Part 2

_Mining and Milling._—Fluorspar and its associated ores are mined in different ways in the Rosiclare and Cave in Rock districts because the types of deposits differ. However, in both areas most of the larger mines are entered by vertical shafts. In the mines, explosives are placed in holes drilled by machines and are then detonated to shoot down the ore (fig. 12). Mine cars, trucks, or conveyor belts carry the ore to the bottom of the mine shaft where hoists raise it to the mills at the surface. In the mills a variety of ore-classifying machines separate the galena, sphalerite, and fluorspar from the waste mineral materials (chiefly limestone and calcite) with which they occur. Some fluorspar after the separation process is almost flour-fine. To increase its use, much of this spar is mixed with a binder and made into pellets or briquets one-half to one inch in size.

_Geological and Chemical Studies._—Because much of the fluorspar produced in southern Illinois has come from veins along faults, geologists have mapped the faults of the area by investigating the distribution and nature of the various bedrock outcrops. The work was complicated by the mantle of earth and vegetation that covers the bedrock at many places. However, a geologic map was made that shows where the various rock formations—sandstone, limestone, and shale—lie beneath the surface and where the numerous faults crisscross the district.

The first geologic map of the fluorspar district was made in 1920 by the Illinois and U. S. Geological Surveys. New maps on a much larger scale have been made recently by the Illinois Survey to meet the needs of the modern fluorspar industry.

The Illinois Survey also has studied the ores and ore deposits of southern Illinois to determine how they were formed. The records of many borings and pits sunk to find ore have been collected and filed at the Survey to guide future prospecting.

Survey chemists are finding new uses for Illinois fluorspar. Their research has produced new organic fluorine compounds that are being tested for use in agriculture, medicine, and industry. They also have worked out easier and cheaper methods of making certain fluorine compounds. Survey chemical engineers have helped to obtain needed information about the physical properties of the pellets made from fluorspar powder.

Origin of Illinois Ore Deposits

The ore deposits of northwestern and southern Illinois were formed so many millions of years ago that it is possible to propose only theories of their origin. Most geologists think that the minerals, dissolved in warm or hot water, came from deep in the earth. Perhaps the mineral-bearing water came from, or was associated with, rocks that were or had been molten (igneous rocks), but it may have had some other source. Why the ores occur where we now find them is not fully known. The cooling of the solutions and the lessening of pressure as the solutions rose toward the surface may have had a part in ore deposition. Faults and the nature of the rocks encountered by the depositing solutions also appear to have had an influence.

Illinois as a Mining State

Although Illinois is not usually thought of as a mining state, northwestern and southern Illinois together produced in 1963 nearly $5,000,000 worth of zinc, about $600,000 worth of lead, and $6,500,000 worth of fluorspar. The annual total value is about $12,000,000.

The southern Illinois fluorspar district has another distinction—for many years its mines have been the major domestic source of the nation’s fluorspar.

SILICA SAND

About 450,000,000 years ago, a shallow ocean covered Illinois. Its waves and currents carried clean white sand and deposited it as curving beaches, sand bars, and dunes. This sand differed from many sands in that it was composed almost exclusively of grains of the mineral quartz instead of being a mixture of quartz and other minerals.

Quartz is composed of silica (SiO₂), and sands such as the ancient Illinois sand that are composed of quartz are known as silica sands. Quartz is very hard and will scratch glass and some steel. Perfect quartz crystals, which are rare, are longer than they are thick and end in pyramids. Probably not many grains of the ancient sand came from perfect crystals; they more likely resulted from the decaying and breaking down of rocks such as granite, which are mixtures of quartz grains and other mineral particles.

The quartz grains probably did not come directly from their source to Illinois. Instead, it is likely they first were deposited elsewhere and formed into sandstone. That sandstone was subsequently broken down by weathering agents and the grains transported to the ancient Illinois sea by streams.

As a result of the erosive action of the agents that transported them, many of the originally angular grains, particularly the coarser ones, were rounded and their surfaces dulled like that of frosted glass (fig. 13). Consequently, they appear white, although they actually are colorless.

Since the ancient sea deposited its silica sand, other seas have covered Illinois at various times and each has left deposits of sand, mud, or limy materials. The silica sand thus was buried by hundreds of feet of other sediments and became sandstone. This sandstone is called the St. Peter Sandstone. It is named from the St. Peter River, now the Minnesota River, in Minnesota where the sandstone was first described and named by geologists. The overlying deposits also were consolidated into rock.

St. Peter Sandstone is exposed at the surface at many places in northern Illinois and in one small area in the western part of the state. The sandstone exposed in northern Illinois generally varies from 125 to 300 feet thick. The fact that it crops out at the surface indicates that the materials that formerly covered it have been removed.

