Supergene enrichment of copper deposits since the onset of modern hyperaridity in the Atacama Desert, Chile

Abstract

Artículo de publicación ISI

Supergene enrichment of Cu deposits in the
 Atacama Desert has played a critical role in making this
 the prime Cu-producing province of the world. Previously,
 this has been believed to have occurred exclusively over a
 long period from the middle Eocene to the late Miocene,
 which ended when climatic conditions changed from arid to
 hyperarid. Here, we report U-series disequilibrium ages in
 atacamite-bearing supergene assemblages that provide a
 new conceptualization on both the supergene enrichment
 process and the onset of extreme hyperaridity in the
 Atacama Desert. 230Th–234U ages of gypsum intergrown
 with atacamite in supergene veins from Cu deposits cluster
 at ~240 ka (Chuquicamata), 130 ka (Mantos Blancos,
 Spence), and 80 ka (Mantos de la Luna, Michilla). When
 coupled with previous data, these results indicate that
 supergene enrichment of Cu deposits did not cease after
 the onset of hyperaridity. We propose that supergene
 enrichment in the Atacama region developed in two main
 stages. The main phase, caused by downward circulation of
 meteoric waters in a semi-arid setting, was active from 45
 until ~9 Ma, with a last pulse ca. 5 Ma in the southern
 Atacama Desert. During this phase, atacamite-bearing
 supergene assemblages were not preserved because atacamite
 requires saline water for its formation and rapidly
 dissolves when contacted by meteoric water. This was
 followed by a second stage starting at ~2–1.5 Ma and
 continuing until at least the late Pleistocene, when deep
 formation waters derived from the basement passed up
 through and modified the pre-existing supergene Cu oxide
 minerals. Atacamite has then been preserved in the
 prevailing hyperarid climate.

FONDECYT grant no. 1070736.