Flow-controlled reactions in rock fabrics

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Abstract
The percolation of fluids through rock fabrics or through fracture networks, continued over millions of years, is associated with selective dissolution, cementation, fabric alteration in metamorphosis and the formation of certain massive ore deposits in specific locations. The degree of mineral alteration and its spatial distribution are both controlled by the patterns of interstitial flow and three distinct types of flow-controlled reactions are reviewed and analyzed. Isothermal reaction fronts propagate from mineralogical boundaries in the direction of flow at a speed proportional to but less than the fluid transport velocity; their occurrence can be recognized in banded or bimodal mineralogical patterns. Gradient reactions occurs pervasively throughout a fabric at rates proportional to the temperature and pressure gradients and the fluid velocity; they produce gradually changing mineral assemblages throughout, though their rates of reaction are greatest in high permeability lenses and in thermal boundary layers. Mixing zone reactions occur when two fluid masses intermingle and are usually highly localized. In each case, simple but general analytical expressions are given that express the rates of reaction in terms of the flow and geochemical variables.