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Topic 1: Ore mineralogy and orebodies


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Topic 1: Ore mineralogy and orebodies

  1. 1. Metals, minerals, mining and (some of) its problems A short series of lectures prepared for the London Mining Network 24 April 2009 by Mark Muller
  2. 2. Acknowledgments: I acknowledge gratefully the extent to which I have leant on the work contained in several good text books: Mine Wastes: Characterization, Treatment and Environmental Impacts, 2nd Edition, by Bernd Lottermoser, 2007. Springer, Berlin Heidelberg. Mining and the Environment: From Ore to Metal, by Karlheinz Spitz and John Trudinger, 2009. CRC Press, Leiden. Introductory Mining Engineering, 2nd Edition, Howard Hartman and Jan Mutmansky, 2002. Wiley, New Jersey. Thank you also to CAFOD, London, for suggesting and organising the workshop, and for covering my travelling expenses to London for the event.
  3. 3. Outline of lectures: Topic 1: Ore mineralogy and orebodies Topic 2: Mining Topic 3: Ore processing and metal recovery Topic 4: Mine wastes Topic 5: Environmental and social concerns
  4. 4. Specific mining problems examined in some detail: • Surface subsidence above underground longwall-mining panels • Rockbursts in deep underground mines • Tailings dam failures • Riverine and submarine tailings disposal • Cyanidation wastes and their attenuation (destruction) • Radioactive uranium wastes and water contamination • Sulphide wastes and acidification of waters
  5. 5. Topic 1: Ore mineralogy and orebodies From a series of 5 lectures on Metals, minerals, mining and (some of) its problems prepared for London Mining Network by Mark Muller 24 April 2009
  6. 6. Outline of Topic 1: • Elements and metals • Types of minerals • Radioactive elements, minerals and radioactive decay • The process of oxidation • Acids and alkalis • Types of rocks and orebodies • Examples of typical orebodies
  7. 7. Elements – minerals – rocks (orebodies) METALLURGICAL Elements are the building blocks of minerals EXTRACTION e.g., iron, zinc, sulphur and oxygen are elements. Recover target metal from mineral concentrate LIBERATION Minerals are the building blocks of rocks PROCESSING e.g., silicon-oxide (silica), iron-sulphide (pyrite) and tin-oxide Liberate target minerals from (cassiterite) are minerals. rock and concentrate them Metal-bearing minerals are the target of mining. Non metal-bearing minerals are referred to as gangue minerals. Rocks are aggregates of minerals MINING e.g., granite, limestone, sandstone and gneiss are rocks. Recover orebody from host rock “Orebodies” are rocks containing an enhanced percentage of metal-bearing minerals, high enough to be economic (i.e., mined at a profit), and a lower percentage of gangue minerals.
  8. 8. Metals enrichment factors Metals require significant enrichment above their normal background levels in the Earth’s crust to form a mineable orebody. Minerals are enriched to form orebodies through a wide range of different geological processes. The enrichment factor required to make a mine viable (i.e., profitable – within today’s economic framework for minerals exploitation) will vary from time to time, depending on commodity prices, and the ease of extraction of both the orebody from the ground and the target metal from the orebody. Figure from Spitz and Trudinger, 2009.
  9. 9. World production of non-fuel mineral commodities in 1999. Table from Lottermoser, 2007.
