Geochemistry of rare-earth elements in surface lateritic rocks and soils from the Maicuru complex, Para, Brazil

Journal of Geochemical Exploratton, 47 ( 1993 ) 165-182

165

Elsevier Science Pubhshers B V , A m s t e r d a m

Geochemistry of rare-earth elements in surface lateritic rocks and soils from the Maicuru complex, Para, Brazil

R f m u l o S Ang6hca t and Marcondes L da Costa Centro de Geocldnclas, UmversMade Federal do Para, 66 075-900 ~ P 1611, Belem Para Braztl (Received 15 July 1991, accepted after revision 21 July 1992)

ABSTRACT

Ang6hca, R S and Da Costa M L , 1993 Geochemistry of rare-earth elements m surface latentlc rocks and soils from the Malcuru complex, Para, Brazil In F W Dlckson and L C Hsu (E&tors), Geochemical Exploration 1991 J Geochem Explor 47 165-182 The Maicuru area contains a typical alkahne/ultramafic rocks-carbonatlte complex that ts part of the Guyana Craton, northern Brazil, and exhibits thick laterltiC profiles Tops of profiles are hardened ferrugmous horizons frequently called "iron crust" or "ferrlcrete" which gives rise to landforms called "iron-hat" The area is being explored for titanium and phosphate resources bv Docego, a subsidiary of Companhla Vale de Rio Doce (CVRD) This paper discusses rare-earth elements (REE) mineralization related to aluminum phosphates m Tl-nch surface latentes (iron crusts and laterltlc soils ) containing anatase over the Maicuru Complex Some samples show total REE concentrations as high as 20 0 wt % that is contained m crandallitegoyazite-floreneite solid solutions, with the last end member predominant Mean X REE contents are shghtly greater in soils than in iron crusts Chondrlte-normahzed plots for REE in soils and iron crusts reveal strong fractlonatlons with high (La/Lu)N mean values, where the heavy rare-earth elements (HREE) are strongly depleted compared to light rare-earth elements (LREE), a pattern typical of carbonatlte and alkaline rocks Strong positive and negative Ce anomalies were observed m ~ron crust samples, but were less common in soils LatentlC tron crusts distinctly differ from latentlc soils m geochemical properties Latermc iron crusts over minerahzed rocks are enriched m T~, Fe, P, REE and reslstate minerals, but soils are not Anomalous concentrations of REE in latentes strongly indicates the presence of phosphates m laterires and in parent rocks The study illustrates the importance of understanding laterlt~zatlon processes for geochemical exploration in tropical terrains

Correspondence to M L da Costa, Centro de Geocl6nclas, Unlversldade Federal do Para, 66 075900, C P 1611, Bel6m, Para, Brazil ~Present address C o m p a n h l a de Pesqulsa de Recursos Mlnerals-CPRM, Av Dr Freltas, 3645 CEP 66 095-110, Belem Parfi Brazil

