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The Canalian M ineralogist Vol. 34, pp. 1211-1222 (1996)
SELF.ORGANIZEDBANDED SPHALERITE AND BRANCHINGGALENA IN THEPINE POINT ORE DEPOSII NORTHWESTTERRITORIES
ANTHOI\Y D. FOWLER'
Oxawa-Carleton Geoscience Cente and Departrnent of Geology, University of Ottatva, 140 L ryAN L'HET]REUX Ottawa-CarletonGeoscience Centre and Depanmenrof Physics,University of Oxawa, 150 lt;uis PasteurStreet. Oftawa. Ontario KIN 6Ns ABSTRACT Botryoidal arraysof banded-acicularsphalerite are intergrownwith dendritic andbranching galena at the Pine Point deposit, Northwest Territories. Scanningelectron microscope(SEM) and electron-microprobeanalyses (EMP) demonstratethat the bandingin sphaleriteis due to an alternationin Fe and Zn content.A time-seriesanalysis constructed from the measurement of band widths in a doubly polished thin section is consistentwith the hypothesisthat the bands are self-organizedas a result ofthe operationofa nonlinearchemical oscillator. The branchingcrystals ofgalena areshown to be anothermanifestation of far-from-equilibrium crystallization.Accordingly, a qualitative model is proposedthat demonstratebhow the interplay of reactionand diffrrsion kinetics can lead to the bandingand branching.Models whereinthe bandingin sphaleritein Mississippi- Valley-gpe (MVT) depositsis solely consideredto be an artifact of bulk chemicalchanges in the fluid within the systemneed to be re-examined. Keywords: Mississippi-Valley-typedeposit, MVT, sphaleritebanding, zoning, branchinggalen4 self-organization,nonlinear dynamics,reaction-diffirsion kinetics, Pine Point, NorthwestTerritories. Somaann Nous d6crivonsI'intercroissance d'amas bulbeux de sohaldriteruban6e et aciculaireavec desdendrites et desramifications de galbneprovenant du gisementde Pine Point, dans les Territoires du Nord-Ouest.Des analysesau microscopeI balayage 6lectroniqueet i la microsonde6lectronique d6montrent que les rubansdans la sphal6ritesont I'expressiond'une variation de la teneuren fer. Une analysede s6rietemporelle construite h parrir de la mesuredes 6paisseurs de bandesdans une sectionmince doublementpolie concordeavec I'hypothbse que les bandessont auto-organis6es et le r6sultatde I'op6rationd'un oscillateur chimique non lin6aire. l,es cristaux de galbne en branchesmanifestent 6galement la pr6senced'une cristallisation hors d'6quilibre. Par cons6quenqnous proposons un moddlequalitatif de I'effet r6ciproqueque peut avoir la cin6tiquede r6action et la diffusion sur la formation des ramificationset des rubansobservds. l,es moddlesselon lesquelsles rubansde sphal6rite des gisementsdu type de la vall6e du Mississippi seraientcaus6s uniquement par des changementsa vaste 6chelle dans la compositionchimique desfluides hydrothermauxsont d revoir. Mots-cl€s: gisements du type de la va1l6e du Mississippi, sphal6rite ruban6e, zonation, branchementsde la galbne, auto-organisation,dynamique non lin6aire, cin6tiquede r6actionet diffusion, Pine Point, Territoires du Nord-Ouest. IwrnooucloN hecambrian Shield in the southernGreat Slave Lake area, Northwest Territories. The Pine Point deposits The Pine Point depositslie at the easternmargin of share characteristicswith many lead-zinc deposits theWestern Canada Sedimentary Basin, in a Paleozoic the world over. and are classified as membersof the section approximately500 m thick dominatedby MississippiValley type (MVT) of deposits.In general, evaporite and carbonate rocks that overlie the MVT deposits were formed by circulating low- temperature(50-100'C) hydrothermal fluids asfillings within carbonaterocks at shallow depths( Y- i'r FIc. l. Plane-polarized-ligbt photomacrographof part of a thin section showing sphalerite banding in a sample from the Pine Point deposit. The band- ing varies from clear, to pale white, to brown, to deep red-orangein color. The concentric bands form coalescent arcuate arays that emanatefrom centersofgrowth, In three dimensions. these form the characteristic hum- mocky, so-called botry- oidal textures. Note that some bands toward the top of the image do not correlate over the entire section. Black blebs