The Almadén mercury mining district, Spain

The Almadén 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 different 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 Almadén can be classified into two main types: type 1, early stratiform type ores characterized by cinnabar deposition on the lower Silurian quartzites (Criadero quartzite; e.g. the Almadén 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 Concepción) to Upper Devonian (e.g. Corchuelo). Received: 23 October 1998 / Accepted: 4 January 1999     

Geological context and plumbotectonic evolution of the giant Almadén Mercury Deposit

The Almadén mine has been the largest among several mercury deposits that represent the biggest mercury concentration in the world. The deposits form a mining district which is located in a 30 km long and up to 15 km wide WNW–ESE oriented syncline, where a thick Lower Ordovician–Upper Devonian siliciclastic sedimentary sequence outcrops. Most of the deposits are located in the south subvertical syncline flank, which has an opposite vergence to the rest of the region. Of special note is the presence of important NW–SE to WNW–ESE crustal structures that played a major role at several times during the regional geological history and controlled the sedimentary unit distribution, volcanism and deformation. One of these structures seems to have played an important role in the Almadén area, probably having been responsible for the anomalous syncline geometry. This structure acted during the E–W Variscan shortening as a ductile–fragile sinistral shear zone that resulted in a subvertical attitude of the southern Almadén Syncline flank, affecting the sedimentary sequence longitudinally. The Hg deposits in the region correspond to two types, stratabound and stockworks. The former are hosted in well-defined “Criadero Quartzite” orthoquartzite levels of Ordovician–Silurian age. These deposits were folded and sheared during the Variscan deformation. The stockwork deposits filled fractures and veins and partially replaced the volcanic rocks affected by the Variscan shear zones. The replacement process took place at the end of the E–W Variscan shorteningThe Almadén deposit belongs to the stratabound type and has three mineralized levels, one located in the lower part and the other two in the upper part of the “Criadero Quartzite”. Of minor relevance, other small stockwork bodies, replacing a volcanic breccia-tuff known as “Frailesca” rock, have also been exploited. This rock formed massive lenticular bodies that have been interpreted as pre-Variscan diatremes. On the basis of field criteria we conclude that the “Frailesca” rock emplacement took place later than cinnabar mineralization. After the “Frailesca” rock was formed, it was cut by sills of quartz-diabase that resulted from a new magmatic event. Both volcanic materials affect the mercury ore, developing small aureoles of contact metamorphism and volatilizing the cinnabar. The deposit shows three sectors, separated by two straight dextral faults, which cut the sinistral WNW–ESE shears bands. The latter affect the mercury ore, mostly in its western area.Lead isotopes from Almadén cinnabar deposits show a broad range of values, higher than those predicted for the Stacey and Kramers and Cumming and Richards crustal Pb evolution models but largely tallying with the Sardinia evolution line for this sector of the Variscan basement quite well. The data set plotted along the Sardinia curve in several well defined clusters that could be interpreted as a lead extraction by means of large scale convective hydrothermal systems from a lead reservoir located in the upper crust at a time indicated by the Sardinia curve. The estimated ages for this lead model evolution indicate lead extraction as having occurred during the late Silurian–Devonian (420–375 Ma), late Variscan (300 Ma,), Permian–Triassic (290–220 Ma), late Jurassic–Early Cretaceous (200–150 Ma) and Eocene–Oligocene (50–25 Ma), and are coincident with the main extensional tectonic episodes (from late Ordovician to Devonian, Permian to Triassic and Late Jurassic to Early Cretaceous). This shows that cinnabar is likely to have been mostly remobilized–crystallized during the regional extensional tectonic events, capturing lead from the host sedimentary sequence. This lead was mobilized by large scale, long term hydrothermal convective cells at various times, constituting a complex geotectonic history for the ore-forming processes.     

Spurenelemente und Schwefel-Isotope in Zinkblenden der Blei-Zink-Lagerstätte von Gorno

Zusammenfassung Im Zusammenhang mit einer feinstratigraphischen Bearbeitung der schichtgebundenen Blei-Zink-Lagerstätte von Gorno wurde eine engmaschige Probenahme in allen Erzhorizonten ausgeführt. Die Zinkblenden dieser Proben wurden auf Spurenelemente und Schwefel-Isotope untersucht. Sowohl die Spurenelemente wie die Schwefel-Isotope zeigen eine weite Streuung. Sie lassen aber beide gesetzmäßige Abhängigkeiten von der sedimentären Fazies der Nebengesteine (Becken- oder Schwellenfazies) erkennen, die sich nur aus dem gleichzeitigen Absatz von Erz und Nebengestein erklären lassen.

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In connection with stratigraphic research in the stratabound lead-zinc-deposit of Gorno a closemeshed sampling in the different ore horizons was carried out. The minor elements and the sulfur isotopes in the sphalerites of these samples have been determined. The minor elements as well as the sulfur isotopes showed a very large spread of values, but an accurate dependence on the sedimentary facies of the bedrocks (swell- and basin-facies) could be observed. This dependence may only be explained by the contemporanëity of the precipitation of the ore minerals and the sedimentation of the bedrocks.

