- © 2016 Clay Mineral Society
The Biga Peninsula of NW Turkey is host to many kaolin and halloysite deposits with mineralization occurring at the intersections of fault zones in contact with Late Eocene–Miocene calc-alkaline volcanic rocks. Distinguishing between the relative overprinting of hypogene by supergene processes in these deposits is a challenge and important because they affect the physical-chemical properties of minerals and their potential for industrial applications. This study examines the Sarıbeyli-Sığırlı and Bodurlar kaolin deposits in NW Turkey, which were formed from similar volcanics as evidenced by 40Ar/39Ar. Late Eocene (34.2 ± 0.20 Ma) to Early Oligocene (32.7 ± 0.17 Ma) ages for both primary volcanic rocks and alunites are consistent with surrounding rocks in the Çanakkale region. Criteria used to distinguish hypogene alteration from supergene alteration processes come from X-ray diffraction (XRD), Fourier-transform infrared (FTIR) and Raman spectroscopies, thermal gravimetric analysis (TGA), scanning and transmission electron microscopy (SEM, TEM), and elemental analyses. Isotopic δ18O depletion and δD enrichment of the Sarıbeyli-Sığırlı deposit suggests that it was more influenced by magmatic waters than was the Bodurlar deposit. The Bodurlar deposit contains a paucity of dickite compared to the Sarıbeyli-Sığırlı deposit, which is evidenced by lower TGA endotherms, higher ratios of XRD intensities for reflections at 1.316 Å and 1.307 Å, distinctive FTIR absorbance bands at 3620 cm−1 and 3652 cm−1, and relative Raman intensities of the ν1 and ν5 vibrational modes.
A genetic model is proposed whereby these deposits are mainly formed through an acid-sulfate hydrothermal alteration, in what appears to be a volcanic-hydrothermal system. The extent of hydrothermal alteration was controlled by fault density and the initial texture of the volcanic rocks. These steam-heated environments included sulfide-enriched vapors and groundwater mixed to varying degrees in the vadose zone. The Sarıbeyli-Sığırlı and Bodurlar deposits, respectively, contain mineral assemblages that reflect both hypogene (kaolinite, alunite, dickite) and supergene (kaolinite, halloysite, jarosite) processes. These observations offer a basis for comparing and discriminating the relative influence of these two important alteration processes responsible for the formation of kaolin deposits in NW Turkey and around the world.