Economic Geology
Vol. 54, 1959, pp. 573-587
THE
ORIGIN
OF
HARD
CHARLES
HEMATITE
IN
ITABIRITE
F. PARK, JR.
ABSTRACT
Great confusionexistsin the interpretationof field evidenceconcerning
the origin of massesof hard hematitein itabirite. The evidenceis briefly
reviewed, and someadditional observationsare recorded. The conclusion
reachedis that depositsof hard ore in itabirite commonlyresult from
normal weatheringprocegses.Supergenedepositsof hard ore are more
numerousand more widespreadthan are depositsof metamorphicor of
other modesof origin.
Ta•. presence
of lensesandirregularmasses
of hard,compact,
generallyblue
hematitehasbeenrecorded
frommanydeposits
of itabirite* throughout
the
World. The genesisof thesehard oreshas provokeda great deal of discussion,
andthe fieldevidence
concerning
their originhasbeeninterpreted
in
severaldifferentways. Amongthe originssuggested,
eachof the following
hasreceivedconsiderable
support,andis still currentlyadvocated
by its proponents.
1. Syngenetic. The hard ores are original sediments.
2. Weathering.
a. The hard ores result directlyfrom weatheringby the leachingof
silicaand other impurities,followedby consolidationand reconstitution of the remainingiron.
b. The hard oresformedby the substitution
of iron carriedby ordinary
ground waters,for silicaand carbonatein itabirite.
3. Weathering followed by mild metamorphism. Silica and other impurities are leachedby meteoricwaters to form soft ores. The hard
oresresult from mild metamorphismof the soft materials.
4. Regionalmetamorphism.
a. The hard oresresultfrom enrichmentof the iron causedby leaching
of silicaand other materialsby heatedgroundwatersduring periods
of regional metamorphism.
b. The iron carried in heated ground waters replacessilica and other
impuritiesin the itabirite. This is especially
operativeduringperiods
of regionalmetamorphism.
• Itabirite was defined by Eschewegein 1822 (GeognostischesGem•ilde von Brasilien) as
the massive,pure iron ore of which the peak of Itabira do Campo, Minas Gerais, Brazil, is
composed.Derby (7, p. 817) later usedthe term to indicatespecularireschist. By common
usageitabirite is now a synonymfor taconite,as usedin the Lake Superiorregion. Much of
the rock is schistose,but this is not an essentialfeature.
573
574
CHARLESF. PARK,JR.
5. Magmaticemanations.
a. The hard ores result from leachingof silica and other impurities
by magmaticemanationsassociatedwith deep-seatedbodiesof igneous materials
or with lava.
b. Replacement
of silica,carbonates,
and othermineralsby iron carried
in solutionsof magmaticorigin or affiliation.
Under favorableconditionsboth iron and silica move readily in either
meteoricor magmaticwaters, hence broad generalizationsabout the origin
of hard ore are likely to be in error. Each of the listed modesof origin is
possible,
andprobablyno two are mutuallyexclusive. Greatconfusion
exists
in the literature,largelybecauseof the complexityof the problemsand becauseof the differencein interpretationof field observations.
The hard oresof the well known districtsin Minas Gerais,Brazil, present
an interestingexample of confusion. Gathmann (16), working in a small
area, consideredthat the hard ores were formed directly from weathering
processes
by the leachingof silica. He pointedout that tunnelsdriven.beneathhard outcropsencountered
soft sandymaterials. Harder (23) denied
Gathmann'sthesis. He thought that the hard ores were continuousin
depth. He alsoemphasized
the fact that thin (ñ 2 m) bedsof hard ore are
continuousalong strike for several hundred meters, and are apparently
conformable
with the enclosing
beds. Thin layersof hematiteare interbedded
with limestones,
and structurallythe ore bedsbehaveexactlythe sameas do
the nearbysandstone
and limestonebeds. Harder calledattentionto numerous outcropsof limestoneas evidencethat extensive surface solution and
decomposition
had not beenwidespread. He believedthat the ores are syngenetic,possiblysomewhatmetamorphosed.A syngeneticorigin, followed
by mild metamorphism,likewisewas proposedby Freyberg (14), by Leith
and Harder (31), and by Harder and Chainberlin(22).
Guild (20, 21) consideredthat the hard ores of Casa de Pedra, in Minas
Gerais, were formed by heated waters, possiblyof meteoric origin, that
ascendedalong faults. Silica and carbonateswere replacedby iron carried
in the solutions. Sanders(40) refrainedfrom direct discussion
of the origin,
but he emphasizedthe considerablehorizontaland vertical extent of the ores.
