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The geological singularities of aworld-wide patrimony: Thegiant mercury deposits ofAlmaden (Spain)/ Dr. Fernando José Palero Fernández

Abstract

The recent declaration of the Almaden Mine as Patrimony of the Humanity has been a recognitionto an exceptional ore deposit. In the Almaden town surroundings special geological events occu-rred that gave rise to some deposits of a scarce metal that are surely unique at crustal levels.The Hg deposit of Almaden is located in a Hercynian WNW-ESE bearing syncline where a very com-plete Palaeozoic sedimentary sequence is outcropping, from Lower Ordovician to Upper Devonianages. These rocks were formed in a marine shelf basin, with some episodes when an importantbasic and ultrabasic volcanic activity took place, including the formation of diatremic explosivebodies. The Hercynian tectonic has given rise to a synclinal configuration by means of a generali-zed folding event and another one of shearing with irregular spatial distribution that specially affec-ted the Southern flank. The deposits of Hg are distributed in the Eastern half of the syncline andmainly throughout the Southern flank, in relation to the shear-bands. Two ore-deposit types arerecognized, stratiform seams and irregular stockwork bodies. The stratiform deposits are clearlypre-tectonic with respect to the Hercynian events, whereas the second ones show a certain sin-tectonic character in relation to the deformation by shearing. The biggest deposit was the AlmadenMine, belonging to the stratiform type, whose huge Hg mineral resources have allowed remainingin industrial operation during over 450 years. This has been possible thanks to an exceptional geo-logy that originated an extraordinary deposit whose long mining history has left a marked stampin the region and its people.

Resumen

La reciente declaración de la Mina de Almadén como Patrimonio de la Humanidad ha sido un reco-nocimiento a un yacimiento mineral excepcional. En el entorno de la localidad de Almadén se handado unos acontecimientos geológicos peculiares que han dado lugar ciertos depósitos de unmetal escaso que seguramente son irrepetibles a nivel cortical. El yacimiento de Hg de Almadénse halla en un sinclinal hercínico orientado WNW-ESE en donde aflora una secuencia de rocaspaleozoicas muy completa que va desde el Ordovícico Inferior hasta el Devónico Superior. Estasrocas se formaron en medios marinos de plataforma, que en ciertos momentos tuvo una impor-tante actividad volcánica de composición básica y ultrabásica principalmente, con la formación decuerpos explosivos diatrémicos. La tectónica hercínica ha sido la que ha configurado el sinclinalmediante una fase de plegamiento generalizada y otra de cizallamiento de distribución heterogé-nea en bandas que son especialmente importantes en el flanco meridional. Los yacimientos deHg se distribuyen en la mitad oriental del sinclinal y especialmente a lo largo del flanco meridio-nal, en buena parte relacionados con las bandas de deformación por cizallamiento. Se recono-cen dos tipologías distintas de depósitos, que son estratiformes y cuerpos irregulares a modo destockworks. Los primeros son claramente precinemáticos respecto a las deformaciones hercíni-cas, mientras que los segundos muestran un cierto carácter sincinemático en relación con lasdeformaciones por cizallamiento. El mayor de todos ellos ha sido la mina de Almadén, de tipo

Geotrex Gestión Minera, S.L. Calle Ancha 10, 13500 Puertollano (Ciudad Real), Spain

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estratiforme, cuyos enormes recursos de mineral de Hg han permitido que haya estado en explo-tación industrial durante unos 450 años. Esto ha sido posible gracias a una geología excepcio-nal, que ha dado lugar a un yacimiento extraordinario, cuya larga historia minera ha dejado unamarcada impronta en la región y en sus habitantes.

Key-words: : mercury, quicksilver, Almadén, giant ore deposits, geology heritage, mining heritage,world-wide patrimony.

The geological singularities of a world-wide patrimony: The giant mercury deposits of Almaden (Spain)Fernando José Palero Fernández

1. Introduction

The declaration of Almaden mining heritage asPatrimony of the Humanity took place the last30th of June. This has clearly been a very remar-kable fact in the history of this spanish town andit supposes an event of great relevance inas-much as that is the first time that a mining cen-tre received this type of consideration.

The recognition has been granted to a joint can-didacy of the two main production centres ofliquid metal in the world, the mines of Almadenin Spain and Idrija in Slovenia, and has beennamed as “the Patrimony of Mercury”. The can-didacy has tried to reflect the world-wide impor-tance that this metal has had in modern eco-nomy and to the technological innovations deri-ved from the mining techniques used for itsextraction. It is also outstanding the stamp thatmercury has historically left in the architecture,customs, idiosyncrasy and traditions, many ofwhich are unique and exclusive of the peoplewho have sustained their lives on their extrac-tion and its commercialization.

The marked stamp that the mining has left inthese places is due to a very long history settledin that extractive activity, that in the case ofAlmaden has extended during 2000 years withoutalmost no uninterrupted. In its beginnings and fora long time, the operation was made on smallscale and was focused to obtain the “red vermi-lion” that ennobled paintings. The discovery in theXVIth Century of the amalgamation as an extrac-tion method of noble metals, caused the extrac-tion be centred in mercury metal and the produc-tion rate has been on great scale ever since.These rates have been high or very high during the450 years in which the mine has remained activeon an industrial scale. Almaden has always beenat the world-wide top in production of this strangerand scarce metal.

