| Abstract |
The Sorikmarapi Volcano (2145 m asl.) and its geothermal field are located approximately 22 km to the north part of Panyabungan, the capital city of Mandailing-Natal District, part of North of Sumatra Province, Indonesia. After more than 400 years, there was no magmatic eruption occur. However, the phreatic eruptions are usually occur until present. The last phreatic eruption at the summit crater was occur in1986. The next year (1987), the eruption occurred at Fumarole field, on the northeast flank of volcano. The released small amounts of gases from Sorik Marapi solfatara on the low part crater rim are function of deep magma process, eq. vapour melt separation during coming up generating magma. The existence SO2 indicates a high temperature magmatic component, rising from underlying magma. On the other hand, the flank of this volcanic fumaroles have no SO2 gas, high H2S and CO2. The existence H2S assumed a low temperature gas presenting at the reservoir. The H2, O2+Ar, CH4, and NH3 gases probably indicate a secondary hydrothermal component slowly rising from a two phase, saline brine vapour, and covering the magmatic system. The H2 to CH4 and NH3 shows a decreasing in equilibration for individual species, interpreting slow rate varieties (CH4 and NH3) are formed at depth within the hydrothermal zones. The rapidly rising magmatic component are controlled by the SO2-H2S bupper. The H2S and CH4 seems to be stable in low temperature rock conditions, particularly at the geothermal reservoir, outside of Sorik Marapi volcanic vent. The acidic conditions within the hydrothermal zones of the Sorik Marapi Crater Lake indicate a high temperature fluid interaction toward surface, where SO2 and H2S reacts with rocks. However, deep drillings to the geothermal reservoir, which is located northern part of outside the Sorik Marapi volcano represent neutral chloride fluids, supporting 2 x 20 MW power plants. Gas ratio and geothermometry indicate steam derived from a deep source, with temperature 200oC-230oC. While, at the lower part of the crater, the subsurface temperature is probably more than 400oC, indicating by present of SO2 gases. |