沉积矿床中重要(变价)金属元素与古海洋环境关系综述——以新元古代—早古生代扬子板块周缘为例

对氧化—还原敏感元素和生命必需元素地球化学性质和相关沉积矿产形成机制及其时空分布特征的分析、总结和归纳,有助于对古海洋环境演化与相关沉积矿产形成关系的深入认识,这对沉积矿床学理论的发展具有重要意义。新元古代南华纪—早古生代早志留世,扬子板块周缘黑色岩系中依次沉积了举世瞩目的南华纪锰矿、震旦纪—早寒武世沉积磷块岩、早寒武世重晶石矿、镍—钼—钴多金属矿、钒矿和早志留世铀矿的完整沉积矿床系列,不同时代地层、同一时代不同沉积环境地层赋存不同类型的沉积矿产,有必要对其成矿机理进行系统研究、总结和归纳。笔者等在对氧化—还原敏感金属元素、生命必需元素和海底热液元素等元素地球化学性质综述的基础上,兼论了它们在结合黄铁矿矿物学对氧化—还原环境、生物生产力和盆地限制性分析方面具体作用;进一步结合新元古代—早古生代扬子板块古环境演化特征,对扬子板块周缘特定时代中特定沉积矿产的分布、成因机理进行分析、总结和归纳;最终得出扬子板块周缘相关沉积矿产,是在全球氧化事件、冰期—间冰期、Rodina大陆裂解大背景下,海洋中成矿金属元素得到长期积累,在特定时代区域性海水—海底热液—生物作用(根据不同矿种各有侧重)影响下,...     

扬子地块西北缘后龙门山南华纪—早古生代沉积地层特征及其形成环境

通过对扬子地块西北缘后龙门山构造带南华纪—早古生代沉积地层的分析研究,综合运用岩石学、沉积特征分析的手段,结合与前龙门山构造带、碧口地块、米仓山构造带和汉南地块的沉积特征对比,探讨后龙门山构造带南华纪—早古生代的沉积环境,查明其形成环境。结果表明：在南华纪—早古生代后龙门山构造带沉积环境可分为南华纪—震旦纪裂解-稳定沉积和早古生代伸展裂陷沉积两个阶段,其中在早古生代伸展裂陷阶段后龙门山构造带经历了裂解→抬升→局部裂解→抬升→裂解的反复过程,最终形成了志留系茂县群裂陷槽沉积,主要沉积了一套代表伸展裂陷环境的沉积岩系,并有少量的陆内火山岩系。研究结果进一步证实,龙门山构造带在南华纪—早古生代总体是一个陆内裂谷带,不发育与古缝合线相关的构造混杂岩和蛇绿混杂岩带,是在陆内裂谷的基础上于印支期—燕山期形成的陆内造山带。     

湘东南南华系—寒武系砂岩地球化学特征及对华南新元古代-早古生代构造背景的制约

湘东南位于扬子板块与华夏地块的交接地带,南华纪—寒武纪期间形成了巨厚的复理石、类复理石连续沉积。其中的砂岩总体上以中等的SiO2含量、高的K2O/Na2O值和Al2O3/(Na2O CaO)值、较高的(Fe2O3* MgO)含量及低的CaO含量为特征。稀土元素球粒陨石标准化曲线与典型的后太古宙页岩和上陆壳相似,以轻稀土富集、重稀土平坦、铕负异常显著为特征。主量元素和微量元素特征显示出被动大陆边缘、活动大陆边缘和大陆岛弧等多种环境信息。从不同构造背景下的剥蚀原岩及风化条件和搬运沉积过程来看,大陆岛弧和活动大陆边缘形成的砂岩应具有显著区别于被动大陆边缘的地球化学特征,而被动大陆边缘形成的砂岩则可能包含较多的大陆岛弧和活动大陆边缘环境的地球化学信息。因此,湘东南南华系—寒武系砂岩应形成于被动大陆边缘环境。这一判断与古元古代—中元古代岛弧与活动大陆边缘形成岩石为重要原岩,以及沉积盆地因强烈陆内伸展断陷而具一定活动性等客观事实相吻合。华南板溪群与冷家溪群间至少存在40～60Ma的沉积缺失,江南造山带新元古代碰撞造山成因的岩浆活动,板块交接带南华纪—早古生代沉积物中岛弧火山物质的缺乏,以及湘(东)南中元古界(甚至更早)结晶基底横向上的连续性等,表明扬子板块与华夏地块之间新元古代—早古生代期间的沉积盆地不是洋盆,而是陆内裂谷盆地。上述基于湘东南砂岩地球化学研究,和根据区域资料分析所得出的关于扬子板块与华夏地块间新元古代—早古生代盆地构造性质的结论,彼此提供了良好的约束。     

