Spier C.A., Barros de Oliveira S.-M., Rosi?re C.A., Ardisson J.D. Mineralogy and trace-element geochemistry of the high-grade iron ores of the ?guas Claras Mine and comparison with the Cap?o Xavier and Tamandu? iron ore deposits, Quadril?tero Ferr?fe

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ARTICLE

Mineralogy and trace-element geochemistry

of the high-grade iron ores of the Águas Claras Mine

and comparison with the Capão Xavier and Tamanduá iron

ore deposits, Quadrilátero Ferrífero, Brazil

Carlos Alberto Spier & Sonia Maria Barros de Oliveira &

Carlos Alberto Rosière & José Domingos Ardisson

Received: 25 September 2005 /Accepted: 10 June 2007 /Published online: 2 October 2007

Springer-Verlag 2007

Abstract Several major iron deposits occur in the Quad-

rilátero Ferrí fero (QF), southeastern region of Brazil, where

metamorphosed and heterogeneously deformed banded iron

formation (BIF) of the Cauê Formation, regionally called

itabirite, was transformed into high- (Fe >64%) and low-

grade (30%<Fe<64%) hematite ores. Based on their

mineralogical composition, three major types of itabirites

occur in the QF: siliceous, dolomitic, and amphibolitic

itabirite. Unlike other mines in the QF, the Águas Claras

Mine contained mainly high-grade ores hosted within

dolomitic itabirite. Two distinct types of high-grade ore

occurred at the mine: soft and hard. The soft ore was the

most abundant and represented more than 85% of the total

ore mined until it was mined out in 2002. Soft and hard

ores consist essentially of hematite, occurring as martite,

anhedral to granular/tabular hematite and, locally, specular-

ite. Gangue minerals are rare, consisting of dolomite,

sericite, chlorite, and apatite in the hard and soft ores, and

Mn-oxides and ferrihydrite in the soft ore where they are

concentrated within porous bands. Chemical analyses show

that hard and soft ores consist almost entirely of Fe

with a higher amount of detrimental impurities, especially

MnO, in the soft ore. Both hard and soft ores are depleted

in trace elements. The high-grade ores at the Águas Claras

Mine have at least a dual origin, involving hypog ene and

supergene processes. The occurrence of the hard, massive

high-grade ore within “fresh” dolomitic itabirite is evidence

of its hypogene origin. Despite the contention about the

origin of the dolomitic itabirite (if this rock is a carbonate-

rich facies of the Cauê Formation or a hematite–carbonate

precursor of the soft high-grade ore), mineralogical and

geochemical features of the soft high-grade ore indicate that

it was formed by leaching of dolomite from the dolomitic

itabirite by meteoric water. The comparison of the Águas

Claras, Capão Xavier and Tamanduá orebodies shows that

the original composition of the itabiritic protore plays a

major role in the genesis of high- and low-grade soft ores in

the QF. Under the same weathering and structural con-

ditions, the dolomitic itabirite is the more favorable to form

high-grade deposits than siliceous itabirite. Field relations

at the Águas Claras and Capão Xavier deposits suggest that

it is not possible to form huge soft high-grade supergene

deposits from siliceous itabirite, unless another control,

such as impermeable barriers, had played an important role.

The occurrence in the Tamanduá Mine of a large, soft,

high-grade orebody formed from siliceous itabirite and

Miner Deposita (2008) 43:229–254

DOI 10.1007/s00126-007-0157-z

Editorial handling: S. Hagemann

Electronic supplementary material The online version of this article

(doi:10.1007/s00126-007-0157-z) contains supplementary material,

which is available to authorized users.

C. A. Spier (*)

19 Colin Road,

Scarborough,

WA 6019, Australia

e-mail: caspier@terra.com.br

S. M. B. de Oliveira

Instituto de Geociências–USP,

Rua do Lago 562,

05508-900 São Paulo, SP, Brazil

C. A. Rosière

Instituto de Geociências–UFMG,

Av. Antônio Carlos 6627,

31270-901 Belo Horizonte, MG, Brazil

J. D. Ardisson

Centro de Desenvolvimento de Tecnologia Nuclear–CDTN,

Campus da UFMG, Av. Antônio Carlos 6627,

31270-901 Belo Horizonte, MG, Brazil

closely associated with hypogene hard ore suggests that

large, soft, high-grade orebodies of the Quadrilátero

Ferrífero, which occur within siliceous itabirite, have a

hypogene contribution in their formation.

