3D numerical modelling of copper leaching, transport and deposition by convective groundwater flow in sedimentary basins

Sediment-hosted deposits are a major global source of copper. This study presents 3D reactive transport simulations to identify critical controls on copper leaching, transport, and deposition. Numerical experiments are performed using the open-source IC-FERST code (http://multifluids.github.io/), integrating buoyancy-driven groundwater flow, heat and salt transport, and copper leaching, transport, and deposition. The code implements dynamic mesh optimisation to improve computational efficiency.

 

The 3D geological model is based on the pre-orogenic stratigraphy of the Katangan basin, noting that many aspects of this stratigraphy are common to other sedimentary basins hosting copper deposits. We model flow in two snapshots of basin geometry at different times: early during evaporite formation and later when the basin has opened further, the evaporites have been buried, and there is a thicker basin fill. Modelling flow in these different snapshots allows us to test (i) the conditions for mineralisation at early and late diagenetic stages of basin evolution, (ii) the impact on flow of changing hydrogeological basin architecture, and (iii) the impact of heating during burial. Copper leaching from potential red-bed and basement source rocks is governed by a partition coefficient, while deposition is assumed to occur at a constant rate within an interval overlying the red beds representing a redox boundary.

 

Results demonstrate convective cells are established at two scales. Large (km) - scale convection occurs within permeable faults, allowing dense, saline groundwater to percolate downwards from the accumulating basin fill into the basement, where heating drives upwards flow back into the basin. Initially, convective cells form within individual faults, emphasizing the flow's three-dimensional nature. Later, some faults become dominated by downwards flow, others by upwards flow. These large-scale, fault-controlled convective cells are a major driver of copper transport: hot, saline brines leach copper from red-bed and basement source rocks and transport it upwards for deposition. Stratabound, lateral flow of copper-rich brine creates deposits near faults dominated by upwards groundwater flow. Additionally, small (10s–100s m) - scale convection occurs within red-beds, provided they have sufficient permeability. These small-scale convection cells drive local copper leaching, allowing upward migration and deposition. Vertical flow of copper-rich brine creates patchy deposits not spatially associated with faults.

 

Key controls on mineralisation are the efficiency of leaching from red-beds and basement source rocks, fault permeability controlling large-scale convective flow, red-bed source rock permeability, and the presence of a salt source in the basin. Early mineralisation can occur only if cool, low-salinity brines effectively leach copper from source rocks, because hot, saline brines do not reach the source rocks until later in basin evolution. Moreover, mineralisation can only occur without a salt source if low-salinity brines can effectively leach copper. Mineralisation does not occur in a single pass of copper-enriched brine but gradually, as convection supplies enriched brine that deposits its copper, removes the depleted brine, and circulates this to the source rocks to be enriched again over numerous cycles.