The Archean-Proterozoic craton of West Africa hosts numerous gold deposits, which are spatially and temporally related to the Eburnean orogeny that took place between 2250 and 1980 Ma, and included multiple deformation events. The majority of these gold deposits are located along shear zones. The structural history is relatively well established for most gold deposits, but absolute timing of the mineralization is commonly lacking. Five deposits hosted in the Baoulé-Mossi domain, located in the southwestern part of the West African craton, were studied to better constrain timing of gold events: Nassara and Kiaka in Burkina Faso and Wassa, Damang, and Obuasi in southern Ghana. Gold mineralization was structurally characterized at each deposit, and dated by the Re-Os method on pyrite, arsenopyrite, and pyrrhotite grains that were coeval with the gold deposition. Combined structural and Re-Os geochronological constraints allow two groups of gold deposits to be distinguished.
Early orogenic gold formed during the Eoeburnean orogeny, i.e., between 2190 and 2125 Ma, as represented by Kiaka1 at 2157 ± 24 Ma, and Wassa1 at 2164 ± 14 Ma. These early gold occurrences are expressed as early disseminated gold enrichment (1–3 g/t Au) at Kiaka, and gold hosted in folded and boudinaged quartz veins that are transposed along the primary S1 foliation at Wassa. These Re-Os ages on the early sulfides at Wassa and Kiaka are significant because they definitively provide the first direct age constraints on gold-only Eoeburnean mineralization in the Birimian of West Africa.
Late orogenic gold, hosted by brittle structures and with higher gold grades (up to 60g/t at Kiaka), formed during late Eburnean deformation (D3–D5 events) between 2120 and 2000 Ma, as represented by the Re-Os dating of these ores at Wassa at 2055 ± 18 Ma and Obuasi at 2045 ± 40 Ma. Similar-style ores dated at Nassara at 2131 ± 99 Ma and Damang at 2080 ± 135 Ma, but both with large uncertainties, and an undated younger event at Kiaka, are also suggested to be part of this later mineralizing episode. The high uncertainties on the Nassara and Damang ages may be directly linked to the low rhenium and osmium contents of the studied samples.
These results highlight the polyphase character of the widespread gold mineralization in the West African craton. Although the late stages of the Eburnean orogeny constitute a prolific period for the formation of high-grade gold mineralization, identification of less well-studied early-stage gold deposits, which can also contain large quantities of gold, is critical for mineral exploration in the West African craton.