Use of laser ablation ICP-MS to determine trace element distributions in coals, with special reference to V, Ge and Al

Laser ablation ICP-MS has been used to produce element profiles from polished coal samples. The elements analysed (V, Ge, Ni, Cu, Zn, Sr and Ba) were those known to have an organic association, with the addition of Al and Fe as controls on the clay and pyrite abundances. The element profiles were compared with the petrography and a statistical analysis was also carried out. Sporinite was found to have low trace element concentrations, inertinite higher concentrations of detrital elements and vitrinite higher concentrations of V, Ge and Al. The relationships between these elements are explored and all are thought to be present in the organic matter and not as minerals. It is thought that these elements became concentrated during diagenesis, with the possibility that some of the Al could be residual. The Cu/Ni ratio in the pyrite is relatively constant, suggesting constant ionic proportions during diagenesis, and possibly a seawater source.     

Petrology and geochemistry of chert on the marginal zone of Yangtze Platform,western Hunan,South China,during the Ediacaran–Cambrian transition

The Ediacaran–Cambrian transition was one of the most critical intervals in Earth history. During this interval, widespread chert was precipitated, commonly as a stratal wedge in carbonates, along the southern margin of the Yangtze Platform, South China. The chert wedge passes into a full chert succession further basinward to the south‐east. Four lithotypes of chert are identified across the marginal zone in western Hunan: mounded, vein, brecciated and bedded chert. The mounded chert is characterized by irregular to digitiform internal fabrics, generally with abundant original vesicles and/or channels that mostly are lined by botryoidal chalcedony cements with minor quartz and barite crystals. The host chert (or matrix) of these mounds is dominated by amorphous cryptocrystalline silica, commonly disseminated with pyrite. The vein chert, with minor quartz locally, generally cross‐cuts the overlying dolostone and chert horizons and terminates under the mounded and/or bedded chert bodies. The brecciated chert commonly occurs as splayed ‘intrusions’ or funnel‐shaped wedges and cross‐cuts the topmost dolostones. The bedded chert, the most common type, generally is thin to medium‐bedded and laminated locally; it is composed of amorphous silica with minor amounts of black lumps. Microthermometry of fluid inclusions from vein and void‐lining minerals (mainly quartz and barite) revealed homogenization temperatures from 120 to 180°C for the trapped primary fluids. Compositionally, these chert deposits generally are pure, with SiO₂ > 92 wt%, and only minor Fe₂O₃ and Al₂O₃ contents, most of which show positive Europium anomalies in rare earth element patterns, especially for the mounded chert. All these data suggest that the marginal zone chert deposits resulted from a low‐temperature, silica‐rich hydrothermal system, in which the mounded chert was precipitated around the releasing vents, i.e. as silica chimneys. The vein and splayed brecciated chert, however, was formed along the syndepositional fault/fracture conduits that linked downward, while the bedded chert was precipitated in the quieter water column from the fallout of hydrothermal plumes onto the sea floor. These petrological and geochemical data provide compelling evidence and a new clue to the understanding of the extensive silica precipitation; rapid tectono‐depositional and oceanic changes during the Ediacaran–Cambrian transition in South China.     

桂北泗里口老堡组硅质岩的常量、稀土元素特征及成因指示

桂北泗里口老堡组为一套埃迪卡拉纪—寒武纪过渡时期（大约550～540 Ma）深水盆地沉积的硅质岩。它们的SiO2含量普遍高（平均93.8%）;Al2O3含量为0.17%～4.92%,沿剖面自下而上明显增加,上部超过2%;Al/(Al+Fe+Mn)比值多高于0.42;Fe/Ti比值大都小于16.3;Al2O3/( Al2O3+Fe2O3)比值多高于0.4,剖面上部样品的比值为0.8～0.9;Y/Ho比值为26.4～36.9,中、下部样品较高（多高于32）,上部样品的比值接近地壳值（27）;Eu/Eu*平均值为1.0,正异常不明显。剖面下部样品的∑REE含量低（15.9×10-6～27.1×10-6）,具有与现代海水相近的REE配分,没有正的Eu异常,不同于海底的热液流体和与其有关的碧玉的REE配分;中部样品的∑REE含量为26.2×10-6～49.4×10-6,由于所含陆源碎屑的增加,REE配分变得平坦,但仍有海水REE的某些特征;上部样品的∑REE含量为40.5×10-6～59×10-6,显示与平均页岩相似的平坦的REE配分,但∑REE含量仅为平均页岩的大约1/4～1/3。这些常量和稀土元素特征表明,海底热液和陆源碎屑都不可能成为泗里口老堡组硅质岩的重要物源。埃迪卡拉纪—寒武纪过渡时期华南深水盆地厚层硅质岩沉积反映了这一时期大气高CO2浓度,大量陆源化学风化的硅质流入海洋和大量生物的降解可能是造成这些硅质岩形成的基本原因。     

