Bonnet, A-L., and Corriveau, L., 2007, Alteration vectors to metamorphosed hydrothermal systems in gneissic terranes, in Goodfellow, W.D., ed., MineralDeposits of Canada: A Synthesis of Major Deposit-Types, District Metallogeny, the Evolution of Geological Provinces, and Exploration Methods: GeologicalAssociation of Canada, Mineral Deposits Division, Special Publication No. 5, p. 1035-1049.

ALTERATION VECTORS TO METAMORPHOSED HYDROTHERMAL SYSTEMS

IN GNEISSIC TERRANES

ANNE-LAURE BONNET1 AND LOUISE CORRIVEAU2

1. INRS-Eau, Terre, Environnement, 490 de la Couronne, Quebec, Quebec G1K 9A92. Geological Survey of Canada, 490 de la Couronne, Quebec, Quebec G1K 9A9

Corresponding author’s email: lcorrive@nrcan.gc.ca

Abstract

Alteration mapping is recognized as an effective tool for the discovery of hydrothermal ore deposits in both green-field and brownfield exploration terranes. Similar to their non- or weakly-metamorphosed equivalents, metamorphosedhydrothermal alteration zones are also key elements in vectoring to ore. However, in gneiss terranes, the recognition ofmetamorphosed hydrothermal alteration is severely hampered by their resemblance to metasediments and metamor-phosed paleosoils. This paper presents strategies and observation protocols to recognize, during fieldwork and with areasonable degree of confidence, hydrothermal alteration zones that have been metamorphosed to upper amphibolite-and granulite-facies conditions. The geological and mineralogical criteria discussed are useful vectors to mineralization,especially among felsic gneisses. These criteria are adapted from strategies developed for volcanogenic massive sul-phide deposits but can be applied to any deposit type where alteration zones are sufficiently large to be recognized dur-ing regional mapping, such as epithermal, porphyry, or iron oxide Cu-Au deposits. Once a hydrothermal system is sus-pected, more detailed geological mapping can reveal enough information to determine the intensity and the spatial dis-tribution of the system, establish targets for systematic lithogeochemical and geophysical exploration, and test differ-ent metallogenic models, thereby increasing the probability of discovery. A proposed practical guide is included on theDVD at the back of this volume to further develop concepts presented in this paper. It includes some case examples anda photo atlas to illustrate rock types, outcrop aspects, mineral assemblages, and textures of alteration zones and theirpotential host volcanic rocks.

Résumé

La cartographie de zones d’altération est reconnue comme un outil efficace d’exploration pour la découverte degîtes hydrothermaux en contextes d’exploration et de cartographie régionales ainsi qu’au sein de camps miniers. À l’in-star de leurs équivalents peu ou faiblement métamorphisés, les zones d’altération hydrothermale métamorphiséesreprésentent elles aussi des vecteurs de minéralisation. Toutefois, leur identification lors de la cartographie de terrainsgneissiques est sévèrement entravée par leur ressemblance avec des roches métasédimentaires et des paléosols méta-morphisés. Cet article présente des stratégies et des clés d’observation pour reconnaître, sur le terrain et avec un degréde confiance raisonnable, les zones d’altération hydrothermale métamorphisées aux conditions du faciès des amphibo-lites supérieur et du faciès des granulites. Les critères géologiques et minéralogiques discutés fournissent des vecteurspour la découverte de minéralisations, particulièrement au sein de vastes étendues de gneiss felsiques. Ils s’appuient surdes stratégies développées pour les gîtes de sulfures massifs volcanogènes, mais sont aussi applicables à tout type degîte qui s’accompagne de zones d’altération hydrothermale cartographiables à l’échelle régionale tels que les gîtesépithermaux, les gîtes porphyriques ou les gîtes d’oxydes de fer-Cu-Au. Une fois la présence d’un système hydrother-mal supposée, une cartographie plus détaillée permet de déterminer l’intensité et la configuration du système, d’établirdes cibles pour des programmes d’exploration lithogéochimique et géophysique systématiques, de mettre à l’essai dif-férents modèles métallogéniques et, ce faisant, d’augmenter la probabilité de découverte. Un guide pratique a été ajoutésur le DVD joint au présent volume afin de développer plus en profondeur les concepts abordés dans cet article. Il com-prend un atlas photographique et des exemples types afin de bien illustrer les types de roche, les aspects mégascopiques,les associations minérales et les textures des zones d’altération et de leurs roches hôtes volcaniques potentielles.

Introduction

Felsic volcano-plutonic environments represent first-order targets for mineral exploration of volcanogenic mas-sive sulphide Cu-Au (VMS Cu-Au), epithermal, porphyryCu (Mo-Au), and iron oxide Cu-Au (IOCG) deposits (e.g.Ohmoto, 1996; Corbett and Leach, 1998; Hedenquist et al.,2000; Sillitoe, 2000, 2003; Large et al., 2001). Once meta-morphosed at high grade, these settings form large gneissicterranes, some hosting deposits with well documented meta-morphosed alteration zones that provide effective vectors toore (Table 1; Hodges and Manojlovic, 1993). Among thoseare the commonly recognized indicators for metamorphosedVMS deposits, the chloritic alteration zones expressed ascordierite-anthophyllite schists at amphibolite facies and themeta-exhalites (Tables 1, 2; Schreurs and Westra, 1985;Trägårdh, 1991; Bernier, 1992; Spry et al., 2000 and refer-ences therein).

Sericitic, argillic, and advanced argillic alteration zonesare also key alteration types of many deposits, includingVMS deposits (Tables 1, 2). However, as their gneissicderivatives commonly resemble metapelites or metamor-phosed paleosoils (Bonnet et al., 2005), challenges remainconcerning their recognition among gneissic terranes wheremineralization has not been found or explored for (e.g.Allard, 1978; Froese, 1985, 1998). This can also be truewithin ore deposits themselves. For example, early papers onthe Challenger gold deposit (e.g. Tomkins and Mavrogenes,2002) interpreted the host to mineralization as metapelites,whereas more recent work is highlighting hydrothermal pro-toliths with a volcanic precursor and an epithermal origin (C.McFarlane, written comm., 2006; Table 1). Though the min-eral potential of gneissic terranes and the importance ofadapting alteration vectors for their successful explorationhave been recognized for decades (Allard, 1978; Gauthier et

A-L. Bonnet and L. Corriveau

1036

Dep

osit

Met

amor

phic

Typ

eFa

cies

Bon

dyG

neis

sC

ompl

ex

Gre

nvill

e Pr

ovin

ce

(Cen

tral

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ased

imen

tary

Bel

t, so

uthw

est

Que

bec)

Kfs

, Bt-G

rt, C

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px, S

il-G

rt,

Sil-Q

tz, o

r Bt-O

px-M

ag g

neis

s;

garn

etite

; alb

itite

; mag

netit

ite;

calc

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cate

rock

; tou

rmal

inite

; C

u an

d A

u m

iner

aliz

atio

n

Pota

ssic

, ser

iciti

c,

chlo

ritic

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illic

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vanc

ed a

rgill

ic,

sodi

c an

d Fe

-oxi

de

alte

ratio

n zo

nes

Rhy

oliti

c to

bas

altic

vo

lcan

ic ro

cks

1.4-

1.3

Cu-

Au

VM

S an

d/or

Fe

oxid

ehy

drot

herm

alsy

stem

Gra

nulit

e

Vol

cani

sm a

nd p

luto

nism

in a

ba

ck-a

rc a

nd is

land

arc

setti

ng

deve

lope

d on

a th

in c

ontin

enta

l su

bstra

te o

f Lau

rent

ia

Cor

rivea

u et

al.,

19

97; B

lein

et

al.,

2003

, 200

4

La R

omai

ne

Supr

acru

stal

Bel

t

Gre

nvill

e Pr

ovin

ce

(Nat

ashq

uan

dom

ain,

sout

heas

t Q

uebe

c)

Qtz

-Crd

gne

iss,

mig

mat

ized

Grt-

Bt-S

il gn

eiss

, Qtz

-Sil-

Ms-

Feox

no

dule

s and

vei

ns in

qu

artz

ofel

dspa

thic

gne

iss,

irons

tone

, car

bona

te, a

nd c

alc-

silic

ate

rock

s, ga

rnet

ite,

diop

sidi

te, e

pido

site

, Cu

min

eral

izat

ion

Seric

itic,

adv

ance

d ar

gilli

c, si

licic

, Ca-

Fe a

ltera

tion

zone

s

Fels

ic-in

term

edia

tepy

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astic

dep

osits

, m

afic

lava

, maf

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ll1.

