Economic Geology
Vol. 95, 2000, pp. 405–428
Geochemistry and Petrology of Some Proterozoic Banded Iron-Formations
of the Quadrilátero Ferrífero, Minas Gerais, Brazil
C. KLEIN†
Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131
AND
E. A. LADEIRA*
Instituto de Geociências, and Museu de História Natural, Universidade Federal de Minas Gerais, Belo Horizonte, CEP 31270-901, Brazil
Abstract
This study presents detailed geochemical and petrologic data for 16 samples collected from four different
iron-formations in the Early Proterozoic Minas Supergroup of the Quadrilátero Ferrífero. These banded ironformation samples represent the best preserved, unweathered, and least altered iron-formation precursors to
the major hematite-rich ore deposits of the region. Since the 17th century, the Quadrilátero Ferrífero has been
recognized as a major gold- and iron-rich province, which has resulted in it being the best-studied Precambrian
region in Brazil.
The banded iron-formations, locally known as itabirites, and associated rocks, including dolomites, all are
part of the Proterozoic Minas Supergroup with a minimum Pb/Pb age determined on the dolomites of 2.4 Ga.
The samples were obtained from fresh outcrops in the field, from fresh exposures in open pit mines or from
deep diamond drill cores in four localities: the Santuário da Serra da Piedade, and the open pit iron ore mines
of Pico do Itabirito, Águas Claras, and Mutuca. All these have hematite as the only or principal iron oxide;
maghemite was found in only one sample. This is in contrast to the common occurrence of magnetite as a major
iron oxide in many Archean and Early Proterozoic iron-formations. Although all four localities have been affected by low-grade metamorphism and complex tectonics, none of the assemblages studied show any metamorphic reaction between the oxides and quartz, or between the oxides and carbonates and/or quartz to form
metamorphic iron-rich silicates. All of the samples studied (except one that contains maghemite) consist of
quartz-hematite, quartz-hematite-carbonate, or hematite-carbonate. It is unknown whether all or only part of
the hematite represents a primary phase. In view of the deep lateritic conditions in much of Brazil it is likely
that not all of the hematite reflects a primary Fe3+-rich oxide or hydroxide phase, but that some of the hematite
is a secondary oxidation product.
The overall major oxide bulk chemistry of these four iron-formations is very similar to that of most Archean
and Proterozoic iron-formations studied except with respect to their very high Fe2O3 (due to hematite) and correspondingly very low FeO contents. Many of the REE patterns of the iron-formation samples show a considerable depletion in light REE, a small negative Ce anomaly and a positive Eu anomaly. Such positive Eu anomalies are common in most Archean and Proterozoic iron-formations and are considered to reflect the result of
deep-ocean hydrothermal input into an otherwise highly seawater-dominated system. Such a hydrothermal origin is corroborated when the sum of the REE is plotted against Co + Ni + Cu with the resulting data points
clustering in the hydrothermal deposits for metalliferous ocean bottom sediments. The carbon isotope compositions of the carbonate-containing iron-formations range from δ13C of –1.055 to –5.083, which coincides
with a range of values for other Proterozoic iron-formations of South Africa (Kuruman iron-formation sequence).
Introduction
THE QUADRILÁTERO Ferrífero (“Iron Quadrangle”) lies in the
central part of Minas Gerais, southeastern Brazil (Fig. 1). Belo
Horizonte, with 2,500,000 inhabitants, is the capital of Minas
Gerais, and is located 490 and 700 km by highway from Rio de
Janeiro and Brasilia, respectively. The Quadrilátero Ferrífero
is a mountainous region, 650 to 1,500 m above sea level, and
forms the divide between two major Brazilian drainage systems, the São Francisco and Doce Rivers. It has been one of
the most important gold producing regions in Brazil for the
past 200 to 300 years and for iron ore in the last 50 years.
The Quadrilátero Ferrífero is recognized as one of the classical Precambrian areas of the world because of the regional
geologic mapping project, at the scale of 1:25,000, undertaken
jointly by the Departamento Nacional da Produção Mineral
† Corresponding
author: [email protected]
*Correspondence address: Rua Muzambinho, 355, Apt. 404, Mangabeiras,
CEP 30310-280, Belo Horizonte, Minas Gerais, Brazil.
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and the U.S. Geological Survey. This resulted in the standard
stratigraphy of the region (after Dorr, 1969). A simplified version of this stratigraphy is given in Table 1. The Precambrian
includes from oldest to youngest:
1. Tonalite, throndjemite, granite-gneiss basement;
2. The Archean Rio das Velhas Supergroup, which includes
the Rio das Velhas greenstone belt, and hosts the major gold
deposits of the region, many of which are associated with
banded iron-formations;
3. The Proterozoic Minas Supergroup, which overlies the
older rocks with a tectonically transposed angular and erosional unconformity, and includes the Cauê Formation or
Cauê Itabirite* that hosts the hematite-rich iron ore deposits
of the region; and
* The term “itabirite” is extensively used in Brazil and denotes a metamorphosed iron-formation composed of iron oxides (hematite, magnetite,
martite), abundant quartz, very rarely mica, and other accessory minerals. It
may be schistose or compact.
405
406
KLEIN AND LADEIRA
FIG. 1. Location of the Quadrilátero Ferrífero in the central part of the state of Minas Gerais, southeast Brazil. Several
of the major iron mines and gold mines, associated with Precambrian iron-formations, are located on the map.
4. The Itacolomi Group, which rests on the Minas Supergroup
with a slight angular and profound erosional unconformity.
The overall metamorphic grade in the western part of the
Quadrilátero Ferrífero, where the banded iron-formation
samples were collected, is primarily lower greenschist facies,
with amphibolite and granulite facies rocks along the eastern
edge (Guimarães, 1951, 1966; Hoefs et al., 1982).
The Rio das Velhas Supergroup that makes up the Rio das
Velhas greenstone belt was deformed and metamorphosed at
about 2.8 Ga, as determined from a minimum Rb/Sr age from
muscovite in a mica schist near the contact of the Bação Com0361-0128/98/000/000-00 $6.00
plex (Herz, 1970; see Fig. 2). Subsequently, Cordani et al.
(1980) defined a Rb/Sr isochron with a minimum age of 2.0
Ga for the Minas Supergroup and for the Minas or Transamazonian orogenic cycle. More refined dating methods have
produced more precise results. U-Pb single zircon ages
(Ladeira and Noce, 1990; Machado et al., 1992, 1996) are as
follows: Rio das Velhas Supergroup felsic metavolcanics, 2776
+23/–10 Ma, with zircon xenocrysts of 2900 to 3000 Ma possibly related to older sialic crust; granite bodies with 2776
+7/–6 Ma and 2730 ± 10 Ma ages indicating coeval plutonism
related to the Rio das Velhas greenstone belt volcanic, metamorphic, and deformational evolution.
406
BANDED IRON-FORMATIONS, QUADRILÁTERO FERRÍFERO, BRAZIL
407
TABLE 1. Proposed Precambrian Stratigraphic Column for the Quadrilátero Ferrífero, Minas Gerais, Brazil
Brasiliano Orogeny (?): 0.8 Ga
Transamazonian Orogeny: 2.0 to 1.125 Ga
Rio das Velhas Orogeny: 2.8 to 2.7 Ga
Minas Supergroup as based on Dorr, 1969, and the Nova Lima Group as based on Ladeira, 1980a, and Ladeira and Viveiros, 1984; a composite column
meaning that some stratigraphic units may be absent in some localities; age data from Babinski et al. (1991), Machado et al. (1992), and Renger et al. (1994);
Thicknesses are approximate
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407
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KLEIN AND LADEIRA
FIG. 2. Generalized geologic map of the Quadrilátero Ferrífero showing several of the iron and gold mines (after Dorr,
1969, Herz, 1970; Thorman and Ladeira, 1991; updated for this paper).
U-Pb ages (Machado et al., 1992) on sphene and monazite
(from pegmatites) place the main Early Proterozoic
Transamazonian metamorphic, deformational event between
2030 and 2060 Ma, and also give a minimum age for the
Minas Supergroup. Detrital zircons from metagraywackes allegedly at the top of Sabará Formation of Dorr (1969), which
has been raised to the rank of Sabará Group (Ladeira, 1980a),
show an age of 2125 ± 4 Ma (Machado et al., 1992, 1996).
