ECONOMIC
GEOLOGY
AND
BULLETIN
OF
THE
ECONOMIC
VoL.
59
THE
SOCIETY
OF
GEOLOGISTS
No.
NOW:•R•:R, 1964
SUPERGENE
IRON ORES OF MINAS
JOHN VAN
7
GERAIS, BRAZIL •
N. DORR II
CONTENTS
PAGE
Abstract .............................................................
Introduction
.........................................................
1204
1204
Backgroundand acknowledgments......................................
Geologic framework ..................................................
Topographyand physiography......................................
1205
1207
1208
Itabirite
..........................................................
1209
Types of supergeneore ................................................
1211
Chemicalcomposition
of itabirite and supergeneores .....................
1212
The supergeneores ...................................................
1213
Enriched itabirite ..................................................
Contacts .......................................................
1213
1215
Disaggregation and enrichment ..................................
Causesof disaggregationand enrichment ..........................
Intermediate grade ore .............................................
Mineralogy ....................................................
1215
1216
1217
1218
Internal
structure
...............................................
Secondary enrichment ...........................................
Canga ............................................................
Nature of canga ................................................
Mineralogy and variation in composition..........................
Types of canga .................................................
Contacts
1219
1219
1219
1219
1220
1221
.......................................................
1222
Ore genesis ..........................................................
Mechanismof secondaryenrichment .................................
1223
1226
Controlsof supergeneore formation .................................
Influenceof physiographicenvironment...........................
Influenceof climatic regimen ....................................
x Publication authorized by the Directors, U.
Nacional da Produq•o Mineral.
1203
1229
1229
1229
S. Geological Survey, and Departamento
1204
JOHN VAN N. DORR II
Grain
size of itabirite
...........................................
1230
Compositionof itabirite .........................................
Origin of canga ...................................................
Significance of data ...................................................
1231
1233
1236
References
1239
...........................................................
ABSTRACT
The iron ores of Minas Gerais, Brazil, fall into two categories: (1)
hypogenehematite ore averaging 66 percentor more Fe, and (2) lowergrade supergeneores. Most ore now extracted is high-grade hypogene
ore; lower-gradesupergeneoreswill be of muchfuture value.
All supergeneores formed by weatheringof itabirite, a metamorphosed
oxide-faciesiron formation averaging about 38 percent Fe and 44 percent
SiO2. The Caug Itabirite crops out for about 540 linear kilometersin
central Minas Gerais. Supergene ores fall into three intergradational
categories: (1) enriched itabirite, averaging 49 percent Fe, easily concentratable,with reserves about 25,000 million tons; (2) intermediate
grade ores,averagingperhaps63 percentFe, with indicatedand inferred
reservesmorethan 600 milliontons; and (3) canga,averagingbetween57
and 62 percentFe, with reservesin the hundredsof millions of tons.
Disaggregationof hard and brittle itabirite by solutionprimarily of
quartz and secondarilyof other solubleconstituentscausesresidualenrichment in iron with minor hydration of hematite. As weatheringcontinues,
most of the quartz is removedand more of the hematite is hydrated,producing intermediategrade ore. Secondaryenrichmentby limonite is important. The final weatheringproductis canga. In canga,almostall the
i,'on is hydrated,and the rock is still further impoverishedin SiO2 and
residuallyenrichedin AtsOa and P. Canga also forms by cementationof
iron-rich detritus by limonite.
Four factorscontrolthe supergeneore-formingprocess:(1) physiography, for ores occur on ridges and plateaus; (2) climate, for seasonal
rainfall is apparentlyneededfor the formationof extensivecangablankets;
(3) grain size of the original itabirite, for quartz with a grain size much
greater than 0.1 millimeter is not readily soluble;and (4) compositionof
the iron formation.
The chemicallyinert and physicallyresistantcangablanketis essential
to supergene
ore formationbecause
softweatheringproductswouldotherwise be removedas fast as formed. Thus, canga gives time for the formation of other ore types.
Iron fixed as cementinglimonite in cangaand as enrichinglimonite in
intermediategrade ore was derivedby leachingand hydrationof hematite
from itabirite. It probablymovedin the ferrousstateand was precipitated as insolublehydrousferric oxide primarily by oxygenationof the
solutions,to a lesser extent by their evaporationat or near the surface,
and to a still smallerdegreeby pH changes.
The data show that the high-grade hematite ore, 66 percent Fe or
higher, cannothave formeddirectlyby supergene
action. Geochemical
processes
resultingin supergene
concentration
of iron also concentrated
aluminaandphosphorus.
The high-grade
ore contains
aboutthe samelow
percentages
of thesematerialsas unweathered
itabirite.
INTRODUCTION
BR.aZIL
haslongbeenknownfor its high-grade
ironoresthathavebeenmined
in increasing
quantities
forthegrowingBraziliansteelindustry
andfor export.
SUPERGENE IRON ORES OF BRAZIL
1205
In 1962, about 3 nilIlion tons were consumed in Brazil and more than 7
million tons of high-gradehematiteaveragingmore than 67 percentFe were
exported; it is probablethat thesequantitieswill be more than doubledin the
next decade. Most recent workers attribute thesehigh-gradeores to metasomatic replacementof iron formation, some by hydrothermal replacement
(17, 28), othersby synmetamorphic
metasomatism(11, 9, 8).
Becauseof the spectacularlyhigh tenor of the high-grade hematite ore,
relativelylittle attentionhas been devotedto lower grade ores of supergene
origin, which are also found in the Quadrilfi,
tero Ferrlfero of Minas Gerais.
Currently only high-gradeore, presentin suchabundancethat it will be used
for many decades,is exportedand domesticconsumptionis rising rapidly.
However, concentratesmade from supergeneores mined as a byproductof
the metasomaticores 1naywell enter both markets. Concentratesrunning as
high as 68 percentFe have beenmadefrom one supergeneore type in laboratory and pilot plant tests(37). Someof the other supergeneoresare entirely
comparableto the Cerro Bolivar ores of Venezuelaand to someof the African
and Indian ores.
The severaltypes of supergeneores, their geologicenvironment,and the
controls governing their formation are herein discussed. Descriptionsof
individualdepositsappearin a numberof recentlypublishedU.S. Geological
Surveyreports(9, 17, 23, 28).
BACKGROUND
AND
ACKNOXeVLEDGMENTS
A joint Brazilian-Americanprojectfor mappingthe (•uadriNtero Ferrifero
of Minas Gerais,an area of about7,000 squarekilometers(Fig. 1), xvasundertaken by the DepartamentoNacional da Produg'•o Mineral, now of the
Minist•rio de Minas e Energia of the Brazilian Government, and by the
GeologicalSurvey,U.S. Departmentof the Interior, under the Agencyfor
InternationalDevelopmentof the Departmentof State and its predecessor
agencies.
This paper is the result of work by the writer and 16 other Brazilian and
Americangeologists,
who have investedmore than 100 man-yearsin mapping
the geologyand ore deposits. These geologistsare: B. P. Alves, Burton
Ashley, A. L. M. Barbosa,J. E. Gair, P. W. Guild, Norman Ylerz, R. M.
Johnson,C. H. Maxwell, S. L. Moore, J. E. O'Rourke, J. B. Pomerene,G. A.
Rynearson,R. G. Reeves,G. C. Simmons,R. M. Wallace, and Amos White.
Guild, Pomerene, Simmons,and the writer were particularly interestedin
geneticproblemsrelatedto supergeneiron ores. Becauseeachgeologistde-
velopedhis own interpretations
of the data availableat the time he mapped,
differencesin viewpointexist and the conclusions
presentedin this paper are
not all subscribed
to by eachof my colleagues.It is hoped,however,that the
summarypresentedherein may be useful for the clarificationof problemsin
the formation of supergeneiron ores, particularly those forming in a semitropicalenvironment.
The paper results also from the magnificentcooperationextendedto the
writer and to his colleagues
by companiesand individualsprospectingfor and
1206
.tOHN
VAN N. DORR II
SUPERGEN'EIRON ORES OF BRAZIL
1207
developing
theironoresin Brazil. Thesecompanies
andpersons,
toolmmerousto nameindividuallyand of manynationalbackgrounds,
sharedfreelyof
their knowledge
and supplieddrillingand tunnelingresults,resultsof concentrationtests,and all otherneededinformationwithoutstint. As a result,
the reachof the individualgeologists
involvedin the projectwas increased
severalfold. For cooperation
of thisnature,thanksare inadequate
and it is
hopedthattheresults
obtained
by thejointDNPM-USGSironprojectwill
in somemeasurerepaytheassistance
rendered.
GEOLOGIC
FRAMEWORK
The mostcomplete
summaryof the geologyof the (•uadril•teroFerrifero
nowin printis SpecialPaper1 of theDepartamento
Nacionalda Produgao
LOCATIONS
I Serra da Piedade
3 Serra de
4
ltablra
District
5 Serra do Curtal
6 Rto de Peixe c•nymm
7 CapenemaD•pa•it
$ Aleg•a Depomt
9 Lag6aGrande
10 C•
da pedra Depamt
11 Feeho do Fuml
12. Morro do Chapeu
13 Monlevade District
14 Jo•o Perttara
15 Ualna
16 BoaEsperane•Mine
17 Congonha$Dbtnet
18 Tutamea Deposit
19 Gandarela syncline
20 Serra da Moeda
Fro. 2. Generalizedgeologicmapof the Quadril/tteroFerrifero.
Mineral,Rio deJaneiro,1960,in EnglishandPortuguese
(12), titledEsbogo
Geologico
de Quadril'3tero
Ferriferode MinasGerais,Brazil,with contributionsby variousgeologists
of the DNPM andGeological
Survey.
Figure2 illustrates
the disposition
of the majorrockrefitsin the region.
The metasedimentary
rock,dominantlyclasticexceptfor the Itabira Group
of the Minas Series,whichis composed
of oxide-facies
iron formationand
1208
JOHN
VAN N. DORR II
dolomite,is divided into three series separatedby angular unconformities.
Granitic rocksof both igneousand metasomaticorigin are present,and all the
rocksare cut by youngerunfoliatedmafic rocks. Metamorphosed
ultramafic
rocks,now soapstoneand serpentinite,occurin small bodies. All theserocks
exceptsomeof the maficintrusivesare Precambrianin age.
Two small areas of Tertiary continentalrocks are known and help date
someof the supergeneores.
The metasedimentaryrockshave been deformedinto complexfolds, many
overturned and isoclinal. Large-scale thrust, normal, and tear faults were
mapped,but haveno geneticrelationto the supergeneores.
The Precambrianrocks were regionally metamorphosedin Precambrian
time to the almandine-amphibole
faciesin the easternpart of the region and
to the greenschistfaciesin the rest of the region. This metamorphismaffectedthe grain size of the iron formationand thus the subsequent
formation
of the supergeneores,as will be discussed
below.
All the rocksof the area exceptcertain monomineralicrocksare commonly
weatheredto considerabledepths,in mostareasmore than 50 meters.
Topographya•d Physiography
The general area of the QuadrilfiteroFerrifero is one of the highestin
this part of Brazil. The highest elevation is in the Serra do Carasa, over
2,100 meters. The lowest elevation is about 650 meters.
The topographyis generallysteepto oversteepened,
and high areasof subdued relief separatedeep valleys. Master streamsflow in incisedmeanders
and crossthe geologicstructure. Less importantstreamsare generallywell
adjustedto rock structurebut have rapidsand waterfalls,somevery spectacular, where they enter lithologiesof low resistanceor crosshard zonesin the
metasedimentary
rocks. Many terraces,pediments,
shoulders,
and remnants
of wide valleysat high levelattestthe intermittentnatureof the dissection
of
older high plateaus.
