Estudio litoestratigráfico y de alteraciones hidrotermales en los testigos de perforación de los pozos TTC-2, TTC-3 y TTC-9, del interior de la Caldera Chacana, provincias de Napo y Pichincha

Abstract

The Chacana caldera complex is located approximately 65 km east of Quito, at the Napo and Pichincha provinces. In 2012, EPMAPS carried out 10 drillings as part of the Ríos Orientales project. The TTC-2, TTC-3 and TTC-9 wells are located inside the caldera, in a SE-NO trend between La Virgen area and Papallacta village; those wells are at an altitude of 3253, 3750 and 4047 meters above sea level, and they have a depth of 160, 250 and 880 meters, respectively. This study provides a detailed analysis of lithostratigraphy and hydrothermal alteration of the drill core of these 3 drills reaching a total of 1290 m of rock logged. Additionally, 192 samples were selected for macroscopic studies and Terraspec analysis, and 73 thin sections were prepared for petrographic and mineralogical study of protholith and hydrothermal alteration. Aphanitic lava flows of dacitic composition were identified in the TTC-2 well, interlayered by sequences of clastic breccias, containing fragments of metamorphic rocks from the Cordillera Real basement and fragments of volcanic rocks, and by lacustrine sequences. The TTC-3 well consists of a series of porphyritic lava flows that vary in composition from basic andesites (with bytownite and pyroxene) to dacites (with biotite, amphibole, plagioclase and quartz), interlayered with a clastic sequence of polymictic breccias. The TTC-9 well shows at its base a sequence of porphyritic lava flows of andesitic composition, cut by phreatomagmatic breccias (diatreme), whose apparently surface manifestation is a sequence of polymictic breccias and tuffs, which probably corresponde to a maar ring structure. The upperest unit in all three wells are late quaternary unaltered till deposits formed during the Pleniglacial and Lateglacial periods. The action of chlorinated alkaline fluids over this rocks caused hydrothermal alteration, characterized by a mineral assemblage formed by: clays (smectite, illite-smectite and illite; defined by Terraspec), chlorite, calcite, quartz, silica, pyrite, hematite and zeolites; corresponding to a zone of mainly sub-propylitic alteration.