The Aluminium in CD discs is vapor-deposited, it's  deposition rate being an exponential function of temperature above the evaporation point of aluminum and thus the temperature of the aluminum source has to be, and if I may say, is easily controlled to a high degree of accuracy.

If iron was present in the vapor source, the fact that iron's vaporization temperature (3135 deg C) is some 340 deg C higher than that of aluminum (2795 deg C) precludes any such contamination.