Origin of saline groundwaters in the Carnmenellis Granite (Cornwall, England): Natural processes and reaction during Hot Dry Rock reservoir circulation

Chemical Geology, 49 (1985) 287--301

287

Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

ORIGIN OF SALINE GROUNDWATERS IN THE CARNMENELLIS GRANITE (CORNWALL, ENGLAND): NATURAL PROCESSES AND REACTION DURING HOT DRY ROCK RESERVOIR CIRCULATION W.M. E D M U N D S 1, R . L . F . K A Y 1 and R . A . M c C A R T N E Y 2 1Hydrogeology, Research Group, British Geological Survey, Wallingford, Oxon OX10 8BB (Great Britain) 2 Camborne School of Mines, Redruth, Cornwall, TR15 3SE (Great Britain) (Accepted for publication July 16, 1984)

Abstract Edmunds, W.M., Kay, R.L.F. and McCartney, R.A., 1985. Origin of saline groundwaters in the Carnmenellis Granite (Cornwall, England): Natural processes and reaction during Hot Dry Rock reservoir circulation. In: Y. Kitano (Guest-Editor), Water--Rock Interaction. Chem. Geol., 49: 287--301. Saline groundwaters (up to 19,100 mg 1-I total mineralisation) issue in tin mines in the Carnmenellis Granite in Cornwall (U.K.) at depths up to 800 m. Their stable-isotope composition rules out seawater as a contributor to salinity. Circulation experiments carried out during Hot Dry Rock (HDR) reservoir development in the same granite also produce return fluids with enhanced salinities. Acid hydrolysis of plagioclase and biotite are proposed as the main sources of salinity in the groundwater. Experimental studies carried out on biotites from a borehole used for HDR evaluation demonstrate the reactivity of the biotite and confirm the hypotheses of the field studies. Mg, Li, K and silica levels in reacted solutions reflect the stoichiometric composition of the biotite. Chloride, Na and Ca in solution, on the other hand, are enriched between 1 and 3 orders of magnitude over that of biotite, reflecting the strongly incongruent nature of the reaction. Quartz and chalcedony saturation of the groundwaters encourages silica (or silicate) deposition rather than dissolution of rock-forming quartz; this argues against fluid inclusions as the source of salinity and suggests that new inclusions might be formed. The proposed model for the genesis of saline water therefore links together, or explains, several processes -- groundwater movement, convective heat transport, the chemistry of the water, water--rock interaction, secondary mineral (including kaolinite) formation and fluid inclusion formation and stability.

1. I n t r o d u c t i o n

The occurrence of saline groundwaters in granitic rocks has been discussed in several recent papers and some controversy or uncert a i n t y exists concerning their origins. Brines with salinities of up to 550 g 1-~ are found in mines and drillholes in the Precambrian plat0009-2541/85/$03.30

form of the Canadian Shield and the U.S.S.R. for example. These are f o u n d at depths down to 10 km and are t h o u g h t to originate from intensive rock--water interactions, possibly starting from one or more precursors -- sedim e n t a r y basin brines, h y d r o t h e r m a l solutions (residual ore-forming fluids) evaporated seawater (Frape and Fritz, 1982; Fritz and

© 1985 Elsevier Science Publishers B.V.

288

Frape, 1983). Nordstrom ( 1 9 8 3 ) h a s proposed an origin for enhanced salinity in groundwater (up to 630 mg 1-1) derived from fluid inclusions in granitic rocks of the Fennoscandian Shield in N.W. Europe. In granitic rocks from Los Alamos in New Mexico, U.S.A., groundwater salinities typically around 600 mg 1-1 C1- are considered to be derived mainly b y displacement o f an indigenous pore fluid, although some compositional changes, e.g. SIO2, HCO~, SO~- and F-, are ascribed to rock--water reactions (Grigsby et al., 1983). Vovk (1981) has proposed that the concentration of original salinity by radiolysis over geological time scales could also account for the anomalous stable-isotope compositions c o m m o n l y observed. Thus the explanation of increases in salinity in igneous rocks contrasts with sedimentary basins, where the presence of formation waters, bedded evaporites or compaction processes can, as a rule, account for the mineralisation. Saline (up to 19,300-mg-1-1 total mineralisation) and thermal (up to 52°C) groundwaters occur in active and disused tin mines in the north o f the Carnmenellis Granite, Cornwall, U.K. Their occurrence has been considered in detail as part of a research programme connected with exploration of the geothermal potential of the U.K. (Burgess et al., 1982; Edmunds et al., 1984) and it has been shown that the saline groundwaters are a mixing series between recent meteoric waters and a much older brine derived from the granite. The same granite, ~ 12 km south of the area of active tin mining, is n o w the site o f the first European H o t Dry R o c k (HDR) experiment. A geothermal doublet has been drilled to 2000 m and subjected to hydraulic and other tests (Batchelor, 1985). Samples have been obtained during the first 4000-hr. circulation period and experimental studies have been carried o u t on minerals taken from drillhole cuttings. The purpose of this paper is to compare the chemistries o f naturally occurring groundwaters {modified by mixing) in the north of the granite

