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Canadian MineralogistYol. 27, pp. 275-292 (1989)

MANGANESE-RICH GARNET ROCKS ASSOCTATED WITH THE BROKEN HILLLEAD_ZINC_SILVER DEPOSIT, NEW SOUTH WALES, AUSTRALIA

PAUL G. SPRY AND J. DAVID WONDER"Department of Geological and Atmospheric Sciences, 253 Science I, Iowo State Unive$ity, Ames, Iowa 50il1, U.S.A.

ABSTRACT

Rocks rich in manganoan gamet are spatially and geneti-cally related to metamorphosed sulfide deposits and repre-sent exploration guides. At the Broken Hill Pb-Zn-Agdeposit, New South Wales, Australia, three varieties ofgarnet-rich rock are found at wallrock margins or as blockswithin ore. These three rock tlpes can be distinguishedeither on the basis of mineralogy or by their geneticrelationshipto features of prograde orretrograde deforma-tion. Quartz-bearing garnetite, the most abundant garnet-rich rock type, contains up to 8090 garnet and can bedivided into at least seven subtypes on the basis ofmineral-ogy. Garnetite contains between 8090 and 9590 garnet andoccurs on the margins of the Lead lodes and as blocks inthe A lode. Layering in quartz-bearing garnetite and gar-netite is concordant to a schistosity that formed in thesurrounding country-rock during a prograde metamorphicevent. "Garnet envelope", a narroril zone of gamet-richrock at orebody margins, is variable, from discordant to aprograde schistosity, or is concordant to a retrograde schis-tosity in the wallrocks, or it surrounds late-stage quarlz-fluorite veins. The precursors to quartz-bearing garnetiteand garnetite formed when Mn-rich emanations fromhydrothermalhot springs mixedwithpelagic claysand weresubsequently metamorphosed. Patterns of garnet zonationand structural relationships suggest that ' 'garnet envelope''was produced during a retrograde metamorphic event bythe reaction of fluids from the ore horizon and Al-rich wallrocks. Calculation of equilibria between aqueous solutionsand Fe and Mn minerals at ore-forming temperaturesreveals tliat Fe oxides and sulfides are stable at lower oxida-tion potentials than Mn minerals. A model for mixing of ahydrothermal solution and seawater demonstrates that Feminerals normally precipitate from solution before Mnminerals, regardless of the nature of the aqueous speciespresent. This fractionation of Fe and Mn can be used toexplain the increase in Mn,/Fe from the stratigraphic foot-wall relative to the hanging wall of the Broken Hill deposit.

Keywords: manganoan garnet rocks' Broken Hill, Aus-tralia. massive sulfides, rrhole-rock analyses, electron-microprobe data, Fe-Mn fractionation.

Present address: Eugene A. Hickok and Associates, 10550New York Avenue, Suite C, Des Moines, lowa 50322,u.s.A.

SOMMAIRE

Des roches riches en grenat manganifdre sont associ6es'tant dans I'espace que dans le temps, aux glsements desulfures metamorphises, et representent ainsi des guides

utiles pour I'exploration de ceux-ci. A Broken Hill (New

South Wales, en Australie), gisement de Pb-Zn lg' nousdistinguons trois vari€tds de "grenatite" situds tout prbs des

roches encaissantes ou au sein des amas de minerai aumoyen des assemblages mindralogiques et des signes -ded6formation rdtrograde. La gxenatite quartzifdre' la plus

abondante, peut contenir jusqu'd 8090 de gxenat' et com-prend au moins sept sous-types diff6renciables par critCresmin6ralogiques. La grenatite, qui contient entre 80 et 9590de grbnat, caract6rise les bordures des zones min6ralisdes enplomb, et forme aussi des enclaves dans la zone A. Unrubanement dans la grenatite quartzifCre et la gxenatite estconforme e la schistosite qui marque un 6v€nement m6ta-morphique prograde dans les roches encaisantes. Une6troite envelope riche en grenat d la limite des zones mind-ralisdes prdsente un aspect assez variable; elle est discor-dante par rapport au plan de schistositd prograde ou

concordante avec le plan de schistosit6 rEtrograde, ou elleentoure les fissures i quartz-fluorite tardives. La grenatitequartzifbre et la grenatite repr€senteraient des pr€cipit6s

manganifdres issus des €vents hydrothermaux auxquels ont6t6 m€lang6s des argiles p6lagiques, le tout m€tamorphis€par la suite. La zonation des cristaux de grenat et les rela-tions structurales font penser que l'enveloppe riche en gre-

nat resulte du metamorphisme r6trograde qu'a causd lacirculation des fluides delazone min6ralis6e dans les rochesalumineuses de I'encaissant. Un calcul des dquilibres entresolution aqueuse et min6raux riches en Fe et Mn aux tem-p6ratures appropri6es pour la formation du minerai montreque les oxydes de fer et les sulfures sont stables d de plus

faibles potentiels d'oxydation que les mineraux riches enMn. Notre modOle de la prdcipitation anticipde suite i unmdlange d'une solution hydrothermale avec del'eau de merpredit la formation de min6raux riches en fer normalementavant ceux qui sont enrichis en Mn, quelle que soit la naturedes complexes aquerx. Cette s6paration du Fe et du Mnexpliquerait I'augmentation du rapport Mn/Fe A partir desroches sous-jacentes du gisement de Broken Hill vers lesroches stratigraphiquement plus 6levdes, qui recouwentcelui-ci.

Clraduit par la R€daction)

Mots-clds: roches riches en grenat manganifbre, BrokenHill, Australie, sulfures massifs, donndes chimiques(roches totales), donn6es par microsonde electronique,fractionnement Fe-Mn.

