Differential behavior of components of the <Superscript>238</Superscript>U-<Superscript>206</Superscript>Pb and <Superscript>235</Superscript>U-<Superscript>207</Superscript>Pb isotopic systems in polymineralic U ores

U-Pb systems were examined in samples (ranging from 4 to 10 cm³ in volume) of ore material taken from along a 3.5-m profile across a zone of U mineralization exposed in an underground mine at the Strel’tsovskoe U deposit in eastern Transbaikalia. The behaviors of two isotopic U-Pb systems (²³⁸U-²⁰⁶Pb and ²³⁵U-²⁰⁷Pb) are principally different in all samples from our profile. While the individual samples are characterized by a vast scatter of their T(²⁰⁶Pb/²³⁸U) age values (from 112 to 717 Ma), the corresponding T(²⁰⁷Pb/²³⁵U) values vary much less significantly (from 127 to 142 Ma) and are generally close to the true age of the U mineralization. The main reason for the distortion of the U-Pb system is the long-lasting (for tens of million years) migration of intermediate decay products in the ²³⁸U-²⁰⁶Pb(RD²³⁸U) in the samples. This process resulted in the loss of RD²³⁸U from domains with high U concentrations and the subsequent accommodation of RD²³⁸U at sites with low U concentrations. The long-term effect of these opposite processes resulted in a deficit or excess of ²⁰⁶Pb as the final product of ²³⁸U decay. The loss or migration of RD²³⁸U are explained by the occurrence of pitchblende in association with U oxides that have higher Si and OH concentrations than those in the pitchblende and a higher ⁺⁶U/⁺⁴U ratio. The finely dispersed character of the mineralization and the loose or metamict texture of the material are the principal prerequisites for RD²³⁸U loss and an excess of ²⁰⁶Pb in adjacent domains with low U concentrations. Domains with low U contents in the zone with U mineralization serve as geochemical barriers (because of sulfides contained in them) at which long-lived RD²³⁸U(²²⁶Ra, ²¹⁰Po, ²¹⁰Bi, and ²¹⁰Pb) were accommodated and subsequently caused an excess of ²⁰⁶Pb. The ²³⁵U-²⁰⁷Pb system remained closed because of the much briefer lifetime of the ²³⁵U decay products. This may account for the significant discrepancies between the T(²⁰⁶Pb/²³⁸U) and T(²⁰⁷Pb/²³⁵U) age values. RD²³⁸U was most probably lost via the migration of radioisotopes at the middle part and end of the ²³⁸U family (starting with ²²⁶Ra). The heavy Th, Pa, and U radioisotopes (²³⁴Th, ²³⁴Pa, ²³⁴U, and ²³⁰Th) that occur closer to the beginning of ²³⁸U decay, before ²²⁶Ra, only relatively insignificantly participated in the process. Our results show that the loss and migration of RD²³⁸U are, under certain conditions, the main (or even the only) process responsible for the distortion of the U-Pb system.     

Precise geochronology of phoscorites and carbonatites:: The critical role of U-series disequilibrium in age interpretations

We present the results of a comparative study of several geochronometer minerals (baddeleyite, zircon, apatite, phlogopite and tetraferriphlogopite) and isotopic systems (U-Pb, Th-Pb and Rb-Sr) from phoscorites (magnetite-forsterite-apatite-calcite rocks) and carbonatites of the Kovdor ultramafic-alkaline-carbonatite massif, Kola Peninsula, Russia. Uranium, thorium and their decay products are extremely fractionated by minerals that crystallise from carbonatite and phoscorite magma. We obtain high-precision ages from different chronometers, compare their accuracy, and evaluate the role of geochronological pitfalls of initial radioactive disequilibrium, differential migration of radiogenic isotopes, and inaccurate decay constants.Apatite yielded concordant U-Th-Pb ages between 376 and 380 Ma. The accuracy of the apatite ²³⁸U-²⁰⁶Pb ages is, however, compromised by uncertainty in the amount of radiogenic ²⁰⁶Pb produced from initial excess ²³⁰Th. The ²³⁵U-²⁰⁷Pb ages are relatively imprecise due to large common Pb correction and the uncertainty in the initial Pb isotopic composition. The Th-Pb system yields a more precise age of 376.4 ± 0.6 Ma.Zircon from two carbonatite samples is characterised by moderate to low U contents, high Th contents, and very high Th/U ratios up to 9000. The ²⁰⁶Pb*/²³⁸U systems in the zircon are strongly affected by the presence of excess ²⁰⁶Pb*, produced by decay of initial ²³⁰Th. The ²⁰⁸Pb*/²³²Th ages of zircon from both carbonatite samples are uniform and yield a weighted average of 377.52 ± 0.94 Ma.Baddeleyite U-Pb analyses are 3 to 6% normally discordant and have variable ²⁰⁷Pb*/²⁰⁶Pb* apparent ages. Eleven alteration-free baddeleyite fractions from three samples with no evidence for Pb loss yield uniform ²⁰⁶Pb*/²³⁸U ages with a weighted average of 378.54±0.23 Ma (378.64 Ma after correction for initial ²³⁰Th deficiency), which we consider the best estimate for age of the phoscorite-carbonatite body of the Kovdor massif. The ²⁰⁶Pb*/²³⁸U ages of baddeleyite fractions from five other samples spread between 378.5 and 373 Ma, indicating a variable lead loss up to 1.5%. The anomalously old ²⁰⁷Pb/²³⁵U and ²⁰⁷Pb/²⁰⁶Pb ages are consistent with the presence of excess radiogenic ²⁰⁷Pb* in the baddeleyite. We interpret this as a result of preferential partitioning of ²³¹Pa to baddeleyite.Fifteen phlogopite and tetraferriphlogopite fractions from five carbonatite and phoscorite samples yielded precise Rb-Sr isochron age of 372.2 ± 1.5 Ma, which is 5 to 7 m.y. younger than our best estimate based on U-Th-Pb age values. This difference is unlikely to be a result of the disturbance or late closure of Rb-Sr system in phlogopite, but rather suggests that the accepted decay constant of ⁸⁷Rb is too high.Comparative study of multiple geochronometer minerals from the Kovdor massif has revealed an exceptional complexity of isotopic systems. Reliable ages can be understood through systematic analysis of possible sources of distortion. No single geochronometer is sufficiently reliable in these rocks. Th-Pb and Rb-Sr can be a very useful supplement to U-Pb geochronometry, but the routine use of these geochronometers together will require more precise and accurate determination of decay constants for ²³²Th and ⁸⁷Rb.     