He impliedthat the Brazilian hard ores are of replacementorigin, thoughhe
doesnot say that iron was carried in either hot or cold waters.
Dorr (8, 9) has emphasizedthat the localizationof the Brazilian hard
ores was effectedby replacementduring metamorphism.The migrationof
iron was apparentlycontrolledby stressesset up during tight folding. Under
this theory,hot fluidsare thoughtto have carriediron, and to have migrated
from the flanks of folds toward lower energy levels at the crests of folds,
where the iron replacedsilica and carbonates. Dorr minimizesthe effects
of surfaceactivities,and he emphasizes
hot waters and metamorphism.He
statesthat hard ore alters to powder ore (jacutinga), but does not consider
the possibilitythat powderore may changeto hard ore. In many placesin
Brazil hard ore changesin depth to jacutinga,and why a crust of hard ore,
supposedly
formedby hot waters,shouldbe alteredby cold meteoricwater
ORIGIN OF HARD HEMATITE IN ITABIRITE
575
to powderore in depthhasnot beenanswered. Isn't the hard ore possiblya
type of "casehardening"relatedto the presentor an old surface? Dorr
further states (9, p. 296) "... many small (1-10 cm thick) hard, pure
hematitelensesin itabiriteare closelyassociated
with and clearlyrelatedto
minorquartzveins,indicatingthe passage
of hot watersor gaseswhichcould
mobilizeboththe iron and silica." The merepresenceof minor quartzveins
and hard hematitedoesnot necessarilyindicatethe passageof either hot water
or gases;suchveinsmay be formedby cold water. Emphasison the lack
of quartzveinsin eitherhard or powderore is alsoopento question. Locally
chert appearsto be leachedmore readilythan doescrystallinequartz, and in
an iron ore depositin French Equatorial Africa, small quartz veins remain in
otherwisehigh-gradehematite. Eventuallyboth chert and quartz veins are
removedby surfaceleaching.
That replacement
is commonin the Brazilianhard oresis amply demonstratedby the preservation
of the very fine bandingtypicalof itabirite,where
minutelycrumpledbeddingis preservedin great detail. It is difficult to
think of hard hematiteas beingfolded-in this fashionwithout resultingdestructionof the bedding. Cross-cuttingveinsof hard blue hematitehave also
beenseenat severalplaces,thoughthey do not appearto be abundant. Other
evidenceof replacementabounds,and iron has unquestionably
beenaddedto
itabiriteafter deformation. At leastpart of the hard ore was formedby replacementprocesses,
and is not an original sediment. It is likewisedifficult
to seehow hard ore couldform by simpleleachingof the silica. In this case
slumpshouldbe recognized,and the overallthicknessof the ore bed would
probablybe lessthan that of the adjacentbedsof itabirite. Slump has not
been describedby any of the many observersin Brazil, and the ore bodies
appear to be at least as thick, and locally thicker, than the nearby beds of
itabirite.
The answeras to the origin of the Brazilian hard ores will not be fully
known until a thoroughstudy of the geologicalhistory and geomorphology
has beenmade. Most geologistswho have workedon the depositsin recent
yearsagreethat the oresare at leastin part of replacement
origin. Nevertheless,the necessityof callinguponhot watersto implementthis replacement
has not been demonstrated. Outcrops of hard ore at both Itabira and at
Casade Pedraseemto be givingway to softore and to powderore as mining
becomes
deeper. How and why this soft materialcan be derivedby normal
weatheringprocesses
from an overlyingcappingof hard ore is difficult to
understand. Extensivebodiesof hard ore havenot yet beenfoundat depths
below which supergeneagenciesare active. Hard ore may possiblyhave
resultedfrom the actionof hot waters,but cold waters may also have been
the medium by which iron and silica were rearranged. Ore such as that
shownin Figure 1, which containsbrokenparticlesof goethitecementedby
hard blue crystallinehematite,is difficultto think of as having formed as a
resultof hydrothermalactivitiesduring regionalmetamorphism.
