Journal of African Earth Sciences (1983)最新文献

The locality of Bondoukou, NE of Ivory Coast, gave its name to a granite type, opposite to the Baoulé-type granite in Bodin's classification (1951, Bull. Dir. Mines AOF, Dakar, N° 12). This classification has been progressively enlarged to the whole of West Africa. The Bondoukou granite is subcircular in form, intrusive and discordant in the tufaceous schists and had not been dated previously.

Recent age determinations on 15 samples from this massif have been done with the Rb/Sr isochron method on whole rocks. The age obtained is approx. 2170 Ma. Consequently, these new data yield a Burkinian age (ex early Eburnean) which was not expected since a lot of works on Birrimian granites of West Africa assigned a post tectonic character and consequently a later age to the Bondoukou granite, related to the Baoulé-type granites.

Other age determinations with the same Rb/Sr method were done on granitic massifs which were assigned to the Bondoukou type; they yielded ages slightly younger than 2000 Ma in Senegal and Burkina Faso respectively (Bassot, J. P. and Caen-Vachette, M. 1984. In: 12th coll. Inter. Géol. africaine, Bruxelles, et géol. africaine (Edited by Klerkx, J. et Michot, J.),pp. 191–209. Tervuren, Belgique; and Gamsonre, P. E. and Rossy, M. 1974. Esquisse géologique de la région de Ouahi Gouya (N. W. de la Hte Volta). Chronologie des divers types de granitoîdes, p. 181. 2ème Rast, Nancy).

So, these few results point out the complex position of the Bondoukou type granites. Their meaning in the main Eburnean orogeny must be considered.

A combined heavy mineral and stream sediment survey was carried out in the area north of Isanlu to delineate potential areas for a detailed search for gold deposits. The geology of the area is typical of the basement of SW Nigeria with gneisses, schists, amphibolites and granites. Mineralization comprises gold/sulphide-bearing quartz veins and amphibolites.

Data obtained from the stream sediment survey were subjected to statistical interpretation and it was found that the log-normal pattern appears to be the one most applicable to the element distributions. No indicator for gold has been identified, but the significant correlation that exists between Pb/Cu and Pb/Zn was taken to distinguish vein mineralization from disseminated mineralization in amphibolites.

The heavy mineral survey, which was intended to simply check for gold occurrences in the field, also proved useful in detecting other anomalies later confirmed by the stream sediment survey. It was found to be a better exploration tool for gold at the reconnaissance stage. The overall high level of gold in the drainage indicates a metallogenic area favourable for gold. The survey succeeded in delineating possible gold targets worthy of follow-up.

Field mapping carried out along the escarpment bordering the southern Afar has revealed the presence of trachyte lava flows and tuffs up to 30 m thick and extending in an east-west direction parallel to the strike of the escarpment. The trachyte is intercalated with the Lower Sandstone Unit, which suggests a Late Triassic-Early Jurassic age for the volcanics.

The regional Mesozoic sedimentary sequence includes the Jurassic Limestone Unit and the Cretaceous Upper Sandstone Unit. These are absent from the eastern part of the study area but are extensive on the western side.

Tensional tectonics is expressed by block-faulting in the region underlain by the Lower Sandstone-trachyte intercalation whereas warping is characteristics of the region underlain by extensive Mesozoic-Tertiary units.

It is suggested that the study area constitutes part of an uplited landmass which was only peripherally affected by Mesozoic transgression. This uplift resembles the Tertiary doming in the region and may have been its precursor.

The Eocene limestones of the Pyramids plateau are characterized by landforms of stepped terraced escarpment and karst ridges with isolated hills. The carbonate country rocks are also dominated by minor surface, surface to subsurface and subsurface solution features associated with karst products.

The systematic field observations eludicate the denudation trend of the minor solution features and suggest the origin of the regional landscapes.

The lithologic and structural characters of the limestone country rocks comprise the main factors controlling the surface and subsurface karst evolution.

The development of the karst features and the associated sediments in the study area provides information on the paleohydrolic, chemical and climatic environments involved in the origin of the karstification.

This suite is located in the Kedougou inlier where a Lower Proterozoic sequence is surrounded by unconformable Upper Proterozoic. This suite is a member of the Daléma supergroup which also includes some epicontinental sediments with numerous carbonate layers. In contrast, the Mako supergroup (located further to the west) is characterized by submarine volcanic or plutonic rocks (mainly tholeiitic) intruded by trondhjemites at around 2200 Ma.

After deposition of carbonate rocks alternating with sandstones or greywackes, the Dalé with some volcanism. The volcanic belt is characterized by dacites or andesites, by a lot of hypovolcanic rocks (accumulative microdiorites) and by its elongation along a N-S trend. This belt was intruded 1990 Ma ago by a plutonic intrusion with petrographic composition ranging from diorite to monzogranite. The two magmatic events are spatially linked but the granitoid develops a strong thermic metamorphism in volcanic and sedimentary rocks which contrasts with the very low grade regional metamorphism. Despite a hydrothermal alteration, petrographic and geochemical studies show that the Dalé,a volcano-plutonic suite is calc-alcaline. Some large amounts of iron ore have been found in skarnoid rocks along the belt near the granitoid.

