Sedimentary mineral deposits

Sedimentary mineral deposits are accumulations of economically valuable minerals that occur in sedimentary rocks ranging in age from 2.2 billion to less than 2 million years old. Such deposits have been widely exploited in the past and continue to be important sources of ores.

Placers

In its broadest sense, a sedimentary mineral deposit is an abnormal accumulation of valuable minerals in sedimentary rocks. When such an accumulation is rich enough to warrant mining, it is referred to as a sedimentary ore deposit. Most geologists would agree, however, that further restriction should be applied to the term. In this discussion, the term “sedimentary mineral deposits” refers only to those deposits that formed through sedimentary processes. For example, hydrothermal deposits would not be considered sedimentary ore deposits even if they occurred in sedimentary rocks. Such ores form by fluids circulating through the rocks long after deposition, not during the deposition or burial of the rocks.

Several types of sedimentary processes may lead to the formation of sedimentary mineral deposits. Primary deposits form at the same time the host sediments are deposited. They can form either as chemical precipitates or as placers. Placers form when ore minerals are eroded and transported along with sand and silt by rivers. When the sand and silt are deposited as sediments, the ore minerals are deposited along with them. Most placers form in riverbeds and tend to be small. Placer accumulations are also found in some sands deposited on beaches.

The best-known placers are those of gold. Although small by comparison to lode deposits (deposits in bedrock), placer gold deposits were responsible for setting off the many gold rushes in the United States, including those to Colorado, California, and Alaska. Other placer deposits occur as paleoplacer deposits—accumulations that, millions or billions of years ago, were placer deposits and have since been hardened into sedimentary rocks. Of particular interest to geologists are large deposits of quartz pebbles with sand-sized grains of pyrite (iron sulfide) and uraninite (uranium oxide). Pyrite and uraninite are unstable in the presence of oxygen and quickly break down during erosion. They are common, however, as detrital minerals (minerals that have been eroded and transported by streams) in rocks of Archean age (older than about 2.5 billion years). Geologists have deduced that such accumulations could have formed only if oxygen were not abundant as a free gas (not combined with other elements) in the earth’s atmosphere at that time.

Chemical Sediments

Many chemical sediments form when minerals precipitate directly from seawater or saline lakes onto the sea floor or lake bottom. The most common type of chemical sediment forms when sea or lake water is concentrated by solar evaporation. Such deposits are known as evaporites. Large deposits of anhydrite (calcium sulfate), gypsum (hydrated calcium sulfate), and halite (sodium chloride, also known as table salt) have formed in small seas in areas with arid climates. As water is lost from these bodies because of high rates of evaporation, new seawater enters through narrow straits, connecting the sea with the open ocean. Dissolved salts in the water do not evaporate. They are concentrated until the brine becomes saturated, at which time minerals begin to precipitate. Gypsum forms first, followed by halite. Anhydrite forms when gypsum deposits are buried by later sediments and heat expels water from the gypsum. Potash (potassium chloride) can also form in this manner if arid, evaporative conditions persist for a long enough time, but most seawater does not evaporate completely enough to precipitate potassium salts.

Saline lakes also undergo the evaporative concentration of dissolved salts, from which evaporite deposits may form as well. Lake-water chemistry is controlled by the chemical composition of the surrounding bedrock, and lake water is often quite different in chemical composition from seawater. Thus, lacustrine evaporites (evaporites forming in lakes) commonly contain minerals not associated with marine evaporites. Such deposits include various borate (boron oxide) deposits in California and Turkey, and trona (hydrated sodium carbonate) deposits of the western United States.

Chemical sediments also form when hydrothermal solutions are expelled onto the sea floor. The hot, mineral-laden hydrothermal water mixes with seawater and is rapidly cooled. The solubility of most metals decreases drastically as the temperature decreases, and various sulfide minerals precipitate, including minerals of copper, lead, zinc, and iron, as well as barite. Deposits of this type occur in many regions of the world, including North America (especially Canada), and are forming today in the Red Sea and the Pacific Ocean along the midocean ridge system.

Primary deposits also form when slight changes in the physical or chemical composition of ocean water cause dissolved minerals to precipitate. For example, the mineral apatite (calcium phosphate) is soluble in cold, deep-ocean water that contains only small amounts of dissolved oxygen. Along continental margins, upwelling of this water to the surface causes the apatite to precipitate out as sediment. If the influx of other types of sediment, such as sand and silt, is very low, rich accumulations of phosphate can result.

