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Geothermics (x97o) - SPECIALISSUE 2

U. N. Symposiumon the Developmentand Utilization of GeothermalReso~s, Pisa I97O. "VoL",, Part ",

Chemistry in the Exploration and Exploitation of Hydrothermal Systems W. A. 1. M~ON *

ABSTRACT Geochemistry plays an increasingly important role in the investigation of hydrothermal systems. Experimental information on the geochemistry of individual elements in high temperature rock/water environments, and thermodynamic data for solution equilibria and gas solubilities has enabled detailed interpretations to be made of natural hot water and steam compositions. Examples are given from particular hydrothermal areas where the chemical composition of hot springs and fumaroles has been used to obtain an estimate of the deep hydrothermal conditions. The lateral extent and chemical uniformity of the deep system can often be judged and zones of high rock per meability, connecting the aquifer at depth to the surface, outlined. Minimum estimates of the deep water temperature may be made from the concentration of constituents such as silica, magnesium, and fluoride, and from the ratio of sodium to potassium in the spring waters. The tendency for deposition of minerals, particularly calcite, in the deep aquifer during exploitation, can sometimes be assessed from the carbon dioxide and ion concentrations in the surface flows. Information on rock types in the deep system may be obtained from the surface water chemistry. For areas which were subsequently drilled, a comparison is made of deep conditions interpreted from surface chemistry measurements, and those actually found by drilling. The chemistry of fluids discharged from driliholes during exploration and production drilling provides information on the temperature of the water supplying the drillholes, the migration of fluids and the distribution of steam and water in the system, changes in temperatures and pressures in the aqui. fer during exploitation, and the possibility of minerals depositing. New sites for drilling production holes can be chosen, the interaction between drillholes discussed, measurements of mass outputs and discharge enthalpies of drillholes made, and heat exchange and dilution processes occurring at depth discussed through study of the chemistry of drillhole discharges. Examples are given of interpretations of this type from the N.Z. hydrothermal areas of Wairakei, Broadlands, Waiotapu, Orakeikorako, and Ngawha. Introduction In the earJy 1950s the first detailed investigations were carried out in the geothermal areas of New Zealand to assess their potential as possible sources of energy. Wairakei, situated close to Lake Taupe in the centre of the North Island, was the first area selected for detailed scientific study, and investigation by deep drilling. In the last eighteen years the hot water aquifer at Wairakei has been harnessed and at present 160 to 170 megawatts of electric power are being generated. During the same period investigations were made in six other geothermal areas, five in the Taupe volcanic zone * Chemistry Division, DSIR, New Zealand. 1310

(Orakeikorako, Waiotapu, Taupe, Broadlands-Ohaki and Rotokaua), and at Ngawha in the northern part of the North Island. Investigations included both scientific survey and drilling. Broadlands, an area located some 20 miles north of Wairakei, appeared the most promising area after Wairakei for the generation of electricity. Considerable work has been carried out in this area during the last four years. At first little was known about the chemical cornposition and chemical equilibria in deep, natural, hot waters and the interpretation of the chemical composition of hot springs and fumaroles was difficult. In the last eighteen years over a hundred deep holes (mainly 2000 to 4000 ft) have been drilled at Wairakei and up to fifteen holes in each of the other areas. Detailed chemical studies of the fluids discharged from the holes, aided by the increasing amount of published geochemical information on various chemical systems, enabled considerably improved physical and chemical interpretations. New Zealand geothermal areas are characterised by relatively shallow aquifers of hot water (200-300 °C) at depths to 6000 ft. In the areas investigated by deep drilling, steam systems as at Larderello, Italy, and The Geysers, U.S.A., have not been found, nor have large pockets of free steam been found in the water aquifers before exploitation. Hot springs at temperatures of 70°C to 100°C, fumaroles and large areas of steaming and altered ground are characteristic surface features of New Zealand geothermal areas. The extent of surface activity varies from one area to another but seems to depend largely on the permeability of the surface rocks. Considerable scientific information is required before a geothermal area can be developed. Chemical studies of fluids discharged from springs, fumaroles, and drillholes are made to assess the uniformity and extent of the aquifer or aquifers, to estimate the deep water temperature and zones of highest rock permeability, to investigate mineral deposition prior to and during exploitation, and to determine the gas and mineral content of the waters to aid power plant design and field management. The application of chemistry to these studies is based on the behaviour of the solutes present and the chemical processes operating in the aquifer. The kinetics of rock/water interaction, the volatility of different

