Geochemical Journal, Vol. 36, pp. 391 to 407, 2002

391

*Corresponding author (e-mail: k136icb@kins.re.kr)

The geochemical behavior of altered igneous rocks inthe Tertiary Gampo Basin, Kyongsang Province, South Korea

CHANG-BOCK IM,1* SANG-MO KOH,2 HO-WAN CHANG3 and TETSUICHI TAKAGI4

1Nuclear Safety Research Department, Regulatory Research Division, Korea Institute of Nuclear Safety (KINS),P.O. Box 114, Yusung, Taejon 305-600, South Korea

2Geology Division, Korea Institute of Geoscience and Mineral Resources, Yusung, Taejon 305-350, South Korea3School of Earth and Environmental Sciences, Seoul National University, Seoul 151-742, South Korea

4Research Center for Deep Geological Environments, National Institute of Advanced Industrial Science andTechnology (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, Tertiaryrhyolite, granite, and basalt in ascending order.

The mineralogy, isotopic composition (δD and δ18O) of clay minerals, and chlorite geothermometrywere performed to interpret the alteration history of these rocks. Clay minerals occurring in basalt have alimited range and are heavier in δ18O than the clay minerals in granite and rhyolite. Chlorite occurring inbasalt 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, Fe2O3, Na2O, MgO, TiO2,MnO, and P2O5 of basalt are relatively immobile in most altered zones. In contrast, these oxides in graniteand rhyolite are relatively mobile, and show some irregular variations in the most altered zones. Sometrace elements in basalt have less prominent variations than in granite and rhyolite. REE distributions ofaltered basalt and granite do not show prominent variations with increasing alteration degree, whereasrhyolite is enriched in REE and has a positive Eu anomaly.

These results indicate that altered basalts that have experienced only low temperature alteration suchas weathering, have indistinct variations of major oxides, some trace elements and REE according toalteration intensity. Granite and rhyolite that have experienced both hydrothermal alteration (low tempera-ture) and weathering process are characterized by relatively prominent variations of major oxides, sometrace elements and REE according to alteration intensity. The results coincide well with those of minera-logical studies, isotopic compositions of clay minerals, and chlorite geothermometry.

Chesworth et al., 1981; Gascoyne and Cramer,1987; Middelburg et al., 1988). In a thermody-namic sense, systems of chemical weathering andhydrothermal alteration are invariably open andirreversible (Cramer and Nesbitt, 1983; Fritz andMohr, 1984).

The purpose of this study is to investigate theelemental behavior and mineralogical changes inzones of variable degree of alteration, and to esti-mate the physicochemical environments that pro-duced these changes. It is expected that these types

INTRODUCTION

The variations of chemical composition andmineralogy with increasing degree of alterationreflect the nature of specific alteration processes.The relative mobility of elements is controlled notonly by primary factors such as the mineralogyand texture of the parent rock, but also by sec-ondary processes such as dissolution of primaryminerals, formation of secondary phases, redoxprocesses, and ion exchange (Nesbitt et al., 1980;

392 C.-B. Im et al.

of study will contribute to the selection processesof a preliminary site for Korea’s nuclear wastedisposal in the near future. In addition, these typesof study can provide geological and geochemicalinformation such as the behavior and mobility ofradiogenic nuclides that might be derived from thewaste disposal facilities. The study area is situ-ated in the construction site of the Wolsung Nu-clear Power Plant (WNPP) on the southeasterncoast of the Korean Peninsula, as shown in Fig. 1.

GENERAL GEOLOGY

The detailed geology of the study area con-sists mainly of Cretaceous shale and Tertiary vol-canic and plutonic rocks such as rhyolite, granite,andesitic lapilli tuff, and basalt in ascending or-der (Fig. 2). The geology of the study area and itsvicinity has been studied by many geologists (Choiet al., 1988; Moon et al., 1989; Lee et al., 1992).KEPCO (1995, 1996) has carried out variousgeotechnical investigations to select the most suit-

able site of the WNPP. The area is characterizedby complicated geological structures developed bytectonic movements during late the Cretaceous tolate Tertiary period (Yoon, 1992; Kee and Doh,1995; Chang and Baek, 1995).

Basalt (the youngest rock) mostly occurs asdikes of various widths and intrudes Tertiaryrhyolite, granite, and andesitic lapilli tuff. Jin etal. (1988) and Lee et al. (1992) reported that theage of the rock is early Miocene (18.50 Ma to21.07 Ma) based on K-Ar whole rock methods.This rock appears to be the least fractured, com-pared with other rocks, and has experienced themost severe weathering processes. The basalt usu-ally seems to have intruded along fault zones.

