EARLY PROTEROZOIC ORE-DEPOSITS AND TECTONICS OF THE BIRIMIAN OROGENIC BELT, WEST AFRICA

The Early Proterozoic of the West African craton comprises a series of volcanic troughs and sedimentary basins (the Birimian) with granitic terranes accreted on Archean nuclei in the Man and Reguibat shields. Studies of the Birimian of the Man Shield indicate a model of polycyclic evolution, with a major collision event (D1; 2.1 Ga) thrusting part of the Proterozoic terrane over the Archean before individualization of numerous volcanic troughs and clastic-infill basins. The proposed evolution for the Birimian orogenic belt comprises: (1) deposition of the sedimentary Lower Birimian (B1) with minor tholeiitic volcano-sedimentary intercalations (containing chert and/or Mn-formations), and with most of the detritus being derived from Early Proterozoic sources, apart from contamination near the Proterozoic/Archean contact; (2) pre-B2 crustal thickening related to D1 thrusting; (3) formation, over about 40 Ma, of the Upper Birimian (B2) with numerous volcanic troughs of different composition (tholeiitic and rare komatiitic, bimodal tholeiitic to calc-alkaline, volcano-plutonic) and Tarkwaian clastic-infill basins; and (4) major transcurrent tectonics with sinistral (D2) and dextral (D3) strike-slip faults. Such a tectonic evolution from a collision (D1) phase to a transcurrent (D2, D3) phase is typical of collision belts, and in the present case the evolution of the Birimian orogenic belt can be "tended into Guyana. The metallogenic history of the Birimian shows a three-phase evolution coinciding with the orogenic evolution, and extends over almost 150 Ma from the Perkoa massive (Zn-Ag) sulfides (2.12 Ga) with a clear mantle affinity to the late-mesothermal Au quartz veins (approximately 2 Ga) with (according to lead isotopes) a high crustal participation. The economic mineralization of belt thus consists of: (1) "Pre-orogenic" (pre-D1) deposits related to early extension zones. This was diverse with stratiform Au tourmalinite (type 1 Au: Loulo in Mali; Dorlin in Guyana), stratiform Fe (Cu) (Faleme in Senegal) and Mn (Nsuta in Ghana; Tambao in Burkina Faso), and a single massive Zn-Ag sulfide deposit (Perkoa in Burkina Faso) associated with regional volcano-sedimentary (variably tholeiitic) stratigraphic marker beds; (2) "Syn-orogenic" (post-D1 to syn-D2/D3) deposits with disseminated Au-sulfides (type 2 Au: Yaoure in the Ivory Coast) in extensional zones of the B2 followed by auriferous paleoplacers (type 3 Au) in B2 extensional zones (Tarkwaian Banket conglomerate) or syn-D2 transtensional zones (debris flow of Orapu in Guyana). (3) "Late-orogenic" (post-peak D2/D3) deposits with mesothermal Au mineralization evolving from a "disseminated gold-bearing arsenopyrite and Au-quartz lode" type (type 4 Au: Ashanti in Ghana) to a "quartz-vein" type with free gold and Cu-Pb-Zn-Ag-Bi paragenesis. Most of the gold in West Africa comes from this phase. Finally, the metallogeny of the Birimian appears as rich in Au- and Mn-formations and poor in volcanogenic mineralization and BIF. It differs from the Archean metallogeny of West Africa and other regions, through the presence of certain deposit types (such as Au-stratiform tourmalinite, gold-bearing conglomerate with Au and Fe-Ti-oxides but no uranium or sulfides, and Au-arsenopyrite-rich shear-zone deposits) that are very common in, but not exclusive to, the Proterozoic and Paleozoic.