suits together with the composition of waters discharged

solutes, the solubility of different materials, and the type of hydrothermal mineral alteration are of prime importance in these studies. Preliminary chemical thermal field

investigations

of

a

from drillholes in the different areas, are shown in Table 1. Most springs with flows of over 0.5 litres/sec discharge close to neutral (pH 6 to 9) water containing as major constituents, in decreasing order of concentration, chloride, sodium, silica, potassium, boron, sulphate, calcium, lithium, bicarbonate (this constituent varies considerably), fluoride, arsenic, bromide, rubidium, ammonia, caesium, magnesium, iodide, and antimony. The extent of mineralisation of the springs in an area is generally quite variable, although in some cases, e.g.,

hydro-

SPRINGS AND FUMAROLES

A summary of the chemical types of water discharged from hot springs in New Zealand was given by ELLIS and MAHON (1964). A selection of these re-

TABLE I. - - Analysis o[ waters /rom springs and drillholes in New Zealand geothermal areas. • Geetkem2

Ge~eeig~atgou i a waters t~ Area

~1

~-L

Ha

g

~

Ci

Ga

NI

g

C1

Dr

I

~m 804

t/1 Ile£ceke4

?.9

12.6

I~00

192 2.9

2.2

23

0.02

6.9

~11~0 5.7

0.2

33

ROAm 20 ga£ra~ai

8.4

15.8

1~30 220 3.1

2.6

18

0.0~

8.2

2215 %$

C.~

35

8,~

I~,2

1520 225 2,8

2.3

17

0 , O 3 8.25 2260

G p r ! q 97 I~L~Me2

6.8

6.8

665

61, 0,?

1.7

45

4.2

lalrml

7.~

10.0

5~0

~

%0

n.d

20

8,9

6,6

8~0 155 2.4

0.8

U~O~p~

8.8

6,4

790

90 0.7

WalOt~pu

.q.7

9.0

12.?.0 160 2.3

~ p r ~ 20 W-4ot~pu

8,6

k.o

4~0

~ole 7~ Ka,.~ra~

8.6

~.35

743

5.0

3.3

740 13o 0.9

8.6

3.3

(~8

6,2

2*7

330

49 P,*d

m.d

13

0.9

1.1'

Kawera~

7,~

3.~

398

33 o.27

o.25

~3

1.o

HoZe 219 Rotor~i

9.4

2.5

375

~

0.27

0.2)

9

9,4

t,4

~6~

31

0.26

0.31

9,o5

4.7

~5

~0.5

9.1

3.t

5~O

~

loll

froze 44

Xel* 6 W~lota~a Note 7

~oZe 8 Kaueraa

~ole ~7 Xale~u

~

0.3

123 0.75

119 0.3

Tota2

Total

ds

Tot a l

$102

R~O2

MR5

4.3

590

112

~etella

s p r i q 83 ~oterma Hole 2

Or~eAke~ake

0.~

1.7

10.O

125

24,9

145

19

1.4

10,O

155

23.9

145

1.8

235

~7

0.22

88

1,9

16.6

26

24.1

135

2.8

245

82

0.37

38

2.0

26

85

24.O

145

~.

3.1

62o

.',6

o.9

65

2

9.8

I~0

52

105

%0

86

2.9

600

63

0.8

90

2

14. 9

140

25.7

1~O

7.2

0.4

145

4.9

490

117 11.5

235

6

13.0

60

21.1

195

2.0

O.S

53

1.1

380

27

0.4

58

3

35

87

31

9.0

780

.?35

0.7,5

6

IO.3

~O0

6.0

I~R.

lO.5

815

248

0.3

4

g.?

117o

6.)

