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Chapter 15 The Cratonic Environment
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Chapter 15 THE CRATONIC ENVIRONMENT E. SUSZCZYNSKI
INTRODUCTION
The crystalline basement in both the Amazonian and Brazilides structural provinces is overlain by sedimentary and volcanic cover sequences, ranging in age from about 2000 Ma to Phanerozoic, that are now confined t o a great number of isolated, remnant basins. The main areas covered by these rocks in the Amazonian shield are: (1)Roraima plateau, and extending through northern Brazil into Venezuela, Guyana and Colombia (locality 31, Fig. 15.1). (2) Cubencranqubm-Gorotire plateau covering the mid-Xing6 River (Fig. 13.2). ( 3 ) Tapaj6s basin and Cachimbo plateau covering a large area south of the Amazonian aulacogen (localities 26', 26'' and 35, Fig. 15.1) and Prosperanca north of the Amazon River (locality 29, Fig. 15.1). (4)Mapuera-Uatuma area north of the Amazon River (locality 36, Fig. 15.1). (5) Scattered outcrops of the Palmeiral Formation and other sedimentary sequences in Rondonia (locality 30, Fig. 15.1). (6) Caiabis-Apiacas, Serra Formosa-Minissau6 and Rio Liberdade basins in northern Mato Grosso (localities 25, 24 and 23 respectively, Fig. 15.1). (7) Carajas plateau in Para state (locality 26, Fig. 15.1). (8) Amazonian aulacogen covering the Amazon basin (locality 28, Fig. 15.1). The age of the cover sequences is often uncertain, but those of the Roraima plateau have a minimum age of about 2000 Ma based on dating of intrusive dolerite dykes and sills (McDougal and Compston, 1963). A minimum age of about 920 Ma is indicated for the cover rocks in Rondonia on the basis of their approximate contemporaneity with granitic ring dykes and rhyolites of that age. The age of the sediments in the Cubencranqubm and Gorotire plateau in the mid-Xingii River, is older than 1350 Ma. Lithologic similarities between these rocks and those in the Caiabis-Apiacas, Serra
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Formosa-MinissauA and Rio Liberdade basins leads t o the provisional conclusion that the latter are of a similar age. An Upper Precambrian age has been proposed for the cover rocks on the Caraj6s plateau. The folded cover rocks in the Mapuera-Uatuma area have yielded fossils of Lower Silurian age, and younger sequences placed in the Lower Devonian. The Amazonian aulacogen has a lithic fill that accumulated during Ordovician and Silurian times. The Roraima cover rocks have been subdivided into three formations from the base up consisting of:
853 (1)Kaieteur Formation has a thickness in excess of 500 m and consists of conglomerate and sandstone, from which detrital diamonds have been reported in lensoid mglomerate beds. ( 2 ) Surumu Formation consists of rhyolites and ignimbrites with a thickness greater than 1000 m. (3) R o r i m a Formation, 900 m thick, composed of arkosic sandstones with minor intercalations of conglomerate, argillaceous schists, jaspilites and basic lavas. Sheets of gabbro and norite, several hundred metres thick, together with dolerite dykes and sills intrude the cover sequence causing it t o be domed and folded. On the Cubencranqukm and Gorotire plateaux Barbosa (1966) described from the base to the top the following formations: Rio Fresco, Gorotire, and Cubencranqukm. Subsequently the author has changed the stratigraphic order as follows from the base upwards: (1) Gorotire Formation consists of conglomeratic sandstones that are micaceous and koalinized with some intercalated blue and black marine shales. (2) Rio Fresco Formation is a volcano-sedimentary sequence with carbonaceous beds, micaceous arkosic sandstones and andesites intruded by dolerite. ( 3) Cubencranqukm Formation comprises sandstones and fine-grained arkoses with lenses of chert and jaspilite. Fig. 15.1. The structural framework of Brazil. Numbers in circles refer to the following areas: 1 = Santa Barbara; 2 = Camaqua; 20 = Upper Balsas; 3 = Itajai; 21 = Tocantins basin; 4 = CampoLargo; 22 = Paraguayan-Araguaian; 5 = Gurutubinha; 23 = Rio Liberdade; 6 = Vale d o Ribeira, including 24 = Serra Formose-Minicua; Cerro Azul-Malo Preto; 25 = ApiacasCaiabis; 7 = Castro-Iapb; 26l = Cachimbo plateau; 8 = EleutBrio; 2611 = Old Tapajbs basin; 9 = Carandai-Veloso; 26 = Carajas plateau; 27 = Jamanxim Area; 10 = Rio CuitC-Tumirintinga; 28 = Amazonian aulacogen; 11 = “Bambui System’‘ 29 = Prosperanca; -SZo Francisco basin; 30 = “Palmeiral Formation” 12 = Jacar&-Lenqois; and others; 13 = Rio Pardo; 31 = Roraima; 14 = Miaba-Canudos ; 32 = Serra da Borracha; 1 5 = Iara ; 33 = Itaberai-Rio Bonito; 16 = Cococi; 17 = Jaibaras; 34 = Estlncia ; 35 = Sucunduri area; i a = Rio Paraim; 1 9 = Pindorama; 36 = Mapuera-VatumT Area.
