Intraplate mafic magmatism, degasification, and deposition of mercury: the giant Almadén mercury deposit (Spain) revisited

The giant Almadén mercury deposit (Spain) is hosted by the Lower Silurian Criadero Quartzite; in turn this ore-bearing rock unit is cross-cut by the so-called Frailesca unit, a diatreme body of basaltic composition. The geochemical characteristics of the Silurian to Devonian Almadén District volcanic units indicate that these rocks originated from an enriched, evolving mantle source that ultimately yielded basanites?nephelinites to yolites, through olivine-basalts, pyroxene-basalts, trachybasalts, trachytes, very scarce rhyolites, and quartzdiabases. The Silurian intraplate alkaline volcanism developed in submarine conditions which triggered widespread hydrothermal activity resulting in Hg ore formation and pervasive alteration to carbonates. The #948;18O, #948;13C, and #948;34S isotopic signatures for carbonates and pyrite suggest different sources for carbon and sulfur, including magmatic and organic for the former and magmatic and sea water for the latter. The most important and efficient natural source of mercury on Earth is by far the volcanic activity, which liberates mercury via quiescent degassing and catastrophic (Plinian) events when eruptions can overwhelm the atmospheric budget of Hg. Thus, we suggest that CO2 degasification and coeval distillation of mercury from the volcanic rocks fed the huge hydrothermal system that led to massive deposition of mercury at Almadén. Build up of Hg0 gas in magmatic chambers during waning rifting in the Late Ordovician, followed by renewed volcanism in the Early Silurian, would have resulted in massive degasification of the accumulated mercury. Part of this mercury went into the Criadero Quartzite leading to formation of the huge Almadén deposit and others (e.g., El Entredicho) along the same stratigraphic level. Progressive depletion of the deep seated magmatic Hg stock would have resulted in a drastic reduction in ore deposit size after the Lower Silurian when smaller deposits formed (e.g., Las Cuevas)

An estimation of mercury concentrations in the local atmosphere of Almadén (Ciudad Real Province, South-Central Spain) during the 20th Century.

We show the first estimations of long-term (100 years, 1900 to 2000) total gaseous mercury concentrations (TGM) in the urban area of Almadén. The estimation was carried out by comparing data on known metallic mercury production with measured TGM concentrations. The estimated diurnal background level ranges from 60 to 120 ngm#8722;3 and corresponds to periods when the metallurgical complex (cinnabar roasting plant) was shut down. The average TGM concentration during the period from 1900 to 2000 was about 600 ng m#8722;3 (with peaks above 1,200 ng m#8722;3). Additionally, a 24-h-based TGM monitoring program has highlighted significant differences between the diurnal and nocturnal concentrations,particularly during the warmer months. In this regard, given that the average nocturnal to diurnal ratio is 2.12, we suggest that the average nocturnal concentrations must have exceeded 1,200 ng m#8722;3, and peak nocturnal concentrations could have reached levels up to 2,400 ngm#8722;3. Our estimations indicate that most parts of the town of Almadén were generally exposed to TGM concentrations in air that exceed the World Health Organization air quality guideline for Hg (1,000 ng m#8722;3) for countryside and urban areas

The mersade (European Union) project: testing procedures and environmental impact for the safe storage of liquid mercury in the Almadén district, Spain.

The MERSADE Project (LIFE--European Union) tested the Las Cuevas decommissioned mining complex (Almadén mercury district, Spain) as a potential site for the installation of a future European prototype safe deposit of surplus mercury from industrial activities. We here present the results of a baseline study on the distribution of mercury in soils and air in the Las Cuevas complex and surrounding areas, and show the results of a plume contamination model using the ISC-AERMOD software. Despite restoration works carried out in 2004, the Las Cuevas complex can still be regarded as hotspot of mercury contamination, with large anomalies above 800 microg g(-1) Hg (soils) and 300 ng Hg m(-3) (air). In the case of soils, high, and persistent concentrations above 26 microg g(-1) Hg extend well beyond the complex perimeter for more than 2 km. These concentrations are about three orders of magnitude above world baselines. The same applies to mercury in air, with high concentrations above 300 ng Hg m(-3) inside the perimeter, which nonetheless fade away in a few hundred meters. Air contamination modelling (Hg gas) predicts formation of a NW-SE oriented narrow plume extending for a few hundred meters from the complex perimeter. The geographic isolation of Las Cuevas and its mining past make the complex an ideal site for mercury stocking. The only potential environmental hazards are the raising of livestock only a few hundred meters away from the complex and flash floods

