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CHAPTER-I
INTRODUCTION:
,Pakkanadu and its environs are comprised of rocks
of late Pre-cambrian age that are characterised by migmatites,
zoned pegmatites, syenites, ultramafics, pyroxenites and
carbonatites. Till now, a few causal references and a report
on radioactive carbonatite has been made by earlier workers and
its detailed geology has been neglected. Therefore, the present
investigation has been undertaken to present a detailed account
of the geology around Pakkanadu in order to decipher its
geological history.
LOCATION:
Pakkanadu (Lat.11 °41'55" and Long.77°49'30") is a small
tillage in the Sankari taluk of Salem district of Tamil Nadu.
The area under investigation is 20 square miles in extent and
falls within the toposheet 58E/14 of the one inch to one mile of
the Survey of India published in the year 1926-27. Its precise
location is between Lat.11°41' and 11°45' N and Long. 770 48' and
77° 51' 30" E. and it is shown in Fig.1.
GENERAL STATEMENT:
The study area was mapped during the summer
seasons from 1985 and 1986. The northern region was mapped from
the field camps set up at Aduvapatti. The Southern region was
mapped from the field camps set up at Pakkanadu. Due to the
Fig.I
SKETCH MAP SOING THE LOCATIONOFTHE STUDY AREA
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"PAKKANADU VILLAGE" SALEY DISTRICT,
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presence of talus on the hill slopes and the soil cover on the
plains, contacts between the rock typos are visible only in a
few places. To trace the contacts between different lithologies,
outcrop mapping technique was employed. When the outcrops
between two different lithologies were within 5 metres, the
boundary drawn between them was considered as an observed contact.
In cases where outcrops of two different lithologies were more
than 50 metres apart, the contact between them was regarded as
inferred.
Inspite,of soil cover and vegetation, the outcrops
encountered in the study area provide wealth of information about
the rock types.
The geological map of the terrain around Pakkanadu was
prepared on one inch to a mile scale by utilizing one inch to
one mile toposheet as the base map. The geological map of the
terrain around Pakkanadu is depicted in Fig.2
The conclusions put forth in the study area are the result
of intensive field observations of several outcrops and the
laboratory investigation of 80 thin sections. Chemical analysis
reports from other sources have been used.
PREVIOUS WORK:
In 1971, Grady (U.N.Photogeologist, Project for
Mineral Development in Tamil Nadu) dealt on "Deep Main Faults
in South India". The study comprised photogeological investigations combined with the results of an airborne geophysical
MAP SHOWING 1HE GEOLOGY AROUND PArKANADU
AND MUL. AK KADU , SALEM DIST . TAMIL NADU
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Fig.2. (After Suryanarayana Rao et al, 1978)
survey and field work(Fig.3). Ho has described a system of d OD
main faults containing carbonatite complexes and ultrabasics in
South India. He regards that the Main Fault at 45°E is made
evident by a northeast-southwest alignment of carbonatite
complexes, ultrabasic to alkaline intrusions, zones of metasomatic
alteration, pegmatite, quartz veins, brecciation and recent
seismic activity, Regarding Pakkanadu carbonatite he opines that
1. It is associated with zones of alteration of gneisses
and ultrabasic intrusions.
2. The intrusions occur near the junction of the N 45° E
faults with the strong Evil. Godumalai shear zone.
3. Between Pakkanadu and Salem occur 20 ultrabasic intrusions
including dunite, one of them is at Chalk hills near Salem and
contains magnesite.
Vemban et al (1971) points out that
1. The main fault at 45°E compares well with those of G.S.I.
maps. Fault 'C' of Grady compares well with Fault No.20 as
shown in G.S.I. map.
2. Godumalai shear zone compares with that of the SalemAttur Fault, F-3 as shown in G.S.I. map (Fig.4).
Dealing on Pre-cambrian carbonatites of Tamil Nadu, South
India, Borodin et al (1971) states that both geological setting
and petrological features support the view that the Tamil Nadu
carbonatite province differs in many respects from Narmada valley
province in India as from other carbonatito provinces all over
the world.
