dong yoon yang, yukihiro utsugi* and tadashi mariko****

RESOURCE GEOLOGY,43(1),11•`22,1993

Fluid Inclusion Study on Magnesian Fe Skarn-type

Deposit of the Shinyemi Mine, Republic of Korea*

Dong Yoon YANG**, Yukihiro UTSUGI*** and Tadashi MARIKO****

Abstract: The Shinyemi magnesian Fe skam-type deposit develops in carbonate rocks of the Ordovician Makkol Lime-

stone Formation in contact with Cretaceous porphyritic rocks intruded along the NNW trending faults which could have

been magmatic fluid and meteoric water inflow paths. The exoskarn is mineralogically divided into three types, magne-

sian, intermediate and calcic skarns, which were formed simultaneously by replacement of dolostone, dolomitic limestone

and limestone, respectively.

Microthermometric analysis of primary 2-phase (liquid+vapor) NaCI-H2O fluid inclusions in the skarn and ore

minerals including forsterite, clinopyroxene, garnet, dolomite, calcite and sphalerite indicates that the fluid responsible for

the Shinyemi magnesian Fe skam-type deposit is a medium-salinity (<23 equiv.wt% NaC1) and hot fluid with little CO2.

The formation temperatures, calculated from homogenization temperatures (360•‹-590•Ž) by pressure (500 bars) correc-

tion of prograde skarns were between 510•‹to 620•Ž (at the early first stage) and 400•Ž to 540•Ž (at the late first stage).

The temperature range agrees well with that (400•‹- 610•Ž) by isotopic geothermometry. The early (440•‹- 540•Ž) and late

(340•‹- 400•Ž) magnetite mineralizations in magnesian skam are considered to have occurred simultaneously with early

and late sulfide mineralizations in calcic skarn, respectively. The general trend of decrease in fluid salinity and tempera-

ture with time, resulted from continuous fluid-rock reaction, and/or interaction and dilution of the hydrothermal fluid with

more dilute fluids in retrograde skarn evolution, agrees well with the results of oxygen and hydrogen isotope study.

1. Introduction

Magnesian skarn deposits are known from sev-eral countries including the former Soviet Union,

the United States, Spain and Korea, of which some deposits are economically very important: Sheregeshi and Teya, the former Soviet Union

(SOCOLOV and GRIGOR'EV, 1977) have 234 and 144 million tons ore reserve, respectively. But de-tailed geochemical studies of these have , hardly

been reported. Also, the Shinyemi magnesian Fe skarn had scarcely studied till 1987 when the

present writers started its investigation, except the mineralogical study by LEE (1987).

Since then, the occurrence, succession and zonal arrangement of minerals in the upper mag-netite orebodies and a detailed mineralogical study on the magnetite series minerals and spinel have been reported by MARIKO and YANG (1989) and YANG et al. (1990). Systematics of the oxy-

gen and hydrogen isotope of the Shinyemi magne-sian Fe skarn was discussed by MARIKO and YANG(1990). YANG (1991) documented the min-eralogy, petrology and geochemistry of the depos-its in detail.

In current, integrated studies involving fluid in-clusion and isotopic studies have made to find the

pressure-temperature-fluid composition (P-T-X) conditions of skarn formation environment. Most fluid inclusion studies, however, are focused on calcic skam deposits including tungsten deposits,

porphyry copper related deposits, and zinc-lead deposits (e. g., EINAUDI et al., 1981; MATHIESON

* A part of this study was presented at the Joint Meeting of

the Mineralogical Society of Japan, Society of Mining Geologists of Japan, and the Japanese Association of Mineralogists, Petrologists and Economic Geologists held in Yamaguchi University, on October 3, 1990.

** Science and Engineering Research Laboratory, Waseda University, Tokyo 162, Japan.

*** Department of Mineral Resources, School of Science and Engineering, Waseda University, Tokyo 169, Japan.

****Institute of Earth Science, School of Education, Waseda University, Tokyo 169,Japan.

Keyword : Korea, Shinyemi mine, Magnesian skarn, Forma-tion temperature, Salinity, Isotopic geothermometer

11

Fig. 1 Geological map of the Shinyemi mining area. (a) Map of tectonic division of South Korea and location of the Shinyemimine . 1-Kyeonggi massif, 2-Okcheon belt,

3-Ryeongnam massif, 4-Kyeongsang basin. (b) Main map. (MARIKO and YANG, 1993).

