Zincian Greenockite in Stratiform

Canadian Mineralogist Vol. 23, pp. 89-94(1985)

ZINCIANGREENOCKITE IN STRATIFORMLEAD_ZINC_SILVER MINERALIZATIONAT LADY LORETTA,NORTHWEST OUEENSLAND

DAVID J. PATTERSON Researchand Development Division, Mount Isa Mines Limited, Mount Isa, Queensland4825, Austalia

ABSTRAcT INTRODUC"TIoN

At the Lady Loretta stratiform lead-zinc-silverdeposit During metallurgical evaluation of the Lady in northwest Queensland,Australia, zincian greenockite Loretta sulfide deposit, tracesof apparently primary occursin a thick, lead-rich,cadmium-anomalous zone in zincian greenockitewere noted in polishedsections laminated sulfide mineralization. It is intergrown with of diamond-drill core. This occurrenceis unusual galena,sphalerite and minor pyrite in galena-richlaminae because primary cadmium sulfides ile seldom microscale and veinlets.Electron-microprobe analyses show ore deposits,and the mineral hasnot significant zinc-for-cadmiumsubstitution in greenockite(7.1 reportedfrom - 18.4mol.Vo ZnS, average13.l 9o);usually cadmium was been previously recorded in the Proterozoic lead- found only in tracesin coexistingsphalerite and was not zinc-silver depositsof northern Australia. In addi- detectedat all in galena.Textural and geochemicalcon- tion, cadmium usually shows a close geochemical siderationssuggest that the cadmium-anomalouszone and coherencewith zinc (Rankama& Sahama1950, p. the occurrenceof zinciangreenockite are primary deposi- 708-714)and typically occursas a trace elementin tional featuresofthe mineralization. This indicatesunusual sphalerite;its occurrencein a separatesulfide phase physicalor chemicalconditions during sulfide deposition, in such a zinc-rich environment is surprising, and resultingin a decouplingof the geochemicalcoherence of suggestsunusual geochemical conditions during for- zinc and cadmium during the formation of a part of the deposit. mation of the deposit.

Keywords: cadmium, zinc, lead, greenockite,sphalerite, Gsoloclcel AND CHEMICALSETTING microprobe data, Lady Loretta deposit, Australia. The Lady Loretta deposit is a small but high-grade stratiform lead-zinc-silver deposit of Proterozoic SoMrr4aRs age,situated about 115km northwestof Mount Isa atLat. 19"46' S, Long. 139o03'E in northwest Dans le gisementstratiforme de plomb-zinc-argent de The style,geological setting and ageof Lady Loretta dansle Queenslanddu Nord-Ouest,en Aus- Queensland. tralie, la greenockitezincifdre sepr6sente i I'int6rieur d'une the depositare similar to thoseof other Proterozoic zone 6paisse,riche en plomb et anomaleen cadmium, au stratiform lead-zinc-silver deposits in northern sein d'une min6ralisationde sulfureslamin€s. Elle s'y trouve Australia, suchas Mount Isa, Hilton and McArthur en intercroissanceavec la galdne,la sphal6riteet un peu River. The regionalgeological setting is discussedby de plrite dansdes lamelles et filonnets microscopiquesriches Plumb & Derrick (1975), Plumb e/ a/. (1980)and en galbne.Les analysesi la microsonde6lectronique r6vb- Derrick (1982),and the geologyof the deposit has lent, dansla greenockite,un remplacementnotable du cad- beendescribed by Alcock & Lee (I974) and Loudon - mium par le zinc (7.1 18.4V0mol. de ZnS, 13.190en et al. (1975). Aspects of the geology and isotope moyenne);mais d'ordinairele ne setrouve qu'en cadmium geochemistry depositwere discussed by Carr tracesdans la sphal6ritecontigue, et n'a pas 6t6 d6cel6du of the tout dansla galbne.Sur la foi de consid€rationstexturales & Smith (1977). et g6ochimiques,la zone anomaleen cadmium et la pr6- Hole Pl16 was drilled on section2420N into the sencede la gxeenockitezincifdre seraient des indices de d6pdt easternlimb of the deposit(see Fig. 4 of Loudon el primaire de la min6ralisation.Ces observationssignalent al, 1975); it intersectedtwo zonesof relativelyhigh- desconditions physiques et chimiquesanormales au cours gradelead-zinc mineralization. Down-hole profiles du ddpOtdes sulfures, provoquant une dissociationde la of lead, zinc and cadmium abundancesin mineral- coh6rencegdochimique du zinc et du cadmiumpendant la ized parts of P I 16 (Fig. 1) show a generallyclose cor- partie gisement. formation d'une du respondencebetween zinc and cadmium, exceptin (estimated (Traduit par la R6daction) a 25-m-thicklead-rich zone true thickness about 15 m) in which cadmium concentrationsare Mots-clds: cadmium, zinc, plomb, greenockite, sphal€rite, consideredhigher than expectedfrom zinc abun- analysed la microsonde,gisernent de Lady Loretta, Aus- dances.A scatterdiagram showsa generallygood tralie. correlation betweenzinc and cadmium throughout 89 90 THE CANADIAN MINERALOGIST

