Fluid inclusion studies of the Kenticha rare-element granite-pegmatite, Southern Ethiopia

In the Kenticha area, a series of barren to rare metal-bearing pegmatites intruded into the Neoproterozoic Adola Belt. The pegmatites host world-class Nb and Ta deposits and significant Li and Be reserves. In this contribution, fluid inclusion data and feldspar geothermometry have been combined to define the crystallization condition of the Kenticha rare-metal pegmatite. Primary and complex assemblages of secondary fluid inclusions representing episodic fluid circulations have been identified in quartz and spodumene. A primary aqueous-carbonic fluid of low salinity aqueous solution with liquid and vapour CO2 phases, secondary carbonic fluid rich and carbonic-only fluids, and multiple generations of secondary aqueous inclusions that represent sub-solidus hydrothermal circulation have been identified. All aqueous inclusions were homogenized into the liquid phase between 100 and 290 °C. Aqueous-carbonic inclusions were homogenized, usually via a critical transition [Th(LV → SCF)] between 241 and 397 °C, or less commonly, via a dew-point transition [Th (LV → V)] between 213 and 264 °C. Crystallization of the rare-element pegmatite is certainly associated with the late-stage magmatic or early hydrothermal low-salinity aqueous-carbonic fluid that homogenizes to critical conditions. A combination of microthermometric data and existing experimentally determined solidus from flux and volatile bearing haplogranite suggests exsolution of fluids from hydrous silicate melt, perhaps during crystallization of the aplitic layer. The fluids were then trapped and isobarically cooled along a reasonable geothermal gradient within the pegmatite unit down to a temperature of around 397 °C.