with waters injected during relatively short circulation periods (< 1500 hr.) during the H D R testing, as well as laboratory experiments carried o u t at low temperature on biotites taken from the H D R borehole. 2. Saline groundwaters in the Carnmenellis Granite The Carnmenellis Granite forms a nearcircular outcrop of the Cornubian batholith (Fig. 1) which was intruded ~ 290 Myr. ago into Devonian argillaceous sediments. It is predominantly a biotite adamellite with primary quartz, plagioclase (zoned oligoclase with An-rich centres), orthoclase (often perthitic) and biotite. Muscovite is usually present, either as a late magmatic or hydrothermal mineral (Exley and Stone, 1964); typical chemical and modal analyses o f the

O0

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Legend

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150

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Fig. 1. Location map showing the Carnmenellis Granite, the distribution of thermal and saline groundwaters and site of Hot Dry Rock (HDR) experiment.

289

granite are also given by these authors. The granite is enriched in volatile elements (B, C1, F, Li) compared with m a n y other granites (Fuge and Power, 1969). Tourmalinisation and greisen formation has been brought about as the result of boron and fluorine metasomatism. There is extensive hydrothermal mineralisation, also of Variscan age (Jackson et al., 1982) which has produced economic vein deposits of Sn, Cu, Pb and Zn, although there is evidence of hydrothermal activity at stages later than this (Durrance et al., 1982). The principal mineral lodes strike ENE--WSW or E--W in the mineralised belt north of the Carnmenellis Granite, but laterstage mineralisation m a y also occur as

NW(NNW)--SE(SSE) cross-courses. Saline groundwaters occur in four active mines in the granite or its thermal aureole as well as in several presently disused mines all at the northern margin (Figs. 1 and 2). They generally issue from cross-courses with discharges between 1 and 10 1 s -1 at depths between 200 and 700 m below surface. The discharge temperatures, up to 52aC, are typically in excess of the average regional thermal gradients of 29.8°C km -1 in the granite and 50oC km -1 in the aureole (Wheildon et al., 1980). This implies that warmer saline fluids are upwelling by convective circulation (Fig. 2). This is borne out by the SiO2 and Na/K g e o t h e r m o m e t r y which suggests an equilibra-

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Wheal Jane

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derived from granite-water reactions stored in fracture system

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Fig. 2. Conceptual cross-section through the Carnmenellis Granite showing the location of active tin mines and saline groundwaters. Isotherms in the granite and its aureole show convective distortion due to groundwater circulation and the suggested circulation route of groundwater flow is shown.

290

tion temperature of around 54°C in the granite, equivalent to a m a x i m u m circulation depth o f ~ 1200 m. The geochemistry o f the saline groundwaters has been discussed in Edmunds et al. (1984) and only a background summary is provided here. Chemical analyses o f the four most saline groundwaters from each of the active mines and two shallow-groundwater analyses are given in Table I. Even the saline

mine waters contain tritium and it was shown that the observed saline chemistry must result from mixing b e t w e e n recent non-saline, shallow groundwaters and a much older saline component. The driving force for the current circulation system is the hydraulic sink created by the mining operations. Circulation o f the groundwater must also have been taking place at a very slow rate over a geological time span in response to changes in

TABLE I Representative chemical analyses o f saline and thermal groundwaters f r o m the f o u r working tin mines in the Carnmenellis Granite or its aureole Site

Mount Wellington mine (Moor cross-cut)

Pendarves mine (Harriet East)

D e p t h (m) F l o w rate (1 s -1) T e m p e r a t u r e (°C) pH

240 15 21.6 5.6

260 0.5 21.4 6.9

Na (rag 1-1) K Li Ca Mg Sr

125 12 3.55 93 11.9 1.43

HCO 3 (mg l -l) NO 3 SO 4 C1 F Br B SiO 2

9 11.2 275 287 0.29 0.9 0.80 19.2

Fe Mn Cu Ni

Wheal Jane mine (Clemmows)

South Crofty mine (HDR)

300 10 39.5 6.4

690 3.5 41.5 6.5

29 3.1 0.06 18 5.0 0.06

1,250 72.0 26.0 835 22.0 12.8

4,300 180 125 2,470 73.0 '40.0

67 8.3 38 32 2.90
21 10 148 3,300 3.30 -3.3 28.4

68 <0.2 145 11,500 2.70 43.0 11.0 34.2

43.0 2.90 0.024 0.134

0.62 0.30 0.002 0.007

T o t a l mineralisation 885

230

Ionic balance +1.3 82H (°/00, SMOW) -35 ~80 (°/oo, SMOW) -5.4 3H ( T U ) 4He (108 cm 3 cm-3 H2 O 1,360

-4.1 -38 -5.2 -

-

24

22.4 4.00 0.005 0.027 5,747 +4.2 -31 -5.7 6.6 --

* T h e t w o shallow-groundwater sites are s h o w n in Fig. 1.

