Encyclopedia of Global Resources part 65 pdf

See Geothermal and hydrothermal energy Hydrothermal solutions and mineralization Categories: Geological processes and formations; mineral and other nonliving resources Hydrothermal solut

Trang 1

U.S Geological Survey

Water Science for Schools: What Is Hydrology and

What Do Hydrologists Do?

http://ga.water.usgs.gov/edu/hydrology.html

See also: Aquifers; Atmosphere; Biodiversity;

Geo-chemical cycles; Glaciation; Groundwater; Lakes;

Oceans; Streams and rivers; Water pollution and water

pollution control; Water rights

Hydroponics

Categories: Scientific disciplines; environment,

conservation, and resource management

The term “hydroponics” literally means water culture

and originally referred to the growth of plants in a

liq-uid medium It later applied to all systems used to grow

plants in nutrient solutions with or without the

addi-tion of inert material (synthetic soil) for mechanical

support.

Background

The ability to produce food and fiber for an

ever-growing population is the most fundamental of all

re-sources, and hydroponics has become an important

method of crop production The increase in the

num-ber of commercial greenhouse operations has

re-sulted in a tremendous increase in the use of

hydro-ponic systems Greenhouses are utilized in the

production of a wide array of bedding plants, flowers,

trees, and shrubs for commercial as well as for home

and garden use Cash receipts from greenhouse and

nursery crops total billions of dollars annually In

some arid regions, the vast majority of vegetable crops

are produced in greenhouses

Types of Hydroponic Systems

The four most commonly used hydroponic systems

are sand-culture systems, aggregate systems, nutrient

film techniques, and floating systems While these

sys-tems are similar in their use of nutrient solutions, they

vary in both the presence and type of supporting

me-dium and in the frequency of nutrient application In

sand culture, coarse sand is used in containers or

spread over an entire greenhouse floor or bed on top

of a recirculating drain system A drip irrigation

sys-tem is used to apply nutrient solution periodically,

and a drainage system is used to collect the excess solution as it drains through the sand In an aggre-gate open system, plants are transplanted into plastic troughs filled with an inert supporting material, and nutrient solution is supplied via drip irrigation The aggregate and sand culture systems are open systems because the nutrient solution is not recycled In the nutrient film technique, there is an absence of support-ing material Seedlsupport-ings are transplanted into troughs through which the nutrient solution is channeled, and the plants are in direct contact with the nutrient solution In this closed system, the nutrient solution is channeled past the plant, collected, and reused The floating hydroponic system involves the floating of plants over a pool of nutrient solution

While the nutrient film technique and floating hy-droponic systems are primarily used in research ap-plications, the sand culture and aggregate systems are commonly used in commercial plant production These two systems require the use of a nutrient solu-tion and synthetic soil for mechanical support Al-though a variety of nutrient solutions have been for-mulated, one of the earliest was developed in 1950, and this solution and slight modifications of it remain popular Beginning in 1950, other nutrient solutions with different concentrations of chemical salts were developed, but the elemental ratios remained similar

to the original solution

Materials Used for Mechanical Support

A large variety of both organic and inorganic materi-als have been used to formulate the synthetic soils used for mechanical support in hydroponic systems Commonly used organic materials include sphagnum moss, peat, manures, wood, and other plant residues Sphagnum moss, the shredded, dehydrated remains

of several species of moss in the genus Sphagnum, is

specifically harvested for the purpose of producing synthetic soil “Peat” is a term normally used to de-scribe partially decomposed remains of wetlands veg-etation that has been preserved under water Moss peat is the only type of peat suitable for synthetic soil mixes Moss peat is harvested from peat bogs, dried, compressed into bales, and sold Animal manures are almost never used in commercial synthetic soil mix-tures because they require costly handling and steril-ization procedures Wood residues such as tree bark, wood chips, shavings, and sawdust are generally pro-duced as by-products of the timber industry A variety

of other plant residues, including corn cobs,

Trang 2

cane stems, straw, and peanut and rice hulls, have

been substituted for peat in synthetic soil mixtures in

localities where there is sufficient supply of these

ma-terials

Commonly used inorganic materials include

ver-miculite, sand, pumice, perlite, cinders, and calcined

clay Vermiculite is a very lightweight material

pro-duced by heating mica to temperatures above 1,090°

Celsius Sand is one of the most preferred materials

for formulating synthetic soils because it is both inert

and inexpensive, but it is heavier than other

com-monly used materials Pumice, a natural glasslike

ma-terial produced by volcanic action, provides a good

inert supporting material when ground into small

particles Perlite, a porous material that will hold

three to four times its weight in water, is produced by

heating lava at temperatures above 760° Celsius

Cin-ders are derived from coal residues that have been

thoroughly rinsed to remove harmful sulfates

Cal-cined clay is derived from the mineral

montmorillo-nite baked at temperatures above 100° Celsius

Future Use of Hydroponics The use of hydroponics will increase in the future as the population continues to grow and as more and more farmland is converted to urban use Modern greenhouses can be constructed almost anywhere—

