Oxygen and hydrogen isotope geochemistry of thermal springs of the Western Cape, South Africa: Recharge at high altitude?

Journal

Pergamon

of African

Pll:SO899-5382(00)00002-1

Earth Sciences,

Vol. 31, No. 3/4, pp. 467-481. 2000 2001 Elsavier Science Ltd All rights reserved. Printed in Great Brdain 0699-5362/01 5. see front matter

0

Oxygen and hydrogen isotope geochemistry of thermal springs of the Western Cape, South Africa: recharge at high altitude? R.E. DIAMOND and C. HARRIS* Department of Geological Sciences, University of Cape Town, Rondebosch 7700, South Africa

ABSTRACT-A number of thermal springs with temperatures up to 64°C are found in the Western Cape Province of South Africa. The average 613C value of gas (CO,+CH,) released at three springs is -22%0, which is consistent with an entirely biogenic origin for the C and supports previous investigations which showed that the springs are not associated with recent or nascent volcanic activity. Most springs issue from rocks of the Table Mountain Group, where faulted and highly jointed quartzites and sandstones of the Cape Fold Belt act as the main deep aquifer. The 6D and 6’*0 values of the springs range from -46 to -18% and from -7.3 to -3.9%0, respectively. Although the thermal springs have isotope compositions that plot close to the local meteoric water line, their 6D and VO values are significantly lower than ambient meteoric water or groundwater. It is, therefore, suggested that the recharge of most of the thermal springs is at a significantly higher altitude than the spring itself. The isotope ratios decrease with increasing distance from the west coast of South Africa, which is in part related to the continental effect. However, a negative correlation between the spring water temperature and the 6180 value in the thermal springs closest to the west coast indicates a progressive increase in the average altitude of recharge away from the coast. o 2001 Elsevier Science Limited. All rights reserved. RESUME-La province Ouest du Cap (Afrique du sud) contient plusieurs sources thermales, dont certaines atteignent des temperatures de 64°C. A I’exutoire de trois de ces sources, la valeur moyenne du 613C des gaz (CH,+ CO,) est de -22%. Ces mesures isotopiques correspondent a la decomposition de mat&es organique, ce qui est en accord avec les precedentes etudes. Ces dernieres indiquerent que ces sources ne sont pas associees a des circulations d’eaux juveniles ou bien liees a une activite volcanique. La plupat-tdes sources emergent dans des roches appartenant au Groupe de ‘Table Mountain’ et correspondent a des zones de plissement fortement silicifiees (quartzite). La zone de gres de la ceinture plissee du Cape sousjaccent joue alors le role d’aquifere profond. Les valeurs 6D et de VO de ces sources sont respectivement comprises entre -46 a -18960 et -7,3 et -3,9%. Bien que ces sources thermales aient des compositions isotopiques proches des valeurs de la droite des eaux meteoriques locales. Elles sont significativement plus basses que les eaux meteoriques et les eaux souterraines. Ces resultats suggerent que la plupart des zones de recharge des sources se situent a une altitude superieure a la source. De plus, ces valeurs isotopiques decroissent avec I’eloignement de la c&e ouest de I’Afrique du sud, ce qui indique que ces variations sont partiellement Ii&es a un effet de continentalite. Cependant, la correlation negative entre la temperature des sources et la valeur de VO de la source thermale la plus proche de la c&e indique une augmentation progressive de I’altitude moyenne de la zone de recharge loin de la zone c&i&e. o 2001 Elsevier Science Limited. All rights reserved. (Received 13/l 2/99: revised version received 29/5/00:

accepted 24/l O/00)

*Corresponding author [email protected]

Journalof African Earth Scimcas 467

R. E. DIAMOND and C. HARRIS INTRODUCTION Most hot springs world-wide, are associated with the waning stages of volcanic activity (e.g. Kent, 1949). Hot springs, which are not associated with volcanic activity, are often associated with recent uplift, for example in the Pakistan Himalayas (e.g. Chamberlain et al., 19951, where meteoric water is heated by cooling magmatic rocks. There are over 87 thermal springs in South Africa ranging in temperature from 25-64°C. None of the springs are associated with recent volcanic activity, which is unknown in this part of Africa. The geology and chemical composition of the springs has been described by Kent (1949) and Hoffmann (19791, respectively. The aims of this paper are as follows: il to establish the degree of variation in 0 and H isotope data for the 12 thermal springs (Table I) from the Western Cape Province. Mazor and Verhagen (I 983) reported stable isotope data from seven of the Western Cape springs (but both 0 and H isotope data from only four); ii) to determine the monthly isotope variability of the spring waters by analysing samples collected from four of the springs every month for a period of eight months. Long term variability can be assessed by comparing data for samples from this study collected in 1995-7 with those of Mazor and Verhagen (I 983) whose samples were collected in 1971-2; ii. to compare the isotope composition of the springs to meteoric water and cold groundwater in the area; and iv) to use the stable isotope data to constrain the nature of the recharge and the mechanism(s) of the heating of the thermal springs.

