Revista Brasileira de Oeociências 12(1-3): 193-214, Mar.-Sel., 1982 - Slo Paulo

GEOLOGICAL AND METALLOGENIC PATTERNS lN THE ARCHEAN AND EARLY PROTEROZOIG OF BAHIA STATE, EASTERN BRAZIL

JURACY DE FREITAS MASCARENHAS' and JOSÉ HAROLDO DA SILVA SÁ'

ABSTRACT The territory of Bahia State is part of the São Francisco structural province. The basement of the province is characterized by granite-greenstone and high grade metamorphic tcrrains developed during the Archean and Early Proterozoic. The Early Archean is represented by small tonalitic-granodioritic and enderbitic-charuockitic nuclei 3,500 Ma old. Several portions of gneissic-migmatitic and granulitie terrains have ages of 3,000 Ma and Rb/Sr isochronic values of 2,700 Ma that represem lhe principal phase of lhe Jequié geotectonic cycle. During lhe Early Protcrozoic. extensive arcas of the basement of the São Francisco province underwent intensive remobilization, through granitization and Kcmetasornatism related to the Transamazônico geo­ tectonic cycle (2.200-1,800 Ma). Besides greenstone belts the granite-greenstcne terrains contain volcano-sedimentary (green­ stone belt like) and platform sequences developed since lhe Archean to the Early Proterozoic. The high grade metamorphic terrains contain remnants of supracrustal sequences and basic­ -ultrabasic complexes. The metallogenic Framework of Bahia shows that more important mineral deposits such as ALI, Cr, Cu, Fe-Ti-V, Pb-Zn-Mn , rnagnesite and tale are related to Archean and Early Proterozoic evolution. Gold deposits are located in vclcanic rocks of greenstone belt environments, and in conglomerates of Witwatersrand type. Eeonornie deposits 01' Cr. Cu, Fe-Ti~V are associated with maflc-ultramafic complexes in high grade metamorphic terrains. Pb-Zn and Mn minerallzations occur in volcano-sedimentary (greenstone bel! like} sequences and the platform sequences contam large deposits of m~gnesite and tale.

INTRODUCTION Bahia State, with an area of about r------.------, 500,000 km '. lies almost entirely within the São Francisco Province (Almeida (!1 at., 1977), an extensivo cratonic nu­ clcus stabilized at the cnd ofthc Transamazónico geotecton­ ic cyc!c (2.700-1.800 Ma). lt is surroundcd hy regions whieh suffered rcgeneration in geosynclinal environments during J"~~ the Brasiliano geotectonic cycle (1,100-450 Ma). Situated in the middle of the most easterly part of Brazil, it is limitcd by the Borborema Provinre, to the north; the "~(~~ Parnaiba, to the northwest; the Tocantins to the west; the Manttquetra to the south and southeast ; and, finally, by the Coastal Province ond continental margin to the east (Fig. I i. As cornponeni parts of the Borborema; Tocantins and Mannqueívu Pravinces, which , howcvcr, represem thc geolcg­ ical limits of lhe São Francisco Province. bordering geo­ synclinal structures occur. with polarity directed towards the interior of the Provincc. These structures are the folded belts ojSergipe. Riacho do Ponto/ (Brito Neves, 1975), Rio Prelo (Ioda and Barbosa, 1978l, Brasília and Araçuai (Al­ meida, 1968. 1977), which were established during lhe Upper Proterozoie Brasiliano Cycle (Fig. 2). --_._------The geological Iimits of lhe Province, as defined by Al­ meida et a/o (1977) are convenüonal. being based 00 tran­ Figure 1 - Situation oflhe São Francisco provínce (I) Relatívely to sitional tectonic belts. Surface gravimetric surveys, under­ Tocantins (2), Mantíqueíra (3), Borbotemo (4), Parnaiba (5), coast taken by Gomes and Morta (1978, 1980). however. cstab­ and continental margin (6) Provinces (after Almeida et al., 1977). lishcd good idcntification with lhe gcological fcatures, and confirrned these ideas , particularly those related to the Ser­ As far as lhe Sergipe belt is concerned, lhe limit was es­ gipe, Riacho do Pontal, and part of lhe Brasília folded belts. tablished at the point of lhe great deflcction of direction of Sigoifieaot displaeemeols belweeo surlace geology aod io­ lhe slruclural aod isogalic aligomeols, which to the soulh ferrcd gravirnetric structllrcs were fOllnd only in the north~ have a N-S orieotalioo which chaoges to NW-SE lo the ern par< of lhe Araçuai be/I (Fig. 3I. oorth of the limit.

* Coordenação da Produção Mineral da Secretaria das Minas e Energia do Estado da Bahia/Companhia Baiana de Pesquisa Mineral, Centro Administrativo da Bahia, 4a. Avenida s/no 40000 ~ Salvador, BA, Brazil 194 Revista Brasileira de Geoctênctos, Volume 12 (I~3). 1982

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F

ft-f-I-f----i----f--\-fi-ft+-++---ISo ---- GEOLOGIC LIMITS ___ GRAVIMETRIC LIMITS , ,• THRUST FAULT --. VERGENCE OIRECTION ----TRANSCORRENT FAULT

o 200 400km ~•••••• ;;;;J

Figure 2·- Sítucníon (~r lhe São Francisco provínce reküívelv TO Brazitían [old beíts: 1 - Sergipana: 2·- Riacho do Pontal .. 3 - Rio Preto,' 4 - Brasilía .. 5·- AI"(/çllaí .. citíes and vílíages, A - Juazciro ; B _.. Campo Alegre de Lourdcs .. C - Reli/aliso," D- Uallá .. E- São Domingos; F - Solvador: G - Vitória da Conquísta; H - Formosa do Rio Preto .. 1-- Patos de Minas; J - Belo Horizonte

For lhe Riacho do Pontal belt, the limit was established ize the Riacho do Pontal limit, continue into Goiás State, as a function ofthe deep, WSW-ENE crustal faults, showing and accompany reasonably well the geological limit up to maximum Bouguer anomalíes of + 5 mgal. which are re­ latitude I3°S. peated only at the coast of Bahia (Malta, in Mascarenhas To the north of the Araçuai bett, the limit is marked et al., 1982). ln the folded belt, the depth of the Moho is by an accentuatcd gravimetríc gradíent, with values chang­ approximately 32 km (Malta, op. cit.). ing rapidly from - 50 mgal to the north of Itambé, For the Brasília belt , the gravímetric data are insufficient, to - 75 mgal at Vitória da Conquista, which corresponds but high Bougucr values, similar to thosc whích charactcr- to I mgal/krn. Revista Brasileirade Geod~ncias, Volume 12 (1.3), 1982 195

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20 Abrolhos •

LEGEND

-.../"80--"'"""" 0 20 - -- -T-T------~--\-- Isoqams Geo109 i c Bounda ry-­ -----. Gravimetric Boundary----­ ...... -.- S'te t e Llml te .-_.._.".._-- --- G"ravimetric ThrOugh'~------~

200km 200km o .

Figure 3 _ Bouguer map ofBahia a/ter: DNPM CPRM - Proj. Levantamento Gravímétrico no Estado da Bania. Reffinal 1980 A C. MOrTA

1981 rifts and intracratonic basin-s, which correspond to the Es­ CHARACTERISTICS OF THE sao FRANCISCO pinhaco Supergroup, were deposited. PROVINCE lN BAHIA STATE Referring to the Ar­ Generical1y, thc Pr(!~É.\pinh(fço Assemtdage includes grau­ chean and Lower Proterozoic, in Bahia State the Pre-Espi­ ite~greenstone and high-grade mctamorphic terrains, Be­ nhaço Assemhlage is defined as the products ofalllithostrat­ cause of the particularities of each one, they are described igraphic processes as late-as the start of the Middle Prote­ separately (Fig. 4). rozoic (1,800-1,100 Ma), when paraplatform sequences of 196 Revista Brasileira de Oeocéncíes, Volume 12 (1-3), 1982

"l"-~ __".__..:;~

-----1&°

PROVINCE LI t1ITS

---20" BOUGUER 'LIMIT -----1---TECTON IC LIMIT covered,ancertain,probable FAULTS ~-,-,,~--- thrust transcorrent

o 200 400km ..:::-- -:::::::..- ._":::.:.= :.:::::-=

Figure 4 ~ SÜo Francisco province simplífied RC'O{ORic map H<;(?.j I. Phanevozoic covers S II. Upp~1' oP:'orerowic: sao Francisco Supergroup; J) Barnbui Group; 2) Una; 3) Macaúbas, 4) Rio Pardo; 5) Canudos~Vaza Barris; 6) Rio Preto.Éoo:oQ:ol III. Milldle Proterozoic: Espinhaço Supergroup: 7) Septentríonal and meridional Espinhaço; 8) Chapada Díámantína; 9) Arai Group; 10) Natividade Group. ~1mmm~~~'12 0013 8 14 R:::E=:=~15 IV. Ear/y Profero:::oic.'o Archean: /2) Probable Grernstone Belts andfor Magmatíc-Sedírnentary and Sedimentar)' Scquences: /3) greenstone Beíts: 14) hígh-Grade Metamorphlc Terrains; 15) graníte-Greenstone Terraíns. Revista Brasi/e;r~ de Geocilncias, Volume 12 (1-3), 1982 197