4.75 4.50 0.023 0.190 19,002 -0.4 -29 -5.2 7.4 21,100

Shallow-groundwater site* 1

9

30

41

10.8 4.92

10.5 --

14 12 2.8 4.0 <0.01 <0.01 7 13 2.7 2.4 <0.06 0.23 4 2.1 15 25 <0.1

9 26.0 17 21 0.11

0.280 0.031 0.004 0.002 73 -4.9

0.014 0.008 0.027 0.0015

105 -1.3

291 hydraulic gradients (Durrance et al., 1982). The proportion of recent groundwater as indicated by tritium must be between 6% and 65%, implying that saline fluids with salinities higher than those measured could exist. The saline c o m p o n e n t has been shown from the radiogenic 4He contents and uranium series geochemistry to have had a likely residence time of at least 5 • 104 yr. and probably of the order 10 ~ yr. The stable-isotope compositions o f all samples o f recent and saline waters, analysed b y Edmunds et al. (1984), cluster near the meteoric line (~2H, - 2 9 to -38%0 ; 6z80, - 5 . 2 to -6.1%o). There is no systematic variation with salinity. This indicates that all groundwaters in the granite are o f meteoric origin. The most important features of the chemistry, in addition to the high chloride concentrations, are t h e depletion of Na ÷ relative to CY, the enhanced Ca 2÷ levels and especially the significantly enriched Li ÷, with values as high as 125 mg 1-1. The unusual chemistry combined with the oxygen and hydrogen stable-isotope compositions demonstrate that seawater can be ruled o u t as the source of any o f the salinity. It was concluded that hydrolysis of biotite could account for the build-up of CI-, Li ÷, K ÷, and other species in these groundwaters: K~(Mg,Fe)4(Fe,A1,Li) 2[ Si6A12020] (OH)2(F,C1)2 + 3H20 + 12H ÷ -~ A12Si2Os(OH)4 + 4H4SiO4 + 2K ÷ + 2(Mg2*,Fe 2÷) + 2(Fe3÷,A13÷,Li ÷) + 2(F-,C1-) The Li* and CI- can be considered as conservative products of any biotite alteration, unlikely to be assimilated b y reaction products. The fact that the Li/C1 ratio was only 1 : 100, compared with the likely 1 : 2 in unaltered granite (Exley and Stone, 1964; Fuge and Power, 1969), was interpreted as showing that C1- release is a relatively easy and rapid process and could readily account for the observed salinities over prolonged time scales, whilst Li ÷, Mg 2÷ and other ions

are only released upon a breakdown of the trioctahedral biotite structure. Mg 2. depletion in the most saline groundwater is consistent with the observed biotite alteration to chlorite. Acid hydrolysis o f plagioclase was proposed as the principal source of Na ÷ and Ca 2÷ in these groundwaters and contributes markedly to their salinity: 5(Na0.8,Ca0n) (All.~, Siz8Os) + 6H ÷ + 19H20 -~ 3A12Si2Os(OH)4 + 4Na ÷ + Ca 2÷ + 8H4SIO4 Although kaolinite is the preferred reaction product under these conditions, other reaction products are possible. Molar Na*/Ca 2÷ ratios (3.03) observed in the saline groundwater are rather low compared with the stoichiometry of the reaction. The plagioclase shows preferential alteration in the centres of most zoned crystals and this could account for the enhanced aqueous Ca 2÷ levels. Alternatively, incongruent dissolution of the oligoclase might account for some Ca 2÷ enrichment. Both reactions produce significant amounts of silica as well as kaolinite or other clay minerals. Silica levels in these groundwaters represent saturation or supersaturation with respect to chalcedony. The strong tendency for silica deposition as well as the Na/Ca ratios was used (Edmunds et al., 1984) as a strong argument~ against fluid inclusions being a source o f salinity. The converse should in fact be true -- that fluid inclusions should be forming during these reactions. This hypothesis is supported by the fluid inclusion studies o f Alderton and Rankin (1983), for example, who record many monophase inclusions, indicating temperatures of origin from 170°C down to temperatures less than 70°C. 3. Chemistry o f groundwater after H D R reservoir circulation Drilling of a H D R doublet was completed during 1981 in granite which was similar both

292

in texture and degree o f alteration to that near to surface. Details of the drilling, rock mechanics, reservoir development and testing programme can be f o u n d in Batchelor (1985). Following explosive pretreatment of the borehole and hydrofracturing, circulation of water was carried o u t for a total of 4000 hr. and the return flow rates normally ranged between 0 and 8 1 s -I, occasionally up to 35 1 s -1. The make-up water for these experiments was derived from a local flooded quarry and later from a nearby stream. Representative analyses of the make-up and return waters are given in Table II; the low levels o f Na ÷, CI-, Li ÷, etc., in the make-up water should be noted. No data are reported here for the first 300 hr. o f the test during which time contamination by drilling fluid (pH 11.5) and casing cement gave unrepresentative results; the possible presence of in situ saline fluids could not be detected. During the period o f the test 2.3 • l 0 s m 3 o f water was injected but only 25% o f this was returned. A tracer experiment was conducted between 720 and 1500 hr.

