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Souuarnr IMPLICATIONS of the PRESENCE of AMAZONITE in the BROKEN HILL and GECO METAMORPHOSED SULFIDE DEPOSITS ROSS K. STEYENSON

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Souuarnr IMPLICATIONS of the PRESENCE of AMAZONITE in the BROKEN HILL and GECO METAMORPHOSED SULFIDE DEPOSITS ROSS K. STEYENSON Canadian Mineralogist Yol. 24, p. 729-745 (1986) IMPLICATIONSOF THE PRESENCEOF AMAZONITEIN THE BROKENHILL AND GECOMETAMORPHOSED SULFIDE DEPOSITS ROSSK. STEYENSON*eNo ROBERTF. MARTIN Department oJ Geological Sciences,McGill University, 3450 University Street, Montreal, QuebecH3A 2A7 AsstRAcr d uneperte de soufrependant le m6tamorphismedes gise- ments,et aurait 616incorpor6 dans les liquides granitiques Deep green amazonite from granitic pegmatites in the anatectiquesform$s in situ dansles zones min6ralis6es. Les Broken Hill Pb-Zn-Ag deposit, Australia, and pale bluish indicesd'amazonite peuvent servir comme guides aux zones greenamazonite from granitic pegmatitesin the Geco Cu- mindralis€esen mdtaux de base dans les socles soumis i un Zn deposit at Manitouwadge, Ontario, are compared. The mdtamorphismemoyen ou €lev6,quoique les d6fauts dans amazonite is notably enriched in lead (Broken Hill: I ro sa structurepeuvent 0tre 6limin6spendant le refroidisse- 2t/o Pb, Geco: 0. 1 to l 9o), The lead content is corelated mentpar interactionavec une phase fluide tardive porteuse with intensity of coloration, although exceptionsare com- de soufre.On peututiliser la min€ralogiedes feldspaths mon. The amazoniteoccurs in both monoclinic @roken pourpr6ciser les diff6rences dans l'6volution thermique ou Hill) and triclinic forms (Geco); the lead content appears le taux de soulbvementde deuxblocs crustaux. to exert no control on degree of Al-Si order, which can mainly be attributed to the thermal history of the crustal Ctraduitpar la R6daction) blocks after emplacementof the pegmatites.The Pb-isotope composition of the amazonite is very similar to that of the Mots-cl4s:amazonite, plomb, microcline, orthose, Broken galenafrom the orebodies,indicating a common sourcefor Hill, Australie,Geco, Ontario, isotopes de plomb, degd the lead. The lead appearsto have beenreleased by desul- d'ordre Al-Si, r6actionde d6sulfurisation,anatexie. furization of the orebodiesduring metamorphism and was incorporated in anatectic granitic liquids, which probably INTRoDUCTIoN formed iz sda in the ore zone. The occurrenceof amazonire may be used as a guide to base-metalminerelization in Amazonite green medium- to high-grademetamorphic tenanes, although its is usually defined as a bluish var- defect structure may be annealedduring cooling by inter- iety of triclinic potassium-rich feldspar..(1.e., action with a late sulfur-bearingfluid phase.Inferences may microcline). The definition was extendedby Cechet be made concerning the different thermal and uplift histo- al. (1971)to include greenK-rich feldspar that ap- ries of the two crustal blocks on the basisof feldsoar miner- pearsto be monoclinic in symmetry(i.e., orthoclase). alogy. Data are herepresented on the composition,degree of Al-Si order and lead-isotopegeochemistry of Keywords: amazonite,Iead, microcline, orthoclase,Broken amzvoniteand associateduncolored K-feldspar from Hill, Australia, Geco, Ontario, lead isotopes, degreeof two similar occurrences:l) Broken Hill, Australia, Al-Si order, desulfurizationreaction, anatexis. and 2) the Gecomine, Manitouwadge,Ontario. In Souuarnr both cases,the amazoniteisaconstituent ofgranit- ic pegmatitesemplaced near a metamorphosedbase- metal orebody. Amazonite is not only a decorative On compareI'amazonite vert foncd de pegmatitesgra- curiosity; it appears nitiques du gisement Pb-Zn-Ag de Broken Hill, en Aus- to be a direct consequenceof tralie, avecl'amazonite vert bleudtrepdle de pegmatitesgra- sulfide-silicateequilibria, and as such can be used nitiques du gisementCu-Zn de Geco, i Manitouwadge, en as a guide to sulfide mineralization in high-grade Ontaxio. L'amazonite est trds enrichieen plomb (entre I metamorphicterranes. et 20/oPb pour les €chantillons de Broken Hill, 0.1 et I 9o pour ceuxde Geco).La teneur en plomb montre une cor- BacrcnouNo INFonuanrou oN THENATURE rdlation avecl'intensit6 de la coloration, quoique les excep- OF AMAZONITE tions sont courantes.On trouve I'amazonitesous formes monoclinique (Geco); @roken Hill) et triclinique la teneur Amazonitic K-feldspartypically occursin granit- en plomb ne semble pas d€terminer le degr6 d'ordre Al- ic pegmatites. Si, qui r6sulterait plut6t de l'6volution thermique de blocs There,it may be associatedwith "nor- crustaux aprEsla mise en place despegmatites. La compo- mal" K-feldspar; comparedto the latter, it is syste- sition isotopiquedu plomb de I'amazoniteest trbs sembla- matically enriched in lead (e.9., Zlirov & Stishov ble d cellede la galEnedes gisements, indication d'une source 1965,Cech et ol. 1971,Foord & Martin 1979,Hof- communedu