Geochemical Journal, Vol. 36, pp. 391 to 407, 2002
The geochemical behavior of altered igneous rocks in
the Tertiary Gampo Basin, Kyongsang Province, South Korea
CHANG -BOCK IM,1* SANG-MO KOH,2 HO-WAN CHANG3 and TETSUICHI TAKAGI4
1
Nuclear Safety Research Department, Regulatory Research Division, Korea Institute of Nuclear Safety (KINS),
P.O. Box 114, Yusung, Taejon 305-600, South Korea
2
Geology Division, Korea Institute of Geoscience and Mineral Resources, Yusung, Taejon 305-350, South Korea
3
School of Earth and Environmental Sciences, Seoul National University, Seoul 151-742, South Korea
4
Research Center for Deep Geological Environments, National Institute of Advanced Industrial Science and
Technology (AIST), Chuo-7, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567, Japan
(Received May 17, 2001; Accepted March 16, 2002)
The area that is located in the Tertiary Gampo basin is mainly composed of Cretaceous shale, Tertiary
rhyolite, granite, and basalt in ascending order.
The mineralogy, isotopic composition ( δD and δ18O) of clay minerals, and chlorite geothermometry
were performed to interpret the alteration history of these rocks. Clay minerals occurring in basalt have a
limited range and are heavier in δ18O than the clay minerals in granite and rhyolite. Chlorite occurring in
basalt has lower formation temperatures than chlorite in granite or rhyolite.
Geochemical studies were carried out to identify the behaviors of major, trace and rare earth elements
(REE) during alteration processes. Most major oxides such as SiO2, Al2O3, Fe 2O3, Na2O, MgO, TiO2,
MnO, and P 2O 5 of basalt are relatively immobile in most altered zones. In contrast, these oxides in granite
and rhyolite are relatively mobile, and show some irregular variations in the most altered zones. Some
trace elements in basalt have less prominent variations than in granite and rhyolite. REE distributions of
altered basalt and granite do not show prominent variations with increasing alteration degree, whereas
rhyolite is enriched in REE and has a positive Eu anomaly.
These results indicate that altered basalts that have experienced only low temperature alteration such
as weathering, have indistinct variations of major oxides, some trace elements and REE according to
alteration intensity. Granite and rhyolite that have experienced both hydrothermal alteration (low temperature) and weathering process are characterized by relatively prominent variations of major oxides, some
trace elements and REE according to alteration intensity. The results coincide well with those of mineralogical studies, isotopic compositions of clay minerals, and chlorite geothermometry.
Chesworth et al., 1981; Gascoyne and Cramer,
1987; Middelburg et al., 1988). In a thermodynamic sense, systems of chemical weathering and
hydrothermal alteration are invariably open and
irreversible (Cramer and Nesbitt, 1983; Fritz and
Mohr, 1984).
The purpose of this study is to investigate the
elemental behavior and mineralogical changes in
zones of variable degree of alteration, and to estimate the physicochemical environments that produced these changes. It is expected that these types
I NTRODUCTION
The variations of chemical composition and
mineralogy with increasing degree of alteration
reflect the nature of specific alteration processes.
The relative mobility of elements is controlled not
only by primary factors such as the mineralogy
and texture of the parent rock, but also by secondary processes such as dissolution of primary
minerals, formation of secondary phases, redox
processes, and ion exchange (Nesbitt et al., 1980;
*Corresponding author (e-mail: [email protected])
391
392
C.-B. Im et al.
Fig. 1. Location map of the study area.
of study will contribute to the selection processes
of a preliminary site for Korea’s nuclear waste
disposal in the near future. In addition, these types
of study can provide geological and geochemical
information such as the behavior and mobility of
radiogenic nuclides that might be derived from the
waste disposal facilities. The study area is situated in the construction site of the Wolsung Nuclear Power Plant (WNPP) on the southeastern
coast of the Korean Peninsula, as shown in Fig. 1.
GENERAL G EOLOGY
The detailed geology of the study area consists mainly of Cretaceous shale and Tertiary volcanic and plutonic rocks such as rhyolite, granite,
andesitic lapilli tuff, and basalt in ascending order (Fig. 2). The geology of the study area and its
vicinity has been studied by many geologists (Choi
et al., 1988; Moon et al., 1989; Lee et al., 1992).
KEPCO (1995, 1996) has carried out various
geotechnical investigations to select the most suit-
Fig. 2. Geologic map and sample location. A-1 and
similar symbols represent the drilling sites.
able site of the WNPP. The area is characterized
by complicated geological structures developed by
tectonic movements during late the Cretaceous to
late Tertiary period (Yoon, 1992; Kee and Doh,
1995; Chang and Baek, 1995).
Basalt (the youngest rock) mostly occurs as
dikes of various widths and intrudes Tertiary
rhyolite, granite, and andesitic lapilli tuff. Jin et
al. (1988) and Lee et al. (1992) reported that the
age of the rock is early Miocene (18.50 Ma to
21.07 Ma) based on K-Ar whole rock methods.
This rock appears to be the least fractured, compared with other rocks, and has experienced the
most severe weathering processes. The basalt usually seems to have intruded along fault zones.
Granite with irregular zonal shapes is intruded
into older Cretaceous shale and Tertiary rhyolite,
and is intruded by the younger basalt. Jin et al.
(1988) presented K-Ar ages of early to middle
Eocene (46.48 ± 2.47 Ma to 58.32 ± 7.82 Ma).
This rock is less fractured than rhyolite. In fact,
the frequency of the fracture occurrence in gran-
Tertiary Gampo Basin, Kyongsang Province, South Korea
393
Fig. 3. Cross section (A–B in Fig. 2) and projected sample locations.
ite increases with proximity to rhyolite contacts.
