The Jammi area consists of Archaean mafic volcanic rock and tonalitic gneiss, which were intruded by the Devonian Sokli carbonatite, causing large-scale and intense fenitization of the country rocks. The rocks are albite-fenites, crosscutting late carbonatite veins, with the mineral assemblages of calcite-dolomite-aegirine-albite-apatite-phlogopite.
Geochemical analyses indicate that the total content of rare earth elements (REE) in the carbonatite veins is 0.11–1.83%, including 0.11–1.81% LREE and 0.002–0.041% HREE. Seven rare earth minerals have been analysed in detail from the area, including Sr-apatite, monazite, ancylite (Ce), bastnäsite (Ce), strontianite, baryte and brabanite.
The Jammi carbonatite and fenite samples display similar REE patterns, with highest enrichment of LREE, and steep slopes towards the HREEs. The REE distribution in the whole-rock data is controlled by carbonates and apatite. REE-rich accessory phases such as monazite and ancylite (Ce) influence the REE pattern only when present in large amounts.
Mineralogical and chemical evidence demonstrates that hydrothermal processes were responsible for the REE mineralization. During late-stage processes, apatite and carbonate minerals were replaced by various assemblages of REE–Sr–Ba minerals. Apatite could be a potential source for REE as a by-product of phosphate production in the Sokli area.