Classification of gem deposits of Sri Lanka
Abstract
Approximately 25% of Sri Lanka's landmass is gem-bearing. Most of the gem deposits are located in restricted zones
(approximately 15 000 sq km) within the area occupied by rocks of the Highland/Southwestern Complex. Over 90%
of Sri Lanka's gem mining is from secondary placer deposits that can be classified as sedimentary gem deposits of
residual, eluvial and alluvial types. Primary or in-situ gem occurrences are located mainly in contact-metamorphic
zones comprising of skarn and calcium-rich rocks. Corundum occurrences have also been found in aluminous-rich,
silica-deficient metasedimentary formations. Gem minerals that are frequently found in pegmatites within the
Highland/Southwestern Complex include corundum, zircon, beryl, quartz varieties, feldspar and chrysoberyl. A
special feature of many secondary gem deposits of Sri Lanka is their location on morphotectonically controlled
sites.
Introduction
Sri Lanka is known the world over for its vast potential
and exquisite varieties of gem minerals. Gem mining
in Sri Lanka has a history of over 2000 years and con-
tinues unabated even at present. In a recent study of the
gem potential of Sri Lanka, the authors (Dissanayake
& Rupasinghe 1993) have observed that nearly 25%
of Sri Lanka's landmass is gem-bearing. Their study
clearly indicates the vast gem potential of Sri Lanka
yet to be unearthed. However, in spite of the very long
history of gem mining in Sri Lanka, there had been no
major study of the gem deposits aimed at their scientific
classification.
Among the earliest accounts on the precious gem-
stones of Sri Lanka is that by Wadia & Fernando
(1945). Subsequently several workers dealt with dif-
ferent aspects of the geology and mineralogy of gem
deposits of Sri Lanka (GLibelin 1968, Katz 1972, Sil-
va 1976, Dahanayake et al. 1980, Munasinghe & Dis-
sanayake 1981, Rupasinghe & Dissanayake 1984, Dis-
sanayake & Rupasinghe 1993).
The sedimentary gem deposits of Sri Lanka were
classified by Dahanayake et al. (1980) into three
types, namely residual, eluvial and alluvial forma-
-tions. Even though sedimentary gem deposits form
a major constituent of Sri Lanka's gem resources, lat-
er work has shown the importance of other types of
gem deposits (Silva & Siriwardena 1988, Mendis et al.
1991, Kumarathilake & Ranasinghe 1992). There is,
therefore, a real need to classify all the gem deposits
of Sri Lanka on a scientific basis as it would provide
background information for future exploration efforts.
At present, exploration for gem minerals is based on
mere 'hearsay' and chance, and a proper understand-
ing of the nature of the different types of gem deposits
is therefore timely. This paper aims at a scientific clas-
sification of the gem deposits of Sri Lanka based on
their geological occurrence and inferred origin.
Geological setting of gem-bearing terrains
Approximately 90% of Sri Lanka is comprised of meta-
morphic rocks of Precambrian age. These rocks have
been classified in different ways by a number of authors
(Adams 1929, Cooray 1984, Vitanage 1985, Kroner
1986). Field work as well as isotopic, geochronolog-
ical, petrological and geochemical data obtained by
a German-Sri Lanka Research Consortium have ledto substantial revision of the existing classifications(Kroner et al. 1991). Figure 1 illustrates the new clas-sification of Kroner et al. (1991) which is used in thistudy. Four lithotectonic units are identified in accor-dance with the rules set forth in the North AmericanCode of Stratigraphic Nomenclature (Hattin 1991).
Lithotectonic units
The Highland/Southwestern Complex
The Highland/Southwestern Complex (HSWC; Kroner
et al. 1991) is the largest unit forming the back-
bone of the Precambrian rocks of Sri Lanka. Included
in it are the supracrustal rocks of the former High-
land Series (Group) and Southwestern Group (Cooray
1962, 1984) together with a variety of igneous intru-
sions of predominantly granitoid composition that now
occur as banded gneisses. This is the granulite ter-
rain of Sri Lanka, the prominent rocks present being
varieties of granulites, charnockites, quartz-feldspar-
garnet-sillimanite-graphite schists, quartzites, marbles
and calc-gneisses. Based on field and mineralogical
information, Kroner et al. (1991) infer that a significant
proportion of the rocks in the Highland/Southwestern
Complex is of granitoid origin.
