4D evolution of the orogenic gold district of Siguiri, Guinea (West Africa)

The West African Craton hosts some of the biggest gold deposits in the world such as Obuasi, Sadiola, Morila or Siguiri. However, the Craton is still largely unexplored and its formation, evolution and how and when the gold deposits it hosts developed during the Eburnean orogeny remain unclear. The Siguiri district is one of West Africa's largest Paleoproterozoic sedimentary basins and sits in the northern part of the Siguiri Basin. The district and its hosting Basin have received limited attention from the scientific community to date. This thesis makes use of a multi-scale and multi-disciplinary approach in order to constrain the evolution of the Siguiri district and Siguiri Basin architecture and hydrothermal activity through time. The objective is to provide the backbone of future exploration strategies for orogenic gold deposits in the Siguiri Basin and, potentially, in the rest of the West African Craton.

Fieldwork was undertaken in eleven deposits of the Siguiri district. Particular attention was given to the Sintroko PB1, Kosise, Kami, Bidini and Sanu Tinti deposits due to their structural complexity, lithostratigraphic position, outcropping conditions and accessibility. Work was focused on structural mapping, lithofacies recognition, core logging, geochemical sampling, GIS and 3D modelling. In addition to the data collected in the Siguiri district, regional fieldwork was also carried out in the rest of the Siguiri Basin for geochronological sampling and regional mapping.

Regional and district scale geochronological sampling and lithofacies observations indicate that the central Siguiri Basin is of Lower Tarkwa Group age (ca. 2015 Ma) and deposited late during the Eburnean orogeny. Lithostratigraphic observations from the Siguiri district indicate that the district is hosted in three distinct sedimentary Formations (the Balato, Fatoya and Kintinian Formations). Stacks of polymict conglomerate in the Kintinian Formation, interpreted to be olistostrome deposits, marks a change in the tectonic setting of the Siguiri Basin that may correspond to the onset of a period of orogenic compression during the Eburnean Orogeny and highlight the early architecture and fundamental structures controlling the morphology of the Siguiri Basin. These fundamental structures are viewed as the first order structures controlling the location of the world-class Siguiri district in the eponym Basin.

District to deposit scale structural work indicates that the Siguiri district underwent four deformation events: a N-S compression (D1S), an E-W compression (D2S) progressively evolving into a transpression and later on into a transtension (early- and late-D3S respectively), and a NW-SE compression (D4S). While D1S deformation is cryptic and commonly expressed as F1S recumbent folds with E-W to NW-SE and NE-SW striking axial traces, D2S is typically characterised by F2S upright folds with NNE-SSW to NNW-SSE striking axial planes that refolds F1S folds and are responsible for the bulk of the deformation and structural grain observed in the Siguiri district and Siguiri Basin. The F2S folds are overprinted by four main orientations of structures that develop (or are reactivated) during the early stages of D3S deformation and are consistent with a transpressional deformation: N-S thrusts, E-W normal faults, NE-SW dextral shear zones and WNW-ESE sinistral shear zones. In field exposures and in drill core, these structures are typically discreet and only expressed as sub-vertical damage zones ten to fifteen meters wide and characterised by an increase in vein density or by disseminated pyrite. The veins that developed along these structures are commonly found to be conjugate and their orientation is consistent with a transtensional deformation. These late-D3S veins are overprinted by the sub-vertical NE-SW striking S4S cleavage that characterises the D4S deformation event. Based on finite strain analysis of D2S structural elements and paleo stress-field reconstruction from early-D3S faults and late-D3S conjugate veins, two stress-switches were identified in the Siguiri district. These stress-switches are also documented at a regional scale and highlight the homogenisation of the stress-tensor prior to gold mineralisation. This process is postulated to occur in numerous orogenic gold deposits around the world and may be a pre-requisite for orogenic gold mineralisation.

Deposit to microscopic scale work indicate that hydrothermal activity in the Siguiri district was polyphase and that gold mineralisation developed over three distinct periods. Gold was first found to be associated with early ankerite-pyrite veins, developed late during D3S deformation. In these veins, LA-ICP-MS data shows that gold is locked in the pyrite crystal lattice. The second and main episode of mineralisation also occurs late during D3S deformation and is associated with the later quartz-ankerite-arsenopyrite-(pyrite) veins, which crosscut the early ankerite-pyrite veins and are coeval with pyrite dissemination in conglomerate layers. Gold is either found native in the veins or invisible and locked in the vein-hosted arsenopyrite crystal lattice. The last gold event is associated with the D4S deformation, which fractures previously developed sulphides and precipitate free gold in their fractures. The geochemical footprint of gold mineralisation in the Siguiri district was recognised to be associated with a wide halo of at least fifteen meters, enriched in: Ag, Au, As, Bi, Co, LOI, Mo, Sb, Se, SO3, Te and W, typical of orogenic gold mineralisation. Comparison of the characteristics and timing of gold mineralisation in the Siguiri district with other gold deposits from the West African Craton indicates that gold mineralisation in Siguiri is coeval with mineralisation in other gold deposits. However, other orogenic gold deposits in the West African Craton are associated with later gold events that may have formed economic concentrations in the rest of the Siguiri Basin.

The conclusions from this multi-scale multi-disciplinary work have implications for exploration targeting in and around the Siguiri Basin. This thesis highlights the importance of the early architecture as a first order control on orogenic gold systems. The expression of the second and third order structures controlling the mineralisation are thought to develop in response to regional stress-switches that can be recognised in the field. Finally, at a deposit scale, high grade zones can be targeted by geochemistry. This thesis hence shows the power of multiscale and multi-disciplinary studies to unravel the complexity of mineral systems and provide a critical understanding that can be used as a foundation for new exploration strategies.