Keywords Banded iron

Formation

Itabirite

Iron ore

Quadrilátro Ferrífero

Águas Claras

Capão Xavier

Tamanduá

Brazil

Introduction

The Águas Claras iron-ore mine is located in the Quadrilátero

Ferrífero (QF), southeastern region of Brazil, within the state of

Minas Gerais (Fig. 1). Open-pit mining at Águas Claras

commenced in 1973 and, by 2002, when the mine was closed,

approximately 288 Mt of high-grade iron ore (average >

68% Fe) had been mined by Minerações Brasileiras Reunidas

(MBR) on the crest and along the slopes of a segment of the

Serra do Curral.

The majority of the iron orebodies in the QF are hosted in

itabirites (oxidized, metamorphosed, and heterogeneously

deformed BIFs) of the Cauê Formation, Itabira Group.

Itabirites of the Cauê Formation are similar in lithology to

most major oxide-facies BIFs known worldwide (Dorr

1969). Based on their mineralogical composition, three

major types of itabirites occur in the QF: siliceous, dolomitic,

and amphibolitic.

Unlike other mines of the QF, the Águas Claras Mine

contained mainly high-grade ore (Fe >64%) hosted within

dolomitic itabirite. Although siliceous and dolomitic itabirite

crop out at the mine, the high-grade ore was developed

mainly within the latter. Two distinct types of high-grade ore

occur at the mine: soft and hard ore. The soft ore was the

most abundant and accounted for more than 85% of the total

ore mined.

The genesis of the high-grade (Fe >64%) and low-grade

(30%<Fe<64%) iron ores within the Cauê Formation has

been the object of debate since the beginning of the

twentieth century and continues to be the focus of studies

by several researchers (e.g. Harder and Chamberlain 1915;

Guild 1953;Park1959; Dorr 1964, 1965; Eichler 1968;

Cabral et al. 2003; Rosière and Rios 2004; Spier et al.

2006). Most studies concentrated on ores that originated

from siliceous itabirite, the most common type of itabirite

in the region, probably because dolomitic itabirite is more

restricted to, and concentrated along, the Serra do Curral

(Fig. 1). Recently, Spier et al. (2003) described the geology

of the Águas Claras and Pico iron mines and presented data

for the main chemical constituents of the iron ore at those

mines.

This study complements the previous work of Spier et al.

(2003) presenting a detailed study on the mineralogy and

trace element geochemistry of the iron ores of the Águas

Claras Mine and aims at contributing to the discussion

about the genesis of the iron ore in the QF and worldwide.

To illustrate the complexity of processes associated with the

genesis of iron ore in the QF, we have briefly described the

geology of the Capão Xavier (CPX) and Tamanduá (TAM)

iron deposits and compared the orebodies of these deposits

to those of the Águas Claras deposit.

Geological setting

The Paleoproterozoic metasedimentary rocks of the Minas

Supergroup form the mountain ranges of the QF, whereas

the lowlands are occupied by Archean granite-gneiss

basement rocks and the Rio das Velhas Greenstone Belt

sequence (Fig. 1). The giant iron ore deposits of the QF

are hosted in the platformal sediments of the Minas

Supergroup and consist of three metamorphic sequences

(Dorr 1969). The lowest unit is the Caraça Group, which

contains clastic sedimentary rocks. The middle Itabira

Group comprises itabirites with minor dolomite and

phyllite of the Cauê Formation and carbonate rocks of

the Gandarela Formation. Itabirites grade u pward to

dolomites and dolomitic/manganiferous itabirites of the

2.4 Ga Gandarela Formation ( Babinski et al. 19 9 5 ). The

Piracicaba Group comprises clastic sed imentary rocks

(with subordinate dolomites) separated by an unconfor-

mity from the underlying Itabira Group.

