Phosphatic Rocks Research Articles

Geochemical changes in the Early Cambrian interval of the Yangtze Platform, South China: Implications for hydrothermal influences and paleocean redox conditions

The possible triggering mechanisms of the Ediacaran–Cambrian transition in South China have been debated for many years. In this study, 80 samples from Liuchapo, Niutitang and Mingxinsi Formations were collected from Well YK-1, located in southeastern Chongqing. A high-resolution chemostratigraphic study, including evaluation of rare earth elements (REEs) and trace elements, was used to elucidate the paleoenvironments of the Upper Yangtze Platform margin. The Mingxinsi gray shale and the Liuchapo Formation exhibit flat Post-Archean Australian-normalized REE patterns with weakly negative Ce anomalies, which is typical of detritus. Obvious positive Eu anomalies in chert samples of the basal Niutitang Formation imply a short-term influence of hydrothermal fluid. The REE patterns of the basal Niutitang phosphatic rocks show significant enrichment of heavy REEs, significantly negative Ce anomalies, and positive Y anomalies, similar to the REE-distribution pattern in seawater, demonstrating that these sediments were produced under oxic seawater conditions. Moderately positive Eu anomalies and positive Y anomalies of the lower Niutitang shales may indicate mixing of hydrothermal fluids and seawater. The flat REE patterns with weak Eu anomalies of the upper Niutitang shale may have been generated by hydrothermal inputs diluted by detrital materials. Dramatic increases of Ce anomalies in the basal Niutitang Formation reveal a significant decrease of the oxygen level in seawater. Trace elements, such as Ba, U, V, U, Cu, Cr, Zn, Ni and Sr, are commonly enriched in these formations compared to the upper continental crust. Authigenic abundances of Al-normalized U and Al-normalized V as well as ratios of Th/U and V/Sc are used as indicators of redox conditions. These distributions demonstrate that the redox conditions of the bottom water during the Ediacaran–Cambrian transition abruptly changed from oxic in the Nemakit–Daldynian age to anoxic in the early Tommotian age, and gradually became less reducing in the late Tommotian age and the Atdabanian age. A strong correlation between total organic carbon (TOC) concentrations and ratios of U/Al and V/Al in the entire section (except in the phosphatic rocks) implies non-sulfidic conditions. Most trace elements cannot be used as proxies for redox conditions associated with the phosphatic rocks in the basal Niutitang Formation because abundant trace elements were especially absorbed in apatite. The TOC concentrations correlated well with the redox conditions of bottom water. We infer that the section deposited in the platform margin was influenced by episodic hydrothermal activities that began at the beginning of the Tommotian age, and these activities may have terminated at the beginning of the Atdabanian age. Enrichments of organic matter that were mainly derived from the photic zone were controlled by the redox conditions of the bottom water.

Phosphatic rocks in the Lower Miocene Tomikusa Group from Central Japan

Phosphatic rocks in the Lower Miocene Tomikusa Group from Central Japan

Diverse mode of phosphatic rocks in the environs of Proterozoic Bijawar and Sonrai basins and its relevance to uranium mineralisation — A case study from central India

The Bundelkhand massif comprising a variety of Archean-Paleoproterozoic granitoids along with low grade and high-grade metamorphites and located in the centre of the Indian Plate, underwent extension during Paleoproterozoic period, resulting in the formation of homotaxial intracratonic Bijawar and Sonrai basins in the south and Gwalior basin in the northern margin. The Bijawar and Sonrai basins are typified by their characteristic sediments and basic volcanic rocks. A feature common to both the basins, is the overwhelming occurrence of phosphatic rocks across stratigraphy and lithotype in the Bijawar basin and its confinement to the basal part of the sedimentary column in Sonrai basin. Most of these rocks are primarily of marine origin, and later subjected to periods of repeated phosphatic redistribution. Multiple episodes of such phosphatisation culminates in the proliferation and enrichment of phosphate in the upper Bijawar rocks of Bijawar basin (phosphatic breccia of Hirapur-Mardeora) and lower Bijawar rocks of Sonrai basin (phosphatic breccia of Lalitpur). Apart from these established phosphatic rocks in both the basins, quartz reefs occurring in the basement as well as the lower Bijawar Malhera Chert Breccia Formation in Bijawar basin at places are endowed with anomalously high phosphate content. The phosphatic component in all the lithotypes is in the form of apatite varying in form from microcrystalline to well formed coarser crystal aggregate comprising cement, veins and botroidal encrustations. Irrespective of its spatial, temporal and paragenetic position, it invariably registers weak to moderate radioactivity, due to the presence of uranium within it, as is evident from microprobe data. Although intra-grain and inter-grain distribution of uranium is found to be random and erratic, in general, it is observed that uranium tends to be enriched in the later generation phosphates, due to secondary process of dissolution and reprecipitation. The present paper, with fresh inputs from petrological, geochemical, minerochemical and isotope data pertaining to apatite from all these diverse units, not only explores the already established association of uranium and phosphate in these basins but also provides new insight to the phosphatic quartz reef within the basement and the phosphatised arenaceous sediments of the lower Bijawar Formation.

