Bullman’s two properties are located within the Birimian-age Siguiri Basin which underlies a portion of northeastern Guinea and southern Mali. This large basin is comprised of sedimentary and volcanic units which have been subjected to greenschist facies metamorphism. Birimian units are intruded by narrow, regionally extensive, mafic dykes and sills, and also locally by late granitic bodies.
The gold mineralization found in the Birimian units is likely to be related to late tectonic plutonism and related hydrothermal events which remobilized gold along fractures and fault zones. Two main fracture and fault zones have probably played an important role in the control of the mineralization. North-south and north-northeast structures are prominent and associated with intense ductile deformations and strong hydrothermal alteration. A strong east-northeast structure system crossing the Siguiri Basin can be interpreted; these systems have associated mylonite and intrusive bodies injected in the structures. The Mesozoic mafic dykes and sills appear predominantly controlled by east-west structures. The Siguiri Basin hosts the three producing gold mines in Upper Guinea: Siguiri (AngloGold Ashanti), Lefa Norgold/Severstal) and Kéniéro (SEMAFO), as well as two other major gold mines in neighboring Mali: Kalana (Avnel) and Kodiéran (PearlGold Mining).
A long period of weathering and deflation is reflected by extensive development of lateritic soils and saprolite horizons across the region.