275

276 THE CANADIAN MINERALOGIST

INTRoDUcTIoN

Garnet-rich rocks, generally manganese-bearing,are associated with a variety of metamorphosedbase-metal sulfides at Broken Hill, Australia (Stan-ton 1976, Barnes et al. 1983), Gamsberg, SouthAfrica (Stumpfl 1979, Rozendaal & Stumpfl 1984),Aggeneys, South Africa (Ryan e/ al. 1982), Peg-mont, Australia (Vaughan & Stanton 1986) andMount Misery, Australia (Stanton 1982), goldmineralization at various localities (Valliant & Bar-nett 1982, Wonder et al. 1988) and stibnitemineralization in the Kreuzeck Mountains, Austria(Reimann & Stumpfl I 98 l). Although quartz-garnetrocks, or coticules, are commonly associated withthese ore deposits, additional minerals form sig-nificant petrological variants. Such variants includegahnite-garnet quartzites at Oranjefontein, SouthAfrica (Hicks et al. 1985) and Aggeneys, SouthAfrica (Spry 1987), garnet-magnetite-quartz rocksat Gamsberg, South Africa (Rozendaal & Stumpfl1984), Pegmont (Vaughan & Stanton 1986) andBroken Hill, Australia (Stanton 1976) and in westernGeorgia (Wonder et al. 1988), garnet-quartz-biotiterocks at Pegmont (Vaughan & Stanton 1986), andgarnet-quartz-muscovite rocks at Bousquet, Que-bec (Valliant & Barnett 1982).

Jacquet ( I 894) described a garnet-quartz rock anda friable garnet rock at the margins of the BrokenHill deposit and named them "garnet quartzite' ' and"garnet sandstone", respectively. The widely dis-cussed genetic relationship of these rocks to thedeposit has resulted in a variety of interpretations,including: l) derivation from manganiferous chemi-cal sediments (Richards 1966, Stanton 1976, Bil-lington 1979, Spry 1979), Q) isochemical metamor-phism of an original manganiferous sediment (Segnit1961, Haydon &McConachy 1987), and (3) relation-ship to either ore-forming solutions or to metaso-matic interaction between the deposit and the wall-rocks (e.g., Henderson 1953, O'Driscoll 1953,Stillwell 1959). Hodgson (1975) and Lee (1977) con-sidered that this metasomatic interaction occurredduring a high-grade metamorphic event.

The term "garnet rim" was introduced by Jones(1968) for narrow zones, 1 to 50 cm wide, formed oforange-brown garnet and qu artzthat occur intermit-tently along the contacts of the orebody and that arediscordant to a high-grade schistosity in the adjacentwallrocks. Jones suggested that ,,garnet rim,' and"garnet sandstone" formed by metasomatic ex-change of Mn and Ca between the orebody and theadjacent gneiss during a retrograde metamorphicevent. Further descriptions of ,.garnet rim,' havebeen made by Ransom (1969), Maiden (1972),Hodgson (1975) and Billington (1976). In conrrasnoJones (1968), Billington (1976) proposed that ,,gar-

net rim" formed during a prograde metamorphicevent by metasomatic exchange between theorebodies and the wallrocks.

Analyses by Hawkins (1968), Stanton er a/. (1978)and Plimer (1979) have shown that a manganeseanomaly is spatially associated with the Broken Hilldeposit. The data of Hawkins show that there is ageneral increase in Mn,/Fe from the footwall to thehanging wall of the deposit inthe southern end of thefield. The absence of data on Fe concentrations inthe northern orebodies precludes an accurate deter-mination of the variation of Mn/Fe in individualorebodies along strike. Despite this, the abundanceand nature of gangue minerals in the individualorebodies suggest ageneral increase of Mn/Fe in theorebodies from south to north @limer 1979).

The current study attempts to classify various gar-net-rich rock types associated with the Broken Hilldeposit and to resolve previous conflicts in inter-pretations ofthe origin ofthese rock types. A ther-mochemical study is used to explain the apparentincrease in the ratio Mn/Fe from the stratigraphicfootwall to the hanging wall of the deposit. Becauseof the association of rocks containing manganese-bearing garnet with othersulfide deposits, the ex-ploration potential of these rocks also is discussed.

GEoLocY oFTHE BRoKEN HILL Deposn

The Broken Hill deposit occurs in a distinctivesuite of rocks of the Purnamoota Subgroup, whichforms part of the Broken Hill Group within theProterozoic Willyama Complex (Willis el a/. 1983).The Complex is composed of metasediments withminor quartzofeldspathic, basic and ultrabasic rocks(Stevens et ol. 1980, Willis e/ al. 1983). AlrhoughStevens el a/. and Willis el a/. suggested that some ofthe minor rock types have volcanic precursors,Haydon & McConachy (1987) suggested tbat theywere all sediments originally. The Broken Hill Groupconsists of metapelites, amphibolites, felsic gneiss,garnet-plagioclase gneiss ("Potosi" gneiss),quartzitic gneiss and lode-horizon rocks @ig. I ). Thelast unit consists of garnet-rich rocks ("garnetquartzite", "garnet sandstone" and "garnet rim"),quartz-gahnite lode and lode pegmatite (Johnson &Klingner 1975, Barnes et al. 1983). "Garnetquartzite" is the most common garnet-rich rock typeand is often not associated with Pb-Zn mineraliza-tion (Barnes et al. 1983).

The Broken Hill Pb-Zn-Ag orebodies (300 mil-lion tons of >5Vo combined metals) were depositedat about 1,800 Ma and underwent a granulite-grademetamorphic event at about 1,700 Ma (Pidgeon1967, Shaw 1968). K-Ar and Rb-Sr mineral datesindicate an amphibolite-grade metamorphic event atabout 500 Ma @vernden & Richards 1961, Richards

ffig+t",t+++

I++

J

.B.l

+++*++

++

+++

+ + + + + h6++

+++

+ + +

+

c I l0n2000

SCALE metres

LEGEND

ttil fletrograde schistr--;4 Eonded tron

I I Jttttmanrc gneEs >'- | formation

ffi Granite gneiss 7- Potosi gneiss

- *!(!Y,',8[l'lode [*"{''Iitr A nph ibolite

. _ . _ . J

& Pidgeon 1963). Structural work by Laing et al.(1978) demonstrated four episodes of folding. Themine sequence occurs on a single inverted limb of afirst-generation fold, which overturned the ore-bodies. The lode horizon contains six separateorebodies, each of which has a characteristic gangueminefalogy and base metal content (Plimer 1979).Only in the Zinc Corporation (2.C.) mine are all sixlenses present. In order of stratigraphic successionthey are:

TopLead lodes

I lensA lodeB lodeC lode

277

A cross-section through five of the lenses in theNew Rroken Hill Consolidated (N.B.H.C.) mine isshown in Figure 2. The C lode occurs as weakstringer-type mineralization in the Z.C. mine.Several intersections of zinc-rich mineralizationknown asZinc lode have been exposed by workingsat the North Broken Hill (N.B.H.) mine.

