elements, pitchblende and arsenides is a decrease in pH during fluid mixing processes. REE patterns in fluorite and the occurrence of Bi in all stages suggest a granitic source of some ore-forming elements, whereas, e.g. Ag, Co and Ni probably have been leached from the redbeds. The greater importance of Cu and isotope data indicates that the Miocene ore stage III is more influenced by fluids from the overlying redbeds and limestones than the earlier mineralization stages.     

Uranium Behavior in the Process of Primary Pitchblende Ores Alteration by the Post‐ore Hydrothermal Solutions: An Application to Assessment of Uranium Migration from Underground Spent Nuclear Fuel Repositories

It has been shown that the main uranium ore mineral, pitchblende (uranium dioxide), is a natural analog of synthetic uraninite (also uranium dioxide), which constitutes 96% of spent nuclear fuel (SNF). Geochronological studies of the U‐Pb isotope systems in unaltered pitchblende from the orebodies reveal that these systems remained completely closed over the entire period (approximately 135 Ma) since the formation of the deposits. The bulk of the primary uranium ores within the Streltsovskoye ore field was influenced to various degrees by post‐ore hydrothermal solutions that led to pitchblende spherulites being replaced by pseudomorphs of an amorphous phase with a U‐Si composition; this phase also re‐precipitated in veinlets proximal to the pitchblende pseudomorphs. A technique specially developed by the authors was used to carry out quantitative counts of the abundance of uranium minerals by calculating the uranium mass balance in one of the orebodies subjected to hydrothermal alteration. The calculations reveal minimal uranium loss from the orebody. Uranium liberated in the process of the pseudomorphic replacement of pitchblende was immediately fixed, in situ, in the newly formed coffinite‐like amorphous U‐Si phase as a result of the development of an efficient geochemical barrier that prevented the long‐distance migration of uranium. In assessing the long‐term safety of underground SNF repositories, the results of the present study give us confidence that SNF uraninite, in terms of the preservation of its integrity as a mineral phase, provides for the reliable long‐term isolation of uranium, transuranium elements, and fission products that are “sealed” in the uraninite matrix. In the case of the mineral transformation of the uraninite matrix by hydrothermal solutions, the liberated uranium would be efficiently immobilized by the newly formed amorphous U‐Si phase.     

粤北青嶂山岩体江头矿区铀矿微区矿物学、年代学特征及其成矿动力背景制约

粤北诸广和贵东是华南最重要的两个花岗型铀矿密集区,青嶂山(龙源坝)岩体位于两者之间,是华南花岗岩型铀矿研究薄弱地区。江头铀矿区地处青嶂山岩体北部与南雄断陷盆地的结合部位,该矿区的铀成矿年代学研究几为空白。本文通过电子探针方法研究了青嶂山岩体、及与该岩体密切相关的江头矿区中的铀矿物微区矿物学特征,获得岩浆成因的晶质铀矿与热液成因的沥青铀矿的U-Th-Pb化学年龄,探讨了华南铀成矿作用动力学背景及成矿地质体。研究表明:青嶂山岩体粗粒斑状黑云母花岗岩和中粒斑状黑云母花岗岩中的铀矿物主要有晶质铀矿、铀石,部分晶质铀矿存在明显铀释放的特征,其晶质铀矿化学年龄分别为246.8±8.8Ma、161.5±8.0Ma,与前人获得的锆石U-Pb年龄结果在误差范围内一致,分别代表了区内印支期与燕山期花岗岩体的成岩年龄,表明在南雄断陷盆地形成之前,青嶂山岩体与诸广岩体可能为一有机整体,有着相同的成岩、成矿环境。江头矿区矿石中铀矿物主要为沥青铀矿,伴有少量钛铀矿、铀石等,沥青铀矿化学年龄分别为121.3±9.8Ma、98.8±8.0Ma、73.2±8.8Ma,分别代表区内3期铀成矿作用的时代,结合华南中生代以来...     