Rhyolite occupies the largest part of the study
area, intrudes the Cretaceous shale, and is intruded
by granite and basalt. In some drill-core profiles,
rhyolite and shale are usually in fault contact.
During the rhyolitic volcanic activity between late
Paleocene and early Eocene, rhyolite and related
pyroclastics such as welded tuffs and volcanic
breccias were erupted (Yoon, 1992). In the most
of areas, the rock is highly fractured and contains
several intersecting faults associated with
brecciated zones. Hornfels often occurs in the contact part of Cretaceous shale and rhyolite.
FIELD AND L ABORATORY METHODS
Sample preparation
Thirty-seven samples were collected from ten
drill-core profiles ranging from EL. –8.04 m to
EL. –61.84 m and one wall of excavated blocks
for reactor site. Each sample was divided into five
zones such as fresh parent rock (F), slightly altered zone (SA), moderately altered zone (MA),
highly altered zone (HA), completely altered zone
(CA), which are based on the “ISRM (1975)’s
scale”. Sample locations and cross section are illustrated in Figs. 2 and 3, respectively. Each sample of the selected drill-cores is traversed by openfractures at various angles, and all appear to be
water-saturated given the observed groundwater
level (EL. +2~+10 m from KEPCO, 1995). These
5 chips (total weight is about 0.1–0.5 kg) in each
altered zone were collected. Five groundwater
samples were also taken from five drill-holes between EL. –70 m to EL. –100 m.
Analytical techniques
Each drill-core sample was cut or handpicked
along the short axis parallel to the fracture surface. Small pieces were prepared for making about
100 thin sections for microscopic observation and
electron probe micro-analysis (EPMA) at KIGAM
(Korea Institute of Geoscience and Mineral Resources) and KBSI (Korea Basic Science Institute), respectively. Larger pieces were used for
analysis as follows: being properly crushed, powdered and homogenized, each piece was analyzed
by X-ray diffractometer (XRD), and then analyzed
394
C.-B. Im et al.
at the Activation Lab., Canada, by Inductively
Coupled Plasma-Atomic Emission Spectrometry
(ICP-AES) for major elements and by Inductively
Coupled Plasma-Mass Spectrometry (ICP-MS) for
trace elements. δD and δ 18O isotopes of separated
clay minerals were analyzed by mass spectrometry
(MS) at KBSI and at Geochron Lab., U.S.A., respectively. The clay minerals under 2 µm size fraction were separated using Stock’s method, and
bulk samples were analyzed by X-ray diffraction
powder method. The samples were measured
through 3° to 45° (2θ) using Cu-Kα radiation with
a Rigaku X-ray diffractometer at KIGAM.
The temperature, pH, Eh, electric conductivity (EC), alkalinity, anion and cation for
groundwater were measured by the in-situ Hatch
Field Chemistry Kit and Ion-chromatography (IC)
of geochemistry lab. of SNU (Seoul National University), and major elements and trace elements
including REE by ICP-MS at Activation Lab. in
Canada. Stable isotopes ( δ D, δ 18 O) of
groundwaters were also analyzed at KBSI.
Table 1. Mineral constituents of fresh and altered basalt, granite, and rhyolite
Major minerals
Parent rock
Hydrothermal alteration stage
Weathering stage
Fresh rock
SA
MA
HA
CA
䊉
䊉
×
×
×
×
䊐
䊎
䊊
䊊
×
×
䊊
䊐
䊊
䊐
×
×
×
䊊
䊐
䊐
䊊
×
×
䊊
䊐
䊐
䊊
䊊
Basalt (16.22~22.70 Ma)
pyroxene
feldspar
chlorite
smectite
kaolinite
calcite
䊉
䊉
×
×
×
×
Granite (46.48~58.32 Ma)
quartz
biotite
muscovite
feldspar
䊉
䊉
䊉
䊉
×
×
×
×
䊉
䊉
䊉
䊉
䊉
䊎
䊎
䊎
䊎
䊐
䊐
䊐
䊐
×
䊊
䊊
䊊
×
×
䊊
chlorite
epidote
illite
smectite
calcite
×
×
×
×
×
䊉
䊉
䊉
䊉
䊉
䊊
×
×
×
×
䊊
×
䊊
䊊
×
䊐
×
䊊
䊊
×
䊐
×
䊐
䊊
×
䊊
䊊
䊊
䊊
䊊
quartz
biotite
muscovite
䊉
䊉
䊉
×
×
×
䊉
䊉
䊉
䊉
䊎
䊎
䊎
䊐
䊐
䊐
䊊
䊊
䊊
䊊
䊊
feldspar
chlorite
䊉
×
×
䊉
䊉
×
䊎
䊊
䊐
䊐
䊊
䊐
䊊
䊐
epidote
smectite
illite
×
×
×
䊉
䊉
䊉
×
×
×
×
䊊
䊊
×
䊊
䊊
䊊
䊊
䊊
䊐
䊐
䊊
Rhyolite (?)
䊉: very abundant, 䊎: abundant, 䊐: moderately, 䊊: low, ×: not contains. SA: slightly altered zone, MA: moderately altered
zone, HA: highly altered zone, CA: completely altered zone.
Tertiary Gampo Basin, Kyongsang Province, South Korea
395
Mineralogical change
Major minerals of fresh and altered rocks on
the basis of the results of both laboratory methods such as microscope, XRD, EPMA and field
observation are reported in Table 1.
The relatively unaltered basalt is mainly composed of pyroxene, feldspar, and opaque minerals. The pyroxene (augite and diopside) and
plagioclase (composition between labradorite and
bytownite) occur mainly as phenocrysts in glassy
groundmass. In the slightly altered zone, pyroxene
and plagioclase is partly replaced by clay minerals along the cleavages. In the highly altered zone
and completely altered zone, clay minerals are
sharply increased and feldspar is absent. Calcite
and clay minerals coat mostly the fracture surfaces
with accessory Fe-oxides. The clay minerals are
dominantly smectite, commonly kaolinite, and
rarely chlorite (Fig. 4(a)).
