Biogenic components of the Baltic Sea sediments

The contents of biogenic components in 1511 samples of the Baltic Sea sediments (depth range 0–5 cm) are studied, and maps of their distribution are compiled. The sediments contain < 13.03% C_org, < 1.33% N, < 9.0% SiO_2am, < 5.0% CaCO₃, and < 1.45% P. The maximum and elevated contents of components are found in the mud of the sea deeps. The more fraction < 0.01 mm the sediments contain, the higher are the contents of components. Four facies types of carbonaceous mud, precursors of shales, have been recognized: (1) shallow-water (lagoon) lime sapropel, (2) carbonaceous mud of the shallow-water Gulf of Finland, (3) carbonaceous mud of the middle-depth Baltic Sea, and (4) laminated carbonaceous metal-bearing mud. The latter type of mud is strongly enriched in manganese and ore-forming trace elements, which points to its formation in the stagnant environment. In composition the Baltic Sea mud is similar to petroliferous mudstones of the Bazhenov Formation in West Siberia and to ancient black shales.     

Comparative geochemistry of shell phosphorites and dictyonema shales of the Baltic

Major and trace element composition of the Ordovician Obolus phosphorites and associated Dictyonema shales were determined by ICP-MS and chemical and microchemical elemental analyses. Relative to the phosphorites, the Dictyonema shales are substantially enriched in a variety of trace elements, except for As, Be, Co, Y, REE, Sr, and Pb. The Obolus phosphorites show enrichment of As, Bi, Hg, Mo, La, Y, Pb, and Sr and depletion of Ag, Ba, Be, Cd, Cr, Cu, Hf, Ni, Sc, Sn, U, V, Zn, and Zr relative to the world average phosphorite composition. The average trace element composition of the Dictyonema shales is close to the mean shale composition, except for higher contents of Mo, Hg, Pb, Se, Ta, Te, Th, V, and U and lower contents of Ba, Bi, Cd, Co, Re, Sr, and Zn. The results suggest that the change from phosphate sedimentation in aerated environments to anoxic carbonaceous sedimentation was accompanied by changes in the composition and concentration of trace elements in the sediment. Both facies show similar trends of trace element distribution indicative of the stability of the composition of seawater and terrigenous sediment input.     

Conditions of the accumulation of organic matter and metals in the bottom sediments of the Chukchi Sea

The chemical composition of bottom sediments in the Chukchi and, partly, East Siberian Seas was studied. In the south and west of the Chukchi Sea, a zone has been detected with the accumulation of sediments rich in organic carbon, an increased background content and anomalies of sulfophile metals (Mo, Zn, Hg, Ag, Au), iron-group metals (V, Ni, Co), and some PGE (Ru, Pt). This zone is confined to the neotectonic active system of rift troughs extending from the Bering Strait and eastern Chukchi Peninsula to the continental slope, where it is bounded by the Cenozoic Charlie rift basin of the Canadian hollow. The geochemical features of the carbon-enriched sediments evidence that they formed under oxygen-deficient conditions and, sometimes, in suboxic and anoxic environments near endogenic water and gas sources. The high carbon and metal contents suggest that the very fine-grained sediments in the rift troughs of the Chukchi Sea are a possible analog of some types of ancient highly carbonaceous sediments belonging to black shales.     

广东三水盆地古近系(土布)心组地球化学特征及其所指示的氧化还原条件

古近系心组红岗段是广东三水盆地的主要生油层,以一套黑—灰黑色页岩沉积为主,其富含有机质、纹层理发育良好的岩层与相对贫有机质、含微体生物化石的微扰动层交替出现。根据纹层状页岩与生物扰动页岩的C-S-Fe关系以及对氧化还原条件敏感的痕量元素(Mo、U、V、Co、Ni、Pb和Cu)地球化学记录,对这两种不同岩相的地球化学特征及其所反映的水体氧化还原条件进行了分析。纹层状页岩的TOC、S含量和DOPT值均较高,TOC-S、TOC-DOPT显示较弱的相关性;而生物扰动页岩的TOC、S含量和DOPT值相对较低,TOC-S、TOC-DOPT相关性显著。痕量元素在两种岩相中的分布也有差别,各元素在纹层状页岩中的丰度均高于生物扰动页岩,尤其是Mo和U具有较高的丰度水平、且在不同岩相中显示明显差别。黑色页岩中的C-S-Fe关系和痕量元素(特别是Mo和U)分布反映了心组红岗段沉积时期湖盆水体的氧化还原条件变化。生物扰动页岩沉积期间,底部水体主要为氧化环境,间歇性出现贫氧条件,硫酸盐还原作用发生于沉积物/水界面及其以下沉积物中。纹层状页岩形成于缺氧的底部水体条件下,水体和沉积物中均可含H2S,为有机质保存以及对氧化还原条件敏感的痕量元素的富集提供了有利条件。     

