Canadian Mineralogist Vol. 30, pp. 145-152(1992) A GAHNITE_GARNETRETROGRADE REACTION FROM THE PINNACLESDEPOSIT, BROKENHILL, NEW SOUTHWALES, AUSTRALIA JOANNA PARR Deporlmentof Geologt, Universityof Newcastle,Newcastle, New South Woles2308, Australia ABSTRAcT INTRODUcTIoN A retrogradereaction involving the breakdown of the The spinel gahnite representsthe zincian end- zincian spinel gahnite [(Zn,Fe,Mg)Al2O4]to almandine member of the gahnite - hercynite solid-solution garnet occurs in samples from the Pinnacles mine, series Although gahnite is southwestof Broken Hill, New [(Zn,Fe,Mg)Al2Oa]. South Wales, Australia. it has been reported in Grains of prograde gahnite are rimmed by a retrograde relatively rare in nature, corona of gamet in a quafiz matrix. The gahnite grains association with metamorphosedmassive sulfide exhibit no compositional zoning, which suggeststhat deposits(Plimer 1977,Williams 1983,Sheridan & homogenization occurred during prograde metamor- Raymond1984, Spry & Scott1986, Spry 1987a,b). phism. The retrograde rim of garnet, however, exhibits The high Zn content of the spinel, together with some compositional zoning away from the gahnite - its hardness(8) and stability in metamorphicrocks, garnet interface and contains trace amounts of Zn. The make it a potential indicator mineral in base-metal assemblagewas probably created by the formation of programs(Sheridan & Raymond 1984, garnet gahnite presence exploration from in the of solutionsenriched have been in Fe, Mn, Mg, Ca, Na and Si, with the loss of Zn. These Spry & Scott 1986). Several theories observations, together with sillimanite-bearing pelitic proposedfor the formation of gahnitein metamor- .assemblages,are consistentwith isobaric cooling during phic rocks. These include the desulfurization of retrogrademetamorphism. sphalerite(Sangster & Scott 1976),the breakdown of Zn-bearing silicates such as biotite (Dietvorst Keywords: zincian spinel, gahnite, electron-microprobe 1980) and staurolite (Stoddard 1979) during data, retrograde metamorphism, Broken Hill, prograde metamorphism, and the metamorphism Australia. of opaline silica and hydrothermal clays with adsorbedZn and zincian carbonate(Plimer 1988). SouuarnE Quartz - gahnite rock is the most common gahnite-bearinglithology in the Willyama Super- Une r€action r€trogradeimpliquant la d6stabilisation group, New South Wales; other rock typesinclude de la gahnite [(Zn,Fe,Mg)Al2Oa]pour donner un grenat psammitic to pelitic metasedimentsand quartz - riche en almandin fait l'objet de cette contribution e la feldspar- biotite - garnet- gahnitegneisses (Barnes min6ralogiede la mine Pinnacles,au sud-ouestde Broken et al. 1983), Gahnite is commonly observedas a Hill, au New South Wales, en Australie. Les cristaux de ganguemineral in the base-metalores of the Broken gahnite, prograde, d'origine ont une couronnede grenat Hill deposit,within the Willyama Supergroup(e.9., r€trograde dans une matrice de quartz. L'absence de Segnit 1961, Richards 1966, Plimer 1976, 19'77, zoflation en compositiondans la gahnite fait penserqu'il y a eu homogdndisationlors d'un 6pisodede m6tamor- Spry 1978, 1984, 1987a,Barnes et ol. 1983).Inthe phisme prograde. Les liser€s de grenat, par contre, main Broken Hill lode, there are at Ieast two montrent une zonation d partir de