Manganoan Fayalite Is Usually Found Associatedwith Sedimentary Iron-Manganeseore Deposits

American Mineralogist, Volume 76, pages 288-292, 1991

Manganoanfayalite [(Fe,Mn)rSiOol:A new occurrencein rhyolitic ash-flow tuff, southwesternNevada, U.S.A. Jauns G. Mrr-r.s,Jn.r* Trnrorrrv P. Rosr Department of Geological Sciences,Michigan State University, East I-ansing,Michigan 48824-1115,U.S.A.

Ansrru,cr Manganoan fayalite is usually found associatedwith sedimentary iron-manganeseore deposits. Phenocrystsof manganoanfayalite were recently discovered in high-silica rhyolite pumice fragmentsfrom the Ammonia Tanks Member of the Timber Mountain Tuff in the SouthwesternNevada Volcanic Field. Twenty-one electron microprobe analyses (major-element oxides, NiO, BaO) are reported for the newly discoveredphenocrysts. The slighfly zoned phenocrystsrange in composition from Fa.rFooTerrla", to FarrFoo.rTerrfa",.

INrnooucrroN of anall"tical data for some of the iron fayalite specimens Manganoanfayalite, a member ofthe FerSiOo-MnrSiOo reported by other workers (Warshaw and Smith, 1988; solid solution series,was originally thought to be restrict- Mahood, l98l; Carmichael, 1967), revealed that several ed in its paragenesis,occurring almost exclusivelyin iron- of them are Mn- and Mg-enriched fayalite, whereasthe manganeseore deposits (Deer et al., 1982). Rare occur- remainder of the phenocrystslack significant Mn or Mg rencesof manganoanfayalite as a primary igneousphase and are near end-member fayalite in composition. have been reported in the literature. These occurrences In this paper we report a new occurrenceand chemical include manganoan fayalite in nordmarkite at Shefford analysesfor manganoanfayalite phenocrystsin high-silica rhyolite pumice fragments from Ammonia Mountain, Quebec(Frisch, 1972), and,in a pegmatite lo- the Tanks cated in the Fukushima Prefecture, Japan (Omori et al., Member of the Timber Mountain Tuff, southwesternNe- 1950;Omori and Hasegawa,1951). A compilation of the vada. more notable occrurencesof manganoanfayalite is given in Table 1. Gnor,ocrc sETTrNc Fayalitic olivine has been noted in the rhyolitic por- The Ammonia Tanks Member of the Timber Moun- tions of several ash-flow tuffs (cf. Bacon et al., l98l). tain Tuffis a large-volume (approximately 900 km3)com- Deer et al. (1982) sLggestedthat many of the reported positionally zoned ash-flow sheetthat formed from erup- fayalite samples in igneous rocks are actually Mn-rich tion of the Timber Mountain caldera complex fayalite. Reexamination by Mills and Rose (this paper) approximatelyI l.l Ma (Fig. l) (Byerset al., 1976;Brox- ton et al., 1989). Compositionally, the pumice fragments * Presentaddress: Geology Department, Monmouth College, within the Ammonia Tanks Member range from trachy- Monmouth,Illinois 61462, U.S.A. andesire (58 wo/o Sior) to high-silica rhyolite (78 wto/o

TABLE1, Reportedoccurrences of manganoanfayalite

Locality Deposit type or host rock Reference East Ghat. Andkra Pradesh. lndia manganeseore belt, nonvolcanogeniccalc-granulite, garnetiferous Bhattacharyya,1986 quartzite Broken Hill. Australia Gu-Pb-Znsulfides and carbonates Birch. 1984 New Broken Hill Claim, Australia Cu-Pb-Zn sulfides and carbonates Hodgson, 1975 BluebellMine, Kootenay, British Columbia massive sulfide, Fe-Pb.Zn, Precambrianlimestone, Pb-Zn massive Gunning,1936 sulfide and limestone North Vancouver lsland. British Columbia unknown Gunning,1936 Dannemora,Tunaberg, Dalekarlia,Sweden skarn Gunning,1936 Santa Eulalia,Mexico skarn(?),Lower Cretaceouslimestone Gunning,1936 Primorye, USSR Fe-Pb-Znmassive sulfide in Mesozoic sandstones + siltstones Kazachenkoet al., 1979 Kaso Mine, Japan rhodonite veins intruded into rhodocrositeore Yoshimura, 1939 Kyurazawa,Japan unknown Watanabe and Kato. 1957 Tochigi Pretecture,Japan p€gmatite Omoriet al., 1950 Hakozaki Mine, Japan unknown Nambu€t al., 1966 Hijikuza Mine, Japan bedded manganeseore associatedwith Jurassic quartzite Keankeoet al., 1986 Shefiord Mountain. Ouebec normardkite Frisch,1972 Lewisianarea, Scotland manganiferousschists and sandy carbonate Tilley,1938 Tamworth, New South Wales, Australia metasediments Segnit, 1962

