ORIGINAL PAPER

The composition of garnet in garnet-rich rocks in the southernProterozoic Curnamona Province, Australia: an indicatorof the premetamorphic physicochemical conditions of formation

Adriana Heimann & Paul G. Spry & Graham S. Teale &

Colin H. H. Conor & Norman J. Pearson

Received: 9 November 2008 /Accepted: 7 September 2010 /Published online: 6 October 2010# Springer-Verlag 2010

Abstract Garnet-rich rocks occur throughout the Prote-rozoic southern Curnamona Province, Australia, wherethey are, in places, spatially related to Broken Hill-typePb-Zn-Ag deposits. Fine-scale bedding in these rocks,their conformable relationship with enclosing metasedi-mentary rocks, and their enrichment in Mn and Fesuggest that they are metamorphosed chemical precip-itates. They formed on the floor of a 1.69 Ga continentalrift basin from hydrothermal fluids mixed with seawaterand detritus. Garnet in garnet-quartz and garnet-amphibole rocks is generally light rare earth element

(LREE) depleted, and has flat heavy REE (HREE)enriched chondrite-normalized REE patterns, and nega-tive Eu anomalies (Eu/Eu*<1). Garnet in garnet-richrocks from the giant Broken Hill deposit has similar REEpatterns and either positive (Eu/Eu*>1) or negative Euanomalies. Manganese- and Mn-Ca-rich, Fe-poor garnets ingarnetite, garnet-hedenbergite, and garnet-cummingtoniterocks at Broken Hill have Eu/Eu*>1, whereas garnet in Mn-poor, Fe-rich quartz garnetite and quartz-garnet-gahnite rocksfrom Broken Hill, and quartz garnetite from other locationshave Eu/Eu*<1. The REE patterns of garnet and its host rockand interelement correlations among REEs and major elementcontents in garnet and its host rock indicate that the Euanomaly in garnet reflects that of its host rock and isrelated to the major element composition of garnet andits host rock. The value of Eu/Eu* in garnet is related toits Mn, Fe, and Ca content and that of its host rock, andthe distribution of REEs among garnet and accessoryphases (e.g., feldspar). Positive Eu anomalies reflect highamounts of Eu that was preferentially incorporated intoMn- and Mn-Ca-rich oxides and carbonates in theprotolith. In contrast, Eu/Eu*<1 indicates the preferentialdiscrimination against Eu by Fe-rich, Mn-poor precursorminerals. Precursors to Mn-rich garnets at Broken Hillformed by precipitation from cooler and more oxidizedhydrothermal fluids compared to those that formedprecursors to Mn-poor, Fe-rich garnet at Broken Hilland the other locations. Garnet from the Broken Hilldeposit is enriched in Zn (> 400 ppm), Cr (> 140 ppm),and Eu (up to 6 ppm and positive Eu anomalies), anddepleted in Co, Ti, and Y compared to garnet in garnet-rich rocks from other localities. These values, as well asMnO contents >15 wt. % and Eu/Eu*>1 are only foundat the Broken Hill deposit and are good indicators of thepresence of Broken Hill-type mineralization.

Editorial handling: J. Raith

Electronic supplementary material The online version of this article(doi:10.1007/s00710-010-0130-x) contains supplementary material,which is available to authorized users.

A. Heimann : P. G. SpryDepartment of Geological and Atmospheric Sciences, Iowa StateUniversity,Ames, IA, USA

A. Heimann (*)Department of Geological Sciences, East Carolina University,101 Graham Building,Greenville, NC 27858, USAe-mail: heimanna@ecu.edu

G. S. TealeTeale & Associates,P.O. Box 740, North Adelaide, South Australia 5006, Australia

C. H. H. ConorGeological Survey Branch, Primary Industries and ResourcesSouth Australia,GPO Box 1671, Adelaide, South Australia 5001, Australia

N. J. PearsonGemoc Centre, Macquarie University,New South Wales 2109, Australia

Miner Petrol (2011) 101:49–74DOI 10.1007/s00710-010-0130-x

Introduction

Trace element compositions of individual minerals revealdetails of genetic processes in modern and ancienthydrothermal systems (e.g., Bach et al. 2003; Bau et al.2003). In particular, rare earth element (REE) and traceelement studies of garnet have been used to understand: theevolution of the mantle and melt-rock interactions (e.g.,Schröter et al. 2004; Scully et al. 2004), the origin of skarnsand metasomatic processes (Smith et al. 2004; Gaspar et al.2008), metamorphic and volcanic processes (e.g., Irvingand Frey 1978; O’Reilly and Griffin 1995; Pyle and Spear2000), and geochronological problems (e.g., Hensen andZhou 1995). Trace element compositions of garnet havealso been utilized to define the chemical signatures of rockshosting diamonds (e.g., Davies et al. 2004; Viljoen et al.2004) and Broken Hill-type Pb-Zn-Ag sulfide mineralization(e.g., Lottermoser 1988; Schwandt et al. 1993; Rozendaaland Stalder 2000; Stalder and Rozendaal 2005). Rare earthelement compositions of garnet in Broken Hill-type depositshave been used to evaluate the physicochemical conditionsof formation of their metamorphic precursors (e.g., Lotter-moser 1988; Schwandt et al. 1993; Rozendaal and Stalder2000; Stalder and Rozendaal 2005; Spry et al. 2007).

Lottermoser (1988) and Schwandt et al. (1993) obtainedthe REE compositions of four garnets in the same samples ofgarnetite (garnet-quartz rocks) that were proximal and distal tothe giant Proterozoic Broken Hill Pb-Zn-Ag deposit, southernCurnamona Province, Australia, and made genetic interpreta-tions that were based, in large part, on the shape of chondrite-normalized REE patterns and the positive or negative sign ofthe Eu anomaly. Spry et al. (2007) also evaluated the REEcomposition of garnet in garnet-rich rocks from the BrokenHill deposit and showed that garnet in most samples ofgarnetite (> 80% garnet) and quartz garnetite (< 80% garnet)proximal to the deposit have positive and negative Euanomalies, respectively. The present study extends the workof Spry et al. (2007) in that it considers the trace elements(including REEs) of garnet in the southern CurnamonaProvince in garnet-rich rocks at Broken Hill (including garnet-cummingtonite and garnet-rhodonite rocks in ore), small BrokenHill-type deposits (Mutooroo, Iron Blow, Thunderdome,Polygonum), Zn-bearing metasedimentary rocks at HuntersDam and Doughboy, and locations where Broken Hill-typemineralization is yet to be found (Raven Hill, Meningie Well,and Weekeroo). Bulk-rock major and trace element data,including REE, that were obtained by Spry et al. (2007) andHeimann et al. (2009), are used to understand the distributionof trace elements between garnet and the host rock.

The compositional constraints that determine differencesin the chondrite-normalized REE patterns and the size andthe sign of the Eu anomaly in garnet in garnet-rich rocks inthe southern Curnamona Province are evaluated here using

the major and trace element composition of garnet, interele-ment correlations among major and trace elements, the sizeof the Eu anomaly in garnet, and differences in the REEpatterns and Eu anomalies of garnet and its host rock.Where pertinent, the distribution of REEs, in particular Eu,between garnet and coexisting minerals is evaluated tounderstand the difference between garnet and bulk-rock Euanomalies. The shape of the REE patterns, the sign of theEu anomaly, and the major element composition of garnetare utilized to characterize the relative physicochemicalconditions (T, fO2) of the hydrothermal fluids from whichthe premetamorphic phases precipitated. The size of the Euanomaly and the trace element content of garnet in garnet-rich rocks are also evaluated as potential exploration guidesin the search for Broken Hill-type deposits in the southernCurnamona Province and elsewhere in the world.

Geological setting

The southern Curnamona Province was subdivided by Conorand Preiss (2008) into three lithostratigraphic domains(Broken Hill, Olary, and Mulyungarie) based on recentgeochronological investigations and geological mapping. Itis dominated by rocks of the Paleoproterozoic WillyamaSupergroup (~1,720-1,640 Ma), which hosts the giantstratabound 280-Mt Broken Hill Pb-Zn-Ag deposit andapproximately 400 minor Broken Hill-type deposits(Fig. 1). The exposed part of the Willyama Supergroupconsists of a 7 km-thick package of poly-deformed andmetamorphosed clastic sedimentary rocks, lesser bimodalvolcanic and volcaniclastic rocks, minor chemical sedimen-tary rocks (e.g., garnet- and gahnite-rich rocks, banded ironformation, tourmalinite, calcsilicate and metacarbonate), andgranitoids (Willis et al. 1983; Laing 1996; Burton 1994). Thesedimentary succession was deposited in an evolving intra-continental rift system (Willis et al. 1983; Page et al. 2005a),which is preserved in the three geological domains (Conorand Preiss 2008): the Broken Hill Domain, hosts most of theknown Broken Hill-type Pb-Zn occurrences and occurs nearthe center of the main rift basin; the Olary Domain isinterpreted to be near the margins of the rift system andcontains stratigraphically restricted playa lake and evaporiticdeposits (Cook and Ashley 1992); whereas the MulyungarieDomain is transitional between the other two domains.

The stratigraphic nomenclature differs for each of thelithostratigraphic domains with stratigraphic correlationsbetween the Broken Hill and Olary Domains being based onfield relations and SHRIMP U-Pb zircon age dating (e.g.,Conor 2000a, b; Conor and Page 2003; Page et al. 2005a, b;Conor and Preiss 2008). The Mulyungarie Domain, which isalmost entirely under cover, is constrained by dates from atuff layer in the Portia Formation suggesting that part of the

50 A. Heimann et al.

sequence is equivalent to the lower Thackaringa or upperRantyga Groups (Conor and Preiss 2008). Those parts of theWillyama Supergroup that contain sulfides and manganiferousgarnet-rich rocks not necessarily spatially associated with theBroken Hill deposit include the Basso Suite volcanic rocks,the Portia Formation, the Cues Formation (which hosts thePinnacles Pb-Zn-Ag deposit), the Bimba Formation-EttlewoodCalc-Silicate Member, and various horizons in the Broken HillGroup including the Broken Hill Pb-Zn-Ag deposit.

TheWillyama Supergroup was subjected to several periodsof deformation (locally noted as D1, D2, etc.) during theOlarian Orogeny (Laing et al. 1978; Berry et al. 1978; Williset al. 1983; Clarke et al. 1986; Plimer 2006). U-Pb dating ofzircon yielded a range of dates from ~1,620 to 1,575 Ma forthe orogeny (Page and Laing 1992; Nutman and Ehlers1998; Teale and Fanning 2000; Page et al. 2005a; Forbes etal. 2008), with D2 and D3 fabrics being constrained to 1,597±3 and 1,591±5 Ma, respectively (Page et al. 2005a). AtBroken Hill, peak metamorphic conditions during D2

reached granulite facies (~700–800°C and 5-6 kb; Phillips1980; Phillips and Wall 1981) and were higher southeast ofthe mine (> 850°C and 5–7 kb; Frost et al. 2005). D3

produced retrograde metamorphic assemblages near BrokenHill at approximately 500–600°C and 5–5.5 kb (Phillips1980; Stevens 1986; Plimer 2006). Pressure-temperatureconditions decrease northwards in all three domains (BrokenHill Domain: granulite to lower amphibolite facies; OlaryDomain: upper amphibolite to upper greenschist facies;Mulyungarie Domain: lower amphibolite to upper greenschistfacies (Phillips 1980; Phillips and Wall 1981; Webb andCrooks 2005; Conor and Preiss 2008). Extensive partialmelting and migmatization suggests that peak metamorphicconditions in the Olary Domain exceeded the estimated 450–550°C and 4-5 kb of Clarke et al. (1987).

The Broken Hill deposit is~8 km long and is composed ofat least six separate Zn-Pb-Ag orebodies (the A, B, and CLodes and 1, 2, and 3 Lenses) spatially associated with adistinctive package of rocks that consist of quartz garnetite,garnetite, blue quartz-gahnite lode, and lode pegmatite(Johnson and Klingner 1975), and characteristic metalcontents (Pb, Zn, and Ag) and gangue minerals (quartz,gahnite, garnet, calcite, fluorite, hedenbergite, pyroxenoids,and apatite). Structural studies by Laing et al. (1978) suggestthat the orebodies were overturned during deformation and thatthe Pb-rich Lodes (2 and 3 Lenses) lie stratigraphically abovethe Zn-rich Lodes (1 Lens, and the A, B, and C Lodes). Bluequartz, blue quartz-gahnite (ZnAl2O4) lode, and quartzgarnetite are the most abundant lithologies spatially associatedwith minor Broken Hill-type deposits (Barnes et al. 1983;Barnes 1988; Burton 1994), whereas quartz garnetite andminor garnet-amphibole rocks are the most common garnet-rich rocks in the Olary and Mulyungarie Domains.

