Pergamon Journal of African Earth Sciences, Vol. 31, No. 3/4, pp. 467-481. 2000 0 2001 Elsavier Science Ltd Pll:SO899-5382(00)00002-1 All rights reserved. Printed in Great Brdain 0699-5362/01 5. see front matter 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-t des 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 charris@geology.uct.ac.za Journalof African Earth Scimcas 467