Chapter -2

GEOLOGICAL SETTING

17

Fig.2.1 Geological Map of Dharwar craton, India (modified after French 2007; Heaman, 2008).

Study Area

CHAPTER-2

GEOLOGICAL SETTING

2.1 INTRODUCTION

This Chapter deals with the regional geology, geological setting of the study

area, field relations of Peninsular Gneisses and U-bearing fracture zones in the

closepet equivalent basement granites, radiometric signatures and mylonitisation of

granites, field guides in locating fracture zones, views of the fracture zones

supplemented by satellite images. Field photographs and geological maps of each

fracture zone are also provided in this Chapter.

2.2 REGIONAL GEOLOGY

2.2.1 DHARWAR CRATON

18

Fig.2.2 Simplified geological map of southern Indian Shield (after GSI and ISRO 1994). Metamorphic isograds between green schist facies and amphibolite facies and between amphibolite and granulite fracies are after Pichamuthu (1965).

The area under investigation (Fig. 2.1) forms a part of the Dhrawar craton. The

Archaean – proterzoic Dharwar craton is made up of granite-gneiss-greenstone belts.

The craton occupies a little less than half a million sq. km area. It is bounded in the

south by the Neoproterozoic Southern Granulite Belt (SGT) or Pandyan Mobile

belt of Ramakrishnan (1993); in the north by the Deccan Trap (late Cretaceous);

19

in the northeast by the Karimnagar Granulite belt (2.6 Ga old) which occupies the

southern flank of the Godavari graben; and in the east by the Eastern Ghats Mobile

Belt (EGMB) of Proterozoic age (Fig.2.1). The boundary between the Craton and the

SGT is arbitrarily taken as Moyar-Bhavani Shear (M-Bh) Zone (Fig. 2.2), while the

boundary between the Craton and the EGMB is demarcated by the Cuddapah

Boundary Shear Zone. Besides these shear zones at the contact between the craton

and the stated terrains/belts, there are many sub-parallel NNW to N-S trending shear

zones within the main Dharwar Craton, mostly at the eastern boundaries of major

schist belts.

Swami Nath and Ramakrishnan (1981) divided the Dharwar Craton into

two blocks—the Western Block and the Eastern Block, separated by the

Chitradurga Shear Zone (ChSz) (Fig. 2.2). Later, Naqvi and Rogers (1987)

designated these blocks as Western and Eastern Dharwar Cratons. According

to Ramakrishnan (2003), the Chitradurga shear zone, separate the Eastern and

Western blocks of the Dharwar craton. Lithological variations, differences in

volcano-sedimentary environment, magmatism and grade of metamorphism, the

Dharwar Craton is recorded in these two domains (Radhakrishna and Vaidyanadhan,

1997).

2.2.1.1 Western Dharwar Craton

Western Dharwar Craton (WDC) is characterized largely by

supracrustals/schist belts, where as the Eastern Dharwar Craton (EDC) is

characterized by voluminous juvenile granites and remobilized gneiss containing

remnants and slivers of schist belts (Fig. 2.2). The Older Gneiss complex (Peninsular

Gneiss), of basically tonalitic–trondhjemitic–granodioritic (TTG) composition and

ranging in age from 3400 to 2900 Ma, forms the basement in the WDC (Bhaskar Rao

20

Fig.2.3 .Sketch map of the Western Dharwar Craton showing major lithologic

boundaries after Naqvi and Rogers (1987) and Ramakrishnan and Vaidyanadhan

(2008).

et al., 1991; Chadwick et al., 1997). The Late Archaean volcanic and sedimentary

rocks (Dharwar supergroup/schist belts) were deposited in the period 2900–2600 Ma

(Taylor et al., 1984; Nutman et al., 1996; Kumar et al., 1996). In contrast, the

Younger.

The Gneiss complex, mostly granodioritic and granitic in composition and

ranging in age from 2700 to 2500 Ma (Balakrishnan et al., 1990; Friend and Nutman,

1991; Nutman et al., 1996), forms the basement in the EDC. It hosts a series of linear

and irregular Dharwar Schist Belts consisting mainly of volcanic rocks. The

voluminous plutonic intrusion of K-granite suggests the Late Archaean juvenile

crustal accretion of the EDC (Jayananda et al., 2000).

21

The Western Block (Fig.2.3) is characterized by the presence of 3000

Ma old TTG (tonalite-trondhjemite-granodiorite) gneisses—the Peninsular Gneiss

Complex. The Peninsular Gneisses contain 3400–3580 Ma old basement tonalitic

gneiss enclaves (Nutman et al., 1992), and ancient Supracrustals designated by the

term Sargur Group (3000– 3200 Ma old) by Swami Nath and Ramakrishnan (1990).

The main Dharwar Schist Belts of the Western Block are: (1) Shimoga-Western Ghat-

Babubudan, and (2) Chitradurga (Fig. 2.2). Their western margin preserves the

depositional contact marked by basal quartz-pebble conglomerate and abundance of

platform lithologies, whereas the eastern contact is tectonized and marked by

mylonitic shear zones.

2.2.1.2. Eastern Dharwar Craton

The Eastern Dharwar Craton (EDC) is bounded to the north by the Deccan

Traps and the Bastar Craton, to the east by the Eastern Ghats Mobile Belt, and to the

south by the Southern Granulite Terrane (Fig.4) (Balakrishnan et al., 1999). The

craton is composed of the Dharwar Batholith (dominantly granitic), greenstone belts,

intrusive volcanics, and middle Proterozoic to more recent sedimentary basins (Naqvi

and Rogers, 1987; Ramakrishnan and Vaidyanadhan, 2008).The western boundary of

the Eastern Dharwar Craton (EDC) is poorly defined and is constrained to a 200 km

wide lithologic transitional zone from the Peninsular Gneisses of the Western

Dharwar Craton to the Closepet Granite (Fig. 2.4). The Closepet Granite is a good

approximation of the western boundary (Ramakrishnan and Vaidyanadhan, 2008).

Greenstone belts of the EDC are concentrated in the western half of the craton and are

stretched into linear arrays. The belts continue to the east where they are covered by

the Proterozoic Cuddapah Basin. The general trend of the belts is N–S and related

belts are classified into supergroups (Ramakrishnan and Vaidyanadhan, 2008).

22

Fig. 2.4. Sketch map of the Eastern Dharwar Craton. Archaean assemblages associated with cratonization are shown here. Abbreviations for schist belts are as follows:Sa = Sandur, KKJH = Kolar–Kadiri - Jonnagiri–Hutti, RPSH = Ramagiri–(Penakacherla–Sirigeri)–Hungund, and VPG = Veligallu–Raichur–Gadwal superbelt. Dyke intrusions into the EDC are HandB = Harohalli and Bangalore swarm; A = Anantapur swarm; M = Mahbubnagar swarm; H = Hyderabad swarm. (Modified from Naqvi and Rogers, 1987).

Metamorphism of the belts is generally limited to greenschist to amphibolite facies

with lower grades occurring in the larger belts and in the interior of smaller ones

(Chadwick et al., 2000). Balakrishnan (1990) used whole rock Pb/Pb dating to

constrain the age of the Kolar Schist Belt between 2900 and 2600 Ma. Nutman et al.

(1996) and Nutman and Ehlers (1998) used SHRIMP U–Pb zircon methods to obtain

23

ages of 2725–2550 Ma in the Kolar Belt. Age trends in the belts generally infer a

younging trend from west to east. These schist belts are all, to some degree, intruded

by syn- and post-tectonic felsic rocks (Chadwick et al., 2000). Some of the more

important greenstone schist belts of EDC include: the Kolar–Kadiri–Jonnagiri–Hutti

superbelt (KKJH), and the Veligallu–Raichur–Gadwal superbelt (VRG). the

Ramagiri–(Penakacherla–Sirigeri)–Hungund superbelt (RPSH), and the Sandur schist

belt. There is no recognizable basement to these schist belts because these belts

are engulfed on all sides by 2500–2600 Ma old granites and gneisses.

