CHAPTER- III
MINEROGRAPHIC STUDY OF IRON ORES/BHQs
OF KAMATGI AND CODLI AREA
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CHAPTER-III
MINEROGRAPHIC STUDY OF IRON ORES/BHQs
OF KAMATGI AND CODLI AREA
INTRODUCTION
The study oftextures of any ore deposit is important as it throws much light on their genesis
and the environment in which the constituting minerals are formed and deposited. Different types
of ores and the associated BHQ are studied to understand their mode of genesis. The specimens
of iron ores and BHQ are prepared suitably by polishing the following precise conditions enumerated
byKehl(1955).
A comprehensive account ofthe textural and minerography of the iron ores and the associated
BHQ are described in this chapter. Further, etch test, microhardness and both quantitative and
semiquantitative reflectance have been determined and are given in the Table 3.1 Representative
samples of Kamatgi Iron ores were subjected to ore-microscopic investigations namely, texture,
colour, crystallinity, pleochroism and isotropism Vs anisotropism. The reflectivity of the minerals
present in the ores was measured.
Both the massive and laminated iron ores rarely take good polish, due to soft nature of the
ore. The ore microscopic technique and instruments used are the same as those employed while
studying the polished sections of the BHQ. Therefore a few-polished specimen of BHQ are
studied under ore microscope. The BHQ are polished at right angles :c the banding. The minerals
identified are hematite, magnetite, martite, specularite and goethite.
PHYSICAL PROPERTIES
Detailed mineralogical and physical characterisation of ores is a efficiency of separation.
The types of iron ores occurring in Hungund schist belts are classified based on their physical
characters. The ores are mainly classified into the following types;
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Massive hematite ore
Flakey or biscuit) ore
Laminated ore
Blue dust or powdary ore
Lateritic ore
Limoniticore
Goethitic ore
Shaley ore
BHQ
Specularite
Massive hematitic ore
Massive, iron black to steel grey in colour. Conchoidal fracture, cherry red streaks. Banding
and laminations are absent. Specific gravity varies from 1.09 to 2.91. The ore is entirely composed
of hematite, with subordinate amount of goethite and specularite. (Plate-3.1 and 3.2)
Flakey / biscuity ore
Thin shale like, hard and compact giving cherry red streak. It is typically banded or laminated
in nature. The original silica layers are however leached. The space between laminae may be open
or filled by ferruginous material. The thickness is few mm to few cms. Due to brecciation, bands
are dislocated and contain small irregular shaped vugs filled with fine white crystalline sacchoroidal
quartz/secondary iron hydroxide minerals. (Plate-3.3)
laminated ore
Laminated ore exhibits distinct laminations of iron layers, steel grey in colour. Thicker laminae
are more hard and compact. The specific gravity varies from 1.62 to 2.66, which gives a
characteristic cherry red streak. The laminations having a thickness of few mm to one cm. The
minerals of laminated ore are abundantly hematite, with subordinate amount of goethite and
magnetite. Hematite is predominant mineral where magnetite occurs as relict core.
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Blue dust (Powdery ore)
Soft ore crumples to powder when touched. Laminations are distinct when in situ. When
compact, it exhibits laminations and resembles biscuity ore. It occurs in a small vein of a few mm
thickness and run over a distance of 3 to 5 meters. Several gradations may be seen. The ore
consists ofhematite and magnetite. Blue dust occurs as pockets. This is unconsolidated powdery
ore. The ore is bluish in colour with a specific gravity 3.6.
Lateritic ore
Small patch of laterite is noticed only on the top of the hill. It occurs as a blanket over the
hematite ore, having a thickness of about 2 meters. The ore is brown in colour w.th specific gravity
2.19. Usually it is dumped as waste during mining. This ore predominantly consists of hematite and
goethite. Quartz occurs as gangue mineral. (Plate-3.4)
Limonitic ore
These are well developed and clearly recognisable occurrences found in natural caves that
are located in the hill ranges. Limonite is yellowish to brownish yellow in colour and gives similar
streak. Hematite alters to hydroxide of iron. Specific gravity varies from 2.8 to 3.0.
