Abstract

Geological mapping of the Gulani area revealed that the area consists of Precambrian Basement Complex rocks, Cretaceous sediments and the Tertiary/Quaternary basalts of the Biu plateau. The Basement Complex rocks are represented by diorite and granites while the sediments include the Bima, Yolde, and Pindiga Formations. Epigenetic baryte-copper mineralizations occur with the baryte veins restricted to the Bima and Yolde sandstones while the Copper mineralizations occur in granites. Petrographic study indicates that the diorite is largely composed of andesine plus hornblende and accessory quartz and iron oxide. The granites, on the other hand, consist essentially of quartz, microcline and orthoclase with minor biotite and iron oxide. The Bima and Yolde sandstones are composed of quartz, microcline, plagioclase and accessory biotite and opaque crystals. The large quartz and microcline crystals in both granites and sandstones are fractured. The Kanawa member of the Pindiga Formation is made up of shales interbedded with Limestones. The limestone interbeds are both crystalline and fossiliferous types. The basalts consist of labradorite laths, olivine, augite and opaques. Transmitted light petrography of the hydrothermally altered Bima and Yolde sandstones show the presence of quartz, baryte and microcline. The barite occurs as pore space in-filling mineral between the quartz and microcline and within the fractures of these brittle minerals. The Cu (chalcopyrite/malachite) mineralizations occur within highly fractured zones of the granites. Reflected light microscopy indicates that only relicts of the primary chalcopyrite exist while the veins/veinlets are now almost entirely occupied by the secondary mineral of malachite and its associated limonite which are the alteration products of the chalcopyrite.

Key words: Petrography, fractures, baryte-copper, mineralization, Gulani, Nigeria.