epythermal arribas 1995 minassoccanada23

8/9/2019 Epythermal Arribas 1995 MinAssocCanada23

1/33

Chapter

9

CHARACTERTSTICS

F

HIGH.SULFIDATION

EPITHERMAL

DEPOSITS,

AND THEIR

RELATTON

TO

MAGMATIC

FLUID

Antonio Arribas Jr .

Mineral Resources

Department,

Geological

Survey

of Japan,

l-l-3 Higashi,

Tsukuba

305,

Japan

[NTRODUCTIoN

A consequence

f the increased

xploration or

gold

deposits

dur ing

the Iate 1970s

and

early

1980swas

tlre revision

of the

classification

of

epithermaldeposits

n

order to account

or the

variations bservedn stylesof mineralization nd

inferred

genetic

environments.

Among

th e

numerous

lassifications

hat followed,

one

group

of deposits

clearly

showed a

common set

of

features,

his deposit

ype is

characterized y

th e

presence

of

minerals

diagnostic

of

high-

sulfidation

tates

e.g.,

enargiteand luzonite)

an d

acidic hydrothermal

conditions

(e.g.,

alunite,

kaolinite,

pyrophyll i te).

The

terms enargite-gold

(Ashley

1982),

Goldfield-type

Bethke

1984,after

Ransome 1909),

high-sulfur

(Bonham

1984,

1986), quartz-alunite u (Berger 1986), acid-

sulfate

Heald

et

crl. 1987),and alunite-kaolinite

(Berger

Henley 198 9) were appl ied

o th is

group

n reference

o someof its mineralogica l

r

inferred

geoclremical

attributes.The

term high-

sulf idat ion

HS) (Hedenquist

987) s now

widely

used; he term was

proposed

riginally o refer

o

a fundamental genetic

aspect,

the relatively

oxidized state

of sulfur contained

in the

hydrothermal

ystem

i.e.,

nitial ly

SO2-rich). his

aspect s

significantbecause

t links HS deposits

with

one of the two

main types of terrestrial

magma-related ydrothermal

ystems

Henley

El l is 1983), those

associatedwith

andesit ic

volcanoes

whose

surface manifestation ncludes

high-temperature

umaroles and

acid sulfate-

chloridehot

springsand

crater

akes.

By contrast,

Iow-sulfidation

deposits orm from

neutral-pH,

reduced

H2S-rich)

hydrothermal

luids similar o

thoseencountered

n geothermal

ystems

Henley

Ellis

1983),

with

surface manifestation

including

i l ica

sinter-deposit ing

ot

springs

nd

steam-heated

cid-sulfate

lteration.

The main

objective

of this

review

is

to

summarize

he characteristics

f HS

minerali-

zation

formed primarily

within

the

epithermal

environment,

lrough ecognizing

he

potential

or

HS conditions o occur at greaterdepths.Earlier

studies

have

argued for

a magmatic

fluid

component

n

HS deposits

e.g.,

Si l l i toe

1983,

1989,

1991a;Hayba

et

al. 1985;

Henley

t99t

White

1991;Rye

1993;

Hedenquist

t al.

1994a),

and the identif ication

nd characterization

f HS

deposits as

contributed

o a re-evaluation

f the

role

of magmatic

fluids

in

other

types

of

hydrothermal

ystems

Hedenquist

Lowenstern

1994;

Simmons

this volume;

de Ronde

this

volume). In

this

context, particular

aftention

s

given to the characteristicshat are helpful in

determining

he

nature

of the

magmatic

contri-

bution to the

hydrothermal

ystem

hrough

time

and

space. his

review

considers

eatures

f many

of

the

deposits

isted in

Table l,

with

locations

shown n Figure

1,

but is based

on a

selection

f

fourteen

eposits or

which

the results

of

detailed

geological

and

geochemical

tudies

are available

(Tables

2,

and 3). For

simpl i f icat ion,

bibl io-

graphic

references

re

not

given

in the

text for

general

deposit eatures;

hese references

may

be

found in

Table 1. For

regional

studiesof HS

deposits, articularly

with respect

o

other ypes

of

magmatic-hydrothermal

ase-

and

precious-metal

deposits,

he reader is

referred

to reviews

by

Heald

et ul .

(1987),

Bonham

(1989),

Si l l i toe

(1989,

99la),

Berger Bonham 1990),

Camus

(1990),

White

Hedenquisr

1990),

Mitchel l

Leach

(1991),

Mitchell

(1992),

and

White

et al.

(

I

99s).


2/33


3/33

H gh-sulfidation

Epithermal

Depos ts

Figure

l. Worldwide

distr ibution

f

high-sulfidation

eposits

nd

principal

documented

rospects.

he

main high-

suifidation

metallogenic

rovinces

re

ndicated.

ee

Table

for deposit

ames

nd selected

eferences.

OPSNTNC

EMARKS

ONGENETIC

ENVIRONMENT

Based

on

detailed

esearch

f

the

Summiwille

Au-Cu-Ag

deposit,

Stoffregen

(1987)

demon-

strated

that

a

nearly

ubiquitous

feature

of

HS

deposits,

fracture-controlled

uggy

sil ica

rock

(intensely leached volcanic rock consisting

dominantly

of

quartz;

Fig.

2)

is

the

product

of

very

acidic

conditions

pH


4/33

A. Arribas, Jr.

Table

2. Main

geological

haracteristics f

l4

selected

igh-sulfidation

pithermal

deposits

Deposit /disrict .

Ag e

locat ion

{Ma)

Metals.

( t onnes )

Local volcanic

sett ing

Principal host

roc Ks

Genetically

rclated rtxks

Time

betwecnhost

rock

& deposit Dcposit l i)rm

Motomboto.

Indonesia

Na lesb i t i n .

Phi i ppincs

Lcpanto.

Ph ippincs

Chi

nkuash ih .

k iwu

Z i i i n s h a n .

Ch ina

Nansatsu,

Japiur

Sunrnr in l lc .

Color:rdo

Goldl ic ld.

Ncvadir

Central-vcnt

volcanir

Small cenral-

vent volcano

Diatreme

complex

Dome

complex

Domc

akrng

caldcra m:trgin'l

Snrall volcanos

in

a c:rldrra'i

Dome

along

preexrst lnS

c:ildera margin

Dac

donr, zrnds/dac/rhy

l-1ows.

yr

and volx

Ands

pyr

+ l lows

Ands/dac vol.

Mioccnc +

older

volx

+ metavol

Dac volc

Mioccnc

se d

Jurassic

granite.

Cretaceousdac

porpyhry

+pyr

Ands

pyr.

l lows +

vo lx

Qtz-lat ite

porphyry

Miocene

andesitc

Diorit ic.

qtz-

diorit ic stocks

None

observed

Qtz{iorite

porphyry

Dacite

domes

md l lows

Not reported

Horblende

:rrds

(Middlc

Volcs)

< 1 . 0n r . y .

N/A


5/33

H gh-sulfidalion

Epithermal

Deposits

Table

2

(continued)

Dcposit /district

locat lon

Control

on mineralizat ion

Vertical ext-

ent of epiri .

ore

(m)2

Relat ion to

porphyry

systen)

Relerences

Motomhoto.

lndurcsia

Nalcshitan.

Ph ippi

ne s

Lcpanto.

Ph

i

ppi

nc s

Ch in luash

h,

Taiw;rrr

Z i i i nshan.

Ch ina

Nansatsu.

Japiut

S u n r m r t v i l l e .

Colorado

Goldlrcld.

Nevalir

Paradisc cak.

Nev:da

P uch loV ie . jo .

Don) in i can

Rcp.

Ju lcan i .

Pcru

El

lnd io .

Ch i le

La

Me.j ic:na &

Ne -

vadosdcl

Fantat ina

AJScntlna

Rrxllr i t1uiIar.

