5. Transport of Metals by Hydrothermal Fluids

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

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

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))

SOLUBILIDADE DE METAIS EM FLUIDOS HIDROTERMAIS SOLUBILIDADE DE METAIS EM FLUIDOS HIDROTERMAIS O fator temperaturaO fator temperatura

Hydrothermal transport of gold

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?

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

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


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)

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


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

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?

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

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

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

5. Transport of Metals by Hydrothermal Fluids

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