The relatively unaltered granite is mainly composed of quartz, alkali feldspar, plagioclase,
biotite, muscovite, and chlorite. Zircon and
(a)
(b)
RESULTS AND DISCUSSIONS
Fig. 4. (a) X-ray diffraction patterns of 9 clay fractions separated from basalts (1 to 9). S: smectite, K: kaolinite,
Ch: chlorite, Q: quartz, F: feldspar. Sample numbers are the same as in Table 2. (b) X-ray diffraction patterns of
8 clay fractions separated from granites (10 to 12) and rhyolites (13 to 17). S: smectite, I: illite, Q: quartz,
F: feldspar. Sample numbers are the same as in Table 2.
396
C.-B. Im et al.
opaques occur as accessory minerals. Plagioclase
comprises mostly euhedral to subhedral
phenocrysts of albite composition. Biotite and
feldspar appear to be replaced by clay minerals
along the surfaces. The clay minerals increase as
alteration increases. In intensively altered zones,
the formation of illite and smectite from the breakdown of the feldspars and micas is characteristic.
In contrast with basalt, the dominant clay minerals are illite rather than smectite (see Figs. 4(a)
and (b)). Epidote and calcite veinlets commonly
occur in altered zones, which are inferred to have
originated by hydrothermal alteration. Fault and
joint surfaces contain Fe- or Mn-oxides and calcite.
The relatively unaltered rhyolite is characterized by a fine-grained texture, and appears to be
severely fractured. The rock is mainly composed
of quartz, feldspar, muscovite, biotite, and opaque
minerals. The feldspar phenocrysts are often zoned
and corroded by alteration. Epidote veinlets are
distinctively injected in intensively altered zones.
In highly altered zone especially, quartz occurs as
a major constituent mineral through silicification,
and its surrounding matrix comprises mostly
smectite and illite (Fig. 4(b)), which are formed
from the decomposition of plagioclase and muscovite. Silicification and sericitization which are
characteristically caused by hydrothermal alteration in intensively altered zones are dominant with
epidote veinlets. Major clay minerals are dominantly smectite and commonly illite.
Stable isotopic compositions (δD and δ 18O)
The isotopic compositions of the separated clay
minerals and clay fractions of each altered rock
are given in Table 2. Hydrogen and oxygen isotopic compositions of clay minerals provide a useful clue for the origin of water contained in them,
and the conditions of their formation (Savin and
Epstein, 1970; O’Neil and Kharaka, 1976; Yeh and
Savin, 1977). Hydrogen and oxygen isotopic com-
Table 2. Rock types, sample numbers, isotopic compositions, constituent minerals, and drillcore numbers of 17 separated clay fractions
Rock type
Basalt
Sample No.
1*
2
3
4
5
6
7*
8
9
Isotopic compositions
Clay minerals
Others
Drill-core No.***
δ1 8 O (‰)
δD (‰)
Major
Minor
+20.0/+20.4
+18.4
+17.0
+17.0
+16.9
+14.7
+13.8/+13.8
+13.5
+13.2
–74
–76
–92
–90
–73
–75
–98
–87
–87
S
S
S
S
S
S
S
S
S
—
—
—
—
K
K
K
K, Ch
Ch, K
—
—
—
—
F, Q
F
F
F, Q
F, Q
A-6
A-5
A-3
**
**
A-6
A-7
A-5
A-1
Granite
10
11
12
+7.1
+6.9
+10.3
–89
–85
–69
I
I
I, S
—
—
—
—
—
F
G-2
G-2
G-1
Rhyolite
13
14
15
16
17
+14.0
+11.5
+10.8
+8.8
+7.8
–61
–82
–87
–68
–72
S
S
S
S
S
—
—
I
I
I
F
F, Q
Q, F
F, Q
F, Q
R-1
R-2
R-2
**
**
*: duplicate analyses on separate aliquots of original sample, **: sampled from excavated road walls near study area, ***: are
same as Fig. 2. S: smectite, I: illite, K: kaolinite, Ch: chlorite, F: feldspar, Q: quartz.
Tertiary Gampo Basin, Kyongsang Province, South Korea
positions of 17 pure clay minerals and clay fractions were analyzed to estimate the origin of fluid
and their formation temperatures. Their mineralogy was confirmed by X-ray diffraction analysis
(Figs. 4(a) and (b)). The clay minerals were separated from 9 basalt, 3 granite, and 5 rhyolite samples occurring in relatively highly and completely
altered zones. They contain mainly smectite, illite,
and kaolinite with accessory quartz and feldspar,
which depends on the types of parent rocks. Altered basalts (sample 1 to 9) mainly contain
smectite with subordinate kaolinite, chlorite,
quartz, and feldspar (Fig. 4(a)), whereas altered
granites (sample 10 to 12) are mainly composed
of illite with accessory smectite and feldspar (Fig.
4(b)). In the altered rhyolites (sample 13 to 17),
smectite occurs with illite, quartz, and feldspar
(Fig. 4(b)).
In the clay minerals and clay fractions occurring in altered rocks of study area, the oxygen iso-
Fig. 5. δD versus δ18O diagrams of the pure clay minerals and clay fractions occurring in basalt and rhyolite
(A) and granite (B). Smectite and illite lines are values
calculated by Kyser (1987). Closed circles (1 to 4): pure
smectites of basalts, open circles (5 to 9): clay fractions of basalts, closed squares (10 to 11): pure illites
of granites, open square (12): clay fraction of granite,
and open triangles (13 to 17): clay fractions of rhyolite.
