REDOX CONTROL ON THE TI-IN MAGNETITE CONTENT: CONSTRAINTS FOR THE ORIGIN OF MAGNETITE APATITE (MTAP) DEPOSITS

We evaluate the use of magnetite geochemistry to trace the origin and evolution of magnetite-apatite (MtAp) deposits. Our results suggest that the large spread in trace element composition of magnetite, the abundance of nanoinclusions, and the great capability of magnetite to re-equilibrate with post-formation metasomatic fluids, cannot be used reliably to distinguish magnetite of magmatic vs. hydrothermal origins.

One of the most intriguing features of MtAp deposits is the relatively low Ti content of the magnetite forming the bulk of the magnetite ore. Magnetite in these deposits on average typically has Ti contents below ca. 0.6 wt. % and often lacks evidence of any ilmenite-ülvospinel exsolution. However, magnetite in some MtAp deposits can be highly enriched in Ti. Magnetite in such deposits often coexists with titanite and/or rutile suggesting that the systems were saturated in titanium and the Ti-in-magnetite content depends on the fO₂-T conditions of the system when the magnetite crystallized.

At El Laco, Chile, systematic sampling of drill core to depths of up to 600 m shows that the massive magnetite ore (which occur primarily as magnetite “lava flows”) – interpreted in the present study as the product of crystallization of an iron-rich melt – the ore is always Ti-poor and chemically similar in Ti content to that of the magnetite feeder dikes, nearby strata-bound magnetite orebodies, and MtAp mineralization elsewhere in the world. However, Ti-rich magnetite is widespread in the metasomatically altered host rocks in drill core from El Laco and can be significantly enriched in the residual melts, where in some cases the stable Fe-Ti-O phases are ilmenite and hematite.

Our results suggest that the main controlling factor in the trace element composition of magnetite is the type of rock in which the magnetite either initially crystallized, or was later re-equilibrated through some post-crystallization metasomatic process. Ti-in-magnetite does not have an unequivocal and unique interpretation and as such any genetic models based solely on the composition of magnetite in MtAp deposits must be evaluated critically. Magnetite geochemistry is, however, a powerful tool for tracking the evolution of igneous and metasomatic systems, and the formation of different generations of magnetite and post-depositional modifications.