CONTROLS ON CARBONATION REACTION PROGRESS IN ULTRAMAFIC ROCKS

Carbonated ultramafic rocks represent a natural analog to in situ CO₂ sequestration via mineral carbonation and are often associated with economically significant lode gold deposits (e.g., Mother Lode, Abitibi). Carbonation mineral reactions and their progress are strongly controlled by the extent of serpentinization and by the permeability system that delivers the CO₂-bearing fluid to the reaction front. A comparison of the massively carbonated ultramafic complexes at Atlin (British Columbia) and Linnajavri (Norway) highlight the processes that promote pervasive carbonation. Both field sites exhibit clearly defined metasomatic zones resulting from infiltration of CO₂-bearing fluids. These zones comprise the unaltered precursor and its carbonated counterparts, listed in the order of increasing CO₂ consumption: serpentine-magnesite (ophicarbonate), talc-magnesite (soapstone), and quartz-magnesite (listvenite) assemblages. The separation of the mineral reaction zones and the variable extent of serpentinization of host rocks allow reaction processes to be examined in isolation. Volume strain associated with carbonation reaction is highly variable and depends on protolith composition, degree of serpentinization, and reaction stoichiometry. Reactions that proceed with small solid volume change are regionally extensive. These carbonation reactions may have been limited by CO₂ supply. In contrast, reactions with large volume strain exhibit a more complex and variable spatial distribution. Mineral reaction textures are consistent with reaction-driven micro-fracturing and posit a correspondingly more dynamic interplay between reaction progress and permeability evolution. In some instances, these reactions generate permeability ahead of the reaction front and thus may promote more extensive carbonation. The impact of these processes on carbonation reaction at and beyond the reaction front may be critical to the success of industrial carbonation by subsurface injection of CO₂ into ultramafic rocks.