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Chalcedony in our region and the Formation of Deposits

 

          The Saricakaya district of Eskisehir city in the mid-west of Turkey is a very rich gemstone deposit including unique and ideal blue colored translucent chalcedony specimens. They are also called Blue Turk Chalcedony or Saricakaya Blue Chalcedony in the world gem market. The chalcedony in the region was mainly deposited as nodules filling the pores. The average diameter of the chalcedony nodules ranges between 10-15 cm, maximum 60- 70 cm. The nodules are covered with an outer crust of about 5 mm in thickness which has a smooth and flat surface with a brownish-yellow color, and some white color. In addition, purple colored chalcedony specimens that are rarely found in the world were also deposited in the same region.

          Chalcedony coagulation occurred with the fault-controlled. The region has a strong and continuous tectonic activity. Because of both the rising granitoid pluton which cut the ophiolitic complex from the basement, and the overthrusting limestones from North to South, a large number of faults developed in the sandstones and arkoses of the Mihalgazi Formation. Next, the additional faults also developed during both the deposition of the Paleocene aged sedimentary Mihalgazi Formation and the spread of the Eocene aged volcanic andesite lavas, and also during the following period. So, a wide and long cracked zone formed in the region. Fault breccias were come into existence within the sandstone-arkose units found in the margins of two normal faults, running parallel to each other and laying in a northeast-southwest direction. During and after the chalcedony coagulations, tectonic activity continued in the region.

          The most plausible assumption is that the medium in the blue chalcedony formation of the region is the colloidal silica system in the presence of hydrothermal circulation under lower P-T and alkaline pH conditions. Because the tectonic activity continued for a long time, chalcedony nodules were deposited in five different periods from Paleogene to Neogene. So, their colors and appearances are different from each other. These are (1) massive deep blue colored chalcedony, (2) massive light blue colored chalcedony, (3) whitish-blue colored chalcedony with brown colored crack fillings, (4) chalcedony whose central part is white and outer part is light blue colored, and (5) massive violet colored chalcedony.

          In addition, in the west of the cracked zone, there are some gem quality silica nodules such as opal, chrysoprase and agate materials and perlitic occurrences.

          On the other hand, the other fault lines in the south of the main cracked zone caused the occurrence of a wide mineralization zone, including the metallic ores such as gold, pyrite, chalcopyrite, sphalarite, galenite, stibnite and manganite.

          Mining activities taking out the chalcedony in the region have continued since the Roman Period. The area is one of the unique places where the longest gemstone mining happened in the history. The mining techniques employed in the chalcedony mines of the ancient periods and those of today are highly similar, despite the long time that has passed in between. Many mining licenses for the region have given by "Migem". In the Saricakaya region, visual concentration of chalcedony has been estimated at nearly 3.000.000 tons with an average tenor of 30 Kg per cubic meter, according to the geological investigation in this study (on a surface of nearly 10 km2 and at a depth of nearly 10 m).

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