Sedimentary Iron Formation

Another type of chemical sediment is known as sedimentary iron formation. During the Phanerozoic eon, which spans from 544 million years ago to the current geologic eon, deposits of this type have formed when iron was leached from rocks and transported by groundwater to shallow, restricted seas, where it precipitated as hematite (iron oxide). The iron ores of northern Europe and eastern North America, which were very important during the Industrial Revolution, are of this type.

A unique type of sedimentary iron formation is found in rocks between 1.6 and 2.5 billion years old. These deposits consist of alternating bands of iron minerals and chert (a hard rock composed of silica). These deposits formed in many parts of the world as iron precipitated from seawater. The source of this iron is the subject of much debate, but one thing is clear: The deposits, though widespread, are clearly restricted to the period in Earth’s history when free oxygen was becoming abundant in the atmosphere. It is believed that this change caused the solubility of iron to decrease (iron is soluble in water only if no free oxygen is present), thereby triggering the precipitation of the iron dissolved in seawater. Upwelling events similar to those envisioned for phosphates may have brought poorly oxidized, iron-rich water upward, to levels where it oxidized and precipitated.

Iron is precipitating on the ocean floor today, along with manganese, to form widespread deposits of manganese nodules. These nodules are found in all the world’s oceans and contain considerable amounts of other metals, such as copper, nickel, and cobalt. It is thought that much, if not most, of the metals are derived from hydrothermal activity along midocean ridges. Because the dissolved oxygen content in the deep oceans is low, the metals can travel considerable distances before precipitating from solution.

Secondary Deposits

Secondary deposits form after the host sediments have been deposited but during the processes related to burial. When sediments are buried beneath new sediments, water located between the various sediment grains is squeezed out. As the sediments are buried deeper and deeper, they are warmed by heat from the earth’s interior. As the temperature rises, the water being expelled from the compacting sediment mass reacts with the various grains and becomes increasingly saline. Such reactions, involving pore fluids and the enclosing rocks, are collectively known as diagenesis and occur in all sedimentary environments. Brines formed in this manner are well known from oil and gas exploration. As these brines move toward the earth’s surface, the temperature decreases; in many instances, they also mix with other groundwater

Several important types of secondary deposit are known. In northern Europe, Africa, Australia, and elsewhere, thin (up to several meters thick) but widespread (up to 10,000 square kilometers) layers of shale are enriched in sulfides of copper and, in some cases, other metals, such as lead and zinc. Although there is still much controversy regarding the origin of brines, most evidence indicates that brines formed during diagenesis reacted with the shale shortly after it was deposited.

Secondary deposits are found in limestones (rocks made up of calcium carbonate) and dolomites (calcium-magnesium carbonate). Brines that were formed during diagenesis migrated into cavities in the host sediment, and sulfides (minerals that contain sulfur), particularly of lead, zinc, and, in some cases, barite and fluorite, were precipitated. These deposits exhibit many similarities to shale-hosted deposits but differ in that copper is rare or absent. They are commonly called Mississippi Valley-type deposits because they are similar to the extensive accumulations in the Mississippi River drainage area. Recent research suggests that warm fluids originating deep under the Michigan and Illinois Basins traveled hundreds of kilometers before precipitating the ores. The fluids were also responsible for converting limestone in the region to dolomite.

Secondary deposits also form when oxidized groundwaters flow through sandstones located near the earth’s surface. Some metals, such as uranium, are present in small quantities in the sandstone. As the water percolates downward through the sandstone, the metals dissolve and are concentrated in the water. Eventually, the dissolved oxygen reacts with organic matter in the rocks. This reaction lowers the solubilities of the metals, and they precipitate. Most of the uranium ores of the western United States formed in this manner. Certain ores of copper are thought to have formed in a similar fashion.

Field Study

Economic geologists (geologists who study ore deposits) have a number of methods available to them for studying sedimentary mineral deposits. Perhaps the most important method is detailed mapping and description of ore deposits exposed in mines. During mining, rock material is continuously removed, thereby exposing new rocks. This exposure allows the mine geologist (a geologist who works with miners to ensure efficient mining) to observe and describe the deposit from a three-dimensional perspective not readily available to geologists conducting other types of studies. (For example, a geologist examining the face of a mountain can only guess at what the rocks beneath the mountain are like; a mine geologist examining the walls of a mine need only wait until these are stripped away to see what lies behind them.) This three-dimensional viewpoint allows the geologist to determine the spatial relationships of the minerals to one another and to the enclosing strata. Drill cores of rock are normally obtained around the periphery of the deposit, and the geologist can make similar observations in the cores. These relationships can then be used to interpret how the deposit formed.