Pure dolomites, related to itabirites of the middle section,
that is the Gandarela Formation of the Minas Supergroup,
give an age of 2.4 Ga for this Supergroup (Babinski et al.,
1991). Renger et al. (1994) interpreting zircon U-Pb data,
bracket the age of the Minas Supergroup between 2580 Ma
(base of the Moeda Formation) and 2050 Ma, an interval of
500 Ma. They correlate its overall features with those of the
Transvaal Sequence of South Africa.
Therefore, the Minas and Itacolomi sequences are Early
Proterozoic in age. Post-Itacolomi Group pegmatites and related granitoids have K/Ar and Rb/Sr dates, respectively, on
biotite and feldspar, ranging from 0.8 to 1.0 Ga (Guimarães,
1966; Herz, 1970, 1978; Teixeira, 1985).
Younger granites and unfoliated pegmatites of the Quadrilátero Ferrífero have been dated at about 500 to 600 Ma and
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this last period of metamorphism, with lesser deformation, is
related to the Brasiliano cycle (Brazilian orogenic cycle)
(Herz, 1970; Cordani et al., 1980; Teixeira, 1985).
Thus, the Quadrilátero Ferrífero consists of a polymetamorphic, highly deformed terrain, which masks its primary
nature and has caused repetition and omission of strata.
It is the purpose of this study to present a comprehensive
set of detailed analytical data for some of the best preserved
Early Proterozoic iron-formations of the Quadrilátero Ferrífero. In this regard Dorr (1973, p. 1007) wrote “despite the
economic importance of the ore deposits, the iron-formations
are relatively poorly known, for exploration has been directed at the high-grade ores and leached material. The area
is one of deep weathering and most of the iron-formations
are covered either by a carapace of canga* or by soil and debris. Outcrops of unweathered iron-formation are widely
scattered, small and generally difficult to find. Little research
has been done on the unweathered iron-formations and
* Canga is a Brazilian term for a tough, well-consolidated, unstratified,
iron-rich rock composed of varying amounts of fragments derived from
banded iron formation (or itabirite). It occurs as a near-surface or surficial
deposit and blankets older rocks. It is very resistant to erosion and chemical
weathering.
408
BANDED IRON-FORMATIONS, QUADRILÁTERO FERRÍFERO, BRAZIL
chemical analyses of the fresh rocks are scanty and widely
scattered.”
The Minas Supergroup
Here we describe in some detail only the Minas Supergroup from which we obtained the banded iron-formation
samples (itabirite) for the present study. A general, simplified
stratigraphic column is given in Table 1.
The Minas Supergroup rests unconformably on granite,
gneiss, and migmatite, or is in sheared contact with these
rocks; it is also in unconformable, disconformable, or sheared
thrust and transcurrent contact with the Rio das Velhas Supergroup. The Minas Supergroup consists of three metamorphic sequences:
1. The lowest unit, the Caraça Group, made up of clastic
sedimentary rocks;
2. The middle unit, the Itabira Group, consisting mainly of
chemical sedimentary rocks, is economically important because it contains the huge iron deposits of the region that are
hosted in its basal unit, the Cauê Itabirite or Formation. This
is overlain and partly interbedded with the Gandarela Formation, made up of dolomites and intercalated dolomite-rich
itabirites, manganiferous itabirites, carbonaceous phyllites
and dolomites; and
3. The upper unit, the Piracicaba Group, clastic in nature,
with subordinate dolomites. Part of this unit is the Sabará
Formation for which the stratigraphic position is not well established. It consists of mica-chlorite schist, thin interbedded
mafic volcanic quartzose rocks, banded iron-formations, and
fragmental quartzose schists.
Metamorphosed mafic dikes cut the Minas Supergroup in
several iron ore deposits of the region. They show considerable foliation and have been metamorphosed to greenschist
facies, but have not been dated.
Younger rocks of the Quadrilátero Ferrífero include diabase dikes and gabbros (which are thought to be of Cretaceous age, because they are not affected by regional metamorphism), unconsolidated surficial deposits and laterite and
bauxite of Tertiary and Quaternary age, and alluvium.
Structural Geology and Deformational Events
The polygonal outline of the Quadrilátero Ferrífero results
from a pattern of structurally controlled, delimiting mountain
ranges that trend north, northeast, and east, reflecting a complex deformational history (Fig. 2).
A regional structural analysis, integrating geological studies
done since the beginning of the 1970’s, allowed Ladeira and
Viveiros (1984, 1986) to establish a structural model in which
six deformational events were proposed that may have affected the Quadrilátero Ferrífero. Other authors have suggested only two deformational phases (Belo de Oliveira and
Teixeira, 1990; Chemale et al., 1990) whereas Marshak and
Alkmim (1989) concluded there had been several. Further integration of data from the iron ore and gold mining districts,
as based on the works by Grossi Sad and Piva Pinto (1986),
Vieira (1987), Simões (1988), Junqueira and Ladeira (1990),
Ladeira et al. (1990), Prado et al. (1991), enabled Ladeira
(1988, 1991) to infer four deformational events.
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409
The first event En–1, which was ductile, affected the basement and the Rio das Velhas Greenstone belt, and produced
Fn–1 recumbent isoclinal, reclined and inclined folds; their
hinges Bn–1, have a Sn–1 axial plane foliation that parallels, at
the limbs of folds, the bedding So of banded iron-formation
and graphitic phyllite. The intersection of Sn–1 with So, is a Lon–1
lineation parallel to mineral alignment on Sn–1, and to boudin
lines. All lineations of En–1 plunge approximately east at 45°.
The second, third, and fourth events affected the basement, the Rio das Velhas greenstone belt, the Minas Supergroup and Itacolomi Group. The second deformational event
En, was the first to affect the Minas Supergroup, and caused
Fn folds with similar style as Fn–1, with axial plane transposition foliation Sn and corresponding Ln lineation. Parallel to Ln
are hinges Bn, mineral alignment, and boudin lines. The hinge
lines of the first two events are predominantly coaxial and
have about the same plunge. Shear zones are roughly parallel
to Sn foliation.
The third brittle-ductile event En+1 produced Fn+1 open and
kink folds and Sn+1 spaced cleavage. Hinge lines Bn+1 trend
about north-south and have small plunges, approximately to
the north or south, and are roughly perpendicular to the previous folds. An intersecting lineation Ln+1 parallels Bn+1. Highangle, normal faults parallel to Sn+1 are common.
The fourth event, En+2, occurred in a brittle regime and
caused widespread fracturing and joints with trends northnortheast, north-northwest, north-south, and east-west. Some
of these contain quartz and quartz-carbonate veins or veinlets, particularly in the carbonate-rich units of the Rio das
Velhas Greenstone belt and of Minas Supergroup, and some
bear specularite and magnetite.
The three-dimensional structural model of Ladeira and
Viveiros (1984) is based on Dorr’s (1969) geologic map of the
Quadrilátero Ferrífero and additional mapping over a tenyear period. On Dorr’s map (Fig. 2) the Moeda, Dom Bosco,
Santa Rita, and Gandarela synclines are separate structures.
In Ladeira and Viveiros’ (1984) model (Fig. 3) these synclines, as well as the folded structure of the Serra do Curral,
would constitute two major antiforms developed by recumbent folding during the first deformational event of the Minas
Supergroup and refolded during subsequent events into reclined and inclined folds.
Sampling and Analysis
Sixteen samples of iron-formation and associated lithologies were analyzed (from four different iron-formation sequences) for major, trace, and rare earth elements. Six of
these (containing carbonate) were analyzed as well for total
organic carbon and carbon and oxygen isotopes. All samples
selected for analysis were chosen in the field (or from deep
diamond drill cores) on the basis of the appearance of freshness, lacking any visible secondary alteration or oxidation. As
such, these banded iron-formation samples represent the best
possible iron-formation precursor materials (to iron ore) in
the Minas Supergroup of the Quadrilátero Ferrífero.
