Abstract
ABSTRACTThe Fen Complex in Norway consists of a ~583 Ma composite carbonatite-ijolite-pyroxenite diatreme intrusion. Locally, high grades (up to 1.6 wt.% total REE) of rare-earth elements (REE) are found in a hydrothermally altered, hematite-rich carbonatite known as rødbergite. The progressive transformation of primary igneous carbonatite to rødbergite was studied here using scanning electron microscopy and inductively coupled plasma-mass spectrometry trace-element analysis of 23 bulk samples taken along a key geological transect. A primary mineral assemblage of calcite, dolomite, apatite, pyrite, magnetite and columbite with accessory quartz, baryte, pyrochlore, fluorite and REE fluorocarbonates was found to have transformed progressively into a secondary assemblage of dolomite, Fe-dolomite, baryte, Ba-bearing phlogopite, hematite with accessory apatite, calcite, monazite-(Ce) and quartz. Textural evidence is presented for REE fluorocarbonates and apatite breaking down in igneous carbonatite, and monazite-(Ce) precipitating in rødbergite. The importance of micro-veins, interpreted as feeder fractures, containing secondary monazite and allanite, is highlighted. Textural evidence for included relics of primary apatite-rich carbonatite are also presented. These acted as a trap for monazite-(Ce) precipitation, a mechanism predicted by physical-chemical experiments. The transformation of carbonatite to rødbergite is accompanied by a 10-fold increase in REE concentrations. The highest light REE (LREE) concentrations are found in transitional vein-rich rødbergite, whereas the highest heavy REE (HREE) and Th concentrations are found within the rødbergites, suggesting partial decoupling of LREE and HREE due to the lower stability of HREE complexes in the aqueous hydrothermal fluid. The hydrothermal fluid involved in the formation of rødbergite was oxidizing and had probably interacted with country-rock gneisses. An ore deposit model for the REE-rich rødbergites is presented here which will better inform exploration strategies in the complex, and has implications for carbonatite-hosted REE resources around the world.
Highlights
THE Fen Complex in southeastern Norway (Fig. 1) is one of the world’s classic carbonatite complexes; it was here that the igneous nature of carbonatite was first recognized (Brøgger, 1921) and significant advances in the understanding of the petrologyThis paper is part of a special open access issue in the Mineralogical Magazine entitled ‘Critical-metal mineralogy and ore genesis’
The detailed mineralogical and geochemical investigations of the progressive transformation of primary carbonatite to rødbergite has provided new insights into the rare-earth elements (REE) concentration processes during the alteration of carbonatite: (1) The progressive alteration is associated with a 10-fold enrichment of REE in the altered samples relative to the primary igneous carbonatite
The main cause of this enrichment is the precipitation of secondary monazite-(Ce) in the matrix, and the occurrence of monazite and allanite micro-veins, all associated with the rødbergiteforming process
Summary
THE Fen Complex in southeastern Norway (Fig. 1) is one of the world’s classic carbonatite complexes; it was here that the igneous nature of carbonatite was first recognized (Brøgger, 1921) and significant advances in the understanding of the petrologyThis paper is part of a special open access issue in the Mineralogical Magazine entitled ‘Critical-metal mineralogy and ore genesis’. Of carbonatites (Brøgger, 1921; Sæther, 1957; Ramberg, 1973; Griffin and Taylor, 1975; Mitchell and Brunfelt, 1975) and their REE contents (Andersen, 1984) were based on studies in the Fen Complex. REE are a resource of critical and strategic importance for present and future technology (European Commission, 2014). Carbonatite complexes such as Bayan Obo (China), Araxá and Catalão (Brazil) and Phalaborwa (South Africa) are the main sources for light rare-earth elements (LREE) and Nb, and contain significant reserves of Cu, Ti, baryte, fluorite, vermiculite, Sr, V, Th, U and P (Mariano, 1989; Groves and Vielreicher, 2001; Cordeiro et al, 2011; Smith et al, 2015). Rødbergite (‘red rock’ in Norwegian) is usually defined as a calcite-dolomite carbonatite stained red by disseminated fine crystals of hematite (Andersen, 1984)
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