Sulfides in alkaline and peralkaline rocks: textural appearance and compositional variations

摘要

Several alkaline rock associations from Greenland, Canada, Morocco and Germany were investigated with respect to their sulfide mineralogy and the compositional variations of the different sulfide minerals. The investigated rock types range from primitive melilitites, pyroxenites and glimmerites through gabbroic, dioritic and syenitic rocks to highly evolved, partly peralkaline nepheline syenites and associated late-stage veins. The sulfide inventory changes systematically with rock type: In the primitive rocks, Fe-sulfides (pyrrhotite and pyrite) are common and occur with minor chalcopyrite and sphalerite, but they are rare and in cases absent in the more evolved rocks. Sphalerite is abundant in syenites and nepheline syenites, with the latter rocks containing additional galena (along with unusual phases such as lollingite, djerfisherite, and thalcusite), especially in highly evolved rocks from the Ilimaussaq complex (Greenland). The stoichiometry of pyrrhotite (Fe1-XS; where typically 0 = x = 0.17) roughly correlates with the redox conditions determined for the various rocks, with stoichiometric FeS being restricted to very reduced rocks from Greenland and Fe-deficient pyrrhotite being typical of the more oxidized rocks from Canada and Morocco. The presence of sphalerite and galena in these evolved rock types is attributed to a combination of the geochemically enriched character of their magma sources (lithospheric mantle), melt degree during magma formation and differentiation history of the magmas.