Three-phase flow modelling of immiscible melt segregation in the genesis of magnetite-apatite deposits

Summary Magnetite-apatite (MtAp) deposits have attracted considerable attention due to their complex genesis and economic importance. These deposits are rich in magnetite ore and can bear significant rare-earth elements, but their exact formation mechanisms remain controversial. This study aims to understand the formation processes of MtAp deposits by investigating the role of iron-rich magmatic liquids. Focusing on the El Laco deposit, northern Chile, we follow the hypothesis that iron-rich liquids separate from silicate magma through liquid immiscibility. Building on previous research, this study employs a three-phase one-dimensional (1D) mechanical model to simulate the separation and accumulation of immiscible iron-rich melts within increasingly crystalline magma. The model reproduces the previously suggested transition from isolated droplet settling to an interconnected drainage network and quantifies the relative efficiency of both modes of phase separation. Using scaling analysis, we define porous, mush, and suspension flow regimes and construct a regime diagram for three-phase flow. The results indicate that the separation efficiency of immiscible iron-rich melts is maximised in the mush flow regime at intermediate crystallinity. The model-derived accumulation rate of iron-rich melts can be used to estimate the time required to form magnetite deposits of a given scale. Our findings support the physical viability of the liquid immiscibility hypothesis for the genesis of MtAp deposits, offering new insights into the mechanical efficiency of melt separation and contributing to a broader understanding of the formation mechanisms of other valuable deposits that have been linked to immiscible melts.