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MPM-04 Platinum-group mineralogy

Platinum-group minerals in chromitite schlieren from the ouen island, new caledonia ophiolite

José María González-Jiménez, University of Granada (Spain)
Thierry Augé, BRGM (Bureau de Recherches Géologiques et Minières) (France)
Fernando Gervilla, University of Granada (Spain)
Laurent Bailly, BRGM (Bureau de Recherches Géologiques et Minières) (France)
Joaquin A Proenza, University of Barcelona (Spain)
 

Chromitite schlieren bearing platinum-group minerals (PGM), occur in wehrlite and pyroxenite at Ouen Island (southernmost part of New Caledonia). PGM occur as small (<20m across), isolated, diphase or polyphase grains, included in cores, altered rims, or fractures of chromites as well as in the interstitial silicate matrix of chromitites. The PGM assemblage mainly consist of: Pt-Fe-(Ni-Cu) alloys [isoferroplatinum (Pt₃Fe), ferronickelplatinum (PtFe_0.5Ni_0.5) and tetraferroplatinum (PtFe)], Pt-Pd-Rh and Ru-Os-Ir sulphides [malanite (Pt₂CuS₄), cooperite (PtS), rhodian pentlandite (Ni,Fe,Rh)₉S₈, prassoite (Rh₁₇S₁₅), laurite-erlichmanite (RuS₂-OsS₂) solid solution series and undetermined Ir-sulphides and ruthenian pentlandite (Ni,Fe,Ru)₉S₈] and Pt-Fe-Ni-Cu oxides (Pt-Fe-Cu and Pt-Fe-Ni-Cu oxides). Other minor PGM are complex undetermined alloys made up of PGE±Ni±Fe±Cu [comprising Pt-Fe-Cu, Pd-Pd-(Cu-Ru-Rh-Fe) and Ir-(Rh-Ru-Os-Pt) alloys], native PGE (Os, Pd, Ru and Pt), Ir-Rh sulpharsenides [members of the irarsite (IrAsS)-hollingworthite (RhAsS) solid solution series], Pt-Pd-Cu tellurides [unknown Pt-Pd-Te and Cu-Pd-Te ], Pd antimonides [sudburyite (PdSb)] and Pd-amalgam [potarite (PdHg)]. Base-metal minerals (BMM, including Ni-Fe-Cu sulphides, alloys and sulpharsenides) occur associated with PGM. Likewise minute crystals of Cr-diopside, enstatite and minor olivine can be found within unaltered chromite crystals.

These primary silicate phases also form the interstitial silicate matrix, where they occur partly replaced by hydrous silicates like pargasite, hornblende, hydrogrossular garnet, chlorite and/or serpentine. The different mineral assemblages and textures suggest a multistage genetic model in which an early magmatic stage is followed by several successive hydrothermal events.

The primary, magmatic assemblage, preserved in fresh, unfractured chromite and in anhydrous primary silicates, is characterized by the coexistence of PGE-alloys (isoferroplatinum and tetraferroplatinum) and sulphides [laurite, cooperite, malanite, Ir-sulphide and (Pd-Pt)-rich sulphide], which suggest variable fS₂, T and, at lesser extent, O₂ conditions in the melt during chromite formation. The first high temperature, hydrothermal event is identified by the presence of a great number of Pt-, Pd- and Rh-bearing PGM (e.g malanite and minor laurite) with pargasite, hornblende and hydrogrossular garnet in fractures or in altered zones of chromite.

Serpentinization of the host rocks promoted changes in physicochemical conditions of the hydrothermal fluids towards lower oxidation and sulfidation states, resulting in the formation of new stable phases like PGE±Ni±Fe±Cu alloys, native PGE, Ir-Rh sulpharsenides, Pd antimonides and amalgams. Finally, the evolution of fluids towards oxidizing conditions during late stages of serpentinization and/or weathering gave rise to the formation of abundant of PGE oxides.