Showing papers on "Ankerite published in 2013"

Formation of the world's largest REE deposit through protracted fluxing of carbonatite by subduction-derived fluids

Abstract: Rare Earth Elements (REE) are essential to modern society but the origins of many large REE deposits remain unclear The U-Th-Pb ages, chemical compositions and C, O and Mg isotopic compositions of Bayan Obo, the world's largest REE deposit, indicate a protracted mineralisation history with unusual chemical and isotopic features Coexisting calcite and dolomite are in O isotope disequilibrium; some calcitic carbonatite samples show highly varied δ26Mg which increases with increasing Si and Mg; and ankerite crystals show decreases in Fe and REE from rim to centre, with highly varied REE patterns These and many other observations are consistent with an unusual mineralisation process not previously considered; protracted fluxing of calcitic carbonatite by subduction-released high-Si fluids during the closure of the Palaeo-Asian Ocean The fluids leached Fe and Mg from the mantle wedge and scavenged REE, Nb and Th from carbonatite, forming the deposit through metasomatism of overlying sedimentary carbonate

The Composition and Depositional Environments of Mesoarchean Iron Formations of the West Rand Group of the Witwatersrand Supergroup, South Africa

Abstract: This paper documents the sedimentological setting, mineralogy, and geochemistry of several iron formation units interbedded with siliciclastic strata of the Mesoarchean Witwatersrand Supergroup, well known for its world-class conglomerate-hosted Au-U deposits. Four major iron formation beds, with associated magnetic mudstones, are present in two distinctly different lithostratigraphic associations, namely shale- and diamictiteassociated iron formation. The shale association is represented by the Water Tower and Contorted Bed iron formations in the Parktown Formation of the Hospital Hill Subgroup in the lower part of the succession and the diamictite association by the Promise and Silverfield iron formations in the overlying Government Subgroup. The iron formation units have been subjected to lower greenschist facies metamorphism. Oxide (magnetite and limited hematite), carbonate, and silicate facies iron formations are recognized. The iron formations typically overlie major transgressive flooding surfaces in the succession and, in turn, form the base of progradational coarsening-upward increments of sedimentation comprising magnetic mudstone, nonmagnetic shale, and interbedded siltstone-quartzite. The upward transition from iron formation into magnetic mudstone is accompanied by a change in mineralogical composition from hematite-magnetite iron formation at the base in the most distal setting through magnetite-siderite- and siderite-facies iron formation in the transition zone to magnetic mudstone. The siderite with associated ankerite displays highly depleted δ13C values, suggesting crystallization via iron respiration in presence of organic carbon. The iron formations display positive postArchean Australian shale-normalized Eu and Y anomalies with depletion in light rare-earth elements relative to heavy rare-earth elements, indicating precipitation from marine water with a high-temperature hydrothermal component. Integration of sedimentological, petrographic, and geochemical results indicates that the shale-associated iron formation was deposited during the peak of transgression, when reduced iron-rich hydrothermal waters entered the Witwatersrand Basin over a limited vertical extent due to neutral buoyancy, with the top of the plume occurring below the photic zone. It is suggested that chemolithoautotrophic iron-oxidizing bacteria, which would have been able to exploit the difference in chemistry between the iron-enriched plume water and ambient ocean water to fuel metabolic activity in the presence of limited free molecular oxygen, were responsible for precipitation of initial ferric iron oxyhydroxides. The vertical facies associations in the iron formations most likely developed in response to the limited vertical extent of the hydrothermal plume, with (from distal to proximal) hematite preserved where the base of the plume was not in contact with the basin floor, magnetite where the plume water was in contact with bottom sediment, iron-rich carbonates where organic carbon input was high, iron-rich alumosilicates where siliciclastic input became significant in more proximal settings, and iron-poor sediment above the top of the plume. Diamictite-associated iron formations in the Witwatersrand are inferred to have been deposited in a fashion similar to the shale-associated iron formations, with the exception that major transgressions and hydrothermal plume invasion were preceded by glacial ice cover. The climate warming and increased volcanic activity required could have been related to increased tectonic activity inferred for the Witwatersrand Supergroup during deposition of the glacially associated iron formations.

