Obtaining stable redox potential readings in gneiss groundwater and mine water: difficulties,meaningfulness, and potential improvement

Natural samples were run in three parallel flow-through cells in the laboratory in order to study the difficulties of obtaining stable redox potential readings, especially in gneiss groundwater and mine water with little redox buffer capacity. Redox potentials were recorded every 2 min for up to 5 days. Measured redox potentials were compared to means of partial potentials, modeled based on species-selective determinations of the most predominant redox-sensitive elements iron and nitrogen. Redox potentials stabilized reproducibly within minutes for a synthetic redox buffer solution and two well-buffered acid mine water samples at pH 2.3 and 3.5. For waters in redox disequilibrium, species-selective analytics might still be the better alternative compared to measuring the redox potential as summary parameter for modeling species distribution. In cases where the redox potential is the only readily available parameter, filtration and possibly acidification is recommended for quicker stabilization of redox potential readings and less deviations between measured and modeled values. A minimum of 2% was achieved in the samples investigated, however, often after measuring significantly longer than the previously suggested 30 min. Final stabilization could take up to several hours in waters with low buffer capacity.

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Résumé Afin d’étudier les difficultés à obtenir des lectures stables des potentiels redox, notamment sur les eaux souterraines présentes dans des gneiss et dans les mines, qui présentent un faible pouvoir tampon, des échantillons ont été prélevés régulièrement en laboratoire, dans trois cellules d’écoulement parallèles. Les potentiels redox ont été enregistrés toutes les deux minutes durant cinq jours. Ces potentiels redox mesurés ont été comparés aux moyennes des potentiels partiels, modélisés à partir des déterminations sélectives des espèces chimiques des éléments sensibles aux conditions redox les plus prédominants, le fer et l’azote. Les potentiels redox se sont systématiquement stabilisés en quelques minutes pour une solution tampon synthétique et pour deux échantillons d’eaux de mines acides bien tamponnées à pH 2.3 à 3.5. Pour les eaux en déséquilibre redox, des analyses sélectives des espèces chimiques seraient toujours la meilleure alternative aux mesures sommaires du potentiel redox, pour modéliser la distribution des espèces. Dans les cas où le potentiel redox est le seul paramètre directement disponible, une filtration et éventuellement une acidification sont recommandées pour obtenir une stabilisation plus rapide des lectures des potentiels redox, et pour réduire les écarts entre les valeurs mesurées et modélisées. Un minimum de 2% a été atteint parmi les échantillons étudiés, mais en prolongeant souvent les mesures au-delà des 30 minutes suggérées initialement. La stabilisation finale pourrait prendre jusqu’à plusieurs heures dans des eaux à faible pouvoir tampon.

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Resumen Se han llevado a cabo experimentos en laboratorio sobre tres muestras naturales en tres células de flujo paralelas, para estudiar las dificultades en la obtención de medidas de potencial redox estables, especialmente en aguas subterráneas procedentes de gneisses y en agua de minas con capacidades tampón redox bajas. Los potenciales redox fueron medidos cada dos minutos durante más de cinco días. Los potenciales redox medidos fueron comparados con medias de potenciales parciales, modelizados en base a determinaciones selectivas de especies de hierro y nitrógeno, los elementos predominantes redox-sensitivos. Los potenciales redox se estabilizaron de forma reproducible en minutos en una solución redox tampón y dos muestras de agua ácida de pozos de mina a pH 2.3 y 3.5. Para aguas en desequilibrio redox los análisis selectivos por especies deberían ser todavía la mejor alternativa comparados con la medida del potencial redox como una suma de parámetros para modelizar la distribución de especies. En casos en los que el potencial redox sea el único parámetro fácilmente obtenible, se recomienda una filtración y una posible acidificación para una estabilización más rápida de las medidas de potencial redox y que haya menos desviaciones entre los valores medidos y los del modelo. En las muestras investigadas se consiguió un mínimo de un 2%, sin embargo, a menudo incluso después de medidas significativamente más largas que las previamente sugeridas de 30 minutos. La estabilización final tuvo lugar después de más de varias horas en aguas con capacidad tampón baja.