Granite~greenstone terrains These are characterizcd itic magma being responsible for the rafting 01' greenstone by large arcas 01' gncissic ando migmatitic grauiric rocks in xenoliths and their subsequent modification by processes which there occurs an infinity 01' rock types the natures of which include granitization, metasomatism, assimilation and which ref1eet the presence of'volcanic, plutonic and sedimen­ partial fusion. tary components under various forrns and over various exten­ sions. The better-preserved sequences, which forrn keels Greenstone belt structures At prcsent, only one within host rocks of amphiboJite or granulite grade, show structure \\ uh typical grcenstone make-up is known in Bahia contact metamorphic aureoles, which are the result of the Statc, This is lhe Serrinha (Mascarenhas, 1973) or Hapicuru granitizing processes operating in an environment 01' lower (Kishida. 1979; Kishida and Riccio. 1979) belt. (greenschist) grade. These sequences are of greenstone-belt The latter authors grouped the volcano-sedimentary se­ type, or are "probable" greenstones, or are volcano-sedi­ quence into three principal rock units: at the base, the mafic mentary sequences 01' uncertain genetic origino metavolcanic unit ; in the middle, the felsic metavolcanic Less well-preserved fragments occur as lenses or leveis unit; and, at the top, the metasedirnentary unit (Fig. 5). disposed along narrow, elongated orests, or in ameboid, These units enclose domes of granodiorite-tonalite the oval or curved patterns which suggest the former existence borders of which show potassification, with abundant peg­ oftrue magmato-sedimentary sequences, now well scattered. matite veins. Contact metamorphic effects are observed Since they have been afTected by granitization, gneissifi­ locally. cation Dr migmatization, over and above tectonic forces, The mafic metavolcanic unit is composed ofbasalts, usually these remnants are structurally concordant with the regional schistose, with ocean-floor tholeiite geochemical charactcr­ foliations, and have a metarnorphic grade similar to that istics, and also small sedimentary intercalations, with a of thc host rocks. predominance ofchemical components (banded iron forma­ The granite-gneiss-migmatite rocks show structural and tions, metachert and carbonate beds) which vary from 0.5 textural patterns which vary from isotropic to highly foliat­ to 100m in thickness, ed. ln general. they are fine to coarse-graincd or augen varieties, and are leucocratic, showing variable compositions ," of granite, adamellite, granodiorite and tonalite. Usually, II o'"o'~ biotite is more abundant than hornblende, Metamorphic .~~'''.' .: ~":'. grade ranges from greenschist to high amphibolite or even '.~ .. 'granulite, but medi um-grade associations are more frequento Banded, foJiated ophthalmitic and migmatitic gneisses occur, the latter with stromatic, folded, agmatic and dictionitic structures, Diatexites with nebulitic and schlicren structures are also observed. Close to the magmato-sedimentary sequences, the granite­ -gneiss-migmatites develop oval structures ar mantled gneiss domes which result from diapiric processes. ln those areas where isochron Rb(Sr ages are older, there is a general tendency for the Na,O(K,O ratio to be high; where the ages are younger, the inverse is true, although there are exceptions. Where extensive K-metasomatism has occurred, the ages of the rocks tend to be Transamazônico. The few geochemical studies on these rocks lead to the conclusion that, in part, they are derived by parametamor­ phic transformations, as is the case 01' the gneisses to the east of the Jacobina Mountains and in the Caraiba area (Figueiredo, 1980). Similar problems have been the subject of world-wide discussion by several authors, who consider either the exis­ tence of thick arkose-graywacke sequences with intercalated marly horizons (Ket et ai., 1976; Arth, 1976) or ofvolcano­ -sedimentary sequences with predominance of ultrabasic, basic and intermediate volcanics and tuffs (Clark, 1979). Yardley (1978) showed that gneisses might be produced by lit-par-lit injection, but Myers (1978) assumed that many fine-grained gneisses are the products 01' tectonic distortion of rnagmatic and volcanic bedding, of bodies such as pillow lavas, of fragments such as igneous-derived crystals or rock fragments in volcanic and plutonic breccias, and 01' igneous LEGEND cumulate textures or 01' dyke swarms.: b,;':j I~I 1::::1 [

The felsic metavolcanic unir occurs as lenticular bodies, e) They have suffered from a strong Transamazônico in­ and is formed ofcalealkaline rocks similar to those ofactive fluence, continental margins and ineluding andesite and daeite lavas, f) Because of the deformation and remobilization, it is pyroelastics and intercalated metasediments. difficult to establish their limits with the older rocks, and Preserved magmatic structures ineluding pillow lavas, flow to deduce the stratigraphic column. breccias, and variolitic, porphyritic and amygdaloid tex­ g) The geochronological data do not yield Lower Pro­ tures are present in the mafic rocks. ln the felsic rocks, terozoic ages, with the exception of some imprecise or un­ breccias, agglomerates, crystal tuffs, tuffaceous chcrt. and reliable values between 1,900 and 2,200 Ma. pyroclastic and chernical components occur. h) Although, locally; some features may be absent, the The metasedimentary unit is a flysch, which includes: sequences generally have a lower magmatic component, with I) schistoseintraformational polymict conglomerates, com­ serpentinized uitramafic rocks, some ofwhich with the char­ posed of milky quartz, felsic voleanics, chert and pelites; acteristics of flows, passing to basalts in the middle part and rare fragrnents ofmetabasites; granitic fragments are absent; massive, porphyritic and voleanielastic felsic rocks at the 2) arkosic meta-arenites ; 3) rneta-arkose. argilite and siltite ; top, intercalated with fine-grained pelitic and chemical me­ 4) graywackes, alternating with dark shales; 5) intraforma­ tasediments including phyllites, graphitic schists, local tional breccias; 6) well-Iaminated, dark or Iight argilites and phyllite-graywacke sequences metacherts, calc-silicates, siltite, with chlorite and rich in graphite, and with thin marbles, banded iron formations etc. chert beds. i) The basaltic rocks have tholeiitic, and the felsic, cale­ The unit can be divided into arkosic-conglomeratic and -alkaline atTinities. graywacke-pelitic associations, both of which are typical Locally, where wéak deformation has permitted, char­ turbidites. acteristic igneous features, such as massive and porphyritic Small bodies of ultrarnafic rocks are composed of cumu­ lavas, voleanic breceias, lapilli-tuffs, vesicular and amygda­ late euhedral serpentinized olivine grains in a post-cumulate loidal textures, etc., are preserved. clinopyroxene matrix. Needles and plates of altered clino­ Nevertheless, unti! now, no examples of pillow lavas 01' pyroxene give a texture similar to that described in some of spinifex textures have becn found. The most important . peridotitic komatiites of the Belingwe greenstone belt by examples of such structures are as follows: Nesbit et ai. (1977, cited by Kishida, 1979). Serpentinized • ln the Rio Capim complex, identified by Winge and Danni and chromitiferous ultramafic rocks are also found near (1980) and Winge (1981) as a greenstone belt-like struc­ Santa Luz and Queimadas (Fig. 5). ture, the basal parts are composed of an association of According to Kishida (op. cil.) the calc-alkaline metavol­ amphibolitized mafie flows or tuffs, with intercalations canic unit can be correlated geochemically with the Yilgarn of silico-ferruginous and silicic sediments (metachert, ita­ greenstone belts, which are characterized by a lack of ba­ birites with magnetite and amphibole) and, 'al lhe IOp, saltic andesites and by the geochemical gap between the voleaniclastic (agglomerates, lapilli-tuffs and ignimbrites) tholeiitic metabasalts and the upper volcanic rocks. and volcanochemical rocks, related to an explosive, inter­ Mascarenhas et ai. (1976, 1979), Pires et ai. (1976), Inda mediate to felsic phase. and Barbosa (1978), Mascarenhas (1979) and Figueiredo (1980)assumed that the volcano-sedimentary sequence, asso­ • ln thé Contendas-Mirante complex, there occurs a Iower ciated with gneisses, migmatites and granulites, which ex­ uni! of mainly ultramafic-mafic rocks, and an association tends to the north of the Serrinha greenstone belt into the ofmafie and felsíc volcanics with immature metasediments Uauá and Caraiba areas.imay be partly correlated withthe (Cunha et al., 1981; Pedreira and Marinho, 1981). The units rclated to this belt, and partly, with its basernent. but forrner is represcntcd by serpentinites, tremolitites-actino­ in a highcr metamorphic grade than greenschist. which IS litites, hornblendites and metabasites with intercalations typical of the bclt. of oxide 01' silicate-facies banded iron formations, carbon­ ates, metacherts and graphitic schists. ln the latter, the Probable greenstone belts and/or magmato­ basal subunit is composed mainly of fine tuffs and lapilli -sedimentary and sedimentary sequences Within tuffs, with less expressive massive and porphyritic felsic the granite-grecnstone tcrrains, therc occur many localized lavas, massive and amygdaloidal mafic lavas, metachert assemblages which show similarities to grcenstone belts. horizons, banded iron formations, carbonates, graphitic Other sequences, with a greater similarity to supracrusta!s. schists and graywackes, The upper subunit has fine pyro­ are frequently deposited over these assemblages. Yet others c1astics with metachert, banded iron formations and have unknown geotectonic significance, but are always graywacke intercalations, which become more abundant present in predominantly amphibolitc grade (Fig. 6). towards the topo Petrochemically, the rocks of the lower Similarities to, and differences from greenstone belts are unit have low-K tholeiitic character with a predominance .as follows: of basalts and andesites over rhyolites. a) The sequences are older than the Espinhaço Super­ The upper unit is formed of fine to coarse metaelastics, group. meta-arenites with abundant cross-bedding, phyllites and b) They are situated in granite-greenstone terrains, in the schists, which were deposited in environments from delta­ sense that they show many of the features of such terrains, -front to marine, with prograding resulting from periods such as a gneiss-migrnatite environment, mobility-plasticity, of basin subsidence (Menezes Filho et ai., 1978). internal 01' peripheral oval domes wíth, sometimes, contact • The complexes of Umburanas, Brumado, Urandi-Licinio metamorphic aureoles. de Almeida, Boquira, Riacho de Santana, Salitre, Barreiro c) Their metamorphic facies is greenschist. and Jacobina are somewhat similar to the assemblages d) The volcano-sedimentary association is compatible already described, each with particularities which are a with known crustal evolution during the Archean and Lower function ofthe area ofOccurrence and ofthe predominance Proterozoic. of some lithologies over others. Revista Brasileira de Oeocténctas, Volume 12 (1-3), 1982 199

