the geology, mineralogy and rare element geochemistry of the gem

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5
THE GEOLOGY, MINERALOGY AND RARE ELEMENT GEOCHEMISTRY
OF THE GEM DEPOSITS OF SRI LANKA
C.B. DISSANAYAKE#, ROHANA CHANDRAJITH and H.J. TOBSCHALL
DISSANAYAKE, C.B., CHANDRAJITH, ROHANA and TOBSCHALL, H.J.
2000. The geology, mineralogy and rare element geochemistry of the gem deposits of Sri Lanka. Bulletin of the Geological Society of Finland 72, Parts 1–
2, 5–20.
The gem deposits of Sri Lanka are studied from the point of view of their
geology, mineralogy and geochemistry. Nearly all the gem formations are located in the central high-grade metamorphic terrain of the Highland Complex.
The gem deposits are classified as sedimentary, metamorphic and magmatic; the
sedimentary types being the most abundant. The mineralogy of the gem deposits varies widely with, among others, corundum, chrysoberyl, beryl, spinel, topaz, zircon, tourmaline, garnet and sphene being common.
Rare element concentrations in sediments from the three main gem fields of
Sri Lanka, namely Ratnapura, Elahera and Walawe, were studied. It was found
that some sediments are considerably enriched in certain elements compared to
their average continental crustal abundances. The Walawe Ganga sediments show
anomalous enrichments of the high field strength and associated elements, particularly Zr, Hf, W and Ti. This is attributed to the presence of accessory minerals such as zircon, monazite and rutile. Some of these heavy minerals comprise as much as 50 wt% of sediment. The geochemical enrichment of some trace
elements compared to their average crustal abundances indicates that highly differentiated granites and associated pegmatites have provided the source materials for enrichment.
Key words: gems, sediments, geochemistry, trace elements, enrichment, mineralogy, Sri Lanka
C.B. Dissanayake: Department of Geology, University of Peradeniya, Peradeniya, Sri Lanka.
E-mail: [email protected]
Rohana Chandrajith* and H.J. Tobschall: University of Erlangen-Nürnberg, Institute of Geology and Mineralogy, Chair of Applied Geology, Schlossgarten 5,
D-91054 Erlangen, Germany
# Corresponding Author
* Present Address: Department of Natural Resources, Faculty of Applied Sciences, Sabaragamuwa University of Sri
Lanka, Buttala, Sri Lanka
6
C.B. Dissanayake, Rohana Chandrajith and H.J. Tobschall
INTRODUCTION
It has been estimated that nearly 25% of the total
land area of Sri Lanka is potentially gem-bearing,
making Sri Lanka one of the countries richest in
gems (Dissanayake 1991, Dissanayake & Rupasinghe 1993). During the past few decades, many
new gemstones and hitherto unknown, yet interesting, gem quality minerals have been discovered.
The gem fields of Sri Lanka contain about 75 varieties and sub-varieties of gemstones, some in
abundance and some as rarities, which occur both
among gem gravels and as components of numerous rock types (Gunaratne & Dissanayake 1995).
Many of these gem minerals are unique and have
recently been the subject of much research.
The geology, mineralogy and geochemistry of
the gem-bearing terrains of Sri Lanka have not
been fully investigated. These gem deposits and
occurrences are set in a high-grade metamorphic
Precambrian terrain, and display unique geological and mineralogical features that are worthy of
thorough scientific investigation. Up to now very
little work has been carried out on these trace element-enriched stream sediments that drain the
gem-bearing terrains in Sri Lanka. Of particular
interest are the occurrences of minerals that are
abundant in rare elements and the probable mechanisms of their enrichment in source rocks and
sediments.
Recently, the gem-bearing sediments of some
of the rivers draining the gem fields of Sri Lanka
have been shown to be highly enriched in certain
trace elements, including rare-earth elements
(REE), Zr, Hf, Ta and Nb (Rupasinghe & Dissanayake 1984, Dissanayake & Rupasinghe 1986,
Dissanayake et al. 1994). Investigations on the
stream sediment geochemistry of the Walawe
Ganga Basin of Sri Lanka (Chandrajith 1999) have
shown very high enrichment factors for some of
the trace elements mentioned above, thus indicating the existence of mineralized terrains within the
gem fields. Therefore, the origin of the gemstones
and the enrichment of these elements appear to be
related genetically.
This paper reviews the present understanding
of the geology, mineralogy and geochemistry of
the gem deposits of Sri Lanka. Further, it is anticipated that research on the stream sediment geochemistry of the gem-bearing terrains of Sri Lanka will contribute to the further understanding of
their mineralization mechanisms and to the development of mineral exploration strategies.
