Carbonate and sulphide ores are usually converted into oxides during the process of extraction because metals can be easily extracted by the reduction of their oxides rather than from their carbonates and sulphides.
Example:
Extraction of zinc: ZnS is converted into ZnO by roasting process.
2ZnS (s)+2O2→(g)Heat−−→2ZnO(s)+2SO2(g)