The uncovering was not a single, simple event but rather a series of events that took place at various times during the many years since the St. Peter sand was deposited. Among these was the up-bowing of the rocks of central northern Illinois into a broad arch. Streams then began to cut across the arched rock, slowly but persistently stripping away the top layers until the core of the arch was laid bare. Among the rocks thus exposed was the St. Peter Sandstone, which may be seen in northern Illinois in the valleys and tributaries of the Rock River near Dixon and Oregon and along the Illinois and Fox Rivers and some of their tributaries near Ottawa, Wedron, Millington, and Troy Grove. The St. Peter Sandstone at Starved Rock and Matthiessen state parks near LaSalle and along the highway between Dixon and Oregon is eroded into scenic bluffs and canyons.

Silica Sand Industry

The Illinois silica sand industry is based on the St. Peter Sandstone. It centers around Ottawa, Wedron, Troy Grove, and Utica in LaSalle County and in Oregon in Ogle County. Two principal grades of silica sand are produced—washed and crude. The value of the silica sand produced in Illinois in 1963 was about $9,000,000.

_Washed Sand._—Although the St. Peter Sandstone is composed almost entirely of quartz grains, a small amount of clay is present. For some uses it is not necessary to remove the clay, for others its elimination is important and is achieved by washing the sand.

In the mining of silica sand that is to be washed, the sandstone is first blasted loose from the parent deposit to break it into sand or pieces of various sizes. Some of the larger pieces may require a second blasting to disintegrate them.

At some pits the material is loaded mechanically and transported to the washing plant. At others a powerful stream of water is directed against the broken sandstone (fig. 14) and the resulting mixture of sand and water flows to a collecting basin from which it is pumped through large pipes to the processing mill.

In both types of operation the sand is thoroughly washed at the plants. After it is washed, the sand is further processed to suit the needs of its users. Much of it is screened into different size grades.

_Uses of Washed Silica Sand._—The washed silica sand produced in Illinois has many uses, some of which are briefly mentioned below. The suitability of the sand for some purposes depends in part on its having been screened to specified sizes.

The high purity of Illinois washed silica sand makes it suitable for making glass, which is more than half silica sand. Each year over a million tons is used for this purpose. The purity of the sand also is of importance for chemical and metallurgical uses such as the manufacture of sodium silicate and silicon carbide and in alloying.

The hardness of the sand makes it useful for grinding large sheets of plate glass to prepare them for polishing and also makes it an effective abrasive agent for sandblasting. Metal castings in foundries and the exteriors of buildings are cleaned by this process. Illinois produces thousands of tons of sand yearly for such abrasive purposes.

Because the coarser grains of the washed silica sand are rounded, strong, and available in uniform sizes, oil operators use thousands of tons of it annually in the hydraulic fracturing of oil-bearing strata. The sand is mixed with oil, other petroleum products, or water and is forced by powerful pumps into sandstone or limestone formations that contain oil. The great force thus exerted opens fractures in the rock strata and pushes the liquid and sand into them. When the pressure is relieved, the sand grains serve as props to hold the fractures open. The oil can then flow more easily into the wells and oil production is thus increased.

The washed sand, because it is clean and does not dissolve in water, is used to filter impurities from drinking water. Its whiteness makes it a desirable constituent in plaster, mortar, and precast building panels.

Because it is round grained and withstands high temperatures without melting, large tonnages of the washed silica sand are used to make molds into which molten metal is poured to make various kinds of castings.

A special type of coarse silica sand from Illinois that is carefully prepared so that it is always of the same grain size is used throughout the world as a standard in laboratories that test cement and other commercial products.

Some silica sand is ground to a fine, white powder. The powder, called ground quartz, ground silica, silica flour, or potter’s flint, has many uses. It is an ingredient in paints, potters use it in making pottery and china, it goes into scouring powders, into molds used for precision types of metal castings, and into enamels.

_Crude Silica Sand._—The crude silica sand produced from the St. Peter Sandstone generally is yellow or yellowish white and is not washed before it is used. It probably originally was white, but iron oxide, similar to the rust that forms on iron, now coats many of the sand grains and colors the small amount of clay in the sand. Thousands of tons of crude silica sand are mined annually (fig. 15). Because it is highly heat resistant, foundries buy much of it to make the molds used for castings, especially steel castings, and for automobile engine blocks, train wheels, and a variety of other metal products. Crude silica sand also is used around industrial furnaces to seal cracks and openings to prevent the loss of heat, in certain ceramic products, and for adjusting the silica content of the raw materials used for making portland cement.

