Origin of the brines associated with the lead–zinc–barite–fluorite deposits in Benue Trough, Nigeria: Evidence from stable isotope studies

Nigerian sedimentary lead–zinc–barite–fluorite-deposits which covers along 600 km long Benue-Trough extends from south of Abakaliki District in Lower-Benue to Zurak-Wase Districts in Upper-Benue. Highly saline subsurface waters issuing out as springs, streams and wells are quite ubiquitous throughout the mineralogical-belts. This research intends to determine the origin of hydrothermal fluids responsible for deposition of lead–zinc–barite–fluorite deposit in basin. Brine samples were collected from three principal sources (springs, wells and stagnated-streams) and hydrogen and oxygen isotope composition of the brines were analysed using well established analytical methods. Isotopic ratios were then determined on H₂ and CO₂ gases in Micromass Mass-Spectrometer. H- and O-isotopic studies of these subsurface brine indicate fairly large range of both δD and δO¹⁸ values between − 13 and + 10‰ and − 14 and + 0.8‰, respectively. Salinity of these waters were also highly variable from 0.1 to 14.2 g/L of Cl. If brine with Cl contents less than 1 g/L are considered, the δD values fall within − 15‰ to − 5‰ which closely represent the isotopic composition of present day meteoric waters in Benue-Trough. Heavier δD values were interpreted to represent fossil meteoric or basinal brine. The close similarity between isotopic compositions of brine and lead–zinc deposits suggest that present day meteoric waters is probably replica of paleo-formation waters from which the sulphides were deposited.