Supergene Silver Enrichment Reassessed_Sillitoe

15

IntroductionDURING the early decades of the 20th century, several eminentinvestigators proposed that silver sulfide enrichment beneathzones of oxidation is a widespread and economically impor-tant supergene process in silver-only and other silver-rich de-posits (e.g., Weed, 1901; Cooke, 1913; Ravicz, 1915; Em-mons, 1917; Lindgren, 1933), a concept that has sincebecome entrenched in the literature (e.g., Bateman, 1942;Boyle, 1968; Guilbert and Park, 1986; Fig. 1). The concept ofsilver enrichment is unsurprising given the geochemicalaffinities between silver and copper, the latter a metal thatcommonly undergoes several-fold sulfide enrichment in thesupergene environment (e.g., Emmons, 1917; Titley andMarozas, 1995). A few deposits have been repeatedly cited astype examples of particularly well-developed silver sulfide en-richment, beneath thick oxidized zones, with Chañarcillo innorthern Chile arguably being the most famous (Whitehead,1919, 1942; Lindgren, 1933; Segerstrom, 1962; Ruiz et al.,1965; Guilbert and Park, 1986).

Nevertheless, the volumetric and economic importance ofsupergene silver sulfide enrichment was extensively debatedduring the early decades of the last century, and some expertsdownplayed its significance, especially in epithermal vein

deposits (e.g., Bastin, 1922). In such deposits, some of thewidely proposed enrichment zones were reinterpreted asproducts of paragenetically late hypogene processes, particu-larly where silver sulfosalts were the putative supergenespecies. Furthermore, argentite (now known to be predomi-nantly acanthite, its lower temperature dimorph) and nativesilver are not necessarily indicators of a supergene origin,since they have been long recognized as valid and importanthypogene species (e.g., Bastin, 1925; see below).

The supergene parts of the deposits in which silver sulfideenrichment was originally defined (e.g., Chañarcillo) werefully exploited as long ago as the early 1900s and, therefore,are no longer accessible to detailed study. However, duringthe last 30 years or so, a number of low-grade, bulk-tonnagesilver deposits have been discovered and explored, and someexploited. Although such deposits, by analogy with porphyrydeposits in the case of copper, should provide optimal sites forsupergene profile development, little mention has been madeof appreciable upgrading due to silver sulfide enrichment.

Therefore, in view of the obvious economic implications, itis considered timely to reassess the importance of supergenesilver enrichment across a broad spectrum of the world’smajor silver-dominated and other silver-rich deposits (Table1). The deposits selected, markedly concentrated in westernNorth and South America, span most of the world’s climatic

Chapter 2

Supergene Silver Enrichment Reassessed

RICHARD H. SILLITOE†

27 West Hill Park, Highgate Village, London N6 6ND, England

AbstractSupergene silver sulfide enrichment has been widely accepted for the last 100 years, but has warranted little

or no mention in descriptions of several silver-rich, bulk-tonnage orebodies defined over the last three decades.This dichotomy is addressed by reassessing the importance of enrichment in 40 of the world’s premier silver-dominated and other silver-rich deposits, including several of historical significance. The deposits are of high-grade vein and low-grade, bulk-tonnage styles and varied genetic types, but are dominated by representativesof the intermediate-sulfidation epithermal and carbonate-replacement, chimney-manto classes.

The results of this preliminary analysis show that only 12 (30%) of the deposits contain(ed) appreciableamounts of silver ore generated by silver sulfide enrichment, mainly in the form of acanthite and argentianchalcocite-group minerals in the cases where its mineralogic characteristics are recorded. Silver-rich oxidizedzones are, however, well developed in 60 percent of the deposits and, locally, display silver enrichment of ei-ther residual or chemical origin. Irrespective of whether oxidative weathering takes place under acidic or alka-line conditions, a factor controlled mainly by hypogene iron sulfide and carbonate contents, silver tends to beretained in oxidized zones, with comparatively little remaining available in solution to generate underlying sil-ver sulfide enrichment. The extreme insolubility of the silver halides (chlorargyrite, embolite, bromargyrite, io-dargyrite) over broad pH and climatic ranges, besides efficient silver fixation as native silver, argentojarosite,or silver-bearing manganese oxides under the appropriate chemical conditions, explains the metal’s relative su-pergene immobility.

The efficient dissolution and downward transport of copper under acidic supergene conditions, as exempli-fied by porphyry copper leached cappings and underlying multicyclic enrichment blankets, appears to have nocounterpart in either silver-only or other silver-rich deposits. Nor are the silver equivalents of exotic oxide copper deposits, the products of lateral metal transport in the acidic supergene environment, considered likelyto exist. Furthermore, the processing benefits accruing from supergene oxidation and enrichment of copperdeposits are not as evident in the silver environment, in which the main supergene oxidation products, espe-cially the silver-bearing manganese oxides and argentojarosite, commonly present metallurgical difficulties.

©2009 Society of Economic GeologistsSpecial Publication 14, 2009, pp. 15–32

† E-mail: [email protected]

and physiographic regimes (Fig. 2) besides representing allthe main deposit types with their correspondingly differentmineralogic constitutions. In this latter regard, the amount ofacid- and Fe3+-generating sulfide minerals and acid-neutraliz-ing gangue minerals are critical to supergene processes, asthey also are in copper deposits (e.g., Emmons, 1917; Ander-son, 1982; Sillitoe, 2005).

This review should be considered as a first attempt at re-assessment of the importance of silver sulfide enrichment be-cause of the difficulty of obtaining reliable data for many ofthe deposits selected. The problem is two-fold: supergene oreswere commonly exploited and described, some rather poorly,as much as a century ago (e.g., Chañarcillo, Comstock Lode),since when only their relative importance vis-à-vis the deeper,hypogene parts of the deposits has been addressed; and sev-eral of the more recent discoveries await full documentation,including detailed mineralogic study. Although this analysisfocuses on silver sulfide enrichment, silver enrichment in theoxidized parts of supergene profiles is also considered. Theeconomic consequences of the resulting supergene mineralassemblages are also the subject of brief comment.

Major Silver-rich DepositsThe 40 silver-only and other silver-rich deposits selected

for consideration (Table 1; Fig. 3) either rank among theworld’s largest silver concentrations or are particularly wellknown, because of either their prominence as major produc-ers during the 19th and early 20th centuries (e.g., Chañarcillo,Comstock Lode, Tintic, Tonopah) or their relatively recent

16 RICHARD H. SILLITOE

0361-0128/98/000/000-00 $6.00 16

Ag halides

Native Ag

Ground water table

Surface

Acanthite

Oxi

diz

ed z

one

Enr

ichm

ent

zone

Hyp

ogen

ezo

ne

Pyrargyrite,proustite

FIG. 1. Idealized supergene silver profile presented by Boyle (1968),which is at variance with the conclusions reached herein. Note importanceassigned previously to silver sulfide enrichment and the supergene origin ofsilver sulfosalts.

60°N

30°N

0°

30°S

150°W

120°W

90

°W

60

°W

30

° W

30°E

60°E

90°E

12

0°E

15

0°E0°

Cove

Leadville

Cobalt

Imiter

Cannington

Fankou

Broken Hill

Dukat

TinticPark City

Greens Creek

RochesterComstock Lode

Paradise PeakTonopahHardshell

FresnilloZacatecasPachuca

Cerro de PascoColquijirca

CoraniOruroSan CristóbalEl PeñónChañarcillo

Pascua-Lama

Navidad

Cerro Rico de Potosí

Pulacayo

La Coipa

TayoltitaSanta Eulalia

PeñasquitoProvidencia

Real de Angeles

Guanajuato

Keno Hill

Eskay CreekCoeur d’Alene

FIG. 2. Locations of the 40 major silver and other silver-rich deposits selected for consideration. Note the concentrationsin western North and South America and distribution across a broad spectrum of physiographic and climatic zones.

SUPERGENE SILVER ENRICHMENT REASSESSED 17

0361-0128/98/000/000-00 $6.00 17

TAB

LE

1. S

elec

ted

Supe

rgen

e an

d O

ther

Cha

ract

eris

tics

of M

ajor

Silv

er-O

nly

and

Oth

er S

ilver

-Ric

h D

epos

its

Supe

rgen

e C

onta

ined

A

vera

ge

Supe

rgen

e en

rich

men

t D

epos

it,A

g,gr

ade,

Su

perg

ene

Hyp

ogen

e A

g ox

idiz

ed A

g A

g m

iner

al(s

) lo

catio

nD

epos

it ty

pe1

Moz

g/t A

gco

ntri

butio

nm

iner

al(s

) (m

inor

)m

iner

al(s

) (m

inor

)(m

inor

)M

ain

data

sou

rce(

s)

Cer

ro R

ico

de

Hig

h-su

lfida

tion

3,70

0N

A95

% o

xidi

zed

(to

Aca

nthi

te, a

ndor

ite,

Chl

orar

gyri

te,

Lin

dgre

n an

d Po

tosí

, Bol

ivia

epith

erm

al s

tock

wor

k-30

0 m

); su

lfide

py

rarg

yrite

, tet

rahe

drite

, na

tive

Ag,

ioda

r-C

reve

ling

(192

8),

diss

emin

ated

Ag-

Snen

rich

men

t abs

ent

mat

ildite

, mia

rgyr

itegy

rite

, em

bolit

e St

eele

(19

96),

(arg

ento

jaro

site

, B

arto

s (2

000)

man

gane

se o

xide

s)

Cer

ro d

e C

arbo

nate

1,

600

150

50%

oxi

dize

d (u

p to

Te

nnan

tite-

tetr

ahed

rite

, Pl

umbo

jaro

site

Cha

lcoc

ite

Geo

logi

cal S

taff

Pa

sco,

Per

ure

plac

emen

t Zn-

Pb-A

g90

m th

ick)

