OXYGEN ISOTOPE VARIATIONS Ln POLYMETAMORPHIC IRON ORES from the QUADRILÁTERO Ferrífero, BRAZIL
Total Page:16
File Type:pdf, Size:1020Kb
Revista Brasileira de Gecciências 12(1-3): 348-355, Mar.-Sel., 1982 - São Paulo OXYGEN ISOTOPE VARIATIONS lN POLYMETAMORPHIC IRON ORES FROM THE QUADRILÁTERO FERRíFERO, BRAZIL OEORO MÜLLER', ALFRED SCHUSTER' and JOACHIM HOEFS" ABSTRACT This study deals with the oxygen isotope variations in rich iron ore bodies and itabirites ofthe Iron Quadrangle (Quadrilátero Ferrífero), Brazil. The (5180 values of 136 quartz -iron oxide pairs have been determined and temperatures of formation have been calculated. The (5180 values of quartz vary distinctly between + 5°/ and + 20°/ , whereas 80/,0 of the iron oxide values fali in the small range between + 4°/ and -l}.s0/ . inlthat way the ô-value variations of the quartz-hematite pairs are dominantly ru~ed by the ,tuartz. The Quadrilátero Ferrifero can be divided into two different regions : in the large western zone of greenschist facies the quartz is distinctly more enriched in heavy oxygen than that from the smaller eastern amphibolite facies. The variation of oxygen isotope Fractionations between quartz and the iron oxides is obviou sly related to the complex deformation history of the iron ores. Sam pies with only primary schis tosity (SI) represent peak metamorphic conditions. ln the Ecregion these samples yield temperatures of formation > 700 "C and seem to correspond to orogenic events in the Archean basement 2,700 Ma ego. ln the w-rcgton o5l-temperatures betwecn 460 "C and 560 "C seem to represent peak metamorphic conditions .of the Proterozoic Minas metamorphism 2,000 Ma ago. lron ores which have been overprinted by later deformation events (052, 053) are selectively reset to lower isotopic temperatures. The more closely spaced the schistosity planes the larger the extent of a temperature lowering. The genetic processes which formed the rich iron ore bodies appear to be sedimentary-meta morphic rather than metasomatic events. Furthermore, there is no evidence for secondary leaching by weathering solutions with respect to the genesis of the blue dust ores. INTROOUCTION I Precipitation of banded quartz-iron metamorphosed 2.7 Ga ago in the Jequié Cycle, As is well formations - known as itabirites in Brazil ~ was a dominam known, this orogenic cycle forms a worldwide pattern process in the Late Archean and the Early Proterozoic (Moorbath, 1976) and is documented by Rb/Sr determina oceans, Among the problems associated with these forma tions in the Quadrilátero Ferrifero (Herz, 1970; Cordani tions is the genesis of hematite rich ores such as those found et al.. 1980). ln other regions of Central Brazil Hasui and in the Iron Quadrangle, Minas Gerais, Brazi!. Almeida (1970). Cordani et ai. (1973), Cordani and Iyer According to Dorr (1965) the high-grade metamorphic (1979), Brito Neves et ai. (1979), and Sighinolfi et ai. (1981) hematite-rich ores were enriched by synmetamorphic repla yie1d similar ages betwcen 2.7 and 2.8 Ga. cement of quartz in the host itabirite by hematite derived On top of this basement a series of older sediments (Rio from the iron formation itself. Dorr assumed that transport das Velhas) and of younger sediments (Minas) is deposited. of iron to and quartz from the sites of replacement was The rocks 'of these two series were metamorphosed some performed by high-temperature fluids. Dorr also proposes 2.0-1.8 Ga ago. ln the Quadrilátero Ferrífero, to the regional that the hard massive hematite ores were partially converted metamorphism of this orogenesis was given the local name to soft ores by the leaching aetion of surface waters during "Minas metarnorphism" since the Minas sediments are the Mesozoic and Tertiary time. If metasornatic solution of youngest deposits which recrystallized during this meta quartz and replacernent by iron oxides took place, huge morphic evenl. As shown by Hurley (1967) this orogenic quantities oftluids were required to dissolve immense masses cycle ean be detected in Venezuela, in lhe Guyana States l of quartz which would amount to more than 50 ,l " of the and in Brazil north of the Amazonas River and was named itabirite volume. Really, there is no evidence for quartz "Transarnazônico Cycle". -leaching in the country rocks, as schists, gneisses and 50 Fig. I shows a rough geologic sketch of the Quadrilátero on. If metasomatic fluids caused the formation of the hard Ferrífero which is divided into a low-grade western and a ores and surface watcr leaching produced the soft ores we high-grade metamorphic eastern region, The boundary bet should find different isotope fractionation values in -the ween the two zones is marked by the breakdown ofchloritoid different types. This was the idea to start our investigations. and the appearance of staurolite. With respect to the meta morphic grade Simmons (1968), Eichler (1968), Moore (1969), Schorscher (1975), and Lauenstein (1981) investiga GEOLOGV ANO PETROLOGV Table I shows a sim ted parageneses of coexisting silicates in sedimentary iron plified stratigraphic column of lhe Quadrilátero Ferrífero. ore bearing