Kremeňovo-karbonátové žily s U-Mo-Cu mineralizáciou v permských intermediárnych až bázických vulkanitoch hronika na lokalite Kravany (Kozie chrbty, východné Slovensko)

Historical uranium ore deposit Kravany is located in the eastern part of Kozie Chrbty Mts., approximately 9 km SW of the district town Poprad. Stratiform, infiltration U-Cu-Pb mineralization is bound to the Upper Permian clastic sediments (Kravany Beds, member of Malužiná Formation, Hronicum Unit), which are enriched in fragments of carbonized flora. Vein U-Mo-Cu mineralization was found in the Upper Permian intermediate to basic volcanics intersecting the sediments of the Kravany Beds (also ore lenses). Vein filling originated in the following development stages: I.) Quartz-pyrite (quartz, pyrite, marcasite), II.) Dolomite-pyrite (dolomite, pyrite, marcasite, galena), III.) Copper (tetrahedrite, tennantite, chalcopyrite), IV.) Uranium-molybdenum (uraninite, Pb-Mo-S phase, coffinite, quartz), and V.) Calcite (calcite, chalcopyrite). The formation of mineralization can be explained by the geological position: random emplacement of the diorite porphyrite, resp. basalt-andesite dikes, directly in the preexisting U,Mo-bearing sediments. Vein U-Mo-Cu mineralization could thus most likely have formed according to the following scenario: I.) sedimentation of Kravany Beds in the Permian riftogenic basin: formation of beds of arkoses and arkosic sandstones with abundant fragments of charred flora, II.) formation of infiltration U mineralization: reduction and accumulation of U in sediments rich in organic matter, III.) emplacement of dikes of intermediate to basic volcanics: intersection of sediments with organics and with high U and Mo content, mobilization of formation fluids, assimilation of U and Mo into intermediate-basic magma, IV.) cooling of volcanic bodies → their contraction (formation of contraction cracks) → filling of contraction cracks with quartz, carbonates and ore minerals (crystallization from residual magmatic solutions mixed with formation waters). From this point of view it is syngenetic volcanogenic vein U-Mo-Cu mineralization, originally of the Permian age, with subsequent Alpine (most probably Cretaceous) reworking (this is evidenced by the variable composition of uraninite). It belongs to the Neohercynian late- to postorogenic metallogenetic stage. The possible younger, post-Permian age of mineralization from alpine hydrothermal solutions must also be assumed, but this consideration has several inconsistencies.

ESPINHAÇO - QUO VADIS ? (ONDE ESTÁ? - AONDE VAI?) A EVOLUÇÃO DOS CONHECIMENTOS SOBRE A CORDILHEIRA DO ESPINHAÇO MERIDIONAL EM MINAS GERAIS ENTRE 1979 E 1995

In this paper a brief review is given of master and doctoral thesis as well of articles published duringthe last 16 years, concerning the geology of the Serra do Espinhaço in Minas Gerais, Brazil. The differentproposals of a subdivision of the Archean basement and sedimentary sequences of Pre-Espinhaço age arereviewed. The existing geochronological data are insufficient for a complete understanding of these units.The Espinhaço rift started its evolution about 1800 Ma ago with continental sediments with intercalationof acidic and basic volcanics evolving in the upper part of the sequence to shallow marine sedimentation.Aspects of stratigraphy and sedimentology of the Espinhaço Supergroup as well as those of structuraland economic geology are briefly discussed. Diamonds washed in the Sopa-Brumadinho conglomeratesand in alluvials remains the principal mineral resource as about 160 years ago predicted by L.W. vonEschwege. In recent years exploration work was successful to identify the gold deposit at Riacho dosMachados in the northern extension of the Serra do Espinhaço.A chapter resumes the geodymamic models presented during the considered period. Finally, suggestionsfor future investigations are listed.

Lithostratigraphic comparison of three Diamictite successions of Nepal Lesser Himalaya

ABSTRACT The Proterozoic Blaini-type of diamictite sequences have been identified from the Lesser Himalaya (Brookfield, 1987). A tillitic sequence called the Sisne Formation of Gondwana age was reported by Sakai (1983) from Tansen, the Western Nepal Lesser Himalaya. The newly reported Kokaha Diamictite and Sallyan Diamictite situated to the east and west of Tansen, respectively, exhibit some lithostratigraphic similarity with the Sisne Formation, though the underlying and overlying rocks are quite different The Kokaha Diamictite from the Barahakshetra area of Eastern Nepal is underlain by grey quartzite with coal seams and conglomerate lenses. Sometimes tuffs and agglomerates are also found together with the diamictite in some places. It is followed upsection by grey calcareous sandstone, siltstone and shale. The Sisne Formation from the Tansen area of Western Nepal overlies grey dolomite with a disconformity and is followed by the Taltung Formation separated by another disconformity. The Taltung Formation itself is composed of basic volcanics, conglomerate, sandstone and siltstone. A non-Blaini type of diamictite horizon called the Sallyan Diamictite from the Sallyan area of West Nepal occurs in low -grade metamorphic rocks. It disconformably overlies quartzite, meta-conglomerates and phyllites, and is transitionally followed by black carbonaceou s slates. Lack of reliable fossils, metamorphism and strong deformation pose difficulties in correlating it with other diamictites.

The effect of vertical crustal fractures on the rifting process

In the past, rifts have mainly been identified in terms of sediment troughs. They account for many of the elongate gravity lows distributed in a coherent rectilinear manner over the continent. Other gravity lows can be attributed to granites intruding rift compartments, and some gravity highs can be attributed to basic volcanics in compartments. The total number of rifts which can be thus inferred from gravity and magnetics is very large, and suggests rifting is pervasive over the whole continent and controlled by a systematically distributed "Cardinal" system of ancient vertical crustal fractures.The extensional concept of rifting is based on a finite number of rifts, all of which have "failed" to split the continent. When a far greater number of rifts is recognised, it becomes difficult to accept that all these rifts have "failed" to reach full opening by extensional processes. In view of the known horizontal compressive forces acting in the crust, it is more probable that rifting is caused by compression. The compartmentalization of rifts, clearly observed in gravity data, also implies compression.Closely spaced rectilinear dyke systems in shield areas may also represent the pervasive "Cardinal" fracture system. In general, this system of orthogonal fractures poses problems for the detatchment rifting concept which assumes that transfer faults are formed at the time a rift forms, whereas they in all probability predate the rift, and owe their existence to a fundamental process operating when continental crust first formed.Two types of compressive rift models are discussed. One is associated with shear couples between widely spaced parallel fractures. The other is based on the concept of a crust cut by closely spaced fractures in which compression is propagated along a network of linked blocks. In both cases the development of basement ridges is a key issue.