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Revista Brasileira de Geociências 12(1-3):.414-425, Marv-get., 1982 - São Paulo

GEOLOGIC SETTING AND GENETIC INTERPRETATION OF THE BOQUIRA Pb-Zn DEPOSITS, STATE,

ILSON O. CARVALHO·, HALF ZANTOp·· and JOAQUIM R.F. TORQUATO···

AB8TRACT The stratabound~straflform:Pb~Zn-AgMCd deposits of Boquira, located ln south-central Bahia State, occur in metamorphic rocks ofthe Boquira Formation. This formation is composed ofaltered volcanic rocks, schists, , formation, and dolomitic marbles which are the metamorphiclequivalents of intermediate to acidic volcanic rocks, volcani­ clastic sediments, and iron-rlch chemical sediments. These rocks were intruded by granitic magmas in the late time. The massive to semimassive lenscs are conformably enclosed in the silicate facies of lhe Contendas-Boquira Member. The primary ore is composed of and in a of , maghemite, martite, and minor , , , and amphi­ boles. Thelassociation of the iron formation with volcanic rocks suggests that it is of Algoman type, and the conformable relationships between the iron formation and thc sulfide lemes suggest that the ' are also volcanic exhalative. ln addition, isotcpic analyses of suggest a marine ar the vicinities of subaqueous centers of discharge of hydro­ thermal brines.

INTRODUCTION TheBoquiraPb-ZnDistrictissituated The contact between the B.F. and the basement is not sharp in lhe south-central area of Bahia State, about 450km west and it is inarked by transitional rock types, and diffused of the city of Salvador. Its area is about 170 km' localized metasomatic effects. The appears caused between coordinates 12'OO'-13'15'S and 42'30'-43'W (Fig. I). mainly by migmatization of the basement since it is more production has been active since 1956 and the cu­ frequently observed close or around the altered intrusive mulative ore production is over 4.6 million tons of primary and in the transitional contact with the gneiss­ plus secondary . -migmatitic basement. The granitic bodies are remobilized The district is morphologically well defined by a narrow products of the basement that intruded the B.F. The mini­ longitudinal belt of volcanic and volcanic-derived rocks mum age for these granites, yielded by the K/Ar method, is which stand up as discontinuous ridges bordering the eas­ between 440 ± 20 to 520 ± 15 Ma, however theymay be tern flank of the Macaúbas Range, over 100 m higher than older(Nagell et ai., 1967; Távora et ai.. 1967; De Sá, 1981). the Valley to the east, Ridges made up of, ar As a result, the B.F. displays different ranks of metamor­ enclosing, similar volcanic derived rocks are Iined up west phism, zoned from high amphibolite (­ of the Paramirim River and parallel the regional trend grunerite-almandine) to low green-schist assemblage (saus­ direction (Figs. I and 2). The regional geological framework surite-epidote--sericite). The latter is mainly observed comprises the old gneiss-migmatitic complex ar Paramirim in the less altered volcanic rocks that compose the Tiros Gneiss (Kegel, 1959); the Late Archean volcanic and vol­ Member. cano-sedimentary rocks of the Boquira Formation (B.F.) (Kaul, 1970); the metasedimentary rocks, mostly quartzites, Undivided Member IU.M.) The U.M. exhibits a moderare of the Proterozoic Santo Onofre Group (Porcher, in Kaul, to high-grade amphibolite facies meramorphism. with local 1970), intrusive gabbros and granites, and the metaphoresis and low grade facies. The U.M. marks the cover. ln this paper especial attention is given to the B.F., lower transitional contact of the B.F. witb the gneiss-mig­ because it hosts the massive sulfide orebodies. • matitic basement. It is composed of: A) a complex assem­ blage of banded and schistose materiais, which in some places have a gneissic appearance making it difficult to distinguish them from the gneisses of the basement; B) in­ THE GEOLOGY Of THE BOQUIRA DISTRICT terbedded narrow lenses ar discontinuous thin beds of the Kaul (1970) divided the B.F. into four members which Contendas-Boquira Member and Tiros Member which re­ he called Undivided, Contendas, Boquira and Tiros. ln this semble preserved ar less altered parts of these members; work the Contendas Member and the Boquira Member are and C) interbedded thin leveis of chert-derived , considered as simpIe fades variations of the sarne strati­ epidotite, and a conglomerate-Iike polymict material which graphic unit because the relationships and the petro­ may represent an altered polylithologic volcanic . graphic features suggest that they were formed coevally in Evidence that these rock types underwent metasomatism the sarne sedimentary basin. are: A) the frequent gradual change from one rock type to The B.F. is a near vertical to steep northeast-dipping, another; B) the presence of porphyroblasts of garnet; and north-northwest-trending sequence which was isoclinally C) the presence of small pegmatoid bodies, usually bearing infolded into the gneiss-migmatitic basement (Figs, 2 and 3). quartz, graphic myrmekitic feldspar and schor!. The major

"Departamento de Geoquímica, I.G. UFBA, Salvador, Bahia, Brazil **Department of Earth Sciences, Darlmouth College, Hanovcr. N.H., USA ***Departamento de Gcoclências, U.F. Ceará, Fortaleza," Ceará, Brazil Revista Brasileira de Geocíências, Volume 12 (1-3), 1982 415

~ AJlu'o'ium

~ Bomtx.lI' GrOup

~ Ecstem cuorteitee

Espinhoço ~ Melosediments ~:J-+-I--II+---tl-~--I,---+>I""-I-+------11'" ~( Supergroup [Sõ{iJ sento onctre Group ~\ \ í mg ~.aom Jesus ~ ~I do Lopa Ecstern Melo'o'olconlcs I~j" ~ Boquiro rormoncn ./"" ~ Migmolific Gnetss Complex AI ~J srcoltes cno sverures

ao --f-- Anlicline -t- Synctine Town or vifloqe

"

