Composition of Weathering Crusts in the Upper Amur Region

摘要

According to the traditional viewpoint, weathering crusts represent a rock assemblage formed in the upper lithosphere owing to transformation of magmatic, metamorphic, and sedimentary rocks and ores in terrestrial environments under the influence of different weathering factors. By occurrence patterns, they are subdivided in two types: aerial weathering crusts that blanket bedrocks and are tens of centimeters to a few tens and hundreds of meters thick and linear weathering crusts that extend laterally in a single direction and fill fissures tens to hundreds of meters deep. Both these types are developed in the Upper Amur region, although best studied among them are widespread weathering crusts of the aerial type. Most of them are characterized by kaolinic hypergenesis products. Chlorite-montmorillonite (beidellite) weathering crusts are usually developed after basic and, less commonly, intermediate rocks, while hydromicaceous and hydromicaceous-kaolinic varieties occur after acid and intermediate sequences. Linear weathering crusts are less known. They are developed along fault zones after acid, intermediate, and basic rocks of mineralized dikes and also above ultramafic intrusions with copper-nickel mineralization. The weathering crusts of the region under consideration are presumably Pliocene-Pleistocene in age [1].