Ankerite

About: Ankerite is a research topic. Over the lifetime, 859 publications have been published within this topic receiving 23960 citations.

Pirometamorfismo em carbonatos cretácicos da Formação Jandaíra, Bacia Potiguar, Nordeste do Brasil

Abstract: This work focuses on the thermal effects caused by hypabyssal basic intrusions hosted in cretaceous carbonatic rocks of the Jandaira Formation, onshore Potiguar Basin, Northeastern Brazil. For this purpose, we used data from field work, microscopic, X-ray diffraction, electron microprobe, and whole rock lithogeochemistry of carbonates. The preserved limestones, located nearby the studied area, have carbonates grains, matrices, cements, pores, and, mineralogically, they have calcite, ankerite, dolomite, minor amount of clays (montmorillonite), and traces of quartz, microcline, pyrite and limonite. Pyrometamorphosed limestones are coarser, the fossiliferous components disappear, and the porosity is irrelevant. The modal composition of carbonates suggests as protoliths calciferous to dolomitc carbonates. Lizardite and spinel minerals can be formed within recrystallized carbonates in little to moderate affected samples, and spinel, spurrite and forsterite in strongly affected samples. Considering the geological context, the mineralogical composition and petrological diagrams from the literature, maximum temperatures and pressures of 1050 to 1200oC and 0.5 to 1.0 kbar, respectively, are suggested. Besides the petrological importance for the characterization of the pyrometamorphism, the conclusions of this research open the possibility of changing the petroliferous system of hydrocarbon reservoirs in the Potiguar Basin and other Brazilian sedimentary basins with associated magmatic rocks

Redox State of Atmosphere and Ocean at the Archaean-Palaeoproterozoic Boundary: A Case Study from the Sausar Belt, Central India

Abstract: The Sausar belt on the southern flank of the Satpura Mountain Belt of Central India comprises several generations of granite gneisses and migmatites, and the metasedimentary rocks of the Sausar Group. A suite of ~3200–2450 Ma old rocks comprising of an older Tonalite-Trondhjemite-Granodiorite (TTG) gneiss and younger quartz-monzonite plutons (Tirodi Gneiss—I) constitutes the basement complex for the Sausar Group. The contact of Tirodi Gneiss—I and the Sausar Group records the geological history at Archaean–Palaeoproterozoic boundary. A paleosol horizon at this contact contains unusual minerals such as siderite, ankerite, uraninite, and alumino-silicate minerals. The geochemical data of the paleosol indicate a reducing environment of formation and oxygen deficient conditions in the atmosphere at the time of development of this paleosol at the Archaean–Palaeoproterozoic boundary. The Sausar Group is reported to contain coarse clastics, volcanics, glaciogenic sediments, cap carbonates, and fine clastics with manganese ore deposits. Geochemical data from the cap carbonate horizon above the glaciogenic unit and manganese bearing horizons above the cap carbonate confirm presence of reducing environment in the shallow ocean.

Genesis of Pyrite Concretions: Constraints from Mineral and Geochemical Features of Longtan Formation in Anhui Province, Eastern China

Abstract: The occurrence of pyrite concretions in the Permian Longtan Formation sheds light on the paragenesis, formation conditions and regional paleoenvironment. We analyzed the mineral and geochemical characteristics of pyrite concretions using scanning electron microscopy-energy dispersive spectrometer (SEM-EDS), X-ray diffraction (XRD) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) from the Longtan Formation shales in Anhui, Eastern China. These pyrite concretions consist of two types, each with a distinct nucleus and outer layer: The former is mainly made up of quartz, bivalve fragments and minor gypsum, ankerite, siderite and pyrite, the latter consists of pyrite (FeS2) in the voids of quartz. Based on the correlation matrix and geochemical/mineralogical affinity, trace elements in the pyrite concretions fall into three groups, that is, I (Sr, Ba, Rb and K) in calcic minerals from bivalve-bearing nucleus, II (Nb, Ta, Zr and Hf) in certain heavy minerals and III (V, Cr, Co and Ni) in pyrites. Mineral assemblage and paragenetic analysis show that the formation of pyrite concretions can be divided into three stages: (1) deposition of bivalve-bearing nucleus, (2) lithification of diatoms and (3) diagenesis of pyrite. Mineral and geochemical indicators suggest that the formation environment of pyrite concretions has undergone a major shift from lagoon with intense evaporation, to strong reducing marsh.

Thermohaline and Hydrocarbon Related Diagenesis of Lower Miocene and Upper Oligocene Sediments, West Hackberry Field, Cameron Parish, Louisiana: Implications for Mass Transport and Fluid Flow.

Abstract: The diagenesis of Lower Miocene and Upper Oligocene sediments flanking the West Hackberry salt dome are documented in order to determine the effect the presence of salt domes may have in driving diagenetic reactions in detrital siliciclastic sediments. These sediments have undergone significant chemical diagenesis during progressive burial resulting in a reduction in porosity and permeability. Precipitation of analcime, siderite, and calcite plus the alteration of detrital volcanic lithic fragments and replacement of aluminosilicate framework grains by calcite are the dominant reactions which have altered the Upper Oligocene sands. Analcime and siderite are also present in the Oligocene mudstones. The isotopic composition of the Oligocene calcites is quite variable suggesting multiple generations of calcite cement, an early one precipitated from seawater and perhaps several later ones precipitated during progressive burial by warm, hydrocarbon enriched, diagenetic pore fluids similar in composition to present day brines. Petrographic evidence suggests and extraformational source is required to account for the occurrence of these authigenic minerals. The most likely sources include the dissolution of halite in the adjacent salt dome, the dissolution and alteration of volcanic lithic fragments and aluminosilicates, and the smectite to illite transformation in more deeply buried sediments. The dominant reactions effecting the composition, porosity, and permeability of the Lower Miocene sands are precipitation of calcite and iron sulfides and replacement of detrital framework grains. Iron sulfide nodules of complex mineralogy and paragenesis occur in interbedded mudstones. The isotopic composition of the calcite is relatively constant and is consistent with the hypothesis that dissolution of Jurassic anhydrite in the adjacent salt dome and oxidation of methane are the primary sources for Ca and CO$\sb3$ respectively. Salt dome anhydrite is also the likely source for the sulfur required for Miocene iron sulfide cements. The diagenesis of Lower Miocene and Upper Oligocene sediments results from the interaction of high NaCl, hydrocarbon enriched brines with the detrital mineral assemblage during progressive burial. These deposits require a dynamic open geochemical system where large quantities of material were transported into the system, most likely due to a dynamic fluid flow regime.