Melals like Cu, Ag, Zn, Hg and Pb occur in nature as sulphide rather than oxides. Why ?

The cations of these metals i.e. Cu+2, Ag+, Zn+2, Hg+2 (pseudo inert gas configuration), and Pb+2 ( inert gas configuration) have high polarising power and hence can easily polarise the bigger S-2 ion than the smaller O-2 ion. Hence sulphides are more stable than oxides of these metals.

Cu+2, Ag+, Zn+2 and Hg+2 are soft Lewis acids and S-2 is soft Lewis base . Compounds obtained by reaction between soft Lewis acid and soft Lewis base is stable. O-2 is hard Lewis base . Adduct of soft Lewis acid and hard Lewis base is not very stable compounds.