FORMATION OF LOW-TEMPERATURE CHALCOPYRITE IN PLUME DERIVED MAGMA: INSIGHTS FROM PYROXENITE HOSTED SULPHIDES FROM SUNG VALLEY, MEGHALAYA, NE INDIA

The distribution of chalcophile elements in Earth’s mantle is greatly controlled by mantle- derived sulphides. The sulphide minerals (mainly chalcopyrite) hosted by pyroxenite from an ultramafic-alkaline-carbonatite complex of NE India, related to the Kerguelen Plume, were analysed in this work. The sulphur (S) concentration (in ppm) at sulphide saturation obtained in pyroxenitic magma was considerably high compared to other pyroxenites occurring at different geological settings globally. Petrography and mineral chemistry of sulphide and diopside were also combined to determine the crystallization conditions of the sulphides. Two types of sulphide, with similar compositions, were detected in pyroxenites: Type A are multi-faceted polygons, elliptical and spherical in shapes occurring as poikilitic inclusions in diopside and Type B are intergranular sulphides of irregular shapes in silicate grains. Mineral chemistry suggested that these sulphides are products of re-equilibration of high-T mono-sulphide solid solution and represent a low temperature (~400° C) mineral phase of Cu-Fe-S system. Petrographic observations further suggest that the sulphides were separated as immiscible melt droplets at the time of S saturation and fractionation of diopside in the coexisting silicate magma. These sulphides were also partially replaced by magnetite. We further suggest that low temperature sulphides, which can also be originated from mantle can be replaced by magnetite due to changes in oxygen fugacity caused by the rapid ascent of ultramafic rocks during upwelling of the plume.