n the cyanide extraction process of silver from argentite ore, the oxidizing and reducing agents used are

In extraction of silver, ${\mathrm{Ag}}_2\mathrm{S}$ is leached with $\mathrm{KCN}$ in presence of air :

${\mathrm{Ag}}_2\mathrm{S}+\mathrm{NaCN}+{\mathrm{O}}_2\rightleftharpoons \mathrm{Na}\left[\mathrm{Ag}(\mathrm{CN}{)}_2\right]+{\mathrm{Na}}_2{\mathrm{S}}_2{\mathrm{O}}_3$

Thus, ${\mathrm{O}}_2$ is oxidant. 

$2{\left[\mathrm{Ag}(\mathrm{CN}{)}_2\right]}^{-}+\mathrm{Zn}\to {\left[\mathrm{Zn}\left({\mathrm{CN}}_4\right)\right]}^{2-}+2\mathrm{Ag}\downarrow$