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Chapter 21: Resources of Mineralsand Energy


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I ntroduction: Natural Resources AndHuman History (1)

Over one hundred sixty thousand years ago, our

ancestors probably began to use flint, chert, andobsidian to make tools.Metals were first used more than 20,000 years ago.

Copper and gold were the earliest metals used.

By 6000 years ago, our ancestors extracted copper bysmelting.Before another thousand years had passed, they haddiscovered how to smelt lead, tin, zinc, silver, andother metals.


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Mineral Resources (1)

Mineral deposits are any volume of rock containing

an enrichment of one or more minerals.Mineral resources have three distinctivecharacteristics:

Occurrences of usable minerals are limited in

abundance and localized at places within the Earthscrust.The quantity of a given mineral available in any onecountry is rarely known with accuracy.Deposits of minerals are depleted by mining andeventually exhausted.


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F igure 21.1


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F igure 21.2


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Ore is an aggregate of minerals from which one or

more minerals can be extracted profitably.Ore is an economic term, whereas mineraldeposit is a geologic term.The economic challenges of ore are to find it, mineit, and refine it as cheaply as possible.The lowest-grade ores ever minedabout 0.5

percent copperwere worked only at a time of high

metal prices.


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I n 2002, lowest grade of of mineable copper ore is closer to 1 percent.

Over production of copper around the world,combined with economic recession, has resultedin the closing of many mines, particularly thoseexploiting the lowest grades of ores.


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S phalerite, galena, and chalcopyrite are ore minerals

from which zinc, lead, and copper respectively can be extracted.Ore minerals rarely occur alone.

They are mixed with other nonvaluable minerals,collectively termed gangue .

G angue may include quartz, feldspar, mica, calcite, or dolomite.


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O rigin O f Mineral Deposits (1)

All ores are mineral deposits because each of them

is a local enrichment of one or more minerals or mineraloids.

Not all minerals deposits are ores.I n order for a deposit to form, processes must bringabout a localized enrichment of one or moreminerals.


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Minerals become concentrated in five ways:

1. Concentration by hot, aqueous solutions flowingthrough fractures and pore spaces in crustal rock to formhydrothermal mineral deposits.2. Concentration by magmatic processes within a body of

igneous rock to form magmatic mineral deposits .


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3. Concentration by precipitation from lake water or sea

water to form sedimentary mineral deposits.4. Concentration by flowing surface water in streams or along the shore, to form placers.5. Concentration by weathering processes to formresidual mineral deposits.


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Hydrothermal Mineral Deposits (1)

S ome solutions originate when water dissolved in

magma is released as the magma rises and cools.Other solutions are formed from rainwater or seawater that circulates deep in the crust.Mineral deposits formed from midocean ridgevolcanism are called v olcanogenic massi v e sulfidedeposits.


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F igure 21.3


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The pyroxene-rich rocks of the oceanic crust yield

solutions charged with copper and zinc.As a result, volcanogenic massive sulfide deposits arerich in copper and zinc.

I n black smokers, the rising hydrothermal fluidappears black due to fine particles of iron sulfideand other minerals precipitated from solution as the

plume is cooled by contact with cold seawater.The chimney-like structure is composed of pyrite,chalcopyrite, and other ore minerals deposited by

hydrothermal solution.


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When a hydrothermal solution moves slowly

upward, as with groundwater percolating through anaquifer, the solution cools very slowly.I f dissolved minerals were precipitated from such aslow-moving solution, they would be spread over alarge volume of rock and would not be sufficientlyconcentrated to form an ore.


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When a solution flows rapidly, as in an open

fracture, or through a mass of shattered rocks, or through a layer of porous tephra where flow is lessrestricted, cooling can be sudden and can occur over short distances.

Rapid precipitation and a concentrated mineral depositare the result.

Veins formed when hydrothermal solutions depositminerals in open fractures.

Many such veins are found in regions of volcanic

activity.


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F igure 21.5


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The famous gold deposits at Cripple Creek,

Colorado, were formed in fractures associated witha small caldera.The huge tin and silver deposits in Bolivia are infractures that are localized in and aroundstratovolcanoes.Many famous ore bodies are associated withintrusive igneous rocks.

Tin in Cornwall, England,Copper at Butte, Montana, Bingham, Utah, and Bisbee,Arizona.


