Sٍedimentary manganes and iron ore deposits

Topic 3: Sedimentary Manganese and Iron Ore Deposits
Hassan Z. Harraz
hharraz2006@yahoo.com
2012- 2013
22 November 2015
Prof. Dr. H.Z. Harraz Presentation
Sed. Mn & Fe Ore Deposits
1

Outline of Topic 3:
We will explore all of the above in Topic 3.
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 MANGANESE ORE DEPOSITS
 Sedimentary Manganese Deposits
 Types of Sedimentary Manganese
 Classification
 Manganese Nodules
 EGYPTIAN MANGANESE ORE DEPOSITS
 IRON ORE DEPOSITS
 Cycle of Iron
 Ironstone (Sedimentary iron) Ore Deposits
 Bog Iron Ore Deposits
 Principal iron-bearing minerals
 Geochemical stability of iron-rich minerals
 World Resources Iron Deposit
 EGYPTIAN IRON ORE DEPOSITS
 Iron ore deposit of sedimentary nature
 Sinai: Gabal Halal iron ore deposit
 Western Desert:
 Aswan iron Ore Deposits
 Bahariya iron Ore Deposits

SEDIMENTARY MANGANES And IRON ORE DEPOSITS
 Most of the world's iron and manganese are derived from deposits of
this type.
 These deposits are very large in size (thousands of millions of tons) and
are usually mined by open-cut methods.
 Sedimentary iron and manganese ores are deposited in both fresh and
marine water, in bogs, swamps, marshes, lakes, lagoons, and in the
ocean.
 The iron and manganese ores occur as distinct sedimentary layers
alternating with iron and manganese-poor sedimentary layers - the
whole mass is sometimes mined.
 All ore minerals in these deposits are oxides and hydroxides.
 The most common iron ore minerals are hematite, lepidocrocite and
goethite
 The most common manganese ore minerals are braunite, manganite
(Mn2O3*H2O) and hausmannite (Mn3O4).
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Sedimentary Manganese Deposits
 Mn behaves chemically Much the Same way Fe does, mixed concentrations are easy to imagine/explain
 But how do we get Mn-poor BIF and even harder, Fe-poor Mn deposits
 Ave. Mn/Fe = 1/40 ‐ 1/60
 We can’t call on Precambrian atmospheres
 Separate Mn from Fe
 Manganese may be deposited:
 as a minor constituent of iron ores or
 separately as sedimentary manganese deposits relatively free from iron.
 Iron oxides precipitate at a lower pH than manganese oxides; thus iron and manganese may be separated but often are not.
 Also, separation of iron and manganese occurs under oxidizing conditions because iron oxide or; hydroxide precipitates at a lower oxidation potential (Eh)
than the comparable manganese compounds at a given pH.
 Manganite (Mn2O3*H2O) and hausmarmite (Mn3O4), the respective counterparts of goethite and magnetite, are common, but the dioxide (pyrolusite
MnO2), which is the chief ore mineral of manganese, has no iron counterpart.
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Mn crystal chemistry
 Mn+2 = 0.83VI (0.66 in IV coord.)
 Silicates, carbonates, sulfides
 Mn+3 = 0.65VI
 Oxides, substutes for Al+3,VI = pink color
 Mn+4 = 0.53VI (0.39 in IV coord.)
 Oxides
 Also +5, +6, +7 possible (0.33-0.25 IV)

