Mineral-petrographic study of greenstone cobbles from Quaternary alluvial deposits and Oligocene conglomerates of the Lemme valley (northwestern Italy): Comparison with analogous Neolithic polished stone implements and archaeometric implications

Rare HP meta-ophiolites of archeologic interest occur in the western Alps, both as small primary outcrops at high altitude located in the Monviso or Voltri massifs and as boulders/pebbles in the derived secondary conglomeratic deposits and/or alluvial beds downhill/downstream, originated after erosion of the former ones. Despite their scarceness, these rocks – in the archaeologic literature still grouped under the term “greenstones” – are of paramount importance for archaeologists, since they were used to produce polished stone tools rediscovered in prehistoric sites spread all over Europe. Tracing the sources of these raw materials is an important issue for reconstructing the migratory fluxes and trade routes of our ancestors. A certain number of “greenstone” geologic samples were retrieved from the alluvial beds of the Lemme valley streams (southeastern Piemonte, Italy) and in the Oligocene conglomerate units of the Tertiary Piemonte Basin, presently exposed in the same valley. Representative samples were analyzed with a classic mineralogical and petrographic approach, which includes XRPD, polarizing microscope and SEM-EDS. These rocks compositions and microstructures were then compared with those of analogous implements found in archaeological sites located nearby and already described in the literature, searching for analogous traits suggesting a common origin. The obtained outcomes show that these secondary deposits of the Lemme valley – originated from the dismantlement of upstream primary outcrops in the Voltri massif (or from a close, analogous palaeo-unit, nowadays completely eroded) – might have represented, during the early-to-middle Neolithic, one of the preferential supply sources of raw “greenstones” for the archaeologic sites of Brignano Frascata, Momperone, Villaromagnano and Rivanazzano.