; up

to 3

0 m

ar

amoy

oite

, pol

ybas

ite,

(str

omey

erite

)of

the

Cor

pora

tion

sulfi

de e

nric

hmen

tac

anth

ite(1

950)

, Am

stut

z an

d W

ard

(195

6)B

roke

n H

ill,

Bro

ken

Hill

-typ

e 1,

400

148

60-1

20-m

oxi

dize

d Te

trah

edri

te, d

yscr

asite

, E

mbo

lite,

nat

ive

Stro

mey

erite

, St

illw

ell (

1953

), A

ustr

alia

Zn-P

b-A

gzo

ne; s

ulfid

e en

rich

-py

rarg

yrite

, nat

ive

Ag,

A

g, M

n ox

ides

, ac

anth

ite,

van

Moo

rt a

nd

men

t up

to 1

m th

ick

step

hani

te (

acan

thite

, io

darg

yrite

, ja

lpai

te,

Swen

sson

(19

82),

mck

inst

ryite

, mia

rgyr

ite,

iodo

embo

lite,

na

tive

Ag

Plim

er (

1984

)po

lyba

site

, pro

ustit

e,

chlo

rarg

yrite

, al

larg

entu

m)

brom

argy

rite

Pach

uca,

Inte

rmed

iate

-sul

fidat

ion

1,36

435

0M

inor

oxi

datio

n A

cant

hite

(m

iarg

yrite

, C

hlor

argy

rite

, N

ativ

e A

g,

Bas

tin (

1948

), M

exic

oep

ither

mal

vei

n A

g-A

u(u

p to

30

m);

man

y py

rarg

yrite

, pro

ustit

e,

brom

argy

rite

, ac

anth

iteT

horn

burg

(19

52)

vein

s bl

ind;

sul

fide

nativ

e A

g)M

n ox

ides

, en

rich

men

t abs

ent

Coe

ur d

’Ale

ne,

Mes

othe

rmal

1,

208

500-

850

Shal

low

ly o

xidi

zed

Fre

iber

gite

(ga

lena

, N

ativ

e A

gR

anso

me

and

Idah

o(m

etam

orph

ogen

ic)

(0-6

0 m

); su

lfide

po

lyba

site

, pro

ustit

e,

Cal

kins

(19

08),

vein

Ag-

Pb-Z

n-(C

u-Sb

)en

rich

men

t abs

ent

pyra

rgyr

ite)

Hob

bs a

nd

Fry

klun

d (1

968)

Gua

naju

ato,

Inte

rmed

iate

-sul

fidat

ion

1,14

027

0M

inor

oxi

datio

n A

cant

hite

, agu

ilari

te,

Chl

orar

gyri

te,

Wan

dke

and

Mex

ico

epith

erm

al v

ein

Au-

Ag

(20-

30 m

); su

lfide

po

lyba

site

, pyr

argy

rite

, em

bolit

e,

Mar

tínez

(19

28),

enri

chm

ent a

bsen

tel

ectr

um, g

alen

a,

brom

argy

rite

Que

rol e

t al.

(199

1)st

epha

nite

, mia

rgyr

iteF

resn

illo,

Inte

rmed

iate

-sul

fidat

ion

910

425

9% o

xidi

zed

(to

300

m

Pyra

rgyr

ite, p

olyb

asite

, C

hlor

argy

rite

, G

emm

ell e

t al.

Mex

ico

epith

erm

al v

ein

and

at C

erro

Pro

año)

; pe

arce

ite, a

cant

hite

na

tive

Ag,

(1

988)

; Tre

jo (

2001

)ca

rbon

ate-

repl

acem

ent

SE v

eins

blin

d an

d (s

teph

anite

, nat

ive

Ag)

acan

thite

, A

g-A

u-Zn

-Pb-

Cu

unox

idiz

ed; s

ulfid

e br

omar

gyri

teen

rich

men

t abs

ent

Peña

squi

to,

Inte

rmed

iate

-sul

fidat

ion

864

2913

% o

xidi

zed

Aca

nthi

te (

frei

berg

ite,

Chl

orar

gyri

te(?

)B

ryso

n et

al.

Mex

ico

epith

erm

al d

isse

min

ated

(4

0-12

0 m

thic

k);

poly

basi

te)

(200

7), B

row

n (d

iatr

eme

brec

cia-

sulfi

de e

nric

hmen

t (2

008)

rela

ted)

Ag-

Au-

Zn-P

bab

sent

Can

ning

ton,

Bro

ken

Hill

-typ

e 75

853

8N

o ox

idat

ion;

F

reib

ergi

te, g

alen

a, p

yrar

gy-

Bai

ley

(199

8)A

ustr

alia

Ag-

Pb-Z

nco

ncea

led

bene

ath

rite,

alla

rgen

tum

, aca

nthi

te,

post

min

eral

cov

erdy

scra

site

, nat

ive

Ag

Zaca

teca

s,In

term

edia

te-s

ulfid

atio

n 75

012

0M

inor

oxi

datio

n A

cant

hite

, pol

ybas

ite,

Nat

ive

Ag

Aca

nthi

te,

Bas

tin (

1941

), M

exic

oep

ither

mal

vei

n (u

p to

40-

60 m

); py

rarg

yrite

, fre

iber

gite

(b

rom

argy

rite

, na

tive

Ag

Ponc

e an

d A

g-Pb

-Zn-

Cu-

Au

min

or s

ulfid

e (m

iarg

yrite

, ste

phan

ite,

chlo

rarg

yrite

)C

lark

(19

88)

enri

chm

ent

frei

esle

beni

te)

San

Cri

stób

al,

Inte

rmed

iate

-sul

fidat

ion

685

6310

% o

xidi

zed

(10-

35

Aca

nthi

te, g

alen

a A

rgen

toja

rosi

te

Aca

nthi

te,

Buc

hana

n (2

003)

, B

oliv

iaep

ither

mal

dis

sem

inat

ed

m th

ick)

; sul

fide

(pol

ybas

ite, p

earc

ite,

(nat

ive

Ag)

nativ

e A

gL

ozan

o (2

007)

, an

d br

ecci

a-ho

sted

en

rich

men

t min

or

pyra

rgyr

ite)

L. B

ucha

nan

(wri

t. A

g-Zn

-Pb

(avg

4 m

thic

k)co

mm

un.,

2008

)


0361-0128/98/000/000-00 $6.00 18

Cob

alt,

Ag-

Co-

Ni-A

s ve

in: A

g60

0N

AM

inor

oxi

datio

n an

d N

ativ

e A

g (s

teph

anite

, N

ativ

e A

gA

cant

hite

, Pe

truk

(19

71),

Ont

ario

sulfi

de e

nric

hmen

t2py

rarg

yrite

, aca

nthi

te,

nativ

e A

gPe

truk

et a

l. (1

971)

(gla

ciat

ed te

rrai

n)pr

oust

ite, p

olyb

asite

)Pa

scua

-Lam

a,H

igh-

sulfi

datio

n 58

566

20%

oxi

dize

d3 (u

p to

Py

rite

(ot

her

min

eral

s)Io

darg

yrite

, chl

or-

Cho

uina

rd e

t al.

Chi

le/A

rgen

tina

epith

erm

al d

isse

m-

350

m th

ick)

; sul

fide

argy

rite

(na

tive

Ag,

(2

005)

inat

ed A

u-A

g-(C

u)en

rich

men

t abs

ent

Ag

sele

nide

s)D

ukat

,In

term

edia

te-s

ulfid

atio

n 56

950

010

% o

xidi

zed

Nat

ive

Ag,

aca

nthi

te,

Nat

ive

Ag,

A

cant

hite

, B

elko

v et

al.

(199

2),

Rus

sia

epith

erm

al v

ein

Ag-

Au

(ave

rage

100

m,

poly

basi

te, p

yrar

gyri

te,

arge

ntoj

aros

ite(?

), na

tive

Ag,

K

onst

antin

ov e

t al.

up to

400

m);

min

or

elec

trum

(st

rom

eyer

ite,

acan

thite

mck

inst

ryite

(1

995)

sulfi

de e

nric

hmen

tst

epha

nite

)(s

trom

eyer

ite)

Nav

idad

,In

term

edia

te-s

ulfid

atio

n 45

711

0<5

% o

xidi

zed

(0-5

0 m

); A

cant

hite

, pro

ustit

e,

Chl

orar

gyri

teL

hotk

a et

al.