Goethitic ore
This is brownish in colour and botryoidal in habit. It is a secondary mineral. It is predominant
in the lateritic type of iron ores. (Plate-3.5)
Shaley ore
This is closely associated with high-grade iron ores and appears to be enriched in ferruginous
shales. Shales vary in colour from buff to white, grey ferruginous at the top and pass on to iron ore.
These shales are also manganiferrous with concentration of manganese ore in places. It is a lowgrade ore might have been formed due to enrichment of iron during the period of leaching of iron
formations.
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BHQ
The occurrences of BHQ’s are noticed on the hill ranges of Kamatgi area. The BHQ’s form
long and narrow ridges and is very hard, compact exhibiting intersecting joints. The thickness ofthe
iron bands varies from 0.2 cm to 1 inch in thickness and so also of silica bands. The BHQ’s are
regarded as the source rocks for the formation of iron ore deposit of this region, as it is evidenced
by the higher concentration of total Fe in BHQ. (Plate-3.6 and 3.7)
Specularite
It occurs as bladed and needles like plates and flakes co-existing with massive hematite ores.
The mineral seems to be developed only in the cavities of hematite. The mineral is as hard as
massive hematite and gives characteristic cherry red streak. Its specific gravity is 2.185.
Table No: 3.1 Physical and optical characters of iron ores of Kamatgi area.
Etch tests
SI
Mineral
Habit
Colour
Bireflectance
Anisotropy
no.
Hardness
HO,
VHN
HCl+SnCl2
SnCi2
1.
Hematite
Anhedral
Greyish
Very weak
white
2.
Magnetite
Anhedral
Grey
Strong shades
125-912
-Ve, -Ve, -Ve
700-840
Darkens,
of grey and blue
Absent
Isotropic
to subhedral
3.
Goethite
Colloform
tarnishes,-Ve.
Grey
Absent
Distinct
500-540
-Ve stains brown,-ve.
Etch tests, microhardness and reflectance have been quantitatively and semiquantitatively
determined and are given in above Table.
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MINEROGRAPHIC CHARACTERS
The study ofthe polished samples shows the different textural features. The iron ore exhibits
the replacement texture of magnetite to martite (Plate 3.8) and also into laths of specularite. Rao
(1969) studied the iron ores and associated BHQ’s of Donimali and brought out clearly where
hematite replaces magnetite and the replacement proceeds from the periphery to inwards. In
some of the cases another type of replacement texture is the break up of martite crystals in which
when the martitization of magnetite is completed, the martite crystals tend to break along (111)
planes. After the break up of martite crystals the original shape of the crystal is distorted. Some
times the laths of specularite show a roughly a triangular arrangement.
The BHQs are medium to fine-grained. The iron bands show a variation in grain size. The
most common texture of BHQ is the development of banded texture. (Plate 3.9). The banding is
nearly parallel to the alternate layers of iron minerals and silica (quartz). (Plate 3.10)
Form the above textural study it is observed that relict magnetite occurs as a primary and
hematite is considered and regarded as a secondary mineral. The minerals martite, goethite and
specularite are also secondary minerals.
Based on the above textural study, the following scheme of iron mineral paragenesis may be
drawn from the ore microscopic characters.
Mineral
Magnetite
Hematite
Martite
Goethite
Specularite
primary
Secondary
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A detailed study of the polished samples of the iron ores and BHQ’s was carried out. The
study reveals that the ore minerals frequently encountered are magnetite, martite, hematite,
specularite, goethite and limonite.
Magnetite
Magnetite is the most dominant mineral in BHQ and isotropic in nature. The replacement
process starts form periphery inwards and passes through the stage of martite to the final product
of hematite (Plate 3.11). The BHQ was showing the altered magnetite to martite crystals is noticed
in Plate 3.12.