Peak tracer return occurred after 70 hr. but this was only 8% o f t h a t injected. During the course of this experiment only 46% was returned, which indicated t h a t the mean reservoir residence time o f the returned percentage was less than 780 hr. The changes in chemistry between injection and return are summarised for Na ÷, Ca 2÷, Li ÷, C1- and Si over the duration of the test (Fig. 3). These results are described in detail by McCartney (1984). Return water values for Na ÷, Li* and Si were enhanced by factors of 3, 8 and 4, respectively, over the input water compositions once stability was reached at ~ 600 hr. Slight oscillations occurred as functions of small changes in the circulation rate but otherwise the ionic concentrations remained constant for these elements. An increase in flow rate can be interpreted as a decrease in mean residence time and vice versa. Chloride concentrations remained high during the first 600 hr. of the test and then declined gradually to near steady-state values. Chloride was more respon-

TABLE II Representative chemical analyses (in mg l -~) of injection and return waters during HDR circulation experiments Injection water Time (hr.) Flow (1 s -1) Stored volume (m 3) pH Na Ca K Mg Li Cl SO 4 HCO 3 Si A1 B Total mineralisation

1,008 25 -7.00 23.1 12.7 3.3 2.16 0.037 26.9 17.2 28.2 4.05 0.14 0.02 118

Return water (a)

(b)

(c)

1,008 6 79,000 9.10

2,590 4.5 158,000 9.07

4,032 3.5 175,000 9.11

66 10.2 2.0 n.d. 0.195 43.1 19.4 72 14.0 0.16 0.14 227

HCO 3 refers to total alkalinity, n.d. = not detected.

62 10.5 2.0 0.06 0.194 37.7 22.3 66 13.5 0.26 0.09 215

63 10.2 1.8 0.06 0.194 45.6 20.1 68 13.5 0.13 0.10 223

293

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sive to changes in flow rate than the other elements, shown, for example, in the rise between 3 5 0 0 and 4 0 0 0 hr. Overall, the chloride in the return fluid shows an enhancement of 1.5 times over the injected fluid representing a gain o f between 15 and 20 mg 1-1. Ca 2. is generally depleted in the return fluid, compared with input values. In absolute terms the returned fluid shows a gain (in mg 1-1) of: 40, Na*; 12, C1-; 0.160, Li*; 9.0, Si; and 0.090, B. The enhanced and near-constant Na ÷, Li ÷ and Si levels must indicate that significant mineral--water reactions are taking place within the reservoir over a relatively short time period (< 1 5 0 0 hr.). If displacement of a saline pore fluid were controlling the composition o f the return fluid, then declining concentrations would have been expected with time due to dilution by an increasing injected volume. High initial C1- return values may indicate some dilution of in situ pore fluids or possibly reaction of casing cement which was made up with saline water. The absence of sympathetic changes in Na ÷ a n d CI-, however, tends to rule o u t these possibilities and mineral--water reactions are also favoured to explain the C1- increase. The loss o f Ca may be explained in part by forma-

294 tion o f calcite which was observed on recovered samples of cement. When the H D R circulation results are compared with the saline groundwaters in the north of the Carnmenellis Granite (Table I) it can be seen that Na, Li and B enrichment is of the same order as observed in the lowersalinity groundwaters. Thus, within relatively short periods of time (< 2 months) there is significant chemical release from the rock which is comparable ionically with that observed in the natural system. H o w are they related? The intergranular (bulk) permeabilities of intact rock are low (10-s--10 -9 D; Brace, 1980) and porosities are correspondingly small (Alexander et al., 1981), which rules o u t either significant storage or convective transfer o f fluids through the bulk of the granite. Any circulation must therefore be controlled by secondary permeability developed via the joints, veins and cross,courses as a result of stress-relief in the granite. The secondary permeability is relatively high within the northern mineralised zone and has allowed convective circulation to have developed over a geological time scale, producing the saline groundwater. This fluid cannot be considered as static or inert, however, as it would have responded to changes in hydraulic head, developed in response to tectonic, eustatic and climatic events, most recently the effects of Pleistocene glaciation. The large storage o f saline groundwater is indicated b y the near-constant composition of underground springs over the last 100 yr., but the salinity must eventually decrease as the storage is replaced by younger less saline groundwater. Various lines of geological evidence, such as the infrequency o f mineral lodes, suggest that secondary permeability and porosity and thus the storage o f saline groundwater are n o t well developed in the central and southern areas o f the granite. Prior to hydraulic or explosive development, the permeability was measured as 1--2 pD around the t w o boreholes (Batchelor, 1984). The

