Hydrothermal alteration-related kaolinite/dickite occurrences in ignimbrites: an example from Miocene ignimbrite units in Avanos, Central Turkey

Abstract

Miocene-Quaternary volcanic complexes associated ignimbrites from the Cappadocia Volcanic Province in Central Anatolia have been subjected to hydrothermal alteration which produced several industrial clay mineralizations. This study focuses on the hydrothermally altered Late Miocene dacitic to trachy-andesitic ignimbrites in the Avanos region and aimed to determine the alteration in terms of conditions and origin. Hydrothermally altered samples consist mainly of quartz and kaolin group clays, and minor amounts of goethite/limonite, alunite, jarosite, illite, and mixed-layered illite-smectite. The kaolinite and dickite associations were principally determined from the diagnostic XRD peaks and confirmed by FTIR and SEM data. They have fine-grained and pseudo-hexagonal uniform-kaolinite and prolonged-dickite-shaped platelets as tightly packed vermicular booklets. The kaolinite and dickite association with fine-grained alunite and jarosite point out the steam-heated igneous environments at shallow depth level. Hydrothermal alteration-related minerals occurred as replacement of volcanic glass/groundmass and neoformation in the pores from the acidic hydrothermal solutions. The mineralogical associations suggest that the hydrothermal alteration occurred in acidic and moderate temperature conditions. Major and trace element compositions of altered samples exhibit distinct differences with respect to the host-rock composition. Chondrite normalized LREE contents increase, whereas HREE contents decrease in altered ignimbrites with respect to unaltered ignimbrite. The oxygen and hydrogen isotope data of kaolinite/dickite demonstrate that magmatic vapor phases mixed with meteoric waters. The steam-heating-related igneous environment seems to be responsible for magmatic vapors which circulated along the crack zones within the acidic volcanogenic rocks, and therefore the acidic conditions caused kaolin-dominant alteration.