鄂西地区上二叠乐平统大隆组硅质岩成因及有机质富集机理

晚二叠世是古生代环境、生物演化的关键时期,发育广泛的富有机质沉积,并成为南方重要的烃源岩层位。为了解该时期的富有机质堆积机理,我们选择了鄂西台内盆地晚二叠世大隆组富有机质硅质岩(TOC平均值为5.8%,峰值为18%)作为研究对象。硅质岩中常量元素Al/Fe均值为0.79,Mn/Ti均值为0.21,显示为大陆边缘环境下的生物成因,即非热水成因。∑REE值与Al2O3含量有着良好的正相关关系,表明当时陆源物质的输入对硅质岩的形成有很大影响。微量元素U/Th、V/(V+Ni)均说明硅质岩沉积环境为缺氧水体环境。通过对草莓状黄铁矿粒径的统计表明,大多数(64%～89%)的草莓状黄铁矿粒径小于5μm,反映缺氧甚至硫化的环境。而且,U、V、Mo的富集以及Ni、Cu元素的亏损说明水体的缺氧主要是由于海水分层,水体循环受阻造成的。TOC与氧化还原参数的变化步调基本一致,两者有着较好的正相关关系,而与生产力指标Ba/Al相关性很差,说明有机质富集主要是由水体的缺氧而不是初级生产力控制的。     

The Ge/Si ratio as a tool to recognize biogenic silica in chert

Germanium and silicon, dissolved in seawater, are considered to be incorporated into biogenic opal with no or little fractionation, which permitted to use diatoms as reliable recorders of seawater Ge/Si. Does some fractionation occur during diagenesis, preventing the use of Ge/Si in ancient sediments? We examined the Ge/Si ratio of fossil sponges and flint nodules of the Cretaceous Chalk Formation of northern France. Though disputed, silica in this formation is considered to originate from sponges. No fractionation is observed between sponges and diagenetic flints, which allows us to observe whether Ge/Si bears a biogenic or detrital signature. We may thus confirm that sponges were the main silica supplier during the chalk deposition. The Ge/Si ratio may be used to identify a biogenic signature in cherts where the origin of silica is dubious.     

Acanthomorph Biostratigraphic Succession of the Ediacaran Doushantuo Formation in the East Yangtze Gorges, South China

Large acanthomorphic acritarchs have been found in chert of the Ediacaran Doushantuo Formation at several localities in South China, including the East Yangtze Gorges of Hubei Province, Weng’an area of Guizhou Province, and elsewhere. However, their potential for biostratigraphic subdivision and correlation of Ediacaran successions is limited by facies control, taphonomic biases, and taxonomic problems. In the Yangtze Gorges, the Doushantuo Formation is generally subdivided into four lithologic members. However, in the Weng’an area, the Doushantuo Formation comprises just a lower and upper part separated by a mid-Doushantuo erosional surface. In the Yangtze Gorges at the Zhangcunping section, the Doushantuo succession is similar to that at Weng’an. So far, the correlation between the Yangtze Gorges and Weng’an area, and elsewhere has been an issue of debate. To resolve the debate, we selected eight sections in the Yangtze Gorges area and systemically sampled chert nodules of the Doushantuo Formation, focusing in particular on the upper Doushantuo Formation. Our data confirm two different assemblages appearing separately in the second and third members, which are separated by a negative δ13C excursion (EN2). The lower assemblage is characterized by Tianzhushania and a diverse suite of large acanthomorphic acritarchs. The upper assemblage is distinguished from the lower assemblage, by (1) absence of Tianzhushania; (2) occurrence of abundant, 100–150 μm, smooth-walled spherical microfossils; (3) occurrence of highly diverse acanthomorphic acritarchs including species extending from the second member and new forms in this member; (4) occurrence of unnamed new forms of protist; and (5) occurrence of the tubular microfossil Sinocyclocylicus guizhouensis. Since the Tianzhushania-dominated assemblage is not present in Australia, it seems that only the upper acanthomorph assemblage is present and thus the lower Doushantuo acanthomorph assemblage is missing in Australia.     