Geochemistry of phosphate bearing sedimentary rocks in parts of Sonrai block, Lalitpur District, Uttar Pradesh, India

Major, trace and rare earth elements of phosphatic rocks around Sonrai block of Paleo-Mesoproterozoic age having phosphatic breccia, quartzite, shale, sandstone, limestone and ironstone, have been determined to evaluate their correlation, relationship with the phosphorus content, the nature of possible substitution of various elements and regional distribution pattern over the area. The study indicates that the number of elements is substituted in the apatite structures; few of them are associated with phosphate and carbonate minerals. The variable concentration of major, trace and rare earth elements in the phosphatic rocks has been influenced by various physico-chemical processes involved during weathering and leaching of the source rocks. The distribution of the major, trace and rare earth elements is controlled by the environmental variations in the sediment water interface. The majority of trace elements were mainly influenced by the principle adsorbents like the phosphate minerals in addition to clay, iron oxides and silicate minerals. The PAAS normalized REE patterns of Sonrai block of phosphorites are characterized by negative Ce anomalies and Positive Eu anomalies. It is inferred from the distribution and interrelationship of major, trace and rare earth elements that the deposition of phosphate minerals might have occurred in highly oxidizing to slightly reducing conditions in supratidal to intertidal continental margins and shallow marine environment. The deposition was controlled by marine upwelling leading to excess charge of phosphate in certain zones of phosphogenic basins, lithologic facies variations in restricted circulations of basinal waters and electrochemical factors such as negative Eh, pH and other factors, which influenced the deposition of phosphates. The replacement, precipitation in voids and fissures and diagenesis were also important mechanisms of phosphate generation in Sonrai basin. The main source for various elements may be the minerals of cratonic mass of Bundelkhand Granitic Complex, a basement of Bijawar Basin, which also provided land derived phosphorus through weathering of the terrestrial cover.

Analytical Constraints on the Measurement of the Sulfur Isotopic Composition and Concentration of Trace Sulfate in Phosphorites: Implications for Sulfur Isotope Studies of Carbonate and Phosphate Rocks

The aim of this study was to evaluate the existing methods for extracting trace sulfate from francolite and measuring its concentration and sulfur isotope composition. Phosphatic rocks were chemically and thermally treated to remove non-structural SO(4)2- in francolite, which would otherwise be inadvertently included in geochemical analyses of the structurally-bound sulfate. Acetic acid (10% v/v) proved to be effective in removing calcite, dolomite and ankerite without affecting francolite. To remove all 'easily' soluble sulfates, such as Ca-sulfates and adsorbed sulfate, rinsing with 10% v/v NaCl had to be repeated several times for most samples. For subsequent S isotope determination sample combustion at 600 degrees C was found to be an efficient way to remove non-francolite S-bearing phases. From a number of SO(4)2- detection methods tested, ICP-AES proved to be the most accurate. For francolite-sulfate recovery, our recommended protocol involved repeated rinsing of powdered phosphorites with 10% NaCl as well as NaOCl, and testing of the filtrate for SO(4)2- in each wash. If only S isotope compositions are needed, combustion at 600 degrees C with a subsequent de-ionised water rinse could be undertaken instead of repeated NaOCl rinsing for studies of both francolite and carbonate. Re-analysis of previously published data, using the new protocol, provided evidence that the use of this protocol considerably improves data quality.

