Richards PCD Shortcourse5(Epithermal)

8/13/2019 Richards PCD Shortcourse5(Epithermal)

1/7

Porphyry Short Course Part 5

Richards (2011)1

Porphyry Cu-Mo-Au Systems

Part 5: Epithermal Deposits

Jeremy P. Richards Dept. Earth & Atmospheric Sciences

University of Alberta Edmonton, Alberta, T6G 2E3, Canada

[email protected]

The fumarolicepithermal environment

Hedenquist et al. (1996)

Active fumaroles on Volcn Lastarra, Chile

Richards (2011)

Mina Julia sulfur mine, NW Argentina

Richards (2011)

Sulfate fumaroles, Corrida de Cori, NW Argentina

Left: Gypsum fumarole spiressurrounded by volcanic bombs (black).

Right: Anydrite veins feeding fossilfumaroles.

Richards (2011)

Two main types ofepithermal deposits:

(a) High sulfidation(acid-sulfate)

(b) Low sulfidation(adularia-sericite)

Newly recognized class:

(c) Intermediate sulfidation(basemetal sulfides, illite).

Evans, A.M., 1993, Ore geologyand industrial minerals, anintroduction, 3rd edn: BlackwellScientific, 390 p.


2/7


Richards (2011)2

The epithermal environment

Hedenquist et al. (1996)

.

1 1 / / / .

Intermediate-sulfidationepithermalAu-Ag

High-sulfidation epithermaldisseminatedAu Ag Cu

High-sulfidation

lode Cu-Au AgCarbonate-replacementZn-Pb-AgAu (or Cu)

DistalAu/Zn-Pbskarn

Marblefront

ProximalCu-Au skarn

PorphyryCu Au Mo

Base oflithocap

1km

1km

Subepithermalvein Zn-Cu-Pb-

Ag Au

Sediment-hosted distal-disseminated

Au-As Sb Hg

Late-mineral porphyry Phreatic brecciaLITHOCAP

PORPHYRYSTOCK

PRECURSORPLUTON

HOSTROCKS

MAAR-DIATREMECOMPLEX

Dacite porphyry plug-dome

Lacustrine sediment

Late phreatomagmatic breccia

Early phreatomagmatic breccia

Late-mineral porphyry

Intermineral magmatic-hydrothermal breccia

Intermineral porphyry

Early porphyry

Equigranular intrusive rock

Dacite dome

Felsic tuff unit

Andesitic volcanic unit

Subvolcanic basement / carbonate horizon

V

V

V

V

V

V

V

V

V

V

. .,

,

. ,, .

, .. 1

, , , , .1 , 1 , .

Relationshipbetween

porphyry and

HSISepithermal

deposits

Sillitoe, R.H., 2010, Porphyrycopper systems: EconomicGeology, v. 105, p. 341.

Epithermal deposit characteristics:

Shallow environment: Typically "1 km depth offormation.

Ore fluid: 50300C, typically


3/7


Richards (2011)3

HS fluids

Possible pathwaysfor magmatic fluids

to contract tomoderate salinityliquids, as seen inHS systems (0.2 to4.5 eq.wt.% NaCl

(Mancano andCampbell, 1995).

Mancano, D.P., andCampbell, A.R., 1995,Microthermometry ofenargite-hosted fluidinclusions from theLepanto, Philippines, high-sulfidation CuAu deposit:Geochimica etCosmochimica Acta, v. 59,p. 39093916.

Richards (2011)

Possible pathwaysfor magmatic fluids

to contract tomoderate salinity

liquids:(1) condensation,

(2) non-condensationpaths of Heinrich et

al. (2004) andHedenquist et al.

(1998), respectively.

Hedenquist, J.W., Arribas, A., Jr.,and Reynolds, J.R., 1998,

Evolution of an intrusion-centeredhydrothermal system: Far

SoutheastLepanto porphyry andepithermal Cu-Au deposits,Philippines: Economic Geology, v.

93, p. 373404.

Heinrich, C.A., Dreisner, T.,

Steffnson, A., and Seward, T.M.,2004, Magmatic vapor contraction

and the transport of gold from theporphyry environment toepithermal ore deposits: Geology,

v. 32, p. 761764.

