Hyogo University of Teacher Education Journal Vol. 14, Ser. 3, (1994) 25

Chemical Compositions of Skarn Minerals from the Kiwada Tungsten Deposit,

Yamaguchi Prefecture

Yasuhiro SHIBUE", Kantaro FuJIOKA日, and Teruaki IsHII…

(Received September 22, 1993)

Abstract

Chemical Compositions of garnet, clinopyroxene, calcite and amphibole from the

Kiwada tungsten deposit, Yamaguchi Prefecture, are obtained in order to document

and characterize the chemistry of these minerals.

Molecular proportions of andradite in garnet are less than 17 mole percents, and

those of johannsenite molecules in clinopyroxene are less than 14 mole percents.

Compositional data on these minerals show the feature of "reduced-type" (Einaudi

et al, 1981) skarn. Fe2+/(Fe2++Fe3+) ratios of amphibole range from 0.889 to0.976, which also shows the reduced feature.

Introduction

Tungsten skarns were classified into "reduced-type and "oxidized-type by

Einaudi et al. (1981). In "reduced-type skarns", abundant ferrous iron assem-

blages, such as hedenbergitic clmopyroxene and almandme-rich garnet are observed,

On the contrary, abundant ferric iron assemblages, including andradite-rich garnet,

are found in "oxidized-type skarns". Shimazaki (1974, 1980) discussed that the

Fujigatani and Kuga tungsten deposits were formed under relatively reducing

condition based on the facts that grossular+hedenbergite skarns are formed

accompanying pyrite and pyrrhotite (no hematite) with less common occurrence of

epidote in these deposits. His conclusion should further be clarified by considering

chemical compositions of skarn minerals.

Shibue and Fujioka (1991) indicated that skarn minerals from the Fujigatani

tungsten deposit show the features of the reduced-type based on the chemical

compositions of garnet, clinopyroxene, and amphibole. Present work follows their

study to obtain the chemical compositions of the minerals from the Kiwada

tungsten skarn deposit which is located 3km northwest of the Fujigatani deposit

and 15 to 20km west of the Iwakuni city of the Yamaguchi Prefecture.

The outline of the geology of mining area was described by Shibue and Fujioka

(1991) and is not repeated here in detail. The Kiwada deposit occurs in the Kuga

group of Jurassic age (Metal Mining Agency Japan, 1987). The sediments of

the Kuga group consist of pelitic rock and chert with small blocks of lenticular

sandstone and limestone. The limestone lenses as well as the pelitic rock are

GeoscienceInstitute,HyogoUniversityofTeacherEducation,Yashiro-cho,KatoJapa君un,Hyogo673-14,MarineScience&究panchn。l。gyCenter,Y。k。suka,Kanagawa237,JapanOceanResearchInstitute,TheUniversityofTokyo,Tokyo164,Japan

26

important host rocks for the skarns. Drilling cores revealed the presence of a

granitic body beneath the Fujigatani mining area (Metal Mining Agency

Japan, 1981). This granitic rock and other exposed granitic rocks, which were

formed at the Cretaceous age (Kawano and Uyeda, 1966; Shibata and

Ishihara, 1974), intruded and metamorphosed the sediments of the Kuga group.

The metamorphic aureole consisting of plagioclase+white mica+biotite十cordiente

+ andalusite ± garnet spreads about 559m distant vertically from the contact

(Higashimoto et al., 1976). K-Ar ages of muscovites in quarts veins were

reported to be 98.0 Ma or 101.6±5.0 Ma (Shibata and Ishihara, 1974; Ishihara

et al., 1988; Watanabe et al., 1988). Thus, the Kiwada deposit is considered to

be genetically related to the Cretaceous granitic activity.

