The Almaden mercury mining district, Spain

The Almaden district is the largest mercury concentration in the world, wit h a total content of about 250 000 t of mercury, nearly one third of the kn own total mercury resources of the Earth. Mercury has been exploited since the Celtic and Roman times, with peak production during the Renaissance and between 1939-1945. The district is hosted by a Paleozoic synclinorium over lying Precambrian rocks. The Paleozoic sequence comprises epicontinental qu artz arenite rocks, including black shales and quartzites. Diatremes, alkal ine lavas of different composition, and late tholeiitic diabases account fo r the Ordovician to Devonian magmatism. The tectonic setting of this comple x suite corresponds to the intraplate type. The mercury deposits of Almaden can be classified into two main types: type 1, early stratiform type ores characterized by cinnabar deposition on the lower Silurian quartzites (Cria dero quartzite; e.g. the Almaden and El Entredicho deposits), and type 2, l ate discordant orebodies (e.g. Las Cuevas), largely hosted or related to di atremes (the 'frailesca rocks') of alkaline basaltic composition. In type 1 cinnabar was deposited during diagenesis, in relation to hydrothermal circ ulation driven by magmatic activity. Type 2 include a variety of deposits h aving in common the discordant character of the orebodies (e.g. veins, stoc kworks, massive replacements), and their wide dispersion along the stratigr aphic column, i.e. from Lower Silurian (e.g. Nueva Concepcion) to Upper Dev onian (e.g. Corchuelo).