Geological evolution of the Neoproterozoic Corumbágraben system (Brazil). Depositional context of the stratified Fe and Mn ores of the Jacadigo Group

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Journal of South American Earth Sciences, Vol. 11, No. 6, pp. 587±597, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0895-9811/98/$ - see front matter S0895-9811(98)00036-4

Geological evolution of the Neoproterozoic Corumba graben system (Brazil). Depositional context of the strati®ed Fe and Mn ores of the Jacadigo Group 1

R. TROMPETTE, 2C.J.S. DE ALVARENGA and 2D. WALDE

1

CEREGE, Universite Aix-Marseille III, CNRS Fu 017, EuropoÃle meÂditerraneÂen de l'Arbois, B.P. 80, 13762 Les Milles Cedex, France 2 Instituto de GeocieÃncias, Universidade de Brasõ lia, Campus UniversitaÂrio, 70910-900 Brasilia D.F., Brazil (Received August 1997; accepted September 1998)

Abstract Ð The Corumba region is located near the junction of two young (545±480 Ma) Brasiliano provinces: the Chiquitos-Tucavaca aulacogen which cross cuts the Amazon craton-Rio Apa block, and the Paraguay fold belt. Formation of the aulacogen and the Paraguay basin, which is interpreted as a foreland basin, was roughly synchronous with the 590 Ma metamorphism in the neighbouring old Brasiliano Brasilia fold belt. Deformation and metamorphism in the Paraguay belt are bracketed between 545 and 500 Ma, whereas folding in the aulacogen occurred between 500 and 480 Ma ago. The NE±SW-trending, Corumba graben system is located on the eastern part of the WNW±ESE-trending, ChiquitosTucavaca aulacogen. The extensional structures of the Corumba graben system are attributed to ¯exure of the Amazon craton along the western border of the Paraguay basin. Stratigraphic successions in the Corumba graben system resemble those of the adjacent cover rocks of the Amazon craton: a lower detrital unit (Jacadigo Group and Boqui, Puga, and Cadieus formations), partly of glacial origin, and an upper carbonate-rich unit, (Corumba Group and PororoÂ, Cerradinho, Bocaina, and Araras formations) with local preservation of an Ediacara-like fauna. The whole sequence is probably Vendian. In the cover sequence around CuiabaÂ, the carbonate unit is capped by siliciclastic sediments. In comparison to the cratonic cover rocks, the Jacadigo Group, which is ®lling the Corumba graben system, di€ers on three points: it is thicker, contains chemical manganese and iron sedimentary intercalations and was a€ected by a diagenetic to epimetamorphic event during which temperatures reached 250±2808C. These features are in good agreement with sedimentation of the Jacadigo Group in an extensional environment where Fe and Mn, of probable hydrothermal origin, would be generated by leaching of hypothetical, hidden intrusive ma®c plutons associated with graben formation. # 1998 Elsevier Science Ltd. All rights reserved Resumen Ð A regiaÄo de Corumba esta localizada proÂximo aÁ junc° aÄo de duas provõ ncias brasilianas jovens (545±480 Ma): o aulacoÂgeno de Chiquitos-Tucavaca, que corta o craÂton Amazonas±bloco Rio Apa, e a faixa dobrada Paraguai. A formac° aÄo do aulacoÂgeno Chiquitos-Tucavaca e da bacia Paraguai, que foi interpretada como uma bacia de tipo foreland, e cronocorrelata do metamor®smo da faixa Brasõ lia dobrada no Brasiliano antigo (590 Ma). A deformac° aÄo e o metamor®smo na faixa Paraguai ocorreram entre 545 e 500 Ma atraÂs, enquanto que os dobramentos no aulacoÂgeno foram considerados mais jovens, com idades entre 500 e 480 Ma. O graben de CorumbaÂ, com direc° aÄo geral NE±SW e largura de 10±20 quiloÃmetros, esta localizado no extremo leste do aulacoÂgeno Chiquitos-Tucavaca. Este uÂltimo apresenta aproximadamente 500 quiloÃmetros de extensaÄo na direc° aÄo WNW±ESE. Estruturas extensionais ao redor de Corumba saÄo atribuõ das aÁ ¯exura do craÂton do Amazonas ao longo da borda oeste da bacia Paraguai. A sucessaÄo estratigra®ca que ocorre no graben de Corumba e nas rochas adjacentes que recobrem o craÂton do Amazonas e muito semelhante: uma unidade siliciclõ stica inferior (Grupo Jacadigo e formac° oÄes Boqui, Puga e Cadieus), parcialmente de origem glacial, e uma unidade carbonaÂtica superior (Grupo Corumba e formac° oÄes PororoÂ, Cerradinho, Bocaina e Araras) com uma localizada fauna de tipo Ediacara. Toda a sequÈeÃncia e provavelmente de idade vendiana. No aulacoÂgeno Chiquitos-Tucavaca e na porc° aÄo norte da faixa Paraguai (regiaÄo de CuiabaÂ) a unidade carbonaÂtica encontra-se recoberta por faÂcies siliciclaÂsticas. Com relac° aÄo aÁs coberturas cratoÃnicas, o preenchimento do graben de Corumba se destaca em treÃs pontos: e mais espesso; conteÂm intercalac° oÄes de sedimentos quõ micos de ferro e manganeÃs; foi afetado por eventos diageneÂticos com temperaturas que chegam a alcanc° ar 250±2808C. Estas caracterõ sticas reforc° am a hipoÂtese de a sedimentac° aÄo do Grupo Jacadigo ter ocorrido em um ambiente extensional, onde o Fe e o Mn saÄo provavelmente de origem hidrotermal, gerados pela lixiviac° aÄo de hipoteÂticos corpos ma®cos profundos associados com a abertura do graben. # 1998 Elsevier Science Ltd. All rights reserved

