ARE MAGNETITE-(APATITE) SYSTEMS RELATED TO CONVENTIONAL SILICATE-RICH INTERMEDIATE MELTS?

Magnetite-(apatite) (MtAp) deposits occur in discrete magmatic and metamorphic belts, in many localities globally and from the Proterozoic to the Pleistocene. They are usually coeval with felsic, and to a lesser degree, intermediate composition magmatism, spanning a wide range of tectonic settings. Excluding nelsonites in anorthosite, and tholeiitic complexes, or related to carbonatites, most MtAp systems are located in active continental margins or in basins with shallow marine or continental sediments. The magnetite-(apatite) deposits of the American Cordillera show a systematic increase in the initial ⁸⁷Sr/⁸⁶Sr ratios of the mineralization when compared with associated silicate igneous rocks. The melt inclusions in the most fractionated pegmatites are enriched in sulfates and carbonates and in many cases contain abundant anhydrite.

These features can be attributed to interaction with a basement made up of shallow marine Paleozoic rocks (e.g., southern Peru), or with more recent Jurassic-Cretaceous sediments (e.g., Utah [Iron Springs], Nevada [Buena Vista]), the Paleocene (e.g., Cerro de Mercado [Durango], Mexico), and the Quaternary (e.g., El Laco, Chile). In the Coastal Cordillera of the Andes this systematic isotopic shift could also be due to the influence of fluids inherited from the subducting slab. Similar geochemical evidence involving sediment-melt interaction has been also traced using triple oxygen isotopes in the Bafq district of Iran.

Interaction of primitive magmas with oxidized sediments enriched in P-F-B or fluids equilibrated with those elements promotes the separation and segregation of immiscible iron-rich melts. Crystallization of large amounts of magnetite promotes the formation of complex iron-poor melts. Melt inclusions in the pegmatites in the last stages of these deposits track the coexistence of different silicate- and carbonate-sulfate melts a complex evolution. Numerical fluid dynamics modelling and widespread geological field evidence show that these mineral assemblages cannot crystallize from a melt/magmatic-hydrothermal fluid in equilibrium with conventional intermediate to felsic melts.