f Mineralogical and Petrographic Features of the Hydrothermal Alteration Around Sandıklı (Afyon) Geothermal Field

The study area is located in the Sandıklı district, 40 km southwest of Afyon city. The area was intensely affected by hydrothermal alteration and geothermal systems related to tectonic and volcanic activities during the western Anatolian Neo-tectonic period. Miocene volcanic activity in the Sandıklı district produced alkaline lavas and calc-alkaline pyroclastic rocks. There are many examples of hydrothermal alteration and mineralization occurrences within the investigated area. The basement rocks consist of low-grade metamorphic rocks of the Afyon zone. The hydrothermal alteration can be traced from Hüdai thermal water to the north and east in the Sandıklı volcanic rocks. The borehole data indicate that the effects of hydrothermal alterations continue further depths. Mineral paragenesis of the alteration is studied and revealed by mineralogical and petrographic studies from the borehole samples. Silicification and chalcedony plus hematite occurrences are extensive at the depth of 50-60 meters of AFS 12 and 13. Marcasite and sericite with pyrite and hematite abundances increases further depths. Euhedral Fe-Ti oxides are seen at the interval of 110-180 m interval. In addition to these, sanidine and euhedral pyroxene (augite) with primary apatite and titanomagnetite minerals are also observed. The Seydişehir formation is cut at the depth of 316 m of AFS 15. Furthermore, fractured and sulfured zones are cut at the depth of 136, 179, 217, 452, 484 and 598 meters, respectively. The Seydişehir formation is cut at the depth of 312 m of AFS 16, and fractured and sulfured zones are cut at the depth of 244, 246, 396, 478, 634, 638, 654, 678, 916 and 920 meters, respectively. In the AFS 17, the Seydişehir formation is cut at 332 m and sulfured zones are observed at the depth of 156, 260, 304, 380, 762 and 960 meters. The borehole data indicate that Hüdai quartzite is second reservoir rock. Hüdai quartzite is observed at 500 meter. The project aims are targeted to obtain higher temperatures at 7 holes studied, in addition to the previous bore holes. The basement rocks show different sequences in these 7 holes. Main production zone is revealed from Hüdai quartzite. In the Hüdai quartzite, hydrothermal mineralization riches and thermal water temperature increases with depth. Production temperatures are given depend on productions. The study is still on progress in order to obtain further data from the geothermal field. Key words: Hydrothermal alteration, Alteration mineralogy, Geothermal resources, Western Anatolia, Sandıklı geothermal field.