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The Geology of Gemstone Occurence in Malawi

March 01, 2019 / Grain W. P. Malunga
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Abstract

Gemstones have always been at the centre of poverty alleviation in rural areas of Mzimba, Rumphi, Chitipa, Ntcheu, Neno, Mangochi, Zomba, Mulanje, Chikwawa and Nsanje.  Most of the information we have on these stones came from small prospectors who made most of the discoveries.  These stones have been found in mica pegmatites, metamorphic rocks, ultramafic rocks and Karoo volcanic rocks.

1.0 GEMSTONES IN MALAWI

Malawi has a wide variety of gemstones including ruby, sapphire, aquamarine, emerald, various garnets, amethyst, rose quartz, rock crystal, tourmaline, chalcedony (agate), spinels, cordierite and jade.  These stones are found in a variety of host rocks including pegmatites, volcanic and basic rocks.  Most gemstones are associated with a particular group or family and may only differ in colour due to inclusion of elements such as iron (Fe), chrome (Cr), lithium (Li) and manganese (Mn).

Pure quartz (SiO2), is colourless, whereas amethyst, a purple variety of quartz, has its purple colour caused by traces of the element iron.  Iron is usually responsible for dark red or brown colours, manganese and cobalt for pink, and chromium for deep green.  

2.0 PEGMATITES

Pegmatites are very coarse crystalline rocks composed of quartz, alkali feldspar and muscovite.

Generally the core of a pegmatite is composed of quartz, with feldspar and muscovite on the outside.  These rocks form the greatest variety of gemstones.

In Malawi, the most important pegmatite belts containing gemstones are found in Chitipa, Mzimba- Kasungu (very wide pegmatite swarm) Ntcheu-Mwanza (e.g. Senzani area), Nsanje (Lulwe – Makoko area). These pegmatites intruded directly into the Basement Complex gneisses. Pegmatites in Malawi Basement Complex contain aquamarine, almandine garnet, rose quartz, tourmaline, amethyst, and sunstone.

In the Zomba –Malosa Massif and Mulanje Massif, the vein pegmatites are associated with intrusive quartz syenite. Common stones in these rocks are smoky quartz, mosaic of orthoclose and microcline feldspar; and aegerine. The pegmatites also have potential for discovery of gem tourmaline, topaz and zircon. In the nepheline gneisses of Thambani (Mwanza), pegmatites contain industrial corundum and zircon.

In other countries pegmatites are known to be also a source of other gemstones including topaz, gem tourmaline, zircon, spessartite garnet, lepidolite, epidote, spodumene and apatite, chysoberyl, fluorite, lazulite, sphene, spinel and a few more others. Therefore we must be on the lookout for these other minerals when mining the more usual gemstones. 

3.0 ULTRAMAFIC ROCKS

Ultramafic rocks are crystalline igneous rocks consisting of dark (mafic) minerals including olivine, pyroxene, amphibole and serpentine.  In Malawi theserocks includeserpentinizedperiodotite, metapyroxenites and tremolite/Actinolite – talc bodies.

Ultramafic rocks are mostly found in the Shire Highlands (e.gMpemba Hill), serpentenizedperidotites are common in the Kirk Range (e.g. Chimwadzulu Hill and Likudzi).  Metapyroxenites are common in Rumphi (Engucwini), Nkhotakota and other parts of the central region while amphibolites of igneous origin are in the Chitipa area.  These rocks can be a source of ruby and sapphire as shown by Chimwadzulu hill. Ultramafic rocks have for years been a source of gem quality ruby and sapphire.

Chimwadzulu rubies are associated with amphibole, mica and feldspar in a metasomatisedperidotite. 

The sapphires are mostly orange, pale green, blue and yellow. Cabochon quality ruby has also been found in the Likudzi area.  Heating and irradiation have been seen to enhance their colour.

In other countries ultramafic rocks have been known to also host jade (nephrite), jadeite, rhodolite and pyrope garnets, green garnet, epidote, diamonds (in kimberlites), diopside, and other gem pyroxenes and olivine. 

4.0 KARROO BASALTS (VOLCANICS)

Most Karroo extrusive rocks (basalts) are exposed in the Shire Valley to the south of Ngabu, west of Sorjin and West of Bangula extending to the Mozambique border. The   volcanics cover an area of about 1000 square kilometers. These basalts are host to gem quality chalcedony including blue agate, chrysoprase (green), variegated agate, and carnelian (pink-red).

The chalcedony was formed from aqueous solutions by infilling of the cavities (amygdales) which formed in the upper parts of the basalt lava pile. 

5.0 METAMORPHIC CALCAREOUS ROCKS

This group of rocks refers to metamorphosed limestone (marble and calc-silicate).  These rocks are widespread over the southern and central parts of Malawi. Green garnet and gem spinel have been found in the Bwanje Valley and Makoko marbles.  Calc-silicates and marble of the Makoko area host malachite.

However in other parts of the world these rocks can also be a source of lazulite, gem spinels, epidote, sphene, scapolite, and glossularite and andradite garnets.

6.0 NON-CALCAREOUS METAMORPHIC ROCKS

This category refers to gneisses and schists which are widespread in the Basement Complex of Malawi.  In certain areas these rocks are a source of mainly almandine and spessartite garnets, and cordierite.

In other parts of the world they also host aquamarine, emeralds, gem andalusite and staurolite, topaz and some of the gemstones found in pegmatites. 

7.0 CONCLUSION

The gemstone industry in Malawi has potential to support the economy if it is properly explored and regulated.  The artisanal miners need to be supported through a properly baked policy, technical and financial support mechanism. These three pillars can bring sustainable development in the gemstone sector.

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