570矿床主要矿石矿物的成分特征

本文对５７０矿床的主要矿石矿物（沥青铀矿、铀石、钛铀矿及胶硫钼矿）的化学组成进行了研究。沥青铀矿的铀含量较高（ＵＯ２＋ＵＯ３）＝８４．４％￣９１％，并含有一定量的Ｃａ、Ｓｉ、Ｐ和Ｔｉ等，而Ｔｈ和ＲＥＥ的含量低，表明沥青铀矿的低温成因。在电镜下观察到钛铀矿与钛的氧化（金红石或锐钛矿）成交代关系，说明钛铀矿是交代成因的。胶硫钼矿中的银含量很高（０．５％￣１．６％），是该矿床伴生银的富集矿物及携带矿物，     

Genetic implications of rare uraninite and pyrite in quartz-pebble conglomerates from Sundargarh district of Orissa,Eastern India

Petrographic and EPMA studies reveal the presence of discrete grains of uraninite and pyrite are being reported for the first time in quartz-pebble conglomerates from western margin of Bonai granite pluton, Sundargarh district, Orissa. Uraninite grains (2–3 μ in size) are subrounded to muffin shaped which show variation in UO₂ (63.86 to 71.73 wt %), ThO₂ (5.48 to 6.42 wt %) and RE₂O₃ (1.57 to 2.23 wt %). Pegmatitic source of uraninite is revealed by comparing UO₂, ThO₂, PbO, CaO content and ratios of UO₂/ThO₂ and CaO/ThO₂ in uraninites from pegmatite and other environments and areas. Subrounded to muffin shape of uraninite, their association with subrounded pyrite and heavies like zircon, tourmaline, chromite, monazite, magnetite and their comparable chemistry with well established quartzpebble conglomerates of India and world are indicative of their detrital origin. Pyrite, minor chalcopyrite and rare galena are observed as sulphide phases in conglomerate. Variable shapes of pyrite, their low Co (up to 0.16 wt %) and Ni contents (up to 0.09 wt %) and Co/Ni ratio less than 1.0 (mean= 0.63) favours sedimentary/diagenetic origin.     

鄂尔多斯盆地东北部砂岩型铀矿伴生矿物组合及其地质意义

特征矿物是地质作用的直接记录,研究铀矿物伴生组合类型和特征为探讨铀矿床成因提供直接信息.本文以鄂尔多斯盆地东北部杭锦旗-纳岭沟地区含铀岩系中侏罗统直罗组为研究对象,通过岩心观察、显微观察、扫描电镜和电子探针分析等,系统研究了含铀砂岩中铀矿物种类、赋存特征及典型矿物伴生组合类型,在此基础上,结合铀成矿过程中流体作用探讨了...     

Mineralogic investigation into occurrence of high uranium well waters in upstate South Carolina, USA

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).     

Mineralogy and geochemistry of the sodium metasomatism-related uranium occurrence of Aricheng South, Guyana

The Aricheng South uranium occurrence is associated with Na metasomatism that affected the granitoids of the Kurupung Batholith in western Guyana. The mineral paragenesis indicates that late-magmatic albitization was followed by chlorite alteration of biotite. A minor amount of uraninite occurs in fractures in the newly formed albite crystals, often in company of calcite. The main mineralization stage occurred later than albitization and chloritization and is represented by brannerite disseminated in a groundmass of fine-grained hydrothermal zircon. Whole rock geochemistry supports the temporal dissociation of albitization from the main ore stage. Brannerite, zircon, and uraninite are often partially altered to secondary brannerite, zircon, and coffinite, respectively. Stable oxygen (chlorite, calcite) and hydrogen (chlorite) isotope compositions suggest that a highly evolved meteoric fluid, or at least one corresponding to a very high rock/fluid ratio (δ¹⁸O of approx. 3.4% to 4‰ and δD of approx. −80‰) may have caused the pre-ore alteration assemblage. The fluids in equilibrium with main ore stage zircon have δ¹⁸O of approx. 6.8‰ and appear to be of magmatic origin. The Aricheng occurrence geochemically, mineralogically, thermally, and paragenetically resembles the Valhalla U deposit in northern Australia despite differences between the deposits’ host lithologies, whereas the Lagoa Real and Espinharas U deposits in Brazil have host rock lithology that resembles that of Aricheng.     

