Geophysical prospecting in the Lake Victoria goldfields

Two areas of the Lake Victoria Region in Tanzania, where most of the old gold prospects are situated, were chosen for a comprehensive mineral survey. Since most of the country is covered by tropical soils, geophysical methods were applied. The gold occurs in quartz veins, shear zones and impregnation deposits, sometimes closely associated with sulphides or graphites, mostly in the archean country rocks of the Nyanzian System. The Nyanzian rocks form irregular shaped rafts in the roof of the central granite and have nearly the same physical properties as the gold occurrences. This obstacle was overcome by a special combination of geophysical methods:Airborne magnetic and electromagnetic surveys were flown in order to distinguish major fault zones, conducting horizons and profound changes in lithology.In promising areas, selected from the airborne results or from the presence of old mine workings, special grids were cut out and pegged. These grids were surveyed with magnetometer and E. M. Gun. The results, evaluated in contour and derived structural maps, produced the important tectonic features of the areas.These informations formed the background for the finalTuram survey, which gave in some instances direct indications of the mineralized quartz veins. This combination revealed also so many particulars of the structural pattern, that extensions of the deposits could be established and it became possible, to carry out trenching and drilling programmes successfully. The application of the combined methods is explicated on the example of one particular mineralized quartz vein of Sarama Area.

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Zusammenfassung Zwei Gebiete am Victoriasee in Tanzania, in denen zahlreiche alte Goldbergbaue liegen, wurden durch eine Mineralprospektion erfaßt. Da ein großer Teil des Landes von tropischen Lehmen überdeckt wird, mußten geophysikalische Methoden angewendet werden. Das Gold kommt in Quarzgängen, Scherzonen und Imprägnations-Lagerstätten vor, teilweise in enger Verbindung mit Sulfiden oder Graphit, zumeist innerhalb der archaischen Gesteine des Nyanzian, die als unregelmäßig gestaltete Flöße im Dach des Zentralgranites schwimmen. Diese Serie besitzt nahezu dieselben physikalischen Eigenschaften wie die Goldvorkommen. Dieses Problem wurde durch eine besonders geeignete Kombination geophysikalischer Methoden gelöst:Magnetische und elektromagnetische Flugvermessungen wurden gleichzeitig durchgeführt, um Hauptverwerfungen, gut leitende Horizonte und die Verbreitung der wichtigsten Gesteine festzustellen.In höffigen Gebieten, die nach den Ergebnissen der Flugvermessungen oder nach der Lage alter Goldbergbaue ausgewählt wurden, sind besondere Netzwerke ausgeschlagen und verpflockt worden. Magnetometer und E.-M.-Gun-Messungen erfolgten zuerst; ihre Ergebnisse, in Isolinienkarten und abgeleiteten Strukturkarten ausgewertet, spiegelten die wichtigsten tektonischen Einheiten des Untersuchungsgebietes wider.Diese Resultate bildeten die Grundlage für die abschließendenTuram-Messungen, die direkte Hinweise auf die vererzten Quarzgänge erbrachten. Diese Verbindung unterschiedlicher Untersuchungsmethoden gab außerdem so viele Einzelheiten des geologischen Aufbaues wieder, daß die Ausdehnung einiger Erzkörper festgestellt und Schürf sowie Bohrprogramme erfolgreich ausgeführt werden konnten. Die Anwendung dieses Verfahrens wird am Beispiel einer typischen Quarzgang-Goldlagerstätte aus dem Sarama-Gebiet erläutert.

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Résumé Une recherche minière a été effectuée dans deux secteurs de la région du Lac Victoria en Tanzanie, où se trouve située la majorité des anciennes mines aurifères. De différentes méthodes géophysiques furent appliquées au cours de cette recherche, compte-tenux de l'importante altération de type tropical existant dans cette région. L'or se rencontre dans des veines de quartz, des zones de cisaillement ou en imprégnations diffuses et se trouve fréquemment associé à des sulfures et du graphite. La minéralisation est intrusive dans les formations précambriennes du Nyanzien. Ces formations constituent des îlots de forme irrégulière répartis dans le granite et ont, géophysiquement parlant, sensiblement les mêmes propriétés physiques que les formations aurifères. Cet obstacle fut levé en utilisant une combinaison spéciale de méthodes géophysiques:Une campagne de géophysique aéroportée (magnétométrie et électromagnétisme) fut effectuée en vue de repérer les failles principales, les horizons conducteurs et les changements lithologiques majeurs.Une étude au sol par magnétométrie et électromagnétisme (E. M. Gun) fut alors entreprise dans les zones favorables, choisies sur la base des résultats de la géophysique et de l'importance des anciens travaux miniers, suivant une maille spéciale qui avait été jalonée au préalable sur le terrain. Les résultats furent consignés sous forme de cartes de contours et révélèrent les principales marques caractéristiques tectoniques de la région.Ces informations servirent de base à une campagne auTuram qui donne en certains endroits des indications précises sur les veines de quartz aurifère.Cette combinaison de méthodes géophysiques permit d'établir l'aspect structural de la région à un degré tel qu'il fut possible de déterminer l'extension des gisements et il devint alors possible de réaliser avec succès les programmes de prospection superficielle et de sondage. A titre d'exemple, l'application de ces méthodes combinéses est détallée pour une veine de quartz aurifère de la région de Sarama.

