Towards a characterisation of iron oxide-rich rocks used during the Archaic period on the Costa Arreica in Northern Chile

Iron or manganese oxide rich rocks were constantly and intensively used by hunter-
gatherers from around 300 kya. Nonetheless, few is none concerning the supply in
raw ferruginous materials. The mine San Ramón 15 in northern part of the Chilean
coast reveals an exceptional evidence of the extraction of iron and probably
manganese oxide rich materials by groups of hunters-fishers-gatherers. Two
extraction phases were determined during the excavation of the mine trench: the
oldest one during the Pleistocene-Holocene transition (11000-8000 cal. BP) and the
second one from 4300 cal. BP. A rich set of lithic pounding stones and hammer
stones were recorded in the mine refus.
The mine trench follows a various meters wide lenticular vein in the granodioritic
bedrock from the Jurassic. The vein is principally made of hydrothermal pyrolusite,
magnetite and goethite which are extremely hard materials and yellow to brown and
black. Thus we suppose that the prehispanic miners intensively extracted a peculiar
part of the vein, between the hard magnetite and the bedrock, so that few evidence
of the material extracted in the mine were recorded.
Our investigations focus on the determination of the characteristics of the quite
messing materials which were extracted and we try to identify the following phases of
transformation and utilisation. In order to address these issues, we sampled and
document the lithology of the vein and of the numerous geological formations which
provide iron rich materials in the neighbourhood. Furthermore, fragments of iron
oxides from the mine refus, as well as red or black residues on tools from divers
Archaic sites in the area (hammer stones in the mine, lithic weapons, grinding-stones
and shells in the shellmiddens and rock-shelters) in order to compare their
mineralogical and geochemical composition.