Polygenetic processes in the genesis of clay deposits of Poços de Caldas alkaline massif in southeastern Brazil

• The clay deposits of Poços de Caldas alkaline massif were identified as polygenetic.
• Two processes, weathering and sedimentation, were involved in the clay formation.
• The processes produced deposits formed by residual and sedimentary clay.
• The residual and sedimentary clay are found in the same clay deposit.
• The mineralogy of the residual clay was conditioned by the parent rock.

The purpose of this research is to characterize the clay deposits occurring in the central plateau of the Poços de Caldas alkaline massif, aiming for a better understanding of the involved genetic processes. The results showed that the formation of clay deposits was associated with two processes related to external geodynamic, in situ alteration (weathering) and sedimentation. These processes produced two different types of clay layers: residual and sedimentary clay. The mineralogical characterization of the residual clay was conditioned by the parent rock. Thus, in the deposits formed from alkaline rocks, the clay material is mainly kaolinitic, whereas the clay deposits formed from potassic rocks are dominated by halloysite. The sedimentary deposits were formed by the erosion of both weathering products (saprolite) of alkaline rocks and bauxite formations, originating from the upper part of the topography and deposition over the residual clay in a clear unconformity. This is a very interesting situation because these two types of clays with different properties and utilization occur in the same deposit, reflecting a polygenetic origin.