Lurg et a l. (1 978) considered that the Broken Hillorebodies were deposited at the end of a period ofvolcanism and were succeeded by a thick unit ofnonvolcanogenic sediment (i.e., sillimanite gneiss).Lung et al. (L984) proposed the presence of ignim-brites and pumice-fall deposits in the enigmaticPotosi grreiss. However, the identityof these depositsmust be questioned since they have been subject togranulite-grade metamorphism. A volcanogenic-ex-halative source for the sulfides has been favored bya number of workers (e.9., Stanlon & Russel 1959,Plimer 1979, Willis e/ ql. 1983), and Both & Rutland(1976) considered'that sulfides were deposited whenmetal-rich saline brines migrated through a sedimen-

Mn?RICH GARNET ROCKS AT BROKEN HILL. AUSTRALIA

Flc. l. Geological map of the Broken Hill lode.

3 lens2 lens

Zinc lodes

F Q le6ls

f- a batLJ Alode

l::ii':!il 1 lsng

[f[fl z len"


Ftc. 2. Cross-section (No. 62) through the New BrokenHill Consolidated mine haulage shaft, looking southl8o east.

tary or volcanic pile onto the floor of a sedimentarybasin. Recently, Haydon & McConachy (1987) andWright et ol. (L987) suggesred rhar Pb-Zn-Agmineralization was related to compactive expulsionof metal-bearing brines during accumulation of athick sedimentary pile. Additional hypotheses sug-gest that ore fluids were derived from formationwaters (Johnson &Klingner 1975), downwardly con-vected seawater (Russel 1983), and metasomatizedmantle (Plimer 1985).

MINERALocY AND PETRoLoGYoF GARNET.RICH RocKs

Ithough we recognize the three garnet-rich rocktypes at Broken Hill that have been proposed byearlier workers (e.g., Johnson & Klingner 1975), wesuggest that the terms "garnet quartzite", "garnetsandstone" and "garnet rim" be replaced by"quartz-bearing garnetite", "garnetite", and "gar-net envelope", respectively. Quartz-bearing gar-netite with more than 3 9o of each accessory mineral

is further distinguished bythe nature of the accessoryminerals. Previous attempts at a classification of thequartz-bearing garnetites were made by Spry (1978)and Billington (1979), but are unsatisfactorybecauseboth classification schemes use a combination ofstructural and mineralogical terms. Furthermore,these authors refened to the rocks as garnet-bearingquaftzites, which implies that they were formed bymetamorphism of sandstone or chert. As will bediscussed later, such precursors are unlikely becauseof the high Al content of some of the quartz-bearinggarnetites. The main characteristics of the seventypes of quartz-bearinggarnetites are summarized inTable 1.

The term "garnet sandstone" is a misnomer thathas been retained in the literature on Broken Hillsince it was introduced by Jacquet (1894). The rockis not an unmetamorphosed sandstone as the nameimplies, but is a friable metamorphic rock in whichgarnet comprises over 8070 by volume. To avoidconfusion between a garnet-rich rock-type and theedge of an individual grain of garnet, the term, "gar-net rim" is considered unsatisfactory for discordantgarnet-rich rocks at the edge of orebodies. It is forthis reason that the term "garnet envelope" has beenintroduced. The main characteristics of garnetiteand garnet envelope are summarized in Table2, aadthe spatial relationship of garnet-rich rock types tothe orebodies is shown in Figure 3.

Quartz-bearing garnetite and garnetite containmany of the same minerals and exhibit similar tex-tures. For example, they are commonly massive @g.4a), exhibit a granoblastic texture @ig. 4b), or arelayered. These layers, 2 mm to l0 cm thick and up to10 m long, are defined by alternations ofgarnet andsulfide (Fig. 4c), variations in the size and color ofgarnet, and alternations ofgarnet with other silicates(Fig. ad). These layers reflect original compositionalbanding similar to that present in banded iron for-mation @ig . 5a) and sillimanite gneiss adj acent to thelode.

Quartz-bearing garnetite and garnetite are charac-terized by minerals that were stable during theprogrademetamorphic event. These minerals consistof quartz, garnet, biotite, apatite, gahnite, sphal-erite, pyrrhotite, plagioclase, orthoclase, hedenber-gite, and wollastonite, and commonly exhibit sharpcontacts between each other. The most conrmonassemblage is quartz-garnet and quartz-garnet-biotite (Figs. 4a,d). Because of the absence ofprimary muscovite or K-feldspar in most varieties ofquartz-bearing garnetite, it is not possible.to repre-sent the prograde assemblages on AFM diagrams.Staurolite (Fig. 5b), cummingtonite, muscovite, talc,pyrite and chlorite cut across grain boundaries andhave formed at the expense of prograde assemblages

Mn-RICH GARNET ROCKS AT BROKEN HILL, AUSTRALIA

tABLs l. CqARACTERISIICS 0P QUARTZ-8EABrtrC CARXEi?rtEeaFet Cortent, l,tllgralogjr Tstule Relatlvo Locatloaeraltr Slr€ ' sd (tn apprdl@to AiuadanceGan6t Color order of abundanco)