2.2.1.3 Dharwar Batholith

According to Narayanaswami (1970), there are numerous elongated granites

running parallel to the schist belts of the Eastern Block but they have not been fully

delineated on geological maps. These granites occur in a series of parallel plutonic

belts which have been collectively designated as Dharwar Batholith (Fig.2.5 a & b)

(Chadwick et al., 2000) in order to distinguish them from the widely used

term Peninsular gneiss, scarcely exposed in the EDC. Age constraints from the

WDC Peninsular gneisses suggest an early Archaean age, whereas the granitic

gneisses of the Dharwar Batholith are of Late Archaean age. The plutonic belts are

approximately 15–25 km wide, hundreds of km long and separated by greenstone

belts (described above). They trend NW to SE except for in the south where the trends

become predominately north–south. The belts are mostly mixtures of juvenile

multipulse granites and diorites, and are wedge-shaped with steep granitic dyke

intrusions (Chadwick et al., 2000; Ramakrishnan and Vaidyanadhan, 2008).

Geochronologic information for this unit comes from SHRIMP U–Pb zircon

measurements that constrain the emplacement of the Dharwar Batholith to 2700–

2500 Ma (Friend and Nutman, 1991; Krogstad et al., 1995; Nutman et al., 1996;

24

Fig..2.5a Model of cratonization of the Dharwar batholith. The WDC is believed to be the

overriding plate and foreland contribution. The Dharwar batholith represents a series of

juvenile granites sutured at the schist belts. (b) Superplume model for creating the granitic

gneisses of the EDC. The mantle transitions from an enriched area near the plume to a

depleted zone beneath the Kolar schist belt. Partial metling of the lower lithosphere

creates plutons that form the mainland of the EDC (Modified from Jayananda et al.,

2000).

25

Nutman and Ehlers, 1998). Ages for granitic units appear to decrease from west to

east; however, gneissic protolith ages of >2900 Ma are inferred from inherited zircons

within younger dykes near Harohalli intruding the gneissic rocks (Pradhan et al.,

2008).

The Dharwar Schist Belts have N-S trend and show a gradual increase of

metamorphic grade from N to S. The metamorphic isograd between greenschist facies

(low grade) to amphibolite facies (medium grade) is defined by a line (outcome of

intersection of isothermal surface with the ground surface) on a regional

geological map.(Fig. 2.2).

The Opx-in isograd between the amphibolite and granulite facies is not a line

but a zone of up to 30 km wide in which mineral assemblages of both

amphibolite facies and granulite facies (characterized by the presence of hypersthene

(a mineral of orthopyroxene group) are found together. This zone is called the

Transition Zone (TZ). It is in this zone that excellent outcrops of "charnockite in-

making" (Pichamuthu, 1960; Ravindra Kumar and Raghavan, 1992 and 2003) or

arrested charnockite (also called incipient charnockite) are seen to have formed from

gneisses due to reduced water activity at same prevailing P and T conditions (Peucat

et al., 1989). From the disposition and wavy nature of the isograds, it is inferred that

the isothermal surfaces had gentle inclination towards north.

Further south of the TZ occurs the main granulite terrane, called the Southern

Granulite Terrane (SGT) (Fig. 2.2), where massive charnockite-enderbite rocks

dominate amidst high-grade supracrustals.

26

2.2.1.4 Mafic Magmatism

The Dharwar craton experienced widespread mafic magmatism during the

Proterozoic along with the intrusion of Meso-Neoproterozoic diamondiferous

kimberlites and lamproites (Rao and Pupper, 1996; Murthy and Dayal, 2001;

Chalapathi Rao et al., 2004). In addition to the kimberlites and lamproites, there are

other mafic intrusives including metadolerites/ metanorites, tholeiitic and alkali-

olivine basaltic dykes forming dense E–W and NNW to NW trending swarms

crosscutting the greenstone and gneissic basement of the Dharwar craton (Murthy et

al., 1987; Kumar and Bhalla, 1983; Radhakrishna and Joseph, 1996; Chalapathi Rao

et al., 2005).

Many of the clusters occur around the Cuddapah Basin and have three main

trends: NW–SE, E–W, and NE–SW. These trends are associated with various

paleostress orientations during the Proterozoic to Late Cretaceous (Srivastava and

Shah, 2008). Most of the dykes disappear beneath the Cuddapah Basin, indicating that

intrusion of the host granitic gneiss took place before the basin developed. These

dykes all formed after the migmatitic activity of the host granitoids and are virtually

free of any effects of metamorphism and deformation (Chakrabarti et al., 2004). Five

major dyke clusters of the EDC are identified which include: (1) Hyderabad, (2)

Mahbubnagar, (3) Harohalli/Bangalore, (4) Anantapur and (5) Tirupati (Fig. 9).

The Hyderabad cluster is located to the north of the Cuddapah Basin (Fig

No2.1). Widely spaced NNE–SSW to N–S trending dykes traverse ENE–WSW and

WNW–ESE oriented dykes. The majority of the dykes present are doleritic in

composition (Murthy, 1995). Whole rock K–Ar ages of local dykes indicate

emplacement between 1471 ± 54 Ma and 1335 ± 49 Ma (Mallikarjuna et al., 1995),

27

but these may reflect a younger isotopic disturbance as at least some of the dykes in

the Hyderabad cluster may be related to either the 1.9 Ga swarm in the Bastar Craton

(French et al., 2008) or the ~2.2 Ga swarm near Mahbubnagar (French et al., 2004).

The Mahbubnagar dyke swarm located to the NW of the Cuddapah Basin and

intrudes local granitic gneisses with Rb–Sr ages of 2.5–2.4 Ga and 2.2–2.1 Ga. The

mafic dykes are predominantly gabbroic; however, dolerite and metapyroxenite are

also present. They are oriented NW–SE and can be up to 50 km long and 5–30 m

wide. Chilled margins are common with coarse aphyric or plagioclase-rich interiors.

Pooled regression results from Sm–Nd analysis gives an emplacement age of 2173 ±

64 Ma (Pandey et al., 1997). These results are duplicated by French et al. (2004), who

obtained ages of ~2180 Ma using U–Pb techniques on nearby dykes. In light of the

Sm–Nd and U–Pb ages for the dykes, it appears that the 2.2–2.1 Ga Rb–Sr ages cited

above for the gneisses in the region may reflect disturbance due to dyke intrusion.

The Anantapur dyke swarm located just west of the Cuddapah Basin is and the

Tirupati swarm is confined to south of the Cuddapah basin is (Fig.2.1). The NE–SW

and ENE–WSW oriented dykes of the Anantapur swarm are dated using K–Ar

measurements and are poorly constrained between 1900 and 1700 Ma and 1500 and

1350 Ma, respectively (Murthy et al., 1987; Mallikarjuna et al., 1995). More recently,

Pradhan et al. (2010) dated the NE–SW trending ‘Great Dyke of Bukkapatnam’ of the

Anantapur swarm to 1027 ± 13 Ma using U–Pb methods suggesting either a third

phase of intrusion or more probably excess argon not discerned in the K–Ar data.

Dykes in the Tirupati swarm show two trends, the dominant trend is E–W and there

are subordinate NW–SE trending dykes. There are K–Ar and Ar–Ar age

determinations on dykes in the Tirupati swarm. The E–W trending dykes have K–Ar

ages of 1073 and 1349 Ma and one Ar–Ar total fusion age of 1333 ± 4 Ma. NW–SE

28

trending dykes have K–Ar ages of 935 and 1280 Ma (Mallikarjuna et al., 1995).

Although there is a bit of agreement between two of the E–W dyke ages at ~1340 Ma,

there was no clear plateau in the argon spectra making it likely that the reported K–Ar

ages are integrating multiple episodes of disturbance.

