Title: Hyrdogeochemistry modelling of Chiweta geothermal prospect, Nornern Malawi
Abstract
The geothermal and non-geothermal water chemistry, structures and geology associated with Chiweta
geothermal prospect was studied. The aim was to identify structures and associated geology
controlling the geothermal system recharge and discharge. It also necessitated to quantify main
chemical and physical characteristics of the geothermal water.
Litho-structural surface mapping and remote sensing data was used to delineate structures and
associated geology. Hydrogeological, geothermal mapping coupled with water chemistry was used to
track water movement and quantification of related processes.
Tectonic structures play an essential role in enhancing water flow from recharge area, within the
geothermal system and out flow zone of the Chiweta. Driven by hydraulic gradient at an elevation of
1200 m, the recharged water percolates underground through reactivated pre-Cenozoic and Cenozoic
faults and fractures of NW-SE and NE-SW trending that sliced Karroo sedimentary beds and
metamorphosed basement rock. At depth, the faults and fractures are assumed to create a geothermal
reservoir by enhancing the permeability of the subsurface rocks. Thermal water hosted by
metamorphic and Karroo sedimentary rocks emerge along the NW-SE fault lineaments as hot springs,
thermally altered grounds and shallow hot water borehole. Surface temperatures of thermal springs
are about 80°C. The discharged thermal water from the 32 m depth well registered a temperature of
46°C.
The proposed preliminary conceptual model of the geothermal system suggests the Chiweta geothermal
prospect is a low temperature fault controlled geothermal system in sedimentary environment. The
recharge waters belong to Ca-Mg-HCO₃ type with temperature between 23 to 33°C and pH of 5.7-7.5.
The inflow waters attain the heat from elevated geothermal gradient at depth, an anomaly assumed to
be associated with crustal thinning due to Malawi Rift spreading. All thermal waters belong to
Na-Cl-SO₄-HCO₃ facies. Chemical geothermometers suggest the subsurface reservoir temperature of
about 132-157°C. Multiple mineral equilibria and mixing models are in good agreement with the
solute geothermometers estimated subsurface temperature range. The elevated temperature is enough
to drive dissolution of the host rock and ion exchange reaction in the reservoir of the geothermal
system that modify chemical composition of reservoir and thermal springs water to Na-Cl-SO₄-HCO₃
facies from recharged Ca-Mg-HCO₃ water type. The thermal water Cl/B ratio approached that of Cl/B
rock ratio. No boiling is occurring during the ascent of the thermal waters but steaming at the
surface causes minimal δD and δ¹⁸O isotopic fractionation. Reservoir pH is slightly lower than in
situ pH in the liquid phase due to loss of acid gases, mainly CO₂. Saturation of calcite and quartz
is low in thermal spring waters indicating limited scaling potential. Reconstructed reservoir
waters are saturated in talc, chrysotile, quartz and calcite indicating high chanced of scaling.
Both thermal and non-thermal waters of the Chiweta system originate as precipitation in the western
highlands as indicated by depleted δD and δ¹⁸O stable ily enriched δD and δ¹⁸O isotope lake water
does not
contribute to recharge of the geothermal system.