Hydrothermal Alteration of Volcanic Rocks in the Hakone and Northern Lzu Geothermal Areas

Home , Hakone, Yugawara

Hydrothermal Alteration of Volcanic Rocks in the Hakone and Northern lzu Geothermal Areas

Tomio HIRANO

Hot Springs Research Institute of Kanagawa Prefecture*

Contents

Abstract ....-.-....2(74 ) 1. Introduction""""...... 2(74)

2 . Acknowledgements ...... -.-..2(74 )

3. Previous studies ..4(76)

4 . Geologic settins """"' ...... 5(77 )

5. Isothermal structure ..'...'...... 8( 80 )

6. Modes of occurrence of thermal waters .'.... 17( 89 )

7. The geothermal system of the Hakone volcano 34(106)

8 . Classification of clay minerals 41(l 13)

9. Hydrothermal alteration and hydrothermal minerals of the Hakone volcano 43(115)

10. Hydrothermal alteration and hydroyhermal minerals of the Yugawara geothermal area """""" ..... 55(127)

Clay minerals of the Atami-Ajiro geothermal area ...... 60(132)

Altered basaltic rocks in the Yugashima group of lower Miocene, Izu peninsula...... '...... 63(135)

３ . Water chemistry of the Yugawara thermal waters....'.'.. 80(152)

４ Summary ...... 89(161) References"" ...91(163)

Bulletin of the Hot Springs Research Institute of Kanagawa Prefecture. Vol. 17, No.3, 73-166, 1986.

r 997 Hakone-Youmoto, Hakone , Kanagawa, JAPAN, 2SO-03. -1- 74

(Abstrastl Hydrothermal alteration of volcanic rocks in the Hakone, Yugawara and Atami-Ajiro geothermal areas is studied. The isothermal maps of the Hakone and adjacent areas at sea-level or 400m below sea-level show that each geothermal area is clearly related with the present volcanic activity. Thermal waters discharged in the high temperature zones, center of the geothermal areas, are mostly of the chloride type. Thermal waters of each geothermal area are classified by their major dissolved minerals and defined their origins. Mineralogical properties of clay minerals in drill cores from each geothermal area show systematic variations with depth. Zonal mapping of hydrothermal alteration is based on the mineralogical change of clay minerals. Four zones are recognized in the Hakone geothermal area ; the kaolinite, smectite, smectite-chlorite and chlorite zones. The Yugawara and Atami-Ajiro geothermal areas are divided into three zones ; the smectite, smectite-chlorite and chlorite zones. Hydrothermally altered basaltic rocks of the lower Miocene, the Yugashima group, which belong to the tholeiitic rocks series are lower than fresh rocks in MgO, total FeO (FeO*0.9Fe,O,),and very high in AlzOs, H,O with a high Fe,O,/FeO ratio. Therefore the chemical composition of the altered rocks cannot be due to the influence of the present- ly saline thermal waters. Extraction of MgO and total FeO from original rocks must have occurred during diagenesis and low grade metamorphism of the Yugashima group. The stability relations among calcium-bearing minerals in the Yugawara geothermal area are discussed in terms of pH, Pco, and activity of Ca'*.

1. lntrodustion The Izu-Hakone area is known as one of the intensive geothermal zones connected with the Quaternary volcanoes. More than 40 geothermal fields are present in this area extending 80km north-south and 30km east-west (Fig.1-1). The intense hydrothermal activities take place in the central portion of each volcanoes,such as Hakone,Yugawar- a,Atami and lto. Hot springs of Kawazu,Mine and Shimokamo issue through fractures of the Neogene Tertiary rock. The heat source for these hot springs must be the crypto- volcanic activity of the Quaternary age. Various kinds of thermal waters are formed by interaction of country rocks. Hydrothermal alteration must reflect the physicochemical condition of the geothermal area. The purpose of this article is to describe the geologic features of the Hakone,Yugawara and Atami-Ajiro hydrothermal systems and to give a petrological in- terpretation of the genesis of hydrothermal alteration combined with water-chemistry.

2. Acknowledgements I would like to express my sincere gratitude to Proffessor Yoshio Ueda of Tohoku University, Professor Yotaro Seki of Saitama University and Professor Hitoshi Onuki

-2- 75 of Hirosaki University for their encouragement, constructive discussions and critical reading of this manuscript. I am much indebted to Dr.Yasue Oki of the Hot Springs Re- search Institute for his guidance in field work and in laboratory experiments, and for his valuable suggestions, encouragement and critical reading of the manuscript. Finally my thanks due to Dr.Koya Shimosaka of the Geological survey of Japan for his help in D.T.A. analyses of clay minerals and to Mr. Shigeru Hirota of the Atugi Health Center for his help in drawing figures. Mr. Kenneth MacDonald kindly read through the manuscript with a lot of advice for the improvement.

Fig. 1-1 Distribution of Quaternary volcanoes and geothermal discharge by thermal waters and steam. The outer circle of Hakone is thermal discharge by thermal waters and steam. The inner one is by thermal waters only (Oki and Hirano, 1974).

-3- 3. Previous studies Volcanoes of Hakone and Izu belonging to the Fuji volcanic zone have been studied geologically by many scientists, particularly by Kuno, who made many important dis- coveries regarding the origin of volcanoes and volcanic rocks through his study of these areas. The general geology and stratigraphic sequence of the northern four volcanoes.i.e. Usami.Taga.Yugawara and Hakone volcanoes, were established by Kuno (Kuno,1950a,b, 1951, 1952), who found that volcanic activity becomes progressively younger toward north. The Atami geothermal area has been studied by many scientists. Otuka and

Photo. 1 One of the famous geysers in Mine geothermal area.southern Izu peninsula. 77

Kuno (1932) mentioned that the source of the Atami thermal waters is in the Yugashi- ma group,lower Miocene,because of close relation in the distribution between the thermal waters and Yugashima group. The thickness of the Yugashima group is not yet clear, but is thought to be a few kilometers (Matsuda,1968). Detailed geological inves- tigation of the Atami geothermal area was carried out by Otuka (1943), who determined that faults lead the thermal water from far underground to shallow formations. Fukuto- mi (1937) classified the Atami thermal waters into two groups based on the differences in water chemistry. The former is characterized by a high temperature, a high discharge and chloride content, rising through fissures from deep to shallow, while the latter waters have a relatively low temperature, considerable content of sulfate and thermal waters belonging to the latter group spread widely. Yuhara (1961) noticed that the Atami thermal waters near sea are contaminated by seawater intrusion caused by over-discharging. Nakamura et al. (1969) observed that low temperature area was expanding yearly over the Atami geothermal area. Kanroji et al. (1979) divided the Ajiro thermal waters into two types, a chloride-dominated type of high salinity and a sulfate type of low salinity. The excellent work on the genesis of thermal waters by White (1957) exerts a deep influence in the study of geochemistry of hot springs in Japan' Sato (1961,1962) emphasized a zonal distribution of hot springs in Hakone. Yuhara et al' (1966, 1969) measured mass and heat discharge in the active fumaroles Owakudani and Sounzan and made a major contribution to our understanding of the heat budget of the Hakone geothermal system. Since 1961, Oki and Hirano have engaged in field work mostly on newly opened drill holes in the Hakone, Yugawara and Atami-Ajiro geothermal areas. They established a genetic model of the thermal waters of the Hakone and Yugawara geothermal areas. Sakurai and Hayashi (1952) discovered Yugawaralite as well as various kinds of zeolites in the Yugawara geothermal area (Sakurai,1953'1955).

4. Geologic setting The Hakone -lzu area has been the site of intense volcanism both below and above the sea since early Miocene time. The stratigraphic sucession of the rocks exposed here is summarized in Table 4-1 (Kuno,1950b). The Yugashima group, lower Miocene,the oldest unit forming the foundation of the volcanic eddifces of the area is a submarine volcaniclastic pile, several thousand meters thick. Basalt and andesite are predominant in the lower Miocene sequence. The Yugashima group is widely exposed in the backbone mountains of the central Izu penin' sula. Many small exposures of the Yugashima group are found in the bottoms of deeply dissected calderas and along the coast of the Izu peninsula. In the upper Miocene and Pliocene formations, dacite and rhyolite are common (Kuno,1952). The Shirahama group overlies in unconformity with the Yugashima group.

-5- 78

Shirahama means "white beach" in Japanese and comes from the light-coloured dacite. The Yugashima and Shirahama groups are broken into many btocks by faults. Although the displacement of each block is small, a considerable difference in hydrothermal alteration is seen between the rocks of the Tertiary and of the Quaternary ages, suggesting a regional hydrothermal alteration might have taken place during the Miocene and Pliocene ages. During the Pliocene era, the lzu area was gradually uplifted. By the end of that era,

Fig. 4-1 Geologic map of Hakone volcano and adjacent areas (Kuno, 1950b, simplified and parrly revised by Oki et al., 1978).

-6- 79 the coast line of the present peninsula had been settled. In the younger or middle Pliocene there was an effluence of basaltic magma in many places along the eastern coast of the northern Izu. They are identified as the Tenshozan basalt group, the Hata basalt group and the Ajiro basalt group. Thick piles of andesitic rocks such as the In- amura andesite group, the Awarada andesite group and the Tanna Tunnel andesites are contemporaneous with these basalt piles (Kuno,1950b,1952). In the Quaternary age, this area was again the site of intense volcanism which built up a number of the volcanic eddifices of the Fuji volcanic zone. The Quaternary volcanoes in the Izu peninsula are aligned into parallel lines running north to south. The eastern row is made up of the volcanoes of Hakone, Yugawara, Taga,Usami and Amagi.

Table 4-1 Stratigraphic succession ol Hakone volcano and adjacent areas(Kuno,1950b, partly revised by Oki et a1.,1978).

Lake deposits, talus and river gravels F Fuji volcano Hakone volcano Kanmuri-gatake pyroclastic flow and lava spine 2,900y B.P. CCg Kami-yama avalanche debris dammed up Ashinoko 3, l00y B.P. CCr-e Central cone lavas, including nuee ardente and mud-flow deposits Second caldera collapse and erosion 30,000y. B.P. P Pumice flows 45,000-70,000y. B.P. YS Young Somma Lavas 130,000y. B.P. First caldera collapse and erosion 200,000y. B.P. OSs Satellite cones OS+ Kintoki-san lavas .,, Somma Lavas OSz Andesite lavas OSr Basalt lavas and agglomerates ) 400,000y B.P. Erosion YV Yugawara volcano - Erosion TV Taga volcano Erosron B Basalts and andesites ol Pliocene(Younger or middle Pliocene) Qd Quartz diorite plug(Pliocene) Erosron Ta Ashigara group Older Pliocene -Pleistocene Ar Sukumo-gawa andesite group to Tz Haya-kawa tuff breccias Middle Miocene Erosron Mz Yugashima group Older Miocene

-7 - The western row is made up of the volcanoes Ida, Daruma, Tanaba, Jaishi and Nanzaki. Kuno (1952) recognized that the volcanic activity of this district in the Quaternary age had taken place from south to north with the exception of the Amagi and Tenshi volcanoes. Many geothermal areas have appeared in the central portion of each volcano. Fig.l ―1 shows the distribution of geothermal areas of the Hakone and Izu district together with thermal energy discharges given by circles. The total energy discharge from the areas amounts to 11 X 107 cal/sec (6.6 X 10fi kcal/min), of which Hakone has 27%, followed by Atami and then Ito.

5. Isothermal structure 5.1 Isothermal structure of the Hakone volcano In Hakone, there are about 300 deep wells for thermal waters. Most of the wells are located along the deep valleys and the eastern half of the Hakone caldera. Their average depth is about 500m, but several wells reach a depth of 1000m. A measurement of temperature has been made in more than 70 drill holes, and isothermal map at sea- level is drawn (Fig.5―1, Oki and Hirano,1970). The isothermal lines are almost concen-

Photo. 2 Owakudani steaming ground,in the bottom of an explosion crater of Kamiyama. 81

Fig. 5-1 lsothermal map of the Hakone volcano at sea level and distribution of drill holes (Oki and Hirano, 1970). a : Isothermal line(C) b : Caldera wall c : Fumarole d : Deep drill hole for thermal rvater tric and parallel to the topographic contour-lines of the present volcano. The interval between the isothermal lines of the western side is narrower than that of the eastern side. The isothermal lines are expanding toward the east, suggesting a flow mechanism of thermal water from west to east. Fig.5-2, is an east-west cross section illustrating the geologic structure and isothermal profile. Isothermal planes dip to the west more sharply than to the east. The isothermal inversion at 0.5-1km depth of the Ubako area may correlate with the inflow of colder water from the west to the east. Kuno et al. (1970) revealed that the basement rocks of the Hakone volcano are at a fairly high elevation within the western half of the caldera, slightly tilting to the east, are directly covered with a thick pile of central cones in the middle, and are exposed in

-9- 82

the bottom of Hayakawa valley in the east. This subsurface structure of the basement rocks seems to play an important role in the hydrological behavior of thermal waters. As shown in Fig.5-2, a considerably permeable zone acting as a major aquifer of thermal water is widely recognized in the basal part of the central cones. Most of the deep wells within the caldera reach this zone. By increasing the depth of a hole in drilling, the water level falls rapidly and finally becomes stable as the hole reaches the major aquifer. The trace of the water table of the zone is very flat in the western side of the central cones and become steeper toward the east. The elevation of the water table is essential- ly controlled by the water level of Lake Ashinoko (725m) in the west and that of Hayakawa valley in the east. The evidence of the water table again suggests that meteoric water infiltrates through the western side of the caldera, passing through the

500 ヽ

ＢＣ

ξ 。ＯＥご．２３ｐ〓一Ｃ及 ‥ ＥＯＮ〓５壺Ｃヨ言０ ∽

-500m― | Tlお Lゞ c ― _500m ‐1000m― ぶI 0 1 2 Кm BR 回

Fig. 5-2 East- weat cross section illustrating geologic structure (upper) and isothermal profile (lower) along A-B-C line shcnvn in Fig. 5-1 (Oki and Hirano, 1970). WT : Water table of the major reservoir of thermal waters, Aq : Aquifer, BR : Basement rock- s(Yugashima formation and Hayakawa tuff breccia)

-10- high temperature region of the central cones, and appears as hot springs in the eastern foot of Kamiyaraa.

5.2 Isothermal structure of the Yugawara geothermal area

The Yugawara geothermal area is at the bottom of the deeply dissected caldera of the

Yugawara volcano. Many deep-drill wells have recently been dug to pump up thermal waters for bath use. There are about 120 deep wells, the average depth of which is about 500m. The deepest one reaches 1200m. Thermal waters are discharged by means of air-lift pumps and their total discharge amounts to 6000mVday.

A measurement of temperature has been made in more than 15 drill holes and an isothermal map at sea ―level is drawn in Fig.5 ―3. Fig.5 ―4 is the NW ―SE cross section

of the isothermal map (Oki.Hirano and Suzuki,1974). The highest temperature area marked by the 70 °C isotherm occupies the Fudotaki area, the central part of the caldera.

5.3 Isothermal structure of the Atami ―Ajiro geothermal area

The Atami-Ajiro geothermal areas are situated in the bottom of the largely dissected caldera of the Taga volcano. The main geothermal field, Atami, covers a circular area

Photo. 3 Fudotaki.the center of the Yugawara geothermal area, is thought to be the vent of the Yugawara volcano. 84

―一」L_可__FKm Hakone/(1ldlilラ万 Δン▲▲▲ l▲ ●▲ `▲ `▲ `|▲ a a a aJ){^6 6 a 6 a a oV a a a Ｊ 6 A 6 d-)Z6l('666 A a a a \o a a £

t卿 :::i::ぶ :

▲ △ ` ` ``

aIビ

V V鞘 VVVI

囲皿%囲 □□日□ 05

Fig. 5-3 lsothermal map of the Yugawara geotheramal area at sea level(Oki, Hirano and Suzuki, 1974) with geologic map after Kuno (1950b). Mz : Yugashima group, Az : Inamura andesite group, Bz : Tensyo.zan basalt group, TV : Taga volcano, YV : Yugawara volcano, OS : Old somma of Hakone volcano, CR : Caldera rim, TR : Talus and river gravels Ａ＼ Yugoworo 800 600 400 200 0 -200 -400 -600

8 Km 」

Fig.5-4 :sotherma:prorile Of the Yugawara geotherma:area a!ong A‐ B‐ C:ine shown in Fig.5- 3(Oki,Hirano and Suzukl,1974).

