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