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A new gem corundum occurrence has been discovered in the Naryn-Gol Creek placer of the Dzhida volcanic field (Russia). In this placer deposit, sapphire associates with large crystals of garnet, spinel, augite, olivine, enstatite, ilmenite, Ti-magnetite, and alkali feldspar. Such a combination of minerals is typical for the placer deposits associated with alkali basalts widely distributed in Southeastern Asia and Australia. We have also found sapphire crystals in phonotephrites of the nearby Cenozoic alkali-basalt paleovolcano Barun Khobol Pravyi, and in basalt sample and trachybasalt from the valley flood basalts. The chemical composition of sapphire is generally typical for 'basalt' corundum: it is rich in Fe, and depleted in Ti and Cr. The δ18O SMOW values of corundum and related megacrysts range from 4.6 to 6.8 ‰, thus corresponding to the isotopic signature of igneous rocks. Etched and corroded surfaces of sapphire and other megacrysts indicate that they are in non-equilibrium with their host alkali basalts. Volatile components, CO2 in particular, played a significant role during sapphire formation as gas inclusions reveal. [ABSTRACT FROM AUTHOR]

Sulfur isotope in sulfides from the Paleoarchean and the Neoarchean sedimentary rocks evidence microbial sulfur metabolism in Archean sulfur cycle. However, sulfur metabolism for the Mesoarchean interval is less obvious since evidence for a large range in sulfur isotope values has not yet been observed in Mesoarchean samples. We report the results of multiple sulfur isotope measurements for sulfide minerals from ~2.8 Ga sedimentary rocks in the southeastern part of the Karelian Craton. In situ isotope analysis of sulfide grains have been performed using a femtosecond laser-ablation fluorination method. Sulfide samples studied here yielded Δ33S values between −0.3 and +2.7‰ and δ34S values between −10 and +33‰. The Δ33S dataset was interpreted to indicate the incorporation of sulfur from two coexisting sulfur pools, photolytic sulfate and photolytically derived elemental sulfur. We suggest that the relative contributions of these Δ33S different pools to the pyritic sulfur could be controlled by the metabolic activity of coexisting sulfate-reducing and sulfur-disproportionating bacteria during pyrite formation. We therefore suggest the operation of different metabolic pathways of sulfur in Mesoarchean sedimentary environments. [ABSTRACT FROM AUTHOR]