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Acid-sulphate hydrothermal alteration of andesitic tuffs and genesis of halloysite and alunite deposits in the Biga Peninsula, Turkey
- Ö. I. Ece, P. A. Schroeder, M. J. Smilley, J. M. Wampler
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- Journal:
- Clay Minerals / Volume 43 / Issue 2 / June 2008
- Published online by Cambridge University Press:
- 09 July 2018, pp. 281-315
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The Biga Peninsula of NW Turkey is host to six major halloysite deposits in the Go¨nen, 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.
Alteration of volcanic rocks and genesis of kaolin deposits in the Şile Region, northern İstanbul, Turkey. Part II: differential mobility of elements
- O. I. Ece, Z.-E Nakagawa
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- Journal:
- Clay Minerals / Volume 38 / Issue 4 / December 2003
- Published online by Cambridge University Press:
- 09 July 2018, pp. 529-550
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In the area of S° ile, NW Turkey, Upper Cretaceous calc-alkaline volcanic rocks with compositions ranging from andesite to rhyolite have been recognized. The most widespread rocks of the suite are andesites, which can be grouped into altered and fresh. The oldest altered andesites are the parent rocks for the kaolin deposits of the study area. The Upper Cretaceous volcanic suite consists of spilite, basalt, andesite, trachyandesite, trachyandesitic and hyaloandesitic dacite, rhyolite lavas, tuffs and agglomerates. The highly altered andesites are composed of plagioclase, pyroxene, hornblende, biotite, augite and very fine opaque minerals. During the Turonian, an E –W trending extensional magmatic arc was developed in the Istanbul Tectonic Zone of the oceanic Western Black Sea basin and intermediate volcanic rocks were emplaced, mostly calc-alkaline andesites, suggesting multi-stage magmatism. The significant features of the andesites are: (1) enrichment of LILE (Rb, Ba, K) over HFSE (Zr, Nb, Hf, Ti, Th, U, Y) and LREE (La –Sm), resulting in high Ba/Nb, Th/Nb, Ba/La, K/Ti and Th/La ratios; (2) depletion of LREE over HFSE, MREE and HREE, generating high La/Nb, Ce/Ti, La/Sm and La/Y values; and (3) depletion of Nb, Sr and Ti; all of which are typical of island arc magmatism, with possible back arc signature. The Th-Hf-Ta diagram for tectonomagmatic classification shows that the S° ile calc-alkaline rocks are similar to volcanic rocks from the Mariana Arc, the Aeolian Arc of Salina, Italy, the Skaros island in the Aegean Sea and Sardinia's ignimbrites. Moreover, relatively low La/Th and Ce/Pb ratios suggest that the source region of volcanism was enriched in LILE with respect to REE, indicating crustal contamination during melting.
Highly weathered andesitic rocks, rich in smectite, were transported gradually and deposited in a lacustrine basin, a coal-forming dysaerobic environment, in which they were subject to post-depositional alteration, or in situkaolinization, to form a kaolin deposit in the presence of humic and fulvic acids. The mobility of major and trace elements and REEs during the progressive kaolinization of andesitic materials has been investigated to reveal the geochemical characteristics of Upper Cretaceous volcanic parent rocks and to explain mineralogical processes in a kaolin deposit as a daughter rock ‘end-product’ . Alteration is characterized by the loss of Si, Fe, Ca, Na and K, and by the gain of Al, Ti, Zr and LOI. Moreover, Ho, Er and Yb are immobile, and Hf, Zr and Nb are mobile. Th and U are slightly enriched in clay horizons with respect to the andesitic rocks. In addition, Cr, Ga, Nb and Ta enrichments indicate variable sources of terrigenous sediments and differential mobilities of elements in lake waters rich in organic acids. The anatase concentration increases in the <2 mm size fractions as subspherical particles and these precipitate at acidic conditions (pH ≈ 5) during early diagenesis.
Element mobility during the formation of the Uzunisa-Ordu bentonite, NE Turkey, and potential applications
- Ş. Özdamar, Ö. I. Ece, B. Uz, F. Boylu, H. Ü. Ercan, G. Yanik
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- Journal:
- Clay Minerals / Volume 49 / Issue 5 / December 2014
- Published online by Cambridge University Press:
- 27 February 2018, pp. 609-633
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Dacitic andesitic calc-alkaline lavas and their pyroclastic rocks of Upper Cretaceous age are widespread in the Ordu area, eastern Black Sea Coast of Turkey. Ca-bentonite and Ca/Nabentonite deposits with significant economic potential formed in the broader region. The mineralogy and geochemistry of these deposits were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and chemical analyses. The parent rocks (PR) and bentonite samples were analysed for the major and trace element contents, including rare-earth elements, to understand the effect of differential alterations on element mobility. The bentonites contain mainly di-octahedral Ca-montmorillonite with minor amounts of illite, quartz, calcite, dolomite and feldspar. Primary K-feldspar and plagioclase phenocrysts were altered in the bentonite horizons, and were completely leached at greater depth. For this reason, the higher quality bentonites are found at deeper zones. Bentonite horizons are characterized by relatively high loss on ignition (LOI: 9.8–20.8%) and MgO content (3–5%) and low K2O (<0.5%) and Na2O (<2%) contents compared with the least altered parent rocks. Silicon, Ca, Mg, Mn and P are markedly enriched whereas Fe, Na, K and Ti are depleted in the bentonite samples. The chondrite-normalized rare earth element (REE) patterns show identical trends characterized by strong light rare earth element (LREE) enrichment in both the PR and bentonite samples. The LREE contents are higher than those of their heavy counterparts (HREE) in the samples. Alteration of the PR to bentonite is associated with intense chemical leaching and subsequent removal of K and Na in open-system conditions.