Spai t r

Contact

bctwcendome and

volcmic Krk. steep

ault

Stecp

strike-sl ip ault

Major

steep+ minor faults.

diatrcn)e

ontact.unc0mlor

mrty,

permeable

ayers

Stecp

normal laults

+

thcir intcrscct ions,

bedding

plancs

Steep srike-sl ip

fault

zones

+

contact 0f

volcztntc ent

Stecp ractures

+

permeable

pyroclast ic

ayers

Steep

ldial fracturcs

+

dtxnc contact

Modcratcly

+

shallow

dipping

faults & f issures

Stccp

Iaults

ant l

permeable

pyroclast ic

ayers

Diatreme

rinq fault +

permcable

ayers

Steep

iactures

Stccp normal faults

LOcal aults

Caldera

ring faults

+

nornral local faults

Porphyry Cu-Au

prospects

earby.

ag e

wi thin 1.0 m.y .

hoposcd,

none lo)owr)

Above +

adjaccnt

samc age

porynyry

Cu-Au

dcposit

Nonc k-nown

None

known

None known

Inrusion-ccncred

scricit ic, ow

grade

stk mineralizat ion

Nonc

lnown

Sericit ic.

stk Au

minerahzation

East

Zone)

Ntne l*rown

None klown

Porphyry Cu-Mo

mineralizat ion

nearby

HS

ore il

Nevado del

Famatina s a

pirt

of a

porphyry

Cu

prospect

Nrne Lrown

Pcrcll6

(

I 994)

Sil l i toe ?r a/.

(

I 990)

Garcia

(

99l

),

Anihas et a/.

(

I 995b)

Huang 1955) ,

Tan et al.

(1993)

Rcn er a/ .

(

1992) ,

Zhang et ul.

(1994)

Izawa & Cunningham

(

I 9f l9).

Hedentluist et al.

\1991a)

Steven

& Rat t i

(1960) .

Menhc r l

et al.

(19'7

).

Stoffregen

I

987

.

Rye

(199, j )G ray

& Coo lbaugh

(

1994)

Ransomc

1909) ,

A sh ley

1974) .

Ashlcy & S i l berman

1976) .

V ikc

(1989.

wr i t ten

conrmun.

I 995)

John?/ a/ .

(

1991

.

S i l l i toe &

Lorson

(1994)

Russel l

&

K es le r 1991

.

Muntean et a1. I990)

Petersen

I

al.

(19'11\.

Noh le & S i lberman

198,+) .

Dccn

(

I

990)

Siddcley

&

Araneda 1986).

Jannasel a1.

1990)

Losurda-Calder(xr

McPhail

(

I 994). Losada-Caldcr6n l a/.

{

994)

Anibas

e/

d/.

(

I

995a)

25 0

15 0

500

800

60()(')

< 1 5 0

250

400

< 1 5 0

4UX

)

600

>l(x)

< 1 5 0

principal

geologic environments

Bethke

1984;

Rye el

al.

1992):

l

)

by

the

disproportionation

f

magmatic SOz to

H2SO4 and

HzS

following

absorption

by

groundwater

(magmatic-

hydrothermal),

2)

by atmospheric

xidation

of

H2S

in the

vadose

zone over

the

water

table,

associated

with

fumarolic

discharge

of vapor

released

y deeper

boiling

fluids

(steam-heated),

and

(3)

by atmospheric

oxidation

of sulfides

during

weathering

(supergene). Magmatic-

hydrothermal

lunite

occurs

with

mir-rerals

uch

as

d iaspore,

y rophy l l i te ,

ao l in i te ,

d ick i te ,

and

zunyite,

which are

ypical of

hypogene

T

:

200-

350

C)

acidic

condit ions

(advanced

argi l l ic

assemblage;

eyer

& Hemley

1961).

This type of

alunite

s characterist ic

f

HS deposits,

ut it may

alsoappear

n areas

f advanced

rgillic

alteration

void of

ore

mineralization

e.g.,

wao 1962;

Flall

1978).

Alunite

in steam-heated

nvironments

forms

with kaolinite

and

interlayered

illite-

smectite

t about

100 o

160

C

where

umarolic

vapor

condenses

above

the boiling

zone of

neutral-pH,

H2S-rich

luid,

typical of

geothermal

s y s t e m s

h a t

f o r m l o w - s u l f i d a t i o n

e p o s i t s .

423


6/33

A. Arribcts,,Ir.

Propylitic

Argillic

+

Adv.

argillic

rock

rock

rock

Because

of the

relatively shallow

and

dynamic

environment

of

mineral izat ion,

overprint ing

among

the three

types of

acid-sulfate

lteration

(including

sLrpergene)

s

possible; owever,

he

spatial

relation

of each

ype of

alunite

o ore is

different,

and correct

identif ication

s important

for exploration

(Rye

et al.

1992:. White

.

Hedenquis t

995) .

DISTRIBUTION,

GEAND

ECONOI\{IC

StcNInrcaNcB

In common

with other

magmatic-

lrydrothermal

deposits

(e.g., porphyry copper

deposits),

HS deposits

oincide

worldwide

with

plutonic-volcanic

rcs.

This associat ion

s

best

observed

n the Cenozoic

deposits

f

the Circurn-

Pacific

and he

Balkanbelt

of

southeastern

urope

(F-ig

I

) .

These deposits

occur

in two main

settings:

n islandarcs

and at continental

margins.

The tectonic

regime during

formation of

the

deposits

seems

to be dominantly

extensional

(Si l l i toe 1993). Some deposits e.g., Goldf ield,

Rodalquilar,

Summitville)

formed

in

intra-

continental

egionsduring

periods

of

extensiot l

that

followed

regional compression

and sub-

ductiorr

y

several

m.y.

Tertiary

HS deposits

redominate, nd only

a

few deposits

are

Mesozoic

(e.g.,

Pueblo Viejo,

Zijinshan),

Paleozoic

e.g.,

Temora

and others

n

southeastern

ustralia), r

PreCambrian

the

early

Quartz

alunite

Mineralized

uggy

quartz

rocl(

ProterozoicEnAsenAu deposit located in the

Balt ic shield

of central Sweden;Fig. I

).

The

youngest

eposits re Pleistocene


7/33

H

g

h-s

ulfi dat

on Ep

he

ma Dep

os s

Figure

3. K2Oversus

SiO,

variation

diagram

for rocks

hought

o be

genetically

elated

o

high-sulfidation

eposits.

he

samples

ro m

12 deposits

or districts

r

:

140)

define

a

small

compositional

ield,

which

contrasts

sharply

with

the large

field

defined

by

volcanic rocks associated rvith low-

sulfidation

or intrusion-related

u

deposits

(>

100 samples

rom

l6

distr icts;

Si l l i toe

1991b,

1993;Mri l ler

&

Groves 1993).

The

degree

f alteration

f the

rock

samples

nd

precision

of the

analytical

data

are Iargely

unknown; however,

according

to

the

individual

datasources,

most

of the samples

are unaltered

r very weakly

altered.

Circles

indicate

average

values

for each

high-

sulfidation

deposit or

district:

Ch

Chinkuashih,Cq

=

Choquel impie,

Go

-

Goldf ie ld, n

:

El Indio.Ju

:

Julcani . a

Laurani ,

Le

:

Lepanto,

Mo

-

Motomboto,

Na

-

Nansatsu, P Paradise

eak,Ro

:

Rodalquilar,

u

-

Summitville.

Compositional

ields

afrerKeith

et al.

(

199 ).

See

Appendix

br referencesnd nformation

n data

plotted.

50

60 70

SiO2 wt / )

simi lar

o that of mineral izat ion.

hereabundant

radiometric

gesare available, he ageof

the

host

rocksand the age of mineral izat ion

re within

analyt ical

precision: where

a di f ference is

indicated,t is

typical ly ess han

1.0

m.y.

Table

2). A comrnon

spatialassociation xists

between

the

deposits

and shal low. ypical ly

porphyri t ic

intrusions. hese ntrusions re interpretedo be

the

roots

of volcanic domes

or

the feeders

of

central-ventvolcanoes or maar-diatrerne

com-

plexes,

he t hree rnain volcanic

settings or HS

deposits

1'able

2). Some

depositsare hosted

entirely

within a

singledome

(Summitvi l le),

r

within a

dornecomplex

Julcani).

n most

cases

tfre

mineralization

extends rorn the

subvolcanic

intrusion nto

country ro cks, such as

the Main

Vein Cu-ALr-Agdepositand associated

reccia

depositsn the Penshan

reaof the Chinkuashih

distr ict.Somedeposits, owever, o not showany

(known)

spatial associat ionwith

subvolcanic

intrusions hought to

be

genetical ly

elated

o

mineral izat ion

e.g.,

Nalesbitan. ansatsu).

n the

Rodalqui lar Au deposit , dykes

and small

intrusions of hornblendeandesitewhich

are

interpreted to be temporally related

to the

mineral izat ion eprcsent

nly a fract ion of the

altered and mineralized

area exposed

at

the

present

epth

of erosion; larger ntrusive

ody

s

tlrought o

exist at depth

(Arrrbas

et al. 1995a).