Granite with irregular zonal shapes is intrudedinto older Cretaceous shale and Tertiary rhyolite,and is intruded by the younger basalt. Jin et al.(1988) presented K-Ar ages of early to middleEocene (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-

Fig. 1. Location map of the study area.Fig. 2. Geologic map and sample location. A-1 andsimilar symbols represent the drilling sites.

Tertiary Gampo Basin, Kyongsang Province, South Korea 393

ite increases with proximity to rhyolite contacts.Rhyolite occupies the largest part of the study

area, intrudes the Cretaceous shale, and is intrudedby granite and basalt. In some drill-core profiles,rhyolite and shale are usually in fault contact.During the rhyolitic volcanic activity between latePaleocene and early Eocene, rhyolite and relatedpyroclastics such as welded tuffs and volcanicbreccias were erupted (Yoon, 1992). In the mostof areas, the rock is highly fractured and containsseveral intersecting faults associated withbrecciated zones. Hornfels often occurs in the con-tact part of Cretaceous shale and rhyolite.

FIELD AND LABORATORY METHODS

Sample preparationThirty-seven samples were collected from ten

drill-core profiles ranging from EL. –8.04 m toEL. –61.84 m and one wall of excavated blocksfor reactor site. Each sample was divided into fivezones such as fresh parent rock (F), slightly al-tered zone (SA), moderately altered zone (MA),highly altered zone (HA), completely altered zone

(CA), which are based on the “ISRM (1975)’sscale”. Sample locations and cross section are il-lustrated in Figs. 2 and 3, respectively. Each sam-ple of the selected drill-cores is traversed by open-fractures at various angles, and all appear to bewater-saturated given the observed groundwaterlevel (EL. +2~+10 m from KEPCO, 1995). These5 chips (total weight is about 0.1–0.5 kg) in eachaltered zone were collected. Five groundwatersamples were also taken from five drill-holes be-tween EL. –70 m to EL. –100 m.

Analytical techniquesEach drill-core sample was cut or handpicked

along the short axis parallel to the fracture sur-face. Small pieces were prepared for making about100 thin sections for microscopic observation andelectron probe micro-analysis (EPMA) at KIGAM(Korea Institute of Geoscience and Mineral Re-sources) and KBSI (Korea Basic Science Insti-tute), respectively. Larger pieces were used foranalysis as follows: being properly crushed, pow-dered and homogenized, each piece was analyzedby X-ray diffractometer (XRD), and then analyzed

Fig. 3. Cross section (A–B in Fig. 2) and projected sample locations.

394 C.-B. Im et al.

at the Activation Lab., Canada, by InductivelyCoupled Plasma-Atomic Emission Spectrometry(ICP-AES) for major elements and by InductivelyCoupled Plasma-Mass Spectrometry (ICP-MS) fortrace elements. δD and δ18O isotopes of separatedclay minerals were analyzed by mass spectrometry(MS) at KBSI and at Geochron Lab., U.S.A., re-spectively. The clay minerals under 2 µm size frac-tion were separated using Stock’s method, andbulk samples were analyzed by X-ray diffractionpowder method. The samples were measured

through 3° to 45° (2θ) using Cu-Kα radiation witha Rigaku X-ray diffractometer at KIGAM.

The temperature, pH, Eh, electric conductiv-ity (EC), alkalinity, anion and cation forgroundwater were measured by the in-situ HatchField Chemistry Kit and Ion-chromatography (IC)of geochemistry lab. of SNU (Seoul National Uni-versity), and major elements and trace elementsincluding REE by ICP-MS at Activation Lab. inCanada. Stable isotopes (δD, δ18O) ofgroundwaters were also analyzed at KBSI.

Major minerals Parent rock Hydrothermal alteration stage Weathering stage

Fresh rock SA MA HA CA

Basalt (16.22~22.70 Ma)

pyroxene � � � � × ×feldspar � � � � � �

chlorite × × � � � �

smectite × × � � � �

kaolinite × × × × � �

calcite × × × × × �

Granite (46.48~58.32 Ma)

quartz � × � � � � �

biotite � × � � � × ×muscovite � × � � � � ×feldspar � × � � � � �

chlorite × � � � � � �

epidote × � × × × × �

illite × � × � � � �

smectite × � × � � � �

calcite × � × × × × �

Rhyolite (?)

quartz � × � � � � �

biotite � × � � � � �

muscovite � × � � � � �

feldspar � × � � � � �

chlorite × � × � � � �

epidote × � × × × � �

smectite × � × � � � �

illite × � × � � � �

Table 1. Mineral constituents of fresh and altered basalt, granite, and rhyolite

�: very abundant, �: abundant, �: moderately, �: low, ×: not contains. SA: slightly altered zone, MA: moderately alteredzone, HA: highly altered zone, CA: completely altered zone.