Original fragmentation and crushing of the samples were
accomplished in a Plattner mortar and pestle; all fine grinding was done in an agate mortar and pestle. Petrographic thin
sections were cut from a rock chip adjacent to the analytical
sample.
409
410
KLEIN AND LADEIRA
FIG. 3. Structural sketch of Proterozoic deformation in the Quadrilátero Ferrífero, Minas Gerais (modified after Ladeira
and Viveiros, 1984, 1986).
Major element analyses, using a combination of wet chemistry and X-ray fluorescence techniques, were made in the
chemical laboratory of J. Husler, in the Department of Earth
and Planetary Sciences of the University of New Mexico.
Trace element analyses for Cr, Co, Ni, Cu, Zn, Rb, Sr, Y, Zr,
and Nb were also made by J. Husler with an X-ray fluorescence spectrometer (Rigaku 3064M) using lithium tetraborate fused discs. The accuracy for major element determinations is estimated between 1 and 3 percent; for minor and
trace elements above 100 ppm between 5 and 10 percent; and
for lower ppm values it is more variable.
Rare earth element analyses were obtained by instrumental
neutron activation analysis (INAA) in the laboratory of L.A.
Haskin, Department of Earth and Planetary Sciences, Washington University, St. Louis, Missouri, using procedures described by Korotev (1987). Concentration values for Sc, As,
Br, Ag, Sb, Cs, Ba, Hf, Ta, Au, Th, and U, also obtained by
INAA analysis, are reported such that they represent 95 percent confidence limits. For further discussion of precision in
INAA results see Korotev (1996).
Total organic carbon and carbon and oxygen isotope analyses
were obtained in the Biogeochemical Laboratories of Indiana
University, Bloomington, Indiana, using procedures described
by Wedeking et al. (1983) and Wachter and Hayes (1985).
Electron microprobe analyses of carbonates were made
using a Jeol Superprobe 733 in the electron microbeam analysis facility of the Department of Earth and Planetary Sciences
and the Institute of Meteoritics of the University of New
Mexico.
Iron-Formations of the Serra de Piedade
The iron-formations of the Serra de Piedade are well exposed in the Santuário de Serra de Piedade at the northern
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edge of the Quadrilátero Ferrífero (see Figs. 1, 2). They are
part of the northeastern end of the range of the Serra de Curral (Figs. 2, 3), which exhibits a synclinal structure trending
from southwest to northeast (Dorr, 1969). At its northeast end
it swings to the north where it is folded into a sharp inverse
syncline, the Serra da Piedade Syncline (Alves, 1961). According to the map of the Serra da Piedade Quadrangle (by
Alves, in Dorr, 1969, plate 3), the rocks inside the nucleus of
the syncline (see Fig. 4), which are deeply eroded, belong to
the Sabará Formation (in Dorr’s 1969 stratigraphic column)
whereas the limbs are made up successively of the Cercadinho, Cauê Itabirite, and discontinuous segments of the
Caraça Group. The southern inverted limb stands out in relief due to the canga capping the Cauê Itabirite. This results
in an elevation of 1,700 m for the Serra de Piedade with the
surrounding areas at 800 to 900 m in elevation. This locality is
one of the few places in the Quadrilátero Ferrífero where totally unaltered iron-formations can be surface sampled in natural outcrops.
Four widely spaced iron-formation samples were selected
from various large outcrop exposures (for detailed geochemical analyses see Table 2; analyses 1–4). The iron-formation
samples are unusually hard and very fresh looking. No analytical data on these formations are available in the literature.
These iron-formation samples have a simple mineralogy
consisting of quartz and hematite. They are fine- to mediumgrained, granular, very well banded, with quartz-rich bands
having an average thickness of 0.5 cm, and hematite-rich
bands thinner and thicker than 0.5 cm (Fig. 5a). The average
grain size of the granular quartz is about 0.1 mm with some
more coarsely recrystallized patches reaching about 0.3 mm
in grain size. Most of the finely recrystallized hematite has an
overall grain size that is about one third smaller than that of
410
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411
FIG. 4. Geologic map of the Serra de Piedade region and location of banded iron-formation samples used in this study
(reproduced from Alves, in Dorr, 1969, plate 3, with some structural modifications by Ladeira).
BIF Sample and location number
Strike and dip of schistosity
Strike and dip of overturned beds
Strike and dip of beds
Inferred fault
Inferred thrust zone
Contact, approximately located
metamafic dikes and rocks
Basement: tonalitic, granitic gneisses and
migmatites with granites and pegmatites of
various Precambrian ages
Nova Lima Group undivided: mainly
metasedimentary and metavolcanic schists
(sheared greenstones) and phyllite, locally
with thin interbeds of Algoma-type BIF
and lenses of siliceous dolomite
Sheared Unconformity
Moeda Formation: mainly quartzite,
locally interbedded with
thin layers of schist and phyllite
Batatal Formation: phyllite, locally graphitic
Sabará Formation: phyllite, schist
Cercadinho Formation: mainly phyllite,
locally graphitic with lenses
of terruginous quartzite, qt
Cauê Itabirite: mainly itabirite with lenses of
compact hematite, h
Canga
LEGEND
BANDED IRON-FORMATIONS, QUADRILÁTERO FERRÍFERO, BRAZIL
411
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48.90
0.02
0.25
49.33
0.63
<0.01
0.04
0.06
0.15
<0.01
0.07
0.00
0.11
<0.05
99.63
45.01
78.3
Ctotal
Corganic
δ13CPDB(‰)
δ18OSMOW(‰)
FeOtotal
Fe2O3/FeO
1
SP-2
Wt %
SiO2
TiO2
Al2O3
Fe2O3
FeO
MnO
MgO
CaO
Na2O
K2O
P2O5
H2O(-)
Vols
S
Total
Analysis no.
Field number
Rock type
412
48.95
96.0
44.62
0.02
0.11
53.78
0.56
<0.01
0.05
0.07
0.19
<0.01
0.06
0.01
0.07
<0.05
99.60
2
SP-3
55.46
180.1
38.66
0.014
0.17
61.26
0.34
<0.01
0.02
0.11
0.16
<0.01
0.10
0.00
0.10
<0.05
101.00
3
SP-4
42.86
53.6
51.12
0.01
0.13
46.67
0.87
<0.01
0.06
0.06
0.22
<0.01
0.06
0.03
0.08
<0.05
99.38
4
SP-5
41.11
26.6
53.02
0.01
0.09
43.86
1.65
<0.01
0.07
0.06
0.17
<0.01
0.07
0.01
0.07
<0.05
99.15
5
Pico-1
45.70
180.2
48.05
0.01
0.05
50.48
0.28
<0.01
0.03
0.06
0.16
<0.01
0.07
0.06
0.06
<0.05
99.38
6
Pico-5
53.68
191.3
39.72
0.01
0.07
59.32
0.31
<0.01
0.06
0.07
0.16
<0.01
0.07
0.00
0.05
<0.05
99.91
7
AC-6
31.06
89.7
64.06
0.01
0.11
34.10
0.38
0.022
0.17
0.38
0.19
<0.01
0.10
0.01
0.39
<0.05
99.98
8
AC-8
Quartz-rich
0.85
0.10
89.07
0.01
0.18
0.08
0.78
0.066
2.0
3.13
