105

Canadian MineralogistVol. 31, pp. 105-l 17 (1993)

TEM STUDY OF INDIUM. AND COPPER-BEARINGGROWTFI-BANDED SPFIALERITE

RICHARDA.D.PATTRICK,MARTADORLINGANDDAVIDA.POLYADepar.tntntofGeotogy,ThzUniversityofManchester,Maru:hesterMl39PLEnglanl

ABSTRACT

An investigation by transmission elecEon microscopy of (Cu, In)-rich bands in sphalerite from base-metal veins in West

Shropshire, Englancl, revealed these elements to be preierentially substituted P.ar tlt: inner margins of the (Fe'cd)-rich layers

within the zoned grains. The iron-rich bands, furthermore, comprise several iron-rich and iron-poor ryb,bq$' the former

containing oriented tows of "chalcopyritid' particles, p.oguUiy firmed by exsolution'.Opaque zones of sphalerite developed

adjacent to late chAcopyrtte are,#cf$ in coiper Uut contain oo -i"tot"opi" particles. High*esolution microscopy of the opaque

sphalerite reveals the presence of Guinier-feiton (GP) zones representilqrire eartl de19lofm9,nt of chalcopynte' Eqlilibrium

precipitationinresponseto changes in solution chlorinity,remperature"Td4o, "*lgtsatisfactorily

explain the observedbanding'

Instea4 the repetitiu" ct "micai

iepa.ation of (cu,In)- aid edcol-ri.rt uuirdsis ascribed to the preferential irrevenible adsorption

ofthese ions overcitreringranges'oipu O*ing "y"i..frung"r

oi'rotutlon pH' The pattern ofbanding caused by such a mechanism

is consistent with that expectei from a considlration of the fint hydrolysis constants of the trace element cations'

Keyvords:transmission electron microscopy, sphalerite, indium, banding, hydrolysis, adsorption, cyclicity, West'Shropshire'

England.

SoMMarns

Nous avons utilis6 la microscopie dlectronique par transmission a.fin de caractdriser les couches riches en cuiwe et indium de

la sphal6rite provenant de tissule, -ine*fre"iOo district de West Shropshire, en Angleterre. Ces 6l6ments semblent incorpor6s

prdf6rentiellement prds de la bordure inteme de couches riches en Fe et Cd. De plus, les -ouches riches en fer sont faites de plusieurs

sous-couches enrichies oo uppuunri"r en fer. Celles qui sont les plus riches contiennent, en rang6es orient6es, des particules

..chalcopyritiques,, qui r6sulteLient d'une exsolution. i,es ,on t opuqu.t qui se sont ddveloppdes dans la sphaldrite tout pras de

la chalcopyrite tardive tbnt preuYe d'un enrichissement en Cu, .ui. n" "onti"nn"nt

pas-de Oarticules fines' Une 6tude des zones

opaques par microscopie d haute resolution d6monrre b ;ili6p;nt de zones. de duinier-Preston, qui reprdsentent un

d6veloppement pro"oce oe ra .trJ.opyrlt". u* ptgapitmon a req-,iiiibre en fonction de chansements en chlorinit6 de la solution'

en temp6rature et enflo2) n,explique pas la zonation en couches successives. Nous .toyonJplototque la s6paration r6p6t6e de

Cu+In et Fe+Cd en couches est le r6sultat d,une adso.ptioo ioJuer.ible prdf6rentietle de ces ions d des valeurs de pH diffdrentes

au cours dechangements cycliques dupHde la solution. L'agencement cyclique des couches qu'acaus6 un tel phdnombne concorde

uuaa ""

qo" nooi un"nao* a pu.tit des constantes de la premidre hydrolyse des ions de ces m6taux'(fraduit Par la R6daction)

Mots-cl\s:microscopie 6lectronique par tr-ansmission, sphal€rite, indium, zonation en couches, hydrolyse, adsorptron' cyclicitf'

West ShroPshire, Angleterre.

1980). Results ofelectron-microprobe analysis reveal a

corrJlafion between the In and Cu content in ZnS,

indicating either the coupled substitution of Cu + In or

the incluiion ofroquesite, CuInS2 (Johan 1988, Rurke

& Kieft 1980, Hak er al. 1983). Substitution would be

in the foim Cu* + In3+ -> 2Zn2* urd is facilitated by the

structural similarity between roquesite and sphalerite'Indium in sphalerite also has been observed to concen-

trate in specific zones within the chemically variable

srowth-bands commonly developed in hydrothermaliohaterite (Johan 1988, Oen et aI. 1980)''In

this study, banded crystals of indium-bearing

IN.IRODUCilON

Trace amounts of indium commonly have beendetected in sphalerite, the marn source for the metal. Thenature of the substitution of the indium is' however, notwell understood. Incorporation as In2S3 was suggestedby Anderson (1953), but the very low solubilities-ofindium in ZnS as observed in experimental studies ledKissin (1972) to suggest the presence of submicroscopicparticles of an indium-bearing phase. Similarly' experi-mental studies have demonstraied the low solubility ofCu in sphalerite at low temperatures (Wiggins & Craig