-12- 85

Fig.5-5a :sotherma! map of the Atam卜 Aiiro Fig.5-5b lsothermal map of the Atami‐ Ailro geotherma:area at 100m be:ow the sea geotherma:area at 200m be!ow the sea !evel(Oki et al.,1974). :eve!(Oki et al,1974)

map of the Atami-Ajiro geothermal area at 400m below the sea level(Oki

-13- 86

Togo Coldero-

Ａｍ 9 1 7肺

Tag。 Coldr.'ro Yugoworc Cctderc-l

S―

AJIRO KAMI― TACA --i-

2 1 ＼６ 80 10080 9 1 7h

Flg. 5-6a (upper) lsothermal profile of the Atami-Ajiro geothermal area along E-W line shovyn in Fig. 5-5 (Oki et al., 1974).

Fig. 5-6b (lower) lsothermal profile of the Atami-Ajiro geothermal area along N-S line shourn in Fig. 5-5 (Oki et al., 1974).

6km in diameter. Ajiro, 6km south of Atami and located at the southern margin of the Taga caldera, is a minor geothermal field attached to Atami. There are about 400 deep wells in these areas. Isothermal maps of the Atami-Ajiro geothermal areas at levels of 100m, 200m and 400m below sea-level are given based on the bottom-hole temperature measured at several long breaks in the drilling operation (Oki et a1.,1974, Fig.S- Sa,b,c,). Fig.5-6a,b, are the vertical profiles running N-S ans E-W through the highest temperature area.

5.4lsothermal structure of Hakone and adiacent areas Fig.5-7 is an isothermal map of Hakone and adjacent areas at 400m below sea - level. Fig.5-8 is a N-S cross section of the isothermal map (Oki and Hirano,1974). The map suggests that important tectonic lines, through which a large amount of thermal water is discharged, lie parallel with the maximum compressional stress axis of this area. The map shows that several heat sources are aligned in north-south directions which are parallel to the east row of Quaternary volcanoes. Within each source area,

-14- 87

＝レユー

一 ¨ 一 ¨ 一一 ¨ 一

一ｒ， ‐ ，

Fig. 5-7 lsothermal map of the Hakone and adjacent area at 400m belovtt sea level (Oki and Hirano, 1974).

Stfi!------<]

D G a lalmD

Fig. 5-8 North-south cross section sholring the isothermal structure of the Hakone and adjacent area (Oki and Hirano, 1974).

-15- high temperature areas elongate in a NW-SE direction, a phenomenon best observed in the Atami area. This elongation, also observed at the Hakone and Yugawara areas, may indicate preferred development of open cracks in the NW-SE direction for the transfer of thermal water. The center of the geothermal activity where thermal waters discharge is encircled by many isotherms. The high temperature zones are surrounded by a low temperature zone, in which the temperature gradient is very small, sometimes less than 3 °C/100m. It is also important that many of the high temperature zones are found in the bottoms of the dissected calderas. The low temperature zones actually lies on the caldera rim and high mountains, from which infiltration of groundwater compensates for the discharged from the high temperature zones. The young volcanoes, such as Fuji, composed of thick piles of volcanic materials, do not display hydrothermal activity on the surface, because the thick piles perform the role of the low temperature zone and prevent the activity of the high temperature zone. A large, deeply dissected caldera will be the best for the development of hydrothermal systems. When the geothermal activity is very strong, separating the vapor ―dominated hydrothermal system from the hot water system (White et al.,1971), the high temperature zone will be found in central cones such as Hakone.

Photo. 4 We can not expect a hydrothermal activity on the surface of Mt. Fuji. 89

6. Modes of occurrence of thermal waters Thermal waters discharged in the center of the intense geothermal area are mostly of the chloride type. Table 6-1 gives the chemical composition of the high temperature waters from the intense geothermal areas along the east coast of the Izu peninsula. Two triangular diagrams (Fig.6-1a,b) show that only the Hakone zone III waters are sodium chloride type. The Atami waters are sodium-calcium chloride type. The high temperature waters of Yugawara belong to the sulfate-bearing sodium chloride type. The zone of high temperature water of chloride type is commonly surrounded by the zone of low temperature water of sulfate type, which sometimes has bicarbonate. Some warm waters in the coastal areas have an extreamely large content of dissolved salts, undoubtedly due to the percolation of sea water into the hot water system. The compositional variation in major anions and cations may reflect the intensity of geothermal activity and variation of hydrothermal alteration as well as the structure of hydrothermal systems.

Table 6-1 High temperature thermal waters of the lzu peninsula.

(ppm)

No. 1 2 3 Atagawa Mine Shimokamo

Temp.(℃ ) 100. 100. 100. pH 8.0 8.5 8.6

Na+ 820. 729.7 3815.9 K十 103.4 37.50 242.2 ca2+ 103.6 61.91 2184.1 Mg2+ 1.851 0.681 19.0 Fe2+ 1.80 0.004 0.9 A13+ 4.0 4.0 0.3 Cl~ 1032. 1058. 9820. S042- 582.8 161.9 261.3 HC03~ 122.8 77.64 8.3 C032- 21.90 39.93 H3B03 9.84 H4Si04 195. 48.7 104.5 Total 2989. 2220. 16466.

I . Atagawa Hot Spring(Table of mineral waters, Japan. 1954) 2. Mine Hot Spring(Table of mineral waters, Japan. 1954) 3. Shimokamo Hot Spring(Table of mineral waters, Japan. 1954) See : Hakone, Table 6-2a Yugawara, Table 6-3 Atami, Table 6-4

-17- 90

Fig. 6-1a Na+-Ca2+-Mg2+ diagram of Hakone and lzu thermal waters.

Fig. 6-1b Cl--SOr'z--total COz diagram of Hakone and lzu thermal waters.

Chemical types of thermal waters found in the Hakone, Yugawara and Atami-Ajiro geothermal areas can be classified by their major dissolved materials.

6.1 The Hakone geothermal area Four types of thermal waters were recognized in the Hakone geothermal area. Che- mical characteristics of the four types based on the relative abundance of major anions such as Cl-, SOI- and HCOa- are given below. Chemical compositions are shown in Table 6-2a,b.

-18- 91

ｏｎ type pH mttOr anions ｌ waters 3 SOlビ >Cl Acid-sulfate Ⅱ Bicarbonate-sulfate waters 6-8 HCO.,S012 >Cl Ⅲ waters 7-85 Cl~>SOi2,HC03~ Sodium-chloride Ⅳ Sodium chloride-bicarbonate- 7-9 Cl,SOF~,HC03 sulfate waters (lvlixed type)

Table 6-2a Chemical composition of Hakone thermal waters. (ppm)

Zone I Ⅱ Ⅲ Ⅳa

No. 1 2 3 4 Type AS. BS NaCl M Temp.(t) 49.7 57.5 91.5 65.5 pH 2.9 8.1 7.7 8.4

H+ 1.18 Li+ 0.0 0.068 2.43 0.27 Na+ 42.7 88.5 1490. 441. K+ 8.90 12.1 154. 39.8 ca2+ 87.4 140。 114. 106. Mg2+ 24.3 84.9 0. 16.8

Fe2‐ 0.099 0.56 0.105 0.257 A13+ 22.6 0.22 0.12 0.05

Mn2+ nd. 0. 0.007 0.44 Cl~ 7.15 19.8 2568. 617. HS04~ 52.4 S042- 526. 381. 81.5 226. HC03~ 0. 590. 29.7 287. C032- 1.72 2.11 H2B03~ 0.50 5.17 3.05 H3Si01~ 7.32 5.10 17.3 H3B03 8.17 172. 22.7 HlSi04 456. 293. 506. 347. C02 14.2 2.19 2.75 Total 1229. 1642. 5130. 2130.

Li+/Na+ 0.00077 0.0016 0.006 K+/Na十 0.21 0.14 0.10 0.09

Depth of wen Hot Spring 525m 506m 351m

A.S. : Acid sulfate water B.S. : Bicarbonate-sulfate water NaCl : Sodium chloride water of central cones area M. : Mixed type(Sodium chloride-bicarbonate-sulfate water of central cones area)

-19- 92

Fig.6-2 shows the zonal mapping of four types of thermal waters (Oki and Hirano,1970). Fig.6-3a,b are triangular diagrams of the three major cations and anions of Hakone thermal waters (Oki and Hirano,1970). Zone I : Zone I , characterized by acid-sulfate waters associated with solfataric fields, is found in the highest part of the central cones, Kamiyama and Komagatake. Zone II : Zone IL which is distinguished by its bicarbonate-sulfate waters with moderate temperature and pH, is widely distributed in the western half of the caldera. The depths of wells are usually several hundred meters. The content of chloride is very low as in the acid sulfate water of zone I . As deduced from the evidence of drilling, this type of water is reserved in the zone of the major aquifer developed in the basal part of the central cones. The groundwater discharged in the western side of the caldera has properties similar to zone II waters (Hirano et a1.,1971). The distribution and mode of the occurrence of zone II waters strongly suggest that the major part of HCO.- is supplied by the decomposition of fossil plants, which are commonly intercalated in the volcanic deposit.

Table 6-2b Chemical composition of Hakone thermal waters of Zone lVb(Yumoto area).

(ppm)

Zone Ⅳb・ H Ⅳb‐ L Ⅳb‐ C Ⅳb‐D

No. 101 25 94 68 Temp.(t) 67.1 45.4 62.2 48.5 pH 8.3 8.6 8.0 8.4

Evap.Res. 1472. 288. 4116. 468. K+ 7.95 0.93 17.9 1.69 Na十 468. 95.4 993. 139. ca2+ 51.6 4.98 432. 18.0 Mg2+ 0.46 0.044 0.60 0.020

sr2+ 0. 0. 1.84 0.05 Cl~ 694. 85.2 1845. 143. S042- 101. 20.2 616. 107. HC03~ 78.9 76.5 24.5 39.2 C032- 1.16 7.98 0.58 OH~ 0.07 H2B03~ 3.57 0.98 3.86 1.55 H3Si04~ 7.15 6.42 1.57 3.04 H3B00 23.8 8.21 64.3 10.4 H4Si04 144. 80.5 79.3 61.1 C02 0.59

Total 1582. 387. 4080. 525。

Depth of well 700m 212m 550m 364m

-20- Fig. 6―2 Zonal mapping of Hakone thermal waters. Zone I : Acid sulfate waters Zone II : Bicarbonate-sulfate waters Zone ID : Sodium chloride waters, Zone IV : Mixed type (Oki and Hirano. 1970)

Photo. 5 Ubako hot spring located in 1000m west of Owakudani discharges an acid sulfate water of zone I. Photo. 6 Many hot spring resorts are distributed in the eastern foot of Kamiyama.one of the central cones.

Zone HI : Thermal water over 90 °C is characterized by a considerable content of sodium chloride and an extremely low content of sulfate and bicarbonate. Three high temperature streams of sodium chloride water are found as zone ID in the eastern flank of Kamiyama. These waters originate as subsurface streams which start from a depth of 300m beneath an active solfatara, Sounzan (Soun ―jigoku), trend to the east, and finally appear as hot springs on the steep slope of Hayakawa valley. No sodium chloride water can be seen on the western side of Kamiyama.

Zone IV : In the eastern half of the caldera, most of the thermal water below 90 °C belong to zone IV. Some minor differences in chemical compositions are evident between the waters from the central cones deposits (zone IV a) and those from the basement rocks (zone Wb). Zone IV a : Zone IV a is mixed type waters restricted to the basal part of the central cones. As seen in Fig.6-3a and 6-3b, this type of water may well be explained by the mixing of high temperature sodium chloride water and low temperature bicarbonate ― sulfate water. There is a linear relation between temperature and chloride content (Fig. 6 ―4). The distribution of the mixed type of zone IVa waters is actually limited to the eastern side of the caldera. No mixed type can be seen in the western side of the caldera. 95

Zone lVb:Zone IVb is waters restricted to the basement rocks of the Hakone volcano. The thermal waters of the Yumoto area issuing from the basement rocks are classified as zone IVb (taUte 6-2b). The thermal waters of the present area are subdivided by their temperature and dissolved minerals. One is zone IV b-H waters characterized by high temperature and considerable amounts of sodium chloride together with a subordinate amount of sulfate and bicarbonate. The other is zone IVb-C waters marked by moderate temperatures and considerable amounts of calcium sulfate and sodium chloride (Fig. 6-5). Thermal waters discharged from drill wells along the Sukumogawa river, usually low in temperature and in dissolved materials, are formed by the mixing of zone IVb-H water with the surface water and can be classified as zone IVb-L group.

Zonc I

Zonr II

Zon. lll

Zone M

Fig. 6-3a Na+-Ca2+-Mg2+ diagram of Hakone thermal waters (Oki and Hirano, 1970).

Fig. 6-3b Cl--So12--total COz dlagram ol Hakone thermal waters (Oki and Hirano, 1970).

-23- 96 ィ ● Zone r o Zoneロ

ｇ o Zorp lV ヽ一Ｏ＃ＥＥ才

℃

Fig. 6-4 Temperature-chloride diagram of Hakone thermal waters.

The thermal waters discharged from wells along the Hayakawa river are also explained by the mixing of the salt-rich water of zone IVI-C with the surface water and can be classified as zone IVU-O group (Fig. 6-6a,b, Hirano et al.,lg?21.

… /

Fig. 6-5 Zonal distributlon ol zone I\l b waters in the Yumoto area (Hirano et al., Lg72l.

-24- 97

1鷲 ::

|:I:::

Fig.6-6a Na+― Ca2+― M92+diagram of zone Ⅳb waters in the Yumoto area,Hakone(HiranO et al.,1972).

0 。。 O° D 。 17b― 。

EC02 0 ｀υ ` Fig.6-6b Cr― So42-_tota:C02 diagram of zone Ⅳb waters in Yumoto area,Hakone(Hirano et al,1972).

Total discharge of thermal waters in the Yumoto area amounts 6000m'/day, which causes decreasing of the water level by 0.7 to 1.3 m/year, indicating over-pumping of the thermal waters.

6.2 The Yugawara geothermal area About 120 thermal wells in the Yugawara geothermal area are all drilled into the Yugashima formation, lower Miocene. Total discharge of thermal waters amounts to 6000m'/day, which causes a serious decrease in the water level 2 to 3 m/year, indicating over-pumping of the thermal waters. Three types of thermal waters, sulfate-bearing sodium-calcium-chloride type, calcium-sulfate type and high-salinity coastal thermal water are distinguished by the re-

-25- 98

lative amount of major dissolved minerals and their geographical distribution. Chemical characteristics of the three types of waters are given below. Chemical compositions of the three types are given in Table 6-3a,b. type pH maJor cations and anions

Sodium-calcium-chloride- 7.5-8.5 Na十 >ca2+>Mg2+ sulfate waters (Na-Ca-Cl- Cl~>S042 >Hc03~ so.) Calcium-sulfate waters 7.5-8.5 ca2+>Na+>Mg2+ (Ca-SO.) S042_>cl― ,HC03 Coastal thermal waters 7.5-8.5 High salinity Na+>Ca2+,Mg2+ Cl~>S042 >Hc03~

Table 6-3 Chemical composition

No. Tvpe NaCl・ CaS04 Intermediate

Temp.(t) 74.0 83.5 55.5 65.2 pH 8.3 8.1 8.5 8.3

Evap.Res. 2941. 1846. 790. 1212. Li+ 0.46 0.47 0.08 0.03 K+ 38.7 27.8 5.28 5.50

Na+ 717. 488. 145。 175. ca2+ 263. 125. 94.0 196. Mg2+ 0.91 0.27 0.0 0.39 Fe2+ 0.33 0.37 Mn2+ 0.06 A13+ 0.13 0.05 0.11 0.11 Cl~ 1286. 666. 117. 218. S042- 373. 426. 326. 517. HC03~ 70.7 44.7 60.4 40.4 C032- 0.67 0.33 0.56 0.38 HP042- 0.39 0.13 0.07 H3B03 37.17 14.59 5.37 14.0 H4Si04 148.4 185.5 68.4 92.7 H2C03 1.21 0.65 Total 2937. 1979. 823. 1260.

