诸广南部产铀花岗岩长江岩体中的绿泥石和铀源矿物研究

长江岩体是诸广南部地区重要的产铀花岗岩体之一,此次研究运用电子探针和扫描电镜对长江岩体新鲜花岗岩和 蚀变花岗岩中的绿泥石和有关含铀矿物进行了精细对比,揭示花岗岩中铀的活化与成矿前期或早期致使花岗岩发生绿泥 石化的还原性热液蚀变作用关系密切,黑云母等的绿泥石化蚀变,使其中包裹的一些含铀副矿物也发生蚀变,导致原来 以类质同象形式存在于副矿物中的惰性铀转变成活性铀,并在绿泥石附近沉淀成铀石等铀含量高且在成矿期低度氧化性 热液作用下容易释放铀的矿物。长江岩体中的副矿物有锆石、磷灰石、褐帘石、铀石-钍石、晶质铀矿、独居石等,其 中,晶质铀矿、铀石、铀钍石中铀含量高且铀容易释放,是长江岩体的主要铀源矿物;独居石中铀含量较高,当其周围 矿物绿泥石化时,独居石蚀变形成直氟碳钙铈矿并释放铀,因而也是长江岩体的潜在铀源矿物;锆石中铀含量虽高,但 因其结构稳定,铀难以释放,因此它不是长江岩体中重要的铀源矿物;磷灰石、褐帘石中铀含量均低于检测限,作为铀 源矿物的可能性很小。     

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.     

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构造运动特征,认为区内铀成矿作用是受中-新生代盆地边缘深大断陷活动、产铀花岗岩体分布的双要素成矿动力学背景制约,青嶂山岩体应与诸广、贵东岩体具有相似的找矿前景。     

化尖地区伟晶状花岗岩与铀矿化

通过铀矿综合区调,在化尖地区发现伟晶状花岗岩脉群长约8—17km,宽约2—4km,特别在尖扎附近分布集中。主要矿石矿物为晶质铀矿、铀石,铀矿化主要富集在伟晶状花岗岩内黑云母集中区、伟晶状花岗岩脉与黑云母斜长片麻岩接触带处。     

鄂尔多斯砂岩型铀矿床古层间氧化带中铀石的产状和形成

鄂尔多斯盆地的东胜铀矿床是一个重要的大型砂岩型铀矿床,业已查明铀石是该铀矿床的主要含铀矿物,铀石的颗粒十分细小.大量的显微镜薄片和电子显微镜观察发现,铀石通常以不规则的集合体产出于蚀变黑云母裂隙和黄铁矿边缘.电子探针分析表明,铀石中w(UO2)变化较大,在50%～70%间.与铀石相关的黑云母已经强烈蚀变,转变为水黑云母、水白云母和绿泥石.黑云母在蚀变过程中K+逐渐流失,直到基本流失殆尽,与原始黑云母相比,水黑云母w(K2O)总体减少了6.54%[占原始蚀变黑云母中w(K2O)的74%],同时w(FeO)减少了7.40%[占原始蚀变黑云母中w(FeO)的29%];水白云母与原始黑云母相比,矿物成分也发生了很大的变化,其中,w(K2O)总体减少了7.87%[占原始蚀变黑云母中w(K2O)的89%],同时w(FeO)减少了19.22%[占原始蚀变黑云母中w(FeO)的76%],w(Al2O3)由16.64%增加至32.72%,增加了16.08%.根据铀石产出的特征和蚀变水黑云母和蚀变水白云母成分特征,探讨了铀酰离子(UO2+2)被还原成U4+和形成铀石[U(SiO 4) 1-x(OH) 4x]的机理,指出古层间氧化带中铀石的形成与黑云母的蚀变以及黄铁矿的形成有着密切的联系.     

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.     

