Metallogenic relationships to tectonic evolution ^the Lachlan Orogen, Australia

Frank P. Bierlein a;�, David R. Gray a;1, David A. Foster b

a VIEPS, School of Geosciences, Monash University, P.O. Box 28E, Melbourne, Vic. 3800, Australiab Department of Geological Sciences, University of Florida, P.O. Box 112120, Gainesville, FL 32611, USA

Received 8 March 2002; received in revised form 5 June 2002; accepted 5 June 2002

Abstract

Placing ore formation within the overall tectonic framework of an evolving orogenic system provides importantconstraints for the development of plate tectonic models. Distinct metallogenic associations across the PalaeozoicLachlan Orogen in SE Australia are interpreted to be the manifestation of interactions between several microplatesand three accretionary complexes in an oceanic back-arc setting. In the Ordovician, significant orogenic gold depositsformed within a developing accretionary wedge along the Pacific margin of Gondwana. At the same time, majorporphyry Cu^Au systems formed in an oceanic island arc outboard of an evolved magmatic arc that, in turn, gave riseto granite-related Sn^W deposits in the Early Silurian. During the ongoing evolution of the orogen in the LateSilurian to Early Devonian, sediment-hosted Cu^Au and Pb^Zn deposits formed in short-lived intra-arc basins,whereas a developing fore-arc system provided the conditions for the formation of several volcanogenic massivesulphide deposits. Inversion of these basins and accretion to the Australian continental margin triggered another pulseof orogenic gold mineralisation during the final consolidation of the orogenic belt in the Middle to LateDevonian. 5 2002 Elsevier Science B.V. All rights reserved.

Keywords: metallogeny; Tasman Orogenic Zone; plate tectonics; mineral deposits, genesis; plate boundaries

1. Introduction

Orogenic belts, throughout space and time, andactive collisional plate margins host a wide range

of base and precious metal deposits. The generalcharacteristics, spatial distribution and timing ofemplacement of these diverse types of mineral de-posits relate directly to the tectonic setting andgeodynamic framework [1^4]. Placing mineralisa-tion within an overall tectonic framework there-fore provides important constraints for the devel-opment of plate tectonic models for orogenic beltevolution.

The Lachlan Orogen (Fig. 1), part of the Pa-laeozoic Tasmanides orogenic system of easternAustralia, has developed by stepwise accretionof deformed oceanic sequences, volcanic arcs

0012-821X / 02 / $ ^ see front matter 5 2002 Elsevier Science B.V. All rights reserved.PII: S 0 0 1 2 - 8 2 1 X ( 0 2 ) 0 0 7 5 7 - 4

* Corresponding author. Tel. : +61-3-9905-1643;Fax: +61-3-9905-4903.E-mail address: bierlein@mail.earth.monash.edu.au

(F.P. Bierlein).

1 Present address: VIEPS, School of Earth Sciences,University of Melbourne, Melbourne, Vic. 3010, Australia.

EPSL 6296 20-8-02

Earth and Planetary Science Letters 202 (2002) 1^13

www.elsevier.com/locate/epsl

and possible micro-continents (e.g. [5^8]). The na-ture and form of the tectonic engine that droveaccretion and structural thickening to form crustof continental character are issues that are stillhotly debated (e.g. [9,10]). The nature of orogen-esis and magmatism, timing of mineralisation,and whether deformation across the LachlanOrogen was diachronous or episodic are alsocontentious [8,11^15]. Powell [16] placed theevolution of the Lachlan Orogen in a back-arcsetting, leading Fergusson and Coney [6] and,more recently, VandenBerg et al. [17] to arguethat deformation and metamorphism had to beintra-plate. Questions relate, however, to howthe deformation of the Lachlan Orogen is

transferred across the former oceanic tractand as to what processes closed the ‘back-arc’basin [10]. On structural grounds some form ofunderthrusting in a convergent plate setting is re-quired, but di¡ering vergences require a compli-cated setting [18^20]. The role of plate conver-gence in the evolution of the orogen has beenquestioned on geochemical grounds by Chappellet al. [21]. These authors argued that the evolu-tion of the orogen was driven by vertical redistri-bution of components of older crust through par-tial melting of thickened crust, in an essentiallyneutral stress regime. The palinspastic base, how-ever, after restoration/removal of deformation, ison the order of 2000 km [6], so the average of

Fig. 1. Structural trend map of the Lachlan Orogen in SE Australia, highlighting mineral deposit styles and their distribution ineach of the three sub-divisions. Tectonic vergence and implied younging of deformation indicated by arrows (modi¢ed from Grayand Foster [18]). WL=western Lachlan Orogen; CL= central Lachlan Orogen; EL= eastern Lachlan Orogen. W-OMC=Wagga^Omeo Metamorphic Complex. SZ=Stawell Zone; BZ=Bendigo Zone; MZ=Melbourne Zone; MWFZ=Mt Wellington FaultZone; TZ=Tabberabbera Zone; CB=Cobar Basin (including Elura, CSA, Queen Bee and Cobar deposits) ; KF=Kiewa Fault;GIF=Gilmore^Indi Fault; WTB=Wagga Tin Belt; S=Stawell; B=Bendigo; BT=Ballarat; M=Maldon; C=Castlemaine;SC=Steels Creek; N=Nagambie; W=Walhalla; A=Ardlethan; K=Kikoira; ML=Mammoth Lode; MM=Mt Murphy;BW=Beechworth; MW=Mt Wills; CS=Cassilis; WI=Wilga; CU=Currawong; CD=Cadia; PK=Parks; CH=Copper Hill ;LC=Lake Cowal; PH=Peak Hill ; CR=Clarkes Reef; BU=Buchan; CW=Cowarra; MC=Majors Creek; HE=Hill End;CF=Captains Flat; WL=Woodlawn.

EPSL 6296 20-8-02

F.P. Bierlein et al. / Earth and Planetary Science Letters 202 (2002) 1^132

about 70% shortening strongly argues againstthis.

This paper uses a correlation of metallogenicdeposit type with tectonic setting to support aparticular tectonic evolution in the Lachlan Oro-gen. We apply comparisons with more recent ana-logues and take into consideration recent geo-chemical and geochronological constraints ofdeformation, magmatism and mineralisation todeconstruct the orogen over time. Even if suchan attempt cannot accurately re£ect all of the im-mense geological and structural complexities ofthe orogenic system, the work presented herehas important implications for the metallogenicpotential and overall mineral endowment of theLachlan Orogen.

