Indian Journal of Geo-Marine Sciences
Vol. 39 (4), December 2010, pp. 562-571
Relationship between heavy mineral placer deposits and hinterland rocks
of southern Kerala: A new approach for source-to-sink link from
the chemistry of garnets
G. R. Ravindra Kumar* & C. Sreejith
Centre for Earth Science Studies, Akkulam, Trivandrum 695 031, India
* [E-mail: [email protected]]
Received 20 August 2010; revised 27 December 2010
Beach sediments of the Kerala coast contain rich economically important heavy mineral deposits. Most previous
studies have traced the source of heavy minerals to the Precambrian crystalline formations in the hinterland based on
comparative mineral occurrences. No previous study has attempted to utilize petrological and geochemical characteristics
and mineral chemistry of source rocks to effectively compare and determine sediment provenance. A clear knowledge on
the composition of minerals from source and sink is important in precise recognition of source rock. Present study
consists the geological, geomorphologic setting and mineralogical characteristics of hinterland rocks to trace their
connection to placer deposits. It is recognized that garnet as the abundant heavy mineral in the placer sediments and in
source rocks of different ages and petrogenetic affinity. Garnet composition varies between different source rock types
due to its strong dependency on the bulk rock composition. In order to decipher the provenance we have compared major
element composition of garnet in the source rocks and placer deposits. Compelling similarities in mineralogical and
mineral chemical characteristics of garnets (Alm68Prp28Grs3Sps1) is noted between khondalites and placer sediments
suggesting latter as major source rock for placer deposits of southern Kerala. The study demonstrates excellent potential
of garnets in identifying placer mineral source.
[Keywords: Placer deposits, Source rocks, Garnet chemistry, Provenance, Southern India]
Introduction
Heavy minerals occur in source rocks either as
primary (like pyroxene) or as accessory components
(e.g., zircon). Weathering regime, which acts upon the
source rocks for a long period of time, removes
detrital heavy minerals and transports through river
channels eventually to the sea. Removed minerals
occur as accessories in most sediment but in placer
deposits, they (e.g., ilmenite, rutile, zircon, monazite,
garnet, and magnetite) become valuable minerals
and may achieve economic grades. Knowledge on
these minerals and their characteristics help us
in unraveling the extrabasinal (e.g. source area
weathering) and intrabasinal (e.g. hydraulic)
processes, which influences the formation of
sedimentary rocks and heavy mineral deposits.
On the basis of physical resistance of rock types
and also between minerals to chemical weathering,
heavy minerals have been classified1 into two main
groups as ultrastable (e.g., zircon, rutile, tourmaline)
and metastable (e.g., augite, chlorite). Detailed
petrographic, mineralogical, and compositional
analyses of these minerals are useful in understanding
the provenance, paleoenvironment, and diagenetic
processes. Even after several episodes of recycling,
many mineral varieties may preserve their diagnostic
characteristics inherited from their parent lithologies2.
In recent years, heavy mineral analysis has also found
useful in constraining the P-T regime of host rocks
and in studying the unroofing history of rocks and
thereby removal of minerals3-5. Electron microprobe
analysis allows determination of the chemical
composition and its variability among grains of a
certain mineral phase for the purpose of provenance
discrimination and lithological fingerprinting3,6.
Single-grain techniques are potentially capable
of revealing the spatial distribution and proportions
of different source rocks contributing to a drainage
system or rates of exhumation and erosion in
source area.
In Kerala, most studies on heavy mineral beach
placers were aimed to answer beach morphology
and hydrodynamic mechanisms of wave sorting
that concentrate heavy mineral grains on the beach
KUMAR & SREEJITH: RELATIONSHIP BETWEEN HEAVY MINERAL PLACER DEPOSITS & HINTERLAND ROCKS 563
faces or on aspects concerning ilmenite. Provenance
studies, which reconstruct the history of sediments
from the initial erosion of parent rocks to the
final burial of their detritus, encompassing all
factors such as the geologic, physiographic, and
climatic context of the source area are very few.
The objectives of early provenance studies were
focused on determining the parent rocks of single
minerals or mineral varieties, based on detailed
inventories of the accessory mineral assemblages
of source rocks. The few studies, which have
addressed the origin and source/provenance of these
deposits, have expressed varying views7-9, vaguely
delineating the crystalline rocks of the Western Ghats
as provenance. This observation was ambiguous
because hinterland is made up of rocks with very wide
variation in composition and age (600 to 3300 Ma)
and there is no clarity on the relative contribution
of heavy minerals from khondalite and charnockites.
Some studies6,10 have suggested that particular
associations of heavy minerals do not necessarily
reflect the mineralogy of the source area since
transport, weathering, and post-depositional solution
may modify them.