Types of Sedimentary Manganese
Regards to mineral composition, four types of sedimentary
manganese deposits can recognize:
(1) Hydroxide ores of continental lacustrine deposits consist of
psilomelane, pyrolusite (MnO2), limonite, clay-minerals, and
opal.
(2) Oxide ores of marine origin include manganite (Mn2O3*H2O)
in addition as a major mineral.
(3) Carbonate ores consist of rhodochrosite, manganocalcite,
opal, marcesite, pyrite, glauconite, and barite.
and
(4) Silicate ores include rhodonite, buetamite, manganous-
garnets (usually mixed with manganese carbonates), quartz,
hematite, and magnetite.
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Classification
Classification
 Volcanogenic
 Associated with tuffs, agglomerates, hot springs
 Associated with BIFs‐ distal submarine exhalite
 Non-volcanogenic
 Bog ore, lacustrine, fluvial
 Continental-terrigenous sediments in
geosynclinal or shelf settings
 Hybrid
 Ferromanganese nodules
Deposition of sedimentary manganese ore in the form of
carbonate or oxides may occur in lakes or bogs, or in the sea.
Marine depositions, chiefly in the form of the dioxide, have
been formed under shallow water conditions and in deep-sea
sediments, where it is widely distributed as nodules, as
coloring matter, and as coatings on fossils.
Under near-shore conditions, as in the case of iron,
manganese oxide hydrosol, or bicarbonate solutions,
precipitate oxide, or carbonates or both.
The oxides commonly form oolites, which are made up
chiefly of psilomelane and pyrolusite.
These, along with included marine fossils, indicate a
marine origin of the manganese.
The associated rocks are shales, limestones, and, less
commonly, sandstones.

Associated with BIFs
Ex: Associated with BIFs
Kalahari Mn-field in South Africa
In Transvaal Basin at stratigraphic levels just above
those hosting large BIF ores
May simply be the result of relative oxidation of Fe
and Mn
Fe oxidized first and deposited lower
Main ore: 5-45m thick, 20‐48% Mn

Continental-terrigenous
Ex: Continental-terrigenous
 These are the largest deposits known
 Near shore basins on continental margins
 Southern Russia‐ Ukraine
 1.7 billion tons of 15-35% Mn
 Shallow water marine/estuary‐ island dohed sea
 Concordant thin lenses
 Oxides dominant
 Common carbonate and carbonate/oxide facies
 Derive Mn by weathering nearby continents
 Major deposition coincided with climate change
(humid subtropical to cold temperate)
 Deposition controlled by both:
 Eh-pH constraints and
 Local control by organic activity

Modern Black Sea
Modern Black Sea Example
 Active sedimentation of MnO2
 Marked stratification
 Euxinic below 200 meters
 Pyritic muds remove Fe from water column
 Mn2+ concentrated in deep water
 Oxidized Mn-rich sediments form around the bathtub ring

 Economic Aspects
 Vast resources: Mn, Fe, Cu, Ni, Co, Zn
 Contain economically important metals (25% Manganese, 15% Iron, 2% Nickel, 2% Copper and
cobalt) (but too expensive to harvest).
 Much interest in mining them during the 1960s and 1970s .
 Problems with mining them as they are on the abyssal plains and in international waters.
Manganese or ferromanganese (Fe and Mn) Nodules
 Discovered by the HMS Challenger since 1872-6
 Authigenic deposits
 Potato-size nodules “Burnt baked potato”
Layered, concentric concretions around seed
Internal shrinkage cracks related to changes in mineralogy during “diagenesis” in place
 Formed by chemical or biochemical reactions and by precipitation of minerals in seawater & on ocean floor.
 Best ones from equatorial Pacific
 Accumulate only in areas of low sedimentation rate (e.g., the Pacific, Red Sea).
 Concentric layers of metal oxides accrete on particles over millions of years (1-10 mm per 106 y).
 Ocean basins with very low sedimentary rates: Develop (growth) extremely slowly (1 - 10 mm/1 Ma years).
 Surficial deposits of manganese, iron, copper, cobalt, and nickel .
 They are hydrogenous - they precipitate from deep ocean water when the Mn oxidizes:
Mn+2 (dissolved) + O2 + 2e- = MnO2 (pyrolusite).
 Often perception is biologically enhanced, probably related to microbe activity.
 Origin uncertain (biological?).

http://teachers.sduhsd.net/hherms/herms/oc
ean/sedimentation/nodules.gif
http://content.answers.com/main/content/img/McG
rawHill/Encyclopedia/images/CE403150FG0010.gif
Manganese Nodules
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Contours in Cu %
Cross-section
Floor of South Pacific Ocean.
Nodule size 1-5 cm diameter
Ferromanganese nodules