Richards (2011)

Physico-chemical

conditions of

epithermal oreformation

HS

LS250C, !S = 0.02 m,

salinity = 1 m

Heald, P., Foley, N.K., and Hayba, D.O.,

1987, Comparative anatomy of volcanic-hosted epithermal deposits: acid-sulfateand adularia-sericite types: EconomicGeology, v. 82, p. 126.

Physico-chemical

conditions of

epithermal oreformation

HS

LS

250C, salinity= 1 m

Heald, P., Foley, N.K., and Hayba,D.O., 1987, Comparative anatomy

of volcanic-hosted epithermaldeposits: acid-sulfate andadularia-sericite types: EconomicGeology, v. 82, p. 126.

Gold dissolution asbisulfidecomplexes

(e.g., Au(HS)2):

[Au]measured in natural fluids# 1.5 ppb

Experimentally determined solubilities:

[Au(HS)2] % 11.1 ppb

[AuCl2

] %

1.2 x 10

7

ppb

Therefore, Au is mainly dissolved as

bisulfide rather than chloride complexes.

Gold transport and deposition

Richards (2011)

Precipitationof Au caused by destabilization of Au-

bisulfide complexes:

Reduction acidification:Au(HS)2

+ H++ 1/2H2O "Au+ 2H2S +1/4O2

Oxidation neutralization: Au(HS)2

+ 33/4O2+1/2H2O "Au+ 2SO4

2+ 3H+


Richards (2011)


4/7


Richards (2011)4

Gold solubility in relation to acidity and oxidation state Boilingis an effective ore forming process. Gases such as

CO2and H2Sare lost to the vapour phase:

Au(HS)2+ H++ 1/2H2O "Au+ 2H2S(g)#+

1/4O2

HCO3

+ H+"H2O + CO

2(g)#(pH increases with loss of CO

2

gas)

Zn2++ HS"ZnS+ H+(sulfides and calcite ppt with increasing pH)

Ca2++ HCO3"CaCO3+ H

+

Ca2++ 2HCO3"CaCO3+ H2O + CO2(g)#(bladed calcite)

Gold and Ag-telluridespseudomorphing

bladed calcite in Py,Porgera, PNG

Bladed calcite andvuggy Qz,

Pachapaqui, Peru Richards (2011)

Casts of bladed

calcite in quartz

Casposo LS epithermalAu deposit,

NW Argentina

PPL

XPL

Richards (2011)

Boiling pointdepth relationships for pure and saline waters,and CO2-bearing waters

CO2increases the depth(pressure) or lowers thetemperature of boiling (phaseseparation or effervescence)

Henley, R.W., Truesdell, A.H., Barton, P.D., Jr., and Whitney,J.A., 1984, Fluidmineral equilibria in hydrothermal systems:

Reviews in Economic Geology, v. 1, 267 p.

Bodnar, R.J., Reynolds, T.J., and Kuehn, C.A., 1985,Fluid inclusion systematics in epithermal systems:

Reviews in Economic Geology, v. 2, p. 7397.

Wallrock sulfidationis also effective:

FeO + 2H2S +1/2O2"FeS2+ 2H2O

2Au(HS)2+ FeO + 2H+"2Au+ FeS2+ 2H2S#+ H2O


Pyritized Mt,

Porgera Audeposit, PNG

Richards (2011)

Relationship between epithermal systems and

volcanism (Sillitoe, 1973)


5/7


Richards (2011)5

Lepanto HS epithermal system linked to FSE porphyry

Shinohara, H., and Hedenquist, J.W., 1997, Constraints on magma degassing beneath the FarSoutheast porphyry Cu-Au deposit, Philippines: Journal of Petrology, v. 38, p. 17411752.

HS: Advanced argillic

alteration: alunite-silica

body surrounded byquartz-kaolinite

(Aras, Iran)

Richards (2011)

Vuggy silicalithocap

overlyingalunite-

cemented

breccia,

Richards (2005)

Co. Laguna Pedernal,NW Argentina

Richards (2011)

5000m

3500m

6409m

Silicified lithocap above high-sulfidation

epithermal Au mineralization, with marginal

intermediate sulfidation Pb-Zn-Ag veins(Volcn Antofalla, Argentina)

Lithocap

Alunite-clay

alteration

Pb-Zn-Ag veins

Richards (2011)

Distal Pb-Zn-Ag intermediatesulfidation epithermal veins in

underlying sedimentary rocksbeneath volcanic sequence

Quebrada de las Minas,Volcn Antofalla

Richards (2011)

Bonanza-type (low sulfidation) epithermal deposits

Panteleyev, A., 1988, A Canadian Cordilleran model for epithermal gold-silver deposits, inRoberts, R.G., and Sheahan,P.A., eds., 1988, Ore deposit models: Geoscience Canada Reprint Series 3, Geol. Assoc. Canada, p.3143.