5

Hig

h-su

lphi

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MS-

type

hydr

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em

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5 G

a vo

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ism

and

se

dim

enta

tion

with

in a

n in

tra

cont

inen

tal a

rc ri

ft (P

inw

aria

n)

Cor

rivea

u an

d B

onne

t, 20

05;

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net e

t al.,

20

05

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lleng

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awle

r cra

ton,

A

ustra

liaA

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inou

s gne

iss q

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h gn

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itic

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ratio

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nic

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al?

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nulit

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ncer

tain

C. M

cFar

lane

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ritte

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mm

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06

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taub

an

Gre

nvill

e Pr

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ce

(Mau

ricie

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rtneu

f sec

tor,

cent

ral Q

uébe

c)

Nod

ular

Sil

gnei

ss, C

rd-A

th

gnei

ss, G

rt-G

hn q

uartz

itic

gnei

ss, A

u-A

g m

iner

aliz

atio

n

Chl

oriti

c, c

hlor

ite-

seric

ite a

ltera

tion

zone

s, si

liceo

us

exha

lite

Fels

ic a

nd m

afic

vo

lcan

ic ro

cks,

sedi

men

ts1.

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MS

Upp

erA

mph

ibol

iteIs

land

arc

(bac

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c) v

olca

nism

an

d hy

drot

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ctiv

ityB

erni

er a

nd

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Lean

, 199

3

Prie

ska

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aqua

Prov

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th

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thite

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ss, C

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rt ro

ck

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oriti

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tera

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si

licifi

catio

n, a

nd

seric

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tion

Dac

itic

tuff

, se

dim

ents

1.3

VM

SG

ranu

lite

Bac

k-ar

c m

agm

atis

mTh

eart

et a

l.,

1989

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stes

uoV

ihan

ti-Py

häsa

lmi

dist

rict,

cent

ral

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and

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bear

ing

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itic

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ratio

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cani

c ro

cks,

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lt an

d ba

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g of

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nd a

rcR

ober

ts e

t al.,

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03

Cou

lon

Bel

tSu

perio

r Pro

vinc

e (n

orth

-wes

tQ

uebe

c)

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odul

ar g

neis

s; C

rd-A

th a

nd

Bt g

neis

s

Chl

oriti

c an

d se

riciti

c al

tera

tion

zone

s

Fels

ic a

nd m

afic

vo

lcan

ic ro

cks;

se

dim

enta

ry ro

cks

? V

MS

Gra

nulit

eA

rche

r et a

l.,

2004

; Huo

t et

al.,

2004

Gec

oSu

perio

r Pro

vinc

e (Q

uetic

o, O

ntar

io)

Oam

-Crd

-Grt

gnei

ss, S

il-M

s-Q

tz sc

hist

, iro

n fo

rmat

ion

Chl

oriti

c an

d se

riciti

c al

tera

tion

zone

s, se

a-flo

or

prec

ipita

te /

near

-se

aflo

orre

plac

emen

t

Fels

ic v

olca

nic

rock

s 2.

7V

MS

Upp

erA

mph

ibol

iteZa

lesk

i and

Pe

ters

on, 1

995

Abb

revi

atio

ns fr

om K

retz

(198

3), e

xcep

t Ghn

= g

ahni

te, O

am =

orth

oam

phib

ole,

Feo

x =

iron

oxid

e.

Nam

eL

ocal

ityM

etam

orph

osed

Alte

ratio

nPr

otol

ithPr

ecur

sor

Age

secnerefeR

txetnoC ci

manydoeG

) aG(

TA

BL

E1.

Exa

mpl

es o

f m

etam

orph

osed

hyd

roth

erm

al d

epos

its

and

occu

rren

ces

in g

neis

s te

rran

es f

rom

mos

tly

the

Gre

nvil

le a

nd S

uper

ior

prov

ince

s.

Alteration Vectors to Metamorphosed Hydrothermal Systems in Gneissic Terranes

1037

al., 1985; Allard and Carpenter,1988; Thompson et al., 1997; Spryet al., 2000), it is through the rou-tine recognition of all gneissicalteration types that effectivediversification of mineral explo-ration will be achieved from theknown, greenstone belt-hosted,mining camps into the largelyuncharted gneissic and graniticbelts of shield terranes, the ‘pink-stone belts’.

This paper presents a strategyfor the recognition and charac-teri-zation of metamorphosed hydro-thermal alteration zones, in partic-ular fossil sericitic, argillic, andadvanced argillic alteration zones,that can point to potential VMS-type mineralization in gneissicpaleo-volcanoplutonic belts (e.g. Boliden: Hallberg, 2001;Gossan Hill: Sharpe and Gemmell, 2002). The strategy isbased on methodical use of geological and mineralogical cri-teria, and observation protocols to prognosticate potentiallyfertile area during fieldwork and from existing reports andmaps. Once a hydrothermal system is identified in the fieldwith a reasonable degree of confidence and mapped appro-priately, exploration programs can follow their usual course.Detailed discussion and illustration of method and caseexamples are provided in the accompanying photo atlas andguide (Bonnet and Corriveau, 2007).

Gneissic Terranes as Exploration Targets

The identification of mineralized volcanic belts withingneissic terranes is a constant reminder that these settingsinclude tectonically juxtaposed surface and near-surfaceenvironments that can be as prospective as any other onesthat were never deeply buried during their evolution (Fig.1A, Table 1; Zaleski and Peterson, 1995; Archer et al., 2004).However, there remains a lingering perception that they rep-resent large crustal segments formed at and exhumed fromgreat depth where large-scale hydrothermally derived oredeposits tend not to be formed (Fig. 1B) and where the like-lihood of preserving high-level deposits such as epithermal,

porphyry Cu, and high-sulphida-tion VMS deposits is consideredpoor due to rapid denudation(Sillitoe, 1973; Gustafson andTitley, 1978). That porphyry andepithermal deposits are beingfound in ancient terranes, even atgranulite facies, is a reminder thattectonic burial may be so effectiveas to preserve high-level mineraldeposits (Dubé et al., 1998; Kontaket al., 2002). A last but not leastproblem is that partial melting,remobilization, and obliteration ofprimary textures and structuresassociated with regional high-grademetamorphism and deformationare, in some cases, sufficiently sig-nificant to conceal the presence ofentire volcanic belts and VMS-typehydrothermal systems withinundifferentiated gneiss complexes(Blein et al., 2003; Corriveau andBonnet, 2005). Initial lithostrati-graphic geometry and primary tex-tures and structures of volcanicbelts can, however, be preserved inareas of low strain, where meta-morphic events were transient or in

Alteration type Diagnostic minerals greenschist facies

Diagnostic minerals granulite facies

Diagnosticcomposition

Similar rocks (at granulite facies)

Advancedargillic

Kaolinite,Pyrophyllite,Andalusite,

Corundum, Topaz

Sillimanite, Kyanite, Quartz

Al2O3, SiO2 Laterite

Argillic Sericite, Illite, Pyrophyllite

Sillimanite, Kyanite, Quartz, Biotite, Cordierite,

Garnet

Al2O3, SiO2, K2O,Fe2O3, MgO

Pelite

Sericitic Sericite, Illite, Quartz Biotite, K-feldspar, Sillimanite, Kyanite,

Quartz, Cordierite, Garnet

K2O, Al2O3,Fe2O3, ±MgO,

±SiO2

Pelite

Chloritic Chlorite, Quartz, Sericite

Cordierite, Orthopyroxene, Orthoamphibole,

Phlogopite, Sillimanite, Kyanite

Fe2O3, MgO, ±Al2O3, ±SiO2

Pelite

CarbonatePropylitic

Carbonate (Fe, Mg), Epidote, Chlorite, Sericite, Feldspar

Carbonate, Grossular Epidote, Hornblende,

Diopside, Orthopyroxene

Fe2O3, CaO Calc-silicate rock of sedimentary origin,

marble or mafic rock

TABLE 2. Diagnostic mineralogy and geochemistry of greenschist- and granulite-grade metamor-phosed alteration associated with volcanogenic massive suphide deposits.