     

On transient electric potential variations in a standing tree and atmospheric electricity

Transient electric potential variations have been observed in a standing poplar tree equipped with electrodes up to a height of 10.5 m. The simultaneous signals at all electrodes have the same shape and their amplitude grows linearly with height, up to values of 10 to 50 mV. This corresponds to an electric current through the tree of the order of a few μA. The frequency of appearance of the signals does not depend on the season or on the time of the day. It is suggested that the potential variations are caused by the passage of thunderstorm clouds, of little activity, whose electrically charged base could induce charges in the ground and give rise to a current flowing through the tree and discharging at its top by point discharge.     

The redox state of subduction zones: insights from arc-peridotites

Spinel peridotites from a variety of island arcs have been utilised to calculate the redox state of the mantle wedge above subduction zones. Oxygen fugacities (f_O2 values) calculated from the ferric iron content of spinels, measured by Electron Microprobe (EMP) using secondary standards [Wood, B.J., Virgo, D., 1989. Upper mantle oxidation state: ferric iron contents of lherzolite spinels by ⁵⁷Fe Mössbauer spectroscopy and resultant oxygen fugacities. Geochim. Cosmochim. Acta, 53, 1277–1291.], yield values which range from 0.3 to 2.0 above the fayalite–magnetite–quartz (FMQ) buffer. These data provide further evidence that the mantle wedge is ubiquitously oxidised relative to oceanic and ancient cratonic mantle. There is no correlation between f_O2 values and the presence of hydrous phases and, in fact, the most oxidised samples contain no hydrous phases. Within individual suites there is no correlation between f_O2 and degree of depletion as indicated by spinel Cr#, except for a suite of reacted forearc-peridotites. However, when the data is viewed as a whole there is broad a positive correlation between f_O2 and spinel Cr# suggesting that partial melting processes may influence the redox state of the mantle wedge. We suggest that the ultimate source of the oxygen which oxidises the mantle wedge is from the subducted slab. It is not clear whether this oxidising agent is a solute-rich hydrous fluid or a water-bearing silicate melt. However, our data does indicate that silicate melts are effective oxidisers of the depleted shallow upper mantle. Simple mass balance calculations based on the ferric iron content of primitive subduction zone magmas indicates that the source region must contain 0.6–1.0 wt.% Fe₂O₃. This amount of Fe₂O₃ in a fertile spinel peridotite yields an oxygen fugacity of 0.5–1.7 log units above FMQ in the IAB source. If water is the sole oxidising agent in the mantle wedge then 0.030–0.075 wt.% H₂O is required which is considerably less than the 0.25% H₂O envisaged by Stolper and Newman [Stolper, E.M., Newman, S., 1994. The role of water in the petrogenesis of Mariana trough magmas. Earth Planet. Sci. Lett., 121, 293–325.], suggesting water is not necessarily an efficient oxidising agent. Alternatively, ferric iron may be added to the mantle wedge by addition of a ferric iron-rich sediment melt or more likely as a solute-rich hydrous fluid. This model would produce spinel, orthopyroxene or amphibole in the wedge with only a slight increase in f_O2 of the source region. Although it is unclear which model is correct the maximum f_O2 of the fertile mantle wedge is unlikely to be above FMQ+2 and therefore some decompression melting in the mantle wedge is required to explain the higher f_O2 values of primitive arc lavas than arc-peridotites.     

Thermal conductivity of soft Bangkok clay from laboratory and field measurements

This paper presents the results of a study on the thermal conductivity of a soft saturated clay (Bangkok clay) carried out in relation to an investigation into using thermal treatment to enhance the consolidation process of soft soils. The thermal conductivity of clay specimens was measured in the laboratory using a steady state method (divided bar test) and a transient state method (needle probe test). In general, the laboratory test results show that the thermal conductivity increased with the increase in soil density. However, the needle probe test was found to yield greater thermal conductivities than those derived from the divided bar test. Furthermore, to assess the validity of the laboratory test results, the heat transfer results obtained from a full-scale embankment test that employed prefabricated vertical thermo-drains (PVTD) were simulated numerically using the laboratory determined thermal conductivity values. The numerical analysis indicates that the field thermal conductivity was close to the value obtained from the needle probe test. However, it was also found that the changes in thermal conductivity values obtained from the two laboratory methods did not impact significantly on heat flow behaviour, suggesting that the two methods are acceptable for characterizing the thermal conductivity of soils.     