Studies of the St. Peter Sandstone

The Illinois Geological Survey has made field studies and prepared maps showing where the St. Peter Sandstone is exposed in northern and western Illinois. Many samples have been screened and examined under a microscope to determine how the sand of different deposits, or different parts of the same deposit, varies in grain size and mineral composition. The possibility of using Illinois silica sand for making silica brick also has been investigated.

GRAVEL AND SAND

Some 225,000 years ago, most of what is now Illinois was buried under the ice of the Illinoian glacier. Two earlier glaciers had covered large parts of Illinois, and another, known as the Wisconsinan glacier, came into the state later, about 50,000 to 70,000 years ago (fig. 16).

The relatively small glaciers in the United States today, such as those in the northern Rocky Mountains, are concentrated in valleys and are called valley glaciers. The glaciers that covered Illinois were parts of huge ice sheets that extended over much of the North American continent and are called continental glaciers. They spread over most of Canada, then pushed southward to bury New England and a great area in the north-central part of the United States north of the Ohio and Missouri Rivers.

Formation of Gravel and Sand Deposits

As the glacial ice edged slowly southward from Canada, it froze fast to and picked up soil and loose pieces of rock, with enormous force tore away huge chunks of bedrock, and mixed and ground these materials together (fig. 17).

Into Illinois the glacier carried rock materials from Canada, Wisconsin, Minnesota, and Michigan; other rock fragments were picked up in Illinois as the ice front advanced. When the glacier melted, it left behind its load of rock flour and rock fragments, much of it as a gray clay containing pebbles, cobbles, and boulders. Geologists call such deposits glacial till.

UNGLACIATED WISCONSINAN GLACIER Freeport Fulton Peoria Decatur Charleston ILLINOIAN GLACIER Kewanee Waterloo Carbondale Harrisburg KANSAN GLACIER UNGLACIATED Hardin

The ice in the continental glaciers usually crept forward, sometimes slowly, sometimes more rapidly. Whether the front of a glacier moved forward or back depended on the balance between the rate of forward motion of the ice and the rate of melting. When the ice advanced faster than it melted, the front of the glacier moved forward. When the glacial ice melted faster than it moved forward, the front of the glacier receded. When the rates of melting and advance were about equal, the front of the glacier stood still or moved back and forth in a narrow zone.

When such a more or less stationary front existed, an enormous amount of clay, silt, sand, pebbles, and boulders was deposited in a belt only a few miles wide along the front of the glacier, creating a line of hills and ridges that extended for many miles. Such belts, called end moraines, can be seen today in many parts of Illinois.

The building of end moraines often was accompanied by the release of great quantities of water (meltwater) from the melting ice. The water, laden with rock debris, flowed from the front of the glacier in many streams.

As the meltwater flowed away from the glacier it sorted its load, although the sorting was rarely perfect. The heavy boulders and pebbles usually were dropped first, then the sand, next the silt, and finally the clay. In general, the farther the deposits were from the glacier the finer they were. The major streams frequently carried pebbles 50 to 100 miles from the glacier and it was many more miles before all the sand was dropped. They carried some of the fine silt and clay as far as the Gulf of Mexico.

Sometimes the floods of glacial water were greater and flowed faster than usual and so were able to carry coarse rock materials farther. As a result, gravel was laid down on top of earlier sand deposits. Later there may have been further sand deposition.

The debris-laden meltwater that flowed into valleys often deposited in them a considerable filling of sand and gravel. Some valleys were filled to a depth of as much as 100 feet. Such deposits are called valley fills or valley trains. Modern streams have cut their courses into many of these fills and even worn away large parts of them. Remnants of valley train deposits are now large terraces or benches along streams, many of them well above the present stream channels.

Where many small streams flowed from the glacier, they deposited sand and gravel as a large apron in front of the glacier. Such deposits are called outwash plains and many of them extend for miles.

Two other types of sand and gravel deposits made by glacial meltwaters also are significant. One was formed where water issued from the front of a glacier or poured into holes or crevasses in the ice. The sand and gravel in the water formed a deposit that now appears as a rounded hill associated with a terminal moraine and is called a kame. The second type of deposit was laid down in beds of streams flowing under, through, or on the glaciers and was left as a more or less continuous ridge of sand and gravel when the ice melted. Such a deposit is called an esker. Some eskers in Illinois are about a quarter of a mile wide and several miles long. Typical are the Kaneville Esker northwest of Aurora, the Adeline Esker south of Freeport, and the Exeter Esker west of Jacksonville.

The deposits of both the Illinoian and Wisconsinan glaciers are widely distributed throughout the state. Melting of the Illinoian glacier caused comparatively little flooding; consequently, extensive gravel deposits were formed in only a few places. The ice of the Wisconsinan glacier, however, melted rapidly and produced great floods laden with sand and gravel. Thus, most major gravel deposits in Illinois are related to the Wisconsinan glacier.