0375-6742/93/$06 00 © 1993 Elsevier Science Publishers B V All rights reserved

166

R S ANGI~LICA AND M L DA COSTA

INTRODUCTION Ultramafic/alkallne rocks-carbonatite complexes are important sources of Nb, P, TI, U, Zr, and also REE In the Amazon regton, where intense tropical weathenng conditions create thick lateritic profiles, important secondary mineralizations commonly are associated with and derived from these rocks The Maicuru ultramafic/alkahne rocks-carbonatlte complex is overlain by thick, mature and complex laterltes, which are among the better known in the Amazon region (Lemos et al, 1988, Lemos and Costa, 1987, Costa et al, 1991; Ang61ica and Costa, 1990, Angelica, 1991 ). Docegeo w a subsidiary of CompanhIa Vale do Rio Doce (CRVD), the operator of the Caraj~is Mining District - - has mining rights in the Maicuru area. This company did exploration and research to find phosphates and titanium resources, which resulted in sampling the laterltiC cover. In conjuction with exploration efforts of this company, a collaborative research program was established with the Geosclences Center of the Federal University of Pardi ( U F P a ) This paper, which describes the rare-earth element geochemistry in surface laterltes (iron crust and lateritic soils) from the Malcuru Complex, is part of research for a M Sc. thesis by the first author at UFPa (Ang6hca, 1991 ) under the direction of the second author PHYSIOGRAPHIC FEATURES The Malcuru Complex is located in the northwestern part of Parfi state, northern Brazil, about 700 km from Bel6m city (Fig 1 ), and within the remote Amazon jungle Access from nearby cities is only by helicopter or small boat The climate is equatorial, of the Amazon type, with an average rainfall of 1500 m m and an annual mean temperature of 26°C Vegetation includes dense, humid, evergreen forests on the slopes and valleys and savanna-like coverings on elevated areas underlain by iron crust. The topography developed on the Malcuru structure is undulating, with high rehefcharacterized by elevated, incurved, "iron hat" elliptical hills, with major and minor axes of aproxlmately 9 and 6 km GEOLOGICALSETTING Rocks of the Complex were emplaced into gnelsses and granites of the Guyana Craton, which is the regional Archean/Proterozolc basement A study of matenal from 18 dnll holes, 38 pits, and a few fresh rock outcrops in areas underlain by Al-phosphates and tltanlferous laterites (Costa et al, 1991 ), indicates that the subsurface rocks are mainly pyroxenItes with subordinate dunltes, and commonly, carbonatlte veins that cut all rocks types, Including

GEOCHEMISTRY OF REE IN SURFACELATERITICROCKS AND SOILS, MA1CURUCOMPLEX

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isolated apatltic bo&es. Syenltes that resemble fenites occur in a narrow zone along the border of the Complex The surface (Fig 2) is underlain mostly by latentlC materials Iron crusts that occupy the central part of the Complex represent the uppermost hardened horizons of laterltlC profiles, called ferrlcret by the French authors (e g, Nahon, 1986, Beauvals, 1989), and duncrust, ferncrust, culrasse, canga, by others Latentes can be subdivided into at least three crust types magnetic, tltanlferous, and phosphatic Soils border the central structure and are derived from iron crusts and primary alkahne rocks (Fig 2 ) SAMPLING AND ANALYTICAL METHODS

Docegeo Implemented a geochemical sampling survey for surface laterltes and soils that covered most of the Maicuru Complex, with about 1500 sam-

168

R S ANGI~LICA AND M L DA COSTA

N

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Fig 2 Slmphfiedgeologicmap of the Malcuru area (after Costa et al, 1991 ) and samphnggrid used by Docegeo pies collected in a semi-regular 100)<200 m grid (Fig 2) For this study, 86 samples were taken from this grid m a second semi-regular 600)< 400 m grid Minus 80 mesh soil fractions, screened directly from the sod samples, and 80 mesh ~ron crust fractions, screened from pulverized material, were used Analyses were done by the Geosol laboratories, for REE, by inductively-coupled plasma-atomic emission spectrometry (ICP-AES), for Ba, Ga, Nb, Sc, Sr, V, Y and Zr, by emission spectrometry, for Fe203 total, by titration, for T102 and P205, by colorlmetry, and for Mn, Nl, Co, Cr, Cu, Pb and Zn, by fusion and AAS. Minerals of the 86 samples were identified at UFPa by use of the X-ray &ffractomer with conventional techmques -

GEOCHEMISTRY OF REE IN SURFACELATERITICRO( KS AND SOILS,MAICURUCOMPLEX

169

G E N E R A L G E O C H E M I C A L A N D M I N E R A L O G I C A L ASPECTS

Surface latentes from the Malcuru Complex contain iron crusts and sods w~th contrasting chemical composltmns Iron crusts have high mean levels of Fe203 (60 0%), T102 (16 59%) and P205 (1 34%), and soils contain high concentratmns of A1203 ( 13 13%) Some tltamferous ~ron crusts are as high as 53.3% m T102, reflecting unusually h~gh contents of anatase Latente varlTABLE 1 Summary statistics for REE m the ~ron crust (c) and soils (s) samples from the Malcuru Complex (m ppm )

