Germanium in Icelandic geothermal systems

AC-iQ Vol. 48. pp. Z489-2%x2

Printed in U.S.A.

Germanium in Icelandic geothermal systems STEF~ ARN~RSSON Science Institute, University of Iceland, Dunhagi 3, tOi’ Reykjavlk, Iceland

(ReceivedMay 24, 1984; accepted in revised form August 29, 1984) A~~~~~~urn ~~~nt~~o~s in geothermal waters in Iceland lie mostly in the range 2-30 ppbThere is an overall positive relation between the germanium cmtent of the water and its temperature. Most of the germanium occurs as GeQH); in solution but Ge(OHb may also be present in sign@ant amounts in saline waters when above 200°C. Evidence indicates that aqueous ~~aniurn inanitions are controlled by exchange reactions where it substitutes for silica in silicates and iron in subhides. It is the rate of dissolution and the relative abundance of the alteration minerals which take up germanium to a variable extent that ultimately fix Ge(OH)4 concentrations in the water. This, together with water pH, fixes total dissolved geraniums It is mostly the primary rock com~t~on that dictates the relative abundance of the alteration minerals. Conductive cooling in upflow zones favours removal of germanium from solution, During the initial stages of boiling of rising hot water dissolution is enhanced but precipitation at later stages, Thermodynamic data of various aqueous germanium species and several minerals are summarized and dissociation constants and solubilities estimated at elevated temperatures using available predictive mtxhods.

ppm ~~~iurn whem the feldspar phenocrysts contained less than I ppm. The germanium content GERMANIUMIS enriched in geothermal waters and uf all minerals increases from basic to acid rocks some non-thermal ground waters relative to waters (H&MANN, 1970) but no significant variation is in rivers and sea-water. Geothermal waters in Japan observed in the total germanium content of dif&rent have been studied rather extensively with respect to types of igneous rocks (ARN&USON, 1969; BURTON germanium (KAWAKAMI et al., 1956; SAKANOVE, et al.. 1959; EL-WARDANI,1957; MICKEWC,1962; 1960; UZUMASA and SEO, 1959). Concentrations are ONISHI, 1956) due to decreasing abundant in the most frequently in the range of l- 15 ppb but values less linked silicates when going from basic to acid as high as 40 ppb are reported. Comparable concenrocks. trations have been observed in geothermal waters in In hydrothermal deposits germanium is concene other countries such as the Taupe Volcanic Zone in trated in sulphides where it occurs in the divalent New Zealand and Iceland (KOOA, 1967; AKN&SSQN, state and ~n~nt~tions run in tens to hundreds uf 1970). %&AYNOV(1965, 1967) found germanium to ppm (I&EwER et & 1955; FRI_JTHand MAUCEIER, be enriched in carbonate-nitrogen thermaI waters of 1966; FRyKttMD and FLETGuzR, 1956; WARREN Caucasus and Pamirs. KRAYNQV(1965) found that and TEK&#PsON,1945). KOGA(1967) found gennanhighest germanium occurred in waters with one or ium to be extinsively depleted in some hydrothermally more of the following characteristics: 1) high temperaltered rocks in active geothermal systems in tha! ature, 2) high dissolved solids content and high Taupe VaIcanic Zone in New Zealand. aIkalinity. PENTCHEVA(1965) found saIine underGermanium has been fuund to be pint as finely ground waters in Bulgaria to contain as much as 2 disseminated sulphide, sometimes as renierite or gerppm 3e~a~ium. manite in severA sulphide minerals (&%LENOV d ai(., Germanium fevels are exceedingly low in sea water, 1962). This could reflect exsolution of a germanium or about 0.05 ppb (EL-WARDANI,1957) and changes component. The experimental work of hd.4LEvSKw with depth appear to h insignificant (BURTON d d, (1966) did nut reveal solid salution of germanium 1959). Available data from river waters indicate sulphide in sphalerite, yet limited substitution, comparably low concentrations or 0.03-0.10 ppb The study -bed in this cozmibution is primarily (HEZDEand K&WER, 1962). aimed at elucidating q~~~~~ely the m which MineraIs which contain germanium as an essentiai govern the mobility of germanium in geothermaI constituent are very rare in nature (H&MANN, 1970). systems. It is based on analyses of over 130 samples Germanium is not homogeneously distributed amoq of thermal water from most of the geothermal tieI& silicate minerals of igneous racks (HARRIS, 1954; in Iceland (Fig. 1) collected in 1979-82, 13 samples H&MANN, 1963; &IHoN, 1968; %%XMAN, 1943), of surface and non-thermal waters as weIl as over It is most concentitd in tie least linked silicate 100 analyses reported by ARN~~N ff969), The structures. The germanium content of the magma thermal waters raae in temperature From digbtly also influences its concentration in silicates (HARRIS, above ambient to 325OC and from 100 ppm dissolved 1954; HCIRIMANN, 1963). According to HARRIS(1954) solids up to sea water salinity, yet most of them are divine phenocrysti in acid glassy contain 1.5-3.4 relatively dilute. Most of the watem issue from basaltic 2489

2490

S. 4rnbw_m

FIG, 1. Location of sample sites; dots rock and mineral samples. circles water samples. Numbers correspond with those in Tables 2 to 4. For waters only the last two figures are given except for the Nesjavellir sample. Water samples from I98 1 are underlined. Areas studied in some detail are indicated.

rocks but some from acid volcanics, particularly in the Landmannalaugar field (Fig. 1). The major element chemistry of most of the waters for which germanium data are reported in Tables 2 and 3 were given by ARN~RSSON et ul. (1983) ARN~RSSONand GUNNLAUGSSON ( 1984) and ARN~RSSON (1984). Some data are also presented for rocks, fresh and hydrothermally altered, as well as few common geothermal minerals in Iceland (Tables 4 and 5). ANALYTICAL Germanium in water samples was determined by two analytical methods. One involved pr~ipi~tion, together with other trace elements, by 8-hydroxyquinoline, tannic acid and thionalide. The other method involved calorimetric dete~ination by the phenyIfluorone method following the procedure described by SCHNEIDER and SANDELL (1953). All rock and mineral samples were analysed by this latter method. The procedure adopted for the pr~ipi~tion technique is a modification of that described by ARN~RSSON(1969) but developed at D.S.I.R., New Zealand, and kindly made TABLE 1. Precision for germanium determinations in waters Obtained by duplicate analyses of selected samples. C

D

E

12.9 14.5

9.6 8.9

A

B

Emission spectrography: Synthetic samples Natural waters

20 22

0.5

0.5-100 0.8-46.0

Colourimetric analysesa

13

-b

3.2-42.9

7.8 6.8

XRF analysesa*c

73

0.5

3.0-67.3

9.2

Method

8.9

A: no. of determinations. 6: detection limit (ppb). C: concentration ranqe (oobf. D: mean % deviation. E: standard deviation. aktural waters. bDepands an the quantity of sample that is evaporated. For a 200Ml

saatple the detectton limit is about 0.5 ppb. CThe average deviation for duplicate samples containing less than 3 ppb was 0.4 ppb.

