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 gisements,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

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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 slowly (approximately 3oC Mrr) until about the baseof the sequence.The orebodiesappear to 1570Ma ago, when the temperaturefell below about be laterally equivalent to the felsic g:reisses,which 5fi)'C (Harrison & McDougall 1981);the 4oArl3eAr may represent recrystallized felsic pyroclastic age spectra also indicate that the region remained material (Willis el a/. 1983,Plimer 1984,1985). Some relatively cold until affecled by a thermal pulse 520 rocks associatedwith the orebodieshave an unusual t 40 Ma ago. This caused temperatures to rise compositioninterpreted in terms of a mixture of clas- locally to 350"C. The emplacementof a suite of tic material and chemicalprecipitates (exhalites). The ultramafic intrusive bodies exposedin the area ulti- rocks of the Broken Hill Block are complexly folded; mately may be found to be responsiblefor the Early three major episodesof deformation have been Paleozoicthermal event.

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The Geco mine, Manitouwadge, Ontorio The pegmatitescut all lithologies except the massive portion of the Main Orebody and younger dykes The Manitouwadge area in north-central Ontario of diabase.As the ore is regardedto be syngenetic is host to the Geco massiveCa-Zn sulfide deposit with the host lithologies, Suffel el a/. (1971)proposed (Friesen et al. 1982). The deposit is located in the that the pegmatitesdo not cut the orebodiesbecause Manitouwadge pegmatite series,which straddlesthe the ore flowed plastically during the waning stages contact betweenthe metamorphosedstrata of the of metamorphismand shearedoff the cross-cutting greenstonebelt and the surrounding Archean base- pegmatites.This interpretation is further supported ment, The pegmatitesabut againstthe orebody, and by the occurrenceof blocks of pegmatite dykes sus- in this specific environment, specimensof amazonite pended in the ore (Suffel et al. l97l). may be found. The amazonite is relatively common in the mine The Manitouwadgegreenstone belt is composed (F.J. Bakker, per$. corlm., 1985);it was sampled of metamorphosedvolcanic and sedimentaryrocks in two pegmatitedykes (denoted6 and l0), within that haveundergone at leasttwo phasesof folding l0 cm of the ore-pegmatitecontact. (Thesedykes to producean east-northeast-plunSngsynform @ak- may consist of medium-grained granite or aplite ker et al. 1985).The strata havebeen metamorphosed to the upper amphibolite facies (James et al. 1978) or lower granulite facies @etersen& Friesen 1982) during the Kenoran orogeny. The core of the synform and the country rocks are composed of granitic and granodioritic gneisses,with patchesof massive granodiorite and associated pegmatites. Late-stage granodiorite, pegmatites and diabase dykescross-cut the synform. A lack of deformation features indicates that they were emplacedafter the secondepisode of deformation. The pegmatitesare likely the product of anatexis. Metamorphic conditions at Manitouwadge were cer- tainly conduciveto melting. Jams et al. (1978)determined the maximum pressure and temperature of metamorphism to have been in the range }to 6 kbar [P(HzO) ( P,o,a]and 625"C using the garnet - biotite - cordierite assemblage.More recently, Peter- sen (1983)proposed 6 kbar and 650"C on the basis of various sulfide, silicate and oxide equilibria. The presenceof migmatitesis consistentwith the proposal that favorable conditions for anatecticreactions did prevail. The gaseousfraction trapped in cordierite in a pegmatite from the Manitouwadge areais much richer in H2O than COr, and was trapped above 600"C (Aines & Rossman1984). Robinson (1979) proposed the formation of extensive quartzofeldspathic melts by ultrametamorphism of wacke-type sedimentary rocks in the area. The Superior Province in this area last underwent an episode of regional metamorphism during the Frc. l. Sampling sites in a coarse single crystal of Kenoran orogeny, some 2700 Ma ago. Extensive amazoniteand in the fine-grainedretrograde assemblage investigations of the radiometric age of the gneisses (stippled) at tle contact witl massive galena + and ore mineralsat Gecohave been made by Rb,/Sr sphalerite;specimen E9155, Broken HiIl. The retrograde and U-Th-Pb methods (Tilton & Steiger 1965,1969, zone contains white K-feldspar, albite, quartz, musco- Oversby1978). Tilton & Steiger(1969) obtained an vite and very fine-grained galena. Site l8 is an inclu- averageage of 2720 + 50 Ma from mineral and sion of white K-feldspar. At site I I, a white halo is developed around an inclusion of galena. Loss of color in whole-rock data. The acceptedmodel age for Geco the amazonite correspondswith decreasingPb content. galena (Stacey is 2700Ma & Kramers 1975).Ratios Concentrations at the different sites are as follows (in of the isotopesof common Pb in non-amazonitic K- ppm; seealso Table 2)l I 13870,2 1840'3 9310,4 I46iYJ, feldspar from pegmatites from the Manitouwadge 5 t4/,30,6230,7 10370,81284A,9 13100, l0 9350,ll areavary widely (Iilton & Steiger1969, Sinha 1969, 24(t0,12 60, 13 13030,14 14670,15 640' 16 150fi)' l7 Stacey& Kramers 1975,Oversby 1978). 13760,18 1140.Width of field of view: 2.5 cm.

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rather than true pegmatite, but to be consistentwith (grain to grain) from pale green to apple green to mine terminology they are all referred to as pegma- a medium-darkgreen with a bluish tint. Not all the tites.) Dyke 6 is locatedon the 2l-52 sublevelof the K-feldspar grains in a single hand-specimenare mine. The dyke coarsensfrom an aplite to a medium- green;many are white, grey-white or pink. Electron- grained granite at the ore-pegmatite contact. The microprobe analyseswere carried out on samples second amazonite occurrence is found in a pegma- from dykes 6 and l0; X-ray-diffraction, X-ray- tite lens enclosedtnthe 8/2 zinc zonein the 28-63 fluorescenceand lead-isotopeanalyses were carried stopeon the 28-50level in the mine. The lens is about out on mineral separates6F, 6D, 10Al (amazonite- 1.5 m long, 0.5 m acrossand unzoned;on average, bearing)and 6P, 10-16and 30-4(white K-feldspar). grains are 8 mm across. Note that pegmatitesamples 10-16 and 30-4are taken The grains of amazoniteand white K-feldspar at from locations distal with respectto the ore zone. Geco exhibit grid twinning typical of microcline, and are nonperthitic to microperthitic. A graphic inter- ANALYTICAL TECHNIQUES growth of quartz and K-feldsparis commonboth in the aplitic groundmassand in the coarser-grained Major- and trace-elementdata were determined portions. Discretegrains of quartz, plagioclaseand by X-ray fluorescenceusing a Philips PW 1400spec- muscovitecomprise the rest of the rock. Sulfide veins trometer.Analytical precisionfor.major elementsis are common in the pegmatitesnear the ore contact. l9o absolute;for trace elements,it is 590 absolute. Robinson(1979) also found that interstitial sulfides Electron-microprobe data were obtained on a in the bodiesof pegmatitesoccur almosl exclusively CamecaMicrobeam MB-l usingwavelength disper- near contactswith the orebody. Common sulfides sion. The beam current was 10 nA, and counting include chalcopyrite,sphalerite, pyrite and pyrrho" timeson samplesand standardswere l0 secondsfor tite. Microscopic grains of galenaare occasionally all elementsexcept Ba (50 s) and Pb (150s). The fol- found in fractures in the pegmatite as well as in lowing standardswere used: orthoclase (Si, Al, K), microfissuresin the amazonite.Gahnite and garnet andradite (Fe, Ca), MgO (Mg), albite (Na), BaSOn are also found locally. (Ba) and vanadinite (Pb). Average levelsof detec- tion for Ba and Pb werefound to be about 900 and DESCRIPTIoNoF THESavpTTs SELECTED 400 ppm, respectively.The isotopic compositionof the lead in K-feldspar and galenaconcentrates was Broken Hill determinedon a Micromass 30 massspectrometer at the University of Alberta, under the supervision Electron-microprobeanalyses were made on a of ProfessorG.L. Cumming. Grainsof galenawere gemmysample of amazonite(89155) from the Sepa- hand-pickedand dissolvedin 2N HCI overnight, and ration pegmatite. This sample is associatedwith the solution evaporatedto dryness.The chloride quartz, albite, apatite, sphaleriteand galenaand is residuewas washed in HCI to removethe more solu- nonperthitic to microperthitic. Retrograderecrystal- bleZr', Fe and Ag chlorides.The feldsparseparates lization has produceda rim of q\arlz, albite, mus- were digested overnight in a covered HF-HNO3 covite and fine-grained white K-feldspar around mixture. The residueafter evaporation was dissolved many of the amazonitecrystals (Fig. 1). Random twice in concentratedHNO. to remove fluoride inclusions of galena in the altered areas of the 'I. amazonile and disseminatedgalena in the retrograde TABLE IHE CHEMICALCOI'IPOSIIION OF AMAZONITEFROM BROKEN HILL rim are common. The color of the feldsparchanges )-?.3.4!.!.2C.9. from greento white to grey or black as the galena- Sl02 wt.% 63,00 63.25 64.29 63.67 64.03 66.98 63.93 62.73 64.54 portions A1,0"- 18.5618.96 19.05 19.00 18.93 17.18 19.13 18.44 18.79 bearing are approached,as noted by Plimer Fe6 - 0.06 0.06 - 0.03 0.06 0.14 0.02 0.04 (1976)and Cechet al. (1971).In the samplecollec- MSo - trace nd trd nd 0.07 nd Cao - 0,40 0.05 0.04 0,34 0.30 0.22 0.21 0.18 tion, the color of the amazonitevaries from pale Na,o 0.59 0.88 0,81 0.85 2.20 2,52 2.00 2.80 1.68 grey-green green K,0 14.9414.95 14.43 14.75 12.23 10.28 13.07 14.65 13.94 through apple to "bottle" green; Bao 0.04 trace 0.07 0.09 - 0.02 0.'14 nd in any one sample,however, the intensity of the color pbo 1.79 l.19 1,48 1.62 1.39 1.95 1.10 l.l5 0.30 is uniform. totdl 98.92 gg,69100.24100.0299.15 gg.Zg99.73n0.0g99.47 Three additional examplesof amazonite(8H4, 6 Pb ppm 1661011050 13760 15000 12900 18100 10210 10770 2780 and 7) and a grey-whitemegacryst (BH3) from the Ab rle 0r 94.3 88.1 91.7 91.6 75.8 70.4 79.3 81.2 83.0 Separation pegmatite were used for X-ray- An - 4.0 0.5 0.4 3.5 3.4 2,3 2.0 1.8 (major diffraction, X-ray-fluorescence and minor All sanples are grey-green to elBrald green except number9, elements)and lead-isotopestudies. whlch ls grey-trhlte. Total lrcn ls expressedas Feo; nd: not detemlned. Composlilon I was detemlned by mlcroprobe analysis by Hofmlster (1984). Conpositlon2 lncludes 0.041 5$' 0,30% Rb20,0.10% 1120, detemlned by wet-chemlcalanalysls (Ceche, Geco aL. (1971). Conposltlons 3 and 4 detemlned by el€ctrcn nlcroprobe, thls study. composltions 5-7, 9 detemdned by x-ray fluorescence, thls study. Conposltlon8 detemlned by X-ray The color of amazonitefrom the Gecomine varies fluorescenc€(Pllrer 1976).