on land that is unsuitable for agriculture and wildlife and on the tops of buildings in metropolitan areas Improved technology will result in the development

of better hydroponic systems as well as an increase in the economic feasibility of greenhouse production

D R Gossett

Further Reading

Brady, Nyle C., and Ray R Weil The Nature and

Prop-erties of Soils 14th ed Upper Saddle River, N.J.:

Prentice Hall, 2008

Bridgewood, Les Hydroponics: Soilless Gardening

Ex-plained Marlborough, England: Crowood Press,

2003

Janick, Jules Horticultural Science 4th ed New York:

W H Freeman, 1986

A hydroponic farmer displays a head of lettuce grown in an Illinois greenhouse (AP/Wide World Photos)

Trang 3

Jones, J Benton, Jr A Guide for the Hydroponic and Soilless

Culture Grower Portland, Oreg.: Timber Press, 1983.

_ Hydroponics: A Practical Guide for the Soilless

Grower 2d ed Boca Raton, Fla.: CRC Press, 2005.

Resh, Howard M Hydroponic Food Production: A

Defini-tive Guidebook of Soilless Food-Growing Methods, for the

Professional and Commercial Grower and the Advanced

Home Hydroponics Gardener 6th ed Santa Barbara,

Calif.: Woodbridge Press, 2001

Web Site

U.S Department of Agriculture

Perlite and Hydroponics: Possible Substitute for

Methyl Bromide?

http://www.ars.usda.gov/is/np/mba/apr99/

perlite.htm

See also: Horticulture; Monoculture agriculture;

Plant domestication and breeding; Soil

Hydrothermal energy See

Geothermal and hydrothermal

energy

Hydrothermal solutions and

mineralization

Categories: Geological processes and formations;

mineral and other nonliving resources

Hydrothermal solutions are “hot-water” solutions rich

in base metals and other ions that create deposits of

minerals Most hydrothermal solutions are

exhala-tions from magmas, but some hydrothermal deposits

have no identifiable magma source Hydrothermal

processes are responsible for the major part of the

world’s base metals upon which modern society is so

de-pendent They have given rise to many of the great

min-ing districts of the world.

Background

Essential conditions for the formation of

hydrother-mal mineral deposits include metal-bearing

mineral-izing solutions, openings in rocks through which the

solutions are channeled, sites for deposition, and

chemical reaction resulting in deposition The term

“ore” is used for any assemblage of minerals that can

be mined for a profit “Gangue” is the nonvaluable mineral that occurs with the ore

During the crystallization of igneous rocks, water and other volatile fluids concentrate in the upper part

of the magma These volatiles carry with them varying amounts of the ions from the melt, including high concentrations of ions that are not readily incorpo-rated into silicate rock-forming minerals If the vapor pressure in the magma exceeds the confining pres-sure of the enclosing rocks, the fluids are expelled to migrate though surrounding country rock These so-lutions travel along natural pathways in the rock such

as faults, fissures, or bedding planes in stratified rocks

As the solutions migrate away from their source re-gion, they lose their mineral content through deposi-tion in natural openings in the host rock (forming open space-filling deposits) or by chemical reaction with the host rock (forming metasomatic replace-ment deposits) A part of these solutions may make it

to the surface to form fumaroles (gas emanations) or hot springs In addition, some hydrothermal solu-tions may be derived from water trapped in ancient sediments or by dehydration of water-bearing miner-als during metamorphism

The observed volatiles from magmas, as seen dur-ing volcanic eruptions and at fumaroles, are 80 per-cent water Carbon dioxide, hydrogen sulfide, sulfur, and sulfur dioxide are also abundant Nitrogen, chlo-rine, fluochlo-rine, boron, and other elements are present

in smaller amounts In addition, metal ions are car-ried in this residual fluid Especially abundant are the base metals—iron, tungsten, copper, lead, zinc, mo-lybdenum, silver, and gold Quartz is the most com-mon nonore, or gangue, mineral deposited Calcite, fluorite, and barite are also common as gangue erals Base metals combined with sulfur as sulfide min-erals, with arsenic as arsenides, or with tellurium as tellurides form the most common ore minerals Gold often occurs as a native mineral