Magma evolution and mineralization of Longmenshan lithium-beryllium pegmatite in Dahongliutan area,West KunlunSCIEI北大核心CSCD

西昆仑大红柳滩地区相继发现了众多伟晶岩型锂铍矿床,已成为我国新的锂资源基地。目前关于这些锂铍花岗伟晶岩的成因多强调其源于地壳深熔形成的二云母二长花岗岩的结晶分异,但研究区出露的同时代的黑云母花岗岩与成矿的关系没有被讨论和关注。为了探讨黑云母花岗岩与成矿的关系,作者对龙门山矿区黑云母花岗岩、二云母二长花岗岩、花岗伟晶岩以及与成矿相关的细晶花岗岩开展了详细的地球化学及年代学研究。结果显示：1)黑云母花岗岩与二云母二长花岗岩具相似的地球化学特征,富集Rb、La和Nd,亏损Ba、Nb、Sr、P和Ti元素,均表现出S型花岗岩的特征;2)从黑云母花岗岩→二云母二长花岗岩→细晶花岗岩,表现出连续分异演化的特征;3)黑云母花岗岩的锆石LA-ICP-MS U-Pb年龄为216.8±0.85Ma,二云母二长花岗岩的锆石SIMS U-Pb年龄为216.0±1.5Ma,细晶花岗岩的锆石LA-ICP-MS U-Pb年龄为209.5±1.2Ma,花岗伟晶岩的锡石LA-MC-ICP-MS U-Pb年龄为211.3±5.0Ma,这意味着从黑云母花岗岩到二云母二长花岗岩与细晶花岗岩的形成时间是连续的并且是接近的。基于此...     

Characterization of uraninite nanoparticles produced by Shewanella oneidensis MR-1

The reduction of uranium(VI) by Shewanella oneidensis MR-1 was studied to examine the effects of bioreduction kinetics and background electrolyte on the physical properties and reactivity to re-oxidation of the biogenic uraninite, UO_2(s). Bioreduction experiments were conducted with uranyl acetate as the electron acceptor and sodium lactate as the electron donor under resting cell conditions in a 30 mM NaHCO₃ buffer, and in a PIPES-buffered artificial groundwater (PBAGW). MR-1 was cultured in batch mode in a defined minimal medium with a specified air-to-medium volume ratio such that electron acceptor (O₂) limiting conditions were reached just when cells were harvested for subsequent experiments. The rate of U(VI) bioreduction was manipulated by varying the cell density and the incubation temperature (1.0 × 10⁸ cell ml⁻¹ at 20 °C or 2.0 × 10⁸ cell ml⁻¹ at 37 °C) to generate U(IV) solids at “fast” and “slow” rates in the two different buffers. The presence of Ca in PBAGW buffer altered U(VI) speciation and solubility, and significantly decreased U(VI) bioreduction kinetics. High resolution transmission electron microscopy was used to measure uraninite particle size distributions produced under the four different conditions. The most common primary particle size was 2.9-3.0 nm regardless of U(VI) bioreduction rate or background electrolyte. Extended X-ray absorption fine-structure spectroscopy was also used to estimate uraninite particle size and was consistent with TEM results. The reactivity of the biogenic uraninite products with dissolved oxygen was tested, and neither U(VI) bioreduction rate nor background electrolyte had any statistical effect on oxidation rates. With MR-1, uraninite particle size was not controlled by the bioreduction rate of U(VI) or the background electrolyte. These results for MR-1, where U(VI) bioreduction rate had no discernible effect on uraninite particle size or oxidation rate, contrast with our recent research with Shewanella putrefaciens CN32, where U(VI) bioreduction rate strongly influenced both uraninite particle size and oxidation rate. These two studies with Shewanella species can be viewed as consistent if one assumes that particle size controls oxidation rates, so the similar uraninite particle sizes produced by MR-1 regardless of U(VI) bioreduction rate would result in similar oxidation rates. Factors that might explain why U(VI) bioreduction rate was an important control on uraninite particle size for CN32 but not for MR-1 are discussed.     

