by Sam Broom-Fendley, Michael Marks, Kathryn Goodenough, John Spear

Alkaline-carbonatite igneous complexes host the world’s largest and highest grade rare earth element (REE) deposits and can also contain deposits of other critical raw materials, such as P, Nb, Ti, Zr, Sr, fluorite, and barite. They are critical for the global transition from fossil fuels towards a low-carbon society. Many aspects of these complexes remain poorly understood, particularly relating to variation in composition and structure with depth. A significant question for this project is the spatial and temporal variability in fluids associated with alkaline-carbonatite complexes, and their influence on microbial deep life and on groundwater systems. We propose to undertake deep drilling of the well-characterised REE-rich alkaline-carbonatite complex Songwe Hill in Malawi, to investigate how fluid-rock interactions have evolved over time to control mineralisation geometry, groundwater chemistry, and deep subsurface life, and to understand how REE-rich carbonatites, unmineralized carbonatites and associated alkaline rocks relate in depth to each other. This proposed new scientific drilling will be integrated with existing shallow cores and geophysical data provided by our industry partner, Mkango Resources, in order to understand the architecture of REE-bearing alkaline-carbonatite systems, the depth-dependent evolution of carbonatitic magmas and associated fluids, and the wider subsurface environment. This will be a unique research project, uniting economic geologists, petrologists and geochemists, hydrogeochemists and microbiologists to build a holistic model of the whole subsurface environment at an alkaline-carbonatite complex. We will work with colleagues in Malawi to integrate this research with knowledge exchange for the wider community, and to ensure long-term societal benefits from the proposed research. The pre-proposal for this project was approved in 2021, followed by a successful workshop proposal in 2022. The workshop was held in May 2023, in Malawi, with 50 attendees from 10 countries. Two days of wide-ranging discussions and one-day visiting the field site provided the basis for a full proposal submitted in 2024. This revised full proposal takes all the SAG comments on the initial version into account. The proposed project will work with the Malawi government, universities, and local school communities. Our PI/co-I/science team includes experts covering all science questions, and is drawn from 15 nations, of which 10 are current ICDP members. We aim for two deviated boreholes, cored to ~1200 and ~1450 m measured depth, that are expected to represent two inclined slices through the inferred vertically zoned structure.

• To understand the formation and preservation of REE mineralisation in an alkaline-carbonatite system, and the effect this has on the surrounding environment and deep-seated subsurface microbial life, we will consider four major, closely-linked, societally relevant science objectives that can be addressed through drilling. Objective 1: Magmatic-hydrothermal evolution of alkaline-carbonatite complexes and their associated fluids, and the development of REE-mineralisation. Objective 2: Structural pathways, tectonic setting, and how these control magma emplacement, and subsequent uplift and erosion or preservation. Objective 3: Interactions with groundwater and the influence this has on preservation of the mineralisation and on the chemistry of the wider hydrogeological system. Objective 4: The influence of deep microbial communities on the fluid-rock system

Alkaline Rocks, Carbonatite, REE, Groundwater, Deep Life

Latitude: -15.70155, Longitude: 35.8215