The generally held view is that the Broken Hill deposit andthe associated lode horizon rocks formed by exhalations orinhalations of hydrothermal fluids on or directly below thefloor of a rifted basin either in a marine or continental lakesetting and were subsequently metamorphosed (e.g., Stanton1976; Willis et al. 1983; Parr and Plimer 1993; Spry et al.2000; Parr et al. 2004; Plimer 2006; Phillips 2008). Aprominent premetamorphic footwall alteration zone beneaththe Broken Hill deposit was identified by Plimer (1979) andGroves et al. (2008). However, the syngenetic hydrothermalmodels contrast to that proposed by White et al. (1995),Rothery (2001), and Roache (2004), who suggested that basemetal sulfides were introduced during high-grade metamor-phism, a concept not supported by the Pb isotope data ofParr et al. (2004) that indicates syn-depositional or syn-diagenetic mineralization.

-

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+ - -

910

11

43

21

5

SOUTH AUSTRALIA NEW SOUTH WALES

MANNAHILL

OLARY

EUROWIEINLIER

0 10 20

Kilometers

7

6

Broken Hill

Pinnacles

8

Iron Blow

Meningie WellRaven Hill

Doughboy

WeekerooMutooroo

Polygonum

Thunderdome

HuntersDam

14101400

320

Neoproterozoic to RecentMesoproterozoic graniteand volcanic rocks

Willyama SupergroupUpper Willyama Supergroup

Broken Hill Group, Bimbaand Plumbago FormationsBasso Suite (~1.71 Ga)Lower Willyama Supergroup

++ Silver City Suite

- - -

1000 Km

BrokenHill

Fig. 1 Geological map of thesouthern Curnamona Provinceshowing the location of theBroken Hill and Pinnaclesdeposits and other study sites: 1.Meningie Well. 2. Raven Hill. 3.Doughboy. 4. Weekeroo. 5.Mutooroo. 6. Polygonum. 7.Thunderdome. 8. Hunters Dam.9. Iron Blow. 10. Broken Hill.11. Pinnacles (modified afterClark et al. 2004; Conor 2006;Conor and Preiss 2008)

The composition of garnet in garnet-rich rocks 51

Sampling and analytical methods

Garnet-rich rocks were collected from outcrops at RavenHill, Meningie Well, Mutooroo, Iron Blow, Doughboy, andWeekeroo, from outcrop, underground locations, and drillcore at Broken Hill, and from drill core at Polygonum,Thunderdome, and Hunters Dam (Fig. 2a-f). Rock sampleswere made into polished and polished-thin sections andexamined with an Olympus BX-60 dual reflected-transmitted light microscope. Analyses of garnet were

obtained at the University of Minnesota and Iowa StateUniversity (ISU) using JEOL 8900 and JEOL 8200 electronprobe microanalyzers, respectively. A range of mineralstandards including pyrope (Si, Al, Mg), hornblende (Ti),ilmenite (Fe), spessartine (Mn), and apatite (Ca) were usedfor garnet analysis, whereas for other minerals hornblende(Si, Al, Ti, Fe, Mg, Ca), ilmenite (Ti, Fe), gahnite (Zn), K-feldspar (K), and apatite (F) were used. Conditions for bothinstruments included an accelerating voltage of 15 kV, abeam current of 20 nA, and a counting time of 20 s. The

a b

c d

e f

Fig. 2 Photographs that show some of the characteristics of thelocalities and outcrops of garnet-rich rocks. a Layering in metasedi-mentary rocks that host the garnet-quartz rocks, Raven Hill. b Outcropof garnet-quartz rocks, Raven Hill. c Outcrop of one of the lenses of

garnet-amphibole rocks, Weekeroo. d Blackwoods open pit, BrokenHill. e Banded garnet-quartz rock, Broken Hill. f Mundi Mundi Plain,where the Polygonum prospect is located under a thick sedimentarycover

52 A. Heimann et al.

composition of garnet is reported in terms of the almandine(Alm), spessartine (Sps), and andradite (Adr)+grossular(Grs) components in mole % using the following format:Alm50–60Sps10–20Grs+Adr5–15.

Laser ablation-inductively coupled plasma-mass spec-trometry (LA-ICP-MS) analyses of garnet were carried outusing an Agilent 7500 ICP-MS system interfaced with aMerchantek EO-UV laser ablation system at MacquarieUniversity, Sydney. The Merchantek instrument uses aQuantel Nd:YAG laser at a frequency of 266 nm. Sampleswere ablated in a single spot with 100 shots at a pulseenergy of 1 mJ, a repetition rate of 5 Hz, and using a spotdiameter of ~30–50 μm. CaO was used as internal standard(measured as 43Ca) for CaO concentrations >1 wt. %, andAl2O3 (measured as 27Al) was used as the internal standardfor CaO concentrations <1 wt. %. NIST 610 glass, a garnetfrom Mongolia (MONGOL), and a basaltic glass (BCR2G)were used as external standards. These standards wereanalyzed after measuring 9 or 10 unknown samples.Methodology, accuracy and standards used follow thoseof Norman et al. (1996, 1998). The carrier gas was amixture of He and Ar. The raw ICP-MS data were exportedin ASCII format and processed using GLITTER! (vanAchterbergh et al. 2001) (http://www.glitter-gemoc.com).Using this program, plots of the ablation analysis weremonitored to extract the signal derived from small mineralinclusions in garnet from that produced by garnet. Theconcentration of 42 trace elements (including REEs) wasdetermined for garnet in 52 garnet-rich rocks from 10locations. A total of 331 analyses were obtained (at leastfive points/sample). Of these samples, 12 (59 analyses)were from various garnet-bearing rocks in the Broken Hilldeposit. REE compositions of garnet from quartz garnetite,garnetite, garnet-gahnite rocks, and garnet-sillimanitegneiss from the Broken Hill deposit that were previouslyobtained by Spry et al. (2007) using LA-ICP-MS techni-ques, are compared to the data obtained here. The traceelement, including REE, data were normalized to thechondrite values of McDonough and Sun (1995).

Local geological setting and petrography

The mineralogy of each sample studied is given in Table 1while additional details and a summary of the maingeological characteristics of each location are presented inHeimann (2006) and Heimann et al. (2009). Details of thepetrology, mineralogy, garnet composition, and spatialrelationship of garnet-rich rocks to sulfide mineralizationat Broken Hill are given in Spry and Wonder (1989), Plimer(2006), and Spry et al. (2007). Information about thegeographic location of these sites, including local geologicalmaps, is given in Conor (2006) and references therein.

Manganoan garnet-rich rocks are intercalated with meta-sedimentary rocks at various stratigraphic levels in differentdomains in the Willyama Supergroup (Table 1). They aregenerally conformable to bedding in the enclosing metasedi-mentary rocks; exceptions to this include the cross-cutting,“remobilized” quartz-garnetite and garnet envelope of Spryand Wonder (1989) at Broken Hill, which formed during D3.Garnet-rich rocks are spatially related to sulfides at BrokenHill (Pb-Zn-Ag), Mutooroo (Pb-Zn-Ag), Iron Blow (Cu-Pb-Zn-Ag-Au), Doughboy (Cu), Polygonum (Zn-Pb-Ag), andThunderdome (Zn-Pb-Ag), and to altered amphibolite atBroken Hill, Raven Hill, Mutooroo, Meningie Well, Polyg-onum, Iron Blow, and Thunderdome. At Weekeroo, garnet-rich rocks are proximal to the Montstephen Metabasalt, whichconsists of mafic metavolcanic rocks. There is also a spatialrelationship to felsic volcanic rocks at Raven Hill, Doughboy,and Weekeroo (Conor 2003).

The Raven Hill, Meningie Well, Weekeroo and Doughboyrocks occur in the Olary Domain in the Ethiudna andWiperaminga Subgroups of the Curnamona Group. Thegarnet-rich rocks at Hunters Dam are in the Portia Formationwithin the Mulyungarie Domain (Conor and Preiss 2008).Prospects (Dingo Ridge, Emu Ridge, Two Mile Ridge,Horseshoe Ridge, and Berta Tank) at Mutooroo and theBroken Hill deposit are located in the Broken Hill Group ofthe Broken Hill Domain, whereas the Polygonum andThunderdome prospects likely occur in the stratigraphicequivalents of the Broken Hill Group in the MulyungarieDomain. The Iron Blow deposit, which also occurs in theBroken Hill Domain, is hosted in rocks of the ThorndaleComposite Group or the Lady Brassey Formation. Themetamorphic grade is granulite facies at Broken Hill,Mutooroo, and Iron Blow, middle amphibolite facies atRaven Hill, Meningie Well, Doughboy, Weekeroo, andThunderdome, lower amphibolite facies at Polygonum, andupper greenschist-lower amphibolite facies at Hunters Dam.

Garnet-rich rocks are primarily quartz garnetite and lessergarnetite at Broken Hill (Fig. 2d,e), Iron Blow, Thunderdome,Mutooroo, and Raven Hill (Fig. 2a,b), whereas quartzgarnetite is the only garnet-rich rock present at Polygonum(Fig. 2f). At Thunderdome, laminated bands of quartzgarnetite and garnetite (up to 1 cm thick) contain magnetiteand grade into a laminated iron formation composed ofmagnetite, grunerite, garnet, apatite, and minor fayalite(Fig. 3a), whereas at Polygonum layers of garnet up tocentimeters wide occur in laminated metapsammopelite withquartz, biotite, muscovite, and accessory gahnite (Fig. 3b).At Hunters Dam, millimeter-scale laminations of garnet andquartz are present within metapsammopelite.

At Raven Hill, massive or laminated quartz garnetite(Fig. 2b) contains garnet (up to 0.7 mm in diameter)coexisting with quartz, ilmenite, minor biotite, and second-ary chlorite (Fig. 3c). Massive and banded garnet-grunerite,

The composition of garnet in garnet-rich rocks 53

Table 1 Mineralogy and geological characteristics of garnet-rich rocks from the southern Curnamona Province, Australia

Site Domain Host Unit Metamorphic facies Sample Mineralogy

Doughboy OD Wiperaminga Subgroup-Curnamona Group Amphibolite Doughb4 Grt-bt-fl-qtz

Doughb11 Qtz-grt-pl-ms-gah

Doughb12 Qtz-grt-bt-ilm-ms-fsp-gah

Doughb15 Qtz-grt-msa-hem-mag

Hunters Dam MD Portia Formation? Greenschist HDDO2-425.8 m Grt-qtz-pl-bt-py-ccp-fsp

Iron Blow BH Thorndale Composite Gneiss or LadyBrassey Formation

Granulite IBlow5 Qtz-grt-ilm

IBlow6 Grt-qtz-bt-ilm-mnz

IBlow10 Qtz-grt-bt-ilm

IBlow12 Grt-qtz-bt-ilm-mnz

Meningie Well OD Oonartra Creek Formation-Saltbush Group Amphibolite MenWS2 Grt-gru

MenWS3 Grt-gru-trace ilm-po-ccp

MenWS12 Grt-gru-ilm-mag

MenWS13 Grt-gru (<1%)

MenWS14 Grt-gru-ilm

Mutooroo BH Broken Hill Group Granulite M3B Grt-qtz

M4D Grt-qtz-ilm

M4E Grt-qtz

M7A Qtz-grt-bt-msa

Polygonum MD Broken Hill Group equivalent Amphibolite IN2B-309 m Grt-qtz-bt-chla

IN2B-393.1 m Grt-qtz-gah

IN2B-414.8 m Grt-qtz-bt

IN2B-540.9 m Qtz-grt-fsp-bt-chl-ms

IN2B-593.3 m Qtz-grt lens in qtz-bt-grt-ms-chl-ga-py

IN2B-604.5 m Grt-qtz-cal

Raven Hill OD Oonartra Creek Formation-Saltbush Group Amphibolite CatR04-3A Qtz-grt-ilm-bt-chla

CatR04-6A Qtz-grt-bt-ilm-chl-sp

CatR04-8 Qtz-ms-grt-tur-chla

CatR04-10 Qtz-grt-bt-ilm

CatR04-13 Grt-qtz-pl-ms-ilm-bt-ttn

CatR04-18 Grt-qtz-ilm-bt

CatR04-19 Grt-qtz-bt-chla

CatR04-20 Grt-qtz-chla-ilm

CatR04-21 Qtz-grt-chla-ilm-bt-ms-sil(?)