If Almaden has been able to maintain so highlevels of production during such a long period of

time, it has been due to the nature of this excep-tional deposit. It is a clear example of what inmetallogeny is described as a Giant Ore Deposit.Moreover, it is not disrespectful to say thatAlmaden is a unique case in the terrestrial crust.The mining district of Almaden has, by far, thegreater accumulation of mercury resources inthe world, having produced over 270,000 metrictons of the liquid metal. In other words, morethan 7.7 million mercury flasks have beenextracted from the district, of which 7 millionhave come from the Almaden Mine itself.

The exceptional nature of this mining district ona world-wide scale, and particularly of theAlmaden giant deposit, lies on the geologicalconcentration process that has been able toaccumulate in such a specific place this hugeamount of mercury, an element with a ratherlow average content at cortical level (about 67ppb). So much so that Almaden means morethan 35% of the recognized Hg resources in theworld, with higher grades than any other greatdeposit in the world (Idrija, Monte Amiatta,McDermitt, Huancavelica, etc.). Besides thespanish deposits appear in a geological contextand show metallogenic characters very diffe-rent to the rest of mercury deposits, whichleads to speak of the Almaden type deposits.

This paper tries to show the geological peculiari-ties of this mining district and particularly of theAlmaden Mine. The Patrimony of the Humanityrecognition has hardly considered the geologicalfacts, but it has been due to an exceptionalchain of geological events that the ore depositwas created. Because of singular geological pro-cesses have had a great deposit with very longtime in production that gave rise to a rich andspecific historic mining industry, which has beennow awarded with this declaration.

2. Location and geological setting in theIberian context

The Almaden town is located about 300 km


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SSW from Madrid, and about 260 km to the NEfrom Seville. It belongs to Ciudad Real Provincein Castilla-La Mancha Region. From a morpho-logical point of view Almaden is in SierraMorena range spurs in the southern limit of theSouthern Castilian plateau.

From a geologic point of view, Almaden is loca-ted in the South flank of an important synclinestructure that it is known with the name of themining town. The outcropping rocks from thissyncline are formed by a detritical sedimentarysequence that includes volcanic intercalations.The whole sequence is Palaeozoic age, fromLower Ordovician to Upper Devonian.

In the regional geological context, the AlmadenSyncline is located near to the southern edgeof Central-Iberian Zone (fig. 1), according to thesubdivision of the Hercynian Iberian Massifmade by Julivert et. al. (1972).

The regional geology around Almaden is charac-terized by a succession of large Hercynian foldswith sub-vertical WNW-ESE bearing axial planes(fig. 2). The general macrostructure of thesefolds is defined by a litho-stratigraphic unit for-med by orthoquartzites that has an importantthickness and good extensive regional conti-nuity. This unit is named “ArmonicanaQuartzite” and it has Arenig age.

The rocks that are outcropping in the region aremainly siliciclastic, were formed in diverse mari-ne environments and they have hardly undergo-ne metamorphic processes that significantlyaltered their original textures. The Oldest rocksthat appear in the anticlines include the“Schist-Greywacky Complex” (CEG) that out-crop in large areas in the centre and the West

part of the Iberian Peninsula (Lozte, 1970;Bouyx, 1970). These rocks constitute a mono-tonous multilayered sequence of shales andgreywackes formed in a turbidity environment.A sequence make up by siltstones, black-sha-les, conglomerates and carbonate lens that isin unconformity over the turbidity multilayeralso appears in some sectors (Bouyx 1970;Ortega and Lodeiro; 1986; San José et. al.,1990; Palero, 1993). The age of these rocks isVendian (San José et. al., 1990), althoughsome authors indicate that the age would alsoreach Lower Cambrian (Lorenzo and Solé,1988; Vidal et. al., 1994).

A thick Palaeozoic sedimentary sequenceunconformably overlies the Precambrian mate-rials. These rocks would have been depositedin a marine shelf basin throughout 140 millionyears. Finally, at South of Almaden there is awide and monotonous sequence formed byblack shales, siltstones, greywackes and con-glomerates of Lower Carboniferous age. Thisseries receives the name of “Los PedrochesCulm” and was deposited in an unstable mari-ne platform basin (Pérez Lorente, 1979; Miraet al., 1987).

At the end of the Namurian or beginnings of the

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Fig. 1. The Almadén Syncline in the Hercynian Iberian Domain

Fig. 2. Geological map around Almadén Syncline. (From Mapa Xeolóxicodo Macizo Hespérico of Parga et al., 1982). Pink: Precambrian rocks;green and brown: Lower and Middle Paleozoic; grey: LowerCarboniferous; yellow: Tertiary and Quaternary; red: granitoids


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Westphalian, the Hercynian Orogeny started inthe region. It has been the main tectonic eventthat is still recognizable nowadays. TheHercynian Orogeny started quite late in thisregion compared to other zones of the IberianMassif. The southern part of the Central-IberianZone corresponds to an external zone of theHercynian orogeny, and in consequence suffe-red the deformation effects of the collision bet-ween ancient continents later than inner zones.The Hercynian deformation around Almadenoccurred in two stages, of which the first wasthe main one (Roiz, 1979; Ortega, 1988). Inthis first stage (F1) the more important macros-tructures and folds were formed, as well as anaxial plane slaty-cleavage (S1). This stage tookplace by a nearly N-S shortening and theireffects are found widespread. The second tec-tonic stage (F2) probably took place duringStephanian times, and it consisted in a hetero-geneous deformation produced by an E-W shor-tening. It gave rise to crossed-axes tectonicsthat originated interference fold figures such asdomes and basins. However, fragile shearbands were the main structures formed, ofwhich those with NW-SE direction and anticlock-wise displacement, predominate over conjuga-ted clockwise NE-SW ones. Occasionally, anincipient slaty-cleavage was generated (S2)that is transected plain to F1 folds.

