The Saxothuringian zone in eastern Germany is part of the Central European Variszides
and located at the northwestern edge of the Bohemian Massif. This zone is of interest
for geothermal research as metamorphic rocks of different composition and
petrophysical properties are invaded by (partly) voluminous masses of high heat
production (HHP) igneous rocks ranging from monzonite/syenite to alkali-feldspar
granite. Data from three regions with a significant geothermal potential are
investigated: the Erzgebirge and Vogtland, the Granulite massif, and the Meissen
massif. These regions display different geological features and complexity, and
therefore different degrees of suitability for geothermal utilization from the
thermal point of view. All are reasonably well investigated for the chemical
composition of the igneous rocks and their metamorphic cover, but only the
Erzgebirge/Vogtland is well analyzed in terms of heat flow and petrophysical
properties. Although there is a lack of borehole data in the Granulite massif and the
Meissen massif, a basic appreciation is developed for rock thermal conductivity and
heat flow so that thermal models can be generated for the evaluation of the thermal
potential of these areas. 
Especially the Erzgebirge/Vogtland region has undergone intensive exploration for,
and exploitation of, mineral deposits in the last several decades, resulting in
substantial amounts of data from surface outcrops, underground mines, wells, and
geophysical investigations comprising gravity, electric, and seismic surveys that may
be used in the development of an EGS. Knowledge from the near-surface thermal
situation was gained from continuous temperature logs, measured in 39 boreholes to
maximum depths of 1200 m.
The granites in this region are responsible for the rise in surface heat flow from
about 60 mW/m² to a maximum of 112 mW/m². U−Th−K2O data show granite heat-production
rates on the order of 4 to 12 µW/m³, dependent on chemical type and degree of
fractionation. The Erzgebirge HHP granites are silica-rich (73−77 wt% SiO2) and,
thus, display relatively high thermal conductivities, between 2.8 and 3.6 W/mK. To
achieve temperatures of about 100°C in the development of an EGS, boreholes need to
be drilled in granite to depths of 2.5−3 km. Since the thermal gradient is inversely
proportional to the thermal conductivity for a given heat flow, those igneous rocks
might be preferred in general that show high abundances in radiogenic elements
combined with a low thermal conductivity. These conditions are met in the Granulite
massif, where monzogranites (69−73 wt% SiO2; 4−14 µW/m³), with a calculated thermal
conductivity between 2.2 and 2.8 W/mK, are intrusive into felsic to mafic granulites.
Even more prospective are monzonitic to syenitic rocks (52−64 wt% SiO2) from the
Meissen massif, which exhibit heat-production rates between 4 and 13 µW/m³ and
calculated thermal conductivities of < 2.2 W/mK.

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