REE Distribution Pattern in Plants and Soils from Pitinga Mine—Amazon, Brazil

doi:10.4236/ojg.2012.24025

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Open Journal of Geology, 2012, 2, 253-259

http://dx.doi.org/10.4236/ojg.2012.24025 Published Online October 2012 (http://www.SciRP.org/journal/ojg)

REE Distribution Pattern in Plants and Soils from Pitinga

Mine—Amazon, Brazil

Maria do Carmo Lima e Cunha1*, Vitor Paulo Pereira1, Lauro V. Stoll Nardi1, Artur C. Bastos Neto1,

Luiz Alberto Vedana2, Milton L. L. Formoso1

1Centro de Estudos em Petrologia e Geoquímica, Instituto de Geociências, Universidade Federal do Rio Grande do Sul,

Porto Alegre, Brazil

2Programa de Pós-Graduação em Geociências, Universidade Federal do Rio Grande do Sul,

Porto Alegre, Brazil

Email: *[email protected]

Received July 19, 2012; revised August 17, 2012; accepted September 14, 2012

ABSTRACT

The rare earth element contents of plant specimens of the families Rhamnaceae, Ampelozizyphus amazonicus Ducke

(local name: Saracura-Mirá) and of Pteridófitas from genus Gleichenia sp. e Adiantum sp. (ferns) were determined and

compared to those of the soils, in the Pitinga Mine area, Amazon, Brazil. The Pitinga mine district has large tin reserves

genetically related to two granite bodies, Agua Boa e Madeira, both intrusive in volcanic rocks included in the Iricoumé

Group. This deposit contains, also, bodies of cryolite and rare metals, such as Zr, Nb, Ta, Y and REE. The REE bio-

geochemical signatures, shown by the collected plants, reflect the patterns of the respective soils. The Eu and Ce

anomalies shown by some plant samples are inherited from soils, as well. The higher contents of REE observed in fern

samples confirm they are accumulators and reflect the abundance of REE in the soils of Pitinga Mine region. Addition-

ally, that supports their potential use in geochemical exploration and bioremediation. The results of this study stress the

importance of biogeochemical research integrated with geochemistry of soils, rocks and minerals.

Keywords: Biogeochemistry; Ree; Amazonian Environment

1. Introduction

Rare Earth Elements (REE) have a particular importance

in geochemistry, for all have very similar chemical and

physical properties [1,2]. In low-temperature geochemi-

cal systems the mobility of REE depends mainly on the

solubility of residual minerals that concentrate REE and

on the capacity of fluids to transport them. REE concen-

trations in natural waters are very low and depends on

the complexes which are capable to be formed. Clay

minerals can concentrate relatively high amounts of REE

[3]. HREE and LREE are generally fractionated during

weathering as demonstrated by Nesbit [4].

Under natural conditions, the absorption of REE by

plants is very low. Plants show total concentrations of

REE in leaf ashes, in the range from 1 to 45 ppm [5] al-

though, some species can accumulate up to 500 ppm.

Such variations are explained by the abundance of REE

in the soil or, by the specific capacity of absorption

shown by some species [6]. The light and heavy REE can

be fractionated by internal processes of the plants, as

admitted by Ding et al. [7] and Lima e Cunha et al. [8].

The bioaccumulation processes of REE have, nowa-

days, an increasing importance in geochemistry and en-

vironmental sciences due to their wide use in non-nuclear

industry and agriculture, which can result in environ-

mental contamination [6]. As far as mineral exploration

is concerned, some authors [9-11], have discussed the use

of vegetation in the identification of anomalous concen-

trations of REE in subsurface deposits. An advantage of

using phytogeochemistry instead of soil geochemistry

for mineral exploration is that the roots of plants can

absorb metals from several cubic meters of substratum,

so that, they represent a large volume of sampled mate-

rial.

