Dissolved inorganic and organic selenium in the Orca Basin

The vertical distributions of Se (IV), Se (VI) and dissolved organic Se have been determined in the oxic and non-sulfide-bearing anoxic zones of the Orca Basin. In the oxic waters, the concentration of Se (IV) increases with depth gradually from 0.25 nmole/kg at the surface to a maximum of 0.46 nmole/kg at 750 m and then decreases with depth to a relatively constant concentration of 0.39 nmole/ kg below 1,230 m. The concentration of Se (VI) is rather uniform in the top 250 m at about 0.24 nmole/ kg. Below 250 m it increases with depth to 0.50 nmole/kg at 1.230 m, and it stays relatively constant below this depth. The concentration of organic Se increases from 0.50 nmole/kg at the surface to 1.39 nmole/kg at 78 m. A pronounced broad maximum of organic Se exists between 78 and 250 m. The concentration decreases with depth below 250 m, dropping sharply between 250 and 380 m and more gradually at greater depths. It becomes undetectable at 1,230 m. Organic Se is the dominant species above 250 m. Se (IV) is the most abundant between 250 and 1,000 m while Se (VI) becomes the dominant species below 1,000 m. The distributions of these three species can be explained by the biological uptake of Se in the surface waters and the multi-step regeneration of Se from biogenic particles at greater depths. In suboxic waters at the oxic-anoxic interface, the concentration of Se (IV) increases while that of Se (VI) decreases reflecting a change in redox conditions in the environment. In the anoxic brine, the concentration of Se (IV) is around 0.25 nmole/kg while Se (VI) is undetectable. The concentration of organic Se increases sharply in the suboxic waters and reaches 2.6 nmole/kg in the anoxic brines probably as a result of the decomposition of organic matter and/or a diffusive flux from the underlying sediment.     

Diphytanyl glycerol ether distributions in sediments of the Orca Basin

Archaebacterially produced diphytanyl glycerol ether (DPGE) was examined in core sediments from the Orca Basin, an anoxic hypersaline basin in the northwestern Gulf of Mexico, to observe its spatial variability and potential origin. A differential extraction protocol was employed to quantify the isopranyl glycerol ethers associated with unbound, intermediate-bound, and kerogen-bound lipid fractions. Archaebacterial lipids were evident at all depths for the unbound and intermediate-bound fractions. Concentrations of DPGE ranged from 0.51 to 2.91 micrograms/g dry sediment at the surface and showed secondary maxima deeper in basin sediments. Intermediate-bound DPGE concentrations exhibited an inverse relationship to unbound DPGE concentrations. Kerogen-bound DPGE concentrations were normally below detection limits. Earlier studies describing the general homogeneity of lipid components within the overlying brine and at the brine/seawater interface suggest that the large-scale sedimentary DPGE variations observed in this study result from spatial and temporal variations in in situ production by methanogenic or extremely halophilic archaebacteria.     

Biogenic hydrocarbon gases and sulfate reduction in the Orca Basin brine

The composition of light hydrocarbon gases in the Orca Basin, an anoxic, hypersaline intraslope depression on the continental slope of the northern Gulf of Mexico, indicates that both methane and ethane are biogenic in nature with a $\text{C}{}_1\text{(C}{}_2\text{+ C}{}_3\text{)}$ ratio of 730 and a δ¹³C of methane of ?73%. relative to the PDB standard. The concentrations of methane (750 mM) and ethane (1300 mM) in the Orca Basin brine are higher than any other marine anoxic basin. These high levels result not from high rates of productivity, but from the long residence time of the brine in the basin, due to its high stability toward mixing with overlying seawater ($\text{Δσ}{}_1\text{ΔZ}\text{=}\text{3.2}\text{m}$). Both methane and ethane show well mixed distributions in the brine. These distributions probably result from convective mixing of the isohaline brine pool due to normal heat flow from the basin sediments. Methane and ethane maxima above the pycnocline at the brine/seawater interface reflect in situ production and/or consumption in the aerobic water column. Concurrent maxima in suspended particulate material distributions in this region suggest methane may be produced there in anaerobic microenvironments associated with the suspended matter. Reduced rates of anaerobic decomposition (including sulfate reduction) in the brine sediments are inferred from preserved Sargassum fronds in the sediments, vertical sulfate profiles in most cores, and the sediment organic carbon content which is two to three times higher in sediments below the high salinity brine than in the normal Gulf sediments nearby.     

