Geology

Can Tsavorite be used as an indicator mineral Tanzanite Exploration?

Left to right: 4.8ct Tsavorite, 3.0ct Tsavorite, 2.4ct Tsavorite, 4.5ct Tsavorite and 6.8ct Tanzanite

Left to right: 4.8ct Tsavorite, 3.0ct Tsavorite, 2.4ct Tsavorite, 4.5ct Tsavorite and 6.8ct Tanzanite

Farfetched? Let us see…

Land-based diamond exploration uses the presence of the dense minerals, garnet, ilmenite and olivine minerals as an indicator for the presence of kimberlite, which we know as the source rock for diamonds. Exploration geologists use these minerals because they are hard and dense and they are common minerals found in the kimberlite rock. These minerals are also more common than diamonds.

What do we know about Tsavorite and Tanzanite? Well, Tsavorite is rarer than Tanzanite. And the scale of the mines producing Tanzanite are much larger than the smaller scale Tsavorite mines, and this results in significantly more Tanzanite. The amount of discovered crystals per pocket also favors Tanzanite.

But what about the actual surface area exposure and deposits of Tsavorite versus Tanzanite? From one of my previous blog post Origins: East African Tsavorite (23 April 2017); we can see that known Tsavorite sources between Kenya and Tanzania cover a region way larger than the single Merelani Tanzanite source.

We also know now that Tsavorite and Tanzanite are geologically related as we saw in my previous blog post The Family Tree: Tsavorite and Tanzanite (7 May 2017).

If we use the same proven concept for diamond exploration, we know that Tanzanite is younger and is found in the graphite-rich Tsavorite garnet-bearing host rocks. Tsavorite and green garnet is hard and dense and is preserved near its hardrock source. Can exploration geologists now use the Tsavorite green garnet as an indicator mineral for Tanzanite? I believe so.

Okay, great idea, but what is the point if Tanzanite is only found in one place, you ask? Ask Bruce Bridges from Bridges Exploration about the Tanzanite and blue zoisite he found on his Tsavorite Mines in Kenya sometime…

The Family Tree: Tsavorite & Tanzanite

Green Tsavorite Garnet with Violet Tanzanite (Zoisite) on the outer edge (Photo: B. Olivier). Destruction of the Tsavorite to form Tanzanite.

Green Tsavorite Garnet with Violet Tanzanite (Zoisite) on the outer edge (Photo: B. Olivier). Destruction of the Tsavorite to form Tanzanite.

This year marks the 50th Anniversary of two of the most beautiful gemstones in our industry. The first discovery of green Grossular Garnet and Tanzanite, both took place in 1967 in Tanzania. Tanzanite was named in 1968. Only after the discovery of the green Grossular Garnet in Kenya, was the gemstone named Tsavorite in 1973. Both of these gemstones were named after the locations they were found by Henry Platt, the president of Tiffany & Co.

Most people know this. But how many people know that Tanzanite was created by the recrystallization of Tsavorite? Simply put, Tanzanite was created by the destruction of Tsavorite.

As vanadium is already present in the green grossular garnet (Tsavorite), the equation can be written as:

3 tsavorite + H2O + 5CO2  -  2 tanzanite + 3 quartz + 5 calcite*

which reads: Tsavorite and High Pressure Fluids resulted in Tanzanite & Quartz & Calcite

PhD dissertation of B.Olivier (University of Stellenbosch, RSA, 2006) 

Origins: East African Tsavorite

I have spent over 7 years working as a geologist in East Africa, focusing on the exploration and mining of both the blue-violet variety of Zoisite, named Tanzanite, and the green Grossular Garnet, named Tsavorite, respectively. Here I present some of the origins of both present day and past, major and minor sources of the green Grossular Garnet. The locations shown are both exploration areas and mines that I have worked at, and personally seen both mineral and/or gem green Grossular Garnet.

Map of East Africa: The yellow line is the border between Tanzania (to the left) and Kenya (to the right). Mount Kilimanjaro lies to the north of Marangu, and Mount Meru lies to the north of Arusha. The numbers 1-7 show an origin where either exploration and/or mining has taken place for green Grossular Garnet.

Map of East Africa: The yellow line is the border between Tanzania (to the left) and Kenya (to the right). Mount Kilimanjaro lies to the north of Marangu, and Mount Meru lies to the north of Arusha. The numbers 1-7 show an origin where either exploration and/or mining has taken place for green Grossular Garnet.

Here is a summary of the origins 1-7 (number; country/area/owner; description; mining* style):

  • 1 – Tanzania; Merelani (from the Tanzanite Mines); light green tone (mint) garnet, both mineral and gem material; Underground

  • 2 – Tanzania; 16km north east of Merelani; light to medium tone green garnet, only mineral seen; Underground

  • 3 – Tanzania; Shamberai Area; Exploration area, historic underground mining (only origin I never saw any material)

  • 4- Tanzania; Lemshuku Area; light to medium tone, medium saturation green garnet; mineral and gem material; Open Pit

  • 5 – Tanzania; Lemshuku Area (Saul’s Tsavorite Mine); light, medium and dark tone, generally medium saturated green garnet; mineral and gem material; Open Pit

  • 6 – Tanzania; 14km south of Lemshuku Area; medium to dark tone, medium to high saturation green garnet; mineral and gem material; Underground

  • 7 – Kenya; Tsavo (Bridges Tsavorite) – light to medium dark tone, medium to high saturation green garnet; mineral and gem material; Underground

Geologically, what is consistent across the border of the two countries is that all the green garnet is forming in the graphite-rich schists and gneisses of the Neoproterozoic Pan-Africa Orogenic Belt. I have seen green grossular garnet form in two completely different rocks types both containing graphite from the Belt.  The importance of the graphite is significant, as this is the source of the Vanadium, the trace element giving the Grossular Garnet its green color.  And like in all gems, the amount of Vanadium, does vary resulting different tones and saturations.

Is all of the gem green Grossular Garnet, Tsavorite? I am both a geologist and gemologist, but I am guided by what I was taught at the GIA, and that when a gem has a tone of 4 and above (out of 7), then it is Tsavorite Grossular Garnet, irrespective of geographical origin.

*mining includes small scale, unsupported, blasted shafts up to 20m underground, as well large scale, supported, shafts beyond 20m underground.

Green Grossular Garnet and Tsavorite from a small scale, underground mine in Tanzania.

Green Grossular Garnet and Tsavorite from a small scale, underground mine in Tanzania.

Real Treasures Within The Earth...pt 3

Real Treasures Within The Earth...pt 3

You can only imagine what it must be like waking up 6 am every morning 5 days a week, sometimes 6 days a week, on a colored gemstone mine. Its not like a commercial commodity mine where the goal is to move thousands, sometimes tens of thousands of tonnes of rock a day....