Trace elements in supergene phosphorites

Supergene phosphorites were analyzed for Sr, Ba, Zn, Cd, Sc, Cr, Ag, and V, i.e., elements incorporated in carbonate-apatite by isomorphic substitution. The phosphorites were subdivided into four groups: (1) phosphorites related to the weathering of sedimentary rocks, (2) phosphorites related to the weathering of endogenous rocks, (3) lacustrine coprolite phosphorites, and (4) phosphorites of ocean islands. In all the phosphorites groups, Sr, Zn, and Ba were the most abundant of the trace elements, whereas Cd, Ag, and Sc showed the lowest concentrations. Variations in trace element contents between supergene phosphorites of different genetic groups or within a single group can be explained by the different compositions of weathered rocks and geochemical environments of supergene phosphorite formation. At the same time, the contents of some trace elements are correlated with the structural type of phosphorite. In particular, phosphorite crusts or only their outer parts show elevated contents of chalcophile elements (Cd, Zn, and Ag), whereas massive phosphorites and inner parts of crusts are often enriched in such lithophile elements as Sc, V, and Cr. It was found that Cd, Zn, Ag, Sr, and Ba are positively correlated with CO₂ but show negligible correlations with other constituents of carbonate-apatite.     

Rare earth elements in supergene phosphorites

An analysis of rare earth elements in various types of supergene phosphorites established the following sequence of increasing average total contents (ppm): phosphorite from Christmas Island in the Indian Ocean, 3.89; spelean coprolitic phosphorite, 21.98; phosphorite from the weathering zone of sedimentary rocks, 27.41; phosphorite from the weathering zone of endogenous rocks, 372.32; and lacustrine coprolitic phosphorite, 461.59. Supergene phosphorites, especially the most common among them from the weathering zone of sedimentary rocks, are significantly depleted relative to marine phosphorites both in average and maximum REE contents. The REE contents of supergene phosphorites are controlled by several factors, including the REE contents in the primary rocks affected by weathering, the physicochemical conditions of phosphorite formation, the presence of a biogenic component in the phosphatogenetic system, and the structural type of the phosphorites. There is a strong positive correlation within the group of light and, in part, middle REEs (La, Ce, Nd, Sm, and Eu) and between the heavy REEs Yb and Lu, whereas the correlation between these two groups is weaker or insignificant. Gd and Tb are well correlated with the elements of both groups.     

Uranium in supergene phosphorites

The distribution of uranium was studied in supergene phosphorites from the zones of the weathering of sedimentary and endogenous rocks, as well as in nonmarine coprolitic phosphorites and, to a lesser extent, phosphorites from ocean islands. These phosphorites show a diversity of the composition of their carbonate-apatite and structural characteristics. The uranium content ranges mostly from 5 to 100 ppm, with minimum and maximum values of 0.5 and 790 ppm. There is no correlation between the uranium content of a phosphorite and the type of rock with which it is connected. Lacustrine coprolitic phosphorites show elevated uranium contents (about 200 ppm). The maximum uranium content was detected in finely laminated phosphorite encrustations. The correlation analysis of the whole data set (63 samples) showed that uranium content is not correlated with any other component of phosphorites at a confidence level of 0.95. In contrast, there is a correlation between U and P₂O₅, CaO, and F for the combined set of samples from southern Siberian deposits. The significant correlation of U with Na₂O and CO₂ is variable both for southern Siberia on the whole and for particular deposits from this region.     

Problems of Iron and Phosphorus Geochemistry in the Precambrian

The localization of economic sedimentary iron ore and phosphorite resources is discussed in comparative aspects. It is shown that the major economic resources of iron ore are hosted in Precambrian rocks, whereas the phosphorites are related to Upper Phanerozoic. High-temperature hydrothermal solutions served as an important source of iron for jaspilite ores. The low P₂O₅content therein indicates that the phosphorus deposition was only weakly related to the hydrothermal activity. Thus, the hydrothermal origin of phosphorite is denied from the geochemical standpoint.     