Once studies of individual sedimentary mineral deposits are completed, they can be compared with studies of other, nearby sedimentary mineral deposits. A regional interpretation of how the deposits formed can be gathered from such comparisons. Models of this type are extremely useful in deciphering the overall geologic history of an area. They are useful also in the search for similar sedimentary mineral deposits.

Microscopic Study

The examination of rock samples using microscopic techniques allows geologists to observe relationships not visible to the naked eye. Several types of microscopic study are available. Standard petrographic microscopy involves observation of a thin wafer or rock. This wafer, known as a thin section (0.03 millimeter thick), is mounted on a glass microscope slide and ground until it is thin enough for visible light to pass through. Polarized light is passed through the thin section and the geologist notes how the light is affected by passing through different minerals.

Some minerals, particularly those containing valuable metals and sulfur, are opaque to visible light no matter how thin they are ground. These minerals are studied by reflected light microscopy. The thin section is polished until a very smooth surface is obtained. Then, light from above is reflected through the microscope for observation. Reflected light microscopy is often combined with standard petrographic microscopy. Together, these techniques are useful for examining the boundaries between the various minerals present in the sample, and for determining the order in which they formed.

Additional microscopic techniques are often used to supplement these studies. Epifluorescence microscopy involves illuminating the sample with violet or ultraviolet (“black”) light. Under these conditions, certain minerals fluoresce, like the colors on a black-light poster, and they can be easily identified through a microscope. In cathode luminescence microscopy, the sample is put in a vacuum chamber and bombarded with an electron beam. The beam excites certain minerals, causing them to emit visible light. Fluid-inclusion microthermometry involves putting thin chips of the sample into a special heating-cooling microscope stage. The sample is then heated with hot nitrogen gas or cooled with liquid nitrogen. Tiny bubbles of water trapped in the minerals are observed as they freeze, thaw, and expand as the temperature changes. This technique yields information about the temperature and chemical composition of the hydrothermal fluids from which the minerals formed.

Geochemical Study

Geochemistry is the study of the chemical characteristics of rocks. Economic geologists use several geochemical techniques, including the study of isotopes. An isotope is an atom of an element that contains a greater or lesser number of neutrons than are usually present in that element. Determining the amounts of the various isotopes of an element present in the sample can yield significant information about the origin of the deposit. Radioisotopes, those isotopes involved in radioactive decay, can be used to determine the age of a deposit; radioisotopes of lead minerals can be used to determine the source of lead that has been concentrated in the deposit as well. Isotopes of sulfur (present in most sedimentary mineral deposits) can be useful in determining the source of the sulfur and the temperature and pressure under which the deposit formed. Isotopes of hydrogen and oxygen can yield information about the source of water in hydrothermal fluids (seawater, rainwater, and water released by the melting of rocks) as well as the temperature at which the deposit formed.

Continuing Value

During the last century, exploitation of the earth’s mineral wealth has expanded at unprecedented rates as advances in technology have led to improvements in mining and extraction methods and the development of new and more expansive uses of minerals and other natural resources. Sedimentary mineral deposits have been, and continue to be, the source of a major portion of the world’s mineral commodities. At one time or another, the largest lead, zinc, gold, uranium, barite, boron, and iron mines in the world have exploited deposits in sedimentary rocks. For these reasons, sedimentary mineral deposits have had a profound effect on the life of every person in the developed world. As technology continues to advance, and as the number of people with access to these resources increases, the demands on Earth’s mineral deposits will grow as well.

Principal Terms

deposition: the physical or chemical process by which sedimentary grains come to rest after being eroded and transported

diagenesis: changes that occur in sediments and sedimentary rocks after deposition caused by interaction during burial with water trapped between the sediment grains

evaporite: a mineral formed by direct precipitation from water resulting from supersaturation caused by solar evaporation in an arid setting

hydrothermal: characterizing any process involving hot groundwater or minerals formed by such processes

midocean ridge: a large, undersea chain of volcanic mountains encircling the globe, branches of which are found in all the world’s oceans

ore: a concentration of valuable minerals rich enough to be profitably mined

placer: an accumulation of valuable minerals formed when grains of the minerals are physically deposited along with other, nonvaluable mineral grains

strata: layers of sedimentary rock

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