GENERAL

GEOLOGY

[The section on Geology is contributed by Shri S. S. Ghodke, Geologist (Sr) of the Geological Survey of India, Maharashtra Circle, Pune.]

Introduction.—The varied lithology, intricate structure and important mineral deposits of this district have drawn the attention of several prominent geologists since 1906. The relief is characterised by the presence of lenticular narrow ridges trending in NNW-SSE direction with occasional spurs rising to the heights of 2310' (Janda dongar) about 9 km north-east of Dewalgaon railway station, and 1996' (Karra hills) about 8 km southwest of Gondia surrounded by the gently undulating country with occasional hillocks among the plains.

The stratigraphic sequence based on available information on the geology of the area is as follows: —

The oldest rocks are the crystalline complex consisting of granite, granite-gneiss, followed by the mica schists and hornblen-deschists, quartzites, crystalline limestones, calc-granulites and calciphyres of Sausar series and is exposed in the northern part of the district. They are usually arranged in parallel bands and are very often continuous for many miles. It is possible to trace a passage along the strike from calc-granulite, through banded calciphyres into marble. It is among the gneisses, schists and quartzites that the manganese ore deposits and the associated manganese silicate rocks are enclosed in the form of elongated lenticular bands conformable in strike and dip to the surrounding rocks. Near Bhamsar hill manganese ores occur in unusual schistose quartzite containing lazurite, rutile, ilmenite. Another quartzite is of green colour and contains chrome-vanadium-mus-covite.

The southern half of Bhandara is covered with members of the Sakoli formations arranged in a triangular pattern which has also been designated as Bhandara triangle (Bhattacharya, 1953). The various members of formations comprising of phyllites, slates, chlorite schists and sericitic quartzites and sillimanite-kyanite bearing schists are exposed in parallel bands like Sausars to the north. The country separating the Sausar and Sakoli belts is largely covered with alluvium in which small exposure of the Sausars occur; and the mutual relationship of these two important formations of the Dharwar group of rocks is not yet known with certainty. The inferred boundary between the two units has been drawn more or less in a northeast-southwest direction, not far from the town of Tirora in Bhandara tahsil and a mile or two northwest of Bhandara town the Sakolis lying to the southwest and the more metamorphosed Sausars to the northwest. The rocks of the two groups appear to show a difference in chemical composition, characterised chiefly by a scarcity of lime bearing rocks among the Sakolis; lithologically the Sakoli groups are more predominantly argillaceous and siliceous than the Sausars, which include such rocks as calc-granulite, marbles, man-ganiferous rocks and manganese ores. At the same time, mice-schists, gneiss and quartzites are very prominent in the Sausars series. The mineralogical difference as summarised by Sir L. Fermor (1936), is that the rocks of the Sausar group commonly contain felspar and biotite but not chlorite whereas those of Sakoli group contain invariably chlorite, rarely biotite and with no felspars. Amongst the rock types, the quartzites are at places highly ferruginous and at other places chloritic; others comparatively pure and massive; red and yellow phyllites and slates sometimes with knots of sericite and crystals of magnetite and garnets can be observed frequently. The chlorite-muscovite schists constitute the predominating unit in the Sakoli group of rocks. Somewhat higher grade of metamorphism as shown by the schists of Sakoli group can be summarised due to the proximity of the tourmaline bearing granite, tourmaline being always abundant in the schists. Kyanite and sillianite bearing rocks of Bhandara are associated with chlorite-muscovite schists, the parent material of which seems to have been clay-perhaps the result of the surface decomposition of an ancient basalt (E. Pascoe, 1950).

A new system of rocks called Dongargarh system of post-Sakoli but pre-Cuddaph age has been suggested by Sarkar (1952). This system occupies a belt of about 88 km. width stretching in a NNE direction for more than 128 km. between the Sakoli synclinorium on the west and the Chhatisgarh-Cuddapah basin on the east. The main rock types of this system are rhyolite and andesite, Dongargarh granites and sandstones.