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"037,F,P r.A.IN FAULTS IN SOUTTT TNDIA" ( After Grady, 1971)
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The location of the Pre-Cambrian carbonatites occuring in
Tamil Nadu are shown in Fig.5. The carbonatites at Koratti was
initially considered as a band of crystalline limestone with
plenty of magnetite and apatite crystals, adjacent to the
vermiculite bearing pyroxenites, which were prospected for
vermiculite in 1965-66. In 1967, Deans inspected the carbonate
rocks of Koratti and confirmed them as really belonging to the
carbonatite group. The samples collected by him were analysed
and trace element contents in the carbonatites were found to
be readily distinguished and different from the local limestones.
It was pointed out by Deans and Powell (1968) that all the
elements reported are much more abundant in the carbonatites;
butcertain of them, notably barium and niobium, are rather
capricious in their abundance. The high abundance of strontium
and phosphorous is perhaps more striking but the persistence of
lanthenum and cerium is equally significant. The strontium
isotope ratios of Koratti carbonatites (Sevathur carbonatites
according to Deans and Powell) were determined to be lower than
of Tamil Nadu crystalline limestones and hence to be
characteristic of carbonatites. K/Ar dating of biotite in
biotite pyroxenites adjacent to Koratti carbonatites was
carried out by Deans and Powell (1968). An age of 720± 30
million years (Upper Pre-cambrian) for Koratti carbonatites has
been suggested by them.
During 1968, Semenov, U.N.D.P. petrologist, discovered the
7 0°
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Figure, 5 Location map of carbonatitc occurrences in Tamil Nadu.
L Koratti, 2. Jogipatti, 3. Reddipatti, 4. Karapattu, 5. Pallasulakkarai, 6. Paklcanadu.
(After Borodin et al, 1971)
presence of pyrochlore minoralization in the Koratti carbonatites
(Borodin 1971). In 1969, zones of pyrochlore mineralization were
identified by Borodin. The various types of analytical work on
minerals were carried out by Borodin in the Institute of
Mineralogy, Geochemistry and Crystal chemistry of Rare Elements,
Moscow under U.N.D.P. programme.
Borodin classified the carbonatites into several types
(Table. I) and determined them as para-ankeritic (also known as
beforsites) and calcitic (known as sovites). Taking into
consideration all possible variations and occurrences of
carbonatites,,Boradin has proposed a genetic classification of
the carbonatites in general. The carbonatites of Tamil Nadu can
be assigned to the IV type associated with alkali syenite and
granosyenites.
However the dolomitic nature and presence of
uraniferous pyrochlore are rather strong in carbonatites of
Koratti, (Borodin 1971). Though Borodin 1971 has not done
detailed study on carbonatites of Pakkanadu, from his preliminery
field and laboratory investigations he observes that,
1. The carbonatite located at Pakkanadu is away from all the
other five occurrences which form a definite carbonatite area
(Refer Fig.5).
2. Even though the carbonate rocks of Pakkanadu are very
different from these of Koratti and Jogipatti, the possibility of
their being some kind of carbonatite cannot be ruled out.
3. The carbonate rock and associated rocks have suffered
regional tectonic activity and minor folding can be seen.
4. Carbonatite rocks are rich in monazite and barite.
5. The associated albitite or oligoclasite rocks contain
plenty of sphene and allanite which are not common to carbonatite
complexes.
From the above observations he concludes that the carbonate
rocks of Pakkanadu can represent some late phase of hydrothermal
or carbonatite activity.