availabie to the author by Dr. A. J. Ellis. This precipitation method was originally developed by SI~VEYand BRENNAN (1953). The precipitate was analysed by emission spectrography (ARN~RSON,1969) or by X-ray fluorescence spectrography. GUNNLALJGSSON and ARN~RSSON(1982) have described the procedure for the X-ray fluorescence method. Table 1 shows the analytical error obtained on duplicate samples by the different analytical met&da. An average error of 50. I5 ppm was obtained for 30 duplicate samples of rocks and minerals. ARN~RSXIN(1969) demonstrated that the germanium content of untreated water samples containing I-20 ppb was not affected by storage as long as 6 months. The details of the procedure for the coprecipitation technique and subsequent analysis are described below. To 2.5 kg of water sample the following reagents were added in the order specified immediately after collection and mixed thoronghiy after each addition: 1. IO ml of 6 M HCl containing 5 ppm of Co (dissolved as CoCl,). 2. IO ml of KAISO&oIution (18.35 g per litre of water). 3. 15 ml of 8-hydroxyquinoline solution (dissolve 75 g in 300 ml of glacial acetic acid and dilute to 1500 ml with water. 4. 25 m1 of glacial acetic acid. 5. Ammonia solution to give pH of 5.2-5.5 (about 25 ml, sp. gr. I. 19). 6. 3 ml tannic acid solution (dissolve 30 g in 300 ml of water). 7. 3 ml of thionalide solution (dissolve 6 g in 300 ml of glacial acetic acid). 8. Ammonia solution to give pH 5.2-5.5 (about 5 ml, sp. gr. 1.19). 9. Leave for 72-96 hours, then filter. The reagents used were from E. Merck, Darmstadt, W. Germany, analytical reagent grade. Deionized water was used to prepare all reagents. All the samples were filtered using a Buchner funnel. The i&r paper used was Whatman 42. The precipitate was dry ashed at 600°C overnight. The amount of KAISO, solution added is equivatent to 20 mg AlZ03 and aluminium is used as a carrier for the trace

2491

Ge in Icelandic geothermal systems TABLE 2. Germanium concentrations in Icelandic geothermal well waters. The concentrations of other selected components are included which characterize the water chemistry and relate to germanium mobilitya.

Sample no.

Location

Ge ppb 19.4 4.8 2.7 4+2 1.8 1.8 2.0 1.3 2.0 2.6 1.9

79-001 72-031 74-008 79-008 72-003 79-009 79-010 79-011 79-012 79-015 81-028 79-016 79-017 79-018 79-019 79-020 79-022 79-027 79-028 79-030 72-205 74-006 79-032 81-016 81-017 79-033 79-034 79-035

Klausturhdlar 1 Svartsengi 2 Svartsengi 3 Svartsengi 4 Reykjanes 2 Reykjanes 8b Arbaer 1 Selfoss gb Husatottir 4 Laugaland 3 Laugaland 4 Reykir 17

79-036 81-003 79-038 79-039 81-005 81-006 79-040 79-041 79-042 81-015 81-011 79-044 81-014 81-013 79-045 79-046 79-048 79-049 81-020 81-021 81-029 81-033

3d %Ii Reykir Reykjabraut Laugarbakkar 3 Varmahlid 1 217 Saudarkrokur 1 1.8 Saudbrkrbkur 12 1.8 Reykir 1 1.2 Siglufjbrdur 7 0.6 blafsfj6rdur 3 1.6 Dalvlk 10 Laugahlid 1 0.6 Glerargil 7 a.5 Laugaland 7 1.2 Svalbardseyri 2 0.6 Grytubakki 1 al.5 Tjarnir 4d 3.0 Laugar 2 1.4 32.5 Krafla 9 Krafla 7b 3.4 Krafla 6 26.7 Krafla llb 10.9 52.5 Krafla 8 Krafla 13 310 Urridavatn4i! Nlmsfjall 26.0 Ndrnafjall5 22.7 Namafjall 6 27.1 Namafjall 7 22.3 N&rafjall 8 17.5 Namafjall 9 31.3 Namafjall 11 9.9 Ndnmfjall 11 16.5 Left-a4b 11.0 Saelingsdalur 10 2.0 Reykholar 2 1.6 Lysuholl 6d 20.4 Hvalstird1 12.9 20.8 _~ 20.4 __~ 18.2 9.7 5.2 2.6 10.7

79-050 79-051 73-057 73-152 73-161 79-052 73-155 81-022 81-023 79-055 79-056 79-057 79-063 81-002

;:::j:v:k 30b Reykjavik 11 Seltjarnarnes 4 Baer 3 Spoastadird Reykholt 1 Reykjabol 1 Nesjavellir 5 Hveragerdi 2 Hveragerdi 4 Hveragerdi 6 Hveragerdi 7 Oxnalaekur 1 Bakki 1 Laugarbakkar ld

z 1:4 1.7 1.8 2.2 9.6 11.8 16.4 30.6 16.0 21.3 19.2 25.4 8.6 24.2 2.3

sio2 ppn 248.9 460.4 474.9 424.5 291.3 575.3 83.7 62.2 71.8 79.7 105.1

Cl ppm

107.7 13185 12918 13506 16743 20811 24.0 246.7 171.7 57.5 89.0 14.2 10.5 75:': 18.4 94:3 36.4 147.8 112.1 670.8 112.2 109.9 48.7 104.0 75.7 175.2 40.9 248.5 7.1 492.0 244.7 134.4 270.8 105.5 392.4 157.8 390.0 170.6 231.9 152.2 133.6 658.5 102.3 139.5 143.0 7.6 ip::i 25.6 20.7 80:5 23.5 77.0 23.7 101.8 9.6 98.7 9.6 86.5 10.2 95.5 7.9 61.7 10.8 83.0 11.3 96.5 18.6 72.6 9.2 38.9 18.8 105.0 4.7 79.3 23.6 486.3 575.8 44.4 20.9 507.0 372.8 20.9 337.5 43.8 24.0 5::*: 42.2 13.4 365:1 336.2 22.1 481.8 23.6 485.6 18.4 378.5 14.1 504.2 17.3 347.1 28.9 467.1 21.1 237.2 263.8 63.5 10.6 100.5 32.5 177.9 69.5 57.6 51.3 222.0 102.0 220.0 118.0 .~ 164.0 52.0 205.0 1234 514.0 1094 304.0 914.0 332.0 246.4