854 Certain of the sedimentary features have been interpreted as the consequence of deposition in a prograding deltaic environment. Further south, in Mato Grosso state the preserved cover rocks are lithologically similar t o the Cubencranqukm Formation with which a correlation is proposed. The Cachimbo plateau is formed by sandstone sequences with basal conglomerates, locally containing placer gold. In the Tapaj6s basin two formations, called the Juruena-Teles Pires and Middle Tapajbs, have been recognized, each consisting of sediments reflecting transgressive conditions followed by regression. No volcanic rocks are known but the sedimentary rocks are cut by basic dykes and sills. In the mid-Aripuaiia valley, de Almeida (1959) described the Beneficente Group consisting of orthoquartzites with basal conglomerate lenses. The sequence is distinguished by the presence of rhyolites and ignimbrites. The Caraj6s plateau is built by the Carajas series consisting of conglomerates, quartzites, phyllites, manganiferous slates, mica schists and basaltic volcanic rocks overlain by banded siliceous rocks with high-grade iron-ore concentrations. These later have been described as itabirites but the author considers that they owe their origin t o differentiation within the magma chamber that was the source of the early basaltic flows coupled with later deuteric and telethermal processes. The Palmeiral Formation and other sedimentary remnants in Rondonia consist of arkosic sandstone, locally conglomeratic, that are red in colour. Conglomerates are also present, that may contain fossil cassiterite placers. Manganese and iron ore are associated with the arkosic facies. Carbonate sequences are found in the Sucunduri area (locality 35, Fig. 15.1), and along the Tapaj6s River. At the last named locality these rocks constitute the Acari Group, which is over 350 m thick. Stromatolite structures have been discovered in the carbonate sequence known as the Sucunduri Group. Two types of preserved basins of cover rocks can be distinguished within the Brazilides domain. The first and most prominent type are the elongated basins in which subsidence occurred, and the second type are small tectonic basins with the lithic fill preserved in old graben structures. The principal basins of the first type are: (1)The SZo Francisco basin which contains the Bambui System, the maximum age of which is 1350 Ma, but an age of 600 Ma is suggested (Cordani, 1968) (locality 11, Fig. 15.1). (2) The Tocantins basin lying close t o the boundary between the Amazonian shield and the Brazilides structural province. The basin is filled mainly by the Tocantins Series. It is open northward (locality 21, Fig. 15.1). (3) The Paraguayan-Araguaian basin whose lithic fill is subdivided into the Upper Paraguay, Caacupk, Araras and Bodoquena groups. I t is open southward (locality 22, Fig. 15.1).
855 (4) The Lenqois-Jacar6 basin located in north-central Bahia which is regarded as a portion of the Sgo Francisco basin, from which separation occurred during the closing stages of sedimentation. Several lithologic groups such as the Paraguaqu, Tombador and Morro do Chap& have been identified in this basin. There are twenty of the second type of structurally preserved basins: (1) Camaqua and Santa Barbara basins, distinguished by the presence of cupriferous ores (localities 1 and 2, Fig. 15.1). (2) Itajai basin filled by clastic sediments and volcanics (locality 3, Fig. 15.1). ( 3 ) Castro-Iap6 basin filled by clastic sediments and volcanics (locality 7, Fig. 15.1). (4) Vale d o Ribeira in which low-grade metamorphism is superimposed on a volcano-sedimentary sequence (locality 6 , Fig. 15.1). (5) Rio Bonito-Itaberai basin filled by a volcano-sedimentary sequence (locality 33, Fig. 15.1). (6) Carandai-Barroso basin that contains limestone and black shales (locality 9, Fig. 15.1). ( 7 ) Rio Pardo basin in which conglomerates contain detrital diamonds and the early Palaeozoic sediments include evaporite deposits (locality 13, Fig. 15.1). (8) Estgncia formation which contains red beds (locality 34, Fig. 15.1). (9) Miaba-Canudos and Itabaiana groups of low metamorphic grade (locality 14, Fig. 15.1). (10) Serra da Borracha (locality 32, Fig. 15.1). (11)Rio Paraim basin containing limestones and black shale (locality 18, Fig. 15.1). (12) Cococi basin of clastic sediments and volcanics (locality 16, Fig. 15.1). (13) Jaibaras basin consisting of clastic and carbonate sediments, black shales and volcanics. Stratiform copper deposits are associated with the sediments (locality 17, Fig. 15.1). (14) Pindorama basin (locality 19, Fig. 15.1). The following small structurally preserved basins comprise clastic sediments with or without intercalated volcanics: (15) Campo Largo (locality 4 , Fig. 15.1). (16) Guaratubinha (locality 5, Fig. 15.1). (17) Eleutkrio (locality 8, Fig. 15.1). (18) Rio Cuit6-Tumiritinga (locality 10, Fig. 15.1). (19) Iara (locality 15, Fig. 15.1). (20) Upper Balsas area (locality 20, Fig. 15.1). It is difficult to be precise about the absolute ages of isolated basins containing similar lithologies. It seems that the western remnants are younger in age than those t o the east. Thus the Paraguayan-Araguian basin is younger
856 than the Bambui System in the Sgo Francisco basin. A further difficulty arises from the fact that close lithological similarities exist between the Bambui System in the SZo Francisco basin and the Araras, Corumba and Bodoquena Groups in the Paraguay-Araguaian basin. An Eocambrian age is regarded as probable for certain of the coarse clastic sedimentary formations, some of which were deposited under palaeodeltaic conditions. Fundamental differences can be distinguished between the nature of the lithic fill of the basins resting on the Amazonian shield and those within the Brazilides structural province. The characteristic feature of the folded sedimentary cover rocks in the Brazilides structural province is the widespread presence of carbonate rocks that are scarce in the sedimentary cover sequences resting on the Amazonian shield. In the Jacar&-Lenqois basin carbonate sequences are over 1300 m thick. The cover rocks on the Amazonian shield are characterized by the presence of thick volcanic flows, usually rhyolitic in composition but also including dacites and andesites. Ignimbrites and granophyres are also present and several types of granitic stocks intrude the cover. In contrast volcanic rocks play only a minor role in the cover sequences within the Brazilides province and have a restricted development when they are present. With the exception of the cover rocks in the UatumZ-Trombetas area which are confined t o a tectonic depression within the Amazonian shield all the remaining cover sequences build prominent plateaux, bounded by steep scarps, that rest with a clear unconformable relationship on the crystalline basement. The folded