DUNITE HOSTED NI-CU-PGE DEPOSITS IN THE REINFJORD ULTRAMAFIC (UM) COMPLEX, SEILAND IGNEOUS PROVINCE (SIP), NORWAY

The Reinfjord UM complex formed when picritic-komatiitic melts were emplaced in to a layered gabbro in the 5000 Km² Seiland Igneous Province (560-570 Ma). The complex formed from three pulses of UM-melts progressively forming more MgO-rich cumulate series and culminating in the formation of dunitic cumulates. A 10-20 metres thick contact deposits with 0.3% Ni and 0.3% Cu in c. 2 % total sulphides was known previously. Several new mineralizations with 1-2 % Cu-Ni sulphides were located throughout the complex. Helicopter TEM-surveys “saw” a substantial conductor c. 100 metres below surface and two exploratory drill holes confirmed two reef deposits of 5 and 7 metres thickness, respectively, and separated by 20 metres of dunite. The reefs only contain 1.2-1.6 wt% sulphides and both deposits occur in the dunitic series of the complex. The lower occurrence (5 m) is a PGE-reef with 0.64 ppm Pt+Pd+Au, 0.2 ppm Os, and 0.27 wt% Ni. The upper Cu-Ni reef carry 0.38 wt% Ni and 0.12 wt% Cu and traces of PGE. The chondrite normalised PGE spectrum shows a trough pattern with positive Os, Pt, Pd and Au anomalies. In-situ, ion-probe, sulfur-isotope analysis yielded juvenile values for all sulphide deposits in the UM complex and the country rock gabbros, however, with conspicuous variations amongst each deposit. The PGE-reef yielded a δ³⁴S value of -0.40, the Cu-Ni reef, -4.56, the contact deposits gave +0.02, and gabbro-hosted sulphides gave an average of +2.19. Finally, sulphides in the country rock gneisses yielded a value of +9.09. Accordingly, the reef deposits did not achieve their sulphur by local country rock assimilation of neither gneisses nor the gabbros. It may also be concluded that the source of sulphur for the PGE-reef is distinctively different from that of the Cu-Ni reef only 20 m’s higher up. What arguably makes the Reinfjord reefs an unusual type of Ni-Cu-PGE deposit is; i) the great thickness of the reefs with low total sulphides; ii) clear separation of a PGE-Ni and a Ni-Cu PGE reef; iii) strongly contrasting S-isotope signatures over only 20 metres of cumulates; iv) the conspicuous trough shaped PGE-pattern and v) setting in a pure dunite formed from UM-melts.