Future
Deep Sea Mining ?
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EGYPTIAN MANGANESE ORE DEPOSITS
Manganese ore deposits are widely scattered in various
districts in Egypt.
They occur at some localities in Sinai Peninsula and at
a few localities in the Eastern Desert.
Manganese deposits are known:
1) in the Um Bogma district in west central Sinai; and
2) in the Halaib "Elba" district in the southern portion
of Eastern Desert.
3) In addition, minor occurrences are known in Wadi
Mialik near Abu Ghosun and Ras Banas in the
Southern Eastern Desert, and Wadi Abu Sharm El
Qibli to the north of Hurghada (Fig. ).

Um Bogma District
 Um Bogma region west central Sinai are considered to be the most important area in Egypt for
manganese deposits.
 Extensive workable manganese deposits contributed significantly to the Egyptian economy up to
1967, when the mines were abandoned.
 Reopening the best mines is being considered and evolution of newly discovered occurrence.
 It is also producing ferromanganese alloys at the plant installed at nearby Abu Zeneima, a port on
the Gulf of Suez.
 Manganese ore deposits occur wide spread at eight localities of the manganese deposits from the
Um Bogma region, west central Sinai. These localities are:
1) Abu Hamata left,
2) Abu Hamata right,
3) Abu Thor,
4) Abu Zarab,
5) Rass EI-Homara ,
6) Area 10,
7) Area 9, and
8) Area 8.
 Manganese ore deposits occur in Paleozoic sediments of Lower Carboniferous age.

Geologic setting
The oldest exposed sedimentary rocks at Um Bogma district belong to the Pre-Carboniferous and
Carboniferous ages.
These rocks unconformably overlie the Precambrian rocks.
Three stratigraphic rock units in the Um Bogma region starting from the base:
i) Lower sandstone unit (Carbo-Ordovician to Devonian)
ii) Middle carbonate unit (Um Bogma formation, Lower Carboniferous):
 This is represented by dolomite and limestone rocks and are covered conformably the
lower sandstone unit. Four members are differentiated from base to top:
 Dolomite and manganese-bearing member,
 Silt-shale member,
 Marly dolomite and silt member, and
 Upper dolomite member.
iii) Upper sandstone unit (Visean) this is represented by medium to coarse grained sandstone.
Some beds are almost snow-white, friable sands with three kaolinitic clay layers.
 Structural pattern of the west central Sinai shows that faulting is much more pronounced than
folding. Faults are the result of successive movements which affected the area in different ages.
Faulting has taken place periodically since the late Paleozoic, increased in intensity and areal
extension progressively and reached its climax in the Oligo-Miocene period.
•

Ore deposits
 The manganese ore is a stratiform type
occupying more or less the same
stratigraphic horizon in the dolomitic
limestone member of the Um Bogma
formation which caps the clastic Adedia
formation.
 Ore deposits always tend to occupy a
particular stratigraphic horizon (i.e. Dolomite
and manganese-bearing member),
representing the base of the middle
carbonate (dolomitic limestone) unit, which
belong to Lower Carboniferous.
 The manganese bodies are usually
surrounded by a zone of calcareous shale,
siltstone or sandstone that form the
transition with the surrounding dolomite.
 The ore bodies usually show abrupt contacts
with the dolomite and are frequently found
to fill depressions in the underlying Adedia
formation.
Forms
 The individual ore bodies vary in length and
thickness, and are often surrounded by a
transition zone of calcareous shale or
sandstone between the surrounding
dolomite. The ore bodies are irregular in
shape, tending to be lenses or lenticular
beds. The thickness varies from 10 cm to 8 m
and the extent of the beds may reach 100 m.
 In some occurrences, the ore bodies are
present as veins cutting the calcareous shale
that forms a transition with the dolomite.
 Several forms characterize the constituents
of the ore deposits such as massive
crystalline, granular, nodular, botryoidal,
reniform, fibrous, radiating, need-like
crystals, earthy soft and ochreous varieties.

