oliveira et al 1992

Mineral. Deposita 27, 137-146 (1992)

s ta �9 Springer-Verlag 1992

Lateritic nickel deposits of Brazil S. M. Barros de Oliveira ~, J. J. Trescases 2 and A. Jos~ Melfi 3

1 Universidade de S~o Paulo, Instituto de Geoci~ncias C.P. 20.899 - S~o Paulo, Brazil 2 Universit6 de Poitiers, Laboratoire de Prtrologie de la Surface, 40 Avenue du Recteur Pineau, 86022 Poitiers, France 3 Universidade de S~o Paulo, Instituto Astron6mico e Geofisico, C.P. 30.627, S~o Paulo, Brazil

Abstract. Many nickel deposits are known in Brazil, accounting for about 350.106 tons of ore with an average of 1.5% Ni. All are of the lateritic type. These deposits are scattered throughout the country, being rarer in the Northeastern Region and in the South, below 25~ latitude. They are mainly associated with mafic-ultramafic massifs of large dimensions and ultramafic alkaline complexes, and occur in climatic regions of contrasting seasons. The weathering profile developed over the fresh rock consists, from bottom to top, of the following horizons: altered rock, coarse saprolite, argillaceous saprolite, ferruginous saprolite and lateritic overburden. The thickness of each horizon varies from one deposit to an- other, the whole profile generally exceeding 20 m. The saprolitic horizons with inherited minerals (serpentine, chlorite) or neoformed minerals (smectites) constitute the silicated nickel ore and are thicker were climatic conditions are drier; the ferruginous upper horizons made up of iron oxide-hydroxides are more developed in more humid regions. In Brazil, the silicated ore generally prevails over the oxidized ore. The main Ni-bearing minerals are serpentine, smectite, garnierite and goethite. The lat- critic nickel deposits of Brazil may be correlated with two erosion surfaces, corresponding to the Sul Americano (Lower Tertiary) and Velhas (Upper Tertiary) levelling cycles. The degree of dismantling of the higher and more ancient surface and the consequent development of the Velhas Surface control the position of the nickel accumulation in the landscape. Thus, the deposits may be found either in the lowlands or in the highlands, where they are always covered by a silcrete layer. The alteration profiles in the Brazilian lateritic nickel deposits are broadly similar to those described elsewhere in the world. However, they present two characteristic features: the silicated ore prevails over the oxidized ore, and a silicified layer covers the profies developed on the highlands.

Two kinds of deposits are responsible for the production of nickel: the magmatic sulfides and the nickeliferous

laterites. World resources total 97.106 tons of contained nickel; about 50% are located in New Caledonia, the remaining being mainly in Indonesia, Canada, the USSR, Cuba and Australia. About 72% of this total is of the lateritic type (DeYoung Jr. et al. 1985).

Nickel production was about 630000 tons of contained nickel in 1982, 65% coming from magmatic ores. More than 80% of the production is derived from only eight countries: the USSR, Canada, Australia, New Cale- donia, Indonesia, Cuba, South Africa, and the Philip- pines (DeYoung Jr. et al. 1985).

Brazil has important resources of nickel adding up to more than 4.106 tons of contained metal. All this nickel comes from lateritic deposits. Only two sulfide deposits are known in Brazil: Americano do Brasil in the State of Goi/ts, and Fortaleza de Minas in the State of Minas Gerais, both recently discovered. In 1987 the production reached 22092 tons of contained nickel (An. Min. Br. 1988).

The lateritic nickel deposits have been known in Brazil since the last century. The first quotation in literature was about the Jacupiranga district, in the State of S~o Paulo, in 1889. Since then other deposits have been mentioned: Niquelgmdia, in the State of Goifis (1906), Liberdade, in the State of Minas Gerais (1916), and Morro do Niquel in the same state (1922). In the second half of this century, through systematic prospection, many other deposits have been discovered:

Nickel was exploited on a small scale in Jacupiranga during the Second World War, and in Liberdade from 1927 to 1975. At the moment two deposits account for the nickel production in Brazil: Niquel~ndia, which provides at least 5000 tons of electrolitic nickel per year, and Morro do Niquel, with a production of about 10000 tons per year of Fe-Ni alloy. Brazil exports Fe-Ni alloy, but imports manufactured products of nickel. The importa- tion has decreased meaningfully in the last few years, since Niquel~ndia mine has been operating (1980).

There are lateritic nickel deposits scattered all over the country; they are rarer in the Northeastern Region and absent in the South, due to climatic reasons. In the Area-

138

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Fig. 1. Lateritic nickel deposits of Brazil

zonian Region the small number of known deposits is probably a consequence of the scarce geologic knowledge of the area. Figure 1 shows the main deposits of lateritic nickel in Brazil, grouped in four regions: North, North- east, Southeast and Central, the latter accounting for the more important resources.

A lot has been written about nickel in Brazil. The economic aspects have been discussed by Ferran (1974), Berbert (1977) and by the Brazilian Mineral Yearbooks. The inventory of the deposits and their general description is the work of Schobbenhaus et al. (1984). The detailed geochemical and mineralogical characterization of the mineralized profiles began in the mid 1970s; at the end of that decade a preliminary synthesis was published by Melfi et al. (1979, 1980). Since then many papers have been published about case histories; these will be cited in this paper.

