THE MORPHOLOGY AND DISTRIBUTION OF SULFIDE BLEBS IN KOMATIITIC OLIVINE CUMULATES, AND IMPLICATIONS FOR ORE GENESIS

The morphology and distribution of magmatic sulfide aggregates or “blebs” in igneous rocks is governed in part by their wetting properties against silicate phases such as olivines, a property which is manifest in the 3D morphology of the blebs. This is revealed by 3D computed X-ray tomographic images of Ni-rich magmatic sulfide blebs in some komatiitic olivine-rich cumulates. Blebs in pure olivine-sulfide adcumulates form interconnected frameworks along olivine triple boundaries at sulfide abundances of less than 4 volume %, with dihedral wetting angles greater than 60 degrees. The connectivity decreases dramatically in orthocumulates, where an intercumulus silicate liquid component is present. Sulfide blebs in orthocumulates tend not to wet olivine crystals but rather to form isolated sub-spherical blebs. This implies that the mobility of sulfide liquid droplets is low in olivine-dominated crystal mushes. Sulfide-rich ores are unlikely to form by accumulation of originally disseminated sulfides within porous crystal piles. This conclusion has implications for the origin of Mt Keith-type (Type 2) disseminated nickel sulfide ores. These ores typically have a relatively uniform sulfide abundance of around 2-3 weight % on a ten meter scale, but this proportion is much more heterogeneous on a centimeter scale. Ores typically contain domains a few cm across of interconnected sulfide, separated at sharp interfaces by sulfide-poor intervals with coarser olivine grain size and small disconnected sulfide blebs. This distribution is unlikely to be due to post-cumulus migration of sulfide liquid through the limited intercumulus porosity. The observed textures are more likely to be the result of in-situ accumulation at the base of the flowing magma. The overall abundance of sulfide is several times higher then would be expected for purely cotectic precipitation of olivine and sulfide from a magma saturated in both phases, and hence much of the sulfide must have been mechanically deposited. The size of the sulfide-rich domains in Mt Keith ores indicates that the size of transported sulfide droplets in komatiite magmas was of the order of mm to cm. Our observations support experimentally-based predictions that sulfide liquid droplets would not be entrained by silicate melt during partial melting of the mantle.