Geological Context and Plumbotectonic Evolution of the Giant Almadén Mercury Deposit
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Ore Geology Reviews 64 (2015) 71–88 Contents lists available at ScienceDirect Ore Geology Reviews journal homepage: www.elsevier.com/locate/oregeorev Geological context and plumbotectonic evolution of the giant Almadén Mercury Deposit Fernando J. Palero-Fernández a, Agustin Martin-Izard b,⁎, Mar Zarzalejos Prieto c, Luis Mansilla-Plaza a a Escuela de Ingeniería Minera e Industrial de Almadén (Universidad de Castilla-La Mancha), Pza. Manuel Meca 1, 13400 Almadén (Ciudad Real), Spain b Departamento de Geología, (Universidad de Oviedo),C. Jesús Arias de Velasco s/n. 33005 Oviedo, Spain c Departamento de Prehistoria y Arqueología, (Universidad Nacional de Educación a Distancia-UNED), Pº Senda del Rey 7, 28040 Madrid, Spain article info abstract Article history: The Almadén mine has been the largest among several mercury deposits that represent the biggest mercury con- Received 4 March 2014 centration in the world. The deposits form a mining district which is located in a 30 km long and up to 15 km wide Received in revised form 9 June 2014 WNW–ESE oriented syncline, where a thick Lower Ordovician–Upper Devonian siliciclastic sedimentary sequence Accepted 13 June 2014 outcrops. Most of the deposits are located in the south subvertical syncline flank, which has an opposite vergence Available online 20 June 2014 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 dis- Keywords: Lead isotopes tribution, volcanism and deformation. One of these structures seems to have played an important role in the Central Iberian Zone Almadén area, probably having been responsible for the anomalous syncline geometry. This structure acted during Mercury the E–W Variscan shortening as a ductile–fragile sinistral shear zone that resulted in a subvertical attitude of the Cinnabar southern Almadén Syncline flank, affecting the sedimentary sequence longitudinally. The Hg deposits in the region Almadén correspond to two types, stratabound and stockworks. The former are hosted in well-defined “Criadero Quartzite” Giant ore deposits orthoquartzite levels of Ordovician–Silurian age. These deposits were folded and sheared during the Variscan de- formation. 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 shortening The 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 volatil- izing the cinnabar. The deposit shows three sectors, separated by two straight dextral faults, which cut the sinis- tral 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 severalwelldefined clusters that could be interpreted as a lead extraction by means of large scale convective hy- drothermal 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 cin- nabar is likely to have been mostly remobilized–crystallized during the regional extensional tectonic events, cap- turing 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. © 2014 Elsevier B.V. All rights reserved. 1. Introduction ⁎ Corresponding author. In the Iberian Peninsula two main mercury districts are found, the E-mail address: [email protected] (A. Martin-Izard). giant Almadén area (Higueras et al., 2000, 2013; Ortega and http://dx.doi.org/10.1016/j.oregeorev.2014.06.013 0169-1368/© 2014 Elsevier B.V. All rights reserved. 72 F.J. Palero-Fernández et al. / Ore Geology Reviews 64 (2015) 71–88 Hernández Sobrino, 1992; Saupe, 1973, 1990) and the Cantabrian area. (CEG) that occurs in large areas in the center and the western part of Numerous small deposits are found in the latter (Martín-Izard et al., the Iberian Peninsula (Bouyx, 1970; Lotze, 1970). These rocks constitute 2009 and references therein). The former mining district has by far a monotonous multilayered sequence of shales and greywackes formed the greatest accumulation of mercury in the world. Just 6 mines in a turbiditic environment. A sequence made up of siltstones, black (Almadén, El Entredicho, Las Cuevas, Vieja Concepción, Nueva shales, conglomerates and carbonate lenses unconformably overlies Concepción and Guadalperal) accounted for more than 35% of world- the turbidite sequence in some sectors (Bouyx, 1970; Ortega and wide Hg production. Of these 6 mines, the Almadén Mine was an excep- González Lodeiro, 1986; Palero, 1993; San José et al., 1990). The age of tional deposit and is a clear example of what in metallogeny is known as these rocks is Late Precambrian (Vendian in San José et al., 1990), al- a Giant Ore Deposit. Moreover, it is no exaggeration to say that Almadén though some authors indicate that the age could also reach Lower Cam- is a unique case with regard to mercury deposits. The exceptional nature brian (Lorenzo Álvarez and Solé, 1988; Vidal et al., 1994). of this mining district, particularly the Almadén giant deposit, on a A thick Paleozoic sedimentary sequence unconformably overlies the world-wide scale lies in the geological concentration process that has Precambrian rocks and the Almadén Syncline shows one of the most led to the accumulation of this huge amount of mercury, an element complete Ordovician to Devonian sequences that can be found in the with a rather low average content at crustal level (about 67 ppb). More- Central-Iberian Zone (Almela et al., 1962; García-Sansegundo et al., over, the Spanish deposits represent a unique geological context and 1987a,b; Saupé, 1973). These rocks were formed in a marine shelf show very different metallogenic characteristics from the rest of the basin and the sequence includes abundant volcanic intercalations. The world’s mercury deposits, enabling us to speak of the “Almadén type” different types of sediment are mainly the results of variations in the deposits. sea level in a stable basin, during Ordovician, Silurian and Lower Devo- The huge geochemical anomaly represented by the Almadén de- nian times, allowing us to define several litho-stratigraphic units based posits and the lack of other deposits of such size and equivalent geolog- on the predominance of one of the following three lithologies: ical features make it difficult to establish a genetic model to explain their orthoquartzites, siltstones and black shales. Four orthoquartzite units formation. In this respect, two hypotheses, both taking into account the with good regional continuity outline the general structure on the geo- important Paleozoic volcanic activity spatially related to the ores in the logical map. These units are, from the bottom to the top (Almela et al., Almadén area, have been postulated. The first (Saupé, 1973, 1990)as- 1962; Pardo and García Alcalde, 1984; Vilas et al., 1999) sumes a mercury concentration in the host shallow marine sediments that is later remobilized by the volcanic processes and then concentrat- “Armonicana Quartzite” of Lower Ordovician (Arenig) age. ed in a number of epigenetic structures. The second (Hernández Sobrino “Canteras Quartzite” of Upper Ordovician (Caradocian) age. et al., 1999; Higueras et al., 2000, 2013; Ortega and Hernández Sobrino, “Criadero Quartzite” of Ordovician–Silurian transition (Hirnantian– 1992) proposed a deep mantle origin for the mercury, which rises to the Llandovery) age. fi surface together with the volcanic rocks, including ultrama c rocks of “Basal Quartzite” of Lower Devonian (Gedinian–Siegenian) age. mantle affinity. The use of analytical tools to understand the geological processes in- The Middle Devonian is not present in the Almadén Syncline (Pardo volved in the genesis of ore deposits gives us information that helps us and García Alcalde, 1984), the basin having been unstable during the understand the formation conditions of these mercury mineralizations. Devonian, with strong volcanic activity reflecting extensional condi- Up to the present moment several authors have published S, C and O tions. In this respect, it should be pointed out that the magmatic rocks stable isotope data (Eichmann et al., 1977; Jébrak et al., 2002; Rytuba found in the Silurian sequence are mainly dikes, sills and diatremes, et al., 1989; Saupé and Arnold, 1992). In all cases the results obtained while in the Devonian sequence they are mostly volcano-sedimentary. show a broad range that has made it impossible to confirm either of To the south of Almadén and beyond the syncline, there is a wide the two proposed hypotheses.