Wind sweeping across the sand and gravel deposits blew the sand into hills or sand dunes near such places as Havana, Prophetstown, Kankakee, and Watseka. Even today the wind shifts sand of long-forgotten glacial floods.

Studies of Glacial Deposits

The foregoing discussion of glaciers and their deposits is greatly simplified. For some time geologists of the Illinois Geological Survey have been mapping the moraines, valley trains, outwash plains, and other glacial deposits of the state. Because the Illinoian and Wisconsinan glaciers advanced and retreated several times, they built many moraines. The Survey has made a map (fig. 18) that shows the complexity of the moraines left by the Wisconsinan glacier. They are roughly concentric, indicating that the general shape of the glacier front remained about the same.

Principal Commercial Sources of Sand and Gravel

The sand and gravel industry is widely distributed throughout Illinois. The principal commercial sources of sand and gravel are valley trains and outwash plains. The Fox, Rock, Illinois, Mississippi, and Wabash Rivers and many smaller streams have terraces in their valleys that are parts of valley trains. In these deposits are some of the largest sand- and gravel-producing operations in the state.

Composition

An examination of glacial gravel deposits in Illinois reveals pebbles and larger pieces of many kinds of rock. Some are gray, others white, pink, brown, or black. They commonly include limestone, dolomite, granite, and many rocks with less common names such as quartzite, schist, and basalt. The limestone and dolomite were picked up by the glaciers from outcrops in northern Illinois, Wisconsin, and Michigan. Some of the granite pebbles resemble outcrops in Wisconsin; others look like granite that crops out in Canada. The quartzite probably came from Wisconsin, and black shale fragments found in some gravel deposits came from the floor of Lake Michigan or from western Michigan. Occasionally pieces of metallic copper are found that probably had their source in the Lake Superior copper-bearing area.

In addition to the sand associated with gravel deposits, extensive deposits of sand alone are found at many places in Illinois. Most of the sand grains are pieces of minerals that were constituents of rocks until weathering, the grinding action of the glaciers, and other erosive agencies broke the rocks into sand. The principal mineral in glacial sand is quartz, but many others occur in lesser amounts, including calcite, dolomite, feldspar, pyroxene, tourmaline, garnet, magnetite, and hornblende. Most of these are foreign to Illinois, although the calcite and dolomite may be native.

Modified from George E. Ekblaw, 1960 _ILLINOIS STATE GEOLOGICAL SURVEY_

Uses of Sand and Gravel

In 1963 more than 27 million tons of sand and gravel, directly or indirectly of glacial origin, was sold by Illinois producers for almost 25 million dollars. The sand alone would have filled a child’s sand box, with an area of 8 square miles, to a depth of 1 foot. The gravel would have covered an even larger area.

The gravel was used in making concrete for roads and buildings, for surfacing roads, for ballast for railroad tracks, and for other purposes. The sand found its way into plaster, mortar, concrete, and a variety of other products and uses. Some of it was produced for use as molding sand.

Production of Sand and Gravel

The production of sand and gravel from its deposits may be a relatively simple operation or one of considerable complexity. Gravel for surfacing a road may be dug from a conveniently located pit and loaded mechanically into trucks that haul it to the road. A large sand- and gravel-producing operation, however, may include not only mechanical equipment to load and transport the material but also a processing plant where it can be washed if necessary and screened to various sizes.

In a “dry pit” operation, mechanical cranes or shovels pick up the gravel and sand and load it into trucks, railroad cars, or conveyor belts to be transferred to the processing plant. There clay and dirt may be washed out and the sand and gravel is sized by screens. Conveyor belts carry sand and gravel to the various processing operations and to storage bins or piles on the ground.

A “wet pit” operation produces sand and gravel from an artificial pond or lake. In some operations the sand and gravel is mined by a dredge that floats on the water (fig. 19). In some pits, a stream of water is directed from the dredge against the bank of gravel to wash the gravel into the lake. A large metal pipe at the front of the dredge slants down into the water and sucks up the sand and gravel from the underwater part of the deposit. The gravel, sand, and water is then pumped through a pipe at the rear of the dredge to the processing plant on the shore.

In some types of wet pit operations a large scoop or bucket operated from a crane on the shore, or by cables fastened to the shore, is used to dig sand and gravel from beneath the water.

Dredges are used to pump up sand and gravel from the beds of Illinois rivers, especially the Mississippi, Ohio, and Wabash.

The sand and gravel resources of many parts of the state have been mapped and studied by the Illinois Geological Survey, and work of this kind is continuing.

SILICA (TRIPOLI) AND OTHER MINERAL MATERIALS OF EXTREME SOUTHERN ILLINOIS