Similar controversiesand confusionexist in regard to most of the other
large itabirite or taconitedepositsin the World where hard hematitemasses
576
CHARLES F. P,•IRK, JR.
are found. In Venezuela, Alcaino and Ascanio (1) thought that supergene
action on iron-rich rocks developedcompactmassesof hard hematite. The
enrichmentwas effectedby a processof substitutionrather than by one of
leachingalone. Burchard (3) thoughtthat the ores of E1 Pao were of metamorphicorigin, thoughhe was not willing to dismissthe possibilitythat they
were syngenetic. Zuloaga (47) and Zuloagaand Tello (48) thoughtthat
the hard ores of the Sierra de Imataca were formed by the reconstitutionof
itabirite by hot solutionsthat accompaniedgranitic intrusions. Lake (30)
statedthat the oresof Cerro Bolivar were probablysyngenetic,combinedwith
partial leachingof the silica.
Fro. 1. Photograph of a slab of iron ore from the Angu property, Minas
Gerais, Brazil. Clusters of radiating needles of brown goethite are brecciated
and recementedby blue hematite. Length of speci•nenis 8 cm.
In the Lake Superiorregionthe hard oresof the Marquetteand Vermilion
Rangeshavelongbeenconsidered
to be metamorphosed
softore, as notedby
Fulton (15) and Royce (38). The soft ores were formedby leachingof
silicaand carbonates,
eitherby hot or by coldwaters. Robertsand Bartley
(36, 37) statedthat the ores of Steep Rock Lake, Canada,as well as the
hardoresof theMarquetteandVermilionRanges,wereformedby theactivity
of iron-richsolutionsof magmaticorigin. This involvedthe introduction.
o{
ironandthe replacement
of impurities,.
ratherthanthe simpleleachingof silica
andcarbonates.Schofield(41) suggested
that the watersthat formedSteep
Rock Lake, MarquetteRange, and the Vermilion Range, might be heated
groundwatersratherthanwatersof magmatic
origin. Joliffe(25) remapped
the SteepRock Lake depositafter considerable
mininghad beendone. lie
ORIGIN OF HARD HEMATITE IN ITABIRITE
577
interpretedthe ore as an originallimoniticsedimentary
bed,in part mildly
metamorphosed,
and with reworkingof part of the iron.
Moss (33) described
the high-gradeoresof Labradoras beingsecondary,
causedby the removalof silicaand the additionof iron to a formationthat
originallycontainedabout30 percentof iron. Someiron extendsinto adjacent
formationsthat containedlittle or no iron originally,thus indicatingthat much
iron has beentransported. Moss did not considerit necessaryto postulate
that hydrothermalsolutionsplayeda role in the formationof the iron ore.
In Australia, Lockhart Jack (32) interpretedthe hard ores of South
Australia as having a hydrothermalreplacementorigin. Edwards (12), in
discussing
the oresof the MiddlebackRanges,concludedthat they might have
a hydrothermalreplacementorigin, but he was inclinedto favor a syngenetic
origin as beingmore probable. He thoughtthat a sedimentaryorigin was
favoredby the gradationof iron formationinto quartzites,by the lack of sulfide in the ore, and by the abrupt and linear characterof the Middleback
Ranges, which suggest a narrow somewhat intermittent coastal deposit.
Later Edwards (13) attributed considerableenrichmentto supergeneprocesses. He statedthat iron, taken into solution,migrateddown dip or down
pitch, into structuraltraps, where it either replacedquartz, or was deposited
in voids left by the solutionof quartz, in the deeper-lyingportionsof the
banded iron formations.
Limonite
derived in this manner was converted to
hematitebeforeexposureto erosion. Rudd and Miles (39) suggestedthat
the iron oresof the MiddlebackRangesformed by removalof silicafrom the
itabirite and enrichmentof the iron by supergenesolutions. They stated,
however,that the ground water is now leachingiron, so the solutionsthat
ß implementedenrichmentdifferedfrom the presentday groundwaters. Similarly, in the Yampi Sounddistrict of Western Australia, Canavanand Edwards (5), considered
the oresto be probablyof syngenetic
origin. Canavan
(6) later preferredto considerthe ore as an originalsediment,thoughhe
admittedsomeevidenceto the effectthat iron replacedthe sediments.
In the Singhbhumdistrict,India, Krishnan (29) favoredthe replacement
of hematite-jasperformation by hydrated ferric oxide, which later became
hematite. This was brought about by descendingmeteoric waters. As
supportingevidenceKrishnanpointedout that the ore at the surfaceis hard
and compact,but at depth graduallychangesto shaly and powdery ores.
Dunn (10, 11) thoughtthat early enrichmentof the ores resultedfrom hot
waters,but that later enrichmentwasby colddescending
waters. He pointed
out that supergene
enrichmentis goingon in India today.
Joubert(27), in discussing
the hard oresof SouthAfrica and of South
West Africa, consideredthat thermalactionwas significantin their formation.