Águas Claras Mine

The Águas Claras Mine is located in the northern segment

of the Serra do Curral, a northea st–southwest trending,

∼100-km-long ridge that limits the northwestern edge of

the QF (Fig. 1). Units o f the Minas Supergroup crop out

along m ost of its extension as an inverted sequence without

repetition. In the northern segment of the Serra do Curral,

the rocks are strained, sinistrally rotated from their original

position, and b eds are overturned (Chemale et al. 1994).

Extensive shear zones occur at the contact between the

Minas Supergroup and the underlying A rchean Rio das

Velhas Supergroup. In this region of the Q F, the r ocks of

the Minas Supergroup were subjected to low-grade

greenschist faci es metamorphism (Pires 1995).

At the Águas Claras Mine, quartzite and phyllite of the

Moeda a nd Batatal Formations, as w ell as dolomite,

itabirite and iron ore of the Cauê Formation, are exposed

(Fig. 2a,b). Within the open pit, the dolomitic itabirite is

more common than the siliceous itabirite, which occurs

only on the northern wall at the top of the sequence

(Fig. 2a).

230 Miner Deposita (2008) 43:229–254

The dolomitic itabirite

The dolomitic itabirite occurs as a continuous 150- to

400-m-thick layer. Its petrography and geochemistry were

described in detail by Spier et al. (2007) and will be briefly

addressed herein. Banding of centimeter-thick iron-rich and

dolomite-rich bands is the most conspicuous macroscopic

feature of the dolomitic itabirite. At the base of the

sequence, the dolomitic bands are thick and predominate

over the iron-rich bands. The dolomitic bands, centimeter to

decimeter thick, display color shades ranging from deep red

to pink. The iron-rich bands are gray and range from less

than one millimet er to 3 cm in thickness. Fifty meters up

the sequence, the thickness of the dolomitic band is

reduced, thus, forming the typical dolomitic itabirite where

sub-centimetric bands of dolomite alternate with hematitic

bands (Fig. 3a) throughout ∼250 m of itabirite. Towards the

top, hematitic bands gradually disappear, and a ferruginous

dolomite predominates. A millimeter thick banding of iron-

and dolomite-rich laminas (microbanding) also occurs

throughout the entire itabirite, especially within dolomite-

rich mesobands (Fig. 4a).

The carbonate bands of dolomitic itabirite consist of

microsparitic dolomite and hematite with grain sizes

ranging from 5–30 μm (average 20 μm, Fig. 4b). The

volume of hematite within the carbonate bands is highly

variable (5 to 50%) from band to band. Hematite occurs as

individual crystals of tabular shape intergrown with

dolomite or forming aggregates of granular and tabular

crystals of irregular shapes (Fig. 4b,c). Randomly oriented

Belo

Horizonte

Itabirito

Moeda

Syncline

Ouro Preto

Pico Mine

Águas Claras Mine

Ouro Branco

Itabira

20°00' 20°00'

Gerais

Minas

BRAZIL

Itacolomi Group

Minas Supergroup (black = BIF)

Rio das Velhas Supergroup

Basement

Alegria

Mine

Nova Lima

19°45'

20°15'

10 Km

Tamanduá Mine

Capão Xavier Mine

Conceição

Mine

Capitão do Mato

Mine

Fig. 1 Location and geological sketch map of the Quadrilátero Ferrífero region (after Alkmim and Marshak 1998)

Miner Deposita (2008) 43:229–254 231

phyllite

0 N

-500N

-1000N

1500E

2500E

3000E

0 N

-500N

-1000N

1500E

2500E

3000E

2000E

100 m

Dolomitic phyllite

Metachert

Ferruginous

Hard high-grade ore

Dolomitic itabirite

Siliceous itabirite

Dolomite

Soft high-grade ore

Sericitic

Quartzite

Canga

Cauê Formation

Moeda Formation

Batatal Formation

Gandarela Formation

45 Foliation

1000N

500N

100m

Current Topography

dolomite

Debris ore

Clays

Weathering level

Fig. 2 Águas Claras Mine. a Cross section 2400E. b Geological map (modified after Spier et al. 2003)

232 Miner Deposita (2008) 43:229–254

red-brown microcrystals of hematite (<1 μm) within

dolomite crystals in the carbonate bands account for the

characteristic deep red color of these bands.

