Calcite trace element chemistry and in-situ measured 87Sr/86Sr composition as a recorder of hydrothermal interaction of carbonatite: a case study from the Kamthai complex (western India)

Calcite is one of the primary host for rare earth elements (REE) in carbonatites. The Kamthai carbonatite complex contains the largest REE budget among Indian carbonatite complexes. This study reports new ⁸⁷Sr/⁸⁶Sr isotope ratios of six textural types of calcite in carbonatite and calcite-quartz veins in phonolite of Kamthai along with their major and trace element composition to constrain the magmatic and hydrothermal evolution of carbonatite. Four textural types of calcite (magmatic: Cal^M and Cal^PR; and secondary: Cal^SK and Cal^S) in carbonatite show consistent mantle-like ⁸⁷Sr/⁸⁶Sr ratios. The Cal^M has restricted ⁸⁷Sr/⁸⁶Sr (0.70437±0.00005) and contains high concentrations of ƩREE, Sr, Ba, and Mn without any Ce anomaly. It probably crystallized from the late-stage brine-melt with primary carbocernaite. The Cal^M is partially re-equilibrated into Cal^PR (0.70425±0.00024) during interaction with syn-magmatic fluid, resulting in the loss of a significant amount of REE and Sr. The two secondary varieties of calcite show overlapping and marginally higher ⁸⁷Sr/⁸⁶Sr (Cal^SK: 0.70469±0.00041; Cal^S: 0.70478±0.00025; δCe*: <0.7 for both) than Cal^M, indicating that they were altered by syn– to para– magmatic fluids with partial contribution from external hydrothermal fluid. The fluid-induced re-equilibration led to the expulsion of most of its original Sr and REE content. Two types of secondary calcite (Cal^S1 and Cal^S2) are identified in three veins within phonolite. These contain the lowest abundance of ΣREE+Y, Sr, Ba, and Mn. One type of calcite (Cal^S1) defines two clusters of ⁸⁷Sr/⁸⁶Sr ratios: 1) one cluster (0.70443±0.00035) is identical to Cal^M and crystallized from late-stage syn– to para–magmatic fluids. 2) Calcite defining the second cluster has significantly more radiogenic ⁸⁷Sr/⁸⁶Sr (0.70768±0.00063) compared to the other varieties of calcite. Calcite of both clusters is characterized by prominent negative Ce (δCe*: <<0.034) anomalies, indicating their crystallization from oxidized fluids that removed Ce as Ce(IV). Mixing calculations indicate that mixing of 40–70% post-magmatic fluid with syn-magmatic fluid can account for the higher ⁸⁷Sr/⁸⁶Sr composition. The other type of calcite (Cal^S2) is characterized by LREE-depleted REE patterns without any anomaly and mantle-like ⁸⁷Sr/⁸⁶Sr (0.70434±0.00073). It possibly crystallized from late-stage para-magmatic fluids exsolved from the carbonatite melt after primary LREE mineralization.