In spite of long history of commercial heavy
mineral exploration and mining, data available
from Kerala on the heavy mineral characteristics
and chemistry is inadequate for provenance analysis.
Understanding of provenance, processes and
determining the relative contribution of different
hinterland source rocks is vital to resolving the
issue of deposition in sedimentary basins and in
the understanding of erosion/exhumation history of
different blocks of the Western Ghats in the
geological past. It is known that compositional
signatures of minerals faithfully reflect various
petrographic units of the source area. Our recent
petrological and geochemical studies on granulites
of southern India have suggested pronounced
variation in mineral compositions. The petrogenetic
diversity noted in the hinterland rocks demands
an understanding of mineral chemical variation
with respect to protolith composition in order to
examine the source-to-sink relation between heavy
minerals in the placers and provenance. In this
paper, we present an outline of geology and
geomorphology of the source rocks and attempt to
examine source to sink link by integrating available
mineral chemistry of source rock with those from
the beach sediments.
Status of heavy mineral studies in India
In India, beach sand mineral exploration and
exploitation started in the beginning of the
20th century after the accidental discovery of
monazite from the beach sands of then Travancore
state, present Kerala. The heavy minerals found
to include ilmenite, magnetite, monazite, sillimanite,
zircon, and garnet. In recent times, the coastal
tract of India has become one of the most
prospective heavy mineral deposits in the world.
Several groups have attempted to study the placer
minerals in terms of their occurrence, distribution
chemical composition, texture, provenance, and
to understand the transportation trends of sediments.
Gillson11, who did a pioneering study of these
deposits, suggested basic rocks as the likely
source of the deposits. Soman8 suggested the
khondalite migmatite complex of southern
India as the source rocks for the placer deposits.
The studies of Mallik et al.9 proposed the existence
of specific petrographic provinces in the coastal
placers. Sabeen et al.12 examined the use of garnet
composition for the reconstruction of sediment
dispersal from well known parent rocks.
Geomorphologic evolution and placer formation
Spatial distribution of placers and many of
their mineral characteristics are dictated by various
geomorphological processes operating on the surface
of the earth. Different geological agents, in
addition to geomorphological processes, play a
vital role in shaping the landscape and development
of landforms. Accumulation of heavy minerals
and formation of placer deposits are directly related
to the large-scale carving of the landforms linked
to different continental and shoreline depositional
environments13. The sediments deposited in
continental margins are connected with hill-slope
processes. Understanding of the geomorphological
evolution of the hinterlands is essential to address
the functions like erosion rates and sediment
load debouching into fluvial system. A majority of
publications on placers in southern India has
addressed the final repositories of heavy minerals
in lakes or along the coast and data on the
weathering and denudation patterns that help in
delineating the provenance are limited.
Great escarpment and denudation rate
Evolutionary history of great escarpments can
provide direct evidences on the roles of tectonics
and erosion in the formation of passive margins and
564
INDIAN J. MAR. SCI., VOL. 39, NO. 4, DECEMBER 2010
subsequent sedimentation in foreland basins. The
present morphology of the western continental margin
of India with great escarpment is the result of major
rifting events related to the break up and dispersal
of eastern Gondwana. The onshore topography of
India’s western margin displays many features that
are characteristic of other rifted passive margins14,15.
The onshore erosional signature and offshore
depositional histories during passive margin evolution
is complex. Conflicting models have been proposed
for the generation of escarpments at high elevation
passive margins16,17. They differ in the nature of
rift tectonics, the spatial distribution of the amount of
post-break up erosion across the margin and the
position of the drainage divide prior to break up.
The rate of escarpment formation exerts an important
temporal control on denudation rates and sediment
removal. The Western Ghats escarpment has been
the subject of numerous morphological studies15,18-21
and some authors have suggested that it was
essentially a fault scarp19,20. However, no convincing
evidence of any topography at the Western Ghats
scarp face having been generated by recent fault
is reported so far. There is now, a general consensus
in favour of an erosional origin for this great
escarpment15,21.
et al.25. Apatite fission track thermochronometry
(AFTT) analyses of samples from the inland and
coastal geomorphic units have revealed smaller
magnitudes of denudation for the highlands than
at the coast24,26, which might be the reason for
the retreat of escarpment and formation of lowlands
in the coastal plain.
The lithological units in the Western Ghats, even
within the same levels of contours, are diverse in
mineralogy, corresponding to distinct petrogenetic
segments. Thus, the variation in topography of the
Western Ghats is partly attributed to the current
morphotectonic setting and climate-controlled
geomorphic regime, but a large amount of the
variability is directly related to petrogenetic structural
aspects,
particularly
fracture
patterns
and
mineralogical composition. In the geomorphological
context, lithological and mineralogical variability is
of major importance27,28. The observed differences
in mineralogy and fabric are attributed to the
variable scope of selective weathering and sediment
contribution to the fluvial system. The widespread
occurrence of weathering mantles and laterite capping
on the mountain tops indicate that pre-weathering of
rock has been the key component of landform
evolution.