Massive Volcanism in the Altiplano-Puna Volcanic Plateau and Formation of the Huge Atacama Desert Nitrate Deposits: A Case for Thermal and Electric Fixation of Atmospheric Nitrogen

The origin of the giant nitrate deposits of the Atacama Desert (Chile) is a controversial issue. At a global scale, the Atacama Desert nitrates constitute a rare singularity because no equivalent deposits are found anywhere else. Previous hypotheses for origin of the Chilean nitrates have failed to recognize the importance of the 70,000 km2 Late Miocene to present Altiplano-Puna volcanic plateau (APVP), only 50-200 km eastward from the nitrate deposits. We argue that the extrusion of a volume of over 104 km3 of pyroclastic rocks at the APVP may have created the conditions to induce thermal and electric fixation of an estimate of 2800 Mt of atmospheric nitrogen in the form of NO3. This figure exceeds the amount of nitrogen required to account for the Atacama Desert nitrate deposits. Thus, the origin of the nitrate deposits may be found in an unusual combination of hyper-arid conditions (vital for the final stabilization and preservation of the NaNO3 mineral phase) and massive volcanism (key to fixation of large amounts of atmospheric nitrogen). Volcanic eruptions have far more environmental implications than usually assumed, decisively contributing to the global cycles of many chemical elements and compounds. For example, nitrogen fixation by volcanic activity could also explain the current excess of NOx compounds in the pristine marine atmosphere of the mid-Pacific, a realm conspicuously surrounded by the world's largest concentration of active volcanoes.Ministerio de Educación y CienciaDepto. de Mineralogía y PetrologíaFac. de Ciencias GeológicasTRUEpu

Mineral deposits and Cu–Zn–As dispersion–contamination in stream sediments from the semiarid Coquimbo Region, Chile

This paper presents Cu–Zn–As geochemical data from stream sediment surveys carried out in the three main watersheds of the Coquimbo Region of Chile. This mountainous semiarid realm occupies an area of 40,656 km2 between 29° and 32°S. Given that the area has a long historical record of mining activities, important environmental disturbances were expected. However, despite the detection of three major geochemical anomalies for Cu, Zn, or As, only one can be unmistakably linked to the development of mining–metal recovery procedures (Andacollo–Panulcillo). An investigation of the other two anomalies (Elqui and Hurtado) reveals three major causes that fully or partially account for them: (1) the type of ore deposit and associated hydrothermal alteration; (2) the regional structural setting (intensity of fracturing); and (3) climate–landscape. Cu–Au–As epithermal deposits/prospects along the so-called El Indio belt are here regarded as the sources of both the Elqui and Hurtado anomalies. The strong advanced argillic alteration present in some of the epithermal deposits/prospects of the El Indio belt may have induced the loss of the buffering capacity of rocks, and therefore favoured metal dispersion during later oxidation–leaching of sulphides. This applies to the Elqui and Hurtado anomalies. Conversely, given that the potassic, propylitic and phyllic alterations do not affect the buffering capacity of rocks, only minor metal dispersion is observed in relation to the Los Pelambres porphyry copper deposit. Besides, the epithermal belt is located within a highly fractured Andean domain (3,000–4,000 m of altitude), which may have conditioned the fast unroofing of ore deposits, contributed to enhanced circulation of meteoric waters, and eventually, to strong oxidation, and leaching of metals. Metal dispersion is aggravated during rainy years in response to strong El Niño episodes.Ministerio de Educación y CienciaDepto. de Mineralogía y PetrologíaFac. de Ciencias GeológicasTRUEpu