43(1), 1993 Fluid inclusion study on magnesian Fe skam-type deposit of the Shinyemi mine 13

Fig. 2 Geological cross sections of the Shinyemi mining area (The symbols are the same as Fig . lb). The section A-A' and B-B' are shown on Fig. lb. (MARIKO and YANG, 1993)

and CLARK, 1984; ROEDDER, 1984; MEINERT, 1987; Lu et al., 1992). Fluid inclusion studies on magnesian skarn, particularly on olivine skarn, are scarcely reported.

The present work discusses the formation envi-ronment of the Shinyemi magnesian skarn-type Fe

deposit, using a fluid inclusion data together with the previously reported geological and geochemi-cal data by the present writers.

2. Geology of the Shinyemi ore Deposits

The Shinyemi mining area, located in the north-

eastern part of the Okcheon belt, is composed

mainly of Paleozoic sedimentary rocks and Creta-

ceous felsic intrusives (Fig. 1 a and b).

The Ordovician Makkol Limestone Formation,

part of the Choseon Supergroup (KIM, 1987), is

the oldest unit in the mine area. It generally

strikes N20-30•‹E and dips 20-25•‹NW. As shown

in Fig. 1 and 2, the formation distributed in the

mine area is divided into two parts by a NNW

trending fault. The formation in the western part

strikes from NNE to NS, and dips rather steeply

(30-60•‹). It is further cut by several faults trend-

14 D.Y. YANG, Y. UTSUGI and T. MARIKO RESOURCE GEOLOGY:

ing NNW along which porphyritic rocks intrude. These faults may have been a path of the ore-forming fluid, because most orebodies are emplaced in carbonate rocks contacted with por-

phyritic rocks intruded along the faults (YANG, 1991). The Makkol Limestone can be divided lithologically into three units, in ascending order

(Fig. Ib): the lower limestone unit, dolostone unit, and upper limestone unit. The lower limestone unit consists of gray massive impure limestone-dolomitic limestone and an intercalated light gray dolostone (D1, Fig. 1b) with some dolomitic lime-stone. The overlying dolostone unit is composed of two dolostone layers (D2 and D3), calcareous slate layers and gray massive impure dolomitic limestone-limestone. The upper limestone unit consists of a lower thin white limestone bed over-lain by light to dark gray siliceous and argilla-ceous limestone with some intercalated thin cal-careous slate layers. It is unconformably overlain by the Carboniferous Hongjeom Formation of the Pyeongan Super-group. The formation consists of shale, sandstone and conglomerate.

The Shinyemi granodiorite and porphyritic rocks, intruded into the Makkol Limestone For-mation and the Hongjeom Formation. The grano-diorite occurs as a small stock near the east sulfide deposit. Porphyritic rocks form large dike-like masses about 100 to 300 m thick and 400 to 1000 m long. Numerous small dikes of the porphyritic rocks occur near the periphery of the dike-like masses. Intrusive bodies of porphyritic rocks con-sist mainly of quartz porphyry and granite por-

phyry, with small amounts of rhyolite and felsite, although quartz porphyry and rhyolite or felsite

are intergradational. Granite porphyry occurs at deeper levels.

The Makkol Limestone Formation and the Hongjeom Formation subjected locally to contact metamorphism near intrusive rocks. Dolostone marble in the Makkol Limestone Formation con-tains metamorphic skarn minerals such as forsterite, phlogopite and minor amount of spinel. In dolomitic limestone marble, clinopyroxene and tremolite occur.

The Shinyemi mine consists of a magnetite de-

posit and the west and east sulfide deposits (Fig. 1). They occur at or near contacts between car-bonate rocks of the Makkol Limestone Formation

and the Shinyemi granodiorite or porphyritic rocks. Skarn alteration occurs in both carbonate rocks of the Makkol Limestone Formation

(exoskarn) and the related intrusive rocks

(endoskam). The composition of the protolith ex-hibits a strong control on the resultant skarn alter-ation. Magnesian, intermediate and calcic

exoskams occur in dolostone, dolomitic limestone and limestone, respectively (MARIKO and YANG, 1989).