46°

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I I

FAULTS --+----If-----.J\--t--I--ISO ----- PROVINCE GEOLOGIC LIMIT

----- PROVINCE GRAVIMETRIC LIMIT

--

Figure (i - D Phancrozoic and earlv to 'midelle proterozoic ('mers:. Greenstane belt: Sctrínha: BGr('('nSfOlle beít: Rio das Velhas Lajaiete, t~:?:::]:::::::l.probab'e greenstone belts andfor magmatic-sedimeruarv sequences: I) Capim: 2) Jocoblna .. 3) Salitre: 4) Barrçlro, -,:.:..•:.:.:. .Cotonn; 5) Riacho de San/ana: (i) Boquira: 7) Urandifl.icinio de Almeida: 8) BrumadoiSerru das Éguas,' 9) Umburanas: lO) Guajeru; I/) Contendas/Mirante,' 12) Bote-pé, 13) Ca'\'~1 Nova: 14) fbÔ; 1,5) Minas SupergrouplSão JoÜo dei Rei Group L(~fÚ;ele Formatíon ffi~G/'{/l1ile­ -greenstone terraíns, D High grade tnotomorphic terrains 1-8-88 - Blt;~IÔ;-E-CAl 200 Revista Brasileira de Geocíências, Volume 12 (l-3), 1982 "II'

As was described in the case of the Contendas-Mironte ln the high-grade rocks, magmato-sedimentary sequences complex, in the Brumado, Barreiro and Salitre complexes are common and preserved as remnants in narrow belts, intracratonic platform basins developed, with thick elastic Thus, they appear tç represent subcrustal zones, brought and chemical sequences, and some volcanic contributions, to the surface by tectonic-orogenic means. deposited over a basal rnagmatic-sedimentary unit, repeating Theparagcneses represented in the high-grade terrains are: the eontrast between upper and lower units. Although the quartz-orthoclase-perthite and antiperthite, rnicrocline-pla­ upper sequenees to the east of'Bahia State have larger elastie gioclase (oligoclase to labradorite), orthopyroxene, clinopy­ components", to the west chernical eomponents tend to roxene, garnet, hornblende, biotite, sillirnanite and cordie­ predominate, with thíck carbonatic and iron formation se­ rite. Biotite and hornblende can be either primary, or along quences, which sometimes contain thick magnesite deposits with bastite, chlorite and white mica, represent the products (Serra das Éguas and Colomi Groups). of retrograde metarnorphism. ln this latter case, similarities with sequenees ofthe Dhar­ The compositions are syenitic, granític, granodioritic and war region in southern India as described by Naqvi, Rao tonalitic, which under the influence of the high-grade, pro­ and Narain (1978) are noted, whilc for the former cases an duce charnockites, charnoenderbites, enderbites, leptynites, analogy with the thick elastie sequenees of the Moodies pyroxene granulites, hornblende-garnet-pyroxene granulites Formation of the Barberton greenstone belt of South Africa and hornblende granulites the textures of whieh vary from (Anhaeusser, 1971) are suggested. isotropic to oriented, massive 01' foliated, in large part with Given the difficulties faeed in establishing geotectonic rnigmatitic features, in which schlieren, nebulitic, folded, models for the sequenees in qucstion, it is believed that the ophthalmitic and stromatitic structures are prominent. sub-division established by Naqvi, Rao and Narain (op. cit) Assoeiated with this assemblage, basic-ultrabasic parts, ean be used, and they ean be eonsidered as being "pseudo­ represented by amphibolites, gabbro-norites, anorthosites, -greenstone belts", or rather, "greenstone belt-like geosyn­ pyroxenites and serpentinites oceur as enclaves, lenses, thin e1inal piles". horizons, wíth or without transitional contacts, Many of As well as the sequenees previously deseribed, there exist the clemcnts are differentiated intrusive products with loeal­ lithostratigraphic units with wide area of exposure and ized Fe-Ti-V or Fe-Ti mineratizations. Others eould be morphological prornincncc which allow them to be char­ correlated, with some doubts, with the original simatic erust, acterized. Situatcd within granite-grcenstonc tcrrains, their according to the geochronological data. essential featurcs are a lithostratigraphic assemblage in high Many continuous Dr diseontinuous belts 01' metasedimen­ greenschist or amphibolite facies represented by chlorite­ tary rocks are rccognizcd, and volcano-scdimentary se­ -muscovite - biotite - garnet - plagioclase - staurolite - kyanite quences suspected, through the cxistcnce ofassociations such schists, lenses of marble, quartzites wíth thín conglomeratic as arnphibolite, metabasite, quartzite and/or metachert, leveis, which suffered intense granitization, migmatizauon calc-silicates and banded iron formations with magnetite/he­ and pegmatization. matite, rnanganiferous formations and graphitic schists, They are defined loeally as the Casa Nova, and Ihó Groups, ln the area of the Jequié eomplex and the Atlantic belt, and the Bale-pé unit, There. have been various attempts to petraehemical analysis show that Ihcre are important differ­ correlate them to the lower or upper units of the pseudo­ ences 01' behaviour, with 011 one hand, igneous tendences -greenstone belts (Pedreira and Marinho, 1981), or as units with a predominance of granitic components, and on the eontemporary with the greenstones, but laid down in plat­ other, a metasedimentary tendcncy with a large basic form environments (Lima et 01., 1981) or as platform units component (Sighinolfi, 1970, 1971l. related to the granite-greenstone terrains but of obseure Enrichmcnt of trace elements such as Rb, Y, Ba, REE, eonnotation (Mascarenhas, 1979) or even are eorrelated with etc. in comparison with normal granites, and non depletion the Minas Group ofthe Quadrilátero Ferrífero ofthe Lower 01' others, such as Cr, V, etc., considered mobile during gran­ Preeambrian (Jordan, 1973). More reeently, Mascarenhas ulitc facies metamorphism, lead Sighinolfi et al., (1981) et al. (1982) related them to the schísts belts deseribed in the to consider that regional metarnorphism oeeurred after the Canadian shield, following the ideas elaborated by Pettijohn granulite rnetamorphism. Another possibility is that there (1972). must be doubts about the eommonly used criteria for the distribution of heat-producing elements with depth (Sighi­ High-grade terrains These are charactcrized by rocks nolfi et 01., 1981). 01' the charnockitc suite, granulitcs and migmatites whicn ln the Curaçá river valley, a metamorphic sequence in extend as a contimious belt along the Atlantic coast 01' the high to medium grade was described (Barbosa el 01., 1964, State of Bahia. Close to the parallel of Salvador, the belt 1970; Ladeira and Broekes Jr., 1969; Souza and Delgado, bifurcates into the western branch, which continues almost 1975) which, according to Lindenmayer (1981) "is compos­ to the São FranciscQ river,and the eastem branch, which con­ ed'01' a gneissie basemcnt of granodioritic to quartz-dioritic tinues along the Atlantic coast in a northeastern dircction. compositíon, with tonalitic eomponents predominating, and Other lesser occurrences ofhigh-grade metamorphie rocks . gabbroic intercalations. This underlies a sequenee composed are found in restricted areas within lhe granite-greenslone of fine elastic sedimcnts at the base and chemieal sediments terrains, to the cast ofthe Rio Capim complex, in the Bruma­ at the top, similar to those of Archean platform sequenees do-Ibitira region. and in the Sanla Isabel complex of the as visualised by Sutton (1973) and Windley (1977). Within Guanambi region. lhe supraerustals, subconcordant differentiated intrusions (hypcrsthenites-norites-gabbros-anorthosites, and norite­ -gabbro-anorthosite) and sills oceur, whieh are mineralized with eopper, and were formed by the fraetional erystalliza­ '" ln the specific case of lhe Jacobina Group (which should not tion of a tholeiitie basalt magma". be confused with the Jacobina camp/ex), the fealures are similar to Based on petroehemieal data, Figueiredo (I 980) concluded those of Witwatersrand, as shown by Sims (1977) that the region of the Curaçá river valley is the lower part Revista Brasileira de Oeociências, Volume 12 (l·3), 1982 201 of a sequence which continues to the greenstone belt of GEOCHRONOLOGY Analysis ofthe geochronological Serrinha. This situation is similar to the model developed data available for Bahia State (Távora et ai., 1967; Cordani for the Botswana region of souihern Africa by Key et ai. et al., 1969; Cordani and Iyer, 1979; Jardim de Sá et al., (1976), who supported the hypothesis of the occurrence of 1976; Agreements SME/lO. USP-CPOeo, 1978 to 1982; thick meta-arkoses and magmato-sedimentary sequences at and others) shows the existence ofspecific periods ofabsolu­ the base of the Vurnba, Tati, Matsitama and Maítengwe te ages, as follows: >3,500 Ma (Lower Archean); 3,500 to greenstone belts. 3,000 Ma (Middle Archean); 3,000 to 2,700 Ma (Upper On the other hand, taking into consideration the contact Archean): 2,700 to 1,800 Ma (Lower Proterozoic); and relations between the granite-greenstone terrains and those 1,100 to 450 Ma (Upper Proterozoic to Lower Paleozoic). ofhigh-grade, and the regional deforrnation and geochrono­ As observed in Figs, 7, 8 and 9, the Rb/Sr isochrons, in logical information avai1able, Mascarenhas (1979) and Mas­ general, can be criticized for the srnall number ofdata points, carenhas et ai. (1982) assumed a vertical rnetamorphic gra­ The isochrons obtained are, however, systematic and should dation from granulite to greenschist facies. therefore have the sarne significance as is given to isochron Identical conclusions were achieved by Figueiredo (1980) ages ín general terms, . for the northeastern region of Bahia.

