The Significance of Cumulus Chlorapatite and High-temperature Dashkesanite to the Genesis of PGE Mineralization in the Koitelainen and Keivitsa-Satovaara Complexes, Northern Finland
Several PGE mineralizations are known from the 2435 Ma-old Koitelainen layered intrusion and the adjacent Keivitsa-Satovaara (K-S) complex. In Koitelainen, transient sulphide separation gave rise to sulphidic PGE mineralizations (Pd-Pt- Au) in the lower zone and upper main zone. PGE, without visible sulphides, occur in Lower Chromitites (Pd-Pt) in the lower zone, in ultramafic pegmatoid pipes (Pt) in the main zone, and in the Upper Chromitite layer (Ru-Pt-Pd) of the lower upper zone. A peculiar peridotite-mixed rock in the main zone is underlain by a Pt-enriched layer, a setting reminiscent of the Stillwater PGE reef. PGE removed by these mineralizations had little effect on the overall buildup of PGE in the rest of the magma; by far the most of PGE precipitated with the last sulphide saturation at 95% level, in magnetite gabbro (Pt-Au-Pd). In the upper part of the K-S complex, assimilation of sedimentary sulphides triggered the deposition of massive sulphides. PGE occur only in overlying meagre disseminations (Pt-Pd-Au), and reach the highest concentrations in chromite-rich rocks (Os-Pt-Pd) at the roof. The distribution of PGE in these intrusions has an unusual pattern. Disseminated and massive sulphides in lower parts, with low PGEsulph (down to ≤10 ppb), are overlain by sulphide-poor mineralizations with high PGEsulph(from 20–13 000 to ∞ ppm). This feature persists, and even increases during crystallization, the principal PGE concentrations being located at or near the roof. Association of PGE with primary Cl-minerals suggests that the odd behaviour of PGE was due to an association between PGE and chlorine.