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 NB economic minerals: major ores for Cu, Zn, Pb, Hg, Bi, Co, Ni, As, Sb NB mineral properties in Table 24.1

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, CuFeS 2 –Sulfur ions are usually polarized to form large diversity of crystal structures –Most sulfides have combination of metallic, covalent and also ionic bonding ZnS

Crystal chemistry Stibnite Pyrite Molybenite

Important sulfide minerals Chalcosite Cu 2 S Galena PbS Sphalerite (Zn,Fe)S Cinnabar HgS Pyrrhotite Fe 0.83 S to FeS Pentlandite (Ni,Fe,Co,Ag) 9 S 8 Nickeline NiS

Important sulfide minerals Realgar As 4 S 4 Orpiment As 2 S 3 Stibnite Sb 2 S 3 Molybdenite MoS 2 Chalcopyrite CuFeS 2 Bornite Cu 3 FeS 4

Important sulfide minerals Tetrahedrite Cu 12 (SbS 3 ) 4 S Pyrite FeS 2 (isometric) Cobaltite Co(AsS) Marcasite FeS 2 (lighter yellow; orthorhombic) Arsenopyrite Fe(AsS)

Sulphide genesis: Magmatic ore forming processes Hydrothermal ore forming processes and deposits

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

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

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)

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

Hydrothermal ore-forming processes: Plutonic hydrothermal deposit

Hydrothermal ore-forming processes: Porphyry copper

Hydrothermal ore-forming processes: Volcanogenic massive sulfides

Black smokers

Riftia pachyptila (Tube worms)

Calyptogena magnifica (Mollusks)

Alvinella pompejana

Crabs

GLG 214 Mineral Project Available 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 words Due: Tuesday 26 April 2011 Typed report Evaluation criteria in your study guide. THERE IS ALSO AN ASSIGNMENT FOR GLG212 ON BLACKBOARD.