GEOLOGICAL DISTRIBUTION
The ores of quicksilver, like those of most metals, show on the whole a close association with igneous rocks and with zones of fissuring. More commonly than with other metals, with the possible exception of antimony, they are associated with volcanism as opposed to plutonic igneous activity and were deposited comparatively near the surface. It follows that quicksilver deposits as a rule are found in regions of Tertiary and Quaternary volcanic activity which have not been subjected to long and deep erosion, that they are more likely to be in the younger geologic formations than in the older rocks, and that as a class, compared for example with the hypogene ores of gold or copper, do not extend to great depth. It must be noted, however, that there are some conspicuous exceptions to these generalizations. Although the California deposits are in a region of late volcanic activity and many of them are closely associated with active hot springs, the ore bodies that are now most productive, those at New Idria (Idria post office) and the great deposits, at New Almaden, that formerly yielded so richly, have no obvious connection with volcanism. The greatest quicksilver mine in the world, that at Almaden, Spain, has no known connection with volcanism or massive igneous rocks, has been worked to a depth of 1,150 feet, and the ore bodies have been found to grow larger and richer downward. The deepest quicksilver mine in the world is the New Almaden in California, worked to a depth of 2,200 feet. The part of the mine below the 600-foot level was abandoned at a time when the price of quicksilver was low, but it is doubtful whether, under any conditions that can now be foreseen, it will be profitable to reopen and work the deep levels of this mine.
Although most of the known quicksilver deposits are in regions of geologically late volcanic eruptions it is probable that ores of quicksilver were deposited during or closely following epochs of similar igneous activity in the older geologic periods, but that many of them have been removed by erosion. Some of the deposits in the older rocks, which do not appear to be related to Tertiary or later volcanic eruptions, may have had such earlier origin.
The quicksilver deposits of the Adriatic region in Europe, including those at Idria, in Austria; Avala, in Serbia; and Monte Amiata, in Italy, have been shown by De Launay to belong to a single metallogenetic province characterized by Tertiary eruptions. Similarly, the somewhat scattered occurrences of quicksilver in Alaska, Washington, Idaho, Montana, Oregon, Nevada, Utah, California, Arizona, Mexico, Peru, and Chile coincide in part with the belt of Tertiary and Quaternary volcanic activity along the western sides of the continents of North and South America. The deposits at Almaden, Spain, in the Donetz basin, Russia, in Asiatic Turkey, and in China appear to be isolated occurrences that can not at present be assigned to recognizable provinces of eruptive activity and metallization.
Quicksilver deposits are not confined to rocks of any particular kind or of any particular geologic age.
At Oviedo the ore averages 0.33 per cent. and yields arsenic compounds as by-products. At Idria the ore yields 0.65 per cent. The ore of the Abbadia-San Salvatore, the principal mine in the Monte Amiata district, in Italy, yielded about 0.9 per cent. in 1915. In California few mines have over 2 per cent. ore, and the average yield of the ore worked is about 0.5 per cent. The lowest yield that was profitably obtained in that state in 1917 was 0.185 per cent. The ores worked in Texas are generally of higher grade than those mined in California. In the principal mine of the Terlingua district, Texas, the won tenor of the ore in 1916 was 2.5 per cent. and in 1917, 3.9 per cent.
GEOGRAPHICAL DISTRIBUTION
Europe.
—The largest and richest deposit of quicksilver ore known is at Almaden, in central Spain. There are three nearly parallel ore bodies standing vertically side by side, each consisting of a portion of a bed of quartzite of Silurian age, impregnated with cinnabar. The ore bodies have been mined to a depth of 350 meters. The production in 1917 was probably about 25,000 flasks. The mine is said to have ore opened up that insures a future production of at least 40,000 metric tons of quicksilver. Other productive deposits in Spain are those near Oviedo, where the ore, which contains cinnabar, pyrite, orpiment, and realgar, is said to average about one-third of 1 per cent. of quicksilver, with arsenic compounds as by-products. According to a report from Vice Consul General H. A. McBride, written in Barcelona in 1911, the principal companies operating in the Oviedo districts were the Oviedo Mercury Mines Co., Ltd., of London, the Sociedad Fabrica de Mieres, of Oviedo, and the Sociedad Union Asturiana, of Mieres. The production from the district in 1915 was 608 flasks (20.7 metric tons). A third group of deposits lies on the south slope of the Sierra Nevada in the provinces of Granada and Almeria, southern Spain. The production from Granada in 1915 was 41 flasks (1.4 metric tons).
A small quantity of quicksilver was produced in Portugal in the nineteenth century from a mine not far from Lisbon. Cinnabar occurs at a number of localities in France and also in Corsica, but the deposits are not of economic character.