fluid inclusion study on the itaga tungsten ... - j-stage home

J. Japan. Assoc. Min. Petr. Econ. Geol. 82, 353-361, 1987

Fluid inclusion study on the Itaga tungsten deposit , Ashio district, Japan

YASUHIRO SHIBUE

Geoscience Institute, Hyogo University of Teacher Education, Yashiro-machi, Kato-gun, Hyogo 673-14, Japan

Fluid inclusion study on the Itaga tungsten deposit is carried out in order to characterize the

hydrothermal solution responsible for this deposit . Polyphase fluid inclusions and vapor-rich ones

as well as liquid-rich inclusions are found in quartz of the disseminated ore in the Itaga granite.

Vapor-rich and liquid-rich inclusions are found in vein quartz, scheelite, and topaz from this

deposit, but polyphase inclusions are not found in these minerals .

Homogenization temperatures of primary fluid inclusions in quartz of the disseminated ore,

vein quartz, scheelite, and topaz range from 518•K to 261•Ž , from 388•K to 265•Ž, from 336•K to 301•Ž,

and from 420•K to 370•Ž, respectively. Salinities of primary fluid inclusions in quartz of the

disseminated ore, vein quartz, scheelite, and topaz range from 45 .1 to 0.9, from 21.0 to 8.3, from 7.9

to 5.3, and from 14.3 to 3.1 NaCl eq. wt%, respectively. The highest temperature and salinity

reported in the present study are higher than any other data previously obtained for the Japanese

tungsten deposits.

Based on the plots of homogenization temperature against salinity, it is suggested that there

were at least two hydrothermal solutions, i.e., very saline and dilute ones, responsible for the

mineralization of the Itaga deposit.

Introduction

There have been many studies on the

Japanese tungsten deposits related to the

granitic activities. Physico-chemical condi

tions for the formation of several Japanese

tungsten deposits have been studied by the fluid

inclusion technique, which is one of the most

useful methods for the characterization of

hydrothermal solution with respect to tempera

ture and salinity. Previous fluid inclusion data

on the Japanese tungsten deposits are summa

rized in Table 1 . These works show that tem

peratures of the hydrothermal solutions for

several Japanese tungsten deposits are higher

than 300•Ž during some stages of these mineral

izations.

Among the tungsten deposits in Japan, the

Kaneuchi, Takatori, Fujigatani, Kiwada,

Kagata, and Yaguki deposits are hosted by

sedimentary rocks (e. g., Shibue, 1984, 1986a).

Shibue (1984) and Shimazaki et al. (1986) con

sidered that the hydrothermal solutions for the

Kaneuchi, Kuga, and Kagata deposits were

interacted with the surrounding sedimentary

rocks preceding the mineralizations. Mori

shita et al. (1982), on the other hand, showed

that the hydrothermal solutions for the Kaneu

chi and Ohtani deposits were mixed with mete

oric water during the mineralizations. Inter

action with sedimentary rocks and/or mixing

with meteoric water obscure the chemistry of

hydrothermal fluid in equilibrium with the

granitic magma. Further studies on the nature of hydrothermal fluid in equilibrium with

granitic magma are required in order to consider the relationship between the environment

for the formation of the Japanese tungsten

(Manuscript received, June 27, 1987; accepted for publication, August 29, 1987)

354 Yasuhiro Shibue

Table 1. Summary of the previous fluid inclusion data on Japanese tungsten deposits#

#

: Major ore minerals are wolframite and/or scheelite.: Data on primary fluid inclusions.

**: As measured values were not shown, those data are obtained by graphical interpolation .

N. A.: Not analyzed.

Fluid inclusion study on the Itaga tungsten deposit 355

deposits and granitic activities.

The Itaga tungsten deposit is located about

100km north of Tokyo (Fig.1). Wolframite,

pyrite, and topaz are disseminated in porphyritic granite (Shibata, 1967). Quartz veins

containing wolframite is found within the gran

ite body. Microscopic observations and X-ray

microprobe analyses revealed the presence of

minor amounts of scheelite in the disseminated

ores. Bulk composition of the granite

(Shibata, 1955), and the absence of magnetite and the presence of ilmenite in trace amounts in

10 sheets of thin section of the granite (present

study) indicate that the granite belongs to the

ilmenite-series defined by Ishihara (1977).

Main purposes of the present study are to

report the fluid inclusion data for the Itaga

tungsten deposit, and to compare the data with

those for other Japanese tungsten deposits.

Sample Materials and Fluid Inclusion Tech

nique

Sample materials examined are quartz,

scheelite, and topaz from the disseminated ores

and vein quartz within the granite body. Dou

bly polished plates, approximately 0.1mm

thick, were prepared from the sample mate

rials. In each hand-specimen, one to five fluid

inclusions were measured.