Microbial reduction of iron(III)-rich nontronite and uranium(VI)

To assess the dynamics of microbially mediated U-clay redox reactions, we examined the reduction of iron(III)-rich nontronite NAu-2 and uranium(VI) by Shewanella oneidensis MR-1. Bioreduction experiments were conducted with combinations and varied concentrations of MR-1, nontronite, U(VI) and the electron shuttle anthraquinone-2,6-disulfonate (AQDS). Abiotic experiments were conducted to quantify U(VI) sorption to NAu-2, the reduction of U(VI) by chemically-reduced nontronite-Fe(II), and the oxidation of uraninite, U^(IV)O_2(s), by nontronite-Fe(III). When we incubated S. oneidensis MR-1 at lower concentration (0.5 × 10⁸ cell mL⁻¹) with nontronite (5.0 g L⁻¹) and U(VI) (1.0 mM), little U(VI) reduction occurred compared to nontronite-free incubations, despite the production of abundant Fe(II). The addition of AQDS to U(VI)- and nontronite-containing incubations enhanced both U(VI) and nontronite-Fe(III) reduction. While U(VI) was completely reduced by S. oneidensis MR-1 at higher concentration (1.0 × 10⁸ cell mL⁻¹) in the presence of nontronite, increasing concentrations of nontronite led to progressively slower rates of U(VI) reduction. U(VI) enhanced nontronite-Fe(III) reduction and uraninite was oxidized by nontronite-Fe(III), demonstrating that U served as an effective electron shuttle from S. oneidensis MR-1 to nontronite-Fe(III). The electron-shuttling activity of U can explain the lack or delay of U(VI) reduction observed in the bulk solution. Little U(VI) reduction was observed in incubations that contained chemically-reduced nontronite-Fe(II), suggesting that biologic U(VI) reduction drove U valence cycling in these systems. Under the conditions used in these experiments, we demonstrate that iron-rich smectite may inhibit or delay U(VI) bioreduction.     

Determining uraniferous host rocks and minerals as a source of dissolved uranium in granite aquifers near the central Ogcheon metamorphic belt,Korea

Korea has suffered from high U concentrations in some groundwater, especially in Daejeon near the central Ogcheon metamorphic belt. However, clear explanation has not been yet reported for the uranium sources in groundwater. This study is focused on identifying the potential uraniferous host rocks and finding the exact U-minerals serving as uranium sources in groundwater near Daejeon, Korea. For this work, we performed an in situ gamma ray spectrometer survey and a laboratory microscopic study with electron micro-probe analysis. The measurements for radioactive element content did not show any prominent K- and Th-anomaly spots, but they did result in several observations of considerable U-anomalies in pegmatite and hydrothermally altered granite, in which uraninite, coffinite and uranophane were found by electron micro-probe analysis. The occurrences of U-minerals are the first such discovery except in the low-grade U deposits of the Ogcheon metamorphic belt in Korea. All observed U-minerals were intimately associated with hydrothermal alteration. The remarkably low Th/U ratios in the studied uraniferous rocks also suggest that the U enrichment was genetically related to a post-magmatic hydrothermal process. However, many of the uraniferous parts were controlled by a reducing environment and occurred near the border between graphite-rich mica-schist and Jurassic leucocratic granite. The findings on these uraniferous rocks can be used in elucidating the source of highly enriched U groundwater in granite aquifers as well as in understanding the occurrence of igneous U-minerals, which has not been previously reported in Korea.     

The major ion, δCa, δCa, and δMg geochemistry of granite weathering at pH = 1 and T = 25 °C: power-law processes and the relative reactivity of minerals