Frc. l. Down-holeprofiles of abundancesof lead, zinc and cadmiumin mineralized sectionof drill hole Pl16, Lady Loretta. Samplespacing is I metrewithin metal- rich zones.Shaded zone on cadmium profile is calculatedcontribution of cadmium containedin sphaleriteover this interual, basedon regressionequation for Fig. 28. the hole, but with severaloutlying points (Fig. 2A). showstypical morphologiesand relationshipsof the Omitting sampleswith over 590 lead, zinc and cad- grains. mium show an excellentlinear relation$hip @ig. 2B), In reflectedlight, greenockitehas a slightly lower but there is no simplerelationship between lead and polishing hardnessand slightly higher reflectance excesscadmium in lead-rich samples(Frg. 3). than the accompanyingsphalerite, and is grey with a distinct blue-greentint; sphaleritehas a faint pink tint in comparison,and this color contrastis accen- OCCURRENCE AND PROPERTIES oF GREENOCKITE tuatedunder oil immersion.Most grainsshow abun- dant and intenseinternal-reflections, mostly clearyel- Only a few sampleswere available from the lead- low with occasionalorange or red tints. rich sectionof drill hole Pl16; greenockitewas first Electron-microprobeanalyses of greenockiteshow observedin one polishedsection from that interval. significant substitution of Zn for Cd, with a com- Subsequentre-examination of all availablematerial positionalrange of 7.1to 18.4mol.Vo ZnS (average from the deposit revealedfurther greenockitein a l3.lt/o; Table l, Fig. 5). Only traces of iron are lead-rich sample from an unknown location. present,and elementsother than cadmium, zinc, iron In both samplesgreenockite occurs in galena-rich and sulfur were not detected;in particular, man- laminaein finely bandedmassive sulfides, and to a ganeseand indium were sought but not found. minor extentin crosscutting,microscale veinlets of Individual grains of greenockiteare homogeneous galena.Greenockite forms disseminatedgrains and within the precision of the analyses. patchesfrom l0 to 1000pm in diameterin galena, X-ray powder-diffraction examinationof a mix- and is commonly intergrown with sphaleriteand fine- ture of greenockiteand its host galenagave a pat- grained euhedralor spheroidalpyrite. Sphaleriteand tern similar to that of synthetichexagonal CdS, with fine-grainedpyrite alsooccur disseminatedthrough- some displacement of lines. Calculated cell- out galena laminae and veinlets. Greenockite- paramereri[a 4.08(1),c 6.64(l) A1 correspondto a sphaleritecontacts are irregular to smoothlycurved, ZnS contentof about 15mol.9o (Hurlbut 1957),in whereas greenockite-galenacontacts vary from good agreement with the range obtained from irregular to planar, with occasional grains of electron-microprobeanalyses. No diffraction lines greenockiteapproaching euhedral forms. Figure 4 attributable to hawleyitewere detected. ZINCIAN GREENOCKITE IN Pb-Zn-Ag MINERALIZATION, QUEENSLAND 9l

Sphaleriteintergrown with greenockiteand in iso- 000 b lated grains in laminae of greenockite-bearinggalena o E containsminor amountsof iron and generallyonly qe trace amountsof cadmium. It is similar in composi- 600 tion to sphaleritein nearby sphalerite-richlaminae a (Table l, Fie. 5). Occasionalanalyses of sphalerite revealedover I wt.9o Cd; this may reflectinclusions ao a. aa of greenockiteat or below the surfaceof the polished ' rrt aala sections,although none were seenin the areasana- aa o l.a lyzed. Etching of sphaleritewith l: I HNO, did not o ll-.. revealany patchesthat might be cadmium-enriched (cl, Chen & Dutrbac 1978).Electron-microprobe analyses of galena intergrown with greenockite t0 50 detectedonly lead and sulfur. wt.o.6Zn