After the Hercynian deformations some laterstructures are recognized, mainly new faultsand reactivations of pre-existent ancient faults.This activity must have extended until recenttimes as evidenced by the Pliocene-Quaternarybasaltic volcanic activity at “Campo deCalatrava” (Gallardo et al., 1998), with out-crops located a few kilometres to the East andNE of Almaden.

The magmatic activity in this part of the Central-Iberian Zone is represented by Palaeozoic sin-sedimentary volcanism and Hercynian acidmagmatism. The first one appears distributedin several position in the Ordovician toDevonian sequence, but without a doubt, it isamong the Silurian and Devonian sedimentswhere there are more abundant, especially inthe Almaden Syncline.

The Hercynian magmatism has its exposures inthe Fontanosas and Garlitos granite stocks,

and in the great Los Pedroches Batholith. Thefirst ones are respectively to the East and theNW of Almaden, whereas the great batholithoccupies a large band at the South maintainingthe WNW-ESE Hercynian orientation. In thenorth border of this batholith there are twosatellite stocks known as Santa Eufemia and ElGuijo. These intrusive bodies are allochthonousand they give rise to a hectometric aureole ofthermal metamorphism. This magmatism hastwo granitic facies: granodiorites and monzogra-nites. The granodiorites are the oldest onesand they comprise the totality of theFontanosas stock, most of Garlitos stock andpart of the Los Pedroches Batholith. Its empla-cement took place between the two Hercyniandeformation stages (Coupez et al., 1988,Escuder & Lorenzo, 2002). The monzogranitesintrude into the granodiorites (Donaire andPascual, 1992). They appear in the LosPedroches Batholith and forming the totality ofthe satellite stocks. Its emplacement is clearlysubsequent to F2 Hercynian stage. Severalabsolute datings of these granitoids have beenpublished, as the 302±10 million years old forthe Fontanosas granodiorites (Leutwein et. al.,1970); the 291±15 Ma of the Los Pedrochesmonzogranites (Penha and Arribas, 1974); the300±3 Ma (Fernández et al., 1990) and the294±22 Ma (García de Madinabeitia, 2002) toEl Guijo Stock; and the 293±6 Ma to SantaEufemia Stock (García de Madinabeitia, 2002).

3. The Geology of the Almadén Syncline

The Almaden Synclinal can be defined as amacrostructure that occupies a 30km longstretch with a maximum width of 15km. It hasthe shape of an asymmetric WNW-ESE basin,with a wider Eastern part than Western one.The fold's south flank is verticalized, whereasthe north one is gently dipping. This is a pecu-liar fact of this great fold, because it is opposedto the regional vergency of the Hercynian struc-tures, which is towards the South.

The rocks that form the syncline show one ofthe most complete Ordovician to Devoniansequences that can be found in Central-IberianZone (Almela et al., 1962; Saupe, 1973;García Sansegundo et al., 1987). The sequen-ce shows different types of sedimentary detriti-cal contributions mainly driven by variations of



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the sea level. Several litho-stratigraphic unitscan be outlined based on the predominance ofone of following three lithologies: orthoquartzi-tes, siltstones and black shales. Four units for-med by orthoquartzites with a good regionalcontinuity can be defined and they serve as ageological mapping reference to draw the gene-ral structure of the syncline. These units are,from base to top (Almela et al., 1962; Pardo,1983; Vilas et al., 1999)

• “Armonicana Quartzite” of Lower Ordovician(Arenig) age.

• “Canteras Quartzite” of Upper Ordovician(Caradocian) age.

• “Criadero Quartzite”, of Ordovician-Siluriantransition (Hirnantian-Llandovery) age.

• “Basal Quartzite”, of Lower Devonian(Gedinian-Siegenian) age.

Multilayers of siltstone and shale beds alongwith abundant volcanic and sub-volcanic rocksare placed among these orthoquartzite units,especially in the sequence over the “CriaderoQuartzite”. These igneous materials are mainlyof basic composition (alkaline basalts), butultrabasic and intermediate compositions arealso present.

The igneous rocks appear as sub-intrusivebodies; as dikes and sills; as diatremic bodies(mainly explosive breccia-tuffs); or as volcano-sedimentary layers. In general is possible tosay that in the Silurian levels there is a predo-minance of sub-intrusive volcanic materials,whereas in the Devonian levels the volcano-sedimentary materials are more abundant. Atpresent, the explosive breccia bodies appearlimited by faults and tectonized bands, thus itis not possible to determine the original timingwith the host rocks. However, it is worth men-tioning the fact that the known explosive brec-cia bodies appear among rocks of ages rangingfrom Upper Ordovician to Devonian. Theserocks are very characteristic of the Almadensyncline and they are named “Frailesca Rock”.

The “Frailesca Rock” (fig. 3) is mostly formedby volcanic material, either as fragments or asmatrix, although also quartzite and black shale

fragments can be found. The volcanic materialis mainly basaltic with minor ultramaphic rockfragments. The rock displays a distinctivestrong carbonatization and sericitization, asseen by optical microscope on thin sectionswere widespread ankerite carbonate and serici-te are the only phases that can be observed.Traces of its original mineralogy are conservedonly in certain volcanic fragments. The com-mon aspect of the “Frailesca Rock” is a massi-ve breccia-tuff, but sometimes a certain bed-ding arrangement can be observed, whichdoesn't mean a true stratification. The interpre-tation of this peculiar rock has been changingsince it was described for the first time, andnowadays it is understood as diatremic bodiesafter the studies made during the fall of the XXCentury by the Geology Service of the miningcompany. They constitute, without a doubt, avery peculiar case in the European HercynianDominion.