广西苗儿山铀矿田张家铀矿床成矿时代:沥青铀矿微区原位测定

张家铀矿床是苗儿山铀矿田北部的代表性铀矿床之一,对其开展详细的成矿年代学研究不仅对认识区域铀成矿规律十分重要,也对探讨华南花岗岩型热液铀矿床的成矿大地构造背景及动力学机制具有重要意义.在详细的镜下观察基础上,采用电子探针U-Th-Pb化学法、LA-ICP-MS U-Pb同位素方法对脉状沥青铀矿进行了年代学研究.20个点的U-Th-Pb化学年龄为55.3~81.1 Ma,其中19个点的加权平均年龄为71.4±1.9 Ma.根据稀土元素特征的不同,将34个点的LA-ICP-MS U-Pb同位素年龄分为2组,第一组共15个点,其中13个点的206Pb/238U加权平均年龄为69.4±4.9 Ma;第二组共19个,其中16个点的206Pb/238U加权平均年龄为94.1±3.0 Ma.电子探针U-Th-Pb化学法加权平均年龄（71.4±1.9 Ma）与LA-ICP-MS U-Pb同位素法较年轻的一组206Pb/238U加权平均年龄（69.4±4.9 Ma）一致,代表张家铀矿床的成矿时代,LA-ICP-MS U-Pb同位素法较老的一组206Pb/238U加权平均年龄（94.1±3.0 Ma）可能是沥青铀矿受后期改造和/或样品剥蚀过程杂质矿物影响而地质意义不明确.晚中生代期间,古太平洋板块向欧亚大陆俯冲可能是控制华南大规模花岗岩型铀矿床形成的动力学机制,包括张家铀矿床在内的华南花岗岩型铀矿床大规模成矿作用可能受80~50 Ma古太平洋板块对亚洲东部的斜向俯冲动力体制的控制.      

Constraints on the U-Pb isotopic systematics of Mars inferred from a combined U-Pb, Rb-Sr, and Sm-Nd isotopic study of the Martian meteorite Zagami

Uranium-lead, Rb-Sr, and Sm-Nd isotopic analyses have been performed on the same whole-rock, mineral, and leachate fractions of the basaltic martian meteorite Zagami to better constrain the U-Pb isotopic systematics of martian materials. Although the Rb-Sr and Sm-Nd systems define concordant crystallization ages of 166 ± 6 Ma and 166 ± 12 Ma, respectively, the U-Pb isotopic system is disturbed. Nevertheless, an age of 156 ± 6 Ma is derived from the ²³⁸U-²⁰⁶Pb isotopic system from the purest mineral fractions (maskelynite and pyroxene). The concordance of these three ages suggest that the ²³⁸U-²⁰⁶Pb systematics of the purest Zagami mineral fractions have been minimally disturbed by alteration and impact processes, and can therefore be used to constrain the behavior of U and Pb in the Zagami source region. The μ value of the Zagami source region can be estimated, with some confidence from the ²³⁸U-²⁰⁶Pb isochron, to be 3.96 ± 0.02. Disturbance of the U-Pb isotopic systems means that this represents a minimum value. The μ value of the Zagami source is significantly lower than the μ values estimated for most basaltic magma sources from Earth and the Moon. This is surprising given the high initial ⁸⁷Sr/⁸⁶Sr ratio (0.721566 ± 82) and low initial ε_Nd value (−7.23 ± 0.17) determined for Zagami that indicate that this sample is derived from one of the most highly fractionated reservoirs from any known planetary body. This suggests that Mars is characterized by a low bulk planet U/Pb ratio, a feature that is consistent with its relatively volatile-rich nature.The leachates contain terrestrial common Pb that was probably added to the meteorite during handling, curation, or sawing. The mineral fractions, particularly those with significant amounts of impact melt glass, contain a second contaminant. The presence of this contaminant results in Pb-Pb ages that are older than the crystallization age of Zagami, indicating that the contaminant is characterized by a high ²⁰⁷Pb/²⁰⁶Pb ratio. Such a contaminant could be produced by removal of single-stage Pb from a relatively high μ martian reservoir before ∼1.8 Ga, and therefore could be an ancient manifestation of hydrous alteration of martian surface material.     