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TABLE2. CHEI'IICALC0I1PoSITI0N 0F K-FELDSPARIN SPECIMEN E9155" BR0KEN HILL 12345678 9t0il121314151617l8 Si02 wt.% 64.1765.59 64.40 63.46 63.51 65.76 64,4264.12 64.66 65.37 66.34 64.69 63.78 63.58 65.8263.67 64.29 65.82 41203 18.7718.75 I8,82 19.0218.69 18.88 18.78 18.96 19.05 19.06 I8.68 18.6618.89 18.75 18.92 19.00 19.05 18.77 Fe0 0,04 0.07 - 0.04 0.06 0.07 Mgo - 0.02 0.01 - 0.01 - 0.02 - 0.05 0.03 Ca0 0.06 0.01 0.08 - o,o2 - 0.08 0.04 0.02 0.01 0.0I 0.04 0.05 0.04 0.66 0.85 0.81 0.7s Na20 '14.690.85 0.54 0.86 0.75 0,96 0.33 0.73 0.96 0.85 0.55 0.30 0.25 0.92 0.73 Keo 15.3314.46 14.95 l5.ll 16.2913.84 14.49 t4.58 14.t3 14.5117.10 15.05 15.15 15.87 14.75 14.43 15.74 Ba0 0.09 0.28 0.04 0.07 0.0i 0.tl 0.04 0.01 0.07 0.07 0.01 0.07 0.04 0.03 0.05 0.09 0.07 0.15 Pb0 0.49 0.20 1.00 1.58 1.55 0.02 I.l2 1.38 1.41 1.03 0.26 0.01 1.40 I.58 0,07 1.62 1.48 0.12 tota l 99.12100.7299.71 99.83 99.85101.39 99.02 99.99100.64100.29100.12100.79100.14 gg.as]01.39t00.02100.24.t01.46 '13870 Pb ppm 1840 931014690 14430 230 10370t2840 13100 9350 2460 60 1303014670 640 1500013760 Il40 Ab mole% 8.0 5.1 8.2 7.1 8.8 3.0 7.3 9.1 8.t 5.6 3.1 ?.2 8.5 6.8 5.9 8,0 7.8 6.7 0r 91.4 94.8 90.9 92.9 91.0 97.0 91.8 90.9 91.9 94.0 96.7 97.7 91.4 93.? 94.1 91.6 91.7 92.9 An 0.6 0.1 0.9 - 0.2 - 0.9 0.4 0.2 0.I 0.I 0.4 0.5 0'4 Compositlonsdetennined by electron-ndcroprobeanalysis. Total iron is reported as Feo. ComposltlonsI to 18 pertaln to locatlons shownln Flgura l.

TABLE3. CHEI'IICALC0I'{P0SIT]0N 0F AMZoNITEIN PEGMTIIEFRoM llANlTolJllADGE

L Z ! g !. 0 z c 9. lo ll 12 13 14 15 5102 wt.% 65.1465.53 65.32 65.35 65.53 64.50 65.15 64.84 63.75 64,58 64.U 64.6664.31 64.11 63.92 A1,0.' 18.72t8.56 18.8318.72 18.69 18.74 18.89 18.68 18.79 18.83 18.61 18,62 18.85 18.70 18,77 Fed o,o2 0.32 0.01 0.01 0.02 0.02 0.07 - 0.01 0.04 0.01 0.01 0.01 t'{90 trace - trace 0.03 0.01 0.02 0.04 - 0.02 - 0.04 0.04 - 0.01 0.02 Cao 0.01 0.03 0.03 0.01 0.03 0.01 0.06 0.01 0.05 0.01 0.01 0.02 0.04 0.02 0'02 Na20 0.45 0.54 0.65 0.62 0.67 0.48 1.03 1.04 0.89 0.96 0.42 0.84 0.42 0.88 0.69 Kzo 15,4814.85 13.78 14.06 14.91 t4.89 14.83 15.37 16.03 15.13 16.50 15.50 17.06 16.17 16.35 Bao 0.13 0,65 - 0.09 0.09trace trace .0.04 0.02 0.14 0.01 0.05 0.26 0.25 0.09 Pbo 0.50 0.49 0.56 0.20 0.33 0.36 0.69 0.39 0,64 0.40 0.4{l 0.72 0.37 0.62 0.37 total 100.55100.0799,18 99.09100.28 ga.72100.76]00,:2100.19100.05100.92100.49101.32100.77100.24 Pb ppn 5570 4550 5200 1850 3060 3340 6400 3520 5940 3710 4455 6680 3435 5755 3435 Ab moteX 4.2 5.2 6,7 6.3 6.4 4.7 9'5 9.3 7.7 B.8 3.6 7.6 3'6 7'6 6.0 0r 95.7 94.5 93.0 93.6 93.3 95.2 89'9 90.6 91.8 9l.l 96.3 92.2 96,0 92,2 93.8 An 0.1 0.3 0,3 0.1 0,3 0.1 0.6 0.1 0'5 0,1 0.1 0,2 0.4 0.2 0.2 Averagechtrical corFosltlonof amzonlte fron lndlvldual salples detemlnedby electron nlcroprobe. samples:5A0c (l and2),6D (3 to 7), l0A0(8), 10820(9 to 12), R2150(13 to l5). Total lrcn ls exprcssedas Feo.