The main

controlon location

f mineral izat ion

t

Rodalquilar

s the

structural nargin

of two nested,

resurgent

calderas. With

the

exception

of

Rodalquilar,

he role

of calderas n

the formation

of HS

deposits eems o

be l i rni ted

o faci l i tat ing

the emplacement

f late

intrusive

magrna

along

preexistingcaldera ing-fractures Rytuba cl rzl.

1990) .

The

magmas

hought o be

genetically

elated

to

HS deposits

have a remarkably

limited

compositional ariation.

The ranges

f

wt.%

K2O

and SiO2 or

twelve deposits

verlapgreatly

and

show a

dominance f

calc-alkal ine

ndesit ic

nd

dacit ic

composit ions, i th

subordinate

hyol i te

(Fig.

3). Intermediate

alcic volcanic

rocks

are

limited

to

porphyritic

intrusions

n

the Lepanto

and Motomboto

Cu-Au-Ag

districts,

and

intermediate-to-felsicalkali-calcic rocks ar e

characterist ic f the

Summitvi l le

and Laurani

distr icts

Fig.

3). Interest ingly,

o

deposits ave

been

discoveredn associat ion

ith

alkal ineor

mafic magmas,

ven hough

hesemagmas

an be

genetical ly

related

to low-sulf idat ion

and

intrusion-related

u

deposits

(Si l l i toe

1991b,

1993; Mii l ler

&.

Groves 1993; Richards

this

volume).

The data

shown in Figure

3 suggest

a

relation

exists between nagma

cornposition

an d

/

\

ca\c.,^

t

3\Katt

*t '


8/33

A Arribas,

Jr .

Table 3. Main

alteration and mineraliza tioncharacteristics

f 14 selected

igh-sutfidation

epithermal

deposits

Dcposit

Lateral

alteration

oning

(outward

rom nrinem-

lizcd

xxlies)

Vertical

altcration

zonin-9

(shallow

() dccp) Pnncial renrinerals

(

)re

mineralization

rn: Ag/Au

Silica

corc

Vcry

Io* As

N/A

Si l i cu ore


9/33

A.

Arribus,

r.

is diff icult,

but

useful

for discussion

of the

differences

among

deposits

and

design

of

exploration

strategies.

n this

context,

White

(1991)

dist inguished

hree

end-member

tyles

of

HS deposits,

amed

after

deposits

f

the Circum-

Pacific:Temora,El Indio,andNansatsu.rregular

bodies

f disseminated,

i l ic i f ied

res

dominate

n

the

Temora-style.

Cavity-filling

veins

with

sericitic

and

clay-rich

haloes

are

characteristic

f

El

Indio-style

ALr

deposits.

A

large

group of

deposits

al ls

into

White's

1991)

Nansatsu-style,

which

is

characterized

by

wallrock-alteratiort

zoning

similar

o

that

shown

n Figure

2, and

by

the occurrence

f

enargite-bearing

res

within

a

si l ica

core

consist ing

f

vuggy

or

massive

i l ica

rock

(Table

3).

Mineralization

n this

style

of

deposit orms irregular

stratabound

odies

(e.g.,

Nansatsu,

Lepanto)

or

subvertical

vein-like

masses

r

ledges

(e.g.,Chinkuashih,

oldf ield,

Lepanto,

Rodalqui lar,

Summitvi l le).

These

deposits

ontain

breccia

bodies,

eins,

stockworks

of small

veins.

and

disseminated

res

hat

replace

or

irnpregnate

ntensely

altered

country

rock'

Ericksen

Cunningham

1993)dist inguished

wo

styles

of

HS deposits

n the

Andean

province:

Ag -

and

Au-rich

polymetallic

base-metal

eins'

and

low-grade

uggy

silica

and

breccias;

he wo

types

are

broadly

comparable

with

El

Indio-

and

Nansatsu-styles,

esPectivelY.

l,ocal

subvertical

aults

and

fractures

re

the

dominant

control

on

HS

mineralization

nd they

are

present n

rnost

deposits

(Table

2).

Other

examples

f

structural

ontrols

observed

n some

districts

arnong

the

foufteen

selected

nclude:

rnoderately

o shallow-dipping

aults

(Goldfield)'

caldera

ing

and

radial

faults

(Rodalquilar),

he

di lat ional

og

of

a

str ike-sl ip

ault

(Nalesbitan),

diatreme

ing-faults

Lepanto,Pueblo

Viejo),

th e

contact

between

a dome

or

volcanic

conduit

and

country rock (Motomboto, the Missionary

orebody

at

Summitvi l le),

and

a

l i thologic

unconfbrmity

PuebloViejo,

Lepanto).

n three

of

the

fburteen

deposits,

he

principal

control

is

l i thological

maar

sediments

t

Pueblo

Viejo,

and

interbedded

yroclastic

ayers

at

Paradise

Peak

and

Nansatsu;

able

2).

A

unique

cornbination

of

the

structural

an d

lithological

controls

characteristic

f

HS deposits

is

exhibi ted

by

the

Lepanto

Cu-Au-Ag

deposit .

The deposit

s

3

km

long and

consists f a

tnain

zone of

breccia

and

replacement

mineralization

along

he

Lepanto

ault

Fig.

4A).

Mult iple

veins

associated

with smaller

diagonal

faults branch

from

the rnain

zone and

extend

into both

the

hangingwall and foot'ivall

(Garcia

l99l).

The

characteristic

mushroom-shaped

ross-section

f

many of

the orebodies

t

Lepanto

s related o

th e

intersection

f

the steeply

dipping

Lepanto

ault

and

branch

veins

with the

unconfonnity

at

th e

base

of Imbangui la

acite

Fig.

aB). Li thologic

variations

in the

host

rocks

also

played

an

important

ole

in the

fonnation

of the

deposit.

as

shown

by

lensesof

stratiform

enargite-luzotrite

ore

which

resulted

rom

replacernent

f detrital

layers

within

volcaniclastic

and

sedirne

tary

basement

nits

Garcia

99l

) .

AITEN.ITION

MINERALOGY

ND

ZONING

As mentioned

above,

the

lateral

alteration

zoning

that

is characteristic

of

HS

deposits

reflects

he

reaction

and

neutralization

f

high-

temperature

acidic

fluids

with

wallrock.

The

innermost

zone

of

vuggy or

tnassive

si l ica

alteration

ommonly

has sharp

boundaries

ith a

zone

that

may

contaitr

quartz, alurrite,

kaolinite,

dicki te,

pyrophyl l i te,

iaspore,

nd

zunvite'.

'his

advanced rgillic assemblagerades nto a second

envelope

of argi l l ic

al terat ion,

composed

of

minerals

uch

as

quartz, aol ini te,

l l i te,

serici te,

and smectite,

nd

an outermost

alo of

propylitic

al terat ion,

with

chlori te.

i l l i te,

smecti te.

and

carbonate

Fig. 2,

Table 3).

The width

o1'eacl.t

zone

varies

widely;

or example,

uggy

si l ica

and

advanced

argillically

altered

rock

fonn

narrow

(


10/33

@

NW

High-sulfidation

Epithermal

Deposits

Figure

4. Longitudinal

A)

and ransverse

B)

cross-sections

f the Lepanto-FSE

Cu-Au-Ag

deposits

phitippines),

showing

tructural nd lithologic

controls

on formation

of the high-sulfidati on

nd

porphyry-type

res

(simplif ied

fromGarcia 99l

).

Potassium-argon

ating

of country ocks

andalteration

minerals

ssociated

ith

the

porphyry

an d

high-sulfidation

epositsndicates

hat hydrothermal

u-Au

mineralization

ook

place

n

the middle

of a

pliocene

to

Pleistocene vent of dacitic-andes itic

agmatism

Arribas

et al. 1995b).

Note

the overall

spatial

overlap

of the

magmatic

nd hydrothermal

plumbing

systems

i.e.,

volcanic

ventsof Pliocene

acite,

quartz

diorite

ntrusions.

porphyry

eposit, nd

deeper

arts

of epithermal

mineralization).

The zones

of alterationwith

increasing

epth

typically

grade

from

a shallow

silicic zone

through

advanced

argi l l ic, argi l l ic,

argi l l ic/

serici t ic, nto a serici t ic or phyl l ic zone with

quartz,

sericite, and

pyrite.

This

alteration

sequence

occurs over a

vertical interval

that

ranges rom

a

few

hundredmeters

o more

than

1000

m, and has

been best documented

y deep

dri l lholesn the

deposits f smal ler

ize, n which

the vertical

span of rnineralization

s

less than

about300 m

(e.9.,

Rodalqui lar,

ummitvi l le; ig.