(200

5)A

rgen

tina

epith

erm

al b

recc

ia,

sulfi

de e

nric

hmen

t st

rom

eyer

ite, p

yrite

, st

ockw

ork,

and

dis

sem

-ab

sent

tenn

antit

e, n

ativ

e A

gin

ated

Ag-

Pb-C

u-Zn

Sant

a E

ulal

ia,

Car

bona

te-r

epla

cem

ent

436

367

50%

oxi

dize

d (c

om-

Gal

ena,

aca

nthi

te,

Chl

orar

gyri

te,

Pres

cott

(19

16),

Mex

ico

Ag-

Pb-Z

n-(S

n)m

only

to >

450

m);

prou

stite

, pyr

argy

rite

arge

ntoj

aros

ite,

Mal

dona

do (

1991

)su

lfide

enr

ichm

ent n

ot

nativ

e A

g, M

n ox

ides

repo

rted

Oru

ro,

Bol

ivia

n-ty

pe v

ein

349

>500

Oxi

dize

d fr

om 2

0-15

0 F

reib

ergi

te, a

ndor

ite

Chl

orar

gyri

te,

Cha

ce (

1948

)B

oliv

iaA

g-Sn

-(Pb

-Cu-

Sb)

m; s

ulfid

e en

rich

men

t (m

iarg

yrite

, ste

phan

ite,

nativ

e A

gab

sent

pyra

rgyr

ite, o

wyh

eeite

)Ta

yolti

ta,

Inte

rmed

iate

-sul

fidat

ion

320

460

Min

or o

xida

tion;

man

y A

cant

hite

, jal

paite

, N

ativ

e A

g,

Cla

rk (

1991

)M

exic

oep

ither

mal

vei

n A

g-A

uve

ins

blin

d; s

ulfid

e st

rom

eyer

ite, n

ativ

e M

n ox

ides

enri

chm

ent a

bsen

tA

g, e

lect

rum

Fan

kou,

Car

bona

te-r

epla

cem

ent

292

102

Supe

rgen

e G

alen

a (p

rous

tite)

Song

(19

84)

Chi

naA

g-Zn

-Pb-

Hg

prof

ile a

bsen

tC

oran

i,In

term

edia

te-s

ulfid

atio

n 27

851

Oxi

datio

n to

30-

40 m

; F

reib

ergi

te, p

rous

tite,

C

hlor

argy

rite

(?)

D. V

olke

rt (

pers

. Pe

ruep

ither

mal

vei

n an

d su

lfide

enr

ichm

ent

pyra

rgyr

ite (

mia

rgyr

ite,

com

mun

., 20

07),

stoc

kwor

k A

g-Pb

-Zn-

Au

not r

epor

ted

nativ

e A

g)Vo

lker

t et a

l. (2

007)

Gre

ens

Cre

ek,

VM

S A

g-A

u-Zn

-Pb-

Cu

278

540

Supe

rgen

e pr

ofile

Te

nnan

tite-

tetr

ahed

rite

, Ta

ylor

et a

l. (1

999)

, A

lask

aab

sent

frei

berg

ite, a

cant

hite

, M

Sat

re (

wri

t. na

tive

Ag,

pyr

argy

rite

, co

mm

un.,

2008

)pr

oust

iteTi

ntic

,C

arbo

nate

-rep

lace

men

t 27

448

670

% o

xidi

zed

(300

-A

cant

hite

, nat

ive

Ag,

C

hlor

argy

rite

, A

cant

hite

, L

indg

ren

and

Uta

hA

g-A

u-Pb

-Cu-

Zn70

0 m

); m

inor

sul

fide

prou

stite

, pea

rcite

, na

tive

Ag,

na

tive

Ag

Lou

ghlin

(19

19),

enri

chm

ent

poly

basi

te, s

trom

eyer

ite,

arge

ntoj

aros

ite,

Mor

ris

(196

8),

frei

berg

ite, p

yrar

gyri

te,

acan

thite

Mor

ris

and

step

hani

teL

over

ing

(197

9)L

eadv

ille,

Car

bona

te-r

epla

cem

ent

260

320

Oxi

dize

d fr

om 1

20-

Aca

nthi

te, f

reib

ergi

teE

mbo

lite,

A

cant

hite

, E

mm

ons

et a

l. C

olor

ado

Ag-

Pb-Z

n-C

u-A

u18

0 m

; min

or s

ulfid

e ch

lora

rgyr

ite,

chal

coci

te,

(192

7), T

wet

o en

rich

men

t in

vein

sbr

omar

gyri

te,

nativ

e A

g(1

968)

, Cap

pa a

nd

nativ

e A

g, a

cant

hite

B

arto

s (2

007)

(ioda

rgyr

ite)

Park

City

,C

arbo

nate

-rep

lace

men

t 25

448

525

% o

xidi

zed;

Te

nnan

tite-

tetr

ahed

rite

, C

hlor

argy

rite

Bar

nes

and

Uta

hA

g-Pb

-Zn-

Cu-

Au

sulfi

de e

nric

hmen

t ac

anth

ite, g

alen

a Si

mos

(19

68)

not r

epor

ted

(mat

ildite

)

TAB

LE

1. (

Con

t.)

Supe

rgen

e C

onta

ined

A

vera

ge

Supe

rgen

e en

rich

men

t D

epos

it,A

g,gr

ade,

Su

perg

ene

Hyp

ogen

e A

g ox

idiz

ed A

g A

g m

iner

al(s

) lo

catio

nD

epos

it ty

pe1

Moz

g/t A

gco

ntri

butio

nm

iner

al(s

) (m

inor

)m

iner

al(s

) (m

inor

)(m

inor

)M

ain

data

sou

rce(

s)


0361-0128/98/000/000-00 $6.00 19

Pula

cayo

,B

oliv

ian-

type

vei

n (+

23

4~1

,000

Supe

rgen

e pr

ofile

F

reib

ergi

teA

hlfe

ld a

nd

Bol

ivia

stoc

kwor

k-di

ssem

inat

ed)

abse

ntSc

hnei

der-

Ag-

Zn-P

b-(C

u-A

u)Sc

herb

ina

(196

4)K

eno

Hill

, M

esot

herm

al v

ein

234

1,41

2O

xidi

zed

from

5-1

50

Fre

iber

gite

, gal

ena,

B

euda

ntite

, N

ativ

e A

gB

oyle

(19

65),

Yuko

n Te

rrito

ryA

g-Pb

-Zn-

(Cd)

m; i

ll-de

fined

sul

fide-

pyra

rgyr

ite (

poly

basi

te,

plum

boja

rosi

te,

Lyn

ch (

1989

)en

rich

men

t zon

est

epha

nite

, aca

nthi

te,

arge

ntoj

aros

ite

nativ

e A

g)(n

ativ

e A

g, a

cant

hite

)R

eal d

e A

ngel

es,

Inte

rmed

iate

-sul

fidat

ion

215

8510

% o

xidi

zed

(2-2

0 m

); G

alen

a, fr

eibe

rgite

C

hlor

argy

rite

, C

halc

ocite

Pear

son

et a

l. M

exic

oep

ither

mal

dis

sem

inat

ed

sulfi

de e

nric

hmen

t (a

cant

hite

, ste

phan

ite)

brom

argy

rite

, (1

988)

, Bra

vo

Ag-

Pb-Z

n-(C

d)<8

5 m

thic

kar

gent

ojar

osite

(199

1)R

oche

ster

,In

term

edia

te-s

ulfid

atio

n 19

438

90%

oxi

dize

d (u

p to

Te

trah

edri

te,

Chl

orar

gyri

te,

Vik

re (

1981

)N

evad

aep

ither

mal

vei

n an

d 20

0 m

); su

lfide

st

rom

eyer

ite, p

olyb

asite

, em

bolit

e, a

cant

hite

, st

ockw

ork

Ag-

Au

enri

chm

ent a

bsen

tpy

rarg

yrite

, aca

nthi

te,

nativ

e A

g,

pyri

te (

owyh

eeite

), ar

gent

ojar

osite

Com

stoc

k In

term

edia

te-s

ulfid

atio

n 19

231

0M

inor

nea

r-su

rfac

e A

cant

hite

, ste

phan

ite,

Nat

ive

Ag,

B

astin

(192

2), S

mith

L

ode,

Nev

ada

epith

erm

al v

ein

Ag-

Au

oxid

atio

npo

lyba

site

, ele

ctru

m

chlo

rarg

yrite

, an

d Ti

ngle

y (1

998)

, M

n w

adH

udso

n (2

003)

La

Coi

pa,

Hig

h-su

lfida

tion

190

119

100%

oxi

dize

d (u

p to

N

ativ

e A

g, a

cant

hite

, C

hlor

argy

rite

, O

vied

o et

al.

(199

1)C

hile

epith

erm

al d

isse

m-

100

m th

ick)

; sul

fide

elec

trum

, pro

ustit

e,

nativ

e A

g, e

mbo

lite,

in

ated

Au-

Ag

enri

chm

ent a

bsen

tpy

rarg

yrite

, ten

nant

ite-

ioda

rgyr

ite,

tetr

ahed

rite

arge

ntoj

aros

iteIm

iter,

Ag-

Co-

Ni-A

s ve

in,

190

700

Oxi

dize

d to

100

m;

Nat

ive

Ag,

am

alga

m,

Nat

ive

Ag

(?)

Bar

oudi

et a

l. M

oroc

cost

ockw

ork,

and

bre

ccia

: su

lfide

enr

ichm

ent

acan

thite

, pol

ybas

ite,

(199

9), C

heill

etz

Ag-

Hg

abse

ntpy

rarg

yrite

, im

iteri

te,

et a

l. (2

002)

pear

ceite

, pro

ustit

eTo

nopa

h,In

term

edia

te-s

ulfid

atio

n 17

264

0O

xidi

zed

to 2

00 m

; A

cant

hite

, pol

ybas

ite,

Chl

orar

gyri

te,

Aca

nthi

te,

Bur

gess

(19

11),

Nev

ada

epith

erm

al v

ein

Ag-

Au

min

or s

ulfid

e en

rich

-py

rarg

yrite

embo

lite,

ioda

rgyr

ite,

poly

basi

te (

?),

Bas

tin a

nd

men

t pos

sibl

eM

n ox

ides

, nat

ive

Ag

pyra

rgyr

ite (

?)L

aney

(19

18)

Col

quiji

rca,

Car

bona

te-r

epla

cem

ent

170

180

5-10

% o

xidi

zed

(50-

Tenn

antit

e, s

trom

eyer

ite,

Nat

ive

Ag,

C

halc

ocite

Ahl

feld

(19

32),

Peru

Zn-P

b-A

g12

0 m

thic

k); m

inor

m

atild

ite, n

ativ

e A

g,

arge

ntoj

aros

ite

Lin

dgre

n (1

935)