279

l. q@rtz up to 80ZrBemetl lo 0.1 @ to I cE,

oraage-broi toptuL

2. q@!tz- up to 802,btot l te 0.1 to 0.5 @,Sanetlte orsnge and ptok

3. quartz- 20 ro 8OZ,Sahnlto 0.5 to 5 @'ganotLte rod

4. Qurtr- l0 ro 202,oiUl,@ntte 0.1 to 0.3 @,gane!!,!o phl

5. quartz- lO to 2OZ,f,oldgpar 0.1 to 0.2 @'Saetlte otaBt€

6. Quartz- up to l5Z,corAlerl,t€ up to 5 @,San,alltet plnL

7 . qurtz- up to 502 r@acovlt6- !p to 0.5 @,atautoltlo oraage-redtoganall.to. ptnt'

Qte, crt, Bt, chli' @asiv€ or @€t abuq- all lens€stIh ' Gah' Ft ' Ap' . layeEed deot type horeverPl, Sl1, Mrs+, st t , ( iarely layeted etrdEbl+, Act+, Col, Gr, croee- aDd croas-sp' Po' ccp, Py+' cutttng) cuttlng!ias, Apy, Io, Ed

:ffi:J::i.-cen! lead

lodes

Qtz ' 8t, Cr!, gb1a, reak c@oduuet' Ap' c@t, ca' f,oltattousp, Po' Ccp, T€t 'Pyr ' Apy' Lo, Nic 'rlD' Gah, (?) Dys

a lL l€naea

Qtz' Crt , Oab, Bt 'Chl+' Po' gp' CB'st+, !ius+, Ib

Q t z , C ! t , 8 t , s i l ,Mu6i, sf , o! , Po,I13' ccp' Sp' Cb,Ca, Apy

qtz' P1' 0i ' Crt 'Bt' Chlr, Zo' Sp'en' Po, Ccp, Py'

Qtz, Crd, Bt, U!s+,Grt, Kfs, Cht+, ?1c+,Sp' Ga, Ccp' Po, Py+

Qtz, -!{ue+, sta, crt,Br, Cah, St l , Crd,t{o, ch1+, sp, cn,Ccp' Po' Py

@asi9e c@n all 1e!664,

EO6t C@Od

Ln @d

arouod B

and C Lod€s

Layeied ralein placestreak folt-atlon

@ggl?o aalo

roakly l6!e1ay€rod

B lods ana

I 16as ooly

neat @t81ns

1en6

1 lons atrd

C lode

reallt f,ale ietrogtade

foLl.at€d 6h€a! zoneg

.Aftor B:,lllngtd (1979)i Act acllnolr,te; Ap apatlt€i Apy alsenoptrltoi Bt btotltsiCb cubanltei Ccp chalcopyfl.t€; Chl chlorlte; Crd cordlerl.te; Cm cu@tngtotrlteiDys .lyscra6l.te3 fl f,l.uoltte; Gah Sahnit€; Go 8al€n6i tlbl honbl€nd€i Bd hedenbe!8lteills lh€aLte; Kfs R foldBpar; Lo 161-ltugtte; Mag @gBetlto; llus @6c@lteiNic nlctelln€a 0r orthoclasoi PI plagloclasoS Po pyrlhotlte; .Py pyrtte; Qtz q@rtziSt l stUt@nttei Sp sphaler l tei $t staurol l te; T6t tet lahedl l tei T1 ralc;t{o.s1la6t@1tG; Zo zol.6ite, + secondary Elneral

TABLE 2. CtrARACTERISTICS OT CARNEIIIE AND GARNET ENVELOPE*Cam6! Co!t6ut, Ulaela1ogy Texture Relatlvs LocatlouCraln Slze' ed (l,n approxlste Abudance

Canetl ta 80-952, 0.05 to0.5 m1 orange-brom to pl,!L

oanot' up to 80?r I meDvoLope to I 6, o!aa8s-

brm

Sratroblas t Lcnoealc oflBterlockl.lggraloa'16ye!ed inplacea

ganet f,onsLn graao-blstl.cuoa4ic 1!tlalns con-coldant toret!ogradeschl6tosi ty

co@! @lgttrs of 'and pods La1 , 2 a n d 3lenses atrdA lods

uu@on @lglos ofLead Lodea,and sutrouud-1o8 late-atage veidaln 2 leo6( N . B . u , C . )aod zlnqlode( N . B . U . )

Ab albttei Af,s alkalt f,eldspari BL boulangorltei CaL calcltei cub cubanrteiCzo c11@zoL6lte; Ep e,ildote; G! graphttei f,@ he@titei !{a! @rcaaltei Rt lutlle;Sc sche€llt€; Tl._ tLtultei Tro trollltot Ves vesuvlaol.t€; Zn zllgoii * otherBlberal abbrevLatLoE are llstsd i.o Tsb16 I

during a retrograde metamorphic event. The Globe-Vauxhall, British and De Bavay shear zones, whichwere active during the 500 Ma amphibolite-gademetamorphic event, contain an abundance of theseretrograde minerals.

In garnet envelope, garnet characteristically iseither discordant to a wallrock schistosity thatformed during a prograde metamorphic event, or ispresent in trains concordant to wallrock sshistositythat formed during a retrograde metamorphic event


Fic . 3 . Schematic reconstruction of the Broken Hill lode showing the spatial relation-ship of the most abundant garnet-rich rocks to the orebodies (modified fromHaydon & McConachy 1987).

Frc.4.a. Garnet (G) with quartz inclusions in quartz garnetite. The white mineral is quartz (Q), plane-polarized light. b.Garnet overgrowth on pre-existing garnet in garnetite, plane-polarized light. c. Folded sample of garnetite showingalternating layers ofgarnet (light-coloredbands) and sphalerite-galena intergrowths (dark-colored bands). d. Bandingproduced by biotite and garnet in quartz-biotite garnetite, plane-polarized light.

%

N.B.H.C. lz .c . l BH.S.t l

Zinc-l2

I N.B.H.