The presence
of a numberof ancienterosionsurfacesin this part of Brazil
is immediately
apparent(18, p. 371-376), and, after a reconnaissance
of the
geomorphology
of easternBrazil in 1954,King (24) namedfour surfacesas
follows,from highestto lowest: Gondwana,post-Gondwana,Sulamericano,
and Velhas. Destructionof these surfacesbegan, accordingto King, in the
early Cretaceous,
late Cretaceous,
early Tertiary, and late Tertiary respectively,sothe surfaces
formedbeforeandbetweentheseepochs.Recentwork
by Somers(writtencommunication,
1963) indicates
that plantfossilsdated
manyyearsagoby Gorceix(14) asMioceneor Pliocene
cannotbedatedmore
closelythanlate Tertiary. King correlatedtheserockswith his Sulamericano
cycle;thuseitherthiscorrelation
or the ageassigned
to the surfaceis called
into question.Other plant fossilsfoundin the valleyof the Rio de Peixe
(Macacosquadrangle)
at lowerelevation
thanthe Sulamericano
surfacewere
tentativelyidentified(28) as Tertiary or Quaternary. No elementsfor
establishing
the age of the older surfacesare knownwithin the regionhere
discussed,
but the evidencefor their agescitedby King seemspersuasive.
•?UPERGENE
lEON
OEES OF BRAZIL
1209
The Gondwanasurfaceis generallybetween1,400 and 1,600 m elevation
in mostof the areaand the post-Gondwana
surfaceis between1,000and 1,400
m; the highersurfacesseemto havebeenwarpedupwardtowardthe Serra
do Caracaand there is overlapin absoluteelevation. The oldestsurfaceis
represented
by accordantridge linesand smallhigh plateaus,the post-Gondwana by fairly broaddissected
surfacesin somecases,the Sulamericano
by
pediments
and broaddissected
streamand river valleys,and the Velhasby
lower terracesand abrupt changesin slope,the old valleyshaving been
cleavedby canyonsduringthis cycle. Monadnocks,
suchas part of the Serra
do Cara•a, Serra do Piedade,and Pico de Itacoloml,projectabovethe
Gondwanasurface,indicatinga still oldersurfacewhichKing suggests
may be
as old as late Paleozoic. Most of the higher surfacesand ridge lines are on
quartziteor iron formation,in mostcasesthe latter owing to the fact that the
surficial weathering product of iron formation, canga, is inert to further
chemicalweatheringand very resistantto mechanicalweathering.
The movementsthat rejuvenatedthe drainageof the area and resultedin
these erosion surfacesand in the formation of the supergeneores were
epeirogenic,not orogenic. No evidenceof Tertiary or Quaternary faulting
has been found during the DNPM-USGS mapping, despitereports of such
faulting in two recentpapers(1, 2). No signsof folding stressesare recorded
in Tertiary or Quaternaryrocks. Steepdips in certainTertiary rocksin the
Gandarelabasinare believedto be dueto sinkholesubsidence
and compaction.
The highestgradeand most abundantsupergeneores are closelyrelatedto
the Gondwanaand post-Gondwanasurfaces,and thoseores at lower elevations
are poorerand thinner.
ltabirite
The
mother
rock of all the iron ores is itabirite.
This
rock is defined
(9, p. 18) as a "laminated,metamorphosed,oxide-faciesiron forlnation, in
whichthe originalchertor jasperbandshavebeenrecrystallizedinto granular
quartz and in which the iron is present as hematite, magnetite,or marrite.
It shouldincludeonly rocksin whichthe quartz is megascopically
recognizable
as crystallinein order to differentiateit from unmetamorphosed
oxide-facies
iron formation." Dolomite and amphiboles,the latter commonlycummingtonite or tremolite, metamorphicproducts,locally substituteto a greater or
lesserdegreefor quartz in itabirite. Where dolomiteor amphiboleis abundant, magnetiteand its oxidation productsare abundant. Normally the iron
mineral is hematite.
Fresh itabirite is hard, dense,brittle, and resistant to mechanicalerosion
but not to chemicalweathering. The Caua Itabirite (6), the only important
sourceof supergeneore, and its weatheringproductscrop out, generally on
ridge lines,for more than 540 linear kilometersin the Quadril•tteroFerrlfero.
The original stratigraphricthicknessof the formationmay have been in the
order of 250-300 m, but it now rangesin apparentthicknessfrom a few
metersto more than 1.400in, rovingto tectonicrock flowage.
The mineral constituentsare generallyxvellseparatedinto bands,as illus-
1210
JOHN
VAN
N. DORR
II
FIG. 3. Typicaloutcropof harditabirite. Scalenumbered
at
2 centimeter intervals.
he'
FIG. 4. Photomicrograph
of itabirite. The onlymineralsidentifiableare
hematiteandquartz. MagnificationX 25, crossednicols.
SUPER'GENEIRON ORES OF BR.ZlZIL
1211
tratedby Figures3 and 4. Normallythereis no signof detrital•nineralsin
this rock,whichwas shownby Tyler (36) to be a chemicalsediment?
Dolomitic itabirite is more commonin the upper part of the Cau• Itabirite
in mostof the area but may occurat any stratigraphichorizonin that formation. The overlyingand intergradationalGandarelaFormation (7) is dominantly composedof dolomiticcarbonaterocks of varying compositionand
dolomiticphyllitebut containslensesof dolomiticitabiriteand normalitabirite.
TYPES
OF
SUPERGENE
ORE
The supergene
iron oresof the (•uadril/tteroFerrifero may be dividedinto
three intergradationalcategories:enricheditabirite, intermediategrade ore,
and canga. Theseare definedasfollows:
E•richcd itabiritc is ital)irite from which varying quantitiesof quartz and
other constituentshave been leachedby supergenefluids, resultingin a rock
that is disaggregatedwholly or in part and is residually'• enriched in iron.
Near the surface,someiron has commonlybeen introducedas limonite, resulting in secondaryenrichment. Some itabirite has been metasomatically
enrichedby the partial replacementof quartz by hematite;this materialis not
here considered. Metasomaticallyenriched itabirite can usually be distinguishedby the occurrenceof quartz as concordantor crosscuttingveins and
by a low percentageof HO2-k accompanied
by high iron content.
Intermediate grade ore is an arbitrary, not a natural, geologicclassification. It is, however, a useful category, as large quantities of rock of great
potentialvalue are of this type, which representsan intermediatestage of a
geologicprocess. It is here definedas material with an upper limit of iron
contentof about 65.5 percentand an arbitrary lower limit of 57 percentiron
and 7 percentor lessof SiOn. It is formedby strongresidualand secondary
enrichment
of itabirite.
Canga is a rock composedof varying quantitiesof detrital material derived from itabirite or high-gradehematiteore cementedby limonite. Canga
forms extensiveblanketdepositsat or near the presentor ancienterosionsurfaces. It is tough,moderatelyhard, well consolidated,
slightlypermeable,and
very resistantto erosionand chemicalweathering.
All the supergeneore types result from the weatheringof itabirite. They
all result from the more-or-lessintenseand extendedapplicationof the same
geologicprocesses
and thereforeare mutually intergradational. Clear lines of
demarcationbetweenthe types are impossibleto draw in the field or by
definition.
2 Rocks correlated with the Itacolomi Series by Guild (16) and Wallace (oral communlcation) in the Congonhas area, that appear to be itabirite but that locally contain abundant
conglomerate, are believed to have been formed from highly ferruginous clastic sediments
similar
to the unconsolidated
Recent
alluvial
sediments
near
Catas
Altas
and Itabira
which
are
also banded hematite and quartz.
a In this paper, resi&tal e•richme•t means enrichment of a rock by removal of gangue
minerals, the valuable elements remaining. Secondary enrichmerit means that valuable ele-
ments have been introduced into the rock by supergene solutions. Thus residually enriched
rock may also be secondarily enriched.
1212
JOHN VAN N. DORR II
CHEMICAL
COMPOSITION
OF
ITABIRITE
AND
SUPERGENE
ORES
The averagecomposition
of unleacheditabirite and of the three ore types
derived from this rock by supergeneenrichment,as shownby samplescollectedaway from areasof metasomatic
enrichment,is givenin Table 1.
Samplingof unleached
itabiriteconsisted
of carefullytakensurfacechipand
grab samplesfrom mostaccessible
outcropsover an area rangingfrom the east
side of the Serra do Caraqato west of the Rio Paraopeba. Samplesof fresh
itabirite from mine dumpsand analysesfrom about 40,000 tons of mined
unleacheditabirite in the Congonhas
area as well as from 256 m of adit in the
Itabira district were included in the average. Becausevariation in tenor
from onesampleto anotheris not greatandbecause
the samplesare sowidely
Table 1. AveragecomposœCion
of unleachediCabœrœCe
and œCsweecherinAproduct:a,
Unleached iCabirlCe
Enriched lcabiriCe
InCerm•diaCe•rade ore
14t. percent Conc2n- •t. p2rcenC Conc2n- •t. p2rcenC Concertf•cCor
factor
CenJ•a
S•p12d a'•2ra•e
Gen2ral 2v2•
f•ctor
•C.
percent
•oncen-
•t.
percent
tr•ion
rector
•e
37.9
I
•e20•
/ 51.1
•iO2
18.7
1.3
69.6
•.7
1
25.i
63.3
1.7
90.i
62.2
1.6
•7.0
•.69
0.0•
2-5
88.8
0.6
2.35
0.05
82.•
&1203
0.5
I
1.35
2.7
2.67
5.3
2.85
5.7
3-6
p
0.•7
1
0.057
1.2
0.0•
1.8
0.1•6
3.1
0.1-0.2
g20plus
0.3
1
1.58
15.3
5.37
17.9
10
99.6
•
96.5
100.1
98.9
1/ Fe203
is calculated,
aesumin8
ell FetobeFe
+•. Analyeee
of11smnples
ofunleached
itebirite
averaSe
0.5/• percentFe0and55 percentF•203.
i/
Dol•tic
itibirite
•nd •hibolitic
itibirite
not inclen.
"Page 32" in last column should read page 1220.
and randomlyspaced,the resultsvery probablygive a reasonable
approximation of the averagecompositionof the rock in place. Fresh itabirite is expensiveto drill and tunnel,and outcropsare scarce;thereforerelativelyfew
data are available.
Sampling of the enriched itabirite is more satisfactory. Several large
bodieshave beenexploredin detail. Roadcutsand severalthousandmetersof
editsand drilling from the wholearea gavemuchinformation. Tenor varies
widelyand the averagecitedappliesto the regionrather than to any one deposit. The mostcompletelyexploredsingledeposittotaledsome300 million
tons; no attemptto weightthe averageswas madebecausesu•cient data on
tonnageare availalJefrom only a few bodies.
Samplingof the intermediategrade ore is alsovariablein validity. One
body,estimatedby companygeologists
who exploredit with drill holes,adits,
SUPERGENE IRON ORES OF BRAZIL
1213
andshaftsmanyyearsagoto contain
300 milliontonsof ma•fial averaging
about64 percent
iron,is nowbeingexplored
moreintensively.Some8,000m
ofadditional
drillingand4,000moremeters
oftunneling
havenotsubstantially
changed
theoriginaltonnageor gradeestimates
(F. Pintode Souza,oralcom-
munication,
1963)usedin thispaper.Twodeposits,
eachtotaling
over100
milliontonsof ore, havebeenexploredin somedetailby aditsand drilling.
Anothertwo bodiesusedin computingthe gradeof intermediateore havenot
beenwell explored.The totaltonnagerepresented
by the analyses
of Table 1
is thoughtto be about600 milliontons.