Paleoceanographic constraints on Precambrian phosphorite accumulation, Baraga Group, Michigan, USA

The Palaeoproterozoic Baraga Group (ca. 1850 ± 1 Ma) of northern Michigan is a ∼ 1200 m thick sedimentary succession of marine clastic, iron formation, chert, and phosphatic sedimentary rocks that accumulated at the peak of the world's first major phosphogenic episode. Lithofacies stacking patterns are interpreted to record the flooding of the Nuna continental margin during two sea-level cycles. The base of the first sequence is marked by a transgressive lag on Archean basement that is transitional into a highstand accumulation of sandstones deposited in peritidal environments. The bottom of the second sequence is characterized by a chert and carbonate unit with numerous subaerial exposure surfaces deposited in intertidal to supratidal environments. This chert grades upward into subtidal deposits composed of interbedded organic-rich mudstone and sandstone. The highstand and falling stage systems tracts are composed of progradational deltaic deposits. Sequences were framed using the newly discovered Sudbury impact ejecta horizon as a datum. Its emplacement approximately coincides with unrelated environmental changes that increased the delivery of river-borne sediment to the Nuna margin. Iron bearing minerals ankerite, siderite, and pyrite show that ferrous iron was present in peritidal to deep marine environments. Ankerite dominates nearshore settings where pyrite is absent; pyrite occurs in deeper, progradational deltaic deposits where sulfide was produced by bacterial sulfate reduction. Phosphorite is restricted to shallow-water sediments of the first sequence and the lowstand of the second sequence. Precipitation is interpreted to be the result of Fe-redox pumping just below the sediment–water interface where photosynthetically-produced, nearshore oxygen oases impinged on the seafloor. Such shallow-water phosphorite accumulation is in stark contrast to many Phanerozoic depositional systems in which phosphogenesis occurs across the shelf. This difference likely reflects the dissimilarity in the oxygenation state of the seafloor. In the Precambrian, Fe-redox pumping and thus, phosphogenesis, was restricted to shallow-water settings with a suboxic seafloor. In the Phanerozoic, phosphorite forms in the full spectrum of shelf environments because the entire seafloor is generally well oxygenated. The concentration of bioavailable P in neritic environments during the Proterozoic may have played a major role in the development of benthic microbial ecosystems and evolving eukaryotes.

Models of the REE distribution in the black shale Bazhenov Formation of the West Siberian marine basin, Russia

Rare-earth elements abundance in black shales of the Upper Jurassic (Tithonian Stage)–Lower Cretaceous (Berriassian Stage) Bazhenov Formation is discussed. This formation is the principal oil source rocks of West Siberia. The deposits within the formation can be subdivided into two main marine groups: (a) moderately hemipelagic deposits (clayey-siliceous, including phosphatic and carbonate rocks) and low-density distal clayey turbidites (argillites), both are considered as normal and (b) silty argillites and clayey-silt rocks, which are channel deposits and considered as anomalous. The hemipelagic rocks of normal sections, which are enriched in the rare-earth elements (REE), accumulated under both slow rates of sedimentation (clayey-siliceous rocks) and faster rates of sedimentation (argillites). The channel deposits of anomalous sections, which are impoverished in the REE, accumulated exclusively under fast rates of sedimentation. Within the hemipelagic group the rate of sedimentation of the argillites was faster than of the clayey-siliceous rocks, but the REE concentration in the former rocks (140.4 ppm) is higher than in the latter group (97.4 ppm). The argillites are more than twice enriched in clayey material than clayey-siliceous rocks. It is likely that the clay fraction was the main carrier of REE in these rocks. In the channel group of rocks, the REE abundance in clayey-silt rocks (21.2 ppm) is lower than in the silty argillite (84.6 ppm), in which the clay content is elevated. With respect to redox potential the Bazhenov Formation rocks can be subdivided further into three groups, based on the degree of pyritization index (DOP): (1) the highly reducing clayey-siliceous rocks of normal sections, with high DOP; (2) the substantially reducing argillites and carbonate rocks of normal sections, with intermediate DOP; (3) the moderately reducing rocks of anomalous sections with low DOP. The rocks with the high DOP (group 1) are characterized by ΣLREE/Σ( M+ H)REE ratios between 7.37 and 7.5, whereas the rocks with the lower DOP (group 2 and 3) are characterized by ΣLREE/Σ( M+ H)REE ratios between 12.8 and 13.5. Negative Ce anomalies are either small or absent in all deposits, which is typical for reducing conditions. Thus, the Bazhenov Formation exemplifies the complex depositional conditions that influence the REE concentration in black shale. However, it is this very complexity that has contributed to the development of six separate depositional models (REE contents in ppm are given in brackets). (1) Phosphatic clayey-siliceous rocks of normal sections (367.95); (2) argillites of normal sections (130.73); (3) clayey-siliceous rocks of normal sections (85.97); (4) carbonate rocks, largely dolomites of normal sections (23.23); (5) silty argillites of anomalous sections (78.7) and (6) clayey-silt rocks of anomalous sections (19.66).

