Michel Piboule

Differentiation of platinum-group elements (PGE) and of gold during partial melting of peridotites in the lherzolitic massifs of the Betico-Rifean range (Ronda and Beni Bousera)

Abstract The Ir, Ru, Rh, Pt, Pd and Au contents of 28 peridotites of the Ronda and Beni Bousera lherzolitic massifs were determined using ICP-MS after hydrolytic extraction. Mantle-normalized NiPGECu distribution in these peridotites varies from almost flat patterns to patterns depleted in Pt, Pd and Cu. This distribution is principally controlled by partial melting which governs the differential behaviour of the PPGE (Pt, Pd) and Cu with respect to the IPGE (Ir, Ru) and Ni. The PPGE and the Cu are extracted and concentrated in silicate melt as a dissolved sulphide complex. On the other hand, the IPGE are not mobilized and are retained in the residues, probably in intergranular refractory phases. Rh behaves like a chalcophile element in the Beni Bousera peridotites (as do Pt and Pd), unlike the Ronda peridotites. The oxygen fugacity ( f O 2 ) conditions (determined by using a solid electrolyte cell) and the low Rh content in the spinel prevent any suggestion of incorporation of this element into the spinel as assumed by the thermochemical data. Unlike the Beni Bousera peridotites, the non-chalcophile behaviour of Rh in the Ronda peridotites tends to imply lower sulphur fugacity ( f ( S 2 ). The absence of a ‘pinel effect’ on the incorporation of Ir, Ru and Rh is probably related to the petrogenetic history of this mineral. It could represent the heritage of pre-existing phases which have no influence on the partitioning of these elements within the mantle.

Experimental study of the solubility of platinum and iridium in basic silicate melts—Implications for the differentiation of platinum-group elements during magmatic processes

This study presents evidence for the dissolution of two platinum group-elements, (PGEs) (Pt and Ir) in silicate melts of basic composition in the absence of any associated fluid phase. The dissolution of these PGEs is attributed to the formation of a complex ion in which the noble metal is uncharged. An increase in oxygen fugacity in equilibrium with the melt brings about a considerable decrease in Ir solubility. In this way, we can explain the early enrichment in Ir-group PGEs (IPGEs) (Ir, Ru, Os) observed during the crystallization of ultrabasic cumulates. By contrast, however, Pt remains soluble in the melt. An increase in the fs2 results in a pronounced increase in the solubility of Pt in the silicate melt. The sulfur saturation which leads to the precipitation of the base-metal sulfides (BMSs) also brings about the fixation of Pt-group PGEs (PPGEs) (Pt, Pd, Rh) which are dissolved in BMSs. This accounts for the presence of PPGes in the nickel sulfide horizons of stratiform deposits. The occurrence of platiniferous Fe, crystallizing with chromite in Alaskan-type complexes at oxygen fugacities of ∼ 10−6 bar, can be explained by the low activity of Fe in FePt alloys. Such alloys have been formed in our laboratory at such fo2.

Réseau de lithothèques en Rhône-Alpes

En 2007, ce projet collectif de recherche (PCR) entrait dans sa deuxieme annee de fonctionnement. Il a pour objectifs de realiser un bilan cartographique, geologique et photographique actualise des ressources siliceuses (silex essentiellement) en Rhone-Alpes en federant les acteurs de ce domaine de recherche (professionnels, amateurs). La creation d’une base de donnees regionale permettra a terme de coordonner les operations de prospection des gites siliceux en harmonisant et en ameliorant le...