Vol 71, No 6 (2016)

The lithology, sedimentation conditions, tectonic evolution features, and mineralization of molassoid complexes in southwestern Ghana that fill the Tarkwa and Bui basins located within the Birrimian Paleoproterozoic orogenic belt are described on the basis of an analysis of published and our own data. The formation of these sediments in geodynamic conditions of pull-apart basins is described. The deposits of the Tarkwa group are compared with the Archean Timiskaming group of the Superior Province and similar younger complexes. The Tarkwa group is considered as the Paleoproterozoic equivalent of molasse-like complexes that formed at the last stage of accretion orogens of different ages in pull-apart structures.

Surface waters are subject to intense contamination with trace elements in ore mining areas. A complex study was performed for the state of waters and bottom sediments from the Bogachukha and Urup Rivers in the area of the abandoned tailing dump of the Urupsky Ore Mining. The impact exerted by the tailing dump on the concentration of elements in the water, suspended particulate matter, and bottom sediments of the rivers is evaluated. The major contaminants of surface waters and bottom sediments, as well as the forms of their element distribution are revealed.

The possible reservoir type of the Bazhenov Formation relative to the intervals enriched in phosphates (apatite varieties) is described in this work. The phosphate rocks are characterized by a highly developed pore space; their porosity can reach up to 14%. Along with this, these rocks have explicit geochemical characteristics of oil-reservoir rocks in comparison with other rock samples in the well. As an example, the productivity index and oil-saturation index for phosphates are twice as high as the background values. The composition of such rocks can slightly vary: fluorine can be observed in phosphate minerals, while the rock is always enriched in organic matter (>8 wt %).

The Riphean rocks of the Baikit anteclise have been examined using pyrolysis Rock–Eval 6 to evaluate the subsidence history and erosion level. The studied Riphean rocks have the МK₃–МK₄ catagenesis grade. Based on the catagenesis of organic matter we propose a model of maximum burial before the beginning of the accumulation of Vendian deposits. Estimated calculations of subsidence and erosion have shown that the assessed catagenesis grade could be reached at a depth of 7 km, while the erosion level was approximately 5–7 km.

This paper reports on the total results obtained in solving morphological, retrospective, and engineering geological forecast tasks for loess-soil massifs. It is shown that the maximum relative subsidence ranges from 0.17 to 0.21, the thickness of subsiding loess massifs is 55 m under a natural load, the maximum number of cyclites composed of subsiding loess soils reaches ten, and more often the number of buried subsiding soils does not exceed four to five in a section of a loess massif. The main result in solving these retrospective engineering-geological tasks was the development of eight hypotheses and mechanisms of loess-soil subsidence, on the basis of which a general theory and four particular theories on the formation of loess subsidence were formulated. The result of solving the forecast tasks was the elaboration of methods for the calculation of the expected settlement of loess massifs under different conditions of wetting and the development of hydrogeochemical, geochemical, geotechnical, and integrated techniques for the improvement of the properties of loess-soil massifs.

Dispersed and concretionary pyrite in chert–clay–carbonate and carbonate rocks of the Abalak Formation (Salym oil field) have been studied. The study was conducted using Scanning Electron Microscopy (SEM), Electron Probe Microanalysis (EPMA), and high spatial resolution Secondary Ion Mass Spectrometry (Nano-SIMS). As a result, three morphological groups of pyrite have been distinguished: large cubic crystals, framboidal pyrite, and fine-crystal aggregates that replace organic remnants. The sulphur isotope ratio allows one to distinguish two genetic types of pyrite. The source of the sulphur for the first genetic group was H₂S produced by bacterial sulphate reduction, while the second group pyrite was formed with sulphur as a product of thermochemical sulphate reduction.