Plutonic rocks and mineral deposits
Plutonic rocks are igneous rocks that form through the slow crystallization of molten magma beneath the Earth's surface. They can create significant geological features known as plutons, which vary in size from small stocks to massive batholiths that span thousands of square kilometers. Common types of plutonic rocks include felsic-intermediate rocks, such as granite, and mafic-ultramafic rocks, like gabbro. These rocks are notable not only for their geological significance but also for their role as sources of valuable mineral deposits.
Plutonic rocks are particularly important in the mining industry, providing key resources like gold, copper, molybdenum, and various rare metals. Notably, granitic plutons can yield minerals directly or create placer deposits through erosion, while mafic-ultramafic plutons often contain layered ore deposits formed by the settling of dense minerals. Major locations for these rock formations and their associated mineral deposits can be found across all continents, particularly in ancient shield areas and mountain ranges. Overall, the study of plutonic rocks and their mineral deposits is crucial for understanding Earth's geology and for the extraction of essential resources used in modern industries.
Plutonic rocks and mineral deposits
Plutonic rocks are formed by slow magma crystallization below the Earth’s surface. Erosion can expose plutonic formations containing valuable mineral deposits, including metallic resources vital to industry.
Background
Plutonic rocks (from Pluto, Roman god of the underworld) form by slow crystallization of molten silicatemagma intruded below the Earth’s surface. Subsequent erosion may expose “plutons” that have the area of a large house or huge granitic “batholiths” that encompass thousands of square kilometers. Many plutons are sources of valuable industrial and metallic mineral deposits, including granitebuilding stone, gold, copper, molybdenum, chromium, and other metals. Plutonic bodies occur on all the major continents of the Earth. They are common in Precambrian shield areas, the ancient cores of continents composed of rocks formed billions of years ago, such as the Canadian Shield of North America. They also occur in younger mountain ranges such as the Rockies and the Appalachian Mountains of North America.
Most of the major metallic resources consumed by modern industries—and thus many commercial products—have their origin in plutonic rocks. The rocks themselves may be cut as building stones or used for monuments and art objects. Plutons are also an important source of metal ores, including gold, silver, platinum, chromium, copper, molybdenum, lithium, beryllium, and nickel.
Plutonic rocks crystallize from igneous intrusions of molten magma that cool slowly beneath the Earth’s surface. Plutonic intrusions underlie many volcanic areas where magma has made its way to the surface. Later erosion by water or glacial scouring removes most or all traces of the overlying volcanic material and other overburden to expose the pluton at the surface. Plutonic igneous rocks can be distinguished from volcanic rocks by their grain size: The slowly cooled plutonic rocks allow for relatively large grain sizes (from a few millimeters to several centimeters in diameter) compared with very fine-grained, quickly cooled volcanic rocks.
Types of Plutons and Plutonic Rocks
Major plutonic rock bodies in the world can be divided into two major compositions based mainly on silica (SiO2) content: “felsic-intermediate” rocks, in which silica content ranges from about 52 to more than 70 percent (examples include granite and diorite), and “mafic-ultramafic” rocks, in which silica is much lower, 52 down to about 45 percent SiO2 (examples are gabbro and peridotite).
Light-colored felsic-intermediate, or “granitic,” plutons are the most common type and include relatively small “stocks” (surface exposures of tens of square kilometers) up to immense “batholiths” that cover hundreds or thousands of square kilometers. The largest of these in North America are the Idaho, Boulder (Montana), Sierra Nevada (California), and Southern California batholiths. Many large stocks and batholiths also occur in the Canadian Shield of Canada and the northern states of Minnesota, Wisconsin, and New York. Granitic plutons also occur in every continental shield area (ancient core) of every continent and in most of the world’s major mountain ranges.
Less common are the iron-rich, dark-colored mafic-ultramafic plutons. Most of these bodies consist of relatively iron-rich basaltic magma (like the dark lava flows in Hawaii) that, upon intrusion, crystallize dense minerals that settle to the bottom of the magma chamber to form mineralogically distinct layers. These layered rocks may contain economically important ore minerals. Prominent examples are the Muskox intrusion (Canada), the Skaergaard complex (Greenland), the Stillwater complex (Montana), and the metallic ore-rich intrusions in South Africa (Bushveld, Great Dyke).
Ore Deposits in Felsic-Intermediate Plutons
Granitic plutons are the source of many valuable mineral commodities, either directly from the rock itself or indirectly as placer minerals (for example, gold or cassiterite in stream sediments). Some important sedimentary ore deposits form by deposition of minerals leached from granitic plutons by groundwater (for example, many uranium ores). Ore minerals may also occur in hydrothermal (deposited by hot water) quartz veins as in the case of native gold. These veins may occur within the plutons themselves or penetrate adjacent “country rocks.” Most North American copper mines (principally in New Mexico, Arizona, and Utah) obtain their ore from so-called porphyry copper deposits. These are low-grade deposits (about 0.65 percent copper) of widely disseminated copper sulfide grains within the granitic pluton or in adjacent rocks mined from large open-pit mines. Typical of these mines is the Kennecott Utah Copper mine at Bingham, Utah, the largest copper mine in the world.
Perhaps the richest source of valuable minerals in granitic plutons is pegmatite deposits, generally small exposures of large crystals, the largest measuring many meters in diameter. Pegmatites are the source of many rare metals (beryllium, lithium, zirconium, boron, tantalum, and niobium) and some valuable gemstones.
Ore Deposits in Mafic-Ultramafic Plutons
Because the magma associated with these plutons is low in viscosity (“thin”) compared with felsic-intermediate bodies, many important ore deposits in mafic-ultramafic intrusions arise by gravity settling of minerals. South African chromite deposits, for example, form as thick-layered accumulations of crystallized chromite grains that have settled on the floor of the plutonlike sand falling through thick motor oil. Such ore-rich layers are called “cumulates” (from “accumulation”), and include platinum-palladium deposits commonly associated with chromite layers.
Similar layered ore bodies involve the formation of large blobs of dense sulfide liquids that separate from the silicate liquid and accumulate on the pluton floor. These “segregation” deposits include some of the richest nickel and copper mines in the world, including the nickel mine at Sudbury, Ontario, the Duluth complex in Minnesota, and the Bushveld of South Africa. Gold, silver, and other valuable metals are also mined from these rich deposits. Some iron and titanium deposits were created this way as well but involve the segregation of titanium or iron-rich fluids in mafic plutons, which eventually crystallize the mineral magnetite (Fe3O4; iron ore) or ilmenite (FeTiO3; titanium ore). An example of this type of iron deposit is the Kiruna district in Sweden. Allard Lake, Quebec, is a good example of a segregation titanium mine.
Bibliography
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U.S. Geological Survey.