内蒙古敖汉旗岱王山金矿田地质特征和资源潜力预测

岱王山金矿田是著名的赤峰-朝阳金矿化集中区内新近发现并勘查的一个重要金矿田,包括岱王山金矿及其东西两侧的峰水山金矿和中井金矿。它们并不是常见的产在老变质岩或岩浆岩中的高品位石英脉型金矿化,而是产在石炭纪砂板岩内的层控-层状的低品位蚀变岩型金矿化,是赤峰-朝阳金矿化集中区内一种新的金矿化类型。矿田内共划分出四条北东向矿化蚀变破碎带,圈定了82条金矿体。矿体多呈似层状、透镜状产在层间构造中,赋矿围岩主要为石炭纪轻微变质的砂板岩,金属矿物主要为黄铁矿和自然金。由于工作程度低,对矿床成因还无法做出较深的讨论。本文把矿田内的金矿成矿期次划分为3期:沉积期、热液成矿期和表生期。对该矿田内矿床地质勘查资料的综合研究表明,岱王山、峰水山和中井矿区的深部、矿区间连接部位以及岱王山矿区北侧和中井金矿西侧均有较好的找矿潜力。本文的结果将为今后同类金矿的找矿勘查提供重要参考。     

江苏宜兴周铁地区地质地球物理场特征及找矿潜力

分析研究了江苏宜兴周铁地区的地质背景和地球物理场特征.周铁地区既有寻找矽卡岩型铁、铅锌银多金属或热液充填型铜多金属矿产的成矿地质背景条件,又有良好的重磁异常反映.结合宣州—苏州Ⅳ级成矿带找矿模式和铁、铜铅锌银多金属矿产典型矿床重磁场特征,探讨了周铁地区铁、铜铅锌银多金属矿产的找矿潜力.     

Bridging the gap between laboratory measurements and field estimations of silicate weathering using simple calculations

Weathering rates of silicate minerals observed in the laboratory are in general up to five orders of magnitude higher than those inferred from field studies. Simple calculations show that even if the field conditions were fully simulated in standard laboratory experiments, it would be impossible to measure the slow rates of mineral dissolution that are observed in the field. As it is not possible to measure the dissolution rates under typical field conditions, one should extrapolate the available data to the field conditions. To do this, a rate law for the dissolution of plagioclase in the field was formulated by combining the far from equilibrium dissolution rate of weathered natural oligoclase at 25°C with the effect of deviation from equilibrium on dissolution rate of fresh albite at 80°C. In contrast to the common view that laboratory experiments predict dissolution rates that are faster than those in the field, the simulation based on this rate law indicates that laboratory dissolution experiments actually predict slower rates than those observed in the field. This discrepancy is explained by the effect of precipitation of secondary minerals on the degree of saturation of the primary minerals and therefore on their dissolution rate. Indeed, adding kaolinite precipitation to the simulation significantly enhances the dissolution rate of the plagioclase. Moreover, a strong coupling between oligoclase dissolution and kaolinite precipitation was observed in the simulation. We suggest that such a coupling must exist in the field as well. Therefore, any attempt to predict the dissolution rate in the field requires knowledge of the rate of the secondary mineral precipitation.     

