A model for the evolution of hematite carbonatite, based on whole-rock major and trace element data from the Fen complex, southeast Norway

Abstract

REE-rich hematite carbonatites are found in numerous carbonatite complexes worldwide, commonly associated with ankerite-bearing carbonatites. The mineralogy of the hematite carbonatites contradicts a primary magmatic origin. Hematite-calcite-dolomite carbonatite in the Fen complex (southeast Norway) was formed by postmagmatic oxidation and partial dissolution of ankeritic ferrocarbonatite. In this process, major components such as CaO, MgO and CO 2, and trace elements such as the light rare earths, were removed from the rock, while more insoluble components (Fe 2O 3, Th, Y and the middle rare earths) were retained in the solid residue. It is possible to model the alteration of the rocks in terms of a two-stage recrystallization and leaching process, corresponding to two distinct stages of the reequilibration of the carbonatite mineralogy. The first step took place at oxygen fugacities below the hematite-magnetite buffer (ferrous iron in part leached from the system), the second at more oxidizing conditions (Fe immobile). The most strongly altered carbonatite has lost more than 70% of its original volume during reequilibration. Ferrous iron and REE leached from the ferrocarbonatite during early reequilibration precipitated as hydrothermal magnetite-pyrite ores with accessory REE minerals. Reequilibration, leaching and redeposition processes like those recognized at Fen may be important factors controlling the mineralogy and geochemistry of REE-enriched carbonatites in general.