Hematite
Hematite forms the main constituent ofthe opaque minerals (Plate 3.13). It occurs in massive,
friable and lateritic ore. The development of hematite from magnetite is very regular pattern of
replacement; large sized grains of hematite though observed frequently, yet these are without any
regular form. At such places the grains show grayish white colour and are feebly anisotropic under
crossed nicols, the grains show red internal reflection bireflectance is very weak.
Martite
The martite is formed during conversion ofmagnetite to hematite. The mineral has no definite
form or shape of its own. It is octahedral in form. It is brownish white in colour. The martite is -ve
to all etching reagents (Plate 3.14).
Goethite
It is predominant in the lateritic type of iron ores and it is found along with martite. The
mineral is grey in colour with bluish tint under cross nicols; it shows reddish brown internal reflection.
It is positive to aquaregia and negative to the remaining reagents.
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Specularite
Generally, specularite occurs in high-grade ores. Crystals of specularite are bladed and
needle like in habbit and are seen co-existing with martite. The formation of specularite is from the
recrystallisation of martite. The arrangement of specular laths is roughly triangular. The laths of
specularite are anisotropic (Plate 3.15).
BHQ
The polished samples show the iron ore bands to be chiefly composed of martite. Majority
ofmartite coalsced to form a continuous band of hematite. The quartz bands have diversely disposed
laths ofspecularite in quartz matrix. The ultimate bands of hematite and quartz with martite crystals
in hematite can be observed in BHQ (Plate 3.16).
PHYSICAL PROPERTIES OF GOA IRON ORE
The ore is hard and compact while at depth it tends to be friable and powdery. Based on
their physical properties the ores are classified as lateritic ore, hard lumpy ore, friable or laminated
ore, powdery ore or blue dust, platy ore, brecciated ore, mixed ore, earthy ore and concretionary
ore.
Lateritic Ore
Occurs on the surface and is partly pisolitic and cavernous with some nodules, lenses and
bands of hematite and limonitic iron ore. The surface rocks have been subjected to continuous
chemical weathering under tropical environment, which resulted in the formation of lateritic cover.
Massive Ore
Dark brown in colour, compact and some lateritic patches in the upper regions which is
followed by distinct lamination at the lower regions containing mostly hematite and rich in Fe
content. The specific gravity is high of 5.2, occur as blocks broken up along joint planes. Massive
ore occurs in a few deposits of Goa for example Bicholim deposits of Messrs. Dempo and D’Souza
and Messrs. Rajaram Bandecar and Co. etc.
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Bedded Ore
The ore is present near the surface and shows original bedding planes. The bedding planes
are marked by either fine lines or by fine open space partly or wholly filled by shaley matter or
laterite. The ore is hard, compact and colour varies from cherry red to bluish black and steel grey.
Platy Ore
'
The ore consists ofthin plates of iron ore ranging in thickness upto 1 cm cemented together
by ferruginous or clayey material. It occurs as small pockets and lenses with in the laterite near the
surface. Small pockets of this type of ore occur in Bicholim, Pale and also Codli deposit.
Brecciated Ore
Ore consisting of small angular pieces ofhematite cemented in lateritic or ferruginous matrix.
This type of ore extends to adepth of 3 m to 8 m from the surface. These types of ore extensively
occur in the Sacorda, Pissur.em and Codli deposit.
Mixed Ore
At the surface irregular patches, lenses and bands ofhematite mixed with laterite is a common
feature observed in many deposits, as such Bicholim, Pale and other deposits. The mixed ore is
formed on the hanging and footwalls of the main ore band and the thickness of the zone ranges
from 2-3m.
Earthy Ore
Below a lateritic ore occurs a mixed ore zone with a soft, earthy ore, bluish green to yellowish
green in colour. These deposits are found in Chowgule and Co. Conda deposits of Messrs.,
Agarwal and co.
