A total of eighty-five thin sections of cores in 15 wells of Berlin geothermal field was re-analyzed using the petrographic microscope. Most of the cores were taken at different depths from 519 m to 2347.4 m in andesitic and lithic tuff formations. However, recent drilling (2006) of well TR-14A cored the first intrusive rock, granodiorite at 2063 m, while wells TR-19B and TR-19C successively obtained another intrusive rock which is more acidic, a granite.

Two additional hydrothermal alteration minerals were identified: actinolite and prehnite. Actinolite is one of the high temperature amphibole minerals (300?C) and mostly observed in well TR-2, while prehnite is usually associated with epidote in veins and vesicles of the rocks.

Facies zones include argillitic-phyllitic zone to propyllitic zone, with increasing depth. Mineralogical temperature ranges from 150? to 300?C.

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