La Ce Nd Sm Eu Gd Dv Ho Er Yb Lu SREE LREE HREE Ce/Ce* Eu/Eu* (La/LU)N

(c) (s) (c) (s) (c) (s) (c) (s) (c) (s) (c) (s) (c) (s) (c) (s) (c) (s) (c) (s) (c) (s) (c) (s) (c) (s) (c) (s) (c) (s) (c) (s) (c) (s)

n

Mean

Std dev

Minimum

Maximum

64 22 64 22 64 22 64 22 64 22 64 22 64 22 64 22 64 22 64 22 64 22 64 22 64 22 64 22 64 22 64 22 64 22

159 64 254 54 426 38 519 80 225 07 233 99 36 32 36 46 8 45 8 56 2093 21 34 90 10 14 1 35 1 64 261 3 32 1 34 1 92 0 21 0 23 891 59 1081 91 854 65 1043 34 35 43 38 57 2 00 1 07 0 83 0 86 72 113

202 26 161 51 455 04 327 81 359 7 169 52 56 99 26 70 12 77 6 61 2951 16 57 11 16 7 47 1 66 1 16 297 2 23 1 11 0 96 0 13 0 097 1042 81 710 57 999 9 684 66 45 94 28 32 4 79 0 24 0 07 0 04 54 40

6 41 87 12 43 97 142 40 8 83 66 43 0 86 9 68 0 23 2 17 055 5 65 0 27 2 55 0 045 0 45 001 0 88 0 12 0 59 0 027 0 078 67 37 318 0 66 35 307 8 1 022 10 2 0 33 0 24 0 71 0 77 5 49

1122 0 703 9 2167 0 1339 0 2414 0 758 5 389 3 125 4 82 2 31 4 1834 78 9 57 2 36 1 86 57 136 11 1 64 54 06 0 5 5200 3 3095 9 4933 3 2958 3 267 1 137 7 38 97 1 86 0 99 0 91 253 196

Ce anomaly=Ce/Ce*=3(Ce/Ce)N/[2(La/La)N+ (Nd/Nd)N] ratio which indicates the REE fractmnatmn

(La/LU)N= (La/La)N / (Lu/LU)N

170

R S ANGI~LICA AND M L DA COSTA

atlons in composmon in general can be related to mineralogical composmon The extremely ferrugmous iron crusts contain major hematite, goeth~te, maghemlte and dmenlte, and lesser but high anatase and Al-phosphates (crandalhte-goyazlte-florenclte and augehte) The sods are composed of clay minerals, Al-goethlte, g~bbslte and Al-phosphates, and lesser ~ron and tltamum Oxl-hydrox~des. Iron crusts and sods also &ffer in trace elements Iron crusts average, in ppm" Mn, 700, Cr, 1868, N1,450; and V, 396 Soils average, m ppm Cu, 101; Ba, 889; Sr, 367, Nb, 237, and Zr, 816 RARE-EARTH ELEMENT GEOCHEMISTRY

Element concentrattons

Selected statistical parameters for 86 samples (64 iron crust and 22 sod) are summarized in Table 1 Average total REE concentrations m laterltes, 892 ppm, are htgh compared to laterltes elsewhere Soils average somewhat higher in total REE, 1082 ppm, than crusts (Fig 3) The light rare earth elements, LREE, (mainly La, Ce and Nd) are more strongly enriched in both soils and crusts than the heavy rare earth elements, HREE

5001 (ppm)

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171

GEOCHEMISTRY OF REE IN SURFACE LATERIT1C ROCKS AND SOILS, MA1CURU COMPLEX

(%)

