3. Hydrothermal Fluids

8/12/2019 3. Hydrothermal Fluids

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Roberto Perez Xavier

Departamento de Geologia e Recursos Naturais

Instituto de Geocincias UNICAMP

Campinas (SP) - Brasil

HYDROTHERMAL FLUIDS: HOW THEY FORM ANDEVOLVE IN THE EARTHS CRUST

Minerals Volatiles in the composition (wt %)

Chlorite 10% H2O

Actinolite/

Hornblende

2% H2O

Biotite 3,2 % H2O

Phlogopite 3,2 3,5% H2O

Epidote 1% H2O

Calcite 45% CO2

Dolomite 40% CO2

Pyrite 50% S

Pyrrhotite 30% S

Hydrous silicates (OH) + carbonates (CO2) + sulphides (H2S) - sulphates(SO4)

FLUIDS IN THE LITHOSPHERE: INDIRECT EVIDENCE ?FLUIDS IN THE LITHOSPHERE: INDIRECT EVIDENCE ?


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1 m

Au-bearing quartz boudins Morro do Ouro (Paracatu/Brazil)

FLUIDS IN THE LITHOSPHERE: INDIRECT EVIDENCE ?FLUIDS IN THE LITHOSPHERE: INDIRECT EVIDENCE ?

Average grade of 0.3 g/t

1 50 m

Loriesfontein area (Karoo Basin,South Africa). Bright blobs hydrothermal pipes filled withbrecciated and metamorphosedshale CH4 degassed duringcontact metamorphism of black,organic-rich shale, some 182.5million years ago.

Jamtveit & Austrheim 2010, Elements, 6

Mesh texture generated duringserpentinization of olivine in aserpentinized peridotite from theLeka ophiolite complex, Norway


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ACTIVE GEOTHERMAL SYSTEMS IN THEOCEANIC CRUST

NEPR, SEPR = East Pacific Rise

MAR = Mar Crest

RR = Iceland

SWIR and SEIR = Southwest andSoutheast Indian Ridge

Fumaroles or black smokers

FLUIDS IN THE LITHOSPHERE: DIRECT EVIDENCE ?FLUIDS IN THE LITHOSPHERE: DIRECT EVIDENCE ?

HOT SPRINGS OR GEYSERS

V

L

S1

S3

S2

S4

ACTIVE GEOTHERMAL SYSTEMS CONTINENTAL CRUST

FLUID INCLUSIONS

FLUIDS IN THE LITHOSPHERE: DIRECT EVIDENCE ?FLUIDS IN THE LITHOSPHERE: DIRECT EVIDENCE ?


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WHAT IS A FLUID?

H2O + salts + volatiles (CO2, CH4, N2, H2S, etc.) lIquid +

vapor/gases + silicate melts (magma)

Above critical point(Pc), there is no

distinction betweenliquid and vaporphases similardensities =supercritical fluid

Viscosity of magmabasltico 103 104 Pwhereas H2O = 10-2 P)

Reviews in Mineralogy & GeochemistryVol. 76 pp. 165-218, 2013

Pirajno (2009)

FLUIDS IN THE LITHOSPHEREFLUIDS IN THE LITHOSPHERE


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Wyborn 2005

HYDROTHERMAL SYSTEMSHYDROTHERMAL SYSTEMS

from the source to the ore !!from the source to the ore !!

HYDROTHERMALHYDROTHERMAL FLUIDSFLUIDS:: HOWHOW DODO THEYTHEY FORM?FORM?

Figure 1. Known sites of hydrothermal venting along mid-ocean ridges, in back-arc basins, rifted arcs, and at submergedisland-arc volcanoes (red), and areas of activity as indicated by mid-water chemical anomalies (yellow). EPR = East PacificRise. TA G= Trans Atlantic Geotraverse, MEF = Main Endeavour Field, and GR-14 = Sea Cliff hydrothermal field on thenorthern Gorda Ridge.