At Thabazimbi, in the Rustenburg district of Northwestern Transvaal, he
attributed the thermal action to solutionsaccompanyingthe Bushyeld complex, but at Postmasburg,
where no igneousactivity is known,he attributed
the hard ores to thermal activities during regional metamorphism. His rea-
sonsfor insistingon thermalactivitiesare not clear. Boardman(2) thought
that field evidenceindicatedthat the hard ores at Postmasburgshouldbe
578
CHARLESF. PARK, JR.
ascribedto prolongedactionof weatheringagencies. At Thabazimbi,Strauss
(43) consideredthe hard ore to be formed by supergeneagencies. Here the
hard ore fingersout with increasingdepth and gradesinto ordinary itabirite.
It becomesmore siliceousand lesscompactat depth. Tyndale-Biscoe(46)
in describingthe hard ore depositsin SouthernRhodesia,quoted Swift to
the effect that the itabirite beds had been squeezedinto a seriesof lenses.
CO2 and H20 were driven off by heat, which also convertedthe limonite
into hematite and much of the hematite into magnetite. The silica was redepositedas quartz, and someof the ore was largely desilicifiedduring the
metamorphism.
Discussionsof this type could be considerablyextended,but these few
examplessufficeto point out the problemsof disagreementand confusion
concerningthe origin of hard hematiteores in itabirite.
SYNGENETIC
IRON
ORE
The formation of large massesof relatively pure massivehematite by
normalsedimentary
processes
hasnot beenconvincingly
demonstrated,
though
it hasbeenadvocated
by severalcarefulfield observers(3, 5, 12, 22). Beds
up to 2 metersor more thick, but generallymuchthinner,suchas were describedby Harder (23) in Brazil, and which extend along the strike for
severalhundredmeters,are very probablyof sedimentaryorigin. However,
manybedsof thistype containthin scatteredlayersof silicaor otherimpurity.
They grade into typicalitabirite,and commonlythey containdiagnosticsedimentarystructuresand textures. They are usuallyrecognizable
as sediments.
Where thick, and where composed
of relativelypure hematite,the bedscommonly show evidenceof reconstitutionand introductionof iron. It is believedthat large massesof hard pure hematiteare seldomformedas a direct
productof sedimentaryprocesses.
WEATHERING
OF IRON
FORMATION
The depthof weatheringis variableand dependsuponmanyfactors,not
the leastof whichis time. Most, if not all, itabirite depositsare Precambrian
in age. They are commonly
withinshieldareaswherelongperiodsof crustal
stabilityhavebeenfavorablefor the development
of maturetopography
and
deepweathering.Undertheseconditions,
andespecially
in tropicalcountries
with abundantrainfall, silicais frequentlyremovedto depthsas much as 100
meters,and soft sandyitabirite extendsto unknowndepths,but probablyas
deepas500metersor evenmore. That ironis transported
readilyis shown
bythedevelopment
of cangaandhardlaterite,andby deposition
of quantities
of limohirewhere undergroundwaters are exposedto air. The iron ap-
parentlytravelsasa ferrousion (Fe+*) or asfinelydividedferrichydroxide.
Why, undertropicalconditions,
iron shouldbe precipitated
on the surfaceas
anhydrous
hematiteratherthan as oneof the hydrousoxidesis not clear,
thoughfieldevidence
confirms
the process.Raymond(35) has suggested
a possible
answer. He showed
thatthe redpigmentof manysoilsin warm,
moistregionsis hematite. This hematiteis not the productof dehydration
ORIGIN OF HARD HEMATITE
IN ITABIRITE
579
of limonite,but probablyresultsfrom the combinedactionsof putrefactive
and nitrifying bacteria,which could produce ferric nitrate and ammonia.
The reactionbetweenthesewould yield hydrousferric oxide, which spontaneouslyloseswater and becomeshematite.
Fro. 2. Slab of cangafrom Minas Gerais, Brazil. Both the fragmentsand
the cementare hematite. The white patcheswere determinedby Kurt Servosas
metahalloysite.
580
CH.4RLES' F. P.4RK, JR.
Cangais a term widely usedin Brazil, and one that is becomingincreasingly commonelsewhere,especiallyin the tropics. Unfortunately the term
canga,•vhilea usefulone,hascometo meandifferentthingsto differentpeople.
Probably the best definition,and one that shouldbe retained,was given by
Derby in 1910 (7). "Cangais a rock formedby the cementationof hematite of rubble ore into a hard ironstoneconglomerate,"Figure 2. Custom
FIG. 3.
