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Understanding Water-Rock Interactions in Ore Deposit Formation

Explore the intricate processes of water-rock interactions that lead to ore deposit formation, including mechanisms like magmatic cumulate deposits and hydrothermal mineralization. Discover the importance of metal sulfide mineral solubility and geochemical traps in concentrating valuable metals. Leverage these insights for better ore deposit formation understanding and metal remediation strategies.

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Understanding Water-Rock Interactions in Ore Deposit Formation

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  1. Water-rock interactions • To concentrate a material, water must: • Transport the ions • A ‘trap’ must cause precipitation in a spatially constrained manner • Trace metals which do not go into igneous minerals easily get very concentrated in the last bit of melt • Leaching can preferentially remove materials, enriching what is left or having the leachate precipitate something further away

  2. 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

  3. Metal Sulfide Mineral Solubility • Problem 1: Transport of Zn to ‘trap’: ZnS + 2 H+ + 0.5 O2 = Zn2+ + S2- + H2O Need to determine the redox state the Zn2+ would have been at equilibrium with… What other minerals are in the deposit that might indicate that?  define approximate fO2 and fS2- values and compute Zn2+ conc.  Pretty low Zn2+

  4. Must be careful to consider what the conditions of water transporting the metals might have been  how can we figure that out?? • What other things might be important in increasing the amount of metal a fluid could carry? More metal a fluid can hold the quicker a larger deposit can be formed…

  5. How about the following: ZnS + 2 H+ + 0.5 O2 + Cl- = ZnCl+ + S2- + H2O Compared to That is a BIG difference…

  6. 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 • Sulfide (reduced form of S) strongly binds metals  many important metal ore minerals are sulfides!

  7. Piquette Mine • 1-5 nm particles of FeOOH and ZnS – biogenic precipitation • Tami collecting samples

  8. cells ZnS

  9. Piquette Mine – SRB activity • At low T, thermochemical SO42- reduction is WAY TOO SLOW – microbes are needed! • ‘Pure’ ZnS observed, buffering HS- concentration by ZnS precipitation

  10. ZnS ZnS y Pb2+ x Zn2+ ZnS PbS Fluid Flow and Mineral Precipitation • monomineralic if: • flux Zn2+ > HS- generation • i.e.  there is always enough Zn2+ transported to where the HS- is generated, if • sequential precipitation if: • Zn2+ runs out then HS- builds until PbS precipitates z HS- generated by SRB in time t

  11. Model Application • Use these techniques to better understand ore deposit formation and metal remediation schemes

  12. Sequential Precipitation Experiments • SRB cultured in a 125 ml septum flask containing equimolar Zn2+ and Fe2+ • Flask first develops a white precipitate (ZnS) and only develops FeS precipitates after most of the Zn2+ is consumed • Upcoming work in my lab will investigate this process using microelectrodes  where observation of ZnS and FeS molecular clusters will be possible!

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

  14. Ore deposit environments • Sedimentary • Placer – weathering of primary mineralization and transport by streams (Gold, diamonds, other) • Banded Iron Formations – 90%+ of world’s iron tied up in these (more later…) • 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)

  15. Ore Deposit Types I • Placer uranium gold • Stratiform phosphate • Stratiform iron • Residually enriched deposit • Evaporites • Exhalative base metal sulphides • Unconfornity-associated uranium • Stratabound clastic-hosted uranium, lead, copper • Volcanic redbed copper • Mississippi Valley-type lead-zinc • Ultramafic-hosted asbestos • Vein uranium • Arsenide vein silver, uranium • Lode Gold

  16. Ore Deposit Types II • Clastic metasediment-hosted vein silver-lead-zinc • Vein Copper • Vein-stockwork tin, tungsten • Porphyry copper, gold, molybdenum, tungsten, tin, silver • Skarn deposits • Granitic pegmatites • Kiruna/Olympic Dam-type iron, copper, uranium, gold, silver • Peralkaline rock-associated rare metals • Carbonatite-associated deposits • Primary diamond deposits • Mafic intrusion-hosted titanium-iron • Magmatic nickel-copper-platinum group elements • Mafic/ultramafic-hosted chromite

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