0003-004x/9l /0 l 02-0288$02.00 288 MILLS AND ROSE: MANGANOAN FAYALITE IN RHYOLITIC TUFF 289

Trau 2, Representativechemical compositions and molepro- portionsof manganoanfayalite grains, whole pumice, andwhole pumice glass in the AmmoniaTanks Mem- beroftheTimber Mountain Tuff. southwestern Nevada tYAU.tTr{ nAc0rPt tr Samole A21-18 A5€ A5-3 A5-3 sil.illT cAllY0li Grain no. 4 8 glass pumice 't4 Il z;-(CALDttA Analysis no. 23 13 15 Position- c E Oxide (wP/o) sio, 30.25 30.41 72.97 75.87 Tio, 0.05 0.06 0.19 0.13 Alros 0.04 0.02 12.32 11.80 FeO" 42.47 49.28 0.72 0.56 Mgo t 0.08 0.10 t Cao 0.11 0.08 0.39 0.35 Naro ll 3.85 3.17 KrO t o.oz 5.34 5.35 MnO 24.46 18.89 0.13 0.08 ++ Nio t 0.04 ++ BaO 0.04 t 0.13 0.01 Total 97.42 98.88 96.18 97.33 Molepropodions Fa 63.17 71.90 Fo 0.0 0.21 Te 33.63 27.75 (ellrrClRn, cl al.,l9EO) LA 0.21 0.15 t E : grain edge, C : grain @re. N ". FeO: FeO + Fe,O3. t Not detected. + Not analyzed. @: snmprrsrTE r.ocATron

Cnrnmsrnv Fig. l. I-ocation map of the Timber Mountain-Oasis Valley Analytical techniques caldera complex, southwestern Nevada, and the sampling site where manganoan fayalite-bearing pumice fragments from the The manganoan fayalite phenocrysts were initially Ammonia Tanks Member of the Timber Mountain Tuff were found and identified with an electron microprobe while collected. compositional studies on phenocrysts from pumice fragments of the Ammonia Tanks Member (samples A5-3 and A2l-18) were being performed. The manganoanfayalite phenocrysts were separated from glassypumice frag- SiOr) (Mills et al., | 987 ; Rose, I 98 8). Manganoanfayalite ments by hand crushing the pumice in a ceramic mortar occurrencesare limited strictly to the high-silica rhyolite and pestle and sieving the resulting powders. The mesh pumice fragments that are most commonly found near fraction of -60 to + 140 was then concentratedinto light the base of the ash-flow sheet,but it may occur throughout and heavy mineral fractions by bromoform heavyJiquid the ash-flow sheetboth vertically and laterally. Basedon separation. Manganoan fayalite phenocrysts were con- previous studies ofash-flow tuffs, it is generallyaccepted centrated in the heavy fraction along with biotite, mag- that high-silica rhyolite magmasoccur in the extreme up- netite, ilmenite, and titanite fragments. The heavy min- per portion of magma chambersbecause of their low den- eral fractions were mounted in standard metallurgical sities and high volatile contents (cf. Smith, 1979; Hll- epoxy disks for microprobe analysis. Hand separation of dreth, l98l). These magmas are most likely formed by the grains was not possible becauseof the small modal fractional crystallization of a more mafic parent magma percentage of manganoan fayalite present in the pumice (i.e., dacite) or partial melting of crustal material (cf. fragments, the small size of the grains (<0.2 mm), and Hyndman, 1982). the difficulty of visual identification. Manganoan fayalite chemical compositions were de- MrNnrul ASsEMBLAGE termined by an automated JEOL 7334' Superprobeat the Thin sections of pumice fragments and microprobe Lawrence Livermore National Iaboratory. Operating pa- analyses of grain mounts for two samples, A5-3 and rameters for the microprobe beam were 15 KeV, 15 nA, A2l-18, revealed similar mineral assemblages.The phe- and a beam size of 100 pm2. The systernwas calibrated nocryst assemblagesin order of decreasingmodal abun- periodically with the oxide and silicate mineral stan- dance are sanidine, plagioclase, qtrarlz, biotite, magne- dards. The Bence-Albeecorrection procedurewas applied tite, ilmenite, titanite, zircon, chevkinite, and manganoan to all quantitative analyses@ence and Albee, 1968). fayalite. To date, phenocrystsof manganoan fayalite have not 290 MILLS AND ROSE: MANGANOAN FAYALITE IN RHYOLITIC TUFF