  10. 10. Elements: Elements are made up of atoms which consist of protons, neutrons and electrons. The number of protons (the “atomic number”) defines the “element”. For example oxygen (O) has 8 protons, Uranium (U) has 92 protons. In a well ordered fashion through the periodic table, the number of protons, neutrons and electrons increases, and the atoms (elements) become heavier and larger in diameter. Electron 1 “valence” 3 Electrons Negative electrical charge electron in the 3 Protons No mass outer electron - 4 Neutrons - “shell” - + + + + Neutron No charge Proton Positive electrical charge - Hydrogen (H) atom Lithium (Li) atom
  11. 11. Periodic Table of Elements “Metalloids” “Rare Earth Elements”
  12. 12. Some definitions regarding metals: • Metals are elements that are malleable, ductile, and conduct heat and electricity well – gold (Au), silver (Ag), copper (Cu), platinum (Pt) etc. • Metalloids (or “semi-metals”) are elements with both metallic and non-metallic properties, and have a lower ability to conduct heat and electricity – boron (B), arsenic (As), antimony (Sb), bismuth (Bi), selenium (Se) and tellurium (Te). • Heavy metals are those metals with a density greater than 6 g/cm3: Fe, Cu, Pb, Zn, Sn, Ni, Co, Mo, W, Hg, Cd, In, Tl. (Gold ~18 g/cm3) • Base metals are those metals that tend to be used in industry by themselves, rather than alloyed with other metals – Cu, Pb, Zn, Sn.
  13. 13. Making minerals from elements: The sharing of electrons by different elements forms the basis of the creation of compounds. Minerals are compounds – combinations of elements held together by the forces established through the sharing of electrons. Gold (Au) is stable and unreactive, and forms no compounds in nature. An ion is an atom or molecule (compound) which has lost or gained one or more electrons, giving it a positive or negative electrical charge. Anions a negative charge (e.g., CN-). Cations have a positive charge (e.g., H+) H+ cation lies at the root of acid mine drainage CN- (cyanide) anion is the basis of cyanidation waste problems.
  14. 14. Minerals: A mineral is often crystalline in form. The crystal lattices of minerals hold metal elements very tightly. Aggressive chemical means, or large amounts of thermal or electrical energy, are therefore required to liberate the metals from their host minerals The mineral pyrite Model of the crystal (FeS2) in its form of the titanium characteristic oxide mineral rutile cubic crystal form (TiO2) O atoms Ti atoms
  15. 15. “Classes” or groups of minerals: Significant metal-hosting minerals - Native metals: pure metals or metal alloys - Oxides: compounds with oxygen (O) - Sulphides: compounds with sulphur (S) Minerals primarily of “industrial” interest - Silicates: Si-O - Carbonates: CO3 - Halides (salts): Cl Minerals hosting interesting metals (and with some industrial interest) - Sulphates: SO4 - Phosphates: PO4 - Borates: B-O
  16. 16. Metal-bearing minerals: Native metals A native metal is a metal found in its metallic form in nature. Only gold, silver, copper and platinum metals occur in nature in exploitable amounts. All mined gold is native metal, alloyed with up to 15% silver. There are no common naturally occurring gold oxides, sulphides or other minerals. Prospector B. O. Holtermann with 286 kg solid gold nugget Native silver (Ag). Source: US Native copper (Cu) about 4 cm in found in 1872 at Hill End, Government. size. Credit: Jonathan Zander. NSW, Australia. From Spitz and Trudinger, 2009.
  17. 17. Metal-bearing minerals: Metal oxides Are simple compounds with the element oxygen (O). Metals are relatively easily extracted from oxide minerals. Examples include: Hematite: Fe2O3 Ilmenite: FeTiO3 Rutile: TiO2 Cassiterite: SnO2 Coltan (Columbite-Tantalite): (Fe,Mn)(Nb,Ta)2O6 Hematite (FeO2) “kidney ore” from Michigan. The yellow is the reflection of a lamp Cassiterite (SnO2). Source: US used for lighting. Government.
  18. 18. Metal-bearing minerals: Metal sulphides Are simple compounds with the element sulphur (S). Metals are less easily extracted from sulphide minerals, and are often oxidised first, as the initial stage in metal recovery. Examples include: Chalcocite: Cu2S Sphalerite: ZnS Galena: PbS Pyrite: FeS2 Cinnabar (HgS), Buckskin Mnts., Aggregate of Sphalerite (ZnS) Galena (PbS) crystal. Nevada. Credit: Chris Ralph. crystals. Credit: Andreas Früh
  19. 19. “Rock-forming” minerals: Silicates Are compounds with silicon-oxygen (Si-O) and occur in many different crystal forms. Silicates all contain metallic elements, but it is currently not possible to extract the metals from them, so interest in silicate minerals lies in their industrial uses. Examples include: Quartz (silica): SiO2 Beryl (emerald): Be3Al2(SiO3)6 Muscovite (mica): KAl2(AlSi3O10)(F,OH)2 Beryl Be3Al2(SiO3)6 Crocidolite (blue) asbestos variety emerald Na2Fe2+3Fe3+2Si8O22(OH)2 from the now closed mine at Wittenoom, Western Australia. Credit: John Hayman.
