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GEOLOGIC SETTING and GENETIC INTERPRETATION of the BOQUIRA Pb-Zn DEPOSITS, BAHIA STATE, BRAZIL

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GEOLOGIC SETTING and GENETIC INTERPRETATION of the BOQUIRA Pb-Zn DEPOSITS, BAHIA STATE, BRAZIL 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, BAHIA STATE, BRAZIL ILSON O. CARVALHO·, HALF ZANTOp·· and JOAQUIM R.F. TORQUATO··· AB8TRACT The stratabound~straflform:Pb~Zn-AgMCd sulfide deposits of Boquira, located ln south-central Bahia State, occur in metamorphic rocks ofthe Archean Boquira Formation. This formation is composed ofaltered volcanic rocks, schists, quartzites, iron formation, and dolomitic marbles which are the metamorphiclequivalents of intermediate to acidic volcanic rocks, volcani­ clastic sediments, chert and iron-rlch chemical sediments. These rocks were intruded by granitic magmas in the late Proterozoic time. The massive to semimassive ore lenscs are conformably enclosed in the silicate facies of lhe Contendas-Boquira Member. The primary ore is composed of galena and sphalerite in a gangue of magnetite, maghemite, martite, and minor pyrite, pyrrhotite, chalcopyrite, quartz 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 'sulfides are also volcanic exhalative. ln addition, isotcpic analyses of carbonate suggest a marine depositional environment 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 metasomatism 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 Mining 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 granites and in the transitional contact with the gneiss­ plus secondary ores. -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 Paramirim 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 (cummingtonite­ 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-calcite-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 Phanerozoic 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 quartzite, considered as simpIe fades variations of the sarne strati­ epidotite, and a conglomerate-Iike polymict material which graphic unit because the field relationships and the petro­ may represent an altered polylithologic volcanic breccia. 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; OXIDE 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 graphite in the garnet. Banded texture is frequent1y ob­ yellowish laminae of quartz and bright brown, and grey served in core samples from diamond 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 iron oxide minerals 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 hematite ) ferent content of lhe minerais in each bando The mineral (> 70:/0 and quartz (> 30:/;,). ln some thin sections hema­ assemblage varialions are not repetitive, Noticeable changes tite is evident as a product ofmartitization.
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