The Shinyemi magnetite deposits consists of the upper and lower orebodies (Fig. 2). Morphol-ogy of mineralization in the upper orebodies is

pipe-like, vein-like or massive at the contact be-tween the upper dolostone layer (D3) of the dolostone unit and quartz porphyry or granite por-

phyry. Orebody B, the largest of the upper mag-netite orebodies, forms a pipe-like mass with

about 60 to 70 m diameter and 120 m long trend-ing N-NE. The magnetite orebodies consist mainly of magnetite series minerals, spinel, forsterite, clinohumite-chondrodite, phlogopite, chlorite and serpentine, forming magnesian skarn,

except in the intermediate skarn of the upper part of Orebody B. The lower magnetite orebodies, about 500 m north of the upper orebodies, were discovered recently (Fig. 2). They are divided into magnesian and intermediate skarns. Magne-

sian skarn replaces the lower dolostone layer

(D2). The mineral composition of magnesian skarn is similar to that of the upper orebodies. On the other hand, magnetite, clinopyroxene and gar-net are the main constituent minerals of the inter-mediate skarn which is formed at the contact be-

tween the dolomitic limestone-limestone layer of the lower limestone unit and quartz porphyry.

The west sulfide deposit, situated at the west end of the upper magnetite orebodies, is devel-oped in and along the bedding planes of the lime-stone of the upper limestone unit near the quartz

porphyry. Mineralization occurs in calcic skarn which are composed dominantly of garnet, vesu-vianite, clinopyroxene, wollastonite, tremolite

and epidote with economically important amounts of sphalerite, chalcopyrite and molybdenite and minor arsenopyrite, pyrite and pyrrhotite. The east sulfide deposit, approximately 600 m south-east of the magnetite deposit, is emplaced at or near the contact between limestone-dolomitic


limestone of the lower limestone unit and the

Shinyemi granodiorite. It forms veins and small

pipes along fissures. Orebodies consist mainly of garnet, epidote and sphalerite with minor galena, chalcopyrite and molybdenite (Mm and Kim,

1978; Kim et al., 1981).

3. Skarn Formation and Ore Mineralization

of Fe deposit

3. 1 Endoskarn

Both the quartz porphyry and the granite por-

phyry dikes are altered near their contacts with

carbonate rocks. The degree of development of

endoskarn in the porphyry dikes varies widely.

The width of endoskam ranges from several centi-

meters to meters. The alteration is characterized

by extensive alteration of plagioclase to potassium

feldspar indicating potassium metasomatism. The

endoskarn mineralogy varies to some extent as a

function of the chemical composition of the car-

bonate rocks in the contact with the altered dikes.

It contains the assemblages K-feldspar•} wollasto-

nite, diopsidic pyroxene, grandite garnet (with

dolostone), K-feldspar+diopsidic-salitic pyrox-

ene •}phlogopite (with dolomitic limestone) and K

-feldspar±grandite garnet , diopsidic pyroxene

(with limestone).

3.2 Exoskarn

3.2.1 Magnesian skarn

The exoskarn formed in four mineralogical

stages. During the first, prograde skarn formation

stage, the metasomatic process yielded the follow-

ing mineral zonation from the dike contact : diop-

sidic-hedenbergitic pyroxene•}grandite garnet•¨

forsterite+early magnetite (magnetite I)•¨

dolostone. Magnetite I and forsterite exhibit a

subparallel banded texture or a compact massive

texture of granular aggregates. Under the micro-

scope, magnetite I generally contains exsolution

bodies of spinel (YANG et al., 1990). Forsterite is

subdivided into two types, olivine I and II, based

on its mode of occurrence. Olivine I, formed in

the early first stage, is fine grained and euhedra to

anhedra. In contrast, olivine II is medium to

coarse grain in size and euhedra to subhedra in

shape. It is considered to be crystallized in the late

first stage. Olivine II is somewhat richer in Fe and

Mn components than olivine I.