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172

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Fig 5 Chondnte normahzed plots for total samplesof the iron crust and soils from the Maicuru Complex A few samples from the phosphatic crust of the Malcuru Complex (Lemos and Costa, 1987) contained as much as 20 0 wt % o f Z R E E m the composltlonally highly variable Al-phosphate group minerals, the crandalhte series, especmlly the rare earth-rich end member, florenote The Al-phosphates are c o m m o n accessory minerals of latentes that have been extensively stud~ed worldwide (Vlelllard et al, 1979, Ollvelra, 1980, Costa, 1982, Schwab et al, 1989) The generalized formula for the crandalhte group minerals is AB3 (XO4)2 (OH,F) 5_6,in which A=Ba, B1, Ca, Ce, La, Nd, Pb, Sr, Th, B=A1, Fe3+; and X = A s , P, $1 (Roberts et al, 1990) C o m p o s m o n s of the more important crandalhte minerals in the Malcuru Complex can be more s~mply written AAI3 (PO4) 2 (OH) s" H20 End member minerals with p o s m o n A occupied by different elements are Ca (crandalhte), Sr (goyazlte), Ba (gorcelxlte), and REE, Y (florenclte) Similar high REE concentrations are also found in latentes derived from other alkahne rock-carbonatlte complexes m the Amazon region at Sels Lagos (Issler and Silva, 1980); m southern Brazil at Arax~i and Catal~o I (Rodrigues and Lima, 1984), m western Africa at

173

GEOCHEMISTRY OF REE IN SURFACE LATERITIC ROCKS AND SOILS, MA1CURU COMPLEX

Maboum6 (Laval et al, 1988), and in Austraha at Mt Weld (Lottermoser, 1990)

Frequency dlstnbuttons Frequency dlstnbutlons for all elements in both sample types (F)g 4) show a positively skewed asymmetry that approaches lognormahty Some show blmodahty. The distributions show slmdar patterns for both ~ron crust and soils, with ~ron crusts exhibiting more pronounced posmve skewness

Chondrtte-normahzed plots The REE d]stnbutlon patterns for all samples are shown m Figure 5, normalized according to Evensen et al ( 1978 ) Iron crusts and soils exhibit different patterns In REE dlstnbutlons" ( 1 ) Iron crusts show wider ranges in REE concentrations (shaded areas ), as shown by greater standard deviations (Table 1 ) Sods not only have less

I0000'

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Fig 6 Chondrlte normahzed plots for selected samples of the iron crust and soils from the Malcuru Complex

174

R S ANGI~LICAAND M L DA COSTA

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Fig 7 P e a r s o n ' s h n e a r c o r r e l a t i o n c o e f f i o e n t s for the R E E for the iron crust a n d soil s a m p l e s f r o m the M a l c u r u C o m p l e x

GEOCHEMISTRY OF REE IN SURFACE LATER1rlC ROCKS AND SOILS MA1CURU COMPLEX

175

TABLE 2 Mare hnear correlation coefic~ents between REE and other trace elements m 64 ~ron crust and 22 sod samples from the Malcuru Complex P

Y

Sc

Ba

Sr

Nb

Ti

Zr

(rust

LREE HREE

0 65 0 64

0 61 0 76

0 83 0 68

0 79 0 82

0 80 0 85

0 73 0 64

0 33 0 25

0 22

Sod

LREE HREE

0 69 0 67

0 63 0 75

0 40 0 52

-

0 60

0 64 0 61

0 56 0 37

0 43

Note Coefficients marked as ( - ) are considered not sagmficant at a 95% level of confidence

