"Rock of the Month # 4, posted October 2001" --- Sample 0949.
Almaden, some 225 km southwest of Madrid, is a famous and much-described locality (see, e.g., Crawford, 1988; Saupe, 1990), yet the exact nature, modes of origin and age have been subject of a lively debate which continues to this day. The historical deposits are all located in a middle Paleozoic succession, in the lower Silurian Criadero quartzite. Ordovician black shales may be source rocks for the abundant Hg and S. The region may have seen two episodes of Hg mineralization, an earlier epithermal phase associated with Silurian magmatism and a late epigenetic remobilization associated with the onset of the Hercynian orogeny at the close of the Devonian period.
The widespread use of mercury as an amalgamation agent for the recovery of gold and silver in small-scale alluvial mining has been a serious local environmental problem for some centuries in parts of Latin America and elsewhere (Nriagu, 1993). In addition, the release of Hg both in industrial processes and in areas of naturally Hg-rich bedrock, as in southern China, have focused attention on this intriguing but toxic heavy metal, most familiar in classic thermometers.
These two smaller images are photomicrographs of polished mounts of two mercury ores, taken in reflected light:
CRAWFORD,JW (1988) Famous mineral localities: the Almaden mines, Ciudad Real, Spain. Mineral.Record 19 no.5, 297-302.
NRIAGU,JO (1993) Legacy of mercury pollution. Nature 363, 589.
SAUPE,F (1990) Geology of the Almaden mercury deposit, province of Ciudad Real, Spain. Econ.Geol. 85, 482-510.
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