2. Geological framework

On the basis of rock type, metamorphic gradeand structural history, the Lachlan Orogen hasbeen separated into three distinct sub-divisions[18] (Fig. 1). The western portion of the orogenis an imbricated, chevron-folded Cambro^Devo-nian turbidite sequence cut by linear fault-

bounded belts of ophiolitic Cambrian-aged frag-ments [17,20]. Regional metamorphism is domi-nantly greenschist to sub-greenschist with rela-tively high P/T ratios [22]. The central portion isdominated by a Ordovician^Silurian, shear-bounded high-T/low-P metamorphic complexconsisting of mylonitic schists, phyllites, gneissesand granites (Wagga^Omeo Metamorphic Com-plex). The high-grade metamorphic complex isjuxtaposed with a low-grade, southwest-vergentOrdovician^Silurian turbiditic fold-thrust belt(Tabberabbera Zone). The boundary betweenthe central and the western Lachlan Orogen isde¢ned by the up to 2 km wide, poly-deformedMount Wellington Fault Zone. Deformation inresponse to the terminal collision between thewestern and the central portions of the LachlanOrogen took place at between 400 and 380 Ma,and again at V360 Ma [19]. The eastern Lachlanconsists of an imbricated, chevron-folded turbi-dite sequence of Ordovician age and abundantSilurian^Devonian granites, but is also character-ised by widespread ma¢c to felsic, Ordovician toSilurian volcanic rocks of convergent margin af-¢nity in the northern segment of the eastern sub-province [7]. The eastern sub-division is separated

Fig. 2. Sectional view of tectonic elements for southern part of the Lachlan Orogen (modi¢ed from Foster and Gray [8]) atV440^445 Ma, combining multiple subduction zone scenario envisaged for the tectonic evolution of the orogen with metallogenicsettings of the three portions of the orogen. Numbers (1, 2 and 3) refer to subduction zones as per text.

EPSL 6296 20-8-02

F.P. Bierlein et al. / Earth and Planetary Science Letters 202 (2002) 1^13 3

Table 1Characteristics of selected mineral deposits in the Lachlan Orogen, illustrating links between style of mineralisation and tectonic framework

Structuralsub-division

Deposit Style Mineralisation Category (based onproduction)

Host rock Probable tectonic setting

Western Ballarat Orogenic lode gold (metamorphic) Au World-class (s 100 t) Turbidites Fore-arc, accretionary prismBendigo Orogenic lode gold (metamorphic) Au World-class Turbidites Fore-arc, accretionary prismCastlemaine Orogenic lode gold (metamorphic) Au World-class Turbidites Fore-arc, accretionary prismStawell Orogenic lode gold (metamorphic/

magmatic)Au( UCu^Pb^Zn)

World-class Turbidites ; basalts Fore-arc, accretionary prism

Maldon Orogenic lode gold (metamorphic/magmatic)

Au( UCu^Mo^Te)

Major (s 10 t) Turbidites ; felsic dykes Accretionary prism

Walhalla Orogenic lode gold (metamorphic) Au Major Turbidites ; ma¢c^felsic dykes Fore-arc, accretionarycollisional zone

Steels Creek Orogenic lode gold (magmatic) Au ( UMo^Te) Very small (6 1t) Granite Accretionary prismNagambie Orogenic lode gold (epizonal) Au^Sb Small (1^10 t) Turbidites Accretionary prism

Central Ardlethan Intrusive-hosted Sn^W Small Granite Supra-subduction zonemagmatic arc

Kikoira Intrusive-hosted Sn^W Very small Granite Supra-subduction zonemagmatic arc

Mammoth Lode Intrusive-hosted Sn^W Very small Granite Supra-subduction zonemagmatic arc

Mt Murphy Intrusive-hosted Sn^W Very small Granite Supra-subduction zonemagmatic arc

Beechworth Intrusive-hosted Sn^W Small Granite Supra-subduction zonemagmatic arc

Mt Wills Orogenic lode gold (magmatic) Au Small Turbidites ; granite Accretionary collisional zoneCassilis Orogenic lode gold (magmatic) Au Small Turbidites ; felsic dykes Fore-arc, accretionary

collisional zoneWilga Volcanic-hosted massive sulphides Cu^ZnUAg Major (s 1 Mt Cu) Andesitic^rhyolitic volcanics;

turbiditesIntra-arc rift basin

Currawong Volcanic-hosted massive sulphides Cu^ZnUAu^Ag Major Andesitic^rhyolitic volcanics;turbidites

Intra-arc rift basin

Elura Sediment-hosted gold/base metals(epigenetic)

Pb^ZnUAg Major Siltstone Intra-arc rift basin

CSA Sediment-hosted gold/base metals(epigenetic)

Cu^Pb^ZnUAg Major Siltstone Intra-arc rift basin

Cobar Sediment-hosted gold/base metals(epigenetic)

Au^Cu Major Slate Intra-arc rift basin

Queen Bee Sediment-hosted gold/base metals(epigenetic)

Cu Small (6 1 Mt) Siltstone Intra-arc rift basin

Eastern Cadia Porphyry Cu^FeUAu Major Andesitic^shoshonitic volcanics;monzonite

Evolving oceanic island arc

Parks Porphyry Cu^Au Major Intermediate-felsic volcanics ;siltstone

Evolving oceanic island arc

EPSL

629620-8-02

F.P.Bierlein

etal./E

arthand

Planetary

Science

Letters

202(2002)

1^134

from the central portion of the orogen by theGilmore^Indi Fault, an up to 6 km wide, poly-deformed mylonitic shear zone [23]. Main folia-tion development within this steeply westerly-dip-ping, major transform fault occurred during theLate Silurian and Middle Devonian [24].

3. Metallogenic provinces in the Lachlan Orogen

The Lachlan Orogen is host to four major anddistinct metallogenic provinces [25] (Figs. 1 and2). A ¢fth metallogenic association is superim-posed on existing provinces due to periods of lo-calised extension, which was followed by inver-sion and closure of short-lived rift basins to giverise to sediment-hosted but thrust-related, epige-netic mineralisation. Each of these provinces isdominated by a speci¢c deposit type, character-istic metal parageneses and diagnostic host wall^rock alteration patterns. The dominant metal as-sociations also vary systematically across theorogen (Fig. 1 and Table 1), with styles of min-eralisation including, from west to east, turbidite-hosted orogenic lode Au ( UAs^Sb), granite-related magmatic Au (UMo^Sb^Cu^Te), sedi-ment-hosted (epigenetic) Cu^Au and Pb^Zn,granite-hosted Sn^W (+skarn^greisen), porphyryCu^Au (+Au^skarn, mantos Pb^Zn and epither-mal Au^Ag) and volcanogenic-hosted massive sul-phides.