Tivey, M. K.2007,Oceanography,20 (1)


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P > 22 Mpa =0.22 kb

T= 00C

4000

C

pH ~ 3

Seawater evolution within the oceanic crust

Humphris et al. (1996)



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Seawater Hydrothermalfluid

T (C) 2 360 - 365pH (25C) 7.8 3.35Na (mM) 464 537K (mM) 9.8 17.1Ca (mM) 10.2 30.8Mg (mM) 52.7 0Si (mM) 0.2 20.75

Cl (mM) 541 636

SO4(mM) 27.9 0H2S (mM) 0 2.3 3.5

Fe (M) > S2-

High Mg2+

Metal poor

3.2% NaCl

Hydrothermal Fluid

350C

Acidic (pH 4.6)

Reducing S2-

>>SO42-

Poor Mg2+

Enriched in metals

3.2% NaCl



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1. Acid pH2Ca2+ + Fe3+ + 2Al3+ + 3SiO2 + 7H2O = Ca2FeAl2Si3O12(OH) + 13H+

Silicates in mafic fluid epidote

rocks

2. Oxidizing to reducing

11Fe2SiO4 + 2SO42- + 4H+ = FeS2 + 7Fe3O4 + 11SiO2 + 2H2O

Mafic fluid pyrite magnetite

minerals

3. Fluid- rock interaction = loss of Mg2+

2NaAlSi3O8 + 5Mg2+ + 8H2O = Mg5Al2Si3O10(OH)8 + 2Na+ + 8H+ + 3SiO2

Albite fluid chlorite quartz


SUBMARINE HYDROTHERMAL SYSTEMSSUBMARINE HYDROTHERMAL SYSTEMS

Sulphides = 3.9 x 106 ton Fe = 2.3 x 106 ton Cu = 30 - 60 x 103 ton Zn = 15.2 x 103 ton Hydrothermal alteration zone = 0.4 0.7 km3

Required energy = 1000 MW

Humphris & Tivey (2000), GSA 349

TAG = Trans-Atlantic Geotraverse (Cadeia Meso-Atlntica 2608N)


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TYPES OF HYDROTHERMAL FLUIDSTYPES OF HYDROTHERMAL FLUIDS

Barnicoat , A. (2009)

8.7 x 1011 kg/year = mass of H2O incorporated into thelithosphere in subduction zones

GEOTHERMAL FLUIDS: EVOLVED METEORIC HGEOTHERMAL FLUIDS: EVOLVED METEORIC H22OO

high geothermal gradients orogenic (convergent margins) and non-orogenic areas (intracontinental rifts)Geothermal waters (250C. 1-2 km) are the present-d ay analogues ofepithermal Au-Ag deposits

Mostly recycled rainwater

nzic.org.nz/ChemProcesses/water/13A.pdf


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omposition(mg/kg) Na-ClNearneutralchloride

SO4 ClAcidsulphate

Na-CO3Alkalicarbonate

Seawater RiverH2O

SiO2 660 490 175 0.005 0.01

13

Na 1200 485 220 10760 4.8Ca 17.5 1.2 37 411 15K 200 58,5 43 399 2Cl 2156 560 57 19350 5.7

HBO2- 115 21.6 1.2 - -

SO4- 25 88 < 1 2710 6,7

HCO3- 32 167 3177 142 23

pH 4.8 3.1 6.2 8.1 8.3 5 6.5

Na Ca Mg - HCO3- alumino silicate rocks

Ca Mg - HCO3- limestone/dolomite

nzic.org.nz/ChemProcesses/water/13A.pdf



Size of meteoric systems may be huge

Transit time for fluid ~2 Myr

Generally of low salinity (< 3.2 wt% NaCl) and dont carry a lot of metals,but can be directly important - mixing with other fluids = dilution

2H+(aq) + 2KAlSi3O8 + 9H2O2K+(aq) + Al2Si2O5(OH)4 + 4H4SiO4(aq)feldspar rock kaolinite dissolved silica

Similar reactions provide Na+(aq) and Ca2+(aq) to the geothermal waterthat becomes more acidic and oxidising.