Phosphogenesis of phosphorite from the Sinian Doushantuo Formation in Guizhou Province and its coupling relation with the Neoproterozoic Oxygenation Event

Phosphate deposits of the Ediacaran Doushantuo Formation accumulated in the central Guizhou Province are the typical phosphate-rich sediments during the Neoproterozoic Phosphogenic Episodes,which occurred after the “Snowball Earth”period and Neoproterozoic Oxygenation Event. However,the mechanisms of phosphate enrichment in seawater have always been hotly debated,and the research for correlations between phosphogenesis and transition of Ediacaran palaeo-ocean environments is still unsubstantial. This study focused on the sedimentological,petrological,mineralogical and geochemical analyses on the Doushantuo pristine phosphorite in Weng'an,Zunyi and Danzhai area. Documented by spherulitic phosphorites in the Lower Phosphorite beds from Weng'an area that contain abundant autogenetic Fe-bearing minerals such as pyrite and glauconite and show weak Ce negative anomaly,Fe-redox pumping in low-oxygen environments are the important phosphogenesis mechanism but only limited in coastal waters in the Early Doushantuo Period. Phosphorite in the Upper Phosphorite beds deposited within organic-rich beds and contain massive biological fossils suggests that phosphogenesis might have been triggered by degradation of organic matter and biological action in the Late Doushantuo Period,and distribution of phosphatic sediments extended to the deeper shelf to slope setting. Obvious Ce negative anomaly implies the increase of oxygen content in seawater. The transition of phosphogenesis mechanisms and the expansion of phosphorite deposits are the sedimentary response of ocean oxygenation,and the associated evolution of metazoans also changed the redox conditions of the deep seawater. These sedimentary and geochemistry data reflect the closed coupling relation between Doushantuo phosphorite in Guizhou Province and the Neoproterozoic Oxygenation Event.     

贵州震旦系陡山沱组磷块岩成磷作用及与新元古代末期氧化事件(NOE)的耦合*

新元古代末期历经“雪球地球”和大气、海洋增氧事件后,发生了全球性成磷事件,贵州省震旦系陡山沱组大规模磷矿床沉积是本次成磷事件的典型代表。然而现阶段对贵州陡山沱组磷块岩成磷作用机制研究存在诸多争议,成磷事件与同期古海洋环境转变之间的关联研究也较为薄弱。作者以贵州省瓮安、遵义和丹寨地区陡山沱组原生磷块岩为研究对象开展的沉积学、岩石学、矿物学和地球化学特征研究表明: 陡山沱早期成磷作用被限制在浅水海岸,瓮安地区A矿层球粒磷块岩中大量的含铁自生矿物黄铁矿、海绿石以及无Ce负异常指示的贫氧沉积水体环境均表明,Fe-氧化还原泵成磷模式在富磷过程中发挥了重要作用;陡山沱晚期磷块岩分布扩散至较深水陆棚—斜坡沉积相区,磷块岩与富有机质岩层共生,矿物晶体形态特征与矿石内富含的大量生物化石均表明,有机质沉降聚磷作用和生物成磷作用促使磷块岩大量沉积,较明显的Ce负异常值也指示了海水氧气含量的提升。成磷作用模式的转变和磷块岩分布的扩展是对海洋增氧事件的沉积响应,同时造成的多细胞动物演化也影响了深部水体的氧化还原状态,进而反映了贵州省陡山沱组磷块岩大规模沉积与新元古代末期氧化事件(NOE)密切的耦合关系。     