The Age of the Neoproterozoic Dapingliang Skarn Copper Deposit in Kuruketage,NW China

The Dapingliang Cu deposit is located at the eastern part of the Kuruketage block in NW China. Igneous rocks are widely distributed in the district and skarn are formed at the contact zone between igneous rocks and the carbonates of Beiyixi Formation. The ore is distributed in the skarn. Zircon U-Pb isotopic ages of the plagiogranite, which is related to the Cu deposit, indicate that the lower and upper intercept ages are 826 ± 23 Ma and 1886 ± 61 Ma, respectively. The lower intercept age corresponds to a weighted mean ²⁰⁶Pb/²³⁸U age of 826 ± 13 Ma, yielded by ten analysis points. The upper intercept age may represent the age of the source rock, from which the plagiogranite originated. Re-Os isotopic analysis of six molybdenite samples from L7 orebody in the Dapingliang deposit shows an isochron age of 830 ± 26 Ma, which corresponds to the weighted mean model age of 829.4 ± 9.5 Ma. The concordant ages obtained by zircon U-Pb dating and molybdenite Re-Os dating are compatible with the skarn-type mineralization at Dapingling and confirm that the deposit formed during the Tarim orogeny.     

新疆铁木尔特铅锌铜矿床锆石U-Pb和黑云母40Ar/39Ar年代学及其矿床成因意义

新疆铁木尔特铅锌铜矿床位于阿尔泰造山带南缘克兰盆地内,矿体呈脉状产于康布铁堡组火山岩地层中.为准确厘定其成岩成矿时代,作者分别对矿区赋矿火山岩和含矿石英脉中的云母进行了年龄测定,获得2件火山岩样品的锆石LA-ICP-MS U-Pb年龄分别为396±5Ma和405±5Ma,2件黑云母样品的40 Ar/39 Ar坪年龄分别为240±2Ma和235±2Ma,相应的39Ar/36Ar-40Ar/36Ar等时线年龄分别为238±3Ma和233±3Ma,与坪年龄在误差范围内一致.据此,认为矿区内康布铁堡组火山岩形成于396～405 Ma,成矿作用发生于235～240Ma;成岩年龄早于成矿年龄约165Ma.因此,铁木尔特铅锌铜矿为典型的后生矿床,而不可能是同生VMS型矿床.考虑到成矿年龄稍晚于区域大规模变质作用(约250Ma),推测成矿作用与阿尔泰造山带碰撞造山作用有关.结合矿床地质特征和流体包裹体特征,认为铁木尔特铅锌铜矿为典型的陆陆碰撞体制下形成的造山型矿床.     

Isotopic composition (U/U) of some commonly used uranium reference materials

We have determined ²³⁸U/²³⁵U ratios for a suite of commonly used natural (CRM 112a, SRM 950a, and HU-1) and synthetic (IRMM 184 and CRM U500) uranium reference materials by thermal ionisation mass-spectrometry (TIMS) using the IRMM 3636 ²³³U-²³⁶U double spike to accurately correct for mass fractionation. Total uncertainty on the ²³⁸U/²³⁵U determinations is estimated to be <0.02% (2σ). These natural ²³⁸U/²³⁵U values are different from the widely used ‘consensus’ value (137.88), with each standard having lower ²³⁸U/²³⁵U values by up to 0.08%. The ²³⁸U/²³⁵U ratio determined for CRM U500 and IRMM 184 are within error of their certified values; however, the total uncertainty for CRM U500 is substantially reduced (from 0.1% to 0.02%). These reference materials are commonly used to assess mass-spectrometer performance and accuracy, calibrate isotope tracers employed in U, U-Th and U-Pb isotopic studies, and as a reference for terrestrial and meteoritic ²³⁸U/²³⁵U variations. These new ²³⁸U/²³⁵U values will thus provide greater accuracy and reduced uncertainty for a wide variety of isotopic determinations.     

Lead and uranium isotopic behavior in diagenetic and epigenetic manganese nodules, Timna Basin, Israel, determined by MC-ICP-MS

Isotope ratios of U and Pb were measured in two types of Mn nodules from the Cambrian Timna Formation, Israel. Type A nodules are mainly composed of pyrolusite and hollandite, with Mn, Ba, Pb and U concentrations of 30–60%, 0.2–2.5%, 0.2–1.0% and 500–3500 ppm, respectively, whereas type B nodules were formed by alteration of the former, and contain mainly coronadite, with Mn, Ba, Pb and U concentrations of 7–48%, 0.2–7%, 0.6–5% and 10–160 ppm, respectively. The isotopic composition of U and Pb was measured by MC-ICP-MS on Mn-rich solutions (up to 100 mg/L) without and with chromatographic separation. The values for the 207/206 and 208/206 ratios have been determined with precisions of up to 50 ppm and those of 206/204, 207/204 and 208/204 – up to 200 ppm. The values for the 234/238 ratios have been determined with precisions of 0.4–1%. The results of the separated and unseparated solutions were shown to be equal within the error. Thus there is no significant matrix effect while measuring U and Pb in Mn rich solution using the MC-ICP-MS.The isotopic composition of Pb and U support the distinction between the two types of Mn nodules. Type A nodules have a wide range of ²⁰⁶Pb/²⁰⁴Pb ratios (18.278–19.776), and an almost constant ratio of ²⁰⁸Pb/²⁰⁴Pb. In contrast, type B nodules have almost constant ²⁰⁶Pb/²⁰⁴Pb ratios and a wide range of ²⁰⁸Pb/²⁰⁴Pb ratios (37.986–38.079). Type A nodules form a linear array on a ²⁰⁷Pb/²⁰⁴Pb vs ²⁰⁶Pb/²⁰⁴Pb diagram, while type B nodules form a tight group characterized by lower Pb isotope ratios that slightly deviate from the type A array. The ²³⁴U/²³⁸U ratio differs between the two types of nodules; type A nodules exhibit a uniform and close to equilibrium ²³⁴U/²³⁸U ratio while type B nodules show a wide range of ²³⁴U/²³⁸U ratios above and below the equilibrium value. The isotopic composition of Pb in type A nodules might reflect Pb contributions from plutonic rock weathering, exposed at the time of deposition or later, to the Cambrian sea. These nodules have remained unaffected by processes that occurred since the Cambrian. The higher ²⁰⁸Pb/²⁰⁴Pb values of type B indicate that these nodules were formed from a Th-enriched solution probably during epigenetic processes which occurred also during the last 1 Ma.Thus the two isotopic systems of U and Pb can record formation, leaching and redeposition of Mn ores.     

