Abstract

The Tertiary (22 Ma) carbonatite centre at Buru hill is located towards the eastern end of the Nyanza rift, western Kenya. As revealed by geochemistry and petrology of drill core samples, the Buru centre ranges in composition from surficial lateritized pyroclastic ferrocarbonatite to recrystallized ferruginous calciocarbonatite at depth.

Mineralogically the lateritic cover of the Buru carbonatite centre is characterized by goethite, hematite, psilomelane, baryte, fluorite with minor calcite, bastnäsite and significant amounts of monazite. A transitional middle zone consists of ferrocarbonatite grading into ferruginous calciocarbonatite containing siderite, baryte and fluorite, a greater proportion of bastnäsite replacing calcite, but lesser proportions of monazite. The lowermost zone consists of ferruginous calciocarbonatite containing intimate mixtures of siderite and calcite, with parisite and synchysite existing as major replacive components of calcite down to depths of 200 m. Synchysite appears to be the more stable form of lanthanide fluorcarbonate at depths below 150 m.

Oxygen isotope measurements on separated calcite concentrates from three of the Buru drill cores reveal significant deviations from primary igneous carbonatite values suggesting isotopic re-equilibration with meteoric water. Accepting that the African plate may have been stationery during the past 30 Ma, it is possible to use a modern sample of water from the Kenya rift to estimate the equilibration temperature at which Buru carbonatite in western Kenya has recrystallized. Temperatures of ~60 to 90°C suggest that the hot-spring activity encouraged the formation of replacive lanthanide fluorcarbonate in ferruginous calciocarbonatite. Lanthanide enrichment also occurred closer to the surface within the pyroclastic ferrocarbonatite at temperatures of ~40 to 60°C corresponding to waning stages of hot-spring activity. Further lanthanide enrichment redistribution and oxide formation occurred at surface supergene temperatures of ~20 to 30°C forming the lateritized top of the carbonatite.

The wider implications from these carbonatite studies are the recognition that lanthanide enrichment can be preserved in volcanic-subvolcanic centres, and that lanthanide fluorcarbonates can replace calcite in response to circulating hot-spring waters. From a classification point of view, ferrocarbonatites should be seen as products of meteoric hydrothermal systems unrelated to carbonatite magmatic processes.

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