NICKELIFEROUS LATERITES OF "

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 pro- ducts 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 con- centration 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 However, three areas show a higher density of ultra- mafic rocks (Fig.1) (Berbert, 1977). According to their conditions of formation nickel A central band, elongated N-S: in Goias and deposits are of two types (Lombard, 1956 and Boldt, continuing to the state of Para; the largest Brazilian 1967): sulfide deposits of hypogene origin, and massifs and the best nickel ore reserves are here lateritic deposits of supergene origin. Nickeliferous (Fig.2) (Angeiras, 1968; Godoy, 1968, Lindenmayer laterites constitute a thick weathering cover of ultra- and Lindenmayer, 1971; Vasconceilos, 1973). mafic rocks in tropical areas. A second area in Northeast Brazil, mainly in the Some sulfides of copper and nickel have been state of Bahia. prospected in Brazil (for example in the Sao Joao And a third area in the state of Minas Gerais in massif, at Americano do Brazil-Goias). But all the Sou theas tern Brazil. Brazilian nickeliferous reserves are in lateritic The Amazon Basin is very poorly known, which weathering products of ultramafic rocks. explains the apparent absence of ultramafic rocks in Ultramafic massifs occur in the majority of the the northwestern part of the country, covered by a Brazilian states, from the equator to 32"s latitude. great equatorial forest.

'This work is a Dart of a scientific uroiect.- between Geo sciences Institute of the University of Sao Paulo (Brazil) and i the ORSTOM (France). I **Mission ORSTOM (Over Seas Scientific and Technical Office), Instituto de Geocien;ias, Univerjidade de Sao Paulo, Brazil. l *'*Instituto de Gcociencias, Universidade de Sao Paulo, C.P. 20S99-Sao Paulo, Brazil. I-

+a i” - , *c -2 ;i L li2 LATËRITSATION PROCESSSS NICKELXFEROUS LATERITES OF BRAm 173

layered mafic-ultramlìc rocks with a probable Morro do Nique1 (MG), Sao Joao do Piaui, Catin- Archean age (perhaps as old as 4 billion years- gueira (Paraiba). Mato Grosso ... The “Alpine” mas- Cordani et al., 1973), and great size. There are three sifs are in general strongly or wholly serpentinised. massifs: Barro Alto, Niquelandia and Canabrava (4) The 1st type is much younger, with a Cre- (and perhaps a fourth to the north in Natividade- taceous age, and can be related tothe volcanism which Berbert, 1977). each being about one hundred km followed the opening of the Atlantic Ocean. The long, and forty km wide, although the ultramafic majority of occurrences surround the Parana sedi- zone is much smaller. Four main layers can be dis- mentary Basin, in Southern Brazil. These massifs tinguished in the massifs (Araujo et al., 1972; are circular, with sizes up to 10 km in diameter. Figueiredo et al., 1972; Souza, 1973), with a west- Sometimes they have a nucleus of carbonatite, ward dip; although the majority has a dunitic nucleus, with (i) A noritic basal zone (with some Cu and Ni thin shells of peridotites, pyroxenites, gabbros and disseminated sulfides) nephelinic syenites (Barbour, 1976; Pena and Flgue- (¡i) An ultramafic zone, 2000 m thick (dunite, iredo, 1973; Chaban and Santos, 1973). The main peridotite, pyroxenite, always more or less ultramafic alkaline massifs occur in Goias and serpentinised, and some pod-shaped bodies Mato Grosso (Ipora and Porto Murtinho provinces). of chromite-White et al., 1971). (5) Some massifs have been recently discovered, (iii) A central zone with norite (Niquelandia) or and their classification is difficult: Americano do anorthosite (Barro Alto). Brazil and Sanclerlandia (Go), or Serra de Carajas (iv) A top zone with gabbro. (PA). The old massifs show both “Alpine” features (mainly in the bottom part) (Thayer, 1972), and (B) Economic considerations “stratiform” features (at the top) (Fleisher and The nickel appears accumulated as ore in the Routhier, 1970). weathering profiles of: (2) The second type of ultramafic rocks occurs (i) The great Archean massifs of Goias (Barro in the state of Bahia, associated with the Transama- Alto and Niquelandia) zonian Cycle (about 2000 million years). These mas- (i) The small Alpine-type massifs of varied ages sifs can be stratiform, as at Campo , with in many states (Goias, Minas Gerais, Piaui) serpentinised peridotites, chromitites, pyroxenites (iii) The ultramafic-alkaline massifs of the Cre- and gabbros (Hedlung. 1974; GonCalves et al., 1972). taceous (Goias, Sao Paulo). But the majority of these massifs is attributed to a Chrysotile asbestos deposits occur in the Archean geotectonic model of a “greenstone belt” with a massifs (Canabrava) and in some Alpine-type N-S orientation. These small massifs contain horn- massifs (, Sao Joao do Piaui). blendites, serpentinised peridotites, actinolite-tremo- The stratiform massifs contain great reserves of lite-talc-schists. Two Transamazonian massifs of chromite (Campo Formoso), and the “greenstone pyroxenites and gabbroic gneisses are considered belt” type shows indications of Cu,Ni and Zn sulfide:. similar to the ultramafic rocks of Bahia: Goiaira- In the ultramafic-alkaline massifs, deposits of Trindade in Goias (Lindenmayer and Oliveira, 1970) vermiculite, phosphate, niobium, rare earths and and Pien in Parana (Girard and Ulbrich, 1978). uranium occur. They could have resulted from the fractional crystal- Table I gives the size, reserves, and average Fig. 2 lisation of a tholeiitic magma. nickel grade of the main nickel deposits. It empha- (3) The third type, which constitutes the majority sises the importance of the state of Goias. In spite of the ultramafic massifs of Brazil, is the “Alpine” of the presence of ultramafic rocks elsewhere, therr The published works on weathering of Brazilian ENVIRONMENT OF THE DEPOSITS type. Small bodies, from a few metres to several kilo- are few nickel deposits in other states; as will i ultramafic rocks and nickel deposit formation are shown later, recent morpeo-climatic evoluf!,u Geological environment metres in size, are disposed in “serpentine belts”. few, with the exception of some mining reports They are of different ages (Almeida, 1978): explains the distribution of the Brazilian nickel. (Andrade and Botelho, 1974; Berbert, 1977; Ferran, The classifications of ultramafic rocks proposed by Thayer (1960), Wyllie (1963, Naldrett and 3000 Million Years in the Minas Gerais belt (N-S 1974; Lages et al., 1975, 1976). The geochemical direction) Recent morpho-climatic conditions study of this process in different climatic zones of Gasparini (1971), and Naldrett (1973) are not The economic accumulation of nickel in a always appropriate in the Brazilian context. We use 2000 M.Y (Transamazonian Cycle) in Rio Grande Brazil began three years ago at the Geosciences In- weathering profile (nickeliferous laterite) is a result here the classification of Berbert (1970) and (1977). do Sul (Alto Vacacai, Bahia (Jacobina), and Sergipe. stitute ofthe University of Sao Paulo, by a French- of the convergence of several morpho-climatic. with some modifications. 1000-2000 M.Y. in the double, NNW-SSE and Brazilian team. The present work is a preliminary ”E-SSW, serpentine belt of Goias (Almeida, 1967). lithological. and structural factors: ultramafic bcd- inventory of the characteristic features of the so- (A) Distribution offhe niussFs (Figs. I and 2) 600 M.Y. (Brazilian Cqcle) in Para. Some isola- rock, tropical climate with contrasting seasons. called *‘nickeliferous laterites” of Brazil. (I) The oldest massifs occur in Goias. They are ted Alpine massifs occur outside of these belts: and levelled relief (with karstic drainage) with -i

