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. O--J..j"t-t"-t'l~~~~~'r*-f+'M''h-,,..f'l-r- 11 Oxygen was extracted from the minerais by reacting the b. samples with RrF, (Clayton and Mayeda, 1963). The ,1"0_ -values are defined in the usual way and given relative to ,
SMOW. The analytical reproducibility is within ±0.2"/". 6 Temperatures were caiculated with a computer program (Hoernes, 1980). We have used for the calculation the experimentally determined fractionation curves for quartz -water according lo Matthews and Beckinsale (1979) and for magnetite-water according lo Bertenrath et ai. (1973). The lalter curve has been applíed also lo lhe caiculalion of quartz-hematite and quartz-martite temperatures. Further 15 a. more for a comparative discussion (see "Discussion") for 1ron o.uadrangle mation temperatures were also caiculated using lhe curve 12 n = 136 compiled by Friedman and O'Neil (1977), which is based on the data given by Becker (1971). 9
RESULTS Fig. 3 shows the ó"O-val.ues of'l36 investi 6 .. gated quartz samples which range between + 6"/00 and 3 - +20"/00 except one heavy value near +23"/00 (histogram a). With respect to regional differences (histogram b and c) o it is interesting that although the overall range of the ó-values is the sarne, on average the quartz of the low grade -4 -2 O 2 4 6 8 metamorphic W-area is isotopically heavier than in the SMOW 85~" high-grade E-area. ln the W-region of the values are Figure 4 - Ól 80 values oftron oxides (dominantly hematítes relative above + 12"/00' whereas in the E-region only 46;;, are in this SMOW from the whofe Iron Quadrangle (a), regionally subdtvided range. b and c; n = number of samples Revista Brasileira de Geociências, Volume 12 (l-3), 1982 351
To elucidate the differences in the fractionation factors n . of quartz and iron oxide and in the temperatures of forma O d. W-regian tion it is necessary to consider the cornplex deformation n= 35 02 history ofthe iron ores in the Quadrilátero Ferrifero (Fig. 2). 6 . 51+52+53 Fig. 5 shows the !l-values of the quartz-hernatite pairs of both regions subdivided after their predominant fabrics which were formed by different metamorphic and tectonic events. ln the E-regioo SI represents coarse-grained samples . of high metamorphic grade and primary foliation. The secondary slaty fabrics (",) cutting (s,) were produced by C. W-regian n" 22 recrystallization under- stress, ln the west region the three 6 02 samples near 8'/" (Fig. 5e) represent probably relictic pre 51 kinematic structures whereas most samples with A-values between 8%0 and 11.5%0 were recrystallized by metarnor 'phic schistosity (s,) parallel to layering of itabirite. Later the mineraIs wcre straightened out to axial plane cleavages and strain slip eleavages by ",. As Fig. 5 demonstrates, E- regian !l-values diminish generally from ",-samples in the west 01 to ",-samples in the east with a partial overlapping of"2-!l % 52+ 5 _ 51+ 3 -values in bothregions in the rangebetwecn 10°/00 and 14 0 As mentioned above, two different experimental deter mined CUrves for !l q-hem l'er"aS T ('e) exist, Fig. 6a (apper cune) shows our data ca1culated wíth a computer program after Hoernes (1980) whereas the lower curve represents the calculated formation temperatures using the curve com E-regian piled by Friedman and O'Neil (1977). As seen, the upper 01 curve indicares higher temperatures than the lower curve. 51 More specifically in Fig. 6h the difference between the two curves amounts to 140"C 00 the right side and reduces to 50 'e on the left low-temperature side.· Besides these dis crepancies, the most important point is that in the high-grade 10 12 14 18 20%0 metamorphic E·region as well as in the low-grade W-area 6 8 16 the ",-affected ore sam pies are always associated with lower temperatures than the unaffected. Obviously the eloser the Figure 5 - ô-volues quartz-hematiteoflhe W. and E-region, Quadri cleavage the more internal reaction surfaces favor isotopic látero Ferrífero, subdividedafter their predominant fabrics, according exchange at lower temperatures. Ali s,-samples of the to Fig. 2
6q-hm • hydrothermol minerQls °/00 W-region 20 .. • 52 a 16 5, • E- region 16 '0
14
12
lO
6
6
250 300 350 400 450 500 550 600 650 700 750 600'(
Figure 6a - Quartz-hematite õ-values versus calculatedtemperatures offormatíon. For the upper curve experimental curves accordingto Mat thews and Beckinsale, 1979, (quartz-water) Bertenrath et al., 1973,. ímagnetíte-waterv were used, whereas lhe lower curve contaíns lhe tempe rature values caículated after lhe curve compiled by Frtedman and O'Neil (/977) 352 Revista Brasileira de Geccíências, Volume 12 (l-3), 1982
6 q- hrn 0/00 10 W- reqion 18 • $1
16 O $, E- region
14 ... $1 S, 11
10 8 ....-..-.._._._._. 6 _139°(_. .-.
