First U-Pb LA-ICP-MS in situ dating of supergene copper mineralization: case study in the Chuquicamata mining district, Atacama Desert, Chile

Since the second half of the twentieth century, exotic copper mineralization represents a prime target for many mining exploration companies operating in the hyperarid Atacama Desert, in northern Chile. Although there is evidence that the emplacement of such deposits took place during specific Tertiary climatic periods and relief formation, many uncertainties remain regarding the exact timing for their deposition and/or the genetic link between the exotic deposits and the primary porphyry copper deposits. We present a first attempt of U-Pb dating of copper-rich minerals from the Mina Sur exotic deposit from the Chuquicamata mining district. A suite of Mn-rich black chrysocolla clasts surrounded by pseudomalachite bands has been characterized and dated in petrographic context using both nanosecond and femtosecond in situ laser ablation ICP-MS analyses. U-Pb dating on pseudomalachite bands yields a crystallization age of 18.4 +/- 1.0 Ma. For the Mn-rich chrysocolla clasts, the Pb-206/U-238 apparent ages range from 19.7 +/- 5.0 Ma to 6.1 +/- 0.3 Ma, a spread interpreted as the result of U and/or Pb mobility linked to fluid circulation following crystallization. This study demonstrates that supergene copper mineralization can be directly dated by the U-Th-Pb method on pseudomalachite. Furthermore, the age obtained on pseudomalachite indicates that Mina Sur copper deposition took place at ca. 19 Ma, about 11 m.y. after the unroofing and hydrothermal alteration of the Chuquicamata deposit, a result that is consistent with the supergene ages already known in the Atacama Desert.