Carbonatite metasomatism drives PGE enrichment in cratonic roots

Cratons host diverse metal deposits, including Cu-Ni-PGE deposit systems, and their lithospheric mantle roots have recently been proposed to contain sulfide-hosted metal reservoirs that can provide a potential metal source for ore-forming systems. The base of cratonic mantle lithosphere (cratonic roots) have been suggested to be metasomatized by carbonate-rich magmas episodically over long periods of time. However, whether carbonated cratonic roots are ubiquitously enriched in Platinum-group elements (PGEs) and related metal elements remains debated. The Aillik Bay intrusive suite in Labrador, Canada, preserves magmatic rocks formed by the melting of cratonic roots in two stages: carbonate-poor lamproites in the Mesoproterozoic (~1.37 Ga) and carbonate-rich ultramafic lamprophyres (aillikites) in the Neoproterozoic (~590-555 Ma). These were succeeded by nephelinites during the Early Cretaceous (~142 Ma) by melting at shallower levels after the craton had been split. These samples constitute an ideal natural archive to test the hypothesis of whether carbonated melts drive PGE enrichment in cratonic roots. Here we present a systematic petrographic, whole-rock PGE, and Re-Os isotopic study of these alkaline silicate rocks and associated carbonatites, aiming to evaluate the temporal evolution of PGE budgets within cratonic roots. Rocks from all three periods contain well-preserved magmatic sulfides with negligible alteration, indicating that the observed PGE signatures are controlled by magmatic processes rather than post-emplacement overprinting or secondary alteration. Geochemical constraints further suggest that these magmas were generated under sulfide-saturated (or near-saturated) conditions in their source regions, establishing a basis for assessing sulfide control on PGE behavior. The lamproites formed in reduced, metal-bearing rocks and display MORB-like PGE patterns with depletion of IPGEs and enrichment of PPGEs, with IPGE contents slightly higher than MORBs. In contrast, the aillikites show significant IPGE enrichment, markedly different from MORB patterns. The lamproites and aillikites yield low and primitive mantle-like initial ¹⁸⁷Os/¹⁸⁸Os ratios (0.078 and 0.130), respectively. The Cretaceous nephelinites originated from melting of mantle source metasomatized by aillikite magmas and show MORB-like PGE patterns and initial ¹⁸⁷Os/¹⁸⁸Os ratios typical for metasomatized mantle sources. These observations point to a key control of CO₂ concentrations in magmas on PGE signatures. Therefore, we suggest that carbonatite metasomatism can enrich cratonic roots in IPGE.