Rubefaction processas prospection criteria within the roman goldmining context of the EriaGold Mining District (León, NW Spain)

Abstract

The Eria Gold Mining District represents one of the most extensive Roman mining remains in the northwest of Spain. Gold mineralization in this area is associated with quartz veins and dykes in Paleozoic rocks (primary) and Miocene and Quaternary sediments (secondary). Hitherto, ancient prospection works in Miocene materials were focused on the location of alluvial fan deposits according to the Roman literary descriptions. However, the finding of mining areas and remains of hydraulic structures located in reddish-orange hillside deposits suggests that there may have had other prospective indicators. We present a mineralogical study of optical microscopy and X-Ray diffraction for the provenance analysis of iron oxides studied in pebbles (within these sediments) and ironstone deposits located next to the main mining areas. The formation of secondary goethite, probably related to the recent chemical weathering of chlorites (formed during Variscan metamorphism), could be the responsible for the intense reddish coating. This could have served to Roman miners as gold signs, beyond other indications such as texture and roundness of rockpebbles in sediments.