Areas of hydrothermal alteration are found on Ollagüe, including in the summit crater, on its northeastern and northwestern rim and low on the northwestern slope. Alunite, gypsum and sulfur were formed by the alteration on the summit and the northwestern slope, and chalcedony, clay, kaolinite and opal are found as well. These areas have been mined, with the Santa Cecilia mine located on the northwestern rim and the Santa Rosa mine in the centre of the crater. Mining was still underway in 1988. In 1990, it was estimated that of sulfur can be mined at the Santa Rosa mine. According to a report in 1894, fumes released from sulfur beds on the volcano can incapacitate a man in seconds, making ascents difficult.