m) of the Cape Fold Belt (Fig. I) (Broquet, 1992; Halbich, 1992). Multiphase deformation of the basement Kango and Malmesbury Groups occurred during Pan-African orogenesis between 600 and 500 Ma (Gresse et al., 1992). Many of the structures were reactivated during the Cape Orogeny (250 Ma: HIlbich et al., 1992) and during the break-up of Gondwana during the Mesozoic (Gresse et a/., 1992). Metamorphic conditions during the Cape Orogeny reached greenschist-facies grade, and much of the sandstone in the Table Mountain Group recrystallised to quartzite. Movement of water through rocks of the Cape Supergroup is, therefore, primarily via these fractures because cementation destroyed the primary porosity. Climate The Western Cape is the small portion of South Africa (Fig. 1) which experiences a Mediterranean-type climate. To the north, this climate regime grades into semi-desert. To the east, the climate becomes less seasonal and tends towards subtropical on the coast. The essence of a Mediterranean climate is cold wet winters and warm dry summers. The generally mountainous nature of the Cape Fold Belt results in the entire region having sharp changes in climate. Rainfall is highly variable and ranges from low summer (December-March) monthly means of 1O20 mm in the wide inter-montane valleys and on the coastal plains and + 50 mm in the mountains, to winter (June-August) monthly means of 40-I 00 mm and over 200 mm, respectively. Temperatures vary from winter mean minimum daily temperatures of < 5°C in the inland valleys and * 10°C on the coastal plains to summer mean maximum daily temperatures of > 30°C inland and +25’C on the coastal plains (SAWB, 1996).

REGIONAL BACKGROUND Geolosy The geology of the Western Cape is dominated by the PalaeozoicCape Supergroup, of which the resistant sandstones and quartzites of the Table Mountain Group are the most prominent. The basement consists of Late Precambrian low-grade metamorphic rocks of the Malmesbury and Kango Groups and the - 540 Ma plutons (Armstrong et a/., 1998) of the Cape Granite Suite. The Table Mountain Group forms the lower part of the Cape Supergroup, above which lie the shale and sandstone formations of the Bokkeveld and Witteberg Groups. The Cape Supergroup is overlain by the varied sedimentary succession of the Karoo Supergroup. The great thickness and wellcemented character of the Table Mountain Group sandstones and quartzites results in them being the major component of the high relief areas (up to 2000

468 Journal of African Earth Sciences

THERMAL SPRINGS All groundwater that sinks to any appreciable depth will become heated because of the geothermal gradient. Mazor (1991) suggested a purely arbitrary temperature divide between cold springs and thermal springs of 6’C above average annual surface temperature. The Western Cape valleys and coastal plains experience annual average temperatures between 15% and 20°C, so any water discharging at or above about 26“C can be classified as a thermal spring. In the Western Cape, there is a full gradation from the cold ( < 2O’C) to the hottest spring in the country, Brandvlei, at 64°C. All of the well-known thermal springs in the area were sampled during this work (Table 1). The majority are above 40°C, with two

Oxygen and hydrogen isotope geochemistry of thermalsprings of the Western Cape, South Africa

’ Victoria West

Area of study LTulbagh

Baden-Baden

Warmwaterberg 100 LI-

60

80

100

20 40

I

0

I 200km

Figure 1. Sketch map of the Western Cape showing the location of thermal springs sampled. The location of rainfall monitoring stations at the University of Cape Town (UCTI, Cape Town International Airport IIAEAJ, Citrusdal, Tulbagh and Oodtshoom are also shown. The thermal spring at Citrusdal is known as ‘The Baths’, but to avoid confusion it is referred to as Citrusdal in the text. The area of outcrop of the Caoe SuDergroup forming the Cape Fold Belt Mountains is indicated (taken from Theron et al., 1991a).

springs, Witzenberg (28“C) and Rietfontein (27OC), just falling within the classification of thermal. Most of the springs are found at relatively low altitude (~300 m), with three springs found at 700 m or above (Toowerwater, Rietfontein and Witzenberg). The constancy of discharge temperature and volume is a generally known fact (Kent, 1949) and was confirmed by discussion with the resort managers at The Baths (Citrusdal), Calitzdorp Spa (for which measurements go back to the 19th century), Caledon and Goudini. The 12 springs sampled have yields that vary from < 5 I s-l to 126 I s-l. The spring with the highest yield (Brandvlei) is also the hottest, whereas most of the springs with low discharge are relatively cool. This may in part be due to more effective cooling by heat loss to the surrounding rock in the case of the springs with low yield. All but two of the thermal springs described in this paper occur within, or close to, rocks of the Table Mountain Group, which, as described above, has very limited residual primary porosity. Deep groundwater movement in the Table Mountain Group is via fractures, which are either horizontal bedding planes or vertical joints. The joints occur in three roughly

parallel sets throughout the Cape Fold Belt: northwestsoutheast, northeast-southwest and east-west. These and the bedding planes provide a network of interconnecting fractures through which water can flow. The Table Mountain Group contains two main aquifers separated by the thin, but impermeable, shales and siltstones of the Cedarberg Formation; the lower is the Peninsula Formation and the upper is the Nardouw Subgroup. Faults punctuate the stratigraphy and are present at nearly all the springs. It seems, therefore, that faults are critical in providing channelways through the otherwise impermeable Cedarberg Formation for heated water to percolate upwards. Geological cross-sections for Brandvlei, Calitzdorp and Citrusdal are shown in Fig. 2. The geothermal gradient of the Cape Fold Belt area is not well established. An estimate can be made from two boreholes drilled into the Karoo Supergroup, north of the Cape Fold Belt, about 50 km from the spring Rietfontein (Fig. 1). The first borehole penetrated 850 m of rock with an average geothermal gradient of about 18°C km-‘, The second borehole reached to 1760 m below s&ace; and a geothermal gradient of about 21’C km-’ was observed in the

