Migration of dissolution front in a fracture network − Implications for weathering of fractured bedrock systems and boulder formation

•Chemical weathering profiles can be predicted from Peclet and Damköhler numbers.•Fractures with a sufficient flow rate contribute to delineating weathering zones.•Boulder size is controlled by the competition of multiscale fracture networks. Fracture networks contribute to the differential weathering of fractured bedrock at multiple scales. The influence of competing fracture networks is discussed in light of hydrogeochemical conditions to predict the dissolution pattern resulting from spheroidal weathering. The investigation of the dimensionless Péclet (Pe) and Damköhler (Da) numbers on a simple case of a dual-scale fracture network and its application to three fracture network weathering reveals the influence of fluid flow and reaction rate conditions on the dissolution patterns. The conceptual model was numerically validated and compared with observations on lateritic nickel deposits in New Caledonia. A fracture network will contribute to boulder development if the associated fluid flow regime reaches a Pe threshold of around 10. On the other hand, predominant diffusion will inhibit the fracture set in favour of a higher permeability fracture network. The Da drives, for its part, the width of the dissolution halo. Owing to the dependency of the fluid flow regime on the fracture network characteristics, scaling laws were established to estimate the size of the boulders at different scales by predicting the fracture contribution as a function of their Pe.