Extent of diagenetic transformations in severely altered biogenic silica deposits from Tunisia: new insights from mineralogy and geochemistry

Sedimentary biogenic silica from Redeyef in Gafsa basin (southern Tunisia) was analysed for its ²⁹Si and ²⁷Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectra and complemented by X-ray diffraction and SEM observations. The ²⁹Si MAS NMR spectrum is characterized by the abundance of hydroxylated silicon, displayed in resonance intensities and reflects a clear tendency towards dissolution of diatomaceous amorphous silica and the occurrence of the hydrated silica, which is the main component that ensures the diagenetic transition via the mechanism of dissolution–precipitation to other more crystalline silica phases, after the lost of its hydroxyls groups (water) by heating (burial). ²⁷Al MAS NMR reveals two coordinations of Al; the octahedrally coordinated Al suggests the presence of clay relics trapped during crystal growth or a microcrystalline zeolite (clinoptilolite detected by SEM observations), while the tetrahedrally coordinated Al suggests the presence of minor quantities of minerals with tetrahedral Al, such as an Al-rich fluid and/or minerals such as feldspars.     

塔里木下寒武统富有机质沉积层段地球化学特征及意义

塔里木盆地寒武系下统玉尔吐斯组底部富有机质沉积层段分布广泛,层位稳定,其中的硅质岩发育,并伴有磷矿产出.硅质岩的Al(Al Fe Mn)和Si/(Si Al Fe)比值分别在0.0023～0.0046和0.965～0.98之间,表明其形成于海底热水沉积环境,远离陆源区.富有机质沉积层段明显富集As、Hg、Pb、Zn、Cu、Co、P、V、Ba等微量元素,富集系数F(N)远大于1.Ba/Sr比值远远大于1,与现代海底热水沉积物中的Ba/Sr比值相似,具有明显的海底热水沉积特征;Th/U和V/Sc比值显示其形成与海洋缺氧事件有关,而导致缺氧事件发生的主要原因则是海底火山作用及其与之相伴的海底热水流体活动.     

滇东南弗拉斯期放射虫硅质岩地球化学特征及沉积环境探讨

在滇东南麻栗坡地区广泛分布的榴江组薄层状硅质岩中.发现包含放射虫动物群Helenifor robustum和Heleniore laticlavium的重要分子,地质时代为晚泥盆世弗拉斯早期.硅质岩SiO2含量在80.2%-95.72%之间,Al2O3含量集中在0.32%-3.83%之间,平均为2.07%,Si/Al值...     

Determining Biogenic Silica in Marine Samples by Tracking Silicate and Aluminium Concentrations in Alkaline Leaching Solutions

This study introduces an alkaline leaching technique for the simultaneous analysis of biogenic silica and aluminium in sediments. Measuring aluminium facilitates the discrimination between silica from the biogenic (BSiO₂) and the non-biogenic fraction, because it originates almost solely from the lithogenic phase. The method was tested using fine-grained silicagel, standard clay minerals, artificial sediments, and natural samples ranging from fresh diatoms to aged sediment from different depositional settings. To determine the BSiO₂ content, four different models each describing the dissolution curves, but of increasing complexity, were applied and for each different type of sample the optimum model was selected on the basis of F-test statistics. For mixtures of silicagel and clay minerals, the contribution of Si from the dissolution of clay was negligible compared to Si originating from silicagel. For natural samples with high clay content, complex dissolution curves were observed and single-phase first order dissolution was the exception. This deviation from `ideal' behavior could only be recognized because of high-resolution sampling, especially in the first 20 minutes of the experiment. For most of the samples, the distinction between the biogenic silica fraction and the silica originating from dissolution of clays could be made on the basis of the Si/Al ratios and reactivity constants of the dissolving phases calculated with the models. Clay minerals typically dissolve slowly at a Si/Al ratio close to 1–2, depending on the type of clay mineral. In contrast, biogenic silica displays a wide range of reactivities and Si/Al ratios. Fresh biogenic silica from the water column usually has a high reactivity and a low Al content. Aged biogenic silica from the sediments has a lower reactivity, but Si/Al ratios as low as 5 were found. The method as described here therefore presents an accurate method to analyze biogenic silica in marine sediments with a relatively high clay mineral content.     