5B). At Lepanto,

sericiticalteration

at depths

of

400 o

500

m

below he

epithermal eposit

ives

way, laterally

towards the

south, to K-sil icate

alterationof the FSE porphyry Cu-Au deposit.

Porphyry-type

stockwork mineralization

at

Paradise eak s

containedwithin the

sericiticores

of the East Zone

deposit which,

according

o

Sillitoe & Lorson

(1994),

ormed

underneath

he

main HS ore bodies rr

the area.A

quartz-sericite-

pyrite

zonewith trace

amounts f chalcopyrite

nd

molybdenite

urrounds n intrusion

f monzonite

porphyry

>300

m below

the HS deposit

at

Summitvi l le

Grav

& Coolbaush

994\.

The

lateral

and vertical

alteration

zones

described

above

correspond

o a

generalized

model.

They

are useful

in exploration

because

they help in understandinghe geneticenviron-

ment of

a deposit

and

provide

spatial

markers

within

the

extinct

hydrothermal

system.

Experimental

data on

the relative

stability

of

rninerals

uch as

alunite, kaolinite, pyropliyllite,

and diaspore

Hemley

et al. 1969,

1980),

oupled

with

the

temperature

angesnoted

for

these

an d

other related

acid

minerals

in active

systems

(Reyes

1990; Reyes

et al. 1993),

also

provide

information

hat

contributes

o definition

of the

paleoconduits

n extinct

systems.

If studied n detailed,severalsuperimposed

and

crosscutting

tagesof

pervasive

as well

as

fracture

(conduit)-related

mineralization

may

be

recognized

n the majority

of deposits.

hese

ar e

the expected

esult

ofvariations,

during

he course

of mineralization,

n

temperature, ressure,

nd

composition

of the hydrothermal

luid

and

the

degree

f wallrock

interaction.

Detailed

ield

an d

petrographic

tudiesat the

Monte

Negro

orebody

in the

Pueblo

Vieio deposit

have

resulted

n


11/33

A. Arribas,

.Jr.

Vuggysilica

Advanced

rgillic

Argillic

S€ricitic

Propylitic

Inlense upergene

cij-sulfate

vsrprint

-100

I K M

Au-(Cu-Te-Sn)

htgh-

sulfidation

deposits

particular

eatures

f the

deposits

isted n Table

3.

Pyrite

and enargite

and

its low-temperature

dimorph

uzonite)

re

he

dominant

sulfides

n

HS

deposits;pyrite is abundantbut the amount of

enargite

and

luzonite

is

variable.

Common

ore

minerals,

isted by

decreasing

abundance

rom

variable

to

very

minor,

include

tennantite-

tetrahedrite,

ovellite,

native

gold and argentian

gold

(electrum),

marcasite,

chalcopyrite,

spha-

lerite,

and

galena.

Famatinite

s

locally abundant

in some

deposits

Goldfield,La Mejicana).

Sparse

ore

minerals

nclude

bornite,

cassiterite,

tnnabar,

molybdenite,

orpiment,

realgar,

stibnite,

an d

wolframite

the

last

locally

important

at

Julcani).

Other minerals

present in

minor amounts

in

several

eposits

nclude

Pb-,

Ag-Pb,

Bi- and

Sn -

bearing

ulfbsalts

Table

3).

Fine-grained

uartz s the

dominant

gangue n

HS

deposits.

Other

comrnon

but

minor

gangue

minerals

include

bari te,

kaol ini te,

alunite,

pyrophyllite,

iaspore,

nd Ca-,Sr-,

Pb- and

REE-

bearing

phosphate-sulfate

mineral(s)

such

as

svanbergite-woodhouseite

r crandall i te

(Stoff-

regen

&

Alpers

1987).

For example,

igh-grade

Elsvat ion

m)

|

5 0 0 m

I

I

ffi

l ^

^

a

f - ' - ^ l

tl

m

@

Figure

5. Generalized

urface

lteration

map

(A)

and

cross-section

B)

of

the Rodalquilar

HS deposit

n the

Rodalquilar

nd

Lomilla

calderas,

outheastern

pain

fiom Arribase/

at.

1995a).

he

boundaries

hown

between

alteration

ones

are

rregular

and

gradational.

identification

of

two

stages

of

mineralization,

interpreted

o

correspond

o

two

distinct

magmatic

pulses

(Muntean et al.

1990).

During

the

first

stage (responsible or -600/oof the Au in the

deposit),

hallow

kaolinite-quartz-pyrite

nd deep

alunite-quartz-pyrite-quartz

zones

were

de-

veloped,

with

gold mineral izat ion

n associat ion

with

disseminated

yrite

in the

wallrock;

during

the

second

tage

responsible

or about

40 ofthe

Au),

an extensive

oneof

silicification

with

pyrite

+

sphalerite

+

errargite

eins

formed

at

shallow

levels.

above

a

zone of

pyrophyll i te-diaspore

alteration

Munteanet

al.

1990).

Ono aNu GANGUEMINERAL0GY' ND

TIMING

OF

MINERALIZATION

White

et

ul .

(1995)

and

White

&

Hedenquist

(1995)

presented

etai led

iscussions

n

various

aspects

f

epithermal

gold mineralization

n the

basis

of

observations

rom

a

large

number

of

deposits

round

he

Pacif ic;

heir

conclusions

ith

respect

to

ore

and

gangue

mineralogy

in HS

deposits

are

included

here,

in addition

to the


12/33

veinspecimens

rom Chinkuashih,

Goldfield,

and

La

Mejicana

have

spectacular

ntergrowths

f ore

minerals

with kaolinite,

alunite,

or

pyrophyll i te.

This

observation

implies that

ore

formation

occurred

under

moderately

acidic to

acidic

conditions, hich are inconsistent ith transport

of

Au as

a bisulfide

complex

(Seward

1973).

Recent

studies

of

Au solubi l i ty

in high-

temperature

cid sulfide

solutions

ave

esulted

n

identif ication

f

AuHS" a s one

of

the

principal

gold

complexes

n HS

mineral izat ion

Bening

&

Seward

1994),

he other

possibilitybeing

AuCl2

(e.g.,

Hedenquist

/ al.

1994a).

The

number

and order

of

mineralizing

vents

providecritical

information

or reconstruction

f

the

hydrothermal

system

that

results

in HS

mineral izat ion.

minimum

of

two stages

of

alteration/mineralization

as been

recognized

n

most

deposits

on

the

basis of

crosscutting

relations

Table

3).

The most

common

evolution

is from

an early

leaching

nd

alteration

tage

o a

laterore-forming

stage.

Vuggy

sil ica

ock and

he

advanced

argillic

assemblage

ith disseminated

pyrite orm

typically

early-stage

cidic

alteration,

and are

followed

by Cu

+

Au

+

Ag

deposition.

Detai led

tudies

n some

distr icts

e.g.,

El

Indio,

Lepanto),

owever,

have

resulted

n identif ication

of two

metal

stages,

an

early

Cu-rich,

Au-poor

stage,dominatedby enargite-luzonite,nd a late

Au-rich,

Cu-poor

stage,

associated

with

intermediate-sulfidation-state

ulfides

such

as

tennantite-tetrahedrite

nd

chalcopyrite,

and

tellurides.

The

transition

from

quartz-alunite-

pyrite

alteration

o enargite-pyrite

nd

finally

to

tennantite-tetrahedrite,

he

last typically

without

sulfate

(alunite)

but

with

quartz-sericite

angue

and

wallrock

alteration,

indicates

a fluid

progressively

more

reduced

and

less

acid.

At

Summitvi l le

and

Chinkuashih

also

Tambo

and

Furtei-Serrenti; able l), a late stageof barite-

gold

has

been

documented.

CsaRactnRISTICS

ANDSoURCES

F

HvuRorsnRMAL

FI-utos

Results

f

recent

detai led

luid-inclusion

nd

stable-isotopic

studies

reveal

much about

th e

composition,

temperature

and

sources

of

hydrothermal

luids n

HS deposits" ombination

H gh-suffidation pithermalDeposts

of these

data with

geological

and mineralogical

observations

mentionedabove

allows the nature

of the

altering and ore-forming

fluids to be

determined.

The framework

for the interpretation

has benefited

from information on

the compo-

sitionand luxesof volcanicdischargesnd active


ystems

Hedenquist

&

Lowenstern

1994; Giggenbach

this volume;

Hedenquist

his volume).

