MINERAL PRECIPITATION AT THERMAL SPRINGS IN THE KENYA RIFT VALLEY

Many hot springs in the Kenya Rift Valley flow into wetlands or alkaline lakes, either at the surface or directly from the lake floor. Those springs commonly precipitate suites of authigenic minerals that provide a record of their activity and chemistry, and can give clues to the basin paleohydrology. Most springs discharge alkaline fluids (pH 7-10) of Na-HCO₃-Cl composition, similar to other alkaline waters in the rift. Consequently, it is important to distinguish minerals precipitated at hot springs from those precipitated in saline, alkaline lakes. Acid sulfate waters are limited to areas of fumarolic activity near central volcanoes.

Siliceous sinter is rare at most modern hot springs because the discharged fluids are initially undersaturated with amorphous silica, but they may precipitate opal-A crusts where fluids evaporate and cool. Some chloride springs precipitate sublacustrine sinter. Carbonates are widespread, and include calcite tufas at low-enthalpy springs, and travertines composed of aragonite and calcite at hot and boiling springs. Crystal fabrics imply that most carbonate precipitation is episodic and rapid. Many alkaline hot springs have little to no alkaline earths and are fed by waters with low silica concentrations. These springs leave no mineral record of their surface activity.

Many other authigenic minerals precipitate around the thermal springs. An Al-rich halo envelopes the vents of many boiling springs. Al is preferentially enriched in proximal opal-A crusts, but also is present in authigenic zeolites and clay minerals. Minute (1 μm) analcime crystals and other zeolites have precipitated on calcite substrates in some travertines. Clay minerals with a reticulate morphology that resembles that of desiccated bacterial exopolymers are common in proximal travertines and silica crusts. Most of these Mg-rich clays, which can form laminae 1-10 mm thick, appear to be poorly crystalline smectites and (or) sepiolite. Magadiite is present as veins in hydrothermal sediments at Lake Bogoria, Na-Al gels at Nasiki Engida. Fluorite, a common accessory mineral in some travertines, cements or locally replaces the carbonates, including those of other origin (rhizoliths, stromatolites, calcrete) in sites of thermal water outflow. Fe and Mn oxihydroxides are common accessories.