106 THE CANADIAN MINERAIOGIST

sphalerite from a base-metal vein deposit are investi-gated by analytical electron microscopy (AEM) with theaim of determining the nature of indium and coppersubstitution and examining the causes of theii conien-tration. The indium and copper are associated with (Fe+ Cd)-rich bands in the sphalerite. possible equilibriumand kinetic mechanisms for the origin of thirchemicalzoning are discussed.

MINERALOGY

The samples come from an example of unmetamor-qhgsed base-metal vein-type mineralization hosred byOrdovician greywackes in the West Shropshire Orefield,England. Galena, sphalerite and minor, parageneticallylate, chalcopyrite are present and are assoCiated withbarite, calcite, witherite and, very rarely, fluorite (pat-trick & Bowell 1991). Three chemically and texturallvdistinct generations of sphalerite have been recognized.An optical study showed the second generation ofsphalerite to be color-banded; analysis of samples fromfour localities revealed a sphalerite stratigraphy of the

Ftc. I . Photomicrograph ofbanded sphalerite showing iron-richbands, which appear black.

type recorded by Barton et al. (1977) and Mclimans eral. (198O) throughout the orefield. The color-bandedgrowth zones are typically pale (yellow) bands 100-200pm wide separated by sharply defined dark (brown;bands, usually 10-100 pm in width (Fig. 1). Some ofthese dark bands comprise several dark and lightsub-bands (- 3-10 pm across) parallel or at a very lowangle to the margins of the main bands. Associated withthe darker sub-bands are lines of tiny particles. Theseparticles are either parallel to the sub-bands or formparallel arrays at a low angle (d") to the sub-bandmargins, giving the impression of cross-stratification(Frg. 2). Adjacent to late chalcopyrite-bearing veins, thesphalerite commonly is opaque, although no inclusionswere observed in it.

SpneLsR[E Crnvrsrny

There is a relationship between color and chemistry,as demonstrated by electron-microprobe analysis(ElvPA), with the darker bands containing higherconcentrations of Fe (up to 6.0 wt%o) andCd(0.89 wtEo)than the pale bands (Fe - 2.7 wtTo and Cd * 0.28 wtVo)(Table l). EMPA also revealed zones enriched in bothIn and Cu coincident with the dark Fe-rich bands. Thesezones are typically less than 8 pm across, occurringeither within or at the margins of the dark bands. Whereassociated with thicker dark bands, they occur at or closeto the inner (earliest formed) margin of the band.

The results (Table I, Fig. 3) reveal a clear positivecorrelation between Fe and Cd, and between Cu and In.but no direct correlation between the In and Fe contents(Fig. 3). In the indium-rich zones, the indium values risefrom below detection limits (200 ppm) up to 2.4 wt%o,although values of 0.4-1.0 wtVo we more typical.Background values of Cu in the sphalerite are - 0.05wtVo but increase to 0.5 wt%o in the indium-rich zones;the highest levels of copper coincide with the highestlevels ofindium (Fig. 3). At low concentrations ofln andCu (- 0.3 atomic %o),theratio between these elements isI : l, but at higher concentrations of both elements, theproportion of indium exceeds that of Cu, commonly bymore than 2:1.

Although the indium-rich zones show la0eral vari-ations in concentration over a few millimeters, they are

TABLE I. REPRESENTATIVE coMPostTIoNsI oF BPHALERTTE

E1.36 2 ,93 0 ,89 2 ,46 0 .41 32 .84 i00 .6A Xr r ich banq69.90 4 ,43 0 ,69 0 ,06 0 .18 93 .34 99 .10 t r r l ch bed01,85 2 .81 0 .26 2 ,75 0 .48 32 .60 , t00 .71 In - r l ch band61.28 4 .06 0 .32 1 .61 0 .32 33 . , t6 100.64 In - r tch zona64,67 3 .07 0 .17 0 .00 0 .06 33 .05 10 t .01 pa le band42.84 5.49 0,80 0.02 0.08 32.34 .t0i,E6 oark bsd

rEl6trcn-mlcrcprobs data, @pclilone glven In wt*.

