The deposition of iron ore in Goa is over an area of 90 km2 strike a length of 25 km. Goa is situated on the west coast of India between the Arabian Sea and the Western Ghats. The state has a geographical area of 3610 sq. kms, less than the area of a district of many states.
Goa is essentially a highly hilly and rugged track with narrow valleys and linear sandy planes along the coast. The iron ore range from Pale in South to Advalpale in North for the North Goan Iron ore deposit. Almost the entire North Goan iron ore deposits are of Haematitic in nature except minor occurrence of magnetite.
The Western Ghats, which are clad by evergreen forest, have a general altitude ranging from 600m to 1000m. In the western margin of the territory along the Arabian Sea there are long and narrow strips of sandy plains and low flat topped laterite hills ranging in altitude from 50m to 100m. Between the Western Ghats on the east and the coastal plains on the west, the territory is undulating with alternate chain of hills and long narrow valleys. The hills range in altitude from 250m to 400m.
The Vageri hills near Valpoi however have an anomalous position as these are shifted much to the west of the Western Ghat chain of hills. But yet there is a narrow link between hills and the Western ghats of Surla ghat.
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The geology and mineral resources of Goa had not received much attention when the State of Goa was under the Portuguese occupation. There were, however, sporadic attempts made by some geologists and mining engineers of private firms, to study the geology and mineral deposits, after the commencement of iron and manganese ore mining in Goa, in 1947.
The Goan iron ores have been explored extensively by core drilling in 50mx 50m grid interval due to complex folding of ore body. At some places drilling at 25m x 25m grid interval also has been carried out for pre-production and quality control purpose.
Field Disposition of Iron Ore:
The northern boundary of Goa runs along the Terekhol River (the mouth of the river lies in Goa) on the eastern side, the boundary is demarcated by the Sahyadris, on the west by the Arabian Sea. Southern section is closed off by an elevation about 111m high near Colem.
The principal geological feature of the land is the extensive lateritization. It is mainly due to Goa’s position in the tropical moist climate subject to vast seasonal changes. Lateritic capping is associated with iron and manganese ore deposits.
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The iron ore area of Goa belongs to Dharwar (Archaean) System and associated with Banded Hematite Quartzite and Ferruginous Phyllites. Iron ore deposits occur predominantly towards north-north west.
The mode of occurrence of iron ore deposit is of lensoid and blanket type of varying dimensions in the pink phyllite horizon. Superficially, they occupy the crest and slopes of hill ranges, low lying hills and mounds, and are considered to be formed due to the enrichment of the banded ferruginous quartzite and phyllites.
The regional distributions of the iron ore follow a general northwest-southeast trend and varying thickness. These rocks are primarily of sedimentary origin and later intruded by igneous intrusions, which resulted in formation of folds and faults.
The iron ore formations of Goa indicate two sets of sequence and transitional epochs and environments of deposition. Iron ore formations of North Goa, are of banded nature formed in geosynclinals basins while those of South Goa are of simple undulating surface forming several indiscrimination indicative of a platform or a carbonic environment.
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Available literature on structure, mineralisation and mineralogical maturity of deposit indicate possibility of a zone where a clear-cut separation could be made out. Example of such zone is seen in the region between Sancordem and Mollem.
Practically the entire territory of Goa is covered by rocks of the Dharwar Super Group of the Archaean Proterozoic age except for a narrow strip in the northeastern corner of the territory which is covered by Deccan trap of the Upper Cretaceous- Lower Eocene age.
The Dharwar rocks which extend in a general north west-south east trend are represented by metamorphosed basic and acid volcanic rocks and sediments at the base overlain by greywacke suite of rocks, which in turn, if followed by pyroclasts and tuffs with the associated chaemogenic precipitates of lime, manganese and iron and ultimately again overlain by greywacks suite of rocks. These rocks have essentially suffered greenschist facies of metamorphism and have been intruded by granitic rocks followed by basic dykes.
Nature of Iron Deposits:
The North Goan iron ore has been divided into three blocks based on field observations and geological setup.
a. Bicholim Block:
The iron ore bearing rocks of Bicholim form a part of the entire length of) the western limb of the synclinal fold, trending NW-SE direction. This area is characterized by a prominent NW-SE trending dip slip fault which passes along the axial plane of the Bicholim-Sirigao and form iron ore body small local faults have also been observed in the mine working.