250 300 350 400 450 soo 550 600 650 700 750 1300 -c
Figure 6b - Ó. c-hm-T curves deducedfrom the sample poínts given in Fig. õa. The curves show convergence to lower temperatures. The dashed curve would gíve lhe best agreement with lhe field evtdence in the Iron Quadrangle
E-region with coarse-grained granoblastic textures are res lhe relatively dry mineral assemblages in lhe iron ores and tricted lo a high temperature field accompanied by a few in lhe adjacenl country rocks do not favor lhe idea that s,-samples from the southernrnost part of lhe W-region. large quantities of melamorphic waters have interacted with It is noteworthy that this high-temperature field is very the rocks in an open system environment, but favor the different depending upon which curve is used in Fig. 6: closed system environment. upper curve 815"C-635"C, lower curve 675"C-500"C. Hard ores, soft ores and lhe itabirites show identical A-values > 14.5"1"" and temperalures < 375"C ,(uppe/' oxygen fractionation values and the same temperatures of curve) and respectivcly < .:\I0°C tlower cune) represent formalion. Obviously ali rock types of lhe banded iron sam pies of extreme schistosity (s)) and in addition four formations were formed under the conditions of regional specularite-quartz pairs from hydrothermal veins and fis metamorphism, and Dorr's hypothesis of soft ore genesis sures. by quartz leaching in Mesozoic and Tertiary limes is not supported by our data, DISCUSSION There isa general agreemenl that lhe band Concerning oxygen isotope thermometry in metamorphic ed iron formations are of sedimentary origin and have been rocks there is considerable discussion about whether the precipitated in sea water. Assuming for the Precambrian calculated temperatures may represent peak metamorphic ocean a b180-value ofzero like today's ocean and lhe tempe conditions or whether they are intluenced by some retro ratures were around 25°C, we may estimate that the precipi- . grade isotope exchange during cooling (Deines, 1977; Dahl, tating iron oxides and hydroxides should have b180-values 1979; Graham, 1981). around - 5%" (Becker and Clayton, 1976) and for lhe chert As demonstraled above oxygen isotope fractionations
, were around + 35°/00 If such an assemblage is metamor correspond to the various deformation and recrystallization phosed, fractionations between the two phases musl decre phases and to the temperature record obtained from other ase with increasing temperatures. If the amount of oxygen field evidence. Relatively good agreement can be achieved bound in both .minerals is about lhe sarne and lhe water-rock when using the magnetite-water curve of Bertenrath et a/o ratio is sufficiently low lhe isotopic cornposition of both (1973) and the quartz-water curve of Matthews and Beckin minerais should move towards each other. Really, this is sale (1979). Staurolite, for exarnple, is a widespread mineral observed (Figs, 3 and 4), but lhe b180-values of lhe quartz in lhe country rocks adjoining lhe ore bodies in lhe E-region sam pies (+ 6%" lo + 17%") are lowered in a larger extent (Fig. 7). The work of Hoschek (1967, 1969) indicates that than the b180-values of lhe iron oxides (-2%" to +6%") the formation of staurolite needs minimum temperatures of are raised. The reason for this relationship might be that lhe Some 550"C. The breakdown of hornblende and lhe forma amount of oxygen in the iron oxides of the rich ores deter tion of clinopyroxene observed by Schorscher (1975) take mines the isotopic composilion .of lhe subordinate quartz place in the same temperature range. Schorscher found which reaches in some high-temperature sam pies very unu sillimanite in the eastern border zone where its formation suai values of as low as +6%". ln other words, during also needs relatively high