INDruM- AND COPPER.BEARING GROWTI{- BANDED SPHAITNTE

ftc. 2. photomicrograph of an iron-rich band showing the microstructure of the band. The darker areas contain high concentrations

of tiny parncles.

to7

nonetheless identifiable in sphalerite from the differentmines in the area and are particularly useful in defining

go.h h ln

Ftc. 3. Results of electron-probe microanalyses aqoss aniron-rich band, showing the distribution of Fe, Cd, In andCu (after Pattrick & Bowell 1991).

tle band suatigraphy, effectively forming indium-richmarker horizons.

{tao

I

itI

ELEc"rRoN MIcRoscoPY

Samples that had been subjected to investigation byoptical microscopy and EMPA were prepared,fgr-a squdfuiing transmission electron microscopy qEy) btlolFfeari thinning using the technique described by Tighe(1976). The study employed a Philips EM430T insfru-mentoperating at300kV. Analysis was carried outusingan EDAX energy-dispersion systemusing the correctionorocedures for thickness decribed by Cliff & Lorimer

ifSZS). By photographing the samples before EMPA-O ut"t ttrinning, the exact position of the areas ofinterest could be iocated. The darker (iron-rich) bandsare easy to identify because their higher content of.Fe(Z-+'i?o) made these much more resistant to thinningthan the iron-Poor bands.

The ind.iam-rich zones

The innerboundary of the Fe-rich bands is sharp, witha rise in Fe occurring over a distance less ttran 200 A'EDAX analysis of the bands produced results similar to

--*-\\

\.----.."-el'

108 TTIE CANADIAN MINERALOGIST

those derived by EMPA. Several indium-rich zonesidentified by EMPA were invesrigated by TEM, includ-ing bands in which the Cu:In ratio is 1 : 1 and bands withindium in excess over copper. Although the EDAXsystem has higher limits of detection (for indium andcopper, - 0.4 wtTo) and lowerprecision than the electronmicroprobe, it was possible to locate the high-indiumzones. These are 1-3 pm in width and usually associatedwith lower contents of iron within the high-iron bands.In the six bands investigated, zo particles were observedin the indium-rich zones, either In-rich particles, such asroqley,E, or Cu-rich particles. At magnifications of upto 50,000 times, the sphalerite seems clear and structure-less. High-resolution microscopy (500,000 x) revealedthe sphalerite to have a perfect lattice, although somedislocations were observed.

Thz band microstructure and chzmistry

The TEM investigation was carried out on the Fe-rich

bands to determine the nature of the microstrucfure(sub-bands) seen in thin section. The iron-poor bands,bands with a low relative level of iron (<4vttvo Fe) andthin Fe-rich bands proved to be largely featureless.Dislocations and twinning were only rarely seen, al-though twin-related domains are locally intense, espe-cially in the Fe-rich bands. Fractures also were observed:these may be decorated by euhedral inclusions (<l pm,rof calcite. Diffraction analysis revealed crystallographiccontinuity across the bands.

In concordance with the optical observations, thewider iron-rich bands were found to be inhomogeneous.At lower magnifications (2100 x), the clearly defineddark and lighter "sub-bands" were revealed, caused bydifferential ion-beam thinning. In addition, thinningaccentuated 120' grain boundaries formed during re-crystallization of the sphalerite. A set of parallel, late,deformation-induced twins also ale present, these corre-sponding to a cross-cutting linear feature observed inthin section (Frg. 4). At higher magnifications" a htgh

iw . '

g pm

Flc' 4' Electron micrograph ofan iron-rich bandin sphalerite revealing alternatingparricte-rich (A) andparticle-free @) sub-bands.The particles are probably th$copynte and have developed in rJlatively iro?rich sub-bands. rhj tzo. grain boundaries dueto recrystalization of the sphalerite and late deformation twins are evident (see text).

IN'DruM- AND COPPER.BEARING GROWTH- BANDED SPHALERITE 109

density of particulate inclusions become clearly visiblein the darker sub-bands. The inclusions are aligned inrows parallel to the sub-band margins (Fig. 4). Singlerows of inclusions may be present within the bands,whereas thicker sub-bands may contain many rows ofsuch particles. EDAX analyses reveal that the darkersub-bands contain approrimately 0.6 wt%o more ironthan the lighter bands, which explains the differentialthinning.

In a 2@-pm-wide Fe-rich band, a well-developedrepetitive sequence of particle-rich and particle-poorsub-bands is developed. This sequence comprises (in thedirection of growth) 2-pm-wide sub-bands with anincreasing number of particles that abruptly terminate,

followed by a 0.7-pm-wide particle-free band (Fig. 5).Analysis ofthe sphalerite revealed variations in the ironcontent, with the base of the particle-free zone contain-ing 3.2 wt%o Fe, increasing to 3.6 wtVo in the middle ofthe clear zone, and 4.8 wtTo just before the particle-richzone. The matrix in the particle-rich area contains - 5.0wtVo Feo and this drops abruptly to 3 wlvo again in thenext particle-free zone. Cadmium also displays a posi-tive correlation with the iron, but the concentration ofcopper is too close to its detection limit for the methodto provide meaningful data.