3.1. Turbidite-hosted orogenic lode gold(western Lachlan Orogen)

The western Lachlan Orogen is host to copiousgold mineralisation, having produced in excessof 2500 t of gold since 1851 [26]. The vast major-ity of production from primary mineralisation(i.e. other than placers) has come from quartzvein-hosted, ‘gold-only’ deposits (e.g. at Stawell,Bendigo, Ballarat) that formed synchronouslywith regional metamorphism and thrusting be-tween 455 and 440 Ma, with remobilisation andsecondary phases of mineralisation occurring be-tween 420 and 400 Ma [14,20]. The auriferousveins, or ‘lodes’, occur in chevron-folded turbi-dites and are associated with reverse faults andT

able

1(C

ontinued).

Structural

sub-division

Deposit

Style

Mineralisation

Category(based

onprod

uction

)Hostrock

Proba

bletecton

icsetting

Cop

perHill

Porph

yry

Cu^

Au

Major

Dacitic^d

ioriticintrusions

Evo

lvingoceanicisland

arc

Lak

eCow

alPorph

yry

Au

Small

Interm

ediate

volcan

ics

Evo

lvingoceanicisland

arc

PeakHill

Porph

yry(epithermal)

Au

Small

Shosho

nitican

desites

Evo

lvingoceanicisland

arc

Gidginb

ung

Porph

yry

Au^

Ag

Major

Interm

ediate

volcan

ics

Evo

lvingoceanicisland

arc

Clarkes

Reef

Sediment-ho

sted

base

metals

Pb^

Zn

Small

Siltston

eIntra-arcrift

basin

Bucha

nSediment-ho

sted

base

metals

(syn

-to

epigenetic)

Cu^

Zn^

PbUAg

Small

Siltston

e;lim

estone

Intra-arcrift

basin

Woo

dlaw

nVolcanic-ho

sted

massive

sulphides

Pb^

Zn

Major

Interm

ediate-felsicvo

lcan

ics;

siltston

eInverted

fore-arc

rift

basin

Cap

tainsFlat

Volcanic-ho

sted

massive

sulphides

Cu^

Ag^Au

Major

Interm

ediate-felsicvo

lcan

ics;

siltston

eInverted

fore-arc

rift

basin

Cow

arra

Orogeniclode

gold

(metam

orph

ic/

magmatic)

Au

Small

Turbidites

Accretion

arycollision

alzone

MajorsCreek

Orogeniclode

gold

(magmatic)

Au^

Ag

Verysm

all

Granite

Accretion

arycollision

alzone

Hill

End

Orogeniclode

gold

(magmatic)

Au

Major

Volcano

genicsand

ston

eAccretion

arycollision

alzone

EPSL 6296 20-8-02

F.P. Bierlein et al. / Earth and Planetary Science Letters 202 (2002) 1^13 5

hinge zones of anticlinal structures [26,27]. Typi-cally, mineralisation is associated with broad,bleached zones of carbonate, sulphide and seri-cite, suggesting involvement of CO2-rich, aqueousore £uids [28].

The geochronological data for Au mineralisa-tion in the western Lachlan Orogen in Bierlein etal. [14] and Foster et al. [20] indicate that quartzvein-hosted gold-only deposits are unrelated tomagmatism. In contrast, a distinct style of poly-metallic gold mineralisation, commonly accompa-nied by elevated Sb, W, Mo and Cu, is associatedwith both Early Devonian and Middle to LateDevonian, ma¢c to felsic magmatism [14,15].Such occurrences post-date the emplacement offelsic dykes throughout the western portion ofthe Lachlan Orogen and are spatially (if notgenetically) related to crustal melting processes.This geochemically distinct and ‘intrusive-related’phase of gold mineralisation is economicallysubordinate to the earlier, metamorphism-asso-ciated phase of ‘orogenic’ (as per the widelyaccepted de¢nition by Groves et al. [29]) gold de-position in the Stawell and Bendigo zones, buttends to be the dominant style in the MelbourneZone. Notably, the dykes and plutons in centralVictoria post-date metamorphism-associated goldmineralisation there by as much as 80 Ma[14,20,30].

A third type of gold deposit is characterised byquartz-carbonate stockworking and disseminatedsulphides such as stibnite. This ‘epizonal’ Au^Sbstyle of mineralisation is particularly prevalent inthe eastern part of the western Lachlan Orogen(Melbourne Zone), where it is generally associatedwith, and post-dates Middle to Late Devonianfelsic intrusives [14].

In contrast to the western Lachlan Orogen, thecentral and eastern portions of the orogen arecharacterised by a striking lack of more wide-spread orogenic lode gold mineralisation. Wherepresent, the deposits can be shown to be spatiallyand temporally related to post-tectonic magma-tism. With the exception of Hill End, occurrencesof orogenic gold in the central and eastern Lach-lan Orogen (e.g. Mt Wills, Cassilis, Cowarra) arelargely minor and have produced only smallamounts of the ore [31].

3.2. Sn^W7 skarn, greisen(central Lachlan Orogen)

The metallogeny of the central Lachlan Orogenis dominated by occurrences of widespread grano-phile Sn^W^Mo mineralisation, porphyry- tovein-style Sn systems and genetically related en-dogreisens. These deposits (e.g. Ardlethan) are as-sociated with a series of north-northwest trendingI- and S-type igneous suites of Early to Late Si-lurian age which are collectively known as theWagga Tin Belt [25]. Throughout the belt, Ordo-vician-aged, tightly folded metasedimentary rockswere intruded by two distinct suites of high-leveland deeper-level granitoids during a V20 Ma in-terval between 420 and 400 Ma [32]. Mineralisa-tion typically occurs within hydrothermally brec-ciated, chlorite^sericite or tourmaline^topaz-altered granites and quartz^feldspar porphyries,with subordinate deposits also hosted within au-reoles in siliciclastic and calcareous sedimentaryrocks. The formation of high-level breccia-hostedSn mineralisation, for example, at Ardlethan isconsidered to be related to the emplacement ofhighly fractionated, late-stage granite phases,roof collapse and brecciation of overlying rocksby a CO2-rich and relatively saline £uid, and de-position of cassiterite, base metal sulphides andgangue minerals in response to pressure decreaseand unmixing of the ore-bearing £uid [33].