We dissolved Boulder Creek Granodiorite in a plug flow reactor for 5794 h at pH = 1 and T = 25 °C. The primary purpose of the experiment was to identify controls on dissolved δ^44/40Ca, δ^44/42Ca, and δ^26/24Mg values during granite weathering. Herein, we also examine the origin of Ca and Mg isotopic variability among minerals composing the Boulder Creek Granodiorite, and we constrain fundamental characteristics of granite weathering important for quantifying the elemental and isotopic geochemistry of the reactor output. Nine Ca-bearing minerals display an 8.80‰ range of δ^44/40Ca values and a 0.51‰ range of δ^44/42Ca values. Three Mg-bearing minerals display a 1.53‰ range of δ^26/24Mg values. These ranges expressed at the mineralogical scale are higher than the ranges thus far reported for bulk igneous rocks. Most of the δ^44/40Ca variability reflects ⁴⁰Ca enrichment in K-feldspar, and to a lesser extent, biotite, due to the radioactive decay of ⁴⁰K over the 1.7 Ga age of the rock, whereas the entire range of δ^44/42Ca values reflects mass-dependent isotope fractionation during igneous differentiation and crystallization. The range of δ^26/24Mg values may represent either fractionation during the chloritization of biotite or interaction of the Boulder Creek Granodiorite with Mg-rich metamorphic fluids having low δ^26/24Mg values.The elemental and isotopic composition of the reactor output varied substantially during the experiment. We synthesize the mineralogical and fluid data using coupled mass-conservation equations solved at non-steady-state. Model calculations reveal an intricate balance between increasing specific surface area and decreasing mineral concentrations. While surface area normalized dissolution rate constants were time-invariant, specific surface area increased as a power-law function of time through positive feedbacks between mechanical disaggregation, chemical dissolution, and mineral depletion. Variations in dissolved δ^44/40Ca, δ^44/42Ca, and δ^26/24Mg values reflect conservative mixing rather than fractionation. Apatite and calcite initially control δ^44/40Ca and δ^44/42Ca values, followed by biotite, titanite, epidote, hornblende, and plagioclase. The release of radiogenic ⁴⁰Ca clearly defines the period where biotite dissolution dominates. The brucite layer of chlorite initially controls δ^26/24Mg values, followed by biotite, the TOT layer of chlorite, and hornblende. Through direct isotopic tracking, these results demonstrate that trace minerals, such as apatite and calcite in the case of Ca and brucite in the case of Mg, dominate elemental release during the incipient stages of granite weathering. The results further show that biotite dissolution dominates the middle stages of granite weathering and that plagioclase dissolution only becomes important during relatively late stages. The Ca and Mg isotope variations associated with these stages are distinct and potentially resolvable in soil mineral weathering studies.     

Geochemical fixation of rare earth elements into secondary minerals in sandstones beneath a natural fission reactor at Bangombé, Gabon

To study geochemical processes for migration and fixation of fissiogenic rare earth elements (REE) in association with uranium dissolution, in situ isotopic analyses using an ion microprobe were performed on U- and REE-bearing secondary minerals, such as coffinite, françoisite, uraniferous goethite, and uraninite found in a sandstone layer 30 to 110 cm beneath a natural fission reactor at Bangombé, Gabon. Phosphate minerals such as phosphatian coffinite and françoisite with depleted ²³⁵U (²³⁵U/²³⁸U = 0.00609 to 0.00638) contained large amount of fissiogenic light REE, while micro-sized uraninite grains in a solid bitumen aggregate have normal U isotopic values (²³⁵U/²³⁸U = 0.00725) and small amount of fissiogenic REE components. The proportions of fissiogenic and non-fissiogenic REE components in four samples from the core of BAX03 vary in depth ranging from 30 cm to 130 cm beneath the reactor, which suggests mixing between fissiogenic isotopes from the reactor and non-fissiogenic isotopes from original minerals in the sandstone. Significant chemical fractionation was observed between Ce and the other REE in the secondary minerals, which shows evidence of an oxidizing atmosphere during their formation. Pb-isotopic analyses of individual minerals do not directly provide chronological information because of the disturbance of U-Pb decay system due to recent geologic alteration. However, systematic Pb-isotopic results from all of the minerals reveal the mobilization of fissiogenic isotopes, Pb and U from the reactor in association with dolerite dyke intrusion ∼0.798 Ga ago and the formation of the secondary minerals by mixing event between 2.05 Ga-old original minerals and reactor materials due to recent alteration.     