GEM DEPOSITS OF SRI LANKA AND
THEIR GEOLOGIC SETTING
General geology of Sri Lanka
Geologically, Sri Lanka is dominated by Precambrian high-grade metamorphic rocks and can be
divided into three major lithotectonic units, namely the Highland Complex, the Vijayan Complex
and the Wanni Complex (Cooray 1994). Among
these, the Highland Complex is the largest unit and
forms the backbone of the Precambrian bedrock
of Sri Lanka. Included in this unit are the supracrustal rocks and a variety of igneous intrusions, predominantly of granitoid composition,
that are represented by banded gneisses (Kröner
et al. 1991). The rocks comprising the Highland
Complex were mostly metamorphosed under granulite facies conditions. There has been widespread
formation of incipient (arrested) charnockite within this unit (Hansen et. al. 1987) Elsewhere, other
granulite types, quartz-feldspar-garnet-sillimanitegraphite schists, quartzites, marbles and calc-silicate gneisses are prominent.
The Vijayan Complex lying to the east of the
Highland Complex (Fig. 1) consists of biotitehornblende gneisses and scattered bands of metasediments and charnockitic gneisses. It also comprises small plutons of granite and acid charnockites (Jayawardena & Carswell 1976) and a NWtrending suite of dolerite dikes. Milisenda et al.
(1991) have described the gneissose granitoids of
the Vijayan Complex as tonalitic to leucogranitic
in composition. The Vijayan Complex, which
comprises rocks mostly in the amphibolite facies,
has not been subjected to granulite facies metamorphism and this fact has been interpreted by
Kröner et al. (1991) to infer that the charnockitic
bodies within the Vijayan domain are klippes and/
The geology, mineralogy and rare element geochemistry of the gem deposits of Sri Lanka
7
Fig. 1. Map of Sri Lanka showing the main lithotectonic units and the gem fields studied. The boundary between
the Highland Complex and Wanni Complex is uncertain and is denoted by a broken line. The numbered sequences
represent the topographic sheets. 1: Polonnaruwa, 2: Nalanda, 3: Elahera, 4: Kurunegala, 5: Rangala, 6: Kandy,
7: Hanguranketa, 8: Nilgala, 9: Avissawella, 10: Hatton, 11: Nuwara Elliya, 12: Passara, 13: Panadura-Horana, 14: Ratnapura, 15: Haputale, 16: Buttala, 17: Alutgama, 18: Rakwana, 19: Timbolketiya, 20: Kataragama,
21: Ambalangoda, 22: Morawaka, 23: Ambalantota, 24: Galle, 25: Matara. The palaeopressure contours of the
Highland Complex of Sri Lanka in relation to the locations of the gem fields are also shown. Data sources: Prame
(1991), Schenk et al. (1991).
8
C.B. Dissanayake, Rohana Chandrajith and H.J. Tobschall
or unfolded or intersliced fragments of the Highland Complex. These are similar to the Kataragama Klippe which is derived from the latter complex (Cooray 1984, Vitanage 1985).
The Wanni Complex consists of granitoid
gneisses, charnockitic gneisses and granites.
Milisenda et al. (1991) showed that these rocks are
mainly amphibolite to granulite facies metasedimentary rocks of predominantly pelitic to semipelitic composition. Studies of detrital zircons
from metapelites have shown that the Wanni Complex is younger than the Highland Complex, even
though the boundary between these units is poorly defined.
Geological conditions of gem mineral
formation
The centrally located granulite grade Highland
Complex is about 30 000 km2 in area and presumably contains the host rocks for the gems. The
study of Schenk et al. (1991) showed that metamorphic pressures are zoned within the Highland
Complex. Cordierite- and garnet-cordierite-bearing metapelitic assemblages, indicative of relatively low pressures, are restricted to the western and
northwestern part of the complex, whereas garnetsillimanite ± biotite-bearing assemblages are predominant in the southeast, in the east and in the
Kataragama Klippe. Notably, the garnet-sillimanite ± biotite-bearing assemblages are absent in the
western part. Schenk et al. (1991) concluded that
the peak pressures in the southeastern Highland
Complex were between 8 and 10 kbar and decreased to 7 and 6 kbar in the west (Fig. 1). They
showed that the Highland Complex does not represent a single level in a former lower continental crust, but a more or less continuous section of
the lower crust with a vertical thickness of about
15 km. In the southeast, the exposed crust is estimated to have been originally at a depth of about
30–35 km, whereas the exposed rocks in the west
were originally at a depth of about 15–20 km.