174 LATERITISATION PROCESSES NICKELIFEROUS LATERITES OF BRAZIL 175

Table I. Nickel deposifs of Brazil. ' S Stare Nanie of the massif Aproximate size of Reserves-IO't are Content Bibliographic hl the ultrabasic massif (measured and estimated) %Ni reference Niauelandia 100km' (39 X 2.6 km) 38000 1.40 Barro Alto 45km' (24X 1.9 km) 73000 Canabrava ? 9500 Santa Fe 38 km' 61000 Goih Serra Agua Branca .35 km' 60000 Morro do Engenho . I2 km' 18000 Morro dos Macacos 30 km' loo00 Rio dos Bois Montes Claros ? 15000 A-MORRO DO NíQUEL . 13- SÄ0 Job DO PIAUí (Salobinha) Sao Luis Montes Belos 12 km' ? Morro do Niquel 0.5 km* 2300 1.7 (1) E Minas Gerais Liberdade 0.2 km' 7m 1.7 (1) : Ipanema 4 k" 7300 1.24-1.59 (1) i f' . Paulo Jacupiranga 6 km' 3200 1.47 (1) . I Sao I Piaui Sao Joao d3 Piaui 6 km' Zoo00 1.57 (1) : Bahia Serra &s Marrecas ? 900 1.5 (4) l1 Sao Felix do Xingu ? 13000 2.1 (2) i ParS. Quatipuru 90 km' (45X 2 km) 13000 1.3 (5) I Carajas ? 1 ? - C-NIQUEL6NDIA SUD (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 1900 mm). The vegetation is of the tropical forest type. the Amazon Basin of Northern Brazil, the (Trescases, 1975; Lelong et al., 1976). All these In AAAAA favourable 'conditions are not simultaneously satis- climate is equatorial, hot, and very humid (average fied for each Brazilian nickeliferous deposit. And temperature above 26"C, and precipitations above 550 each deposit shows characteristic features, accor- 1800-2000 mm). The vegetation is of the great equatorial forest. ding to its particular recent evolution. D- NIQUELANDIA- NORD (iv) The climate of Southern Brazil is t'ransitional (A) Chtate to a temperate climate. The following main bio-climatic zones can be Nickel deposits occur in the first four climatic distinguished in Brazil (Romaris, 1974). zones and mainly in the second one (tropical cli- (i) In the Northeast, the climate is hot and semi- mate with contrasting seasons). arid. The vegetation is a dry brushland, with thorny shrubs and cacti ("caatinga"). The average temp- (B) Relief erature is 24" to 26"C, and precipitation is helow Most of the ultramafic massifs of Brazil comprise hills, sometimes tabular, or mountains. These 800 mm, with 8 to 9 months of very dry season. E-SANTA Fe (i¡) In Central Brazil, the climate is hot and elevations look down on broad levelled plains. The humid, with contrasting seasons. The vegetation ultrabasic reliefs are of three types: ÉCHELLEHORIZONTALE CGMML%E O I km consists of savannah with some trees ("cerrado" (i) Tabular hills and mountains. This is the most or "campo limpo"). The average temperature is frequent topographic form. Hill tops and plateaus between 22" and 25"C, and the rainfall isbetween1200 are capped by a hard layer of silcrete. The slightly and 1800 mm, with only 3 to 4 months of dry season. weathered rock crops out on steep slopes. Figure 3 (iii) On the Atlantic Coast of Southeastern Brazil, shows some cross-sections of Alpine ultramafic the tropical climate is semi-hot and semi-humid massifs, with the silcrete horizon and the weathered (average temperature 20T, and rainfall 1500 to zone; Morro do Niquel (MG)and Sao Joao (Piaui). , Fig 3 176 LATERITISATION PROCESSES NICKELIFEROUS LATERITES OF BRAZIL 177 A similar landscape can be presented by the ultra- DIFFERENT TYPES WEATHERING OF B 111 mafic-alkaline massifs (Serra Agua Branca, G0,- PROFILES ... m Lessa Sobrinho et al., 1971; Justo, 1973). The largest massifs show a contrast of reliefs, with high- Weathering profiles of the highlands lands, foothills, and lowlands. It is possible, how- (Plateaus, tops of hills) ’ ever, to identify the highlands as plateaus, as in the These highlands are relicts of the South Ameri- case of the Southern part of Niquelandia (Fig. 3), can Surface. Their weathering profiles were the where the dunitic zone is a relict of an old plateau first to begin their evolution. Several types of with silcrete (the pyroxenite bands correspond to weathering can be distinguished according to the valleys, slightly carved into the old peneplain) climate. NIOUELANDIA(Dunite) (Pecora and Barbosa, 1944; Pecora, 1944; Barbosa, 1968; Costa, 1970). (a) Hot aiid humid tropical climate, with contrast- (i¡) Plains and hills. The largest massifs show ing seasons (Go, .+.íg, Central Brazil) characteristic features of a dissected, old plateau; This climate is considered as the most propiti- foothills and plains are dominant, with ferruginous ous for the genesis of a thick lateritic profile with detrital material, sometimes with ironcrust near the nickel accumulation (Trescases, 1975). Indeed, it creeks; around these low zones are some hills with is in this climate zone that the biggest Brazilian deposit occurs. Most of the bed-rocks are dunites steep and silcrete on their tops (Santa Fe and the BARRO ALTO Northern part of Niquelandia, Fig. 3). or serpentinites, sometimes harzburgites. Figure 4 (iii) Finally, some massifs have been wholly level- shows some weathering profiles at Morro do Niquel led by erosion, mainly in the semi-arid Northeast. (Griffon and Richer, 1976; Langer, 1969, Santi- vanez, 1965; Trescases and Oliveira, 1978), Barro MORRO DO N~OUEL Alto and Niquelandia and gives for comparison one (q Interpretation of the evolution of the relief profile on pyroxenite in Niquelandia. The profile The suggested course of evolution of the relief appears really different only on Pyroxenites. On si -Roche silicifi6e (silcrete) rocks rich in olivine and/or serpentine, the profiles is as follows: NIOUELANOIA si -Bloc de silcrete (i) During the late