U-Pb systematics of an Upper Carboniferous black shale from South Yorkshire, UK

Open-system behaviour of uraniferous shales, which has been known for many years, has discouraged attempts to use U-Pb geochronology to date sedimentary systems. Techniques now available can facilitate better understanding of their geochemical evolution and their possible use in geochronometry. For the U-rich Alton (G. listeri) Marine Band, a combined fission track mapping, electron optical and sequential chemical extraction study confirms that uranium is incorporated into francolite, an early diagenetic precipitate. U-Pb analyses of uranium-rich (>1000 ppm) francolite nodules are discordant and imply ages ∼50-150 Ma younger than the date of sedimentation. Pb isotopic analysis suggests that uranium daughters continually leaked from the francolite, ²³⁸U daughters being released more efficiently than those of ²³⁵U. Extrapolation of the U-Pb data to concordia produces an age consistent with the time of sedimentation. These features are also displayed by other uranium-rich shales such as the Swedish Kolm Measures, despite uranium being incorporated into different phases. Preferential loss of ²³⁸U daughters from fine-grained particles due to alpha recoil could explain the unusual U-Pb isotopic composition, in both examples. Further work would be justified to investigate the application of U-Pb isotopic analysis of such material to date sedimentary sequences.     

粤北石角围花岗岩型铀矿床沥青铀矿LA-ICP-MS原位U-Pb定年研究

石角围花岗岩型铀矿床位于粤北下庄铀矿田东部,沥青铀矿是矿床的主要矿石矿物,也是厘定成矿年龄的理想对象。前人采用同位素稀释法(ID-TIMS)和电子探针U-Th-totalPb化学定年法获得的成矿年龄为38～138Ma,但前人年龄变化范围大,可靠性有待考究,难以有效约束矿床的成矿时代。本文利用LA-ICP-MS原位微区分析技术,对石角围矿床矿石中沥青铀矿开展了原位U-Pb定年。研究表明:沥青铀矿的206Pb/238U年龄为52. 46～56. 89Ma,加权平均年龄为54. 68±0. 53Ma(MSWD=1. 19,n=18)。本次沥青铀矿原位U-Pb定年与前人相比更好地避免了矿物包裹体、后期次生变化、显微裂隙等因素的影响,获得的沥青铀矿原位U-Pb同位素年龄代表矿床的成矿年龄。本研究获得的石角围矿床成矿年龄(～55Ma)与华南花岗岩型铀矿床主成矿期(～50Ma)相一致,指示石角围矿床铀成矿作用与华南岩石圈局部伸展作用下的断裂构造活动密切相关。     

新疆白杨河铍铀矿床萤石Sm-Nd和沥青铀矿U-Pb年代学及其地质意义

白杨河矿床是我国类型独特的一个特大型铍、铀多金属矿床,铍矿物主要确定为羟硅铍石,铀矿物主要发现沥青铀矿和次生的硅钙铀矿以及少量的铌铀矿,伴生矿物主要是萤石。为恢复铀和铍的成矿过程,划分成矿阶段,本次工作通过系统采集钻孔中的萤石样品,进行了Sm-Nd同位素测年研究,获得了三组等时线年龄,分别为291±16Ma、265±33Ma和207±37Ma,代表了成矿前、成矿期和成矿后萤石的形成;采集中心工地、新西工地和九号工地平巷内的沥青铀矿样品,进行了UPb同位素测年研究,获得了~(206)Pb/~(238)U表观年龄237.8±3.3Ma、224±3.1Ma、197.8±2.8Ma、97.8±1.4Ma和30.0±0.4Ma,利用U-Pb表观年龄将铀矿化划分为四个阶段:中三叠世、晚三叠-早侏罗世、晚白垩世和古近纪中期。因此,白杨河矿床具有铍早铀晚的成矿特点,铀成矿经历了四个阶段。     