Petrography and paleoecology of carbonate- phosphatic rocks of Ratga formation (Damlouk member) Akashat Area, western desert of Iraq

This research covers the detail petrographic study of the rocks of Damlouk member, which represented the middle part of Ratga formation (Eocene) of Akashat area in the western desert of Iraq. This study including the knowldgment of the lithologic and microfacies aspects of this member and also the determination of environmental deposition for each of these microfacies obtained from petrographic analysis. From the field observations and study the rocks in thin sections under the microscope,it is clear that the studied member consist generally of about 40 meter of carbonat_dominated sequence_Rocks types are chalky,marly limestone,fossiliferous limestone with ocasional phosphatic horizons.Detailed petrographic examination of these rocks show that they are characterized by nine distinctive microfacies which include: 1 - L ime mudstone microfacies. 2 - S helly lime wackstone packstone microfacies. 3-Nummulitic lime packstone microfacies . 4-Nummulitic lime packstone to grainstone microfacies . 5-Shelly nummulitic lime packstone microfacies . 6-Foraminiferal lime wackstone microfacies. 7-Oyster_shelly lime packstone microfacies. 8-Bioclastic lime packstone microfacies. 9-a-Phosphatic lime wackstone microfacies . 9-b-Phosphatic wackstone and packstone microfacies . 9-c-Shelly phosphatic lime packstone microfacies.

Application of multivariate statistical analyses in the interpretation of geochemical behaviour of uranium in phosphatic rocks in the Red Sea, Nile Valley and Western Desert, Egypt

Factor and cluster analyses as well as the Pearson correlation coefficient have been applied to geochemical data obtained from phosphorite and phosphatic rocks of Duwi Formation exposed at the Red Sea coast, Nile Valley and Western Desert. Sixty-six samples from a total of 71 collected samples were analysed for SiO 2, TiO 2, Al 2O 3, Fe 2O 3, CaO, MgO, Na 2O, K 2O, P 2O 5, Sr, U and Pb by XRF and their mineral constituents were determined by the use of XRD techniques. In addition, the natural radioactivity of the phosphatic samples due to their uranium, thorium and potassium contents was measured by gamma-spectrometry.The uranium content in the phosphate rocks with P 2O 5>15% (average of 106.6 ppm) is higher than in rocks with P 2O 5<15% (average of 35.5 ppm). Uranium distribution is essentially controlled by the variations of P 2O 5 and CaO, whereas it is not related to changes in SiO 2, TiO 2, Al 2O 3, Fe 2O 3, MgO, Na 2O and K 2O concentrations.Factor analysis and the Pearson correlation coefficient revealed that uranium behaves geochemically in different ways in the phosphatic sediments and phosphorites in the Red Sea, Nile Valley and Western Desert. In the Red Sea and Western Desert phosphorites, uranium occurs mainly in oxidized U 6+ state where it seems to be fixed by the phosphate ion, forming secondary uranium phosphate minerals such as phosphuranylite.In the Nile Valley phosphorites, ionic substitution of Ca 2+ by U 4+ is the main controlling factor in the concentration of uranium in phosphate rocks. Moreover, fixation of U 6+ by phosphate ion and adsorption of uranium on phosphate minerals play subordinate roles.

Phosphate Prospecting Using Natural Gamma Ray Well Logging in the Khneifiss Mine, Syria

Research Article| January 01, 2002 Phosphate Prospecting Using Natural Gamma Ray Well Logging in the Khneifiss Mine, Syria JAMAL ASFAHANI JAMAL ASFAHANI Department of Geology, Atomic Energy Commission, Damascus, Syria Search for other works by this author on: GSW Google Scholar Exploration and Mining Geology (2002) 11 (1-4): 61–68. https://doi.org/10.2113/11.1-4.61 Article history received: 20 Jun 2001 accepted: 16 Jul 2003 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation JAMAL ASFAHANI; Phosphate Prospecting Using Natural Gamma Ray Well Logging in the Khneifiss Mine, Syria. Exploration and Mining Geology 2002;; 11 (1-4): 61–68. doi: https://doi.org/10.2113/11.1-4.61 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyExploration and Mining Geology Search Advanced Search Abstract Natural gamma ray well logging, an effective tool in geophysical prospecting, is used to investigate the radioactive and phosphatic layers in the Khneifiss mine in Syria. The interpretation of the gamma ray measurements, using numerical methods of analysis developed previously and applied successfully in some phosphatic areas in Syria, make it possible to define precisely the phosphate thickness from place to place in the study area. This technique has been successfully applied while studying seven boreholes in the area. Sixty-three core samples from phosphatic layers in the boreholes have been analyzed, using gamma ray spectrometry for the determination of P2O5 , U, Th, and K. Good correlation between P2O5 content and U concentration has been found. The total count gamma logs correlate reasonably well with the U core analysis, suggesting that radioactive equilibrium exists in the U decay series. These gamma logs can be therefore used effectively to quantitatively map the distribution of P2O5 and U. The characteristics of both subsurface phosphatic sand and phosphatic rocks have been investigated and outlined using a statistical approach. The affinity of uranium to some trace elements such as V, Sr, Cu, and Ni has been verified using correlation matrices of these elements. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Eumetazoan fossils in terminal proterozoic phosphorites?