INTRODUCTION

tal or tilted cratonic cover rocks lie over the basement of the Amazon craton-Rio Apa block. These strata include the Jacadigo Group, which caps inselbergs, up to 1000 m high, and the carbonate Corumba Group

Around Corumba in western Brazil and south to Puerto Suarez in Bolivia, Neoproterozoic subhorizon587

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which crops out in the surrounding lowlands of the swampy Pantanal peneplain. The Jacadigo Group is made of coarse siliciclastic rocks, which are dominant in its lower part, and contains iron-formation and strati®ed Mn ore bodies in its upper part. The Jacadigo Group, and especially its Fe and Mn ore deposits, was studied during the second World War (Almeida, 1945; Dorr II, 1945). However a few problems concerning the geology of the Corumba region are still under debate. They are: Ðthe age of the cover sequence, which has been considered either to be correlative with the 1000±600 Ma Neoproterozoic Bambu|¨ Group of the neighbouring SaÄo Francisco craton (Schobbenhaus et al., 1981), or Vendian (600±545 Ma); Ðthe geodynamic signi®cance of these successions which have been long considered to be remains of a widespread cratonic cover characterized, around CorumbaÂ, by very peculiar facies (Fe jaspilites and Mn ores) or, more recently, as ®lling of a graben system developed in the southern part of the Amazon craton; Ðthe origin of the huge sedimentary Fe and Mn ore bodies (43 billion tons of iron with 50±58% of Fe and 250 million tons of manganese with 25±50% of Mn according to Haralyi and Walde (1986). Fe and Mn sources are thought to be either the adjacent basement of the Amazon craton-Rio Apa block, a volcanic episode recently discovered in Bolivia and correlative with the lowest part (?) of the Jacadigo Group, or hydrothermal circulation associated with the graben formation. The purpose of our contribution is, using available stratigraphical and structural data along with our own structural observations, to show that around CorumbaÂ, the Jacadigo Group was deposited in a set of NE±SW-trending grabens. Based on this reconstruction, we then suggest a hydrothermal origin for Fe and Mn ores. STRATIGRAPHICAL FRAMEWORK The main geological features of the Corumba area were established during the Brasiliano/Pan-African (about 600±500 Ma) orogeny. Since that time, the South American platform has been devoid of deformation except for faulting and dike intrusion associated with the opening of the Atlantic ocean to the east, and minor folding and fault reactivation associated with the building of the Andean belt to the west. In Western Gondwana, two types of Neoproterozoic basins have been distinguished (Trompette 1994). The oldest and most important opened at about 1000±900 Ma ago and was tectonized and metamorphosed by the main Brasiliano/Pan-African orogeny at about 600 Ma. The youngest were formed at around 600 Ma or later. Some of them are molassic

