Igneous processes, rocks, and mineral deposits

Igneous rocks and mineral deposits, created by the crystallization and solidification of magma, are found all over the world. Many of the world’s most economically important mineral deposits result, directly or indirectly, from igneous activity.

Background

Igneous rocks are created by the crystallization and solidification of hot, molten silicate magma. magma consists of silicate liquid (the major component is the silica molecule SiO₄^-4), solid crystals, rock fragments, dissolved gases such as carbon dioxide, water, and various sulfurous oxides. Familiar examples of igneous rocks are granite (an “intrusive” or “ plutonic” rock that is crystallized at depth) and basalt (as in dark “extrusive” lava flows, such as those in Hawaii). Igneous rocks are found worldwide on all continents, on oceanic islands, and on the ocean floors. They are particularly common in mountain ranges or other areas where the Earth has undergone tectonic activity. Oceanic islands, such as Hawaii and Iceland, are nearly exclusively igneous in origin, and the world’s oceans are floored by basalt lava flows.

Metallic ores produced by igneous activity may be mined directly from the igneous rocks or obtained through the injection of hydrothermal (hot water) veins into adjacent rocks. Some of the most important commodities obtained from igneous sources include copper, nickel, gold, silver, platinum, iron, titanium, tungsten, and tin. Nonmetallic products include crushed stone, construction stones for buildings and monuments, and some precious and semiprecious gemstones.

Igneous (from the Latin word ignis, meaning fire) rocks form by the crystallization of hot, molten magma produced by the heat of the Earth’s interior. Surface exposures of igneous rock bodies are widespread throughout the globe. On continents they mostly occur in mountainous areas or ancient “ Precambrian shield” areas where billions of years of erosion reveal the roots of old mountain ranges. In the oceans, igneous rocks cover the floors of ocean basins below a thin layer of sediment. Most oceanic islands owe their very existence to ocean floor volcanic eruptions that produce volcanoes of sufficient stature to project above the waves. Familiar examples are the Hawaiian chain, the Galápagos Islands, and Iceland.

Types of Igneous Rocks

Igneous rocks are divided into two major categories defined by their mode of emplacement in or on the Earth’s crust. If molten magma cools and solidifies below the surface, the rocks are called “intrusive” or “plutonic.” Because these rocks generally take a long time to cool and solidify (a process called “crystallization”), their component minerals grow large enough to see with the naked eye (coarse-grained rocks). On the other hand, if magma flows out onto the Earth’s surface, it forms “extrusive” or “volcanic” rock. These rocks lose heat rapidly to air or water, and the resulting rapid crystallization produces tiny, nearly invisible crystals (fine-grained rocks). Some volcanic rocks cool so quickly that few crystals have time to form; these are glassy rocks such as obsidian. Two kinds of volcanic rock exist: lava flows and “pyroclastic” deposits formed by explosive volcanism. Pyroclastic materials (volcanic ash) are deposited as layers of particles that have been violently ejected into the air.

Igneous rocks are also classified according to chemical composition. At one extreme are the light-colored “felsic” rocks that contain high concentrations of silica (up to about 75 percent silicon dioxide, SiO₂) and relatively little iron, magnesium, and calcium. Examples of felsic rocks are granite, a plutonic rock, and its volcanic equivalent, rhyolite (obsidian glass is rapidly cooled rhyolite).

At the other extreme are the dark “mafic” rocks with relatively low silica (as low as about 46 percent SiO₂) but with higher concentrations of iron, magnesium, and calcium. Examples of mafic rocks are gabbro (plutonic) and its volcanic equivalent, basalt. Rocks of intermediate composition also exist, for example plutonic diorite and its volcanic equivalent, andesite. It is andesite (and a more silicic variety called “dacite”) that is expelled from the potentially explosive volcanoes of the Cascade range in the American Pacific Northwest (Mount St. Helens, Mount Rainier, Mount Hood, and others).

Intrusive (Plutonic) Structures

Intrusive igneous rock bodies come in many shapes and sizes. The term “pluton” applies to all intrusive bodies but mainly to granitic rocks (granites, diorites, and related rocks). Specific terms applied to plutons mostly describe the size of the body. “Stocks” are exposed over areas less than 100 square kilometers, whereas “ batholiths” are giant, commonly lens-shaped, bodies that exceed 100 square kilometers in exposed area. The Sierra Nevada range in eastern California is a good example of a batholith.

Some specialized pluton varieties are “laccoliths,” commonly mountainous areas (for example, the Henry and La Salle mountains in Utah) in which intrusive granitic magma has invaded horizontal sedimentary layers and has bowed them up into a broad arch. A “phacolith” is similar to a laccolith only the magma has invaded folded sedimentary rocks so that the pluton itself appears to have been folded.

Minor intrusive bodies include “sills,” tabular bodies intruded parallel to rock layers (a laccolith can be considered a “fat sill”), and “dikes,” tabular bodies that cut across rock layers. Sills and dikes are common features around the margins of plutons where they contact “country rock” (older, pre-intrusion materials).