R. E. DIAMOND and C. HARRIS Table 1. General information about sampled thermal springs Spring

Temp (OC)

Baden-Baden

38

Brandvlei

64

Flow (I s-11

Altitude (ml 280

Distance (km) 150

Geological environment

Fe, Mn and Si mineralisation

TMG-Bokkeveld TMG

Group contact + near regional fault in

TMG-Bokkeveld

Group contact + regional fault in TMG

126

220

90

10

360

100

TMG-Bokkeveld Group contact + regional fault in TMG; Fe, Mn and Si mineralisation

200

310

TMG-Bokkeveld unconformity

Caledon

53

Calitzdorp

52

Citrusdal (‘The Baths’)

43

30

250

80

Fault in Nardouw Subgroup of TMG

Goudini

39

3

290

80

Regional fault in TMG (Peninsula Formation/Nardouw Subgroup faulted together)

Malmesbury

34

120

40

Fault in Malmesbury

Montagu

45

280

155

TMG-Bokkeveld TMG

700

260

Dwyka Group-Prince Albert Formation contact

800

455

Regional fault in TMG (Peninsula Formation/Enon Formation faulted together); Fe, Mn and Si mineralisation

*5

500

225

Near top of Nardouw Subgroup + regional fault in TMG; Fe, Mn and Si mineralisation

*1

800

105

Peninsula Formation

Rietfontein

-27

Toowerwater

49

Warmwaterberg

44

Witzenberg

-28

i4

*2

Group contact + TMG-Uitenhage

Group

Batholith of Cape Granite Suite

Group contact + near regional fault in

Distance: distance from the West Coast measured in a straight line with an east-west orientaion; TMG: Table Mountain Group; f : flow rate was estimated; -: temperatures were measured on one occasion only.

upper 1450 m, which then increased to about 27% km-’ in the deep section of the hole (Theron et al., 1991a; Jones, 1992). In the Cango Caves, near Oudtshoorn, the air temperature is constant at 17% (Doel, 1995). If this temperature were typical of shallow groundwater in the area, then Brandvlei thermal water has been heated to at least 47OC above that of shallow groundwater. If an average geothermal gradient of 20% km-’ is assumed, then the thermal water at Brandvlei must come from an average depth of 2.35 km. Although this is a minimum estimate, because the water must have cooled on its way to the surface, the rate of flow (I 26 I s-l) is large and the degree of cooling must be slight. The geological cross-section is consistent with this interpretation (Fig. 2).

ANALYTICAL Sampling methods Water samples were stored in 1DO ml plastic (‘medical flats’) bottles and analysed as soon as possible after collection. Some springs were sampled only once, but four [Brandvlei, Calitzdorp, Citrusdal (The Baths) and Malmesburyl were sampled every month. At

470 Journal of African Earth Sciences

Citrusdal, samples were taken at the eye of the spring; and at Baden-Baden, Caledon, Rietfontein, Goudini, Montagu, Toowerwater and Warmwaterberg, samples were taken from pipes which directly tapped the spring. The springs at Brandvlei, Calitzdorp and Malmesbury issue directly into pools, and samples were collected from as close to the source as possible in order to minimise the influence of evaporation. Gas bubbling up through the source pools at Brandvlei, Calitzdorp and Malmesbury was collected in November 1995. Gas bubbles were caught in a plastic funnel before being allowed to expand into evacuated glass vessels. Isotope analysis For 0, the CO, equilibration method of Socki er al. (1992) employing disposable pre-evacuated 7 ml glass vials was used. For H, 2 mg of water contained in a microcapilliary tube was dropped into a Pyrex@tube containing - 10 grains of Indiana Zn. The tube was attached to the vacuum line, frozen in liquid N, evacuated and then sealed using a torch. Once a large enough batch of samples had been prepared, they were placed in a furnace at 45O’C to reduce the water to H,. Isotope ratios of CO, and H, were measured

Oxygen and hydrogen isotope geochemistry of thermal springs of the Western Cape, South Africa W

The Baths r 3 km / Kouebokkeveldberge / Warmbadberg . ~ - . , / ? S o .......... ~ " ' ~ . , ! ,-.

I

6 km

E

~ .... ~

~..~...,.

I

1. Citrusdal ("The baths") N

S

I" 4 km

Klein Swartberge

\

- 6 km ,

Calitzdorp spdng

BS

H.,,,vie org0

\

,

2. Calitzdorp

NE

$W Brandvlei sprtng

"-I.' Malmesbury Group

4 km |

5 km

granite

~

'--. TMG

J

3. Brandvlei Ftgure 2. Cross-sections which illustrate the sub-surface geology at Citrusdal, Ca#tzdorp and BrandvleL Sections were drawn from published survey maps (Diamond, 1997J. MG: Malmesbury Group; hiS." Nardouw Subgroup; CF: Cederberg Formation; BG: Bokkeveld Group; WG: Witteberg Group; PF: Peninsula Formation; BS: Bidouw Subgroup; CS: Ceres Subgroup; TMG: Table Mountain Group.

