Third in rank among the copper-producing States is Michigan, the mines being located on Keweenaw Peninsula, which extends into Lake Superior. For fully fifty years this district has been one of the most famous and important copper producers in the world. A unique feature is that the ore is native copper, associated with some native silver. The rocks containing the ore are steeply tilted lava sheets and conglomerate (cemented gravel) strata of Proterozoic Age. Openings in porous lava and spaces between the conglomerate pebbles have been filled by metallic copper, which was carried off in hot solutions from the cooling lavas. Certain of the mining shafts have been sunk more than 5,000 feet below the surface, these being next to the deepest in the world.
Utah ranks fourth among the copper producers, the greatest mining district being at Bingham Canyon, southwest of Salt Lake City. The rocks are late Paleozoic strata, pierced by a large body of igneous rock. Some of the sulphide ores (mainly chalcopyrite) occur in veins in the igneous rock and some in large tabular masses in the adjacent limestone. Hot solutions from lower portions of the uncooled igneous rock carried the ore in solution into the limestone and into cracks in the upper cooled igneous rock.
Lead. Lead must surely be counted among the five or six most useful metals. As in the case of nearly all the other most important natural resources, the United States is the world’s greatest producer of lead, the output of metallic lead having been 552,000 tons in 1916 and somewhat less in 1918. Most of this came from Missouri, Idaho, Utah, and Colorado. The leading other countries are in order—Spain, Germany, Mexico, and Australia. Nearly all the lead comes from the mineral galena (a sulphide of lead), which is described in the chapter on “Mineralogy.” Among the many uses of lead are the following: manufacture of certain high-grade paints from lead compounds; making alloys such as pewter, type metal, solder, babbit metal; in plumbing; in glass making; and in the manufacture of shot.
The greatest lead-mining district is in the vicinity of Joplin, Mo., where the ore (galena), associated with much zinc ore, occurs as veins and great irregular deposits in limestone of early Paleozoic Age. It is generally agreed that underground waters dissolved the ores out of the limestone in which they were disseminated as tiny particles and deposited them in concentrated form at lower levels.
In the famous Cœur d’Alene district of northern Idaho the great output of lead is really obtained from a lead-silver ore; that is, galena rich in silver. This ore is in composition a lead-silver sulphide. It occurs in great fissure veins, mostly following fault fractures in highly folded strata of Proterozoic Age. Igneous rocks cut through the strata, and it is believed that hot ore-bearing solutions given off from the highly heated igneous rocks rose in the fissures and deposited the ores.
The Park City and Tintic districts of Utah are great producers of lead. The lead ore (galena) is usually rich in silver. It occurs mainly in veins and irregular deposits in limestone of Paleozoic Age closely associated with certain igneous rocks.
One of the most famous mining districts in the world is that around Leadville, Col., where ores of four metals—gold, silver, lead, and zinc—have been extensively mined. The salient points in the rather complex geology are the following: Paleozoic strata, including much limestone, rest upon a foundation of pre-Paleozoic granite. Sheets of igneous rock are interbedded with the strata and many dikes of igneous rocks cut through the whole combination. After the last igneous activity all the rocks were somewhat folded and notably faulted in many places. The ores were dissolved out of the igneous rock and deposited in large masses mostly in the limestone and in fissure veins, especially along and near the fault zones.
Zinc. Another of the few most useful metals is zinc. It never occurs in metallic form in nature, but most of it by far is obtained from the ore mineral sphalerite (sulphide of zinc) described in the chapter on “Mineralogy.” A red oxide of zinc ore, called zincite, assumes great economic importance in New Jersey. In 1917 the United States produced 686,000 tons of metallic zinc and was easily the world’s leader. Since 1917 the production has fallen notably. The four greatest producing States are Missouri, Montana, New Jersey, and Colorado. Germany and Belgium are the greatest foreign producers.
Most important of all in the United States is the district around Joplin, Mo., where the ore is closely associated with lead ore. The mode of occurrence and origin of these ores are above referred to in the discussion of lead.
In Montana some of the great east-west fissure veins in granite are rich in silver ores in the upper levels, and in zinc ores (mainly sphalerite) at depths of from some hundreds of feet to nearly 2,000 feet, that is as far down as they have been mined. They, like the great copper veins of the same general district, were carried by hot solutions which rose from the lower still very hot granite and deposited the ores in fissures of the same cooler rock higher up.