Mineralogy
The ore body varies in composition from pure manganese ore to pure iron ore but it generally
represents a mixture of the two ore in variable concentrations. Small lenses are richer in Mn than the
lenticular beds, where Mn occurs admixed with Fe. Multistage formation of the Mn minerals is
noticed especially in the regeneration and recrystallization of pyrolusite. In the large ore bodies (i.e.
layer ore lenses, i.e. those with diameter >50 m), a division into three mineralogical zones is
recognized. These include:
i) The inner manqaniferous zone: essentially composed of psilomelane and pyrolusite with rare
manganite, hausmannite, polianite and pyrochroite. Hematite and clay minerals usually
<25%. The structure is massive, but concretions of pyrolusite may be present.
ii) The intermediate ferruginous-manganese zone: consists of psilomelane, pyrolusite and
hematite with up to 15% goethite, quartz, barite, and clay minerals. The ore is massive and
constitutes the main ore reserves of Um Bogma.
iii) The outer ferruginous zone: composed mainly of hematite and goethite with minor
psilomelane. Detrital quartz is common and spherulitic concentrations are frequent.
The transition between the ore bodies and the surrounding dolomite is abrupt distinguished by
enrichment (up to 73%) of quartz and grains. The ore deposits are all in a highly oxidized state, the
bulk of them being composed of pyrolusite, psilomelane, hematite and goethite. In addition,
polianite, manganite, cryptomelane, hausmannite, and ramsdellite are present in subordinate
amounts, while chalcophanite turquoise, malachite, alunite and pyrochroite occur as rare minerals.
Gangue minerals include quartz, dolomite, calcite, barite, gypsum, and some clay minerals.

Origin
The problem of the origin of the of the studied manganese deposits, which are carbonate-hosted, remains in dispute. Two trends have been more or less
defined:
a) Manganese ores are of epigenetic origin: Manganese ore is a result of the activity of ascending mineralized hydrothermal solutions through the host rocks.
The following arguments can be used as evidence of this:
i. The ore deposits are found in the immediate neighborhood of faults and are thicker and richer in manganese at points close to the faults.
ii. Wherever ore occurs, the dolomitic limestones have partially or wholly disappeared. Where only a part of the limestone series had disappeared in the
vicinity of the ore deposits, it is always the lower part of the series which has vanished with the upper beds being left.,
iii. The induced effect of weathering of the clastic rocks from Adedia formation capping the manganese-dolomite layer (Um Bogma formation) is thought to
have epigenetically added turquoise, malachite and alunite to the mineralogy of manganese ore deposits.
iv. The presence of hausmannite, manganite, turquoise, malachite and alunite are indicative of hydrothermal deposits.
v. Criteria for replacement textures are reported with the manganese-iron ore deposit (e.g., relict, core and rim replacement textures). Besides, the
foraminiferal tests of Fusilina sp. are shown to be completely replaced by polianite with their internal structures mostly obliterated.
•
b) Primary sedimentary-type of manganese ore: They gave the following considerations to support their theory:
i. The ore deposits always occupy the same stratigraphic horizon. These deposits are older than the predominant faulting and folding in the district. In
some cases, the deposits are cut and displaced by faults.
ii. There is no link between the ores and faults and where Mn deposits are present in fault, they are introduced as fillings from above,
iii. The association of pyrolusite, manganite and psilomelane with goethite and hematite characterizes the sedimentary deposits.
iv. The difference in the kind of insoluble residue between the inner and outer zones of the ore lenses indicates that the major part of the ore was not
formed at the expanse of the surrounding sandy dolomite, but rather associated with a different lithology.,
v. There is no transition in mineralization between the ore bodies and the overlying unconformable strata, in contrast with the narrow transition zone
between the ore and the laterally surrounding dolomites.
vi. The dolomitization is contemporaneous with the mineralogical and chemical reconstitution of the zoned deposits,
vii. The zoned pattern in the larger ore body indicates a low pH - high Eh conditions at the rims, and high pH- low Eh in the cores. The mineralogy also
indicates a low temperature of formation in a sedimentary environment (i.e., a shallow marine origin of the Mn ore deposits).