The gem fields of Sri Lanka are mostly located within the high-pressure region in the southeast (see Fig. 1) and in some northern regions (e.g.
Elahera). They are much less prominent in regions
of lower pressure. Gem minerals are absent in the
Vijayan Complex which does not have pyroxene
± garnet-bearing assemblages. The temperatures of
formation of the gem minerals within the Highland Complex are estimated to range from about
700 °C to 900 °C (Prame 1991).
Nearly all the gem deposits of Sri Lanka are
derived from the granulite facies rocks of the
Highland Complex (Fig. 1), clearly indicating that
there were petrological conditions suitable for the
formation of gemstones. Gemstones found within the Vijayan domain have been transported by
rivers from the Highland Complex.
The source rocks of the gem minerals are
skarns, marbles, pegmatites, garnetiferous gneisses
and the contact rocks of charnockites (Dissanayake & Rupasinghe 1995). Recent research (Silva
& Siriwardena 1988, Mendis et al. 1993) has
shown that these calcium-rich rocks were particularly suitable as source rocks for the gemstones.
Rupasinghe and Dissanayake (1985) discussed the
importance of charnockites as a heat source for the
contact metamorphism of limestone and aluminous
metasediments. Earlier, Munasinghe and Dissanayake (1981) constructed a sequence of events
that they thought were significant in bringing
about gem mineralization:
1. Deposition of argillaceous sediments in the
Highland Basin (the present Highland Complex) during the Archaean. These were presumably derived from weathering and transportation of material from a continental crust.
2. Deformation and metamorphism of sediments
during a collisional orogeny. The granulite
facies conditions attained favoured the formation of gem minerals such as garnet, sillimanite, andalusite and cordierite.
3. Basement remobilization associated with collision and the emplacement of basic and ultrabasic igneous rocks. Subsequent desilication,
caused by the contact metamorphic effects of
charnockites and other basic intrusions, formed
corundum and spinels.
4. Intrusion of pegmatites enriched in Be and F
into the basic and ultrabasic rocks within the
metasediments of the Highland Complex. Gem
The geology, mineralogy and rare element geochemistry of the gem deposits of Sri Lanka
minerals such as beryl, chrysoberyl, topaz and
tourmaline are assumed to have been formed
in this manner.
The presence of Al-rich metasediments, regimes
of high P and high T, contact metamorphism, and
extensive fluid activity were prerequisite for gem
formation within the Highland Complex of Sri
Lanka.
9
sis. The advantage of the genetic classification of
gem deposits lies in its predictive value. For example, contact metamorphic zones associated with
calcium-rich rocks are likely loci for certain gem
deposits in Sri Lanka and identification of such
features assists in the location of target areas for
detailed exploration.
Sedimentary gem deposits
CLASSIFICATION OF THE GEM
DEPOSITS OF SRI LANKA
Fig. 2 illustrates the classification scheme for the
gem deposits of Sri Lanka proposed by Dissanayake and Rupasinghe (1995). The scheme follows
the general classification of the three main rock
types and classification is based on deposit gene-
Sedimentary gem deposits are by far the most
important of all gem deposits in Sri Lanka and
were classified by Dahanayake et al. (1980) into
residual, eluvial and alluvial types. The sedimentary placer gem deposits occur in thin layers or
lenses of gravel and sand, termed locally as Illam,
in river beds and alluvial plains and on hillslopes
and hillsides. Among the most important factors
that govern the depositional nature of these gem
Fig. 2. Classification of the gem deposits of Sri Lanka with examples of locations of different types (modified
after Dissanayake & Rupasinghe 1995).
10
C.B. Dissanayake, Rohana Chandrajith and H.J. Tobschall
deposits are the intensity and distance of transportation from the source and the topographical suitability of the sites for deposition.
The residual gem deposits occur as beds containing gem minerals mostly deposited in-situ and
are found at depths ranging from a few centimetres to about 10 metres. These deposits mostly
occur on the flood plains of rivers and streams and
their sources are assumed to be in the close vicinity. A characteristic feature of the residual gem
deposits is the presence of layers of alternating
sand, clays and laterites containing angular fragments, as exemplified by the Elahera gem deposit.
The eluvial sedimentary gem deposits are found
on hillslopes and flat areas incised by valleys.
Often, the eluvial deposits grade into alluvial deposits making identification difficult. The presence
of rock fragments and the angular to sub-rounded nature of the gem minerals are characteristic
of the eluvial beds.
