Platinum Group Element Alloys Research Articles

Palladium and gold mineralization in podiform chromitite at Kraubath, Austria

Summary We report, for the first time, the occurrence of five palladium-rich, one palladium bearing and two gold-silver minerals from podiform chromitites in the Eastern Alps. Minerals identified include braggite, keithconnite, stibiopalladinite, potarite, mertieite II, Pd-bearing Pt-Fe alloy, native gold and Ag-Au alloy. They occur in heavy mineral concentrates produced from two massive podiform chromitite samples (unaltered and highly altered) of the Kraubath ultramafic massif, Styria, Austria. Distribution patterns of platinum-group elements (PGE) in these chromitites show considerable differences in the behaviour of the less refractory PGE (PPGE-group: Rh, Pt, Pd) compared to the refractory PGE (IPGE-group: Os, Ir, Ru). PPGE are more enriched in chromitite showing pronounced alteration features. The unaltered chromitite displays a negatively sloped chondrite-normalised PGE pattern similar to typical ophiolitic-podiform chromitite. Except for the Pd- and Au-Ag minerals that are generally rare in ophiolites, about 20 other platinum-group minerals (PGM) have been discovered. They include PGEsulphides (laurite, erlichmanite, kashinite, bowieite, cuproiridsite, cuprorhodsite, unnamed Ir-rich variety of ferrorhodsite, unnamed Ni-Fe-Cu-Rh- and Ni-Fe-Cu-Ir-Rh monosulphides), PGE alloys (Pt-Fe, Ir-Os, Os-Ir and Ru-Os-Ir), PGE-sulpharsenides (irarsite, hollingworthite, platarsite, ruarsite and a number of intermediate species), sperrylite and a Ru-rich oxide (?). Three PGM assemblages have been recognised and attributed to different processes ranging from magmatic to hydrothermal and weathering-related. Pd-rich minerals are characteristic of both chromitite types, although their chemistry and relative proportions vary considerably. Keithconnite, braggite and Pd-bearing ferroan platinum, together with a number of PGE-sulphides (mainly lauritePresent address: Federal Institute for Geosciences and Natural Resources (BGR), Stilleweg 2, D-30655 Hannover, Federal Republic of Germany

Ir, Ru, Pt, and Pd in basalts and komatiites: new constraints for the geochemical behavior of the platinum-group elements in the mantle

The concentrations of the platinum-group elements (PGE) Ir, Ru, Pt, and Pd were determined in 18 mantle-derived basalts from a variety of tectonic settings and six komatiites from three locations. All analyses were performed using isotope dilution, Carius tube digestion, and the precise technique of multiple collector inductively coupled plasma mass spectrometry. Multiple analyses of two samples indicate external reproducibilities, based upon separate dissolutions, of approximately 2–9% in the ppt to ppb concentration range. Mid-ocean ridge basalts from the Kolbeinsey Ridge, tholeiites from Iceland and alkali basalts from the Cameroon Line define three individual sample suites that are characterized by distinct major, trace, and platinum-group element systematics. All three-sample suites display correlations of the PGE with MgO, Ni, and Cr. The new analytical results are employed to constrain the geochemical behavior of the PGE during the formation and differentiation of mantle–derived melts. The PGE are inferred to be compatible in sulfides during partial melting with sulfide-silicate melt partition coefficients of ∼1 × 10 4. The fractionated PGE patterns of mantle melts are a consequence of the incompatibility of Pd in nonsulfide phases, whereas Ir and Ru must be compatible in at least one other mantle phase. Model calculations indicate that PGE alloys or spinel may be responsible for the higher compatibility of the latter elements during partial melting. It is further demonstrated that the shape of the melting regime has a profound effect on the PGE systematics of mantle magmas. The systematic trends of the three sample suites in plots of PGE against Ni and Cr are the result of magma differentiation processes that involve fractional crystallization of silicate minerals and the concurrent segregation of an immiscible sulfide liquid. The behavior of the PGE during magma fractionation indicates that the segregated sulfides probably equilibrate with >90% of the silicate magma and that PGE scavenging by sulfides is best described by a combination of batch and fractional equilibrium partitioning.

Platinum genesis; new mineralogical evidence

The two distinct types of platinum deposits in the Bushveld Complex (reefs and pipes) differ in relative age, geological position, and mineralogy. Reefs are characterized by the association of braggite, cooperite, laurite, and sperrylite with platinum-iron alloy and minor Pt-Pd bismuthotellurides and pipes by platinum-iron alloy, sperrylite, geversite, and Rh-Ir sulfarsenides. There are distinct regional variations in platinum mineralogy within the Merensky Reef; these are, however, largely confined to the relative proportions of the constituents. The pipes differ in the actual minerals present.The Sudbury and Stillwater deposits show a prevailingly higher content in compounds of Pd, Pt, Bi, Te, and S. This can be explained by the general tendency of Pd to associate preferentially with sulfides in the earlier stages and to form independent platinum-group minerals in the later stages of the magmatic cycle. Serpentinization plays an active role not only in the conversion of silicate nickel into sulfide nickel, but also in the extraction of platinum-group elements from silicates, oxides, and sulfides, and in their localization as independent and economically significant platinum-group minerals. Hydrothermal copper-platinum ores not spatially related to basic igneous rocks emerge as a separate genetic type.Investigation of placer deposits from worldwide localities shows that (1) placers linked to alpine-type ultrabasic massifs carry significant amounts of Pt as well as iridosmine and osmiridium; (2) inclusions in native alloys and sperrylite may provide information on the mineralogy of primary deposits--michenerite and hollingworthite in Urals placers are a case in point; and (3) alteration of platinum-group element alloys in placers is a widespread process which takes place at surface temperatures. Placers derived from Neogene ultrabasics in Japan carry textural types similar to those found in Precambrian Witwatersrand platinum-group mineral concentrates.The formation of platinum deposits is dominated by polyphase processes which also include transport of platinum-group elements in aqueous solutions.