The strike of the ore body as well as of the associated formations is about N50°W – S50°E with overall dip varying from 45 to 60 degree due NE. These rocks are affected by small folding and a few block faults. The powdery ore bands around 20 – 45m thick occur below low grade lumpy ore underlying lateritic capping.
The ore body at places is affected by basic intrusive. Weathering of phyllites has given rise to laterites and this has protected the soft iron ore bodies down below. The iron ore deposits are strata bound; wherein the concentration of iron ore is mainly because of weathering and peneplanation. The iron ore deposits are enclosed within pink phyllite of manganiferous clay lenses. The footwall is generally phyllitic clay and the hang wall is manganiferous clay.
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The succession comprises:
Laterite/Lateritic Ore-15-20 meters
Iron Ore Lumpy-20-45 meters
b. Velguem-Surla Block:
This is the biggest block having about 27 mines. The entire area was originally covered with thick capping of laterite below which iron ore is present. Lower portions of the eastern slope are covered with clay formed from weathering of phyllites and laterite. However, following is the normal succession of litho units observed from bench excavations and inferred from geological mapping and exploration data and show in in Figure 8.4.
c. Geological Sequence Average Thickness in Metres:
1. Soil Cover 0 to 01
2. Laterite/Lateritic Clay 8 to 10
3. Ferruginous laterite /lumpy ore 5 to 10
4. Limonitic/Phyllitic /Mn Clay 5 to 30
5. Low grade Powdery Ore 2 to 10
6. High Grade Powdery Ore 2 to 20
7. BHQ Siliceous 2 to 10
8. Siliceous Clay/Mn Clay mix with silica Foot wall
The ore body occurs in two ridges i.e. Dignem and Sonshi ridge. At present working are concentrated in both the ridges.
It is revealed from the geological map and exposed rock formation in the area that the ore body and the associated rocks are folded. The ore formations are associated with the ferruginous phyllitic and limonitic clays on the hanging wall side and manganiferous clays and schists form towards footwall side. Structurally ore body represents a symmetrically folded, doubly plunging anticline. Pitch of the fold roughly follows the topography of the area.
The ore horizon generally consists of parallel ore laminate intercalated with bands of hard ore, limonitic clays. The general strike NNW – SSE with local variation of 10° on either side. Dip of the ore body is 45 – 60° NE. Mineralogically, the ore body consists of mixture or haematite,/Limonite ore upto 62 per cent Fe in case of haematite, martite combination.
The northern part of this block consists of three ore bodies locally named as Bombay, Calcutta and Delhi. These are one below the other. They are 50m, 15m and 20m thick respectively. Calcutta ore is of high grade followed by Delhi and Bombay.
The ore bearing horizons are separated by thick manganiferous clays. Bombay forms the youngest and Delhi forms the oldest ore body. Physico-chemically, they exhibit distinct characters. The Bombay ore body has 5 – 10m thick manganiferous ore, which is transitional zone between Bombay ore body proper and manganiferous clay. They are separated by 50m thick manganiferous clay horizon.
The Delhi ore body has phyllitic foot wall. In between Delhi proper ore body and phyllitic basement there is 10m to 40m thick limonitic ore body of 55 per cent Fe grade. In southern portions weathered dolerite intrusions are observed.
d. Gavanem Block:
The ore deposit of Gavanem Mine occupies the northern part of Gavanem- Malpona iron ore deposit of central Goa. Ore body extends over a strike length of 100 m striking NW – SE and dipping 20 – 40° towards west into the hill. In the Central part of the area the ore body is flatter.
The ore body is intruded by dykes at certain places overlying formations below laterite are pinkish brown phyllitic clays. Manganiferous clays form the underlying formation. Lumpy and friable ore columns are followed by powdery ores.
The thickness of ore body is varying from 15m to 20m. The beds show minor undulation in the form of gentle anticline and syncline. The deposit has a limb of syncline dipping north-east phyllite, forms both the hanging wall and foot wall side.