Radiometric data on these doleritic dykes suggest at least three major episodes

of dyke emplacement in the region that occurred at 1.9–1.7, 1.4–1.3 and 1.2–1.0 Ga

(Murthy et al., 1987; Padmakumari and Dayal, 1987; Mallikarjuna Rao et al., 1995;

Chatterji and Bhattacharji, 2001). A baddeleyite age of 1885.4±3.1Ma for the

Pullivendla mafic sill from the Cuddapah basin has been interpreted to coincide with

the widespread 1.9 Ga basaltic magmatism occurred in the widely separated Large

Igneous Provinces (French et al., 2008)

2.2.1.5. Closepet Granite

The Closepet granite is located on the western margin of the EDC (Fig 2.1)

and is a linear feature trending ~N–S. The granite is 400 km long and approximately

20–30 km wide with shear zones on both sides. Recent studies suggest that the similar

convexity of adjacent schist belts and granitic plutons may indicate that the Closepet

Granite is a ‘stitching pluton’ formed during the suturing of the Eastern and Western

Dharwar Cratons (Ramakrishnan and Vaidyanadhan, 2008). The exposed rock is

divided into northern and southern components by a part of the Sandur Schist Belt;

however, both sections appear to be lithologically similar at the outcrop level (Naqvi

and Rogers, 1987). The Closepet Granite is dated to 2513 ± 5 Ma (Friend and

Nutman, 1991) and appears to be part of a widespread Neo-Archaean phase of

plutonism (Moyen et al., 2003) in both the Eastern and Western Dharwar Cratons that

we consider to mark the stabilization age for the WDC and EDC.

29

Balakrishnan et al. (1999), Manikyamba et al. (2005), and others propose that

the eastern portion of the Dharwar Craton formed as a result of island arcs accreting

to an older (>3500 Ma), solid western craton through transpression. The linear schist

belts represent back-arc basin environments that were metamorphosed during the

accretion. The Eastern Ghats Mobile Belt is thought to mark the closure point during

amalgamation of the EDC. Chadwick et al. (1996, 1999, 2000) suggest a similar idea;

however, their model involves the ‘Dharwar Batholith’. This idea suggests that

already formed island arcs and granitic plutons constituted a single landmass. The

Dharwar Batholith then obliquely converged with the WDC causing sinistral

transpressive shear systems along the margins. Greenstone belts developed as a result

of intra-arc basins associated with the batholith. The proposed timing of this collision

history is between 2750 and 2510 Ma. These models suggest that the Closepet Granite

was accreted onto the WDC, in contrast with the arguments presented below.

Jayananda et al. (2000) and Chardon et al. (2002), propose that the most likely

mechanism of formation of the EDC was through vertical tectonics. The plume model

suggests a large mantle plume situated just beneath the EDC/WDC boundary in an

enriched mantle. Further east, the plume began the melting of a colder and more

depleted mantle. Induced melting from the plume is suggested to emplace juvenile

magmas around 2500 Ma in the EDC. In this model, the greenstone belts result from

inverse diapirism and resulting metamorphism. In this case, the Closepet granite is

simply a batholith rather than an accreted island arc or a stitching pluton between the

EDC and WDC.

30

2.3 GEOLOGICAL SETTING OF THE STUDY AREA

The area around Lakkireddipalle (Fig.2.6) in the southwestern margin of the

Cuddapah basin where the fracture-controlled uranium mineralisation occurs, is

largely occupied by the granitoids which is emplaced by dolerite dykes and dissected

by fracture system. Geomorphologically two phases of granitoids are identified.

Peninsular gneisses form the peneplane area where as younger granitoids of Closepet

form ridges and mounds. The younger granitoids are of two types with pink and grey

colours. There is no distinct boundary between the two. These younger granites akin

to Closepet granite and are more potassic and intrusive in nature and contain higher

uranium and host uraniferous / fracture zones.

2.3.1 PENINSULAR GNEISSES

Peninsular gneisses form the peneplane area (Fig. 2.7) where as the younger

granites form the ridges and mounds. There is no distinct boundary between the two.

The granite gneiss crops out in the eastern and western part of the study area.

Exposures are small and tend to be limited to the top of the weathered hill slopes. The

rocks are mostly medium- to coarse-grained grey granite (Fig. 2.8), comprising

quartz, microcline and oligoclase. Muscovite, biotite, chlorite, epidote, occur in

small but varying proportions. At places they are migmatitic and with strong

gneissosity. Amphibolites (Fig.2.9) are the commonest enclaves within the granite

gneiss. These dark coloured rocks, made up of green hornblende prisms and quartz,

show a xenoblastic granular texture. A few dolerite dykes traverse the gneisses.

31

Fig.2.7. Field photograph showing topographic expression of Peninsular gneisses.

32

79° 00'45'30'15'78° 00

15'

'

GENARALISED GEOLOGICAL MAP OF LAKKIREDDIPALLE AREAKADAPA AND ANANTPUR DISTRICT, ANDHRA PRADESH

PART OF TOPOSHEET NO. 57 J5 km 2.5 0 5 km2.5

URANIFEROUS FRACTURE ZONE

QUARTZ REEF

PENINSULAR GNEISSGREENSTONE BELTGRAY/PINK GRANITE

CUDDAPAH SEDIMENTS

(CLOSEPET PHASE)

INDEX

15'

14°14°

79° 00'45'30'15'78° 00

VEERABALLE

PULLIVENDLA

VEMPALLE

CUDDAPAH

RAYACHOTI

BIRJUPALLE

CHAKRAYAPETTATALPULA

KADIRI

VELLIGALU

DORIGALLU

GONDIPALLE

LAKKIREDDIPALLE

KAMAGUTTAPALLET. SUNDAPALLE

CHENCHELAPALLE

MULAPALLE

VARIKUNTAPALLE

SAMPLE LOCATION

STUDY AREA

Fig.2.6. Generalised geological map of Lakkireddipalle area, Kadapa district, Andhra Pradesh.

33

Fig

Fig.2.8. Field photograph showing grey granites-Peninsular gneisses.

Amphibolite

Granitic Veins

Fig.2.9. Field photograph showing amphibolite enclaves-Peninsular Gneisses.

34

2.3.2. CLOSEPET GRANITE EQUVIVALENTS

The younger granite plutons of Closepet granite equivalents form isolated

nearly circular to elongated masses of high and rugged relief (Fig. 2.10). They are

emplaced as steep-sided plutons and many of them record anomalous radioactivity.

The younger granitoids are of two types with pink clour (Fig. 2.11) and grey colour

(Fig.2.12). There is no distinct boundary between the two. Micro-granite phase

occurring as mounds have been identified in this area by Dhanaraju et al. (2001). The

studied granites are generally in lower areas are weathered and locally covered by

granitic soil. These granites are also marked by syn-magmatic aplitic veins (Fig.2.13),

xenoliths of older units (Fig.2.14).They acquire pink to red colours and range in

composition from granite to granodiorite to alkali feldspar granite. The plutons are

unfoliated.

Granites are medium to coarse- grained, dominantly equigranular, but in some

places pinkish potash feldspar and quartz may develop as larger phenocrysts giving

the rock a local porphyritic texture. The main mineral constituents are quartz, K-

feldspar, biotite and some iron oxides. The granite is typically leucocratic with

quartzo-feldspathic phases>90%.The voluminous plutonic intrusion of K-granite is

attributed to the Late Archaean juvenile crustal accretion of the EDC (Jayananda et

al., 2000). The granitoid terrain is criss - crossed by number of basic intrusives mainly

of doleritic composition.

35

Fracture zone

Fig.2.10. Field photograph showing topographic expression of Closepet granite.

36

Fig.2.12 Field photograph showing exposure of grey granite in a quarry in Vaddepalle village near Konampeta.

Fig.2.11 Field photograph showing exposure of pink granite in a quarry near Chakrayapeta Village.

37

2.3.3. FRACTURE SYSTEM IN THE BASEMENT GRANITOIDS

Geophysical and remote sensing studies have indicated several lineaments,

deep-seated faults and three sets of major fracture systems viz. ENE-WSW,NNE-

SSW and NW-SE (Geol.Surv. India 1994). ENE-WSW systems are more pronounced

where NNE-SSW in the eastern part NW-SE in the wetern part of Lakkireddipalle are

well represented. This may be attributed to the margin of the Cuddapah basin. In a

majority of these fractures, the deformed variants of the granitoids in the form of

cataclasite, mylonites and phyllonites are noticed (Zakaulla et al., 1995). Both the

fractures and dykes appear to be of pre-Cuddapah age. Deformation, fracturing and

brecciation due to closely spaced faulting is common among granitoids of Closepet

granite equivalents especially at the margin of the Cuddpah Basin around

Lakkireddipalle (Fig.2.15 & 2.16). Quartz reefs emplacement in granite with NW-SE

trends parallel to the regional structural trend (NW-SE) are seen near Kadiri.

