Using weathering and alteration minerals to constrain water table movement and supergene enrichment of porphyry copper deposits in the Central Andes

Abstract

The Atacama Desert of northern Chile hosts many of the world's largest porphyry copper deposits (PCDs), many of which have been upgraded through supergene enrichment (Sillitoe and McKee 1996). Enrichment of exhumed PCDs occurs in the near-surface weathering environment, when copper is leached from primary sulphides and reprecipitated beneath the water table to form a concentrated blanket of secondary copper minerals (e.g. chalcocite). The Atacama is one of the driest regions on Earth, but supergene enrichment requires precipitation to drive the aqueous redistribution of metals. Previous alunite dating studies have suggested enrichment stopped in the middle Miocene, due to an increase in aridity. This conclusion is supported by the youngest supergene alunite ages from several PCDs clustering around 14 Ma (Sillitoe 2005). Hematite also forms in the leached caps of PCDs and hematite (U-Th)/He geochronology provides a tool to track the downward progression of weathering fronts (e.g. Cooper et al. 2016). This study will combine Ar-40/Ar-39 dating of supergene alunite (a proxy for the timing of copper enrichment) with hematite geochronology (as an indicator of the progression of weathering) to better understand the link between water table movement and supergene enrichment of Central Andean PCDs.