using a Finnegan MAT252 mass spectrometer, and the fractionation factor between CO 2 and water at 25°C was assumed to be 1.0412 (Coplen, 1993). Data are reported in the familiar 5 notation, relative to SMOW, where 5 = (R mp,,/RsMow-1)'lOOO, and R = 180/180 or D/H. The average difference between duplicates of internal water standard (CTMP) over the course of this research was 0.48%0 for H (n = 23) and O. 10%o for O (n = 18). These correspond to values of 2a of 0.74%0 and O.14%o, respectively. The standards V-SMOW and SLAP were analysed to determine the degree of compression of raw data, and the equations of Coplen (1993) were used to

convert raw data to the SMOW scale. Our internal water standard (CTMP 5D = - 9 % ; 5180 =-2.85%o), which had been calibrated against V-SMOW and SLAP and independently analysed, was run with each batch of samples and used to correct for drift in the reference gases. The gas samples were analysed as follows: the sample bottle was placed onto the vacuum line and the condensable gases were collected in a U-trap immersed in liquid N. The line was then opened to a furnace containing CuO at 700°C so that any CH 4 present would be converted to CO=. The liquid N was replaced by frozen isopropyl alcohol, and the dry CO 2 Journal of African EarthSciences471

R. E. DIAMOND and C. HARRIS Brandvlei, all of which are found in the belt of mountains closest to the coast (the’coastal group’). The springs, which plot on the lower group, are found in the mountain belts further inland (see Fig. 1 I. This negative correlation is less strong for 6D versus temperature. There is no correlation between isotope ratios and height above sea level. However, the three highest altitude springs (700-800 m) have a significantly lower mean 6D and 6180 than the other springs. For those springs less than 200 km from the west coast, there is a good correlation between isotope ratios and distance from the west coast (Fig. 41, which is clearly not influenced by the altitude of the spring. Rietfontein (700 m asl), which is the only spring in this study from the Karoo region, has an anomalously high 6180 compared to the other springs > 200 km from the west coast. The variation in 6D and 6’*0 values of the Brandvlei, Calitzdorp, Citrusdal and Malmesbury over eight

was collected in a second U-tube. From there, the CO, was frozen into a break seal tube for analysis. The Brandvlei and Calitzdorp gas samples appeared to be dominantly CO, based on the relative proportion of gas frozen directly into liquid N, The Malmesbury gas contained about 20% CH,. The standards NBS1 9 (calcite) and NBS21 (graphite) were used to convert the raw data to the PDB scale. The 613Cmeasured in this way is that of the total C present (CH, + CO,).

RESULTS Thermal springs Water 6D and 6180 values are presented in Table 2. There is no correlation between isotope ratios and temperature or altitude of the spring (Fig. 3). However, on the V’O versus temperature plot, there are two distinct groups of samples, which show a negative correlation. The springs plotting in the upper group are Malmesbury, Goudini, Caledon, Citrusdal and

’ -20

\Ma

-

\

Cit

‘.\

. .

-i ! -! _I

Wi

1,

\

‘1

‘1

-30

F----~)ci, ma

--..

‘\

-

‘\

\

‘\

! 1%Coastal group \

G o’*\

‘\

O-

Wi

0

0

i

Bad

-40 - i R

a

Bad

0 Cali

- 10 -

\

‘\ --_

ww

-.-._._. -.-._-___./ ,....l....l....l....

I

*

I

I..‘\o*--..

n

I

I

I

-

xb,.\Coastal group

Ma\

\

i

\

iiCit 0 i

., ‘\

k

‘\ BA,,o o,j Cale ._._.--0.

0

z

Go

0

Wi

I

v Bad 0 “Qyw Il....lly group .-.__ I....I....l....I..~I 30 Tern;erature

ww-

Cali 0

--.-__/

I

50

60 (“C)

200

-0 IO

0

-._

,

I

400

I

I

600

I

I

J ,

600

Heiaht a.s.1 (rn)

Figure 3. Plot of 6D and 6”O values of thermal springs versus temperature and height of spring above sea level. The coastal and inland groups of thermal springs are indicated. Ma: Malmesbury; G: Goudini; Cale: Caledon; Cit: Citrusdal; Br: Brandvlei; Bad: Baden-Baden; Cali: Calitzdorp; MO: Montagu; R: Reitfontein; To: Toowerwater; Wi: Witzenberg; WW: Warmwaterberg.

472 Journalof African Earth Sciences

-31

-31

-28

Aug

Sept

Ott

during

collected

Samples

were

-31

-37

M&V

-37

March

mean

-30

-28

July

-6.9

-6.1

-30

June

1997

-5.5

-32

the

-6.1

-5.6

-5.9

-5.6

-5.0

-5.8

-5.5

Mav

-5.6

s’%

-27

6D

Brandvlei

-33

-6.9

6’8o

April

6D

IBaden-Baden

March

Feb

1995

month

-31

-31

6D

M&V:

-35

-6.2 indicated.

