Stratigraphy, petrology, and platinum-group element mineralization of the early Proterozoic Penikat layered intrusion, northern Finland

Abstract

The Penikat layered intrusion is part of an early Proterozoic belt of layered intrusions, 2,440 m.y. in age, which extends across northern Finland. The intrusion is located between late Archean granitoids and early Proterozoic supracrustal rocks. The Marginal series, with slightly contaminated chilled margin, separates the Penikat intrusion from the underlying basement complex. The Layered series is divided into five megacyclic units, I to V from bottom to top, each unit comprising an ultramafic lower part and a gabbroic upper part.Three major mineralized zones, enriched in platinum-group elements (PGE), the Sompujaervi (SJ), Ala-Penikka (AP), and Paasivaara (PV) mineralizations, have been delineated along almost 23 km of strike and occur in megacyclic unit IV. The Sompujaervi mineralization, about 1 m thick, is mostly associated with pyroxenitic rocks at the lower contact of the unit, whereas the Ala-Penikka zone (about 0.35 m thick) is associated with gabbronorite and poikilitic-textured anorthosite in the middle part and the Paasivaara zone (about 1 m thick) with leucogabbro and anorthosite very close to the upper contact.The ratios of platinum-group elements are relatively constant for each mineralization. The Paasivaara mineralization represents the most platinum-rich variety and the Ala-Penikka the most palladium-rich one, the ratio of Pd to Pt being typically about 2 for the sulfide-free type, and 1.5, 3.5, and 0.7 for the sulfide-bearing type for the Sompujaervi, Ala-Penikka, and Paasivaara mineralizations, respectively.The sulfur-free platinum-group minerals (PGM), sperrylite and isomertieite are the dominant species encountered in the Sompujaervi mineralization, whereas sulfur-bearing platinum-group minerals, together with sperrylite, dominate in the Ala-Penikka and Paasivaara mineralizations. These are represented by braggite and vysotskite in the Ala-Penikka one and by braggite and cooperite in the more platinum-rich Paasivaara mineralization.According to the new classification of platinum-group element deposits proposed by Cabri and Naldrett (1984), the typically sulfide-free Sompujaervi mineralization may be included in the oxide-silicate association as a possible new type, whereas the Ala-Penikka and Paasivaara mineralizations may be classified as Merensky reef-type of the sulfide associations.