LOCATION MAP

Figure I --- General geologv o] Central Bahia (modijied ajter Távora ct al., 1967)

EX.e_,,""~_NATION

QUATERNARY r., Sedlmenls cn AlIuvium UPPER PROTEROZOIC mg v, v v Amphibolite l/1/rusive Rocks dio Dioríle + gr t Gronite MIDDLE PROTEROZOIC M"o,,',m.'" b@Ê ~',;:~;~~ ~o:~:::" "".'.', Serro do voreoc rcrmoucn ARCHAEAN rtrce M.mber SChIStS, meta­ I vctcco.cs ora Marb,le , IrOIl farmatlall Soqulro Member Undivided Member rnordo Gnelsses ond Migmolilic Gneiss migmaliles Comptel'/PorcilT11 rim Gneiss -rr- Overturned anticlinol -P-- Overturna d bed Faull tnt ter red foult ,\ .,~um Contcc t Inferred contcct __ "" "-\..,. Strecm farm 00 Town f//// Linealíon

L. -.l eecrcçtc CfOSS secucn

CROSS ª-EETION A- B _c.co,,"3 4 km MACAÚBAS RANGE /" se~~~-;----__ ~j~~~~~TO f;~:O~

Figure 2 - Geologíc Map of the Boquira Distríct, Brazil 416 RevistaBrasileira de Geociências, Volume 12 (l·3), 1982

, I, . ,'\ I' I\I\ I \I\ r\ 1 \ I\ I \II I \ (METERS) W I\I \ I\ I\I\ i \ E r 975 I\I " \ r I\I 'J J I ' I I', I I ": o , o­ ~ :-;~, I v , 900 o , o o , o I- " '\ , , i !\ I I , , ,'- I\ , o , I SOBRADO .\ , , ,,, , o t::: ,, f675

rrn ~ rctce f77"'"l \ L..:...:...:J Espinhoço quor t zite - Serra da Vereda Formotíon \ ,....., \ l.!....!....LI IntrUSIVO granites \ ~ L...d Volconic -ocxs - Tiros Member

Per ruqinaou s cno pure quer t zit es ond silve r vlus tr ous mica $chist -Unnomed Member

ê oquu c I~:;.:tl Melaconglomerole (? ) - unncmed Member o 100 200 300m For mc no n b ::±:::=d [;TI Bonded iron formolion (o~ide, eor boncte cno silicole facies} - êocu.rc Member

g ~''ls,ive sulfide ore EI Mica scntsts DOá çeeiesee - Undivided Member-Boaement

Figure 3 - Cross sectíon ojBoouiraDlstrict, Bahta, BrazU (Modified after Fleischer; 1976; Espourteille and Fleischer, 1980)

types ofschists (Johnson, 1962; Cassedanne and Melo, 1966; FACIES Previous authors (e.g. Bruni ando Bruni, Nagell, 1970; and this work) are: a) biotite-garnet schist: 1973; Cassedanne and Melo, 1966; Espourteille and Fleis­ porphyroblasts of biotite and garnet and matrix composed cher, 1980; Fleischer, 1976; Kaul, 1970; Nagell et ai., 1967; of biotite and subordinate chlorite, quartz and sericite. The Schobbenhaus Filho, 1971) called "itabirites". Those rocks micaceous components are deformed and bowed around that represenl the oxide facies of lhe iron-rich sequence of the porphyroblasts. A poikiloblastic texture is represented the B.F. They present a banded texture of fine grained by inc1usionsofquartz and zircon in the biotite, and quartz minerais. The bands are well marked by alternating white ar and in the garnet. Banded texture is frequent1y ob­ yellowish laminae of quartz and bright brown, and grey served in core samples from drill holes. b) Quartz­ or black laminae of magnetile. These bands are frequent1y -sericite and sericite schists with porphyroblasts of tourrna­ thinner than 5 mm and display sub-banding (Photo I). line: crystals of tourmaline (schorl) are oriented in a matrix Graded textures in these bands are not observed, though of chlorite-derived rnuscovite, quartz, sericite and traces of the proportions of quartz and may chlorite. The blades of sericite are bent and their contours change from band lo band even on a meter scale. AIso, are frequent1y corroded by quartzo A banded texture is there is a gradual change of this facies into both carbonate present in planes where biotite-dcrived chlorite (up to lO/li) and silicate facies and also into hematitic quartzite, Under forms mm-thick bands. c) Banded amphibole-biotite-chlori­ the microscope the boundary between the bands may not te-quartz-magnetite (martite) schisl: the bands (up lo 15cm be c1earlydefined, but in some cases, it is sharp. The typical thick ) in this kind of schist are made prominent by the dif­ ·iron oxide-rich bands are most1y composed of ) ferent content of lhe minerais in each bando The (> 70:/0 and quartz (> 30:/;,). ln some thin sections hema­ assemblage varialions are not repetitive, Noticeable changes tite is evident as a product ofmartitization. This replacement in the amount ofthese minerais are observed in the successive is mainly observed in the zones of gradual transition into bands even over a short distance. the carbonate facies. ln lhe bands with high proportions of quartz, the iron-bearing mineral phase in the rock is hematite Contendas-Boquira Member (C.B.M.) This member cor­ (Ilaky specularite) and there is no evidence ofmarlitization. responds to a sequence of banded iron forrnation, char­ Moreover, there is.a direct correlation between the magne­ acterized by lhe presence of the oxide, carbonate, silicate tite and iron-amphiboles. ln these transitional zones the and sulfide facies. bands show rnartitization on several scales of frequency, Revista Brasill#radeQeocilncios, Volume 12 (l~3). 1982 417

