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Chapter 24

Chapter 24. Sulfides and related minerals Hydrothermal processes. Introduction. 500 sulfide minerals: most metal or semi-metal: pyrite, chalcopyrite, sphalerite Simple sulfides, salts of HS: ZnS Salts of thioacids, such as H 3 SbS 3 : Ag 3 SbS 3 Salts of polysulfuric acid, H 2 S 2 : FeS 2

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Chapter 24

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  1. Chapter 24 Sulfides and related minerals Hydrothermal processes

  2. Introduction • 500 sulfide minerals: most metal or semi-metal: pyrite, chalcopyrite, sphalerite • Simple sulfides, salts of HS: ZnS • Salts of thioacids, such as H3SbS3: Ag3SbS3 • Salts of polysulfuric acid, H2S2: FeS2 • NB economic minerals: major ores for Cu, Zn, Pb, Hg, Bi, Co, Ni, As, Sb • NB mineral properties in Table 24.1

  3. Crystal chemistry • Bonding between metal and sulfur • Sulfur has large ionic radius relative to small radius of most metals • Sulfides mostly NOT in simple close packing • Exceptions are: ZnS, PbS, HgS, FeS, CuFeS2 • Sulfur ions are usually polarized to form large diversity of crystal structures • Most sulfides have combination of metallic, covalent and also ionic bonding ZnS

  4. Crystal chemistry Stibnite Pyrite Molybenite

  5. Important sulfide minerals • Chalcosite Cu2S • Galena PbS • Sphalerite (Zn,Fe)S • Cinnabar HgS • Pyrrhotite Fe0.83S to FeS • Pentlandite (Ni,Fe,Co,Ag)9S8 • NickelineNiS

  6. Important sulfide minerals • Realgar As4S4 • Orpiment As2S3 • Stibnite Sb2S3 • Molybdenite MoS2 • Chalcopyrite CuFeS2 • Bornite Cu3FeS4

  7. Important sulfide minerals • Tetrahedrite Cu12(SbS3)4S • Pyrite FeS2(isometric) • Cobaltite Co(AsS) • Marcasite FeS2(lighter yellow; orthorhombic) • Arsenopyrite Fe(AsS)

  8. Sulphidegenesis: • Magmatic ore forming processes • Hydrothermal ore forming • processes and deposits

  9. Magmatic ore-forming processes Layered complexes, e.g.: Bushveld Complex Immiscible liquids, e.g.: sulfide deposits: Ni-Cu at Sudbury

  10. Hydrothermal ore-forming processes • Hypothermal (500 – 300ºC) • Mesothermal (300 – 200ºC) • Epithermal (200 – 50 ºC) • Hypogene: • Deposits formed during the upward migration of mineralising solutions • Supergene • Deposits formed by descending solutions

  11. Hydrothermal ore-forming processes • Most common sulfide deposits are hydrothermal • Precipitation from aqueous or carbonaceous-aqueous fluids and liquid solutions • Porosity or fracturing required to have sulfide-bearing solutions mobilised through rocks • Alter composition of rocks • E.g.: Greisens: enriched in tin (cassiterite), tungsten (scheelite, wolframite) and gemstones (beryl, topaz, tourmaline)

  12. Hydrothermal ore-forming processes • Plutonic hydrothermal deposits • Solution from crystallizing granitic magma, enriched in heavy metals: forms pegmatites (veins) surrounding granite • Pneunomatolitic deposits • Fractured zone adjacent to granitic intrusion; quartz, cassiterite and scheelite precipitate from supercritical water in fractures – Sn and W • Skarns • At contacts of granitic intrusions and limestone/marble; calcsilicate minerals form; altered by hot supercritical liquid; W, Sn, Zn, Cu, Pb, Be • Porphyry copper deposits • Shallow granitic intrusion underlying stratovolcanoes; alteration zone and vein network with metal ores; Cu and Mo deposits • Volcanogenic massive sulfides • Hydrothermal systems vent on seafloor depositing sediments with Fe, Cu, Zn, Pb sulfides – Black smokers • Hydrothermal sedimentary deposits • During deposition and diagenesis metal rich water replace pore water; sulfides precipitate during compaction; Cu, Pb-Zn-Fe, Fe-Mn deposits • Stratiform deposits • Sedimentary & volcanoclastic rocks; brine precipitate Cu-Pb-Zn sulfides in intergranular spaces of shales, sandstones, carbonate rocks

  13. Hydrothermal ore-forming processes:Plutonic hydrothermal deposit

  14. Hydrothermal ore-forming processes:Porphyry copper

  15. Hydrothermal ore-forming processes:Volcanogenic massive sulfides

  16. Black smokers

  17. Riftia pachyptila(Tube worms)

  18. Calyptogena magnifica(Mollusks)

  19. Alvinella pompejana

  20. Crabs

  21. GLG 214Mineral ProjectAvailable on Blackboard • You have been assigned a mineral to investigate. Describe this mineral optically, physically, crystal chemically and chemically and describe the formation process(es) whereby this mineral is formed. Give the typical minerals that occur associated with your mineral and give the rock types in which it occurs. Give the most important uses for this mineral and state at least two locations in Southern Africa where this can be found. If the mineral does not occur in Southern Africa, give two international locations for it. • NB: Use at least three different non-internet references for this study and use scientific referencing of all the information. Websites can also be used as extra references, make sure to add this referencing in the correct format. Remember to include ‘in-text’ referencing. • 1000-1500 words • Due: Tuesday 26 April 2011 • Typed report • Evaluation criteria in your study guide. THERE IS ALSO AN ASSIGNMENT FOR GLG212 ON BLACKBOARD.

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