Chapter 3. Release of Geothermal Energy: Hot Springs and Tufa-Travertines

Abstract

Heat and groundwater flow through a rift basin are an integral part of its geodynamics, but predicting what is happening below the surface is often difficult due to a lack of direct information. Field observations on the occurrence, subaerial distribution, temperature and geochemistry of freshwater springs may help to form an idea of groundwater flow through the basin and what it is, or has been, flowing through. Hot springs in the Albertine Rift are common and their occurrence is directly linked to deep-seated main rift bounding faults, or major intra-basinal fault intersections. The majority of Lake Edward and Lake Albert active, or paleo-springs, are also associated with precipitation of localised tufa-travertine limestones. The cooler tufas may contain calcitised plant roots, leaf imprints and freshwater gastropods. Active scavenging of uranium can be demonstrated in the algae and cyanobacteria that inhabit active spring mouths and corresponding tufa-travertines are depleted in radioactive K, U and Th elements. The source for concentrated HCO3- in groundwater at depth, needed to precipitate limestones at the surface, remains problematic. However, REE+Y geochemistry of the tufa-travertines suggest end-member sources of either carbonatites or marine limestones, indicating the possibility of a pre-Neogene rift sequence beneath the Albertine Rift.