扬子克拉通南华纪-早古生代的构造-沉积旋回

根据影响克拉通盆地演化的板块活动所经历的威尔逊旋回,扬子克拉通南华纪-早古生代的构造-沉积旋回可分为5期,即南华纪-震旦纪的均衡调整期、寒武纪的扩张期、早-中奥陶世的汇聚期、中-晚奥陶世的碰撞期和志留纪新一轮的均衡调整期。发生在晚奥陶世的中加里东运动是加里东期最强烈的一次造山运动,而发生在晚志留世的晚加里东运动是一次地壳上升运动;相应的扬子古板块志留纪盆地的构造背景为造山期后的裂谷伸展环境,其原型盆地表现为周缘裂谷盆地与弱伸展的克拉通内盆地相间发育的格局。     

安徽石台地区下寒武统黑色岩系微量元素地球化学特征

黑色岩系成因复杂,且与多金属元素成矿密切相关。对安徽石台地区下寒武统黑色岩系微量元素地球化学研究表明,其富集Ag、V、U、Mo、As、Sb等多种金属元素。且在这套黑色岩系地层剖面中,其中部碳质页岩的金属元素富集程度比其它岩性的富集程度明显偏高。微量元素组成及相关元素比值显示这套黑色岩系形成于海相热水沉积作用,代表了一种非正常海水沉积。沉积环境由早期缺氧的还原环境逐渐向后期的半还原-氧化环境演化,反映早寒武世安徽下扬子海由海侵向海退的逐渐过渡。     

贵州南华纪—震旦纪沉积大地构造及其对沉积矿产的控制作用

新元古代早期(820 720 Ma),华南由扬子地块、华夏地块和江南造山带3个构造单元组成。新元古代晚期,南华纪-震旦纪(720 541 Ma)沿新元古代早期的江南造山带发育一裂谷盆地-南华裂谷,贵州东部是南华裂谷的重要组成部分。传统认为黔东地区该裂谷盆地为北北东向,越来越多的证据证明该裂谷盆地为北东东向。黔东地区南华纪裂谷盆地具有典型的地堑、地垒结构,并控制着"大塘坡式"锰矿的分布。震旦纪之后,裂谷盆地处于沉降阶段,沉降中心向南迁移到凯里-玉屏一线。震旦纪时期,扬子地块和南华裂谷存在明显的滨浅海磷矿-碳酸盐岩和深水泥质岩-硅质岩的沉积分异。扬子地块和南华裂谷控制着扬子地块震旦系陡山沱组磷矿和南华裂谷老堡组重晶石等沉积矿产的分布。富磷矿位于松桃-贵阳同沉积断裂以北的扬子地块南缘,重晶石矿发育于裂谷盆地强烈沉降区的天柱-岑巩一带。     

安徽铜陵志留系—三叠系地层层序地层和微量元素地球化学:对沉积环境的约束

在安徽铜陵地区广泛分布有志留系—三叠系沉积地层,它们与区内的层控矽卡岩矿床关系密切。作者对这些地层进行了层序地层学和微量元素地球化学研究,并在此基础上分析了铜陵地区古生代—中生代的沉积环境。层序地层学研究显示,铜陵地区在古生代—中生代期间发生了3次海平面变化事件,形成了3个沉积序列:(1)早志留世—晚泥盆世海退序列;(2)晚泥盆世—早二叠世海侵序列;(3)晚二叠世—早三叠世海退序列。微量元素地球化学研究表明,铜陵地区在古生代—中生代地层沉积过程中底层海水的物理化学条件(氧化还原条件)发生过3次明显的波动,并与3次海平面变化事件相对应。有意义的是,铜陵地区的大部分层控矽卡岩矿床都产在与后两次海平面变化事件相对应的地层中。     