Alluvial gem deposits are the most widely distributed gem deposit type in Sri Lanka, the Ratnapura gem deposits being a good example. They
often reach depths of more than 20 metres and
usually contain two or three gem-bearing layers.
They occur mostly in old stream terraces and flood
plains and are characterized by well-rounded
grains. The gem-bearing layers in these alluvial
deposits are markedly heterogeneous exhibiting a
variety of shapes and sizes that indicate frequent
changes in stream courses and velocity.
Metamorphic gem deposits
Most of the gem deposits in Sri Lanka are clearly
of metamorphic genesis reflecting the fact that
approximately 90% of Sri Lanka comprises highgrade metamorphic rocks. Intense tropical weathering has decomposed and disintegrated the gembearing rocks to form sedimentary gem deposits.
sions in gem corundums from Sri Lanka and noted
that all the fluid inclusions are pure CO2. Thus
CO2 is an important indicator of the genesis of the
gem minerals. The microthermometry results for
the primary inclusions suggested that these corundums formed under granulite facies metamorphism
(> 630°C, 5.5 kbar), while the presence of secondary fluid inclusions indicated retrograde postmetamorphic cooling and uplift of the source areas. Further, the high density of the fluid inclusions (average density d = 1.05 g/cm3) was considered as being compatible with the formation of
corundum under granulite facies metamorphism.
Silva and Siriwardena (1988) described an example of a corundum-bearing skarn deposit, located
at Bakamuna near the main Elahera gem field.
Cooray (1984), and Wadia and Fernando (1945)
describe some other examples of this type, at Elahera and at Ohiya respectively.
Aluminous metasedimentary rock types
One of the characteristic features of the Highland
Complex is the abundance of aluminous metasedimentary rocks. These have the chemical composition required for the formation of corundum and
other aluminous gem minerals. It is clear that in
the Highland Complex a combination of the P-T
conditions and a suitable chemical composition
has yielded voluminous sources for gems of this
type. Katz (1986) suggested that these gemstones
have an origin related to granulite facies metamorphism involving CO2 flooding, the purging of
H2O-rich fluids and partial melting. Cooray and
Kumarapeli (1960) studied the occurrence of corundum in biotite-sillimanite gneiss and ascribed
its origin to recrystallization and metamorphic differentiation with the formation of aluminium-rich,
silica-poor bands in a semipelitic gneiss.
Gems of pegmatitic origin
Skarn and calcium-rich rock types
Recent research has shown that calcium-rich bedrock is a source for gem minerals within the metamorphic terrain of Sri Lanka. Maesschalck and
Oen (1989) studied the mineral and fluid inclu-
Pegmatites are common in the Highland Complex
and they are also considered as important
sources of gem minerals. One of the best known
pegmatitic gem deposits is the moonstone deposit
at Meetiyagoda, southern Sri Lanka (Spencer
The geology, mineralogy and rare element geochemistry of the gem deposits of Sri Lanka
1930, Malley 1989). In addition, moonstones have
been located in regions around Balangoda and
Kundasale near Kandy. Pegmatites in Sri Lanka
also contain gem minerals such as beryl, chrysoberyl, zircon and corundum (Rupasinghe et al.
1994).
GEM LOCALITIES IN SRI LANKA
The Ratnapura gem field
The Ratnapura gem field is by far the most important gem field in Sri Lanka. The gem deposits
of this vast field are of alluvial and eluvial types.
As shown in Fig. 3, except for scattered patches
of alluvium, the areas covered by the main Ratnapura gem field consist of Precambrian metamorphic rocks of charnockite-metasedimentary type.
The main rock types are charnockites, garnet-sil-
11
limanite granulites, amphibolites and perthitebearing garnet-biotite granulitic gneisses. Of these,
charnockites and pelitic garnet-sillimanite granulites are the most abundant. The occurrence of intrusive rocks of zircon-bearing granites, vein
quartz and pegmatites is of particular significance.
The Ratnapura gem field consists of Pleistocene
or sub-recent alluvium with patches or streaks of
gravel of heavy minerals laid down in flood plains
of streams, either in the beds of abandoned tributaries or in talus fans at the foot of steep hillslopes
(Wadia & Fernando 1945). The heavy minerals
including gems were deposited during periods of
intense flooding that caused their mechanical removal from their source areas.