The retrograde process began with alteration of

the early formed skarn minerals to phlogopite,

clinohumite-chondrodite, amphibole, dolomite

and calcite, although they are not widespread (the

second stage). In the upper orebodies, late magne-

tite (magnetite II) associated with Mg-rich chlo-

rite (chlorite I)•}dolomite and calcite extensively

replaced the previously formed magnetite I and

forsterite, clinopyroxene - garnet skarns and

dolostone (the third stage). Although magnetite II

+ chlorite I skarn generally exhibits a massive tex-

ture, it occurs infrequently as an interstitial filling

of the brecciated magnetite I + forsterite skarn

which apparently indicates two episodes of mag-

netite mineralization. At the following stage (the

forth stage), previously formed minerals includ-

ing olivine I and II, clinopyroxene, garnet, magne-

tite I and II, and chlorite I were altered to serpen-

tine, Fe-rich chlorite (chlorite II) and carbonate

minerals such as calcite, dolomite, rhodochrosite

and siderite.

3. 2. 2 Intermediate skarn

During the early first stage, magnetite I and di-opsidic pyroxene associated with a small amount of grandite garnet were produced. The diopsidic

pyroxene (clinopyroxene I) crystals were partly to largely replaced by johannseno-hedenbergite

(clinopyroxene II) associated with a small amount of magnetite I and/or grandite garnet during the late first stage. The later garnet is distinguishable from the early garnet by optical anisotropism, euhedral shape, and clear appearance under the microscope although the two types of garnet are similar in chemical composition. Minor amounts of early sulfides including troilite, hexagonal pyr-rhotite, sphalerite, chalcopyrite and galena, which were formed in the late first stage, occur in car-bonate rocks around the magnetite orebodies and in contact areas between limestone and calcareous slate. During the second stage, the early-formed skarn minerals altered to phlogopite, amphibole,

quartz and feldspar. In the lower orebodies, mag-netite II associated with amphibole, quartz and feldspar formed extensively during this stage re-

placing the clinopyroxene skarn. In the upper orebodies, a magnetite II - chlorite I assemblage replaced previously formed skarn minerals during the third stage.

The magnetite II - chlorite I ore occurs as mas-sive aggregates and veins in pyroxene skarn. Pyr-


rhotite associated with chlorite I is found at the pe-riphery of the magnetite II - chlorite I ore replac-ing the pyroxene skarn. It seems to be a product

of the third stage. A minor amount of late sphal-erite usually occurs as interstitial fillings between

garnet crystals which were formed in the third stage. During the latest stage of the retrograde

process (the forth stage) chlorite II and calcite were formed as alteration products in the interme-diate skarn.

3. 2. 3 Calcic skarn

The calcic skam in the magnetite deposit oc-

curs as small intercalations in the magnesian

skarn at the periphery of orebodies. The products

during the first stage are composed of grandite

garnet associated with a subordinate amount of diopsidic pyroxene and a minor amount of vesuvi-

anite. Early mineralization of sulfides including

of small amounts of pyrrhotite, chalcopyrite, ga-

lena and sphalerite is recognized as an event in the

late first stage. The retrograde process in the cal-

cic skarn is characterized by mineralization of

sphalerite and small amounts of pyrrhotite and

molybdenite associated with chlorite I and calcite

during the third stage. Chlorite II and calcite formed during the third and fourth stages.

4. Fluid Inclusion

Fluid inclusions were examined in representa-

tive minerals from three different types of skarn : forsterite and dolomite from the magnesian skarn, clinopyroxene and calcite from the intermediate

skarn, and garnet, sphalerite and the calcite from calcic skam (Table 1) in order to delineate trends of the change in temperature and fluid composi-tion during hydrothermal activity.

4. 1 Analytical methods

A total of 530 inclusions on 36 doubly polished

wafers with a thickness (500-60ƒÊm) suitable for

inclusion observation were measured for homog-

enization temperatures (Th) and freezing tempera-

tures (Tf). Detailed sketch maps of each sample

were prepared so that specific fluid inclusions

could be located and relocated for subsequent

studies. The measurements were made with

Linkam TH 600 RMS heating-freezing stages on a

Nikon Optiphot-pol microscope. Both stages

were calibrated using freezing and melting point

standards described by ROEDDER (1984). The ac-

Table 1 List of samples with their locations and mineral

compositions.

