A Review of the Behaviour of Platinum Group Elements within Natural Magmatic Sulfide Ore Systems

Abstract

The largest and most significant type of geological deposit of platinum group elements (PGEs) is that associated with magmatic base metal sulfide minerals in layered mafic or ultramafic igneous intrusions. The common association of PGEs with sulfide minerals is a result of processes of magmatic and sulfide liquid segregation and fractionation. The mineralogical nature of the ores is dependent on a number of factors during sulfide liquid fractionation. The most significant of these with regard to the mineralogy of the two most important metals, platinum and palladium, is the presence and concentration of semimetals such as bismuth and tellurium within the mineralising sulfide liquid. Whereas rhodium, iridium, osmium and ruthenium are almost always present in solid solution within the resultant base metal sulfide minerals; should sufficient semimetals be present, Pd and especially Pt will form discrete minerals (such as platinum bismuthides) around the margins of, and possibly away from, the sulfides.

Platinum-group elements

“…Textural and experimental evidence indicate that PGE-enriched mineralization forms when mafic to ultramafic magma becomes saturated with sulfur and an immiscible sulfide liquid exsolves from the silicate magma (Barnes and others, 2008;Holwell and McDonald, 2010;Naldrett, 2010a). The solubility of sulfur in mafic magmas is affected by changes in the bulk composition of the magma, the fugacity of sulfur and oxygen, temperature, and pressure (Ripley, 1999).…”

“…recrystallization after solidification (Barnes and others, 2008;Holwell and McDonald, 2010). At high temperature (about 1,000 °C), osmium, iridium, and ruthenium preferentially partition from the immiscible sulfide liquid into a mineral called monosulfide solid solution (MSS).…”

Paradoxical co-existing base metal sulphides in the mantle: The multi-event record preserved in Loch Roag peridotite xenoliths, North Atlantic Craton

“…6c-d) suggests they exsolved during cooling and fractionation of the sulphide droplets. Arsenic, Te, Bi and Sb levels are systematically low in both BMS groups (Table 3) indicating that semi-metals were present at insufficient concentrations to scavenge Pt (or PGE in general) into a late-stage semimetal-rich immiscible liquid (e.g., enriched in Te, Bi, As and Sb) to exsolve as Pt-Bi-Te-Sb-As PGM (Helmy et al, 2007;Holwell and McDonald 2010). The absence of Te-bearing microphases and the lack of any correlation between Te and elevated Cu or Pd concentrations (c.f.…”

Extreme enrichment of Se, Te, PGE and Au in Cu sulfide microdroplets: evidence from LA-ICP-MS analysis of sulfides in the Skaergaard Intrusion, east Greenland

“…Microinclusions of other minerals are recorded as small spikes in individual elements across the TRA profile. Typically, in magmatic sulfide minerals, microinclusions are commonly PGM such as PGEbearing tellurides, bismuthides and arsenides (e.g., Holwell and McDonald 2007), which form due to the tendency for telluride, bismuthide, arsenide melts to separate from the sulfide liquid on cooling, sequestering PGE and forming PGM (e.g., Helmy et al 2010Helmy et al , 2013aHolwell and McDonald 2010).…”

“…4e, j). The behavior of Te, Bi, As and Sb contrasts with that of Se which remains in solid solution in sulfide, by generally fractionating into immiscible telluride/bismuthide/arsenide and possibly antimonide melts on cooling and crystallization of a sulfide liquid (Helmy et al 2010(Helmy et al , 2013aHolwell and McDonald 2010;Prichard et al 2013). Furthermore, these melts sequester precious metals more effectively than sulfide melt (Helmy et al 2010).…”