Table J - Analyses of íron formation and sulftde ore from lhe Boquíra District

I. F. Fades Sample SiO, AI,O, CaO MgO MnO Fe203 FeO TiO, P20S Na,O K,O LO! S

I. Oxide G-B 73 40.96 0.56 0.68 0.44 0.02 52.18 2.52 0.04 0.12 0.05 0.24 0.34 <0.03 2. Oxide G-B 57 46.73 0.48 1.92 1.42 0.58 42.63 1.88 0.09 0.23 0.21 0.07 2.30 <0.03

3. Silicate G-B 52 58.79 1.28 4.15 7.92 0.40 2.48 2Q.62 0.11 0.05 0.21 0.12 0.17 0.62 4. Silicate G-B 108 46.09 0.26 0.75 5.14 0.29 23.85 20.64 <0.04 0.24 0.07 0.03 •••• <0.03 5. Silicate G-B 109 47.90 0.22 1.52 7.69 0.40 19.58 18.45 <0.04 0.16 0.05 0,03 •••• <0.03

6. Carbonate G-B 63 27.36 1.15 28.86 4.49 0.33 5.71 3.61 0.09 0.02 0.56 0.18 27.53 0.03 7. Carbonate T B-1-- 11.21 0.60 32.50 9.22 9.63·.... 0.08 0.23 0.02 32.17 8. Carbonate G-B 84 8.88 0.14 47.86 3.50 1.14 0.22 1.13 0.04 <0.01 0.03 0.01 37.22 <0.03

Sulflde ore (%l Pb Zo Cu 810, Fe 8 CaO MnO CO, AI,O, Ag"

9. (Data from Rocha, 1973) 9.03 2.40 tr. 0.34 21.10 5.65 0.50 4.40 2.05 2.10 28

-values given in ppm "Pb = 180 ppm, Zn = 186 ppm ···Total iron as FeZ03 ····The sampie gained weight after ignition at 1,000°C

and an increase of amphibole in both iron oxide-rich and blende may occur as a subordinate or accessory mineral. carbonate-rich bands. The chemical composition of this Toward the silicate facies there is an increasing content 01 facies is shown in Table i (I and 2). amphiboles (cummingtonite and hornblende) in the carbo­ nate-rich bands, and ofless-martitized magnetite in the iron oxide-rich bands. Unbanded types are also found, displaying greenish and grey shades of colour, massive texture, and ;{~~~: locally, exhibiting porphyroblasts of actinolite and plagio­ elase. Microscopícally, these marbles show a granoblastic texture of or ferroan dolomite and subordinate A calcite or dolornite, magnetite and/or martite, quartz and amphiboles. Carbonate grains normally amount to less than 90%, but exceptionally may be more. Chlorite, biotite­ -derived sericite, biotite, feldspar, tourmaline, pyrite and zircon are common accessory components. Table I (6, 7 and 8) shows the chemícal composítion of rocks of this facies.

Photo I - Oxide faeies. The photomícrograph shows a quartz-hema­ tite-rich band (A - upper portion), and a thin hematític bond (B -r Io­ wer portion). Subbanding is observed ín A (a, a'). The interlayered grey material corresponds to quartz-rich bands. Btack areas = holes. Reflected plane light

CarbonaM Facles This facies corresponds to the banded carbonate-rich rocks (Photo 2) the bands ofwhich are com­ posed of ; carbonates-amphiboles; carbonates­ -amphíbotes-magnetite-martite; quartz-carbonates-amphi­ boles-magnetite-martíte (Photo 3); and amphiboles. The carbonates range in composition from dolomite to ferroan dolomite, with calcite and locally abundant. The Photo 2 - Carbonate fades. Photograph ofa bouider detached from amphiboles belong to the tremolite-actinolite series. Horn- a nearby outcrop. Note lhe banded texture. Contendas Region 418 RevistaBrasileira de Geocilncias. Volume 12 (1.3), 1982

lhe banding. The bands are composed ofgrunerite (> 60%), magnetite and cummingtonite « 10% each) and subordi­ nate quartz, actinolite-trernolite, hornblende and accessory fine grained sulfides. Clots of magnetite may be present in the thicker bands.

• Cummingtonite-rich bands: Clear green to brownish green. The mineral composition is a complex assemblage of cum­ mingtonite (> 50%), magnetite (up to 30%), hornblende (may be over 10%), and trernolite-actinolite, quartz and grunerite « 10% each) (Photo 4).

Photo 3 - Carbonate fades. The photomicrograph shows the repta­ of magnetite by hematlte. The comrols of the martítization process are: planes, grain boundaríes, and thc [III] spínel taw twinning. Reflected plane lighr

