Un paleoplacer métamorphisé d'hématite — rutile uranifère et thorifère dans l'Atacorien (Précambrian supérieur) du Togo,Afrique occidentale

Résumé Dans l'Atacorien (Akwapimian) du Togo, qui représente probablement la partie la plus inférieure du Précambrien supérieur, on a trouvé en 1970 une minéralisation sédimentaire à hématite et rutile uranifère et thorifère. Cette minéralisation contient des minéraux radioactifs du groupe de brannérite — davidite, aussi bien que l'uranothorite et le pyrochlore. Elle a été deposée dans un paleoplacer qui fait part d'un niveau flyscheux et a été transformée sous l'action du métamorphisme régional. La minéralisation représente un nouveau type génétique des minérais titanifères en Togo, aussi bien qu'en Afrique occidentale. Cette découverte change le but de la prospection detaillée de certaines séries dédritiques du Précambrien de l'Afrique occidentale.

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In the Atacorian (Akwapimian) of Togo, which probably represents a lower part of Upper Precambrian, a sedimentary mineralization of Hematite and urano-thoriferous rutil was discovered in 1970. In this mineralization occur some radioactive minerals of the davidite-brannerite group, as well as uranothorite and pyrochlore. It is built up by paleoplacers, located in a flysch series of the Atacorien, which was exposed to a later metamorphosis. These mineralizations represent a new genetic type of titaniferious ores in Togo, as well as in West Africa. This discovery changes the scope of detailed prospection of certain detritic series of the Precambrian of West Africa.

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Institut géologique Dionýz túr.     

Hydrothermal alteration in volcanic rocks,eastern part of the Lukavice Group, Železné Hory Mountains,Czech Republic

Many rocks mapped as felsic metavolcanics in the eastern part of the Lukavice Group are shown to be altered mafic metavolcanics, similar to those in the Noranda and Flin Flon-Snow Lake mining districts, Canada. The relatively fresh rocks of the Lukavice Group are rhyolite, dacite-andesite, and andesite-basalt of calcalka-line character. Assuming no substantial volume change during alteration, Ti, P, La, Ce, Yb, Lu, Th (partly), Sc and V contents remained unchanged. Altered rocks are enriched in (Fe + Mg), K and Si and depleted in Na, Ca and Zr. Some elements show both increased and decreased contents in altered rocks (Mg, Ba, Sm, eu, Tb and Hf). Although hydrothermal alteration in the Lukavice Group is of large extent, it is of the proximal Kuroko style and not of regional Amulet Rhyolite style. Implications for a large hydrothermal system within a volcanic pile are discussed in relation to the Ordovician Lukavice Group and its mineral deposits and to some other parts of the Bohemian Massif with volcanosedimentary sequences of the same age.     

Constructing Site-Specific Climate Change Scenarios on a Monthly Scale Using Statistical Downscaling

Summary Monthly mean temperature and monthly precipitation totals in two small catchments in the Czech Republic are estimated from large-scale 500 hPa height and 1000/500 hPa thickness fields using statistical downscaling. The method used is multiple linear regression. Whereas precipitation can be determined from large-scale fields with some confidence in only a few months of the year, temperature can be determined successfully. Principal components calculated separately from the height and thickness anomalies are identified as the best predictor set. The method is most accurate if the regression is performed using seasons based on three months. The test on an independent sample, consisting of warm seasons, confirms that the method successfully reproduces the difference in mean temperature between two climatic states, which indicates that this downscaling method is applicable for constructing scenarios of a future climate change. The ECHAM3 GCM is used for scenario construction. The GCM is shown to simulate surface temperature and precipitation with low accuracy, whereas the large-scale atmospheric fields are reproduced well; this justifies the downscaling approach. The observed regression equations are applied to 2xCO₂ GCM output so that the model’s bias is eleminated. This procedure is then discussed and finally, temperature scenarios for the 2xCO₂ climate are constructed for the two catchments. Received December 3, 1998 Revised December 4, 1999     

Groundwater-quality monitoring—effective method of hydrogeological system pollution prevention

Groundwater-quality monitoring plays an important role in groundwater protection and quality conservation. National, regional, and site-specific monitoring programs operating in Czechoslovakia concern collection of back-ground data on groundwater quality and regional diffusion pollution problems. Site-specific monitoring is concerned with point sources of pollution. Pilot monitoring stations with automatic data acquisition systems optimize monitoring methods, design of monitoring networks, sampling techniques, and frequency and selection of the variables analyzed. Ground-water-quality monitoring in Czechoslovakia supports decision and policy making in areas of intensive farming. Time and spatial changes in shallow vulnerable aquifers are evaluated in several areas of national territory and support integrated land-use planning and groundwater protection management.     

Book reviews

Geographical, seasonal, and day-to-day variations in the vertical distributions of atmospheric density and temperature must be considered in the design and operation of reentry vehicles. This work is part of a continuing effort to compile, analyze, and present information on the distribution of the atmospheric temperature in a form suitable for use in the design and operation of aerospace vehicles. We have used temperature data obtained from rockets which are mostly based on the datasonde system throughout the decade 1969–1978. Profiles and the individual arrays of coefficients of correlation of the temperature at 20 km with temperature at higher altitudes up to 60 km are shown for each season for locations in the low, middle, and high latitudes.     

Two Periods of 1999 HF1—Another Binary NEA Candidate

Photometric observations of 1999 HF₁ reveal that its lightcurve has two components of low amplitudes (0.10-0.12 mag) and different periods (2.3191 and 14.02 h). It is likely another binary near-Earth asteroid; its lower limit on the secondary-to-primary-diameter ratio is ≈0.2, the radius of the mutual orbit is (2.0±0.3)× the effective primary diameter, the primary's bulk density is >2.0 g/cm³, and it belongs to the E/M/P taxonomic class.     

A Non-Planar Circular Model for the 4/7 Resonance

An adiabatic approximation for the non-planar, circular, restricted 3BP is presented for the external resonance 4/7. It can be used as a model for resonant Kuiper belt objects. The Hamiltonian is truncated at the fourth order in eccentricities and inclinations. After averaging, we have a system of two degrees of freedom with two frequencies. Numerical calculations show that the ratio of these frequencies is ~10². Having introduced suitable canonical variables, we used the adiabatic approach introduced by Wisdom in a different context. We left slow variables frozen and after solving the pendulum problem for fast variables, we used the averaged effect of fast variables on slow variables. In this way we obtained the guiding trajectories for slow variables as contour lines of adiabatic invariant. We discuss the existence of a chaotic region which is formed by trajectories crossing a critical curve which corresponds to the separatrix of fast pendulum motion, where the assumption of sharp division between fast and slow frequencies is not correct and the adiabatic theory fails. The model works well for e ~ 0.1 and can be used for finding the chaotic regions, but for e~ 0.17 it becomes unsatisfactory due to truncation and bad convergence of the Laplace expansion. Qualitatively it can, however, help us to understand how the protective mechanism works as the interplay of mean motion and Kozai–Lidov resonance.