The geotectonic significance is discussed; the Daléma suite could result from the subduction of oceanic crust as shown by Mako supergroup. However, it is more probable that the suite has spatial relations with large transcurrent faults according to a geodynamic model proposed for the Stephano-Permian calc-alcaline volcanism of western Europe. The sedimentary and tectonic context, the strongly linear position and the coincidence of a linear positive gravity anomaly with the volcano-plutonic belt are more consistent with the second interpretation.

The study of two of the intermontane molassic basins of the ‘Série pourprée de l'Ahnet’ shows that they developed independently both in time and space. The characteristics of their thick sedimentary infillings are quite different. The Ouallen basin is filled by essentially fine-grained sediments which were deposited in continental then marine or lacustrine environments; these sediments thicen from east to west. The In Semmen basin is characterised by coarser sediments which were deposited from south to north, in alluvial fan, fluvial, deltaic and slope environments. This basin is also characterised by an episode of carbonate sedimentation leading to the formation of a remarkable thin layer of carbonate, covering the entire sedimentary area, and perhaps related to a volcanic rhyolitic event. The history of the two basins is also marked by obvious tectonic events simultaneous with the sedimentation and related to the recurrent faulting of major Pan-African faults. These led to the formation of very coarse fanglomerates located near the fault scarps, and are probably responsible for the shape and the evolution of the basins.

Progress within mining exploitation and recent drilling provides additional data on the stratigraphic and structural setting of the Akjoujt's copper mine. We confirm the synclinal pattern of the mineralized carbonate layer. The mineralization is linked to the sediments of a volcano-sedimentary basin, reactivated during metamorphism which occurred during the late Proterozoic or the early Paleozoic time. Then, two folding phases, hercynian to late hercynian, generated a trap-shape structure owing to their local dispositions. The supergene processes enhance the Cu-enrichment of the oxidized zone. The mineralogical and geochemical features of metavolcanic rocks together with the geological environment may be related to an active plate margin.

Sediments of Maastrichtian and Paleocene age, deposited in the Trans-Saharan seaway, indicate littoral tidal flat conditions predominating, with periodic extensions of more fully marine conditions over the area. An unconformity between the Maastrichtian and Paleocene is described.

Evidence is given of a climate becoming increasingly humid through the Maastrichtian and Paleocene, giving rise to increased rates of chemical weathering. This led to the deposition of marine limestones and oolitic ironstones when less clastic sediment was available. The oolitic ironstones were not, therefore, formed within a soil profile as various authors have claimed.

More than 70 small younger granite masses and 10 ring complexes are exposed in the Southeastern Desert of Egypt (about 80,000 km²). The granites comprise three main varieties: the Sn mineralized; stanniferous and Sn barren granites. the Sn mineralized granites are usually smaller in size; circular to elliptical in outline and contain higher concentrations of Sn, Nb, Be, Mo, Bi, Y, Cu and Pb and are usually subjected to greisenizaiotn and albitizationn processes which changed their original texture and composition with the development of lode and disseminated cassiterite and beryl mineralization. The stanniferous granties may contain only few accesories of cassiterite. the sn-barren granites are usually larger in size and show hghier Zr values. The granties are calcalkaline, highly aluminous, enriched F with Na_aO > K₂O and low CaO and MgO. The ring complexes comprise alkaline rocks mostly of Cretaceous age (although some complexes are older) and range in composition from ultramafic to acidic and from undersaturated to quartz bearing. Disseminated U, Th, Pb, W, ?Nb, Y, Be and Mo are recorded in some ring complexes.

The Nigerian basement consists of Eburnean granitic and metamorphic rocks into which are folded Upper Proterozoic supra-crustal low grade metasedimentary and metavolcanic rocks forming N-S elongate belts. Pan-African granitoids mark the last major event and they have intrusive and/or tectonic relationships with the earlier units. Structural features are often complex. The Basement can be divided into the Eastern and Western Provinces, where the latter is marked by the occurrence of a variety of metallogenetic types and greater development of the supracrustal N-S belts, while the Eastern province is dominated by Pan-African granitoid rocks and a paucity of metallic mineral occurrences.

The principle metallogenetic types that are (or have been) exploited are iron deposits, gold vein mineralization and SnTaNb-bearing pegmatites. Small occurrences are known of chromite, asbestos, CuNi sulphides, manganese and a variety of industrial minerals including talc, kyanite and magnesite. Metallogenetic types found in other Pan-African terrains that are markedly absent in Nigeria include basemetal deposits of plate accretion type (e.g. Saudi Arabia) and uranium (e.g. of Rossing type). It is suggested that Nigeria's relatively low metal production from basement deposits is attributable to a regional low metallogenetic inheritance which in turn has discouraged intensive prospecting but there is good potential especially for industrial mineral deposits.