Frances Cooper, University College London. Mineralogical Society Distinguished Lecturer 2024-25.

Описание: Tracking groundwater flow in the hyperarid Central Andes: Insights from copper enrichment

Frances Cooper, University College London. Mineralogical Society Distinguished Lecturer 2024-25.

The Atacama Desert on the western margin of the Central Andes has the dual honour of being one of the driest places on Earth and host to the richest porphyry copper province in the world, accounting for 24% of global copper production. This high concentration of copper is partly due to secondary “supergene” enrichment of many of the deposits, driven by leaching and reprecipitation of copper by oxygenated groundwater during surface weathering. However, the Atacama has been dry since at least the middle Miocene, so how and when did this enrichment take place?

In this talk, I will discuss how the age and composition of hematite left behind by the leaching process can tell us about the history of surface weathering and the sources of the fluids driving enrichment. I will focus on two deposits in northern Chile – Spence and Cerro Colorado – both of which have experienced significant supergene enrichment, but under quite different circumstances. By combining (U-Th-Sm)/He geochronology with δ18O isotopic analysis of hematite in the leached zones of both deposits, I conclude that: (1) continuous hematite precipitation at Cerro Colorado since ~31 Ma reflects a protracted period of weathering, and captures progressive lowering of the water table driven by aridity-induced canyon incision in the late Miocene (~11 Ma); and (2) hematite at Spence precipitated later (≲10.5 Ma) when the climate was already dry, and has a heavy δ18O signature suggesting that weathering was not driven by in-situ precipitation, but from groundwater sourced far away in the high Andes. This means that direct precipitation is not required to drive weathering (and thus supergene enrichment), which can continue at depth beneath appreciable cover even in a hyperarid climate.