2H2S + 3O22SO2 + 2H2OH2S + 2O2 SO42- + 2H+

Both hydrogen sulfide ions (HS-) and chloride ions (Cl-) can form

complexes with metal cations derived from the magmas or by fluid-rockreactions stable at high temperatures and high concentrations ofmetals.

PbCl3-(aq) + H2S(aq) PbS(s) + 2H+(aq) + 3Cl-(aq)


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MAGMATIC FLUIDSMAGMATIC FLUIDS

Vulcanos Augustine(Greece)

Etna

(Italy)

St. Helens(USA)

Magma andesitic basaltic dacitic

T(C) 870 928 710

H2O 83.9 91.9 98.6

CO2 2.4 1.4 0.8

SO2 5.72 2.8 6.7x10-2

H2S 1.00 - 9.0x10-2

HCl 6.0 0.1 7.6x10-2

HF 8.6x10-2 0.5 0.03

NaCl 1.4x10-3 1.3x10-3 4.1x10-4

Analyses in moles/100 moles de gsSymonds (1992)

MAGMATIC FLUIDSMAGMATIC FLUIDS


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MAGMATIC FLUIDS AND VOLCANICMAGMATIC FLUIDS AND VOLCANICERUPTIONSERUPTIONS

Hedenquist & Lowenster (1994)

Etna (Italy) 1975 to 1987

T (C) 900

H2O 50 x 106 t / yr

CO2 13 x 106 t / yr

HCl 0.1 0.5 x 106 t / yr

S 0.2 0.75 x 106 t / yr

Cu 480 580 t / yr

Au 80 1.200 kg / yr

Salinity dependson DEPTH(PRESSURE) ofmagmacrystallization

MAGMATICMAGMATICFLUIDS:FLUIDS:

SALINITYSALINITY


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MAGMATIC FLUIDS: VOLATILES (COMAGMATIC FLUIDS: VOLATILES (CO22 AND Cl)AND Cl)

CO2 is 10 X less soluble thanH2O in silicate melts

CO2 solubility increases linearlywith pressure CO2-rich fluidsare generated by magmas thatcrystallize at deep crustal levels

Solubility plot for system rhyolite-H2O-CO2 at675C.

Cl is less volatile than CO2 limited solubility in exsolvingvapors Fe, Na, K chloride complexes remain in the meltuntil saturation of an aqueous phase

Addition of H2O to a meltdecreases the solubility of CO2

Baker (2002)

CO2- H2O Brine

+

v vv vvvv

+++

++

+

+++

+++

++

+

+++

+

++

+

0 km

5 km

10 km

MAGMATIC FLUIDS: VOLATILES (COMAGMATIC FLUIDS: VOLATILES (CO22 AND Cl)AND Cl)


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FLUID PHASE SEPARATION: BOILING -IMMISCIBILITY

Audtat et al. (2008)

Efficient mechanism for the precipitation of metals byh drothermal fluids !!

MAGMATIC HYDROTHERMAL SYSTEMS


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Formation waterwater present in pores and fractures no

significance to origin or age

Connate water

water trapped with the sediment andsubsequently unmodified.

Sedimentary basin waters have a range of originsconnate,(modified) meteoric water

BASINAL FLUIDS (OILBASINAL FLUIDS (OIL--FIELD BRINES)FIELD BRINES)

Most basinal fluids (formation waters) are ofrecent meteoric origin

Conc.(ppm)

Louisiana Mississippi Seawater

TDS 235.700 230.000 35.200Na

+78000 54200 10760

Ca+

10250 27600 412K

+1065 485 399

Fe+

84 181 0,002Mg

+1140 1770 1294

SiO2 48 42 6,2Zn+

- 143 0,02Pb

+- 28 0,00003

Cl- 143000 143600 19350H2S 0.4 - -

SO4-

0.4 248 2712HCO3

- 450 - 45T(C) 150 150 2pH 6.2 6.2 8,2

salinity up to 6.6higher than

seawater:origin of highsalinity?