Laminated Ore
The ore follows the hard massive ore and commercially called as biscuity ore. It is very
porous with thin laminae and polygonal cracks apparently derived by extensive leaching. Often as
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soon as the ore is dislodged from the mine face it crumples resulting in a mixture of small pieces of
powdery ore. An intermediate thin zone of laminated ore about 2-4 m thick. The alternate
depressions in the ore are clearly the result of either leaching away of silica and inadequate
concentration and cementation of iron or the presence of thin laminae of phyllitic matter owing to
its friable nature.
Powdery Ore
This ore is formed below the zone of laminated ore at lower depth and popularly called as
blue dust and contains fine grains of hematite and magnetite crystals, ranging in size up to 6 mm
and varying in colour from blue to bluish black, loosely packed with considerable amounts to
voids and pore spaces between them. The powdery ore is rich in iron content than the hard lumpy
ore lying above it nearer the surface.
MINEROGRAPHY OF CODLI IRON ORES
Polished sections of representative ore samples collected from Codli deposit were prepared
and studied. The study has revealed that the ores are essentially made up of hematite, magnetite,
limonite and goethite. The presences offine euhedral crystals ofmagnetite :n a fine groundmass of
hematite are common.
Three representative ore samples were collected and subjected to IR analysis. It can be
broadly said that the lumpy ore is essentially made up of hematite and partly of magnetite, goethite
and limonite whereas, powdery ore is essentially made up of magnetite and hematite.
Generally the lumpy ores of Codli are comparatively soft, porous and low in iron content
owing to the varying proportions of laterite and shaley materials. The colour of the ore varies from
blue to cherry red and yellowish red. The porous nature of the ore is also responsible for its low
specific gravity that ranges from 3 to 3.5 consequently when the ore is soft and/or porous.
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GENESIS OF KAMATGI IRON ORE DEPOSITS
The Precambrian iron ore deposits are closely associated with BHQ or IFs. Many researches
scanned the genesis of iron ores and exposused there views like; Weld (1915), Gruner (1937),
Guild (1953), Ruckmik (1963), Dorr (1965), Sarvanan (1969), Krishnan (1973), Sahu and Gurav
(1972), Mishra (1976), Devaraju and Ananthmurthy (1976), Jayasheela (1982). Different units
exist regarding the source, mcde oftransportation, nature ofdeposits, mineralogy and environmental
conditions:
Mac Leod (1970) was ofthe opinion that the hematitic ore bodies ofHamersley iron province,
western Australia have originated as a result of enrichment of parent IF under supergene conditions.
Grass (1973) was ofthe opinion that metamorphic differentiation and supergene processes
were responsible for the development of iron ores of itabirities of Siberia.
Sims (1973) advocated two hypotheses for the origin of Belgium iron ores Viz., a)
metasomatic replacement o: quartz by specularite (specular hematite) and b) leaching of silica by
meteoric waters.
Syngenetic sedimentary deposition of iron ores has been advocated by Harder and
Chamberlin (1915). Hydrothermal replacement of iron fonnation was supported by Guild (1953).
Magmatic origin was also advocated by Park (1961), Bennett (1962) and Geijer (1967).
Leaching of silica from itabirite by meteoritic water resulting in residual enrichment of iron
ores is proposed by Tolbert et.al (1973) for the high-grade hematite ores of Sierra dos Carajos,
Brazil.
*
Dunn (1935); Krishnan (1954); Schmidt (1963); Symons (1966); James et.al. (1968);
Percival (1970) and Mishra (1976) and a host of others support the residual enrichment of iron
ores from the IFs.
The iron ores of Kamatgi are not due to Hydrothermal activity, as there is no evidence of
wall rock alteration and also the process metamorphic replacement, as the evidence of replacement
of silica by iron is not noticed.