The Elahera gem field
The Elahera gem field, located in northeastern Sri
Lanka (Figs. 1 and 4), has produced a significant
Fig. 3. Geological map of the main Ratnapura gem field. Inset shows the location of the Ratnapura and Elahera
gem fields in relation to the main lithotectonic units of Sri Lanka (modified after Rupasinghe & Dissanayake
1985).
12
C.B. Dissanayake, Rohana Chandrajith and H.J. Tobschall
Fig. 4. Geological map of the Elahera gem field (modified after Rupasinghe & Dissanayake 1985).
amount of gemstones and the deposit is mostly of
the residual type, although alluvial deposits are not
uncommon. The Elahera gem deposits also lie
within the Highland Complex and consist mainly
of quartzites, marbles and garnet-sillimanitebiotite gneisses. Silva (1976) reported the occurrence of granites and pegmatites in the Highland
Complex and these have a special significance
because they are considered source materials for
the gemstones.
Silva and Siriwardena (1988) described a corundum-bearing deposit at Bakamuna in the Elahera gem field (Fig. 5). According to these authors, the skarn body was formed by the reaction
of pegmatitic fluids with marble. Hydraulic fractures in the rock, an increase in CO2 pressure and
dedolomitization had made the rock permeable to
fluids. The marble reacted with these fluids to
form the corundum.
Fig. 5. Geology of the Bakamuna area and the detailed
geology of the skarn deposit (after Silva & Siriwardena 1988).
Corundum-bearing gem pockets
of pegmatitic origin
An interesting occurrence of corundum-bearing
gem pockets of pegmatitic origin was described
by Kumaratilake and Ranasinghe (1992) from
Avissawella and Getahetta, northwest of the main
Ratnapura gem field. There are two types of gem
pockets: corundum-bearing pockets and hollowtype. The former contains treatable corundum and
lesser quantities of yellow and blue corundum
while the latter type only contains pyrite. These
gem pockets are of pegmatitic origin.
The geology, mineralogy and rare element geochemistry of the gem deposits of Sri Lanka
The moonstone deposit at Meetiyagoda is one
of the best examples in Sri Lanka of a pegmatitic
type of gem formation and is associated with a
large pegmatite vein that crosscuts metamorphic
rocks. Malley (1989) has shown that the mineral
composition of the deposit is approximately 50%
clay, 40% feldspar, and 5% quartz, smoky quartz
and opaline silica with traces of sulphides (mostly marcasite) and tourmaline.
MINERALOGY AND GEOCHEMISTRY
OF THE GEM DEPOSITS
Sri Lanka has a wide variety of gem minerals including, among others, corundum, chrysoberyl,
zircon, tourmaline, kornerupine, garnets, topaz,
spinel and taaffeite. Table 1 gives the key gem
minerals of Sri Lanka listed by locality. Fig. 6
gives a general classification of the minerals found
in the washed gem gravels of Sri Lanka. Among
the heavy minerals found in gem-bearing stream
sediments are zircon, garnet, monazite, ilmenite,
magnetite and rutile. Recent studies in the Walawe
Ganga Basin in southwestern Sri Lanka (Chandrajith 1999) show that some stream sediments contain as much as 50 wt% of these heavy minerals.
Studies by Rupasinghe et al. (1994) indicate that
some minerals in the stream sediments are potential indicator minerals for gems: notably Mg-rich
ilmenite, geikielite, Mg-rich spinel, Ca-rich scapolite, Ca-Mg pyroxene (salite), Ca-rich garnet
(grossular) and minerals containing REE such as
sphene, davidite and monazite. These appear to be
closely associated with the gems themselves and
are common in the stream sediments.
The gem-bearing sediments of Sri Lanka are
also rich in certain rare minerals as shown in Table 2. These minerals have a unique geochemical
signature and are rich in REE, Ta, Nb, Zr, Th, U,
Ti, Be and F (Dissanayake & Rupasinghe 1992).
The occurrence in sediments of anomalous concentrations of such elements leads to the discovery of rare minerals such as zirkelite, niobian rutile, gadolinite, chevkinite, samarskite, aeschynite,
anatase and fergusonite. Some of the gem minerals, namely serendibite Ca2(MgAl)6(Si,Al,B)6O22,
13
sinhalite MgAlBO4, ceylonite MgAl2O4, taprobanite Mg3Al8BeO16, ekanite ThCa2Si8O22 and uvite
WX3Y6(BO3)3Si6O18 were discovered in Sri Lanka. These are special varieties of larger families
of gem minerals and have unique properties. Within the heavy mineral fractions of the stream sediments, some radioactive minerals such as monazite, zircon, thorianite, thorite and allanite are also
abundant. They contain significant concentrations
of U, Th, REE and some other trace elements.