F uid-in

c usion

Ev dence

Suitable

hosts

or fluid-inclusionstudies

are

scarce

n HS deposits,

s the

ganguemineralsare

typically

fine-grainedand

even mill imeter-size

hydrothermal

uartz

crystals

re usually

ate

stage

and vug-filling.

Satisfactory

esults

are obtained

on secondary

luid-inclusions

n igneous

quartz

phenocrysts rom

altered

wallrocks; although

lacking

temporal

information,

these inclusions

seem o

provide a representativeross-section

f

the fluids

involved.

The most

reliabledataon the

ore-forming

luids are obtained hrough

infrared

microscopy

directly on

ore minerals, such

as

enargite

(Deen

1990;

Mancano & Campbell

1ee5).

The temperatures

nd salinitiesestimated

or

HS deposits e fine

a

wide range,

rom 90o o

48 0

oC

and

300

"C )

fluids

of

variable

salinity,

which have

been

documented

n several

eposits

nd are

generally

431


13/33

A. Arribas,

Jr .

Table 4.

Summary of

fluid-inclusionmicrothermometric

data

for high-sulfidation

deposits

Deposit

Host-mineral

studicd

Tcmpcrature

Salinity

Asstriatcd

( C) t (cquivwt .%NaCl )

al tcrat ion

Mrxoniboto, Indoncsia

Nalcsbi tan,Phi l ippines

Lcpanto,

Phi l ippines

Chinkuashih, a iwan

Zi . l inshan.

hina

Nansatsu,

apan

Akaiwa,

Japan

Mitsumori-Nukeishi,

Japan

Sunimi tv i l le ,Colorado

Coldlielcl,

Nevada

Pradise Peak,

Nevada

Julclni,

Peru

Ccarhuaraso,

Peru

Colqui.j irca,Peru

Can-Can

(La

Coipa),

Chi lc

El Indio, Chile

La Mejicana

LM)

an d

Ncvados Famatina

NF),

Argentina

Rrxlalquilar,Spain

Furtei-Serrenti, taly

Barite

Quartz

Enargitc

Quartz,

baritc.

a luni te

Qu:rtz

(no

dctails

rcfx)rtcd)

Quartz

DiasJnre

Quartz.

ba-ritc,

quanz

pnen(x

Quartz

phcnoc

Baritc

Quartz-

phenoc

Quartz,

baritc

Quartz,

barite

Quartz

Quartz

phenrr

Quartz

phenoc

Wol, ena,

quartz

Sidcritc

Quartz

phenoc

Qufiz

phcnoc

Sphalcritc. quartz

hiibnerite

Quartz

phenoc

N/A

Quartz

quartz

phcnm

Quartz,

arite,

quartz

hentr

3(n)

2(XI'+60

l6(i-340

230-480

17F300

22(}.450

I 9(),320

9(I 140

(390-5m)

o .2 - t2

a 1 a

3 , 9

0-5

(3-2(

)

< l

up

o

30

0.5-

1. 1

2 - 1 8

(up o 9)

5 1 8

0.2-8


14/33

H

gh-sulfdation

Epithermal

Deposits

Table 4.

(continued)

Dcposit Commcnls

Rcl'crcnccs

M() tornboto,ndoncs ia

n*alcsbi lan. h i l ippincs

Lcpanto,

Phi l ipprncs

Chinkuashih.

a iwan

Zi . j inshan, h ina

Nansatsu,

lpan

Akaiwa,Japan

Mi tsumor i

Nuke: ishi ,

apan

Su

nmi tv i l c . Colorat io

Coltl l iekI,

Ncvatla

Piiraclise

Pcrk, Ncvatla

Julcani ,Pcru

Ccrrhuaraso,

Pcru

Colc lu i l i r ca,

c ru

Can-Can

La

Coipa) .

Ch i l e

El Indiu, Chi lc

La Mc.jicana

LM)

an d

Ncvacirs Famatina

NF),

Argcnt ina

Rrxlalquilu,

Spain

Furtci-Scrrcnti, taly

Rcconnaisancc

rudy n latc-stagc

arite

Reconnaissancc

tudyi

iquid

CO2 observcd

Samplcd intcrval

3

knl long

by 0.5 kn hieh

t

ctnling tluit ls

awav fionr subjaccntporphyry Cu-Au degrsit, whcrc

Th >.150'C

& salinity up to 5.1eq wt.rl NaCl

Prx r r ly -documented

amples

longa'15( lnr

ver t i cal

ntc rval :

the highcr Ths in

sanples lt

-7,50

m

dcpth: CO2 ohserved

Asstrciatcdwith

main stagc

Cu

Dorrp

altcrationzonc

(>6(X)

nr

depth)

Associatcdwith late.

shallow sil ica-Au

Assrriated

with

carly sil ica and

quartz-dickite

Late, vug-li l ling quirtz

Qtz

in

brcccia. salrne iquid and krw-salimty vapor

cmxist

Vein quartz

-4(X)

m

helow Kasugadeposit

Coarsc-grainedclilsgrre

Not

(known)

Au

or Cu

mincralization,

but high salinity

l lu ids

Lic lui t l - r ich:

a l in i ty>6

eq

wl .7 NaCl

only

in vuggy

si l ica

associatedwith

Cu

mineralization:

CO2

obscrvcd

Lrquid- and vapor-rich nclusions: alsopolyphase nclusions

Latc

barite-Au

assemblagc

Truc T5 is interpreted

o be 25(1290 C

Hydrostatic

and ncar-l ithostatic

rcssures

uggested

Latc, vug-li l l ing

crystals n hydrothermal

brcccia:

Frorn

stockwork Au East Zonc

dcoosit: COr observed

Quaru-alunitetpyrite

Pro-ore

ourmalinc

brcccia dykes, ithostatic

pressures

ikely.

Main-stagc

orc

fluicls,

also nner veins, iquid-rich inclusions

Latc-stage

ore fluids,

also

n

outcr

vcinsl P

correction applied

Quartz-alunitctpyrite

Quartz-al

u

ni

etpyrite

Two

generations

dcntil lcdl

both

may

be

very

salinc. Evidcncc

firr P abovc hydrostatic

and

higher

salinit iesat

dcplh

Coppcr and gold stages

Late stage

Interprctcd

as carly,

with vapor-rich

nclusions,

CO2 observetl

LM

& NF. includes iquid-,

vapxrr-rich

nd

potyphasc

nclusions

NF:

complctc transiLion iom

porphyry-type

fluids in K-

sil icatc stage

30(),6(X)+ C,

up to 67 eq wtq, NaCl)

through sercit ic

o epithcrmal

f ' luids n HS

(AA)

stage;

vapor-rich nclusions

ypically

less

saline

Vcrtical

temperature

and salinity

gradient:

high-lcmperature

brines coexist with low

-;Llinity

vapor

inclusions:

hydrostatic

and

near-lithostatic

pressures

suggested

Includes

hi-eh+ low-salinity f luids

(22-23,


15/33

A. Arribas,

r.

and Julcani

Deen

1990)

are broadly

similar, but

their sal ini t iesare

dist inct ly di f ferent

(0.2-4.5

equiv.wt.% NaCl versus

-18equiv.wt.%

NaCl,

respectively),

roviding

constraints

n the role

of

a sal ine magmatic l iquid

(versus

Iow-sal ini ty

vapor) n the

generation

f HS deposits.

Group 3. Lower temperature

e.g.,

90-180

C),

dilute

(typically

600-m

vertical

interval

extending

00

m below he ore zone;Fig.

6) shows a

gradient

which

correlates

with the

change n dominant alteration, rom silicic an d

advanced rgi l l ic

Q

:

170-300

C,

sal ini ty

2-15

equiv.

wt.% NaCl at

the

elevation f the orebody)

to

serici t ic

T:

220-450

C,

sal ini ty

2-45

equiv.

wt.% NaCl)assemblages.

The transition

rom

advanced

argillic alteration,

through

quartz-sericite-pyrite,

to

K-silicate

alteration and typical

porphyry-type

high-

temperature

600+

C)

and

high-salinity

up

to 67

equiv.

wt.% NaCl) f luids of magmatic r igin

is

displayed,among he examples

eviewed,

at

th e

Lepanto-FSE and La Mejicana-Nevadosdel

Famatina epithermal-porphyry opper systems.

