Almadien 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 CO 2 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 Hg 0 (sub (aq)) . The deposit and its host rocks were folded vertically and faulted during the Hercynian orogenesis. The regional metamorphism is of very low grade (210 degrees -270 degrees C, 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 paleocurrent 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 sediment-water interface and impregnated porous quartzites.Other mines and occurrences in the Almaden 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 Almaden.