Rare earth element and stable sulphur (δ 34S) isotope study of baryte–copper mineralization in Gulani area, Upper Benue Trough, NE Nigeria

• Baryte and copper mineralizations have been located within sandstones and granites.
• REE patterns of the unaltered rocks show high LREE, low HREE and weak negative Eu.
• REE patterns of the separates show separation of the copper and baryte sub-systems.
• The patterns indicate negative and positive Eu for the copper and baryte sub-systems.
• Stable sulphur isotope range of 12.3–13.1‰ shows formational source of the sulphur.

The geology of Gulani area comprises of inliers of diorite and granites of the Older Granite suite of the Pan-African (600 ± 150 Ma) age within Cretaceous sediments of the Bima, Yolde and Pindiga Formations and the Tertiary/Quaternary basalts of the Biu Plateau. Epigenetic baryte–copper mineralization occurs as baryte veins within the Bima and Yolde sandstones and fracture-filling malachite in Pan-African granites. Unaltered (distal), hydrothermally altered (proximal) granites and sandstones and vein materials (mineral separates of baryte and chalcopyrite/malachite mineralized rocks) were analysed for rare earth elements (REE) and stable sulphur isotopes. The REE patterns of the unaltered rocks (both granites and sandstones) indicate background values before mineralization, depicted by enriched LREE, depleted HREE and weak negative Eu anomalies typical of Pan-African (calc-alkaline) granites and sandstones derived from them. On the other hand, the hydrothermally altered and mineralized rocks and mineral separates show a distinct baryte and copper mineralization sub-systems characterized by similar high LREE and corresponding low HREE abundances. However, the negative Eu anomalies of the copper sub-system hosted by granites are typical of Pan-African (calc-alkaline) granites. The sandstone host rocks of the baryte sub-system are marked by positive Eu anomalies interpreted as reflecting the injection and subsequent deposition of the baryte-bearing hydrothermal solutions under oxidizing conditions. The baryte mineral separates show δ (34S) isotope range of 12.3–13.1‰ (CDT) indicating sulphur from sedimentary formation sources. This ruled out magmatic source of the sulphur from the nearby Tertiary/Quaternary volcanic rocks of the Biu Plateau as well as ocean water. However, the stable sulphur isotopic determination of the sulphides (chalcopyrite/malachite mineral separates and mineralized rocks) did not yield peaks and therefore no inferences drawn in this regard.