Widespread arrested charnockite formation has
been observed in the Central Highland regions in the
districts of Colombo, Kurunegala and Galle (Hansen
et al. 1987).
The most important observation is that most of the
gem deposits of Sri Lanka occur within the High-
land/Southwestern Complex. This complex, which
has calciphyres, charnockites and cordierite-bearing
gneisses, contains abundant sedimentary gem deposits
such as those at Ratnapura, Avissawella, Balangoda
and Rakwana.
The source rocks of the gem minerals in the sedi-
mentary gem deposits are still a subject of much debate.
Among probable source rocks that are mentioned are
skarns, marbles, pegmatites, garnetiferous gneisses
and contact rocks of charnockites. Recent research
(Mendis et al. 1993) appears to indicate calcium-rich
rocks as being important source rocks for gemstones of
Sri Lanka. Charnockites may be considered important
as a heat source for contact metamorphism of limestone
and aluminous sediments.
The Vijayan Complex
The Vijayan Complex (VC) consists of biotite-
hornblende gneisses and scattered bands of metased-
iments and charnockitic gneisses. Small plutons of
granites and acid charnockites also occur close to
the east coast (Jayawardena & Carswell 1976). A
prominent feature in the area is the NW-trending
suite of dolerite dykes. Milisenda et al. (1991) have
described the gneissose granitoids of the Vijayan Com
plex as having compositions ranging from tonalite to
leucogranite. Kroner et al. (1991), commenting on the
fact that the Vijayan rocks have not experienced gran-
ulite facies metamorphism, interpret the charnockitic
bodies occurring within the Vijayan domain as tecton-
ic klippen and/or unfolded or intersliced fragments of
rocks of the Highland/Southwestern Complex, similar
to the Kataragama klippe of which the derivation from
the latter complex has been established.
The Wanni ComplexThe Wanni Complex (WC) includes the rocks of the
former Western Vijayan Complex and consists of a
suite of granitoid gneisses, charnockitic gneisses and
granites. Recent work by Milisenda et al. (1994) has
shown that there is a large diversity of amphibolite to
granulite-facies rocks such as metasediments of pre-
dominantly pelitic to semipelitic composition. The
depositional age of the Wanni Complex metasediments
is yet unknown, but age data obtained from detrital zir-
cons from metapelites show that the Wanni Complex
is younger than the Highland/Southwestern Complex.
The boundary between the Wanni Complex and the
Highland/Southwestern Complex is poorly defined.
Scheme of classification of gem deposits
Figure 2 shows the classification scheme of the gem
deposits of Sri Lanka. The scheme follows the accept-
ed petrological classification of the main three rock
types and the genesis of the deposits therefore forms
the basis of the classification. In addition a special cate-
gory which could be denoted as structurally controlled
deposits can be considered. Gem deposits very often
occur in exploitable structures within metamorphic for-
mations and these have been found to be common in
the Highland/Southwestern Complex. A further advan-
tage of a genetic classification of gem deposits lies in
its predictive value. Contact-metamorphic zones asso-
ciated with calcium-rich rocks for example are likely
loci for gem deposits and identification of such fea-
tures thus helps in locating target areas for detailed
exploration.
Sedimentary gem deposits
Dahanayake et al. (1980) have classified the sedimen-
tary gem deposits in Sri Lanka in three types, namely
(a) residual, (b) eluvial, and (c) alluvial. This classifi-
cation has genetic inferences and hence is of predictive
value. It is therefore proposed to adhere to this classi-
fication. The reader is referred to the original paper of
Dahanayake et al. (1980) and only a brief outline of the
different types of sedimentary gem deposits indicated
in Fig. 2 is given here. The localities mentioned are
shown in Fig. I.
Residual gem deposits
These deposits are represented by beds containing gem
minerals mostly deposited in-situ and are found at
depths ranging from a few centimeters to about 10
meters below the surface. The deposits are generally
found on flood-plains of rivers and streams and the
source of the gem minerals is located nearby. The
residual gem deposits are characterized by layers of
alternating sands, clays and laterites containing angu-
lar rock fragments. Such residual deposits are very
common in the Elahera gem field.