plomb. Ce plomb sembleavoir 6t6lib6r€ suite meister& Rossman1985). There are two waysto ac- commodatelead in the structure of K-feldspar: l) EPresentaddress: Department of Geosciences,University coupled substitution of Pb2+ + Al3+ for K+ * of Arizopa, Tucson, Arizona 85721,U.S.A. Si4+, the type of mechanismthat accountsfor the 729 Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/24/4/729/3446257/729.pdf by guest on 30 September 2021 730 THE CANADIAN MINERALOGIST plagioclaseseries, and 2) Pbz+ * n for 2K+, lead- recognized(Laing et al. 1978,Laing 1980).There is ing to a defect structure, suggestedby Plyusnin a generalincrease in metamorphic grade from north- (1969).However, Pb2+ cannot causethe color, be- west to southeast;the orebodiesand the granitic causeits electronic transition lies in the ultraviolet massesoccur in the high-gradeterrane. The regional part of the spectrum(e.9., Hofmeister & Rossman gradient may reflect regional tilting of the block to 1985).Marfunin & Bershov (1970)proposed that reveal a deeper crustal section to the southeast. Pb+ centresare a more likely causeof the charac- Pegmatite lensesoccur throughout the district in teristic color. Hofmeister & Rossman(1985) used sillimanite gneissas well as within and betweenore- resultsof visible, infrared and electron paramagnetic bodies.They are unzoned@limer l97O and loca[y resonancespectroscopy to show that Pb3+ could be deformedwith the enclosingorebodies (Johnson & the coloring ion, and that the unusual chargeresults Klingner 1976).The peematitesare thus syn- to post- from a reaction of the Pb2+ with products of dis- kinematic. The so-called Separation pegmatite, sociation of structurally bound water (unchargedOH located between the two lowermost stratiform ore- and H) upon radiation. bodies of the Broken Hill deposit, consists of A grven sample of amazonite contains "astive" microperthitic to nonperthitic grey-white to green lead, intimately associatedwith a molecule of water orthoclasein crystalsup to l0 cm across,quartz and and causingthe blue-greenor greencolor, and "in- minor plagioclase, muscovite, garnet, galena, active" lead, which may be locatedtoo far from a sphalerite,apatixe and zircon. Pegmatitestringers cut moleculeof water to allow the reaction to take place, through parts of the main body. Close to the ore- or which may residein the structure accordingto the body, the pegmatite stringers contain type-G schemeofsubstitutionPbz+ + Al3+ : K+ + Si4+ amazonite and minor amounts of sulfides; away (Hofmeister & Rossman1985). from the orebody, the stringers contain grey-white The degreeofAl-Si order ofthe K-feldsparexerts K-feldspar, and are free of sulfides @limer 1976). an influence on the wavelengthof light absorbed. The granitic pegmatites at Broken Hill are likely Hofmeister & Rossman(1985) found that samples the product of in situ partial metling of quartzofeld- with the bluest hues are invariably microcline, spathic sediments during the culmination of whereas yellowish green samples all consist of metamorphism,which reachedthe upper amphibo- orthoclase. They also found that the bluest amazo- lite to granulite facies in the area @hillips & Wall nitic microcline (their type-B amazonite) typically 1981,Corbett & Phillips 1981).The rocks of the containsless than 0.2 weight VoPbO, whereasthe Willyama Supergroup are inferred locally to have green orthoclase (type-G amazonite) contains more attained 8@oC at a confining pressurebetween 5 and than l9o PbO, and may attain 1.890. 6 kbar @hillips & Wall l98l). The generally accepted age of high-grade Gnor,ocrclr SETTTNcoF TIIE Surres ExeMrNEl metamorphismof the Willyama Complex is 1660Ma (Stacey& Kramers 1975).Shaw (1968)obtained a Broken Hill, Australio Rb/Sr ageof 1660 t 10 Ma for rocks of the Mine Sequence,and Reynolds(1971) determined an age The Broken Hill Block, in southernAustralia, con- of 1660 t 16 Ma from a MPb/zMPb versus sists of an apparently conformable successionof NPb/2uPb whole-rock isochron. Gulson (1984) Early Proterozoicgroups and formations that com- derived U-Pb dates of 1565 + 2.0Ma for apatite prise the Willyama Supergroup(Willis el a/. 1983). from rocks of the Lode Horizon and Mine Sequence, The world's largesthigh-grade Pb-Zn-Ag deposit 1595Ma for monazite and titanite, and 1663 t 9 occurs ilr the Broken Hill Group (500 - 1500 m Ma for zircon from granitic gneiss underlying the thick), which consistsof pelitic, psammopeliticand Broken Hill Group.The 1565and 1595Ma datesare psammitic metasedimentsdeposited approximately interpreted to be cooling ages. The orebody and 1800Ma ago (now quartz-feldspar-biotite-garnet enslssingrocks probably remainedabove the block- paragneiss,K-feldspar > plagioclase),with minor ing temperatureof apatite (takento be approximately felsic and basic gneissicrocks and banded iron- 600"C) for 60 or 70 Ma (Gulson 1984).The area formation. Concordant granite gneissoccurs toward cooled
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