Siderite precipitation

An increase in carbonate alkalinity in the NR subzone enhances the precipitation of carbonate cements with 6 0 compositions similar to oxic carbonates, but with a slight enrichment in Mn and Fe and depletion in C. Rhodochrosite and siderite precipitate in the MnR and FeR subzones of sediments containing large amounts of Mn- and Fe-oxides, respectively. Because the three subzones overlap, it is common to observe, such as in deep-sea sediments, that suboxic siderites and rhodochrosite are enriched in Mn and Fe, respec-... 

Siderite precipitates from reducing, non-sulphidic pore waters that evolve in the suboxic and microbial methanogenesis zones of all depMDsitional environments. These geochemical conditions occur in organic-rich sediments containing appreciable amounts of reactive iron minerals and in which the pore waters are S04 -poor meteoric or brackish (Postma, 1982). 

In marine turbidite sandstones of the central basin, following minor siderite precipitation, dolomites formed early in the zone of methanogenesis. These have Ca-rich compositions similar to dolomites reported from contemporaneous fine-grained rocks of the Monterey Formation, coastal California. The dolomites are an example of young (< 6 Ma) dolomite formation at shallow burial depth in marine pore water, and they may have undergone some recrystallization during shallow burial without resetting their initial Sr/ Sr values. 

Sporadic occurrences of minor amounts of early siderite formed at low temperature (20-40 °C), frequently in association with detrital biotites. The fluid from which siderite precipitated had a consistent oxygen, carbon and strontium isotopic composition over great distances, which is best explained as representing homogeneous mixing of Jurassic seawater and meteoric water at field scale. Carbon was predominantly supplied by organic sources. 

Three of the samples with heavy calculated carbon isotope ratios (Table IV) could be matched with the selected hypothetical dilution water. The calculated dilution ranged from 45 to 85%. For a given pH, the range was smaller (Table IV). The dilution was also calculated for siderite precipitation at pH 7, and the dilution values were similar to those calculated for pH 5 without siderite precipitation (Table IV). For three samples whose calculated isotope ratios were not greatly different from background, and one sample whose mixing curves did not intersect, a different background water composition needs to be selected. One sample (S-20) had an observed carbon isotope ratio that was heavier than observed. This sample may have been diluted by a more contaminated solution with heavier carbon. 

Of the mechanisms suggested, dilution is a likely influence. Dilution certainly occurred during water level recovery after the mines closed. The amount and influence of dilution cannot be precisely quantified, but isotope modeling suggests dilution factors of 45 to 85%. These dilution factors could saturate the water with respect to siderite, gypsum, anhydrite and cerussite, but not smithsonite or anglesite (2). The pH during oxidation is unknown, but pH 5 or below in the mine environment is not an unrealistic hypothesis, and low pH makes the calculated carbon isotope values shift toward the observed ratios. Siderite precipitation... 

Based on microbial siderite precipitated by thermophilic Fe(III)-reducing bacteria in culture. 

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