epithermalau

7/31/2019 EpithermalAu

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Epithermal Au-Ag

Products of large-scale hydrothermal convectivesystems driven by magmatic heat in the upper 1-6

km of the Earths crust.

The term epithermal was coined by Lindgren(1922, 1933).

Subdivision into: 1. high-sulfidation (alunite-kaolinite or acid sulfate), 2. low-sulfidation (adularia-sericite), [3. hot spring deposits]

Low-sulfidation Deposits


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Midas, NevadaHigh-sulfidation and low-sulfidation

epithermal Au-Ag deposits

The two deposit styles form from fluids of distinctlydifferent chemical composition in contrasting volcanic

environments.

The ore ofHS deposits is hosted by leached silicic rockassociated with acidic fluids generated in the volcanic-

hydrothermal environment. The presence of high

sulfidation state sulfide minerals indicates high-oxidation

states typical of acidic hypogene fluids.

In contrast, the fluid responsible for formation ofLS oreveins is similar to waters tapped by drilling beneath hot

springs into geothermal systems; low sulfidation state

minerals form from those reduced, neutral-pH waters.

Low Sulfidation Deposits

2km

2 km

Magmatic heatsource (plus

volatiles?)

Magma

acid sulfate steam-heatedwaters mud pools, fumaroles

Steam-heated acidsulfate waters

CO2-rich steam-heated waters

Peripheralbicarbonate

waters

chloride watersboiling springs, silica sinter

200C250C300C

400CNeutral chloride

LS waters

cold groundwatersrecharge Meteoric

convection


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Magma

High Sulfidation Deposits

2 km

2km

Volcanism maydisrupt or

destroy

hydrothermal

system

300C

300C

400C

200C

acid sulfate waterssolfatara

crater lake

acid chloridewaters / brines

Acid alterationin upflow &

lateral outflow

zones

Magmatic heatand volatilesource

Low

sulfidation

deposits

Highsulfidation

deposits

Modified after Sillitoe,

1997

0 200 400 600 800Kelian

Waihi

Pachuca-Real

Hishikari

McDonald

Comstock Lode

El Indio

Round Mountain

Ladolam

Porgera

Pueblo Viejo

Baguio

Yanacocha

Cripple Creek

Au (t)

Giant Epithermal Deposits

Pascua-Lama

Pierina


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Selected

styles and

geometries of

epithermal

deposits


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Ore DepositionLow sulfidation

Boiling is the principle mechanism Mixing occurs during collapse of thesystem

High sulfidation

Unequivocal evidence for mixing at somedeposits

Boiling is a viable mechanism for depositswhere gold is transported as a bisulfide

complex

Electrum, tellurides & base metal sulfides, Acupan, Phillipines

Depositional Mechanisms

Boiling leading to loss of H2S Au(HS)2- + H+ + 0.5H2 Au + 2H2S Mixing with oxidized meteoric water

Au(HS)2- + 8H2O Au + 2SO42- + 3H+ + 7.5H2 Dilution of saline fluid destabilizing Cl-

complexes (AuCl2-) and raising pH

General characteristics of epithermal gold deposits

associated with subaerial volcanic rocks

Low Sulfidation High Sulfidation

Open-space veins dominant,stockwork ore common

Disseminated and replacement

ore minor

Veins, cavity filling (bands,colloforms, druses), breccias

Pyrite, electrum, gold,sphalerite, galena (arsenopyrite)

Quartz, chalcedony, calcite,adularia, illite, carbonates

KAlSi3O8 Au, Ag, Zn, Pb (Cu, Sb, As,Hg, Se)

Disseminated ore dominant,replacement ore common

Stockwork ore minor, veins

commonly subordinate

Wallrock replacement, breccias,veins

Pyrite, enargite, chalcopyrite,tennanite, covellite, gold,

tellurides

Quartz, alunite, barite,kaolinite, pyrophyllite

KAl3(SO4)2(OH)6 Cu, Au, Ag, As (Pb, Hg, Sb,Te, Sn, Mo, Bi)


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Alteration characteristics of epithermal

gold deposits

Low sulfidation alteration near-neutral pH thermal waters Core : ore vein Halo : smectite, illite, adularia (argillic alteration) High sulfidation alteration acidic pH thermal waters Core : most acid altered rock is a silica residue, termed

vuggy quartz Halo: acid stable minerals such as alunite, dickite,pyrophyllite, diaspore (advanced argillic alterationassemblage) Outwards: illite/smectite (propyllitic alterationassemblage)


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Midas

Bladed

CalciteTemperature

stability of

hydrothermal

minerals

Alunite:

KAl3(SO4)2(OH)6

Jarosite:KFe3(SO4)2(OH)6

Frequency and

abundance of

ore and gangueminerals in Au-

rich epithermal

deposits

Schematic cross-section showing the main features

of a hot-springs sub-type epithermal deposit.


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Solubility of Au,

Ag, Zn as a

function of S

and Cl

concentrationsat pH and redox

of LS mineral

assemblages. Cl-

poor solutions

typical of Au-

rich LS

ore deposits

transport Au as

bisulfide

complexes, but

cannot transport

much chloride-

complexed base

metals.

Broadlands Geothermal Fluids Low-Sulfidation Systems


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High-Sulfidation Systems

HS Deposits - Genesis

300

400

Cool meteoricwater

0(km)

2

1Heated ground-

water

Magmaticbrine

Magmatic vapors(incl. SO, HCl)

2 Vuggy quartz

Alteration

AluniteKaolinite

Fumaroles

ALTERATION ORE DEPOSITION

A

Sericite K-silicate

OresAlteration

envelope

Heatedground-water

convective cell

Magmaticbrine

Absorption of high-Pvapor producesreduced, acidlow salinity waterwith high Ausolubilities asAuHS

(aq)

B1

Acid brine

transports gold

as AuCl2-??

Mixing withshallow meteoric

water

B2

Heatedground-water

Gas phasemetal transport

Acid sulfate waterwith low Au solubility

Modified after Arribas, 1995; & White, 1991

Meteoric water

transports gold

as AuHS(aq)??

epithermalau

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