Physicochemical study of the hubnerite-ferberite (MnWO4-FeWO4) zonal distribution in wolframite (Mn X Fe(1−X)WO4) deposits

A thermochemical approach to the problem of zonal structure in metallogenetics involving the use of Ellingham diagrams and minimization of free energy (ΔG) agrees closely with the “normal” zonal structure as defined by Fersman (1934). This approach is applied to the hubnerite-ferberite (MnWO₄-FeWO₄) solid solution in order to elucidate the repartition law of the Mn/Fe ratio in wolframite (Mn _X Fe_(1?X)WO₄). The study requires experimental determination of thermodynamical data, such as enthalpy ΔH of the solid solution, which is obtained using a high-temperature calorimetric method developed for this purpose. The calculation leads to an equation which gives the Mn molar value of wolframite with respect to temperature and the Mn/Fe ratio of the mineralizing solution. It is theoretically demonstrated that a high manganese content in wolframite is connected with high deposition temperature, but the decrease of the Mn/Fe ratio is a consequence of the decrease of the Mn/Fe ratio in the mineralizing solution, as can be observed in the Borralha deposit from which the geochemical study has been made. Conversely, a rapid lowering of the temperature of deposition leads to an increase of the Mn/Fe ratio in wolframite. Consequently, this ratio cannot be used as a geologic thermometer, but rather as an indicator of the position of the source of the mineralization.