Halaib "Elba" Region
Manganese occurs:
 in sedimentary rocks of Miocene age in 24 areas within the Halaib region, situated in the
southern extremity of the Egyptian eastern Desert near the Red Sea coast.
 In a few cases, manganese deposits occur as fracture fillings in basement rocks, especially
granites.
Manganese minerals occur either in veins trending within a range of N 28o-40°W in a belt ~70 km
long and less than 7 km wide, or occasionally replacing the Miocene conglomerates and limegrits.
Mineralogy:
 The ore minerals include: pyrolusite, psilomelane, cryptomelane, ramsdellite, todorokite,
and nsutite, in addition to occasional goethite and hematite.
 The gangue minerals include: quartz, barite, black calcite, opal, and chalcedony.
Manganese mining is restricted to elementary operations due to the remoteness and unfavorable
conditions of the area. Extraction of Mn began in 1955 and -66,000 tonnes of high-grade ore have
been produced.
Origin: This manganese deposits formed by weathering of Precambrian rocks and supergene
deposition in fissures accompanied by replacement along the walls of the fissures. An epigenetic low
temperature origin was proposed based on the predominance of stable higher oxides of manganese
and the absence of Mn silicates, carbonates, and sulphides which reflected near-surface deposition of
the ore.

Other manganese occurrences
Minor manganese occurrences are recorded
from:
a) Wadi Mialik near Abu Ghosun and Ras Banas in
the Southern Eastern Desert: The ore occurs as
fillings in fault zones and fissures in
Precambrian amphibolites.
b) Wadi Abu Sharm El Qibli, in the southern part
of Esh El Mellaha range north of Hurghada: a
thin Mn deposit (~50 cm thick) occurs in the
Miocene sediments.

IRON ORE DEPOSITS
Economic iron ore deposits occur in three natures:
 Ironstone deposit
 Bog Iron ore deposits
 Iron Formation deposits (IFs)
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The cycle of iron:
 Because of the low solubility of ferric iron (Fe3+), it is probably ferrous iron (Fe2+) that is dissolved during rock
weathering and moves largely in streams to favorable sites of deposition.
 The iron may be lost during transportation:-
(1) if the solutions traverse limes one, where reactions cause deposition of ferrous carbonate or ferric oxides;
(2) if the solutions come to rest in an enclosed basin undergoing evaporation;
(3) by contact with organic matter; or
(4) by decrease in carbon dioxide content of the solutions.
 By mineral composition, sedimentary iron ore deposits are divided into three groups:
(1) Oxide Group: The oxide group of brown iron ores consists mainly of limonite, hydrogoethite, goethite,
hematite and occasionally of magnetite with an admixture of other minerals.
(2) Carbonate Group: The major ore-forming mineral of carbonate ores is siderite.
(3) Silicate Group: Silicate, ores are composed of ferruginous chlorites like chamosite {Fe4 Al(Si3 Al10)(OH)6}
and thuringite {Fe3.5 (Al, Fe)1.5 x(Si2.5 Al1.5O10)(OH)6}.
 In addition, the composition of all three groups is supplemented to varying degrees by manganese hydroxides and
oxides, quartz, chalcedony, feldspar, calcite, barite, gypsum, and clay minerals and occasionally sulphides, mainly
pyrite.
 An oolitie texture of the ore is highly typical of sedimentary iron deposits.
 The most of marine hematites were deposited directly as ferric oxide. Glauconite, chamosite, and greenalite are
less common forms.
 In the presence of air, ferric oxides form:
 At lower PH and Eh, siderite forms.
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Oxidation (addition of oxygen):
3
3
224
3
2
2
O3Fe0.5OOFe2Fe 

Hydration (addition of H2O):
Magnetite Hematite
OnHOFeOnHOFe 232232 
Hematite Limonite