The

cooler and less sal ine

inclusion f luids

documented

n the ore zoneof the HS deposits

re

interpreted o reflect mixing

of magmatic and

meteoric

luids

in an environment hallower

han

that

of

porphyry

mineralization.

urthermore,n

common

with

porphyry-type deposits, high-

temperature,

vapor-r ich.

low-sal ini ty f luid

inclusionscoexist

with high-temperature,iquid-

434

Temperature C)

200

300

400

Figure

6. Elevation versus temperature

diagram

showing the range

(horizontal

line)

and average

(vertical

l ine)

of fluid-inclus ion homogenization

temperatures easuredn

the RodalquilarAu deposit,

Spain.Also shownare he temperatures

alculated, n

the basisof

63aS

urfide-surrare

or four

coexisting alunite-

pyrite

samples

large

il led

circles),

eference

oil ing-

point

curves,and vertical spansof the alterationzones

mentioned n

the text. Estimatedsalinit iesof f luid

inclusionsn the

shallowadvanced rgillic/silicic one

and deepsericit iczone rangebetween2 to 30

equiv.

wt.% NaCl and2 to 45 equiv.wt.% NaCl, respectively

(modified

from

Arribas et al. 1995a).

r ich hypersal inenclusions

i.e.,

with

Groups 1

and4, above).These luids may be the resultof

boiling of

a high-temperatureiquid,

or they may

reflect immiscible

vapor

and hypersaline

iquid

derived directly

from shallow-emplaced

magma

(Rye

1993;

Hedenquist & Lowenstern

1994

Shinohara

994;Hedenquisthis

volume).

Sulfur-is otope

Ev den ce

The abundance

of coexisting hydrothermal

sulfidesand

sulfates,

n addition o the

possibility

o

0)

q)

dno

6

'

( g

o

(s

3

q)

Ann ;

- '

o

a,

E

q)

o

H 2 O + 5 w f / . N a C l


16/33

2-6

4

5

Lepanto

Chinkuashih

Nansatsu

Summitvi l le

Goldfield

PuebloViejo

Julcani

El Indio

Rodalquilar

of measuring

oS/"S

in host rock and

genetically

related

gneous ock

(Sasaki

et al.

1919),allows

sulfur-isotope

tudies

o

provide

information

on

the composition, emperature, nd sulfur sources

of

the hydrothermal luids. The resultsof

detailed

studies

n nine HS districts show a

remarkable

consistency

(Fig.

7). In

agreement

with

the

observations in active

volcanic-hydrothermal

systems

e.g.,

Kiyosu

&

Kurahashi 983), ul f ide

and sulfate

minerals

are

mainly in isotopic

equil ibrium,

and, therefore, heir overall

'oS/1'S

depends.on

he temperature

f mineralization

nd

the

'"S/"S

of

total sulfur in the

hydrothermal

system. Only

the data

for alunite from the

Campana ein in El Indio (Fig. 7) are different. f

the

measuredEl

Indio alunites are

not

steam-

heated

r supergene

unlikely

as hey contain ine-

grained

pyrite;

Jannas t al. 1990),

he most ikely

explanation

s a

"magmatic-steam"

(Rye

et al.

1992)origin,

n which

he 63aS

f alunite

s close

to the compositionof total sulfur

in the system

(e.g.,

Alunite Ridge n Marysvale;

Cunningham

l

al.

1984: Rve

el al. 1992\

. Combined

with the

p0

-

420

20

-270

200

-

240

200

390

200

350

180

26 0

210 270

220

330

'(minerat

pairs)

63aS

alues

of

pyrite

and enargite rom the same

vein, these values indicate

drastic changes n

H2S/SO4during the

course of mineralization

(similar to those for the Red Mountain alunite

deposit;Bove

et

al. 1990;Rye 1993).

The main

conclusionsof the sulfur-isotope

studies n HS deposits are:

(

I

)

sulfur in

the

deposits s magmatic,

but

the magmatic

sulfur is

overallheavier han mantlevalues

from

63aS

2

+2olooat

ummi fv i l l e ,

o 9

+2o/noat

Rodalqu i la r ;

Fig. 7). This is not surprising

given

the most

common

geological

setting of

the

deposits;

isotopicallyheavy igneoussulfur is co mmon in

volcanic arc

environments

e.g.,

Ueda & Sakai

1984). (2) A simple mass-balancealculat ion

done

n

severaldeposits sing

the

3oS/"S

values

of the igneous rocks and the average

oS/"S

values of sulfides and sulfates indicates

that

H2S/SO4n the hydrothermalluids was

generally

about

4

*

2

(Fig.

7; Rye

et

al. 1992;Hedenquist

l

ctl.

1994a; Arribas

et al.

1995a).

This is a

minimum value for ore-forming luids

because t

applies

mainly to the early stageof hydrothermal

-Sultides

-

*

Sulfates

^

V& V=

634515

r V

--F

I

'1-

- t v

I

v

- l i

r s z

--.--*

r J

Y

l v

-

I

- ;

o

r Y l

t - ' l

t l

l r f f i

t l

I

@

I

I

I

ry

I

I

@"f"

I

H gh-sulfidation

Epithermal

Depos

ts

aSSHzs-sor

Temp.

"C)'

H2S/

SO 4

10 20

6345

%",

CDT)

-igure

7.

Range of 63o5

per

mil) values

or

sulf ides

and

sulfates rom nine high-

sulfidation eposits.

lso shownare he values

alculatedor

5'oS

or

total sulfur n the

hydrothermal system

(triangles),

H2S/SO4. nd the

range

of temperatures

determined

from sulfide-sulfate

mineral

pairs.

Solid triangles ndicatedeposits n which

6toS*

was

calculatedon

the basis of

isotopic analysesof samples

of unalteredwhole rock

genetically

related o mineralization.

See Appendix for references

and information

on

data

plotted.

43s


17/33

.4. 4rrihns.

/r .

al terat ion.

hich s characterizedv

a sulfate-r ich

alunite-pyri te

ssemblage

3)

lsotopic equi l ib-

rium between

sulfide and

sr-rlfate n the

hydrothermalolut ions

esults,n a nrajori ty f

the

deposits.

n rel iable emperatures

alculatedn

he

basis on A3aSrr:s-so+Fig 7). Pyri te-alunite

rnineral

pairs

were used

most commonly,

and

rvhere ampling

vith depth

s available,hev shorv

a

thermal

radient:

.g.,220

o 330

oC

over200-m

clevation

at Rodalqui lar

Arribas

et al . 1995a).

200 to 390

C

over

.--900

m at S'.tmmitvil le

R1'e

1993)1

20 to 420

'C

over 500

m at l -epanto

(Hedenquist

nd

Carcia

1990:J

\\r. Hedenquist.

unpr-rb. ata).

Other

mineral

prirs

used

with

consistent

results

include

p1'rite-barite

Vikre

1989:

Deen

1990), phaleri te-bari te

Venncmann

et al . 1993), ndplr i te-g1'psurnVikre 1989). he

rangeof

isotopic

emperalures

s consistent

vi th

temperatures

stimated

rom

fluid inerlusionsnd

alteration

mineralogy

e.g.,

Flemley'

t ul. 1980;

Reyes

1990;

Rey'es t ul.

1993).

-he range s also

consistent

with

formation

of

altrnite at

temperatures

elorv

400

C,

rvhenSO2

gas

starts

to dispropottionate

n the

h1'drothermal

olution

(Sakai

&

Matsubaya

911;,

ethke

1984).

Oxygen-

and

Hydrogen-isotope

Evidence

In terms

of oxygett

and

hydrogen

isotopic

composition,

he

fluids that

form

HS deposits

re

arguably

some

of the better

documented

an d

understood

n ore-deposit

tudies.

his si tuation

contrasts

sharply

witli t hat of a

decade

ago, at

which time

no data

were available

o corroborate

the

affinity

suggested

etween

luids

in active


ystems

and

HS deposits

(e.g.,

Heald

et al .

1987;Hedenquist

987).Stable-

isotope

studies

of

HS deposits

are

particularly

i l luminating

because

f :

( l )

the abundance

nd

variety

of oxygen-

and

hydrogen-bearing

inerals

(e.g., lunite, l l i te,kaol ini te),2) thedevelopment

of

analytical

procedures

or complete

stable-

isotope

nalysis

f alunite,

ncluding l8oroo

and

6' tOu'

that help

o dist inguish

he various

ypes

of alunite

and

associated

cid-sulfate

alteration

(Rye

et al.

1992;

Wasserman

t

ctl.

