Abstract

Iron oxide-apatite (IOA) deposits, also known as magnetite-apatite or Kiruna-type deposits, are a major source of iron and potentially of rare earth elements and phosphorus. To date, the youngest representative of this group is the Pleistocene (similar to 2 Ma) El Laco deposit, located in the Andean Cordillera of northern Chile. El Laco is considered a unique type of IOA deposit because of its young age and its volcanic-like features. Here we report the occurrence of similarly young IOA-type mineralization hosted within the Laguna del Maule Volcanic Complex, an unusually large and recent silicic volcanic system in the south-central Andes. We combined field observations and aerial drone images with detailed petrographic observations, electron microprobe analysis (EMPA), and Ar-40/Ar-39 dating to characterize the magnetite mineralization-named here Vetas del Maule-hosted within andesites of the now extinct La Zorra volcano (Ar-40/Ar-39 plateau age of 1.013 +/- 0.028 Ma). Five different styles of magnetite mineralization were identified: (1) massive magnetite, (2) pyroxene-actinolite-magnetite veins, (3) magnetite hydrothermal breccias, (4) disseminated magnetite, and (5) pyroxene-actinolite veins with minor magnetite. Field observations and aerial drone imaging, coupled with microtextural and microanalytical data, suggest a predominantly hydrothermal origin for the different types of mineralization. Ar-40/Ar-39 incremental heating of phlogopite associated with the magnetite mineralization yielded a plateau age of 873.6 +/- 30.3 ka, confirming that the emplacement of Vetas del Maule postdated that of the host andesite rocks. Our data support the hypothesis that the magnetite mineralization formed in a volcanic setting from Fe-rich fluids exsolved from a magma at depth. Ultimately, Vetas del Maule provides evidence that volcanic-related IOA mineralization may be more common than previously thought, opening new opportunities of research and exploration for this ore deposit type in active volcanic arcs.