Sr B7/Sr86 MUTUípE CONTENDAS / MIRANTE 2.00 0.80

1,75 1.50 1.25 Q.70+--.--,----.---- • 1.00 I=3.16'O±60 m.a9 2 3 0.75 •• 0.70 RI=0.711±0.0040 MSWD = 3.3 I=3.180±191 I'l.a. N=7 N=9 r=l RI=0.704±0.003 10 20 30 / N=6 r=1.00 101 MSWD=2.3 O.70-'------r-----r--- 2 (o I

0.800· 0.7'2. • SETE VOLTAS TONALITE / 0.70 ..x----r---.,--- '.0 2.0 e/ B"t MUTUípElIPIAIJ... / '.,~/ 1.00 . / / // I=3,018±31.9 m.a. '/ 0.750 RI=0.7017±0.0024 0/ ~/ N=4 0.900 :' /. ~ /I/l' A=I=3.S00 mva , MSWD=2,3452 //// RI=O. 7015 f B=I=2,870 m.a. 0800 RI=O.7ü4 ._- /.; C=I=2,700 m.a. ~ >~ I RI=O.706 / 0.100 i 0.700 - , 00 2.50 5.00 7.50 0.5 ( e ) (b)

LAGOA DO MORRO/ANAGÉ TONALITE BOA VISTA TONALITE ...... liEGMATITE ..... ~~DIKE Ri:õ:~gg 0.80 . 0,800 /' mv a • / / • ••)V N=4 MSWD=O,4780 • I=3,145±100 mva , I=3,433±61 m.a. 0;150 RI=0.7023±0.0021 0.75 RI=0.7006±0.0008 • N=6 / • MSWD=1.6457 0.70 L __--.,-__-, _ 0.700 -1'.---,----,-----,,------2 2 3 101 If) Figure> lA: - RblSr isochronic reference ages for rocks of Ear/y to Middle Archean 202 Revista Brasileira de Geocíências, Volume 12 (1·3), 1982

Sr87/Sr 86 SUL DE V. DA. CONQUISTA LARGO/M.NOVO/SANTA ROSA 1=3,lOO±72 mva, / RI=0.702±0.002 ~. 1.00 N=7(±3) ~" 0,90 r=1.00 AG '1.-' 0.90 MSWD=2,5 /: '/ 1=2,700 n;e, 0.80 0.80 RI=0.706

0.70..L.----.,-----,-- 87 86 5 10 Rb /Sr (o) 0.70 -'--,--,,---r---,---,--- 2 3 4 5 (e) RIO CAPIM TüNALITE 0,80 1=3,120 mva • RI=0.702 • ARACATU /BRUMAOO ~ -' / (GAVIAO BLO// 0,75 PEGMATITE VEIN (FELDSPAR) 0.80 .>: • /./ 1=3,100 m s a , 1=2,700 m, a. ~ RI=0.704 RI=0.70S 0.70' 0.70-'------,-----+------,------3 0.5 1.0 1.5 (b ) (d)

Figure 78 - Rh Sr isochrnnic rejercnce ages [or rock:s afkne 10 Middle Archean, Brito Neves et al.• 1980: Fig. 1/Aa, d; llRa,lb, c. SME//G. USP-CPGeo, (/979/8/); Fig. lIA, b, c. e, f; IIBd Thc valucs larger than 3.500 Ma are found in small, isola-­ cd nuclci 01' charnockitic and endcrbitic compositions in 8 7 25 Si ; <:",,86 JEQU1É BLOcK high-grade metarnorphic rocks of the Jequíé granulitic com­ 1"'2,700:1:60 m..a, plex, and, within the graníte-greenstone terrains, in tonali­ Kl=0.707"0.0014 tic-granodioritic bodies with the features of mantled gneiss N=12 (+3) domes, in the Contendas-Mirante complex (Fig. 7Aa, c). r"'1.00 • These are the oldest rocks known in South Amcrica, .'ISlm= 2.40 • The possibility that rocks older than 3.500 Ma are present is seen in Fig. 7Aa. where the result for a gabbro-norite falis well above the isochrone, 1.5 The values situated in the interval 3,500-3.000 Ma al- ihough representative of small nuclei, are rather widely dis­ tributed in the granite-greenstone terrains, as is seen from 1.0

Figs. 7Ad. e andf; 7Ba. b, c. d).lln thelhigh-grade terrains, Rb 87/Sr86 they were found as well in the region of Mutuipe (Brito Ne­ o 1.0 2.0 ves et a/.. 1980) (Fig. 7Aa) with a very high initial ratio (o I (R o=0.711 ±0.00040; MSWD=3,3; N=9). Values in the interval 3,000-2,700 Ma occur over wider A regions, well represented in the Jequié complex (the type area JEQUIE' B for the Jequié geotectonic cycle), and in the Brumado-Anajé region (Figs. 8A, B, e). A=I=2,810 rnc a , Ages in the interval 2,700-1.800 Ma are distributed RI=0.706 /,..;/,..::) throughout the entire São Francisco province. thus showing B=1=2,SlO rnva • / the grcat influence of the Transarnazônico geotectonic cycle RI=0.704 /,/ in the State 01' Bahia, Intermediate values of 2,500, 2,400, 2,300 Ma etc. lack 0.7"0 a systematic distribution which would allow a specific phase or geotectonic cycle to be defined, and therefore these ages are believied to be the result of incomplete isotopic rehomo­ genization ofthe Rb/Sr systern. On the other hand, the great concentration ofvalues around 2.000 Ma (Figs. 9A, B) shows OJO) i this to be the age of the main phase of the Transamazônico 1.0 2.0 3.0 cycle. (b) Revista Brasileira de Oeocíêncías, Volume 12 (1-3), 1982 203

Figure 8A - (continuation) BOA NOVA/ DARIO MEIRA JEQUI( / MANOEL VITORINO 080

1.10 015

0.90 .. 0.10 --1":..,-,-.-,--,--,--,-,----­ O 2.0 4.0 6.0 8.0 (, I

• • FIRMINO ALVES ITAPETINGA

0.80 .' 1=2,690 m.a. RI=0.708

0.70·t--,-,--,-,--,--r­ 0.0 2.0 4.0 6.0 0.70 -f------,------,--­ (fi O 1.0 2.0

(o I Figure 8A - RbjSr isochronic reference ages for roces oflate Archean (Jequié Cycle). Brito Neves, et al., 1980: (a). SME/IG. USPwCPGeo, 1980/82: 12A (b, c. d, e. f)

Sr 87/SrS6 JITAÚNA / JEQUIE'

1.00 1=2,700 m..e , EAST OF IPIAÚ RI=0.70S y 0,750 1=2,705 m.a. • RI=0.7015

x • y=X=Il =2,891 m.a. 0.725 • RIl =0.7015 0.90

O.700+---_.-~-,---_.-­ O 0040 OBO 1.20 (c)

ITABUNA BLOCKSOUTH OF SALVADOR AND ADJACENT AREAS / r- -- •. \ / :: / (.',0;/ v- _ 1,00 / c '\ .> ,'l.v /Í'2lL:~"/ • -: ',0 ./// L· -: L/ d óV' 0,8 ,ç;< /' 0,7 O 6 12 1.0 2.0 3.0 4.0 (di 204 Revista Brasileira de Geociêncías, Volume 12 (1·3), 1982

Figure 88 - ícontínuationv-

SANTA ROSA/CARArnA 1 / /2 2 1 "'11=2 ,550±200 fi. a. CURAÇÁ VALLEY •.,-- / / 0.90' Y RI,=0.705±0.002 / / Il=2,89Q±200 m.a. '~ N= 8 / ./ r=0.986 1 .:«: 0.85 MSWD=5.9 // /y 0.80 ~/ 0.80 // // 12.=2,160±50 mva, //2=12= 2,000 ± 70 mva , /'// RI • 0.705 ± 0.001 0.75 0.70+--,----,,---,---,----,---r ~/ O 2 4 5 6 / 0.70-F-,------,-----,--,---,---­ O 1.0 3D 5.0 7D lei

Figure 8B - Rb Sr ísochronic reference ages for rocks oflate Archean (Jequié Cycle). SMEIIG-USP - CPGeo, 1981182.' 12~ (a, b, c; Brito Neves et ai.. 1980: 128 (d. e); Líndenmaver, 1981: 128 (f}; Brito Ne.'esetal., 1980: 12C(a,b,c,e); .SMEIIG-USP-CPGeo,198081: 12C (di; Brito Neves, et al.. 1979: 12C (fi; Cordani et al., 1980: 12C (gl

Sr 87;Sr86 BERNARDA GRANITO IDE REMANSO REGION, ,/ /1/2_ .. (CONTENDAS/MIRANTE) •? LENÇÕIS CRATON / 1::::2,600 m, a. « 0.75 RI"0.705 / / . ~ 12=2,040±68 m.a. 1=2,686±98 m, a. 1.0 / RI,=0.705±0.006 RI=0.7053±0.0008 /. N =9 r=0.999 N=4 /- MSWD =10.5 MSWD=O . 2665 0.71 0.7 ~--...,-----...,-----...,-----,------= 0.70+------r------, O 5 10 15 20 Rb 87 ISr86 O 1 2 101 {d l

PARAMIRIM VALLEY

1.0

0.9 1=2,680±58 m.a. RI=0.700±0.002 0.8 N=5 r:;ol.O ST.ISABEL!GUANAMBI MSWD=O .66 1.1 COMPLEXES . 0.7 • 1.0 O 1 5 10

í b ) 1.9

SW OF JEQUIÉ BLOCK 1=2,680±83 mv.a , 1.1 SER~/ 0.8 RI=0;705±0.0015 (S.OF PÉ DE N=12 r=0.999 MSWD=B,4 1.0 0.7

0.9 O 2 3 4 5 6 7 8 le I

I=2,700±60 m s a , 0.8 RI"0.707±0.0014 N=6 0.7 O 1 5 10 lei

Figure 8e ~ Rb Sr isochranic I'eférence ages for rock» oI/a/(' Atchcan Uequié C.I'c!e) RevistaBrasileira de Geocilnclas, Volume 12 (1-3), 1982 205

Sr 87/Sr86 / 1 ITABERABA ./' METARYOLlTE / ",,7- JUREMA - TRAVESSÃO /' /2 0.90 ~ ~ 1.0 i::~ (CONI/MIRANTE) / ~ . I=2,450±20 m.a. 1I=2 ,350±173m. a.;:;: Y 0.9 r? RI=0.714±0.0004 RI=0.738±0.00&é/' e N=4r=099 N'*9 r l . 0 0 MSWD=0.58 1''"2.800 m , a. 0.8 MSWD=0.0008 0.80 RI,=0.716 1,=2,000 m c a , 0.7 RI,"0.750