H D R experiments have shown, however, that markedly increased permeabilities are developed at right angles to the principal stress direction (NE--SW) during the hydraulic stimulation. There is good mineralogical evidence that fluids have circulated at some time within this joint system at > 2000-m depth. This is shown by the secondary mineralisation -- significant chloritisation of biotite and the mineralisation of joint surfaces (A.V. Bromley, pers. commun., 1983). Therefore we are witnessing two processes: (1) dilution of a very slow moving ancient saline groundwater in the north of the granite with flow n o w enhanced by the hydraulic sink of the mine; and (2) short-term reaction of granite or secondary minerals by circulation induced by hydraulic stimulation which has " o p e n e d u p " at least some of the otherwise dormant joints and also exposed some fresh mineral surfaces. It is likely that the dynamic H D R system is a means of accelerating, within the limits of mineral stability and solubility, those processes which have occurred more slowly in the natural system. The same processes can be identified in the composition of shallow groundwaters, indicating their importance in near-surface granitic weathering.

4. Experimental studies with biotite Following the effective elimination of seawater and fluid inclusions as sources of the salinity, rock-forming minerals remain the only sources of salinity, especially chloride, in the granite. Fuge (1979) has demonstrated that a significant proportion of C1 is in a water-soluble state in the Cornubian batholith. Biotite is clearly the principal potential source and was chosen for limited experimental studies (McCartney, 1984). Two biotites (1 and 2) from one of the H D R doublet boreholes were sampled from drillhole cuttings at 1000 and 2000 m, respectively. The samples were separated and cleaned b y wet sieving with tap water, followed by magnetic and heavy-liquid separation.

295

The cleaned samples were examined petrographically, by X-ray diffractometry (XRD) and thermogravimetric analysis (TGA) to determine their purity. A lamellar chlorite

intergrowth was found which constituted an impurity of between 3% and 6% of the sample. Other minor inclusions in fresh biotite (~ 1% in total) consisted of andalusite,

T A B L E III Analyses of biotites from the Cornubian batholith Dartmoor Big Feldspar Granite

Dartmoor poorly megacrystic granite

Carnmenellis adamellite (Bosahan)

SiO 2 Al203 Fe203 FeO MnO MgO CaO Na20 K20 Li20 TiO~ P20s H20 F C1

36.43 16.0 6.13 16.1 0.33 4.66 0.16 0.40 9.71 0.372 3.58 0.29 3.69 0.937 0.413

38.64 21.90 3.62 13.38 0.55 1.93 -0.44 10.47 0.993 2.13 0.137 2.64 1.759 0.177

Total

99.202

98.766

0.488

0.781

O ~ F, C1 Total

98.71

Rosemanowas borehole

(I)

(2)

1000 m

2000 m

35.06 18.40 4.11 18.83 0.65 5.72 0.40 0.36 8.68 -2.16 -2.75 1.96 --

36.3 22.2 3.06 18.0

34.9 21.3 1.49 22.1

3.52 0.69 0.27 7.94 1.86 2.52

3.86 0.45 0.15 7.70 1.45 2.48

4.28 1.33 0.040

5.19 0.93 0.060

99.08

102.01

102.06

0.57

0.41

101.47

101.65

97.98

Si P AI IV A1V1 F e 3÷ F e 2÷ Mn Mg Li Ti

5.593 0.037 2.370 0.522 0.708 2.066 0.043 1.066 0.230 0.413

5.628 0.018 2.354 1.403 0.397 1.628 0.068 0.419 0.581 0.234

5.43 -2.57 0.80 0.48 2.44 0.08 1.33 0.22 0.25

5.197 -2.803 0.942 0.325 2.150 -0.755 1.070 0.268

5.049 -2.951 0.679 0.154 2.674 -0.830 0.840 0.270

Ca Na K

0.027 0.120 1.901

-0.124 1.945

0.07 0.11 1.71

0.105 0.076 1.452

0.068 0.039 1.429

OH F Cl

3.775 0.454 0.108

2.562 0.810 0.044

2.85 0.96

4.089 0.602 0.010

5.010 0.425 0.019

-

-

Biotite analyses f r o m t h e D a r t m o o r G r a n i t e are average values t a k e n f r o m A1-Saleh e t al. (1977). Biotite analysis f r o m B o s a h a n is f r o m Butler (1958). The b i o t i t e s f r o m t h e R o s e m a n o w a s b o r e h o l e f r o m 1000- a n d 2 0 0 0 - m d e p t h s are r e f e r r e d t o in t h e t e x t as b i o t i t e s 1 a n d 2, respectively.