G e o l o g i c a n d c l i m a t i c s e t t i n g

The lateritic nickel deposits are formed by weathering alteration of ultramafic rocks under favorable morphoclimatic conditions.

In Brazil there are a lot of ultramafic massifs scattered around different climatic zones. In many of them, the weathering alteration led to the formation of thick blankets enriched in nickel, known as nickeliferous laterites.

cently, Danni and Teixeira (1981) and Nilson (1984) pre- sented two classification proposals at regional and global levels.

The mafic-ultramafic complexes differ in age and type. The nickel-bearing massifs can be grouped into three cat- egories:

Mafic-ultramafic bodies of large dimensions, associated with granite-gneiss. Three massifs of this type occur in the State of Goi~ts following the SW-NE direction: Barro Alto, Niquelgmdia and Cana Brava. They are about 100 km long and tens of kilometers wide, but their ultramafic zones are much smaller. For Niquel~ndia and Barro Alto, four unities, metamorphosed in the granulite facies and dipping to the west, have been identified (Figueiredo et al. 1975):

- a gabbroic basal zone; - an ultramafic serpentinized basal zone; - a gabbroic central zone; - a gabbro-anorthositic upper zone.

The total thickness of these unities is more than 20 km, only 2 km corresponding to the ultramafic zone. Danni and Teixeira (1981) and Danni et al. (1981) suggested for Niquelgmdia the individualization of the upper zone as an independent body. Whether this interpretation is valid and can be also applicable for Barro Alto are subjects under discussion.

Small serpentinized massifs of "alpine" type. These form the "serpentine belt" (Almeida 1978) and are from 3000 to 600 m.y. old, being associated with several orogenic cycles. They occur along alignments in the State of GoiAs, Parfi, Bahia and Minas Gerais. Some of them have nickel deposits, like Silo Jogo do Piaui, and the small complexes of the State of Minas Gerais (Angeli and Choudhuri 1985).

These massifs generally contain an associated mafic zone; their ultramafic zones are derived from dunites and peridotites and are always strongly serpentinized.

Cretaceous ultramafic-alkaline complexes. These are related to the volcanism which occurred at the time of the opening of the Atlantic Ocean (Ulbrich and Gomes 1981). They form a belt surrounding the Paranh Basin, from the State of Rio Grande do Sul up to the State of Mato Grosso. In general they have a circular shape of about 10 km diameter; the ultramafic zones are generally in their cores. Santa F6, ,~gua Branca, Jaupaci, Iponi and Morro do Engenho, in the State of GoiAs, and Jacupi- ranga, in the State of $5o Paulo, belong to this category.

Some massifs do not fit into this classification, for instance Vermelho and Quatipuru, both in the Parer State.

Mafic-ultramafic complexes in Brazil

In the last few years, the geological knowledge of the various types of mafic-ultramafic complexes has in- creased mainly in the Central and Eastern Regions. Re-

Climatic conditions

Being a very large country, Brazil shows a great variety of climates (Fig. 2). The lateritic nickel deposits are found in four bioclimatic domains associated with four geo- graphic regions:

139

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""x.

climatic zones. The deposits are associated mainly with the large massifs and the ultramafic-alkaline massifs and, to a lower degree, with the "alpine" type massifs. As to the climate, it is principally in the Central Region, with contrasting seasons, that the large deposits are concentrated. There are also some important deposits in the Amazonian Region and in the Southeastern Littoral. There is just one deposit in the semi-arid zone and none in the Southern Region beyond 25~ latitude, from where the climate becomes sub-tropical to temperate.

Description of the deposits

Central Region deposits

REGION CLIMATE ~ Northeast Semi-arid tropical

~ Central Tropical with contrasting seasons

--~ Southeasl Sub - worm and sub-humid tropical

~ North Worm and humid equatorial

~ South Sub- tropical to temperate

Fig. 2. Climatic domains in Brazil (Romaris 1974)

Northern Region warm and humid equatorial climate with equatorial forest. The precipitation is more than 1800 mm per year and very well distributed. The dry season never exceeds two or three months. The temperature, constant during the year, is about 26 ~

Central Region warm and humid tropical climate of contrasting seasons with savannah vegetation. The precipitation varies between 1200 and 1800 mm per year and is concentrated in at least six months. The dry season is not longer than four months. The average annual temperature ranges from 22 to 25 ~

Northeastern Region tropical semi-arid climate with "caatinga" vegetation. The average temperatures are between 24 and 26 ~ The average annual precipitation is less than 800 mm, concentrated in only three or four months, being completely dry the rest of the year.

Southeastern Region sub-warm and sub-humid tropical climate with tropical forest. This is a transition region to sub-tropical climate with an average temperature of about 20 ~ with great variations according to the altitude. The precipitation is well distributed during the year, amounting to between 1200 and 1900 ram.

