New exploration methods for platinum and rhodium deposits poor in base-metal sulphides.

M. Ohnenstetter, Z. Johan, A. Cocherie, C. Fouillac, D. Ohnenstetter, M. Chaussidon, O. Rouer, E. Makovicky, M. Makovicky, Sven Karup-Møller, D. Vaughan, G. Turner, R.A.D. Pattrick, A.P. Gize, I. Lyon, I. McDonald

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    Platinum-group elements (PGE) are typically associated with mafic and ultramafic intrusive rocks and the main exploration targets are layers and zones rich in PGE-bearing sulphides. Some PGE occurences, however, are in sulphide-poor situations and this raises the possibility that PGE deposits may be present in parts of mafic and ultramafic intrusives courrently considered to have low exploration potential.A multidiciplinary study was undertaken on four subeconomic deposits of platimum-group metals to develop a model of formation for PGE-deposits lacking significant base-metal sulphides. Two of the deposits occur in Albania, in the Tropoja and Bulqiza massifs, and are part of an ophiolitic belt created in an oceanic environment during the Upper Jurassic. The other two deposits occur in Madagascar, in the Andohankiranomena and Lavotrafo ultramafic massifs, and are within a Pan-African rifted zone.A Pt-rich chromitite style of mineralization was identified in the Andohankiranomena and Tropoja deposits, where the PGE are mostly included in chromite. A Pt- and Pd-rich silicate (dunite) style of mineralization was identified in the Lavatrafo and Bulqiza massifs, where PGE mineralization is assocciated with interstitial material between olivine grains. The four deposits have contrasting patterns of PGE distribution and individual elememnt ratios, suggesting that different mineral species (alloys, arsenides and sulphides) host the PGE. No primary geochemical halos were detected around any of the deposits and weathering has littel effect on the distribution of the PGE.The study showed that alloys and arsenides are the main carriers for platinum in all the deposits. Pt-Fe alloys, in particular, are often present in PGE deposits poor in base-metal sulphides and two phase systems were investigated experimentally: Pt-Ir-Fe-S (and the related subsystem Ir-Fe-S) and Fe(Cu)-Pt-Rh-S. A comparison of experimental results with natural phases in the depoists suggests that fluid-assisted exsolution of Pt, Ir and other elements from original higher-temperature solid solutions could be wide-spread. This supports the fluid-driven multistage mineralization concept suggested by field data. The experimental work also indicated that Pt-Rh-Fe alloys can coexist with two types of immiscible sulphide melt (one low in Rh and high in Cu, the other Rh-rich); both types of sulphide melt may have been present in the Tropoja deposit.
    Original languageEnglish
    JournalTrans. Instn. Min. Metall. (Sect. B: Appl. earch sci.).
    Pages (from-to)B119-B150
    Publication statusPublished - 1999

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