PH

H2S ppm

Aquiferc yH3SiOa temp.Y

0.88 0.863 6.91 1.45 0.503 5.66 2.49 0.501 5.66 5.40 0.490 5.42 48.2 4.15 0.474 5.33 8.56 7.64 8.17 a.94 8.63 8.67 8.50 a.31 8.il 1.1.10-2 :*;:: 2.8910-2 01824 7.43 8.1.10-3 0.905 7.75 8.05 3'.7*10-30.921 7.58 0.13 0.901 7.34 0.75 0.898 6.57 333*9 0.838 9.03 0.873 7.02 8.41 7.00 0.876 17.6 0.861 6.80 26.9 0.854 6.74 7.04 0.85 0.876 7.16 0.11 2.5.10-2 ::::"9 8.06 7.88 9.0.10-; 0.889 8.34 l.8W2 0.925 8.42 _ 0.927 9.00 :::.10-3 0 937 9.13 1.3.10-3 0:935 9.29 1.9.10-4 0.941 9.04 2.1*10-4 0.948 9.25 l.0*10-4 0.948 9.32 1.4.10-4 0.944 9.82 2.6*10-5 0.953 9.28 1.7.10-4 0.943 8.59 8.5.10-4 0.936 9.39 1.2.10-4 0.944 9.89 4.5.10-5 0.958 8.74 7.5*10-3 0.936 9.62 5.8.10-5 0.943 6.86 48.7 0.836 35.0 5.98 0.833 6.20 99.4 0.839 342.3 5.45 0.844 6.90 35.7 0.846 6.66 251.4 9.34 1.4.10-4 X: 7.59 58.3 0.850 7.22 64.8 0.863 7.27 142.2 0.846 6.99 297.0 0.846 6.75 296.3 0.852 7.16 157.4 0.851 6.67 1034 0.847 6.63 406.8 0.865 7.40 1.47 8.89 2.2.10-3 o"% 8.13 1.0.10-2 0:931 7.14 4.6.10-3 0.800 7.97 3.4.10-2 0.915 7.55 0.40 0.855 7.64 0.36 0.862 7.67 0.10 0.a51 7.26 0.761 7.58 2:47 0.716 7.29 0.55 0.762 7.20 0.830

o.jo

160 225 230 240 232e 248e t65e 70 ::, 8': :22ie 114 109, 1:: 152 271 la2 181 215 225 157 134, 1:: 108e :: :: 67 :: :z z;: 22 e :: 240 261e 300 262e 215 3:: 25Te 242e 264e 26ge 246 270e 320 250e 210e 1:: 170e 1;; 151 151 183 258 218 192

aptl.H S and YH SiO4 = YGe(OH 5 were calculated with the program of ARNdRSSON et al. (1682) at ti! e reported re4erence temperature. Ge. Si02 and Cl values have been corrected for steam loss. bnixed discharge from two or more aquifers of significantly different temperature (>lO"C). cIf not otherwise specified the reference temperature is the measured aquifer temperature. dShallow well. Conductive cooling is lil+elyto have occurred in the feeding aquifer. eNa-K geothermometry temperature. Downhole sample. Figures in brackets indicate sampling depth.

TABLE 3. Germanium concentrations in hot spring watersa. Sample TlO.

79-002 79-003 79-007

79-013 79-014 79-021 79-023 79-024 79-025 79-026 79-027 79-031 79-037 79-043 79-047 79-058 79-059 79-060 79-061 79-062

Ge

SfO2

wb

pm

Laugarvatn 5.0 Sydri Reykir 6.4 Tungufell Thjorsardalur 1::: Skamnbeinsstadalaugar 2.4 Reykir Lund. 2.8 Hu'safell Reykholt Skrifla ::: Deildartunga Thorlikshver ::A Vatidlahver Reykjabol 1% Hrolfsstadir co.5 Tjdrn 0.8 Uxahver 5.4 Reykhblar 1.5 Bjarnastadir 1.0 Reykjanes Isafjardard. 0.8 Gjogur Hveravik Klljka

GEYSIR FIELO 79-004 Geysir 79-005 Sisjodandi 79-006 Smidur 82-005 Blesi 82-006 otherrishola 32-007 Litli Strokkur 82-008 Litli Geysir 82-009 Strokkur 82-010 Marteinslaug 82-011 Kaldilaekur 82-012 82-013 Nedridalur 82-014 Mtili 82-015 82-016 82-017 Laugarfell 82-018 82-019 Helludalur LANDMANNALAUGAR FIELD 79-053 Sullur 79-054 Land~nnalaugar 82-093 82-095 Graenagil 82-097 Eyrarhver 82-099 Vondugil 82-106 Vondugil 82-113 Landmannalaugar 82-114 Landmannalaugar 82-115 Landmannalauaar 82-116 Land~nnalau~ar 82-117 Landmannalaugar 82-118 Vondugil 82-119 Vondugil HVEIJAVELLIR 82-124 Braedrahver 82-125 Fagrihver 82-126 Eyvindarhver 82-127 82-128 Boluhver 82-129 82-130 82-131 82-132 82-133

19.1 14.9 14.0 18.8 21.3 19.6 24.0 24.3 17.5 CO.5 0.6 :*: 1:e 2z 10:4 3.4 23.0 15.6 32.6 CO.5 36.6 22.7 28.2 6.2 0.9 i:; 5.6 26.2 23.2 23.8 15.7 12.3 7.4 21.4 2.3 4.2 8.6

Cl Ppm

143.6 33.3 182.2 42.0 37.5 8.1 58.0 49.4 82.1 20.2 155.5 18.6 64.4 29.5 173.3 32.3 125.3 35.4 111.7 53.3 151.0 24.7 223.8 35.9 29.9 6.0 50.2 6.4 156.1 13.4 106.9 28.8 36.5 18.8 70.9 390.2 49.0 2460 33.1 15.6 360.1 282.0 260.4 370.3 330.0 309.5 337.0 340.8 290.9 30.6 35.2 146.2 169.6 60.2 145.7 279.6 192.2 79.7

98.1 99.0 92.2 97.3 93.7 90.5 98.2 98.5 86.6 7.4 10.6 46.4 36.0 18.9 43.6 83.1 64.5 26.4

Ph 7.32 7.45 9.31 7.88 9.14 7.81 7.91 7.23 7.91 8.10 7.85 7.81 10.01 10.07 7.79 8.23 9.45 7.92 6.76 9.46

-_____...___.___ H2S Ppm

%3SiOi

0,916 0.910 0.959 2.3*1O-2 0.882 0.931 314.10-2 0.922 ;.;;10-3 0.925 2.99 0.33

2:4*10-2 :.::: 2.7.10-2 0:920

TemPb '5

14od 1;;:

Xc 0.~3 0.909

93

88d 113d a5d 14ld 10gd

0.921 0.982

lo3d lo7d

0.994 0.987

1;;;

0.889

l?d 5.5.10-2 0.926 0.930 0.956 0.875 0.748 0.95a

7.45 0.42 7.78 0.20 6.97 0.78 7.53 0.60 6.80 3.26 6.11 14.3 6.63 0.66 6.51 0.77 7.70 0.10 8.21 7.74 7.39 7.46 7.73 7.27 6.56 7.35 7.87

0.838 0.852 0.854 0.843 0.851 0.862 0.847 0.846

0.858 0.969 0.964

0.885 0.895 0.944 0.928 0.860 0.906 0.929

2%

83d 83d

34d 233 201 203 236 227 221 230 231 206 5e 17e 68e 46e e $e 210 39e 30e

0.741 0.806 0.802 0.736 0.754 0.766 0.748 0.745

0.788

174.8 6.57 15.8 307.0 7.09 3.9.10-2 471.5 7.34 1.11 160.6 192.0 6.51 241.4 369.5 8.34 0:32 134.0 138.1 7.77 0.71 259.8 229.6 8.24 0.49 51.8 6.18 186.2 81.8 6.51 79.3 6.48 165.5 49.8 36.9 13.6 7.36 173.1 45.0 7.67 231.6 244.6 8.10 0.86 257.6 237.9 8.20 0.75

