Nickeliferous laterites of Brazil - Cadic intégralehorizon.documentation.ird.fr/.../b_fdi_20-21/28794.pdf · 2013-10-16 · NICKELIFEROUS LATERITES OF BRAZIL" JEAN-JACQUES TRESCASES*"

NICKELIFEROUS LATERITES OF BRAZIL"

JEAN-JACQUES TRESCASES*" ADOLPHO JOSE MELFI***

SONIA MARIA BARROS DE OLIVEIRA***

ABSTRACT

Nickliferous deposits have been known in Brazil from about a century ago. These ores are lateritic weathering products of ultramafic rocks. They occur between 8' and 25' South; most are in Goias S!ate, central Brazil.

The Brnzilian ultramific rocLs can be related to at least three omgenic cycles: Early Precambrian; Late Precambrian and Crelaceous. All of these ultramafic rocks are panially or completely serpentinised.

There are other small Precambrian (age not established) min;ralised massifs: Americano do Brasil, Go: Sanclerlandia, Go, Carajas, PA.

.Brazilian nickeliferous deposits occur in varied conditions of relief and climate. Their formation can be related to the Early Tertiary '.South American erosion cycle". The weathering products of this cycle are laterilic, with residual concentration of iron, but show siiica accumulztion (chalcedony, quarfz) as silicreles over ultramafic rocks.

This silcrete, often of lhe "box-wark" type. apparently formcd at the bottom of Tertiary' profiles and was later exposed by erosion ("Velhas cycle", Late Tertiary). Beneath the silcrcte. a new profile, lateritic or saprolite. formed. Erosion slopcs and especially the lower. new-formed plains bave weathered according to more recent climatic conditions. forming laterites in humid central Brazil, and saprolites in the arid Northeast.

Brazil occupies the seventh position in the world in its estimated reserves of nickel ore (4.10' tons Ni in 1975). The main features of these deposits are:

-initial stage of lateritic weath:ring in Early Terliary. -important silicification related to this cycle. -the silicated ore-type predominates over the limonitic type, as a result of post-Tertiary' tectonomorpho climatic

evolution. . :. Thus, Brazilian nickelilermr deposits are different from the majority of other lateritic deposits in the world.

INTRODUCTION

According to their conditions of formation nickel deposits are of two types (Lombard, 1956 and Boldt, 1967): sulfide deposits of hypogene origin, and lateritic deposits of supergene origin. Nickeliferous laterites constitute a thick weathering cover of ultramafic rocks in tropical areas.

Some sulfides of copper and nickel have been prospected in Brazil (for example in the Sao Joao massif, at Americano do Brazil-Goias). But all the Brazilian nickeliferous reserves are in lateritic weathering products of ultramafic rocks.

Ultramafic massifs occur in the majority of the Brazilian states, from the equator to 32"s latitude.

However, three areas show a higher density of ultramafic rocks (Fig.1) (Berbert, 1977).

A central band, elongated N-S: in Goias and continuing to the state of Para; the largest Brazilian massifs and the best nickel ore reserves are here (Fig.2) (Angeiras, 1968; Godoy, 1968, Lindenmayer and Lindenmayer, 1971; Vasconceilos, 1973).

A second area in Northeast Brazil, mainly in the state of Bahia.

And a third area in the state of Minas Gerais in Sou theas tern Brazil.

The Amazon Basin is very poorly known, which explains the apparent absence of ultramafic rocks in the northwestern part of the country, covered by a great equatorial forest.

'This work is a Dart of a scientific uroiect between Geo sciences Institute of the University of Sao Paulo (Brazil) and . - the ORSTOM (France).

**Mission ORSTOM (Over Seas Scientific and Technical Office), Instituto de Geocien;ias, Univerjidade de Sao Paulo, Brazil.

*'*Instituto de Gcociencias, Universidade de Sao Paulo, C.P. 20S99-Sao Paulo, Brazil.

i I l

l i 2 LATËRITSATION PROCESSSS

Fig. 2

The published works on weathering of Brazilian ENVIRONMENT OF THE DEPOSITS ultramafic rocks and nickel deposit formation are few, with the exception of some mining reports (Andrade and Botelho, 1974; Berbert, 1977; Ferran, 1974; Lages et al., 1975, 1976). The geochemical study of this process in different climatic zones of Brazil began three years ago at the Geosciences In- stitute ofthe University of Sao Paulo, by a French- Brazilian team. The present work is a preliminary

Geological environment The classifications of ultramafic rocks proposed

by Thayer (1960), Wyllie (1963, Naldrett and Gasparini (1971), and Naldrett (1973) are not always appropriate in the Brazilian context. We use here the classification of Berbert (1970) and (1977). with some modifications.

inventory of the characteristic features of the so- called *‘nickeliferous laterites” of Brazil.