, su

lfide

enr

ichm

ent

acan

thite

, gal

ena

(pro

ustit

e)

(chl

orar

gyri

te)

McK

inst

ry (

1936

)E

l Peñ

ón,

Low

-sul

fidat

ion

165

242

100%

oxi

dize

d (u

p to

E

lect

rum

, aca

nthi

te

Chl

orar

gyri

te,

Rob

bins

(20

00),

Chi

leep

ither

mal

vei

n A

u-A

g40

0 m

thic

k); s

ulfid

e (s

ulfo

salts

, inc

ludi

ng

embo

lite,

ioda

rgyr

ite,

War

ren

et a

l. (2

004)

enri

chm

ent a

bsen

tpr

oust

ite)

nativ

e A

gC

ove,

Se

dim

ent-

host

ed16

456

Oxi

dize

d up

to 3

00 m

; G

alen

a, te

nnan

tite-

Mn

oxid

es,

Em

mon

s an

d N

evad

aA

u-A

g-(Z

n-Pb

)su

lfide

enr

ichm

ent

tetr

ahed

rite

, pro

ustit

e-ch

lora

rgyr

ite,

Eng

(19

95),

abse

ntpy

rarg

yrite

, ste

phan

ite,

ioda

rgyr

iteJo

hnst

on (

2000

)pe

arce

ite-p

olyb

asite

, ca

nfie

ldite

, str

omey

erite

, el

ectr

um, n

ativ

e A

g,

acan

thite

(m

iarg

yrite

)Pr

ovid

enci

a,

Car

bona

te-r

epla

cem

ent

107

440

70%

oxi

dize

d (t

o 15

0-50

0G

alen

a, te

trah

edri

teU

nrep

orte

d, b

ut

Unr

epor

ted

Trip

lett

(19

52),

Mex

ico

Zn-P

b-A

gm

); su

lfide

enr

ichm

ent

halid

es p

roba

ble

Map

es e

t al.

(196

4)st

rong

ly s

uspe

cted

Esk

ay C

reek

V

MS

Ag-

Au-

Zn-P

b-C

u10

12,

930

Supe

rgen

e pr

ofile

F

reib

ergi

te (

elec

trum

)R

oth

et a

l. (1

999)

(21B

), B

ritis

h ab

sent

(bl

ind

depo

sit)

Col

umbi

a

TAB

LE

1. (

Con

t.)

Supe

rgen

e C

onta

ined

A

vera

ge

Supe

rgen

e en

rich

men

t D

epos

it,A

g,gr

ade,

Su

perg

ene

Hyp

ogen

e A

g ox

idiz

ed A

g A

g m

iner

al(s

) lo

catio

nD

epos

it ty

pe1

Moz

g/t A

gco

ntri

butio

nm

iner

al(s

) (m

inor

)m

iner

al(s

) (m

inor

)(m

inor

)M

ain

data

sou

rce(

s)

discovery (e.g., Corani, Navidad, Peñasquito, San Cristóbal).Cerro Rico de Potosí, where the colonial Spanish producedan estimated 13 Moz Ag in 1580, is by far the world’s largestsilver deposit, and probably more than twice the size of thesecond largest (Cerro de Pasco; Fig. 3). The Cannington de-posit is currently the largest producer of mined silver, nearly40 Moz per year, but both Pascua-Lama and Peñasquito willchallenge its leading position once these low-grade depositsattain full production, as will Fresnillo once the planned ex-pansion is complete.

In Table 1 and Figure 3, the deposits, arranged in order ofdecreasing size, are assigned to widely accepted geneticclasses rather than to the more descriptive categories used inthe seminal review of silver deposits by Graybeal et al. (1986).Furthermore, instead of attempting a subdivision into silver-dominant, coproduct silver, and byproduct silver deposits(Graybeal et al., 1986), deposits containing >~100 g/t Ag areselected for analysis, although several lower-grade, but silver-dominant deposits are also included because of their largesize (e.g., Corani, Peñasquito, Rochester, San Cristóbal).However, Table 1 does indicate the relative importance of anyaccompanying economic metals.

As summarized in Figure 4, epithermal deposits of inter-mediate-sulfidation vein and bulk-tonnage types (cf. Heden-quist et al., 2000) constitute 42 percent and intrusion-related,carbonate-replacement, chimney-manto Zn-Pb-Ag deposits,18 percent of the total. Several deposits assigned to the inter-mediate-sulfidation epithermal category, including the dia-treme-related Peñasquito, manto-type Hardshell, and vein-type Fresnillo deposits (Table 1), display transitions to thecarbonate-replacement type. Bulk-tonnage, high-sulfidationepithermal; Ag-Co-Ni-As vein (cf. Bastin, 1939; Graybeal etal., 1986); Bolivian-type, polymetallic, semimassive sulfidevein (cf. Ludington et al., 1992); Broken Hill-type Zn-Pb-Ag;mesothermal Ag-Pb-Zn vein (of possible metamorphogenicorigin; Beaudoin et al., 1999); low-sulfidation epithermalvein; volcanogenic massive sulfide (VMS); and sediment-hosted (Carlin-like) deposits make up the remainder (Table 1;Figs. 3, 4). Interestingly, the five largest deposits representfive different deposit types.

Viewed together, the deposits selected contain a wide vari-ety of hypogene silver minerals (Table 2), with acanthite(commonly reported originally as argentite) and silver sulfos-alts, both occurring in approximately three-quarters of thedeposits (Table 1), being the most widespread. The epi-thermal deposits, particularly those of high-sulfidation type,along with most carbonate-replacement, VMS, and Bolivian-type vein (Pulacayo, Oruro) deposits are highly pyritic,whereas the Ag-Co-Ni-As veins, stockworks, and breccias(Chañarcillo, Cobalt, Imiter) and Broken Hill-type deposits(Broken Hill, Cannington) are deficient in iron sulfides butdo contain arsenopyrite. Argentiferous pyrite is an importantore mineral at Pascua-Lama (Chouinard et al. (2005), and isalso reported at Rochester (Vikre, 1981), Navidad (J.J.Chulick, pers. commun., 2007), and Broken Hill (Plimer,1984).

The high-sulfidation deposits are devoid of carbonate min-erals because of formation under acidic hypogene conditions,whereas the intermediate- and low-sulfidation epithermal,


0361-0128/98/000/000-00 $6.00 20

Cha

ñarc

illo,

A

g-C

o-N

i-As

vein

: Ag

>100

NA

70%

(?)

oxi

dize

d (5

0-N

ativ

e A

g, a

cant

hite

, C

hlor

argy

rite

, W

hite

head

(19

19,

Chi

le19

0 m

); su

lfide

enr

ich-

dysc

rasi

te, p

earc

eite

, io

dobr

omar

gyri

te,

1942

), M

oest

a m

ent m

inor

to a

bsen

t4pr

oust

ite, f

reib

ergi

te,

nativ

e A

g, a

cant

hite

(1

928)

poly

basi

te, p

yrar

gyri

te,

(bro

mar

gyri

te,

step

hani

te, s

trom

eyer

ite

embo

lite,

ioda

rgyr

ite)

(am

alga

m)

Har

dshe

ll,In

term

edia

te-s

ulfid

atio

n 54

161

100%

oxi

dize

d (6

0 m

Su

lfosa

ltsM

n ox

ides

, em

bolit

e,

Kou

tz (

1984

), A

rizo

naep

ither

mal

dis

sem

inat

ed

thic

k); s

ulfid

e en

rich

-ac

anth

ite (

nativ

e A

ddis

on e

t al.

(man

to)

Ag-

Pb-Z

n-m

ent a

bsen

tA

g, li

mon

ite)

(200

7)M

n-(C

u)Pa

radi

se P

eak,

Hig

h-su

lfida

tion

3812

610

0% o

xidi

zed

(to

Aca

nthi

teC

hlor

argy

rite

, Si

llito

e an

d N

evad

aep

ither

mal

bre

ccia

and

13

0 m

); su

lfide

em

bolit

e, n

ativ

e L

orso

n (1

994)

diss

emin

ated

Au-

Ag

enri

chm

ent a

bsen

tA

g, a

cant

hite

1 M

etal

s lis

ted

in o

rder

of e

cono

mic

impo

rtan

ce; p

aren

thes

es s

igni

fy o

nly

min

or e

cono

mic

con

trib

utio

n2

Oxi

dize

d zo

ne a

nd m

inor

und

erly

ing

sulfi

de e

nric

hmen

t are

pre

serv

ed o

nly

in W

oods

vei

n (B

oyle

and

Das

s, 1

971)

3 C

houi

nard

et a

l. (2

005)

con

clud

ed th

at th

e ox

idiz

ed A

g or

e is

hyp

ogen

e in

ori

gin,

an

inte

rpre

tatio

n no

t sub

scri

bed

to h

erei

n4

Whi

tehe

ad (

1919

, 194

2) c

onsi

dere

d th

e up

per

part

s of

the

sulfi

de z

one

to b

e a

prod

uct o

f sup

erge

ne e

nric

hmen

t, an

inte

rpre

tatio

n ar

gued

aga

inst

by

the

wri

ter

(Sill

itoe,

200

7; s

ee te

xt)

TAB

LE

1. (

Con

t.)

