The Cachiyacu Geothermal Prospect, Chacana Caldera, Ecuador

Abstract

Cachiyacu is located at about 65 km ESE of the capital city of Quito, in the high ranges of the Eastern Cordillera at 4000 masl, on ragged topography in a wet and cold climate. Paved accesss road and high voltage transmission line cross along its northern border; the main load center is Quito. The heat source is the southern part of the resurgent silicic Chacana caldera, which has been persistently active since late Pliocene, producing numerous rhyolitic domes, andesitic to dacitic lava flows and ignimbrites of high-K calc-alkaline affinity; latest volcanic activity comprises two large Si-andesite lava flows as recent as 240 yr BP. The 35 by 15 km diameter caldera structure is hosted in the Paleozoic-Mesozoic metamorphic basement and is likely controlled by regional NNE-striking strike-slip faults. A hot water–dominated geothermal system, with deep temperatures in excess of 200 °C is assumed to be hosted in the late Tertiary to Quaternary volcanic pile filling the caldera and in its lower wall rocks. 30 to 65 °C, highly saline hot springs are common inside the area and its waters belong to the Cl-SO4-alkaline type. Hot spring water along a NE-oriented outflow at 5 to 10 km distance, is used mainly for bathing purposes and less for space heating. Older hydrothermal alteration is widespread in the area. A preliminary geothermal model is proposed in the Cachiyacu area with a self-sealed reservoir and permeability sustained by active faults. A comprehensive magnetotelluric survey is recommended to locate the geothermal reservoir and to site the first deep drilling targets in Cachiyacu.