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Ores • Principally we discuss ores as sources of metals • However, there are many other resources bound in minerals which we find useful • How many can we think of? Ore Deposits • A deposit contains an unusually high concentration of particular element(s) • This means the element(s) have been concentrated in a particular area due to some process • What sort of processes might concentrate these elements in one place? Gold Au • Distribution of Au in the crust = 3.1 ppb by weight 3.1 units gold / 1,000,000,000 units of total crust = 0.00000031% Au • Concentration of Au needed to be economically viable as a deposit = few g/t 3 g / 1000kg = 3g/ 1,000,000 g = 0.00031% Au • Need to concentrate Au at least 1000-fold to be a viable deposit • Rare mines can be up to a few percent gold (extremely high grade)! Ore minerals • Minerals with economic value are ore minerals • Minerals often associated with ore minerals but which do not have economic value are gangue minerals • Key to economic deposits are geochemical traps metals are transported and precipitated in a very concentrated fashion – Gold is almost 1,000,000 times less abundant than is iron Economic Geology • Understanding of how metalliferous minerals become concentrated key to ore deposits… • Getting them out at a profit determines where/when they come out Ore deposit environments • Magmatic – Cumulate deposits – fractional crystallization processes can concentrate metals (Cr, Fe, Pt) – Pegmatites – late staged crystallization forms pegmatites and many residual elements are concentrated (Li, Ce, Be, Sn, and U) • Hydrothermal – Magmatic fluid - directly associated with magma – Porphyries - Hot water heated by pluton – Skarn – hot water associated with contact metamorphisms – Exhalatives – hot water flowing to surface – Epigenetic – hot water not directly associated with pluton Ore deposit environments • Sedimentary – Placer – weathering of primary minerals and transport by streams (Gold, diamonds, other) – Banded Iron Formations – 90%+ of world’s iron tied up in these – Evaporite deposits – minerals like gypsum, halite deposited this way – Laterites – leaching of rock leaves residual materials behind (Al, Ni, Fe) – Supergene – reworking of primary ore deposits remobilizes metals (often over short distances) Geochemical Traps • Similar to chemical sedimentary rocks – must leach material into fluid, transport and deposit ions as minerals… • pH, redox, T changes and mixing of different fluids results in ore mineralization • Cause metals to go from soluble to insoluble • Sulfides (reduced form of S) strongly binds metals many important metal ore minerals are sulfides! • Oxides – Oxidizing environments form (hydroxy)oxide minerals, very insoluble metal concentrations (especially Fe, Mn, Al) Hydrothermal Ore Deposits • Thermal gradients induce convection of water – leaching, redox rxns, and cooling create economic mineralization Massive sulfide deposits • Hot, briny, water leaches metals from basaltic ocean rocks • Comes in contact with cool ocean water • Sulfides precipitate Vermont Copperbelt • Besshi-type massive sulfide deposits • Key Units: – Giles Mountain formation – More siliciclastic, including graphitic pelite, quartoze granofels (metamorphosed greywacke), hornblende schist, amphibolite – Standing Pond Volcanics – mostly a fine grained hormblende-plagioclase amphibolite, likely formed from extrusive basaltic rocks (local evidence of pillow structures in St. Johnsbury). Felsic dike near Springfiled VT yielded a U-Pb age of 423± 4 Ma. – Waits River formation – Calcareous pelite (metamorphosed mudstone), metalimestone, metadolostone, quartzite. Minerals associated with economically recoverable metals • • • • • Elemental forms Sulfides Oxides Carbonates Sulfate salt Cuprite, Cu2O Elemental copper Chalcocite, Cu2S Chalcanthite, CuSO4*5H2O Malachite, Cu2CO3(OH)2 Sulfides Part 1 • Substitution into sulfides is very common • As and Se substitute for S very easily • Au can substitute in cation sites (auriferrous minerals) • Different metals swap in and out pretty easily Cu and Fe for instance have a wide range of solid solution materials Sulfide Minerals • Minerals with S- or S2- (monosulfides) or S22- (disulfides) as anionic group • Transition metals bonded with sulfide anion groups Iron Sulfides • • • • • • • • Mackinawite – FeS Greigite – FexSy Pyrite – FeS2 (cubic) Marcasite – FeS2 (orthorhombic) Troilite – FeS end member Pyrrhotite – Fe1-xS (slightly deficient in iron) Arsenopyrite – FeAsS Chalcopyrite – CuFeS2 Other important sulfides • • • • • • • • • Galena – PbS Sphalerite/wurtzite – ZnS Cinnabar – HgS Molybdenite – MoS Covellite – CuS Chalcocite – Cu2S Acanthite or Argenite – AgS Stibnite – Sb2S3 Orpiment – As2S3 ; Realgar – AsS Sulfides are reduced minerals what happens when they contact O2? • This is the basis for supergene enrichment and acidic mine drainage Actively Oxidizing Pyrite • FeS2 + 3.5 O2 + H2O Fe2+ + 2 SO42- + 2 H+ • FeS2 + 14 Fe3+ + 8 H2O 15 Fe2+ + 2 SO42- + 16 H+ • 14Fe2+ + 3.5 O2 + 14H+ 14 Fe3+ + 7 H2O • Sulfur species and H+ generation: – FeS2 + 2 Fe3+ 3 Fe2+ + ¼ S8 + 0 H+ – FeS2 + 7 Fe3+ + 3 H2O 8 Fe2+ + 0.5 S4O62- + 6 H+ AMD neutralization • Metals are soluble in low pH solutions – can get 100’s of grams of metal into a liter of very acidic solution • HOWEVER – eventually that solution will get neutralized (reaction with other rocks, CO2 in the atmosphere, etc.) and the metals are not so soluble but oxidized S (sulfate, SO42-) is very soluble • A different kind of mineral is formed! Ely Mine