Eléments traces dans quelques argiles des altérations et des sédiments

Abstract

The content of trace elements in kaolinites and smectites formed by alteration under equatorial and tropical conditions is closely similar to that observed for the parent rocks. The content of Ni, Co and Cr increases from clays formed on acid rocks to those formed on basic rocks. The same variation is observed with V, but it is less regular. The content of Ba, Rb, Li, Ga and Pb, on the other hand, decreases in clays developed on progressively more basic rocks. Two mineral groups, the kaolinites, and iron oxides and hydroxides, are found in the <2μ fraction of kaolinitic alteration products. Trace elements are shared between these two mineral groups. Mainly only smectite is found in the <2μ fraction of smectitic alteration products and trace elements are then localized with the dominant smectite component. During transport out of the alteration profiles and during subsequent sedimentation, kaolinites and iron compounds can be separated. The trace elements associated with these two minerals are then also separated. This is observed with kaolinites sedimented in the Charentes (France). These kaolinites are nearly free of iron (1.5%Fe 2O 3), most of which occurs as sulfide minerals, in which Pb and Zn are also concentrated. The trace elements from some sedimentary clay minerals allow us to distinguish between original characteristics of alteration and later sedimentation environments. Two examples are given: kaolinites sedimented in the Charentes and tertiary illites and smectites from Cormeilles en Parisis. The high level of Ga and the low content of other trace elements in the Charentes kaolinites suggest that these clays are alteration products which may have developed on granite-like acid parent rocks. A similar parent rock type is suggested for the illites and smectites on the basis of the content of V, Ni, Co, Zn, Ga, Cu, Pb and Sn. The very high Li level (500 ppm) of the Charente kaolinites is indicative of the influence of the depositional environment; the content of Sr, Cr, B, Li and Rb of the Cormeille illites and smectites are indicative of the laguno-marine sedimentation environment.