varlabihty in distribution patterns, but also are more hmlted in local variations, a consequence of more homogenizing processes during laterltizatlon (2) All laterltes are distinctly enriched in LREE, with chondrlte-normalized (La/Lu)N ratios ranging from 5 to 253, mean 72 for iron crusts, and from 49 to 196, mean 113 for soils (Table 1 ) Highly variable (La/LU)N ratios of laterxtes point to the fractionatlon of REE during the weathering processes (Lottermoser, 1990) Strong fractionatlon and high REE concentrations is typical for fresh carbonatltes and alkaline rocks as described for carbonatItes from Lages - - Brazil (Scheibe and Formoso, 1982), and Mt Weld - - Australia (Lottermoser, 1990), for syenltes from Akongo - - Africa (Braun et al, 1990), and for carbonatites and syenltes previously described from the Malcuru Complex (Lemos and Costa, 1987) These similarities in REE patterns between primary rocks and overlying laterltes can be explained in two ways (a) the laterltes (mainly Iron crusts) may have inherited these patterns from their respective parent rocks, or (b) the homogenizing laterltlZatlon processes may have completely disturbed patterns in different horizons of profiles, in which the similarities observed in Malcuru are merely coincidental (3) Some iron crusts show strong positive Ce/Ce* values, a e > 1, and others show negative values, 1 e Ce/Ce* < 1, a mean Ce/Ce* value of 2 0, with standard deviations as high as 4.79 (Table 1 ) Values of Ce/Ce* range from 0 33 (sample A0-1999) to 38 97 (sample A0-1919) (Fig 6) Sample A01999 has the highest REE contents (5200 ppm) Soils from Malcuru rarely show Ce anomalies, with a mean Ce/Ce* value of I 07, a low standard deviation, 0 24, and a range from 0 84 to 1 86 (sample 603-632, Fig 6) Laterltes at Akongo, Africa, above syenltes had Ce/Ce* values of 0 9 for iron crusts and 0 87 for overlying loose nodular horizons derived from iron crusts (Braun et al, 1990) These values are smaller than those from the Maicuru Complex Strongly positive Ce anomahes in the Akongo profile were observed only in tops of saprohte horizons (Ce/Ce*= 7 01 ) ,

176

R S ANGI~LICA AND M L DA COSTA

(4) Europium anomahes, Eu/Eu*, commonly observed in chondrite plots for different rocks and laterltes, were not seen in the Malcuru samples Mean Eu/Eu* values for iron crusts and soils are 0 83 and 0.86, respectively, for iron crusts the values range from 0 71 to 0.99 (Table 1 ) Sample A0-1865 (Fig. 6) showed the most significant negative Eu anomaly (Eu/Eu*= 0 71 )

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Fig 8 Scattergrams and linear correlation coefficients for (a) T102 X Nb, (b) T~O2× Ce t c ) Ce X Nb for the iron crust and sod samples from the Malcuru Complex

GEOCHEMISTRY

OF REE IN SURFACE LATERITIC

ROCKS AND SOILS MAICURU

177

COMPLEX

Correlatton coefficients The REE analyses from the Maicuru Complex show high linear correlation coeficzents (Fig 7) Among the REE, Ce shows the largest and more erratic correlations, because of the relatively great mobihties of Ce In the lateritic environment The behavlour of Lu correlations, which are systematically smaller, is probably due to low concentrations and analytical problems caused thereby (Hall and Plant, 1990) Table 2 summarizes significant correlations between the REE and trace elements Phosphorus, Ba and Sr show strong positive correlations with LREE and HREE in iron crusts, representing their presence in crandalhte group minerals. Yttrium correlates more strongly with heavy REE than with light REE, for both iron crusts and soils Niobium correlates well with LREE, especially with Ce Cerzum

dlsperszon

Cerium data confirm that this element, among the lanthanides, shows different behavior in surface environments, probably related to the ability o f Ce

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R S ANGI~LICA AND M L DA COSTA

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to transform to C e 4+ under oxidizing condmons Braun et al (1990) gave detailed d~scusslons of Ce anomahes in African laterltes However, many questmns remain unanswered on Ce mobdlty m latentlc environments Posrove Ce anomalies were explained by the formation of cenamte (CeO2) under oxidizing conditions that are also favorable for formation of crandalhte group minerals (Manana, 1989, Braun et al., 1990) NegaUve Ce anomalies occur m low anatase materials from the Maicuru Complex. The Cea+ions can substitute for Ti or Nb m Nb-rlch anatase Strong T102-Ce and Nb-Ce correlations (Fig 8) both in iron crusts and soils support the exlstance of such a subsUtutlon Thus, low anatase m laterltes can produce negative Ce anomahes Surface distributions of Ce anomaly values (Fig. 9 ) show that Ce/Ce* values less than one (negative anomalies) are clustered in central parts of magnetic iron crust, exactly where anatase ~s lowest (Fig 10), and where T102 is present mainly m llmemte