The following studies described the chemical compositions of skarn minerals

from this deposit: Nagahara et al. (1978), garnet from No. 10 ore body; Sato

(1977), wolframite (rarely observed in this deposit); Shibue (1987), muscovite;

Shibue (1988), sphalerite. Thus, it is still necessary to obtain the chemical

compositions of the skarn minerals from this deposit. Because both of the

Fujigatani and Kiwada deposits were formed at the Cretaceous period (Shibata

and Ishihara, 1974) and are located adjacently, it is not unreasonable to assume

that the Kiwada deposit belongs to the "reduced-type skarn deposit. Present

paper aims to examine this probability by describiIng the compositional data on

skarn minerals.

Analytical Methods of Minera一s

Electron probe micraonalyses

Chemical compositions of minerals are determined with use of electron probe

microanalyzer JXA-733 at an accelerating voltage of 15 kV and a beam current of

12 nA. Analyses were screened by rejecting those with totals outside the ranges

from 99.0 to 101.0 wt.% for isotropic garnet and chnopyroxene, and from 98.0 to

100.5 wt.% for amphibole. Partial analyses of Fe, Mn, Mg, and Ca are carried

out for carbonates by the method of Bence and Albee (1968); CO2 contents are

calculated by subtracting the weight percents of the oxides from 100 wt.%.

Analytical results are then screened by rejecting those with (Fe+Mn+Mg+Ca)/C

atomic ratios outside the range from 0.9 to 1.1.

Ferric/ferrous ratios and structural formulae of garnet and amphibole

Ferric/ferrous ratios of garnet are calculated by an empirical scheme shown by

Knowles (1987); this method is splitting total iron into the two valence states by

considering the weight percents of the other cation oxides. Using ferric/ferrous

ratios, cations are allocated to endmembers by the method of RiCKWOOD (1968).

Analytical results are further selected from the percentages of cations allocated to

endmembers; the critical percentage for the selection is assigned to be 95 percent

in this study. This percentage is chosen from the statement that "a deviation

from perfect structural balance of as much as 5 percent may be expected in natural

Skarn minerals from the Kiwada deposit 27

garnets (Rickwood, 1968)". Water contents in anisotropic garnets are not

included in the present study.

Structural formulae of amphibole are calculated from microprobe analyses,

following the method described by Robinson et al. (1982) and the procedure by

Pe-Piper (1988). First, the structural formulae were calculated assuming total

cations to be 13, exclusive of K, Na, and Ca. The resulting values for ferric iron

are the maxima consistent with the stoichiometry. Second, the structural

formulae were calculated assuming total cations to be 15, exclusive of K and Na.

The resulting values for ferric iron are the minima consistent with the

stoichiometry. After the exclusion of the results violating the crystal-chemical

constraints (shown in Robinson et al., 1982), mean values from the two methods

of calculation (13 and 15 cations) are used for discussing the compositions of

amphibole.

Occurrences of Skarn Minerals

Previous works on the Kiwada deposit focused on the zonation of skarn minerals

(e.g., Nagahara, 1978). Nagahara (1978) described the zonal arrangement of

skarn minerals in No. 10 ore body as follows: (slate) - (clinopyroxene+garnet)

- (quartz+calcite) - (vesuvianite) - (limestone). In Na ll ore body, Hiiragi

et al. (1984) described the zonation as follows: (slate) - (clinopyroxene) -

(clinopyroxene+garnet) - (calcite+quartz+wollastonite). Present study

confirms these observations on the zonation of skarn minerals, with some

additional exceptions.

Unreacted limestones are encrusted by (garnet+clinopyroxene) skarns (No. 10 ore

body). Although distinct zone of neither garnet nor clinopyroxene is observed in

this ore body, concentrated parts of either mineral are observed at several

outcrops. Furthermore, quartz+calcite zone is recognized between limestone and

(garnet+clinopyroxene) skarn in several outcrops of No. 10 ore body, but this zone

is absent from many outcrops. In both No. 10 and ll ore bodies, quartz veins fill

the cracks within the limestone, accompanying hydrothermal alterations such as

chlontization. Scheelite, the tungsten mineral mined in this deposit, occurs not

only m skarns but also in quartz veins. Skarn scheelite occurs mainly in

(garnet +clinopyroxene) skarns or their garnet-rich parts.