The comparison between Fig. 1 and 2 indicates that the nickeliferous lateritic deposits are preferentially related to certain types of ultramafic massifs and to certain

The most important Brazilian resources of lateritic nickel are in the Central Region, adding up to 195.1 �9 1 0 6 tons of ore with 1.1 to 2.1% Ni. In this region, as elsewhere in the country, the ultramafic massifs are generally in the higher parts of the landscape, forming elevations capped by a layer of silicified material (silcrete). The lowlands correspond to the country rock or to the more dissected parts of the ultramafic massifs. Melfi et al. (1979/1980) suggested that the lowlands belong to the Velhas Erosion Surface, formed in the Upper Tertiary, and that the silici- fled tops can be interpreted as relics of a more ancient Erosion Surface, Sul Americana, formed in the Lower Tertiary (King 1956; Braun 1971). The degree of incision and dismantling of the Sul Americana Surface and the consequent development of the Velhas Surface can be different depending on the lithology and on the dimensions of the massifs. Thus, the nickel-enriched profiles may be found either in the lowlands or in the highlands, under the silicified layer.

At Niquelrndia (Pecora 1944; Barbosa 1968; Costa 1970; Oliveira and Trescases 1982; Colin etal. 1985; Girardi et al. 1986), despite the enormous dimensions of the massif, the ultramassif zone is not well developed, forming a narrow belt about 40 x 2.5 km. In the southern zone of this belt, there are serpentinized dunites and peridotites, interlayered with pyroxenites, forming a high un- dulated plateau related to the Sul Americana Surface. The dunitic areas capped by silcrete correspond to the tops and the pyroxenite bands correspond to the valleys slightly carved in the plateau (Fig. 3). The northern zone of the ultramafic belt, where there are only serpentinized dunites, is almost entirely eroded down to the Velhas level (Fig. 3).

In the dunitic zones, the complete weathering profile, generally developed in the lowlands, varies in thickness from 8 to 20 m, and shows two main groups of horizons (Fig. 4). The first group, developed at the bottom of the profile, is composed of weathered ultramafic rock which conserves its original structure (saprolite); upwards weathering becomes more and more intensive: the weathered hard rock (saprolite 1, about 3 m thick) evolves to a friable material (saprolite 2 = coarse saprolite), and then to layers in which a brown to greenish-brown argillaceous matrix contains smaller and smaller rocky frag- ments (saprolite 3 = argillaceous saprolite). The thickness

140

,,%

9 0 0

8 O 0

1

NIQUE LJ~NDIA NORTH

~ Silcrete I - - ] A l t e r a t i o n p r o f i l e

~ Dunite ~ Pyroxenite ~ Gabbro

IKm

SANTA F~-

Fig. 3. NiquelAndia and Santa 1:6 geologic sections

profile

v v v v v v v

v v

size of the bulk M g 0 =/=

portic(es density

u

_o x-- 1.0 I - 2

0 . 9 0 - 1 , 2 0 15 - 2 0 o2

1 .20 - 1 . 3 5 21 - 2 4 ~eq

~- F - ~ ,.3s - ,.7o 25-29

"x-- 1 .70 - 2 . 5 0 3 0 - 3 3

f r e s h 2 . 6 - 3 . 0 4 0 - 4 5

rock t ,m /2 s~ola.~z ~

Fig. 4. Schematic weathering profile in lowlands area on dunite- peridotite zone of Niquel~ndia (Central Region). Saprolite 1 = altered rock; Saprolite 2 = coarse saprolite; Saprolite 3 = argillaceous saprolite; Laterite 1 = yellow laterite = ferruginous saprolite; Laterite 2 = red laterite = lateritic overburden

of coarse and argillaceous saprolite is respectively 2 to 4 m, and 2 to 6 m. In these saprolitic layers, bulk density progressively decreases from 2.5 to 3 at the bottom to 1.1 to 1.3 at the top. The MgO content, and, later, the SiO 2

content decrease as well; in contrast, the Fe, Cr, A1 and Ni contents increase with increasing weathering. From a mineralogical point of view, the saprolites are mainly made up of magnesian to ferro-magnesian silicates: serpentine, chlorites and smectites. Garnierites can occur in

veins, associated with quartz, inside the saprolitic layers, or just beneath, in the rock cracks. The saprolitic layers represent the silicated nickel ore (NiO between I and 5%).

The second group of horizons is essentially ferruginous (laterite). From bottom to top, the color varies from yellow (laterite 1) to red (laterite 2). In the laterite 1 the original structure is not entirely destroyed; for this reason it can be considered as a saprolitic ferruginous horizon; its thickness is about 2 m. The size of particles, first very, very small (1 gm), increases to the top, with formation of millimetric pisolitic concretions. In the lateritic layers, goethite is the predominant mineral, accompanied by hematite at the top of the profile (red laterite, 2 m thick). When the NiO content is higher than 1% the ferruginous saprolite constitutes the oxidized or lateritic nickel ore. The superficial red material is considered a barren lat- critic overburden due to its low content of Ni.

On the highlands, instead of the red lateritic overburden, the ferruginous saprolite is capped by an extremely silicified ferruginous material (silcrete). On the steep slopes, a thin layer of ferruginous saprolite lies directly over the slightly altered rock.

