Canadian Mineralogist Vol. 23, pp. 83-88(1985)

ANISOTROPICSPHALERITE OF THEELMWOOD_GORDONSVILLE DEPOSITS, TENNESSEE

ROBERT R. SEAL II*, BRIAN J. cooPER AND JAMES R. CRAIG Departmentof GeologicalSciences, Virginia Polytechnic Institute and State University, Blacksburg,Virginia 2406 I, U.S.A.

ABSTRACT morph, to sphalerite @leet l977a,b), mechanical deformation @leet 1977b),thermal stress(Akizuki Sphalerite,the dominant ore-mineralin the Elmwood- 1970, l98l), and contamination of sphalerite by GordonsvilleMississippi-Valley-type deposits in centralTen- impurities that stabilizea hexagonalstructure (Geilik- nessee,exhibits a distinct anisotropy betweencrossed nicols. man 1982).In this paper, we describethe characteris- This anisotropy results from the presenceof small domains tics of thesespecimens, the conditions of their ori- of hexagonal symmetry in the normal cubic structure of gin, and the investigation of their sphalerite.These domains result from primary growth- anisotropy by meansof optical polished defectsthat are probably stabilized by local higher ioncen- studiesof doubly thin sec- trations of cadmium and iron. tions and by X-ray-diffraction methods.

Keywords: sphalerite, anisotropy, wurtzite, cadmium, GToLocTcaT-SETTING AND PARAGENESIS Elmwood-Gordonsville,Tennessee. The Elmwood-Gordonsville Mississippi-Valley- type deposits,located in the Central Tennesseezinc sourrlanE district, occur in the Lower OrdovicianMascot For- mation, associatedwith paleokarst topography in sphaldrite,principal _-La mindral du minerai des gites dolomites (Kyle 1976,Kearns & Campbell 1978,Gay- Elmwood-Gordonsville (Tennesseecentral) du typJ dit lord & Briskey 1983).Structurally, these deposits lie "Mississippi Valley", est nettementanisotropique entre on the northeastern flank of gently nicols crois6s,Cette anisotropieest due d la prdlencede the deformed petits domaines de sym€trie hexagonaledans la structure NashvilleDome (Fig. 1). The mineralizationoccurs (cubique)de la sphal6rite.Ces domaine r&ultent de d6fauts as infillings of massive breccia and vugs and con- de croissance primaires, probablement stabilis6s par la sistsof coarse-grainedsphalerite with minor amounts teneur localement plus 6levdeen cadmium et en fer. of galena,pyrite and marcasite.The gangueminerals consist of calcite, fluorite and slrontium-bearing (Traduit par la Rddaction) barite, with lesseramounts of quartz, dolomite, celestiteand anglesite(Kyle 1976,Kearns & Campbell Mots-cl6s: sphaldrite, anisotropie, wurtzite, cadmium, 1978,Gaylord & Briskey 1983). Elmwood-Gordonsville,Tennessee. Data collected from an examination of doubly polishedthin sectionsand hand specimenssuggest INTRoDUCTIoN a parageneticsequence dominated by severalgenera- tions of sphaleriteand calcitedeposition. The com- Sphalerite is the dominant (and commonly the positeparagenetic sequence in Figure 2 attemptsto on$ ore-mineral extracted from carbonate-hosted reconcilethe observationsof Kyle (lW6) andKearns (Mississippi-Valley-type) zinc deposits. The sphalerite & Campbell (1978)with thoseof the presentstudy. of the Elmwood-Gordonsville mines in the Central An important addition is our recognitionof two dis- Tennesseezinc district is distinctive in appearance, tinct episodesof sphalerite deposition (described both megascopically and microscopically. It occurs below and shown in Fig. 3). as very coarsely crystalline, deep brownish, resinous masse$ that are invariably, and often strongly, SAMPLES AND METHoDS oF INvESTIGATIoN anisotropic between crossed nicols in transmitted light. Anisotropy in sphalerite has been attributed Eighteen typical samples of ore from the to the presenceof nonisometric domains. The origin Elmwood-Gordonsville deposits were kindly of these domains has been attributed to the partial providedfor this study by Dr. Fred Main of the Jer- inversion of wurtzite, the high-temperature poly- seyMinidre Zinc Company, The samplesconsist of massivecoarse-grained sphalerite. Somesamples are attachedto host dolomite, and others are overgrown *Present address:Department of GeologicalSciences, Uni- by minor amounts of calcite and purple fluorite. versityof Michigan, Ann Arbor, Michigan 48109,U.S.A. Severalsamples consist of clustersof interpenetrat- 83 84 THE CANADIAN MINERALOGIST

