The geology of the area is characterised by volcanism. Almadén is home to the world's greatest reserves of cinnabar, a mineral associated with recent volcanic activity, from which mercury is extracted. Cinnabar was first used for pigment by the Romans.
Cinnabar is a mercury sulfide mineral (HgS). It is one of the few sulfide minerals that lacks a metallic luster (other examples are orpiment & realgar). In its crystalline form, cinnabar has an intense adamantine luster. Massive, fine-grained specimens generally have an earthy luster. Cinnabar has a reddish color reddish streak, 3 different cleavage planes, is quite soft (H = 2 to 2.5), and is heavy for its size (high specific gravity).
Cinnabar generally occurs as a vein-filling mineral associated with recent volcanic activity and alkaline hot springs. The mineral resembles quartz in symmetry and in its exhibiting birefringence. Cinnabar has a mean refractive index near 3.2, a hardness between 2.0 and 2.5, and a specific gravity of approximately 8.1. The color and properties derive from a structure that is a hexagonal crystalline lattice belonging to the trigonal crystal system, crystals that sometimes exhibit twinning.
The exceptionality of this ore deposit lies in the unique geological characteristics that led to the high concentrations and large accumulations of mercury, which constitutes its own metallogenetic model. The mining and metallurgical complex, together with part of the rest of the museum facilities, currently constitute the Almaden Mining Park, which is open to the public since 2008. The park includes a Visitor Center, the Mining Interpretation Center, and the Mercury Museum. It offers tours in real tunnels in the 16th-century inner mine. Almadén is included into the World Heritage List.
The giant Almadén mercury deposit is hosted in 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 Almadén district is the largest mercury geochemical cluster on Earth, having produced one third of the total world mercury. It is a stratabound mineral deposit and is composed of three mineralized levels in the “Cuarcita del Criadero”. Formation that correlates with the Upper Ordovician to lower Silurian. 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 deposit consists of impregnated and infilled joints with cinnabar in siliceous sandstone beds. It appears to be linked to the presence of explosive volcanic tuffs (Roca Frailesca). It is located on the southern limb of the Almadén syncline where it appears in a vertical position and with an E-W strike. In addition, the good outcrops of the mine tunnels allow excellent observations of the sedimentology, volcanic petrology, and Variscan tectonic structures, and are classic sites for fossil occurrences, particularly Silurian graptolites. The Almaden deposit has mined out about 7,000,000 flasks (= 241.500 Tm) with an average grade of 3.5% Hg. This is about a third part of all the mercury consumed by humanity.