In the extraction of gold, the cyanide solution acts as a

When the gold pass into the solution of sodium or potassium cyanide forming the respective soluble complex cyanides while the impurities remain unaffected which are filtered off. 

$4\mathrm{Au}+8\mathrm{KCN}+{\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O}\to \underset{\begin{array}{l}\text{Pot. dicyanoaurate(I)}\\ \text{(soluble complex)}\end{array}}{4\mathrm{K}\left[\mathrm{Au}{\left(\mathrm{CN}\right)}_2\right]}+4\mathrm{KOH}$