FORMATION OF PGE-CU-NI DEPOSITS IN A HIGH-YIELDING LOWER CRUSTAL PLUMBING SYSTEM: THE SEILAND IGNEOUS PROVINCE, N-NORWAY

The Ediacaran Central Iapetus Magmatic Province (CIMP, Ma 610-560) is a prominent example of a worldwide LIP that mostly is exposed in Greenland, Labrador and the Baltic shield, either in dense dykes swarms or in subvolcanic layered gabbros (including the Sept Iles, Canada). The Seiland Igneous Province (SIP, Ma 570-520) in NW-Norway formed during the CIMP episodes and comprises a deep-seated (0.5-0.8 Gpa) high yielding mafic-ultramafic-alkaline plumbing system conveying thousands of km³ of melts from the asthenosphere to the continental Lithosphere (Larsen et al. 2018, https://doi.org/10.1016/j.lithos.2017.11.013).

Here, we have located several Cu-Ni-PGE reefs in one of the ultramafic plumbing systems. The reefs carry up to 1.7 g/t total PGE with c. 75% PPGE+Au and 25 % IPGE with up to 0.3 g/t Os+Ir. Other reefs have c. 0.1 wt% Cu and up to 0.4 wt% Ni. PGE and Cu+Ni reefs are decoupled i.e. PGE-reefs are situated 10-20 metres below the Cu+Ni reefs. PGM’s are dominated by PPGE minerals mostly moncheite, merenskyite, sperrylite and Au-Ag phases but has a conspicuous proportion of IPGE minerals mostly hollingworthite and irarsite.

PGE-reefs are consistently situated in dunitic cumulates forming from picritic melts, however, being open long living plumbing system, multiple evidence for magmatic remobilization and in conduit enrichment and decomposition processes are observed.

Of particular importance we observe that the parental melts at these depths are enriched in volatile constituent. On the average, barren as well as mineralised ultramafic cumulates comprises 1-2 wt% sulphides and 1-2 wt% carbonic and hydrous phases and ultramafic volatile-rich melts are involved in both the emplacement processes and remobilization of PGE-Cu-Ni deposits.

Essentially, SIP provides are rare glimpse in to the very deep-seated parts of high yielding volcanic systems where fertile asthenospheric mafic-ultramafic melts are mixed (and un-mixed!) before ascending to the upper lithosphere during the formation, upgrading as well as destruction of ore-deposits.