Tertiary Gampo Basin, Kyongsang Province, South Korea 395

RESULTS AND DISCUSSIONS

Mineralogical changeMajor minerals of fresh and altered rocks on

the basis of the results of both laboratory meth-ods such as microscope, XRD, EPMA and fieldobservation are reported in Table 1.

The relatively unaltered basalt is mainly com-posed of pyroxene, feldspar, and opaque miner-als. The pyroxene (augite and diopside) andplagioclase (composition between labradorite andbytownite) occur mainly as phenocrysts in glassy

groundmass. In the slightly altered zone, pyroxeneand plagioclase is partly replaced by clay miner-als along the cleavages. In the highly altered zoneand completely altered zone, clay minerals aresharply increased and feldspar is absent. Calciteand clay minerals coat mostly the fracture surfaceswith accessory Fe-oxides. The clay minerals aredominantly smectite, commonly kaolinite, andrarely chlorite (Fig. 4(a)).

The relatively unaltered granite is mainly com-posed of quartz, alkali feldspar, plagioclase,biotite, muscovite, and chlorite. Zircon and

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 of8 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.

(a) (b)

396 C.-B. Im et al.

opaques occur as accessory minerals. Plagioclasecomprises mostly euhedral to subhedralphenocrysts of albite composition. Biotite andfeldspar appear to be replaced by clay mineralsalong the surfaces. The clay minerals increase asalteration increases. In intensively altered zones,the formation of illite and smectite from the break-down of the feldspars and micas is characteristic.In contrast with basalt, the dominant clay miner-als are illite rather than smectite (see Figs. 4(a)and (b)). Epidote and calcite veinlets commonlyoccur in altered zones, which are inferred to haveoriginated by hydrothermal alteration. Fault andjoint surfaces contain Fe- or Mn-oxides and cal-cite.

The relatively unaltered rhyolite is character-ized by a fine-grained texture, and appears to beseverely fractured. The rock is mainly composedof quartz, feldspar, muscovite, biotite, and opaqueminerals. The feldspar phenocrysts are often zonedand corroded by alteration. Epidote veinlets are

distinctively injected in intensively altered zones.In highly altered zone especially, quartz occurs asa major constituent mineral through silicification,and its surrounding matrix comprises mostlysmectite and illite (Fig. 4(b)), which are formedfrom the decomposition of plagioclase and mus-covite. Silicification and sericitization which arecharacteristically caused by hydrothermal altera-tion in intensively altered zones are dominant withepidote veinlets. Major clay minerals are domi-nantly smectite and commonly illite.

Stable isotopic compositions (δD and δ18O)The isotopic compositions of the separated clay

minerals and clay fractions of each altered rockare given in Table 2. Hydrogen and oxygen iso-topic compositions of clay minerals provide a use-ful clue for the origin of water contained in them,and the conditions of their formation (Savin andEpstein, 1970; O’Neil and Kharaka, 1976; Yeh andSavin, 1977). Hydrogen and oxygen isotopic com-

Rock type Sample No. Isotopic compositions Clay minerals Others Drill-core No.***

δ18O (‰) δD (‰) Major Minor

Basalt 1* +20.0/+20.4 –74 S — — A-62 +18.4 –76 S — — A-53 +17.0 –92 S — — A-34 +17.0 –90 S — — **5 +16.9 –73 S K F, Q **6 +14.7 –75 S K F A-67* +13.8/+13.8 –98 S K F A-78 +13.5 –87 S K, Ch F, Q A-59 +13.2 –87 S Ch, K F, Q A-1

Granite 10 +7.1 –89 I — — G-211 +6.9 –85 I — — G-212 +10.3 –69 I, S — F G-1

Rhyolite 13 +14.0 –61 S — F R-114 +11.5 –82 S — F, Q R-215 +10.8 –87 S I Q, F R-216 +8.8 –68 S I F, Q **17 +7.8 –72 S I F, Q **

Table 2. Rock types, sample numbers, isotopic compositions, constituent minerals, and drill-core numbers of 17 separated clay fractions

*: duplicate analyses on separate aliquots of original sample, **: sampled from excavated road walls near study area, ***: aresame as Fig. 2. S: smectite, I: illite, K: kaolinite, Ch: chlorite, F: feldspar, Q: quartz.