397
tope compositions ( δ 18 O) have relatively large
variations, whereas the range of hydrogen isotopic
compositions (δD) is limited, regardless of rock
types. The δ18O ranges of clay minerals and clay
fractions occurring in altered basalt, granite and
rhyolite are from +13.2 to +20.4‰, +6.9 to
+10.3‰, and +7.8 to +14.0‰, respectively. The
δ D ranges of each altered rock are from –73 to
–98‰, –69 to –89‰, –61 to –87‰, and –77 to
–78‰, respectively (Table 2). Notably, four pure
smectites in basalt (sample 1 to 4) have relatively
variations from +17.0 to +20.4‰, and two pure
illites in granite (sample 10 and 11) range from
+6.9 to +7.1‰. The δ 18 O of pure smectite in
rhyolite (sample 13) is +14.0‰.
As shown in Fig. 5, the clay fractions separated from altered basalt (sample 5 to 9) plot to
lower δ 18 O values compared with four pure
smectites separated from altered basalts (sample
1 to 4). They are affected by the existence of feldspar and quartz. The pure smectites (sample 1 to
4) plot near the smectite equilibrium line at 25°C
proposed by Kyser (1987). Two pure illites (sample 10 and 11) separated from altered granites have
very similar δ D and δ18O compositions with the
exception of one sample that contains smectite and
minor feldspar (sample 12), but δ 18O is much
lower than in basalt. The clay fractions (sample
13 to 17) containing mainly smectite and the small
amounts of illite, quartz and feldspar, which are
separated from altered rhyolites, show the widest
range of variations in δ18O, and a pure smectite
sample (sample 13) containing a small amount of
feldspar has a much lower δ 18O than that of basalt.
The physical, chemical, and isotopic compositions of groundwaters taken from five drill-holes
are listed in Table 3. The isotopic compositions
of five groundwater samples show a considerably
narrow δ18O and δ D range from –7.5 to –7.8‰
and –45 to –47‰, respectively (Table 3). This indicates that the groundwaters originated from relatively shallow meteoric water, and the isotopic
values correlate well with the meteoric water line
proposed by Craig (1961). The δ18O and δD compositions of the groundwater are similar or a little
398
C.-B. Im et al.
Table 3. Physical, chemical, and isotopic compositions of five
groundwater samples
Sample No.
S-1
S-2
S-3
S-4
S-5
Temp. (°C)
pH
Eh (mv)
TDS (mg/l)
Alkalinity (mg/l)
17.20
6.82
121.00
212.50
110.78
16.60
7.19
123.00
239.20
81.00
17.90
7.01
90.00
247.60
81.98
16.00
7.09
111.00
247.20
111.14
17.40
8.19
101.00
199.50
98.20
Na + (mg/l)
K+
Ca 2+
Mg2+
Fe 2+
HCO3 –
CO3 2–
Cl–
F–
SO4 2–
14.81
1.57
15.63
5.01
0.05
110.80
0.03
14.88
0.11
10.65
19.51
2.24
31.01
9.66
0.02
109.30
0.08
15.32
0.18
10.74
21.32
2.23
30.65
8.18
0.05
81.00
0.04
45.25
0.10
14.07
14.62
1.65
17.71
4.92
0.03
110.30
0.06
37.95
0.09
15.50
15.72
2.39
22.09
4.90
0.01
94.90
0.68
15.83
0.11
11.74
δ18 O (‰)
δD (‰)
–7.70
–45.00
–7.70
–46.00
–7.50
–46.00
–7.50
–45.00
–7.80
–47.00
Each cation and anion was calculated from NETPATH computer program.
higher than those reported by Lee (1991), who
proposed that the initial δ18O and δD compositions of meteoric waters contained in late Cretaceous to early Tertiary granitic rocks in the southern part of Kyongsang Basin are from –8 to –9‰
and –50 to –60‰, respectively. In summary, the
clay minerals separated from basalts have higher
δ 18 O than those separated from granite and
rhyolite, whereas the δ D contents are very similar to each other.
O-isotope geothermometry
The O-isotope fractionation between two
phases under isotopic equilibrium depends on the
temperature and not on pressure. Therefore, it can
be used to estimate the approximate temperatures
at which isotopic equilibrium between those
phases occurred (Clayton and Steiner, 1975; Yeh
and Savin, 1977; Hoefs, 1980; Sheppard and Gilg,
1996).
The calculated results were plotted on a diagram of δ 18 O fractionation factor (1000ln α
δ18Omineral-water) and temperature (106/T2) proposed
by Kyser (1987), as shown in Fig. 6. The forma-
tion temperatures of four pure smectites occurring
in basalts (sample 1 to 4) are estimated to have
been between ~14°C to 29°C if δ18O composition
of the waters of study area are –7.5 to –7.8‰,
whereas, if they are –8.0 to –9.0‰ by Lee (1991),
the formation temperatures are estimated to have
ranged from 8°C to 27°C. The two pure illites
(sample 10 and 11) occurring in granites are calculated to have equilibrated in the range from 68°C
to 83°C, which are much higher temperatures than
those of basalts. Therefore, they plot “leftward”
of illite line for 25°C, as shown in Fig. 5. The formation temperatures of smectite (sample 12) occurring in granite, which contains a small amount
of feldspar, are estimated to be between about
53°C to 71°C and about 44°C to 67°C, respectively, whereas clay fractions from five rhyolites
(sample 13 to 17) are estimated to have equilibrated in the range from 37°C to 98°C. In the case
of granite and rhyolite, the calculated temperatures
do not necessarily reflect formation temperatures
of each clay mineral because they could have been
formed by various processes such as hydrothermal alteratoin, weathering, or a combined of these.
Tertiary Gampo Basin, Kyongsang Province, South Korea
399
Fig. 7. Al (IV) of chlorite octahedral site versus temperature ( °C). Closed and open symbols indicate values calculated from equations of Kranidiotis and
MacLean (1987) and Jowett (1991), respectively.