Garnets from both ore bodies often show birefringence at their peripheral parts

(Fig. 1(1)), and are replaced by quartz, calcite, chlorite, or muscovite. Sulfide

minerals (chalcopyrite and pyrrhotite, or less commonly sphalerite ) are sometimes

precipitated within garnet crystals or their intragranular voids (Fig. 1(2)).

Muscovite and K-feldspar also sometimes fill the cracks or intragranular voids

within garnet-rich parts (Fig.1(3)). On the other hand, clinopyroxene is often

replaced by amphibole+quartz+calcite during the late hydrothermal activity.

In quartz veins, scheelite, muscovite, sodic plagioclase (No. ll ore body), apatite,

chlorite, fluorite, chalcopyrite, pyrrhotite, and sphalerite are commonly observed.

Fig.1(2)

Fig.1(3)

Fig.1(4)ト-'-ォ;。滋近辺齢盗泉雌し'l :心績■齢Bl j>

Fig. 1. Photomicrographs of skarn minerals from the Kiwada

garnet (G) band is observed at the peripheral part. Quartz (Q) and muscovite (M)

replace garnet. (2) Chalcopyrite (CP) fills the intragranular void of garnet crystals.(3) Muscovite (M) and K-feldspar (P) fill the intragranular void of garnet crystals.

(4) Clinopyroxene (C) is replaced by amphibole (A)+ quartz (Q) + calcite (CO.

Samples specimens (1), (2), and (4) are collected from No. 10 ore body, and (3) from Na ll

ore body. White bars indicate 0.05mm.

Among these minerals, chlorite is concentrated along the borders to the

(clinopyroxene+garnet) skarn of No. ll ore body, and is closely associated with

muscovite. Ilmenite (Fea. 02- 。Mn。. 98-。. 9。Ti03) and siderite are observed in

quartz veins of this ore body although these sporadically occur in the Kiwada

deposit.

Chemical Compositions of Skarn Minerals and Their Implications

for Skarn-forming Fhid

Representative analyses of garnet, clinopyroxene, calcite, feldspars, and

amphibole are listed in Tables 1 to 4.

Garnet

Andradite mole percents in garnet examined in this study are less than 17%;

almandine and spessartine molecules in the garnet range from 0 to 13% and from

10 20 〕0ん0 50

Spessarline mole%

Fig. 2. Spessartine mole percents in garnet.

Skarn minerals from the Kiwada deposit

Table la. Representative analyses of garnet froi No. 10

ore body of the Kiwada deposit.

n o core riォ core

Site 40.01 37.78 38.07 38.25 36.80

TiO2 0.73 0.09 0.12 0.04 0.20

Ah O3 18.91 19.92 20.47 19.71 20.35

FeOto 6.12 7.01 7.ll 5.13 8.34

NnO 2.82 10.04 17.20 1.70 5.63

Mg0 0.00 0.03 0.05 0.03 0.09

Ca0 31.68 23.78 17.54 33.75 27.52

Na?O N.A. 0.00 0.00 0.00 0.00

K20 N.A. 0.00 0.00 0.㈱ 0.00

CrsOa 0.03 0.00 0.00 0.00 0.00

Total '100.27 98 .65 '100.55 98.61 ' 98.91

Hole percents of endieabers'

Ala

And 8 12 2 9 15

Gr0 87 57 50 86 65

Pyr 0 0 0 0 0

SPe 4 23 40 4 13

UVa 0 0 0 0 0

'See text for the calculation sethod.

Table 2. Representative analyses of clinopyroxene

and feldspars fron the Kiwada deposit.

