Petrogenesis of Uivfaq Iron, Disko Island, Greenland

Massive blocks of native iron and iron carbide, weighing up to 22 tons, have been found at Uivfaq, Disko Island, western Greenland. The iron blocks are associated with Tertiary basalts that erupted during rifting between Greenland and Baffin Island, early in the opening of the Davis Strait. The `iron' is an asemblage of iron and iron carbide (cohenite) and various silicate, sulphide, and oxide inclusions containing highly reduced and iron-rich phases, including fayalite, wustite, troilite, ulvospinel, ilmenite, ferriferous pigeonite, and FeO-rich glass. The chemistry and petrography of the inclusions are used to derive a paragenetic history for the Uivfaq iron assemblage. Petrographic study of a massive Uivfaq iron block shows that the block formed through accumulation of partly molten spherules of iron and cohenite,∼0.5 to 1.0 mm in diameter. Basaltic liquid was trapped by the accumulating metal spherules and reacted with the metal to form various highly reduced and iron-enriched immiscible liquids. These liquids solidified as inclusions in the iron mass. Various models are considered for the origin of the iron-cohenite spherules; at present, a carbon-reduction mechanism is preferred. The association of graphite-rich plagioclase-spinel xenoliths (thought to be derived from sediments) with the iron masses suggests a genetic relation between the two. We suggest that graphite was assimilated by mantle-derived basaltic magma. The carbon reduced iron oxide components of the basalt at a depth of 3 km or less and formed small immiscible droplets of high-carbon iron-carbon liquid. These droplets were erupted with the magma. The droplets settled and accumulated near or at the base of a flow or sill to form massive iron blocks.