Origin of giant stratabound deposits of magnesite and siderite in Riphean carbonate rocks of the Bashkir mega-anticline, western Urals

In the Bashkir mega-anticline (western Urals) stratabound magnesite, siderite, fluorite and base-metal deposits are hosted by a sequence of Riphean sediments with a thickness of more than 12 km. The giant deposits of siderite (Bakal) and sparry magnesite (Satka) belong to the largest known mineral deposits of this type on Earth but are still disputed with respect to their origin. Both the Fe- and Mg-carbonate ores are clearly characterized by mimetic preservation of sedimentary and diagenetic textures of the host carbonate sediments, giving evidence of epigenetic metasomatic replacement. In the stratiform magnesite deposits of Satka, O- and C-isotopes, REE pattern and the lithostratigraphic position of the ore, point to the reflux of early diagenetic Mg-rich brines being responsible for the selective replacement of brecciated dolomite. The Bakal siderite deposits are hosted by Lower Riphean carbonate rocks and are controlled by a Lower to Middle Riphean unconformity marked by deep erosion and subsequent transgression-related sedimentation of coarse clastics. Their independence of carbonate lithofacies and their trace element distribution are indicative of metasomatic processes. Fe-bearing fluids have probably been generated by low-grade metamorphic (catagenetic) devolatization from underlying argillites, causing the metasomatic formation of large siderite ore bodies in the Bakal carbonates due to the focusing of the fluid flow by the overlying Mid-Riphean coarse clastics.     

鄂尔多斯盆地西缘二叠系事件地层及其地球化学特征

通过钻井岩心的观察及镜下岩矿鉴定,同时依据地球化学和沉积学特征,把区内发育的事件地层分为两类:火山事件层和海退事件层,其中太原组火山事件层和海退事件层各1层,山西组火山事件层1层,石盒子组火山事件层3层。所体现的常量、微量和稀土元素质量分数及岩石学特征具明显的差异。研究事件地层获得的成果,可以为区域地层划分和对比提供更加准确的依据。     

辽西义县组早期火山事件层地球化学特征及意义

义县盆地早白垩世义县组为一套火山-沉积岩系,沉积层中多含凝灰质成分。为了解沉积期火山活动对沉积层产生的影响,对砖城子层火山碎屑泥灰岩(火山事件层)和纹层状泥灰岩夹层、大康堡层纹层状泥灰岩进行岩石学和地球化学的研究。研究表明:火山事件层中含大量的火山碎屑(20%±),其他各层均含凝灰质成份。样品中主量元素Ca O、Mg O和Si O_2含量较高,Si O_2含量变化较大,Al_2O_3含量较低;微量元素与上地壳(UCC)元素比值多小于1,大多数微量元素(Rb、Th、Nb、REE等)处于亏损状态,火山事件层富集Li、Sc、As、Sr和Ag元素。沉积期火山活动对沉积层中Si O_2和K_2O含量影响较大,并可能导致As元素富集。火山事件层样品化学风化指数(CIW)在82~91之间,呈逐渐增加的趋势,指示风化作用强烈。     

粤西罗定盆地南缘金矿地球化学特征及成矿作用

研究区矿石及近矿蚀变围岩中黄铁矿的硫、铅同位素组成特征,以及金矿石(含金石英脉)和花岗斑岩的稀土元素特征,显示金矿成矿物质来源于地壳.矿体石英中的流体包裹体氢、氧同位素组成特征,显示成矿热液来源于大气降水.结合气液包裹体均一温度,认为本区金矿的成矿作用发生于低温、浅成-超浅成的环境条件,金矿的成因类型为浅成-超浅成低温热液型.     

Geochemical processes and sedimentological characteristics of Holocene lagoon deposits,Alikes Lagoon,Zakynthos Island,western Greece