(A) Distribution of fhe niussFs (Figs. I and 2) (I) The oldest massifs occur in Goias. They are

I- - +a , * c - 2 i”

; i L

NICKELXFEROUS LATERITES OF B R A m 173

layered mafic-ultramlìc rocks with a probable Archean age (perhaps as old as 4 billion years- Cordani et al., 1973), and great size. There are three massifs: Barro Alto, Niquelandia and Canabrava (and perhaps a fourth to the north in Natividade- Berbert, 1977). each being about one hundred km long, and forty km wide, although the ultramafic zone is much smaller. Four main layers can be distinguished in the massifs (Araujo et al., 1972; Figueiredo et al., 1972; Souza, 1973), with a west- ward dip;

(i) A noritic basal zone (with some Cu and Ni disseminated sulfides)

(¡i) An ultramafic zone, 2000 m thick (dunite, peridotite, pyroxenite, always more or less serpentinised, and some pod-shaped bodies of chromite-White et al., 1971).

(iii) A central zone with norite (Niquelandia) or anorthosite (Barro Alto).

(iv) A top zone with gabbro. The old massifs show both “Alpine” features

(mainly in the bottom part) (Thayer, 1972), and “stratiform” features (at the top) (Fleisher and Routhier, 1970).

(2) The second type of ultramafic rocks occurs in the state of Bahia, associated with the Transama- zonian Cycle (about 2000 million years). These massifs can be stratiform, as at Campo Formoso, with serpentinised peridotites, chromitites, pyroxenites and gabbros (Hedlung. 1974; GonCalves et al., 1972). But the majority of these massifs is attributed to a geotectonic model of a “greenstone belt” with a N-S orientation. These small massifs contain horn- blendites, serpentinised peridotites, actinolite-tremo- lite-talc-schists. Two Transamazonian massifs of pyroxenites and gabbroic gneisses are considered similar to the ultramafic rocks of Bahia: Goiaira- Trindade in Goias (Lindenmayer and Oliveira, 1970) and Pien in Parana (Girard and Ulbrich, 1978). They could have resulted from the fractional crystal- lisation of a tholeiitic magma.

(3) The third type, which constitutes the majority of the ultramafic massifs of Brazil, is the “Alpine” type. Small bodies, from a few metres to several kilo- metres in size, are disposed in “serpentine belts”. They are of different ages (Almeida, 1978):

3000 Million Years in the Minas Gerais belt (N-S direction)

2000 M.Y (Transamazonian Cycle) in Rio Grande do Sul (Alto Vacacai, Bahia (Jacobina), and Sergipe.

1000-2000 M.Y. in the double, NNW-SSE and ”E-SSW, serpentine belt of Goias (Almeida, 1967).

600 M.Y. (Brazilian Cqcle) in Para. Some isola- ted Alpine massifs occur outside of these belts:

Morro do Nique1 (MG), Sao Joao do Piaui, Catin- gueira (Paraiba). Mato Grosso ... The “Alpine” massifs are in general strongly or wholly serpentinised.

(4) The 1 s t type is much younger, with a Cre- taceous age, and can be related tothe volcanism which followed the opening of the Atlantic Ocean. The majority of occurrences surround the Parana sedi- mentary Basin, in Southern Brazil. These massifs are circular, with sizes up to 10 km in diameter. Sometimes they have a nucleus of carbonatite, although the majority has a dunitic nucleus, with thin shells of peridotites, pyroxenites, gabbros and nephelinic syenites (Barbour, 1976; Pena and Flgue- iredo, 1973; Chaban and Santos, 1973). The main ultramafic alkaline massifs occur in Goias and Mato Grosso (Ipora and Porto Murtinho provinces).

(5) Some massifs have been recently discovered, and their classification is difficult: Americano do Brazil and Sanclerlandia (Go), or Serra de Carajas (PA).

(B) Economic considerations The nickel appears accumulated as ore in the

weathering profiles of: (i) The great Archean massifs of Goias (Barro

Alto and Niquelandia) ( i ) The small Alpine-type massifs of varied ages

in many states (Goias, Minas Gerais, Piaui) (iii) The ultramafic-alkaline massifs of the Cre-

taceous (Goias, Sao Paulo). Chrysotile asbestos deposits occur in the Archean

massifs (Canabrava) and in some Alpine-type massifs (Pontalina, Sao Joao do Piaui).

The stratiform massifs contain great reserves o f chromite (Campo Formoso), and the “greenstone belt” type shows indications of Cu,Ni and Zn sulfide:.

In the ultramafic-alkaline massifs, deposits of vermiculite, phosphate, niobium, rare earths and uranium occur.

Table I gives the size, reserves, and average nickel grade of the main nickel deposits. I t empha- sises the importance of the state of Goias. In spite of the presence of ultramafic rocks elsewhere, therr are few nickel deposits in other states; as will i shown later, recent morpeo-climatic evoluf !,u explains the distribution of the Brazilian nickel.

Recent morpho-climatic conditions The economic accumulation of nickel in a

weathering profile (nickeliferous laterite) is a result of the convergence of several morpho-climatic. lithological. and structural factors: ultramafic bcd- rock, tropical climate with contrasting seasons. and levelled relief (with karstic drainage) with

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174 LATERITISATION PROCESSES

Table I. Nickel deposifs of Brazil.

the ultrabasic massif (measured and estimated) %Ni reference Nanie of the massif Aproximate size of Reserves-IO't are Content Bibliographic

Niauelandia 100km' (39 X 2.6 km) 38000 1.40

' Stare

Barro Alto Canabrava ? Santa Fe 38 km'

Goih Serra Agua Branca . 3 5 km' Morro do Engenho . I2 km' Morro dos Macacos 30 km'

Montes Claros ?

