Mineralogical and geochemical properties and genesis of kaolin and alunite deposits SE of Aksaray (Central Turkey)

Abstract

The kaolin and alunite deposits, situated approximately 20 km southeast of Aksaray in Central Anatolia, were formed by hydrothermal alteration related to extensional tectonics from the Upper Miocene to Quaternary, mostly from rhyolitic volcanoclastic rocks. The mineralogical and geochemical properties of the deposits were investigated to explain the stages of hydrothermal alteration and the origin of the deposits. X-ray diffraction (XRD), scanning electron microscopy (SEM), and chemical and stable isotope (O, H, and S) studies were carried out on the alteration minerals (e.g., kaolinite, gypsum, alunite, native sulfur and smectite) of the deposits. Kaolinite/halloysite minerals were determined along with mostly alunite- and silica-group minerals (alpha-quartz, opal-CT, and tridymite), feldspar, native (elemental) sulfur, and partial hematite, goethite, gypsum, cinnabar and realgar. The delta S-34 isotope values for alunite, gypsum and native sulfur are in a fairly narrow range and vary from 5.03 to 6.62 parts per thousand, 4.5-5.24% and 5.97-7.49 parts per thousand, respectively. The delta O-18 values of the kaolinites and smectites vary from +1.8 to +13.3 parts per thousand and from 98 to 152 parts per thousand, respectively, and their delta D values range from 4.2 to 15.7 parts per thousand and from -93 to -191 parts per thousand, respectively, showing a spread that is very close the primitive magmatic water line. From the delta O-18 values, the calculated formation temperatures of the kaolinites and smectites range from 38 to 135 degrees C and from 29 to 189 degrees C, respectively. The delta O-18 values of the alunites and amorphous silica vary from 4.4 to 8.9 parts per thousand and from -1.1 to 8.4 parts per thousand, respectively, and the delta D values vary from -72 to -87 parts per thousand, and from -104 to -191 parts per thousand, respectively. In addition, the formation temperatures of alunite and amorphous silica minerals range from 57 to 100 degrees C and from 139 to 281 degrees C, respectively. Hydrothermal alteration progressed in different stages between 29 and 281 degrees C according to the calculated modal formation temperatures. The isotopic mineral data suggest that the original hydrothermal waters were a mixture of magmatic and meteoric waters. Oxygen and deuterium analyses indicate that kaolinite and alunite formed at lower temperatures than smectite and silica minerals. The determined stable isotopic compositions of the minerals indicate that hydrothermal solutions at different compositions and temperatures played important roles in the formation of minerals. The determined neoformed minerals may have been formed by steam-heated hydrothermal solutions sourced from rhyolitic magma and are mostly of hypogene origin.