贵州省下寒武统黑色岩系型多金属矿床研究进展

文章从矿物学、矿床地球化学等方面对贵州省下寒武统黑色岩系型多金属矿床的研究进展进行综合评述,研究表明：贵金属主要以硫化物形式均匀散布于黄铁矿等硫化物中,超常富集的PGE、Au可能主要以不可见有机结合态、无机结合态和吸附态等赋存于矿石干酪根中;碳硫钼矿是Mo的主要存在形式,钒则主要以类质同像形式存在于粘土矿物水云母中;镍钼层矿石成矿年龄521±54Ma-542±11Ma,与赋矿地层成岩年龄531±24 Ma基本一致;硫主要来源于生物对海水硫酸盐的还原,高度富集的多金属元素层主要为海底热水沉积作用形成;矿床成矿流体主要是盐度较高的海底含矿热卤水与正常海水混合沉积的产物;南华纪-早古生代阶段,湘黔地区南华裂谷海槽洋壳向华夏古陆俯冲及扬子陆块向南华裂谷海槽洋壳俯冲的构造环境及其诱发的挤压-伸展构造机制的多次转换等事件为矿床形成提供了有利的成矿通道与就位场所。     

扬子西北缘碧口微地块南华系碎屑锆石物源示踪及其地质意义

扬子板块西北缘碧口微地块华严寺地区碧口群之上发育一套含砾沉积地层,其确切沉积时限对研究该区域的沉积—构造演化具有重要意义。本文对该套地层中的2件砂岩样品进行了碎屑锆石LA-ICP-MS U-Pb测年分析,结果显示,锆石晶型较好,无色透明,磨圆度较差,具典型岩浆生长振荡环带和韵律结构,Th/U比值为0.28～3.6,锆石U-Pb年龄介于706～2 489Ma之间。存在3个主要的年龄组：新元古代年龄组（706～951 Ma）,占92.9%,显著峰值为850 Ma和843 Ma;中元古代年龄组（1 017～1 080 Ma）,占2.6%;古元古代年龄组（1 628～2 489 Ma）,占4.5%。最小年龄组为706～715 Ma（峰值为711 Ma）,结合区域地质和研究资料,华严寺地区沉积地层时代应属于南华纪,沉积时限约为720～635 Ma,物源区主要包括扬子板块西北缘碧口微地块以及南东侧的后龙门山构造带和汉南—米仓山微地块。新元古代晚期碧口微地块及扬子板块西北缘后碰撞—裂解阶段,华严寺地区南华纪沉积为碧口微地块及邻区新元古代岩浆岩在边缘裂谷环境中快速堆积形成。     

黔东南三穗下寒武统黑色岩系中钒富集机制研究

扬子板块南缘下寒武统黑色页岩地层中分布有一套富含Ni、Mo、V、U、Ba、PGE等的多金属层,并在贵州多地形成Ni—Mo—V—Ba矿床。黔东南三穗钒矿是扬子板块南缘黑色页岩型多金属矿床的重要组成部分,矿体主要赋存于早寒武世九门冲组底部黑色炭质、硅质泥岩中。针对三穗钒矿含矿岩系,通过沉积学和元素地球化学研究,结果表明,赋矿硅质岩、含钒炭质泥岩中存在热水物质贡献的明显信号,指示热水端元输入很可能是钒的重要来源。环境敏感元素及相关参数分析显示,局限盆地内的缺氧水体环境更有利于钒的富集。此外,有机质与钒富集之间具有明显的耦合关系,说明有机质的吸附对钒的沉淀和富集起到重要控制作用。     

Transition from confined to phreatic conditions as the factor controlling salinization and change in redox state, Upper subaquifer of the Judea Group, Israel

An increase in salinity and change from oxic to anoxic conditions are observed in the Upper subaquifer of the Judea Group in the Kefar Uriyya pumping field at the western foothills of the Judea Mountains, Israel. Hydrogeological data indicate that the change, which occurs over a distance of only a few kilometers, coincides with a transition from confined to phreatic conditions in the aquifer. The deterioration in the water quality is explained as a result of seepage of more saline, organic-rich water from above, into the phreatic "roofed" part of the aquifer. The latter is derived from the bituminous chalky rocks of the Mount Scopus Group, which confine the aquifer in its southeastern part. In this confined part, water in perched horizons within the Mount Scopus Group cannot leak down and flow westward while leaching organic matter and accumulating salts. However, upon reaching the transition area from confined to phreatic conditions, seepage to the Judea Upper subaquifer is possible, thereby allowing it to be defined as a leaky aquifer. The incoming organic matter consumes the dissolved oxygen and allows bacterial sulfate reduction. The latter accounts for the H₂S in the aquifer, as indicated by sulfur isotopic analyses of coexisting sulfate and sulfide. Thus, from an aquifer management point of view, in order to maintain the high quality of the water in the confined southeastern part of the Kefar Uriyya field, care should be taken not to draw the confined-roofed transition area further east by over pumping. Electronic Publication     