O 1 5 (o I 0.70 -j---,---,--,.---,---r­ O 3 4 5 (e I

.-' '. ARACATU I BRUMADO /' /2 (GAVIÃO BLOCK) ~ ~3 /' // 1.0- »<>: Il~2,600 m.a/Y...K: RII"0.707 ~~ / UMBURANAS 1.300 (ACIDIC EFUSlVES) ~/2 ~~ 1,"1,800 m.a. /;:::0 RI,"0.75.0 1,=2,940 m.a. / 0.8 ~/ x =2,000 m s a , 1,150 RI,=0.706 /' 0.7 /' O 5 10 ;/ (bl 1.000 12=2,420 m.a. • RI2=0.730 (r e r e r e n ce ' 0.850 CARA SUJA SYENITE • 0.50 0.700+----,----.----.-_ 0.85 • O 5 10 15 (fi

0.80 1=1,850±60 m.a. RI=0.709±0.0016 N=9 r=0.997 0.75 MSWD=4,7B

0.70 -/-----,------.-_ O 5 10 (c) RIOGAYiÃO~HYIj.ITE 1 2 I (CONT. IMIRANTE) / I,=2,230m.e . II RII=0.705 I,. STA. ISABEL J GUANAMBI 0.800 COMPLEX 0/ COMPLEX t ~ 0.90 '/ 12=1,960 m v a , teii f ' RI,=0.710 'Y 0.750 I=1,970±50 m.a. 0.80 RI=0.705±0.001 .;'" N=7 r=0.9996

0.70 -f!-----,------,-­ 0.700+----.-----.-­ O 5 10 O 2 4 (di 1'1

Figure 9A - RbjSr isochronic reference ages for rocks 01ear/y Proterozoic Brito Neves et al., 1980: (a, b, c, d, e, f); SME IG. USP- CPGeo 191J(): (8) 206 Revista Brasileira de Geocíêncuu, Volume 12 (1-3), 1982

Sr 87/Sr 86 InúBA SYENITE /' 1.30 JACOBINA - GRANITES 0.90 Il"'2,100±50 m.a. ~ RIt=0.70S ,/ 2 N =6 ~ I=I,960±16 rru a , : ~ RI=0.708±0.00IS /. 1.l0~ N=12 rol. 00 0.80 -: MSWO=0.72 ,/ • ~ 12"'1,SOO±5 m c a • RI,-0.7049±0.0004 0.90 (rs oc . Mineral)

3 6 te I

0.70 -f'::'::"--~-----,----,--­ SERRINHA COMPLEX O 5 10 15 (ACIDIC EFUSIVES lo)

0.76 0.900 LOWER JACOBINA (I!ZTO.+ MIGMAT.) 1 -: /' 1J=2,300±60 mc a , A ...... 2 0.74 RI'=0.7066 0.0017 ,/ /' • N=8 r=0.998 . /' I=2.0S0±90 m- a, 0.800 RI=0.7017±0.0014 MSWO=7.24 ~.'/ N-S r=O. 998 ... <> I" = 2,000 m..e, MSWO=l.03 -z>: RI, =0.705 0.72 ~~ 0.700f::::--,---,----,---,---,-­ O 2 3 4 5 (bl 0.70-l'---~---,---_- O 0.82 SALVI\DOR /FEIRA/CAPIM GROSSO (GRANULITE+MIGMATITE) MANSIDÃO 0.75- m- a , ~ (MIG + GRANITE) 0.78 12:::2,000±70 RI ,'0. 710 .-.:: .-- J 0.74 N= 6 ..--.: 0.74 .....---=-:J I "'2 , OOO±70 rru a , .---;j ---- 1 • ~.-<"i N =21 RI,=0.70S±0.001 0.72 • 1=2,016 m.a. r =0.991 MSWD =l.31 RI=0.7033

O 2 25 0.70+----,---~--_-­ te I O 0.5 1.5

106 SERRINHA BLOCK(CRATON) 1.00 CORRENTINA 0.98 (HIG.+GRANITE) (MIG.+GNEIS)

0.90- 0.90 I"'2,090±27 rns a • RI=0.704±0.0008 1=2,000 m.a. 0.82 N=13(6+7) r=l.OO 0.80 RI=0.704 MSWD= 1,8 0.74 0.70•.,iI!L--,-----,----,- 0.70-1"=---,-----,-­ O 4 8 12 O 4 8 Ihl (d)

Figure 9B-Rb(Sr isochronic reference ages for rock:s of Early Proterozok: Brito Neves et al.. 1980: ,(a, b, c, d, e, O; SME/IG USP - CPtieo, 1981: (g, h)

This cycle, through its wide-spread occurrence, causes se­ senta tive of the Archean to Lower Proterozoic in Bahia is rious difficulties in the separation, in time and space, of the presented, ln this, it can be seen that lhe location oftheolder geological processes which acted until the end of the Lower nuclei does not follow a pattern of decreasing ages, which Proterozoic. It is not possible to separate the particular would result from evolution by accretion, and it is therefore events which must have occurred since the Archean, and more realistic to consider the oldest arcas simply as having it is therefore impossible to avoid speculation. been preserved from the continuing deformation in, and ln Fig. 10. a synthesis ofthe geochronological data repre- additions to, an already extensive crust. Revi!/Q Brasileira de Oeocténcías, Volume 12 (l~3), 1982 207

42°

\ ,;i ".

o 400 km

Figure 10 - Actua! representation 0/ Archean regíons, nuc!eí and fragmente ln lhe São Francisco provincecfrom RblSr isochronic data Figure II - Domínant folíatíon dírectíon pattern in lhe São Fran­ o >3,5 b.y, .. ~ ,;;;3i! b.v. "1I'''<;1~i) lo 2.7 b.v., 1:-::112,7 /O cisco provínce, in gneíss, migmatites anel granulítíc rocks 2,5 b.v., D. Phanerozoic and Early to Middle Proterozoic ('0­ f:::;::::::::;::::::::~ CO\ws vers, and Pre-Espinhaço assemblage stabííized duríng lhe Transa­ (:;·~;;::/;.ja Gneíss, migmatítíc and granulltic rocks mazônico Province límits: _J__ ~....L rectonic: a) Covercd: b) Cvcle...... Thrust [ault Uncertain: lc] Probab/e. ~. __ .,!Gral'imnric ---- Stríke-slíp andtor ohlique [auhs (Generally associares my­ lonitic zones): I - Ptanalto-Ponraguá fault : 2 - Poções-Itororá fault On the other hand, assuming (Brito Neves et at., 1980) (1,2 - ltapebí ttneomentv. 3 - Maracàs foult "4 - Conquista plateau that changes of'Rb/Sr ages only occur in open physical-chern­ [ault .. 5 - Jacobino fault SH'arm .. 6 - ttacamblra fàult .. 7 - Caetíté ical systerns, which .allow lasses or gains af material. and fault ; 8 - Monte Santo mylonitic zone; 9 -Itabuna shear zone taking into consideration the fact that the observed ages Geologic provínce timits occur practically every 100Ma (with ages 2,000Ma, 2,100, _._-- Gravímetric 2,300, etc., to 3,500 Ma with intervals which represent the peaks of the geotectonic cycles) it is reasonable to accept confirm the existence of these areas, where the sialic crust that the crustaI evolution was a continuous process in ao is thicker (Morta. in Mascarenhas et al., 1982; Davino, 1980). extremely active crust. ln the eastern part of the State. they appear to control, or were controlled by, the formation ofstructures ofmobile-belt GEOTECTONIC ELEMENTS The structural outline type, and this could also be true of the area represented by of Bahia shows that the regional foliation directions possess the Santa Isabel cornplex between Urandi-Guanambi, which divergences, and sinuosities and curves, apparently defining separated the Remanso craton from that of Guanambi, local "barriers" which behave more rigidly during the de for­ although more detailed study is necessary. mations (Fig. 11). These more rigid areas were defined by According to Almeida (1979), both the Jequié and Serri­ Mascarenhas et ai. (1976, 1979) as proto-cratons, anil lhe nha cratons participated in the processes which led to the various examples are Jequié, Aracatu, Medina, Serrinha formation of the Coastal mibile belt at about 2,700 Ma, in and Gavião-Riachão do Jacuípe-Ipecaetá (Marinho, in Sei­ which there occurs the majority of the high-grade melamor­ xas et ai., 1975). phic rocks of Bahia. These protocratonic arcas are interpreted as being Arche­ To Mascarenhas (1979), at about 2,700 Ma the Jequié ao nuclei which, nevertheless, were not completely immune craton was an extension of the Remanso craton, and the to mig-magmatic processes and polycyclic deformation, Coastal mobile belt skirted the Serrínha craton with its while retaining the older tectoníc configuration. At present, western (in the direction of the Curaçá river) and eastern they are termed the Serrinha, Remanso, Guanarnbi and (aiong the Atlantic coast, starting as Salvador) branches. Medína cratons. Wíth the exception of the Medina craton, As was demonstrated by the geochronological data, Tran­ where the data are sparse, the gravimetric interpretations samazônico Rh/SI' vaJues are pratically inexistent in the 208 Revista Brasileira de Geocíêncías, Volume 12 (1-3), 1982

EAST DF IPIRO bZ-FOLO AXIS \.11TH STEEP OlP TO SOUTH BA~ GANOU QUARRV SI/SOl/I SLACK ZONES '" IC (8 5Km SOUTH GANOU) •• QUAflTZlTE I / GRANULlTES

V VERTlei SECTlON' c-v I /1 1~)/50 I 1!l1 /II QUARTZ VE1N SUBCONCOR- \\ IlIlt!1 OANT WITH St TRANSPOSEO NCLlNE I( I( I," I I OURING F2 PLANE U II NI 11\ 11111)1 VERTlCAL SECTION } I 1t 11 II, I • / I I II I o 50 N I / II' \ II I' J '-----' cm I I II ! 1.1 I / 1\ /11 I / 11\\' I, JI PAj (TRACE) I , I I II 1. I)) I ,I II /'J II (e) A REoueEO VERTICAL S, 52 \ II) J I I \11/1 II (A.C M.6.) ARE lAL PHOTOGIIAPH S2 Ib) {A. C./'1. B

EAST OF IPIÃll ~ '-.') A.P.l GANOU QUARRY 51/S ,.' . 2/ o GANDU QUARRY ,,' / \.,',\ , , S2/S 1 /( ) VElN '","" , s,/so ~ s, 'I ", 2 II' I I/ • ,. . \:~\, {'/ o 30cm \ \\ I' /, ~ .~%\:~~

[{1ST DF IPIAO

WESr o. USATA

-n, ,/I II I I FOLOEO LEPTYNITES HIGHLV TRANSPOSEO 6Y 52 'O} (A.C .M.S AT THE OUTCROP 01P OF FOLO AXES (j) VARI ES FROM 0° TO 20°- 300 N DR S (A.C .M.6.