A comparative study of Pleistocene phosphorites from the continental slope off western India

Two types of phosphorite recovered from the continental slope off western India are described. The first type, phosphorite 1, comprises a hard, grey nodule composed of carbonate fluorapatite (CFA) and calcite as major minerals. The phosphorite consists of light‐brown microcrystalline apatite containing a few skeletal fragments and planktonic foraminifera. Scanning electron microscope (SEM) studies show evidence of dissolution of skeletal calcite and filling of the resulting cavities by phosphate composed of ovoid to rod‐shaped apatite microparticles. Apatite also occurs as coatings on these particles. The P₂O₅ content of the phosphorite is 29%, and the CO₂ content of the CFA is about 4·5%. The rare‐earth element (REE) abundance (ΣREE=2·02 μg g^–1) is lower than in other modern phosphorites. The ⁸⁷Sr/⁸⁶Sr ratio and ?Nd value of this sample are 0·70921 and –9·9 respectively. The ¹⁴C age found through accelerator mass spectrometry (AMS) dating (18 720 ± 120 years BP) is much younger than that determined by the U‐series method (100 ka). The second type, phosphorite 2, comprises a friable, light‐brown nodule consisting of CFA as the only major mineral, with a CO₂ content of the CFA of 4·5%. In thin section, the phosphate is light brown and homogeneous, and a few bone fragments are present. The P₂O₅ content is 33%, and REE contents (ΣREE = 0·18 μg g^–1) are lower than in phosphorite 1. The age of phosphorite 2 is >300 ka. Phosphorite 1 appears to have formed during the late Pleistocene through replacement of carbonate by phosphate; phosphorite 2 is also of Pleistocene age but is much older than phosphorite 1. The initial substrate for phosphorite 2 was a fish coprolite, which was subsequently phosphatized during slow sedimentation under low‐energy conditions. Microbial mediation is evident in both phosphorites. The colour, density and P₂O₅ content of the phosphorites are found to be dependent on the nature of the initial substrates and physico‐chemical conditions during phosphatization. The CO₂ content of the CFA is not related to the precursor carbonate phase. The nature of sediments, rates of sedimentation and the time spent undergoing phosphogenesis at the sediment–water interface may control REE concentrations in phosphorites.     

Anatomy and origin of a Cretaceous phosphorite-greensand giant, Egypt

Late Cretaceous epicontinental phosphorites, porcelanites/cherts, dark-coloured shales, glauconitic sandstones and bioclastic and fine-grained carbonate rocks in Egypt are examined in terms of their overall depositional and diagenetic framework and stable isotopic and organic geochemical characteristics. Two main depositional realms are interpreted and correlated through sequence stratigraphic analysis: (1) a shallow hemipelagic environment accompanying initial stages of marine transgression and conducive to the formation of organic carbon-rich shales, biosiliceous sediments and thick phosphorites, and (2) a relatively high energy depositional regime accompanying sea-level fall during which deltas advanced, glauconites were reworked seaward and prograding oyster banks became periodically exposed to episodes of fresh water diagenesis, thereby promoting solution-collapse phenomena in associated cherts. Lenticular to massive phosphorites are viewed as the result of current winnowing and concentration of authigenic grains initially precipitated in associated reducing shales and biosiliceous sediments. In eastern Egypt the phosphorites form winnowed lag layers, some of which may have been redeposited down slope in structural lows. In the west, these sands were concentrated into giant phosphorite sand waves built by reworking of penecontemporaneously deposited phosphatic muds during marine transgression. Carbon isotopic results substantiate interpretations from modern deposits for limitation of phosphate mineral precipitation with depth in sediments as a result of lattice poisoning. However, direct desorption of phosphorus to pore waters from detrital iron-oxyhydroxide phases also may have been important in the Cretaceous setting, the iron reduced in this process being available for incorporation in glauconites. The main locus for authigenic glauconite precipitation appears to be where iron fluxes from regions of lateritic weathering were highest and near the boundary between oxygenated and reduced waters. This study suggests a model for the common coexistence of glauconites and phosphorites in the geological record. Although upwelling is often advocated as the origin of nearly all giant phosphorite deposits, we suggest that some of these may have been strongly influenced by fluvially derived phosphorus borne on particulates and desorbed from these compounds upon flocculation and/or reduction in bottom waters or pore waters.     