鄂尔多斯盆地北部铀矿床铀矿物赋存状态及富集机理

为了进一步深化铀矿物的富集机理.利用α径迹放射性照相、扫描电镜、电子探针等方法对鄂尔多斯盆地北部铀矿床中铀矿物的赋存状态进行了系统研究.发现该区铀矿物主要为铀石，少量沥青铀矿和含铀钛矿物.沉积-成岩期碎屑铀矿物赋存在碎屑颗粒内部，吸附在锐钛矿周围，为铀储层中预富集的铀.成矿期铀矿物大部分赋存在碎屑颗粒填隙部位，与黄铁矿、碳质碎屑相伴生，与石英颗粒及方解石胶结关系密切；部分吸附在包裹碎屑颗粒的蒙脱石薄膜上.另外发现了，沥青铀矿-赤铁矿-黄铁矿的矿物组合，以及硒铅矿（PbSe）和白硒铁矿（FeSe₂）与铀矿物相伴生，并伴有REE含量明显升高.分析得出，沥青铀矿形成于成矿早期，氧化酸性流体与还原碱性流体的过渡界面，偏向于氧化酸性一侧；而铀石主要形成于成矿晚期的还原碱性环境.双重铀源供给、丰富的还原介质、多源流体的耦合，局部的热液流体叠加改造，共同造就了鄂尔多斯盆地北部大矿、富矿的形成.      

Geochemical,isotopic, and geochronlologic constraints on the formation of the Eagle Point basement-hosted uranium deposit,Athabasca Basin,Saskatchewan, Canada and recent remobilization of primary uraninite in secondary structures

The Athabasca Basin hosts many world-class unconformity-related uranium deposits. Recently, uranium reserves for the Eagle Point basement-hosted deposit have increased with the discovery of new mineralized zones within secondary structures. A paragenetic study of Eagle Point reveals the presence of three temporally distinct alteration stages: a pre-Athabasca alteration, a main alteration and mineralization comprised of three substages, and a post-main alteration and mineralization stage that culminated in remobilization of uraninite from primary to secondary structures. The pre-Athabasca alteration stage consists of minor amounts of clinochlore, followed by dolomite and calcite alteration in the hanging wall of major fault zones and kaolinitization of plagioclase and K-feldspar caused by surface weathering. The main alteration and uranium mineralization stage is related to three temporally distinct substages, all of which were produced by isotopically similar fluids. A major early alteration substage characterized by muscovite alteration and by precipitation Ca–Sr–LREE-rich aluminum phosphate-sulfate minerals, both from basinal fluids at temperatures around 240°C prior to 1,600 Ma. The mineralization substage involved uraninite and hematite precipitated in primary structures. The late alteration substage consists of dravite, uranophane-beta veins, calcite veins, and sudoite alteration from Mg–Ca-rich chemically modified basinal fluids with temperatures around 180°C. The post-main alteration and mineralization stage is characterized by remobilization of main stage uraninite from primary to secondary structures at a minimum age of ca. 535 Ma. U–Pb resetting events recorded on primary and remobilized uraninites are coincident with fluid flow induced by distal orogenies, remobilizing radiogenic Pb to a distance of at least 225 m above the mineralized zones.     

Mineral composition of uranium ore at the Dalmatovo deposit,Russia

The mineral composition of hydrogenic uranium ore of the Dalmatovo deposit was studied with analytical scanning electron microscopy. The results correspond to earlier known data only in general terms. Phosphosilicate uranium mineralization, which is predominant in the samples, is similar to P-bearing coffinite in elemental composition but differs in morphology. The quantitative analysis of microcrystals corresponds to the formula (U,Ca)[(Si,P)O₄]₂, where U/Si ratio is twice as low as in coffinite. The occurrence of oxide pitchblende mineralization has been confirmed. The initial stage of the formation of uranyl minerals has been revealed. The mineral species of Ti-U substance that determines geochemical attributes of the Dalmatovo deposit is considered.     