0.859 0.831 0.839 0.903 0.835 0.877 0.852 0.929 0,934 0.924 0.952 0.927 0.853 0.853

167 233 162

433.3 422.4 354.0 174.0 406.4 42.3 78.4 280.9 303.0 50.6

0.854 0.856 0.865 0.893 0.857 0.937 0.925 0.917 0.876 0.939

246 248 234 i61 243 e 43% e

0.714 0.709 0.739 0.884 0.720

2:: 20e

0.787

183.3

258.4 176.9

50.0 51.8 49.7 48.2 53.3 29.0 38.6 39.6 46.4 26.4

8.02 7.79 7.08 7.39 7.80 a.48 8.47 8.42 7.73 7.34

0.74 0.85 2.49 0.08 0.89 0.06 0.22 -

0.872 0.882

de

159 142 169 59e 24e 58e

0.888 0.922 0.869

ioe

49 167 170

0.873 0.866

"See footnote a in Table 2. bIf not otherwise speciffed reference temperature is quartz equilibrium temperature. cWater fraction. (1-X) represents the fraction of deep water which has evaporated by adiabatic boiling from the reference t~perature temperature. eMeasured temperature. These waters are to lOO~C* dNa-K ~othe~try cold or they contain a large cold water component and it is, therefore,likely that they have not equilibrated with quartz nor feldspar (see ARNdRSSDN, 1984).

metabs that precipitate. The ashed precipitate was always weighted and it was found that recovery was most o&en 9C!-95%. The precipitate was placed in a shallow socket of a specially made Ha03 pill and pressed at 40 tonnes/cm”. The K, peak was selected and the count ratioed to the

count of the K, peak for cobalt. The cobatt radiation intensity is taken to be a measure of the quantity of precipitate radiated. Standards were prepared in the same way as samples by adding variable amounts of germanium (and other trace

2493

Ge in Icelandic geothermal systems

.

4

0

.

0

D

:.. .bo I

200 Temperature “C

.

O

I

300

FIG.2. Relation between germanium and temperature of Icelandic geothermal waters. Large dots: drillholes fed by a single aquifer. Circles: drillholes with mixed discharge and hot springs from low temperature fields. Na-K temperature was used for reference. Stars and filled stars represent boiling hot spring waters and mixed waters, respectively, from high temperature fields. Quartz equilibrium and measured discharge temperature used for reference. Small dots: Data from ARN~RSSON(1969); Na-K temperature was used for reference.

metals) to deionized water followed by the various reagents as described for samples above. Standards covered the concentration range of l-100 ppb. The coprecipitation method and subsequent analysis are rather time consuming compared with other methods. However, it has the advantage that samples are prepared on the sampling site so problems which relate to losses of trace metals upon sample storage are eliminated and up to 17 trace elements can be analysed in the same sample. DISTRIBUTION

IN WATERS

thermal water. The waters containing highest germanium, at Husafell (Fig. 3), issue from acid volcanits. Geothermometry indicates that subsurface temperatures are below 100°C. Waters associated with acid rocks appear to be somewhat higher in germanium than waters of the same temperature and in basaltic rocks. Between low temperature fields of similar temperatures and with similar major element composition there may be significant differences in the germanium content of the water. Thus, in the

Germanium was detected in 98% of the geothermal waters analysed but it was below detection limit (0.5 ppb) in the 13 samples of river water and nonthermal ground water analysed for the present study. Concentrations in the geothermal waters lie mostly in the range 2-30 ppb and there is an overall relation between analysed germanium concentrations and temperature (Fig. 2) and, therefore, a relation with the major element

composition

(see ARN~RSSON et

1983). Within individual low temperature geothermal fields germanium concentrations either remain rather constant or they vary regularly across the field in harmony with variations in the major element chemistry. This is exemplified in Fig. 3 for the Reykholtsdalur field in western Iceland (Fig. 1). In this field geothermometry indicates highest underground temperatures at Reykholt (about 150°C) and a general decrease westwards to less than 100°C at the boundary of the field. Chloride increases somewhat from east to west (from 30 to 100 ppm), presumably due to increasing amount of sea-water component in the al.,

FIG. 3. Germanium in thermal waters from the Reykholtsdalur low temperature field, western Iceland. Dots: data from ARN~RSSON(1969); circles: data from this study.

2494

S. Amhson

Southern Lowlands and Reykjavik, where temperatures are similar to those in Reykholtsdalur, germanium levels are in the range of 5-10 and l-2 ppb respectively. The constancy or regular geographic variation within individual geothermal areas is taken to be indicative of a control of the aqueous germanium concentration by a solute/mineral equilibrium and/ or dissolution rates from the primary rock constituents. The observed variation between the Reykholtsdalur, Reykjavik and Southern Lowlands fields would accordingly be explained by involvement of different minerals in the dissolution process or in the exchange equilibrium. Larger variations are observed in the germanium content of high temperature waters than in low temperature waters (Fig. 2). The saline waters at Svartsengi and Reykjanes are much lower in germanium than dilute waters of comparable temperature from other fields. Some of the hottest well waters from Krafla and Nbmafjall, northern Iceland are lower in germanium than discharges from cooler and therefore shallower aquifers within the same fields. Of the waters sampled by ARN~RSSON (1969) in 1967 and 1968, 34 were resampled in 1979-1982. The mean difference between the earlier samples and the 1979-1982 samples is 27%, the latter being on average 9% lower. Most of the difference is due to three samples, the emission spectrograph results of ARN~RSSON (1969) giving higher values. If these three samples are excluded the corresponding figures become 17% and 0.5% respectively. By comparison with the precision obtained from analyses of duplicate samples (see Table 1) it appears that germanium concentrations in individual discharges do not, at least in general, vary significantly within the relatively short period of a few years.

TABLE 4. Germanium in fresh and hydrothermali,y altered rocks fran Iceland. ___-.___ ._ Locality

Aa

Bb

,:i

BASALT 1 Na'mafjall 4 2 Reykjanes 5 3 Kerlingarfjdll 2 4 Krisuvfk 8 5 TorfajUkull 2 6 Hallmundarhraun 1 7 yy'rdalshryggur (Nesjavellir)1 8 Crater row west of Stampar 1 9 Hengill. Brennisteinstindur 1 10 Askia, lava 1961 11 Vadalda ; 12 Trlclladyngja 1 13 Kerlingarhraun 14 Lava at JUku1s.a' FjUllum 1 1 15 KolUttadyngja 16 ReykjavfkurgrSgry'ti 17 Eldgjdrhraun ; 18 Breiddalur 2

1x5

ANDESITE 1 Nbmafjall 3 KerlingarfjUll 5 TorfaiUkull 19 Hekla; lava 1970

2 1 2 1

1.1-1.3 1.2 1.0 1.4-1.6 1.5 1.9

ACID VOLCANICS 1 Nhmafjall 3 KerlingarfjUll 5 TorfajUkull 20 Hrafntinnuhryggur 21 Pdlsfjall, Vatnajukull

3 0.9-1.2 1.1 2 1.2-1.3 1.2 4 1.4-1.5 1.5 1 ld ;:i;