No. 47-42-53. The Vickers microhardnesswas meas- ured on a Tukon microhardnesstester with a 100g ELMWOOD/ LE weight and a contact time of 20 to 25 seconds.Fluid- inclusion studieswere carried out using a gas-flow stagemodified from the designof Weneet al. (1979). Sphaleritebirefringence was calculatedfrom retarda- tion estimatedfrom a doubly polishedthin section mounted on a universalstage. To obtain theseesti- o 50 loo l\.'!lLEs matesof retardation, the thin sectionwas rotated on o 150KM a univeralstage to the point that gavethe maximum retardation. The birefringencewas then calculated FIG. 1. Locationmap for the Elmwood-Gordonsvilleby taking a visual estimateof the highest-orderinter- depositsalong the noflheast flank of theNashville Dome ferencecolor (the retardation) in conjunction with in centralTennessee. a thin sectionthickness that was trigonometrically correctedfor the amount of rotation on the univer- sal stage. These data were then applied to an ing crystals of sphaleritethat protrude into open interference-colorchart as found in Bloss (1961). rugs. Coarselyground surfacesreveal that the mas- sive sphalerite consists of angular to subparallel lamellar single crystalsthat range from I to 3 cen- PsrnocnePHY timetresin length. Doubly polishedthin sectionsof all sampleswere preparedin the manner described The Elmwood-Gordonsville depositshave milli- by Craig & Vaughan(1981), so that examinationby metre-sizedquartz euhedrapresent as a discontinu- both transmitted and reflected light could be con- ous thin coating around fragmentsof dolomitic brec- ducted. cia. Dolomite occursboth beforeand after sphalerite The composition of selectedsamples was deter- precipitationbut only consitutesa minor portion of mined by electron-microprobeanalysis on an ARL- the gangueminerals (Kyle 1976).Calcite has been SEMQ electron microprobe using syntheticbinary recognizedin threegenerations, one beforeand two sulfide and pure metal standards.Measurements of after sphalerite formation (Kyle 1976, Kearns & the unit-cell parameterswere carried out on an auto- Campbell 1978).Calcite in open vugsmay occur as mated Norelco diffractometer using Ni-filtered white to slightly yellowishscalenohedral crystals up CuKcuradiation and CaF2 as an internal standard. to tens of centimetresin length. Purple fluorite fol- The 133,024and224reflections were usedto calcu- lows sphaleriteand crystallizedin cubesup to l0 cen- late cell dimensions.Single-crystal-precession pho- timetresacross. Strontium-bearing barite appearsas tographs were obtained using a Huber precession roughly hemispherical porous white crystalline camerawith Ni-filtered CuKo and Zr-filtered Mo.l(a massesup to severalcentimetres across that rest on radiation. sphalerite, calcite and fluorite crystals. The rare Reflectancewas measured on a McCroneMPA-I galenais interpretedby Kearns& Campbell (1978) photometer calibrated against pyrite, galena and as forming before sphalerite,but Kyle (1976)stated magnetite standards which, in turn, had been that it postdatesthe sphalerite.Pyrite occursbefore calibratedagainst Zeiss WTiC reflectancestandard the sphaleritemineralization in the intersticesof car-

Quortz - _KTKAC - K Dolomito K.KAC Colcife KAC Goleno Spholerite Pyrife _K - Morcoslle _K - Fluorlle Borlfc Eorly LATG

