Showing papers on "Ankerite published in 1983"

Origin of Precambrian iron-formations in the Lake Superior region

Abstract: The Precambrian banded, cherty iron-formations in the Lake Superior region were deposited on broad continental shelves, and deposits of the supratidal, intertidal, and subtidal zones can be recognized in the well-known facies of the iron-formations. The supratidal deposits were primarily calcitic, aragonitic, and sideritic muds. Other minerals, in order of decreasing abundance, may have been gypsum, anhydrite, ankerite, dolomite, quartz, apatite, and halite. Intraclasts were produced by desiccation-granulation, and some of this material was redeposited in the intertidal and subtidal zones. The intertidal zone was characterized by the development of extensive algal reefs and digital-sized and larger stromatolitic mounds. Desiccation-granulation and tidal rip of mineralized stromatolites further added to the accumulation of intraclasts. The intertidal zone was the domain of granules that were derived by abrasion and rounding of intraclasts. The original mineralogy of the intertidal deposits was similar to that of the supratidal zone; however, because of photosynthesis, the sea water was oxygenated. Lozenge-shaped grains are considered to be secondary, silicified, and iron-oxide replicates of original gypsum crystals, and, by comparison with recent evaporites, some anhydrite and halite may also have been present. The deposits of the intertidal and subtidal zones are considered to be comparable to the present-day Abu Dhabi complex in the Persian Gulf. Spherical microstructures are abundant and well preserved in the chert of the subtidal deposits. Pyrite, which characterizes these deposits, probably was formed largely by the bacterial reduction of sulfate derived from the intertidal zone. The laminations in the iron-formations resulted from several causes that were controlled by chemical and mechanical processes, such as the rates and localization of the precipitation of calcitic, aragonitic, and sideritic muds and the mechanical breakup and redistribution of these sediments as sand, silt, and mud. These processes resulted in compositional and textural laminations. The diversity of the banded, cherty iron-formations, recognized in the different facies, then, can be explained largely in the diagenetic changes. The large-scale pervasive diagenetic changes were the replacement of calcite and aragonite by chert and the formation of iron silicates by the reactions between ionic silica and siderite and iron-bearing dolomite. Vein-like structures and the development of magnetite along stylolites indicate mobility of iron in late diagenetic stages, and iron-organic chelates are suggested to account for the mobility. Depending on the composition of the laminae and the chemical environment, silicification during diagenesis produced the hematite, magnetite, siderite, pyrite, and mixed-mineral facies.

Iron ores in the Fen central complex, Telemark (S. Norway): petrography, chemical evolution and conditions of equilibrium

Abstract: The Fen central complex (Brogger 1921, Saether 1957, Barth & Ramberg 1966) is an early Cam­ brian alkaline intrusive complex, situated 119 km S. W. of Oslo, ca. 20 km W of the late Paleozoic Oslo Rift (Fig. 1). The intrusive rocks range in composition from alkaline lamprophyres ('dam­ tjernite') and rocks of the urthite-ijolite-meltei­ gite association, to calcite carbonatite (sovite), dolomite carbonatite (rauhaugite l) and ankerite­ ferrocarbonatite (rauhaugite Il). They have pen­ etrated Precambrian granitic gneisses, which are fenitized along the western margin of the central complex. Iron ores associated with the carbonatites were worked from the mid-seventeen th century until 1927 (Vogt 1918, Bjorlykke & Svinndal 1960) . Vogt (1918) estimated the production during this period to one million tons of ore, with present reserves of the same order of magnitude. Mining took place at several localities in and around the central complex. The most extensive workings were in the Gruveasen area, dose to the eastem margin of the complex, where operations by the 1920's had reached a leve! 225 m below the sur­ face of lake Norsjo. The Fen iron ores are typically veins, lenses or dyke-like bodies, found within carbonatites or in carbonated gneisses in their immediate surround­ ings. (Saether 1957). They are most frequently associated with rauhaugite Il or metasomatized hematite-dolomite-calcite carbonatite (rodberg) in the central and eastem parts of the complex.