Reporting on the major and trace element compositions of the
Sevathur carbonatites, Krishnamurthy (1977) shows that there is
an enrichment of total iron, MgO, MnO, P205 and Na20 with
successively' younger phases within the main carbonatite body and
such a feature is comparable with the liquid lines of descent
observed in synthetic carbonatite systems. Further fenitisation
of the country rocks (granitic gneisses) along the southern
border of the carbonatite body is indicted from the enrichment
of K20, Na20, A1203 and impoverishment in Si02, total iron and
MgO in the fenites when compared with the country rocks,
Krishnamurthy further observes that
1. Sr (4109-9375, mean 6427) exceeds Ba (1000-2420 mean 1663)
in all types of carbonatites of Sevathur.
2. La and Ce are enriched in the calcite types when compared
to the beforsites.
3. Beforsites are generally rich in Ba, Nb, Ta and Se.
4. Early calcites are much enriched in Sr (5500-6200
ppm).
TABLE
. (After 9orodin et a', 1 077:
VI
GENETIC CLASSIFICATION OF CARBONATITES (SUBVOLCANIC AND DEEP-SEATED COMPLEXES)
I. Alkali-ultrabasic rocks and
nepheline-syenites
Genetic features of carbonatites
typomorphic
elements
rare-metal
composition minerals and
of carbona- examples of
tites
deposits
11. Alkali-gabbroids and
nepheline-syenites
rare-metal
composition minerals and
of carbonaexamples of
tiles
deposits
III. Nepheline—and
feldspathoid- syenites
rare-metal
composition minerals and
of carbona- examples of
tites
deposits
Stage
mode of
formation
High
temperature
Intrusive bodies
(stocks, dikes) and
related metasomatic
zones in rocks of
any composition
Ca, Mg
Ti, Nb
Ta, Zr
U, Th
CO2
P205
Calcite,
Dolomite
Uranpyrochlore
(` hatchettolite')
pyrochlore, dysanalyte, baddeleyite,
niobozirconolite,
calzirtite, thorianite
—Sove (Norway)
Alno (Sweden),
Iron Hill (USA)
Shava, Dorovo,
Spitskop (Africa),
Siberia (USSR)
Calcite,
Pyrochlore,
dysanalyteKaiserstuhl (W.
Germany)
Stjorna (Norway), East
Siberia (USSR)
Mediumtemperature
( (-390 500)
Injection
metasomatic
zones and
vein fillings
in rocks of any
composition
Ca, Mg
Ti, Nb
REE,
TO U
CO2
P20,
SiO2
Calcite,
dolomite,
ankerite,
feldsparcarbonate
Pyrochlore,
dysanalyte, rutile,
monazite, zircon—
Magnet Cove, Iron
Hill (USA), Sove
(Norway), Kola
Peninsula (USSR)
Calcite,
dolomite,
feldsparcarbonate
Pyrochlore,
monazite
burbankite—
Kaiserstuhl
(W. Germany)
Rocky Boy,
Mountain Pass
(USA)
Calcite
apatitecalcite,
feldsparcalcite,
dolomite
Pyrochlore,
zircon,
cerianiteLueshe (Africa)
Nemegosenda
(Canada),
Urals (USSR)
Lowtemperature
Metasomatic
zones in early
carbonatites and
vein fillings in rocks
of any composition
Mg, Fe
Ca, Ba
Sr, REE
Th
CO /
SO4
Si0/
F, S
Ankerite,
dolomite,
siderite
baryte
carbonate,
zeolitecarbonate
Burbankite,
ancylite,
monazite, REE
Fluorocarbonates,
columbite, fersmite,
zircon—Iron
Hill (USA) Kola
Peninsula (USSR)
Dolomite,
ankerite,
siderite,
barytecarbonate,
fluorite
barytecarbonate
Bastnasitc and
other REE carbonates, thorite,
cerite, orthite,
zircon—Mountain
Pass (USA)
Barytecarbonate
fluoritecarbonate
siderite
REE carbonates
and fluorocarbonates—Bearpaw Mountain
(USA); Kalkfeld
(Africa); Urals
(USSR)
500700)
100 300)
Calcite
IV. Alkali syenites
and granosyenites
composition
of carbonatites
rare-metal
minerals and
examples of
deposits
Pyrochlore,
bad deleyite,
lueshiteLueshe,
Kalkfeld (Africa)
Gronnedal (Greenland)
Feldsparcarbonate,
fluoritecarbonate
fluoritebarytecarbonate
(siderite
and others),
hematitesiderite
Columbite.