  20. 20. Other minerals of interest: Sulphates Formed with sulphur-oxygen (SO4). Gypsum: CaSO4∙2H2O - used in cement Borates Formed with boron-oxygen (B-O) and are exploited for the metalloid element boron (B). Borax: Na2B4O7∙10H2O Ulexite: NaCaB5O9∙8H2O Carbonates Formed with carbon-oxygen (CO3). They are easily dissolved in acids, and are able to neutralise acids. Calcite, limestone: CaCO3 - limestone is the main component of cement
  21. 21. Other minerals of interest: Phosphates Formed with phosphorous-oxygen (PO4). Phosphates exploited for Rare Earth Element (REE) metals and thorium (Th): Monazite: (Ce,La,Pr,Nd,Th,Y)PO4 - radioactive, due to thorium, and the most common ore of thorium Apatite: (Ca,Sr,Ce,La)5(PO4)3(F,Cl,OH) - a non-radioactive source of REEs Phosphates are also mined to obtain phosphorus for use in agriculture and industry: Phosphate: H3PO4 Apatite, variety fluorapatite (Ca5(PO4)3F from Mexico. Credit: Chris Ralph
  22. 22. Other minerals of interest: Halides (salts) Salts are “evaporite” minerals formed with chlorine (Cl). They are easily dissolved in water and are often mined in-situ using solution-mining methods. Halite (rock salt): NaCl Sylvite: KCl - fertiliser industry Sylvite (KCl). Credit: Luis Miguel Bugallo Sánchez.
  23. 23. Oxidation: Oxidation is a reaction with oxygen that results in the breakdown of minerals. Metallic sulphide minerals (e.g., pyrite) oxidise in the presence of water and oxygen to: • produce acids and • release dissolved metals into water. Note: “Oxidised” sulphide minerals are not the same as “primary” oxide minerals. A primary oxide of iron is hematite: Fe2O3 Oxidation of pyrite (FeS2) produces iron-hydroxide: Fe(OH)3
  24. 24. ACID Acids and alkalis and pH: Anything that reacts with an “acid” is called an “alkali”. pH is a measure of the acidity or alkalinity of a solution. Acidic pH less than 7 (lemon juice = 2, battery acid = 0) Neutral pH equal to 7 (distilled water) Alkaline pH greater than 7 (household ammonia = 11) They neutralise each other through the following reaction: H+ + OH- H 2O Acid Alkali Water ALKALI Figure from: Credit Stephen Lower
  25. 25. Rocks and orebodies: Rocks and orebodies are aggregates of different minerals. Orebodies have high concentrations of metal bearing minerals and are hosted in barren “country” rock. Mined country rock is referred to as gangue or waste. Volcanic, sedimentary and metamorphic processes form rocks and minerals. Hydrothermal fluids associated with volcanic and metamorphic processes contain high concentrations of dissolved metals and also form ores
  26. 26. Igneous rocks Igneous rocks are formed when molten magma cools and crystallises either on the surface or at depth in the crust. Examples: granite, basalt, kimberlite. An outcrop of orbicular granite. Locality: Orbicular Granite Nature Sanctuary, near Caldera, Chile. Photo credit: Herman Luyken
  27. 27. Sedimentary rocks Sedimentary rocks are formed by deposition of • clastic sediments derived from the erosion of other rocks (mud, gravel, sands) • organic matter • chemical precipitates (evaporites) followed by burial and compaction of the material. Examples: Sandstone, conglomerate, limestone, coal, potash. An outcrop of conglomerate overlying sandstone. Locality: Point Reyes, Marin County, California.
