U-Pb LA-ICP-MS in situ dating of chrysocolla in copper deposits in Coastal Cordillera, northern Chile

Abstract

The study of supergene minerals has been used as a proxy to unravel the palaeoclimatic conditions that prevailed when it occurred. The youngest age of supergene mineralisation is interpreted as the last time with sufficient moisture; therefore, it will reflect the transition from semi-arid towards hyperarid conditions. The history of dating supergene minerals, mainly alunite and far less common copper-bearing minerals such as atacamite and pseudomalachite, in the Atacama Desert is mainly restricted to the Central Depression and Precordillera indicating that supergene processes were active from 44 to 6 Ma. In contrast, there are only four ages in the Coastal Cordillera, one reported by Sillitoe & McKee, 1996, and three obtained by Reich et al., 2009. This lack of information makes it impossible to constrain the onset of hyperaridity in the Coastal Cordillera and how it relates with the previously mentioned physiographic units. The Coastal Cordillera in northern Chile correspond to a Jurassic-Early Cretaceous magmatic arc consisting mainly of andesites and basaltic andesites intruded by numerous plutonic bodies. It hosts the metallogenic belt with the largest number of mineral deposits in the Antofagasta Region, the majority of which are copper deposits. We test for the first time, the potential of the LA-ICP-MS in situ U-Pb technique to date the deposition of the copper deposits in the Coastal Cordillera and use it as a new proxy to understand its palaeoclimatic evolution. For this purpose, we selected chrysocolla samples from manto- and vein-type deposits hosted in the west side of the Coastal Cordillera. Chrysocolla is an amorphous hydrated copper silicate that precipitate from gel-like material. Furthermore, the chrysocolla may occurs as a replacement of other copper minerals such as malachite and atacamite. Macroscopically it is possible found it in crust, in veins filling cracks along the host rock or in amygdales of andesites. The second most common mineral is atacamite which presents a complex textural relationship with chrysocolla. The amorphous structure of chrysocolla makes difficult that the U and Pb concentrations will be consistent along the same sample. Due to this, we apply a pre-scan with the laser to measure the U-Pb ratio in the sample. After that, the laser spots were defined in tree main areas: highest, intermediate and lowest U-Pb ratio to be secure that we will get a dispersion of the ratios that can help to obtain an isochron. The preliminary results show that the chrysocolla is extremely young (<1 Ma). The oldest age obtained is 7 Ma. It is possible that these ages are the consequence of a reset of the system. We need to consider the possibility that the amorphous structure of chrysocolla allows U- and Pb-loss that can give a younger age than the real deposition age.