3.3. Porphyry Cu^Au (eastern Lachlan Orogen)

Several major porphyry Cu^Au deposits (e.g.Cadia, Copper Hill, Parkes, Peak Hill, Lake Cow-al), as well as associated Au^skarns and epither-mal Au^Ag systems are both spatially and genet-ically associated with ma¢c to intermediatevolcanics and related intrusives in the northernpart of the eastern Lachlan Orogen (Fig. 1). Thesedeposits formed during one or several peaks ofmagmatic activity between 440 and 420 Ma [32].The fault-bounded volcano-intrusive belts are rel-atively undeformed and have been metamor-phosed to lower greenschist facies. In many in-stances, emplacement of the alkaline intrusiveswas facilitated and localised by the developmentof dilation zones. Mineralisation, for example at

EPSL 6296 20-8-02

F.P. Bierlein et al. / Earth and Planetary Science Letters 202 (2002) 1^136

Cadia and Parks, is hosted by Late Ordovicianvolcanics and sub-volcanic intrusions of basalticandesite to shoshonite composition [34,35]. Thecopper sulphides and gold at Cadia are sited with-in, as well as forming haloes of disseminated orearound, sheeted, narrow quartz ( U chlorite^car-bonate) veins. In the Endeavour deposits nearParks, a bornite-dominated quartz stockwork iscentred on vertical, ¢nger-like monzonitic por-phyry intrusions. The porphyry alteration^miner-alisation systems are dominated by weak to per-vasive propylitic metasomatism, with subordinatepotassic, phyllic and carbonate alteration also de-veloped. Genesis of the porphyry systems and as-sociated epithermal Au^Ag deposits is consideredto be related to the late magmatic^early hydro-thermal exsolution from crystallisation of frac-tionated, alkaline and high-level intrusives duringthe Late Ordovician to Early Silurian. Heithersayand Walshe [34] suggested that mineralisation inthe Endeavour deposits was driven by the mixingof highly saline ‘ortho-magmatic’ and evolvedmagmatic £uids at temperatures between 500and 800‡C. Concurrent hydrothermal alterationand mineralisation of impure limestones inter-bedded with volcaniclastic sandstones resulted inthe formation of hematite^magnetite^chalcopyritecopper^gold skarns, for example at Big and LittleCadia [34,36].

3.4. Volcanogenic^exhalative sulphides(eastern Lachlan Orogen)

Several inverted rift basins in the central andsouthern parts of the eastern Lachlan Orogenare host to stratabound, Kuroko-style volcano-genic^exhalative Pb^Zn^Ag sulphide deposits(e.g. Captains Flat, Woodlawn), as well as genet-ically related sub-sea£oor replacement- and skarn-style mineralisation. The deposits are hosted in avolcano-sedimentary sequence consisting of calc-alkaline volcanics (rhyolites, andesites), siliciclas-tics, limestones and tu¡aceous sediments [37]. Theonset of rifting during the Middle to Late Silurianresulted in widespread volcanism along a series ofnorth-trending normal faults. Although the timingof mineralisation remains controversial (e.g. [38]),similarities between most of the volcanogenic^ex-

halative deposits regarding stratigraphic position,association with (inverted) growth faults and oreparagenesis strongly suggest that the depositsformed concurrently with the deposition of dom-inantly marine volcanoclastics and tu¡aceous£ows, close to the hiatus of rift-related volcanicactivity during the Late Silurian. The larger de-posits (e.g. Woodlawn) consist of several closelyspaced lenses of massive sulphides (predominantlypyrite, sphalerite, galena, chalcopyrite), withstratigraphic bands of mineralisation, stockworksystems, stringers and disseminated ore accumu-lations also present in many deposits. Character-istic footwall and hangingwall alteration sequen-ces are well developed and include silici¢cation,sericitisation, chloritisation and sulphidation ofthe wallrocks.

3.5. Rift basin-related sediment-hosted Cu^Pb^Zn(central and eastern Lachlan Orogen)

Several thrust/reverse fault-bounded, sedimen-tary basins of Silurian to Devonian age that aresituated in both the northern and southern por-tion of the central, and also in the southern partof the eastern Lachlan Orogen contain substantialCu^Au and Pb^Zn mineralisation [25,38,39].These deposits (e.g. Wilga, Currawong, Cobar de-posits, Clarkes Reef) are hosted in volcano^sedi-mentary sequences and characterised by basemetal sulphide^magnetite^pyrite^pyrrhotite^goldassemblages in stringer veins, stockwork systemsand massive sulphide lenses. The bulk of mineral-isation is localised along faults near or at basinmargins and displays crosscutting relationshipswith stratigraphy and evidence for remobilisation.Alteration in these deposits is dominated by per-vasive silici¢cation and chloritisation, with vari-able degrees of sericite, carbonate and Fe-oxidealteration. Considerable uncertainly remains re-garding the timing of mineralisation, but strati-graphic, structural and geochronological con-straints strongly favour a syn-deformational (i.e.epigenetic) origin of these base metal deposits inrift basins of the central and eastern Lachlan Oro-gen. As argued recently by Stegman [39] for theCobar deposits, metal deposition was probablycontrolled by depressurisation, £uid mixing and

EPSL 6296 20-8-02

F.P. Bierlein et al. / Earth and Planetary Science Letters 202 (2002) 1^13 7

fault-valve behaviour along active faults zonesduring compressional tectonics.

4. Metallogeny and geodynamic settings

Independent of the Lachlan Orogen, each ofthe ¢ve metallogenic associations has been linkedto speci¢c tectonic and/or geodynamic settings.The diagnostic features of these associations arebrie£y summarised in the following:

b Orogenic lode gold deposits have been equatedwith sub- to medium-grade greenschist metamor-phism during structural thickening and accretionat active continental margins involving subduc-tion^underthrusting [29]. Modern accretionarysettings that host major orogenic lode gold depos-its are developed in the North American Cordil-lera of SE Alaska, the Mongol^Okhotsk SutureZone in Far East Russia, the Sierra de Rinconadain NW Argentina and the Buller Terrane andOtago Schist in New Zealand [40]. All of thesesettings are characterised by relatively low geo-thermal gradients in keeping with low-T/high-Pconditions typically associated with subduction^accretion settings. Subduction of hydrated oceaniccrust and crustal delamination processes are con-sidered crucial in triggering crustal melting pro-cesses via the transfer of thermal energy from themantle into the overlying crust [41]. Phanerozoicorogenic gold deposits are primarily hosted in sev-eral kilometre-thick sequences of marine sedimen-tary rocks which accumulated on pre-collisioncontinental margins and/or in prograding arc^trench complexes. The sedimentary successionsare commonly underlain by imbricated (structur-ally thickened) ma¢c oceanic crust related tospreading, arc formation, plate collision and sub-duction.

b Sn^W greisen deposits, skarns and associatedAg^Bi mineralisation develop in highly di¡erenti-ated granites that have been emplaced in evolvedcontinental margin arcs, inboard of associatedporphyry Cu^Au systems and above the deepersections of the Benio¡ Zone [42]. The host intru-sives (e.g. tin provinces of Alaska, China, Malay-sia, Chile, central Europe) are both S- and I-typegranites, possibly representing anatectic melts that

derived their tin from recycled continental crustand igneous processes following the closure of anoceanic basin [43].