Nature of Open Spaces Hydrothermal solutions find ready-made escape routes through the surrounding country rock in the form of faults and fissures Ore and gangue minerals

of cavity-filling deposits are found in faults or fissures (veins), in open spaces in fault breccias, in solution openings of soluble rocks, in pore spaces between the grain of sedimentary rocks, in vesicles of buried lava

Trang 4

flows, and along permeable bedding planes of

sedi-mentary strata The shape of the mineral deposit is

controlled by the configuration of structures

control-ling porosity and permeability Fracture patterns, and

therefore veins, may take on a wide variety of

geomet-ric patterns, ranging from tabular to rod-shaped or

blanketlike deposits

Some deposits are characterized by ore minerals

that are widely disseminated in small amounts

throughout a large body of rock such as an igneous

stock These igneous bodies undergo intense

fractur-ing durfractur-ing the late stage of consolidation, and

resid-ual fluids permeate the fractured rock to produce

massive deposits of low-grade ores In such deposits,

the entire rock is extracted in mining operations The

famous porphyry copper deposits of the

southwest-ern United States—including those of Santa Rita,

New Mexico; Morence, Arizona; and Bingham, Utah—

are of this type, as are the molybdenum deposits of

Cli-max, Colorado

Metasomatic Replacement

Some hydrothermal deposits are emplaced by

reac-tion of the fluids with chemically susceptible rocks

such as limestone or dolostone Metasomatic

replace-ment is defined as simultaneous capillary solution

and deposition by which the host is replaced by ore

and gangue minerals These massive deposits or lodes

take on the shape and the original textures of the

host Replacement is especially important in

deep-seated deposits where open spaces are scarce

Re-placement deposits of lead-zinc are common in

lime-stones surrounding the porphyry copper of Santa

Rita, New Mexico, and at Pioche, Nevada

Classification by Temperature and Depth

Veins are zoned, with higher-temperature minerals

deposited near the source and lower-temperature

min-erals farther away Hypothermal or high-temperature

and high-pressure mineral assemblages include the

minerals cassiterite (tin), scheelite and wolframite

(tungsten), millerite (nickel), and molybdenite

(mo-lybdenum), associated with gangue minerals quartz,

tourmaline, topaz, and other silicates The mineral

deposits of Broken Hill, Australia, the tin deposits

of Cornwall, England, and Potosí, Bolivia, and the

gold of the Homestake Mine, South Dakota, are

hypo-thermal

Mesothermal, or moderate-temperature and

mod-erate-pressure deposits consist of pyrite (iron sulfide),

bornite, chalocite, chalcopyrite and enargite (cop-per), galena (lead), sphalerite (zinc), and cobaltite or smaltite (cobalt) Gangue minerals include calcite, quartz, siderite, and rhodochrosite The zinc-lead-silver replacement deposits of Leadville, Park City, and Aspen, Colorado, and the Coeur d’Alene, Idaho, lead veins are mesothermal

Epithermal or low-temperature, near-surface de-posits are often associated with regions of recent vol-canism The ore is characterized by stibnite (anti-mony), cinnabar (mercury), native silver and silver sulfides, gold telluride, native gold, sphalerite, and ga-lena Gangue minerals include barite, fluorite, chal-cedony, opal, calcite, and aragonite The extensive silver-gold mineralization of the San Juan Mountains

of Colorado, including Cripple Creek, Ouray, and Creede, are epithermal deposits

Telethermal deposits are formed by hydrothermal solutions that have cooled to approximately the same temperature as the near-surface rocks These solu-tions may originate as mobilized connate and deeply circulating meteoric waters rather than fluids expelled from magma The principal ore minerals are sphalerite and galena, with gangue minerals marcasite, fluorite, calcite, and chalcopyrite The Mississippi Valley-type deposits of the tristate district of Missouri, Kansas, and Oklahoma exemplify this low-temperature mineral-ization

René A De Hon

Further Reading Barnes, Hubert Lloyd “Energetics of Hydrothermal

Ore Deposition.” In Frontiers in Geochemistry:

Or-ganic, Solution, and Ore Deposit Geochemistry, edited

by W G Ernst Columbia, Md.: Bellwether for the Geological Society of America, 2002

_, ed Geochemistry of Hydrothermal Ore Deposits.

3d ed New York: John Wiley & Sons, 1997

Guilbert, John M., and Charles F Park, Jr The Geology of

Ore Deposits Long Grove, Ill.: Waveland Press, 2007.