Phosphatic sedimentary rocks preserve a record of early animal life different from and complementary to that provided by Ediacaran fossils in terminal Proterozoic sandstones and shales. Phosphorites of the Doushantuo Formation, South China, contain eggs, egg cases, and stereoblastulae that document animals of unspecified phylogenetic position; small fossils containing putative spicules may specifically record the presence of sponges. Microfossils recently interpreted as the preserved gastrulae of cnidarian and bilaterian metazoans can alternatively be interpreted as conventional algal cysts and/or egg cases modified by diagenetic processes known to have had a pervasive influence on Doushantuo phosphorites. Regardless of this interpretation, evidence for Doushantuo eumetazoans is provided by millimeter-scale tubes that display tabulation and apical budding characteristic of some Cnidaria, especially the extinct tabulates. Like some Ediacaran remains, these small, benthic, colonial fossils may represent stem-group eumetazoans or stem-group cnidarians that lived in the late Proterozoic ocean.

Sedimentology of Triassic Dounan-type manganese deposits, western margin, Yangtze Platform, China

Manganese (Mn) deposits formed during the Ladinian (Middle Triassic) are widely distributed along the western margin of the Yangtze Platform. All the Mn deposits occur in condensed sections (CS) and are formed in similar depositional and structural settings related to late-stage incipient spreading of the Tethys seaway. Differences in the degree of development and sedimentary facies in different areas determined the size, ore-body strike and dip, and ore composition. Four sedimentary facies occur: (1) dolomite-rich Mn rocks, (2) glauconite-bearing oolitic Mn rocks, (3) phosphatic Fe-rich Mn rocks, and (4) silica-rich Mn rocks. Type 1 formed along the margin of a passive, stable margin; type 2 formed on basin slopes with relatively low topography and deep water; and types 3 and 4 formed mainly in the Ma'erkang Basin, a deep depression on the shelf. Ore constituents partly derived from a western source area formed medium- to low-grade ores in offshore areas during the Ladinian maximum sea-level rise. Ocean currents and lower-cycle sea-level changes produced composite ore beds. Seafloor hydrothermal systems typical of Tethyan rifting may have provided Mn-bearing fluids that ascended along faults and mixed with bottom currents. Those fluids and/or diagenetic oxidation reformed and concentrated the previously formed medium- to low-grade ore bodies.

Recursos minerais não-metálicos do Estado de Mato Grosso do Sul, Brasil

A Planície do Pantanal, a maior área inundável da América do Sul, é limitada por três planaltos onde estão concentrados os recursos minerais não-metálicos do Estado de Mato Grosso do Sul. O Planalto da Bodoquena é sustentado por rochas carbonáticas proterozóicas do Grupo Corumbá, enquanto os outros dois, os planaltos de Maracaju - Campo Grande e Taquari Itiquira, são desenvolvidos sobre rochas sedimentares e vulcânicas da Bacia do Paraná. No Planalto da Bodoquena as rochas carbonáticas têm emprego na fabricação de cimento e como corretivo de solo, e para esta finalidade, são também lavrados calcários pulverulentos (tufa) em Bonito. A argila é outro importante recurso, mas é utilizada apenas na indústria cerâmica, atualmente concentrada em dois pólos, nas regiões de Rio Verde de Mato Grosso - Coxim e Bela Vista - Jardim. As rochas ornamentais estão restritas aos mármores de Bonito e aos granitos de Porto Murtinho. Existem algumas restrições com respeito aos materiais de construção, como areia e brita, devido à baixa qualidade dos produtos. No caso da areia, a lavra vem causando problemas ambientais, especialmente nos rios vizinhos aos principais núcleos urbanos. Problemas ambientais similares decorrem também da atividade dos garimpos de diamante nas regiões de Aquidauana e Coxim. Rochas fosfáticas ocorrem no Grupo Corumbá na parte leste do Planalto da Bodoquena. A Formação Botucatu, a oeste de Campo Grande, apresenta bom potencial para areia de uso industrial; entretanto, são necessários estudos geológicos específicos para a melhor avaliação econômica destes depósitos. O aproveitamento dos minerais industriais do Estado de Mato Grosso do Sul é ainda incipiente. Entretanto, as novas perspectivas econômicas e a melhoria do transporte para o Estado de São Paulo, principalmente através da Hidrovia Tietê-Paraná, deverão criar condições favoráveis ao incremento desta atividade. Por outro lado, a curto prazo deverão ser tomadas medidas para o adequado planejamento da atividade de mineração visando à redução dos impactos ambientais.

