During extraction of copper, iron oxide present as impurity is removed by adding ${\mathrm{SiO}}_2$ and converting it into ${\mathrm{FeSiO}}_3$

$$\begin{gathered} \mathrm{FeO}+{\mathrm{SiO}}_2\to {\mathrm{FeSiO}}_3 \\ \mathrm{Flux} \mathrm{Slag} \end{gathered}$$

Hence,$\mathrm{FeO}$ is gangue and ${\mathrm{FeSiO}}_3$ is slag.