Rare earth element geochemistry during weathering of S‐type granites from dry to humid climates of Brazil

Abstract

S‐type granites are igneous rock types that are widespread in all continental crust. These granites play a key role in global weathering patterns and can be important economic ores of rare earth elements (REEs). This study provides insights into the geochemical and mineralogical signatures of REEs in S‐type granites and their derived soil profiles along a climosequence from dry to humid climates of Brazil. Mineralogical, petrographic, geochemical and soil analyses (physical and chemical) were used to evaluate the geochemistry of REEs in rock and soil samples. The granites were characterized as strongly peraluminous, being classified as typical S‐types. Both the granites and their derived soils contained high contents of REEs, with monazite and apatite being the major source of REEs. The high enrichment of light REEs (LRREs) in monazite and apatite explained the REE fractionation evidenced in the S‐type granites. The normalized REE distribution patterns for soil profiles along the climosequence revealed the fractionation between light and heavy REEs (HREEs). The ratio of La/YbN > 1 also confirmed the LREEs/HREEs fractionation. The mineralogical composition of the parent material and the clay minerals (kaolintes) seemed to govern the fractionation between LREEs and HREEs in these soils. Chemical index of alteration (CIA) values, REE fractionation and kaolinite proportions in soil profiles increased from dry to humid climates. These results suggest that the degree of weathering plays an important role in the type and volume of secondary minerals, as well as the REE fractionation. Among the phyllosilicates, kaolinite seems to be the most important adsorbent of light REE. The higher positive Eu anomalies with advanced weathering are explained by the breakdown of plagioclase. The weathering of monazite and apatite was the main reason for the slight negative Ce anomalies in the soil profiles. This study can be used to comprehend the geochemistry of REEs in other parts of the tropics underlain by S‐type granites.

Petrology of weathering materials developed on granites in Biou area, North-Cameroon: implication for rare-earth elements (REE) exploration in semi-arid regions

“…SiO 2 /Al 2 O 3 and Na 2 O/K 2 O ratios confirm the dominance of SiO 2 , Al 2 O 3 and to a lesser extent K 2 O over the other major elements ( Table 1 ). The LOI (<1 wt.%), CIA (50.63 %), IOL (16.57 %) values ( Table 1 ) show that the granite (parent rock) is still fresh ( Nesbitt and Young, 1982 ; Babechuk et al., 2014 ; Silva et al., 2018 ) as expected. The binary diagram of A/CNK (Al 2 O 3 /CaO + Na 2 O + K 2 O) versus SiO 2 for granite sample ( Figure 4 ; Chappell and White, 1974 ) show that the rock is peraluminous and belongs to S-type granites (A/CNK> 1.1).…”

“…Therefore, fresh rocks have the lowest CIA values; about 50% or less. CIA values close to 100% indicate a total removal of mobile elements (Ca 2+ , Na + and K + ) relative to immobile residual Al 3+ ( Nesbitt and Young, 1982 ; Silva et al., 2018 ). CIA = [Al 2 O 3 /(Al 2 O 3 + Na 2 O + CaO∗ + K 2 O)] × 100 …”

Quality reference values for rare earth elements in soils from one of the last agricultural frontiers in Brazil

“…The ClA values increase with the depletion of cations (Ca 2+ , K + , and Na + ). Aluminum is relatively immobile; therefore, it is often used as an indicator to evaluate the intensity of chemical weathering of rocks and their respective soils (Alfaro et al, 2018;Silva et al, 2018b;Santos et al, 2019). Muscovite has CIA values around 75 % and kaolinite has a value close to 100 % (Nesbitt and Young, 1982).…”

“…The contents of REEs of the upper continental crust (UCC; Taylor and McLennan, 1985) were used to normalize the concentration of REEs in soils of the Gurguéia watershed. The UCC has been commonly used to evaluate the enrichment and depletion patterns of REEs in soil samples (Cunha et al, 2018;Silva et al, 2018b;Mihajlovic et al, 2019). The La N /Yb N , Gd N / Yb N and La N /Sm N ratios were used to calculate the fractionation between LREEs/HREEs, HREEs and LREEs, respectively.…”