Crustal-Extension Ag-Pb-Zn Veins in the Xiong’ershan District, Southern North China Craton: Constraints from the Shagou Deposit

Abstract: The Shagou vein-type Ag-Pb-Zn deposit in the Xiong’ershan district, southern margin of the North China craton, is hosted within amphibolite facies metamorphic rocks of the Late Archean to early Paleoproterozoic Taihua Group. The Ag-Pb-Zn veins are localized in NE- to NNE-trending brittle faults and typically display symmetrical zoning consisting of siderite, quartz + sphalerite, galena, and quartz + calcite from the margin toward the center of each vein. Ore-related hydrothermal alteration is well developed on both sides of the veins, dominated by silicification, sericitization, chloritization, and carbonatization. Sericite separates extracted from a major Ag-Pb-Zn vein yield a 40 Ar/ 39 Ar plateau age of 140.0 ± 1.0 Ma (1 σ ) and isochron age of 141.1 ± 1.6 Ma (1 σ ), indicating that mineralization occurred at the beginning of Early Cretaceous. Field and textural relationships indicate four hydrothermal stages marked by assemblages of quartz + siderite (stage I), quartz + sphalerite + ankerite (stage II), quartz + galena + silver minerals + ankerite (stage III), and quartz + calcite (stage IV), respectively. Silver minerals are abundant in all veins and are composed of, in paragenetic order, argentiferous tetrahedrite, polybasite, jalpaite, argentite, and native silver. These silver minerals commonly occur as replacements of galena, chalcopyrite, and other sulfides, or as fillings of microfractures in sulfides and quartz. Microthermometric measurements of primary fluid inclusions in quartz, carbonates, and sphalerite from various hydrothermal stages indicate that ore minerals were deposited at intermediate temperatures (267°–157°C) from aqueous-carbonic to aqueous fluids with moderate salinities (7.2–15.9 wt % NaCl equiv). Coexisting galena-sphalerite pair yields sulfur isotope equilibrium temperatures of 205° to 267°C, consistent with the overall homogenization temperatures of fluid inclusions. The microthermometric data also indicate that both fluid mixing and fluid-rock interaction were important mechanisms for ore precipitation. Carbonate minerals (siderite, ankerite, calcite) spanning the entire mineralization history have δ 13 C V-PDB values of −5.2 to −1.4‰ and δ 18 O V-SMOW of 10.9 to 15.0‰, corresponding to calculated values for the ore fluids of −6.5 to −1.8‰ and 1.4 to 5.4‰, respectively. δ 34 S V-CDT values of sulfide minerals (pyrite, sphalerite, galena) range from 1.1 to 5.5‰, consistent with a deep-seated sulfur source. Galena separates have 206 Pb/ 204 Pb ratios of 17.472 to 17.813, 207 Pb/ 204 Pb ratios of 15.411 to 15.498, and 208 Pb/ 204 Pb ratios of 38.178 to 38.506. The isotope data, together with geological and geochronological evidence, favor a primary metamorphic source for sulfur and other components in the ore fluids. A synthesis of available data suggests that the Shagou deposit is a typical vein-type Ag-Pb-Zn deposit that formed under an extensional geodynamic setting associated with tectonic reactivation of the North China craton during the late Mesozoic, a time period that is manifested by pervasive magmatism, widespread formation of metamorphic core complexes, and development of faulted basins throughout the eastern part of the craton. Metamorphic devolatilization of the Meso-Neoproterozoic marine sedimentary rocks previously subducted beneath the Xiong’ershan district, facilitated by extensive magmatism and elevated heat flow due to lithospheric extension, could have provided large amounts of ore fluids responsible for the Ag-Pb-Zn mineralization. The NE- to NNE-trending faults affiliated with the transcrustal Machaoying fault may have acted as pathways for the upward migration of deep-seated metamorphic fluids. Mixing of the metamorphically derived fluids with meteoric waters ultimately resulted in deposition of the Ag-Pb-Zn veins in brittle extensional structures.

The application of P–T–X(CO2) modelling in constraining metamorphism and hydrothermal alteration at the Damang gold deposit, Ghana

Abstract: Orogenic gold mineralization at the Damang deposit, Ghana, is associated with hydrothermal alteration haloes around gold-bearing quartz veins, produced by the infiltration of a H2O–CO2–K2O–H2S fluid following regional metamorphism. Alteration assemblages are controlled by the protoliths with sedimentary rocks developing a typical assemblage of muscovite, ankerite and pyrite, while intrusive dolerite bodies contain biotite, ankerite and pyrrhotite, accompanied by the destruction of hornblende. Mineral equilibria modelling was undertaken with the computer program thermocalc, in subsets of the model system MnO–Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–CO2–H2O–TiO2–Fe2O3, to constrain conditions of regional metamorphism and the subsequent gold mineralization event. Metapelites with well-developed amphibolite facies assemblages reliably constrain peak regional metamorphism at ~595 °C and 5.5 kbar. Observed hydrothermal alteration assemblages associated with gold mineralization in a wide compositional range of lithologies are typically calculated to be stable within P–T–X(CO2) arrays that trend towards lower temperatures and pressures with increasing equilibrium fluid X(CO2). These independent P–T–X(CO2) arrays converge and the region of overlap at ~375–425 °C and 1–2 kbar is taken to represent the conditions of alteration approaching equilibrium with a common infiltrating fluid with an X(CO2) of ~0.7. Fluid-rock interaction calculations with M–X(CO2) diagrams indicate that the observed alteration assemblages are consistent with the addition of a single fluid phase requiring minimum fluid/rock ratios on the order of 1.