b Porphyry Cu^Au deposits worldwide conformto a generalised descriptive model (e.g. [44]) in-volving their generation at convergent plateboundaries during or immediately following thesubduction of oceanic lithosphere in Cordilleran-type arc and island arc settings. Most deposits,such as those in Chile, Peru and the Philippines,are associated with subduction-related volcano-in-trusive arcs. These occurrences are also character-ised by the presence of associated epithermal Au^Ag systems. Periods of extension within the over-all compressive regime above shallow-dippinglithospheric slabs (e.g. Bingham) can enable thegeneration of Au^Cu porphyry systems in back-arc settings. In response to variations in the re-gional setting (e.g. arc^continent collision, sub-duction of an aseismic ridge), the volcano-intru-sive host rocks span a range of compositions fromlow-K calc-alkaline diorites, through high-Kquartz monzonites to alkaline syenites, monzo-nites and shoshonites [44].

b Volcanogenic^exhalative sulphides of all agesoccur in tectonic settings associated with riftedarcs, inter-arc rift volcanic basins and subduc-tion-related back-arc basins (e.g. [45]). The hostrock sequences typically comprise calc-alkaline,basaltic to rhyolitic volcanics, associated tu¡sand volcanic breccias that were deposited in asub-aerial or shallow-depth to deep submarine en-vironment. Variations in the above result in aspectrum of volcanogenic^exhalative sulphide de-posits, with metal associations ranging from Zn-rich polymetallic through pyritic Cu^Au to Au-only deposits [45,46]. Formation of these depositsduring the Palaeozoic peaked worldwide in theCambro^Ordovician and Silurian. In all cases(e.g. Mt Morgan, Rosebery, Kuroko), mineralisa-tion formed as synvolcanic sea£oor depositsabove hydrothermal vents (cf. black smokers atactive oceanic rises) or as synvolcanic, sub-sea-£oor replacement deposits.

b Rift basin-related sediment-hosted Cu^Pb^Znmineralisation. Although these deposits have a⁄n-ities with volcanic-hosted massive sulphide depos-its and Mississippi Valley-type deposits, rift basin-

EPSL 6296 20-8-02

F.P. Bierlein et al. / Earth and Planetary Science Letters 202 (2002) 1^138

related base metal deposits di¡er in that they oc-cur in sedimentary graben structures that devel-oped in intra-arc and intra-continental settings,possibly during periods of extension and/or trans-pressional deformation. The deposits are distin-guished from SEDEX deposits by their metal as-sociations and higher temperatures of oreformation [39]. Yet there is no general agreementas to the exact timing of ore genesis which mayrange from syngenetic to epigenetic, and conse-quently, the style of mineralisation in these occur-rences cannot be regarded diagnostic of a partic-ular setting without taking into account othergeological-structural and tectonic considerations.

5. Evolution of the Lachlan Orogen ^ tectonicimplications from metallogenesis

The distinct metallogenic provinces outlined inthis paper are di⁄cult to reconcile with geody-namic models for the Lachlan Orogen based ei-ther on simple single plate or intra-plate tectonics,

or the migration of a deformation front that wascontrolled by mid-crustal plutonism in a neutralstress regime. Neither of these models can fullyexplain the spatial and temporal distribution ofmineral deposits within the Lachlan Orogen.Moreover, there are no known analogues to sup-port the feasibility of a genetic link between the¢ve metallogenic provinces observed in the Lach-lan Orogen and single plate or intra-plate settings.Based on the established links between metallo-genic associations and speci¢c geodynamic set-tings (cf. Section 4), we argue that the metallo-genic provinces in the Lachlan Orogen stronglysupport a tectonic framework whereby the oro-genic system developed within a complex oceanicaccretion^subduction setting not unlike the south-west Paci¢c today (Figs. 2 and 3).

To generate a world-class orogenic lode goldprovince in the western portion of the LachlanOrogen, some form of accretionary setting involv-ing subduction (subduction zone 1, Fig. 2) wasrequired, possibly similar to the present-day Alas-kan margin but perhaps much smaller in scale. In

Fig. 3. Interpretative plate tectonic evolution of the Lachlan Orogen incorporating con¢guration and metallogenesis of the tecton-ic segments for the Middle Ordovician to Early Silurian (450^430 Ma), Late Silurian to Early Devonian (420^400 Ma) and Mid-dle to Late Devonian (380^360 Ma). WL=western Lachlan Orogen; CL= central Lachlan Orogen; EL= eastern Lachlan Oro-gen.

EPSL 6296 20-8-02

F.P. Bierlein et al. / Earth and Planetary Science Letters 202 (2002) 1^13 9

such a scenario, structural thickening of a sedi-ment wedge above oceanic crust occurs due tounderthrusting in an oceanic environment (cf.[40,47]). This process leads to the generation oflarge volumes of metamorphically derived £uidsduring devolatisation of a hydrated successionwithin the internally heated, thickened crust[47,48]. Structural and geochronological evidencesuggests that the generation of a major orogeniclode gold system in Phanerozoic orogens is fa-voured by episodic in¢ltration of metamorpho-genic £uids over protracted periods of time, thusre£ecting the prolonged nature of deformation(‘tectonic pulses’) in an evolving accretionarywedge that is undergoing active erosion duringprogressive deformation [14,40,47] (cf. Section3.1).

In the western Lachlan Orogen, structuralthickening has been related to the eastward prop-agation of a tiered oceanic thrust system devel-oped within the lower part of the turbidite pile.This led to closure of a marginal oceanic basin vialow-angle underthrusting [18] with possible colli-sion with an intervening continental platform [17](Fig. 3, middle). The presence of fault-boundedbelts of ophiolitic Cambrian-aged fragments[17,19] and relatively high P/T ratios of the dom-inantly greenschist to sub-greenschist regionalmetamorphism support this setting [22].

The absence of steep-angled subduction, hydra-tion of the mantle wedge, and therefore associatedcrustal melting during the Silurian, would pro-duce a time di¡erential between prograde meta-morphism at shallow versus deep crustal levels inthe western Lachlan Orogen by as much as 50 Ma(inferred recently by Bierlein et al. [14]). This isthe observed di¡erence between Late Ordovicianhydrothermal alteration and Early Devonianmagmatism. Arguments, however, have been pre-sented against a tectonic setting involving subduc-tion; these include the lack of a volcanic arc, thelack of distinctive shallow level accretionarywedge features (e.g. me¤langes) and lack of volca-niclastic sediment (for discussion see [8^10]).