296

tions were not anoxic. Two types of experiment were performed: (1) Five successive 5
ilmenite, sulphides, zircon, monazite and uraninite. Some tourmaline grains were present (< 1%), b u t no fluorite or fluid inclusions were observed; rutile was observed as intergrowths in the chlorite impurity. Chemical analyses and unit cell formulae for the biotites used in experiments are compared in Table III, with other previously published analyses from the Carnmenellis Granite and average biotite analyses from the Dartmoor Granite in the east of the batholith. The analyses are low in K and have excess water, confirming the results of the physical examination. Compared with unaltered biotites the partially altered biotites used in the experiment have high Li content b u t are significantly depleted in chloride. The reaction fluid was 20-ml aliquots of typical HDR-injection water (Table II) and the water/mineral ratio was 10 : 1 b y weight. Experiments were performed in PTFE-lined autoclaves and agitated at 80°C and 1 atm. The reaction fluid was not degassed b u t the head space (< 1 ml) was filled with 02free N~: therefore the initial reaction condi-

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297

TABLE IV Molar ratios of specified elements vs. lithium for the two biotites (1000 and 2000 m, respec.) and solutions after reaction runs Run No.

Na/Li

C1/Li

K/Li

Ca/Li Mg/Li

Si/Li

0.02 0.8 0.8 0.3 9.0 27 0.9

1.3 1.3 2.3 2.0 3.3 0.9 0.8

0.2 1.8 2.9 4.1 6.0 5.8 2.0

1.4 0.4 0.7 0.9 0.9 0.2 0.7

9.7 2.5 4.7 7.5 7.5 8.4 3.0

0.04 1.1 3.8 1.6 16 24 1.1

1.7 2.1 3.0 2.8 3.6 4.6 1.0

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Biotite 1 (1000 m):

1 2 3 4 5 34-Day run

0.07 1.1 1.7 0.7 6.1 14 0.6

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0.05 1.7 5.1 2.9 10 16 1.1

energy-dispersive X-ray analysis (EDAX). In addition, one polished slice of granite core, taken from 2000-m depth in the H D R borehole, was subjected to continuous leaching under the same conditions as those in the single-mineral experiments to permit qualitative comparison o f the effects on other minerals with those on biotite, when compared to a control sample. The results are discussed in full b y McCartney (1984) b u t the data are summarised in the cumulative curves o f Fig. 4. It is observed that: (1) reacted fluids are enhanced over starting compositions for all elements; (2) biotite 1 (from 1000 m) is more reactive than biotite 2 (2000 m); (3) the cumulative yields from successive leachings are generally greater than the yield from the single reaction; (4) the yields are generally consistent with the chemical analyses o f the biotites, e.g. Li ÷ and C1- are enriched in the fluid derived from biotite 2 which has higher levels o f these elements;

(5) the first produced fluid is enriched in most solutes compared with successive extracts; (6) there is an increase of around ten times C1~ over Li ÷ despite the much higher biotite Li levels; the chloride yield in the successive leaching experiments is 27% and 24% of that initially contained in biotites 1 and 2 and in the single leaching 11% and 8%, respectively. Therefore, the results demonstrate that significant reaction of the biotite has taken place in each set o f experiments. The extent to which dissolution is taking place m a y be assessed by comparing molar ratios against Li of solutions with the same molar ratios in the two biotites (Table IV). The K/Li, Si/Li and Mg/Li ratios are of the same order of magnitude in both biotites and in solution. This indicates that structural breakd o w n is occurring and further confirms that Li ÷ is contained in structural sites, probably in octahedral coordination (Robert et al., 1983). In contrast, Ca/Li is enriched b y up to 60 times in the fluid compared with the

298 biotites and Na/Li and C1/Li are enriched between 1 and 3 orders of magnitude. The Na ÷ and Ca 2÷, which constitute lattice impurities in the biotite, are being readily exsolved during the low-temperature (80°C) reactions in preference to the K ÷ which is retained due to strong bonding with AI in the tetrahedral layer. There is a decrease in pH in all samples from ~ 7.3 to 5.8. The possible sources of H ÷ are the initial sample fluid, sulphide oxidation or the internal generation of H ÷ as a result of C1- ~ OH- exchange (Edmunds et al., 1984). Increase of SO~- in all samples is interpreted as resulting from sulphide oxidation; assuming 0.05% pyrite impurity in the biotite, the cumulative sulphate would only require reaction o f 1% of the pyrite, which must be the main source o f H ÷ in the experiments. Since cumulative leaching exceeds the single long-term leach, and since the latter experiment released comparable amounts of most species to the first of the cumulative runs, it seems likely t h a t the procedure adopted between runs considerably influences the leachability of the biotite. It is not clear from this set of experiments whether this is a chemical or mechanical response. An additional input of oxygen, for example, would give a renewed impetus to sulphate production. Alternatively, the drying operation could leach to opening up of the biotite cleavages, creating new reactive sites and allowing easier access for new reagent. Plateaux are observed in the cumulative release curves (Fig. 4) notably for Na ÷, CIand SO~- which could be real phenomena, indicating a two-stage release. Alternatively, these might result from some presently unexplained difference in procedure. It is important to observe that the order of magnitude differences in the a m o u n t o f Li ÷ released compared with Na ÷ and C1- should lead to progressive increases down the columns of Table IV. Because of the anomalous run-3 results, the expected sequence is interrupted.