Another intrusive body, mostly produced by mafic (gabbroic) magmas, is the “lopolith.” Lopoliths are relatively large funnel-shaped bodies (on the order of large stocks or small batholiths) in some cases created where magma fills the down-warped part (syncline) of a fold structure. An excellent example is the Muskox intrusion of northern Canada; another possible one (one limb is unexposed under Lake Superior) is the Duluth gabbro intrusion of northeastern Minnesota.

Extrusive (Volcanic) Structures

The nature of volcanoes and volcanic rock deposits in general is greatly influenced by the composition of their parent magmas. Basalt magma is a low-viscosity liquid (it is thin and flows easily) and thus produces topographically low, broad volcanic features. Typical of these are the “fissure flows” (also known as plateau basalts) in which basalt lava issues from fractures in the Earth and spreads out almost like water in all directions. Examples are the Columbia River basalt plateau in Oregon and Washington, the Deccan plateau in India, and the Piraná basalt plateau in Brazil. The basalt flows that floor the oceans are underwater versions of fissure flows.

Basaltic volcanoes tend to have low profiles but laterally extensive bases typified by the “shield” volcanoes of Hawaii and other areas. These volcanoes resemble giant ancient shields lying on the ground. Pyroclastic eruptions of basalt, powered mostly by the violent release of dissolved carbon dioxide, produce cinder-cone volcanoes, otherwise known as “Strombolian” volcanoes, after the Italian volcano Stromboli.

In contrast to mafic magmas, the more silica-rich felsic and intermediate magmas are more viscous, and thus flow less readily. This magma tends to pile up in one place, producing towering volcanoes of mountainous proportions. Because felsic-intermediate magmas also tend to contain significant dissolved water, steam trapped during eruption may explode violently, producing thick blankets of volcanic ash near the volcano. The best North American example of these potentially violent volcanoes, called “ stratovolcanoes” or “composite” volcanoes, is the Cascade Range in the Pacific Northwest. The terms for these volcanoes reflect their tendency to have layers of mud and lava flows (generally andesite or dacite) that alternate with pyroclastic ash deposits. Stratovolcanoes occur worldwide, particularly at continental margins and in the oceans near continents where “lithospheric plates” (thick horizontal slabs of crust and upper mantle) collide, with one plate moving under the other (subduction zones). Volcanism associated with subduction zones has produced the Andes of South America as well as islands such as Japan, the Philippines, New Zealand, the Aleutian islands of Alaska, and the islands of Indonesia.

Another important volcanic feature is the “rhyolite complex,” or “caldera complex,” exemplified by Yellowstone National Park in Wyoming and the Valles Caldera (Jemez Mountains), New Mexico. When fully active, these areas produce violently explosive volcanism and rhyolite lava flows that blanket many square kilometers. The most violent activity occurs when the roof of a large underground magma chamber collapses into the shallow void created by expulsion of magma during previous eruptions. The crater formed during this process is called a caldera. Roof collapse during caldera formation has the effect of ramming a large piston into the heart of the magma body, violently expelling gas-charged, sticky rhyolite into the atmosphere, from which it may cascade along the surface as a nuée ardente (French for “glowing cloud”). These roiling infernos of hot noxious gases, bubbling lava fragments, and mineral crystals are capable of speeds in excess of 300 kilometers per hour and temperatures in excess of 400° Celsius. They deposit ash blankets (welded ashflow tuffs) over wide regions, as in the case of Yellowstone. Stratovolcanoes (described above) can also form calderas and ashflow deposits, as exemplified at Crater Lake, Oregon.

Ore Deposits of Felsic-intermediate Rock

Granite and related rocks are the source of many metals and other products that are the foundation of an industrial society. Quartz veins intruding granite may contain gold and other precious metals, as in the “mother lode” areas of the Sierra Nevada Range in California. These veins originate as hydrothermal deposits, minerals precipitated from hot-water fluids flowing through fractures in cooling granitic bodies. Felsic and intermediate composition igneous rocks contain significant dissolved water in their magmas (called “juvenile” water), which is finally expelled as hydrothermal fluids in the late stages of plutonic crystallization. Hydrothermal veins occur in the parent granite itself or are injected into the surrounding rocks. Many important metallic ore bodies formed as hydrothermal deposits.

So-called porphyry copper deposits such as those of the American southwest (Arizona, New Mexico, Colorado, and Utah) are low-grade deposits of widely scattered small grains of chalcopyrite (CuFeS₂) and other copper minerals in felsic plutonic and volcanic rocks, mostly residing in a multitude of extremely thin hydrothermal veins. Some porphyry copper deposits also have considerable deposits of molybdenite (in the sulfide molybdenite, used in high-temperature alloys), especially at the Questa mine in New Mexico and at Climax, Colorado.