The volcanic activity seems to take place in atleast 2 pulses. The first one would be an explo-sive volcanic episode of basaltic compositionthat originated the “Frailesca Rock”. A networkof dikes and sills with ultramaphic xenoliths isrelated to the volcanic breccia-tuffs. Fragmentsof the same ultramaphic material, characterizedby the presence of fuchsite, also appear incorpo-rated on the “Frailesca Rock”. The dikes andsills often show peperitic structures, especiallywhen hosted in black shales, evidencing thattheir emplacement took place when the sedi-ments still had important amounts of water.


Fig. 3. The “Frailesca Rock” on a tunnel wall of 5th Level in AlmadénMine. The outcropping shows false bedding


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The second volcanic pulse is represented bybasic rocks, mainly quartz-diabases. Theserocks mostly appear as sub-intrusive bodieswith an extended form that follows the generalstructure of the syncline. They display clearrelations of intrusion with the “Frailesca Rock”,which indicates a later formation than the vol-canic breccia-tuffs.

The volcano-sedimentary materials are difficultto relate to a specific volcanic episode. Thecommon heterolithical composition of tuffs andbreccias along with clear evidences of reworkedmaterial hinder the establishment of its origin.

Intermediate volcanic rocks (mainly trachytes)appear sporadically in the core of the AlmadenSyncline. These materials reflect a clear evolu-tion of the volcanism towards more acid com-position and they could either represent a thirdindependent volcanic pulse or an acid evolutionof the second one.

The geological structure of the Almaden syncli-ne is interpreted as the superposition of the dif-ferent Hercynian and post-Hercynian deforma-tions. The first Hercynian stage (F1) gave rise to

folds of cylindrical geometry, sub-horizontalWNW-ESE axes and sub-vertical axial planes.Folds were formed on all scales and they weregenerated by perfect buckling. This type ofdeformation does not bring an important volu-me reduction and only very incipient develop-ment of slaty-cleavage on incompetent rocks(black shales) occurs. The metamorphism ispractically absent and only certain basic volca-nic rocks, with very susceptible components,gave rise to neo-formed minerals in the zeolitefacies (Saupe, 1973; Higueras et al., 1995).

The second Hercynian stage (F2) is mainlyrepresented by fragile shear-bands. They areaffecting with different intensity various partsof the syncline. Conical geometry folds can befound too, which are the result of the interfe-rence between the E-W shortening with F1folds. The shear bands can be recognized bystrong dipping faults that are longitudinal to thegeneral structure of the syncline, with WNW-ESE to NW-SE direction and anticlockwise dis-placement. Often these structures are not evi-dent on medium scale mapping, even thoughmore detailed studies showed a very complexstructure that is produced by shearing on zones


Fig. 4. Geological sketch of Eastern part of Almadén Syncline, delighted traces of orthoquartzite Palaeozoic units and putting the mainmercury deposits.


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with apparent slight deformation. This fact isespecially important when faults interfere inlongitudinal direction with the verticalized multi-layers. This happens, for example, throughoutthe Southern flank of the synclinal, where thegeological maps do not succeed reflecting thisreally complicated structure.

Finally, it is also important to point up the exis-tence of faults cross cutting litho-stratigraphicunits. They are easily recognized in the compe-tent units (like quartzite levels). These faultsmake up two conjugated systems, one of themNW-SE with clockwise displacement, and theother one NE-SW with anticlockwise movement.These faults are the result of a new N-S bearingshortening that has been considered as late-Hercynian tectonic movements. Most of thesefaults are likely to be inherited, produced duringHercynian times deformation and reactivatedunder later stress fields.

4. The mercury ore deposits

The name of Almaden is worldwide known forits extraordinary mercury deposits. Almadenis a crustal geochemical anomaly unrivalledby any other place. The huge concentration ofthis scarce metal encountered in Almadenarea is equivalent to all mercury contained in4,03*1012 metric tons of crustal material.This enormous amount of metal has comemostly from only 6 mines: Almaden, ElEntredicho, Las Cuevas, Nueva Concepción,Vieja Concepción and Guadalperal. In addi-tion to these mines there are a dozen ofsmall occurrences and mining registries, aswell as a new deposit, found during the 80s,named Nuevo Entredicho that remains unex-ploited. Altogether these mines and occurren-ces form a mining district whose geologiccontext is limited to the Almaden Syncline(fig. 4).

The mercury deposits respond to two diffe-rent typologies that can be grouped under theterms of stratiform deposits (fig. 5) andstockwork deposits (fig. 6). The stratiformsare the biggest ones and they are theAlmaden Mine, El Entredicho and ViejaConcepción deposits. The stockworks aresmaller but they have higher ore grades,being outstanding examples the mines of

Nueva Concepción, Las Cuevas and the unex-ploited deposit of Nuevo Entredicho.

The stratiform deposits are hosted exclusi-vely in the “Criadero Quartzite” unit. The oreappears impregnating and/or fisural infillingsome specific layers, located at the footwalland the hanging-wall of the orthoquartzitesunit. The impregnation occurred by filling withcinnabar of the primary rock porosity, essen-tially the intergranular spaces. The fisural infi-llings (joins) surely represented the sealing ofthe porosity originated by cracking during late-diagenetic compaction processes of thequartzite rock. The paragenesis in thesedeposits is very simple, with cinnabar as themain mineral, followed by small amounts ofpyrite and native mercury. Practically at tracelevels there are crystalline quartz, dolomite-ankerite, baryte, siderite, and small crystalsof cinnabar and pyrite. These trace mineralsare occupying fractures that cut through themineralized quartzite beds and they formedlater than the main mineralization.