The increase in chloride over Li* in solution by up to 2 orders o f magnitude must represent a strong preference for the biotite to release the larger C1- ion during the lowtemperature alteration despite the relatively low initial chloride levels. The anomalously low C1- in these two biotites (Table III) probably indicates that some chloride has been removed during previous water--rock interaction including the production of chlorite. Examination of the polished biotites indicated no significant change in biotite chemistry following the experiments, although possible physical changes were disguised by polishing. Biotite in the leached slice showed swelling following reaction as well as alteration to vermiculite along cleavages. Plagioclase grains were pitted, especially in their Ca-rich centres, providing a possible source of Ca required for vermiculite formation. Examination of biotite in the unreacted granite slice indicated chloride to be uniformly distributed and no evidence was found for chloride enrichment along grain boundaries or cleavages. Within the limits o f resolution of EDAX, chloride levels in biotite appeared to be similar to those in the muscovite (and possibly vermiculite). Muscovite appeared to be unaltered both physically and chemically although Fe-hydroxide spherules had nucleated on grain surfaces. Although biotite contains higher amounts of Li than all other minerals in Cornish granites, muscovite could still be considered a possible source of Li (Wilson and Long, 1983; Bray and Spooner, 1983) and of chloride. Further work is necessary to demonstrate its reactivity compared with biotite notwithstanding the observation of its inertness in the granite slice.

5. General discussion The results from each of the above three studies independently indicate that significant reaction of rock-forming minerals is taking place during groundwater circulation in the

299

Cammenellis Granite and during experimental studies which simulate the in situ conditions. It is clear that biotite, if n o t the only source o f chloride and Li, is in fact a major source o f b o t h elements. Na ÷ and Ca 2÷ are prin-

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cipally derived from analogous reaction of plagioclase feldspar, although it is shown that biotite also can be a significant contributor of these t w o elements. The results from each line of research are compared and summarised in Fig. 5 using plots of Na ÷ and Li ÷ plotted against chloride. The field data from mines in the Carnmenellis Granite show good linearity which demonstrates the mixing between mature saline groundwaters and recent lower-salinity waters derived either from natural or induced circulation. Saline groundwaters are likely to exist naturally in the granite with levels of chloride higher than the more saline endmember of the mixingseries so far observed. The compositions of H D R fluids at the end of the 4000-hr. circulation experiment show element ratios similar to those observed in the natural system. The molar Na+/Ca 2+ ratio (10.7) is considerably higher than in the mature saline groundwater (3.0) and this may be due to congruent solution of albiteenriched feldspar margins, to an exchange equilibrium with the biotite or secondary clay minerals, or to loss of some Ca b y calcite precipitation. Element ratios from the biotite experimental studies m a y also be compared with the other sets of data. Although the experimental studies confirm the nature of the biotite breakdown under simulated field conditions, the extrapolation of the results for a single experiment on one mineral to the field processes must be considered to be qualitative at this stage. The Na*/C1- ratios produced from biotite are similar close to seawater and rainfall values and the possible contribution o f both elements from biotite must therefore be taken into account in further discussion o f the meteoric water evolution. Further work is needed to identify the relative contribution o f Na ÷ from biotite relative to that from plagioclase. The Li+/C1- ratios from H D R circulation are close to those observed in the natural system. The groundwater Li*/CY ratios are

300

at least an order of magnitude lower than would be expected from congruent solution of unweathered biotites with Li/C1 around 1 : 2 by weight (e.g., Dartmoor average Big Feldspar Granite; Table III). This has been explained (Edmunds et al., 1984) as due to the preferential loss of C1- during prolonged groundwater circulation, without the structural breakdown of the biotite. The aqueous Li÷/C1- ratios from biotite experiments are even higher than in the natural system. This is consistent with the composition of biotites used for experiment having Li/C1 ratios up to 2 orders of magnitude higher than the likely average for the granite.