and filigees within the spha- lerite bands are elements of what are interpretedto be a branchingcrystals of galena that radiate from the middle of the thin section. Although it appearsdiscontinuous, in three dimensions, all parts are presumably connected. The arrow demarcates a point cornmon to that of the more detailedFigure 3. BANDED SPIIALERITE AND BRANCHING GALENA. PINE POINT" NORTHWESTTERRITORIES L2t3 Ftc. 2. SEM imagecaptured from nearthe gro*th centerofthe galenausing electronback- scattermode. The galenaclosest to the centerof growth is dendritic andrelatively thick in comparisonto the later galena,which hasa branchinghabit and is characterizedby tipspliuing. The branchingopens in the growth direction,toward the top of the figure. to light brown, to red, to almost black. The bands Close examination of the bands shows that they consist of bulbous, so-called botryoidal arrays consistof networksof many acicular,platy and stubby composedof sheavesof numerous acicular crystals crystalsof sphalerite.Acicular crystalsradiate from the oriented normal to the bands. Figure I illustrates a inner margin to the outer margin of the band (Fig. 3). typicalsequence ofbands from coreto rim ofa doubly- In addition to tlte color, both texture and grain size polishedthin sectionof a sphaleritesample from Pine changebetween bands @igs. 3, 4). Unlike the galena Point. Note that the banding is discontinuousat the crystals,there appearsto be no evidenceof branching, small scale,and that many bandsare intergrown with Le.,tip splitting.Some individual crystals can be traced branchingand dendritic crystalsof galena.These show across more tfian one band. Examination with a evidenceof tip splitting, and their branchingopens in scanning electron microscope(SEM) and an optical the growth direction (Frg. 2). microscopeindicated that the sphaleriteis intergrown 12t4 T}IE CANADIAN MINERALOCIST Frc. 3. Plane-polarized-lightphotomicrograph illustrating the growth habit of sphalerite. The arrow demarcatesthe samepoint as the correspondingarrow in Figure l. In all bandsobserved, the sphaleriteis composedof small acicularand platy crystalsthat are orientednormal to the banding.The relativd coarsesphalerite of one bandis clearly shownto be arrangedin a fasciculatetexture (i.a., bunchesof crystalsthat radiatefrom a commonarea witlout evidenceof tip-splitting). with a hexagonal mineralo presumably the ZnS tions for iron and zinc, respectively.For dark bands, polymorph wurtzite. SEM evidence and electron- the meanvalues are 6.0 wt.Voond 61,.2wt.7o, whereas microprobe (EMP) analysis demonstratethat the for the light bands,they are 1.9 *.Vo and64.2 *.Vo. bandingcorrelates with a fluctuation in the content of Figure 4a, an Fe X-ray map, demonstratesthe alter- Fe and Zn in sphalerite.For instance,SEM analysis nation in Fe contentbetween the bands.Note that the of 56 bands vielded the followine atomic concenfa- banding is well characterizedby the altemationin Fe BANDED SPHALERTTE AND BRANCHING GALENA, PINE POINT, NORTI{WEST TERRITORIES t2t5 contentand that the distributionof Fe anticorrelates into reaction vesselsat varying rates, and the nature of with that of Zn @ig. 4b). Cu, Sn, Cd, Ni, and Co the products are monitored. Depending upon the concentrationsall provedto be below detectionlimits particular rate of pumping, product compositions may for the electronmicroprobe (500,362,321,293 and be steady, or fluctuate in an oscillatory or chaotic 281 ppm,respectively). manner (Gray & Scott 1990). Deterministic nonlinear systems may exhibit INTERpRETATToNoF THE Tnxrunrs complex irregular (chaotic) behavior. We can distinguish their output from purely random signals lntergrowths of sphalerite and galena form the because there is an underlying order in deterministic botryoidal textures. The galena is characterizedby nonlinear systemsthat is absentfrom random systems dendritic and branchingcrystals. Under conditions (Packardet al. 1980).In nature, systemsare commonly close to equilibrium, galenaforms equanteuhedral both stochastic and nonlinear, but data analysis may cubiccrystals. The mineralmorphology observed here allow