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

The Biga Peninsula of NW Turkey is host to six major halloysite deposits in the G6nen, Yenice and Balya districts. Mineralization took place in areas of Permian limestone blocks where the Triassic Karakaya Complex is in contact with early Miocene cale-alkaline volcanic rocks. Hypogene halloysite mineralization was controlled by tile 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 hydrotherrual 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 hydrotherinal waters contained major H2S and S02 acids. Only halloysite 16 mineralization occurred in outflow areas of the same fossil geothermal field. Pyrite and alunite samples froin the Turphl deposits have 634S values of 0.6-1.8%) and 4.8-7.9%, respectively, with values 1`01- gypsum of 3.1 -3.5%o. The 634S values of Pyrite suggest that local meteoric waters had partially mixed with the dominant fluid during tile closure stage of fossil hydrothermal activities. The range of 6D values of halloysite samples frorn Turplu is -58.4 to -68.6%,,. The 6'0 values for halloysite are in the range 16.7-18. 1 %o. 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 tile S02-bearing thermal waters indicate the importance of acidic waters and the continuous leaching of elements in forming relatively pure hydrated halloysite. A stearn-heated dissolution-precipitation model is proposed for the occurrence of all halloysite and alunite deposits. Sulphur gases (H2S-SO_1) 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 1-12SO4 in this hydrotherinal system enhanced the acidity of geothernial waters provoking advanced argillic alteration. Hypogene alUnite deposits also have large PI-05 contents, suggesting a parent material with a magmatic origin deeper than the alkaline WITS. lialloysite is a fast-forming metastable precursor to kaolinite.