Canadian Mineralogist Yol. 27, pp. 275-292(1989)
MANGANESE-RICHGARNETROCKSASSOCTATEDWITH THE BROKENHILL LEAD_ZINC_SILVERDEPOSIT,NEW SOUTH WALES, AUSTRALIA PAUL G. SPRYANDJ. DAVID WONDER" Department of Geological and Atmospheric Sciences,253 ScienceI, Iowo State Unive$ity, Ames, Iowa 50il1, U.S.A.
ABSTRACT
SOMMAIRE
Des rochesriches en grenat manganifdre sont associ6es' tant dans I'espace que dans le temps, aux glsements de sulfures metamorphises, et representent ainsi des guides utiles pour I'exploration de ceux-ci. A Broken Hill (New South Wales,en Australie), gisementde Pb-Zn lg' nous distinguonstrois vari€tdsde "grenatite" situdstout prbsdes roches encaissantesou au sein des amas de minerai au moyen des assemblagesmindralogiques et des signes-de d6formation rdtrograde. La gxenatite quartzifdre' la plus abondante,peut contenirjusqu'd 8090de gxenat'et comprend au moins sept sous-typesdiff6renciables par critCres min6ralogiques. La grenatite, qui contient entre 80 et 9590 degrbnat, caract6riselesbordures deszonesmin6ralisdesen plomb, et forme aussi des enclavesdans la zone A. Un rubanement dansla grenatite quartzifCre et la gxenatiteest conforme e la schistositequi marque un 6v€nementm6tamorphique prograde dans les roches encaisantes. Une 6troite enveloperiche en grenat d la limite deszonesmindralisdesprdsenteun aspect assezvariable; elle est discordante par rapport au plan de schistositdprograde ou concordante avecle plan de schistosit6rEtrograde, ou elle entoure les fissuresi quartz-fluorite tardives. La grenatite quartzifbre et la grenatite repr€senteraientdes pr€cipit6s manganifdresissusdes€ventshydrothermaux auxquelsont 6t6 m€lang6sdes argiles p6lagiques,le tout m€tamorphis€ par la suite. La zonation descristaux de grenat et les relations structurales font penserque l'enveloppe riche en grenat resulte du metamorphisme r6trograde qu'a causd la circulation desfluides delazone min6ralis6edanslesroches alumineusesde I'encaissant.Un calcul desdquilibres entre solution aqueuseet min6raux riches en Fe et Mn aux temp6raturesappropri6espour la formation du minerai montre que les oxydes de fer et les sulfures sont stablesd de plus faibles potentiels d'oxydation que les mineraux riches en Mn. Notre modOlede la prdcipitation anticipde suite i un mdlanged'une solution hydrothermale avecdel'eau demer predit la formation de min6raux riches en fer normalement avant ceuxqui sont enrichis en Mn, quelle que soit la nature des complexesaquerx. Cette s6paration du Fe et du Mn Keywords: manganoangarnet rocks' Broken Hill, Australia. massivesulfides,rrhole-rockanalyses,electron- expliquerait I'augmentation du rapport Mn/Fe Apartir des du gisementde Broken Hill vers les rochessous-jacentes microprobe data, Fe-Mn fractionation. roches stratigraphiquement plus 6levdes,qui recouwent celui-ci. Clraduit par la R€daction) Rocks rich in manganoangamet are spatially and genetically related to metamorphosedsulfide depositsand represent exploration guides. At the Broken Hill Pb-Zn-Ag deposit, New South Wales, Australia, three varietiesof garnet-rich rock are found at wallrock margins or asblocks within ore. Thesethree rock tlpes can be distinguished either on the basis of mineralogy or by their genetic relationshipto featuresof prograde orretrograde deformation. Quartz-bearing garnetite, the most abundant garnetrich rock type, contains up to 8090 garnet and can be divided into at least sevensubtypeson the basisofmineralogy. Garnetite contains between8090and 9590garnet and occurson the marginsof the Lead lodesand as blocks in the A lode. Layering in quartz-bearing garnetite and garnetite is concordant to a schistosity that formed in the surrounding country-rock during a prograde metamorphic event. "Garnet envelope", a narroril zone of gamet-rich rock at orebodymargins,is variable,from discordantto a progradeschistosity,or is concordantto a retrogradeschistosity in the wallrocks, or it surrounds late-stage quarlzfluorite veins. The precursors to quartz-bearing garnetite and garnetite formed when Mn-rich emanations from hydrothermalhot springsmixedwithpelagic claysand were subsequentlymetamorphosed.Patterns of garnet zonation and structural relationshipssuggestthat ' 'garnet envelope'' was produced during a retrograde metamorphic event by the reaction of fluids from the ore horizon and Al-rich wall rocks. Calculation of equilibria betweenaqueoussolutions and Fe and Mn minerals at ore-forming temperatures revealstliat Fe oxidesand sulfidesare stableat lower oxidation potentialsthan Mn minerals.A model for mixing of a hydrothermal solution and seawaterdemonstratesthat Fe minerals normally precipitate from solution before Mn minerals, regardlessof the nature of the aqueous species present.This fractionation of Fe and Mn can be usedto explain the increasein Mn,/Fe from the stratigraphic footwall relative to the hanging wall of the Broken Hill deposit.
Presentaddress:EugeneA. Hickok and Associates,10550 New York Avenue, Suite C, Des Moines, lowa 50322,
u.s.A.
Mots-clds: roches riches en grenat manganifbre, Broken Hill, Australie, sulfures massifs, donndeschimiques (rochestotales),donn6espar microsondeelectronique, fractionnement Fe-Mn.
