Carwdian Mineralogist Vol. 15, pp,74-80 (19711
GRO$$UTAR.SPESSARTINE-ALMANDINEGARNETS FROM SOMEJAPANESE SCHEELITE SKARNS
HIDEHIKO SHIMAZAKI GeologicalLtstitute, Faculty of Science,University of Tokyo, Hongo, Tokyo 113, Iapan
AssrRAct cate that the garnets form a continuous solid solution which extends from grossular to spes- Garnets from the Kuga, Fujigatani and Kagata sartine-almandine. paper mines, typical scheelite skarn deposits in Japan, are In this occurrencesand garnets found with tle electron probe to be close to €ro.gsu- analysesof these will be described. lar in composition with more or less substitution The recent development of techniques for of Mn and FC+ for Ca. Compositional zoning ob- high-temperafure and high-pressure experiments served in some ouhedral to subhedral crvstals con- has allowed syntheses of a wide range of garnet sists of a gradual increase in the Mn'and Fez+ solid solutions. One of the problems at present contents from core to rim. The compositional varia- in garnet mineralogy is the determination of the tion of garnets, given by Brown (1969), Ackermand dependence of the solid solubifity among garnet et al, (L972) and the present study, confirms the formation of solid solution among grossular, end members and the conditions of formation. spessartine and alnandine" even ulder conditions The present paper will demonstrate that nearly of relatively low tem4rerature and pressure. complete solid solution is possible among al- mandine, spessartine and grossular even under relatively low-temperature and low-pressure con- Sorvnvrenr ditions. Les grcnatsprovenant des mines Kuga, Fujigatani et Kagata, gftes typiques de skarn i scheelite,s'avd- PRBvrous WoRK rent, i la microsondg prochesd'uo grossulairedont les atomes de Ca sont partiellement remplac6spar Complete solid solution between pyrope and des atomesde Mn et de Fe2+. Irs zones de compo- almandine, almandine and spessartine, and sition observ6esdans certains cristaux idiomorphes- grossular and andradite is well-established.How- i-hlpidiomorphes traduisent l'accroissement de la ever, some conventional wet-chemical analyses teneur en Mn et en Fez+, du centre vers la p6ri have given "unusualo' compositions, such as ph6rie. La variation dans La composition des gre- those rich in both Mn and Ca, or rich in Feu" nats, observ6epar Brown (1969), par Ackennand and Ca (seeTrdger 1959 and N6mec 1,967).N- et al. (1972) et daos le pr&ent travail, confirme la formation de solutions solides entre grossulaire, though these garnets were reported as single spessartineet almandin, m6me i temp6rature et phases having characteristic refractive indices pression relativement basses. and/or unit-cell constants,it was not considered in most papers that the garnets might exhibit (lraduit par la R6daction) compositional zoning or heterogeneity that could not be distinguished with wet-chemical INrnonucrtoN techniques. With the development of the elec- tron probe, it has become possible to study in It is well known that garnet forms solid solu- detail the compositional variation and zoning tion series with various end members, among of garnets. Brown's (1969) paper, on zoned which the following five are common: pyrope, garnets from quartzo-feldspathic schists of almandine, spessartine,grossular and andradite. several greenschist-faciesterrains in the world, The first three are called the pyralspite series could be cited as an example of such detailed and the last two, the grandite series. Although study of compositional variation. continuous solid solution seemed to be absent Brown's (1969) results cover a wide area in (e.g. Deer et aI. 1962), recent studies of syn- the almandine-spessartine-grossulardiagram as thetic and natural garnets in the two series are shown in Figure 1. The Figure also includes the revealing that various degrees of solid solution microprobe-determined compositional range of are possible, depending on the conditions of almandine-grossular garnets from low-grade formation. metamorphic roctrs of the Tauern area, Austria Microprobe analyses of grossulars from (Ackermand et al. 1972), The results indicate scheelite-bearingskarn deposits in Japan, indi that almandine-spessartinegarnets can incor- 74 GARNETS FROM SCHEELITE SKARNS 75
Spes.(Mn-) tween the grossular corner and the analyses of Brown and Ackermald et al. in Figure t have not yet been confirmed, to the present wdter's knowledge, by electron probe microanalysis.
