• Abstract and Figures
• Public Full-text

FIELD REPORT MOZAMBIQUE: A RUBY DISCOVERY FOR THE 21ST CENTURY Merilee Chapin, Vincent Pardieu, and Andrew Lucas he Republic of Mozambique is located in south- eastern Africa along the Indian Ocean. Separated fromT Madagascar by the Mozambique Channel, it is Pemba bordered by Tanzania to the north, South Africa to Montepuez Namahumbire N the south, and Zimbabwe, Malawi, and Zambia to Namahaca the east (figure 1). t The country boasts rich resources of natural gas, 10~ 150 mi 1 coal, titanium, and hydroelectric power. Agriculture 0 150 km is also important to the economy and includes ex- I~ tensive production of cotton, cashew nuts, sugar- cane, tea, manioc root, tapioca, fruits, potatoes, beef, A F R I C A and poultry. Ruby mining is one of the newer eco- nomic sectors, with major developments in the northeastern part of the country in the mid to late 2000s. MOZAMBIQUE The Montepuez ruby deposit was discovered in May 2009 (Pardieu et al., 2009). Shortly thereafter, Maputo thousands of miners, supported by foreign traders, • began to illegally work the area. This continued until June 2011, with the formation of the Montepuez Figure 1. Mozambique, located in southeastern Ruby Mining company (MRM), a partnership be- Africa, is the world’s newest major ruby source. Its ruby mines are concentrated in the country’s north- tween Mozambique’s own Mwiriti Ltd. and the eastern corner. Adapted from Hughes (2014). multinational British gemstone mining company Gemfields. MRM acquired a 25-year concession in March 2012 and has expended considerable effort and capital to improve the mine’s infrastructure, machin- MRM ruby deposit, as the entire inventory was the di- ery, staffing, security, and public relations. rect result of the ongoing bulk sampling at Mon- While the public is generally unaware of Mozam- tepuez. This bulk sampling will guide further bique as a ruby source, the deposit near Montepuez is development of large-scale mine operations. considered by the trade to be the world’s largest sup- The company employs about 400 people at the plier. Gemfields’ first Mozambique ruby auction took mine, plus another 300 contractors. Along with ex- place in Singapore in June 2014, marking a milestone perienced personnel imported from other countries, for the global trade (figure 2). The US$33.5 million rev- the company also trains Mozambican locals, who enue generated by the auction put the spotlight on the make up the vast majority of the workforce, to attain high skill levels. In September 2014, GIA researchers Tao Hsu, Vin- See end of article for About the Authors and Acknowledgments. cent Pardieu, and Andrew Lucas visited the Mon- Note: The full version of this article was originally published as Hsu et tepuez ruby deposit (figure 3). Hosted by MRM, the al. (2014). expedition’s purpose was to witness the evolution GEMS & GEMOLOGY, Vol. 51, No. 1, pp. 44–54, http://dx.doi.org/10.5741/GEMS.51.1.44. and operation of this major deposit and to gather re- © 2015 Gemological Institute of America search samples for study by GIA’s laboratory. 44 FIELD REPORT GEMS & GEMOLOGY SPRING 2015 Belt (figure 4). Both are “treasure-bearing” Neoprotero- zoic (approximately 500–800 million years old) oro- genic belts within the global Pan-African tectonic framework. Several major geologic complexes are sep- arated by major thrusts and shear zones. Complex thermal and deformational events provided ideal tem- perature and pressure for the formation of ruby, garnet, and other minerals of economic importance. The Montepuez ruby mine lies about 150 km west of the beautiful coastal city of Pemba (again, see figure 1). It is located within the wedge-shaped Mon- tepuez Complex (Boyd et al., 2010). Its mineral as- semblages indicate that the whole complex Figure 2. The large amount of ruby produced by MRM underwent amphibolite-grade metamorphism, gen- allowed Gemfields to offer goods in a wide variety of qualities at their 2014 Singapore auction. Photo by erally at a pressure of 0.4–1.1 GPa and a temperature Andrew Lucas/GIA. of 550–750°C. Ruby forms only within a very limited range of pressure and temperature conditions, and only in the AREA GEOLOGY presence of a sufficient supply of aluminum, Northeastern Mozambique is located at a geologically chromium, and oxygen. Around Montepuez, ruby critical junction between the north-south trending formation seems to have resulted mainly from a Mozambique Belt and the east-west trending Zambezi metasomatic process, when fluid derived from the Figure 3. The GIA team that visited the Montepuez ruby deposit in September 2014 consisted of (left to right) Tao Hsu, Vincent Pardieu, and Andrew Lucas. Photo by Stanislas Detroyat. FIELD REPORT GEMS & GEMOLOGY SPRING 2015 45 40° A F R I C A N TANZANIA Palma 11° 11° t 39° MOZAMBIQUE Mucda Figure 4. The Mon- tepuez ruby deposit is MAIN GEOLOGICAL UNITS at the geologically criti- 12° 12° IN NORTHEASTERN MOZAMBIQUE cal location between