Thin layers of noble metals (M = Cu, Ag, Pd, Pt, Au) embedded in silica were submitted to irradiation with increasing fluences Phi of 4.5 MeV Au ions. Concurrently to the mixing of metal and silica atoms, the irradiation promotes the clustering of the metallic phase. Metallic particles were observed in transmission electron microscopy in order to determine if their size contributes fur a large part to the spreading of the M distribution measured by Rutherford Backscattering Spectrometry (RBS). The part of this spreading due to the recoil implantation of M atoms into the silica matrix and to their radiation-enhanced diffusion is yet larger. The variance of the distribution increases in proportion to Phi or to its square, depending on whether it is controlled by the diffusion (thin films) or the recoil implantation process (thicker films).