TABLE4. COMPOSITIONOF IHITE K-FELDSPAR are 1.002907for 2o6Pb/2MPb, IN PEB.4ATITEFROi,I I,IANITOIJI,IAOGE i dard NBS SRM 981 1.00411 for 207Pb/2oaPb,and 1.006506 for 6P 2A 6AD 6AP l0A0 6D 10820R2150 6F 208Pb/2i4Pb.Error limits calculated from repeated 5'102 wt.% 64.?8 65.11 64.32 64.75 64.30 65.14 64.48 63.56 64.76 A1203 18.5518.57 18.68 18.80 18.75 18.70 18.73 18.70 18.49 analysesof standards arc ox : 0.O2lt/0, oY : Feo 0.03 0.03 0.04 0.01 0.01 0.03 0.01 0.01 0.01 : 0.029V0(where x, y and z refer to the Mgo 0.02 O.29 0.02 trace 0.02 0.01 0.01 0,01 0.01 0.026t/o,az Cao 0.03 0.02 0.08 .0,05 0.04 0,02 0.04 0,03 0.02 above three ratios). Na20 0.63 0.58 0.85 0.6I O.72 0.66 0.72 0.65 0.68 KzO 16.21t6.33 15.67 15.8615.93 14.74 15.76 16.26 14.83 BaO 0.07 0.03 0.10 0,07 0.07 0.09 0.07 0.39 0.ll Pbo 0.08 o.lt 0.62 0,32 0.42 0.36 0.55 0,46 0.53 CHsr{rcat, CouposrrroN oF THE K-Fuospen total 100.40,^,^-100.38,^^ ,-100,26 ^- ,.100.37,^^^- 99.44 tut.ut ruu,a, tt.ta tuu.u/ Pb ppm 740 1020 5755 2970 3900 3340 5105 4270 4920 Broken Hill Ab mle Z 5,6 5.1 7.5 5.5 6,4 6.4 6.5 5.7 6.5 0r 94.'l 94.7 91,7 94.0 93.2 93.4 93.1 94.0 93.3 Table I shows the repre$entativeanalytical data An 0.3 O.2 0.8 0.5 0.4 0.2 0.4 0.3 0.L collected in this study and by previous investigators. Averagechemlcal composltlon of whlte K-feldspar frcm lndivldual samples,detemlned by electron mlcrcprobe. Total lrcn ls ex- The data obtained by X-ray fluorescenceindicate a pressed as Feo. higher Na content in view of the impossibility of removing albite impurities. However, the effect of suchimpurities on the quoted contentsof lead aud salts. The precipitated nitrate salts were dissolvedi.n barirrm is negligible. The microprobe and XRF data 2N HCl, and ion-exchangeresin columns were used on lead contentsare in good agtreementwirh pub- to obtain a pure PbCl2 solution. All operations Iished information on Broken Hill material (Cechel wereperformd in clean-airlaminar-flow hoods. The al. 1971,Plimer 1976,Hofmeister & Rossman1985). lead chloride solutionswere mounted on singlefila- The amazonitein E9155 rangesfrom Ore to Or95 ments in a silica-phosphate mixture. Total lead (microprobe data; seeTable 2); Fe, Mg, Ba and Ca blanks for the feldspar semplesare I ng. Fractiona- are either very low or negligible. The lead content tion corrections calculated from the isotopic stan- of the amazoniteis high, ran.grngfrom I Io 2 wt.olo

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IABLE5. COIiIPOSITIONOF K-FELDSPAR FROMMANITOI'IIADGE

30-4 10-16 6F 6p l0Al si02 rt.g 65.37 65.96 64.37 64.39 67.10 41203 18.77 18.69 18.92 18.8 "17.27 Fe203 0.03 0.04 0.06 0.08 0.04 Ca0 0.14 0.10 0.15 0.lt 0.13 Na20 2.48 2.57 1.84 "1.23 l.14 Kzo 13.04 12.68 13.89 14,65 13.57 Pz0s 0.01 0.01 0.03 0.03 0.01 Ba0 0,06 0.02 0.08 0.07 0.06 sr0 0.01 0.01 0.02 0.oz 0.02 Pb0 trace trace 0.52 0.53 0.45 total 99.91 100.08 99.88 99,99 99,79 Pb 108 4802 4904 4214 PPN '16.5 'l't.2 Ab mle % 22.1 23,3 11.2 0r 82.0 87.7 87.4 An 1.4 1.0 1.5 l.l 1,4 Bulk conpositlon detemlned by X-ray fluorescence. Total iron expressed as Fe203.

Geco

The chemicalcompositions of amazonite(Table 3) and white K-feldspar(Table 4) are consistentand range from Orseto Orrr, with sampleaverages from Ore2 to Ore5. Calcium content i3 negligible in all cases.The Ba content is highly variable (0 to 8000 ppm), although the majority of the grains contain lessthan 1000ppm. Associatedwhite K-feldspar and amazonitecontain a similar concentrationof barium. 0 3000 6000 9000 The average Pb contents of the K-feldspar are illustrated in hist6gram form (Fig. 2). The general LtADC0tCtllTRlll0ll, lprn trend defined by the white K-feldspar is mimicked by that qf the amazonite.This is good evidencethat FIc. 2. Histogam sho\ryingthe frequencydistribution of whereaslead may play a role in the formation of the leadconcenhation in K-feldsparfrom the Gecosuite. Amazoniticand non-amazoniticK-feldspar was ana- amazonitecolor, it is not the determining factor. The lyzed;the dataindicate that whiteK-feldspar may con- amazonite samples are concentrated in the middle tain asmuch or morelead tlran amazoniticK-feldspar. range of the histogram, revealingthat in many cases The lone sampleof amazonitethat containsbetween at Geco, pink or white K-feldspar is just as rich if 1000and 2000 ppm Pb is enclosedin galena.Ordinate: not richer in lead. However, it is true that the numberof samplesanalyzed, amazonite color occurs at Geco only if the lead content exceeds1000 ppm. The XRF data on K-feldspar (Table 5) are in quite good agreementwith the microprobe data. Albite (average 1.5190). Spots selectedfor analysis by lamellae and small grains of plagioclase in the microprobe are free of galena(Fig. l). Thesecom- separate account for the difference. The small qositionsseem to be the most Pb-enrichedrecorded. amount of sodic feldspar dilutes the measuredPb Cechet al. (1971)listed the lead content of a num- concentrationonly slightly. ber of amazonite samples, the highest being an Samples10-16 and 30-4 are fist-sizedcrystals of amazonitefrom Pack, Austria (l.35Vo Pb). pink-white K-feldspar taken from pegmatitedykes Electron-microprobeanalyses across the retro- distal to the orebody. The samplescontain up to 230/o grade rim reveala distinct decreasein lead content albite owing largely to perthitic lamellae and sodic of the feldsparfrom the amazonite(1.3 - 1.590Pb) plagioclaseincluded in the K-feldspar. Of particu- through the white halo around galenainclusions to lar note is the much lower Pb content of the two sam- the retrogradezones (60 - 2460ppm Pb). Further- ples of pink-white K-feldspar, 108 and 74 ppm. more, a turbid, grey K-feldspar within the massive There is no compositionalzoning in K-feldspar galenaore containsonly ll40 ppm Pb comparedto with respectto Na, K, Ba or, generally,lead. One 130@ppm in the nearby amazonite.Table 2lists the exceptionis discussedhere. In many caseswhere core compositionsrecorded at the sitesindicated in Figure and rim areas of amazonite and white K-feldspar I . The loss of lead and of the greencolor occqrsonly grains were analyzed (or grains proximal and distal in areasthat appear to have undergonea retrograde to galenawhere grain sizeis small),the lead content structural modification (seebelow). decreaseswhere the feldspar is adjacent to galena

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TABLE6. Pb CONTENTOF K-FELDSPARVERI decreasein lead content cLosET0 ('Rlt4tr)AND REI10VED FRoM Clable 6). In eachcase the (.CORE")CONTACT IIITH GALENA occurs oyer a small distance. Only those parts of the samplenumber 'coreu feldspar directly adjacent to the galenaare affected, 6F 2350 and there is no visible physical alteration of the feld- 5060 2220 '10820 spar there. The galena is found either in fractures 5230 2850 6250 1890 or is interstitial to the feldspar grains. One possible 6AD 3797 3440 explanaxion for this phenomenon is that the inter- 64P 5930' 160 face betweenthe feldspar and galenamay have been 6530* 240 4560 2980 physically open owing to unequal shrinkage during 5410. 760 '10A0 cooling, thus allowing very local retrogradeequilibra- 3133 2840 3920 3700 tion. This could involve removal of lead from the 4?90 2510 feldsparand its transferto the fluid phaseresponsi- 6D 2420 1360 ble for the supply of sulfur and deposition of the 6490 6370 3910 1840 galena. 4710 4670 As already noted, both white and greenK-feldspar * indlcates discrete gralns of K-feld- may have a high content of lead. There are also spar, one adacent to and one renDved from a graln of galena. Concentra- individual grains of anazonite that vary in color tlon! of Pb ln ppn. from white to gteen. This zonation is never soncen-