Redox history of the Three Gorges region during the Ediacaran and Early Cambrian as indicated by the Fe isotope

The Ediacaran-Cambrian transition is characterized by numerous events such as the emergence of large multi-cellular metazoans and surface environmental disturbances.Based on geological evidence,it has been proposed that this transition coincided with the increase in the atmospheric oxygen level that was key to the evolution of life.Even though ancient redox conditions can be inferred from the composition of sedimentary iron mineral species,this method is not necessarily applicable to all rocks.In the Earth system,the cycling of iron is of considerable interest owing to its sensitivity to redox conditions.Information regarding the paleo-oceanic iron cycle is revealed in the iron isotopic composition of ironbearing minerals.Unfortunately,only limited iron isotopic data exists for Ediacaran-to Cambrianperiod oceans.To circumvent this deficiency,we drilled a fossiliferous Ediacaran to Early Cambrian sedimentary succession in the Three Gorges region,South China.We analyzed the iron isotope ratios(δ~(56/54)Fe)of pyrite grains in the drill cores using laser ablation multi collector inductively coupled plasma mass spectrometry.The results demonstrate large variations inδ~(56/54)Fe,from-1.6 to 1.6‰,and positive iron isotope ratios are observed in many successions.The presence of positiveδ~(56/54)Fe in pyrite indicates that the ferrous iron in the seawater was partially oxidized,suggesting that seawater at Three Gorges was ferruginous during the Ediacaran and Early Cambrian periods.However,aggregated pyrite grains in organic carbon-rich black shales at Member 4 of the Doushantuo Formation and the base of the Shuijingtuo Formation yield near-zeroδ~(56/54)Fe values;this suggests that the ocean was transiently dominated by sulfidic conditions during these periods.Notably negativeδ~(56/54)Fe values,lower than-1‰,can be interpreted as a signature of DIR.The DIR also might contribute in part to the re-mineralization of organic matter during the largest negative carbon isotope anomaly in the Ediacaran.     

湖南保靖地区震旦系陡山沱组页岩气成藏地质条件分析

为评价湖南保靖地区震旦系陡山沱组页岩气勘探潜力,利用野外地质调查及页岩气井勘探成果资料,结合钻井岩心样品测试结果,研究了湖南保靖地区陡山沱组页岩储层发育的特征及其分布规律。湖南保靖震旦系陡山沱组发育被动大陆边缘浅水陆棚斜坡相灰色白云质灰岩、黑色碳质泥岩、黑色硅质岩、黑色硅质岩、深灰色泥晶灰岩、深灰色含钙碳质泥岩等。该页岩层分布稳定,厚度巨大(150～200m);有机碳含量较高(4.07%～10.25%),热演化程度较高(3.78%～4.97%),达到过成熟阶段;页岩孔隙度中等(0.19%～2.355%),渗透率极低(0.0005257～0.5910081 mD);脆性矿物含量较高(55.9%～84.5%),主要以石英、方解石、黄铁矿、斜长石为主;黏土矿物含量较低(16.5%～44.1%),以伊利石和伊蒙混层为主,经历了晚成岩作用阶段;储集空间主要有矿物溶蚀孔、有机质溶蚀孔、黄铁矿晶间孔、黏土矿物晶间孔、成岩解理缝及构造微裂缝等。综合研究分析认为,湖南保靖地区陡山沱组具有较好的页岩气成藏条件,可继续加大勘探力度。     

Silicification of sulphate evaporites and their carbonate replacements in Eocene marine sediments, Tunisia: two diagenetic trends

