Ancient extractive metallurgy of copper in the Aosta Valley (Western Alps, Italy): new evidence from pre-Roman slags from the Misérègne site

Many metallurgical sites are scattered across the Aosta Valley territory (e.g. Tumiati et al., 2005). The age of some of them is unknown and often the slag heaps are the sole vestiges of metallurgical activities. Such is the case of Miséregne (Fénis), where the village is built on an enormous slag deposit. We performed radiocarbon dating on charcoal fragments included in the slags and obtained ages comprised between the 4th and 1st century BC. So far, these are the most ancient ages for early mining activity in the Aosta Valley, probably carried out by the local population of the Salassi. We studied the slags following a petrologic approach. The occurrence of matte inclusions in the slags indicates that the processed raw minerals were Cu-Fe sulfides. Relying on morphologic and micro-textural features, we classified the slags into three categories, i.e., coarse, massive and flat, as suggested by Addis (2013). In all the slag classes the most abundant phase is olivine. Other common phases are spinel group minerals and sulfides, mainly pyrrhotite and bornite solid solutions. The olivine crystal shapes allowed us to qualitatively estimate the degree of undercooling (ΔT) and the rate of cooling of the slags: the coarse slags show a high ΔT; the massive slags seem to record an initial, long-lasting low-ΔT stage, followed by an increase in the cooling rate; the flat slags underwent a brief low-ΔT stage, followed by a sudden marked increase in ΔT. Combining this information with the slag bulk chemistry, the sulfide compositions, the slag morphology and the presence of inclusions of charcoal, quartz and other slag fragments, we hypothesize that the massive and coarse slags come from the lower and, respectively, the upper portion of the same slag contained in the furnace, while the flat ones were tapped slags. Minimum furnace working temperatures estimated by means of olivinespinel geothermometry are in the ranges 932-964°C, 968-1037°C, 1202-1239°C for the coarse, massive and flat slags, respectively. According to the above observations, the flat slags should record conditions that are the closest to the actual furnace working temperature.