BP、Hopfield神经网络在位场反演中的应用比较

比较了BP、Hopfield二种神经网络模型的特性及其运行机制,分别用于位场反演,还比较了各自在位场反演中的应用效果。结果表明:这二种神经网络模型虽然都可用于位场反演,但由于Hopfield网络缺乏学习能力,不能较好地利用已知地质、地球物理信息而受到限制。而BP神经网络具有较强的学习能力,能从已知的信息中得到有利于解决最优化问题的结论,比Hopfield神经网络更加适合于位场的反演问题。     

攀枝花钒钛磁铁矿深部成矿潜力及找矿方向的航磁资料研究

攀枝花钒钛磁铁产出于侵入震旦系大理岩中的海西期辉长岩体,受断裂切割为朱家包包、兰家火山、尖包包、倒马坎等6个区段,累计探明铁矿石储量(A+B+C+D)67.29亿t,是我国重要的矿集区,重要矿段均开展了钻探工作。在航磁异常分析中,攀枝花航磁异常的原因没有得到完全的解释,说明还有一定的找矿潜力。本文以1∶5万航磁数据交互式反演为基础,部分由已完成的由钻孔控制的实测地质剖面资料为约束,结合物性参数和地质信息,对矿体空间分布进行反演,利用10条2.5D反演剖面模拟3D模型,估计了该矿区的深部成矿潜力,为下一步地质勘探提供依据。     

Phosphorus transport in shallow groundwater in peri-urban Kampala, Uganda: results from field and laboratory measurements

To understand Phosphorus (P) sources and transport processes in the subsurface in Bwaise III Parish, Kampala, P attenuation and adsorption capacities of soils were studied in situ and from laboratory measurements. Relationships between sorption parameters and soil matrix properties, rates and mechanism of the adsorption process and soil P fractions were also investigated. P was generally higher in the wet than the dry season, but for both seasons, the maximum was 5 mgP/l. P transport mechanisms appeared to be a combination of adsorption, precipitation, leaching from the soil media and by colloids with the latter two playing an important role in the wet season. The sorption process comprised two phases with the first stage rate constants being about fourfold those of the second stage. The Langmuir isotherm described the sorption data well (R ² ≥ 0.95) with the second soil layer exhibiting the highest sorption maximum (C _max) (average value 0.6 ± 0.17 mgP/gDW). The best prediction of C _max had organic carbon, Ca, available P and soil pH. Residual P consisting mostly of organics was the main fraction in all the layers followed by inorganic HCl-P and NaOH-P in the top and middle layers, respectively. Loosely bound P (NH₄Cl-P) was the least fraction (<0.4% of total P) in all layers indicating the high binding capacity of P by the soils. The study results suggest that P dynamics is related to Ca, Fe and organic carbon content of the soils.     

贺根山蛇绿岩带航磁异常特征及成矿潜力分析

铬铁矿贺根山蛇绿岩带地处西伯利亚板块和华北板块的汇聚部位,该带出露中国北方最大的超基性岩体群,大小有超过超过30个超基性岩块,其中自西向东分布的朝根山,贺根山,崇根山,乌兹尼黑为最大的蛇绿岩块。与超基性岩关系最密切的矿产莫过于铬铁矿,数年来很多学者针对贺根山蛇绿岩的含矿性做了大量工作(白文吉等1993,1995; Miao Laicheng et al.,2008;段明,2009,2015;黄竺,2015;王成,2016),该带分布的铬铁矿床（点）众多,有代表性矿床(点)为赫根敖拉、733和3756,然而能达到工业开采的矿床只有3756。近些年在该地区的找矿勘查工作未取得突破,笔者认为很多原因在于该区覆盖面积太大,找矿靶区定位不准以及对隐伏的矿体勘查力度不够。2013-2015年天津地质调查中心在2009~2011年度新飞的航磁资料基础之上对该区进行了航磁异常查证与评价研究工作,对该区的成矿潜力与找矿方向提出了心得认识。     

江西南岭成矿带自然重砂异常特征及其找矿潜力

南岭成矿带位于江西省西南部,是该省重要的钨、锡、稀土自然重砂异常带。带内单矿物重砂异常形态复杂,受自然地理条件控制显著,与花岗岩体和已知矿产地之间关系密切,组合异常总体呈北北东向展布,与区域构造线方向一致。根据地质构造背景及自然重砂组合异常特征,进一步划分出2个次级自然重砂异常带和14个自然重砂异常集中区。区内自然重砂异常充分展现了矿产资源潜在前景,为开展矿产资源调查与评价提供了重要的重砂异常信息标志。     