Sao Luis Montes Belos

45km' (24X 1.9 km)

Rio dos Bois

(Salobinha) 12 km'

73000 9500

61000 60000 18000 loo00

15000

?

Morro do Niquel 0.5 km* 2300 1.7 (1) Minas Gerais Liberdade 0.2 km' 7 m 1.7 (1)

Ipanema 4 k" 7300 1.24-1.59 (1)

Sao Paulo Jacupiranga 6 km' 3200 1.47 (1) .

Piaui Sao Joao d3 Piaui 6 km' Zoo00 1.57 (1)

Bahia Serra &s Marrecas ? Sao Felix do Xingu ?

900 1.5 (4) 13000 2.1 (2)

ParS. Quatipuru 90 km' (45X 2 km) 13000 1.3 (5) - Carajas ? 1 ?

(1) M.M.E. 1978. (2) Lages et al., 1976. (3) Berbert. C. O., 1978 (Conference at the Instituto de Geociencias deRio Claro). (4) Bruni et al., 1976 (Carta geologica do Brasil ao milionesimo, Folha Aracaju, 226 p.. DNPM, Brasilia). ( 5 ) Cordeiro & McCandlcss, 1976.

secondary rejuvenation after a tectonic uplift (Trescases, 1975; Lelong et al., 1976). All these favourable 'conditions are not simultaneously satis- fied for each Brazilian nickeliferous deposit. And each deposit shows characteristic features, according to its particular recent evolution.

(A) Chtate The following main bio-climatic zones can be

distinguished in Brazil (Romaris, 1974). (i) In the Northeast, the climate is hot and semi-

arid. The vegetation is a dry brushland, with thorny shrubs and cacti ("caatinga"). The average temperature is 24" to 26"C, and precipitation is helow 800 mm, with 8 to 9 months of very dry season.

(i¡) In Central Brazil, the climate is hot and humid, with contrasting seasons. The vegetation consists of savannah with some trees ("cerrado" or "campo limpo"). The average temperature is between 22" and 25"C, and the rainfall isbetween1200 and 1800 mm, with only 3 to 4 months of dry season.

(iii) On the Atlantic Coast of Southeastern Brazil, the tropical climate is semi-hot and semi-humid (average temperature 20T, and rainfall 1500 to

1900 mm). The vegetation is of the tropical forest type. In the Amazon Basin of Northern Brazil, the climate is equatorial, hot, and very humid (average temperature above 26"C, and precipitations above 1800-2000 mm). The vegetation is of the great equatorial forest.

(iv) The climate of Southern Brazil is t'ransitional to a temperate climate.

Nickel deposits occur in the first four climatic zones and mainly in the second one (tropical climate with contrasting seasons).

(B) Relief Most of the ultramafic massifs of Brazil comprise

hills, sometimes tabular, or mountains. These elevations look down on broad levelled plains. The ultrabasic reliefs are of three types:

(i) Tabular hills and mountains. This is the most frequent topographic form. Hill tops and plateaus are capped by a hard layer of silcrete. The slightly weathered rock crops out on steep slopes. Figure 3 shows some cross-sections of Alpine ultramafic massifs, with the silcrete horizon and the weathered zone; Morro do Niquel (MG) and Sao Joao (Piaui).

NICKELIFEROUS LATERITES OF BRAZIL 175

hl S

13- SÄ0 J o b DO PIAUí A-MORRO DO NíQUEL .

E : f ' .

:

i I I I

1 l

i I

C-NIQUEL6NDIA - SUD

A A A A A

D- NIQUELANDIA- NORD

E-SANTA Fe

550

ÉCHELLE HORIZONTALE CGMML%E O I km

, Fig 3

176 LATERITISATION PROCESSES

A similar landscape can be presented by the ultramafic-alkaline massifs (Serra Agua Branca, G0,- Lessa Sobrinho et al., 1971; Justo, 1973). The largest massifs show a contrast of reliefs, with highlands, foothills, and lowlands. It is possible, however, to identify the highlands as plateaus, as in the case of the Southern part of Niquelandia (Fig. 3), where the dunitic zone is a relict of an old plateau with silcrete (the pyroxenite bands correspond to valleys, slightly carved into the old peneplain) (Pecora and Barbosa, 1944; Pecora, 1944; Barbosa, 1968; Costa, 1970).

(i¡) Plains and hills. The largest massifs show characteristic features of a dissected, old plateau; foothills and plains are dominant, with ferruginous detrital material, sometimes with ironcrust near the creeks; around these low zones are some hills with steep and silcrete on their tops (Santa Fe and the Northern part of Niquelandia, Fig. 3).

(iii) Finally, some massifs have been wholly levelled by erosion, mainly in the semi-arid Northeast.

(q Interpretation of the evolution of the relief

The suggested course of evolution of the relief is as follows:

(i) During the late Cretaceous, the uplift of South America began. A very long period of erosion established an extensive peneplain: the South American erosion surface (King, 1956). The weathering products of this cycle are lateritic (Braun, 1971), but show silica accumulation (silcrete) over ultramafic rocks (Santos, 1974; Trescases and Oli- veira, 1978).