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F igure 21B1


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F igure 21B2


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Magmatic Mineral Deposits (1)

The processes of partial melting and fractional

crystallization are two ways of separating someminerals from other.The processes involved are entirely magmatic, andso such deposits are referred to as magmaticmineral deposits .


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Pegmatites formed by fractional crystallization of

granitic magma commonly contain richconcentrations of such elements as:

Lithium.Beryllium.Cesium.

Niobium.


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Much of the worlds lithium is mined from

pegmatites such as those at Kings Mountain, NorthCarolina, and Bikita in Zimbabwe.The great Tanco pegmatite in Manitoba, Canada,

produces much of the worlds cesium, and pegmatites in many countries yield beryl, one of themain ore minerals of beryllium.


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Crystal settling , another process of fractional

crystallization, is especially important in low-viscosity basaltic magma.One of the first minerals to form is chromite, themain ore mineral of chromium.

The dense chromite crystals settle to the bottom of the magma, producing almost pure layers of chromite.

The worlds principal deposits of chromite are in theBushveld igneous complex in S outh Africa and the G reatDike of Zimbabwe.


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S edimentary Mineral Deposits

The term sedimentary mineral deposits is applied

to any local concentration of minerals formedthrough processes of sedimentation .One form of sedimentation is the precipitation of substances carried in solution.

There are three types of sedimentary mineraldeposits:E v aporite deposits.I ron deposits.S

tratabound deposits.


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E v aporite Deposits (1)

Evaporite deposits are formed by evaporation of

lake water or seawater.The layers of salts precipitate as a consequence of evaporation.

S alts that precipitate from lake water of suitable

composition include sodium carbonate (Na 2CO 3), sodiumsulfate (Na 2S O4), and borax (Na 2B4O7.1OH 2O).


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Huge evaporite deposits of sodium carbonate were

laid down in the G reen River basin of Wyomingduring the Eocene Epoch.

Oil shales were also deposited in the basin.

Borax and other boron-containing minerals aremined from evaporite lake deposits in Death Valleyand S earled and Borax Lakes, all in California; andin Argentina, Bolivia, Turkey, and China.


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Much more common and important than lake water

evaporites are the marine evaporites formed byevaporation of seawater.The most important salts that precipitate fromseawater are:

G ypsum (Ca S O4.2H 2O).Halite (NaCl ).Carnallite (KCl.MgCl 2.6H 2O).


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Low-grade metamorphism of marine evaporite

deposits causes another important mineral, sylvite(KCl ), to form from carnallite.Marine evaporite deposits are widespread.

I n North America, for example, strata of marine

evaporites underlie as much as 30 percent of the landarea.


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Marine evaporites produce:

Most of the salt that we use.The gypsum used for plaster.The potassium used in plants fertilizers.


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F igure 21.6


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I ron Deposits (1)

S edimentary deposits of iron minerals are

widespread, but the amount of iron in averageseawater is so small that such deposits cannot haveformed from seawater that is the same as todaysseawater.


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I ron Deposits (2)

All sedimentary iron deposits are tiny by

comparison with the class of deposits characterized by the Lake S uperior-type iron deposits.These remarkable deposits, mined principally inMichigan and Minnesota, w ere long the mainstay

of the U.S

. steel industry.They are declining in importance todaybecauseimported ore is replacing them.They are of early Proterozoic age (about 2 billionyears or older ).


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I ron Deposits (3)

They are found in sedimentary basins on every craton(Labrador, Venezuela, Brazil, Russia, I ndia, S outhAfrica, and Australia ).They appear to be the product of chemical precipitation.They are interbedded layers of chert and several differentkinds of iron minerals.

The cause of precipitation remains uncertain.


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I ron Deposits (4)

Many experts suspect these evaporites formed from

seawater of a different composition than todaysseawater.The grade of the deposits ranges from 15 to 30

percent Fe by weight.


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I ron Deposits (5)

Two additional processes can form iron ore:

First, leaching of silica during weathering can lead tosecondary enrichment and can produce ores containing asmuch as 66 percent Fe.The second way a Lake S uperior-type iron can become an oreis through metamorphism.

First, grain sizes increase so that separating ore minerals from thegangue becomes easier and cheaper.

S econd, new mineral assemblages form, and iron silicate and ironcarbonate minerals originally present can be replaced bymagnetite or hematite, both of which are desirable ore minerals.


