ULTRAMAFIC HOSTED NI-CU-PGE DEPOSITS IN THE SEILAND IGNEOUS PROVINCE (SIP) NORWAY: ORE-FORMING PROCESSES AT THE ROOTS OF A GIANT MAGMATIC CONDUIT SYSTEM

English Abstract:

We recently discovered Ni-Cu-PGE deposits with high Os contents at two localities in the 5000 km² Seiland Igneous Province (560-570 Ma). The Reinfjord Peridotite Complex formed when picritic to komatiitic melts were emplaced in 3 pulses and formed pyroxenitic to peridotitic cumulates. The 1st melt-pulse, formed a 10-20 metres thick contact deposits with 2% sulphides wt. 0.3% Ni and 0.3% Cu. The third pulse formed two reef deposits of 5 and 7 m’s thickness, respectively, 20 m’s apart and with 1-2 % sulphide. The lower deposit (5 m) is a PGE-reef with 0.64 ppm Pt+Pd+Au, 0.15 ppm Os, and 0.27% Ni. The upper Cu-Ni reef has 0.38% Ni and 0.12% Cu and traces of PGE. The other locality, The Lokkarfjord Hornblendite Complex, comprises a vertical sulphide dike (0.2-0.4 m thick) intersecting the cumulus layering. It is massive sulphides with 0.9% Ni, 1.5% Cu, 1.1 ppm Pt+Au+Cu, 0.15 ppm Os and, contrary to Reinfjord, also 0.15 ppm Rh.

The chondrite normalised PGE spectrum shows a trough pattern with positive Os, Pt, Pd and Au anomalies for both deposits and a positive Rh anomaly for Lokkarfjord. In-situ, ion-probe, S-isotope analysis yielded juvenile values for all sulphide deposits in both Reinfjord and Lokkarfjord and the country rock gabbros. The PGE-reef yielded average δ³⁴S value of -0.40, the Cu-Ni reef, -4.56, the contact deposits +0.02. Lokkarfjord gave -1.9. Gabbro-hosted barren sulphides gave +2.19 and barren sulphides in country rock gneisses yielded +9.09.

Accordingly, sulphur is not assimilated from country-rock gneiss or gabbro. The PGE-reef sulphur is distinctively different from the Cu-Ni reef only 20 m’s higher up. The conspicuous trough shaped PGE-pattern with positive Os but negative Ir and Ru is a rare signature for PGE-deposits. Also conspicuous we note the great thicknesses of the reefs, a clear separation of a PGE-Ni and a Ni-Cu reef and strongly contrasting S-isotope signatures over only 20 metres of cumulates.

Our studies imply that the ultra-mafic complexes comprises volcanic conduits of a giant magmatic system accomodating deep-seated plume related melts that were at or close to sulphide-saturation at emplacement. Ore-genesis is a complex interplay of multiple replenishment episodes, interstitial melt migration and dissolution-reprecipitation of silicates as well sulphide phases.