Paleo-environment evolution

VMS

P-CuIOCG

E-Au

High-grademetamorphic terrane= roots of an arc

P,T

VMS

P-CuIOCG

E-Au

erosion

erosion

STERILEFERTILEFERTILE

Top to bottom erosion of an arc

High-grademetamorphic terrane

= tectonic juxtaposition of sedimentaryand volcanic belts, plutons, faults

Polyphase juxtaposition at different crustal levels

P-CuIOCG

E-Au

P,T

FERTILE

FERTILE FERTILE

Deformation

VMS

FIGURE 1. Schematic diagrams of a magmatic arc metamorphosed at granulite facies. (A) Settings that includefertile surface and near-surface volcanoplutonic belts that have been tectonically buried relatively intactthough thoroughly metamorphosed and juxtaposed to other settings during orogenesis. Red dots locate poten-tial settings for epithermal gold (E-Au), porphyry copper (P-Cu), volcanogenic massive sulphide (VMS Cu-Au), and iron oxide Cu-Au-U-REE (IOCG) deposits. (B) Top to bottom erosion of an arc and its impact onthe preservation of different deposit types.

A

B

cases that competency was particularly uniform or high (e.g.Nadeau et al., 1999; Mueller and Corcoran, 2001; Gauthieret al., 2004). Such preservation can extend to very delicatetextures, including those of quartz phenocryst (Stevens andBarron, 2002), phenoclast, lapilli (Fig. 2A), pumice (Fig.2B), and scoria (Fig. 2C), not only at amphibolite facies butalso at granulite facies (Figs. 2C, 3A; Corriveau and Bonnet,2005). Where observed, relic primary textures permit theidentification of metamorphosed volcanic environments dur-ing regional mapping and subsequent lithogeochemicalinterpretation of paleo-environments (Slagstad et al., 2004;Bonnet et al., 2005; LaFlèche et al., 2005). High-grade meta-morphism and deformation may also be beneficial in termsof mineral potential by concentrating ore in structural trapsand increasing grain size and purity (Gauthier, 1993;Marshall et al., 2000; Mavrogenes et al., 2001; Gauthier andChartrand, 2005).

Influence of Metamorphism on Alteration Zones

Hydrothermal fluids involved in the genesis of VMSdeposits are neutral to acid (pH 7 to 2), hot (T = 150 and400ºC), and derived mostly from seawater (e.g. VMS) withor without a magmatic component (Lydon, 1988; Yang andScott, 1996). As they circulate within the Earth’s crust,hydrothermal fluids react with country rocks through min-eral leaching and chemical precipitation and, due to theirhigh fluid/rock ratios, may lead to the formation of signifi-cant alteration zones. These include chloritic (+Mg, +Fe, -Ca, -Na, -K), sericitic and argillic (+K, +Mg, ±Fe, -Ca, -Na),advanced argillic (+Al, +Si, -Fe, -Mg, -Na, -Ca, -K), andpropylitic or carbonate (+Fe, +CO2, +Mg, -Na) alteration.Through the chemical changes associated with hydrothermalleaching and precipitation, the composition of alterationzones and consequently their mineral assemblages com-monly acquire a diagnostic character that differs from that oftheir unaltered precursor (Galley, 1995; Large et al., 2001).Following high-grade metamorphism and despite the associ-ated obliteration of primary mineralogy and dehydration,rock compositions remain largely unchanged except in rare

cases where anatectic melt remobilization is significant dur-ing partial melting (Sawyer, 2001) or where hydrothermalalteration took place during metamorphism through externalinflux of fluids. Consequently, in most cases, metamor-phosed alteration zones preserve the imprint of the chemicalchanges that the original protolith has undergone duringpremetamorphic hydrothermal processes (Trägårdh, 1991;Hannington et al., 2003; Roberts et al., 2003; Blein et al.,2004; Stanton, 2004).

Rock composition and physical conditions, such as pres-sure and temperature (e.g. Rosenberg et al., 2000) constrainmineral assemblages, mineral composition, and modal min-eral abundance of metamorphic rocks, including those form-ing metamorphosed hydrothermal alteration zones and thoseaffected by synmetamorphic sulphidation and oxidationprocesses. As pressure and temperature change, mineralphases react with each other to form new mineral assem-blages. Such mineralogical changes obey the phase rule;hence the pressure-temperature stability field of differentmetamorphic assemblages can be predicted, and experimen-tally reproduced and calibrated for the most common rockcompositions and then graphically represented using petro-genetic grids and phase diagrams (AFM, A’CF, A’KF, etc;see Chapter 5 in Spear, 1993; Chapter 3.2 in Bonnet andCorriveau, 2007). The plotting of the composition of alteredrocks and unaltered precursors in phase diagrams allowstheir respective stable mineral assemblages and modal min-eral abundance to be determined. As grain size is enhancedby high-grade metamorphism, mineral assemblages andmodal mineral abundance can be established routinely dur-ing fieldwork, thereby serving as a user-friendly tool to rec-ognize different types of alteration during mapping. This isdone by positioning the observed rock assemblage in achemically appropriate chemographic phase diagram andvisualizing potential chemical departure of a suspectedhydrothermally altered rock from the composition of a ‘nor-mal’ protolith (Fig. 3, Tables 2, 3). This exercise provides ameans of identifying hydrothermal alteration in the field, astrategy that can be greatly facilitated where primary tex-

A-L. Bonnet and L. Corriveau

1038

Lac Musquaro sector, amphibolite facies La Romaine sector, granulite facies

FIGURE 2. Volcaniclastic and pyroclastic rocks (1.5 Ga) at amphibolite and granulite facies from the Musquaro Lake and the La Romaine area in the easternGrenville Province, Quebec (location in Fig. 1 of Corriveau et al., 2007). (A) Polygenic lapillistone. (B) Primary moulding of pumices of intermediate com-position around more competent felsic lapilli; a texture diagnostic of a pyroclastic origin and proximal volcanic environment. (C) Polygenic lapillistone atgranulite facies from La Romaine.

A B C

Alteration Vectors to Metamorphosed Hydrothermal Systems in Gneissic Terranes

1039

FIGURE 3. Photographs of metamorphosed argillic (A), chloritic (B), and sericitic (C to F) alteration types. (A) Granulite-facies biotite-garnet-sillimanitegneiss with preserved fragments interpreted as volcanic, observed in the alteration zones of the Bondy Gneiss Complex (location in Fig. 1 of Corriveau etal., 2007). Note that the volcanic precursor may have been similar to that of the lapillistone in Figure 2C. Relict textures are particularly difficult to find inthe polydeformed Bondy Gneiss Complex (Harris et al., 2001). (B) Granulite-facies cordierite-orthopyroxene-bearing white gneiss, observed in the BondyGneiss Complex (Blein et al., 2004). (C) Network of nodular aluminous veins in a tuffaceous unit metamorphosed to upper amphibolite facies in theMusquaro Lake area. (D) Detail of a nodular aluminous vein seen in photo C. (E) Aluminous nodules with fibrolite-rich cores and feldspar-rich rims dis-seminated within a tuffaceous unit in the Musquaro Lake area. (F) Network of aluminous veins in a tuffaceous unit metamorphosed to granulite facies in theLa Romaine area. Abbreviations: Crd=cordierite, Krn=kornerupine, Ms=muscovite, Opx=orthopyroxene, Pl=plagioclase, Qtz=quartz, Sil=sillimanite.

