Abstract

The Vammala nickel deposit is located in southwestern Finland in an ultramafic intrusion with a distinct layered structure. Occurring in mica gneiss, the deposit is one of the Svecokarelian nickel occurrences that form a ring around the central Finland granitoid area. The ultramafite is composed of three layers: the lower and upper ones are ultramafic and the middle one hornblenditic in composition. The magma was sulfur saturated and the bulk of the sulfides were carried in suspension and sank to the basal part of the lower ultramafic layer where they formed economic orebodies. The other layers contain only low-grade disseminated sulfides.When the first magma cycle was over, the partial oxygen pressure increased significantly and, as shown by microscopic examination and the relationship between the Fe and Ni contents in olivine, the sulfides of the upper ultramafic layer were oxidized soon after crystallization.The isotope composition of the sulfur in the lower and middle layers suggests magmatic origin. The sulfur of the sulfides in the upper layer, however, is heavy and probably from a nonmagmatic source. The magma seems to have intruded the clay sediments close to or even on the sea floor.The age of the zircon and monazite fractions separated from the mafic pegmatoidic portions in the ultramafite is 1,890 m.y.

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