TABTE7. CELLPARAMETERS AND IND1CATORS OF COMPO5ITTONAND DEGREE OF A].SI ORDER,K-FELDSPAR FROM BROKEN HILL, AUSTRALIA

g b e d B 1 ! a* bt a* c* Br f g"q 0r Pb BH3 8.573212.9693 7.2128 90 116.03690 720.590.129816 0.077105 0.t54301 90 63.96490 0.458 98,9 27950R 0.00110.0016 0,0009 0.011 0.130,000020 0.000010 0.000018 0.01t 93.5 BH4 8,570812.9812 7.2023 90 n6.02890 720,M0,1298/,5 0.077035 0.154517 90 63,97290 0.405 97.8 129190R 0.00070,0008 0,0005 0,006 0,070,000014 0.000005 0.0000t0 0.006 92.0 BH6 8.563712.9852 7.2018 90 116.02790 719.630.129951 0.077011 0.154525 90 63,97390 0.398 98.718081 0R 0.00140.0022 0.00t3 0.014 0,170.000026 0.000017 0.0000?6 0.014 90.9 BH7 8,568412.9763 7.2149 90 116.01590 720,920.129ffi7 0,077063 0.154229 90 63.98590 0.456102.1 10163 0R 0.00120.0017 0.0010 0.011 0.140.000019 0.000010 0.000019 0.01r s4.4 BHlo* 8.570 12.9877.209 90 116.00090 720.91 0.421 111000R 0.004 0.003 0.003 0.57 94.4 E9t55 8.579713.0043 7.2013 90 116.02290 722.020.129703 0.076898 0.154529 90 63.97890 0.371103.5 15500 0R (l) 0.00170.0028 0.0012 0.016 0.170.0000?8 0.000016 0.000020 0.016 97.5 E9155 8.565112.9904 7.1989 90 l16.0l790 719.810.129917 0.076980 0.154574 90 63.98390 0.388 91.0r2000 0R (2) 0.00050.0008 0.0004 0.004 0.050.000007 0.000005 0.000009 0.004 91.4 8.566212.9805 7.2040 90 116.01690 7t9.870.129901 0.077039 0.154462 90 63.98490 0.418 91.212000 0R (3) 0.00070.0009 0.0006 0.006 0.070.000013 0.000006 0.000012 0.006 91.6 BHI5 8.576112.9952 7.2084 90 116.07290 72'1.620.129813 0.076951 0.154443 90 63,92890 0.415 96.1 300000R 0.00140.0032 0,0014 0.0t5 0.200.000027 0.000019 0.000027 0.0t5 96.4 NBH2 8.569612.9928 7.2006 90 116.00990 720,540,1?9W1 0.076966 0.154528 90 63.99190 0.384100.6 ? 0R 0,00050.0007 0.0004 0.005 0.050.000007 0.000004 0,000009 0.005 93.4 KLl,l-V48.594213.0009 7.20ep. 90 116.05490 723.570,129519 0.0769t8 0.154419 90 63.94690 0.389104.8 ? 0R 0.00170.0031 0.0011 0.014 0.190.000027 0.000019 0.000023 0.014 102,0 BH'I3 8.585212.9605 7.2197 90.578 115.969 87,920 72"t.72 0.129644 0.077210 0.154070 90.370 64.034 92.032 0,945 96.4 ? tM 0.00070.0009 0.0006 0.010 0.006 0.010 0.070.000011 0.000006 0.000012 0.009 0.006 0.009 97.5 BHl4 8.580612.9646 7.2144 90.468 115.955 88.195 72"t,24 0.129677 0.077173 0.154163 90,359 64,047 91.780 0,862 95.0 ? IM 0.00r0 0.00120.0007 0.013 0.010 0.015 0.100.000016 0.000007 0.000017 0.013 0.010 0.015 95.3

SpeclnEnsBH3, E9155(3), BHl3 and BHl4 consist of grey-white K-feldspar; the others are amazoQltlc. 0l- orthoclase, lM^inter- medlate mlcmcline' LM low nlcrocline. Lead concentr;tlon quoted ln ppm. Unlts: a, b, c in A, / ln A3, a*, tr*, a* 16 A-r, c, Br'yr c*r B*' y* ln degrees,0r ln %. 0r content quotedon first line ls or(b*c*), calculated uslng the expresslonof Blasl (1977). 0r content quotedon secondllne ls 0r(f), calculated uslng the expresslonsof Krol'l & Rlbbe (1983), for the serles LA - LM ln the case of BHl3 and for lnternediate degrees of A1-Sl order ln all other cases. t Cechet al. (1971).

TABLT8. CELLPAMMETERS AND INDICATORS OF COMPOSITTONAND DEOREE OF AI-SI ORDER,K.FELDSPAR FROM GECO, ONTARIO

a b 9 9 g L v at bt c* ot Br f hq 0rPb 30-4 8.576012.9638 7.2244 90.683 115.944 87.652 721,63 0.129775 0.077204 0.153935 90,382 64,062 92,279 r.010 101.6 74 LM 'r0Al 0.00080.0011 0.0006 0.010 0.009 0.009 0.080.0000t3 o.000006 o.o0oo]4 0.010 0.007 0.009 97,2 8,574312.9655 7.2226 90.664 115.970 87.647 72'1,23 0.129831 0.077195 0.154009 90.407 64.035 92.?93 1.A02 r00.6 4214 LM 0.00i0 0.00110.0010 0.009 0.009 0.009 0.100.000016 0.000007 0.000020 0.009 0.009 0.010 96.1 6D 8.566512.9641 7.2207 90.677 115.954 87.648 720.41 0.129931 0.077203 0.154028 90.391 64.051 92,286 0.997 99.9 4909LM 0.00110.00t2 0.0008 0.011 0.009 0.009 0.100.000019 0.000007 0.000016 0.011 0.009 0.009 oa7 6F 8.572012.960? 7.2222 90.670 I15.952 87.680 720.83 0.129843 0.077225 0.'153995 90.384 64.053 92.254 1.00r 99.6 4802LM 0.00060.0009 0.0005 0.008 0.006 0.006 0.070.000010 0.000005 0.000010 0.009 0,006 0.007 94.9 Geco 8.570612,9651 7.2173 90.667 1'15.946 87.657 720.52 0.129860 0.077197 0.154090 90.398 64.059 92,281 0.9U 98.73900 Ll'l 0.00140.0026 0.0011 0.02] 0.016 0,020 0.180.000@5 0.000015 0.000028 0.020 0.016 0.020 94.1

Speclmen30-4 is whlte; the others are amzonltlc. The speclrnenlabeled rrceco'rwas donated by F. Edkker, and lts Pb content was detennlned by spectrographlc analysls by Nancyconklln of the U.5. Geologlcal Survey ln Denver. 0r{r) ls calculated using the expression of Kro'll & Rlbbe (1983) for the serles LA - LM. Unlts and symbols are the sameas In Table 7.

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7.r9 y-

91.5'

7.11 --"is

t20"t2m c.

I tI trltl

Frc. 3. A. Plot of the cell dimensions D yerszs c for the eighteen samples of K-feldspar included in this study. The five data-points representinglow migocline from Geco (black trianeies) plot closeto the LM (ow microcline) corner of the D - c quadrilateral. Two samplesof microcline also form part of the Broken Hill suite (black dots). Most of the K-feldspar at Broken Hill is orthoclase (starred black dot); these data points illustrate a large variability in the degreeof Al-Si order attained in orthoclase, reflecting degreeof recrystallization above a temperature of 400oC. HS high sanidine, HA high albite. Two setsof error bars are shown, one referring to the low microcline data-points' the other to the orthoclase data-points. B, Plot of the interaxial anglesa* and 1* for the triclinic samples(all samples of monoclinic K-feldspar plot at o* : ?* = 90'). Triangles:Geco suite, black dots: Broken Hill suite. HS high sqnidine, LM low microcline, LA low albite. This plot indicates that the microcline from Broken Hill, formed in an evolving retrograde environment, is clearly less well ordeied than that in the Geco orebody. C. Part of the triangular plot tlo - tlm - (t2O + t2m), showing the important differences between the K-feldspar at Broken Hill' in which t1O = t1m in most casesand in which the microcline falls short of end-member low microcline (l1O = l). The symbolsare the sameas in A and B.