cover rocks in the Brazilides province are found principally along regonal tectonic axes. Nearly all the outcrops of cover rocks are confined to the area of tectonic downwarping known as the P&ran&Sa"o Francisco-Parnaiba axis which has a general north-northeasterly or northeasterly trend. Similarly the cover rocks in the Paraguay-Araguaian and Tocantin basins occupy clearly defined structural depressions, the latter having a general northward strike. There are significant age differences between the sedimentary cover of the two structural provinces, those resting on the Amazonian shield being older. The volcano-sedimentary sequences forming the folded cover to the Amazonian shield appear to represent at least four cycles, starting with an initial period of marine transgression and terminating with regression during which sandstones and conglomerates were deposited. The final stages of regression were marked by periods of continental volcanism, there being no evidence of submarine volcanics. Attention must be drawn to the lithologic similarities between the Bambui System or the Araras and Bodoquena Groups with the Katanga System of central Africa which is emphasized by the occurrence of stratiform copper mineralization in some areas. In Minas Gerais, the following succession has been recognized from the base upwards, forming collectively the classic "Minas Series":
857 (1) Caraqa Group: with the Moeda Quartzitic Formation, about 200 m thick, rests on the crystalline basement that is invaded by granites at -1350 Ma and by dykes at -1110 Ma; and the Batatal Formation of slates with minor cherts, about 150 m thick. (2) Itabira Group consisting of the CauE Formation with banded ironstones, and the Gandarela Formation with dolomites, about 300 m thick. (3) Piracicaba Group, 3000 m thick, comprising several geological formations with quartzites, ferruginous quartzites, pelites and dolomites. (4)Itacolomi Group overlies the “Minas Series” unconformably and consists of cross-bedded quartzites with conglomerates, phyllites and an upper cross-bedded quartzite. It is in this series that the flexible sandstone, itacolomite, is found. A palaeodeltaic regressive environment is here clearly represented, with some scattered gold occurrences. The crystalline basement and older cover sequences are overlain in both the Amazonian and Brazilides provinces by largely unfolded sediments and volcanics of Phanerozoic age. In the Jacobina area, thin-bedded and cross-bedded quartzites, containing about 27 auriferous conglomerates and cross-bedded deltaic quartzites overlie micaceous quartzites, schists and phyllites. The conglomerates that consist of white quartz pebbles set in an arenaceous matrix are largely barren and low gold values are associated with pyrite-bearing conglomerates. There are many similarities between the Jacobina Series and the Witwatersrand succession of South Africa. The Jacobina Series is considered t o be Early Proterozoic in age, and it has a net .regressive metasedimentary character. Only regressive clastic sequences have developed gold-pyrite mineralization in the Brazilian platform. Primitive transgressive sequences are barren. STRUCTURE
Although the preserved cover sequences on the Amazonian shield are found now as isolated fragments it is possible t o re-construct their probable original distribution and identify three major structural trends south of the Amazon River. The oldest one is bounded by the upper X i n d and the mid-TapajGs rivers and includes the sedimentary-volcanic sequences on the Cubencranqu6m, Gorotire, and CarajAs plateaux, where volcanic rocks are well-preserved. T‘ne oldest rocks are found along the Xing% River. A prograding deltaic sequence at the end of a major sedimentary cycle is represented by the sediments on the Cubencranqukm-Gorotire plateau. One hundred kilometres south of this plateau, marine sediments are preserved but imperfectly known in the Liberdade-Minissaui area (localities 23 and 24, Fig. 15.1), extending into northern Mato Grosso state. The dominant structural trend is northwest-southeast.
858 Post-dating this trend is one that affects the remnants of lower rocks in Rondonia. It is possible that old sediments preserved in the Caiabis basin (locality 25, Fig. 15.1) also belong t o this event, which, if proved correct, would extend considerably the area affected. The main characteristic of this group of rocks is their consistent westward dip so that the youngest members of the stratigraphic column are found along the western border of Rondonia Federal Territory. Superimposed on these earlier trends is the west-northwesterly aligned Cachimbo-Tapaj6sProsperanca axis (localities 26l, 2611and 29, Fig. 15.1) the cover sequence associated with which rests with strong discordance on older rocks involved in the earlier events. The sediments contained within this structure have been subdivided into two series, both consisting of regressive and transgressive sequences. The southernmost comprises the JuruemaTeles Pires series and the younger, Middle Tapaj6s series lies towards the northwest. The Apiachs sedimentary sequence (locality 25, Fig. 15.1) represents an isolated southeasterly extension of this event, which is represented north of the Amazon aulacogen by the outcrops in the Prosperanqa area. The main structural expression is similar to a megasynclinorium plunging to the west-north west. North of the Amazon River one of the most prominent structural trends is that which involves the rocks of the Roraima plateau. In this area clastic sedimentary and volcanic sequences present arcuate structures with northwesterly directed plunges on the southern side of the Amazon aulacogen. The author suggests that the Roraima cover rocks young t o the south. If this interpretation is correct it implies that the central core or nucleus of the Amazonian shield was the site of successive volcano-sedimentary depositories the depositional axes of which migrated radially away from the central nucleus. The major feature of the Amazonian shield is the early Phanerozoic Amazonian aulacogen, trending N60" E and cutting transversely across all the Precambrian structures. Significant geophysical anomalies, attributed to the presence of ultrabasic rocks, have been found along the central axis of this megastructure in addition to large basic intrusions of late Mesozoic age. The structural patterns within the Brazilides province are quite different from those just described for the Amazonian shield. The identification of continuous structural trends associated with basins of preserved sedimentaryvolcanic sequences, aligned generally northwards, can be made readily in the Brazilides province. The preserved basins now have the form of synclinoria that consistently plunge northwards throughout the Brazilides domain. The lithologic sequences in each preserved basin reflect an upward coarsening along their western flanks that is interpreted as the response to regression. In the larger basins regressive cycles are commonly terminated by volcanism. The sediments accumulated in a series of basins, the depositional axes of which migrated towards the west with the result that lithic sequences found