us to deducethe relative importance of noisy and suggestsfar-from-equilibrium conditions of growth deterministic processes. (e.9., Fowler et al. 1989). As is typical in these For this pu{pose, we performed time-series analyses circumstances,the interplaybetween rates of crystal on a measrue of the sphalerite banding in order to test growth,diffusion of solute,and surface tension results for nonlinear growtl-modes. This was accomplished in self-organizedpattems. Dendrites arise when a by measuring the thickness of successive bands of protuberanceon a planarsurface invades the solution, sphalerite. We found that the best way to observe the enhancing the local concentration-gradient.This textures was through the use of doubly polished thin augmentsthe diffusiveflux and,correspondingly, the sections and an optical microscope.'Nomarski phase- rate of growth of the tip. However,surface-tension contrast rnicroscopy using various etchants (dilute effects limit the growth process,and eventually the HC| HNO3, and NaOCI) was inferior to viewing in large cost in sudace energy results in dissolution. transmitted light in terms of the contrast between Tip-splitting occurs when small protuberances(at adjacent bands. Back-scattered electron images the atomic scale) form on the growing tip. These generatedusing the SEM also did not offer sufficient protuberancesare stabilizedrelative to their inter- contrast. X-ray maps generated on the EMP have vening embayments,as the probability of aggregation sufficient resolution, but are of such restricted areal is highestat their tips (Nittmann & Stanley 1986). extent that the technique is impractical. It is unlikely that diffusing (i.e., random-walking) Figure 5 illustrates the variation in band thickness growth-speciescould penetrate between protuberances acrossthe botryoidal texture. Figure 6a is a return map without striking them and sticking. Once initiated, produced by plotting the (n+l)th band's thickness as a the branches shield their embayments from function of the thickness of the previous one (nth). By growth-species,so that in-filling growthrarely occurs. plotting thicknessesof successivebands against each The resulting patiern of gowth is said to be self- other, the retum map monitors the influence of one organizedin the sensethat there is a pattern that is a event upon another, l'.e., determinism. Although it is spontaneousreaction to the growthconditions and not difftcult to detect a simple structure in the distribution the result of growth on a pre-existingtemplate e.g., ofpoints, one can see a clustering near the origin, and a unit cell (Ortoleva1994). the appearance of a degree of determinism. lndeed, a Galenacrystals such as those of FiguresI and2 are typical successionof points on the return map forms extremely delicate in that they are made up of a a cycle whereby a point in the upper part of the map is hierarchyof fine brancheswhich, in threedimensions, followed by one in the lower right corner and then by a are connected.The texturesobserved have three orders point close to the origin. In contrast, Figure 6b shows of branches.The fragile nature of the texture is the analysis of a seriesof uniform random numbers and evidence that they were formed in quiescent results in uniform scattering ofpoints acrossthe retum conditions. map. Having recognized the acicular habit of the We can further analyze the data using a predictor sphalerite crystals and the importance of far-from- technique (Sugihara & May 1990, Fowler & Roach equilibriumconditions for the formationof branching 1993), which searches for correlations within the crystals such as the galena, we investigatedthe series.Because systems may be describedby numerous possibilitythat color bandingin sphaleritealso was independent variables, a simple two-dimensional plot the result of a self-organizationprocess. The succes- such as the retum map may not be sufftcient to resolve sion of dark and light bandsthat vary with Fe content any underlying pattern within the dynamics. We suggeststhe existence of a nonlinear chemical examine the system in higher dimensions by searching