Precious Metals and Gems

Kirbie Brown

Mandy Daigle

Aimee Porter

Scarce Metals

• Present in Earth’s crust <0.1%• Special properties led to technological

marvels• Four groups of geochemically scarce

metals– Ferro-alloy metals– Base metals– Precious metals– Special metals

Precious Metals• Focus

– Gold – Silver– Platinum group elements (PGE)

• Highlights– Geology– Mining– Production and the environment– Production and reserves

www.joellessacredgrove.com

A Few Facts…• Soft and malleable• Extremely resistant to chemical attack• Corrosion-free• Better records for gold production than any other metal• 43% held by banks• 57% in bullion, coins, jewelry, and art • Annual gold production of approximately 2,200 metric tons ($25

billion)• 1991

– 83%+ of world consumption went into jewelry– 6% in medals and official coins– 6% in electronic equipment– 2.2% in dental materials– 2.8% in industrial applications

http://resourcescommittee.house.gov/subcommittees/emr/usgsweb/photogallery/images/Gold%203_jpg.jpg

Geology of Gold Deposits

• Found largely in native state or with silver in electrum

• Also forms telluride minerals

• Found as hydrothermal deposits (low grade gold) and paleoplacer deposits

Hydrothermal Deposits - Epithermal

• Consist of gold-containing veins, veinlets, and disseminations– Created by cool hydrothermal solutions

(<250°C) that circulated through shallow crust• Further divided into adularia-sericite deposits and

acid-sulfate deposits

Hydrothermal Deposits – Epithermal: Adularia-Sericite

• Characterized by minerals adularia and sericite

• Near neutral hydrothermal solutions• Linked to felsic and intermediate

volcanism– Form massive veins with precious metal

accumulations called shoots

• Western North America, western Pacific volcanic arcs, Saudi Arabia, and Ontario

Hydrothermal Deposits – Epithermal: Acid-Sulfate

• Characterized by minerals like alunite and pyrophyllite

• Created from acid hydrothermal solutions

• Confined to small fracture systems– Direct gases (CO2, SO2, and HCL) upward

• Forms acidic hydrothermal solution

• Nevada, Chile, and Dominican Republic

Hydrothermal Deposits – Epithermal: Sediment-Hosted

Micron Gold Deposits• Created by channeling of epithermal

waters or brines through carbonaceous limestone– Gold dispersed through changed limestone

• Extremely fine-grained and found with optical and electron microscopes

• Concentrated in thin layers

• Nevada, Utah, and Sonora

Hydrothermal Deposits – Epithermal: Hot-Springs

• Adularia-sericite or acid-sulfate liquids from hot springs at surface– Encompassed by silica-rich deposits

(sinter) or carbonate-rich deposits (travertine)

– Easily removed by erosion• Few deposits known

• California

http://fargo.itp.tsoa.nyu.edu/~pwv203/hotsprings/fales.jpg

Hydrothermal Deposits – Mesothermal

• Gold-containing quartz veins– Created deep within crust

• Deposition by fluids >250°C

• Enclosed by changed rocks that contain carbonates

• Divided into intrusion related, greenstone hosted, and turbidite hosted

Hydrothermal Deposits – Mesothermal: Intrusion-Related

Veins• Quartz veins with gold, silver, and base-

metal sulfides

• Created around felsic intrusions– Liquid mixture of magmatic water and

meteoric water– Depths of 5+ km

• Korea and Russia

Hydrothermal Deposits – Mesothermal: Greenstone-Hosted

• 10+ km

• Originate in metamorphosed mafic volcanic rocks– Contain chlorite

• Found as quartz veins adjacent to offshoots from huge crustal fractures

• California

Hydrothermal Deposits – Mesothermal: Turbidite-Hosted

• 10+ km

• Found in turbidites– Created by erosion of volcanic rocks

• Gold concentrated in iron-rich wallrocks

Kesler, Stephen E. Mineral Resources, Economics, and The Environment. Macmillan College Publishing Company, Inc., NY. 1994, 235-262

Placer and Paleoplacer Deposits

• Placer: alluvial deposit containing particles of a valuable mineral

• Primary gold source for thousands of years• Most placers nearing exhaustion• Witwatersrand paleoplacers of South Africa

– Discovered in 1886– Several gold-containing conglomerate layers– Mined approximately 32,000 metric tons of gold with

an estimated reserve of 20,000 metric tons

Kesler, Stephen E. Mineral Resources, Economics, and The Environment. Macmillan College Publishing Company, Inc., NY. 1994, 235-262

By-Product Gold

• Important by-product of many base-metal mines– Porphyry copper deposits

• Low grade gold– Grasberg mine in Indonesia

• Utah– 16 metric tons annually

• Western Pacific

Gold Mining• Placer mining

– Stream gravels containing gold moved over riffle system to catch gold

• Riffle made of Astroturf

• Open-pit mining• Bulk mining

– Allowed for recovery of low grade ores

www.oxbowriver.com

Gold Mining cont.

