The ores of iron are
widely distributed all across the United States. The following is a summary
of some of the best known iron ore mining areas. Many are deposits of
taconite, or banded iron formations.
Lake Superior Region: The most productive deposits are those of the Lake
Superior region. These deposits are in long and relatively narrow belts, the
so-called iron "ranges." The ores are carried by rail to upper Lake Ports
and shipped by boats to lower Lake Ports, from which they are distributed to
iron furnaces. Practically all the ores mined in the Lake Superior region
are superficially enriched products of pre-Cambrian sedimentary protores.
The principal iron-bearing formations are the Soudan (Keewatin), of the
Vermilion range; the Negaunee (middle Huronian), of the Marquette range; and
the upper Huronian or Animikie formations, of several other ranges. The
upper Huronian is the most productive. The iron-bearing formations are
stratified sedimentary rocks composed chiefly of iron oxide, silica, iron
carbonate, and iron silicates. Such rocks are called jasper, ferruginous
chert, taconite, greenalite, cherty iron carbonate, etc. By weathering and
enrichment they become ore. The process is chiefly solution and removal of
silica and carbon dioxide, although locally iron may be dissolved and
precipitated as oxide by ground water.
In all the ranges the iron-bearing formations have been tilted, and in some of them closely folded. The ore-bearing formations have been exposed to weathering through many geologic periods. At some places removal of silica and concentration of iron began in pre-Cambrian time. In the Vermilion range parts of the ore-bearing formation were weathered and metamorphosed to schist in the pre-Cambrian. In the Marquette district the Negaunee formation was altered by weathering before upper Huronian time. All phases of the iron-bearing formation shown in this region, except specular hematite, had been formed, for they are represented by pebbles in the upper Huronian. The specular hematite was developed by deep-seated metamorphism of portions of the iron-bearing formation already weathered and enriched. This metamorphism, which gave a secondary cleavage to the iron ore, was accomplished by deformation that took place after the deposition of the upper Huronian sediments.
In the Mesabi Range concentration by weathering had taken place before the Cretaceous period, for pebbles of weathered ore are found in the conglomerate at the base of the Cretaceous. During long geologic ages large parts of the Lake Superior region have been land; for much of this time the land surface has been relatively low, a condition favoring extensive chemical denudation and deep weathering. The weathered parts of the ore-bearing formations are found in various positions with respect to the geologic structure. On the Mesabi Range the ore deposits are for the most part blankets that lie below the mantle of drift. Hero and there the ores extend down the dip of the beds below interstratified lean beds or below Virginia slate. Some of the deposits are due to leaching along enlarged joints. In some of the iron ranges the rocks associated with the ores are complexly folded; the ores are in places found in pitching troughs where ground-water circulation has been controlled by impermeable beds or intrusive rocks, or by both. Circulating ground waters conducted along restricted paths through long periods have leached out silica to depths far below the surface. Locally also they have deposited iron oxide, cementing the ore and further enriching it.
Mesabi Range, Minnesota: The iron deposits of the Mesabi Range are formed by local concentration in a ferruginous sedimentary formation, which extends from a point about 12 miles southwest of Pokegama Lake to Birch Lake, a distance of over 100 miles. The rocks strike about N. 73E. and have low dips toward the south. The rocks of the Algonkian system rest uncomformably upon the Archean rocks and are not so greatly metamorphosed. The oldest member of this system is a sedimentary series of conglomerates, graywackes, and slates, of lower-middle Huronian age which strike approximately with the axis of the range and stand nearly vertical. The iron formation is of upper Huronian age. It rests unconformably on steeply dipping lower-middle Huronian and older rocks. The upper Huronian is composed of (1) the Pokegama quartzite, consisting mainly of quartzite but containing also conglomerate at its base; (2) the Biwabik formation, which rests upon the Pokegama and consists of ferruginous cherts, iron ores, slates, greenalite rocks, and limestone carbonate rocks, with a small amount of coarse detrital material at its base; and (3) the Virginia slate.
Some acidic and basic intrusive igneous rocks are associated with the upper Huronian sediments. All the upper Huronian rocks were formed after the close folding which affected the lower and middle Huronian sedimentary rocks. They dip at low angles. The Biwabik formation extends along the entire length of the range. Its thickness is about 620 feet, but owing to the prevailing low dips, the width exposed ranges from a quarter of a mile to 3 miles. The formation is generally covered with glacial drift, which ranges in thickness from 20 to 200 feet. Where it is not too thick the drift is removed by stripping, and the ore is loaded into cars with steam shovels. The bulk of the Biwabik formation exposed is ferruginous chert, with which are varying amounts of amphibole, some lime and iron carbonate, and bands and irregular deposits of iron ore.
Associated with the slaty layers in the iron-bearing formation or closely adjacent to the overlying Virginia slate are green rocks made up of small granules of a ferrous silicate called greenalite. The greenalite has at some localities been replaced by cherty quartz, magnetite, hematite, limonite, and other minerals, and associated with the greenalite rocks are small quantities of lime and iron carbonates. At the east end of the range, near Birch Lake, the iron-bearing formation has been metamorphosed to a rock composed of magnetite, amphibole, olivine, and quartz. There are large quantities of this material, which can be easily concentrated to a high-grade Bessemer product by magnetic concentration. In the future, as iron-ore reserves decrease, it will become an important economic asset. The Biwabik formation may be subdivided into four members from bottom to top: the lower cherty member, the lower slate member, the upper cherty member, and the upper slaty member. The principal ore bodies are in the cherty members and in the lower slaty member. The iron ores are formed by local concentration in the iron bearing formation. This formation contains conglomerate and quartzite layers near the base and here and there thin layers of slate or other sedimentary rocks. The ferruginous layers grade laterally into slate bands, and upward the formation grades into the Virginia slate. The formation as a whole is extensive laterally and it has a comparatively uniform thickness like many other beds deposited in water. Only small proportions of the Biwabik formation are rich enough to constitute iron ore. These are patches here and there along the eroded surface of the iron-bearing formation. The workable deposits are due to secondary concentration. The ore rarely extends to depths of more than 400 feet below the bedrock surface, although at some places it lies deeper.