GABBRO 22 GeitafellsbjUrg 23 Vidbordsfjall

;d

1.2 1.6

GRANOPHYRE 24 Reydardrtindur

ld

1.2

HYDROTHERMALLY ALTERED BASALTIC ROCKS 1 Ndmafjall 25 Hveragerdi 26 Nesjavellir 4 Krfsuvfk 27 Krafla 18 Breiddalur 28 Setberg, Snaefellsnes

21 7 2 10 7 17 6

1.1-1.4 1.3-1.4 1.1-1.2 1.1-1.8 1.4 1.2 1.4 1.4 1.7 ._ 14 1:1 1.1

i.3 1.4 1.2 1.3 1.4

;:: ;::

0.4-1.9 0.4-1.6 0.6-1.3 0.2-1.6 0.3-1.6 0.7-1.5 1.3-2.0

1.5

0.9 1.0 1.0 1.0 1.0 1.2 1.6

aNumber of samples analysed. bconcentration range, ppm. cAverage, ppm. dAverage of 4, 2 and 5 samples fran no. 21, 23 and 24 respectively.

DIS’IlUBUTION IN ROCKS AND MINERALS The average concentration of germanium in 37 samples of fresh basalt and gabbro from geothermal fields and other localities in Iceland was found to be 1.3 ppm. The concentration range is 1. l-l .8 ppm (Table 4, Fig. 1). The andesitic and acid rocks average 1.4 ppm (6 samples) and 1.3 ppm (12 samples) respectively, the concentration range being 1. l-l .9 and 0.9-1.5 ppm (Table 4, Fig. 1). The Icelandic data do not indicate any increase of germanium in the intermediate and acid rocks over the basalts and gabbros; a result which conforms with those obtained by many previous investigators in other regions (see e.g. H~RMANN, 1970). Some of the altered rocks recovered from drillholes in active high-temperature geothermal systems show considerable depletion in germanium relative to the fresh rocks. Comparable results have been obtained for some New Zealand systems. (KOGA, 1967). Germanium depletion was not found to be so prominent in some samples of propylitized basalts from fossil high-temperature geothermal systems at Setberg in

western Iceland and Breiddalur in eastern Iceland. These areas have been studied geologically by SIGURDSSON (1966) and WALKER ( 1963) respectively. In the active geothermal system it is observed that the germanium tends to be most depleted at shallow levels where flashing of the rising hot water occurs. Germanium was analysed in some vesicle minerals from the Tertiary central volcano of Breiddalur in eastern Iceland. The results are presented in Table 5. All the quartz and zeolite samples are from a relatively limited area on the southern slopes of Breiddalur just above and below the Graenavatn porphyry basalt group which forms stratigraphicaIly the lower limit of the core of the volcano (WALKER, 1963). In the core the rocks are strongly propylitized with laumontite being present with other zeolites, quartz and calcite in its outer part. The zeolite mineral assemblage indicates formation temperatures of less than 2OO’C. The zone of propylization within the core of the Breiddalur volcano represents a fossil high-temperature geothermal system.

Ge

in

Icelandic geothermal systems

TABLE 5. Germaniun in some minerals from the fossil geothermal system at Breiddalur, eastern Iceland, and in silica sinter and travertine from the active geothermal fields at Geysir, Hveravellir. Hengill and Lysuholl on Snaefellsnes. Mineral

A

B

C

quartz chalcedony calcite thompsonite scolecite stilbite heulandlte chabacite fluorite silica sinter travertine "silica gel"a

2 1

0.1-1.3 0.6 (0 1 0.5lO.6 0.4 0.1-0.6 CO.l-0.2 0.1 co.1 (0.1-1.6 ~0.1-2.5
0.7

: 2 3 2 1 1 5 5 1

0.6 0.4 0.2 0.1 0.8 0.9

aprecipitate from the saline geothermal water at Svartsengi in SW Iceland. The associated water contains about 3 ppb Ge. A: no. of samples. B: range, ppm. C: average, ppm.

The germanium content of the zeolites varies from x0.1 to 0.6 ppm. No correlation can be observed between the density of these minerals and their germanium content. It appears, however, that the fibrous zeolites contain highest germanium (thomp sonite, scolecite), the platy zeolites somewhat less (stilbite, heulandite) and the “cubic” zeolite, chabasite, least germanium. The data are, however, far too meager for this correlation to be anything more than suggestive. Germanium substitutes for silicon in the zeolites, as it does in other silicates, and should on the basis of its larger ionic radius (0.53& as compared with 0.42A for silicon) find it most favourable to enter the fibrous zeolites where the crystal growth is strongest in one direction. On the other hand substitution for silicon should be least favourable in the “cubic” zeolites where crystal growth occurs in three dimensions. The germanium content of the zeolites would be expected to vary with their temperature of formation. In this respect the available data are far too limited to allow the study of such correlation for individual minerals. Germanium was not detected in five samples of calcite collected within and outside the core of the Breiddalur central volcano. On the basis of crystal chemistry considerations one would not expect germanium to enter the crystal structure of calcite. Its ionic radius is too large to fit into the triangular planar coordination of calcite. The germanium content in three quartz samples was found to be highly variable or between 0.1 and 1.3 ppm (Table 5). The highest concentration was from the only sample collected outside the zone of propylitization. In comparison with the zeolites the germanium content of the quartz is high. The variable content in quartz may be the result of different formation temperature. The germanium content in several samples of silica sinter and travertine was found to be highly variable, or in the range of x0.1-1.6 ppm and ~0.1-2.5 ppm,

2495

respectively (Table 5). Silica sinter from New Zealand contains considerably higher concentrations of germanium (KOGA, 1967). The ratio of germanium to silica is always much lower in the sinters than in the associated thermal waters. The sinters consist in part of silica truly deposited from solution in an amorphous form, but also of windblown dust and evaporated residue from drops of water thrown out from vigorously boiling springs and geysers. This inhomogeneous nature of the sinters explains, it is believed, the variable germanium content of sinter samples. The travertine samples are precipitates from warm water with an intermediate pH of 6-7 and all contain some amorphous silica. It seems likely that the germanium is contained in this silica impurity rather than in the calcite. AQUEOUS SPECIATION AND MINERAL SQLUBILITIES NAUMOVet al. (197 1) and GARRELS and CHRIST (1965) present free energy data at 25°C for several germanium hydroxy and fluoride complexes as well as enthalpy and entropy data for the fluoride complexes (Table 6). GARRELSand CHRIST (1965) express the germanium hydroxy complexes in analogous form as carbonate complexes. In crystals it is known that germanium exhibits coordination numbers of 4 and 6, the former being more common (H~RMANN, 1970). Germanium does not occur in carbonate minerals as is expected by crystal chemistry considerations (HARRIS, 1954). By analogy with crystals, it is expected that germanium prefers fourfold or sixfold coordination with hydroxide in aqueous solution and accordingly it would be appropriate to express HzGeOJ , HGeO; and GeO;’ as I&G&&, H&eO; and HzGeO;* or as Ge(OH)4, Ge(OH); and Ge(OH)i2 as chosen by NAUMOV et al. (197 1). However, in order to distinguish data on aqueous species from GARRELS and CHRIST (1965) from those given by NAUMOV et al. (197 1) the former have been expressed throughout the text and in Table 6 as H2Ge03, HGeO: and GeO;* but the latter as Ge(OH),,, Ge(OH); and Ge(OH)i2. Note that in thermodynamic terms AG$‘(T&-, + AGP(Tr)H20 is taken to be equal to AGr)(Trhouk etc. for other hydroxy complexes. The free energy data for the hydroxy complexes given by NAUMOVet al. (197 1) are based on solubiity experiments for Ge02,hex and a known value for the free energy of formation of the solid. The data of GARRELS and CHRIST (1965) indicate much lower solubility. Also the absolute free energy values differ greatly. Their value for Ge02,hcx is 1704 1 J/mole less negative than that selected by NAUMOV et al. (1971) which is the same as reported by ROBIE et al. (1979)., 497059 J/mole. This later value was selected for the present study. Entropy data for the aqueous hydroxy species and free ions at 25°C have been estimated by the method

