Showing papers on "Ankerite published in 1981"
TL;DR: Based on the regular sequence and burial depth of precipitation of authigenic carbonates in a specific sedimentary facies, three diagenetic stages of carbonates are proposed in this paper.
Abstract: Carbonate concretions, lenses and bands in the Pleistocene, Palaeogene and Upper Triassic coalfields of Japan consist of various carbonate minerals with varied chemical compositions. Authigenic carbonates in freshwater sediments are siderite > calcite > ankerite > dolomite >> ferroan magnesite; in brackish water to marine sediments in the coal measures, calcite > dolomite > ankerite > siderite >> ferroan magnesite; and in the overlying marine deposits, calcite > dolomite >> siderite. Most carbonates were formed progressively during burial within a range of depths between the sediment-water interface and approximately 3 km. The mineral species and the chemical composition of the carbonates are controlled primarily by the initial sedimentary facies of the host sediments and secondarily by the diagenetic evolution of pore water during burial. Based on the regular sequence and burial depth of precipitation of authigenic carbonates in a specific sedimentary facies, three diagenetic stages of carbonates are proposed. Carbonates formed during Stage I ( 2000 m) are Fe calcite and extremely high Ca-Mg siderite; the latter is exclusively in marine mudstones. The supply of Ca is partly from the alteration of silicates in the sediments at elevated burial temperatures. After uplift, calcite with low Mg content precipitates from percolating groundwater and fills extensional cracks.
105 citations
TL;DR: Banded iron-formation is a widely distributed although relatively minor lithology in the Archean Yilgarn Block, Western Australia, occurring in supracrustal sequences composed mainly of metamorphosed volcanogenic rocks in the northern and eastern parts of the Yilgaran Block and also in highly deformed and metamorphised predominantly metasedimentary sequences in the gneissic terrain in the southwest.
Abstract: Banded iron-formation is a widely distributed although relatively minor lithology in the Archean Yilgarn Block, Western Australia, occurring in supracrustal sequences composed mainly of metamorphosed volcanogenic rocks in the northern and eastern parts of the Yilgarn Block and also in highly deformed and metamorphosed predominantly metasedimentary sequences in the gneissic terrain in the southwest. In the central part of the Yilgarn Block structural mapping and stratigraphic reconstruction indicates that individual banded iron-formation units extend over an area of at least 40 X 60 km and could extend over a much larger area. Most banded iron-formation units appear to be 5 to 50 m thick and units of 100 to 150 m are uncommon. Three scales of layering occur in iron-formation units: (a) interlayering of banded iron-formation and Fe shale. Fe shales range in thickness from several millimeters to 5 to 6 m; (b) mesobanding is the most prominent structure in the banded iron-formations; (c) millimeter scale laminations, including microbanding, occur in both banded iron-formation and Fe shales. Other structures in the banded iron-formation include pods, macules, and small spheroidal nodules. Despite modification by metamorphic and deformation processes which, among other effects, have caused thinning of mesobands, all the structures can readily be equated with those in the Hamersley Basin iron-formations. There is a wide range in metamorphic grade. At the lowest grades banded iron-formation contains quartz, magnetite, greenalite, stilpnomelane, minnesotaite, siderite, and ankerite. Fe shales contain chamosite, stilpnomelane, siderite, and pyrite. The highest grades are characterized by quartz-magnetite-clinopyroxene-orthopyroxene in iron-formation and hornblende-almandine + or - biotite assemblages in Fe shale. Mineral assemblages are essentially identical to those in many metamorphosed Proterozoic iron-formations.The bulk chemical compositions of the Yilgarn Block iron-formations are very similar to other Precambrian iron-formations from very different stratigraphic settings. Fe shales have highly variable compositions characterized by a marked positive Al 2 O 3 -TiO 2 correlation and high Cr contents.The similarity in the bulk chemical composition of iron-formations in the Yilgarn Block and other Precambrian sequences implies that deposition of these iron-formations occurred in very similar chemical environments. It is suggested that although Fe shales may be related in some way to volcanic activity, the main constituents of the banded iron-formations are not directly derived from volcanic sources. Banded iron-formations appear to have been precipitated from ocean waters which, relative to younger oceans, contained higher concentrations of Fe and Si in solution.
45 citations
TL;DR: The detection limit of iron, expressed as wt.% FeCO 3 component in the dolomite structure, is of the order of 1%. The presence of Fe in the mineral siderite may be clearly distinguished from the Fe present as ferroan dolomerite/ankerite.
39 citations
4 citations