New Mexico Mineral Symposium — Abstracts

The Lake Superior copper deposits of Michigan's Keweenaw Peninsula are unique. The Keweenaw Peninsula is the only place in the world where pure elemental or "native" copper naturally occurs in large, economically recoverable amounts.

The Lake Superior copper district is, in fact, the "cradle" of American copper mining. Prehistoric Native Americans mined native copper at least 5,000 years ago and produced an estimated 500 million pounds of copper using crude hand methods. They used the copper to make ornaments, tools, and weapons. By the seventeenth century, European missionaries and explorers became aware of the Lake Superior copper deposits. Later, in 1843, the region became the scene of the first mining boom in U.S. history. From 1850 to 1887 Michigan's copper mining industry was the largest in the United States and in the world. Michigan's native copper mines produced 11 billion pounds of copper from 1845 to 1968.

Because of its longevity, the Lake Superior copper district is a unique place to study the history of technological change in hard-rock mining methods. Historians can trace how manual mining methods, used by skilled Cornish miners and learned by apprenticeship, gave way to mechanical methods employed by a larger, unskilled labor force that was supervised by scientifically trained mining engineers. Immigrants from more than thirty nationalities came to Lake Superior to work in the copper mines. By 1907, 21,000 workers were employed in the Michigan mining industry. At that time, 95,000 persons lived in the Keweenaw Peninsula. Approximately 30,000 people live there today. Only twenty-four of some 400 mining ventures were successful. During the first century of operation shareholders in these companies received some $350 million in dividend payments. The most famous of the Lake Superior copper mines was the Calumet and Hecla mine. Calumet and Hecla alone accounts for 50% of the total Michigan copper production. From 1871 to 1887 Calumet and Hecla was the most profitable metal mine in the world.

The Lake Superior copper deposits occur in the Portage Lake Volcanics Formation, which constitutes the "spine" of the Keweenaw Peninsula. The Portage Lake Volcanics consists of some 200 flood-basaltic lava flows, sporadically interbedded with conglomerate and sandstone beds. These rocks are late Precambrian and have been dated to be 1.5 billion years old. The native copper and associated minerals were deposited from hydrothermal solutions into faults and fissures, into the brecciated and vesicular flow tops of basaltic lava flows, and into the pore spaces and voids of sandstones and conglomerates. These solutions are now believed to have originated from formational brines that were heated at depth and remobilized. The resulting mineralization is associated with very low grade prehnite-pumpellyite metamorphism.

Crystallized native copper and native silver specimens from the Lake Superior copper deposits can be found in most fine museum and private mineral collections the world over. Calcite crystals with native copper inclusions occur in great variety of crystal form and habit and are by themselves sufficiently important to establish the region as a major mineral locality. In addition, exhibit-quality specimens of analcime, apophyllite, datolite, natrolite, powellite, prehnite, and pumpellyite have been found in the Michigan copper mines.

Michigan Technological University's Seaman Mineral Museum is the official "Mineralogical Museum of Michigan" and is on the Fifth Floor of the Electrical Energy Resources Center. Founded in 1902 by geology department head Arthur Edmund Seaman, the museum collection contains some 60,000 mineral, gem, and rock specimens, of which 20,000 are on display. The Seaman Mineral Museum conserves the world's finest collection of minerals from the Lake Superior copper and iron mining districts. In addition, the museum's collection contains many fine exhibit- and reference-quality specimens from most classic mineral localities in North America and around the world.

Michigan's Keweenaw Peninsula, home of the famous Lake Superior copper deposits, possesses a remarkable blend of natural beauty, historic significance, and scientific importance that is characteristic of a world-class mineral deposit.