OUTCROP SURFACE

Figure 12 - Folding stvtes and deformatíons on high-grade rocks of Jequié Complex (J.C.) and Atíantíc Coast Mobiíe belt (A,C.M.B.) (Drawing based on photographs of outcrops and on vertical aerial photographs); a) Interference panem type 3 (Ramsay, 1967) of regional character (h, e, d, e) FJ!'F2 coaxial ínterference patterns on vertical to subvertícaí sectíons with varíous degrees of deformation intensity ofSoiS I f, g, h, i,j)fo/dconfíguratíons on horizontal surfaces ofoutcrop íf, g) F) vertical phase gives transversal deformation of SoiSdS2, local/y wtth drag [olds. k) vertical section on lhe Ipiaú archean fragment (3.5 b.y. see Fig. 7Ab) showing gent/y dipping VlI'TtCAL SECTI ON 1'1 catacíastíc foliation not deforrned by F2/F) phases. Xenolith ofbasíc u.e. ) granulite in actdic granulite. Ohs: Every fo/ding is 01 similar type Revista Brasileira de Geociências, Volume 12 (1-3), 1982 209

Jequié complex but the deformations which affected the Atlantic Coastal mobile belt. It is, nevertheless, problematic Atlantic coastal region affected this cornplex as wel!. On whether the phase ofintense transposition, which also caused lhe other hand, the coastal region, and ali the central. west­ the formation ofmylonites, ultramyíonites and blastomylon­ em and northeastern arcas were very strongly subjccted to ites as well as the S2 foliation, occurred in synchrony with isotopic hornogenization during the Transamazônico, and the normal folding or whether it was later. these processes seem to have beco related to intense crustaI On the other hand, the isoclinal character, which is observ­ reworking, granitization and migmatization events. ed locally (Fig. 12) could also be related to this phase of ln lhe coastal region, and including ali central-easl Bahia, intense transposition. the Transamazônico geological activity remobilized par! of The undu1ating nature of the fold axes, which can dip the Coastal mobile belt and the pre-existing cratonic areas north or south, could be related lo a wide-spread deforma­ (Serrinha), forming the Atlantic Coast mobile belt (Masca­ tion, usually opcn but loeally tight and orthogonal to the renhas, 19790; Mascarenhas et al., 1982) during which re­ two earlier phases, which, in part, allowed the preservation lrometamorphism of the granulites ("decharnockitization" of lhe supracrustal sequences in synform keels, according according to Moutinho da Costa and Mascarenhas, 1982) to Mascarenhas (1979). occurred. Local evidence, both in the high-grade rocks and in the To this phase can be related the intensc vertical transposi­ granite-greenstone terrains, stress the patterns of transverse tions found in the cntire domain ofthe Atlantic Coast mobile deforrnation, the meaning, intensity and expression ofwhich belt, which also affected lhe Jequié cornplex. The styles of need further analysis. deformation are visualized in Fig. 12. To these geotectonic elements are related a number 01' ln this figure, an earlier folding phase is shown, the style aproximalcly N-S faults, and NW-SE fault systems which of which is similar to a "nappe de charriage" which appears produce various geotectonic blocks, and brought the high­ to be related to Archean events. ln the context ofthe Archean -grade rocks to their present positions, as can be seen in and Lower Próterozoic regions of Bahia, this phase of re­ Fig. II. These faults served, as well, as controls for the cumbent folds or folding with horizontal axes, which produc­ intracratonic basins of the Middle and Upper Proterozoic. ed the gneissic foliation S[ and in which intrafoliai folds GEOTECTONIC SKETCH The geotectonic model prc­ with N-S axes folds are observed, is always present. This scnted in Fig. 13 stands for the up to date geological knów• phase was followed by symmetrical or asymmetrical to iso­ ledge since 1979, when first elaborated by Mascarenhas. e1inal folds, locally with intense vertical transpositions, pro­ At present, the sketch ineludes the gravimetric data, as ducing a type 3 interference pattern of Rarnsay (1967) interpreted by Morta (in Mascarenhas et al.. 1982) and the (Fig. 12a). geochronological information acquired mainly by the Pro­ The intcnsities ofthe second foldingand the vertical trans­ gramme of Geological Dating of Bahia State, underlaken positions are locally variablc, and may develop or not an by the Secretariat of Mines and Energy under an agreement S2 foliation. This latter is a prominent structural element with the Geochronological Research Center of São Paulo and is considered as correlated with the formation of the University.

46" 44 Q 42° 40° I I I I ~"''-~ J, ' *~ ~\"\I"-\.. J;' ~ XA/ :;r'.~ ~~ f~~ .0" ~ { ~ --'"""'... '>,. s····· 0 00 d~: ""--'I'" "':::~i':::'.\ ",\~cy,O G~~. ~~iI~l; :'\.~::::"'%:i.i~~.,", )i1:~/x x xA<." x x x X x\"'!I";~·f'O.-~.;-';"--- LEGEND IOO-;~~ ,-;;-~ \i11 ~ x 1 ;E~A~S; x::~. .fi!:~\!'::;::~~?(~:-- ~100 Arc~e<;1n cratons{y3.000m.y.)highly r e- I/ \ ~c --<){ / . 1,(, ".~~'.SERRINHA.. ./ ~rnoblllzed at t r-ansemaaont an times _~' : ': '-:>\ 0 /1 / CRATON: '\~' '/', ::CRATON::: ~ (""2,OOOm.y.)with vol çenosedf ment arv \.."' - ', . x'x\·",' .. ··...... sequences of the early prote roaol ç ze r- .• :::::::::::;, fOY,; \ . '-,!(ESPINHAÇO/SÃO: ·\·\·\\~::,::::::,0::.j archean\l).and t rue greenstone belts ~( tz) . . );~::::::~~~:::\ E \FRANCISCO COVER'I \~\~\~~::t:::.{;:~ ",-,", Upper archean mobile belt(2.700m.y.)par- 12°F(/:::~;:::;::;:':'! \ .. ·:;x\x\::c\x\~:·l,x·ft..;!~120 ,,'::,~':::,':: tially r-emobilized in the t r ansamazonl an ~\E::::::::::::':::::;;\ ::~: ~"x\<'x~~/·>x rol , '" ::. 'x,· ·\·\·,/·,,,; orogeny <:J: , • •• .. •• ..1 E .. \".. "'<,,,x\x,x~.~ ~f§J A-Early archean nuclei(""3.500m.y.) - (::;GUANAMBJ CRATON~? .: ~x y-";",_ x\,!) \J AB 'ddl h 1 .( J 000 ) ,~II""""''''''''''':t.·t<' x x '\:::>9:" \. SALVADOR e-MI e are ean nuc e r "", m.v • ;;;1· 'S'~ / ç.-E"'",l Espinhaço rlf t ~, (:·::;::::::::::::::;.:-,x\,/ \, 8- 'CJX\x,,\1:I:y'x.\x\: I " , .J9.\ ::::::::;:::::::::I'::~\'l'\ x x'x"x\(\lxiX\I_' 14 .•••••• , ••••••••••••••• ..1\ \\A' 14 ~ .-Cl(2700) --.....' :; \'1i; x X\V;X X I C.J. Jequr e omp exe , m.v . \. l: .•...... :---<' r--->~:"" P A\ .>, \' .... "',." I" "~'\>fl X x'x IX'x) I r.::::;..:. 4~o"",,---'~"" " g ,,:\. '1' Geochronologic Pr ovl nces 1.... ,..- I "x:x\x x' ~ \ '...... JI B x x X I" XIX II 1- Jequié Province{3,OOO-2.700m.y.) ~ (: '"----y X/XIX',·1X ~160 ,41?4f...... :.:...~ Ix,rtIK~ x x "Il-Transamazônico Prov!nee(2,600-1.800m.y.) // (;4/!J~ 1,1}11 _ ~Anomalous gravimetric zone : 160 ~j ~+ MEDINA Á. /7, RIO PARDO Positivei - Negative CRATON 1\:1 BASIN 1 1\ "_--'2tl. ,}~O"" (7) • 1:;1; 1\11 BRASILIAN BELTS (~50-1.000rn.y.)LIMITS: r \;\\\jl~.' --Tectonic \ 1\;\1\1\\\/ll\ l)I __,_ Gravimetric ~ j -16" __, __ Fault .\"

Figure 13 - Geotectonic modoljor the Archean and Protero;oic in Bohia Stote, Il'irh geoc/mJl/%gic prol'inces. AdaptedFom Mascarenhas, 1979 210 Revista Brasileira de Geociêncías, Volume 12,(l-3), 1982

