Geochemistry of Diagenesis of Organogenic Sediments: An Example of Small Lakes in Southern West Siberia and Western Baikal Area

Organogenic sediments (sapropels) in lakes are characterized by a reduced type of diagenesis, during which organic compounds are decomposed, the chemical composition of the pore waters is modified, and authigenic minerals (first of all, pyrite) are formed. Pyrolysis data indicate that organic matter undergoes radical transformatons already in the uppermost sapropel layers, and the composition of this organic matter is principally different from the composition of the organic matter of the its producers. The sapropels contain kerogen, whose macromolecular structure starts to develop during the very early stages of diagenesis, in the horizon of unconsolidated sediment (0–5 cm). The main role in the diagenetic transformations of organic matter in sediments is played by various physiological groups of microorganisms, first of all, heterotrophic, which amonifying, and sulfate-reducing bacteria. SO₄^2? and Fe²⁺ concentrations in the pore waters of the sediments are determined to decrease (because of bacterial sulfate reduction), while concentrations of reduced Fe and S species (pyrite) in the solid phase of the sediment, conversely, increase. Comparative analysis shows that, unlike sapropels in lakes in the Baikal area, sapropels in southern West Siberia are affected by more active sulfate reduction, which can depend on both the composition of the organic matter and the SO₄^2? concentration in the pore waters.     

Estimation of the time of magma chamber solidification beneath the Strel’tsovka caldera and its effect on the nonstationary temperature distribution in the upper crust,the eastern Transbaikal region,Russia

The hydrothermal Mo-U deposits of the Strel’tsovka ore field, unique in reserves, are localized in the Late Mesozoic caldera of the same name. The consideration of geochemical processes that controlled uranium transfer by ore-bearing fluids and its precipitation in orebodies has shown that a nonstationary temperature distribution could have exerted a substantial effect on ore formation. The temperature field in the Strel’tsovka caldera, which was caused by a shallow-seated magma chamber that existed beneath the caldera by the onset of the ore stage, was simulated by mathematical modeling. A one-dimensional nonstationary model of conductive heat transfer taking into account the latent heat of magmatic melt crystallization was used. The problem was solved with the finite difference method. It has been established that, at optimal parameters of the model, the magma chamber would have completely crystallized in 56 ka; the maximum estimate is 133 ka. Three million years after emplacement of the granitic intrusion, the related thermal anomaly in the upper crust should have disappeared. The results obtained indicate that granitic melt of this chamber could not have been a source of uranium-bearing solutions that formed deposits 5 Ma after the cessation of magmatic activity.