位场数据小子域滤波法处理效果对比与优选

重磁位场数据处理解释的主要任务之一是研究推断地质体的边界位置,而通过对重磁位场数据进行一定方法滤波处理后的相关图件可直观显示地质体的边界。小子域滤波法是众多滤波方法中的一种,前人利用小子域滤波法识别地质体边界位置有较多的研究,目前已有10种类型的小子域滤波法,但是鲜有对10种方法滤波效果的系统研究,给使用者带来诸多不便。因此,设计2组理论模型对比了10种方法的处理效果,并根据试验分析优选了最佳的滤波参数。理论模型与实际资料处理结果表明,滑动形子域滤波法对地质体边界增强的效果最好,且滤波窗口选择5×5数据点、迭代1次即可;对称形子域滤波法在滤除随机噪声的同时能够较好的保持原始异常场的梯度变化特征,滤波窗口选择5×5数据点、迭代2次较好。     

地热场与深部铀矿的关系研究及应用

根据地热学原理,本文从放射性元素富集程度、铀矿石产热数值模拟和地热资料铀矿信息识别等方面较深入地研究了地热与深部铀矿的关系,采用热红外遥感资料在若尔盖地区进行试验。结果表明,深部铀矿与地热场的分布有密切关系,可利用地热资料指导深部铀矿勘查。     

基于区域重磁信息的湖南怀化地区金属成矿预测

根据湖南怀化地区1∶20万区域重力资料和1∶5万航磁资料,研究了怀化地区深部断裂及岩浆岩特征,圈定了隐伏-半隐伏岩体和断裂构造,并根据地质矿产和物化探综合信息,分析了成矿区带与岩体和构造的关系,总结了成矿区带的区域重磁场特征,圈出5个成矿远景区,为怀化地区寻找深部矿和隐伏矿提供了重要信息。     

利用铝饱和指数评价中酸性岩浆岩的含矿性——以山西省210个中酸性岩体为例

利用常量元素地球化学特征参与评价岩体的含矿性具有重要意义.通过计算山西境内210个中酸性岩体的铝饱和指数,发现含矿岩体与非矿岩体的铝饱和指数存在一个明显的分界值,即铝饱和指数大于1.7的岩体基本与矿有关,小于此值的岩体绝大多数尚未发现与矿有关.铝饱和指数可能是反映岩体中矿元素活化释放程度或能力的一项重要指标,是本次探讨的重点,对评价其他区域中酸性岩的含矿性有重要参考意义.     

川东北普光地区新型杂卤石钾盐矿的物源:Sr、S同位素证据

新型杂卤石钾盐矿是近年来发现于四川盆地东北部早—中三叠世的一种可溶性海相固体钾盐,因其可利用性及低成本可开采特征,及其对海相找钾实现突破性进展提供的指示意义,引起了工业界及学术界的广泛关注。但当前以新型杂卤石钾盐矿为载体的海相成钾理论研究尚处于探索阶段,本文聚焦川东北普光地区中下三叠统新型杂卤石钾盐矿,将锶、硫同位素研究方法应用于杂卤石碎屑、石膏及"绿豆岩"等样品,以探讨新型杂卤石钾盐矿中不同组分的物源特征。分析结果显示,杂卤石碎屑和矿层底部硬石膏样品具有相似的δ³⁴S和⁸⁷Sr/⁸⁶Sr同位素特征,δ³⁴S分布在27.9‰～28.9‰之间,⁸⁷Sr/⁸⁶Sr介于0.70826～0.70829之间。该研究结果与同时期全球海相硫酸盐的δ³⁴S和全球海水的⁸⁷Sr/⁸⁶Sr的可对比性表明上述组分的主要物源应为同时期海水。该研究结果为进一步新型杂卤石钾盐矿成矿模型的建立及海相成钾理论的补充和完善提...     