川西甲基卡锂矿床伟晶岩结构分带及多期岩浆成矿作用

LCT(lithium- cesium- tantalum)伟晶岩的分带有两种形式,一种是单个伟晶岩脉的内部结构分带,这种伟晶岩脉通常体积巨大,内部结构显示出成分分带的特征,单一脉体中不同结构带中可能发育不同的稀有金属矿化带,例如新疆可可托海3号伟晶岩脉;另一种分带性表现为伟晶岩和成矿系统的区域分带,通常以高分异的过铝质花岗岩为中心,围绕花岗岩数以百计的伟晶岩脉显示出不同的伟晶岩类型和矿化类型,越向外围,伟晶岩的演化程度越高。区域分带内的伟晶岩单个脉体的规模不大、内部分带性差、含矿性各不相同,甲基卡锂矿就是这一类伟晶岩矿床的典型代表。对甲基卡矿区伟晶岩的野外观察发现,含Be的9号脉、含Li的133号脉和含Nb、Ta的528号脉具有相对较好的内部结构分带性,而含Li的104号脉和308号脉的内部分带性则相对较差,推测与脉体内部多期岩浆- 热液活动有关。甲基卡矿区伟晶岩脉的相互穿插现象说明,矿区存在多期次的岩浆与成矿作用。科学钻探在JSD- 2和JSD- 3两个钻孔中均发现含锂辉石的伟晶岩脉和细晶岩脉侵入在不含矿的马颈子花岗岩中,佐证了多期岩浆与成矿事件的存在。伟晶岩中不同矿物U- Pb定年结果表明,全区伟晶岩的形成至少有两期,分别发生在213~206 Ma和199~191 Ma。依据钻孔JSD- 1的研究表明,岩浆由深至浅逐步向高分异方向演化。从下向上,花岗岩与伟晶岩同步演化,花岗岩依次出现黑云母花岗岩、含白云母的黑云母花岗岩、二云母花岗岩、白云母花岗岩和钠长石花岗岩,伟晶岩依次出现微斜长石型伟晶岩、微斜长石- 钠长石型伟晶岩、钠长石型伟晶岩和钠长石- 锂辉石型伟晶岩。根据不同深度伟晶岩的矿化特点,矿床的垂向分带大致划分如下：0~105 m的Li- Be- Nb- Ta矿化带;0~860 m的Be- Nb- Ta矿化带;0~1730 m的第一期Nb- Ta矿化带;1730~3170 m的无矿伟晶岩带;3170~3211 m的第二期Nb- Ta矿化带。     

Hydrothermal mineralization in the sandstone–hosted Hangjinqi uranium deposit,North Ordos Basin,China

Tabular–type uranium ore deposits (the Hangjinqi and Daying deposits) have recently been found in the Middle Jurassic Zhiluo Formation, north of the Ordos Basin, China. Petrographic observations, the chemical composition of U minerals determined by EMPA and fs–LA–ICP–MS, whole rock geochemistry and the microthermometric study of fluid inclusions have been integrated to characterize the genetic conditions of the U mineralization in the Hangjinqi sandstone–hosted deposit. Two different groups of U minerals have been identified. One group includes coffinite(I) associated with vanadium–rich micas. Coffinite(I) is enriched in vanadium (V) and devoid of iron (Fe) and yttrium (Y) and has a LREE–enriched chondrite–normalized REE pattern. The U minerals of this group are similar to meteoric fluid infiltration related deposits. The second group has coeval coffinite(II) and coarsely crystalline calcite cement. Coffinite(II) is enriched in Y and Fe and depleted in V and is marked by a flat chondrite–normalized REE pattern, which is compatible with typical hydrothermal genetic deposits with high–salinity mineralizing fluids. The temperature and salinity of the primary aqueous inclusions in the ore–stage calcite are 120–180 °C and 8.00–16.34% (eq. wt% NaCl), respectively. These mineral assemblages, temperatures and salinities indicate that the Hangjinqi deposit was affected by two distinct types of ore–bearing fluids: low–salinity meteoric waters and high–salinity hydrothermal fluids. The meteoric fluids event began at 97 ± 5 Ma with the titling of the northern Ordos Basin and the uplift of the Hetao region to the north. Hydrothermal U mineralization occurred since 39 ± 2 Ma with the rifting of the Hetao graben. Thus, the previous biogenic model for the U mineralization should be modified in the uraniferous region of the north Ordos Basin.     