The Sn^W^Mo-bearing granites of the high-T/low-P Wagga^Omeo Metamorphic Complex rep-resent the deeper, more geochemically evolvedlevels of a supra-subduction environment [2] (sub-

duction zone 2, Figs. 2 and 3). Although the tec-tonic setting and evolution of the granites contin-ue to be controversial (compare, for example,Chappell et al. [21] with Collins [49]) the highgeothermal gradient recorded in the central Lach-lan Orogen [50], the elongate nature of thebatholiths [18], the Pb^Pb and Sm^Nd signatures[51,52] and their geochemical signatures are allconsistent with the formation of these intrusivesin crust above a subduction slab with some con-tributions from the asthenospheric wedge.

Multi-component mixing, with varying propor-tions of recycled Ordovician sediments, oceaniccrust of the Lachlan basement and subductionzone mantle contributions incorporated into themelt, resulted in the end-member ma¢c to felsic,I- and S-type character of the igneous suites [49].Unroo¢ng and removal of the volcanic edi¢ce andstrata during the Middle to Late Silurian[17,19,50] could account for the general lack ofdeposit types in the central Lachlan Orogen thatare broadly associated with magmatic and hydro-thermal processes at shallow levels in an di¡er-entiated arc system.

Concomitant regional extension and intra-arcrifting in response to periods of transpressionand strike-slip movement along major regionalfaults (e.g. Kiewa and Gilmore^Indi fault sys-tems), associated with magmatic ‘£are-ups’, dur-ing the Late Silurian and Early Devonian prob-ably produced the sediment-hosted (but epigenetic[38]) Cu^Pb^Zn mineralisation in short-lived riftbasins, such as the Cobar Basin.

The porphyry Cu^Au deposits (north-centralNSW) are associated with a now structurally seg-mented Ordovician arc system related to the lon-ger-lived subduction system outboard of Gond-wana. ‘Primitive’ lead isotopic signatures pointto a largely mantle-derived source for the oresand their corresponding host rocks [51] and areconsistent with their formation during the calc-alkaline stage of an evolving oceanic island^arcsystem [53]. This is related to westward subduc-tion underneath, but outboard of, the ‘pre’-Wa-gga^Omeo Metamorphic Complex (subductionzone 3, Figs. 2 and 3) during the Ordovician toEarly Silurian.

Rifting in the southwestern part of the eastern

EPSL 6296 20-8-02

F.P. Bierlein et al. / Earth and Planetary Science Letters 202 (2002) 1^1310

province, east of the metamorphic complex, couldhave been due to transpression at about 410^400 Ma [19] or progressive roll-back of the under-thrusted oceanic crust towards the east [16,54].Whatever the cause, various parts of the fore-arcregion underwent rifting and rapid subsidence inLate Silurian to Devonian time [55]. The prevail-ing geological and structural conditions in thisextensional setting created the ideal environmentfor the formation of Kuroko-style volcanogenic^exhalative sulphide mineralisation. Althoughthese deposits were clearly a¡ected by structuraloverprinting, during subsequent inversion and de-formation (e.g. [38]), geochronological and radio-metric constraints reported in Carr et al. [51] alsofavour a model where initial ore formation oc-curred contemporaneously with the developmentof the transient rift basins in the Late Silurian toEarly Devonian.

Understanding the nature and distribution ofthe various gold deposits in the Lachlan Orogen(Fig. 1) also provides important tectonic con-straints. The close association of the orogeniclode gold deposits with meta-sedimentary rocksabove ‘primitive’ oceanic crust in the majority ofmajor lode gold provinces, including the westernportion of the Lachlan Orogen, implies that theserock sequences play an important role in the ore-forming process. Granites can be ruled out aspossible source rocks for gold-only deposits, ex-cept where complex gold UMo^Te^Cu^Sb^Bi^Wparageneses point to a magmatic £uid contribu-tion. In the eastern portion of the Lachlan Oro-gen, and in contrast to the western portion of theorogen, contractional deformation was governedby the diachronous closure of several sub-basinsof rather limited extent. These basins developedon thickened crust of oceanic arc origin that hadalso been punctuated by voluminous granite in-trusions during extensional tectonics. These con-ditions are generally considered unsuitable for theformation of orogenic lode gold deposits as theydo not favour the release of massive £uid volumesfrom underplated hydrated rocks within a rela-tively short time [40]. Similarly, P^T^t paths ofrocks exposed in the Wagga^Omeo MetamorphicComplex as calculated by Morand and Gray [50]appear to have higher T/P ratios than those suit-

able to generate lode gold occurrences. This isstrikingly di¡erent when compared to the low-grade metamorphic-dominated peripheral colli-sional orogen accretionary wedge that developedto the west.

6. Conclusions

The presence of several distinct metallogenicassociations across the Palaeozoic Lachlan Oro-gen in SE Australia strongly support a tectonicframework whereby the orogen developed withina complex oceanic accretion^subduction setting.Within the larger collisional framework appliedhere, litho-tectonic segments include, from westto east, a Cambro^Devonian accretionary wedgethat was undergoing active erosion during pro-gressive deformation, an Early Silurian magmaticarc, an Ordovician oceanic island arc and a Silu-rian developing fore-arc system. The ore-formingprocesses that operated in each segment of theorogen were identical to those responsible forthe formation of magmatic^hydrothermal ore de-posits in present-day analogues such as the volca-no-plutonic belts of the circum-Paci¢c. Identifyingthese commonalities in analogous terrains has im-plications for the assessment of fertility and over-all exploration potential of the Lachlan Orogenand demonstrates the usefulness of linking metal-logenic associations to the interpretative geody-namic framework of an orogenic system understudy.

Acknowledgements

We are grateful to D. Arne, S. Maher, R. Glen,D. Taylor, R. Cayley, C. Willman and W. Collinsfor input and many fruitful discussions on thesubject of this paper. P. Cawood and J. Walsheare thanked for providing formal reviews andcomments that helped clarifying the ideas ex-pressed herein. F.P.B. acknowledges the supportof a Monash University Logan Research Fellow-ship and a 2001 Monash University Small Grant.Funding for D.A.F. was provided by NSF GrantEAR 0073638.[RV]

EPSL 6296 20-8-02

F.P. Bierlein et al. / Earth and Planetary Science Letters 202 (2002) 1^13 11

References

[1] F.J. Sawkins, Metal Deposits in Relation to Plate Tecton-ics, Springer, Berlin, 1984, 261 pp.

[2] E.P. Nelson Suprasubduction mineralization: Metallo-tec-tonic terranes of the southernmost Andes, in: Subduc-tion: Top to Bottom, Am. Geophys. Union Monogr. 96(1996) 315^330.

