Droplets and Bubbles: Solidification of Sulphide-rich Vapour-saturated Orthocumulates in the Norilsk-Talnakh Ni–Cu–PGE Ore-bearing Intrusions

Abstract

A common feature of the mineralised chonolith intrusions of the Norilsk-Talnakh camp is the presence of globular disseminated Ni–Cu–Fe sulphide ores, particularly well developed within the olivine orthocumulate layers of the intrusions, locally referred to as picritic gabbrodolerites. In many cases these globules partially fill subspherical intercumulus spaces within a framework of crystals of olivine and/or plagioclase. The sulphide-free component of these intercumulus spaces is partially or completely infilled with crystallization products of highly fractionated residual trapped liquid derived from the interstitial liquid of the original crystal mush; these intergrowths typically develop as ‘caps’ above the globules. Similar partially-filled cavities are well known within basaltic and komatiitic lava flows and are referred to as ‘segregation vesicles’; they have also been observed as caps above sulfide globules in quenched dolerite dyke margins in other localities. In the NorilskTalnakh intrusions, where the caps are incompletely filled by magmatic material the remaining space is occupied by amygdaloidal linings, which in some cases include anhydrite. These relationships provide evidence that the caps represent original gas bubbles and the close association with the sulphide globules arises from the strong tendency for sulphide droplets to attach to vapour bubbles in magmas. The original outlines of the vapour–bubble pairs are preserved by the rigidity of the surrounding crystal framework, which is interpreted to have developed largely by simultaneous in situ nucleation and growth of olivine and clinopyroxene on the basis of detailed textural observations. Pressure increase during solidification coupled with gas filter pressing forced residual silicate melt into the bubbles, while gas filter pressing and vapour–liquid capillary attraction drove differentiation of the droplets themselves. Whether the globules were deposited already attached to bubbles, or whether the bubbles were generated by volatile saturation within the crystal mush after deposition, cannot be determined definitively, but the geometry of flattened bubble– droplet pairs tends to favour the former option. The distinctive nature of the Norilsk globular ores, along with other unusual aspects of these deposits, is interpreted to be the result of low confining pressures and syn-crystallization degassing, an unusual condition in intrusion-hosted magmatic sulfide ores.