Heavy mineral-based provenance analysis of Mesozoic continental-marine sediments at the western edge of the Bohemian Massif,SE Germany: with special reference to Fe–Ti minerals and the crystal morphology of heavy minerals

During the Mesozoic, the epicontinental Germanic Basin and the Regensburg Strait the latter being an embayment of the Tethys Ocean that had subsided into the Moldanubian Zone of the Central European Variscides were filled with terrigenous continental-marine sediments. Both sediments’ heavy mineral (HM) grains and aggregates have been studied in a drill section in the Wackersdorf area, SE Germany. The majority of them belong to the (semi)opaque group of Fe–Ti minerals. In Wackersdorf, the entire stratigraphy of the basin fill, which occurred between the Triassic and the Late Cretaceous, is well exposed. In addition to the chemical composition of HM, the morphology and texture of zircon, apatite and Fe–Ti compounds have been studied in a provenance-related mineral classification. Provenance analysis has yielded five discrete source rock lithologies: (1) Moldanubian H-T-metamorphics, (2) late Paleozoic (sub)volcanic rocks, (3) gneisses of the Tepla-Barrandian unit, (4) ophiolites of the Tepla-Barrandian unit, (5) silicified shear zones and quartz cores of pegmatites. The detrital minerals include zircon, tourmaline (dravite-schoerl), apatite, monazite (Ce–Th–La–Nd), xenotime, biotite, rutile, ilmenite, “nigrine” (ilmenite-rutile intergrowth), sphene, amphibole, staurolite, garnet and spinel (Cr–Mg–Al). Based on the allogenic Ti and Fe minerals, a magnetite-type source area (Eh > 0, near-surface felsic to intermediate (sub)volcanic rocks) was distinguished from an ilmenite-type source area (Eh < 0), deeply eroded crystalline basement rocks (gneiss, granite, shear zones). The latter may be subdivided into “nigrine –I” (deep) and “nigrine-II” (intermediate) subtypes, according to the level of erosion in the source area. At the Jurassic–Cretaceous transition, extrabasinal erosion provoked a noticeable variation of allogenic heavy minerals with the incisions of rivers into source rock lithologies (4) and (5). Uplift and erosion along the western edge of the Bohemian Massif took place contemporaneously with spreading and closure in the central parts of the adjacent Tethys Ocean.     

Fate of trace elements during alteration of uraninite in a hydrothermal vein-type U-deposit from Marshall Pass, Colorado, USA

Alteration of uraninite from a hydrothermal vein-type U-deposit in Marshall Pass, Colorado, has been examined by electron microprobe analysis in order to investigate the release and migration of trace elements W, As, Mo, Zr, Pb, Ba, Ce, Y, Ca, Ti, P, Th, Fe, Si, Al, during alteration, under both reducing and oxidizing conditions. The release of trace elements from uraninite is used to establish constraints on the release of fission product elements from the UO₂ in spent nuclear fuels. Uraninite occurs with two different textures: (1) colloform uraninite and (2) fine-grained uraninite. The colloform uraninite contains 1.04-1.75 wt% of WO₃, 0.16-1.70 wt% of As₂O₃, 0.06-0.88 wt% of MoO₃; whereas, the fine-grained uraninite retains 2.25-4.93 wt% of WO₃, up to 5.76 wt% of MoO₃, and 0.26-0.60 wt% of As₂O₃. The near constant concentration of incompatible W in the colloform uraninite suggests W-incorporation into the uraninite structure or homogeneous distribution of W-rich nano-domains. Incorporation of W and Mo into the uraninite and subsequent precipitation of uranyl phases bearing these elements are critically important to understanding the release and migration of Cs during the corrosion of spent nuclear fuel, as there is a strong affinity of Cs with W and Mo. Zoning in the colloform texture is attributed to variation in the amount of impurities in uraninite. For unaltered zones, the calculated amount of oxygen ranges from 2.08 to 2.32 [apfu, (atom per formula unit)] and defines the stoichiometry as UO_2+x and U₄O₉; whereas, for the altered zones of the colloform texture, the oxygen content is 2.37-2.48 [apfu], which is probably due to the inclusion of secondary uranyl phases, mainly schoepite. The supergene alteration resulted in precipitation of secondary uranyl minerals at the expense of uraninite. Four stages of colloform uraninite alteration are proposed: (i) formation of an oxidized layer at the rim, (ii) corrosion of the oxidized layer, (iii) precipitation of U⁶⁺-phases with well-defined cleavage, and (iv) fracture of the uraninite surface along the cleavage planes of the U⁶⁺-phases.     

