Tertiary and Quaternary alluvial gold deposits of Northwest Spain and Roman mining (NW of Duero and Bierzo Basins)

Abstract

During the 1st and 2nd centuries AD, more than 500 gold mines were operating in the northwest (NW) of Spain. The region was then a part of the Roman Empire and known as Iberia. Corresponding excavations totaled 600million cubic meters (Mm3), with about 300Mm3 coming from alluvial mines. Miocene sediments (mainly alluvial fans) derived from the Cantabrian Mountain and León Mountain uplifts, and Quaternary deposits (principally regolith, fluvial terraces, moraines and fluvio-glacial fans) were exploited by the Roman miners, using hydraulic mining methods.The paper attempts to incorporate the following numerous themes. (a) The time-successive, global relationship of the auriferous deposits. (b) A brief geological overview of the Miocene and Quaternary placers in the NW Spain. (c) A description of the composition of the gold grains, both in the hardrock source and in the various alluvial placers. The latter is necessary to determine possible source areas. Pre-Miocene age of gold precipitation helps to understand the possible recycling processes. (d) The size, grade and gold grain size distribution of the studied deposits. (e) A brief description of Roman mining methods, including new viewpoints on how the operations were conducted. Special reference will be made to the Las Médulas mine (included in the World Heritage List in, the largest earth movement mining operation in Iberia up until the 20th century. (f) An update on earth moving techniques and gold recovery in Roman times.The present work draws heavily on data collected from some 6500m3 of alluvium washed from 300 trenches and drill holes on Miocene and Quaternary deposits, and the Las Médulas Archaeological Zone project.