1~

11.0

7~0

255

1.2

79

~

9.8

600

6°3

198

2~5

85

4.O

cob ~--

10

11.5

44

6.5

1?0

102

4.0

110

6

12.8

53 "

6,6

t~o

23.4 0.05

206

36

18.Z

314

32.3 0.05

1~'3

74

s.I

49o

21.6 0.2

167

14

O.8

k~O

31.5 0.1

ly96

s.d

i.4

to

o.o~

7,5

14~o

~.7

o,2

0.~

50

0.05

3.3

1510 3.7

%7

)~

s.d

5,5

2000

0.6

9

o.0~

5,2

688

o.3

2.6

o.16

4.3

1254 5.o5

o.2

0.5

1.1

0.39

5.2

1262 6.2

o.2

2.0

0.32 3.3

12~0 5.1

0.2

445

1.4

0.7

1~

1.9

Y~

%6

o.8

%

0.06

a.d

333 0,1

0.2

12

0.(~

k03

1

0.22

4.0

65,?. 2.1

0.7

30

0.~0

0.4

1.2

a.d

6,4

~o

..d

~,~

aS

O.3

1.O 0.2

5,7

~

O.8

1.O

142

2.5 0.08

1.Z

1.2

82

0.1

1275 223

2.5

11.k

Role 10 |roadlalds

8.9

9.6

890 9~0

$r~dlaads

7.0

7.4

860

]to].e 1 ?amp,

8.0

13.8

165

17

5.8

1.9

8.5

23*6

o.2o

0.05

284

100

O.15

o,4

8,5

IOr(Ad]~ndl

0.t5

11*3

117

1110 2.5

2.~ 0.~

4.0

1.3

11o

%4

0.22

280

42

8,3

Orakelkorak*

25

640

7~

2~

Hole

~pr£ag 98

0.25

Me/Ca

640

$prla8 4

h i e 137

~

Cl/y

12~

4.7

0,3

0.45

C1/~

CO2

4,§

6.0

~pr&nI 2 ~werau

T a i l 1 Total

"~ 24

"~

I~..4 0.1

70

t5.6

0.33

100

0.$

0.2

7.2~ ~

6.4

0.3

3.5

5,6

8~8

216

1.6

~

2.1 0.1

6.3 1262

3.9

0.4

6.0

3.1

695

2205

1.2

.%2

1.2

2.6 0.48

5.2 1060

3.0

0,6

lOO

1.o

338

lpO

5.8

65o

1,9

14.O 0.07

6.8 2222

4.2

Z,1

30

k.1

?26

153

O.1

220

0.3

280

71

700

17.2

31

1000

24.2

85

14

?00

32

47

6

17.3

955

al.4

51,p

5

10.?

4~0

,25.8

%3

1.3

11.5

190

29,8

1,o

8.2

480

lO.6

136

2.0

10.1

737

6,9

1Oh

1.o

17.8

573

10,1

10~

18

1.0

9.7

160

18.0

175

0.4

14.7

64

17.5

260 61

17,9

Hale 2

9075 2 ~ , ~ :'.~

8pria~ 1

337

a.d

u*d

101

0,4

2)3

38

0.1

CO2 ~

3*7 O,15 9.& 11OO

s.d

O.2

46

a.d

?60

154

1.6

~

1.0

9.8

326

10.1

1o6

CO2 ~

0.2

16.5

lk0

9.7

$priq Taupe

7.4

4.6

~05

47

8.45

2.~

693 120

0*23

0.13

O. 5

O.5

11.0 2.3

1.1

Motet $priq 6 Ilele 1 X~ba

2.3

?.It

990 102

1.7

2.0

12 10

8.1

10,7

900

6.4

8.0

830

1.0 1433

k.O

0.3

78

s*d

z.d

29

65

0.3

0.55

1,4

~o4. 1658

aed

n.d

7.8 2.5

0.3 12~0

2.6

1.0

3#7

7.25 22.3

17~0 1~5

1.1

2.8

36

0.5

1,5 3064

5.0

2.4

65

7.3

13J0

0.9

1.5

~0

0.6

0.43 23q9 %1

I.~

39

520 L~O

a*d

3~0

183

a.d

475

4800

~;~

0.2

178

~69o

14o

5.~

505

564

3.7

24o

279

~

1.0

19,6

55

0.1,3

co 2 ~

a.(t

22.o

185

O.42

1*2

IIco2 -TO

1.O

18.O

849

1.?