275
276
THE CANADIAN MINERALOGIST
INTRoDUcTIoN Garnet-rich rocks, generally manganese-bearing, are associatedwith a variety of metamorphosed base-metalsulfides at Broken Hill, Australia (Stanton 1976, Barnes et al. 1983), Gamsberg, South Africa (Stumpfl 1979,Rozendaal& Stumpfl 1984), Aggeneys, South Africa (Ryan e/ al. 1982), Pegmont, Australia (Vaughan & Stanton 1986) and Mount Misery, Australia (Stanton 1982), gold mineralization at various localities (Valliant & Barnett 1982, Wonder et al. 1988) and stibnite mineralization in the Kreuzeck Mountains, Austria (Reimann& Stumpfl I 98l). Although quartz-garnet rocks, or coticules,are commonly associatedwith these ore deposits, additional minerals form significant petrological variants. Such variants include gahnite-garnet quartzites at Oranjefontein, South Africa (Hicks et al. 1985) and Aggeneys, South Africa (Spry 1987),garnet-magnetite-quartzrocks at Gamsberg,South Africa (Rozendaal& Stumpfl 1984), Pegmont (Vaughan & Stanton 1986) and BrokenHill, Australia (Stanton1976)andin western Georgia(Wonderet al. 1988),garnet-quartz-biotite rocks at Pegmont (Vaughan& Stanton 1986),and garnet-quartz-muscoviterocks at Bousquet, Quebec(Valliant & Barnett 1982). Jacquet( I 894)describeda garnet-quartz rock and a friable garnet rock at the margins of the Broken Hill depositand namedthem "garnet quartzite' ' and "garnet sandstone", respectively.The widely discussedgenetic relationship of these rocks to the deposithas resultedin a variety of interpretations, including: l) derivation from manganiferouschemical sediments(Richards 1966, Stanton 1976, Billington 1979, Spry 1979),Q) isochemical metamorphism of an original manganiferoussediment(Segnit 1961,Haydon &McConachy 1987),and (3) relationship to either ore-forming solutions or to metasomatic interaction betweenthe deposit and the wallrocks (e.g., Henderson1953, O'Driscoll 1953, Stillwell 1959).Hodgson(1975)and Lee (1977)consideredthat this metasomaticinteraction occurred during a high-grade metamorphic event. The term "garnet rim" was introduced by Jones (1968)for narrow zones,1 to 50 cm wide, formed of orange-browngarnet and qu artzthat occurintermittently along the contactsof the orebody and that are discordantto a high-gradeschistosityin the adjacent wallrocks. Jonessuggestedthat ,,garnet rim,' and "garnet sandstone" formed by metasomaticexchangeof Mn and Ca betweenthe orebody and the adjacent gneissduring a retrograde metamorphic event. Further descriptionsof ,.garnet rim,' have been made by Ransom (1969), Maiden (1972), Hodgson(1975)and Billington (1976).In conrrasno Jones(1968),Billington (1976)proposedthat ,,gar-
net rim" formed during a prograde metamorphic event by metasomatic exchange between the orebodiesand the wallrocks. Analysesby Hawkins (1968),Stantoner a/. (1978) and Plimer (1979) have shown that a manganese anomaly is spatially associatedwith the Broken Hill deposit. The data of Hawkins show that there is a general increasein Mn,/Fe from the footwall to the hangingwall of the depositinthe southernendof the field. The absenceof data on Fe concentrationsin the northern orebodiesprecludesan accuratedetermination of the variation of Mn/Fe in individual orebodiesalong strike. Despitethis, the abundance and nature of gangue minerals in the individual orebodiessuggestageneralincreaseof Mn/Fe in the orebodiesfrom south to north @limer 1979). The current study attempts to classifyvarious garnet-rich rock types associatedwith the Broken Hill deposit and to resolve previous conflicts in interpretationsofthe origin ofthese rock types.A thermochemicalstudy is used to explain the apparent increasein the ratio Mn/Fe from the stratigraphic footwall to the hangingwall of the deposit.Because of the associationof rocks containing manganesebearing garnet with othersulfide deposits,the exploration potential of theserocks alsois discussed. GEoLocYoFTHEBRoKENHILL Deposn The Broken Hill deposit occurs in a distinctive suite of rocks of the Purnamoota Subgroup,which forms part of the Broken Hill Group within the ProterozoicWillyama Complex(Willis el a/. 1983). The Complex is composedof metasedimentswith minor quartzofeldspathic,basicandultrabasicrocks (Stevenset ol. 1980, Willis e/ al. 1983). Alrhough Stevensel a/. and Willis el a/. suggestedthat someof the minor rock types have volcanic precursors, Haydon & McConachy (1987)suggestedtbat they wereall sedimentsoriginally. The BrokenHill Group consistsof metapelites,amphibolites,felsic gneiss, garnet-plagioclase gneiss ("Potosi" gneiss), quartziticgneissand lode-horizonrocks@ig. I ). The last unit consists of garnet-rich rocks ("garnet quartzite", "garnet sandstone"and "garnet rim"), quartz-gahnite lode and lode pegmatite (Johnson & Klingner 1975, Barnes et al. 1983). "Garnet quartzite" is the most common garnet-rich rock type and is often not associatedwith Pb-Zn mineralization (Barneset al. 1983). The Broken Hill Pb-Zn-Ag orebodies(300 million tons of >5Vo combinedmetals)weredeposited at about 1,800Ma and underwenta granulite-grade metamorphic event at about 1,700 Ma (Pidgeon 1967,Shaw 1968).K-Ar and Rb-Sr mineral dates indicate an amphibolite-grademetamorphic eventat about 500Ma @vernden& Richards1961,Richards
277
Mn?RICH GARNET ROCKS AT BROKEN HILL. AUSTRALIA
t + + + + + + + + + + * + + + I + + + + + + + + J
ffig+t", +++++ +++
h6
.B.l
c Il 0 n 2000
metres
SCALE
LEGEND ttil I
schist fletrograde I Jttttmanrc
r--;4 gneEs >'-
Eonded tron | formation
Potosi gneiss
ffi
Granite gneiss
-
Anphibolite *!(!Y,',8[l'lode[*"{''Iitr
7-
. _._.J
Flc. l. Geologicalmap of the Broken Hill lode.
& Pidgeon 1963).Structural work by Laing et al. A cross-sectionthrough five of the lensesin the (1978)demonstratedfour episodesof folding. The New Rroken Hill Consolidated(N.B.H.C.) mine is mine sequenceoccurson a singleinvertedlimb of a shown in Figure 2. The C lode occurs as weak first-generation fold, which overturned the ore- stringer-type mineralization in the Z.C. mine. bodies. The lode horizon contains six separate Several intersections of zinc-rich mineralization orebodies,eachof which hasa characteristicgangue known asZinc lode havebeenexposedby workings minefalogy and basemetal content (Plimer 1979). at the North Broken Hill (N.B.H.) mine. Only in the Zinc Corporation (2.C.) mine are all six Lurg et al. (1978)consideredthat the Broken Hill lensespresent.In order of stratigraphicsuccession orebodieswere depositedat the end of a period of they are: volcanism and were succeededby a thick unit of nonvolcanogenic sediment (i.e., sillimanite gneiss). Lung et al. (L984)proposedthe presenceof ignimTop 3 lens 2 lens Lead lodes brites and pumice-fall deposits in the enigmatic Potosigrreiss.However,the identityof thesedeposits must be questionedsincethey have beensubjectto granulite-grademetamorphism. A volcanogenic-exI lens halativesourcefor the sulfideshasbeenfavoredby A lode (e.9., Stanlon & Russel1959, B lode Zinc lodes a number of workersql. 1983),and Both & Rutland Plimer 1979,Willis e/ C lode (1976)considered'thatsulfidesweredepositedwhen metal-rich salinebrines migrated through a sedimen-
278
THE CANADIAN MINERALOGIST
F
Q le6ls
f-
a bat
LJ
Alode
l::ii':!il1 lsng
[f[fl z len"
(No. 62) throughthe New Broken Ftc. 2. Cross-section Hill Consolidatedmine haulageshaft, looking south l8o east.