SILICATE FACIES The so-called amphibolites of the Boquira District represent the silicate facies of the Boquira iron formation. The term amphibolite has been here applied by the geologists who have worked in the area (e.g. Bruni and Bruni, 1973; Cassedanne, 1972; Cassedanne and Melo, 1966; Espourteille and Fleischer, 1981; FIeis­ cher, 1976; Johnson, 1962; Kaul, 1970; Nagell, 1970; Nagell et ai., 196.7; Schobbenhaus Filho, 1971). It is sugges­ Photo 4 - Silicate fades. The photomtcrograph shows a magnetite­ ted here that this term should not be used because of the -rích band (lower portion) and a silicate-rích band (upper portion). lack of feldspar and the clear spatial correlation of these ln decreasing order ofmineral abundance, the latter is composed of: rocks with those previously described. The chemical com­ cummingtonite, gruneríte, magnetite, ferroan-actínotíte, hornblende, and quartzo Note the angíebetween the directíons ofgrain eíongatton position of the rocks of this facies is shown in Table I (LIl and bandtng (Lo). Transmitted /ight, nearly crossed nicols (3, 4 and 5). The rocks are texturally banded, displaying the same appearance as that of the rocks of the oxide and carbonate facies. The dark or black bands are mainly com­ posed of magnetite with varying amounts of amphiboles. The less dark bands have variations in their mineral assem­ Pb-Zn Sulfide Bearing Fades: James (1954), Gross (1980), blage as follows: Gross and McLeod (1980), and Gross et ai. (1972)depicted the sulfide facies iron formation and defined it as the pyrite • Actinotue-rtch bands: Dark green. Actinolite (> 60%) and/or pyrrhotite-rich mineral assemblage. They considered is tabular or acicular. Its parting and planes are theseminerals as formed in the deepest reducing zone of filled by iron , quartz and calcite. Magnetite (> 10%) the depositional basin and that they underwent later diage­ may form sub-banding or discontinuous seams which para­ nesis and . Other sulfide mineraIs would have lIel the banding. Pyrite ( < 3%) is fine grained and occurs been able to form if enough of their metallic components in both actinolite and magnetite-rich bands. Some magne­ were supplied to the depositional environment. This ex­ tite grains are almost entirely martitized. plains the frequent chalccpyrite, sphalerite and galena de­ posited as minor or trace amounts of sultide mineraIs. Ho­ • Pargasite-rtch bands: Several shades of greenish brown. wever, so far as the authors are concerned there is no re­ The composition is pargasite (> 55%), quartz « 20%), ported sultide facies iron formation in which chalcopyrite, pyrite (> 10%); hornblende plus grunerite and biotite sphalerite and galena, separately or together, overwhelm amount to less than 15%. the iron sultide mineraIs. The Boquira ore has little pyrite and pyrrhotite, but abundant magnetite, galena and spha­ • Cummingtontte-grunerite-rtcn bands: From Iight brown lerite. AIso, the presence of magnetite accompanied by ga­ to greenish brown. This change reflects the different pro­ lena and sphalerite has been reported as the normal parage­ portion of each amphibole in these bands, Ifcummingtonite nesis of the sulfide ore of Central (Frietsch, 1982). is less than 10%, they are classified as grunerite-rich bands This kind of Pb-Zn ore is hosted in the silicate-carbonate and display an almost black colour. If grunerite is less than facies, spacially distinct from, but close to the sultide facies 10%, they are classified as cummingtorute-rich bands. These which occupied the deepest, most strongly reducing zone of two types are to be described next, the depositional basin. The iron and abundance in the depositional basin easily explain the formation of • Grunerise-rtch bands: Very dark, almost black. Sometimes pyrite, sulfur-defíciency or later metamorphism would the darkness of the rock colour makes it difficult to observe explain the formation of pyrrhotite. Depletion of sulfur Revista Brasileira de Oeoci8ncias, Volume 12 (1-3), 1982 419

(in lhe aqueous phase) will prohibit any phase from forming (Garrels, 1960, p. 161; Garrels and Christ, 1965). Therefore lhe question arises, does lhe Bo­ quira ore really represenl a sulfide facies iron formation? To answer this question', the first point lo be considered is, was the original solution rich enough in sulfur lo react with ali available cations? The second point is lo explain the abundance of and subordinale amounls of sphalerite in the ore. Consideration of these points requires that the phase relations between lhe ore mineraIs and the sea water be ralher well understood, Thermodynamic data (Barnes and Czamanske, 1967; Helgeson, 1964, 1969; Nriagu and Anderson, 1970) indicate that lhe sulfides observed in lhe ore resulted from decornposition of sulfide complexes in a solution under conditions of high activity of sulfur. Moreo­ ver, these complex compounds possess dilferent chemical properties under lhe conditions of a varying acidity-alka­ Iinity regime of lhe aqueous environment. The question seems lo be related rather lo lhe pH-Eh conditions than lo Photo 6 - Ore mineral assemblage: chalcopyrite (cp), sphaterite (sp), lhe degree of chalcophility of lhe metals, or lhe amounl gaiena (g),and gangue (darkarea in lheupper porttoní. Blackareas= of sulfur involved in the iron deposilion processo The ore holes. Reflected plane ttght itself is ao iron forrnation, its iron oxide content averaging over 25%, Table I (9), most of which is in lhe oxidized spinel structured form of magnelite (Pholo 5). This implies that there was probably suffícient sulfur in lhe system lo precipitate abundanl iron sulfides, but that there was also is composed of pyrite, chalcopyrite, pyrrhotite, magnetite sufficient lead and lo precipitate galena and sphalerite, magnetite (lhe most abundant mineral of gangue), martite, The preference for lead and zinc is related to lhe con­ maghemite, amphiboles, asbestos, , quartz, and dition of lhe depositional environrnent. This was not suffi­ locally chlorite, biotite, calcite, and sericile (Photos 5, 6, ciently reducing lo cause iron sulfide deposition. So, we and 7). Most of the sphalerite is a dark brown colored must call lhe sulfide ore zones of Boquira Pb-Zn sulfide marmatite type. Locally sphalerite may form concordant bearing facies and not sulfide facies because iron sulfides are bands or elongated seams in lhe galeria-magnetite rieh not lhe dominant mineral phases. ore. Fine indusions of pyrite and chalcopyrite occur in The width ofthe ore bodies ranges from a few centimeters well recrystallized sphalerite. lo 6 meters, and the length can be as much as 1.5 km. The ore continuity is frequently broken by low-grade zones or barren zones. This occurs where lhe host silicale facies grades along strike into lhe carbonate and/or oxide facies. The ore conlinuity is also interrupted by theintrusive granitic bodies and is displaced by oblique transcurrent faults. The sulfide ore is mainly composed ofgalena and sphalerite. The gangue