Samples of chert nodules, diagenetic carbonates and evaporites (gypsum/anhydrite) collected from the gypsiferous limestones of the Kef Eddour Member (Ypressian‐Priabonian) near Metlaoui and Sehib (Tunisia) show selective silicification with great variety in the silicified by‐products. Based on δ¹³C values, which support an organic origin for the carbon, carbonates replaced evaporites microbially through bacterial sulphate reduction. Observations and results suggest two scenarios for chert formation that are related to the rate and timing of diagenetic carbonate replacement of the evaporites (anhydrite/gypsum). In the absence of early diagenetic carbonate phases, silica with δ¹⁸O values from +25 to +28·6‰ [standard mean ocean water (SMOW)] replaced the outer parts of anhydrite nodules at pH < 9. In contrast, pore‐fluid pH values > 9 in the innermost parts of the anhydrite nodules prevented silica precipitation. The record of this chemical barrier is preserved in the microquartz rims and geode features that formed in the inner parts of the nodules after dissolution of the anhydrite nucleus. The microbial diagenetic replacement of evaporites (bacterial sulphate reduction) by carbonates (calcite, aragonite and dolomite) favoured silica replacement of carbonates rather than evaporites. Silica, with δ¹⁸O signature of +21 to +26‰ (SMOW), replaced carbonates on a volume‐for‐volume basis, yielding a more siliceous groundmass, and accounting for 90–95% of the nodules. The relatively higher δ¹⁸O values of quartz replacing anhydrite can be explained by a diagenetic fluid in equilibrium with mixed (meteoric/marine) to marine water. The lower δ¹⁸O values of the quartz that replaced the diagenetic carbonates are ascribed to flushing by meteoric water in a later diagenetic stage. The silica supply for chert formation could be derived from the reworked bio‐siliceous deposits (diatomites) to the west of the basin [vestiges of an opal‐CT precursor undetectable by X‐ray diffraction (XRD) were revealed by δ²⁹Si magic‐angle‐spinning nuclear magnetic resonance investigations], diagenesis of the extraformational and overlying clay‐rich beds (the host limestones are clay‐poor as shown by XRD measurements), and minor volcanogenic and hydrothermal contributions during early diagenetic stages.     

峡东伊迪卡拉纪陡山沱组顶部重晶石特征与地质意义研究

陡山沱组顶部潮间带泻湖相泥页岩中发现重晶石, 其宏观、微观特征以及测试分析结果表明为早期成岩成因。结合产出层位沉积和地球化学特征认为, 陡山沱组沉积中期和末期发生显著海平面下降, 导致沉积速率降低至无沉积或沉积间断, 使已埋藏有机质暴露、氧化和陆源物质输入增加, 从而导致早期成岩重晶石和大型钙质结核发育和δ13C高负异常等。深水环境的变迁可能是导致陡山沱组下部发育的大型具刺疑源类生物灭绝的原因。野外沉积学特征考察显示, 陡山沱组底部、内部以及与灯影组接触部位可能存在沉积间断、饥饿沉积或平行不整合。上述研究对深入理解陡山沱组顶部年龄、早期生物演化、辐射以及Ediacaran地质历史演化具有新的启示作用。     

The significance of cherts as markers of Ocean Plate Stratigraphy and paleoenvironmental conditions: New insights from the Neoproterozoic–Cambrian Blovice accretionary wedge,Bohemian Massif

The Ediacaran to early Cambrian Blovice accretionary complex, Bohemian Massif, hosts abundant chert bodies that formed on an oceanic plate and were involved in subduction beneath the northern margin of Gondwana. Field relationships of cherts to their host, their microstructure and elemental as well as isotopic compositions revealed diverse processes of chert petrogenesis reflecting depositional environment and position on the oceanic plate. The deep-water cherts formed through a hydrothermal precipitation of silica-rich gels on outer trench swell of the subducted slab with none or only minor addition of terrigenous material. On the contrary, the shallow-water cherts formed in lagoons on seamount slopes, and at least some of them represent a product of hydrothermal replacement of former carbonate and/or evaporite precursors. For both chert types, the hydrothermal fluids were of low temperature and continuous pervasive hydrothermal alteration of oceanic crust, together with an elevated Si content in Neoproterozoic seawater, served as the major source of silica. On the other hand, minor carbon enrichment in chert is mostly linked to variable incorporation of organic matter that was deposited on the seafloor. Rare earth element (REE) systematics of the cherts indicate predominantly oxygenated environment for the shallow-water cherts whereas the deep-water cherts were deposited in diverse redox conditions, depending on their distance from hydrothermal vent. Using these data, we demonstrate that the cherts once formed a part of Ocean Plate Stratigraphy (OPS) now dismembered and mixed with terrigenous siliciclastic material to form OPS mélanges. Combining our data with those from the existing literature, we show that cherts can serve as significant markers of OPS since the Archean, recording a complex interplay between seafloor-related volcanic (production of MORB- and OIB-like magmas) and sedimentary processes, hydrothermal activity at mid-ocean ridges and seamount chains as well as at outer slopes of subducting slabs. However, the cherts also exhibit a secular change in composition and petrogenesis most profoundly affected by an overturn in seawater silica cycle across the Precambrian–Phanerozoic boundary.     