• Underground mining– Over large areas– Extremely deep levels

• Hostile conditions– Newly broken rock up to 65°C– High humidity– Mud rushes if chilled water is used as a cooling element– Rock bursts

» Some felt as earthquakes at the surface

Gold Production and the Environment

• Hydraulic mining problems– Sediment is disturbed, creating wastewater

that was once dumped into river systems

• Amalgation– Mercury never recovered

• Remains a pollutant in old mining locations

– Process still being used in Latin America and the Pacific Rim

Gold Production and the Environment cont.

• Cyanide process– Extremely toxic compound

• 50 to 250 mg can cause death

– Yields gold-containing solution (pregnant solution)– Heap leaching was used once process was complete

• Cyanide solution leaks through bottom of abandoned heap-leach pads

• Roasting– Required to mine deep, sulfide-rich ores in the USA

– Releases SO2 and As gas

Gold Production and Reserves

• Measured in units of grams or troy ounces (31.104 grams)

• Entire world production approximated at 130,000 metric tons– 40% mined in last 30 years– 85% since 1900

• Produced in 67 countries– 30% in South Africa– Nevada is leader in USA

• World reserves estimated at 44,000 metric tons– Adequate for only 20 years of production at present

rates

www.joellessacredgrove.com

A Few Facts…

• Corrosive with high electrical conductivity• Evolving from precious metal to industrial metal• Main market is photographic film• Also used in electrical and electronic

applications, highly reflective mirrors, pharmaceuticals, batteries with zinc, dentistry, coins, solders, jewelry, silverware, and as an edible silver foil in India

• Annual production of about 15,000 metric tons– Approximately $2 billion

Geology of Silver Deposits

• Occurs in electrum, argentite, and many complex sulfide minerals

• Hydrothermal deposits

• Placer deposits

• By-product – Copper and lead mining

Epithermal Vein Deposits

• Most familiar deposit• USA and Mexico• Cerro Rico deposit (Bolivia)

– Largest deposit in world– Quartz-silver-tin veins cutting silica-rich dome

of volcanic rock– Deposit at 500°C from extremely saline liquids

• Precipitate ore down to <100°C• Solution becomes less saline

Cobalt-Nickel-Arsenide Deposits

• Ontario– Veins consisting of native silver, cobalt,

nickel, and iron arsenides within calcite and quartz

• Within sediments above and below massive gabbro intrusion

• Created from brines heated by the intrusion

Kesler, Stephen E. Mineral Resources, Economics, and The Environment. Macmillan College Publishing Company, Inc., NY. 1994, 235-262

By-Product Silver

• Mostly from gold and base-metal deposits• Silver forms small inclusions and are difficult to

separate– Australia, Utah, Ontario, and Alaska

• Chimney-manto deposits– Mexico and Peru

• Lead-zinc vein deposits– Idaho and Missouri

• Sediment-hosted copper deposits– White Pine in Michigan – Kupferschiefer in Germany and Poland

Silver Production and the Environment

• Underground mining• Open pit mining• Production dependent on presence with

gold or base-metal sulfides– With gold

• Cyanide leaching

– With base-metal sulfides• Specific step in smelting process

• Relatively low recovery in each process

www.pangea.stanford.edu/.../ kurt-mining-methods.html

Silver Production and Reserves

• 56 countries• Industrial markets

– Silver-free photographic film, video tape, and xerography

• Industrial, jewelry, and silverware encompass 95% of world silver consumption

• Production fallen short of consumption• Reserves at 280,000 metric tons

– Dependent on by-product silver

Platinum-Group Elements (PGE)

www.theodoregray.com

A Few Facts…

• Platinum, palladium, rhodium, ruthenium, iridium, and osmium– Occur together in geological settings– Can substitute for one another by atomic substitution– Similar chemical and physical properties

• First discovered in placers• Form steel-gray nuggets• Malleable • High melting temperature• Resistant to corrosion• 300 metric tons annually

www.angloamerican.co.uk/. ../platinum.asp

A Few More Facts…

• Used for:– Increasing speed in

chemical reactions through catalysis

– Highly corrosive environments

– Extremely high temperature situations

– Catalytic converters– Diesel-powered vehicles– Catalyst in oil refining– Production of nitric acid– Fuel cells

- Electrical and electronics- High-resistance wires- Memory devices- Special solders- Automotive oxygen sensors- Dental and medical applications- Nozzles for glass and ceramic fiber extrusion