Nature of Ore Goan in Iron Ores:
(a) Lateritic Ores:
Weathering under tropical conditions has resulted in formation of laterites. The pieces of iron ore are seen embedded within the lateritic mass in this type of ore. This type of ore occurs on the surface. The ore is partly pisolitic and cavernous with some nodules, lenses and bands of haematite and limonitic ore. The recovery of iron ore from this type is lumpy and is low varying from 10 to 20 per cent. The grade of the lumps is low varying from 55 to 58 per cent Fe.
(b) Hard Lumpy Ore:
It is dark brown in colour, compact and shows some lateritic patches in upper horizon, followed by distinct lamination at lower horizons containing mostly haematite. This ore zone occurs below laterite and above friable zone.
This zone is encountered when the iron ore body comes near the surface due to folding, mainly plunging. The lumpy ore is hard and its grade varying from 55 to 60 per cent Fe. This ore type is found in some of the mines, and friable ore directly occurs below the laterite. The recovery of lumps from this zone varies between 40 – 50 per cent.
(c) Friable Ore:
This ore when extracted from the mine face crumbles into powdery ore and generally is a mixture of lumpy and fine ore. This ore is commercially known as biscuity ore. It is very porous with thin lamina and poly-ground cracks apparently derived by intensive leaching. The Fe content varies from 55 to 60 per cent in friable ore with lumps recovery of 10 to 20 per cent.
(d) Powdery Ore:
This type of ore occurs predominantly in all the deposits of Goa. This ore is formed below the zone of friable ore at lower depths and indicates dark greyish brown to brownish grey colour, containing fine haematite crystals. This powdery ore is intercalated with that of yellowish to pink phyllites. This is the main ore zone. It is fine-grained greyish blue to brownish earthy in colour. The Fe content varies from 58 to 65 per cent in powdery ore.
(e) Siliceous Ore:
The ore is soft and high in silica and low in Fe. The ore is found in some of the mines in Velguem-Surla block. The average grade of this ore is + 40 per cent Fe and SiO2 ranging from 18-25 per cent. This is being stacked separately and is consumed on demand directly.
All the three i.e. topographical, lithological and structural controls of mineralisation have been observed in iron ore mines of the study area.. It is observed that hills with steep slopes are favourable locales for iron ore concentrations in the form of bands, having distinct lithological control.
The iron ore mineralisation is confined only to the ferruginous phyllites or altered siliceous formation i.e. B.H.Q. The peculiarity of Goa deposits are that the ore is present in the synclinal trough, fairly deep seated and which are wider and more open.
The origin of iron ore deposits of Goa, is due to enrichment of pre-existing rocks i.e. B.H.Q. The corrugation banding so typical of BHQ clearly indicates that silica bands are leached by rain water, resulting in enrichment of iron ore formation.
The replacement of quartzite bands in the BHJ by secondary ferric hydroxide has resulted into secondary hematite on hydration is demonstrated by presence of fine banding in the hard massive ore.
The descending meteoric water becomes poorer in secondary ferric hydroxide and is thus capable only of leaching silica bands from BHQ but do not deposit any of iron hydroxide. Because of the fineness of grain size and banding, slightly acidic descending meteoric water is capable of disabling inter-granular ferric iron within the hematite bands, thus loosening the hematite grains which crumble into fine powder known as blue dust.
This is evident from the high percentage of water molecules in form of moisture, which is due to the fact that the water molecule occupies the voids left over by the leaching of quartzite band.
The theory of replacement in ferruginous phyllite also holds good where pH and eH conditions are similar. From the field study the corrugation bands of BHQ indicates that silica bands are removed by rain water with the more resistant haematite bands protruding out.
The replacement of quartzite bands in the BHQ by secondary ferric hydroxide which as dehydration has given rise to secondary haematite is amply demonstrated by the preservation of very fine banding in the hard massive ore.
Resource Outlook of Iron Ore:
The iron ore deposits of the region are of complex nature. Structurally, the deposits are cross-folded. The deposits are occurring in low lying and in synclinal basins and are spread more in horizontal planes.