temperatures. From this point prograde metamorphism of lhe iron ore lhe tluid: rock ratio of view the s,-temperature field between 635"C and 81O"C was progressively reduced by dehydration reactions and in (Fig. 6b, upper curve) shows better agreement wíth field lhe highest metarnorphic grade the amount of lhe fluid observations than the lower curve in Fig. 6b, which reaches may be so small that its isotopic composition may be buffe as low as 510"C. red by that of lhe mineral assemblage especially by that of If we use other isotopie fractionation curves such as the lhe iron oxides, and lhe tluid rnay acl as a kind of catalyst quartz-rnagnetite curve drawn by Friedman and O'Neil to perform isotope exchange between minerais. However, (1977) (based on the data of Becker, 1971) the calculated Revisto Brasileiro de Geocíêncías, Volume 12 (I·3), 1982 353 temperatures become considerably lower especially in the grade metamorphic iron ores as well as high grade silicate high temperature environment, and then the data must be rocks of the eastern border zone were not completely over interpreted' in terms of retrograde effects. As we know printed by the recrystallization and tectonics of the Minas from experimental petrology reactions, rates below 4OO'C orogenesis. This is also documented by the staurolite-silli are very low. If the lower curve in Fig. 6 is correct the tem manite assemblages in lhe Esregion. Fig. 7 shows the course peratures for one third of the samples would fali into the of the staurolite isograd together with the highest tempera field ofzeolite facies which is not consistent with the mineral tures of the rich ore bodies. East from Ouro Preto the cal assemblages of the adjoining phyllites and schists of culated temperature is 595'C (Fig. 7) whereas west from greenschist facies. However, the upper curve of Fig. 6 shows Ouro Preto it is 520'C, which agree with petrological data only a few cataelastic s]·aITected sam pies in the low tempe given by Hoschek (1967, 1969) who showed that the for rature range and in addition four hydrothermal mineral mation of staurolite takes place at 540 'C-565'C ± 20'e. pairs, On the other hand, two samples of the sl·type in the This relationship favors peak temperature for the Wcregion W·region indicate temperatures in the range 560'C-590'C in the triangle Ouro Preto - Itabirito - Congonhas do but should correspond to temperatures below 550'C accor Campo. ln addition, field obscrvations by Simmons (1968), ding to field evidence (Eichler, 1968). Schorscher (1975) and our own microscopic studies of rocks We thus propose that the Matthews and Beckinsale (1979) between Mariana and Ouro Preto revealed the two follo and Bertenrath et ai. (1973) calibration curves should be wing mineral reactions: ~ used for quartz-iron oxide isotope thcrmometry. Wc are Chloritoid + kyanite staurolite + quartz + H 20 fully awarc, however, that this is in contrast to some extent Chlorite + muscovitc-estaurolite + biotite + quartz+ H20 to the recent publication (Downs et ai.. 1981) and directly The course of the staurolite isograd in the northern branch determined isotopic fractionation values for quartz and is unelear (see "Regional aspecrs"). magnetite at 600'C and 800'e. The fractionation curves given by them, although lying between the Bertenrath et ai. (1973) and Becker (1970) curve, fit more elosely the data given by Becker (1970). ~ 5I18"DC61O:C. A ,I ln many cases, past discussions about lhe importance of 660--Co/1 ~bí;'a retrograde effects might be due to the fact that other oxygen , ,/, curves as lhe magnetite-water curve . 780"-Pi