Trace elements in the sediments
Rupasinghe and Dissanayake (1985) studied the
geochemistry of the stream sediments that form
the Ratnapura and Elahera gem fields and Chandrajith (1999) investigated the geochemistry of
those of the Walawe Ganga Basin. The trends in
element enrichment in the Ratnapura, Elahera and
Walawe Ganga gem fields are put in perspective
in Fig. 7, which shows the enrichment factors of
elements of the sediments against their average
crustal abundances.
In the stream sediments from the three gem
fields, most alkali, alkaline earth and transition
elements show closely similar enrichment factors.
However, in stream sediments from the Elahera
gem field, the enrichment factors for most transition elements, Sr and Ba are lower than those for
stream sediments from the Ratnapura gem field
and the Walawe Ganga Basin. Samples from the
latter, which is situated southeast of the main Ratnapura gem field (Fig. 1), are particularly enriched
in Ti, Zr, U, Th, W, La, Ce, Hf and Ta.
The enrichment or depletion of the elements in
the various gem fields reflects the geology, geochemistry and mineralogy of the source regions.
In the Ratnapura and Walawe Ganga gem fields,
the mineralogy of the sediments, particularly the
abundance of minerals such as zircon, monazite,
rutile and spinel, mainly controls the geochemistry of the elements. Because of this, those sediment fractions coarser than 63 µm are particularly enriched in trace elements, notably REE, Zr,
Mo, U, Th and Ta (Chandrajith 1999). It is apparent that the high abundance of zircons in the
Walawe Ganga gem field has a marked influence
14
C.B. Dissanayake, Rohana Chandrajith and H.J. Tobschall
Table 1. Key gem minerals of Sri Lanka listed by locality. Topographic sheet numbers are those shown in Fig. 1
(after Dissanayake & Rupasinghe 1993).
Topographic Sheet
No.
Name
Gem minerals
01
Polonnaruwa
corundum, garnet
02
Nalanda
apatite
03
Elahera
chrysoberyl, corundum, garnet, iolite (cordierite), kornerupine,
sinhalite, sphene, spinel, zircon
04
Kurunegala
amethyst, apatite, citrine, fluorite, iolite (cordierite), topaz, tourmaline
05
Rangala
no known deposits
06
Kandy
amethyst, aquamarine
07
Hanguranketa
corundum
08
Nilgala
corundum, garnet, spinel, tourmaline, zircon
09
Avissawella
amethyst, andalusite, beryl, chrysoberyl, corundum, diopside, epidote, iolite (cordierite),
kornerupine, garnet, sinhalite, spinel, tourmaline, zircon
10
Hatton
andalusite, corundum, garnet, iolite (cordierite), spinel, topaz
11
Nuwara Eliya
amethyst, corundum, spinel, zircon
12
Passara
corundum, ekanite, garnet, kornerupine, spinel, taaffeite, topaz, tourmaline, zircon
13
Panadura-Horana
aquamarine, axinite, beryl, chrysoberyl, corundum, garnet, vesuvianite, phenakite,
scapolite, sillimanite, spinel, taaffeite, topaz, tourmaline, zircon
14
Ratnapura
amethyst, andalusite, apatite, beryl, chrysoberyl, citrine, corundum, diamond, danburite,
diopside, ekanite, garnet, iolite (cordierite), kornerupine, scapolite, sillimanite, sinhalite,
spinel, taaffeite, topaz, tourmaline, zircon
15
Haputale
andalusite, axinite, beryl, chrysoberyl, corundum, diopside, garnet, vesuvianite, spinel,
topaz, tourmaline, zircon
16
Buttala
corundum, ekanite, garnet, spinel, tourmaline
17
Alutgama
chrysoberyl, corundum, spinel, zircon
18
Rakwana
apatite, aquamarine, axinite, beryl, chrysoberyl, corundum, danburite, diopside, ekanite,
enstatite, fluorite, garnet, kornerupine, spinel, tourmaline, zircon
19
Timbolketiya
garnet
20
Kataragama
corundum, hiddenite (spodumene), sphene, spinel
21
Ambalangoda
moonstone (feldspar)
22
Morawaka
aquamarine, beryl, chrysoberyl, corundum, danburite, diopside, garnet, sillimanite,
sphene, spinel, tourmaline, zircon
23
Ambalantota
beryl, chrysoberyl, corundum, garnet, vesuvianite, iolite (cordierite), scapolite,
sillimanite, sinhalite, spinel, tourmaline, zircon
24
Galle
beryl, chrysoberyl, corundum, sphene
25
Matara
aquamarine, chrysoberyl, corundum, garnet, zircon
on the concentration of some of these elements.
