Authigenic vivianite and siderite microconcretions were found, respectively, in hemipelagic and deltaic facies of 600-m-long BDP-98 sediment section from Lake Baikal. Textural investigations of these microconcretions show that they are typically <1 mm in size, irregular in shape and composed of aggregated crystallites. Dissimilar orientation of vivianite and siderite crystallites suggests formation at different depths in the sediment; up to tens of centimeters for vivianite and tens of meters for siderite. Chemical analyses of both the vivianite and the siderite indicate cation composition characterized by minor amounts of Mn, Ca and Mn apart from the dominating Fe. Rather limited and distinctive carbon isotopic composition of the siderite, with δ13CVPDB values between about +13 and +16‰, implies formation of the mineral in the methanogenic zone of diagenesis. Isotopic composition of oxygen in the siderite (δ18OVPDB values between about −10 and -11‰ ) is consistent with crystallization temperature at about 10–30°C and water δ18OSMOW values between about −10 and -16‰ . The distribution of the authigenic minerals in the section suggests changes in both sedimentary facies and climate, where vivianite formation was controlled by hemipelagic depositional conditions during the Pliocene and Quaternary, whereas siderite reflects impact of deltaic conditions during the Miocene. 相似文献
Rb–Sr isotope data for siderite and fluorite from sediment-hosted epithermal mineral veins in the eastern Harz Mountains (Germany)
are presented. Several fluorite and siderite-bearing paragenetic stages have been proposed for these veins, with the most
important mineralization being related to a quartz–sulfide and a subsequent calcite–fluorite–quartz stage, which occurred
at 226±1 and 209±2 Ma, respectively. Our Rb–Sr data do not permit the identification of distinct generations of siderite and
fluorite, but rather reveal straight internal mixing relations, reflecting mixing of fluids or differential fluid–rock interaction
processes. This indicates merely two significant phases of mineral deposition related to the quartz–sulfide and calcite–fluorite–quartz
stages. It is shown that the Paleozoic sedimentary host rocks of the veins are the most likely source for the siderite Sr,
whereas fluorite displays a two-component mixture between sedimentary Sr and radiogenic Sr derived from locally occurring
Permian metavolcanic rocks.
Editorial handling: B. Lehmann 相似文献
Sediments from shallow aquifers in Bengal Delta, India have been found to contain arsenic. Rivers of Ganga-Brahmaputra system, responsible for depositing these sediments in the delta, have created a store of arsenic. Geomorphological domains with different depositional styles regulate the pattern of distribution of zones with widely different content of groundwater arsenic. The high arsenic zones occur as narrow sinuous strips confined to channel deposits. A few iron-bearing clastic minerals and two post-depositional secondary products are arsenic carriers. Secondary siderite concretions have grown on the surface of the clastic carriers in variable intensity. The quantity of arsenic in all clastic carriers is in excess of what is generally expected. Excess arsenic is contributed by the element adsorbed on the concretion grown on the surface of the carriers, which adds up to the arsenic in the structure of the minerals. Variable abundance of concretions is responsible for the variable quantity of arsenic in the carriers and the sediment samples. Fe2+ for the growth of siderite concretions is obtained from the iron-bearing clastic carriers. The reaction involves reduction of trivalent iron to bivalent and the required electron is obtained by transformation of As3+ to As5+. It is suggested that oxidation of As3+ to As5+ is microbially mediated. In the Safe zone arsenic is retained in the carriers and groundwater arsenic is maintained below 0.05 mg/l. In the Unsafe zone sorbed arsenic is released from the carriers in the water through desorption and dissolution of concretion, thereby elevating the groundwater arsenic level to above 0.05 mg/l. 相似文献