Stable carbon isotope ratio variations of organic matter in Orca Basin sediments

Orca Basin is a highly reducing basin on the slope of the Gulf of Mexico. Stable carbon isotope ratios and total organic carbon percentages were determined for two cores within the basin and one control core outside the basin. The results show that the organic carbon content of the basin cores is consistently 2–3 times greater than that of the control core. The Pleistocene-Holocene boundary, indicated by a break in the δ¹³C depth profile, occurs at a greater sediment depth in the basin cores than in the control core. A small sampling interval has made it possible to detect an unexplained fine structure in the δ¹³C profile not previously observed.     

Uranium geochemistry in groundwater from tertiary sediments

Dissolved U concentrations and activity ratios (ARs) of the U isotopes in the ²³⁸U decay series were measured in ground and surface waters as part of an investigation to delineate the water quality in a proposed uranium mining area of northwest Nebraska. In oxidizing groundwaters from 67 wells completed in the Tertiary sediments, increasing U concentrations in the direction of groundwater flow generally were associated with a maturation of the formation water as evidenced by evolutionary trends in major ion character. The increased U levels probably are associated with leaching as shown by the positive correlation between U concentrations and total dissolved solids (TDS) (r = +0.83). The inverse relationships between TDS and U ARs (r = ?0.73) and U levels and ARs (r = 0.72) indicate that the decay of excess U-234 is related to maturation of the formation water and to sediment leaching along the flowpath. The data are described by a model which incorporates etching, decay and recoil and suggests that aquifer residence time can be estimated from the TDS level.The levels of soluble U in a reducing uraniferous hydrogeologic unit near Crawford, Nebraska are affected by the proximity of the sample collection to ore. In groundwater samples having similar chemistries (Na-SO₄ + Cl type), similar Ehs, and collected from a close-knit pattern, U concentrations ranged from 0.01 to 2,037 μg l^?1 and ARs ranged from 0.75 to 12.6. This high variability in U levels and ARs is indicative of uranium ore in small areal studies where low ARs almost always are associated with high U concentrations.     

Uranium geochemistry and dating of Pacific island apatite

Uranium-series disequilibrium dating of island phosphate deposits is evaluated in terms of known associated coral ages, uranium geochemistry, and stratigraphic sequences as well as the concordance between the geochronometers ²³⁴U/²³⁸U, ²³⁰Th/²³⁴U and ²²⁶Ra/²³⁸U. U(VI) is the predominant oxidation state of uranium in island phosphorites and by analogy to the youngest surficial deposits, most of the uranium initially bound is in the form of U(VI) sorbed by surfaces from seawater. Insular deposits contain more organic matter than even very young ocean floor samples and this leads to a greater probability of reduction of available recoil uranium than occurs in marine deposits. As a consequence, R(VI) ? R(T) ? R(VI), where R represents the ²³⁴U/²³⁸U activity ratio. This situation is completely opposite from that observed for marine-origin phosphorites. We determined that a fraction of U(VI) in ancient insular phosphorites is very labile and lost to alkaline carbonate solutions with a uranium activity ratio even more depleted in ²³⁴U than the bulk R(VI).Most younger samples appear to have no more discordance between ²³⁴U/²³⁸U, ²³⁰Th/²³⁴U and ²²⁶Ra/²³⁸U than marine phosphorites of similar apparent age. Young, surficial atoll-rim apatite cements and unconsolidated phosphorites date in the range of 1500–8000 years B.P., consistent with the concept of partial submergence of low-lying coral islands prior to 2000 years B.P. Sub-surficial samples in the same environment date older at about 20,000 years B.P. Violation of the closed system assumption occurs in at least 6 out of 13 ancient (> 800,000 year) samples. Uranium-series disequilibrium dating of insular apatite shows some promise as a recorder of climatic/sea level events, but the assumptions necessary for valid ages must be carefully evaluated for each occurrence.     

