Note pr�liminaire concernant la gen�se du gisement de mercure d'Almad�n

Saupé, Francis

doi:10.1007/bf00206819

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“…commun., 1975). Thus, part of the cinnabar must have been deposited at a sediment-water interface, that is, syngenetically.The cinnabar grains between the quartz grains and their diagenetic overgrowths suggest a prediagenetic deposition for part of the mercury(Saup(•, 1967). The same applies to the cinnabar quartz grains contained within the pyrite frambolds (Maucher and Saup(•, 1967; Arnold et al, 1971; Fig.…”

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confidence: 81%

“…A random distribution indicates that none of the three minerals was selectively replaced. Redrawn from Saup(• (1973).nabar between rounded quartz grains and diagenetic growth rims were observed in a few thin sections of mineralized quartzite from S. Pedro(Saup•, 1967;Fig. 5Gand H).…”

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confidence: 90%

“…This situation is best explained by a cinnabar impregnation of unconsolidated sands, prior to the sedimentation of the shaly interlayer and of the following sand layer (now the white barren quartzite;Saup(•, 1967). Samples from private collections show textures that can be interpreted as mechanically reworked cinnabar fragments having induced load casts in unconsolidated cinnabar muds by settling and compaction (Fig.…”

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

Saupé¹

1990

Economic Geology

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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.

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