Attaque au feu au Bronze ancien sur le gîte de cuivre de Saint-Véran (Hautes-Alpes)

Abstract

The copper deposit of Clausis Saint-Véran (Hautes-Alpes, France) is a lens of ore included between an oceanic substratum, with serpentinite and shale, and a sedimentary cover with marble and quartzite with riebeckite. The sulphide ore is a high-grade massive bornite (45/ 65% copper) associated with native copper. Prehistoric mining activity was located between 2 400 and 2 600 metres, and thus represents the highest mine in Europe for this period. The miners excavated the ore systematically in the oxidation rich outcrop, from the soft shale, in contact with the ore, up to a wall of quartzite. The mined ore was then taken down into the valley, 250 metres below the level of the mine, to the mineralogical/ metallurgical workshop of Cabane des Clausis for treatment. The metallurgists used all the technical processes necessary to change the ore into copper metal : hand-picking, washing, crushing and smelting. The earliest date obtained is 3815 ± 50 BP (2460-2140 cal. BC [ 2 σ] : Tucson AA22621), cf. Barge, 2002 and 2003 ; Barge ed., 2006. On the main vein, the most important extraction was from an open cut called Tranchée des Anciens. In places, the irregular and rounded rock faces show tool-prints (fine grooves, hammer marks) resulting from green stone picks and grooved mauls. Numerous well-conserved waterlogged wood fragments (pit-props, floor planks, sticks from torches) have dated the exploitation to between 3656 ± 44 BP (2189 -1888 cal. BC [ 2 σ] : Tucson AA36604) and 3418 ± 43 BP (1877-1618 cal. BC [ 2 σ] : Tucson AA36603). However the presence of charcoal remains is not necessarily proof of fire-setting in the mine, as they could come from torches. A second old mine working was discovered on the eastern branch of the copper deposit, at the top of the outcrop at an altitude of 2 650 m. Here, short excavations were dug into small secondary mineral veins (native copper ?). They show the prehistoric workers’ intensive search to collect copper ore from the slightest irregularities of the rock. Contrary to all expectations, traces of the fire-setting technique were found in this second zone, little studied previously. This exploitation, situated near a modern prospection (gallery no. 0) on a quartzite outcrop between shale and ophiolite, has been dated to the Early Bronze Age, between 3366 ± 42 BP (1750-1520 cal. BC [ 2 σ] : Tucson AA58833) and 3524 ± 41 BP (1960-1730 cal. BC [ 2 σ] : Tucson AA58831), which remains exceptional in the Southern Alps. This open cut, excavated over a surface of 5 m2, show a succession of circular or oval thermal hollows containing broken stones and charcoals. These hollows are the result of successive and contiguous fires with a diameter of 0.60 m. A hearth structure was preserved against the rock face and represents the last stage of an abandoned fire-setting attempt. The cross-section shows three particularly well characterised levels : a superficial filling material above a dense level of brown and grey sediments with numerous small plates of shale and quartz, then an unstable level with small plates of shale and quartz and charcoal. The ore was extracted with stone tools like those discovered in situ, including a riebeckite maul bearing percussion marks. Numerous small and dense cup-like marks are also visible on the green shale. More than one kilo of charcoals was collected during the excavation of the fire-setting open cut and from thermal hollows or hearths. From the point of view of species selection, it is obvious that the anthracological assemblage is not very rich. Only three species were identified, all trees. They were Pinus Cembra, largely dominant in our results, larch and pubescent birch. No shrubs, such as juniper, bilberry or rhododendron, were identified, pointing to a deliberate choice of the wood used. Pinus Cembra was especially selected for fire-setting, while larch and birch were merely occasional. On the contrary, the results of the anthracological study of the underground workings showed a preference for Norway pine/ Mugho pine. These results confirm those of the anthracological analysis of the fire-setting open cut, the underground workings and the mineralogical/ metallurgical workshop. Therefore, anthracological analysis of the material found in the fire-setting open cut at Saint-Veran provides interesting information about the choice of the combustible used preferentially for fire-setting, i. e. Pinus Cembra. It also proves that the alpine vegetation around the site was more wooded than at present and that prehistoric men used the wood found nearby. Thus, the mining activity largely contributed to deforestation of the mountainside and to rarefaction of the Pinus Cembra.