To lhe geological limits of lhe São Francisco cralon were a) with lhe sialic crust established during lhe Archean, added lhe gravimetric limits, which agree reasonably well lhe Jequié geotectonic cyele produced a new cruslal sub­ with lhe former. -division by forming lhe Coaslal, and possibly lhe Guanam­ A new gravimetric element, the anomalous Buritirama­ bi-Urandi mobile belts, giving nse lo lhe Serrinha, Remanso, -Vitória da Conquista be1t, was brought lo light as a resu1t Guanarnbi and, perhaps, lhe Medina cratons ; of lhe interpretations of Davino (1980) and confirmed by b) these cralonic áreas ineluded sequences ofgreenstone­ Moita (op. cil). This anomalous belt reflects lhe presence -belt type, geosynelinal accuinulations similar lo greenstone ofa crustal discontinuity, with prominent gravimetric highs be1ts, heavily granitized and migmalized, as well as baste­ to lhe exlreme NW, and gravimelri.c lows lo lhe SE, in lhe -ultrabasic and gabbro-anorthositic complexes. The geosyn­ region to the east of Macaúbas and west of Paramirim. elinal accumulatíons, with their producls of paraplatform This belt coincides with lhe limil belween the Salvador sedimentation, could have partly evolved during the Lower and São Francisco cratons, as defined by Cordani (1973). Proterozoie. AI presenl, it seems lo rel1ecllhe lirnit between lhe Guanam­ c) the Transamazônico geoteetonic cycle, with a similar bi and Remanso cratons, even though lhe possible Guanam­ intensity (O that of Jequié, remobilized lhe larger part of bi-Urandi mobile belt, defined in a prelirninary fashion by the crusl in Bahia State, through deformation, granitization Mascarenhas (1979), is situated to the easl oflhe anomalous and migmatization. Loeally the intensity was sufficient to gravity be1t. rework lhe rocks ofthe Coaslal mobile belt and lhe cratonic . If this be1t represenls Archean and/cr Lower Proterozoic areas, establishing lhe Atlantic Coastal mobile belt, elements, activated during lhe Brasiliano ; if it is simply an There are insufficient criteria yet available to evaluate element formed at lhe end of lhe Precambrian by the colli­ whether lhe Brasiliano marginal fold belts around lhe São sion of lwo paleoplates or during the evolutions of a mobile Francisco craton were established in Archean or Lower belt (Cordani, 1973, 1978); or if it was produced during Proterozoic mobile belts, although this idea was proposed the evolution of the Espinhaço Supergroup, as was suggesl­ by Mascarenhas (1979) on lhe basis of lhe polycyclicity ed by Jardim de Sá et aI. (1976), Moutinho da Costa and concepl of formation oiimobile belts (Anhaeusser et 01., Silva (1980) and Moutinho da Costa and Inda (1982); or, 1969; Krõner, 1977). ifít evolved during the Brasiliano cyele as a result ofjostling of crustal blocks (Torquato and Kishida, in Pedreira et aI., MINERALlZATION Mosl of'the mineral deposíts in lhe 1978)are suggestions for which it is not possible to establish State of Bahia occur within lhe Archean and Lower Prole­ any conclusive ehoiee. rozoie terrains. Many of these deposits 'aré' intimately con­ . With reference lo the mobile belts, at present dou bts re­ nected with lhe origin and evolution of lhe hosl rocks. main as to lhe preferred model ofevolution, with two possi­ The deposits of greater economic importance are those of bilities to be considered (Mascarenhas et al., 1982): gold, chromium, copper, magnesite, lead-zinc, rale, manga­ a) lhe Jequié complex did nol form during the Upper ncse. iron-titanium-vanadium and uraniurn. Smaller depo­ Arehean as a eonsequenee of structural processes resulting sits of'barite, asbestos, beryl (ernerald ), rnolybdenite and ver­ in lhe forrnation of mobile belts, but is only a highly-eroded miculite are also found within lhe domains of lhe Archean continuation of the Remanso Craton; and Lower Proterozoic terrains. The most important min­ b) the Jequié complex formed as part of a mobile be1t at erai deposits of the State of Bahia are rclaled and localized about 2,700 Ma, forrning part of lhe Coaslal mobile belt of in Table I and Fig. 14. Almeida (1979), and preserved from the processes of remo­ bilization during lhe Transarnazônico (2,000 Ma) which gave Gold The largest deposils of this element are in the dis­ rise lo lhe Atlantic Coastal mobile belt (Mascarenhas, 1979a). tricts of lhe river Itapicuru and lhe Jacobina hills, lhe latter Present doubts concern two items: a traditional gold producer in the State, lhe working of a) lhe age of the event which produced lhe penetrative which commenced in lhe mid-Xv'lflth. century. foliations and vertical transpositions which affeet extensive­ ln lhe !tapicuru river region occurs lhe greenstone be1t Iy both lhe Jequié complex, and the Atlantic Coastal be1t of the sarne name, where, during lhe last decade, lhe first up lo the Curaçá river valley. They could either belong lo signs of gold rnineralization were discovered, leading lo lhe a final event within lhe Jequié cyele, or could have generaled finding of lhe gold-bearing body within lhe so called Faixa during lhe Transarnazônico, in viewofthe following factors: Weber. The main rnineralizations are of slralabound type, • They exisl in both regions. but it is still not possible to contained in two principal leveis, parallel and folded, and define a specific Transarnazônico deformative event, the composed of chloritic schists and subordinare quartz-felds­ characteristics of which are differenl from lhose which pathic breccia and irregular quartz-carbonate segregations afTected lhe Jequié complex . (Teixeira, 1981).The mosl important hosl-rock for lhe miner­ • The geochronological interprelalion for the areà of the alization, ealled the magneric schisr, is composed ofchlorite, Atlanlic coastal be1t (Brito Neves el aI., 1980; Moulinho quartz, carbonates and plagioelase, wilh large quanlities of da CosIa and Mascarenhas, 1982) assumes the presence magnetite and several accessory minerais, such as epidote, of Archean rocks, remobilized during the Transamazóni• biotile, apatile, arsehopyrile, pyrile, ilmenile, pyrrhotite and co, the' remobilization being intrinsically related to gran­ gold. Of lesser importance, the gold occurs disseminated in itizing processes, such as injeetion, migmatization, and quartz-carbonate masses, in pegmatite veins, in recrystalliz­ metasomatísm. cd chert beds, and in zones of silicification along fractures b) the Rb/Sr isochron ages may not have been afTecled and faults (Teixeira, op. cil.). The economic ore bodies are by lhe deformational or melamorphic processes, iflhe phys­ always related to zones of breccias, myloniles and silicifi­ ico~chemical system remained c1osed, i.e., if there were cation within lhe mineralized portion. Aparl from lithologi­ no gains or losses of either Rb and/or Sr. cal and slrucloral conlrols, lhe mineralizalion is slratigraph­ Under these cireunstances, the geotectonic outline of ical1y c.onlrolled, since magnelic schisl is limiled lo two Bahia Slale can be synlhesized as follows: leveis, localized at the top and lhe bollom of a complex maf- Revista Brasileira de Geociências, Volume 12 (1·3), 1982 211

Table I - Major f!1ineral deposíts in lhe Archean and Lower-Proterozoic terraíns of Bahia State

a) Mineral deposits in predominantly sedimentary complexes

Mineralization Host Rock

Au Magnetic chlorite schist of the Itapicuru -greenstone belt Pb, Zn (Ag) Magnetite-bearing amphibolite of the Boquira eomplex Mn (Fc) Carbonatie metasediments of the Urandi-Licínio de Almeida complex Mn Phyllites of the Jacobina complex

b) Mineral deposits in volcano-sedimentary complexes

Mineralization Host Rock

Au Metaconglomerates and quartzitos of the Jacobfnaltjaoup Magnesite, Tale Metacarbonates of the serra das Éguas and Colomi groups

c) Mineral deposits in high grade rnetamorphic terrains

Mineralization Host Rock

Fc-Ti-V Gabbro-norite-anorthositic intrusions Cu (Au} Pyroxeni te-gabbro-noritic intrusions Cr Peridotite-pyroxenite-gabbroic layered intrusions Mn Basic granulites with supergenic alteration --_._------(Ag, Fe, Au): Minor quantities and by-products

