Mineralogical and chemical characterization of supergene copper-bearing minerals: Examples from Chile and Burkina Faso

Using optical microscopy, SEM, EPMA and LA-ICP-MS, we analyzed and characterized the textural features of Cenozoic supergene Cu-bearing minerals from three exotic and two in situ supergene copper mineralization from the Atacama Desert in northern Chile. In addition, we analyzed their major and rare earth elements compositions. We then compared these data to those obtained from the in situ supergene copper mineralization from the Gaoua Cu-Au porphyry district, emplaced during the Cenozoic in a different geodynamic setting in the West African craton. In both the in situ and exotic supergene copper mineralization, chrysocolla is the dominant supergene copper-bearing mineral, followed by pseudomalachite with minor amount of copper wad. Chrysocolla and pseudomalachite show distinct textural features. Chrysocolla appears either as black Mn-rich clasts or lightblue to green masses, filling the fractures and coating the non-mineralized clasts. Pseudomalachite occurs as green color bands or thin coatings filling empty spaces. All the deposits share some common features with regard to their major element and REE compositions, i.e. i) same range of chemical compositions suggesting similar conditions of formation and ii) strong Ce anomaly indicative of oxidant conditions during the crystallization of these supergene copper minerals. Our results reflect similar conditions for the formation of both supergene copper minerals in all the mining districts and lead us to propose that both areas (i.e. the Atacama Desert and southwestern Burkina Faso) underwent similar geological and climatic conditions in order to form and preserve supergene copper mineralization, i.e. exhumation of the porphyry copper deposit and weathering of the primary copper sulfides, downward and laterally moving of copper-bearing solutions to form in situ and exotic SCM and finally, arid to hyperarid climate to prevent mechanical abrasion and leaching of the newly formed supergene copper mineralization.