Mineralogy of the Mahogany marker tuff of the Green River Formation, Piceance Creek Basin, Colorado

Abstract: The Mahogany marker tuff is a chronostratigraphic marker which was deposited in Eocene Lake Uinta approximately 45-46 million years ago when the lake was at its maximum size. The Mahogany marker lies 3 to 6 meters above the Mahogany oil shale bed in the upper part of the Parachute Creek Member of the Green River Formation. The mineralogy of the marker was studied in drill cores by X-ray diffraction and hand specimen examination. The Mahogany marker consists of authigenic sodium feldspar, analcime, quartz, ankerite, dolomite, potassium, feldspar, calcite with lesser amounts of siderite, hematite, pyrite, undifferentiated clays, pyrrhotite, biotite, marcasite, and locally dawsonite. Analcime is not present in all samples and in samples which are analcime-free, K-feldspar shows a greater abundance. Dawsonite is locally present only in analcime-free samples. The presence or absence of analcime and K-feldspar is attributed to the geochemical conditions that existed in the lake at the time of deposition of the Mahogany marker. The evidence supports a stratified lake model of oil shale deposition, with extremely alkaline pH values existing in deeper central portions of Lake Uinta.

Mineralogy and geochemistry of Green River Formation Oil Shales, C-A Tract, Colorado

Abstract: Whole-rock chemical data are used to obtain the normative mineralogical composition of 71 samples obtained at 10-ft intervals from a drill core from the C-a Tract in the Piceance Basin, Colorado. The vertical variation of quartz, analcite, albite, orthoclase, illite, pyrite, calcite, dolomite, ankerite, magnesite, Mg-siderite, dawsonite, and nahcolite is discussed. With depth, three mineralogical ''zones'' are distinguished: Mahogany Zone (and stratigraphically higher) oil shale is characterized by dolomite > quartz > calcite, analcite, albite, orthoclase, illite; L-6 to middle of R-5 Zone oil shales are characterized by dolomite > albite, orthoclase > quartz, ankerite; R-5 to R-4 Zone oil shales are characterized by quartz > dolomite > orthoclase, albite, dawsonite, illite. As, Se, and Mo exhibit cyclic trends with depth and are generally more abundant in higher grade zones. No significant correlation exists between organic C and As, Se, or Mo on a core-wide basis. Significant positive correlations between pyrite and As, Se, and Mo suggest that these heavy metals are associated with pyrite, especially with disseminated, fine grained framboidal pyrite. Hg shows a general decrease with depth; no significant correlations exists between Hg and pyrite or organic C. F is most abundant in the feldspar-rich shales and ismore » probably present as fluorite.« less

Evidence for superstructuring in ankerite

Abstract: Ankerite from the Radmer-Buchegg and Erzberg mines, Austria, were studied by high resolution electron microscope imaging and selected area diffraction. Electron diffraction data suggest the presence of a periodic antiphase structure. The unit cell of the periodic antiphase structure of ankerite consists probably of three units: CaMg(CO3)2 as a basal unit, CaFe(CO3)2 and CaCO3. The last CaCO3 unit is connected with excess of Ca in ankerite. The upper size limit of CaFe(CO3)2 domains is 20–40 nm according to dark-field image study. Other rhombohedral dicarbonates (Zn-dolomites, Fe-poor ankerites) have probably also domain structure. The size of domains seems to be controlled by the value of misfit of domain lattice to the host lattice and by the value of octahedral distortion of units of the periodic antiphase structure.

Petrology of Quartzo-Feldspathic Schists/Phyllites Associated With Manganese Formations of North Kanara and Kumsi, Karnataka

Abstract: Rocks associated with manganese formations in Supa, Dandeli and Bisgod (North Kanara district), and Kumsi (Shimoga district) areas consist of quartz ± muscovite± biotite ± plagioclase ± chlorite ± calcite ± ankerite ± ilmenite ± pyrite ± pyrrhotite ± K feldspars ± epidote ± rutile ± apatite. Whole-rock compositions correspond to greywacke, clay/shale and suggest deposition in reducing environments. Muscovite is phengitic, Chlorite is mainly ripidolite variety. Plagioclase is dominantly albite. Ilmenite in some rocks is manganoan. Phase relations between coexisting minerals suggest that they have formed with a close aporoach to equilibrium. Compositions of coexisting pyrite and pyrrhotite, and of phengite suggest temperatures of formation of 300-400°C and pressures 2-4 kb corresponding to greenschist facies conditions.