GEOCHRONOLOGY OF EPITHERMAL Au-Ag MINERALIZATION, MAGMATIC-HYDROTHERMAL ALTERATION, AND SUPERGENE WEATHERING IN THE EL PENON DISTRICT, NORTHERN CHILE

Abstract

The El Penon epithermal gold-silver deposit, Region II, Chile, comprises multiple epithermal veins, which contain a geologic resource (measured + indicated + inferred) of 7 million oz (Moz) Au and 199 Moz Ag. The size, combined with bonanza Au grades, makes it the most important precious metal deposit in the Paleocene metallogenic belt of northern Chile. Quartz-adularia-carbonate veins are hosted in Paleocene and early Eocene mafic to felsic volcanic rocks that are hydrothermally altered up to 100s of meters from mineralized veins. Late Cretaceous and Eocene intermediate composition intrusive rocks also occur in the district and locally preserve alteration associated with magmatic-hydrothermal systems that are not related to precious metals mineralization of the El Penon deposit. Timing of hydrothermal activity in the district and age and duration of weathering are constrained by ages for vein adularia, hypogene alunite, and supergene alunite determined by Ar 39 /Ar 40 step-heating experiments. Adularias from the three largest veins of the El Penon deposit are dated at 52 to 53 Ma ( 40 Ar/ 39 Ar), indicating formation 1 to 3 m.y. later than host rhyolite domes. Magmatic-hydrothermal quartz-alunite alteration spatially associated with Late Cretaceous and Eocene intermediate composition intrusive rocks is dated at 73 to 65 and 50 to 49 Ma, respectively. Supergene alunite dates from the El Penon district indicate that weathering within a semiarid to arid climate occurred from 23 to 17 Ma, prior to the onset of hyperaridity.