Abstract

The Cerro de Pasco epithermal Cordilleran base metal deposit in central Peru consists of two mineralization stages which are spatially and genetically related to a mid-Miocene diatreme-dome complex. In order to constrain the duration of the magmatic-hydrothermal system at Cerro de Pasco, U-Pb age determinations and Hf isotope determinations have been conducted on single zircon grains. Biotite from intrusive rocks, and sericite and alunite related to the first and second mineralization stages, respectively, have been used for stepwise IR-CO2 laser 40Ar/39Ar age determinations. This study has been complemented by Rb-Sr analyses of sericite and associated pyrite.

Single crystal zircon U-Pb analyses yield ages of 15.36 ± 0.03 Ma for an accretionary lapilli tuff from a collapsed block within the diatreme, 15.40 ± 0.07 Ma for a dacite porphyry dome in the diatreme, 15.35 ± 0.05 and 15.16 ± 0.04 Ma for two quartz monzonite porphyry dikes emplaced into the diatreme-dome complex that are within a time span of 350,000 yr. There is no field evidence for any magmatic activity predating the formation of the diatreme-dome complex.

Stepwise IR-CO2 laser 40Ar/39Ar ages have been determined on sericite and alunite. The latter formed during the second mineralization stage in advanced argillically altered domes and from halos of oxidized enargite-pyrite veins in the Venencocha and Santa Rosa area, located on the northwestern margin of the diatreme-dome complex and on the southern part of the Cerro de Pasco open pit, respectively. 40Ar/39Ar alunite ages cluster between 14.54 ± 0.08 and 14.41 ± 0.07 Ma. This age consistency suggests that advanced argillic alteration related to the second mineralization stage at Cerro de Pasco formed during a period of about 100,000 yr. Three significantly younger alunite ages of 12.39 ± 0.06, 12.13 ± 0.07 Ma, and 10.94 ± 0.10 M, probably reflect partial resetting by late circulating fluids. Therefore, it appears that magmatic-hydrothermal activity at Cerro de Pasco lasted about 1 m.y., from 15.4 to 14.4 Ma, similar to the duration in the nearby mid-Miocene Colquijirca district.

An attempt to more precisely date the first mineralization stage, bracketed by the quartz monzonite dikes (15.1 Ma) and the second mineralization stage (14.5 Ma) failed: the 40Ar/39Ar ages of sericite in the alteration halo of the pyrite-quartz body are not reliable, probably due to inherited argon derived from precursor micas in Paleozoic clasts of the diatreme breccia. This is indicated by Rb-Sr data on sericite which indicate binary mixing between Miocene magmatic and Paleozoic basement sources and do not carry any age information.

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