DIScUSSIoN

The Lady Loretta greenockiteis clearly primary with respectto the presentcycle of weathering.It occurswell below the baseof significant oxidation, in samplesshowing no original oxidation or leach- ing, and only slight post-drilling oxidation. Mineral texturesdo not resemblethose of secondarysulfides. Some recrystallizationof sulfides has occurred; sphaleriteand galena show an approach to poly- 'o m 30 lo hedral "foam" textures, sphalerite has planar rt.%zn* annealing-typetwins, and there is somemicroscale Frc. 2. Zinc-cadmium scatterplots for drill hole Pll6' veiningof sphaleritelaminae by galenaa greenock- Lady Loretta. A. All samples;shaded area contains 44 ite + sphalerite.This slight metamorphismis insuffi- points. Least-squatesregression equation is: amountof cient to explain the thick cadmium-anomalouszone, Cd (ppm)= 10.69(Znwt.s/o)+37, correlation coeffi- and equivalentor more intenserecrystallization at cient 0.802.B. Sampleswith lessthan 590Pb; shaded Mount Isa and Hilton has not generatedcadmium areacontains 42 points. Least-squaresregression equa- sr-rlfidesin oresof similar bulk-composition.There- tion is: amount of Cd (ppm)= ll.l2 (Zn wt.t/0)+2, fore, the occurrenceof greenockitein a cadmium- correlation coefficient 0.980. enrichedzone is most likely an original depositional ! 8OO feature of the mineralization. o The sulfur isotope data of Carr & Smith (1977) R suggestthat depositionalor metamorphictempera- 600 ture at Lady Loretta probablydid not exceed300oC. Compositionsof zinciangreenockite and coexisting 100 sphaleriteare far from the boundariesof the two- phasefield in the ZnS-CdSsystem at theselow tem- peratures(Tauson & Chernyshev1977); thus the two 200 minerals are not in equilibrium. This occurrenceof apparently primary zincian 'o 20 30 Q greenockite in the Lady Loretta deposit is of *.% pt'o geochemicalas well as mineralogicalinterest. The (total Cd minusCd Pl 16intersection has a higher Zn/Cd ratio than the Frc. 3. Leadverszs "excess cadmium" altributedto sphalerite,based on regressionequation deposit as a whole, and the deposit is not enriched for Fig,28)for sampleswith 590 Pb or greater,in drill in cadmium in comparison to other Proterozoic, holePl16. sediment-hostedlead-zinc-silver deposits in northern Australia (Table2). In thesedeposits, cadmium typically occurs as a trace elementin sphaleriteat At Lady Loretta, there has been significant concentrationsof a few thousandparts per million, decoupling of the geochemicalcoherence of cad- and there is a good linear correlation betweencad- mium and zinc during the formation of a substan- mium and zinc abundances(Smith & Walker 1971, tial thicknessof mineralization, which presumably Croxford & Jephcott 1972). reflects changesin physicalor chemicalconditions 92 THE CANADIAN MINERALOGIST

,W.dJl

Ftc. 4. Textural relationshipsof greenockite,sphalerite and galenaat Lady Loretta. Scalebar represents50 pm and appliesto all photographs. All photographs taken under oil immersion. A. Irregular greenockite(medium grey) intergrown with galena(white) and sphalerite(dark grey, somewith internal reflections).White euhedraof plrite are included in greenockite. B. "Euhedral" greenockitein galena, intergrown with sphaleriteand with fine-grained inclu- sionsof pyrite. C. Irregular greenockiteintergrown with galenaand sphalerite;both greenockiteand sphaleritehave locally planar boundarieswith galena. D. Graphic intergrowth of greenockite(with minor included pyrite) and galena.

during sulfide deposition.These may be relatedto ACKNOWLEDCEMENTS local oxidizingconditions, as hematite and barite are common accessoryminerals in the Lady Loretta P.D. Cheyne,W. van Oosterom,G. Wood and mineralization,in contrastto other similar deposits M. Young are thanked for their assistancewith the in the region. Efficient separationof cadmium and work. This note is publishedwith the permissionof zinc can occur in relatively oxidizing environments, the Lady Loretta joint venturepaftners, Mount Isa as shownby the typical occurrenceof cadmiumsul- MinesLimited and Triako (Aust.) Pty. Ltd. The text fides as supergeneminerals (Oen et ql, 1974, 'rri.asimproved by commentsfrom two anonymous Ramdohr 1980,p. 582). referees. ZINCIAN GREENOCKITE IN Pb-Zn-Ag MINERALIZATION, QUEENSLAND 93

TABLEI. COMPOSITIONOFCOEXISTINO GREENOCKITE ANDSPHALERITE FROM LADY LORETTA (ELECTRON-MICROPROBE DATA)

! ? ! 2 a 7G) 8(a) e(b) 10(c'e) 11(c) 12(c) ]r(d) 14(d) Zn 7.O 7.4 5.2 a,9 7.5 64.2 64,0 64.9 62,7 65.4 65.1 65.5 65.2

Fe lr na t1 t.1 L.0 na na na 1.1 I.0

70.7 74.5 74.2 68.0 0.1 0.] t.6 0.) 0.5 0.1 0.1 s 72.4 2'1 2..5 2.1 22.8 13.a, t2.8

99.6 [email protected] [email protected] 99,9 99.7 100.4 99,o 99.0 99.8 99.7 l-6 oleenocklte, 7-14 sphaterite. Anai-yses ln reight percent, tr = trace, na = mt analyzed. (a) Intergrom wlth gleenockl.te 1. (b) Intergflnn rlth greenocklte 2. (c) In sane gal-ena-rlch lamlna as qn€nockites 4-6. (d) F dn massive sphaLerlte Lanlna adjacent to galg|a-rlch band contalnlng dlsseminated greenocklte and sphalelite. (e) Hlqh Cd ln thls salple nay be due to incluslons of greenocklte, al-though mne were seen in the ilea analyzed. Analytlcal cordltLons: Pure Zns, Fe, cd standards, m ku, o,OE r[A, JEoL zAF corr€ctlon pr@ran. Analyses by P.D. cheyre' l|ont lsa l,llE Llmited.

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