The iron-rich bands contain hematite (>80%), interstitial

carbonates, and pores (Fig. 4d). The average hematite grain

size is simila r to hematite inside of the dolomitic bands

(∼20 μm) ranging from 5– 30 μm. Hematite occurs mainly

as tabular and granular crystals growing ove r martite

(hematite pseudomorphs after magnetite), forming a dense

network and locally as specularite. Relics of magnetite are

observed in the nuclei of some hematite crystals, especially

in the coarser iron-rich bands.

Accessory minerals consist of talc, chlorite, and apatite.

Talc occurs intergrown with dolomite (Fig. 4e) and

associated with chlorite in thin veins. Apatite forms

monomineral layers (<1 mm) or occurs as isolated euhedral

crystals (4–10 μm) intergrown with or, included in,

hematite and dolomite crystals (Fig. 4f). Chlori te occurs

along veins and shear planes.

The iron ore

The Águas Claras orebody comprises soft and hard high-

grade ores (Fig. 3b,c). The orebody is a mostly bedding-

concordant, 2,500-m-long, roughly tabular-shaped lens with

a maximum thickness of 300 m formed essentially by soft

ores (Fig. 2b). The hard ores occur within the soft ores as

minor lenses concordant to the bedding. The depth of the

orebody is variable, extending to more than 400 m below

the surface at the center of the pit and then grading into the

parental fresh dolomitic itabirite deeper down (Fig. 2a). In

this contact region, pinnacles of fresh dolomitic itabirite are

surrounded by the soft ore (Fig. 3d). Higher levels of

contaminants in the iron ore, especially apatite, occur in this

zone. Strike and dip of the orebody are conformable with

regional trends, i.e., strike is northeast–southwest, dip is

65° to 80° SE.

The soft ore is gray, friable, and has a laminated

appearance. It preserves partially the original structure of

the protore, particularly meso- and microbanding, which is

characterized by the presence of massive and porous bands

within the ore parallel to the banding of the itabirite

(Fig. 3e,f ). Adjacent bands are rarely of similar thickness.

Field relations show that iron-rich bands of dolomitic

itabirite grade into massive bands in the iron ore, whereas

dolomite-rich bands grade into porous bands. Kink bands

(not present in the protore) are locally observed in the soft

ore.

The main hard ore occurrences are concentrated on the

west side of the mine, close to the top of the sequence.

Here, they are in contact with siliceous itabirite forming

strike parallel, 50- to 60-m-thick, up to 200-m-long lenses

(Fig. 2b). Small concordant lenses of hard ore from a few

centimeters to nearly 4 m thick also occur within the fresh

dolomitic itabirite on the east side of the pit. These lenses

show banding conspicuously concordant with banding of

the dolomitic itabirite (Fig. 3g).

The hard ore is dense, compact, dark bluish gray, and

has a dull to metallic luster (Fig. 3c). Three main subtypes

of hard ore are found at the Águas Claras Mine: massive,

banded, and schistose (Spier et al. 2003). Massive hard ore

has low porosity, lacks internal structure, but usually shows

conchoidal fracture. Banded hard ore is the maj or type of

hard ore, being characterized by conspicuous banding and

varied porosity. Schistose hard ore generally has low

porosity, does not preserve the original structures, and

exhibits a conspicuous foliation. This type of hard ore is

5 m

Fig. 3 a Hand specimen of dolomitic itabirite. Mesobanding is

preserved as dolomite-rich (d) and iron oxide-rich (h) layers. b, c

Mine exposures of the soft (SO) and hard (HO) ores. d Deeper contact

zone of the soft ore and dolomitic itabirite (DI). Note the pinnacles of

dolomitic itabirite surrounded by the soft ore. e Detail of the contact

between soft ore and dolomitic itabirite. f Detail of the soft ore

showi ng massive bands (MB) and porous bands (PB). g Mine

exposure of hard ore (HO) within unweathered dolomitic itabirite

(DI). Note the gradation from dolomitic itabirite to hard ore and that

hard ore is concordant with banding of dolomitic itabirite

Miner Deposita (2008) 43:229–254 233

associated with the shear zones, which are mainly observed

on the northern wall of the pit.

