Hydrothermal alteration of the Late Eocene Caetano ash-flow caldera, north-central Nevada: A field and ASTER remote sensing study

Geologic mapping and analysis of ASTER remote sensing data were used to define the effects of a large hydrothermal system in the 12–18 by 22 km Caetano caldera. The caldera formed at ~33.8Ma during eruption of >1100km³ of the rhyolitic Caetano Tuff that left a 1 km deep basin which was partly filled by a lake. Magma resurgence resulted in shallow(<1 km) emplacement of the Redrock Canyon granite porphyry intrusion into caldera fill in the southwestern part of the caldera. Field and petrographic studies indicate that heat from the pluton or from deeper residual Caetano magma caused a large hydrothermal system to form that altered both the pluton and intracaldera Caetano Tuff throughout the western half of the caldera. ASTER-based mineral mapping shows that the most intense alteration, including local alunite, developed primarily along the southwestern and western caldera margins. Alteration was most intense in the Redrock Canyon intrusion itself and in the upper unit of the Caetano Tuff, which is a sequence of thin ash flows interlayered with finely bedded sedimentary rocks and mesobreccia. Hydrothermally altered intracaldera tuff was domed and crosscut by the unaltered 33.78 ± 0.05 Ma Carico Lake pluton, indicating that caldera formation, magma resurgence, pluton emplacement, and hydrothermal activity all occurred in less than about 100 ka.