Photo 7 - Gangue-rích ore mineral assemblage; galena (g), spha­ letite (sp), maghemite (mh) (b/uish marmatite) from martitization ofmagnetite (dark areas in the maghemite groins), and omphiboles (o). Note the ínclustons of sulfides in the amphibole (a). R = epoxy resino Reflected plane Iight

Tiros Member (T.M.) The T.M. has been considered as younger than lhe B.F. (V.M. pIus CB.M.) by several Piloto 5 - Common ore minerais assemblage: ga/ena (g); sphalerite authors (e.g. Fleischer (1976), Espourleille and Fleischer (sp); quartz (q); amphibole (a); martite (mr); and residual magnetite (1980), De Sá (1981), McRealh et ai. (1981) and Da Cosia (mi). Reflected plane light and Inda (1982) on lhe basis of geochronoIogical data, 420 Revista Brasileira de Oeocíênctas, Volume 12 (l·3), 1982 which suggest anage of about 1.0 Ga and by the presence tamorphism. De Sá (1981) considered the B.F. as supra­ of discontinuous leveis of polymict conglomerare described crustal remnants of the old sialic crusl. as being placed between this unit and the V.M. The T.M. Two major sets of faults aITected the isoclinally folded is basically cornposed of voleanic rocks which extend along B.F. (Fig. 2). The older set is formed by the North-trending the lower part of the deep slope of the Macaúbas Range. strike-slip faults. This faulting is well exhibited between the Cassedanne and Melo (1966) believe that this unit is formed sulfide facies and the silicate facies layers. ln the contact by repeated massive flows of lavas which also originated between the ore body and the host banded amphibole-mag­ the interbedded lenses in the schists. McReath et ai. (1981) netite rocks, a slikenside chlorite-gouge is frequently obser­ suggest that it forms extensive dikes, 100 to 350 m thick ved. The other set is referred to as the transcurrent faults. and up to 3 km long, aligned parallel to the scarp of the They strike roughly E-W. Dislocations in the fault plane Macaúbas Range. These may represent the feeder zones are both vertical and horizontal. Ore was remobilized into to now-eroded flows. The voleanic rocks have a peralkaline these planes up to 50 m from the main ore body. This fault character (McReath et ai. (op. cit.)). Here, we separate them system also aITected the Proterozoic metasedimentary se­ on mineralogical and textural grounds into rhyolites, tra­ quence of the Santo Onofre Group and it is presumably chyrhyolites, dacites, and trachyandesites.The studied rhyo­ related to the emergence of the Paramirim Gneiss, lites and trachyrhyolites correspond to the comendites des­ ln the Boquira District, metamorphism aITected the area cribed by McReath et ai. (op. cit.). with different intensities and at diITerent times. Migrnatiza­ tion took place in the older basement and materiais intruded the B.F. ln many places in the Boquira Formation, STRUCTURES AND METAMORPHISM Da Costa metasomatism was a common process of rock alteration. and Inda (1982) recently regarded the structural fra­ However, it appears chiefly caused by migmatization be­ mework of the region of the Paramirim Valley as resul­ cause it is more common around the altered intrusive gra­ ting from the development of a Middle Proterozoic au­ nitic bodies and(or close to the gneiss-migmatitic basemenl. lacogen system. According to their model, the be­ Tale, chlorite, epidote, saussurite, albite, sericite, quartz, ginning of the aulacogen was marked by North trending schorl, caleite and some actinolite, at least in part, resulted faulting, resulting in a NNW rift basin which was filled with from the metasomatism. volcanic rocks (the Eastern Metavoleanics and the Tiros Member (Figs, I and 2)). The rift stage is marked by rene­ wed rifting, resulting in the enlargement of the rift basin STABLE ISOTOPES INVESTIGATIONS ON CAR­ and deposition of the Espinhaço Supergroup. The post­ BONATES Twelve carbonate-bearing rock samples and -rift stage is marked by emergence of the aulacogen, erosion five handpicked carbonate mineral sam pies wereselected and and finally the deposition of the Bambui Group in post­ analysed for carbon and stable isotopic cornpositions. The -rift basins formed eastward and westward from the uplifted specimens were crushed and split to obtain uniform samples. central region (Fig. I). The rift-aulacogen system is also CO, was evolved by the phosphoric acid method modified discussed by De Sá (1981) who proposed a vertical model from McRea (1950). Kinetic fractionation factors used for related to a rifting episode, followed by a compressive tecto­ correcting bISO values are from Blattner and Cooper (1974). nometamorphic event, with accompanying low grade me- The bISO and b' 'C values have a reproducibility oí ±0.20

Table 2 -.Carhon and ox)'gen isatope data oI carbonato materiais

Group Average values (j13C Sample N.!> pOB (i180pOB (i180SMOW l 3 18 Ó C ÓI80PDB Ô O SMOW