草莓状黄铁矿形成机制及其研究意义

黄铁矿是沉积物中较为常见的矿物之一,而草莓状黄铁矿是由等粒度的亚微米级黄铁矿晶体或微晶体紧密堆积而成。基于已有的草莓状黄铁矿相关研究成果,本文综述了草莓状黄铁矿的形成机制,阐述了其对古环境恢复的意义。目前普遍认为草莓状黄铁矿的成因主要有两种,即有机成因和无机成因,前者认为有机质或细菌参与草莓状黄铁矿形成;而后者主要认为过饱和的黄铁矿浓度是微球粒草莓状黄铁矿形成的必备条件。草莓状黄铁矿粒径的大小是其古沉积环境的直接反映,被作为比较可靠的古氧化还原条件判别指标,已被广泛应用于古代及现代海洋沉积物的古环境恢复。目前,虽然草莓状黄铁矿粒径判别古环境的大小及分布范围还不统一,但普遍认为封闭的水体环境（硫化环境）草莓状黄铁矿粒径较小且变化不大,且随着含氧程度增加,草莓状黄铁矿的粒径大小增大且分布范围趋于加宽。     

Gas pressures and redox reactions in geothermal fluids in Iceland

The gas and redox chemistry of 100–300 °C geothermal fluids in Iceland has been studied as a function of fluid temperature and fluid composition. The partial pressures of CO₂ in dilute (mCl<500 ppm) and saline (mCl>500 ppm) geothermal fluids above 200 °C are controlled by the mineral buffer clinozoisite+prehnite+calcite+quartz. Two buffers are considered to control the H₂S and H₂ partial pressures above 200 °C depending on fluid salinity, epidote+prehnite+pyrite+pyrrhotite for dilute fluids and pyrite+prehnite+quartz+magnetite+anhydrite+clinozoisite+quartz for saline fluids. Below 200 °C, the partial pressures of CO₂, H₂S and H₂ also seem to be buffered but other minerals must be involved. Zeolites are expected to replace prehnite and epidote. Redox potential calculated on the assumption of equilibrium for the H⁺/H₂ redox couple decreases in dilute geothermal fluids with increasing temperature from about −0.5 V at 100 °C to −0.8 V at 300 °C, whereas saline geothermal fluids at 250 °C display a redox potential of about −0.45 V. A systematic discrepancy between redox couples of about 0.05–0.09 V is observed in the redox potential for the dilute geothermal fluids, whereas redox potentials agree within 0.02–0.04 V for saline geothermal waters. The discrepancies in the calculated redox potential for dilute geothermal fluids are thought to be due to a general lack of equilibrium between CH₄, CO₂ and H₂ and between H₂S, SO₄ and H₂. It is, accordingly, concluded that an overall equilibrium among redox species has not been reached for dilute geothermal fluids whereas it appears to be more closely approached for the saline geothermal fluids. The latter conclusion is based on limited database and should be treated with care. Since the various redox components are not in an overall equilibrium in geothermal fluids in Iceland these fluids cannot be characterised by a unique hydrogen fugacity, oxygen fugacity or redox potential at a given temperature and pressure.     

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.     

Chemical contaminant reactions and assessment of soil cleanup levels for protection of groundwater

About 70 percent of hazardous waste sites listed in the National Priority List (NPL) have some groundwater contamination that may require remediation. Such remediation is inadequate if the unsaturated soils above will continue to act as a source of groundwater contamination. Consequently, for most of these sites, it becomes necessary to determine what the cleanup levels for contaminants in soils should be so that subsequent contribution of contaminants from these soils to groundwater would not exceed groundwater protection levels. Representation of the dynamics of interactions between contaminants and soils is very complex, requiring among others, a thorough understanding of the chemical processes that influence the behavior of the contaminant once it enters the subsurface. Because of such complexities, environmental professionals frequently utilize methods with very simple assumptions that tend to err on the conservative side. While the public may feel protected, the needless spending of dollars could be avoided if attempts are made to incorporate, where possible, such complexities in the modeling efforts so that the system is represented as accurately as possible.     