Analysis of the particles proved difficult because oftheir small size. Even in nanoprobe mode, the volume ofthe particle is too small (<5Vo of the analyzed volume).

Frc. 5. Electron micrograph of part of a well-developed repetitive sequence of parncle-rich (B) and particle-free sphalerite (A).

The direction of crystal growth was from bottom to top.

1 1 0 TT{E CANADIAN MINERALOGIST

However, by comparing results of analysis of theparticles and the adjacent matrix, it was possible todemon$trate an enrichment of Cu and Fe in the particlesin a l: I ratio, suggesting them to be either Cu+Fe sulfide(chalcopyrite) or, much less likely, Cu+Fe+Zn sulfides.

Opaque sphalerite

EMPA analysis of the opaque zones demonsffated anenrichment in Cu of up to 1.5 wt%o. Tlte presence ofchalcopyrite in the cross-cutting veins suggests thatthese zones might contain a dusting of chalcopyriteparticles formed by replacement of the iron-bearingsphalerite (chalcopyrite disease, see Barton & Bethke1987). TEM observations showed that particles areabsent; however, the sphalerite was found to be intensely

twinned, with voids developed at the twin intersections(Fig. 6). This development of twins may well be relatedto the fracturing of the sphalerite during the formationof the cross-cutting vein. Examination of twinned areasusing high-resolution microscopy (500,000 x) did,however, reveal zones of twinning developed in thesphalerite in t'wo directions parallel to { I I I } (Figs. 7, 8).These features have all the characteristics of Guinier-heston (GP) zones, as described in silicate minerals byChampness & Lorimer (1976) and Nord (1980), whichrepresent the first stage ofphase separation in theprocessof exsolution. A diffraction pattern (SAED) from thisarea (Fig. 9) shows intense streaking between positionsof the matrix reflections. This corresponds to thepresence of very thin platelets in the sphalerite matrix,such as is seen in GP zones.

Ftc. 6. A higb density of rwins in opaque, copper-rich sphalerite with voids at rhe rwinintersections. The trvins are developed on { I 1 1 }.

INDIIJM- AND COPPER-BEARING GROWTTI- BANDED SPHALERITE

Frc. ?. Electron micrograph dispkying a system ofplatelets on { I 1 I } in the copper-rich sphalerite ttnt have the characteristics of

GP zones.

1 1 1

L12 TTIE CANADTAN MINERALOGIST

FIG. 8. Electron micrograph showing a high-resolution lattice image of the copper-rich sphalerire (detail of Fig. 7). The Gp zonesdeveloped on { 1 I I } are clearly visible, as is the sphalerite latrice.

Flc. 9. Selective-area diffraction pattem of the copper-richsphalerite (Frg. 8), the streaking coresponding to thedevelopment of GP zones on { I I I }.

DIscussIoN

Subsrtntion ofindium and copperfor zinc

The solubility of indium in sphalerite under experi-mental conditions is very low: only 25 ppm at 500.C(Kissin 1972). Similar experimental studies of coppersolubility in sphalerite in the system Cu-Zn-Fe-S haveshown that at arnbient temperatures, it is less than 0.5wtTo (Wiggins & Craig 1980). However, several fi7o ofboth copper and indium have been detected in sphaleriteusing EMPA, where both elements are present in thestructure (Burke & Kieft 1980). In this study, the clearcorrelation between copper and indium enrichment hasagain been demonstrated. The absence of any particlesin the indium-rich zones and the undisturbed structureshows that the indium and copper are genuinely incor-porated into the sphalerite. The structural similaritiesbetween roquesite and sphalerite indicate this to be asimple substitution of Cu+ + In3+ = 2Zn2+. Presumably,a similar mechanism involving copper and feric iron(chalcopyrite and roquesite are isostructural) is inhibitedby incorporation of iron in ZnS as Fe2+.

