Stable isotope constraints on the origin of kaolin deposits from Variscan granitoids of Galicia (NW Spain)

Abstract

Kaolinite-rich fractions of <45, <2 and <1μm size from deposits of Galicia (NW Spain) at Ramón-Fazouro and Nuevo Montecastelo to the north, Vimianzo, Barilongo and Lendo in the center, and Santa Tecla to the south show significant ranges in δ18O from 16.5 to 22.4‰ and in δD from −99 to −40‰. These widely spread values question a single crystallization process as no statistically significant overall correlation between δ18O and δD can be obtained. The analytical spread is such that 11 of the 43 analyzed separates plot below the kaolinite weathering line, and one at the line separating the supergene from hypogene evolution field. Noteworthy for this wide scatter is the fact that some separated size fractions, especially from Ramón-Fazouro deposit contain also up to 65% halloysite, as well as varied amounts of contaminating minerals generally considered as relicts from parental igneous rocks, such as quartz, illite (=muscovite) and feldspars. However, the addition of these minerals does not appear to significantly bias the isotopic data of the <2μm size fractions. Mean values for the <1μm kaolin fractions of the five deposits for which there is more than a single sample have linearly correlated δ18O and δD values with a best-fit line: δD=8.75 δ18O−248 (R=0.896, n=5, >95%) rather similar to the kaolinite weathering line of δD=7.5 δ18O−220. However, the overall spread of values and particularly those for coarser material, together with occurrences of halloysite, strongly suggest that higher temperature process(es) also occurred.Kaolinite, and also likely halloysite, crystallized from waters, whose δ18O values were of meteoric signature from about −5 to −1‰, at temperatures ranging from about 20°C to 50°C. The higher crystallization temperatures, especially at the sedimentary Lendo deposit, imply fluids at higher temperatures than during a climatic-controlled weathering process, probably of a low-temperate hydrothermal character with a fluid isotopically close to meteoric origin. However, interaction with fluids at higher temperatures and with higher δ18O cannot be ruled out, because the relatively wide ranges of kaolin δ18O and δD values are difficult to reconcile with a narrow range of δ18O and δD for the interacting parental fluids. The most likely scenario for the kaolinite and halloysite precipitation is then a two-stage process of interaction with meteoric waters at close Earth-surface temperature, but with one of the two episodes occurring at temperatures slightly above those of weathering processes even under tropical conditions. There is no compelling evidence for an episode of significantly high temperature, such as in most hydrothermal fluid–rock interactions, but a low-temperature episode of about 50°C cannot be ruled out unequivocally, especially for the <2μm material. Following local continental weathering, percolation of meteoric waters at temperatures up to ~50°C, and even higher, through the granitoids and felsites could have favored halloysite crystallization and affected the isotopic composition of earlier precipitated kaolinite.