The Benue trough is Nigeria's main, but not exclusive source of barite mineralisation. There are at least ten barite fields in the trough, each containing swarms of veins or concordant stratiform minerals flats of hydrothermal origin. There are only two vein trends in the trough: the NW-SE trend, which tends to be orthogonal to the axis of the trough; and the N-S to NNE-SSW trend, which is younger than the former. Both vein sets are formed from ac tension joints reflecting different post-sedimentary deformation phases in the trough. The NW-SE veins are also more frequent than the N-S veins, almost in the ration of 2:1. Although 86 veins have been studied, not one was found oriented parallel to the axis of the trough, a good indication of structural control on mineral vein orientations. The dips of the veins tend to be high (>80 o ), a condition that favours manual mining. Veins tend to display a simple block profile and their widths vary from a few centimetres to as much as 6m, though the most frequently occurring widths are between 50cm and 1m. The implication of this is that BaSO 4 concentrations in hydrothermal fluids in the trough were probably low during the Cretaceous period. In many veins, there is barite quality contrast between the top part and the lower portions. Higher quality barite tends to come from the lower portions. Hence it is expected that much of the low quality stigma associated with Nigerian barite from the trough would be relieved when mining gets down to good depths. For this reason, mining in those veins of low quality barite should proceed by vertical stripping.
The Precambrian polycyclic basement terrains of Oban and Obudu in southeastern Nigeria contain a plethora of fractures, which on analysis fall into only four sets or trends. The most prominent fracture set in Oban Massif is the NNW-SSE, with a trend of 1500-1600 from the north. Others are NNE-SSW, E-W and NW-SE sets. The most prominent fracture set in Obudu is the NW-SE which trends 140-1500 from north. Minor sets occur in the NNE-SSW, E-W and ESE-WNW directions. While E-W fractures are interpreted as ‘ac’ tensile or extension fractures, NE-SW and NW-SE fractures are shear fractures. The orientation difference of 100 between the major facture sets in Oban and Obudu is interpreted as the amount of fracture induced angular rotation between the two basement blocks, leading to the formation of the Ikom-Mamfe embayment. The most frequently occurring fracture length in both areas is ~ 2km, while the fracture frequency decreases exponentially as fracture length increases. This is depicted by the exponential relationship n(l) = noe-kl. There appears to be compensation between fracture length and frequency, such that directions of long fractures have lower density and vice versa. Whenever fracture lineaments less than 5km are ignored in the Nigerian basement, it could represent over 60% loss of data in the analysis
The evaluation of an intra-plate earthquake that occurred on Friday, 11 th September, 2009 and felt in parts of Abeokuta, Ago-Iwoye, Ajambata, Ajegunle, Imeko, Ijebu-Ode, Ilaro and Ibadan, all in south western Nigeria is presented. This event has been the largest inland earthquake recorded since the inception of digital seismological recording in Nigeria in 2008 was incepted. The event was recorded by three seismological stations operated by Centre for Geodesy and Geodynamics (CGG), Toro. Data obtained from the CGG stations and others distributed around the world were analysed to determine precise earthquake locations and focal mechanism and to assess the regional tectonic stress. The data recorded in MiniSEED format at a sampling rate of 40 samples per second (sps) were analyzed using the SEISAN earthquake analysis software. The result showed an epicentral location situated about 108 km west of Lagos, a focal depth of 10.0 km and an origin time of 03:10:21.60 GMT. The local and moment magnitudes were 4.5 and 4.2 respectively. The P-wave to S-wave velocity ratio was 1.72. The fault plane solutions obtained for the rupture process indicated that a normal dip-slip fault with median solution of strike 325˚, dip 40˚ and rake −90˚ was the probable trigger mechanism for this earthquake. It suggested that the event was a reactivation of a buried high-angle fault in the Precambrian basement represented by the contemporary northeast-southwest trending regional horizontal compressive stress. Generally, the seismotectonics of the region was linked to the fracture zones in the Atlantic Ocean.
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