FLUID INCLUSION AND CATHODOLUMINESCENCE STUDIES ON FLUORITE FROM THE KERIO VALLEY, KENYA

The Kerio valley lies between the Elgeyo escarpment and the Tugen hill s which mark the western margin of the Kenya rift valley. The main flu orite deposits are located in the southern part of the valley at Kimwa rer, Choff and Kamnaon. Three types of inclusion fillings were identif ied: Liquid+Vapour, Liquid+Daughter Minerals and Liquid. The L+V type is dominant. Inclusions occur as clusters, trails along the crystal gr owth zones and as isolated ones. Low salinities, apparently lower than the 5% wt. NaCl equivalent, were established. Homogenization temperat ures suggest that fluorite mineralization took place at different stag es and at temperatures between 120 and 180 degrees C. Isolated reading s above 180 degrees C may be referring to the original inclusions in l imestone. These measurements and the absence of CO2 in the inclusions, as well as the occurrence of vugs and crustifications within fluorite , suggest that mineralization took place at relatively shallow depths. Emission spectrum lines representing Eu2+ Dy3+, Tb3+ and Sm3+ in fluo rite were identified. Sm3+ was detected only in the pinkish luminescen ce of veined fluorite: whereas the pinkish zone in banded fluorite con tains Tb3+. Eu2+ which gives the strongest emission lines in the blue part of the visible spectrum, apparently is responsible for the strong blue cathodoluminescence (CL) in fluorite. The dominance of Eu2+ peak s further points to the fact that fluorite mineralization in the Kerio valley took place in an environment that was enriched in Lanthanide R are Earth Elements (LREE). The presence of rare earths and radioactive elements in fluorite points towards their enrichment in the environme nt of fluorite mineralization. A juvenile origin of mineral forming so lutions is proposed. Two generations of fluorite were established: all otriomorphic fluorite, forming the matrix, and the idiomorphic variety , occurring either in barite or in druzes in early fluorite. Barite in turn farms idiomorphic crystals in allotriomorphic fluorite. Relies o f calcite occur in both K-feldspars and in early fluorite. Oxides and hydroxides of Fe, Mn,Ti and Al commonly occur in open spaces in fluori te. Of significance is the presence of gold in fluorite. Fluorite mine ralization is of hydrothermal origin in the post-Miocene era and was f ormed as a result of metasomatic replacement of marble and open space fillings.