oscillator.Chemical oscillators have compositions that for correlations within progressively higher vector fluctuatein time or spacein responseto feedbacks space. [n principle, this allows us to determine the betweenreaction and transport rates. [n one class of embedding dimension, i.e., the minimal number of chemical oscillators,reaciant solutions are pumped dynamic variables sufficient to describe the evolution l2t6 TIIE CANADIAN MINERALOGIST Frc. 4 a. X-ray map showingthe distribution ofFe acrossseveral bands. Brightness is proponional to Fe contenl.The image showsthe samefield of view as Fisure 4b. Notice the anticorrelationwith Fizure 4b. of the system. For instance, we can completely found for deterministic systems exhibiting low describe the dynamics of a periodically forced dimensionalchaos (Fig. 7). pendulum by monitoring two dynamic variables,the Figure 7 plots the results of 130 measurements phase and its first time-derivative. Knowledge of of band thicknessesand demonstratesa maximum theseallows for calculationof otler variables,e.9., correlation (0.3) at embeddingdimension 4. This momentum,potential energy, kinetic energy,etc.Tlte meansthat there are only a few independentdynamic result of the sphaleriteanalysis is a plot of correlation variablesresponsible for the banding.It also signifies coefficient versus embedding dimension. No that the complex pattern of banding is the result significant correlations are expected for random of theaction 6f n nsnlingarsystem. Stochastic systems, systems.In contrast, near-perfectcorrelations are on the other hand,have so many independentdynamic BANDED SPHALERITEAND BRANCHING GALENA' PINE POINT, NORTHWESTIERRITORIES FIc. 4 b. X-ray map shov/ingthe distribution of Zn acrossseveral bands. Brighmess is proportionalto Zn content.The image showsthe samefield of view as Figure 4a. Notice the anticonelationwith Figure 4a. variables that results of such analyses never yield due to random fluctuations in the growth any significant correlations (see line connecting x parameters. points in Fig. 7). Deterministic systems have bjgh The best interpretationof the dali'based upon :are correlationsbecause future values of the svstem t{e requrn pap and the preilictor analysis is that related to those of the past by fixed rulei such as the sphaleripbandiing,'although noisy, is largely the equations(see line connectingsqwres in Fig. 7). The result of a low-dimensional nonlinear deterministic analysis of the sphalerite data (Fig. 7) also process.In other words, we expectthat the sphalerite demonstratesthat in comparison to an exclusively modified the solution from which it grew, thus deterministic nonlinear system, there is also providing a feedback or nonlinearity in the growth considerablenoise within the signal. This is likely conditions. 12t8 T}IE CANADIAN MINERALOGIST 0.5 E tr o o .r< .9 0.25 o c o N 20 40 60 80 100 120 144 zonenumber Frc. 5. Variation in thickness of sphalerite bands along the thin section shown in Figure la. The data are from 130 bands (abscissa). The scale of the ordinate ftand thickness) is in mm. DtscussloN this idea (Haynes & Kessler 1994, Plumlee et al. 1994' Baines e/ at.1993,Anderson 1991,Samson & Russell Our analysis of the galenaand the sphaleritetextures 1987). imply that: 1) The crystals grew in an environment that Models for galena and sphalerite growrh in accord was far from equilibrium, 2) the system was relatively with the mixing hypothesis and our observations and quiet, l.e., the fragile galena precluded turbulent data analysis are proposed. Branching galena can be mixing, 3) the sphalerite banding is described by linked to a process known as diffusion-limited alternations in Fe and Zn content at the -0.1-1.0 mm aggregation (DLA) (Witten & Sander 1981). Here, scale, and 4) the organized deposition of the sphalerite growth is computer-simulated by the release of a and galenais likely dominated by a nonlinear far-from- random walker from a point far from a nucleus particle. equilibrium process. When the walker eventually touches the nucleus it Models for deposition of the sulfides in MVT anaches, and a new walker