Heating and cooling measurements were

made on a Chaixmeca apparatus (Poty et al.,

1976). The stage was calibrated for the melt

ing temperatures of pure reagents and Merck

standards. Temperature measurements are

Fig. 1. Locality map of the Itaga deposit and the geologic map of the surrounding area after Shibata

(1967) and Kawata and Isomi (1977). Basement is composed of Permian to Jurassic sedimentary rocks.

356 Yasuhiro Shibue

accurate within •}5•Ž between 150•K and 550•Ž.

Repeated measurements of homogenization

temperatures showed a reproducibility

within •}2•Ž. The cooling stage was calibrat

ed for the NaCI solutions of known concentra

tions. Repeated measurements of melting tem

peratures of ice showed a reproducibility

within •}0.5•Ž between 0•K and -23•Ž. The

uncertainty of determination is correspondingly

within 1 NaC1 eq. wt%. Salinities of fluid

inclusions composed of liquid and vapor phases

were calculated from the melting temperatures

of ice, using the equation given by Potter et al.

(1978). Salinities of salt-bearing fluid inclu

sions were determined by the disappearance

temperatures of the solid phase under the pres

sure condition of the coexistence of vapor and

liquid phases upon heating (Potter et al., 1977).

Primary fluid inclusions were identified by the

empirical criteria proposed by Roedder (1984).

Results and Discussion

Three types of primary fluid inclusions

were recognized in the present study (Fig. 2).

Type ‡T: Liquid-rich two-phase inclusions

which homogenize to liquid phase upon heating.

Type ‡U: Liquid-rich polyphase inclusions

which homogenize to liquid phase upon heating.

Solid phase is transparent and isotropic, and

dissolves completely before the disappearance

of vapor phase upon heating.

Type ‡V: Gas-rich two-phase inclusions which

homogenize to vapor phase upon heating.

Neither liquid CO2 nor CO2 hydrate was

Fig. 2. Photomicrographs of fluid inclusions. White bars show 0 .05 mm.

(1) Type ‡T and Type ‡V inclusions in scheelite. (2) Type ‡U inclusions in quartz. (3) Type

‡V inclusions in quartz. (4) Type ‡V inclusions in topaz . Classification of the type of fluid

inclusion is shown in the text.


observed in all inclusions upon freezing.

Primary inclusions occurred randomly, and

distributed three-dimensionally throughout the

examined crystals. It was failed to observe

fluid inclusions occurred in planar arrays or

those regularly distributed in growth zones

within the examined crystals.

Fluid inclusions in scheelite and topaz are

of type ‡T and ‡V. Inclusions of these types are

distributed within the same grain of scheelite

and/or topaz. Type ‡U inclusions are not found

in these minerals. Fluid inclusions of the three

types are often found within the same grain of

quartz of the disseminated ore. It is some

times observed that type ‡U inclusions are adja

cent to type ‡T or type ‡V inclusions within the

same grain of quartz of the disseminated ore.

Type ‡T and ‡V inclusions are observed in vein

quartz, but type ‡U inclusions are not observed.

As the fluid inclusions occurred in the growth

zones are not observed, entrapment of fluid

inclusions from a heterogeneous fluid or a

changing fluid is not confirmed based on the


Homogenization temperatures and salin

ities of fluid inclusions are listed in Table 2.

Homogenization temperatures and salinities of

primary fluid inclusions in quartz of the dis

seminated ore range from 518•K to 261•Ž, and

from 45.1 to 0.9 NaCl eq. wt%, respectively.

Most homogenization temperatures are above

350•Ž, and the highest temperature is above

500•ŽC. This temperature is higher than any

other data reported for the Japanese tungsten

deposits (Table 1). Present measurements

quantitatively show that very saline hydro

thermal solution was responsible, in part, for

the crystallization of quartz of the disseminated

ore from the Itaga deposit. Fluid inclusions of

type I in vein quartz show almost the same

range of homogenization temperature and salin

ity as type I inclusions in quartz of the dis

seminated ore. Homogenization temperatures

and salinities of primary fluid inclusions in

scheelite range from 336•K to 301•Ž, and from 7.9

to 5.3 NaCI eq. wt%, respectively. Fluid inclu

sions in scheelite are lower in homogenization

temperature and salinity than the average

homogenization temperature (358•Ž) and salin

ity (17.0 NaCl eq. wt%) of fluid inclusions in

quartz of the disseminated ore and vein quartz.

