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
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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.
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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.
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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
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Vempalle – Lakkireddipalle road section recorded higher uranium concentration
(>100 ppm) (Fig2.67a, b & c). Similar concentration is recorded in a dyke at
Payalapalle area.
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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.
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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.
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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.
.
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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.
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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
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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.
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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.
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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.