Supe

rgen

e C

onta

ined

A

vera

ge

Supe

rgen

e en

rich

men

t D

epos

it,A

g,gr

ade,

Su

perg

ene

Hyp

ogen

e A

g ox

idiz

ed A

g A

g m

iner

al(s

) lo

catio

nD

epos

it ty

pe1

Moz

g/t A

gco

ntri

butio

nm

iner

al(s

) (m

inor

)m

iner

al(s

) (m

inor

)(m

inor

)M

ain

data

sou

rce(

s)

carbonate-replacement, Ag-Co-Ni-As veins, stockworks, andbreccias, Coeur d’Alene veins, Broken Hill-type deposits, andCove sediment-hosted deposit typically contain abundant car-bonate gangue, to which is added carbonate wall rocks in thespecific case of the carbonate-replacement deposits and a fewothers (Chañarcillo, Cove). Importantly, the intermediate-sulfidation epithermal, Broken Hill-type, and some carbon-ate-replacement deposits as well as the Keno Hill veins andCove sediment-hosted bodies contain manganoan carbonate(± rhodonite ± alabandite) gangue.

Supergene Silver GeochemistryDissolution, migration, and reprecipitation of silver in the

supergene environment are less well documented than is thecase for copper, in part because of the large number of po-tentially stable silver complexes under low-temperature,aqueous conditions (e.g., Webster, 1986; Renders and Se-ward, 1989; Akinfiev and Zotov, 2001). However, a few keysolubility- and redox-controlled processes appear to explainthe observed distribution of silver and silver-bearing minerals

in most supergene profiles developed in the upper parts ofsilver-rich deposits (Ravicz, 1915; Emmons, 1917; Boyle,1968; Shcherbina, 1972; Fig. 5).

The solubility of Ag+ increases dramatically with increasingEh and decreasing pH (e.g., Gammons and Yu, 1997) so dis-solution of the native metal and most silver-bearing sulfidesand sulfosalts takes place readily in oxygenated water undernear-surface, acidic conditions. Molecular O2 and/or Fe3+ ionsact as the oxidants. Oxidation of pyrite and other iron-bearingminerals (pyrrhotite, arsenopyrite, siderite) produces Fe2+,which then oxidizes, commonly with the catalytic assistance ofacidophilic bacteria (e.g., Nordstrom and Alpers, 1999), toproduce the Fe3+. The potential for appreciable mobility ofsilver in such sulfate-rich solutions is confirmed by the highsilver contents of some efflorescent sulfate salts in mine open-ings (Morris and Lovering, 1952). At the iron redox front, theoxidation of Fe2+ to Fe3+ is accompanied by reduction of anyAg+ to Ag0. Hence, supergene native silver tends to be moreabundant on approach to underlying sulfide zones, as cor-rectly shown in Figure 1 (Boyle, 1968).


0361-0128/98/000/000-00 $6.00 21

Intermediate-sulfidation epithermal

High-sulfidation epithermal

Low-sulfidation epithermal

Carbonate replacement Zn-Pb-Ag

Ag-Co-Ni-As vein

Mesothermal Ag-Pb-Zn

Broken Hill type Zn-Pb-Ag

Bolivian-type vein

VMS

Sediment-hosted

0 500 1000 1500 2000 2500 3000 3500 4000 M oz

Cerro de PascoBroken Hill

Coeur d’AlenePachuca

GuanajuatoFresnillo

PeñasquitoCanningtonZacatecas

San CristóbalCobaltPascua-LamaDukat

Santa EulaliaOruro

Navidad

Tayoltita

CoraniFankou

Greens CreekTinticLeadvillePark CityPulacayo

Real de AngelesKeno Hill


ImiterTonopahColquijircaEl PeñónCove

La Coipa

Hardshell

ProvidenciaEskay CreekChañarcillo

Paradise Peak


FIG. 3. The 40 silver and other silver-rich deposits selected for consideration, showing deposit types and total silver con-tents (in million oz). Deposits arranged in decreasing order of size, as listed in Table 1, which also indicates deposit locations.

Under neutral to alkaline, oxidizing conditions, consequentupon deficiency of iron sulfides and/or abundance of carbon-ate gangue (Fig. 5), silver tends to be much less mobile, al-though limited transport as the slightly soluble hydroxycar-bonate is possible because of the abundant bicarbonate ionsproduced by acid attack of carbonate gangue or wall rocks.Thiosulfate and other metastable sulfur species, generatedduring the oxidative conversion of sulfide minerals to sulfate,may also solubilize silver under alkaline (i.e., pyrite-deficient)conditions (Webster, 1986), but the resulting Ag(S2O3)3– islikely to be only a transient species.

In saline, oxygenated groundwater, silver is also readily sol-uble as chloride complexes (AgCl0, AgCl2–, AgCl23–; Gam-mons and Yu, 1997). Nevertheless, the chloride, bromide, andiodide anions are also the most effective precipitants of silverat ambient temperatures over a wide range of redox and pHconditions (Gammons and Yu, 1997; Fig. 6) because of the ex-treme insolubility of the resultant silver halides, particularlywhere the supply of descending groundwater is relatively lim-ited. Indeed, under even moderately saline supergene condi-tions and irrespective of solution pH, silver sulfides and sul-fosalts as well as native silver may undergo direct replacementby the halide minerals (Fig. 5). Vertical silver halide zoning,from chloride near surface through bromide to iodide nearthe base of the oxidized zone, as described at Tonopah(Burgess, 1911) and Chañarcillo (Moesta, 1928), is a result ofthe initial formation of the least soluble halide, iodargyrite,

and its subsequent conversion to bromine- and chlorine-bear-ing species under higher oxidation states (Gammons and Yu,1997; Fig. 6). In stark contrast to the case of copper, othernaturally occurring silver compounds, including oxides, sili-cates, hydroxycarbonates, hydroxysulfates, arsenates, andphosphates, are either rare or unknown.

Precipitation of native silver is favored by the progressiveneutralization of acidic, silver-bearing supergene solutions(Gammons and Yu, 1997; Fig. 6), conditions that also lead tosorption of Ag+ by ferric oxyhydroxide (goethite) or its copre-cipitation as argentojarosite or argentian plumbojarosite/beu-danite (Table 2) during hydrolysis of ferric sulfate in solution(Fig. 5). Under near-neutral or alkaline conditions, inmanganoan carbonate-bearing deposits, either biotic or abi-otic oxidation of Mn2+ to Mn4+ (Mills, 1999) causes efficientprecipitation of silver, which may be an integral component ofseveral of the Mn4+ oxides that constitute manganese wad. Al-ternatively, the silver may be directly adsorbed onto thesenegatively charged minerals, especially at high pH values(Nicholson, 1992), or finely intergrown with them as the na-tive metal.


0361-0128/98/000/000-00 $6.00 22

Low-sulfidationvein

Bolivian-type vein

Ag-Co-Ni-As vein

Mesothermal Ag-Pb-Zn

Broken Hill type Zn-Pb-Ag

VMS

Sediment-hosted

Intermediate-sulfidationvein

Intermediate-sulfidationbulk tonnage

Carbonate replacementZn-Pb-Ag

High-sulfidationbulk tonnage

1 2 3 4 5 6 7 8

FIG. 4. Bar graph showing the relative importance of the different silverand other silver-rich deposit types listed in Table 1 and Figure 3. Referencesto less widely known deposit types are provided in the text.

TABLE 2. Chemical Compositions of Main Supergene and Hypogene Silver-Bearing Minerals Referred to in the Text and Table 1

Supergene oxidized zoneChlorargyrite (cerargyrite) AgClEmbolite Ag(Cl,Br)Bromargyrite AgBrIodargyrite AgIIodembolite Ag(Cl,Br,I)Argentojarosite AgFe3(SO4)2(OH)6

Argentian plumbojarosite (Pb,Ag)Fe3-6(SO4)2-4(OH)6-12

Argentian beudantite (Pb,Ag)Fe3AsO4SO4(OH)6

Manganese oxides and oxyhydrates K1.2(Mn3+Mn4+)8O16 · xH2O(e.g., cryptomelane)

Supergene and hypogene zonesNative silver AgArgentite αAg2SAcanthite βAg2SStromeyerite Ag1-xCuSMckinstryite Ag1.2Cu0.8SJalpaite Ag3CuS2

Hypogene zoneElectrum (Au,Ag)Amalgam (Ag,Hg)Allargentum AgSbDyscrasite Ag3SbAguilarite Ag4SeSPolybasite (Ag,Cu)10Sb2S11

Pearceite (Ag,Cu)10As2S11

Canfieldite Ag8SnS6

Stephanite Ag5SbS4

Pyrargyrite Ag3SbS3

Proustite Ag3AsS3

Tetrahedrite-tennantite (freibergite) (Cu,Fe,Ag)12(Sb,As)4S13

Freieslebenite Pb3Ag5Sb5S12

Owyheeite Pb5Ag2Sb6S15

Miargyrite AgSbS2

Aramoyoite Ag(Bi,Sb)S2

Matildite AgBiS2

Andorite PbAgSb3S6

Imiterite Ag2HgS2

Under reducing conditions at and immediately below thewater table, Ag+ in solution progressively substitutes for cop-per, zinc, and iron in less-soluble sulfides to form acanthite(Figs. 5, 6), the process of silver sulfide enrichment. Theprocess has been replicated experimentally at 25°C (Scaini etal., 1995) and its results observed microscopically (e.g., Gref-fié et al., 2002). If copper accompanies the silver in solution,stromeyerite or argentian chalcocite-group minerals may ei-ther coprecipitate with or form instead of the acanthite. Manysulfide and sulfosalt minerals, including chalcocite, enargite,galena, sphalerite, pyrite, chalcopyrite, tetrahedrite, ar-senopyrite, and even cobalt-nickel arsenides, whether belowthe water table or as remnants above it, reportedly also causeprecipitation of native silver under neutral to slightly acidicconditions (Palmer and Bastin, 1913), although the quantita-tive importance of such reactions in naturally formed super-gene profiles is difficult to ascertain.