白云鄂博群黑色岩系微量元素地球化学特征及地质意义

白云鄂博群位于华北地台北缘,是一套中—新元古代裂谷沉积的产物。尖山组和比鲁特组是其中主要的黑色岩系。对以上两组中的碳质页岩和板岩的Au、Ag、As、Ba、Co、Cu、Hg、Mn、Mo、Ni、P、Pb、V、Zn、U、B、Sb等微量元素和稀土元素进行了较为详细的研究。通过元素含量特征、w(V)/w(V+Ni)、w(Zn)-w(Co)-w(Ni)三元图、稀土配分曲线、w(Ce)/w(La)、Ce和Eu异常、w(La)/w(Yb)-w(Ce)/w(La)和w(La)/w(Yb)-w(ΣREE)图解等方法对其沉积环境特征进行了探讨。微量元素含量分析显示白云鄂博群黑色岩系以富亲铜元素和亲石元素、贫亲铁元素为特征,其中富集Hg、B、As、Mn、Sb、Au、Ag、Pb、P、Ba、U、Mo等多种元素;较高的w(B)反映了黑色岩系沉积于盐度较高的水体中;高的w(V)/w(V+Ni)、w(Ce)/w(La)反映了黑色岩系形成于缺氧的沉积环境中;黑色岩系中Hg、Sb、Ba的富集和w(Zn)-w(Co)-w(Ni)图解中投点大部分靠近热水沉积区域指示了黑色岩系中有热水沉积物的参与;稀土元素配分模式、Ce和Eu异常及w(La)/w(Yb)-w(ΣREE)和w(La)/w(Yb)-w(Ce)/w(La)图解投点在两岩组中的不同特点表明尖山组沉积物以陆源沉积为主,有少量热水沉积物参与;而比鲁特组中热水沉积组分所占比例较大,形成时海水较深。     

Sedimentology,geochemistry, and biota of Volgian carbonaceous sequences in the northern part of the central Russian Sea (Kostroma region)

Lithological, geochemical, stratigraphic, and paleoecological features of carbonaceous sediments in the Late Jurassic Volgian Basin of the East European Platform (Kostroma region) are considered. The shalebearing sequence studied is characterized by greater sedimentological completeness as compared with its stratotype sections in the Middle Volga region (Gorodishche, Kashpir). Stratigraphic position and stratigraphy of the shale-bearing sequence, as well as the distribution of biota in different sedimentation settings, are specified. It is shown that Volgian sediments show a distinct cyclic structure. The lower and upper elements of cyclites consist of high-carbonaceous shales and clayey-calcareous sediments, respectively, separated by transitional varieties. Bioturbation structures in different rocks are discussed. Microcomponent composition and pyrolytic parameters of organic matter, as well as distribution of chemical elements in the lithologically variable sediments are analyzed. Possible reasons responsible for the appearance of cyclicity and accumulation of organicrich sediments are discussed.     

Formation conditions and rare-earth mineralization of Riphean carbonaceous shales of the Upper Nyatygran Subformation,Russian Far East

Graphitic and graphite varieties are distinguished in the carbonaceous shales of the Riphean Upper Nyatygran Subformation in the Melgin fragment of the Turan block, eastern Bureya Massif. The protolith of the graphitic shales had a terrigenous source related to island-arc volcanism. Pelagic sedimentation played a great role in the formation of the protolith of the graphite shale. These rocks were juxtaposed during the formation of an accretionary wedge on an active continental margin. The carbonaceous shales are characterized by high (>600 ppm) REE + Y contents, especially in the zones of brecciation and hydrothermal reworking. Detrial monazite enriched in LREE and MREE is the main carrier of REE mineralization in the graphitic shales. The main REE carrier in the graphite shales is REE phosphate (xenotime) formed during lithogenesis of sediments. Preliminary experimental treatment of the graphite shales of the Upper Nyatygran Subformation by ammonium hydrofluoride shows their potential for economic extraction of REE and Y.     

