Structure and Evolution
Located wholly within N. Chile, Aucanquilcha volcano is the site of the world's highest mine (sulphur) and permanent human habitation (Figure 17.1 and 17.3). Topographically, it forms a ridge extending about 10 km in an E-W direction, made up of several distinct cones and craters. It is one of the largest volcanic edifices in N. Chile. A well preserved débris flow (DF) is conspicuous on TM images (Francis & Wells, 1988), but its source region on the volcano is obscured because its proximal portions are largely obscured by extensive moraines (M). This indicates an age in excess of 10,000 yr for the avalanche. During glacial times, the ice cap on the volcano was extremely extensive (~45 km2), since moraines can be traced continuously over the high plateau south of the main ridge, reaching down to approximately 4, 600 m. Although the summit region is obscured by snow and ice on the TM image, a few lava flows (LF) are apparent on the upper flanks. Worner et al. (2000) dated the lava flows on the northeast side and southwest slope. which are the youngest, revealing K-Ar ages of 0.78±0.12 Ma (feldspar), 0.52±0.12 Ma and 0.40±0.20 Ma (whole rock and replicate measurement). Sulphur is currently mined in the summit region of the edifice, which is extensively altered by fumarolic activity (Figure 17.2). Feeble fumarolic activity is encountered at the present day within pits and shafts worked for sulphur. There are no records of historic eruptions. Extrusion of lava from a summit vent appears to have been the most recent activity.
Feeble fumarolic activity persists to the present day and is most obvious within shallow sulphur mine workings. Fumarolic activity persisting long after the most recent Pleistocene eruption is unusual.
Samples from Aucanquilcha were analysed by Roobol et al. (1976) during the course of a reconnaissance of the volcanoes between 21º and22ºS. They describe Aucanquilcha as being composed "almost entirely of hornblende andesite lavas." Clark (1970) reported an unsual assemblage of native sulphur-covellite "Cu5.5x FexS6.5" in the fumarolic sulphur deposits.Body
Clark, A.H. 1970 An occurrence of the assemblage native sulphur-covellite "Cu5.5x FexS6.5" , Aucanquilcha, Chile. Am. Mineral. 55, 913-918.
Francis, P.W., & Wells G.L., 1988 Landsat Thematic Mapper observations of débris avalanche deposits in the Central Andes. Bull Volcanol. 50:258-278.
Roobol, M.I., Francis, P.W., Ridley, W.I., Rhodes, M., & Walker, G.P.L., 1976. Physico-chemical characteristics of the Andean volcanic chain between 21° and 22° S. In: I.A.V.C.E.I. Proc. Symp. Andean and Antarctic problems. (Santiago, Chile, September 1974.), p. 450-464.
Worner, G., Hammerschmidt, K., Henjes-Kunst, F., Lezuan, J., Wilke, H., 2000. Geochronology (40Ar/39Ar, K-Ar and He-exposure ages) of Cenozoic magmatic rocks from Northern Chile (18-22¡S): implications for magmatism and tectonic evolution of the central Andes. Rev. geol. Chile, 27(2):205-240. ISSN 0716-0208.