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The Kamatgi deposit might have formed due to leaching of silica, redeposition of iron in
open spaces, residual enrichment of iron from IF for the origin of iron ores. The chemistry of IF
and iron ores throw an evidence of residual concentration of iron was obtained by comparing the
trace element analysis, common trace elements are noticed both in IF and iron ores. Heterogeneity
is observed in the concentration of trace elements; Pb, Ni and Cu contents are high in IF and iron
ores. However from the above facts more oxidising atmospheric conditions prevailed during the
formation of iron ores and the predominance of hematite and the high concentration of trace
elements support it.
GENESIS OF GOA IRON ORE DEPOSITS
The deposits of Goa occur as reefs and lensoid bodies on the ferruginous quartzites and
ferruginous phyllites and extend up to a depth of 35 m. to 40ms. The iron ore occupies the crusts
and slopes of hillocks and close to the valley portions. The water bodies in the valley is close to the
surface, at this fracture the concentration of iron ore is poor or absent. Relict unaltered ferruginous
quartzite and phyllites are noticed. Replacing the iron oxide are observed in iron ore. From the
above facts the iron ores are formed due to residual concentration by leaching down of silica and
concentration of iron oxide and replacement of the associated phyllites. The iron oxide has the
ability move upwards, downwards or along horizontal planes and in deposited as ferric hydroxide
which on hydration it becomes hematite.
The increase in the phyllitic concentrations with ferruginous quartzites, then the iron ore
concentration is moderate to poor lumpy ore occurs, below the lumpy ore, soft, low grade or
friable ore exists. The process of blue dust ore at depth below the lumpy ore is directly related to
the richness of the deposit.
From the above facts :he iron ore deposits of Goa are mainly due to residual concentration.
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CONCLUSIONS
The minerographic study of Kamatgi and Codli iron ores has revealed the following points:
•
Hematite is dominant in BHQ and magnetite is primary.
•
Martite, hematite, specularite are secondary minerals.
•
Relict magnetite is common in iron ores of Kamatgi as well as Codli mines.
•
Replacement texture is observed where hematite replaces the magnetite.
•
Formation of specularite is observed in association with magnetite and martite.
• The Kamatgi deposit might have formed due to leaching of silica, redeposition of iron in open
spaces, residual enrichment of iron from IF for the origin of iron ores.
• The iron ores of Goa formed due to residual concentration by leaching down of silica and
concentration of iron oxide and replacement of the associated phyllites.
Plate No: 3.1 Massive hematitic ore/ laminated ore.
Plate No: 3.2 Massive hematitic ore.
Plate No: 3.3 Flakey / biscuity ore.
Plate No: 3.4 Lateritic ore.
Plate No: 3.5 Goethitic ore.
Plate No: 3.6 Banded Hematite Quartzite.
Plate No: 3.7 Folds in association with Quartzites (in BHQ)
Plate No: 3.8 Photomicrograph of the goethitic ore showing the
replacement texture of magnetite to martite. (Under reflected light)
Plate No: 3.9 Photomicrograph of the BHQ showing the banded texture,
thin bands of hematite and quartz. (Under reflected light)
Plate No: 3.10 Thin section showing the chert grains in BHQ. (Under cross nicols)
Plate No: 3.11 Thin section showing the relict magnetite grains in BHQ.
(Under ordinary light)
Plate No: 3.12 Photomicrograph of the BHQ showing the altered magnetite to martite
crystals. (Under reflected light)
Plate No: 3.13 Photomicrograph of the BHQ showing the hematite ground mass.
(Under reflected light)
Plate No: 3.14 Photomicrograph of the BHQ showing the martite crystals.
(Under reflected light)
Plate No: 3.15 Photomicrograph showing the magnetite and specularite.
(Under reflected light)
Plate No: 3.16 Photomicrograph of the BHQ showing the banded texture,
alternate bands of hematite and quartz. (Under ordinary light)