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Fission-track dates of zircon and apatite from porphyry and country rock in and near the Aspen mining district of central Colorado form clusters at about 52 and 30 Ma and reveal some complications in the thermal history of the rocks. Zircons from three varieties of Late Cretaceous porphyry and from both altered porphyry in the mining district and less altered and unaltered porphyry outside the district have fission-track dates of about 52 Ma. Apatite fission-track dates in the area are about 30 Ma. These dates furnish no definitive evidence for the age of the ore deposits. Fission-track dates of zircon from Proterozoic granite and Penn-sylvanian sandstone near the mining district are much older, do not significantly reflect a Tertiary thermal event, and show that the Aspen mining district is not marked by a regional paleothermal anomaly. In that respect the Aspen district differs from the Leadville, Gilman, and Tennessee Pass districts.

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