Characteristic Features of REE and Pb–Zn–Ag Mineralizations in the Na Son Deposit,Northeastern Vietnam

The Na Son deposit is a small‐scale Pb–ZnPb–Zn–Ag deposit in northeast Vietnam and consists of biotite–chlorite schist, reddish altered rocks, quartz veins and syenite. The biotite–chlorite schist is intruded by syenite. Reddish altered rocks occur as an alteration halo between the biotite–allanite‐bearing quartz veins and the biotite–chlorite schist. Allanite occurs in the biotite–allanite‐bearing quartz veins and in the proximal reddish altered rocks. Rare earth element (REE) fluorocarbonate minerals occur along fractures or at rim of allanite crystals. The later horizontal aggregates of sulfide veins and veinlets cut the earlier reddish altered rocks. The earlier Pb–Zn veins consist of a large amount of galena and lesser amounts of sphalerite, pyrite and molybdenite. The later Cu veins cutting the Pb–Zn veins include chalcopyrite and lesser amounts of tetrahedrite and pyrite. The occurrences of two‐phase H₂O–CO₂ fluid inclusions in quartz from biotite–allanite‐bearing quartz veins and REE‐bearing fluorocarbonate minerals in allanite suggest the presence of CO₂ and F in the hydrothermal fluid. The oxygen isotopic ratios of the reddish altered rocks, biotite–chlorite schist, and syenite range from +13.9 to +14.9 ‰, +11.5 to +13.3 ‰, and +10.1 to +11.6 ‰, respectively. Assuming an isotopic equilibrium between quartz (+14.6 to +15.8 ‰) and biotite (+8.6 ‰) in the biotite–allanite‐bearing quartz vein, formation temperature was estimated to be 400°C. At 400°C, δ¹⁸O values of the hydrothermal fluid in equilibrium with quartz and biotite range from +10.5 to +11.7 ‰. These δ¹⁸O values are consistent with fluid that is derived from metamorphism. Assuming an isotopic equilibrium between galena (+1.5 to +1.7 ‰) and chalcopyrite (+3.4 ‰), the formation temperature was estimated to be approximately 300°C. The formation temperature of the Na Son deposit decreased with the progress of mineralization. Based on the geological data, occurrence of REE‐bearing minerals and oxygen isotopic ratios, the REE mineralization is thought to result from interaction between biotite–chlorite schist and REE‐, CO₂‐ and F‐bearing metamorphic fluid at 400°C under a rock‐dominant condition.     

江西石城海罗岭花岗岩型铌钽矿床岩石地球化学、 年代学特征及其地质意义

稀有金属矿产是江西省优势矿产资源,成矿类型以花岗岩型为主,主要分布于赣西和赣北地区,以宜春414超大型钽铌矿为代表,而花岗岩广泛分布的赣南地区鲜有关于燕山期花岗岩型稀有金属矿床的报道。本文以赣南石城海罗岭铌钽矿床为研究重点,结合详细的野外调查,开展花岗岩的岩石学、岩石地球化学和同位素年代学等研究,厘定了海罗岭的中粒斑状黑云母二长花岗岩- 中细粒黑云母二长花岗岩岩石组合,明确了钠长石化叠加白云母化的中粒斑状黑云母二长花岗岩与铌钽矿密切相关的成矿专属性。海罗岭的成矿作用具两阶段特征,早阶段以蚀变花岗岩型钽铌矿为主,赋存于钠长石化白云母化中粒斑状黑云母二长花岗岩中,晚阶段则以花岗伟晶岩型锂矿为主,赋存于云母锂辉石伟晶岩中。海罗岭的花岗岩主要经历了钠长石化、白云母化、黄玉化、绢云母化、硅化等蚀变作用,呈现碱性长石化→云英岩化的演化过程。海罗岭花岗岩具富硅、富碱、富铝,贫钛、镁的特征,其中钠长石化白云母化中粒斑状黑云母二长花岗岩（富钽花岗岩）中F含量为8330×10-6～13076×10-6,平均为10475×10-6,具极低的Nb/Ta值（0. 34~0. 49）、Zr/Hf值（3. 73~4. 19）、稀土总量低（ΣREE为16. 3×10-6~23. 2×10-6）和“四分组”效应等特征,显示其成矿经历了岩浆- 流体相互作用的过程。研究显示,Li矿化富集程度与F含量呈明显的正相关,与稀土总量、K/Rb值呈负相关;Ta矿化富集程度与F含量呈明显的正相关,与Nb/Ta值、Zr/Hf值呈明显的负相关。中细粒黑云母二长花岗岩锆石U- Pb年龄为141. 9±1. 1 Ma,云母锂辉石伟晶岩和碱性长石伟晶岩独居石U- Pb年龄分别为141. 68±0. 69 Ma和137. 62±0. 73 Ma,均归属于早白垩世。研究表明,赣南地区140 Ma左右可能存在一次与钠长石化叠加白云母化中粒斑状黑云母二长花岗岩相关的独特的铌钽矿成矿事件和与花岗伟晶岩相关的锂成矿事件。这一发现打破了以往华南稀有金属主要赋存于燕山期复式岩体晚期二云母花岗岩- 白云母花岗岩中的认识,拓宽了找矿思路,为赣南乃至华南地区稀有金属找矿提供了新的方向。     

