Abstract
This paper provides insight into large-scale kaolinization of Variscan granitoids in the Iberian massif, based on an integrated study combining mineralogical, geochemical (with emphasis on trace elements) and stable isotope data from the Nuevo Montecastelo kaolin deposit (NW Spain). Mineral composition of the kaolinized granite is dominated by well-ordered kaolinite with minor amounts of quartz, K-feldspar, mica (illite/muscovite), accessory phases (rutile, ilmenite, zircon and monazite), and gibbsite locally. The abundance and structural order of kaolinite decrease with depth. The isotope composition of the kaolin (δ 18O values around + 20.5‰, and δD values around −60‰) is consistent with a supergene origin. Kaolinite was formed by extensive hydrolysis of feldspars (preferentially plagioclase), in equilibrium with meteoric waters at about 21 °C. Differential mobilization of major and trace elements during kaolinization has been inferred by using a mass-balance approach. Significant amounts of Na, Ca and minor Mn, Sr, P and U were removed from incipiently leachable minerals (plagioclase and apatite). In advanced stages of weathering, K, Rb, Cs and Ba were released from partial dissolution of K-feldspar and mica. By contrast, Ti, Zr, Th, Hf and the REE were retained in the resistant trace minerals that appear residually concentrated in the fine fractions of the kaolins.
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