0.20
0.03
0.08
0.00
4.73
<0.05
100.41
9
AC-2
12.170
0.023
–2.702
29.156
3.99
0.21
3.44
0.017
0.11
0.72
3.34
0.197
18.04
29.35
<0.01
<0.01
0.15
0.04
44.29
<0.05
99.76
10
AC-1
8.274
0.019
–1.819
29.657
29.03
43.0
2.06
0.025
0.26
31.46
0.73
0.340
13.74
22.27
0.09
<0.01
0.12
0.01
29.38
<0.05
100.54
50.35
81.2
1.84
0.058
0.77
55.21
0.68
0.242
8.76
14.16
0.16
<0.01
0.20
0.03
18.01
<0.05
100.18
11
12
AC-3
AC-5
Carbonate-rich
7.237
0.019
–1.055
29.754
35.59
68.3
1.40
0.065
0.20
38.92
0.57
0.311
11.97
19.64
0.04
<0.01
0.21
0.01
25.58
<0.05
98.98
13
AC-7
7.432
0.023
–2.175
27.680
29.62
17.4
2.66
0.07
1.84
30.94
1.78
0.341
13.98
20.68
0.09
<0.01
0.09
0.04
27.44
<0.05
100.01
14
M1
8.583
0.027
–5.083
26.946
27.48
19.8
1.68
0.045
0.40
28.92
1.46
0.536
13.17
23.48
0.10
<0.01
0.10
0.01
30.09
<0.05
100.05
15
M2
TABLE 2. Chemical Composition of Proterozoic Banded Iron Formations and Some Associated Lithologies from Four Different Localities in the Quadrilátero Ferrífero
9.961
0.509
–1.530
27.432
4.95
0.15
16.83
0.10
2.93
0.64
4.37
0.559
15.43
22.24
<0.01
0.83
0.11
0.04
35.86
<0.05
100.00
16
M3
412
KLEIN AND LADEIRA
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413
0.30
115
280
23
37
42
12.6
<1.0
32
9
3
3
6
0.20
149
229
20
37
43
10.7
<0.8
28
8
11
1
6
<1.0
1.7
<0.10
59
0.279
0.36
<3
0.115
0.071
0.106
0.68
0.116
0.09
<0.15
1.8
<3
0.07
0.55
1.8
0.10
56
0.525
0.46
<5
0.128
0.050
<0.04
0.152
0.025
<0.11
<0.06
1.0
2.3
0.10
1.05
2
SP-3
1
SP-2
47
41
12
3
6
7
129
337
18
40
41
3
SP-4
2.5
<0.15
60
0.504
0.8
<10
0.441
0.175
0.258
0.182
0.031
<0.10
<0.07
1.20
2.0
0.09
2.93
0.26
114
198
21
32
45
4.9
<0.4
31
12
5
3
7
0.28
118
209
17
30
42
12.9
<0.4
30
9
5
4
6
2.0
<0.10
71
0.472
0.8
<5
0.109
0.050
0.046
0.355
0.062
<0.09
<0.07
1.1
<2
0.07
0.86
5
Pico-1
4
SP-5
0.96
1.5
<0.04
0.44
4.7
<0.20
45
0.473
0.48
<2
0.083
0.041
0.026
0.158
0.031
<0.08
0.04
113
271
20
31
40
6.6
<0.4
35
10
3
0
6
6
Pico-5
55
38
11
8
0
6
119
330
16
35
42
7
AC-6
9
AC-2
10
AC-1
0.035
0.32
0.53
127
104
119
127
5
12
16
6
8
22
31
18
39
30
36
1.8
3.4
2.2
0.2
0.3
4.5
20
4
10
8
4
30
0
1
10
2
5
10
7
8
7
0.5
0.08
0.2
0.33
0.80
0.22
0.05
0.03
0.04
81
89
111
0.302
1.76
4.61
0.41
4.00
9.65
<2
2.0
5.9
0.106
0.448
1.276
0.031
0.176
0.834
0.017
0.065
0.236
0.045
0.187
1.005
0.0161
0.0290
0.151
<0.04
0.04
0.16
<0.04
0.007
0.02
0.90
0.29
<0.3
332
0.8
2.2
0.03
0.16
0.37
1.13
0.25
0.81
8
AC-8
Quartz-rich
Abbreviations: AC = Águas Claras; M = Mutuca; Pico = Pico do Itabirito; SP = Sierra de Piedade
1
Au values in ppb; all other trace elements in ppm; open spaces = concentration values not obtained
Sc1
Cr
Co
Ni
Cu
Zn
As
Br
Rb
Sr
Y
Zr
Nb
Ag
Sb
Cs
Ba
La
Ce
Nd
Sm
Eu
Tb
Yb
Lu
Hf
Ta
W
Au
Th
U
Analysis no.
Field number
Rock type
TABLE 2. Cont.
0.21
2.33
0.50
0.08
86
2.28
3.80
3.5
0.565
0.231
0.118
0.731
0.122
0.07
<0.07
<0.8
0.55
149
104
24
28
59
4.1
1.4
25
17
9
16
6
1.47
154
285
39
44
61
11.8
0.7
44
50
12
13
7
<2.0
1.4
0.11
58
4.32
9.3
5
1.10
0.50
0.215
1.13
0.180
0.18
<0.3
1.3
<3
0.74
4.40
11
12
AC-3
AC-5
Carbonate-rich
0.8
0.20
72
2.64
5.67
2.7
0.652
0.239
0.133
0.704
0.114
0.35
0.05
0.05
<2
0.82
1.09
0.86
124
161
26
29
54
4.5
1.3
30
47
9
17
6
13
AC-7
2.6
0.4
52
3.96
8.41
5.1
1.311
0.505
0.241
1.41
0.225
0.56
0.09
2.51
1.7
1.37
2.55
1.88
120
131
51
24
64
4.9
0.5
26
29
11
29
7
14
M1
0.95
0.07
65
2.73
5.05
2.0
0.628
0.341
0.155
0.991
0.164
0.22
0.05
0.7
<1.6
0.60
7.25
0.97
145
100
33
33
53
8.6
1.1
28
64
9
15
6
15
M2
4.07
128
16
21
6
44
4.5
3.3
29
16
16
36
12
<1.0
0.38
0.86
153
6.74
14.5
6.3
1.84
0.363
0.400
2.03
0.310
1.31
0.44
0.37
<1.0
6.05
2.03
16
M3
BANDED IRON-FORMATIONS, QUADRILÁTERO FERRÍFERO, BRAZIL
413
414
KLEIN AND LADEIRA
the associated quartz. Some, however, is much coarser with a
thin, tabular habit, especially in laminae that are locally parallel to schistosity. The outcrops commonly show a highly contorted metamorphic deformational fabric (see Fig. 5b). Their
bulk chemical composition averages 45.93 wt percent SiO2,
ranging from 38.66 to 51.12 wt percent (see Tables 2, 3), and
52.89 wt percent Fe2O3, ranging from 46.67 to 61.26 wt percent (see also Tables 2, 3). Their average FeO content is only
0.60 wt percent. All other major oxide components are less
than 0.30 wt percent (see Tables 2, 3). Their overall bulk
composition is in the range of most Archean and Proterozoic
iron-formations (plotted in Fig. 6) except for their very high
Fe2O3 and very low FeO contents, and their overall low CaO
and MgO values.
The Serra de Piedade banded iron-formation samples, as
well as all of the iron-formation samples listed in Table 2, are
very much depleted in all elements except Si, Fe, Mg, Ca, and
C. Concentrations of some of the transition metals (especially
Cr and Co) are slightly elevated over those reported by
Dymek and Klein (1988) for the Archean iron-formations of
Isua, Greenland. The Cr results in this study range from 104
to 154 ppm. The Co values range from 5 to 337 ppm and show
some of the highest concentrations in the Serra da Piedade
samples. Similar ranges of concentrations of trace elements
are reported by Klein and Beukes (1989) for the Proterozoic
iron-formations of South Africa (Kuruman Iron-Formation).
Most of the iron-formation samples of this study are anomalously low in Au, particularly those of the Águas Claras iron
deposit, where two quartz-rich samples have values of 0.8
(sample AC-2) and 332 ppb (sample AC-8). Ag is present in
some samples and correlates with Au. These are interesting
anomalies because some important gold deposits are hosted
by itabirites in the Quadrilátero Ferrífero (Ladeira, 1988,
1991; Cabral and Pires, 1995; Olivo et al., 1995).
The rare earth element (REE) data, given in Table 2, for
the Serra de Piedade samples, are plotted in Figure 7a normalized to the North American Shale Composite (NASC) and
chondrites. These patterns show a general depletion in the
light REE, and a small negative Ce anomaly, but lack a positive Eu anomaly that is present in most of the iron-formations
of Archean and Proterozoic age (see Klein and Beukes, 1992,
fig. 4.2.3). When the sum of the REE results is plotted against
Co + Ni + Cu, as in Figure 8, all samples of this study plot in
the general region of REE-poor hydrothermal deposits as
outlined by Bonnot-Courtois (1981) for the FAMOUS area
and in the Galapagos mounds. The field labeled “hydrothermal deposits” in Figure 8, represents data from green muds
and/or nontronite and is concluded to be the result of mixing
of hydrothermal solutions from suboceanic basalts with ocean
water.
