Geology and mineralogy of sapphire-rich metasomatites (sapphirites) deposit at the Portezuelo de Pajas Blancas, northern Chile: Genetic implications for unusual metasomatic processes in the Central Andes

Abstract

The Portezuelo de Pajas Blancas deposit, located in northern Chile, represents a unique opportunity to constrain unusual geological processes involved in the genesis of sapphire-rich metasomatites in the central Andes. For this purpose, geological mapping was combined with conventional and high-resolution petrographic and mineralogical analysis (light microscope, SEM-EDS and automated mineralogy). Results demonstrate that this uncommon occurrence is genetically related to eight main lithological units that host variable proportions of corundum (sapphire), sillimanite, andalusite, diaspore, böhmite, dravite, magnesiofoitite, olenite, alunite, and calcite. Microtextural relationships between mineralogical assemblages demonstrate that the origin of this deposit was associated with two main genetic stages. The early stage is linked to the initial crystallisation of abundant sapphire, promoted by an extreme desilication during high-temperature prograde metasomatic processes due to the intrusion of a Cretaceous granodioritic magma body (Si-Al-rich) into basaltic rocks from the La Negra Formation (Al-rich Si-poor) through the Atacama Fault System. The late-stage corresponds to hydrothermal alteration of sapphires to böhmite and diaspore in the deep and shallow zones, respectively, where dravite and alunite were precipitated directly from these fluids. The formation of this late hydrothermal assemblage can be interpreted as an overprinting of a metasomatic retrograde system. This study demonstrates that the sapphire-bearing rocks at Portezuelo de Pajas Blancas correspond to primary metasomatic sapphirites related to middle- to high-temperature contact metasomatism at estimated pressures of ∼ 2 kbar.