In dolomite formation, which dolomitization model involves a mixing zone between fluids?

• A. Evaporative
• B. Seepage Reflux
• C. Mixing-Zone
• D. Seawater

Answer: (C)

Dolomitization via a mixing zone occurs when two fluids with different chemistries meet in the rock, and their interaction shifts the activities of calcium and magnesium. This change drives the replacement of calcite by dolomite as magnesium-rich water mixes with calcium-rich pore waters at the interface. The result is a localized zone where dolomite precipitates—a mixing front that forms dolomite along fractures, channels, or boundaries between fluid masses. This is why the model that explicitly involves a mixing zone between fluids is the Mixing-Zone model. Other models rely on evaporation concentrating brines, seepage-reflux pathways, or dolomitization by seawater itself, which don’t hinge on a distinct mixing front.