by: Bob Linnen
Brazil produces the greatest variety of gems and semi-precious stones in the world, including diamonds, emeralds and amethyst. Mining is very much a part of the Brazilian culture and prospecting is a way of life for many people. If a mineral deposit is found in an area where no exploration license has been granted then prospectors, called garimpeiros, can mine the area using manual tools to extract the minerals. Of course many of the mines in Brazil are modern operations, owned by mining companies, similar to Canada.
Some of the most spectacular gems come from the region near the city of Belo Horizonte, in the state of Minas Gerais. This area produces emerald, aquamarine, rubellite (pink tourmaline), green tourmaline, imperial topaz, alexandrite and amazonite. These gems are commonly associated with pegmatites and several of these deposits were the subject of a field trip sponsored by the 31st International Geological Congress in 2001. This article will first provide some background on the origin of pegmatites, then look at some specific deposits and how they came to contain gems.
What are Pegmatites?
Strictly speaking pegmatites are not actually a type of rock, but instead are the description of a texture. This texture is a rock with variable crystal sizes including some very large crystals'. Generally these rocks are igneous in origin and are most commonly granitic in composition. An example of how large the crystals can be is shown by Figure 1, where the K-feldspar (potassium-feldspar) crystal is roughly 3m long and the beryl in Figure 2 is about 0.7 m across. The origin of pegmatite and the explanation of how crystals grow so large are controversial. However, the most widely accepted origin is that the crystals grow from water-rich melts, that are also rich in fluxing elements (elements that lower the melting temperature of silicate minerals) such as boron, fluorine and phosphorous. The high concentrations of these fluxing elements also increases diffusivities in the melt, allowing cations to move more quickly to sites of growing crystals, and this results in the growth of very large crystals. The question of why some pegmatites contain gems whereas others do not is also the subject of debate. Part of the explanation is that the gems emerald, aquamarine and alexandrite are beryllium minerals, tourmaline is a boron mineral and gem quality tourmaline also contains lithium, and topaz a fluorine mineral. These elements do not easily substitute into the major rock-forming minerals (quartz, feldspar, biotite etc.), therefore granitic melts that have undergone extensive crystallization will have high concentrations of the gem elements' lithium, beryllium, boron and fluorine, as well as rare metals' which also do not easily substitute into crystals, e.g., tantalum, niobium and tin. It is important to note that not all elements build in concentration in the melt as minerals crystallize. Many elements, such as chromium readily substitute into certain minerals and thus crystallization will result in the impoverishment of these elements. It will be shown below that Cr is in fact very important to the formation of many gem deposits, so extensive crystallization alone will not result in the formation of a gem pegmatite.
The Colour of Gemstones
There are several reason why gemstones have their colour. The gem industry commonly heat treats or irradiates minerals to impart or enhance colour in minerals. Naturally occurring colour is usually due to the presence of impurities. The green colour of emerald is the result of chromium and/or vanadium, green in tourmaline is also due to chromium, whereas aquamarine owes its colour to iron and rubellite's pink colour is likely due to the presence of magnesium. Alexandrite (gem variety of chrysoberyl) is one of the rarest gems and is unusual in that it changes colour. It is green in natural or fluorescent light and is red in incandescent light. Alexandrite also owes its colour to the presence of trace amounts of chromium.
The geology of Minas Gerais in brief can be simplified to a core of Precambrian basement (S‹o Fransisco craton) surrounded by Neoproterozoic mountain belts (Brasiliano orogeny). Most of the pegmatites are associated with the Brasiliano event, 700 to 450 million years ago. Brazil also experienced major uplift during the opening of the South Atlantic during the Cretaceous, 124-89 million years ago, then extension during the Tertiary. The famous imperial topaz of Oro Preto is likely related to this uplift and extension. The locations of some of the gemstone deposits in Minas Gerais are shown by Figure 2.
The discovery of the Capoeirana garimpo is a classic case of a lucky person being at the right place at the right time. This person was observing two bulls fight each other on the Capoeirana farm. One of the bulls missed goring the other and stabbed the ground with its horn, exposing a green crystal in the red soil that turned out to be emerald. The emeralds here, and at the nearby Belmont mine are found in ultramafic biotite schists. The emeralds are in quartz veins cutting the schists (Figure 3), or in the schists adjacent to the veins. The ultramafic host is important because these rocks have the high Cr contents that is necessary to form emerald. The source of Be is not well understood, but potentially was from derived from fluids that emanated from pegmatites in the area.
Alexandrite is mined in alluvium at the Itaitinga deposit. On average only 250 carats per month are produced. About 55-65% of this weight is lost in cutting, but the best quality stones are worth up to 3500 $US/carat. The alexandrite is recovered from a gravel horizon that also contains emerald, aquamarine, chrysoberyl. This suggests a pegmatite source for the alexandrite. Exploration in the area has also discovered alexandrite crystals associated with ultramafic biotite schists that are cut by pegmatite dykes. Thus the origin of alexandrite deposits involves the interaction of a beryllium source, pegmatites, with chromium-rich rocks, ultramafic biotite schists, similar to emerald deposits.
Aquamarine and Tourmaline
The Grota da Generosa pegmatite is an important aquamarine deposit, and also a producer of feldspar for the ceramic industry and of industrial beryl for beryllium. Since the colour of aquamarine is derived from iron, the origin of this gem pegmatite does not involve ultramafic rocks, in contrast to emerald and alexandrite. This pegmatite is zoned and Figure 4 shows a large beryl crystal at the contact between the mica-rich contact zone (above) and quartz-feldspar zone (below). The Jonas pegmatite is no longer in production but it was perhaps the most important producer of gem tourmaline (rubellite) in the world. Of note, four large crystals were found in pockets. These crystals were named: Joaninha, Roguete, Tarugo and Flor-de-Liz and they weighed 325, 135, 80 and 32 kg, respectively. Figure 5 is a photo of the Forguete rubellite, which sold for $US 9 million.
Coloured Gems In Canada
From the description of the gems of Brazil, Canadians might easily wonder "why are there no coloured gemstones in Canada?". Considering that the discovery of diamonds in Canada is very recent, the answer to this question might be "because we haven't looked hard enough". There are some occurrences of rubellite and emerald in northern Ontario and emeralds have recently been discovered in the Yukon.
Cesar-Mendes, J., 2000, Guide to the major coloured gemstone deposits in the vicinity of Belo Horizonte, Minas Gerais State, Brazil, 31st Internat. Geol. Cong. Field Trip Aft-20.