Capão Xavier Mine

The Capão Xavier orebody is located at the eastern limb of

the Moeda Syncline close to the junction of the Moeda

Syncline with the Serra do Curral (Fig. 1). Mining at Capão

Xavier began in 2004 by MBR. The orebody consists of a

large concordant lens of high-grade ore hosted within

dolomitic itabirite (Fig. 5a) quite similar to Águas Claras.

Soft high-grade ore is largely dominant and represents 82%

of the 233 Mt of geological iron resources (average 67%

Fe). Table 1 shows statistics regarding the major element

composition of about 969 samples of run-of-mine soft ore

collected by MBR. Hard high-grade ore occurs as local,

metric to decimetric lenses within the soft ore. Low-grade

ores (30%<Fe<64%), formed from siliceous itabirite, are

subordinated wi thin the pit of the mine, but widespread

outside this limit (Fig. 5a). In contrast to the Águas Claras

and Tamanduá mines, the Capão Xavier’s orebody dips

gently, with a maximum angle of 45° to the northeast. The

weathering front varies at depth from a few meters to more

than 300 m. Soft high-grade ore reaches a maximum depth

of 280 m. A thick (5–50 m) transition zone, characterized

by higher contents of Mn (up to 5% of MnO

, average

1.8%) compared to soft ore (average 0.4% MnO

), is

usually observed at the contact with the dolomitic itabirite.

Weathering of the siliceous itabirite at the Capão Xavier

deposit produced a set of rocks with varied goethite content

and mechanical strength, ranging from a friable goethitic

rock, called limonitic itabirite, to the partially weathered,

goethite free, medium itabirite. Limonitic itabirite occurs

immediately below the canga cap and consists of hematite

and goethite with subordinated quartz. Limonitic itabirite

varies in thickness from few meters to up to 20 m and

grades in depth into the iron-rich siliceous itabirite accord-

ing to the gradual reduction of the goethite content. The

iron-rich siliceous itabirite is a low-grade iron ore (52%<

Fe<64%) and consi sts of a soft, powdered, siliceous

80 µm

1 mm

10 µm

100 µm

20 µm

Fig. 4 Photomicrographs and

SEM images of dolomitic

itabirite. a Microbanding of

dolomite-rich (white) and iron

oxide-rich (black) laminae;

transmitted light. b Detail of the

dolomite-rich microband.

Aggregates of hematite crystals

(h) between the dolomite crys-

tals (d); transmitted light. c SEM

image of the dolomite-rich band

showing aggregates of hematite

(h) between dolomite (d); back-

scattered image. d SEM image

of an iron oxide-rich laminae;

backscattered image. e, f Detail

of a dolomite-rich microband

showing talc crystal (t) inter-

grown with dolomite (d) and

hematite (h)ine and apatite

between dolomite and

hematite in f

234 Miner Deposita (2008) 43:229–254

100 m

DH 42-00

DH 03-00

DH 86-15

DH 98-07

DH 86-07 DH 90-06

DH 91-01

DH 87-01

DH 06-85

DH 17-04

-1250 N -1050 N

-850 N -650 N -450 N

Tamanduá Mine

Section -8500

RL 1000

Ultimate pit

Current topography

-100 N

100 N

300 N 500 N

DH 03/78

DH 02/78

DH 09/79

DH 10/91

DH 13/91

DH 06/93

Ultimate Pit

RL 1200

100 m

Capão Xavier Mine

Section -1900

Weathering level

Moeda Formation

Quartzite

Batatal Formation

Phyllite

Hematitic phyllite

(Argillaceous

Siliceous

DolomiticMetavolcanic

Friable siliceous

Friable iron-rich

siliceous itabirite

Limonitic itabirite

(30<% Fe <64)