Iron-Formations from the
Pico de Itabirito Iron Ore Open Pit Mine
The Pico de Itabirito deposit is located approximately 44
km south of Belo Horizonte in the Serra do Itabirito (Fig. 1).
The main orebody is located in the eastern inverted limb of
the Moeda Synform (Fig. 3) one of the largest structures of
the Quadrilátero Ferrífero and consists of a large lens of
hematite, which in plan view, has an ovoid shape amidst friable, iron-poor and iron-rich itabirites. It is located at an elevation of 1,585.8 m and is one of the highest peaks in the
Quadrilátero Ferrífero. The mine wall rocks are composed of
siliceous itabirites of the Cauê Formation, graphitic phyllites
of the Batatal Formation, and quartzites and minor phyllites
of the Moeda Formation (see Fig. 9). The general strike of
the orebody is about N 30° to 35° E and the dip of beds
ranges from 20° to 70° toward the southeast or northwest.
The Pico main orebody, which is made up of compact
hematite, has a spindle shape due to a pronounced lineation
plunging almost vertically to the southeast. This ore lens is
1,000 m in length, with an average thickness of 190 m, and
reaches a maximum known depth of 235 m.
Total ore reserves in 1986 were about 135 Mt of high-grade
hematite with reserves of very iron-rich itabirites amounting
to 300 Mt. Gomes (1986) reports that the Pico Mine, at that
time, was the third most important mine for Minerações
Brasileiras Reunidas, S.A. and in 1982 its annual production
reached 939,576 t. The ore is mined through open pit methods and is treated at a local ore dressing plant. Thirty percent
is sold from the mine itself and 70 percent is exported. Since
a
b
FIG. 5. Hematite-quartz iron-formation from Serra de Piedade. a. Photomicrograph, in doubly polarized light, of hematite
(black) and quartz (white to gray) bands. Length of bar is 1 mm. Sample SP-2. b. Outcrop photograph showing complex folding (squares on scale represent centimeters).
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TABLE 3. Averages and Ranges of Major Components in Bulk Analyses (see Table 2) of Four Different Iron-Formations of This Study (recalculated to
100% on an H2O- and volatile-free basis)
SP (Serra de Piedade)
n= 4
Pico
n= 2
AC (Águas Claras)
quartz-rich
n= 3
AC
carbonate-rich
n= 4
M (Mutuca)
n= 3
SiO2
45.93
38.66–51.12
50.96
48.05–53.02
65.36
39.72–89.07
2.73
1.40–3.44
9.11
1.68–16.83
TiO2
0.016
0.01–0.02
0.01
0.01
0.01
0.01
0.05
0.02–0.07
0.09
0.045–0.100
Al2O3
0.16
0.11–0.25
0.07
0.05–0.09
0.12
0.07–0.18
0.41
0.11–0.77
2.22
0.40–2.93
Fe2O3
52.89
46.67–61.26
47.57
43.86–50.48
31.68
0.08–59.32
39.60
0.72–55.21
26.01
0.64–30.94
FeO
0.60
0.34–0.87
0.97
0.28–1.65
0.50
0.31–0.78
1.67
0.57–3.34
3.28
1.46–4.37
MnO
<0.01
<0.01
<0.01
<0.01
0.03
<0.01–0.066
0.34
0.20–0.34
0.62
0.341–0.559
MgO
0.04
0.02–0.06
0.05
0.03–0.07
0.75
0.06–2.0
16.47
8.76–18.04
18.31
13.17–15.43
CaO
0.07
0.06–0.11
0.06
0.06
1.21
0.07–3.13
26.78
14.16–29.35
28.55
20.68–23.48
Na2O
0.18
0.15–0.22
0.17
0.16–0.17
0.18
0.16–0.20
0.09
<0.01–0.160
0.09
<0.01–0.100
K2O
<0.01
<0.01
<0.01
<0.01
0.02
<0.01–0.030
<0.01
<0.01
0.36
<0.01–0.830
P2O5
0.07
0.06–0.10
0.07
0.07
0.08
0.07–0.10
0.21
0.12–0.21
0.13
0.09–0.11
S
<0.05
<0.05
<0.05
<0.05
<0.05
<0.05
<0.05
<0.05
<0.05
<0.05
C
n.d.
n.d
n.d.
11.58
7.24–12.17
8.66
7.43–9.96
37.46
34.02
22.55
29.00
20.74
0.99
0.98
0.98
0.95
0.88
Wt %
Total Fe
Fe+
(Fe2++ Fe3+)
n.d. = not determined
1996 the ore has been transported from the mine to a nearby
homogenization plant by conveyor belt and from there it is
transported by train to the port of Sepetiba, from where it is
exported abroad.
Because all surface materials in this region (be it in outcrop
or in the open pit mine area) are deeply weathered and
leached, an extensive search was made among available exploration diamond drill cores to locate fresh iron-formation
material for this study. Although many partial chemical analyses are available for the ore types, no detailed chemical data
are available in the literature for precursor iron-formations of
this region.
Two fresh core samples were analyzed (see analyses 5, 6 in
Table 2). Both are finely banded with quartz-rich bands ranging in thickness from 1 mm to a maximum thickness of about
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0.5 cm. These quartz-rich bands alternate with hematiteand/or maghemite-rich bands of more variable thickness (Fig.
10a). The quartz grain size is most commonly about 0.10 mm
in diameter, although some quartz bands show granular, recrystallized grain sizes of up to 0.2 mm. The hematite (and/or
maghemite) grains are recrystallized to a somewhat smaller
grain size than that of the quartz. The hematite grains are
closely interlocked and show no platy or tabular habit. In
none of our samples was magnetite observed, which is in accordance with the observations of Rosière (1981).
Their SiO2 content ranges from 48.05 to 53.02 wt percent;
their Fe2O3 content from 43.86 to 50.48 wt percent; and their
FeO content from 0.26 to 1.65 wt percent. All other major oxide
components are less than 0.17 wt percent. The averages for
these lesser oxide components of the two Pico iron-formation
415
416
KLEIN AND LADEIRA
Legend
Serra de Piedade
Pico
Weight %
Águas Claras
Total
FIG. 6. Plot of the major chemical components of the iron-formations listed in Table 2, with the analytical results recalculated to 100 percent on an H2O- and CO2-free basis as given in Table 3. The shaded region represents the range of chemical oxide components for 204 complete chemical analyses of Archean and Proterozoic iron-formations listed in Table 4.2.1
of Klein and Beukes (1992).
samples (see Table 3), plot below (for CaO and MgO) or
within (for MnO, Al2O3, and Na2O) the range of 204 Archean
and Proterozoic iron-formations (Fig. 6). Their Fe2O3 content
is extremely high and their FeO content very low as compared to the banded iron-formation samples that define the
analytical data band in Figure 6. The REE data for the two
Pico de Itabirito iron-formation samples are plotted in Figure
7b. Both plots show a strong depletion in light REE with respect to the heavier REE. Both also show a small negative Ce
anomaly, and analysis 5 (Table 2) lacks a Eu anomaly, whereas
analysis 6 (Table 2) shows a considerable positive Eu anomaly
in both the NASC- and chondrite-normalized graphs. When
the sum of the REE results are plotted against Co + Ni + Cu
as in Figure 8, both analyses plot in or close to the field outlined as hydrothermal deposits.
Iron-Formations of the Águas Claras Iron Ore Deposit
These iron-formations are located to the southeast of Belo
Horizonte (Figs. 1, 2) and are part of the Cauê Formation.
The ore deposit consists of very large, pure hematite lenses
within a stratigraphic succession that has been overturned.
The Gandarela Formation (dolomites with interlayered
lenses of dolomitic and siliceous itabirites) makes up the footwall and the Cauê Itabirite (siliceous and dolomitic itabirites)
is the hanging wall (see Fig. 11). On top of the latter lies the
Batatal Formation (carbonaceous-graphitic phyllite; see Fig.