Crystalline rocks and variable weathering
The Western Ghats form a distinctive
physiographic province, running parallel to the
west coast of India. The Ghats traverse many
different geological formations of differing
physical and structural characteristics. Three
distinct structural and petrogenetic segments are
demarcated in the Western Ghats; the Deccan
volcanic, Archaean granite–greenstone (Dharwar
craton), and southern granulite provinces15. The
crystalline rocks exposed in Western Ghats
have imprints of high P-T conditions representing
lower crustal origin22,23. This indicates that a great
volume of crustal section has been removed
from Western Ghats in the recent geological past,
yet some portion of the mountains still have high
mean elevation and local relief. Gunnell et al.24
conducted an extensive apatite fission track study
to resolve the thermal history of the Western
Indian margin. Modelled thermal histories indicate
that a sharp increase in denudation occurred at
the start of the Cenozoic, which is contemporaneous
with the increase in sedimentation recorded in the
offshore sedimentary record proposed by Campanile
Regional geology and heavy mineral source
The Western Ghats, the possible source rock terrain,
forms almost continuous chain (except for the
altitudinal lows at Palghat) of lofty mountains
composed of varying lithounits represented by
high-grade gneisses, charnockites, and granitoids
(Fig. 1). It is dissected by crustal scale shears like
Palghat-Cauvery shear zone (PCSZ) and Achankovil
shear zone (AKSZ) and several lineaments.
Geologically, Kerala forms the southwestern fringe
of the south Indian Peninsular shield and consists of
two major terrains,29 the cratonic part lying north
of PCSZ and 2) mobile belt– the Pandyan mobile
belt formed of gneiss, charnockite and khondalite.
The charnockite and khondalite, making up the hill
ranges of the Western Ghats, are thought to be the
main supplier of heavy minerals, which are now
seen as beach placers along the Kerala coast. For the
Kerala beach placers, five heavy mineral provinces
with specific mineral assemblages were recognised
by Mallik et al.9 (Fig. 1). The spatial distribution of the
mineral associations, delineated as petrographic
provinces on the coast, reflect the composition of
KUMAR & SREEJITH: RELATIONSHIP BETWEEN HEAVY MINERAL PLACER DEPOSITS & HINTERLAND ROCKS 565
Gneissic Rocks
Fig. 1—Generalised map showing geology and drainage basins of
Kerala and Tamil Nadu. The Roman letters indicate different
beach mineral provinces (after Mallik et al.9): I – mixed mineral
province with opaques, zircon, hornblende, garnet, sillimanite,
epidote; II – garnet-hypersthene province with epidote-sillimanite;
III – opaques-zircon province; IV – hornblende-hypersthene
province; V – opaque-zircon-monazite province with central
garnet-sillimanite-epidote patch.
parent rocks. Thus, each province corresponds with
a petrographic unit of the source complexes and their
petrogenetic characteristics.
A large part of south Kerala and southern parts of
Tamil Nadu is occupied by a group of metasediments
referred in earliest literature as Khondalite group.
In recent years they are being referred to as Kerala
Khondalite belt (KKB) as these are well exposed
and well developed in the Kerala region. In probable
order of abundance the khondalite group consists of
garnet – biotite ± graphite gneisses and intimately
associated garnetiferous charnockite–khondalites s.s.
(graphite – sillimanite – garnet - biotite ± cordierite),
cordierite gneisses (garnet – biotite – cordierite ±
orthopyroxene) and less abundant calc-silicates, basic
granulite and quartzites. Recent petrological studies
(authors’ unpublished data) on these source rocks show
remarkable compositional variations in rock-forming
and accessory minerals in different lithounits. The
tectonothermal/exhumation paths presented are also
dissimilar for different lithospheric blocks in southern
India30. This petrogenetic diversity provides an ample
scope to reexamine the source-to-sink relation between
heavy minerals in the placers and provenance.
Granites and gneisses constitute the most common
rock types in regions south of Achankovil shear
zone (AKSZ). The rock is made up of irregular grains
of quartz and feldspar with variable amounts of
biotite. Garnet is common to all the rocks south of
AKSZ. Zircon, apatite and ilmenite occur as
accessory minerals. Chacko et al.31 and Braun et al.32
based on petrography and chemistry have classified
garnet-biotite gneisses into two varieties as sodic
and potassic.