Abbreviations:Up=Upper, Low=Lower, Sul=Sulfide, Dep=Deposits, Mt=magnetite, Fo=forsterite, Cpx=clinopyroxene, Gt=garret, Cal =cal-cite, Chl=chlorite, Dol=dolomite, Ms=magnesite, Phl=phlogopite, Se= serpentine, Act=actinolite, Qz=quartz, Ves=vesuvianite, Sph= sphalerite, Py=pyrite, Gn=galena, Mo=molybdenite,

Cp=chalcopyrite, Po=pyrrhotite, *=measured mineral

curacy are as follows: between -38.86•‹ and 0•Ž,

•}1.54•Ž., between 70•‹ and 398•Ž, •}4.2•Ž. Both

Th and Tf of fluid inclusions were measured at

heating rates of 4•Ž/min. and 0.1•Ž/min., respec-

tively. Replicated measurements showed

reproductivities within •}2•Ž for Th and •}0.1•Ž

for Tf. Salinities in units of equivalent wt % NaCI

are based on freezing point depression in the sys-

tem NaCl-H20 (POTTER et al., 1978). All in cases,

only inclusions considered to be the criteria of

ROEDDER (1984), were used for microthermo-

metric analysis.

4. 2 Sample description and fluid inclusion

morphology

Table 1 shows locality and mineral composition

of the measured samples. Representative fluid in-

clusions from the Shinyemi magnetite skarn are

exhibited in Fig. 3.

Several skarn minerals including forsterite,

clinopyroxene, garnet, sphalerite, dolomite and

calcite were available for fluid inclusion studies.

Care was taken to select samples for fluid inclu-

sion measurements, because the inclusions in

forsterite, pyroxene and garnet are not numerous

enough to random choice of sample are as and then it is common to find only a few inclusions of

43(l), 1993 Fluid inclusion study on magnesian Fe skarn-type deposit of the Shinyetni mine 17

Fig. 3 Representative fluid inclusions from the Shinyemi magnetite deposits (scale bars=50ƒÊm): a) olivine II, 88107: b)

clinopyroxene I, A3619; c) early dolomite, 5L211:d) late dolomite. 5L212: e) early sphalerite, WH2: f) late sphalerite,

2S 10: g) early calcite. WH 13: h) late calcite, 8SA09.


workable size on a chip.

Most inclusions in skarns from the Shinyemi

mine are exceedingly small size, <1 to 15ƒÊm , and

occur in various shapes. They are two phases

(liquid+vapor), except only one inclusion in cal-

cite (88A09) containing daughter mineral consid-

ered to be halite. No vapor-rich or CO2-bearing

inclusions were observed under the microscope in

the samples.

in forsterite: As described in section 3, early

forsterite (olivine I) is so fine-grained (mostly 20-

320ƒÊm) that inclusions of workable size are not

present (<lƒÊm). Inclusions in late forsterite (oli-

vine II), which is medium- to coarse-grained (0.2-

2.5mm), were analyzed in this study. Size of fluid

inclusions ranges from <1 to 15ƒÊm, and mostly

less than 10ƒÊm. Most of them have irregular

shapes, some are ellipsoid, or rectangular in

shape.

in clinopyroxene: Pyroxene used in this study is

medium-to coarse-grained (0.1-3mm) and colum-

nar in shape. Early pyroxene (clinopyroxene I) in

the sample 6M09 and A4883 (see Table 1) were

partly recrystallized to, or replaced by late pyrox-

ene (clinopyroxene II). Both of clinopyroxene I

and II were analyzed in this study. Inclusions in

clinopyroxene are usually larger in size than those

in olivine II, mostly 5 to 15ƒÊm and 50ƒÊm in maxi-

mum. The inclusions are various in their shapes

such as irregular, ellipsoid, or columnar.

in garnet: One sample of garnet (WH13) formed

in the late first stage was analyzed in this study.

Analyzed garnet is optically anisotropic, euhedra

in shape and ranging from 1 to 7mm in size. It was

partly replaced by interstitial calcite. Both garnet

and calcite were measured in this study. Most

fluid inclusions in garnet are rectangular in shape,

and less than 5ƒÊm in size.

in dolomite: Two stages of dolomite were ob-

served in the sample 5L22: crystalline dolomite of

the second stage and dolomite associated with

magnetite II of the third stage. The former is

white to greenish white in hand specimen, and the

latter is transparent with pinkish tint. Their grain

sizes are 0.2 to 3mm and 10 to 20mm, respec-

tively. Inclusions in early dolomite occur as clus-

ters of irregularly shaped inclusions and range

mostly from 10 to 50ƒÊm in size. In contrast, those

in late dolomite array along crystal planes, and

have ellipsoidal, rectangular or columnar shapes

and size of 10 to 30ƒÊm.

in calcite: Sample 88A09 includes intergranular

calcite filling up between pyroxene grains. Cal-

cite in sample WH13 replaces garnet and pyrox-

ene. Inclusions in 88A09 are about 10ƒÊm in size

and columnar shaped, aligning along planes.