(iii) In the late Tertiary, a new erosive cycle began; the Velhas Cycle (King, 1956). A new, lower surface was formed, after incision and dismantling of the South Amercian Surface. In areas of ultramafic rocks, however, silcrete impeded the levelling of the relief. Thus plateaus and tabular hills with silcrete are relicts of the South American Surface.

(iii) In the Quaternary, erosion has continued, restricting the hills even more. The landscapes of Niquelandia and Santa Fe are in a more advanced stage of erosion than the others. During the Qua- ternary, the weathering attached at the same time both

-the relicts of the South America Surface (Highlands), where weathering profiles are developed under the silcrete:

-and the new base level (Velhas Surface), with deep weathering profiles. This weathering is con- trolled by recent climatic cònditions.

DIFFERENT TYPES OF WEATHERING PROFILES

Weathering profiles of the highlands (Plateaus, tops of hills) ’ These highlands are relicts of the South Ameri- can Surface. Their weathering profiles were the first to begin their evolution. Several types of weathering can be distinguished according to the climate.

(a) Hot aiid humid tropical climate, with contrasting seasons (Go, .+.íg, Central Brazil)

This climate is considered as the most propiti- ous for the genesis of a thick lateritic profile with nickel accumulation (Trescases, 1975). Indeed, it is in this climate zone that the biggest Brazilian deposit occurs. Most of the bed-rocks are dunites or serpentinites, sometimes harzburgites. Figure 4 shows some weathering profiles at Morro do Niquel (Griffon and Richer, 1976; Langer, 1969, Santi- vanez, 1965; Trescases and Oliveira, 1978), Barro Alto and Niquelandia and gives for comparison one profile on pyroxenite in Niquelandia. The profile appears really different only on Pyroxenites. On rocks rich in olivine and/or serpentine, the profiles always appear as one of the two extremetypes: Morro do Niquel and Barro Alto. Two principal layers can be distinguished:

Weathered zone, with preservation of the structure of the rock.

Silicated and/or reworked top zone. Weathered zone. The weathering of the hard,

dark, dense fresh rock begins with the modification of the colour during the “slightly weathered rock stage” (R-SG in Fig. 4). The thickness of this layer is betweeen 2 and 5 m. Next, cohesion and bulk density decrease, with densities between 2 and 1.5: this is the coarse saprolite stage (SG). When cohesion disappears, the rock is transformed into a brown argillaceous mass, with conservation of the rock structure, but with very low density (under 1.5). This is the fine saprolite stage (SF), never very thick (1 to 2 m,) and sometimes absent.

In the Brazilian profiles, the coarse saprolite is the thickest horizon of the weathered zone. This stage is intermediatc between the slightly weathered rock and the fine saprolite. But depending on the case, it maintains some features of the former (rocky type, as a t Morro do Niquel), or it quickly loses all cohesion (argillaceous type, as a t Barro Alto). Some silicification can occur in the coarse saprolite as vertical veins (Barro Alto, Fig. 4) or fractures fillings.

...

NICKELIFEROUS LATERITES OF BRAZIL

B m

BARRO ALTO

MORRO DO N~OUEL

-Roche silicifi6e (silcrete)

-Bloc de silcrete

-Bloc de cuiro$se Ferrugineuse

-‘Latérite rausë

si

si

LR L;a

SF -Saprolite Fine (“latérite jaune“) (rache très altdrbe)

Saprolite grossière, facihs “argileux” LW SG- SF ,,/ ,,, (rache altdrde)

+/,* (rache altéde) SG,R-SG -Saprolite grassiire, facibs ”rocheux”

B R - S G -Roche trèspeualtérée

111

NIOUELANDIA (Dunite)

NIOUEL ANOIA (P yroxénite)

R - Dunite, Serpentinite 4 Pyrbxinite

177

Fig. 4

The top zoize is almost always composed of dissected by erosion, and on the slopes. the surface blocks of silicified ultramafic rock this is the silcrete horizon is a powdery red material (“red laterite”) layer. The blocks are a quartz and chalcedony box with fragments of silcrete, and sometimes ferrugi- work, with cavities f ikd by red ferruginous mate- nous fine gravels: this “red laterite” is a residual rial; or they can be massive and hard, like a chert. soil formed from the silicrete. Each block results from the epigenetic silicification Mineralogical evolution of rhe profiles. In the of the rock. But the silcrete itself is an accumula- weathering zone, the mineralogical evolution is a tion of silcified, and later reworked, blocks. transformation of the magnesian silicate into

The silcrete layer is 2-4 m thick, but locally its goethite, with ephemeral precipitation of a little bit thickness exceeds 20 m. This layer always covers of chalcedony, quartz and garnierites.* (Trescases the tabular summits. But, where the plateaus are and Oliveira, 1978; Melfi, 1974).

*The garnierite is a mixture of nickeliferous silicates (hydrated-talc, serpentine, niontmorillonitc, chlorite, Sepial¡tÇ, chalcedony. (Faust, 1976; Brindley and Phon Thi Hang. 1973).

LATERlTlSATlON PROCESSES 178

Calcite, brucite and olivine disappear first (in the slightly weathered rock).