In this paper we present sedimentological, geochemical, mineralogical and ¹⁴C Accelerator Mass Spectrometry (AMS) dating data of a 21.0‐m core obtained from Alikes Lagoon, Zakynthos Island. It is the first time that sedimentological and geochemical data are presented, up to 21‐m depth, from an Holocene coastal lagoon environment of the Ionia Sea, western Greece. The sedimentological properties and the geochemical composition of sediments were studied for the time period between 8540 years bp to present. Samples were analysed for their particle size, calcium carbonate and total organic carbon (TOC) content. Moreover, bulk sample chemical analyses for major and trace elements were carried out, as well as bulk, oriented mineralogical analyses for the clay fraction (<2 µm) were determined by powder X‐Ray diffraction. The grain size characteristics, statistical parameters and TOC, for the Holocene analysed samples, suggest a coastal environment (restricted‐shallow) with reduced salinity such as a tidal flat and/or particularly marsh in a lagoon margin. Sediment characteristics as well as trace element records may contain additional new palaeoclimate information that provide important new constraint on sediment depositional environment and Holocene climate. Stratigraphic variation of geochemical indices, such as Rb/Ti, Cs/Ti, Eu/Eu*, Th/Ti, La/Ti, Ta/Ti, Yb/Ti and Y/Ti, for the sediments with ages from 8540 to 7210 years bp , remain relatively constant indicating that the provenance of the Alikes Lagoon remained similar throughout the mid Holocene. However, geochemical indices in sediments with ages from 7110 years bp to present, indicate the transition from a warm and wet climate in the middle Holocene to a relatively cold climatic conditions in modern times. Copyright © 2012 John Wiley & Sons, Ltd.     

新疆且日克其菱铁矿床稀土元素地球化学特征

新疆且日克其菱铁矿床赋存于下志留统温泉沟群中低级变质岩系中,由三个矿段组成,总体呈层状、似层状、透镜状.矿石稀土含量较高(∑REE=740.97 ×10-6～820.21×10-6),轻稀土富集(LREE/HREE=13.17～14.74)的特征暗示了矿床形成于弱-中碱性的含矿溶液;明显的Ce正异常(δCe=3.25～...     

湖南省沉积型锰矿床地球化学特征

湖南省锰矿资源丰富,类型多样,沉积型锰矿是最重要的类型,主要沉积成锰期集中在早南华世大塘坡期、中奥陶世烟溪期及晚二叠世孤峰期.通过对三个主要成锰期代表性锰矿石电子探针分析显示,三个主要成锰期所成锰矿床,矿石组分类似,组成矿石的锰矿物主要为锰的碳酸盐,包括菱锰矿、钙菱锰矿、镁菱锰矿、锰白云石、锰方解石等.三个主要成锰期代表性锰矿石及地层稀土元素地球化学标准化参数与配分模式显示,各成矿期含锰地层与锰矿石之间无显著差异,暗示了它们是相似的沉积环境下形成的产物,为正常沉积与热水沉积的复合.一方面,各成矿期岩、矿石稀土元素总量中等偏高,表现为正常沉积物的稀土元素特征.另一方面,在北美页岩标准化配分曲线上,呈现出水平或重稀土相对富集的左倾配分曲线,Ce的负异常,又是典型热水沉积成因呈现出的稀土元素地球化学特征.δCe异常特征,指示了成锰作用处于波动的缺氧还原/氧化沉积环境.     

Geochemical characteristics of tertiary oil-generating strata in Jiyang Sag

The Jiyang Sag was a near-sea basin in Oligocene time of mild and wet climate. The flourishing living things in this basin and the continuous depressions created a favorable environment for oil generation. Occasional transgressions resulted in special water-medium conditions, giving rise to a unique biological group of abundan terrestrial and fresh-water biota and organisms characteristic of semi-saline environments as well as small amounts of marine biota. Discussions are also made of the evolution of organic matter on the basis of changes of geochemical indexes for oil-generating rocks at different depths in the Dongying Depression.     

柴达木盆地西部地区咸化湖盆天然气地球化学特征及成因类型判识指标——以古近系-新近系为例

根据天然气地球化学基本理论,对柴达木盆地西部(柴西)地区天然气的地球化学特征进行分析。测试和收集该地区18个油气田及含油气构造的83个天然气样品,分析其组分和C同位素数据,结果显示,该区天然气以烃类气体为主,烃类含量介于79.24%~99.81%之间,天然气比重介于0.72~1.36之间,天然气干燥系数(C1/C2+)介于1.04~617.8之间,甲烷C同位素值介于-51.4‰~-24.2‰之间,乙烷C同位素值介于-34.2‰~-17.4‰之间。在此基础上,对该区不同构造带的天然气成因类型进行研究。研究结果表明,柴达木盆地西部地区天然气类型可划分为腐泥型气、腐殖型气、混合气3类,柴西北部天然气主要为腐泥型气和混合气,少量腐殖型气;柴西南部主要为腐泥型气,少量混合气。同时,建立了天然气成因类型的判识指标,对该区的天然气地球化学特征有了较为深入的认识,对今后的天然气勘探具有指导意义。     

