Abstract

The Soledad Caldera is a low-volume, nonresurgent “ash flow”-type caldera, located on the Altiplano northwest of Oruro in the central Andes of Bolivia. The first stage in the evolution of the composite volcanic field was the intrusion of a cluster of small dacitic stocks around La Joya on the southwest edge of the caldera at ca. 15 Ma. These have related, strong hydrothermal alteration and epithermal vein and disseminated Au-Ag-Cu-Pb-Zn mineralization with minor As, Sb, Mn, Bi, W, and Sn. At 8.8 Ma, a line of small rhyolite domes formed along an outer ring fracture ∼15 km west of the caldera margin. Caldera collapse took place at ca. 5.4 Ma, accompanied by the eruption of the Soledad Tuffs. These make up a sequence of interbedded, dacitic, air-fall and nonwelded ash-flow tuffs. They crop out in a ring around the nonresurgent caldera center, which is elongated northwest and is 22 km × 14 km (240 km2). Post-collapse ring-fracture volcanism formed the voluminous dacitic lavas of the Esquentaque Massif, which overlie the tuffs on the south side of the caldera. There is a zone of advanced argillic alteration in the lavas, formed beneath a hot-spring system. The caldera is considered to be the volcanic manifestation of the episodic rise and emplacement of a batholith.

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