The role of the subducting slab and melt crystallization in the formation of magnetite-(apatite) systems, Coastal Cordillera of Chile

• Published in: Mineralium deposita Vol. 56; no. 2; pp. 253 - 278
• Main Authors: Tornos, Fernando, Hanchar, John M., Munizaga, Rodrigo, Velasco, Francisco, Galindo, Carmen
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer Berlin Heidelberg 01-02-2021; Springer Nature B.V
• Subjects: Apatite; Ascent; Breccia; Computational fluid dynamics; Crystallization; Dehydration; Deposits; Diorite; Earth and Environmental Science; Earth Sciences; Fluids; Genetic relationship; Geology; Hydrothermal alteration; Hydrothermal systems; Ilmenite; Iron; Jurassic; Lava; Magnetite; Melts (crystal growth); Mesozoic; Mineral Resources; Mineralization; Mineralogy; Oceanic crust; Pegmatite; Rock; Rocks; Strontium 87; Strontium isotopes; Subduction (geology); Veins (geology); Chile; Andes; Chile; Magnetite-(apatite) deposits; Isotope geochemistry; Atacama Fault System
• ISSN: 0026-4598; 1432-1866
• DOI: 10.1007/s00126-020-00959-9

The Mesozoic magnetite-(apatite) deposits of the Coastal Cordillera of Chile are interpreted as the product of the crystallization of oxidized iron-rich melts and subsequent hydrothermal alteration produced by related magmatic-hydrothermal systems. These deposits form a regional-scale mineral system controlled by the Atacama Fault System and where the mineralization spans more than 10 km in vertical extent. Individual sub-vertical bodies of massive magnetite coexist with and evolve vertically into pegmatite-, breccia-, and vein-like apatite-actinolite-magnetite/ilmenite rock. The mineralization is always hosted by a hydrothermal aureole of alkali-calcic-iron alteration that includes stockwork-like to disseminated mineralization. The deposits cluster in two groups. Those located in the northern part are mostly vein-like, and are hosted by Jurassic diorite. They have
87
Sr/
86
Sr
i
and εNd
i
values of 0.7042–0.7062 and + 5.1 to + 7.2, respectively. The southern group includes shallowly emplaced ore lenses in broadly coeval (sub-)volcanic intermediate rocks. They show similar
87
Sr/
86
Sr
i
signatures (0.7033–0.7065, with one value up to 0.7097) and more variable εNd
i
values (+ 3.9 to + 8.6). As a whole, the Sr-Nd data do not seem to be influenced by the type of crust intruded, but rather, likely track the mixing between a MORB-like reservoir and another source with elevated
87
Sr/
86
Sr
i
(≥ 0.706). The genetic model proposed involves the dehydration of variably altered subducted oceanic crust, the interaction of fluids released from the mantle wedge, the separation of iron-rich melts, and their ascent along transcrustal faults. The broadly coeval intermediate host rocks show a lesser contribution of subducted crust, something that perhaps excludes a genetic relationship between these rocks and the magnetite-(apatite) mineralization.