Deposits related to

chemical

sedimentation –

Sedimentary Iron

Ores

GLY 361

Lecture 14

World Iron Ore Production

USGS, 2014

Crude Steel production

South Africa

ranks 21

Crude Steel production

South African Iron-Ore Mines and Deposits

South African Iron-Ore Mines and Deposits

SA’s Main Ore Producers

1. Sishen - Postmasburg area in Northern Cape (Est.

Reserves: 4200 Mt)

2. Thabazimbi area in Northwestern Province (Est.

Reserves: 100Mt)

3. Bushveld Complex (titaniferous magnetite 55 – 57% Fe)

mainly from Mpumalanga.

4. Beach Sand Deposits (by-product).

Sedimentary ores

Ore components localized by processes of

sedimentation or diagenesis.

High tonnage, medium to high grade

Sedimentary iron deposits:

Banded Iron Formation (BIF)

Oolitic ferruginous deposits (e.g. Clinton ores, USA; Minnette

ores; Alsace-Lorraine, France)

Bog ores

Iron carbonate beds (black band ores)

Banded Iron Formation

Banded Iron Formation

Synonyms:

Taconite (Lake Superior district), itabirite (Brazil), jaspilite

(Australia)

1 billion tons of iron ore is produced and consumed each year.

Banded Iron Formations, now serving 99% of the industrial

need for iron, were only discovered in the 19th century and

initially exploited at the beginning of 20th century.

(Bog Iron and ironstone deposits served for a long time as the main

source of iron and were the base for the development of the Iron Age

and for the beginning of industrialisation of Europe and N-America!)

Banded Iron Formation

Banded Iron Formations (BIFs) are sedimentary, quartz-rich rocks (cherts), with a relatively high Fe content.

According to various definitions, BIFs consist mainly of chert, with a minimum of 15% and up to over 50% of FeO, as haematite (Fe2O3), magnetite (Fe3O4), siderite (FeCO3) and Fe-silicates and sulfides. Per definition these rocks are banded, e.g. finely laminated and regularly bedded on regional scale.

Accessory minerals in BIF are riebeckite (Na), dolomite, stilpnomelane as detritus, and, in metamorphic BIF, acmite, green biotite and other. Chemistry is thus very simple and characteristically lacking K, Al, and other common elements.

Most economic iron formations contain 25-35% iron.

Regular BIFs are not iron ores!

Banded Iron Formation

Iron is provided by several characteristic minerals including: Granular magnetite (Fe3O4)

Hematite (Fe2O3) or limonite (Fe2O3 nH2O, Fe(OH)3, FeOOH)

Siderite (FeCO3)

Chlorite (Fe6Si4O10(OH)8)

Greenalite [(Fe2+,Fe3+)2-3Si2O5(OH)4]

Chamosite [(Fe2+,Mg)5Al(AlSi3O10)(OH)8]

Minnesotaite [(Fe2+,Mg)3Si4O10(OH)2]

Grunerite (Fe7Si8O22(OH)2)

Stilpnomelane (K(Fe2+,Mg,Fe3+)8(Si,Al)12(O,OH)27)

The olivine fayalite (Fe2SiO4)

Ferruginous chert (jasper) (SiO2)

Pyrite (FeS2)

Pyrrhotite (Fe(1-x)S)

Banded Iron Formation

BIF mineral facies:

Oxide Facies (30-35% Fe)

- hematite subfacies

- magnetite subfacies

Silicate Facies: chamosite, minnesotaite, stilpnomelane

Carbonate Facies: siderite (ankerite)

Sulphide Facies: pyrite (± pyrrhotite)

The stability field of haematite is

much larger than that of

magnetite that requires low

reducing and alkaline (low Eh and

high pH).

Siderite and pyrite require

relatively neutral pH-Eh and

reducing conditions.

BIFs contain mainly haematite,

sometimes also high magnetite

and some siderite but little to no

pyrite.

On the other hand, SiO2 is only

stable below pH of 9-10, and goes

into solution above it.