Cangavariessoirregularlyin tenorthat the figurescitedare of valueonly
in discussing
grossrelations,and any one body may vary widely from the
figurescited. No sizabledepositaveragingless than 50 percentiron is
known, however.
T•
SUW•N•
Enriched
Ol•ES
Itabirite
Most enricheditabirite (Fig. 5) has beendisaggregated
into a soft, noncoherent,rock in which the individualmineral grains no longer adhere to
lOO
•
•
•
•---••
, ,//•
ß/
•
• ..................
•
n•
k
X /''" /••
Mesh+1)2•
..... '''
Iron
Phosphorus
Cumulative
weight,
m
percent
-- •S•ica ....
• Analysis.
in
percent
Fe
_'t•
20
l)• •, 6 8 10 18 35 45 70 lOO1202OO<;200
53.0
SiO,
23.6
P
.020
Fro. 5. Graphof screen
sizevschemical
composition
ofenriched
itabirite.
Averageof 5 samples
fromtheItabiradistrict.
eachotherexceptwhererecemented
by introduced
limonite. Residualenrichmentis dominant. Becauseindividualgrainsare angularand interlock
eventhoughnotmutually
bound,the materialstands
well in opencutsand
aditsanddoesnotshowslumpstructures,
although
it doessettlewithoutsignificantdeformation.It can be removedwith pick and shoveland without
theuseof explosives.
Mostof thequartzis in discrete
grainsaftermining
andmuchof thehematite
• is alsofreeexceptwherehydrationis far advanced.
ßIn this paper,the iron mineralof itabiritewill be referredto generically
as hematite,
eventhoughvaryingpercentages
are knownto be magnetite,
maghemite,
or martire. Except
locally,the lattermineralsare verysubordinate.
Similarly,the hydratedsesquioxides
of iron
will be referred to collectivelyas limonite,althoughthe dominanthydratedoxide is goethite,
with otherhydrousiron mineralspresent. "Hydratedhematite"of early authorsis referred
to as limonite.
6
F*GS. {•, 7, AND 8.
1214
SUPERGENE
IRON
ORES OF BR.4ZIL
1215
The disaggregation
resultsfrom the leachingof quartzaroundthe boundariesof the individualquartzgrains,of dolomitewherepresent,and of minor
amountsof hematite. The radicallyhighersolubilityof quartzcomparedto
hematiteundersurfaceconditions
has resultedin a morecomplete
disaggregationof the quartz bands; screentests of enricheditabirite from the
Itabiradistrictshowthat at plus18 mesha concentrate
runninghigherthan
65 percentiron canbe madefrom rockaveraging
about50 percentiron. A
typicalcurveof the chemicalcomposition
of disaggregated
itabiritefrom the
Itabira districtplottedagainstsievesizeshowsan increase
in SiO2between
45 and100mesh,andin ironat thecoarsest
andfinestgrainsizes(Fig. 5).
This is characteristic
of enriched
itabiritein theeastsideof the regionandis
perceptible
evenwherethe rock hasbeenresiduallyenrichedto morethan 64
percentiron. In thecentralpart of theregion,for whichmanyscreenanalyses
are available,and probablyin the west part, where no screenanalysesare
available,the maxima for SiO2 are displacedtoward the 200 meshand even
325 meshscreensize,but the finestfractioncommonly
contains
lessSiO=and
more Fe•Oa. These data have strong economicimplications.
Hematitein enricheditabiriteis hydratedto varyingdegree. As a generalizationwith manyexceptions,
it maybe venturedthat thegreaterthe leaching
of quartz, the greater the degreeof hydrationof the iron. Disaggregated
siliceous
itabiritenot notablyenrichedis rarely hydrated,whereasdisaggregated dolomitic itabirite is generally strongly hydrated, even where not
notably enriched. Unleacheditabirite is rarely even stainedby limonite
exceptin streambeds.
Contacts.--Contactsbetweendisaggregateditabirite and hard unleached
itabiriteare extremelyirregular. Long thin fangsof hard itabiriteprojecting
upward into soft disaggregated
itabirite are notablywell shownin Grota de
Esmeril in the Itabira
district and on the north flank of Serra da Piedade.
Ruckmick(31, p. 228) foundthe samerelationin the Cerro Bolivar deposit
in Venezuela,and attributedit thereto tight foldsin the iron formation. Such
folds undoubtedlyinfluencethe irregularityof the interfacein the Quadri1/tteroFerrifero. Minor textural or compositional
changesin the unleached
itabiritethat affectpermeabilityare thoughtto be equallyimportant.
Contactsof enricheditabirite with the overlyingrocks,cangaand intermediategrade ore are normallygradationaland marked by increasinghydrationof the iron mineral;locallytheyare abrupt.
Disaggregation
and Enrichment.--Disaggregation
of itabiriteby leaching
extendsto great depthsbelowthe surface. Guild and Pomerenereport softeneditabirite more than 200 m below the surface(16, 28). Disaggregated
itabiritehasbeenencountered
in drill holesat evengreaterdepth.
FIG. 6.
Soft itabirite, Serra do Moeda (Photographfurnished by R. M.
Wallace).
FXG.7. Intermediategradeore, Tutameadeposit.
Fic. 8. Canga cap on Serra de Rola Moqa, showing subduedrelief on an
ancient canga-covered
surface (King's post-Gondwana
surface), topographicunconformity with rapidly eroding softenedand enriched itabirite, and typically
sparsecover of soil and vegetation.
1216
JOHN VAN N. DORR
The averagedepthof this disaggregation
and of residnale1•richlnent
is not
known,for explorationof this ore type is rndimentaryand a byproductof
explorationfor the high-gradeores. Basedon informationgained in several
tens of thousandsof metersof undergroundworkingsand drill holesin the
region,a minimum averagedepth of 50 m is conservativelyestimated;an
estimate of 100 m might well be closer to the facts. Unleached itabirite has
beenencountered
in only a few localitiesoutsideof streamcanyonsand erosion
blowouts,a few rather restrictedareas of ridge tops and monadnocks,and a
few adits and deepdrill holes. Of the 540 linear kilometersof itabirite outcrop, more than 90 percentis itabirite leachedto unknowndepths.
Table 1 shows that the average enrichmeutof the enriched itabirite is
about 10 percentin iron and the impoverishmentin quartz is about 19 percent
of the original rock. Unfortunately, few of the assaysof leacheditabirite,
most of which were made by companiesprimarily interested in Fe and
SiO2, determinedwater of crystallization; undoubtedlymuch of the unaccountedmaterial in this column of Table 1, 3.5 percent of the rock, is such
water.
Causes of Disaggregationand JEnrichment.--Allrecent writers agree
that the disaggregation
and residualenrichmentof itabiriteare dueto leaching
of one or more solubleconstituentsfrom fresh itabirite by meteoric waters.
Much discnssionhas revolvedaround the problemof exactly what materials
were removed,however. The difficulty of finding fresh unleacheditabirite
in the regionhas beenan effectivebarrier to agreement,for somegeologists
(17, 28) arguethat the fresh itabirite that so rarely cropsout is not representativeof all itabirite and is fresh on the outcropbecauseit is not typical.
They believethat typicalitabiritecontainssignificantquantitiesof dolomite,
whichhasbeenleachedto disaggregate
and enrichthe originalitabirite.
Other geologists
believethat quartz is solubleenoughin a semitropical
environmentto accountfor the disaggregation
and that it is not necessary
to
assume
the formerpresence
of a mineralthat is not nowpresentin the rock.
Both schools
of thoughtadmit overalllarge-scale
leachingof quartzand the
latterschooladmitsthe presence
of minorquantities
of dolomiticitabirite,the
dolonfitein which,of course,wouldbe leachedundersupergene
conditions.
An opportunity
to testthe two theorieswaskindlyprovidedby the St.
Johndel Rey Mining Co., whichshutdownits hydroelectric
powersystem
for a daysothata criticalwatertunnelthatcrosscuts
thewholeitabiritesectionnearthe bottomof the Rio de Peixecanyoncouldbemappedandsampled.
The itahiriteat the top of the nearlyverticalwallsof this canyonis disag-
gregated,
butthetunnelcutsabout200m of steeply
dipping
freshitabirite.
Threemembers
of the DNPM-USGS partymappedandexaminedthe rock,
and17 representative
samples
of therockwereexamined
in thinsection.No
signof carbonate
was foundin the rock (R. M. Wallace,writtencommunication).
This evidence
is in accordwith manyscattered
surfaceobservations
else-
wherein the Quadrilfi.
tero Ferriferothat freshitabiritewithoutdolomite
gradesintonormaldisaggregated
itabirite.
In this context,the DNPM-USGS staff found that carbonate-free
SUPERGENE IRON ORES OF BR.4ZIL
1217
quartzites in the region characteristicallyleach at the surface into friable
sandstonesand may be completelydisaggregated,locally to considerable
depths. Reed, Bryant, and Hack (30, p. 1186) have recently shown that
very significantsolutionof quartz from quartzite may take place in such a
relativelycoolclimateas that of the mountainsof North Carolina.
The analysesin Table 1 prove that quartz is easily solublein the semitropical environmentof Minas Gerais. The averageintermediategrade ore
grading into and derived from the enricheditabirite containsless than onetenth as much quartz as the enricheditabirite, which in turn containsonly
57 percentasmuchquartzastheaveragefreshitabirite.
Ruckmick'swork at Cerro Bolivar (31) bearsout the mobilityof quartz
under supergeneconditions,analysesof spring waters from iron formation
there averaging10.5 ppm SiOn. Siever (33, p. 132) has shownthat the
solubilityof quartz in water at 25ø C is about 10 parts per million, correspondingcloselyto Ruckmick'sdata.
For theseand other reasons,the presentwriter believesthat the solution
and removal of quartz from itabirite is the primary causeof the disaggregation and residualenrichmentof that rock and that the presenceor absenceof
dolomitein itabirite is not an importantfactor.
I,termediate
Grade Ore
Intermediategradeore, definedon page1211,is an unsatisfactory
term for
a categoryof iron ore of muchpotentialvalue. A conventional
namefor this
rockis not available.It haslostthe distinguishing
featuresof enricheditabirite
and not yet gainedthe firm indurationof canga,into both of which it grades.
One companycalledit "direct shippingore" but this is not preciseenough
becausehigh-gradehematiteis also direct shippingore. Another called it
"laminated ore," but not all of it is laminated. The term here chosen must
sufficeuntil a better one is devised,althoughit is subjectto criticismbecause
the ore would be high grade in someeconomiccontextsand has commercial
as well as geologicimplications.The extraordinarysimilarityin composition
of a numberof large bodiesof this ore type, in contrastto other supergene
ore types,makesit a usefulgeologicunit, however.
The widespreadexistenceof this ore type has been recognizedonly recentlyin Brazil and little is knownaboutit. It rarely cropsout, beingcovered
by canga almost everywhere,and is seen only in tunnels, shafts,and pits.
The writer walked many times over a deposit containing more than 100,000,000 tons of this ore without realizingthat there was anythingother than
cangaand enricheditabirite beneathhis feet.
Intermediategrade ore has a natural upperlimit of tenor and an arbitrary
lower limit. The upper range is set by the degreeof hydrationof the iron
mineralsandof leachingof quartz. No sampleshigherthan 67.5 percentiron
are known and suchhigh values,representingspecimensrather than tonnage,
are very exceptional. Commonlythe upper limit is 65.5 percentiron, with
a percentor even lessof quartz. The lower limit is arbitrarily set at 57
percentiron and 7 percentSiO2. It would probablybe more economicin
1218
JOHN VAN N. DORR II
the ore-blast furnace-flux-energy-transportation
equation to concentratematerial belowthis tenor. With this arbitrary lower linfit as a cutoffpoint,major
bodiesof this ore type average63.3 percent iron, 2.35 percent SiO2, 2.67
percent-A12Oa,and 4.58 percentwater of crystallization. Significantly,for
the five bodiesfor whichenoughthree-dimensional
data are availableto perinit
someestimateof grade and tonnage,the grade of the individual bodiesvaries
only slightly. The highestgrade body averages63.9 percentFe, the lowest
62.1 percentFe.
