Geology and genesis of gold-bearing quartz veins at Bini Yauri and Okolom in the Pan-African domain of western Nigeria

Abstract

The Bini Yauri and Okolom primary gold occurrences are localized within the Precambrian to Lower Paleozoic schist belts of western Nigeria. These belts consist of gneisses, migmatite, quartzite, mica schist, phyllites, amphibolite, and granite which represent suites of metasedimentary, metavolcanic and intrusive rocks that are infolded into the Nigerian basement complex. Gold-bearing veins in the Bini Yauri lode occur as lenticular bodies within altered mica schists at the contactzone with a granite porphyry. At Okolom, the veins are hosted in sheared zones within a sequence of silicified biotite gneiss, amphibolite and schist. Vein contacts in the two deposits are generally sharp, steeply dipping at ca. 80°E and commonly contain stockworks and discordant stringers adjacent to the wall rocks. Vein constituents are essentially quartz, sericite, chlorite, albite, tourmaline calcite, magnetite and hematite. These are commonly intergrown with pyrite, pyrrhotite, chalcopyrite, arsenopyrite, galena, sphalerite and argentite which altogether may constitute up to 3% of the vein systems. Alteration minerals like sericite, chlorite, epidote, calcite and quartz are common in wall rocks adjacent to veins. The alteration minerals are commonly associated with quartz, magnetite, ilmenite, hematite, zircon, rutile and limonite. Fluid inclusion studies in vein quartz reveal a bimodal distribution of filling temperatures which suggests at least 2 temperature regimes centred on 170°C and 240°C up to a maximum of 320°C during mineral deposition. Salinity estimates for the ore fluid average 1.5 equivalent weight percent NaCl and ore precipitation appears to have taken place at a minimum depth of about 1.4km. Our study of the contacts, shape, petrography and fluid inclusion aspects of the Bini Yauri and Okolom vein systems suggests that gold mineralization in the two localities and in the Nigerian schist belts in general may have evolved as a result of the metamorphic dewatering of thick sequences of clastics, shales and their associated volcanic rocks within the Precambrian to Lower Paleozoic basement complex. Several stages of remobilization and reconcentration of vein constituents appear to have taken place during succeeding thermotectonic events.