On the origin of (4)He and (40)Ar in natural gold

In a first report on our investigations of noble gases in native gold we demonstrated that placer gold contains an excess of radiogenic (4)He and (40)Ar relative to the concentrations expected from in situ decay of U, Th, and K, respectively, during the geologic age of about 30 Ma of the samples. We also showed that the U/Th-(4)He age of 36 Ma of vein-type gold from the Southern Alps agrees with its K-Ar formation age derived from associated muscovite and biotite. We now studied the question of the origin of the (4)He and (40)Ar excesses of placer gold. We conclude that gold contains two components of noble gases, a low-temperature component from fluid inclusions or phases which release noble gases at less than 800 C and a high-temperature component released when gold melts (1064 C). In some samples extremely high U and K concentrations or an unreasonably high formation age would be required to explain the observed (4)He abundances. Thus, trapped (4)He and (40)Ar must be present in gold.