This paper describes the age and geology of 13 centers of Miocene magmatism with related epithermal precious and base metal mineralization on the eastern side of the northern Altiplano of Bolivia, part of the central Andean polymetallic belt. Twenty-eight new K-Ar ages are reported, together with a compilation of 12 from the literature. The data show four magmatic episodes. The two middle Miocene episodes (17-15 Ma, 13.5-12 Ma) are characterized by clusters of small volcanic and hypabyssal porphyries, which host the most important epithermal mineralization. In contrast large volcanic centers formed during the two late Miocene episodes (10-8 Ma, 6-5 Ma). No significance is attached to the secondary-age groups. The middle and late Miocene magmatic-mineralization episodes correlate with those of the southern part of the tin belt of the Eastern Cordillera and formed during the main-arc broadening of the central Andes.