Eluvial gem deposits
Hillslopes often expose gem-bearing layers of the elu-
vial type. Depending on the location with respect to the
topography of the hill slopes and the flat areas incised
by valleys, the eluvial deposits often pass impercepti-
bly into the alluvial deposits described below.
Alluvial gem deposits
Alluvial gem deposits are by far the most prevalent
and are characteristic of the main Ratnapura gem field.
They can reach depths of 25 meters and may even have
more than one gem-bearing layer (illam). The allu-
vial gem deposits generally lie in old stream terraces
and flood plains and are characterized by well-rounded
heavy minerals indicating their provenance. The gem-
bearing beds in alluvial deposits are invariably het-
erogeneous and show different shapes and sizes. The
presence of a gem-bearing layer in one pit may not
necessarily be an indication of its presence in an adja-
cent gem pit. Figure 3 illustrates a geological section
of gem beds in an alluvial gem deposit in the Ratna-
pura gem field. Figure 4 shows a photograph of such a
deposit.
Mineralogy of the sedimentary gem deposits
Gem mining in Sri Lanka is almost entirely confined
to the sedimentary deposits except for a few miner-
als such as moonstones and topaz that are mined from
pegmatite deposits. The sedimentary gem deposits are
abundant in the gem fields of the Ratnapura, Elahera,
Rakwana, Balangoda, Opanayake, Hasalaka, Bibile,
Passara, Okkampitiya and Deniyaya areas. Mineralog-
ically the sedimentary gem deposits of Sri Lanka con-
tain a wide variety of gemstones and these include
corundum, zircon, spinel, chrysoberyl, garnet, beryl,
tourmaline, topaz, sillimanite and cordierite.
Rupasinghe et al. (1993) carried out a detailed
study of over 200 samples obtained from alluvial gem
deposits located on a granulite basement in Sri Lan-
ka and showed the potential use of indicator minerals
in gem exploration. Geikielite, sphene and davidite,
monazite, scapolite, spinels and garnets and saute in
stream sediments were found to be suitable for further
investigation as indicator minerals of gem deposits.
Careful mineral analyses in stream sediments pro-
vide the necessary information on the mineralogy of
gem fields, particularly in cases where there is a good
mineralogical correlation between the stream sedi-
ment and the gem deposit. Rupasinghe & Dissanayake
(1984) showed that Sri Lanka's gem-bearing stream
sediments contain high amounts of rare-earth elements
with the light rare-earth elements being particularly
enriched. Rare minerals such as fergusonite, gadolin-
ite, samarskite and niobian rutile are also considered
as potential indicators of gem minerals (Rupashinghe
et al. 1993). Many of these indicator minerals, which
may escape detection in the field, merit detailed inves-
tigation in the laboratory.
Metamorphic gem deposits
Classified under this heading are all primary gem
deposits that have a metamorphic genesis. In view
of the fact that 90% of Sri Lanka's rocks are of the
high-grade metamorphic type, it is conceivable that a
metamorphic parentage could be attributed to many
of Sri Lanka's gem deposits. The intense tropical
weathering decomposed and disintegrated these gem-
bearing host rocks of the Highland/Southwestern Com-
plex and yielded the secondary gem deposits in river
sediments.
Several workers have studied the origin of the pri-
mary gem deposits of Sri Lanka. Katz (1972), fol-
lowing his study on the cordierite gneisses from the
southwest of Sri Lanka, suggested that the Ratnapura-
type gem deposits are derived mainly from cordierite
gneisses. Munasinghe & Dissanayake (1981) showed
that cordierite is more likely to represent a product of
an intermediate stage of a desilication process culmi-
nating in the formation of corundum. They presumed
that the desilication process had been caused by the
contact-metamorphic effects of charnockite and other
basic intrusions such as diopside-rich dykes and sills,
and had affected aluminous metasediments. Rupas-
inghe & Dissanayake (1985) elaborated on this premise
and emphasized the possible role of charnockites in
the genesis of gem minerals. More recent work, how-
ever, points towards calcium-rich rocks such as skarns
as being important in the origin of corundum-bearing
gem deposits (Mendis et al. 1993).