in the east are older than those in the west. The most extensive outcrops of the cover rocks are found in the Siio Francisco basin where there is arcuation of the Brazilides trend, concave to the east. Like all the other preserved basins the SZo Francisco basin was originally open to the west. The absence of carbonates in certain of the smaller preserved basins is attributed to the lack of preservation of marine environments during regressive phases, which each time finished in continental environments. The similarity in lithologic and structural features in successive basins which migrated from east to west emphasizes the essential tectonic unity that prevailed during the evolution of the Brazilides tectono-orogenic cycle. MINERALIZATION
Some Brazilides manganese ores may be similar to those found in India. The classic primary manganese protores of Minais Gerais (for example, that from Conselheiro Lafaiete) were developed within volcano-sedimentary sequences spatially associated with amphibolitic-garnetiferous and calc-silicate horizons. To the west towards the G o i h region the manganese deposits are exclusively associated with thin amphibolitic zones on top of the Volcanogenic Schist Series, i.e. the so-called Red Schist belts. Specific carbonatic manganese mineralization is found only in some facies of the Bambui System, near the Iron Quadrangle, in Minais Gerais state. Toward the top of the Bambui System in the Goi6s region the manganiferous sedimentary facies grades into slate, quartzite and metaconglomerate, strongly developed at the top of the regressive sequences, as in the case of the manganiferous area around SZo JoZo da Alianqa. Near the Atlantic coast in Bahia, Espirito Santo and Rio de Janeiro the manganese protore is associated with highgrade metamorphic rocks (leptites and anorthosites) derived from impure metasedimentary sequences. The itabirites are found with rocks that were originally limestones and dolomites, calc-pelitic rocks and chert-quartzites giving rise to three types of itabirites, namely, dolomitic, amphibolitic and quartzitic. All of these lithologies were formed under regressive sedimentary conditions. No itabirites related to transgressive sedimentary sequences have been found in the Brazilian platform. Another type of manganese mineralization is that represented by iiie mixed iron-manganese ore found especially in the Iron Quadrangle. This type, predominantly uneconomic, is developed in fine detrital metasedimentary sequences which cover the lower, most iron-rich sequences formed by chemical precipitation. The classic iron-manganese association in the Iron Quadrangle represents one of the branches of the well-known “Jacutinga” gold mineralization. In this case impure, manganese-bearing strata are developed within ferruginous quartzite. Graphite at low stratigraphic levels,
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manganese at intermediate levels and iron in the higher stratigraphic levels form a very common metallogenetic association in Precambrian areas of the Brazilian Shield. The Brazilian iron-ore deposits are located in several kinds of metasedimentary series where thin layers of haematite are intercalated rhythmically with quartzite of which itabirite is the classic type. The “Minas Series”, now subdivided by the author into several other “groups” with similar lithologic characteristics due t o the cyclic nature of the old marine environments, in the Iron Quadrangle, is well-known for this type of deposit. They are also found in many other areas on the Atlantic side of the Brazilian shield. A correlation with the iron ore deposits in the Transvaal Supergroup of South Africa may be possible. Within the Amazonian shield important deposits of manganese and iron have been discovered since 1967 on the Carajas plateau. Supergene enrichment of siliceous, magnetite-rich protore related directly t o old basic and hybrid volcanism, has resulted in the formation of large and economically important deposits in this area. Manganiferous slates and shales stratigraphically below the iron protore have been prospected with success in recent years. Large amounts of primary, banded magnetite and titaniferous magnetite ore in basic volcanic rocks have not yet been taken into consideration in the Carajas plateau. The gold deposits of the Jacobina district are similar to those of the Witwatersrand. Gold placers associated with pyritic conglomerates have been mined from the lower stratigraphic levels of a thick, clastic, regressive sequence which in the west overlie various schists and micaceous quartzites. In GoiAs, Mato Grosso, Iron Quadrangle where similar geologic environments are found further gold mineralization is known. Recent discoveries of economic lead and zinc sulphide ores have been made in the western outcrops of the Bambui System in the Vazante-Morro Agudo-Paracutu area. The mineralization is associated with brecciated calcarenites in the lowest stratigraphic levels of a 350-400 m-thick dolomite sequence which is overlain by phillites, slates and quartzites reflecting a low metamorphic grade, representing a classic lithologic profile of a regressive metasedimentary environment open toward the west. The many significant features of mineralization in sedimentary-volcanic sequences in Brazil indicate their association with regressive mega-cycles of sedimentation. It must be recognized, however, that detailed studies and exploration of transgressive cycles are rare because they are almost completely unknown within the Brazilian Precambrian. In very few known cases only poor, uneconomic and scattered mineralization is found in this environment which still needs more studies t o confirm that it is in fact a primitive transgressive sedimentary environment. It is important t o emphasize this point because it leads to a better understanding of metallogenesis in the Brazilian platform. This part of the Earth’s crust is almost entirely formed by
861 rocks types characteristic of regressive, metasedimentary sequences, and associated volcanism. In addition to authigenic placer gold deposits derived from the precipitation of colloidal gold under specific, Recent conditions with pH lower than 5 . 3 , mainly in the Amazonian areas, detrital tin is also mined in Rondonia and other regions. Placer tin and gold deposits are also known on the Amazonian shield north of the Amazon River in AmapA state but these deposits have been superseded in importance by the recent discovery of extensive cassiterite placers in northeastern Goi6s. (References are listed at the end of Chapter 18).