intramontane graben or foreland basins. Others are probably of impactogen-type (Sengor et al., 1978) i.e. formed as result of the strain generated within the foreland of a collisional orogen. All were slightly deformed and metamorphosed by the young Brasiliano/Pan-African orogenic phase, and locally intruded by granite at about 520 Ma, near the EarlyMiddle Cambrian limit, according to the stratigraphic scale of Bowring et al. (1993) and Grotzinger et al. (1995). So the geological evolution of the youngest basins straddles the Neoproterozoic±Paleozoic boundary. In central Brazil, on the SaÄo Francisco craton, the Bambu|¨ Group was deposited during the 1000±600 Ma interval. It starts with glacial deposits, poorly dated at about 900 Ma, and continues with alternating limestones, commonly stromatolitic, and generally ®ne, well-sorted, siliciclastic rocks. In western Brazil, as we shall see, the cover rocks of the Amazon craton-Rio Apa block are younger and were deposited during the 600±520 Ma interval. The Corumba area is located on the eastern edge of the Amazon craton-Rio Apa block, about 20± 30 km west of its tectonic contact with the Brasiliano Paraguay belt (Fig. 1A). In this region, two sets of extensional structures occur: the poorly exposed eastern extremity of the WNW±ESEtrending, 500 km long, Chiquitos±Tucavaca aulacogen and the NE±SW-trending, 10±20 km long, Corumba graben system. The geodynamic signi®cance of the Paraguay belt is still under debate. It could result from the deformation of a mega-graben developed during the separation of Laurentia from Gondwana (Dalziel, 1992) or, as suggested by Trompette (1994), of a foreland basin developed in front of a prolongation of the Brasilia fold belt which would be hidden beneath Paleozoic deposits. Metamorphism in the Brasilia belt is well dated at 590 Ma (Pimentel and Fuck, 1992, Pimentel et al., 1996). Cover sequences of the southern part of the Amazon craton±Rio Apa block Cover sequences crop out extensively to the west of Cuiaba (Fig. 1A) and in the Serra da Bodoquena, west of Bonito (Almeida, 1964, 1965, 1984; Alvarenga, 1990; Alvarenga and Trompette, 1992). West of CuiabaÂ, monoclinally dipping to subhorizontal cover rocks, lying disconformably over the basement, grade eastwards into a gently folded cover sequence. Decakilometric upright folds are outlined by the massive and thick carbonate Araras Formation (Fig. 2). A subvertical slaty cleavage appears in argillaceous intercalations in anticlinal hinges. The lithostratigraphic succession is roughly similar west of Cuiaba and in the Serra da Bodoquena (Fig. 2). It starts with the Puga Formation, consisting

Geological evolution of the Neoproterozoic Corumba graben system

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Fig. 1. (A) Geological map of the Brasiliano Paraguay fold belt and the Chiquitos±Tucavaca aulacogen which cross-cuts the eastern part of the Amazon craton near the Brazil±Bolivia boundary. According to Schobbenhaus et al. (1981), Litherland, et al. (1986), Alvarenga and Trompette (1994) and Trompette (1994). The contact between the two geological provinces is generally characterized by thrusting of metasediments onto the folded cratonic sequences. (B) Geological map of the eastern part of the WNW±ESE-trending Chiquitos±Tucavaca aulacogen with location of the NE±SW-trending Corumba graben system. According to O'Connor and Walde (1985), Litherland et al. (1986) and Walde (1988).

of diamictites with beds of sandstones, conglomeratic sandstones, arkoses and a few shaly or silty intercalations with basement cobble-sized and pebble-sized lonestones, generally interpreted as ice-rafted dropstones. West of CuiabaÂ, the Puga Formation is interpreted as a gravitationally reworked glacio-marine deposit (Alvarenga, 1990; Alvarenga and Trompette, 1992). The coarse siliciclastic facies of the Puga Formation are capped locally by thick, stromatolitic limestones and dolostones of the Araras, Cerradinho and Bocaina formations (Fig. 2). In the Serra da Bodoquena, the cover succession starts either with the Puga Formation or the Cerradinho and/or Cadieus formations made up of ®ne to coarse siliciclastic rocks. West of CuiabaÂ, the Araras Formation is overlain (Fig. 2) by the 1500 m thick, Alto Paraguay Group (sensu Almeida, 1964), made up of ®ne sandstones followed by medium to coarse reddish sandstones. The age of this cover sequence is poorly known. The signi®cance of the Rb/Sr 569 220 and 6602 60 Ma ages (Cordani et al., 1978, 1985) from two shale units of the Diamantino Formation (uppermost part

