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

Abstract

New U-Pb zircon and 40 Ar/ 39 Ar 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 2Sigma errors. At the Chimborazo porphyry Cu prospect, a weighted mean 40 Ar/ 39 Ar plateau age of 38.09 + or - 0.30 Ma has been obtained for igneous biotite from a syn- or postmineralization feldspar-biotite-quartz porphyry intrusion. At the Zaldivar porphyry Cu mine, zircon and biotite have been analyzed from a similar synmineralization feldspar-biotite-quartz porphyry (the Llamo porphyry): the zircon U-Pb age is 38.7 + or - 1.3 Ma, whereas the weighted mean 40 Ar/ 39 Ar plateau age of igneous biotite is slightly younger at 37.40 + or - 0.18 Ma. U-Pb dating of two other rhyolitic quartz-feldspar porphyry bodies that host much of the ore at Zaldivar shows that these rocks are Paleozoic in age (Antigua porphyry: 290 + or - 4 Ma; Zaldivar porphyry: broadly coeval with Antigua but age poorly defined). At the Escondida deposit, the reportedly synmineralization Escondida porphyry is dated by U-Pb at 37.9 + or - 1.1 Ma, whereas the reportedly postmineralization Rhyolitic porphyry returned a U-Pb age of 34.7 + or - 1.7 Ma.These results indicate a single pulse of synmineralization magmatism in the Escondida district at approximately 38 Ma, followed by minor largely nonmineralizing intrusive events. The timing of main-stage porphyry mineralization at Escondida is apparently bracketed by the ages of the Escondida (37.9 + or - 1.1 Ma) and Rhyolitic porphyries (34.7 + or - 1.7 Ma). Previously published K-Ar dates for intrusive rocks and alteration minerals at Escondida, which range from 39 to 31 Ma, are interpreted to reflect variable degrees of disturbance of the K-Ar system by later intrusive events and/or by supergene weathering processes.An age of approximately 38 Ma for ore-forming porphyry magmatism in the Escondida region is significantly older than previously accepted best estimates based on K-Ar and U-Pb dating (32-34 Ma) but still falls within the age range of other large porphyry systems within the north-south-trending West Fissure Zone of northern Chile (31--42 Ma). A common feature of the porphyry intrusions at Zaldivar and Escondida, and also at Chuquicamata and El Salvador, is the presence of zircons with inherited upper Paleozoic cores. These cores are interpreted to reflect interaction between arc magmas and crustal rocks during melting, assimilation, storage, and homogenization (MASH) and assimilation and fractional crystallization (AFC) processes at various crustal levels.