By far the greatest concentration of valuable minerals associated with granitic rocks comes from pegmatite deposits. Like hydrothermal deposits, pegmatites form in the late stages of granite crystallization after most of the other rock-forming minerals have already crystallized. Another similarity to hydrothermal fluids is their high volatile content—materials that tend to melt or form gases at relatively low temperatures, such as water, carbon dioxide, and the halogens fluorine and chlorine. Elements with large atomic sizes (ionic radii) and valence charges also tend to concentrate in pegmatitic fluids because the majority of minerals in granites (mostly quartz and feldspars) cannot accommodate these giant atoms in their mineral structures. Thus, pegmatite deposits may contain relatively high concentrations of uranium, thorium, lithium, beryllium, boron, niobium, tin, tantalum, and other rare metals. The high water content of pegmatite fluids, some of it occurring as vapor, allows minerals such as quartz, feldspar, and mica to grow to enormous sizes, the largest of which are on the order of railway boxcars. Pegmatites are generally fairly small bodies; some deposits are no larger than a small house. They may also occur as veins or dikes. Excellent North American examples containing rare and exotic minerals are located in the Black Hills of South Dakota, Maine, New Hampshire, North Carolina, the Adirondacks of New York state, Pala and Ramona in California, and Bancroft and Wilberforce, Canada. Notable international occurrences are in Brazil (Minas Gerais), Russia (the Urals and Siberia), Greenland, Italy, Australia, Germany (Saxony), Madagascar, and Sri Lanka.

Ore Deposits in Mafic and Related Rock

Owing to their low viscosity, mafic magmas produce some unique mineral deposits compared with thicker felsic magmas. In plutonic settings formed early, heavy mineral crystals can easily sink through the magma to form crystal-rich layers on the bottom of the magma chamber. These gravitationally deposited layers are called “cumulates” (from the word accumulate) and, depending on their mineralogical makeup, may constitute important ore bodies. Because cumulates are generally enriched in iron and depleted in silica compared with their mafic parent magma, they are termed “ultramafic,” the common rock type being “peridotite,” a rock rich in olivine [(Fe,Mg)₂SiO₄]. Most of the world’s chromium that is used in high-temperature, corrosion-resistant alloys comes from cumulate layers of the mineral chromite (FeCr₂O₄), mostly mined in South Africa. The other major commodities recovered from cumulates are the precious metals platinum and palladium, mined in South Africa and Russia.

Intrusive mafic magmas may also form layers of sulfide-rich minerals called “late-stage immiscible segregations” that constitute some of the richest copper and nickel ore bodies in the world. As some mafic magmas cool and change chemically, sulfur and metal-rich fluids may separate from the silicate liquid, just as oil would from water. These “immiscible” (incapable of mixing) sulfide droplets then sink through the lower density silicate magma to form thick layers of “massive sulfide” deposits on the magma chamber floor. The major minerals in massive sulfide copper-nickel mines are chalcopyrite, bornite (Cu₅FeS₄), pyrrhotite (Fe_1-xS), and pentlandite [(Fe,Ni)₉S₈]. Platinum, gold, and silver, among minerals, are commonly recovered as by-products. Major magmatic segregation sulfide mines are located in South Africa (Messina and Bushveld districts, Transvaal) and Norway, and at Sudbury, Ontario, Canada, which has ore rich in nickel.

Titanium and iron ores may also form as magmatic segregations. Massive titanium ores, mostly the oxide ilmenite (FeTiO₃), are mined from anorthosite rock, a plagioclase [(Ca,Na)AlSi₃O₈] feldspar-rich variation of gabbro. Typical examples of these deposits occur in the titanium mines in the Adirondacks of New York state and at Allard Lake, Quebec. Iron deposits of this type, mostly the mineral magnetite (Fe₃O₄), are located at Kiruna, Sweden; the Ozarks of Missouri; Durango, Mexico; and Algarrobo, Chile.

Other Important Igneous Commodities

Some valuable mineral commodities are recovered from igneous rocks that do not lend themselves to simple classification. For example, diamonds occur in deposits called “ kimberlites,” a type of general deposit called “diatremes,” explosively injected mixtures of mantle (mostly serpentine) and crustal materials that in rare localities contain diamonds. The diamonds form deep in the upper mantle, where pressures are sufficiently high to produce them by the reduction (removal of oxygen) of carbon dioxide. They are then injected into more shallow crustal levels upon the carbon dioxide-powered eruption of kimberlite. Diamonds are mostly mined in South Africa, Ghana, the Democratic Republic of the Congo, Russia, Brazil, India, and the United States (Murfreesboro, Arkansas).

Two other deposits with chemical affinities to kimberlites are “nepheline syenites” and “ carbonatites.” Like kimberlites, these bodies are rare, and their magmas probably originate deep in the Earth’s mantle. Nepheline syenites contain mostly the mineral nepheline (NaAlSiO₄) and are sources of apatite (phosphate mineral) and corundum (Al₂O₃), used as an abrasive. Nepheline itself is used to make ceramics. Carbonatites are unusual igneous deposits in that they are composed mostly of the carbonate mineral calcite (CaCO₃). They have become increasingly important as sources of the rare elements niobium and tantalum, used in the electronics industry.

Bibliography

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U.S. Geological Survey. Igneous Rocks. http://vulcan.wr.usgs.gov/LivingWith/VolcanicPast/Notes/igneous‗rocks.html