FORMATION OF MAGNETITE-(APATITE) SYSTEMS BY CRYSTALLIZATION OF ULTRAMAFIC IRON-RICH MELTS

Magnetite-(apatite) systems occur in open space fillings in igneous rocks and are dominated by a high temperature (>600°C) mineral assemblage that is dominated by magnetite with variable amounts of pyroxene-amphibole and fluorapatite. Andradite, anhydrite, fluorite, and scapolite can be also present.

These deposits formed from within deep metamorphic zones with widespread anatexis to shallow (sub-)volcanic environments. When not extrusive, these systems show a marked vertical zonation with an enrichment in silicates and apatite that cap and surround a dominant magnetite-rich lower region. Perhaps the most characteristic and diagnostic rock of this mineral system is a pegmatite made up of large crystals of fluorapatite and/or actinolite or pyroxene that probably reflects the magmatic-hydrothermal transition. The systematic presence of P, F or B-bearing minerals is not casual but mirrors the fact that these elements flux iron-rich melts and drop the solidus of magnetite to temperatures that can be attained in the upper crust.

Widespread geological, experimental, and melt inclusion evidence support the existence of ultramafic iron-rich melts. The most evolved of these systems are seriously depleted in silica and alumina (<5 wt%) but enriched in Fe and, in lesser amounts, in Ca, Mg, SO₄, CO₃, P-F-B(?) and some HFSE elements. O, Sr-Nd and Pb isotopes are consistent with an interaction of primitive melts with a sedimentary component. Our interpretation is that the iron-rich melts form when the primitive melts assimilate continental or marine sediments enriched in P, F, B or Fe that could be located beneath the mineralization or even inherited from the subducting slab.

Gradual crystallization of anhydrous magnetite is predicted to enrich the remaining melt in water, promoting the separation and segregation of large amounts of magmatic-hydrothermal fluids. Reaction of this fluid with host rocks is responsible of the formation of silica-poor and Na/K-Ca-Fe-rich assemblages with adularia/albite-actinolite/diopside or magnetite and replacive mineralization of massive hydrothermal magnetite.