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tric, but up to half the crystal may be green,and may amazoniteconsist of orthoclase,whereas the triclinic grade to white in the other half. In a few casesthe K-feldspar is non-amazonitic. However, it is evident amazoniteis mottled greenand white. Grains of such from the two suites that amazonite exists in both color-zoned amazonite were analyzed by electron triclinic and monoclinic structures. microprobe. The resultsshow that both green and Values of Or content calculated from D* and c* white portions of the samecrystal have similar con- for Broken Hill (91-10390)and Geco (99-10190) tents of lead: e.9., 6F (white) 4010 ppm and 6F (Iables 7,8) indicatethat the K-feldspar from both (green) /y{)Q pp1n. The difference in lead content localities is rather potassic, in agreement with the barely exceedsanalytical error. The changein color nonperthitic to microperthitic nature of the samples. is likely due to some other factor than Pb content The sampleshave equilibrated successfullyto a low becausethe difference in lead content is no greater temperature, and more so at Geco than at Broken than the variation found in other grains of amazonite Hill. Values grealer than 10090(Tables 7, 8) iudi- that are uniformly colored. The H2O content may catethat a large cation like Rb, Cs and Pb(?) causes be lower in the white portion (Hofmeister & Ross- a slight expansionin D* and c*. Compositions deter- man 1985). mined by electronmicroprobe (Tables2-4) arevery similar to those inferred by XRD data; the small X-RAy-DIFFRACTToNStuty differencesprobably reflect the influence of the same Iarge cations on the cell dimensions. Six samplesof K-feldspar were chosenfrom the 'The presenceof low microcline and the well- Broken Hill suite for an X-ray-diffraction study. developed grid twinning at Geco are indicative of Data from six other refinements (R.F. Martin, relatively slow cooling in the presenceof an aque- unpubl. data) and the resultsof Cech et al. (197I) ous fluid that calalyzed the inversion. Such condi- are also plotted. Four samplesof K-feldspar from tions are characteristic of a pegmatite system. The Geco were selected; three are amazonite (6D, 6F, persistenceof orthoclase at Broken Hill, then, l0Al) and one is pink (30-4). A fifth refinementof appearssomewhat uncharacteristic of pegmatites;the amazonite was carried out on a sample contributed lange of Al-Si order found amongstthe Broken Hill by F. Bakker. Cell edges,angles, ordering parameters samplesrequires closerscrutiny. The low microcline and lead content are listed in Tables 7 and 8. sample(BHl4) is a fine-grainedpink K-feldspar that The refinements for the Broken Hill samplescon- rims and penetratescoarser crystals of grey-white K- firm previous work in that the amazonite is feldspar along fractures and cleavages.The distri- orthoclase(definition of Ribbe 1983).The white or bution of the pink microcline suggeststhat it results pink K-feldspar, however,may be more orderedQow from the modification of the grey-whiteK-feldspar, microcline, intermediatemicrocline or orthoclase). which is intermediate microcline. If the fluid phase As noted previously,the grid fyinnilg in the Geco that produced the low microcline rim on the inter- samplesindicates a triclinic structure, and the XRD mediate microcline had interacted with the resultsconfirm the absenceof orthoclase.The sam- widespreadorthoclase at Broken Hill at a tempera- ples from both suites range from nonperthitic to ture lessthan 400oC (approximatetemperature of microperthitic, and this is confirmed by the absence the monoclinic - triclinic inversion), then we could or relativeweakness of the albite peaksin the XRD study.

Plots of b versusc and cu*versus y* @gs. 3A,B) TABTE9. LEAD.ISOTOPEDATA FOR K-FELDSPAR AND GALENA were made to determinethe degreeof Al-Si order FROMBROKEN HILL in the samples.Values of A(bc) and A(a*7*) were 206pb 207pb 208pb calculatedusing the approach of Blasi (1977)and, zu4F5 2TEF6 ZT'FF6 from (Tables Anazonlt€ 1 BH4 16.0ll 15.392 36.668 thesevalues, l1O was calculated 7, 8). BH6 16.013 15.396 35.698 Regardlessof color or lead content, the samplesfrom repeat 16.012 15.396 35.690 3 BH7 16.015 Geco form a tight cluster around the low microcline repeat 16.015 t5.399 35.707 end-member@igs. 3A, B, C). The Brokeq Hill data BBH 16.002 15.387 35.667 5 E9155 . 16.004 tc. Jdo J5.ood reveal a relatively large spread in the degreeof A1- repeat 16.004 15.388 35.681 Si order; the orthoclasesamples show a spreadin llhlte K-feldspar o BH3 16.045 15.389 35.677 ItO from 0.38 to 0.46, and a small difference in Galena 7 BHI 16.004 tt.J6 J9.O|Z 8 NBHI 16.005 'r5.39515.393 35.684 degreeof order existsbetrveen the low microcline and 9 NBHz 16.049 35.735 the intermediate microcline. For the orthoclase, this repeat 16.048 I 5.395 35.739 l0 BH6 16.032 t5.395 35.698 indicates that the mineral had locally progressed 1l BH7 16.010 15.39t 35.679 toward the monoclinic-triclinic inversion before final JRR 16.005 15.389 35.651 retrograde equilibration. Ilowever, the retrograde r Broken Hlll standard fron J.R. Rlchards (pere. com. to G.L. Cuming). Concemingthe galena samples,BHI is fron the maln conversion to low microcline evidently did not pro- orebody, NBHIis from the north orebody, NBH2ls suspect, BH6 ceedin many cases.Also, a1lsamples of BrokenHill and BH7are dlsseninated in the pegmtite.

Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/24/4/729/3446257/729.pdf by guest on 30 September 2021 738 THE CANADIAN MINERALOGIST 15,.42 A Arnozonlte o K-Feldrpor o Goleno g E Rlchor& stondord - C' C' 0 s I tlu=9.9) - 26 .{tat ,tr (D -o ro 9 TL !t c! N 15.36 -o o- lht4.71 Ct 6a

, Stgrtr Errorr /

16.00 *pa/ -Pb

Frc. 4. A plot of the ratios 2o7Pb/2MPbversw26Pb/2MPb for common lead in samplesof amazonitic orthoclase, white orthoclaseand galenafrom Broken Hill. The standardof J.R, Richards(pers. comm. to G.L. Qumming)is added for comparison. The data for the amazonite and most of the data for galena cluster about this value (indicating approximately 1612Ma). The solid lines are growth curvesfor different valuesof aW l2MPb (Mu) calculatedaccord- ing to Stacey & Kramers (1975). The dashed line is a l60GMa isochron. The numbers are those used in Table 9.

expectefficient ordering of the monoclinic K-feldspar sampleof grey-whiteorthoclase (Table 9). Samples to microcline. In the extreme case,recrystallization BH3, 4, 5,6 andT arehigh-purity mineral separates, would result in the perfectly orderedstructure of low whereasBBH and E9155are chip samples.Galena microcline, as is virtually achieved in the pink rim samplesBH6 and 7 were separated from the same at Broken Hill, and possiblyin the removal of lead rock as amazonite BH6 and 7, and are interstitial from the feldspar structure (as noted previously). to the amazonite. Galena samplesBHl ' NBH-I and This would in turn cause a discoloration of the -2 are derived from massiveore in the Lode horizon. amazonite and formation of disseminatedgalena in Plotted with the results @ig. 4) is a value for the altered areasif sulfur were locally available. We Broken Hill galena determined by J.R. Richards contend that at Geco, the ordering is complete on (G.L. Cumming, pers. comm. 1984)that is similar a regional scale;the relative scarcityof amazonite to the acceptedcomposition for the Broken Hill sys- there may indicate that the hydrothermal fluid tem, quotedby Stacey& Kramers (1975). All of the presentduring the ordering reaction did contain the amazonite data-points and most of the galena data- necessaryanions to remove the lead in almost all points cluster around the value of Richards. The cases. trend of the cluster is exactly that of the error ellipse. Using a two-stagegrowth curve for a238u/2n4Pb LEAD-ISoToPEGEoCHEMISTRY valueof 9.90,the resultsindicate an ageof resetting of the galena-amazonite system of approximately Broken Hill 1625Ma (Fig. 4). Note that the chip samples(4, 5) also plot in the clusterand are evenslightly lessradio- To comparethe isotopic composition of lead in genic than the galena and other amazonite, but still amazonite,galena and white K-feldspar, we analyzed within error. Three samplesplot significantly out- five samplesof amazonite,five of galena and one side the cluster: two representgalena @H6, NBH-2)