Pirajno, Franco Hydrothermal Processes and Mineral

Sys-tems London: Springer/Geological Survey of

West-ern Australia, 2009

Thompson, J F H., ed Magmas, Fluids, and Ore

De-posits Nepean, Ont.: Mineralogical Association of

Canada, 1995

See also: Magma crystallization; Open-pit mining; Pegmatites; Secondary enrichment of mineral depos-its; Underground mining

Trang 5

Ickes, Harold

Category: People

Born: March 15, 1874; Frankstown Township,

Pennsylvania

Died: February 3, 1952; Washington, D.C

Ickes, U.S secretary of the interior from 1933 to 1946,

expanded the responsibilities and powers of the

Depart-ment of the Interior in the areas of conservation and

preservation of the nation’s natural resources.

Biographical Background

Harold L Ickes was a lawyer, journalist, and municipal

reformer in Chicago before his appointment as

secre-tary of the interior His selection was political;

Presi-dent Franklin D Roosevelt, a Democrat, was eager

to gain the support of progressive Republicans and

chose Ickes, who quickly became one of the most

pow-erful figures in the nation Always contentious and

ready to battle for his beliefs, Ickes’s enemies and

ad-mirers were legion

Impact on Resource Use

As interior secretary, Ickes administered the Biological

Survey, the Bureau of Fisheries, and the Grazing

Divi-sion Particularly committed to the wilderness ideal, he

added several parks and monuments to the National

Park System and opposed their overdevelopment He

fought to have the Forest Service transferred to the

Department of the Interior but lost; he also failed to

obtain his ultimate dream: to turn the Department of

the Interior into the Department of Conservation

In the enduring struggle within the conservation

movement between preservationists and utilitarian

conservationists, Ickes personified both strains but

leaned toward the former Nevertheless, as head of

the Works Progress Administration (WPA), one of the

New Deal agencies, he supported the building of

dams and other massive public works projects that

re-made the land and provided jobs during the

Depres-sion Still, like few others in American government,

Ickes exemplified the importance of the wilderness to

the human spirit

Eugene Larson

See also: Conservation; Department of the Interior, U.S.; National Park Service; Roosevelt, Franklin D.; Roosevelt, Theodore; Taylor Grazing Act

Igneous processes, rocks, and mineral deposits

Categories: Geological processes and formations; mineral and other nonliving resources

Igneous rocks and mineral deposits, created by the crys-tallization and solidification of magma, are found all over the world Many of the world’s most economically important mineral deposits result, directly or indi-rectly, 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 var-ious sulfurous oxides Familiar examples of igneous rocks are granite (an “intrusive” or “plutonic” rock that is crystallized at depth) and basalt (as in dark “ex-trusive” lava flows, such as those in Hawaii) Igneous rocks are found worldwide on all continents, on oce-anic islands, and on the ocean floors They are partic-ularly common in mountain ranges or other areas where the Earth has undergone tectonic activity Oce-anic islands, such as Hawaii and Iceland, are nearly ex-clusively 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 com-modities obtained from igneous sources include cop-per, nickel, gold, silver, platinum, iron, titanium, tung-sten, and tin Nonmetallic products include crushed stone, construction stones for buildings and monu-ments, and some precious and semiprecious gemstones

Trang 6

Global Resources Igneous processes, rocks, and mineral deposits • 593

Typical Ore Minerals Associated with Igneous Rocks

Metal or Other Commodity Obtained

Felsic—Intermediate

aquamarine)

rare-earth elements Columbite, Tantalite Niobium, tantalium, used in

electronics

Amazonite (microcline feldspar) Gemstone

Sphene (titanite) Titanium, gemstone Muscovite mica Electrical insulation

Mafic—Ultramafic

Labradorite (plagioclase feldspar) Gemstone

Trang 7

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

oc-cur in mountainous areas or ancient “Precambrian

shield” areas where billions of years of erosion reveal

the roots of old mountain ranges In the oceans,

igne-ous 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

pro-duce 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

be-low the surface, the rocks are called “intrusive” or

“plutonic.” Because these rocks generally take a long

time to cool and solidify (a process called

“crystalliza-tion”), 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

result-ing 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

volca-nic 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 concentra-tions of silica (up to about 75 percent silicon dioxide, SiO2) and relatively little iron, magnesium, and cal-cium 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, magne-sium, and calcium Examples of mafic rocks are gab-bro (plutonic) and its volcanic equivalent, basalt Rocks of intermediate composition also exist, for ex-ample plutonic diorite and its volcanic equivalent, an-desite It is andesite (and a more silicic variety called

