Micro-PIXE study of gold archaeological objects

Several fragments of ancient gold objects coming from an Eneolithic hoard and from Pietroasa 'Closca cu Puii de Aur' ('The Golden Brood Hen with Its Chickens') hoard, unearthed on Romanian territory, and two Romanian native gold nuggets samples were analyzed using micro-PIXE technique. The purpose of the study was to clarify the metal provenance, establishing if the hypothesis of local gold holds. To reach this goal, trace and minor elements (Cu, Te, Sn, Pb, Ti, Cr, V, Mn, Ta) and PGE (Platinum Group Elements) concentrations were estimated. The presence of inclusions (micrometer size areas of composition different from the surroundings) was also checked. The results obtained by microprobe experiments on gold ancient artifacts, especially the inclusion findings, provided some useful hints regarding the possible provenance of the manufactured metal. The micro-PIXE elemental mapping and point analyses were made using the Nuclear Microprobe Facility at the Institute of Ion Beam Physics and Materials Research, Forschungszentrum Rossendorf, Germany, in the frame of an European Union Large Scale Facility Access (LSFA) action. This microprobe is based on a 3 MV Tandetron accelerator and a Danfysik magnetic quadrupole triplet for beam focusing. A 3 MeV proton beam was used and the beam current was about 400 pA. The beam was focused down to 6 x 6 mm2, rastering an 800 x 800 mm2 area (128 x 128 pixel elemental map). The X-rays were detected using a Si(Li) detector positioned at 120 angle with respect to the incident beam and mylar absorbers of different thickness were employed to reduce the soft X-ray region of the spectra. The total accumulated charge for the scanned areas was of the order of 3 μC. PIXE spectra were analyzed using GUPIX code. Ta inclusions on two samples from Pietroasa hoard were found. These findings give strong support for the hypothesis of using Ural Mountains gold ore as a source for manufacturing these objects. The investigated Pietroasa hoard artifacts were proved to be of different origins - different composition and even different trace elements were present, confirming the stylistic arguments. We found some Si and Ca inclusions on two Eneolithic samples. The measurements gave further support to the hypothesis of the use of alluvial gold to manufacture these Eneolithical artifacts. The lack of Ir in the Romanian gold nuggets is confirmed by the present measurements. An archaeological work is in progress with respect to these findings, containing the archaeological conclusions of our measurements. Further analyses performed on other artifacts belonging to the same hoards are to be done. One must admit that a correct answer to the question of the native metal provenance used for each artifact will be a difficult task as long as a comprehensive data bank for the composition of Euro-Asian native gold will not be available. (authors)