CatR04-23 Qtz-grt-bt-chla-fsp

Thunderdome MD Broken Hill Group equivalent Amphibolite DDHP2-632 m Grt-pl-qtz-mag-ilm-bt-py

DDHP2-724.2 m Grt-qtz-ilm-bt-fsp(?)-sc-gah-mag

Weekeroo OD Bewooloo Formation-Curnamona Group Amphibolite W1 Grt-gru-qtz-calc-ap-tit

W4B Grt-gru-qtz-gru-grt-ap

W6 Grt-gru-qtz-pl-mag-ilm-ap

W12 Grt-fac-ep-fsp-qtz-tit-calc

Broken Hill BH Broken Hill Group Granulite 532-83 Grt-qtz-gn-ilm-po-ccp

532-343 Grt-qtz-ccp-ilm-po-sp-gn

532-45 Grt-sp-qtz-apy-po-ccp-gn

532-104 Grt-sp-qtz-ccp-gn-py-po

532-283 Grt-qtz-sp-po-gn-ccp

532-313 Grt-qtz-sp-gn-po-ccp

6844-47.8 m Grt-cal-gn-qtz-bt-ilm-ccp-po-chl-sp

532-284 Grt-qtz-gn

532-298 Qtz-grt-gn-po-ilm-gah-ccp-apy

532-303 Qtz-grt-bt-sp-gn-cp-po-py-hm

532-311 Qtz-grt-ilm-gn

532-343 Grt-qtz-ccp-ilm-po-sp-gn

532-599 Qtz-grt-ilm-gn-cp

54 A. Heimann et al.

Table 1 (continued)

Site Domain Host Unit Metamorphic facies Sample Mineralogy

532-17 Grt-bt-gah-qtz-sp-gn-po-ccp-apy

532-19A Gah-grt-qtz-bt-po-sp-ccp-ilm

S77-11 Qtz-grt-gah-bt-mgt-po-ccp-apy-chl-ilm

6925-89.1 m Qtz-grt-gah-bt-ilm-sp-po-gn-ccp-hm

Z3293-77.9 m Grt-qtz-gah-bt-ms-ilm-gn-po-ccp

532-222B Grt-sil-bt-qz-st-ap-ms-po-cp-ilm

Minerals listed in approximate order of abundance. Abbreviations after Kretz (1983); fsp feldspar; gah gahnite; sc sericite; a retrograde; OD Olarydomain; BH Broken Hill domain; MD Mulyungarie domain. Stratigraphic position and domains from Conor and Preiss (2008) and this study

6844-47.8m

0.5 mm 0.5 mm

IN2B414.8m

CatR04-10

0.25 mm

TH-36

0.5 mmM4D

Thunderdome

Mutooroo Broken Hill

Raven Hill

Polygonum

Meningie WellMWS120.25 mm

WeekerooW6

grt

grugrt

grt

gru

ilm

ilm

bt

grt qtz

grt

qtz mag

grtqtz

cal

Iron BlowIBlow10

grtqtz

0.5 mm

a b

c d

e f

g h

grtqtz

ilm

1 mm

0.25mm

Fig. 3 Photomicrographs(plane-polarized light) of garnet-rich rocks from the southernCurnamona Province. a Lami-nated garnetite and quartz (qtz)-garnet (grt)-magnetite (mag)rock, Thunderdome, sampleTH-36. b Quartz-garnet,garnet-quartz, and quartz-biotite(bt)-garnet laminations in quartzgarnetite, Polygonum, sampleIN2B-414.8 m. c Garnet-quartz-ilmenite (ilm) rock, Raven Hill,sample CatR04-10. d Garnet-grunerite (gru) rock with minorilmenite, Meningie Well, sampleMWS12. e Garnet-gruneriterock with minor ilmenite,Weekeroo, sample W6. fGarnet-quartz rock, Iron Blow,sample IB10. g Massive quartz-garnet rock, Mutooroo, sampleM4D. h Garnet and minorquartz and calcite (cal) ingarnetite, Broken Hill, sample6844-47.8 m. Abbreviationsafter Kretz (1983)

The composition of garnet in garnet-rich rocks 55

grunerite-rich, and magnetite-grunerite rocks at MeningieWell consist of varying proportions of garnet, grunerite,quartz, and minor ilmenite (Fig. 3d). At Weekeroo, garnet-amphibole and garnet-amphibole-calcite rocks (Fig. 2c) areexhalative components genetically related to amygdaloidalbasalt, syndepositional volcanic breccias, banded quartz-grunerite rock, calcsilicate, and Mg-carbonate (Pointon1980; Conor and Preiss 2008). The garnet-bearing rockscontain compositionally zoned garnet, Mn-bearing gruner-ite (Fig. 3e), ferroactinolitic hornblende, accessory carbon-ate (included in amphibole), quartz, trace ilmenite, feldspar,apatite and titanite, and secondary epidote.

At Doughboy, garnet-bearing rocks are interlayeredwithin a lens of metapsammopelite enclosed by felsicvolcanic rocks (Zdziarski 1997; Conor and Preiss 2008).Garnet coexists with quartz, carbonate, amphibole, feldspar,gahnite, fluorite, ilmenite, and biotite. Quartz garnetite atIron Blow occurs as lenses in a sequence of psammopelite,psammitic gneiss, pegmatite, chlorite schist, banded ironformation, amphibolite, quartz-magnetite rocks and quartz-gahnite rocks (Leyh and Larsen 1983; Leyh et al. 1992).Quartz garnetite contains quartz, garnet (up to 1.6 mm indiameter) and ilmenite with minor biotite, trace monazite,and secondary chlorite (Fig. 3f). Blue quartz-gahnite-garnetrock, quartz garnetite, and garnetite at Mutooroo occur in apackage of psammitic and pelitic schists, garnet-bearingquartzo-feldspathic gneiss, and amphibolite that resemblesthat found at Broken Hill. Massive quartz garnetite andgarnetite locally contain pyrrhotite, galena and sphaleritebut are dominated by quartz and garnet (Fig. 3g). Garnetiteat Broken Hill (Fig. 3h) is almost mineralogically andtexturally identical to that found at Mutooroo.

Major element composition of minerals

Representative major element compositions of garnet arepresented in Table 2 and shown in Figs. 4 and 5, whereas thecomposition of some of the minerals coexisting with garnetare given in Table 3. The composition of garnet in garnet-rich rocks reflects a solid solution predominantly betweenalmandine and spessartine with variable amounts of grossu-lar and andradite, and trace quantities of pyrope. In contrastto garnet in garnet-quartz layers, garnet in calc-silicate rocksfrom Polygonum and Hunters Dam and some fromWeekeroo that coexist with grunerite and carbonate are notablyenriched in Ca, as is garnet from 2 Lens and A Lode from themain Broken Hill lode (Figs. 4 and 5). The Ca-enrichedcomposition of these garnets at Broken Hill was previouslyreported by Spry and Wonder (1989). The Fe-rich composi-tion of garnet from Iron Blow resembles that of garnetcoexisting with gahnite from the B and C Lodes at BrokenHill (Spry et al. 2007). At Broken Hill, garnet in garnetite is

generally Mn-rich, whereas garnet in quartz garnetite andgarnet-gahnite rock is most commonly enriched in Fe(Fig. 5b).

In view of the potential use of Mn- and Zn-rich-bearingminerals in the search for Broken Hill-type deposits (Spry etal. 2000), considerable attention was paid to determining theMn and Zn content of ferromagnesian silicates and oxidesthat coexist with garnet in the garnet-rich rocks (Table 3).Gahnite is common in parts of the Broken Hill Domain,including Mutooroo, and is known from Doughboy in theOlary Domain. In the Mulyungarie Domain, elevated Zncontents are present in spinels in garnet-bearing rocks fromPolygonum (gahnite57–72hercynite24–37spinel3–8) and Thun-derdome (gahnite47–64hercynite33–50spinel2–4) (Table 3).Chlorite and biotite from Polygonum contain up to 0.7 and0.9 wt. % MnO, respectively, whereas secondary chlorite atRaven Hill contains up to 1.4 wt. % MnO. Ilmenite-pyrophanite contains up to 26 wt. % MnO and 1.0 wt. %ZnO at Raven Hill, up to 20 wt. % ZnO atMutooroo, 13 wt. %MnO at Weekeroo, 10 wt. % MnO at Meningie Well,9.5 wt. % MnO at Polygonum, and up to 14 wt. % MnOand 15 wt. % ZnO at Broken Hill. However, ilmenite fromIron Blow contains no measurable amounts of Zn and Mn.Grunerite and ferroactinolite contain up to 6 wt. % and0.8 wt. % MnO, respectively, in garnet-amphibole rocks fromMeningie Well and Weekeroo. Elevated quantities of Mn arealso found in calcite and epidote in garnet-grunerite rocksfrom Weekeroo (up to 1 and 10 wt. % MnO, respectively).

Trace element compositions of garnet

Given the large number of trace element data collected here,only representative analyses are presented in Table 4. Thecomplete data set is listed in the Electronic Supplement thataccompanies this paper (Tables ES-1 and ES-2). It should benoted that the elements La to Sm and Gd to Lu are referredto as LREEs and HREEs, respectively, and that Y is insertedin the trace element patterns between Gd and Ho.

Rare earth elements

Figures 6a-d and 7a-d show representative chondrite-normalized trace element (including REE) patterns ofgarnet in garnet-rich rocks from Mutooroo, Iron Blow,Raven Hill, Meningie Well, Doughboy, Weekeroo, Polyg-onum, Hunters Dam, and Thunderdome. Selected REEpatterns of garnet in quartz garnetite from Raven Hill(Fig. 8a, b), garnet-grunerite rock from Meningie Well(Fig. 8c), and in quartz garnetite from Doughboy (Fig. 8d),as well as a REE pattern of grunerite from Meningie Well,are shown separately for clarity given some of theoverlapping patterns in Figs. 6 and 7.

56 A. Heimann et al.

Tab

le2

Representativemajor

elem

entcompo

sitio

nsof

garnet

from

thesouthern

Curnamon

aProvince,

Australia

Sam

ple

M4D

IBlow-5

CatR04

-6MWS14

Dou

ghb11

W1

IN2B

414.8m

DP2-72

4m

Blackw5

532-29

8S77

-11

532-22

2BCatR04

-8

wt.%

N=8

N=1

N=5

N=7

N=12

N=11

N=15

N=17

N=9

N=4

N=6

N=8

N=8

SiO

237

.11

38.16

37.13

36.71

36.92

37.58

36.14

36.94

37.10

37.17

37.46

37.00

36.94

TiO

20.02

00.06

0.11

0.03

0.21

0.12

0.02

0.05

0.05

0.03

0.08

0.01

Al 2O3

20.94

22.14

20.62

21.16

20.60

20.80

21.47

20.84

21.16

21.53

21.74

20.94

21.37

FeO

21.91

35.94

28.53

16.00

26.66

12.81

26.83

34.55

13.71

33.34

33.86

35.62

32.18

MnO

16.00

1.06

10.97

22.96

10.46

11.47

12.30

4.09

14.54

3.43

4.36

3.01

5.98

MgO

1.96

3.94

0.50

0.73

0.61

0.13

1.74

1.07

0.48

3.26

2.90

2.78

1.19

CaO

1.55

0.63

2.82

2.95

3.78

17.37

1.11

1.74

12.98

1.59

0.85

0.63

2.90

Total

99.51

101.87

100.63

100.62

99.06

100.35

99.71

99.25

100.02

100.37

101.20

100.06

100.63

apfu

1212

1212

1212

1212

1212

1212

12

Si

2.99

73.00

13.01

62.97

43.03

42.97

72.94

73.03

62.96

22.97

82.98

32.99

22.98

4

Ti

0.00

20.00

00.00

40.00

70.00

20.01

20.00

70.00

10.00

70.00

30.00

20.00

20.00

1

Al

2.02

12.05

01.97

42.02

11.99

41.94

22.06

42.01

71.99

22.03

22.04

11.99

62.03

3

Fe

1.55

02.36

31.93

81.08

41.83

20.84

91.82

82.37

50.91

52.23

32.25

52.40

92.15

1

Mn

1.06

20.07

10.75

51.57

60.72

80.77

00.85

20.28

50.98

30.23

30.29

40.20

70.40

9

Mg

0.22

00.46

20.06

10.08

90.07

50.01

50.21

20.13

10.114

0.39

00.34

40.33

40.14

3

Ca

0.13

40.05

30.24

50.25

60.33

31.47

40.09

70.15

31.110

0.13

60.07

20.05

50.25

1

Total

7.98

78.00

07.99

38.00

77.99

78.03

88.00

67.99

98.08

38.00

57.99

17.99

57.97

2

N32

indicatesthenu

mberof

analyses,C

atR04

Raven

Hill,M

WSMeningieWell,M

Mutoo

roo,

IBlowIron

Blow,W

Weekeroo,

Dou

ghbDou

ghbo

y,IN2B

Polyg

onum

,DP2Thu

nderdo

meDDHP2,

Blackw5,

532-29

8,S7

7-11,and53

2-22

2BBrokenHill

The composition of garnet in garnet-rich rocks 57

All the garnet REE patterns are LREE depleted and HREEenriched, and have variable HREE shapes. Garnet in garnet-rich rocks, other than those at Broken Hill, have negative Euanomalies (Eu/Eu*<1; Eu/Eu*=EuN/([Sm+Gd]N)/2, where

N refers to normalized values to the chondrite of McDonoughand Sun 1995) in all samples except for those from RavenHill (Fig. 8a, b). Garnet in garnet-quartz rocks fromMutooroo has pronounced negative Eu anomalies andgenerally flat to slightly decreasing HREE patterns(Fig. 6a). REE patterns of garnet in quartz garnetite fromIron Blow have the largest negative Eu anomalies of garnetin garnet-rich rocks from all the sites studied here and flat toslightly convex-up HREE shapes (Fig. 6b). Garnet in quartzgarnetite from Raven Hill has variable REE trends from Euto Lu, and small positive (Eu/Eu*>1) to no Eu anomalies inmost samples (Figs. 6c and 8 a, b). The REE patterns of

these garnets are not smooth but are variable due to the likelypresence of microinclusions of other minerals that wereablated, and perhaps related to compositional zoning in thegarnet. Garnet in a sample (CatR04-19) which containsapproximately 5 volume % of secondary chlorite hasdecreasing REE contents from Gd to Lu (Fig. 8a). Garnetin garnet-rich rocks that were subjected to brittle shearing,has similar bell-shaped REE patterns with decreasing REEcontents from Eu to Lu (Fig. 8a, b). The REE pattern of aLREE-rich mineral, apatite, which is included in garnet isshown in Fig. 8b.