Cl~/S042- 3.45 1.56 0.36 0.422

I中 0.0559 0.0359 0.0169 0.0280 Depth Of well 650m 600m 500m 500m Discharge 89.0 78. 43. 12. (1/min)

-26- 99

The major salts dissolved in high temperature waters are sodium-calcium - chloride-sulfate (NaCl-CaSO.), whereas those in low temperature ones are calcium- sulfate (CaSO.) with subordinate amounts of bicarbonate and sodium-chloride (NaCl). High salinity coastal thermal waters are yielded in coastal area. A zonal distribution of the thermal waters is observed (Fig. 6-7). Thermal waters, discharged from high temperature area, are markedly high in Cl -/SO.'- with the ratio ranging from 1.5 to 4.1 , whereas those from the low temperature area are low,their ratios ranging from 0.005 to 0.1. The dissolved content of NaCl and the Cl-lSO.'z- ratio are higher in the high temperature thermal waters than in the low temperature waters. The distribution of the Cl-lSO.'- contours seems to suggest a fracture zone extending along the direction of the Fuzikigawa river, through which high temperature waters are coming up and flowing down at shallow depths, mixing with groundwater.

ol Yugawara thermal waters. (ppm)

CaS04 Coastal thermal water

52.5 60.0 39.2 8.2 8.2 7.50

1951. 1710. 32535. 0.03 0.01 0.72 1.77 2.03 188.

47.6 45。 7 8385.

514. 436. 2435。 0.24 0.08 943.1 0.18 0.15 0.05

11.1 13.6 17989. 1293. 1109. 2267. 19.3 21.2 80.7 0.09 0.20 0.15 0.04 3.49 13.58 55.8 63.7 0.41 9.70 1947. 1705. 32298.

0.009 0.012 7.94 0.0541 0.0463 0.686 651m 570m 820m 183. 98. 109. lt : Ionic strength

-27- 100

′ 。15く ‰く20 比 +ワ Q2 `Qく

Fig. 6-7 Zonal distrlbution ol Yugawara thermal waters based on Cl-lSOr2- ratio. lsotherms are at sea level (Oki, Hirano and Suzuki, 1974).

6.3 The Atami geothermal area The thermal wells in the Atami geothermal area are all drilled in the Yugashima formation, lower Miocene. There are about 400 wells and the average depth is about 300m. In the central part of Atami, the highest temperature area (Fig.5-5),there are about 20 wells, most of which are shallower than 100m in depth. Drill wells deeper than 500m are distributed in the marginal portion of the area. The deepest one reaches 900m. Thermal waters are discharged by pumping, and their total discharge amounts to 21000m'/day. Thermal waters of the Atami area can be classified into three types by the relative amounts of their dissolved materials and their geographical distribution. Chemical characteristics of the three types of waters are given below.

-28- Photo. 7 Na―Ca ―Cl―SO4 type water is discharged in the central part of the Yugawara geothermal area.

type pH major cations and anions

Sodium-calcium-chloride 7.5-8.5 Na+, Ca2+ > Mg2+ waters (Na-Ca-Cl) Cl" > SO,2" > HCCV

Mixed type of Na-Ca-Cl 7.5-8.5 Na+, Ca2+, Mg2+ waters and Ca ―SOi waters CP, SOr", HC03~

Coastal thermal waters 7. -8.5 High salinity Na+, Ca2+, Mg2+ cr > so,2- > HCOa-

Chemical compositions of the three types are given in Table 6 ―4. Fig.6 ―8 shows the distribution of the three types of thermal waters. Na ―Ca ―Cl type waters : Thermal waters over 90 °C are characterized by a relatively high content of Na+ (1500-2000ppm), Ca2+ (1000-1600ppm), Cl" (4000-5000ppm), a lower content of K+ (100-200ppm), SO.2" (200-300ppm) and a remarkabl low content of Mg2+ (l-25ppm). Thermal waters of this type are restricted to the Oyu area, in the central part of the Atami geothermal area. Predominant faults extending NW ―SE direction in the Oyu area (Otuka,1973, Kuno,1950) are accompanied with the high 102

temperature Na-Ca-Cl type thermal waters. Otuka (1943) made clear that NW-SE faults of the Oyu area lead the high temperature thermal waters from deep. Thermal water of Na-Ca-Cl type is understood to be the original thermal water in this area (Nakamura et a1.,1969). They reported that Na-Ca-Cl type waters showed no remark- able change in temperature and chemical compositions during past 30 years. Mixed type of Na-Ga-Gl waters and Ga-SO. waters : Mixed type thermal waters are distributed in the area surrounding the central part of geothermal area. The temperature of mixed-type waters ranges from 50 to 90 t. This type of water is well explained by the mixing of high temperature Na-Ca-Cl type waters and low temperature Ca-SOr type waters. It is supposed that Ca-SO, type waters may be yielded in the marginal region of the geothermal area. But typical Ca-SO. water just as in Yugawara area is not yet discharged. Goastal therma! watels : Yuhara described (1961) that some thermal waters near the

Table 6-4 Chemical composition of Atami thermal waters.

(ppm)

No. 291 292 242 290

Type Na・ CaoCl Mixed type Coastal thermal water

Temp.(℃ ) 95.0 94.0 50.0 60.5 78.0 54.6 pH 8.2 8.3 8.2 8.2 8.1 7.8

Evap Res. 2158. 2129. K+ 164.0 159.6 3.90 3.39 97.5 24.5 Na+ 1666. 1624. 319.2 328.4 3595. 2850. ca2+ 1081.4 1089. 336.0 325,9 3184. 3070.

Mg2+ 1.6 6.2 0.68 0.59 215。 9 93.3 Fe2+ 0.10 0.17 0.60 Mn2+

Cl~ 4521. 4405。 224.4 236.1 11187. 9114. Br~ 15.2 S042- 225.5 223.9 1163. 1141. 999.9 1313. HCOo~ 16.0 22.9 27.4 32.2 27.5 29.6 C032- 4.5 6.0 0.22 0.53 3.0 H4Si04 76.5 70.7 59.2 H£ 03 0.20 0.26 Total 7680. 7552. 2152. 2139. 19310. 16570.

Cl~/S042- 20.1 19.7 0.193 0.207 11.2 6.94

Depth(m) 112. 168. 495。 445。 243.2 493.

Discharge 80. 95。

(1/min。 )

(Nakamura et al.,1969)

-30- 103

Fig. 6-8 Zonal distribution ol Cl- contents in the Atami geothermal area (Nakamura et al., (1969). sea are contaminated by seawater. Seawater intrusion has been followed by a serious decrease in the water table due to over-discharging of thermal waters. However, no high-salinity thermal u/ater was not discharged 40 years ago. The present coastal thermal waters have more than 8000ppm of Cl- content. This type of waters shows a mod- ified ratio deviated from simple mixing of seawater and meteoric water. Thermal waters contaminated with seawater are widely distributed along the coast from Yugawara through Atami-Ajiro to Ito.

6.4 The Aiiro geothermal area The Ajiro geothermal area is located on the southern rim of the Taga caldera. There are about 40 deep wells and their average depth is about 600m. The deepest one reaches 1000m. Thermal waters are discharged by pumping with the total discharge of 3400m'/day. Thermal waters in this area can be classified into three types by the relative

-31- 104

amounts of their major dissolved materials combined with their geographical distribution. Chemical characteristics of the three types of waters are given below.

type pH maJor cations and anions

Coastal thermal waters 7.5-8.5 High saHnity Na+>Ca2+,Mg2+ Cl~>S042->Hc03~

Mixed type of coastal thermal 7.5-8.5 Na+>Ca2+>Mg2+ waters and Ca-SOo waters Cl~>S042 >Hc03~

Ca-SO, waters 7.5-8. ca2+>Na+,Mg2+ S042->cl―,HC03~

Table 6-5 Chemical composltion of Allro thermal waters.

No. 58 10 35S Tvpe Coastal thermal water Mixed type CaSO+type

Temp.(C) 45.0 46.5 54.0 43.0 53.0 pH 7.4 8.3 8.1 8.3 7.7

Evap.Res. 29518. 31334. 6258. 480. 1457. K+ 304. 150. 14.8 3.60 1.67 Na+ 8826. 8449. 1323. 43.0 74.0 ca2+ 767.6 1728. 748.3 70.0 366.9 Mg2+ 994.4 849.3 29.4 1.60 20.44 Fe2+ 1.12 0.70 0.57 1.00 0.40 Cl~ 16845. 16890. 3134. 10.59 11.41 Br~ 9.45 S042- 2171. 2339. 888.5 228.8 909.3 HC03~ 95.0 122.4 48.7 40.9 38.8 H3B03 2.33 H4Si04 64.2 62.4 72.0 52.8 Total 30078. 30591. 6268. 454.6 1423.

Cl~/S042- 7.76 7.22 3.53 0.046 0.013

Depth(m) 500. 4∞ . 5∞ . 700. 800. Discgarge 150. 13. 200. 70. 80. (1/min.)

No.5,8 and 10:Shizuoka Prefecture(1975) No.35 and S:Kanroii et al.,(1979)

-32- 105

Cl-:2500ppm

SACAMI BAY 7500

:0000 coaStal

l:38θ thermat water

Fig. 6-9 Distribution ol Cl- content in the Ajiro geothermal area (Kanroji et al., 1979).

Chemical compositins of the three types are given in Table 6-5. Fig.6-9 shows the distribution of these thermal waters. Coastal thermal waters : Coastal thermal waters are a chloride-dominant type of high-salinity waters. Thermal waters of this type show chemical properties caused by the interaction of seawater, meteoric water and t,olcanic rocks. K - and Mg'* in the coastal thermal waters are extremely low compared with mixed components of seawater and meteoric water, but Ca'- is high. The Ajiro hydrothermal system provides a good example of the interaction among present seawater, meteoric water and volcanic rocks at temperatures ranging from 40 to 100C. This problem is discussed in detail in chapter 12. Mixed type of coasta! thermal waters and Ca-SO. waters : Nlixed type waters are distributed in the inland area surrounding the coastal thermal waters. Ca-SO. waters : Typical Ca-SO, type water was found in deep well (800m) in 1978, in the marginal portion of this geothermal area (Kanroji et a1.,1979). Three deep well waters in the marginal area are classified into Ca-SO. type waters. Ca-SO, type water is moderate in temperature, relatively low in Cl- content and low in Cl-lSO,=- (0.013- 0.046). Dissolved contents of Ca'* and SO.'- are close to the saturation of gypsum (Kanroji et a1.,1979).

-33- 106

7. The geothermal system of the Hakone volcano As described in the previous chapter,in the Hakone,Yugawara and Atami-Ajiro geothermal areas characterized by large thermal discharge, brines rich in sodium chloride are usually found in the highest temperature area. The zone III waters of Hakone are similar to typical sodium chloride water (White,1957). Among these geothermal areas, the Hakone volcano provides the best geothermal system for the genetical study of volcanic thermal waters.

7.1 Compositional trend of therma! waters The compositional trend of the three major anions of the Hakone thermal waters is shown in a triangular diagram (Fig.7-1). Zone III waters are in the Cl- corner and zone I waters are in the SOn'- corner. Groundwater infiltrated through central cones tends to be richer in bicarbonate with increasing depth of burial. Cold groundwater restricted to the bottom of the caldera, having no obvious relation to the geothermal activity, is quite high in bicarbonate (Hirano,et a1.,1971). This may also suggest that bicarbonate dissolved in zone II waters are formed by the decomposition of fossil plants in the volcanic edifice.

Fig. 7-1 Compositional trends ol Hakone thermal waters (Oki and Hirano, 1970).

Zone IVa waters are well explained by the mixing of zone II and zone III lraters. The trend of zone IVa waters in the diagram is convex toward total CO, apex, suggesting the formation of bicarbonate during the mixing and flowing of thermal waters. The content of bicarbonate in zone III waters is extreamely low.

7.2 Genetic model of the Hakone geotherma! system Fig.7-2 is a genetic model of the Hakone geothermal system (Oki and Hirano,1970). Asymmetric patterns of the isothermal structure (Fig.5-1) and zonal distribution of

-34- 107

volconlc steom mctcorlcwotcr cond.rEcg H25。 C02 ∝ :d sulfate^lx107 sec↓ water l ◆i hm

scc bic 'col sullcte wat er Ucorbmtc- sullote wctr 0 t - lmircd typcl 3rl0'cqUsec I aaa NoCl wotcr - _lkm 野α :: :: 1 b super crit:cct GpB“ wlthはC: -4 hn ::

猛淋 ‐lom?

Fig.7-2 Genetic model ol Hakone hydrothermal system. : Repeated processes of vaporization and condensation of volcanic steam resulting in concen. tration of volatile components such as HzS and CO: b : Sodium chloride water(zone III) C : Super critical gases(steam) with NaCl(Oki and Hirano,1970) thermal waters (fig.6-2) as'well as the eastward inclination of the water table (Fig. 5- 2) all suggest the following mechanism for the genesis of the Hakone geothermal system. Groundwater which infiltrates through the western side of the caldera is flowing eastward, passing through the basal part of the central cones, and then contacts high temperature volcanic steam coming up through the volcanic conduit. At the depth of a few kilometers below the central cone Kamiyama, temperature and vapor pressure are high enough to dissolve a considerable amount of sodium chloride in steam (Sourira- jan,et al., 1962). By mixing of low temperature groundwater with high temperature dense steam, high temperature streams of sodium chloride water are formed that run through the permeable zone and mix with groundwater percolating down from the surface, and then finally appear as hot springs on the steep slopes of Hayakawa valley. At the top of the major reservoir being penetrated by the steam vent, the thermal water boils. The confining pressure on the thermal water decreases as it approaches the

-35- Fig. 7―3 Distribution of Hakone micro-earthquakes and the depth frequency relation plotted on an east-west cross section (data fom Minakami 1960, Minakami et al..l969 and Hiraga,1972), (Oki and Hirano.1974). surface. This means that most salts dissolved in the gas phase are left in the liquid phase. Condensation of secondary steam derived from depth may take place repeatedly within local layers of thermal waters in the body of the central cone above the major

Photo. 8 Sounzan(Soungigoku) steaming ground. 109 reservoir. With repeated processes of vaporization and condensation of thermal water, volatile components such as hydrogen sulfide and carbon dioxide are enriched in the gas phase, which finally appears as volcanic gases in solfatara. Local seismic activity sometimes takes place in the Hakone caldera (Minakami,1960, Minakami et a1.,1969, Hiraga et a1.,1971,1972,1973.1974). N{inakami reported that the Hakone earthquakes are of A type occurring in a narrow area bounded by the isothermal line of 100C at sea-level (FiS.7-3). Depths of the foci are generally shallower than 4km, mostly I to 2km below the surface. The generation of Hakone earthquakes may correlate with boiling of thermal water at various depths within the central cones.

7.3 Sodium chloride waters White (1957) emphasized the importance of sodium chloride in the origin of volcanic thermal waters. The genesis of the Hakone sodium chloride waters and the temperature-pressure conditions of dense steam rich in sodium chloride are considered. As shown in chemical anal-,-ses, zone III waters are extremely rich in sodium chloride, but poor in SO"- and HCO.-. The content of SO''- seems to reflect the degree of dilution with low temperature groundrvater.

50 t@ pp',.'pP,n 150 m SO;,

Fig.7-4 Cl--SO4'?- diagram of zone III waters (Oki and Hirano. 1974).

Fig.7-4 is a Cl --SO.'- diagram of zone III waters. For convenience of description, subscripts a,b,and c are put to each branch of zone III (Fig. 7-5). Zone III waters of each branch are placed on each straight line on this diagram (Fig. 7-4). Precipitation of Cl--bearing minerals is unlikely to occur in thermal water of Hakone. Neither is precipitation of sulfates such as gypsum and anhydrite expected from thermal waters, because they are unsaturated with calcium sulfate.

-37- lt0 ― 口月 ′い

Numbers attached to the thickFc:s;,ki,".?'lXltilTr?l,t"lr"#;.r tabre or the major reservoir in meter(Oki and Hirano, 1970).

