| Abstract |
Stable, radioactive isotopes and chemical concentrations have been used to investigate the origin, circulation time and subsurface history of geothermal waters of the Tawau region, particularly the Apas Kiri area. The Tawau region has the highest concentration of young volcanics rocks of Late Tertiary (Pliocene) to Quaternary age, which therefore offers the best possibility of a heat source sufficient for a substantial geothermal resource. Thermal manifestations are found in seven areas, namely Tawau (T1), Upper Tawau (T2), Apas Kiri (A1, A2 , A4 and A5), Balung (B1 and B2), Sungai Jepun (J1 and J2), Mostyn and Tingkayu. An old steaming ground and hot mud pools are also found in the Apas Kiri area. Discharge of springs was up to 2.0 l/s with the surface temperature from 41.1 to 77.6oC. A maximum of eleven sites were selected for collection of first set of water samples and analysed for various isotopes (18-O, 2-H, 3-H of water, 13-C and 14-C of dissolved inorganic carbon; 34-S and 18-O of dissolved sulphates) and water chemistry. Isotopic and geochemical data show that the origin of thermal waters is meteoric water. The δ34S (in SO4) value shows that the sulphate of all the sampled hot springs originated from a modern oceanic water (δ34-S of about +20%o VCDT). Tritium contents and δ18O-δ2H diagram indicate that there is no mixing with shallow young groundwater. The Apas Kiri shows δ18O significant shift of up to 2%o VSMOW ; one of the processes involved for the significant enrichment in isotopic and chloride contents in Apas Kiri hot springs is steam separation/evaporation. As for the water chemistry, generally the Apas Kiri hot springs appear to have consistency in the levels of chloride, silica and cations, as well as spring temperatures that the water geothermometers reflect its good potential. The tritium content of all the hot springs is about zero showing circulation time more than 60 years, which suggests, these thermal waters were recharged before 1952 (nuclear weapon test). Reservoir temperatures estimated by δ18O (SO4-H2O) model are very encouraging; Apas Kiri shows 152-196oC, Sungai Jepun 124-156oC and Tawau 112oC. By using chemical geothermometers Na-K-Mg, the temperature obtained is ranging about 190-236oC, in which the Apas Kiri is the highest. |