li \ \ ll\ OccunneNcn or i\\ Gnossur-en-SpEssARTINE-ALMANDINEGenNBrs lro \ 9i. \. of scheelite- .l One of the characteristic features ;\ bearing skarn depositsin Japan is that the main 1.8 \. a. \ constituent minerals are generally poor in ferric ai (Shimazaki In accordance with this I iron 1974). fact, garnets from the skarns are poor in Fe3+' '1 Brown(1969) and rich in Al. The Kuga" Fujigatani and Ka- gata mines are typical scheelite skarns in lime- stone, and have genetic relations to granitic in- '){ L'li;.*.,'i:. (1972) trusions. The grossular garnets in these deposits were found to contain considerable amounts of crca.(ca') Atm.(Fe") Mn and Fe'+. The following is a brief descrip- tion of the occurrence of these garnets, Frc, 1. Plot of some garnets with intermediate com- positions between the almandine-spessafrine joir The Kuga mine is a tungsten-copper deposit and grossular. Dashed line and dotted line repre- 30 km southwest of Hiroshima City, southwest- sent microprobe compositional ranges of ganrets ern Japan. The area consists of Upper Pala- from several greenschist-facie$ t€rrains (Brown eozoic to Lower Mesozoic formations of slate, rocks of 1969) and from low-grade metamorphic sandstone,chert and limestone intruded by Late the (Ackermand el al. Tauern area, Austria ganitic to Imai & 1972), respectively. 1: regronally metamorphosed Cretaceous rocks. According garnet skarn, Kadov, Moravia (N6mec 1967). Ito (1959), the main constituents of the Kuga 2: Franklin, New Jersey (Frondel & Ito 1965). deposits include andradite, Ca-clinopyroxene' 3: regionally metamorphosed Mn-rich sediment, wollastonite, epidote, actinolite, quart4 scheelite, Basttj[rn, Sweden (Magnusson L940). 4-52 sheared cassiterite,chalcopyrite, pyrrhotite, arsenopyrite parts of granodiorite, Victory mine, Nevada and pyrite. (tre 1962)8. 6: quartzose pelitic schist, Katsuya- Although Imai & Ito (1959) recognized andra- (Ilashimoto ma, soutlwestern Japan 1968). 7: dite as one of the main skarn minerals, all gar- rock, Kotakarra, India K-feldspar-garnet-apatite studied by the present (Fermor 7934)**. 8: quartz-wolframite vein, nets from the deposits Kaneuchi mine, central Japan (Ashida & Onuki writer are grossular with more or less Mn and 1963), 9: Mn deposit, Guettara, Algeria (Byram- Fe'+. As there are many ore deposits in this jee & Meindre 1956). 10: garnet-quartz rock, mine, and the samples studied were randomly Sakrasanhalli, India (Fermor 1934)x*. collected from several, the occurrence of grossular garnets with Mn and Fez+ could be xAccording data, A" to unit-cell dimension "Garnet Under the by Lee (1962) seems to be a mixture. It is omitted regarded as a common feature. in the present plot. microscope, the garnets often show distinct zoning with anomalous anisotropism. Clinopy- **Data for tlesd t"ro garnets from India are from roxenes associatedwith garnet are hedenbergites (1959) (1962). Trdger and Lee with considerable amounts of Mn. The Fujigatani mine is 5 km southeast of the porate considerable amounts of Ca under meta- Kuga mine, and has a similar geological setting. morphic conditions of greenschist to low-tem- The main constituentsof the deposits are heden- perature amphibolite facies. bergite, grossular, plagioclase, wollastonite, In the literature most garnets reported to have vesuvianite, amphibole, quatlz, calcite, fluorite, intermediate compositions between the alman- chlorite, sericite, stilpnomelane, scheelite, pyr- dine-spessartine join and grossular ate close to, rhotite, chalcopyrite, arsenopyrite, sphalerite, or