Residence of REE, Y, Th and U in Granites and Crustal Protoliths; Implications for the Chemistry of Crustal Melts

A systematic study with laser ablation—ICP-MS, scanningelectron microscopy and electron microprobe revealed that 70–95wt% of REE (except Eu), Y, Th and U in granite rocks and crustalprotoliths reside within REEYThU-rich accessories whose nature,composition and associations change with the rock aluminosity.The accessory assemblage of peraluminous granites, migmatitesand high-grade rocks is composed of monazite, xenotime (in low-Cavarieties), apatite, zircon, Thorthosilicate, uraninite andbetafite-pyrochlore. Metaluminous granites have allanite, sphene,apatite, zircon, monazite and Thorthosilicaie. Peralkaline graniteshave aeschinite, fergusonite, samarskite, bastnaesite, fluocerite,allanite, sphene, zircon, monazite, xenotime and Th-orthosilicate.Granulite-grade garnets are enriched in Nd and Sm by no lessthan one order of magnitude with respect to amphibolite-gradegarnets. Granulitegrade feldspars are also enriched in LREEwith respect to amphibolite-grade feldspars. Accessories causenon-Henrian behaviour of REE, Y, Th and U during melt—solidpartitioning. Because elevated fractions of monazite, xenotimeand zircon in common migmatites are included within major minerals,their behaviour during anatexis is controlled by that of theirhost. Settling curves calculated for a convecting magma showthat accessories are too small to settle appreciably, beingseparated from the melt as inclusions within larger minerals.Biotite has the greatest tendency to include accessories, therebyindirectly controlling the geochemistry of REE, Y, Th and U.We conclude that REE, Y, Th and U are unsuitable for petrogeneticalmodelling of granitoids through equilibrium-based trace-elementfractionation equations. KEY WORDS: accessory minerals; geochemical modelling; granitoids; REE, Y, Th, U     

Comparative dissolution kinetics of biogenic and chemogenic uraninite under oxidizing conditions in the presence of carbonate

The long-term stability of biogenic uraninite with respect to oxidative dissolution is pivotal to the success of in situ bioreduction strategies for the subsurface remediation of uranium legacies. Batch and flow-through dissolution experiments were conducted along with spectroscopic analyses to compare biogenic uraninite nanoparticles obtained from Shewanella oneidensis MR-1 and chemogenic UO_2.00 with respect to their equilibrium solubility, dissolution mechanisms, and dissolution kinetics in water of varied oxygen and carbonate concentrations. Both materials exhibited a similar intrinsic solubility of ∼10⁻⁸ M under reducing conditions. The two materials had comparable dissolution rates under anoxic as well as oxidizing conditions, consistent with structural bulk homology of biogenic and stoichiometric uraninite. Carbonate reversibly promoted uraninite dissolution under both moderately oxidizing and reducing conditions, and the biogenic material yielded higher surface area-normalized dissolution rates than the chemogenic. This difference is in accordance with the higher proportion of U(V) detected on the biogenic uraninite surface by means of X-ray photoelectron spectroscopy. Reasonable sources of a stable U(V)-bearing intermediate phase are discussed. The observed increase of the dissolution rates can be explained by carbonate complexation of U(V) facilitating the detachment of U(V) from the uraninite surface. The fraction of surface-associated U(VI) increased with dissolved oxygen concentration. Simultaneously, X-ray absorption spectra showed conversion of the bulk from UO_2.0 to UO_2+x. In equilibrium with air, combined spectroscopic results support the formation of a near-surface layer of approximate composition UO_2.25 (U₄O₉) coated by an outer layer of U(VI). This result is in accordance with flow-through dissolution experiments that indicate control of the dissolution rate of surface-oxidized uraninite by the solubility of metaschoepite under the tested conditions. Although U(V) has been observed in electrochemical studies on the dissolution of spent nuclear fuel, this is the first investigation that demonstrates the formation of a stable U(V) intermediate phase on the surface of submicron-sized uraninite particles suspended in aqueous solutions.     

