Mineral Resources Chapter 12 2011 Pearson Education Inc

Mineral Resources Chapter 12 © 2011 Pearson Education, Inc.

You will learn • What makes minerals a resource for people • The roles played by exploration, mining, and processing in providing people with mineral resources • The environmental concerns associated with mineral resource production • How negative environmental impacts can be avoided or mitigated • The future challenges of mineral resource use © 2011 Pearson Education, Inc.

Mineral Use in the U. S. Figure 12 -6 Each year, mineral consumption in the United States amounts to nearly 21, 700 kilograms (47, 800 lb) person. © 2011 Pearson Education, Inc.

Mineral Resources Naturally occurring solid materials in or on Earth’s crust from which we can currently or potentially extract a useful commodity – Example: Lead—Pb—widely distributed in small amounts – Ave. concentration in Earth’s crust is 12. 5 ppm, or 0. 00125% – Cube of ave. crust 100 m/side = 3 million tonnes (6. 6 billion lb) with an ave. Pb content of 0. 00125% • Cube will have over 3. 7 tonnes (8200 lb) of Pb – At 2008 ave. price of $1. 31 per pound – Pb recovered from this cube could equal more than $10, 000, equivalent to approx. 0. 3 cents/ton of crust processed – Difficult to process amounts this low – Pb concentrations 1000 s of times greater than the crustal average are needed if it is to be recovered economically © 2011 Pearson Education, Inc.

Making Mineral Deposits • Ore deposits—mineral deposits with high enough concentrations of valuable elements to allow profitable mining and recovery of a saleable commodity Ore deposits form by: – crystallization processes in igneous rocks – surface processes that concentrate heavy metal -bearing minerals in sediments such as beach sands – chemical precipitation on the seafloor – interaction of water-rich fluids and crustal rocks © 2011 Pearson Education, Inc.

Types of Ore Deposits—Igneous • Hot oceanic crust heats ocean water, which dissolves metals (e. g. , Cu, Pb, Zn) distributed through the crust – Metal-bearing hot water migrates to the ocean floor to form springs – Dissolved metals react with sulfur, create sulfide minerals – Sink downward and accumulate on the seafloor – (e. g. , ore deposits in ancient Cu mines on the island of Cyprus) • Magmas that solidify in the shallow crust may release hot fluids rich in H 2 O and dissolved metals – These fluids precipitate sulfide minerals containing metals such as copper • (e. g. , Bingham Canyon Cu deposit in Utah) © 2011 Pearson Education, Inc.

Figure 12 -8 The Bingham Canyon Mine in Utah © 2011 Pearson Education, Inc.

Types of Ore Deposits— Sedimentary • Sediments deposited on the ocean floor commonly contain seawater in the pores between sediment grains • This salty water becomes part of the sedimentary sequence and can contain dissolved metals, especially Pb and Zn • Mineral deposits on the seafloor form when metalbearing water escapes along permeable pathways (e. g. , faults) and emerges on the seafloor • (e. g. , Pb and Zn deposits at the Red Dog mine in Alaska) © 2011 Pearson Education, Inc.

Types of Ore Deposits— Metamorphism • During metamorphism rocks may dehydrate and release H 2 O-rich fluids containing Au and other metals • Metal-bearing waters can escape to shallower crustal levels—and localize along permeable structures • Precipitate quartz and disseminated Au • (e. g. , California Mother Lode region) Figure 12 -10 A Gold-Bearing Vein in the Mother Lode Region, CA. (a) This gold-bearing white quartz vein is exposed underground in the Lincoln Mine. (b) Gold in quartz. © 2011 Pearson Education, Inc.

Ore Deposits That Can Form in Stratovolcanoes • Deposits containing Cu, Pb, Zn, Au, and Ag commonly form within or near stratovolcanoes. The Au and Ag deposits tend to be veins • The Pb, Zn, and Cu deposits in carbonate rock tend to be lens-like or podlike in shape • Largest Cu-rich deposits form in the interior of the volcano near the top • Ore deposits that extend deep into Earth’s crust need to be mined by underground methods, and need to Figure 12 -11 be of a relatively higher grade—that is, have a higher concentration of The large copper deposit at the top of the intrusion can be mined by open-pit valuable minerals—to be profitable. methods. Such ore deposits range from hundreds to thousands of meters. © 2011 Pearson Education, Inc.

Finding, Mining, and Processing Mineral Resources • Steps before mining: • More exploration to determine whether site contains an ore deposit • Permission to mine must be obtained from one or more government regulatory agencies • If the deposit can be mined in an environmentally responsible manner, permits will be approved and mining can start • Mining commonly recovers material that needs to be processed to separate minerals containing useful elements • This step is called beneficiation. The separated minerals are then further processed in another step, called metallurgy, to separate useful elements from their mineral hosts © 2011 Pearson Education, Inc.

