Evolution of stress fields in the Por’ya Guba dike field (Kandalaksha Gulf, White Sea)

The tectonic evolution of the Por’ya Guba segment of the White Sea Rift System began in the late Paleoproterozoic, i.e., soon after completion of the Svecofennian collision. The fracture system that controlled localization of the lamproite dike complex was formed under conditions of horizontal compression combined with shear. Subsequently, this system predetermined the location of a rift-graben segment that formed as a result of simple shear. The reactivation of the rift system in the Middle Paleozoic proceeded in two stages. The first stage, when strike-slip movements along previously formed faults predominated, resulted in formation of quartz-carbonate veins bearing base-metal mineralization. The veins that filled the shear fractures opened owing to local reorientation of the stress field. The second stage fitted the transtension conditions, and the Late Devonian alkaline ultramafic dikes of this stage introded into the already existing fracture system, which was oriented at a roughly right angle to the predominant stress orientation.     

Manganoan garnet rocks associated with the Broken Hill Pb–Zn–Ag orebody, Australia

Summary The Palaeoproterozoic Broken Hill Pb–Zn–Ag stratiform orebody is intimately associated with manganoan garnet-bearing rocks. On stratigraphic and chemical grounds it is argued that garnet-rich metasediments below, equivalent to and above massive sulphide were hydrothermal precipitates. Other manganoan garnet rocks formed during pre-metamorphic hydrothermal alteration, syn-metamorphic dehydration and reaction of manganese with prograde pelitic rocks, reaction between cataclastic manganese-bearing sulphide rocks injected along axial planes, shears and faults and pelitic wall rocks and reaction between dolerite dykes and sulphide rocks.     

New data on hydrocarbons in auriferous conglomerates of the Witwatersrand ore region, Republic of South Africa

Genesis of deposits in the world’s largest Witwatersrand auriferous region (South Africa) remains a debatable issue over several decades. Based on geological and mineralogical-geochemical investigations, we previously proposed a sedimentary-exhalative hypothesis for its origin. In the present communication, we have attempted to develop and scrutinize this concept based on the study of organic matter in ore and nonore conglomerates, quartz in conglomerate pebbles, and fluid inclusions in quartz from pebbles and cement of ore and nonore conglomerates.     

Isotopic signatures of deposition and transformation of Lower Cambrian saliferous rocks in the Irkutsk Amphitheater: Communication 3. Carbon and oxygen isotopic compositions in carbonates

In previous communications based on the study of sulfur and strontium, we showed that Lower Cambrian rocks of the Irkutsk Amphitheater underwent a significant epigenetic transformation. All postsedimentary alterations of rocks are related to the influence of water solutions that provoke the direct (physical) dissolution of material and its chemical transformation. In particular, an appreciable portion of anhydrites disappeared from the section due to reduction. Probably, these processes took place several times in the past and they are continuing at present. A similar conclusion was deduced from the strontium isotopic composition of carbonate rocks. Their strontium isotopic composition is usually shifted relative to the primary composition, although the prevalence of sulfate and carbonate rocks in the section promotes the stability of strontium isotopic composition with respect to secondary alterations. The carbon isotope system is even more stable due to the abundance of carbonate rocks in the section. This circumstance is probably responsible for the incapacity of isotope data to serve as obvious evidence of the epigenetic transformation of carbonates. The major elements of the evolution of carbon isotope signature could be retained since the sedimentation stage.     

Lower Miocene sediments of central and western Ciscaucasia

The composition, structure, and geochemical properties of the thickest, relatively deep-water Lower Miocene sediments developed in western Ciscaucasia are considered. Of particular interest are materials from the Kuban superdeep borehole SG-12000 that recovered the uppermost layers of the Maikop Group at 3148–3961 m in the central Indol-Kuban Trough west of Krasnodar. However, the borehole did not penetrate the whole Lower Miocene section of the Maikop Group. Therefore, characteristics of the Maikop Group are supplemented with new materials from several other boreholes drilled in the eastern Kuban region. Thus, the typical (reference) Lower Miocene section of central and western Ciscaucasia has been sufficiently well described.