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ZusammenfassungMagnesit ist zwar ein relativ h~iufiges Mineral, seine wirtschaftlich bedeutenden Lagerst~itten erscheinen abet nur in zwei Typen: Manuscript received: 10. 5. 89; accepted: 6. 1.90. AbstractMagnetite is relatively frequent as a mineral, there are, however, only two types of economically important deposits:The world's largest reserves are found in deposits of the Veirsch type. These are strata-bound lensoid and nearly monomineralic bodies consisting of coarsely crystalline ,,spar,, magnesite hosted by marine platform sediments. Conspicuous are sedimentary and diagenetic features of the magnesite bearing suites, which indicate formation within a subtidal, evaporitic environment closely associated with sabkhas, mudbars or other inter-to supratidal highs. The coarse crystallinity of these magnesites is a product of diagenesis.As spar magnesites as a rock are mainly characterized by diagenetic features it is now often concluded that the main phase of magnesium concentration would also have taken place during late diagenesis or early metamorphism from fluids formed by these processes. In that case, their origin would be by hydrothermal replacement.Contrary to this trend it is here concluded by integrating important data that Veitsch type magnesites are essentially of syngenetic origin. A genetic model is presented, which stresses their kinship with shallow water marine chloride type evaporites.Deposits of cryptocrystalline magnesite of the Kraubath type are much smaller and less frequent than spar magnesites, although they are quite important because of their high quality product. Deposits include veins and stockworks of snow-white magnesite ii) ultramafic country rocks.Genetic models of Kraubath type magnesites include the extremes of supergene alteration versus hypogene-hydrothermal formation. In this paper, arguments are discussed which favour the second interpretation, illustrated by a model as near-surface epithermM deposits. R6sum6Bien que Ia magn&ite soit un mineral relativement fr6-quent, elle ne constitue que deux types de gisements d'importance 6conomique:Le premier est le type Yeitsch dont les gisements comportent les plus grandes r6serves de magn6site du monde. I1 s'agit de corps lenticulaires ,,strata-bounds,,, form6s de magn6site grossi~re (,spathique,,) presque mononain6rale et contenus 292 W. POHL dans des s6diments matins de plate-forme. Diverses structures s6dimentaires et diag6n&iques dans les roches de ces s&ies indiquent une formation dans un milieu sub-&aporitique, en association &roite avec des sabkhas, des ,,mud bars,, et d'autres milieux inter-~ supratidaux. La cristallinit6 grossi6re de ces magn&ites est d'origine diag6n&ique. Comme ces magn6sites spathiques portent surtout l'empreinte du stade diag6n6tique, on tend aujourd'hui ~ rapporter aussi ~ ce stade la phase principale de concentration du Mg. Ceci impliquerait une formation par m&asomatose hydrothermale de carbonates pr6-existants, les fluides responsables de ce m&aso-matisme &ant d'origine diag6n&ique ou m&amorphique. Co...

The four metals of economic significance in the Central Africa or Great Lakes region, i.e. gold, tin, tantalum and tungsten, are part of one composite metallogenic system that operated about 980¡20 Ma. The main driving agent was peraluminous ilmenite-series granite magmatism, synchronous with intracratonic compression and associated with the final amalgamation of the supercontinent Rodinia. The granitic melts were emplaced at intrusive levels of §2 kbar ( §8 km); the intrusions display a variable and often advanced degree of fractionation, including abundant Sn-Ta-Li-Be-Rb-Cs pegmatites, and are associated with hydrothermal systems enriched in tin, tungsten and/or gold. Based on cumulative past production and present metal prices, gold in hydrothermal quartz veins is the major commodity, followed by tin either in rare metal pegmatites or in sheeted, hydrothermal quartz veins. Many deposits in the province occur in siliciclastic metasedimentary, or metabasaltic roof rocks above parental granites; mainly in its western part, the zone of mineralisation retracts into the granite roof. Typically in the first case, antiformal sites acted as fluid escape zones, with carbonaceous or metabasaltic rocks as chemical traps for tungsten and gold. Examples of pegmatitic and magmatic-hydrothermal deposits are presented in some detail in order to illustrate characteristics and genetic controls, and to support the metallogenic hypothesis here advanced. Impeding strategic exploration, published elements of understanding the evolution and mineralisation of the Kibara belt are contradictory and essential links are missing, foremost an understanding of the 1 Ga flare up of fertile granites. Towards solving this conundrum we suggest that the key is delamination of the mantle lithosphere and dense mafic lower crust, residual after extraction of voluminous 1?38 Ga granitic melts. During pan-Rodinian orogenic events, the Tanganyika spur of the Tanzania craton acted as an indenter whose impact caused foundering of the early Kibaran lithosphere. Consequent influx of asthenospheric heat triggered large-scale crustal melting that resulted in the tin granites. The stress state was largely compressive but possibly punctuated by short or local extensional events. The correlation of geological evolution and mineralisation substantiates the formal recognition of a Kibara Metallogenic Domain, which is composed of two units: The Mesoproterozoic (1?4 Ga) Kabanga-Musongati nickel (¡copper, cobalt, platinum) province; and the early Neoproterozoic (1 Ga) Kibara rare metal and gold province that is the main subject of this paper. The present understanding of the operating metallogenic systems remains limited. Regarding the application of modern concepts and technologies, this province is drastically underexplored.

Abstract. We present a suite of extensions for ARCVIEW GIS™ (ESRI) that allows to map the spatial distribution of first-order mechanical slope-properties in hard rock terrain, e.g. for large slope areas like water reservoir slopes. Besides digital elevation data, this expert-system includes regional continuous grid-based data on geological structures that might act as potential sliding or cutoff planes for rockslides. The system allows rapid automated mapping of geometrical and kinematical slope properties in hard rock, providing the basis for spatially distributed deterministic sliding-susceptibility evaluations on a pixel base. Changing hydrostatic slope conditions and rock mechanical parameters can be implemented and used for simple predictive static stability calculations. Application is demonstrated for a study area in the Harz Mts., Germany.