FJ.lOuartz garnetite I Ore lenses

l.'.'lcarnet ewelope El Gametite

7 tode pegmatite l:-:T,? g66n11s-tuartz garnetite

l-lp"litt", p"ummitlc and psammopelltic metasediments

.Niltl

s-;* ):::#

Mn-RICH CARNET ROCKS AT BROKEN HILL, AUSTRALIA

Frc. 5.a. Banded quartz gametite Qeft) and sutfide-bearing banded iron-formation (right). b. Intergrowth of staurolite(St), sillimanite (S), garnet and biotite in sillimanite-quartz garnetite, plane-polarized light. c. Garnet envelope (E) onthe margin of the 2 lens orebody. Note the rows of garnet parallel to a schistosity in the wallrocks that formed duringa retrograde metamorphic event. d. Garnet envelope surrounding a quartz vein in quartz garnetite .

281

(Fig. 5c). A second type of garnet envelope, pre-viously undescribed, occurs around late-stagequartz-fluorite veins in 2lens (N.B.H.C. mine) andquartz veins inZinc lode at N.B.H. mine (Fie. 5d).

CHEMISTRY OF GARNET.RJCH ROCKS

Mojor elements

Compositions of 17 samples of quartz-bearinggarnetite (predominantly quartz-garnetite) areshown in Figure 6a, and representative compositionsare shown in Table 3. MgO was omitted from Figure6a and 6b because it is generally less than l9o.Despite the small number of analyses and thepredominance of quartz-garnetite, it is apparent thatquartz-bearing garnetites from 2 lens contain a highproportion of Ca and Fe, those from 3 lens, a highproportion of Mn, and those from B lode and 1 lens,a high proportion of Fe. Additional iompositions of

some quartz-bearing garnetites from the southernmines are reported in Billington (1979).

Results of analyses of 16 samples of garnetite werereported by the Geological Subcommittee (1910),Andrews (1922), Stillwell (1922, 1959) and McKay(1974). McKay (197 4) aaalyzedtwo samples adjacentto the 2lens (N.B.H.C. mine), and Stillwell (1959)indicated that one sample was collected adjacent tothe 3 lens at Broken Hill South (B.H.S.) mine. Theother 13 samples came from the B.H.S. mine, butthere is no record of the nature of the orebody as-sociated with these samples. Despite this, Black(1953), in his description of the geology of B.H.S.mine, indicated that garnetite is confined to the 3lens. Results of one new analysis of garnetite rockfrom the 3 lens (ZC. mine, anal. ll, Table 3) areplotted with the previous I 6 analyses of garnetite inFigure 6b.

The only analyzed sample of garnet envelope thatsurrounds a quartz vein from the Zinc lode (N.B.H.mine) contains 7.690 MnO and 0.890 CaO (anal. 12,Table 3). Billinglon (1976) reported approximately

l l

/l

oo o o o o


Ftc. 6.a. Triangular CaO-MnO-FeO plot showing com-positions of quartz-bearing garnetite. b. Triangular plotshowing garnetite compositions; data for 2 lens samplesare from McKay (1974). All but two of the open circlesrepresetrt data from Black (1953), which are likely forgarnetites from the 3 lens @.H.S.). The other twodata points are samples from 3 lens (Stillwell 1959,this study).

1090 CaO and 1090 MnO in two samples of garnetenvelope adjacent to the 2lens (N.B.H.C. mine).

Composition of garnet

Previous studies of the composition of garnetwithin or in close proximity to the Broken Hill lodeinclude those of Hodgson (L975), Stanton (1976),Stanton & Williams (1978) and Plimer (1976, 1979).Hodgson (1975) examined the pompositional rela-tionship between garnet and coexisting pyroxenoidminerals in the lode, and Stanton (l 976) analyzed thegarnet in a transect across the B lode. In a 1000-msection enclosing the Broken Hill deposit, garnet inpelitic, psammitic and quartzofeldspathic rocks wasshown by Plimer (1976, 1979)to be Fe-rich.

Quartz-bearing garnetite and garnetite constituteapproximately 9590 of the garnet-rich rocks as-sociated with the Broken Hill deposit. Since theyconsist of garnet and quartz mainly, the variation inthe composition of garnet will be an excellent in-dicator of the variation of Mn, Fe, Ca, Mg, and Alin these rocks.

Garnet in 2l samples of quartz-bearing garnetite,7 of garnetite, and 3 of garnet envelope was analyzedusing an Etec Autoprobe at the University of Sydney.Operating conditions, using wavelength dispersion,were: accelerating voltage of 14.2 kV and a specimencurrent of between 0.04 and 0.07 p.A. Garnet (Mg,Al, Fe, and Si), rhodonite (Mn), wollastonite (Ca)and rutile (Ti) were used as standards. The electronmicroprobe was connected to an Interdata computersystem providing on-line datareduction. The operat-ing programs are written in BASIC, and data correc-tion is based on a ZAF-type program. Electron-microprobe data were corrected for dead-time, back-ground and beam-current-monitored drift prior toZAF correction. The garnet compositions arepresented in Figures 7a artd b, and representativecompositions are shown in Table 4.

Garnet in quartz-bearing garnetite from B and Clodes is almandine-rich (anal. 8, 9, 10, Table 4), andsimilar in composition to garnet analyzed by Plimer(1976) from country-rock gneisses and schists sur-rounding the deposit. However, garnet in garnet-richrocks from other lenses contains lower proportionsof the almandine component. Orange or browngarnet from the 3 lens and A lode quartz-bear-ing garnetites and garnetites generally contains ahigh proportion of spessartine (anal. l, 2, 6, 7, T able4), whereas garnet from the same type of rock in the2lens is rich in the grossular component (anal. 4, 5,Table 4). Although pink garnet is rare within theLead lodes, it is rich in the almandine component(anal. 3, Table 4).

Garnet in garnet envelope is optically and com-positionally zoned (Fig. 8). Small inclusions ofquartz typically separate overgrowths of garnet onearlier-formed cores. Overgrowths are generallydepleted in almandine and pyrope and rich in spes-sartine and grossular relative to the cores (anal. L2,Table 4), regardless of the mineral in contact with ir.In contrast, garnet in quartz-bearing garnetite andgarnetite is generally homogeneous in composition(anal. ll, Table 4). Exceptions exist where theserocks are spatially associated with garnet envelope,late-stage qua^rtz veins, and shear zones.