12), which in turn is overlain by the Moeda Formation, both
of which make up the base of the Minas Supergroup. The
structure of the deposit is part of the overall Serra do Curral
homocline (Dorr, 1969), which is the limb of a major regional
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fold with antiformal shape. Overlying this sequence, with a
shear thrust contact (Ladeira, 1991), is the Nova Lima Group,
which is represented by weathered metamorphosed mafic
volcanic rocks interlayered with volcanoclastic sediments.
The mine is reaching its ultimate pit size and depth. Due to
this deepening, formerly unknown fresh dolomitic lenses interlayered with itabirites, as well as interlayered black to gray
metachert beds, have become exposed.
The general strike of the main planar structure, a schistosity parallel to and transposing the bedding, is N 45° E with
dips ranging from 30° to 70° to the southeast. According to
Gomes (1986), the lensoid orebody is 1,650 m in length, 250
m in average thickness and about 500 m below the surface.
Details on the rock succession and origin of the Águas Claras
ore deposit are given in Viel et al. (1987).
The total ore reserves of the Águas Claras deposit, estimated in 1987, are 177 Mt (with an average grade of 68%
total Fe) and between 1973 to 1983 about 108 Mt of iron ore
have been mined. The ore is extracted through open pit
methods, treated at a local ore dressing plant and subsequently transported to the port of Sepetiba, near Rio de
Janeiro, from where it is exported. This deposit, which is
owned by Minerações Brasileiras S.A., is scheduled to be
mined out by 2003.
The iron-formations and associated lithologies sampled at
the Águas Claras deposit were obtained from very fresh appearing pods and lenses at lower benches of the iron ore
(hematite) open pit mine (see Fig. 10c). These insular occurrences range up to a maximum size of several tens of meters
in overall dimensions. They are hard, lacking in secondary
416
Sample/NASC
Sample/Chondrite
BANDED IRON-FORMATIONS, QUADRILÁTERO FERRÍFERO, BRAZIL
Sample/NASC
Sample/Chondrite
(a)
(b)
FIG. 7. Abundances of REE normalized to both the NASC (North American Shale Composite) and chondrites. a. Ironformation samples from the Serra de Piedade; b. Iron-formation samples from the Pico do Itabirito iron ore mine; Numbers in figure refer to analyses in Table 2.
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417
417
418
Sample/NASC
Sample/Chondrite
KLEIN AND LADEIRA
Sample/NASC
Sample/Chondrite
(c)
(d)
FIG. 7. (Cont.) c. Iron-formation samples from the Águas Claras iron ore mine; d. Iron-formation samples from the
Mutuca iron ore mine.
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BANDED IRON-FORMATIONS, QUADRILÁTERO FERRÍFERO, BRAZIL
Co+Cu+Ni (ppm)
Hydrothermal Deposits
Metalliferous Deep-sea
Sediments
REE (ppm)
FIG. 8. A plot of Co + Ni + Cu abundance versus total REE content (La + Ce + Nd + Sm + Eu + Tb + Yb + Lu) for
analyses listed in Table 2. The field labeled “Hydrothermal Deposits” encloses data for deposits from the FAMOUS and
Galapagos regions, which are mostly green muds and/or nontronite, whereas the field labeled “Metalliferous Deep-Sea Sediments” represents mostly deep-sea drilling project (DSDP) samples from East Pacific sites (see Bonnot-Courtois, 1981, for
an extended discussion of these data). Data points for all the iron-formation samples are generally within, or close to the field
labeled “hydrothermal,” suggesting a possible common origin.
Itabirito Peak
Cauê
Formation
Cauê
Formation
Batatal
Formation
(phyllites)
Moeda
Formation
(quartzites)
CANGA
SOFT ORE
MEDIUM ITABIRITE
DOLOMITE
SILICEOUS ITABIRITE
INFERRED CONTACT
HARD ORE
POWDERY SILICEOUS ITABIRITE
INFERRED FAULT
FIG. 9. Simplified geologic cross section of the Pico de Itabirito iron ore open pit mine (section no. 700, used with permission from Minerações Brasileiras Reunidas S.A.).
0361-0128/98/000/000-00 $6.00
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KLEIN AND LADEIRA
a
b
c
d
FIG. 10. Hematite-quartz iron-formations from the Pico de Itabirito, Águas Claras and Mutuca iron deposits. a. Photomicrograph in doubly polarized light of well-banded hematite (black)–maghemite (black)-quartz (white to gray) ironformation at Pico de Itabirito (length of bar is 1 mm) deposits. Sample Pico-1. b. Photomicrograph of well-banded hematite
(black)-quartz (white to gray) iron-formation at Águas Claras (length of bar is 1 mm). Sample AC-8. c. Outcrop photograph
in the Águas Claras iron ore pit of well-banded, broadly folded medium-grained carbonate (mainly dolomite with some calcite)-hematite iron-formation (squares on scale represent centimeters). d. Photomicrograph, in doubly polarized light, of irregularly banded, granular carbonate (mainly dolomite; ranging in color from white to gray) and coarser-grained hematite
iron-formation (length of bar is 1 mm) from the Mutuca iron ore mine. Sample M-2.
weathering, and display primary sedimentary features such as
pronounced mineral banding. In Table 2 the analyzed samples are grouped under two headings: quartz-rich (analyses 7,
8, 9) and carbonate-rich (analyses 10–13). Analyses 7 and 8
(Table 2; AC-6, AC-8) are of well banded quartz-hematiterich iron-formations in which quartz-rich bands show a maximum thickness of about 3 mm (Fig. 10b). The thickness of
the hematite banding is more variable ranging from less than
1 mm to somewhat thicker than the associated quartz-rich
bands. Analysis 8 (Table 2; AC-8) is from a sample in which
both the granular quartz and hematite are fine grained with
an average grain size of about 0.05 mm for both minerals.
Analysis 7 (Table 2; AC-6) consists of granular hematite and
quartz with the size of the hematite grains, on average, about
0.20 mm and the quartz at about 0.05 mm. Analysis number
9 (Table 2; AC-2) is of a dense, hard, black chert containing
finely dispersed graphite (Table 4), that is interbanded with
the quartz-hematite-rich iron-formation. The iron-formation
samples consist essentially of two main components: SiO2,
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which ranges from 39.72 to 64.06 wt percent, and Fe2O3,
which ranges from 34.10 to 59.32 wt percent (see Table 2).
FeO ranges from 0.31 to 0.38 wt percent. All other major
oxide components are below 0.78 wt percent. Analysis 7 has a
somewhat low SiO2 content (39.72 wt %) and a high Fe2O3
content (59.32 wt %, which recalculates to 41.5 wt % total
iron). These values suggest that this sample may have undergone some effects of the ore-forming process such as possible
leaching of quartz. The black chert sample (analysis 9 in Table
2) consists of 89.07 wt percent SiO2 with several percent of
enclosed dolomite. The four carbonate-rich samples (Table 2;
analyses 10–13) range from essentially pure carbonate
(dolomite with some intergrowths and overgrowths of ferroan
dolomite; see Table 4) to samples of interbanded dolomite and
hematite. The purest dolomite sample (analysis 11 in Table 2)
is coarse grained, off-white to light beige in color and was obtained from a 5-cm-thick carbonate band interlayered between hematite (see Fig. 10c). The banding in the dolomitehematite samples is clearly visible due to the red-brown color
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BANDED IRON-FORMATIONS, QUADRILÁTERO FERRÍFERO, BRAZIL
COMPACT HEMATITE
DOLOMITIC ITABIRITE
FRIABLE HEMATITE
PHYLLITE
SOIL
STRIKE & DIP OF BED
LATERITIC INDURATED CRUST
APPROXIMATE LOCATION OF SAMPLES
COLLECTED BY CK & EAL AND SAMPLE NO.
