Fluid–rock reactions in the 1.3 Ga siderite carbonatite of the Grønnedal–Íka alkaline complex, Southwest Greenland

Abstract

Petrogenetic studies of carbonatites are challenging, because carbonatite mineral assemblages and mineral chemistry typically reflect both variable pressure–temperature conditions during crystallization and fluid–rock interaction caused by magmatic–hydrothermal fluids. However, this complexity results in recognizable alteration textures and trace-element signatures in the mineral archive that can be used to reconstruct the magmatic evolution and fluid–rock interaction history of carbonatites. We present new LA–ICP–MS trace-element data for magnetite, calcite, siderite, and ankerite–dolomite–kutnohorite from the iron-rich carbonatites of the 1.3 Ga Gronnedal–Ika alkaline complex, Southwest Greenland. We use these data, in combination with detailed cathodoluminescence imaging, to identify magmatic and secondary geochemical fingerprints preserved in these minerals. The chemical and textural gradients show that a 55 m-thick basaltic dike that crosscuts the carbonatite intrusion has acted as the pathway for hydrothermal fluids enriched in F and CO2, which have caused mobilization of the LREEs, Nb, Ta, Ba, Sr, Mn, and P. These fluids reacted with and altered the composition of the surrounding carbonatites up to a distance of 40 m from the dike contact and caused formation of magnetite through oxidation of siderite. Our results can be used for discrimination between primary magmatic minerals and later alteration-related assemblages in carbonatites in general, which can lead to a better understanding of how these rare rocks are formed. Our data provide evidence that siderite-bearing ferrocarbonatites can form during late stages of calciocarbonatitic magma evolution.