INDIUM- AND COPPER.BEARING GROWTTI- BANDED SPHALEzuTE 1 1 3

Cu-rich particles

Three processes could account for the developmentof the chalcopyrite particles: primary precipitation'exsolution, and replacement. The distribution of theparticles parallel to the growth banding suggests thatthey are a primary feature, related in some way to thegrowth of the iron-rich sub-bands. The regular distribu-tion and ellipsoidal shape ofthe particles, however, havesimilarities to textures seen in the exsolution of chalco-pyrite in natural and synthetic sphalerite, as discussedbyWiggins & Craig (1980). Solubilities of CuS in (Zn,Fe)Sarelow, less than2.4mol.Vo at300'C (Kojima& Sugaki198fl and at25"C, as low a$ 0.5 wtVo Cu (Wiggins &Craig 1980). Nonetheless, the particles observed in thisstudy are volumetrically a very small component of thesphalerite (<0.5 wt%o) and could represent the amountof copper that can be dissolved in sphalerite at thetemDerature of mineralization, 160'C (Pattrick & Bow-ell I 99 1 ) . On cooling, the excess copper reacted with fteFeS component ofthe sphalerite to form chalcopyrite.This would indicate a higher initial concentration ofcopper in these iron-rich sub-bands than in the rest ofthesphalerite.-

The development of chalcopyrite blebs in sphaleriteby replacement is a well-recognized phenomenon, as isthe selective development of these particles in iron-richbands (Barton & Bethke 1987, Eldridge et al.1983).Inthe Shropshire veins, a paragenetically late generationof chalcopyrite does occur, and there is evidence of somereplacement of the sphalerite in the form of crystal-lographically controlled rows of particles, especially iniron-rich bands. It is therefore possible that all thesphalerite has been affectedby apervasive Cu-rich fluid'wtrich has resulted in very selective replacement ofiron-rich bands (c/ Barton & Bethke 1987). However'the regularity of the rows of inclusions observed in thisTEM investigation strongly suggests that replacement isnot the mechanism of formation of the particles de-scribed here.

The opaque sphalerite

The development of the opaque sphalerite is clearlyrelated to the reaction with later fluids; the GP zones areconsidered the product of this process. GP zones areusually formed by exsolution, but in this sample itappears to be the introduction of copper into thesphalerite that has caused their formation. The twinnednature of the sphalerite and the development of voidswill have greatly aided the innoduction of the copper,but the pervasive nature of the chalcopyrite plateletsindicate that copper diffirsion through the sphalerite alsooccurred. Thus, the GP zones may have developed by achange in composition causing the system to move intothe two-phase field, rather than by a decrease intemperature.

Although there is no direct information on the exact

composition of the GP zones, they may well represeffthe eartest development of chalcopyrite in thesphalerite. This is a very common replacement phe-nb."non involving the reaction of copper with the ironcomponent of (Zn,Fe)S @ldd.dge et al. 1983) that leadsto the formation of chalcopyrite particles in thesphaleriie. Diffraction analysis indicates the GP^zonesand the host sphalerite to have tlle same structure, furthersupporting the case for chalcopyrite.

The nature of the voids is uncertain, but they mayrepresent the location of secondary fluid inclusions attwin intersections.

The generation of the Fe-Cd-In-Cu-rich bands

Studies of fluid inclusions (Pattrick & Bowell l99l)suggest that fluids precipitated sphalerite at tempera-tuiei ranging from 200 to 120'C (mostly around 1 50'C)'The fluids were highly saline CaCl2-dominated brines,with chloride concentrations of several molal. Althoughthere are insufficient data to deduce fluid pH or lo EflO) ,Mclimans et at. (1980) and Giordano & Barnes (1981)

have deduced pH values from neutral to 2 log units acid(5.8 to 3.8 at 150'C) and log fiO) of around -50 ati50"C (i.r., well within the field of predominance ofreduced sulfur species) for similar Pb-Zn-Ba deposits'

The compositional banding suggests that periodicchanges in the chemical or physical properties of theore-iorming fluid must have taken place' The observeddistribution of trace-element-enriched zones may be dueto either equilibrium or kinetic effects.

ttre equiliUrium distribution of trace elements insphaleriteis governed by the composition of .thelty-drotfrermal fluid and by the thermodynamic equilibriumconstant for the exchange reaction:

ZnS (sp) + UP*1aq1=uS (sp) +Znz+(aq)

Thus Fe-rich sphalerite could be produced either by anincrease in the ratio of total dissolved iron to dissolved

zinc in solution,ZFelDTn, or by a change in concentra-tion of potential metal-complexing ligands in solution'Indeed, Mookherjee (1962) proposed that temperaJure-and ionic-strength-dependent differences in the stabili-ties of Zn and Cd chloride complexes may account forthe large variations that may be found in the Cd contentsof sphalerite. Zinc is more effectively complexed bychloride ligands over the temperature range I 00 - I 50oCthan is iron (Frg. 10). Thus, an increase in chlorideconcentration of a sphalerite-precipitating solution will

lead to an increase in (ap;z+la7nz*)l(l'Fel7-Zn), which inturn will result in a more Fe-rich sphalerite being

precipitated [provided (DFel7-Zn) remains constant]'Finor" t t shows how the relative free concentrations ofthi metal ions Fe2*, Cu2+ Cd2+ and In3* vary withchloride ion concenfration at 150oC. An increase inconcentration of the chloride ion from I to 4 molal willgreatly increase the Fe and Cu contents of any precipi-

n4 THE CANADIAN MINERALOGIST

Relatlve free metal lon concontratlons at l5OoC

.-n' ' / cu

--cd

l r 2 4 6 I 10 " t2p H

Ftc. I l. The frst hydrolysis constant for In, Cu, Zn, Fe and Cd at temp€ratures from 25o to150"C. Plotted from data presented in Table 2. Each curve represents the boundarybetween the predominancefields ofagiven free metal ion and its monohydroxy complex.

