Abstract :
Deuterium ((delta)D) and oxygen ((delta)18O) isotope data from the Rio Capim kaolin, northern Brazil, were combined with optical studies in order to better understand the genesis and evolution of the kaolinites. The results show that (delta)18O values from a lower soft kaolin unit range from 6.0(per mille sign) to 19.2(per mille sign) for Ka (size ranging from 1 to 3 (mu)m) and Kb (size ranging from 10 to 30 (mu)m) kaolinites, and from 15.4‰ to 24.9‰ for Kc (size < 200 nm) kaolinites. The (delta)D values range from - 63.1(per mille sign) to 79.5(per mille sign) for the Ka + Kb kaolinites, and from - 68.8(per mille sign) to - 244.35(per mille sign) for the Kc kaolinites. An upper semi flint kaolin unit, dominated by Kc kaolinites, displays (delta)18O and (delta)D values ranging from 15.1(per mille sign) to 21.8(per mille sign), and - 71.3(per mille sign) to - 87.4(per mille sign), respectively. Based on these data, and on the (delta)18O and (delta)D values obtained for the surface meteoric water and groundwater, it can be concluded that the kaolinites are not in equilibrium with the modern weathering environment, but they reflect isotopic compositions of the formation time, probably due to the interaction with fossil groundwater. However, mineralogical contaminations derived from replacements of framework grains also had great influence in the isotopic composition of these kaolinites. In addition, the isotope values of the Kc kaolinites from the semi-flint kaolin unit is variable, which is due to the presence of Kc kaolinites of different origins, including kaolinites derived from the underlying soft kaolin unit, kaolinites formed during different phases of paleoweathering, as well as later phases of coarse-grained kaolinites formed along fractures. Due to these complexities, binary diagrams contrasting (delta)18O and (delta)D values, worldwide applied for distinguishing supergenic from hypogenic kaolinites, as well as those formed under weathering conditions, can not be applied to interpret the origin of the kaolinites in the Rio Capim Kaolin.