  28. 28. Metamorphic rocks Metamorphic rocks are formed when any rock type is subjected to high temperature and pressure. Examples: marble (from limestone precursor), quartzite (from sandstone precursor), gneiss (from granite precursor). Banded gneiss, formed by high pressure compression that aligned minerals, forming a layered fabric. Locality: Skagit Gneiss Complex, North Cascades Range, Washington, USA. Credit: US Geological Survey
  29. 29. Ore genesis: Enrichment of metal-bearing minerals occurs in specific geo-tectonic settings in response to specific geological processes. These geological settings and processes produce different types of orebodies, with “classic” mineral assemblages/combinations, e.g.: Massive iron-ore Placer (alluvial) gold Massive copper sulphide + gold Massive lead-zinc sulphide Layered igneous intrusions: platinum, palladium, chromium Nickel laterite and bauxite Diamondiferous kimberlite Alluvial diamond Mineral sands Coal
  30. 30. Massive sulphide lead-zinc deposit, Black Angel Mine, Greenland (1973 – 1991) Black Angel Mine exploited a SIMPLIFIED CROSS-SECTION THROUGH BLACK ANGEL MINE massive sulphide lead-zinc deposit (sphalerite, galena and Massive pyrite) hosted in marble and sulphide orebodies metasediments. Ore-grades of 12.5% Zn, 4.1% Pb, 30 ppm (g/ton) Ag were reported (Asmund et al., 1994). The massive sulphide 600 m orebodies are developed sub- parallel to metamorphic banding in the country rock, and were mined using a room-and-pillar method. Approx. 9 km 3m Cable car access point into mine Massive sulphide ore (dark band) showing in a support pillar left remnant after cessation of mining in 1990. (From: Black Angel News, 2005).
  31. 31. Kimberlite diamond deposits Kimberlite volcanic pipes are the hosts of Diagram showing “primary” diamond deposits. the structure of a kimberlite volcanic Both the volcanic magmas and the contained pipe. diamonds originate at depths of about 170 to 200 km below the Earth’s surface, and are brought to surface during a very rapid and explosive eruption events. Kimberlite pipes are subsequently eroded through geologic time, exposing deeper parts of the pipe, and developing “secondary” deposits of alluvial diamonds that are found in river beds, flood plains, and offshore as marine deposits. Figure from McCarthy and Rubidge (2005) Diamond grades in kimberlite pipes are highly variable, and some pipes are completely barren (for good geological reasons). Some Udachnaya Pipe, Sakha Republic, reported grades lie in the range 0.28 – 7.5 Russia, in the carats per ton (Roberts, 2007, pg 68). summer of 2004. Credit: Alexander Secondary alluvial diamond deposits may be Stepanov. significantly enriched in diamonds as the process of erosion “concentrates” heavy, resistant minerals.
  32. 32. Palaeo-placer gold deposit - Witwatersrand Basin, South Africa The Witwatersrand Basin in an ancient (2.8 billion years old) palaeo-placer deposit, consisting of multiple stacked and alternating shale, sandstone and thin conglomerate sedimentary bands. The gold mineralisation is found in the conglomerate bands (called “reefs”), typically between 5 to 100 cm thick. The gold was either introduced at the time the sediments were deposited, or was introduced later by gold-bearing hydrothermal fluids (or both). Geological cross-section through the Welkom Goldfield. Figure from McCarthy, 2006 The sedimentary basin subsequently suffered Carbon extensive deformation, producing folds and faults that disrupt the deposit. Faults impact Pyrite significantly on safe (and efficient) mining. Gold Underground mines operate up to a maximum Quartz depth of about 4,000 meters. Mineable grades in a deep goldmine operations are of the order of 10 – 20 g/ton. 1 cm Many of the reefs contain accessory Gold and carbon nodules with “buckshot” pyrite in conglomerate reef from the Witwatersrand uranium, which is processed as by-product on Basin, South Africa. Figure from McCarthy and several mines. Rubidge, 2005. Photo credit: Goldfields.