草莓状黄铁矿与古海洋环境恢复

草莓状黄铁矿是指由等粒度的亚微米级黄铁矿晶体或微晶体紧密堆积而成,形似草莓的黄铁矿球形集合体.它们在氧化和缺氧海洋环境中形成的机理不同,沉积岩中草莓状黄铁矿的粒径分布特征是恢复古海洋的氧化还原状态行之有效的方法之一.然而,草莓状黄铁矿粒径统计仅能区分出氧化和缺氧硫化的环境,不能进一步区分缺氧程度及状态(如次氧化、缺氧含...     

Nuclear waste isolation: an urgent international responsibility

The isolation of nuclear wastes for at least thousands of years is an urgent world problem. It is an international problem, for in many nations with such wastes their geology is not suitable for long-term isolation.

A lead geo-question involves which are the best host rocks, with guaranteed long-term low permeability and the best ion-exchange, redox systems for the capture, retention, of the most dangerous nuclides. In general mud rocks on land or the sea floor must be considered.     

A procedure to identify natural arsenic sources,applied in an affected area in North Rhine-Westphalia,Germany

The aim of the study was to identify the geogenic source for elevated arsenic (As) concentrations recently discovered in soils of the Heubach plain, North Rhine-Westphalia, Germany. Therefore, a catalogue of conditions that an As source has to fulfil in order to be considered as geogenic was formulated, including the source’s linkage to the sink, its mineralogy, As total content, As mobilization potential, groundwater redox conditions and As output. Accordingly, mineralogical, geo- and hydrochemical investigations were carried out, including X-ray diffractometry, microscopy, ICP-OES and AAS analysis and a sequential extraction procedure. Paleo bog iron ores (PBIOs) of Tertiary age, occurring within unconsolidated sands (Haltern-layers, Santonian–Lower Campanian), and glauconitic marlstones (Dülmen-layers, Lower Campanian) were examined. Results indicate that output from the PBIOs is responsible for the elevated As levels. Accounting their diverse mineralogy, five types of PBIOs were defined. Type-dependent, they at least partly fulfil all of the formulated requirements. The relations and behaviour of As sources and sinks in space and time could be clarified for the area of interest. The approach presented in this paper may offer a tool for identifying natural As sources worldwide.     

Changes in tourmaline composition during magmatic and hydrothermal processes leading to tin-ore deposition: The Cornubian Batholith,SW England

To investigate the potential of tourmaline as a geochemical monitor, a comprehensive dataset on major, minor and trace element concentrations as well as Fe³⁺/ΣFe ratios of tourmaline is presented. The dataset includes samples from five plutonic complexes related to diverse magmatic to hydrothermal stages of the Cornubian Batholith (SW England). Tourmaline composition found in barren and cassiterite-bearing samples include all three primary tourmaline groups and tourmaline species with the general endmembers schorl, dravite, elbaite, uvite, feruvite, foitite and Mg-foitite.Based on textures and compositions, it is possible to distinguish not only between late-magmatic and hydrothermal tourmaline, but also between several formation stages. Hence, tourmaline monitors late-magmatic processes and the partitioning of elements during exsolution of an aqueous phase. For example, in hydrothermal tourmaline Sn is strongly enriched, while Ti, Cr, V and Sc are depleted compared to late-magmatic tourmaline of the same sample. Several tourmaline generations that precipitated from magmatic fluids can be distinguished with differing major and minor elements and REE patterns depending on the composition of the melt from which they were expelled from. Strongly zoned tourmaline allows for unraveling the hydrothermal history of a distinct location including ore precipitation. The precipitation of SnO₂ in the study area was probably caused by mixing between acidic, reduced, Sn-bearing magmatic fluids and oxidized meteoric fluids, which is in agreement with London and Manning (1995) and Williamson et al. (2000). Hence, the ability of tourmaline composition to monitor changes in Sn concentration and redox conditions in hydrothermal fluids has potential as an exploration tool.