179

GEOCHEMISTRY OF REE IN SURFACE LATERITIC ROCKS AND SO|LS MAICURU COMPLEX

Anomaly maps F,gure 11 shows that I REE distributions above 3000 ppm dehmlts an elongate zone that overlaps areas of anomalous phosphate (phosphatic iron crust) (Fig 12) Drill holes in this area intersected carbonatltes and apattterich rocks (Costa et al, 1991 ) This anomalous phosphate-REE zone also extends toward areas underlain by tltanlferous iron crusts, which are higher in REE than magnetic iron crusts High concentrations of REE that occur scattered throughout the complex, in some places far from the central zone (Fig 11 ), probably stem from poorly exposed, isolated apatlte-rlch cross-cutting bodies pervasively present in all rock types

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180

R S ANGI~LICAAND M L DA COSTA

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Fig 12 Dlstnbutlon o f phosphorus concentrations xn surface laterltes

CONCLUSIONS

High concentrations of REE in surface laterltes from the Malcuru Complex, and the strong fractlonatIons of HREE relative to LREE, confirm the presence o f underlying alkallne-carbonatite rocks The typical association P-Sr-Ba-Y-Sc, together with the REE, is clear evidence o f the presence of Al-phosphates o f the crandalhte group in laterites and of primary apatite in underlying rocks The crandalhte solid solution series (especially REE-nch florencite) plays an important role as a scavenger of REE and other elements assooated with phosphates The compositions o f laterltes, therefore, especially iron crusts, closely reflect the chemical nature of the substratum Thus, the laterltes can be Important in sampling for geochemical exploration in tropical terrains. Their study constitutes a geochemIcal tool, useful as an adjunct to geological mapping These characteristics,

GEOCHEMISTRY OF REE IN SURFACE LATERITIC ROCKS AND SOILS MAICURU COMPLEX

181

therefore, cause laterltes to be very important wherever intense weathering creates thick laterltes over poorly exposed rocks, such as in the Amazon region Finally, the results also show that iron crusts and associated sods are geochemically d~stlnct latentlc materials Iron crust compositions reflects bedrock contents of T1, Fe, P, REE, and reslstate minerals, whereas the soil compositions in the same area do not ~.CKNOWLEDGEMENTS

The authors want to express their thanks to Docegeo and to Luoval R da Fonseca, for field and laboratory assIstence Financial support has come from Brazilian PADCT/FINEP program and CNPq We also thank Anselmo dos Santos for drawing the figures and two anonymous reviewers for their comments and suggested improvements to the manuscript

