Archaeomagnetic dating of prehistoric mining-firesetting in the southern Alps (Roua, France).

The native copper mines dug on Permian sandstones in the Dôme du Barrot (southern Alps, France) provide vestiges of ancient firesetting associated with prehistoric tools (3000 - 450 BCE). The objective of this study is to obtain an archeomagnetic dating and a relative chronology of 6 galleries among the forty mapped up to now. In a preliminary study, we demonstrated the feasibility of the archaeomagnetic dating of firesets by overprinting at different temperatures a laboratory partial Thermal remanent Magnetization (pTRM) of normal polarity on the Detrital Remanent Magnetization (DRM) of reversed polarity recorded in the Permian rocks. For these rocks, we showed that a pTRM can be successfully acquired and then retrieved from a set temperature as low as 200°C. At total of 24 oriented blocks were taken for the archaeomagnetic dating, this way, one fireset is sampled on each gallery by means of 4 oriented blocks. Their orientation is achieved in situ with a magnetic compass and then refined in the laboratory with a 3D photogrammetric analysis. A minimum of 10 specimens were prepared from each block. The NRM components were retrieved from a stepwise paleomagnetic cleaning by heating, the remanence is measured with a cryogenic magnetometer (2G) at Géosciences Montpellier laboratory. We were able to isolated a well-defined pTRM for 5 firesets among the 6 sampled. The averaged pTRM directions at a fireset level are compared with BIGMUDI4K and SHA.DIF.14K reference curves calculated at the location of the mines. In addition, we also developed a new local directional reference curve based on a Bayesian approach from a compilation of archaeomagnetic and speleothems data located within a distance of 1000 km around the mines with the hope to refine the global models. We obtained prehistoric ages on 4 firesets. To our knowledge it is the first time that the archeomagnetic dating approach is successfully applied to vestiges of mining-firesetting.