Garnet in garnet-amphibole rocks at Meningie Well hasrelatively low REE contents (10 times chondrite), middle REE(Sm-Gd) enrichments, Eu/Eu*<1, and slightly decreasing

Alm0 10 20 30 40 50 60 70 80 90 100

Adr+Grs

0

10

20

30

40

50

60

70

80

90

100

Sps

0

10

20

30

40

50

60

70

80

90

100

3 lens (n=16)2 lens (n=3)A lode (n=5)1 lens (n=4)B lode (n=22)C lode (n=2)

Pb

Zn

a

b

Alm0 10 20 30 40 50 60 70 80 90 100

Adr+Grs

0

10

20

30

40

50

60

70

80

90

100

Sps

0

10

20

30

40

50

60

70

80

90

100

Garnetite (n=12)Qtz garnetite (n=5)Garnet-gahnite rock (n=5)

N=693

Fig. 5 Ternary diagram of garnet compositions in garnet-rich rocksfrom the Broken Hill deposit. a Garnet in garnetite and quartz-garnetite from various orebodies in the Broken Hill deposit (dataderived from this study and Spry 1978). b Average compositions ofgarnet in garnetite, quartz garnetite, and garnet-gahnite rock in theBroken Hill deposit (analyzed in the present study)

Alm0 10 20 30 40 50 60 70 80 90 100

Adr+Grs

0

10

20

30

40

50

60

70

80

90

100

Sps

0

10

20

30

40

50

60

70

80

90

100

Raven Hill (n=10)Meningie Well (n=5)Mutooroo (n=4)Iron Blow (n=4)Weekeroo (n=4)

Alm0 10 20 30 40 50 60 70 80 90 100

Adr+Grs

0

10

20

30

40

50

60

70

80

90

100

Sps

0

10

20

30

40

50

60

70

80

90

100

Thunderdome (n=2)Polygonum Dam (n=6)Hunters Dam (n=1)Doughboy Well (n=3)

b

a

Fig. 4 Ternary diagram of garnet compositions in garnet-rich rocksfrom the southern Curnamona Province, Australia. a Raven Hill,Meningie Well, Mutooroo, Iron Blow, and Weekeroo. b Thunder-dome, Polygonum, Hunters Dam, and Doughboy

58 A. Heimann et al.

patterns from Gd to Lu (Fig. 6d). By comparison, grunerite inthe same rocks is depleted in REEs (between 1 and 10 timeschondrite), has a relatively flat REE pattern, and weaknegative Eu anomalies (Fig. 8c). Garnet from Doughboy ischaracterized by high total REE contents (up to 1,000 timeschondrite; Fig. 7a), Eu/Eu*<1, and flat HREE patterns fromDy to Lu (Figs. 7a, 8f), although garnet in one sample fromDoughboy has a bell-shaped REE pattern (Fig. 8d). Garnet ingarnet-grunerite rocks from Weekeroo has Eu/Eu*<1 andrelatively flat to slightly grunerite decreasing REE patternsfor the HREEs (Fig. 7b). Garnet in garnet-bearing rocks fromPolygonum and Thunderdome contains abundant smallunidentified mineral inclusions. The REE patterns of garnet,which were obtained by filtering elemental peaks caused bythe presence of mineral inclusions, are not as smooth asthose for garnets from other localities. The REE patterns ofgarnet in quartz garnetite from Polygonum have Eu/Eu*<1,and flat to increasing and slightly convex HREEs (Fig. 7c).

In general, garnet from Thunderdome has Eu/Eu*<1 andHREE patterns that are either flat or increase with atomicnumber (Fig. 7d). The total REE content of some garnetsfrom Raven Hill, Doughboy and Polygonum are >600 ppm(Tables ES1-ES2; Fig. 8e, f). Garnet in garnetite fromBroken Hill has LREE-depleted, HREE-enriched patternswith flat HREE trends, and, in most cases, Eu/Eu*>1(Fig. 9a), whereas garnet in quartz garnetite has the sameoverall patterns but Eu/Eu*<1 (Fig. 9b). Garnets in bluequartz-garnet-gahnite rock from B Lode at Broken Hill haveboth flat and slightly increasing heavy HREE trends(Fig. 9c).

For comparative purposes, coarse-grained garnet from amuscovite-garnet schist from Raven Hill and a garnet-sillimanite gneiss from Broken Hill were also analyzed(Fig. 9d). Garnet in the garnet-sillimanite gneiss adjacent tothe Broken Hill deposit has LREE values lower thandetection limits, total REE contents similar to those of

Table 3 Representative major element compositions of minerals that coexist with garnet

Ilm Ilm Ilm Chl Chl Chl Gru Gru GahMWS14 CatR04-21 W6 IN2B-309 m CatR04-21 CatR4 W6 MWS14 IN2B-393.1 m

wt. % N=1 N=7 N=3 N=3 N=1 N=1 N=1 N=7 N=12

SiO2 0.02 0.02 0.00 25.24 25.29 24.07 50.71 50.97 0.15

TiO2 50.52 50.32 52.47 0.07 0.06 0.02 0.00 0.00 0.02

Al2O3 0.06 0.05 0.04 20.67 22.18 20.99 0.20 0.18 56.09

FeO 38.66 25.39 33.73 32.24 35.10 36.06 32.89 29.31 12.09

MnO 8.89 23.19 13.42 1.16 0.59 1.36 6.01 6.23 0.20

MgO 0.02 0.01 0.02 7.63 4.73 2.18 8.68 9.71 1.63

CaO 0.00 0.03 0.01 0.21 0.03 0.73 0.60 0.62 0.02

K2O 0.01 0.01 0.00 0.40 0.75 0.36 0.00 0.02 0.01

ZnO 0.90 0.21 0.05 0.09 0.00 28.06

Na2O 0.19 0.13 0.03

F 0.02 0.00 0.00 0.01 0.00 0.05 0.00 0.00 0.01

Cl 0.00 0.00 0.01 0.06 0.05 0.09 0.01 0.01 0.01

Total 99.10 99.23 99.75 87.88 88.87 86.04 99.10 99.08 98.29

apfu 6 6 6 12 12 12 24 24 4

Si 0.001 0.001 0.000 2.379 2.381 2.381 8.225 8.308 0.004

Ti 1.953 1.942 1.995 0.005 0.004 0.002 0.000 0.000 0.000

Al 0.004 0.003 0.003 2.298 2.461 2.448 0.038 0.034 1.988

Fe 1.662 1.090 1.426 2.546 2.764 2.984 4.462 3.996 0.304

Mn 0.387 1.008 0.575 0.093 0.047 0.114 0.825 0.860 0.005

Mg 0.002 0.001 0.001 1.071 0.664 0.321 2.099 2.358 0.073

Ca 0.000 0.002 0.001 0.021 0.003 0.077 0.104 0.108 0.001

K 0.001 0.000 0.000 0.048 0.090 0.045 0.000 0.003 0.000

Zn 0.034 0.008 0.002 0.006 0.000 0.623

Na 0.035 0.025 0.010

F 0.003 0.000 0.001 0.003 0.000 0.016 0.000 0.002 0.001

Cl 0.000 0.000 0.000 0.009 0.008 0.014 0.003 0.002 0.001

Total 4.047 4.055 4.004 8.508 8.428 8.427 15.756 15.681 3.000

Abbreviations after Kretz (1983), Gah gahnite, CatR Raven Hill, MWS Meningie Well, W Weekeroo, IN2B Polygonum

The composition of garnet in garnet-rich rocks 59

Tab

le4

Representativetraceelem

entcompo

sitio

ns(inpp

m)of

garnet

ingarnet-richrocksfrom

thesouthern

Curnamon

aProvince

Sam

ple

12

34

56

78

910

1112

13

P50.7

131

231

50.8

53.3

37.2

1,329

105

243

93.3

165

244

85.3

Sc

24.7

11.9

17.8

15.2

5.2

4.8

77.8

38.9

265

56.5

115

72.7

37.5

Ti

72.6

132

344

1,141

35.5

783

1,186

6.0

919

195

132

85.6

70.3

V80.5

68.1

309

91.1

6.6

143

133

19.0

497

31.7

17.9

31.8

24.3

Cr

82.4

65.9

81.2

91.7

<3.1

29.4

18.9

32.8

579

179.8

30.4

52.2

Co

18.6

17.9

63.7

38.6

3.3

3.1

2.5

5.9

49.2

10.0

19.8

11.3

10.4

Nit

92.7

4.5

0.21

1.3

0.65

1.9

Zn

227

219

126

53.9

855

3.0

554

194

710

505

80.2

38.0

Gat

33.4

29.8

41.8

31.8

27.1

49.7

45.1

10.6

80.6

6.5

9.6

14.6

10.2

Srt

0.05

8.1

0.10

0.54

0.41

0.03

0.15

Y92.6

57.9

34.9

109

940

72.3

644

137

646

137

274

111

1,188

Zr

4.4

13.0

101

22.9

3.8

0.79

6.4

29.1

32.8

8.5

5.1

9.4

3.1

Nb

1.9

<0.02

0.79

2.9

<0.04

0.07

25.5

<0.05

<0.10

<0.04

<0.01

0.09

<0.06

La

<0.04

<0.02

3.1

<0.03

<0.05

0.02

0.24

<0.04

<0.09

0.02

<0.01

<0.09

<0.04

Ce

0.08

0.03

6.5

0.11

<0.03

0.03

2.2

<0.05

0.22

0.14

0.01

<0.11

<0.04

Pr

<0.04

0.05

1.1

0.04

<0.03

0.02

1.5

<0.04

0.22

0.11

0.02

<0.08

<0.04

Nd

0.57

0.81

5.3

1.8

0.26

0.54

17.4

0.36

7.2

1.8

0.48

<0.47

<0.19

Sm

1.8

4.2

5.4

6.6

3.7

3.3

20.2

0.93

19.4

4.8

1.8

1.4

0.53

Eu

0.32

0.33

3.0

2.1

3.4

1.6

7.3

0.41

0.94

0.20

0.41

0.16

0.47

Gd

8.5

12.0

14.3

25.1

49.8

11.7

44.6

3.2

49.7

13.4

8.0

5.0

11.8

Tb

2.6

2.2

1.9

4.4

18.5

2.1

11.6

1.1

12.0

3.2

1.9

7.1

Dy

18.1

11.9

9.0

25.0

162

13.5

102

14.3

98.8

25.1

36.5

17.8

114

Ho

2.9

1.9

1.3

3.9

30.9

2.5

23.8

6.0

26.2

5.1

10.7

4.1

37.6

Er

5.5

4.4

2.9

6.3

65.1

6.2

67.5

30.7

87.5

12.9

35.4

12.5

135

Tm

0.60

0.61

0.44

0.65

7.5

0.78

8.8

7.0

14.0

1.7

5.7

1.5

22.5

Yb

3.3

3.9

3.1

3.3

37.1

5.9

50.2

64.5

14.0

10.8

40.3

9.5

185

Lu

0.40

0.57

0.44

0.49

4.5

0.79

5.7

13.0

14.0

1.4

6.1

1.1

30.9

Pb

<0.17

<0.06

19.2

0.10

<0.25

0.11

4.2

<0.17

0.27

<0.07

<0.04

<0.21

<0.18

Th

<0.09

<0.05

17.8

0.92

<0.13

<0.03

0.88

0.12

<0.12

<0.03

0.02

<0.15

<0.10

U0.34

0.05

15.7

1.7

<0.12

0.07

66.3

0.86

<0.12

0.10

0.08

<0.13

<0.09

∑REE

40.9

38.4

56.8

75.9

341

42.5

307

64.0

316

65.3

102

44.4

328

Eu/Eu*

0.21

0.13

0.98

0.43

0.44

0.70

0.71

0.66

0.09

0.07

0.28

0.16

0.26

Ce/Ce*

0.83

0.34

0.44

0.34

Com

positio

nsob

tained

atMacqu

arie

University

Not

allelem

entsanalyzed

areshow

nhere;theentiresetof

data

with

completeanalyses

appearsin

theElectronicSup

plem

ent

<indicatesvalues

below

detectionlim

its;Gat=

69Ga+

71Ga;

Nit=

60Ni+

62Ni;Srt=

86Sr+

88Sr;Eu/Eu*

=Eu N

/((Sm

N+Gd N

)/2);Ce/Ce*

=Ce N/((La N

+Pr N)/2)

Sam

plenu

mbers

are:

1qu

artz

garnetite

(Mutoo

roo,

M4D

-1);2qu

artz

garnetite

(IronBlow,IBlow5-2);3qu

artz

garnetite

(Raven

Hill,CatR04

-6A2);4garnet-grunerite

rock

(MeningieWell,

MWS14

-1);5qu

artz-garnet-plagioclase-muscovite-gahnite

rock

(Dou

ghbo

y,Dou

ghb11-1);6garnet-grunerite-quartzrock

(Weekeroo,

W1-1);7qu

artz

garnetite

(Polyg

onum

,IN

2B41

4.8m-3);

8qu

artzgarnetite

(Thu

nderdo

me,DDHP2-72

4.2m-3);9garnetite

(BrokenHill,B

lackw-5-1);10

quartzgarnetite

(BrokenHill,5

32-298

-2),11

quartz-garnet-gahn

iterock

(BrokenHill,S

77-11-2);

12sillimanite-garnetgn

eiss

(BrokenHill,53

2-22

2B-1);13

meta-pelite(Raven

Hill,CatR04

-8-3)

60 A. Heimann et al.

garnet in garnet-rich rocks (~ 100 times chondrite, total REEs~45 ppm), and Eu/Eu*<1, whereas that in the schist fromRaven Hill contains high amounts of REEs (> 100 timeschondrite, total REEs~800 ppm), and has LREE-depleted andflat HREE (Dy-Lu)-enriched patterns and Eu/Eu*<1.