Iodine in karst-type phosphorites from the Lahn region,Germany

The concentration of iodine in 31 Tertiary continental phosphorites from the Lahn region in Germany ranges from about 0.01% to 0.18%, the mean value of 0.044% being quite compatible with that of typical commercial-grade iodine deposits. Microscopic and X-ray diffractometric studies manifested three main mineral phases in these phosphorites viz. francolite, clay minerals (kaolinite, illite), and ferric oxides (goethite, hematite). These mineral phases were investigated for iodine distribution, mode of its fixation and possible source. Sample heating at 600°C evinced no appreciable organic matter in the sample and further showed that the iodine was non-volatile in character. Regression analysis revealed a good correlation between I and Al (Ti), practically no correlation with Fe and negative correlation with P₂O₅. It was thus deduced that the iodine content is inorganic in nature and bound principally and probably internally to Al-bearing minerals. An iodine content of about 150 ppm is believed to be associated with the apatite phase of the phosphorite.Devonian volcanic rocks are assumed to be the ultimate source of both phosphorus and iodine. Those elements were released during a Tertiary weathering period, their accumulation being controlled by chemical environment and karst topography of a Devonian reef limestone.     

Uranium in Phosphorites

The uranium concentration in phosphorites on continents and modern seafloor varies from 0.nto n· 10²ppm (average 75 ppm). The average uranium concentration is 4–48 ppm in Precambrian and Cambrian deposits, 20–90 ppm in Paleozoic and Jurassic deposits, 40–130 ppm in Late Cretaceous–Paleogene deposits, 30–130 ppm in Neogene deposits, and 30–110 ppm in Quaternary (including Holocene) deposits. On the whole, the variation range is almost similar for phosphorites of different ages. The U/P₂O₅ratio in phosphorites ranges from less than unity to 24 · 10^–4(average 3.2 · 10^–4). Major phosphorite deposits of the world with ore reserves of approximately 250 Gt (or 58 Gt P₂O₅) contain up to 19 Mt of uranium. Uranium is present in phosphorites in the tetra- and hexavalent, i.e., U(IV) and U(VI) forms, and their ratio is highly variable. At the early diagenetic stage of the formation of marine phosphorites in a reductive environment, U(VI) diffuses from the near-bottom water into sediments. It is consequently reduced and precipitated as submicroscopic segregations of uranium minerals (mainly uraninite) that are probably absorbed by phosphatic material. During the subsequent reaction between phosphorites and aerated water and the weathering in a subaerial environment, uranium is partly oxidized and lost. The uranium depletion also occurs during catagenesis owing to a more complete crystallization of calcium phosphate and replacement of nonphosphatic components.     

陕南天台山、茶店一带磷块岩的时代和成因特征

陕西南部汉中天台山、勉县茶店和略阳何家岩、金家河一带磷块岩的时代,目前有三种看法,即:泥盆纪、震旦纪和寒武纪。这一带磷块岩的成因也比较复杂。在磷酸盐沉积、成岩以后,又受到变质、褶皱、断裂和水溶液活动影响。本文对天台山、茶店一带的磷块岩的成矿时代和生成特点提出一些初步意见。     

The incorporation of uranium into diagenetic phosphorite

The behavior of U during the diagenetic formation of marine phosphorite has been modelled. The model examines a dissolution-reprecipitation replacement of skeletal hydroxyapatite, calcium carbonate and earlier generated francolite by francolite. The amount of organic matter consumed relative to the mass of francolite formed, the replacement reaction progress, and the concentration of U in the replaced phases are the important parameters which dictate the concentration of U in the phosphate rock.A partition coefficient between apatite and interstitial solution was calculated, and is $\text{λ}{}_{\mathrm{U}}{}^{\mathrm{F}}\text{= 0.57}$.Natural phosphorites have been examined and are discussed in the light of the proposed model. The U mass-balance in a Recent phosphorite is in good accord with theoretical predictions. Differences in U concentrations between sea-floor phosphorites are explained either by the (original) variation in the organic matter distribution in the corresponding sediments and/or by mineralogical differences (CaCO₃vs. hydroxyapatite) therein.Senonian phosphate rocks which were formed via the francolite → francolite transformation, demonstrate that during that process the organic matter content in the sediment was approximately 50%.The model supports the idea that phosphorite rock formation is a multistage process.     