Cretaceous, the uplift of always appear as one of the two extremetypes: (P yroxénite) South America began. A very long period of ero- Morro do Niquel and Barro Alto. Two principal -Bloc de cuiro$se Ferrugineuse layers can be distinguished: sion established an extensive peneplain: the South LR -‘Latérite rausë American erosion surface (King, 1956). The Weathered zone, with preservation of the struct- L;a ure of the rock. weathering products of this cycle are lateritic (Braun, SF -Saprolite Fine (“latérite jaune“) (rache très altdrbe) 1971), but show silica accumulation (silcrete) over Silicated and/or reworked top zone. Weathered zone. The weathering of the hard, Saprolite grossière, facihs “argileux” ultramafic rocks (Santos, 1974; Trescases and Oli- LW,,/ ,,, SG- SF (rache altdrde) dark, dense fresh rock begins with the modification veira, 1978). SG,R-SG -Saprolite grassiire, facibs ”rocheux” of the colour during the “slightly weathered rock +/,* (rache altéde) R- Dunite, Serpentinite (iii) In the late Tertiary, a new erosive cycle began; 4). stage” (R-SG in Fig. The thickness of this BR-SG -Roche trèspeualtérée 4 Pyrbxinite the Velhas Cycle (King, 1956). A new, lower sur- layer is betweeen 2 and 5 m. Next, cohesion and face was formed, after incision and dismantling of bulk density decrease, with densities between 2 and Fig. 4 the South Amercian Surface. In areas of ultrama- 1.5: this is the coarse saprolite stage (SG). When fic rocks, however, silcrete impeded the levelling of cohesion disappears, the rock is transformed into the relief. Thus plateaus and tabular hills with sil- a brown argillaceous mass, with conservation of the The top zoize is almost always composed of dissected by erosion, and on the slopes. the surface crete are relicts of the South American Surface. rock structure, but with very low density (under blocks of silicified ultramafic rock this is the silcrete horizon is a powdery red material (“red laterite”) (iii) In the Quaternary, erosion has continued, 1.5). This is the fine saprolite stage (SF), never layer. The blocks are a quartz and chalcedony box with fragments of silcrete, and sometimes ferrugi- restricting the hills even more. The landscapes of very thick (1 to 2 m,) and sometimes absent. work, with cavities fikd by red ferruginous mate- nous fine gravels: this “red laterite” is a residual Niquelandia and Santa Fe are in a more advanced In the Brazilian profiles, the coarse saprolite is the rial; or they can be massive and hard, like a chert. soil formed from the silicrete. stage of erosion than the others. During the Qua- thickest horizon of the weathered zone. This stage Each block results from the epigenetic silicification Mineralogical evolution of rhe profiles. In the ternary, the weathering attached at the same time is intermediatc between the slightly weathered rock of the rock. But the silcrete itself is an accumula- weathering zone, the mineralogical evolution is a both and the fine saprolite. But depending on the case, tion of silcified, and later reworked, blocks. transformation of the magnesian silicate into -the relicts of the South America Surface it maintains some features of the former (rocky The silcrete layer is 2-4 m thick, but locally its goethite, with ephemeral precipitation of a little bit (Highlands), where weathering profiles are deve- type, as at Morro do Niquel), or it quickly loses thickness exceeds 20 m. This layer always covers of chalcedony, quartz and garnierites.* (Trescases loped under the silcrete: all cohesion (argillaceous type, as at Barro Alto). the tabular summits. But, where the plateaus are and Oliveira, 1978; Melfi, 1974). -and the new base level (Velhas Surface), with Some silicification can occur in the coarse saprolite deep weathering profiles. This weathering is con- as vertical veins (Barro Alto, Fig. 4) or fractures *The garnierite is a mixture of nickeliferous silicates (hydrated-talc, serpentine, niontmorillonitc, chlorite, Sepial¡tÇ, trolled by recent climatic cònditions. fillings. chalcedony. (Faust, 1976; Brindley and Phon Thi Hang. 1973). -

NICKELIFEROUS LATERITES OF .BRAZIL 178 LATERlTlSATlON PROCESSES . 179 Calcite, brucite and olivine disappear first (in the forms) can precipitate in the coarse saprolite (Brind- slightly weathered rock). ley and Souza, 1975; Esson and Carlos, 1978). Residual iron and nickel hydroxides precipitate The silcrete consists mainly of quartz and chalce; in the cleavages of the serpentinous network (in dony. In the reworked red laterite, goethite, and the coarse saprolite); next, this network is replaced hematite appear abundantly. When the rock includes by a nickeliferous goethite network (fine saprolite). pyroxenes, the minerals are transformed into However, prior to this, quartz, chalcedony and smectites (nontronite), and the profile is of the nickeliferous silicates (mainly hydrated talc-like argillaceous type.

A 1 A’ Table 2. Average chernicol composition of the two t).pes of weotherincprofiles in the Highlonds. Ear de versant A-“Rocky“ type profile: Marro do Niquel (ofter TreAcoses and Oliveira 1978). Piedmont 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 Plaine LR Collurian de blocs di silerit. - si 2.8 85.7 2.4 6.8 0.95 0.19 0.44 0.034 0.003 SF 10.1 50.3 22.7 10.4 0.99 0.24 3.37 0.078 0.002 embollis doni un maliriou brm-roup. R/SG-SG 132 41.5 30.6 9.8 0.78 0.1s 2.40 0.033 0.003 R-SG 13.3 45.2 35.2 4.15 0.62 0.053 1.08 0.016 0.003 R 16.6 35.7 38.5 5.7 0.75 0.052 0.41 0.015 0.002

B-“Argillaceorrs” rype profile: Borro Alto (after Stache. 1974). 3.