In regions north of AKSZ, referred to as
Cardamom hill charnockite massif, hornblende-biotite
gneisses, and quartzo-feldspathic biotite gneisses
are relatively minor constituents, largely occurring
close to intrusive bodies like granite/pegmatite.
Hornblende-biotite and biotite gneisses are regarded
as retrogressed equivalents of the granulites
of the charnockite-enderbite massifs. Retrogressed
charnockites produce grey gneisses which occur
as layered rocks showing fine-grained, biotite
rich elongated aggregates mimicking the former
anhydrous phases aligned in the foliation plane.
In addition to these grey gneisses a variety of granites
are also noted as pink and grey types of granites,
which are referred in the literature as Chenganoor
granite, Munnar granite. Zircon, magnetite, and
ilmenite are accessory minerals making up to a
maximum of 2-5 vol% of the rock.
Khondalites
Khondalites are the most widely seen rock types in
the southern part of Kerala. Due to their dominance in
regions south of AKSZ, the entire region is described
as Kerala khondalite belt. Khondalite are made up
of a mineralogy of quartz + biotite + sillimanite +
K-feldspar (Or 80-90) + plagioclase (An 22-37) + graphite ±
cordierite ± spinel + rutile + graphite. In highly
migmatised zones they form the restite portions.
Approximate modal abundance of minerals in
khondalite is garnet (10-25%), biotite (20-40%),
sillimanite (5-25%), feldspar (10-35%) and graphite
(0.2-1%). Cordierite may be present with varying
proportion of 2 to 15%. Khondalites are variously
described as metamorphosed weathering crust33 or
acid volcanics and tuffs, but Chacko et al.22, regard
them as metasediments (shales and sandy shales).
Charnockites
Charnockites occur as fringing masses to the
north and south of Khondalite Group of rocks. Minor
bodies of massive charnockites also occur within
566
INDIAN J. MAR. SCI., VOL. 39, NO. 4, DECEMBER 2010
the khondalite belt but whether these are similar
by age, protolithic composition, and metamorphism
to the massif charnockites occurring outside the
supracrustal belt or different is not yet clear. Foliation
in massif charnockite is not conspicuous but
weathered portion reveal gneissic banding. Garnet
and graphite are normally absent. The dominant
mineralogy is orthopyroxene (5-10%), amphibole
(2-15%), clinopyroxene (2-8%), plagioclase (10-40%)
and quartz in minor amount. Biotite is scarce and
is usually of secondary origin. Magnetite, rather
than ilmenite, is the common opaque phase in
charnockites of KKB.
Opaque is fairly abundant in charnockites of
Cardamom massif. They are medium sized grains
with mostly straight to curved boundaries, but grains
with secondary overgrowths have serrated grain
margins. Opaques are mostly ilmenite. Ilmenites
adjacent to mafic minerals have patches and exsolved
discontinuous streak/beads of hematite. In some cases
opaque adjacent to pyroxene are ferrian ilmenite with
more than 60% hematite rich portion, the ilmenite
rich portions have streaks and exsolved hematite
bands/beads in them. Ilmenite within felsics have
no exsolution grains, they show simple contacts with
ilmenite rich and ilmenite poor zones.
Materials and Methods
Tracing the source-to-sink link
Detailed petrographic analysis of major rock types
in the source region and an analysis of grain mounts
of beach sediments reveal that provenance indicators
are present and can be traced by comparing the
orthopyroxene, hornblende, ilmenite, garnet, and
zircon populations. Such an analysis would limit to
physical comparison but would not help in tracing to
the specific source as these minerals are important
constituents in a majority of rocks in the hinterland.
In such a scenario, chemical compositions of
heavy minerals attain importance. Mineral chemical
composition is available for all these important
constituents (except ilmenite) in hinterland rocks as
they are vital in petrogenetic and metamorphic
studies. Same is not true for the beach sediments
as most early works have concentrated on the
composition of ilmenite, as it is dominant constituent
in beach sediments while other minerals like garnet,
pyroxene, sillimanite, and rutile are scarce. Of all
the minerals garnet is known to be most useful in
provenance studies as it is present in all the rock types
of source region, possesses wide compositional range,
strong relation to whole rock composition, and
stability and ability to withstand. The composition of
garnet can be successfully used due to its composition
potential as solid-solution between seven principal
end-members, showing significant differences in
composition between different types of garnet bearing
lithology. Few earlier studies have used garnet to
constrain the sediment provenance12.