Since the Cretaceous, the covertures of the Amazon

region are being modified by weathering processes ori-

ginating deep alteration profiles, derived from a variety

of source rocks [12]. The current climate, humid tropical,

causes intense soil leaching and significant losses of

chemical elements. In this way, bioprospection programs

in the Amazonian region have, in many cases, some ad-

vantages in relation to soil sampling, since the B horizon

is frequently covered by thick layers of humus or lateritic

crusts with up to 200 m of thickness.

This paper, which focuses on the analysis and inter-

*Corresponding author.

M. C. LIMA e CUNHA ET AL.

254

pretation of environmental behaviour of the REE, aims to

compare the representativeness of the biogeochemical

method in relation to soil geochemistry in the Pitinga

Mine, consisting the data integration a pioneering initia-

tive in this knowledge area.

2. Geological Setting

Located in the State of Amazonas, the Pitinga mine dis-

trict has large tin reserves genetically related to two

granite bodies, Agua Boa e Madeira, both intrusive in

volcanic rocks included in the Iricoumé Group, the lar-

gest geologic unit in this area [13-15]. The mine, in addi-

tion to being one of the world’s largest producers of tin,

contains expressive deposits of cryolite and rare metals,

such as Zr, Nb, Ta, Y and REE. There also some mine-

rals rich in Li, Th, Be, Rb and sulfides [16,17].

The sampling of plants and soils was concentrated in

the Madeira granite soils and in areas where the volcanic

rocks of Iricoumé Group are predominant. The Madeira

Granite is composed of by four facies: biotite granite,

porphyritic hypersolvus granite, rapakivi granite and, the

albite granite facies, which is subdivided in core and

border sub-facies [18]. According to Costi [19], these

facies show significant variations of ∑REE contents,

from 180 to 1100 ppm. The Madeira Granite tin deposit

has 164 million tons of disseminated ore, at a grade of

0.17% of Sn (cassiterite), 0.20% of Nb2O5 and 0.024 of

Ta2O5, both contained in pyrochlore and columbite. The

disseminated ore contains 0.17% of REE, concentrated

mostly in xenotime. The volcanic rocks which are pre-

dominant in the region of Pitinga Mine consist of effu-

sive and pyroclastic sequences with associated hypabys-

sal bodies [14,15] (Figure 1).

3. Materials and Methods

Eighteen pairs of soil-plant samples were collected in the

vicinity of Pitinga mine, in areas of granitic substratum,

more precisely in the Albite granite (ABG) and Biotite

Granite (BG) facies and, also, in areas dominated by the

volcanic sequences of Iricoumé Group.

Plant specimens of the families Rhamnaceae, Am-

pelozizyphus amazonicus Ducke (local name: Saracura-

Mirá) and of Pteridófitas from genus Gleichenia sp. e

Adiantum sp. (ferns) were chosen for sampling, for they

show wide distribution in the studied area and are of easy

recognition.

The samples of leaves were dried in an oven at 80˚C

and, then, subjected to calcination (ashing) under tem-

peratures of 450˚C - 500˚C for a period of 6 to 8 h. The

ashes (0.25 g) were digested with HClO4-HNO3-HCl-HF

and analyzed by ICP-MS at Act Labs (Canada). The ob-

tained results are expressed as weight ash. The soil sam-

ples, about 50 g each, were collected at a depth of ap-

proximately 20 cm, at the same point where the sample

plants were obtained. Soil samples were dried in an oven

at 80˚C, broken up in a porcelain grail and the sieved

grain fraction under 250 mesh was used. The samples

were analyzed in ActLabs (Canada) by INAA and ICP

(4A-Exploration methods and 4B-Lithium Metaborate

Figure 1. Geological map of Madeira Granite and associated volcanic rocks from Pitinga Mine with the sample location

points.

M. C. LIMA e CUNHA ET AL. 255

Fusion, respectively). The REE contents were norma-

lized against the C1 chondrite values [20].

4. Results

The statistics representing the REE contents in plant

(leaf-ashes of fern and Saracura-Mirá) and in soil sam-

ples collected in the Pitinga Mine area (Table 1), show

that the sum of REE contents (∑REE) is higher in plants

than in soils. The exception is the Saracura-Mirá col-

lected over the biotite-granite soils.