Residual iron and nickel hydroxides precipitate in the cleavages of the serpentinous network (in the coarse saprolite); next, this network is replaced by a nickeliferous goethite network (fine saprolite).

However, prior to this, quartz, chalcedony and nickeliferous silicates (mainly hydrated talc-like

forms) can precipitate in the coarse saprolite (Brind- ley and Souza, 1975; Esson and Carlos, 1978).

The silcrete consists mainly of quartz and chalce; dony. In the reworked red laterite, goethite, and hematite appear abundantly. When the rock includes pyroxenes, the minerals are transformed into smectites (nontronite), and the profile is of the argillaceous type.

Table 2. Average chernicol composition of the two t).pes of weotherinc profiles in the Highlonds. A-“Rocky“ type profile: Marro do Niquel (ofter TreAcoses and Oliveira 1978).

HOf SO. MrO . Fe-O.” ALO. Cr.0. NiO’* COO CILO - . = . . - --2- - - I

3.9 74.3 0.3 17.2 3.79 0.84 0.16 0.042 0.003 0.44 0.034 0.003 3.37 0.078 0.002 2.40 0.033 0.003

13.3 45.2 35.2 4.15 0.62 0.053 1.08 0.016 0.003 16.6 35.7 38.5 5.7 0.75 0.052 0.41 0.015 0.002

LR

SF 10.1 50.3 22.7 10.4 0.99 0.24

R-SG R

s i 2.8 85.7 2.4 6.8 0.95 0.19

R/SG-SG 1 3 2 41.5 30.6 9.8 0.78 0.1s

B-“Argillaceorrs” rype profile: Borro Alto (after Stache. 1974).

H*O* SiO: MgO Fe,O: AI,OO, C,,O, NiO” MnO, Co0 Cu0

9.2 17.0 1.5 51.7 14.2 3.19 1.18 0.91 0.092 0.037 2.58 0.67 0.059 0.041

LR SF 10.8 20.3 12.1 41.5 7.9 2.68 SGSF 14.8 34 25.8 16.8 4.1 1.04 2.51 0.40 0.030 0.026 R-SCI 1.63 34 31.1 13.6 2.9 0.86 1.56 0.27 0.027 0.027

‘Total FL as F~,o,. **The nickel content shows a strong variation from one sample to another. LR. Reworked red-laterite, partially developed from the silcrete. Si: Silcrete. Sl? Fine saprolite (-;yellow laterite). SG-SF: Greenish-brown coarse slprolite (ar$llaceous type). R“ SG-SG: Mainly friable weathered rock (rocky type). R-SG: Hard, slightly weathered rock. R: Fresh rock.

Cliearical evolution of the profiles. Table 2 gives the mean chemical composition of each layer of the two types of highland profiles: “rocky” type (Morro do Niquel) and “argillaceous” type (Barro Alto). The magoesium content decreases with weathering. This progressive leaching is correlated with a relative concentration of the residual elements: iron, aluminium, chromium, cobalt. The silica content increases a little in the Morro do Niquel saprolite, but the ratio SiO,/Fe,O, decreases with increased weathering. In the Barro Alto profile (as in the majority of the profiles), the content of silica of the sspro!ite decreases. Nickcl is strocgly conccn- trated in the uppcr part of the weathered zone. The silcrcte is very rich in silica and poor in nickel. In the Barro Alto profile, the “red laterite” is con- stituted mainly by thc residual elements (Fe, AI, Cr, &o), and nickel content is medium (1%).

The isovolumetric balance (gain and Iöss) of the weathering confirms that magnesium and part of the silica are progressively leached in the weathered zone (part of the silica precipitates ‘as amorphous compounds). Nickel shows absolute gains. The other elements are strictly constant. In the silcrete, the residual elements are again constant, but here, there is a loss of nickel and an absolute gain of silica.

Conclusion on the evolutiori of fite higlIand profiles in a tropical climate with contrasting seasons. The evolution of the highland weathering profiles in a tropical climate with contrasting seasons compriscs the progressive, but sometimes incomplete, destruc- tion of thc silicates in favour of a goethitic residue: this is an evolution with a lateritic tendency.

However, some residual free silica remains; and this evolution, in the recent morpho-climatic condi-

-

. 179 NICKELIFEROUS LATERITES OF .BRAZIL

A 1 A’ Ear de versant

Piedmont

Plaine - Collurian d e blocs d i silerit.

embollis doni un maliriou brm-roup.

3 .

, -.- - , O SO 100 2 0 0 250m ---

A A n n a . . A n n a *

Fig. 5

tions of Central Brazil, does not allow the develop- ment of thick ferruginous horizon of fine saprolite typical of the lateritic profiles of New Caledonia, for example (Trescases, 1975).

The presence of silcrete a t the top of the profile is in contradiction with the lateritic tendency. This layer shows an absolute gain of silica without parent source. This silcrete could not be formed from the underlying saprolite. I t represents the silicified basal part of theold profiles of the South American Surface. Some roots of this silicified layer remain, now in the new weathcrcd zone, beneath the silcrete (as at Barro Alto, Fig. 5). The upper part of the old South American profiles was removed by erosion, thus supplying the colluvial formation of red laterites on the slopes (and in the plains and lowlands of the Velhas Cycle).

