Check option A

In extraction of lead from galena, some $PbS$ is oxidized to $PbO$ during roasting: $2PbS + 3O_2 \rightarrow 2PbO + 2SO_2$ Then self-reduction occurs between $PbS$ and $PbO$: $PbS + 2PbO \rightarrow 3Pb + SO_2$ Thus, a mixture of $PbS$ and $PbO$ produces $Pb$ and $SO_2$.

Hence, A is correct.

Check option B

In extraction of copper from copper pyrites, silica is added as a flux to remove iron oxide impurity: $FeO + SiO_2 \rightarrow FeSiO_3$ The slag formed is iron silicate, not copper silicate.

Hence, B is incorrect.

Check option C

In copper extraction from sulphide ores, partial roasting converts part of the sulphide into oxide: $2Cu_2S + 3O_2 \rightarrow 2Cu_2O + 2SO_2$ Then self-reduction occurs: $2Cu_2O + Cu_2S \rightarrow 6Cu + SO_2$ The copper obtained is blister copper.

Hence, C is correct.

Check option D

In the cyanide process, gold dissolves as sodium dicyanoaurate(I), $Na[Au(CN)_2]$. Zinc powder is added to precipitate gold: $2Na[Au(CN)_2] + Zn \rightarrow Na_2[Zn(CN)_4] + 2Au$ Therefore zinc displaces gold from the complex.

Hence, D is correct.

Final selection

Correct statements are: $\boxed{A,\ C,\ D}$

Comparison with stored answer

Stored correct answer: $A, C, D$

My derived answer matches the stored answer.