An appraisal of an iterative construction of the endmembers controlling the composition of deep-sea manganese nodules from the Central Indian Ocean Basin

This paper describes an estimation of endmember compositions followed by the assessment of those results by log-ratio variance analysis. As an appraisal, it deals only with the first objective of an endmember analysis namely, to identify endmembers if they exist by estimating their compositions. Following the creation of the endmember estimates, the computation of an array of log-ratio variances was a key innovation in this type of study. Log-ratio variances revealed intrinsic linear associations between the dominant elements on each of the estimated endmember compositions, largely confirming the endmember analysis. The dataset under study contained the concentrations of 16 elements in 93 samples of deep-sea manganese nodules from the Central Indian Ocean Basin. Many previous analyses of these nodules were undertaken to assess the economic potential of the deposits. This study by contrast, quantified the inter-element associations that account for the nodule compositions. Four endmembers were identified. The elements loaded on each were: (1) Mn, Zn, Ni, Cu, Mn-rich, (2) Fe, Ti, P, Co, Fe-rich, (3) Si, Al, Na, K, clay minerals, (3) Mg, ultramafic material, possibly including Mn, Cr, V, Ca, Na. These latter elements were also detected by their log-ratio variances to be associated with Mg on the 4th endmember.     

南秦岭震旦系中发现微细浸染型金矿

南秦岭震旦系中发现微细浸染型金矿隗合明（西安地质学院，西安７１００５４）关键词震旦系，微细浸染型金矿，矿源层，韧－脆性剪切带，南秦岭８０年代以来，黔东南、桂西北、川西北、南秦岭等地区相继发现了微细浸染型金矿带和一批较大规模的金矿床，由此我国的金矿地质...     

Submarine silica-rich hydrothermal activity during the earliest Cambrian in the Tarim Basin,Northwest China

Siliceous rocks were widely deposited in many continents during the Ediacaran–Cambrian (E–C) transition. Based on detailed field investigations in the Aksu area of the Tarim Basin in Northwest China, this study presents evidence of a submarine silica-rich hydrothermal system preserved in the E–C boundary successions. This system consists of a lower stockwork silica-dominant vein swarm zone in the karstified dolostone of the uppermost Ediacaran Qigebulake Formation, which terminates directly under the overlying bedded chert and black shale succession of the lowermost Cambrian Yurtus Formation. The stockwork vein swarms were filled dominantly by a wide spectrum of silica precipitates (amorphous silica, chalcedony, spherulite, fine to coarse quartz) with subordinate pyrite, Fe-(oxyhydr)oxide, and barite. The host dolostones that were dissected by the vein swarms also suffered extensive silicification and recrystallization. The vertical stacking relationship of silica-dominant vein swarms and overlying bedded chert suggests they were formed by an identical low-temperature, silica-rich diffusive submarine hydrothermal system in the earliest Cambrian. This suggestion is further supported by fluid inclusion microthermometry (T_h 40–200°C) of the quartz-barite vein fills. In this case, silica-rich hydrothermal fluids were channelled and precipitated partially along the stockwork veins in the antecedent karstified dolostone and vented mostly into seawater, promoting widespread deposition of bedded chert on the seafloor of Tarim Basin in the earliest Cambrian. This study provides a useful clue and analogue to understand the widespread silica deposition and coeval vast oceanic and geochemical changes during the E–C transition in the Tarim Basin and elsewhere.