华北北部中元古界洪水庄组页岩地球化学特征： 物源及其风化作用

华北北部洪水庄组黑色页岩是中元古代的富有机质沉积,它可能记录了当时重要的地球化学信息。通过对洪水庄组页岩中的常量和微量元素特征的分析,研究了洪水庄组的物源及其风化作用。高Th/Sc、Al2O3/TiO2、La/Sc、La/Cr、La/Co、Th/Cr和Th/Co值,低Cr/Zr和TiO2/Zr比值,Euan值、Co/Y Ti/Zr关系和La Th Sc组成表明洪水庄组页岩物源主要为上地壳中的长英质花岗闪长岩。洪水庄组页岩的Al、Ca、Na和K组成表明其具有较高的化学蚀变指数（CIA）,同时,元素组成的化学风化作用轨迹反映了洪水庄组页岩受到钾交代作用的影响,可能导致CIA值被低估,其原始CIA值应在90以上。高的原始CIA、化学风化指数（CIW）和斜长石蚀变指数（PIA）表明洪水庄组物源区经历了强烈的化学风化作用。化学风化作用强度以及微量元素组成特征揭示了中元古代洪水庄组沉积时期可能处于温暖潮湿的气候条件,这与中元古代时期大气高CO2浓度以及华北板块古大陆当时位于低纬度地区重建的结果不谋而合。     

中、下扬子地区下寒武统黑色页岩微量元素富集特征

中国南方扬子地区下寒武统黑色页岩分布广泛,主要由黑色页岩、黑色白云质页岩,黑色粉砂质页岩和黑色硅质页岩等组成;并伴生有较厚的磷块岩矿床、钒矿床和镍、钼多元素硫化物矿床。对黑色页岩岩石学特征、元素地球化学特征进行系统研究,并详细分析微量元素的富集成因,结果表明：黑色页岩主要形成于静水还原的浅海-半深海缓坡环境,层状元素富集带为沉积成因,海底热流体提供了丰富物质来源。总体上,Co、Be、Mn、Ga等元素含量较低;Cu、V、Ni、Mo等元素的含量较高,多数超过北美页岩平均值;以Ni、Mo为主的多元素富集层通常位于磷块岩之上,以黑色页岩、黑色白云质页岩中的硫化物和硫酸盐矿物为主要载体,其中有机碳（TOC）含量可达12.2%以上,远高于其他元素富集层,随着页岩内硅质成分增加,Ni、Mo、Fe、Co等元素含量明显降低;以V为主的多元素主要赋存于黑色硅质页岩中的水云母,与页岩内硅质成分具有较好的正相关关系;黑色页岩中稀土元素含量为(76.22~290.67)×10-6,轻重稀土比值LREE/HREE介于1.21~4.22,说明轻稀土更为富集,Sr/Ba值为0.04~0.34,δEu正异常,且北美页岩标准化配分曲线呈现平缓左倾,为沉积过程中存在海底热水流体提供了进一步证据。     

Lake evolution and its influence on the formation of oil shales in the Middle Jurassic Shimengou Formation in the Tuanyushan area,Qaidam Basin,NW China