Principal iron-bearing minerals
From Boggs, Principles of Sedimentology and Stratigraphy, 4th ed., p. 219
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Ironstone (oolitic) Deposits
are dominantly Phanerozoic sedimentary
deposits that form poorly-banded or nonbanded
bedded successions.
commonly have an oolitic texture and may
contain fossils that have been partially or
completely replaced by iron.
Sedimentary structures (i.e. cross-bedding) are
common.
All ore minerals in these deposits are oxides and
hydroxides
The most common iron ore minerals are
hematite, lepidocrocite and goethite
IRON ORE DEPOSITS
Economic iron ore deposits occur in two natures:
Bog Iron ore
 was formerly mined in Europe hundreds of
years ago when the large iron ore deposits had
not been discovered.
 consists of iron hydroxide (goethite) deposited
in swamps and lakes as a product of bacterial
action.
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Origin of Iron Deposits =
Geochemical stability of iron-rich minerals
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From Boggs, Principles of Sedimentology and Stratigraphy, 4th ed., p. 222

www: mdsresourcesirondeposit
World Resources Iron Deposit
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EGYPTIAN IRON ORE DEPOSITS
In Egypt economic iron ore deposits occur in two natures:
i) Iron ore deposit of sedimentary nature
(Sedimentary iron ore deposit is a very limited occurrence, being found
only in the 2 localities in the Western Desert and one locality in Sinai) and
ii) The Precambrian Banded Iron ore deposits (BIFs)
(BIFs have being found only in the 13 localities in the central Eastern
Desert)
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Sinai: Gabal Halal iron ore deposit
 It is located ~4 km NW Sir Hadhira, Sinai (El-Far, 1965).
 This area contain oolitic iron ores of lower Cretaceous age that extending ~8
km.
 The iron ore were found in two beds separated by 14 m sandstones:
 The lower bed (~2.65 m in thickness) is a yellowish-brown and compact
bed mainly oolitic.
 The upper bed (~5 m in thickness) is a typical oolitic iron ore.
Main ore minerals: goethite and hematite.
Gangue minerals: clay minerals, quartz, calcite-dolomite, and sulphate minerals.
I) Iron ore deposit of sedimentary nature
Sedimentary iron ore deposit is a very limited occurrence, being found only in
the:-
 One locality in Sinai
Two localities in the Western Desert
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Economic iron ore deposit of sedimentary nature, being found in the 2 localities in
the Western Desert.
Sedimentary iron ore types only occur in
 Cretaceous (Senonian) sediments  East of Aswan
 Middle Eocene sediments  north of the Bahariya oases
Iron ore deposit in Western Desert
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Western Desert:
i) Aswan iron Ore Deposits
Economic iron has been produced from East Aswan
regions since Pharaonic times (1580 to 1380 B.C.) until 1973.
The main occurrence located east of Aswan,
while small deposits are also encountered in the
variegated shales along the Nile Valley to the south at
Kalabsha, Garf Hussein, Kurusko, and Abu Simbil.
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Iron ore deposits occur in Senonian (Cretaceous) sediments ~7,000,000 tonnes were
produced between 1956 and 1973.
The estimated reserves are 121 to 135 million tonnes with 20 million tonnes proved
reserves (Attia. 1955).
The ore is a bedded oolitic type of Senonian age in the form of three bands
interbedded with ferruginous sandstone and clay capping Precambrian rocks.
The thickness of the bands varies from 20 up to 350 m.
Ore
The ore is oolitic hematite, dark red with a bluish metallic tinge in place, compact
and dense (Sp.gr. 3.45-4.35 gm/cc).
Ore minerals are mainly hematite with minor goethite. The hematite is occur in
oolitic form range from 1-1.5 mm in diameter and is cemented by a compact
hematitic matrix.
Gangue minerals include quartz, gypsum, halite, glauconite and clay minerals.
The Fe-content of this ore ranges 31.2-62.3 % (average 46.8% Fe),
SiO2 ranges 5-31% (average 14.1%).
P ranges 0.4-3.5%!!, Mn up to 1.3%, and S up to 0.3%!!.
The oolites themselves contain 60% of Fe while matrix contains 40% of the iron.
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Gneiss:
Ore is considered to have formed under sedimentary
lacustrine to fluviomarine conditions during the deposition of
Senonian sediments.
The iron is mostly dissolved from bottom sediments and
mobilized in so-called "carbon-dioxide zone" as ferrous
bicarbonate, then precipitated in an oxidizing environment as
ferric hydroxide.
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II) Western Desert:
Bahariya iron Ore Deposits
The Bahariya oasis is located in central plateau of Western Desert between 27°
