The role of magma degassing in sulphide melt mobility and metal enrichment

International audienceA strong association between sulfide droplets and gas bubbles has been observed in both experimental and natural samples (e.g. the picrogabbrodolerites of the Norilsk-Talnakh ore-bearing intrusions), suggesting that the fluid phase may have a major control on the segregation and mobility of the sulfide melt. Here we show how the association between sulfide melt and gas bubbles during magma degassing can have consequences for the accumulation of the sulfide liquid but also for its metal enrichment. Two types of experiments are presented involving magma degassing in sulfide-bearing mafic magmas at 1200°C and variable pressure: experiments simulating the interaction of the magma with coal, and experiments simulating magma decompression. Despite a marked difference in the composition of the fluid phase of the two types of experiments, the sulfide melt and the fluid phase are always associated, with each gas bubble being generally connected to one or several sulfide droplets. By facilitating the coalesce of the sulfide droplets attached to the same gas bubble, the fluid phase therefore seems to play a crucial role on the accumulation of the sulfide liquid. Large sulfide blebs are produced by this process in experimental samples, suggesting that magma degassing can be a potentially critical mechanism for sulfide mobility and accumulation in magmas. Moreover, magma degassing can also play a role on metal content of the sulfide melt: metal enrichment of the sulfide droplets is observed in the degassed samples, due to the consumption of the sulfide melt consequent upon magma degassing. In experimental samples, limited degassing produces metal-enriched sulfide melts, while extensive degassing generates PGMs. The possible implication of this process in the formation of ore-deposits and particularly in those of Norilsk-Talnakh is discussed, with a particular attention to the disseminated sulfides in the picrogabbrodolerites and the S-poor, PGE-rich ores