Chapter 15 THE CRATONIC ENVIRONMENT E. SUSZCZYNSKI
INTRODUCTION
The crystalline basement in both the Amazonian and Brazilides structural provinces is overlain by sedimentary and volcanic cover sequences, ranging in age from about 2000 Ma to Phanerozoic, that are now confined t o a great number of isolated, remnant basins. The main areas covered by these rocks in the Amazonian shield are: (1)Roraima plateau, and extending through northern Brazil into Venezuela, Guyana and Colombia (locality 31, Fig. 15.1). (2) Cubencranqubm-Gorotire plateau covering the mid-Xing6 River (Fig. 13.2). ( 3 ) Tapaj6s basin and Cachimbo plateau covering a large area south of the Amazonian aulacogen (localities 26', 26'' and 35, Fig. 15.1) and Prosperanca north of the Amazon River (locality 29, Fig. 15.1). (4)Mapuera-Uatuma area north of the Amazon River (locality 36, Fig. 15.1). (5) Scattered outcrops of the Palmeiral Formation and other sedimentary sequences in Rondonia (locality 30, Fig. 15.1). (6) Caiabis-Apiacas, Serra Formosa-Minissau6 and Rio Liberdade basins in northern Mato Grosso (localities 25, 24 and 23 respectively, Fig. 15.1). (7) Carajas plateau in Para state (locality 26, Fig. 15.1). (8) Amazonian aulacogen covering the Amazon basin (locality 28, Fig. 15.1). The age of the cover sequences is often uncertain, but those of the Roraima plateau have a minimum age of about 2000 Ma based on dating of intrusive dolerite dykes and sills (McDougal and Compston, 1963). A minimum age of about 920 Ma is indicated for the cover rocks in Rondonia on the basis of their approximate contemporaneity with granitic ring dykes and rhyolites of that age. The age of the sediments in the Cubencranqubm and Gorotire plateau in the mid-Xingii River, is older than 1350 Ma. Lithologic similarities between these rocks and those in the Caiabis-Apiacas, Serra
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Formosa-MinissauA and Rio Liberdade basins leads t o the provisional conclusion that the latter are of a similar age. An Upper Precambrian age has been proposed for the cover rocks on the Caraj6s plateau. The folded cover rocks in the Mapuera-Uatuma area have yielded fossils of Lower Silurian age, and younger sequences placed in the Lower Devonian. The Amazonian aulacogen has a lithic fill that accumulated during Ordovician and Silurian times. The Roraima cover rocks have been subdivided into three formations from the base up consisting of:
853 (1)Kaieteur Formation has a thickness in excess of 500 m and consists of conglomerate and sandstone, from which detrital diamonds have been reported in lensoid mglomerate beds. ( 2 ) Surumu Formation consists of rhyolites and ignimbrites with a thickness greater than 1000 m. (3) R o r i m a Formation, 900 m thick, composed of arkosic sandstones with minor intercalations of conglomerate, argillaceous schists, jaspilites and basic lavas. Sheets of gabbro and norite, several hundred metres thick, together with dolerite dykes and sills intrude the cover sequence causing it t o be domed and folded. On the Cubencranqukm and Gorotire plateaux Barbosa (1966) described from the base to the top the following formations: Rio Fresco, Gorotire, and Cubencranqukm. Subsequently the author has changed the stratigraphic order as follows from the base upwards: (1) Gorotire Formation consists of conglomeratic sandstones that are micaceous and koalinized with some intercalated blue and black marine shales. (2) Rio Fresco Formation is a volcano-sedimentary sequence with carbonaceous beds, micaceous arkosic sandstones and andesites intruded by dolerite. ( 3) Cubencranqukm Formation comprises sandstones and fine-grained arkoses with lenses of chert and jaspilite. Fig. 15.1. The structural framework of Brazil. Numbers in circles refer to the following areas: 1 = Santa Barbara; 2 = Camaqua; 20 = Upper Balsas; 3 = Itajai; 21 = Tocantins basin; 4 = CampoLargo; 22 = Paraguayan-Araguaian; 5 = Gurutubinha; 23 = Rio Liberdade; 6 = Vale d o Ribeira, including 24 = Serra Formose-Minicua; Cerro Azul-Malo Preto; 25 = ApiacasCaiabis; 7 = Castro-Iapb; 26l = Cachimbo plateau; 8 = EleutBrio; 2611 = Old Tapajbs basin; 9 = Carandai-Veloso; 26 = Carajas plateau; 27 = Jamanxim Area; 10 = Rio CuitC-Tumirintinga; 28 = Amazonian aulacogen; 11 = “Bambui System’‘ 29 = Prosperanca; -SZo Francisco basin; 30 = “Palmeiral Formation” 12 = Jacar&-Lenqois; and others; 13 = Rio Pardo; 31 = Roraima; 14 = Miaba-Canudos ; 32 = Serra da Borracha; 1 5 = Iara ; 33 = Itaberai-Rio Bonito; 16 = Cococi; 17 = Jaibaras; 34 = Estlncia ; 35 = Sucunduri area; i a = Rio Paraim; 1 9 = Pindorama; 36 = Mapuera-VatumT Area.