Controlling of cements and physical property of sandstone by fault as observed in well Xia503 of Huimin sag, Linnan sub-depression

Abstract: Cement content of carbonate in tight sandstone near section is much higher than that of the normal sandstones far away from the fault of well Xia503, in the Huimin sag in Linnan sub-depression. In order to understand the origin and its impact on fault sealing, analyses of the whole-rock minerals, casting thin sections, cathodoluminescence, isotope and physical properties are conducted on cores from well Xia503. It is found that δ13C varies from 0.1‰ to 0.6‰ with the average value of 0.42‰, δ18O varies from ∼13.5‰ to ∼12.3‰ with the average of ∼13.1‰, and C-O isotope plotting points are distributed in the low to moderate temperature area of the hydrothermal dolomite. According to the occupied relationship, cathodoluminescence, and C-O isotope feature, the carbonate cementation could be divided into four stages: calcites, dolomite, ankerite, and ferrocalcite. It is discovered that the carbonate cementation is negatively related to reservoir physical property, with the porosity of 4.8%, permeability of 0.37 mD, and displacement pressure of 1.97 MPa in the tight sandstone, which have increased by almost one order of magnitude compared to the porosity of 14.3%, permeability of 3.73 mD, and displacement pressure of 0.27 MPa in the normal sandstone, which is far away from the fault. Regardless of the lithology of the counterpart wall of the fault, only the displacement pressure difference caused by carbonate cementation between the tight sandstone and the normal sandstone could seal 41 m high oil column.

Investigation on Inorganic Constituents in Indian Coal and Emission Characteristics of the Particulates (PM2.5 and PM10)

Abstract: The micro-analysis of the inorganic constituents was performed on a tertiary Indian coal sample using computer-controlled scanning electron microscopy (CCSEM). A laboratory combustion experiment of the coal was also carried out to reveal the emission characteristics of particulate matters (PM2.5 and PM10). The results show the presence of major minerals like clay minerals, quartz, pyrite, and pyrrhotite forming the bulk of the mineral matter including some minor minerals, such as calcite, dolomite, ankerite, barite, oxidised pyrrhotite, and gypsum in the sample. The particle size distribution (PSD) of the included minerals is generally observed to be finer than that of the excluded ones in the coal. As a consequence, the coals rich in included minerals have smaller particles, which may affect its reactivity. The ratio of included to excluded minerals is found to be higher in majority of cases. It is found that Si mostly occurs as quartz and clay minerals, while Al mostly occurs as silicate minerals. Fe is...

The microbial precipitation of lacustrine dolomite from Permian Formation, Urumchi, Xinjiang, China

Abstract: Middle Permian Lucaogou Formation is characterized as lacustrine dark gray or gray carbonate rocks,interbeded with sandstone,thick balck mudstone and oil shale in Yangniuchang section,Urumchi,Xinjiang Province.The carbonate rocks were mainly composed of dark gray dolomicrite,then gray micrite.The dolomicrite comprised micro-crystalline dolomite,ankerite and calcite with mixture of clay-sized quartz and enrichment of organic matter.Three micromorphologies of dolomite were found under SEM,including microspheroids(9μm in diameter),micro-rods(2μm) and micron-sized(5μm) anhedral dolomite,and the micro-anhedral dolomite held the majority in dolomicrite.There were also found large amount of nanoparticles(70~150nm in diameter) in the micrite with shape very similar to micrococcus.Compared with the micrite,dolomicrite has higher Sr value,Sr/Ca ratio,Mn value,and δ18Ovalue,which indicate that the dolomicrite is formed in deeper anoxic water with higher salinity than micrite,and the dolomicrite lacks of replacement textures from micrite.The δ 13 C PDB of dolomicrite ranged from 9.2 ‰ to 15.6‰.The higher δ 13 C value may be the result of isotope fractionation led by methanogens.Such characteristics indicate that the dolomite precipitation had close relationship with the bacterial methanogenesis in methane-producing zone.

