Ores

1. 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?

2. 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?

3. 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)!

4. 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

5. 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

6. 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

7. 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)

8. 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)

9. Hydrothermal Ore Deposits • Thermal gradients induce convection of water – leaching, redox rxns, and cooling create economic mineralization

10. Massive sulfide deposits • Hot, briny, water leaches metals from basaltic ocean rocks • Comes in contact with cool ocean water • Sulfides precipitate 

11. 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.

12. Minerals associated with economically recoverable metals • Elemental forms • Sulfides • Oxides • Carbonates • Sulfate salt Cuprite, Cu2O Elemental copper Malachite, Cu2CO3(OH)2 Chalcocite, Cu2S Chalcanthite, CuSO4*5H2O

13. 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

14. Sulfide Minerals • Minerals with S- or S2- (monosulfides) or S22- (disulfides) as anionic group • Transition metals bonded with sulfide anion groups

15. 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

16. 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

17. Sulfides are reduced minerals  what happens when they contact O2? • This is the basis for supergene enrichment and acidic mine drainage

18. 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+à 3 Fe2+ + ¼ S8 + 0 H+ • FeS2 + 7 Fe3+ + 3 H2Oà 8 Fe2+ + 0.5 S4O62- + 6 H+

19. 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!

20. Ely Mine