Fracture Orientation Significance in the Study of Pb-Zn Mineralization of Lower Benue Trough, Abakaliki Se Nigeria

Abstract

The Lower Benue Trough, in South-eastern Nigeria, hosts several lead-zinc deposits. The mineralization is frequently hosted by Cretaceous black shales that appear to have undergone low grade metamorphism, and subordinate sandstones. Most parts of the deposits have been epigenetically formed, and occur as blocky shaped, discordant veins, fracture, and possibly open-space fillings, of hydrothermal origin. For this reason, structural evaluation constitutes an important criterion for the investigation of the mineralization. In particular, the orientation of fractures occurring at the Pb-Zn deposits can be useful in the differentiation of the relative ages of the mineral vein deposits. A total of one thousand, one hundred and forty-seven fractures around mineralized zones, at the study localities, were measured and analysed. Results indicate that the vein deposits are structurally controlled, have four main orientations, and most likely formed by crack-seal process. The most prominent fracture sets show NW-SE, NNE-SSW to N-S orientations, while the less prominent ones are displayed in the NE-SW and ESE-WNW directions. The NW-SE, NNE-SSW to N-S fracture sets is interpreted as ‘ac’ extension fractures from two different deformation episodes that affected the trough, while the NE-SW, ESE-WNW to E-W sets is the two ‘bc’ tensile fractures, respectively parallel to the axes of F1 and F2 folds which occur in the trough. Small variations of the NNE-SSW and ESE-WNW brought about the occurrences of N-S and E-W sets respectively. Indication are that the two deformation episodes affected the trough: the first episode produce the NE-SW (F1) fold axes, while the second less intensity episode produce the ESE-WNW (F2) folds. The most dominant mineral vein trends in the study area are the NW-SE and NNE-SSW to N-S orientations, in which mineral veins are loaded in the ‘ac’ extension fractures. The less dominant mineral vein trends are the NE-SW and ESE-WNW orientations, which are in the ‘bc’ tensile fractures. At the NW-SE trend, vein minerals mainly occur along 150-160o from N, although it ranges from 135-175o from N. Vein width up to 7.0m can be obtained from both the NW-SE and NNE-SSW directions. The Lower Cretaceous sediments appear to be characterized by the NW-SE (major) and NE-SW (minor) sets of veins, while the upper Cretaceous sequence is most likely characterized by the NNE-SSW (major) and ESE-WNW (minor) vein sets. The widest Pb-Zn vein in the study area is 11.5m wide, located in the Enyigba axes sub-area and trends in the NE-SW direction with evidence of slickensides which indicates faulting.