豆乍山地区产铀与非产铀岩体黑云母成分及赋存副矿物特征

豆乍山岩体和香草坪岩体成岩时代相近,空间关系密切,为苗儿山中段豆乍山地区产铀与非产铀岩体。文章选择豆乍山和香草坪新鲜花岗岩中黑云母为研究对象,结合诸广、贵东地区产铀与非产铀岩体中黑云母的电子探针数据开展综合分析,并在此基础上进行了黑云母中包裹副矿物种类的对比研究。结果表明:产铀岩体黑云母基本以铁叶云母为主,非产铀岩体黑云母为铁质黑云母,且前者具有相对富铝、铁、锰,贫镁、钛特征;产铀豆乍山岩体黑云母中副矿物为锆石-磷钇矿-独居石-钛铁矿-金红石-方钍石-晶质铀矿,非产铀香草坪岩体黑云母中副矿物为锆石-磷灰石-褐帘石-独居石,缺少钍石和晶质铀矿。不同亚种黑云母具有不同的成岩成矿环境,与各矿化类型往往具有直接对应关系,铁叶云母的出现有可能可以作为产铀与非产铀岩体(铀成矿潜力)的间接判别标志,对于后期找矿具有较大指导意义。     

Ages and compositions of primary and secondary allanite from the Lala Fe–Cu deposit,SW China: implications for multiple episodes of hydrothermal events

Numerous Fe–Cu deposits in southwestern China form the Kangdian Iron-Oxide Copper-Gold (IOCG) metallogenic Province. These deposits have a close association of Fe-oxides and Cu-sulfides formed at different stages, which are possibly related to multiple hydrothermal events. In this paper, U–Pb dating and chemical analyses on allanite from different stages of the Lala deposit were used to constrain timing and origin of such events. Allanite occurs as disseminated grains or patches in Fe–Cu ores and is closely associated with chalcopyrite, molybdenite, calcite and minor titanite, postdating magnetite and apatite. High-resolution backscattered electronic (BSE) imaging, electron microprobe compositions and X-ray scanning profiles demonstrate that REE-rich primary allanite was replaced by later, relatively porous and REE-poor secondary allanite. Such a replacement was promoted by interaction between primary allanite and fluid fluxes infiltrating the minerals, following an exchange scheme of REE³⁺ + Fe²⁺ → Ca²⁺ + Al³⁺. The secondary allanite has higher Fe³⁺/(Fe³⁺+Fe²⁺) ratios and U contents, indicating involvement of relatively oxidized fluids during alteration. The alteration has also produced unidentified secondary REE minerals in fractures, indicating re-deposition of some of the removed REEs. The primary and secondary allanites are dated by in situ LA-ICP-MS technique and have U–Pb ages of 1,067 ± 41 Ma and 880–850 Ma, respectively. The ~1.07 Ga primary allanite was contemporaneous with the main Mo–Cu–LREE mineralization with a molybdenite Re–Os age of ~1.08 Ga. The 880–850 Ma secondary allanite is comparable with the Ar–Ar ages (890–830 Ma) of biotite from hosting schists and undeformed sulfide veins occurring throughout the Kangdian Province, suggesting that such an event was possibly syn-deformational and represents a younger hydrothermal event. Occurrences of both primary and secondary allanites suggest that the mineralization may have involved multiple tectonothermal events including the ~1.05–1.1 Ga intra-plate and subsequent 960–740 Ma arc magmatism in the Kangdian region.