-40

-41

-42

-36

-40

-39

-37

-44

SD

Data

-7.4

-7.3

-8.5

-6.8

-5.8

-9.0

-7.7

-6.5

-6,s

S”C

Calitzdorp

-5.5

-5.5

8°C

Caledon

from

-20

-22

-18

-22

-19

-19

-22

-16

6D

Mazor

-4.1

-4.9

-5.0

-6.0

-4.1

-5.7

-4.6

-4.8

6°C

Citrusdal

and Verhagen

-4.4

S’s0

Goudini 6D

-26

T

Table 2. Hydrogen and oxygen isotope data for sampled thermal springs

(1983)

-18

-18

-17

-33

-34

-33

6D

collected

-7.1

-6.4

-6.4

-6.4

S’s0

Montagu

for samples

-4.2

-3.9

-4.3

-4.5

-3.8

-20

-4.3

-17

-3.8

-2.8

-3.7

-3.7

S’s0

-18

-18

-15

-20

-23

6D

Malmesbury

-5.2

-5.2

6’*0

in 1971-1972.

-42

-42

6D

Rietfontein

-41

-41

SD

-6.9

-6.9

6°C

Toowerwater

-46

-46

SD

-7.0

-7.0

S’*O

Warmwaterberg

-30

-30

6D

-5.6

-5.6

S”O

Witzenberg

R. E. DIAMOND and C. HARRIS

-20

-

y

0 \. &

-30

-

a

is

-40

= o.aa

‘10 o‘\

-

0

0

0

0 -50 -

1.

_ -4 -

9.

I I I.

I I I I .,.

Q ., 290m r = 0.95

-5 -

\O

250m

700m

0

360m

,o k

I I.,.,

120m

220m tk!A

-6-

8\ m

280m

0 -7 -

500m

280m

-0 -

I. 0

100

.I.

8OOm

0

0

200m

0 I. 200

I I *I,,.,,,,.,, 300

400

500

Distance from West Coast (km) F&we 4. Plot of 6D and 6180 of thermal spring water versus distance from the west coast of southern Africa measured in an east-west direction. Lines of best fit and correlation coefficients are given for thermal springs situated at altitudes <2DD m.

months of 1995 is shown in Fig. 5. All four springs show variations that are larger than the expected analytical errors of f 1 .O and + 0.1 %Ofor 6D and 6180, respectively. In the case of Calitzdorp and Malmesbury, there is a reasonable degree of correspondence between the 6D and PO values, which indicates that analytical error alone is not the cause of the variation, since the methods of analysis for 0 and H are completely separate. These springs were sampled from pools fed by the spring, and the variations in 6D and PO values could have been caused by varying degrees of evaporation from the pool. There is no evidence for any systematic difference in isotope ratios between summer and winter, which indicates that the spring waters originate from extensive aquifers which are unaffected by seasonal changes in the SD and al80 values of rainwater. The springs show a good correlation between the average 6D and 6180 values (Fig. 51, Rietfontein again being a significant outlying point. The line of best fit through the data calculated using the reduced major axis method (RMA: Rock, 1988) has the equation 6D = 7.816’*0 + 12.45. If Rietfontein

474 Journalof African Earth Sciences

is excluded from the data, the equation becomes SD = 7.48PO + 11 .74 . Gas data The 613C values obtained for samples of gas discharged with the spring water are given in Table 3. The gas was collected at all the springs where the water discharges directly from the ground upward into a pool above and the collection of gas bubbles was possible. The quantities of gas bubbling up appear to be proportional to the water discharge, with Brandvlei releasing on the order of a litre or so of gas every second, Calitzdorp significantly less and Malmesbury releasing streams of bubbles every few seconds of up to only a few millilitres each. The 613C values range from -21.5 to -23.2%0 compared to typical P3C values for volcanic and geothermal gas CO, of 0 to -11960 (Taylor, 1986).

Dl8ClJ88lON Carbon isotopes in gas bubbles The gas from the three springs analysed (Brandvlei, Calitzdorp and Malmesbury) yielded 613C values

Oxygen and hydrogen isotope geochemistry of thermalsprings of the Western Cape, South Africa

Calitzdorp

0

Ciirusdal -20 >

/ 8’”

kJ\~~@+-&

cl Malmesbury P erg -so

0

/“\o,o~o~o_olo 0

Figure 5. Variation of 6D and Sr80 values of Malmesbury, Citrusdal, Brandvlei and Calitzdorp thermal springs with the month each was samDIed.

between -21.5 and -23.2%0, which clearly labels the C as being of organic origin (Dai et a/. , 1996). Mazor and Verhagen (I 983) obtained a range of 613Cvalues from -16.6 to -24.5% for dissolved bicarbonate in some Western Cape springs. The data for Malmesbury (-16.6960) and Brandvlei (-18.9%0) of Mazor and Verhagen are significantly higher than the data obtained during the present work. This is probably due to differences in the material analysed, viz. gas bubbles (this work) versus dissolved bicarbonate. The large C isotope fractionation between CO, and CH, (&J~_cH~= +70%0 at 20°C: Bottinga, 1969) means that in a system where the 613C value of the total C present remains constant, the 613Cvalues of the dissolved bicarbonate will increase as the CH,/ CO, ratio increases. The 613Cvalue of bicarbonate in Malmesbury water (Mazor and Verhagen, 1983) is the least negative, which is consistent with our observation that the gas sample contained significant quantities of CH,. Despite the problems in interpreting the 613Cvalues of the mixtures of CO, and CH, without knowing the quantities of each gas present, these data are important in the context of the present study because they confirm a non-volcanic origin and support the conclusions of Mazor and Verhagen (1983) that the

C is of an entirely biogenic origin. Mazor and Verhagen (1983) concluded that “no significant exchange with 14C-free aquifer materials has taken place”. This seems reasonable given that rocks of the Cape Supergroup contain very little carbonate material. The Cape Mountains are known for their nutrient poor, structureless and nearly topsoil-free soils. There are, however, flat areas that become waterlogged in winter and have black organic-rich soils. These soils would tend to be reducing, as well as having a large supply of C. The fynbos (heath-like) vegetation that grows on the Cape Mountains is distinctive in producing fulvic and humic acids, which, if present in sufficient quantities, stain the water Table 3. Stable isotope data for gas Spring

6%

Brandvlei

-22.7

Calitzdorp

-21.5

Malmesbury

-23.2

Carbon isotope ratios were measured on the total C present in gas bubbles collected from the spring water.