Photograph of canga slab (hematite), in •vhich the outlines of individual fragments are obscure. The fragments fade into hard ore. French
Equatorial Africa.
now includesrubble of limonite ore as well as of hematite. Many fragments
in cangaare sharplyangular;othersare sub-rounded
to ,veil-rounded. The
use of suchterms as cangabrecciato denotethe presenceof original angular
fragments,is confusingand is undesirable. It shouldbe avoided. Canga
gradesinto hard laterite, and the originsof thesetwo ,veatheringproducts
have many similar features. In placescanga cannot be distinguishedfrom
laterite. Canga is most abundantin the tropics,,vhere it forms extensive
ORIGIN OF tt.4RD HEM.4TITE
IN IT.4BIRITE
581
Itabirite
ß
Fro. 4. Sketch of an outcropof hard hematiteore, showingthe relationship
betweenore and underlyingitabirite. Lines in the itabirite are parallel to bedding.
French Equatorial Africa.
layersof "pavement,"mostlyoverlyingitabirite, but overlappingas well on
other rocks. The "pavement" is composedof hard compacthematite, cementedby red or blue hematite,or by limonite. In French Equatorial Africa
typical cangagradesinto hard compactblue ore, Figure 3. This hard ore
ordinarilyshowsno relict fragments,but is in layerson the surfacein exactly
the sameway as the more characteristiccangaof Brazil, Figure 4. The covclusionhere is inesca'>able
that the hard ore is of supergeneorigin.
FIG. 5. Photographof a specimenof ore taken from a surface exposure.
Shows crystalsof specularitein a rug. The principal material is soft red hematite. French Equatorial Africa. Length of specimen6 cm.
582
CH,dRLESF. P/IRK, JR.
FIG. 6. Sameas Fig. 5. Vug showingspecularitecrystals. Length of
rug is 1.0 cm.
Many geologistsconsiderthat hematite, and more especiallyspecular
hematite,is indicativeof hypogeneorigin--it resultsfroin hydrothermalactivities. This is basedin part upon the presenceof specularitein igneous
metamorphicand in other unquestionedhydrothermaldeposits,and in part
upon laboratoryexperiments(18, 42, 45). Nevertheless,field evidence
abundantlyindicatesthat hematite--red,blue,or specular--initabirite,is very
commonlyof supergene
origin. Figures5 and 6 are photographs
of a specimen of surficialsoft red hematitefroin French EquatorialAfrica. This specimen is cut by veinletsof specularite,and containssinallrugs lined with specularite crystals. Also in French Equatorial Africa, hard blue hematite,
thoughtto be obviouslyof supergene
origin, is foundalongjoints and water
courses in weathered itabirite and in soft red hematite near the surface.
Simi-
lar sealnsare found near the borders of weathered dikes, on both walls, but
especiallyalongthe hangingwalls. Possiblyconditionsexistingin a heavy
tropicalrain forestinfluencethe formationof hematiterather than goethite.
Waters near the surfaceare commonlybrown froin tannin and organicmaterials dissolvedfroin decayingvegetation. They are acidic, and in places
are reducing. It is believedthat, under properconditions,magnetiteas well
as hematite is a near-surfaceproduct.
Notably absentfroin most hard ore bodiesis evidenceof slump,which
wouldbe expectedif the ores•vereformedby compaction
followingleaching
ORIGIN
OF HARD
HEMATITE
IN ITABIRITE
583
of silica and carbonates. Rather the processnear the surfaceis one of substitution. Iron carriedby groundwater replacesother materials.
Junner (28), in discussingthe hard ore cappingin Sierra Leone, thought
that ore was formedby actionof downwardpercolatingwaterson the original
schistose hematite
and associated rocks.
Silica and other soluble constituents
were leached,and the remainingmaterial was later cementedby hematiteand
.• •i•½:.
'.•
:•
ß
:•:.'•'•:..•.•
,•'.'•.5-,•.-•?i•:
.•.
• ?.• .•:"•'::.•5
.:•?•i•:.
'.
..•
•..... :.•':
...... .•...
."
3:':?:.,,..;..
%73::-1..,•":
; r•v*:'': :' :::•4 :'"•;'•..
'
'
.:
• X'• ......
ß.4 :•:: '•'
......
'•
...:
?•:.'
'
•.:2•
.ß
:'.•:,.
:
Fro. 7. Photograph of a specimen of tightly folded itabirite showing the
banded nature of the hematite and silica. French Equatorial Africa.