Finding Mineral Resources— Exploration • Geologists identify areas that may be favorable for mineral deposits • Fieldwork to make more detailed geologic maps • Extensively sample surface materials (e. g. rocks, soils, vegetation) • Surface samples are analyzed by geochemical techniques • Subsurface relations are investigated with geophysical techniques • If a mineral deposit is found by initial exploration efforts it is called a prospect until it is shown to contain ore that can be profitably mined • To determine whether a mineral deposit contains ore: • Combination of trenching/drilling • Significant infrastructure needed to support this stage of exploration © 2011 Pearson Education, Inc.

Mineral Exploration: Drill Rigs Figure 12 -14 Mineral Exploration Drill Rig This drill rig cuts and collects rock chips at a gold prospect in Australia. The white bags are full of rock chips recovered from specific intervals underground. These are the samples that are analyzed to determine the amount of gold that is present. © 2011 Pearson Education, Inc.

Mining Mineral Resources • Mines recover ore using a variety of technically complex and highly mechanized operations, and large facilities are needed to support them • Facilities include roads, buildings, power systems, and water systems • Open-pit mines commonly excavate and process large amounts of rock. • Waste rock from open-pit mines must be processed • These dumps are huge piles of processed rock—classified as toxic waste Figure 12 -17 Open-Pit Mines Produce Huge Quantities of Waste Rock The hill in the background is all waste rock from the Bingham Canyon mine in Utah. (The area in the foreground has already been reclaimed. ) © 2011 Pearson Education, Inc.

Underground Mining • Underground methods are used to mine deposits extending deep into the subsurface. Ore deposits can be accessed in a variety of ways: – Adits = vertical shafts driven into the side of slopes – Declines = inclined from the surface downward – Process much less ore than open-pit mines • Waste rock may be disposed of underground • The Mining Law passed by Congress in 1872 allowed prospectors to obtain the right to exploit the mineral resources of an area by staking a claim—physically marking the corners of the area • Physical disturbances such as prospect pits, trenches, exploration shafts or adits, and small waste rock dumps are common • Environmental consequences can be significant © 2011 Pearson Education, Inc.

Underground Mining (cont. ) Figure 12 -18 (a) This diagram of a zinc mine in Tennessee shows the basic components of an underground mine. (b) Underground mine operations can use large equipment like this scaler, which removes loose materials left on the walls and roof after blasting © 2011 Pearson Education, Inc.

Abandoned Mine Land Figure 12 -19 The waste rock dump at this abandoned mine in Colorado is the gray pile of loose rock at upper left. © 2011 Pearson Education, Inc.

Processing—Mineral Resources • Typical copper ore contains the valuable Cu-bearing sulfide mineral chalcopyrite along with other minerals such as quartz, mica, and the iron-sulfide mineral pyrite • Beneficiation separates and concentrates the valuable minerals. The key steps in this process, milling and flotation, produce a waste material called tailings • Milling grinds the ore into particles the size of silt or fine sand • The objective is to break the ore down into separate mineral grains • The mills that do this are cylinders containing steel balls that grind the ore • Water is mixed with the ore in the mill • Slurry—ground-up ore suspended in water removed for further processing Figure 12 -21 Milling The large cylinders (ball mills) hold steel balls that grind the ore as the mill rotates. © 2011 Pearson Education, Inc.

Flotation • Flotation concentrates valuable minerals —separated from nonvaluable minerals • Vats—contain the slurry from the mill and special bubble-forming reagents • Specific sulfide minerals such as chalcopyrite will selectively adhere to a bubble and float to the surface, where they can be collected • Mineral-bearing bubbles are dewatered/ filtered • Leaving behind a concentrate of valuable minerals • Tailings—minerals left behind in the slurry after flotation (e. g. , quartz, pyrite) © 2011 Pearson Education, Inc.

Tailings Disposal • Tailings are the principal focus of the environmental concerns • Commonly contain large amounts of sulfide minerals (e. g. , pyrite) • Oxidation of these creates acidic conditions—degrade soil and water quality • For large mines like that at Bingham Canyon, Utah, the associated tailings ponds can be huge, over thousands of acres in area and about 100 meters deep. Figure 12 -23 The waste after milling and flotation, called tailings, is pumped to a disposal area (a). The disposal area can be very large and collect pools of water on its surface where the tailings are less permeable (b). © 2011 Pearson Education, Inc.