Geology of PGE Deposits

• Minor production from placer deposits

• Major production from magmatic deposits correlated with mafic igneous rocks– Layered igneous complexes – UG-2 chromitite– Nickel-copper sulfide ore

Layered Igneous Complexes

• Dominant deposit• Merensky Reef of the Bushveld complex (South

Africa)– Between chromitite and vanadium-containing

magnetite layers– Consists of coarse-grained, mafic silicate minerals– Result of magmatic immiscibility– Hypothesized that PGEs scavenged by hydrothermal

solutions from deeper parts of Bushveld complex

UG-2 Chromitite Deposit

• Consists largely of chromite

• Three times as much rhodium per metric ton as Merensky Reef

• Platreef (Bushveld)– Veinlets of PGE-bearing sulfide minerals in

ultramafic rocks

Nickel-Copper Sulfide Ore

• Least important source

• Created by separation of immiscible sulfide magmas

• PGE production sufficient only where a large nickel production exists– Australia

PGE Production and the Environment

• Almost exclusively underground mining– 500 to 1000 meters– Problems

• Removing enough ore to meet production requirements• Need to mine large areas• Potholes• Geothermal gradient causes hazardous conditions

• Separation of sulfide mineral-PGE concentrate– Matt rich in PGE is created

• Dissolved and different metals are separated by ion exchange

PGE Production and the Environment cont.

• Similar environmental problems as base-metal smelters

• Some forms of PGEs are toxic

• Main problem– Platinosis

• Respiratory and dermatological symptoms

PGE Production and Reserves

• Production in South Africa, Russia, USA, Zimbabwe, Australia, Canada, Finland, Colombia, Ethiopia, and Japan

• Future lies in jewelry and investment markets• 56,000 metric tons in reserves

– Platinum 40%– Palladium 40%– Rhodium 9%– Iridium 6%– Ruthenium 4%– Osmium 1%

•Craig, James R. et al. Resources of the Earth: Origin, Use, and Environmental Impact. 3rd ed. Prentice Hall, NJ. 2001, 312-325.

Gems• 150 natural compounds used as gems

– Diamonds, emeralds, rubies, alexandrite, and sapphires sell at highest prices

– Followed by amber, aquamarine, jade, opal, pink topaz, spinel, and tourmaline with intermediate values

– Agate, amethyst, and zircon with lower values• No distinctive chemical composition• Crystal structure defines clarity, color, and

brilliance• Retailed in cut and polished forms• Formed by many different processes

Gems cont.

• Focus– Diamonds– Beryl Group (emeralds and aquamarines)– Corundum Group (rubies and sapphires)

• Highlights– Geology– Mining and Production– Classification, Trade, and Reserves

Diamonds

Background picture from www.mwdiamonds.com

A Few Facts…

• Measured in carats (0.2 grams)

• Annual world production – 50 million carats worth $5 billion

• Would fit in a cube 2.86 meters on one side

• Metastable at Earth’s surface

Geology of Diamonds

• Found as xenocrysts in kimberlite– Found in thick continental crust

• Forms kimberlite pipes that shoot upward from deep, dike-like bodies

– Pipes include rock and mineral fragments and diamonds held together by magma

» Magma originated in mantle and brought up diamonds

» Rose rapidly

• May be hosted by lamproite– Similar to kimberlites

• Also found in metamorphic rocks

Geology of Diamonds cont.

• Diamond placer deposits– Wider geographical distribution– Deep erosion of diamond pipes– Some placers traced to mines that cannot be

mined• Arkansas and Atlantic Coast of USA

– May be derived from deeply eroded kimberlites

• Angola, Chana, Guinea, Sierra Leone, Tanzania, and Zaire

Kesler, Stephen E. Mineral Resources, Economics, and The Environment. Macmillan College Publishing Company, Inc., NY. 1994, 235-262

Diamond Mining and Production

• Early days– Recovered ore by rotary pans and hand-

sorting• Security problems, limited extent of operation, and

resulted in poor recoveries

• 1896– Discovered that diamonds stick to grease

• Allowed for large-scale mining and processing

Diamond Mining and Production cont.

• Open pit and underground mining– Placer mining very thorough

• Beach placers– Mined using large dikes to inhibit surf

• Offshore mining– By divers using suction tools placed on shore

or small boats

• Deeply submerged beach zones– Mined from offshore ships

Diamond Mining and Production cont.

