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Acid-sulphate hydrothermal alteration of andesitic tuffs and genesis of halloysite and alunite deposits in the Biga Peninsula, Turkey

¹ Istanbul Technical University, Faculty of Mines, Department of Geological Sciences, Mineralogy-Petrography Division, Maslak 34469 Istanbul, Turkey, ² The University of Georgia, Department of Geology, Athens, GA 30602-2501, USA, and ³ Georgia Institute of Technology, School of Earth and Atmospheric Sciences, Atlanta, GA 30332, USA

^* E-mail: ece{at}itu.edu.tr

(Received 23 January 2007; revised 24 July 2007)

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 ³⁴S values of 0.6–1.8 and 4.8–7.9%, respectively, with values for gypsum of 3.1–3.5. The ³⁴S 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 D values of halloysite samples from Turplu is –58.4 to –68.6. The ¹⁸O 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.

KEYWORDS: calc-alkaline volcanism, acid-sulphate thermal waters, hydrothermal alteration, halloysite, alunite, sulphur isotopes, Turkey