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Alkaline igneous rocks host many High Field Strength (HFSE) and Rare Earth Element (REE) deposits, which are key for the global low-carbon energy transition. To better target these, a better understanding of alteration associated with silicate-related ore deposits is required. Fluid-rock interaction around syenite forms altered haloes ('fenite'), normally uneconomic, but hosting many of the normally immobile HFSE expelled from the magmatic hearth. The fluid parameters which control the transport and deposition of these elements in the crust, their composition and the element speciation within need to be better constrained. Here a field study of an exceptionally exposed fenite (Gardar Province, SW Greenland) was followed by major and trace element, and stable isotope (O-H-S) analyses of whole rocks and minerals, and the investigation of fluid inclusion assemblages. The field data show the volcano-sedimentary wall-rocks to the west of Illerfissalik centre were altered by the ejection of metasomatic fluids. Fenitized sediment comprises a skarn-like (garnet-absent) calc-silicate assemblage developed interstitially, often along relict structures, later subjected to short-lived Na- Fe(III) -Ti alteration. Using oxygen isotope equilibria, the temperature of the causal fluid was estimated at ~750°C suggesting a magmatic origin. The differences in O, H and S isotopic data also indicate the system underwent fluid-driven exchange with the country rocks, potentially mixing with externally derived meteoric water. Elements including Ti, Zr, Nb and REE are mobile during alteration, but they precipitate as silicates and phosphates mainly in the late-stage in the basement granite, while rare chevkinite and titanite host much of the early-stage HFSE in the Eriksfjord. An estimated total REE-oxide cargo mobilized from Illerfissalik (~43 Mt) rivals the tonnages of many major ore deposits (i.e. Kringlerne, Ilímaussaq) and underscores how critical fenitization is as a tipping point during evolution of fertile magmas in the upper crust.