Journalof Afrfcan EarthSciences475

R. E. DIAMOND and C. HARRIS reddish-brown. These organic compounds and possibly others could allow the water to contain appreciable dissolved organic C, which is released during heating of the water at depth. Long-term changes in 6D and 6180 The data presented here are similar, but not identical, to the data of Mazor and Verhagen (I 983) obtained on samples collected in 1971 and 1972. A limited amount of H isotope data (four springs) are available for comparison, and on average the 6D values of Mazor and Verhagen are slightly higher. The 6180 values reported in this paper are generally 0.3-0.5% lower than those reported by Mazor and Verhagen (1983). It is possible that these differences reflect long-term changes in the isotope composition of recharge due to climate change, but any shift in S’*O values with time ought to be accompanied by a similar shift in 6D values, and this is not observed. Thus, differences in 6D and PO between the 1971/ 2 and 1995/7 samples are far more likely to be a function of analytical procedures employed by the two laboratories involved.

Comparison with meteoric water One of the main conclusions of Mazor and Verhagen (1983) was that the thermal springs have systematically lower 6D and ZPO values than rivers sampled in the same area at the same time and, hence, ambient meteoric water. However, as acknowledged by Mazor and Verhagen (19831, this conclusion is weakened by the probability that seasonal variations in the 6D and 6’*0 values of the rivers exist, as well as possible isotope gradients, with water depth. The 6D and PO values of rivers might not, therefore, be a good approximation to the integrated annual rainfall in a particular area. The isotope data for the springs had been chosen to be compared with data for ambient meteoric water. The ideal comparison would be with rainwater collected at the spring site over a period of several years, but such data are not available. The International Atomic Energy Agency database (IAEA, 1997) has a monthly record for Cape Town International (formerly D.F.Malan) Airport from 19621974, and Diamond [I 997) and Diamond and Harris (I 997) reported monthly 6D and PO values for the University of Cape Town (UCT) and elsewhere in the Western Cape. The rainfall data are compared to the thermal spring data on Fig. 5, and it can be seen that the springs have systematically lower 6D and 6180 values compared to the rain. The weighted mean annual 6D and 6180 values for UCT and the IAEA data are plotted, and it can be seen that they are

476 Journal of African Earth Sciences

significantly higher than the thermal spring values. Rain data from inland stations at Oudtshoorn, Citrusdal and Tulbagh are not complete annual records; nevertheless, they all include the winter months when rainfall is highest and temperatures are lowest. Hence, the weighted mean 6D and PO values from these rainwater collecting stations ought to be somewhat lower than the weighted mean annual values. The Malmesbury spring has 6D and S180 values, which are only slightly lower than the mean annual rainfall value for UCT. Malmesbury is 70 km north of Cape Town and further inland. Hence, the data are consistent with the spring being recharged by ambient rainwater. The situation is similar for both the Citrusdal and Witzenberg Springs. The spring water has slightly more negative 6D and PO values than the measured rain data. The average spring 6D and 6’*0 values for Citrusdal are -20 and -4.9% compared to the weighted mean for rain (Diamond, 1997) of -11 and -4.4%0. The average spring 6D and 6’*0 values for Witzenberg are -30 and -5.5960 compared to the weighted mean for rain (Diamond, 1997) for Tulbagh of -20 and -5.1960. The Calitzdorp Spring has the lowest 6D and 6180 values of all the springs analysed and these values (6D and 6180 equal to -40 and -7.3%0, respectively) are considerably lower than rainfall at Oudtshoorn, 40 km east of Calitzdorp Spa, at the same altitude (weighted mean 6D and al80 equal to -11.6 and -4.1 %o,respectively). No data for rainfall in the vicinity of Montagu, Baden-Baden, Warmwaterberg, Toowerwater and Rietfontein exist, but there is no reason to suppose that it should be significantly different from the analysed rainfall samples. It is, therefore, concluded that most of the thermal springs have isotope ratios that are significantly lower than ambient rainfall. Isotope exchange between rock and water As discussed above, the 6D and 6180 values of the hot springs are generally lower than ambient rainfall. In addition, the springs plot slightly below the local meteoric water line (Fig. 6). One possible explanation for this is that the 6’*0 values of the springs increased as a result of the exchange of 0 between the water and the rocks through which they passed. This is commonly observed in geothermal waters of volcanic regions (e.g. Sheppard, 1986). Water-rock interaction usually affects 6’*0 values but not 6D values because rocks generally consist of 50 wt% 0 and very little H. The potential shift in 6180 value of the thermal water is dependent on the 0 isotope fractionation factor between the rock and water, temperature and the 6180 value of the rock. The fractionation factor between quartz (the dominant mineral in the rocks) and water is large at low