The profile developed over pyroxenites, up to 50 m thick, is essentially of smectitic composition, cut by garnieritic veins. In the upper levels smectite changes into kaolinite.

In the Niquelfindia massif, the ore is mainly of the silicated type, associated with the coarse saprolite and the argillaceous saprolite, even when of pyroxenitic origin. In this case, the nickel included in the smectites came from the dunites that are in higher topographic positions: the nickel content in the fresh pyroxenites is not sufficient to produce economic concentrations. A less important part of the ore is of the oxidized type, related to the ferruginous saprolite.

The Barro Alto massif (Melfi 1974; Stache 1974; Trescases and Oliveira 1981), like Niquelgmdia, has a narrow elongated ultramafic zone (30 x i km), formed by serpentinized peridotites. The relief of this zone is characterized by plains (Velhas Surface) at the extremities and by a hilly area in the middle portion. In the hilly area, one can distinguish remains of a plateau capped by silcrete, convex slopes and steep foothills. In any case, the anorthositic zone of the massif is situated in higher topographic positions.

The weathering profile is well developed in the plains (up to 18 m thick) and in the convex slopes (about 25 m thick). It is formed by altered rock and coarse saprolite cut by garnieritic veins, argillaceous saprolite, ferruginous saprolite and a lateritic overburden. On the hill tops and steep foothills the profile is interrupted at the level of the slightly altered rock.

The main mineralized level is the argillaceous saprolite because it is the thickest and the most enriched level (NiO between 2.2 and 2.7%). The coarse saprolite and the ferruginous saprolite can be considered ores as well. As for the topographic position, the deposit is situated on the convex slopes and on the plains.

The ultramafic massif of Santa FO (Barbour 1978; Oliveira and Trescases 1980) is an oval body of 38 km 2

141

surface, formed by a core of serpentinized unites (26 km2), surrounded by concentric rings of peridotites, pyroxenites and alkaline gabbros. The landscape (Fig. 3) is in an advanced stage of erosion, with an extensive levelled plain (Veihas Surface) with a few elevations capped by silcrete, The alteration profile is well developed in the lowlands and in a lower degree on the slopes. The complete profile consists of four zones, namely, altered rock, coarse saprolite, ferruginous saprolite and, at the top, lateritic overburden. Here the argillaceous saprolite is absent. The coarse saprolite is about 15 m thick and the ferruginous saprolite varies from 0 to 2 m. Finally, a red lateritic material, sometimes pisolitic, 1 to 4 m thick, covers the profile. On the tops, the slightly altered rock lies directly under the silcrete. Thus, at Santa Fr, the deposits is located essentially in the lowlands and the ore consists of coarse saprolit.e (NiO between 0.4 and 1.6%), ferruginous saprolite (NiO between 0.2 and 2.1%), when it occurs, and the lower part of the lateritic overburden (up to 1% NiO). The pyroxenites alter into a smectitic material, whose nickel content never exceeds 0.6%.

At the .]gua Branca massif (Justo 1973), the deposit consists of a level of silicated ore developed under a silcrete larger that caps an extensive plateau (35 km2). At Morro do Engenho (Chaban and Santos 1973), the massif corresponds to a tabular hill (5 km); the silicated type of ore is dominant.

It can be said that the nickeliferous lateritic deposits of the Central Region of Brazil are mainly of the silicated type and that they lie either on the higher parts of the landscape (plateaus, convex slopes, suspended valleys), where they have a more argillaceous composition, or on the lowlands, where the more intensive lateritic evolution has led to the formation of oxidized ore associated with the ferruginous saprolite.

Northern Region deposits

In the Serra dos Carajfis Region, in the State of Pardi, between the Araguaia and Xingu rivers, five deposits of lateritic nickel are mentioned in the geological literature: Vermelho, Quatipuru, Onqa, Puma and Jacarr, making up far more than 100.106 tons of ore.

Vermelho deposit (Bernardelli et al. 1983; Alves et al. 1986) is formed by two ultramafic bodies (serpentinized peridotites), VI (2.5 • 1.5 km) and V2 (1.5 x 0.5 kin), which are located in the central zone of a mafic-ultramafic massif. These bodies form hills with tabular tops emerging from a plain (Velhas Surface). Pyroxenitic lenses included in the serpentinite are common. In V1, for instance, a thick pyroxenitic lens crops out in a suspended valley between two serpentinic crests (Fig. 5).

On the tabular tops, the alteration profile, up to 50 m thick, is developed under an important layer of silicified material. It comprises an altered rock layer, a coarse saprolite horizon (serpentine and chlorite with NiO between 0.5 and 3%) of 20 m thickness, and a ferruginous saprolite horizon (goethite and quartz with NiO between 0.4 and 5%) from 0 to 30 m thick. All these layers can be nickel ore. At the bottom of the profile, garnieritic veins

s m: N

4 5 C

4 0 0 , 4 ~ 0

^ ^ ~ ^ ^ ^ ^ �9 .