It has been reported that zircon is a sink for more
than 50 elements (Speer 1982). Chandrajith (1999)
reported nearly identical average Zr/Hf ratios of
73, 75, 75 and 71 in four stream sediment size
fractions from the Walawe Ganga gem-bearing
area. Because of their similar ionic radii, coordination numbers and ionic charges, Hf4+ (r = 83 pm)
readily substitutes for Zr4+ (r = 84 pm) in zircon,
which occurs as a solid solution series with the
The geology, mineralogy and rare element geochemistry of the gem deposits of Sri Lanka
15
Fig. 6. A general classification of the minerals found in washed gem sediments (after Rupasinghe et al. 1986).
end-members zircon (ZrSiO4) and hafnon (HfSiO4). Medenbach (1976) reported that the sum of
the concentrations of REE and Y in zircon can be
as high as 25 wt%. The enriched concentrations
of REE in the Walawe Ganga sediments are due
not only to the presence of zircon, but also due
to the presence of other minerals such as monazite. The enrichments of Nb, Th and U are attributed to the occurrence of minerals such as niobian rutile, which acts as a sink for Nb, Ta, Ce and
La, and fergusonite, which is a sink for Y, Nb, Ta,
Th and U. The presence of minerals such as thor-
ite and thorianite accounts for the enrichments of
Th and U in the sediments. The enrichments of
base metals (Co, Ni, Cu) in the Ratnapura gem
sediments, though of minor significance, are presumably due to the basic rocks in the terrain,
which are hosts for these metals.
Origin of rare metal enrichments
in the sediments
The above discussion has shown that the stream
sediments, particularly those in southwestern and
16
C.B. Dissanayake, Rohana Chandrajith and H.J. Tobschall
Table 2. Rare minerals found in the gem sediments of Sri Lanka (after Dissanayake & Rupasinghe 1992).
Mineral
SG
Chemical formula
Other elements found
Ekanite
Serendibite
Anatase
Allanite
Perovskite
Zirkelite
Rutile
Gadolinite
Geikielite
Microlite
Zircon
Chromite
Ce-Monazite
Columbite
Baddeleyite
Tantalite
Samarskite
Thorite
Fergusonite
Scheelite
Cassiterite
Thorianite
3.28
3.4
3.9
4
4
4
4
4–4.7
4.05
4.2
4.3–4.5
4.5–4.8
4.8–5
5
5
5–8
5.24
5.3
5.6–5.8
5.9–6.1
6.8–7.1
9.7
K(Ca,Na)2Th(Si8O20)CaTi(O/SiO4)
(Ca,Mg)5(AlO)5(BO3/(SiO4)3)
TiO2
(Ca,Ce)(Fe3+,Fe2+)Al2O(SiO4)(Si2O7)(OH)
CaTiO3
(Ca,Ce,Y,Fe)(Ti,Zr,Th)3O7
TiO2
Y2Fe2+Be2(O/SiO4)2
MgTiO3
(Ca,Na)2(Ta,Nb,Ti)2O6(OH,O,F)
ZrSiO4
(Fe,Mg)Cr2O4
CePO4
(Fe,Mn)(Ta,Nb)2O6
ZrO2
(Fe,Mn)(Nb,Ta)2O6
(Y,U,Ca)2(Nb,Fe2+)2(O,OH)6
ThSiO4
Y(Nb,Ta)O4
CaWO4
SnO2
(Th,U)O2
Nb, Ta, REE
Nb, Ta, REE
1–14% U2O8, REE
Nb, Ta, Fe
U, Th, Hf, REE
U, Th, REE
Ta, REE
U
4% U3O8, REE
Nb, Ta, Ti, Mn, Zr, W, Fe
Fig. 7. Comparison of the enrichment factors of the elements in the –63 µm sediment fractions with their average crustal abundances of Wedepohl (1995) for Ratnapura, Elahera and Walawe gem fields (after Rupasinghe
& Dissanayake 1985, Chandrajith 1999)
southeastern Sri Lanka, are rich not only in gem
minerals but in trace elements as well. The mineral sources, particularly those rich in trace elements such as Zr, Hf, REE, Ta, Nb, U and Th,
are associated with late stage magmatic events
that brought about metal-rich solutions through
igneous activity that included pegmatite emplace-
ment. The origin of the sources is closely related to the geologic and tectonic history of the
Highland Complex. The restriction of both the
gem mineralization and the metal enrichments
mostly to this area indicates late magmatic activity, which possibly post-dated the main granulite facies metamorphism. Structural and geo-
The geology, mineralogy and rare element geochemistry of the gem deposits of Sri Lanka
17
Fig. 8. P-T-t paths for the Highland Complex (modified after Prame 1995).
logical investigations support this view (Voll &
Kleinschrodt 1991).
