U-Pb dating of chrysocolla from supergene copper deposits in the Coastal Cordillera of northern Chile, Atacama Desert

Abstract. The dating of supergene copper minerals has been widely used as a proxy to investigate the evolution and onset of hyperaridity in the Atacama Desert. However, investigation of supergene copper mineralisation in the Atacama Desert has been restricted to two physiographic units favourable for the industrial extraction of copper: the Central Depression and the Precordillera. Furthermore, these studies dated the timing of supergene mineralisation by secondary non-copper minerals like alunite. In this study, we present new results of LA-ICP-MS U-Pb dating of chrysocolla from supergene deposits hosted in the western part of the Coastal Cordillera of northern Chile. The obtained U-Pb ages range from 8.0±1.2 to 0.045±0.027 Ma. Supergene mineralisation ages point to significantly reduced precipitation, necessary for leaching and mineral precipitation process, since the Late Miocene to Pleistocene in the Coastal Cordillera, later than the secondary supergene mineralisation ages from the Precordillera. The data point to repeated phases of sufficient moisture along the Coastal Cordillera that promoted chrysocolla mineralisation during the Pliocene and Pleistocene. We propose that due to the position of the study areas near the coastal escarpment, and the predominant hyperarid environment in this part of the Coastal Cordillera since at least the Mid-Miocene, pluvial periods and/or intensification of coastal fog events caused alternating phases of supergene activity.