3 5 0

~ Altered dunite ~ Coarse sapmlite ~ Ferruginous saprolite

[ ~ $11crete ~ Altered pyrolenite

Fig. 5. Geologic section of Verrnelho deposit (Northern Region)

have not been found. A lateritic overburden (hematite and goethite) improverished in nickel (less than 1% NiO) caps the profile. On the gentle slopes, on the northern side of V1, the alteration profile in not covered by silcrete (Fig. 5). On the steep slopes, the profile is absent and the slightly altered rock crops out. In the suspended valley, developed on pyroxenites, the profile is essentially smectitic, covered by a lateritic level (goethite and kaolinite). The NiO content never exceeds 0.5%.

From the 40.10 6 tons of ore, about one half is of the silicated type, the remaining being of the oxidized type. In the ferruginous silicified saprolite (silcrete), high contents of nickel occur only locally; this horizon is therefore a barren overburden. The ferruginous saprolite thickness is, in Vermelho, much bigger than in the other deposits of Brazil and its composition is much more oxidized. These features are probably controlled by the present climate with equatorial characteristics.

Quatipuru is an ultramafic body (45 x 1 kin) which forms a ridge in contrast with the surrounding rocks (Cordeiro and McCandless 1976). The ultramafic is a serpentinite derived from dunites and harzburgites containing Fe-Ni-Co sulfides. The alteration profile, 6 m thick, is formed by a coarse saprolite horizon (2 m), that underlies a ferruginous saprolite horizon and is capped by a lateritic concretionary soil. The nickel concentrations are in coarse and ferruginous saprolites. The upper levels of the profile are silicified (S~i 1980).

The lateritic nickel deposits of the Northern Region of Brazil are related to profiles that show a very silicified layer on their tops. Their characteristic feature is the importance of the oxidized ore, much greater than in other Brazilian deposits.

Northeastern Region deposits

In the Northeastern Region, there are two deposits of lateritic nickel: Serra das Marrecas, in the State of Bahia, and S~o Jo~o do Piaui, in the State of Piaui. From both, only the latter represents important resources (20.10 6

tons of ore with 1.6% Ni). The Sdo Jodo do Piaui deposit (Santos 1974; Trescases

et al. 1986) is developed over the ultramafic zone of a mafic-ultramafic massif. The mafic zone is levelled at the same topographic level as the country rock (Velhas Sur-

142

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440 -̂ ^ --̂ ~- 2~ ~ '~/,., ^

^ ~ ~ ~ ? ; ~ ' ? ^ ~ ~ ~- . . . . . . _/~ ... . . . . ,.%

~ Coorso soproliteArQi}loce~ soprolite ~5'~ii) : J : : J ^ .. . . ^ A '~'~' ^ "̂ ~:̂ ~ i ~ ~~!....::.::,..~'1~1~1~_~ " ~ ' ~ . ~ ~:~!~ ??'i~;: ! " ~ ' g : ' ~'~'1: ~:':':::': " ;~:'~!' ....... ' 57j!

^ ^ Fig. 6. Geologic section ofS~_o Jofiodo Piauideposit (Northeastern Region)

face); the ultramafic zone (7 kin2), formed by serpentinites, emerges from this surface as a hill with tabular top (silcrete) and extensive and gentle slopes (Fig. 6).

The alteration profile can be divided into two layers. At the bottom, there is a saprolitic horizon made up of serpentine in the lower part (altered rock and coarse saprolite with NiO between 0.4 and 2%) and smectite in the upper part (argillaceous saprolite with NiO between 1 and 1.6%). This horizon is not much silicified or inten- sively silicified and is about 20 m thick. At the top, there is a 10 m thick horizon formed by silicified serpentinite blocks mixed with ferruginous concretions (silcrete).

On the plateau, at the top of the hill, the saprolitic horizon is thicker and dominated by the argillaceous level. Toward the lowlands, the argillaceous level is less important and the saprolitic level, less developed as a whole, is mainly formed by the coarse saprolite facies. On the steep slopes, the profile is almost absent (Fig. 6). The degree of silicification of the saprolites is stronger on the top and on the upper parts of the slopes, and less intensive toward the plains.

At Silo Jo~.o do Piaui, the evolution is not of a lateritic type, except in some restricted zones on the plateau, where a ferruginous material forms the matrix that embodies the silicified blocks.

The ore consists of the coarse and argillaceous saprolite and presents the best grades when these materials are not very silicified. It can be found only in the lower parts of the slopes and on the plains; on the plateaus, the saprolitic levels always have nickel contents inferior to 0.6%. A characteristic feature of this deposit is the absence of garnierite at the bottom of the profile.

A semi-arid climate like that of the Northeastern Re- gion of Brazil favours erosion instead of chemical alteration. As a result, most of the ultramafic massifs in this region do not contain nickel deposits. In the State of Bahia, the small occurrence of lateritic nickel in Serra das Marrecas (Schobbenhaus 1976) is no more than an eroded relict of an accumulation, which could have been similar to that of S~o Joao do Piaui. In the State of Paraiba, at Catingueira, the alteration profile developed

on ultramafic rocks is formed only by a strongly silicified level with some garnierite, directly overlying the fresh rock (Farina 1969). At Andorinha, in the State of Bahia, the ultramafic massif is completely eroded, levelled to the Velhas Surface. The alteration profile, only some decime- ters thick, is formed by a silicified smectitic material.