1044 -4.03 - 8.65 +21.46 A -4.03 - 8.65 + 21.46

1041 +3.28 -15.12 + 14.79 B +3.31 -15.10 + 14.81 1045 + 3.34 -15.08 + 14.83

1047 -0.88 -16.42 + 13.45 C -0.82 -16.13 + 13.75 1052 -0.97 -16.11 + 13.77 1057 -0.61 -15.85 + 14.D4

1049 - 3.85 -16.90 + 12.96 D -4.71 -16.15 + 1'3.73 1053 ·-4.18 15.81 + 14.08 1054 - 5.55 .. 15.69 + 14.20 1056 -5.24 -16.21 + 13.67 - 1042 -1.03 - 20.68 + 9.06 E -1.90 -19.87 + 9.90 1043 -2.16 -19.53 + 10.25 1046 -2.66 -18.70 + 11.10 1048 -3.58 ··20.47 + 9.28 1051 -2.23 - 21.15 + 8.58 1055 -0.39 -19.80 +'9.97 1058 -1.26 -18.77 + 11.03 Revista Brasileira de Geocténctas, Volume 12 (1-3), 1982 421 per mil. Ali results given in Table 2 are reported in parts GENETIC DISCUSSIONS Metemorphie Evolution of per mil deviation from the PDa standard. The iso­ SedimentaryFaeies of IronFormation - A Briel Diseussion tope analyses are also presented as converted SMOW values Harder (1919) described chamosite, greenalite, minneso­ by using the equation given by Craig (1957). taite, and stilpnomelane asthe normal. silicates that The b"C values show a range of - 5,55 to + 3,34 per are present in the . James (1954) mil and tive groups are categorized in the b1 3C versus 0 180 suggested that stilpnomelane, chlorite, fuchsite, greenalite, diagram (Fig. 4). Group A is formed by sampie 1044 which , minor carbonate, and iron oxide compose is a sparry calcite from a carbonate veio that cross-cuts the the silicate faeies iron formation. Gross et ai. (1972), on the silicate-facies irou forrnation nearby the ore zone, Group basis of fades composition and depositional environment, Bis formed by samples 1041 and 1045, which are from silica­ defined Algoman type iron formations as the result ofeugeo­ te-bearing carbonate. This rock type has a white colour and synclinal deposition within a sequence of prominent volca­ is mostly composed ofcarbonate, less than 10% oftremolite, nism and sediments, this type of deposition less than 5:;; of chlorite and accessory martite. Group C being typical of Archean greenstone belts. Oolitic texture is formed by samples 1047, 1052, and 1057 which are from is absent. Except for chlorite, ali iron silicate minerais are greenish carbonate and are richer in tremolite (over 25~~) poor in , because of the absence of chamosite than the samples of Group B. Sam pies of both Group B in those terranes. French (1973) stated that the silicate mi­ and C are from the Contendas Marble which is the local neraI assemblage in diagenetic and low-grade metamorphic name for the rocks of the carbonate faeies that outcrop in iron formation is characterized by generally fine-grained the Contendas region. Group D is formed by samples 1049, quartz or chert; iron-rich phyllosilicates (greenalite, chlo­ 1053, 1054, and 1056, which are coarse sparry calcite in rite, minnesotaite, and stilpnomelane) and Na-Fe-amphi­ association with well-crystallized stratiform ore. Group E boles (riebeckite and crocidolite). The appearance of gru­ is formed by samples 1042, 1043, 1046, 1048. 1051, 1055, nerite fixes the upper boundary of the low-grade metamor­ and 1058, which are finely crystalline carbonate from lhe phism. Chert, and progressively quartz, résult from mega­ transitional zone between the banded carbonate and the diite - a Na-Si gel. Minnesotaite - a tale structured mineral. silicate fades. stilpnomelane and riebeckite might also result from silica-

+ 5

B ;::"\.·~045

o .1055 .1057 c ••1047 .1042 1052 .1058

E .1043 1051• .1046

10048 • • An • 1049 1053 V D C ARBONATIT IC -5 .1056 .1054 FIELD

-5 -10 -15 -20 - 25

18 Figure 4·- 013C versus 0 0 pio! o] carbonates from Boquíra. The field ofcarbonatítc is based 011 Torquato (/980) 422 Revista Brasileira de Geociéncias, Volume 12 (1-3), 1982