Heap Leach Operations for the Recovery of Gold • Some metals—are removed from rocks and concentrated by leaching • Ores are piled in large heaps • Specially formulated chemical solutions percolate through them • Dissolving the metals they contain • The metal-bearing solutions are collected from the base of the heap • Processed to recover the metals • Acidic (for Cu) and cyanidebearing (for Au) solutions are restricted from entering groundwater by impermeable liners of synthetic or natural clay Figure 12 -24 Gold ore is being placed in a pile (heap) for leaching at this mine in Ghana. Solutions will be sprinkled on the surface that dissolve gold as they percolate through the heap. The solutions are trapped above an impermeable liner (the black sheet) at the base of the heap, and collected for processing to remove the dissolved gold © 2011 Pearson Education, Inc.

Recovering Metals—from Ore Concentrate Metallurgy removes desired elements from the valuable minerals that are concentrated by beneficiation • Smelting—melting sulfide minerals—common metallurgical technique – Smelters—heat the ore to its melting point – Molten metals sink to the bottom and are removed – Impurities (mostly Fe, Si. O 2) rise to the top; cool to glassy slag – Large dark piles of slag—commonly mark the location of smelters – Because it is glassy, slag is not highly reactive in the environment – Some does contain metals (e. g. , Pb, As) that need proper disposal – Smelters release gases (SO 2) and particulates to the atmosphere – Slag can be used for abrasives, base material for railroads, and even in traps on golf courses © 2011 Pearson Education, Inc.

Environmental Concerns • Physical disturbances – Exploration—trenches and roads – Extraction—open-pit mines—deep excavations—large waste rock dumps – Beneficiation—waste materials (tailings) or smelting (slag) – Underground mining—adits, declines, waste piles • Steps for mitigation—reclamation – Reshaping of the land surface to resist erosion – Covering it with soil – Planting new vegetation to help stabilize the land surface – At mine closure, it is common practice to demolish, salvage, or otherwise remove all mine support facilities • Reclamation is capable of changing large piles of bare rocks, tailings, or slag into stable, vegetated landscapes. – Does not restore the land to its pre-mining conditions © 2011 Pearson Education, Inc.

Environmental Concerns (cont. ) • Surface water quality • Surface and ocean waters can be degraded by: • Accidental spills of toxic chemicals • Erosion of waste materials • Discharge of contaminated water from mines or related facilities. • Spills • Accidental spills of toxic chemicals from storage or processing facilities • Modern facilities are surrounded by berms to contain potential spills • International Cyanide Management Code—best practices include: • Strong, impermeable barriers at the base of the leach pads • Effective collection systems for the leach solutions • Rinsing, physical isolation, detoxification of heap leach pads © 2011 Pearson Education, Inc.

Environmental Concerns (cont. ) • Erosion – Erosion of waste materials can affect surface water quality • Metal-bearing materials can be eroded into bodies of water • Materials react with water and oxygen to release metals – Dissolved metals are more bioavailable to organisms – Current mining operations may not dispose of waste rocks or tailings where they can be eroded into surface bodies of water • Proper disposal requires placement outside active floodplains • Reclamation that stabilizes the waste rock or tailings – Many of these wastes can oxidize—generate acidic soils or waters • Add materials (e. g. , lime) to neutralize acidity • Cover the rock with topsoil to promote vegetation growth – Revegetation and surface contouring to control runoff are additional reclamation steps that will inhibit water infiltration, stabilize slopes, and prevent erosion © 2011 Pearson Education, Inc.

Discharge of Acid Rock Drainage • Water that collects in mines or drains through them can become acidic and contaminated with toxic metals. Where this water is discharged to the surface, it can degrade nearby surface water quality. • This happens especially where the ore deposit is rich in sulfide minerals. The sulfide mineral that has the greatest effect on water quality is pyrite (iron sulfide). • Pyrite (Fe. S 2) oxidizes (with bacteria) to form Fe oxides + sulfuric acid. FIGURE 12 -28 Generation of Acid Rock Drainage Reaction of oxygen with pyrite, catalyzed by certain bacteria, produces sulfuric acid that can mix with and contaminate surface and groundwater. © 2011 Pearson Education, Inc.

Acid Rock Drainage (ARD) • Acid rock drainage (ARD)—acidic water – Produced by the oxidation of pyrite (and other sulfide minerals) – Dissolves metals such as copper, zinc, and silver – Must be properly treated and disposed of • Prevention of ARD, avoiding oxidation of sulfide minerals (e. g. , pyrite) • Disposal of pyrite-bearing wastes in appropriate places – With impermeable materials at their base; inhibit water infiltration • Other prevention techniques include: • Flooding underground mine openings • Capping pyrite-bearing rock in mines with impermeable coatings • Filling unused mine openings with material that neutralizes acid (e. g. , limestone) © 2011 Pearson Education, Inc.

ARD Being Released from a Mine Adit FIGURE 12 -30 This ARD, flowing from a small underground mine in Colorado, contains high levels of dissolved metals including Cu, Fe, Al, Zn, and As. © 2011 Pearson Education, Inc.