854 Certain of the sedimentary features have been interpreted as the consequence of deposition in a prograding deltaic environment. Further south, in Mato Grosso state the preserved cover rocks are lithologically similar t o the Cubencranqukm Formation with which a correlation is proposed. The Cachimbo plateau is formed by sandstone sequences with basal conglomerates, locally containing placer gold. In the Tapaj6s basin two formations, called the Juruena-Teles Pires and Middle Tapajbs, have been recognized, each consisting of sediments reflecting transgressive conditions followed by regression. No volcanic rocks are known but the sedimentary rocks are cut by basic dykes and sills. In the mid-Aripuaiia valley, de Almeida (1959) described the Beneficente Group consisting of orthoquartzites with basal conglomerate lenses. The sequence is distinguished by the presence of rhyolites and ignimbrites. The Caraj6s plateau is built by the Carajas series consisting of conglomerates, quartzites, phyllites, manganiferous slates, mica schists and basaltic volcanic rocks overlain by banded siliceous rocks with high-grade iron-ore concentrations. These later have been described as itabirites but the author considers that they owe their origin t o differentiation within the magma chamber that was the source of the early basaltic flows coupled with later deuteric and telethermal processes. The Palmeiral Formation and other sedimentary remnants in Rondonia consist of arkosic sandstone, locally conglomeratic, that are red in colour. Conglomerates are also present, that may contain fossil cassiterite placers. Manganese and iron ore are associated with the arkosic facies. Carbonate sequences are found in the Sucunduri area (locality 35, Fig. 15.1), and along the Tapaj6s River. At the last named locality these rocks constitute the Acari Group, which is over 350 m thick. Stromatolite structures have been discovered in the carbonate sequence known as the Sucunduri Group. Two types of preserved basins of cover rocks can be distinguished within the Brazilides domain. The first and most prominent type are the elongated basins in which subsidence occurred, and the second type are small tectonic basins with the lithic fill preserved in old graben structures. The principal basins of the first type are: (1)The SZo Francisco basin which contains the Bambui System, the maximum age of which is 1350 Ma, but an age of 600 Ma is suggested (Cordani, 1968) (locality 11, Fig. 15.1). (2) The Tocantins basin lying close t o the boundary between the Amazonian shield and the Brazilides structural province. The basin is filled mainly by the Tocantins Series. It is open northward (locality 21, Fig. 15.1). (3) The Paraguayan-Araguaian basin whose lithic fill is subdivided into the Upper Paraguay, Caacupk, Araras and Bodoquena groups. I t is open southward (locality 22, Fig. 15.1).
855 (4) The Lenqois-Jacar6 basin located in north-central Bahia which is regarded as a portion of the Sgo Francisco basin, from which separation occurred during the closing stages of sedimentation. Several lithologic groups such as the Paraguaqu, Tombador and Morro do Chap& have been identified in this basin. There are twenty of the second type of structurally preserved basins: (1) Camaqua and Santa Barbara basins, distinguished by the presence of cupriferous ores (localities 1 and 2, Fig. 15.1). (2) Itajai basin filled by clastic sediments and volcanics (locality 3, Fig. 15.1). ( 3 ) Castro-Iap6 basin filled by clastic sediments and volcanics (locality 7, Fig. 15.1). (4) Vale d o Ribeira in which low-grade metamorphism is superimposed on a volcano-sedimentary sequence (locality 6 , Fig. 15.1). (5) Rio Bonito-Itaberai basin filled by a volcano-sedimentary sequence (locality 33, Fig. 15.1). (6) Carandai-Barroso basin that contains limestone and black shales (locality 9, Fig. 15.1). ( 7 ) Rio Pardo basin in which conglomerates contain detrital diamonds and the early Palaeozoic sediments include evaporite deposits (locality 13, Fig. 15.1). (8) Estgncia formation which contains red beds (locality 34, Fig. 15.1). (9) Miaba-Canudos and Itabaiana groups of low metamorphic grade (locality 14, Fig. 15.1). (10) Serra da Borracha (locality 32, Fig. 15.1). (11)Rio Paraim basin containing limestones and black shale (locality 18, Fig. 15.1). (12) Cococi basin of clastic sediments and volcanics (locality 16, Fig. 15.1). (13) Jaibaras basin consisting of clastic and carbonate sediments, black shales and volcanics. Stratiform copper deposits are associated with the sediments (locality 17, Fig. 15.1). (14) Pindorama basin (locality 19, Fig. 15.1). The following small structurally preserved basins comprise clastic sediments with or without intercalated volcanics: (15) Campo Largo (locality 4 , Fig. 15.1). (16) Guaratubinha (locality 5, Fig. 15.1). (17) Eleutkrio (locality 8, Fig. 15.1). (18) Rio Cuit6-Tumiritinga (locality 10, Fig. 15.1). (19) Iara (locality 15, Fig. 15.1). (20) Upper Balsas area (locality 20, Fig. 15.1). It is difficult to be precise about the absolute ages of isolated basins containing similar lithologies. It seems that the western remnants are younger in age than those t o the east. Thus the Paraguayan-Araguian basin is younger
856 than the Bambui System in the Sgo Francisco basin. A further difficulty arises from the fact that close lithological similarities exist between the Bambui System in the SZo Francisco basin and the Araras, Corumba and Bodoquena Groups in the Paraguay-Araguaian basin. An Eocambrian age is regarded as probable for certain of the coarse clastic sedimentary formations, some of which were deposited under palaeodeltaic conditions. Fundamental differences can be distinguished between the nature of the lithic fill of the basins resting on the Amazonian shield and those within the Brazilides structural province. The characteristic feature of the folded sedimentary cover rocks in the Brazilides structural province is the widespread presence of carbonate rocks that are scarce in the sedimentary cover sequences resting on the Amazonian shield. In the Jacar&-Lenqois basin carbonate sequences are over 1300 m thick. The cover rocks on the Amazonian shield are characterized by the presence of thick volcanic flows, usually rhyolitic in composition but also including dacites and andesites. Ignimbrites and granophyres are also present and several types of granitic stocks intrude the cover. In contrast volcanic rocks play only a minor role in the cover sequences within the Brazilides province and have a restricted development when they are present. With the exception of the cover rocks in the UatumZ-Trombetas area which are confined t o a tectonic depression within the Amazonian shield all the remaining cover sequences build prominent plateaux, bounded by steep scarps, that rest with a clear unconformable relationship on the crystalline basement. The