ilmenite,
rutile,
thorite,
xenotime,
monazite,
REE fluorocarbonates—
Siberia,
Ukraine
(USSR)
Karonga
(Africa)
C
14;
Srinivasan (1977) while comparing the carbonatite complex
(pyroxenite-syenite-carbonatite) of Hogenakal area of Tamil Nadu
with the adjoining parts of Karnataka states that both of them
were emplrced along rim-ssw fracture zones within the Pre-cambrian
gneissic complex. He compares the carbonatite of Hogenakal with
Carbonatites of Koratti, Pakkanadu and Strangways Range of
Australia (Table-II). He points out that the Hogenakal
carbonatite can be distinguished from metamorphosed limestone by
its genetic connection with crustal fractures having been emplaced
along deep fracture zones. The events relating to the emplacement
of the carbonatite are:
i) Faulting.
ii) Emplacements of a) pyroxenite, b) syenite and
c) carbonatites and
iii) Metasomatism (Fenitisation) (Ramakrishnan et al 1973).
According to Krishnamurthy (1971) carbonatites of Hogenakal
differs from carbonatites of Pakkanadu in many respect. He
observes that the carbonatites of Hogenal:al has certain similarity
with that of Strangways Range area of central Australia and
differs in certain respects from other carbonatite occurrences
of Tamil Nadu.
While surveying the previous work conducted at Pakkanadu and
Mullakkadu(5 Km.North of Pakkanadu) Suryanarayana Rao et al (1978)
states that
TABLE
TI.
CHARACTERISTICS OF HOGENAKAL CARBONAT1TES COMPARED WITH
CARBONATITES OF KORATTI, PAKKANADU AND STR A.NGWAYS RANGE
Tamil Nadu (India) Carbonatite Province
Hogenakal
Koratti •
Australia
Pakkanadu •
Strangways Range ••
I. Only calcite sovite
Sovite, para ankeritic
and dolomitic carbonatite (beforsite)
Dolomite
and calcite
sovite
Calcite and dolomitic
carbonatites
2. Long sinuous lenses
within the linear bands
(dykes) of mixed rock
pyroxenite and
syenite
Arcuate (wider) outcrops associated with
syenites and pyroxenites
Arcuate outcrops associated with
albitite or
oligoclasite
Lenses within linear
bands of mixed rock of
pyroxenite and syenite
3. Rich in apatite, less so
in magnetite (apatite
fluoroapatite ; magnetite—rich in Ti, V & P)
Rich in apatite, magnetite, pyrochlore (apatite= fluorapatite ;
magnetite rich in Ti,
V & P)
Rich in
monazite
and baryte
Rich in apatite, magnetite, zircon and pseudomorphs of columbite
after pyroch lore
4. Little or no fenitization
(development of biotite
seen from pyroxenite
due to syenite intrusion
and limited further formation of phlogopite/
vermiculite due to carbonatite activity)
Little fenitization but
strong development of
biotite, phlogopite/
vermiculite in pyroxenite due to the intrusion
of both syenite and
carbonatite
Not studied
Little or no fenitization
5. Associated with NNESSW faults.
Associated with NE-SW
faults
Associated
with NE-SW
faults
Data not available
(however, the alignment of outcrops is
reported to be in a
north-easterly (NE-sw)
direction)
• after Borodin, etal.. (1971); **
" after Crohn and Gellately (1968)
(After Srinivasan, 1977)
1. Radiometric surveys were carried out by Rama Rao, in
1959 and allanitc and monazite bearing pyroxenites and calcsilicate rocks are associated with ferruginous stained limestone
to the ':lest of Pakkanadu.