A B

C D

E F

tures of the precursor rocks are alsopreserved, such as in Figure 3A, incontrast to the more current situa-tion where they are not (Fig. 3B).

Above upper amphibolite facies,partial melting and leucosome for-mation become intrinsic parts ofmetamorphic reaction products(Fig. 4). Under these conditions,the mineral assemblage of the leu-cosomes is an important tracer ofthe metamorphic reactions thattook place in a rock, informationthat in turns permits the in situcharacter of observed granitic veinsversus those extraneous to the hostrock to be determined with a rea-sonable degree of confidence.Appraisal of the extent of meltingand melt extraction is important todistinguish altered protolith fromrestites derived from metasedi-ment, as discussed below. It is alsoimportant during sampling for geo-chemistry. Of particular interest ismild to intense depletion of Na or K or enrichment of Al or Si during premetamorphic hydrother-mal activity since these chemicalchanges increase the temperature atwhich significant melting mayoccur and reduce metamorphicremobilization of not only melt butalso potentially of premetamorphicore. The lowering of melting tem-peratures also increases the proba-bility of preserving primary tex-tures (e.g. Fig. 3A), which facili-tates the study and lithogeochemi-cal sampling of protoliths.

Advanced Argillic, Argillic,Chloritic, and Sericitic AlterationZones

The principal oxides present in advanced argillic, chloritic, and sericitic alteration zones are SiO2-Al2O3-FeO-MgO-K2O-H2O.In these alteration zones, CaO andNa2O are very low in abundanceand exert little influence on the overall mineral assemblage(beyond hindering the formation of Ca and Na minerals). Asa result, the effects of metamorphism can be predicted usinggrids and phase diagrams designed for metapelites (alumi-nous metasedimentary rocks). These grids include those calibrated for the systems SiO2-Al2O3-H2O (ASH) andSiO2-Al2O3-(FeO or MgO)-K2O-H2O (KFASH, KMASH,KFMASH; Fig. 3.1 in Bonnet and Corriveau, 2007). TheASH-type grids and relevant phase diagrams are used tomodel metamorphic evolution of advanced argillic alter-ation, which is characterized by chemical compositions that

are low in Fe, Mg, and K. In contrast, the KFASH grids areuseful to portray argillic and sericitic alteration that is char-acterized by low Mg but high Fe and K. Finally the KMASHand KFMASH systems may be used to better characterizemetamorphosed chloritic alteration zones that are rich in Mgand, in some cases, Fe.

Modeling of argillic alteration mineralogical variations asa function of pressure and temperature is shown in Figure 4using a particularly well documented argillic alteration of theouter margin of a porphyritic microgranite hosting theMaronia porphyry Cu-Mo deposit (Melfos et al., 2002). The

A-L. Bonnet and L. Corriveau

1040

1

2 3

4P-T path

A

F

Ky+ Ms+ Qtz+ H O2

Bt M

MPA

F M

Kln+ Ms+ Qtz+ H O2

MP

Argillic alterationAdvanced argillic alteration

M

A

F

Sil+ Kfs+ Qtz+ H O2

MP

Bt

A

F M

Sil

Grt

Grt

Crd

Crd

+ Kfs+ Qtz+ H O2

Bt

MP

300

2

4

6

8

10

P(kbar)

T (ºC)

500 700 900

Bt

Bt

BtBt

PrlPrl

Prl And

And

Sil

Sil

Ky

Ky

Qtz

Qtz

QtzQtzKln

Sil

Sil

Qtz

Qtz

QtzQtz

Qtz

Qtz

AndMs

Ms

V

V

L

L

Pl

Pl

PlPl

Pl

Pl

Crd

CrdOpx

Opx

CrdSpl

Grt

Grt

Grt

Grt

L

L

L

±Kfs

±Kfs

±Kfs

Kfs

Kfs

KfsAls

1

2 3

4

FIGURE 4. Petrogenetic grid for metapelite-like rocks showing the modelled pressure-temperature path for anargillic (dark grey square) and an advanced argillic (medium grey triangle) alteration types. The examplesplotted here are from the Maronia porphyry Cu-Mo deposit (square; data of Melfos et al., 2002) and theBondy Gneiss Complex (triangle; data of Blein et al., 2003, 2004). The composition and the mineralogy ofthese alteration types are represented in AFM diagrams (A = Al2O3-K2O, F = FeO, M = MgO; molar pro-portion) at stages 1, 2, 3, and 4 of the pressure-temperature path (projection from muscovite for stages 1 and2, and from K-feldspar for stages 3 and 4). In these AFM diagrams, the composition of the hydrothermallyaltered rock is compared with that of their precursor (empty square and triangle) and of metapelites (MP). Theposition of samples will only change from one diagram to the other if the projection is done from a differentmineral as it is the composition that ultimately governs where samples plot on these diagrams. In contrast, thestability of minerals changes at each stage, governed by metamorphic reactions that are represented in the pet-rogenetic grid as calibrated by Spear and Cheney (1989) and Pattison et al. (2003). Mineral abbreviation isafter Kretz (1983). L = liquid.

A

B

Alteration Vectors to Metamorphosed Hydrothermal Systems in Gneissic Terranes

1041

altered rocks are metamorphosed toa low-grade mineral assemblagedominated by kaolinite, quartz, andin lesser amount sericite (Stage 1 inFig. 4). These rocks are enriched inSi, Al, and K but depleted in Fe, Ca,Na, and Mg. The chemical compo-sition of the alteration is distinctfrom that of its precursor and is sig-nificantly more aluminous than thatof a pelite (Fig. 4). During progrademetamorphism, kaolinite breaksdown to pyrophyllite and subse-quently to kyanite, in both casesforming metamorphic quartz as aproduct (from Stage 1 to Stage 2 inFig. 4) (Hemley et al., 1980). If thetemperature increases further, silli-manite replaces kyanite betweenStage 2 and Stage 3 (e.g. Powell etal., 1999). At upper-amphibolite-facies temperature conditions, mus-covite in the presence of quartz isdestabilized to form K-feldspar andsillimanite with a granitic leuco-some (Stage 3 in Fig. 4). AvailableFe and Mg in the system are largelydeposited as biotite. Garnet orcordierite can also be stabledepending on their composition,although they are unstable togetherunder upper-amphibolite-faciesconditions. The modal proportionof sillimanite in the hydrothermallyaltered rock will be particularly high, reflecting the alumi-nous composition of the protolith. In fact, the modal value ofsillimanite will be higher than that of a metapelite as the Alcontents is significantly higher than that of average compos-ite shale (Gromet et al., 1984) and pelite (Fig. 4) (using thelever rule to calculate modal proportion of AFM phases, sil-limanite/biotite would be in a ratio of 3 to 1 for the Maroniaargillic alteration). Between Stage 3 and Stage 4, the biotiteoriginally present reacts with sillimanite to form garnet andcordierite until one of the reactants disappears (Stage 4 inFig. 4). A granitic melt is formed and will commonly host thegarnet and cordierite produced. With sillimanite being inexcess, biotite will be consumed and the stable mineralassemblage will consist of sillimanite, K-feldspar, quartz,garnet, and cordierite (Table 2).