The lateritic tendency of recent weathcring is hampered by the introduction of silica from the silcrete; and the time of evolution has been too short for true laterite formatioq.

For mining, the silcrete is poor in nickel and considered barren. The “red laterite” layer contains medium grade of nickel (1% Ni) where it is not derived exclusively from the silcrete; it is an “oxidized” ore. The weathered zone represents the main part of the ore. The more complete the weathering, the higher is the grade of nickek, the best layer is at the top of the saprolite zone. This ore is a silicated ore.

(b) Semi-orid tropical clitnate (Bahia, Piarri, Nor& eastern In spite ßrazil) of the abundance of ultramafic rocks

in the state of Bahia, there is only a small number of nickel deposits in the Brazilian semi-arid region. This climate favours mechanical processes of erosion, rather than chemical weathering. A lot of the ultramafic massifs of this zone are strongly eroded, as at Andorinha (BA), where the relief is wholly levelled. Here, the weathering profile is not thick, and shows only some smectitisation and veinlets of

LATERITISATION PROCESSES . 180

silica. The small occurrences of Serra das Marrecas, in

Central Bahia and Catingueira (Paraiba) are dis- mantled relicts of an old deposit.

In the Sao Joao do Piaui deposit, however, a strong silcretecapprotected the plateau relief (Santos, 1974). A new profile is developed beneath the silcrete, but here, the evolution does not show a lateritic tendency: magnesia is again leached away, silica and iron are strictly residual. The saprolitic weathered zone shows the massive transformation of the ultramafic rock into smectite, with local high nickel content (2% Ni). In some places the silica dissolved in the upper part of the silcrete precipitates in the saprolite. This silicified saprolite is less rich in nic- ke1(0.5%-1% Ni). Thus here, there is a slow and gradual descent of the silcrete, which is destroyed at its top and regenerated at its bottom.

(c) Permarrently httntid climatic cortditiorts, (Ivorfh and Soutlt B r a d )

The nickel deposits in the Amazonian forest were discovered only a short time ago and they are not well known. Cordeiro and McCandless (1976) write about silicification lateritisation and peneplanation in the massif of Quatipuru (Pars).There seems to be strong analogies with the deposits of Goiãs.

In southern Brazil, there is only one small deposit at Jacupiranga (State of SBo-Paulo, 25"s Latitude). The evolution of the weathered zone has a lateritic tendency at the top without total leaching of silica. The top .layer is a very thick horizon of powdery "red laterite", with some blocks of silcrete. The nickel content is very low (SI. 5% Ni) (Ferran, 1974). The localisation 3f the massif of Jacupiranga in a tectonic graben could explain a lower degree of erosion: the thick cover of red laterite could come from the evolution of the old weathering products of the South American surface.

In Parans, Santa Catarina and Rio Grande do Sul (South of 25" S Latitude) the recent weathering is not lateritic and the profiles are not thick. Vermi- culite and smectites are the dominant minerals and nickel concentration does not occur. Even during the Tertiary, the weathering probably was not lateritic in the South.

Profiles of the plains and lowlands

(a) Tropical climate with contrasting seasons (Goi& The weathering profiles observed in the low parts

of the massifs of SantaFe and Niquelsncia (North- ern part) are very similar. The weathered zone show the same sequence of horizons present in the highlands: slightly weathered rock, coarse saprolite, and fine-grained ferruginous saprolite which is thicker here.

The superficial reworked layer is very differenl; the profiles of the lowlands never show the silcrete cover characteristic of the highlands. The upper part of the lowland profiles is always made up of about 3m (or more) of powdery red laterite, with ferruginous fine gravels.

The mineralogical evolution of the profile is the same in the lowlands as that previously described in highlands. The mean chemical composition of each layer for the massif of Santa Fe is given in Table 3. The similarity between these compositions and the corresponding horizons of the highland profiles (Table 2) isvery high, with the evident exception of the top layer of red ferruSinous laterite. A part of the accumulated nickel of the lodand profiles is of residual origin; but the rest comes from the highlands either through mechanical processes with the colluvial red laterite or by chemical processes, through lateral migration.

In conclusion, in the humid tropici1 region of Brazil with contrasting seasons, the weathered zones

Table 3. Average coniposition of each horizon in the inassif of Sailto Fe.

HtO* SO, MgO Fc,O,' A1200, Cr,O, Ni0 Co0 CuÒ

LR 5.2 10.1 1.6 70.8 3.5 6.2 1.19 0.142 0.011 8.9 32.9 9.9 39.0 2.4 4.1 1.40 0.063 0.010

0.028 0.005 SF SG 12.2 40.8 28.3 15.5 0.7 0.64 1.0

R-SG 12.8 41.0 33.4 10.4 0.5 0.44 0.33** 0.07.4 0.004 R 7.4 36.3 41.1 13.2 0.2 0.35 0.23 0.020 0.003

(0.3-1.9)

- *Total Fe as Fe.0, (Fe0 for the fresh rock only). '*Sample witho& the garnicritic veinlets. LR: Red laterite. S F Fine saprolite=yellow lateriie. S G Weathered mck=Coarse saprolite. R-SG: Hard, slighily weathered rock. R: Fresh rock.