Circle: basalt, square: granite, and triangle: rhyolite.
Fig. 6. 1000lnα δ18Omineral-water versus 106/T 2 diagram.
Equilibrium lines of each mineral are from Kyser
(1987). Symbols are the same as in Fig. 5.
Chlorite geothermometry
Chlorite has the potential to record invaluable
information about the physicochemical conditions
under which it formed. From the relationship of
crystallochemistry between formation temperature
and chemical composition, many researchers have
noted a systematic increase of AlIV(or conversely
a decrease in SiIV), ΣIV and (Fe + Mg) in chlorite
as the formation temperature increases
(Cathelineau, 1988; Jahren and Aagaard, 1989;
Hillier and Velde, 1991; De Caritat et al., 1993).
Superscript IV and VI mean the cation charge of
tetrahedral and octahedral sites, respectively.
The formation temperature of chlorite was calculated using the empirical geothermometer of
Kranidiotis and MacLean (1987) and Jowett
(1991). Tables 4(a) and (b) show the chemical
compositions and calculated formation temperatures of 19 chlorites taken from 12 basalt, 4 granite and 3 rhyolite samples. Despite the considerable differences in calculated absolute temperatures using these two thermometers (Fig. 7), there
is a consistent difference between the chlorite
formed in basalt on the one hand (lower T) and
that formed in rhyolite or granite (higher T) on
the other hand. The differences in formation temperatures indicate that chlorite in basalt has typically experienced a lower temperature alteration
than those in granite and rhyolite. It is possible
that chlorite in granite and rhyolite formed during a slightly higher temperature alteration event
than the calculated temperatures because of overprinting during lower temperature alteration in a
continuous water-rock interaction episode. However, chlorite formation temperatures in basalts
show much higher temperatures relative to weathering conditions. De Caritat et al. (1993) noted
that chlorite composition depends not only upon
temperature but also upon the nature of the coexisting mineral assemblage and the bulk rock composition or the physicochemical characteristics of
its surrounding groundwater. Accordingly, the formation temperatures calculated by chlorite
geothermometry do not represent the temperature
of major alteration process for the study area.
However, it is clear that selected chlorite from
basalt and other rocks (granite and rhyolite) has
experienced different types of alteration.
Chemical change variation
To identify the behavior of chemical elements
in each zone as the degree of alteration increases,
400
C.-B. Im et al.
Table 4(a). Chemical composition (wt%), structural formulae (half-cell), and estimated formation temperatures
of selected chlorites occurring in basalts
37 representative samples (15 basalts, 11 granites,
and 11 rhyolites) were selected. The total contents
of TiO2, MnO, and P2O5 were selected as reference components for normalization of each major
oxide. The increasing or decreasing percentage of
any element X in a sample relative to each freshparent rock is calculated according to the equation below, where Y denotes the total contents of
TiO 2, MnO, and P2O5:
Change [%]
= [(X/Y)sample/(X/Y)fresh parent rock – 1]·100.
Middelburg et al. (1988) have suggested that the
parameter “Degree” represents an independent
measure for the weathering degree of each sample.
Degree = (1 – Rsample/Rfresh parent rock),
where R indicates the ratio of (CaO + Na2O + K2O)
to (Al2O3 + H2O). The parameter “Degree” approaches 1 whenever clay minerals such as
smectite and kaolinite prevail and is equal to 0
for unaltered rocks. Figure 8 shows the relationship between the “Degree” (Middelburg et al.,
1988) and the geoengineering scale of ISRM
(1975). Positive trends are indicated from the fresh
parent rock (F) to the highly altered zone (HA).
However, the completely altered zone (CA) has
an irregular pattern, which is attributed to the enrichment and leaching of some elements from
more active water-rock interaction.
Major oxides
Figure 9 shows the compositional variations
of major oxides in each altered zone except for
the reference oxides of TiO2, MnO, and P2O5. The
SiO2, Al2O3, Fe2O 3, Na2O, and MgO in basalt are
Tertiary Gampo Basin, Kyongsang Province, South Korea
401
Table 4(b). Chemical composition (wt%), structural formulae (half-cell),
and estimated formation temperatures of selected chlorites occurring in granites and rhyolites
T1 and T2 denote temperatures (° C) estimated by Kranidiotis and MacLean (1987), and Jowett (1991), respectively. FeO
indicates total iron content. Cations were calculated on the basis of 14 oxygens per half-cell.
relatively immobile in all zones of alteration,
which reflect a greater resistance to leaching and
dissolution of groundwater than oxides such as
K2O and CaO. On the other hand, SiO2 in granite
and rhyolite shows mobile behavior, that might
be attributed to the breakdown of silicate minerals such as feldspar and micas by active waterrock interactions under weathering and hydrothermal alteration. Depletion of CaO and enrichment
of K2O in basalt are observed. These patterns reflect the greater alteration rate of plagioclase compared to K-feldspar, and the formation of illite
from plagioclase and micas (Nesbitt et al., 1980).
The considerable decrease of CaO relative to Na2O
is due to the greater decomposition rate of Carich plagioclase in basalt and granite. Na2O and
K2O in granite and rhyolite are similarly decreased
over all profiles. For total iron as Fe2O3, a gradual
increase exists for granite and rhyolite. This pattern seems to reflect the formation of Fe-oxides
by oxidation of Fe-bearing minerals from active
water-rock interaction. MgO in granite and especially rhyolite is increased as alteration degree
increases. It is attributable to the formation of Mgrich clay minerals like chlorite from the alteration of plagioclase and biotite.
Clearly, the variation patterns for SiO2, Al 2O 3,
Na 2O 3, K2O, and MgO as a function of alteration
in basalt are different from those of granite and
rhyolite. We suggest that the former are caused
by low temperature alteration processes, whereas
the latter result from the combined effects of both
402
C.-B. Im et al.