FORSTERIIE O Amnia lanks ltr. (this studyl I xahood (198t) I tarshd lnd Srith (1988) P1o 11 D Canichael (1967) o 1?'19 o

SCALE BAFI APPFTOX. C,.1rvlM I M rcFtoPFlctElE PclslTloN o ANtrl ANALYSISi NUMBEFI TEPHROITE Fig. 3. Locationof electronmicroprobe analyses on a chemically zoned(with respectto FeO, MnO, and CaO) manganoan fayalitephenocryst (sample A5-3). Fig. 2. Plot of compositionsof olivine from the Ammonia TanksMember of the Timber Mountain Tuff, southwesternNe- vada (this study), and those of olivine from other high-silica tained up to 8.2 wto/oZnO, which was shown by Mason rhyolitesreported in the literature.(See text for referencesto data (1973) to be the result ofsphalerite inclusions. Although sources.) ZnO was not determined for the manganoan fayalite samples that were analyzedduring this study, Ni (up to 0.43 wto/oNiO, analysis l0) and Ba (up to 0.17 wtoloBaO, been positively identified in thin section owing to the analysis 22) were detegted(Table 2). Evidence of inclu- small size of the host pumice fragments and the corre- sions or possible exsolved phases was not observed in sponding paucity of manganoan fayalite grains. Several manganoan fayalite grains from the Ammonia Tanks ash- potential phenocrystshave been located that have similar flow sheet during backscatteredelectron imaging. optical properties to those noted by Henriques (1956, Chemical analysesof the high-silica pumice fragments 1957), but they are too small for any further determina- that are hosts to the manganoan fayalite grains show them tive analyseson a petrographic microscope. to be Mg poor (<0.01 wt%) with moderate amounts of Fe (Fe'?tand Fe3*reported as FeO,.,) (0.56 to 0.72 wto/o), Rnsur,rs MnO (0.08 wto/o),and CaO (0.35 to 0.41 wto/o).Analyses Representative chemical analyses of the manganoan of glass separatesfrom the pumice fragments are slightly fayalite grains, pumice fragments,and pumice glasssep- higher in MgO, FeO, MnO, and CaO relative to the pum- arates are reported in Table 2. Analyses of the manga- ice fragment analyses.This is most likely due to the ab- noan fayalite samples cluster in the FerSiOo-MnrSiOofield senceoffeldspar and quartz phenocryststhat causea de- (Fig. 2). The compositional range of the grains is from creasein these elemental concentrations in the analyses FaurFooTerrlao, to FarrFoo,Terrlao ,. of the pumice fragments.Pumice fragment and glass-sep- Low analytical totals for the glassand pumice fragment arate alkali and Fe concentrations were most likely af- analysesare probably due to secondaryhydration ofthe fected by secondary hydration and probably do not reflect pumiceous glass that was produced during periods of preeruptive magmatic concentrations (Lipman, 1965; ground-water saturation or surface weathering. Deer et Noble, 1965, 1967). Although FeO,o,and MnO concen- al. (1982) reported that manganoanfayalite may contain trations are low in the host rhyolite when compared to significant concentrationsof H2O* and COr. Since HrO* enrichments in iron-manganeseore bodies where man- and CO, were not included in our analyses,this may ac- ganoanfayalite is most commonly found, the enrichment count for the low totals on some of the analyses.Alter- in volatile phasessuch as HrO and CO, in the high-silica natively, partial oxidation ofFe2* to Fe3*could also ac- rhyolitic magma (T. Vogel, personal communication) may count for the low analytical totals; however, backscattered have contributed to the stability of manganoan fayalite. electron imaging is not capable of distinguishing between Of the grains analyzed, only grains no. I and 8 (corn- Fe2* and Fe3*. Thus it is not clear whether this process plete data set available from authors) in sample A5-3 has occurred.More detailed chemical analysesneed to be were largeenough for zoning studies.Grain no. I exhibits performed to resolve this problem. Significant concentra- negligible zoning, whereas grain no. 8 is zoned with re- tions of ZnO (1.54 wto/o)have been reported by Stillwell spectto FeO (47.7| wto/oto 50.03 wto/o),MnO (20.63 wt06 (in Deer et al., 1982) in manganoan fayalite at the Broken to 19.48 wto/o),and CaO (0.08 wto/oto 0.05 wto/o),as mea- Hill lode, New South Wales, Australia. Other manganoan sured from the edge to the center of the grain. The crys- fayalite samples at the Broken Hill lode however, con- tallographic orientation ofthis grain could not be deter- MILLS AND ROSE: MANGANOAN FAYALITE IN RHYOLITIC TUFF 29r