The ore producesmuch fine material on mining and handling. It is
porousand very reductable,however,and would make a very adequateblast
furnaceore if carefullysizedand the finesagglomerated.Figure 7 illustrates
the mixednatureof the ore andthe characteristic
slumping.
Explorationof depositsof intermediategrade ore has been thoroughly
doneby many drill holesand aditsin the caseof one deposit,by a few drill
holesand adits in the caseof two other deposits,and in only a rudimentary
mannerin the caseof four other known significantdeposits. Presentindicated and inferred reservesof 5 of these 7 deposits(no reasonabletonnage
estimateis possiblefor 2) are in the order of 600,000,000tons and it is
probablethat otherdeposits
will be foundby furtherprospecting
in favorable
localities. The writer believesthat eventuallymore than one billion tons of
such ore will be found.
_Although
the intermediate
gradeore is somewhat
highergradethan the
"laminatedore" of the Noamundimine in India (25, p. 277-278), the general correspondence
in analysesbetweenthesetwo ore types (and alsobe-
tweenthehigh-grade
massive
ores)in Indiaandin Brazilis mostinteresting.
Mineralogy.--Mineralogically,
the intermediate
gradeore is dominantly
composed
of the simplehydrous
iron oxidesplushematiteresidualfromthe
itabirite. Perhaps40 percentof the iron is presentas goethite,as shownby
theaverage
content
of 4.6 percent
waterof crystallization.
The materialis so
mixedthat it is not possible
to isolateminorindividualconstituents,
but most
of the -A-120
a is probablypresentas gibbsite,boehnfite,
or in the latticeof
goethite
(3). The SiO2is largelypresent
asfine-grained
quartzcoated
by
goethite.Phosphorus
is relatively
highin thisoretype,averaging
0.084percent;it maywellbepresent
aswavellite
andstrengite,
theotherconstituents
of whichare presentandwhichhavebeenobserved
in otheriron-richrocks
affectedby supergeneaction.
Internal oCtructure.--Intermediate
gradeore is characterized
in some,not
all,deposits
bystrong
slump
structures,
in contrast
toalltheotheroretypes
of
the area. _Adits
in suchdeposits
showthat thereis no consistent
structure;
theslumped
material
looks
likeanagglomeration
of surface
rubble
dumped
in
at random.It consists
of fragments
of somewhat
hydrated
andthoroughly
leached
itabiriteloosely
cemented
by limonite.The platynatureof thefragmentscanstillbeperceived
in someplaces
withoutdifficulty._Afew of the
slumped
blocks
areverylarge,10toeven100m,butcommonly
theyarea few
tens of centimeters
or a few centimeters
in maximumdimension.Such
material
grades
irregularly
downward
withvarying
abruptness
intomuch
hydrated
andthoroughly
leached
itabirite
retaining
original
structures.
The
SUPERGENE IRON ORES OF BR.4ZIL
1219
zone of slumpedore varies from a meter or so to as yet unknown depth, cer-
tainly morethan 25 m, and variesgreatlyfrom depositto depositand within
individualdeposits. Somedepositsshowno slumpedore, the ore apparently
havingformedin placefrom itabiriteand enoughlimonitehavingbeenintroducedto maintainstrengthin the rock.
SecondaryEnrichment.--In intermediategradeore, secondary
enrichment,
in contrast to residual enrichment, is much more important than in most
enricheditabirite. Limonite cementsslumpedfragments of somewhathy-
dratedand thoroughlyleacheditabiriteand fills voidsleft by the leachingof
quartz in unslumpedore. Small-scalebotryoidalforms are common. This
ore type is grosslyporousand it is not possibleto quantitativelyevaluatethe
degreeof secondaryenrichment,althoughit is clearlyimportant. The high
iron contentof intermediategrade ore is due to the introductionof limonite,
addedto unhydratedhematiteresidualfrom the originalitabirite, as well as to
the nearlycompleteremovalof quartz, althoughthe highestgrade specimens
are due almostentirelyto residualenrichmentwithout muchhydration.
Ca•ga
Cangahasbeenusedas an iron ore in Brazil for many years. Now that
agglomeration
of the higher grade ores has becometechnicallyand economically feasible,the use of cangais declining. As an ore type, it will become
morea byproductof miningof othertypesand lessimportantin its own right,
as it is too variable in compositionand too high in phosphorusto be used
exceptin the smallcharcoalblastfurnacesproducingfrom 15 to 50 tons per
daythat are scatteredin andnearthe QuadrilSteroFerrifero.
Cangaformsa capover all other supergene
ore typesand alsoover many
other rocks. It coversmore than one hundredsquarekilometersin the region
to depthsrangingirregularlyfrom a few centimeters
to more than 30 m,
averagingperhapsa meter or two. Ore reservesare measuredin the
hundredsof millions of tons. Figure 8 illustratesa typical cangacap over
itabirite.
Nature of CaJ•a.wCangawasdefinedasa lithifiedsurficialor near-surface
iron-rich rock formedof widely varying quantitiesof detrital fragments,commonly high-gradehematiteor iron formation,cementedby limoniteinto a
blanketingbody. It may gradeinto ferruginous
lateritewith decreasing
iron
content,into ferruginousconglomerate•vith increasingquantitiesof coarse
nonferruginousdetrital material and into high-grade rubble ore with decreasingquantitiesof limonite cement. It also may grade into ferruginous
bauxite with increasingalumina and decreasingdetrital material. It also
gradesinto enricheditabirite and intermediategrade ore. Some cangacontains only a percentor two of detrital material, the rest being limonite. The
arbitrary lower limit of iron in cangais about 40% Fe; below this amount
the material is generally not lithified or is better called ferruginous conglomeratebecauseof the high content of nonferruginousdetrital material.
The definition used here containsall the essentialelementsof that used by
Derby, one of the first geologists
to definecanga(4, p. 818, 820821).
1220
JOHN VAN N. DORR
AlthoughPark (27) onceemphasized
the importance
of hematitecement
in canga,he later stated(personalcommunication,
1963) "(•uantitatively
speaking,
in theBrazilianandVenezuelan
deposits
I wouldsaythathematite
is a minorcementing
constituent
in canga,butis locallypresent."The writer
andPark are in complete
agreement
on thispoint. The writer hasobserved
muchmaterialin IndiaandVenezuela
whichis cangaaccording
to the above
definition,and it undoubtedlyexists in other parts of the world where
conditions
arepropitious.
Ruckmick's"crustalore" (31, p. 229) at Cerro Bolivar is identicalto
muchcangain Brazil.
Mineralogyand Fariationin Composition.--Canga
containsboth detrital
and nondetritalfractions;the greater part of both is derived from itabirite
and the high-gradeore it contains. The detrital fractionis commonlycomposedof fragmentsof high-gradehematiteand of itabirite, the latter leached
andhydratedto varyingdegree. In manyplacesquartzis in coarseangular
fragmentsderived from veins in the itabirite. Fine quartz derived from
itabiriteis subordinate
exceptwherecangais very thin or the itabiritevery
coarsegrained.
The nondetritalpart of cangais largelycomposed
ot•the simplehydrous
oxidesof iron. Alunfina is disseminated
in the limonitecementand very
rarely can aluminousmineralsbe found; it may be presentlargelyas gibbsite
and boehmite,althoughCorrens (3) has shownthat goethitemay contain
much A10 (OH) in the lattice. Phosphorusis commonlyconcentratedin
cangato more than 0.157oP; it is probablypresentas wavellite or strengite;
Simmonsreportsleucophosphite
in bat caves(35) in cangabut this is a local
occurrence.
Althoughin someareasand for reasonsunknownto the writer the hydrous
iron oxides seemto dehydratesuperficiallyto form a thin crust of hematite
and maghemite,at depthsof more than a few' centimeters'
in theselocalities
the secondaryoxidesare generallyhydrous. Maghemiteis locallyabundantat
the surface. While surveyingthe Itabira District, the writer foundhiscompass
radicallydeflectedovercanga,in placesas muchas 180ø, in a mannerunrelated
to the bedrockgeology. Somedeflection
wasfoundin aditsin high-grade
hematiteand itabirite, but it was quite minor comparedto surfacemanifestations. Magnetic concretionscollectedby the writer from mappablezonesin
soiloverphyllitein the Lag6aGrandequadranglewere identifiedas maghemite
by Mary Mroseof the U.S. Geological
Survey,soit is certainthat maghemite
can form at the surfacein this environment. The control causingformation
of maghemitein preferenceto goethiteor hematitecouldnot be deciphered.
Little explorationfor cangaas suchhas beendone; thereforethe compositiongiven in Table 1 in the columnheaded"Sampledaverage"is not
representative
of all cangain that this materialwas sampledduringexploration for highergrade ore types. This sampledcanga,representing1,290 m
of scatteredsamplingplus651,000tonsof productionfrom variousdeposits,
includedmany areasin which hypogeneoresare present,and thereforecontainsmuchhigh-gradehematiteas detritalfragmentsand cobbles. Compared
SUPERGENE
IRON ORES' OF BRAZIL
1221
to the usualcangafound over itabirite not affectedby hypogeneenrichment,
the iron contentis raisedand the water of crystallization,A12Oa,and SiO2
content are lowered. These data do indicate, however, the tenor of iron to
be expectedin miningcangawhichmustbe extractedto securethe high-grade
ores.
The writer believesthat the normalcanganot incorporatingdetrital highgrade hematitemay averageabout 57% Fe, 10 % water of crystallization,
1-5% SiO2, 2-5% Al=Oa,and 0.1-0.2% P. The averagegrade of 34,589
tons of cangaderivedonly from itabirite in the Itabira District and shipped
to Cia. Ferro e Acosin Vit6ria was 56.75• Fe, 4.81% SiOz, 3.19% Al=Oa,
and 0.128% P. Water of crystallizationwas not determinedbut must have
been not less than 10 percent,probablymore, as other constituentsare not
visiblein that thoroughlyhydratedore.
Types of Ca•ga.--Canga may be subdividedinto many types in detailed
mapping,but for the purposesof this paperfour will suffice. They are intergradational. These are "structurecanga,"5 formed by the hydration of iron
in itabirite and partial leachingof quartz, preservingthe original sedimentary
structure of the rock; "normal canga" (Fig. 9), generally structurelessor
grosslylaminatedparallelto the surface,composed
of fragmentsof enriched
itabirite and/or high-gradehematite cementedby limonite; "canga rica,"
composedof large and abundant fragments of high-grade hematite ore ce-
mentedby limoniteandcontainingmorethan64% Fe; and "chemicalcanga,"
material containingvery little detrital material and consistinglargely of limo~
nite. All these types may contain fragments of rocks other than those
specified.By far the greaterpart of cangais normalcanga. Chemicalcanga
was sonamedby Harder and Chamberlin(18, p. 393) and hasbeendiscussed
in detailby Pomerene(28) and Simmons(34).