U−Ti phases from Precambrian quartz-pebble conglomerates of the Elliot Lake area,Canada, and the Pongola basin,South Africa

Summary In the present study inhomogeneous, partly uraniferous leucoxene/rutile aggregates and brannerite grains from the Elliot Lake area, Canada, and the Pongola basin, South Africa, were identified by ore microscopy. This observation accords with the fact that U–Ti phases are second only to uraninite as the most important uranium minerals in Precambrian conglomerates. The observed U–Ti phases, however, are somewhat indefinitive. They often merge into each other and represent authigenic constituents pseudomorphous after detrital titanium minerals. Microprobe work has confirmed the existence of a continuous mineral series recognized optically which ranges from uranium-free leucoxene/rutile to uranium-enriched brannerite. However, the presence of metamict brannerite can only be verified by X-ray diffraction after prolonged heating, a process which may synthesize the latter mineral. The actual occurrence of brannerite in Precambrian conglomerates must, therefore, be regarded with reservation and it is suggested that redistribution and subsequent adsorption of uranium on Ti phases during diagenesis and/or metamorphism of the conglomerates did not lead to the formation of authigenic brannerite, but rather resulted in microcrystalline leucoxene/rutile admixtures containing uranium in varying amounts.

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U–Ti-Phasen aus präkambrischen Quarz-Geröll-Konglomeraten des Elliot Lake-Gebietes, Canada, und des Pongola-Beckens, Südafrika

Zusammenfassung Im Untersuchungsmaterial, das aus dem Elliot Lake-Gebiet, Canada, und dem Pongola-Becken, Südafrika, stammt, konnten Leukoxen/Rutil-Aggregate, die teilweise Uran-führend sind, und Brannerit mikroskopisch bestimmt werden. Dies stimmt mit vielfach mitgeteilten Beobachtungen überein, daß in präkambrischen Konglomeraten U–Ti-Phasen, nach Uraninit, die wichtigsten Uranminerale darstellen. Die festgesteilten U–Ti-Phasen sind recht wenig definiert und gehen oft graduell ineinander über. Es sind authigene Konglomeratkomponenten, die im allgemeinen pseudomorph nach detritischen Titanmineralen sind. Ein kontinuierlicher Übergang von Uran-freiem Leukoxen/Rutil zu Uran-reichem Brannerit konnte mittels der Mikrosonde ebenfalls mineralgeochemisch bestätigt werden. Wegen des metamikten Charakters des Probenmaterials kann jedoch die Anwesenheit von Brannerit, mittels Röntgendiffraktometrie, erst nach längerem Glühen der Proben verifiziert werden, was möglicherweise zu einer Synthetisierung von Brannerit führt. Sein Auftreten in präkambrischen Konglomeraten ist deshalb nicht gesichert, und es wird vorgeschlagen, daß diagenetische oder/und metamorphe Umlagerungen von Uran eher zur Bildung uranhaltiger mikrokristalliner Leukoxen/Rutil-Aggregate als von authigenem Brannerit führen.