Fig. 4. Backscattered electron photomicrographs (a and b) of manganoan fayalite phenocrysts from samples A5-3 and A2l-18. Note difference in scale bar size. Iabels are g : glass, k : manganoan fayatte.

mined, but a sketch ofthe grain orientation and electron (Carmichael, 1967) to FarrFonTeu(Warshaw and Smith, microprobe traverse is shown in Figure 3. Becauseof the 1988). This suggeststhat many of the reported occur- random nature ofthe cut through the manganoanfayalite rences of fayalite by workers in highJevel silicic mag- grain, the range of chemical zonation in grain no. 8 should matic systems may in fact have a significant proportion be considereda minimum. as it is uncertain whether the of the manganoan component present. The manganoan actual core ofthe grain was analyzed.Backscattered elec- fayalite of the Ammonia Tanks Member, however, is more tron (BSE) photographs ofrepresentative grains are pre- enriched in Mn than any other fayalitic olivine thus far sentedin Figure 4. Comparison of the BSE photographs documented by workers in silicic magmatic systems. and Figures 4a and,4b of Mossman and Pawson (1976, pages484 and 485) reveal similar grain morphologies. Acxxowr.BocMENTs DrscussroN We would like to thank I-ee Younker of the Iawrence Livermore Na- tional Iaboratory and the Basic Energy Sciences Program for a grant The manganoan fayalite phenocrysts are interpreted by supplied to Tom Yogel rhat funded this study and allowed us a@essto us to be primary magmatic phasesthat crystallized within the electron microprobe. We would also like to thank Tom Vogel for the Timber Mountain magmatic system.The occurrence assistancein operation ofthe electron microprobe. The authors also thank preparation of manganoanfayalite phase Monmouth College for assisting in of the final draft of rhis as a xenocrystic can be re- rnanuscript. jected becauseofthe lack ofother xenocrystic phasesor rock fragments in the pumice fragments (Rose, 1988). RspnnpNcns crrED The use of whole pumice fragmentsas opposedto whole- rock tuffsamples to study magmatic compositions in the Bacon, C.R., Macdonald, R., Smith, R.L., and Baedecker,P.A. (1981) Pleistocene high+ilica rhyolites ofthe Coso Volcanic Field, Inyo Ammonia Tanks Member Coun- of the Timber Mountain Tuff ty. Journal of Geophysical Research, 86, 10223-1024L. is based partly upon this condition (seeSchuraytz et al., Bence, A.E., and Albee, A.L. (1968) Empirical correction factors for the I 986, and Schuraytzet al., I 989, for a detailed discussion electron microanalyses of silicates and oxides. Journal of Geology, 76, of sample analysesof pumice vs. whole-rock tuff). Other 382-403. (1986) petrology xenocrystic phases should be present if the magma had Bhattacharyya, S. Mineral chemistry and ofthe man- ganesesilicate rocks ofVizianagaram manganesebelt, Andhra Pradesh. partially assimilated or mechanically incorporated meta- Journal Geologiail Society of lndia, 27, 169-184. somatized country rock containing manganoan fayalite. Birch, W.D. (1984) Roepperite from Broken Hill (New South Wales) is Although Paleozoic carbonate forms a cover l-2 km thick ferroan tephroite. Mineralogical Magazine, 48, 137-139. in the southern Nevada region, there is no evidence of Broxlon, D.E., Warren, R.G., Byers, F.M., Jr., and Scott, R.B. (1989) within Mountain-Oasis manganese-bearing Chemical and mineralogic trends the Timber sulfide deposits in the area. Valley caldera complex, Nevada: Evidence for multiple cycles of chem- Fayalite may be present in highly evolved silicic ash- ical evolution in a long-lived silicic magma systerr. Joumal of Geo- flow tuffs. Several analyses taken from the literature are physical Research, 94, 5961-5986. plotted in Figure 3. The fayalite found in silicic ash-flow Byers, F.M., Jr., Can, W.J., Orkild, P.P., Quidivan, W.D., and Sargent, (1976) Volcanic tuffs has a composition of approximately FaroFo,Te, K.A. suites and related cauldrons of Timber Moun- tain-Oasis Valley caldera complex, southern Nevada. U.S. Geological (Carmichael, 1967; Mahood, 1981). Fayalite from other Survey Professional Paper 9.19. rhyolitic tuffs ranges in composition from Fa.rFouTe. Carmichael, I.S.E. (1967) The iron-titanium oxides ofsalic volcanic rocks 292 MILLS AND ROSE: MANGANOAN FAYALITE IN RHYOLITIC TUFF

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