sequences of the Iron Quadrangle and found which is divided into four divisions by unconforrruties. mineral assemblages in the W-region which indicate meta name1y the Archean basement, the Proterozoic Rio das Ve morphic temperatures of greenschist facies whereas the lhas series, the Minas series and the Post-Cretaceous sedi E-region is marked by mineral parageneses of the arnphi ments. The rock sequences of the basement were strongly bolite facies. "Mineralogisch-Petrographisches Institut der Technischen Universitãt, C1austhal, Adolf Roemer Str. 2 A, D~3392 Clausthal-Zellerfeld, German Federal Republic **Geochemisches Institut der Universitãt Gôttingen, V.M. Goldschmidt Str. 1, D~3400 Gôttingen, German Federal Republic Revista Brasileira de Geocténctas, Volume 12 (1·3). 1982 349 Table 1 - Simp/ified stratigraphic column ofthe Quadrilátero Ferrí• Phases of fero (afier Dorr, 1969; Roeser and Müller, 1977; Guerra, 1979; deformation Phenomena of deformation Glõckner /98l) Age Series Lithology and orogenic cycles o, Canga, lacustrine and stream sands, c1ays, sands, alluvium and colluvium -', Minas" 12 _..... " - - --' /" _ !. !.2 ~o~ ~1 r kl Rio das Velhas sI Basement Figure 2 - Deformatíon and recrystallization history of the Iron Quadrangle metasedímentary rocks (after Guba, 1981) • ltacolomi after Glõckner, 1981, part of the Minas series Schorscher (1975) found in high-grade melamorphic ro cks ofthe eastern border zone the paragenesis: sillimanite + + quartz + garnet + cordierite and interpreted the break down of amphibole and the formation of c1inopyroxene in amphibolites as the beginning of a granulitization processo ln some areas of the Quadrilátero Ferrifero three phases ofrock deformation can be distinguished (Fig. 2) but mainly the folding of only two (D, and D,) were accompanied by strong recrystallization of minerals even if lhe relationships in delaiI may be more complex (Harder and Chamberlin, 1915; Dorr, 1964, 1965; Dorr and Barbosa, 1963; Eichler, 20· 00' 1968; Hackspacher, 1979; Rosiêre, 1981; Guba 1981). Samples which show only s occur as well in lhe sarne out t tacotom Serles uncJlVlded crops and mines as those which were affected by s, +s, + S,. Sometimes lhe distance between cornpact ores and strongly Minas Series wifh schistose samples çovers only a few meters. This is very Cauê Formalian importam for lhe understanding of the different t.-values and "isotopic temperatures of forrnation" in closely neigh bouring sam pies (Fig. 6). um Bosemenl SAMPLlNG AND ANALYSIS Mosl of lhe rocks in Central Brazil are overlain by a thick weathered cover, but Thrust taul f cuts la eo km due to large mining excavations and road a lot Df . fresh sam pies are available. 375 samples of 3-5 kg weíght Fou/l were laken in lhe Iron Quadrangle (Fig. I) and supplemented Figure 1 - Rough geologic sketch after Dorr et at. (/969) and map by five samples frorn the Southern Serra do Espinhaço. of samplíng points. Dashed Une = "staurolite in" after Schorscher, Amidsl 380 sarnples, 143 (see Table 2) were selected for 1975 .. Roeser, 1977; Glõckner, 1981,' and own observattons near mineral separation through jaw crushing, pulverizing in a Ouro Preto. W = greenschist facíes area; E = amphibolite and gra disc mill, sieving. magnetíc separation in series of different nulite facies area; partially retrograde recrystallízed steps and fractionations, enrichment of specularite and tale 350 Revista Brasileira de Geociências, Volume 12 (1-3), 1982 Table 2 - Raw-materia! classtfícatton of investígated ítabirítes and rich ores • c ~"9i,"n ,,59 Nwnber Nome Fe(%) Physlcal propertles ofsamples 1:' i I I I i I I ",flfl" ',r;=;l",,'fl, Quartz-itabirite <30 Compact and hard or " arenaceous or dusty b. E-region weak . 2 n" 77 Itabirite 30-50 Compact and hard or (sensu stricto) arenaceous or dusty we~ ak, partially carbonace- o ous and silicate-bearing 30 n Rich itabirite 51-63 Compact and hard or dusty weak 29 " Iron Quodrongle n" 136 Hard hematite >63 Lumpy, hard, coarse ore ± magnetite grained, blueish-gray, magnetite ore sometimes very coarse grained 39 4 Soft hematite >63 fine flaky, platy, soft ore sliding,"blue dust" 39 Hydrothermal Variable Very coarse grained 4 veins on a vibrating table, heavy liquid separation, grinding in an Fig. 4 summarizes lhe ó"O-values of 136 hematite and agate mortar for separating the finest intergrowth, and in magnetite samples coexisting with the above mentioned alI cases stereo-microscopical selection of pure mineral quartzo The iron oxide samples range between + 10"/00 and ~ , concentrates by hand picking. 3,5°/00 ln contrast to quartz the iron oxides do not show Minerais used in the oxygen isotope investigations were: any regional differenccs. quartz, hematite. magnetite. martite and a few concentrates n of tale, , w- region Initially, sampies with small amounts of martite and mag n= 59 netite were separated from the main constituent hematite and measured separately. Since the concentrates of martite, magnetite and hematite did not show any difference in .,1"0 to that of the total iron oxide fractions, this cornplicated and time consuming procedure was considered unnecessary.