Geology of the Almaden mercury deposit, Province of Ciudad Real, Spain

Abstract

Almadan is the largest known mercury deposit in the world. It is formed by three lenticular strata-bound cinnabar impregnations in the two quartzite members of the Criadero quartzite formation of Lower Silurian age, adjacent to a lens of a phreatomagmatic explosion breccia (Frailesca) filling a crater piercing this quartzite. Cinnabar is the dominant ore mineral, with some native mercury, pyrite, and subordinate, late metacinnabar. The underlying, the intercalated, and the overlying shales (positioned with respect to the quartzites) are not hydrothermally altered. Spilitization of the mafic volcanic rocks is a regional process and thus is unrelated to ore deposition, whereas the mafic volcanic rocks overlying the ore deposit are sericitized and strongly carbonatized. The C isotope data do not indicate whether the carbonate CO2 was derived from the mantle or from hydrolysis of organic matter. The most probable source of mercury is the Ordovician black shales rather than a positive mercury anomaly in the mantle. The source of sulfur could not be determined by means of the S isotopes. These data suggest that mercury may have been transported as a sulfide complex and/or as HgPaq/.The deposit and its host rocks were folded vertically and faulted during the Hercynian orogenesis. The regional metamorphism is of very low grade (210ø-270øC, 1 kbar).The following sequence of events accounts best for the various field observations and laboratory work: (1) lying abruptly on top of clays, sands were deposited near a submarine delta on a shallow epicontinental platform (shelf) periodically subjected to incipient rifting; (2) these sands (the lower quartzite member) were impregnated by cinnabar and (3) underwent diagenesis, possibly accelerated by hydrothermal fluids; (4) the detrital (sand and clays) sedimentation resumed, subsequently leading to barren quartzite, shales, and the quartzite of the upper quartzite member; (5) the upper quartzite member was laterally impregnated by cinnabar and two conformable ore lenses formed--the impregnation followed zones of greater permeability parallel to palcocurrent directions and was possibly controlled by hydraulic fracturing; (6) a phreatomagmatic breccia, the Frailesca, formed in shallow volcanic explosion centers situated in the footwall shales without visible roots and intersected the two quartzite members; and (7) this sequence ended by volcano-sedimentary alternations including mafic intrusions and flows. During events (2) and/or (5) muddy cinnabar was deposited at a sedimentwater interface and impregnated porous quartzites.Other mines and occurrences in the Almadan district have similar features, although much smaller tonnages. Altogether, five morphologic types of mercury mineralization exist in the district. Outside of the district no other mine is similar to Almad(•n.

Chronic mercury exposure in Late Neolithic/Chalcolithic populations in Portugal from the cultural use of cinnabar

“…One of the largest natural sources of mercury in the world is located at cinnabar mines near Almadén, Spain 31 32 33 , approximately 300 km ENE of Perdigões ( Fig. 1 ).…”

Time and space variations in mercury and other trace element contents in olive tree leaves from the Almadén Hg-mining district

“…This area corresponds to the biggest mercury geochemical anomaly in the world and has yielded almost one third of the total global production of this element (Hernández et al, 1999). Descriptions of the geological characteristics of the area can be found in reports by Saupé (1990) and Hernández et al (1999), among others, while Lindberg et al (1979), Huckabee et al (1983), Higueras et al (2003), Millán et al (2004Millán et al ( , 2006Millán et al ( , 2011, Molina et al (2006), Moreno-Jiménez et al (2006), Martínez-Coronado et al (2011) have described environmental concerns regarding mercury in the area and include local soil/plant relationships. In particular, Molina et al (2006) reported mercury contents in olive tree leaves, which is the main aspect discussed in this paper.…”

XANES speciation of mercury in three mining districts – Almadén, Asturias (Spain), Idria (Slovenia)

“…On the other hand, Asturias has a more complex mineralization, with high proportions of arsenic in its paragenesis. It is important to highlight that Almadé n is the largest cinnabar (HgS) deposit in the world and it has been active since the Roman times until the present days, having accounted for about one third of the total Hg world production (Herná ndez et al, 1999;Saupé, 1990). Metallurgical processing in the study area evolved from Bustamante furnaces, with roasting temperatures over 873 K, to Pacific furnaces in the last century, reaching temperatures of up to 1073 K.…”