Nd isotopic variations of Chinese seawater during Neoproterozoic through Cambrian

Samples of sedimentary phosphatic rocks and manganese deposits were collected from the Neoproterozoic Sinian System and the Lower Cambrian Series in the Yangtze region, China. The results from SmNd isotopic determinations show a considerable change in Nd isotopic composition of the Chinese seawater during the Neoproterozoic Period, which was similar to that of the Panthalassa ocean, buth with smaller swing. The ε Nd( T) values of the Chinese seawater were about −4.5 in the Nantuo ice age (corresponding to the Varanger ice age), falling within the range of the Panthalassa ocean, then gradually decreased and separated from the Panthalassa ocean after the Nantuo ice age, and finally reached the lowest point (about −8.0) at the beginning of the Early Cambrian, obviously distinct from the Panthalassa ocean (−20 to −10). As one might infer, the Chinese seawater was not co-oceanic with the Panthalassa ocean in terms of Keto and Jacobsen (1988) and belonged to another major ocean as a result from reconstruction of the global palaeoceanographic pattern during the Sinian (Vendian) through the Cambrian.

A contribution to the characterization of the aerosol sources in São Paulo

Fine and coarse particles in suspension in the atmosphere were collected on two Teflon filters with a dichotomous sampler. The concentration of Al, Br, Ca, Ce, Cl, Co, Cr, Fe, K, La, Mn, Na, P, Pb,, S, Sb, Sc, Se, Si, Sm, Th, V and Zn was determined by ED-XRF and INAA. The elemental concentration was analyzed by means of linear correlation coefficients, enrichment factors and principal factors analysis in order to identify the aerosol sources. The main sources of the aerosol particles were the marine ones, resuspended soil, fuel-oil combustion, phosphatic rocks, refuse incineration and residual high-temperature processes.

Phosphorites of Rocinha Mine; Patos de Minas (Minas Gerais, Brazil); genesis and evolution of a middle Proterozoic deposit tectonized by the Brasiliano Orogeny

In central Brazil (the states of Minas Gerais and Goias) the Brasiliano orogeny ([asymp]600 Ma) delineates the Sao Francisco craton and, to the west, the Brasilia fold belt. Rocks of the fold belt are thrust to the east and display increasing deformation and metamorphism westward.Almost all the phosphate deposits are found in the Brasilia fold belt, either along its boundary with the craton or in external structural units showing polycyclic deformation and greenschist metamorphism. They occur in the metasedimentary rocks of the middle Proterozoic Paranoa Group and in the upper Proterozoic Sao Francisco Group and its equivalents. Middle Proterozoic phosphorites are economically the more important. Most are spatially and temporally associated with a stromatolite reef barrier. However, the Rocinha-Patos de Minas deposit, the only presently mined, is located south of the reef barrier. It has developed on a shallow to deep ramp between a wide internal basin with reduced oxygen conditions and limited detritus and a more oxygenated marine environment to the west.In the Rocinha mine, phosphate forms grains, angular fragments, lenses, or centimeterthick beds in schists and carbonate or glauconitic schists. The P 2 O 5 content of phosphatic rocks ranges between 10 and 15 percent. Annual production is around 200,000 tons of ore containing 24 percent P 2 O 5 . The mine area comprises four tectonic slices made up of schists, quartzoschists, and phosphatic schists, separated by subhorizontal shear planes. These are defined by tectonic breccias in which secondary apatite has recrystallized. The mine is exposed in a 50- to 75-m phosphatic sequence which is repeated in a lower slice about 100 m below mine level.The phosphatic ore is a homogeneous crypto- to microcrystalline apatite in fairly continuous centimeter-thick beds which were stretched, fragmented, and boudinaged during Brasiliano isoclinal folding and subsequent subhorizontal shearing.X-ray diffraction, IR spectrometry, and chemical analysis indicate that the metasedimentary phosphate is a weakly carbonated fiuorapatite with low F and CO 2 contents. The influence of the Brasiliano orogenesis is marked by partial substitution of F by OH. Secondary prismatic apatite, kaolinite coatings, and wavellite sheaflike structures are products of weathering and are found in mine sections and bore holes to a depth of 10 to 15 m. Secondary apatites are enriched in CO 2 compared to primary metasedimentary apatites.