中国大陆地壳单颗粒锆石U-Pb年代学特征:国内文献数据的初步分析

锆石U-Pb年代学方法已经成为地质学研究必不可少的方法。本文收集整理了二十世纪八十年代以来分散在期刊论文、学位论文等多种出版载体中的锆石U-Pb年代学数据,建成中国大陆单颗粒锆石数据库中文子库。该数据子库涉及截至到2017年底的文献2331篇,有效数据154768条目,数据总量已经能够用来进行数据的初步分析和相关地球科学研究。数据子库中年龄-年龄绝对误差关系的分析表明,Age(~(206)Pb/~(238)U)、Age(~(207)Pb/~(235)U)、Age(~(207)Pb/~(206)Pb)在不同的地质年代区间误差表现有所不同。在小于1684.4Ma、1684.4~2855.2Ma、大于2855.2Ma年龄区Age(~(206)Pb/~(238)U)、Age(~(207)Pb/~(235)U)、Age(~(207)Pb/~(206)Pb)的误差最小、置信度最好,除了Age(~(207)Pb/~(235)U)测试方法的原因外,Age(~(206)Pb/~(238)U)和Age(~(207)Pb/~(206)Pb)可分别作为不同年龄段的推荐年龄。将选用的推荐年龄运用于LA-ICP-MS、SHRIMP、SIMS三种方法的比较,得出其适用于不同地质年代的范围。推荐年龄运用于年龄-频数图中并使用高斯多峰拟合,则可发现中国大陆锆石存在6个生长峰期,分别为131.71Ma、255.17Ma、442.42Ma、811.56Ma、1868.36Ma和2505.31Ma等;更小尺度下的新生代则存在七个峰期,分别为16.99Ma、27.64Ma、35.26Ma、43.44Ma、48.27Ma、52.74Ma和62.07Ma等,峰期及其对应测试点的位置可与中国大陆地壳演化重大历史事件对应。     

Genesis of the South Zhuguang Uranium Ore Field,South China: Pb Isotopic Compositions and Mineralization Ages

U–Pb isotopic analyses indicate that ores from the South Zhuguang uranium ore field, south China, have high common (non‐radiogenic) Pb contents, with variable and relatively radiogenic initial Pb contents. The U–Pb isochron method was used to date these ores, with plots of ²⁰⁸Pb/²⁰⁴Pb and ²⁰⁷Pb/²⁰⁴Pb versus ²⁰⁶Pb/²⁰⁴Pb being used to identify sample suites with similar initial Pb isotopic ratios and to normalize variable initial Pb isotopic ratios. The resulting U–Pb isochrons indicate two substages of uranium mineralization at ~57 and 52 Ma, with a later hydrothermal reformation at ~49 Ma, which homogenized Pb isotopic compositions. Initial Pb isotopic systematics indicate that the ore‐forming fluid was characterized by high ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁷Pb/²⁰⁴Pb ratios and low ²⁰⁸Pb/²⁰⁴Pb ratios, suggesting that the ore‐forming fluid was sourced from Cretaceous–Paleogene red‐bed basins, rather than from magma or the mantle, with consideration of mineralization ages.     

Trace-element study and uranium-lead dating of perovskite from the Afrikanda plutonic complex, Kola Peninsula (Russia) using LA-ICP-MS

The U-Pb geochronology of perovskite is a powerful tool in constraining the emplacement age of silica-undersaturated rocks. The trace-element and U-Pb isotopic compositions of perovskite from clinopyroxenite and silicocarbonatite from the Afrikanda plutonic complex (Kola, Russia) were determined by laser-ablation inductively-coupled mass-spectrometry (LA-ICP-MS). In addition, the Sr isotopic composition of perovskite was measured by isotope-dilution mass-spectrometry to better constrain the relations between its host rocks. Perovskite from the two rock types shows a different degree of enrichment in Na, Mg, Mn, Pb, Fe, Al, V, rare-earth elements, Zr, Hf, Th, U and Ta. The perovskite ⁸⁷Sr/⁸⁶Sr values are within analytical uncertainty of one another and fall within the range of mantle values. The ²⁰⁶Pb/²³⁸U ages (corrected for common lead using ²⁰⁷Pb-method) of perovskite from silicocarbonatite statistically yield a single population with a weighted mean of 371?±?8 Ma (2σ; MSWD?=?0.071). This age is indistinguishable, within uncertainty, to the clinopyroxenite weighted mean ²⁰⁶Pb/²³⁸U age of 374?±?10 Ma (2σ; MSWD?=?0.18). Our data are in good agreement with the previous geochronological study of the Afrikanda complex. The observed variations in trace-element composition of perovskite from silicocarbonatite and clinopyroxenite indicate that these rocks are not related by crystal fractionation. The Sr isotopic ratios and the fact that the two rocks are coeval suggest that they were either produced from a single parental melt by liquid immiscibility, or from two separate magmas derived at different degrees of partial melting from an isotopically equilibrated, but modally complex mantle source.     

Direct dating of hydrothermal W mineralization: U-Pb age for hübnerite (MnWO4), Sweet Home Mine, Colorado

