In the electrolytic refining of Copper (electrolyte is CuSO4 solution + dil H2SO4 ;Pure Cu cathode and Impure Cu anode), metallic impurities having lower oxidation potential than Cu are deposited as
The whole process of electrolysis should be carried out with precision. This is because we need to keep the electrode potentials (for the desired reactions) in mind and choose the electrodes and electrolyte accordingly. Also, there should be no competing reactions.
In this case (electrolyte is CuSO4 solution + dil H2SO4 ; Pure Cu cathode and Impure Cu anode), metallic impurities having lower oxidation potential than Cu are deposited as just below the anode. This is called the anode mud or anode sludge. Some of the impurities could be precious metals like Gold or Silver.