This study will use a combination of textural evidence and geochemical modeling to define the nature and origin of carbonate alteration at the Eskay Creek Massive Sulfide deposit, located in the Iskut River area of Northwestern BC. Preliminary mineralogical and geochemical investigations show that the carbonaceous mudstone hosting the stratiform sulfide mineralization has been affected by widespread carbonate alteration, with abundant ankerite proximal to mineralization, and calcite in distal zones. Kaolinite formation accompanies the carbonate alteration. Emphasis will be paid to the evaluation of three possible mechanisms for carbonate alteration in the host mudstones: 1) alteration by cooling hydrothermal fluids, 2) alteration by hydrothermal fluids that have lost significant amounts of CO2 due to boiling, and 3) alteration by seawater that has been heated by CO2 derived from a boiling hydrothermal fluid. Based on the modeling outcome, a set of guidelines will be formulated that will permit interpretation of carbonate alteration halos recognizable in the field by simple and readily applicable exploration techniques.