A LITTLE BIG HISTORY OF IBERIAN GOLD: GEOLOGICAL ORIGINS AND HUMAN CONSEQUENCES

Big History is the attempt to understand broadly all of the past that lies behind the human situation. We present a “little Big History” of the gold that played a critical role in the history of Iberia (now Spain + Portugal). This gold came from three geologic sources via four routes: Northwest Iberia during Roman times; Sub-Saharan West Africa via camel caravan in the Middle Ages, and via Portuguese ships beginning in the 15th century; and from Colombia during and after the discovery and subsequent conquest of the Americas.

The geologic setting and age of gold mineralization is completely different in each of these regions. Gold-bearing parts of the West Africa Craton (the Man-Leo Shield) of Paleoproterozoic age (2195 Ma to 2072 Ma) are predominantly syn-deformational, with deposits in shear zones hosted by metamorphic terrains and greenstone belts of the Birimian volcanic and sedimentary sequence, separated by granitic bodies of the Eburnean tectono-thermal event. Northwest Iberian gold deposits formed during the Variscan-Appalachian orogeny ca. 300 Ma. Epithermal gold in quartz veins was exploited by the Romans, but the most important gold sources were placer deposits, like Las Medulas Miocene conglomerates, derived from the erosion of Variscan deposits. Gold mineralization in Colombia is of Mesozoic-Cenozoic age and of a variety of types of deposits, and it affects all three Andean Cordilleras, despite their very different geological character. The different geology in Africa, Iberia, and Colombia provides no obvious answers to the ultimate origin of gold within the earth. As a highly siderophile element, the vast majority of Earth’s gold is likely in the core. It has been suggested that whole-mantle plumes may provide an effective way to continue transporting small amounts of gold from the deep earth to the near-surface. Limited geochemical studies indicate that magmas derived from mantle plumes may have a higher gold content than mid-ocean ridge type magmas.

Given the very different geologies and geologic histories, we must look for mechanisms that concentrated gold and made it accessible for exploitation and extraction in some parts but not others, and the social, cultural and economic impact of this at a regional and global scale.