Southeastern Region deposits

Despite the existence of several ultramafic massifs in the Brazilian Southeastern Region, there are only four nickeliferous lateritic deposits: Jacupiranga, in the State of SaD Paulo, and Liberdade, Ipanema and Morro do Niquel, in the State of Minas Gerais. Jacupiranga district, at 24~ latitude, is the southernmost known occurrence of lateritic nickel in Brazil. Other ultramafic massifs situated at more meridional latitudes, like Pien (State of Paranfi) and Pedras Pretas (State of Rio Grande do Sul) do not show lateritic alteration profiles.

In the Jacupiranga complex (Oliveira and Trescases 1985; Oliveira et al. 1988), the alkaline ultramafic massif contains an ultramafic zone formed by serpentinized dunites (20 km2). The morphology of this zone consists of a plateau slightly dissected by erosion. Only in the western part is the plateau more incised, resulting in a set of round hills separated by narrow valleys. The alteration profile (Fig. 7) is very thick, but the mineralized zone is restricted only to the coarse and argillaceous saprolitic horizons (NiO between 0.4 and 2%). These levels, composed by serpentine, smectite and chlorite, cut at the bottom by garnieritic veins, are capped by a thick layer of silicified ferruginous saprolite with low nickel contents (less than 1% NiO) and by a kaolinitic overburden, whose origin is at least partially allochtonous.

The deposits of Liberdade, Morro do Niquel and Ipanema are developed over small ultramafic "alpine" type massifs. In Morro do Niquel (Trescases and Oliveira 1978), the deposit consists of a small hill (0.3 kmZ), the fresh serpentinite cropping out at the bottom. The weathering profile (Fig. 7) is formed by altered rock (serpentine, chlorite and garnieritic veins with NiO between 0.4 and 3.9%) covered by a thick ferruginous silicified level (NiO between 0.4 to 0.5%). Discontinuous and thin ferruginous layers may occur between these two horizons. An overburden material derived from the silicified level caps the profile.

At Ipanema (Angeli et al. 1984), there are several small mineralized bodies, the most important of them being Santa Cruz. This is a small hill (0.5 km 2) formed by serpentinized peridotites. The alteration material is a more or less argillaceous saprolite enriched in nickel (NiO between 1.1 and 3.5%). This level underlies a silicified ferruginous horizon and a thick layer of ferralitic soil (Fig. 7).

At Liberdade (Esson and Santos 1978 a) the ultramafic body, a hill of 1.5 km 2 of surface, is formed by serpentinites. The alteration profiles (Fig. 7) is not very thick, and has at its bottom altered rock and a coarse saprolite horizon (NiO between 1.1 and 2.9%) with abundant chlorite and garnieritic veins; at the top, there is a silicified

Depth ( rn ) Jacugiranga Liberdode O.

~ Loteritic overburden

20.

40.

60.

I m Morro do Nique~

�9 ).

'1 Silcrete ~ Argillaceous saprolite

~ Altered rock ~ Coorse saprolite

Fig. 7. Alteration profiles in the Southeastern Region

ferruginous saprolite horizon and about 0.5 m of ferralitic soil.

In all the four massifs described, the saprolitic level constitutes the ore and is more or less argillaceous, depending on the content of smectite. At Jacupiranga, this content is maximum and at Morro do Niquel and Liber- dade there are no smectites at all.

The deposits of the Southeastern Region are small with moderate amounts of nickel. The resources add up to 9.2.10 6 tons with 1.2 to 1.7% Ni. Their topographic position is always in the highlands and they are covered by a thick barren overburden, which makes their ex- ploitation difficult.

Genesis of the lateritic profile

Mineralogical evolution

The nickel lateritic profiles described above are derived from ultramafic rocks formed essentially by olivine, serpentine and a certain amount of pyroxenes. At the begin- ning of the alteration process (altered rock), olivine is gradually hydrolised with a nearly total removal of magnesia and a partial removal of silica. In this oxidizing and alkaline environment, iron and nickel are insoluble and precipitate with silica as amorphous or badly crystallized products in the cavities left by the dissolution of the olivine. If the drainage conditions in the profile are ineffi- cient, Ni-Mg-smectites may be formed. The pyroxenes alter into Mg or Fe-smectites (Bosio et al. 1975; Brindley and Souza 1975b; Souza et al. 1978; Oliveira and Melfi 1979; Colin et al. 1985). The serpentine seems stable but, in reality, part of its octahedral magnesium is replaced by nickel. The amorphous material derived from olivine can also fill up the void spaces between the fibres of the serpentine, giving them a yellow color and a higher nickel content. In the cracks of the altered rock, neoformed minerals precipitate from solutions carrying Si, Mg and Ni. These minerals are the garnierites: 7A and 10A Mg-Ni-silicates (Brindley 1978). In Brazil, garnier-

143

ites have been described in detail in Morro do Niquel, Barro Alto and Liberdade (Esson and Santos 1978b; Souza et al. 1978). Irregular accumulations of chlorite and vermiculite of probable hypogeneous origin, slightly altered and enriched in nickel, have been mentioned in Sante Fr, Jacupiranga, Morro do Niquel and Barro Alto (Brindley and Souza 1975a and b; Souza et al. 1978; Trescases and Oliveira 1981).