in the area between the towns of Itabira and Nova Era. He since s2~atTected ores are restricted to the greenschist facies, concluded that the Minas metamorphism generally increa < 520 'C and d-values > 10'/00 (Fig. 6). Only a minority sed frorn west to east and changed from high-pressure Barro of the samples which seem to be s2-unaffected shows d-va vían-type to low-pressure Abukuma type. lues < 10.5'/00' like samples from Córrego de Feijão and Our data offer a different interpretation ofthe petrological others, Another sample group with no clear-cut evidence and geological setting in the eastern border zone. During the of S2 ranges between 10.5'/ and II "] and indicates for Minas orogenesis high-grade metamorphic rocks ofArchean mation temperatures betwee;; 525 'C and 500 'e. Thus, a age were dislocated in the region east from the Iron Qua lot of samples yield peak formation temperatures in the drangle and thrust westwards over the sedimento sequences upper greenschist facies. of the Proterozoic Minas trough. The thrust nappes include Ali samples ofthe mines Cauê, Conceição and Dois Córre• itabirites and rich ore bodies which were partially stressed gos were attached on the diagrams to the W-region (Figs. by tectonic transport and recrystallized. 3 to 5), since the staurolite isograd after Schorscher (1975) ln many high-grade metamorphic ore samples the old runs some kilometers east from these mines. The presenta sl~foliation is crossed by c1eavages of S2' which is accom tion of the oxygen data and metamorphic temperatures in panied by an increase ofthe d-values ofs,-samples from an the above mentioned graphs would be clearer, if lhe com average vaiue of 7.5'/00 to 8.5'/00-14.0'/00 depending on the pact and s2-unaffected sarnpies of this mining district be extent of secondary recrystallization. The newly equili longed to the sample group of the E-region. brated samples give now mixed d-values. ln case ofcomplete Schorscher (1976) discussed discrepancies of sorne mela recrystallization .d-values reflect lhe temperatures of Minas morphic reactions with respect to metamorphic PT-relation metamorphism (Transamazônico Cycle, after Hurley et al., ships in that area. He observed that chloritoid disappears 1967). Besides the allochthonous ores of Prc-Minas age far to the west of Itabira whereas staurolite is evident only autochthonous Minas ores do also occur in the E-region. several kilometers east of Itabira. ln the intermediate area These itabirites and other sediment sequences of Minas age chloritoid exists only in armored relics included in garnet. were only affected by lhe Minas metamorphism and there Torres et ai. (1969), Geisel et ai. (1972) and Schorscher (1975) fore the bulk of lhe samples reflects in both regions the tem described parautochthonous and allochthonous nappes from peratures and A~values of this metamorphic event. the Itabira rnining district, namely frorn the mines Cauê, The samples of the mine Piçarrão are with one exception Conceição and Dois Córregos, where tectonic units of quite (n." 161) s2-unaffecled and have very low d-qu-hem values different stratigraphic positions were thrust West and North between 8.4%, and 6.2'/00 which is equivalent to high me west during the Minas orogenesis. Probably lhe high grade tarnorphic temperatures in the range of 640 "C-780'e. Simi metamorphic ores of the Itabira area belonged erigi lar relationships show lhe samples ofthe mine Morro Agudo. nally to the E-region and were displaced by tectonic trans There is no doubt that these rich ore bodies belong lo the port into their present position. Additional investigations Archean. 