1 2 3 4 5

SiOs 48.89 53.23 48.34

An

65.31 68.58

TiOa 0.00 0.01 0.00 0.00 0.00

AIpOs 0.08 0.13 0.17 18.00 19.41

Pe0 25.24 7.37 25.22 0.14 0.00

Nn0 2.77 0.59 1.90 0.00 0.01

Mg0 0.35 13.29 0.88 0.01 0.00

Ca0 22.54 24.23 22.60 0.08 0.17

NasO N.A. 0.06 0.06 0.27 ll.80

K20 N.A. N.A. 0.00 15.59 0.08

Total ' 99.87 9.11 ' 99.17 99 .40

0

100.05

1

Mole percents of en血eiibers

J0 10 2 7

Hd 88 23 88 Ab 3 99

Di 2 75 5 Or 97 0

l: Clinopyroxene froi No. 10 ore body.

2, 31 Clinopyroxene froォNo. ll ore body.

4: K-feldspar fron garnet-skarn of No. 10 ore

body (alteration product).

5: Plagioclase froi quartz vein of No. ll ore body.

29

Table lb. Repre部ntative analyses of

garnet froサNo. ll ore body

of the Kiwada deposit.

rn core

Si02 37.63 37.23 37.19

TiOs 0.10 0.08 0.10

A1203 20.16 20.13 20.05

FeOto 7.01 8.51 8.80

Mn0 15.02 9.13 9.31

Mg0 0.05 0.04 0.05

Ca0 19.10 23.92 23.38

Na?O N.A. 0.00 0.01

KsO N.A. 0.㈱ 0.00

CrsOa 0.00 0.00 0.04

Total ' 99.07 99.03 98.91

Hole percents of endieibers'

Ala 10 i 10

And 8 15 15

G㌻O 87 55 53

Pyr 0 0 0

SPe 4 21 22

Uva 0 0 0

蝣See text for the calculation aethod.

Table 3. Representative analyses of

calcite froI蝣the Kiwada deposit.

1 2 3

Fe0 0.00 45.02 1.12

Nn0 0.29 3.6 2.08

NiO 0.00 0.22 0.ll

Ca0 55-.55 6.51 51.08

Nuibers of Aton (3 oXygens)

Fe 0.00 0.79 0.02

Kn 0.02 0.07 0.04

Ng 0.00 0.01 0.00

Ca 0.99 0.13 1.00

C 1.00 1.00 0.97

l: Garnet+clinopyroxene skarn of No. 10

ore body.

2: Quartz vein of No. ll ore body,

associated with chalcopyrite.

3: Quartz vein of No. ll ore body,

associated with chalcopyrite.

30

Table 4. Representative analyses of

anphiboles from the Kiuada

deposit.

1, 2: No. 10orebody.

3, 4: No. ll orebody.

【4IAK Al in tetrahedral site.

【6】kll Al in octahedral site.

See text for the calculation method.

GPO SpeAm

And

Gro Speトイ

Aim nd ◆・・◆

Fig. 3. Compositional heterogeneity in garnet

from No. 10 ore body.

Fig・ 4. Compositions of garnet from the

Kiwada deposit, expressed as mole pro-

portions of grossular (Gro) +pyrope

(Pyr), andradite (And), and almandine

spessartine (Aim + Spe). The dashed line

shows the boundary between "reduced-

type and "oxidized-type" skarns after thedata of EINAUDl et al. (1981).

Fig. 5. Compositions of clinopyroxene

from the Kiwada deposit, expressed as

mole proportions of diopside (Di),

hedenbergite (Hd), and johannsenite (Jo).

The dashed line shows the boundary be-

tween "reduced-type and oxidized-typeskarns after the data of EINAUDI et al.

(1981).

Skarn minerals from the Kiwada deposit

詛

7

3

J

+

6

w

)

/

6

n

7.5 7.25 6.5

Si

酢Fe2+/( Fe2+* F e〕◆)

31

a No.ll

□ No.10

Fig. 6. Plots of Mg/(Mg+Fe'+) versus Si atoms in Fig. 7. Frequency diagramamphibole from the Kiwada deposit. Nomenclature for Fe +/(Feや+Fe3+) ra-

is after LEAKE (1978). See text for the calculation tios of amphibole.

method.