Fig. 5. Stratiform type ore of San Francisco Seam.14th Level, AlmadénMine.


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The stockwork type deposits are hosted in dif-ferent lithologies but they show affinity to volca-nic materials, even though some minor amountof mineralization can be found in detriticalrocks. In these cases ores are always relatedto nearby volcanic rocks. The paragenesis ismore complex than in the stratiform type, withcinnabar and pyrite as the major minerals,accompanied by native mercury, pyrophylite,kaolinite, sericite, ankerite and quartz. A latermineralization with baryte, siderite, dolomite-ankerite, chalcopyrite, pyrite and recrystallizedcinnabar is also present. This later paragenesishas scanty volumetrically importance. The mainmineralization appears infilling small veins andreplacing volcanic rocks, with both kinds beingintimately related. Clear examples can beobserved where the replacement degree decre-ases with the distance from the massive cinna-bar vein. In detritical materials the mineraliza-tion only occurs as infilling veins. The vein andreplacement textures allow to establish asequence of crystallization to the main minera-

lization process, so that pyrite is the first one tocrystallize, followed by pyrophylite and kaolini-te, quartz afterwards, and finally the cinnabar.

The timing between both types of deposits canbe established on the basis of the relationsbetween mineralizations and Hercynian defor-mations. Thus, the stratiform bodies are clearlyaffected by the Hercynian deformations as theyare folded, sheared and cut by the structuresformed during those tectonic events. On thecontrary, the cinnabar veins in stockwork mine-ralizations, are cutting the Hercynian foliations(slaty and milonitic). In these deposits, ore tex-tures and relations between the mineralizationand the host rocks, suggest that their emplace-ment has taken place in slightly sin-tectonic orlate-tectonic conditions with respect to the E-WHercynian F2 shortening. These relations evi-dence that stratiform ores are older than stock-work ores, which could be considered to be epi-genetical from the stratiform ones.

For a long time the spatial coincidence of Hgores with the “Frailesca Rock” has been obser-ved, settling down a cause-effect relation betwe-en sin-sedimentary volcanism and mineraliza-tion process (Almela at al., 1962; Saupe, 1973and 1990; Hernández, 1984; Ortega andHernández, 1992). The exploration criteria inAlmaden mining district has long been basedupon this idea. The places where the volcanicbreccia-tuffs are in contact with “CriaderoQuartzite” were the most solid candidates forthe exploration targets. This spatial relation isclear, but it has been verified that stratiformdeposits are previous to the emplacement ofthe “Frailesca Rock”. Indeed, in the ElEntredicho Mine, a sill that includes ultramaphicxenoliths related to the “Frailesca Rock”, couldbe observed in contact with the mineralizedquartzite level. The sill produced a narrow “ther-mal metamorphism” band on the mineralizationseam by effect of the igneous rock heat. Thisheating triggered the removal of cinnabar andproduced the recrystallization of the quartzitetoo. This proves that “Frailesca Rock” volca-nism is younger than the stratiform mineraliza-tions, but they have probably been produced bythe same geological process and that they arenot a consequence from one another. The stock-work mineralizations are post-volcanism, sincethey host in the volcanic breccia-tuffs and they


Fig. 6. Stockwork type ore in a replacement body hosted in volcanicbreccias. Western Massif, 2nd Level, Las Cuevas Mine.


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produce alterations that affect them as alreadyconsolidated rocks (Higueras, 1994).

The origin and metallogenic process responsibleof having generated such great concentration ofa scarce element still remains a geologicalmystery. Two main hypotheses have been sug-gested. The first one considers a significant geo-chemical Hg increase in the Palaeozoic marinesedimentary environment, with the metal beingfixed by the abundant black shales. A geologicalevent (like volcanism) would have been able tomobilize Hg from this pre-concentration, andstore it in levels with adequate porosity and che-mical conditions (Saupe, 1973 and 1990). Thesecond hypothesis supposes a great geochemi-cal mantellic anomaly with an ascent, canalizedby great crustal accidents, of Hg and relatedultramaphic volcanism (Ortega, 1988; Ortegaand Hernández, 1992; Higueras et al., 2000).

5. The Almadén Mine

The Almaden Mine exploited the Western mostHg deposit of a fringe of occurrences locatedthroughout the southern flank of the syncline

(to see fig. 4). It has been the greatest strati-form deposit in the mining district and, thus thebiggest mercury deposit ever found. It is remar-kable as much for the great volume of metalaccumulation as for the high grades of its ores.In order to give an idea of the deposit magnitu-de, in the last years of production the cut-offwas 1% of Hg and the average grade ore was3.5%. Most Hg deposits in the world exploit oregrades lower than 1% while ores above 3.5%are truly exceptional.

When the mine closed, its tunnels not only offe-red a catalogue mining methods used duringthe last 300 years, but also showed a specta-cular geology, with unique elements and excep-tional examples for diverse subjects like strati-graphy, tectonic, volcanism and metallogeny.The part that has been adapted for tourism userepresents one reduced expression of the trulypotential that the mine could offer.

5.1. Mine description

The basic mine infrastructure at its closure, con-sisted of two main shafts: The San Teodoro (522


Fig. 4. Geological sketch of Eastern part of Almadén Syncline, delighted traces of orthoquartzite Palaeozoic units and putting the mainmercury deposits.