By extrapolating the best fitting lines to SOn'- equals zero, a possible content of sodium chloride in the original steam can be derived. It is seen from the diagram that the Cl - content of original steam for zone IIIa is 6.010g/kg, which is equivalent of 9.907e/ks of sodium chloride. Similarly, that for zone IIIb is 3.406glkg, corresponding to 5.615g/kg of sodium chloride. Thus, the sodium chloride content of volcanic steam responsible for zone III waters ranges from 0.6 to 1% and the contribution of volcanic dense steam can be evaluated at 50 to 30%. An alternate check on the sodium chloride content of original volcanic steam can be obtained by a study of the total discharge of thermal energy and sodium chloride. The total discharge of sodium chloride by thermal waters directly related to the hydrothermal activity of Kamiyama has been determined to be O.22kglsec. The contribution of zone I and II waters to the discharge of sodium chloride is fairly small, about 0.01kg/ sec, and can be neglected. The greatest majority of sodium chloride is transferred by means of the zone III and IYa waters. Most sodium chloride is therefore supplied by high temperature dense steam coming up though the volcanic conduit. Yuhara et al. (1966) determined the thermal discharge from the solfataric fields of Kamiyama to be 0.7XlO'cal/sec, not including thermal waters from deep wells. Oki and Hirano (1970) established that the energy discharge by thermal u/aters from deep wells directly related to the hydrothermal activity of Kamiyama (zone III and a part of zone IV ) is f.S X l0?callsec. The total energy discharge by thermal waters and solfataric activity in Kamiyama thus amounts to 2.2X lO'cal/sec. If the enthalpy of steam is assumed to be 600kcal/kg as the first approximation, the

-38- steam discharge of 36.5kg/sec can provide the energy of the Hakone geothermal system. One third of thermal discharge is liberated through solfataric fields, and the other two thirds are provided by the hot water system. The quotient of the total discharge of sodium chloride (0.22kglsec) is close to 0.6% of sodium chloride in steam, which gives good agreement with the sodium chloride content of 0.56-0.91% estimated by Cl--SO.'- relation (Fig. 7-4).

7.4 Estimation of the temperature-pressure conditions of dense steam The temperature-pressure conditions of the volcanic dense steam have been ex- amined by Sourirajan and Kennedy (1962). The phase transformation of liquid to gas is the major cause of Hakone earthquakes. This means that the system has two fluid phases and the thermal brines are not saturated with sodium chloride. In Fig. 7-6, su- perheated dense steam at around 385C and 230 bars can dissolve about 0.5 to 1% of sodium chloride. When temperature decreases to 374T, the critical temperature of water, sodium chloride is hardly dissolved in steam, but mostly remains in the liquid phase. With minor variation of temperature and pressure at around 385 to 374T and 230 to 220 bars, about 1% of sodium chloride can be allowed in the gas phase or alternately in the Iiquid phase. The variation of depth of earthquake foci may be related to the variation of dissolved salts in thermal brines. At depths of I to 2km below sea-level, temperature and pressure of the geothermal system may be 385C and 220 bars or slightly larger. Assuming the pressure to be caused by the water column, pressure at a depth of 4km, the lowest limit of Hakone earthquakes, is 400 to 500 bars. On the diagram (Fig.7-6), isotherms of the two fluid phases at 400 to 500 bars should range from 450 to 480C , with several percent of dissolved sodium chloride.

o G.a coryllhn o Liala a crlllrol tolnl

Fig.7-6 lsotherms of HzO-NaCl system between 350'-450t, showing compositions of coexist- ing gases and liquids(Sourirajan and Kennedy, 1962).

-39- Photo. 9 A lava spine Kanmurigatake.Hakone volcano.appeared after the explosion crater was formed.

Photo. 10 Panoramic view of Lake Ashi and central cones. Table mountain named Byobuyama on the right side is Young Somma, Hakone volcano. 113

8. Glassification of clay minerals Clay minerals and chlorite are studied as follows. Rock specimens crushed into fine powder are dispersed in water and left to settle for about three hours. Grains less than two microns are collected from the 10cm layer at the top of the water column. The sus- pended grains are centrifugied, transfered to a glass slide and dried for X-ray diffraction. Clay minerals are classified into several categories which have been proposed by Seki et al. (1980) to indicate the temperature at which clays were formed. Based on the variation of basal spacing of d (001) treated with the ethylene glycol, Seki et al. (1980)

type ossociot ion Cu― K3 20(001)

メ

untrmted(uN) 昴

＝ olkoline mont- : mori[[onite 滞…d脳 ω

I : ボ| smectite smectite に : :°γ 4 5 6 7 0 9

(η 70 olkoline mont. F‐ + 0 ・° smec tite 1/

ωN)(T)「 ° 9‐ olkoline mont. l肝 :° smectite :・ H + smectite-cf{orite + mixed loyer smectite- T.T:楊ソ 1 2 3 4 5 6 7 3 9 chlorite mixed loya ° smectite + :T ポl 卜 H ilnectite- cHorite mixed loyer ''"1 (30)t,ro,{ur-u.r,

smectite- U‖ 0. chlorite 7..∫町 nixed loyer 'mixed loyer (P 156‐ 59

(uN) (60-61)

chlorite c hiorit e 1 ０一 (EC) １ ■ 川

' 2 0 4 5 6 , o 9 Fig.8-l C:assification of clay minera:s and ch:orite(Seki,Oki and Hirano,1980)

-41- 一日ヽ

Table 8-1 Classification of clay minerals and chlorite(Seki, Oki, and.Hirano, l9g0)

D.T.A. Type Association Cu‐ Kα 2θ (001) Cu_Kα 2θ (001) Endothermic Temp.(℃ ) (untreated) EG

I alkaline montmorillonite 7.0 5.0 70, 150, 650

I' smectite 6.0 5.0 70, 150, 650 smectite I * I' alkaline montmorillonite 6.2, 6.9-7.0 5.0 70, 150, 650 + ― smectite 卜ＮＩ二十 1 alkaline montmOrillonite 3.0, 6.0, 6.9-7.0 3.0, 5.0, 5.6-5.9 70, 150, 550, 650 smectite 十 + smectite-chlorite mixed layer smectite- chlorite I'+ I smectite 3.0, 6.0 3.0, 5.0, 5.6--5.9 70, 150, 550, 650 mixed layer + smectite-chlorite mixed layer

smectite- chlorite II smectite-chlorite mixed layer 3.0, 6.0-6. 2 3. o, 5.6-s. g zo, 150, sso mixed layer

chlorite I chlorite 6.0-6.1 6.0--6.1 550,

rEG I ethylene glycol treatment 115 divide clay minerals into seven types. Characteristic features of these types of clay minerals associations are given in Table 8-1 and Fig.S-1. Type I clay is alkaline-montmorillonite, type I 'clay is smectite, type II clay is smectite-chlorite mixed layer and type III is chlorite. Type I + I ', type I * II and type I ' ' t II clays are defined by the certain associations of type I , type I and type II clays. Together with the ethylene glycol treatment, treatment of the ammonium nitrate and heating at 550C are also made (Kimbara et a1.,1973' Sudo'1974).

9. Hydrothermal alteration and hydrothermal minerals of the Hakone volcano 9.1 Drill holes There are about 300 drill holes in the Hakone geothermal area. Core samples from 13 drill holes are subject for study of hydrothermal minerals (Fig.9-1). Drill holes O- l, O-2 and O-3 were bored in the Owakudani solfataric field. Mo-24 and Mo-28 drill holes were bored in the area 500m west and Mo-19, Mo-23, Mo-26 drill holes were bored in the area 1200-1600m west of Owakudani. Drill holes Mi-112 Mi-116 and On-131 were bored in the eastern foot of the central cone, Kamiyama. Drill holes Yu-103 and Yu-112 were bored in Yumoto where basement rocks of the Hakone volcano are exposed. Table 9-1 shows the classification of the thermal waters discharged from these drill holes.

0 Yu―:12

Fmgoyomo ノ 9 1 7h

Fig. 9-1 Locations of core samples collectted form drill holes in the Hakone geothermal area.

-43- 116

Table 9-1 Samples collected frorn drill holes in the Hakone geothermal area.

Thermal water Samples collected drill holes Depth (m)

0-1 59. 0-2 49.5 Zone I Acid-sulfate 0-3 132. Mo― -24 183. Mo― -28 190.9

Mo-19 409. Zote I Bicarbonate‐sulfate Mo―-23 210. MO―-26 557. Zoae il NaCl Mi-112 468. Mi-116 482. Zone IVa Mixed・ type 0n-131 655. Zone IYb Mixed・ type Yu-103 647. Yu-112 587.3

9.2 Zonal mapping of hydrothermal alteration Zonal mapping of hydrothermal alteration was done based on the mineralogical change of clays and four zones are recognized (Fig. 9-2). Clay minerals and chlorite occur as fibro-interstitial films, small radiating aggregates, microcrystals replacing glass shards and pumice or pseudomorphs replacing pyroxenes ０メＵＢ一一ＯＣＯｏｔＯ０っコ c 〓ｏ〓Ｅ o ００ｏＣ N ″ 〇〓Ｏ c ０一ま０一Ｏ o 一』〇 1400m 二 a 一ＺＡｏ０一０のＯｎｅゝま ■ ZoneA Ｎ３ 1200 ｏ Σ ｏ〓一〓 Σ ― ｏ 1()()0 一、一 Σ ｏエ 800 ″ 一 600 川一 400 ― 一 200 出一 0 30℃ ZilD 一 -200 9 1 7Km

Fig。 9-2 :sothermal profi:e and zonai distribution of hydrotherma:aiteration in the Hakone geoth‐ ermai area.Isothermal profile along A・ B‐ C line shown in Fig.9-1

-44- 117

ZONE 20ne A 20ne B zone c zone 0 kcothite s― tite chlorite

smedne{距 :.I 翼端Q{糠II i朧柑距品 ittite mordenite thomsonite dachiardite stilbite ctinoptilotite loumontite wai『 okite cristobolite quortz o lbl te tolc colcite 9yPsum onhydriie olunite mqgnetite hemotite py ri te

ep i dote odulorio prehnite

Fig. 9-3 Mineralogical sequence of hydrothermal minerals in the Hakone geothermal system.

The paragenetic relations among the hydrothermal minerals are summarized in Fig.9-3. Zone boundaries of hydrothermal alteration are roughly consistent with the isothermal profile (Fig. 5-2).

9.3 Hydrotherml minerals Zone A : Zone A is characterized by the appearance of kaolinite. Alunite and gypsum also present. Solfatalic fields are included in zone A. In zone A, acid sulfate water derived from the oxidation of HsS in the volcanic gases actively performs the leaching alteration of rocks. The rocks exposed are changed in colour to reddish brown or yellow- ish brown. Table 9-2 shows chemical analyses of the original and altered rocks. CaO,Mgg and FeO are almost leached out, followed by the decrease of Na,O, K,O, Al,O,

-45- 118

Table 9-2 Comparison of chemical component in original and altered Kamiyama lavas.

Kamiyama lava' Altered Kamiyama Comparison of each component in 1 cm3 (original rock) lavat' Original Altered

% % mg Si02 57.93 62.51 1627.8 812.6 Ti02 0.31 1.12 8.4 14.6 A1203 15.96 11.90 448.5 154.7 Fe203 2.83 0.17 79.5 2.2 FeO 5.08 0.00 142.7 0.0 MnO 0.19 0.00 5.3 0.0

MgO 4.93 tr. 138.5 0.0

CaO 7.87 tr. 221.1 0.0 Na20 2.84 2.17 79.8 28.2

K20 0。 70 0.38 19.7 4.9

H20+ 0.21 14.68 5。 9 190.8 H20~ 0.48 3.79 13.4 49.3 P205 0.16 0.15 4.8 2.0 S042- 2.83 36.8

Total 99.49 99.70 t Kamiyama lava ! Kuno(1962), Density, 2.81 rr Altered Kamiyama lava in Owakudani solfataric field:Density, 1.3 (Hirano et al., 1965).

Table 9-3 X-ray diffraction analysis and mineralogical composition of an altered Kamiyama lava in Owakudani sollataric field.

Altered Kamiyama lava d(A) I kaolinite alunite crystobalite residual plagioclase

4.92 13 4.31 89 4.20 3 4.11 100 4.05 38 3.81 62 ＊ 3.25 9 ＊ 3.22 15 2.98 31 ＊ 2.966 23 ＊ 2.951 15 2.490 14

-46- Table 9―4 Mineral associations in 0―1 drill cores.

Photo. 11 Owakusawa steaming ground,Hakone. An acid sulfate water performs the leaching alteration of rocks. 120

ギ+ +キ +≠キキキ

一０ ● 一、一 ● ｏ＋０＋ ● ２一Ｃ． ● 工ｔ一ＦＺ〓 ● ‘ ＋〓〓を＋〓 ´ さ ″ ０ ● つ一０〇 “ ０ ● 」 ● 一、 ¨ （一（〇一０〇）００）つ

A " L(Om)untほ。ted “ Fig. 9_4 Variation of the basa: spacing d(001) Fig.9-5 Variation of the basa:spacing d(001) of clay minera:s of Mo‐ 19, On‐ 131 and of c!ay nlinera:s of Mo‐ 19, On‐131 and Mi‐ 112 dri‖ ho:es treated with ethy:ene Mi-112 dri‖ ho!es treated with lN‐ g:yco:. NH4N03

一● ０ヽ一 ● ● ０Ｏ２Ｃ． ● 工一ヽＺこ ¨ ● ‘ 〓 ‘ ， ” 電５，２０３ ● ０一」Ｏ “ ｏ」（ ++幸 + ０ ¨ ０（０一０） ● ，）０

13 14 15,ted 6 ム d(001)untre( Fig. 9_6 Variation of the basai spacing d(001) Fig. 9-7 Varlation or the basa: spacing d(001) of ciay minera!s of Yu… 103 and Yu‐ 112 of ciay nlinera:s of Yu‐ 103 and Yu‐ 112

drl‖ ho:es treated w:th ethylene g:ycoi. dri‖ ho:es treated with l N‐ NH4N03.

-48- 121

and SiO:. H,O and SO,=- are added as new composition. Table 9-3 shows X-ray diffraction analysis of the altered Kamiyama lava. Soda-alunite. kaolinite and crystobalite are identified as altered minerals. Small amount of plagioclase and pyroxene remained. Clay minerals of O-1 drilled cored in the Owakudani solfataric field are summarized in Table 9-4. Together with kaolinite and alunite. smectite (t1'pe I') appears from 20m in depth. Zone B : Zone B is characterized by the appearance of smectite type I bnd type I * I'. However, smectite type I * I' is not common. In the western part of Kamiyama where bicarbonate-sulfate water is yielded, all clay minerals from the surface to the bottom (700m in depth) of Mo-19.Mo-23 and NIo-26 drill holes are classified as smectite type I ' (Fig. 9-a). In the eastern side of Kamiyama, clay minerals from the surface to 219-240m in depth in Mi-112 and Mi-116 drill holes which discharge high temperature sodium- chloride water are classified as zone B (smectite type I ' and type I + I '). In the 0n- 131 drill hole, clays from the surface and to 700m in depth are also classified as zone B. Bicarbonate-sulfate-chloride water (mixed type) is discharged from this drill hole. In the Yumoto area, the rocks from the surface to 515.5m in depth are defined as zone B by the clays of Yu-103 and from surface to the bottom (587.3m in depth) are defined as zone B by the clays of Yu-ll? (Fig.9-6,9-7). Zeolites of mordenite and clinoptilolite appear with smectite type I '. Calcite commonly occurs in zone B. Zone C '. Zone C is characterized by the clay mineral assemblages of type II (smectite- chlorite mixed layer clay), type I + II (alkaline montmorillonite associated with smectite-chlorite mixed layer clay) and type I'* IJ (smectite associated with smectite-chlorite mixed layer clay) (Table, 9-5. 9-6). In Mi-112 drill hole, cla,v minerals from 278to 386m in depth are classified as zone C (Table,9-5). In Mi-1126 drill hole, clay minerals from 230 to 382m in depth also fall inside zone C (Table, 9-6). In the Yumoto area, the rocks from 536m in depth to the bottom (647m) of Yu-103 drill hole belong to zone C (Fig. 9-6,9-7). Calcium zeolites of mordenite and laumontite appear in the clay mineral assemblages of type IL type I + II and type I '+ [ . Mordenite disappears in the middle horizon of zone C (Table, 9-5, 9-6). Calcite commonly occurred in zone C. Zone D : Zone D is characterized by the appearance of chlorite type III. Although calcium zeolites of laumontite and wairakite appear in zone D, wairakite is predominant. Calcite is also a common mineral in this zone. In the Hakone geothermal area, zone D is found only in the deepest horizon of Mi- 112 and Mi-l16 drill holes from which high temperature sodium-chloride waters are discharged. In the Yumoto area, zone D rocks have I'et to be observed.