l'interface gahnite * distinct gahnite-bearing assemblages:quartz - grenat, et contiennentdes traces de zinc. L'assemblage gahnite - pyrrhotite - galena- garnet - biotite (- r6sulterait de la formation du grenat au d6pens de la staurolite, muscovite, ilmenite, sphalerite) and gahniteen pr6sencede solutionsenrichies en Fe, Mn, Mg, garnet - biotite - chlorite - sphalerite- gahnite - Ca, Na et Si, et capables de mobiliser le Zn. Ces galena - pyrrhotite - q\aftz (+ ilmenite); mus- observations, et la pr6sence de sillimanite dans les covite, chlorite and staurolite are retrograde assemblagespdlitiques, concordent avec un moddle de minerals(Spry 1978).This contribution documents refroidissementisobare pendant le m6tamorphismer6tro- grade. one pafiicularly unusual occurrence of gahnite from the Pinnacles Mine Sequence,New South Wales,in which retrogradereactions in sulfur-poor (Traduit par la R6daction) quartz - garnet - gahnite - biotite pelites Mo*-clds: spinelle zincifbre, gahnite, donn6es i la (sillimanite-deficient)have produced b garnet rim microsondedlectronique, m6tamorphisme rdtrograde, on grainsofgahnite. The associationof retrograde Broken HiIl. Australie. g:unet and prograde gahnite in sillimanite- and t45 Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/30/1/145/3435381/145.pdf by guest on 27 September 2021 r46 THE CANADIAN MINERALOGIST sphalerite-deficientrocks has not been described deposit, in the Cues Formation of the early previously and allows some speculation on Proterozoic Willyama Supergroup (Willis el a/. metamorphicconditions in the Broken Hill area. le83). The Willyama Supergroup has undergone two GsolocrceL SsrrrNc eventsof high-grademetamorphism and deforma- tion, followed by at leastone later retrogradeevent The Pb-Zn Pinnaclesmine lies 15 km southwest (Rutland & Etheridge1975, Glen et al. 1977,Laing of Broken Hill, westernNew South Wales(Fig. l), et al. 1978,Corbett & Phillips 1981,Stevens e/ a/. in the ProterozoicWillyama Supergroup.The mine 1988). Prograde regional metamorphism in the is the largest "Broken Hill"-type deposit in the Willyama Supergroup increasesfrom andalusite Broken Hill Block, after Broken Hill itself: grade in the northeast, through sillimanite - approximately 200,000 tonnes of ore have been muscovite and sillimanite - K-feldspar zones, to extracted,with averagegrades of l09o Pb, 420g/t two-pyroxenegrade in the southwest(Phillips 1978, Ag and 2.5W0Zn in the main lead lode, and 3.590 1980) (Fig. l). The Pinnacles deposit lies in the Pb, 100g/t Ae and 7.5-ll.59o Zn, in the smaller trvo-pyroxenezone and is characterizedby high- Zn lodes. Remainingore-reserves are estimatedat temperature,low- to intermediate-pressuremineral 200,000to 600,000tonnes (Departmentof Mineral assemblages.Retrograde metamorphism is most Resources1981). The depositconsists ofa seriesof strongly developedwithin, but not confined to, highly deformed strataboundsulfide lenseshosted zones of retrograde schists that transect the in pelitic schists. Stratigraphically, it lies ap- Willyama Supergroup (Stevens1986). There is a proximately 700 m below the main Broken Hill transition from lower-gradechloritoid-bearing as- semblagesin the central and northern regions, to staurolite-bearingassemblages in the central and southern regions, and zones of kyanite-bearing retrograde schist in the south (Corbett & Phillips Andaluslte + l98l). In the Pinnacles area, the geology is mugcovllo zons dominated by the kyanite - staurolite-bearing Thackaringa-PinnaclesRetrograde Shear Zone to Sllllmanlte + muocovllo zono the south ofthe deposit(Stevens 1986). The deposit itself also is cross-cutby severalretrograde shear Sllllmrnlto + K-loldspat zono zones. 