' DISCUSSIoN

Origin of the garnet-rich rocks

Hodgson (1975) and Lee (1977) suggested that

Mn-RICH GARNET ROCKS AT BROKEN HILL. AUSTRALIA

rM 3. rm-tmr (:mm DAIA or m-Rrm mfiS

283

r 2 3 4 5 6 7 A 9 1 0 r t 1 23 2U 303 343 2l9A 23t 24t 46 107 3r9 t20 315s@le Xo.

8tO2 I

It02

4ry3162O3*

M

!t8o

m

nazo

E2o

P205

803

Co

h

b

59.65 63.95

0.71 0.52

12.02 l ! .4t

13.?t s. !0

0.80 3.69

t.32 0.19

0.2! lo.?7

0.14 0.06

1.20 0.77

0.03 0.05

0.00 0.00

0.01 0.00

0.01 0.00

0.00 0.ro

o . t i 0 . 1 6

36.42 55.75

0.57 0.67

1 8 . 9 9 1 7 . ! 5

t0.17 t4.74

0.23 0.86

6.19 0.79

0.04 0.14

0.00 2.98

0.02 0.00

0 . r 7 0 . 0 0

1 . 4 0 0 . 0 r

o.oJ 0.00

0.01 0.02

75.86 56.35 65.32 57.U

0.33 0.r4 0.30 0.436.57 t3.77 9.81 12.60

9.9e 8.47 t8.75 23.38

l.3a t6.02 1.30 1.68

t.70 0.16 L?4 2.59

0.15 5.61 1.04 0.32

0.23 0.02 0.07 0.o2

t.67 0.00 0.99 0.61)

0.02 0.02 0.09 0.060.o2 0.00 0.00 0.010.05 0.00 0.00 o.o30.07 0.07 0.02 0.03

.036 0.@ o.za 0.o2

0.09 0.@ 0.17 0.00

55.70 S.97 40.30 75.71

0.0, 0.48 0.44 0.75

t2.r6 15.t0 18.26 rr .26

7.t1 14.39 L2.51 5.42

5.22 16.00 22,J8 3.02

0.13 0.54 0.97 0.60

t4. 3.17 5.29 0.46

0.03 ̂ 0.03 0.07 0.09

0 . r 2 0 . 0 3 0 . r 7 2 . 4 1

2.3\ 0.16 0.07 0.05

0,00 0.00 0.00 0.00

0.00 0.04 0.00 0.00

o.79 0.10 0.01 0.@

0.o1 0.r4 0.01 0.m

0,03 0.@ 0.00 0.20

99.€ 100,63 99.84 99.61 99.96 99.81 99.66 100,03 100.44 99.93 99.96 100.44

qotd trd €rp!$sed s tra2o3i fttulo! 1Gs3 l. q6rts 8a@ttt6, B lod€ (X.B.f,.C. tue)i 2. q@rtz 8an€ttta,A lds (!.B.f.C. lfu); 3. @ttr aatEttta, t 16@ (2.C. d!€); 4. quilr 8ana!lt€, I l€G (I.B,E.C. dre);5. 81u'd!a{|let @!l!d, A 1de (I.B.E.C. &s)t 6. srts samtlta, 216m (il.8.[. ea)37. Wtr gam!1t6' 2 &6 ([.4.8. Etus)t 8. qErtr Fn6tltd, 316G (tr.8.8. tuo)i 9. q@rtr Sar€ilrd,3l3B (r.8.f,. dn6); 10. qe!, @tl!6, Zh 1od6 (r.B.S. tu)i !1. quitr Satutlle, 3 los (2.C.e1!6)t 12. fnot 6ea1oF, Zls 1od6 (!.8.8. &.). rTazo dys6s d6rsdiad by fl@ phot@try, Cu-Z!-ndya€6 d6!6d6d bt l€chtr@ &-6 S&eth spetropbt@tet, suled @lys€ by L6co Aut@tlc SdtutD6t6tutol' dl olht c&B d€todod bt X{ay fhor$cma &cd$os atug a Sl@Ea 8s @hido(HveFlty of Ad€htd6).

O 3 lensI 2 lensa 1 lgnga Alodea BlodeEl ClodeO Zlnc lod€-N.B.H.mlre

a Garnet gnFlope

Frc. 7.a. Composition of garnet in quartz-bearing g.rnetite in terms of almandine (ALM), spessartine (SPES), pyrope(PYR), and andradite + grossular (ANDR + GROS). Areas outhned by dots enclose garnet compositions from thesame orebody. b. Composition ofgarnet in garnetite and garnet envelope.


TABLE 4. TEUICAL COI'IPOSI?IONS OT GARNET 1X GARNET-RICII ROCKS- 4 i 6 7 I 9 l o l l 1 2

cglte! ceDter center conte! c€nte! cente! centet celter center centel c€ntet edge center edSe

sahp le No. 71 119 l l7 350 241 279 284 3 l l 142 296 316 318

St02 Z 36,97 36.76T102 0 .09 0 .10A1203 19.75 20.30Re2O3. 1.18 0,79r€o 10 .19 8 .42!,tno 29.71 26.79ltgo 0.16 0.29CaO 2 .61 6 .31ToTAL: 100.66 99,75

A13+Fe3+rc*Uolrs

37 .54 37 .82 38 .07 38 . 150.02 0 .18 0 .10 0 .00

20.88 20 .54 20 .79 i8 .190 . 1 9 0 . 9 3 r , 2 5 3 . 7 0

29.38 16 .61 9 .98 11 .277,23 12 .77 9 ,O7 21.572 . 9 2 0 . 1 8 0 . 1 7 0 . 7 01.58 l l .3 l 20 .58 6 .27

99.74 rOO.34 100.11 .99.85

64.66 37.450.86 3 .133,85 29.54

13.54 0 .7 r1.7.08 29.t7

36.24 31 .06 37 .01 37 .34 37 .O30 . I I 0 . 0 4 0 . 0 0 0 . 0 1 0 . 0 5

2t .04 21 . I I 21 .45 21 .61 2r .290.00 0 .00 0 .00 0 .00 0 .00

11.06 33 .9r 31 .52 32 .73 23 .1322.63 3 .38 4 .86 4 ,23 15 .740 . 3 0 2 . 7 8 4 . 4 4 3 . 3 9 r . 9 48 . 6 9 1 . 9 5 0 . 9 1 0 . 7 0 0 . 9 2

