Geological Setting, Mineralization and Deposit Types
Burkina Faso is largely underlain by the West African craton. This cratonic domain is bounded to the west by the Panafrican mobile zone of the Mauritainides-Rokellides, and to the north and east by the transgressive cover of the Taoudeni and Volta basins. The latter forms the foreland of the Panafrican Dahomeyide mobile belt.
The craton is made up of an Archean core, the Man Shield in the southwest, and the Proterozoic Baoule-Mossi domain to the east, with the latter containing relics of Archean basement. Birimian gold-bearing greenstone formations of prime economic importance were formed in the Baoule-Mossi domain during the Early Proterozoic between 2400 and 2100 Ma. These are illustrated in Figure 8. The craton was reworked by the Eburnean orogenic cycle from 2400 to 1200 Ma. The Archean is comprised of extensive granite-gneiss complexes and greenstone belts including basic, ultrabasic, and iron formations, which were largely affected by two major orogenic events, the Leonian (3.5-2.9 Ga) and the Liberian (2.9-2.6 Ga) cycles. Archean rocks are known to host gold deposits, such as Ity in Côte d’Ivoire; however, the potential has been found to be much less than that of later Proterozoic rocks.
Early Proterozoic Birimian terrane has been divided by the BRGM of France into two units (Wright, et al., 1985); a volcanic-pyroclastic and fluvio-deltaic unit (Type I), and a flysch-type (turbiditic) sedimentary unit with minor volcanic rocks (Type II). The predominantly sedimentary unit is composed of phyllite, tuffs, and greywackes, and is extensively intruded by early Eburnean granite batholiths. The volcanic unit is a diverse assemblage of basic and intermediate maficvolcanics, metarhyolite, porphyries, chlorite schists, tuffaceous greywackes, quartzite, conglomerates, and sandstone. This unit is intruded by a series of late Eburnean plutonic complexes composed of gabbro to granitic rocks, which post-date the earlier granite batholiths. In practice it has been found that this type of lithostratigraphic division is underpinned by little practical geologic mapping and is therefore of little use in regard to identifying terrain prospective for gold mineralization.
These Birimian formations were deformed by the most active period of the Eburnean orogeny, which took place in three major tectono-metamorphic phases between 2150 and 2190 Ma. After the Eburnean orogeny, most of West Africa formed a stable craton (about 1600 Ma) and was bounded on the east and west by the Panafrican mobile zones. Overthrust units in the Dahomeyides range produced late Proterozoic foreland sedimentation in the Volta Basin of eastern Burkina Faso.
Figure 8: Regional Geology Map (after Curien, H. 2012)
The permit in underlain by rocks of the Djibo greenstone belt. Very little outcropping is present on the permit so the property geology is based on a compilation of litholgies identified in drilling and from interpretations of the geophysics (magnetics, radiometrics and IP) conducted on the property. The different lithologies are described as basic volcanic basalts, andesites and dacites; basic intrusive (gabbro, dolerite) and acid intrusive (granite, granodiorite porphyry usually with feldspar). There was also some mudstone / siltstone intercalated volcanic rocks in the west boreholes. The property shows a complex faulting pattern.
Figure 9: Property Geology Map (after Curien, H. 2012)
Gold mineralization on the property is associated with quartz veins occurring in foliated mafic volcanics. Drilling has identified a series of vertical veins set hosted in mafic volcanics, proximal to an intrusive plug.
The type of deposits in greenstone belts in this area are the shear hosted, multi-stage epigenetic vein gold deposits (Huot, et al., 1987) which usually contain polymetallic sulphides and an assemblage of hydrothermal alteration minerals such as sericite, kaolinite, carbonate, silica, and chlorite. Most of these deposits occur in volcanic or volcano-sedimentary terranes but a few small granitic stocks, intruded into volcanic piles, may contain structurally controlled gold- bearing veins as well (e.g., Bagassi, Aribinda, Semapoum, Margo). The eluvial and placer gold deposits are found adjacent to the syngenetic and vein type deposits in the Birimian belts and are usually exploited by small-scale traditional methods.
All of the greenstone belts in Burkina Faso are northerly extensions of auriferous belts in Ghana and Côte d’Ivoire. Huot, ET al. (1987) have recognized three different genetic types of gold occurrences in these belts:
- Syngenetic gold associated with manganese in carbonaceous schists and stratabound sulphides, exhalites, felsic tuffs, and pyritic cherts (e.g., Kiere).
- Structurally controlled gold vein deposits and gold-bearing shear zones with associated hydrothermal alteration minerals (e.g., Bouroum, Dossi, Poura, Koupela, Arbinda and Essakane).
- Gold in eluvial, placer, and paleo-placer deposits (e.g., Essakane, Gambo, Seguenega).