We have investigated the potential of hübnerite for U-Pb dating. Hübnerite forms typically at medium to low-temperatures in a wide range of pneumatolytic-hydrothermal mineral deposits, particularly porphyry molybdenum and Sn-specialized granites. Hübnerite from the Sweet Home Mine (Alma, Colorado) formed in a Pb-rich, U-poor environment, but still developed relatively radiogenic Pb isotopic compositions. The low Pb_common contents in hübnerite (0.075 to 0.155 ppm) demonstrate that Pb is efficiently excluded from the crystal lattice. In contrast, U may substitute for Mn. The U-Pb data of hübnerite scatter. Most of the scatter originates from samples with ²⁰⁶Pb/²⁰⁴Pb values below 50, where Pb_blank contributes up to 30% to Pb_total. Using the least radiogenic galena Pb, samples with ²⁰⁶Pb/²⁰⁴Pb values above 70 have overlapping ²⁰⁶Pb∗/²³⁸U and ²⁰⁷Pb∗/²³⁵U values and yield a ²⁰⁶Pb/²³⁸U age of 25.7 ± 0.3 Ma (2σ). Late stage apatite from the Sweet Home Mine yields a ²⁰⁶Pb/²⁰⁴Pb-²³⁸U/²⁰⁴Pb isochron corresponding to an age of 24.8 ± 0.5 Ma (2σ). A comparison of the U-Pb hübnerite ages with literature ⁴⁰Ar/³⁹Ar ages on earlier sericite and the U-Pb age on later apatite suggests that (i) hübnerite yields accurate U-Pb ages and (ii) the evolution of the Sweet Home mineralization from greisen-type mineralization to medium-temperature hydrothermal vein mineralization took place in a few hundred thousand years at most. Aqueous low-N₂-bearing and aqueous inclusions in the dated hübnerite have homogenization temperatures between 325 and 356 °C and moderate salinity (up to 6.7 wt% NaCl equiv.). Thus, hübnerite represents one of the rare examples of a mineral that can be dated accurately and carries petrological information.     

北京平原区晚中生代安山岩锆石U-Pb年代学及其地质意义

本文对北京平原区Jr173井和Jr176井安山岩进行了SHRIMP和LA-LCP-MS锆石U-Pb定年,用以精确限定北京平原区火山岩时代、盆地形成与地貌转变之间的关系。2个安山岩中的锆石多晶型良好,具有典型的生长环带和较高的Th/U比值,显示其为岩浆锆石。Jr173井底部安山岩14颗锆石的206Pb/238U年龄可分为两组,其加权平均年龄分别为122.6Ma和138.4Ma;Jr176底部安山岩53颗锆石的206Pb/238U加权平均年龄为122.63Ma,表明该期火山岩的形成时代为早白垩世晚期,与前人认为的侏罗纪有较大区别。结合前人资料,北京地区沿着黄庄-高丽营断裂一线形成断陷盆地,西部隆升为山脉,东部下降为平原,就其安山岩喷发时代、空间展布和盆地发育特征而言,可视为中国东部晚中生代构造演化的一个缩影。     

The Ranger uranium deposit,northern Australia: Timing constraints,regional and ore-related alteration,and genetic implications for unconformity-related mineralisation