48/ -28° 30/ N and 28° 55/ - 29° 10/ E.
Its northern edge is located along the contact between the stable and unstable
shelves.
Economic iron ores confined to the lower part of the middle Eocene limestone (El
Naqb formation) in four major occurrences north of Bahariya oasis.
Area Reserves Fe SiO2 Mn S P Cl
(M.Tonnes) %
El Gedida 126.7 53.6 8.9 2.3 0.9 0.2 0.6
Ghorabi 57.0 48.0 9.0 3.0 0.7 0.9 0.8
Nasser 29.0 44.7 6.7 3.9 0.6 0.1 1.3
El Harra 56.6 44.0 12.5 2.9 1.0 0.1 0.8
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These occurrences are called El Gedida, Ghorabi, Nasser and El Harra,
extending over 11.7 km2; and the ore thickness varies from 2 to 25 m (averaging 9
m).
The deposits are under laid unconformably by the Bahariya formation
sandstones and overlaid by the Redwan formation.
The iron ore deposits are generally irregular in outline. They form a succession
of beds which are concordant with local dips (~4°).
The ore is thought to be localized in the crests of two major anticlines trending
in a NE-direction. El Gedida and El Harra ore deposits are localized on the eastern
anticline, while Ghorabi and Nasser are on the western anticline.
The high-grade ores exist in the crests and that low-grade ores are localized in
the limbs of the anticlinal structures.
Major faults disturb the peripheries of the ore bodies, forming the major wadis
which surround the area of the iron ore deposits. Many small faults affect the iron
beds in the four areas. These structure natures of the folds apparent to be
generated by faulting affiliated with the Pelsuium mega-shear, along which the
Bahariya oasis are located (Neev et al., 1982).
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1.1.1. Forms, Shapes and Textures
Several forms characterize the constituents of the ore deposits such as
massive crystalline, crystal aggregates, granular, botryoidal-shape, kidney-
shaped, oolitic, pisolitic, pseudoolites (spheroids), subspherulitic, and sponges.
Therefore, several textures are recognized in the ore deposits such as
banded, disseminated, cavity filling, cavernous, and replacing.
1.1.2. Mineralogy
Main ore minerals: hematite, goethite, and hydrogoethite, with occasional
pockets of softly ochre and lepidocordite, chamosite, magnetite,
psilomelane, and pyrolusite.
Pyrite and chalcopyrite occur as rare minute single grains.
Gangue minerals: barite, kaolinite, glauconite, alunite, chert, gypsum,
calcite, chlorite, and Tripoli,
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1.1.3. Ore Types
Generally, four types of ore are distinguished based on texture, constituents and chemical composition
namely:
Hard-massive ore type: This type is relatively massive hard crystalline and has a deep reddish-
brown color. It consists mainly of hematite (>80%) with minor amounts of goethite and limonite.
Micro- and macro-fossils which are replaced by hematite (and/or goethite) are common.
Manganese minerals (mainly psilomelane) are rare in this ore type.
Banded-cavernous ore type: It has a brown or yellowish color, generally banded and cavernous.
The cavities being filled with red or yellow ochre or manganiferous powder. It consists mainly of an
intergrowth of goethite and hematite together with a little amorphous limonite and minor amounts
of manganese minerals. The pyrite and chalcopyrite are present as minute grains within limonite or
in the core of subspherulitic goethite bodies. This banded texture is attributed to pre-existing
laminations in the original limestone.
Friable-ore type: Generally, bright yellow, soft, friable and has an earthy luster. The ore minerals
consist mainly of goethite and limonite together with minor amounts of hematite. Glauconite is the
most common gangue mineral and result in the appreciable increase Al2O3 content of these ore
type.
Oolitic-pisolitic ore type: Low to moderate grade ore (49-45 % Fe) has a yellow to yellowish-brown
color and oolitic to pisolitic texture. It is mainly formed of goethite, Iimonite and quartz, minor
amounts of hematite, glauconite and Fe-rich chlorite.
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How Geologist do this mixture???
They classified ore blocks according to Fe-content into three categories as
following:
Poor ore (17-35% Fe): Low-grade iron ore, highly ferruginous sandstones
and hydrogoethite ore.
Normal ore (35-45% Fe): Oolitic and pisolitic hydrogoethite ore, banded
hydrogoethite, and hydrohematite ore
Rich ore (>45% Fe): Colloform hydrogoethite ore and massive
hydrogoethite-hematite ore.
It is necessary to blend the various types to obtain:
Fe 53%, SiO2 7.5%, Cl 0.7%, and MnO 1.98%,
for use in the metallurgical plants at Helwan Iron and Steel Co., Cairo.
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39