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

LR 9.2 17.0 1.5 51.7 14.2 3.19 1.18 0.91 0.092 0.037 SF 10.8 20.3 12.1 41.5 7.9 2.68 2.58 0.67 0.059 0.041 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. O SO 100 200 250m Sl? Fine saprolite (-;yellow laterite). , -.- - , --- SG-SF: Greenish-brown coarse slprolite (ar$llaceous type). A Anna.. A n na* R“ SG-SG: Mainly friable weathered rock (rocky type). Fig. 5 R-SG: Hard, slightly weathered rock. R: Fresh rock. tions of Central Brazil, does not allow the develop- For mining, the silcrete is poor in nickel and ment of thick ferruginous horizon of fine saprolite considered barren. The “red laterite” layer con- Cliearical evolution of the profiles. Table 2 gives The isovolumetric balance (gain and Iöss) of the typical of the lateritic profiles of New Caledonia, tains medium grade of nickel (1% Ni) where it is the mean chemical composition of each layer of the weathering confirms that magnesium and part of for example (Trescases, 1975). not derived exclusively from the silcrete; it is an two types of highland profiles: “rocky” type (Morro the silica are progressively leached in the weathered The presence of silcrete at the top of the profile “oxidized” ore. The weathered zone represents the do Niquel) and “argillaceous” type (Barro Alto). zone (part of the silica precipitates ‘as amorphous is in contradiction with the lateritic tendency. This main part of the ore. The more complete the weath- The magoesium content decreases with weathering. compounds). Nickel shows absolute gains. The layer shows an absolute gain of silica without parent ering, the higher is the grade of nickek, the best This progressive leaching is correlated with a rela- other elements are strictly constant. In the silcrete, source. This silcrete could not be formed from the layer is at the top of the saprolite zone. This ore is tive concentration of the residual elements: iron, the residual elements are again constant, but here, underlying saprolite. It represents the silicified a silicated ore. aluminium, chromium, cobalt. The silica content there is a loss of nickel and an absolute gain of basal part of theold profiles of the South American increases a little in the Morro do Niquel saprolite, silica. Surface. Some roots of this silicified layer remain, (b) Semi-orid tropical clitnate (Bahia, Piarri, Nor& but the ratio SiO,/Fe,O, decreases with increased Conclusion on the evolutiori of fite higlIand profiles now in the new weathcrcd zone, beneath the easternIn spite ßrazil) of the abundance of ultramafic rocks weathering. In the Barro Alto profile (as in the in a tropical climate with contrasting seasons. The silcrete (as at Barro Alto, Fig. 5). The upper part majority of the profiles), the content of silica of the evolution of the highland weathering profiles in a of the old South American profiles was removed in the state of Bahia, there is only a small number sspro!ite decreases. Nickcl is strocgly conccn- tropical climate with contrasting seasons compriscs by erosion, thus supplying the colluvial formation of nickel deposits in the Brazilian semi-arid region. trated in the uppcr part of the weathered zone. the progressive, but sometimes incomplete, destruc- of red laterites on the slopes (and in the plains This climate favours mechanical processes of ero- The silcrcte is very rich in silica and poor in nickel. tion of thc silicates in favour of a goethitic residue: and lowlands of the Velhas Cycle). sion, rather than chemical weathering. A lot of the In the Barro Alto profile, the “red laterite” is con- this is an evolution with a lateritic tendency. The lateritic tendency of recent weathcring is ultramafic massifs of this zone are strongly eroded, stituted mainly by thc residual elements (Fe, AI, Cr, However, some residual free silica remains; and hampered by the introduction of silica from the as at Andorinha (BA), where the relief is wholly &o), and nickel content is medium (1%). this evolution, in the recent morpho-climatic condi- silcrete; and the time of evolution has been too levelled. Here, the weathering profile is not thick, short for true laterite formatioq. and shows only some smectitisation and veinlets of '< -- "2. *a y 180 LATERITISATION PROCESSES . NICKELIFEROUS LATERITES OF BRAZIL 18 1 silica. In Parans, Santa Catarina and Rio Grande do in the lowlands are very similar to those in the high- The small occurrences of Serra das Marrecas, in Sul (South of 25" S Latitude) the recent weathering lands: the weathering is in equilibrium with recent Central Bahia and Catingueira (Paraiba) are dis- is not lateritic and the profiles are not thick. Vermi- climatic conditions. The lateritic evolution is more mantled relicts of an old deposit. culite and smectites are the dominant minerals and complete in the lowlands. The reserves of nickel ore In the Sao Joao do Piaui deposit, however, a nickel concentration does not occur. Even during are more important in the lowland profiles, where strong silcretecapprotected the plateau relief (Santos, the Tertiary, the weathering probably was not lateri- three layers constitute ores: 1974). A new profile is developed beneath the silc- tic in the South. --coarse saprolite, silicated ore, with 1 to 2% Ni rete, but here, the evolution does not show a lateri- -red laterite, oxidised ore, with 1% Ni tic tendency: magnesia is again leached away, silica Profiles of the plains and lowlands -and fine saprolite, transititional between silicat- and iron are strictly residual. The saprolitic weath- ed and oxidized type, with content of Ni (1 to ered zone shows the massive transformation of the (a) Tropical climate with contrasting seasons (Goi& 4% Ni), but with restricted thickness. ultramafic rock into smectite, with local high nickel The weathering profiles observed in the low parts content (2% Ni). In some places the silica dissolved of the massifs of SantaFe and Niquelsncia (North- (b) Semi-arid tropical climate (Piaut) in the upper part of the silcrete precipitates in the ern part) are very similar. The weathered zone show A foothill fringe is devgloped around the central saprolite. This silicified saprolite is less rich in nic- the same sequence of horizons present in the high- plateau of the Sä0 JOBO do Piaui massif. These foot- ke1(0.5%-1% Ni). Thus here, there is a slow and lands: slightly weathered rock, coarse saprolite, and hills are connected with the "Velhos" plain. Figure gradual descent of the silcrete, which is destroyed at fine-grained ferruginous saprolite which is thicker 5 shows the schematic disposition of the horizons its top and regenerated at its bottom. here. for all the topographic sequence slope-foothills- The superficial reworked layer is very differenl; plain. (c) Permarrently httntid climatic cortditiorts, the profiles of the lowlands never show the silcrete The colluvial surface horizon contains a lot of (Ivorfh and Soutlt Brad) cover characteristic of the highlands. The upper part crumbled blocks of silcrete within a powdery brown- The nickel deposits in the Amazonian forest were of the lowland profiles is always made up of about red material. Exactly on the topographic knick point, discovered only a short time ago and they are not 3m (or more) of powdery red laterite, with ferrugin- these blocks have a ferruginous cortex, and are well known. Cordeiro and McCandless (1976) write ous fine gravels. cemented by iron hydroxides (iron crust appearance). ------?------,-----Ancien 6vem 1o~ogro~hi,~us about silicification lateritisation and peneplanation The mineralogical evolution of the profile is the The weathered zone shows a saprolite horizon (argil- in the massif of Quatipuru (Pars).There seems to be same in the lowlands as that previously described laceous type in the plain; rocky type on the slope strong analogies with the deposits of Goiãs. in highlands. The mean chemical composition of and at the bottom of the profiles). On the lower part In southern Brazil, there is only one small depo- each layer for the massif of Santa Fe is given in of the slope, this sarprolite issilicified. On the slope, sit at Jacupiranga (State of SBo-Paulo, 25"s Latitude). Table 3. The similarity between these compositions only slightly weathered rock crops out. The evolution of the weathered zone has a lateritic and the corresponding horizons of the highland pro- The mineralogical evohtion is the same as that tendency at the top without total leaching of silica. files (Table 2) isvery high, with the evident exception described for the plateau: transformation of the ser- The top .layer is a very thick horizon of powdery of the top layer of red ferruSinous laterite. A part pentine into smectite, mainly in the foothills and "red laterite", with some blocks of silcrete. The nic- of the accumulated nickel of the lodand profiles is on the plain; incipient formation of goethite at the Fig. 6. kel content is very low (SI. 5% Ni) (Ferran, 1974). of residual origin; but the rest comes from the surface; predominance of quartz and chalcedony in all The localisation 3f the massif of Jacupiranga in a highlands either through mechanical processes with the colluvial cover and in the coarse saprolite in the Brazil presents good conditions for having nickeli- tectonic graben could explain a lower degree of ero- the colluvial red laterite or by chemical processes, lower part of the slope. ferous laterites. The estimated reserves of the nickel sion: the thick cover of red laterite could come from through lateral migration. The chemical evolution is marked by the preser- ore are 4.106 tons Ni, which .places Brazil 7th-in the evolution of the old weathering products of the In conclusion, in the humid tropici1 region of vation of iron and silica (and even by the introduc- in world. South American surface. Brazil with contrasting seasons, the weathered zones tion of silica coming from topographically higher The common features of the Brazilian deposits, silcrete). The magnesium is quickly leached as soon and also the differences between the deposits located Table 3. Average coniposition of each horizon in the inassif of Sailto Fe. as the serpentine disappears. The nickel migrates in diffirent bio-climaticzones, are in accord with the . HtO* SO, MgO Fc,O,' A1200, Cr,O, Ni0 Co0 CuÒ laterally from the plateau to the plain through the following interpretation of the successive stages of LR 5.2 10.1 1.6 70.8 3.5 6.2 1.19 0.142 0.011 weathered zone. Without this contribution the lea- their evolution (Fig. 6). ching of the magnesium alone could not produce a SF 8.9 32.9 9.9 39.0 2.4 4.1 1.40 0.063 0.010 (i) The. redistribution of elements began in the SG 12.2 40.8 28.3 15.5 0.7 0.64 1.0 0.028 0.005 significant relative concentration of the residual nic- Early Tertiary, with the South American levelling (0.3-1.9) kel of the rock. The colluvial cover is barren (0.2 cycle. In already levelled relief, a semi-arid climatic R-SG 12.8 41.0 33.4 10.4 0.5 0.44 0.33** 0.07.4 0.004 to 0.3% Ni). The coarse saprolite is nickel ore: 0.8 period induced the formation of a silicified layer at 7.4 36.3 41.1 13.2 0.2 0.35 0.23 0.020 R 0.003 - to 1%Ni (withlccally 3% Ni) in the rocky type and the bottom of the weathering profiles on ultramafic *Total Fe as Fe.0, (Fe0 for the fresh rock only). 1.2 to 1.5% Ni in the argillaceous type. rocks. '*Sample witho& the garnicritic veinlets. (i¡) Later, a more humid climate promoted the LR: Red laterite. GENETIC INTERPRETATION lateritisation of these paleo-profiles, and a prelimin- SF Fine saprolite=yellow lateriie. SG Weathered mck=Coarse saprolite. ary (residual) concentration of nickel. However, to R-SG: Hard, slighily weathered rock. Being a tropical country with numerous ultrama- the south of 25"s latitude, silicification and lateritisa- R: Fresh rock. fic massifs submitted to several erosional cycles, tion seem not to have occurred. LATERITISATION PROCESSES NICKELIFEROUS LATERITES OF BRAZIL 183