Adopting remote sensing for public health

As part of a series of geoenvironmental studies on the mobilization and fate of selenium (Se) and other potentially toxic trace elements in southeast Idaho phosphate mining areas, trace element concentrations (mg kg1 dry mass) in plant samples collected along transects at the Wooley Valley Unit 1,3, and 4 waste-rock dumps were compared with samples collected from undisturbed sites at Dairy Syncline, Deer Creek, Dry Valley, Maybe Canyon, and Rasmussen Ridge. Additionally, trace-element concentrations in veg- etation samples collected from wetlands associated with mine waste-rock piles were compared with samples collected from a single reference wetland.In undisturbed areas, Se in vegetation growing in soils overlying and derived from Phosphoria Formation phosphatic rocks tended to be higher than vegetation in undisturbed Wells Limestone or Rex Chert soils. Vegetation growing in highly disturbed soils, such as those comprising waste-rock dumps, had the highest tissue Se. Vegetation in a wetland at the base of Wooley Valley Unit 4 waste-rock dump accumulated decreasing concentrations of Se with increasing distance away from the waste-rock dump along the wetland flow path. Iron oxides were observed coating wetland sediment surfaces and helped control Se bioavailability. Plant uptake, as well as coprecipitation and sorption of Se by iron oxides, were key processes in the natural attenuation of Se in this wetland.Legumes at the rock dumps contained higher Se (mean = 80 mg kg-1) than trees (mean = 52 mgkg1), grasses (mean = 18 mgkg1), shrubs (mean = 6 mgkg1), and forbs (mean = 3 mg kg-1). However, grasses were among the highest Se accumulators among plant lifeforms in contaminated wetlands, with a mean value of 53 mg kg1 Se. In most places, uptake of Cd, Cr, Cu, Mn, Mo, Ni, and Zn was below critical high levels for plants. However, Se, Cd, Cr, and Zn uptake by some plants may have been large enough to affect their growth. Several plant lifeforms had Se concentrations that surpassed the acute and chronic toxicity thresholds for grazing livestock and wildlife, posing a lethal risk to these animals. Forages, particularly legumes, sampled from waste-rock dumps had increased Mo concentrations, resulting in Cu/Mo ratios below 1. High Mo (above 10 mg kg1) and Cu/Mo ratios below 2 may cause molybdenosis in ruminants. There were instances where tree Zn content exceeded upper chronic intake for livestock/wildlife. This may pose some concern for browsing animals feeding upon trees, particularly in winter months. Based on the vegetation survey, possible remediation strategies via physical, chemical, and biologi- cal manipulations of the contaminated sites include removal of the most contaminated soils, capping contaminated soils and revegetating capping materials, application of selec- tive herbicides to remove legumes from reclaimed waste-rock dumps, and fencing some contaminated areas to better manage grazing.

Vanadium accumulation in carbonaceous rocks: A review of geochemical controls during deposition and diagenesis

Published data relevant to the geochemistry of vanadium were used to evaluate processes and conditions that control vanadium accumulation in carbonaceous rocks. Reduction, adsorption, and complexation of dissolved vanadium favor addition of vanadium to sediments rich in organic carbon. Dissolved vanadate (V(V)) species predominate in oxic seawater and are reduced to vanadyl ion (V(IV)) by organic compounds or H 2S. Vanadyl ion readily adsorbs to particle surfaces and is added to the sediment as the particles settle. The large vanadium concentrations of rocks deposited in marine as compared to lacustrine environments are the result of the relatively large amount of vanadium provided by circulating ocean water compared to terrestrial runoff. Vanadium-rich carbonaceous rocks typically have high contents of organically bound sulfur and are stratigraphically associated with phosphate-rich units. A correspondence between vanadium content and organically bound sulfur is consistent with high activities of H 2S during sediment deposition. Excess H 2S exited the sediment into bottom waters and favored reduction of dissolved V(V) to V(IV) or possibly V(III). The stratigraphic association of vanadiferous and phosphatic rocks reflects temporal and spatial shifts in bottom water chemistry from suboxic (phosphate concentrated) to more reducing (euxinic?) conditions that favor vanadium accumulation. During diagenesis some vanadium-organic complexes migrate with petroleum out of carbonaceous rocks, but significant amounts of vanadium are retained in refractory organic matter or clay minerals. As carbon in the rock evolves toward graphite during metamorphism, vanadium is incorporated into silicate minerals.