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Herrn Professor Dr.Roland Brinkmann zum 70. Geburtstag gewidmet.

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Annotation note of the editorial staff: Due to lack of time we have not been able to alter the text of this paper.     

Deformation und Umwandlung von Plagioklas durch Stoßwellen in den Gesteinen des Nördlinger Ries

Plagioclase from fragments of crystalline basement rocks in breccias found in the area of the Nördlinger Ries crater displays characteristic plastic deformation and phase transition phenomena due to shock metamorphism at different pressures in the range of 100 to 1000 kilobars.These phenomena are discussed in the scope of a progressive impact metamorphism the degree of metamorphism reflecting a radial gradient of pressure and temperature diminishing outward from the point of meteorite impact.Within the lowest pressure range of about 100 to 300 kilobars (shock stage I) strong fracturing and plastic deformation such as bending of crystals, deformation bands and planar features (lamellae of lowered refractive index and of lowered or no birefringence) are to be found. The lamellae which are mostly isotropic, are interpreted as slip bands the glide planes of which are low indices planes of the plagioclase lattice such as (001), (010), (100), (1¯20), (130) and others. These slip bands are unknown from feldspar formed by normal processes within the earth's crust. Plagioclase of such a stage of deformation shows an unusual strong decrease of refraction and birefringence. Its optical properties are those of a highly disordered plagioclase. It may be called diaplectic plagioclase.Total isotropization of plagioclase is a typical feature of the pressure range from 300 to 500 kilobars (shock stage II). This glass which is called diaplectic glass differs strongly from the normal glass in physical properties and structural state. It is formed by a kind of solid state transformation without actual melting.Shock pressures in the order of 500 to 650 kilobars (shock stage III) are able to cause selective melting of plagioclase grains in a crystalline rock. Normal glasses with vesicles and streaks are formed by this process.Within the pressure range of about 650 to 1000 kilobars (shock stage IV) residual temperatures are so high that total melting of rocks occurs. Plagioclase melts are mixed inhomogenously with other silicate melts forming rock melts which can be found in suevite as flat glassy bombs. Vaporization of silicates must be expected in the upper pressure range of this shock stage.Statistical universal stage measurements on the fabric of plagioclase support theoretical considerations after which the deformation pattern of a single crystal should depend on the fabric relations to the surrounding minerals and on their physical properties. Strongly inhomogenous deformation of plagioclase minerals within the microscopic rock scale was observed because polycrystalline rocks are disorganizing a unique shock front by interaction of wave fronts at interfaces and free surfaces and perhaps by multiwave shocks. Directions of compressive and tensile stresses on a mineral are therefore changing from grain to grain.     

A linear solution of the equations of motion of an earth-orbiting satellite based on a Lie-series

Using Hill's variables, an analytical solution of a canonical system of six differential equations describing the motion of a satellite in the gravitational field of the earth is derived. The gravity field, expanded into spherical harmonics, has to be expressed as a function of the Hill variables. The intermediary is chosen to include the main secular terms. The first order solution retains the highly practical formal structure of Kaula's linear solution, but is valid for circular orbits and provides of course a spectral decomposition of radius vector and radial velocity. The resulting eccentricity functions are much simpler than the Hansen functions, since a series evaluation of the Kepler equation is avoided. The present solution may be extended to higher order solutions by Hori's perturbation method.     

XENON,OSMIUM, AND LEAD FORMED IN O-SHELLS AND C-SHELLS OF MASSIVE STARS

In this paper we show that the explosive products from O-shells of massive stars which contain ¹²⁴Xe with large overproduction factors do not contain any of the naturally occurring isotopes of Os and Pb. We show further that the explosive products from C-shells (explosive carbon burning) do contain Os and Pb along with Xe which is strongly enriched in r-Xe of anomalous isotopic composition. The composition of Os in this matter is probably s-like rather than r-like. Pb in this matter is enriched in ²⁰⁸Pb. The results and arguments of this paper have implications for studies of isotopic compositions of Xe, Os, and Pb in residues of the Allende and other carbonaceous chondrites.     

Chemical Mechanism Development: Laboratory Studies and Model Applications

Within the German Tropospheric Research Programme (TFS) numerous kinetic and mechanistic studies on the tropospheric reaction/degradation of the following reactants were carried out: oxygenated VOC, aromatic VOC, biogenic VOC, short-lived intermediates, such as alkoxy and alkylperoxy radicals.At the conception of the projects these selected groups were classes of VOC or intermediates for which the atmospheric oxidation mechanisms were either poorly characterised or totally unknown. The motivation for these studies was the attainment of significant improvements in our understanding of the atmospheric chemical oxidation processes of these compounds, particularly with respect to their involvement in photooxidant formation in the troposphere. In the present paper the types of experimental investigations performed and the results obtained within the various projects are briefly summarised. The major achievements are highlighted and discussed in terms of their contribution to improving our understanding of the chemical processes controlling photosmog formation in the troposphere.     