1992),

(3)

fewer limitations

on

the

interpretation

of the

isotopicdata

because

f the

relatively

young

age

of mineralization

f

most

HS deposits

nd

general

lack

of

post-deposit ional

f fects hat disturb he

stable-isotopeystematics.

nd

4)

the avai labi l i ty

of detai led information

on

the

isotopic

composit ion f

f luids in act ive

geothermal

nd

volcanic-hvdrothermal

ystems. which

al lows

fluids estimated n HS deposits o be compared

with

hose

n thei ract ive quivalents.

Some

limitat ionssti l l exist.

fhese

rnay be

rndependentf obvious actors uchas sampling

or

mineral-preparation

rocedures

fundamental

for

achieving

epresentat ivcnd

rel iable

esults).

analyt ical mprecision. nd

naturalvariat ions, s

observed

n active ystems

c.g.,

Aoki 1991,1992,

Rowe

1994). mpo rtant imitat ions hat rnustbe

taken

nto accourrtor optimum useof the stable-

isotope data

are related to

(

l

)

the choice of

temperature f mineral

formatiott br

calculation

of

the l luid isotop ic ompos it ion.

2)

thc lack

of

mineral-water

lractionation

factors for

some

minerals

(e.g

pyrophyl l i te),

and

(3)

the

disagreement among

fractionation constants

proposed

br evencommon

minerals uchas l l i te

(see

Dil les er a/.

1992, for a discussion) nd

kaol ini te. For examole.

at 200

oC

there is a

difference

of

-20

no

between

tlte D/lI fiac-

tionationconstants

or kaolinite

water

as

given

by Marumo et al.

(1980)

on the basisof samples

of

minerals nd rvater

rom activesystems, nd

by

t, iu & Epstein 1984) n the basis f experimental

results.

For these reasons.discussion

of the

sourcesof

water during acidic

alteratiorr

n

the

deposits onsidered

ere

s

basedon

the average

of

the data

collected for alunite,

for which

fractionation

actors are

well-known

(Stoffregen

et al.

1994).

The magmatic-hydrothermal

lunite

typical

of HS deposits

ives good resultsbecause

it is relatively

coarse-grained

post-mineral

D- H

exchange

s not a

problem;

Stoffregen t al.

1994)

and commonly

s closely

associated

ith

ore,

hus

recording qui l ibr iurn ondit ions f a f- luid loser

in composit ion

o

the ascending

mirreral iz ing

solution

than the

kaolinite

or illite

from outer

alteration

ones.

Oxygen

and

hydrogen

sotopic

compositions

of

water n HS deposits

re

clearly

consistent

it h

mixing

between a

high-temperature

magmatic

f lu idof

6180:9

+

1o/nond6D:

-30

+

20 /oond

meteoric

groundwaters

Fig.

8).

In

part

because f


18/33

H gh-su(idation

Epithermal

Depos

ts

-1

00

- t z v

-1

40

6180

% ,

Mow)

Figure 8. Summary

diagramshowingvariation

n

oxygen-and

hydrogen-isotope

omposit ionof hydrothermal

fluids in high-sulfidation eposits.

The average sotopiccomposition

or

the main stagesof acidic alteration

(squares)

nd ore-mineralizat ion

circles)

luids are shown.Where

possible,

nly alunitedata were used

or the

alteration

tage

6D

and

6r8O5eo);'tOo,

is not

usedbecause

ydroxyloxygen equilibrates ith the hydrothermal

fluid during

cooling

(Rye

et al. 1992),

Tie-lines

befween

data

points

connectsamples rom the same

deposit. nset

shows

he isotopic compositionof

fields defined by waters rom active

geothermal

systemsand high-temperature

fumarolecondensates

n

subduction-related

ndesit ic

olcanoes

from

Giggenbach 992b).Go:

Goldfield,Ju :

Julcani,

Le-

Lepanto,

Nansatsu

istrict:

Ka

-

Kasuga, w

:

Iwato, NF

:

Nevados

del Famatina,PV

:

Pueblo

Veijo, Ro

:

Rodalquilar,RM

:

Red Mountain,

Lake City , Colorado, Su

:

Summitville. The

approximate

compositions

of

groundwaters

suggested

or severaldepositsare

indicatedby the intials

parallel

to the meteoric

water

ine. SeeAppendix for references nd

nformationon data

plotted.

the

very

light isotopic composition of local

relations are identical to those

of

volcanic-

meteoric

water,

this meteoric-magmatic water-

hydrothermal and

geothermal

systems associated

mixing

trend

is displayed

particularly

well by

the with subduction-rel ated

volcanism

(Giggenbach

three

stages

of alterationlmi neralization at

Julcani 1992b;

Fig.

8,

inset). The

similarity is even closer

(Deen

1990;

Rye

1993): from

a

magmatic-water- between

he composition of aci dic

alteration

fluids

dominated

early

stage of

(alunite)

acid-sulfate

(large

shaded

field, Fig.

8) and the

vapor

alteration

(Ju,

Fig. 8),

through main ore-stage condensates

rom high-temperature fumaroles of

fluid-inclusion

waters

(Ju1

and Ju2), o meteoric-

andesitic volcanoes

(dark

shaded field, Fig. 8 ,

water-dominated

late ore-stage

fluid-inclusion

inset), such as Nevado del Ruiz,

Satsuma

-40

3

>

-ou

a

d

t9

-eo

o

ta

waters

Ju3).

n

addition

o Julcani,

he

ore

fluids

at Summitville

(Rye

et

al. 1990:'Rye

1993) and

Rodalquilar

Arribas

et al.

1995a) lsohave

ower

6180

values

han thoseof

acidic alteration

luids,

indicating

greater

dilution

by

groundwater

Fig.

8).

The extent of an O-shift

in the

groundwater

component

due to

water-rock

interaction,

as

typically

seen in some

neutral-pH

geothermal

systems,

s not known, but

such

a shift

is not

indicated

y

the Julcanidata.

The overall oxygen-

and hydrogen-isotope

Iwojima, or White Island, he last documentedo

have

a

geochemical

nvironment imilar o that of

HS mineralization

Hedenquist

et al. 1993).

The

origin

of the D-enrichedmagmatic

end-

member) luid of HS deposits asbeen nterpreted

in two ways. Most workers conclude that the

acidic fluid in HS dep osits

is

derived from

absorptionof magmatic vapors outgassing rom

arc volcanoes r

felsic magmas n

crustalsettings

(e.g.,

Hedenquist Aoki

1991;

Matsuhisa 992;

Giggenbach

1992q' Vennemann et

al. 1993;

n

Alunite

lterationtg.

Q

Ore

mineralization

tg.

O

Alteration/

. ^?y,

Subduction-related

volcanrc apor

437


19/33

A. Arribas, Jr.

ALTERATION

Figure 9. Model showing he two main stages f evolutionof HS deposits. : Early stageof advanced rgil l ic

alteration

dominated

by

magmaticvapor.

B, and

Bt: Two

genetic

hypotheses

roposed

or the

stageof ore

formation.

B,

-

absorption

f high-pressure

aporby entrainment

n meteoric

water cell at depth o explain ow-

salinity,

mixed

magmatic-meteoric

re

fluid

(Hedenquist

his volume).B,

-

ascending

metal-bearing

agmatic

brine

with shallow

cooler

meteoricwaters

o explain

high-salinity,mixed magmatic-meteoric re

fluid

(White

I 99

;

Rye

I 993;

Hedenquist t al.

1994a).

metals

strongly

partit ioned nto the

high-density

l iquid

(Hemley

et al .

1992; Hedenquist

his

vo lume) .

At this

early ntrusive

stage, everal

modes

of

magma

degassing

may occur

which wi l l lead o

different styles of magmatic-hydrothermal

systems

with or

without associated

ineralization

(Giggenbach

1992a).

To

form the styles

of

alteration

and he spatial

distribution

of alteration

zones characteristic

of

HS deposits,

degassing

must be

very efficient,

with oxidized

high-

temperature

magmatic

vapor

reaching shallow

depths

with little

reaction

with rock or

dilution by

groundwaters t

greater

epths

Fig.

9A).

Dilution

with

groundwater s s unlikely

because

he high

temperatures

surrounding

the

cooling

magma

causemeteoricwater cells to be displaced rom

the

magma core

(Fig.

9A). In

addition to

the

relat ively

ow

pressure

t

the depth

of intrusion,

effective

degassing

will be

favored by

the

structural

actors

characteristic

f

HS deposits,

such

as fractured

volcanic

domes

or roots of

domes,

aldera

or diatreme

aults,

volcanic

vent

contacts.

and active

faults

with a dilational

comporrent.