129

1.0

15.9

46

Jvb~1 ee $prtag

~pvk-

Sprll~ 6

?olme.lm

10.3

8~

2200 1100

S p r t q 23A 1?.3

142

1311

Orakeikorako, they are of similar mineral content. Maximum constituent concentrations in different areas vary by a factor of ten and, for example, at Orakeikorako the chloride concentrations are approximately 300 to 400 ppm while at Tokaanu concentrations of 3000 ppm are common. The composition of the neutral pH spring waters, allowing for surface evaporation and dilution, is similar to the deeper hot water (Table 1). Many stagnant and semistagnant springs and a small number of flowing springs are acid, and their compositions are determined to a variable extent by reaction of the acid waters with the confining surface rocks. In some cases they retain some remnant of the underground hot water composition but in others there is no relationship at all. The steam discharged from fumaroles and steaming ground in the Taupo volcanic zone areas originates from shallow levels and is produced from boiling of the underground hot waters. The composition of the steam is quite different from that discharged from active volcanoes. Gases commonly present are carbon dioxide, hydrogen sulphide, nitrogen, hydrogen, ammonia and methane. Magmatic steam gases such as HC1, HF and SO._, are absent. During the early chemical surveys samples are collected from many neutral pH springs. Samples from acid springs, due to the limited information they supply, are second in priority. Most of the larger fumaroles in an area are sampled, since the steam they discharge is often representative of the underground steam. A lesser number of the smaller fumaroles are sampled. The use of individual constituents or ratios of constituents for chemical investigations depends on their behaviour in the rock/hot water environment. The geochemistry of many constituents present in thermal waters has been studied in laboratory experiments in New Zealand and elsewhere, and certain deductions can be made concerning their likely behaviour in a hydrothermal system (ELLIS, MAHON 1964 and 1967; MAriON 1967; ELLIS 1968). For example chloride, boron and caesium behave as soluble elements, concentrating in an aqueous phase. Once liberated from a rock they remain in solution and do not readily enter into secondary mineral structures. Sodium, potassium, lithium, and rubidium are controlled in natural hot waters by temperaturedependent mineral equilibria, while the concentrations of silica~ calcium, magnesium, fluoride, and sulphate in high temperature solution are determined by the solubility of minerals such as quartz, calcite, chlorite, anhydrite and fluorite. The chloride ion concentrations are used extensively during preliminary chemical surveys. In areas where the possibility of extensive evaporation and subsequent concentration of constituents is small (high output springs) the highest chloride springs are regarded tentatively as being supplied most directly from a deep source. Changes in constituent concentrations as water migrates 1312

from depth to the surface comes mainly from dilution or boiling and evaporation. The amount of evaporation between the local boiling point and the temperature of sample collection is usually very small. Construction of iso-chloride lines across a map of the area may define zones which can be correlated with geophysical and geological data. At Waiotapu and Broadlands iso-chloride lines encompassing zones of high chloride springs were correlated with low values of resistivity at 1000 to 1800 ft, and with surface traces of what appeared to be major faults. Similarly springs with lower chloride concentrations appeared in areas where the resistivity of the deeper water was higher. Figure 1 shows iso-chloride lines drawn across the Waiotapu field (LLOYD 1959). The chloride tracer may give a rough outline of the deeper system, useful for exploratory drilling. The chloride concentrations in boiling springs have been used to estimate minimum water temperatures in the deep aquifers. The highest chloride springs are assumed to be similar in concentration to the deep water while boiling springs with the lowest chloride concentrations, but lacking characteristics which would indicate steam heating, are assumed to have been diluted by low temperature (15-20°C), low chloride ground water. The amount of dilution in going from the high to low chloride concentrations allows a minimum temperature for the deeper water to be calculated. This calculation for Orakeikorako, Wairakei, Waiotapu and

FIG. 1. -- Iso-chloride lines, drawn from hot spring analysis at Waiotapu (a#er LLOgV1959).