2496

S. Arnbrsson TABLE 6. Basic thermodynamicdata for some aqueous species, gases and minerals of germanium.

AG;(T$

Average heat capacityb"

Species

60" 100" 150" 200" 250" 300°C

Get2

-3176gd

Get4

-22564d

H2GeOgh z;i,; ;:33

-709188' ~~~~~~ :;;',',$

Ge(OH);' -496097d

-95.of

+201 +268 +272 +297 +314 *331

-103985e -497.gg

+6941e

+385 +510 +536 *577 +607 +636

-795496j t179.99

+238 +335 +293 +335 +347 +36Ek

-752965e +159.8'

+109 +155 +117 +130 +i‘%? +155

-713514e

+lOO +146 +?09 1121 +138 +146

+49.41

-337770e +158.21

-351 -385 -377 -406 -431 -452

-340176e -264.0"

+318 +473 +389 +397 +402 +439'

-78.7"

+431 +582 +519 +548 +569 +61gk

-590095e

-730317d

-B3852Ee +106.7m

+527 +682 +649 +695 +736 +795k

-963450d Ge(W4 Ge(OH)i -1150767'

-3117132j t187.0'

+477 +607 +569 +619 +661 +707k

-1356252e +246.4m

+406 +498 +456 +498 +536 +565

Ge(OH)i2-1315324d

-i603853e +200.Em

+318 +402 +360 +393 +423 +448

Ge(OH);

Gef;

-1527754d'n -1694938' +125.0d

GeFS2

-1828902d'n -2026311d +190.4d

GeF40H- -145783gd*" -1618016d +214.7"' tl+

0

0

0

+96 +130 +138 +146 +155 +163p

OH-

-1572934

-229994p

-lO.Ep

-197 -243 -255 -272 -289 -31Ep

F"

-279993d

-333841d

-13.Ep

-197 -243 -255 -272 -289 -3141°

cl-

-131260'

-167151p

+56.5p

-213 -243 -259 -276 -293 -31Ep

-37083q +129.2R

+192 +163 +146 +134 +138 +14gp

H2Saq R201

-27865d -237178'

+69.gp

+75

-551033d

t55.3d

+62.76 +11.97 +12.64d

-189535'

+87.4d

a

Minerals,gases

d Ge02,hex -497059 GeS2

-187443d

+75

-285830p

t36.74

+75

b*103 +0.42

+75

+2.84d

-12916Ehid+104.3t

+103.51 +14.98 t12.34"

Ca2Ge04 -1879076'

-1995015d +135.5t

+128.66 t50.04

-1150140d

-1189804d +303.4d

-457311d

-495804d +347.6d

GeE4,g GeC'4,g

+75p

c lo-5.r

-1247083'

CaGe03

+75

+e.12u

t95.06 +11.97 +14.90d t106.52

t1.34

+9.66d

-25% bJ/moledeg. cIf not otherwiseindicated,averageheat capa~~~~~~a~~ been calculatedfrom the data gfven by CRISS and COBBLE (1964a.1964b).

volumeof CaSiO and Ca S104 by the mathodof NELGESONet al. (1978).Walculated by tk methodo HELGE N et al. (1978)from the heat capacitiesof CaSiO3and Ca2SiO4respectfvely. lia

of COBBLE(1953) and LATIMER(1952) and the entropy correlation plot for hydroxy complexes given by ARN&SSON et al. (1982) (Table 6). Dissociation constants at elevated temperatures have been estimated applying the electrostatic approach of HELGESON (1967) and using Eqn. (1 la) in ARN~RSSONet al. (1982) (Table 7, Fig. 4). Average heat capacities have also been estimated for the hydroxy anions and simple cations using the relationship given by CRIS and COBBLE( 1964a,b). From these and the predicted dissociation constants average heat capacity values

have been derived for neutral and positively charged hydroxy species. The derivation makes use of the fact that a~(T)/aT = -A&$‘(T). It can be seen from Fig. 4 that there is a considemble discrepancy between predicted dissociation constants of anions by the electrostatic method of HELGESON (1967) and values obtained from estimated average heat capacities. The shape of the temperature functions by the two methods are most sensitive to A$(T& yet in a different way. The two sources of data (GARRELS and CHRIST, 1965 and NAUMOV et

2491

Ge in Icelandic geothermal systems TABLE 7. Themdynamic data at 2VC and 1 bar abs. and temperature functions for dissociational eguilibrra and mineral solubilities for some aqueous gersmnium Spies and geMniUm mineralsc. logK(T,) AH;(T,)" AS;(Trlb

Reaction

-6.50

H2Ge03'H++HGe0

; HGeO; = H+ + GeOi2

-12.68 -1.27

Ge(Otl),2= Ge(OH); + OH-

GeF5 = GeFqBg + F

-20.1

+39447 -110.5

-1.38 -2932/T -16.45*10-6*T2

+17606

t34.7

tO.43 -645/T +5.17.10-6*T2 -0.88 -1032/T -10.47*106*T2

-5.27

+9125

-70.3

-13.28

+48610

-91.2

-13.48

+18439 -196.2

-2.45 -2513/T -29.22*106*T2

-13.74

+19924 -196.2

-2.45 -2590/T -29.22*10-6*T2

-13.87

+6197 -244.8

-3.06 -2257/T -36.45*106*T2

-2092

-1.12 -601/T -13.39.10-6.T2

-4.34

GeFi2 = GeF; + OH-

+42530

Telrperaturefunction “K

-0.25 -2380/T -2.99*10-6.T2

-90.0

-1.14 -3259/T -13.58*10-'*T2

-19.36 +172506 tl98.3

_

-30.65+198154

GeF40H-= GeF4,g+ OH

tl42.7

GeF4 g + 4H20 = Ge(OH)4 + 4F- + 4H+ -3.51 -114520 -451.2 f + 4H20 = Ge(OH)4 + 4Cl- t4Hf14.50 -146612 -214.6 GeC'q,g t25.92 -129365 +62.3 HGe02 + Hz0 = HGeO; + H2,g

