Abstract

There is much current interest in the critical resource potential of Greenland. The Motzfeldt deposit is being tipped as one of the world’s largest Ta deposits, also containing significant volumes of Nb and Rare Earth Elements. Initial studies identified pyrochlore has the host of Nb and Ta, and the coexisting U in the pyrochlore was used to map the deposit using radiometrics. Early mineralogical studies reported bastnasite (CeCO3F) in addition to pyrochlore, leading some to classify it (mistakenly) as a carbonatite-type deposit. However field studies showed that the host was a quartz syenite and that the mineralisation was spread over a wider area than the radiometric survey suggested. It became clear that Motzfeldt was a complex geological conundrum, a deposit type for which a clear genetic model did not exist.

Big questions in economic geology include the roles of extreme fractionation vs partial melting in generating extreme melt compositions as well as the role and timing of fluid interaction. This talk will run through how we picked apart the story behind the novel mineralisation. The target elements at Motzfeldt are hosted in the magmatic roof zone and key to the siting of the mineralisation is the interaction between the magma and arenite in the roof. The mineralised units come from multiple sources, including both extreme fractionates and partial melts, plus or minus hydrothermal interaction involving highly aggressive fluids that transported REE and Nb. We were helped by the fact that Motzfeldt is deeply dissected by recent glaciation giving superb 3D slices through the roof zone. We now have a genetic model for syenite-hosted Ta-Nb mineralisation that is being applied to other similar deposits elsewhere in the world.