Hard

Medium

Soft

(% Fe >64)

Dolomite

dyke

Canga

Tertiary Basins

Low-grade iron ore

High-grade iron oreGandarela Formation

Metamafic

Cauê Formation

iron formation)

Clay

itabirite

Fig. 5 Cross-sections of the a Capão Xavier and b Tamanduá mines

Miner Deposita (2008) 43:229–254 235

itabirite made up of hematite, with subordinate quartz. The

goethite content is variable. With the gradual increase of

quartz in the iron-rich itabirite, the iron content proportion-

ally d ecreases, and the rock is called soft itabirite. Soft

itabirite is also considered a low-grade ore (30%<Fe<

52%), with an average Fe content of 46% at the Capão

Xavier deposit. Soft itabirite has low goethite content and

grades to hard, but partially weathered siliceous itabirite,

called medium itabirite.

Tertiary basins, up to 200 m long and 80 m deep, occur

locally at the Capão Xavier deposit, blanke ting both the soft

ore and the iron-rich soft itabirites (Fig. 5a). These basins

were originally filled with kaolinitic sediments that were

almost totally transformed into bauxite through weathering

processes (Pomerene 1964; Maizatto 2001).

Tamanduá Mine

The Tamanduá deposit is located in the Tamanduá

Syncline, which is a second-order fold of the Moeda

Syncline (Fig. 1). Production of iron ore at Tamanduá has

been carried out by MBR since 1996. The orebody, hosted

within siliceous itabirite, strikes NW/SE, sub-parallel to the

eastern limb of the Moeda Syncline (Fig. 1). In this area,

the Cauê Formation is 2,000 m thick due to folding. The

orebody is transversally cut by two strike-s lip faults hosting

dykes mostly of basic composition, which limit the orebody

and divide it in three domains. The richest and thickest

high-grade bodies are located in the central domain

(Fig. 5b).

The Tamanduá orebody is composed of soft high-grade

ore, with lenses of hard ore and low-grade ore. Soft and

hard ores represent 80 and 20%, respectively, of the 297 Mt

of high-grade iron resources (average 67% Fe). Statistics of

the major element composition of about 2,139 samples of

soft ores obtained during production by MBR are shown in

Table 1. Low-grade ore comprises 480 Mt of iron-rich

itabirite, soft-itabirite, and limonitic itabirite. The orebody

is approximately 2,800 m long and 400 m thick.

High-grade ores occur mainly in the syncline hinge

zone, in contact with the underlying phyllite of the Batatal

Formation where lenses of strongly fractured hard ore are

located within medium and soft high-grade ores and iron-

rich/soft itabirites, forming a complex arrangement

(Fig. 5b). The soft ore extends to more than 500-m depth.

The hard orebodies may be massive or porous, and their

shapes are controlled by mesoscopic, acylindric, SE-

plunging folds. Hard schistose ore also occurs in the

central domain of the Tamanduá deposit. This hard

schistose ore is rela ted to oblique shear zones. Subordinate

high-grade orebodies occur immediately below the canga

surface on the western side of the deposit as well. Contacts

between hard ore and unweathered itabirite observed in

drill cores are sharp or grade into the protore within a few

decimeters (Fig. 6a,b). Generally, this contact is highly

fractured favoring the circulation of ground water. The

contact between the soft ore and the low-grade ore is

gradual where it occurs close to the topographic surface

(outsi de of the Tamanduá Syncl in e ) and sharp wher e it

occurs at greater depths within the Tamanduá Syncline

(Fig. 6c).

Low-grade ore consists of limonitic, iron-rich, and soft

itabirite and predominates on the western side of the deposit

where the same transition limonitic itabirite, iron-rich

itabirite, soft and medium itabirite, and hard siliceous

itabirite is observed as at the Capão Xavier deposit. The

weathering front in the siliceous itabirite in this area varies

from few meters below the topographic surface to up to

250 m at depth (Fig. 5b). Iron-rich and soft itabirites have a

high content of goethite close to limonitic itabirite, which

gradually decrease (and locally disappear) toward the hard

siliceous itabirite.