-49- 122

9.4 Ctay minerals of Mi-112 and Mi-116 Table 9-5 and 9-6 show the clay minerals and Ca-zeolites change in Mi-112 and Mi-116 drill holes with the depth. The genetic sequences of hydrothermal alteration of volcanic rocks are as follows.

Tabie 9-5 Minera:associations in Mi‐ -112 dri‖ cores.

，， ● lt― t・ ● ● ■ ● ― 電〓 ● 〓 ● “ ● 〓〓〓， ● ，〓籠〓「５璧 ‥ ピ ■ 。日〓〓Ｌ一０と〓Ｏ・一〓日Ｅ彙 “ 口０こ口， ■ 〓 “ ■ 一ｏ ● ２ｒ ● 留〓こＪＣ ● 〓一一〓 ■ 〓３ＥＬＥ ■ ｔ１１Ｅ ● Ｅ ‘ ０Ｆ１Ｏ口Ｊ ■ 一＾〓 ●，・８〓西ｒ ■ 〓 ● ０５ョ「Ｈ ■ Ｅ ■ 〓Ｅ ■ ” ＦＯ ■ ● 電〓 ‘ ” ３お３ヨ，お燿Ｅ里ａ０ ● ０ ● ● ” 一 ● ” ） ● 一）ｍ０００ 0 ｍ０００００ 00 ００００ 00 ００００ 00 ０００００ 00 ００００ 00 ● ０００００ 00 ● ００００ 00 ● ００００ 00 ● ● ０００ O 00 ０００ ● 00 Ｏ ● ００００ 00 ● ００Ｃ００ 00 ｏ ● ００００Ｎ 0 00 ００ 00 １０００００に 00 Ｄ０００Ｌ 00 Ｐ００００Ｉ 0 Ⅳ ＾０００ 00 Ｌ０Ｆ０００ 00 ｂ０００ 0 ● ００００ 00 ● ● ０００ ● 00 ● ● ００００ 0 0 ● ０００ ● ０００ 00 ● ００００ 0 ０ ● ００ ● ００００ 0 0 ● ００００ 00 ００００００ ● ００００ 3260 00 ● ００００ 330J 000 ● ０００００Ｃ 00 3詢００ｏ００ 00 “ 3御０００００鰤 ● ００００ 0 00 34“ ０００００ 3000 0 00 ００００ 3m ０ 00 0 ００００ 3700 0 0 ● ００００ 0 00 3m0 ０００ 3ma ● ０ 0 卿 ● ０００ 0 0 0 側 ● ０００ 0 0 00 側 ● ０００ 00 側 ● ００００ 00 00 00 ０００ ● 颯０ 000 ● 側０００Ｃ 00 00 000 ０００Ｏ枷０ 0 0 0 0 ” ● ００００釧 ‐ 00 0 “ ● ００００ 066 66 0

-50- 123

( clay ) ( zeolite ) ZoneB Typel', I+I' mordenite ZoneC Typel+ll, I'+LI laumontite Zone D Type III wairakite

The stability of hydrothermal minerals reflects a distribution of temperature in drill holes(Tomasson et a1,1972, Kristmannsdottir, 1979). With increasing depth and temperature of drill holes, the basal spacings of clay minerals become smaller and clay minerals change from low temperature type to high temperature types. D.T.A. curves of clays of Mi-112 and IUi-l16 drill holes show their mineralogical change from low temperature types to high temperature types (Fig. 9-8,9-9). Endothermic peaks of shallow clays are lower in temperature than those of deeper clays. The chemical analy-

Table 9-6 Mineral associations in Mi-1 16 drill cores.

● （ ● ‐dOrlte “ ● ● ● ● Ｅ〓 ● ■ ” 〓Ｏ ― ・・〓，● “ ● 「 ● 営ｙ「 ” Ｏ ● ）〓【一 ¨ ” “ たＦ ● 一一２Ｘ・管】﹇Ｅ〓贅一Ｌ０一」ち一一〓Ｏ〓Ｎ】 ◆ ´ 【〓〓Ｘ ● ヨ〓〓Ｌ零Ｃ ¨ 呂 ¨ ０ａＸ ” Ｏ “ ０豊３ＯＣＣ “ “ 】 ● ０ “ 一昴二Ｖ α ■ 」０〓Ｏ０ＯＯ００ σ ヒヒ〓一Ｃぃ一〓 ● ００ α 缶営と一０ α げＬ，ｏｍ一０ α スＬ０ｋＣョ０コｎ一，こ』一０，０ ● ヽ０〇ヽＣ ● む〇つＮ二、、』卜ヽ，〓メ一０こ０一一Ｅ贅 ● ● σ “ 〓一 ■ 〇い〇０ ‘ ０ ” 一 “ “ ” 一一 ● 〇一 ´ “ ０〇Ｈ Ц◎ ０００Ｏ０珈０００Ｏ０Ｏ００狐◎ ０００００００ α０００００００珈◎ ００ＯＯ加◎ ００Ｏ OO 『◎ ０００Ｏ OO ｍ剛０００Ｏ 00 剛◎ ０００Ｏ 00 鰤◎ ０００ ◎ ◎ Ｏ 0 ｃ脚００００ＯＯ 00 剛Ｏ０ＯＯＮ O OO O 000 脚００ＯＯ OoO 0 ｍ００ O OO ｍ００Ｏ ◎ 0 00 獅００ ◎ ◎ 000 劉Ｏ００Ｏ O OO 劉 ◎ Ｏ０００Ｏ OO 劉 ◎ Ｏ００ O 側 ◎ Ｏ０Ｏ０Ｏ０剛０ＯＯ００Ｏ 0 ● 四 ◎ ００００ＯＣ蜘 ◎ ＯＤ０００ＯＯＮ剛 ◎ ＯＣ００００剛 ◎ ＯＣ００００ O 釧０００００ｍ００ OO 00 知００Ｏ O 0 00 剛００ OO Ｏ 0 o o ０００ 4100 OO 00 00 ◎ ００ 4200 O 0 0 ０００ 4250 O 0 00 0 0 ◎ ００ 4400 O 0oo o o Ｃ００Ｏ 44" O O OOO O o Ｎ０００ 4600 O 0 0 00 o ◎ ００ 4700 0 Ｏ００ 0 000 0 4020 O 0 000 0

-51- 124

9-8 D.T.A. curves Temp('C) for clay minerals from Mi-112 drlll

L6 hole.

/-\so, \/:ia---\-/-->.'

9-9 D,T.A. curves for clay minerals form Mi-l16 drill hole.

-52- Table 9-7 Chemical analyses ol clay minerals frorn Mi-l12 drill hole(Hakone).

(Wt%)

Depth(m) 151.0 198.0 166.0 326.0 330.1 366.0 344.0 420.0

Type I' I' I+1' I+Ⅱ I十 Ⅱ

Si02 57.26 47.05 47.84 58.06 45。 24 45.12 49.85 50.49 Ti02 0.54 0.61 0.80 0.77 1.08 1.21 1.54 0.27 A1203 18.29 10.43 14.48 16.01 19.96 15.18 17.73 17.71 | Fe203 5.99 6.13 5.50 3.83 4.42 5.13 3.86 2.44 ω FeO 1.11 3.46 1.75 3.52 4.85 10.34 10.56 9.75 | M■ 0 0.18 0.21 0.09 0.12 0.17 0.33 0.37 0.28 MgO 1.74 7.54 1.53 2.02 3.08 7.26 4.55 4.34 CaO 7.10 1.92 3.91 1.67 3.05 4.41 2.33 2.99 Na20 3.28 1.20 2.80 1.60 1.23 1.90 3.62 3.37 K20 0.66 0.20 0.27 0.89 1.51 0.91 0.34 2.11 H20+ 2.33 6.20 6.50 4.81 7.19 4.99 4.24 4.56 H20~ 2.04 14.40 13.45 6.45 8.60 3.02 1.16 1.31 P205 0.10 0.06 0.52 0.07 0.06 0.18 0.12 0.14

Total 100.62 99.41 99.44 99.82 100.44 99.98 100.27 99.76

ｄＮ９ Photo. 12 Mi―116 thermal well.Kowakudani.Hakone from which high temperature Na ―Cl water of zone E is discharged.

ses of clays support the fact that waters (H2O ― and H2O + )contained in the shallow clays are higher than that of the deep clays (Table,9 ―7), due to the dehydration that occurred at higher temperature. The pattern of zeolites distribution in increasing depths is also related to the dehydration due to the increase in temperature (Seki,1968, Steiner,1968, Ellis et al.,1977). 127

10. Hydrothermal alteration and hydrothermal minerals of the Yugawara geothermal area 10.1 Drill holes In the Yugawara geotherml area, there are about 120 drill holes. Among these holes, hydrothermal minerals of six drill holes together with some specimens collected from 日目ｏ

Fig. 10-1 Locations of core samples collected from drill holes in the Yugawara geothermal area. a : Caldera Wall b : Yugashima group c : Drill hole

ｍ８。。 Yugoworo 〓一６。。０一０ ● ４。。，」２。。

。

-2∞

-400 50 -600 6δ、ZONE B ZONE A

2 4 6 8Km

Fig. 10-2 Zonal mapping ol hydrothermal alteration in the Yugawara geothermal area. Isothermal profile along A-B-C line shown in Fig. 10-1. the outcrops of this area are studied. Three drill holes, TO (Tomita), OT (Otaki) and AM (Amano) are in the Fudotaki area, which is thought to be the vent of the Yugawara volcano and the center of the Yugawara geothermal area. The other three drill holes are located along the Titose river. HI (Hirogawara) is in Hirogawara, the northern mar-

-55- gin of the geothermal area and MO (Motor boat) is in Yoshihama, the southern margin of the geothermal area and very close to the sea shore (Fig.10-1).

10.2 Zonal mapping of hydrothermal alteration The Yugashima group (lower Miocene) exposed here is strongly affected by hydrothermal alteration and penetrated by dykes and veins. It is difficult to ascertain their original structures. Most of the mafic minerals such as olivines and pyroxenes are altered to chlorite, clay minerals and calcite. Plagioclase also recrystallized, as albitic plagioclase, zeolites, prehnite and sometimes calcite and anhydrite. Zonal mapping of hydrothermal alteration can be carried out with the help of the mineralogical change of clays,and three zones are recognized (Fig. 10-2). The paragenetic relations among the hydrothermal minerals are summarized in Fig.10 ―3. Zone boundaries of hydrothermal alteration are roughly consistent with the present isothermal profile (Fig. 5-4).

10.3 Hydrothermal minerals Zone A : Zone A is characterized by the appearance of smectite type I'. In the six drill holes, clays from the surface to a depth of 788m in MO-hole are classified as zone A. Zone B : Zone B is marked by the clay minerals assemblages of type II (smectite ―chlorite mixed layer clay) and type I ' + II (smectite associated with smectite ―chlorite

Photo. 13 Sampling of thermal water in the Yugawara geothermal area. 129

ZONE zone A zone B zone c smectite smec.-chlo chlorite

s認:fibo w"卜 H smec.― chiQ type:: chiorite typeHI muscovite mordenite stilbite loumontite yugoworolite woirokite prehnite o lb ite quortz co lc ite onhydrite gyp sum sphen e pyrite hemotit e Fig. 10-3 Mineralogical sequence of hydrothermal minerals in the Yugawara geothermal system.

一Ｏ υ 、ぃ一ぃＯＺＯ．Ｃ工 ● ２一ヽ ‘ ‘ ¨● 〓を ‘ ´ 〓 ≧ ● ´ ● 一 ■ ● ● ● 』一〇一０」（ ¨ 一０〈 ● ”０）００）０

‐ d(001) untrected d(001) untrected ~

Fig.lo-4 Variation of the basal spacing d(001) Fig。 lo-5 Variation of the basa:spacing d(001) of ciay minera:s(Yugawara)treated with of clay minerals(Yugawara〕 treated wnh ethy:ene g!yco:. lN‐ NH4NOI.

-57- 130

cdc ortygrp qt hrnuni epst 一一一一一一一一一一一一一一一一一一一一一一一一一一一一一一一三一一三一〓一一一一一

一一一一一

一一一一一一一一一一〓一

一〓一一一一一一一一一

Fig. 10-6 Occurence of vein minerals observed in drill cores from TO-drlll hole.

mixed layer clay). Clays of TO-hole from surface to a depth of 291m belong to zone B. Most of rocks exposed in the Yugawara geothermal area belong to zone B. Zone G :Zone C is characterized by the appearance of chlorite (type III).At AM-hole, about 400m south of Fudotaki,chlorite type III appeared at the depth of 250m. Core samples of OT-hole were collected from depths of 460m to 537m. Chlorite is pre- dominated over their hydrothermal minerals. In TY-hole, about 1300m south of Fudo- taki, transformation of the mineral assemblages from zone B to zone C was recognized at the depth of 617m. In the center of geothermal area, Fudotaki, the boundary between zone B and zone C is thought to be at 300m beneath the surface (Fig. l0-Z). No expo- sure of zone C was observed in the Yugawara geothermal area. Fig.10-4 and 10-5 show the variation of basal spacing of d (001) for clays of these

-58- 131 six drill holes together with those treated by ethylene glycol and ammonium nitrate' Ca-zeolites and other hydrothermal minerals : Mordenite and stitbite are common in the highest grades of zone A and zone B respectively. Yugawaralite occurs at the lower horizon of zone B. Laumontite is the most common calcium zeolite of zone C. Wairakite also is sometimes found in zone C. Veins and amygdals filled with calcite, anhydrite, gl'psum and calcium-zeolites are easily found in zone B and zone C. Fig.10-6 is an example showing the occurence of vein minerals in drill cores. A special feature is that calcite and anhydrite are the pre' dominant vein minerals with subordinate amounts of calcium-zeolites and quartz'

10.4 Mineral paragenesis The mineralogical sequence of hydrothermal alteration in the Yugawara geothermal area summarized as follows.

( clay ) ( zeolite )

Zone A Type I' mordenite Zone B Type I'+ [, I mordenite, stilbite, laumontite (yugawaralite)

Zone C Type III Iaumontite, wairakite

The mineralogical tendency in the Yugawara geothermal area is almost the same as that of the Hakone geothermal field. With increasing depth and temperature, the basal spacing of clay minerals becomes smaller and changes from low temperature types to high temperature types. D.T.A. curves of clays of four drill holes (MO,HI,TO,TY) clearly show this latter phenomenon (Fig.10-7). Endothermic peaks of clays move from low temperature to high temperature in accord with the mineralogical sequence from type I' to type III. ´´´~11]＼＼ |ヽJ`I][]}一＼

TY‐ 640m

Fig. 1O-7 D.T.A. curves for clay minerals from the Yugawara geothermal area.

-59- 132

11. Glay minerals of the Atami-Aiiro geothermal area 11.1 Drill holes Localities of three drill holes in the Atami-Ajiro geothermal area, Taga caldera, are depicted in Fig.11-1. IT (Itokawa)-hole was drilled to a depth of 300m in the center of Atami area through which the main tectonic fault runs in a NW-SE direction. BU (Busyari)-hole, situated in the southern margine of Atami, was drilled to 700m in depth. SH (Shimotaga)-hole, drilled to 603m in depth, is in the Ajiro geothermal area, the southern margin of the Taga caldera.

Fig. 11-1 Locations ol core samples collected lrom drill holes in the Atami-AJiro geothermal area. a : Caldera wall b : Yugashima group c : Drill hole

ZONE zone A zone B zone C smectite smec-chlo. chlorite smectite type:' 昌留ξftthiQ type卜 H smecrchia type ll :] 」 chlorite type III

Fig. 11-2 Sequence of clay minerals in the Atami-Aliro geothermal system.