6. Conclusions The results of these preliminary biotite experiments are consistent with the hypotheses from the two field investigations that biotite can be a major source of solutes in granite--groundwater interaction over short periods as well as over geological time scales. If the biotite reaction is considered together with the breakdown of plagioclase feldspar then it is possible to account for almost all of the salinity found in the Carnmenellis Granite groundwater. Silica derived from these reactions would be maintained at, or above, saturation levels and deposition of silica must be favoured rather than dissolution of rock-forming quartz. Therefore, original fluid inclusions should not be exposed; indeed, the formation of contemporaneous fluid inclusions would be expected. The proposed model for the genesis of saline water in this granite therefore links together several processes often considered separately including groundwater circulation and convective heat transport, water chemistry, water-rock interaction, secondary-mineral formation (including kaolinisation) and fluid inclusion formation and stability. These processes are considered to be as important under the present low-temperature conditions as at previous periods of elevated temperature and we share the view of Sheppard (1977) that

earlier events are important in preparing the rock for lower-temperature interaction. The reaction sequence has probably been continuous over the past 200 Myr. up to and including the present day. The model proposed here for one highlevel granite boss, strongly enriched in volatile elements, needs confirmation elsewhere. When projected to the very long residence times of groundwaters in continental shield areas it is conceivable that comparable water-rock interaction could account for the very high salinities observed, enhanced by soluteconcentrating processes such as ultrafiltration or radiolysis. As with the controversy over the evolution of granites themselves, it must be recognised that there are "granites and granites" (Read, 1957) and that different lines of evolution of circulating groundwater are likely. In the context of HDR development, the dispersion of secondary products, notably silica and clay minerals, might be expected to lead to some reduction of permeability. Further research is required to understand in detail the processes presented qualitatively in this paper, so that the chemical mass balance and reaction kinetics of real HDR systems over considerable periods of time can be predicted.

Acknowledgements The authors wish to thank B.G.S. colleagues: D. Hutchinson, J. Bain, for carrying out chemical analyses of biotites and their XRD and TGA characterisation; D.L. Miles and colleagues for their assistance with the ICP analysis; and A.E. Milodowski for his assistance with SEM work. We thank C. Neal and A.V. Bromely for their critical reading of the manuscript and also E. Althaus for a constructive review. This work was partly funded by the U.K. Department of Energy and C.E.C. (Contract EG-084-76-UK). One of us (R.A.M.) acknowledges a CASE award from the Natural Environment Research Council (N.E.R.C.). We wish to thank A.S.

301

Batchelor for access to unpublished results from the HDR experiments and for his support of the geochemical studies. This paper is published with the permission o f the Director, British Geological Survey (N.E.R.C.). References Alderton, D.H.M. and Rankin, A.H., 1983. The character and evolution of hydrothermal fluids associated with the kaolinized St. Austell granite, S.W. England. J. Geol. Soc. London, 140: 297-309. Alexander, J., Hall, D.H. and Storey, B.C., 1981. Porosity measurements o f crystalline rocks by laboratory and geophysical methods. Inst. Geol. Sci., London, Rep. ENPU 81-10, 45 pp. A1 Saleh, S., Fuge, R. and Rea, W.J., 1977. The geochemistry of some biotites from the Dartmoor granite. Proc. Ussher Soc., 4: 37--48. Batchelor, A.S., 1985. Hot Dry Rock geothermal exploitation in the United Kingdom. Proc. 3rd Int. Semin. on Results of EC Geothermal Energy Research. Nov. 29--Dec. 1, 1983, Munich (in press). Brace, W.F., 1980. Permeability of crystalline and argillaceous rocks, status and problems. Int. J. Rock Mech. Min. Sci. Geomech., Abstr., 17: 241--251. Bray, C.T. and Spooner, E.D., 1983. Sheeted vein Sn--W mineralisation and greisenisation associated with economic kaolinisation, Goonbarrow China clay pit, St. Austell, Cornwall, England: Geological relationships and geochronology. Econ. Geol., 78: 1064--1089. Burgess, W.G., Edmunds, W.M., Andrews, J.N., Kay, R.L.F. and Lee, D.J., 1982. The origin and circulation of groundwater in the Carnmenellis granite: the hydrogeochemical evidence. In: Investigation of the Geothermal Potential of the United Kingdom. Inst. Geol. Sci., London, Rep. Set., 102 pp. Butler, L.R., 1958. The geochemistry and petrology of rock weathering - - The Lizard area, Cornwall. Geochim. Cosmochim. Acta, 4: 157--178. Durrance, E.M., Bromley, A.V., Bristow, C.M., Heath, M.J. and Penman, J.M., 1982. Hydrochemical circulation and post magmatic changes in granites of south-west England. Proc. Ussher Soc., 5: 304--320. Edmunds, W.M., Andrews, J.N., Burgess, W.G., Kay, R.L.F. and Lee, D.J., 1984. The evolution of saline and thermal groundwaters in the Carnmenellis granite. Mineral. Mag., 48: 407--424. Exley, C.S. and Stone, M., 1964. The granitic rocks of southwest England. In: Present Views of Some Aspects of the Geology of Cornwall and Devon.

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