of the Alto Paraguay Group) is disputed, as are many ages from sedimentary material. Around CorumbaÂ, an Ediacara-like fauna (Fairchild, 1984) has been found in the limestones of the Tamengo Formation, correlative with the upper part of the Araras Formation. Based on these geochronological and paleontological data, the cover rocks of the Amazon craton±Rio Apa block have been placed in the Vendian±Early Cambrian interval, i.e. 590 Ma (deformation in old Brasiliano/Pan-African belts) to 520 Ma (deformation in young belts), as suggested by Dorr II (1973). Metasediments of the Paraguay belt Metasedimentary sequences in the Paraguay belt represent a metamorphic equivalent of the cover sequences described above. Around CuiabaÂ, a highangle, east to southeast dipping, reverse fault outlines the contact between the metasedimentary Cuiaba Group and the adjacent cratonic cover. The Cuiaba Group is mainly a thick equivalent of the Puga Formation. It consists of glacio-marine facies, more or less reworked by gravitational processes

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Fig. 2. Lithostratigraphic correlations of Vendian to early Cambrian sequences along the southeastern margin of the Amazon craton. Transgression of the limestones and dolostones of the Araras, CorumbaÂ, Pororo and Bocaina Formations and Groups is of glacio-eustatic origin and supposedly synchronous over the whole studied area. ÐW-Cuiaba log according to Almeida (1964) and Alvarenga (1990). ÐSerra da Bodoquena log according to Almeida (1965) and Alvarenga and Trompette (1994). ÐChiquitos±Tucavaca log according to Litherland et al. (1986). ÐCorumba log according to Almeida (1945), Dorr II (1945), Walde (1988) and Zaine (1991).

(Alvarenga, 1990; Alvarenga and Trompette, 1992). Eastward it grades into debris ¯ows, conglomerates and proximal coarse turbidites and then to ®ne distal turbidites with a few debris ¯ow intercalations that were deposited in a ¯at internal part of the basin. The coarse facies were deposited along the foot of the western continental slope of the Paraguay basin. In the central part of the basin, ®ne calcareous turbidites with a few limestone intercalations are capping the siliciclastic turbidites. They are possible equivalents of the cratonic Araras Formation. The whole Cuiaba Group is interpreted as a submarine fan system developed along the southeastern border of the Amazon craton. East of Bonito, in the Serra da Bodoquena (Fig. 1A), metasedimentary equivalents of all the formations described in the cratonic cover sequence, with the exception of the lenticular Cadieus Formation (Fig. 2), have been identi®ed in the Paraguay belt (Alvarenga and Trompette, 1994). Cratonic platformal carbonate facies of the Cerradinho Formation grade eastwards to metasedimentary calcareous turbidites. Greenschist facies metamorphism in the Paraguay belt is poorly dated. It is older than 504 24 Ma (Rb/

Sr) and 504 2 12 Ma (K/Ar), based on datations of the SaÄo Vicente cross-cutting granite (Almeida and Mantovani, 1975), but younger than the Ediacarian or late Vendian (570±545 Ma). Brasiliano polyphase deformation, well studied in the Paraguay belt around Cuiaba (Alvarenga, 1990; Alvarenga and Trompette, 1993), shows no clear tectonic vergence. The ®ll of the the graben system around Corumba According to Jones (1985), the Corumba area is located on a triple junction developed above a hot spot where three basins met, separated by angles of about 1208. They are the North and South Paraguay basins and the Tucavaca or Chiquitos±Tucavaca aulacogen. Geodynamic reconstructions (O'Connor and Walde, 1985; Litherland et al., 1986; Hoppe et al., 1987; Walde 1988; Trompette, 1994), using recent mapping by Litherland et al. (1986), indicate that the Chiquitos±Tucavaca aulacogen joined the Paraguay belt a few tens of kilometres east of Corumba (Fig. 1A). Deformation in the aulacogen is younger than in the Paraguay belt and dated at between

