K-Ar ages of mineral separates of biotite, plagioclase, and sanidine from one of a group of volcanic domes of quartz latite to dacite composition and two quartz latite dikes, all of which predate mineralization at the Cerro de Pasco mining district, are in the range of 14 to 15 m.y., with an overall average of 14.5 + or - 0.2 m.y. A large (6 cm long) sanidine phenocryst from one of a group of late dikes that cut the major mineral deposits gives a K-Ar age of 15.2 + or - 0.2 m.y. This age, slightly but apparently significantly, greater than those of the older volcanic rocks that predate mineralization suggests that the sanidine crystal contains about 1 X 10 (super -11) moles/g of extraneous radiogenic argon.The radiometric ages establish the time of volcanism and mineralization at Cerro de Pasco as middle Miocene and suggest that the various igneous and hydrothermal events took place within a very short period of geologic time. Available geological and radio-metric data support the concept of an intimate genetic relation between hydrothermal activity and the pulse of magmatic activity represented by the exposed volcanic rocks.

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