William E. Stone

Partitioning of palladium, iridium, and platinum between sulfide liquid and basalt melt: Effects of melt composition, concentration, and oxygen fugacity

Abstract The partitioning of Pd, Ir, and Pt between immiscible (Fe, Ni)-monosulfide liquid and basalt melt has been investigated at 1300°C and at low pressure over the concentration range 40 to 20,000 ppm platinum-group element(s) (PGE) in the sulfide liquid and at oxygen fugacities from the C-CO-CO2 to the wustite-magnetite buffers. The experiments used sealed silica-glass tubes with internal oxygen buffers: PGE in glass were determined by radiochemical neutron activation analysis (RNAA). Partition coefficients (D) vary markedly with compositions of the sulfide liquid and silicate melt, increasing with decrease in oxygen fugacity, S, Fe, and possibly Ni, and with increase in total concentration of PGE. For 5 ppb PGE in the silicate melt and the iron-silica phase-fayalite (IQF) buffer, D(Pd) and D(Pt) are about 2 × 103, and D(Ir) is about 3 × 103; whereas, at the maximum concentration of PGE investigated, D(Pd) and D(Pt) are about 2 × 104, and D(Ir) is about 3 × 104. A single experiment confirms the marked fractionation of Pt from Pd predicted for partitioning with alloy in S-bearing and S-saturated silicate melts. The experimental D(PGE) values for low concentration of PGE are similar to D(PGE) calculated for many sulfide ore deposits, but are several orders of magnitude lower than calculated values for concordant sulfide PGE deposits in layered complexes.

Partitioning of platinum-group elements in the Fe-Ni-S system and their fractionation in nature

Abstract The partitioning of minor amounts of platinum-group elements (PGEs: Ru, Rh, Pd, Os, Ir, and Pt) has been investigated in the Fe-Ni-S system at 1000 to 1400°C and low pressure. Revising previous experimental results indicating lack of specificity of PGE, distinctly different partitioning behavior is demonstrated for heavy and light platinoids; Os, Ir, and Pt are preferentially concentrated in alloy phases, whereas Pd is depleted in alloys and relatively enriched in sulfide liquid. Partition coefficients for Fe alloy/sulfide liquid at 1000°C are 1 to 2 for Pd (comparable to values for Ni), 30 to 110 for Rh, and in excess of 1000 for Os, Ir, and Pt. Ruthenium and, to a lesser extent, rhodium are the only PGEs with significant solubility in troilite coexisting with sulfide liquid. The marked fractionation of Pt from Pd in the presence of coexisting alloy and sulfide liquid is contrasted with the minimal fractionation of PGEs in the primary differentiation of planetary material and of Pt from Pd in the generation of basic magmas in the upper mantle. The characteristic fractionation patterns for PGEs in upper mantle and crustal rocks are most likely related to the presence of a combination of host phases (alloys, spinel, diopside) for the refractory PGEs (Os, Ir, and Ru) during partial melting in the upper mantle.

Partitioning of palladium, iridium, platinum, and gold between sulfide liquid and basalt melt at 1200°C

Abstract Iron-nickel monosulfide and basalt glass containing trace amounts of PGE equilibrated at 1200°C, and fo 2 = 10 −9.2 (close to the wustite-magnetite buffer) and fs 2 = 10 −0.9 , have been analyzed for noble metals by radiochemical and instrumental neutron activation analysis. The average contents of PGE in coexisting Fe-Ni sulfide and basalt glass, respectively, are Pd, 50 ppm and 0.5 ppb; Ir, 50 ppm and 0.5 ppb; Pt, 100 ppm and 10 ppb; and Au, 0.7 ppm and 0.8 ppb. The sulfide liquid-silicate melt partition coefficients ( D values) for the noble metals are (9 ± 7) × 10 4 for Pd, (1 ± 0.7) × 10 5 for Ir, (9 ± 6) × 10 3 for Pt, and (1 ± 0.9) × 10 3 for Au. The noble metals are strongly partitioned into sulfide liquid, but the affinity of Pd and Ir for sulfide liquid is about 50 times greater than that of Pt and about 500 times greater than that of Au. The D values indicate that equilibrium partitioning between immiscible sulfide liquid and basalt magma would result in fractionation of the noble metals, which differs significantly from that generally observed in nature.

Experimental partitioning of osmium, iridium and gold between basalt melt and sulphide liquid at 1300°C

The partitioning of Os, Ir and Au between FeNi sulphide liquid and basalt melt was determined at 1300°C and low pressure using internally buffered charges in alumina containers sealed in silica tubes. Six runs were at the oxygen fugacity of the iron‐silica phase‐fayalite (IQF) buffer and one at iron‐wustite (IW), with Os, Ir and Au concentrations in the sulphide ranging from approximately 45 to 400 ppm for each metal. The monosulphide liquid (FeNi) varied from 0.02 to 8.9 at % Ni. The major element composition of silicate glass and sulphide run products was quantitatively determined by electron microprobe analysis. The noble metal content of the sulphide phase was calculated from the initial platinum‐group element (PGE) content added and corrected for dilution by constituents derived from the silicate melt, which was mainly iron. Noble metals in silicate glasses were determined by radiochemical neutron activation. Average distribution coefficients (D) for the IQF buffered runs were D(Os) = 3720 ± 1320, D(...

Geochemistry of metasedimentary rocks in the late Archean Hemlo-Heron Bay greenstone belt, Superior Province, Ontario : implications for provenance and tectonic setting

The late Archean Hemlo-Heron Bay greenstone belt of the Superior Province near the Hemlo gold deposit, Ontario, consists mainly of fine-grained clastic metasedimentary rocks with subordinate amounts of metavolcanic rocks. Cordierite-anthophyllite rocks are also locally present, but restricted to a single metasedimentary sequence, and most likely represent metamorphosed Mg- and Fe-rich sedimentary rocks derived from predominantly mafic-ultramafic sources. The clastic metasedimentary rocks are geochemically highly immature. Some of their geochemical characteristics, such as low Al2O3/Na2O ratio, low CIA, K2O/Na2O < 1.0, uniform Th/U ratio (3.6), and strong correlations of transition metals and MgO with Fe2O3∗, are similar to those of typical Archean sedimentary rocks. However, their REE compositions (high contents of LREE and steep chondrite-normalized patterns) are different from those of most Archean sedimentary rocks and similar to those of more mature Proterozoic sedimentary rocks. The clastic metasedimentary rocks from the Hemlo-Heron Bay greenstone belt were derived mainly from felsic (or intermediate) volcanic sources, admixed with variable amounts of mafic-ultramafic volcanic components and possibly some continental weathering products, and deposited rapidly as turbidites. The geochemical uniformity of all clastic metasedimentary rocks indicates that the Hemlo-Heron Bay greenstone belt represents a single tectonic environment (possibly equivalent to a modern island-arc system) rather than a juxtaposition of suspect terranes.