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and one is the grey-whiteorthoclase (BH3). Repli- & Martin (1979) and others in that amazonite con- cate analyses of several samples indicate that the tains high amounts of lead. In the two suites exa- measurementsare reproducible within error. mined, no amazonite with less than 1000 ppm Pb was found. Geco The correlation noted between intensity of coloration and lead content @oord & Martin 1979)is evi- Mineral sepaxatesof amazonite(6F, 6D, 10Al) dent in both suites. A decreasein Pb content and and white K-feldspar (6P, 10-16, 30-4) were ana- a change in color occur in altered portions of the lyzed, along with a sample of galena randomly amazonite, which may or may not contain galena. chosen from the orebody. Sample 6P was leached This is best seen in the "altered" amazonite from with HCl, HNO3 and HF to produce three addi- Broken Hill. However, the trend is also apparent tional samples.The results(Table 10,Figs. 5, 6) use among unmodified samples.A dark greenamazonite the growth curve of, Stacey& Kramers (1975).The typically contains more lead than a pale gxeenone. model agesfor the galena are calculated according For example, pale amazonite at Geco contains on to the two-stagemodel of Stacey& Kramers(1975). average 0.4890 Pb, whereas the emerald green The first leach from sample 6P [6P-Ll(5)] is amazonitefrom Broken Hill contains l.5l9o on aver- enriched in radiogenic lead compared to the rest of age. Apparent exceptions to this correlation are the data (Frg. 5). This fraction presumably contains found at Geco. The amazonite-bearing pegmatites the lead generatedby U decay in the vicinity of the of Gecoare, overall, Pb-rich. Eventhe fine-grained sampling site. In Figure 6, note how closely the data white K-feldspar in the pegmatite matrix may con- from amazonite and galena overlap. Also, the tain in excessof 1000ppm lead. However, on a amazonitepoints overlap despitethe separateoccur- broader scale, K-feldspar in pegmatitesnot in con- renceof 10Al and 6D, F. The samplesof white K- lact with the orebody and lacking amazonite con- feldspar(6P, 10-16, 30-4) are somewhatscattered. tains lessthan 100 ppm Pb (e.g., 10-16, 30-4). The subsamplesof 6P (6P-Ll, 6P-L2 and 6P-R) In light of the findings of Hofmeister& Rossman define a linear trend (Fig. 5) that correspondsto an (1985), the occurrence of fine-grained Pb-rich ageof 2726 t 25 Ma. Specimens10-16 and 30-4 (greaterthan 1000ppm) white K-feldspar in both are not included in the regression, as they are der- suites may result from an insufficient amount of ived from different pegmatites. The model age for structurally bound water to form the necessaryPb the amazoniteand galenaclusters are about Zl@Nla, - H2O pair required for the color centre. However, which is the accepted age of the rocks in the the mode of formation of coarse-grained,Pb-rich Manitouwadge area (Stacey& Kramers 1975). white K-feldspar (BH3) or mottled green-and-white amazonite (6F) remains an enigma. Their lack of DIScUSSIoN color could stem from insufficient or excessnatural irradiation (Hofmeister& Rossman1985) or, again Although thesetwo occurrencesare separatedby from a lack of Pb - HrO pairs. thousandsof kilometres and some 1100Ma, they are quite similar in many ways. Both depositsare strati- Degree of Al-Si order form massive-sulfidedeposits that have undergone high-grade metamorphism and at least two phasgs Hofmeister & Rossman(1985) noted that increased of deformation. Pegmatites of probable anatectic lead content may promote increaseddisorder in K- origin are common to both areas,and wherethese feldspar, such that Pb-rich (290) K-feldspar is pegmatites are associatedwith the orebodies, they monoclinic, and Pb-poor K-feldspar is triclinic. The may contain amazonite. These similarities argue high-Pb monoclinic amazonite at Broken Hill and strongly for a similar envilonment of formation of the amazonite in the two suites. The environmenr 'IO. TABLE LEAD-ISOTOPEDATA FOR K-FELD5PAR AND must allow for the presenceof lead in the pegma- GALENAFROM GECO tites and the K-feldspar and explain 11e5imilar iso- 206pb 207pb 208pb topic compositionof the lead in the amazoniteand zTtrF zTqpb 2 o-Epb galena.The hypothesisshould also explain why Pb, Amzonite I I0Al 13.221 14.400 33.081 260 13.223 14.405 33.11 3 normally chalcophile,substitutes in a silicatemineral 36F 13.226 14.410 33.118 in close proximity to an ore deposit (where there is [hite K-feldspar 4 6P 13.562 14.479 33.279 an abundanceof sulfur). 5 6P-L',l 17.374 15.186 35.283 6 6P-L2 13.665 14.483 33.281 7 6P-R',10-16 13.312 14.423 33.131 8 13.664 14.551 33.273 Chemistry 9 30-4 13.819 14.616 33,361 Galena l0 grab 13.215 14.339 33.103 Our resultson the compositionof the K-feldspar Ll leach in 6N HCl, L2 leachin HN03wlth a fs drops of con- from both suitessubstantiate the findings of Foord centrated HF, R residue.

Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/24/4/729/3446257/729.pdf by guest on 30 September 2021 740 THE CANADIAN MINERALOGIST o 0 = = ct ct ,o o ct "to = ct ls.6 ct ct ro o = 13.4 ct cl cl C\I 15.2 -o g 5 = o- CD tt c, o 15.0 rf, N N

-o 14.8 o- F C' o N 14.6 = ct 8, c, ct 4u, rt ,/'' 6 2 Slgmo Errors 14.4 7

14.2 12.O 14.0 16.0 18.0 '*P b/'*Pb Frc. 5. A plot of the ratios 207Pb/204Pbversus MPbPMPb for common lead in samplesof amazonitic microcline, white microcline and galena from the Geco deposit, Manitouwadge district. The numbers are those used in Table 10. The isochron, based on leaches from sample 6P, gives an age of 2726 t 25 Ma. The growth curve is from Stacey & Iftamers (1975).

the lower-Pb triclinic amazonite at Geco could be block is above400oC, then the K-feldsparmay order construed as evidencefor the above correlation. part way but remain metastably in the monoclinic However,the fact that Pb-poor K-feldsparat Geco form, such as is found in the Broken Hill pegna- has the samedegree of order as its amazonitic coun- tites. There, introduction of a fluid medium during terpart arguesagainst the Pb - degreeof order corre- the cooling of the pegmatites or during the mild lation. Rather, the difference in degreeof order Early Paleozoic thermal event resulted in the local betweenthe Broken Hill and Geco amazonite is conversionof orthoclaseto intermediatemicrocline attributed to differing thermal histories and degrees and, very locally, to incompletely ordered low and timing of fluid interaction. microcline. In bulk compositionssuch as these,plutonic K- The occurrenceof galena in the modified areasof feldsparcrystallizes from the melt in the disordered, the Broken Hill amazonite and in fractures in Geco monoclinic form, and begins to order (and unmix) amazonite, along with a decreasein lead content of during cooling by interaction with a fluid medium. the amazonite in contact with the galena, reflect a If cooling proceedsslowly and if the fluid medium re-equilibration of the Pb-bearingK-feldspar with is availableto catalyzethe processbelow 4@oC, then later sulfur-bearingfluids. the orderingreaction will go to completion, suchas at Geco.However, if the cooling period is terminated Isotopic implicotions rather abruptly, possibly as a result of uplift, or if the fluid catalyst is present only when the crustal The closerelationship between"primary" galena

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0 = ct o (\o

0 = o o o 14.s ot o_ N 3 al \-o o_ R 14.4

Slgmo

14.3 13.0 13.5 14.0 'o"Pb/'*Pb

Frc. 6. Expanded view of plot shown in Figure 5 to clarify relationships in the lower part of the isochron. Note that the amazonite and tie galena overlap, but the white microcline is more radiogenic. The sample numbers are those used in Table 10.