folded cover rocks in the Brazilides province are found principally along regonal tectonic axes. Nearly all the outcrops of cover rocks are confined to the area of tectonic downwarping known as the P&ran&Sa"o Francisco-Parnaiba axis which has a general north-northeasterly or northeasterly trend. Similarly the cover rocks in the Paraguay-Araguaian and Tocantin basins occupy clearly defined structural depressions, the latter having a general northward strike. There are significant age differences between the sedimentary cover of the two structural provinces, those resting on the Amazonian shield being older. The volcano-sedimentary sequences forming the folded cover to the Amazonian shield appear to represent at least four cycles, starting with an initial period of marine transgression and terminating with regression during which sandstones and conglomerates were deposited. The final stages of regression were marked by periods of continental volcanism, there being no evidence of submarine volcanics. Attention must be drawn to the lithologic similarities between the Bambui System or the Araras and Bodoquena Groups with the Katanga System of central Africa which is emphasized by the occurrence of stratiform copper mineralization in some areas. In Minas Gerais, the following succession has been recognized from the base upwards, forming collectively the classic "Minas Series":
857 (1) Caraqa Group: with the Moeda Quartzitic Formation, about 200 m thick, rests on the crystalline basement that is invaded by granites at -1350 Ma and by dykes at -1110 Ma; and the Batatal Formation of slates with minor cherts, about 150 m thick. (2) Itabira Group consisting of the CauE Formation with banded ironstones, and the Gandarela Formation with dolomites, about 300 m thick. (3) Piracicaba Group, 3000 m thick, comprising several geological formations with quartzites, ferruginous quartzites, pelites and dolomites. (4)Itacolomi Group overlies the “Minas Series” unconformably and consists of cross-bedded quartzites with conglomerates, phyllites and an upper cross-bedded quartzite. It is in this series that the flexible sandstone, itacolomite, is found. A palaeodeltaic regressive environment is here clearly represented, with some scattered gold occurrences. The crystalline basement and older cover sequences are overlain in both the Amazonian and Brazilides provinces by largely unfolded sediments and volcanics of Phanerozoic age. In the Jacobina area, thin-bedded and cross-bedded quartzites, containing about 27 auriferous conglomerates and cross-bedded deltaic quartzites overlie micaceous quartzites, schists and phyllites. The conglomerates that consist of white quartz pebbles set in an arenaceous matrix are largely barren and low gold values are associated with pyrite-bearing conglomerates. There are many similarities between the Jacobina Series and the Witwatersrand succession of South Africa. The Jacobina Series is considered t o be Early Proterozoic in age, and it has a net .regressive metasedimentary character. Only regressive clastic sequences have developed gold-pyrite mineralization in the Brazilian platform. Primitive transgressive sequences are barren. STRUCTURE
Although the preserved cover sequences on the Amazonian shield are found now as isolated fragments it is possible t o re-construct their probable original distribution and identify three major structural trends south of the Amazon River. The oldest one is bounded by the upper X i n d and the mid-TapajGs rivers and includes the sedimentary-volcanic sequences on the Cubencranqu6m, Gorotire, and CarajAs plateaux, where volcanic rocks are well-preserved. T‘ne oldest rocks are found along the Xing% River. A prograding deltaic sequence at the end of a major sedimentary cycle is represented by the sediments on the Cubencranqukm-Gorotire plateau. One hundred kilometres south of this plateau, marine sediments are preserved but imperfectly known in the Liberdade-Minissaui area (localities 23 and 24, Fig. 15.1), extending into northern Mato Grosso state. The dominant structural trend is northwest-southeast.
858 Post-dating this trend is one that affects the remnants of lower rocks in Rondonia. It is possible that old sediments preserved in the Caiabis basin (locality 25, Fig. 15.1) also belong t o this event, which, if proved correct, would extend considerably the area affected. The main characteristic of this group of rocks is their consistent westward dip so that the youngest members of the stratigraphic column are found along the western border of Rondonia Federal Territory. Superimposed on these earlier trends is the west-northwesterly aligned Cachimbo-Tapaj6sProsperanca axis (localities 26l, 2611and 29, Fig. 15.1) the cover sequence associated with which rests with strong discordance on older rocks involved in the earlier events. The sediments contained within this structure have been subdivided into two series, both consisting of regressive and transgressive sequences. The southernmost comprises the JuruemaTeles Pires series and the younger, Middle Tapaj6s series lies towards the northwest. The Apiachs sedimentary sequence (locality 25, Fig. 15.1) represents an isolated southeasterly extension of this event, which is represented north of the Amazon aulacogen by the outcrops in the Prosperanqa area. The main structural expression is similar to a megasynclinorium plunging to the west-north west. North of the Amazon River one of the most prominent structural trends is that which involves the rocks of the Roraima plateau. In this area clastic sedimentary and volcanic sequences present arcuate structures with northwesterly directed plunges on the southern side of the Amazon aulacogen. The author suggests that the Roraima cover rocks young t o the south. If this interpretation is correct it implies that the central core or nucleus of the Amazonian shield was the site of successive volcano-sedimentary depositories the depositional axes of which migrated radially away from the central nucleus. The major feature of the Amazonian shield is the early Phanerozoic Amazonian aulacogen, trending N60" E and cutting transversely across all the Precambrian structures. Significant geophysical anomalies, attributed to the presence of ultrabasic rocks, have been found along the central axis of this megastructure in addition to large basic intrusions of late Mesozoic age. The structural patterns within the Brazilides province are quite different from those just described for the Amazonian shield. The identification of continuous structural trends associated with basins of preserved sedimentaryvolcanic sequences, aligned generally northwards, can be made readily in the Brazilides province. The preserved basins now have the form of synclinoria that consistently plunge northwards throughout the Brazilides domain. The lithologic sequences in each preserved basin reflect an upward coarsening along their western flanks that is interpreted as the response to regression. In the larger basins regressive cycles are commonly terminated by volcanism. The sediments accumulated in a series of basins, the depositional axes of which migrated towards the west with the result that lithic sequences found