Advanced argillic alteration types consist predominantlyof Si and Al, although K, Fe, Mg, Na, and Ca are present inminor amounts. In low-grade metamorphic terranes, thedominant minerals are quartz, kaolinite, dickite, and alunite.During prograde metamorphism, the aluminous hydrother-mal minerals are destabilized to form, according to the pre-vailing pressure-temperature conditions, pyrophyllite (dur-ing Stage 1, Fig. 4), andalusite (if at low pressure, Fig. 4),kyanite (along the pressure-temperature path of Fig. 4,reaching Stage 2), or sillimanite (Stage 3, Fig. 4). The alu-minosilicates are very abundant compared to other AFMminerals, as the composition of the rock is particularly rich

in Si and Al (Fig. 4). Biotite in Stage 1 to Stage 3 is in a pro-portion of 1 to 9 with respect to sillimanite. At granulitefacies, the assemblages will be dominated by sillimanite andquartz (Table 2). In contrast to argillic alteration, garnet andcordierite would be absent or very minor in abundance, asthe system does not contain enough Fe and Mg to form sig-nificant amounts of these minerals (Stage 4, Fig. 4); biotitewould have completely broken down. Thus, at granulitefacies, metamorphosed advanced argillic alteration formsvery atypical quartz- and sillimanite-rich rocks, such asthose observed at the granulite-facies Challenger golddeposit (Tomkins and Mavrogenes, 2002) and in thehydrothermal system of the Bondy Gneiss Complex (Blein etal., 2003, 2004). Topaz, allanite, and rutile can also be pres-ent in such peraluminous rocks and are associated with ironoxides and sulphides. They are not, however, a product ofreactions in ‘normal’ pelite and consequently are notreported on Figure 4 used to conceptually model the evolu-tion of aluminous alteration zones in this example.

In low-grade metamorphic terranes, chloritic alteration(Si+Al+Mg+Fe± K) forms a ferromagnesian schist domi-nated by chlorite, quartz, sericite, and minor amount of sul-phides or tourmaline. The petrogenetic grids calibrated forthe KFMAS system indicate that muscovite, phlogopite, andquartz appear above 425ºC from the breakdown of chloriteand sericite (Wang et al., 1986; Trägårdh, 1991).Assemblages with cordierite+phlogopite, and subsequently

Mineral Potential indicator ofOrthoamphibole (Mg,Fe)7(SiO22)(OH,F)2= Anthophyllite, GedriteOrthopyroxene (Mg,Fe) SiO3

Cordierite Al3(Fe, Mg)2(Si5AlO18)Talc Mg6(Si8O20)(OH)4

Magnetite Fe23+Fe2+O4

Hematite Fe2O3

Biotite K(Mg,Fe)3AlSi3O10(OH)2Muscovite K2Al4(Al2Si6O20)(OH,F)4

Phlogopite KMg3(AlSi3O10)(OH)2Phengite K2(Al,Mg)4(Al2Si6O20)(OH,F)4

Sillimanite Al2SiO5Topaz Al2(SiO4)(OH, F)2

Spinel (Mg, Fe, Zn)Al2O4

Garnet (Fe, Mg, Mn)3 Al2Si3O12 Al enrichment through leaching of more mobile elementsCarbonates Ca(Fe,Mg)(CO3)2Epidote Ca(Al,Fe)3(OH)(SiO4)3

Diopside CaMg(SiO3)2

Grossular Ca3Al2Si3O12

Plagioclase Na-Ca(Si3AlO8) Ca or Na alterationTourmaline NaSi6O10(BO3)3(OH,F)4 B enrichmentSulp nemhcirne Ssedih tRutile TiO2 Ti enrichmentBarite BaSO4 Ba enrichmentFluorite CaF2 F enrichmentApatite Ca5(PO4)3(F,Cl,OH) P enrichmentMonazite (PO4)(Ce, La, Di) P and REE influx or leaching of more mobile elementsZircon Zr(SiO4 stnemele elibom erom fo gnihcael ro xulfni rZ)

K and Mg enrichment in aluminous gneiss

Al influx or severe leaching of more mobile elements

Ca alteration

Fe-Mg alteration

Mg alteration

Fe alteration

K alteration in particular in mafic rocks

TABLE 3. Main indicator minerals of high-grade metamorphosed hydrothermal alteration.

cordierite+K-feldspar or cordierite+orthoamphibole (antho-phyllite, gedrite)±garnet appear following the destabilizationof chlorite, muscovite, and phlogopite (Schade et al., 1989;Trägårdh, 1991). If there is enough Al and Fe in the system,chlorite reacts with muscovite and garnet to form staurolite.Staurolite-bearing rocks are recognized in the Linda deposit(Zaleski et al., 1991) whereas rocks withcordierite+quartz+orthoamphibole+garnet+phlogopite areobserved in the alteration halos surrounding VMS depositsin the mining district of Vihanti-Pyhäsalmi and Ruostesuo inFinland (Roberts et al., 2003) or with the Prieska deposit inSouth Africa (Theart et al., 1989). Rocks with anthophyl-lite+garnet+orthopyroxene-sulphide are associated also withthe Coulon prospect in Quebec (Huot et al., 2004). Theserocks are interpreted as chloritic alteration products of felsicor mafic volcanic rocks, metamorphosed at upper amphibo-lite-granulites facies. Finally at granulite facies, theorthoamphibole is destabilized completely and garnet-cordierite-orthopyroxene or cordierite-orthopyroxeneassemblages are formed (Schreurs and Westra, 1985; Bleinet al., 2004; Fig. 3B; Table 2, Plate X in Bonnet andCorriveau, 2007). Moore and Waters (1990), Bernier andMacLean (1993), and Hallberg (1994) suggest thatcordierite-phlogopite and phlogopite-rich gneisses are diag-nostic of magnesian alteration zones.

Sericitic alteration (Si, Al, K, and Fe or Mg) metamor-phosed at low grade consists mainly of sericite, quartz, andsulphides. These minerals are destabilized during progrademetamorphism to form biotite, K-feldspar, aluminosilicates,and quartz at amphibolite and granulite facies (Table 2).Theart et al. (1989) illustrate that, at the Prieska deposit, alu-minous gneisses with a quartz, K-feldspar, and sillimaniteassemblage (containing 70-80% SiO2) correspond to silici-fied and sericitized sediments. Gneisses with sillimanitenodules near the Montauban (Grenville Province), Coulonand Geco (Superior Province) deposits and prospects areinterpreted as metamorphosed sericitic alteration zones(Bernier and MacLean, 1993; Zaleski and Peterson, 1995;Huot et al., 2004). Such nodular rocks are also found withinthe volcanic units of the La Romaine Supracrustal Belt andMusquaro extension of the Wakeham Group (Fig. 3C-F;Plate XI in Bonnet and Corriveau, 2007; Corriveau et al.,2007). The spatial association of such nodular units withmineralized zones at Montauban led Gauthier et al. (1985) touse these units as lithological vectors to volcanic-hosted Aumineralization. They are now being recognized at theChallenger deposit providing further evidence for significantpremetamorphic hydrothermal activity at this Au deposit (C.McFarlane, written comm., 2006). Similar nodular units, for-mally interpreted as diatexites (Perreault and Heaman,2003), may represent metamorphosed sericitic alteration andsilicification, and provide a potential target for VMS explo-ration in the Brador’s Bay supracrustal belt (easternGrenville Province) (Corriveau et al., 2007; Plate X inBonnet and Corriveau, 2007).

Carbonate and Propylitic Alteration ZonesCarbonate and propylitic alteration is composed of vari-

able proportions of Ca, Al, Mg, Fe, and Si. The petrogeneticgrids used to predict the mineral assemblages formed duringtheir metamorphism are those calibrated for carbonate or

mafic rocks (Fig. 3.1 in Bonnet and Corriveau, 2007). Inlow-grade metamorphic terranes, the dominant minerals arechlorite, various carbonates, epidote, plagioclase, and quartz.At amphibolite facies, amphibole (actinolite and at highergrade hornblende) and anorthite appear from the breakdownof chlorite and epidote, although epidote can remain stableunder high pressure and temperature conditions in rocks witha very calcic composition (Spear, 1993). At the transitionbetween upper amphibolite and granulite facies, hornblendeand quartz are destabilized and a new mineral assemblage oforthopyroxene, clinopyroxene, and plagioclase appears. InFe-rich systems, Fe-rich silicates, such as ferrous garnet andclinopyroxene, prevail (Spear, 1993) whereas in Ca-rich sys-tems, Ca-rich silicates, such as grossular, diopside, and epi-dote, are dominant (Table 2). Chemical changes associatedwith carbonate or propylitic alteration are not very large, andthe composition of these alteration types is commonly verysimilar to that of calc-silicate rocks of sedimentary or meta-somatic origin (Pan and Fleet, 1992). Thus high-grade meta-morphosed carbonate or propylitic alteration is difficult toidentify in the field and is not commonly documented in theliterature.

