Tzt1 SELF.ORGANIZED BANDED SPHALERITE and BRANCHING

tzt1 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<tuis Pasteur Street. Ottawa. Ontario KIN 6Ns 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(<l km). The oreoccurs in breccias,solution channels or othervoids, r E-mnil address.' [email protected] and may be accompaniedby pyrite, fluorite, barite or r2t2 THE CANADIAN MINERALOGIST bitumen. Many MVT depositscontain ore and gangue equilibrium conditions. We also propose possible minerals that are banded. Banding in sphalerite of models that consider the growth and banding as MVT depositshas been the focus of much attention, self-organizingprocesses resulting from the action of and its origin has been attributed to mineralization nonlinear chemical oscillators (Ortoleva 1994). We from pulses of hydrothermal fluids of variable contrastthe natureand origin ofbanding in sphalerite compositionover time (e.9.,Mclimans et al. 1980). from MVT depositswith that of vein-typedeposits. In this paper,'we examine the details of the sphaleritebanding and its intergrowth with branching Drscr,rrnoN oF THETExrIiREs galena at Pine Point. We analyze the banding using modernmethods of nonlineardynamics, and conclude ln commonwith otherMVT deposits,the bandingin that both the sphalerite banding and the branching Pine Point sphalerite is characterizedby alternation galena are due to crystallization under far-from- in the color of the zinc sulfide from clear to pale white, 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 banding 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 botryoidal textures. Note that some bands toward the top of the image do not correlate over the entire section. Black blebs and filigees within the sphalerite 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

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