-60- 133

B A fTogo Coldero

600m

AJIRO KAMl― TACA 400 2∞ 0 べヽ ′ -200 25°C l------zs - ′ ‐ ｏｏ ‐ -4∞ 30 406000 60℃ Zone 汎 Lnec 80 -600 =3″`h≠ or-----l------2K'

Fig. 11-3 lsothermal profile and zonal distribution of hydrothermal alteration in the Atami-Ajiro geothermal area. Isothermal profile along A-B line shown in Fig. ll-1.

11.2Zonal mapping of hydrothermal minerals Thermal waters of this geothermal area are restricted to the rocks of the Yugashima formation which are strongly affected by hydrothermal alteration as in the Yugawara geothermal area. Zonal mapping of hydrothermal alteration was carried out by the mineralogical changes of clay minerals summarized in Fig.11-2. Three zones are identified. Zone boundaries of hydrothermal alteration are roughly consistent with the present isothermal pofile (Fig. 11-3).

一● ● ヽ一一一〇２ ● Ｃ． ● 工Ｚ一、こ。 “● ‘ 。〓 ‘ ■ 〓一Ｄを ● Ｄ ´ ● ” つ００ ● “ 』０一 ● 』（一一〇（００一） ● 「）つ

13 :4 15 ted i6 A 13 14 :5 16 A d(00:) untrec d(001) untrected Fig.11_… 4 Variation of the basai spacing d(ool) Fig. ll-5 Variation of the basa!spacing d(001) of c:ay minerals(Atama‐ Aioro)treated of clay minera:s{Atama‐Aiorol treated with ethy:ene g:yco:. wnh lN‐ NH4N03.

-61- r34

11.3 Glay minerals The mineralogical features of clays for each zor,e are the same as in the Yugawdra geothermal area. Zone A : Zorre A is characterized by the appearance of smectite type I'. Clays from the surface to a depth of 300m in BU-hole and from the surface to 466.2m in SH-hole are classified under zone A.

lTOKAWA

Fig. 11-6 D.T.A. curves tor clay minerels from lT-drill hole(Atami).

SHIMOTACA

Fig. 1 1-7 D.T.A. curves for clay minerels from SH-drilll hols(Ajiro).

-62- 135

Zone B : Zone B is marked by the clay mineral assemblages of type I '+ [ (smectite associated with smectite-chlorite mixed layer clay) and type II (smectite-chlorite mixed layer clay). Clay of lT-hole from surface to depth of 199m , BU-hole from 300 to 600m in depth and SH-hole from 485m to bottom (603m) are classified as zone B. Zone C : Zone C is characterized by the appearance of chlorite type III. Zone C occurs at a depth of 300m drill core in lT-hole and at a depth of 600m in BU-hole. Fig.11-4 and 11-5 show the variation of basal spacing of d (001) for clays of these three drill holes together with those treated by ethylene glycol and ammonium nitrate. D.T.A. curves of clay of IT and SH-holes clearly show their mineralogical change with increasing depth (Fig.l1-6, lI-7). With increasing depth and temperature, the basal spacing of clay minerals become smaller and clays change from low temperature types to high temperature types. D.T.A. curves are useful to define the zone boundaries because endothermic peaks of clays move from low temperature to high temperature in accord with the mineralogical sequence from type I ' to type III.

12 Altered basaltic rocks in the Yugashima group of lower Miocene,lzu peninsula The Izu peninsula has been the site of vigorous volcanism since Miocene age. During the period from early to late Miocene, Honshu had been split into two blocks, the eastern and western halves, by the development of a huge volcano-tectonic depression call- ed the Fossa Magna. This event was followed by an extensive transgression of seawater with an accu- mulation of thick sedimentary pile made of submarine volcanic material. The early stage of the volcanic activity in the southern Fossa Magna region rvas characterized by tholeiitic basalt and andesite. The later stage of the volcanism was marked by the presence of rocks of the calc-alkaline series. The total thickness of these volcaniclastic sedimentary piles reached more than 10,000m and widely suffered burial metamorphism ranging from zeolite facies to pum- pellyite-prehnite facies. The Yugashima group, the thick submarine volcanic formation of lower Miocene, makes up the basement of the whole region of Izu. These rocks were hydrothermally altered in colour to dark green and are often referred to as the "green tuffs". As described in the previous chapters, the Yugashima group exposed in the bottom of the dissected caldera, the Yugawara and Taga volcanoes where the intense hydrothermal activity has taken place, is mostly composed of submarine pyroclastic rocks of basalt and andesite strongly affected by hydrothermal alteration. The major hydrothermal systems of the Yugawara and Atami-Ajiro areas are restricted to fractures of the Yugashima group. It is supposed that the hydrothermally altered Miocene rocks may be affected by the present thermal activities, but chemical composition of rocks of the Yugashima group have scarcely been studied from the view point of rock-water in-

-63- 136

teraction.

12.1 Ghemical compositions of altered rocks Eighteen chemical analyses were carried out on the hydrothermally altered basaltic rocks collected as drill cores from the Yugashima group lying in the Yugawara and Atami-Ajiro areas (Fig.12-1). Two specimens, one from the outcrop of Fudotaki, the center of the Yugawara geothermal area and the other from Yugashima, the middle part of the lzu peninsula, are analyzed together. As indicated above, hydrothermal minerals of these altered rocks have been studied and three zones of hydrothermal alteration are identified. In particular, to examine the present seawater-rock interaction, SH,IT,AT and MO drill holes are important because these wells are very close to the sea and are discharging high salinity coastal thermal waters. Igneous primary phases such as pyroxenes and volcanic glass were altered into smectites and chlorite. Calcic plagioclase was altered into albite and Ca-zeolites. Cal-

t__i___i___rK,,,

Flg. 12-1 Distribution of the Yugashima group and drill holes lrom whlch altered basaltic rocks were collected for chemical analyses. a : Caldera wall b : Yugashima group c : Drill hole

-64- 137

Table 12-1a Chemical analyses and norms of altered basalts and andesites. -Ajiro, Atami area- (Wt%)

Speciment Shimotaga Busyari Depth (m) 390 600--650

Si02 50.27 47.04 47.69 56.87 46.04 49.30 T102 0.76 0.93 0.64 0.60 0.69 0.65 A1203 18.02 14.57 16.43 16.58 21.14 17.31 Fe203 3.43 9.09 5.73 4.21 4.40 4.53 FeO 6.64 4.55 3.49 1.81 4.50 5.03

M■ 0 0.16 0.20 0.14 0。 13 0.13 0.17 MgO 3.53 4.33 6.13 2.75 3.48 6.95 CaO 11.20 7.02 9.56 7.96 12.10 10.85 Na20 2.66 3.00 1.53 2.97 1,75 1.89 K20 0.17 0.23 0.22 0.36 0.35 0.36 H20+ 1.31 2.55 2.35 4.07 2.83 1.78 H20~ 1.31 6.25 5.01 0.93 1.08 1.44 P205 0.03 0.03 0.04 0.05 0.03 0.03 C02 0.00 0.00 0.35 0.24 0.00 0.00 S03 0.00 0.00 0.11 0.20 1.07 0.00

99.49 99,79 99.42 99.73 99.60 100.29

Ｑ 5.37 10。 19 11.90 20.60 5.00 5.00 Ｏｒ 1.03 1.47 1.40 2.20 2.10 2.30 Ａｂ 23.22 27.90 14.00 26.60 14.80 17.10 Ａｎ 37.90 28.16 40.60 32.60 49.00 40.20 Ｄｉ 15.90 7.69 6.10 4.90 11.80 9。 90 Ｈｙ 9.89 8.28 14.40 4.90 9.50 17.10 ＣＨ 1.93 1.30 1.20 1.30 1.30 Ｍｔ 13.87 9.00 4.80 6.40 7.00 Ｈｅ 0.42 1.20 Ａｐ 0.08 0.10 0。 10 0.10 0.20 Ｃａ ‐ 0.90 0.60 Ａｈ 0.20 0.30 1.80

-65- 138

Table 12-1b Chemical analyses and norms of altered basalts and andesites. -Atami, Yugashima area- 山 (Wt%) Speciment ３００ Atamikaigan Yugashima Depth (m)

Si02 47.11 48.19 48.18 49.82 T102 0.62 0.62 0.50 0.83 A1203 18.22 18.57 16.47 17.68 Fe203 5.07 7.53 6.67 3.77 FeO 4.48 2.44 2.44 5.57 MnO 0.17 0.17 0.15 0.10 MgO 4.49 3.31 4.76 3.76 CaO 9.33 9.70 8.26 9.47 Na20 2.25 2.38 2.22 2.17 K20 0.16 0.31 0.17 0.61 H20+ 5.40 2.20 3.09 3.73 H20~ 1.22 4.46 5.89 1.02 P205 0.03 0.02 0.03 0.08 C02 0.93 0.17 0.67 1.36 S03 0.34 0.00 0.00 0.∞

99.82 100.07 99.50 99.97

Ｑ 8.40 10.34 13.59 10.99 Ｏｒ 1.∞ 1.97 1.11 3.80 Ａｂ 20.30 21.55 20.91 19.26 Ａｎ 41.70 41.81 38.40 38.50 Ｄｉ 6.56 0.97 0.99 Ｈｙ 16.50 5。 77 12.37 15.62 Ｃｎ 1.30 1.25 1.05 Ｍｔ 7.90 7.11 7.71 Ｈｅ 3.16 2.11 Ａｐ 0.05 0.08 ｃ ‐ ａ 0.42 1.69 Ａｈ

-66- 139

Table 12-1c Chemical analyses and norms ol altered basalts and andesites. -Yugawara area-

(Wt%)

ＭｂＫｂｅｎｅＴｏｋ Speciment ・ｙ。 Makigami ６４５ ‐ ９０２３２ Depth (m) 401--430

Si02 49.11 49.71 49.44 47.07 48.21 50.53 T102 0.54 0.61 0.81 0.79 0.87 0.67

A1203 15.12 16.04 13.99 18.72 19。 91 18.89 Fe203 5.43 4.93 9.75 4.10 4.83 4.13 FeO 4.03 5.35 2.30 6.02 4.92 4.91 MnO 0.12 0.15 0.22 0.12 0.15 0.20 MgO 3.83 5.34 5.43 5.63 3.25 3.51 CaO 9.09 10.19 5.18 11.17 11.34 10.29 Na20 1.95 2.14 3.52 1.73 1.72 2.26 K20 0.29 0.28 0.88 0.23 0.22 0.35 H20+ 2.55 1.22 2.25 1.20 2.72 2.51 H20~ 4.98 2.78 6.10 0.63 1.27 0.52 P205 0.02 0.03 0.04 0.04 0.03 0.05 C02 2.58 0.43 0.13 1.78 0.61 0.00 S03 0.00 0.00 0.00 1.06 0.54 0.74

99.64 99.20 100.04 100.29 100.59 99.56

Ｑ 19.02 8.90 8.67 7.50 10.50 6.30 Ｏｒ 1.88 1.75 5.65 1.30 1.30 2.30 Ａｂ 17.94 19.00 32.48 14.80 15.00 20.50 Ａｎ 31.16 35.00 21.57 40.80 47.70 43.40 Ｄｉ 12.07 4.08 3.60 7.00 Ｈｙ 12.79 13.45 12.86 21.00 10.30 11.10 Ｃ 1.16 0.90 Ｉｌ 1.12 1.21 1.67 1.50 1.80 1.40 Ｍｔ 8.54 7.51 6.31 6.10 7.30 6.50 Ｈｅ 6.28 Ａｐ 0.07 0.11 0.10 0.10 0.10 Ｃａ ‐ 1.03 0.33 4.20 1.50 Ａｈ 1.80 0.90 1.40

-67- 140

Table 12-ld Chemical analyses and norms of altered basalts and andesites. -Yugawara area2- (Wt%)

Speciment Tomita Depth (m) 752

Si02 44.65 47.04 44.85 42.81 T102 0.76 0.62 0.49 0.58 A1203 18.26 18.80 13.41 19.45 Fe203 3.36 4.13 4.36 7.47 FeO 6.37 3.94 6.42 2.10 MnO 0.23 0.18 0.15 0.03 MgO 4.77 2.96 3.87 5.50

CaO 10。 77 12.80 7.74 10.54 Na20 1.00 1.57 0.29 1.41 K20 0.11 0.20 0.07 0.18

H20+ 4.30 1.84 4.74 3.21

H20~ 1.11 0.89 5.62 5.88 P205 0.03 0.03 0.02 0.02 C02 3.56 4.50 2.60 0.37 S03 0.26 0.43 5.63 0.00

99.54 99.93 100.26 99.55

Ｑ 15.60 17.40 36.25 5.54 Ｏｒ 0.70 1.20 0.43 1.17 Ａｂ 8.90 13.70 2.73 13.16 Ａｎ 31.70 34.20 2.56 51.12 Ｄｉ 3.11 Ｈｙ 21.20 10.80 19.37 13.70 Ｃ 6.00 3.90 13.33 Ｈ 1.50 1.20 1.03 1.22 Ｍｔ 5.20 6.20 7.02 5.71 Ｈｅｍ 4.32 Ａｐ 0.10 0.03 0.03 Ｃａｌ 10.50 6.57 0.93 Ａｈ 0,70 10.67

-68- 141

SiO,Cwt,お )

′ ｃＡ ′′ ′ＴＨ 0ヽ ′ .小。／ 0 浄 ∴ ．、｀ “ 、TH CA｀、・・ヽ一ヽヽ

~ 1 2 5 231 予3 4 5 reolMso FeO′ M90 Fig. 12-2a SiOz vs. FeO r /MgO diagram of Fig. 12-2b FeOtt vs FeO・ /MgO d:agram of altered basaltic rocks. altered basa:tic rocks.

cite, anhydrite and gypsum are very common minerals replacing the primary minerals, and magnetite and hematite are also common. The analyses of the altered Nliocene rocks are shown in Table 12-la,b.c,d together with their C.l.P.W.norms. The altered rocks have a high Fe,O./FeO ratio (0.52-4.24) and large content of HrO and AlrOr. CO: and /or SO. are contained in many specimens because calcite and anhydrite occur as hydrothermal minerals. In the C.l.P.W.norms, magnetite is particularly high (4.80-13.9%). Fig. l2-2a,b show the chemical behaviour of the altered rocks in the SiO,-FeO * /MsO and FeO* -FeO*Ztutgo diagrams (FeO*=FeO* 0.9Fe,O,) (Miyashiro,1974). The plots of the altered Miocene rocks lie in the tholeiitic rocks field.

12.2 Original chemical compositions of the Miocene rocks From already published chemical analyses of the rocks of Neogene Tertiary of the lzu area (Geol.Surv.Japan, 1962), rocks of the tholeiitic series are selected. Using these data, Haker's variation diagram is given in terms of water-free presentation. The general trend of each oxide calculated by the least squares method is taken to be the baseline of the original Miocene tholeiitic rocks in the Izu area (Fig.l2-3). Tholeiitic rocks of Quarternary volcanoes of the Izu-Hakone area (Kuno,1976) show a trend similar to that of the Miocene tholeiitic rocks (Fig.12-3).

-69- 日ヽＮ

(wtOr.) ＡＢｍｏｃＯｍＲ０ｏ

ｏ ◎ Ｍ９。

Ｘ０Ｃａ。 ¨ ▲ ０ｍ０Ｋ２。

A1203 A1203 15

・Totαl FeO ,0

● Cα0

MgO fNα20 K20 45 5o s,o(rrfl,) 55 60 Fig. 12-4 Variation diagram ol altered basaltic rocks(water free). Fig. 12-3 Variation diagram ol Miocene tholeiitic rocks in the lzu peninsula(water free). A : Miocene tholeiitic rocks(Geol. Surv. Japan,1962) B : Tholeiitic rocks of Quaternary volcanoes of Izu-Hakone area(Kuno,1976) 143

12.3 Gompositional change of hydrothermally altered rocks Converting the analytical data of the altered rocks to water-free presentation, their variation diagram is made with the general trend of each oxide of original rocks (Fig.12-4). It is noted that MgO and total FeO (FeO*0.9FerO') contents of altered rocks are significantly reduced than the original trend by I to 4% (Fig.12-5b,d). Na,O is slightly in' creased (Fig.12-5e). But most of the altered rocks do not change significantly in CaO (Fig.12-5c), and no significant change is seen in K,O (Fig. 12-5f). These compositional changes of the altered rocks are hard to interprete due to the present hydrothermal alteration caused by the coastal thermal waters, low in Mg'* and K* and high in Ca'*.

