GEOLOGICAL AND GEOTECHNICAL ASSESSMENT OF SOME DERELICIT BARITE FIELDS IN THE ABAKALIKI BASIN, SOUTHEASTERN NIGERIA
The proliferation of abandoned surface excavations in the Benue Rift not only contributes to land degradation and landscape disruption but also results in loss of economic deposits, dearth of mining data and generation of unprotected spoils with known potential to contaminate ground and surface water sources. This study employed field description and measurements, laboratory testing and numerical simulation techniques to evaluate some derelict barite fields in the rift with the aim of factoring geological and geotechnical issues that will enable the reclaiming of untapped reserves and forestall the creation of abandoned excavations. Field studies indicate a consistent rock sequence in which exposures of arkosic sandstone are overlain by profusely fractured shale and both rock types play host to barite ore deposits, brine ponds and intrusives. The area displays some major lineaments which exhibit, in order of decreasing magnitude, N-S ,NE-SW, NW-SE, and E-W structural styles. Ore deposits occur in varied modes as disseminated nodules, strata-bound deposits as well as in two dominant NW-SE and N-S trending vein sets with steep dips. The modes of occurrence, structural styles and lithofacies associations predispose the ore deposits to manual extraction and vertical stripping using surface excavations. The complex and varied geologic setting of ore incidences predicate significant errors in interpretation of their geophysical signatures and obtaining reliable ore quantity estimates. Ore grades vary widely due to the varying geologic framework and appear to depend on such factors as mining depth, presence of gangue mpinerals and location within the barite fields. Groundwater flow is constrained in an unconfined setting by the fractured and weathered sections of the lithologies, driven by structural dips and topographic gradient with the ores and intrusive rocks forming as seals. The extraction of the permeable barriers by excavation inadvertently reverses and directs flow towards the excavation leading to groundwater incursion and slope failures. The use of diversion ditches, sump and pump technique and slope unloading will achieve stable excavations. Unreliable reserve estimates, ore grade migration, groundwater invasion and the associated slope distress are the major impediments that render surface excavations derelict. Optimum location of excavations should be preceded by exploratory drilling and analysis of groundwater flow regime.