Homogenization temperatures and salinities of

primary fluid inclusions in topaz range from

420•K to 370•Ž, and from 14.3 to 3.1 NaCl eq.

wt%, respectively. These temperatures are

higher than the average value of those in quartz

of the disseminated ore and vein quartz, and are

also higher than those in scheelite. Salinities

of fluid inclusions in topaz are lower than most

of those in quartz of the disseminated ore and

vein quartz, but similar to those in scheelite.

At present, the cause for the large scatter

of the homogenization temperature and salinity

data is not obvious. Based on the facts that

fluid inclusions of different types are sometimes

found to be located adjacently within the same

grain of quartz of the disseminated ore, it is

difficult to consider the different stage as the

cause for the entrapment of the fluid inclusions

of different types. Boiling phenomenon as this

cause is not confirmative on the basis of the


One of the possible reasons for the existence of

the inclusions of different types within the same

grain and the large scatter of the homogeniza-

tion temperature and salinity data is that the

hydrothermal fluid for the Itaga deposit became

mixture of saline solution with dilute one

locally during the course of the mineralization.

Two distinct trends of hydrothermal solu

tions are at least documented on the plots of

homogenization temperature against salinity

(Fig. 3). Two of the homogenization tempera-

tures of very saline inclusions (type ‡U) almost

overlap with the lowest temperature among

those shown by the low-salinity inclusions (type

358 Yasuhiro Shibue

Table 2. Homogenization temperatures and salinities of pri

mary fluid inclusions from the Itaga deposit.

*: Classification of the type of fluid inclusion is shown in the text.**: Extrapolated values, using the equation given by Potter at al. (1978) .

‡T and ‡V). It can be considered that heteroge

neous solutions, composed of saline and dilute

ones, caused the crystallization of quartz of the

disseminated ore. Decrease in salinity with

decrease in homogenization temperature sug

gests a possibility that hydrothermal fluid for

the Itaga deposit was originally high in temper

ature and salinity, and that the fluid was mixed

with the other water, e. g., circulated under

ground water, resulting in the decrease in tem

perature and salinity.

Conclusions

1. Polyphase and vapor-rich fluid inclu-

sions as well as liquid-rich ones are found in

quartz of the disseminated ore in the Itaga

granite. Vapor-rich and liquid-rich inclusions

are found in vein quartz, scheelite, and topaz,

but polyphase inclusions are not found in these

minerals.


Fig. 3. Plots of homogenization temperature (T,,) against salinity of fluid inclusions. Open and solid

circles show the data for quartz and vein quartz, respectively. Crosses and squares show the

data for scheelite and topaz, respectively. Two fluid groups are shown by enclosing areas.

2. Very saline hydrothermal fluid, whose

temperature is higher than 500•Ž, is responsible,

in part, for the mineralization of the Itaga

deposit. The highest homogenization tempera

ture and salinity obtained in this study are

higher than any other data reported for the

Japanese tungsten deposits.

3. It is shown that there were at least two

distinct hydrothermal solutions, very saline and

dilute ones, during the mineralization based on

the plots of homogenization temperature

against salinity. Quartz precipitated both

from saline and dilute ones, whereas topaz and

scheelite precipitated from dilute solution.

Acknowledgements : The author wishes to

thank Professor Akira Tokuyama of the Geo

science Institute of the Hyogo University of

Teacher Education for his encouragement dur

ing the preparation of the manuscript. The

author also wishes to thank Professor H.

Shimazaki and Dr. N. Shikazono of the Geolog

ical Institute of the University of Tokyo for

their valuable suggestions. Professor H.

Shimazaki, Drs. N. Shikazono and M. Shimizu,

and Mr. T. Shiozawa of the University of

Tokyo collaborated in the sampling at the

Itaga granite. Topaz samples are from the

University Museum of the University of Tokyo.

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足尾地区,板荷タングステン鉱床の流体包有物の研究

澁江靖弘

板荷鉱床を形成した鉱液の特徴を調べるため,斑状花崗岩の鉱染鉱中の石英,灰重石,黄玉,及び花崗

岩体中の脈石英に含まれる流体包有物の均質化温度と塩濃度を測定した。

流体包有物には,液相包有物,気相包有物,多相包有物の3種類のものがあり,石英中にはこの3種類

の包有物が見られる。また,他の鉱物中では前二老の種類が見られる。石英,脈石英,灰重石,黄玉中の

流体包有物の均質化温度はそれぞれ,518゜-201℃,388゜-265℃,336.一301℃,410゜-376℃ であり,塩濃度はそ

れぞれ45.1-0.9,21.0-8.3,7.9-3.3,14.3-3.1NaCI eq. wt%であった。均質化温度と塩濃度の最高値

は他の日本のタングステン鉱床について得られている値よりいずれも高い。均質化温度と塩濃度の関係か

ら見ると,二つの果なった熱水のトレンドが存在すると思われる。

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