Composition of garnets
In recent years electron microprobe as been a boon
in mineral chemical analysis and such data in
conjunction with conventional (optical) data, have
provided important information on the mineralogical
composition of source rocks and processes in their
transport and deposition. A wealth of mineral
chemical data primarily developed to work on the
P-T conditions of metamorphism of the granulites,
on the probable source rocks for sediments, are
available22,34-36, whereas only sporadic data are
reported for sediments. Very little effort has been
made in the past to compare mineral chemical data
of source rocks with the sediments to understand
the provenance. The only exception is the study
conducted by Jayalakshmi et al.37 to a narrow
stretch of coast at Ambalapuzha. As the available
database is not comprehensive for the beach
sediments, we use here only garnet chemistry
from possible sources and compare with similar
data available for sediments to establish source-tosink link and physical characteristics. Garnets are
expected to preserve their original composition,
from the point of removal to the point of deposition,
as they are inert to dissolution and are shielded by
the lower temperature processes that are active in
alteration, removal and transport.
Garnet is a common minor accessory mineral
accounting to about 10-15% in the hinterland rocks.
They are important accessories in rocks occurring to
the south of AKSZ. In beach sediments of the
southern province of Kerala garnet is very abundant.
Their population starts decreasing from south to north
and become conspicuously absent from central
province. This feature correlates very well between
sediments and hinterland rocks. Colour of garnet can
vary from light to dark pink and shades of red,
depending on the composition in that pyrope types are
dark pink and almandine rich types are red in colour.
Grains may vary from rounded to irregular shapes.
KUMAR & SREEJITH: RELATIONSHIP BETWEEN HEAVY MINERAL PLACER DEPOSITS & HINTERLAND ROCKS 567
Results and Discussion
Chemistry of garnets from various source rocks is
presented in Table 1 and from beach sediments
in Table 2. They are also graphically illustrated
in figures 2 and 3. Although all are almandine garnet
(Alm 62-82%) there is subtle to distinct difference in
grossular and pyrope composition. Most important
information can be noted is that garnet composition is
dissimilar for different rock types, in other words
source rock garnets are of diverse types and their
compositions are largely controlled by the bulk
composition of the host rocks and garnet composition
can be used very effectively to precisely suggest the
type of source rock. Figure 3 illustrates compositional
differences in garnets from the three geological
provinces like northern province, central province,
and southern province. When figure 2 and 3 are
viewed together a striking comparison can be seen
between garnets composition from khondalite with
those of garnets from southern province suggesting
khondalite as the primary source of garnets in the
southern province. This is apparent in figure 3d which
presents a comparative line plot of average chemistry
of garnet in sodic gneiss, khondalite with average
composition of garnets from the southern province.
The composition of garnets from source rocks
and beach sediments are plotted in (almandine +
spessartine)–pyrope–grossular trilinear diagrams
(Fig. 4). The garnet data from source rocks are shown
as fields of different lithological compositions for
easy understanding. The source rocks plotted are
khondalites, sodic, potassic and augen types of
gneisses, charnockites. Available beach garnet data
for southern province (Fig. 4a) and central and
northern province (Fig. 4b) are plotted onto the fields
of the source rocks for comparison. Khondalites,
sodic gneisses and a part of charnockites in the
Table 2—Average garnet compositions from different beach provinces
[NP, CP, and SP correspond to northern, central, and southern beach
provinces respectively. 1-2 and 4: data from Bernstein et al.38;
3: Jayalekshmi et al.37; 5: Sabeen et al.12. n = number of analysis]
NP
CP
SP
1
2
3
4
5
(4)
(5)
(4)
(5)
(7)
Major elements in wt%
SiO2
37.63
33.64
36.72
38.92 38.57
TiO2
0.05
0.05
0.04
0.03
Al2O3
21.51
20.89
21.09
22.26 22.08
Cr2O3
0.02
0.02
0.02
0.00
0.05
FeO
33.41
37.50
37.29
29.91 31.57
MnO
0.45
0.58
0.49
0.39
0.44
MgO
6.27
2.21
2.75
8.33
7.06
CaO
1.28
2.83
2.42
0.92
0.68
Na2O
0.02
0.02
0.02
0.03
Total
100.64 97.74 100.84 100.73 100.51
End member compositions based on 12 Oxygen
Alm
71.85
81.70
81.57
64.62 69.40
Grs
3.36
7.72
7.11
2.53
1.90
Prp
23.66
9.17
10.06
32.01 27.39
Sps
0.98
1.29
1.13
0.84
1.06
Uv
0.08
0.06
0.05
-
Beach province
Ref:
n=
Table 1—Average garnet chemistry from different rock types in southern and central Kerala [Ref.1- Sodic gneiss; 2: Potassic gneiss;