(Sims, 1977). The principal mineralizations are localized in this part, distributed in at leasl four leveis. The gold occurs in the free form within the matrix of the conglomerates, usually accompanicd by recrystallized pyrite and, in some leveis, uranium rninerals. The primary mineralization appears to have been synge­ netic, although subsequent remobilization and redistribu­ tion occurred as a result of metamorphic and supergene processes. Mineralizations within discordant quartz veins, along fracture surfaces and within mafic-ultramafic intru­ sions which cut the sequence are examples of the remobili­ zation. Similar gold-bearing conglomerates are found in the Witwatersrand, South Africa, especially the Elsburg bodies. Magnesite and tale ln thc ccntre-south ofBahia, prin­ cipally in the region of the Serra das Éguas, one of the largest magnesite deposits in the world is found. It forms I 46" part of a folded sequence of rnetasedimcnts, which include mctacarbonates, metapelites, metarenites and iron forma­ tions in the lower part, passing gradually to calc-silicates and mctabasites. The magnesite deposits form beds and concordant lenses, with thicknesses of up to hundreds of meters, intimately associated with dolomitic marbles. The genesis of these has been discussed in terms of, either, direct precipitation of magnesite and calcium-magnesium carbonate undcr ocean­ Figure 14 - Geologicsketch map showíng lhedistvtbutions of Archean ic conditions, or as the result of progressivo metasomatism and Lower-Proterozoíc terrains and the major mineral deposits of of dolomites by magnesian hydrothermal solutions. ln the Ballia Stare. latter case, the source of the magnesium would be formed through retrometamorphism of the basic volcanics which ic sequence, in contact with carbonaceous, pelitic and chem­ occur at the base of the sequence (Moraes et ai., 1980). ical sedimcnts. Several talc deposits are associated with the magnesite The Jacobina includes a sequence 01' mctaconglomeratcs, of Serra das Éguas, and many of thern are economically metarenites and rnetapelites which were deposited in a Lower viable. The origin of these bodies has been attributed to Proterozoic basin established in the granite-greenstone base­ processes of retrometamorphism of magnesite, with addi­ ment. At present. the sequence has a monoclinal structure. tion ofsilica and water, or to hydrothermal alteration, stea­ The lower part of the sequence is composed of conglome­ tization of ultrabasic rocks (Moraes et ai., op. cil.). rates and quartzites which were deposited in a Iluvial palco­ ln the northern part of the state, in the municipaJity of system, the current direction of which was east to west Sento Sé, metasedimentary sequences ofthe Colomi Group, 212 Revista Brasileira de Geociêncías, Volume 12 (1-3), 1982 correlalable with those of Serra das Éguas occur, and also cornposed of exsolved ílmenite-hematite grains ando contain thick stratiform magnesite bodies. The ore is mas­ magnetite, with a gangue of feldspar and mica. sive, with thin concordant lenses of chlorite-talc schists and The hosl rocks are well-deformed and relrometamorphos­ fine lale veios filling fraclures. ln this region, lhe iron for­ ed. ln spite of the tectonic and metamorphic modifications, mations are quite prominent and in some places form-Ienses the mineralizations preserve textural and structural features of compact hernatite. which indicate an origin by rnagmatic segregation. These The stratiform character of lhe magnesite deposits, their deposits presenl features intermediate between those of AI­ great thickness and the nature of the associated rocks are lard Lake and of Sanford Lake (Soares et ai., 1977). strong arguments in favour of a purely sedimentary origino ln the south and centre-south of Bahia, various iron-tita­ nium-vanadium deposits are known, and are ín the explo­ Lead and zinc The Boquira cornplex is an association of ralory phase. mctamorphozed volcano-sedimentary rocks, and contains one of lhe most importam lead-zinc deposits of Brazi!. The host rock lo the ore is a magnetíte-bearing amphibolile, Manganese Somedozens ofsmall and medium mangane­ with carbonate and siliceous leveis. The mineralization is se deposits occur in Bahia State, concentrated mainly in the vein-Iike and, above ali, is concordanl with lhe banding districts of Urandi-Licinio de Almeida (south-west), lhe Ja­ in lhe arnphibolites. This banding strikes north-south, with cobina hills (centre-norlh) and in lhe south of lhe State steep dips, cither to the west or to the east. Thc orebodies . (Fig. 14). ln lhe firsl two districts, lhe manganese occurs have a mean thickness of 1.5 to 2 m, and Iengths of up lo within volcano-sedimentary associations in·Iow to medium some hundreds of meters. metamorphic grade. ln lhe last district, the deposits are Two types of ore exist, oxidized and sulphide. The first hosted by granuliles. ln ali cases, surface enrichment by is composed essentially of cerussite and lesser quantities of supergene processes has formed economic ore grades, in srnithsonite, pyromorphite and anglesite. This ore has an lhe form of manganese oxides, principally psilomelane and irregular distribution, and can altain depths of up to 50 m. pyrolusite. The sulphide ore is composed of galena, sphalerite, pyrile ln the Urandi-Licínio de Almeida district , lhe manganese and small quantities of chalcopyrite. Aparl from lead and deposits are associated with a volcano-sedimentary sequence zinc, the Boquira deposits bear the recovery of silver. as which conlains hydrothermally-altered basic-ultrabasic a by-product. rocks, calc-silicates, metapelttes, mctacarbonatcs, meta­ The mineralization is stratiform, with a distinct lithologi­ cherts and iron formations. cal contrai, and is considered to be syngenetic, although Thc rnanganiferous rocks within the sequcnce are carbon­ regional metamorphism and tectonism have caused modi­ atic.sornetimes have high iron contents and are structurally fications (Espourteille and Fleischer, 1980). The source of concordant with the host rocks. A volcanogenic ôrigin for lhe mineralization is still discussed, either in purely sedimen­ lhe mineralization has been proposed (Moraes et ai.. 1980) tary terms, or in terms of a volcanogenic origino on the basis of the geological environmenl and lhe nature of lhe associaled rocks, but a purely sedimentary origin Copper The principal copper mineralizalions in lhe State should nol be discarded. are' found in lhe Curaçá river valley. Of these, lhe Caraíba ln lhe Serra de Jacobina region, lhe manganese deposits Mine deposits are lhe most prominent, being lhe second are distributed along almost ali lhe eastern side, contained largest in lhe counlry. The mineralized bodies are associaled in phylliles of lhe Jacobina volcano-sedimenlary complex. wíth mafic-ultrarnafic complexes with approxímately len­ The prímary mineralization is believed to have been deposit­ ticular shapes, folded and stretched in a north-south direc­ ed as manganese oxides and hydroxides (Mascarenhas et aí., tion. Mineralized petrographic types inciude pyroxenites 1976,1979). Nevertheless, lhe present-day ores are possibly (hypersthenites), norites, gabbro-norites. gabbros and, more of supergene origin. rarely, anorthosites. The mineralizations occur mainly as The manganese deposits of the south of Bahia are related chalcopyrite, magnetite, pyrite. bornite, pyrrhotite and to supergene alteration ofbelts wilhin the granulite complex. ilmenite. These minerais are usually disseminated, but can These belts are concordant with the regional structures, and also occur as veinlets along foliation and fracture planes. sornetimes extend for over IOkm along a NNE-SSW gen­ Massive mineralizations are restricted to the pyroxenites and eral direction. ln most cases, the host rocks are basic gran­ to fracture ~ones. The tectono-metamorphic processes which ulites, which led to an interprctation of the original rocks affected lhe rocks caused remobilizations and redistribu­ as marls, now metamorphosed in the granulite facies (Silva tions of the mineralization, and this may explain the great et al., 1980). These metamorphic conditions must have mo­ variation of copper content within lhe ore-bodies (Delgado dified considerably most of the original characteristics mak­ et aI., 1975). ing the recognition of the primary mineralization very The origin of'the mineralizations is attributed lo magrnatic difficult. The present mineralization is within secondary differentiation within layered basic intrusions, through lhe enrichment crusts composed of manganese oxides, usually formation of an immiscible sulphide liquid (Lindenmayer, accompanied by isolaled crystals and veinlets of graphite. 1980). Chromium The State of Bahia possesses almosl ali Iron-titanium-vanadium These mineralizations are as­ the known chromium reserves of our country ; they are sociated with gabbro-norite-anorthosite bodics within gneiss­ centralized in the Campo Formoso andJacurici river valley -rnigmatite and granulite terrains. The largest reserves are Districts, located in the northeast portion of" lhe state in lhe Campo Alegre de Lourdes region, in lhe extreme north (Fig. 14). of'Bahia (Fig. 14), distributed in eight deposits which totalise The Campo Formoso deposits are found in metarnor­ almost 100 million tons of ore wilh mean conlenls of phosed mafic-ultramafic rocks, nearly always changed inlo Fe = 47.40%. Ti = 21.10% alid V = 0.75%. The ores are serpentiniles and chlorililes, which are exposed aI lhe fOOI Revista Bra$ilejr~ de Geociências, Volume 12 (1-3), 1982 213

of the west flank of the Jacobina range, with an extension AI lhe Medradojlpueira Mines, lhe main chromitile body ofabout IS km and mean width of600 m. The ore occurs as occurs at lhe base of the layer, consisling oforthopyroxene­ disseminated and massive forros composed of chromitite -olivine-spinel (Barbosa de Deus, 1982, unpublished). The layers with thicknesses ranging from centimetres to 2 m. chrome ore reserves in the Jacurici river valley are evaluated These deposits are regarded as of stratiform type, and it in lhe order of II million lons. is believed that the original mafic-ultramafic body was sub­ stantially larger than the present-day one, being largely erod­ Other deposits The State ofBahia has lhe largest reser­ ed before thc deposition of the Jacobina Complex (Thayer, ves of barite of our country, mostly occurring as veios 1970, unpublished). The ore reserves of this district are esti- inserted both in gneissic-migmatitic çomplex rocks and in mated to be over 30 million tons. . volcano-sedimentary complexes. These deposits are beJived The chrome deposits ofthe Jacuricilriver valley are within to be a result of hydrothermal processes. mafic-ultramafic sills, which are emplaced in granulites, ln lhe region ofCarnaiba, in lhe west flank ofthe Jacobina gneisses and migrnatites and, in synform structures, they range, granitic intrusions dating from the end of the Early show the sarne deformation patterns of the enclosing rocks. Proterozoic (1,800 Ma) caused metamorphic-rnetasomatic The sills have ticknesses lesser than 300 m and very variable effects, which, in the contact zones, give rise to beryl (emer­ lengths, sometimes reaching a few kilomelres, and in general, ald) and molybdenite mineralizations. Within lhe high they are composed of peridotite-pyroxenite, gabbro-serpen­ grade' terrains (gneisses, migmatites and granuliles), mafic­ tinite ando eventually chromitite layers. -ultramafic bodies often occur, showing sometimes evidence Orebodies are quite thick (in general ranging from 5 to of slrong hydrothermal alteration, Some of these bodies 8 mi, as compared to the whole thickness of the hosl sills. contain arnianthus, vermiculite and tale economic deposits.

REFERENCES

ALMElDA, F.F.M. de ~ 1968 - Evolução tectônica do Centro Oeste Bra­ CORDANI, V.G., ISOTTA, C.AL. and ABREU, AC.S. -1969- Reco­ sileiro no proterozóico superior. Anais da Academia Brasileira de Ciências. nhecimento gcocronológico do embasamento na região oriental do Es­ Rio de Janeiro, Suplemento 40; pp. 285·295. tado da Bahia. ln: Congresso Brasileiro de Geologia. 23.", Salvador, ALMEIDA, F.F.M. de - 1977 ~ O Craton do São Francisco. Revista Bra­ Anais. Salvador, 1969, pp. 159·165. sileira de Geoctênctas, São Paulo, 7(4):349-364. CORDANI, V.G. - 1973 - Evolução geológica precambriana da faixa coso ALMEIDA, F.F.M. de, HASUI, V., BRITO NEVES, B.B. and FUCK, teira do Brasil entre Salvador e Vitória. São Paulo, Instituto de Geociên­ R.A - 1977 - Províncias estruturais brasileiras. ln: Simpósio de Geo­ elas da USP. Tese de Doutoramento. logia do Nordeste VIII, Campina Grande, Alas. Campina Grande, 1977. 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