Abstract
The Birimian of the West African Craton comprises a Palaeoproterozoic granitoid-greenstone terrain. Among the greenstone belts is the NE-SW trending Bole-Nangodi belt located in the northern part of Ghana. It consists of volcanic and volcaniclastic rocks (basalt to rhyolite) and immature sedimentary rocks (greywacke, shale), and is flanked on both sides by extensive granitoid complexes. These rocks have been affected by the Eburnean orogeny at ca. 2120-2095 Ma, which deformed and metamorphosed the rocks under greenschist facies conditions, contemporaneous with gold mineralization. Until the recent gold deposit discovery by Cardinal Resources (Namdini Gold Project) in the Nangodi belt, very little attention had been given to the rocks in the belt and the associated gold mineralization. This study presents U-Pb zircon ages, (ore) petrographic observations, whole rock major and trace element data, and mineral chemistry data on sulphides and carbonates, from outcrop and drill core samples (including drill cores from the Namdini Gold Project). The rocks in the area are predominantly arc-related metavolcanic rocks of basaltic to andesitic origin and associated metasedimentary rocks (organic-rich shales), amphibole-bearing alkali feldspar granite and muscovite-bearing granite. The rocks are deformed to variable degrees and altered by CO2-rich hydrothermal fluids. Gold mineralization is associated with disseminated sulphides occurring at the selvages of deformed quartz-carbonate veins or along foliations, usually within high-strain zones in the metavolcanic rocks and shales. Compositional zoning in the sulphides implies changes in the chemistry of ore-forming fluids resulting from two main stages of ore deposition; stage I is an earlier pyrite-arsenopyrite deposition and stage II is a late-stage pyrite deposition. Gold is refractory and hosted in arsenian pyrite and arsenopyrite as a solid solution. U-Pb zircon ages indicate that the volcanic units were deposited at ca. 2160 Ma followed by muscovite-bearing granite and alkali feldspar granite intrusions at ca. 2119 Ma and 2100 Ma, respectively. Gold mineralization took place in the time interval between the emplacement of the granites.