THE GEOLOGY OF GEMSTONE OCCURRENCE IN MALAWI

Fellow of the Institute of Materials, Minerals, and Mining (UK) and Distinguished Fellow of the Africa Institute of Administration (Ghana)

1. 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. The gemstones are found in or are associated with a variety of host rocks including pegmatites, volcanics, basic rocks.

2. PEGMATITES

Pegmatites are very coarse crystalline rocks composed mainly of quartz, alkali feldspar and muscovite.  Generally the core is composed of quartz, with feldspar and muscovite on the outside. Of all the rock types, pegmatites contain the greatest variety of gemstones.

In Malawi the most important pegmatite belts as regards gemstones are found in Chitipa, Mzimba- Kasungu (very wide pegmatite swarm), Ntcheu-Mwanza (e.g. Senzani area), and Nsanje (Lulwe – Makoko area). These pegmatites are intruded directly into the Basement Complex gneisses. However, this does not imply that other parts of Malawi do not have pegmatites containing gemstones. It should be noted that not every gemstone mineral is gem quality. So far in Malawi pegmatites in the Basement Complex have yielded aquamarine, almandine garnet, rose quartz, rock crystal, gem tourmaline, amethyst, and sunstone.

In the Zomba –Malosa Massif, the vein pegmatites are associated with intrusive quartz syenite. Gemstones (loosely used) found in these pegmatites include smoky quartz, mosaic of orthoclase and microcline feldspar and aegerine. The pegmatites also have potential for gem tourmaline, topaz, and zircon. Mulanje Mountain has similar geology.

The nepheline gneiss of Thambani (Mwanza) has pegmatites that contain industrial corundum and zircon. Surely, the pegmatites must have potential for gem varieties of the two minerals. In other countries pegmatites are known to also be a source of other gemstones including topaz, gem tourmaline, zircon, spessartite garnet, lepidolite, epidote, spodumene and apatite, chrysoberyl, fluorite, lazulite, sphene, spinel and a few more others. There is need to be observant to detect these other gemstones.

3. ULTRAMAFIC ROCKS

Ultramafic rocks are crystalline igneous rocks consisting of dark (mafic) minerals including olivine, pyroxene, amphibole and serpentine. In Malawi these include serpentinized peridotite, metapyroxenites and tremolite/actinolite – talc bodies. Ultramafic rocks are found in Shire Highlands (e.g. Mpemba Hill) and Kirk Range area.  Metapyroxenites are found in Rumphi (Engucwini), Nkhotakota and other parts of the central region. Some amphibolites of igneous origin are found in the Chitipa area in the north.

So far in Malawi, ultramafic rocks have for years been a source of gem quality ruby and sapphire, in particular Chimwadzulu in Ntcheu district.  Chimwadzulu rubies are associated with amphibole, mica and feldspar in a metasomatised peridotite.  The sapphires are mostly orange, pale green, blue and yellow. Cabochon quality ruby has also been found in the Likudzi area.

All the other ultramafic bodies e.g. Choma Hill to the south of Chimwadzulu and in other areas mentioned have not been prospected in detail for ruby and sapphire or other gemstones for that matter.

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. KARROO BASALTS (VOLCANICS)

The Karroo extrusive rocks (basalts) are exposed to South of Ngabu, west of Sorjin and West of Bangula extending to the Mozambique border.  The volcanic rocks cover an area of about 1000 square kilometers.

The 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. 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. Gemstones have been found in the Bwanje Valley, Makoko marbles and calc-silicates. These include green garnet and gem spinel in Bwanje, and malachite in the Makoko area.

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

6. 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.  

• CONCLUSION

Malawi has relied much on artisan and small scale miners (ASM) to prospect for gemstones. We have hardly began a formal gemstone industry in Malawi. Systematic exploration and formalizing the sector will benefit the country in its quest for wealth creation through the mining sector. The potential is unlimited.