included in, the ranges given by Brown (1969) galena and so on Qto L962; Sato 1975, and the and Ackermand et al. (1972), Published results present study). Based on unit-cell size and re- of wet-chemical analyses of some garnets which fractive index, Ito (1962) suggestedthat garnets are close in composition to those studied in the are generally grossulax with 5 b 2A mol. Vo present paper are plotted in Figure l. Natural Fe3+.In the presentstudy, garnetscollected from occurrences of continuous solid solution be- the Akemidani deposit (the main deposit in this 76 THE CANADIAN MINERALOGIST mine) are confirmed to be close to grossular, of the garnets. Count-ratios of ttre sample to and some have considerable amounts of Fe2+ the standard were measured for Si, Al, Fe, Mn, and Mn, especially at the margins of euhedral Mg and Ca. The ratios were converted into grains. Under the microscope, the garnets are oxide weight percentages using the method of generally distinctly to weakly anisotropic. Bence & Albee (1968). Detailed descriptions of the ana$cal procedure and the correction The Kagata mine is about 90 km north-north- factors used in this study have been given by east of Tokyo. The area consists of Late Pala- Nakamura & Kushiro (1970). In the present eozoic shale, sandstone, chert and thin lenses study, the totals of tle corrected weight per- of limestone intruded by i Late Cretaceous centagesof SiO:, AlrOs, FeO, MnO, MgO and biotite granite. Although the mine was closed CaO, are usually in the about 20 years ago, specimens langes of 98.5 to were collected lOL.SVo. from old dumps. The main constituent minerals include garnet, Ca-clinopyroxene, wollastonite, Ratios of ferric and ferrous iron are important a CasFe!+SisOrsphase with the bustamite struc- to determine the composition of garnets. Num- ture (so-called iron-wollastonite), actinolite, bers of cations including Fe'+ and Fee+ are cal- scheelite, pyrrhotite, pyrite and chalcopyrite culated to satisfy the following two condifions: (Shimazaki & Yamanaka 1973). Garnets from (l) total number of oxygens is 12.00, and (2) this mine are generally close to grossular in some Fe is taken as ferric to fill the six-coor- cornposition, and show very weak anisotropism dinated position so that Al+Fee+=2.00. The under the microscope. fn some specimens,Fez+ remainder of the Fe is assumed to be ferrous and Mn-rich portions of the garnets have a (see Table 1). After these calculations, mole- compositional heterogeneity or zoning. cular percentages of end members are calcu- lated as follows. The amounts of andradite are ANALYTICALPRoCEDTTRE ervo Rrsur-rs determined from the proportion of ferric iron, pyrope from Mg, almandine from ferrous iron, About 30 polished- and polished thin-sections and spessartinefrom Mn. The remainder of Al of skarns from the Kuga, Fujigatani and Kagata and Ca gives the amounts of grossular. The mines were prepared for electron probe analyses proportion of Si is always in the range of. 2.9O
TABLEI. EXA}IPLESOF GARNETANALYSIS UITH THE ELECTROI1MICROPROBE '10 'll No. 1Z 34 5789 12 sl02 37.71 36.8 35.9 37.6 37.0 37.1 37.0 &.7 37.t 38.6 37.8 37.3 '19.4 A1203 20,9 20.9 20.5 2A.7 ?0.9 19.8 |9.8 19.0 20.0 t9.3 19.4 19.9 F€0. 3.5 7.0 ,.o o.u 5.5 7.1 7.5 12.4 13.7 7.2 9.9 15.1 l4n0 't4.3 3.5 9.5 t5.7 7.6 IJ.a ]1.2 17.1 22.3 10.6 2.7 4.6 7.4 il90 0.0 0.0 0.1 0.0 0.1 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 '15.0 C!0 v..9 24.3 19.l 27.6 20.7 24.1. 17.8 t3.3 19.l 32,5 27.3 18.7 Total 98.5 98.5 99.9 99.5 99.2 98.7 99.4 98.8 99.2 98.9 100.3 99.0 99,4 Nw6€E of cations on the basls of lZ orygens
