MINERALOGIA, GEOQUÍMICA, FERTILIDADE E ORIGEM DOS SEDIMENTOS DE PRAIA (BARRA EM PONTAL) DAS BACIAS DOS RIOS PURUS E JURUÁ NOS TERRENOS DO ESTADO DO ACRE

Abstract

The Acre state is in the southwestern part of the Brazilian Amazon region with an area of 164.123.738 km2 and a low population density (5.52 inhabitants/km2 for 2021). The geological context is mainly related to recent units that outcrop at the Acre basin and associated with the Solimões Formation, besides old alluvial terraces that follow main drainages. The Solimões Formation comprises fossil-bearing clayey siltstones intercalated to fine sandstones. The old alluvial terraces are composed by sand, silt, clay, and occasionally fossil fragments. The area of the Acre state is crossed by two main hydrographic basins: The Purus and Juruá rivers in the eastern and western part, respectively. Both rivers are the so-called “white-water rivers”. They exhibit a typical dendritic drainage pattern and are extremely meandering what give rise to lakes formed after abandoned meanders. During dry season, several beaches (point bars) are exposed on river‘s meanders, frequently used for swimming but also playing a very important role in the small-scale agriculture. These beaches are also used for planting corn (Zea-Mays) and beans (Vigna unguiculata (L) Walp). The main purpose of this work is to investigate the mineralogical and geochemical aspects of those beach sediments to understand their high fertility and to identify their primary source. Fourteen stations were selected for sampling purposes throughout the river valley close to BR-364 highway. Laboratory analyses were carried out as follow: grain sizes, x-ray diffraction, scanning electron microscopy and optical microscopy of heavy minerals concentrates; sediment whole chemistry (major and traces elements analyses including rare-earth elements) by means of ICP-MS; macronutrients (P, Mg, Ca, and K) besides available Na and Al and organic matter. The river sediments are mainly fine-grained, angular sub-angular and selected moderately. Mineralogical assemblage is represented by quartz, clay minerals (smectite, illite and kaolinite) and feldspars (K-feldspars and albite). The heavy minerals contents ranged from 0.5 to 4.6% and are mainly represented by zircon, epidote, kyanite, tourmaline, garnet, rutile, apatite, staurolite, ilmenite and hematite. Chemical results shown that the sediments are mainly composed by SiO2, besides minor amounts of Al2O3, Fe2O3, K2O, CaO and MgO, what is compatible with mineralogical composition. High SiO2 contents are mainly related to quartz and Al2O3 contents reflects the presence of clay minerals. Contents for alkaline and alkali-earths are relatively high for such beach environment related to feldspars and smectites. P2O5 is related to apatite and can also be adsorbed on smectite. Fe2O3 contents are mainly associated to smectite and a minor amount to ilmenite and hematite, the most common opaque minerals found in the beach sediments. Strong correlation coefficients were observed between Al2O3 and most major and trace elements, except for Zr, Hf, Ni and As. On the other hand, strong negative correlation was obtained between SiO2 and major and trace elements what points to the importance of smectites as scavenger of trace elements in such environment. The chemical results were compared with the mean of the Upper Crust and post-Archean Australian Shales (PAAS) and show that they are impoverished in MgO, CaO, Na2O and K2O in comparison with the former and MgO, in comparison with the latter. Such impoverishment is higher in samples enriched on sand fraction. The same comparison was carried out with the Chinese Changliang and Huanghe river sediments, showing impoverishment in CaO and MgO in comparison with the Changjiang River and MgO, CaO, Na2O and Sr, in comparison with the Huanghe River. The small value obtained for the Chemical Index of Alteration (CIA) shows that these sediments have experienced small degree of chemical weathering and consequently, physical weathering has prevailed in the source area. The beach sediments also exhibit high immature index as evidenced by the following very low rations: SiO2 / Al2O3 (11,64) and K2O/ Na2O (2,08). Rare-earth elements were normalized to the Upper Crust and to the PAAS patterns. They show similar patterns of LREE depletion and HREE enrichment. On the other hand, normalized to the chondrites present enrichment in LREE and horizontalization of the HREE, with similar standards of distribution between each other, presenting typical Eu negative anomalies of granitic and intermediary nature rocks. Analytical results for macronutrients exhibited high concentrations for P, Mg, Ca and K, and low contents of available Na and Al. Organic matter contents in below 1%. The sediments show high cation exchange capacity, base saturation over 70% and low Na and Al saturation. Accordingly, those sediments can be interpreted as representing a redistribution of the recent Miocene sediments (Solimões Formation) and the own sediments that were deposited in old alluviums. Chemical and mineralogical characteristics with a silty granulometry indicate that beach sediments are like the eutrophic cambisols.