[3] R. Kerrich, R.J. Goldfarb, D.I. Groves, S. Garwin, Thegeodynamics of world-class gold deposits: Characteristics,space-time distributions, and origins, in: S.G. Hagemann,P.E. Brown (Eds.), Gold in 2000, Soc. Econ. Geol. Rev.Econ. Geol. 13 (2000) 501^551.

[4] M. Solomon, D.I. Groves, The Geology and Origins ofAustralia’s Mineral Deposits, Oxford University Press,New York, 1994, 951 pp.

[5] P.J. Coney, The Lachlan fold belt of eastern Australianand Circum-Paci¢c tectonic evolution, Tectonophysics214 (1992) 1^25.

[6] C.L. Fergusson, P.J. Coney, Convergence and intraplatedeformation in the Lachlan fold belt of southeastern Aus-tralia, Tectonophysics 214 (1992) 417^439.

[7] D.R. Gray, Tectonics of the southeastern AustralianLachlan fold belt : Structural and thermal aspects, in:J.P. Burg, M. Ford (Eds.), Orogeny through Time,Geol. Soc. Spec. Publ. 121 (1997) 149-177.

[8] D.A. Foster, D.R. Gray, The structure and evolution ofthe Lachlan Fold Belt (Orogen) of eastern Australia,Annu. Rev. Earth Planet. Sci. 28 (2000) 47^80.

[9] D.H. Taylor, R.A. Cayley, Character and kinematics offaults within the turbidite-dominated Lachlan Orogen:implications for tectonic evolution of eastern Australia:Discussion, J. Struct. Geol. 22 (2000) 523^528.

[10] D.R. Gray, D.A. Foster, Character and kinematics offaults within the turbidite-dominated Lachlan orogen:Implications for the tectonic evolution of eastern Austra-lia. Reply, J. Struct. Geol. 20 (2000) 1691^1720.

[11] A. Soesoo, I.A. Nicholls, Ma¢c rocks spatially associatedwith Devonian felsic intrusions of the southern LachlanFold Belt: a possible mantle contribution to crustal evo-lution processes, Aust. J. Earth Sci. 46 (1999) 725^734.

[12] A.H.M. VandenBerg, Timing of orogenic events in theLachlan Orogen, Aust. J. Earth Sci. 46 (1999) 691^702.

[13] A.H.M. VandenBerg, Timing of orogenic events in theLachlan Orogen: Reply, Aust. J. Earth Sci. 47 (2000)818^822.

[14] F.P. Bierlein, D.C. Arne, D.A. Foster, P.R. Reynolds, Ageochronological framework for orogenic gold mineralisa-tion in central Victoria, Australia, Miner. Depos. 36(2001) 741^767.

[15] F.P. Bierlein, M. Hughes, J. Dunphy, S. McKnight, P.R.Reynolds, H.M. Waldron, Trace element geochemistry,40Ar/39Ar ages, Sm-Nd systematics and tectonic implica-tions of ma¢c^intermediate dykes associated with orogen-ic lode gold mineralisation in central Victoria, Australia,Lithos 58 (2001) 1^31.

[16] C.McA. Powell, Tectonic relationship between the LateOrdovician and Late Silurian palaeogeographies of south-eastern Australia, J. Geol. Soc. Aust. 30 (1983) 353^373.

[17] A.H.M. VandenBerg, C.E. Willman, S. Maher, B.A. Si-mons, R.A. Cayley, V.J. Morand, D.H. Taylor, D.Moore, A. Radojkovic, The Tasman Fold Belt Systemin Victoria, Geol. Surv. Vic. Spec. Publ., 2000, 462 pp.

[18] D.R. Gray, D.A. Foster, Orogenic concepts ^ applicationand de¢nition: Lachlan fold belt, eastern Australia, Am.J. Sci. 297 (1997) 859^891.

[19] D.A. Foster, D.R. Gray, M. Bucher, Chronology of de-formation within the turbidite-dominated Lachlan oro-gen: Implications for the tectonic evolution of easternAustralia and Gondwana, Tectonics 18 (1999) 452^485.

[20] D.A. Foster, D.R. Gray, T.A.P. Kwak, M. Bucher, Chro-nology and orogenic framework of turbidite hosted golddeposits in the Western Lachlan Fold Belt, Victoria: 40Ar/39Ar results, Ore Geol. Rev. 13 (1998) 229^250.

[21] B.W. Chappell, A.J.R. White, I.S. Williams, D. Wyborn,L.A.I. Wyborn, Lachlan Fold Belt granites revisited:high- and low-temperature granites and their implications,Aust. J. Earth Sci. 47 (2000) 123^138.

[22] R. O¥er, S.W. McKnight, V. Morand, Tectono-thermalhistory of the western Lachlan Fold Belt ^ insights fromwhite mica studies, J. Metamorph. Geol. 16 (1998) 531^540.

[23] K.A.W. Crook, C. McA. Powell, The evolution of thesoutheastern part of the Tasman Geosyncline, FieldGuide for Excursion 17A, 25th International GeologicalCongress, VVV International, Sydney, 1976, 122 pp.

[24] P.G. Stuart-Smith, The Gilmore fault zone ^ the defor-mation history of a possible terrane boundary within theLachlan fold belt, New South Wales, BMR J. Aust. Geol.Geophys. 12 (1991) 35^50.

[25] J.L. Walshe, P.S. Heithersay, G.W. Morrison, Toward anunderstanding of the metallogeny of the Tasman FoldBelt System, Econ. Geol. 90 (1995) 1382^1401.

[26] W.H.R. Ramsay, F.P. Bierlein, D.C. Arne, A.H.M. Van-denBerg, A review of turbidite-hosted gold deposits, Cen-tral Victoria: Regional setting, styles of mineralisationand genetic constraints, Ore Geol. Rev. 13 (1998) 131^151.

[27] S.F. Cox, V.J. Wall, M.A. Etheridge, T.F. Potter, Defor-mational and metamorphic processes in the formation ofmesothermal vein-hosted gold deposits ^ Examples fromthe Lachlan fold belt in central Victoria, Australia, OreGeol. Rev. 6 (1991) 391^423.

[28] F.P. Bierlein, D.C. Arne, S. McKnight, J. Lu, S. Reeves,J. Besanko, J. Marek, D. Cooke, Wallrock petrology andgeochemistry in alteration haloes associated with meso-thermal gold mineralization, central Victoria, Australia,Econ. Geol. 95 (2000) 283^312.

[29] D.I. Groves, R.J. Goldfarb, M. Gebre-Mariam, S. Hage-mann, F. Robert, Orogenic gold deposits: A proposedclassi¢cation in the context of their crustal distributionand relationship to other gold deposit types, Ore Geol.Rev. 13 (1998) 7^27.

