Kris Sokol PhD work on Fenite Completed

Congratulations to PhD student Krzysztof (Kris) Sokó­ł who has submitted, successfully defended and now had his PhD thesis confirmed. His title was: THE NATURE OF FLUIDS ASSOCIATED WITH THE SUBVOLCANIC ALKALINE MAGMAS AND THEIR ROLE IN ’HI-TECH’ METAL TRANSPORT AND MINERALIZATION, and the work explores the solubility of high-field strength elements (HFSE) and rare earth elements (REE) in natural hydrothermal fluids. We use an area of Southern Greenland adjacent to the Illerfissalik intrusion (previously spelled “Igdlerfigssalik”) as a natural laboratory. Here, the country rocks have been profoundly chemically and isotopically modified by a fluid exiting from the adjacent igneous body to create an altered rock known as “fenite”. Kris used chemical and isotopic means to reconstruct the composition of the fluid and showed that it had transported significant volumes of HFSE and REE. He was able to estimate the temperature and provenance of the fluid as a function of time.

The importance of his work is that it shows that those fluids transported truly astounding volumes of HFSE and REE. In the case of Illerfissalik, the fluid exited the magmatic body, dissipating tonnages of HFSE and REE equivalent to some of the world’s largest ore bodies into the surrounding country rock. Had this not happened, Illerfissalik would surely have evolved to be a globally significant agpaitic critical metal deposit. The work shows us that fluid make – or break – critical metal deposits.

Parts of Kris’s thesis have already been published in his Geology paper in 2021 but the whole project will be made public after an embargo period so that we can continue to bring this important research into the public domain. However, we are happy to respond to any queries about the work in the interim. His abstract is given in full below.

THE NATURE OF FLUIDS ASSOCIATED WITH THE SUBVOLCANIC ALKALINE MAGMAS AND THEIR ROLE IN ’HI-TECH’ METAL TRANSPORT AND MINERALIZATIONAbstract

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 target these better, an improved 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 (e.g. Kringlerne, Ilímaussaq) and underscores how critical fenitization is as a tipping point during evolution of fertile magmas in the upper crust.

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