The ideal way of exploring such type of deposits is by means of drilling in grid pattern. Delineation of ore body by field mapping becomes very difficult. This necessitated detailed exploration of the deposits at closure grid intervals by drilling at 50mx50m and at places at 25m x 25m depending on the complexity.
The exploration carried out in the area is systematic. In most of the mines core drilling has been carried out in a closed grid regular interval in mineralized area. However the area used for dumping within the ML areas are proved barren by drilling few bore holes. The core logs are prepared and analysis & records are properly maintained. Slopes are not stable in some mines of Bicholim & Velguem-Surla block as wall rocks are not strong.
Goa under Portuguese rules had granted Title concessions to a maximum of 100ha each which have been subsequently converted to mining leases from 1987. The moderately steep dipping ore-bodies and the narrow lease areas restrict the depth of mining and the ultimate pit limit. Therefore exploration at deeper depth has not been carried out. Though mineral occurrence continues at depth it cannot be mined.
Therefore common boundary working is a routine feature in Goa. The footwall and hanging wall being weak exposure of large area are not permitted from safety angle and sometimes backfilling is carried out to maintain the safety of the pits. This restricts deeper exploration.
The North Goa Iron ore deposits are classified under stratiform, strata bound and tabular ore deposits of regular habit with particular change in trend with sharp to moderate physical contact with bounded surfaces and moderately steep dip. The requirement of exploration as per the field guidelines prescribed under UNFC to categorise reserves under different code applicable for Goan Iron ore deposits
Petrography & Mineralogical studies including gangue and its liberation characteristics have been carried out in all the major working mines like Bicholim, Velguem-Surla and Cudnem etc.
The threshold value of the Goa Iron ores is at present 55 per cent Fe. However, for siliceous ore, the threshold value has been fixed as + 40 per cent Fe. The Fe content for the powdery ore exported is + 60 per cent Fe with Al2O3 < 2.8 per cent & silica < 3 per cent and lumps is in the range of 59 – 60 per cent Fe with Al2O3:2.5 per cent.
The ROM grade generally contains on an average 58 per cent Fe which in turn is beneficiated to obtain marketable grade. The low grade ore with 55 per cent Fe is usually blended with high grade to get feed grade of 58 per cent ROM which is fed to beneficiation plants. Thus, it is seen that the quantity of sub-grade generated is almost meagre and all the ore above threshold value +55 per cent Fe is utilised fully.
The Iron ore reserves and remaining resources as on 01.04.2005 as per UNFC for North Goa and South Goa districts of Goa are as presented in Table 8.1a-b.
From the above table it is seen that the total reserves of haematite in three iron ore blocks of North Goa amounts to 250 million tonnes. Out of total 250 million tonnes more than 60 per cent reserves are in Velguem-Surla block. However the demonstrated reserves (both proved and probable together) constitute 245 million tonnes (more than 98 per cent of total geological reserves) in both Bicholim and Velguem-Surla Clusters together.
Further blockage of reserves 1.5 million tonnes of haematite was observed in Bicholim block due to the passage of high tension line and PWD Road through the lease area. Similarly in leases with common boundary, also a considerable quantity is blocked in Velguem-Surla block. These three blocks contribute more than 50 per cent of the total Goan haematitic proved Iron Ore Reserves.
Exploration in the form of core drilling in 50m X 50m grid interval is completed in the acquired areas of all the major mines. However in some mines/leases where the ore body is intricately folded 25m. × 25m grid interval is taken up.
The un-acquired area which is about 30 per cent of the total lease area may be taken up for exploration after acquiring them. The total 5,505 bore holes with cumulative meterage of 2.73 lakh m in iron ore mines carried out in the area, reveals that the exploration has been carried out in the three blocks viz. Bicholim, Velguem-Surla and Gavanem.
The reserves of iron ore estimated in all mines of these three blocks were about 250 million tonnes of all categories out of which 180 million tonnes of ore of proved category of haematite have been estimated (Figure 8.5).
The Goan iron ore are fast depleting. As against a total proved reserves of 458 million tonnes of iron ore the state has been producing at rate of about 33 million tonnes, therefore the reserves shall not last long unless very low grade ore is marketed.