The porphyry copper system: hydrothermal alteration€¦ · Porphyry Cu (Au, Mo), epithermal, VMS,...

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Arribas & Hedenquist (2020) 1

Antonio ArribasThe University of Texas at El Paso

Ambientes genéticos de alteración argílica avanzada: relevancia en la exploración en sistemas pórfido-epitermal

The Grand Canyon of the Yellowstone (~1895), by T. Moran

Epithermal deposits - Alteration is ever present

Hedenquist et al. (2000)

Arribas (1995)

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The porphyry copper system: hydrothermal alteration

Sillitoe (2010)

… includes aluminosilicates (kaolinite, dickite, halloysite, pyrophyllite), plus alunite, and diaspore, andalusite, zunyite, and topaz”

Advanced argillic = advanced hydrolytic alterationHemley & Jones, 1964; Meyer & Hemley, 1967

Hedenquist et al. 1996

Al+3 Si+4SO4

-2K+

OH- F-Cl-PO4-3

Ca+2

H+HCO3-

Na+

Forms in acidic conditions: pH~2-<5 Residual quartz (vuggy silica): pH ~1

Shallow: typically <1 km

Epithermal deposits -Alteration is ever present

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Five environments: simplified chemical reactions

2 KAl3Si3O10(OH)2 + 2 H+ + 6 SiO2 à 3 Al2Si4O10(OH)2 + 2 K+

muscovite quartz pyrophyllite

4 SO2 + 4 H2O à 3 H2SO4 + H2S

H2SO4 à H+ + HSO4- HCl à H+ + Cl-

alunite, kaolinite, dickite,

pyrophyllite, diaspore, zunyite,

residual vuggy silica, etc.

CO2 + H2O à H2CO3 kaolinite (illite-smectite, Fe carbonates, pyrite)

H2S + O2 à H2SO4 kaolinite, alunite, native S, opal

Fe sulfide + H2O + 02 à Fe oxy-hydroxides + H2SO4kaolinite, halloysite,

alunite, jarosite

Minerals formed in acid aqueous (alteration) environments

Same 5 environments of acid alteration minerals

Cooling of a magmatic-hydrothermal fluid

Hypogene acid sulfate-chloride (SO2 + H2O, HCl, HF)

Steam-heated CO2-rich (CO2 + H2O)

Steam-heated acid-sulfate (H2S + O2)

Supergene oxidation of pyrite (FeS2 + O2)

320-260°

300-200°

170-120°

120-70°

<40°C(approx.)

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Five environments with ‘advanced argillic’ minerals

Modified from Sillitoe (2010)

Subepithermal

Porphyry

IS Epithermal

Sed-hosted disseminated

SkarnCarbonate replacement

Au-Cu HS Epithermal

HS Lode

1. Cooling of a magmatic-hydrothermal fluid

(white mica to pyrophyllite)

2. Hypogene acid sulfate-chloride

3. Steam-heated acid sulfate 4. Steam-heated

CO2-rich

5. Supergene oxidation of pyrite

<40°C(approx.)

70-120°120-170°200-300°260-300°

Forms: Above a porphyry system, cooling of white mica-stable fluid

Simplified reaction:2 KAl3Si3O10(OH)2 + 2 H+ + 6 SiO2

à 3 Al2Si4O10(OH)2 + 2 K+

2 muscovite + 6 quartz à 3 pyrophyllite

1. Cooling of white mica to pyrophyllite

Typical minerals:White mica (muscovite), pyrophyllite, diaspore (dickite) (nodular – patchy – replacement texture if in lithocap)

white mica

Porphyry intrusion

~1 k

m

à

pyrophyllite

white mica

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Damiana exotica

El Salvador porphyry Cu, Chile Looking east, late 1950s

Muscovite

Pyrophyllite

Watanabe & Hedenquist, 2001

Cu ore at 2600 m

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1

2

2) Hypogene vapor condensation Lithocap environment,

residual qtz, alunite halo, hotter pyrophyllite (roots)

Hemley & Jones, 1964; Watanabe & Hedenquist, 2001

pyrophyllite

muscovite± andalusite

Two environments of hypogene pyrophyllite:

2. Vapor condensation

El Salvador porphyry Cu deposit

K-Al-Si-H-O

ßlower pH

1) Fluid cooling (retrograde)Muscovite (A) to pyrophyllite (B) to dickite (C)

1. Cooling

Forms: Adjacent to volcanic vents, discharge of high-temperature magmatic vapors

Simplified reactions:4 SO2 + 4 H2Oà 3 H2SO4 + H2S,H2SO4 à H+ + HSO4

-

HCl à H+ + Cl-Cooling and dissociation leads to increased reactivity (<250–300oC)

2. Hypogene magmatic vapor condensation

Typical minerals: Core of residual (vuggy) quartz

(potential ore host)Alunite-kaolinite-dickite halo

(pyrophyllite in feeder zone)

~1 k

m

AluniteDickite- Kaolinite

Pyrophyllite

H2SO4 , HCl

White mica

K-silicate

H2O, CO2, SO2, H2S, HCl, metals

⤳ ⤳Hedenquist & Taran (2013)

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Hemley et al., 1969, Stoffregen, 1987

KA

l 3(S

O4)

2(O

H),

Alu

nitepH

~1, AlO

H soluble;

residual quartz

Residual quartz (vuggy)

Res.Qtz Alun Kao Mica Clay Chl

Steven & Ratté, 1960

~1 2 to 4 4 to 6 >6 pH

ß lower pH

Tabular, platy alunite

Martabe, Indonesia

Arribas et al., 2000

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Arribas et al., 2000

Forms: Within the vadose zone, above water table, by oxidation of H2S (pH = 2-3, ~70-120 °C, up to 150 °C)