3 to 41%, respectively. These results agree well with the previous results (N

agahara et al., 1978). Spessartine molecules, on the other hand, show larger

variation than four analytical results of Nagahara et al. (1978). Molecular

proportions of spessartine differ grain by grain (Fig. 2), but the systematic

difference about their modes of occurrences are unknown.

Line step scans are carried out for more than 30 garnet crystals in order to

examine the compositional heterogeneity and the representative results are shown

in Fig. 3. Spessartme molecules are sometimes enriched in the cores of garnet

crystals accompanying the depletion of grossular molecules. However, it should be

noted that the compositional heterogeneity is not always observed for garnet

crystals.

Garnet analyses are recast onto the triangular diagram showing the relation

among andradite, grossular+pyrope, almandine+spessartine mole percents (Fig.

4). Almost all of the garnet compositions fall within the "reduced-type" zone.

Although spessartine molecules vary in their proportions, the whole range does not

invalidate the compositional limits for the "reduced-type skarn.

Cl inopyroxene

Molecular proportions of johannsenite in clinopyroxene are less than 14%. In

contrast to garnet, clinopyroxene shows no compositional heterogeneity. Mole

percentages of johannsenite, diopside, and hedenbergite are plotted in Fig. 5. Most

of clinopyroxenes ・are hedenbergitic (Fe/(Fe+Mg)>0.89), but two analyses show

the existence of diopsidic clinopyroxene which coexist with grossularitic garnet.

Amphibole

Amphibole analyses revealed ferro-hornblende to ferro-actinolite compositions

(Fig. 6), and their Fe2+/(Fe2++Fe3+) ratios range from 0.889 to 0.976 (Fig. 7).

Because calcic amphibole often replaces clinopyroxene and is associated with

interstitial quartz and calcite (see compositional data 1 tabulated in Table 3),

formation of the amphiboles could be expressed by the following reaction:

5Ca(Fe,Mg)Si206 + H20 + 3CO2-Ca2(Fe;Mg)5Si8022(OH)2+ 2Si02 + 3CaCO3

32

This reaction is not only sensitive to temperature, pressure, and CO2 fugacity but

also to oxygen fugacity of the hydrothermal solution {e.g., Shoji, 1980).

Magnetite in stead of actinolite appears as the product at higher oxygen fugacity

than FMQ buffer (Fig. 5 in Shoji, 1980). Absence of magnetite in the

clinopyroxene zone suggests that the oxygen fugacity of the hydrothermal solution

for the Kiwada deposit was below FMQ buffer.

Conclusions

Compositional data on garnet and clinopyroxene from the Fujigatam deposit

reveal that these minerals show the feature of the "reduced-type skarn defined by

Einaudi et al. (1981). Fe2十/(Fe2++Fe3+) ratios of amphiboles are from 0.889 to

0.976, supporting the reduced feature.

The assemblage of calcic amphibole, quartz, and calcite after clmopyroxene

suggests that the hydrothermal solution was in a reduced state (the oxygen

fugacity was below FMQ), considering the actinolite stability.

Acknowledgements

This work was supported by the funds of the Cooperative programs (Na 90115

and No. 91035) of the Ocean Research Institute of the University of Tokyo.

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34

山口県喜和田タングステン鉱床産のスカルン鉱物の化学組成

浪江靖弘・藤岡換太郎・石井輝秋

山口県喜和田タングステン鉱床産の柘梱石、単斜輝石、角閃石およびスカルン中の方解

石の化学組成を求めた。

柘相石中の灰鉄柘櫓石分子は17モル%より少なく、単斜輝石中のヨ-ンゼナイト分子は

14モル%より少ない。これらの鉱物の組成値はEinaudi et al. (1981)の「還冠型スカルン」

瑚胡経示してい右脚e 2+/(Fe2++Fe3+)他部.889か功.976であり、やはり還i甜勺な特徴を示

している。