Provenance of heavy and clay minerals in bottom sediments of Green Lake, an Amazonian fluvial lake in Brazil

Abstract

<jats:p>ABSTRACT Studies on provenance of minerals in Amazonian rivers focus mostly on suspended sediments, while processes that control bottom-sediment production and distribution are still little known. We determined the provenance of the bottom sediments of Green Lake, a micro-basin draining into the Tapajós River, in the eastern Brazilian Amazon. We used X-ray powder diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscope (SEM) and, cathodoluminescence techniques to analyze clay, light and heavy minerals of 22 samples. The lake is L-shaped, with 5.5 m maximum depth, and predominance of mud over sand in the center. Quartz and feldspar were dominant in the light fraction, while zircon, tourmaline, kyanite, rutile, and staurolite were dominant in the heavy fraction. The clay fraction was dominated by kaolinite, with morphology and degree of crystallinity indicative of a detrital origin related to weathering. The Alter do Chão Formation (ACF) is suggested as the main source of sand sediments and heavy minerals, due to their mineralogical and percentage similarity. The provenance of kaolinite was mainly the ACF, with a minor contribution of the Amazon and Tapajós rivers. The primary origin of the heavy minerals in the ACF indicates the basement of the Amazonas Basin as source rock and this formation as a source of sediments for Green Lake through weathering and erosion processes under current tropical conditions. The presence of Aulacoseira granulata and Aulacoseira ambigua indicates the importance of current erosive processes on sediment production.</jats:p>

Distribution and provenance of heavy minerals from recent sediments of Green Lake, North Brazil, revisited

“…The grain size of the Green Lake sediments varies between mud and coarse sand (Mendes et al, 2020), which is a sedimentary characteristic of Amazonian lakes (Souza-Filho et al, 2016). To determine the spatial distribution of heavy minerals, the application of classical and modern analytical techniques was required.…”

“…Downstream, near the Green Lake, rutile (33%) and tourmaline (50%) occur. The Green Lake heavy mineral assemblage is represented by medium to high-grade metamorphic minerals together with igneous minerals (Mendes et al, 2020) (Fig. 3; Table 2).…”

“…3). This assemblage is considered moderate to stable even though the calculated ZTR index value, above 90%, indicates an abundance of stable heavy minerals (Mendes et al, 2020). Mineralogical similarities, variations in spatial distribution, and significant changes in the heavy mineral assemblage were noted…”

“…Cretaceous siliciclastic rocks of the Alter do Chão Formation have been considered the probable source of the heavy minerals of Green Lake (Mendes et al, 2020;Ribeiro et al, 2017). However, these researches failed to demonstrate a preferred spatial distribution of its heavy minerals and a relation between the Green Lake bottom sediments and the ones from the Amazon River or the Tapajós River.…”

“…Heavy minerals in the Amazon River are of immature Andean origin (Landim et al, 1983) whereas the ultrastable mineral assemblage of the Tapajós River interpreted as cratonic (Gozzi, 2019). In the Green Lake, sediments of the Amazon River and the Tapajós River, are mixed (Mendes et al, 2020), which makes it difficult the determination of the source and the heavy mineral spatial distributions. The main objective of this research is to determine the spatial distribution of the heavy minerals in order to assess their provenance and testing the applicability of multivariate and geostatistical analysis.…”