The armorican Brittany tin production : a rich region exploited since Bronze age

International audienceThe Armorican Massif is an old geological feature whose last structuration dates back to the formation of the hercynian chain leading to the setting up of many granites fracturing the surrounding rocks. Hydrothermal fluids bearing mineralized elements (including tin), were then injected into these major vein systems. Some of these veins remained untouched, while others did suffer the effects of erosion. In this latter case, grains of cassiterite (hardly alterable), were swept by water and deposited in more quiet areas thus forming alluvial and marine placers. These concentrations of variable size are extremely numerous in Brittany, and many of them could easily be exploited since the protohistoric periods. Field surveys begun in order to identify old tin production workshops, and first evidences were indeed observed. Tin wastes were characterized by laboratory studies including microscopic observations and chemical analysis. Some slags, witness of the reduction process from ore to metal, were found in mixed layers of charcoal, last remnants of the alluvial workshops. The chronology for these workshops is variable, form Bronze Age to Iron Age, Antiquity and medieval period. Few other slags were found isolated, indicating probably the location of nearby workshops, however obviously no dating is possible for now. Similarly, some mining excavations have been spotted close to the veins, some of these are dated back to Antiquity, but many of them can’t be dated for the moment without archaeological excavations. The Armorican Massif is therefore a particularly tin-rich region, but these important mining resources seem to have never been overexploited, leading to a continuous extraction during about two millennia. This region, in the west end of continental Europe was at all time part of exchange networks and long distance trade. Perhaps it could correspond to the mythical Cassiterides islands…