The granitoid basement is overlined unconformably by Cuddapah sediments in

the northern part. Gulcheru quartzite representing the basal unit of Papaghni Group of

sediments commences with basal conglomerate with non-confomity contact

(Fig.2.17).

38

Fig.2.13. Field photograph showing exposure of pink granite with synmagmatic aplitic veins.

Fig.2.14. Field photograph showing exposure of 14 amphibolite xenoliths in pink granite.

39

Fig.2.16 Field photograph of exposure showing Faulting in pink granites.

Fig.2.15 Field photograph of exposure showing Faulting in pink granites.

40

Basement granite

Cuddapah Sediments

Fig.2.17 Field photograph showing the unconformity contact between Gulcheru quartzite and the granitoid basement.

41

2.3.4. GEOLOGICAL FIELD GUIDES IN LOCATING FRACTURE ZONES

The field and the geological guides for exploration of fracture controlled type

of uranium mineralisation in the south-western margin of the Cuddapah basin can be

broadly grouped in to three general categories, viz.

(1) Topographic guides

(2) Mineralogical guides and

(3) Structural guides.

2.3.4.1. TOPOGRAPHICAL GUIDES

Fracture zones in the granitoid basement of southwestern margin of the

Cuddapah basin occur as continuous rectilinear ridges (Fig.2.18 & 2.19). These

fracture zones have a characteristic association of siliceous zone (Zakaulla et al.,

1998). Since these siliceous zones are more resistant to weathering than the associated

host rock, due to differential erosion they stand out as rectilinear ridges as the host

rock is more easily removed in erosional process. The fracture zones are developed

in both the Peninsular Gneisses and the Closepet granite. The fracture zones

developed in areas of Peninsular gneisses are well pronounced due to the low relief

of Peninsular gneisses, and since Closepet granite in the southwestern margin of the

Cuddapah basin occur as N-S trending hillocks and mounds within the Peninsular

gneisses, and the fracture zones developed in these rocks form less pronounced

ridges on these hills and mounds. It has been found that the fracture zones developed

within the Clospet granite are more favourable in hosting uranium mineralisation than

the ones developed in the Peninsular gneisses (Zakaulla et al., 1995 & 1998).

Thereby using the topographical guides, targets for finding fracture controlled type of

uranium mineralisation can be narrowed down to areas with Clospet granite.

42

T. Sundupalle – Sanipaya Fracture

Kamaguttapalle Fracture

Fig.2.18. Satellite image showing T. Sundupalle fracture zone as rectilinear ridge.

Fracture zones in Lakkireddipalle area

Fig.2.19. Satellite image showing number of fracture zones in the study area.

43

2.3.4.2. MINERALOGICAL GUIDES

Fracture zones form conduits for passage of the hydrothermal solutions.

When these solutions invade the rock, the chemical and mineralogical readjustment

the rock undergoes results in the alterations along these fracture zones. These

alterations can be identified based on colour, texture and their spatial relation to the

fracture zones. The alterations observed are mainly haematitisation (Fig.2.20),

silicification (Fig.2.21), chloritisation, sericitisation, apatitisation and argillisation(

Zakaulla et al., 1998). These are not necessarily associated with all the fracture zones,

rather a combination of two or rarely three are prevalent in a single fracture zone.

They are mainly confined to the fracture zone and do not extend much laterally away

from the contact of the fracture zone. It has been found that Uranium concentration

increases with the increase in the intensity of alteration. Thus the alterations

associated with the fracture controlled uranium mineralisation provide an important

insight into the chemical processes responsible for the uranium mineralisation.

Silicification is represented by a band of recrystallised quartz veins ranging from a

cm thick to as thick as 25 m depending on the intensity of the mylonitisation in the

fracture zone. Fractures filled with Chlorite runs just parrallel to the this silicification

alteration., where as alterations like sericitisation and argillisation occur as

discontinous lenses. Alterations like haematitisation is recorded with charecteristic

colour of haematite where as alterations like apatitisation can be seen under the

microscope only.

44

Fig.2.20. Field photograph showing haematitisation along fracture zone in the granite.

Fig.2.21. Field photograph showing silicification along fracture zone within the granite.

45

2.3.4.3. STRUCTURAL GUIDES

The Archaean basement comprising granite and gneisses are traversed by

different set of fracture zones involving dynamothermal and dislocation

metamorphism in the host rock. Imprints of deformation is evidenced by development

of cataclasite and mylonite. The best method of observing fractures on the surface is

by stereoscopic view of aerial photographs and study of satellite imageries. It has

indicated three major sets of fracture system viz. ENE-WSW, NNE-SSW and NW-

SE, with the first one being closely spaced. Based on the field relationship on a local

scale ENE-WSW appears to be the oldest . Most of these fractures are intruded by the

basic dykes. Both the basic dyke and the fracture zones are pre-Cuddapah and extend

below the Cuddapah sediments without disturbing the cover rocks. Drury (1984) is of

the view that N-S extensional forces followed by N-S compression is responsible for

the development of these basement fractures that are responsible for the development

of the Cuddapah Basin. Deformation signatures are seen in most of the fracture zones.

Intensity of deformation is much more pronounced along ENE-WSW fractures. It is

indicated by the development of cataclasite, mylonite and phyllonite. The fractures

transecting the Closepet granite appears to have a higher concentration of uranium.

This can be seen in the fracture zones in Nagarai- Kuravapalle in the east to

Tapetavaripalle in the west. The blocks within these fracture zones exhibit large scale

metamorphic (green schist effect) and metasomatic effects indicated by conspicuous

alterations like chloritisation, sericitisation, haematitisation, apatitisation etc.

46

2.3.5. U-BEARING FRACTURE ZONES

Though about 68 fracture zones associated with uranium mineralisation are

reported, but for the present study a total of nine fracture zones are taken up for a

detailed investigation. Chkrayapeta fracture zone forms the western and Sundupalle

fracture zone the eastern boundary of the study area (Fig.2.6). Most of the fracture

zones fall in the central part of the investigated area. The fracture zones at Mulapalle

and at Sundupalle area records very conspicuous signatures of uranium mineralisation

in response to deformation and alterations. Overall, the lithostructural features like

mylonites, phyllonites and sheared granites exposed in areas of Chenchelapalle,

Burjupalle and Timmareddigaripalle fracture zones records features more or less

comparable with Mulapalle fracture zone, whereas the fracture zones at

Kamaguttapalle and Varikuntapalle fracture zones are comparable with T.

Sundupalle fracture zone. Fracture zones at Chkrayapeta, Palagundam gollapalle and

Bidiki are of similar type.

2.3.5.1. MULAPALLE FRACTURE ZONE

The Mulapalle fracture zone (Fig.2.22) trends in N70°E - S70°W that is

manifested by the trend of shear schitosity. The shear zone is about 1.5 km long and

has an average width of about 25 meters. The exposed radioactive outcrop of the

mineralised shear zone extends for about 400 m with a width of 1-50 m. At the

northeastern end of the radioactive outcrop are exposed unsheared massive granite

where as the southeastern end is covered by cultivated fields. The exploratory drilling

in the soil covered southeastern end has indicated not only the extension of the

fracture zone but also the continuity of the mineralised bands. The deformation zone

varies in width from 1 m to approximately 50 m. They are planar and have sub

parallel boundaries and are characterized by penetrative foliation defined primarily by

47

phyllosilicates. Narrow zones are single shears, where as the wider areas contain

several high strain zones with intervening relatively undeformed zones characterized

by low strain. Shear zone boundary is sharp and are characterized by penetrative

schistosity and an increase in abundance of phyllosilicates mostly chlorite and

sericite. Low strain rocks within the deformation zone preserve original texture.