Banded Iron Formation

Three great periods

of Precambrian iron

formation

development:

3800-2800 Ma,

Algoma BIF

2500-2300 Ma,

Lake Superior BIF

1000-500 Ma, Rapitan BIF

Banded Iron Formation

Banded Iron Formation Three broad classes of BIF: Algoma-type (Archean age; 3.8-2.8 Ga)

- related to submarine volcanic processes

- within greenstone belts; oceanic volcanic arcs

Superior-type (Proterozoic age; 2.5-2.3 Ga) - may include some volcanic input but need not

- stable continental platforms and margins

Rapitan-type (Proterozoic age; 1.0-0.5 Ga) - in aftermath of global glaciations

- passive continental margins and continental setting

Models for BIF

sedimentation, not

including Mn-rich BIF

Banded Iron Formation

Relative

abundance of

BIF deposits

Oldest large carbonate

platform: Wit Mfolozi

2.9Ga

First giant

carbonate platforms

Banded Iron Formation

Algoma BIF: closely related to volcanic rocks

rarely more than 50 m thick

more magnetite than hematite

deposits are fairly small but mineable; e.g. Abitibi greenstone belt, Canada;

directly connected to volcanic exhalations

Banded Iron Formation

Different BIF mineral facies:

oxide and carbonate facies

minerals interlayered with chert,

jasper, and finely granular quartz

silicate facies is sparse:

greenalite-chamosite-

minnesotaite

carbonaceous sulphide facies

common

Algoma BIF: 40-55% SiO2, 28-37% Fe.

Trace elements that might be consistent with volcanic exhalation:

Mn, Ba, Co, Ni, Cu, Cr, As, Sr, Au (all in ppm)

Banded Iron Formation

Adjacent rocks:

Graywackes with prominent

volcanically derived components

Pyroclastic flow units

Immediate footwall units may

include altered-fractured, or

brecciated zones resembling

pipes or solution conduits (black

smokers)

Lake Superior BIF:

gigantic BIF deposits. The most important Fe-ore deposits distributed

on Precambrian cratons; Hamersley basin, W.A.; Transvaal basin,

S.A.; Quadrilatero Ferrifero, Brasil; Labrador trough, Canada; Lake

Superior district, Canada; Krivoi Rog - Kursk, Ukraine; Singhbhum

craton, India, etc.

no apparent connection to volcanic setting!

several 100 metres thick

magnetite still predominant but slightly more hematite than in Algoma-

type deposits.

Banded Iron Formation

Rapitan BIF:

smaller deposits of large extend but minor thickness (tens of

metres): Rapitan Group - McKenzie Mountains, Canada;

Scandinavia. Connection to upper Proterozoic glaciation.

Banded Iron Formation

Greenstone Belt BIF -

Submarine Volcanic

Spreading Centres

Deep Shelf

BIF

Granular -

shallowing

reworked GIF

Glacial, Snowball

Earth BIF

Banded Iron Formation

Lake Superior BIFs are usually older than 2.2 Ga

and show no clastic input or reworking

GIFs are usually younger than Lake Superior BIFs

and are associated with clastic rocks

Banded Iron

Formation

Banded Iron Formation

At least five different theories about the formation of BIF’s are discussed in literature:

Silica and iron associated with volcanism were poured out on

the seafloor from springs of magmatic origin (van Hise and Leith, 1911; Trendall, 1965; Gross, 1980).

Iron and silica carried in true solution from nearby landmasses were rhythmically deposited as sediments in water, probably in response to seasonal variations in the composition of the water and involving either direct inorganic precipitation of silica and iron or one of several biochemical processes (Baarghorn and Tyler, 1965; Eugster and I-Ming, 1973).

Banded Iron Formation

Iron formation beds were originally deposited as more thickly bedded fine-grained ferruginous tuffs and other iron-rich sediments that were diagenetically oxidized and silicified under the influence of solutions that were partly volcanic in origin. The silification caused separation to finer beds of banded cherts and jaspers that alternate with more iron-rich layers (Dunn, 1935, 1941).

Iron formations were deposited as end members, or final products, of carbonate sedimentary cycles (Button, 1976).

Banded Iron Formation

Deposition of iron formation resulted after buildup of iron concentration in the sea. Joliffe (1966) envisioned a primitive Archean acidic sea with a pH of 6 or less, an Eh of about 0, and with seawater in equilibrium with an atmosphere rich in CO2. Under these conditions, iron released by erosion and by volcanism would remain as ferrous iron in the sea. As time progressed, the CO2 of the atmosphere was gradually depleted, and an increase in the pH of the sea resulted in removal of H2CO3. A point of saturation was ultimatively reached, and FeCO3 and Fe3O4 started to precipitate. The gradual buildup of oxygen and the depletion of CO2 in the atmosphere eventually led to the wholesale precipitation of iron in seawater as magnetite, hematite, and siderite.

Depositional environments proposed for BIF:

• shallow marine, restricted

• evaporitic

• biogenic Fe and Si precipitation

• deep marine basins (shelf)

• submarine plateaus (deep)

• ocean spreading centres (or their Archean

equivalents)

• oceanic, connected to global glaciations

(post snow ball earth deposits)

3 major types of BIF are distiguished:

• In submarine volcanic spreading centres,

Algoma type

• In shelf environments below storm wave

base, Lake Superior type, including

Granular Iron Formations (GIF) -

shallowed and reworked BIF)

• Rapitan BIF, deposited as a consequence

of O2 enrichment after global glaciations.