柴达木盆地西部地区咸化湖盆天然气地球化学特征及成因类型判识指标——以古近系—新近系为例

根据天然气地球化学基本理论,对柴达木盆地西部（柴西）地区天然气的地球化学特征进行分析。测试和收集该地区18个油气田及含油气构造的83个天然气样品,分析其组分和C同位素数据,结果显示,该区天然气以烃类气体为主,烃类含量介于79.24%~99.81%之间,天然气比重介于0.72~1.36之间,天然气干燥系数（C1/C2+）介于1.04~617.8之间,甲烷C同位素值介于-51.4‰~-24.2‰之间,乙烷C同位素值介于-34.2‰~-17.4‰之间。在此基础上,对该区不同构造带的天然气成因类型进行研究。研究结果表明,柴达木盆地西部地区天然气类型可划分为腐泥型气、腐殖型气、混合气3类,柴西北部天然气主要为腐泥型气和混合气,少量腐殖型气;柴西南部主要为腐泥型气,少量混合气。同时,建立了天然气成因类型的判识指标,对该区的天然气地球化学特征有了较为深入的认识,对今后的天然气勘探具有指导意义。     

灵台风尘堆积中钙质结核的地球化学研究

通过对甘肃灵台风成黄土－古土壤－红粘土序列中钙质结核的野外分布及形态特征观察,并对主元素和微量元素进行化学分析,发现结核中氧化物活动给分和惰性组分含量的比值在剖面中自下而上逐渐变大,反映了结核形成时其上覆土层所经历的淋溶程度逐渐增强;而ＭｇＯ含量校正后的ＭｇＯ／Ｃａｏ比值则逐渐减小,批示结核形成时环境温度逐渐降低。灵台剖面自下而上不同层位结核中Ｓｒ／Ｂａ比值及ＣａＣＯ３含量的变化同氧化物含量及比值     

Geochemical and petrological characteristics of Eppawala phosphate deposits,Sri Lanka

The apatite-bearing carbonate rocks at Eppawala, Sri Lanka occur as massive, discontinuous bodies in a Precambrian, high-grade metamorphic terrain, which weather to form economically important phosphate deposits. The ore bodies at Eppawala contain =42% P ₂O ₅, and citric acid solubility of different components varies from 4 to 6%. The parent rocks are mainly made up of calcite, dolomite and apatite, with lesser amounts of ilmenite, magnetite, pyrite, forsterite, phlogopite, enstatite, magnesite, diopside, tremolite and spinel. Most of minerals show an euhedral habit, with a wide range of crystal sizes (from a few millimetres to several decimetres). The Eppawala rocks are characterised by low silica (=0.41%), high phosphorous (=10.58%) and high strontium content (2,960–6,819 ppm). Concentrations of light rare-earth elements in these rocks are comparably higher than those of marbles. The REE fractionation of these rocks is pronounced, and La/Yb ratios vary between 14 and 43. Both apatite and calcite show markedly elevated strontium levels (=0.6%). The d ¹³C _PDB and d ¹⁸O _SMOW values of the carbonates are in the range of –3.4 to –2.2 and 7.7 to 16.4‰ respectively. The euhedral habit, as well as the presence of major quantities of apatite and considerable amounts of iron-bearing minerals suggest that the ore host rock has genetic links to an igneous source rather than to an intensely metamorphosed limestone. The higher light REE contents of the rocks, compared to marbles, also argue against a metamorphic or sedimentary origin. The Sr/Mn and Ce/La ratios in the apatite are ~40 and ~2 respectively, suggesting that they were formed in a carbonatite magma. The markedly increased REE concentrations in the bulk chemistry of the rocks have been shown to be mainly controlled by the content of phosphate minerals. Compared to most carbonatites, the Eppawala rocks are generally depleted in selected trace elements, particularly Ba, Nb, Th, V, U and Zr. This depletion may be due to either a primary infertility of the parent magma with regard to such trace elements, or it is a result of fractional crystallisation during the rock formation. The stable isotope ratios do not plot within the defined "mantle carbonatite box", but still lie within the broader range of carbonatitic rocks. With these data at hand, it can be readily argued that the mode of occurrence, petrography and geochemistry of the Eppawala apatite-bearing carbonates provide conclusive evidence of their carbonatitic origin.     