In Mulapalle area, the protolith is predominantly granite to granodiorite

although local variation from granite to granodiorite is visible. Texturally, the rock is

usually inequigranular, medium-grained but it is locally fine- or coarse-grained and,

rarely, porphyritic.

A detailed map of the area under study (Fig.2.23) illustrates the structural

heterogeneity of the Mulapalle fracture zone. On the basis of field investigation and

the macroscopic features of the fault zone, at the outcrop scale the fracture zone is

divided into three domains on the basis of observed macroscopic structures (Fig.2.24).

The domains are distinguished predominantly by overall differences in degree of

deformation, but each is internally heterogeneous.

Domain # 1 represents the protolith which is homogeneous and undeformed rock

(Fig.2.24).

Domain # 2 is the most heterogeneous and least deformed (Fig.2.25). It

shows fractures which are randomly distributed. They are cut across by each other,

forming networks of fracture zones. The contact between domain-1 and 2 is irregular.

Domain #3 is characterized by development of cataclasite, like autoclatic

conglomerate (Fig.2.26) and breccia (Fig.2.27)

Domain # 4 include mylonite to ultramylonite (Fig 2.28). They occur as

discontinuous, a meter scale lens to 50 meter long pods. They show sharp contact

48

with the protolith. With increasing strain the amount and sizes of fine-grained matrix

increase and clasts of primary rocks decrease.

Post mylonite deformation is also evidenced by the brecciated mylonite

(Fig.2.29). It represents youngest phase of deformation.

A continuous change in both mineralogical and microstructural properties of

the granites can be followed across the fracture zone (Fig.2.30) while maximal strain

is seen in mylonites. The most obvious parameters, which record the variation, are

the following:

1. Decrease in average grain size

2. Replacement of biotite by chlorite in the granite (Fig.2.31)

3. A mylonitic foliation replacing granitic/gneissic texture/structures

Based on these features the rock column was divided into three rough

intervals. The altered granite zone is only hardly deformed and the original textural

features are clearly preserved. Second zone is brecciated granite zone with strained

rocks characterized by intense fracturing with no pulverisation and it is borderd by

cataclastic rocks indicated by the occurrence of autoclastic conglomerate and

ferruginous breccias. Third zone is mylonite/phyllonite which exhibits a pure

mylonitic texture.

49

Fig.2.23. Geological map of Mulapalle Fracture zone, Kadapa district, Andhra Pradesh.

100m 0 100m

GEOLOGICAL MAP OF MULAPALLE FRACTURE ZONEKADAPA DISTRICT, ANDHRA PRADESH

TOPO SHEET NO. 57J/12

N

QUARTZ VEIN (BRECCIATED & SILICIFIED)

DYKE

MYLONITE/PHYLLONITE

BIOTITE GRANITE (MASSIVE & UNSHEARED)

FAULTS

INFERRED & FIRM LITHOLOGICAL CONTACTSBOREHOLE

LEGEND

SHEARED GRANITE

F F

FF

50

Fig.2.22. Field photograph showing panaromic view of Mulapalle fracture zone.

Fig.2.24. Field photograph showing protolith granite (Domain-1) in Mulapalle fracture zone.

51

Fig.2.26. Field photograph showing autoclastic conglomerate (Domain- 3) developed in the granite along the fracture zone in Mulapalle area.

Fig.2.25. Field photograph showing fractured granite (Domain-2) in Mulapalle fracture zone.

52

Fig.2.27. Field photograph showing brecciated granite (Domain-3) in Mulapalle Fracture zone.

Fig.2.28. Field photograph showing Mylonite lens (Domain-4) in Mulapalle fracture zone.

53

Fig.2.29. Field photograph showing brecciated mylonite in Mulapalle Fracture zone.

Chloritised granite

Chlorite- haematite Mylonite

Fig.2.31.Field photograph showing alterations of the granite in Mulapalle fracture zone.

Altered granite

Brecciated granite

Mylonite

Autoclastic conglomerate

Fig.2.30Fig.2.230. Field photograph showing transformation of granite to cataclasite to

mylonite across the fracture zone in Mulapalle area.

54

2.3.5.2. T.SUNDUPALLE-SANIPAYA FRACTURE ZONE

The uranium mineralisation in T.Sundapalli- Sanipaya area is controlled by

NNE-SSW fracture system (Fig.2.26). It starts in the south near Kanchaputtapalli (lat.

130 59'5”: long. 780 58' 20”) and extends in north-easterly direction for about 16 km

upto the basement of Nagari quartzite near village Sanipaya. The geomorphological

expression of this fracture zone is as linear curved or irregular ridges (Fig.2.33). In the

southern portion, the trend of the N280E 300E which changes to N150 E - N 170 E in

the north near the sedimentary contact. The fracture zone dips steeply towards north-

west (Fig.2.34 and 2.35). The surface field manifestation of the fault/fracture zone is

the presence of breccia, mylonite and abutment of dyke rocks against the fracture

zone.

Sundupalle Fracture Zone

Fig.2.33. Satellite image showing Sundupalle fracture zone.

55

The granite is light grey, coarse to medium grained, hypediomorphic, non-

porphyritic, and shows signatures of cataclasite deformation. The mineral

assemblages include saussuritised plagioclase, subequal portions of K-feldspar and

interstitial quartz. Zircon, monazite, sphene, apatite, anatase, chalcopyrite and

hematite constitute the accessories. The basement granites are traversed by dolerite

dykes. The dykes show two trends viz., NE-SW and E-W where latter one is

predominant. Dolerite dykes with sheared and chloritised margins are common within

the area. In some places, the contact between the dykes and granites is faulted and

shows the development of cataclasites (breccia and mylonite). Dykes show angular

relationship with the fracture zone. The dykes in the hanging wall side make an acute

angle towards N 300 E. Similarly, dykes in the footwall side towards S 300 W may be

indicating strike slip movement.

In the southern portion near Kanchaputtapalli village, field examination of the

fracture zone outcrop (Fig.2.36) shows progressive changes in the associated faulted

rocks from west to east across the fracture zone. The changes are very conspicuous in

the core samples from the boreholes. The sequence consists of -

Granite with biotite

Granite with biotite and chlorite

Granite with chlorite

Chloritic granite

Granite with clast of mylonite

Mylonite with clast of silica

Mylonite

Granite with clast of mylonite

Sheared granite

56

The normal light grey biotite granite shows light green to yellow green

colouration due to alteration of biotite to chlorite closer to the fracture zone.

Immediately in the vicinity of the fracture zone, due to intense alteration, the granite

is almost completely altered to green colour rock. This feature is very clearly

observed in the lithological examination of the core from the bore holes drilled in

T.Sundapalli sector. Silicification and haematitisation of granite are conspicuous,

especially; immediately at the contact with fracture zone.

The mafic alteration is normally found in the crush zones and joints in the

granites. The normal granite grades over a distance of few mm to 1 cm into a fine

grained dark grey to black rock, which can be seen on a radioactive granite outcrop

near K.R. Lake, about 300 metres east of mineralised fracture zone. This type of

alteration in the granite is also very well envisaged in the hanging wall side of the

fracture zone.

57

N

TSU-19TSU-18

TSU-17

TSU-16

TSU-15TSU-14

TSU-13

TSU-12

TSU-1

TSU-22

TSU-3TSU-2 TSU-4TSU-5 TSU-6 TSU-8

TSU-9

TSU-11

TSU-10

TSU-7

BRECCIA/ MYLONITE (RADIOACTIVE)

BASIC DYKE

GRANITE

BOREHOLE

INDEX

80 160 m0

GEOLOGICAL MAP OF T.SUNDUPALLE FRACTURE ZONET.S.No. 57 J/16 & K/13

KANCHIPUTLAPALLE

T. SUNDAPALLE

TSU-20 TSU-21

TSU-1

Fig.2.32. Geological map of T. Sundupalle Fracture zone, Kadapa district, Andhra Pradesh.

58

Fig.2.34. Field photograph showing rectilinear ridges of Sundupalle fracture zone.