Tertiary Gampo Basin, Kyongsang Province, South Korea 397

positions of 17 pure clay minerals and clay frac-tions were analyzed to estimate the origin of fluidand their formation temperatures. Their mineral-ogy was confirmed by X-ray diffraction analysis(Figs. 4(a) and (b)). The clay minerals were sepa-rated from 9 basalt, 3 granite, and 5 rhyolite sam-ples occurring in relatively highly and completelyaltered zones. They contain mainly smectite, illite,and kaolinite with accessory quartz and feldspar,which depends on the types of parent rocks. Al-tered basalts (sample 1 to 9) mainly containsmectite with subordinate kaolinite, chlorite,quartz, and feldspar (Fig. 4(a)), whereas alteredgranites (sample 10 to 12) are mainly composedof 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 occur-ring in altered rocks of study area, the oxygen iso-

tope compositions (δ18O) have relatively largevariations, whereas the range of hydrogen isotopiccompositions (δD) is limited, regardless of rocktypes. The δ18O ranges of clay minerals and clayfractions occurring in altered basalt, granite andrhyolite are from +13.2 to +20.4‰, +6.9 to+10.3‰, and +7.8 to +14.0‰, respectively. TheδD 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 puresmectites in basalt (sample 1 to 4) have relativelyvariations from +17.0 to +20.4‰, and two pureillites in granite (sample 10 and 11) range from+6.9 to +7.1‰. The δ18O of pure smectite inrhyolite (sample 13) is +14.0‰.

As shown in Fig. 5, the clay fractions sepa-rated from altered basalt (sample 5 to 9) plot tolower δ18O values compared with four puresmectites separated from altered basalts (sample1 to 4). They are affected by the existence of feld-spar and quartz. The pure smectites (sample 1 to4) plot near the smectite equilibrium line at 25°Cproposed by Kyser (1987). Two pure illites (sam-ple 10 and 11) separated from altered granites havevery similar δD and δ18O compositions with theexception of one sample that contains smectite andminor feldspar (sample 12), but δ18O is muchlower than in basalt. The clay fractions (sample13 to 17) containing mainly smectite and the smallamounts of illite, quartz and feldspar, which areseparated from altered rhyolites, show the widestrange of variations in δ18O, and a pure smectitesample (sample 13) containing a small amount offeldspar has a much lower δ18O than that of ba-salt.

The physical, chemical, and isotopic compo-sitions of groundwaters taken from five drill-holesare listed in Table 3. The isotopic compositionsof five groundwater samples show a considerablynarrow δ18O and δD range from –7.5 to –7.8‰and –45 to –47‰, respectively (Table 3). This in-dicates that the groundwaters originated from rela-tively shallow meteoric water, and the isotopicvalues correlate well with the meteoric water lineproposed by Craig (1961). The δ18O and δD com-positions of the groundwater are similar or a little

Fig. 5. δD versus δ18O diagrams of the pure clay min-erals and clay fractions occurring in basalt and rhyolite(A) and granite (B). Smectite and illite lines are valuescalculated by Kyser (1987). Closed circles (1 to 4): puresmectites of basalts, open circles (5 to 9): clay frac-tions of basalts, closed squares (10 to 11): pure illitesof granites, open square (12): clay fraction of granite,and open triangles (13 to 17): clay fractions of rhyolite.

398 C.-B. Im et al.

higher than those reported by Lee (1991), whoproposed that the initial δ18O and δD composi-tions of meteoric waters contained in late Creta-ceous to early Tertiary granitic rocks in the south-ern part of Kyongsang Basin are from –8 to –9‰and –50 to –60‰, respectively. In summary, theclay minerals separated from basalts have higherδ18O than those separated from granite andrhyolite, whereas the δD contents are very simi-lar to each other.

O-isotope geothermometryThe O-isotope fractionation between two

phases under isotopic equilibrium depends on thetemperature and not on pressure. Therefore, it canbe used to estimate the approximate temperaturesat which isotopic equilibrium between thosephases occurred (Clayton and Steiner, 1975; Yehand Savin, 1977; Hoefs, 1980; Sheppard and Gilg,1996).