Less common carbonate (siderite) IF

Today, it is largely agreed that BIF are deep water deposits. The Fe is of oceanic hydrothermal

origin. Si is still a problem and precipitation mechanism for both is not clear, however BIFs bear

very little signs for organic influence on precipitation and are regarded as chemical sediments

Banded Iron Formation

How was the CO2 depleted? Where did the

oxygen come from?

Photosynthetic bacteria (cyanobacteria) in shallow basins produced, for perhaps the first time in the young Earth's oceans, free O2 as a waste product.

The free O2 oxidized the dissolved Fe2+-ions to form insoluble iron oxides (Fe3+ is not soluble in water)

Precipitating iron oxides formed thin layers on the seafloor substrate (anoxic mud, forming shale and chert).

Banded Iron Formation

Banded Iron Formation

Depositional-environmental facies concept of

iron formations:

Oxide Facies: Hematite:

accumulated in a strongly oxidizing near-shore environment (e.g., Clinton Formation, USA)

Magnetite (interlayered with silica, carbonates, iron silicates): weakly oxidizing to

moderately reducing conditions (e.g., Lake Superior, USA, Canada).

Depositional-environmental facies concept of

iron formations:

Carbonate Facies: Interbedded

siderite, iron-rich ankerite, chert – oxygen concentration high enough to destroy most organic material but not high enough to permit ferric compounds to form

Depositional-environmental facies concept of

iron formations:

Sulphide Facies: Black

carbonaceous slates with up to 40% pyrite and free carbon content of 5-15% - stagnation and strongly reducing conditions.

Deep-water or volcano-slope proximal depressions.

Depositional-environmental facies concept of

iron formations:

Algoma-SVOP: Proximal

Superior-MECS: distal

Progression from sulfide through carbonate to oxide was considered to reflect

precipitation succession from reducing to progressively oxidising conditions (James,

1954).

However, all facies occur in BIF together, in the same laminae.

Rather, redox potential, alkalinity and depositional environment determine the

mineralogy of BIF in a poorly understood way.

Preston Cloud, 1968:

Cloud Hypothesis:

Archean atmosphere deficient in O2 and

rich in CO2 (O2pP< 1%PAL)

CO2 dominated weathering and

enrichment of Fe2+ in the oceans.

Hematite and magnetite are precipitated

upon oxidation (photosynthesis) at 2.5

Ga.

Contra - arguments:

•BIFs are not synchronous

•BIFs contain little Corg

•Many BIFs are deposited before

significant oxygen enrichment

Banded Iron Formation

Model for BIF deposition on the Kaapvaal craton, South Africa: BIF are

precipitated after transgression and drowning of the carbonate platform

Banded Iron Formation

Model for hematite, magnetite and siderite precipitation via photosynthetic oxidation of

Fe2+. Hematite requires however 1.5 O : 1.0 Fe and magnetite 1.33 O : 1.0 Fe.

Banded Iron Formation

Iron oxidation and BIF

precipitation via:

•oxygenic and

•anoxygenic

photosynthesis.

Haematite is precipitated

at sufficient O2 levels

Anoxygenic

photosynthesis produces

siderite

N.J. Beukes, 2003

Banded Iron

Formation

Transition zone with colloidal Fe3+ brine

and oxy-hydroxy-chloride complexes

Banded Iron Formation

REE patterns like positive Eu anomaly, evidence an oceanic

hydrothermal vent source for the Fe in BIF, this influence is

slightly weaker in Lake Superior (upwelling model) than in

Algoma BIF and disappears in the Rapitan BIF, where the Fe-

content appears to be continental.

It has been demonstrated that continental weathering

can easily supply the required mass of Fe needed to

precipitate world wide BIF deposits.

Precipitation of silica is another problem in BIF

precipitation, as no biological SiO2 precipitation is

known in the Precambrian. Silica is however,

increasingly soluble with increasing alkalinity.

Amorphous silica [Si(OH)4] is nevertheless highly

soluble (~120g/l) and Precambrian oceans might have

been supersaturated in silica in the absence of SiO2

secreting organisms, leading to colloidal or gel

precipitation of SiO2.

Silica precipitation could also be driven by seasonal

alkalinity changes in the surface sea water.

Banding is assumed to result from cyclic variations in available oxygen.