3: augen gneiss (all unpublished data of authors); 4: Chacko et al.22; 5: Morimoto et al.34; 6: Cesare et al.35; 7: KKB Charnockite
(Chacko et al.22); 8: Nagerkovil charnockite (Santosh et al.36); 9: Nagerkovil charnockite and 10: Cardamom charnockite
(data of authors); n = number of analysis]
Rock type
Ref:
n=
1
(11)
Gneiss
2
(18)
SiO2
TiO2
Al2O3
Cr2O3
FeO
MnO
MgO
CaO
Na2O
Total
37.63
0.05
21.51
0.02
33.41
0.45
6.27
1.28
0.02
100.64
33.64
0.05
20.89
0.02
37.50
0.58
2.21
2.83
0.02
97.74
Alm
Grs
Prp
Sps
Uv
71.85
3.36
23.66
0.98
0.08
81.70
7.72
9.17
1.29
0.06
Khondalites
4
5
6
(20)
(12)
(6)
Major elements in wt%
36.72
38.92
38.57
38.64
0.04
0.03
0.06
21.09
22.26
22.08
21.64
0.02
0.00
0.05
0.06
37.29
29.91
31.57
31.62
0.49
0.39
0.44
0.48
2.75
8.33
7.06
7.61
2.42
0.92
0.68
0.67
0.02
0.03
0.02
100.84
100.73
100.51
100.80
End member compositions based on 12 Oxygen
81.57
64.62
69.40
68.19
7.11
2.53
1.90
1.36
10.06
32.01
27.39
29.18
1.13
0.84
1.06
1.05
0.05
0.20
3
(13)
7
(19)
Charnockites
8
9
(14)
(5)
10
(6)
38.61
21.58
30.96
1.08
6.15
1.94
100.32
38.28
0.11
21.48
0.03
33.60
0.51
4.93
1.60
0.01
100.55
37.01
0.05
21.27
32.71
1.17
5.79
1.06
0.02
99.08
35.96
0.06
19.93
0.01
31.89
1.92
0.67
8.20
0.01
98.65
68.16
5.51
23.93
2.39
-
74.71
4.48
19.52
1.16
0.09
71.72
2.99
22.61
2.59
-
70.00
23.02
2.63
4.27
0.04
568
INDIAN J. MAR. SCI., VOL. 39, NO. 4, DECEMBER 2010
Fig. 2. (a-f)—Comparisons of average composition of garnets from different rock types in the hinterland
southern province are dominated by low grossular,
pyrope rich garnets. Garnets in potassic gneisses,
augen gneisses and some of the charnockites in
KKB and Nagerkovil are dominated by almandinespessartine rich varieties. We note overall similarity
and close correspondence between beach garnets
analysed by Sabeen et al.12 and Bernstein et al.38
with garnet compositions from metasediments like
khondalite and sodic gneisses. It is therefore likely
that origin of most of the beach garnets is from
the khondalites and sodic type of gneisses. Potassic
and augen gneisses are less likely sources of garnet.
Chemistry of garnets from central province beach
sediments, on the other hand, suggests that they were
fed from different sources consisting of khondalites
and charnockite. Data for northern province garnets
KUMAR & SREEJITH: RELATIONSHIP BETWEEN HEAVY MINERAL PLACER DEPOSITS & HINTERLAND ROCKS 569
Fig. 3. (a-c)—Comparisons of average composition of garnets from different placer provinces; d: binary plot depicting extreme closeness
between source (khondalite and sodic gneisses) and sink.
Fig. 4 (a & b)—Chemical composition of garnets. Fields for different rock types are drawn based on individual garnet composition plots
for easy comparison end member compositions from source rocks: composition of various type garnets from different source rocks of
(a) southern provenance and (b) northern and central Kerala. Garnet compositions from different placer provinces are plotted on to the
field of possible source rocks for comparison
INDIAN J. MAR. SCI., VOL. 39, NO. 4, DECEMBER 2010
570
are too limited and seem to be widespread, not
conforming to any particular source rock mineral
chemistry.
Conclusions
The mineralogy and petrogenesis of hinterland
rocks are important for deciphering provenance for
placer minerals. Heavy minerals (ilmenite, rutile,
monazite, zircon and garnet) are common accessories
in all the rocks of southern Kerala. Heavy minerals
may have derived from any of the hinterland rocks
of different petrogenetic affinity and age. From
garnet compositions in hinterland rocks and
beach sediments, provenance can be very clearly
distinguished. Garnet compositions of different source
rocks indicate that they are strongly bulk composition
controlled and are therefore potential indicators of
the source rock types for the placers. Garnets from
both source rocks and placers are almandine types
with high pyrope, low grossular and spessartine
components, typical compositional characteristics
for high grade metamorphic rocks. The garnet
composition data in placer deposits of southern
province illustrates strong similarity with those in the
khonadalites of source region, suggesting that may
have largely derived from similar rocks in the source
region. Relatively soft, easily erodable characteristics
and abundance of heavy minerals in khondalites,
compared to other igneous rocks (potassic and augen
gneisses) in the hinterland may have been the main
reason for its large contribution to placer deposit. In
view of varying petrogenetic characteristics of
probable source rocks, detailed study of morphology
and chemistry of minerals, especially trace and REE
of heavy minerals and in-situ dating of zircons, are
required to precisely understand aspects concerning
erosion, transportation, provenance and depositional
history of placer deposits.
