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DJERFISHERITE IN THE GULI DUNITE COMPLEX, POLAR SIBERIA: A PRIMARY OR METASOMATIC PHASE?

Federica Zaccarini^1,, Oskar A.R. Thalhammer¹, Francesco Princivalle², Davide Lenaz², Chris J. Stanley³ and Giorgio Garuti⁴

¹ Department of Applied Geosciences and Geophysics, University of Leoben, Peter Tunner Strasse 5, A–8700 Leoben, Austria
² Dipartimento di Scienze della Terra, University of Trieste, Via Weiss, I–34127 Trieste, Italy
³ Mineralogy Department, Natural History Museum, Cromwell Road, London SW7 5BD, United Kingdom
⁴ Dipartimento di Scienze della Terra, University of Modena and Reggio Emilia, Via S. Eufemia, 19, I–41100 Modena, Italy

The uncommon sulfide djerfisherite, ideally K₆(Fe,Cu,Ni)₂₅S₂₆Cl, occurs in the dunite portion of the Guli complex, located in Polar Siberia, Russia. Distinctive features of the Guli complex are its considerable size, ~2000 km², making it the largest dunite–clinopyroxenite massif in the world, and its complicated geology. The exposed part is composed predominantly (60%) of variably serpentinized dunite. Melanocratic alkaline rocks cover about 30%, and other rock types, including melilitolite, ijolite, alkaline syenite and carbonatite, occupy less than 10% of the area. The sample with the most abundant djerfisherite is a coarse-grained phlogopite–magnetite-rich clinopyroxenite. Djerfisherite most commonly occurs in irregular patches of sulfide composed mainly of pyrrhotite accompanied by minor chalcopyrite and rare galena, in a matrix of Ti-bearing andradite, clinopyroxene, phlogopite, plagioclase, apatite and rare zircon, titanite and pyrophanite. It forms crystals generally less than 100 µm in size and occurs as irregular single-phase grains or it fills fissures of the silicate matrix and infiltrates phlogopite along its cleavage planes. We provide new data on the physical and crystallographic properties of djerfisherite. Its composition is characterized by an extensive substitution among Fe, Cu and Ni; their reciprocal substitutions are mainly controlled by the nature of the host rock. The djerfisherite from the Guli complex is Cu-rich and resembles that reported from alkaline rocks. This feature is consistent with the proposal that djerfisherite in the Guli complex crystallized as a primary mineral, during the late-stage fractionation of a highly alkaline melt portion, derived from successive fractional melting of an ascending mantle plume under metasomatic conditions.

Keywords: djerfisherite, Raman spectroscopy, crystal-structure analysis, Guli dunite complex, mantle, Siberia, Russia.