The largest and richest of these occurrences in that of El Gedida (-127 million tonnes proven ore). At El
Gedida mine, distinguishing three genetic, types:
I) Iron ore of a massive nature and a hydrothermal-metasomatic type (Type I):
•Represented by the high central area in El Gedida mine.
•The ore is high-grade, with high Fe and NaCl contents, and low Si, and high traces of Zn and Cu.
•The mineralized middle Eocene limestone (El Naqb formation) is brecciated and metasomatically replaced
by hydrothermal solutions ascending along NE-SW trending fractures.
II) Iron ore is cavernous, ochreous or massive type (Type II):
•Following the emergence and faulting of the mineralized middle Eocene block, the generated depressions
received reworked rocks including high-grade ore from the high central area.
•Fresh water lakes occupied the depressions where remobilization of Fe and Mn and their redeposition
were effected, possibly through biogenic interference.
•Tripoli earth and kaolinite were authigenetically deposited with the debris.
•Detrital barite is a common associated.
•Abrupt change in grade characterizes the iron ore of this genetic type
III) Iron ore is oolitic or pisolitic type (Type III):
•This follows type II in age and is tied to post-middle Eocene glauconitic succession which caps the reworked
iron ore of type II
•Enrichment of the marine depositional basin in Fe and K promoted the formation of glauconitic.
•Cyclic deposition of glauconitic clays and sands was interrupted by intermittent emergence followed by
lateritic weathering of glauconite sediments
•Profound changes in the mineralogy of these sediments took place resulting in the deposition of low-grade
Fe ore characteristically poor in Mn and Ba.22 November 2015
40

1.1.4. Origin
Ambiguity arises regarding the genesis of the iron ores in the Bahariya oases
area.
Attia (1950) favored a shallow water lacustrine origin during Oligocene time.
Deposition of leached iron under lagoonal environment and subsequent
replacement of the underlying middle Eocene and Cenomanian beds. Evidence of
replacement is apparent where most of the calcareous fossils, especially the
diagnostic nummulites of the middle Eocene, are almost completely replaced by
iron oxides.
Contrary of these opinions, Tosson and Saad (1974) suggested that the ores
were formed by metasomatic replacement associated with impregnations and
cavity filling from ascending solutions affiliated with volcanic activity. The oolitic
and pisolitic iron ore outcropping in the Ghorabi area to be syngenetic, the iron
being supplied by weathering processes and the high grade ores exist in the crests
and that low-grade ores are localized in the limbs of the anticlinal structures.
22 November 2015
41

2-5 Geophysical SurveyingEND OF TOPIC 3

Sٍedimentary manganes and iron ore deposits

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Sٍedimentary manganes and iron ore deposits