s4h

=n

c(ltNEi 0N

TE

8 - 2J

-4

Chlorlde lon concentratlon /mol kg-!Ftc. 10. Relative concentrations of the free metal ions Fe, cu, cd and In relative to zn as a

function of chloride ion concentration at 150oC. This parameter is expressed as log(mMz+lmzj?+)l(Wlrzrr), where W andlZnrcfer to the total dissolved concentrationof metal, M, and zinc respectively. The parameters at l00oC are closely similar to thosepresented here. Plotted from data presented in Table 2.

1 6 0

126

Flrst hydrolysls stop tor In, Cu, Zn, Fe and Cd to i6OoCIn Gu Zn Fe Cd

oo 1 0 O3!to

FzooF

50

tated $phalerite and will slightly increase the Cd and Incontents. A closely similar trend is observed at 100.C.Thus, the observed pattern of trace element zoning heremay be due to changes in the chlorinity ofthe ore-for-

ming fluid. However, it is difficult to envisage amechanism by which changes in chlorinity could pro-duce the equilibrium separation ofFe+Cd- from Cu-FIn-rich bands.

INDruM. AND COPPER.BEARING GROWTH- BANDED SPHALERITE 1 1 5

Changes in the oxygen fugacity or sulfur fugacity ofan ore-forming fluid may lead to changes in the level oftrace elements in the sphalerite. Notably, Barron et al.(1977) invoked periodic influxes of relatively reducedfluid to produce bands of relatively iron-rich sphaleritein the OH vein, Creede, Colorado. The describedrelationship between Fe-content of sphalerite and log

flO) and logJts) assumes saturation of the fluid withpyrite. The absence of pyrite in the sphalerite bandsdescribed in this study, however, means that the modelof Barton et aI. (1977) is not applicable.

The effect of changes in oxygen fugacity on metal-complexing has been considered here as a possibleexplanation for the observed trace-element zoning andhas been rejected. The absence ofbarite suggests that theore-forming fluids were characterizedby oxygen fugaci-ties at which reduced sulfur(tr) species predominated.Under these conditions, Fe, Zn, Cd and In would not beexpected to undergo any significant change in thedominant valence state ofthe dissolved species (Chen etal. 1983). Only Cu might exhibit a change frompredominantly Cu(I) to CuG) valence states; such achange forjust Cu cleady cannot explain the enrichmentin Fe and Cd associated with the Cu-rich bands insohalerite.

Itis likely thatvariauons in ap"z*/a2nz* may bebroughtabout by changes in the bulk composition of the fluid orby changes in the concenfations of ligands notconsidered here, particularly bisulfide ion and variouscarboxylic acids. There are insufficient data to fullyassess these possibilities.

An altemative approach to explain the sphaleritebanding observed in this study is to invoke disequili-brium processes.Oenet al. (1980) suggested nonequili-brium crystallization of a supercooled hydrothermalfluid with difftrsion-controlled cyclic supersaturation atthe sphalerite-fluid interface to explain oscillatory zon-ing in sphalerite at Cartagena, Spain. Roedder (1968)suggested secondary (postdepositional) diffrrsion toexplain compositional banding in an analogous fashionto Liesegang rings; however, such a process would beexpected to produce anhedral bands in contrast to bandsalong crystal faces ubiquitously observed in this study.

Incorporation of trace elements will be enhanced byrapid rates of precipitation (cf Lorens l98l), in turnbrought about by an increase in supersaturation of theore-forming fluid. Such an increase in sphalerite super-saturation might be brought about by a decrease intemperature, by an increase in total dissolved reducedsulfur, or by an increase in pH. Of these changes, a pHincrease may serve to separate Fe+Cd and Cu+Inbecause of the pH dependence of their adsorption ontosphalerite surfaces.