The granulite terrain of Sri Lanka is considered
to be a continuous crustal block representing the
middle to lower crust (Prame 1991). Mineral parageneses indicative of various P-T conditions were
formed either as a result of the regional variation
in metamorphic pressures, or at different stages of
the retrograde P-T path (Fig. 8).
Both thermobarometric studies and detailed
geological and petrological studies have revealed
that southeastern Sri Lanka, particularly near the
Highland Complex-Vijayan Complex boundary,
has undergone maximum deformation and compression, which resulted in intense thrusting
(Kröner et al. 1991). The significant mineralizations occur in these areas. The late stage magmatic
activity, particularly granite and pegmatite formation, presumably took place during the Pan-African geological events around 550 Ma. Such metallogenic activity was common during the PanAfrican period in the Gondwana terrains of India,
Sri Lanka, Madagascar, Eastern Africa and
Antarctica (Santosh & Drury 1988, Yoshida et al.
1990). Pegmatites are widely distributed in Sri
Lanka and are interspersed with granitic intrusions
and other bodies of magmatic origin. The markedly high Zr, Hf, REE and Ta enrichments are interpreted to be due major fractionation of the
source granitic melts.
Voll and Kleinschrodt (1991) considered that
the late and ubiquitous occurrence of pegmatites
is indicative of the presence of granitic bodies
below the present surface level, and that such
granites could have formed at even deeper levels.
The southeastern part of Sri Lanka, i.e. the area
represented by the Walawe Ganga Basin, is particularly rich in pegmatitic material containing rare
trace elements. The fact that the deepest crustal
levels of Sri Lanka are in the southeast and east
suggests that this material could have formed at
deeper crustal levels.
Anomalously high concentrations of fluorides
were noted by Dissanayake and Weerasooriya
18
C.B. Dissanayake, Rohana Chandrajith and H.J. Tobschall
(1986) in a hydrogeochemical survey carried out
on the Highland Complex-Vijayan Complex
boundary, which is known to be a mineralized belt
(Dissanayake 1985). Base metals such as Cu, Zn,
V and Co also showed relative enrichments in this
mineralized belt. Dissanayake and Weerasooriya
(1986) attributed the increase in fluorides to granite magmatism concomitant with the enrichment
of volatiles and mineralizers. The presence of
uraniferous granites, hot springs, serpentinites,
massive sulphide deposits with Cu, Ag, B, Be, Pb,
V, Co and Zn in the mineralized belt, in association with high fluoride concentrations, was taken
to be indicative of a deep-seated fracture, in which
fluoride might have acted as a mineralizing agent.
Fluoride is especially well known as an indicator of mineralization, particularly for occurrences of hydrothermal origin (Lalonde 1976).
Fluorine-rich granitic rocks are known to contain
elevated concentrations of high field strength and
related elements such as U, Th, Zr, Hf, Nb, Ta,
Ti, Sn, Mo, W and REE (Pollard et al. 1987, Keppler 1993).
CONCLUSIONS
The Highland Complex of Sri Lanka, a high-grade
metamorphic terrain, is one of the most prominent
gem-bearing terrains in the world. The gem deposits are mostly of the sedimentary type, and
these are classified as residual, eluvial and alluvial. Mineralogically, there is a wide variety of
gemstones with corundum being the characteristic and most important gem mineral. Sri Lanka’s
gem-bearing sediments, which are most abundant
in the granulitic terrains of the southwest and
southeast, contain markedly enriched concentrations of high field strength and associated elements
such as Zr, Ta, Nb, Hf and REE. Highly differentiated granites and pegmatites contain phases
such as zircon, rutile and monazite that acted as
sinks for these elements. The high concentrations
of mineralizers such as fluoride at the HighlandVijayan boundary close to the metal-enriched regions probably had a marked influence on the enrichment of the elements in these minerals.
ACKNOWLEDGEMENTS. CBD acknowledges
with thanks a grant from the Alexander von Humboldt Foundation and RC gratefully acknowledges
a grant from the German Academic Exchange
Service (DAAD).
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