“dacite”) that is expelled from the potentially explo-sive 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 ex-posed 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

increasing silica increasing iron and magnesium

Extrusive(volcanic)

Intrusive(plutonic)

Felsic

rhyolite granite

Intermediate

dacite/andesite tonalite/diorite

Mafic

basalt gabbro

Ultramafic

peridotite

Simple Classification of Igneous Rocks

Trang 8

trusive granitic magma has invaded horizontal

sedi-mentary 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

bod-ies 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

mate-rials)

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

pla-teau in India, and the Piraná basalt plapla-teau in Brazil

The basalt flows that floor the oceans are underwater

versions of fissure flows

Basaltic volcanoes tend to have low profiles but

lat-erally extensive bases typified by the “shield”

volca-noes of Hawaii and other areas These volcavolca-noes

re-semble 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

“Strom-bolian” 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

moun-tainous proportions Because felsic-intermediate

mag-mas also tend to contain significant dissolved water,

steam trapped during eruption may explode violently, producing thick blankets of volcanic ash near the vol-cano The best North American example of these po-tentially violent volcanoes, called “stratovolcanoes” or

“composite” volcanoes, is the Cascade Range in the Pacific Northwest The terms for these volcanoes re-flect their tendency to have layers of mud and lava flows (generally andesite or dacite) that alternate with pyroclastic ash deposits Stratovolcanoes occur world-wide, particularly at continental margins and in the oceans near continents where “lithospheric plates” (thick horizontal slabs of crust and upper mantle) col-lide, with one plate moving under the other (subduc-tion zones) Volcanism associated with subduc(subduc-tion zones has produced the Andes of South America as well as islands such as Japan, the Philippines, New Zea-land, the Aleutian islands of Alaska, and the islands of Indonesia

Another important volcanic feature is the “rhyolite complex,” or “caldera complex,” exemplified by Yel-lowstone National Park in Wyoming and the Valles Caldera (Jemez Mountains), New Mexico When fully active, these areas produce violently explosive volca-nism and rhyolite lava flows that blanket many square kilometers The most violent activity occurs when the roof of a large underground magma chamber col-lapses 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, vio-lently expelling gas-charged, sticky rhyolite into the atmosphere, from which it may cascade along the

sur-face 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 tem-peratures 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 met-als 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

Trang 9

posits, 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

crys-tallization 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

al-loys), especially at the Questa mine in New Mexico

and at Climax, Colorado

By far the greatest concentration of valuable

min-erals associated with granitic rocks comes from

pegma-tite deposits Like hydrothermal deposits, pegmapegma-tites

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

accom-modate these giant atoms in their mineral structures

Thus, pegmatite deposits may contain relatively high

concentrations of uranium, thorium, lithium,

beryl-lium, 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

min-erals are located in the Black Hills of South Dakota,

Maine, New Hampshire, North Carolina, the

Adiron-dacks of New York state, Pala and Ramona in

Califor-nia, and Bancroft and Wilberforce, Canada Notable

international occurrences are in Brazil (Minas Ge-rais), 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 accumu-late) and, depending on their mineralogical makeup, may constitute important ore bodies Because cumu-lates 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 com-modities recovered from cumulates are the precious metals platinum and palladium, mined in South Af-rica and Russia

Intrusive mafic magmas may also form layers of sulfide-rich minerals called “late-stage immiscible seg-regations” 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” (incapa-ble 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 (Fe1-xS), and pentlandite [(Fe,Ni)9S8] Plat-inum, gold, and silver, among minerals, are com-monly recovered as by-products Major magmatic seg-regation sulfide mines are located in South Africa (Messina and Bushveld districts, Transvaal) and Nor-way, 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

Trang 10

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

lo-cated 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

car-bon dioxide-powered eruption of kimberlite

Dia-monds are mostly mined in South Africa, Ghana, the

Democratic Republic of the Congo, Russia, Brazil,

In-dia, and the United States (Murfreesboro, Arkansas)

Two other deposits with chemical affinities to

kim-berlites are “nepheline syenites” and “carbonatites.”

Like kimberlites, these bodies are rare, and their

mag-mas probably originate deep in the Earth’s mantle

Nepheline syenites contain mostly the mineral

neph-eline (NaAlSiO4) and are sources of apatite

(phos-phate 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

im-portant as sources of the rare elements niobium and

tantalum, used in the electronics industry

John L Berkley

Định dạng
Số trang	10
Dung lượng	168,54 KB