Journal of African Earth Sciences (and the Middle East)最新文献

A study of lithological logs from forty (40) wells drilled in twelve (12) localities in parts of the Niger State of Nigeria and the interpretation results of the parametric vertical electrical soundings (VES) were carried out to identify the geoelectric/hydrogeologic characteristics of the basement complex area.

Five (5) aquifer types were identified. These include the weathered aquifer; the weathered/fractured (unconfined) aquifer; the weathered/fractured (confined) aquifer; the weathered/fractured (unconfined)/ fractured (confined) aquifer and the fractured (confined) aquifer. The mean groundwater yield for the aquifer types varies from 0.83 L/S for the weathered layer aquifer to 3.0 L/S for the weathered/fractured (unconfined)/ fractured (confined) aquifer.

The fracture frequency increases with depth and reaches a maximum at between 25–35 m for granite, gneiss and schist but decreaseswith further increase in depth. The cumulative fracture frequency is maximum in granite and minimum in schist.

The fracture thicknesses are maximum (greater than 3 m) and occur most frequently within the depths of 10–40 m. Minimum fracture thicknesses (≤ 1 m) occur most frequently between the depths of 50–70 m. Fractures rarely occur at depths greater than 90 m An optimum borehole depth for the study area is 60–70 m.

The Umm Rus gold lode is housed along fractures in granitoid-gabbroic rocks, being largely controlled by a NE-SW trending fracture system that affected the Eastern Desert. Mineralogically, the gold lode consists of quartz and carbonate gangue enclosing minor amounts of auriferous pyrite and arsenopyrite. Trace amounts of sphalerite, galena, marcasite and pyrrhotite are also present. The lode can be divided into: (i) Au-poor, pyrite-quartz vein, (ii) Au-rich, pyrite-arsenopyrite-quartz vein and (iii) gangue dominant. Inspection of primary inclusions from the Umm Rus gold lode showed that the ore was formed from CO₂-H₂O-rich fluids (ca. 30–46 mol % CO₂) of low salinity (6.75–7.75 wt. % NaCl equiv.) and alkaline to neutral pH with a density of 0.76–0.85 g/cc. These data are consistent with dissolution of gold as a bisulphide complex. Deposition of Au most likely occurred over a temperature range of 250–300°C and at pressures around 0.35 Kbars. The deposition may have occurred in response to separation of a liquid CO₂-phase from an originally CO₂-H₂O-rich aqueous fluids. The style of mineralization at Umm Rus bears certain resemblances to Au-bearing quartz veins in the Archaean deposits of Canada and Australia and the “Mother Lode” deposits of the U.S.A.

This paper discusses some of the problems related to the palaeoenvironmental interpretation of non-fossiliferous, early Precambrian, recrystallised quartzitic sandstones, using the Early Proterozoic Daspoort Formation, Transvaal Sequence of southern Africa as a case study. These cross-bedded and planar stratified rocks have been interpreted previously as shallow marine deposits, based on limited studies of areas with well-exposed, relatively undeformed outcrops. This postulate rests largely on the apparently mature nature of the recrystallised sandstones and their thin bedding. Examination of outcrops throughout the preserved basin, including those which have been deformed and metamorphosed, reveals the presence of subordinate immature sandstones. Lateral facies relationships permit an alternative distal fan-fluvial braidplain model to be proposed. This is compatible with collected palaeocurrent data, thicknes trends and results of thin section petrography.

Archaean diamictite occurs in the Pongola Sequence, exposed in the southeastern part of the African subcontinent. Four diamictite units are developed in a mudrock-dominated interval which is interbedded with arenites of the 5000 m thick Mozaan Group. The most prominent of these diamictites is 80 m thick. The rock is black and comprises a homogeneous matrix supporting sparse clasts that are characteristically varied in composition. Some clasts are striated and faceted. The diamictite essentially represents a mudflow deposit which was emplaced in a marine shelf environment. Sediment was delivered to a subsiding basin by downslope mass movement from a fault-bounded, elevated margin where highland glaciers are likely to have contributed clastic detritus. The diamictite would thus represent a reworked admixture of glacially-derived debris and argillaceous gravity flow sediment related to tectonic activity along the basin margin. Whereas the final depositional mechanism involved subaqueous mass-flow, the presence of striated stones, the heterogeneous clast composition, and major element chemical data support a glacial interpretation for the diamictite. The sparse record of glaciation during the early history of the Earth warrants documentation of such Archaean diamictite with possible glacial affinities.

Geophysical and sedimentological studies on two lake deltas in Northern Lake Malawi have been carried out. The architecture of subaqueous fan deltas and associated channel-levee complexes indicates that the deltas are fluvially dominated, presumably due to a high influx of detritus relative to the levels of basinal energy. Fan size, thickness and gradients appear to be directly proportional to the amount of sediment funneled to the fan. The data show a consistent pattern of sediment facies extending from the lake shores to the deep basins. This pattern is the gradation of nearshore sands into regions of widespread slumping, then into regions covered with sediment waves, which merge with hemipelagic sediments in open lake environments.