Broadlands gave in each case a deep water temperature of between 180 and 200°C. Boron is another very soluble element, and the atomic ratio of CI/B in spring waters is useful for studying the source of the surface and deep waters. The use of an atomic ratio of this type eliminates the effects of dilution and evaporation of hot waters, enabling a comparison of waters from springs scattered over large areas. Figure 2 shows C1/B ratios over the Waiotapu field. In many areas the hot springs have been shown to originate from a common water source. Experimental rock/water interactions (ELLIS, MAHON 1964 and 1967; MAHON 1967) and field observations have shown that hot water in contact with sedimentary rocks, or with andesites in the Central volcanic area, contains higher concentrations of boron than those in contact with other volcanic rocks. This is illustrated by comparing the CI/B ratios in the surface waters of Ngawha, an area of shales and sediments, of Tokaanu, an area where andesite is common, and Wairakei, Waiotapu and Orakeikorako where rhyolites and ignimbrites predominate. The ratios are respectively 0.5, 10.0, and 20 to 35 (cf. Table 1). The chloride concentrations of the springs, except at Orakeikorako, are of the same order. Analysis of a large number of New Zealand rocks for boron (ELLIS, SEWELL 1963) gave higher average contents in sediments, with the exception of greywacke, than in volcanic rocks. The average content in greywacke was similar to that in rhyolites.

LAKE

NGAHEWA

/

I

t

/ /

//./ ./ t

,o-me~

[ AAIqlGI

FIG. 2. -- Contours o] chloride-boric acid ratio, drawn ]rom hot springs analysis at Waiotapu (alter LLOYD1959).

A significant trend in the C1/B ratio in springs over an area may indicate a change in the rocks with which the hot waters are associated at depth. An example of this occurs at Broadlands where the ratio in springs on the west side of the field is 10 to 11, while on the east side the ratio is 6 to 7 (Figure 3). In the initial survey of Broadlands the low overall ratio was taken to indicate the presence of thick beds of sediments or andesites in the system, and the changing ratio to a thickening of these beds on the east side of the field. Subsequent deep drilling showed this interpretation to be basically correct. The low ratio was due to the presence of a thick sedimentary formation, and the change was due to basement argillites occurring at relatively shallow depths in the east. The Na/K ratio in natural hot waters is controlled by a reversible temperature-dependent rock mineral/ water equilibrium, involving potash mica, potash feldspar and albite (ELLIS, U.N. Geothermal Symposium, Pisa 1970). Experimental high temperature rock/water interactions (ELLIS, MAriON 1964 and 1967), field results and the results of HEMLEV and IONES (1964) have enabled an approximate relationship to be established between the Na/K ratio in natural hot waters and temperature. The reversible relationship appears to hold only at temperatures above about 200°C (ELLIS, U.N. Geothermal Symposium, Pisa 1970). The sodium/potassium equilibrium adjusts after a temperature change relatively slowly, which enables useful information on conditions in the deep aquifer to be obtained from the values of Na/K in spring waters. In many cases the ratio is lowest in springs with the highest chloride concentrations, and iso Na/K lines correspond closely with iso chloride lines. Assumptions concerning the history of hot waters reaching the surface can be made from the Na/K ratios in the springs. Waters which have reached the surface rapidly by faults or permeable formations have rather low ratios e.g. 12 to 15 while waters which have passed through impermeable formations or by a circuitous route, allowing reaction between rock and water as they cool, have higher ratios of 15 to 25. Zones of high or low permeability and sometimes centres and perimeters of fields may be recognized from the ratios. Low temperature surface waters often contain low Na/K ratios and significant concentrations of potassium. Dilution of hot waters from depth with surface water can sometimes result in abnormally low ratios in the mixed water. This misleading factor should be kept in mind when interpreting the ratios in springs. Estimates of underground water temperatures are obtained, from boiling springs with the highest chloride concentrations, using a calibrated Na/K vs temperature graph. Estimates at Broadlands, Orakeikorako, Wairakei, Tokaanu and Waiotapu gave temperatures of 180 to 220°C. Later drilling showed the results to be minimal but of the right order. Estimates of the temperature 1313

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