Stable isotopes of the thermal springs of the Cape Fold Belt
The Cape Fold Belt is a 250Ma orogenic belt comprised of rocks of the Cape Supergroup, an Ordovician to Devonian sedimentary sequence. The mountainous areas, which reach over 2000m, are composed of the faulted and highly jointed quartzites and sandstones of the Table Mountain Group, which acts as the main deep aquifer. It is from the secondary porosity of this aquifer that over ten thermal springs issue forth, ranging in temperature from ~27°C to 64°C. Samples of the integrated total monthly rainfall were taken for several months at each of four stations around the Cape Fold Belt. Values ranged from δ¹⁸O=- 8.3⁰/₀₀ and 6δ=-37⁰/₀₀ to 1.2⁰/₀₀ and 7⁰/₀₀, respectively. Integrated recharge values based on a full year of observation at UCT, Cape Town, are δ¹⁸O =- 3. 7o/oo and 6δ = -10⁰/₀₀. Eleven thermal springs were sampled in 1995, four of which were sampled once a month for several months, over the same period that the rain was being sampled. The average discharge values for the four springs sampled monthly are, for δ¹⁸O and δD respectively, at Malmesbury - 3.9⁰/₀₀, -18⁰/₀₀; at Citrusdal -4.9⁰/₀₀, - 20⁰/₀₀; at Brandvlei - 5.6⁰/₀₀, - 30⁰/₀₀ and at Calitzdorp -7.3⁰/₀₀,-40⁰/₀₀. A meteoric water line for the Cape Mediterranean climate area was calculated by the general form of a structural regression, using the monthly data weighted by rainfall amount. It has the equation δD = 7.38δ¹⁸O + 18.6. Using the same calculation technique, but not weighting the data, the data for the thermal springs yield a water line with the equation δD = 8.32δ¹⁸O + 16.5. The difference in gradient suggests that the springs are recharged from a colder and isotopically more fractionated weather system, such as during a previous colder climate regime, or at high altitude. There is no evidence for isotope exchange between the groundwater and host rocks; rather, the shift of the spring water line to less negative δ¹⁸O values suggests evaporation prior to recharge. Oxygen and hydrogen isotope ratios from the discharged spring water are clearly more negative than those expected for rain falling at the spring, which can be explained by recharge at much higher altitudes. The springs are therefore believed to be recharged high on mountains in the near vicinity. A continental effect was observed in the spring data, with respect to the distance from the west coast, which is the direction from which weather systems approach. Most of the thermal springs of the Cape Fold Belt seem to be recharged at high altitude in nearby mountains, whereafter the water is heated by geothermal gradient upon reaching depths of two to three kilometres via the secondary porosity of the Peninsula Formation, finally reaching the surface by means of various faults, which allow passage through the impermeable Cedarberg Formation. Isotopically light carbon is released at some springs in the form of CO₂ and CH₄ (total carbon δ¹³C≈- 21⁰/₀₀). These gases could come from near surface bog environments, however, at Malmesbury, where H₂S is also released, a possible geological source is indicated for the CO₂ and the CH₄.
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