is further distinguishedbythe natureof the accessory minerals.Previousattemptsat a classificationof the quartz-bearing garnetiteswere made by Spry (1978) and Billington (1979),but areunsatisfactorybecause both classificationschemesuse a combination of structural and mineralogical terms. Furthermore, theseauthors refened to the rocks asgarnet-bearing quaftzites, which implies that they were formed by metamorphism of sandstoneor chert. As will be discussedlater, suchprecursorsareunlikely because of the high Al content of someof the quartz-bearing garnetites. The main characteristics of the seven typesof quartz-bearinggarnetitesare summarizedin Table 1. The term "garnet sandstone"is a misnomerthat has been retained in the literature on Broken Hill sinceit was introducedby Jacquet(1894).The rock is not an unmetamorphosedsandstoneas the name implies, but is a friable metamorphicrock in which garnet comprisesover 8070 by volume. To avoid confusion between a garnet-rich rock-type and the edgeof an individual grain of garnet,the term,"garnet rim" is consideredunsatisfactoryfor discordant garnet-richrocks at the edgeof orebodies.It is for this reasonthat the term "garnet envelope" hasbeen introduced. The main characteristics of garnetite and garnetenvelopeare summarizedin Table2, aad the spatial relationship of garnet-rich rock types to the orebodiesis shownin Figure 3. Quartz-bearing garnetite and garnetite contain many of the same minerals and exhibit similar textures.For example,theyarecommonlymassive@g. 4a), exhibit a granoblastictexture @ig. 4b), or are layered.Theselayers,2 mm to l0 cm thick and up to 10m long, are definedby alternationsofgarnet and sulfide (Fig. 4c), variations in the sizeand color of garnet, and alternations ofgarnet with other silicates (Fig. ad). Theselayersreflectoriginal compositional banding similar to that present in banded iron formation @ig. 5a)and sillimanite gneissadj acentto the lode.
tary or volcanicpile onto the floor of a sedimentary basin. Recently,Haydon & McConachy(1987)and Wright et ol. (L987) suggesredrhar Pb-Zn-Ag mineralizationwas relatedto compactiveexpulsion of metal-bearing brines during accumulation of a thick sedimentarypile. Additional hypothesessuggest that ore fluids were derived from formation waters(Johnson&Klingner 1975),downwardlyconQuartz-bearinggarnetite and garnetite are characvectedseawater(Russel1983),and metasomatized terized by minerals that were stable during the mantle (Plimer 1985). progrademetamorphicevent.Thesemineralsconsist of quartz, garnet, biotite, apatite, gahnite, sphalMINERALocYANDPETRoLoGY erite, pyrrhotite, plagioclase,orthoclase,hedenberoF GARNET.RICH RocKs gite, and wollastonite,and commonly exhibit sharp contacts between each other. The most conrmon Ithough we recognize the three garnet-rich rock assemblageis quartz-garnet and quartz-garnettypes at Broken Hill that have been proposed by biotite (Figs. 4a,d). Becauseof the absenceof earlierworkers (e.g., Johnson& Klingner 1975),we primary muscoviteor K-feldsparin most varietiesof suggestthat the terms "garnet quartzite", "garnet quartz-bearinggarnetite,it is not possible.torepresandstone" and "garnet rim" be replaced by sent the prograde assemblages on AFM diagrams. "quartz-bearinggarnetite", "garnetite", and "garStaurolite(Fig. 5b), cummingtonite,muscovite,talc, net envelope", respectively. Quartz-bearinggar- pyrite and chlorite cut acrossgrain boundaries and netitewith more than 39o of eachaccessorymineral haveformed at the expenseof prograde assemblages
279
Mn-RICH GARNET ROCKS AT BROKEN HILL, AUSTRALIA tABLs l. CqARACTERISIICS 0P QUARTZ-8EABrtrC CARXEi?rtE eaFet Cortent, l,tllgralogjr Tstule Relatlvo (tn apprdl@to Aiuadance eraltr Slr€ ' sd Gan6t Color order of abundanco) q@rtz
l.
Bemetllo
up to 80Zr 0.1 @ to I cE, oraage-broi to ptuL
Qte, crt, Bt, chli' Ih' Gah' Ft' Ap'. Pl, Sl1, Mrs+, stt, Ebl+, Act+, Col, Gr, sp' Po' ccp, Py+'
!ias, Apy, Io, Ed
@asiv€ or layeEed (iarely croeecutttng)
@€t abuqdeot type
Locatloa
all lens€st horever layeted etrd aDd croascuttlng
:ffi:J::i.cen! lodes
lead
up to 802, 0.1 to 0.5 @, orsnge and ptok
Qtz ' 8t, Cr!, gb1a, uuet' Ap' c@t, ca' sp, Po' Ccp, T€t' Pyr' Apy' Lo, Nic' rlD' Gah, (?) Dys
reak f,oltattou
c@od
alL
20 ro 8OZ, 0.5 to 5 @' rod
Qtz' Crt, Oab, Bt' Chl+' Po' gp' CB' st+, !ius+, Ib
@asi9e
c@n
Sahnlto ganotLte
all 1e!664, EO6t C@Od Ln @d arouod B and C Lod€s
4.