The Ranger 1 unconformity-related uranium deposit in the Northern Territory of Australia is one of the world's largest uranium deposits and has ranked in the top two Australian producers of uranium in recent years. Mineralisation at the Ranger, Jabiluka and other major unconformity-related deposits in the Alligator Rivers Uranium Field (ARUF) occurs in Paleoproterozoic metamorphic basement rocks immediately beneath the unconformity with the Paleo- to Mesoproterozoic McArthur Basin.The sites of uranium mineralisation and associated alteration at the Ranger 1 deposit (Number 3 orebody) were fundamentally controlled by reactivated shear zones that were initiated during the regional Nimbuwah tectonothermal event. The timing of shearing at medium metamorphic grade was constrained by ion microprobe U–Pb dating of zircons in two pegmatites, one weakly foliated (1867.0 ± 3.5 Ma) and another that is unfoliated and cuts the shear fabric (1862.8 ± 3.4 Ma). The younger age of ~ 1863 Ma represents the minimum age of D1 shearing during the Nimbuwah event at the Ranger 1 deposit (Number 3 orebody). Titanite within veins of amphibole-plagioclase-apatite yielded an ion microprobe U–Pb age of 1845.4 ± 4.2 Ma, which represents a previously unrecognised hydrothermal event in the ARUF. Based on previous data, retrograde hydrothermal alteration during D2 reactivation of D1 shear zones is interpreted to have occurred at ~ 1800 Ma during the regional Shoobridge tectonothermal event.Detailed paragenetic observations supported by whole-rock geochemical data from the Ranger 1 deposit (Number 3 orebody) reveal a sequence of post-D2 hydrothermal events, as follows. (1) Intense magnesium-rich chlorite alteration and brecciation, focussed within schists of the Upper Mine Sequence in the Cahill Formation. (2) Silicification of Lower Mine Sequence carbonate rock units and overlying schist units, comprising quartz ± Mg-foitite (tourmaline) ± muscovite ± pyrite ± marcasite, and rare uraninite (early U1). (3) Formation of main stage uranium ore and heterolithic breccias including clasts of olivine–phyric dolerite, with breccia matrix composed of uraninite (U1), Mg-chlorite ± Mg-foitite and minor pyrite and chalcopyrite. (4) A second generation of uraninite (U2) veinlets with disordered graphitic carbon and quartz of hydrothermal origin. (5) Late-stage veinlets of massive uraninite (U3). As inferred in a previous study and confirmed herein, olivine–phyric dolerite dykes at Ranger are mineralised and chloritised, and are geochemically similar to the regional Oenpelli Dolerite. A maximum age for uranium mineralisation at the Ranger 1 deposit is therefore set by the age of the Oenpelli Dolerite (~ 1723 Ma).In-situ ion microprobe U–Pb analysis of texturally oldest U1 uraninite yielded a discordia array with a ²⁰⁶Pb/²³⁸U-²⁰⁷Pb/²³⁵U upper intercept age of 1688 ± 46 Ma. The oldest individual ion microprobe ²⁰⁷Pb–²⁰⁶Pb age is 1684 ± 7 Ma whereas the oldest age determined by in-situ electron microprobe chemical dating of U1 uraninite is ~ 1646 Ma. Another sample containing both U1 and U2 uraninite yielded discordant data with a ²⁰⁶Pb/²³⁸U–²⁰⁷Pb/²³⁵U upper intercept age of 1421 ± 68 Ma. When the ²⁰⁷Pb/²⁰⁶Pb ages are considered the data are suggestive of U2 uraninite formation and possible resetting of the U1 age between ~ 1420 Ma and ~ 1040 Ma. All ion microprobe analyses of U1 and U2 uraninite indicate variable and possibly repeated lead loss. In contrast ion microprobe U–Pb dating of the third generation of uraninite (U3) yielded several near-concordant analyses and a ²⁰⁶Pb/²³⁸U–²⁰⁷Pb/²³⁵U upper intercept age of 474 ± 6 Ma. This age is supported by electron microprobe chemical ages of U3 uraninite between 515 Ma and 385 Ma.The new results constrain the timing of initial uranium mineralisation at the Ranger 1 deposit (Number 3 orebody) to the period ~ 1720 Ma to ~ 1680 Ma, which just overlaps with a previous U–Pb age of 1737 ± 20 Ma for uraninite-rich whole-rock samples. Our results are consistent with individual laser-ICPMS ²⁰⁷Pb/²⁰⁶Pb and chemical ages of uraninite as old as 1690–1680 Ma reported from other deposits and prospects in the ARUF.Whole-rock geochemical data in this study of the Ranger 1 deposit (Number 3 orebody) and in other studies in the ARUF demonstrate that zones of intense chloritisation associated with uranium mineralisation experienced large metasomatic gains of Mg, U, Co, Ni, Cu and S and losses of Si, Na, Ca, Sr, Ba, K, Rb, Y and the light REE. More broadly in the ARUF, a regionally extensive illite–hematite ± kaolinite-bearing ‘paleoregolith’ zone in basement beneath the McArthur Basin exhibits depletion of about half of its uranium as well as major losses in Na, Sr, Pb, Ba and minor losses of Mg. These features together with new petrographic observations suggest this zone is a regional sub-McArthur Basin alteration zone produced by interaction with diagenetic or hydrothermal fluids of primary basinal origin, rather than representing a low-temperature paleo-weathering zone before the deposition of the McArthur Basin, as previously suggested.Based on these results and a synthesis of previous work, a new multi-stage model is proposed for the Ranger 1 ore-forming mineral system that may apply to other major unconformity-related uranium deposits in the ARUF and which may be used for targeting new deposits in the region. As in most recent models, oxidised diagenetic brines within the McArthur Basin are envisaged as crucial in mobilising uranium. However, a different architecture of fluid flow is proposed involving the sub-unconformity regional basement alteration zone as a preferential source of leached uranium. Possibly driven by convection during regional magmatism at ~ 1725–1705 Ma, oxidised basinal brines were drawn downwards and laterally through fault networks and fractures in the regional sub-unconformity alteration zone, leaching uranium from hematite-altered basement rocks. Simultaneously within deeper and lateral parts of the hydrothermal system, Mg-metasomatism produced chloritic alteration and brines with increased acidity and silica content (from the desilicification of the basement rock), analogous to processes described in sub-seafloor hydrothermal systems. Silicification occurred locally (e.g., Ranger deposit) within upflow zones of convective systems due to decreases in temperature and/or pressure of the brines and/or CO₂ generation during carbonate dissolution. Interruptions to convection during transient regional extensional or strike-slip tectonic events resulted in generalised lateral and downwards flow of fluids from the McArthur Basin through deepened zones of sub-unconformity alteration, transferring leached uranium into reactivated shear zones within the basement. The main stage of uraninite precipitation at the Ranger deposit and elsewhere in the ARUF is proposed to have occurred between ~ 1720 Ma and ~ 1680 Ma as a result of reduction of oxidised and evolved basin-derived ore fluids during reaction with pre-existing Fe^2 +-bearing minerals and/or mixing of the ore fluids with basement-reacted silica-rich brines.A second, volumetrically minor but locally high-grade, stage of uraninite mineralisation was associated with hydrothermal disordered carbon and quartz of presently unknown origin. Available data suggest formation between ~ 1420 Ma and ~ 1040 Ma. Almost a billion years later at ~ 475 Ma, fluids capable of mobilising uranium again resulted in uraninite (U3) deposition as sparse veinlets in the Ranger deposit, representing the first documentation of uranium mineralisation of this age in the region.     

西天山查岗诺尔和智博铁矿区火山岩地球化学特征、锆石U-Pb年龄及地质意义

西天山东段的查岗诺尔铁矿和智博铁矿赋存于以玄武岩、玄武安山岩、粗面岩以及安山质凝灰岩为主的晚石炭世火山岩中, 对火山岩的形成时代以及构造地质背景的研究是重建成矿过程的关键。本文通过对两个矿区的火山岩进行岩石地球化学和LA-ICP-MS锆石U-Pb测年分析来探讨火山岩形成的构造环境与时代。地球化学分析表明大多数火山岩化学成分从钙碱性、高钾钙碱性变化到钾玄岩系列,富集轻稀土元素(LREE)和大离子亲石元素(LILE; 如Rb、Th、K),重稀土元素(HREE)配分平坦,同时具有Nb、Ta、Ti的强烈亏损,类似于岛弧火山岩的地球化学特征。大多数玄武质火山岩在构造环境判别图中位于火山弧环境。LA-ICP-MS锆石U-Pb测年显示流纹岩和英安岩的²⁰⁶Pb/²³⁸U加权平均年龄分别为301.8±0.9Ma和300.3±1.1Ma。此外,对两件闪长岩样品测年获得²⁰⁶Pb/²³⁸U加权平均年龄介于303.8～305Ma之间。火山岩与闪长岩样品具有类似的地球化学特征以及形成时代,表明它们可能来源于同一母岩浆,形成于相同的构造背景下。结合区域地质资料,本文认为矿区内出露的高钾钙碱性到钾玄岩系列火山岩可能属于俯冲过程末期阶段大陆岛弧岩浆作用的产物。     