A small-scale hydrothermal uranium mineralization hosted within the siderite deposit at Hüttenberg (Eastern Alps, Austria) was re-investigated using modern scanning electron microscope (SEM) and microprobe (EMPA) methods. The uranium mineralization comprises brannerite, coffinite and uraninite, and is spatially associated with Fe-, Ni- and Co-arsenides (loellingite, rammelsbergite, safflorite), bismuth and bismuthinite, as well as rare accessory silver-bearing minerals and gold locally. The U-bearing fluids also carried P, Y and REEs, which precipitated as xenotime and P–Y coffinite. The uranium mineralization paragenetically postdates the metasomatic formation of the host siderite ore.Uraninite allows for precise single spot EMPA ages to be calculated (±2 Ma) due to its high radiogenic lead content. There is an excellent internal consistency in the uraninite data with calculated dates ranging between 77 and 84 Ma. We interpret that these record the crystallization age of the uraninite, and that any influences of lead loss or common Pb are minimal. Brannerite was also analyzed but U-total Pb microprobe data scatter (c.30–80 Ma) with evidence of lead loss, particularly in altered domains. It does, however, provide a minimum constraint on the timing of mineralization that is consistent with the crystallization age of the uraninite. Coffinite proved completely unsuitable for U-total Pb geochronology due to extreme lead loss. However, hydrothermal xenotime yields a U-total Pb age of 78 ± 5 Ma, consistent with the uraninite age. Therefore, the U-total Pb geochronological data support a late Cretaceous age for the U-mineralization of c. 80 Ma. Considering this timing constraint together with other geological and indirect thermochronological aspects, we conclude that the siderite deposit of Hüttenberg is older and formed at c. 90−80 Ma. 相似文献
The Attepe district consists of Precambrian, Lower–Middle Cambrian, Upper Cambrian–Lower Ordovician and Mesozoic formations. It contains several iron deposits and occurrences. Three types of iron-mineralizations can be distinguished in the area; (i) Sedimentary Fe-sulfide in Precambrian bituminous metapelitic rocks, and Fe-oxides in Precambrian metasandstones (SISO), (ii) vein-type Fe-carbonate and oxides composed of mainly siderite, ankerite and hematite including barite in Lower–Middle Cambrian metacarbonates of the Çaltepe Formation (HICO), (iii) karstic Fe-oxides and hydroxides essentially in the Lower–Middle Cambrian metacarbonates and the unweathered Fe-carbonates (KIO). The latter type is more widespread and located at the upper parts of the most important mineable iron deposits like Attepe deposit.
Oxygen-, carbon-, sulfur- and strontium-isotope studies have been performed on siderites and barites in the vein-type ores, and on calcites in the recrystallized Çaltepe Limestones to investigate the sources and formation mechanism of primary ore-forming constituents. The δ13C values of siderites and calcites in limestones of the Çaltepe Formation range from −10.10‰ to −8.20‰, and from −0.8‰ to 2.30‰. Both carbonate minerals show δ18O values between 17.50–18.30‰ and 16.20–23.00‰, respectively. The δ13C and δ18O isotopic variations do not indicate any direct or linear relations between siderites and limestones. However, it is possible that the carbon and oxygen isotopic compositions of carbonate minerals could be changed to some extent, when limestones were subjected to hydrothermal processes or thermal alterations during metamorphism.
The isotopic values of barites display 32.40–38.30‰ for δ34S and 12.20–14.70‰ for δ18O. The strontium isotope ratios (0.717169–0.718601) of barites and the sulfur isotope compositions of barites and pyrites suggest that there are no direct linkages of ore-forming compounds neither with a magmatic source nor with sedimentary pyrite formations in the Precambrian bituminous shales of the Attepe formation.
According to the field observations and the stable isotope data, siderites and ankerites should be formed by interaction between iron-rich hydrothermal fluids and Çaltepe limestones, whereas isotope ratios of barites indicate that they were formed by mixing of sulfur-rich meteoric waters and deeply circulated hydrothermal solutions. 相似文献
Hydrocarbon gases with unconventional carbon isotopic signatures were observed in the Solimões sedimentary basin in north-west Brazil. Siderite contents measured with a new Rock-Eval methodology in the drill-cuttings samples of the Famenian source rock were found to decrease with the increase of gas maturity and with the occurrence of the gas isotopic anomalies. Triassic diabase intrusions induced heating of the source rock, which likely resulted in the gradual oxidative dissolution of siderite as suggested by the observation of etch pits on the siderite surfaces. It is proposed that ferrous iron from the carbonate was involved in a redox reaction with water producing ferric iron and H2, then reducing CO2 and yielding an inverse correlation between siderite content and gas maturity. Alternatively, hydrogenation of highly mature kerogen by H2 derived from siderite could explain the production of 13C-rich CH4. Mass balance considerations suggest that these mechanisms may account for a significant fraction of the hydrocarbon gases generated from the Famenian source rock in the Solimões basin. 相似文献