The non-marine Qaidam Basin is a petroliferous basin in northwest China. The Tuanyushan area is located in the Saishiteng Depression in the northern Qaidam Basin. Coal and oil shales are widely developed in the Middle Jurassic Dameigou and Shimengou formations in this area where the sedimentary sequence and controls on coal accumulation have already been well documented. However, the geochemical characteristics of lacustrine fine-grained sediments, including lacustrine oil shales, in the shale member of the Shimengou Formation and the main controlling factors of the formation of the oil shales are ambiguous. This paper aims to reconstruct the lake evolution history during the Middle Jurassic period and reveal its influence on the formation of the oil shales in this area. Oil shales and fine-grained sediments were systematically sampled to determine their mineralogical and geochemical characteristics (major, trace and rare earth elements; stable carbon and oxygen isotopes). Based on lithological variations and total organic carbon (TOC) contents, a complete third-order sequence is identified and can be further divided into four system tracts (lowstand system tract, LST; transgressive system tract, TST; highstand system tract, HST and regressive system tract, RST) that correspond to four lake evolution stages (A–D). Changes in the lake level show an initial shallow lake, followed by a continuous upward deepening trend, followed by a shallowing trend. Shallow lake facies developed in the LST, TST and RST, whereas semi-deep to deep lake facies developed in the HST. Stable carbon and oxygen isotopes indicate that the Shimengou Lake was semi-closed to closed in the Middle Jurassic. Therefore, the water properties responded strongly to climate changes. According to elemental and mineralogical analyses, a moist climate prevailed, except during the early stages of the TST (stage B1) and HST (stage C1) when there was a semiarid climate. The semiarid climate influenced the water properties and detrital input and was the major controlling factor for the formation of the higher quality oil shales within a saline water environment. In comparison, under the moist climatic condition in the HST, the stable semi-deep to deep-water environment was the major controlling factor for the formation of lower quality oil shales within a fresh water environment.     

Phosphate-rich sedimentary rocks: Significance for organic facies and petroleum exploration

Phosphorus-bearing rocks and sediments can be divided into two genetically distinct classes: phosphatic shales or limestones and phosphorites. Phosphatic shales are primary sediments in which phosphate nodules or micronodules have formed diagenetically by precipitation of calcium phosphates derived mainly from organic phosphorus. The nodules form in reducing environments at shallow depths within the sediments, where loss of phosphate by diffusion to the overlying water column is minimized. Highly biogenic sediments containing large amounts of organic matter and some fine clastic debris provide ideal environments for the formation of phosphate nodules.Phosphorites, in contrast, represent concentrated accumulations of reworked phosphate nodules which originated in phosphatic shales or limestones. Currents, wave action, recrystallization, and erosion and resedimentation are important mechanisms in the concentration process.Phosphatic shales and limestones may become excellent oil source rocks if thermal maturity is achieved. They are useful facies indicators for anoxic or nearly anoxic depositional environments, and are often associated with restricted basins, or, during certain geologic periods, with broad shelves developed during transgressions. Phosphorites, in contrast, are often correlated with sea-level regressions or uplifts. They are modest source rocks because of their low organic carbon contents and the fact that they were reworked under oxidizing conditions. Nevertheless, because phosphorites are derived from, and often grade into, phosphatic shales, they also are of potential utility in the search for oil source beds.     

Comparison of elemental composition of recent and ancient carbonaceous sediments

The concentrations of 13 macroelements and 36 microelements are determined in calcareous deposits from recent basins, namely, the Black Sea (coccolith and sapropel oozes), the Namibian shelf (diatomaceous oozes), and the Peruvian shelf (diatomaceous-terrigenous oozes). The essential similarity of the composition of the microelements is established for all three types of sediments, including calcareous, terrigenous, and siliceous sediments. The comparison of these data with the average composition of the world shales reveals similar trends of microelement distribution, which supports the former hypothesis about the comparable environment of formation of both ancient and some modern basins.     

皖南赣北地区早、中奥陶世含笔石黑色岩系地层地球化学的研究

<正> 地层地球化学是研究地球物质元素组成在地层中元素富存形式及其演化,即地层中物质元素组成的时空规律(侯德封,1959)。元素在地层中的分布和演变反映了自然条件和沉积环境的变化历史,元素的共生组合揭示岩石的成因和形成条件,所以地层地球化学是地层研究中的一个重要内容。同时,地层地球化学研究对于进一步揭示生物群兴衰、演替绝灭的历史,及其与环境介质变化之间的关系也有着十分重要的意义。 皖南赣北地区早、中奥陶世宁国组和胡乐组含笔石黑色岩系发育较好,地层出露完整,笔石化石丰富,笔石带序列齐全,生物地层研究较为详细(许杰,1934;陈旭等,1964;钱     

Pyrite geochemistry in the Toarcian Posidonia Shale of south‐west Germany: Evidence for contrasting trace‐element patterns of diagenetic and syngenetic pyrites