(iii) In the Late Tertiary, with the breaking up of Ynigo, 1961), Venezuela (lurko Vic, 1963). In these S. I. C. E. G., Ouro Preto, BA. N06, 78-79. Tome XV. no 1, 45402. the South American Surface, the lateritised weather- deposits, the weathering profiles are very deep and BARBOUR. A. P. 1976. Geologia do macico ultramätico de GIRARDI.V. A. V. 1975. Geologia e petrologia do complexo Santa Fé. Goiis. Teese USP-Inst. de Geociencias. p. bis¡ Co-ultrabgsico de Pien, PR, Rev. Bras. Gem.. Sa0 ing products were removed, and the silicrete exposed. mainly goethitic; nickel is associated with goethite; the , I. 138 With this protection against erosion, the ultramafic multgr. Paulo, 6 (2), 109-124. nickel content is never very high, but it is a little higher BERBERT,C. O. 1970. Geologia geral dos complexos’ basicos GIRARD. V.A.V. and ULBRICH,H.H. G.J. 1978. Origin massifs appeared little by little as plateaus above the at the bottom (Schellman, 1971). Oxidised-silicated e ultrabasicos. de Goiäs, XXIV Congres. Bras. Geol, and evolution of the Pia mafic-ultramafic-complex. Sou- levelled “Velhas” Surface. deposits occur in New Caledonia (Trescases, 1975), Brasilia Resumos. thern Brazil, Bu//. Geol. Soc. of America (in Press). . (iv) During the ~‘Velhas”cycle, weathering was and in western United States (Hotz, 1964). The sili- BERBERT,C. O. 1977. Rochas bäsico-ultrabäsicas no Brasil GODOY,A. C. 1968. Mapa das ocorrencias minerals doEsta- do de Goias, com localizacao dos principais macicos active beneath the silcrete which covers the plateaus cated layer of coarse saprolite is present here, but the e suas potencialidades eeonömicas, IlQ-Scmana de est. (South American relicts), and in the “Velhas” low- aeol. do Parü, CEGEP-U. F.. Paranä, 1-51. hasicos e/ou nltrabãsicos, XXII Cangr. Bras. Geol., Belo thickest horizon is goethitic. The nickel content is BOLLI, J. R. Jr. 1967. The ivin;iing of nickel* Longmans Horizonte. Anais. lands. In accordance with recent climatic conditions high at the bottom of the coarse saprolite (2-5% Ni), Canada Ltd., Toronto. 487 p. GONÇALVES,J. C. V., MOREIRA,J. F., C. HEDLUND,D. C. this weathering shows a lateritic tendency in Central but the goethitic fine saprolite is also nickel ore BONIFAS. M. 1959. Contribution ä I’étude gCochimique de’ and ALII, 1972. Projecto Cromo DNPM/CPRM, Relat. Brazil (tropical climate with contrasting seasons). In (1-273. The best deposits are located in the high- alteration latéritique (Thése), Mem. Serv. Carre Grol, Final Inedito 181 plrasil. the semi-arid North-East the weathering process is Als. Lon, Starsbourg no 17, 159 p. GntfroN, J. C. and RICHTERH. 1976. Geologia, mineraçao lands. The nickel ore reserves are principally of the e tratamento do minério de niqueldo Morro--- do Niquel. smectitisation. BRAUN.O. P. G. 1971. Contribuicao ä ceomorfologia do --- oxidised type. These deposits are located in the Brasil central, Revista Brasileiriia de Geografia, no. 3, MG. VI Simp Bras. de Mineracao, JX+.jY8. (v) With the progress of erosion, the South Southern Hemisphere (New Caledonia), or to the 3-39. HEOLUNG,D. C., Como MOREIRA,J. F. DE, and ALII. 1974. American relicts disappear. The nickel is progressi- north of the intertropical zone (USA). However, BRINDLEY,G. W. and PHANTHI HANG, 1973. Thenature of Stratiform chromiteat Campo, Formoso Bahia. Brazil. vely transferred from the highlands towards the some small deposits of the Southern Hemisphere garnierites. Clays and Clay m;neral, Vol. 21, 27-57. U. S. Ge01 Snrvey. Jour. Rerearclr, Denver, Co. 2 (5), lowlands, where it enriches the new-formed weather- show similarities with the Brazilian type. In South BRINDLEY,G. W. and SOUZA, V. DE. 1975. Nickel-contain- 551-562. ing profiles. ing montmorillonites and chlorites from Brazil with HOTZ, P. E. 1961. Nickeliferous laterites in Southwestern Africa (De Waal, 1971) and in Australia silicification remarks on schuchudtite, Mineral Mag, Vol. 40. 141- Oregon and Northnestern California, Econ. Geol., Vol. in the highlands and lateritisation in the lowlands 152. 59, no 3. 355-396. CONCLUSION have been mentioned. In these profiles the nickel CHADAN,N. and SANTOS,J. F. DOS, 1973. Intrusive do Morro JURKOVIC,I. 1963. Some geochemical aspects about the gen- content increases toward the upper part. do Engenho (Goigs): consideracees sobre geologice pes- esis of the nickel deposit Loma de Hierro (Venezuela), quisa, Congresso Bras. de Geol., Aracaju, Bol. Geolos., Vjesnlk-Zaghreb-YU-17, The main features of the Brazilian deposits are: This differentiated supergene evolution of the ul- XXVII 103-112. (i) genesis correlated with two levelling cycles in no 1.57-58. JCSTO? L. J. E. C. 1973. Maciço ultramäfico de Agua Branca, tramafic rocks of the two hemispheres could result CORDANI.U. J., DELHAL,J. and LmENT, D. 1973. Oregen- Goiäs: um corpo tabular, XXVII congr. 5”. Gd., the Early and Late Tertiary. from world climatic variations correlative with the &ses superposees dans le Pdcambrien du Brésil sud Aracaju, Bol. no 1. 