Hydrothermal minerals and microstructures in the Silangkitang geothermal field along the Great Sumatran fault zone, Sumatra, Indonesia

Detailed study of core samples of silicic tuff recovered from three geother mal wells along the strike-slip Great Sumatran fault zone near Silangkitang , North Sumatra, supports a model for enhanced hydrothermal circulation adj acent to this major plate-boundary fault. Two wells (A and C) were drilled nearly vertically similar to1 km southwest of the eastern (i.e., the princi pal) fault trace, and the third, directional well (B) was drilled eastward from the site of well A to within similar to 100 m of the principal fault t race. The examined core samples come from depths of 1650-2120 m at measured well temperatures of 180-320 degreesC. The samples collected near the prin cipal fault trace have the highest temperatures, the largest amount of seco ndary pore space that correlates with high secondary permeability, and the most extensive hydrothermal mineral development. Secondary permeability and the degree of hydrothermal alteration decrease toward the southwestern mar gin of the fault zone. These features indicate episodic, localized flow of hot, possibly CO2-rich fluids within the fault zone. The microstructure pop ulations identified in the core samples correlate to the subsidiary fault p atterns typical of strike-slip faults. The geothermal reservoir appears to be centered on the fault zone, with the principal fault strands and adjoini ng, highly fractured and hydrothermally altered rock serving as the main co nduits for vertical fluid flow and advective heat transport from deeper mag matic sources.