SILICEOUS ITABIRITE
421
FIG. 11. Geologic map of the Águas Claras iron ore open pit mine (modified after Gomes, 1986).
of the dolomite, alternating with black, massive hematite. The
dolomite bands are commonly 2 cm in thickness, alternating
with hematite bands of highly variable thickness. Analysis 12
(Table 2; AC-5) consists of fine- to medium-grained carbonate and massive bands of very fine-grained, anhedral
hematite. This sample has an unusually high Fe2O3 content
(55.21 wt %, equivalent to 38.9 wt % total iron) suggesting
that this may be another occurrence of banded iron-formation
that has undergone some effects of the ore-forming process
such as leaching of quartz. Table 4 shows that several of the
samples also contain calcite. Representative electron microprobe analyses of the carbonates in the samples from the
Águas Claras deposit are given in Table 5. All turn out to be
relatively close to the dolomite end member, except for some
ferroan dolomite in sample AC-1. The calcite in some of the
samples is essentially pure CaCO3.
The averages of most major oxide components of the three
quartz-rich samples (two iron-formations and one chert sample) plot well within or close to, the established range for
iron-formations (see Fig. 6), except for their high Fe2O3, and
low FeO contents. The REE results for all sample analyses
from the Águas Claras deposit are plotted in Figure 7c. There
is some light REE depletion in some of the samples when
normalized against NASC, but this pattern is reversed when
normalized against chondrites. Most of the samples show a
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slight negative Ce anomaly and a considerable positive Eu
anomaly. When the Co + Ni + Cu abundances are plotted
against total REE content (Fig. 8), almost all of the Águas
Claras samples are concentrated in, or close to the field for
hydrothermal deposits.
Iron-Formations from
the Mutuca Iron Ore Open Pit Mine
The Mutuca deposit is located near the junction of the
Serra do Curral and the Moeda Synform, about 12 km south
of Belo Horizonte (see Figs. 1, 3). The orebody, hosted by the
Cauê Formation, consists of a lens of hematite that is partly
within an overturned syncline. In plan view it has the shape of
a wedge elongated along a north-south direction. A simplified
geologic map is given in Figure 13, and a cross section of the
mine area is shown in Figure 14. The ore is in sheared fault
contact with western wall rocks that consist of phyllites and
quartzites of the Caraça Group and metasedimentary and
metavolcanic mafic schists of the Nova Lima Group. The
eastern wall rocks are dolomitic phyllites of the Cauê Formation. The southern end of the orebody is delimited by the intrusion of a basic dike. The general strike of the main planar
structure is parallel to a north-south schistosity, with dips variably towards west and east. The area is structurally highly
complex owing to a thrust fault with movements along a
421
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KLEIN AND LADEIRA
FIG. 12. Schematic geologic cross section through the iron ore open pit mine of Águas Claras (modified from Gomes,
1986).
frontal and a lateral ramp and wall rocks made up of tectonic
mélanges (Ladeira, 1991). This has resulted in the complex
fabrics described by Rosière and Chemale (1991) and Rosière
et al. (1993).
The orebody is 1,100 m in length, 200 m in average thickness
and extends to a maximum depth of 270 m below the surface.
Total reserves, according to Gomes (1986), are about 70 Mt of
high-grade blue hematite. He notes that the Mutuca has been
the second most important mine for Minerações Brasileiras
Reunidas, S.A. and in 1982 annual production reached
2,643,247 t. The ore is mined through open pit methods and is
treated at a local ore dressing plant. Thirty percent of the ore is
sold at the mine and 70 percent is now transported by a conveyor belt (partly underground) to a homogenization plant at
Olhos d’Agua. From there it is transported by train to the port
of Sepetiba, near Rio de Janeiro, to be exported.
TABLE 4. Observed Mineral Assemblages in All Samples of This Study
Sample
number
SP-2
SP-3
SP-4
SP-5
Pico-1
Pico-5
AC-6
AC-8
AC-2
AC-1
AC-3
AC-5
AC-7
M-1
M-2
M-3
Quartz
Hematite
X
X
X
X
X
X
X
X
X (~90%)
X (trace)
X
X
X
X (trace)
X (trace)
X
X
X
X
X
X
X
X
X
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X
X
X
X
X
X
Dolomite
Maghemite
Calcite
Biotite and Na amphibole (traces)
Na amphibole (trace)
X
X
X (also ferroan dolomite)
X
X
X
X
X
X
422
Additional minerals/
comments
Graphite
X
X
X
Graphite
0.119
0.021
0.840
1.021
87.59
0.001
0.073
0.904
1.022
99.89
4.49
0.77
17.81
30.12
0.012
0.010
0.013
1.966
52.00
0.04
2.86
20.23
31.83
0.061
0.153
0.916
1.008
93.76
0.42
0.34
0.25
54.21
0.005
0.005
0.004
1.986
44.44
2.35
0.58
19.79
30.30
0.059
0.018
0.934
0.989
94.06
0.036
0.015
0.936
1.013
96.30
0.026
0.000
0.950
1.047
97.33
0.005
0.004
0.964
1.026
99.48
Fe
Mn
Mg
Ca
Mg/Mg+Fe
0.354
0.009
0.616
1.021
63.50
0.119
0.013
0.845
1.023
87.65
Recalculated on the basis of 2 (Fe, Mn, Mg, Ca)
0.026
0.010
0.035
1.929
57.38
0.025
0.016
0.955
1.004
97.45
0.17
0.19
0.08
54.73
0.95
0.60
20.66
30.23
0.84
0.32
0.64
48.80
2.27
0.68
20.11
29.63
0.10
0.00
20.65
31.66
0.20
0.17
21.71
32.16
Wt %
FeO
MnO
MgO
CaO
12.96
0.32
12.67
29.19
4.46
0.48
17.84
30.03
1.36
0.55
19.65
29.58
AC-7
coarse
calcite
AC-7
coarse
dolomite
AC-5
fine-grained
calcite
AC-5
coarse
dolomite
AC-3
fine-grained
dolomite
AC-3
coarse
dolomite
AC-2
coarse
dolomite
AC-1
coarse
dolomite
AC-1
ferroan dolomite
overgrowth
TABLE 5. Representative Electron Microprobe Analyses of Carbonates in Iron-Formation Samples
M-1
coarse
dolomite
M-2
coarse
calcite
M-2
coarse
dolomite
M-3
average
dolomite
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423
In order to escape the very extensive deep weathering and
leaching of rocks in this region, iron-formation samples of the
Mutuca deposit were carefully selected from deep sections of
diamond drill cores. Chemical analyses are given in Table 1
(analyses 14–16). No complete data on these iron-formations
are available in the literature. Two of the samples (analyses
14, 15 in Table 2) are well banded, beige to light pink, and
consist of alternating bands of carbonate (mainly dolomite,
see Table 5) and hematite. The carbonate bands are most
commonly about 1 cm in thickness, whereas the interlayered
bands of hematite range in thickness from a few millimeters
to over a centimeter. Most of the dolomite in analysis sample
14 (Table 2) is fine grained with an average grain size of about
0.02 mm, but coarser grained patches and veinlets also occur.
The hematite in this sample is concentrated in centimeterthick bands in which some dolomite is sporadically dispersed.
The hematite is subhedral, fine- to medium-grained ranging
from about 0.05 to 0.1 mm in grain size. Analysis sample 15
(Table 2) is more finely banded with continuous carbonate
bands of about, on average, 3-mm thickness, interlayered
with discontinuous hematite bands that are about 1 mm thick
(see Fig. 10d). The carbonate is a mixture of dolomite and
calcite (see Table 5) and the hematite is fine-grained and anhedral. The third sample (analysis 16 in Table 2) is a black,
well-banded, fine- to medium-grained dolomite-quartz rock
with finely dispersed graphite throughout. The scale of banding in this sample ranges from less than 1 mm in thickness to
over several centimeters. As shown in Table 2, all three samples have low SiO2 contents ranging from 1.68 to 16.83 wt
percent. The first two samples have high Fe2O3 contents,
ranging from 28.92 to 30.94 wt percent, due to finely interbanded hematite. All three samples are very dolomite-rich
with sample M-2 (see Table 5) consisting mainly of coarse
dolomite as well as lesser coarse calcite. A calculated bulk average for these three analyses is given in Table 3. The REE
data for these three samples are plotted in Figure 7d. All three
show depletion in the light REE when normalized against
NASC as well as a suggestion of a slight negative Ce anomaly.
Two of the samples show a quite pronounced positive Eu
anomaly as well. When the REE data are plotted in Figure 8,
against the sum of Co + Ni + Cu, the data points fall within
or close to the field marked hydrothermal deposits.