Those in WH13 array along crystal planes and ex-

hibit ellipsoidal (5-10ƒÊm) and rectangular (10-

20ƒÊm) shapes.

in sphalerite: Sphalerite in calcic skarn is subdi-

vided into early sphalerite formed at the late first

stage, and late sphalerite at the third stage. The

former usually occurs as intergranular grains in

previously formed garnet and clinopyroxene

skarn. Early sphalerite is 0.2 to 3mm in size, yel-

lowish to reddish brown in color. Late sphalerite,

in contrast, replaces the garnet and pyroxene

skarn, and is usually associated with calcite, 1 to

5mm in size and yellowish to reddish brown.

Both early (WH2 and 6S12) and late (2S10)

sphalerites were tested for inclusion measurement

in this study. Most fluid inclusions in early

sphalerite are columnar, tetrahedron or trigonal

shaped, less than 10ƒÊm in size. On the other hand,

inclusions in late sphalerite are various in shape

and size and they occur as scattered groups of one

or two columnar shaped inclusions, 10 to 50ƒÊm in

size or as clusters of hexagonal prismatic shaped

(about 10ƒÊm) or ellipsoidal shaped (<10pm) in-

clusions.

4. 3 Homogenization temperatures and

salinities

The histograms of homogenization tempera-

tures and salinities of fluid inclusions in each min-

eral are shown in Figs. 4 and 5, respectively.

All fluid inclusions analyzed in this study ho-

mogenized to a liquid phase without any phase

change between the room temperature (25•Ž) and

homogenization temperature (Th). The inclusions

froze between -40•‹ and -60•Ž. The fact that no

expansion of the bubble was recognized when

they were frozen below -70•Ž, indicates that the

inclusions have little CO2.

There are no correlation between Th or salini-

ties and fluid inclusion size. Although only inclu-

sions considered to be primary, according to the

criteria of ROEDDER (1984), were attempted to

analyze, distinction of the primary inclusions


Fig. 4 Histograms of fluid inclusion homogenization

temperatures from the Shinyemi magnetite deposits.

from the secondary origin was very difficult in

skarns, because of their tiny sizes. As shown in

Figs. 4 and 5, The Th and salinity of inclusions in

each mineral show wide ranges resulted from

leakage or necking down of a part of fluid inclu-

sions. Therefore, reasonable Th and salinity

ranges (Table 2) of each mineral were determined

as clusters with one or two modes in Figs. 4 and 5.

The Th and salinities of inclusions in prograde

skarn minerals range from 360•‹to 590•Ž and

from 13 to 29 equiv. wt % NaCl, respectively.

The retrograde skarn range from 190•‹ to 430•Ž

and from 0 to 23 equiv. wt % NaCl, in salinity

(Table 2).

4. 4 Pressure correction of Th

Homogenization temperatures of fluid inclu-

sions should be corrected to the formation tem-

Fig. 5 Histograms of fluid inclusion salinities, as equiv.

wt % NaCl, from the Shinyemi magnetite deposits.

peratures at corresponding pressures. A maxi-

mum pressure of 500•}200 bars for the Shinyemi

skarn has been estimated by YANG (1991) using

the FeS mole fraction in sphalerite from a

sphalerite + hexagonal pyrrhotite + pyrite assem-

blage (SCOTT, 1973). Calculated formation tem-

peratures for Th (190•‹to 400•Ž) of each mineral

can be estimated based on the pressure with the

isochore correction technique of POTTER (1977).

Since the pressure-correction graphs by POTTER

(1977), however, cover only the range between 20

and 400•Ž, the pressure-correction for Th higher

than 400•Ž were made using the extrapolation of

20 D.Y. YANG, Y. UTSUGI and T. MARIKO RESOURCE GEOLOGY,

Table 2 Homogenization and calculated formation tem-

peratures(•Ž), and fluid salinities (equiv. wt% NaCl)

of the minerals from the Shinyemi mine.