Geological characteristics and metallogenesis of iron deposits in western Tianshan,China

The western Tianshan metallogenic belt is one of the most significant polymetallic iron metallogenic belts in China. Important advances have been achieved recently in iron exploration in the Awulale Mountain in western Tianshan, China. These newly-discovered iron deposits are mainly hosted in the basic-medium andesitic lavas and volcaniclastics, often comprising a number of high-grade ores. Magnetite is predominated in ore mineral assemblages, and pyrite, chalcopyrite, pyrrhotite or sphalerite increase in certain deposits. Wallrock alterations are intensively developed, exemplified as sodic–calcic and potassic alterations which display in different patterns as country rocks and ore-controlled structures vary. Skarn assemblages are commonly developed in ore districts like Beizhan, Dunde and Chagangnuoer, and pyroxene + albite + K-feldspar − epidote + actinolite alterations are dominated around ore bodies in Zhibo deposit, whereas the Shikebutai deposit develops alteration assemblages comprising of jasper, barite, sericite, and chlorite. Thus, iron deposits can be divided into three types including volcanic-sedimentary type, volcanic magmatic-hydrothermal type and iron skarn type. Our preliminary interpretation about the tectonic background of this iron mineralization in this area is in the late stage of a collisional–accretional orogenic belt around Carboniferous, with some extrusional–extensional tectonic transition locally. Iron mineralization is likely to have a close genetic relationship with volcanic–subvolcanic activity, syn- or slightly post- the volcanism which took place besides continental arc. Volcanic eruption contributes to majority of mineralizing iron, with minor extracted from hydrothermal replacement from wall rocks.     

豫西黄沟金矿床地质地球化学特征

通过研究黄沟金矿床中岩石地球化学特征,从而探讨太华群地层对于该区金矿床成矿的作用。研究表明(1)本区的片麻岩原岩主要为火成岩,属高钾钙碱性系列,且金矿石一般都明显贫钠而富钾、富钙,SiO2含量普遍较高,Au品位随SiO2的含量增高而呈正相关变化;(2)微量元素特征表明矿石样品以Rb、Ta、Ti等元素强烈亏损,Ba、Sm和Y元素强烈亏损为特征,与太华群片麻岩和熊耳群安山岩的标准化分布曲线型式接近,说明金矿化可能与太华群片麻岩和(或)熊耳群安山岩具有成因上的联系;(3)稀土元素特征表明多数金矿石的轻稀土略亏损、重稀土略富集,弱负铕异常与矿区出露的各类岩石的稀土元素标准化分布曲线(右倾型)不同,而具深源特性;个别金矿石的标准化曲线形态一定程度上具片麻岩或(和)安山岩的特征,呈弱负铕异常的右倾形态,暗示了太华群片麻岩和(或)熊耳群安山岩也贡献了部分成矿物质。     

Geochemical features of ore fluid for gold deposits related to alkaline rocks in China

Fluid inclusion studies of 5 gold deposits connected with alkaline rocks show that quartz separated from auriferous quartz veins contains abundant three-phase CO2-NaCl-H2O inclusions and two-phase CO2-dominated ones,measuring 5-20um in diameter,Homogenization temperatures of the fluid inclusions are mostly within the range of 150-300℃,and the salinities,mainly 0.2wt%-12 wt%(NaCl),Gold mineralizations occurred at depths of 1.4-2.8km,The most striking character of fluid composition is that among the cations,Na^ in dominant,followed by K^ ,Ca^2 ,among the anions,Cl^- is slightly higher than SO4^2-,In the evaporate,H2O is dominant,followed by CO2,The pH values are mainly within the range of 6.5-8.5,indicating that the ore-forming solutions are alkaline in nature.The hydrogen and oxygen isotopic ratios indicate that the ore fluid is composed mainly of magmatic water.With the dropping of temperature in the ore fluid,the contents of CO2 decreased while the salinity increased.The relations between Au and other components of the ore fluid are discussed in the paper,and it is concluded that in these deposits,Chlorides,H2S,SiO2,CO2,etc.in the fluid all are involved in the migration and concentration of Au.     