Acknowledgements
Authours are grateful to the Director, Centre for
Earth Science Studies, Trivandrum for support and
facilities.
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
References
1
2
Rothwell R G, Minerals and Mineraloids in Marine
Sediments: An Optical Identification Guide, (Elsevier
Applied Science Publ. Ltd., London) 1989, pp. 279.
Dewey J & Mange M, Petrography of Ordovician and Silurian
sediments in the Western Irish Caledonides: tracers of a shortlived Ordovician continent-arc collision orogeny and the
evolution of the Laurentian Appalachian/Caledonian margin,
18
19
20
in: Continental Tectonics, edited by C. Mac Niocaill & P.D.
Ryan (Spec. Pub., Geol. Soc. London) 1999, 164, pp. 55–107.
Morton A C, Geochemical studies of detrital heavy minerals
and their application to provenance research, in:
Developments in Sedimentary Provenance Studies edited by
A.C. Morton, A. Kronz & P.W.D. Haughton, (Spec. Publ.,
Geol. Soc., London) 1991, 57, pp. 31–45.
Zack, T., von Eynatten, H. & Kronz, A., Rutile geochemistry
and its potential use in quantitative provenance studies, Sed.
Geol., 171(2004) 37–58.
Dill, H.G., Melcher, F, Fuessl, M. & Weber, B., The origin
of rutile-ilmenite aggregates (“nigrine”) in alluvial-fluvial
placers of the Hagendorf pegmatite province, NE Bavaria,
Germany, Min. Pet., 89 (2007) 133–158.
von Eynatten, H. & Gaupp, R., Provenance of Cretaceous
synorogenic sandstones in the Eastern Alps: constraints from
framework petrography, heavy mineral analysis, and mineral
chemistry, Sed. Geol., 124 (1999) 81–111.
Aswathanarayana U, Origin of the heavy mineral beach sand
deposits of the southwest coast of India, (Proc. Indian
Geophysical Union, Advancing Frontiers in Geology and
Geophysics Hyderabad) 1964, pp. 481-489.
Soman, K., Origin and geologic significance of the Chavara
placer deposits, Kerala, Curr. Sci., 54 (1985) 280–281.
Mallik, T.K., Vasudevan, V., Verghese, P.A. & Machado, T.,
The Black sand placer deposits of Kerala beach, southwest
India, Mar. Geol., 77 (1987) 129–150.
Morton, A.C. & Hallsworth, C.R., Processes controlling the
composition of heavy mineral assemblages in sandstones,
Sed. Geol., 124 (1999) 3–29.
Gillson, J.L., Sand deposits of titanium minerals, Mining
Eng., 11 (1959) 421–429.
Sabeen, H.M., Ramanujam, N. & Morton, A.C., The
provenance of garnet: constraints provided by studies of
coastal sediments from southern India, Sed. Geol., 152(2002)
279–287.
Selley R C, Applied Sedimentology, (Academic Press,
London) 2000, pp. 523.
Lister, G.S., Etheridge, M.A. & Symonds, P.A., Detachment
models for the formation of passive continental margins,
Tectonics, 10 (1991) 1038–1064.
Gunnell Y & Radhakrishna B P, Sahyadri, the Great
Escarpment of the Indian Subcontinent: Patterns of
Landscape Development in the Western Ghats, (Mem., Geol.
Soc. India) 2001, 47, pp. 1054.
Gilchrist A R & Summerfield M A, Tectonic models of
passive margin evolution and their implications for theories
of longterm landscape evolution, in: Process model and
theoretical geomorphology, edited by M.J. Kirby, (John
Wiley and Sons, Chichester) 1994, pp. 55– 84.
van der Beek, P., Summerfield, M.A., Braun, J., Brown,
R.W. & Fleming, A., Modeling post breakup landscape
development and denudational history across the southern
African (Drakensberg escarpment) margin, J. of Geophys.
Res., 107 (2002) 2351.
Ollier, C.D. & Powar, K.B., The Western Ghats and the
morphotectonics of peninsular India, Z. Geomorphol. Suppl.,
54(1985) 37–56.
Krishnan M S, Geology of India and Burma, (Higginbothams
Ltd., Madras) 1960, pp. 604.
Pascoe E H, A Manual of the Geology of India and Burma,
(Govt. India Press, Calcutta) 1964, pp. 230.
