Ultramafic cumulate bodies in the Vammala Nickel Belt, some of which host small magmatic Ni-Cu sulphide deposits, are remnants of synorogenic intrusions that were emplaced into the early Proterozoic Svecofennian arc terrane and became progressively boudinaged by continuing tectonic movements. Mineral and sulphide compositions of mineralised Svecofennian intrusions require that sedimentary sulphides (0.5 wt.% Zn; Se/S almost-equal-to 100 x 10(-6) played an important role in ore genesis. It is proposed that when ascending magmas encountered sedimentary formations containing abundant sulphidic black schists, they assimilated external sulphur, which led to the formation of an immiscible sulphide phase in the magma. The high Zn contents of interstitial sulphides (280-450 ppm) and liquidus chrome spinels (0.7-1.0 wt.%) indicate that the parental magma contained much more Zn than conventional assimilation processes would allow. Probably, S and Zn were selectively transferred by C-O-H-S fluids from the black schists into the cooling magma. Desulphidisation (involving conversion of pyrite to pyrrhotite) in the country rocks was driven by thermal energy provided by both the intrusions themselves and the concomitant regional metamorphism. Magma-country rock interaction during ore genesis is also indicated by the presence of minor phases such as graphite, ZnS, PbS, MoS2, an unknown Re-Mo-Cu-Os sulphide and numerous tellurides among the Fe-Ni-Cu sulphides. During the peak of regional metamorphism small felsic dykes intruded the cumulate bodies and remobilised the interstitial Fe-Ni-Cu sulphides into thin massive ore veins. Compared to interstitial ore, vein sulphides are depleted in Cu, Se, and Zn. Some Fe-Ni-Cu sulphides also migrated, probably due to regional strain effects, into the country rocks and mixed with sedimentary sulphides. Those magmas that formed unmineralised intrusions had already intruded sulphidic black schists and assimilated external S and Zn prior to final emplacement, and had thus become depleted in chalcophile elements and Zn by the segregation of sulphides and chrome spinel, respectively.