Uranium districts defined by reconnaissance geochemistry in South Greenland

A reconnaissance exploration survey over 14 000 km² of Precambrian terrain in South Greenland using stream-sediment and stream-water samples delineated a central uranium district of 2000 km² with enhanced uranium levels and smaller anomalous zones in the south of the field area.The area is underlain by Archean and Proterozoic gneisses, granites and metasediments all of which have been intruded by late Proterozoic alkaline intrusions (Gardar Province). The terrain is mountainous and the streams are either steep torrents or impeded drainages typical of glaciated terrains with boggy organic rich sediments.The central uranium district was defined by a high uranium background in both stream sediments (5–20 ppm) and stream waters (0.5–1.0 ppb) and a markedly higher frequency of very anomalous values in the order of 50–100's ppm in the stream sediments and 1–10 ppb in the stream waters. An areal correlation of uranium, in this district, with high pH and conductivity in the stream water in addition to a higher organic content noted in the stream sediment raised the question of a possible enhancement of uranium values due to secondary environmental effects. On the other hand, an areal correlation of uranium with niobium and other trace elements characteristically associated with alkaline rocks, and the geographic proximity of this uraniferous district to the alkaline intrusions suggested a genetic relationship between uranium mineralization and the alkaline igneous activity.Limited follow-up work located 8 pitchblende occurrences in this extensive district. The pitchblende is in veins which contain quartz, calcite, iron oxide, fluorite and minor sulphides. The isotopic (U-Pb) age of the pitchblende, which ranges from 1180-1090 Ma, corresponds to the late stages of Gardar alkaline igneous activity. It is concluded, therefore, that the reconnaissance geochemistry reflects a district-wide hydrothermal event related to the late volatile differentiates derived from the highly fractionated alkaline magma. A combination of primary and secondary features have complemented each other in enhancing the geochemical reconnaissance data and emphasized its importance but has not materially altered the interpretation.The south of the field area also has a relatively high uranium background in both the sample media with some discrete anomalous zones, usually with a slightly lower order of magnitude than the central area, but still with a distinct contrast of 5–10 times. Fine-grained uraninite has been found in the area occurring as disseminated grains in pegmatitic elements as in the central district. Isotopic ratios (U-Pb) suggest an age of 1728 ± 30 Ma which probably reflects the long cooling of the granite.It is concluded that the geochemical reconnaissance data delineated two uranium metallogenic districts characterized by distinctly different types of uranium mineralization. It is suggested that South Greenland may be part of a much wider uranium geochemical province which includes parts of Labrador. To the present plate-tectonic models, which suggest such a connection (Le Pichon et al., 1977), must be added the comparable reconnaissance geochemical results (G.S.C. Open Files nos. 748 and 749), and the similar 1730 Ma age of the Kitts uranium mineral occurrence in Labrador (Gandhi, S.S , 1978) to that of the uraninite found in the south of the field area in Greenland.     

中国盆地铀资源概述

近20多年来,我国在北方6大沉积盆地中陆续发现了系列大型和超大型砂岩型铀矿床,丰富的资源量昭示了沉积盆地是一个巨大的促使铀汇聚的化学反应器.然而,盆地中的铀矿床类型远不止于砂岩型一种,它们一并构成了宝贵的盆地铀资源.将在沉积盆地发展演化过程中,受沉积、成岩和构造作用制约而促使铀富集形成的系列铀矿床,统称为盆地铀资源.充分考虑铀成矿作用的关键制约要素和矿床形成的发育时序,将盆地铀资源划分为同沉积型、不整合型和成岩型三大类13个亚类型矿床.在盆山耦合的构造体制驱动下,铀的变价属性是铀大尺度循环(汇聚与分散)的基础,这使得各种铀矿床之间既具有成因联系又能相互转化.我国已探明盆地铀资源的矿床成因类型、时空分布和资源量规模极不均衡,但是北方砂岩型铀矿床和南方碳硅泥岩型铀矿床构成了盆地铀资源的主体,从而具有"一北一南"、"一陆一海"、"一新一老"的基本格局.目前,北方砂岩型铀矿是我国勘查和开发的重点,然而铀储层结构和物质成分的非均质性极大,地浸采铀技术亟需革新以适宜多数砂岩型铀矿床的开发.同时,需要在新地区、新层位发现更多新类型铀矿床,还需要依赖技术研发盘活已发现的以南方碳硅泥岩型铀矿床为代表的"...     

