K-Ar Ages of Tin-Polymetallic Mineralization in the Oruro Mining District, Central Bolivian Tin Belt

摘要

K-Ar age determinations were carried out on vein- and rock-forming minerals from five vein-type tin-polymetallic ore deposits of the Oruro mining district in the central part of the Bolivian tin belt. The sericite from vein selvedges and an altered host rock provides good estimates of the ages of hypogene mineralization, and supergene alunite and jarosite provide ages for erosional and weathering episodes. It is concluded that hypogene mineralization in the Oruro mining district took place during the early to middle Miocene: 15.8+-0.8 Ma at San Jose. 20.1+-1.1 Ma at Morococala. 20.5+-1.0 Ma at Avicaya, and 19.6+-1.0 Ma at Llallagua. Fine grained supergene alunite (delta~(34) S = -10.1 per thousand) and jarosite yield K-Ar ages of 6.7+-0.7 Ma at Avicaya and 3.9+-0.7 Ma at Bolivar, respectively, suggesting that erosion and chemical weathering were active at those times.