The calculated results were plotted on a dia-gram of δ18O fractionation factor (1000lnαδ18Omineral-water) and temperature (106/T2) proposedby Kyser (1987), as shown in Fig. 6. The forma-

tion temperatures of four pure smectites occurringin basalts (sample 1 to 4) are estimated to havebeen between ~14°C to 29°C if δ18O compositionof 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 haveranged from 8°C to 27°C. The two pure illites(sample 10 and 11) occurring in granites are cal-culated to have equilibrated in the range from 68°Cto 83°C, which are much higher temperatures thanthose of basalts. Therefore, they plot “leftward”of illite line for 25°C, as shown in Fig. 5. The for-mation temperatures of smectite (sample 12) oc-curring in granite, which contains a small amountof feldspar, are estimated to be between about53°C to 71°C and about 44°C to 67°C, respec-tively, whereas clay fractions from five rhyolites(sample 13 to 17) are estimated to have equili-brated in the range from 37°C to 98°C. In the caseof granite and rhyolite, the calculated temperaturesdo not necessarily reflect formation temperaturesof each clay mineral because they could have beenformed by various processes such as hydrother-mal alteratoin, weathering, or a combined of these.

Sample No. S-1 S-2 S-3 S-4 S-5

Temp. (°C) 17.20 16.60 17.90 16.00 17.40pH 6.82 7.19 7.01 7.09 8.19Eh (mv) 121.00 123.00 90.00 111.00 101.00TDS (mg/l) 212.50 239.20 247.60 247.20 199.50Alkalinity (mg/l) 110.78 81.00 81.98 111.14 98.20

Na+ (mg/l) 14.81 19.51 21.32 14.62 15.72K+ 1.57 2.24 2.23 1.65 2.39Ca2+ 15.63 31.01 30.65 17.71 22.09Mg2+ 5.01 9.66 8.18 4.92 4.90Fe2+ 0.05 0.02 0.05 0.03 0.01HCO3

– 110.80 109.30 81.00 110.30 94.90CO3

2– 0.03 0.08 0.04 0.06 0.68Cl– 14.88 15.32 45.25 37.95 15.83F– 0.11 0.18 0.10 0.09 0.11SO4

2– 10.65 10.74 14.07 15.50 11.74

δ18O (‰) –7.70 –7.70 –7.50 –7.50 –7.80δD (‰) –45.00 –46.00 –46.00 –45.00 –47.00

Table 3. Physical, chemical, and isotopic compositions of fivegroundwater samples

Each cation and anion was calculated from NETPATH computer program.

Tertiary Gampo Basin, Kyongsang Province, South Korea 399

Chlorite geothermometryChlorite has the potential to record invaluable

information about the physicochemical conditionsunder which it formed. From the relationship ofcrystallochemistry between formation temperatureand chemical composition, many researchers havenoted a systematic increase of AlIV(or converselya decrease in SiIV), ΣIV and (Fe + Mg) in chloriteas 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 oftetrahedral and octahedral sites, respectively.

The formation temperature of chlorite was cal-culated using the empirical geothermometer ofKranidiotis and MacLean (1987) and Jowett(1991). Tables 4(a) and (b) show the chemicalcompositions and calculated formation tempera-tures of 19 chlorites taken from 12 basalt, 4 gran-ite and 3 rhyolite samples. Despite the consider-able differences in calculated absolute tempera-tures using these two thermometers (Fig. 7), thereis a consistent difference between the chloriteformed in basalt on the one hand (lower T) and

that formed in rhyolite or granite (higher T) onthe other hand. The differences in formation tem-peratures indicate that chlorite in basalt has typi-cally experienced a lower temperature alterationthan those in granite and rhyolite. It is possiblethat chlorite in granite and rhyolite formed dur-ing a slightly higher temperature alteration eventthan the calculated temperatures because of over-printing during lower temperature alteration in acontinuous water-rock interaction episode. How-ever, chlorite formation temperatures in basaltsshow much higher temperatures relative to weath-ering conditions. De Caritat et al. (1993) notedthat chlorite composition depends not only upontemperature but also upon the nature of the coex-isting mineral assemblage and the bulk rock com-position or the physicochemical characteristics ofits surrounding groundwater. Accordingly, the for-mation temperatures calculated by chloritegeothermometry do not represent the temperatureof major alteration process for the study area.However, it is clear that selected chlorite frombasalt and other rocks (granite and rhyolite) hasexperienced different types of alteration.

Chemical change variationTo identify the behavior of chemical elements

in each zone as the degree of alteration increases,

Fig. 6. 1000lnα δ18Omineral-water versus 106/T2 diagram.Equilibrium lines of each mineral are from Kyser(1987). Symbols are the same as in Fig. 5.