Authigenic pyrite grains from a section of the Lower Toarcian Posidonia Shale were analysed for their trace‐element contents and sulphur‐isotope compositions. The resulting data are used to evaluate the relationship between depositional conditions and pyrite trace‐element composition. By using factor analysis, trace‐elements in pyrite may be assigned to four groups: (i) heavy metals (including Cu, Ni, Co, Pb, Bi and Tl); (ii) oxyanionic elements (As, Mo and Sb); (iii) elements partitioned in sub‐microscopic sphalerite inclusions (Zn and Cd); and (iv) elements related to organic or silicate impurities (Ga and V). Results indicate that trace‐element contents in pyrite depend on the site and mechanism of pyrite formation, with characteristic features being observed for diagenetic and syngenetic pyrites. Diagenetic pyrite formed within anoxic sediments generally has a high heavy metals content, and the degree of pyritization of these elements increases with increasing oxygen deficiency, similar to the degree of pyritization of reactive Fe. The highest gradient in the increase of the degree of trace element pyritization with bottom‐water oxygenation was found for the elements Ni < Cu < Mo = As < Tl. In contrast, syngenetic pyrite formed within a euxinic water column typically is enriched in As, Mo and Sb, but is low in heavy metals, and the geochemical variation reflects changes in sea water composition.     

吉林省桦甸油页岩中稀土元素和微量元素的研究

对桦甸油页岩及其灰渣的矿物成分、主量元素、稀土元素和微量元素含量进行测定。结果表明：油页岩中稀土元素含量低于北美页岩(NASC)中的平均含量,REE球粒陨石标准化的分布模式曲线表现为负斜率,(La/Yb)_N的平均值大于1,属于轻稀土富集型;REE北美页岩标准化的分布模式曲线较平缓,（La/Yb）_S的平均值接近于1,轻重稀土分馏不明显。与球粒陨石和北美页岩相比,Eu有较严重的正异常。油页岩中的微量元素与北美页岩和地壳的平均值相比较,Sb、Nb、Cs、Zn、Bi、W等元素具有较高的富集度。油页岩灰渣中稀土元素和微量元素富集度均高于油页岩。     

Geology, geochemistry and genesis of BIF of Kushtagi schist belt, Archaean Dharwar Craton, India

The Banded Iron-Formation (BIF) of the Kushtagi schist belt, Dharwar Craton is interbedded with metavolcanics. The oxide fades cherty (Al₂O₃ < 2%) and shaley (Al₂O₃ > 2%) BIFs show large-scale variations in their major and trace elements abundance. Cherty Banded Iron-Formation (CBIF) is depleted in Al₂O₃, TiO₂, Zr, Hf and other trace elements like Cr, Ni, Co, Rb, Sr, V, Y and REE in comparison to Shaley Banded Iron-Formation (SBIF). Depleted REE, positive Eu anomalies and the flat to HREE-enriched pattern of CBIF indicate that Fe and SiO₂ for these BIFs were added to ambient ocean water by hydrothermal solutions at the AMOR vent sites. It is inferred that the higher amount of hydrothermal fluid flux with a higher exit temperature provided enormous quantities of iron and silica. Fine-grained sedimentation in the basin gave rise to the observed variability in the composition of BIF. During transgression a wave base was raised up, consequently deposition of CBIF became possible, whereas, during the regressive stage, these chemical sediments were buried by and/or mixed with the terrigenous sediments resulting in deposition of SBIF and interbedded shales. Volcaniclastic activity within the basin appears to have contributed significantly to the composition of some SBIF and shales. The hydrothermal exhalative hypothesis combined with the Archaean miniplate model explains most of the chemical features of the BIFs of greenstone belts.     