100.07 r00 .34 100.23 100.01 100.10

36.76 38.20 18.760.0 i 0 .00 0 .00

21,35 20,49 20.340.00 0 .65 0 .81

23. t6 22 ,89 17 .7415.90 15 .77 17 .29

r . 7 5 1 . 7 2 0 . 8 80.95 0 .76 4 .O7

99.83 100.48 99 .89

5.979 6 . r89 6 .2940.002 0 .000 0 .0004.093 3 ,912 3 .8900.000 0 .088 0 .1103.148 3 . r01 2 .4072.191 2 .163 2 .3780.423 0 .415 0 .2120.166 0 . r37 0 .709

52.68 52,t4 42.140.00 2 .2 r 2 .894 . 7 r 2 . 3 0 9 , 5 36.88 6 .98 3 .72

35.66 36 .37 41 .68

6.016 5 .9840.012 0 .0123.840 3.8930.160 0 .1071.386 1 .1454.094 3,6930.038 0.0660.454 1 .10r

Atonic Proportlons (cation ba6ls 16)6.067 6.058 5.964 6.221. 5.842 5.966 5.900 6.003 6.0570.012 0 .002 0 .012 0 .000 0 .014 0 .004 0 .000 0 .002 0 .0063.975 3 .876 3 .837 3 .495 4 .000 4 .006 4 .031 4 .093 4 .1040.025 0 , t24 0 .163 0 .509 0 .000 0 .000 0 .000 0 .000 0 .0003.743 2 .224 1 .304 r .536 1 .491 4 .563 4 .200 4 .397 3 .1630.989 t . '132 1 ,203 2 .978 3 .087 0 .460 0 .656 0 .575 2 .1800.704 0 .042 0 .062 0 .172 0 .072 0 .666 I .057 0 .810 0 ,4730.273 r .940 3 .453 1 .094 r .499 0 .336 0 .156 0 . r20 0 .016

Ganet Propolt1on62t.6s 23.49 24.25 75.94 69.39 73.16 52.544,01 13 .11 0 .00 0 .00 0 .00 0 .00 0 .00

53.28 5 .8 r 243a 5 .37 2 .35 4 ,27 4 ,241.03 2 .98 1 .17 11 .04 17 .42 13 .19 1 .64

19.97 51 .52 50 .20 7 .65 10 .83 9 .38 35 .39

A1@dli€ 23.2I 19.06Alalradtto 4.15 2.68eroasuLa! 3.58 15.67rJEOpe q. oJ r . r lsp€asartls€ 58.55 61,.49

ale2o3 caLculat€d fto@ tbe F€3+ content, tho latter ras aeteatoed froo A13* defLctenclss lo the octahedral 6l.tei 1. qualtz

ganeitte (olege-colored ganet), 3 1en6 (tr.8.9. Elne)i 2. gan€tlte (oraogo-cololed Sarnet)' 3 lede ([.8.9. Etne);3. ganerl , ta (phl-coXoled ganet), 3 Leas (N.B,E. Elno)i 4. 8alnett te (otaage-colored ganet) ' 2 1oE (N.B.g.C. atre) i

5. quttz gaaetle (oraoge-colored ganet)r 2 l€os (I.B.n.C. ElBe); 6. qualtz Saaetlte (otaoge-colored gaF€8)' A lod€(tr .B.f , .C. oi ,ne) i 7. q@!tz 8a!!et l t€ (olaige-coLored Sanet), A lode (N.3.[ .C. atoe);8. q@!tz gaEeti te ( led-coloied

ganet), B loale (2.C. ol ,oe);9. q@ltz-Sahnlte ganet l t€ (red-color€d garnet)r B lode (N.B.U.C. otue)i 10. qertz-gahnlte

Sanett te ( !€d-color6d Sanet), C lod€ (2.C. dlde) i 11. quartz ganet l te (broo-coJ.ored Sanet) i Z18c lode (N.! .4. Elne)i12. ga@t elvelope (olatr86-brom colorod gaaet)' Zloc loae (N.B.E. nloe).

O.5 mmFtc. 8. Compositional profile across garnet in garnet en-

velope, sample 318, Zinc lode, N.B.H. mine. Symbols:ALM ahnandine, ANDR andradite, GROS grossular,PYR pyrope, and SPES spessartine.

quaftz-bearing garnetite and garnetite at Broken Hillwere derivedby Mn-Ca-rich fluids that were sweatedout of the Mn-Ca-bearing orebodies and thatreacted with the Al-rich wall-rocks during a progrademetamorphic event. The inferenca, therefore' is thatthese garnet-rich rocks should have formed onlywhere economic Pb-Zn-Ag mineralization waspresent. Such a suggestion is untenable because theserocks occur without Pb-Zn-Ag throughout the Wil-lyamaComplex.

Suggested precursors to the garnet-rich rocks arema4ganiferous chamositic beds (Stanton 1976) andvariants of original clean detrital sands (Haydon &McConachy 1987). Stanton considered that pennan-tite (a manganiferous thuringite) has a suitable com-position to be a precursor of spessartine garnet.Although chlorites such as pennantite and thuringitecould produce the variations in Mn, Fe, and Mgcontent in garnet, there i$ no apparent way by whichthese or any other chlorites could give rise to thegrossular or andradite component of garnet in gar-netite or quartz-bearing garnetite. A Ca-bearingmineral such as calcite or plagioclase would have tobe incorporated into the chlorite-rich sediments. Themain problem with Stanton's (1976) suggestion re-lates to the amount of pennantite that would berequired to produce the large volume of garnet-richrock in the Broken Hill lode and throughout the


Wilbama Complex. Pennantite is a rare chlorite innature and has not been reported in the amounts 18necessary to form the extensive Mn-bearing horizonsat Broken Hill. Similarly, unmetamorphosed 14equivalents of extensive manganese-bearing "cleandetrital sand units" as envisaged by Haydon & Mc-Conachy (1987) also areunknown. lo

Possible models to account for manganiferoussediments on the seafloor include halmyrolysis ofbasalt, low;temperature precipitation as nodules andcrusts, with diagenetic enrichment in the sedimentcolumn, and formation from hydrothermal vents(e.g., Wonder et al. 1988). A recent model proposedby Huebner et al. (1986) to account for the man-ganese-rich metasediments in the Buckeye mine,California, envisages Mn-gel forming at the sedi-ment-seawater interface and reacting with biogenicsilica. Although each of these models may accountfor the high Mn and Si content of the garnet-richrocks at Broken Hill, all are unable to account forthe high Al content of many of the garnetites andquartz-bearing garnetites.