Where sulfide ions are present in solution, resulting fromeither pyrrhotite or sphalerite oxidation or bacterial reductionof aqueous sulfate, silver may be precipitated as acanthite,stromeyerite, or, where arsenic and antimony are present, per-haps even as silver-bearing sulfosalts. Sulfur isotope evidencesuggesting bacterial involvement in acanthite formation was

recently obtained from the Pierina high-sulfidation epithermalgold-silver deposit, Peru (Rainbow et al., 2006). Acanthite,whether of hypogene or supergene origin, is commonly re-ported from the oxidized zones of silver deposits (Table 1),probably because of its high degree of resistance to oxidation(Shcherbina, 1972); however, it may also form locally in oxi-dized zones if sulfate-reducing bacterial populations are pre-sent (Rainbow et al., 2006).

Supergene Silver ProfilesEmmons (1917), Lindgren (1933), and Boyle (1968) recog-

nized that supergene profiles developed in silver deposits arefar less orderly than those in copper deposits, but proposedthat despite the appreciable mineralogic admixture and com-plexity, a generalized downward progression from silverhalides through native silver to acanthite and, finally, silver sul-fosalts was typical (Fig. 1). However, essentially all the sulfos-alt minerals, an appreciable proportion of the acanthite, andat least some of the native silver seem likely to be hypogenein many cases (e.g., Bastin and Laney, 1918). For example,Table 1 reveals that 75 percent of the deposits contain one ormore hypogene silver sulfosalts, 72.5 percent contain acan-thite of probable hypogene origin, and 35 percent contain


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Watertable

SurfacePyrite-rich

depositPyrite-poor

deposit

Low pH, Fe3+:sulfide oxidation

giving Ag+ in solutionAg + halide ions givingAg halides

+

Direct AgCl replacementof Ag minerals

Acanthite preservationbecause resists oxidation

S2- ions from FeS or ZnSoxidation or bacterial

SO42- reduction: precipitate

acanthite

Precipitation of anyAg+ by sulfides:

acanthite, argentianchalcocitegroup

Silver sulfosalts ±acanthite ± native Agas hypogene minerals

Fe2(SO 4)3hydrolysis: Ag°precipitation asargentojarosite

Fe2+ reprecipitatesnative Ag

Neutral pH:minor hydroxycarbonate+ thiosulfate transportof silver

Ag° precipitated by Mn2+:Mn4+ oxyhydroxides (wad)

Ag and sulfides:native Agprecipitation

+

FIG. 5. Schematic representation of supergene processes in pyrite-rich and pyrite-poor silver-only and other silver-rich de-posits. Important processes and minerals are highlighted. Compiled from Emmons (1917), Boyle (1968), Shcherbina (1972),and references therein.

native silver that is also judged to be hypogene. In this sec-tion, the world’s premier silver-bearing deposits are used totest the broad-scale validity of the idealized supergene profilereproduced as Figure 1.

Six of the silver deposits listed in Table 1 lack supergeneprofiles (Fig. 7). The main Pulacayo vein deposit in Bolivia islargely blind and, hence, protected from supergene processes(Ahlfeld and Schneider-Scherbina, 1964). Immediately pos-tore rocks cover the Eskay Creek VMS deposit (Roth et al.,1999), whereas much younger sedimentary rocks conceal theBroken Hill-type deposit at Cannington, beneath which nosupergene effects are reported (Bailey, 1998). Geomorpho-logic conditions inhibited development of supergene profilesat Cobalt, Fankou, and Greens Creek (Petruk, 1971; C. Allen,writ. commun., 2007; M. Satre, writ. commun., 2007). How-ever, a single productive vein in the Cobalt district partiallyescaped the widespread effects of glacial erosion and retaineda preglacial supergene profile (Boyle and Dass, 1971; Table1), as indeed did all the veins at Dukat where permafrost con-ditions prevail today (Konstantinov et al., 1995). Most of theintermediate-sulfidation epithermal ore shoots at Pachuca andTayoltita as well as those in the Southeast sector at Fresnilloare also blind and, hence, unaffected by supergene processes(Thornburg, 1952; Clark, 1991; Trejo, 1991). It is importantto emphasize, though, that the silver mineralogy of the six de-posits that lack supergene profiles is not greatly differentfrom that of the sulfide zones in the rest of the deposits inwhich supergene effects are variably developed. For example,

native silver, acanthite, and silver sulfosalts of necessarily hy-pogene origin all occur at Cannington, Cobalt, and GreensCreek (Table 1).

The remaining 34 major silver-rich deposits have super-gene profiles, 70 percent of which may be considered to in-clude reasonably well-developed oxidized zones (Table 1; Fig.7). These attain maximum subsurface depths in the 300- to500-m range at Cerro Rico de Potosí (Lindgren and Crevel-ing, 1928; Fig. 8a), Cove (Emmons and Eng, 1995), Dukat(Konstantinov et al., 1995), El Peñón (Robbins, 2000), Pas-cua-Lama (Chouinard et al., 2005), Providencia (Triplett,1952; Mapes et al., 1964), and Santa Eulalia (Maldonado,1991), but an extreme maximum of 700 m in the karsted car-bonate terrain at Tintic (Lindgren and Loughlin, 1919). Some95 percent of the mined and remaining ore at Cerro Rico dePotosí, the world’s largest silver deposit, is oxidized (Table 1;Fig. 8a), with 50 to 100 percent oxidation reported for at least13 other deposits (Table 1; Fig. 7). Besides being located inarid to semiarid environments, these deeply oxidized depositsalso have high intrinsic permeability. This permeability com-monly results from pyrite and/or carbonate dissolution in car-bonate-replacement and other massive to semimassive sulfidebodies (e.g., Paradise Peak; Fig. 8b) or is provided by steepveins and, in the case of high-sulfidation epithermal deposits,bodies of vuggy quartz (Plumlee, 1999; Sillitoe, 2005). One-third of the deposits affected by supergene processes have ox-idized zones that are rather poorly developed, thin (maximum60 m), and economically unimportant because of inappropri-ate geomorphologic and climatic histories, as exemplified bythe Navidad, San Cristóbal, and Guanajuato deposits (Wandkeand Martínez, 1928; Lhotka et al., 2005; Buchanan, 2003;Table 1; Fig. 7). The oxide silver minerals typically either at-tain or approach the present surface, without development ofthick, silver-deficient leached cappings or gossans although,in this regard, the manganese- and lead-rich gossan at BrokenHill is a notable exception (van Moort and Swensson, 1982).

Silver sulfide enrichment is reliably reported from only 14(35%) of the supergene profiles (Table 1; Fig. 9): Broken Hill(Stillwell, 1953), Cobalt (Boyle and Dass, 1971), Cerro dePasco (Geological Staff of the Corporation, 1950), Colquijirca(R. Bendezú, writ. commun., 2007), Dukat (Konstantinov etal., 1995), Keno Hill (Boyle, 1965), Leadville (Tweto, 1968),Pachuca (Bastin, 1948), Providencia (Triplett, 1952), Real deAngeles (Pearson et al., 1988), San Cristóbal (L. Buchanan,writ. commun., 2008), Tintic (Morris, 1968), Tonopah (Bastinand Laney, 1918), and Zacatecas (Bastin, 1941). However, asnoted above, the silver enrichment at Cobalt and Pachuca isof trivial importance at the deposit scale because most of theore shoots were unaffected; hence, it is excluded from Figure9. The enriched horizons are commonly poorly defined, buttypically thin (Table 1), ranging from ≤1 m at Broken Hill(Stillwell, 1953; van Moort and Swensson, 1982) and 4 m atSan Cristóbal (L. Buchanan, writ. commun., 2008) to 30 m atCerro de Pasco (Geological Staff of the Corporation, 1950).The enrichment factor is also commonly low, only 1.3 at SanCristóbal (L. Buchanan, writ. commun., 2008).

In the high-grade vein deposit at Chañarcillo, however, amajor enrichment zone, up to 150 m thick and, hence, compa-rable to those developed in many copper deposits, was claimed


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AgCl(s)

AgI(s)

Ag(s)

pH

Ag2S(s)

Ag2O(s)lo

gfO

2

0

-20

- 40

-60

-800 2 4 6 8 10 12

FIG. 6. Eh-pH diagram to show the stability fields of chlorargyrite, iodar-gyrite, native silver, and acanthite at 25°C. Ligand concentrations approxi-mate those of rainwater. The bromargyrite stability field, between chlorar-gyrite and iodargyrite, is not shown. Note the broad stability field of the silverhalides and confinement of acanthite to reduced conditions. Taken fromGammons and Yu (1997).