in the east are older than those in the west. The most extensive outcrops of the cover rocks are found in the Siio Francisco basin where there is arcuation of the Brazilides trend, concave to the east. Like all the other preserved basins the SZo Francisco basin was originally open to the west. The absence of carbonates in certain of the smaller preserved basins is attributed to the lack of preservation of marine environments during regressive phases, which each time finished in continental environments. The similarity in lithologic and structural features in successive basins which migrated from east to west emphasizes the essential tectonic unity that prevailed during the evolution of the Brazilides tectono-orogenic cycle. MINERALIZATION
Some Brazilides manganese ores may be similar to those found in India. The classic primary manganese protores of Minais Gerais (for example, that from Conselheiro Lafaiete) were developed within volcano-sedimentary sequences spatially associated with amphibolitic-garnetiferous and calc-silicate horizons. To the west towards the G o i h region the manganese deposits are exclusively associated with thin amphibolitic zones on top of the Volcanogenic Schist Series, i.e. the so-called Red Schist belts. Specific carbonatic manganese mineralization is found only in some facies of the Bambui System, near the Iron Quadrangle, in Minais Gerais state. Toward the top of the Bambui System in the Goi6s region the manganiferous sedimentary facies grades into slate, quartzite and metaconglomerate, strongly developed at the top of the regressive sequences, as in the case of the manganiferous area around SZo JoZo da Alianqa. Near the Atlantic coast in Bahia, Espirito Santo and Rio de Janeiro the manganese protore is associated with highgrade metamorphic rocks (leptites and anorthosites) derived from impure metasedimentary sequences. The itabirites are found with rocks that were originally limestones and dolomites, calc-pelitic rocks and chert-quartzites giving rise to three types of itabirites, namely, dolomitic, amphibolitic and quartzitic. All of these lithologies were formed under regressive sedimentary conditions. No itabirites related to transgressive sedimentary sequences have been found in the Brazilian platform. Another type of manganese mineralization is that represented by iiie mixed iron-manganese ore found especially in the Iron Quadrangle. This type, predominantly uneconomic, is developed in fine detrital metasedimentary sequences which cover the lower, most iron-rich sequences formed by chemical precipitation. The classic iron-manganese association in the Iron Quadrangle represents one of the branches of the well-known “Jacutinga” gold mineralization. In this case impure, manganese-bearing strata are developed within ferruginous quartzite. Graphite at low stratigraphic levels,
860
manganese at intermediate levels and iron in the higher stratigraphic levels form a very common metallogenetic association in Precambrian areas of the Brazilian Shield. The Brazilian iron-ore deposits are located in several kinds of metasedimentary series where thin layers of haematite are intercalated rhythmically with quartzite of which itabirite is the classic type. The “Minas Series”, now subdivided by the author into several other “groups” with similar lithologic characteristics due t o the cyclic nature of the old marine environments, in the Iron Quadrangle, is well-known for this type of deposit. They are also found in many other areas on the Atlantic side of the Brazilian shield. A correlation with the iron ore deposits in the Transvaal Supergroup of South Africa may be possible. Within the Amazonian shield important deposits of manganese and iron have been discovered since 1967 on the Carajas plateau. Supergene enrichment of siliceous, magnetite-rich protore related directly t o old basic and hybrid volcanism, has resulted in the formation of large and economically important deposits in this area. Manganiferous slates and shales stratigraphically below the iron protore have been prospected with success in recent years. Large amounts of primary, banded magnetite and titaniferous magnetite ore in basic volcanic rocks have not yet been taken into consideration in the Carajas plateau. The gold deposits of the Jacobina district are similar to those of the Witwatersrand. Gold placers associated with pyritic conglomerates have been mined from the lower stratigraphic levels of a thick, clastic, regressive sequence which in the west overlie various schists and micaceous quartzites. In GoiAs, Mato Grosso, Iron Quadrangle where similar geologic environments are found further gold mineralization is known. Recent discoveries of economic lead and zinc sulphide ores have been made in the western outcrops of the Bambui System in the Vazante-Morro Agudo-Paracutu area. The mineralization is associated with brecciated calcarenites in the lowest stratigraphic levels of a 350-400 m-thick dolomite sequence which is overlain by phillites, slates and quartzites reflecting a low metamorphic grade, representing a classic lithologic profile of a regressive metasedimentary environment open toward the west. The many significant features of mineralization in sedimentary-volcanic sequences in Brazil indicate their association with regressive mega-cycles of sedimentation. It must be recognized, however, that detailed studies and exploration of transgressive cycles are rare because they are almost completely unknown within the Brazilian Precambrian. In very few known cases only poor, uneconomic and scattered mineralization is found in this environment which still needs more studies t o confirm that it is in fact a primitive transgressive sedimentary environment. It is important t o emphasize this point because it leads to a better understanding of metallogenesis in the Brazilian platform. This part of the Earth’s crust is almost entirely formed by
861 rocks types characteristic of regressive, metasedimentary sequences, and associated volcanism. In addition to authigenic placer gold deposits derived from the precipitation of colloidal gold under specific, Recent conditions with pH lower than 5 . 3 , mainly in the Amazonian areas, detrital tin is also mined in Rondonia and other regions. Placer tin and gold deposits are also known on the Amazonian shield north of the Amazon River in AmapA state but these deposits have been superseded in importance by the recent discovery of extensive cassiterite placers in northeastern Goi6s. (References are listed at the end of Chapter 18).