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F igure 21.7


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I ron Deposits (5)

Ore grade is not increase by metamorphism,

The changes in grain size and mineralogy transform thesedimentary rock into an ore.

I ron ores formed as a result of metamorphism arecalled taconites , and they are now the main kind of

ore mined in Lake S uperior region.


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S tratabound Deposits (1)

S ome of the worlds most important ores of lead,

zinc, and copper occur in sedimentary rock;The ore mineralsgalena, sphalerite, chalcopyrite,and pyriteoccur in such regular, fine layers thatthey look like sediments.

The sulfide mineral layers are enclosed by and parallel to the sedimentary strata in which theyoccur.

For this reason, they are called stratabound mineraldeposits.


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F igure 21.8


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S tratabound Deposits (2)

Most stratabound deposits are diagenetic in origin.

S tratabound deposits form when a hydrothermalsolution invades and reacts with a muddy sediment.The famous copper deposits of Zambia, in central Africa,are stratabound deposits.The worlds largest and richest lead and zinc deposits arealso stratabound:

Broken Hill, Australia. Mount Isa in Australia. Kimberley in British Columbia.


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Placers (1)

A mineral with a high specific gravity will become

concentrated by flowing water.Deposits of minerals having high specific gravitiesare placers .Most placers are found in stream gravels that aregeologically young.


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F igure 21.9


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F igure 21.10


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Placers (2)

The most important minerals concentrated in placers

are gold, platinum, cassiterite ( S nO 2), and diamond.More than half of the gold recovered throughout allof human history has come from placers.


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Placers (3)

The S outh African fossil placers are a series of gold-bearingconglomerates.

They were laid down 2.7 billion years ago as gravels in theshallow marginal waters of a marine basin.Associated with the gold are grains of pyrite and uraniumminerals.

Nothing like the deposits in the Witwatersrand basin has beendiscovered anywhere else.

Mining the Witwatersrand basin has reached a depth of 3600 m(11,800 ft ).

The deposits are running out of ore.


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Residual Mineral Deposits (1)

Chemical weathering leads to mineral concentration

through the removal of soluble materials and theconcentration of a less soluble residue.A common example of a deposit formed throughresidual concentration is bauxite.


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Bauxites are:

The source of the worlds aluminum.Concentrated in the tropics because that is wherelateritic weathering occurs.Found in present-day temperate conditions, such asFrance, China, Hungary, and Arkansas, where theclimate was tropical when the bauxites formed.

Not found in glacial regions. G laciers scrape off the soft surface materials.


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More than 90 percent of all known bauxite deposits

formed during the last 60 million years,All of the very large bauxite deposits formed lessthan 25 million years ago.


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Many of the worlds manganese deposits have beenformed by secondary enrichment of low-grade

primary deposits, particularly in tropical regions.S econdary enrichment zones are produced bydeposition of soluble minerals near the groundwater table, leached from mineral deposits present near the surface.One of the largest nickel deposits ever found, in

New Caledonia, was formed by secondaryenrichment.


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S econdary enrichment has led to large deposits in

the arid southwestern UnitedS

tates and desertregions of northern Chile of:Pyrite (Fe S 2).Chalcopyrite (CuFe S 2).

Chalcocite (Cu S 2).


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U seful Mineral S ubstances (1)

Excluding substances used for energy, there are two

broad groups of useful minerals:Metallic minerals, from which metals such as iron,copper, and gold can be recovered.

Nonmetallic minerals, such as salts, gypsum, and clay.


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G eochemically abundant metals include:

I ron.Aluminum.Manganese.Magnesium.

Titanium.


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Geochemically scarce metals represent less

than 0.1 percent by weight of the crust.They are present exclusi v ely as a result of atomicsubstitution.

Atoms of the scarce metals (such as nickel,

cobalt, and copper) can readily substitute formore common atoms (such as magnesium andcalcium).


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Figure 21.12


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Energy Resources (2)

Most energy used by humans is drawn annually

from major fuels:Coal.Oil.

Natural gas.

Nuclear power.Wood and animal dung.


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F ossil F uels (1)

The term fossil fuels refers to the remains of plants

and animals trapped in sediment that can be used for fuel.The kind of sediment, the kind of organic matter,and the processes that take place as a result of burial

and diagenesis, determine the kind of fossil fuel thatforms.