Geological evolution of the Neoproterozoic Corumba graben system 5062 15 Ma and 4882 12 Ma, according to K/Ar datings on basement gneisses remobilized and highly deformed along WNW±ESE-trending shear zones (O'Connor and Walde, 1985; Litherland et al., 1986). Thought to be located in the eastern part of the WNW±ESE elongate Chiquitos±Tucavaca aulacogen, the Corumba graben system displays dominant NE± SW normal faults, i.e., it lies roughly at right angles to the aulacogen (Fig. 3). The Corumba graben system is probably associated with the ¯exure of the Amazon cratonic basement, as the western border of the Paraguay basin is approached. In the Corumba area (Fig. 1B and 2), two lithostratigraphic units have been distinguished from base to top (Almeida, 1945; Dorr II, 1945). They are the Jacadigo and Corumba groups. The Corumba Group is made of dolostones at its lower part (Bocaina Formation) and limestones at its upper part (Tamengo Formation). The Tamengo Formation contains an Ediacara-like fauna (Vendian) near CorumbaÂ. This fauna contains the syphozoar Corumbella werneri (Hahn et al., 1982; Hahn and P¯ug, 1985), acritarchs such as Babvinella faveolata (Shepeleva) Vidal and Vandalosphaeridium sp., and fragments of calcareous tubes identi®ed as Cloudina lucianoi (Zaine and Fairchild, 1987). The fauna can be compared to that described by Germs (1972) in the lower part of the Nama Group, in Namibia. Two (Almeida, 1945) or three (Dorr II, 1945) formations have been distinguished in the Jacadigo Group. According to Almeida's nomenclature, which is more commonly used in Brazil, the Urucum Formation, lying with an angular unconformity over the basement of the Amazon craton±Rio Apa block, is siliciclastic and consists of coarse arkosic sandstones and conglomeratic sandstones. The overlying Santa Cruz Formation is made up of jaspers capped by banded hematite-rich jaspilites with arkoses and manganese ore intercalations. Beds of manganese ore, up to 5 m in thickness, are presently mined in the underground Urucum Mine. In the Santa Cruz Formation, basement boulders, some of them bigger than 1 m3, have been identi®ed in arkosic intercalations, especially in those capping manganese beds. Barbosa (1949), taking into account the size of the boulders, suggested that they may be glacially `dropped boulders', although no decisive observation supports this interpretation. The Jacadigo Group as a whole is coeval with the Puga Formation (Fig. 2). In Bolivia, possibly at the lower part of the Boqui Group, correlative with the Jacadigo Group, the Pimienta Formation consists of tu€s, agglomerates, lapilli, and volcanic breccias of basaltic composition (O'Connor and Walde, 1985; Litherland et al., 1986). Associated plutonic rocks have been dated by the K/Ar method at 623 215 Ma (Darbyshire, in Walde, 1988). However, inclusion of the Pimienta Formation in the Jacadigo Group remains hypothetical. The CoÂrrego das Pedras Formation is the third lithostratigraphic unit distin-

591

guished by Dorr II (1945). It constitutes a lithological transition between the two formations described above and includes siliciclastic and chemical, mainly manganesiferous, rocks. In Almeida's (1945) lithostratigraphy, these transitional facies are included in the Urucum Formation. Around CorumbaÂ, the Jacadigo Group di€ers from its cratonic equivalent, the Puga Formation, in the development of signi®cant ferruginous and manganesiferous chemical deposits. In the Bolivian part of the Chiquitos±Tucavaca aulacogen (Litherland et al., 1986) the Boqui Group, lenticular but reaching locally 3000 m in thickness, is made of diamictites, conglomerates and sandstones. The dolostones of the Pororo Formation, coeval with the Corumba Group, are capped by the upper part of the thick Tucavaca± MurcieÂlago Group, partly correlative with the Brazilian Alto-Paraguay Group. The Tucavaca± MurcieÂlago Group contains trace fossils of lower Paleozoic, probably Cambrian, age.

CORROBORATION OF DEPOSITION OF THE JACADIGO GROUP IN A GRABEN SYSTEM New unpublished structural observations by Alvarenga and Trompette con®rm and provide details of the structure of the graben system around CorumbaÂ. The existence of this extensional structure was ®rst suggested by Haralyi and Walde (1986) and Walde (1988). As previously stated, the area around Corumba (Fig. 3) consists of two morpho-geological units: a low swampy peneplain, with a few outcrops of the Corumba Group, and high inselbergs made up of basement rocks capped by the Jacadigo Group. Inselbergs are bounded by NE±SW-trending, subvertical faults (Fig. 4). The relative stratigraphic position of the Jacadigo and Corumba groups has been long disputed. According to Almeida (1945), the Jacadigo Group, which starts with conglomerates rich in limestone pebbles, lies over the Corumba Group. The presently accepted lithostratigraphic succession, with the Corumba Group on top, has been proved by Haralyi (1972). South of Corumba (Fig. 3) the Jacadigo Group crops out in two areas. The ®rst is the Jacadigo± Mutum hill (or morraria), which extends into Bolivia. The second is a composite large hill, made up of the Tromba dos Macacos, Urucum, Santa Cruz, Grande and Rabicho hills, where the Brazilian manganese mines are located. In the central composite main hill, the monoclinally tilted Jacadigo Group is about 500 m thick and consists of the Urucum and Santa Cruz formations. Northward and southward, it is covered by the Corumba Group, which is locally folded into open, NNE±SSW-trending synclines and anticlines. Here the Jacadigo Group is reduced to the