On the outcrop,structurecanga,normal canga,and cangarica are tough
but fairly soft rocks,with a rough surfaceranging in color from brown to
blackthroughgray and blue, dependingon the contentof high-gradehematite
and itabirite. Below the surfacethe first two types are commonlylighter
brown and reddishbrown, with flecksof yellow, red, and tan hydrousiron
oxide. These rocks are porous,locally grosslyporous,but not notably
permeable;during the rainy seasonephemeralponds may form in shallow
undraineddepressions.Some very porouscangahas a similar physicalappearanceto vesicularlava, substitutinglimonite for glass,and laymen have
confusedthe two. Botryoidalstructureson a smallscaleare commonin normal canga with a low content of detrital material and much denseglassy
goethite.
Chemicalcanga,on the other hand, is characteristically
denseand flinty,
breakingwith a conchoidalfracture, and may be yellow, tan, or brown. It is
• "Structure canga" is strictly speaking not canga but an altered Precambrian rock, as
pointed out by Pomerene (written communication, 1963). It grades imperceptibly into normal
canga, has the same physiographic expression and a generally similar physical aspect and it
has been included as a practical matter with the other types of canga by all geologists mapping
in the area; hence it is included in this discussion. Quantitatively it is not very important
except locally, as it breaks down into other types of canga with further weathering.
1222
JOHN
VAN N. DORR
II
co•nmonly
morealuminous
thantheothertypesandis usuallyfoundongentle
slopes
andin flatvalleyson rocksotherthantheCau•Itabirite.
Contacts.---Normaland structurecangacommonlygrade downwardinto
enricheditabirite,in manyplaceswith a cavernous
interfaceowingto settling
of the lower rock? Collapseof the canga causedby this settling creates
fissuresand localundraineddepressions.Cangaalsomay gradedownward
into intermediate
gradeore, the contactbeingvery irregularand hard to
define,anddeternfined
largelyby the degreeof lithification
and the physical
Fro. 9. Normalcanga(nc), enrichedby includedblocksof high-gradehematite
ore in foreground,overlainby cementedrubbleore, cangarica (cr), in turn over-
lainby uncemented
rubbleore (ro) in background.This closeassociation
andthe
relationare unusual. Note moisture(darker areas) in normal canga,which
overlies itabirite.
aspectof the rocks. Wherecangaoverliesnonferruginous
rocks,the contact
is generallyabrupt,althoughlocally,as at AguasClaras,phyllitehas been
replaced
by limoniteto sucha degreethat the contactmay be gradational.
Cangacropsout extensively
lint in manylocalities
is coveredwith a sparse
rubbleof highlylimoniticfragments
of itabiriteor by high-grade
orefragments.
Locallyit is covered
by a sparsesoilandmuchmorelocallyby thicksoil.
Simmons(35) describedcavesformed under cangathat are causedby solutionof
dolomtiic itabirite and by mechanicalremoval of softeneditabirite.
SUPERGENE IRON ORES OF BRAZIL
1223
A few drill holesthroughTertiary(?) lake beds(?) showedcangato underlie
theserocks at depthsof as much as 106 m (28). Chemicalcanga rarely
directlyoverliesitabiritebut is foundin soil or alluviumor on other rocks;
contactsare commonlysharp.
Cangais the ultimate productof the weatheringof itabirite and interme-
diate grade ore. It capsthe ridgesand shoulders,and, becauseit is inert
chemicallyand toughphysically,protectsthe underlyingrocksfrom erosion;
it thus has strongphysiographic
effects,preservingthe ridgesand plateaus.
Figure 8 illustratesthis well; the almostcatastrophic
nature of the erosion
of the softeneditabirite where unprotected,undercuttingthe cap, and the
relativelygentleoriginalslopesare apparent.
ORE
GENESIS
The bulk changesin the composition
of itabiritein the processof supergene
ore formation have been indicated in Table 1. The changesin Fe, AI_,O.•,
SiOn, and H20 in the progressionfrom fresh itabirite to canga are here
summarized'
Fresh
itabirite
(percent)
Fe
A1203
SiO2
H•O+
Density
37.9
0.5
44.7
0.3
3.6
Canga (Away from
Enriched
itabirite Intermediate
grade high-grade
hematite
(percent)
ore(percent) bodies)
(percent)
48.7
1.4
25.4
63.3
57.0
2.7
2.4
3.5
2.75 (?)
4.6
10.0
2.75
2.75
2.0]
2.75
The valuesfor densityin placeare estimated,exceptthat of fresh itabirite.
Porosityvaries so widely and nmch of the rock is so friable that densitydeterminationson rock typesother than fresh itabirite have not been made.
The content of Fe, A1203, SiO2, and H20 in 1 cubic meter of these rocks
wouldbeasfollows,in kilograms:
Fresh
itabirite
Fe
A1203
SiO•
HxO plus
Canga (Away from
Enriched
itabirite Intermediate
gradehigh-grade
hematite
ore
1,360
1,340
14
40
1,610
10
700
70 (?)
1,740
7O
7O
130
bodies)
1,570
100
6O
28O
These figures are probably approximatelycorrect, although based on
assumeddensity.
Figures 10, 11, and 12 graphicallyshowthe relationbetweenthe variation
in ore composition
and the presentsurfacein threewidely separateddeposits.
Unfortunately, no drill hole passedfrom enricheditabirite into fresh itabirite,
soa profileof this transitioncannotbe presented. As shownby outcropsand
1224
JOHN VAN N. DORR II
adits,the gradeof the enricheditabiritedeclinesirregularlyuntil the unleached
itabirite is reachedat depthsof 50 to 200 m or more.
Figure 13 is an attemptto representthe resultsof the weatheringprocess
on itabirite. Obviouslythis representsno singleweatheringprofile, being
rather a generalizationof the effect of a dynamicprocesson a rock type.
Figures 10, 11, and 12 are specificexamplesof ore formationunder optimum
condition.
The removal of vast quantitiesof quartz to form the supergeneores is
apparentfrom thesedata.
TUTAMEA
Diamond
drill hole 37
PERCENTIRON (,,,)
0
5
10
20
0
0.05
0.1
0.2
I
I
I
0
AND SILICA (--)
30
40
50
60
I
I
70
PERCENT PHOSPHORUS (--•)
I
Canga
I
Fe=63.1
SfO2=0'8
H•)+=4.9
P=0.O8
ø
AI•:)-•= 3.8
'
"
15
Fe = 62.8
z
SiO2= 5.8
30
(with enriched Itabirite breaks)
AIzO3= 0.6
H20+=3.7
Intermediate
grade
iron
ore
P=O.07
•
45
ß
Fe=51.3
Itabirite
ß
' .'
5•0•=19.1
' ßß
AI203 =nd
HL•:)+=nd
P=O,07
ß
'
Fro. 10. Relation of compositionof supergeneore to depth below surface,
Tutamea deposit,diamonddrill hole 37. Data courtesyCia. Mineraqao Novalimense.
Silica, oncedissolvedby supergenesolutions,is not easily reprecipitated
and is therefore carried out of the system by ground or surface water, as
proved by the characteristiclack of silicificationin this region. In contrast,
after the iron in hematiteis dissolved,it is easily reprecipitatedas relatively
insolublehydrous ferric oxide. Thus, where conditionsare propitious for
long-continued
leachingof quartz and hematiteand reprecipitatiouof iron,
strongsecondaryenrichmentof the residuallyeuricheditabirite producesthe
intermediategrade ore. Where sufficienttime has not beenavailableor con-
SUPERGENE
IRON ORES OF BR.4ZIL
1225
CASA da PEDRA
Churn drill hole J•
PERCENT IRON (o ß ß ) AND SILICA (•)
0
AND ALUMINA (x x x )
10
20
30
40
50
60
I
I
I
I
I
I
Canga rica
10
x
Intermediate grade iron ore
_z
20
•o
•o
'
•,, 30
x
•
Enriched
Itabirite
x
x
ø
ß
x
ß
4O
ß
•
.
x
x
x
x
x
ß
Fro.
Relation of compositionof supergeneore to depth below surface,
Casa de Pedra deposit,chm'n drill hole JI. Data courtesy Cia, Sidurugica
_
Nacional.
ditionsare not favorablefor reprecipitation
of iron in the zoneof residual
enrichment,
residually
enriched
itabiritewithoutmuchsecondary
enrichment
is found. The irontakenintosolution
nearthesurface
mayeitherbe entirely
removedfrom the systemor redeposited
as limonitein cangaor intermediate
ALEGRIA
Shaft 1
PERCENT IRON (oo,)
0
5
10
20
0
0.05
0.1
0.2
I
I
30
AND SILICA (•)
40
50
60
I
•
70
PERCENT PHOSPHORUS (----)
i Cangarica I
oß
ß
ß
ß
ß
Intermediategrade iron ore
8
•
.
50 ß
__
ß ßo
Enriched Itab,rite
Fro.12. Relation
of composition
of SUl)erg-ene
oreto depthbelowsurface,
Alegriadeposit,Shaft1. Data courtesy
C. K. Leith.
_
1226
JOHN VAN N. DORR II
gradeore,asdiscussed
below.The netlossof ironfromthesystem
mustbe
great,butmuchlessthanthenetlossof quartz.
Mechanismof oC½condary
Enrichment
In the formationof limonitein cangaand secondarily
enrichediron ore,
two generalsourcesof the iron are apparent. (1) Somelimoniteis formed
bythehydration
in placeof hematite
in itabiriteor otherrocks. No transporPercent
I
Fe
Percent
SiO2
Mainlyresidual
' enrichment
•
•
•_ ••-
70--
Unieached
Mainly
c
o•
-
45
secondary
enrichment
E• - 40
itabirite
60--
50--
Quartz
and
Quartz
and
hematite
• Hematit
•-20
hematite
40-
-
meters
0-20
meters
I 0-50•
U nweathered
FIG.
tom of
active.
meters
meters
•
3O
10
0
Degreeof weatheringof itabirite increasesto right •
13. Graphicalgeneralization
of weatheringof itabirite. Figuresat botgraphshowapproximate
rangeof depthat whichthe processes
are normally
tation,oxidation,or reductionof the iron is involved. This hydrationtakes
placeat andcloseto thesurface,
it rapidlydiminishes
downward,
and,except
underfavorablestructuralconditions,
is commonly
not importantbelowa few
metersor tens of metersin normal itabirite. (2) Iron is introducedinto the
site of depositionand depositedas the hydrousferric oxides. The ultimate
sourceof this iron is the iron formationitselflint someprobablywas dissolved
andreprecipitated
manytimes. Suchsecondary
enrichment
by introducediron
is alsoconfinedlargely to the surfacezone,as may be expected,and dies out
SUPERGENE IRON ORES OF BRAZIL
1227
downwardin a few metersor tens of meters,exceptin particularlyfavorable
localitieswhere it may penetrateto a depth of 100 m or more.
The leachingof quartz from itabirite to notabledepthwas previouslydiscussed. Deep supergeneleachingof hematitefrom high-gradehematiteore
also occursin the region (17, p. 57; 9, p. 74) and significantquantitiesof
hematitehavebeenremovedfrom this rock to depthsgreaterthan 200 meters.
Solution of hematiteis also indicatedby the disaggregationof hematite-rich
bands in weathereditabirite. It must therefore be presumedthat, in the
leachingof itabirite resulting in residualenrichmentby removal of quartz,
hematite is also dissolvedby the same supergenesolutionsthat removedthe
quartz, althoughin far smaller quantity becauseof the relative solubilitiesof
the two minerals.
The solubility of ferric iron, the form in which that element occurs in
itabirite, is so low (0.01 ppm in pH range of 5-8; 20, p. 23) that it is
generallyassumedthat significantsolutionand transportationof iron probably
involvereductionto the ferrousstate. Many geologistsdiscussing
this region
(32, 18, 27, 17, 9, 34) havecitedthe importanceof organicacidsin increasing
solubilityof iron, althoughthe existenceof organicacidsin water in itabirite,
canga,or soilsoverlyingtheserockshasnot beendemonstrated.