• Kimberlite mining– Begins with open pit but changes to underground as

depth increases• Block caving or sublevel caving

– Crush ore to tiny fragments to free diamonds• Ore passed through pans and cyclones

– Concentrates heavy minerals

» Grease tables and X-ray sorters

– 1906• Largest diamond in history

– 11cm x 6cm, 3,206-carat Cullinan diamond

Diamond Classification, Trade, and Reserves

• Graded according to size, quality, color, and shape

• Divided into 11 groups based on size– Further sorted by shape, quality, and color– Main divisions are gems and industrial

diamonds• Cut into wide range of shapes

– Enhance appearance

Diamond Classification, Trade, and Reserves cont.

• Produced in 21 countries– Including Australia, Botswana, Zaire, South Africa,

Namibia, Brazil, China, India, Russia, Canada, Sierra Leone, Lesotho, and Kazakhstan

• Sold by Central Selling Organization (CSO)– De Beers– Primary function is to alleviate and capitalize on

wholesale prices

• Reserves at about 300 million carats– Only six times higher than annual world production

www.gemsuite.com www.icgems.com

www.gemsuite.com www.worldofrockhounds.com

The Beryl Group – Emeralds and Aquamarines

A Few Facts…

• Beryl: common beryllium-aluminum silicate– Forms many important gem stones– Develops crystals with few imperfections and

good color• Emeralds – green• Aquamarines – pale blue or bluish-green• Heliodor – yellow• Morganite - pink

A Few More Facts…

• Chromophores: trace elements that affect the colors in beryl group gem stones– Emerald

• Chromium and vanadium

– Aquamarine• Iron

– Heliodor• Manganese, iron, and titanium

– Morganite• Manganese and iron

Geology of the Beryl Group

• Found in beryllium deposits– Narrow calcite veins cutting carbonaceous

shale– Where veins near granitic intrusions cut

ultramafic rocks

• Best gems arise from pegmatites and hydrothermal veins– Slow cooling allowed for growth

• Some aquamarines found in placers

Production of the Beryl Group

• Emeralds– Brazil, Colombia, Russia, and Zambia

• Aquamarines– Brazil

• Nearly all production from bedrock deposits and regolith

• Gems irregularly distributed– Most work done by hand

• Reserve estimates not known

• Without detection of a large deposit that can be mechanically mined, production will remain a small business

Beryl Group Reserves

www.aboutgemstones.com

Rubies and Sapphires

The Corundum Group

www.eastgems.com

A Few Facts…

• Corundum: oxide of aluminum– Creates gems when in well-developed

transparent crystals with good color• Red corundum – ruby

– Chromophores - chromium

• Blue corundum – sapphire– Chromophores - iron and titanium

– Wide industrial use

Background picture from www.msgjewelers.com

Geology of the Corundum Group

• Corundum not stable in presence of quartz– Will react to form other minerals– Reactions limit geologic environments

available to corundum• Need quartz-free rocks with copious aluminum

– Bauxite» Metamorphosed

• Low-silica mafic rocks– Peridotite and hydrothermally altered limestones

Production and Reserves of the Corundum Group

• Production primarily in Australia, Cambodia, Myanmar, Nigeria, Sri Lanka, and Thailand– Mostly from placer deposits

• Dominated by poorly funded, small businesses– Extremely dependent on local politics

• Least secure supply of all precious gems• Reserves are not known

The Future of Precious Metals and Gems

• Industrial applications continue to increase• Predictions

– “the world has outgrown the need for mineral commodities as investment vehicles, and possibly even as ornaments and art objects” (Kesler)

– “synthetic gems are supposed to satisfy the world’s gem buyers” (Kesler)

• Mandatory need for exploration– Precious metals and gems has dimmest reserve

outlook of all mineral commodities– Future may be dependent on improved recoveries as

by-products

Works Cited

• Craig, James R. et al. Resources of the Earth: Origin, Use, and Environmental Impact. 3rd ed. Prentice Hall, NJ. 2001, 312-325.

• Kesler, Stephen E. Mineral Resources, Economics, and The Environment. Macmillan College Publishing Company, Inc., NY. 1994, 235-262

• Nevada Commission on Mineral Resources Division of Minerals. “Digging Deep Into Mining.” 21 Mar. 2005. http://minerals.state.nv.us/programs/min_diggingdeep.htm

Sources for Pictures• www.aboutgemstones.com• www.eastgems.com• www.gemsuite.com• www.icgems.com• www.joellessacredgrove.com• www.msjewelers.com• www.mwdiamonds.com• www.oxbowriver.com• www.theodoregray.com• www.worldofrockhounds.com

• http://resourcescommittee.house.gov/subcommittees/emr/usgsweb/photogallery/images/Gold%203_jpg.jpg

• http://fargo.itp.tsoa.nyu.edu/~pwv203/hotsprings/fales.jpg• pangea.stanford.edu/.../ kurt-mining-methods.html

• www.angloamerican.co.uk/. ../platinum.asp