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
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
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+
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…
How about the following: ZnS + 2 H+ + 0.5 O2 + Cl- = ZnCl+ + S2- + H2O Compared to That is a BIG difference…
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!
Piquette Mine • 1-5 nm particles of FeOOH and ZnS – biogenic precipitation • Tami collecting samples
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
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
Model Application • Use these techniques to better understand ore deposit formation and metal remediation schemes
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!
Hydrothermal Ore Deposits • Thermal gradients induce convection of water – leaching, redox rxns, and cooling create economic mineralization
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)
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
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