Proton microprobe results for the partitioning of platinum-group elements between monosulphide solid solution and sulphide liquid

Abstract

Partition coefficients (D) for Ni, Cu, and platinum-group elements (PGE) between monosulphide solid solution (mss) and Fe-sulphide liquid (liq) have been determined experimentally using an electron microprobe (EMP) to analyze experimental run products. The EMP detection limit is approximately 0.05 weight per cent for the PGE, consequently few results were obtained for Pt and Ir and the precision for Pd and Rh at low concentrations was poor. These run products have been reanalyzed using a proton microprobe (PMP), which has a detection limit between 10 and 50ppm for these elements.It is now clear that D mss/liquid for all the elements show a strong dependence on the S content of the run in S-undersaturated and S-saturated runs. However, in S-oversaturated runs the S content of the run does not appear to influence D mss/liq . The greater precision of the PMP data establishes that in S-oversaturated runs D mss/liq at 1000°C are consistently higher than those at 1100°C. In contrast, D mss/liq in the S-undersaturated and S-saturated runs are similar at both temperatures. This difference in behaviour is thought to arise because in the S-undersaturated and S-saturated runs the amount of S in the mss is controlled by the S content of the run. As the S content in the mss increases, the number of vacancies in the structure of the mss also increases, and D mss/liq rises. In contrast, in S-oversaturated runs the mss has absorbed the maximum amount of S and thus the S content of the run no longer influences the structure of the mss and hence does not control D mss/liq . Thus, the effect of temperature on D mss/liq only becomes apparent in the S-oversaturated runs. The tendency for Os, Ir, Ru and Rh to partition into mss and the exclusion of Cu, Pt and Pd from mss maybe used to explain a number of phenomena; the zonation of massive sulphide bodies, the tendency for Os, Ir, Ru and to a lesser extent Rh to be enriched in cumulates with minor sulphides, and the presence of two types of sulphides in mantle nodules (an Os-Ir -rich mss and Cu-Pd rich pentlandite).The tendency of sulphide liquids to crystallize RuOsIr and Pt-Fe minerals at low f S2 may explain the enrichment of RuOsIr in ultramafic mafic cumulate rocks in the following manner. Sulphide solubility increases as pressures falls. Thus, sulphide droplets in rising basalt magma could be partly resorbed. The PGM could crystallize from this liquid. If these PGM survive long enough, then they could be incorporated into the early cumulate phases such as olivine and chromite. This would explain both the presence of PGM in many olivine and chromite cumulates and the tendency of more evolved magmas to have high Pd/Ir ratios.

Crystallization of magmatic sulfides: An empirical model and application to Sudbury ores

“…Although extensive early experimental work in binary and ternary subsystems of the Ni-Cu-Fe-S-O system established the basic topology of most of the important phase boundaries (Craig and Kullerud, 1969, and references therein;Naldrett, 1969), the complexity of the five-component system complicates simple graphical approaches to understanding the phase equilibria, and recent attention has been focussed on the measurement of coexisting phase compositions in quenching experiments (Fleet et al, 1993;Fleet and Pan, 1994;Ballhaus et al, 2001;Li et al, 1996;Naldrett, 1996, 1997;Kosyakov and Sinyakova, 2005). Several other studies have been conducted in various systems including some of the components Ni, Cu, Fe, S and O in addition to other major element components, especially the PGE (Fleet and Stone, 1991;Barnes et al, 2001;Sinyakova et al, 1996;Sugaki and Kitakaze, 1998;Makovicky, 2002;Peregoedova and Ohnenstetter, 2002). Attempts to apply thermodynamic solution models have been limited to superliquidus conditions in the system Fe-S-O (Kress, 1997).…”

Problems of PGE metallogenesis related to mafic–ultramafic complexes in North Xinjiang, China

“…In crystallization differentiation, the distribution coefficients of PGE between the single-sulfide solid solution (solid phase) and residual sulfide melt (liquid) are Ir (3.4e11) > Os (4.3) > Ru (4.2) > Rh (1.17e3.03) > Pt (0.05e0.2) > Pd (0.09e0.2) (Fleet et al, 1999;Barnes et al, 2001). Therefore, Os, Ir, Ru, and Rh preferentially enter single-sulfide solid solutions, but Pt and Pd have priority to remain in the residual sulfide melt under sulfur-saturated conditions (as a segregation process).…”

Platinum-group element (PGE) deposits and occurrences: Mineralization styles, genetic concepts, and exploration criteria

“…The sulfides will all melt and thus be concentrated by a factor of 10 in the magma, yielding 2000 ppm sulfur. This significantly exceeds the sulfur solubility of the magma and it is usually assumed that in this case some dense sulfide melt remains in the mantle during basalt extraction (but note that Barnes et al, 2001, have examined the possibility that the sulfides are entrained in the basaltic magma extracted from the mantle). As the PGE have high partition coefficients (D values) with regard to the sulfide melt (ca 10,000, Barnes and Maier, 1999 and references therein) the PGE would largely remain in the mantle and the basaltic magma would be PGE poor.…”

“…This will eventually drive a sulfur-undersaturated magma to sulfur saturation. (ii) As sulfur is bonded to Fe 2+ in the magma (Haughton et al, 1974;Shima and Naldrett, 1975), fractionation of Fe-rich minerals, notably olivine, pyroxenes, chromite and magnetite causes a decrease in sulfur solubility that may lead to sulfur saturation (Haughton et al, 1974;Li et al, 2001). The appearance of cumulus magnetite in layered intrusions often coincides with a sharp increase in sulfide contents (see Maier et al, 2003b and references therein).…”

“…Further, an increase in oxygen fugacity due to contamination may result in chromite and magnetite precipitation and/or a lowering of the FeO content and thus the sulfur carrying capacity of the magma (Haughton et al, 1974;Buchanan and Nolan, 1979). (iv) It has been suggested that mixing of compositionally contrasting, sulfur undersaturated, magmas could trigger sulfur supersaturation (e.g., Naldrett and von Li et al, 2001). Cawthorn (2002) recalculated the available data and found that hybridization of sulfur undersaturated Mg-basaltic Bushveld magmas with resident magmas of variable composition does not result in sulfur supersaturation.…”