PHYSICAL CONTROLS ON SULFIDE MIGRATION IN CRYSTAL MUSHES

The accumulation of sulfide liquid in magmatic systems is a process of fundamental importance in the generation of magmatic sulfide deposits. At low crystal fractions or when sulfide drops are much smaller than spaces between crystals, sulfide droplets fall freely through the silicate melt. At higher crystal fractions, a drop must either remain trapped in the intergranular space or 'pore body', or must deform its surface and squeeze in between the suspended crystals ('pore throat'). For downward migration of sulfide liquid droplets, the buoyancy force resulting from the density difference between the sulfide liquid and the silicate melt must exert enough pressure on the interface to overcome the change in capillary pressure caused when sulfide drops move from relatively large pore bodies into narrower interconnecting pore throats. We find that for crystal mushes containing 60 to 80% solids, sulfide droplets the size of pore bodies must be as large as ~ 3 mm to be able to migrate through pore throats. This condition corresponds to crystal grain sizes on the order of 1 cm. In finer-grained crystal mushes, isolated sulfide droplets as large as pore bodies are unable to migrate. In contrast, in order for a net-textured domain of sulfide to force its way down through a silicate crystal mush containing 65% solids its vertical extent ζ must exceeed 0.35 Δρga/γ where Δρ is the difference in density between the sulfide and silicate melts, g is the acceleration due to gravity, a is the average silicate crystal size, and γ is the sulfide-silicate melt interfacial tension. ζ increases rapidly with decreasing crystal size for small values of a; for example, when a = 1 mm, ζ = 1 m, whereas for crystals with a > ~6 mm, ζ changes little and maintains a value of ~ 1 cm. Consideration of these relationships indicates that sulfide melt can migrate through silicate crystal mushes only if the sulfide droplets are very small, if the silicate crystals are very large, or if the sulfide has accumulated to a net texture with interconnected sulfide domains considerably larger than the crystal grain size. Formation of the net texture might be accomplished by accumulation of very small individual droplets into large masses, but intermediate-sized droplets are immobile and cannot coalesce.