The Technical File “Economic potential of pegmatites in Malawi” featured below was initially published in Malawi’s Mining Review Issue Number 25, May 2015.
The Technical File series is written by a leading Mining and Environmental Management Expert, Grain Malunga (FIMMM).
Economic potential of pegmatites in Malawi – Grain Malunga
Abstract
Malawi is underlain by basement metamorphic rocks intruded mainly by basic, granitic and syenitic rocks and overlain by sedimentary rocks. Most of these pegmatites are granitic with abundant quartz, feldspar and mica.
So far, valuable minerals such as beryl, tourmaline, zircon and garnets have been observed. There exist potential for discovery of other rare and industrial minerals.
Introduction
Pegmatites are course grained granitic rocks that occur as dykes and form narrow ridges or small hills. Most pegmatites contain abundant quartz, feldspar and mica. It is important to note that there exist other pegmatites derived from plutonic rocks such as gabbro and syenite.
They contain important accessory minerals in form of gemstones or rare minerals.
These accessory and valuable minerals include beryl, tourmaline, spodumene, garnet and topaz. Industrial minerals from these rocks include quartz and mica while rare minerals that can also be formed in them include columbite and tantalite.
This paper tries to inform prospectors that they will definitely encounter these minerals and tries to describe their physical and chemical characteristics.
Pegmatitic Minerals
Gemstones
Gemstones found in Malawi pegmatite include: amazonite, apatite, beryl(aquamarine, chrysoberyl, emerald) garnet, kunzite, lepidolite, spodumene, topaz, tourmaline, zircon and many others. Large crystals of excellent quality material are often found in pegmatite.
Gemmstones obtain their colour from their own makeup (idiochromatic) or from trace elements (impurities) such as chromium, lithium, copper, iron, manganese and vanadium giving them an allochromatic characteristic. Natural dispersion of light (pseudochromatic) through surface reflection can also give colour to gemstones.
Examples of allochromatic characteristics or colour by impurities is given below;
- Beryllium (Be): Emerald (blue-green)
- Chromium (Cr): Emerald, Tourmaline (green)
- Copper (Cu): Malachite (green)
- Iron (Fe): Aquamarine, Tourmaline (green); Chrysoberyl, Citrine, (green-yellow-brown)
- Lithium (Li): Tourmaline (green, pink)
- Manganese (Mn): Tourmaline (pink), Morganite
- Titanium (Ti): Sapphire (blue)
- Vanadium (V): Emerald, (green-red), Tourmaline (green)
Amazonite
Amazonite is an easily fracturing green feldspar that can be cut and polished to be used as gemstone. Its colour is believed to be coming from lead and water.
This mineral is common at Kabumba in Mzimba and Usisya in Nkhata Bay hosted by granitic rocks.
Apatite
Apatite is a calcium phosphate rock containing fluorine, chlorine and vanadium among other elements that substitute for calcium. Its colour varies from green to blue and can be mistaken for aquamarine and tourmaline. The immediate differentiation is that it is softer.
Gem apatite has been mined in Nsanje around Tengani and Lulwe mission.
Beryl
Beryl is a rare mineral that is rich in beryllium and occurs in veins and cavities associated with hydrothermal activity within pegmatites. These minerals are in veins that penetrate quartz rocks in the order of mica, feldspar and beryl in quartz.
Beryl which is non gem quality is a source of beryllium and is sold as “trash”. The gem quality has usually uniform colour, is transparent and with less or no cracks for easy cutting and polishing.
This mineral occurs in eight colours with different following names: emerald (green), aquamarine (greenish blue to blue), morganite (pink to orange), red beryl (red), heliodor (yellow to greenish yellow), maxixe (deep blue), goshenite (colourless), and green beryl (light green). The common gemstone from this mineral is aquamarine whose colour ranges from blue to milky blue.
Garnet
Garnet is found in pegmatites as an accessory mineral. The common garnet in Malawi pegmatite is red garnet called almandine and orange garnet called spessartine. While the author was undertaking limestone exploration on Malowa hill (Golomoti) in 1996, he found green garnet in a pegmatite on eastern summit.
Quartz
Quartz is a hard, durable and common mineral that forms a massive principle component of a pegmatite. Gem varieties include amethyst, smoky quartz and rose quartz. These are grouped as semi-precious stones.
Amethyst forms through radiation from surrounding rocks activating colour centres around iron impurities.
Smoky quartz derives its colour through natural radiation emitted from the surrounding rock. This radiation activates colour centers around aluminum impurities within the crystalline quartz. These gems can be processed and used in rings, cufflinks, pendants, necklaces, earrings, and brooches for people who like the yellowish brown to brown color.
The color of rose quartz is attributed to microscopic inclusions of a pink variety of the mineral dumortierite. Rose quartz isusually cut into cabochons and beads.
Amethyst is common in Mzimba and Chitipa. Smoky quartz is collected from pegmatite cavities on Mulanje Mountain, Zomba Mountain and Nyika plateau where they are sold as mineral specimens. Rose quartz occurs as pockets within massive white quartz in Mzimba.
Tourmaline
Tourmaline is a multi-coloured gem falling within semi-precious stones. It is crystalline boron silicate mineral compounded with elements such as aluminium, iron, magnesium, sodium, lithium, or potassium.
Rare Minerals
Pegmatite hosts many rare minerals. In Malawi a few minerals have been observed so far. These contain beryllium, columbium, niobium, tantalum and titanium, tungsten and many other elements. In most cases these have included columbo-tantalite, pyrochlore, beryl, rutile and ilmenite. If the mine contains nice crystals, the minerals are often more valuable as mineral specimens and faceted or tumbled. Good sources of these minerals are Mzimba, Senzani, Neno, Thambani and Tengani areas.
Industrial Minerals
Pegmatite can be a source of industrial minerals such as;
- Sheets of mica as initially explored in Mzimba pegmatites,
- Feldspar for use in glass and ceramic use for example in the Linthipe and Kirk Range area
- Quartz for use in glass making, abrasive and foundry sand.
Conclusion
Malawi is country that requires a serious business minded leadership if we are to unveil earth’s treasure. A systematic mineral exploration program is required grouped according to geological prospectivity as well as taking into account the financial and human resource allocation. Pegmatites prospecting and support to small scale miners will bring order and sustainable mining that will generate mineral wealth and sustainable environmental management and poverty alleviation.
