Epithermal Ores 11/3/10

1

Epithermal Au-Ag

!  Products of large-scale hydrothermal convective systems driven by magmatic heat in the upper 1-6 km of the Earth’s 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]

Epithermal Ores 11/3/10

2

Low-sulfidation Deposits

Epithermal Ores 11/3/10

3

Midas, Nevada

High-sulfidation and low-sulfidation epithermal Au-Ag deposits

!   The two deposit styles form from fluids of distinctly different chemical composition in contrasting volcanic environments.

!   The ore of HS deposits is hosted by leached silicic rock associated 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 of LS ore veins is similar to waters tapped by drilling beneath hot springs into geothermal systems; low sulfidation state minerals form from those reduced, neutral-pH waters.

Epithermal Ores 11/3/10

4

Low Sulfidation Deposits 2

km

2 km

•  Magmatic heat source (plus volatiles?)

Magma

acid sulfate steam-heated waters mud pools, fumaroles

!   Steam-heated acid sulfate waters

CO2-rich steam- heated waters

!   Peripheral bicarbonate waters

chloride waters boiling springs, silica sinter

2 0 0 ° C 2 5 0 ° C

300ºC

4 0 0 ° C Neutral chloride

LS waters

cold groundwaters recharge !   Meteoric

convection

Epithermal Ores 11/3/10

5

Magma

High Sulfidation Deposits

2 km

2 k m

•  Volcanism may disrupt or destroy hydrothermal system

300ºC 300ºC

400ºC

200ºC

acid sulfate waters solfatara crater lake

acid chloride waters / brines

•  Acid alteration in upflow & lateral outflow zones

•  Magmatic heat and volatile source

Epithermal Ores 11/3/10

6

!   Low sulfidation deposits

!   High sulfidation deposits

Modified after Sillitoe, 1997

0 200 400 600 800

Kelian

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

Epithermal Ores 11/3/10

7

Epithermal Ores 11/3/10

8

Selected styles and geometries of epithermal deposits

Epithermal Ores 11/3/10

9

Ore Deposition Low sulfidation !   Boiling is the principle mechanism !   Mixing occurs during collapse of the

system

High sulfidation !   Unequivocal evidence for mixing at some

deposits !   Boiling is a viable mechanism for deposits

where 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

Epithermal Ores 11/3/10

10

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)

Epithermal Ores 11/3/10

11

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 alteration assemblage)

!  Outwards: illite/smectite (propyllitic alteration assemblage)

Epithermal Ores 11/3/10

12

Epithermal Ores 11/3/10

13

Midas Bladed Calcite

Temperature stability of hydrothermal minerals

Alunite: KAl3(SO4)2(OH)6 Jarosite: KFe3(SO4)2(OH)6

Epithermal Ores 11/3/10

14

Frequency and abundance of ore and gangue minerals in Au-rich epithermal deposits

Schematic cross-section showing the main features of a hot-springs sub-type epithermal deposit.

Epithermal Ores 11/3/10

15

Solubility of Au, Ag, Zn as a function of S and Cl concentrations at 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.

Epithermal Ores 11/3/10

16

Broadlands Geothermal Fluids

Low-Sulfidation Systems

Epithermal Ores 11/3/10

17

High-Sulfidation Systems