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F ossil F uels (2)

I n the ocean, microscopic phytoplankton and

bacteria are the principal sources of trapped organicmatter that are transformed (mainly by heat ) to oiland gas.On land, trees, bushes, and grasses contribute most

of the trapped organic matter, forming coal rather than oil or natural gas.


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F ossil F uels (3)

I n many marine and lakes shales, burial

temperatures never reach the levels at which theoriginal organic molecules are converted into oiland natural gas.

I nstead, an alteration process occurs in which wax-like

substances containing large molecules are formed.This material, which remains solid, is called kerogen ,and it is the substance in so-called oil shale.


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Coal (1)

Coal is the most abundant fossil fuel.

I t is the raw material for nylon, many other plastics,and a multitude of other organic chemicals.

Through coalification, p eat is converted to lignite,subbituminous coal, and bituminous coal.

Anthracite is a metamorphic rock.


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F igure 21.13


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Coal (2)

A coal seam is a flat, lens-shaped body having the

same surface area as the swamp in which itoriginally accumulated.Coal seams are found in Utah, Montana, Wyoming,and the Dakotas.

Peat formation has been widespread and more or less continuous from the time land plants firstappeared about 450 million years ago, during theS ilurian Period.


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Coal (3)

The greatest period of coal swamp formation

occurred during the Carboniferous and Permian periods, when Pangaea existed.These periods produced the great coal bed of Europe andthe eastern United S tates.

The second great period of coal deposition peakedduring the Cretaceous period but commenced in theearly Jurassic and continued until the mid-Tertiary.


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Petroleum: O il and Natural Gas

The major use of oil really started about 1847, when amerchant in Pittsburgh, Pennsylvania, started bottling andselling rock oil as a lubricant.

I n 1852, a Canadian chemist discovered kerosene, aliquid that could be used in lamps.I n Romania in 1856, workers were producing 2000

barrels a year.I n 1859, the first oil well was drilled in TitusvillePennsylvania;

Modern use of gas started in the early seventeenth century inEurope, where gas made from wood and coal was used for

illumination.


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O rigin of Petroleum (1)

Petroleum is a product of the decomposition of organic matter trapped in sediment.

Nearly 60 percent of all the oil and gas discoveredso far has been found in strata of Cenozoic age.Petroleum migration is analogous to groundwater migration. When oil and gas are squeezed out of theshale in which they originated and enter a body asandstone or limestone, they can migrate easily.Because it is lighter than water, the oil tends to glideupward, until it encounters a trap.


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F igure 21.14


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F igure 21.15


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F igure 21.16


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Tars

Tar is made of oil that is exceedingly viscous;

The largest known occurrence of tar sand is in Alberta,Canada, where the Athabasca Tar S and covers an area of 5000 km 2 and reaches a thickness of 60 m.S imilar deposits, almost as large, are known in Venezuelaand in Russia.


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O il S hale

The worlds largest deposit of rich oil shale is in

Colorado, Wyoming, andU

tah.O nly oil shale that produces 40 liters of oil perton are worth mining.The richest shales in the U .S . are in Colorado:

they produce as much as 240 liters of oil per ton.Production expenses today make exploitation of oil shales in all countries unattracti v e bycomparison to oil and gas.


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O ther S ources of Energy (1)

Biomass energy:

Wood and animal dung.Hydroelectric power. Nuclear energy .

Heat energy is produced during controlled

transformation (fission) of suitable radioactiv

eisotopes.Three of the radioacti v e atoms that keep the Earth

hot by spontaneous decay 238 U , 235 U , and 232 Thcanbe mined and used to obtain nuclear energy.


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G eothermal power.G eothermal power is produced by tapping the Earths internalheat flux (Zealand, I taly, Iceland and the United S tates ).

Energy from winds, waves, tides, and sunlight:Winds and waves are both secondary expressions of solar energy.Winds have been used as an energy source for thousands of years through sails on ships and windmills.S teady surface winds have only about 10 percent of the energythe human race now uses.


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Tides arise from the gravitational forces exerted on theEarth by the Moon and the S un.

If a dam is put across the mouth of a bay so that water can be trapped at high tide, the outward flowing water at lowtide can drive a turbine.


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Consumption Rates

I n North America, each person uses approximately

20 tons of crushed rock, cement, sand and gravel,fertilizer, oil, gas, coal, metals, and other commodities per year.For the world as a whole, the consumption rate is

about 9 tons per person per year.A bout 54 billion tons of material is dug up and usedeach year.


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F igure 21 20

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