Once the olivine is dissolved, the alteration solutions become more diluted and the porosity of the material increases. As a result, the Mg-smectites previously formed change into Fe-smectites with loss of nickel. Now the alteration of the serpentine begins, resulting in a Mg-Ni-smectite (Barro Alto, Niquelandia and mainly S~o Jo~o do Piaui) or, as happens more frequently, it slowly dissolves, leaving behind a Ni-goethite residue (Morro do Niquel and Vermelho). The garnierites become unstable, go into solution, leaving in their place, in the cracks, a mixture of Mn-Ni-Co-Fe oxides and hydroxides (asbolanes). The smectites, either derived from olivine or from serpentine, change into goethite as well. All these transformations begin in the fissures where an argillaceous material embodies the residual blocks of slightly altered rock (coarse saprolite). As the argillaceous material becomes predominant, the blocks become smaller, forming the argillaceous saprolite. These horizons, where the silicates are the dominant minerals, constitute the silicated ore with 1 to 4% of Ni.

When the serpentine disappears, it leaves behind a goethitic material maintaining to a certain degree the texture of the original rock (ferruginous saprolite). Nickel is more soluble in this environment and part of it migrates downwards to the underlying saprolite horizons, making them richer. In general, a small amount of nickel, about 1%, remains in the ferruginous saprolite which constitutes the oxidized or lateritic ore.

At the top of alteration profile, the original textures are no longer preserved; goethite turns into hematite with loss of nickel. This horizon changes in colour to red tones and becomes concretionary. In the lowlands or in the depressed zones of the highlands (Niquel~ndia) the evolution may reach the stage of an indurated ferruginous crust.

Chemical evolution

The weathering alteration of ultramafic rocks is characterized by preferential leaching of Mg and with a certain delay of Si. As a result, the more insoluble elements - Fe, A1 and Cr - are residually concentrated. Ni, Co and Mn, with an intermediate solubility, behave in a more complex way. Ni, released from hypogeneous minerals, is retained for a certain period of time by several phases (garnierites, serpentine, smectites, asbolanes, and goethite) in the different levels of the alteration profile, but its preferential horizon of accumulation is the coarse or the argillaceous saprolite, by vertical leaching from the upper levels. The stabilization zone for Mn and Co, less soluble than Ni, is the ferruginous saprolite.

This evolution is similar to that in other nickel deposits of the world (Schellmann 1983; Golightly 1981). A

144

striking characteristic of Brazilian deposits it that the desilicification is never complete due to the presence of silicified material in the upper levels of the landscape. The isovolumetric balances indicate that the high amounts of nickel in the coarse saprolite are absolute accumulations for which the upper lateritic levels cannot be entirely responsible. This excess of nickel can only be explained by considering the general evolution of the deposits since the Tertiary, as described below.

General evolution of the deposits since the Tertiary (MEN" et al. 1979, I980)

The history of the weathering blankets in Brazil began in the Lower Tertiary with the Sul Americano cycle. At that time, two different alteration mechanisms prevailed:

intensive silicification at the bottom of the alteration profiles, under more arid climatic conditions;

lateritisation of the profiles with nickel concentration, under more humid climatic conditions that followed the semi-arid episode.

In the Upper Tertiary, a new erosion cycle began (Vel- has) which dismantled the Sul Americana Surface and elaborated at lower altitudes a new surface. The alteration profiles developed on the ancient surface have been eroded down to the silicified zone. This zone has pro- tected the ultramafic massifs against the general levelling. Part, of these ancient profiles are found today mixed with the upper levels of the profiles developed on the Velhas Surface after mechanical transportation. This hypothesis can explain why these levels are higher in nickel content (> 1%), as compared with the overburden material of lateritic profiles, for instance, in New Caledonia (Trescases 1975) and Cuba (Vletter 1955). This material, derived from a previous erosion cycle, has also been the source for the absolute accumulations found in the saprolite levels.

The alteration continues today both in the highlands, under the silicified layer (Morro do Niquel, Vermelho, etc.), or where it does not exist (suspended valleys in Niquelfindia and convex slopes in Barro Alto), and in the lowlands (Santa F6, etc.). This alteration of lateritic tendency occurs together with a lateral migration of nickel, from the highlands, where it has been concentrated since the Tertiary, towards the lowlands. This migration is more complete when the Sul Americana Surface is more eroded, as happens in Santa F6, and is almost absent in massifs were the Sul Americana Surface is well preserved (Morro do Niquel, for instance). At S~o Jo~o do Piaui, the current evolution is not of lateritic tendency, the accumulation of nickel being a result of the preconcentration, due to the Tertiary lateritisation episode.

Discussion

The investigation of the supergenic nickel deposits of Brazil, developed from several types of ultramafic rocks in different climatic conditions, allows the evaluation of

the role of petrographic and morphoclimatic factors in the genesis of the nickeliferous laterites. The main characteristics of the Brazilian nickeliferous laterites can also be defined.