Hem (19, p. 35) stressedthe role of carbondioxide in increasingthe
solubilityof iron, and Oburn and Hem (26) demonstratethat microorganisms in soil reduceEh and pH of infiltrating waters, in large part through
generationof CO_.. Moreover,Hem states(personalcommunication)"Rainwater in equilibriumwith the atmosphere
shouldhavea pH of 5.4. In passing
through surfacesoil with any biologicactivity and containinglittle soluble
matter to controlthe pH, a decreasein Eh and probablypH couldbe expected
to provide for solution of a modest amount of iron." Thus the rainwater
entering the itabirite or canga directly or through the generally sparsesoil
cover may be expectedto be able to reduceminor amountsof ferric iron and
to transportit to the site of deposition. The presenceof solutionpits in highgrade hematiteore in outcropsso locatedthat they cannothave beencovered
by soil in geologicallyrecent time indicatesthat hematite is solubleto a
significantextentin rainwateralone. If King's (24) age assignments
for the
surfacesunder which most of the important supergeneores are found are
correct,the ore-formingprocesshasbeengoingon in this regionfor 50 to 100
million years and ample time is availableto move much iron even in very
dilute solution.
It is impossible
at this time to evaluatethe relativeefficacyof organicacids
and carbondioxidepresumedto be presentin the solutionsin promotingthe
solubilityandtransportation
of iron, for no quantitativedataare yet available.
This problem,and the related onesof the pH and Eh of surfaceand subsurfacewater as revealedby aditsand drill holesin itabirite,are now being
investigatedfit situ. The generallysterile,thin, and well aerated soil over
canga, supportingonly a sparsexerol)hytic vegetation,and the almost complete absenceof soil over large areasof cangalead the •vriter to believethat
organicacidshave not playeda large role. Hem (personalcommunication,
1228
JOHN VAN N. DORR II
1964) pointsout that organicacidsare weakand supplylittle H+ to solutions;theyare moreeffectivein transportingiron ascomplexed
ions.
Oncetakeninto solution,iron couldbe transportedby thosesolutionsuntil
their pH or Eh was changed. In normal itabirite no minerals exist that
wouldeasilychangethesefactorsby reactionwith the solutionsor the dissolved
materials,thus explainingthe great depth to which residualenrichmentmay
proceedwithout other apparent change in the rocks. On the other hand,
dolomiticitabirite commonlycontainslimonite wherever supergenesolutions
havepenetratedthe rock,probablythe resultof pH changescausedby reaction
of the solution with dolomite.
Solutionscirculatingthrough the porousresiduallyenricheditabirite near
the surface in the topographic,geologic,and climatic environmentof the
QuadrilfiteroFerrifero have two generalcourses. Much of the groundwater
comesto the surfaceon the flanks of ridges as ephemeralspringsand seeps
during the wet seasonand early part of the dry season. The rest of the water
penetratesthe porous rock to the less permeableunaltered rock, thencefollowing channelsdeternfinedby permeabilityand gravity to reappearat the
surfaceas larger permanentsprings,some of them thermal becauseof the
depthof circulation(9, p. 106). Most of the water broughtup in the larger
springsruns off in rivers, removingall dissolvedmaterial from the system,
but muchof the dissolvediron broughtto the surfacein the ephemeralsprings
and seepsis depositedat and near the surface.
The sourceof the iron that secondarilyenrichedthe residuallyenriched
itabiritenear the surfaceof ridgetopscanonly havebeenfrom part of the iron
formationnow removed,for the greatestenrichmentis found at the highest
elevations and lateral movement of iron-rich solutions into those areas would
not be possible. As shownby adits, during the dry seasonand indeedfor
mostof the year the upperpart of the porousenricheditabiriteand intermediategrade ore is rarely saturatedwith groundwater. Drilling for water
wellshasshownthat the permanentwater tableis deep,in mostplacesmore
than 100 m. Thus downward-moving
solutions,which may have been able
to reduceand dissolvesmallquantitiesof iron in the upperpart of the zone,
comein contactwith oxygenas they descend
throughthe rock and the dissolvedferrousiron is oxidizedto insolublehydrousferric oxide,precipitating
at onceor possibly
movingfartherdownwardas colloidalferric hydroxide.
The ephemeral
natureof the springsand seepson the flanksof the slopes,
and possiblythe bandedgoethitein mammillaryform, suggestthat this
accretion
maybeseasonal.The zoneof secondary
enrichment
slowlydescends
with theerosion,
dominantly
chemical,
of theridgelines.The iron in the solutionsis easilyprecipitated
bychanges
in Eh. The dissolved
silica,however,
is
not sensitive
to Eh andpH changes
andthereforeis carriedout of the system.
Clearlythe effectsof long-continued
rains,shortshowers,
intermittent
showers,
andincreases
anddecreases
of totalrainfalloverlongperiodsof time
wouldcause
greatvariation
in thedominance
of deepcirculation,
shallow
circulation,aerationwithinthe residually
enrichedzones,etc. Thusthe dominanceof theseveral
possil•le
modes
of secondary
enrichment
andof different
preferred
sitesofironprecipitation
bychanges
in Ehwonldalsovarywidely.
SUPERGENE IRON ORES OF BR.4ZIL
1229
It is probablethat much of the iron laken into solutionin the lower parts
of the residuallyenrichedzonemay be as ferric iron, for ferrousiron presentas
ferroan dolomite is commonlyoxidized to limohire at depth and magnetite
alters to martite.
Controlsof Super•7ene
Ore Formation
The formation of supergeneiron ore in the Quadril/ttero Ferrifero is
controlledby physiographicdevelopment,the climaticregimen,and the grain
size and compositionof the itabirite. The single most important factor in
the developmentof suchore is, however,the sinmttaneousdevelopmentof the
physically resistant and chemically inert canga, which gives time for the
other less resistant ores to form.
Influence of Physio•lraphicœnvironment.--The remnants of widespread
ancient erosionsurfacesmentionedpreviouslysuggestthat at the time the
physiographic
evolutionof the regioninto its presentform began,the region
was oneof low elevationand slightrelief,a peneplaneprobablycoveredwith a
thick mantle of saproliteover which streamsmeandered. Epeirogenicuplift,
in Early Cretaceoustime accordingto King (24), rejuvenatedthe drainage
and incisionof the major streamsbegan. As the rocks were exhumedfrom
the lnantlingsaprolite,hematitein itabirite hydratedand a thin skin of canga
may haveformed,protectingthe itabirite from further erosion. The lessresistant adjacent rocks were then removed, stimulating lateral subsurface
circulation of the iron-rich ground water and leaching of the quartz from
the itabirite. Uplift was intermittent and canga sheets spread out over
adjacentrocks from the developingitabirite ridges. These sheetsprotected
surfacessuchas the Gondwanaand the post-Gondwana
until they were in part
removedby later erosionafter the topographicunconfornfitybetweenthe old
surfacesand the youngererosionsurfacesbecametoo great for stability (Fig.
8 and also King's discussionof pediplanation(24)).
Remnants of such
sheetsoccurat many levels.
During the long intermittent cycles of erosion and uplift, leaching of
itabirite would continue in favorable localities, most of them under the older
and highercangasheets,until quartz was nearly completelyremoved,and secondary enrichment,forming intermediategrade ore, becamethe dominant
process.Removalof the quartz,morethan 50 percentof the volumeof the
originalitabirite,causedlocalcollapse.
All known important depositsof intermediategrade ore, with two exceptions,occuron relativelyflat canga-covered
ridgesrangingfrom 1,200 to 1,600
m in elevation. Subsurfacedrainageis stimulatedby the abrupt topographic
unconformitywith the adjacent steep slopes. Exceptionsare the Capanema
deposit,whichoccurson a high ridgewith fairly steepslopesin both directions
and the Alegria deposit,which is on a dip slope. Subsurfacedrainage is
excellentin boththesecases. Without strongrelief to promotethe circulation
of groundwater, it doesnot seemprobablethat the enricheditabirite and the
intermediategradeorescouldform on a largescale.
Influenceof ClimaticRe•7imen.--Thepresentclimatein Minas Gerais is
one of pronouncedrainy and dry seasons. In the Itabira District, typical of
1230
JOHN P'.4N N. DORR II
the region,more than txvo-thirdsof the 1,579 millimeters(62.1 inches) of
averageannhall)recipitati(mfalls in the monthsof NovemberthronghFebruary. Rain mayfall nearlycontinuously
for a weekor moreat the heightof the
rainy season,but a large part of the arerealprecipitationfalls as showersbetween sunnyperiods. Often severalshowersfall in a singleday. The sun at
this latitudeand altitudeis very powerful,and surfacerocksxvithhigh thermal
conductivitysuchas hematitemay becomeso hot that they cannotbe held in
the hand with comfort. When showers fall on such warm rocks, the water is
heatedand becomesa nmchbetter solventfor quartz and iron than would be
expectablefrom the averageair temperature,which is about 20 ø C.
Intermittent rainfall has another important effect. Water caught in the
voids of the grosslyporouscangahas more time to penetratethat relatively
impermeablerock to the underlying itabirite. Thus the canga acts not only
as a shieldagainsterosionbut to someextent as a reservoir. Conversely,when
the sundriesthe rock surfaceafter shoxvers,
capillaritycan draw water, carrying dissolvediron, from the saturatedunderlying rocks toward the surface.
(The quartzcontentof the near-surfacerocksis very low.) That this process
is importantis shownby the fact that cangaon ridge topsoverlyingintermediate gradeore commonlyhas a lower tenor than underlyingintermediategrade
ore, owingto the greaterhydrationof the iron oxides. During the dry season,
however,when severalmonthsmay passwith no rainfall, the highly porous
supergene
oreslosetheir entrainedwater and are well aeratedto considerable
depths.
Stronglyseasonal
rainfallhascharacterized
all the areasin whichthe writer
has seenimportantcangadepositsand supergeneenrichmentof underlying
iron formation, including this area, Mato Grosso, Venezuela, and the
Singhbhum
and Orissaareasof India. He considers
it to be a key factorin
near-surfacesupergeneprocesses.
It is perhapsworth mentioningthat the presence
of cangaat the surface
is a strongargumentagainstthe existence
of temperatures
muchbelowfreezing, presentor past. Water caughtin the large poresof this rock would
freeze,pulverizingit and causingits removalby mechanical
erosion. This is
perhaps
theexplanation
for theabsence
of cangain the CanadianShield,where
secondary
ores,similarin manywaysto theotherBrazilianores,are common.
GrainSi•e of Itabirite.--James(22) hasshownthat the grain sizeof iron
formationis a functionof degreeof n•etamorphism,
and the situationin Brazil
is consistentwith his conclusions.On the eastsideof the area, metamorphism
hasreachedthe almandine-amphibole
faciesandthe iron formationis coarsely
crystallized,
whereasin the centraland westernsidesthe rocksare in the
greenschist
faciesandthe itabiriteis mediumto finegrained.
Table 2 tabulatesthe variationin grain sizein differentparts of the area as
established
by members
of theDNPM-USGS team.
No supergene
intermediate
gradeoreis knownin sizable
bodiesin theeast
sideof the area,wherethe grain sizeis large. All deposits
are in the central
area,thusleadingto the conclusion
that,underthe conditions
obtaining
in
the Quadril•ttero
Ferrifero,intermediate
gradeore formsonlyif the average
grainsizeof thequartzis about0.1millimeter
or below. In thewestern
area,
$UPERGENE IRON ORES OF BRAZIL
1231
the itabiriteis relativelythin and is unexplored. Someintermediategradeore
is probablypresent,but significantbodieshave not yet beenfound.
