A rusty record of weathering and groundwater movement in the hyperarid Central Andes

Joe M Shaw, Laura Evenstar, Frances J Cooper, Byron A Adams, Adrian Boyce, Florian Hofmann, Ken Farley

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Abstract

The Atacama Desert, on the western margin of the Central Andes, hosts some of the world's largest porphyry copper deposits (PCDs). Despite a hyperarid climate, many of these PCDs have undergone secondary “supergene” enrichment, whereby copper has been concentrated via groundwater-driven leaching and reprecipitation, yielding supergene profiles containing valuable records of weathering and landscape evolution. We combine hematite (U-Th-Sm)/He geochronology and oxygen isotope analysis to compare the weathering histories of two Andean PCDs and test the relative importance of climate and tectonics in controlling both enrichment and water table movement. At Cerro Colorado, in the Precordillera, hematite precipitation records prolonged weathering from ∼31 to ∼2 Ma, tracking water table descent following aridity-induced canyon incision from the late Miocene onward. By contrast, hematite at Spence, within the Central Depression, is mostly younger than ∼10.5 Ma, suggesting exhumation ended much later. A heavy oxygen isotopic signature for Spence hematite suggests that upwelling formation water has been an important source of groundwater, accounting for a high modern water table despite persistent hyperaridity, whereas isotopically light hematite at Cerro Colorado formed in the presence of meteoric water. Compared with published paleo-environmental and sedimentological records, our data show that weathering can persist beneath appreciable post-exhumation cover, under hyperarid conditions unconducive to enrichment. The susceptibility of each deposit to aridity-induced water table descent, canyon incision and deep weathering has been controlled by recharge characteristics and morphotectonic setting. Erosional exhumation, rather than aridity-induced water table decay, appears to be more important for the development of supergene enrichment.
Original languageEnglish
Article numbere2021GC009759
JournalGeochemistry, Geophysics, Geosystems
Volume22
Issue number8
Early online date20 Jul 2021
DOIs
Publication statusPublished - Aug 2021

Bibliographical note

Funding Information:
This work was funded by a NERC GW4+ UK DTP Studentship (NE/L002434/1) with CASE support from BHP. Additional funding for the (U‐Th‐Sm)/He work was provided by a Student Research Grant from SEG (SRG18‐104; J. Shaw) and a Royal Society Grant (RG140683; F. Cooper). Isotopic analyses were supported by a NERC Isotope Geosciences Facilities Grant (IP‐1752‐1117; F. Cooper). The authors are grateful to BHP for permitting mine visits, Ed Bunker for sample collection, and Guillerme Santos, Reinaldo Guzman, and Lia Ituarte for advice and logistical assistance. The authors thank Martin Smith for feedback on a draft manuscript and Mark Cuthbert for discussion on water tables. The authors thank Pete Reiners and Carlos Marquardt for their constructive reviews, and Peter van der Beek for his editorial handling.

Publisher Copyright:
© 2021. The Authors.

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