Limits Concerning the Use of the Petrogenetic GridsPetrogenetic grids must be used carefully to interpret

metamorphosed hydrothermal alteration zones, as their cali-bration does not take into account many of the trace andminor elements that occur in such rocks. Such elements caninfluence the stability of mineral assemblages if they occurin unusually high abundances relative to rocks used to cali-brate the grids, or if they are contained in the reacting min-erals such as chlorite and biotite (F, Ba), feldspar (Ba, Sr),staurolite and gahnite (Zn), and garnet (Mn) (e.g. Spry andScott, 1986). Pelite generally contains very small quantitiesof Mn, P, S, B, F, Zr, Zn, Ba, Sr, and other trace elements(Gromet et al., 1984). These elements have therefore very lit-tle influence on the stability of the minerals that containthem and are generally not taken into account in the calibra-tion of the petrogenetic grids. However, in aluminous rocksderived from altered protoliths, these elements can be rela-tively abundant. Zaleski et al. (1991) demonstrate that,because of incorporation of F in biotite and Zn in stauroliteand gahnite, the assemblages kyanite-biotite and staurolite-anthophyllite-gahnite in the alteration zones of the Lindadeposit (Manitoba) crystallized at a lower grade than thoseobserved in aluminous rocks of sedimentary origin. Theseauthors conclude that in this case, the use of the petrogeneticgrids calibrated for the pelite is not suitable for rigorousinterpretation of isograds and pressure-temperature meta-morphic reaction conditions. On the other hand, the grids areuseful in predicting the range of possible reactions andassemblages in the field, and where mineral assemblages arenot those expected for normal metamorphosed rocks. Suchdiscrepancies between observed and predicted metamorphicassemblages may indicate unusual compositions related tohydrothermal alteration.

Strategies, Criteria, and Diagnostics

Advanced argillic, sericitic, chloritic, carbonate, andsiliceous alteration are useful exploration vectors for detect-ing VMS deposits. Once deformed and metamorphosed,

A-L. Bonnet and L. Corriveau

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Alteration Vectors to Metamorphosed Hydrothermal Systems in Gneissic Terranes

1043

such alteration is represented byaluminous, ferromagnesian, calc-silicate, or siliceous gneisses com-monly with atypical mineral assem-blages and modal mineral abun-dance with respect to sedimentary,volcanic, or plutonic protoliths.Their diagnostic geological traitsare critical to protolith assessment(Fig. 5) in gneissic terranes as theyprovide criteria to distinguish themfrom metasediments or metamor-phosed paleosoils with whom theyshare field exposure affinities. Inunder-explored areas, mineraliza-tion may not be reported or mayappear insignificant; alterationmapping is required at this stage tolocate mineralized areas. It alsoprovides constraints on the inten-sity and the spatial distribution ofhydrothermal systems, targets for systematic lithogeochemi-cal and geophysical exploration, and supportive evidence formetallogenic model(s), all of which may contribute to themineral resources assessment of the studied area and poten-tially the discovery of new deposits (e.g. Winston Lakedeposit, Ontario; Thomas, 1991).

Field Mineralogical, Textural, and Structural AnalysisA metamorphosed volcano-plutonic belt with VMS-type

hydrothermal alteration zones will have the appearance of afelsic gneiss complex containing minor proportions ofamphibolite as well as aluminous, ferromagnesian, calc-sili-cate, or siliceous ‘metasedimentary’ rocks with atypical tex-tures, mineralogical characteristics, and modal abundances(Figs. 5, 6A-C). Diagnostic lithologies such as sillimanite-quartz rock, garnetite, and cordierite-orthoamphibole/orthopyroxene quartzofeldspathic or maficgneiss (Fig. 3B) may be the first sign that hydrothermalalteration is present and that the geological context may be anear surface environment (Corriveau et al., 1997). In suchcases, searching for volcanic textures among otherwise non-descript quartzofeldspathic gneisses may provide the firstconclusive evidence of volcanic activity within a studiedgneissic complex (Fig. 6A,B; Blein et al., 2003; Corriveauand Bonnet, 2005). Rock types that appear more ‘normal’but have some unusual abundance of certain minerals mayalso be indicators of premetamorphic hydrothermal alter-ation (Table 3). However, different processes may lead to theformation of similar rocks and protolith assessment rarelyprovides a single solution (e.g. Reinhardt, 1987; Chapter 4 inBonnet and Corriveau, 2007). For example, alterationprocesses that involve highly acidic hot fluids severely leachhost rocks, leaving high-alumina assemblages (advancedargillic alteration). Such high-alumina assemblages may beformed also from low-temperature surface weathering, caus-ing lateritization, or from partial melting of pelite with sig-nificant melt extraction. Consequently, peraluminous gneisscan have, as a protolith, a rock affected by an advancedargillic alteration, a laterite, or a restitic pelite (Gromet et al.,1984; Lemiere et al., 1986). Textures, such as aluminous

nodules in quartzo-feldspathic gneiss (Fig. 3C-F) and frag-ments (lapillis or block) in aluminous gneiss (Figs. 3A, 6C)adjacent to lapillistone units with fragments of similar sizeand rock types (Fig. 6A,B), provide firmer evidence ofhydrothermal alteration.

Metamorphosed hydrothermal alteration zones commonlyconsist of rocks that attract the interest of field geologistsand tend to be located and discussed in geological reports orassessment files where they are interpreted as restites basedon the unusually high modal abundance of aluminous ormafic minerals (e.g. the diatexites of Perreault and Heaman,2003). Though such units do not become instant explorationtargets, they warrant closer examination and a search forother key vectors to possible mineralization, such as meta-exhalite (coticule, garnetite, iron formation, tourmalinite,and quartz-gahnite rock; Spry et al., 2000).

Hydrothermally altered rocks that have a bulk composi-tion, hence a mineral assemblage and a modal abundance,similar or fairly similar to that of sedimentary, volcanic, orplutonic units, (Fig. 7) may only be determined as hydrother-mal in origin by lithogeochemistry (Fig. 7D). U-Pb SensitiveHigh Resolution Ion MicroProbe (SHRIMP) dating on zir-con also has potential for constraining genetic models of oreformation in ancient high-grade metamorphosed terranes(Claoué-Long et al., 1990; Yeats et al., 1996; Williams,1998; Hartmann et al., 2000). For example, such data canhighlight the presence of a detrital zircon population (Fig.7B), providing conclusive evidence of a sedimentary originor of a single magmatic zircon population (Fig. 6D) that isconsistent with a volcanic or plutonic origin.

Follow-up lithogeochemistry using alteration indices(Ishikawa et al., 1976; Hashigushi et al., 1983; Hodges andManojlovic, 1993; Large et al., 2001; Piché and Jébrak,2004) or mass-balances techniques (MacLean andKranidiotis, 1987; MacLean, 1990; MacLean and Barrett,1993; Leitch and Lentz, 1994) will help refine field interpre-tations. The NORMAT norm calculation of Piché and Jébrak(2004) is of particular use as it provides a measure of rela-tive alteration intensity and characterization of alterationtypes through a series of alteration indices that are inde-

FIELD OBSERVATIONto recognize metamorphosed hydrothermal alteration

To recognize and/or confirm hydrothermally altered protolith

More efficient and better targeted sampling

GEOCHEMISTRY MINERAL CHEMISTRY

Felsic gneiss,Aluminous gneiss,Calc-silicate rock,Mafic rock

Altered protolith

Mineralogical characteristics(mineral assemblage, modalmineral abundance)

Yes No

Clues supportingunaltered protolith

Altered protolithuncertain

Preserved textures of a sedimentary,volcanic, or plutonic protolith

distinct from what is expected frommineralogical characteristic

Normal withrespect to theinferred protolith

Abnormal withrespect to theinferred protolith

Sedimentary, volcanic,or plutonic protolith

Clues supporting apotentially altered

protolith

FIGURE 5. Field strategy for the recognition of metamorphosed hydrothermal alteration and by extension ofhydrothermal systems possibly affiliated to mineral deposits.

pendent of the composition of the precursor rock as long asthe protolith was not peraluminous. These norm calculationsare currently only available for greenschist-facies settings(Piché and Jébrak, 2006) but are being developed for highergrade rocks.

