Pedogenic siderites fossilizing Ediacaran soil microorganisms on the Baltica paleocontinent

Ediacaran siderite-bearing sedimentary rocks from the western part of the East European craton exhibit features typical of redoximorphic paleosols, including pedogenic siderite (e.g., sphaerosiderite) with uniform and negative δ18O and highly variable δ13C values. The siderite precipitated in water-logged soils in a hot and humid climate, and represents a rare example of pedogenic siderite formed before the rise of vascular plants. Morphology of soil microorganisms was preserved in three dimensions due to the early siderite precipitation. These are mainly filamentous and tube-like threads, which might belong to cyanobacteria or fungi, and spherical structures resembling green algae. This microbial life fostered development of strongly reducing soils in tropical wetlands on the Baltica paleocontinent. The Ediacaran sediments of the western East European craton, traditionally regarded as marine, are reinterpreted as containing sections affected by freshwater conditions, documenting the presence of microbial life on the Ediacaran land.

Arsenic mobilization in the piedmont area of the Hetao basin: an insight from a reactive transport model

Groundwater showed a trend from oxic-suboxic conditions in the alluvial fan to anoxic conditions in the flat plain in the Hetao basin, with increases in As, Fe2+, NH4+, alkalinity and pH, and decreases in Eh and NO3-. A 1D reactive transport model was constructed using a model flow column by PHREEQC to quantitatively interpret geochemical processes related to As mobilization in a 5000 m-long hydrogeological unit. Results indicated that groundwater As concentration was mostly controlled by Fe(III) oxide reduction and As adsorption processes. Observed Fe2+ was the combination of Fe(III) oxide reduction and mackinawite/siderite precipitation. Increasing alkalinity was the result of oxidation of organic carbon, rather than the dissolution of carbonates.

Open versus closed mesogenetic systems in Cretaceous fluvial and tidal sandstones, Sirt Basin, Libya

ABSTRACT This study constrains factors controlling the distribution of diagenetic alteration and their impact on reservoir quality of the Cretaceous sandstones from the Al-Bayda Platform, located in the southern Sirt Basin (Libya). These factors include the presence of early cements as well as the influx of hot basinal brines. The studied samples come from two blocks in the Khalifa Field, which are dislocated by a major normal fault. The deep-burial (mesogenetic) alteration includes the partial to pervasive replacement of early (eogenetic) dolomite and calcite cements by ferroan-dolomite, ankerite and siderite, precipitation of grain-coating chlorite, and cementation by quartz overgrowths, barite and anhydrite, particularly in the downthrown block. The association of quartz overgrowths with barite suggests that deep burial was influenced by the influx of hot basinal brines through faults. Conversely, deep-burial alteration in braided fluvial deposits of the Nubian sandstones of the upthrown block include: illitization of eogenetic smectite, quartz cementation and formation of chlorite. This study shows that deep burial of the studied sandstones did not occur in a closed system, but was affected by the influx of hot basinal brines through faults, which formed during basin rifting. This interpretation is supported by the relatively high homogenization temperatures (100–110°C; corrected to 110–125°C) of primary fluid inclusions within quartz overgrowths, which exceed the maximum burial temperatures experienced by the Cretaceous succession, and by the high salinity of these inclusions.

A fresh look at sideritic “coprolites”

Sideritic “coprolites” from the late Miocene of southwest Washington, the Upper Cretaceous of Saskatchewan and Madagascar, and the Permian of China have often been claimed to be pseudofossils. They are here interpreted as intestinal casts (cololites) prefossilized by bacterial activity and later transformed into siderite with no traces of original food particles left. All occurrences are found within fluvial overbank deposits that carry no other vertebrate remains. Their absence could be due to aquifer roll-fronts that destroyed phosphatic bones and teeth but favored siderite precipitation.

Synmetamorphic Variscan siderite mineralisation of the Rhenish Massif, Central Europe

The Rhenohercynian thrust- and fold-belt of Central Europe hosts a syn- to post-kinematic Variscan vein mineralisation, which is restricted to the Rhenish Massif, Germany. It is formed during four major stages and siderite is the principal ore mineral of the 'main stage'. The latter can be traced throughout the massif, but is mainly developed in the Siegerland district as synkinematic lodes which are hosted by low-grade metamorphic pelites of Lower Devonian age.Fluid inclusion studies prove large-scale homogeneous ore forming fluids of low salinity (≤5 wt.% NaCI equiv.) and Na(-K > -Fe ≫ Mg)-C1 composition, which are CO2 undersaturated (XCO2 = 0.003-0.1). Siderite precipitation at 220−≤320°C and 0.7-1.4 kbar can be deduced by microthermometry, chlorite themometry, REE fractionation and experimental data. Thereby the maximum formation temperature comes close to or even exceeds the peak metamorphic temperature. From Mn contents and δ13C-δ18O variation of siderite a trend is apparent in which formation temperatures gradually decrease from S.E. to N.W. across the belt. This trend correlates with decreasing degree of host-rock deformation and decreasing metamorphic grade. Fluid composition as well as stable isotopes and REE fractionation of siderite point to ore-forming solutions, which were generated and equilibrated at depth during prograde metamorphism.Structural characteristics of the lodes, age determinations, and P-T estimations prove fluid ascent and siderite precipitation during and/or immediately after peak metamorphism predating the postkinematic magmatism of the Rhenohercynian belt. Main(-siderite)-stage mineralisation of the Rhenish Massif is classified as metamorphogeneous.