is released. In-filling deposits can be categorized into two broad types. In growth is rare, because the walkers can seldom the first type, both metals and reduced sulfur are penetratebetween branches.The resulting aggregateis transported to the site of deposition in the same a pattern characterized by several orders of branching. hydrothermal fluid (Anderson 1975). Metals could be DLA can be thought of as the interaction of a precipitated by various processes (e.g., changes tn stochastic process (the Brownian motion of growth- temperature,pH, pressureor concentrationof reactant). species) and a potential field (e.8., temperature, In the secondtype of model, the metals are transported concentration, chemical potential) that obeys the into carbonate host-rocks containing HrS. The H2S Laplace equation (e.g., Nittmann & Stanley 1986). may be locally derived through the reaction of Simply put, this means that the crystal grows in a hydrocarbon gases and sulfate minerals (Anderson chemical potential field that is constant and that the 1991) or could be transported to the site of deposition growth species diffuse down-field to randomly arrive in another brine (Plumlee et al. 1994). We expect that at the growth front. Stiring, advection, turbulence or the second class of model to be more conducive to the other flow would mean that the potential field no generation of far-from-equilibrium conditions, since longer obeys the Laplace equation. Physically, one can such models require the mixing of very different envisage that such flow would enhance the flux of solutions. The most recent findings are consistentwith fresh growth-species at the growth front in a directed BANDED SPIIALERITE AND BRANCHING GALENA, PINE POINT, NORTIIWESTTERRITORIES L2L9 i:rfer that the growth conditions for the galena were quiescent,which is consistentwith ourinterpretation of the fragile nature of the galena texture. Becausethe branchinggalena has a habit characteristicof patterns formed by diffusion-limited growth, we can also assumethat its gowth was diffrrsion-controlled. Consideringthe (Fe,Zn)S system as a continuous sotd-solutionseries, a model for the branchingcan be based on the coupling of reaction and diffrrsion kinetics during growth. This could lead to oscillatory patterns similar to those resulting from the constitutional undercooling model for plagioclase (L'Heureux 1993, L'Heureux & Fowler 1994). x [mm) However,the bandingis composedof aggregatesof many crystals,in contrastto the single-crystalzoning observedin the plagioclasesystem, or indeedto the sphaleriteof higher-temperaturevein-type mineraliza- tion (e.9.,Pattrick et al. 1993). A promisingmodel for growth bandingin sphalerite underfar-from-equilibrium conditions is relatedto that of the Liesegangphenomenon (e.9., Liesegang 1913) for precipitation from colloidal suspensions.In our model, an influx of Fe-Zn metal-bearing solution locally reactswith H2Sto producenuclei of sphalerite, thus locally changing the supersaturationand the Fe:Zn ratio in the solute. These are not immediately restored to their bulk values by diffrrsive influx of new material. It is reasonableto assume that the nucleation rate changesrapidly with supersaturation 1.0 (e.g,.Dee 1986).Consequently, the reactionmay stop "n or be retarded, providing time for the subsequent replenishmentof the reactant concentration.This Fto. 6. a. Return map plotting the (z + 1) band thickness (in mm) as a function of the thicknessof the previousone, successionof nucleateddeposits can be linked to the the zth band. For the most parL the data are restrictedto observed variability in iron content. An order-of- an areaof the plot near the origin. They form a triangular magnitudeestimate of the characteristicband-thickness pattern that spirals in a clockwise direction. In contrast, Z is estimatedfrom I = DlV, whereD is the diffrrsion randomdata (Fig. 6b) scatteracross the ploq and show no constant,and V is the velocity of flow of the incoming suchoatlgm. fluid (Ortoleva L994). Typically, D is approximately equalto 10-5- 10-7cm2 s-1, and a reasonablevalue for Vcould be 100cm a-l (Dewers& Ortoleva1988). This simulationyields a band width Z of approximately1 to 10-2cm, in goodagxeement with our observations.We are currently refining this model. way, thus causingin-fllling go\r'th betweenbranches. Models for the growth should invoke mechanisms The growth of branching crystals can be driven by in which the diffi:sion rates of $owth-species for the vaxiouspotential gradients, e.g., Cu electrodeposition galenaand sphaleriteare relevant.Indeed, the banded driven by an electricpotential (Brady & Ball 1984). textures of many ore minerals led early investigators This type of model is known to be relevant for (Rogers l9l7) to interpret them in terms of classic branchinggxowth of minerals and may be driven by a Liesegangexperiments, in which bandsare formed in a steepconcentration or thermal gradient (FowIer et al. gel, which acts as a barrier to diffusion. They referred 1989, Fowler & Roach 1996). For the branching to the ore textures as colloform and suggestedthat galena,we expect that growth was driven by a sharp ores were precipitatedfrom sols. On the contrary, all gradient in chemical potential as a result of the evidence(e.9., fluid inclusions)demonstrates that ttre introduction of metal-bearing brines into HrS-rich metals were precipitated from aqueous solutions carbonaterocks. During the branching growth, the Orines). The limitation of diffusion in a liquid system potential fields must remainin a quasistationarystate, can be achieved by invoking an interplay between i.e., almost constant, so that the growth species growth rate and diffitsion rate. Systems where the aggregateby diffrrsion. This constraint allows us to growth rate dominatesover diffusion [a.9.,sno*flakes: 1220 T}IE CANADTAN MINERALOGIST bulk compositionof the fluid on a broad scale(e.9., zr Mclimans et al. 1980).We reject this hypothesisas U the direct causefor small-scalebotryoidal sphalerite ii banding at Pine Point for severalreasons. 1) Our L IIJ analysisshows that the bandingof adjacentclusters of sphalerite is not always correlated; indeed, examinationof the figures of Mclimans et al. (1980) z showsthat their correlationsof oresfrom their areaare tr numerous I by no means exact, and that there are UJ discontinuities. 2) Uncorrelated banding between t adjacentgro'er,th-bands, splitting andrejoining of bands (J arehallmarks of chemicaloscillators. 3) Furthermore, once the growth is initiated, a suddenchange in fluid compositionwould deshoythe quasi-stationary growth EMBEDDINGDIMENSION conditions required for branching growth in galena. Frc. 7. Plot showing the result of the wimplex predictor 4) Onewould expectto seechanges in themorphology techniquefor the data on the thicknessof sphaleritebands or chemistryof galenaat the contactsbetween bands in (dots connectedby lines). The graphplots the correlation the sphalerite. predictions points coefficient determinedfor made from However, we recognize the importance of fluid in the seriesabout those elsewhere in the series.uarsas MVT deposits, the embedding dimension. Good predictions result in mixing in the depositionmechanism of correlations.Note that at embeddingdimension (4), the which could be partly responsiblefor the isotopic correlation coefficients are positive and significant (for variation observedin many other deposits(e.g., Heyl 130samples at 2o), suggestiveof a deterministicthough et al. 1966,Deloule et al. 1986).Our work doesnot noisy "signal". In contrast,analysis of 130 uniformly rule out the possibility of multiple sotucesof fluid distributed random dala (x points connectedby lines) or pulsesof mineralizationacting in concert with the showsno significant conelations,and unlike the casefor far-from-equilibriumgrouth processes.Indeed, mixing the sphaleriteanalysis, the "conelations"are enatically of fluids could be necessaryto initially trigger the (squares) distributed. The upper line is the result of gowth. Our proposedmodels could easily include analysis of chaotic data from the Lorenz system of equations(e.g., Moon 1992).Although in time-series these effects. Although our analysisis inconsistent form the data are scaftered,they were