and amazoniteis reflected in their similar Pb isotope The orebodiesrespond to metamorphism through composition. In the Geco samplesthere is a com- recrystallization and remobilization of sulfides if plete overlap of amazoniteand galena. Similarly, all there is accompanyingdeformation (Vokes 1969); but two galenasamples from Broken Hill plot within here, "remobilization" should not be construedto error of the amazonite. Thesefindings indicate that indicatethat melting of the sulfide assemblageshas the ordering reaction, which involved H2O, was occurred @limer 1984).The flow of sulfides at Geco largely a closed-systemprocess with respectto the is demonstratedby the shearing-offof once cross- lead isotopes.The data on the amazoniteand the cutting pegmatitesby rerqobilized ore (Suffel ef a/. bulk of the galena are consistent with previously 1971). Prograde metamorphic reactions produce reported compositionsof galenafrom Broken Hill fluids througlt the dehydration and decarbonation and Geco (Stacey& Kramers 1985,Oversby 1978, of phasesin and near the orebodies.The fluids would Franklin & Thorpe 1982,Franklin el a/. I 983).This likely be a mixture of HrO, CO2 (Aines & Rossman is a strong indication that the pegmatites and their 1984)and, most likely, H2S, F and Cl as well. Cir- constituentamazonite, which postdatethe orebod- culation of tlese fluids aid recrystallizationand result ies, derive their lead from the orebodies.Further- in extensivetransport and redistribution of base more, the white K-feldspar, generallypoorer in lead metalsin the orebodyand, locally, in the wall rock than in the amazonite, is more radiogenic than the as well. amazonite and the orebodies (e.9., BH3, 10-16, In a study of melting relations involving PbS' 30-4, 6P). FeS2and granite(+ H2O),Kullerud & Yoder (1967)

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found that at a temperatureof 650oC,neither sul- scatter in isotopic values may then reflect differing fide phasehad melted, but that extensiverecrystal- initial ratios inherited from their respective paren- lization had occurred, with pyrite replaced by pyr- tal material. The presenceof a large number of "peg- rhotite (Sr lost), accompanied by extensive matite" bodies at Manitouwadge, with variable redistribution of galena and quartz via the vapor chemical and modal compositions,probably reflects phase.At 725"C, melting of the granite occurred, the formation of small batchesof anatectic magma and the resulting quenchedmelt was found to con- in numerous localized sites rather than from a sin- tain disseminatedsulfides. The sulfide-bearing peg- gle magma-chamber.Each batch can have a slightly matites in and around the Broken Hill and Geco different lead-isotope signature owing to differing deposits are believed to have formed in much the combinations of parental material and differing same manner, most likely by the in si/r melting of U/Pb values. The Broken Hill pegmatites can be metamorphosed sulfide-bearing pelitic and described similarly.The Rb,/Sr isotope values of K- quartzofeldspathic material. feldspar from tJrepegmatites exhibit the same type Such a setting for anatexis would explain a num- of scatter(Shaw 1968), and plot abovethe isochron ber of features. The amazonite-bearing stringers, for the whole-rockgneiss samples; further study of whether in the orebody or in the surrounding the Pb-poor pegmatitesof this suitemay show that gneisses,could be intruded from the site of melting. the lead isotopesbehave similarly. Coevalpegmatites formed outsidethe ore environ- The grey-white orthoclase BH3 is taken from the ment would be amazonite- and sulfide-free. Galena same peCnratiteas amazonitic orthoclase BH7, but and amazonitewould sharethe sameisotopic com- contains a quarter of the amount of lead aud is more position becausethey both formed in the samePb- radiogenic than BH7. The possibility existshere also rich environment. A more remote setting for that the isotopic compositionof BH3 has somehow anatexis,with "contamination" of the granitic melt become more radiogenic, presumably though the only upon intrusion into the ore zone, is a lesslikely decay of uranium in nearby accessory phases. hypothesis,in our opinion. Consideredeven less However, it is doubtful that leaching of this sample likely is a proposal that the lead in amazonite is will regless its composition toward the ore-lead addedat a subsolidustemperature (cl, Doe & Hart values.Retrograde alteration of the sampleby a fluid 1963).As the color is intrinsic and ultimately due phase,however, could haveresulted in exchangeof to structural defects,there can be little driving force lead and addition of a radiogeniccomponent. Galena to induce such defects in a normal feldspar during BH6 and NBH-2 may have!ser1 similarly affected, its subsolidusevolution. In fact, we contend that thus resulting in anomalousratios. Galena BH6, for thorough recrystallization of amazonite, such as in example,is from the sarnesample as amazoniteBH6. the Geco mine, will progressivelyremove the color The radiogenic nature ofthis galena may reflect the cenues. incorporation gf "mixed" lead, removedfrom acces- The regressionof isotopic data for the leachfrac- sory phasesas well as from the amazonite by a fluid tions from sample 6P (from the samepegmatite as phase during retrograde metamorphism. amazonite 6F) and the amazonite gives an age of The increasein the proportion of radiogenic lead 2726 ! 25Ma, closeto the acceptedage of 27fi) Ma in microcline may be a consequenceof the feldspar of the rocks in the Manitouwadgearea. This indi- - water interaction Qocalsolution and redeposition) catesthat the more radiogenicK-feldspar of this peg- that leadsto the increasein Al-Si order. It is con- matite had initially the sameisotopic composition ceivable that as monoclinic K-feldspar that is rela- as the galenaand amazonite, thus substantiatingthe tively lead-rich is convertedto microcline, the net loss proposal that anatexis occurred within the ore in lead is accompaniedby an ion exchangeof struc- environment.The more radiogeniccomposition of turally bound lead with lead carried in the fluid sample 6P is then a direct artifact of its lower con- medium. The lead that is preferentially retained in centrationoflead and open-systembehavior during the structure probably is that presentssselding to its recrystallization. the coupled substitution Pb2+ + Al3+ = The other samplesof white K-feldspar (10-16, K+ + Si4+,i.e.,lead that is considered"inactive" 30-4) and those of Tilton & Steieer(1969) show con- from the point of view of the formation of the siderablescatter. This could in part be due to the amazonitecolor. This would provide an explanation addition of radiogenic lead through U decay in for the commonpresence of radiogeniclead in sam- nearby accessoryminerals, and local "contamina- ples of Precambrian microcline, and tbe finding of tion" ofthe lead in the K-feldsparduring the order- Ludwig & Silver (1977\ that in no case are the ing reaction(see below). However,if an isochronof observedU and Th contentssufficient to explain the about 2700Ma is drawn through any one of these buildup in the radiogenic component. Ludwig & Sil- points, an intersectionwith the ore lead is not pos- ver consideredmost of the radiogenic component sible.Assuming that the cross-cuttingpegmatites at "parentless", i.e., it must have found its way into Geco are, broadly speaking,of the sameage, this the feldspar from elsewherein the rock.