The Levant Volcanic extends through northeastern Jordan and Syria to the Golan Heights. Minor volcanic fields also developed in the northern Israel, around the Hula Valley (northern Israel) and in southern Lebanon. This province is a part of an extensive volcanic field which developed during the Cenozoic in the northwestern part of the Arabian plate, aligned roughly in a northwestern direction, subparallel to the Red Sea. The location of the Golan Heights is significant because of its proximity to the Dead Sea Rift Valley, which is assumed to be a transform fault on the Red Sea spreading system.

The volcanic sequence in the Golan Heights and its surroundings, named the “Bashan Group”, overlies the Lower Pliocene regional erosional unconformity. This group is subdivided into five rock units which differ in certain aspects of their morphologic attributes as well as in their radiogenic ages, based on 175 K-Ar age determination made on samples from 54 selected sites:

• 1.

  1. Lower Pliocene basalts (5.0-3.Ma.)

• 2.

  2. Upper Pliocene basalts (2.9-1.7 Ma)

• 3.

  3. Lower Pleistocene basalts (1.6-0.7 Ma).

• 4.

  4. Upper Pleitocene basalts (0.4-0.1 Ma.)

• 5.

  5. Holocene basalts (no data yet).

The Lower Pliocene basalts create a large volcanic plateau from southern Syria to the Lower Galilee. The present Dead Sea Rift in northern Israel began to subside at the end or after this phase, and the Upper Pliocene basalts flowed into it from Lebanon to the Hula Valley. Pleistocene volocanic sources were recognized only in the eastern side of the Rift.

The mean rate of tectonic lowering of the Dead Sea Rift bottom south of the Sea of Galilee is estimated to have been 0.24 mm/yr. The incision rate of the Yarmouk River (the main tributary of the Jordan River) was 0.13 mm/yr, and the rate of the infilling of the Rift in this region was 0.11 mm/yr.

Interpretation of aeromagnetic and radiometric geophysical data is carried out in an area of the Precambrian basement rocks of the south Eastern Desert of Egypt. The area is delimited by lat. 24°00′ and 25°00′N and long. 34°00′ and 35°00′E.

Aeromagnetic anomalies in the area reflect important features on basement tectonics, on deep-seated structures and on detailed geological mapping. Major faults and shear zones, which play an important role in the emplacement of mineralized bodies, have been interpreted and two tectonic blocks are suggested. The statistical analysis of the basement fractures and aeromagnetic lineaments shows major NNW, NW and ENE trends with intersections indicating locations of magmatic intrusions and alkaline ring complexes. The depth computations indicate shallow to near surface magnetic sources as well as deeper ones. The constructed residual, second- derivative, upward-downward continuations and regional maps respectively emphasize these features. The surface rocks of basic -ultrabasic affinity are reflected on the magnetic map. Other rock units give low magnetic effects that indicate variations in lithological composition and/or the degree of metamorphism.

The total count-radiometric map shows a close relationship between the alkaline rocks (e.g. ring complexes) and younger granites as well as strong radioactive indications of uranium and/or thorium mineralization (e.g. G. Abu Khurq and G. Kahfa). The linear radiometric anomalies indicate locations of fault lines which are mineralized with radioactive minerals, such as at G. Hafafit area. These faults are also interpreted from the magnetic map. Additional locations are recommended for further ground geophysical and geological explorations.

The Adola volcano-sedimentary belt of southern Ethiopia has been referred to as a late Proterozoic ophiolitic belt within the Mozambique belt of East Africa. The bulk chemistry of three units - an amphibolite, a tonalite and a calc-alkaline granite - to the west of the main axis of this belt indicate that they are related to an oceanic arc: the amphibolite and the tonalite are chemically similar to low-K tholeitic mafic rocks and calc-alkaline plutons in island arc settings respectively.

From these data, and previous regional works it is inferred that magmatic rocks in the Adola belt were emplaced in a late Proterozoic immature oceanic arc that appears to have evolved over a west dipping subduction zone.

Analyses were made of sulfur isotopes and major and minor element (Fe, Pb, Zn, Cu, Hg, Cd, Bi, S, As, Tl, Sb, …) contents of a well characterized suite of minerals from ore deposits wether associated with vulcanism, metamorphism, Cretaceous sedimentary facies or Tertiary detrital-rich and molasse deposits. Paleogeography, tectonism and ore genesis are closely related: periods characterized by intense deformation (Middle and Upper Jurassic, Middle and Upper Cretaceous and Oligocene-Miocene) enable deposition of polymetallic syngenetic/epigenetic sulfide deposits; in contrast, during tectonically less active periods, the ore genesis appear to be clearly restricted. Mineralogy, sulfur isotopes, abundant sulfosalts and minor element abundances (Ag, Hg, Cd, Bi) point to important remobilization and repetitive mineralization stages the latest of which postdated the Alpine orogeny. The origin of Upper Cretaceous mineralizations hosted in local black shales, is thought to be found in metals brought up from deep-seated, eventually endogenic hydrothermal fluids. These deposits may later act as a transition base metal source for Tertiary deposits, during and after the Alpine tectogenesis, along with the new-additions of metals from presumably endogenic sources. Even though admixtures of diagenetic connat fluids somehow hampered geochemical information, it is concluded that ore deposits from Northern Tunisia are mixed-type and closely related to paleogeographic environments and tectonical history of the region. The previous prominent role of Triassic salt diapirism as a metallogenic factor is discussed but not confirmed.