completely deter- with each sphaleriteband being related to a unique ministically derivedfrom a fixed set of equations,with no period of fluid mixing, mixing may be important for randominputs. The analysisshows very high conelations the initiationof the nonequilibriumconditions. consistentwith the data occupying a low-dimension Finally, for completeness,we comparethe banding deterministicattractor. of vein-typedeposits to that in MVT deposits.The banding of crystalswithin vein depositsis parallel to the faces of individual equant crystals. Our observa- tions of crystals occurring in vein-type depositsshow the bandingto be of two types,sector and oscillatory zoning. The zoning patternsare unique to individual crystals.In contrast,the crystal banding at Pine Point Ben-Jacob& Garik(1990)l will tendto form branching and otherMVT depositsis observedacross a multitude structures. In contrast, for polymers (plastics and of acicular and radiating crystals aligned normal to silicateglasses), the sluggishrate of diffusion domi- banding. The bandedsphalerite in vein-type deposits natesthe branchinggrowth (Keith & Padden1963). showsan alternationin Cu concentrationbetween light Analogously, the feedback between growth and and dark bands(Pattrick et al. 1993),which has not diffrrsion rates should affect the sphaleritebanding. beenreported for MVT deposits.This arisesfrom a Thus the interpretationof Roedder(1968) that the coupled substitution of Cu+ + In3* for 2zf,f+, which galenaand sphaleritetextures are due to rapid growth, is facilitated by the structural similarity between and the older interpretationthat they were the result roquesite(CuInS) andsphalerite (Pattrick et al.1993) of colloidal (i.e., diffrrsionlimited) growth are both and the higher solubility of Cu at the elevated partly correct. We conclude that the sphalerite temperaturestypical of vein-type mineralization crystallizedas a relatively open network of acicular (Pattricket al. 1993,Bente & Doering 1995).Indeed, andstubby crystals in responseto far-from-equilibrium our analysesshow Cu to be below electron-microprobe growth conditions. detectionlimits (500ppm) for the PinePoint samples. It is commonly assumedthat the banding in ores Therefore, we prefer a nucleation-diffr:sion model from the MississippiValley can be correlatedover rather than a reaction-diffrrsion coupled-substitution various scales(from hundredsof m to many km), and modelfor the gtowth at low temperaturesof botryoidal that the alternationin bandingis due to changesin the sphaleritein MVT deposits. BANDED SPHALERITE AND BRANCHING GALENA, PINE POINT, NORTHWEST TERRITORIES L22l CottclustoNs DELor,'r-E,E., Au-EcRE,C. & Dos, B. (1986):lead andsulfur isotope microstratigraphy in galena crystals from 1307- In this paper,.Pine Point sphalerite-galenainter- MississippiValley-type deposits. Econ. Geol. 81, 1321. growthswere analyzed. Sphalerite has an acicularhabit and exhibits an irregular alternation of differently DEwERs,T. & Onror.Eve,P. (1988):The role of geochemical colored bands correlating with Fe concentration. self-organizationin the migration and trapping of Nonlinear dynamic data analyseswere performed on hydrocarbons.Appl. Geochem.3, 287-316. the band thicknesses,as observedfrom a doubly- polished thin section. Results of these analysesand FowLER,A.D. & RoAcH,D.E. (1993): Dimensionality the presenceof branching dendritic galena allow us analysisof time seriesdata: nonlinearmethods. Comput. to concludethat the growth patternis likely the result Geosci.19, 4l-52. of non-equilibrium self-organizationprocesses. A far- & - (1996):A model and simulationof growth potential from-equilibrium modelhaving the to branching mineral growth from cooling contacts and yield spontaneouslyorganized patterns is presented. glasses.M ineral. Mag 60, 595-60 l. Accordingly,the hypothesisthat the small-scalebands in sphaleritecan be correlatedover large distances (l'.a., SreNlrv, H.E. & DAccoRD, G. (1989): sphaleritestrati$aphy) needsto be re-examined. Disequilibrium silicate mineral textures: fractal and non-fractalfeatures. Nanre 341, 134-138. AcrNowtpocnunnrs