  33. 33. Nickel laterite deposits Nickel laterite ore deposits are the surficial, deeply weathered residues formed on top of ultramafic rocks h_New_Caledonia.JPG.JPG that are exposed at surface in tropical climates. They are found widely in New Caledonia, Cuba, Australia, Papua New Guinea, the Philippines, and Indonesia, and are estimated to comprise about 73% of the Limonite zone world continental nickel resource. Two kinds of lateritic nickel ore can be distinguished: limonite (oxide) types and saprolite (silicate) types. Deep downward Near surface upward penetration of water evaporation of water producing weathering precipitates Fe, Ni oxide LIMONITE A Creek in southern New-Caledonia. Goethite ZONE Red colours reveal the richness of the (hydrated oxide) 1- 2% Ni ground in iron oxides, and nickel. SAPROLITE OREBODY Serpentine ZONE (hydrated silicate) 1.5 - 2.5% Ni The process of oxidation and Olivine and weathering depletes the original Mg RICH “ULTRAMAFIC” pyroxene mafic rock of Mg and Si, and ROCK 0.3% Ni (silicate minerals) concentrates Fe and Ni in the weathered zone.
  34. 34. Radioactive elements: In radioactive elements, the configuration of the nucleus is unstable, and breaks down, emitting radioactive “decay” products: alpha, beta and gamma radiation. Isotopes of an element have nuclei with the same number of protons but different numbers of neutrons. Some isotopes are stable, and others subject to radioactive decay. Alpha radiation is readily stopped by a sheet of paper. Helium nucleus Beta radiation is halted by an aluminium plate. Electron Gamma radiation is eventually absorbed as it penetrates a dense material. Lead, being Energy dense, is good at absorbing (electromagnetic gamma radiation – several radiation) centimeters of thickness is needed. Modified from
  35. 35. Radioactive elements: A parent nuclide is an element that undergoes radioactive decay, producing a daughter nuclide, that may be a different element. Parent Daughter U-238 decays to form Th-234 by releasing an alpha particle. 92 protons 90 protons 146 neutrons 144 neutrons The daughter nuclide may itself be stable or unstable (i.e., radioactive). The half-life is the time taken for half the radionuclide's atoms to decay. Half-lives vary between more than 1019 years, for very nearly stable nuclides, to 10−23 seconds for highly unstable ones.
  36. 36. Uranium radioactive decay series – and half-lives Uranium-238 Series starts with radioactive isotope (92 protons, 146 neutrons) Series ends with stable lead isotope The SI unit of radioactive decay is the Becquerel (Bq). One Bq is defined as one decay per second. Table from Lottermoser, 2007, and references therein.
  37. 37. Radioactive uranium minerals: The main “primary” ore in uranium deposits is Uraninite: UO2 Other important “primary” uranium ore minerals are: Brannerite: (U,Ca,Y,Ce)(Ti,Fe)2O6 – a mixed uranium, iron, titanium oxide mineral. Coffinite: USiO4∙nH2O – a hydrated uranium silicate Pitchblende – an amorphous, poorly crystalline mix of uranium oxides often including triuranium octoxide: U3O8.
  38. 38. “Daughter” nuclides are trapped in uranium minerals or escape At the time the mineral is formed in orebody 1 Billion years later Uraninite: UO2 Uraninite: UO2 100% uranium 75% uranium has decayed to daughter radionuclides. Some daughters will remain trapped in the mineral, or they migrate elsewhere in the orebody to form other minerals
  39. 39. Radioactive minerals: The “primary” uranium minerals weather and break down very easily when exposed to water and oxygen, to produce numerous “secondary” (oxidised) minerals, for example carnotite and autunite, which are often mined, but in significantly lower quantities that uraninite. Uranium is also found in small amounts in other minerals: allanite, xenotime, monazite, zircon, apatite and sphene. File:Carnotite-BYU.jpg Carnotite K2(UO2)2(VO4)2∙3H2O, An important “secondary” uranium-vanadium bearing mineral, from Happy Jack Mine, White Canyon District, Utah, USA. Credit: Andrew Uraninite (pitchblende) UO2 Silver.