A1203 °

50 sror(It%) 55 60

Flg. 12-5a AlzOs vs. SiOz diagram ol altered basaltic rocks(water free).

Flg. 12-5b Total FeO vs. SiOz diagram ol altered basaltic rocks(water free)

-71- 144

Fig. 12-5c CaO vs. SiOe diagram ol altered basaltic rocks(water free).

Fig.12-5d MgO vs.Sio2 diagram of anered basanic r∝ ks(water free)

● 50 s,。 .J.1 55 2(嗽 Fig.12-5e Na20 VS.Si02 diadram of anered basanic r∝ ks(water

twt7.l I K20

Fig.12-5f K20 VS.Si02 diadram of anered basaLic rocks(water free).

-72- 145

12.4 Geochemistry of coastal thermal waters Coastal thermal waters in the Atami-Ajiro and Yugawara areas are restricted to the outside of the sulfate zone. They are low in temperature and high in Cl- and SO.'-. These waters are formed by the mixing of meteoric water and sea water. The 0 D- d"O relation of coastal thermal waters supports this view (Fig.12-6). The heat sources of the geothermal activities in these areas are derived from the volcanic activities of the Quaternary age. Fig.l2-7a,b,c,d,e,f and g show that temperature, pH and compositional variation of major chemical dissolved in the Atami-Ajiro and Yugawara coastal thermal waters are

5189晏〆or・ 0) -6 -5 -4 -3 -2

●Yugawara o Atami -10 5DsMow

(Or。。) -20

Fig. 12-6a 6 D vs. 6 160 plot of coastal thermal waters in the Yugawara and Atami geothermal areas (lsotopic data of Atami thermal waters : Matsuba!'a et al., 1973).

O Vugowara -1 oAtami / ヽ独 ¨/

-4

-5

-6 o loo 200 300 400 500 600 C「 (meq) Fig.12-6b δ 180 vs.c!~p:ot Of cOastal therma:waters in the Yugawara and Atami geothermal areas. SW:Seawater

-73- 146

科

o too M 3oo 4oo 5oo 6oo meq 100 200 000 400 500 600 Fig.'12-la Temperature vs. Cl- diagram. Fig.12-7b pH vs.c!~diagram.

# ,+ +

卜£ 3X°

o r@ z@ 3oo 4oo soo 600 100 200 300 400 500 600 Fig. 12-7c K+ vs. Gl- dlagram. Fig.12-7d Na+vs.C:~diagram.

a function of the Cl- concentration. The orifice temperature of waters ranges from 30 to 82C, with the average temperature being 53C (Fig.12-7a). The maximum temperature observed at a bottom of drill well is 95C. Na* , K* and Mg'* of these coastal thermal waters (Fi9.l2-7cd,f) show well below the simple mixing lines of sea water and meteoric water. Enrichment of Ca'* is very pronounced in waters having Cl - concentration between 150 and 450 meq (Fig.lZ-7e). HCO' - of coastal thermal waters are smaller than that of seawater (Fis.t2-7s).

12.5 lsotopic composition of caleites in drill cores from the Yugawara geoth- etmal area Carbon ( d "C) and oxygen ( A '"O) isotopic composition of calcites from drill cores of TO and TY in the Yugawara geothermal area are shown in Table l2-2 (Watanabe,Ku- sakabe,Hirano and Oki, 1977). Carbon isotopic composition of TO calcites ranges from

-74- 147

meq ctr・ AJIRO 240 A TAMI + VU― RA AJ:RO ATAMI ♯ + VU… ＋

0~1ご 2あ 300 400 500 6∞ meq

Fig. 12-71 Mg2+ vs. Cl-' diagram.

loo 2@ 300 4@ 5o0 600 mcq

Fig. 12-7e Ga'+ vs. Cl-- diagram.

Fig.12-7g Hc03~VS.Cl~diagram

300 60O meq

Fig. 12-7a-g Water chemistry of coastal thermal waters in the Atami-Ajiro and Yugawara geothrmal areas.

0- -3%o and TY-calcites ranges from -1- -4Voo (Fig.l2-8). Fig.12-9 shows carbon isotopic comositions of carbonates and carbonaceous materials of different origins (Sasaki, 1977). Carbon isotopic ratios of calcites from the Yugawara drill cores are plotted in Fig.12-9. Variation of carbon isotopic ratios of the Yugawara calcites is fairly small in the range of 0 - - 4%o. These ratios suggest that the carbonate materials were derived from marine sediments but not from the organic materials.

-75- 148

Table 12-2 Carbon and oxygen isotopic composition of calcite in drill cores from Yugawara geothermal area (Watanabe, Kusakabe, Hirano and Oki, 1977).

180sMow l℃ Sample Calcite δ δ PDB (Depth m) (Wt.%) (‰ ) (‰ )

TY‐ 70 8.8 11.16 -4.03 TY・ 104 2.4 6.53 -2.19 TY‐ 150 4.7 8.26 -2.48 TY‐ 223 4.3 8.55 --1.92 TY‐ 530 2.1 6.74 -2.82 TY‐ 550 0.8 9.92 -3.11 TY‐ 628 44.9 5.55 -2.35 TY‐ 647 2.0 9.22 -4.14

TY・ 776 0。 9 6.77 --2.52

TO‐ 97 6.9 6.26 -2.48

TO・ 106 9.4 5。 95 -‐ 0.83 TO・ 149 6.0 5.71 -2.15 TO‐ 169 44.0 4.27 -0.22 TO‐ 208 5.8 5.92 -2.68 TO・ 216 25.5 4.33 -1.31 TO・ 241 3.9 4.37 --1.79 TO・ 279 1.4 7.25 -0.64

O Toyoko -1 O Tomita 0 -2 0 0 ● ・ O : ‐3 6t3c田

(Or。。)

-7

4 5 6 7 3 9 1o ll 12

5100sMow(° r..)

Fig.12-8 610C vs. 6100 plot of ca:cites from dri‖ cores in the Yugawara geothermai area.

-76- 149

-40 -30 -20 -10 10

Carbon dioxidein ctmospLE

Hydrothermol carbonot 幸 Carbonate m heYugowam gedhcmd area■ ■

Ccrbonate in mo日 ne sediment 。百zed m _==‐― :酔配ifry盤轟 Orgo画 c mde面 d

::ょ篇mPFrtttedd in -40 -30 -20 ,. -10 0 l0 5 -CpB(%.)

Fig. 12-9 Carbon isotope ratios ol carbonate and carbonaceous materials of ditterent origin (Sasaki, 1977). {: Carbonate in the Yugawara geotheemal area : Watanabe, Kusaabe. Hirano and Oki (1977).

12.6 Schematic model of the hydrothermal alteration of the Yugashima group Given the progress of present seawater-rock interaction, it may be that Ca=* is being extracted from basaltic rocks, in contrast with the enrichment of K-,Na- and Mg2*. However, the geochemical behaviour of major elements such as Ca, Mg and Fe in

Tab:o12-3a Chemical composition of carbonated waters in the Matsushiro area.

―Na‐ Ca‐ Mg‐Cl・ HC03 type― (mg/`)

15.0 18.3 Temp.(℃ ) 14.8 15.5 pH 7.4 6.6 6.7 6.4

K+ Na+ 171. 256. 240. 550. ca2+ 592. 857. 772. 1727. Mg2+ 286. 275. 300. 416.

Cl~ 1539. 1933. 1890. 3849. S042- 207. 276. 283. 343. HC03~ 708. 941. 915. 2291.

Si02 51. 65. 65. 82.

(1). StatiOn・ 6,1966.1119. (2). Station_7,1966.11.19 (3). Station・ 21,1966.12.25. (4). Station.14,1967.2.25. (Kitano,Y et al.,1967)

-77- 150

Tablo 12-3b Chemica! composition of carbonated thermal waters in the Yunokawa geothermal area(HOkkaido). -Na-Ca-Mg-Cl-HCOr-SOr type- (ppm)

Temp.(t) 65.4 66.3 pH 6.5 6.7

K+ 144.0 145.8 Na+ 2124. 2127. ca2+ 605.8 606.9 Mg2+ 196.5 200.9 Fe2+ 0.02 0. A13+ 0.06 0.07

Cl~ 3931. 3942. S042- 759.8 786.2 HC03~ 905。 9 902.8

Si02 54.6 54.5 HB02 36.0 37.3 C02 162.8 86.2

Hakodate City Samekawa No. G Hakodate City No. D

Nc― M ―Cc― K― Ct― SO`:=

lc― Ccl

Ct-504 Ｃ０ (A,iro) ヽ３ Nc― Ca― Ctご一２ Nc― Cc― Ct 蟹 Mg'-Fe- Ego. 討Ｈ６二＼ Sour ce Fig. 12-10 A model of seawater-rock interaction during Mlocene to Pliocene in the Yugashima group.

-78- 151 altered rocks of the Yugashima group show contradictory tendency to those elements of coastal thermal waters. The depletion of MgO and total FeO (FeO*0.9FerO,) from the basaltic rocks must not be due to the present hydrothermal activity. It is well known that thermal waters with large bicarbonate content are generally high in Ca'*,Mg'* and Fe'- (Table 12-3, Kitano et a1.,1967). Carbonated thermal waters can extract Ca'*, Mg'* and Fe'?t from rocks. However,the thermal waters now discharged in the Izu peninsula are of the chloride and sulfate types. No carbonated wa' ters are found. The hydrothermal alteration which had extracted Mg and Fe from the basaltic rocks might have taken place in the diagenesis and low grade metamorphism of the Yugashima group. Fig.12-10 shows a schematic model of the hydrothermal alteration of the Yugashima * group. Seawater which permeated into the Yugashima group lost Mg'* and K and gained Ca'* by interaction of volcanic rocks and changed into water of Na-Ca-Cl-SO, type like Ajiro thermal waters (Fig.l2-7cd,e,f). The Na-Ca-Cl-SOr water further permeated into a high temperature zone, crystallized CaSO, due to the decrease of solubility (Marshall et a1.,1968) and as a result the Na-Ca-Cl type waters like Atami ther' mal waters were formed (Table 6-4). In the high temperature zone, CO, was supplied from depth to the hydrothermal system. The carbonated Na-Ca-Cl waters extracted Ａ‐Ｏ，枷ｏ

Loumontite - Chlorite Zone

I oumont ite . co lc it e (Mg,Fe)O JI prehnite . quortz. HrOr COr

Woirokite - Chloriie Zone

colcite r chlorlte Jl. Pu,nPelty i t e o oct inoli terHrOr COr

Pumpetlyite - Chtorite Zone

Cql. Act. (Mg.Fe)O CoO Fig. 12-'ll Paragenetic sequence ol Ca-silicates and Ca-carbonates in louv grade metamorphism, shoalng the liberation of COz in the rnetamorphic reactions.

-79- 152

Mg'* and Fe'* from volcanic rocks. The thermal waters initially extracted considerable amounts ofCa"* at shallow zone, consequently could not extract more Ca'* from the basaltic rocks. Isotopic ratios of "C ( d '3C) in the Yugashima group calcites indicate that COz was mostly derived from sedimentary carbonate. CO, might be liberated by metamorphic reaction of calcite and Ca-zeolites or chlorite (FiS.12-11).

13. Water chemistry of the Yugawara thermal waters The most common hydrothermal minerals in the Yugawara geothermal area are, however, calcite and anhydrite (Fig. 10-G), the paragenesis of which with calcium zeolites seems to reflect the chemistry of hydrothermal solutions. The stability relations among calcium-bearing minerals are treated in term of pH, Pco, and the activity of Ca'*. The main discussion will be aimed at estimating the partial pressure of COz in the hydrothermal system.

13.1 Evaluation of pH in the subsurface system As calcite is the predominant hydrothermal mineral, it implies that the thermal waters are in equilibrium with it. The precipitation of aragonite on the pumping devices indicates that the thermal waters are evidently supersaturated with calcium carbonates. Provided that the Yugawara thermal waters at depth are supersaturated with calcite, all the fractures through which thermal waters can flow will be sealed up successively by the preciptation of calcite, resulting in the rapid decrease of water discharge. However, no rapid sealing of the fracture system has been known over these 20 years, except for the precipitation of aragonite in the pipes of the air-lift pumps. This rapid change in the chemical environment in the pipes of the air-lift pumping is explained by the rapid change of pH due to the rapid reduction of the partial pressure of CO, (Suzu- ki et a1.,1971, Awaya et a1.,1974). Table 13-1 gives the saturation indices (Paces, 1969) of the calcite-water system of the Yugawara thermal waters at the elevated temperatures,calculated from the values of chemical analyses only, showing their supersaturation with respect to calcite. If we assume chemical equilibrium between calcite and thermal waters, pH can be calculated under subsurface conditions, using the relations and equilibrium constants given in Table 13-2. The total dissolved carbonic species is

: 2CO. IIlH,coe * IIIHcos- * 11166rr- (6)

where IIli denotes the molality of the species "i". When pH of a system is small enough, less than pH 10, the last term can be neglected. Using the equilibrium constant (Table, 73-2) and the activity coefficients / i, equation (6) becomes:

-80- Table 13-1 Molalities of Ca2+, HCO3-, COs2- and SOr2-, ionic strength and saturation indicies (I : log Q / K) for calcite and anhydrite of Yugawara thermal waters.

Molalities(× 10 1 mole/kg) I cal I anhy Type No. ca2+ HC03~ C032- S042- 1.S. 25R〕 100℃ 25℃ 100℃

1 6.737 0.911 0.0068 3.862 0.064 0.55 1.52 -0.62 0.91 2 6.612 0.869 0.0033 3.935 0.057 0.34 1.30 -0.59 0.23 3 6.562 1.159 0.OHl 3.883 0.056 0.86 1.77 -0.69 0.22 4 4.764 0.875 O.0130 5.018 0.036 0.57 1.55 -0.46 0.38 NaCl‐ 5 8.483 0.870 0.0065 6.642 0.074 0.63 1.60 -0.33 0.48

| CaS04 6 3.119 0.733 0.0055 4.435 0.036 -0.09 0.89 -0.79 0.15 ∞ 7 3.169 1.000 0.0095 4.122 0.034 0.32 1.30 -0.72 0.13 8 6.238 1.565 0.0150 3.102 0.050 0.63 1.59 -0.68 0.12 | 9 5.389 1.493 0.0223 4.622 0.039 0.88 1.80 -0.47 0.36 10 3.044 0.775 0.0073 3.676 0.030 0.23 1.21 -0.72 0.13

Inter‐ 11 2.345 0.990 0.0093 3.394 0.017 0.24 1.23 -0.80 0.06 (3)中 --0.34 mediate 12 4.890 0.662 0.0063 5.382 0.028 0.31 1.30 0.51 (4)中

CaS04 13 12.82 0.316 0.0015 13.46 0.054 -0.07 0.90 0.24 1.07 (5)* 14 10.88 0.345 0.0033 11.55 0.046 0.ll 1.80 0.17 0.99 (6)*

:lc Numbers (l) to (6) correspond to those in Table 6- 3. I. S. : Ionic strength

一Ｏ ω 154

Table 13-2 Equilibrium constants for carbonate and sulfate reactions in aqueous solution.

ｐＫ ℃ 眈 ℃ Source

(1) CaCOs: Ca2'*COs2- Kc= 〔Ca2+〕〔C032-〕 8.37 9.39 10.25 (b) calcite

(2)C02+H20=H2C03 玲=鼎 1.46 1.99 2.07 (al

(3)H2C03=H++HC03~ L= 6.35 6.45 6.77 (al

(4)HC03~=H+十 C032- &=器 10.33 10.12 10.37 (a)

(5)CaS04=Ca2++s042- Ka= 〔Ca2+〕〔s042-〕 4.70 5.63 6.35 (a)

p(: -log K, ( I : denotes the activity (a) Helgeson (1967b) (b) Helgeson (1969)

Kc ΣC03= 十 (7) mca2+ γ ca2+ |1呂響多薫ギリ曇≒Fl

１Ｈ〓一２一毎型 Lげ #+J開崇ギ施 "Ж Q1 0

The substitution of the analysed amounts of Ca'* and the total carbonic species into equation (8) gives the precise pH values. When the pH values of the thermal waters fall between pKr (ca. 6.5) and pKz (ca.10), the pH of the system is simply defined as:

pH■ +γ og寺一bgmc′ c″ ―bgΣ C03γ HCO~ (9)

where ΣC03iS almOst to IIIHc03~. When the pH vcalue is lower than pKl(ca.6.5),the equation becomes

pH=T戸 -10gmca2+ γ ca2十 -10gΣ CC)31 110g蓋 (10)

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Table 13-3 Observed pH values at orifice and calculated pH(I00t and lS()t)at subsurface condition of Yugawara thermal waters.