sl 2.U 2.93 2.U 2.94 2.94 2,95 2,96 2.99 2.97 2.96 2.97 2.97 2.98 '| 't '1.86 'I Al 1.92 1.96 .93 I .91 l.to .86 L87 1.82 1.89 .75 1.79 1.87 Fe3+ o.oa o.o4 0.07 0.09 0.M 0.14 0.13 0.18 0.ll 0. 14 0,?5 0.21 0.13 Fez* o.l5 0.43 o.u 0.30 0.44 0.29 0.35 0.33 0.72 0.79 0.21 0.45 0.95 'I Itn 0.23 0.64 .06 0.50 0,89 0.75 1.20 1.54 0.72 0.98 0. t7 0.31 0.50 Mg 0;00 0.00 0.01 0.00 0.01 0.00 0,00 0.00 0.00 0.00 0.00 0.00 0.00 ',r.53 Ca 2.74 2.O7 f.oj a.Jl 1.77 2.05 l.l5 1.64 I.31 2,8 2,30 1.60 tlol. g of end nE[De6
Prr. 0.0 0.0 0.3 0.0 0,2 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 ' SIe 5.0 14.3 14.5" 10.0 14.8 9.5 ll.7 ll.0 23.8 26.3 6.9 14.8 11 4 50e5s. .7.6 21.3 35.2 16.8 29.7 25.1 40.0 51.2 23.9 5.8 10.2 16.7 Gross. 83.2 62.5 46.4 68.5 53.4 58.3. 41.9 28.7 46.5 33.7 74.5 64.5 45.4 And. 4.1 1.8 3.7 4.6 1.9 7,0 6.4 9.1 5.8 7.2 12.7 lo.s Cmpolttlonal zollng (core)-(rln) (core)+(rln) (core)-(rlm) (core)-(rln) * Total Fe as Feo Nos. l-5: HS72081807gamet-cllnopynene skam trcm the lQnnondeposit, Kugamlne. Scannlngproflle of Al, Mn and ca frcn the core (No. l).to rln (No. 5) ls shom tn Flgure 3. Nos. 5-8: rs72081818garnet-cllnopymrene skam frcm the ratho depostt, Kugantne. Nos. 9-10: HS72081624garnet-amphibole-fluorlte skam frcm the Akemldanideposlt. Fujiqatani nine. Nos. lt-13: HS72072307qamer- clinoplmxene skam frcm the Kagatanlne. GARNETS FROM SCHEELITE SKI\RNS 77
Spess(Mn'1
Grcs.(Ca*) Alm.(Fe") Ftc. 2. Compositionsof analyzed garn€ts. Solid circles: garnets from thq Kuga mine; solid triangles: the Fujigatani mine; solid squaresand hexa- gon: xhe Kagata mine; solid hexagon (No. 14) is the wet-che,mical analysis CIable 2) of tle Kagata mine garnet separatewhich contained the zoned graiu plotted as Nos. 11-13. Dashed and dotted linee are the salte as those in Figuro 1. Arrows show the trend of the comlrositional zoning from core to rim. Numbers correspondwith those in Table 1. to 3.00, and is not included in the above cal- mol Vo was also very low, usually less than 0.5 culations. The results are plotted on the almandins-gpsg- Andradite mol. Vo in the analyzed garnets sartine-grossular triangle (Fig. 2). As shown in from the three mines ranged from 0.0 to 16.6, this Figure, the compositions of garnets from and was less than 10 in most cases. Pyrope these thss mines extend from the grosular
-o Rrm E o nhh\tu/\ '\. l"l-2scso \iMto** /^t$ "/a,-rr^,*/r^^A/rFt l3[#,4 /An$nv I ,' flO Mrto
ql--r-,-'grflr'ln ru-,r,^^(/ [r.,*
Flc. 3. Electron probe scanning profrle of garnet crystal in specimen IIS72081807 of ga,rnet-clinopy- roxene skarn from tle Kuga mine. The rezult of point analysisat points Nos. 1-5 is given in Table 1 and plotted in Figure 2. 78 THE CANADIAN MINERALOGIST corner to the area reported by Brown (1969) as decreasein Mn content from core to rim (e,g. the range of the compositional variation of gar- Brown 1969). nets from greenschist-faciesterrains. The gar- nets studied commonly show compositional DrscussroN variation between and within grains in a single polished section: in some a remarkable com- Little has been reported previously as to tem- positional zoning is observed from core to rim perature-pressure conditions for skarn forma- of euhedral to subhedral crystals. Selected tion in the Kuga, Fujigatani and Kagata mines. analyses to demonstrate such compositional zon- From the intimate association with granitic in- ing are given in Table 1. A crystal in a specimen trusions, the