KUMAR & SREEJITH: RELATIONSHIP BETWEEN HEAVY MINERAL PLACER DEPOSITS & HINTERLAND ROCKS 571
21 Radhakrishna B P, The Western Ghats of the Indian
Peninsula, (Proc. Seminar on Geomorphological Studies in
India) 1967, pp. 5–13.
22 Chacko, T., Ravindra Kumar, G.R. & Newton, R.C.,
Metamorphic P-T conditions of the Kerala (south India)
khondalite belt: a granulite facies supracrustal terrain, Jour.
Geol., 95(1987) 343–350.
23 Ravindra Kumar, G.R. & Chacko, T., Geothermobarometry
of mafic granulites and metapleite from the Palghat Gap,
south India: petrological evidence for isothermal uplift and
rapid cooling, Jour. Meta. Geol., 12 (1994) 479–492.
24 Gunnell, Y., Gallagher, K., Carter, A., Widdowson, M. &
Hurford, A.J., Denudation history of the continental margin
of western peninsular India since the early Mesozoic–
reconciling apatite fission-track data with geomorphology,
Earth Planet. Sci. Lett., 215 (2003) 187–201.
25 Campanile, D., Nambiar, C.G., Bishop, P., Widdowson, M.
& Brown, R., Sedimentation record in the Konkan-Kerala
Basin: implications for the evolution of the Western
Ghats and the Western Indian passive margin, Basin Res.,
20 (2007) 3–22.
26 Kalaswad, S., Roden, M.K., Miller, D.S. & Morisawa, M.,
Evolution of the continental margin of western India:
new evidence from apatite fission-track dating, Jour. Geol.,
101 (1993) 667–673.
27 Twidale C R, Granite Landforms, (Elsevier, Amserdam)
1982, pp. 372.
28 Migoń P, Granite Landscapes of the World, (Oxford
University Press, Oxford) 2006, pp. 384.
29 Ramakrishnan M, Tectonic evolution of the Granulite
terrains of southern India, Continental Crust of India,
(Mem. Geol. Soc. India) 1993, 25, pp. 45–62.
30 Braun I & Kriegsman L M, Proterozoic crustal evolution of
southernmost India and Sri Lanka, in: Proterozoic East
Gondwana: Supercontinent Assembly and Breakup, edited
by M. Yoshida, B.F. Windley & S. Dasgupta, (Spec. Publ.,
Geol. Soc., London) 2003, 206, pp.169–202.
31 Braun, I., Raith, M. & Ravindra Kumar, G.R., Dehydrationmelting phenomena in leptynitic gneisses and the generation
of leucogranites: a case study from the Kerala Khondalite
Belt, southern India, Jour. Petrol., 37(1996) 1285–1305.
32 Dash, B., Sahu, K.N. & Bowes, D.R., Geochemistry and
original nature of Precambrian khondalites in the Eastern
Ghats, Orissa, India, Trans. Royal Soc. Edinburgh, Earth Sci.,
78 (1987) 115–127.
33 Chacko, T., Ravindra Kumar, G.R., Meen, J.K. & Rogers,
J.J.W., Geochemistry of high-grade supracrustal rocks from
the Kerala Khondalite Belt and adjacent massif charnockites,
South India, Precam. Res., 55 (1992) 469–489.
34 Morimoto, T., Santosh, M., Tsunogae, T. & Yoshimura, Y.,
Spinel + quartz association from the Kerala khondalites,
southern India: evidence for ultrahigh temperature
metamorphism, Jour. Min. Petrol. Sci., 99 (2004) 257–278.
35 Cesare, B., Satish-Kumar, M., Cruciani, G., Pocker, S. &
Nodari, L., Mineral chemistry of Ti-rich biotite from
pegmatite and metapelitic granulites of the Kerala
Khondalite Belt (southeast India): Petrology and further
insight into titanium substitutions, Am. Min., 93 (2008)
327–338.
36 Santosh, M., Tagawa, M., Taguchi, S. & Yoshikura, S., The
Nagercoil Granulite Block, southern India: petrology, fluid
inclusions and exhumation history, Jour. Asian Earth Sci.,
22 (2003) 131–155.
37 Jayalakshmi, K., Nair, K.M., Hisao, K. & Santosh, M.,
Mineralogical and geochemical variations as indicators of
provenance in the heavy mineral deposits of Ambalapuzha
beach sands, SW coast of India, Jour. Geosci., Osaka City
Univ., 46 (2003) 157–168.
38 Bernstein, S., Frei, D., Mc Limans, R.K., Knudsen, C. &
Vasudev, V.N., Application of CCSEM to heavy mineral
deposits: Source of high-Ti ilmenite sand deposits of South
Kerala beaches, SW India, Jour. Geochem. Expl., 96 (2008)
25–42.