Skarn and calcium-rich rock type
Several field investigations by the authors and their
colleagues have highlighted the role of calcium-rich
bedrock as a source for gem minerals within the meta-
morphic terrain of Sri Lanka. The presence of pure CO2
inclusions in corundum from four localities in Sri Lan-
ka, including the Ratnapura district, points to the role
of CO2 and reflects the carbonate-rich environment as
being favourable for the occurrence of sapphire, ruby
and spinel (De Maesschalck & Oen 1989). The high
density of these primary inclusions, 1.05 g/cm3, was
considered compatible with the formation of corundum
under granulite-facies metamorphism.
Silva & Siriwardena (1988) carried out field and
laboratory studies in a corundum-bearing skarn deposit
at Bakamuna near Elahera. They concluded that the
skarn body was formed by the reaction of peg-
matitic fluids with marble. Hydraulic fracturing of the
rock with simultaneous increase in CO2 pressure and
dedolomitization had made the rock permeable to the
fluids which reacted with the marble in stages:
fluid I + marble scapolite + corundum + MgCO3 + CO2 (1)
With increasing Mg-activity in the fluids, scapolite and
corundum become unstable, resulting in the reactions:
MgCO3 + corundum ---+ spinel + CO2 (2),
fluid I + MgCO3 + spinel + scapolite phlogopite + fluid II.
The action of the Al-enriched fluid II on the fresh
marble through a reaction analogous to reaction (1)
Cooray (1984) cited the occurrences of spinet and
corundum in skarns formed at the contact of intru-
sive granite and marble in the Elahera area. Wadia
& Fernando (1945) had recorded a similar occurrence
of corundum and spine] at the contact of marble and
syenite at Ohiya, while Hanni & Gunawardene (1982)
noted the occurrence of ferroaxinite that was typically
formed by pegmatitic action on calcareous rocks. Gar-
nets are also of very common occurrence in calcareous
rock types in Sri Lanka.
Figure 5 shows an example of gem quality spinel.
There is now ample evidence that skarn and calcium-
rich rocks are good host rocks for gem minerals. Explo-
ration efforts should therefore be concentrated on local-
ities of metasomatically altered carbonate and Ca-rich
rocks within the granulite belt.
Aluminous metasedimentary type
The abundant aluminous metasedimentary rocks with-
in the Highland/Southwestern Complex are charac-
terised by a chemistry appropriate for the formation
of corundum and other aluminous gem minerals. Katz
(1986), in his review of the geology of the gemstones of
Sri Lanka, noted the following theories that had been
proposed for the origin of gemstones in high-grade
metamorphic terrains:
the isochemical metamorphism of an original-
ly highly aluminous, silica-deficient parent (e.g.
bauxite, anorthosite),
desilication of aluminium-bearing rocks by nearby
mafic intrusions,
metasomatism and partial melting.
Munasinghe & Dissanayake (1981) and Rupas-
inghe & Dissanayake (1985) have suggested that
Fig. 8. Gem quality garnet in a structurally controlled primary
deposit near Elahera (ballpoint pen for scale).
charnockitic rocks which generally occur in regions
of crustal thickening as in the Highland/Southwesten
Complex play a key role in the formation of gem min-
erals. They contend that contact-metamorphic effects
of charnockitic plutonic activity on highly aluminous
metasediments result in the formation of an assem-
blage of gem minerals such as sapphirine, cordierite,
spinel, ruby and corundum.
Katz (1986) however suggested that such a mode of
origin is applicable only locally, and proposed an ori-
gin related to granulite-facies regional metamorphism
involving CO2 flooding, purging of H20-rich fluids,
and partial melting. He further suggested that the gem
deposits of Sri Lanka are probably part of a Precambri-
an Gondwanaland granulite-facies gem province that
includes India, Madagascar and Antarctica.
Corundum in a biotite-sillimanite gneiss from near
Polgahawela was studied by Cooray & Kumarapeli
(1960), who ascribed its origin to re-crystallization
and metamorphic differentiation with formation of
alumina-rich, silica-poor bands in a semi-pelitic gneiss.
A similar occurrence of in-situ corundum had been
recorded by Coomaraswamy as far back as 1903, near
Kandy.