Basic dyke

Mineralised Fracture Zone

Fig.2.35. Field photograph showing the steeply dipping curvicular mineralised fracture zone. Note the presence of basic dyke.

59

Granite

Siliceous Breccioa

Mylonite

Fig.2.36. Field photograph showing progressive changes from protolith granite to siliceous breccia and then mylonite across the fracture zone.

60

2.3.5.3. CHENCHELAPALLE FRACTURE ZONE

Chenchelapalle fracture zone (Fig.2.37) is located 10 kms NNW of

Lakkireddipalle and 1.0 km south of Chenchelapalle village. This fracture zone

forms a very low lying ridge (Fig.2.38) extending roughly 1.5 km in strike

length with a width varying from 5 to 10 m. It trends in ENE – WSW to E – W

with subvertical dips and is dissected by N40oE - S40oW trending basic dyke.

Fracture zone rock comprises of mylonite, phyllonite and sheared granites with

the contact of unaltered granites on either side. Uranium mineralisation in this

fracture zone has been established over 250 m of strike length with a width

ranging from 1.0 m to 5.0 m. N30oE - S30oW, E – W and N10oW - S10oE are

the common joints present in the fracture zone. Slickensides and drag folds are

present in the highly sheared zone.

Fractured and sheared basic dyke of N40oE - S40oW trend also show

intermittent radioactivity of the order of 2 – 5xbg and at places upto 20xbg

over 400 m of strike extent with spotty and patchy nature. The southern

extension of same basic dyke show the similar radioactivity in Laxmipuram,

and it followed the same trend upto Varikondapalle.

Study of the outcrops in this fracture zone shows similar deformation and

lithostructural features (Fig.2.39, 2.40 & 2.41) as noticed in the Mulapalle Fracture

Zone. The lithotypes recorded include sheared granite, mylonites and phyllonites.

These are more or less comparable with Mulapalle fracture zone rocks.

61

Fig.2.37. Geological map of Chenchalapalle Fracture zone, Kadapa district, Andhra Pradesh.

N

GEOLOGICAL MAP OF CHENCHALAPALLE FRACTURE ZONECUDDAPAH DISTRICT, ANDHRA PRADESH

T.S.No. 57 J/12

DOLERITE (RADIOACTIVE)

GRANITE

RADIOACTIVE BAND (>4xbg)

SHEAR / FOLIATION

BOREHOLE WITH DIRECTION & INCLINATION

INDEX

85°

CLP-6

CLP-7(50°)RL 408.12m

CLP-9(50°)RL 419.02m

CLP-8

CLP-1

CLP-5

SHEARED GRANITE (RADIOACTIVE)

CLP-4

CLP-3

CLP-2

F

F

F

F

F F FAULT / JOINTS

m 20 0 40 80 m

Roa

d

62

Fig.32 Fig.33

Fig.2.38. Field photograph showing fracture zone which forms a very low lying ridge of granite with 1.5 km strike length and 5 to 10m width.

Fig.2.38

63

Fig.2.40

Fig.2.40. Field photograph showing brecciated granite (Domain-2) in Chenchelapalle fracture zone.

Fig.2.39.Field photograph showing Protolith granite (Domain-1) in Chechelapalle fracture zone.

Fig.2.39

Fig.2.41

Fig.2.41.Field photograph showing brecciated granite (Domain-3) in Chenchelapalle fracture zone.

64

Fig-34

Fig-35

2.3.5.4 CHAKRAYAPETA FRACTURE ZONE

Chakrayapeta fracture zone is located on the eastern bank of Papagni river between

Gandi and Chakrayapeta villages. This fracture zone forms a very low lying

ridge (Fig.2.42) extending roughly 750 m in strike length with width varying

from 2 to 20 m. It trends in ENE – WSW to E – W with subvertical dips (Fig.

2.42a). Fracture zone rock comprises of sheared granites and ferruginous quartz

breccia. Alterations like haematitisation (Fig. 2.43 & 2.44) and silicification are very

conspicuous.

Fig.2.42. Field photograph of Chakrayapeta fracture zone which forms a very low lying ridge of granite.

65

Fig.2.42a. Geological map of Chakrayapeta Fracture zone, Kadapa district, Andhra Pradesh

GEOLOGICAL MAP OF CHAKARYAPETA FRACTURE ZONELAKKIREDDYPALLE AREA, CUDDAPAH DISTRICT, ANDHRA PRADESH

T.S.No. 57 J/12

SURVEY STATIONE

SOIL COVERGRANITEBRECCIATED/ SHEARED GRANITERADIOACTIVE ZONEMINERALISED ZONEFOLIATION/JOINTSSHEAR/ FOLIATION

INDEX

85°

T-18 T-17

T-21 T-20

T-19T-1

T-2

T-4

T-5

T-7

T-8

T-9

T-10

T-11T-12

T-1T-1

T-1

T-1

N

Nala

OPEN WELL

SHEARED GRANITE

SHEARED GRANITE

RAY

ACH

OTI

N a l a

VEM

PAL

LE

C

A

B

E

GRAB SAMPLE VALUE0.013-0.278 eU3O8

0.019-0.335 eU3O8

0.01 ThO2

0 20 mm 20 10

70°64°

70°

80°85°

150N

100N

50N

00

50S

100S

150S

Roa

d

66

Fig.2.43. Field photograph showing sheared granite with ferruginous quartz breccia.

Fig.2.44. Field photograph of exposure of sheared granite showing haematitisation.

67

Fig-36

2.3.5.5. BURJUPALLE FRACTURE ZONE

This fracture zone is located 13 km NE of Lakkireddipalle. It extends over

1.5 km in ENE – WSW direction with sub-vertical dip due north (2.46). The

fracture zone forms a rectilinear ridge (Fig.2.45). It is characterized by intensely

sheared granite (Fig.2.47) mylonite & brecciated mylonite (Fig.2.48) and

phyllonite(Fig.2.49). The mylonite zone occurs as dark colured discontinous lenses.

Significant radioactivity has been delineated over 650 m strike length

intermittently with width varying from 0.50 to 5.0 m (Fig.2.46). Uranium

mineralisation is associated with the sheared granite, cataclasite and mylonites

(Fig.2.50). Association of specularite is seen in mineralized sheared granite (Fig

2.51). It is mineralised with at least 2 to 5 bands of low order uranium value.

The richer mineralisation is confined to 300 m strike length.

Fig.2.45. Field photograph showing Burjupalle fracture zone forming rectilinear ridge.

68

Fig.2.46 Geological map of Burjupalle Fracture zone, Kadapa district, Andhra Pradesh

BJP-4(50°)RL 478.56m

BJP-1(50°),1A(75°)

BJP-2(50°),2A(75°)RL 472.50m

BJP-4(50°)RL 478.56m

BJP-4(50°)RL 478.56m

BJP-3(50°),3A(60°)RL 456.68m

BJP-12(50°)RL 469.00m

BJP-6(50°)RL 447.93m

BJP-7(50°)RL 448.72m

METADOLERITE (RADIOACTIVE)

QUARTZ VEIN

MINERALISED ZONE

MYLONITE (FRACTURE ZONE)

GREY/PINK GRANITE

MINERALISED INTERCEPT

BOREHOLE LOCATION

FAULT/FOLIATION/JOINTS

INDEX

GEOLOGICAL MAP OF BURJUPALLE FRACTURE ZONECUDDAPAH DISTRICT, ANDHRA PRADESH

T.S.No. 57 J/12 N

SURVEY STATION

FOLIATION/JOINTS

0.018x1.40m

0.013x0.60m

0.011x1.30m0.012x0.80m

BJP-3

BJP-1

BJP-1A

BJP-2

BJP-2A

BJP-3A

F

FF

F

F

F

F

F

0.017x1.20m

0.018x1.10m

020m 20m

E

F I

H

E

85°

85°

80°

85°

80°

83°

BJP-11(50°)RL 440.54m

FF

5 7 8

6

69

Fig.2.47. Field photograph showing intensely sheared granite along the Burjupalle fracture zone zone.

