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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.

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Precious Metals and Gems


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    1. Precious Metals and Gems Kirbie Brown Mandy Daigle Aimee Porter

    2. 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

    3. Precious Metals • Focus • Gold • Silver • Platinum group elements (PGE) • Highlights • Geology • Mining • Production and the environment • Production and reserves

    4. www.joellessacredgrove.com

    5. 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

    6. 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

    7. 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

    8. 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

    9. 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

    10. 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

    11. 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

    12. 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

    13. 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

    14. 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

    15. Hydrothermal Deposits – Mesothermal: Turbidite-Hosted • 10+ km • Found in turbidites • Created by erosion of volcanic rocks • Gold concentrated in iron-rich wallrocks

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

    17. 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

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

    19. 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

    20. 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

    21. 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

    22. 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

    23. 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

    24. 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

    25. www.joellessacredgrove.com

    26. 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

    27. Geology of Silver Deposits • Occurs in electrum, argentite, and many complex sulfide minerals • Hydrothermal deposits • Placer deposits • By-product • Copper and lead mining

    28. 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

    29. 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

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

    31. 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

    32. 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

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

    34. 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

    35. Platinum-Group Elements (PGE) www.theodoregray.com

    36. 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

    37. 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

    38. 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

    39. 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

    40. 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

    41. 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

    42. 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

    43. 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

    44. 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%

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

    46. 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

    47. Gems cont. • Focus • Diamonds • Beryl Group (emeralds and aquamarines) • Corundum Group (rubies and sapphires) • Highlights • Geology • Mining and Production • Classification, Trade, and Reserves

    48. Diamonds Background picture from www.mwdiamonds.com

    49. 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

    50. 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