| Abstract |
The Sandıklı geothermal field is located on the eastern part of western Anatolia (Türkiye). The field is hosted by volcanic rocks. Samples from two wells were kindly made available for this study, namely from AFS-12 and AFS-13 drilled to depths ranging from 450 to 500m. The wells were located in a major fault controlled porphyry hydrothermal system (hydrothermal upflow zone). A widespread and intense porphyry-type hydrothermal alteration zone has developed in the Neogene Sandıklı volcanics at 4.5 km NE of the Karacaören village (Sandıklı), as a result of the intrusion of syenite-monzonite porphyries. From distal zone to the intrusion center, propylitic (epidote, chlorite, carbonate minerals, albite, K-feldspar), argillic (alunitization, illitization), phyllic (sericitic; quartz, sericite, pyrite) and potassic (alkali metasomatic; K-feldspar and biotite) developed, respectively, under wide range of pH and Eh conditions. The reservoir is hosted by interbedded pyroclastic and lavas of trachyandesite composition with some basaltic andesites and trachytic tuffs. The primary minerals present in the Sandıklı subsurface rock samples are mainly sanidine, pyroxene, biotite, plagioclase, nosean and apatite. In general, carbonatization and chloritization are common in the matrix and pyroxene phenocrysts of the trachytic rocks. On mineralogic (XRF, SEM-EDS) and petrographic investigations on the wells at 50-60 m, silicification, and chalcedony and hematite formation are common. In the SEM-EDS studies, 0.75 – 1.25 % Ag was measured from the hydrothermal quartz. At deeper levels, abundance of hematite and pyrite with marcasite and sericite increase due to the hydrothermal alteration. At 110-180 m levels, euhedral Fe-Ti oxide minerals (hematite-Fe2O3 or titanomagnetite-Fe2.5Ti0.5O4, including Ti) are observed. However, primary apatites and titanomagnetite minerals associated with sanidine and euhedral pyroxene (augite) occur within the trachyandesite. In the SEM study, it is observed that the surfaces of the apatites were rounded and decomposed, due to the intense hydrothermal alteration. At 350-400 m levels, depends on hydrothermal alteration, euhedral pyrite crystals were formed within the dolomitic limestones (the Çaltepe formation is not observed on surface). Within this pyrite which replaced the dolomite and at the boundary of dolomite-pyrite, euhedral gypsum minerals were formed. At deeper levels, in sandstone alternating with dolomitic limestone, epidotization is common. |