Journal of African Earth Sciences (1983)最新文献

The Wadatta Limestone, composed of biosparite and biomicrite, is a marine member of the otherwise fluvial Makurdi Formation (Turonian). The carbonate unit exhibits calcite cement precipitation in both primary and secondary pore spaces. Replacement fabrics were created when the original aragonite of shells was dissolved and simultaneously precipitated as calcite crystals of various sizes, in some cases with oriented inclusions left as relics of the former texture. The original microcrystalline aragonite matrix was neomorphosed, mostly beyond the micrite barrier, into microspar and pseudospar.

These diagenetic effects—precipitation of sparry cement and calcitization of both molluscan shells and matrix—are diagnostic of meteoric phreatic environment. The change from marine to freshwater phreatic condition is suggested to have transpired when extensive regimes of meteoric groundwater flushed away marine waters from the carbonate deposit after the Turonian regression led to subaerial exposure in thet part of the Benue Trough.

About 80 large felsic plutons were emplaced in the Midyan region of the northwestern Arabian Shield between 725 and 570 Ma ago. Polymetallic mineralization, including Mo, Nb, REE, Sn, Ta, U, W and Zr, is associated with particular varieties. Five intrusive suites of regional extent and five other plutons have been identified. The oldest extensive unit is the Muwaylih suite, consisting of trondhjemite, tonalite, diorite and gabbro, with generally trondhjemitic compositional affinities and no known mineral potential. The most voluminous is the Ifal suite, a heterogeneous assemblage of biotite—hornblende monzogranite, granodiorite and quartz monzodiorite in large, oval to irregular plutons characterized by numerous intraplutonic dikes, but with no known mineral potential. The Ifal suite is intruded by plutons of the Atiyah monzogranite, with a variable number of dikes. The Muwaylih, Ifal and Atiyah suites are chemically metaluminous, but the Midyan suite consists of alkali granite to alkali-feldspar granite which is metaluminous to peralkaline. Large, irregular plutons of the Haql suite are composed of perthite leucogranite which is metaluminous to marginally peraluminous or peralkaline.

Plutonic rock units of local significance include the Lawz complex, consisting of commonly granophyric syenogranite to monzogranite, the Mowasse quartz syenite and the Sawda (nepheline syenite) complex.

Polymetallic NbZr deposits, such as those at Jabal Tawlah and Ghurayyah, are the most important known mineral resources and are related to alkali granite of the Midyan suite. Plumasitic specialized plutons, such as the Ratamah granite, have weak SnWTaNb mineralization, and are related to the Haql suite. Apical and contact zones of plutons of these suites are favorable exploration target areas.

The Jabal Hamra silexite, a crescent-shaped stock 300 m long by 100 m wide, averages over 6000 ppm combined REE and is the Kingdom's highest-grade resource of these elements. It is anomalously radioactive (total-count gamma radiation 1000–3000 cps), has high average contents of U (75 ppm) and Th (263 ppm) and is also enriched in Nb, Zr, Y, Sn and Ta.

The silexite crystallized as a pressure-quenched rock resembling aplite, and was subsequently pervasively cataclased. It was derived by differentiation of a quartz alkali-feldspar syenite magma. Petrologic continuity can be demonstrated from quartz alkali-feldspar syenite through leucocratic and amphibole-bearing alkali-feldspar granite to silexite. Although the geochemical signature of the mineralization resembles that of mineralized Arabian alkali granites, the nature of the associated rocks and therefore the genesis of the deposit are significantly different.

In eastern Senegal and southern Mauritania, the thrusted and more or less metamorphosed units of the Mauritanides belt involve similar lithostratigraphical sequences including, from the bottom to the top, a green volcano-sedimentary series (Nagara and Bouly groups) then a detrital red series (Ndouméli group) and a quartzitic series (Ndiéo group). In the more external part of the belt these sequences are completed with coarse sandstones which are thought to be the equivalent of upper Ordovician sediments of glacial origin (Sakha group) and, at the top, red fossiliferous sandstones of Devonian age. All of these sequences can be correlated with the upper Proterozoic-Devonian sedimentary sequence of the West African craton. The facies variations as well as the volcanic features allow one to define four main structural zones that are, from east to west: an Autochthonous, which includes the western margin of the Taoudeni basin, a Parautochthonous, displaced but not metamorphosed, an External Zone which comprises basic and ultrabasic rocks, and an Internal Zone marked by acid and andesitic metavolcanism. These zones are more or less linked with palaeogeographic domains which were created during an upper Precambrian distensive phase occurring prior to the Mauritanides orogeny. The detrital sedimentation is regarded as the result of the erosion of structures which arose during different stages of a compressive taconic-hercynian event.