In order to identify residues of partial melting in a reliableway, it is important to empirically establish the degree ofsegregation and remobilization of anatectic melts, as well asthe types of crystallization sites. For this purpose, it is impor-tant to (1) compare the mineralogical assemblages of thehost gneisses and their leucosomes to know if they can beinterpreted in terms of metamorphic reactions generatinganatectic melts (Sawyer, 1999), and (2) document the mor-phology, structures, and textures of the leucosomes to esti-mate their degree of remobilization (Sawyer, 2001). If the

mineralogical assemblages and the textures and structures ofleucosomes are diagnostic of in situ leucosomes, the unitsthat are rich in aluminous and ferromagnesian phases cannotbe restites resulting from significant melt extraction (see Fig.5 in Boggs and Corriveau, 2004) and are candidates for ahydrothermal alteration protolith, which can be tested bylithogeochemistry and U-Pb geochronology. In contrast, iftextures are diagnostic of a significant degree of anatecticmelting associated with severe melt extraction, then a restiticorigin is a likely interpretation. However, special conditionsare required for this to happen (Vigneresse et al., 1996) andit is fortunately more the exception than the rule, at leastwithin the Canadian Precambrian Shield. Lithogeochemicalprotolith studies may lead to spurious results and may not beworth attempting for exploration purposes.

A-L. Bonnet and L. Corriveau

1044

Aluminous fragmental gneiss(advanced argillic alteration of lapillistone)

Hydrothermal alteration

1600

1400

1300

0.21

0.23

0.25

0.27

0.29

2.4 2.8 3.2 3.6

1508 ± 14 Ma

One population of igneous zircons

1500

206 238Pb/ U

207 235Pb/ U

50 um

Lapillistone (1511 to 1491 Ma)

Post -emplacement

Deformation

Compositional changes , igneous zircons preserved

FIGURE 6. Field interpretation of fragmental aluminous gneiss in the granulite-facies La Romaine Supracrustal Belt and subsequent testing. (A) Precursorlapillistone with fabric interpreted as largely primarily based on volcanic textures. (B) Deformed equivalent in the same outcrop. (C) Adjacent garnet-silli-manite-biotite gneiss (aluminous unit) with fine-grained quartzo-feldspathic fragments of similar size, shape, and composition as those of (B). (D) SHRIMPzircon analyses from the garnet-sillimanite-biotite gneiss with well preserved volcanic fragments on the concordia diagram. The U-Pb crystallization age of1508 ± 14 Ma (95%) is consistent with zircon U-Pb crystallization ages obtained on least altered lapillistone.

AB

C D

Alteration Vectors to Metamorphosed Hydrothermal Systems in Gneissic Terranes

1045

Metamorphic Alteration VectorsAlteration mapping based on visual estimates of rock-

types, mineral modal abundance, and mineral assemblagesare effective in qualitatively measuring the variations inmajor elements associated with the premetamorphichydrothermal alteration and in establishing the chemicalzonation with a reasonable degree of confidence. Such zona-tion reflects major element mobility during fluid-rock reac-tion in the upflow zone (e.g. Al, Fe, Mg, Ca, Na, and K) andrepresents geochemical indicators of the type and intensityof alteration (Barrett and MacLean, 1994; Galley, 1995;Large et al., 2001). These tools are also applicable to meta-morphosed, weakly to intensely altered rocks whose precur-sor is known (Hodges and Manojlovic, 1993; Hallberg,2001; Gale, 2003). Alteration vectors may be representedgraphically using chemical diagrams or on AFM, A’CF, orA’KF phase diagrams, as discussed above, in order to visu-alize the whole-rock composition and the mineralogicalassemblages of the altered rocks and their precursors. Thisallows (1) the chemical changes associated with thehydrothermal alteration to be determined by comparing themetamorphic characteristics of the altered rocks and theirprecursors, and by extrapolation, their compositions; (2) theestablishment of the link between the metamorphic assem-

blages observed and the primary hydrothermal mineralogicalassemblages; and (3) the development of metallogenic mod-els (e.g. Roberts et al., 2003). The field models can then berefined with lithogeochemical studies (Large et al., 2001).

The main alteration vectors are represented in Figure 8A.In AFM diagrams, the vectors pointing towards the apices A,F, and M, respectively, model the enrichment of Al, Fe, andMg, and characterize advanced argillic or siliceous alteration(Al-Si), argillic alteration (Al-Fe), and chloritic alteration(Mg ± Fe). The vector moving away from apex A character-izes sericitic- or potassic-alteration types as apex A takes intoaccount Al and K (A = Al2O3 + K2O in mole proportion;Spear, 1993). In this type of diagram, acidic or siliceousargillic alteration is localized close to apex A near the fieldof aluminosilicates (kaolinite, pyrophyllite, kyanite, and/orsillimanite, depending on the metamorphic facies); they arethus characterized by high contents of these minerals.Chloritic alteration is localized close to the apex M, thusclose to the field of magnesian minerals such as chlorite,anthophyllite, gedrite, cordierite, phlogopite, and/ororthopyroxene (depending on the metamorphic facies). Inthe same manner, sericitic alteration is characterized by highcontent of sericite or, at higher metamorphic grade, of biotiteor K-feldspar. Finally, iron-bearing alteration will be domi-

- detrital zircon population

1850

1550

13500.22

0.24

0.26

0.28

0.30

0.32

0.34

2 3 4 5 6

Field observations:

No fragmental texturesMineralogical characteristicsand composition typical ofmetasediments

METAPELITE

Metapelite

Aluminous gneiss (hydrothermally altered protolith)

Field observations:

No fragmental texturesMineralogical characteristics andcomposition atypical with respectto that of metasediments

HYDROTHERMALALTERATION

207 235Pb/ U

206 238Pb/ U

50 um

Ky, Sil

Grt

A

Crd

OpxBt

F M

NASCMSC

+ Kfs+ Qtz+ H O2

Volc

A

B

C D

FIGURE 7. Procedures for recognizing the origin of metapelite-like aluminous gneisses in the granulite-facies La Romaine Supracrustal Belt and subsequenttesting. (A) Garnet-sillimanite-biotite gneiss interpreted as a metapelite based on its mineralogy and textural characteristics.(B) SHRIMP zircon analyses fromthe metapelite on the concordia diagram. The gneiss has a detrital population of zircon, which confirms its sedimentary origin. (C) Garnet-sillimanite-biotitegneiss interpreted as a metamorphosed hydrothermal alteration based on its mineralogy and texture. (D) AFM diagrams (A = Al2O3-K2O, F = FeO, M = MgO;molar proportion) showing the compositional differences between garnet-sillimanite-biotite gneiss of hydrothermal origin and metapelite (MSC and NASCvalues from Gromet et al., 1984).

nated by magnetite, hematite, iron sulphides, and, in certaincases, garnet. The A’ KF and A’ CF diagrams are particularlyuseful to represent carbonate (Ca) or propylitic alteration(Ca-Fe) localized close to the apex C as well as sericiticalteration (K-Al) localized close to the apex K. These alter-ation types are characterized by significant contents of calcicminerals (calcite, epidote, grossular, anorthite, and/or diop-side), iron-bearing minerals (magnetite, hematite, garnet,and/or iron sulphides), and potassic minerals (sericite,biotite, and/or K-feldspar ), depending on pressure-tempera-ture conditions. In such diagrams, the FeO and MgO com-ponents are not differentiated and fall near apex F, hencethey do not permit magnesian alteration to be differentiatedfrom iron alteration.

