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 Chalcocite Cu2S Cuprite Cu2O Chalcanthite CuSO4 5H2O Elemental copper 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 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
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