Fig. 8. The relationship between the weathering “Degree” proposed by Middelburg et al. (1988) and the
weathering scale by ISRM (1975). Dash line represents
fresh parent rock. The arrow indicates line fit by eye
from a slightly altered (SA) to highly altered zones (HA).
MA and CA mean moderately altered and completely
altered zones, respectively.
hydrothermal alteration and weathering.
Figure 10 shows stability diagrams for Na2OAl 2 O 3 -SiO 4 -H 2 O and K 2 O-Al 2 O 3 -SiO 4 -H 2O at
25°C and atmospheric pressure proposed by Harris
and Adams (1966). The two stability diagrams
indicate that the local groundwaters are in equilibrium with kaolinite but not smectite (Fig. 10).
This suggests that the geochemistry of
groundwater during mineralogical alteration was
different from that of the present.
Fig. 9. Variation diagrams showing the percent
changes for major oxides with increasing degree of alteration. The data plotted are average values for each
alteration zone.
Trace elements
The compositional variations of trace elements
in each altered zone are shown in Fig. 11. It is
clear that cations such as Rb, Cs, Sr, Ba, Co, Ge,
and U appear to be more mobile than Ga, Y, Hf,
Zr, Nb, Ta, and Th. The distribution patterns for
basalt are different from those of granite and
rhyolite. Granite and rhyolite have similar patterns, although rhyolite shows somewhat more
prominent variations than granite. Ga, Y, Hf, Zr,
Nb, and Ta in basalt are relatively immobile over
all alteration zones compared with granite and
rhyolite.
The dramatic increases of Rb and Cs in the
highly altered zone of basalt are similar to the
behavior of K2O, and result from the well known
geochemical affinity of K for these trace elements.
Sr is less variable than Rb and Cs over all altered
profiles, and is closely related with the variation
of CaO. The patterns of Sr in granite and rhyolite
are more depleted than those of basalt. This indicates that high Ca groundwater can be attributed
to the breakdown of Ca-bearing minerals such as
carbonates, Ca-plagioclase, and epidote by waterrock interaction (Fig. 12). The general leaching
of Sr in granite and rhyolite is probably caused
by more active water-rock interaction than in basalt along fractures, and by the high degree of alteration of Ca-plagioclase.
Tertiary Gampo Basin, Kyongsang Province, South Korea
403
Fig. 10. System Na 2O-Al2O3-SiO4-H2O (a) and K 2OAl2O3-SiO4-H 2O (b) at 25 °C and 1 atm. total pressure
(Harris and Adams, 1966). The plotted dots and circled field represent the geochemistry and geochemical
range of 5 groundwater samples, respectively.
The distribution patterns for Ba are similar to
those of K2O. The systematic decrease of Ba in
progressive zones of alteration can be attributed
to the alteration of K-feldspar and biotite containing appreciable Ba. Alteration of alkali feldspar
tends to release Ba in solution rather than being
absorbed in altered zone. On the other hand, Ba
in basalt is enriched, which seems to be introduced
from groundwater. The cation exchange capacity
of clay minerals can cause the preferential adsorption of Ba in more strongly altered zones of the
profile (Middelburg et al., 1988). Co in all altered
profiles generally appears to be rather enriched
compared with fresh parent rock. A sharp enrichment in all rhyolite profiles and in the completely
altered granite is attributed to the addition of Co
by hydrothermal alteration.
Ge is more mobile than Ga in all alteration
profiles. The general depletion of Ga in each rock
may be caused by more active water-rock interaction in fissures and joints. The general distribution patterns of Y are very similar to those of REE.
The trend of Y in basalt is an increase as alteration degree increases, whereas the reverse is true
in granite and rhyolite.
Fig. 11. Variation diagrams showing the percent
changes for trace elements in basalt, granite and
rhyolite. The data plotted are average values for each
alteration zone. The dashed lines represent fresh parent rock. Symbols are the same as in Fig. 9.
U is easily mobilized and leached from parent
rocks relative to Th, and its distribution pattern is
more marked in basalt. Absolute U and Th contents in basalt are considerably lower than those
of granite and rhyolite. Generally, U in basalt increases gradually with increasing of alteration
degree and is sharply decreased at fracture surfaces, whereas Th shows a very uniform pattern
over all profiles. U seems to be accumulated rather
than leached in the groundwater-fracture system.
404
C.-B. Im et al.
Fig. 13. REE abundances for alteration zones of basalt, granite, and rhyolite normalized to each parent
rock. The data plotted are average values for each alteration zone. The dashed lines represent fresh parent
rock. Symbols are the same as in Fig. 9.
Fig. 12. Concentrations of major (a) and trace (b) elements of five groundwater samples.
Rare earth elements
The REE have similar chemical properties and
generally show a uniform geochemical behavior
during any given alteration history (Nesbitt, 1979;
Kamineni, 1985; Middelburg et al., 1988). REE
patterns for each altered rock, normalized to the
concentrations of each fresh parent rock, are illustrated in Fig. 13.
In the basalt, the HREE tend to be enriched
with increasing alteration, whereas LREE have
uniform patterns. However, the HREE in the completely altered zone are much less enriched than
in the highly altered zone, and LREE show the
opposite patterns. These imply that the LREE in
leachable phases such as pyroxene, biotite, and
feldspar are easily liberated by water-rock interaction, and subsequently enriched in the completely altered zone. The leaching of the HREE
cannot easily occur relative to those of LREE, and
HREE appear to be gradually enriched from the
slightly altered zone to highly altered zone. Leaching processes of active water-rock interaction may
cause HREE depletion in the completely altered
zone.