depositsare thought to have genetic (IIS72081807) from the Kuga mine showed the relationships to them. As andalusite is found rather complicated zoning pattern given in in the contact aureoles in both Kuga and Fuji- Figure 3. The compositions of five points in the gatani, and Kagata areas (fakimoto 1939; Taka- crystal are given in Table 1 and plotted in hashi & Ueno 1960), pressurecould be estimated Figure 2. as lower than several kbar. Ito (1962) compared The result of conventional wet-chemical the mineral assemblagesof the skarns in the analysis of garnet separated from a specimen Fujigatani and Kuga mines with those of re- (tIS72O723O7) of garnet-clinopyroxene skarn gionally metamorphosed calcareous rocks in from the Kagata mine is given in Table 2, and Abukuma plateau described by Miyashiro (1.953a), and concluded that grandite garnet, clinopyroxene, and wollastonite in the skarns CTIET.IICAL TABLE2. RESI'LTSOF HET AMLYSTSOT EAMN' were formed at lower temperatures than low- st02 37.76 dt. l{os, of catlons on the fiiol. g of end nenber:s grade amphibolite facies in the Abukuma meta- Tt02 0.14 basls of l2 orygens Pyr. I.9 morphic belt. .Al?03 't9,63 5t 2.934 AIm. 9.0 Crystallization conditions for garnets also te- F"Z0S 4,26 Tt 0.008 spass. 6.2 ported by Brown (1969) facies) and Fd) 4.20 Greenschist Al 0.058 Gmss. 70.4 (1972) (greenschist l'ln0 2.85 by Ackermand et aI. to low- Al t.7it0 And. 12.5 [go 0.51 temperature amphibolite facies) seem to fall in Fe3+ o,z4g for the forma- tao 30.31 F"2' 0.213 the same range estimated above
Lrp, D. E. (1962): Grossularite*spessartite garnet. problem of ferric and ferrous stilpnomelanes. from the Victory mine, Gabbs, Nevada. Amer. Neues Jahrb. Mineral. Monash. 1975. t79-192. lltlineral. 47, 747-151. SttrMAzAKr, H. (L974)z Characteristics of tungsten tr4ecxussor,'N. H. (1940): Geology and ore de- mineralization in lapanese skaru depo$ib. fr posits of Ljusnarsberg. Sveriges Geol. (Jnder- Metallization Associated with Acid Magmalism sdkning, Ser. C, No. 30 (in Swedish). {M. Stemprok, ed.), 1, 3L2-3L5. Ustredni Ustav Mrvasnno, A. (1953a): Progressive metamorphism Geologicky. of the calcium-rich rocls of the Gosaisvo-Taka- & YervreNere, T. (1973) : Iron-wollastonite nuki district, Abukuma plateau, Japan. iapan. J. from skarns and its stability relation in the Geol. Geogr. 23, 81-10?. CaSiOs{aFeSisOs join. Geochem. J. Clokyo) 7, (1953b): Calcium"poor garnet in relation 67-79. to motamorphism. Geochim. Cosrnochlm. Acta 4, Tlraueuu, H. & Uwo, M. (1960): On the wol- 179-208. lastonite of the Kagata mine, Ibaragi Plefecture. Monthly Rep. Geol. Sum. Japan 7L, 451453 (1973): Metamorphisa and Metamorphic (in Japanese). Belts. George Allen & Unwin, London, Texrrvroro, K. (1939): Geology and ore deposits Nerervrune, Y. & Kusnrno, I. (1970): Composi- near the Kiwada mine, Yamaguchi Prefecture. tional relations of coexisting orthopyroxene, l. Geol. Soc. Japan 46, 47-57 (in Japanese). pigeonite and augite in a tholeiitic andesite from Hakone Volcano, Contr. Mineral. Petrology 26, Tndcen, E. (1959): Die Granatgruppe: Beziehungen 265-275. zwischen Mineralchemismus" und Gesteinsafi. Neues Jahrb, Mineral. Abh. 99, l-4. Nrvmc, D. (1967): The miscibility of the pyralspite and grandite molecules in garnets. MineraL Mag. ZeuaNN, J. (1962): Zur Kristallchemie der Granate. 36, 389-402. Beitr. Mineral. Petrogr. 8, 180-188. Sa1o, K. (1975): Stilpnomelane from the Fujigatani Manuscript received Aagust 1975, emended lune skarn ore deposit, Japan, and its bearing-on the 1976.