As part of a Benue Trough project, a pseudogravimetric study of the Benue Trough is being carried out and preliminary results of this study are here presented. Several aeromagnetic profiles have been taken across the Benue Trough and transformed to their corresponding pseudogravimetric profiles using the equivalent layer method. Qualitative interpretation and comparison of the pseudogravimetric, aeromagnetic and gravity profiles have been carried out in order to find possible origins of the short and long wavelength anomalies observed over the trough. The results of this analysis point to the fact that the short wavelength anomalies are possibly due to the minor intrusive bodies at shallow depths, while the medium to long wavelength anomalies are possibly due to the combined effects of variations in the magnetic susceptibility and topography of the underlying basement and deeply seated intrusive bodies.

The Tessalit deposit comprises a massive sulphide mineralization (up to 7% Zn, 1–2% Pb and less than 1% Cu) associated with carbonate and magnetite layers. It lies within a submarine acid dominated volcanic and pyroclastic complex belonging to an Upper Proterozoic island arc system (Tilemsi Group). A dome of porphyric rhyolite, brecciated in situ and pyritized occurs 1 km north from the Tessalit deposit; this dome is surrounded by laminated acid tuffs and tuffites. It is proposed that like many massive base metal sulphide bodies, the Tessalit deposit formed by volcanogenic and exhalative processes. The Tessalit mineralization comprises bedded sulphides associated with carbonate and iron rich exhalites and with waterlain acid tuffs. The orebody displays a mineral zoning with an upward change from a magnetite facies to a carbonate-copper sulphide, then a carbonate-sphalerite facies. Pan-African deformations have determined the actual complex geometry of the ore lenses. Other sulphide mineralization occurrences are known in other parts of the Tilemsi Group or equivalent volcano-sedimentary series. Gold occurrences may be also expected in these series.

Ages of potassic, undersaturated lavas from the Karisimbi volcano have been obtained by applying unspiked potassium-argon dating. All age determinations from this study range between 0.14 ± 0.06 and 0.010 ± 0.007 Ma, constraining the formation of the main cone and its parasitic structures to the Late Pleistocene. The history of the volcano, as it has been reconstructed considering the field relationships, can be confirmed by the obtained ages.

The Guilliz volcano is composed of upper miocene and pliocene suites (8.0–4.9 Ma) of shoshonitic and alkaline affinity. The shoshonitic suite is comprised of absarokites, shoshonites, latites and trachytes. Lavas with intermediate compositions are characterized by the presence of biotite and K-feldspar.

Alkaline lavas are, for the most part, hawaiites. The hawaiites are found as aphanitic inclusion in the shoshonitic series. Amphibole bearing absarokites and shoshonites erupted in the pliocene have compositions similar to that of the hawaiite inclusions.

Electron microprobe analyses of oxides, olivines, pyroxenes, amphiboles, biotites, feldspars and glasses indicate a multi-stage evolutionary process. Shoshonitic lavas appear to differentiate through crystal fractionation, leading to a zoned magma chamber. An attempt has been made to calculate the mineral assemblages and the proportion involved in the fractionation process.

The existence of an episode of magma mixing is evidenced by geological observations, and corroborated by chemical evolution of the lavas. The mixing occurs between shoshonitic and hawaiitic liquids. The mixing liberates a K-rich vapor phase from the hawaiitic magma, and this vapor contributes to the K-enrichment of the liquids present in the upper parts of the magma chamber.

The beds with Halimeda nana Pia contain in the type-locality of Timahdite (Middle-Atlas, Morocco) a lot of badly preserved and incomplete Dasycladales like Broeckella, Dissocladella, Neomeris, Acicularia, fragments of Zittelina, Carpentella and unditerminable Bornetellae, as one fragment of Archeolithothamnium, lost within all these green Algae. These beds belong to the Timahdite pink limestone, previously studied by Pia et al. (1932, Notes et Mem., Serv. Géol. Maroc, 20, 13–18) and intercalated in a formation which, assigned to the Paleocene-Lutetian interval, fills up the axis of the El Koubbat syncline.