-12.22 +4281/T -0.01902.T

Ge

-0.0060 W.002206.T -2.12837 10-6.T2sd

=Ge

4 + 2e-

w.0477d

Ge02 hex + 2H20 = Ge(OH)4

,

GeS2 + 4H20 = Ge(OH)4

l

2H2S

CaGe03 + H20 + 2Ht = Ge(OH)4 + Cat2 +2 Ca2Ge04 + 4Ht = Ge(OH)4 + 2Ca

-4.39 -1.40

-110926 -402.9 +41367 +5561

t54.7 -8.1

tl.37 -5785/T -0.02122.T t4.76 +6531/T -0.00253.T t106.37 -4555.68/T +0.03547-T -42.858.logT t202.12 -4073.69/T +0.08377-T -86.804*logT

-23.69 +177364 tl41.3 -20.70 +141557 t78.5

tl77.46 -12884.70/T W.06292.T -71.369.1ogT t273.05 -12396.58/T +0.11125.T -115.268.logT

+9.34 +12.34

-46643 -82450

t22.5 -40.3

t87.57 +604.16/T +0.03223-T -36.308.logT t175.86 +1308.86/T +0.07881-T -77.337.1ogT

t30.55 -171975 t33.54 -207782

t8.0 -54.8

t69.15 +7733.89/T +0.02900-T -29.577.logT t150.77 +8643.24/T +0.07409.T -68.007.logT

aJ/mole. bJ/mole deg. 'Two sets of data are given for mineral solubilities.The first is basgd on data from GARRELS and CHRIST (1965) on Ge(OH)4 but the second on data from NAUMOV et al. (1971). E0 (volts).

al., 197 1) yield very similar predicted values over the whole temperature range for the dissociation of divalent and monovalent anions (Fig. 4). Data from well discharge compositions have been used to evaluate which of the germanium complexes may dominate in geothermal waters. Figure 5 shows that HGeO~/H~GeQ and HGeO;/GeO;’ log mole ratios almost always exceed 5 considerably indicating that HGeO; is the dominating hydroxy bearing species. practically the same ratios would be obtained when using data from NAUMOV et al. (I 97 1) since the values for dissociational equilibrium constants are so similar (Fig. 4). Fluoride, chloride and germanium levels are much too low to cause any significant formation of fluoride bearing complexes or GecL,. The redox potential of the geothermal waters, as deduced from sulphide/sulphate equilibria and hydrogen levels, are much too high to cause significant formation of divalent germanium species such as HGeO;. It is concluded that practically all the germanium in the Icelandic geothermal waters occurs as Ge(OH), and Ge(OH), (or H&OS and HGeO;) the latter being much more abundant except in relatively saline waters with temperature in excess of 200°C such as at Svartsengi and Reykjanes (see Table 2) where Ge(OH), is almost as abundant as Ge(OH);. This conclusion is valid whether average heat capacities or the electrostatic approach of HELGESON (1967)

were used to calculate dissociational equilibria at elevated temperatures. Thermodynamic data on several germanium minerals are given in Table 7. According to the data given by NAUMOV et al. (197 I) GeOz,hu is highly soluble but the GARRELS and CHRKT (1965) data indicate moderate solubility. The calcium germanates are both very soluble but GeSr sparingly soluble. None of these minerals occur in nature. It is only in sulphides that germanium may occur naturally as a major component and sometimes as small grains of germanite or renierite within other sulphide. Thermodynamic data on germanite, renierite and other known naturally occurring germanium minerals are not available. FAaORS

CONTROLLING GERMANIUM MOBILITY

The activity of HrGeOr (or Ge(OH),) in the geothermal waters relates reasonably well to the aquifer temperature (Fig. 6). ARN~RSSONet al. (1983) demonstrated that the concentrations of undissociated weak acids are mostly dependent on water temperature at equilibrium. The temperature dependence of with the regular distribution of aH2Ge03 ) together germanium in waters within individual geothermal fields, previously discussed, is taken to indicate control of germanium concentrations through solution/mineral equilibria. Exchange reactions must be involved as no germanium minerals form. The geothermal

2498

S. Am&son

FIG.6. Calculated concentrations of H2Ge03 in geothermal well waters.

ioo

200 Tamponturn

I

300

evaluation of water pH have a significant effect on the scatter of data points in Fig. 6. ARN~RSSON et al. (1983) showed that uNP+/uH+ratios depended mostly on aquifer temperature. Therefore, uNP+ = uH+KN~

‘I:

and

aNa+ * UHGCOT =

KN~

* K~a.03

* UHZCCOS.

The ratio of germanium to silica is 5-50 times FIG. 4. Predicted dissociation constants for germanium hydroxy complexes at elevated temperature using the elec- higher in the geothermal waters than in the associated trostatic approach of HELGESON (1967). Curves labelled 1 rocks. The overall effect of dissolution of the primary are based on data at 25°C from GARRELSand CHRIST rock constituents and precipitation of alteration min(1965) (see Table 7). Broken curves are based on average erals, therefore, leads to initial enrichment of gerheat capacity data. manium in the water relative to silica. Enrichment continues until precipitation rates of germanium to satisfy exchange equilibrium balance dissolution rates. waters are grossly undersaturated with the minerals The chemical potential of any component in a for which data are given in Tables 6 and I. Substimultiphase system in equilibrium is the same in all tution for silica in silicates and iron in sulphides is phases this component enters. In geothermal systems expected. equilibrium is always closely approached with quartz The activity product of Na+ and HGcO; gives (or chalcedony) (FOURNIER and ROWE, 1966; MAconsiderably better correlation with temperature than HON, 1966; ARN~RS~N, 1975). Defining an exchange H2Ge03 concentrations suggesting that errors in the equilibrium involving quartz as wo&mmio,

.

,.

,

100 200 300 Temperature“c

L-eTemperature “C

FIG. 5. Relative abundance of H2GeOj, HGeO: and GeO? in geothermal well waters. Thermodynamic data on a (?‘,) are from GARRELSand CHRIST(1965) (see Tables 6 and 7).

2499

Ge in Icelandic geothermal systems

= it& &ZS mce(oH), = I!& * Kgl. where Kgtl repreSeUts Mu, the solubility of quartz at a specified temperature. Although all silicate minerals, as well as pyrite and pyrrhotite, will take up some germanium the respective exchange equilibrium with quartz is not affected. Each of these minerals will have their specific exchange equilibrium with the solution. The quantity of germanium in each of these minerals as well as quartz will depend not only on the exchange equilibrium constant but also on the concentration of l&GeG., in the associated water. The formation of all the minerals taking up germanium and their relative proportions will ultimately control the balance between germanium dissolution from the rock and its precipitation. Consequently, for a given dissolution rate and the same relative abundance of the secondary minerals taking up germanium, the H_,GeG4 concentration in the water will be dictated mostly by the water temperature as the exchange equilibrium constants are mostly affected by temperature. Germanium concentrations do not vary much in igneous rocks. Only “incongruent” dissolution would be expected to affect the release of germanium relative to silica, such as dissolution of olivine as germanium is concentrated in this mineral (HARRIS, 1954). It may be that abundant olivine basalts at depth in the Southern Lowlands are the cause of the relatively high germanium content in the waters as compared with the Reykholtsdalur area, but subsurface temperatures are comparable in both fields. The lowering of germanium in the hottest well waters (see Fig. 6) can be explained by abundant epidote formation, but based on crystal chemistry considerations this mineral is expected to concentrate germanium. The reverse, i.e. relatively limited epidote formation, could explain the higher germanium content in the waters issuing from the acid rocks at Landmannalaugar as compared

