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Page 1
Roberto Perez Xavier
Departamento de Geologia e Recursos Naturais
Instituto de Geociências – UNICAMP
Campinas (SP) - Brasil
TRANSPORT AND DEPOSITION OF METALS BY HYDROTHERMAL FLUIDS
Hydrothermal fluids are subjected to changes in T, P, composition, pH, redox conditions during fluid/rock or fluid/fluid interactions in hydrothermal systems ���� affect metal solubilities and may cause deposition of ore and gangue minerals
Page 2
METAL SOLUBILITY IN HYDROTHERMAL
FLUIDS
Seward & Barnes (1997)
Metal concentrations on the basis of fluid inclusions in ore deposits and active geothermal fields. Cu, Pb and Zn (black circles); Ag, Au and Hg (open circles).
Dotted lines = minimum concentration values
Cu – Pb - Zn
Ag - Au
10 p
pm
1 pp
m
1 ppb Au e 10 ppb Ag
Cu – Pb - Zn
Au-Ag << base metal in hydrothermal solutions
May reflect crustal source !!
Yardley (2005)
METAL SOLUBILITY IN HYDROTHERMAL FLUIDS ....
Magmaticfluids
...... depends upon TEMPERATURE
Page 3
magmático
bacinal
geotermal
metamórfico
Yardley (2005)
Metal solubility in hydrothermal fluids depends upon sal inity
Ore-forming fluids are chemically similar to other crustal fluidsControlled by reactions with (crustal) rocks
NaClNaCl DissolutionDissolution
NaNa++ envelopedenveloped byby HH22O O moleculesmolecules ((hydrationhydration))
o)o)
ClCl -- envelopedenveloped byby HH22O O moleculesmolecules por por
moléculas de moléculas de água água ((hydrationhydration))
Page 4
SOLUBILIDADE DE METAIS EM FLUIDOS HIDROTERMAIS SOLUBILIDADE DE METAIS EM FLUIDOS HIDROTERMAIS O fator temperaturaO fator temperatura
Hydrothermal transport of gold
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HYDROTHERMAL FLUIDS: METAL TRANSPORT
Metals ARE NOT transported as single ions but as ionic complexes
What complexes would be more important to transportmetals in hydrothermal fluids?
Ligant must be available in the
fluid
Ionic complex must be stable at high P-T condiitons
AuI 2-
AuBr 2-
AuCl 2-
Au(HS)2-
Au(HS)0
AuNH3)2+
Au(CN)2-
Broadlands Salton Sea
T(oC) 260 320
pH 6,2 4,2
Cl- 1184 118.202
Br- 4,3 91,6
I- 0,5 13,6
NHn 1148 374
CO2+HCO3-+CO3
- 5278 570
HSO4- + SO4
-2 5,4 3,7
H2S + HS- 136 15,9
Complexos Constante de equilíbrio
AuI2- 19
AuBr2- 12,4
AuCl2- 9,2
Au(HS)2- 30,1
Au(HS)0 24,5
AuNH3)2+ 26,5
Au(CN)2- 38,7
What complexes would be more important to transport Au in thesehydrothermalfluids?
Page 6
Fluid inclusions and active geothermal systems reveal tha tCl- and HS - are the most important ligands for a wide range of metals
pH=4, 1m NaCl, aH2S= 10-3, SO4/H2S= 10-1
TEMPERATURA °C
150 200 250 300 350
0
2
ZnCl2-
Au(HS)2- AuCl 2
-
Cu(HS)2-
CuCl2-
SO
LUB
ILID
AD
E (
log
ppm
)
-4
-2
Cu&Zn Au
Seward & Barnes (1997)
HYDROTHERMAL FLUIDS: METAL TRANSPORT
FLUIDOS HIDROTERMAIS: TRANSPORTE E
DEPOSIÇÃO DO OURO
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4 Au(HS)2- + 2 H2O + 4 H+ = 4 Au0 + 8 H2S + O2
temperature ����
����pH
ƒƒƒƒO2����
aH2S ����
HOW DO METALS PRECIPITATE FROM HYDROTHERMAL HOW DO METALS PRECIPITATE FROM HYDROTHERMAL FLUIDS: FLUIDS: AuAu
300°C Seward (1982) e Brown (1986)
Sulphide alteration (reduces < ΣS of the fluid)
FeO (sillicato ou óxido ) + Au(HS) -2 + O2 = Au0 + FeS2 + quartz +H 2O
Interaction with carbonaceous rocks
2C + 2H2O = CH4 + CO2 ���� < fO2
Oxidation : it promotes Au transport until saturation at highƒO2 (H2S + HS- + SO4