Fig.2.48. Field photograph showing development of mylonites and brecciated mylonite along the Burjupalle fracture zone.

Mylonite

Brecciated Mylonite

Fig.2.49.Field photograph showing development of phyllonite along the Burjupalle fracture zone.

70

Fig.2.50. Field photograph showing uranium mineralisation associated with sheared granite, cataclasite and mylonite in the Burjupalle fracture zone.

Cataclasite

Mylonite

Cataclasite

Fig.2.51. Field photograph of uraniferous altered sheared granite with specularite in Burjupalle fracture zone.

Specularite

71

2.3.5.6. KAMAGUTTAPALLE FRACTURE ZONE

This fracture zone is located east of village Kamaguttapalle in the

crystalline basement about 7 kms away from the unconformity contact. This

NNE – SSW trending fracture zone runs for about 3 kms with an average width

of about 15 m. It runs parallel to the T. Sundupalle fracture zone on the western side

(Fig.2.52). In the field this fracture zone is marked by the presence of

cataclasites, mylonites, breccia (Fig.2.53) with siliceous and haematite matrix.

The mylonites are chloritised, intensely sheared and at places are schistose.

The sheared schistosity make an angle with the trend of the main fracture zone

and at the hanging wall and footwall side it merges with the main trend. At

few places veins and vein lets of milky white quartz veins are seen specially

in the south of the fracture zone. At places these vein lets of quartz are

contorted, brecciated and intensely sheared to an extent of development of

platy silica. Radiometrically this fracture zone records radioactivity of the order

of 3 – 10xbg rising upto 15xbg.

The host rock of uranium mineralisation in Kamaguttapalle area is

identified as phyllonite. The major minerals present are limonite, quartz,

sericite, chlorite, biotite and clay. Most of the sericite and chlorite are alteration

products. The ore minerals present are haematite and anatase. Source of

radioactivity identified are secondary uranium minerals and from limonite,

chlorite, clay, sericite and biotite where uranium occurs in adsorbed state.

72

Sundupalle Fracture

Kamaguttapalle Fracture Zone

Fig.2.52. Satellite image showing Kamagutappale fracture zone.

73

Fig.2.53. Geological map of Kamaguttapalle fracture zone.

QUARTZ VEIN

GRANITE

CATACLASITE/MYLONITE

FOLIATION/JOINTS

CHANNEL

INDEX

85°

GEOLOGICAL MAP OF KAMAGUTAPALLE AREACUDDAPAH DISTRICT, ANDHRA PRADESH

T.S.No. 57 J/12

0.016x0.60m

SURVEY STATIONE

P7-2 ASSAY VALUES (eU3O8/U3O8)

30mm 10 0 10 20

N

24

URANIUM MINERALISED ZONE

74

4B22 2.3.5.7. TIMMAREDDIGARIPALLE FRACTURE ZONE

This fracture zone is located 3 km north of Chenchelapalle fracture

zone. It is ENE-WSW trending and steeply dipping fracture. The mineralised

fracture zone extends intermittently over 500 m with a width ranging from 0.50

to 4.50 m (Fig.2.54). Mineralisation is associated with cataclasite and mylonite

(Fig.2.55). Mineralisation occurs as small lenses 10 to 45 m length. It is dissected by

NNW-SE trending dolerite dyke in the eastern part. Alterations recorded include

haematitisation and chloritisation.

2.3.5.8. VARIKUNTAPALLE FRACTURE ZONE

Varikuntapalle fracture zone is located 15 km east of Lakkireddipalle,

which forms the linear ridges of low to medium relief (Fig.2.56). This fracture

zone extends for 4.5 kms with a width varying from few meters to tens of

meters in N55oE - S55oW trend with subvertical dips (Fig.2.57). Radioactivity

has been recorded intermittently over 4.5 km strike length with a width ranging

from 1.0 to 7.0 m in six ridges of 200 m, 150 m, 300 m, 500 m, 1.2 km and

100 m separated by cultivated fields. Dolerite dykes are present, which at

places cut across the fracture zone and at places run parallel to the fracture

zone. Granites are exposed at the contact of fracture zone and show very less

deformation (Fig2.58). Radioactive fracture zone rocks are comprised of

brecciated granites, mylonites and sheared basic rocks (Fig.2.59). Apatite, rutile,

magnetite, limonite, haematite are present abundantly in the U – mineralised

rocks. Potash metasomatism, haematitisation and limonitisation are common

phenomenon observed here.

75

2.3.5.9. GONDIPALLE FRACTURE ZONE

It is located 1 km NNW of Gondipalle village. It is ENE-WSW trending

fracture zone recording radioactivity ranging from 3 to 20 xbg. It extends over a

strike length of 1.30 km with the width ranging from 3 to 10m (Fig.2.60). The rock

types associated with the fracture zone include mylonites and cataclasites with

alterations like haematitisation, chloritisation and silicification. Uranium

mineralisation hosted by the mylonites occurs in three lenses of dimension ranging

from 40 m to 70 m length with a width of 0.5 to 3m.

76

Fig.2.54 Geological map of Timmareddigaripalle Fracture zone, Kadapa district, Andhra Pradesh

TRP-1(50°)&1A(75°)RL 348.24m

TRP-3&3A(50°)RL 344.56m

TRP-7(50°)RL 352.54m

TRP-6(50°)RL 365.50m

TRP-4(70°)RL 342.55m

TRP-5(50°)RL 337.92m

RP-8(50°)RL 361.42m

DOLERITE

QUARTZ VEIN

FRACTURE ZONE

GRANITE

MINERALISED ZONE

FOLIATION/JOINTS

BOREHOLE LOCATION

INDEX

GEOLOGICAL MAP OF TIMMAREDDIGARIPALLE FRACTURE ZONECUDDAPAH DISTRICT, ANDHRA PRADESH

T.S.No. 57 J/12N

C

E

85°

85°

85°85°85°80°

80°

78°

10 20 m0

TRP-1A

85°

77

N

MINERALISED ZONE

GRANITE CATACLASITE/BASIC MYLONITE

BASIC DYKE

BASEMENT GRANITE

SURVEY STATION

INDEX

0.031x1.20m

T-7

T-8

T-14

T-15

VKP-3(50°)RL 457.48m

VKP-4(50°)RL 451.10m

VKP-1(60°)RL 445.69m

VKP-3(50°)RL 457.48m

VKP-5(50°)RL 441.84m

VKP-6

ORL 461.20m

GEOLOGICAL MAP OF VARIKUNTAPALLE FRACTURE ZONECUDDAPAH DISTRICT, ANDHRA PRADESH

T.S.No. 57 J/12

0 50 m50 m

LOCATION OF TRENCH

Q

E

600N

400N

200N

0

600N

400N

200N

0

0200E 400E 600E

200S

200W 800E200S

0200E 400E 600E

200W 800E

T-1

Fig.2.57 Geological map of Varikuntapalle Fracture zone, Kadapa district, Andhra Pradesh

78

GEOLOGICAL MAP OF GONDIPALLE FRACTURE ZONECUDDAPAH DISTRICT, ANDHRA PRADESH

T.S.No. 57 J/11( NOT TO SCALE )

GRANITE

RADIOACTIVE BAND / SPOT

FAULT / SHEAR

BCATACLSAITE/MYLONITE

MILDLY SHEARED BASIC DYKE

UNDEFORMED BASIC DYKE

P1

N

I N D E X

Fig.2.60 Geological map of Gondipalle Fracture zone, Kadapa district, Andhra Pradesh

79

Fig.2.55. Field photograph showing uraniferous sheared granite and mylonite in T.R. Palle fracture zone.

Fig.2.56. Field photograph showing Varikuntapalle fracture zone forming rectilinear ridge.

80

Fig. 2.58. Field photograph of undeformed unaltered granite closer to the margin of Varikuntapalle fracture zone.

Fig. 2.59. Field photograph of deformed unaltered granite, cataclasite and mylonite along Varikuntapalle fracture zone.