鄂尔多斯盆地大营铀矿铀的赋存状态研究

鄂尔多斯盆地大营铀矿是近几年新发现的砂岩型铀矿,详细研究该矿床铀矿物的赋存状态,对其矿床成因认识及之后的选冶开采均具有重要的意义。笔者利用电子探针和逐级化学提取分析以及α径迹分析等方法,研究了大营铀矿铀的赋存状态。结果发现,铀一部分以铀矿物为主,主要是铀石,另外存在少量的水硅铀石和钛铀矿。在电子探针镜下观察,表明铀矿物呈微粒集合体形式存在于黄铁矿、方解石及有机质裂缝中。逐级化学提取分析发现,各形态铀的含量不均一,其中有机质黄铁矿态占40.88%;其次为碳酸盐态,占28.33%;另外残渣态占24.89%,少部分为离子吸附态和铁锰氧化态,分别占3.2%和2.64%。反映其中铀矿物形式的铀占53.22%,分散吸附态的铀占46.72%。分散吸附态的铀以有机质及黄铁矿吸附占主导。综上所述,大营铀矿铀的赋存状态主要为铀矿物和吸附两种形式,二者所占比例大体相当,以铀矿物存在的铀主要为铀石,以吸附形式存在的铀主要赋存在黄铁矿和有机质中。     

新疆三塘湖盆地基底特征及砂岩型铀矿的铀源条件

通过对三塘湖盆地基底地层，特别是其中火山岩岩石组合及其形成的构造环境进行的研究，认为盆地基底主要由岛弧系列的火山岩及火山碎屑岩、正常碎屑岩组成，在早石炭世处于岛孤环境。通过分析与对比．认为盆地的铀源条件局部较好。     

二连盆地西部隆起区卫境岩体铀丢失与盆内铀矿成矿关系探讨

二连盆地在近20年已探明多个砂岩、泥岩型铀矿床,是我国重要的产铀盆地。位于二连盆地边缘西北部隆起区的卫境岩体,是一个多期次(海西—燕山期)的复式岩体。其分布面积广(>1000km²)且有多个铀矿床围绕分布。为查明卫境岩体与盆地内铀成矿之间的关系,开展了详细地野外地质调查,采取了卫境岩体出露地表、钻孔中揭露的岩石以及盆地内含矿目的层砂岩样品。露头观察和镜下鉴定结果表明,卫境岩体主要为中粗粒黑云母似斑状花岗岩。其在地表已被强烈风化、剥蚀乃至夷平。含矿目的层砂岩中主要碎屑颗粒为石英、长石(斜长石与钾长石),部分为花岗岩岩屑,它们主要来自花岗岩岩体。卫境岩体和哈达图铀矿床目的层(赛汉组)砂岩样品稀土元素及其参数对比表明,含矿层砂岩物质成分来自于卫境岩体。卫境岩体与赛汉组砂岩样品Pb同位素分析结果表明,卫境岩体原始铀含量(U0)较高,平均达15.17×10^–6,丢失的铀含量平均达到–45.47%。赛汉组氧化带砂岩中铀明显丢失,平均铀的丢失率为–41.42%,而还原带砂岩中铀多为获得,平均铀获得率达66.80%。由此可知,目的层铀矿化中铀的来源有两...     

鄂尔多斯盆地纳岭沟铀矿床铀矿物特征与形成机理

纳岭沟铀矿床位于鄂尔多斯盆地北东部,是我国近年发现的一个特大型砂岩铀矿床,铀矿体赋存于中侏罗统直罗组下段河流相砂体中,受古层间氧化带控制,呈板状。铀矿物主要为铀石、沥青铀矿和钛铀矿,其中铀石是最主要的铀矿物。沥青铀矿中w(CaO)较高;铀石中w(UO_2)偏低、w(SiO_2)偏高,w(UO_2)/w(SiO_2)达到1:1.29,远低于正常值;钛铀矿则呈现U低Ti高的特点。根据各铀矿物的形成、富集机理,认为纳岭沟铀矿床在古层间氧化发育阶段赋矿砂体属于酸性环境,SiO_2活度较低,形成铀矿物主要为沥青铀矿。始新世晚期及以后,氧化作用不发育,赋矿砂体被二次还原,岩石地球化学环境由酸性转变为弱碱性,SiO_2活度增大,使早期阶段形成的沥青铀矿转变为铀石。钛铀矿主要是在氧化—还原作用下由含铀溶液中的铀(UO_2~(2+))交代重矿物—钛铁矿中的Fe2+而形成。     