Focal Mechanisms and Stress Field in The Region Vogtland/Western Bohemia

We calculated focal mechanisms for 30 of the strongest events (1.5 M _L 3.3) in distinct subregions of Vogtland/Western Bohemia between 1990 and 1998. Our investigations are concerned with events of the swarms near Bad Elster (1991), Haingrün (1991), Nový Kostel (1994 and 1997) and Zwickau (1998), two events from a group of earthquakes near Klingenthal (1997) and eight single events. Seismograms were provided by the digital station networks of the Geophysical Observatory of the University of Munich, the Technical University of Freiberg, the Academy of Sciences of the Czech Republic in Prague, the Masaryk University in Brno and some nearby stations of the German Regional Seismic Network (GRSN). To calculate focal mechanisms two inversion methods were applied. The inferred focal mechanisms do not show a simple, uniform pattern of seismic dislocation. All possible dislocation types – strike-slip, normal and thrust faulting - are represented. The prevailing mechanisms are normal and strike-slip faulting. Considerable differences in the fault plane solutions are noted for the individual subregions as well as in some cases among the individual events of a single swarm. For the Nový Kostel area we succeeded to resolve a change in the orientations of the nodal planes for the two successive swarms of December 1994 and January 1997. Besides this we also observe a change in the mechanisms, namely from strike-slip and normal faulting (December 1994) to strike-slip and thrust faulting (January 1997). Based on the inferred focal mechanisms the stress field was estimated. It turned out, that the dominating stress field in the region Vogtland/Western Bohemia does not substantially differ from the known stress field of West and Central Europe, being characterized by a SE-NW direction of the maximum compressive horizontal stress. We conclude that the seismicity in the Vogtland/Western Bohemia region is not predominantly caused by an independent local stress field, but rather controlled by the dominating stress regime in Central Europe.     

Spatial distribution and bonding forms of heavy metals in sediments along the middle course of the River Elbe (km 287ċ390)

Sediments of eight groyne fields along the middle course of the River Elbe (river km 287?390) were geochemically studied. The 78 sediment samples were analysed for pH and grain size distribution. The grain size fraction < 2 μm was used for mineralogical and chemical analysis: semiquantitative clay mineral analysis; total element content (Al, Ca, Fe, K, Mn, Cd, Cr, Cu, Ni, Pb, Zn), total inorganic and organic C, and bonding form fractionation with a six‐step sequential chemical extraction. The latter was performed on selected samples (n = 32). The sediments along the Elbe's course are still contaminated with heavy metals far above the local geogenic background level. An enrichment factor of more than 15 was calculated for zinc. Cr and Ni are the elements with the lowest enrichment. The bonding form analysis of selected heavy metals shows a dominance of relatively immobile bonding forms, e. g. the moderately reducible and the residual fraction, which implies a relative low mobility potential. Only Zn poses a higher potential threat to the environment, since it has a higher percentage of the first three extracted phases: adsorbed, carbonate, and easily reducible fraction.     

Field Signatures of the SE-Asian Mega-Tsunami along the West Coast of Thailand Compared to Holocene Paleo-Tsunami from the Atlantic Region

The Andaman-Sumatra Tsunami of Dec. 26, 2004, was by far the largest tsunami catastrophe in human history. An earthquake of 9 to 9.3 on the Richter scale, the extension of waves over more than 5000 km of ocean and run-ups up to 35 m are its key features. These characteristics suggest significant changes in coastal morphology and high sediment transport rates. A field survey along the west coast of Thailand (Phuket Island, Khao Lak region including some Similan Islands, Nang Pha mangrove areas and Phi Phi Don Islands) seven to nine weeks after the tsunami, however, discovered only small changes in coastal morphology and a limited amount of dislocated sediments, restricted to the lower meters of the tsunami waves. This is in striking contrast to many paleo-tsunami's events of the Atlantic region. Explanations for this discrepancy are sought in: a. Mechanics of the earthquake. A rather slow shock impulse on the water masses over the very long earthquake zone, b. Shallow water in the earthquake zone, and c. Bathymetry of the foreshore zone at the impacted sites. Shallow water west of Thailand has diminished wave energy significantly. The differences in geomorphological and sedimentological signatures of this tsunami compared with many paleo-tsunami worldwide makes it unsuitable to be used as a model for old and future tsunami imprints by an event of this extreme energy and extension.     

Isoseismals of the strongest Friuli aftershocks of September 1976

Summary Isoseismal maps of the three strongest Friuli aftershocks of September 1976 were compiled using the contributions from European countries within the shaken area. The characteristic features of the macroseismic fields are discussed.