Frc. 2. Compositeparagenetic sequence. Note the two distinct episodesof sphaleritedeposition. K: observationsof Kyle (19?6);K&C: observationsof Kearns & Campbell (1978). ANISOTROPIC SPHALERITE FROM TENNESSEE 85 bonate grains in breccia fragmentsand as a fracture- filling in second-generationsphalerite. Marcasite is found at the contact between second-generation sphalerite and fluorite. The sphalerite occurs as coarselycrystalline infillings in fractures and between angular fragmentsof the host carbonatesin the brec- cia. Vugs are lined with myriadsof centimetre-sized sphaleritecrystals. The sphaleriteis generallydark reddishbrown but rangeslocally to a pale yellowish orange. Doubly polishedthin sectionsof sphaleriteappear a homogeneousgrey in plane-polarizedreflected light and reveal no observablebireflectance; however, internal reflectionsare abundant.In plane-polarized transmittedlight, two generationsof sphaleritecan Frc. 3. Photomicrograph in transmitted plane-polarized be recognized(Frg. 3). Both generationsare clearro light of a Gordonsville specimenillustrating the two light orange in color and display mottled varia- generationsof sphalerite,an organic-inclusion-richfirst tions in color. Both also locally exhibit subtle phaseand an inclusion-freesecond phase, Width of field is 3 cm. growth-banding. The older generation is slightly paler and containsabundant carbonaceousinclusions . (Fig. 3) that aretotally lackingin the youngergener- ation. Theseinclusions, which appearmegascopically as sharply bounded black laths and rods, are seen at high magnification to be aggregatesof small, sometimesdiffuse, black, apparently amorphous carbon. Betweencrossed nicols in transmittedlight, both generationsexhibit a distinct anisotropy, generally of low retardation, in thin sectionsslightly thicker than 0.3 mm. Anisotropic texturesrange from ran- domly distributed gridiron textures(Fig. a) similar in appearanceto those found in microcline to tex- turessimilar in appearanceto polysynthetictwinning (Fig. 5) commonly found in plagioclase.These tex- turesextend apparently undisturbed across the con- Frc. 4. Transmittedlighl photomicrograph taken with tacts betweenfirst- and second-generationsphalerite. 'ogridiron"-type and thus either formed continuouslybecause of an crossed nicols to illustrate the anisotropy. Width of field is I mm. initial growth-defector weresuperimposed after the secondgeneration of sphaleritehad formed. Sphaleritebirefringence calculated from retarda- tions estimatedfrom doubly polishedthin sections average0.002(l) on a universalstage. Birefringence of small domains reachesa maximum of 0.008(2).

EXPERIMENTAL RESULTS r#' The unit-cell parameterwas measured on 14sam- :;, ples of first- and second-generationsphalerite. The ll,,:,9 ** data for^10 samplesfall between5.410(8) and ' 5.412(3) A, but the data for the othel four samples .&* fall between5.418(7) and 5.421(3)A. There is no "u1' correlationwith color, samplelocation or difference in age.The causeof theselarge variations, which are far greaterthan thoseexpected on the basis(Barton Frc. 5. photomicrograph with & Skinner 1967) taken of the small chemical variations crossed to illustrate en dchelon twinnins in (Table 1) are not known. X-ray powder diffraction Elmwood sphalerite. The irregular black of smearmounts revealsonly reflectionstypical of specksare ing inclusions.Width sphalerite,but precessionphotographs of anisotropic of field is I 86 THE CANADIAN MINERALOGIST

TASLE 1. REPRESEMATIVE COMPO5ITIONS OF SPSAT,ERTTE FROII! TEE ELMW@D-CORDONSVILLE DEPOSITS

(relght %) Zn Fa Cd S Total

FIret ggneration 66.L4 0.47 0.47 31.S7 9S.95 66.46 0.st o.56 32.r2 99.65 56.43 0.54 na

Elgh-blrefrlngence - 0.31 o.32 arsa (l'19.7)

SulloudLig area o.20 Frc.7. TransmittedJightphotomicrograph of Elmwood- Gordonsvillesphalerite, taken witl crossednicols, illus- tratinga zoneof stronganisotropy that is alsoricher CoaposltionaL data detemlned by electron-nicroprobe in iron and cadmiumthan the surroundingarea (see analyEls. Slnthetlc Eulflde€ rer6 used a9 Etedards. Tablel). Width of field is 2.5 mm.

