A revised late eocene age for porphyry Cu magmatism in the Escondida area,northern Chile

New U-Pb zircon and Ar-40/Ar-39 single-crystal incremental-heating biotite dates are reported for porphyritic rocks from three mineralized centers in the Escondida region of northern Chile; all dates are reported with 2 sigma errors. At the Chimborazo porphyry Cu prospect, a weighted mean Ar-40/Ar-3 9 plateau age of 38.09 +/- 0.30 Ma has been obtained for igneous biotite fr om a syn- or postmineralization feldspar-biotite-quartz porphyry intrusion. At the Zaldivar porphyry Cu mine, zircon and biotite have been analyzed fr om a similar synmineralization feldspar-biotite-quartz porphyry (the Llamo porphyry): the zircon U-Pb age is 38.7 +/- 1.3 Ma, whereas the weighted mea n Ar-40/Ar-39 plateau age of igneous biotite is slightly younger at 37.40 /- 0.18 Ma. U-Pb dating of two other rhyolitic quartz-feldspar porphyry bod ies that host much of the ore at Zaldivar shows that these rocks are Paleoz oic in age (Antigua porphyry: 290 +/- 4 Ma; Zaldivar porphyry: broadly coev al with Antigua but age poorly defined). At the Escondida deposit, the repo rtedly synmineralization Escondida porphyry is dated by U-Pb at 37.9 +/- 1. 1 Ma, whereas the reportedly postmineralization Rhyolitic porphyry returned a U-Pb age of 34.7 +/- 1.7 Ma. These results indicate a single pulse of synmineralization magmatism in the Escondida district at similar to 38 Ma, followed by minor largely nonminer alizing intrusive events. The timing of main-stage porphyry mineralization at Escondida is apparently bracketed by the ages of the Escondida (37.9 +/- 1.1 Ma) and Rhyolitic porphyries (34.7 +/- 1.7 Ma). Previously published K -Ar dates for intrusive rocks and alteration minerals at Escondida, which r ange from 39 to 31 Ma, are interpreted to reflect variable degrees of distu rbance of the K-Ar system by later intrusive events and/or by supergene wea thering processes. An age of similar to 38 Ma for ore-forming porphyry magmatism in the Escond ida region is significantly older than previously accepted best estimates b ased on K-Ar and U-Pb dating (32-34 Ma) but still falls within the age rang e of other large porphyry systems within the north-south-trending West Fiss ure Zone of northern Chile (31-42 Ma). A common feature of the porphyry int rusions at Zaldivar and Escondida, and also at Chuquicamata and Fl Salvador , is the presence of zircons with inherited upper Paleozoic cores. These co res are interpreted to reflect interaction between are magmas and crustal r ocks during melting, assimilation, storage, and homogenization (MASH) and a ssimilation and fractional crystallization (AFC) processes at various crust al levels.