Fig. 8. Same specimenas Fig. 7. Acetate print brings out the texture of
coarselycrystalline hematite,shown as dark patchesalong the crest of the fold.
584
CH,4RLES F. P,4RK, JR.
FIG. 9. Polished surface of hard ore from the Andrade mine, Minas GerMs,
Brazil. Both the phenocrysts and the groundmass are blue hematite.
limonite. In Sierra Leone,the term lateriticiron ore is apparentlya synonym
for canga.
The developmentof hard hematite ore by surface waters under tropical
conditionsis consideredto be a widespread and common process. It is
unnecessary
to call upon late regionalmetamorphismor upon hydrothermal
fluids to explain hard ore.
HEMATITE
FORMED
DURING
REGIONAL
METAMORPHISM
Many geologistsfavor the theory that hematiteis concentratedby heated
ground waters during regional metamorphism.Dorr, Guild, and Barbosa
(9) pointedout that at Itabira, Minas Gerais, Brazil, ore on the nosesof
tight folds is generallythicker and of higher grade than is itabirite on the
flanks of the folds. This likewiseappearsto be true at Morro Agudo and
at other depositsin Minas Gerais. That the processis entirely possibleis
convincinglyshownin Figures 7 and 8. These figuresare of a specimenof
tightly foldeditabiritefrom French EquatorialAfrica. High grade,coarsely
crystallinehematiteis concentratedon the noseof the fold. The hard material probablyformedin part as a resultof substitutionand not as a result of
leachingof silica. As indicatedpreviously (Figure 1) all of the Brazilian
hard ore is probablynot formed under hydrothermalconditions. Likewise
the specimenof ore from the Andrademine, Brazil, shownin Figure 9, prob-
ORIGIN OF HARD HEMATITE
IN ITABIRITE
585
ably did not form underconditions
inducedby tight folding,wherebothtextures and structureswouldshowa strongtendencytowardalinement. The
specimenis nearly pure hematite,both groundmass
and phenocrysts.The
crystals,as shownin Figure 9 are vaguelyorientedin a northwestdirection,
thoughthe alinementis probablynot rigid enoughto have resultedfrom
deepseated
metamorphism. The crystal orientationis more likely the result
of mild metamorphism,
suchas might be inducedby moderateload.
CONCLUSIONS
The origin of hard hematitein itabirite is a problemof great economic
significance,
as the depositsformedby weatheringwill changedownwardinto
softpowderyore and into itabirite. On the otherhanddepositsformedduring
regionalmetamorphism
are more likely to continuein depth. In areassuch
as thoseof Minas Gerais,Brazil, the depthattainedby miningand exploration
is insufficientto demonstratethe origin of the ores; they may have been
formedby weathering,by migrationduringregionalmetamorphism,
by hydrothermal fluids working upward along fractures,or by combinationsof these
features. The fact that the hard ores of Brazil seem to be giving way at
depthto powderyoresrelatesthe depositsto similaronesin India, Transvaal,
and FrenchEquatorialAfrica, wherehard materialson the surfacefinger out
downwardinto powdery ore and into itabirite. These ores are supergene.
Most of the hard hematiteore bodiesin itabirite throughoutthe World
fall into two principalclasses:1) thoseformedby normal weatheringprocesses,and 2) thoseformed by migration and rearrangementduring regional
metamorphism. The secondclasspossiblyincludessome of the hard ore
pipesin the easternUnited States,thoughthesepipesare not in itabirite, as
well as Morro Agudo and others in Brazil. Other deposits,attributed to
original sedimentation
and to magmaticactivitieson itabirite,.are comparatively of minor significance.A resumeof existingevidenceindicatesthat
depositsof hard ore resultingfrom normalweatheringprocesses
are probably
morenumerousand more widespreadthan are depositsof othertypes.
ACKNOWLEDGMENTS
In the preparationof this paper,discussions
with K. B. Krauskopf,D. M.
Fraser, and Adolph Knopf, are gratefully acknowledged. The idea that
manyhard ore bodiesin itabiriteare supergene
has beendeveloping
over a
periodof years. It has beendiscussed
with many iron ore geologistsin the
field, and to thesemen go thanksfor suggestions
and for directingattention
to helpful evidence. Thanks are also due to Ruperto Laniz for his efforts
and help in obtainingphotographs.
SCHOOLOF MINERAL SCIENCES,
STANFORDUNIVERSITY, CALIFORNIA,
October 28, 1958
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Venezuela.
586
CHARLES F. PARK, JR.
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