In the extraction of copper from copper glance, the step in which the sulphide itself reduces the oxide (self-reduction) is

After partial roasting, $\mathrm{Cu}{}_2\mathrm{O}$ and unreacted $\mathrm{Cu}{}_2\mathrm{S}$ react together and the sulphur of $\mathrm{Cu}{}_2\mathrm{S}$ is oxidised to $\mathrm{SO}{}_2$ while copper(I) in both is reduced to the metal. The second reaction is roasting, the third forms slag and the fourth uses carbon as external reductant.