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Sulfide-silicate equilibria K-feldspar was thorough during this protracted period of cooling, and amazonitewas preservedonly The formation of amazonitedepends on the con- whereionic sulfide specieswere in short supply. Sub- tribution of lead from the orebodies.Lead, however, solidus compositionalazd structural equilibrium was is chalcophile; the concentration of lead in a silicate attained in the K-feldspar at Geco, and relics of the mineral so closeto a sulfur-rich environmentrequires original (or even a more evolved) monoclinic K- an explanation. The breakdown of galena during feldspar should not be exPected. metamorphism is a consequenceof desulfurization, In contrast, the Broken Hill pegmatitesstill retain i.e., loss of sulfur oncethe mineral becomesunsta- the original high-temperature monoclinic structure, ble. Evidence for desulfurization at Broken Hill exceptwhere the effectsof retrogrademetamorphism stems from two assemblages.The first is a pyrrho- or of the Early Paleozoicreheating have beenimpor- tite envelopearound the soutlern end of tle orebod- tant. Although the pressureand temperatureattained ies. The enveloperesults from the desulfurization of in the Willyama crustal block are roughly compara- pyrite by a lowering of the fugacity of sulfur during ble to those in the Manitouwadgeblock, the cool- metamorphism(Plimer 1976).The secondassem- ing history must have differed significantly. In par- blage is a halo of magnetite, biotite and gahnite ticular, the K-feldspar was allowed to order above around the pyrrhotite envelope(Johnson & Klingner the field of stability of microcline, and this $tage 1970. Work on the assemblage(E.J. Essene,pers. lasted at least 90 million years, according to Harri- comm. 1984)reveals that gahnite and quartz (+ S) son & McDouea[ (1981),possibly on accountof a formed through the desulfurization of sphalerite in rift-related environment and high heat-flow @limer a reaction with $illimanite. 1985).This very slow rate of regional cooling was A pyrrhotite envelopemay also exist around parts followed by sufficiently rapid cooling through this of the Geco deposits @.J. Essene,per$. co[tm. field (caused by rapid uplift?) that conversion to 1984).Furthermore, gahnite is also found associated microcline could not be carried out efficiently, in with the deposit. Gahnite at the Geco mine occurs spite of the presenceof water (17,22 and 35 ppm largely in the enclosing cordierite - staurolite - bio- of structural water are noted in three samples of tite - garnet schistsand results from the breakdown amazonitic orthoclase from Broken Hill, amounts of zincian staurolite (Spry 1982).However, the obser- that are completely comparableto the level of struc- vation ofgahnite, sphalerite,muscovite and quartz tural water present in amazonitic microcline; Hof- in veins in pegmatites would seemto indicate that meister & Rossman 1985). The high-temperature desulfurization of sphalerite also occurred during assemblagecan still be found and, by implication, metamorphism(Stevenson 1985, Suffel et al, l97l). so car the original textures and mineral assemblages The breakdown of galenain a fluid medium, then, in the ore zone. Although compositional equilibra- requires a reduction in thb fugacity of sulfur such tion was attainedin the K-feldspar (TableT) via an that galenabecomes unstable with respectto Pb2+ ion-exchange process, structural equilibration was and a sulfur-bearing volatile species.Under mini- not. The Willyama block evidently escapedany sig- mum conditions for anatexis[650oC, P(H2O) : 5 nificant event of regional metamorphic reheating kbarl, only a relatively small decreaseinfiS) below after the 1660Ma culmination that could have elimi- the pyrite-pynhotite boundary would be required for nated the relict high-temperature feldspar. The the galenato 6ssq6s unsttble (Kajiwara 1970).Dur- prominence of amazonite at Broken HilI reflects the ing cooling,the fugacity of sulfur beginsto increase relative unimportance of the retrograde effects and, in localized areas;the pore fluids may then scavenge possibly,a gxeaterproportion of disseminatedgalena lead from the K-feldspar during its recrystallization in the quartz- and feldspar-bearing metamorphic (i.e., ordering and exsolution) and deposit it as fine- assemblagethat underwent melting than at Geco. grained galena in fractures and along the rim of Whether or not the original orthoclase at Geco wa$ amazonitegrains. as lead-rich as at Broken Hill remains unanswered. The presenceof amazonitein pegmatitesin medium- Differences in thermal history to high-grade metamorphic terranes can be considered a useful indicator of potential base-metal Although the two suites share many characteris- mineralization, although from our findings' it seems tics, they differ significantly in their feldspar miner- restricted to the immediate vicinity of the mineral- alogy. The Geco suite contains grid-twinned low ized zone, microcline, as could be expectedof a mesozonal crustal block that cooled very slowly and at a uni- Acmqowr,BpcsN{BNTs form rate from 650'C. Uplift of the Manitouwadge crustal block from a depth of 18 to 20 km probably R.K. Stevensonacknowledges the help of the staff occurred long after the effects of the Kenoran of the GecoDivision of Noranda Mines, and espe- orogeny had ceased.Recrystallization of the primary cially thanks F.J. Bakker and R. Friesenfor their

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guidanceand hospitality.Mr. R.E. Le Messurier,of Dor, B.R. & Hanr, S.R. (1963):The effect of contact lhe Zitc Corporation Limited, Broken Hill, con- metamorphismon lead in potassiumfeldspar near tributed many useful samples(the seriesBH1-7 from the Eldora stock, Colorado.J. Geophys.Rer. 63, the B lode and the Separation peematite) to this 3521-3530. study. SamplesNBH-I to NBH-3 come from the (1979):Amazonite from North mine and were donated FooRo,E.E. & ManrrN,R.F. by the North Broken PikesPeak bathohth. Mineral. Record70, 3'73-382, Hill CompanyLimited. SamplesBHl3 and 14 come from an older collection contributed by The Zinc FnexKLrN,R.W., Roscoe,S.M., Lovemocr,W.D. & Corporation. KLW-V4 was sontributedby Profes- Sarcsrnn, D.F. (1983): Lead isotope studiesin sor KennethL. Williams and Dr. E.E. Foord. Sam- Superiorand SouthernProvinces. Geol. Sum, Can. ple E9155was collectedby ProfessorJohn S. Steven- Bull.35t, son from the B orebody, South Broken Hill. Dr. George L. Cumning graciously made available & Tgonee, R.I. (1982): Comparative metal- laboratory facilities and counselduring productive logeny of the Superior, Slave and Churchill a provinces. stay at the University .In PrecambrianSulphide Deposits @.W. of Alberta; Messrs. Dragan Hutchinson,C.D. Spence& R.W. Franklin, eds.). Krstic, Paul Wagner, Gilles Gauthier and Dr. Cl6- Geol.Assoc. Can. Spec,Pap, 25,3-90. ment Gari6py patiently instructed the flrst author in the fine points of mineral separationand Pb-isotope FnmsEN,R.G., Pmncn,G.A. & Wrers, R.M. (1982): analysis.Mr. Tariq Ahmedali and Dr. M. MacKin- Geologyof the Gecobase metal deposit..InPrecam- non also contributedto the acquisitionof chemical brian SulphideDeposits (R.W. Hutchinson,C.D. data. The flrst author acknowledgesthe support of Spence& J.M. Franklin, eds.).Geol. Assoc. Can. the Natural Sciences and Engineering Research Spec.Pap. ?5,343-364. Council in the form of a scholarship. Both authors B,L. (1984):Uranium-lead and lead-lead thank NSERC for ongoing support of GursoN, researchcosts invxtigatioru of mineralsfrom the BrokenHill lodes (grant 47721to RFM). Vicki LoSchiavo'sattention andMine Sequencerocks. Econ, Geol,79,476490. to detail at the end stagesofthis investigation proved invaluable.Messrs. F.J. Bakker (Geco),B.J. Drew HanrusoN,T.M. & McDoucer-L,I. (1981):Excess foAr and R.C. Haydon (The Zinc Corporation), Drs. A. in metamorphicrocks from BrokenHill, NewSouth Hofmeister (GeophysicalLaboratory), C. Gari6py Wales:imptcations for 4oAr/3eAxage spectra and (Universit6du Qu6beca Montr6al) and E.E. Foord the thermal history of the regjon.Earth Planet. Sci. (USGS,Denver) have provided very useful comments Lett. 55. t23-t49. on an early version of this paper. Horunrsrrn, A.M. (1984):A SpectroscopicStudy of FeldsparsColored by lrradiation and Impurities, REFERENcES Including Water.Ph.D . thesis,California Institute of Technology,Pasadena, California. A.rurs,R.D. & Rossnmll,G.R. (1984):The high tem- peraturebehavior of water and carbon dioxide in - & Rossrraer.r,G.R. (1985): A spectroscopic cordieriteand beryl. Amer. Mineral, 69, 319-327. study of irradiation coloringof amazonite:structurally hydrous, Pb-bearing feldspar, Amer, BaKKER,F., Cavraerl, J. & Fnrrsnr.r,R.G. (1985): Mineral. 70,794-804. Geologyand excursionguide to the GecoCu-Zn-Ag mine and Manitouwadge arca. In Gold and Jaurs, R.S., Grunve,R.A.F. & Peur, L. (1978):The Copper-Zinc MetallogenyWithin Metamorphosed petrologyof cordierite-anthophyllitegneisses and GreenstoneTerrain, Hemlo - Manitouwadge- Win- associated mafic and pelitic gneisses at stonLake, Ontario, [email protected]. McMillan & D.J. Manitouwadge,Ontario. Amer, J, 9ci. 278,4l-63. Robinson,eds.). Geol. Assoc. Can. - Can. Inst. Mining Metall., Spec.Publ., 16-29. Jomtsorl,I.R. & KuNcwsn,G.D. (1976):The Broken Hill ore depositand its environment..lr Economic Brasr,A. (1977):Calculation of T-siteoccupancies in Geologyof Australia and PapuaNew Guinea(C. alkali feldspars from refined lattice constants. L. Knight, ed.).Austral. Inst. Mining Metall., Mon. Mineral, Mag. 41, 525-526. 5, 476491. dncn, F., Mrsan,Z. & Povouona,P. (1971):A green Karrwene,Y. (1970):Some limitations on the physico- lead-containingorthoclase. TschermaksMinerol. chemicalenvironment of depositionof the Kuroko Petrog.Mitt. lS, 213-231. ore. In Volcanism and Ore Genesis(T. Tatsumi, ed.). Univ. Tokyo Press,Tokyo, Japan. Connrrr, G.J. & PHrlues, G.N. (1981):Regional retrogrademetamorphism of a high gradeterrain: Knorl, H. & Rnne, P.Hr{1983): Latticeparameters, WillyamaComplex, Broken Hill, Australia.Zillos compositionsand Al-Si order in alkali feldspars. 14, 59-73. Mineral, Soc.Amer,, Rev.Minerol,2,57-99.

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