In the Monlewtdearea, residualenrichmentof the itabirite by leachingof
quartzis not knownto great depthsor on a large scale. In the Itabira District,
it is knownto at least 100 m depthbut muchhard itabiritehasbeenrevealedat
shallower depths. In the Belo Horizonte area enriched itabirite is found
at depthsof more than 200 m, and the rock is disaggregated
to greater depth
than on the eastside. In drilling the Jo5o Perreira area near Casa da Pedra,
no hard itabirite was found to 164 m (J. Eichler, written communication,
1963). No adequatebasisfor generalizationis known on the west end or
alongthe southborder,as no deepexplorationhasbeendone.
Thus it appearsthat the grain size is a critical control of the formation of
superveneores in this area. Assumingsphericity,areas of individualgrains
vary as the squareof the radiusand volumeas a cubicfunction,so grain size
is a sensitivecontrolin the leachingprocess.
TABLE
2
VARIATION IN GRAIN SIZE OF ITABIRITE IN THE QUADRIL•TERO
FERR•FERO, MINAS GERAIS, BRAZIL
Grain
Area
size in millimeters
•qource
Quartz
East
Northeast
Northwest-central
West-central
Southwest-central
South-central
Far west
Central
• Written
Reeves t
Dorr and Barbosa (9)
Pomerene (28)
Wallace
•
Guild (17)
Johnson (23)
Simmons
O'Rourke
t
t
0.5 (average)
0.2 (average)
0.01-0.06 (range)
0.01-0.1 (range)
0.01-0.1 (range)]
0.10-0.16 (range)
0.04 (average)
0.2 (average?
ttematite
0.4 (average)
0.1 (average)
0.03 (average)
0.05-0.5 (range)
0.001-0.05 (range)
0.01-0.25 (range)
0.03 (average)
0.05 (average)
communication.
2 Anomalously high compared with local metamorphic grade and size of hematite.
fresh itabirite in this area and the figure may not be typical.
Very little
Compositionof Itabirite.--The compositionof itabirite exercisescontrol
over the formationof supervene
iron ore deposits
because
certaintypesof
itabirite are more susceptible
to leachingand enrichmentthan others,and,
particularly,because
the weathering
productsof certaintypescannoteasilybe
utilized.
Itabiritecomposed
dominantly
of quartzand hematitefirst disaggregates
into a soft rock easilyamenableto concentration
by gravity or electrostatic
methods. As the hematitebecomesmore hydratedcloseto the surface,these
means of concentration
become more difficult.
The iron mineralsin dolomiticitabiritecommonlyhydrateas the rock disaggregatesif the ferruginousIaminaecontainmuch dolomite. Furthermore,
the hydrousoxidesare in manycasespowdery,impedinggravityconcentration
or, in somecases,the hydrousoxidesin theserocksbecomeweldedinto very
hard and densemassesof materialthat are not of economicore grade.
1232
JOHN VAN N. DORR II
However,onetyl)eof oreeasilyconcentrated
to economic
grade,chal)inha
,
is thought to have formed from dolomitic itabirite. In this ore the dolomite
was probablyconcentrated
in the siliceous
layers. On dissolving,the qnartzrich layersare left completelydisaggregated
and the iron-richlayerscoherent,
so that, at Pires in the Congonhasdistrict,by a simplewashingprocessan
ore averagingmorethan 62 percentiron is won from rock averagingin place
al)ont38 percentiron.
Amphiboliticitabirite, like the dolomiticitabirite from •vhichit xvasderived
by metamorphism,commonlyis not suitablefor concentrationnor is it known
to be associatedwith intermediategrade ore, except very locally. The amphibole,commonlycummingtoniteor tremolite, breaks down on weathering
into softyello•vto buff hydrousiron oxidemostdifficultto recover.
Both dolomiticitabirite and amphiboliticitabirite commonlycontainrelativelyhighpercentages
of magnetiteßThis mineralalsooccursvery locallyin
high concentrations
in the normalitabirite,as near AntonioPerreira. Conceivablysuchmaterialcouldbe concentrated
by magneticmeans.
It is perhaps
worthyof notethat the carbonate-facies
ironformationthat
occursin manylensesin theoldestPrecambrian
metasedimentary
rocksof the
regiondoesnot become
enrichedto ore grade. Only thin and low-grade
cangacapsare associated
with this material. Whethertiffs is dne to the
mineralfaciesof the iron formation,the thinnessof the lenses(100 m or less),
or the fact that the lensescommonlycropont belowthe post-Gond•vana
erosion surface is not clear.
Origin of Can.qa
The exactmodeof originof cangahasbeenmootamonggeologists
who
havestudiedthe rockin recentyears. Therehasbeenno question
as to the
originof thedetritalfraction
of therock;it obviously
is largelyderived
from
the itabiriteand the high-gradehematitelensescontainedtherein. All
writersalsoagreeonformation
of "structure
canga"by hydration
of hematite
in situandpartialleaching
of quartz. Discussion
hascentered
aroundthe
sourceof the iron in the cementing
limohireand the modesof transportation
anddeposition
of thiscementing
material.
Theclassical
explanation
for thegenesis
of cangaisperhaps
bestexpressed
byHarderandChamberlin
(18,p. 392-393),whosay:
The fragments(of high-grade
ore and itabirite) havebeenmechanically
concentratedandhavebeencemented
togetherby iron oxidedeposited
from solution,both
fragments
andcementing
materialbeingderivedfromthe samesource--the
iron
formation. The cementing
materialis largely hydratedhematiteand limohire.
ß. . The fragments
decrease
in sizeandabundance
as thedistance
fromthe source
increases,
so that in oneplacea cangadepositmay consistlargelyof cemented
fragments
andelsewhere
of finelytexturedchemically
deposited
ironoxide?
The genesis
of cangain MatoGrosso,
Brazil,wasascribed
by thexvriter
(5, p. 44 45) to solution
of ironfromironformation
by descending
and
* From their paper,it is clearthat theseauthorsmeanthydratediron oxide (limonite)
ratherthan anhydrousiron oxide (hematite,martitc,or magnetite).
SUPERGENEIRON ORES OF BRAZIL
1233
laterallynilgratingwaters,which redeposited
the iron wherethey surfaced
in nearbylower valley slopesand in valley bottoms,cementingthe colluvial
and alluvialiron formationfrag•nents. Somecanga,foundon granitetwo km
and morefrom the apparentsourceof iron, was depositedby surfacingwater
whichhadmigratedalongfaults. Dorr, Guild,andBarbosa(11, p. 293) and
Guild (16, p. 673-674) ascribedthe formationof cangain the Quadril/ttero
Ferrifero to the sameprocess,as well as to the hydrationof itabirite in place;
somecangawas ascribedto evaporationof iron-rich waters drawn to the
surfaceby capillarity. Putzer (29, p. 38), writing of both the Mato Grosso
and MinasGeraisoccurrences,
essentially
agreeswith theseideas. Park (27,
p. 578) wrote: "That iron is transportedreadily (in tropicalclimates)is
shownby the developmentof cangaand hard laterite and depositionof
limonite where undergroundwaters are exposedto air."
A new element,.'as addedby Pmnerene(28) who in 1957 evolvedthe
hypothesis
that cangawasformedby the replacement
of soiloveritabiriteand
other rocksby limonite. In supportof this thesishe citedthe well-developed
but fragile botryoidaland mammillarystructuresthat are locally found at
somesoil-cangainterfacesand are rapidly destroyedwhen the fresh canga
face is exposed. He did not statewhetherhe believedthe iron in "normal"
cangato have beenderiveddirectly from the itabirite or from the overlying
soil or both, but implied that at least someof the iron in "chemical"canga
might be derived from soil rather than directly from iron formation. He
emphasizedthe needof a soil coverto forln normal cangaand cangarica.
Simmonsfurther developedtheseideas and carried them a step beyond
Pmnerenein a paperpublishedin 1960 (34, p. 43) saying: "It is advocated
ß . . that cangawas formed by cementationof hematite-rich(largely in the
forln of itabirite) detritusprincipallythroughthe actionof laterally and downward moving meteoricwater. Rain falling on the surfaceof these sediments
dissolvedorganic material to form acid water, and dissolvediron as it
circulated. Ultimately the solutionsreached a new environment where they
becamelessacid becauseof dilution in the groundwater zone or becameless
acid or alkaline becauseof reaction with the materials through which they
passed. Where this happenediron was precipitatedas limonite around the
detrital particlesto form canga."
Thus Simmons advocated that the source of the iron in the limonite
cement
was the alluvial, colluvial,or eluvial material at the surfaceand not primarily
the iron formation itself, which contains no organic material. He also
stressed
precipitationof limoniteby neutralizationof solutionsoriginallymade
acid by organicacidsrather than precipitationby evaporationor oxygenation.
He alsoadvocatedprecipitationfrom downward-movingsolutionsrather than
froin solutionsmoving toward the surface.
Simmonsextendedthe definition of canga to include material considered
by the writer to be better describedas ferruginousconcretions(34, pl. 8, 9,
& 10) and baseshis argumenton analyzedprofilesaboveand below suchconcretionary material (34, p. 48), although trace-elementanalysesto prove
leachingof this alluvial material seemedinconclusive. Leachingof iron from
soil as advocatedundoubtedlymay occur (13, p. 101).
1234
.JOHN VAN N. DORR II
It was pointed out previously that where waters from seasonaland
ephemeralspringscome into contact with the air, the Eh abruptly increases
and containedferrous iron is oxidized to insolubleferric hydroxide. Heln and
Cropper describethe process(20, p. 16):
Water below the land surfaceand not in contactwith air probablyhas Eh valuesof
0.20 volts or less and a relatively low pH (less than 6) if enoughexcesscarbon
dioxideis present. Under theseconditions,ferrous solutionsas high as 50 ppms
are permanentlystable. As the water approachesthe surface of the ground and
dissolvesoxygen from the air, (a) nonequilibrium situation . . . sets in. Water
low in Eh and high in Fe++ is continuallysupplied. Ferrous-iron oxygenation
and ferric-hydroxideprecipitationboth continue.
Thus oxygenationof ferrous iron is believedto be one commonmechanism
by which the limonite is formed in the extensivecanga sheetsfound spreading
out over the surfacesbelowthe ridgetops. Clearly, if the waters stirfaceunder
the thin soil cover in the flat valleys bordering some of the ridges, as along
the Serra do Itabirito and Serra de Moeda, or if the solutions follow the soilsaprolitecontacton pedimentsextendingout from the ridges,limonite will be
deposited,as the soil desiccatesrapidly during the dry season. This limonite
might surroundcoarsedetrital material in the soil or might replacesoil min-
erals, forming norlnal or chemicalcanga, dependingon the local situation.
Commonlydetritus is coarseand iron-rich on or near steepslopesand is fine
and lnorealuminousin the flatter areas,whichthusexplainsthe usualdistribution of normal and chemical canga.
Another factor promoting the depositionof limohire is evaporation of
the waters from seepson soillessslopes. During the early part of the dry
seasonand betweenshowersin the wet season,soillesscangaslopesare locally
moistened,locally continually wet by water seepingthrough from below.
Films of iron hydroxidemay form on pools. Any containedferrousiron is
depositedthrough oxygenation. Iron transportedas a colloidor in solution
in ferric form is depositedthrough evaporationof the carrying medium.
Figure 14 illustratessucha wet soillesscangasurface,here coveredby yellow
limonite.