中新生代盆地深部地质流体及铀成矿作用

随着在哈萨克斯坦楚·萨雷苏盆地坎茹甘、莫英库姆、托尔特库杜克等砂岩型铀矿中发现了成矿溶液(热流体)来自深源的地质证据后,伴随着中新生代盆地后期次造山作用的活化改造,深源热流体在砂岩型铀矿成矿中的重要性越发显现出来。文章简述了中新生代盆地地质流体的来源与性质,归纳了盆地流体的3种类型和运移方式,总结了盆地深源热流体铀成矿的主要特征,并就我国北方中新生代盆地可能存在的深部地质流体及铀成矿作用进行了探讨,以期在勘查工作中开拓视野、创新思路、实现找矿新的突破。     

鄂尔多斯盆地HJQ地区砂岩型铀矿铀赋存状态研究

砂岩型铀矿床是一种具有重要经济价值的铀矿类型,受到世界各国地质勘探部门的高度关注。通过电子探针、α径迹蚀刻、逐级化学提取等方法的联合应用,对鄂尔多斯盆地HJQ砂岩型铀矿铀的赋存状态进行研究。结果表明,该地区铀矿物以铀石为主,还有极少量的钍铀石和钛铀矿,这些矿物主要是以微粒吸附形式存在(1~5 μm),部分呈较大铀石微粒集合体形式存在的铀矿物颗粒主要存在于黄铁矿、绿泥石、 方解石及有机质的裂隙中,还有少数存在于石英、褐铁矿、云母等矿物的裂隙之中。逐级化学提取实验进一步证实,铀矿物占很大比例,约为50%。分散吸附态的铀约占50%。在分散吸附态的铀中又以黄铁矿和有机质吸附为主,其他矿物吸附的铀在分散吸附的铀中约占5%。     

灵泉火山盆地铀成矿条件分析

灵泉火山盆地位于著名的额尔古纳-克鲁伦北东向火山岩铀成矿带中南段,与俄罗斯斯特列措夫铀矿田、蒙古多尔诺特铀矿田处于同一矿带上.通过多年在该盆地的地质工作,在盆内发现了众多的铀矿化异常点（带）,呈现良好的找矿前景.在详细论述了盆地的基底、盖层、火山岩岩石化学和热液蚀变等特征的基础上,分析了盆地的铀矿化特征和成矿条件,认为该盆地是寻找火山岩型铀矿的有利远景盆地,指出了盆地南部是下一步工作的重点地段.     

邬郁盆地铀成矿地质条件

通过对邬郁盆地地质特征和后生改造条件的分析,盆地基底及蚀源区具有铀源层与铀源体,盖层中中酸性火山岩、火山碎屑岩分布广泛且存在含铀层,盆中斯裂构造发育,具有多期活动与热液蚀变,盆地盖层主体宗当村组( N2z)中已发现了铀矿体和异常点(带),提出邬郁盆地具有寻找砂岩型铀矿和火山岩型铀矿的条件.     

鄂尔多斯盆地东胜砂岩型铀矿中铀矿物的电子显微镜研究

鄂尔多斯盆地东胜铀矿是我国一个重要的大型砂岩型铀矿床。通过电子显微技术,对该铀矿床中铀矿物的种类、产出特征进行详细的研究。研究确定东胜砂岩型铀矿床中铀矿物主要为铀石和水硅铀矿,两者紧密共生,它们均为表生铀矿物。铀矿物的颗粒十分细小,一般粒度均为微米级。铀石和水硅铀矿往往呈板状、不规则微粒状产出,铀矿物的表面溶蚀现象明显,有的还具纳米级的溶蚀孔。铀矿物的产出与蚀变黑云母、蚀变黄铁矿密切相关,也见到微脉状铀矿物集合体穿切碎屑石英等现象。研究表明在表生作用下,该铀矿床中的铀元素存在溶解(迁移)-沉淀(富集)的反复过程。     

潮水盆地铀成矿水文地球化学研究

在分析潮水盆地自然地理、地质背景及水文地质特征的基础上，对盆地铀成矿水文地球化学特征进行了探讨。结果表明：潮水盆地为-渗入型自流水盆地，其深层地下水中铀存在形式为UO2(CO3)n^2-2n。；从盆地边缘到盆地中心具有明显从氧化环境到还原环境的水文地球化学分带特征，水中铀经历了由溶滤迁移到还原沉淀的矿化过程；在盆地北缘的照壁山-白芨芨、阿右旗-陶家井地区，西南部的平易-平山湖地区具备十分有利的铀成矿条件和良好的铀成矿远景。     

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.