Geology and timing of gold mineralization in the Nangodi greenstone belt: a case study of the Namdini Gold Project, NE Ghana

AbstractThe Bole-Nangodi belt (northern Ghana) is a NE-SW trending greenstone belt in the Paleoproterozoic (Birimian) part of the West African Craton. The Nangodi greenstone belt, forming the northeastern section of the Bole-Nangodi belt, comprises volcanic (basaltic to rhyolitic) and volcaniclastic rocks, as well as immature sedimentary rocks (including greywacke and shale), which are flanked on both sides by extensive granitoid complexes. These rocks have been impacted by the Eburnean orogeny around 2120–2095 Ma, which deformed and metamorphosed them under greenschist facies conditions, contemporaneous with gold mineralization. Until recent discoveries of gold deposits in the Nangodi belt, there was very little attention given to the rocks in this belt and the associated gold mineralization. This study presents (ore) petrographic observations, whole rock major and trace element geochemistry, mineral chemistry data on sulfides and carbonates, and U-Pb zircon ages from outcrop and drill core samples in the Nangodi greenstone belt (Namdini Gold Project). The results indicate that the rocks are predominantly arc-related mafic to intermediate metavolcanic rocks, together with carbonaceous metasedimentary rocks. These have been intruded by a muscovite-bearing (altered) granite and an alkali feldspar (Bongo-type) granite. The rocks are also deformed to varying degrees and altered by CO₂-rich hydrothermal fluids. Gold mineralization is associated with disseminated sulfides found at the selvages of deformed quartz-carbonate veins or along foliation planes, typically within high-strain zones in the metavolcanic rocks and slates. Compositional zoning in the sulfides and their assemblages suggest two distinct stages of ore deposition resulting from changes in the chemistry of ore-forming fluids; stage I represents early deposition of pyrite-arsenopyrite, whereas stage II involves late-stage deposition of only pyrite. Gold is refractory and hosted in arsenian pyrite and arsenopyrite as a solid solution. U-Pb zircon ages reveal that the volcanic units were deposited at ca. 2160 Ma, followed by intrusions of muscovite-bearing granite at ca. 2120 Ma and alkali feldspar granite at ca. 2102 Ma. These ages, together with the observed deformation and alteration associated with the gold, indicate that gold mineralization occurred during the interval between the emplacements of the granitic bodies.