Download GEOL 301 - Precious Metals and Gems

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
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Geology
Mining
Production and the environment
Production and reserves
www.joellessacredgrove.com
A Few Facts…
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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
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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/photo
gallery/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 basemetal 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
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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 heapleach 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…
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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
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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
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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 basemetal 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
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Platinum
Palladium
Rhodium
Iridium
Ruthenium
Osmium
40%
40%
9%
6%
4%
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, dikelike 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 handsorting
• 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.worldofrockhounds.com
The Beryl Group – Emeralds and
Aquamarines
www.gemsuite.com
www.icgems.com
A Few Facts…
• Beryl: common beryllium-aluminum silicate
– Forms many important gem stones
– Develops crystals with few imperfections and
good color
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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
Beryl Group Reserves
• Reserve estimates not known
• Without detection of a large deposit that
can be mechanically mined, production will
remain a small business
www.eastgems.com
The Corundum Group
Rubies and
Sapphires
www.aboutgemstones.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