我国西南地区震旦系陡山沱组微体遗迹化石的发现

目前世界上发现的最古老并得到普遍承认的动物化石是澳大利亚南部的伊迪卡拉动物群^[15,16]。伊迪卡拉山的晚先寒武纪砂岩中发现有大型的相当进化的水母和蠕虫化石,其中含有直径数十公分的狄更逊虫(Dickinsonin costata)、体长3公分的枝沙蚕(Spriggina floundersi)以及遗迹化石等。其所在地层为寒武纪灰岩之下160米的庞德砂岩^[1]。     

Origin of PhosphoriteNodules of Lebedinsky Iron Deposit in Kursk Magnetic Anomaly （KMA） of the Russian Platform

INTRODUCTION TheKurskmagneticanomaly(KMA)coversnear ly120000km2andhasalengthof600kminSE NW direction,withawidthrangingfrom150kmto250 km.TheKMAbasinislimitedbytheDoneskPaleozoic massifinthesouthandthePrecambrianAzovsk podol skupliftinthesouth westanditsnorthernborderis markedbythecrystallinemassifupliftofVaronesh.0 c s TheLebedinskycomplexdepositislocatedinthecentral partoftheRussianplatformbetweenlatitude50°00′and50° 20′north,andlongitude34°00′and39°00′east,inthefron ti…     

Effect of the depositional environment on the compositional variations among the phosphorite deposits in Egypt

Late Cretaceous economic phosphorites from the Red Sea, Nile Valley, and Abu Tartur areas, Egypt, show distinct variations in the lithology of associated sediments, mineralogy of nonphosphatic constituents, and distributions of major and trace elements. In the Red Sea area, the phosphorite beds are intercalated with laminated black shales, and the nonphosphatic constituents are detrital quartz and calcite, ankerite, and pyrite cements. In the Nile Valley, the phosphorite beds are intercalated with chert, marl, and sandstone and the nonphosphatic constituents are detrital quartz and calcite and chalcedony cements. In the Abu Tartur Plateau, the phosphorite beds are intercalated with laminated black shales, and the nonphosphatic constituents are detrital quartz and ankerite and pyrite cements. The phosphorites studied also show distinct variations in major- and trace-element concentrations. The Abu Tartur phosphorites have higher contents of TiO₂, Al₂O₃, Fe₂O₃, K₂O, Co, Nb, Pb, Sr, Th, Y, and Zr and lower SiO₂, Ba, and U contents as compared to those in the Red Sea and Nile Valley areas. The positive correlations between Al₂O₃ and TiO₂, K₂O, Nb, Y, and Zr suggest the detrital origin of these constituents.Similarity in the phosphatic constituents, which were derived from outside the depositional sites, and variations in the lithology of associated sediments and the mineralogy and geochemistry of the nonphosphatic constituents, which reflect the conditions at the depositional sites, suggest that the variations in the depositional environment of the phosphorites are the potential controlling factor of the compositional variations among these phosphorites. The abundance of black shales in the Red Sea and Abu Tartur areas, as well as the occurrence of ankerite and pyrite as cementing materials for the phosphatic constituents, might reflect reducing conditions in these areas, while the abundance of siliciclastic sediments and calcite and chalcedony cements suggests oxidizing conditions in the Nile Valley. The reducing conditions in the Red Sea and Abu Tartur areas were probably developed within the pre-existing depressions in a shelf environment. These depressions might have formed as a result of a change in the movements of the North Atlantic, Eurasian, and African Plates during the late Santonian, which led to transgressive inversion of rifts along northern Egypt and consequent folding in the continental interior. The higher contents of detrital components in the Abu Tartur phosphorites compared to the Red Sea and Nile Valley areas suggest more detrital inputs during the deposition of the phosphorites in Abu Tartur. The products of the diagenesis and weathering of these deposits also reflect the variations in the depositional conditions.     

Lower Eocene phosphorites of the western desert,Iraq

Marine sedimentary phosphorites of Eocene age (Upper Ypresian) are exposed in the extreme west of Iraq within the Dammam Formation. They are associated with limestone and chert, and their deposition seems to have taken place in a shallow marine environment within a structurally controlled basin open to the sea from the northern and western sides only.The studied phosphorites are granular in texture, coarse-grained and cemented by calcite which is occasionally silicified. Bone fragments are present in small amounts. Carbonate-fluorapatite is the only phosphate mineral detected in these phosphorites, with relatively high amounts of the components SO₄^?2, CO₃^?2, F^?1, H₃O¹ and Na¹ substituting in the crystal structure.The Lower Eocene phosphorites of Iraq are part of the Tethyan phosphorite province, and are comparable in many aspects with those of Paleocene and Upper Cretaceous age in the Western Desert of the country.     