The fractionation of LREE (La-Eu) relative to the

HREE (Gd-Lu) represented by LREE/HREE, is higher in

plants than in soils, which demonstrates that LREE are

more absorbed than the heavy ones by plants. Lima e

Cunha et al. [8] observed the same behavior in previous

studies in this same region. The plant/soil ratio of the

∑REE in areas where the substratum is volcanic is higher

than in areas where it is granitic.

The contents of LREE, in areas with volcanic substra-

tum, in most of the Saracura-Mirá ash samples, is about

ten percent of soil contents and two percent for the

HREE (Figure 2). The mean values (Table 1) for plants

are strongly affected by two samples with very high and

anomalous REE contents (LaN over 1000, Figure 2).

REE patterns of soils and plants in areas with volcanic

substratum are approximately paralell, even in relation to

those described by Horbe a Peixoto [12] for the lateritic

covertures of volcanic areas in this region. The same is

observed for the Eu/Eu* values, which are close to 0.31

and 0.38 for soils and plants, respectively. The contents

of REE in ferns is very high in relation to soils (REE in

plant/REE in soil = 6, Table 1) and, higher than those

referred by Kabata-Pendias & Pendias [5] for plants.

In the area dominated by the biotite granite the sam-

ples of Saracura-Mirá show REE contents lower than

those of soils and, approximately parallel patterns (Fig-

ure 3). The LaN/YbN ratios in the plants is close to 7.55,

whilst in the correspondent soils it is 1.77. The LREE-

segment of normalized patterns (Figure 3) are similar in

plants and soils and show, even, the Ce positive anoma-

lies and the Eu negative ones. Two samples of fern

colected in the biotite granite area, show the same pro-

perty of concentrate LREE relative to the HREE (Table

1).

Table 1. Mean values of REE contents (ppm) in plant ashes and soils from Pitinga Mine area, Amazon.

*Sara 1Volc Soil Volc **Sama VolcSoil VolcSara 2BgSoil BgSama BgSoil Bg Sama 3ABg Soil ABg

LREE 195.66 28.2 2420.24 336.2 8.75 134.2 53.12 20.30 996.35 366.20

HREE 11.40 3.13 920.40 251.22 1.00 38.77 2.09 4.15 148.00 220.57

∑REE 206.06 31.33 3340.64 587.42 9.75 172.9755.21 24.45 1144.37 586.77

LREE/HREE 17.07 9.009 2.62 1.33 8.68 3.46 25.41 4.89 6.73 1.66

LaN 195.53 43.3 357.00 83.10 2.26 30.10 70.88 32.10 994.66 28.70

*Saracura-Mirá; **Fern; 1Volcanic Substratum; 2Biotite Granite; 3Albite Granite.

Figure 2. Biogeochemical signature of plants (Saracura-Mirá) and soils in areas of volcanic substratum from Pitinga Mine

area. Full lines = soil; dashed lines = plant.

M. C. LIMA e CUNHA ET AL.

256

Figure 3. REE-chondrite normalised patterns in soils (full line) and in plants (Saracura-Mirá) in the biotite granite area.

The behavior of REE in soils and plants (ferns) from

the albite granite area, where the mineralization is situ-

ated, is different from the previous areas with volcanic

and biotite granite substrates. The soils are enriched in

HREE and, ferns show concentration ten times higher

than the correspondent soils for LREE. Additionally,

soils and plants show strong negative Eu anomalies. The

albite granites when compared to volcanic and biotite

granites, show the same features, HREE enrichment and

deeper negative Eu anomalies [21]. Differently, also,

from the rocks, the soils and ferns in the albite granite

area show distinct positive Ce anomalies (Figure 4).