2. In 1970, examination of the ,microsections of the cores of
calcite-biotite rocks from depths of 40 to 60 metres from bore
holes Nos.BXN-3,5 and 7 brought to light accessory minerals like
ppatite, barite, strontianite, celestite, ankerite, siderite,
sphene and biotite, besides minute opaque rectangular grains
enclosed in biotite flakes.
3. Paper chromatographic tests on these biotites indicated
the presence of Nb.
4. During 1972-73 Krishnaiah Setty and Suryanarayana Rao
while looking for niobium and tantalum bearing minerals in this
region came across outcrops of radioactive pyroxenite carbonatite
bands emplaced along a fault zone within syenitic rocks one
kilometre to the West of Mullakkadu.
5. Though the carbonatites occur near the N 450 E major
crustal fault of Grady (1971) on a closer examination of
Pakkanadu and Mullakkadu areas, the pyroxenite and carbonatite
complex is found emplaced along a .N 30° E trending subsidiary
fault to the west of the main fault (Fig,').
6. Carbonatites of Pakkanadu and Mullakkadu are associated
with pyroxenites and occur as discontinous lenticular bodies
intruding the syenites.
7. They strike north east to southwest with easterly dips.
8. Large outcrops of serpentinised dunites carrying
accessory chromite, but by asbestos and magnesite veins are also
found along the same fault zone near Pakkanadu.
9. The carbonatites are of the pure calcite rich sovite
type with biotitic and ankeritic variants,
10. Apatite, magnetite, allanite, barite, monazite, zircon
and cerianite are the accessory minerals.
11. Spectrographic data suggest that the carbonatite
pyroxenite complex is Ce-La rich.
12. Nickel is not found in economic concentration.
13. The general radioactivity ranges from 5 to 20 x 8G.
14. Whole rock sample 1 assay e U308 from 0.02% to 0.054% in
carbonatites and from 0.02% to 0.06% in pyroxenites.
15. Allanites assay e U308 with chemical U308 from 0.007% to
0.034% thereby indicating the predominance of thorium over
uranium.
16. Monazite is mostly concentrated in the biotite-rich
portions of the carbonatites to the west of Pakkanadu.
17.
Allanite predominates in the pyroxenites west of
Mullakkadu.
Intensive study on the rare earth beaing minerals in the
carbonatites of Koratti, Samalpatti and Pakkanadu was done by
Semenov at al 1978 at the Institute of Mineralogy, Geochemistry
and crystal chemistry of Rare earths, Moscow. They conclude that,
47
1. The alkaline complexes with carbonatite activity are
enriched in niobium bearing minerals like pyrochlore, fergusonite,
eschynite and Fe/Nb rutile.
2. Eschynite (CaCeNyi012) occurs as brown grains with
pitchy lustre in association with baryte and allanite in
carbonatites of Pakkanadu.
3. Generally the light lanthonaus (LREE) are predominent
over heavier ones (HREE).
4. The pyroxenites contain relatively very small portion of
rare earth minerals.
The other relevant reference on rocks similar to the study
area will be cited in the proper context.
METHODS OF STUDY:
References to Air-Photos on 1:50,000 scale were
made at P.W.D. Water Institute Building, Taramani, Madras. to
decipher the major structures and to trace the contacts between
different lithologies. The geomorphic units, vegetation and
water bodies of the area under investtgation were deciphered by
examining the Landsat imageries of different spectral bands
(available at P.W.D. Water Institute Building, Taramani, Madras.)
The rock slices were examined using polarizing microscopes,
Carl Zeiss and Getner model. Refractive Index of minerals was
determined on grains of -80 mesh size, seperated in bromoform
and Clerci's solution and the refractive index of matching liauids
were read on Leitz Refractometer.
The modal composition of rocks was determined the swift
point counter with a spacing of third of a millimetre following
the method of Chayes (1949). About 6000 points uere counted to
approximte the actual volumetric composition.
Published analytical reports of important rock types of the
area under invest&gation have been used for interpretation.