Melanterite and szomolnokite as weathering products of pyrite from the Bazhenovo Formation

Weathering of pyrite in the core recovered from black shales of the Bazhenovo Formation (Upper Jurassic-Lower Cretaceous) in the West Siberian marine basin promoted the successive formation of melanterite (FeSO₄ · 7H₂O) and szomolnokite (FeSO₄ · H₂O). Szomolnokite was detected in West Siberia for the first time.     

Chemical evolution and petrogenetic implications of loparite in the layered,agpaitic Lovozero complex,Kola Peninsula,Russia

Summary Lovozero, the largest of the world’s layered peralkaline intrusions, includes gigantic deposits of Nb + REE-loparite ore. Loparite, (Na,Ce,Ca)₂(Ti,Nb)₂O₆, became a cumulus phase after crystallisation of about 35% of the ‘Differentiated Complex’, and its compositional evolution has been investigated through a 2.35 km section of the intrusion. The composition of the cumulus loparite changes systematically upwards through the intrusion with an increase in Na, Sr, Nb and Th and decrease in REE and Ti. This main trend of loparite evolution records differentiation of the peralkaline magma through crystallisation of 1600 m of the intrusion. The formation of the loparite ores was the result of several factors including the chemical evolution of the highly alkaline magma and mechanical accumulation of loparite at the base of a convecting unit. At later stages of evolution, when concentrations of alkalis and volatiles reached very high levels, loparite reacted with the residual melt to form a variety of minerals including barytolamprophyllite, lomonosovite, steenstrupine-(Ce), vuonnemite, nordite, nenadkevichite, REE, Sr-rich apatite, vitusite-(Ce), mosandrite, monazite-(Ce), cerite and Ba, Si-rich belovite. The absence of loparite ore in the “Eudialyte complex” is likely to be a result of the wide crystallisation field of lamprophyllite, which here became a cumulus phase. Received November 6, 2000; revised version accepted January 18, 2001     

Variolitic lavas in the axial rift of the Mid-Atlantic Ridge and their origin (Sierra Leone area, 6°18′N)

Altered variolites described for the first time in the axial zone of the Mid-Atlantic Ridge are represented by rounded globules of andesite (icelandite) composition with light trachyandesite rim embedded in a picrobasaltic matrix. The globules were transferred with picrobasaltic melt and then floated to the surface of lava flow, while formation of leucocratic rims was presumably related to thermodiffusion (Soret effect) in a cooling heterogeneous melt. This heterogeneous melt was formed by penetration of ascending column of picrobasaltic magma in already existing small intracructsal magmatic chamber filled with residual icelanditetype andesite melt and involvement of the latter into a general upward movement. The rapid ascent of the melts in the oceanic spreading zones by means of turbulent flowing caused dispersion of the extragenous melt into small drops in a jet of picrobasaltic magma, without their interaction. Variolites were formed during cooling of such heterogeneous lava flow. No signs of liquid immiscibility were found in the studied variolites.     

U-Pb isotopic dating of zircons (SHRIMP-II) from granulites of the Ol’khon region of Western Baikal area

Doklady Earth Sciences -     

Noble-metal mineralization in the Semenov-2 hydrothermal field (13°31′N), mid-atlantic ridge

The porous fine-grained to microcrystalline copper-zinc ore of the Semenov-2 hydrothermal field, a site in the Semenov hydrothermal cluster discovered in 2007 (13°31′N, MAR), is anomalously enriched in Au (22–188 ppm) and Ag (127–1787 ppm). Chalcopyrite, isocubanite, würtzite, and opal are major minerals; sphalerite, marcasite, pyrite, and covellite are auxiliary; and galena, pyrrhotite, native gold, silver telluride, barite, and aragonite are sporadic. Gold containing 0.31 to 23.07 wt % Ag occurs as up to 9-μm-sized subhedral, dendritelike, and elongated grains mostly hosted in opal and less frequently in sphalerite and in pores within isocubanite-chalcopyrite aggregate. An elongated grain (2 × 4 μm in size) of the Ag-Te phase was found in a pore. So far only basalts have been dredged from the Semenov-2 field, but anomalous gold and silver concentrations suggest the influence of ultramafic rocks; the latter were found 1.5 km westward, in the Semenov-1 hydrothermal field. Mineral assemblage and morphology of gold particles indicate its primary origin in contrast to the hydrothermal fields hosted in basalts, where gold is a product of remobilization. Zonal gold grains, found on oceanic floor for the first time, are characterized by low Ag content in the cores and high Ag content in the outer rims, reflecting variation in formation conditions.     

Comparative analysis of marine paleogene sections and biota from West Siberia and the Arctic Region

The analysis of the main biospheric events that took place in West Siberia and the Arctic region during the Early Paleogene revealed the paleogeographic and paleobiogeographic unity of marine sedimentation basins and close biogeographic relations between their separate parts. Most biotic and abiotic events of the first half of the Paleogene in the Arctic region and West Siberia were synchronous, unidirectional, and interrelated. Shelf settings, sedimentation breaks, and microfaunal assemblages characteristic of these basins during the Paleogene are compared. The comparative analysis primarily concerned events of the Paleocene-Eocene thermal maximum (PETM) and beds with Azolla (aquatic fern). The formation of the Eocene Azolla Beds in the Arctic region and West Siberia was asynchronous, although it proceeded in line with a common scenario related to the development of a system of estuarine-type currents in a sea basin partly isolated from the World Ocean.     

Catagenesis of organic matter at the top and base of the Jurassic complex in the West Siberian megabasin

Thermal maturation for the top and base of the Jurassic in the West Siberian megabasin was modeled on the basis of the vitrinite reflectance (R_vt⁰) data using mathematical modeling and computer simulations. The values of thermal maturation are found to vary within substages PC₃-MC₃² for the top (or being equivalent to PC₃-MC₁² on the periphery and southern part of the basin, or to MC₂-MC₃² in the north) and PC₃-AC₃ for the base of the Jurassic (or being equivalent to PC₃-MC₂ on the periphery and southern part of the basin or MC₃¹-AC₃ in the north). Thermal maturity levels of the Jurassic in West Siberia are controlled by depths of burial and peak temperatures which the source rocks were subjected to during this period. The situation is further complicated by high heat flows superimposed on the regional background, which are observed in deep fault zones and in the proximity of numerous igneous bodies.