Fig. 7. Al (IV) of chlorite octahedral site versus tem-perature (°C). Closed and open symbols indicate val-ues calculated from equations of Kranidiotis andMacLean (1987) and Jowett (1991), respectively.Circle: basalt, square: granite, and triangle: rhyolite.

400 C.-B. Im et al.

Table 4(a). Chemical composition (wt%), structural formulae (half-cell), and estimated formation temperaturesof selected chlorites occurring in basalts

37 representative samples (15 basalts, 11 granites,and 11 rhyolites) were selected. The total contentsof TiO2, MnO, and P2O5 were selected as refer-ence components for normalization of each majoroxide. The increasing or decreasing percentage ofany element X in a sample relative to each fresh-parent rock is calculated according to the equa-tion below, where Y denotes the total contents ofTiO2, MnO, and P2O5:

Change [%]= [(X/Y)sample/(X/Y)fresh parent rock – 1]·100.

Middelburg et al. (1988) have suggested that theparameter “Degree” represents an independentmeasure for the weathering degree of each sam-ple.

Degree = (1 – Rsample/Rfresh parent rock),

where R indicates the ratio of (CaO + Na2O + K2O)to (Al2O3 + H2O). The parameter “Degree” ap-proaches 1 whenever clay minerals such assmectite and kaolinite prevail and is equal to 0for unaltered rocks. Figure 8 shows the relation-ship between the “Degree” (Middelburg et al.,1988) and the geoengineering scale of ISRM(1975). Positive trends are indicated from the freshparent rock (F) to the highly altered zone (HA).However, the completely altered zone (CA) hasan irregular pattern, which is attributed to the en-richment and leaching of some elements frommore active water-rock interaction.

Major oxidesFigure 9 shows the compositional variations

of major oxides in each altered zone except forthe reference oxides of TiO2, MnO, and P2O5. TheSiO2, Al2O3, Fe2O3, Na2O, and MgO in basalt are

Tertiary Gampo Basin, Kyongsang Province, South Korea 401

relatively immobile in all zones of alteration,which reflect a greater resistance to leaching anddissolution of groundwater than oxides such asK2O and CaO. On the other hand, SiO2 in graniteand rhyolite shows mobile behavior, that mightbe attributed to the breakdown of silicate miner-als such as feldspar and micas by active water-rock interactions under weathering and hydrother-mal alteration. Depletion of CaO and enrichmentof K2O in basalt are observed. These patterns re-flect the greater alteration rate of plagioclase com-pared to K-feldspar, and the formation of illitefrom plagioclase and micas (Nesbitt et al., 1980).The considerable decrease of CaO relative to Na2Ois due to the greater decomposition rate of Ca-rich plagioclase in basalt and granite. Na2O and

K2O in granite and rhyolite are similarly decreasedover all profiles. For total iron as Fe2O3, a gradualincrease exists for granite and rhyolite. This pat-tern seems to reflect the formation of Fe-oxidesby oxidation of Fe-bearing minerals from activewater-rock interaction. MgO in granite and espe-cially rhyolite is increased as alteration degreeincreases. It is attributable to the formation of Mg-rich clay minerals like chlorite from the altera-tion of plagioclase and biotite.

Clearly, the variation patterns for SiO2, Al2O3,Na2O3, K2O, and MgO as a function of alterationin basalt are different from those of granite andrhyolite. We suggest that the former are causedby low temperature alteration processes, whereasthe latter result from the combined effects of both

Table 4(b). Chemical composition (wt%), structural formulae (half-cell),and estimated formation temperatures of selected chlorites occurring in gran-ites and rhyolites

T1 and T2 denote temperatures (°C) estimated by Kranidiotis and MacLean (1987), and Jowett (1991), respectively. FeOindicates total iron content. Cations were calculated on the basis of 14 oxygens per half-cell.

402 C.-B. Im et al.

hydrothermal alteration and weathering.Figure 10 shows stability diagrams for Na2O-

Al2O3-SiO4-H2O and K2O-Al2O3-SiO4-H2O at25°C and atmospheric pressure proposed by Harrisand Adams (1966). The two stability diagramsindicate that the local groundwaters are in equi-librium with kaolinite but not smectite (Fig. 10).This suggests that the geochemistry ofgroundwater during mineralogical alteration wasdifferent from that of the present.

Trace elementsThe compositional variations of trace elements

in each altered zone are shown in Fig. 11. It isclear 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 forbasalt are different from those of granite andrhyolite. Granite and rhyolite have similar pat-terns, although rhyolite shows somewhat moreprominent variations than granite. Ga, Y, Hf, Zr,Nb, and Ta in basalt are relatively immobile overall alteration zones compared with granite andrhyolite.