Inorganic sorption is highly effective in controllingthe metal ion content of river and groundwaters (Hem1972).The extent of sorptlon is strongly pH-dependent,typically occurring over a narow range of pH. James &MacNaughton (1977) found that heavy metals are

adsorbed onto inorganic minerals over a pH rangeconesponding to the transition between the predomi-nance-of nonhydrolyzed and hydrolyzed metal species.That is, sorption is strongly favored by the formation ofmetal hydroxide sPecies:

Fe2* (aq) + oH- (aq) = Fe(OH)' (aq)

Furthermore, Jean & Bancroft (1986) reported enhancedCd adsorption onto sphalerite when the pH is increasedto a value 2 or 3 log units below the first hydrolysisconstant of Cd. Since ln, Cu, Fe and Cd have differentvalues of the first hydrolysis constant, this mechanismmay provide a means of separating these elements in thesphalerite bands observed in this study.'

The first hydrolysis constants for In, Cu' Fe and Cdover the temperature range 25oC to 150'C are shown inFigure 1 1. Indium is clearly largely hydrolyzed over theentire range ofpH conditions expected for the splal91tebands, whereas an increase in pH from 3 ll2 to 5 ln at150oC would be expected to successively lead to theadsorption of Cu, then Fe, then Cd. This sequence ofmaximum adsorption closely matches the observedcompositional banding. Particularly where the effectiveradius and charge distribution of the adsorbed ion aresimilar to those of Zn2+, the adsorption process is likely

TABLE 2. TTEMODYMNIC $TABILITY OF gEIECTED CHLORO AIID HYDROXY @IiPI'EXES OF

iioiur,'udirin, oppen' rnol mo zrlc rnor uo'c ro tso'c

los (EquJl lbr ' l@ @nstant)2 6 ' C 8 0 ' C 1 0 0 ' c 1 0 0 ' c

2 , 4 3 2 , 8 1 2 . 8 1 3 . 1 93 . 4 3 3 . 7 5 4 . 1 5 4 . 9 23 . 6 3 4 . 8 6 5 . 8 8 7 . 2 41 . 3 7 1 , 4 1 1 . 8 3 1 . 9 82 . 2 1 2 . 2 5 2 , 4 7 3 . 0 f2 . 4 0 2 . 8 3 3 . 3 4 4 . 0 9o . o 1 0 , 7 2 1 , U 2 . 8 7

- 0 . 6 9 0 . 1 8 1 . 1 6 2 . 3 0- 2 . 2 0 - 1 . 2 3 0 . 0 4 1 , 5 2

Equi l lbr l@ R.f

c1-2Cl-3C l-c l -

2Cl-3C l-

3Cl-cl-c l -

2C1-3Cl-4Cl-

H20H*HbHlJH20

tn91Ins'tn3rcd21cd2+cd2,cu2+Cr?l

Fgztzn2+zn2+,.2+liz.Frn3rcd21cu2rFe2+znza

rncl 2+InCl2:Incl 3uMCI'Cdc l 20cdcr 3-cucl tCuClr0cucl a-FeCltcdclrCdC l 20CdCl rzncl;2-HCI U

:r0H2+ + Htcdot+ + HlcuoHr + H+FeoHr l F'znotfr + Ht

0 . 4 20 , 6 20.34o . l 9

- 0 . 1 8 0 . 2 1 0 . 8 81 . 8 3 2 . 8 61 . 8 9 2 . 9 01 . 3 0 2 . 1 02 . 1 0 3 . 0 0

-0,71 -0.04 -0.49-4.00 -3.12 -2,42 -1.76

-to.08 -9.06 -A.13 '7.22-?,03 -?.00 -6. l9 -5.39-9.50 -8.€ -?.60 -0.65-8,90 -7.9? -7.09 -6.24

l{hsrs unavallable, data for mebl-Mplgx dabllltl@ have b@n.extrarclat€d

;;;;;;;;;-16'C dab uelns the eetlmatlon t@hnlque of crrs & cobbls

(!9€4) for average lonlc h€t 6p@ltl@i

cp,r l l r . ( r) + arx st(rr) + zlgH+(abg)(Tr) )

where cor lT. 16 tho h@t €paclty, cP, ol lon' I ' . ov€r. tho t6mperatur€

;il;; T"'6'i' *d *h"." q ild F ar€ tdperaturo depondent @ndanla rcr

a-o;rtlc'ular lon-tvpe (e..g. €tlona, oxygen fr4 Mlons) tabulatod by cds

; A;;6 .'irs4t, 6'ri*(abe\17; lo the ab@lute rcla' ent-@pv or H'(aq) at -rhei"6,iiir'i!iii*t,i*, ri, z1 ts the lonlc ohars€ of t-he ep@l@-,1'-andi,ii.i ii. ti. itot*

"nircpv of thre ep*le, l, at th€ refren@ t€m96dure'

