Toward a global conceptual model for metal enrichment in felsic, mafic-ultramafic, and alkaline-carbonatitic magmas

Abstract

The formation of intrusion-related metal ore deposits commences with a combination of sequential metal and melt interactions in the magma reservoir: (i) metal diffusion within the melt; (ii) melt motion; (iii) metal partitioning between silicate melts and a magmatic immiscible phase (MIP, including immiscible melts and/or immiscible vapor phases, and crystalized solids; (iv) metal advection in the melt; (v) chemical interactions between the MIP and the mush; and (vi) finally metal deposition. Occasionally, these processes may also develop simultaneously, and lead to formation of multiple depositional centers. During those events, the presence of the MIP seems preponderant and a prerequisite to metal segregation and transport with the interactive mush. Based on forward and inverse modeling, novel diffusion-partitioning-advection models are proposed to account for economic metal enrichment (three to five orders of magnitude relative to initial concentrations in parental melts). For felsic to intermediate magmas, metals (Cu, Mo, W, Sn) are enriched in a saline aqueous phase by diffusion and metal partitioning. Progressive stagnation of melt motion enhances metal diffusion according to the tortuosity of a developing mush and destabilization of plagioclases. The metal-rich liquids separate from the magma reservoir by advection and transfer the metals to the country rocks. For mafic/ultramafic magmas, the MIP is a dense, sulfide melt, which preferentially sequesters metals (Ni, Cu, platinum group elements). The sulfide droplets interact with olivine to scavenge more Ni before they sink toward the bottom of the magma chamber due to negative buoyance. The down-going flow may even become turbulent according to the melt viscosity, leading to disperse and chaotic ore. In peralkaline magmas, CO2-rich carbonatitic melts are the MIPs formed by segregation of CO2-poor feldspathoid syenite melts, and attracting rare earth elements. Those are selectively partitioned by fenitization. In all the three different types of magmatic system, metals diffuse from viscous evolved parental melts toward a MIP, which is responsible for metal partitioning, segregation and transports. This global conceptual model for metal enrichment focuses on the competition between metal diffusion within the melt, melt viscosity and metal partitioning between the parental melt and MIP. However, a second stage of metal enrichment occurs during interaction of the already enriched melt and the mush. Such conceptual model is a first order model that could suggests newer ideas to ore formation.