Abstract

AbstractThe 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.

Highlights

  • The Central Andes of northern Chile host many large porphyry copper deposits (PCDs)—hydrothermally generated, sulfide-bearing orebodies centered on felsic to intermediate igneous intrusions (Richards, 2013)

  • We suggest that following the end of exhumation-driven relative water table descent, possibly by ∼30 Ma, and certainly by 19.25 Ma, the water table at Cerro Colorado remained relatively stable, until a reduction in mean annual rainfall (MAR), from ∼130 to

  • We use hematite (U-Th-Sm)/He geochronology and oxygen isotope analysis to compare the timing of weathering and sources of groundwater at two Andean PCDs in different morphotectonic settings—Cerro Colorado, within the Precordillera, and Spence, within the Central Depression

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Summary

Introduction

The Central Andes of northern Chile host many large porphyry copper deposits (PCDs)—hydrothermally generated, sulfide-bearing orebodies centered on felsic to intermediate igneous intrusions (Richards, 2013). For weathering and enrichment to progress, tectonic or climatic factors must trigger relative water table descent for fresh rock to be exposed to oxidation and leaching (Ague & Brimhall, 1989; Alpers & Brimhall, 1988; Anderson, 1982; Brimhall et al, 1985). This could be caused by uplift of rock through the water table (Sillitoe, 2005) (hereon referred to as “exhumation-driven water table descent,” where the water table is the reference datum), or climate-induced canyon incision or reduction in aquifer recharge (Cooper et al, 2016), lowering the water table (hereon referred to as “water table decay,” where the surface is the reference datum). The boundary between the weathered zone and enrichment blanket represents the deepest paleo-water table position—the “ultimate redox front.”

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