The dramatic increases of Rb and Cs in thehighly altered zone of basalt are similar to the

Fig. 8. The relationship between the weathering “De-gree” proposed by Middelburg et al. (1988) and theweathering scale by ISRM (1975). Dash line representsfresh parent rock. The arrow indicates line fit by eyefrom a slightly altered (SA) to highly altered zones (HA).MA and CA mean moderately altered and completelyaltered zones, respectively.

Fig. 9. Variation diagrams showing the percentchanges for major oxides with increasing degree of al-teration. The data plotted are average values for eachalteration zone.

behavior of K2O, and result from the well knowngeochemical affinity of K for these trace elements.Sr is less variable than Rb and Cs over all alteredprofiles, and is closely related with the variationof CaO. The patterns of Sr in granite and rhyoliteare more depleted than those of basalt. This indi-cates that high Ca groundwater can be attributedto the breakdown of Ca-bearing minerals such ascarbonates, Ca-plagioclase, and epidote by water-rock interaction (Fig. 12). The general leachingof Sr in granite and rhyolite is probably causedby more active water-rock interaction than in ba-salt along fractures, and by the high degree of al-teration of Ca-plagioclase.

Tertiary Gampo Basin, Kyongsang Province, South Korea 403

Fig. 10. System Na2O-Al2O3-SiO4-H2O (a) and K2O-Al2O3-SiO4-H2O (b) at 25°C and 1 atm. total pressure(Harris and Adams, 1966). The plotted dots and cir-cled field represent the geochemistry and geochemicalrange of 5 groundwater samples, respectively.

Fig. 11. Variation diagrams showing the percentchanges for trace elements in basalt, granite andrhyolite. The data plotted are average values for eachalteration zone. The dashed lines represent fresh par-ent rock. Symbols are the same as in Fig. 9.

The distribution patterns for Ba are similar tothose of K2O. The systematic decrease of Ba inprogressive zones of alteration can be attributedto the alteration of K-feldspar and biotite contain-ing appreciable Ba. Alteration of alkali feldspartends to release Ba in solution rather than beingabsorbed in altered zone. On the other hand, Bain basalt is enriched, which seems to be introducedfrom groundwater. The cation exchange capacityof clay minerals can cause the preferential adsorp-tion of Ba in more strongly altered zones of theprofile (Middelburg et al., 1988). Co in all alteredprofiles generally appears to be rather enrichedcompared with fresh parent rock. A sharp enrich-ment in all rhyolite profiles and in the completelyaltered granite is attributed to the addition of Coby hydrothermal alteration.

Ge is more mobile than Ga in all alterationprofiles. The general depletion of Ga in each rockmay be caused by more active water-rock interac-tion in fissures and joints. The general distribu-tion patterns of Y are very similar to those of REE.The trend of Y in basalt is an increase as altera-tion degree increases, whereas the reverse is truein granite and rhyolite.

U is easily mobilized and leached from parentrocks relative to Th, and its distribution pattern ismore marked in basalt. Absolute U and Th con-tents in basalt are considerably lower than thoseof granite and rhyolite. Generally, U in basalt in-creases gradually with increasing of alterationdegree and is sharply decreased at fracture sur-faces, whereas Th shows a very uniform patternover all profiles. U seems to be accumulated ratherthan leached in the groundwater-fracture system.

404 C.-B. Im et al.

HREE appear to be gradually enriched from theslightly altered zone to highly altered zone. Leach-ing processes of active water-rock interaction maycause HREE depletion in the completely alteredzone.

The distribution patterns of REE in granitesare similar to those of basalt, but have narrowerHREE and wider LREE ranges than those ofbasalts. This is caused by the formation of clayminerals and the differential dissolution rate ofbiotite and feldspar. Negative Eu anomalies ex-cept for the moderately altered zone are attributedto the breakdown of feldspar (Alderton et al.,1980).

Altered zones of rhyolite show mostly enrich-ment patterns relative to fresh parent rocks, al-though these patterns are a little irregular. REEcontents in the moderately altered zone are theprominently increased. This feature may be dueto sorption from groundwater by secondary min-erals such as Fe-oxides and clay minerals.

Fig. 13. REE abundances for alteration zones of ba-salt, granite, and rhyolite normalized to each parentrock. The data plotted are average values for each al-teration zone. The dashed lines represent fresh parentrock. Symbols are the same as in Fig. 9.

Fig. 12. Concentrations of major (a) and trace (b) el-ements of five groundwater samples.