* Pressure-corrections for Th>400•Ž were made using the ex-

trapolation of Potter's (1977) graphs.

Abbreviation: Cpx I=early clinopyroxene, Cpx II=late clino-

pyroxene, 01 11 =late olivine, Gt II=late garnet, Sph I=early

sphalerite, Sph II=late sphalerite, Dol I= early dolomite, Dol

II =late dolomite, Cal I=early calcite, Cal II=late calcite

the POTTER'S graphs. The calculated formation

temperatures for each mineral are 30•‹ to 50•Ž

higher than the homogenization temperature

(Table 2).

Garnet shows large variation in homogeniza-

tion temperatures and salinities, ranging from

250•‹to 360•Ž with modes of 300•‹ and 310•Ž,

from 6 to 19 equiv. wt % NaCl, respectively. Cal-

culated formation temperatures of garnet shows

the range from 290•‹ to 400•Ž, which are lower

than those (340•‹ - 400•Ž) of interstitial calcite be-

tween the garnet grains. On the basis of field and

microscopic observations, garnet in all three types

of skarn is considered to have been formed during

the prograde skarn formation. The unreasonably

low temperatures possibly indicate that most of

the measured inclusions in garnet may be the sec-

ondary origin which could not be differentiated

from the primary origin under the microscope due

to their tiny sizes.

5. Discussion

The calculated formation temperature range of

olivine II (Table 2) agrees well with the range

(from 420•‹to 550•Ž) of the olivine II-magnetite I

isotopic temperatures (MARIKO and YANG,1990;

1.993; YANG, 1991). It is consistent with the for-

mation temperature range (400•‹- 650•Ž) of the

prograde magnesian skarn based on the T-XC02

phase diagram for the system MgO-Si02-H2O-

CO2 (MARIKO and YANG, 1989). The calculated

Table 3 Formation temperatures (•Ž) and fluid salinities

(equiv.wt% NaCI; in parenthesis) at each, stage in three

types of skam from the Shinyemi mine.

formation temperature ranges of clinopyroxene I

and II in intermediate skarn agree also well with

the range (from 400•‹to 610•Ž) of clinopyroxene-

magnetite I isotopic temperatures. The crystalli-

zation sequence of skarn minerals based on field

and microscopic observations are consistent with

the formation temperature ranges of each mineral.

Table 3 shows the formation temperatures to-

gether with fluid salinities of minerals from three

types of skarn at each stage in the Shinyemi mag-

netite mineralization, which have been deter-

mined by all accounts of fluid inclusion, isotope

and phase equilibrium studies. Plot of formation

temperature vs fluid salinity for minerals at the

variable stages in the three types of skarn are

shown in Fig. 6. The formation temperatures and

fluid salinities exhibit the general trend of de-

crease with time in each types of skarn. Further-

more, the formation temperatures at each stage

are substantially the same in the three types of

skarn as shown in Table 3 and Fig. 6. This is an

important evidence supporting that magnesian, in-

termediate and calcic skams were simultaneously

formed by replacement of dolostone, dolomitic

limestone and limestone, respectively (MARIKO

and YANG, 1989). In addition, the early and late

magnetite mineralizations in magnesian skarn are

also considered to have formed simultaneously

with early and late sulfide mineralizations in cal-

cic skarn, respectively, based on their estimated

formation temperatures (Table 3) and field obser-

vations.

Although the fluid salinities of magnesian skarn


Fig. 6 Formation temperature vs salinity plot of fluid in-

clusions in magnesian (01 II, and Dol I and II), interme-

diate (Cpx I, Cpx II and Cal II) and calcic (Sp I, Sp II

and Cal I) skams from the Shinyemi mine.

Abbreviations: Cpx I=early clinopyroxene, Cpx II=late

clinopyroxene, 01 II=late olivine, Dol I=early dolo-

mite, Dol II=late dolomite, Sp I=early sphalerite, Sp

II=late sphalerite, Cal I=early calcite, Cal II=late cal-

cite.

at the second stage scarcely changed compared

with those at the first stage, the salinities at the third stage reduced to about a half of those at the second stage (Fig. 6). At the forth stage, the fluid in the intermediate skarn had very low salinities corresponding to about one tenth of those at the first stage in the magnesian and intermediate skams. The remarkable decrease of fluid salini-

ties at the third and fourth stages is consistent with hydrogen and oxygen isotope study (MARIKO and YANG, 1990; YANG, 1991).

