The Sompujärvi PGE reef in the Penikat layered intrusion, northern Finland

The Sompujärvi (SJ) PGE Reef is located at the border between the third and fourth megacyclic units, 400–1000 metres above the base of the Penikat layered intrusion. It usually occurs in the basal bronzititic portion of megacyclic unit IV, but can occur in the overlying peridotitic cumulates or the gabbroic cumulates at the top of megacyclic unit III in places. PGE concentrations have been found to occur mainly in association with either base metal sulphide (mainly pyrite-chalcopyrite-pentlandite) or chromite disseminations, the latter type usually representing a distinctly higher grade of mineralisation. When the magma which formed megacyclic unit IV penetrated into the Penikat layered intrusion, its lower part intermixed with the older residual magma. It may be assumed that this mixing of magmas was responsible for the precipitation and PGE enrichment of sulphides in the lower part of the olivine cumulates, accounting for the sulphide-disseminated mineralization type. The entry of the new magma pulse was probably quite powerful, and when this magma spread out over the uppermost, partly consolidated gabbroic cumulate of megacyclic unit III it caused pronounced local erosion in the uppermost crystal layer and excavated elongated channels and/or depressions, as associated with faulting. These channels and depressions then trapped older residual liquid enriched in platinum-group elements and the chromite-disseminated type within the SJ Reef was formed.