QurtroiUl,@ntte gane!!,!o
l0 ro 202, 0.1 to 0.3 @, phl
sil, Qtz, C!t,8t, Mu6i, sf, o!, Po, I13' ccp' Sp' Cb, Ca, Apy
Layeied in placest reak foltatlon
rale
B lods I 16as
5.
quartzf,oldgpar Saetlte
lO to 2OZ, 0.1 to 0.2 @' otaBt€
qtz' P1' 0i' Crt' Bt' Chlr, Zo' Sp' en' Po, Ccp, Py'
@ggl?o
aalo
neat
QuartzcorAlerl,t€ San,alltet qurtz@acovlt6atautoltlo ganall.to.
up to l5Z, up to 5 @, plnL
Qtz, Crd, Bt, U!s+, Grt, Kfs, Cht+, ?1c+, Sp' Ga, Ccp' Po, Py+
roakly 1ay€rod
l6!e
1 lons C lode
up to 502 r !p to 0.5 @, oraage-redto ptnt'
Qtz, -!{ue+, sta, crt, Br, Cah, Stl, Crd, t{o, ch1+, sp, cn, Ccp' Po' Py
reallt
f,ale
ietrogtade 6h€a! zoneg
2.
q@!tzbtotlte Sanetlte
3.
quartz-
6.
7.
l€naea
ana ooly
@t81ns
1en6
foLl.at€d
atrd
.Aftor (1979)i Act acllnolr,te; Bt btotltsi B:,lllngtd Ap apatlt€i Apy alsenoptrltoi Cb cubanltei Ccp chalcopyfl.t€; Chl chlorlte; Crd cordlerl.te; Cm cu@tngtotrltei Dys .lyscra6l.te3 fl f,l.uoltte; Bd hedenbe!8ltei Gah Sahnit€; Go 8al€n6i tlbl honbl€nd€i lls lh€aLte; Kfs R foldBpar; Lo 161-ltugtte; Mag @gBetlto; llus @6c@ltei Nic nlctelln€a 0r orthoclasoi PI plagloclasoS Po pyrlhotlte; .Py pyrtte; Qtz q@rtzi Stl stUt@nttei T1 ralc; Sp sphalerltei $t staurollte; T6t tetlahedlltei t{o.s1la6t@1tG; Zo zol.6ite, + secondary Elneral
TABLE 2. CtrARACTERISTICS OT CARNEIIIE Cam6! Co!t6ut, Ulaela1ogy (l,n approxlste Craln Slze' ed
AND GARNET ENVELOPE* Texture Relatlvs Abudance
Canetlta
80-952, 0.05 to 0.5 m1 orangebrom to pl,!L
Sratroblas t Lc noealc of lBterlockl.lg graloa' 16ye!ed in placea
oanot' eDvoLope
up to 80?r I m to I 6, o!aa8sbrm
ganet f,ons Ln graaoblstl.c uoa4ic 1! tlalns concoldant to ret!ograde schl6tosity
Locatlou
co@!
@lgttrs of ' and pods La 1,2 and3 lenses atrd A lods
uu@on
of @lglos Lead Lodea, and sutrouud1o8 lateatage veida ln 2 leo6 ( N .B . u , C . ) aod zlnq lode (N.B.U. )
cub cubanrtei BL boulangorltei CaL calcltei Ab albttei Af,s alkalt f,eldspari Rt lutlle; !{a! @rcaaltei Ep e,ildote; G! graphttei f,@ he@titei Czo c11@zoL6lte; * other Zn zllgoii Sc sche€llt€; Tl._ tLtultei Tro trollltot Ves vesuvlaol.t€; i.o Tsb16 I Blberal abbrevLatLoE are llstsd
during a retrogrademetamorphicevent.The GlobeIn garnet envelope, garnet characteristically is Vauxhall, British and De Bavay shear zones, which either discordant to a wallrock schistosity that were active during the 500 Ma amphibolite-gade formed during a progrademetamorphicevent,or is metamorphicevent,contain an abundanceof these presentin trains concordantto wallrock sshistosity retrogrademinerals. that formed during a retrograde metamorphic event
280
THE CANADIAN MINERALOGIST
Zinc-l 2 %
BH.S.
N.B.H.C l z.. c . l tl
I
FJ.lOuartzgarnetite
I
l.'.'lcarnet ewelope
El Gametite
7
tode pegmatite
N.B.H.
Orelenses
g66n11s-tuartzgarnetite l:-:T,?
l-lp"litt", p"ummitlcand psammopelltic metasediments Fic . 3. Schematicreconstruction of the Broken Hill lode showing the spatial relationship of the most abundant garnet-richrocks to the orebodies(modified from Haydon & McConachy1987).
s-;* )::: .N
#
iltl
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 showing alternating layersofgarnet (light-coloredbands) and sphalerite-galenaintergrowths (dark-colored bands). d. Banding produced by biotite and garnet in quartz-biotite garnetite, plane-polarized light.
Mn-RICH CARNET ROCKS AT BROKEN HILL, AUSTRALIA
281
Frc. 5.a. Bandedquartz gametite Qeft)and sutfide-bearingbandediron-formation (right). b. Intergrowth of staurolite (St), sillimanite (S), garnet and biotite in sillimanite-quartz garnetite, plane-polarized light. c. Garnet envelope(E) on the margin of the 2 lens orebody. Note the rows of garnet parallel to a schistosity in the wallrocks that formed during a retrograde metamorphic event. d. Garnet envelopesurrounding a quartz vein in quartz garnetite .