湖南锡矿山锑矿矿区煌斑岩的地球化学特征

对湖南锡矿山矿区煌斑岩脉进行了系统的元素和同位素地球化学研究,来探讨煌斑岩的成因。本区煌斑岩形成时代大约为120Ma,早于锑成矿时间,并与锑成矿有密切相关。其微量元素MORB蛛网图,以富集高场强元素,Nb-Ta亏损和Ti不亏损,Th强富集和Ce弱富集为特征,具有岛弧型玄武岩微量元素分配模式,构造环境可能为大陆内部破坏板块边缘。稀土元素具有总量高,无明显的负铕异常,配分模式呈轻稀土富集的右倾型特征。同位素以高^87Sr/^86Sr、低^143Nd/^144Nd为特征。结合元素地球化学、同位素特征和地质发展史分析,煌斑岩可能为富含REE和高场强元素的俯冲带流体交代过的富集地幔部分熔融所产生的岩浆,在上升过程中同化和混合很少量的矿区隐伏花岗岩浆和灰岩的产物。     

鄂尔多斯盆地西缘宁东地区侏罗系含铀地层元素地球化学特征

沉积岩的地球化学特征记录了岩石沉积时的重要地质信息,是研究沉积古环境、源区构造背景及源岩属性的有效手段。采用X衍射荧光光谱（XRF）和等离子体质谱仪（ICP-MS）对鄂尔多斯盆地西缘宁东地区中侏罗统直罗组下段地层砂岩和泥岩样品的主、微量元素及稀土元素含量进行测试分析,旨在对其沉积古环境进行恢复,并对其源区构造背景、源岩属性等特征进行综合研究。研究表明：Sr含量、Sr/Cu值、V/（V+Ni）值、U/Th值、化学蚀变指数（CIA）等地球化学指标综合指示出研究区直罗组下段地层沉积古气候为半干旱-半湿润气候,古水体环境为富氧的淡水环境。根据稀土元素（REE）特征分析,推测研究区直罗组下段地层物源主要来自盆地西北部的阿拉善古陆,源岩岩性主要为片麻岩、石榴石浅粒岩、变不等粒石英砂岩和变中细粒石英砂岩。源区构造背景主要为大陆岛弧,与大洋岛弧也有一定关联。     

琼西抱板群变质沉积岩地球化学研究

琼西中元古代抱板群变质沉积岩可分为白云母石英片岩组和石英二云母片岩组，其原岩为砂岩质泥质沉积岩夹火山物质。白云母石英片岩组和石英二云母片岩组在地球化学成分上的差异是原始沉积化学分异作用的结果。对主元素、微量元素（含稀土元素）及Sm-Nd同位素的综合研究表明，海南岛存在古元古代或更早的古老基底，抱板群变质沉积岩一部分来源于成熟度较低的古老地壳物质，另一部分来源于含地幔火山物质较多的初生地壳，或与研究区大规模造山运动、构造－岩浆活动所伴生的地幔物质加入有关。初步研究显示，琼西抱板群变质沉积岩可能是造山带岛弧和活动大陆边缘区（扩张弧后或弧间盆地）大地构造环境下的沉积产物。     

黔中—川南石炭纪铝土矿的地球化学特征

论述了黔中—川南铝土矿成矿带内早石炭世和晚石炭世铝土矿及其下伏地层的主要化学成分、稀散元素、稀土元素的地球化学特征,进而探讨了铝土矿的成矿母岩———下寒武统、下奥陶统和下志留统的泥、页岩及碳酸盐岩的成矿意义。     

碳酸盐岩中锶元素地球化学特征及其指示意义

应用锶指示古环境的方法很多,其中以锶的含量和与Sr配对的相应的元素比值作为指示剂,具有典型代表性。本文以北京下古生界碳酸盐岩为研究对象,采用ICP-MS分析技术测试了49件样品中Sr、Ba、Cu的含量。以锶的含量、Sr/Ba、Sr/Cu比值作为地化指标,深入探讨了锶的地球化学特征。系统的分析了Sr含量,Sr/Ba、Sr/Cu比值对不同的沉积环境的地质响应。指出从浅海环境到深海环境Sr含量及其相应的元素比值增加趋势明显,对沉积环境具有重要的指示作用。并且,Sr的分布还受到不同沉积旋回的影响,研究沉积作用对Sr分布的影响时,应在同一沉积旋回中进行。此外还深入研究了不同岩性、不同沉积旋回中Sr的分布特征,探讨了影响Sr分布的不同因素。归纳了锶对沉积环境的指示作用。就成岩作用对碳酸盐岩中锶分布的影响,结合前人的工作,进行了探索性的分析。