At tipping point where the oceans became permanently oxygenated, small variations in oxygen production produced pulses of free oxygen in the surface waters, alternating with pulses of iron oxide deposition.

For reasons largely unknown, this was a periodic process resulting in the alternating bands of iron oxide and shale. The periodic process might have been due to seasonal fluctuations or storm surges.

Banding

Banding

Dales Gorge BIF Banding

Bedding and banding:

Trendall & Blockley, 1970:

• Macrobands: in m - range; BIF-macrobands are

intercalated with S-macrobands (stilpnomelane).

Each BIF-macroband consists of an intercalation

of Fe-rich and -poor mesobands.

• Mesobands: cm - range; Chert with abundant

SiO2 -Chert matrix (and subordinate FeO) and

SiO2 (with more FeO). Some mesobands can be

composed of microbands.

• Microbands: mm - range; alternating lamination

of hematite, magnetite, carbonate,

stilpnomelane or any combination of these

minerals with chert. Microbands are about 2.0 to

0.2 mm thick.

AV =Acidic Volcanics; C-M = Chert matrix

Microbands were interpreted by AF Trendall (AFT-

bands) as varvites or nocti-diurnal laminae.

Banding

Chert matrix

Mesoband

Fe-

carbonates

Chert

Fe-Oxide

Fe-Hydrooxide

(Weathering to

limonite)

Microbands

1 cm

Banding

BIF Alteration

Low grade metamorphism of BIF creates veins of fibrous or asbestos riebeckite. Often these asbestos type crystals are replaced by quartz. The formerly fibrous nature of the crystals causes the play of light that is known as Tiger's Eye.

Due to the extreme age of these formations, almost all BIF formations have undergone some faulting, fracturing, folding, compaction, veining, intrusions and metamorphism. Although all BIF formations are probably metamorphosed to some degree their general character is still sedimentary.

BIF Alteration Original BIF precipitates and metamorphic equivalents:

Compound

Inferred initial

precipitate

Now observed

SiO2 Amorphous Chert

Fe2O3 Amorphous Fe2O3

nH2O

Hematite

Fe3O4 Fe3O4 nH2O

hydromagnetite

Magnetite

FeCO3 Siderite Siderite

Fe3Si2O5(OH)4 Amorphous ferrous

silicate

Fe3Si2O5(OH)4

greenalite

Fe7Si8O22(OH)2

grunerite

Fe3Si4O10

Fe sulfide FeS

Na-Fe silicate Na-Fe silicate gels

proto-ore may hardly be regarded as iron ores as the silica/iron ratio of these rocks is too high.

iron content of the BIF’s varies between 25-35%.

A secondary natural process is therefore an essential requirement in the formation of the ores. supergene/ hypogene enrichment upgrades the proto-ore to

above 60% Fe.

Secondary enrichment

Origin of Fe-ore in Banded Iron Formations, supergene vs.

hypogene mineralisation

Sishen, Kathu supergene deposits

Bello Horizonte,

hypogene deposits

Thabazimbi

Supergene

Hypogene

Enrichment of Fe in BIF must be at least 100% to make it a mineable

deposit.

Sishen/Kathu in SA and Mount Whaleback in WA are the world largest

single iron deposits with c. 65-67% FeO.

Mount Whaleback alone produces ~ 170 Mio.t/annually, exported to Japan,

China and S. Korea, with an resource of 1.8 billion t.

The ore hosted in BIF and shales of the Dales Gorge Member of the

Brockman BIF, is interpreted as supergene enrichment after folding

during the Ophthalmian orogeny (~ 2400-2200 Ma) and subsequent rising

and exposure. This correlates to the syndepositional folding of the

Kuruman BIF in SA, and exposure during the Paleoproterozoic.

The BIFs were exposed and subject to supergene alteration with

martitization (oxidation of magnetite to hematite) prior to 2200Ma (age of

the Wyloo Group conglomerates, and Ongeluk/Hekpoort)

Supergene vs. hypogene

mineralisation

After supergene alteration the goethite-martite was buried and

metamorphosed to microcrystalline hematite-martite ore (e.g. Pretoria

Group sedimentation) resulting in lumpy, low P (0.05%), and high Fe

(>65%) ore (Morris, 1985).

However, fluid inclusions and stable isotope data evidence hematite-

martite ore formation above 400°C which is incompatible with supergene,

near-surface conditions, supported by mineralogy evidencing high P/T

conditions.

Mineralisation by hypogene fluids along low angle thrusts has been

therefore suggested (Powell et al., 1999; Barley et al., 1999). This is

supported by occurrence of Fe ores down to a depth of 500m below the

surface, far too deep for supergene enrichment. Thus, orogenic, hot,

oxidising, basinal fluids were proposed as the mineralising medium.