'< -- "2. * a y

18 1 NICKELIFEROUS LATERITES OF BRAZIL

in the lowlands are very similar to those in the highlands: the weathering is in equilibrium with recent climatic conditions. The lateritic evolution is more complete in the lowlands. The reserves of nickel ore are more important in the lowland profiles, where three layers constitute ores:

--coarse saprolite, silicated ore, with 1 to 2% Ni -red laterite, oxidised ore, with 1% Ni -and fine saprolite, transititional between silicat-

ed and oxidized type, with content of Ni (1 to 4% Ni), but with restricted thickness.

(b) Semi-arid tropical climate (Piaut) A foothill fringe is devgloped around the central

plateau of the Sä0 JOBO do Piaui massif. These foothills are connected with the "Velhos" plain. Figure 5 shows the schematic disposition of the horizons for all the topographic sequence slope-foothills- plain.

The colluvial surface horizon contains a lot of crumbled blocks of silcrete within a powdery brown- red material. Exactly on the topographic knick point, these blocks have a ferruginous cortex, and are cemented by iron hydroxides (iron crust appearance). The weathered zone shows a saprolite horizon (argillaceous type in the plain; rocky type on the slope and at the bottom of the profiles). On the lower part of the slope, this sarprolite issilicified. On the slope, only slightly weathered rock crops out.

The mineralogical evohtion is the same as that described for the plateau: transformation of the serpentine into smectite, mainly in the foothills and on the plain; incipient formation of goethite a t the surface; predominance of quartz and chalcedony in all the colluvial cover and in the coarse saprolite in the lower part of the slope.

The chemical evolution i s marked by the preservation of iron and silica (and even by the introduction of silica coming from topographically higher silcrete). The magnesium is quickly leached as soon as the serpentine disappears. The nickel migrates laterally from the plateau to the plain through the weathered zone. Without this contribution the leaching of the magnesium alone could not produce a significant relative concentration of the residual nickel of the rock. The colluvial cover is barren (0.2 to 0.3% Ni). The coarse saprolite is nickel ore: 0.8 to 1%Ni (withlccally 3% Ni) in the rocky type and 1.2 to 1.5% Ni in the argillaceous type.

GENETIC INTERPRETATION

Being a tropical country with numerous ultramafic massifs submitted to several erosional cycles,

------?--------,-----Ancien 6vem 1o~ogro~hi,~us

Fig. 6.

Brazil presents good conditions for having nickeliferous laterites. The estimated reserves of the nickel ore are 4.106 tons Ni, which .places Brazil 7th-in in world.

The common features of the Brazilian deposits, and also the differences between the deposits located in diffirent bio-climaticzones, are in accord with the following interpretation of the successive stages of their evolution (Fig. 6).

(i) The. redistribution of elements began in the Early Tertiary, with the South American levelling cycle. In already levelled relief, a semi-arid climatic period induced the formation o f a silicified layer a t the bottom of the weathering profiles on ultramafic rocks.

(i¡) Later, a more humid climate promoted the lateritisation of these paleo-profiles, and a preliminary (residual) concentration of nickel. However, to the south of 25"s latitude, silicification and lateritisation seem not to have occurred.

.

LATERITISATION PROCESSES

(iii) In the Late Tertiary, with the breaking up of the South American Surface, the lateritised weathering products were removed, and the silicrete exposed. With this protection against erosion, the ultramafic massifs appeared little by little as plateaus above the levelled “Velhas” Surface.

. (iv) During the ~‘Velhas” cycle, weathering was active beneath the silcrete which covers the plateaus (South American relicts), and in the “Velhas” lowlands. In accordance with recent climatic conditions this weathering shows a lateritic tendency in Central Brazil (tropical climate with contrasting seasons). In the semi-arid North-East the weathering process is smectitisation.

(v) With the progress of erosion, the South American relicts disappear. The nickel is progressively transferred from the highlands towards the lowlands, where it enriches the new-formed weathering profiles.

CONCLUSION

The main features of the Brazilian deposits are: (i) genesis correlated with two levelling cycles in

(ii) Restricted lateritisation, with silicated pro-

(iii) Important silicification on the highlands. (iv) Lateritisation more advanced in the lowlands. (Y) Thickness and nickel grade of the mineralised

layer often modest: either because the climate is no longer aggressive (North East), or because the lateritisation acted for a short time (Central Brazil).

(vi) Preferential accumulation of nickel in the Iow- lands.

(vii) Preferential accumulation of nickel at the top of the weathered zone.

(viii) Huge predominance of the silicate type of nickel ore.

Thus, Brazilian nickeliferous deposits are different from the majority of the other lateritic deposits in the world. Oxidised deposits occur in the Northern Hemisphere in Cuba (De Vletter, 1955), Guinea (Bonifas, 1959; Percival, 1965), Philippines (Santos-

the Early and Late Tertiary.

files, without great thickness.