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m deep) and the San Joaquin (700 m deep),along with three auxiliary and vent shafts (SanMiguel, San Aquilino and Robbins). These shaftsonce run the services to a 27 level system, witha separation of about 25 metres between conse-cutive levels. The 1st to 15th levels were thecompletely developed ones along with the 17th,19th, 21st and 23rd levels from the deep part ofthe mine (fig. 7). For a long time the main shaftwas San Teodoro, which got to reach the 19th

Level with auxiliary San Miguel and San Aquilinoshafts. At the beginning of the 60s San Joaquinshaft was sunk at the western part of the depo-sit, and turned out to be the main one. It reachedall the way down to 27th Level, although theexploitation never developed under the 23th

Level. At the beginning of the 80s the Robbinsshaft was excavated by rise boring method tofacilitate the ventilation of the western zone ope-rations.

The first mining works in Almaden would havebeen in outcrops located in the Western part ofthe deposit, where later would become the “Minadel Pozo” and what at deep levels is known as the“Rama Sur”. This zone displays a complex geo-logy and the mineralized bodies appear in irregu-lar form with short lateral continuity.

The Eastern part of the deposit was discoveredin 1697. Since then the main mining works focu-sed into this area, which has been called the“Rama Mina”. This area has easier geology anda good continuity of the mineralized bodies. Fromthe geologic point of view there is another zonenamed “Los Masivos” which was integrated tothe “Rama Mina” in mining planning purposes

5.2. Ore deposit lithology

The ore bodies in the deposit had a sharp stra-tiform character, with cinnabar located in somevery specific levels of the “Criadero Quartzite”.These ore bodies received the names of SanPedro and San Diego Seam (SP), San FranciscoSeam (SF) and San Nicolas Seam (SN).

The “Criadero Quartzite” in the Almaden minewas subdivided in 3 sections. From footwallto hanging-wall are (fig. 8):

• “Lower Quartzite”, with 8 to 15 m thickwhite orthoquartzite beds. This section

includes the San Pedro and San DiegoSeam, with 3 to 8 m wide.

• “Middle black shales”, with 10 to 15 m ofrich carbonaceous matter mudstones andfrequent sandy pillow structures that werealways barren.

• “Upper Quartzites”, composed by 30 to 50m of multilayered thin grey quartzite andfine micaceous siltstones beds. The bedsbecome increasingly thicker and darkertowards the top. “San Francisco” and “SanNicolas” seams are found in this upperpart. These seams are from 2,5 to 5mthick and separated by up to 8 m of thinsiltstone and carbonaceous mudstonesbeds. Tectonic lamination is responsible forvariation, or even absence, of this barreninterbedded.

A black shale unit with marked sedimentarylamination and fine layers of siltstones andmudstones is found underneath the “Criadero


Fig. 8. Stratigraphic column sketch of the “Criadero Quartzite” inAlmadén deposit.


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Quartzite”. This unit is known as “FootwallBlack Shales”. Another black shale unit called“Hanging-wall Black Shales”, formed by foliatedgraphitic black shales is found overlying thequartzites. Fossils are very common in thisshales, mainly graptolites. In fact, the AlmadenMine is a renowned classic world-wide localityfor this kind of fossils.

The graphitic shales are partially or totallyreplaced by intrusive sub-volcanic rock bodies(quartz-diabases) that were called as “Hangingwall Lavas”. Besides these rocks, cutting orconformity among sedimentary rocks, there aredikes and sills of volcanic materials, some ofthem compositionally similar to the “HangingWall Lavas” while there are others with aphani-tic texture that hinders the allocation to one oranother volcanic pulse (fig. 9). Spectacularcases of “thermal metamorphism” of thesedikes could be observed on the mineralizedquartzites.

A heterolithic breccias formed by fragmentsof siltstones, orthoquartzites and volcanicrocks showing peperitic structures (fig. 10)

are found limited by faults in a doubtful stra-tigraphic position. The peperitic structuresare spectacular and they represent anotherpeculiarity of the rich and complex geology ofthe Almaden Mine.

The volcanic “Frailesca Rock” is a massivelenticular body that hosts in unconformitycontact with the rest of the lithological unitsand is limited by very important WNW-ESEand NW-SE faults. The breccia-tuff material iscomposed by fragments of variable size andcomposition, including some quartzites andblack shales.

5.3. The mineralization

The mineralization of Almaden Mine is charac-teristic of the stratiform deposits and is consti-tuted by cinnabar impregnating and fissure infi-lling the orthoquartzite beds. Although in gene-ral terms the aspect of the ore is similar in allthree seams, there are certain textural diffe-rences between San Pedro Seam and the other


Fig. 9. Aphanitic dykes and sill hosted in “Footwall Black shales” and“Lower Quartzite”. This sill in the footwall of quartzite has good continuityand it is a guide in the Almaden Mine geology. 19th Level, Almaden Mine.

Fig. 10. Heterolithic breccia including volcanic blocks with peperiticstructures. 14th Level, Almadén Mine.


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two seams on top of the “Criadero Quartzite”.Thus, while in the first of them the impregnatingores are prevailing (fig. 11) in the SanFrancisco and San Nicolas seams the infillingjoins ores are the most common (fig. 12). Thisis not a general rule, as examples of the diffe-rent mineralization types are found in allseams. The mineral paragenesis of the depositis the common of the stratiform type with cin-nabar, pyrite and native mercury along with tra-ces of quartz, dolomite, ankerite, baryte andsiderite.

Stockwork ores were occasionally found hostedin the “Frailesca Rock” in specific places of theAlmaden Mine. This mineralization has an irre-gular shape and is bounded by WNW-ESE andNW-SE faults. It is uncommon in relation tostratiform ores, but it is the only one that canbe seen “in situ” at present in the zone fit outfor tourism.

