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.

中文翻译：

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干旱中部安第斯山脉风化和地下水运动的生锈记录

阿塔卡马沙漠位于安第斯山脉中部的西部边缘，拥有一些世界上最大的斑岩铜矿床 (PCD)。尽管气候极度干旱，但这些 PCD 中的许多经历了二次“表生”富集，其中铜通过地下水驱动的浸出和再沉淀进行了浓缩，产生了包含风化和景观演变的宝贵记录的表生剖面。我们结合赤铁矿 (U-Th-Sm)/He 年代学和氧同位素分析来比较两个安第斯 PCD 的风化历史，并测试气候和构造在控制富集和地下水位运动方面的相对重要性。在前科迪勒拉山脉的科罗拉多山，赤铁矿降水记录了从 ~31 到 ~2 Ma 的长时间风化，从晚中新世开始追踪干旱引起的峡谷切口后的地下水位下降。相比之下，中央坳陷内的 Spence 赤铁矿的年龄大多小于 10.5 Ma，这表明挖掘结束的时间要晚得多。斯宾塞赤铁矿的重氧同位素特征表明，上升流地层水一直是地下水的重要来源，尽管存在持续的超干旱，但现代地下水位仍然很高，而科罗拉多山的同位素轻赤铁矿是在大气水存在下形成的。与已发表的古环境和沉积学记录相比，我们的数据表明，在不利于富集的超干旱条件下，风化作用可以在明显的挖掘后覆盖层下持续存在。每个矿床对干旱引起的地下水位下降的敏感性，峡谷切口和深层风化受补给特征和地貌构造环境控制。侵蚀性挖掘，而不是干旱引起的地下水位衰减，似乎对表生富集的发展更为重要。