Discussion and Conclusions
The samples from the four different Proterozoic ironformations in this study all have been affected by metamorphic overprint(s) and tectonic deformation(s). The overall
metamorphic conditions that these four iron-formation districts have undergone is generally considered to have been
low grade, within the lower to medium greenschist facies
(Guimarães, 1966; Dorr, 1969; Herz, 1970; Hoefs et al., 1982;
Ladeira, 1980a, b; 1985, 1991). More quantitative estimates
of the metamorphic grade on the basis of the simple mineralogy of these specific iron-formation assemblages are not
possible.
Samples from the Serra da Piedade, the Pico de Itabirito
Mine, and the Águas Claras deposit consist of finely banded,
granular quartz and hematite that show no mineralogical evidence of any metamorphic reaction. Quartz and hematite are
423
424
KLEIN AND LADEIRA
PHYLLITE
BASIC INTRUSIVE
QUARTZITE
FAULT
STRIKE & DIP OF BED
AND/OR SCHISTOSITY
COMPACT HEMATITE
FRIABLE HEMATITE
ITABIRITE
“CANGA” COVER
CARBONATE-RICH PHYLLITE
FIG. 13. Geologic map of the iron ore open pit mine of Mutuca (modified from Gomes, 1986).
indeed a stable mineral pair over a very wide range of metamorphic conditions (Klein, 1983). The dolomite (calcite)hematite-quartz samples of the Águas Claras as well as the
Mutuca mines show no reaction between hematite, quartz or
carbonate, to form metamorphic iron-rich silicates. These assemblages are in contrast to the common occurrence of several low metamorphic grade silicates (e.g., greenalite and/or
minnesotaite, in association with stilpnomelane) in the Proterozoic banded iron-formations of the Labrador Trough,
Canada (Klein and Bricker, 1977; Klein, 1978, 1983), and the
Lake Superior region (French, 1968; Floran and Papike,
1978).
All of the iron-formation samples are hematite-rich (containing no coexisting magnetite; one sample, Pico 1, see Table
4, contains abundant maghemite as well), which means that
they contain almost all of their iron as Fe2O3. In this study,
each sample was studied by reflected light microscopy using
polished sections, but no relict textures of magnetite were
found, nor was martite (hematite pseudomorphous after magnetite) recognized. As such, the hematite may well represent
a diagenetic to very low-grade metamorphic product of an
earlier, primary ferric oxide/hydroxide, of originally sedimentary origin (Klein and Bricker, 1977).
As shown in Table 2 and Figure 6, the overall major oxide
chemistry of these samples is very similar to that of most
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Archean and Proterozoic iron-formations studied in the literature, with the exception of their very high Fe2O3, and corresponding very low FeO contents. Even though every effort
was made in the field to sample only materials from natural
outcrop or artificial exposures (in mines), or deep diamond
drill cores that showed no visible oxidation and/or decomposition, it cannot be concluded on the basis of detailed microscopic observation of our samples that all of the hematite is
truly primary. Indeed, one of the samples (Pico 1) contains
abundant maghemite. Because of the deep lateritic weathering profiles in Brazil one cannot exclude the possibility of
some (or considerable) secondary alteration and oxidation.
Furthermore, as noted in the discussion of the banded ironformation samples from the Águas Claras deposit, a few of the
samples may have been subjected to part of the ore-forming
process such as possible leaching of quartz and oxidation of
iron.
The REE patterns of the Proterozoic iron-formations from
the Águas Claras and the Mutuca mines show well-defined
positive Eu anomalies in most instances (Figs. 7c, d). The
same patterns for the Serra da Piedade and Pico de Itabirito
occurrences are much less clear-cut (Figs. 7a, b). The presence of positive Eu anomalies in many Archean and Proterozoic iron-formation REE patterns (Dymek and Klein, 1988;
Junqueira and Ladeira, 1990; Ladeira et al., 1991; Klein and
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BANDED IRON-FORMATIONS, QUADRILÁTERO FERRÍFERO, BRAZIL
FIG. 14. Typical geologic cross section of the Mutuca iron ore open pit mine (modified after Gomes, 1986).
a. Archean to Early Proterozoic
Hydrothermal
Input
ck
Bla
Oxide BIF
hale
or s
t
r
che
BIF
Siderite
Stratified ocean system. Hydrothermal input
large. Production of organic matter low in
open marine environment.
b. Early to Middle Proterozoic
Hematite - rich BIF
Hydrothermal
Input
Loss of ocean stratification and diminished
hydrothermal input. Deposition of hematite-rich
BIFs on submerged platforms. Low productivity
of organic matter.
FIG. 15. Paleooceanographic model for iron-formation deposition from the Archean through the middle Early Proterozoic (after Beukes and Klein, 1992). a. Archean stratified ocean system with predominantly deep water deposition of microbanded iron-formation (in these magnetite can be abundant). b. Breakdown of ocean stratification in the Early to middle
Early Proterozoic and deposition of hematite-rich iron-formations (some of which can be oolitic in texture)
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KLEIN AND LADEIRA
Beukes, 1992; Bau and Möller, 1993; Pereira, 1995; Raposo,
1996; Junqueira, 1998; Raposo and Ladeira, 1998) is concluded to be the result of the input from sub-oceanic hydrothermal solutions from deep-sea spreading centers into
ocean waters (which are considered the source of the iron as
well as of SiO2 of the iron-formations). Such a hydrothermal
origin is corroborated by plots of REE (Fig. 8) against Co + Ni
+ Cu. Klein and Beukes suggest that iron-formations of Early
to middle Early Proterozoic age were deposited on submerged
platforms in the presence of some oxygen in the surface waters
(Fig. 15). It is possible that the hydrothermal input into the
deep ocean basin at this time was somewhat less than in
Archean times, as deduced from the lesser intensity of the positive Eu anomaly in the REE profiles (see Klein and Beukes,
1992). Bau and Möller (1993) conclude this decrease in the
positive Eu anomaly, with decreasing age, to be the result of
the lowering of the temperature of the hydrothermal solutions, as a reflection of decreasing upper-mantle temperatures.
The carbon isotope compositions for several iron-formations
with abundant carbonate are reported in Table 2. The overall
range of δ13C compositions is from –1.055 to –5.083, which
falls within the variations reported by Beukes and Klein
(1992) for a sequence of Proterozoic iron-formations of South
Africa (the Kuruman Iron-Formation sequence).
Acknowledgments
Research on these Brazilian Proterozoic iron-formations
has been made possible by National Science Foundation
grants EAR-9003552 and EAR-9404617 to C. Klein and
Brazilian Research Council grants 91022389-0 (Cooperation
NSF-CNPq) and 301100/82-9 (CNPq-Scholarship) to E.A.
Ladeira. We are grateful to J. Husler, Department of Earth
and Planetary Sciences at the University of New Mexico, for
the major element and trace element analysis; to R.L. Korotev of Washington University, St. Louis, Missouri for REE
analyses and additional trace elements by instrumental neutron activation analysis; and for the organic carbon and carbon and oxygen isotope analyses we thank J.M. Hayes who
was responsible for the Biogeochemical Laboratories of Indiana University, Bloomington, Indiana.
Thanks are due to the following iron ore mining companies
in the Quadrilátero Ferrífero for allowing us access to their
properties, mining operations, diamond drill cores and geologic information: Minerações Brasileiras Reunidas S.A.
(MBR), Companhia Vale do Rio Doce (CVRD), Rio Doce
Geologia e Mineração (DOCEGEO), FERTECO, Companhia Siderúrgica Nacional (CSN), and Minas do Itacolomy (Itaminas). We also thank the following professionals for stimulating discussions: N.R.A. Borges, J. Eichler, W.S. Fyfe, L.M.
Lobato, P.C.H. Moreira, S.L.M. Pereira, M.B. Prado, M.
Rossi, J.H. Grossi Sad, J. Henrique da Silva, the late Z. do
Pico, L.T. Siqueira, O.J. Tessari, and R.S. Viel.
We are grateful to M.E. Barley, G.A. Gross, and an anonymous reviewer for their comments, which much improved an
earlier version of this manuscript.
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