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Fig. 3. Geological map of the Corumba graben system according to Walde (1988). Names of the main hills or morrarias (M): 1. M. do Jacadigo (Mutum in Bolivia); 2. M. da Tromba dos Macacos; 3. M. do Urucum with the underground manganese mine of the Urucum Minerac° aÄo; 4. M. Santa Cruz with the SaÄo Domingos underground manganese mine (presently inactive), and open-pit iron mine in the northern part; 5. M. Grande with the presently inactive Figueirinha manganese underground mine; 6. M. do RabichaÄo with the presently inactive Santana manganese underground mine; 7. M. do Zanetti; 8. M. Pelada; 9. M. d'Aguassu; 10. M. do SajutaÂ.

Geological evolution of the Neoproterozoic Corumba graben system

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Fig. 4. Geological NW±SE cross sections of the Corumba graben system (location of section on Fig. 3). (A) Schematic section showing the present structure. Partly according to Haralyi and Walde (1986). (B) Reconstruction of the architecture of the paleo-basin in which the Jacadigo Group was deposited. The oldest manganese ore intercalation, outlining the base of the Santa Cruz Formation, is supposedly horizontal. The thickness of eroded cover rocks can be estimated at about 2500± 3000 m, according to a comparison with the more complete sequence of the Chiquitos±Tucavaca aulacogen. [1] South of CorumbaÂ, along the road to Jacadigo. According to Walde (1988), Zaine (1991), and Alvarenga and Trompette (unpublished). [2] South of the main synclinal structure of the Corumba Group. According to Walde (1988) and Haralyi and Walde (1986). [3] Morraria Tromba dos Macacos. According to Walde (1988). [4] Morraria Urucum. According to Dorr II (1945). [5] Morraria Santa Cruz. According to Almeida (1945) and Haralyi and Walde (1986). [6] Santana manganese mine. According to Walde (1988) and Alvarenga and Trompette (unpublished). NB. Attribution of temperature of 250±2808C, deduced from geochemical studies (Hoefs et al., 1987), to burial metamorphism would need a cover sequence of about 7 km thick. (C) Hypothetic reconstruction of the half-graben of Corumba at the end of deposition of the Urucum Formation. The subvertical faults, delineating the present system of inselbergs, are supposed to be the reactivated normal faults of the graben system.

Urucum Formation, a few to 10 m thick in the north and up to 150 m in the south (Fig. 4A). In the central composite hill, the Santa Cruz Formation covers about 30 km2. If we accept the hypothesis that it was not deposited in areas where the

Corumba Group presently lies directly over the Urucum Formation, we can reconstruct the paleogeography of the basin where sediments of the Santa Cruz Formation were deposited. This basin did not reach the southern hills of SajutaÂ, Aguassu, Pelada