The distribution patterns of REE in granites
are similar to those of basalt, but have narrower
HREE and wider LREE ranges than those of
basalts. This is caused by the formation of clay
minerals and the differential dissolution rate of
biotite and feldspar. Negative Eu anomalies except for the moderately altered zone are attributed
to the breakdown of feldspar (Alderton et al.,
1980).
Altered zones of rhyolite show mostly enrichment patterns relative to fresh parent rocks, although these patterns are a little irregular. REE
contents in the moderately altered zone are the
prominently increased. This feature may be due
to sorption from groundwater by secondary minerals such as Fe-oxides and clay minerals.
Tertiary Gampo Basin, Kyongsang Province, South Korea
Landstrom and Tullborg (1990) noted that Feoxyhydroxide and carbonates are associated with
high concentrations of LREE and HREE, respectively.
As described above, the REE in basalt have a
regular fractionation pattern as the alteration increases compared with those of granite and
rhyolite. These trends indicate that the parent basaltic rocks and their altered zones have been
largely affected by a continuous low-temperature
alteration. In contrast, rhyolite has a relatively irregular distribution patterns. These trends can
probably be attributed to the complicated
geochemical processes such as hydrothermal alteration and subsequent lower-temperature alteration by rock-water interaction.
CONCLUSIONS
The area is composed of Cretaceous shale,
Tertiary rhyolite, granite and basalt distributed in
the Gampo Basin. Rhyolite and granite, compared
with basalt, appear to be highly fractured by tectonic movement.
Alteration minerals occurring in altered basalts
are smectite, kaolinite, chlorite, and rarely illite,
whereas calcite is mostly located in fractures. Alteration minerals in granite and rhyolite are
epidote, chlorite, illite, and smectite, whereas Mn
and Fe oxides, and calcite coat fractures. Calcite,
epidote, and quartz veinlets are common in altered
granite and rhyolite. In general, primary minerals
such as pyroxene, biotite, muscovite, and feldspar
decrease as the degree of alteration increases, and
are mostly decomposed in intensively altered
zones. On the other hand, the amount of clay minerals such as smectite, illite, chlorite, and kaolinite
is increased. Clay fractions of basalt have a limited and much heavier δ18O composition than those
in granite and rhyolite. The formation temperatures of smectite in basalt, illite in granite, and
smectite in rhyolite (calculated by oxygen and
hydrogen isotopic compositions) are from 14°C
to 29°C, 78°C to 83°C, and 37°C to 98°C, respectively. Chlorite in basalt was formed at lower temperatures (105°C~205°C) than the chlorite in gran-
405
ite and rhyolite (302°C~362°C). Based on the
above results, it appears that basalt has experienced only low temperature alteration such as
weathering, whereas granite and rhyolite have the
geochemical characteristics of hydrothermal alteration and superimposed weathering. The heat
source for hydrothermal alteration is inferred to
be related with the intrusion of concealed younger
igneous rocks.
Major oxides except for K2O and CaO are relatively immobile in all altered zones of basalt,
whereas most major oxides are relatively mobile
and show some irregular distribution patterns over
all altered zones of granite and rhyolite. Rb, Cs,
Sr, Ba, Ge, and U appear to be easily mobile,
whereas Ga, Y, Hf, Zr, Nb, Ta, and Th are relatively immobile during alteration processes. The
variation patterns of Rb, Cs, and Ba are very similar to that of K, whereas Sr is closely related with
Ca. In the basalt, HREE do not show large variations as alteration increases, and LREE is depleted.
In the granite also, REE do not show a large variation as alteration increases. In contrast, altered
rhyolite has greater REE enrichment and positive
Eu anomalies compared with fresh parent rock.
Furthermore, rhyolite has relatively irregular REE
patterns.
These results mean that altered basalts that
have just experienced low temperature alteration
(such as weathering) have indistinct variations of
major oxides, some trace elements and REE
though basalt does not have the prominent difference of alteration degree and alteration mineralogy compared with granite and rhyolite. Altered
granite and rhyolite, which have experienced 1st
stage hydrothermal alteration (relatively low temperature) and 2nd stage weathering are characterized by prominent variations of major oxides, trace
elements, and rare earth elements.
Acknowledgments—We would like to express our appreciation to Professor R. J. Arculus, Department of
Geology, Australian National University, for his detailed readings of the manuscript and critical comments.
This research has been partially supported by nuclear
research and development program of Korean Ministry of Science and Technology.
406
C.-B. Im et al.
REFERENCES
Alderton, D. H. M., Pearce, J. A. and Potts, P. J. (1980)
Rare earth element mobility during granite alteration:
evidence from southwest England. Earth Planet. Sci.
Lett. 49, 149–165.
Cathelineau, M. (1988) Cation site occupancy in
chlorites and illites as a function of temperature. Clay
Miner. 23, 471–485.
Chang, T. W. and Baek, Y. (1995) Tectonic movement
of the Tertiary between Kyungju and Eoil, Korea.
Jour. Geol. Soc. Korea 31, 200–214.
Chesworth, W., Dejou, J. and Larroque, P. (1981) The
weathering of basalts and relative mobilities of the
major elements at Belbex, France. Geochim.
Cosmochim. Acta 45, 1235–1243.
Choi, W. C., Hwang, J. H., Yoon, O. and Kim, D. H.
(1988) Geological map of Korea, Eoil sheet
(1:25,000). Korea Institute of Energy and Resources,
42 pp.
Clayton, R. N. and Steiner, A. (1975) Oxygen isotope
studies of the geothermal system at Wairakei New
Zealand, Geochim. Cosmochim. Acta 39, 1179–1186.
Craig, H. (1961) Isotopic variations in meteoric waters. Science 133, 1702–1703.
Cramer, J. J. and Nesbitt, H. W. (1983) Mass-balance
relations and trace element mobility during continental weathering of various igneous rocks. Symp. on
Petrology of Weathering and Soils, Sci. Geol., Mem.
73, 63–73.
