Estimation of carbon dioxide fugacity and braunite, II: stability in quartz-free manganese oxide ores from the Kalahari manganese field, South Africa

The maximum carbon dioxide fugacity [f(CO₂)] for the formation of the Wessels-type hydrothermally altered ore from the Kalahari manganese field is estimated to be < 3 bar at 300-400°C and 1 kbar total pressure, based on the andradite - haematite - calcite assemblage. The stability relations in the system Ca-Mn-Fe-Si-C-O are discussed with reference to published phase equilibria in the system Ca-Mn-Fe-Si-O, and making use of the approximate standard Gibbs free energy and predicting thermodynamic properties of braunite II. The relations show that, in the presence of calcite, the stability field of braunite II is enlarged, whereas that of braunite and calcite shrinks, with decreasing carbon dioxide fugacity and silica activity at constant temperature and pressure. Thus braunite reacts to form braunite II below the maximum f(CO₂) at the bixbyite-hausmannite buffer. Carbon dioxide, released from decarbonization in the hydrothermal processes, must have escaped from the rock system, thereby maintaining f(CO₂) below 3 bar. -Authors