To illustrate these points, the chemical changes recordedby the alteration zones of the granulite facies Ruostesuo Zn-Cu deposit are presented on ternary diagrams (Fig. 8B). Thisdeposit comprises gneisses with quartz-cordierite-biotite andquartz-garnet-cordierite-sillimanite assemblages that respec-tively represent magnesian (enriched in Mg-Al) and iron(enriched in Fe-Al) alteration of a felsic volcanic precursor

(Roberts et al., 2003). In an AFM diagram, these gneissesdefine vectors towards the F and M apices and in the dia-gram A’ KF they project along a vector directed towardscordierite (between apices A’ and F). Such a chemographicprojection was also applied to rocks of the La Romainehydrothermal system (Fig. 8B). This system includes meta-morphosed pumaceous tuffs with aluminous veins and nod-ules, and aluminous gneisses of volcanic origin (Bonnet etal., 2005). The aluminous nodules or veins are composedmainly of quartz, muscovite, sillimanite, and iron oxides.These nodules are thus enriched in K, Al, and Fe, which dif-fers from that of the least altered felsic volcanic rocks host-ing them. The metatuffs with aluminous nodules or veins arethus typical of sericitic alteration and define vectors towardsthe apices A and K in the AFM and A’ KF diagrams, respec-tively. The aluminous volcanic rocks are characterized byhigh contents of sillimanite indicating an Al-rich composi-tion compared to that of their volcanic precursor. Such rocksare characteristic of an advanced argillic alteration (Bonnetet al., 2005). These gneisses are thus aligned towards apicesA and A’ in the AFM and A’ KF diagrams respectively.

A-L. Bonnet and L. Corriveau

1046

Ky, SilMs

AnEp

Grs

CalDi-Hd Tr

Hbl

A’

C F

Alm, PrpCaCa

Advanced argillicAdvanced argillic

Kfs

Opx

K

F

A’

FIFI

FIFI

AlAl

AlAl

Crd

F

Bt

K

PropyliticPropylitic

SericiticSericiticKy, Sil

KGrt

Biotite

A

FeFe

AlAl

Crd

M

Bt

K MgMg

FIFI

ChloriticChloriticSericiticSericitic

Advanced argillicAdvanced argillic

OpxFe-alterationFe-alteration

MagHem

MagHem

Composition of the precursor(unaltered felsic volcanic rock)

Quartzofeldspathic gneiss with aluminousnodules (Al-K alteration)

Aluminous gneiss (Al alteration)

Alteration vector

Composition of the precursor (unaltered)

Zn-Cu VHMS deposit, Ruostesuo, Finland(metamorphosed at granulite facies)

Cu hydrothermal system of La Romaine(metamorphosed at granulite facies)

Qtz-Crd-Grt-Sil gneiss (Si-Fe alteration)

Qtz-Crd-Bt gneiss (Mg alteration)

vlcvlc

AdvancedargillicAdvancedargillic

Alteration vector

Ms

Alm, Prp

Kfs

Opx

K

F

A´

Crd

Bt

vlcvlc

Al Al-K

Ky, Sil

KGrt

Biotite

A

Crd

F M

Opx Bt

MgMg

AlAl

vlcvlc

Si-FeSi-Fe

Al-KAl-K

A

B

FIGURE 8. Alteration vectors and their control on modal composition and mineral paragenesis of metamorphosed hydrothermal alteration zones. (A) Ternarydiagrams A’CF (A’=Al2O3+Fe2O3,-(K2O+Na2O), C=CaO, F=FeO+MnO+MgO; molar proportion) A’KF (A’=Al2O3+Fe2O3,-(K2O+Na2O+CaO), K=K2O,F=FeO+MnO+MgO), and AFM (A=Al2O3-K2O, F=FeO, M=MgO; molar proportion) showing geochemical changes of the main alteration zones associatedto volcanogenic massive sulphide deposits. (B) Ternary diagrams A’KF and AFM showing the alteration vectors (dotted arrow) for Si-Fe- and Mg-alterationzones of the granulite-grade Ruostesuo Zn-Cu deposit (square; data from Roberts et al., 2003) and for advanced argillic (aluminous gneiss) and sericitic(gneiss with aluminous nodules and veins) alteration of felsic pyroclastic rocks from the La Romaine hydrothermal system (star; data from Bonnet et al.,2005).

Alteration Vectors to Metamorphosed Hydrothermal Systems in Gneissic Terranes

1047

Conclusions

Volcano-plutonic belts are potentially fertile forhydrothermal ore deposits such as VMS deposits and repre-sent potential targets for exploration. In high-grade meta-morphic terranes alteration zones, where well developed,provide important vectors to ore. In felsic gneiss complexeswhere volcanic rocks are not even known, refinements tomapping strategies and protocols provide a means to recog-nize potential hydrothermal systems even though evidencefor volcanic rocks and subvolcanic intrusions have been par-tially to completely obliterated during high-grade metamor-phism. Metamorphism of hydrothermal alteration zonesgives rise to diagnostic rock types, mineral assemblages, andmodal abundances that can be used as vectors to mineraliza-tion. Widespread hydrothermal alteration that is chemicallydistinct from host rock lithologies can be recognized with areasonable degree of confidence during regional mapping aslong as it is not confused with metasedimentary rocks. Thefield strategies and tools proposed to evaluate gneissic pro-toliths and to recognize hydrothermal alteration zones arebased on the following conditions: 1. Metamorphic characteristics of the gneiss (mineral

assemblages and modal mineral abundance) reflectingthe chemical composition of their protoliths and thephysical conditions of the metamorphism;

2. Primary structures and textures that are locally pre-served;

3. Field indicators diagnostic of hydrothermal activity,such as the presence of meta-exhalite, stockworks, oractual mineralization. The geological criteria presented provide a method of

identifying a fossil hydrothermal system with a reasonabledegree of confidence using vectors to mineralization. Thesefield criteria are not exact tools but represent permissive evi-dence for a hydrothermal origin of certain gneisses. They canalso be applied to any deposit type associated to volcano-plutonic belts where alteration zones are sufficiently largeenough to be recognized during regional mapping such asepithermal, porphyry, and iron oxide Cu-Au deposits. Oncea hydrothermal system is identified, it is possible to targetspecific areas for more detailed mapping and exploration.These geological criteria are also useful in guiding the litho-geochemical sampling of key lithologies and in testing met-allogenic models.

Acknowledgements

Funding for this work was provided by the DIVEXresearch group and the Geological Survey of Canada and isbased on the knowledge gained in the eastern GrenvilleProvince in the course of a Targeted Geoscience Initiativeproject of Natural Resources Canada in partnership with theMinistère des Ressources Naturelles et de la Faune, Québec(MNRF) and the Geological Survey of Newfoundland andLabrador. Financial support was also granted by l’Institutnational de la recherche scientifique - Eau-Terre-Environnement (INRS-ETE) for the doctoral studies of A.-L.Bonnet under the co-supervision of M.R. LaFlèche (INRS-ETE) and Alain Tremblay (University of Quebec atMontreal). The authors thank P. Archer, M. Chapdelaine, andF. Huot for their contributions concerning the Coulon prop-

erty when it belonged to the Virginia-Noranda consortium,C. Guilmette, S. Parson, O. Rabeau, G. Scherrer, and K.Williamson for preliminary research for this paper, L.Nadeau, W. Goodfellow, J. Lydon, O. van Breemen, and B.Dubé (Geological Survey of Canada), G. Beaudoin (LavalUnversity), M. Auclair, V. Bodycomb, M. Gauthier, and M.Jébrak (University of Quebec at Montreal), M.R. LaFlècheand M. Malo (INRS), P. Verpaelst, T. Clark, and S. Perreault(MRNF), S. Cadéron (U. Polytechnique), and the expertcommittee of DIVEX for their logistic and scientific contri-butions, and L. Dubé (INRS) for the digitalization of thephotographs. Special thanks are expressed to G. Beaudoinand D. Lentz, journal reviewers, and W. Goodfellow, editor,for their scientific and editorial comments that significantlyclarified the manuscript.

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