A plot of Mn content versus Eu/Eu* for garnet in garnet-rich rocks shows that Mn-rich garnet in garnetite fromBroken Hill has the largest positive Eu anomalies(Figs. 10a, 5b). Most garnets in quartz garnetite fromBroken Hill have Eu/Eu*<1 and lower Mn contents(Figs. 5b, 10a), whereas for garnets from garnet-rich rocksfrom the other sites in the southern Curnamona Province,other than those spatially associated with the Doughboydeposit, there is no clear relationship between the Eu/Eu*value and Mn content (Fig. 10a). Iron-rich, Mn-poor garnetin garnet-gahnite rocks from Broken Hill has strongnegative Eu anomalies (Fig. 10a). These patterns mimic

those obtained by Barovich and Hand (2008) for metasedi-mentary rocks from the Willyama Supergroup.

Other trace elements

Chondrite-normalized trace element (including REEs)patterns of garnet in garnet-bearing rocks from all thelocations studied here (including Broken Hill) have thesame general shape (Figs. 6 and 7). However, there aresignificant differences in the abundances of some elementsat certain locations, which are indicated by more pro-nounced peaks in some spider diagrams and can be seen inscatter diagrams (Fig. 10b-g). For example, garnet fromthe Broken Hill deposit contains more Zn than garnet fromall other locations (up to 1,070 ppm) (Fig. 10b, c;Electronic Supplement Tables ES-1 and ES-2), eventhough garnet from Doughboy and one sample from

a

b

c

dL

iB

e B PS

c Ti V Cr

Co Ni

Zn

Ga

Ge

Rb

Sr

Zr

Nb

Cs

Ba

La

Ce Pr

Nd

Sm Eu

Gd

Tb

Dy Y

Ho

Er

Tm Yb

Lu Hf

Ta

Pb

Th U

Sam

ple/

chon

drite

Sam

ple/

chon

drite

Sam

ple/

chon

drite

Doughboy

0.000010.00010.0010.010.1

110

1001000

10000

Doughb-11

Sam

ple/

chon

drite

Li

Be B P

Sc Ti V Cr

Co Ni

Zn

Ga

Ge

Rb

Sr

Zr

Nb

Cs

Ba

La

Ce Pr

Nd

Sm Eu

Gd

Tb

Dy Y

Ho

Er

Tm Yb

Lu Hf

Ta

Pb

Th U

Weekeroo

Li

Be B P

Sc Ti V Cr

Co Ni

Zn

Ga

Ge

Rb

Sr

Zr

Nb

Cs

Ba

La

Ce Pr

Nd

Sm Eu

Gd

Tb

Dy Y

Ho

Er

Tm Yb

Lu Hf

Ta

Pb

Th U

Polygonum

1E-50.00010.0010.010.1

110

1001000

10000

DDHP2-724m

Li Be B P Sc Ti V Cr

Co Ni

Zn

Ga

Ge

Rb Sr Zr

Nb

Cs

Ba

La Ce Pr

Nd

Sm Eu

Gd

Tb

Dy Y Ho Er

Tm Yb Lu Hf

Ta

Pb

Th U

Thunderdome

0.000010.00010.0010.010.1

110

1001000

10000

IN2B-414.8m

Incl.

0.0000010.000010.00010.0010.010.1

110

1001000

W-1

Fig. 7 Representative chondrite-normalized trace element patterns ofgarnet from garnet-rich rocks from the southern Curnamona Province. aDoughboy. b Weekeroo. c Polygonum. d Thunderdome. Normalizationvalues after McDonough and Sun (1995)

Sam

ple/

chon

drite

M4D1E-61E-51E-41E-3

0.0010.01

110

1001000

1E-51E-4

a

b

c

d

Li

Be B P

Sc Ti V Cr

Co Ni

Zn

Ga

Ge

Rb

Sr

Zr

Nb

Cs

Ba

La

Ce

Pr

Nd

Sm Eu

Gd

Tb

Dy Y

Ho

Er

Tm Yb

Lu Hf

Ta

Pb

Th U

Li

Be B P

Sc Ti V Cr

Co Ni

Zn

Ga

Ge

Rb

Sr

Zr

Nb

Cs

Ba

La

Ce

Pr

Nd

Sm Eu

Gd

Tb

Dy Y

Ho

Er

Tm Yb

Lu Hf

Ta

Pb

Th U

1E-5

1E-4

0.0010.01

0.11

10100

1000

IBlow-5

Li

Be B P

Sc Ti V Cr

Co Ni

Zn

Ga

Ge

Rb

Sr

Zr

Nb

Cs

Ba

La

Ce

Pr

Nd

Sm Eu

Gd

Tb

Dy Y

Ho

Er

Tm Yb

Lu Hf

Ta

Pb

Th U

Sam

ple/

chon

drite

Sam

ple/

chon

drite

1E-61E-51E-4

0.0010.01

0.11

10100

100010000

CatR04-6A

Li

Be B P

Sc Ti V Cr

Co Ni

Zn

Ga

Ge

Rb

Sr

Zr

Nb

Cs

Ba

La

Ce Pr

Nd

Sm Eu

Gd

Tb

Dy Y

Ho

Er

Tm Yb

Lu Hf

Ta

Pb

Th U

0.0010.010.1

110

1001000

10000

MenWS-14

Sam

ple/

chon

drite

Mutooroo

Iron Blow

Raven Hill

Meningie Well

Fig. 6 Representative chondrite-normalized trace element patterns ofgarnet from garnet-rich rocks from the southern Curnamona Province. aMutooroo. b Iron Blow. c Raven Hill. d Meningie Well. Normalizationvalues after McDonough and Sun (1995)

The composition of garnet in garnet-rich rocks 61

Meningie Well (MWS-13) also have elevated Zn contents(up to 855 ppm). By contrast, garnets from Weekeroo andHunters Dam possess the lowest Zn contents (2–11 ppmand 24–32 ppm, respectively). The cobalt content ofgarnet from Broken Hill is lower (up to 72 ppm) thanthose in all other garnets (Fig. 10b, c), and the highest Cocontents are found in garnet from Meningie Well(350 ppm; sample MWS-13).

A plot of Y versus Sc for garnet in garnet-rich rocksfrom Broken Hill shows that these elements are positivelycorrelated (R2=0.73) even though the Y content of garnetfrom Broken Hill is lower than that of garnet in garnet-richrocks from the other locations in the southern CurnamonaProvince (Fig. 10d). The Y content of garnet is highest ingarnet from the pelitic schist from Raven Hill (up to2,186 ppm Y). There are also high Y and Sc values ingarnet from the Polygonum and Hunters Dam prospects,but these high values (774 ppm and 137 ppm, respectively)might be an artifact of the presence of mineral inclusions,since some were impossible to avoid during garnet ablationand to completely remove from the LA-ICP-MS analysis.The highest Sc content (average of 270 ppm) obtained here

is from an inclusion-free area of garnet in quartz garnetite(sample M7A) from Mutooroo.

Some garnets in garnetite from Broken Hill have higher Crcontents (up to 650 ppm Cr; samples 532-45, Blackw5) thangarnet in quartz garnetite. They are also more enriched in Crthan garnet in garnet-rich rocks from the other localities, withthe exception of two garnets from Weekeroo, which containup to 451 ppm Cr (Fig. 10e, sample W6). Vanadiumanomalies are positive in garnet in garnet-rich rocks fromall sites with the most pronounced and variable being thosefrom Raven Hill (Figs. 6c and 10e). Garnet in samples fromRaven Hill, Meningie Well, Weekeroo, and Broken Hill havethe highest V contents (up to 800 ppmV; Fig. 10e). The Ticontent of garnet is highest in samples from Raven Hill(5,000 ppm), Meningie Well, and Weekeroo (2,960 ppm,sample W6; Fig. 10f, g). However, sample W6 contains Tipeaks that indicate the presence of inclusions of rutile.Phosphorous contents are highest in garnet from Iron Blowand Polygonum, although garnet from the latter containsabundant mineral inclusions and so these data must beconsidered with caution due to the possible ablation of a P-bearing mineral (apatite).

d

a b

c

e

La CePrNdSmEuGdTbDy Y HoErTm YbLu0.01

0.1

1

10

100

1000CatR04-21CatR04-23

Sam

ple/

chon

drite

inclusion

LaCePr NdSmEuGdTbDy Y Ho ErTmYbLu0.01

0.1

1

10

100

1000CatR04-6ACatR04-19

Sam

ple/

chon

drite

Amphibole

LaCePrNdSmEuGdTbDy Y HoErTmYbLu0.1

1

10

100MWS13-5MWS13-7MWS13-2

Sam

ple/

chon

drite

LaCePrNdSmEuGdTbDy Y HoErTmYb Lu0.01

0.1

1

10

100

1000

Doughb-15 grt

Sam

ple/

chon

drite

Sum

RE

Es

(ppm

)

Eu/Eu*0.0 0.5 1.0 1.5 2.0 2.5 4.06.08.0

050

100150200250300350400450500550600800 Iron Blow

WeekerooDoughboyPolygonumThunderdome

f

Eu/Eu*

0.0 0.5 1.0 1.5 2.0 2.5 4.0 6.0 8.00

50100150200250300350400450500550600800 Raven Hill

BH qtz garnetiteBH garnetite

Meningie WellMutooroo

BH qtz-grt-gah rock

Sum

RE

Es

(ppm

)

Fig. 8 Chondrite-normalizedrare earth element (REE) patternsand Eu anomalies (Eu/Eu*)of garnet from selected samplesreferred to in the text andamphibole from Meningie Well.a Raven Hill garnet (samplesCatR04-6A and 19). b RavenHill garnet (samples CatR04-21and 23) and LREE-rich inclusion(in sample CatR04-21).c Amphibole, Meningie Well.d Doughboy garnet (sampleDoughb-15). e and f. Eu/Eu* vs.Sum of REEs

62 A. Heimann et al.

Interelement correlations

Interelement correlations were obtained for major (in wt. %oxide and atoms per formula unit) and trace element (inppm) compositions of garnet from garnetite and quartzgarnetite from the Broken Hill deposit (59 analyses) andfrom Mutooroo (18 analyses), Iron Blow (20 analyses), andMeningie Well (31 analyses) (Tables ES3-ES6). Thecorrelations (R2>0.5) obtained are utilized in the discussionto interpret the significance of Eu anomalies in garnet.Tables ES3 to ES6 of the Electronic Supplement show theelements that are mentioned in the discussion and Table 5shows the summary of main element correlations, whereas

the complete correlation matrices appear in Heimann(2006). Correlation matrices for garnet from Raven Hill,Hunters Dam, Thunderdome, and Polygonum are notshown because garnet in some garnet-rich rocks from theselocations contain abundant mineral inclusions or they werenot much larger than the ablation beam.

Discussion

Major and trace element compositions of garnet

The major element chemistry of garnet in quartz garnetite andgarnetite at Broken Hill is expected to be dictated by the bulkcomposition of the host rock (Spry and Wonder 1989; Spry etal. 2003, 2007; Plimer 2006). This relationship is alsoevident elsewhere in the southern Curnamona Province, forexample, in Ca-rich garnets in garnet-rich rocks fromPolygonum and Weekeroo that occur in Ca-rich rocks. Sucha relationship between bulk-rock and garnet compositionneed not be the case when considering trace elementcompositions of garnet because these elements may beincorporated in the structure of garnet, other mineral phases(in or adjacent to the garnet), or they may occur along thegrain boundaries of various minerals. Therefore, the incor-poration of a specific trace element in garnet may depend onthe partition coefficient among garnet, mineral inclusions,and major phases in contact with garnet. This may be ofparticular importance for rocks that contain accessory toabundant amounts of feldspar, calcite, monazite, amphibole,and ilmenite.