Trace elements such as Cu, Co and Ni are knownto be more concentrated in hydrogenous Fe-Mnsediments than in hydrothermal sediments, largelybecause of the accumulation rate of hydrogenoussediment is much slower than forhydrothermal sediments, allowing scavenging of these seawater-derived elements by Fe- and Mn-mineral particles(Bonatti et sl. 1972, Toth 1980). Data for Cu, Co,Ni, Fe and Mn reported by Billington (1979) andrecent rare-earth-element analyses by Plimer & Lot-termoser(1988) ofgarnet-richrocks fromtheBrokenHill deposit show that they have a chemical signaturecharacteristic of hydrothermal sediments from theRed Sea and the East Pacific Rise.

The most plausible model to explain the presenceof garnetite and quartz-bearing garnetite at BrokenHill is for them to have formed near hydrothermalvents on the ocean floor. Likely precursors to garnetin garnet-rich rocks are minerals such as manganite,pyrolusite, hausmannite, vernadite or pyrochroite;these phases commonly are observed near hydrother-mal vents on the osean floor (e.9., Dymond et al.1913,Hackett& Bischoff 1973). Not only could thistype of setting account for the continuity of themanganiferous rocks, but Fe minerals such ashematite and goethite also are spatially related.Bostrdm & Peterson (1969) have described man-ganese- and iron-rich oxides in calcium carbonateson the ocean floor. If thesg minerals were incor-porated with detrital Al-Mg-bearing clays, all therequisite elements would be present to account forthose present in garnet and most other minerals ob-served in quartz-bearing garnetites and garnetites.

Billineton (1976) suggested that the precursor togarnetite originated by the introduction of Mn intounconsolidated pelitic sediments beneath the basin

285

o ) 2n-

pH

I E

1 4

1 0

3_?

6

pH

FIc.9. Mineral-solution stability diagrams vdth respect tope and pH at 125"C,2.3 bars, total aqueous sulfuractivity of 0.01, and aqueous metal species activities ofl0-) (solid lines) and l0-o (dotted contour). a. Stabilityfields for aqueous Fe and Fe oxides and sulfides, b.Stability fields for aqueous Mn and Mn oxides andsulfide.


1 8

t 4

1 0

A 2o_

- L JH "

PHFIc. 10. Mineral-solution stability diagrams with respect to

pe aj|.d pH at225"C,25.5 bars, total aqueous sulfuractivity of 0.01, and aqueous metal species activities oflO-s (solid lines) and 10-6 (dotted contour). a. Sta-bility fields for aqueous Fe and Fe oxides and sulfides.b. Stability fields for aqueous Mn and Mn oxides andsulfide.

of sulfide deposition. Billington's model, however,does not account for garnetite on both margins ofthe 3 lens at N.B.H. mine, not just on the footwallside. Billinglon (19'7A aho suggested that garnetenvelope was formed during the prograde metamor-phic event, whereas Maiden (1972) considered thatthe envelope was fsrmed during both the progradeand retrograde metamorphic events. Because garnetin garnet envelope forms trains that are parallel toretrograde schistosity or that cut across schistosityformed during a prograde metamorphic event, it issuggested that garnet envelope was formed by themetasomatic replacement of Al-rich wallrocks byMn and Ca during a retrograde metamorphic event.This suggestion is supported by the spatial associa-tion of garnet envelope with late-stage qvartz-fluorite veins (W.P. Laing, oral comm. 1978), theincorporation of late marcasite, pyrite andmonoclinic pyrrhotite in the garnet halo, and thepattern of compositional zonation in garnet. Inquartz-bearing garnetite and garnetite, garnet is typi-cally homogeneous in composition, as is garnetmetamorphosed to granulite grades elsewhere (e.g.,Tracy 1982). However, garnet in garnet envelopecommonly shows an overgrowth enrichedin spessar-tine and grossular and depleted in pyrope and alman-dine, and a garnet core that has the composition ofgarnet in quartz-bearing garnetite across which thegarnet envelope cuts. The garnet core is interpretedto have formed during a prograde metamorphicevent, whereas the overgrowth formed during aretrograde metamorphic event.

Chemical relationship of Mn and Feat the Broken Hill deposit

If we assume a hydrothermal source for the ore-bearing components, including Mn and Fe, and avent-type setting on the ocean floor, we must be ableto explain the increase in the ratio Mn/Fe from thefootwall to the hanging wall of the Broken Hilldeposit under the physicochemical conditions likelyto be encountered at an ore-forming hydrothermalvent.

Experimental studies by Bischoff & Dickson(1975), Seyfried & Bischoff (1977), andMottl et ol.(1979) have shown that heavy metals, including Mnand Fe, can be transported in significant quantitiesby circulation of seawater through hot oceanicbasalt. Furthennore, these studies have shown thatthe relative abundance of Fe and Mn in solutiondepends on parameters such aspfl, 4 the water-to-rock ratio, and the degree ofsolution-to-rock equi-libration.

The stabilities of phases of the system Mn-Fe-O-H in Eh-pH spaca at. 25o C were calculated by Creraret al, (L980). However, the stability of the system has

t 4

t o

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Received October 2, 1987, revised manuscript acceptedAugust 24, 1988.

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