Environment of ore formation and anchizonal metamorphism in Pb-Zn-fluorite-barite deposits of the Benue Trough, Nigeria

Abstract

Lead-zinc-fluorite-barite vein ore-bodies occur in the Benue Trough within the Lower Cretaceous (Albian) sequence of this intracontinental rift structure. The veins are distributed in a series of N/S and NW/SE fracture systems cross-cutting the Trough axis. Ore-hosting sediments range from deep marine carbonaceous shale in the lower Benue area to platform carbonates (middle Benue) and fluvial to deltaic sandstones in the upper Benue area. These sediments are invaded by mafic to felsic intrusives which are either pre or post-ore. Fluid-inclusion temperatures of vein minerals range from about 105°C to over 200°C and ore-fluid salinities vary from 14 to 25 equivalent weight % NaCl. Reflectance (Rm) of finely dispersed vitrinite in the ore-hosting sediments reaches up to 4.3% at vein centres in the lower Benue, decreasing to ca.2.6% at about 2km from known veins. In the middle Benue, Rm values vary from 0.9 to 1.7%. This suggests that ore-hosting shales in the lower Benue are slightly metamorphosed having been heated to an estimated temperature of about 240°C before the Santonian peak of tectonism. Illite-crystallinity indices on these shales in places suggest anchi to epimetamorphic contact aureoles adjacent to intrusive bodies. Our data suggest that the Benue ore-bodies were formed by hot, evaporitic, basinal brines set into motion by the high geothermal gradient accompanying continental rifting. Anchimetamorphic conditions were reached in shales of the lower Benue area where the largest ore-bodies occur.