Political and commercial geology and the world's mineral resources

by Unknown

GEOLOGICAL DISTRIBUTION

Iron ores are associated with many different classes of rocks—sedimentary, igneous, and metamorphic. Where associated with sedimentary rocks the ores may be the result of direct sedimentation or may be later replacements of sedimentary beds by magmatic or meteoric iron-bearing waters. Many iron-ore deposits associated with sedimentary rocks are formed by the enrichment of original iron-bearing beds, either by solution and transportation of iron compounds or by the removal of other associated mineral constituents.

Among those important iron-ore deposits of sedimentary origin that have undergone little or no further enrichment since deposition, except perhaps directly at the surface, are the iron ores of the Clinton type of the eastern United States, the Wabana iron ores of Newfoundland, the “minette” ores of the Lorraine district in northern France, Luxemburg, and southern Germany, the oolitic siderite beds of the Cleveland district in northern England, and the hematite ores of Minas Geraes in Brazil. The most important of the sedimentary iron ores that are the result of further enrichment since deposition are those of the Lake Superior district in the United States.

Iron ores associated with igneous rock are mostly of deep-seated origin, usually having been formed by solutions that accompanied or followed the intrusion of the rocks with which the ores are associated. These ores are of two main classes: (1) Those associated with siliceous igneous rocks; and (2) those associated with basic igneous rocks. The ores associated with siliceous igneous rocks consist either of hematite or, more commonly, magnetite. They occur in granite, syenite, and monzonite, and in gneiss derived from these by metamorphism. Many important ore deposits in different countries belong to this class, among them being the magnetite and hematite deposits of Swedish Lapland and of central Sweden, the magnetite bodies of the Adirondacks and northern New Jersey in the eastern United States, various magnetite and hematite bodies in California and elsewhere in the western United States, the mixed hematite and magnetite deposits of the south coast of Cuba, most of the iron-ore bodies of Chile, and the newly developed iron ores of Manchuria. As a class, the iron ores associated with siliceous igneous rocks rank next in importance to iron ores of sedimentary origin.

Iron-ore deposits associated with basic igneous rocks are nearly all of a distinct type known as titaniferous magnetites. These ores consist of a mixture of magnetite and ilmenite in varying proportions, and therefore carry a variable amount of titanium. Many large ore deposits of this class are found in different parts of the world, among the larger ones being certain ore bodies in Wyoming, in the Adirondack region, and elsewhere in the United States, and several deposits in Norway and in northern and southern Sweden.

An important group of iron-ore deposits has resulted from mineral replacement along the contact of sedimentary rocks with igneous intrusives. These ores usually occur in limestones not far from intrusive masses of granite, monzonite, syenite or diorite, but they may be found within the igneous rocks themselves, near the contact. They are rarely associated with the more basic igneous rocks. These ores are known as igneous contact ores, and their origin is ascribed to iron-bearing solutions that accompanied or followed the intrusion of the igneous rocks with which the ores are associated. Such ores are extremely widespread, occurring in practically every continent. Locally, extensive deposits exist, as in the Cornwall district of Pennsylvania, in the western United States and British Columbia, in Chile, and in China and Japan. Igneous contact ores have furnished only a relatively small percentage of the world’s total production of iron ore, however.

There are also widespread replacement deposits in sedimentary rocks that are not associated with igneous rocks. These are believed to be formed by ordinary meteoric waters which dissolve disseminated iron minerals from certain beds or masses of rock, and redeposit the mineral elsewhere in a more concentrated form. Such ore deposits may be roughly tabular and resemble bedded deposits, or they may be very irregular. Most deposits of this type consist of siderite which has replaced limestone, but hematite and limonite deposits formed by replacement also exist. Among the important deposits of this group are the siderite ores of Bilbao, Spain, largely altered to limonite near the surface; the siderite ore of Eisenerz, Styria; and the hematite deposits near Hartville, Wyoming. Small deposits of siderite, hematite, and limonite of this type are found in many parts of the world.

Besides the classes of iron ores already mentioned, widely distributed iron ores occur as residual products derived from the weathering of either igneous or sedimentary rocks. These ores have been formed by the concentration of iron-bearing materials originally disseminated through the rocks whose weathered products they now constitute. They are mainly in the form of limonite and occur either as large bodies of relatively pure ore or as aggregates of irregular masses of various sizes imbedded in clays. To this class belong the brown iron ores associated with clay in the Appalachian region of the United States, the limonite ores of parts of Russia, and similar ores in Korea. In this class should also be included the extensive limonite deposits derived from the weathering of serpentine which have recently been developed along the north coast of Cuba, as well as the lateritic iron-ore deposits found in many tropical countries. Limonite ores associated with clays have been smelted since early ages, owing to their accessibility and the ease with which they could be smelted by crude methods. They have, however, furnished a decidedly minor percentage of the world’s production of iron ore.