Abstract: Chalcedony, quartz and amethyst vein and veinlets which are emplaced in the cracks and fractures ofparticularly in basaltic andesites around Belkavak village of Yerköy- Yozgat are investigated. Fluid inclusionstudies carried out on in solutions rich in silica for determination of the formation temperatures and theorigins of the salinities. Siliceous veins cut the Middle Eocen basaltic andesites. During the studies, fivedifferent silica zones were distinguished respectively consisting of chalcedony, white quartz, transparentquartz-I, amethyst and transparent quartz-II. Homogenization temperatures and % NaCl equivalentsalinty parameters are dedected. First stage consists of chalcedony bands. Their cryptocrystalline texture have led to no fluid inclusions. The white quartz crystalls are formed after chalcedony bands and theirhomogenization temperatures measured as between 272 and 323°C, salinities between 3.86 and 4.65% NaCl equivalent. During transparent quartz-I formation stage corresponding temperatures weredetermined as ranging between 217 and 280°C and salinities between 3.06 and 4.96 % NaCl equivalent.During the formation of amethyst crystals, homogenization temperatures changing from 162 to 370°Cand salinities of 1.73 to 4.96 % NaCl equivalent are measured. At the the final stage, transparent quartzII crystals are formed. Their homogenization temperatures varied between 120 to 270°C and salinitiesbetween 0 to 6.88 % NaCl equivalent.According to the obtained data, previously ascending siliceous solutions emplaced in the open spaces ofcracks and fractures in a relatively cold environment and hence the formation of chalcedony occurred.Then, the temperatures of the ascending siliceous solutions reached up to 200-370°C and graduallydecreased to 120 °C. Generally, rates of salinity is below %8, therefore it can be postulated that meteoricwater was in interaction with seawater during the formations of quartz and amethyst crystals.