4 H2NO3 + CuS → Cu(NO3)2 + S + 2 NO + 4 H2O
This is an oxidation-reduction (redox) reaction:
S-II - 2 e- → S0 (oxidation)
2 NIV + 4 e- → 2 NII (reduction)
2 NIV - 2 e- → 2 NV (oxidation)
CuS is a reducing agent, H2NO3 is an oxidizing agent, H2NO3 is a reducing agent.