Abstract
Ferrocarbonatites from the lamprophyric Alpine Dyke Swarm, south Westland, New Zealand are composed of dolomite–calcite-albite–hematite and contain interstitial patches dominated by calcite–ancylite–barite–monazite–thorite–albite–aeschynite–analcime, interpreted as modified late-stage segregations. The dominant carbonate in the ferrocarbonatite is a ferroan dolomite that contains vermicular and blocky patches of calcite and a more Fe-rich ankerite. The calcite is interpreted as the product of exsolution or the by-product, with dolomite and hematite, of the oxidation of primary ankerite during interaction with hydrothermal fluids. Late stage ancylite-rich segregations have elevated 87Sr/86Sr compositions relative to host carbonatite suggesting they have crystallised from fluids that have equilibrated with host schist, but with the REE derived from fractionation of ferrocarbonatite. Mineral veining indicates this stage of crystallisation post-dated the ankerite to dolomite replacement.
The carbonatites, as inferred from mineral associations, textures and carbonate geothermometry, have a complex history of recrystallisation from late stage, low temperature carbo-hydrothermal fluids. All carbonatitic phases have convex-upward chondrite normalised REE patterns enriched in Nd, and are relatively depleted in lighter LREE and in HREE. Depletion in La and Ce is attributed to either complementary enrichment in felsic magmas during initial separation of carbonatite, or more likely, to subsequent preferentially enhanced mobility of La– and Ce–Cl− complexes in low-temperature aqueous fluids.
BSE image of sector-zoned ancylite enclosed in late stage calcite (dark) from ferrocarbonatite. Bright zones are Nd-rich (ancylite–(Nd)), darker zones are enriched in Sr (ancylite–(Ce). [Display omitted]
•We describe the middle REE-rich mineralogy and complex crystallisation and fractionation history of ferrocarbonatite•The dominant dolomite-calcite-hematite assemblage has been derived by the secondary hydrothermal oxidation of ankerite•Middle REE enrichment is due in part to selective carbothermal mobility of LREE complexes in aqueous fluids