Groundwater flow in crystalline rock environments with poorly developed or unsaturated saprolite is dominated by fracture flow, whether this be within the upper weathered/fractured zone or deeper in the bedrock where structural discontinuities are present. Predicting flows in discrete fractures can be challenging but by using all available data – hydrogeological, geological, structural and geotechnical – an integrated evaluation can provide enhanced characterisation.
In 2013, SRK undertook a hydrogeological assessment of the high-grade, shear-zone hosted gold deposit at Yaramoko’s 55 zone in Burkina Faso as part of a multidisciplinary Feasibility Study. A field investigation included downhole wireline impellor logging (“spinner testing”), pumping tests, infiltration testing and laboratory core analysis.
The field investigation demonstrated that the fissured, weathered zone was permeable, and that faults identified in the structural interpretation also had the potential to be permeable.
Not all the faults could be tested in the field and so their hydrogeological significance was, initially, not well understood. In the absence of such data, a detailed hydro-structural interpretation completed by SRK hydrogeologists and structural geologists assisted with the hydraulic parameterisation of faults. This included a review of structural parameters such as RQD, fracture frequency and infill composition. This allowed the magnitude of groundwater inflow from each fault intersected by the planned mine workings to be evaluated in greater detail.
A numerical groundwater flow model was constructed using MODFLOW with domains and layers modelled in Leapfrog geological modelling software. The model was calibrated on a limited basis due to limited groundwater level observations and site specific hydraulic parameters. Detailed sensitivity analysis was performed instead with base case and worst case sensitivity scenarios developed, with a focus on the hydrostructural interpretation and identified uncertainties. The base and worst case scenarios presented a range of inflows that were used conservatively depending on the scenario being investigated (i.e. base case which investigated water shortfalls, and worst case for designing the dewatering system). Such an approach was acceptable to the study team, and it is noteworthy that even worst case inflows were insensitive to the financial model.
The zone 55 mine has been operational since late 2015, and dewatering rates at this early stage are broadly comparable to base case predictions. Further review of operational data over the coming years will be important to evaluate the accuracy of the groundwater model.
This study has shown that a hydrogeological feasibility study may be completed successfully without comprehensive in-situ hydraulic testing by using supplementary geological and structural data. However, the risk posed by uncertainties must be incorporated by worst-case scenarios, and this may not be an appropriate approach for all projects.