Garnet in garnet-rich rocks from Doughboy coexists withfeldspar, calcite, fluorite, muscovite, biotite, and gahnite(Fig. 7a), and their high REE contents reflect the highamount of REEs in the host rock (Heimann et al. 2009).Garnet from garnetite and quartz garnetite from the BrokenHill deposit is enriched in Zn and Cr and depleted in Co, Y,and Ti compared to garnet in metasedimentary rocks andgarnet-rich rocks from the other sites. The high Cr (650 ppm)and Sc (270 ppm) contents of garnet in some garnetiterelative to garnet from quartz garnetite either indicates thepresence of accessory or trace minerals in quartz garnetitethat contain Cr and Sc, or, alternatively, the amount simplyreflects the different bulk chemistries of the two rock types.The concentration of these metals, as well as Co, may alsobe dependent on the composition of other minerals such asilmenite (e.g., Irving and Frey 1978). The Cr content of bulk-rock quartz garnetite and garnetite is very low (Heimann etal. 2009). Furthermore, Sc contents are also lower in quartzgarnetite than in garnetite. No Cr peaks indicative ofinclusions were identified in garnet from Broken Hill.Therefore, the lower Sc and Cr contents in garnet fromquartz garnetite compared to that in garnetite directly reflect

a

b

d

Li

Be B P

Sc Ti V Cr

Co Ni

Zn

Ga

Ge

Rb

Sr

Zr

Nb

Cs

Ba

La

Ce Pr

Nd

Sm Eu

Gd

Tb

Dy Y

Ho

Er

Tm Yb

Lu Hf

Ta

Pb

Th U

Sam

ple/

chon

drite

Sam

ple/

chon

drite

Li

Be B P

Sc Ti V Cr

Co Ni

Zn

Ga

Ge

Rb

Sr

Zr

Nb

Cs

Ba

La

Ce Pr

Nd

Sm Eu

Gd

Tb

Dy Y

Ho

Er

Tm Yb

Lu Hf

Ta

Pb

Th U

Sam

ple/

chon

drite

0.001

0.01

0.1

1

10

100

1000

10000

532-298

Li

Be B P

Sc Ti V Cr

Co Ni

Zn

Ga

Ge

Rb

Sr

Zr

Nb

Cs

Ba

La

Ce Pr

Nd

Sm Eu

Gd

Tb

Dy Y

Ho

Er

Tm Yb

Lu Hf

Ta

Pb

Th U

0.000010.0001

0.0010.01

0.11

10100

100010000

RavenHill meta-sedimentaryrock (CatR04-8)

Broken Hillgrt-sill gneiss(532-222B)

Broken Hill garnet in quartz garnetite

Broken Hill garnet in garnetite

Garnet in metasedimentary rocks

0.000010.0001

0.0010.01

0.11

10100

100010000

S77-11

Li

Be B P

Sc Ti V Cr

Co Ni

Zn

Ga

Ge

Rb

Sr

Zr

Nb

Cs

Ba

La

Ce Pr

Nd

Sm Eu

Gd

Tb

Dy Y

Ho

Er

Tm Yb

Lu Hf

Ta

Pb

Th U

Sam

ple/

chon

drite

Broken Hill garnet in quartz-garnet-gahnite rocks

c

0.000010.00010.0010.010.1

110

1001000

10000

Blackw5

Fig. 9 Representative chondrite-normalized trace element patterns ofgarnet from the Broken Hill deposit and metasedimentary rocks,southern Curnamona Province. a Broken Hill garnet in garnetite. bBroken Hill garnet in quartz garnetite. c Broken Hill garnet in quartz-garnet-gahnite rock. d Garnet from a garnet-sillimanite gneiss fromBroken Hill and a garnet-muscovite schist from Raven Hill

The composition of garnet in garnet-rich rocks 63

the chemistry of the host rock. Heimann et al. (2009) showedthat the Co content is lower in garnetite and quartz garnetitefrom Broken Hill than in garnet-rich rocks elsewhere in thesouthern Curnamona Province. Cobalt is a hydrogeneticelement (Bonatti 1975) and is highest in garnet in garnet-richrocks from Raven Hill and Meningie Well (Fig.10b, c),similar to what is observed in bulk-rocks, which indicatesthat the composition of garnet is dictated by its host rock,which, in turn, reflects the variable input of hydrogenetic andhydrothermal components in the protolith.

The highest Zn contents were found in garnet ingarnet-rich rocks from Broken Hill (1,070 ppm) and

Doughboy (855 ppm). The relative amount of Zn ingarnet will depend on whether there is another phasepresent in the same rock (e.g., gahnite, zincian ilmenite,sphalerite) that can incorporate Zn into its structure andhow much of it is present. Although there are traceamounts of gahnite or ilmenite present in garnet-bearingrocks at Doughboy, the relatively high Zn content ofgarnet is consistent with the enrichment of the rocks inZn compared to those from the other sites in the southernCurnamona Province (Heimann et al. 2009). On the otherhand, ilmenite that coexists with garnet at Mutooroocontains up to 20 wt. % ZnO, leaving garnet deficient in

Fig. 10 Binary plots of garnetcompositions (in ppm) fromgarnet-rich rocks from theBroken Hill deposit and nineother sites in the southernCurnamoma Province. Please,note the difference in scales. aEu/Eu* vs. Mn. b Co vs. Zn. cCo vs. Zn. d Y vs. Sc. e V vs.Cr. f Ti vs. Co. g Zn vs. Ti. hTime-resolved LA-ICP-MSsignal of a garnet analysisshowing peaks of Ni, Y, Zr, Ba,La, and Pr indicative of zoningand/or mineral inclusions(Raven Hill sampleCatR04-20-8)

a b

c

e

d

f

SampleCatR04-20-8

Time (sec.)80 100 120 140

coun

ts p

er s

econ

d

0

40

80

120

160

200

240NiYZrBaLaPr

Ce

Ni

La

Y

Zr

60400

g

Y (ppm)0 400 800 1200 1600 2000 2400

Sc

(ppm

)

0

100

200

300

400600800

1000CurnamonaBroken HillMetamorphic garnet

V (ppm)0 200 400 600 800 1000

Cr

(ppm

)

0

100

200

300

400

500

600700

CurnamonaBroken Hill

Eu/Eu*0 1 2 3 4 5 6 7 8 9

Mn

(a.p

.f.u.

)

0.0

0.5

1.0

1.5

2.0

2.5

Raven HillCurnamonaDoughboy 15BH garnetiteBH qtz garnetiteBH grt-gah rock

Co (ppm)0 25 50 75 100 125 150

Zn

(ppm

)

0

200

400

600

800

1000

Raven HillWeekerooDoughboyPolygonumThunderdome

Co (ppm)0 25 50 75 100 125150 300 350

0

200

400

600

800

1000

1200

Meningie WellMutoorooIron BlowBroken Hill

Zn

(ppm

)

h

Ti (ppm)0 1000 2000 3000 4000 5000

Co

(ppm

)

0

50

100

150

400CurnamonaBroken Hill

CatR04-13

Zn (ppm)0 200 400 600 800 1000

Ti (

ppm

)

0

1000

2000

3000

4000

5000

6000CurnamonaBroken Hill

64 A. Heimann et al.

Zn. The partitioning of an element between garnet and acoexisting mineral is clearly exemplified by the partition-ing of Eu between garnet and other minerals, and this isdiscussed in the next section.

REE patterns and Eu anomalies of garnet as indicatorsof genetic processes

The immobility of REEs during high-grade metamorphismwas advocated by, for example, Muecke et al. (1979),Taylor et al. (1986), and Bingen et al. (1996). It is nowwidely accepted that the REE patterns of rocks do notundergo significant modifications during alteration ormetamorphism under conditions of low fluid/rock ratios(i.e., f/r<100; Bau 1993). Europium anomalies in REEpatterns develop due to the variable oxidation state of Eu(i.e., Eu2+ and Eu3+), in contrast to the other REEs (exceptCe) that have only one oxidation state, which allows forthe fractionation of Eu during geological processes. Forexample, Sverjensky (1984) showed, based on theoreticalconsiderations, that at T>250°C Eu2+ prevails over Eu3+

in hydrothermal fluids, while Bau (1991) indicated thatunder these conditions the fluids develop positive ornegative Eu anomalies depending on whether the REEcontent of the fluid is governed by complexation orsorption processes. Other studies have shown that theREE pattern of a hydrothermal fluid and chemicalprecipitate depends on fractionation during precipitationof mineral phases, sorption of REEs onto constituentminerals, precipitation of REE-bearing minerals, temper-ature (T), oxygen fugacity (fO2), and pH of the fluid, andcompositional changes in the source (e.g., Bau 1991;Humphris 1998; Bach et al. 2003; Bau et al. 2003). In

garnet, the trivalent REEs, including Eu3+, substitute forthe major element cations predominantly in the VIII site(Jaffe 1951).

It has been suggested that the REE composition of garnetin metamorphosed exhalites reflects the composition oftheir precursor phases, which were formed from hydrother-mal and detrital sources on the sea floor, and can be used toevaluate the physicochemical conditions under which theyformed (Lottermoser 1988; Schwandt et al. 1993; Stalderand Rozendaal 2005). Lottermoser (1989) used the REEpatterns of metamorphic rocks from Broken Hill to interpretthe physicochemical characteristics under which the proto-liths formed. Lottermoser (1988) and Schwandt et al.(1993) presented four REE patterns of garnet from fourgarnetite samples, three proximal and one distal from thesulfide bodies at the Broken Hill deposit, and showed thatgarnet in the proximal samples has Eu/Eu*>1 whereas thatin the distal sample has Eu/Eu*<1. They considered thesign of the Eu anomaly in garnet to reflect the temperatureand fO2 conditions of the fluid from which the precursorphases precipitated.

Garnet from most samples of garnetite from theBroken Hill deposit has Eu/Eu*>1, whereas that fromquartz garnetite has Eu/Eu*<1 (Spry et al. 2007). Thesame anomalies are also observed for bulk-rock REEpatterns from these same rocks (Fig. 11a-e; Heimann et al.2009). In a minority of samples that contain other phasesthat incorporate REEs, however, the Eu anomaly of garnetand its host rock differ. For example, garnet that coexistswith feldspar in a sample of garnetite (Blackw5) from theBlackwood pit at Broken Hill has a negative Eu anomaly,whereas the bulk-rock has a positive Eu anomaly(Fig. 11c), which indicates that the negative Eu anomaly

Table 5 Main positive correlations for garnet and bulk-rock compositions from Broken Hill, Mutooroo, Iron Blow, Meningie Well, and RavenHill

Garnet compositions

Broken Hill Mutooroo Iron Blow Meningie Well

Mn Ca,Eu,Eu/Eu* Ca,Eu,Eu/Eu* Ca Eu

Eu/Eu* Eu,Mn Mn,Ca,Eu Fe –

Ca Eu,∑REE,HREE Eu, Eu/Eu*,Mn ∑REE,Mn –

Eu Eu/Eu*,Mn,Ca Eu/Eu*,Ca,Mn Eu/Eu*,Fe Mn

HREE Ca Fe Ca –

∑REE Ca Fe Ca –

Bulk-rock compositions

Garnetite, Broken Hill Quartz garnetite, Broken Hill Mutooroo, Iron Blow, and Raven Hill

MnO Al2O3,CaO,P2O5,V,Ga,Eu,Sn,Pb,Sb,Bi,Ag,Eu/Eu* Al2O3,CaO,P2O5,C,Sc,Ga,Y,Sr,Eu,Ta,W,Hg,Nb,HREEs

Al2O3,CaO,Ga,Ba,Eu,Sb,Eu/Eu*

Eu/Eu* CaO, P2O5,MnO,C,Sr,Sn,Pb,Sb,Bi,Ag Pb,As,Sb,Bi,Ag,Au CaO,MnO,Ga,Sn

Eu CaO,P2O5,MnO,C,Sr,Pb CaO,MnO,P2O5,C,Pb CaO,MnO,Co,Ga

C Eu,∑REE,Eu/Eu* MnO,CaO,Eu,W Pb

Correlations listed are >0.5. Only the positive correlations mentioned in the text are listed. The entire correlation matrices for garnet compositionscan be found in the Electronic Supplementary material and those for bulk-rocks in Heimann et al. (2009)

The composition of garnet in garnet-rich rocks 65

in garnet reflects the preferential partitioning of Eu intofeldspar.

Garnet in garnet-rich rocks from the other locations inthe southern Curnamona Province has flat HREE patternssimilar to those exhibited by garnet from Broken Hill,and Eu/Eu*<1. Most bulk-rock REE patterns havesimilar REE contents, HREE shapes, and the same typeof anomalies as the individual garnets within them(Fig. 11a-e). An exception to this similarity in REEpatterns between garnet and host rocks includes garnetsfrom Raven Hill, which have both Eu/Eu*<1 and smallEu/Eu*>1 and bell-shape REE patterns. Although theeffects of the tiny mineral inclusions were largely removedfrom garnet analyses after inspecting time resolved LA-ICP-MS signals, the major variation in REE compositionsof individual garnets at Raven Hill is probably due tocompositional zoning (i.e., higher amounts of Eu in someareas) and to the presence of these inclusions (Fig. 8b).Back-scattered electron imaging shows that such zoning isnot concentric but generally has a patchy distribution. Forgarnet samples from Raven Hill that have Eu/Eu*>1, the

quartz garnetite host rocks, which only contain ilmenite inaddition to garnet and quartz, have Eu/Eu* < or close to 1(Fig. 11d), suggesting that the anomaly in garnet is causedby Eu present in mineral inclusions. Alternatively, thepositive Eu anomaly and the bell-shape REE pattern is theresult of a late process of HREE removal, which wediscuss below.

