Mineralogical and isotopic characterization of the Late Chalcolithic Slags from Bressanone/Brixen (Northern Italy)

In 2008 the excavation activities for the constructions of the Bressanone/brixen (North-Eastern Itlay) west ring-road allowed recovery of an archaeometallurgical site. The finding of Late Chalcolithic slags in the site is particular interesting because it follows the recent discovery of two nearby coeval occurrences: Millan/Milland and Gudon/gufidaun (Colpani et al., 2009). In order to investigate the nature of the extractive process and the type and origin of the minerals used at Bressanone/Brixen, fifteen slag samples were analyzed.The slags are small, mostly with irregular subrounded shape; only three slags are nearly flat with thickness of 1-2 cm. Polished thin sections were analyzed by transmitted and reflected light optical microscopy, textural and chemical analyses were obtained by SEM-EDS, mineralogical phases were identified by XRPD and semi-quantitatively evaluated by the Reference Intensity Ratio method.The samples show rather heterogeneous textures and similar compositions, with features encompassing: high amount of unreacted quartz, often associated with the high temperature polymorph cristobalite; abundant fayalitic olivine displaying euhedral to skeletal morphology in the same slag; widespread occurrence of unreacted or poorly reacted Cu-sulphides; ubiquitous presence of variable amounts of Fe-oxides; widespread occurrence of small copper prills. The chemical analyses show a substantial Zn content in the Bressanone/Brixen slags, as previously observed in the Millan/Milland and Gudon/Gufidaun materials. The immature slags from Bressanone/Brixen (partially molten, not fully reacted) suggest the use of a low efficiency copper smelting process, similar to those observed at Millan/Milland and Gudon/Gufidaun, and in line with the “Chalcolithic process” defined by Bourgarit (2007).Pb isotope ratios were measured on 10 slag samples from the 3 sites and the results, compared to the geochemical database of the alpine Cu-mineralisations (Alpine Archaeocopper Project - http://www.geoscienze.unipd.it/aacp/welcome.html) allow safe identification of the ore source in the post-Variscan polymetallic vein deposits of the Trentino-Alto Adige area (Nimis et al., 2011). Specifically, the isotopic data are fully compatible with those measured on the ores from the relatively near Montefondoli/Pfunderer Berg mine (Chiusa/Klausen, Bolzano/Bozen). The isotopic and trace element data measured on a raw copper fragment found in association with the Millan slags are discussed.