     

Konvergenzerscheinungen der Mineralbildung einiger sedimentärer deutscher Eisenerze

Zusammenfassung Sedimentäre Eisenerze können durch Ausflocken von Fe⁺⁺⁺haltigen Solen und durch Ausfällen von Fe⁺⁺haltigen Lösungen gebildet werden. Aus Solen gebildete Erze werden als Begleitelemente Kieselsäure und Aluminium anreichern, während aus Lösungen gebildete Erze manganreicher sind und Kieselsäure und Aluminium zurücktreten.     

Die Metasomatose in der Eisenspatlagerstätte Hüttenberg,Kärnten

Ohne Zusammenfassung     

Geochemische Untersuchungen von Wässern aus der ägyptischen Sahara

Zusammenfassung Die Arbeit beschäftigt sich mit hydrochemischen Analysen ägyptischer Tiefenwässer und ihren geologischen Deutungsmöglichkeiten. Bestimmungen stabiler Isotopenverhältnisse (0¹⁸/0¹⁶ und C¹³/C¹²) vervollständigen die Untersuchungen.

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The paper deals with chemical aspects of subsurface waters and their geological consequences. Namely, stable isotopes in geohydrological cycles are discussed and geological conclusions are offered.

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Résumé Le travail a trait aux analyses chimiques d'eau de cette région et à leur signification géologique possible. Des discussions détaillées des vues d'auteurs étrangers complètent le travail.

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Publication of the Div. of Geol. Sciences California Inst. of Technology Pasadena, California, Contribution No. 1019.     

II. Mitteilung. Die Zersetzungsprodukte des Olivins aus dem Muttergestein der Lagerstätte dichten Magnesits vom Galgenberg bei Zobten

Ohne ZusammenfassungAbgeschlossen im Juni 1947.     

Ein Wachstums- und ein Rekristallisationsgefüge von Bleiglanz aus einer rheinischen Lagerstätte

Ohne ZusammenfassungMit 3 Diagrammen.     

Die lateritische Zersetzung des Peridotits bei der Bildung der Nickelerzlagerstätte von Frankenstein in Schlesien

Ohne ZusammenfassungAbgeschlossen im Juni 1947.     

Mineralogische Notizen aus dem Mineralogisch-Petrographischen Institut der Universität Heidelberg

Ohne Zusammenfassung     

The Hunsrück Slate Konservat-Lagerstätte

The Lover Devonian Hunsrück Slate in western Germany is world-famous for its exceptionally preserved and beautiful fossils. What makes this Konservat-Lagerstätte so fascinating is its unique style of pyritization that has perfectly captured an entire ecosystem, from delicate echinoderms with preserved soft tissues to the Lovecraftian Mimetaster.     

Sedimentationsverhältnisse im Mesozoikum der nördlichen Kalkalpen

Ohne Zusammenfassung     

III. Mitteilung. Die hydrothermale Zersetzung des Peridotits bei der Bildung der Magnesitlagerstätte am Galgenberg bei Zobten

Ohne ZusammenfassungAbgeschlossen im Juni 1947.     

Umbettung von Fossilien mit dem ursprünglichen Sediment. Mit Beispielen aus dem deutschen und australischen Paläozoikum

Zusammenfassung Umbettung von Fossilien mit noch unverfestigtem Sediment unter unvollständiger Aufbereitung kann zur schwer erkennbaren Mischung wesentlich verschieden alter Faunen führen. Mischung und Wiedereinbettung miozäner und rezenter Fauna wurde am Morsum-Kliff auf Sylt in allen Einzelheiten beobachtet. Als fossiles Beispiel wird das Zusammenvorkommen leitender Fossilien dreier Zonen des rheinischen Devon entsprechend gedeutet. Vermutlich ebenso aufzulösen ist die Gemeinschaft von Psilophyten der Tracht des oberen Unterdevon mit Unter-Ludlow-Graptolithen in Australien; bei Bestätigung entfiele das gotlandische Alter der ältesten Festland-Gefäßpflanzen. Oft läßt nur die phylogenetische Entwicklungsstufe von Fossilien Umlagerung zwingend erschließen und weittragende stratigraphische Fehlschlüsse vermeiden.Vortrag auf der Jahresversammlung der Geologischen Vereinigung in Karlsruhe vom 22. bis 24. März 1958.     

Über den Bleigehalt des Kaolins von Hirschau-Sehnaittenbaeh und die Bleierzlagerstätte von Freihung in der Oberpfalz

Zusammenfassung Der Bleigehalt des Hirschau-Schnaittenbacher Kaolins (ca. 0.1% Pb) wird aus dem Entstehungszyklus der kaohnhaltigen Buntsandstein-Arkosen hergeleitet. Die Bleierzlagerstätte von Freihung kann als örtliche Konzentration des Bleigehaltes der Trias-Arkosen gedeutet werden.Herrn Professor Dr. Dr. h. c.C. W. Correns zum 70. Geburtstag gewidmet.     

Überreste tertiärer Verwitterungsrinden in Deutschland

Ohne Zusammenfassung     

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 rhyolites, through olivine-basalts, pyroxene-basalts, trachybasalts, trachytes, very scarce rhyolites, and quartz-diabases. 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 δ¹⁸O, δ¹³C, and δ³⁴S 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).