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1000 1500 2000 2500 3000 3500 4000

Cerro Rico de PotosíCerro de Pasco

Broken Hill


GuanajuatoFresnillo

PeñasquitoCannington

ZacatecasSan Cristóbal

CobaltPascua-LamaDukat

Santa Eulalia

Greens Creek

Real de Angeles

Comstock Lode

Native Ag

Argentojarosite/plumbojarosite/beudantite

Halides

Mn oxides

Pre- or postmineral cover

Dominant Ag mineral

0 500

Oruro

Navidad

Tayoltita

CoraniFankou

TinticLeadvillePark CityPulacayo

Rochester

ImiterTonopahColquijircaEl PeñónCove

La Coipa

Hardshell


Paradise Peak

M oz

Keno Hill

FIG. 7. Approximate proportion and dominant silver mineralogy of oxidized ore in the 40 silver-only and other silver-richdeposits selected for consideration. Deposits lacking supergene profiles because of presence of pre- or postmineral cover areindicated. Layout as in Figure 3.

a

bb

FIG. 8. Views of classic silver-rich oxidized zones. a. Cerro Rico at Potosí where the 300-m-thick oxidized zone coincideswith an advanced argillic lithocap composed almost entirely of vuggy quartz (darkest brown). Arrows indicate the base of ox-idation within the mountain. Photograph taken in 1973. b. Paradise Peak, where masses of semimassive sulfide were trans-formed to gossanous oxidized ore that was porous, broken, and rubbly as a result of compaction and possible collapse beforebreakage by blasting. Photograph of the basal part of the orebody taken in 1992.

aa

b

by Whitehead (1919, 1942). Notwithstanding its location inthe southern Atacama Desert, a region that was especiallyconducive to the generation and preservation of major coppersulfide enrichment blankets during the last ~40 m.y. (Sillitoe,2005, and references therein), doubt is believed to surroundWhitehead’s (1919, 1942) interpretation of the vertical min-eralogic zoning at Chañarcillo. Although a detailed reap-praisal is now impossible because the mine is depleted and itswaste dumps repeatedly reprocessed, Sillitoe (2007) pro-posed that much of the putative enrichment, beneath a thick(50–190 m) oxidized zone, reflects hypogene zoning in a na-tive Ag-Co-Ni-As-type deposit (Table 1). Four main lines ofevidence combine to argue strongly against appreciable su-pergene sulfide enrichment (Sillitoe, 2007): (1) the low acid-and Fe3+-generation and high acid-neutralization potentialsof the pyrite-deficient and carbonate-rich vein material andenclosing wall rocks; (2) the crystalline nature of some of thesupposedly supergene sulfide minerals, in particular acanthitepseudomorphs after argentite; (3) the several-times highersilver grades in the oxidized ore than in the underlying sulfide

zone (Whitehead, 1919), an unusual situation for supergeneprofiles in either silver or copper deposits; and (4) the physi-cal separation of the oxidized and sulfidic parts of the veins bythick (up to 165 m), relatively impermeable, tuffaceous hori-zons in which the veins are represented only by tight, sulfide-free fractures (Whitehead, 1919; Fig. 10).

Supergene Silver MineralogyThe supergene mineralogy of the 34 oxidized zones consid-

ered herein is characterized by a relatively restricted numberof silver-bearing species (Tables 1, 2), although oxidized min-erals containing lead, zinc, copper, manganese, and othermetals as well as many textural varieties of limonite (mainlycomposed of jarosite, goethite, and/or hematite) are com-monly also abundant. The silver halides, of which chlorar-gyrite is typically the most common (Table 1), are dominantin just over half of the oxidized zones and present in at leastminor amounts in 68 percent of them. Embolite, bromar-gyrite, and iodargyrite, besides chlorargyrite, are widely re-ported. Native silver of assumed supergene origin dominates


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0 500 1000 1500 2000 2500 3000 3500 4000 M oz

Cerro de PascoBroken Hill


GuanajuatoFresnillo

PeñasquitoCanningtonZacatecas

San CristóbalCobaltPascua-LamaDukat

Santa EulaliaOruro

Navidad

Tayoltita

CoraniFankou

Greens CreekTinticLeadvillePark CityPulacayo

Real de AngelesKeno Hill


ImiterTonopahColquijircaEl Peñón

La Coipa

Hardshell


Paradise Peak

Cove

Silver sulfide enrichment

Pre- or postmineral cover


FIG. 9. Approximate proportion of enriched ore in the 40 silver-only and other silver-rich deposits selected for consid-eration. Deposits lacking supergene profiles because of presence of pre- or postmineral cover are indicated. Layout as inFigure 3.

20 percent of the oxidized zones as well as occurring as a sub-sidiary silver mineral in another 43 percent, whereas argento-jarosite, argentian plumbojarosite, and argentian beudantiteor silver-bearing manganese oxides (wad) are the main min-erals in only a few percent each. The wad may be completelyamorphous or contain minerals such as cryptomelane, chalco-phanite, coronadite, and hetaerolite (e.g., Koutz, 1984).Acanthite, of unspecified hypogene or supergene origin, is re-ported from 23 percent of the oxidized zones (Table 1), whereit persists because of its resistance to oxidation (see above).

The silver sulfide enrichment zones (Table 1) are typifiedby the presence of powdery, black sulfide aggregates, which,where studied in any detail, prove to contain acanthite and,where copper is also present, argentian chalcocite-group min-erals and stromeyerite, in some cases accompanied by nativesilver. Jalpaite and mckinstryite, sulfides of silver and copperlike stromeyerite (Table 2), are reported from single deposits(Table 1). The unambiguous presence of supergene silver sul-fosalts, such as pyrargyrite-proustite, pearcite-polybasite, andstephanite (Table 2), as enrichment products (e.g., Emmons,1917; Lindgren, 1933; Bateman, 1942; Boyle, 1996) remainsto be authenticated. Even Boyle’s (1965) detailed descriptionof putative supergene pyrargyrite crystals at Keno Hill ismore in keeping with an end-stage hypogene origin (Lynch,1989).

Silver Enrichment in Oxidized ZonesIn many of the oxidized zones considered herein, silver

contents seem likely to broadly reflect the former hypogenedistribution patterns. Hence, vertical changes, like the up-ward increase in Ag/Au ratios in the completely oxidized LaCoipa deposit (Oviedo et al., 1991), most likely reflect hypo-gene zoning on approach to the base of the partially pre-served steam-heated horizon (i.e., the paleowater table).Where grade distribution patterns for silver and gold in oxi-dized ore are closely similar, as at Paradise Peak (Fig. 11), ap-preciable supergene silver mobilization is essentially pre-cluded (Sillitoe and Lorson, 1994). If upward increases insilver content commence in the hypogene sulfide zone andcontinue upward uninterruptedly into the oxidized zone, as atSan Cristóbal (L. Buchanan, writ. commun., 2008), then hy-pogene zoning is also the most likely explanation. Neverthe-less, in some oxidized zones, silver enrichment consequentupon oxidative sulfide destruction is clearly discernable, andmay be the result of physical and/or chemical processes.

The main physical process seems to be residual enrichment,whereby the specific gravity of the oxidized ore is lowered


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100m

10m

Limestone Tuff

Oxidized

Barren

Barren

High-gradehypogene

Low-gradehypogene

CONSTANCIA

NEGRO

AZUL

DELIRIO

AHUESADO

FIG. 10. Schematic section of a typical silver vein at Chañarcillo, con-structed using data reported by Whitehead (1919) for the southern part ofthe district. Note the marked expansion of the vein within bituminous lime-stone horizons and its contraction to a tight, sulfide-free fracture in interven-ing tuffaceous horizons. The high-grade hypogene vein interval was previ-ously considered to be a supergene enrichment zone. Names down the leftside are those used locally during mining for some of the limestone and tuffunits. Note the horizontal scale is five times the vertical. Taken from Sillitoe(2007).

aa

bb

DRILL HOLES

PARADISE PEAK

PARADISE PEAK

DRILL HOLES

FIG. 11. Gold (a) and silver (b) distributions in the completely oxidizedParadise Peak deposit, constructed on the basis of life-of-mine blast-holeassay data. Note the near coincidence of the two distribution patterns, a fea-ture strongly suggesting that the silver underwent no significant supergenemobilization. Taken from Sillitoe and Lorson (1994).

with respect to that of the former sulfidic material because ofremoval of components, most notably sulfur and carbonate,and, in some cases, also zinc (Fig. 12). Element subtractionduring oxidative weathering leads to volume loss and conse-quent compaction, subsidence, and even collapse, as docu-mented at Broken Hill (van Moort and Swensson, 1982;Plimer, 1984), Cerro de Pasco (Bowditch, 1935), Oruro(Chace, 1948), Paradise Peak (Sillitoe and Lorson, 1994; Fig.8b), Providencia (Triplett, 1952), Santa Eulalia (Prescott,1916), and Tintic (Morris, 1968), although unfilled cavitiesand even caverns commonly remain. Residual enrichment ismost prevalent in sulfide- and manganese carbonate and/orsilicate-rich ores, such as those typical of carbonate-replace-ment, VMS, Broken Hill-type, and Bolivian-type vein de-posits. Graybeal et al. (1986) observed that the silver contentsof oxidized, carbonate-replacement, chimney-manto depositsare typically four times those of the underlying sulfide zones,a situation spectacularly exemplified by production data fromthe Providencia deposit (Fig. 12). Even greater degrees of sil-ver enrichment may have occurred in some oxidized carbon-ate-replacement deposits (e.g., Leadville; Cappa and Bartos,

2007), although it is unclear if this was entirely residual in ori-gin or also had a chemical contribution. At Cerro de Pasco,Bowditch (1935) recorded residual silver enrichment accom-panying a 44 percent decrease in specific gravity consequentupon the oxidative transformation of massive silica pluspyrite, a limestone-replacement product, to friable quartzand limonite: the pacos of colonial Spanish miners. Lead, be-cause of the extreme insolubility of the carbonate (cerussite)and sulfate (anglesite), is also enriched with the silver in suchoxidized zones, but zinc is severely depleted (Sangameshwanand Barnes, 1983; Fig. 12).

The most common forms of chemical enrichment seem totake place as a result of the preferential precipitation of eitherargentojarosite-argentian plumbojarosite (e.g., Cerro de Pasco;Geological Staff of the Corporation, 1950) or silver-bearingmanganese oxides (e.g., Hardshell; Koutz, 1984). The con-tained silver appears to have been coprecipitated during thehydrolysis of ferric sulfate or oxidation of Mn2+ in solution, re-spectively. The basal parts of the gossan above the Rio Tintomassive sulfide deposit in Spain display extreme silver en-richment, much of it also in the form of argentojarosite andargentian beudantite (García Palomero et al., 1986). Silverhalides may also become enriched in the lower parts of a fewoxidized zones, presumably because of progressive downwardflushing resulting from protracted groundwater flux. Exam-ples include the greater than two-fold increase in silver con-tent in the lower compared to the upper parts of the deeplydeveloped oxidized zone at El Peñón (S. Kasaneva, pers.commun., 2008) and the exceptionally high-grade silver ore(up to 9,500 g/t) near the base of the Broken Hill gossan (av-erage 900 g/t Ag; Plimer, 1984).