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With 10 Figures     

电子探针测年方法应用于晶质铀矿的成因类型探讨

电子探针Th-U-Pb测年因其高分辨率与高精度的优势,在独居石、锆石等定年矿物中得到了推广,但在Th、U、Pb含量高的晶质铀矿、沥青铀矿等矿物中则应用较少。本文在铁矿床变质岩绿泥石、阳起石黑云母蚀变岩首次发现U含量高的晶质铀矿,基于此,结合该铁矿床地区的地质背景,利用偏光显微镜与电子探针等分析测试手段,将镜下蚀变现象、年龄计算与其他相关元素分析相结合,重点对晶质铀矿的成矿年龄及成矿规律进行探讨。研究发现:通过镜下观察判断,晶质铀矿的成因类型与澳大利亚著名的变质型铀矿相似,均为古老的变质型,且周围的脉石矿物均为绿泥石,绿泥石皆由黑云母退变质而成,铀矿的赋存位置显示其与黑云母、绿泥石之间有紧密联系,其成矿年龄与黑云母、绿泥石形成年龄息息相关。继而根据电子探针数据计算成矿年龄,判断成矿期次,得出主要成矿期在(1654±17)Ma~(1805±17)Ma,为中元古代中期,且主要成矿期与热液蚀变作用黑云母化有关,后期活化富集时期在(657±17)Ma~(807±17)Ma,为新元古代南华纪时期,此阶段是热液侵入、绿泥石化广泛发育的时期;选取较大颗粒对晶质铀矿的环带年龄进行计算,从年龄分布上证实后期有强烈的流体活动的发生,且主要与绿泥石化相关。另外,对比变质型与沉积型铀矿中Y2O3与UO2含量发现,两者之间存在负相关关系,此关系对判断铀矿成因即是否为变质型或沉积型可能有指示意义,但缺乏大量的数据佐证,需进一步研究。     

Response of xenotime to prograde metamorphism

Xenotime is a widespread accessory mineral in lower greenschist to upper amphibolite facies metasedimentary rocks from the Palaeoproterozoic Mount Barren Group, southwestern Australia. Xenotime is closely associated with detrital zircon, commonly forming syntaxial outgrowths, in samples of sandstone, micaceous quartzite, slate, phyllite, garnet-bearing semi-pelites, and in kyanite-, garnet-, and staurolite-bearing mica schists. In situ geochronology of xenotime from lower greenschist sandstones has previously yielded multiple U–Pb ages with peaks at ~2.0, ~1.7, and ~1.65 Ga, interpreted to represent the age of detritus, early diagenesis, and a later thermal event, respectively. New U–Pb dating of xenotime in slate yields a major population at ~1.7 Ga with a minor population at ~1.2 Ga, reflecting diagenetic and metamorphic growth, respectively, whereas xenotime in phyllite forms a minor age population at ~1.7 Ga and a main peak at ~1.2 Ga. Mid-greenschist facies semi-pelitic schists (quartz-muscovite-garnet) contain xenotime that formed before 1.8 Ga and at 1.2 Ga, representing detrital and peak metamorphic ages, respectively. Xenotime in samples of amphibolite facies schist (650°C and ~8 kbars) yields U–Pb ages of ~1.2 Ga, coinciding with the time of peak metamorphism. A single analysis of a xenotime core from an amphibolite facies schist gave an age of ~1.8 Ga, consistent with the presence of detrital xenotime. Our results suggest that detrital xenotime may be preserved under greenschist facies conditions, but is largely replaced during upper amphibolite facies conditions. Detrital xenotime is replaced through dissolution–reprecipitation reactions forming compositionally distinct rims during greenschist and amphibolite facies metamorphism at 1.2 Ga. Diagenetic xenotime is present in lower greenschist facies samples, but was not observed in metasedimentary rocks that had experienced temperatures above mid-greenschist facies metamorphism (450°C). The apparent disappearance of detrital and diagenetic xenotime and appearance of metamorphic xenotime during prograde metamorphism indicates that some of the yttrium, heavy rare earth elements, and phosphorus needed for metamorphic xenotime growth are probably derived from the replacement of detrital and diagenetic xenotime.