Table 1 Chemical composition (wt%) of the in situ soft high-grade ore of the Águas Claras, Capão Xavier, and Tamanduá mines

Águas Claras Capão Xavier Tamaduá

Avg Std n Avg Std n Avg Std n

68.23 0.78 4,009 66.89 1.40 969 66.80 1.39 2,139

0.17 0.18 4,009 0.11 0.09 969 0.17 0.09 2,139

0.54 0.35 4,009 1.22 0.76 969 1.20 0.75 2,139

SiO

0.63 0.38 4,009 1.25 1.06 969 1.45 1.02 2,139

MnO

0.59 0.25 2,766 0.43 0.65 969 0.50 0.48 2,139

CaO 0.13 0.18 224 0.02 0.05 729 0.02 0.01 1,566

MgO 0.12 0.14 224 0.03 0.07 729 0.03 0.02 1,567

TiO

0.04 0.02 224 0.07 0.06 729 0.06 0.06 1,567

LOI N.A. 1.23 0.79 477 1.08 0.70 1,038

Data for the Águas Claras Mine are from Spier et al. (2003) and for the Capão Xavier and Tamanduá Mines are from MBR.

Total iron, N.A. not analyzed

236 Miner Deposita (2008) 43:229–254

Sampling and analytical procedures

Eighty-six samples representing hard and soft ores types

were collected from diamond drill cores and outcrops along

the benches of the Águas Claras Mine. Electronic supple-

mentary material Appendix A presents the local coordinates

of each sample, followed by the respective analytical

procedure. Detailed sampling was carried out in the contact

zone with the protore to access the transition from the

dolomitic itabirite to the iron ore. All samples were

impregnated with epoxi resin and polished for petrographic

analysis. Samples of each type of ore were examined by

scanning electron microscopy (SEM). The porous band of

the soft ore was examined by transmission electron

microscopy (TEM). The SEM analyses were carried out at

the laboratories of the Geoscience Institute of the University

of São Paulo, Brazil and at the Center for Microscopy and

Microanalysis of the University of Queensland, Australia

where TEM analyses were also carried out.

Eight ore samples were selected for Mössbauer spec-

troscopy (MS), four of which are soft ore and four hard

ore samples. For each sample of soft ore, the massive band

(samples MAC126A to MAC129A) was carefully sepa-

rated from the por ous band (samples MAC126B to

MAC129B). For the har d ore, samples of su btypes

massive ( MAC97), banded (MAC67 and MAC105), and

schistose (MAC68) ores were selected. Besides iron ore,

two samples (MAC96 and MAC100) of fresh dolomitic

band from the dolom itic itabirite were exami ned b y MS.

Transmission

Fe Mössbauer spectra at 300 K were

obtained on a constant acceleration spectrometer using a

Co/Rh source at the Laboratory of Applied Physics of

the Nuclear Technology Development Center (CDTN) in

Belo Horizonte. Mössbauer spectra of four samples

(MAC97, MAC127A, MAC127B, and MAC100) were

also recorded at 50 K. A least-square fit program was

employed to calculate the spectral hyperfine parameters.

All samples studied by MS were characterized by X-ray

diffraction (XRD) at the Institute of Physics at the Federal

University of Minas Gerais. Unit cell parameters of

hematite were determined using the Rietveld refinement

for samples MAC67, MAC97, and MAC127A and B.

Eighty-six samples of the iron ore were analyzed by ICP

for all major and some trace elements (Ba, V, Ni, Cu, and

Zn) at the chemical laboratory of MBR. From the total

number of samples, 48 were selected for analyses of major,

traces, and rare earth elements (REE) at Activation

Laboratories—ACTLABS in Canada. All samples were

prepared for analyses by grinding in a tungsten carbide

vessel. As this method causes contamination by W, Co, and

Ta, concentrations of these elements are not reported. Major

and trace elements were determined by ICP/ICP-MS

after digestion of the sample with a lithium tetraborate/

metaborate fusion. Base metals were determined by ICP-

MS/INAA after total digestion by acid attack. Ferrous iron

was determined at the laboratories of MBR by potassium

dichromate titration. The REE data on the post-Archean

Australian Shale (PAAS) obtained by McLennan (1989)

were used as normalization values. We present REE

Fig. 6 Mine exposure and drill cores of the Tamanduá Mine showing

contacts between iron ore and siliceous itabirite. a, b Contact between

high-grade hard ore (HO) and hard siliceous itabirite (HIB). c Straight

contact between high-grade soft ore (SO) and low-grade soft itabirite

(SIB)