Rare earth elementsThe REE have similar chemical properties and

generally show a uniform geochemical behaviorduring any given alteration history (Nesbitt, 1979;Kamineni, 1985; Middelburg et al., 1988). REEpatterns for each altered rock, normalized to theconcentrations of each fresh parent rock, are il-lustrated in Fig. 13.

In the basalt, the HREE tend to be enrichedwith increasing alteration, whereas LREE haveuniform patterns. However, the HREE in the com-pletely altered zone are much less enriched thanin the highly altered zone, and LREE show theopposite patterns. These imply that the LREE inleachable phases such as pyroxene, biotite, andfeldspar are easily liberated by water-rock inter-action, and subsequently enriched in the com-pletely altered zone. The leaching of the HREEcannot easily occur relative to those of LREE, and

Tertiary Gampo Basin, Kyongsang Province, South Korea 405

Landstrom and Tullborg (1990) noted that Fe-oxyhydroxide and carbonates are associated withhigh concentrations of LREE and HREE, respec-tively.

As described above, the REE in basalt have aregular fractionation pattern as the alteration in-creases compared with those of granite andrhyolite. These trends indicate that the parent ba-saltic rocks and their altered zones have beenlargely affected by a continuous low-temperaturealteration. In contrast, rhyolite has a relatively ir-regular distribution patterns. These trends canprobably be attributed to the complicatedgeochemical processes such as hydrothermal al-teration and subsequent lower-temperature altera-tion by rock-water interaction.

CONCLUSIONS

The area is composed of Cretaceous shale,Tertiary rhyolite, granite and basalt distributed inthe Gampo Basin. Rhyolite and granite, comparedwith basalt, appear to be highly fractured by tec-tonic movement.

Alteration minerals occurring in altered basaltsare smectite, kaolinite, chlorite, and rarely illite,whereas calcite is mostly located in fractures. Al-teration minerals in granite and rhyolite areepidote, chlorite, illite, and smectite, whereas Mnand Fe oxides, and calcite coat fractures. Calcite,epidote, and quartz veinlets are common in alteredgranite and rhyolite. In general, primary mineralssuch as pyroxene, biotite, muscovite, and feldspardecrease as the degree of alteration increases, andare mostly decomposed in intensively alteredzones. On the other hand, the amount of clay min-erals such as smectite, illite, chlorite, and kaoliniteis increased. Clay fractions of basalt have a lim-ited and much heavier δ18O composition than thosein granite and rhyolite. The formation tempera-tures of smectite in basalt, illite in granite, andsmectite in rhyolite (calculated by oxygen andhydrogen isotopic compositions) are from 14°Cto 29°C, 78°C to 83°C, and 37°C to 98°C, respec-tively. Chlorite in basalt was formed at lower tem-peratures (105°C~205°C) than the chlorite in gran-

ite and rhyolite (302°C~362°C). Based on theabove results, it appears that basalt has experi-enced only low temperature alteration such asweathering, whereas granite and rhyolite have thegeochemical characteristics of hydrothermal al-teration and superimposed weathering. The heatsource for hydrothermal alteration is inferred tobe related with the intrusion of concealed youngerigneous rocks.

Major oxides except for K2O and CaO are rela-tively immobile in all altered zones of basalt,whereas most major oxides are relatively mobileand show some irregular distribution patterns overall 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 rela-tively immobile during alteration processes. Thevariation patterns of Rb, Cs, and Ba are very simi-lar to that of K, whereas Sr is closely related withCa. In the basalt, HREE do not show large varia-tions as alteration increases, and LREE is depleted.In the granite also, REE do not show a large vari-ation as alteration increases. In contrast, alteredrhyolite has greater REE enrichment and positiveEu anomalies compared with fresh parent rock.Furthermore, rhyolite has relatively irregular REEpatterns.

These results mean that altered basalts thathave just experienced low temperature alteration(such as weathering) have indistinct variations ofmajor oxides, some trace elements and REEthough basalt does not have the prominent differ-ence of alteration degree and alteration mineral-ogy compared with granite and rhyolite. Alteredgranite and rhyolite, which have experienced 1ststage hydrothermal alteration (relatively low tem-perature) and 2nd stage weathering are character-ized by prominent variations of major oxides, traceelements, and rare earth elements.

Acknowledgments—We would like to express our ap-preciation to Professor R. J. Arculus, Department ofGeology, Australian National University, for his de-tailed readings of the manuscript and critical comments.This research has been partially supported by nuclearresearch and development program of Korean Minis-try of Science and Technology.

406 C.-B. Im et al.

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