It seems that there are no noticeable differences between the fluid salinities at each stage in mag-nesian and intermediate skarns, though lacking enough data. The fluid salinities in calcic skarn, however, are apparently lower at the first and sec-ond stages than those in the other two types of

skarns. It is difficult to explain the low salinities only by the present data. More investigation for this problem is needed.

As described before, C02-rich fluid inclusions were not observed in the Shinyemi magnetite skarn, which indicates that it was formed by C02-

poor fluid. This agrees with the result by TAYLOR and O'NEIL (1977) that XCO2 in fluid permeating carbonate rocks during metamorphism and meta-

somatism is generally quite low, perhaps less than

0.1, based on oxygen isotope and fluid inclusion

data. The XCO2 of the fluid generating the magne-

sian skarn was estimated to be less than 0.07

(MARIKO and YANG, 1989). In addition, serpent-

inization of forsterite-bearing magnesian skarn in

low pressure environments (e. g., 500 bars) im-

plies not only XCO2 less than 0.05, but also tem-

peratures less than 420•Ž (GREENWOOD, 1967).

Acknowledgements: This study was supported in

part by a Grant-in-Aid for Fundamental Scientific

Research from the ministry of Education, Culture

and Sciences in Japan, especially project No.

01303006 awarded to Professor H. SHIMAZAKI,

and also by a Research-Aid Expenses for Speci-

fied Subjects from Waseda University.

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韓国・新礼美マグネシウムスカルン型

磁鉄鉱鉱床の流体包有物の研究

梁東潤・宇津木弘之・鞠子正

要旨:新礼美マグネシウムスカルン型磁鉄鉱鉱床は,オ

ルドビス紀のマッコル石灰岩層とNNW方向の断層に

沿って貫入する白亜紀の斑状質岩との接触部に発達す

る.スカルンは,苦灰岩,苦灰質石灰岩および石灰岩を

各々交代し同時に形成されたマグネシウムスカルン,中

間型スカルンおよびカルシウムスカルンに分けられる.

かんらん石,単斜輝石,ざくろ石,ドロマイト,方解石

および閃亜鉛鉱等のスカルン鉱物に含まれる流体包有物

の均質化温度と塩濃度が測定された.包有物は,殆ど

C 02(<0.07)を含まない気液2相の液相包有物である.測

定された流体包有物の均質化温度は190℃ から590℃ の範

囲であり,その塩濃度(NaCl相当濃度)は12～23%の値を

示す.均質化温度に対する圧力補正(500bar)後の温度は

240℃ から620℃ の範囲を示す.その中で,累進スカルン

の形成温度は400℃ から650℃ の範囲と推定され,同位体

地質温度計による温度範囲(400℃-610℃)と殆ど一致す

る.この温度範囲は他の多くの鉱床におけるスカルン鉱

物形成温度範囲(400℃-650℃)によく一致するが,塩濃度

は他の多くの鉱床の塩濃度範囲(10-50%)より多少低い値

を示す.

マグネシウムスカルンと中間型スカルンには2回の磁

鉄鉱鉱化作用が認められるが,特に,マグネシウムスカ

ルンの早期鉱化作用(第1ステージ;440-540℃)と後期鉱

化作用(第3ステージ;340-400℃)は,カルシウムスカル

ンの硫化物鉱化作用の早期(450-530℃)と後期(340-400℃)

鉱化作用の各々と同時期に形成されたと推定される.

本鉱床のスカルン鉱物中の流体包有物の均質化温度と

塩濃度は,地質調査と鏡下で決められた鉱物の晶出順序

に従って低下する傾向がある.これは,鉱液と岩石問の

連続的反応と共に鉱液への天水の混入率が増加した為で

あると考えられ,この結果はまた酸素・水素同位体の研

究結果とよく一致する.

dong yoon yang**, yukihiro utsugi*** and tadashi mariko****

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dong yoon yang, yukihiro utsugi* and tadashi mariko****