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 log K 9 57 log Zn 2 f S 2 H 2 O H 2 f O02 5 ZnS 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 log K 16 6 log ZnCl f S 2 H 2 O H 2 f O02 5 ZnS Cl Compared to log K 9 57 log Zn 2 f S 2 H 2 O H 2 f O02 5 ZnS 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 cells ZnS 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 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 y Pb 2 x Zn2 z HS generated by SRB in time t ZnS ZnS ZnS PbS 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
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