81

2.3.6. RADIOMETRIC SIGNATURES OF DIFFERENT ROCK TYPES

Radiometric examination of the different rock types suggests that the

background radiation measured from scintillometer shows granitoids of Peninsular

Gneissic Complex records 20 mR/h (Fig.2.61) where as the pink (Fig.2.62 & 2.63)

and grey granitoids (Fig.2.64) records higher background of 40 mR/h. Similarly the

background radiations recorded from the basic dyke rock is still less i.e 10 mR/h.

This variation in the radiation recording is attributed to the intrinsic content of

radioactive elements like U and Th. Average intrinsic content reported ( Devakar Rao

et al., 1972) in Peninsular gneisses is 1.4 ppm uranium and 2.6 ppm and in Closepet

granite it is 3.5 ppm U and 18.2 ppm Th. The radiation recoded in the mineralized

granites is much higher it is 5 to 10 times that of the non-mineralised rock of the same

category. Sometimes the mineralised granites show secondary uranium minerals

occurring as surface encrustations (Fig 2.65) and as fracture fills (Fig 2.66).

The dikes intruding the Archean crust around the Cuddapah basin are

composed of tholeiites and alkali basalts and their differentiates. The largest

concentration of mafic dikes occur as E- W clusters at the southern end of the basin

and are known as the Tirupati dike swarm or cluster. Many of the oldest dikes of this

area precede the formation and evolution of the Cuddapah basin. But there are many

younger E- W trending dikes in this region, which were emplaced during the

formation and evolution of the basin Murty et al (1987). These E-W trending dikes at

the southern end of the basin were correlated with the E-W trending gravity high in

this region by Balakrishna et al (1984) . A few NW-SE and NE-SW trending dikes

are also present which cut across some of these dikes. In general the dykes record

very low radioactivity as their intrinsic content of both uranium and thorium is very

less. However in the sheared and mylonitised basic dykes at Laxmipuram on

82

Vempalle – Lakkireddipalle road section recorded higher uranium concentration

(>100 ppm) (Fig2.67a, b & c). Similar concentration is recorded in a dyke at

Payalapalle area.

83

Fig.37 Fig.38

Fig.39 Fig.40

Fig.2.54.Radiations In Granites Of Peninsular Gneisses

Fig. 2.62. Higher background gamma ray counts in pink granite of Closepet granite.

Fig. 2.63. Higher Gamma ray radiations in mineralised pink granite.

Fig. 2.61. Lower background gamma ray radiations in Granite representing Peninsular Gneissic Complex.

84

Fig-2.65

Secondary uranium minerals as surface encrustations

Fig.2.65. Field photograph showing development of secondary uranium minerals as surface encrustations in the granite near Konampeta.

Fig.2.64. Field photograph showing higher radioactive counts recorded by Scintillometer in the grey granite near Vaddepalle village, Konampeta.

Secondary uranium minerals as Filling along Joints

Fig.2.66. Field photograph showing secondary uranium minerals occurring as fracture fills in the granite near Reddigaripalle, Konampeta.

85

RADIATIONS IN GREY GRANITE OF PENINSULAR GNEISSES

Fig.267a. Field photograph showing sheared and mylonitised basic dyke intruding basement granites near Laxmipuram.

Fig.267b. Field photograph showing exposure of uraniferous basic dyke near Laxmipuram.

Fig.267c. Field photograph of an outcrop of massive basic dyke intruding basement granites.

.

86

2.3.7. MYLONITISATION OF THE GRANITE

The term mylonite was introduced by Lapworth in 1885 considering fine-

grained schistose fault rocks related to the Moine Thrust Zone, Scotland. He has given

the name mylonite (mylone in Greek is equal to Mill) thinking that these rocks are

formed by grinding processes similar to the grinding of flour in a mill and the

dominant deformation mechanism leading to the typical rock fabric of mylonites was

thought to be cataclasis. Different definitions of mylonite and its various sub-types are

proposed based on principally descriptive criteria (Lapworth, 1885, Staub, 1915,

Quensel, 1916, Termier & Maury, 1928, Waters & Campbell, 1935, Christie, 1960,

Higgins, 1971, White, 1982, Heitzmann, 1985). Mylonite development to ductile

deformation processes was introduced by Bell & Etheridge (1973).

The term phyllonite, introduced by Sander (1911), is a contraction of ‘phyllite

mylonite’ and indicates a rock of phyllitic appearance which was created through

deformation. Knopf (1931) suggested it to be phyllite produced by mylonitization of

an originally coarser-grained rock.

Mylonites are found in most of the shear zones developed in the granites

exposed in the southwestern margin of the Cuddapah Basin. These are millimeters

(Fig.2.68.b) to few centimeters (Fig.2.68c) thick, and more than a meter (Fig.2.68d)

thick. It occurs as single or paired bands, or as a network of bands, with sharp planar

boundaries. They appear dark green to light green-grey and consist of fine-grained,

thin layered rocks with a strong foliation parallel to the shear zone boundary. The

foliation of the dark bands subparallel to their margins, and generally dips steeply. A

pervasive mylonitic fabric is defined by aligned phyllosilicates composed of chlorite-

sericite quartz with variable amounts of epidote and ferruginous material. Shear zones

are in sharp contact with the fractured but relatively undeformed granite.

87

Development of low-temperature (sub-greenschist- and greenschist grade) ductile and

brittle/ductile shear zones in granitic rocks have been described by Mitra (1992),

Segall and Simpson (1986), Simpson (1985), O’Hara (1988), Losh (1989), Gibson

(1990) Evans (1990), Fitz Gerald and Sttinitz (1993) and Sttinitz and Fitz Gerald

(1993). Most of these studies stressed the importance of fluids for the promotion of

brittle fracturing and chemical softening that lead to strain localization and the

development of ductile deformation zones.

Post mylonite deformation is also evidenced by the brecciated mylonite (Fig.2.69). It

represents youngest phase of deformation.

A large range of microstructures can be preserved in these deformed rocks that

can potentially be used to reconstruct parts of the deformation history of the rock.

Once it is understood how a microstructure develops, the observed microstructure in a

rock sample can be used as a source of information on deformation mechanisms,

deformation regime and deformation history.

In low strain zones, fractures are randomly distributed (Fig.2.68a) They are cut

across by each other, forming networks of fracture zones. With increasing strain

intensity, fracturing increases, there is preferred orientation of fractures (Fig.2.68b).

The mylonitisation is due to the compression. The essential controls of mylonite

formation are temperature confining pressure, differential stresses and rock

properties (rock chemistry, mineralogy, grain size etc.). In mylonitisation due to

the communition of the rock in the mineral structure and rock texture are

totally destroyed. Frictional movement along the fractures provide both temperature

and pressure. With increase in strain, the amount and sizes of fine-grained matrix

increase and clasts of primary rocks decrease. There by it grows from mm scale to cm

scale (Fig.2.68c) and to m scale (Fig.2.68d). During this mechanochemical

88

metamorphism, mechanical initiation of chemical reaction results in localised

remobilisation of the constituents released due to communition of grains .

The principal changes observed are:-

1. The progressive development of a mylonitic foliation, from incipient

millimetre thin shear and increase in density and thickness into

protomylonites, blastomylonites and, in places, ultramylonites.

2. A gradual grain-size reduction of all principal minerals and the rock in

general, notably with 3 cm-size megacrysts of microcline progressively

reduced to relict clasts less than 1 mm in size.

3. Polygonisation of quartz, initially in microcrystalline lenticular grain

aggregates, or polygonised granular mosaics, with at least two

neocrystallisations.

4. An increasing presence of string- and flame-perthite.

Sericitisation of plagioclase and chloritisation of biotite, in places with new,

tiny chlorite grains.

89

Fig.2.68a. Field photograph of basement granite exposure showing low strain zone-Fracturing.

Fig.2.68b. Field photograph of basement granite exposure showing development of millimeters

thick mylonites along the Fractures.

Fig.2.68c. Field photograph of basement granite exposure showing development of few centimeters thick mylonites along the Fractures.

90

Fig.2.68d. Field photograph of basement granite exposure showing development of more than meter thick mylonites along the fractures.

Fig.2.69. Field photograph of brecciated mylonite exposure showing post mylonite deformation.