GEMSTONES
Introduction.
Tanzania is
endowed with various species of coloured gemstones including the unique,
famous and beautiful tanzanite. Most of the gemstones occur in the
Palaeoproterozoic metamorphic rocks of the Mozambique (Usagaran) belt in
the east and the Ubendian Supergroup in the west. An excellent, although
outdated, review of gemstones in Tanzania ids by Solesbury (1966). Others
include Harris (19610, Keller (1992), Nyelo (1995) and Kashabano (1998)
present an up-to date description of gemstones in Tanzania.
In the past,
small-scale miners have dominated the gemstone mining operations. The
current Mineral Policy (1997) of Tanzania encourages the private sector to
take the lead in developing modern gemstone mines and establishment of a
lapidary and jewellery manufacturing industry.
There has been an
increase in the official gemstone exports from 137,165 tonnes valued at
US$10 million in 1996 to 1,531,547 tonnes valued at US$ 19.05 million in
2004. Nearly all production is by small-scale miners. However, true
exports might be higher than these figures due to some unrecorded
exports.
Tanzanite.
Tanzanite is a variety of zoisite, which is strongly peochroic. In its
rough form it radiates three different colours from each of its
crystallographic axes: blue, violet and burgundy. Once cut and polished,
the colour of tanzanite range from pale blues and violets to a deep
indigo. It was first discovered in the Merelani area, Tanzania in 1967
(see p. 42 for precise definition). The variety was named tanzanite after
the name of the only country in which it is found. Tanzanite's exquisite
violet-blue colour and rarity have made it one of the most popular
coloured gemstones. The only commercial deposit of tanzanite is at
Merelani, located about 40km south of Arusha town and 24 km southwest of
the Kilimanjaro International Airport.
Tanzanite occurs
mainly in boundinaged pegmatitic veins and hydrothermally altered
graphite- bearing diopside gneiss. Malisa and Muhongo (1990) discuss
regional control of tanzanite mineralisation at Merelani. In 2000, African
Gem (AFGEM), A South African Company, conducted feasibility studies in
what is known as "block C" and suggested that the ore body is
characterized by isoclinal folds. Boudins are typically developed in the
hinges of these folds.
Tanzanite is
located within a relatively complex geological environment and is found in
'chocolate-tabletboudin structures typically located in the hinges of
isoclinal folds (folds dipping in the same direction) present in the ore
body. The ore body extends across the mining license area in a
southwesterly to northeasterly direction on a dip of approximately 4.
Geological work, subsequent to the initial feasibility study carried out
in 2000, has indicated that multiple folding is present in the structure
contains significantly more boudins than previously anticipated.
- (Clockwise from
upper left)Tanzanite crystals in host.
Tanzanite crystals
uncut (Photo courtesy of Jewellery International)
Small-scale miners
have been traditionally mining tanzanite at Merelani since it was
discovered in 1967. Exploration work conducted in 2002 by AFGEM calculated
and indicated resource of 4.6 Mt of ore with a grade of 22carats/tone (4.4
g/t), containing 50 million carats of A and B grades tanzanite.
In 2003, AFGEM
completed phase 1 of the development and opened a mechanized tanzanite
mine at Merelani.
In May 2004, The
Tanzaite One Group acquired tanzanite business and assets from AFGEM that
has been involved in the tanzanite industry since 1999. Tanzanite One
commissioned Ddraig Minerals Developments Limited ("DMD") to re-evaluate
the deposit. The result of the work carried out in 2004, place the
indicated resource at between 0.95and 1.26 Mt for between 63 and 83
million carats of tanzanite.
Ruby and Sapphire.
Mining areas are located at Longido in Arusha, Umba River in Tanga,
Matombo and Mahenge in Morogoro and Tunduru and Kitai in Ruvuma. The host
rocks in Tanga and Morogoro Regions are metamorphosed ultramafic rocks or
marbles of the Usagaran Supergroup.
In the Ruvuma
Region sapphire is recovered from alluvial deposits. Blue and pink
sapphires occur in the areas of Amani Makoro, Kitai and Mafurungu in the
Mbinga District.
Chrysoberyl.
Recent discoveries of gemstones, including alexandrite, chrysoberyl cat's
eye and sapphire, are being worked by small-scale and artisanal miners in
the beds of the Muhuwesi, Mtetesi, Lumesule and Nambalapi Rivers, located
in the districts of Tunduru and Nachingwea.
Emerald.
In Tanzania the best-known emerald deposit is in Mayoka Village at the
southern tip of Lake Manyara in the Manyara Region. The deposit has both
emerald and alexandrite. They occur in contact zones between
biotite-actinolite schists/gneisses and pegmatites.
Production
statistics indicate that over one million carats of emerald have been
mined. No official emerald reserve exists, but it is believed that the
western sides of Lake Manyara host more emerald deposits. Another emerald
deposit is at Ponda near Sumbawanga town in the Rukwa Region.
Other gemstones.
Other gemstones known in Tanzania are: Beryl (aquamarine); Garnet
(almandite, pyrope, rhodolite, tsavorite, hessonite, spessartite, Malaya
garnet, color-change granite); Feldspar (moonstone, sunstone, amazonite);
Quartz (amethlyst, citrine, rose quartz, smoky quartz); Tourmaline (green,
yellow, brown, pink, water-melon, black); Spinel (violet, blue, yellow,
pink, black); Scapolite (purple and yellow) Topaz (white, yellow, blue);
Zircon (brown, pink, colorless); lolite; Chalcedony (chrysoprase); Opal
(cat's eye, yellow, brown, white).
A summary of
gemstone occurrences and geological environments in Tanzania is presented
in Table 6
Ruby Crystal in
Zoisite. Total weight of a specimen is 38kg (Photo by Jaeger, Precious
minerals Corporation)
Opportunities.
Opportunities for the investor include systematic prospecting of Usagaran
and Ubendian Supergroup terranes (Fig 47) for previously unrecognized
gemstone localities. Another opportunity is the establishment of value
adding activities in gemstones.
Gemstones
Marketing
The current
gemstone marketing arrangements in the country do not guarantee enough
benefits to the stakeholders including the government due to the absence
of formal marketing system of gemstones. Upon realization of this
situation, the Government has decided to establish formal marketing
systems and promote value-added activities. In 2002 and 2004 the
Government revised its legislation to encourage value added activities to
gemstones.
Cut and uncut
tourmaline varieties found in Tanzania. (From Gemstones of the World)
Cut and uncut
varieties of ruby, corundum and sapphire found in Tanzania. (From
Gemstones of the World)