Miner Deposita (2008) 43:229–254 237

patterns with Y (REEY) inserted between Dy and Ho

according to its ionic radius. The Eu anomaly is defined

quantitatively as:

¼ Eu

PAAS

 Gd

PAAS

ðÞ

where Eu* is the hypothetical concentration of Eu. The

anomalies of Ce and Y were similarly calculated. The

degree of light REE enrichment relative to heavy REE is

presented as the ratios of PAAS-normalized La and Yb (La/

Yb), La and Sm (La/Sm), and Dy and Yb (Dy/Yb).

The bulk density of nine protore and 37 ore samp les was

determined on 5 cm diameter×20-cm-long fragments,

which were waterproofed and weighed in air and water.

Mineralogical and geochemical characteristics

The dolomitic itabirite

To allow comparisons between the parental rock and the

iron ore, we present a brief summary of the dolomitic

itabirite chemical data. For additional information, readers

should refer to Spier et al. (2007) and references therein.

The average major, trace, and REE composition of the

dolomitic itabirite is presented together with statistics of the

iron ore in Table 4. Dolomite within dolomitic bands has a

very uniform composition, with a Ca/Mg ratio around 1:1

and average contents of CaO and MgO ranging from 29.6%

to 30.5 wt% and from 18.4% to 20.6 wt%, respectively. The

average contents of FeO and MnO are low, ranging from

3.7% to 1.0 wt% and from 0.5% to 0.9 wt%, respectively.

The EMP analyses were also carried out on several grains

of hematite and martite. The results did not show any

variation of the chemical composition, resulting in Fe

content that is always >99%.

The XR D analyses of the two samples of the dolomite-

rich bands (MAC-96 and MAC-100) showed only the

presence of hematite and dolomite (Fig. 7a). The MS

identified hem atite and s iderite in the samples (Fig. 7b,

Table 2), represented by a typical sextet and a less defined

doublet, respectively. The values of the hyperfine field of

hematite at 300 K (51.9 T) and at 50 K (54.8 T) are

typical of well-crystallized pure hematite (Murad and

Schwertmann 1986). The isomer shifts of the doublet at

300 K (1.2 mm/s) and at 50 K (1.4 mm/s) match well with

those of siderite and represent the FeCO

content of the

dolomite.

The C and O isotope analyses of 45 dolomite samples

of the dolomitic itabirite show negative δ

Cvalues

ranging from −2.5 to −0.8‰, whereas the oxygen isotope

data range from −12.4 to −8.5 ‰ δ

O(Spieretal.2007).

These ranges of δ

Candδ

O val ues are in the sam e

range of values obtained by Bekker et al. (2003)in

stromatolitic dolomites in the upper part of the Gandarela

Formation and are consistent with those found in well-

preserved Paleoproterozoic carbonate successions (Veizer

et al. 1990;Bekkeretal.2003).

Soft iron ores

Mineralogy and petrology

The soft ore consists of alternating millimetric to centi-

metric massive and highly porous bands both rich in iron

Dol Dol

Dol

Dol/Hm

Dol

10 20 30 40 50 60 70

2θ (Degrees) CuKα

-12-9-6-3 0 3 6 912

0,96

0,97

0,98

0,99

1,00

MAC-100

300K

Velocit

(mm/s)

0,92

0,94

0,96

0,98

1,00

MAC-96

300K

Relative Transmisson

MAC96

MAC100

Fig. 7 a Powder XRD patterns of dolomite-rich bands of the

dolomitic itabirite showing hematite (Hm) and dolomite (Dol). b

Mössbauer spectra at 300 K of the same samples

238 Miner Deposita (2008) 43:229–254