That lilnoniteis depositedby meansother than the neutralizationof acid
solutionsadvocatedby Simmonsis proved by the presenceof stalactitesand
stalagmitesof limoniteas muchas a meterin lengththat forlnedin a tunnelin
cangaat FazendaAlegria in about40 years (Shearer, 32, p. 16). Henwood
(21, p. 225) alsospeaksof stalactitesand tuberculatedcrustsof limonitein
natural crevicesin itabirite revealedby gold mining at Agua (•uente. No
reason for neutralization of solutions in such an environment is apparent,
althoughboth evaporationand oxygenationwouldbe effective.
Cangaon ridgetopsmust be continuallyrenewed,otherwisein the course
of tens of millionsof years of leachingand mechanicalerosion,it would be
removed. Extensivecangablanketsare found on high ridges,where the only
possiblesourceof renewalis from below. Depositionof limonitefrom solutionsbroughtto the stirfaceby capillarityseeinsthe only possiblemeansfor
s No concentration of iron in water
Quadril•tero Ferrifero.
even close to this value is yet known in the
SUPERGENEIRON ORESOF BRAZIL
1235
FIG. 14. Wet surfaceof canga at the beginningof the dry seasonnear Morro
do Chapeu. Water seepsupwardthroughcanga.
Fic. 15. Relatively sharp contactbetween small body of high-grade hematite
and itabirite, near Cau• Peak. Note vein under hand with watch. hgh--highgrade hematite, i--itabirite.
this rene•val, although this mechanism•vould only be effective during the
rainy season. Between sho•versthe surface of the rock dries rapidly, and
moisturecarrying minor quantitiesof dissolvediron •vould be drawn upward
by capillarity to depositthat iron by evaporationand oxygenation. As pointed
out by J. Eichler (personal communication,1963), the presenceof manganiferouscanga vertically above manganiferousbeds in itabirite strongly
indicatesan up,yard movement of solutions.
1236
JOHN
VAN N. DORR II
As emphasized
by Guild (17, p. 59) and others,cangais bestdeveloped
on
the dip slopesideof hogbackand cuestaridges. This is because
the drainage
of surfaceand subsurfacewater is directedby rock structuretoward the dip
rather than the scarpslopes. Such dip slopesin many casesintersectboth
permanentand temporarywater tablesin the itabirite, as attestedby seeps
and springs.
There is an enormousquantitativediscrepancy
betweenamountof finely
dividedhematiteavailablefor leachingin the great massof residuallyand
secondarilyenricheditabirite in the high ridges on the one hand and in the
thin soil over mostcangaon the other hand. Most cangais closelyassociated
with itabirite. It thereforeseemsprobablethat only a minor percentageof
the iron in limonite of the cangawas directly derived from soil or detritus.
Furtherlnore,in certainlocalities,suchas near the airport at Itabira, it can be
observedthat where drainagechangeshave cut off canga formed on nonferruginousrocks from water derived from itabirite, the canga gradually
disintegratesinto nonlithified aluminous laterite, even though covered by
ferruginoussoil.
For these reasonsthe writer agreeswith many other geologiststhat the
major source of the iron found in canga, both the detrital and chemical
fractions, is the iron formation itself.
g)uartzite and phyllite containingappreciablepercentagesof hematite
occur in the region and on weatheringgive rise to small patchesof yellow
to light-brown botryoidaland mamlnallarylimonitic concretionarymaterial,
locallycontainingmaghemite,that is a few centimetersthick in protectedareas.
Suchconcretionarymaterial rarely occupiesas much as a few tens of square
meters;in manyplacesit occursas individualpellets,and is not to be confused
with the large sheetsof canga. It is more closelyrelated to the granjon in
soil near somemanganesedepositsin Brazil (10, p. 14) or to the perdigones
in ferruginouslaterite in Cuba.
In summary,the limonitecementof cangamay form under a soil cover by
replacementof soil minerals, as originally suggestedby Pomerene and Simmons, and also by evaporationof iron-bearing water at the canga-air or soilcangainterfaces. It may alsoprecipitateby oxygenationof ferrous solutions
at the samesitesof deposition. Solutionsmay move downward, laterally, or
upward. Dominance of one or another processis determined by physiographic, climatic, and geologicenvironmentand probablychangeswith the
season. The writer believesthat oxygenationand evaporationof iron-bearing
watershaveprobablybeenthe mostimportanton a regionalbasis.
SIGNIFICANCE
OF DATA
The supergeneores of Minas Gerais are thoughtto be quite similar in
mode of origin to thosein many other parts of the world. Their genetic
controlby physiographic
environmentis matchedin India, parts of Africa,
and Venezuela,where oresalsooccuron the ridge tops,high slopes,and high
plateaushaving vigorous undergroundwater circulation. Genetic control
by climaticregimenis lessclearcut,for climateschangewith time and it is
SUPERGENE
IRON
ORES' OF BR./tZIL
1237
perilousto projectpresentclimaticregimens
far into the past;a clearcorrespondence
existsat present,however,in severalpartsof the world. Genetic
controlby grainsizeof the itabiriteis matched
in Venezuelawheregrainsize
alsoincreases
fromwestto eastandwheresupergene
oresare notpresentin
areasof largergrain size. Geneticcontrolby composition
of the iron formation is probable. Variationsof composition
of oxide-facies
iron formation
havedefiniteeffectson supergene
ore formation,and,moreimportant,such
supergeneores have been observedto form best from oxide-faciesiron forma-
tion in mostplacesin the world. The major exceptionis, of course,the
CanadianShield. Oresthereseemto havehad a differentphysiographic
historyand,asGrunerhasemphasized
(15, p. 207), opinionis notunanimous
on ore genesis.
Two otherimportantgeneralizations
maybe made. The first is that data
fromthe Quadril/ttero
Ferriferoindicatethat a definiteupperlimit existsfor
thetenorof ironthatcanbeachieved
by supergene
enrichment.The ultimate
weathering
productof itabiriteand intermediate
gradeore is canga,and
wherecangais notenriched
by included
fragments
of high-grade
hematiteore
formedbyhypogene
processes,
cangais generally
lowergradethanunderlying
intermediate
gradeore. Table 3 givesspecificexamplesof suchrelationsas
revealed
in the Tutameadeposit.Althoughthe SiO2is lowerin the canga,
indicatingcontinued
leachingof this constituent,
Al,zO
a is residuallyconcentrated.Fe is lowerin cangabecause
the iron hasbeenfurtherhydrated.
With timeand exposure
to the elements,
hydrationproceeds
further,as
shownby theestimated
generalanalysis
of cangaor eventhatof thesalnpled
high-grade
canga(Table1). Werehematite
formedin significant
quantities
at thesurface
in preference
to limonite
or frolnlimonite,
it wouldbeexpectable
that the cangawouldbe higherin tenorthan the underlying
intermediate
grade ore.
A_second
generalization
thatcanbe drawnfromthesedataon supergene
oresis that the high-grade(>66 percentFe, <1.5 percent14_oO+)hematite
ores cannothave formeddirectlyby supergene
enrichment,pre- or postmetamorphic.The analyses
in Table 1, whichrepresenthundredsof millions
of tonsof eachsupergene
ore type,and in Table 3, in conjunction
with the
three-dimensional
exploration
workfromwhichmostof theanalyses
resulted,
showthefollowing
statements
to begenerally
valid: (1) The supergene
ores
are commonly
morehydratedand lowerin tenorin iron at the surface,reach
a maximum
tenorin ironin theintermediate
gradeoresnearthesurface
just
belowthecangacapandthenagaindecrease
in gradedownward.(2) The
contentof silicadecreases
toward the surface. (3) The contentof alulnina
increases
towardthesurface.(4) Thecontentof phosphorus
increases
toward
thesurface
andis verylowin itabiriteunaffected
by supergene
processes.
No reason
is apparent
why,in anyhypothetical
pre-metamorphic
supergene
enrichment,
similarweathering
effectsshouldnothavealsoresulted,
as they
are the result of the normal geochemicalbehaviorof the elementsinvolved.
Specifically,
aluminaandt)hosphorus
wouldbe evenfurtherconcentrated
hy
further supergene
activity.
Analysesrepresenting
lnorethan 20 milliontonsof production
and the
1238
JOHN
VAN N. DORR II
three-dimel•sionalexl)loratiou of many hundredsof 1nillionsof tons of highgrade hematiteore at significantdepth below the surfaceshow the following
statementson suchores to be generallyvalid: (1) High-grade hematiteores
averagelessthan one percentof both water of crystallizationand of alumina.
(2) The tenor in iron of the ores doesnot changesystematicallywith depth
and, to maximum explored depth (270 m below lowest outcrop), bears no
relation to the presentsurface. (3) No systematicspatial relation between
positionin the depositand contentof quartz and alumina exists,exceptat the
very surfacewhere the ores have been modified by superimposedsupergene
weathering. Phosphorusis commonlyand alumina locally enrichedfor the
first 10 to 30 m, and phosphorusmay be enrichedin pocketsand brecciated
ore and alongfaultsor fissuresto greaterdepthsbut rarely is as high as in the
supergene
ores. (4) Contactsof high-gradeore with wall rock are relatively
abrupt, the ore gradingwithin centimetersor a few metersinto unenriched
Table 3.
Relation between grade of canga and that of underlytn• intermediate grade ore, Tutamea deposit.
(Data courtesy of Cœa. Mtneracgo Novaltmense)
Working
Canga
Intermediate
....
(meters)
plus
.....es,.0l ,0
(meters)
plus
(percent)
Adlt
37
(percent)
0-3
63.2
0.91
3.32
0.045
4.4
3-6
65.8
0.97
DH 37
0-9
63.1
0.66
3.60
0.079
5.0
9-21
65.2
2.41
DH 39
0-'6
61.9
1.05
4.58
0.074
6.0
6-9
64.0
DH 44
0-6
55.9
0.63
nd
0.074
nd
6-9
60.3
61.0
0.81
Average
grade ore
3.83 0.068 5.1
0.036
nd
2.62 0.052
4.1
0.60
2.55 0.061
4.8
0.98
4.36 0.039
nd
3.18 0.047
nd
63.6 1.24
nd
wall rock. This is especiallynotable in the smaller bodies of high-grade
hematite,wherecontactsare commonlysharpbetweenmaterialaveraging685/0
Fe andabout50% Fe (Fig. 15).
Thus, although the differencesin tenor in iron between the high-grade
hematiteoresand the intermediategrade ores may be only a few percent,the
differencesin mnountand distributionof ganguemineralsand in the relationof
ore to presentsurfacesmakeit clearthat onecouldnot havebeenderivedfrom
the other. The only supergeneores high enoughin iron to yield high-grade
hematiteore by isochemical
metamorphismare much higher in aluminaand
phosphorusthan the high-grade hematite ores.
The modeof originof cangaandits importancein the development
of other
oresshouldbe more carefullystudiedin other areasand more detailedstudies
shouldbe nndertakenill Brazil by future workers. Data are neededas to the
presenceof organicacidsand CO., in significantquantity iu solutionsin iron
formation,called upon by many geologiststo explain the solutionof iron,
SUPERGENE
IRON ORES OF BR.4ZIL
1239
but demonstrated
by none. The presenceof bacteriashouldbe investigatedin
the field and their role,theoreticallyimportant,shouldbe specificallyevaluated.
pH and Eh conditionswithin the iron formationshouldbe investigatedat
various depthsand at different seasons. The formation of minor hematite and
maghemite under surface conditions needs rational explanation, hitherto
lacking.
International studiesmight be facilitated by the adoptionof a uniform
nomenclature
for the variousiron-bearingrocksand their weatheringproducts,
for the samerocksare now knownby many differentnames. In any case,the
local rock nalnesshouldbe clearly defined.
U.S. GEOLOGICAL
SURVEY,
WASHINGTON, D.C.,
.4pril I0, I964
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