(Fig. 7), and then back to an area of low birefrin- gencerevealed that the Fe and Cd contents were approximately 5090 higher in the area of greater birefringence(Table l). Reflectancesfor first- and second-generation sphaleritewere measured,respectively, as 18.090and 16.3s/oat 460nm, l5.7Voand 16.0t/oat 546nm, and 14.7tloand 19.70/oat 650nm. Thesevalues are con- sistent with those obtained for other samplesof sphalerite with approximately 0.5 wt.9o iron (Vaughan& Craig 1978). Homogenization-and freezing-temperaturestudies wereconducted on primary fluid-inclusionsin first- generationsphalerite and fluorite. Uncorrectedtem- peraturesof homogenizationmeasured on inclusions rangefrom l25o to 132'C in the sphaleriteand from 108' to ll6oC in the fluorite. Freezing-pointtem- peratures of inclusions in sphalerite and fluorite rangefrom -19.6oto -20.1"Cand from -13.9' to -14.8oC, respectively.These temperatures indicate 22.2 and 18.4 wt.Vo Frc. 6. Zero-level[1 l0]-axis precession photograph of sam- salinites of approximxely ple G-4: CluKo,p = 30o,four-day exposure. Note (NaCl equivalent) for the fluid inclusions in the streakingparallel to llllt, aswell asreflections due sphaleriteand fluorite, respectively.These homogeni- to twinning. zation and freezing temperaturesare similar to those reportedby Roedder(1976). Upon heatingof solidly frozen inclusions, the initial melting was first sphaleritedisplay diffraction streaksparallel to one observedat about -33'C for the sphaleriteand about of the four symmetry-equivalent< lll > directions -30"C for the fluorite; this suggeststhat the fluids (Fig. 6). Thesestreaks thus representsmall domains contain additional salts such as CaCl2 or MgClr. of stacking disorder along that direction. Reflections Unfortunately, the appropriatepressure-correction associatedwith a twin axis pafallel to that direction to apply to thesefluid inclusionsis difficult to esti- are also pre$ent. mate becausethe age of the mineralization is not Electron-microprobeanalyses (Table l) indicate known. that the first-generation sphalerite averages0.49 wt.7o Fe and 0.57 tlo Cd; the second-generation DISCUSSION sphaleiiteaverages 0.36 VoFe and 0.30 9o Cd (Craig et al. 1983).A traverse beginning in an area of low The Elmwood-Gordonsville depositsare typical birefringence, acrossan area of higher birefringence Mississippi-Yalley-typeores. The fluid-inclusion data ANISOTROPIC SPHALERITE FROM TENNESSEE 87