with similarly hot waters from Hveravellir and the Geysir field (see Fig. 1 and Table 3). The solubility data of GARRELS and CHRIST ( 1965) on Ge&,_ give results which are more consistent with the geochemical results of the present study than do the solubility data of NAUMOV et al. (1971). The former data and the germanium concentrations in the water indicate that the activity of GeQ,k in quartz and of Ge!& in sulphide would be expected to be of the order of lo-’ and lo-’ respectively, corresponding to 1 and some 10 ppm Ge in these minerals (Fig. 7). No data are available on germanium in Icelandic sulphides. However, H~RMANN (1970) reports values in the range of 10 ppm Ge in pyrite from other regions. Germanium in quartz is of the order of I ppm. COOLING, BOILING AND MIXING PROCES!XS The effects of conductive cooling and boiling of geothermal water on germanium mobility have been evaluated with the aid of the program of ARN~RSSON et al. (1982). Conductive cooling causes an increase in m&o&, and favours, therefore, removal of germanium from solution (Fig. 8A). The effect of boiling is more complicated. During the initial stages, H&&J concentrations decrease strongly but upon continued boiling they begin to increase (Fig. 8). It is the sharp rise in pH which results from degassing which is responsible for the initial drop in mnm,. The later increase results from an increase in the value of K n_, with falling temperature. Degassing of HrS together with the pH increase favours increased germanium mobility with respect to sulphide equilibria (Fig. 8B). The results presented in Fig 8 indicate the tendency for germanium leaching during the early stages of boiling of rising geothermal water but precipitation at later stages and during conductive cooling. It is

Gss2+3H20-

H2&03

l

2H2S

f loo

200 Tempemtwe

300

9:

RG. 7. Saturation with respect to Ga.kl (A) and CieS2(B) in geothermal well waters. Curves 1 and 2 are based on data from GARRELSand CHRIST(1965) and NAUMOVef al. (1971) respectively. The broken lines in part A indicate GeOz activity of 1O-‘.86and 10-4.86per one Si02 which corresponds to 1.0 and 0.1 ppm Ge in quartz as indicated. The broken line in part B corresponds to GeS2 activity in pyrite equalto IOppmGe.

2500

S. Ambson __/~_

_--

--

~-_._~~~_~_~~~_~ 7--- --.----

.--_.-..

Temperature

“c

Tnnp~ature

t

FIG. 8. Changes in H2Ge03 concentrations and Ge& saturation upon adiabatic boiling (solid lines) and conductive cooling (broken lines) of selected geothermal well waters. The heavy solid lines in part A was

&tamed by multiple linear regression through the data points in Fig. 6. The heavy fine in part B represents Gegz activity in sulphide corresponding to 10 ppm Ge. The data of GARREL~and Cnxrsr (1965) on the aqueous germanium species were used.

considered that depletion of germanium observed in the upilow zones in the active high temperature systems included in this study and in some New Zealand systems, as reported by K~GA ( 1967), is due to leaching by boiled water. Water in warm springs and boiling springs in the geothermal fields at Geysir, Hveravellir and Landmannalaugar (see Fig. 1) contain a variable cold water component as a result of mixing in the upflow. The chloride content of the water is a measure of the degree of mixing (ARN~RSSON, 1984). Figure 9 shows how germanium relates to chloride in the mixed waters from these fields. For the Geysir field and Hveravellir the data points tend to form a curve GEYSIR FIELD

which is somewhat convex downwards. This apparent deviation could either be due to precipitation of germanium from the cooler water subsequent to mixing or dissolution of germanium from the rock by the hottest water when it boils. The germaniumchloride relationship is not as clear for the Landmannalaugar waters because both processes, dissolution and precipitation, may be operative. It is noteworthy that one sample from Landmannalaugar (82-095) contains considerable chloride but not detectable germanium. This water is high in iron and ferrihydroxide precipitates occur around the spring outlets. Possibly the germanium has been precipitated with the iron.

LANDMANNALAIJGAR

FIELD

.

.

_ 200400600

cl, wm

Cl, ppm

FIG. 9. Relation between germanium and chloride in variably mixed geothermal waters from the Geysir, Landmsnnalaugar and fiveravellir fields. Open circles: data from ARN~RSSON(1969); Filled circles: this study.

2501

Ge in Icelandic geothermal systems The data plotted in Fig. 9 indicate that the tendency for germanium precipitation upon mixing is not strong and mixed waters containing a high temperature component tend, accordingly, to have high dissolved germanium for their temperature. The data from wells at Nimafjall and Krafla clearly show that germanium is leached from the rock in upflow zones where boiling occurs. Thus, well 8 at Krafla (81-029, Table 2) is fed by a rather shallow aquifer at 215”C, away from the major upflow and its water contains more germanium than deeper wells at Krafla some of which are close to the major upflow. Similar situation is observed at Nimafjall. Water in wells (11 and 12) fed by the deepest aquifers contain less germanium than water from shallower aquifers entering wells 4 to 9. The product of sodium and total germanium concentrations may be taken to be a crude geothermometer. This assumes molalities to be equal to activities, all germanium to occur as HGeO; and ignores sodium complexing. A temperature function which is based on the well data plotted in Fig. 6 is: t”C = 25693 + 7545.76. X + 742.96. Xz + 24.449. X3 where X = log mNa + log MC+.The above temperature function includes the effects of steam loss by adiabatic boiling to 100°C. The product of log mNa and log mGc for the Geysir and Hveravellir fields indicates temperatures of 236” and 225°C respectively for the samples highest in germanium in each field. These are somewhat lower than estimated underground temperatures by various chemical geothermometers and mixing models (ARN~RSSON, 1984). The same approach for Landmannalaugar yields well over 300°C but mixing models indicate 260°C (ARN~RSSON, 1984). These results are taken to indicate that dissolution of germanium in the uptlow at Geysir and Hveravellir is not extensive but at Landmannalaugar the high Na-Ge temperature is likely to be affected by dissolution although differences in rock type may also contribute. Acknowledgements-This study was initiated while the author was working at the University of Oslo as a Postdoctoral Fellow of the Norwegian Technical and Scientific Research Council in 1976-77. A part of the results presented here were reported at that time. Professor Ivan Th. Rosenquist, Professor Per Jijrgensen, Dr. Ellen Roaldset of Oslo University and Dr. Per Aagaard are sincerely acknowledged for their assistance while the author was at Oslo University. Dr. Karl Gr6nvold and Mr. Niels bkarsson of the Nordic Volcanological Institute and Dr. Einar Gunnlaugsson of the Reykjavik Municipal Heating Service are thanked for providing many of the rock samples. I am specially indebted to Miss Gudtin Sverrisdbttir and Mr. Gr&ar ivarsson who carried out a large part of the germanium analyses. I also want to thank Drs. M. Thompson, J. V. Walther and W. F. Giggenbath for reviewing the paper. This study has been supported by the Icelandic Science Foundation.

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