6/7


Richards (2011)6

Low-sulfidation epithermal deposits

Examples: Tonapah (NV), Creede (CO),Pachuca-Real del Monte (Mexico),Hishikari (Japan).

Derived from near-neutral, bisulfide-bearing fluids.

Commonly associated with rhyolitic rocks.

Alteration is characterized by Qz-adularia-carbonate-sericite assemblages.

High Ag/Au ratios, variable concentrationsof Cu, and anomalous Mo, W, Mn, F, Se.

Ore minerals include base metal sulfides,sulfates, sulfosalts, selenides, Au,electrum.

www.jamstec.go.jp/jamstec-e/XBR/suger/en/therod21.html

Hishikari, JapanTypical boiling textures

(bladed calcite, vuggy quartz)

Pachapaqui Pb-Zn-Ag mine, Pru

Richards (2011)

Hedenquist et al. (1996)Distal low-

sulfidationepithermal Aumineralization

Hishikari, Japan

Sakurajima volcano

Richards (2011) Richards (2011)

Efemukuru LS Au deposit,

Western Turkey

High grades (up to 210 g/t Au over 1 m)in quartz-rhodonite-rhodochrosite veins,

with minor sphalerite and galena.

Alkalic-type low-sulfidation

epithermal Au

mineralization

Porgera, Papua New Guinea

Bonanza grades over

1000 g/t

Richards (2011)

A veins cuttingintrusion

Pyrite, galena,

arsenopyrite, andrare chalcopyrite

Au in pyrite

Tetrahedrite,arsenopyrite,

sphalerite, (Au)

Galena, sphalerite,arsenopyrite, (Au)

Porgeraearly (A)

veins(Stage I)

Au

Richards (2011)


7/7


Richards (2011)7

FIs in sphalerite(Th = 300350C)

Hypersaline FIs in earlyquartz with phyllic

alteration

Porgera A veins:Fluid inclusions

Richards (2011)

A vein with late D-

type vuggy cavity,

showingparagenetic

sequence (A "D)

(Note: This AD veinterminology is mine-specific,and not related to Gustafson& Hunt$s (1975) porphyry vein

terminology)

Richards (2011)

Breccia-type D veins with visible Au

Richards (2011)

Typical brecciatedD vein

Roscoelite withpyrite and Au

Au withpyrite,

tetrahedrite

Au-Ag tellurides withpyrite

Au-Ag telluridespseudomorphing

bladed calcite in Py

PorgeraD veins

(Stage II)

Richards (2011)

Typical layered D vein: Auoccurs near the fine-

grained margin

Rare vapor-rich FIs

FIs in growth zonesin vuggy quartz (Th %150C)

Rare CO2-rich FIs

Porgera D

veins:

Fluidinclusions

Richards (2011)

Summary

Epithermal deposits are divided into two main categories:Low sulfidation (a.k.a., adularia-sericite) and highsulfidation (a.k.a. acid-sulfate). Intermediate sulfidation

deposits feature more abundant base-metal-sulfides andillite, and may reflect higher salinity fluids.

HS deposits are closely related to magmatic act ivity; fluidsof direct magmatic origin. Early intense acidic alteration by

magmatic volatiles provides permeability, followed(sometimes) by later less-acidic mineralizing fluids.

LS deposits are commonly distal to magmatic activity, andpost-date it by ~1 m.y. or more. Fluids dominantly of

meteoric origin, although some magmatic fluid may be

present. Metals may be derived from country rocks ormagmas. Boiling is a characteristic ore depositional

mechanism. Richards (2011)

Richards PCD Shortcourse5(Epithermal)

Documents

Transcript of Richards PCD Shortcourse5(Epithermal)