Soil Quality Acidic soils prevent plant growth —leave surface vulnerable to erosion Hypothesis: Pb in ore minerals (e. g. , galena) may be less bioavailable than the type of lead people are exposed to elsewhere (e. g. , Pb in gasoline or in paint) Figure 12 -35 Blood and Soil Lead Levels in Mining Communities Mining communities have high soil lead levels—but blood lead levels have been low, along with low health issues. © 2011 Pearson Education, Inc.

Soil Remediation Techniques • Add chemicals—make elements of concern less mobile and bioavailable • Reactions with these chemicals form new minerals in the soil • Keep the elements of concern from being dissolved in passing water • Phytoremediation—grow plants that take up elements of concern • Harvesting these plants decreases toxic element content of the soil Figure 12 -33 A Repository Design for Metal-Bearing Soil Repositories physically isolate metalbearing soil from interacting with the environment. © 2011 Pearson Education, Inc.

Air Quality • The principal concerns are dust generated at mine sites or blown off tailings ponds and the emissions from smelter operations • Dust – Drilling, blasting, hauling, and crushing rocks—all create dust – Water spray systems and vacuums—used to diminish dust • Smelter emissions – Controlling emissions = biggest challenge at smelters – Sulfur dioxide reacts with water to form sulfuric acid – Contaminates soils and water and kills vegetation • Smelters also emitted concentrations of metals (e. g. , Pb) © 2011 Pearson Education, Inc.

Mineral Resources in the Future • Environmental challenges: – – Proper waste rock disposal Sound tailings pond construction Prevention of ARD Control of smelter emissions • Other issues: – Rapidly increasing demand for mineral resources – Role of recycling in meeting mineral commodity needs – Application of sustainability concepts to use of minerals © 2011 Pearson Education, Inc.

Recycling • Extend the use of a finite resource • Diminish environmental consequences of mineral resource development and production © 2011 Pearson Education, Inc.

Sustainability and Mineral Resource Use • Mineral resources are considered nonrenewable • Consumption may lead to shortages of some mineral commodities • Substitute materials for needed minerals • Use less mineral-intensive technologies • More carefully use and recycling (conserve) • How can sustainability concepts be applied to nonrenewable resources like a mined ore deposit? • Such conversion is a major contribution that mineral resource development can make to a sustainable future for society as a whole • The mineral resource capital, an ore deposit in this case, is converted to another form of capital that can provide sustainable benefits to society © 2011 Pearson Education, Inc.

SUMMARY • Concentrations of valuable minerals may form where valuable minerals precipitate from metal-bearing waters in the shallow crust or on the seafloor. Ore is the part of a mineral deposit that at current or foreseeable commodity prices can be mined at a profit. • Ore deposits are uncommon geologic features. Exploration can be expensive and time-consuming, involving surface and subsurface sampling and observations. • Mining removes ore by underground or open-pit methods. Mining also removes waste rock surrounding the ore. The amount of waste rock is commonly two or three times the amount of ore in open-pit mines, but it can be less than the amount of ore in underground mines. © 2011 Pearson Education, Inc.

SUMMARY (cont. ) • Ore is processed by milling and flotation to separate and concentrate valuable minerals. The leftover nonvaluable minerals are a waste product called tailings. • Leaching removes valuable metals from some types of ores. Leach solutions soak through the ore, dissolve the metal, and carry it to processing facilities where the metal is removed. • The process of smelting recovers metals from the minerals concentrated by milling and flotation. The solid waste from smelting is commonly an iron- and silicon-rich material called slag. Smelters are a source of gas (particularly sulfur dioxide) and particulate emissions to the atmosphere. © 2011 Pearson Education, Inc.

SUMMARY (cont. ) • Exploration, mining, and mineral processes lead to physical disturbances of the landscape, many of which can be reclaimed. • Mining and mineral processing can affect water quality. Containment structures and proper reclamation prevent erosion of wastes, such as tailings, that could contaminate surface water bodies. • Acid Rock Drainage (ARD) can be prevented by various methods and treated by the addition of neutralizing materials. • Dust, generated during mining operations, can be controlled by water spraying, and sound reclamation prevents dust generation from tailings ponds. © 2011 Pearson Education, Inc.

SUMMARY (cont. ) • Sulfur dioxide emissions from smelters react with water to form acid rain, which acidifies surface water (lakes and streams) and ultimately the soil itself. • Population growth and expanding economies will significantly increase demand for mineral resources in years to come. • Recycling can help sustain mineral resources, but is not efficient enough to replace mining or new mineral resource production altogether. • Sustainability can come partly from mineral resource production if the financial capital derived from production is satisfactorily converted to other forms of capital that can sustain society after the mineral resources are depleted. © 2011 Pearson Education, Inc.