Supergene vs. hypogene

mineralisation

Oolitic iron ores: Can make up most of the rock or may be scattered throughout

a clay or limestone matrix.

Most valuable Post-Precambrian deposit.

Oolites of hematite, limonite, siderite, chamosite, with or without calcite or chalcedonic silica.

e.g., Clinton Formation (Silurian age).

Post-Precambrian sedimentary

iron deposits

Ironstones, oolitic and peloidal Fe ores are widespread through certain periods

of the Phanerozoic era and were an important source for iron ore in Europe

(England, France, Germany) and are still mined in eastern USA (Kentucky,

Alabama).

The Silurian-Ordovician and Jurassic-Cretaceous (periods of very low

continental freeboard) deposits formed in shallow marine to deltaic

environments, above lateritic ferricretes, through abrasion, mechanical

reworking to oolites and pellets, and through bacterial precipitation at oolite

surface. They are often associated with glauconite (K,Na,Ca) (Al, Fe2+, Fe3+,

Mg)2[(OH)2|Al0.35Si3,65O10] and chamosite (Fe-rich chlorite).

Continental source of Fe, drowned at transgressions (continental flooding, sea-

level high stand) and laterally linked to deeper water black shales. Or marine

origin, connected to Ordovician-Silurian or Jurassic sea level high stands and

upwelling.

Ironstone deposits (Alsace-Lorraine or

Minette - type ores):

Reworking of lateritic iron deposits and upwelling of Fe-rich, deep oceanic waters

at times of pronounced sea-level highstand and mid ocean ridge activity during

continental dispersion.

Deposition of continental reworked hematitic and goethitic ores and Fe-oolite

formation through biological processes in shallow water (bacterial oxidation of

Fe)

Ironstone deposits (Alsace-Lorraine or

Minette - type ores):

Black band ores (siderite): widely distributed throughout the world.

generally of low grade (successfully mined in Germany, England, USA).

Post-Precambrian sedimentary

iron deposits

Post-Precambrian sedimentary

iron deposits

Bog ore

Bog ores and spring deposits: Limonitic iron ores.

Occur in small low-grade deposits with Mn, P, water, clay, etc.

Not mined.

Good example for biochemical precipitation of iron minerals: iron content in bog waters is higher than that of other surface

waters because iron is stabilized by humic complexes and low pH.

bacterial action causes precipitation of ferric oxides and hydroxides from breakdown of humic iron complexes and ferrous bicarbonate.

iron is delivered by streams and springs.

Post-Precambrian sedimentary

iron deposits

Bog iron ore deposits:

Formed in geologically young and recent swamps and lakes of the

interglacials of northern hemisphere and northern Canada,

Scandinavia and arctic Russia.

Deposits are typically small and thin, comprising Fe-

oxyhydroxides (goethite - aFeOOH, limonite g,dFe(OH)3, and

related minerals) and associated laterally with reduced

carbonaceous black shales.

Fe concentration occurs when Fe2+ (ferrous iron) is oxidised to

Fe3+ (ferric iron) and precipitated as limonite or goethite from a

relatively reducing meteoric water, at contact to relatively oxic

ground water. This usually happens at the ground water table.

Bacterial oxidation of Fe, may play an important role in these near

surface environments.

Meteoric waters in swamp environment are reducing because of

oxidation of Corg.

pH is usually slightly acidic due to humic acids.

Neutral or slightly alkaline, oxygenated ground waters cause

transformation from ferrous to ferric iron and precipitation.

Reducing and

acidic meteoric

water

Exploration guide for BIFs

GEOCHEMICAL SIGNATURE: Elevated values for Fe and Mn; at times elevated values for Ni, Au,

Ag, Cu, Zn Pb, Sn, W, REE and other minor elements.

GEOPHYSICAL SIGNATURE: Electromagnetic, magnetic, and electrical conductance and

resistivity survey methods are used effectively in tracing and defining the distribution of Algoma- type beds, either in exploring for iron and manganese ore, or for using these beds as metallogenetic markers.

OTHER EXPLORATION GUIDES: Discrete, well defined magnetite and hematite lithofacies of iron-

formation are preferred with a minimum of other lithofacies and clastic sediment interbedded in the crude ore. Iron- formations are usually large regional geological features that are relatively easy to define. Detailed stratigraphic information is an essential part of the database required for defining grade, physical and chemical quality, and beneficiation and concentration characteristics of the ore. Basin analysis and sedimentation modeling enable definition of factors that controlled the development, location and distribution of different iron-formation lithofacies.