Bracketing the age of magmatic-hydrothermal activity at the Cerro de Pasco epithermal polymetallic deposit, central Peru: a U-Pb and 40Ar/39Ar study.

The deposit consists of two mineralisation stages which are spatially and genetically related to a mid-Miocene diatreme-dome complex. The first stage includes a large pyrite-quartz body and pyrrhotite bodies zoned to Pb-Zn ores containing Fe-rich sphalerite replacing Pucara carbonate rocks, and the second consists of Cu-Ag-Au-(Zn-Pb) veins hosted in the diatreme breccia and in the pyrite-quartz body on the western part of the open pit and Zn-Pb-(Bi-Ag-Cu) Pucara carbonate-hosted replacement bodies in the eastern and central open pit. Single crystal zircon U-Pb analyses were carried out and stepwise IR-CO2 laser 40Ar/39Ar ages determined on sericite and alunite. The latter formed during the second mineralisation stage in advanced argillically altered domes and from halos of oxidised 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. The age consistency suggests that advanced argillic alteration related to the second mineralisation stage formed during a period of about 100 000 yr. Three significantly younger alunite ages probably reflect partial resetting by late circulating fluids, and magmatic-hydrothermal activity appears to have lasted about 1 m.y., a period similar to the duration in the nearby mid-Miocene Colquijirca district.
The deposit consists of two mineralisation stages which are spatially and genetically related to a mid-Miocene diatreme-dome complex. The first stage includes a large pyrite-quartz body and pyrrhotite bodies zoned to Pb-Zn ores containing Fe-rich sphalerite replacing Pucara carbonate rocks, and the second consists of Cu-Ag-Au-(Zn-Pb) veins hosted in the diatreme breccia and in the pyrite-quartz body on the western part of the open pit and Zn-Pb-(Bi-Ag-Cu) Pucara carbonate-hosted replacement bodies in the eastern and central open pit. Single crystal zircon U-Pb analyses were carried out and stepwise IR-CO2 laser 40Ar/39Ar ages determined on sericite and alunite. The latter formed during the second mineralisation stage in advanced argillically altered domes and from halos of oxidised 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. The age consistency suggests that advanced argillic alteration related to the second mineralisation stage formed during a period of about 100 000 yr. Three significantly younger alunite ages probably reflect partial resetting by late circulating fluids, and magmatic-hydrothermal activity appears to have lasted about 1 m.y., a period similar to the duration in the nearby mid-Miocene Colquijirca district.