IRIDIUM, RHODIUM, AND PLATINUM SULFIDES IN CHROMITITES FROM THE ULTRAMAFIC MASSIFS OF FINERO, ITALY, AND OJÉN, SPAIN

GIORGIO GARUTI and MORENO GAZZOTTI
Dipartimento di Scienze della Terra, Universitá di Modena, Via S. Eufemia, 19, 41100 Modena, Italia

JOSE TORRES-RUIZ
Departamento de Mineralogia y Petrologia, Universidad de Granada, Fuentenueva s/n, 18002 Granada, España

Abstract

Cuprorhodsite, cuproiridsite, malanite, and other unknown sulfides of Ir, Rh, and Pt occurring as primary inclusions (up to about 12 µm) in fresh chromite of chromitites from the ultramafic massifs of Finero (Western Alps, Italy) and Ojén (Betic Cordillera, Spain) have been investigated by electron-microprobe analysis. Compositions obtained for cuprorhodsite, cuproiridsite, and malanite, combined with data from the literature, confirm the existence of extensive solid-solution in the system CuRh2S4-CuIr2S4- CuPt2S4, although Pt-rich representatives close to pure malanite have not yet been found in nature. Among the other minerals, an (Ir,Rh)-rich, Ni-Fe-Cu sulfide (unknown #1) is the most common. Compositions documented here and from the literature are consistent with the stoichiometry (Ni,Fe,Cu)2(Ir,Rh)S3, although it is still an open question whether these compositions represent an independent mineral species or pertain to PGE-rich pyrrhotite and pentlandite. The other sulfides are: (Ir,Rh)3(Ni,Fe,Cu)2S5, probably xingzhongite, (Rh,Cu)3S2 (unknown #2), and a Pt,Rh,Ir,Cu sulfide (unknown #3). Cuprorhodsite, cuproiridsite, malanite, and a (Ir,Rh)-rich, Ni-Fe-Cu sulfide similar to unknown #1 have commonly been reported from placer deposits, mostly derived from Alaskan- type complexes or from other uncertain source-rocks. More rarely, they have been described in situ from ophiolitic chromitites. The occurrences of Finero and Ojén indicate that these minerals can be found in situ also within chromitite from the lithospheric mantle underneath the continental crust.

Keywords: cuprorhodsite, cuproiridsite, malanite, Ir-Rh sulfides, Finero, Western Alps, Italy, Ojén, Betic Cordillera, Spain.