The economic mineralization of AlmadenMine was worked in a maximum length of

about 500m and to a depth of 600m. Oresin the San Pedro and San Francisco seamsgot down beneath the 23rd Level (more than600 m deep), whereas the San Nicolas seamonly reaches the 17th Level (about 425 m).The richest ores of the San Pedro Seam werearranged following a theorical axis with strongplunge to the East that would be going fromthe San Teodoro Shaft at superficial levels, tothe San Miguel Shaft in deep levels. In thecase of San Francisco and San Nicolasseams, the richest ores were following ano-ther theoretical vertical axis located to thewest of San Aquilino Shaft.

Today it is impossible to surely know the ton-nages and grades obtained during the exploi-tation of the deposit, but it is not speculativeto venture a production of about 7 milliontons with an average grade of 3.5% Hg(approximately 1 flask per metric ton). Therehas been a variability of grades, ranging from1% to 40% of Hg. The greater one of the threemineralized seams was the San Pedro andSan Diego, followed by the San Francisco andfinally by the San Nicolas Seam. An estima-tion of the amount of metal produced by eachone of them would be of 50% extracted fromSan Pedro-San Diego Seam, 30% from SanFrancisco and 20% from San Nicolas.

5.4. Ore-deposit structure

The Almaden deposit is hosted in a verticalizedquartzite multilayer that has approximately E-Wdirection and is affected by fragile WNW-ESEshear bands. The shear bands produce displa-cements following the main anisotropy of the


Fig. 11. Cinnabar impregnating quartzite beds in San Pedro Seam, 19th

Level, Almaden Mine.

Fig. 12. . Cinnabar infilling joins in dark quartzite beds of San NicolasSeam. ”Tercer Macizo”, 7th Level, Almaden Mine.


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multilayer which is the stratification planes. Thepresence of a competent body of “FrailescaRock” caused the refraction of the shear bandstowards the NW-SE direction and in consequen-ce a transpresive zone was created. The multi-layer sequence behaves like a rocky massifwith marked anisotropy, while the diatremic vol-canic body tectonically acts as a compact, iso-tropic and rigid rocky body.

There are two important fracturing episodesthat affected the deposit: longitudinal shearbands produced by the E-W F2 Hercynian shor-tening; and transversal fractures originated bytardi-Hercynian movements produced by N-Sshortening.

Longitudinal shear bands structures are inpseudo-conformity with stratification planes.They cause a strong deformation in the depositarea and represent an important network ofsub-vertical anticlockwise strike-slip faults withfragile and fragile-ductile character. They giverise to significant displacements (up to severalhundred meters) and have frequent refractions

and ramifications in the quartzite multilayer,with different adaptation and assimilation dis-placement structures. The mining tunnels offe-red a complete catalogue of tectonic structuresproduced by this type of deformation, withspectacular examples of drag-folds with sub-vertical plunge axes, tectonic wedges (fig. 13),milonitic bands, etc., etc. Together with themain anticlockwise WNW-ESE faults, therewere other conjugate clockwise NNE-SSWsystems, (fig. 14), and also anticlockwise ENE-WSW.

Transversal strike-slip faults with NW-SE bea-ring and usually high dips are cutting at wideangle the "Criadero Quartzite". They produ-ced clockwise movements and they have amore fragile character than the longitudinalones. These faults lead to translations withirregular importance, whose magnitudesrange from a few decimeters to several tensof metres.

The intersection of longitudinal and transver-sal faults with the "Criadero Quartzite" onsub-vertical position caused the configurationand structuration of the deposit. The generalstructure can be defined as a dismemberedlarge sub-ver tical axe fold of “CriaderoQuartzite”, produced by anticlockwise trans-lation of a wide longitudinal WNW-ESE shearcorridor, which encounters the rigid diatremicbody of "Frailesca Rock". This corridor isoverlapped by a clockwise transversal NW-SEfracturing that separates the ore deposit intwo parts by the displacement of the Easternblock Southwards in relation to the Westernblock (fig. 15). These parts would basicallycorrespond with the "Rama Mina” to the Eastand the "Rama Sur" to the West.


Fig. 13. Tectonic wedge producing duplication of a quartzite bed. 14th

Level, Almaden Mine

Fig. 14. Domino structure produced by a dextral NNE-SSW shear bandcutting to “Lower Quartzite”. 14th to 9th Sublevel ramp, Almaden Mine.


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Three sectors can be outlined based on dis-position of the faults, the "CriaderoQuartzite", the volcanic rocks and the ore-bodies. They are called from East to West as:"Rama Mina", "Los Masivos" and "RamaSur". These sectors were separated by twoimportant transversal NW-SE faults, named"C Fault" and "A Fault" (fig. 16). The maindistinctive features of these sectors wouldbe:

The “Rama Mina” Sector was the classicalexploitation sector in the Almaden Mine. The"Criadero Quartzite" appeared a little disa-rranged by few transversal NW-SE faults thatproduced small strike-slip jumps. Only one ofthese faults is worth mentioning the so-calledSan Miguel Fault. Mineralizations appeared inthe classic three seams, but the best gradesand the larger volume of ore came from the"San Pedro and San Diego Seam". The

“Frailesca Rock" appeared South of the ore-deposit, and separated from the "CriaderoQuartzite" by a broad band of "Footwall BlackShales", that became wider towards the Eastand in depth (fig. 17). The contact betweenvolcanic breccia-tuffs and black shales is animportant fault named Southern ("M Fault"),that has a strong dip towards SSW.