荆襄地区陡山沱组磷块岩的类型、成因和沉积环境

本文根据野外调查和室内初步研究成果并结合有关地质资料,分析、讨论了荆襄陡山沱组含磷岩系的磷块岩类型、成因和沉积环境。通过分析可知,组成磷块岩的磷灰石有不同的形成方式;磷块岩沉积在海退旋回中粘土和硅质沉积向碳酸盐沉积的转变阶段;磷块岩矿床形成在陆缘海内与深水有联系的浅水地带。 矿区含磷岩系特征综合于图1。     

Formation of dawsonite mineralization at the Zaozernyi deposit,Belarus

Systems of silty sand-water and clayey rock-water were simulated under open-system conditions with respect to CO₂ and oxygen at 25°C. It was shown that the dawsonite(+ quartz)+kaolinite assemblage at the Zaozernyi deposit was formed by interaction of terrigenous sediments with chloride-sodium solutions containing NaCl >100 g/kg H₂O and saturated in carbonic acid (a partial pressure of CO₂ > 0.5 bar). The modeling results do not confirm the possibility of dawsonite formation after primary bauxites (products of kaolinite weathering crusts), which is presumably related to the inconsistent formation parameters of these minerals during weathering of aluminosilciate rocks.     

朝阳磷矿磷块岩的类型和成因特征

朝阳磷矿的磷块岩有五种类型,即泥晶磷块岩、球粒磷块岩、鲕粒磷块岩、内碎屑磷块岩和细晶磷块岩。其中内碎屑磷块岩占优势。磷酸盐岩沉积在海退旋回中,是处于深水向浅水过渡的层位。磷块岩的沉积环境主要是隆起附近的浅水带。     

Phosphorites of the Chiatura manganese deposit and specific features of their formation

The paper presents results of the detailed study of phosphorites from manganiferous beds of the Chiatura deposit. The relatively high-grade (P₂O₅ 20–28%) phosphorites are represented by various rocks ranging from the variety dominated by massive phosphates with a rare aleuritic admixture of quartz and feldspar grains to rocks mainly composed of terrigenous material with phosphates in the matrix. Phosphates make up the matrix of various organic remains: differently preserved diatom algae and microbial species. Some relatively large organic remains (in particular, sponge spicules) are typically composed of iron minerals (with manganese admixture) rather than phosphates. Manganese ores comprise phosphorite fragments composed of phosphatized cyanobacterial mat. Phosphorites of the Chiatura deposit were likely formed in a shallow-water zone away from the continental land.     

Paleoclimate changes in the late precambrian: Evidence from the upper precambrian section of the South Urals

The climatic impact on the formation of fine-grained rocks from the Riphean stratotype and Vendian Asha Group on the western slope of the South Urals during the time interval lasting approximately 1200 Ma is considered. It is shown that these rocks are largely represented by “tectonosilicate-dominated” shales. This feature combined with changes in the average K₂O/Al₂O₃ values disavows the hypothesis in (Kennedy et al., 2006), according to which the growth of free oxygen concentration in the Late Riphean and Vendian atmosphere was determined by gradual intensification of the organic carbon extraction from the biosphere by clays. The average values of the hydrolyzate module, chemical index of alteration (CIA), and several lithogeochemical parameters calculated for the Riphean and Vendian clayey rocks provide grounds for the conclusion that intensity of weathering in paleodrainage areas during the accumulation of the Upper Precambrian sedimentary successions was low. The curve reflecting changes of the average CIA values in the Upper Precambrian fine-grained siliciclastic rocks of the South Urals is similar to some extent with the “standard” CIA_correct. curve (GonzalezAlvarez and Kerrich, 2012). It is assumed that changes in microand macrobiotic communities during the Late Precambrian were controlled to a variable extent by climate fluctuations as well. At the same time, these fluctuations most likely left the chemical composition of water in the ocean virtually unchanged, which is evident from analysis of the redox conditions in the ocean and the distribution of primary producers with the average CIA_correct. and CIA values.     

扬子地台早寒武世梅树村早期的古地理及其磷块岩展布特征

梅树村早期的地层,是在海退背景下形成的一套富含小壳动物化石的白云岩、磷块岩和硅质岩等岩石组合,主要分布扬子西区。该期的古地理轮廓,是在灯影期古地理的基础上演化而来的,是一个四周被深水环绕的较为特殊的碳酸盐台地。工业磷块岩主要分布于靠近“康滇古岛”东侧较拗陷的地区,成磷盆地呈近于南北向带状依次间隔展布,可能是受近南北向同生断裂和近东西向隆起、拗陷的控制有关。