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and Zanetti (Fig. 3). Eastward, the continuation of the Santa Cruz Formation below the Corumba Group is probable. Northwards its distribution is unknown. Westwards, a continuation in the Jacadigo±Mutum hill remains hypothetical. These paleogeographical data, although lacking in detail, suggest that the iron and manganese ores were deposited in a basin of limited size. We suggest that the reduced extent of the mineralized area is mainly original. Using variations of lithologies and thicknesses registered in the Urucum Formation (Fig. 4B and 4C), it is possible to show that this formation was deposited in a NW±SE-elongated half-graben where the most important faults are the AF and BF normal synthetic faults (Figs 3 and 4C). Mapping of the various lithologies suggest that coarse detrital, immature, continental sedimentation with conglomeratic piedmont facies, representing alluvial fan deposits, borders the shoulders of the half-graben. The fault system (Fig. 4C) as a whole is dissymmetric, indicating an asymmetric rift (Wernicke, 1985). The paleostructure of the basin, in which the Santa Cruz Formation was deposited, cannot be outlined. The true thicknesses of this formation are not precisely known because its upper part has been eroded (Fig. 4B). However sedimentological data suggest the persistence of a half-graben structure during deposition of the Santa Cruz Formation. In the Santa Cruz hill (Fig. 3), the upper part of the Santa Cruz Formation consists of centimetric to decimetric alternations of: (1) vacuole-rich, locally argillaceous banded jaspilite; (2) well-bedded, more or less silty and ferruginous shale, and (3) ferruginous, locally kaolinitic, often graded, arkosic sandstones and conglomeratic feldspathic sandstones containing granite boulders. Lithology 1 represents autochthonous chemical sediments. Lithology 2Ð very ®ne-grained siliciclastic sedimentsÐand lithology 3Ðgraded arkosic sandstones and feldspathic sandstones with granite bouldersÐrepresent turbiditic and debris ¯ow deposits. Paleogeographic reconstructions show a half-graben with the central part dominated by chemical sedimentation, periodically interrupted by distal gravitational ¯uxes arriving from the shoulders of the basin and/or the main internal fault-related scarps. In the conglomeratic arkosic sandstones of the lower part of the Santa Cruz Formation basement boulders are not icerafted elements. Even the biggest, more than 1 m3, do not cross-cut the underlying bedding. Deformation occuring around the boulders is associated with compactation and is generally more pronounced above the boulders than below them. No `ice-dropped-boulders' were identi®ed in the Jacadigo Group around Corumba although, in adjacent areas, its equivalent, the Puga Formation, consists of glacial deposits more or less reworked by gravitational ¯uxes (Alvarenga and Trompette, 1992).

GEOLOGICAL EVOLUTION OF THE CORUMBA GRABEN SYSTEM Stratigraphical, sedimentological and structural data suggest the following history for the Neoproterozoic sequences around CorumbaÂ: ÐJust before Ediacarian time, probably during Varengian, or Early Vendian, time (about 600±570 Ma), extensional tectonics generated a system of graben paralel to the border of the Amazon craton. If we accept the hypothesis that the Paraguay basin is a foreland basin developed in front of the nappes of the Brasilia belt, then the extensional event in the Corumba graben system is synchronous with the deformation dated at 590 Ma in the Brasilia belt (Pimentel and Fuck, 1992; Pimentel et al., 1996). ÐDeposition of the sediments of the Jacadigo Group occurred in a half-graben. This unit consists of a lower siliciclastic unit and a upper, marine and/or lacustrine ®ne chemical unit with periodic contributions of debris ¯ows and turbidity currents. ÐTransgression of glacio-eustatic origin initiated the deposition of dolostones of the Bocaina Formation (Corumba Group), which are, in part, of Ediacarian age. ÐWeak NW±SE Brasiliano folding, probably synchronous with deformation and metamorphism in the Paraguay belt (545±500 Ma), a€ected the Jacadigo and Corumba groups. ÐTectonic inversion of the graben system and development of the present inselberg topography are tentatively correlated to the subsidence of the Pantanal low area which, according to Shiraiwa (1994), may be dated at about 3 Ma (Pliocene).

DEPOSITIONAL CONTEXT OF THE CORUMBA FE AND MN ORES Corumba iron and manganese ores have, for a long time, been interpreted as sedimentary platform deposits. Fe and Mn ions have been considered as originating from the weathering of the basement of the adjacent Amazon craton±Rio Apa block. However, lithological and petrographical studies, published at the end of the 2nd World War, have questioned this interpretation, suggesting deposition of the Jacadigo Group in a more active geodynamic environment. Almeida (1945) pointed to the extreme hardness of arkoses of the Urucum Formation with feldspars that seem to be sericitized in situ and large well-crystallized chlorites that appear in the matrix. Dorr II (1945) also mentioned diagenetic sericite and non-detrital magnetite in ferruginous facies, near principal faults. In addition, tourmaline-rich quartz veins cross cut the Jacadigo Group.