2-pyroxone zone ANALYTICAL PROCEDURE The chemical compositions of 35 gahnite and approximately250 garnet crystalswere determined using a JEOL JSM840 scanning electron micro- scopefiued with a TRACOR TN5500 energy-dis- persion spectroscopyEDS system.An accelerating potential of 15 kV and an emission current of 2 nA were used.The spectrawere analyzedusing the following standards:wollastonite (Si, Ca), corun- dum (AI), hematite (Fe), rutile (Ti), rhodonite (Mn), periclase(Mg), anorthoclase(Na, K), wil- lemite (Zn), V2Os (v) and chromite (Cr). Repre- sentativecompositions are given in Tables I and2. PTTnocnepHYAND MINERAL CHEMISTRY Within the PinnaclesMine Sequence,gahnite- bearingpelites are associatedwith the wallrocks to both the galena-richand sphalerite-richorebodies. The pelites form stratiform units with distinct compositional layering, interpreted as primary Frc. I . Outline of the Broken Hill Block showingprograde bedding (Ss.1)and defined by relative abundances metamorphic zones (after Phillips 1978), retrograde of garnet, gahnite, quartz and feldspar. Two metamorphic zones and location of the Pinnacles foliations (S2 and Sr) can be defined by the mine: modified after Stevens(1986). alignment of sillimanite and retrogrademuscovite Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/30/1/145/3435381/145.pdf by guest on 27 September 2021 A GAHNITE-CARNET RETROGRADE REACTION. BROKEN HILL 147 TABT,EL NEPNBSENTATTVECTIEMICAL COMPOSITTONS OF OAITNIIE s-qple Gha (h4 Ghr4 (h16 Chl? frr8 Ch31 Gh32 Gh3.ll GA1 aatu 6ntor edgs edgs .-_mtar-€dge sior 0.@ 0.00 0.@ o.L? 0.18 0.00 0.18 0.85 0 0.n 0.00 0.00 o.12 0.00 Tio" 0.00 0.00 0.@ 0.16 0.00 0.0o 0.00 0.@ o.r1 0.00 0.(X! 0.ft1 0.@ 0.00 ALO" u.1o 66.02 60.30 64.96 66.(E 66.34 &2 66.6? &6.64 66.94 65.46 66.48 66.@ 6820 Cr2Os 0.00 0.00 0.68 L17 0.00 0.@ 0.16 0.(}0 0.00 0.00 0.(}0 0.00 0.@ 0.00 FeO' 10.89 12.66 9.8n rO,2 13.84 1Ls6 1358 $.?O 14.06 r.106 L3.67 13.60 14.01 L4Ll !,tno 0.00 02o 0.00 0.00 0.OO 0.00 0.@ 0.@ 0.00 0.00 0.@ o.fi) 0.00 0.00 MsO 0.m 031 O.U o.8t L72 029 0.91 0.66 0.U 0.48 0r9 0.U 0.16 027 7^O 83.08 3198 3:1.6? 82.4) .& 31.07 28.8:t W.62 28.8tt 2&68 n.6l 4.6|| 4.68 8.6 T0IAL 98.83 9€.36 [email protected] 99.91 99.& 98.66 [email protected] [email protected] [email protected] 100.76 98.92 Sg28 99.06 9910 Cation prcportione (based oa 4 oxygen atoEs) si 0.00 0.@ 0.00 0.01 0.01 0.@ 0.0r 0.01 0.01 0.01 0.@ 0.00 0.(p 0.00 Ti 0.00 0.@ o.fft 0.00 0.q, 0.(X} 0.m 0.(p 0.00 0.o0 0.@ 0.00 0.00 0.m Al r..98 LS? 1.98 L96 L.97 L90 1.98 LgE L9E L99 L90 L98 L98 L98 Cr 0.q, 0.@ 0.01 o.o:t 0.@ 0.00 0.00 0.@ 0.@ 0.(X) 0.00 0.@ 0.00 0.@ Fee o.28 0.t2 0 026 0.g6 0.80 0.84 0.34 0.36 0.35 0.35 0341 0.36 0.36 l[n 0.@ 0.01 0.00 0.00 0.00 0.00 0.@ 0.@ 0.00 0.q) 0.@ 0.@ 0.(b 0.(X, w 0.00 0.01 0.03 0,04 0.06 0.01 0.04 0.03 0.0:l o.t2 0.01 0.02 0.01 0.01 Za 0.75 0.?0 O.71 0.72 0.6:t 0.70 0.6:t 0.66 0.6t1 0.6:l 0.66 0.60 0.87 0.66 'Total Fe m FeO TABLE 2.