Simplified reaction:H2S+ O2 à H2SO4(bacterial involvement)

3. Steam-heated, H2S oxidation

Typical minerals: Kaolinite, alunite, opal, cristobalite, native S, montmorillonite, jarosite

Sinter Terrace

Hot Springs

Hedenquist et al. (2000)

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Typical mud-pools (Goshogake Geothermal Area, Hachimantai, Akita)


4. Steam-heated, H2S oxidationLa Coipa, Chile, mining district, to west from 4700 m elevation

1 km

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4. Steam-heated, H2S oxidationLa Coipa, Chile, mining district, to west from 4700 m elevation

Coipa Norte: Steam-heated alunite-kaolin blanket, chalcedony, over residual quartz, Ag-Au ore

vuggy silica Ag-Au ore

chalcedony blanket


La Coipa, Chile, mining district: Puren open pit

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The old Sulphur district (Nevada) looking north: S, Hg, clay mined since the late 1800s


Native sulfur and cinnabar

Photo: mindat.org (R.Luetcke, 1978) Photo: JWH (~1982)

Silica sinter

Steam-heated acid-sulfate alteration blanket (overprint due to falling water table)

H2S + 2 O2 = H2SO4 (oxidation in vadose zone)

Qtz vein, ad, cc, Au

massive opal horizon - deeper FeS2

SiO2, Fe+3


Crofoot-Lewis. South pit

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Forms: Below water table where vapor + CO2 & H2S condense (on margins)

Simplified reaction:CO2 + H2O à H2CO3

4. Steam-heated: vapor + CO2 condensation

Typical minerals: Smectite, illite-smectite, illite, kaolinite, pyrite, Fe carbonates

carbonic acid (beer pH ~4-4.5)

CO2-rich

SO4-rich

Taupo Volcanic Zone, New Zealand: rift behind an arc Looking northeast: volcanic rift to west, volcanic arc to east

Waiotapu geothermal system

rhyolite domeandesite cone

basalt dike

Broadlands geothermal system

Ohaaki pool, neutral pH,

sinter

rift margin

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Extent of kaolinite

Extent of siderite

Acid-sulfate kaolin pool,

pH~2-3

Hedenquist, 1990; Simmons and Browne, 2000 Browne, 1971

CO2-rich froth,BR-6

Ohaaki Pool, neutral pH Broadlands,

NZ

Deep upflow

Boiling,

vapor loss

smectite, I/S, kaolinite, Fe

carbonate: CO2-rich vapor condensate, pH 4-5

CO2-rich condensate

Forms: Above water table by supergene oxidation of pyritic rocks

Simplified reaction:Fe sulfide + H2O + 02 =Fe oxy-hydroxides + H2SO4

5. Supergene weathering, sulfide oxidation

Sillitoe (1993)

Assemblage: Kaolinite, halloysite, alunite, jarosite, hydrous Fe oxides

tubular halloysite

rhombohedral alunite

Riaza kaolinite mine (Spain)

rhombohedral alunitewith nearly cubic angles

Rodalquilar Au deposit (Spain)

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Associated deposits:Porphyry Cu (Au, Mo), epithermal, VMS, etc.

Typical occurrence:Irregular veinlets, nodular masses, local pervasive replacement

Texture and Color:Chalky, porcelaneousmasses and fine powdery aggregates; white, cream,yellow-brown (jarosite)

Rodalquilar Miocene epithermal Au (Spain)

Supergene alunite (~4Ma)

5. Supergene weathering, sulfide oxidationRiaza kaolinite mine (Spain)Silurian (~430 Ma) pyrite-bearing black shale

Kaolinite-alunite formed during Miocene (13.5 Ma) weathering

Typical occurrence:Irregular veinlets, nodular masses, local pervasive replacement

Texture and Color:Chalky, porcelaneousmasses and fine powdery aggregates; white, cream,yellow-brown (jarosite)

Assemblage: Kaolinite, halloysite, alunite, jarosite, hydrous Fe oxides

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Features of interest

Refractory sulfides are oxidized:

- Au can be recovered by heap leaching

- Cu by SX-EW (secondary Cu minerals)

Copper Alliance 2018

320-260°

300-200°

170-120°

120-70°

5 environments of acid alteration minerals

Cooling of a magmatic-hydrothermal fluid White mica to pyrophyllite ± diaspore transition

Hypogene acid-sulfate (SO2 + H2O) Residual quartz (±Au); alunite-kaolinite halo, pyrophyllite±dickite proximal to feeder zone

Steam-heated CO2-rich (CO2 + H2O)Marginal (deep) illite/smectite-kaolinite-Fe carbonate

Steam-heated acid sulfate (H2S + O2)Blanket of kaolinite-alunite, no metals (except Hg)

Supergene oxidation of pyrite (FeS2 + O2)Horizon of kaolinite-alunite-Fe hydroxides, no pyrite<40°C

(approx.)

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• Distinguish environment of formation, based on mineral assemblage, textures, morphology

• Map: integrate with lithology, structure, etc. • Interpret location relative to mineralization potential

e.g., lithocap vs quartz veins; erosion level; assessment of metal anomalies, etc.

• Determine mineralogy relevant to exploration and each stage of assessment

• Focus on mineralogy, and avoid names“Horizon of kaolinite – alunite – Fe hydroxides, no pyrite”

• Do not use “advanced argillic”, ”intermediate argillic”, etc.

Practical recommendations

Steam-heated alteration over Pascua-Lama epithermal HS Au-Ag deposit, Chile

BarrickMod. Chouinard et al. (2005)

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Yellowstone River valley alteration/mineralization?

The porphyry copper system: hydrothermal alteration€¦ · Porphyry Cu (Au, Mo), epithermal, VMS,...

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