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 shortening madén 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. sotopes 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.