Tanzanite is only mined one small area in the Mererani Hills of the Manyara Region in Northern Tanzania, near the city of Arusha and overlooked by Mount Kilimanjaro. This region is often called ‘Merelani’ in the gem industry, but it is properly called ‘Mererani’. It takes its name from the Tanzanian word ‘Merera’, meaning ‘trees that grow from the swamp’ – which gives some clue as to the local terrain!
When Manuel de Souza stumbled across the gem on July the 7th 1967, he couldn’t possibly have guessed how quickly the 4km-long area would descend into chaos. By mid-1970, the area was flooded with prospectors, with claims (some official, others less so) pegged in a higgledy-piggledy fashion. In 1971, the Tanzanian government nationalized the mines hoping to create some order, but by 1986, things had become even more chaotic and the government withdrew. In 1990, the area was carved up into four blocks, simply named A, B, C and D, finally bringing some order to the claims.
Licenses for Blocks A and C were given to large-scale mining operations, whereas Blocks B and D were left for local artisanal miners. In the event, Blocks A and B were on the most difficult terrain and very little quantity was found below 100 meters. There is very little information available on current mining output from blocks A and B. It is probably safe to assume it is an extremely limited amount.
Block C has changed hands a number of times, but since 2004, it has been owned by the hugely successful Tanzanite One, who have the most industrialized mining process.
Block D is mined by a number of small to medium-scale operations. This map shows the approximate layout of the site, the main Tanzanite seam itself (which is known as ‘JW Rock’) and two minor parallel seams.
The main seam is not parallel to ground level, but drops away, so that it is closest to the surface in A Block and deepest in D Block.
Money, one of the miners in D Block, has told us that for every 10m further away from C Block he tunnels, the JW Rock seam drops a further 10m. This means that in some places at least, we are looking at a 45-degree angle on the seam. Incidentally, D Block is where we source most of our Tanzanite. Coincidentally, it is also where Manuel de Souza started his first mine, and is the area that can be seen in the original footage of de Souza at the mine.
The angle on the Tanzanite seam is caused by some extraordinary local geology. Tanzanite is thought to be perhaps 550 million years old. It was probably already lying in the ground when the tectonic plate it had formed within collided with an adjacent plate, around 300 million years ago. One plate buckled as it descended past the other. This means that the Tanzanite seam is not in a neat, straight line, but is buckled and broken up into smaller pieces called Boudins. The rough outline of the effect of this buckle on the Tanzanite seam can be seen in this diagram.
This subduction zone forced one tectonic plate to become angled downwards. The impact with the neighboring plate caused the buckling seen in the Tanzanite seam. This explains why the seam has a downward angle as well as a buckle.
Hopefully, this gives some idea of the difficulty mining this gem. Each time a deposit runs out, to find more, they have to dig a lot further down than you might expect to reach the next gem-rich area – usually it is a drop of around 200-300 meters.
The same forces that caused the Tanzanite seam to buckle also created nearby Mount Kilimanjaro. Kilimanjaro is a Stratovolcano, which is a conical volcano made up of many layers of sediments deposited over time. These form on subduction zones, where one tectonic plate is being pulled under another. The huge temperatures created by the friction caused the rock to change density, allowing the magma underneath to rise up, forming the 5,895m Mount Kilimanjaro.
Tanzanite One has now reached a depth of at least 800 meters, which is an astonishing achievement. To put that into perspective, that's around the same depth as the height of the tallest building in the world.
As the miners are descending further, the costs are escalating. Tanzanite One predict they have 30 years of mining left, however, the price of rough stones will undoubtedly increase as the mining depth increases. Interestingly, as they dig deeper, more naturally blue stones are being unearthed. This is probably due to the natural heat from the earth’s core at that depth.
The smaller mines in D Block are currently mined using very simple methods and basic air supply systems, and do not have the ability to dig any further without significant investment and the purchase of mechanized mining equipment – thus their ability to dig much deeper is also restricted. No data for the amount of Tanzanite left in the several hundred small mines situated in D Block exists.
In 2010, the government of Tanzania banned the export of rough stones weighing more than one gram (5 carats). All rough stones weighing more than this must be cut in Tanzania. This again has influenced the wholesale prices for larger stones.
When Gemporia CEO Steve Bennett visited D Block in January 2014, many small scale mines had closed due to the escalating costs. The mines which were still operating, were operating at around 800 meters. In many cases it is simply too difficult to continue mining at these depths. Mark Saul, son of John Saul, the first geologist on the scene after the discovery, tells us more:
The miners’ only hope is that the price per carat will increase enough due to interest from the Chinese and Japanese markets that they will be able to cover the enormous cost of extracting the gems from such depths. However, such price increases would have a knock-on effect for Tanzanite customers globally. This means that there is no better time to buy than right now.
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