Brine–Melts and Fluids of the Fe-F-P-: The Petrogenetic Aspects

The carbonatite complexes of the Central Asian carbonatite province comprise the Siberian carbonatites of the Western Transbaikalia and the Central Tuva regions, as well as those from the Mushugai-Khudag complex in Southern Mongolia. They are confined to Late Mesozoic rift structures and have endured considerable tectono-magmatic processes caused by intense plume activity, which also accompanied their formation. A systematic study of melt and fluid inclusions revealed that these carbonatites formed as a result of immiscibility processes in silicate-carbonate (salt) melts, as well as fractional crystallization. Alkaline-carbonatite rocks crystallized in the presence of brine-melts with different compositions, i.e., alkaline-fluorine, carbonate, sulfate, phosphate, and chloride. These melts are responsible for mineralization during the orthomagmatic stage and the primary phase of Fe-F-P-(Ba)-(Sr)-REE ore formation at temperature ranges of 850-830 °C, 650-610 °C, and 560-440 °C and pressures between 290 and 350 MPa. At a later stage, the brine-melts evolved into saline hydrothermal fluids, which are considered to be the source of the second stage of F-(Ba)-(Sr)-REE ore mineralization. The saline crystal-fluid inclusions consist mainly of fluorine-sulfate-carbonate-chloride and bicarbonate-chloride compositions, with temperatures of approximately 480-250 °C and pressures below 250 MPa. The shift from melt to fluid in carbonatite complexes could occur more frequently in nature than previously believed and could also apply to other F-REE carbonatite complexes that are linked to rifting and plume activity in mountain-building zones.