Ynigo, 1961), Venezuela (lurko Vic, 1963). In these deposits, the weathering profiles are very deep and mainly goethitic; nickel is associated with goethite; the nickel content is never very high, but it is a little higher at the bottom (Schellman, 1971). Oxidised-silicated deposits occur in New Caledonia (Trescases, 1975), and in western United States (Hotz, 1964). The silicated layer of coarse saprolite is present here, but the thickest horizon is goethitic. The nickel content is high at the bottom of the coarse saprolite (2-5% Ni), but the goethitic fine saprolite is also nickel ore (1-273. The best deposits are located in the highlands. The nickel ore reserves are principally of the oxidised type. These deposits are located in the Southern Hemisphere (New Caledonia), or to the north of the intertropical zone (USA). However, some small deposits of the Southern Hemisphere show similarities with the Brazilian type. In South Africa (De Waal, 1971) and in Australia silicification in the highlands and lateritisation in the lowlands have been mentioned. In these profiles the nickel content increases toward the upper part.

This differentiated supergene evolution of the ultramafic rocks of the two hemispheres could result from world climatic variations correlative with the movement of the poles and the continental drift during the Tertiary and Quaternary.

ACKNOWLEDGEMENTS

This work has been supported by grants from the FAPESP (Fundacäo de Amparo ä Pesquisa do Estade de Sao Paulo) and from the CNPq (Conselho Nac- ional de Desenvolvimento Cientifico e Tecnololgico, Brazil).

We thank the mining companiesMorro do Niquel CODEMIN, MONTITA, Vale do Rio Doceand BAMINCO, mineracao e siderurgia, for their help in the collection of field data.

This publication represents a condensed paper ex- tracted from a more complete work to be published in Cahier ORSTOM SOrie Geologie. We thank ORSTOM for the authorisation to reproduce the figures of the main work.

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Rept., pt, 13, p. 247-259.

.. - . . _ . .

LATERITIC FORMATIONS ÌN USSR

D.G. SAPOJNIKOV Institut of Geol of Ore Deposit

Staromoiietny Pet. 35 Moscow 109017

USSR

ABSTRACT

The lateritic formations of the USSR are subdivi d into three main varieties: lateritic bauxites, developed in the crust of weathering of rocks with different composition; iron ores, formed during jaspilite weathering; ochres, which were formed in the crust of weathering on serpentinites.

The lateritic formations are associated with the upper part of crust of weathering profile with zonal structure. The profile Of crust of weathering is subdivided into five zones, distinguished by a degree of source rock reworking (from below upwards): 1-disintegrated; 2- kerolithised; 3-montmorillonitised; 4-nontronitised serpentinites; and 5-clayey- ochreous rocks and ochres. Similar zonation is observed in the profiles of lateritic crusts of weathering, formed on the rocks with different composition.

The lateritic formations of the USSR are distributed over the territory of the Russian platform, the Siberian platform, the Urals, Kazakhstan and some otherregions. Within the areas of crust of weathering distribution. they are associated with the mother rocks of favourable composition. However, lateritic formations are usually overlain by a mantle of younger sediments, they outcrop predominantly as a result of the latter washout and therefore their surface exposures are restricted.

The contemporary climatic conditions in USSR is such that on I t s territory thelateritic formations do not occur. We are aware only of the fossil lateritic formations associated with earlier weathering epoch. They are formed in the weathering zone due to the accumulation of iron and aluminium oxides with some other elements following the leaching of silicon, alkaline earth metals and of other soluble compon- ents in the course of the weathering of parent rocks of silicious or silicic compositions.

Among the lateritic formations of USSR we may mention the following: iron ores of residual origin (lateritic) and lateritic bauxites. The former, in turn, are subdivided into: (1) iron ores rich in hematite and martite which were formed due to the jaspilite weathering, and (2) iron ores of hydrogoethite composition-goethite forming the upper crust of the weathering mantle on serpentinites.

The study of weathering layers, and their metalli- ferous deposits, petrography, mineralogy and geochemistry (IGEM), carried out at the Geologic Institute of the Academy of Sciences in USSR, has established existence of some zonation in the struc-

ture of the alteration layer (1.1. Guinzbourg et al., 1946; A.P. Nikitina et al., 1968). In particular, on the weathering layer of the serpentinites, we can distin- guish few zones characterised by their mineralogic composition. The following are these zones from below upwards.

(1) Zone of serpentinites, slightly weathered and disintegrated; (2) zone of serpentinites kerolithised and lixiviated; (3) zone of montmorillonitised serpentinites; (4) Zone of nontronitised serpentinites; (5) zone of clayey-ochreous rocks and ochres.

The weathered bed of serpentinites is studied in detail at Urals range where its age is determined to be upper Mesozoic era. Normally, the thicknesi of the weathering bed, which has a large lateral extent, varias from 10 to 40 m. In a few areas of ultramafic rocks from the Southern Urals range (those of Kenpisai. Khalilovshi and others), mainly consisting of apoperidotic serpenthites, a clear zonal structure is noticed.

(1) The lowest zone consists of disintegrated rocks invariably passing downwards into serpentinites. According to I.V. Vitovskaia (1976) the disin-

!

i

kational Geological Monument of Laterite. Angadipuram, Kerala Phofo : Geological Survey of Indie ..

. . -s*

- LATERITISATION PROCESSES - i--.-----

, Proceedings of the International Seininar on Lateritisation Processes

(TRIVANDRUM, INDIA 11-14 DECEMBER, 1979)

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