Origin of lead-zinc mineralization in the southern Benue Trough, Nigeria ? Fluid inclusion and trace element studies

Abstract

The epigenetic Pb-Zn deposits of the southern Benue Valley (Nigeria) are localized within Cretaceous sediments of an intracontinental rift basin. Fluid inclusion studies of vein minerals from the Abakaliki and Ishiagu orebodies show that sphalerite and quartz were deposited at relatively low temperatures (102–175 °C), with ore-fluid salinity mostly in the range of 17–25 equiv. wt% NaCl. Trace-element contents of sphalerite and galena are also consistent with the low temperature of formation and epigenetic origin. On the basis of the geotectonic setting, the mode of occurrence and fluid-inclusion characteristics, mineralization is attributed to connate brines set into motion by a high geothermal gradient accompanying continental rifting. Mineral deposition was caused principally by rapid cooling due either to reaction with wall rocks or mixing with meteoric or descending water of low salinity.