Diagenesis of Dawsonite-bearing Tuffite in Beier Sag,Hailar Basin

Abstract: The dawsonite-bearing pyroclastic rock from Damoguaihe-Yimin Formation in Beier Sag of Hailar Basin was taken as the research objectPolarization microscope,Scanning Electron Microscope,Energy Disperse Spectroscope,alizarin red-S staining,Cathode Luminescence and X-ray Diffractomer are used in this research to study its rock type,types of authigenic minerals and paragenetic sequencesIt is ascertained that the host rock type of dawsonite is tuffite and carbonates are the main authigenic mineral such as dawsonite,ankerite and sideriteContent of dawsonite is up to 25%The paragenetic sequence is shown as sideriteⅠ→ kaolinite,overgrowth quartz → dawsonite → microcrystalline quartz → calcite → ankerite → sidriteⅡ→ bitumenDawsonite is characterized by replacing feldspar,quartz,debris and kaolinite matrix,which is different from pore-filling dawsonite found in other field,implicating that these mass in tuffite could provide origin of metal ions,and reacting with diagenetic fluid under the participation of CO2 which results in precipitation of dawsoniteLarge amount of carbonates(15%~44%) developed proves that pyroclastic rock owns a relatively well capacity of CO2 fixation

New petrological data on carbonate mineralogy in the Middle Jurassic siliciclastic deposits of the Kujawy region (Polish Lowlands)

Abstract: Carbonate minerals present in bioclasts, ooids, cements and veinlets of the Middle Jurassic rocks from the Kujawy area have been studied in detail. It has been found that the bioclasts are built of calcite and manganese calcite replaced by ferroan calcite, ankerite and minerals from the siderite group. Chamosite - the primary component of the ooids - is often replaced by siderite and magnesium siderite or ankerite. Cements of sandstones and mudstones are mainly built of ankerite while fillings of the veinlets - of ankerite and ferroan calcite. Widespread cementation with iron, calcium and magnesium carbonates as well as associated metasomatosis of grains and cements are related mainly to activity of salt tectonics.

Microstructure and crystallographic preferred orientation (cpo) of hematite ores from the sishen mine, northern cape province, south africa

Abstract: Banded iron formations (BIFs) are described as banded chemical sedimentary rocks composed of major amounts of magnetite, hematite and quartz, with variable amounts of iron bearing minerals like carbonates (e.g. siderite, ankerite) and silicates (e.g. greenalite, grunerite). The BIFs are transformed to high grade iron ores by removal of quartz and transformation of almost all iron bearing minerals to hematite. On macroscopic and microscopic scale the high grade hematite ores generally reveal a more or less visible lamination caused by alternating layers of different grain sizes. In addition to the microstructure (grain size, grain shape etc.), the crystallographic preferred orientation is a powerful tool to characterize rocks and ores. The overall crystallographic preferred orientation (CPO, texture) of volumes can be determined by neutron diffraction and the local texture on small areas of polished sections by electron backscattering diffraction (EBSD). The CPO of volumes of all hematite ore types (including a conglomerate pebble) of the Sishen Mine is characterized by a weak, usually circular c-axis maximum aligned to the normal of the lamination of the ores, and seems to be typical for this essentially unmetamorphosed and undeformed ores. The crystallographic orientation of grains of the hematite ores determined locally by EBSD shows in general the same preferred orientation. But, it is remarkable that partially the CPO is quite different and inherited from grain orientations of preceding primary minerals (e.g. magnetite, goethite, siderite/calcite, ankerite/dolomite) of the Banded Iron Formation, without any microscopic visible indication.

Facies and depositional environment of the phosphate deposits at el-rashda, el-mawhoob area, dakhla oasis, western desert, egypt.

Abstract: This work which is based on the field and laboratory investigations represents to throw light on the geological, mineralogical, geochemical and environmental deposition of the phosphorite deposits at El-Rashda, El-Mawhoob area, Dakhla, Western Desert, Egypt. fifty stratigraphic sections, 19 borholes representing the study area, are measured, sampled and studied. The laboratory work comprises 69 samples for studying physical properties, 11 samples for XRD analyses, 27 thin sections and 133 samples for chemical analyses using XRF method to detect the major oxides, as well as trace element. The studies revealed that flour apatite, carbonate flourapatite, carbonatehydroxylapat, flourellestadite, ankerite, dolomite, are the main constituents of the studied phosphorite deposits. While the associated minerals are, halite, quartz, anhydrite, montmorillonite, calcite. (table.1). Chemically; the average contents of Al2O3, SiO2 are comparatively low in the study area, while the average content of MgO, Fe2O3 and SO3 are relatively high. The low contents of Al2O3, SiO2 may be attributed to locate them outside the apatite lattic. While the high content of MgO may be due to derivation of MgO from dolomite by the oxidation processes, or resulted from the negative relationship of MgO, CaO where Ca2+ substitute by Mg2+ (McConnell 1980). The iron content in phosphorites deposits is valueless for any postdepositional indications, as iron may enter the site of deposition at any stage, whether syngenetic or epigenetic. Field observations and petrographic, mineralogical, chemical studies revealed that the origin of the phosphorite deposits at the study area points to shallow marine environment and controlled by various physico-chemical conditions.