The evolution of carbonatite melts and their aqueous fluids : evidence from Amba Dongar, India, and Phalaborwa, South Africa

The Amba Dongar complex, India, consists of calciocarbonatite, ankerite-dominated ferrocarbonatite and numerous nephelinitic bodies intruded into Late Cretaceous quartz sandstones and Deccan basalts.Ankeritic (mottled) and calcite +/- barite +/- ankerite (multiphase) melt inclusions were observed in apatite hosted by calciocarbonatite. Mottled inclusions start melting at temperatures of 610°C and are completely molten at temperatures >800°C. The onset of melting, of multiphase inclusions, is at 680°C, and final melting occurs above 1100°C. Based an the coexistence of mottled (ankeritic) and multiphase (calcitic) inclusions, a model is proposed for the genesis of calciocarbonatite and ferrocarbonatite that calls upon separation of immiscible ankeritic and calcitic liquids from a parent carbonate melt.Primary fluid inclusions, in apatite hosted by calciocarbonatite, and in quartz of the surrounding fenitized sandstones, record a complex evolution of aqueous fluids. The fluids (orthomagmatic) were trapped at temperatures between 1000 and 260°C, at pressures ranging from 12.5 kbars to 500 bars, respectively, while fenitizing fluids ranged in temperature 260 to 120°C, and were trapped at <500 bars pressure. The low temperature phase behaviour of all inclusions indicates that the fluids are NaCl-, KCl-bearing brines with salinities between 15 and 1.5 wt.%. The compositions of decrepitate residues indicate that early fluids were dominated by SO4 2- and HCO3-, and had low Na/Na+K ratios (