Why is the extraction of copper from pyrites more difficult than that from its oxide ore through reduction?

Explanation

The group of `Delta_r G^@` vs T in Ellingham diagram for the formation of oxides shows that the copper-copper oxide line is almost at the top. Therefore, it is very easy to reduce oxide ores of copper directly to metal by heating with coke. This is because `Delta G^@` vs T lines for CO has negative slope at higher temperature and therefore, can easily reduce `Cu_2 O` to copper. However, the Gibbs energies of formation of most sulphides are greater than that for `CS_2` . In fact , `CS_2` is an endothermic compound.
Moreover, there is no CS analogous to CO for which `Delta G^@` vs T graphs slope downward. Therefore, extraction of copper from pyrite ore `(Cu_2 S)` is difficult.