Isotopic and elemental analysis of uraninite concentrates using inductively coupled plasma-mass spectrometry (ICP-MS)

Inductively coupled plasma-mass spectrometry (ICP-MS) is capable of measuring isotopic and elemental abundances in geologic materials easily and rapidly. Although the precision of isotope ratio data obtained by ICP-MS is inferior to that by thermal ionization mass spectrometry, it is adequate for application to a number of geochemical exploration problems.National Institute of Standards and Technology (NIST) Pb metal standard reference material 981 (NBS981), was used as the isotopic standard to correct the measured isotope intensities for mass discrimination. The mean relative standard deviation (RSD) of the determinations of the abundances of ²⁰⁶Pb, ²⁰⁷Pb, and ²⁰⁸Pb in the two other NIST Pb isotope reference materials, NBS981 and NBS982, was better than 0.3%, whereas the RSD for the determination of the less abundant ²⁰⁴Pb was 0.4%. Accuracy was demonstrated by repeated analysis of NBS981, NBS982, and NBS983. The Student t-statistic ranged between −1.75 and 2.04 for the abundances of the 4 Pb isotopes in the three NIST materials.Data from a suite of 13 uraninite-rich samples from Labrador demonstrate the ability of ICP-MS to determine age and geochemical information sufficient for regional interpretations. The determined radiogenic ²⁰⁷Pb/²⁰⁶Pb ratios of 12 of the samples give ages between 1697 and 1805 Ma with average uncertainties (one standard deviation) of 4 Ma, whereas one of the samples has an age of 495 ± 4 Ma. The average age of the 12 samples was 1752 ± 27 Ma. Along with the Pb isotope intensities, ²³²Th and ²³⁸U were measured and the U-Pb age determined from a fit of the ²⁰⁶Pb/²³⁸U vs. ²⁰⁷Pb/²³⁵U for 9 of the samples. The concordia intercept age of 1740 Ma for the best-fit line is in good agreement with the mean ²⁰⁷Pb/²⁰⁶Pb age of 1752 Ma.     

U-Pb ages of secondary silica at Yucca Mountain,Nevada: implications for the paleohydrology of the unsaturated zone

Uranium, Th and Pb isotopes were analyzed in layers of opal and chalcedony from individual mm- to cm-thick calcite and silica coatings at Yucca Mountain, Nevada, USA, a site that is being evaluated for a potential high-level nuclear waste repository. These calcite and silica coatings on fractures and in lithophysal cavities in Miocene-age tuffs in the unsaturated zone (UZ) precipitated from descending water and record a long history of percolation through the UZ. Opal and chalcedony have high concentrations of U (10 to 780 ppm) and low concentrations of common Pb as indicated by large values of ²⁰⁶Pb/²⁰⁴Pb (up to 53,806), thus making them suitable for U-Pb age determinations. Interpretations of U-Pb isotope systems in opal samples at Yucca Mountain are complicated by the incorporation of excess ²³⁴U at the time of mineral formation, resulting in reverse discordance of U-Pb ages. However, the ²⁰⁷Pb/²³⁵U ages are much less affected by deviation from initial secular equilibrium and provide reliable ages of most silica deposits between 0.6 and 9.8 Ma. For chalcedony subsamples showing normal age discordance, these ages may represent minimum times of deposition. Typically, ²⁰⁷Pb/²³⁵U ages are consistent with the microstratigraphy in the mineral coating samples, such that the youngest ages are for subsamples from outer layers, intermediate ages are from inner layers, and oldest ages are from innermost layers. ²³⁴U and ²³⁰Th in most silica layers deeper in the coatings are in secular equilibrium with ²³⁸U, which is consistent with their old age and closed system behavior during the past ∼0.5 Ma. The ages for subsamples of silica layers from different microstratigraphic positions in individual calcite and silica coating samples collected from lithophysal cavities in the welded part of the Topopah Spring Tuff yield slow long-term average growth rates of 1 to 5 mm/Ma. These data imply that the deeper parts of the UZ at Yucca Mountain maintained long-term hydrologic stability over the past 10 Ma. despite significant climate variations. U-Pb ages for subsamples of silica layers from different microstratigraphic positions in individual calcite and silica coating samples collected from fractures in the shallower part of the UZ (welded part of the overlying Tiva Canyon Tuff) indicate larger long-term average growth rates up to 23 mm/Ma and an absence of recently deposited materials (ages of outermost layers are 3–5 Ma.). These differences between the characteristics of the coatings for samples from the shallower and deeper parts of the UZ may indicate that the nonwelded tuffs (PTn), located between the welded parts of the Tiva Canyon and Topopah Spring Tuffs, play an important role in moderating UZ flow.