Carbonatites and carbothermalites: A revised classification

Carbonatites (sensu lato) are distinguished on a mineralogical-genetic basis as primary, high-temperature magmatic rocks as opposed to low-temperature, carbothermal, deuteric residual deposits for which we propose the name “carbothermalites”. Carbothermal deposits are analogous to hydrothermal ore deposits and are formed from fluids dominated by CO2 and/or carbonate anions. They can be formed from a variety of parental magmas ranging from primary magmatic carbonatites to diverse sodic or potassic silica-saturated and silica-undersaturated syenites. Each group is characterized by distinctive mineralogy and texture. We propose that the taxonomy of carbonatites and carbothermalites should be undertaken only on the basis of their modal mineralogy. The nomenclature of “carbonatites” (sensu lato) and of compositional classifications is reviewed and it is recommended that otiose names such as sövite, alvikite, beforsite, etc. be abandoned and that both carbonatites and carbothermalites be described using modal mineralogical nomenclature (e.g. apatite calcite carbonatite; bastnäsite sahamalite carbothermalite). The currently used bulk rock major element compositional classifications of carbonatites (sensu lato) are reviewed and it is concluded that these do not reflect the modal mineralogy of either carbonatites or carbothermalites. We conclude that there is little purpose in presenting the bulk rock compositions of such rocks or employing them in compositional classifications. However, recognizing that old habits die hard, we present with considerable reservations a quaternary SiO2-MgO-CaO-(Fe2O3)T diagram as an alternative scheme of representing bulk compositional variation for those who persist in using such an approach as this offers some improvement over those currently in use.