Chapter 3. Release of geothermal energy: hot springs and tufa–travertines

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 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 intrabasinal fault intersections. The majority of Lake Edward and Lake Albert active springs, or palaeosprings, are also associated with precipitation of localized tufa–travertine limestones. The cooler tufas may contain calcitized plant roots, leaf imprints and freshwater gastropods. Active scavenging of uranium (U) can be demonstrated in the algae and cyanobacteria that inhabit active spring mouths, and corresponding tufa–travertines are depleted in radioactive U, potassium (K) and thorium (Th) elements. The source for concentrated bicarbonate ions ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mi mathvariant="normal">HCO</mml:mi> </mml:mrow> <mml:mn>3</mml:mn> <mml:mo>−</mml:mo> </mml:msubsup> </mml:math> ) in groundwater at depth – needed to precipitate limestones at the surface – remains problematic. However, rare earth element (REE) plus yttrium (Y) (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.