-rich (Fe, Mg, Ca)(CO,), gels (French, op. cit.v. Both silicate the low grade metamorphic rhyolitic and ignimbritic rocks, and carbonate "original precipitates" are aluminium-poor agglomerates and tuffs that grade laterally and vertically gels. into the Slate Group (slates, marbles, greywacke and silts­ Klein (1973) suggested that the unleached sedimentary tone). The post-nft stage is initiated by vertical movements oxide facies of unmetamorphosed iron forma­ and emergence of the region. Metabasites occur throughout tions is generally made up of a finely banded assemblage the whole volcanic sequence and also in some parts of the of chert, or quartz, hematite and/or magnetite, and Slate Group. They are represented by layers of albite-epi­ locally some hydrous iron oxide. Recrystallization of chert dote ± actinolite ± chlorite ± carbonate-rich schists associa­ and iron oxide occurs. Under regional metamorphism to ted with NNW feeder dikes, Both acid and basie magmas kyanite-sillimanite grade, the recrystallization is accorn­ ascended along the NNW zone of rifting. panied by a marked increase in grain size of the component Iron, and base metaIs mineralizations occur phases, especially silica. The original sedimentary textures associated with geochemical facies genetically related to (alternation of bands on a scale of less than 2 cm) are the volcanism. ln several pIaces the primary relations are frequently preserved because there are no reactíons among masked by the regional metamorphism and metasomatism these minerais. with generation and, locally, by contact-rnetamor­ For the banded carbonate facies, Klein (ap. cit.) suggested phism caused by younger Svecokarelian granites (Frietsch, two possibilities: I) Ifthe original carbonate facies consisted 1982; Oen et 01.,1982; Zakrzewski, 1982). The western part of carbonate, magnetite and chert, and if the chemical po­ of Bergslagen (Grythyttan-Hallefors area) was affected by low grade metamorphism (greenschist facies). tential of CO 2 during subsequent metamorphism remained locally high enough to prevent the breakdown of the car­ The distribution of the different iron and manganese ore bonates, no reactions occur among the coexisting phases. deposits associated with Cu ores and Pb-Zn-Ag ores reflects Only recrystalization and ao increase in the grain size occur. a systematic change in redox conditions from oxide banded 2) If the chemical potential of CO, is reduced, chert reacts iron formation into highly reduced graphite-bearing slates. with carbonates to form new sílicates, for example: Frietsch (1982) and Zakrzewski (1982) regarded much of the element distribution in the Grythyttan-Hallefors basin Ca(Fe, Mg) (CO,), + 2SiO, = Ca(Fe, Mg) Si,O" + 2CO , (I) as being determined by variations of the oxidation potential If, during the metamorphism, the chemical potential of and, to a lesser degree, of alkalinity. With increasing water water is high and that ofCO, is low, the following reactions depth towards the central part ofthe basin, there is a de­ take place: crease in the oxidation potential and an increase in alkalinity (Fig. 5). As pH and Eh change with lhe depth of the basin, 5Ca(Fe, Mg) (CO,), + 8SiO, + H,O = the depth appears as the main controller of the element = Ca,(Fe, Mg), (Si 0 ),(H,O), + distribution and consequently of the geochemical nature of 4 1 1 the deposit. With rhis in mind, we may consider that the + 3CaCO, + 7CO, (II) depth ofthe basin during the time of the Boquira Formation deposition was insufficient to allow lhe iron sulfide facies ln support of this, Klein iop, cil.) notes that much of the to be formed. banded carbonate facies contains metamorphic silicates, indicating a general loss of CO, during metamorphism to the kyanite-sillimanite facies. Parts 01' the carbonate fades Oxygen and Carbon Isotopic Evidence for theOrigi n of the in some metamorphic arcas may, however, still consist of Boquira lron Formation Keith and Weber (1964), Gariick the original, although recrystallized, quartz-carbonate-rnag­ (1969), Schidlowski et 01. (1975 and 1979), Veizer and netite assemblage. This implies that the ehemieal potential Hoefs (1976), and Torquato (1980) considered that the oxy­ gen isotope composition of carbonate is related to the of CO, has been locally variable in the iron-rich carbonate l 80 rocks and that the CO" therefore, cannot at ali times be following factors: a) Geological lime ofdeposition: the b considered as a perfectly mobile cornponent. The mentio­ values decrease with increasing géological age; b) chemical ned author suggested that the silicate facies shows, more composition: the vaIues increase with increasing content of consistently, the results of general decarbonization and the dolomitic and ankerite-sideritic terms ; and c) lhe rela­ dehydration during progressive metamorphism. ln addition, tive position of lhe subaqueous deposiüonal site which is a reaction to form abundant siIicate as rnembers of the considered in relation to lhe shore margin : the values in­ cumrningtonite-grunerite series, may give way to elino and crease from the shallower, near shore, brackish 01' transi­ orthoamphiboles, gamei, and ortho- and clinopyroxenes. tional marine-continental site, which has a strong contribu­ The original silicate is described as being composed of a tion of fresh water, to the deepest marine depositional site. complex oozy mixture of hydrous Fe-silicates, carbonates, Factor (' did not affect the isotopic composition of the chert and iron oxides, Contendas-Boquira Member. This conclusion is based upon, firstly the assumption of a shallow depositional site for the carbonate facies which is shallower than the site for the The Bergslagen Supracrustal Series - A Similar Case of transitional zone between the carbonate and silicate facies Pb-Zn Sulfide Mineralization Oen et 01. (1982) suggested and for the silicate facics, and secondly, on the (5180 values that the paleo-environment of the Bergslagen Supracrustal obtained for the carbonate facies (Groups B and C) and for Series. Central Sweden, represents a Middle Proterozoic the transitional zone between the carbonate and silicate aulacogen system, The stratigraphic sequence of this series facies (Group E). The b"O values do not shift as expected consists of the Lower Leptite Group, which corresponds because Group E is depleted in 180 compared with Groups to the metamorphosed acid volcanics of the Early Voieanic B and C. For this reason the model proposed in Fig. 5.A Stage; the Middle Leptite Group, which is derived from appears to be false because there was no equilibration acid volcanics, and siliceous iron formation, and with continental meteoric water, 01' fresh water derived car­ the Upper Leptite-hãlleflinta Group which eorresponds to bonate during the time of the deposition of the Contendas- Revista Brasileira de Oeoctênctas, Volume 12 (l.3), 1982 423

'/////, ,.------.-.- --,,~.------.-"---... S, L 1

~"I" Fet~-.-F;3;I--·~-"---,;;tt-:=-FeH "', S, L. 2

Fe+t . /,e

-----jlJl?/~--....-.-.----..------"SL. H ~ ''';~ Fe -...

, I I I , PH e e N 7 7 I I I Oxiaizing I Slighlly Reducing , Slrongly I , Eh \ Reducing ,I I Reducino Fé 2<{Fe:H I Fet 2.:Ç"Fe,t I Fe3t~Fe2t ,I Fe 2 1 chemistr y si : co , Mg, Mn . : co, MO, Mo ,I S CO;~ I Si) Zn) Pb C (g r o trte} I SI I I : co-l EXPLANATION

S.L. 1- S.L.2 - Seo levei in rwo dit ter en t atcces of ° reçresstve morine bosio S.L.· sec l evel. 1- s tub Ie plolform. 1'- Volconic sub strcctum. 2~CollJmnofwOlerldeepesllone) 3 - Oxide focles. 4 - corbcncte to cie s } 5 ~ Silicole (ocies. 6- Sulfide rcc.e s iron formotion (moy ce cbsent inA)

Figure .5 ~ Diagrammatíc schemes of lhe irem cycle under marina conditíons A ..- Lawer Proterozoíc envíronment-miogeosyncline, B - Upper Archean enviranment-eugeosynclíne, Based upon James (1954), Borchert (1960), Elder (/965), and Frietsch (/982)