(Fig. 5c). A secondtype of garnet envelope,pre- some quartz-bearinggarnetitesfrom the southern viously undescribed, occurs around late-stage mines are reported in Billington (1979). quartz-fluorite veinsin 2lens (N.B.H.C. mine) and Resultsof analysesof 16samplesof garnetitewere quartz veinsinZinc lode at N.B.H. mine (Fie. 5d). reported by the Geological Subcommittee(1910), Andrews (1922), Stillwell (1922, 1959)and McKay (1974).McKay (1974) aaalyzedtwosamplesadjacent CHEMISTRY OF GARNET.RJCH ROCKS to the 2lens (N.B.H.C. mine), and Stillwell(1959) indicatedthat one samplewas collectedadjacentto the 3 lens at Broken Hill South (B.H.S.) mine. The Mojor elements other 13 samplescame from the B.H.S. mine, but there is no record of the nature of the orebody asCompositions of 17 samplesof quartz-bearing sociated with these samples. Despite this, Black garnetite (predominantly quartz-garnetite) are (1953),in his descriptionof the geologyof B.H.S. shownin Figure6a,and representativecompositions mine, indicated that garnetite is confined to the 3 areshownin Table 3. MgO wasomitted from Figure lens. Resultsof one new analysisof garnetiterock 6a and 6b becauseit is generally less than l9o. from the 3 lens (ZC. mine, anal. ll, Table 3) are Despite the small number of analyses and the plotted with the previous I 6 analysesof garnetitein predominanceof quartz-garnetite, it is apparentthat Figure 6b. quartz-bearing garnetites from 2 lens contain a high The only analyzedsampleof garnet envelopethat proportion of Ca and Fe, those from 3 lens, a high surroundsa quartz vein from the Zinc lode (N.B.H. proportion of Mn, and thosefrom B lode and 1 lens, mine) contains7.690MnO and 0.890CaO (anal. 12, a high proportion of Fe. Additional iompositions of Table 3). Billinglon (1976)reported approximately
282
THE CANADIAN MINERALOGIST
Quartz-bearing garnetite and garnetite constitute approximately 9590 of the garnet-rich rocks associatedwith the Broken Hill deposit. Since they consistof garnetand quartz mainly, the variation in the composition of garnet will be an excellentindicator of the variation of Mn, Fe, Ca, Mg, and Al in theserocks. Garnet in 2l samplesof quartz-bearinggarnetite, ll 7 of garnetite,and 3 of garnetenvelopewasanalyzed usingan EtecAutoprobe at theUniversityof Sydney. Operatingconditions,using wavelengthdispersion, /l were: acceleratingvoltageof 14.2kV and a specimen o current of between0.04 and 0.07 p.A. Garnet (Mg, oo ooo Al, Fe, and Si), rhodonite (Mn), wollastonite (Ca) and rutile (Ti) wereusedas standards.The electron microprobewasconnectedto an Interdatacomputer systemproviding on-linedatareduction.The operating programsarewritten in BASIC, and data correction is based on a ZAF-type program. Electronmicroprobedatawerecorrectedfor dead-time,background and beam-current-monitoreddrift prior to ZAF correction. The garnet compositions are presentedin Figures 7a artd b, and representative compositionsare shownin Table 4. Garnet in quartz-bearing garnetite from B and C lodesis almandine-rich(anal. 8, 9, 10,Table 4), and similar in compositionto garnetanalyzedby Plimer (1976)from country-rock gneissesand schistssurroundingthedeposit.However,garnetin garnet-rich rocks from other lensescontainslower proportions of the almandine component. Orange or brown garnet from the 3 lens and A lode quartz-bearing garnetites and garnetites generally contains a Ftc. 6.a. TriangularCaO-MnO-FeOplot showingcomproportion of spessartine (anal. l, 2, 6, 7, T able positionsof quartz-bearing garnetite.b. Triangularplot high garnetfrom the sametype of rock in the 4), whereas garnetite showing compositions; datafor 2 lenssamples arefrom McKay(1974). All but two of theopencircles 2lens is rich in the grossularcomponent(anal. 4, 5, represetrt datafrom Black(1953),whicharelikely for Table 4). Although pink garnet is rare within the garnetitesfrom the 3 lens @.H.S.).The othertwo Lead lodes, it is rich in the almandine component data points are samplesfrom 3 lens (Stillwell 1959, (anal. 3, Table 4). this study). Garnet in garnet envelopeis optically and compositionally zoned (Fig. 8). Small inclusions of quartz typically separateovergrowths of garnet on 1090CaO and 1090MnO in two samplesof garnet earlier-formed cores. Overgrowths are generally envelopeadjacentto the 2lens (N.B.H.C. mine). depleted in almandine and pyrope and rich in spessartineand grossularrelativeto the cores(anal. L2, Table 4), regardlessof the mineralin contactwith ir. Composition of garnet In contrast, garnet in quartz-bearing garnetite and garnetite is generally homogeneousin composition Previous studies of the composition of garnet (anal. ll, Table 4). Exceptions exist where these within or in closeproximity to the Broken Hill lode rocks are spatially associatedwith garnet envelope, include those of Hodgson (L975), Stanton (1976), late-stagequa^rtzveins,and shearzones. Stanton& Williams (1978)and Plimer (1976, 1979). Hodgson (1975) examinedthe pompositionalrela' DISCUSSIoN tionship betweengarnet and coexisting pyroxenoid minerals in the lode, and Stanton (l 976)analyzedthe garnet in a transectacrossthe B lode. In a 1000-m Origin of the garnet-rich rocks sectionenclosingthe Broken Hill deposit,garnetin pelitic, psammitic and quartzofeldspathic rocks was Hodgson (1975) and Lee (1977) suggestedthat shownby Plimer (1976,1979)tobe Fe-rich.
283
Mn-RICH GARNET ROCKS AT BROKEN HILL. AUSTRALIA rM s@le 8tO2 I It02
Xo.
3. rm-tmr
r234567A910rt12 3 2U 303 343 75.86
56.35
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2l9A
23t
24t
46
107
3r9
t20
315
59.65
63.95
55.70
S.97
40.30
75.71
36.42
55.75
l!.4t
0.71 12.02
0.48
0.44
9.81
0.43 12.60
0.0,
6.57
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0.30
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0.75 rr.26
0.57 18.99
0.67 17.!5
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9.9e l.3a
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t8.75
23.38
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s.!0
7.t1
14.39
L2.51
5.42
t0.17
t4.74
1.30
1.68
0.80
3.69
5.22
16.00
22,J8
3.02
!t8o
t.70
0.16
L?4
2.59
t.32
0.19
m
0.15
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lo.?7
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0.23
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.036 0.09 99.€
0.00 0.00
0.00
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0.02 o.za
0.@
0.17
100,63
99.84
0.52
0.54
0.97
0.86
5.29
0.60 0.46
0.23
3.17
6.19
0.79
0.03 ^
0.03
0.07
0.09
0.04
0.14
0.r2 2.3\
0.03 0.16
0.r7 0.07
2.41
0.00
2.98
0.03
0.77 0.05
0.05
0.01
0.00
0.00
0,00
0.00
0.00
0.00
0.02
0.00
o.o3 0.03
0.01
0.00
0.00
0.04
0.00
0.o2
0.00 0.ro
o.79 0.o1
0.10 0.r4
0.00 0.@
0.r7
0.01 0.00
0.00 0.01 0.01
0.m
1.40 o.oJ
0.0r 0.00
0.00
o.ti
0.16
0,03
0.@
0.00
0.20
0.01
99.61
99.96
99.81
0.13 t4.