Geological evolution of the Neoproterozoic Corumba graben system Recent mineralogical and geochemical investigations of Corumba Fe and Mn ores (Schneider, 1984; Schreck, 1984; Urban and Stribrny, 1985; Urban et al., 1992; Verissimo et al., 1994), suggest the Jacadigo Group has undergone diagenetic to epimetamorphic transformations. Cryptomelane is the main mineral in manganese ore. In a few facies, braunite is the dominant manganese oxyde. It is cross cut by thin veins of tirodite (a manganiferous amphibole) and nambulite (a silicate similar to rhodonite). Braunite, tirodite, and nambulite are manganese minerals which crystallized at high temperatures, generally in a hydrothermal context. In addition, in the jaspilites of the Santa Cruz Formation, oxygen isotopes indicate that hematites have crystallized at between 2508 and 2808C (Hoefs et al., 1987). Thus petrogenetic, mineralogical and geochemical studies suggest that the Jacadigo Group has su€ered a diagenetic to anchimetamorphic event, the origin of which is disputed: burial metamorphism below a cover sequence of more than 7 km thick (Hoefs et al., 1987) or, in spite of the absence of known intrusive plutons, contact metamorphism (Schneider, 1984; Schreck, 1984). Because this high temperature diagenetic event has not been identi®ed in the overlying Corumba Group, burial metamorphism seems unlikely. According to generally accepted sedimentological models for the Jacadigo Group, siliciclastic materials as well as Fe and Mn ores originated from weathering of the basement of the Amazon craton±Rio Apa block (Walde et al., 1981; Urban and Stribrny, 1985; Urban et al., 1992). The Jacadigo Group is considered to have been deposited in a fjord-like basin, i.e., a glacially carved paleovalley, at least partly covered by sea-ice (Urban et al., 1992). In the upper part of the fjord, under a sea-ice cover and reducing conditions, Fe++ and Mn++ would be extracted from siliciclastic materials, and transported to the lower part of the fjord. Here oxidizing conditions, generated for instance by the entrance of large volumes of continental fresh waters, would favour Fe and Mn precipitation. Glacial climates have been put forward for the deposition of Neoproterozoic iron ores of the Canadian Rapitan Group (Young, 1976, 1988; Yeo, 1981) and of the Chinese late Sinian (0800±545 Ma) manganese ores of Xiangtan (Ye Lianjun et al., 1988; Fan Delian et al., 1922). In the Rapitan Group, Fe is considered to have precipitated close to an ice platform according to Young (1976), whereas Beukes and Klein (1992) and Klein and Beukes (1993) consider that Canadian iron and Chinese manganese were deposited with an eustatic transgression. Two sets of observations allow a re-evaluation of the nature of the depositional environment of Neoproterozoic Corumba Fe and Mn ores. First, they were deposited in a graben system and not on a cratonic platform and, second, they have su€ered a diagenetic-epimetamorphic event responsible for the

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crystallization of hematites at temperature of 250± 2808C. In such a geological context, Mn and Fe ions can result from the hydrothermal alteration of intrusive magmatic rocks by syntectonic brine migrations (Bethke, 1986; Daniels et al., 1990). CONCLUSIONS The Jacadigo Group, which forms the lower part of the ®lling of the Neoproterozoic graben system around CorumbaÂ, di€ers from its adjacent cratonic equivalents by the presence of chemical iron and manganese deposits and by having su€ered a hydrothermal event. The Corumba half-graben is ®lled by a lower coarse siliciclastic unit (Urucum Formation) with deposition of thick conglomerates and piedmont breccias along the graben shoulders and the main internal normal faults. The upper sequence, The Santa Cruz Formation is a mixture of chemical deposits (Fe and Mn) and siliciclastic sediments. The graded arkosic sandstones were deposited by turbidity currents and the arkosic sandstones with granitic boulders by gravitational ¯uxes. Both turbidity and gravitational currents originate from the shoulders and internal faultrelated scarps of the graben system. Corroboration of the deposition of the Jacadigo Group in an extensional basin provides support for an hydrothermal origin for Fe and Mn ores. They could be generated by leaching of hidden plutonic rocks, of probably ma®c composition, associated with Neoproterozoic, probably Vendian, extentional tectonics. Acknowledgements Ð Field work has been partly ®nancially supported by a grant of FAPESP (Fundac° aÄo de Amparo aÁ Pesquisa do Estado de SaÄo Paulo) and logistically and scienti®cally favoured through collaboration with Minerac° aÄo Corumbaense Reunida S.A., Urucum Minerac° aÄo S.A. and Minerac° aÄo Mato Grosso. We thank S. Marshak, R. Trouw, R. Andreis and R.A. Fuck for constructive reviews.

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