Characterization of the Element Redistribution Between Fresh-Altered Pairs of Rocks Associated with Iron-Oxide-Cuau Mineralization Near Copiapó, Chile

Abstract

The general alteration patterns associated with Iron-Oxide-Copper-Gold (IOCG) deposits have been well characterized in the past, but the level of description has often focused on broad characteristics. This study aims to examine alteration on a more detailed mineralogical scale in rocks that have undergone hydrothermal sodic-calcic (Na-Ca) alteration. Four pairs of sample were selected, each with a fresh and altered version of the host rock in the region. These samples span the plutonic rock types present in the Chilean Coastal Cordillera near Copiapό, Chile. These sample were analyzed using petrography to determine mineral assemblages and relative timing, as well as electron microprobe-based X-ray mapping and individual mineral analyses. Mg-rich amphiboles replacing Fe-rich amphiboles and pyroxenes occurs profusely in Na(-Ca) regimes while if the Fe content of the amphibole is retained, much of that amphibole can recrystallize into a more K and Cl rich assemblage. Igneous plagioclase is calcic and is replaced either by more Na-rich compositions or by marialitic scapolite. Fe-rich oxides were often recrystallized as titanite. The micas within the fresh samples was also converted, primarily, to amphiboles and chlorites with much of the K in the system leached from the system during the alteration events.