99.66
100,03 100.44
99.93
99.96
0.02 100.44
qotd
q@rtz trd €rp!$sed s tra2o3i fttulo! q6rts 1Gs3 l. B lod€ (X.B.f,.C. tue)i 2. 8an€ttta, 8a@ttt6, (!.B.f.C. (I.B,E.C. A lds lfu); quilr 3. I l€G dre); t 16@ (2.C. d!€); 4. @ttr aatEttta, 8ana!lt€, (I.B.E.C. (il.8.[. 5. 81u'd!a{|let A 1de &s)t 6. 216m ea)3 @!l!d, srts samtlta, gam!1t6' ([.4.8. (tr.8.8. q@rtr 7. 2 &6 qErtr Etus)t 8. tuo)i 9. 316G Wtr Sar€ilrd, Fn6tltd, (r.8.f,. (2.C. qe!, 3l3B quitr dn6); 10. 3 los Zh 1od6 (r.B.S. tu)i !1. @tl!6, Satutlle, rTazo dys6s e1!6)t 12. phot@try, 6ea1oF, Zls 1od6 (!.8.8. Cu-Z!-n &.). d6rsdiad by fl@ fnot dya€6 d6!6d6d bt l€chtr@ &-6 Sdtut Sð spetropbt@tet, suled @lys€ by L6co Aut@tlc D6t6tutol' dl olht c&B d€todod bt X{ay fhor$cma &cd$os atug a Sl@Ea 8s @hido (HveFlty of Ad€htd6).
O I a a a El O
3 lens 2 lens 1 lgng Alode Blode Clode Zlnc lod€-N.B.H.mlre
a Garnet gnFlope
Frc. 7.a. Composition of garnetin quartz-bearingg.rnetite in terms of almandine(ALM), spessartine(SPES),pyrope (PYR), and andradite + grossular(ANDR + GROS).Areas outhnedby dots enclosegarnetcompositionsfrom the sameorebody.b. Compositionofgarnet in garnetiteand garnetenvelope.
284
THE CANADIAN MINERALOGIST TABLE4.
sahple
No.
St02 Z T102 A1203 Re2O3. r€o !,tno ltgo CaO ToTAL:
A13+ Fe3+
rc*
Uo lrs
A1@dli€ Alalradtto eroasuLa! rJEOpe sp€asartls€
cglte! 71
ceDter 119
36,97 0.09 19.75 1.18 10.19 29.71 0.16 2.61 100.66 6.016 0.012 3.840 0.160 1.386 4.094 0.038 0.454 23.2I 4.15 3.58 q. oJ 58.55
center ll7
36.76 0.10 20.30 0,79 8.42 26.79 0.29 6.31 99,75 5.984 0.012 3.893 0.107 1.145 3,693 0.066 1.10r 19.06 2.68 15.67 r. r l 61,.49
ROCKS TEUICALCOI'IPOSI?IONS OT GARNET 1X GARNET-RICII lo 7 I 9 4 i 6 conte! 350
c€nte! 241
cente! 279
centet 284
celter 3ll
center 142
centel 296
edge
Ganet Propolt1on6 2t.6s 23.49 24.25 0.00 4,01 13.11 243a 53.28 5.8r 1.17 2.98 1.03 19.97 51.52 50.20
75.94 0.00 5.37 11.04 7.65
69.39 0.00 2.35 17.42 10.83
73.16 0.00 4,27 13.19 9.38
52.54 0.00 4,24 1.64 35.39
edSe
center 318
316
3 7. 5 4 3 7. 8 2 3 8 . 0 7 3 8 .1 5 3 6 . 2 4 3 1 . 0 6 3 7 . 0 1 3 7 . 3 4 3 7 . O 3 0.05 0.00 0.01 0.00 0.I I 0.04 0.02 0.18 0.10 21.45 21.61 2r.29 2 0 . 8 8 2 0 . 5 4 2 0 . 7 9 i 8 . 1 9 2 t . 0 4 2 1 .I I 0.00 0.00 0.00 0.00 0.00 0.19 0.93 r,25 3.70 29.38 16.61 9.98 11.27 11.06 33.9r 31.52 32.73 23.13 4,23 15.74 4.86 3.38 7,23 12.77 9,O7 21.57 22.63 r.94 3.39 2.78 4.44 0.70 0.30 2.92 0.18 0.17 0.92 0.70 8.69 1.95 0.91 20.58 6.27 1.58 ll.3l .99.85 1 0 0.10 1 0 0 . 0 1 r 0 0 . 3 4 1 0 0 . 2 3 1 0 0 . 0 7 99.74 rOO.34 100.11 Atonic Proportlons (cation ba6ls 16) 6.067 6.058 5.964 6.221. 5.842 5.966 5.900 6.003 6.057 0.012 0.002 0.012 0.000 0.014 0.004 0.000 0.002 0.006 3.975 3.876 3.837 3.495 4.000 4.006 4.031 4.093 4.104 0.025 0,t24 0.163 0.509 0.000 0.000 0.000 0.000 0.000 3.743 2.224 1.304 r.536 1.491 4.563 4.200 4.397 3.163 0.989 t.'132 1,203 2.978 3.087 0.460 0.656 0.575 2.180 0.704 0.042 0.062 0.172 0.072 0.666 I.057 0.810 0,473 0.273 r.940 3.453 1.094 r.499 0.336 0.156 0.r20 0.016 64.66 37.45 0.86 3.13 3,85 29.54 13.54 0.7r 1.7.08 29.t7
12
ll c€ntet
36.76 38.20 0.0i 0.00 21,35 20,49 0.00 0.65 23.t6 22,89 15.90 15.77 r.75 1.72 0.95 0.76 99.83100.48 5.979 0.002 4.093 0.000 3.148 2.191 0.423 0.166
6.r89 0.000 3,912 0.088 3.r01 2.163 0.415 0.r37
52.68 52,t4 0.00 2.2r 4.7r 2.30 6.88 6.98 35.66 36.37
18.76 0.00 20.34 0.81 17.74 17.29 0.88 4.O7 99.89 6.294 0.000 3.890 0.110 2.407 2.378 0.212 0.709 42.14 2.89 9,53 3.72 41.68
ale2o3 caLculat€d fto@ tbe F€3+ content, 6l.tei 1. qualtz lo the octahedral ras aeteatoed froo A13* defLctenclss tho latter (oraogo-cololed 3 lede ([.8.9. Etne); (olege-colored ganet), ganeitte 2. gan€tlte 3 1en6 (tr.8.9. Elne)i Sarnet)' (otaage-colored ganet)' 2 1oE (N.B.g.C. atre)i (phl-coXoled ganet), 3 Leas (N.B,E. Elno)i 4. 8alnettte 3. ganerl,ta (otaoge-colored gaF€8)' A lod€ (I.B.n.C. qualtz (oraoge-colored quttz ganet)r 6. gaaetle 2 l€os ElBe); 5. Saaetlte (led-coloied q@!tz gaEetite (olaige-coLored Sanet), (tr.B.f,.C. oi,ne)i 7. q@!tz 8a!!etlt€ A lode (N.3.[.C. atoe);8. (N.B.U.C. qertz-gahnlte (red-color€d 10. (2.C. garnet)r otue)i q@ltz-Sahnlte ganetlt€ B lode ganet), B loale ol,oe);9. (broo-coJ.ored Sanet)i Z18c lode (N.!.4. Elne)i (!€d-color6d Sanet), C lod€ (2.C. dlde)i 11. quartz ganetlte Sanettte (olatr86-brom Zloc loae (N.B.E. nloe). 12. ga@t elvelope colorod gaaet)'
O.5mm Ftc. 8. Compositional profile across garnet in garnet envelope,sample318,Zinc lode, N.B.H. mine. Symbols: ALM ahnandine, ANDR andradite, GROS grossular, PYR pyrope, and SPESspessartine.
