The Almadén mercury mining district, Spain

Abstract

The Almade n district is the largest mercury concentration in the world, with a total content of about 250 000 t of mercury, nearly one third of the known 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 overlying Precambrian rocks. The Paleozoic sequence comprises epicontinental quartz arenite rocks, including black shales and quartzites. Diatremes, alkaline lavas of dierent composition, and late tholeiitic diabases account for the Ordovician to Devonian magmatism. The tectonic setting of this complex suite corresponds to the intraplate type. The mercury deposits of Almade n can be classi®ed into two main types: type 1, early stratiform type ores characterized by cinnabar deposition on the lower Silurian quartzites (Criadero quartzite; e.g. the Almade n and El Entredicho deposits), and type 2, late discordant orebodies (e.g. Las Cuevas), largely hosted or related to diatremes (the`frailesca rocks') of alkaline basaltic composition. In type 1 cinnabar was deposited during diagenesis, in relation to hydrothermal circulation driven by magmatic activity. Type 2 include a variety of deposits having in common the discordant character of the orebodies (e.g. veins, stockworks, massive replacements), and their wide dispersion along the stratigraphic column, i.e. from Lower Silurian (e.g. Nueva Concepcio n) to Upper Devonian (e.g. Corchuelo).

First lead isotopic data for cinnabar in the Almadén district (Spain): implications for the genesis of the mercury deposits

“…However, a preliminary investigation of Sm/Nd isotopic systematics on seven samples of Almadén district mafic volcanics and ultramafic xenoliths produced 143 Nd/ 144 Nd values from 0.512578 to 0.512757, with e(Nd) 360 Ma ranging from +2.5 to + 5.7. The inferred long-term LREE depleted mantle source(s) contrasts with the strong incompatible element enrichment seen in the Almadén mafic volcanic rocks (Higueras and Munhá, 1993;Higueras, 1995;Higueras et al, 2000b), being consistent with geological and geochemical data (Hernández et al, 1999) suggesting that rifting processes and plume related mantle metasomatic activity may have shortly predated the extensive mafic magmatism in the Almadén syncline during early Palaeozoic times. By pointing towards the involvement of those mantle derived magmatic sources of mineralization, the new cinnabar lead isotope data lends support to previous studies that proposed genetic relations between the Almadén mercury deposits and the regional alkali basalt magmatism (e.g., Higueras et al, 2000b).…”

“…The different mines of the district have produce almost one third of the total historic production liquid Hg metal, which is more than any other production of any other Hg mining district in the world. Mercury ore bodies are hosted by sedimentary and volcanic rocks belonging to a Lower Palaeozoic synclinal sequence that unconformably overlies the pre-Ordovician basement of the Central Zone of the Iberian Variscan Chain (Higueras, 1995;Hernández et al, 1999). The volcanic-sedimentary rocks, ranging from Ordovician to upper Devonian in age, comprise several packages of black shale and sandstone/ quartzite units and include frequent intercalations of submarine mafic volcanics including the so-called frailesca rock, diatremes that locally cut the sedimentary and volcanic units (Saupé, 1990;Higueras, 1995).…”

“…Two main types of Hg deposits have been described in the district (Hernández et al, 1999): stratabound and discordant ore bodies. The former corresponds to well known Almadén type mineralizations, including the Almadén mine and the "El Entredicho" open pit, where the ore (cinnabar) is disseminated in lower Silurian quartzites (the so-called Criadero quartzite).…”

“…Despite the many differences in size and morphology between these two types of deposits, one feature is shared by both: they occur in close spatial association to frailesca rocks. The nature of mineralizing brines and ultimate origin of mercury has remained a matter of controversy; Saupé (1990) and Saupé and Arnold (1992) postulated the Ordovician black shales as the source of mercury, whereas Hernández et al (1999) and Jébrak et al (2002) suggested a relationship between the source of mercury and the alkali basalt volcanism in the area.…”

Mercury distribution in an abandoned metallurgical plant

“…The Almadén Hg mining district included cinnabar extraction sites (open pits and underground mines), storage areas and metallurgical plants, including mineral furnaces. In two millenniums of activity, this area produced one third of the Hg used in the human history (Hernández et al 1999).…”