Some samples that have been subjected to brittle-shearing or that contain chlorite formed during retrogrademetamorphism, have bell-shape REE patterns and thehighest V contents (Fig. 8a, b). Garnets in chloritizedsamples also have bell-shape REE patterns (sampleCatR04-19, Fig. 8a). These patterns suggest that shearingand chloritization have removed HREEs from the garnets,which probably were incorporated by chlorite and minormineral constituents. Increasing loss of HREEs from Eu toLu in these garnets would create a bell-shape pattern,which also results in a positive Eu anomaly in garnet(Figs. 8a, 11d). These bell-shape patterns are similar tothose obtained by Spry et al. (2007) for garnet inmetasomatic rocks (e.g., remobilized quartz garnetite and

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66 A. Heimann et al.

garnet envelope) from the Broken Hill deposit and indicatethat garnet from the southern Curnamona Province thatformed, or were affected, by post-depositional processes orbymetasomatic fluids have REE patterns that are distinct fromgarnet in unmodified metamorphic rocks. Although attemptswere made to not analyze rims that may have formed as aresult of post-peak metamorphic overgrowths, as has beendescribed by Dutch et al. (2005) for garnet in shear zones inthe southern Curnamona Province, it is possible that someanomalies could be the result of post-peak metamorphicmodification.

Compositional and crystallographic constraintson the REEs and the Eu anomaly in garnet

REE distribution between garnet and host rocksand the significance of the Eu anomaly

The REE distribution in monomineralic rocks is given bythe chemical composition of the bulk rocks and thecrystallographic control that the mineral exerts on thebulk-rock composition, whereas in polymineralic rocks thebulk-rock REE patterns largely reflect physicochemicalconditions (e.g., Corey and Chatterjee 1990; Lottermoser1992; Bach et al. 2003). The REE pattern of a givenmineral in monomineralic rocks formed from hydrothermalfluids will be determined by the crystallography of themineral. However, we emphasize that the bulk-rockcomposition depends on the physicochemical conditionsof the fluid as well (i.e., composition, P, and T). Inpolymineralic rocks, the REE distribution of a givenmineral will be determined by the partitioning of REEsbetween the mineral and coexisting minerals, which alsodepends on the physicochemical conditions of the fluid.Garnet-quartz rocks that do not contain significant amountsof accessory phases are useful for determining whetherthere is a crystallographic control on the REE distributionof garnet. Most importantly, if no other minerals crystal-lized out of the hydrothermal fluid, the major and REEelement composition of garnet will reflect the physico-chemical conditions of precipitation of the precursor phasesout of the hydrothermal fluid.

The REE studies of garnet from Broken Hill by Lotter-moser (1988) and Schwandt et al. (1993) yielded conflictingconclusions. Lottermoser (1988) found high LREE contentsin garnet and considered that there was no crystallographiccontrol on the abundance of REEs in garnet in garnetite. Incontrast, Schwandt et al. (1993) proposed that a crystallo-graphic control was necessary to explain the strongcorrelation (R2=0.97) between Eu2+ and Ca in garnet andthe high abundance of Eu in garnet. This controversy arosebecause the samples analyzed by Lottermoser (1988), incontrast to those utilized by Schwandt et al. (1993), consisted

of garnet and matrix material, where the majority of theLREEs resided. Moreover, Schwandt et al. (1993) found lowLREE/HREE ratios in garnets and that “an intergranularsubstance” located between garnet grains was enriched inthe LREEs. In order to understand the significance of theEu anomaly in garnet, we determined the distribution ofREEs in garnet and its respective host rock (Fig. 11). Inaddition, we used petrographic sample characterization toidentify other mineral phases that could incorporate REEsto understand the Eu anomalies. For example, in additionto garnet, phases present in the garnet-rich rocks such asmonazite, apatite, xenotime, zircon, calcite, and feldsparcan incorporate REEs into their structure. Because the sizeand sign of Eu anomalies have previously been used tointerpret the conditions of formation of the precursorphases at Broken Hill, here we evaluate a possiblecompositional control for these parameters by studyinginter-element correlations among the size of the Euanomaly (Eu/Eu* value), Eu, and major element contentsin garnet, and the relationship between Eu anomalies ingarnet and its host rock.

Garnet is enriched in the HREEs, compared to its hostrock, whereas the elements from La to Sm are depleted ingarnet and are more abundant in the rock compared togarnet (Fig. 11) indicating that HREEs are preferentiallypartitioned into the garnet structure relative to the host rock.This is because the ionic radii of the LREEs are verydifferent from that of Mn, Ca, and Fe, and due to thepresence of LREEs in some inclusions in garnet and inother minerals in the same rock (Fig. 8b). In most cases, thedistribution between garnet and the host rock for eachelement is almost identical for all garnets analyzed in thesame sample (Fig. 11).

A matrix correlation study for major and trace elementcompositions of garnet in quartz garnetite and garnetite fromthe Broken Hill deposit yield moderate to moderately strongpositive correlations between Ca and Eu (R2=0.4), total REEs(R2=0.7), and total HREE contents (R2=0.7), and betweenMn and Eu (R2=0.7) and the Eu anomaly (R2=0.5) (Table 5;Table ES3). These results show that the correlation betweenCa and Eu is considerably lower than the 0.97 correlationreported by Schwandt et al. (1993), but this is probably dueto the small number of samples analyzed by these authors.Moreover, Eu shows a higher correlation with Mn than withCa. In addition, the correlation between the Ca and HREEcontents indicates that the HREEs substitute for Ca in thegarnet structure, in addition to Mn. A moderately strongpositive correlation exists between the Eu/Eu* value and Mn(R2=0.5) contents of garnet, which is also observed for thehost rocks (Table 5; Heimann et al. 2009).

The correlations for the same elements at Mutooroo aresimilar to those for garnets from Broken Hill (Table 5;Table ES4), although garnet in garnet-rich rocks at

The composition of garnet in garnet-rich rocks 67

Mutooroo have large negative Eu anomalies. Garnet inquartz garnetite from Iron Blow, which is Fe rich, also haslarge negative Eu anomalies. In addition, the Eu/Eu* value ofgarnet from Iron Blow correlates positively with Fe (R2=0.98)and negatively with Ca (R2=-0.9) and Mn (R2=-0.8), whichreflects the low Mn and Ca content of garnet, and that thenegative Eu anomaly is related to the high Fe content ofgarnet and its host rock (Table 5; Table ES5; Heimann et al.2009). At Polygonum and Meningie Well, the lack ofsignificant correlations (not shown for Polygonum) betweenthe Eu/Eu* value and major element contents in garnetreflects the effect imparted by the presence of REE-bearinginclusions and amphibole (Table 5; Table ES6). We note herethat garnet in garnetite proximal to ore analyzed bySchwandt et al. (1993) exhibits the largest positive Euanomaly of the garnets analyzed by them and is enriched inMn and Ca, whereas the garnet in garnetite distal to oreshows the largest negative Eu anomaly and is Fe-rich, whichis consistent with our findings. These observations indicatethat the size and sign of the Eu anomaly is dictated bythe major element composition of garnet because as theMn content of garnet increases, the amount of Eu andthe Eu/Eu* value also increase, and as the Fe content ingarnet increases, the Eu/Eu* value decreases. This is thesame as what is observed in bulk rock analyses(Heimann et al. 2009).

This study of the major and REE element composition ofgarnet and its host rock shows that the Mn content and the Euanomaly in garnet are also dependent upon the composition ofthe host rock. The highest Mn contents in garnet are found ingarnetite that has the highest Mn values and Eu/Eu*>1. Incontrast, lowMn contents and Eu/Eu*<1 are found in garnetsin most quartz garnetite fromBrokenHill and other sites in thesouthern Curnamona Province that have low Mn and high Fecontents and Eu/Eu*<1 (Heimann et al. 2009). Inter-elementcorrelations for major and trace element compositions andthe Eu anomaly in bulk-rocks that host the analyzed garnetsare similar to those determined for the individual garnets(Table 5; Heimann et al. 2009). Although this similarity mayseem obvious at a first glance, it may not be the case if therocks contain other phases that host Mn, Fe, and REEs.Therefore, a bulk-rock compositional control, which ulti-mately is a crystallographic control in most garnet-quartz(“monomineralic”) rocks, explains the abundance of Eu andthe size of the Eu anomaly in garnet.

If we consider that the REE composition of the garnet-richrocks is the same as that of the precursor pre-metamorphicrocks, the observation that the Eu content is related to the Mncontent of garnet and its host rock indicates that in most rocksthat consist of garnet and quartz only, higher amounts ofMn inthe protolith allowed more Eu to be incorporated into theprecursor phases, which were later incorporated mostly intogarnet. Considering the difference in ionic radii between VIII-

fold coordinated Eu3+ and Fe2+ (0.146Å) compared to thatwith Mn2+ (0.106Å) and Ca2+ (-0.054Å; Shannon 1976), thelarge negative Eu anomalies in Fe-rich garnets and the strongpositive Eu anomalies in Mn- and Ca-rich garnets in garnet-quartz rocks can be explained by the preferential discrimi-nation of Eu by Fe-rich minerals (e.g., Bau 1993), in contrastto the preferential incorporation of Eu into Ca- and Mn-richminerals. Similar ionic differences exist if Eu2+ is consid-ered. The largest negative Eu anomaly (0.02) was found ingarnet from Mutooroo that contains low Eu (0.1 ppm) andMn (2.1 wt. % MnO), whereas the largest positive Euanomaly (up to 8.1) is present in garnet in garnetite fromBroken Hill that has high Mn (28.4 wt. % MnO) and Eu(6.9 ppm) contents. Therefore, the major and REE compo-sition and the Eu anomaly of the protolith, which wastransferred to the metamorphic rock, determined the majorelement composition of garnet, its REE content and pattern,and the sign and size of its Eu anomaly. In garnet-quartzrocks that do not contain other mineral phases thatsignificantly incorporate REEs, the Eu anomaly in garnetreflects the composition of the bulk-rock, which in turnreflects the REE composition and Eu anomaly of theprecursor precipitate. Following the ideas above, the REEcomposition of the precursor hydrothermal precipitatereflects the physicochemical conditions of the hydrothermalfluids from which they precipitated, and this is discussedbelow.

Implications for premetamorphic physicochemicalconditions

The observation that Eu anomalies in garnet in garnet-richrocks are determined by chemical and crystallographiccontrols on the protolith has implications for the interpretationof premetamorphic physicochemical conditions. It is general-ly accepted that the garnet-rich rocks are metamorphosedmanganese-bearing exhalites (e.g., Stanton 1976; Spry et al.2007; Heimann et al. 2009). In the formation of such rocks,manganese and iron are carried in high-temperature hydro-thermal fluids that contain high amounts of CO2 and underreduced conditions. Manganese can be deposited eitherproximal or distal to a hydrothermal vent under relativelyoxidized conditions (e.g., Borchert 1980). Chemical precip-itates on modern sea-floors, which consist of hydrothermalMn-Fe oxides, silica, carbonates, anhydrite, and baritetypically have Eu/Eu*>1 even at T << 250°C (e.g.,Humphris 1998; Bach et al. 2003; Hein et al. 2005).Considering the factors cited above that determine the REEpatterns, and the compositional data derived here from garnetin garnet-rich rocks, the Eu contents and the Eu/Eu* value ofthe hydrothermal fluid and the Fe-Mn(-Ca) oxides/hydrox-ides/carbonates, which are precursors to garnet in these rocks(Heimann et al. 2009), are dependent upon the: 1) relative

68 A. Heimann et al.

proportion of the hydrothermal fluid or chemical precipitateto the amount of detrital sediments, the last having negativeEu anomalies; 2) T and fO2 of the fluid, the amount of Eu2+

in the fluid, and precipitation rate and mixing of the fluidwith ambient water; and 3) major element composition of thehydrothermal fluid and resulting precipitate. The shape andtotal REE content of the garnet-rich rocks may reflect therelative contribution of hydrothermal versus detrital compo-nents to the precursor phases (e.g., Peter and Goodfellow1996; Spry et al. 2000). However, Heimann et al. (2009)showed that this cannot explain the differences in Euanomalies in whole-rock compositions because garnetiteand quartz garnetite from Broken Hill and garnet-rich rocksfrom other locations studied here have essentially the sameamount of REEs.

Large positive and negative Eu anomalies in normalizedpatterns of garnet in garnet-rich rocks indicate that Eu wasdecoupled from the trivalent REEs during its mobilization andprecipitation, and that Eu2+ likely predominated over Eu3+ (inthe rock and within the structure of the precursor phases togarnet). This indicates that the temperature of the hydrother-mal fluid that precipitated the precursor phases was probably≥250°C, as a minimum of ~250°C is required to producelarge Eu anomalies (Sverjensky 1984). Since positive Euanomalies can also be found in rocks that form from ahydrothermal fluid at T<250°C, other factors may also beimportant. For example, it is known that plagioclaseincorporates Eu, and therefore, a low Eu content and Eu/Eu*<1 in garnet can be explained by a low abundance of Eu2+

in the hydrothermal fluid due to a low degree of alteration ofplagioclase in basaltic source rocks (e.g., Graf 1977;Lottermoser 1992; Bau et al. 1998); in contrast, high Eucontents in garnet with Eu/Eu*>1 may have resulted fromthe alteration of significantly large quantities of feldspar andincorporation of Eu into the protolith. However, no chemicaldifferences are known among meta-basaltic rocks at BrokenHill and other sites that could indicate a different Eu source.Alternatively, a metamorphic fluid that lost Eu to aplagioclase-rich rock would display a negative Eu anomalyand produce a negative Eu anomaly in the final rock.However, it has been shown that metamorphism at BrokenHill was isochemical and mobilization of large volumes offluid did not occur (e.g., Stanton and Williams 1978).