5. Discussion

Volcanic and granitic rocks from Pitinga Mine area show

REE patterns of some samples with M-type tetrad effect

[18,19,21], which are more noticeable in the third and

fourth segments. Soils and plants, in some cases, keep

the same features, even though, not so clear as in rock

patterns. The tetrad effect in soils and plants from Pitinga

Mine area are also suggested by high SmN/NdN ratios and

small positive Gd anomalies. The Gd positive anomalies

have been referred by Ding et al. [6] and Xu et al. [22] in

plant samples from elsewhere. Lima e Cunha et al. [23]

registered these kind of anomalies in plant samples col-

lected from soils over syenitic rocks in southernmost

Brazil and concluded that, the Gd positive anomalies

reflect the presence of M-type tetrad effect.

LREE are more absorbed than the HREE by plants

from granitic and volcanic soils of Pitinga Mine area.

The presence of abundant zircon among the detrital

phases of soils from the Pitinga Mine area, particularly,

over the granitic rocks [12], explains why the HREE

show low availability for plants. Zircon is a very resistant

mineral to weathering and, a powerful concentrator of

HREE. The ratio HREE in zircon/HREE in granitic mag-

mas is close to unity for La and higher than 300 for Lu

[21]. According to Tyler [24] the HREE in soil solu-

tions, in general, come mostly from the dissolution of

xenotime, which is a widespread phase in the mineralized

albite granites from the Pitinga Mine area [25].

Volcanic and granitic rocks in the Pitinga Mine region

show similar REE patterns with LaN ~ 250, LuN ~ 25 and

slight negative Eu anomalies [14]. The soils which cover

both rock types are strongly depleted in LREE in relation

to rocks. The mean value of the ratio REE content in

plant/REE content in soil in the granitic areas is lower

than in the volcanic ones, 2.0 and 6.0 respectively. Such

behavior can be explained, at least in part, by the finer

grain size of volcanic soils, which would make the REE

bearing phases more soluble and, consequently, more

available for vegetal absorption [26]. Additionally, the

REE in granitic rocks are mostly in accessory phases,

such as, zircon, titanite, allanite and apatite [27], which

are resistant to weathering and, therefore, would cause a

decrease in the availability of REE for plants from gran-

itic soils. The REE contents of Saracura-Mirá (Figures 2

and 3) is six times lower than that of the respective soil,

which corroborates the assumption of Tyler [24], who

affirms that the transfer from soil to plant is usually low

and generally unrelated to their total concentrations in

the soil.

M. C. LIMA e CUNHA ET AL. 257

Figure 4. REE-chondrite normalised patterns of plants (fern) and soils in the albite granite area. Dashed lines = plants, full

line = soils.

On the other hand, the REE contents of fern samples

(Figure 4) are higher than those of the respective soils,

which is in good agreement with several authors that

consider the pteridophytes effective accumulators of

REE [28,29]. Tyler & Olsson [29] affirmed that the ca-

pability, of many types of fern, for accumulate REE, is

probably explained by particular absorption or solubili-

zation processes developed by these plants. Ozaki et al.

[30] suggested that the REE can have some positive role

in the evolution of pteridophytes, as, for instance, to con-

tribute for their adaptation to environmental changes.

The negative Eu anomalies shown by plants are a re-

production of soil and rock patterns (Figures 3 and 4),

whilst, the positive Ce anomalies can be related to altera-

tion processes related to soil formation.

6. Conclusions

Based upon the results obtained in this study and, with

support on the research of other authors we conclude:

1) The biogeochemical signatures shown by the plants

collected in the Pitinga Mine area, reflect the patterns of

the respective soils;

2) The high concentrations of REE observed in the

ferns (pteridophytes) are related to their high capacity of

accumulate them and, to the high contents in the soils of

the studied area;

3) The negative Eu anomalies in the studied plants

show that they can indicate the geochemical features of

rocks and soils; whilst, the positive Ce anomalies indi-

cates the action of secondary processes in soils from the

mineralized granites;

4) The relatively high contents of REE in ferns, make

them accumulator species with potential use in geo-

chemical exploration and bioremediation;

5) The results obtained in this study have stimulated

the continuity of biogeochemical research integrated with

geochemistry of rocks, soils and minerals.

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