REFERENCES Angelica, R S, 1991 Dlspers~o mlneral6glca e geoquimlca em crostas e solos laterltlCOS aphcada a caractenzag~o do substrato do Complexo de Malcuru, Para Unpubl M Sc thesis Federal University of Para, Brazil, 168 p (in Portuguese) Angelica, R S and Costa, M L, 1990 Assooa96es geoqmmlcas nos laterltos superficlas do Complexo de Malcuru, Para In 36th Brazlhan Geological Congress Natal Abstracts pp 88-89 (in Portuguese) Beauvals, A , 1989 Etude petrographlque et geochlmlque de profils d'alteratlon laterlhque cuirasses clans le sud-est de la Repubhque centrafrlcame Geodynamlque, 4 (2) 71-91 Braun, J J , Pagel, M , Muller, J P , Balong, P , Mlchard, ,X and Gulllet, B 1990 Cemum anomahesin latentlcprofiles Geochlm Cosmochlm Acta, 54 781-795 Costa, M L, 1982 Petrologlsh-Geochemlsche Untersuchungen zur Genese der Bauxite und Phosphat-Latente der Region Gurupl (Ost-Amazonlen) Unpubl Ph D thesis, Um~ of Erlangen, 189 pp Costa, M L, Fonseca, L R , Angelica, R S, Lemos, V L and Lemos, V P , 1991 Geochemical Exploration of the Malcuru alkahne-ultramafic-carbonatlte complex, northern Brazil J Geochem Explor, 40 193-204 E~ensen, N M , Hamilton, P J and O'Nlons, R K , 1978 Rare earth abundances m chondmtlC meteorites Geochlm Cosmochlm Acta, 42 I 199-1212 Hall, G E M and Plant, J A, 1990 Are your REE results total ~ Explore, 68 18-20 Issler R S and Silva, G G 1980 The Seis Lagos carbonatlte complex In 3 lth Brazilian Geological Congress, Camborlu Proceedings, 3 1564-1573 Laval, M , Johan, V and Tourhere, B, 1988 La carbonatlte de Mabounle exemple de formation d'un glte reslduel a pyrochlore Chron Rech Mln, 491 125-136 Lemos, V P and Costa, M L, 1987 Part19~o dos elementos Tetras Raras nos laterltos fosfatlcos de Malcuru, Para 1st Brazlhan Geochemical Congress, Porto ~,legre Proceedings 1 375384 (in Portuguese) Lemos, R L, Fonseca, L R and Martins, L P B, 1988 Petrografia do Complexo alcahno-uhramafico-carbonatltlCO de Malcuru, Para 35th Brazlhan Geological Congress, Belem Proceedlngs, 3 1400-1411 (m Portuguese)

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R S ANGl~LICAAND M L DA COSTA

Lottermoser, B G , 1990 Rare-earth element mineralization within the Mt Weld carbonatlte latente, Western Australia Llthos, 24 151-167 Marlano, A , 1989 Economic geology of rare earth minerals In B R Llpln and G A McKay (Editors), Geochemistry and Mineralogy of Rare Earth Elements Mineralogical Society of America, pp 309-338 Nahon, D , 1986 Evolution of iron crusts in tropical landscapes In S M Colman and D P Dethler (Editors), Rates of Chemical Weathering of Rocks and Minerals Academic Press, Inc, Orlando, FL, pp 169-191 Ohvelra, N P , 1980 Mlneralogm und Geochemle der phosphatf'uhrenden Laterlte yon Itacuplm und Trauira (Nordbraslhen) Unpubl Ph D thesis, Univ of Erlangen, 149 pp Roberts, W L , Campbell, T J and Rapp, G R J r , 1990 Encyclopedia of Minerals, 2nd ed Van Nostrand Reinhold, New York, NY, 979 pp Rodrlgues, C S and Lima, P R A S, 1984 Complexos Carbonatitlcos do Brasll In Companhla Braslleira de M l n e r a ~ o e Metalurgla - - CBMM (Editors), Complexos CarbonatltlCOS do Brazil Geologla CBMM, S~o Paulo, pp 3-17 (in Portuguese) Schelbe, L F and Formoso, M L L , 1982 Contnbul~O da geoqulmlca das terras raras/l caracterlzaq~o dos carbonatitos da Fazenda Varela, Lages, SC Rev Bras Geoc, 12 (4) 553561 (in Portuguese) Schwab, R G , Herold, H , Costa, M L and Ohvelra, N P , 1989 The formation of alumlnous phosphates through latentlc weathering of rocks In K S Balasubramanlan et al (Editors), Weathering its Products and Deposits Theophrastus, Athens, Vol 2, pp 369-386 Vlelllard, P , Tardy, Y and Nahon, D , 1979 Stablhty fields of clay and aluminum phosphates parageneses in latentlc weathering of argillaceous phosphatic sediments Am Mineral, 64 626-634