Rare earth element- niobium enrichment in carbonatite hosted within the Clay-Howells Alkalic Complex, Kapuskasing, Ontario: Implications of auto-oxidation and magnetite fractionation in ferrocarbonatites

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Michael Richards 2011
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University of New Brunswick
The (auto-) oxidation process of ferrocarbonatite (ankerite) magma to crystallize magnetite with migration to shallow depth appears to involve volatile exsolution (H20, F, C02, H2). This process may explain the crystallization of magnetite and REE-Nb enriched cumulate bodies with calcite at low magmatic temperature and subvolcanic pressure, with increasing oxygen fugacity. These cumulates were derived from extreme fractionation of an Fe-Mg-rich calciocarbonatite (inferred to have originated as a magnesiocarbonatite) with high REE- and Nb- oxide components. H20 exsolution from the crystallizing ferroan carbonatite magma (during high level emplacement), is believed to be one of the possible sources of oxygen required to facilitate the oxidation process, possibly releasing H2 (gas) as a byproduct (eq. 1). The decarbonation of the melt causes Ca, Mg, and Fe carbonate components to react with the silicate component of the melt as well, with C02 as a byproduct further increasing the oxygen fugacity Carbonatite magmas have extremely low viscosities and liquidus temperatures, which enable emplacement to shallow levels and possibly even eruption. 6 (Ca-7+, Mg-7+, Fe-7+) C03 + H20 --> Fe 3+2 Fe-7+0 4 + 3 (Ca2 +, Mg 2+) C03 + 3C02 +H2 (gas) (1) As the carbonatite melt becomes depleted in ferrous iron with oxidation, calcite and dolomite begin to crystallize with magnetite; this cogenetic crystallization relationship between carbonate and magnetite is manifested through the magnetite covariant relationship with Nb, supported by microscopic evidence of Nb-oxide grains hosted as inclusions within magnetite (and vice versa). Geochemically, relationships are shown between total iron content (Fe20n), Nb and REE's using Pearson Product and Spearmans rank correlation coefficients. Areas where large amounts of dolomite and calcite have crystallized contain euhedral niobium oxides (chiefly columbite, but locally manifest as exsolution lamellae in pyrochlore in small amounts). The presence of Fluorine is indicated by ~5% modal fluoroapatite, which also lowers the liquidus temperature of the melt. General rheological properties of carbonatites below the decarbonation threshold of 1 bar have been estimated to have a viscosity of 5x 10-2 poise, density of 2.2g/cm3, thermal conductivity of 1.65x10-2, thermal diffusivity of 4.2x10-3, heat of fusion of 125, heat capacity of 2, and a thermal expansion of 2.3x 10-4. These characteristics - along with mineral densities - have been used to estimate settling velocities for magnetite crystals with diameters ranging from 0.2 to 5 mm within the range of 0. 7 - 25 cm/s. Calcite crystals with diameters rangi ng from 0.1 - 2 mm were also estimated to have settling velocities ranging from 0.22 - 8 cm/s. The calculated settling velocities of magnetite, columbite-pyrochlore, and monazite crystals in fenoan carbonatite are relatively similar; this produces dynamically interlayered massive magnetite and magnetite-carbonate layers, with REE and Nb-oxide abundances most enriched within the magnetite cumulates.