De Caritat, P., Hutcheon, I. and Walshe, J. H. (1993)
Chlorite geothermometry: a review. Clays and Clay
Miner. 41, 219–239.
Fritz, S. J. and Mohr, D. W. (1984) Chemical alteration
in the micro weathering environment within a
spherolidally-weathered anorthite boulder. Geochim.
Cosmochim. Acta 48, 2527–2535.
Gascoyne, M. and Cramer, J. (1987) History of actinide and minor element mobility in an Archean
batholith in Manitoba, Canada. Applied Geochem. 2,
37–53.
Harris, R. C. and Adams, J. A. S. (1966) Geochemical
and mineralogical studies on the weathering of granitic rocks. Am. J. Sci. 264, 146–173.
Hillier, S. and Velde, B. (1991) Octahedral occupancy
and the chemical composition of diagenetic (lowtemperature) chlorites. Clay Miner. 26, 149–168.
Hoefs, J. (1980) Stable Isotope Geochemistry, 143–145,
Springer Verlag, Berlin.
ISRM (1975) ISRM Working Party, suggested methods for the description of rock masses, joints, and
discontinuity. International Society of Rock Mechanics, 2nd ed., August, Lisbon.
Jahren, J. S. and Aagaard, P. (1989) Compositional variations in diagenetic chlorites and illites, and relationships with formation-water chemistry. Clay Miner.
24, 157–170.
Jin, M. S., Kim, S. J. and Shin, S. C. (1988) K/Ar and
fission-track datings for volcanic rocks in the
Pohang-Gampo area. Research on Isotope Geology,
Korea Inst. Ener. Resour. KR-87-27, 51–88.
Jowett, E. C. (1991) Fitting iron and magnesium into
the hydrothermal chlorite geothermometer. GAC/
MAC/SEG Joint Annual Meeting, Toronto, May 27–
29, 1991, Program with Abstracts 16, A62.
KEPCO (1995) Final safety analysis report for Wolsung
nuclear power plant, Units No. 2, 3 and 4. Korea Electric Power Corporation.
KEPCO (1996) Foundation treatment report for
Wolsung nuclear power plant, Units No. 2, 3 and 4.
Korea Electric Power Corporation.
Kamineni, D. C. (1985) Significance of the distribution of rare-earth elements (REE) in the Eye-Dashwa
pluton the migration of radionuclides. Atomic Energy
of Canada Limited Technical Record TR-299, 508–
527.
Kee, W. S. and Doh, S. J. (1995) Geological structures
in the southern part of the Tertiary Eoil Basin.
Korea, Jour. Geol. Soc. Korea 31, 10–20.
Kranidiotis, P. and MacLean, W. H. (1987) Systematics of chlorite alteration at the Phelps Dodge massive sulfide deposit, Matagami, Quebec. Econ. Geol.
82, 1898–1911.
Kyser, T. K. (1987) Stable isotope geochemistry of low
temperature processes: equilibrium fractionation factors for stable isotopes. Miner. Assoc. of Canada,
short course handbook, Vol. 13, p. 1–84, Saskatoon,
Saskatchewan.
Landstrom, O. and Tullborg, E. L. (1990) Mineralogical and geochemical studies of fracture infillings in
drillcore KLJ 1. In: Interdisiplinary study of post
glacial faulting in the Lansjarv area, northern
Sweden, 1986–1988, SKB TR 89–31.
Lee, H. K., Moon, H. S., Min, K. D., Kim, I. S., Yun,
H. and Itaya, T. (1992) Paleomagnetism, stratigraphy
and geologic structure of the Tertiary Pohang and
Changgi Basins: K-Ar ages for the volcanic rocks.
Jour. Korean Inst. Mining Geol. 25, 337–349.
Lee, J. I. (1991) Petrology, mineralogy, and isotopic
study of the shallow-depth emplaced granitic rocks,
southern part of Kyongsang basin, Korea: origin of
micrographic granite. Ph.D. Thesis, Univ. of Tokyo.
Middelburg J. J., Van der Weijden, C. H. and Woittiez,
J. R. W. (1988) Chemical processes affecting the
mobility of major, minor and trace elements during
weathering of granitic rocks. Chem. Geol. 68, 253–
273.
Tertiary Gampo Basin, Kyongsang Province, South Korea
Moon, H. S., Choi, S. K. and Kim, M. Y. (1989) Thermal and mineralogical characterization of Camontmorillonite from Gampo area. Jour. Korean Inst.
Mining Geol. 21, 175–184.
Nesbitt, H. W. (1979) Mobility and fractionation of rare
earth elements during weathering of a granodiorite.
Nature (London) 279, 206–210.
Nesbitt, H. W., Markovics, G. and Price, R. C. (1980)
Chemical processes affecting alkalis and alkaline
earths during continental weathering. Geochim.
Cosmochim. Acta 44, 1659–1666.
O’Neil, J. R. and Kharaka, Y. K. (1976) Oxygen and
hydrogen isotope exchange reactions between clay
minerals and water. Geochim. Cosmochim. Acta 40,
407
241–246.
Savin, S. M. and Epstein, S. (1970) The oxygen and
hydrogen geochemistry of clay minerals. Geochim.
Cosmochim. Acta 34, 25–42.
Sheppard, S. M. and Gilg, H. A. (1996) Stable isotope
geochemistry of two drill-cores from the geothermal
granite (Krakemala test site), southeast Sweden.
Chem. Geol. 51, 55–78.
Yeh, H. Y. and Savin, S. M. (1977) Mechanism of burial
metamorphism of argillaceous sediments: O-isotope
evidence. Geol. Soc. Am. Bull. 88, 1321–1330.
Yoon, S. (1992) Geology of the Tertiary Yangnam and
Pohang Basins, Korea. Bulletin of the Mizunami Fossil Museum 19, 13–31.