quaftz-bearing garnetiteand garnetite at Broken Hill werederivedbyMn-Ca-rich fluids that weresweated out of the Mn-Ca-bearing orebodies and that reactedwith the Al-rich wall-rocks during a prograde metamorphicevent.The inferenca,therefore'is that these garnet-rich rocks should have formed only where economic Pb-Zn-Ag mineralization was present.Sucha suggestionis untenablebecausethese rocks occurwithout Pb-Zn-Ag throughout the WillyamaComplex. Suggestedprecursorsto the garnet-richrocks are ma4ganiferous chamositic beds (Stanton 1976)and variants of original clean detrital sands (Haydon & McConachy1987).Stantonconsideredthat pennantite (a manganiferousthuringite) hasa suitable composition to be a precursor of spessartinegarnet. Although chlorites suchaspennantite and thuringite could produce the variations in Mn, Fe, and Mg contentin garnet,therei$ no apparentway by which theseor any other chlorites could give rise to the grossularor andraditecomponentof garnetin garnetite or quartz-bearing garnetite. A Ca-bearing mineral suchas calciteor plagioclasewould haveto beincorporatedinto the chlorite-richsediments.The main problem with Stanton's (1976)suggestionrelates to the amount of pennantite that would be requiredto producethe largevolume of garnet-rich rock in the Broken Hill lode and throughout the
Mn-RICH GARNET ROCKS AT BROKEN HILL, AUSTRALIA
285
Wilbama Complex. Pennantiteis a rare chlorite in 18 nature and has not been reported in the amounts necessaryto form the extensiveMn-bearing horizons at Broken Hill. Similarly, unmetamorphosed 14 equivalents of extensivemanganese-bearing"clean detrital sandunits" as envisagedby Haydon & Mclo Conachy(1987)also areunknown. Possible models to account for manganiferous sedimentson the seafloor include halmyrolysis of basalt,low;temperatureprecipitationasnodulesand crusts, with diagenetic enrichment in the sediment o ) 2 column, and formation from hydrothermal vents n(e.g., Wonder et al. 1988).A recentmodel proposed by Huebner et al. (1986)to account for the manganese-richmetasedimentsin the Buckeye mine, California, envisagesMn-gel forming at the sediment-seawaterinterface and reacting with biogenic silica. Although eachof thesemodelsmay account for the high Mn and Si content of the garnet-rich rocks at Broken Hill, all are unable to account for the high Al content of many of the garnetites and quartz-bearing garnetites. Traceelementssuchas Cu, Co and Ni are known pH to be more concentratedin hydrogenousFe-Mn sedimentsthan in hydrothermal sediments,largely becauseof the accumulation rate of hydrogenous sedimentis much slower than forhydrothermal sedi IE ments, allowing scavengingof these seawaterderived elements by Fe- and Mn-mineral particles (Bonatti et sl. 1972,Toth 1980).Data for Cu, Co, 14 Ni, Fe and Mn reported by Billington (1979)and recent rare-earth-elementanalysesby Plimer & Lot10 termoser(1988)ofgarnet-richrocksfromtheBroken Hill depositshowthat they havea chemicalsignature characteristic of hydrothermal sedimentsfrom the Red Seaand the East Pacific Rise. The most plausiblemodel to explainthe presence of garnetite and quartz-bearing garnetite at Broken 3 _ ? Hill is for them to have formed near hydrothermal ventson the oceanfloor. Likely precursorsto garnet in garnet-rich rocks are minerals suchas manganite, pyrolusite, hausmannite,vernaditeor pyrochroite; thesephasescommonlyareobservednearhydrothermal vents on the osean floor (e.9., Dymond et al. 1913,Hackett& Bischoff 1973).Not only could this type of setting account for the continuity of the manganiferous rocks, but Fe minerals such as 6 hematite and goethite also are spatially related. Bostrdm & Peterson (1969) have describedmanpH ganese-and iron-rich oxides in calcium carbonates on the ocean floor. If thesg minerals were incorporated with detrital Al-Mg-bearing clays, all the FIc.9. Mineral-solution stability diagramsvdth respectto requisiteelementswould be presentto account for pe and pH at 125"C,2.3 bars, total aqueoussulfur thosepresentin garnetand most other mineralsobactivity of 0.01, and aqueousmetal speciesactivitiesof servedin quartz-bearing garnetitesand garnetites. l0-) (solid lines)and l0-o (dotted contour). a. Stability Billineton (1976)suggestedthat the precursorto fields for aqueousFe and Fe oxides and sulfides, b. garnetiteoriginatedby the introduction of Mn into Stability fields for aqueous Mn and Mn oxides and sulfide. unconsolidatedpelitic sedimentsbeneaththe basin
286
THE CANADIAN MINERALOGIST
18
t4 10
A2
o_
-LJ H"
t4
of sulfide deposition.Billington's model, however, doesnot account for garnetiteon both margins of the 3 lens at N.B.H. mine, not just on the footwall side. Billinglon (19'7A aho suggestedthat garnet envelopewasformed during the progrademetamorphic event, whereasMaiden (1972)consideredthat the envelopewas fsrmed during both the prograde and retrogrademetamorphicevents.Becausegarnet in garnet envelope forms trains that are parallel to retrogradeschistosityor that cut acrossschistosity formed during a prograde metamorphic event, it is suggestedthat garnet envelope was formed by the metasomaticreplacementof Al-rich wallrocks by Mn and Ca during a retrogrademetamorphicevent. This suggestionis supportedby the spatial association of garnet envelope with late-stage qvartzfluorite veins (W.P. Laing, oral comm. 1978),the incorporation of late marcasite, pyrite and monoclinic pyrrhotite in the garnet halo, and the pattern of compositional zonation in garnet. In quartz-bearinggarnetite and garnetite, garnetis typically homogeneousin composition, as is garnet metamorphosedto granulite gradeselsewhere(e.g., Tracy 1982). However, garnet in garnet envelope commonlyshowsan overgrowthenrichedinspessartine andgrossularand depletedin pyropeandalmandine, and a garnet core that hasthe compositionof garnet in quartz-bearing garnetite acrosswhich the garnet envelopecuts. The garnet core is interpreted to have formed during a prograde metamorphic event, whereasthe overgrowth formed during a retrogrademetamorphicevent.
