HFSE enrichment in carbonatites and associated peridotite-melilitolite-foidolite complexes: the role of fractionation versus metasomatism in ore-forming processes

• Applicants: Privatdozent Dr. Michael Marks; Privatdozent Dr. Benjamin Walter
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 521502267

Carbonatites associated with ultramafic rocks, melilitolites and foidolites contain economic HFSE mineralizations in some cases, but are barren in others. In that sense, the Kovdor (Russia) and Gardiner (E Greenland) complexes, which will be studied and compared in this project, represent key examples for such opposite cases: carbonatites at Kovdor are HFSE-mineralized, with only minor HFSE-phases in the associated silicate rocks. In contrast, melilitolites from Gardiner show ore-grade enrichment of perovskite, while the associated carbonatites are barren. Our previous study (DOME 1) suggests that ultramafics and melilitolites from Gardiner are cumulates of a mantle-derived, primitive and Ti-rich silicate melt. In contrast, ultramafic rocks from Kovdor are calcite- and biotite-rich, lack cumulate textures and are strongly depleted in compatible elements (e.g., Ni and Cr), which contradicts a cumulate, but rather supports a metasomatic origin. We hypothesize, that these rocks formed by a reaction between mantle-derived carbonatitic melts and silicic host rocks. This project aims at comparing these two complexes in detail to decipher the processes leading to the HFSE-enrichment in early cumulates (Gardiner) and late-stage carbonatites (Kovdor), respectively. If silicate rocks in such complexes can form by reaction of mantle-derived carbonatites with silicic wall rocks, enrichment of HFSE in such cases would be controlled by carbonatitic melts, and not by fractionation processes in evolving silicate magmas. This probably leads to completely different enrichment and fractionation patterns (e.g., Zr/Hf, Nb/Ta, La/Lu). To test this hypothesis, we will follow two complementary analytical strategies: (1) melt and fluid inclusions in all major rock units will be investigated by a state-of-the-art analytical approach (re-homogenization, microthermometry, confocal Raman spectroscopy, high-resolution X-ray computed tomography, EPMA, LA-ICP-MS), and (2) radiogenic isotope compositions (Sr-Nd-Pb) will be obtained for major minerals using both in-situ methods (LA-MC-ICP-MS) and high-precision analysis by ID-TIMS. The expected results will allow to constrain the nature and composition of the host melt/fluid at variable evolutionary stages and associated element enrichment processes. Combined with radiogenic isotope tracers which resolve isotopic differences from complex-scale to individual grains, this will clarify if silicate rocks associated with carbonatites indeed represent products of variable stages of metasomatism between carbonatite and crustal host rocks. This finding would have major implications for ore-forming processes of HFSE in carbonatite-alkaline silicate rock complexes, which differ strongly in style and timing depending on whether magmatic or metasomatic processes dominate.

• DFG Programme: Priority Programmes
• Subproject of: SPP 2238: Dynamics of Ore-Metals Enrichment - DOME
• International Connection: Canada, Denmark
• Co-Investigators: Professor Roman Botcharnikov, Ph.D.; Tobias Fusswinkel, Ph.D.; Professor Dr. Francois Holtz; Professorin Dr. Marion Tichomirowa; Ilya Veksler, Ph.D.
• Cooperation Partners: Dr. Troels Nielsen; Dr. Matthew Steele-McInnes