for the sphaleriteand fluoriten though limited, are first- and second-generationsphalerite apparently consistentwith an ore-forming fluid that exhibited undisturbed; thus the anisotropy appearsto result a slight decreasein salinity and temperaturewith time from growth defectscausing stacking disorder and during ore deposition. Electron-microprobeanaly- twinning that formed contemporaneouslywith the sisindicates that the compositionsof the Elmwood- deposition of the sphalerite. Gordonsvillesphalerite range from 0.14to 0.60wt.9o Fe (average0.36 Vo)and from 0.09to 0.72wt,9o Cd (average0.36 9o). X-ray powder diffraction reveals ACKNowLEDGEMENTS that most of the unit-cell parametersare consistent with those expectedof sphaleritewith approximately The authorsare grateful to JerseyMinidre Inc. for 0.5 wt.9o Fe and 0.5 wt.9o Cd Garton & Skinner permissionto work on the samplesdescribed and 1967),but that somevalues are unexpectedlylarge. especially to Dr. Fred Main who collected the X-ray-precessionphotographs (Fig. 6) display samples.They alsothank Drs. M. E. Fleet, J. Guha diffraction streaksalong reciprocal lattice rows with and R.F. Martin for their most constructivecriti h+ k +3n (equivalenthexagonal indices), indicating cisms of the original manuscript. stackingdisorder in the sphalerite@eet 1977b).The stacking faults and twinning create domains of nonisometric symmetrythat are optically anisotropic REFERENcES @e* l%7a,b). Using the linear relationship between birefringence 6 and volume 9o hexagonal close- Arzurr, M. (1970):Slip structureof heatedsphalerite. packedlayers o observedby Nelkowski & Pfiitzen- Amer. Mineral. 55. 1302-1312. Reuter(1971) and Fleet (1977a),o averages590 and rangesfrom 0 up to 38%oin small domains in the (1981):Investigation of phasetransition of samplesstudied. natural ZnS mineralsby high resolutionelectron The zinc sulfide mineralizationof the Elmwood- microscopy.Amer. Mineral. 66, 1006-1012. Gordonsville depositsfills collapsebreccias and open (1967): cavities.Sphalerite is the only zinc sulfide found BanroN,P.B., Jn. & SrrmNsn,B.J. Sulfide in mineral In Geochemistryof the deposits, growth stabilities. Hydrother- and ore texturessuch as band- mal Ore Deposits(H.L. Barnes,ed.). Holt, Rine- ing paralleling sphalerite-typecrystal facessuggest hart and Winston. New York. that it wa$ primary; accordingly, transformation from wurtzite seemsto be an unlikely explanation Bross,F.D. (1961):An Introductionto theMethods of for the anisotropy. Furthermore, M.E. Fleet (pers. Optical CrystaUogruphy.Holt, Rinehart and Win- comm. 1984) has pointed out that sphalerite ston, New York. transformed frorn wurtzite would have twin orien- tations presentin equalproportions; the twin orien- Cnerc,J.R., Sorssnc,T.N. & Vaucnan,D.J. (1983): Growth characteristicsof sphaleritesin Appalachian tations in the samplesexamined are unequal. The zincdeposits. InProc. Int. Conf. Miss.Valley Type structural setting,the undistortednature of the crys- Lead-Zinc Deposits(G. Kisvarsanyi,S.K. Grant, tals, and the opennature of the mineralizedcavities W.P. Pratt & J.W. Koenig,eds.). University of apparently preclude structural deformation as the MissouriRolla, Rolla, Missouri. causeof the anisotropy. Uncorrectedtemperatures of homogenizationabove l35oC werenot observed - & VeucHaN, D.J. (1981): Ore Microscopy and in primary fluid-inclusions;thus, the thermal-stress Ore Petrography. Wiley Interscience, New York. mechanismsdescribed by Akizuki (1970,l98l) seem - to be an unlikely explanationfor the anisotropy of Frrrr, M.E. (1977a): The birefringence structural sphalerite,because they arereported to occurat tem- state relation in natural zinc sulfides and its appli- peraturesin cation to a schalenblende from Pribram. Can. excessof 900'C. Mineral. 15, 303-308. Geilikman (1982)suggested that cadmium atoms replacingzinc atomsin zinc sulfide stabilizea hexa- (1977b): Structural transformations in natural gonal (wurtzite) structure becausepure cadmium sul- ZnS. Amer. Mineral. 62,540-546. fide (greenockite)has hexagonalsymmetry at the temperatures in question. Becausethe electron- Gayrono, W.B. & Bmsrry, J.A. (1983):Geology of the microprobe data demonstratethat Cd and Fe are Elmwood and Gordonsville mines, Central Tennes- higher in areas of higher birefringence in the seezinc district. In TennesseeZinc Deposits Field sphalerite,the hexagonals).rnmetry of small domains Trip Guidebook. Dep. Geol. Sci., Virginia Polytech- nic Inst. State Univ., Guidebook 9, l16-151. that impart the anisotropic characterto the sphalerite is probably stabilized by the cadmium and iron GsrLrrN4aN,M.B. (1982): Mechanisms of polytype replacing zinc in the structure. Furthermore, the stabilization during the wurtzite - sphalerite transi- birefringent texturesextend acrossthe contactsof tion. Phys. Chem. Minerols E, 2-7. 88 THE CANADIAN MINERALOGIST

KBanNs,L.E. & CeMparu, F.H., III (1978):The EIm- VaucrnN, D.J. & Cnarc, J.R. (1978):Mineral Chemis- wood and Gordonsville zinc mines near Carthage, try of Metal Suffides. Cambidge Univ. Press, New Tennessee.Mineral. Record 9. 213-218. York.

Kvr-E,J.R. (1976): Brecciation, alteration, and miner- Wpnns,R.W., Jr., BooNen,R.J., Br-rru

Roroorn, E. (1976): Fluid-inclusion evidence on the genesisof ores in sedimentary and volcanic rocks. .Iri Handbook of Stratiform and Stratabound Ore Received February 21, 1984, revised manuscripl Deposits2 (K.H. Wolf, ed.). Elsevier,New York. accepted May 23, 1984.