THE ALA-PENIKKA PGE REEFS IN THE PENIKAT LAYERED INTRUSION, NORTHERN FINLAND

The PGE mineralized zones referred to as the Ala-Penikka PGE Reefs (AP I and AP II) are located about 250 m and 340 m above the base of megacyclic unit IV in the Penikat layered intrusion, both mineralizations being hosted by plagioclase-augitebronzite and narrow poikilitic plagioclase cumulates. A depression structure (pothole) about 300 m long and 100 m deep is encountered in the area of the AP I Reef, and it is in this structure that the AP I Reef, which is normally 30 cm thick, attains its maximum thickness of 20 m. The dominant sulphide paragenesis in AP I is pyrrhotite-chalcopyrite-pentlanditepyrite and that in AP II chalcopyrite-pentlandite-pyrite. The platinum-group minerals identified comprise almost thirty species, the most common being Pd-Te-(Bi) and Pd-As-Sb minerals and sperrylite (PtAs2). The AP Reefs are interpreted as having been formed from an upward-migrating fluid-enriched intercumulus melt in which PGE, S, Ni, Cu and related elements occurred in the fluid phase. The poikilitic plagioclase cumulate in both of the AP Reefs acted as a layer which trapped the upward-migrating intercumulus melt at its lower contact. The depression structure developed when a disturbance of some kind in the magma chamber caused the unconsolidated cumulate layers to collapse. © 1990 Springer-Verlag.