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Geology
The area is mainly occupied by metamorphic and igneous rocks such as amphibolite, pyroxenite, gneisses, granites and charnockites traversed by pegmatite veins, basic dykes and quartz veins.
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Peninsular gneisses
Rocks belonging to this group are the most widespread in the area. They consist of a very heterogeneous mixture of different types of granites intrusive into the schistose rocks. The granitic rocks of Krishnagiri area could be divided into three types:
(i) Grey granite and granodiorite (Krishnagiri type)
(ii) White granite,
(iii) Pink granite (Closepet type)
Further divisions within the main rock types could be distinguished. Three such subdivisions are recognised in each rock type on the basis of the intensity of migmatisation (i) more or less pure granites usually the most massive in structure and definite in colour; (ii) the migmatites of the same granites composed of lenses, patches and bands of rocks of different composition; and (iii) patches and xenoliths of metamorphic rocks of basement with significant quantity of granitic materials.
Grey granite and Pink granite: The grey granite and pink granite are found to be uniform in their characters throughout the area.
Charnockites: Within the charnockite group, two types could be recognised in. the field.
(i) Banded basic charnockite, and
(ii) Acid charnockite. the basic charnockites· are essentially pyroxene granulites and variants, and the acid charnockites are an igneous suite.
White granite : The white granites of the area are not uniform in their character. Two varieties could be recognised on the basis of mineralogical composition. The first is biotite or hornblende-biotite variety mostly seen to the west of Krishnagiri. The second variety is white granite with hornblende or pyroxene-hornblende granite, seen east of Krishnagiri.
The results of the analyses of heavy concentrates, collected in this area, have, shown that zircon is one of the most widely distributed minerals. It is also observed that the quantity of zircon in heavy concentrates and the frequency of occurrence of high zircon content in stream sediments are remarkably more in area of white and. pink granites and migmatites.
Faults systems of the area:
It can be easily seen that the faults observed can be grouped into four systems:
(i) NW-SE faults
(ii) N-S faults
(iii) E-W faults
(iv) NE-SW faults
Among these four systems, the NW-SE faults are the predominant in the area. The following are the criteria based on which the faults are identified in the field.
However, no clear cut evidences as to the relative age of the fault systems cloud be noted. The basic dykes and pegmatites may be of use in the case of a few fault.
NW-SE Faults: The most predominant faults observed in the area are the faults which traverse the rocks in a NW-SE direction. These faults extend to a maximum of 33 miles in length. Crushed rocks and epidotisation are found in evidence of these faults; mylonites are also found along a river course east of Krishnagiri. Dislocation of a basic dyke and a zone of sulphide mineralisation due to the NW-S:S faults are also observed in some places. On a general apprehension the drainage pattern seems to follow the NW-SE fault lines mostly. As regards mineralisation, these faults are found to be mostly barren.
N-S Faults: Next to the NW-SE faults are the N-S faults. These faults can h~ traced up to a maximum distance of 28 miles. Mylonites and sheared rocks were found in these fault zones. Epidotisation is also found in some places. Dislocation of E-W basic dykes, in places, is observed in evidence of these faults. Some of the drainage pattern also follow these fault lines. These fault zones are more or less devoid of any mineralisation.
E-W Faults: The E-W faults of the area are mostly represented by the basic dykes which run to a maximum distance of 28 miles in an E-W direction. Epidotisation is also found, in places, in evidence of these fault zones. Mineralisation of sulphides is found to the east of Krishnagiri, and this shows clear evidence of hydrothermal action along the fault zone.
NE-SW Faults: The NE-SW faults can be seen NW, SE and SW of Krishnagiri. These faults extend up to a distance of 10 miles and more, and are parallel to the strike of the country rocks. The elongation of several hill masses in a NE-SW direction may be considered as an evidence for faulting. The Dharwarian schistos (rocks are affected by these faults. This system of faults gains importance in area south of Krishnagiri where some basic dykes are found to follow the fault lines and mineralisation of sulphides can be seen in some places.