Oxygen and h ydrogen isotope geochemistry of thermat springs of the Western Cape, South Africa

X IAEA

+ UCT 0 Citrusdal A Oudlshoorn

Rain

UCT weighted average

_ IAEA weighted average

0

Rielfontein

Figure 6. Plot of 6D versus 61B0 for thermal springs and rainwater from various places. All rain data are integrated monthly samples; the UCT data are for a two year period (Diamond and Harris, 1997J and the IAEA data for most (but not al// months between 1962 and 1974 (IAEA, 1997); the Citrusdal, Oudtshoorn and Tulbagh data are for March-October 1995. The weighted annual mean values for the UCT and IAEA collection stations are shown and the line of best fit through the rain data is from Diamond and Harris f 1997).

temperatures (Aqu,.__ = 3.38.1 06*T2-3.4, where T is the temperature in K: Clayton et al., 1972). This

translates to a difference between quartz and water 6180 values of 25.3%0 at 70°C. The sandstones and quattzites of the Cape Supergroup have average PO values of 10.91960 (n = 28: Diamond, 1997) and the Malmesbury Basement has an average 6180 of 13.06% (Harris et al., 1997). It therefore follows that the PO values of water in equilibrium, with average Malmesbury Group Basement and Cape Supergroup, would have been -12.3 and -14.4% at 70°C. Any change in PO value of water as a result of interaction with rocks at this temperature would have been to lower, not higher, values and such exchanged waters would plot to the left of the meteoric water line on Fig. 6. In order to cause shifts to higher PO values in the water, interaction would

have had to take place above about 1OO’C because at this temperature the waters are in approximate 0 isotope equilibrium with the average Table Mountain Group. This temperature is much hotter than any of the thermal springs, and it is therefore concluded that water-rock interaction did not affect their 6180 values. In any case, 0 isotope exchange at such low temperatures is likely to have been sufficiently slow that water-rock interaction has no effect on isotope ratios. Comparison with groundwater

In this study, the thermal spring data has been compared with data (Harris et al,, 1999) from cold springs issuing from the lower slopes of Table Mountain (next to UCT; Fig. I) and water sampled from boreholes in the area around Victoria West

Jwmal of African Earth Sciences 477

R. E. DIAMOND

(altitude 1200 m; Fig. 1) in southwest Karoo (C. Harris and S. Peth, unpub/. data). These data give some idea of the range of 6D and 6’*0 values of unheated groundwater as one proceeds from the west coast inland and are compared to the thermal springs in Fig. 7. The Victoria West water samples were taken from various depths (O-250 m) from a number of boreholes drilled by the Department of Water Affairs and Forestry in the area. Those samples from > 1DO m tend to have lower 6D and 6’*0 values than samples from < 100 m and this is most likely to be caused by selective recharge of deep waters by heavy rainfall events. The Table Mountain springs plot close to the Western Cape meteoric water line, whereas the Victoria West borehole waters form an array which

A Table

and C. HARRIS

is approximately parallel to the Western Cape meteoric water line with much lower 6D values for a given 6180 value. The equation of best fit through the Victoria West data has the equation 6D = 6.9@0 -1.8. The negative intercept value is uncharacteristic of meteoric water data arrays and may reflect significant evaporation in the near surface environment during recharge. Note that the Rietfontein thermal spring, which is geographically closest to Victoria West, and which is situated in the southern Karoo region to the north of the Cape Fold Belt, has 6D and 6180 values which lie within the range shown by the Victoria West ground-waters. For the most part, the thermal springs plot between the lines of best fit through the Table Mountain and Victoria West data but generally have lower 6D and 6180 values.

Mountain

0

Victoria

West

0

Victoria

West z 1 OOm

c 1OOm

Line ot best-lit through Vic.Wsst data

Fm 7. Comparison of thermal spring SD and 6180 values with those of cold springs on the lower slopes of Table Mountain (Harris et al., 19991 and groundwater from the area around Victoria West (Harris and Peth, unpubl. data). Meteoric water line for Western Cape is from Diamond and Harris 11997). The line of best fit through the Victoria West data was calculated using the RMA method (see text). The Global Meteoric Water Line of Craig /1961/ is shown for reference.

