Advanced Argillic Lithocaps Above Chilean Iron Oxide-Apatite Deposits

Abstract Advanced argillic lithocaps, defined by zoned residual quartz with silicification, alunite, pyrophyllite, dickite, and kaolinite, are recognized above and/or alongside five major iron oxide-apatite (IOA) deposits in the iron belt of coastal northern Chile. The IOA deposits are centered on massive magnetite bodies formed along the transtensional Atacama fault system during Lower Cretaceous dioritic magmatism. Subsequent erosional degradation reduced the size of the lithocaps as well as removed those that are thought to have formerly overlain other magnetite deposits in the belt. The IOA lithocaps closely resemble those documented from the shallow parts of porphyry copper and subvolcanic tin systems and must have formed by condensation and absorption of large volumes of magmatic vapor into groundwater aquifers. The presence of these lithocaps has important genetic implications: They require involvement of magmatic vapor and not solely hypersaline brine in IOA genesis; imply formation of the massive magnetite bodies at paleodepths of less than approximately 4 km; and render it unlikely that IOA deposits transition upward to either iron oxide-copper-gold (IOCG) deposits or pegmatitic breccias and veins. There are also important exploration implications: IOA deposits elsewhere in the iron belt could be concealed beneath lithocaps, which themselves could contain elements of economic value.