Write the chemical reaction involved in the process of extraction of Gold. Explain the role of dilute NaCN and Zn in this process.

The roasted ore of Gold is leached with the solution of sodium cyanide in the presence of oxygen for many days. The role of NaCN is to dissolve Gold to form aurocyanide complex,
from which the metal is obtained by displacement. 
$\begin{array}{l}4\mathrm{Au}+8\mathrm{NaCN}+2{\mathrm{H}}_2\mathrm{O}+{\mathrm{O}}_2⟶4\mathrm{Na}\left[\mathrm{Au}(\mathrm{CN}{)}_2\right]+4\mathrm{KOH}\\ 2\mathrm{Na}\left[\mathrm{Au}(\mathrm{CN}{)}_2\right]+\mathrm{Zn}⟶{\mathrm{Na}}_2\left[\mathrm{Zn}(\mathrm{CN}{)}_4\right]+2\mathrm{Au}\end{array}$