478 Journal of African Earth Scfences

Oxygen and hydrogen isotope geochemistry of thermalsprings of the Western Cape, South Africa Origin of low 6D and 6’*0 values The comparison of 6D and PO values between the thermal springs and meteoric and groundwater water samples confirms that the thermal springs have significantly lower 6D and Pa0 values than ambient rainwater, Various combinations of the following may be responsible for these low 6D and PO values: i) the continental effect (e.g. Dansgaard, 1964); E) selective recharge during periods of abnormally high rainfall (as suggested by Mazor and Verhagen, 1983); i@)recharge during an earlier period of colder climate; and iv) recharge at higher altitude. The continental effect cannot account for low 6D and PO values of the thermal springs because they have lower 6D and al80 values than the groundwater at Victoria West, which is further inland. Mazor and Verhagen (I 983) concluded that the springs were selectively recharged by direct rain infiltration after heavy rains without any evaporation or averaging associated with rivers. Heavy rain events generally produce rain that has more negative 6D and PO values than normal rainfall at the same place (the ‘amount effect’ of Dansgaard, 1964). Selective recharge by heavy rain events is the likely cause of the differences in isotope composition between the deep and shallow groundwaters at Victoria West, but this effect is too small to account for the observed isotope differences between thermal springs and groundwater. The possibility that the springs were recharged during a colder climate regime was rejected by Mazor and Verhagen (1983) because of the lack of correlation between 14C data (as a proxy for time) and 0 and H isotope ratios. There remains the possibility that high average altitude of recharge is the cause of the low isotope ratios of the thermal springs. It is well known that the 6D and PO values of rainfall decrease as altitude increases (Dansgaard, 1964). Midgley and Scott (1994) reported an altitude effect on PO of -0.32% per 100 m for the Jonkershoek Mountains, about 70 km east of Cape Town. At Calitzdorp, the possibility exists that the zone of recharge of the spring could be in the Klein Swartberg Mountains to the north, which rise up to 2000 m (Fig. 2). The difference between the PO value of the spring and Oudtshoorn rain is 3.2%0, which could be interpreted as the recharge zone being on average 1000 m higher than the spring that is at about 1200 m. Regiial variation The small number of thermal springs available for analysis preclude a detailed discussion on the regional variation of their 6D and PO values. Nevertheless,

the stable isotope data present several interesting features. The most obvious feature is the apparent effect of continentality, whereby the 6D and S’80 values decrease with increasing distance from the west coast. The difference between the Table Mountain Springs data and the Victoria West groundwater data illustrate a second effect, that is a much lower ‘deuterium excess’ (d), where d=6D-84’*0 for a given data point (Dansgaard, 1964; Whelan, 1987) for the inland groundwater. Regardless of whether the low y-axis intercept value for the line of best fit through the Victoria West data is indicative of evaporation prior to recharge, the thermal springs also show a similar decrease in deuterium excess as their distance from the west coast increases. The apparent grouping of thermal springs into coastal and inland groups (Fig. 31, which both show a negative correlation between EPO and water temperature, is more difficult to explain in the light of the observations made above. Within each group, higher temperatures of spring water can only be explained by circulation of water to greater depths. As discussed above, lower 6D and PO values can generally be explained by recharge at higher altitude, thus the data are consistent with the higher temperature springs being recharged at higher altitude. This is to be expected as a greater depth of circulation would be expected in aquifers with a greater hydraulic head of water. The correlation between iY*O values and distance from the west coast in the coastal group must, therefore, reflect an increase in the average altitude of recharge with increasing distance from the coast and is not simply due to the continental effect. The inland group of thermal springs shows a negative correlation between al80 values and water temperature with a similar gradient but with 6180 values about 2%0 lower for a given temperature. This offset is presumably due to the greater ‘continentality’ of these springs. The lack of correlation between distance from the west coast and isotope ratios in those springs > 200 km from the west coast (Fig. 4) may, in part, be due to the change in geometry of the Cape Fold Belt from east to west. The coastal group of thermal springs is located in mountain belts which trend north-south, perpendicular to the movement of weather systems, whereas the inland group is situated in mountain belts which trend east-west.

CONCLUSIONS The authors agree with previous work by Mazor and Verhagen (1983) that the source of water in the Western Cape thermal springs is meteoric in origin and that there is no evidence for water-rock interaction having any effect on 0 isotope ratios. No systematic

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R. E. DIAMOND and C. HARRIS changes in 6D and ZY80 values were detected over a period of eight repeated samplings, suggesting that the aquifers contain significant volumes of water which are not affected by seasonal changes in the 6D and ?Y80 values of rainfall. The main feature distinguishing the thermal springs from ambient meteoric water is the significantly lower 6D and 6180 values. Although the isotope ratios of the thermal springs become progressively more negative with increasing distance from the west coast (for the first 200 km), it appears that high average recharge altitude is the most important factor responsible for the low 6D and 6180 values .

ACKNOWLEDGEMENTS The authors are grateful to the FRD for financial support in the form of a studentship to RED, and a core grant to CH. The authors are indebted to their water samplers, Captain D.C. Taljaard of Brandvlei Prison, Worcester, Mr H. van Huysteen of the Caliizdorp Spa, Mr M. Gordon of The Baths, Cirusdal, Mr B. Beylevelde of Citrusdal, Mrs K. of Oudtshoorn and Mrs V. Humphris of Tulbagh. They are also grateful to all the personnel at the other thermal springs for allowing them to take water samples. K. Faure, P. Dennis, A. Issar, I. Cartwright, B. Verhagen, S. Talma and J. Weaver are thanked for helpful discussions and comments at various stages of this work. This paper was written by CH during periods of sabbatical leave at Monash University, Australia and Universite Jean Monnet, St. Etienne, France. Finally, the authors are again indebted to F. Rawoot for help with the analytical work. Editorial handling - I? Bowden

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Theron, J.N., Gressa, P.G., Siegfried, H.P., Rogers, J., 1991a. The geology of the Cape Town area-explanation of sheet 3318. Geological Survey, Republic of South Africa, 14op. Wefhan, J.A., 1987. Stable isotope hydrology. In: Kyser, T.K. (Ed.), Short Course in Stable Isotope Geochemistry of Low Temperature Fluids. Mineralogical Association of Canada 13, pp, 129-161.

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