Fluid inclusion and stable isotope studies of Pb-Zn-fluorite-barite mineralization in the lower and middle Benue Trough, Nigeria

Abstract

Lead-zinc-fluorite-barite veins in the lower and middle Benue Trough (Nigeria) are located within the Lower Cretaceous (Albian) carbonaceous shales, limestones, and arkosic sandstones of this intracontinental rift structure. The veins in the lower Benue Through consist of sphalerite + galena+marcasite ± chalcopyrite ± barite in a gangue of siderite and quartz hosted by carbonaceous shales, whereas in the middle Benue Trough, fluorite, barite, quartz, and similar sulfide minerals are hosted by limestone and sandstone. Fluid inclusion temperatures in vein minerals range from 95°C to 200°C (without pressure corrections) and salinities range from 14 to 24 equiv. wt% NaCl. Oxygen isotope compositions of limestone wall rocks (middle Benue) have been lowered from premineralization δ18O values of about 25 per mil to approximately 16 per mil. Fluid in equilibrium with vein calcite has a calculated δ18O of +2.6 per mil at 130°C. The 87Sr/86Sr ratio of this calcite (0.71497) suggests that strontium and calcium had a considerably more radiogenic source than the Cretaceous limestone or evaporite did (87Sr/86Sr=0.7073−0.7078). Observed strontium data, lead isotope compositions of galena, and REE patterns in fluorite suggest that the Lower Paleozoic basement rocks in the trough or their weathered equivalents are likely sources for the Benue Trough ore components. Sulfur isotope data suggest that the sulfur was probably contributed from the Cretaceous evaporites in the trough.