BASINAL FLUIDS (OILBASINAL FLUIDS (OIL--FIELD BRINES)FIELD BRINES)

Hanor(1995)

Variablecomposition, butgenerally Na-Ca-Cl-rich

Fe, Zn, Pb 103 to 106 higher thanseawater

Interaction withevaporites= halitedissolution

Seawater evaporationto the halite saturationpoint = bittern fluids


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Sediment-hosted Pb-Zn deposits:SEDEX and MVT

BASINAL FLUIDS AND ORE DEPOSITSBASINAL FLUIDS AND ORE DEPOSITS

PELITIC ROCKS

Clay minerals (15-20 % H2O)

chlorite (10-12 % H2O)

biotite + muscovite (3-4 % H2O)

staurolite + cordierite (2 % H2O)

CaMg(CO3)2 + 2SiO2 = CaMgSi2O6 + 2CO2dolomite quartz diopside

CARBONATE ROCKS

METAMORPHIC FLUIDSMETAMORPHIC FLUIDS

metamorphism

metamorphism


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GERAOGERAO DEDE FLUIDOSFLUIDOS NONO METAMORFISMOMETAMORFISMO

Folhelho granada mica xisto

(Jamtveit, 2010)

Salinities may be higher if evaporites are involved in themetamorphic process

Metamorphic fluids tend to be CO2-rich in highermetamorphic facies (e.g. granulite)

H2O + (C O2 + CH4 + N2 + H2S), 5-6 % NaCl

COMPOSITION OF METAMORPHIC FLUIDS

Maximum fluid flux ~10-11 m3/m2/s (very small)

Duration


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METAMORPHIC FLUIDS AND OROGENIC GOLDDEPOSITS

Low-salinity, mixed aqueous-carbonic fluid broadlyuniform over deposits formed at different crustal levels (6km to 20 km)

Aqueous solutions

Diluted (0.2 - 0.5% salts) to highly concentrated (> 25%salts) solutions abundant Na+ e Cl-

Metals: ionic complexes [e.g. Au(HS)-2 ; AuCl-2 ] T, pHand redox control solubility

Variable temperatures: 50C to >500C

pH: acid to slightly alkaline

Volatiles: CO2 (CH4, N2, H2S, SO2) control the redoxstate (O2) and metal solubility

HYDROTHERMAL FLUIDS ?HYDROTHERMAL FLUIDS ?

NO GENETIC IMPLICATION !!


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MECHANISMS OF FLUID FLUX IN THE CRUST

Etheridge et al. 1983

-8 -6 -4 -2 0

Deformation

Convection

Topographic/meteoric

Metamorphism

Compaction

log fluid flux (m/yr)

Heather Sheldon (2008 )

Intrusions

MECHANISMS OF FLUID FLUX IN THE CRUST

There is considerable overlap in flow rates, making itdifficult to predict which one will dominate.


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HYDROTHERMAL FLUIDS: SUMMARYHYDROTHERMAL FLUIDS: SUMMARY

Kesler (2005)

metamorphic

magmatic +meteoric

basinal

Evolvedseawater

Groves et al. (1998)

Magmatic

+

meteoric

TECTONIC SETTINGS, CRUSTAL FLUIDS AND ORETECTONIC SETTINGS, CRUSTAL FLUIDS AND OREDEPOSITSDEPOSITS


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FLUIDS IN DIFFERENT CLASSES OF GOLDFLUIDS IN DIFFERENT CLASSES OF GOLDDEPOSITSDEPOSITS

Ridley & Diamond (2000)

3. Hydrothermal Fluids

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