Geochemistry of approximately 1.9 Ga sedimentary rocks from northeastern Labrador, Canada

Fifty-eight rock chips from fifteen samples of sedimentary rocks from the Ramah Group (approximately 1.9 Ga) in northeastern Labrador, Canada, were analyzed for major and minor elements, including C and S, to elucidate weathering processes on the Earth's surface about 1.9 Ga ago. The samples come from the Rowsell Harbour, Reddick Bight, and Nullataktok Formations. Two rock series, graywackes-gray shales of the Rowsell Harbour, Reddick Bight and Nullataktok Formations, and black shales of the Nullataktok Formation, are distinguishable on the basis of lithology, mineralogy, and major and trace element chemistry. The black shales show lower concentrations than the graywackes-gray shales in TiO2 (0.3-0.7 wt% vs. 0.7-1.8 wt%), Al2O3 (9.5-20.1 wt% vs. 13.0-25.0 wt%), and sigma Fe (<1 wt% vs. 3.8-13.9 wt% as FeO). Contents of Zr, Th, U, Nb, Ce, Y, Rb, Y, Co, and Ni are also lower in the black shales. The source rocks for the Ramah Group sediments were probably Archean gneisses with compositions similar to those in Labrador and western Greenland. The major element chemistry of source rocks for the Ramah Group sedimentary rocks was estimated from the Al2O3/TiO2 ratios of the sedimentary rocks and the relationship between the major element contents (e.g., SiO2 wt%) and Al2O3/TiO2 ratios of the Archean gneisses. This approach is justified, because the Al/Ti ratios of shales generally retain their source rock values; however, the Zr/Al, Zr/Ti, and Cr/Ni ratios fractionate during the transport of sediments. The measured SiO2 contents of shales in the Ramah Group are generally higher than the estimated SiO2 contents of source rocks by approximately 5 wt%. This correction may also have to be applied when estimating average crustal compositions from shales. Two provenances were recognized for the Ramah Group sediments. Provenance I was comprised mostly of rocks of bimodal compositions, one with SiO2 contents approximately 45 wt% and the other approximately 65 wt%, and was the source for most sedimentary rocks of the Ramah Group, except for black shales of the Nullataktok Formation. The black shales were apparently derived from Provenance II that was comprised mostly of felsic rocks with SiO2 contents approximately 65 wt%. Comparing the compositions of the Ramah Group sedimentary rocks and their source rocks, we have recognized that several major elements, especially Ca and Mg, were lost almost entirely from the source rocks during weathering and sedimentation. Sodium and potassium were also leached almost entirely during the weathering of the source rocks. However, significant amounts of Na were added to the black shales and K to all the rock types during diagenesis and/or regional metamorphism. The intensity of weathering of source rocks for the Ramah Group sediments was much higher than that of typical Phanerozoic sediments, possibly because of a higher PCO2 in the Proterozoic atmosphere. Compared to the source rock values, the Fe3+/Ti ratios of many of the graywackes and gray shales of the Ramah Group are higher, the Fe2+/Ti ratios are lower, and the sigma Fe/Ti ratios are the same. Such characteristics of the Fe geochemistry indicate that these sedimentary rocks are comprised of soils formed by weathering of source rocks under an oxygen-rich atmosphere. The atmosphere about 1.9 Ga was, therefore, oxygen rich. Typical black shales of Phanerozoic age exhibit positive correlations between the organic C contents and the concentrations of S, U, and Mo, because these elements are enriched in oxygenated seawater and are removed from seawater by organic matter in sediments. However, such correlations are not found in the Ramah Group sediments. Black shales of the Ramah Group contain 1.7-2.8 wt% organic C, but are extremely depleted in sigma Fe (<1 wt% as FeO), S (<0.3 wt%), U (approximately l ppm), Mo (<5 ppm), Ni (<2 ppm), and Co (approximately 0 ppm). This lack of correlation, however, does not imply that the approximately 1.9 Ga atmosphere-ocean system was anoxic. Depletion of these elements from the Ramah Group sediments may have occurred during diagenesis.     

下扬子地区早古生代黑色岩系地球化学特征及其地质意义

下扬子地区早古生代发育3套重要的深水相黑色岩系,分别为下寒武统荷塘组和黄柏岭组、上奥陶统五峰组、下志留统高家边组和相当层位的霞乡组。27件样品的微量和稀土元素分析结果显示：此3套黑色岩系具有Li、Be、Rb、Ba、Cs、W、Th、U、Cd、Bi、Pb等元素富集,Sc、Sr,Cr、Cu、Ga、Co、Ni等元素亏损,稀土元素总量较高,轻重稀土分异明显和具负Eu异常等壳源物质特征;微量不活动元素含量和比值特征的构造环境判别表明,下扬子地区早古生代黑色岩系构造环境为大陆岛弧和活动大陆边缘,兼具被动大陆边缘特征;母岩以上地壳的再旋回沉积物和后太古宙地层沉积物为主,部分为未分异的太古宙地壳和玄武岩成分,源岩性质主要为安山岩、英安岩和沉积岩;沉积环境为缺氧的半深海、深海环境。在上述研究基础上,对物源的来源进行了探讨,认为物源主要来自江南造山带。     

