Gemstones formation and alluvial mining
By Jephter Ngwira
(Ravenstein Gems – Africa Office)
We have to understand that most gems form in many different environments in the earth. It is also important to distinguish where gems are formed and where they are found. For example, in Salima district, central region of Malawi, people are mining and picking up rhodolite garnets along and in the Lilongwe river which has a source in Dzalanyama forest, about 200km away. It might happen that the stones were formed from the source of the river or along the way and after hundreds of years it got lodged there.
How and where do gems form? It’s a combination of the following five factors: Temperature, pressure, space, chemical elements and time, which are required for the formation of each kind of gem. This is why gems in general are rare and some are even rarer than others.
Traditionally, we were taught that there are three kinds of rock formation: Igneous, metamorphic and sedimentary and today geologist prefer to describe rock formation as four processes like molten rocks & associated fluids, environmental changes, surface water and gems formed in the earth’s mantle.
Almost all gems are formed below the Earth’s surface crust with notable exceptions of peridot and diamonds which form in mantle.
So, some are brought to the surface through mining. Some are brought to the surface through earth processes (faulting, folding, large scale uplift, volcanism). These processes can move rock up from more than 400 km below the surface.
1. Water surface, water near the Earth’s surface interacts with minerals and dissolves them. The ability of these solutions to maintain elements in solution varies with physical conditions. If the solution conditions change (for example if the solution cools or evaporates), minerals will precipitate. A similar, familiar process is formation of salt crystals by evaporation of sea water. The mineral that forms is determined by what the dissolved elements are. If the water has interacted with silica-rich rocks, gemstone sandstone will form. Other examples are opal and turquoise which can be formed in this way.
2. Molten rocks and associated fluids
- Hydrothermal deposits. The formation of gems by hydrothermal processes is not dissimilar to formation of gems from water near the Earth’s surface. The solutions involve rain water and/or water derived from cooling magma bodies. Gems crystallize from solution when it encounters open spaces such as cracks. As a result, ‘veins’ of minerals fill pre-existing cracks. Minerals such as beryl (e.g., emerald), tourmalineneed unusual elements, and some of these, like beryllium (for beryl) or boron (for tourmaline) are derived from cooling molten rock (magma). Emeralds, rock crystal quartz, amethysts and fluorite are gems commonly formed when hydrothermal fluids solidify (as veins or crystals) in the cracks or pockets within rocks, or between rock layers.
- Pegmatite, are unusual magma bodies. Magmas in the upper part of the mantle becomes concentrated with volatiles, this volatile rich magmas is sometimes forced into cavity where it cools. It differs from hydrothermal vein in that magma is the primary agent rather than water. When the pegmatite magma is rich in beryllium, crystal of beryl form and when rich in boron, tourmaline will crystallise.
- Gas Crystallization, have you ever wondered why some crystals are doubly terminated, whereas most are broken off at the base like smoky quartz?, Most of these grow on solid base of other minerals, but examples of gas crystallisation are Herkimer diamonds(not real diamonds!), have double termination.
3. Environmental changes
Great stresses exist inside the earth. Under the right conditions, the temperature and pressure can rise to the point where existing minerals are no longer stable. Under these conditions, minerals can change into different species without melting. This is called metamorphism. There are two types:
- Contact metamorphism which occurs when magma forces its way into an existing rock formation, under the intense heat, existing rocks begin to melt and eventually re-crystallize as new species that are stable at higher temperature, for examples in contact metamorphism sites you can find garnets, corundum and spinel.
- Regional metamorphism which takes place on a much broader scale and affects much greater variety of minerals.
4. Gems formed in earth mantle which include diamonds and peridots. An explosive eruption of hot fluid brought them near the surface of earth. Weathering and erosion finally brought them close enough to the surface for people to find them. Gems rising to the surface after formation, since crystals form so far under the surface, you may be wondering how they get to the top where people can mine them. A few crystals are brought to the surface during volcanic eruptions. However most reach surface through mountain building and erosion.
Alluvial Gem Deposits
After rock is brought to the surface, gems may be released from the rock by weathering (some minerals dissolve, others are transformed to clay minerals, and some others survive unchanged). The minerals that survive unchanged may be washed into streams, etc., where they are concentrated by river / ocean processes. Gems retrieved from alluvial deposits are often rounded due to rolling around in rivers and oceans. Gems are often those minerals that are resistant to chemical weathering. They are commonly concentrated in stream beds and beach sands in what are known as alluvial deposits. Gems often have quite a high specific gravity (density) compared to other minerals so that they are easily trapped in depressions in stream beds. This causes them to become concentrated and makes it easier to mine them. Other valuable and durable things are also concentrated by these processes. Gold is a well known example. In summary, gems are not always found where they were formed, nor are they formed where they’re found!
So what is alluvial mining? Put simple, it’s the mining of stream bed deposits for minerals. These alluvial deposits are formed when minerals are eroded from their source, and often transported by water to a new locale. Examples are gold, pyrope garnets, topaz, sapphire, spinel, zircon, diamond and watermelon tourmaline.
In some parts of Malawi, like Lilongwe, Salima, Dedza and Nkhotakota, people do some alluvial mining of garnets garnets. Recently people have discovered deposits of gold in Mangochi also.
In some parts of Lilongwe and Salima, when people are tilling the land for the preparation of next growing season, they bring red garnets and rhodolites on the surface, now when the heavy rains come, they wash soil and expose these gems and villagers easily pick them up, and yet some stones are carried away to the nearby rivers and streams. So after the rains they will go back to the river banks and even in the water and continue with the mining. They use sieves to separate common rocks from valuable gems.
Ruby and sapphire miners in Ntcheu do the same by doing some surface alluvial mining during rainy season.
In garnets, most of these gems are round in shape as already explained and are of excellent clarity after surviving some physical conditions.
Some gems are dug from the walls of the river banks and curves are eventually formed. However, this method is very dangerous as there have been incidences of the wall falling on miners and results in deaths. In Makanjira (Mangochi) and Salima such accidents have occurred.
There is a need for government to regulate such unprofessional small scale mining skills to avoid more accidents. People doing small scale mining need to belong to licensed clubs within the villages. There is a need for frequent supervision and some training on small scale mining. It is sad to learn that up to now government is not regulating small scale mining in most part of the country. Also government is losing millions worth of revenue every year through uncharted and dubious mining operation in the country!
However, people in these areas are very happy and are able to buy all necessities at home like soap, fertiliser, sugar, salt, medicine and even pay school fees for their children after selling these gems. Last year, one villager picked up a big piece of pink rhodolite garnet weighing about 20g and they sold it at 500$!