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Gem mining in North America


23 Haz 2018
Konbuyu başlatan #1
NORTH AMERICA. Gem mining in North America has never been extensive, and typically has accounted for less than 1% of the global gem production. Nevertheless, over the years the U.S. has produced more than 60 different colored gemstones, mostly from relatively small, short-lived deposits. Some U.S. colored gem areas and deposits have been known for more than a century. Mining at these deposits generally has been intermittent, however, as new economic zones are found or older zones are reworked. For example, the pegmatite district in San Diego County, California, has been famous for its tourmalines since the 1890s (figure 1). Mines such as the Himalaya, Stewart, and Tourmaline Queen have enjoyed revitalization and supplied fine gem material over the past three decades. Since 1976 at the Himalaya mine alone, 10,000 feet (3,075 m) of underground tunnels have been driven, with about 2 tons of tourmaline recovered to date. The Little Three pegmatites, also in San Diego County, have produced spectacular spessartine and blue topaz. The historic pegmatites in Maine (e.g., at Plumbago and Mt. Mica) have been mined sporadically in the same time frame and also have produced some fine tourmaline. Arizona produces several important gem minerals. Significant deposits of peridot are located on the San Carlos Indian Reservation, where magnificent gem rough up to 40 ct have been recovered, although most of the pieces are small. Nearby, fine andradite and “ruby”-colored chrome pyrope garnets occur. The Four Peaks deposit, located in the rugged Mazatzal Mountains near Phoenix, has recently been reopened and is producing high-quality amethyst. Although the faceted stones are typically small, the of it magnificent but most of it of lower quality (porous, chalky but amenable to enhancement). Some of the copper mines (e.g., at Morenci) still produce malachite and azurite. Chrysocolla is obtained from the Globe-Miami district. These copper minerals are continually being exposed during mining operations. Other important gems presently being mined in the U.S. include sunstone (feldspar) from Oregon; sapphire from two areas in Montana (the in situ Yogo Gulch deposit and several alluvial sources in the western part of the state); and emerald from Hiddenite, North Carolina. Although recent emerald finds at Hiddenite have been encouraging, it remains to be seen whether this deposit can be mined economically. Much scientific and media attention has been given to two rare gem materials that are unique to the United States: red beryl from Utah and benitoite from California. Extensive (and expensive) development work by organized mining companies is currently in progress at both of these deposits. Elsewhere in North America, Mexico has been the source of fine amethyst and iridescent andradite, but it is probably best known for beautiful opal from Querétaro. In the last few years, some extremely fine opal has been produced, but in small quantities. Although only a limited number of colored gems have been mined in Canada, British Columbia has been the world’s principal supplier of nephrite for several decades. Alberta is the world’s only source of “ammolite,” the nacreous outer shell layer of fossil ammonites that can exhibit remarkable iridescence.

-William Larson