The importance of the major element chemical composi-tion of garnet and its precursor phases and the partitioning ofREEs among mineral phases in determining the shape of theREE pattern is supported by other studies. For example, Bauet al. (2003) showed that the effect imparted by complexationand partitioning of REEs between rocks and mineral phasessuch as monazite, apatite, and titanite and hydrothermalfluids, as well as sorption processes and the mineralogy andcomposition of the precipitate strongly control the mobilityand fractionation of REEs in hydrothermal systems. There-

fore, the composition of the precipitate, which depends onthe composition of the fluid, complexing agents present, andthe degree of fractionation among minerals and between thefluid and minerals, determined the nature of the Eu anomalyin garnet in the southern Curnamona Province. The Euanomaly in garnet, which is inherited from the protolith,should be the same as that of the bulk-rock in essentiallymonomineralic (garnet-quartz) rocks.

Because the sign and size of the Eu anomaly of garnet ingarnet-rich rocks reflect the composition of the precursorminerals and the hydrothermal fluid, changing T, fO2, andcomposition of the hydrothermal fluid must have prevailedas sulfides and the precursors to minerals in the spatiallyassociated garnet-rich rocks were precipitated in the BrokenHill deposit. The negative Eu anomalies present in garnet inquartz garnetite in sites away from the Broken Hill depositreflect the low Mn and base metal content of thehydrothermal fluid. Strong negative Eu anomalies in Fe-rich, Mn-poor garnet from the Broken Hill deposit and theother localities reflect high temperature and low fO2

conditions of the hydrothermal fluid near its vent site. Sincestrong positive Eu anomalies are found only in Mn(-Ca)-richgarnet in garnet-rich rocks in the Broken Hill deposit, thephysicochemical conditions of the hydrothermal fluid musthave been significantly different than those prevailing atother sites studied here which were probably small, short-lived hydrothermal emanations. At Broken Hill, a long-livedhydrothermal system was responsible for the formation ofthe large sulfide deposit. In this evolved hydrothermalplume, mixing of the hydrothermal fluids with oxidizedseawater produced a decrease in the T and an increase in thefO2 of the fluid. This oxidized fluid allowed the precipitationof higher amounts of Mn in minerals that incorporated largerquantities of Eu in the precursors to garnet in garnetite atBroken Hill compared to Mn-poor quartz garnetite and bluequartz-garnet-gahnite rocks at Broken Hill and garnet-richrocks from the other sites away from Broken Hill. Suchchanges in the temperature and composition of the hydro-thermal fluids that produced Mn-bearing chemical precip-itates are expected at Broken Hill, as well as variations in thehydrothermal fluid flux with time, since similar fluctuationsare observed in modern hydrothermal environments such asat TAG in the Mid Atlantic Ridge (e.g., Bogdanov et al.1998; Herzig and Hannington 2000).

Ce anomalies in garnet

Modern seawater commonly has a negative Ce anomaly inchondrite-normalized REE patterns, whereas positive Ceanomalies are observed in hydrothermal and hydrogenetic Fe-Mn oxides and crusts deposited in seawater (Elderfield 1988;Bau et al. 1996; Hein et al. 2005). Negative Ce anomalies inseawater are developed because Ce3+ dissolved in seawater or

The composition of garnet in garnet-rich rocks 69

a hydrothermal fluid is oxidized to Ce4+, which is insoluble,and is removed from the fluid and adsorbed onto Fe-Mnparticles. This results in positive Ce anomalies in theprecipitates. Wright et al. (1993) suggested that the absenceof Ce anomalies in the REE patterns of exhalites analyzed byLottermoser (1989) indicate the lack of interaction betweenthe hydrothermal precipitates and basinal brines. REEanalyses of garnet in garnet-rich rocks and bulk rocks(Heimann et al. 2009) from the southern Curnamona Provinceobtained in this study also lack Ce anomalies. In contrast tothese results, Rozendaal and Stalder (2000) interpretednegative Ce anomalies in REE garnet patterns from the orezone at the metamorphosed Gamsberg Zn deposit, SouthAfrica, as an indication of prolonged exposure of theprotoliths of garnet-rich rocks to seawater. However, positiveCe anomalies are also visible in garnet patterns from Ca-Mnmarbles adjacent to that deposit and it is unclear what causedthese anomalies.

The absence of Ce anomalies in garnet and bulkgarnet-rich rocks in the southern Curnamona Provincecould be the result of minimal entrainment of precursorphases in oxidized seawater, or to the low PO2 levels ofmarine or lacustrine Paleoproterozoic deep water in thebasin hosting the iron-manganese-rich sediments, whichmight have been too low for stabilizing Ce4+ compounds(e.g., Bau and Möller 1993; Bau and Dulski 1996).Modern hydrothermal fluids, including black and whitesmoker fluids, however, do not have pronounced Ceanomalies in chondrite-normalized REE patterns (e.g.,Mitra et al. 1994). Therefore, we favor the alternative thatthe absence of Ce anomalies in garnet reflects precipita-tion of the precursor Fe-Mn oxides/carbonates from areduced hydrothermal fluid with low seawater and hydro-genetic contributions, which is consistent with our earlierobservations.

Comparison with compositions of garnetfrom the Gamsberg deposit, South Africa

Rozendaal and Stalder (2000) and Stalder and Rozendaal(2005) showed that garnet in garnet-rich rocks in theGamsberg deposit has LREE-depleted, HREE-enrichedpatterns with flat HREE trends and positive and negativeEu anomalies. REE patterns of garnet from the southernCurnamona Province that do not texturally show the effectsof retrograde metamorphism or shearing are almost identi-cal to those of garnet from Gamsberg. Garnets from ore-bearing rocks at Gamsberg have Eu/Eu* values>1,spessartine-almandine garnets in metamorphosed hydro-thermal rocks that envelope the deposit have values of Eu/Eu*<1, and those from metamorphosed calc-silicate rockshave Eu/Eu* values of 0 and<1. However, garnet frommetamorphosed hydrothermal rocks and ore lithologies

coexists with apatite, which has positive Eu anomalies(Stalder and Rozendaal 2005), and this could explain thenegative Eu anomaly in garnet. It is apparent that garnet ingarnet-rich rocks only has positive Eu anomalies in the orezones of both giant metamorphosed base metal sulfidedeposits.

Stalder and Rozendaal (2005) showed that garnet fromthe Gamsberg deposit is enriched in Zn and V anddepleted in Ti and Zr compared to garnet in metahydro-thermal host rocks (Fe-Mn-Ca-rich silicate and silicate-carbonate rocks, magnetite-quartz-apatite rocks, andquartz-hematite-gahnite rocks) and iron formations. Anenrichment in Zn and a depletion in Ti are also observedin garnets from the Broken Hill deposit. However, garnetfrom Broken Hill is not enriched in V or depleted in Zr.These enrichments and depletions in garnets from bothdeposits compared to those in garnet-rich rocks distal toore reflect the high Zn and the low detrital content of theore-bearing lithologies.

Trace element composition of garnet as an explorationguide

Based on the presence of positive and negative Eu anomaliesin garnet REE patterns in samples proximal and distal to theBroken Hill and Gamsberg deposits, Schwandt et al. (1993),Lottermoser (1988), and Stalder and Rozendaal (2005)proposed that positive Eu anomalies in garnet could be usedas an exploration guide in the search for Broken Hill-typedeposits. Although all samples of garnetite and quartzgarnetite derived from the Broken Hill deposit are consideredto be proximal to a fossil hydrothermal vent, the proximal ordistal position of samples collected from other locations inthe southern Curnamona Province is uncertain. However, theclose association of volcanic rocks at Doughboy (felsic),Weekeroo (mafic and felsic), and Raven Hill, Mutooroo,Meningie Well, Polygonum, Iron Blow, and Thunderdome(mafic) does suggest proximity. Regardless of the distal orproximal location of garnet in garnet-rich rocks to fossilhydrothermal vent systems, Mn-poor, Fe-rich garnet fromthese rocks in the southern Curnamona Province hasnegative Eu anomalies. At Broken Hill, Mn-poor, Fe-richgarnet in quartz garnetite and garnet-gahnite rocks hasnegative Eu anomalies, whereas Mn- and Mn–Ca-rich garnetin garnetite and carbonate- and hedenbergite-bearing rockshas positive Eu anomalies (Spry et al. 2007). Garnetcharacterized by Eu/Eu*>1 is found only in garnet-richrocks in close proximity to sulfides. It generally has high Mn(up to 28.4 wt. % MnO), Zn (1,200 ppm), Cr (650 ppm), andEu (6 ppm) contents, and low concentrations of Co, Ti, andY. Therefore, garnets with similar compositions can poten-tially be used as exploration guides to sulfides elsewhere inthe southern Curnamona Province.

70 A. Heimann et al.

Conclusions

Garnet in garnet-rich rocks from the southern CurnamonaProvince that has flat HREE patterns reflect the REEcomposition of the bulk-rock, which in turn reflects thephysicochemical conditions of the hydrothermal fluid fromwhich the precursor phases precipitated. Exceptions to thisare rocks that have textural evidence for post-peakmetamorphic or deformational effects, or evidence ofmetasomatic fluids being active during or after metamor-phism, in which case garnets, and generally their bulk-rocks, have bell-shape or flat REE patterns.

REE patterns of Mn- and Mn–Ca-rich, Fe-poor garnet ingarnetite and garnet-hedenbergite rocks from the BrokenHill deposit have values of Eu/Eu*>1, in contrast to valuesof Eu/Eu*<1 for Mn-poor, Fe-rich garnets in quartzgarnetite and garnet-gahnite rocks from Broken Hill andgarnet-rich rocks from other locations. The smallest Eu/Eu*values (0.02 for the largest negative Eu anomalies) werefound in garnet from Mutooroo with low Eu (0.1 ppm) andMn (2.1 wt. % MnO) contents, whereas the highest Eu/Eu*values (8.1for the largest positive Eu anomalies) are presentin garnet from Broken Hill garnetite with high Mn (28.44wt. % MnO) and Eu (6.9 ppm) contents. Inter-elementcorrelations and comparison of the distribution of REEsamong garnet, its host bulk rock, and coexisting mineralsindicate that the size of the Eu anomaly in garnet ischemically and, ultimately, crystallographically controlledby the Mn, Fe, Ca, and Eu content of the host rock, whichdetermines that of garnet, and the distribution of REEsamong garnet and accessory phases. In garnet-quartz rocks,the Eu/Eu* value of garnet reflects crystallographic-chemical controls of the bulk-rock, that are, in turn, relatedto the physicochemical conditions of formation of thehydrothermal precursor phases. The partitioning of REEsand other trace elements between garnet and coexistingminerals, and between garnet and its host rock, needs to beconsidered when making interpretations about the origin ofthe rocks or when using Eu anomalies as an explorationguide.

The exhalative or inhalative precursors to Mn-richgarnets with Eu/Eu*>1 formed from hot and more oxidizedfluids that deposited larger amounts of Mn compared tothose in the precursor to Fe-rich, Mn-poor quartz garnetiteand garnet-gahnite rocks at Broken Hill and garnet-richrocks in other sites in the southern Curnamona Province.Precursors to Mn-poor, Fe-rich garnet from quartz garnetiteand garnet-gahnite rocks at Broken Hill and those in quartzgarnetite in other sites that have similar REE patterns andEu/Eu*<1 formed from hot and more reduced fluids.Enrichments in Mn, Zn, Cr, and Eu/Eu*>1 are characteristicof garnet from Broken Hill, whereas enrichments in Co, Ti,and Y in garnet with Eu/Eu*<1 are common in garnet in

garnet-rich rocks in the other sites from the southernCurnamona Province. Garnet in garnet-rich rocks with Eu/Eu*>1, Eu up to 5 ppm, MnO>15 wt. %, Zn>400 ppm, andCr>200 ppm is only found in garnet from the Broken Hilldeposit and can potentially be used as guides in theexploration for Broken Hill-type deposits.

Acknowledgements This study was supported by U.S. NationalScience Foundation Grant EAR 03-09627, Primary Industries andResources South Australia, Havilah Resources, and a Society ofEconomic Geologists McKinstry Student Research Grant. Suzy Elhlouis thanked for assistance with LA-ICP-MS analyses. Many fruitfuldiscussions with Wolf Leyh, Ian Plimer, and Frank Spear aregratefully appreciated. Pedro Oyhantçabal is thanked for help withthe statistical analysis. Jesús de la Rosa kindly provided trace elementdata of garnets for comparison. Critical and constructive reviews byFrank Spear and Berndt Lottermoser helped improve an earlier versionof the manuscript. We thank Kurt Stuewe and Johann Raith forhandling our paper and for many beneficial annotations.

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