This study describes the composition and provenance of dense bodies or structures present in uncharred plant vestiges recovered at Monte Castelo (6000–700 cal. AP-SW Amazonia). It aimed to disclose some aspects of this plant remains’ interactions with the sedimentary matrix of the site over the 200 years (at least) since its initial deposit, from the point of view of the soft tissue mineralization processes. Two specimens were examined using XFCT, X-ray MicroCT, and SEM-EDS techniques to reveal the presence and distribution of Ca, K, Mn, Fe, Ti, Si, S, Cu, Br, Rb, Sr, Zn, and Zr. These attributes were integrated with compositional ED-XRF and XRD measured data from the sedimentary substrate. Results show that some of the chemical elements present in solid bodies and anatomical structures of the plant remains refer to the sedimentary environment, while others have an endogenous origin. These include mainly Rb and Br, which were interpreted as the result of degradation processes of the internal tissues, where they are mainly present. Except Sr and Zr, a portion of all the other elements entered and disperse into the sample structures from the sedimentary substrate. Its presence is attributable to mechanisms such as attachment, diffusion and impregnation through the outermost tissues, where they are mostly concentrated. The composition of most of the dense bodies consists of both endogenous and exogenous elements.