The "Los Masivos" Sector was a structurallyvery complex sector because it was affectedby longitudinal faults that formed two anti-clockwise systems, the main WNW-ESE, andthe other one ENE-WSW (which was calledSan Aquilino Fault). These faults producedimportant translocations in the "CriaderoQuartzite", including duplication of minerali-zed seams, which led to the existence of pla-ces with high grade ores up to 15 meter wide.Besides the longitudinal deformation animportant network of transversal faults wascutting the “Criadero Quartzite”. These faultswere delimitated by two great faults bearingNW-SE, the "A Fault" at the East and the "CFault" at the West. Both faults have oppositedips and they converge at about the 8thLevel, being the "C Fault" the only one rea-ching the surface. This fact entailed theabsence of "Los Masivos" sector at the topof the deposit. The classic three seams appe-ared mineralized, but the “San Pedro andSan Diego Seam” was getting progressivelybarren towards the west and depth. So, bene-ath the 15th Level this Seam was sterile inthis sector. On the other hand, “SanFrancisco” and “San Nicolas” seams had


Fig. 15. Structural sketch at superficial levels of Almadén deposit.

Fig. 16. Geological map of the 13th Level of Almaden Mine.


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extraordinary grades, hence the name of thesector, especially at levels between 10th and14th. Below 14th Level grades started to fallas much horizontally as vertically, even soSan Francisco got down to the 21st Level. The“Frailesca Rock" in this sector is locatedNorth of the quartzites (fig. 17). In deeperlevels volcanic breccia-tuffs practically disap-pear, as they become replaced by the diaba-ses of “Hanging-wall Lavas”. In the Southernpart, there are small disjointed blocks of thevolcanic breccias limited by faults.

The “Rama Sur” Sector comprised the sectorlocated at the West of the "C Fault". It was astructurally complex area affected by longitu-dinal fault tectonics. A distinctive structuralfeature was the existence of several sub-ver-tical axe folds, which settled much of thetranslation of the strike-slip faults. In thatarea there weren't many transverse faults,and they all produced minor displacements.Mineralization was broken off in disconti-nuous irregular bodies due to strong tectoni-zation, but the ores belonged to the SanFrancisco and San Nicolas seams. The mine-ralization finished above the 14th Level. The“Frailesca Rock" was limited to the East by

the "C Fault" and to the South by a great lon-gitudinal accident called “G Fault”. This faultacts as the Northern limit of the "CriaderoQuartzite" and put it in contact with volcanicrocks (figs. 16 & 17). From the 7th floor down-wards, the volcanic breccia-tuffs were intru-ded and replaced by the diabases of the“Hanging-wall Lavas”, with the last outcropsof the “Frailesca Rock” at the 14th Level.There are no more volcanic rocks in this sec-tor, except some small dikes and sills hostedin "Criadero quartzite" and “Footwall BlackShales”. They are compositionally equivalentto the "Hanging-wall Lavas”.

6. Conclusions

The declaration of Almaden, together with Idrija,as Patrimony of the Humanity has implied a consi-deration of its geological singularity. The conjunc-tion of a series of peculiar geological events duringthe Paleozoic and the effects of the HercynianOrogeny in the area, gave rise to a group of mer-cury deposits without counterpart in the world.Among them, the Almaden Mine is highlighted asa true example of giant ore-deposit.

The Hg mineralizations of the Almaden mining


Fig. 17. . Representative geological cross sections of the Almaden deposits between5th and 15th levels..


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district are restricted to a syncline where acomplete Paleozoic sequence outcrops. Thissequence includes volcanic materials of basicand ultrabasic composition, with explosivestructures filled with very characteristic volca-nic breccia-tuffs locally known as "FrailescaRock". These materials contrast with the sedi-mentary deposition environment that mostlywas a shallow and stable marine platform.

The Almaden Syncline is a Hercynian macro-fold with WNW-ESE direction that shows anasymmetric shape with the Southern flank ver-ticalized and a gently dipping Northern one.This fact contrasts the general vergency of theHercynian structures in the region, which issouthward. It is in this anomalous Southernflank where most part of the Hg deposits arelocated. This flank is also intensely affected bylongitudinal anticlockwise shear bands of gene-ral NW-SE direction, formed during an E-WHercynian shortening.

Two types of mercury deposits can be definedin Almaden district, the stratiforms and thestockworks. The stratiform type mineralizationsare clearly pre-tectonic in relation to theHercynian deformations, while the second onesshow a sin-tectonic character respect to thelongitudinal shearing. Thus, these deposits areepigenetic and probably come from the strati-form type.

The Almaden Mine has been the greatest expo-nent of the stratiform type deposits. It hadthree mineralized seams interbedded in thelithological unit named "Criadero Quartzite".This unit has a vertical disposition and appearsinterrupted by the "Frailesca Rock" body. Themineralized seams are arranged in an areastrongly affected by longitudinal faults, whichare cut with other transversal NW-SE strike-slipfaults with clockwise displacement. All thisleads to a partitioning of the deposit in threesectors which are known from East to the Westas "Rama Mina", "Los Masivos" and "RamaSur". The “Rama Mina” has been the moreextended one and also exploited the most con-tinuous ore bodies.

The Almaden Mine, throughout its tunnels, offe-red a compendium of excellent exhibitions ofseveral geological subjects. A minimal expres-

sion of all this is possible to be observed atpresent at the area adapted for tourist use. Itis thanks to this amazing and extraordinarygeology that Almaden has become to be consi-dered and awarded as Patrimony of theHumanity.

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