GEOCHEMICAL PROPERTIES OF THE DÖLLÜK AND GÜMENEK (TOKAT-TURKEY) CHALCEDONIES

Abstract

The study area is located within the borders of Tokat province, which is one of the richest provinces in Turkey in terms of gemstone diversity. The region has very high quality and various gemstone formations such as jasper, opal, chalcedony, agate, chrysoprase, which have both geological and commercial value. Metamorphic, magmatic, and sedimentary units from the Paleozoic to the present day crop out in the study area. While Paleozoic-Mesozoic Tokat metamorphics and Artova ophiolite mélange form the basement, the Almus formation, which is represented by Lower Miocene aged volcano-sedimentary rocks from the central parts of the area to the east, covers these units unconformably. Quaternary slope debris and alluviums in the Yeşilırmak basin are the youngest units in the region. The chalcedony formations are observed along an E-W trending almost vertically dipping fault zone passing through the north of Döllük and Korucak Villages within the metamorphic units of the Tokat Massif. The chalcedonies are in the cracks, fractures and cavities of the recrystallized limestones that exhibit a fractured and brecciated structure along a line of approximately 25 m in the direction perpendicular to this fault zone. In mineralogical, petrographic, and gemological studies, quartz, tridymite, cristobalite, calcite and dolomites were determined together with chalcedony. Chalcedonies have average 96.3% SiO2, 1.96 % Fe2O3, 0.56 % MgO, 1.02 % CaO, 780 ppm Cr2O3, 6.3 ppm Cu, 20 ppm Hg, 4.2 ppm Pb, 18 ppm Ni and 77 ppm Zn. Chalcedony formations are the last product of a hydrothermal system related to volcanic activities after the formation of the wall-rocks. The movement of chalcedony-generating solutions within the fault zone and their inclusions of carbonate wall rock and secondary calcite formations during chalcedony formations ensured that the CaO and MgO contents of the chalcedony were relatively high.