Type No. Obs.pH Calculated pH (OrifiCe) 100℃ 150℃ eq(10) eq(13)

1 8.4 6.5 6.3 6.7 2 8.1 6.5 6.3 6.8 3 8.3 6.3 6.2 6.6 4 8.4 6.5 6.3 7.0 NaCl・ 5 8.1 6.4 6.2 6.9 CaS04 6 8.1 6.8 6.4 7.0 7 8.2 6.6 6.4 6.8 8 8.2 6.3+ 6.2 6.5 9 8.4 6.3+ 6.2 6.7 10 8.2 6.7 6.4 6.9

Inter・ 11 8.5 6.7 6.4 6.8 (3)・

mediate 12 8.3 6.6 6.3 7.1 (4)・

CaS04 13 8.2 6.6 6.4 7.7 (5)°

14 8.2 6.6 6.4 7.7 (6)・

{:Numbers (1) to (6) correspond to those in Table 6-3. * Calculated by equation (10).

The activity coefficient for each ion is calculated by the Debye-Huckel equation, which can be safely applied to the Yugawara thermal waters because of their relatively small ionic strength of 0.01 to 0.07 (Table, 6-3). The temperature of the subsurface system is tentatively assumed to be 100C , considering the isothermal profile (Fig. 5-4). The calculated pH at depth is given in Table 13-3 compared with the observed pH at orifice. An increase in pH up to 8 in the process of air-lift pumping can clearly be seen.

13.2 Partial pressure of GO. Substituting equations (2), (3) and (4) into (1) repeatedly to eliminate the carbonic species, the partial pressure of CO, in equilibrium with the calcite-water system can be obtained from the equation:

ぽ⑩=bg轟 ―わH― bg囲 (11)

The partial pressure of CO, under subsurface conditions will be given by the substitu' tion of the calculated pH (Table 13-3) into equation (11) together with the activity of Ca,* at the given temperature. The results are shown in Table 13-4 and illustrated in Fig.13-1. The estimated partial pressure of co, at depth is 10-'78 to 10-08r atm

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Table 13-4 Partial pressure of COz (log Pco, ) in the subsurface system of Yugawara thermal waters.

10g PC02 Type 100℃ 150℃

1 -1.16 -1.01 2 --1.20 -1.02 3 --0。 94 --0.83 4 -1.19 -0.98 NaCl‐ 5 -1.00 -0.90 CaS01 6 -1.53 -1.00 7 -1.24 -1.01 8 -0.85 -0。 79 9 -0.84 -0.82 10 -1.45 -1.02

Inter‐ 11 -1.27 -0.95 (3)'

mediate 12 -1.36 -1.04 (4)・

--1.78 -‐ CaS01 13 1.52 (5)・

--1.70 -‐ 14 1.50 (6)Ⅲ

*Numbers (l) to (6) correlate with those in Table 6-3.

lot loo lo-r lo{ lo-3

Fig. 13-1 log(ca'+J versus pH diagram shoring estimated pH and partial pressure of COz in equilibrium with calcite at subsurface condition of the Yugawara geothermal area.

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(100e), slightly higher than that in the atmosphere (10-'u atm). The major factor which controls the precipitation of aragonite and calcite should be the abrupt decrease of the partial pressure of CO, with a simultaneous 1.5 unit increase in pH in the pumping pro-

CESS.

13.3 Galcite-anhydrite relataon A combination of equations (1) and (5) yields the following relation for the paragenesis of calcite and anhydrite:

ＫＣ一〓切Ｋａ =Ю ¬ Ю00～ Ю叩∞η (1幼器 where Kc and Ka are the solubility products of calcite and anhydrite at the given temperature (Table l3-2). Although most of the Yugawara thermal waters discharged from the well are unsaturated with anhydrite at 25T, all of them are supersaturated with it under subsurface conditions (Table 13-1), where the pH value decreases to 6.5 accompanied by a decrease of CO.'- species to 10-d mole/kg. Thus, the activitl' ratio of [CO.'-; / (SO,'-J is of the order of 10-', which implies disequilibrium between calcite and anhydrite. The supersaturation of anhydrite in the high temperature water can be explained by a con- vectional flow of groundwater from the low temperature area to the high temperature area. As seen in Table 6-3 and 13-1 the thermal waters confined to zone B, where gypsum is the most common vein minerals, are relatively rich in CaSO.. The migration of the groundwater, saturated with CaSO. at low temperatures, to the certre of the geothermal system may be the major factor which allows the supersaturation and precipitation of anhydrite. The source of SOo'- , precipitated as gypsum and anhydrite, could be seawater or the oxidation product of hydrogen sulfide from volcanic gases. If the chemical equilibrium between calcite and anhydrite is exact, the pH values of thermal waters should increase to values for which equation (12) holds true. The elimina- tion of CO.'- from equation (12), using equation (4), yields

K. : K, [HCO,-J Ka [SO,'-J (H-J or in logarithmic form

一司+崚卜同 (13) 岬=囀 ≪吾聴m∞

If we simply apply equation (13) to the Yugawara thermal waters given in Table 13-1 the pH values of the system in equilibrium with calcite and anhydrite can be obtained

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in Table 13-3. The pH values, calculated from equation (13), will give the maximum pH associated with minimum Pco,, whereas those calculated from equation (9), will give their minimum pH associated with maximum Pco,.

13.4 Laumontite equilibrium Various kinds of calcium zeolites are known in the Yugawara geothermal area, however, here laumontite only will be treated as a representative of the calcium zeolites, because of the scanty thermodynamic data of the other zeolites. Standard Gibbs energy of formation of laumontite : The standard Gibbs energy of formation of laumontite is measured by a dissolution method. The chemical composition of laumontite from Tanzawa used for experiments is shown in Table 13-5. Laumontite

Table 13-5 Chemical composition of laumontite lrom Tanzawa.

Si02 A1203 CaO

52.64 20.58 12.02 0.29 0.68 14.25 100.46

Cao.ses Nao.o.r Ko.oou Ah.s?s Si{oss Orz 3.6HzO a :1.507 | :1.517 D:2.26

carefully separated by means of the three layer heavy liquid method is pulverized to fine powder. Three runs with 0.3g of laumontite were performed in Teflon bottles fixed on a rotating arm of continuous bubbling of CO, through a Teflon tube for 25 days at 25C and 1 atm. with various concentration of Na*, Ca'* and partial pressure of COr. The equilibrium constant of laumontite dissolution is calculated by the following reaction.

CaAl,SioO,,4H,O * zCO, + 8H,O : Ca'* * 2Al (OHh * 4HnSiOn + 2HCO,- (14) (laumontite)

n,, _- lca,*) [H.sio.J. [HCo,-J, lecor;'

The results of the runs are shown in Table 13-6. The standard Gibbs energy of formation of Tanzawa laumontite obtained by each runs is closely consistent wih each other with the value - 1598kca1/mol estimated by Zen (L972\ based on the hydrothermal experiments (Liou, 1971). Laumontite exuilibrium : The temperature of the Yugawara geothermal area in zone C is assumed to be 150C.

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Table 13-6 Experiments of dissolution, solubility and standard lree energy of lormation ol Tan- zawa laumontite.

Exp. pH Na+ K+ ca2+ HC03~ H4Si04 Pcoz pK oF(Kcal / mol

6.90 0.32 0.06 0.88 0.36 7.68 22.47 -1596.81 6.10 0.47 0.17 252. 762. 7.68 22.73 -1597.16 6.22 393. 1.66 380. 1156. 7.68 22.30 -1596.58

(ppm) mean -1596.85 Exp.1 : laumontite 0.39, distilled water 200m1 Exp.2 : laumontite 0.3g, distilled water 200m1, CaCOr 0.3g Exp.3 : laumontite 0.3g, distilled water 200m1, CaCOs 0.3g, NaCl 0.2g Activity coefficient of each ion is calculated by the Debye-Huckel equation

The ionic forms of Al in aqueous solution change significantly with pH. Helgeson de- monstrated the distribution of Al species as a function of temperature and pH. The boundaries between the equal activities of the AI species tend to shift towards lower pH with increasing temperature. The boundary between AI(OH)'* and AI(OH). - at 150C passes through the point pH 4.3 . The major ionic form of Al in terms of pH at 25C and 150C are as follows.

~ A13+ Al(OH)2‐ Al(OH)・

25℃ pH 4.3 pH 6.3 150℃ pH O.4 pH 4.3

Therefore he following three reactions are requireed for the dissociation of laumontite, corresponding to the ionic species of Al for the individual pH intervals. pH<0.4 CaA12Si40124H20+8H+=Ca2++2A13++4HlSiO.

〔ca2+〕〔A13+〕 2〔 Hlsi01〕・ KL-r : 8 (15) 〔H+〕 0.4

2〔〔Ca2+〕〔Al(OH)2+〕 H4SiO■〕・ KL-2= (16) pH>4.3 CaA12Si40124H20+8H20=Ca2++2Al(OH)4~+4HlSiO.

2〔 4 KL-3= 〔Ca2+〕〔Al(OH)4~〕 H4Si04〕 (17)

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Table 13-7 Standard free energies of formation and stndard heats of formation for laumontite reactions in aqueous solution.

State △Ff(kca1/mol) △Hf(kCa1/mol) Source

H+ aq 0. 0. (1) ―‐ -68.3174 H20 I 56.690 ― (2)

ca2+ aq -132.18 -‐ 129.77 (2)

--94.257 -‐ C02 s 94.05 (7) HC03~ aq -140.26 -165.18 (7)

A13+ aq -115. -125.4 (1) ―‐ Al(OH)2+ aq 165.9 -179.98' (3),(4・ ) ―-304.9 Al(OH)3 amorph -271.9 (1)

--310.2 -‐ Al(OH)4~ aq 356.2 (3) -312.9 --348.06* H4Si04 aq (5),(4中 )

CaA12Si40124H20 c -1598.± 4 -1729.± 5 (6) (laumOntite)

(1). Latimer(1952),(2). Rosini et al.(1952),(3)。 Wagman et al.(1968),(4). HelgesOn(1969),(5). SieVer (1957),(6).Zen(1972),(7)Robie et al.(1968)

Table 13-8 Equilibrium constants and standard heats of reactions for laumonllte in aqueous solution.

° 25℃ 150℃ △Hr(kCal)

‐ Kt_z 10*ros 10-046 ―16.335 KL_s 10-3..71 10-2580 +41.129

As quartz is a common vein minerals, the solubility of silica in thermal water may be controlled by quartz (Siever,1962). The standard free energies of formation and standard heats of formation required for the calculation of the laumontite equilibria, are tabulated in Table 13-7. In the calculation of the laumontite equilibria, the standard free energy of formation and the standard heat of formation of laumontite determined by Zen (1972) are used. The equilibrium constants at higher temperature are calculated by the van't Hoff equation (Garrels and Christ, 1965), the heat of reaction is assumed to be constant and the results are given in Table 13-8. The stability field of laumontite in equilibrium with qtartz is drawn with AllSi as the parameter (activity ratio of A1(OHF*/H,SiO.) in the log[Ca'*J versus pH diagram (Fig.13-2). The partial pressure of CO, associated with calcite, as defined by equation (11), is also shown in Fig.13-2. The Yugawara thermal waters,expressed in terms of calculated pH and log (Ca'*] at

-88- 161

lo9[Cd'l

:50° C

-l

i- ro'

-4 =10-'

-6

34s6?89 PH Fig. 13-2 log[ca'+) versus pH diagram shortring the stability field ol laumontit€ associated with quartz, pro\riding the most probable values of activity ratios ol A(OH)4-lH.SiO. ranging from 10-{ to 10-3 together with the partial pressure of COz in equilibrium with calcite. The solubity quartz of at l50t used for the calculation of laumontite equilibrium is l42.3mg/kg as SiOz (2.369X 10-3 mole/kg) (Siever,l962). Dots are the Yugawara theramal waters given in Table 13-3 and l3-4. l50C . are plotted in this diagram (Fig.13-2). It is noted that calcite, anhydrite, laumontite and quartz are still able to crystallize in the present hydrothermal system,whose partial pressure of CO.r may be in the order of 100 to 10-'atm.

14. Summary The isothermal maps of the Hakone and adjacent areas show that each geothermal areas, Hakone,Yugawara and Atami-Ajiro, is clearly related with the present volcanic activity. Thermal waters discharged in the center of each geothermal area are mostly chloride type. The Hakone zone III waters are the sodium chloride type. Atami waters are sodium-calcium chloride type. The high temperature waters of Yugawara are sodium-calcium-chloride-sulfate type. The zone of high temperature water of chloride type is commonly surrouned by the zone of low temperature water of sulfate type sometimes with bicarbonate. In the coastal area, thermal waters are extremely high in dissolved salts undoubtedly due to the percolation of seawater into the hot water system.

-89- 162

Chemical types of thermal waters found in each geothermal area are classified by their major dissolved. Zonal mappings of thermal waters were made based on the chemical compositions of each geothermal area. The compositional variation in major anions and cations may reflect the intensity of geothermal activity and variation of hydrothermal alteration as well as the structure of hydrothermal systems. Among these geothermal areas, Hakone volcano provides the best hydrothermal system for the genetical study of volcanic thermal waters. Chemistry and occurence of thermal waters come to the following conclusion for the genesis of Hakone thermal waters. Groundwater which infiltrates through the western side of the caldera is flowing eastward passing through the basal part of central cones, where high temperature dense steam rich in sodium chloride is coming up through volcanic vents. Subsurface streams of high temperature water thus start from the vent area, run through the permeable zone mixing with groundwater and appear as hot springs on the steep slope of Hayakawa valley. Within high temperature geothermal areas the reaction of the original rocks with thermal water results in a series of recrystallization, solution and deposition reactions which are referred to as hydrothermal rock alteration. In the Hakone, Yugawara and Atami-Ajiro geothermal areas hydrothermal alteration of rocks shows a zoning with increasing depth and temperature. Mineral associations of the clay-chlorite group and Ca-zeolites show systematic variations with depth and temperature. The Yugashima submarine volcanogenic sediments (lower Miocene) are exposed at the basement of the Taga and Yugawara volcanoes (Quaternary). Three zones of hydrothermal alteration were recognized in the drill hole cores. Successively with increasing depth, they are smectite (type I ') * mordenite, smectite-chlorite (type I '+ [, [ ) + stilbite (mordenite), and chlorite (type III ) * Iaumontite (wairakite). Calcite, anhydrite, pyrite and hematite are ubiquitous. Thermal waters due to Quaternary volcanic activity occur mainly along fracture of the Yugashima group. The coastal thermal waters are mixtures of seawater and meteoric water affected by the interaction with the Miocene rocks. Depletion of Mg'* by 20-40meq and enrichment of Ca2* by 50-200meq in thermal waters are very pronounced. Altered basaltic rocks which belong to the tholeiitic rock series are lower than fresh rocks in MgO, total FeO (FeO * 0.9Fe,O,) and very high in Al,O., H,O with high Fe,OrlFeO ratio. Chemical compositions of the altered rocks can not be due to the influence of the present day saline thermal waters. Extraction of MgO and total FeO from original basaltic rocks must have occurred during diagenesis and low-grade metamorphism of the Yugashima group through interaction with bicarbonate-rich thermal waters. The most common hydrothermal minerals in the Yugawara geothermal area are, however, calcite and anhydrite, the paragenesis of which with calcium zeolites seems to reflect the chemistry of hydrothermal solutions. The stability relations among calcium-

-90- t63 bearing minerals are treated in terms of pH, Pcoz and activity of Ca'*. The study of the relations among minerals and solutions is very helpful for estimating the chemical environment of the hydrothermal system. The present Yugawara thermal waters seem to reflect the presence of "fossil thermal brines" at depth, left behind after the separation of high-temperature dense steam rich in NaCl. This separation resulted in the concentration of Ca'* and SO.'- which caused the precipitation of calcite and anhydrite.

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