56-57. (ii) Restricted lateritisation, with silicated pro- movement of the poles and the continental drift Oriental (Etats de Rio de Janeiro et de Minas Gerais), KING,L. C. 1956, A geomorfolosia do Brasil Oriential. files, without great thickness. during the Tertiary and Quaternary. Rev. Bras. Geoc.. 3 (I), 1-22. Rev. Bras. Geografia u” 2. Ano XVIU. 147-265. (iii) Important silicification on the highlands. CORDEIRO.A. A. and MCCANOLESS,O. 1976. Macico Ultra- LAGES,S. M., ANDRADE,M. R. DE, and OLIVA,L. A. 1975. (iv) Lateritisation more advanced in the lowlands. mäfico de Quatipuru, XXIX CongrCs. G~~I.,ouro “Niquel”. Encontro Nacional sobre minérios de metais ACKNOWLEDGEMENTS Preto-Belo Horizonte-re”, 238. alo ferrosor-Goignia-Go; ReI. DNPM. para XXIX (Y) Thickness and nickel grade of the mineralised COSTA,J. L. G. 1970. Nickel deposits of the Sao Jogo do Congr. Bras. Ceo/. Belo Horizonte. 48 P. layer often modest: either because the climate is no This work has been supported by grants from the Tocantins massif, Notes on economic geology and on LAGES,S. M., ANDRADE,M. R. DEand OLIVA.L. A. 1976. longer aggressive (North East), or because the lateri- FAPESP (Fundacäo de Amparo ä Pesquisa do Estade exploration by C. N. T.. XXIX congres. ceol., Economia mineral do niquel. Rel. DNPM para XXIX Corrgr. Bras. Geol., Belo Horizonte, tisation acted for a short time (Central Brazil). de Sao Paulo) and from the CNPq (Conselho Nac- Brasilia-resumos, 142-43. 32p. (vi) Preferential accumulation of nickel in the Iow- DE VLETIER,D. R. 1955. How Cubsn ore was formed; a LANGER,E. 1969. Die Nickellxertitten des Morro do Niquel ional de Desenvolvimento Cientifico e Tecnololgico, lesson in lateritic genesis, Eng. and Min.JoUrn., New in Minas Gerais, Brasilien, Ihre Bemusterung und Bew- lands. Brazil). York, Vol. 156, no JO, 54-87. wrtung. Clausthaler Hefte zur Laserstattenkande und (vii) Preferential accumulation of nickel at the We thank the mining companiesMorro do Niquel DE WAAL, S. A. South African nickeliferous serpentinites, Geochimie des Miner. Rohstoffe Heft 8-Gebruder top of the weathered zone. CODEMIN, MONTITA, Vale do Rio Doceand Minerals Sci. Eng. Pretori., ZA. Vol. 3, no 2, 32-45. Barntrdegcr-Berlin-~tut~gart,64 p. (viii) Huge predominance of the silicate type of BAMINCO, mineracao e siderurgia, for their help in ESSON,J. and CARLOS,L. 1978. The occurrence, mineralogy LESSASOBRINHO, M., ANDRADE,R. S. DE and BERBERT, C.D. nickel ore. and chemistry of some garderites from Brazd, colloque 1971. Projeclo Jasara. Geologia dos Vuadriculos de the collection of field data. sur la mineralogie, goechimie, geologie des minereaux Britania, Santa FC. Amguapaz e (Goiis). DNPM Thus, Brazilian nickeliferous deposits are different This publication represents a condensed paper ex- et minerais nickelifereres lateritiques-IAGGIGCP- CPRM-Goiinia,86 P. from the majority of the other lateritic deposits in tracted from a more complete work to be published BRGM-Odeans Bull. BRGM (2) II. 3, 263-274. LINDENMAYER,D. H. and OLIVEIRA,C. C. DE, 1970. Reaot- the world. Oxidised deposits occur in the Northern in Cahier ORSTOM SOrie Geologie. We thank FARINA,M. 1969. Oltrabasitos niqueliferoui de Catingucira ório de viagem äs regiaes de Inhumäs e Serra de Santa Paraiba, Consideracóes sobre a geoquimica e a geologia Rita. Goris. DNPM Distrito, Reh. Inedito s/ident./ Hemisphere in Cuba (De Vletter, 1955), Guinea ORSTOM for the authorisation to reproduce the 6“ (Bonifas, 1959; Percival, 1965), Philippines (Santos- econümlca. Sup. des. Nordeste-Depart. de recursos Goiania. Brasil. figures of the main work. naturais Seu. Geol. Econ., no 7. 53p. LINDENLIAYER,D. H. and LINDENMAYER,2. G. 1971. Intru- FAUST,G. T. 1966. The hydrous nickel-magnesium soes ultarbasicas-alcalinas e suas mineralizaçees ä niquei- The garnierite group, Am. Mineral, Vol. 51, 279-288. XXV Congre. nras. Geul., Si0 Paulo. Bol. esp. no 1 REFERENCES FERRAN,A. DE. 1974. Panorama do niquel no Brasil, Ceol. (resumo), P. 35. e. Mefa//!lrgia,IV Simposio de Mineraczo) 10-parte), no LOMBARD, J., 1956. Sur la gCochimie et les gisements du 35, 103-124. nickel. Chronique des mines d’Outre-Mer-Paris no 244, ALMEIDA,F.F.M. DE. 1978. Tectonic map of South central de Goiäs, Anais Acad. Bras. Ci& vol. 40, Rio America-1/5.OM).WO and explanatory note, M. M. E.- FIGUEIREDO.A. N. DE, SOUZA.E. P. DE and MELLO.J. C. 2OP. . de Janeiro, 129-136. MkLfI, A. G., Cancteristicas geoquimicas e miner- D. P. M., Brasilia, p. R. DE, 1972. projeto Goianésia-Barro Alto-Relatörio 1974. N. 23 ARAUJO,V. A. DE, MELLOW.J. C. R. DE and OGUINO.K. nlögicas dos estádios iniciais da alterab superficial das ANDRADE,M. R. DE and BOTELHO,L. C. 1974. Perfil anali- 1972, Projeto NiquelSndia. Rapport. ConvCnio D. N. FL~ISCHER,Final ConvfnioR. and DNPM/CPRM,ROUTHIER.p. Goianla,1970, Quelques 129 p. grands rochas ultrzbasicas de Barro Alto (GO), Bol. Inst. tico do Niquel. D. N. P. M., Bol. no 33, Rio de laneiro. P. M./C. P. R. M., Goiãnia. ^^ 2240. thémes de la géslogie du Brésil. Miscellanées geologi- Geoci%nciasUSP, V-5. 117-128. YU p. BARBOSA,A.L.M. 1965. Ambiente geologic das Jazidas ques et mëtallogeniqucs sur le planalto, sc. de la Tetre- NALDWTT, A. J. and GASPANNI, E. L. 1971. Archean nickel ANGEIRAS,A. G. 1968. A faixa de serpentinitos da regiao niquelireras de Niquelandia, GO-VI Semana de estudo ia4 LATERITISATION PROCESSES

