The Biga Peninsula of NW Turkey is host to six major halloysite deposits in the Gönen, Yenice and Balya districts. Mineralization took place in areas of Permian limestone blocks where the Triassic Karakaya Complex is in contact with early Miocene calc-alkaline volcanic rocks. Hypogene halloysite mineralization was controlled by the intersection of minor faults in the vicinity of clay deposits. During the Pleistocene, activity of the North Anatolian Fault (NAF) brought ascending geothermal solutions through the fault zones to the surface, which led to hydrothermal alteration and halloysite formation. N-MORB normalized element values for each halloysite deposit and the volcanic rocks suggest genetic links. Alunite and halloysite were formed in the Turplu area where upwelling hydrothermal waters contained major H2S and SO2 acids. Only halloysite mineralization occurred in outflow areas of the same fossil geothermal field.
Pyrite and alunite samples from the Turplu deposits have δ34S values of 0.6–1.8‰ and 4.8–7.9%, respectively, with values for gypsum of 3.1–3.5‰. The δ34S values of pyrite suggest that local meteoric waters had partially mixed with the dominant fluid during the closure stage of fossil hydrothermal activities. The range of δD values of halloysite samples from Turplu is −58.4 to −68.6‰. The δ18O values for halloysite are in the range 16.7–18.1‰. All halloysite deposits in the study areas are either overlying or adjacent to limestone blocks, and these provide excellent drainage for the discharging geothermal waters. Subsurface drainage systems in the karstic environment and the SO2-bearing thermal waters indicate the importance of acidic waters and the continuous leaching of elements in forming relatively pure hydrated halloysite. A steam-heated dissolution-precipitation model is proposed for the occurrence of all halloysite and alunite deposits. Sulphur gases (H2S-SO2) of hypogene origin rose from deep in the fault zone to the surface where they encountered oxygenated groundwater at the water table. The occurrence of H2SO4 in this hydrothermal system enhanced the acidity of geothermal waters provoking advanced argillic alteration. Hypogene alunite deposits also have large P2O5 contents, suggesting a parent material with a magmatic origin deeper than the alkaline tuffs. Halloysite is a fast-forming metastable precursor to kaolinite.