-Boquira Member. The Groups B, C and E show J I 8 0 process formed the calcite of Group D, but here the JI80 values much lighter than most marine limesroncs and may values may have been adjusted by equilibration with mag­ reflect the geologic time of deposition, equilibration with netite during the spathization of the carbonate and ore metamorphic waters and with the 160_rich magnetite du­ recrystallization. Actually, well recrystallized magnetite is ring the late Archean and Proterozoic orogenies. The iS 13C an abundant gangue mineral. The presence of maghemite, values for samples of Groups B, C and E are within the pseudomorphic after magnetite, and the martitization of range of most marine limestones which have á l3e values lhe magnetite indicate that magnetite did not remain isola­ of O±4 per mil regardless of the age of the formation ted from oxygen exehange making possible the isotopic (Keith and Weber, 1964; Schwarcz, 1969; Torquato, 1980) equilibration with the other gangue minerais. (Figs. 4 and 6). The average of J 13 C values ofGroup A and ofGroup D, CONCLUSIONS On the basis of the volcano-exhala­ -4.03 and -4.71 per mil respectively, are within the range tive origin for AIgoman type iron formation (Hutchinson of most fresh water derived carbonates. which range from et al., 1971), the stratiform Boquira Iead-zinc deposits may - 7.68 to - 2.18 per mil (Keith and Weber (1964)). Though be volcano-exhalative. They were deposited in the deepest Group A is formed only by one sarnple, 'íts vaiue is well zone of a shallow subaqueous environment. The banding explained by equilibration with percolating continental in lhe rocks of lhe Boquira Member cannot be regarded as water which solubilized the carbonate of the Contendas­ a feature of metamorphic segregation in view of the discor­ -Boquira Member and by transport in solution of the cal­ dance between the banding and the schistose plane direc­ cium carbonate that was redeposited in the planes of lhe tions (Nagell, 1970; Kaul, 1970; Schobbenhaus. 1971; cross-cutting fracture system. This process probably took this paper, Photo 4). AIso no clastic relicts have been found place during Upper Proterozoic time. An older but similar in lhe facies iron forrnation. It is suggested that the deposi- 424 Revista Brasileira de Oeocténctas, Volume 12 (1-3), 1982

O - 00 9 U1RA tional basin was cyclicly fed with solutions carrying the ORAlIL { ~-OTl!ERS iron (Fe+'), , lead, zinc and CO,', and comple­ xes of these ions. Furthermore, the deposition probably ~6~~~RIES O - was also controlled by the major specific .supply, If no silica was available to react with the ferrous ion, precipita­ tion of magnetite would took place. Therefore, if each ion is precipitated when its saturation point is reached its depo­ sition will also be cyclical (Photo 8). " The lateral facies variation is outlined in Fig. 5. It is a consequence of pH-Eh variations with the depth of the sea floor, As iron was an abundant ion, iron oxides oecur in ali the facies, magnetite being the most abundan!. This iron oxide oecurs, even in the shallower marginal zone, along with hematite and quartz to form the oxide facies. The pervasive 'character of the magnetite may be related to the primitive reducing atmospheric conditions. Lead and zinc may have been introduced by hydrothermal pro­ cesses accompanying nearly submarine volcanism. That volcanism was active during deposition of the Boquira Formation is shown by the close spatial occurrence of the ore, the iron formation and the volcanic rocks. Heie we " suggest two periods of major voleanic activities. The first is related to the formation of the original rocks of the V.M. and the volcano-exhalative events which originated the deposition of the CB.M. The second period of volca­ nism is related to the early, or rifting, stage ofthe aulacogen system which built the structural framework of lhe District. However, the geological similarities (folding, lithofacies, mineralization) between the B.F. and some other geologic terranes (Central Sweden and the Gamsberg dome in South Africa (Rozendaal, 1982), which developed from an aula­ -10 -2~ -22 cogen system, in addition to the lithochemical similarities between the preserved voleanic rocks of the V.M. and Figure 6 - bI 80 values of lhe Lower Precambrían Carbonates those volcanics of the T.M., are suggestive of a beginning (modified after Torquato. 1980) of the aulacogen system at the end of the Archean or during early Proterozoic. The stable isotope geochemistry ofthe carbonates suggests that the Contendas-Boquira Member was deposited in the late Archean and that the b13C and b 18 0 values were adjus­ ted by equilibration with metamorphic and meteoric waters during Proterozoic metamorphism and uplifting, when car­ bonate was solubilized and redeposition of calcite took place locally. If decarbonation during metamorphism, as exemplified in the equation II, occurred, it was a local pro­ cess and did not affect significantly the original isotopic composition of the analysed materiais. Equilibration with magnetite explains the lower values for sparry calei te from stratiform ore as compared with sparry calcite from the cross cutting calei te veins. Similarly, the biSO values for carbonate from the magnetite-rich banded transitional zone between the carbonate and silicate facies, which give the lowest average value (bISOSMOW = 9.90 per mil), are also explained by equilibration with magnetite.

Acknowledgements The work presented in this paper is part ofthe "Projeto Pb-Zn na Região de Boquira. Vale do Rio Paramirim, Bahia" that is supported by the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq-file n." 30.1592;79). The authors are also indebted to Dr. Ian McReath, of the Department ofGeochernistry of Photo 8 - Transition zone between iron-oxíde and carbonate faeies. The iron oxide-poor band (Ieft portion) is composed o]: quart: VFBA, Brazil, for proof reading and comments on it, the (75~,,), hematíte (15'10)' calcite (5%), amphíboles (5'1~), and accessory Mineração Boquira S.A. for ready access to its mines and minerais ísphene and rutile) (1;-,,). Note a cakíte-rích subband ín for assistance in the field, the graduate student Gelbio Melo the iron oxíde-rích band (right portion). Transmitted líght, near/y in providing part ofthe samples analysed for stable isotopes, para/lel nicoís and Mr. F. Espourteille of the Penãrroya Coo Revista Brasileira de qeociêncías, Volume 12 (l-3), 1982 425

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