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Hydrotherman Solution ore deposits chalcopyrite hydrothermal vein deposits galena sphalerite molybdenite solubility solution sulfides porphyry deposits
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DM Sherman, University of Bristol
Geochemistry D.M. Sherman, University of Bristol
Lithophile = oxides, silicates Siderophile = Fe alloys Chalcophile = sulfides
DM Sherman, University of Bristol
Hypothermal (300-600 oC) Mesothermal (200-300 oC) Epithermal (50-200 oC) Sulfide Ore Minerals Molybdenite MoS 2 Pyrrhotite Fe1-xS Chalcopyrite CuFeS 2 Chalcopyrite, CuFeS 2 Bornite, Cu 5 FeS 4 Galena, PbS Sphalerite, ZnS Arsenopyrite, FeAsS Cinnabar, HgS Stibnite, Sb 2 S 3 Argentite, Ag 2 S Gangue Minerals Quartz Tourmaline Topaz Micas Quartz Carbonates Barite Quartz Chalcedony Opal Calcite
DM Sherman, University of Bristol
How are metals such as Cu, Zn, Au and Pb concentrated into ore deposits? What chemical signatures can we use to find ore deposits? Are there vast resources at depth that we haven’t yet discovered?
DM Sherman, University of Bristol
pK = -(ln K)/2.303 = ΔG^0 /(2.303RT) = ΔH^0 /(2.303RT) - ΔS^0 /(2.303R) € pK( T ) = pK( 298 ) +
If we assume ΔH^0 and ΔS^0 are constant with T, then Since lnK = -ΔG^0 /RT we find,
ZnS + 2H+^ = Zn+2^ + H 2 S Under acidic conditions, we can express the dissolution of sphalerite as For this reaction, pK = 4.44 and ΔH^0 = 14.0 kJ/mol at 298 K. pK = pZn + pH 2 S - 2pH
DM Sherman, University of Bristol
Fluid inclusions in mineral grains preserve samples of hydrothermal solutions. Upon cooling, the hydrothermal brines separate into solid (usually NaCl, gas (CO 2 + CH 4 ) and aqueous phases. The temperature at which the fluid was trapped can be determined by heating the sample and measuring the temperature at which gas + liquid recombine.
Zn+2^ + Cl- = ZnCl+ Zn+2^ + 3Cl-^ = ZnCl 3 - Zn+2^ + 2Cl- = ZnCl 2 Zn+2^ + 4Cl-^ = ZnCl 4 - Zn(H 2 O) 6 + nCl = ZnCln + 6H 2 O pK = -0.2; ΔH = 43.3 kJ/mol pK = -0.25; ΔH = 31.2 kJ/mol pK = 0.02; ΔH = 22.6 kJ/mol pK = -0.86; ΔH = 5.0 kJ/mol (^) Complexation is driven by the entropy increase when solvation waters are released.
DM Sherman, University of Bristol
We can combine the reaction ZnS + 2H+^ = Zn+2^ + H 2 S (pKZnS; ΔHZnS) with each complexation reaction Zn+2^ + nCl-^ = ZnCln2-n^ (pKn; ΔHn) to get the reactions ZnS + 2H+^ + nCl = ZnCln2-n^ + H 2 S with pK = pKZnS + pKn and ΔH = ΔHZnS + ΔHn
pK ( T ) = pK ( 298 ) +
= pZnCln 2 − n
−^ pH 2 S^ +^ 2pH^ +^ npCl
DM Sherman, University of Bristol
DM Sherman, University of Bristol
Phyllic : 3KAlSi 3 O 8 + 2H+^ = KAl 3 Si 3 O 10 (OH) 2 + 6SiO 2 + 2K+ Argillic : 2KAl 3 Si 3 O 10 (OH) 2 + 2H+^ + 3H 20 = 3Al 2 Si 2 O 5 (OH) 4 + 2K+ Potassic Ore zone: CuCl 2 + FeCl 2 + 2H 2 S = CuFeS 2 + 4H+^ + 4 Cl-
