GEOLOGY AND GEOCHEMISTRY OF IPERINDO PRIMARY GOLD DEPOSIT, ILESHA SCHIST BELT, SOUTHWESTERN NIGERIA

Iperindo is one of the few primary gold deposits known in Nigeria and is currently under assessment by the Nigerian Mining Corporation. It l ies in amphibolite-facies biotite granite-gneisses of Proterozoic age in the Ilesha schist belt and some 4 km to the east of a major crustal 'break' known locally as the Ifewara-Zungeru fault. Gold mineralizati on is contained in a zone of quartz-carbonate (mainly calcite) veins a nd hydrothermally altered gneisses with associated small-scale intrusi ons along a second-order, steeply dipping fault zone that roughly para llels the main Ifewara-Zungeru fault. Gold is present in the veins mai nly as discrete particles, up to 100 mu m in size, at grain boundaries between quartz and carbonates, in general association with sparse, sc attered pyrite (two generations), pyrrhotite and rarer base-metal sulp hides. Adjacent to the gold-bearing zone the granite-gneisses have bee n hydrothermally altered under conditions that can be approximated as constant alumina, with consequent minor volume increase. Within the al teration envelope K2O, S, Pb and Cu (and Au) are the most notably enri ched components. Concentrations of Th, Y, Ce and Zr in the granite-gne isses are abnormally high, which implies their derivation from progeni tor S-type granites. It is likely that the (high heat-production) capa city of these host rocks maintained high temperatures and, thus, the c apacity to transport gold in the hydrothermal systems that created the gold deposit and, potentially, others in comparable settings. On the basis of the limited information available it is most probable that th e Iperindo mineralization was formed during the Pan-African orogeny (6 50-450 m.y.).