sulfide deposits in Canada-Their classification, geologi- Mindano Island (Philippines), XXI Inter. Geol. Congr.. cal retting, and genesis with some suggestions as IO ex- New Delhi, Sect. 14 (laterite). 126-141. ploration, Geol. Soc. Ausfralia.Spec. Publ. 3. 201-226. SCHELLMAN,W. 1971: Uber Beziehungen lateritische Eisen LATERITIC FORMATIONS ÌN USSR NALDREIT, A. I. 1973. Nickel sulphide deposils-Their nickel-aluminium-und Manganerze zu ihren Ausgan- classification and genesis, with special emphasis on gsgesteinen, -Mineral Deposifa Bzrlin, Vol. 6, no 4. 275- deposits of vulcanic association, CM[ Transactions, 291. Bull. 16, 183-201. SOUZA,A.. DE.1973. Geologiae geocronologia do complexo PECORA,W. T. 1944. Nickel silicate and associated nickel- Barro Alto, Goiãs-Tese Dout. FFCLRio Claro (SP). D.G. SAPOJNIKOV foballmanganese oxide deposits near Sao José do Toca- THAYER,T.P. 1960. Some critical differences between alpine- Institut of Geol of Ore Deposit ntins, Goias, Brazil, U. S. Geol. Surv. Bull., 935 E, type and stratiform peridotite-gabbio complexes, In: In- Staromoiietny Pet. 35 247-305. ternational Geological Congress, 21, Copenhagen, 1960, Moscow 109017 PECORA,W. T. andBmeosA, A. L. M. 1944. Jazidas de ni- Rept., pt, 13, p. 247-259. USSR quel e cobalto de Sä0 José do Tocantins, Estado de THAYER,T. P. 1972. Some observations and problem con- GoiacDNPM, Bol. no 64. cerning the peridotites and peridotite-gabbro complexes PENA, G. S. andFromcnmo, A. J. DE. 1973. Geologia da of the Goiäs belt, XXYI Congr. Bfasil. Gcol., Beltm, Quardricula de Iporä (Goiäs). XXYII Cortgr. Bras. Ceo/., Vol. 1, 37. Aracajü. resmos, 106-108. TR~SCASES.J. J. 1975. L' évolution geochimique supergbe PERCIVAL,F. G. 1965. The lateritic iron deposits of Conarky, des roches ultrabasiques en zone tropicale, Formation Trans. Inst. Min. and Metall., London, GB, Vol. des gisements nickéliféres de Nouvelle-CalCdonie, MCm- ABSTRACT 74, part 8, 429462. oire ORSTOM, no 78, Paris, 259 p. ROMARIS,D. A. 1974. Aspectos da vegelaçäo do Brasil, Inst. TRESCASES,J. J. and OLIVEIRA,S. M. B. DE, 1978. AlteraÇäo Brasileiro de Geografia e Eslatistica, Rio de Janeiro, des serpentinitos do Morro do Niquel (MG), XXX Con- 60 P. gr. Bras. Ceol., Recife, Vol. 4, 1655-1670. The lateritic formations of the USSR are subdivi d into three main varieties: lateritic bauxites, developed in the SANTVANEZ,A. O. 1965. As Serpentinitas niquiliferas do VASCONCELLOS,I. B. ET AL. 1973. Ocorrencias minerais das crust of weathering of rocks with different composition; iron ores, formed during jaspilite weathering; ochres, which were Morro do Niquel em Pratapolis. Eng. Mineraçä0 e folhas Goias e Goiania-DNPM 6" Distr. Goiania. formed in the crust of weathering on serpentinites. Metalurgia, Vol. XLII, no. 248, 61-64. WHITE,R. W., MOTTA.J. andA~Au10,V. A, DE. 1971. Ph- The lateritic formations are associated with the upper part of crust of weathering profile with zonal structure. The SANTOS,J. F. DES. 1974. Fatores de controle na concentraco tiniferous Chromitite in the Tocantins complex, Nique- profile Of crust of weathering is subdivided into five zones, distinguished by a degree of source rock reworking (from de nique1 lateritic0 condicionado pela evolucäo geolügica landis, Goias. Brazil. U. S. Geol. Surv. Prof. paper, below upwards): 1-disintegrated; 2- kerolithised; 3-montmorillonitised; 4-nontronitised serpentinites; and 5-clayey- a geomorfolögica do complexo bãsico-ultrabgsico de Sä0 750D, D26D33. ochreous rocks and ochres. Similar zonation is observed in the profiles of lateritic crusts of weathering, formed on the Joao do Piaui (PI), XXYIII Congr. Bra;. Ceo/., Porto WYLLIE,P. J. 1967. Ultramafic and ultrabasic rocks-Petro- rocks with different composition. Alegre. 25-32. graphy and petrology. In: Ultramafic and Related Rocks, The lateritic formations of the USSR are distributed over the territory of the Russian platform, the Siberian platform, SANTOS-YNIGO,L. 1964. Distrbution of iron, alumina and John Wiley & Sons, Inc.; New York, 1-7. the Urals, Kazakhstan and some otherregions. Within the areas of crust of weathering distribution. they are associated silica in the Pujada Laterite of Mati, Davao Province, 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 ture of the alteration layer (1.1. Guinzbourg et al., ...... _- such that on Its territory thelateritic formations do 1946; A.P. Nikitina et al., 1968). In particular, on the not occur. We are aware only of the fossil lateritic weathering layer of the serpentinites, we can distin- formations associated with earlier weathering epoch. guish few zones characterised by their mineralogic They are formed in the weathering zone due to the composition. The following are these zones from accumulation of iron and aluminium oxides with below upwards. some other elements following the leaching of silicon, (1) Zone of serpentinites, slightly weathered and alkaline earth metals and of other soluble compon- disintegrated; (2) zone of serpentinites kerolithised ents in the course of the weathering of parent rocks and lixiviated; (3) zone of montmorillonitised of silicious or silicic compositions. serpentinites; (4) Zone of nontronitised serpentinites; Among the lateritic formations of USSR we may (5) zone of clayey-ochreous rocks and ochres. mention the following: iron ores of residual origin The weathered bed of serpentinites is studied in (lateritic) and lateritic bauxites. The former, in turn, detail at Urals range where its age is determined to be are subdivided into: (1) iron ores rich in hematite and upper Mesozoic era. Normally, the thicknesi of martite which were formed due to the jaspilite wea- the weathering bed, which has a large lateral extent, thering, and (2) iron ores of hydrogoethite composi- varias from 10 to 40 m. In a few areas of ultramafic tion-goethite forming the upper crust of the weather- rocks from the Southern Urals range (those of ing mantle on serpentinites. Kenpisai. Khalilovshi and others), mainly consisting The study of weathering layers, and their metalli- of apoperidotic serpenthites, a clear zonal structure ferous deposits, petrography, mineralogy and is noticed. geochemistry (IGEM), carried out at the Geologic (1) The lowest zone consists of disintegrated Institute of the Academy of Sciences in USSR, has rocks invariably passing downwards into serpentini- established existence of some zonation in the struc- tes. According to I.V. Vitovskaia (1976) the disin-

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

, Proceedings of the International Seininar on Lateritisation Processes (TRIVANDRUM, INDIA 11-14 DECEMBER, 1979)

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

-s*