Gnav, P. & Scorr, S.K. (1990):Chemical Oscillations and Press.Oxford, U.K. We acknowledgethe continuing support of NSERC. Instabilities. Oxford University grateful We are to Gary Ansell of the Geological HAyNEs,F.M. & KESsLER,S.E. (1994): Relation of Survey of Canadafor providing one of the samples mineralization to wall-rock alteration and brecciation, usedfor this study.The assistanceofPeter Jones (SEM Mascot - JeffersonCity Mississippi Valley-tpe district, laboratory, Carleton University) and Glenn Poirier Tennessee.Econ. Geol. 89, 51-66. (EMP laboratory, McGill Universiry) was $eatly appreciated.We thank the referees for their useful Hrvl, A.V., DHEVAUx,R.E., ZARTMAN, R.E. & Bnocr, M.R. comments. (1966): Isotopic study of galenas from the upper Mississippi Valley, the lllinois-Kentucky, and some Appalachian Valley mineral districts. Econ. Geol. 61, REFERENCES 933-961. (1975): Ar.IDeRsoN,G.M. Precipitationof MississippiValley- Ksmr, H.D. & PaoonN,F.J. (1963): A phenomenological type ores.Econ. Geol. 70,937-942. theory of spherulitic crystallization. J. Appl. Phys. M, 2N9-2421. (1991):Organic maturation and ore precipitation in southeastMissoui. Econ. Geol. 86, 909-926. L'HEUREUx,L (1993):Oscillatory zoning in crystalgrowth: mechanism.Plrys. Rev. BerNrs,S.J., BuRLEy, S.D. & Gzn, A.P. (1993):Base metal a constitutionalundercooling sulphide mineralization in North Sea hydrocarbon E 48. M60-4469. reservoirs:evidence for masssolute transfer during burial. 1rlGeofluids '93 (J.Parnell, A. Ruffell & N. Moles,eds). & FowLER,A.D. (1994):A nonlineardynamical zoning in plagioclase.Am. Mineral. Departmentof Geology, Queen's Univ. of Belfast, model of oscillatory Belfast U.K, (435-438). 79.885-891. BEN-JAcoB,E. & Gantr, P. (1990):The formationof pattems LESEcANG,R.E. ( 19 13) : Geolo g ische D ffis ionen. Y erlagTlt'. in non-equilibrium growth.Nature 34J, 523-530. Steinkopf,Dresden, Germany. Bryrs, K. & Dornnc, T. (1995): Experimentalstudies MclnaeNs, R.K., Bewes, H.L. & OHMoro, H. (1980): on the solid state diffirsion of Cu + ln in ZnS and on Sphalerite stratigraphy of the upper Mississippi Valley "Disease",DIS (Diffusion Induced Segregations),in lead-zincdistrict, southwest Wisconsin. Econ. Geol.75, sphalerite and their geological applications. Mineral. 351-361. Petrol 53,285-305. MooN, F.C. (1992): Chaotic and Fractal Dynamics. lohn Bnaoy, R.M. & Ball, R.C. (1984):Fractal growth of copper Wiley & Sons,New York, N.Y. electrodeposits.Naare 309 (5965),225-229. Dpr, G.T. (1986): Pauemsproduced by precipitationat a NnrruaNN,J. & Sremt-rv,H.E. (1986):Tip spliningwithout movingfront. Phys.Rev. lett. 57,275-278. interfacial tension and dendritic growth patterns arising from molecularanisotropy. N ature 321,663-668. DEER,W.A., HowrE, R.A. & Zussrraen,l. (1966): An Introduction to the Rock-Fonning Minerals. longman, Onrorpve, Pl. (1994): GeochemicalSelf-Organization. London.U.K. OxfordUniversity hess, Oxford,U.K. t222 THE CANADIAN MINERALOGIST PAcKARD,N.H., Cnucsrruo, J.P.,Fanvnn, J.D. & Snaw, SAMsoN,I.M. & RussFrr, M.J. (1987):Genesis of the R.S.(1980): Geometry from a time series.Phys. Rev. Lett. Silvermines zinc-lead-barite deposit, Ireland: fluid 45.712-71,6. inclusion and stable isotope evidence,Econ. GeoI. 82, 371-394. PATRTcK,A.D., DoRLn{c,M. & Polva, D.A. (1993):TEM study of indium- and copper-bearinggrowth-banded SUcHARA,G. & MAy, R.M. (1990):Nonlinear forecasting as sphalerite.Can. Mineral. 31, 105-117. a way of distinguishingchaos from measurementerror in time series.Nature 34/',73+741. PLUMLEE,G.S., LsecH,D.L., Horsrr.a, A.H., LANDrs,G.P., RowAN,E.L. & Vmrs, J.G. (1994): Chemicalreaction WrrrEN, T.A. & SANDER,L.M. (1981): Diffusionlimited path modelingof ore depositionin MississippiValley- aggregation,a kinetic critical phenomenon.Phys. Rev. type Pb-Zn deposits of the Ozark region, U.S. mid- Iett. 47,1400-1403. continent.Econ. Geol. 89. 1361-1383. RoEDDER,E. (1968):The noncolloidalorigin of "colloform" texturesin sphaleriteores. Econ. Geol.63,45l-471. RoaERs,A.F. (1917): A reviewof theamorphous minerals. "I. Received September I, 1995, revised manuscript accepted Geol.25.515-541. May 2, 1996.