Abstract

Geochronology and paleotopographic reconstruction of the porphyry-copper deposit at La Escondida, Chile, are used to calculate long-term erosion rates and to deduce the timing of Tertiary climatic change for a portion of the Atacama Desert region. Hypogene hydrothermal alteration and protore mineralization at La Escondida took place between 33.7 ± 1.4 and 31.0 ± 1.4 Ma based on K-Ar dating of hydrothermal biotite and sericite. Supergene weathering and copper-sulfide en-richment processes were active from 18.0 ± 0.7 to 14.7 ± 0.6 Ma based on K-Ar dating of supergene alunite, distinguished from hypogene alunite by grain size, color, and sulfur isotopic composition. Reworked lenses of volcanic ash in the vicinity of La Escondida provide useful time-stratigraphic markers at 8.7 ± 0.4, 6.5 ± 0.2, and 4.2 ± 0.2 Ma within present soil profiles.

Long-term average rates of erosion are determined by these age dates and quantitative calculation of eroded leached capping thickness at La Escondida using mass-balance analysis of geochemical profiles coupled with an estimate of unmineralized lithocap thickness based on alteration petrology and fluid-inclusion geobarometry at similar deposits. The observed trend of decreasing long-term average erosion rates with time is consistent with arid to semiarid conditions in the early Miocene changing to hyperarid conditions during the middle Miocene. This climatic desiccation caused termination of significant supergene copper-suffide enrichment at La Escondida and elsewhere in the Atacama region and preservation of surficial features, including the ash horizons and the leached capping.

Middle Miocene climatic desiccation in northern Chile and southern Peru was probably related to a pronounced decrease in temperature of coastal waters supplied by an ancestral Humboldt Current and an increase in upwelling intensity as the Antarctic ice cap became established at approximately 15 to 13 Ma. The Central Andes Cordillera, which now provides a rain shadow protecting the Atacama region from precipitation from the east, must have attained at least half its present elevation prior to about 15 Ma to have played a similar role in the middle Miocene.

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