2- see diagram) ���� interaction withoxidized rocks (hematite-bearing), fluid mixing orboiling/immiscibility
HOW DO METALS PRECIPITATE FROM HYDROTHERMAL HOW DO METALS PRECIPITATE FROM HYDROTHERMAL FLUIDS: FLUIDS: AuAu
Page 8
FLUID PHASE SEPARATION: BOILING - IMMISCIBILITY
Audétat et al. (2008)
< ΣS of the fluid = H2S/HS- fraccionate to the vapour phase
Au(HS) -2 + 1/2 H2 = Au + H2S + HS-
> pH = CO2 removal
���� H2CO3 ���� HCO3- + H+
O SISTEMA H2O - CO2 – NaCl
Imiscibilidade (efervescência)
Gehrig et al. (1986)
Page 9
4 AuCl 2- + 2 H2O = 4 Au0 + 4 H+ + 8 Cl- + O2
temperature
����pH
ƒƒƒƒO2����
aCl- ����
����
2 4 6 8 10
pirita
pirrotita
hematita
300°C
-4-3
-40
-35
-30
-25
ƒƒƒƒO2
pH
AuCl 2-
AuCl 2-: ΣΣΣΣS= 0,5x10-2
Seward (1982) e Brown (1986)
Fluid mixing : < aCl-, >fO2
HOW DO METALS PRECIPITATE FROM HYDROTHERMAL HOW DO METALS PRECIPITATE FROM HYDROTHERMAL FLUIDS: FLUIDS: AuAu
1m NaCl; Kf+muscovite+qtz; hm+mgt+py (HMP) e mgt+py+po (MPP)
400°C – 0,5 kb; hm+mgt+py (HMP) e mgt+py+po (MPP)
Liu & McPhail (2005)
CuCl x1-x + 1/2 H2O + FeCly2-y + S29g) = CuFeS2 + (x+y) Cl - + 3H+ + 0,75 O2
HOW DO METALS PRECIPITATE FROM HYDROTHERMAL HOW DO METALS PRECIPITATE FROM HYDROTHERMAL FLUIDS: Cu FLUIDS: Cu
Page 10
temperature
����pH
ƒƒƒƒO2����
aCl- ����
����
200 250 300temperature (°C)
-8
-6
-4
-2
0
2
4
33
55
3: 103 mg/kg chloride5: 105 mg/kg chloride
log
solu
bilit
y (m
g/kg
)
PbCl 2 + H2S (aq) ���� PbS + 2H+ + 2Cl-
ZnCln 2-n + H2S(aq) ���� ZnS + 2H+ + nCl -
HOW DO METALS PRECIPITATE FROM HYDROTHERMAL HOW DO METALS PRECIPITATE FROM HYDROTHERMAL FLUIDS: FLUIDS: PbPb -- Zn Zn
Why is salinity important?Cl- is the prime ligand for a wide range of metals
400°C – 0,5 kb; hm+mgt+py (HMP) and mgt+py+po (MPP)
Na – K – Ca – Mg chlorides are highly solubles !!
Liu & Phail (2005)
Highly saline fluidstransport metals more efficiently than dilutedfluids at the sameconditions
If Cl - is the main ion whychlorides are not commongangue minerals in ore deposits?
Page 11
But sulphur is also important !Necessary for the precipitation of metals as sulphides
CuCl x1-x + 1/2 H2O + FeCly2-y + S2(g) =CuFeS2 + (x+y) Cl - + 3H+ + 0,75 O2
Barton & Johnson (1996)
Without enoughsulphur, metals tend
to remain in thehydrothermal fluid
Barton & Johnson (2000)
1. fluids with���� Cl e ���� S (metals > lineΣ S) precipitate Fe oxides andlowsulphidationassemblages
3. Modrate salinity fluids (e.g., sea H 2O) = transport Pb, Zn, Fe
2. ���� Cl (meteoric) = not eficient to transport Cu and Fe, butmay transportAu, Pb, andZn
Page 12
The Fe solubility would increase with increasing temperat ure, but if fluids are reduced, the content of dissolved sulphu rwould limit the concentration of dissolved Fe. The lowsulphide-content and abundance of minerals with ferric iro nat a deposit, indicates a more oxidized fluid. In na ox idizedfluid, more of the sulphur would be present as SO 4
2- andwould thus not greatly limit Fe solubility at hightemperatures.
AUAU VERSUSVERSUS BASEBASE METALSMETALS
http://www.unalmed.edu.co
Au, Ag, Zn solubilities as a function of S and Cl con centrations
Page 13
REFERENCESBarnes, H.L..1997. Geochemistry of Hydrothermal Ores Deposits, John Wiley, 972 p.
William-Jones, A.E.; Bowell, R.; Migdisov, A.A. (2009). Gold in solution. Elements, 5: 281-287.
Yardley, B. (2005). Metal concentrations in crustal fluids and their relationship to ore formation. Econ Geol 11th Anniversary Special Paper, 100: 613 - 632