PLATINUM-GROUP ELEMENTS IN BASALTS DERIVED FROM THE ICELAND PLUME –IMPLICATIONS FOR METALLOGENESIS

We report here the PGE, REE and HFSE composition of Holocene (Iceland) and Eocene (East Greenland) basalts derived from melting of the Icelandic plume. The East Greenland basalts erupted during continental breakup above the Eocene precursor of the Iceland plume, whereas the Icelandic basalts were extruded during Holocene plume-ridge interaction. The parental magmas in both cases were PGE-rich and initially S-undersaturated and therefore capable of generating significant PGE mineralization.

Two distinct basalt series are present in East Greenland, a low-Ti series and a high-Ti series, and whereas Ir correlates inversely with Pd, it correlates well with MgO indicating S-undersaturated fractionation. In Iceland the primitive (parental) basalts constitute a mixing array between picritic (74 ppm Cu and 17 ppb Pd) and olivine tholeiite (120 ppm Cu and 6 ppb Pd) magmas. In Iceland and Greenland evolved olivine tholeiites have the highest Pd/Ir ratios and Pd-concentrations (Iceland 18 ppb Pd, mg# 58; Greenland 24 ppb Pd, mg#50). Low PGE concentrations in Icelandic samples with <7 wt% MgO indicate S-saturation and segregation. The moderately PGE-rich and S-undersaturated nature of the Greenland and Iceland basalts contrasts with MORB and reflect relatively deep melting of hot plume material beneath thick lithospheric lids in Greenland and Iceland.

Orthomagmatic Pd-Au-Pt reefs in East Greenland (e.g., the Platinova Reefs in the Skaergaard Intrusion), are hosted by Paleogene Intrusions that are comagmatic with the overlying flood basalts. The Platinova Reefs are atypical of most economic PGE reefs (e.g., Merensky Reef) as they are hosted by evolved gabbros (magma mg#: 25). In contrast to classic PGE reefs such as Merensky, which are products of mixing of a PGE-rich, S-undersaturated magma with a PGE-poor, S-saturated magma, the Platinova Reefs are probably a product of extreme fractionation of a S-undersaturated magma which became S-saturated as a result of fractionation.