Depositional Environments of Permian Phosphoria Formation and Related Rocks, Leach Mountains, Northeast Nevada

In the Leach Mountains, northeastern Nevada, the Phosphoria Formation is represented by phosphatic rocks of the late Leonardian Meade Peak Phosphatic Shale Tongue. The Meade Peak is overlain by dolomitic siltstones and chert, including black bedded chert, of the upper Leonardian( ) to lower Guadalupian Murdock Mountain Formation. The black bedded chert is related to the Rex Chert Member of the Phosphoria Formation. There is little consensus regarding depositional environments of the Meade Peak. Also, little work has been presented on the origin of the black bedded cherts such as in the Murdock Mountain and Rex Chert. Locally, a shallow subtidal to perhaps partly intertidal depositional model for the Meade Peak is based upon characteristics of reworked phosphatic clasts, a crowded and mixed shallow-water fauna, oolite beds, and stratigraphic position between shallow subtidal to supratidal rocks. Supratidal deposition of dolomitic siltstones and units of black bedded chert in the Murdock Mountain is based largely on abundant cauliflower-shaped blebs of chalcedony with shapes resembling modern anhydrite nodules. Also, zebraic chalcedony in the black bedded chert is interpreted as a replacement product of evaporite deposits. The setting in this southwestern part of the depositional basin of the Phosphoria Formation, in the latemore » Leonardian, during the deposition of the Meade Peak, is interpreted as a network of shallow to very shallow marine basins and intervening, perhaps periodically merged shoals. Subsequently, during the late Leonardian( ) to early Guadalupian, there was a marine regression and the supratidal environment of the Murdock Mountain Formation was established.« less

Effects of hydrous pyrolysis on biomarker thermal maturity parameters: Monterey Phosphatic and Siliceous members

Hydrous pyrolysis of immature Monterey Phosphatic or Siliceous rock at progressively higher temperatures causes systematic changes in biomarker thermal maturity parameters of the generated hydrocarbons. Biomarker ratios based on proposed carbon-carbon cracking or aromatization reactions increase during hydrous pyrolysis along similar pathways for both Siliceous and Phosphatic members. An increase in these biomarker ratios is also observed for oils of increasing thermal maturity from the offshore Santa Maria Basin, although the rates of changes for each parameter differ between the hydrous pyrolysis and natural samples. Changes in some cracking parameters during maturation appear to result from differential thermal stability of the compounds rather than conversion of precursors to products.The behavior of isomerization-based biomarker ratios in these experiments is more complex than ratios based on carbon-carbon cracking or aromatization reactions. During heating, kerogen-bound precursors generate steranes and hopanes showing lower levels of thermal maturity based on isomerization ratios than those extracted from the unheated rock. Asymmetric centers in the kerogen-bound steroids and hopanoids appear to be protected from isomerization compared to those of free steranes or hopanes in the bitumen. The Phosphatic and Siliceous rocks can show different sterane or hopane isomerization ratios when heated under the same time/temperature conditions. Further, these isomerization ratios unexpectedly decrease at high hydrous pyrolysis temperatures (> 330°C) for the Phosphatic, but not for the Siliceous samples. This could be caused by the combined effects of isomerization and differential destruction of epimers, apparently mediated by rock mineralogy.Differences between the biomarker compositions of bitumens and expelled oils in these experiments mimic those caused by natural primary migration. Heavier, more polar compounds are preferentially retained in the bitumen. For example, bitumens are enriched in tri- over monoaromatic steroids, hopanes over tricyclic terpanes, and regular steranes over diasteranes compared to expelled oils. No significant fractionation of stereoisomers was observed, such as 22S vs 22R C32 17α(H),21β(H)-homohopanes or 20S vs 20R and ββ vs αα C29 5α(H)-steranes.