Supergene Profile InterpretationThis review of the world’s major silver-bearing deposits

concludes that supergene sulfide enrichment is an economi-cally unimportant process, and that in the majority of depositsit is largely absent or, at best, only incipiently developed.Based on the best estimates used to construct Figure 9, <1percent of the silver contained in the deposits selected hereinis a product of sulfide enrichment.

The extreme insolubility of the silver halides under mostsupergene conditions is the main reason for efficient silverfixation above the water table, although the precipitation ofnative silver, argentojarosite, and silver-bearing manganeseoxides may also be effective in removing silver from descend-ing solutions under certain specific supergene conditions (seeabove). Consequently, little silver seems to remain in solutionto effect sulfide enrichment at and beneath the groundwatertable. Interestingly, where silver sulfide enrichment is mostimportant (e.g., Cerro de Pasco, Keno Hill), silver halides areunreported from the overlying oxidized zones (Table 1).

Therefore, in marked contrast to the case of copper,major silver sulfide enrichment zones overlain by silver-deficient leached capping do not appear to have developed,even in pyritic deposits that generate highly acidic conditionsconducive to silver transport during the oxidation of their silver-bearing minerals. The bulk-tonnage, high-sulfidationepithermal deposits at La Coipa, Paradise Peak, Pascua-Lama, and Cerro Rico de Potosí, for example, are devoid of


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Transitional

Oxi

diz

ed z

one

Sul

fide

zone

2000

2250

2500

200 400 600 800

5 10 15 20 25

Ag, ppm

Pb or Zn, ppm

Elevatio

n, m

Ag

PbZn

FIG. 12. Changes of silver, lead, and zinc contents in supergene and hypo-gene ores with depth in the Zinc West carbonate-replacement chimney ore-body at Providencia, constructed from assay data obtained during pre-1955mining operations. Note the residual enrichment of silver and lead andmarked zinc depletion in the oxidized zone. Triplett (1952, p. 592) describedthe transitional zone as consisting of “semistratified layers of mud, sand,pieces of limestone, chunks of [lead] carbonate ore, and some leachedsiliceous irony material in places overlying semioxidized sulfides, which werelow in grade. Only a part of the muddy sandy material was minable.” Al-though the highest silver values presumably occurred as residually enriched,oxidized minerals in this muddy, sandy material, Triplett (1952) also stronglysuspected silver sulfide enrichment at the top of the sulfide zone. Taken fromGraybeal et al. (1986) after Mapes et al. (1964).

appreciable enrichment beneath their thick, well-developedoxidized zones (Table 1; Figs. 7, 9). The overriding reason isthat silver is not appreciably leached in acidic or alkaline en-vironments if groundwater recharge characterized by evenaverage rainwater halide concentrations is available (Gam-mons and Yu, 1997; Fig. 6). Indeed, silver halide formationcan be effective even in some high-rainfall climatic regimes inthe tropics (e.g., Dominican Republic and Philippines; Rus-sell et al., 1981; Sherlock and Barrett, 2004), probably in re-sponse to the influence of halide-bearing, sea-salt aerosols onvadose water compositions (e.g., Prospero, 2002). Silver de-posits lacking silver halides tend to be located in temperateregions distant from oceanic influences (e.g., Cobalt, KenoHill). It must be stressed that evaporative concentrationunder arid conditions, a requirement for widespread forma-tion of copper hydroxychloride minerals of the atacamitegroup (Sillitoe, 2005), is not needed for silver halide genera-tion. Furthermore, in contrast to oxide copper minerals,which are unstable at pH <5—a common condition in oxidiz-ing, pyritic deposits—irrespective of anion availability (e.g.,Anderson, 1982), the silver halides readily precipitate over abroad pH range (Gammons and Yu, 1997; Fig. 6).

The end result is that silver-only and other silver-rich de-posits retain much of their supergene silver in the oxidizedzones, which may be economically important where ore-zonepermeability is high (see above) and geomorphologic and cli-matic conditions favor deep oxidation and preservation of themineral products, either as a result of low denudation rates orconcealment beneath postmineral sedimentary and/or vol-canic cover. Roughly 50 percent of the total silver in the de-posits selected is present in oxidized ores, although half ofthat is in a single deposit, Cerro Rico de Potosí (Fig. 7). Oro-genic belts subjected to tectonically induced surface upliftand exhumation and semiarid climatic conditions tend to op-timize supergene profile development and preservation (Silli-toe, 2005).

Economic ConsequencesThe absence of major supergene profiles of the type illus-

trated in Figure 1 from the world’s premier silver-only andother silver-rich deposits has important implications for ex-ploration because, unlike in the case of copper deposits, tar-geting of enhanced silver grades hosted by sulfide enrichmentzones is not a viable strategy. By the same token, near-surfacesilver tenors in most silver-rich deposits are likely to bebroadly representative of values at depth, although the up-permost few meters may undergo serious depletion. This con-clusion is, of course, independent of the possible existence ofhypogene vertical zoning, as observed in many epithermal de-posits, including silver concentration in particularly receptivelithologic units concealed below the present surface.

Nevertheless, the economic viability of some silver-richdeposits is a direct consequence of oxidative weathering,most notably the case in high-sulfidation epithermal gold-sil-ver deposits, such as La Coipa and Paradise Peak, where re-fractory hypogene sulfides were broken down to release thegold (and some of the associated silver). The low-gradeRochester deposit benefited similarly, although to a lesserdegree (Vikre, 1981). It should also be recalled that much of

the bonanza-grade silver production in western North Amer-ica early last century was from the oxidized zones of carbon-ate-replacement zinc-lead-silver deposits (e.g., Leadville,Park City, Tintic) that had undergone extreme residual and,possibly, also chemical enrichment (Titley and Megaw, 1985;Graybeal et al., 1986).

However, some oxidized silver ores suffer from severe met-allurgical recovery problems. Halides, argentojarosite, andmanganese oxide-bound silver can be difficult to treat by con-ventional cyanidation, and commonly result in low (<50%) silver recoveries, particularly in the case of low-grade, bulk-tonnage deposits for which heap leaching must be employed.Hence, a variety of experimental hydrometallurgical processeshave been devised as possible means of treating argento-jarosite and manganese-bound silver ores (e.g., Sánchez et al.,1996; Jiang et al., 2003). Therefore, the widely appreciatedmetallurgical benefits that stem from the oxidation of copperand gold ores are commonly not shared by their silver-domi-nant counterparts.

Concluding StatementNotwithstanding some geochemical similarities between

silver and copper, and the ease with which sulfides containingboth metals are broken down by oxidative weathering, it isconcluded that silver sulfide enrichment is both verticallymore restricted and less effective than copper sulfide enrich-ment, and commonly absent altogether. Indeed, it is furtherconcluded that thick, mature silver enrichment zones proba-bly do not exist, a conclusion that requires reinterpretation ofthe few such zones proposed previously, in particular that atChañarcillo (Sillitoe, 2007).

A corollary of the relative immobility of silver in the oxi-dized environment, irrespective of whether acidic or alkalineconditions prevail, is that exotic oxide silver deposits, accu-mulated like exotic copper under arid to semiarid conditionsin piedmont gravel sequences alongside their oxidizing parentdeposits (e.g., Münchmeyer, 1996; Sillitoe, 2005), have notyet been encountered and likely do not exist. In this regard, itshould be emphasized that small silver ± copper depositshosted by siliciclastic sedimentary rocks, like Providencia inArgentina (Segal, 1999) and Paoli in Oklahoma (Shockey etal., 1974), are of red-bed not exotic type. Any oxidized ore insuch deposits is an in-situ oxidation product of hypogene sul-fides and native silver rather than being directly precipitatedfrom laterally migrating supergene solutions.

Therefore, wherever supergene profiles are deeply devel-oped, the silver explorer needs to focus on the oxidized zones,some of which may have higher average silver grades than theunderlying hypogene mineralization because of either resid-ual enrichment or localized silver mobilization and reprecipi-tation that attended sulfide and sulfosalt oxidation. This situ-ation contrasts with that in copper deposits, where oxidizedzones appreciably higher in grade than the former hypogenesulfide mineralization are only generated by the predomi-nantly in-situ oxidation of mature chalcocite enrichmentzones (e.g., Chuquicamata, Chile; Jarrell, 1944). Thick, mul-ticyclic, silver sulfide enrichment blankets and exotic oxidesilver deposits are not viable exploration targets, in markedcontrast to the copper environment. These conclusions offer


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little solace to the metallurgist given the greater difficulty oftreating many oxidized silver ores compared to those contain-ing only silver-bearing sulfides and sulfosalts of either hypo-gene or supergene origin.

AcknowledgmentsCam Allen, Regina Baumgartner, Ronner Bendezú, Larry

Buchanan, David Giles, Sergio Kain, Stabro Kasaneva, PeterMegaw, Carlos Peralta, Stewart Redwood, Michael Satre,Sergei Struzhkov, Dave Volkert, Don Wagstaff, and ChengyuWu kindly provided data on supergene profiles, mineralogy,production, and/or resources for a number of the depositslisted in Table 1. Review comments by Tawn Albinson, PaulBartos, Larry Buchanan, Fred Graybeal, and LouisaLawrance led to improvement of the manuscript.

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Supergene Silver Enrichment Reassessed_Sillitoe