Mineral precipitation in geological formations occurs when reactive fluids of varying compositions mix, altering the porous microstructure of the rock. Basalt rocks are of particular interest for long-term CO2 storage due to their potential to rapidly mineralize CO2 into stable carbonate minerals. We investigated reactive fluid mixing and subsequent carbonate mineralization in porous basalt using time-lapse three-dimensional neutron and X-ray imaging. Two flow-through experiments with different injection rates were performed on basalt cores, where co-injected CaCl2 and Na2CO3 solutions were mixed within the porous network, leading to calcium carbonate precipitation. Time-lapse neutron imaging distinguished the two injected fluids and tracked their mixing. X-ray imaging was used to separate the solid matrix from the pore space to enable fluid analysis in the neutron images. A first experiment with a high flow rate induced a steady transverse mixing pattern, captured by a decay of the concentration variance through the sample, as measured by neutron imaging. A second experiment at a lower flow rate promoted more temporal fluctuations in the fluid distribution due to the multiphase flow of water and air in the rock. The analysis of neutron images showed a significant mixing of reactive fluids driven by these temporal fluctuations. Furthermore, a higher-resolution, synchrotron X-ray image of one of the sample rocks acquired after the experiment showed the formation of additional calcite resulting from long-term diffusive mixing. The results highlight the great potential and challenges of neutron and X-ray imaging in characterizing pore-scale mixing and precipitation in rocks.