18 0 temperatures of granulitic rocks from the in the ore bodies and related country rocks are certainly Archean granulitic belt in eastern Minas Gerais (unpubli needed to solve this problem. shed results by Herberl'and Hoefs, 1980) yield temperature Rosiére (1981) made a very interesling observation near values in lhe sarne range between 600'C and 8oo'e. lhe contact of the Pico de Itabirito ore bodies with the ln other mines of the eastern border zone.isuch as Cauê, Archean basement. He found an intense fracturing s) in Conceição, Dois Córregos, Água Limpa, Morro Jacutinga, lhe s2-affected iron ores due to lhe tectonic uplift of lhe Fazendão, Germano, and Timpopeba, the s2-unaffecled Archean Complexo de Bação, which caused drag tectonics high grade ores range between 590'C and 710'C (Fig. 7). in the adjacent Minas metasediments, Those s,-affecled ore They may represent thrust units of Archean age. sam pies yield d-values about 13'/ and recrystallization tem Of great regional importance is an itabirite pebble, which peratures as low as 310 'C (ng 245 Pico de Itabirito), was found in a conglomcrate near Pico de Itacolomi by 326 'C (no. 268 Águas Claras), 250 'C (no. 112 Fazenda Glõckner (1981). Its formation lemperalure was calculated Alegria), 310 'C (no. 063 Andrade). to be 594 'e. Glôckner (1981) describes occurrences of ln conclusion, oxygen isotope data and ca!culated tem staurolite and kyanite from Chapada and near Passagem peratures of metamorphic recrystallizations support earlier de Mariana. One of lhe authors (Müller) found staurolite results published by Barbosa (1960), Torres et ai. (1969), and kyanite in mica schists east of Ouro Preto and further Geisel et ai. (1972), Schorscher (1975) and Glôckner (1981) more Roeser (1977) described the paragcnesis staurolile + namely that in the eastern and southern border zone of garnet + biotite + quartz in mica schists east from Mariana. the Iron Quadrangle nappes were thrust upon lhe Minas Thus lhe calculated temperature of 594 'C agrees with the seríes, Our results show that units of banded iron formation petrological relationships. Glõckner (1981) points out that in the nappe systems are polymetamorphic. These units the eastern part of the complex Itacolomi nappe system in underwent an earlier high-grade metamorphic evenl and lhe area of Córrego de Belchior very probably consists of later during the Minas metamorphism they became more or Maquiné metasediments, These quartzites and garnet schists less reequilibrated. Some ore samples from the northeastern of Pré-Minas age were thrust over the Minas sequences. border zone show analogous "forrnation temperatures" bet ln the soft ore region near Congonhas do Campo we sam ween 6OO'C and 800'C like granulite facies rocks (chamo pled some big hard bloeks which look like erratics (Mine ckites) from eastern Minas Gerais. Baixa de João Pereira). Normally in this region lhe calculated temperatures belong to the field of greenschist facies bet ween 4OO'C and 520'C, but the "erratic block tempera Acknowledgements This research has been supported tures" are 650'C and 690'C and indicate an allochthonous by grants from: Deutsche Forschungsgemeinschaft, Bonn, position of high grade metamorphic rocks. Kernforschungsanlage Jülich and Conselho Nacional de The rnajority of the samples from the W-region shows Desenvolvimento Cientifico e Tecnológico (CNPq), Brasilia. also a subdivision due to the recrystallization phenomena, We especially thank Domingos Drumont and M.T. Vaz de Revista Brasileira de Oeocíénctas. Volume 12 (1·3), 1982 355
Melo (Companhia Vale Rio Doce), C. Marçal Mendez (Mi and Dr. J. Carvalho da Silva, Universidade Federal de Ouro nerações Brasileiras Reunidas), Dr. J. Büchi (Samitri), Dr. Preto, for logistic help. Dr. Ch. Hoffmann, Freiburg, and J. Eichler (Ferteco), J.J.Stettler(Samarco), and many geolo Dr. H. Glôckner, Clausthal, supplied some additional sam gists of these mining companies, which gave support during pies. For helpful discussions we thank I. Guba, Clausthal, the sampling compaign. We are indebted to Dr. H. Roeser and for some preparation-work S. Rahner, Clausthal.
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