Unconformably overlying the archaeans are present; the Cuddaphs consisting mainly of sandstones, shales, grits in the upper part with alternate bands of quartzites and conglomerates in the lower part. The unconformity is represented by a conglomerate bed which is seen near Ghasitola (20° 56': 80° 25') and Gotanpar (20° 53': 80° 29'). The sandstones and shales form a lower synclinal basin at Mehsuli (20° 58': 80° 18') (P. N. Phadtare 1964) and sandstones and shales of Vindhyan age have been reported by Sahasrabuddhe (1958-59) from Dhanori (20° 45': 79° 41') and Rewani (20° 46': 79° 41'). Bhattacharya (1935) reported an occurrence of conglomerate of Vindhyan age at 2 km. west of Dhanori. Small outcrops of sandstones referable to the Vindhyan age are noticed resting unconformably on the archaean matasediments in the Wainganga river section north of Kodurli (20° 54': 79° 46') Gajbhiye (1962-63).

Beds of Gondwana sediments referred to as Kamthi series comprising conglomerates, arkose and sandstones and lying un-conformably over the archaeans have been noticed from Wainganga and Chulband river section and near Ghapral (20° 42':79° 54') and Soni (20° 41': 79° 55').

The major parts of the Bhandara district are covered with alluvium which consists of Kankar soil and laterites.

Structure.— Repeated folding and faulting has complicated the structure of the rocks in the area. The Sausar series of rocks generally strikes NNE-SSW with occasional variation, some of this being due to the granitic intrusion. Synclinorial folds can be made out in places and isoclinal folding has affected the area in which two large manganese mines exist. The structure is in places marked by copious and intricate intrusion of granites, pegmatites and vein quartz.

The Sakoli series of rocks exposed in this district as a large triangular out crop strike in a direction of NE-SW and are more or less parallel to Sausar formation. The SSE dips of the southern margin of Sausar group rocks, and the NNW dips of Sakoli group rocks suggest the possibility of the presence of a synclinorium with a belt of disturbance along the axis. The less metamorphosed rocks of south belonging to the higher horizons have been thrusted on the older and more metamorphosed rocks to the north (E. Pascoe, 1950). But Sarkar (1964) from critical consideration of the stratigraphy, tectonics and metamorphism in the Sausar and Sakolis concluded that Sausars and Sakolis were deposited in separate basins under different conditions over a basement older than 1400 m.y. and had a different tectonic history, (ii) and that the Sausar may be stratigraphically younger than the Sakolis. Bhattacharya (1935) calls attention to the triangular arrangement (" Bhandara Triangle") which affects many aspects of the rocks of this triangular outcrop. This is seen in the lines of quartz veins, the foliation and disposal of intrusion. The foliation strike in this series around Sakoli is NE-SW but, further to the east, in the eastern margin of the Gaikhuri range it is NW, the rocks being here folded in a synclinorium. The smaller outcrops of Sakoli beds are thought to be the tips of synclinorium. Faulting is common in the main exposures and is oriented in two or three sets of fractures which are usually indicated by fault breccias.

The Dongargad system of rocks occupying the area between the Sakoli synclinorium in the west and the Chhatisgarh-Cuddaphs basin in the east had been subjected to one major phase of oro-genic movements causing generation of first phase of folds, post-Andesites in age, with the axes approximately NNE-SSW, and contemporaneously or at a later stage culmination and depression of these fold axes approximately along WNW-ESE have produced complex types of normal and inverted canoe folds. A number of diagonal faults, block faults, and normal faults have been traced. Along certain zones in the area, the rocks are sheared. The principal shear zone along which the rocks are tightly folded extends SSW-NNE for about 22 km.

The younger rocks overlying the archaean unconformably generally exhibit the presence of current bedding, ripple marks, cross bedding in the grits and sandstone, etc.

Economic Geology.—Of the varied mineral deposits of the district that have been taken up for exploitation by private mining concerns, the following economic minerals are important.