The role of dense brines in the formation of vent-distal sedimentary-exhalative (SEDEX) lead–zinc deposits: field and laboratory evidence

A majority of the world's sediment-hosted exhalative (SEDEX) lead-zinc deposits are vent-distal. They are not underlain by a discordant alteration zone or stockwork vent complex that would indicate the path by which ore fluids reached the seafloor. The absence of a vent complex, together with sulfide mineral replacement of host rock mineral assemblages has led several investigators to suggest that, in spite of the well-layered nature of these deposits, mineralization was formed by sub-seafloor lateral migration of ore fluids along permeable strata. Field observations, supported by simple laboratory experiments, however, suggest an alternative process for characterizing the genesis of vent-distal SEDEX deposits. Cool, saline brines (e.g., ~120 °C and >15 wt% NaCl equiv.) are denser than seawater and, upon discharging into the sea, would flow away from the discharge vent as bottom-hugging fluids, similar to the behavior of turbidity currents. Their high densities and velocities prevent them from mixing with overlying seawater, thereby precluding significant cooling and dilution of the ore fluid. Upon coming to rest in a seafloor depression, the addition of H₂S and/or dilution of the ore fluids to lower salinities result in the eventual precipitation of a vent-distal SEDEX deposit. Furthermore, the dense ore-forming fluid can sink into permeable sediments beneath the brine pool by displacing less dense pore water. The ore fluids are thus capable of effectively overprinting and/or replacing pre-existing minerals in the consolidating sediment pile.     

Delineation of potential exploration targets based on 3D geological modeling: A case study from the Laoangou Pb-Zn-Ag polymetallic ore deposit,China

The Laoangou Pb-Zn-Ag polymetallic ore deposit (LAG) is a hydrothermal vein-type deposit in the East Qinling polymetallic metallogenic belt, which is controlled by NW-, NE- and nearly SN-trending faults and mainly hosted in the dolomite marble of Meiyaogou and Baishugou formations within the Luanchuan ore district (LOD). Although district-scale metallogenic prediction related to the Pb-Zn-Ag deposits in the LOD has been attempted, there have been no studies to formulate a detailed deposit-scale metallogenic prediction of the Pb-Zn-Ag deposit in 3D space. Here we selected the LAG to formulate a metallogenic prediction model for Pb-Zn-Ag deposit based on data from 1:10,000 scale geological map, and 66 boreholes and 39 sections of exploration lines on 1:1000 scale, to extract the salient spatial features of the deposit. We apply ordinary weights and weighted weights of evidence, followed by boosted weights of evidence, logistic regression and information entropy for integrating the features of the ore deposit for exploration targeting. The C–V fractal method is applied to classify the probabilities. Accordingly, three levels of exploration targets are delineated: the first level targets are mainly distributed in the periphery of known orebodies (M2, M3, II-3 and VI), which are also located in the ore-controlling strata and fault zones; the second level targets are located in the region of known orebodies, indirectly confirming that the prediction result is reliable; and the third level targets, which are mainly located at the contact between ore-controlling strata and metagabbro bodies, offer new targets for exploration in this ore district. Combining our results obtained in this study with those in previous studies, we provide some guidelines for exploration targets in the Pb-Zn-Ag deposits in the LOD, which might help in more effectively delineating the target zones.     

The Mg-carbonate-Fe interaction:Implication for the fate of subducted carbonates and formation of diamond in the lower mantle

The fate of subducted carbonates in the lower mantle and at the core-mantle boundary was modelled via experiments in the MgCO3-Fe^0 system at 70-150 GPa and 800-2600 Kin a laser-heated diamond anvil cell.Using in situ synchrotro n X-ray diffraction and ex situ transmission electron microscopy we show that the reduction of Mg-carbonate can be exemplified by:6 MgCO3+19 Fe=8 FeO+10(Mg0.6Fe^0.4)O+Fe7 C3+3 C.The presented results suggest that the interaction of carbonates with Fe^0 or Fe^0-bearing rocks can produce Fe-carbide and diamond,which can accumulate in the D"region,depending on its carbon to Fe ratio.Due to the sluggish kinetics of the transformation,diamond can remain metastable at the core-mantle boundary(CMB)unless it is in a direct contact with Fe-metal.In addition,it can be remobilized by redox melting accompanying the generation of mantle plumes.