Magma mixing and sulfide production in the lower crust: insights into arc metallogenesis

The location and distribution of metal sources for felsic-magma related ore deposits in continental and island arcs (porphyry Cu-Au-Mo, epithermal Au-Ag, and skarn) is contentious. Crustal and mantle sources may both contribute to the metal budgets of these types of deposits, with a chemical signature that is set early. Mixing between mantle-sourced magmas and crustal partial melts in Mixing, Assimilation, Storage, and Homogenisation (MASH) zones, at or near the base of the Earth’s crust, is a possible mechanism to generate fertile magmas for ore development. Magma mixing of this sort may have an important role in sulfur solubility and thus sulfide mineral stability. Sulfide minerals partition Cu, Au, Ag, Mo, platinum group elements, and other chalcophile metals, which are key components in felsic magma-related ore deposits. This study investigates the role of magma mixing in the production of sulfide melts in the lower crust and the role that these sulfide melts have in ore generation in the upper crust. Geochemical data from samples collected in the lower crustal Opirarukaomappu Gabbroic Complex (OGC), southeastern Hokkaido, Japan suggest that sulfide occurrences are associated with magma compositions produced by mixing ~80% gabbro with ~20% tonalite. High temperature, high pressure piston cylinder experiments are used to simulate this mixing and the consequent saturation and exsolution of sulfide melt. A new, redox-controlled, model for sulfide saturation, called the “sulfur fence”, describes a sudden reduction in sulfur solubility caused by the mixing of oxidised, sulfate-rich magmas with magmas containing a strongly reducing component (i.e. graphite). In this model, reduction from sulfate-stable to sulfide-stable decreases sulfur solubility by an order of magnitude (from ~1 wt. % to ~0.1 wt. %). Resulting large scale sulfur oversaturation may allow the generation of pervasive and voluminous sulfide melts in mixing magmas. The greater density of sulfide melts causes them to settle through lower density silicate magmas. Small globules (<0.1 cm radius) are able to be entrained in migrating liquidus magmas of OGC compositions. However, to entrain sulfide globules similar in size to those observed in the OGC (~0.1 cm – 0.25 cm radius), about 20% w/v crystallisation of these magmas must occur. Controlling the settling out of sulfide melts generated during mixing may determine the deposit type generated in MASH zone-affected arc settings. Felsic magma-related ore deposits may occur in arcs where sulfide saturation is prevented both during mixing of magmas at depth and during migration from their source regions to emplacement. These types of deposits may also develop from magmas that are able to continuously entrain sulfides after sulfide saturation. Conversely, sulfide saturation, followed by settling out of the sulfide melt, may result in the accumulation of massive sulfides and the formation of magmatic Ni-Cu deposits. Hence, felsic magma-related ore deposits and massive sulfide deposits may represent endmembers of an ore deposit spectrum: porphyry Cu-Au-Mo, epithermal Au-Ag, and skarn deposits where sulfides were not generated or were kept entrained when they were; magmatic Ni-Cu where sulfides were generated and settled out.