Manganese : The manganese ore belt of Bhandara district is principally made up of intensely deformed and metamorphosed rocks of the Precambrian Sausar series. The most important manganese deposits are associated with a series of rocks known as " Gondites" which are looked upon as metamorphosed man-ganiferous sediments of Dharwar age. Of all the occurrences of manganese deposits in Bhandara district, three manganese belts namely, (i) the Dongri-Buzurg, (ii) The Sitasaongi and (iii) The Chikla are most important and have been taken up for mining.

The Dongri-Buzurg deposit generally strikes NNE-SSW over a length of 600' and dips 50°—70° south. The maximum thickness of the ore body is 86' in the central part and the average thickness is 28'. The Sitasaongi deposit comprises four ore bodies, the southernmost are body having a maximum length of 1,650'. The thickness of the ore body varies from 40' to 140'. The Chikla belt to the north of the Sitasaongi deposits occurs in two limbs of a tightly folded syncline plunging 20° to 25° to the east. The length of the ore body is 2,500', having the average thickness of 8'.

The ore bodies are generally confined to the quartzites and quartz-muscovite-schists and are often well bedded parallel to the strike of the enclosing rocks, sometimes occurring as lenticular intercalations in the schists. The gondite forms the narrow conformable basal bed below the ore body of the abovementioned three belts.

The total measured and indicated reserves of 100' dip extension of all the three belts is put to 45,00,000 long tonnes. The estimates are for an average grade of ore containing 30 to 40 per cent of manganese.

Chromite: Next to Manganese chromite is occurring near Pauni (20° 57': 79° 39'). The chromite occurs as several small bands associated with dunite and serpentinite surrounded by the country rock granite (Sahasrabuddhe 1959). The chromite zone is about 3,000' wide near Wainganga river and is traceable over a distance of about 6,000' in WSW direction from the river; ore body varies from 6" to 10' in width; the average content of Cr2 O3 is 41.88 per cent (Sinha and Shrivastava, 1957). The reserves are estimated to be about ½ million tons of low grade ore (Sahasrabuddhe, 1959).

Kyanite-Sillimanite: The Sakoli series of rocks in Bhandara district also carry important deposits of kyanite-sillimanite associated with chlorite muscovite schists. Of these deposits, massive sillimanite deposits at Pohra occurring in a broad band of about 45 m. wide and extending over a length of 365 m. and the Dahegaon kyanite-sillimanite deposits extending over a length of nearly 4.5 km. in southerly direction up to Pipalgaon village are most important.

Besides the above two important deposits, there are also four or five belts of kyanite-sillimanite bearing rocks. At Girola the belt has a maximum thickness of 122'. Kyanite and sillimanite are also reported from Pardi, Mogra, Dighori, Garkhabhonga, and Miregaon villages.

Corundum: Corundum deposit has been reported at the foot hill of a small hillock at Pohra. Another tiny knoll has also been recorded about 1 km. north of the abovesaid hillock.

Gold: Occurrence of gold as placer deposits has been reported around Kokha village (21° 12': 79° 48') and other localities in Bhandara district.

Radio active minerals: The occurrence of uranium oxide from granophyres, 5 km. NW of Parsori (21° 12': 79° 48') traceable over a distance of about 1 km. has been reported by P. N. Phadtare (1963). The rock is highly fractured and encrustations of bright yellow colour uranium oxide are found along the fracture joints trending N 15° W dipping 70° E.

Iron Ore: The banded hematite-quartzites and the interbedded purple ferruginous phyllites have given rise to high grade lateritic hematite ore in small pockets towards east of Maselli. Small development of hematites has been noticed along the foliation of phyllites near Gogibherh (20° 46': 70° 33'), and Pahugaon (20° 47': 79° 31') (Sahasrabuddhe, 1959).

Besides the above mentioned minerals, other mineral deposits of lesser economic significance include the green mica near Mohal-gaon (21° 01' 30": 80° 02' 30") (Chattergee, 1932), talc and soap-stone in Sakoli tahsil and west of Kausi (P. Nag. 1960), and antimony and lead near Pauni (Sinha, 1950), etc.

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