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The sudden increase in the amount of Celtic gold coins circulating in central Europe during the second century BC coincides with the appearance of platinum-group mineral (PGM) inclusions in the Celtic gold. It has been proposed that large amounts of gold were introduced from the eastern Mediterranean by mercenaries following the conquest by Alexander the Great, but another hypothesis might be the discovery of entirely new gold sources in central Europe during this time. The presence of PGM inclusions in gold objects is usually taken as an indication for an origin from placer deposits (Hartmann 1970; Park 1986). Initially, it was hoped that those inclusions could be characteristic for a certain deposit, but Meeks and Tite (1980) have shown that the compositional variation within a single gold object can be as large as the total variation observed in all investigated gold artefacts. Therefore, the elemental composition of the PGMs in gold artefacts does not provide any reliable information on the provenance of the gold.An alternative to chemical analysis of the inclusions is the determination of their osmium isotope ratios. Similarly to lead, osmium exhibits a variable isotopic composition due to the radioactive decay of 187 Re and 190 Pt. In order to investigate the potential of this new geochemical tool to determine the provenance of ancient gold, a quasi-non-destructive method for the determination of Os isotope ratios with LA-MC-ICP-MS has been developed.The results show that different types of deposits can be distinguished by Os isotope patterns and their distributions when applying a simple descriptive statistical method. In contrast to lead isotope ratios, the variations of Os isotope abundances are rather large in many ore deposits. But, also contrary to lead, the heterogeneity of the Os isotope ratios of the deposits is preserved in the artefacts, because the analysed PGMs are not dissolved during melting of the gold. Therefore, it is feasible to distinguish between different types of gold sources and to define specific deposits provided that the artefact contains several PGMs. The application of this new technique to Celtic gold coins from southern Germany and coins from western Anatolia has been demonstrated in this study.

The chemical and osmium-isotope composition of platinum-group minerals (PGM) [e.g., laurite-erlichmanite (RuS 2-OsS 2), ruarsite-osarsite (RuAsS-OsAsS) series and Os-Ir alloy (Os,Ir)] from variably altered podiform chromitites of the Kraubath and Hochgrössen dunite-harzburgite massifs are reported for the first time. These massifs, the largest dismembered mantle relics in the Eastern Alps of Austria, were interpreted as a strongly metamorphosed ophiolite sequence, which forms part of the Speik Complex. Unaltered podiform chromitites from both localities display negatively sloped chondrite-normalized platinum-group element (PGE) patterns. The highly altered podiform chromitite at Kraubath is dominated by less refractory PGE (PPGE: Pd, Pt, Rh) over refractory PGE (IPGE: Os, Ir and Ru). The chemical composition of chromite varies from a Cr# [100*Cr/(Cr + Al)] of 74 to 87 and a Mg# [100*Mg/(Mg + Fe 2+)] of 44 to 61, values typical of podiform chromitites from the mantle section of an ophiolite. The PGM assemblage in the unaltered podiform ores is dominated by laurite (43% and 75% of all PGM at Kraubath and Hochgrössen, respectively). Sperrylite, PtAs 2 , is the most abundant PGM (61%) in the altered chromitite, whereas minerals of laurite-erlichnmanite series are subordinate (4%). At Kraubath, Os-bearing PGM (laurite, erlichmanite, ruarsite and Os-Ir alloy) occur as (a) single grains and (b) complex polyphase assemblages. At Hochgrössen, laurite and Os-Ir alloy are present as solitary grains only. In situ osmium-isotope measurements of 16 PGM grains from bedrock (e.g., laurite and ruarsite) by laser-ablation multiple-collector inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS) revealed low 187 Os/ 188 Os and ␥Os t=0 values, indicative of a subchondritic source of the PGE in the mantle. Combined with less radiogenic Os isotopic values measured by negative thermal ionization mass spectrometry (N-TIMS), 187 Os/ 188 Os ranges from 0.11580 to 0.12437, and ␥Os t=0 values, §

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