Geochemical characteristics of precious metal-bearing carbonaceous sequences from the eastern Bureya Massif

This paper reports the geochemical characteristics of PGE- and Au-bearing carbonaceous shales of the Sutyr and Kimkan sequences from the eastern Bureya Massif in the Russian Far East. The weakly altered shales are chemically close to the average upper crustal shale (NASC) but differ in the lower contents of iron, manganese, magnesium, phosphorus, calcium, and REE. According to the discrminant diagrams, these sediments are similar to the modern sediments accumulated under the influence of the continental terrigenous runoff, suprasubduction volcanism, and seawaters, which suggest their relation either with the deep-water trench near the active continental margin or with the central part of the marginal sea. The iron-rich sedimentary rocks of the Kimkan Sequence, including the Kimkan iron ore, are characterized by a positive Eu anomaly, which probably indicates their rift origin. The superimposed hydrothermal alterations of the Sutyr Sequence, including sericitization, sulfidization, and formation of quartz veinlets, were accompanied by the removal of silica and the input of potassium, LREE, and MREE with the formation of a well expressed Eu anomaly. The rocks from the weakly altered to metalliferous sediments of the Kimkan Sequence show a decrease in alkalis and especially in potassium. The carbon isotope composition of the Sutyr shales corresponds to that of the biogenic carbon (δ¹³C_VPDB from ?20.7 to ?23.7‰). The shales of the Kimkan Sequence have a heavier carbon isotope composition (δ¹³C_VPDB from ?15.6 to ?19.1‰), which may indicate either partial carbon influx from an endogenous source or its formation during decarbonatization in the presence of iron.     

Geochemistry and genetic model of the ore-bearing sediments of the Parnok ferromanganese deposit, Polar Urals

The Parnok ferromanganese deposit is confined to the black shales of the western slope of the Polar Urals. The deposit area is made up of weakly metamorphosed terrigenous-carbonate rocks formed in a marine basin at a passive continental margin. Ore-bearing sequence is composed of coaliferous clayey-siliceous-calcareous shales comprising beds and lenses of pelitomorphic limestones, and iron and manganese ores. The iron ores practically completely consist of micrograined massive magnetite. The manganese ores are represented by lenticular-bedded rocks consisting of hausmannite, rhodochrosite, and diverse manganese silicates. With respect to relations between indicator elements (Fe, Mn, Al, Ti), the shales are ascribed to pelagic sediments with normal concentrations of Fe and Mn, the limestones correspond to metalliferous sediments, ferruginous sediments are ore-bearing sediments, while manganese rocks occupy an intermediate position. It was found that the concentrations of trace elements typical of submarine hydrothermal solutions (As, Ge, Ni, Pb, Sb, Zn, etc.) in both the ore types are in excess of those in lithogenic component. At the same time, the indicator elements of terrigenous material (Al, Ti, Hf, Nb, Th, Zr, and others) in the ores are several times lower than those in the host shales (background sediments). REE distribution patterns in iron ores show the positive Eu anomaly, while those in manganese ores, the positive Ce anomaly. In general, the chemical composition of the ores indicates their formation in the hydrothermal discharge zone. The peculiar feature of the studied object is the manifestation of hydrothermal vents in sedimentary basin without evident signs of volcanic activity. Hydrothermal solutions were formed in terrigenous-carbonate sequence mainly at the expense of buried sedimentation waters. The hydrothermal system was likely activated by rejuvenation of tectonic and magmatic processes at the basement of sedimentary sequences. Solutions leached iron, manganese, and other elements from sedimentary rocks and transported them to the seafloor. Their discharge occurred in relatively closed marine basin under intermittent anaerobic conditions. Eh-pH variations led to the differentiation of Fe and Mn and accumulation of chemically contrasting ore-bearing sediments.