The incompatible behavior of gold in reduced magmas: a working hypothesis

Gold seems to show contrasting behaviours during magmatic evolution according to the fO₂ of the melts. In more oxidized conditions, characterized by the presence of magnetite as the main Fe-Ti oxide, gold content in plutonic rocks decreases with increasing differentiation. On the other hand, in more reduced conditions, characterized by the predominance of ilmenite, gold content in plutonic rocks increases with increasing differentiation.
The mineral structures of the intervening minerals (magnetite and ilmenite) are compared with the possible different gold ions (aurous, Aut; and auric, Au3+). It is hypothesized that under more oxidizing conditions auric gold is present, which is easily incorporated in the crystallizing magnetite due to its similar size to ferrous and, to a lesser degree, ferric iron; magnetite effectively acting as a sink for the gold and causing its early removal from the melt. Under more reducing conditions, aurous gold is the only gold cation present and its larger size prevents it from being incorporated into the ilmenite (and possibly magnetite). Therefore gold behaves as an incompatible element and tends to concentrate into the late fluids, possibly leading to gold mineralization related to highly evolved fluids.