Origin of djerfisherite from dunite of the Guli massif (Polar Siberia)

In this work, we provide new data on the physical and crystallographic properties of the rare sulphide djerfisherite, ideally K6(Fe,Cu,Ni)25S26Cl discovered in the dunite portion of the Guli complex, Polar Siberia, Russia. The obtained data are used to better understand the origin of djerifisherite as well as those of its host rock. The Guli complex, with an area of about 2000 km2 is possibly the largest dunite clinopyroxenite massif in the world. It is characterized by a complicate geology, that comprises different lithotype, such as variably serpentinized dunite (about 60% of the area), melanocratic alkaline (about 30%), and melilitolite, ijolite, alkaline syenite and carbonatite (about 10%). Djerfisherite was found in a coarse grained clinopyroxenite enriched in phlogopite and magnetite and it generally occurs in irregular patches of sulphide composed mainly of pyrrhotite accompanied by minor chalcopyrite and rare galena. The matrix consists of Ti-bearing andradite, clinopyroxene, phlogopite, plagioclase, apatite and rare zircon, titanite and pyrophanite. It forms crystals generally less than 100 microns in size and occurs as irregular single-phase grains or it fills fissures of the silicate matrix and infiltrates phlogopite along its cleavage planes. The microhardness values, obtained on three grains, vary between 132 and 154 with an average of 149. Reflectance (R%) data of djerfisherite at the standard wavelengths (470, 546, 598 and 650 lambda) of the IMA commission on ore mineralogy (COM) are the following, R% in air 20.2, 23.8, 25.3 and 26.2, R% in oil 10.1, 12.3, 13.2 and 14. Djerfisherite display a very characteristic Raman spectrum with a peak at 300 cm_1. Our crystal of djerfisherite has a cell edge equal to 10.385(5) Å. Literature data on composition of djerfisherite from different worldwide countries, have shown that it is characterized by an extensive substitution among Fe, Cu and Ni. These 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.