Cuyuna Range, Minnesota: The Cuyuna range, southwest of the Mesabi range, extends from Aitkin through Deerwood and Brainerd to a point beyond Fort Ripley. It is about 65 miles long in a northeasterly direction and from 1 to 12 miles wide. This range was discovered by drilling areas showing magnetic attraction. The iron-bearing formation occurs in eight or ten northeastward trending, discontinuous belts. The dip of the beds is usually steep, and the prevailing dip is southeast. The formation is enclosed between walls of sericitic, chlorite, or quartzose schist or slate. It consists mainly of ferruginous chert, but ferruginous slate is abundant, and in parts of the district magnetitic slate and amphibole-magnetite rock, are prominent. The ore bodies are irregularly lens-shaped and lie within the iron-bearing layers; the longer diameter is usually parallel to the bedding of the iron-bearing formation. They may be enclosed within ferruginous chert or slate or other phases of the iron-bearing formation. Many of them are also bounded on one or both sides by schist or slate wall rocks. In places original iron-bearing rocks such as cherty and slaty iron carbonate have been encountered at varying depths below the ore and the associated altered phases of the iron-bearing formation. The ore contains much manganese oxide. Like iron, the manganese has been reconcentrated by weathering. Most of the ore bodies probably do not extend to depths of more than a few hundred feet below the bedrock surface.
Penokee-Gogebic Range, Wisconsin and Michigan: The Penokee-Gogebic range is south of Lake Superior in northern Michigan and Wisconsin. The range trends N. 30 E. for about 80 miles. The principal mines are near Hurley, Wis., and near Ironwood, Wakefield, and Bessemer, Michigan. In a broad way, the iron-bearing rocks correspond to the Animikie iron-bearing series of the Mesabi Range, Minnesota. The Ironwood formation corresponds in age and character to the Biwabik formation of the Mesabi Range. It is succeeded by the Tyler slate, corresponding to the Virginia slate on the Mesabi Range. On the Mesabi the ore-bearing series dip southeast at low angles; on the Penokee-Gogebic the series dip north or northwest at high angles. Low dips on the Mesabi favor the development of the broad, shallow deposits that are worked by open cuts. Steep dips and narrower outcrops on the Penokee-a comparativey simple structure, but on the Penokee-Gogebic Range they are intruded by many dikes, mainly basic in composition. The ore bodies are the portions of the Ironwood formation that have been enriched by surface agencies. Only a small area of the iron-bearing formation is workable, and the ores are found only in the central part of the range, in a belt about 26 miles long. Oxidation extends to a depth of more than 2,000 feet. At that depth in the Newport mine the iron-bearing formation is as thoroughly oxidized and leached as it is near the surface. The downward circulation has been controlled by pitching troughs, and thus oxidation has been carried to extraordinary depths in channels where downward-moving waters concentrated. Some troughs are formed also by westward-pitching dikes intersecting eastward-pitching dikes, and by dikes intersecting slate bands in the ore-bearing formation. Several ore bodies may be formed in troughs one below another.
Menominee District, Michigan: The Menominee district is in Michigan not far west of Escanaba, but outlying areas that may properly be included in this district are the Florence area, in Wisconsin, and the Crystal Falls, Iron River, and Metropolitan areas, in Michigan. The iron ores are in the Vulcan and Michigamme formations of the Animikie group. Greenalite and siderite are not now present, but pseudomorphs of both are abundant. The Vulcan formation and the overlying Michigamme slate are conformable, and the contact is usually sharp. The Michigamme formation is composed of black and gray slates, gray calcareous slates, graphitic slates, graywackes, thin beds of quartzite, local beds of ferruginous dolomite and siderite, and rarer bodies of ferruginous chert and iron oxide. These formations that carry the ore deposits are closely folded. The larger deposits rest upon relatively impervious formations whose folds form pitching troughs
Eagle Mountain, California: The Eagle Mountain region, California, is an area of ancient gneiss, schist, and quartzite, upon which was deposited quartzite with lenses of dolomite. These rocks are intruded by great sills of monzonite. The ores are irregular deposits formed through replacement of the limestone by solutions emanating from the igneous rocks. The ore is hematite, with some magnetite. A considerable proportion of it is very pure and of high grade, containing between 62 and 67 per cent, of iron and less than 0.06 per cent, of phosphorus. The principal gangue minerals are serpentine, mica, amphibole, garnet, epidote, pyroxene, and sphene. The Iron Age deposit is near Dale, San Bernardino County, California is a typical example. The country rock is an intrusive mass of soda granite and granite porphyry. The ores are chiefly hematite with subordinate magnetite and occur as veins cutting the intrusive granite and granite porphyry. Garnet and epidote are locally associated with the ore and rocks. The principal iron-ore veins occur over an area about half a mile square; the larger veins, on account of their resistance to erosion, they form the summit of a hill.
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Precious and Base Metal Ores
Above: Hematite, the chief ore mineral for iron in the last 50 years.