;li;';;;;-6;iit-,i.i, 'pp.-1urdn to b€ 2s&15 K' A varuo.or -s'0

iulTri*l *." aaopiea for sH+(abs)(Tr) aftEr crls md 6bble (19€4)' .-ii,J

rd"iiriv ot th€ meth;d was ohek€d bv @parlns .9u19']1t99,,ad

". . . l r rJni" l

-6tui f r f t" @nstmt6 tor z lnc{hlor ld€ dd lron-chlo' lde @nplox

;,;;;i;:-;g;*"i betvsn calculated and dperlm€ntal valu€ LZn: Ruava

iS".;i.a ii-ge?)fr": Holnrlch & g€rard (19€O)l l5 bsfrer than r-a los unrt

Jxepifor'zncta2-, for whlch the method ldorke p@'ly b@u@ of a change

ii-Jrior"r eymiletry trcm znc\- ro z^ct12- (Ruava 19ag)'

&urc6 ol data: a Extrarclated In thls study frm 26'c data of-Tuck

i ig; : ] . -u extrarcratod In thl6 dudv frcm 26'c data ot carrels & chr lst

i id#i l "-

wJi i .v l rb,s). d Holnr lch a'na *ward (1s90) ' e Ruava Md seward

i i6e6j ' i umlt" i data f Ruava & s€f lard (1986) ' s Extrapolated in thla

;;; rrc; r;tC dut" ol Ba6 & M@mer (19s1), w@t (1970) ed Garrelo &

chrtgt (19€6).

l l 6 T}IE CANADIAN MINERALOGIST

to be very strong and may become ineversible (Hem1972), leading ultimately to the incorporation of theadsorbed ion into the crystal strucrure of sphalerite. Thusthe adsorption of Fe2+ and Cu2*, which have ionic radiivery similar to that of Zn2* in 4-coordinated sites, islikely to be much stronger and more irreversible thanthat for In3* and Cd2*, which are significantly larger ions(Shannon l98l). It further seems likely, therefore, thatthe incorporation of the relatively poorly adsorbedindium may depend critically upon the adsorption ofCu"* and the dissolution of particles of the isostructuralroquesite on and within the sphalerite.

The events that caused the periodic changes in pH ofthe fluid leading to formation of the sphalerite bandingmay be related to (i) changes in source fluid or (ii)changes in degree of mixing of hydrothermal fluid withsurface or near-surface waters. Periodic inputs of mas-matic waters have been suggested by Barton e t al. (1971)as a cause ofsphalerite banding at Creede, Colorado. Theabsence of nearby coeval igneous bodies in WestShropshire leads us to eliminate this as a possible modelin this case. Considered more likely are periodic inputsofhydrothermal fluid due to cyclic reactivation offaults(with transient permeabilities) (cl Sibson 198 l, Sibsonet al. 1988): this would result in the sudden influx ofrelatively rapidly moving high-temperature fluidspoorly equilibrated with the country rock, followed bya gradual decrease in flow rate, temperature and hydro-gen ion activity. Such cycling would be expected toproduce a sharp boundary between new Fe-poorsphalerite and older Fe-rich sphalerite, and a somewhatdiffirse boundary between new Fe-rich sphalerite andolder Fe-poor sphalerite; this set of boundaries was norobserved in the West Shropshire veins.

Seasonal variations in groundwater levels, leading tolarying degrees of mixing, have been proposed byRoedder (1968) to explain basin-wide banding insphalerite. Horbury & Adams (1989) explained bandedcarbonate cements in the late Dinantian of the EnglishLake District as being due to varying degrees of fluidmixing as a result of cyclic changes in sea level.Howevel Mclimans et al. (1980) dismissed ground-water mixing as an explanation for basin-wide bandingin sphalerite on the basis that such a process wouldproduce more irregular textures, without consistentlysharp boundaries between compositionally distinctbands.

In summary, we favor the formation of banding insphalerite as a result of disequilibrium crystallization inresponse to cyclic changes in pH and possibly, lessimportantly, temperature. The variation in the firsthydrolysis step of the various trace elements incomo-rated is the key to the chemical separation of In+Cu frbmFe+Cd in sphalerite, due to preferential adsorption ofthese elements over different ranges of pH. There isinsufficient evidence at present to determine flrlly thegeological processes resulting in such cyclic chemicalchanges, but it appears likely that they are related either

to cyclic pumping (cl Sibson l98l) of source fluid orroperiodic changes in the extent of mixing of the hy-drothermal fluid with groundwaters.

AcKNowLEDGEMENTS

The authors are grateful to Ian Brough, Graham Cliffand Peter Kenway for invaluable assistance with theTEM analysis, and to hofessor G.W. Lorimer and Dr.P.E. Champness for useful discussions. The final manu-script also benefitted greatly from the comments of threeunnamed referees. Financial support was provided byNERC grant GR.3?1'97.

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