D Marrupa Complex two large tectonic Karoo Supergroup structures along two di- Intrusions of Mesozoic to rections at right angles Neoproterozoic age • to each other. Adapted D Nampula Complex • Recent to Palaeogene from Boyd et al. (2010). sediments Pemba D 13° 13° D Meluco Complex Montepuez Ruby Deposit Lalamo Complex worked by D MRM D Nairoto Complex D Xixano Complex D Montepuez Complex Ocua Complex 39° 40° parental magma interacted with the host rocks in a all scales, later cut by a number of shear zones trend- low-silica environment. ing northeast to southwest. The Montepuez operation consists of mostly sec- The MRM concession is located on one limb of a ondary deposits, with only one primary deposit ex- fold with a subvertical axial plane striking east-west. posed so far. The composition of the rocks in the At least four different deformation phases occurred in Montepuez complex ranges from granitic to amphi- this area. The strong deformation history complicated bolitic, while quartzite and marble can be found in- the rock units on all scales, making the exploration side and outside the concession property. The rocks and prediction of the primary ore very complex. are also strongly folded into tight isoclinal folds on The pit walls in Maninge Nice, where the only Figure 5. Left: A clear contact between the topsoil and the ruby-containing amphibolite can be found on the walls of the primary deposit pit. Photo by Andrew Lucas/GIA. Right: The top of the ruby-containing layer has been ex- tensively weathered, and the rock is very fragile. Photo by Vincent Pardieu/GIA. 46 FIELD REPORT GEMS & GEMOLOGY SPRING 2015 Current channel Paleo channel RuRubby traps Ruby traps Figure 6. Left: River channel morphology changes constantly over time. Center: Rubies are trapped and concentrated in certain areas along those channels. Right: Cross-sections of former river channels can be seen in the wall of pit 3. Photo by Andrew Lucas/GIA. primary deposit discovered so far is located, show an channels are quite recognizable and easy to work. obvious contact zone between the topsoil and the MRM targets these as well as the richest paleochan- weathered ruby-containing amphibolite, which is nels, where large reserves of fine rubies are buried. very fragile and easily crumbled by hand (figure 5). Pit 3, the largest being worked by MRM, is lo- Ruby is found in close association with white cated along one of the main paleochannels. On the feldspar, mica, and dark green amphibole. Core wall of this pit, observers can see some superb cross- drilling data indicate that the amphibolite extends sections of former river channels (figure 6, right). about 30 meters below the surface and lies on top of Gravel size increases visibly from the edge to the bot- the basement gneiss. The whole amphibolite unit tom of these “traps,” where gravity caused heavier strikes roughly east-west. gravels to settle and concentrate at the bottom of a Hundreds of millions of years of erosion have depression. made the secondary ruby deposits in Montepuez far Larger gravels are better at slowing the water easier to uncover and work than the primary de- speed. Since the specific gravity of ruby is relatively posits. Rubies liberated from the host rock were high, it tends to settle along the bottom of a depres- transported and concentrated by water and eventu- sion with other heavy minerals. ally settled in the alluvial, colluvial, and eluvial de- Heavy mineral traps occur at different places posits being worked today. along the river channel, wherever the water current slows. The traps themselves work as natural jigs, PALEOCHANNELS AND RUBY TRAPS usually leaving a higher concentration of ruby at the Cycles of weathering and erosion are constantly bottom of the trap. These ruby “traffic jams” are shaping the surface of the earth. Once the ruby-bear- ideal locations for gem hunters. ing rock is weakened and broken down by physical Since there are numerous granitic rocks in the area, and chemical weathering, it is ready for erosion. In the gravels are mainly quartz crystals and aggregates Montepuez, rubies and other minerals liberated from of different sizes. Their shape and sharp corners indi- the parent rocks were picked up and carried by water cate that they were not transported very far from their and, due to their high specific gravity and hardness, source rocks. The focus of secondary deposit explo- trapped and concentrated in certain locations along ration is to find old stream channels and better define current or former river beds, above the weathered the distribution of this ruby-bearing gravel layer. basement rocks. River channel morphology has also changed con- THE MINE stantly over time. Hundreds of millions of years ago, The MRM concession encompasses roughly 400 the stream channels were at different positions; square kilometers, with both primary and secondary traces of their original flows are known by geologists deposits.

Load more

  11

Copy Link