EPSL 6296 20-8-02

F.P. Bierlein et al. / Earth and Planetary Science Letters 202 (2002) 1^1312

[30] D.C. Arne, F.P. Bierlein, N.J. McNaughton, C.J.L. Wil-son, V. Morand, Absolute timing of gold mineralisationin Central Victoria: New constraints from SHRIMP IIanalysis of zircon grains from felsic intrusive rocks, OreGeol. Rev. 13 (1998) 251^273.

[31] R. Woodall, Gold in Australia, in: F.E. Hughes (Ed.),Geology of the Mineral Deposits of Australia and PapuaNew Guinea, Australasian Institute of Mining and Metal-lurgy, 1990, pp. 45^67.

[32] C. Perkins, J.L. Walshe, G. Morrison, Metallogenic epi-sodes of the Tasman Fold Belt System, Eastern Australia,Econ. Geol. 90 (1995) 1443^1466.

[33] S.K. Ren, J.L. Walshe, R.G. Paterson, R.A. Both, A.Andrew, Magmatic and hydrothermal history of the por-phyry-style deposits of the Ardlethan tin ¢eld, New SouthWales, Australia, Econ. Geol. 90 (1995) 1620^1645.

[34] P.S. Heithersay, J.L. Walshe, Endeavour 26 North: Aporphyry copper-gold deposit in the Late Ordovician,shoshonitic Goonumbla volcanic complex, New SouthWales, Australia, Econ. Geol. 90 (1995) 1506^1532.

[35] J.R. Holliday, A.J. Wilson, P.L. Blevin, I.J. Tedder, P.D.Dunham, M. P¢tzner, Porphyry gold-copper mineralisa-tion in the Cadia district, eastern Lachlan Fold Belt, NewSouth Wales, and its relationship to shoshonitic magma-tism, Miner. Depos. 37 (2002) 100^116.

[36] Newcrest Mining Sta¡, Cadia gold-copper deposit, in:D.A. Berkman, D.H. Mackenzie (Eds.), Geology of Aus-tralian and Papua New Guinean Mineral Deposits, TheAustralasian Institute of Mining and Metallurgy Mono-graph 22, 1998, pp. 641^646.

[37] L.W. Davis, Silver-lead-zinc-copper mineralisation in theCaptains Flat - Goulburn Synclinorial Zone and the HillEnd Synclinorial Zone, in: F.E. Hughes (Ed.), Geology ofthe Mineral Deposits of Australia and Papua New Guin-ea, Australasian Institute of Mining and Metallurgy,1990, pp. 1375^1384.

[38] R.A. Glen, Thrusts and thrust-associated mineralizationin the Lachlan orogen, Econ. Geol. 90 (1995) 1402^1429.

[39] C.L. Stegman, Cobar deposits: still defying classi¢cation,Soc. Econ. Geol. Newsl. 44 (2001) 1 and 15^26.

[40] F.P. Bierlein, D.E. Crowe, Phanerozoic orogenic lodegold deposits, in: S.G. Hagemann, P.E. Brown (Eds.),Gold in 2000, Soc. Econ. Geol. Rev. Econ. Geol. 13(2000) 103^139.

[41] W.J. Collins, R.H. Vernon, Palaeozoic arc growth, defor-mation and migration across the Lachlan Fold Belt,southeastern Australia, Tectonophysics 214 (1992) 381^400.

[42] J.F. Davies, Some temporal-spatial aspects of NorthAmerican porphyry systems, Econ. Geol. 84 (1989)2300^2306.

[43] H.-J. Fo«rster, G. Tischendorf, R.B. Trumbull, B. Gottes-mann, Late-collisional granites in the Variscan Erzge-birge, Germany, J. Petrol. 40 (1999) 1613^1645.

[44] R.H. Sillitoe, Gold-rich porphyry deposits: Descriptiveand genetic models and their role in exploration and dis-covery, in: S.G. Hagemann, P.E. Brown (Eds.), Gold in2000, Soc. Econ. Geol. Rev. Econ. Geol. 13 (2000) 315^345.

[45] R.R. Large, Australian volcanic hosted massive sulphidedeposits : Features, styles and genetic models, Econ. Geol.87 (1992) 471^510.

[46] R.R. Large, J. McPhie, J.B. Gemmell, W. Herrmann, G.J.Davidson, The spectrum of ore deposit types, volcanicenvironments, alteration halos, and related explorationvectors in submarine volcanic successions: Some examplesfrom Australia, Econ. Geol. 96 (2001) 913^938.

[47] R.J. Goldfarb, L.W. Snee, L.D. Miller, R.J. Newberry,Rapid dewatering of the crust deduced from ages of meso-thermal gold deposits, Nature 354 (1991) 296^298.

[48] K. Stu«we, Tectonic constraints on the timing relationshipsof metamorphism, £uid production and gold-bearingquartz vein emplacement, Ore Geol. Rev. 13 (1998) 219^228.

[49] W.J. Collins, Evaluation of petrogenetic models for Lach-lan Fold Belt granitoids: implications for crustal architec-ture and tectonic models, Aust. J. Earth Sci. 45 (1998)483^500.

[50] V.J. Morand, D.R. Gray, Major fault zones related to theOmeo Metamorphic Complex, northeastern Victoria,Aust. J. Earth Sci. 38 (1991) 203^221.

[51] G.R. Carr, J.A. Dean, D.W. Suppel, P.S. Heithersay, Pre-cise lead isotope ¢ngerprinting of hydrothermal activityassociated with Ordovician to Carboniferous metallogenicevents in the Lachlan Fold Belt of New South Wales,Econ. Geol. 90 (1995) 1467^1505.

[52] J.L. Walshe, D.J. Whitford, S.-S. Sun, The origin of theWagga Tin Belt, and the Ardlethan porphyry style tinmineralisation, in: P.K. Seccombe, P.M. Ashley (Eds.),CSIRO Centre for Isotope Studies Research Report1993^1994, 1995, pp. 53^55.

[53] R.A. Glen, J.L. Walshe, L.M. Barron, J.J. Watkins, Or-dovician convergent-margin volcanism and tectonism inthe Lachlan sector of Gondwana, Geology 26 (1998)751^754.

[54] K.A. Dadd, Incipient backarc magmatism in the SilurianTumut Trough, New South Wales: an ancient analogueof the early Lau Basin, Aust. J. Earth Sci. 45 (1998) 109^121.

[55] E. Scheibner, A plate tectonic model of the Palaeozoictectonic history of New South Wales, J. Geol. Soc.Aust. 20 (1974) 405^426.

EPSL 6296 20-8-02

F.P. Bierlein et al. / Earth and Planetary Science Letters 202 (2002) 1^13 13