As thc

high-temperature

nagmatic

vapor

440

reaches hallowdepths

of less han a kilometer, t

may be absorbedby

groundwater

f it does not

discliarge

as a

fumarole. The

acidity of

this

groundwater-absorbed

apor condensate

ncreases

as the

liquid cools, first a t temperatur es elow

-400

C

by disproportionation f SO2 to form

H2SO4 nd H2S

(Day

&

Allen 1925; Sakai &

Matsubaya

1971), then by

progressive

disso-

ciation

of H2SOa nd

HCI at lowe r temperatur es

(


20/33

consti tut ing relat ivelv mal l

part

of

the

rnixturc-

(gcneral ly


21/33

A. Arrihas,

r.

vapor is required

for transport

of sufficient

amounts of metals

(Hedenquist

his

volume;

Si l l i toe this volume).

These

condit ions are

consistent ith the low

salinityof the Lepanto

nd

El Indio

f luid-inclusion

ata.

Mineral

deposit ion

in this casemay be caused y mixing with cooler

groundwater

r by boiling,

possibly

esulting

ro m

the

abrupt

pressure

reduction

associatedwith

hydrothermal

recciation.

In the

hypersaline

liquid transporl

hypothesis

Fig.

9B2), ol lowing waning

of the

rnagmatic

vapor

plume

responsible

or early

alteration, he l i thostatic-pressured

ystem frac-

tures and the metal-bearing

hypersaline iquid

ascendsnto the

porous

eached

one

Deen

1990;

White l99l; Rye 1993;Hedenquist

/ al . 1994a).

The dominantore-forming

mechanismn this

case

is

rnixingof the metal-bearingypersal ine

iquid

with

cooler

groundwaters

t the site

of deposition,

not

at depth

n

the meteoricwater onvection

el l .

This hypothesis as

been

proposed

o explain

he

high

sal ini t ies

ecorded

by inclusion

luids in

several eposits

e.g.,

ulcani).

A

part

of the ore-fbrming

componentsmay

originate frorn leaching

of

wallrock,

but both

hypothesesagree on a dominantly

magmatic

source br metals,with an increasen

the meteoric

water

component with t ime. The

principal

differencebetween he two hypothesess in th e

nature of the magmatic

phase

responsible

or

transporting the metals into

the epithermal

environment.and in the site of meteoric water

dilution. A

potential

contributor o ore fbrmation

in

HS deposits

nvolves

remobi l izat ion f the

metals by a meteoric-wat er-dominate dydro-

thermal

system

fiom

a subjacent K-silicate

assemblage nd

porphyry-typeprotore,

such as

that

which may have

ormedclose o the intrusion

(e.g. ,

Brimhal l 1980).This mechanism, owever,

has not beensuggested s the main ore-fbrming

process

n

any of the deposits

eviewed n

this

study.

The three models or formation of HS ores.

assimilated here from the l iterature, are not

mutually exclusive;

on

the contrary, they may

occur

in the same HS

deposit

as the magmatic-

hydrothermalsystem evolves,with complexities

arising from multiple intrusions, variations in

depth

of emplacement, nd

changes n

the

local

442

tectonic

and hydrodynami c

nvironment.

one

of

the hree nodels

atisfies

he

overall

evidence.

or

example, f

metalswere

supplied

only by

a dense,

high-salinity

iquid,

a relation

would

be

expected

among

estimated

salinities,

metal

associations,

and ore gradeor metalabundancesf the various

deposits.

uchseems rot

o

be he case.

Similarly,

if alteration

and mineralization

were

solely

th e

result

of interaction

between groundwater

an d

low-

and high-pressure

apor,

respectively.

ig h

salinities

houldnot be as

comtnon

as hey

unlesshey

areexplained

y local

boi l ing

of di lute

to

moderately

saline meteoric

or seawater-

dominatedluids.

SYNTHESIS

Gold, Cu, and

Ag

(and

in

a few

exceptional

cases

also Hg, W, Bi,

Pb, and

Zn) are produced

from

HS deposits. s

a source

f Au,

and because

their mode

of occurrence

and the potential

to

overlie

porphyry-type

nineralization

have

been

widely recognized

nly within

the past

10

to l5

years,

HS deposits

represent

a

valuable

exploration

arget that

has

been overlooked

n

some egions.

Most known

HS deposits

re

young

in age, Tertiary

and

even

Quaternary.

High-

sulfldation

deposits fbrm

dominantly

in

subduction-related plutonic-volcanic arcs,

commonly

duringcrustal

xtension.

fhe

deposits

form

at a depth ntermediate

etween

he

surface

and shallow

few

kilometersdepth)

ntermediate-

composit ionntrusions.

The nt imate elat ionship

mong

HS deposits,

volcanic

host rocks,and

oxidized magrnatic

luid

derived rom

a degassingntrusion

s supported

y

the ol lowing

observations:

l)

the

volcanic ocks

hosting

HS depositswere

erupted immediately

prior

to mineralization,

(2)

the

ore-fbrming

hydrotherma l ystem ommonly ollows the same

plumbing

as that

of t he magmatic

system

i.e.,

rnineralization

patiallyassociated ith

domes

or

volcanic

onduits),

3)

he sotopic

omposit ion

f

hypogene

ulfides

e.g.,

enargite

and

pyrite)

and

sulfates

e.g.,

lunite

nd.bari te)

ommonly

anbe

model led rom

the

'oS/ S

of sul l ' ur n rg neous

rocks thought

to be

genetically

related,

by

equilibrium ractionation

etweenH2S

and

SOa

n

solutionat

T

-200-400

oC,

and

(4)

on the

basisof


22/33

oxygen

and

hydrogen

sotopic

ratios, he

waters

involved

n formation

of HS

deposits re

dentical

to

waters

in active


ys -

tems,

in

which the

same

HS

geochemical

environment

asbeendocumented.

Ore formation in some HS depositsmay

accompany

cidic

alteration,

nd

recentstudies

f

the

hydrothermal

geochemistryof

Au

provide

preliminary evidence

hat

this

element

may be

transported

n HS

and low-sulfidati on

ystems

s

different

hydrosulfide

complexes

(AuHS

and

Au(HS)2,

respectively;

ening& Seward

1994;

Seward

1913).On

the other

hand, he

presence f

moderate

o high sal ini t ies

n many

HS deposits.

the

intimate

association

with

porphyry copper-

type

deposits,

and the

assumptions

f

the most

recentgeneticmodels transport f Au and Cu

by

either

hypersaline

iquid or

high-pressure

apor)

indicate

that

chloride

complexes

must also

be

considered

or

metal ransport.

Most

HS deposits

volve

rom an early

period

of

acidic

wallrock alteration

o a

late

period

of

precious-and

base-metal

nineralization.

cidic

alteration

s characterized

y

advanced

argillic

assemblages

nd

porous

(leached)

ock, and

th e

hydrothermal

luid

responsible

or this

alteration

is dominated

y

high-temperature

agmatic

apor

containing

SO2,

H2S,

and

HCl. Less

reactiveand

oxidized

fluids

are typically

responsible

or

or e

mineralization.

actors uch

as

multiple

ntrusions

and

opening

or closing

of

fractures

conduits)

result

in variations

n the

temperature,

ressure,

and composit ion

of

the

ascending

solut ions.

Combined

with

the shal low

environment

of

mineralization,

hese

conditions

ead o

a variety

of deposit

styles

(mainly

replacements,

reccias,

and

veins)

that

usually

occupy

a

limited

vertical

span

of

800

m at the

giant Chinkuashih

deposit).

The

geological,

mineralogical, and geochemical evidence,

particularly he

association

etween

he orebodies

and the

lateral

and

vertical

zones of

alteration,

illustrates

the basic

genetic condition

of

HS

deposits,

hat

a magmatic

luid

interacts

xtensive

ly

with

country

rock and

groundwaters

n

it s

relatively

short

path o the

earth's

urface.

High-sulfidation pithermalDeposts

ACKNOWLEDGMENTS

Valuable

nsighton variousaspects elated o

this excitingore-forming

environmentwas

gained

throughdiscussions

nd ield work

with

M. Aoki,

A. ArribasSr.,C. G.

Cunningham, . Hedenquist.

W.C.

Kelly, R. O. Rye, J. J.

Rytuba,andT. A.

Steven.

Earlier versions of

this manuscript

benefited

from constructive

reviews by Phil

Bethke,

Andrew Campbell,

Anne Thompson,

ohn

Thompson,

Peter Vikre, Noe l White, and Jeff

Hedenquist,

who also

provided

abundant

documentation

n HS deposits

worldwide.

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