Role of the parent rock

The alteration process and the genesis of the ore are controlled by the petrographic nature of the rock and by its degree of fracturation.

The nature of the hypogeneous minerals is important for two reasons: their different original nickel content and their resistance to the alteration processes. Their nickel content is about 3000 to 4500 ppm for the olivines, 500 to 1000 ppm for the orthopyroxenes, 150 to 600 ppm for the clinopyroxenes and 2500 and 3500 ppm for the serpentines. It seems, therefore, evident that the pyroxenes cannot be sources for nickeliferous accumulations. However, at Niquelfindia, the smectitic horizon developed over pyroxenites behaved as a trap for the nickel that came by lateral migration from dunites and peridotites. On the other hand, the delay in the alteration of the pyroxenes compared with the olivines and of the serpentine compared with the two other silicates is responsible for the succession of different saprolitic horizons at the bottom of the profiles. These horizons are the silicated nickel ore; their nickel content derives from the in situ alteration of the olivine and from the transformations - silicates ~ goethite ~ hematite - that occurred in the upper parts of the profiles. If the original mineralogy had been simpler, as in serpentinites, the several steps of the evolution would have been superimposed, reducing the thickness of the silicated ore.

The alteration occurs when the solutions percolate the minerals through fractures and fissures. Therefore, weathering proceeds more rapidly in fractured zones and consequently the release of nickel is more complete. The circulation of the solutions being easier, the nickel will migrate downwards and will enrich the inherited minerals not yet altered (serpentines) or the neoformed secondary minerals (smectites and garnierites). Such absolute accumulations of nickel have been observed in all Brazilian deposits.

Role of the climate

A tropical climate, hot and humid, is required for the release of nickel from its primary sites and to its relative concentration by leaching of silica and magnesia. When the climate is characterized by contrasting seasons, the alteration profile is formed by silicated and oxidized saprolites, as in the Central Region. Under more aggres- sive climatic conditions, the relative importance of the silicated horizons becomes smaller and the oxidized horizon is more completely developed (Northern Region). In contrast under more arid climates, the alteration can stop at the smectitic stage (S~.o Jo~o do Piaui), and, in this case, the leaching of silica is not enough to concentrate the nickel. Such conditions must have prevailed for the

145

format ion of the silicified horizons in the Lower Tertiary in Brazil. In more recent times the silicified horizons had an indirect role in the development of the deposits, acting as a protective layer against erosion.

Role of the topographic conditions

The deposits are associated with thick alteration blankets formed during long periods with the consumption of an impor tant amount of rock; otherwise the nickel concentrated in the laterites would not be sufficient. These conditions require a low erosion rate, and therefore low gra- dient surfaces. However in massifs o f large dimensions, several erosion cycles may have produced several surfaces and the stock of nickel accumulated in the highest surfaces played the role of a preconcentrat ion that fed the profiles in the lowest topographics level, even if they were not lateritic, as in S~o Jo~o do Piaui.

General characterization of the Brazilian nickeliferous laterites 1

The lateritic nickeliferous deposits in Brazil may be correlated with two Tertiary erosion cycles. Accordingly, the deposits can be found either in highlands or in lowlands. The degree of dismantling of the higher and more ancient surface controls the preferential situation of the present nickeliferous accumulations.

The alteration profiles in the Brazilian nickeliferous deposits are broadly similar to those described elsewhere in the world with regard to the succession of horizons, but there are some important differences. The silicated horizons are thicker than the oxidized ones as a consequence of the less intensive degree of lateritisation compared, for instance, with deposits in New Caledonia (Trescases 1975), Cuba (Vletter 1955) and the Philippines (Ogura 1977). As a result, in Brazil the silicated ore prevails over the oxidized ore. In the silicated ore, nickel can be found with silica in the garnierites, in the smectites that replace pyroxenes and olivines, and associated with serpentines. Although the garnierites are the richest nickel minerals, their amount is often very restricted and they are not therefore important as ore minerals. The main ore mineral is the enriched serpentine, as happens commonly in other deposits of the world. In many Brazilian deposits, the smectites also play an important role as nickel- containing minerals (Niquel~ndia).

Finally, the thick silicification level at the tops, witness of a biphasic evolution, seems to be a characteristic feature of Brazilian deposits. Similar features, however at- tenuated, have been described in South Africa (Waal 1971) and in Australia (Zeissink 1969; Elias et al. 1981), at about the same latitudes.

1 Detailed data on resources and chemical composition of the nickel ore are available to interested readers

Acknowledgements. This work is a result of a research programme of international cooperation between the Instituto de Geocirncias da Universidade de S~o Paulo and the Institut Frangais de Recherche Scientifique pour le Drveloppement en Cooprration. The authors would like to express their acknowledgements to the organisations which have given financial support as fellowships (CNPq-Brazil; MRT-France) and funds for research (CNPq, FAPESP and FINEP in Brazil; OSTROM and CNRS in France). They are indebted to Rio Doce Geologia e Minera~o S/A, Com- panha Niquel Tocantins, Morro do Niquel S/A Minera~5o, Indfis- tria e Comrrcio, and Mineradora Montita Ltda for fieldwork support.

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