to Chemical relationship of Mn and Fe at the Broken Hill deposit
PH FIc. 10.Mineral-solution stability diagramswith respectto pe aj|.dpH at225"C,25.5 bars, total aqueoussulfur activity of 0.01, and aqueousmetal speciesactivitiesof lO-s (solid lines) and 10-6 (dotted contour). a. Stability fields for aqueousFe and Fe oxides and sulfides. b. Stability fields for aqueousMn and Mn oxides and sulfide.
If we assumea hydrothermal sourcefor the orebearing components,including Mn and Fe, and a vent-typesettingon the oceanfloor, we must be able to explain the increasein the ratio Mn/Fe from the footwall to the hanging wall of the Broken Hill depositunder the physicochemicalconditionslikely to be encounteredat an ore-forming hydrothermal vent. Experimental studies by Bischoff & Dickson (1975),Seyfried& Bischoff (1977),andMottl et ol. (1979)haveshownthat heavymetals,including Mn and Fe, can be transportedin significant quantities by circulation of seawater through hot oceanic basalt. Furthennore, thesestudieshave shownthat the relative abundanceof Fe and Mn in solution dependson parameterssuchaspfl, 4 the water-torock ratio, and the degreeofsolution-to-rock equilibration. The stabilitiesof phasesof the systemMn-Fe-OH in Eh-pH spacaat.25oC werecalculatedby Crerar et al, (L980).However,the stability of the systemhas
Mn-RICH GARNET ROCKS AT BROKEN HILL, AUSTRALIA
B,F. (1986): HurnNnn,J.S.,Fr-oun,M.J.K. & JoNBs, Origin of Mn protore, FranciscanComplex,CA. Geol. Soc.Am, Abstr. ProgramstS,A2, Jacqunr,J.B. (1894):Geologyof the BrokenHill lode and Barrier Rangesmineral field, N.S.W. Geol. Surv.New SouthWalesMem. 5, l-149. JonNson,I.R. & Kuucxrn, G.D. (1975):BrokenHill oredepositandits environment.In EconomicGeologyof AustraliaandPapuaNewGuinea.l. Metals (C.L. Knight,ed.).Austral.Inst.Min. Metall.Mon. 5,47G491. andgarnetrims JoNes,T.R. (1968):Carnetsandstones at orebodycontacts,Broken Htll. In Broken Hill Mines-1968(M. Radmanovich& J.T. Woodcock, eds,\.Austral.Inst. Min. Metall., Mon.3,11l-178.
291
geochronological studyof the WillyamaComplex, BrokenHill, Australia.J. Petrol.8,283-3U. Pruvmn,I.R. (1976):Garnet-biotiterelationshipsin high grade metamorphicrocks at Broken Hill, O, Australia.Geol, Mag. ll3, 263-27 Sulphide rock zonation and hydrother-(1979): mal alteration at Broken Hill ., Austrdia. Inst. Min.
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Sml, S.S.& Nrsnnr, R.W. (1984):Acid volcanicprecursorto Potosi gneissat Broken Hill and its implicationsfor oregenesis.SeventhAust. Geol. Conv., 318-320(abstr.).
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setREIMaNN, C. & Sruuerl, E.F. (1981):Geochemical ting of strata-boundstibnite mineralization in the KreuzeckMountains, Austria. Inst. Min. Metoll. Trans.890,126-132. Rrcranos,J.R. & ProcroN,R.T. (1963):Someage measurementson micas from Broken Hill; Australia.J. Geol. Soc.Aust. 10.24-60. Rrcuenos, S .M. (1966):Thebandediron formationsat BrokenHill, Australia,andtheirrelationshipto the lead-zincorebodies.Econ. Geol.61,257-274. Rosrr,R.A., Heuncwev,B.S.& FtsuBn,J.R. (1978): Thermodynamicpropertiesof minerplsand related ar298.15K and I bar (10'pascals)pressubstances U.S. Geol. Surv. sureand at highertempeiatures. 8u11.7452.
MoncaN,J.J. (1967):Chemicalequilibriaand kinetic RozeNoeaL, A. & Sruuprl, E.F. (1984):Mineral propertiesof manganese in naturalwaters..lz Princhemistryand genesisof Gamsbergzinc deposit, ciplesand Applicationsof Water Chemistry(S.D. South Africa. Inst. Min. Metall. Trans. Br93.16lFaust&J.V. Hunter,eds.).JohnWiley&Sons, New 175. York (561-624). halosurroundingthe Russrrr,M.J. (1974):Manganese MorrL, M.J., Holr-aNo,H.D. & Conn,R.F. (1979): Tynagh ore deposit,Ireland: a preliminarynote. Chemicalexchangeduring hydrothermalalteration Inst. Min. Metall. Trans.B,E3,65-66. of basaltby seawater.II. Experimentalresultsfor Fe, Mn, and sulfur species.Geochim.Cosmochim, (1983):Major sediment-hosted exhalativezinc Acta43,869-884. + leaddeposits:formationfrom hydrothermalconvectioncellsthat deependuring crustalextension. O'DnrscolqE.S.(1953): TheZincCorporationandthe Mineral. Assoc. Can., Short CourseHandbook I, NewBrokenHill Consolidated mines.In Geologyof 25t-282. Australian Ore Deposits(A.8. Edwards,ed.). Awtral. Inst. Min. Metall. 1, 658-673. A.L., Lrrsox,R.D., Moonr, Rvalq,P.J., LawneNce, D.P. & VeNZvr, D. J.M., PerrnsoN, A., SrroruaN, (1982):The Aggeneys basemetalsulphidedeposits, P r o c e o xR , . T . ( 1 9 6 7 ) :A r u b i d i u m - s t r o n t i u m
292
THE CANADIAN MINERALOGIST
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ReceivedOctober2, 1987,revisedmanuscriptaccepted August24, 1988.
