『Abstract
The paper presents a detailed description of the geological structure,
mineralogy, and chemical composition of the Archean weathering
profile of granitoids in the Baltic Shield. Based on model calculations,
the primary composition of granites in the substrate and weathering
profile, as well as elemental composition of solutions draining
the supergene profile, are determined. Model calculations for
the Proterozoic weathering profile of analogous granitoids testify
to the similarity of supergene alterations in the Early Precambrian.
Mineral transformations during the weathering of granitoids are
mainly expressed in the formation of minerals of new micas of
the illite group against the background of large-scale decomposition
of plagioclase. the composition of solutions in the hypergenesis
zone of Precambrian weathering profiles sharply differs from the
modern composition by lower contents of Ca2+ and K+
and the higher content of Fe2+. This is only possible
under reducing conditions at high acidity of the diagenetic environment
(pH<6.5). The results obtained are essential for understanding
the nature of unique Precambrian rocks (sedimentary iron ore rocks).
The subordinate amount of carbonate sequences in Precambrian sedimentary
sequences can be related to the specific composition of continental
runoff primarily governed low-Ca supergene solutions.』
Introduction
Materials and results
Weathering profile at the base of the Okhta sequence (upper
Lopian)
Accessory minerals in weathering profiles
Chemical alteration of plagiogranites during weathering
Composition of solutions
Composition of solutions
Discussion
Conclusions
Acknowledgments
References