Geotourism in Tanzania
February 16 – March 4
Introduction
The Department of Earth and Planetary Sciences holds an annual, student-organised international field trip led by Professor A.E. Williams-Jones. Participants are given the exceptionally valuable opportunity to not only benefit from Dr. Williams-Jones’s scientific expertise in the field, but are able to experience firsthand geological concepts that have been studied in the classroom.
The Tanzanian trip provided students with ample opportunities to study fields as diverse as volcanology, ecology, paleontology and economic geology, and allowed many to experience the unique African culture for the first time.
Volcanology
Tanzania is bisected by the East African Rift Valley, a divergent plate margin that runs from the Red Sea in the north to Mozambique in the south. Several volcanoes are the product of this rifting, including the 'roof of Africa', Mt. Kilimanjaro, famous for its summit glacier.
One of the most active volcanoes in the Rift Valley is Ol Doinyo Lengai, a young (~370,000 year old) stratovolcano near the Kenyan border. This volcano is unique in that it is the only site in the world where carbonatite (natrocarbonatite) lavas are currently erupting. These unusual lavas have viscosities approaching that of water (1-5 Pa.s) and erupt at considerably lower temperatures (500-600 oC) than any other lavas; basaltic lavas such as those in Hawaii typically extrude at temperatures in excess of 1000 oC. When crystallized, natrocarbonatite consists primarily of large crystals of nyerereite [Na2Ca(CO3)2] (a mineral that takes its name from the first president of Tanzania, Julius Nyereri) and gregoryite [(Na,K,Ca)2CO3] set in an extremely fine-grained groundmass consisting of these and a variety of other carbonate, sulphate and sulphide minerals.
On February 25, our group ascended this volcano in the early morning, and spent 24 hours in the active summit crater at around 2800 metres above sea level, observing a variety of volcanic features and processes. The crater has a diameter of approximately 500 metres, and since the volcano's last major eruption in 1966, it has been completely filled in with lava.
The most conspicuous features in the crater are the numerous hornitos, spire-like features formed by the up-welling and solidification of lava, which is only possible because of the extremely low viscosity of the natrocarbonatite magmas (Fig. 1). As activity persists, these are hollowed by the thermal erosion of the lava within, and often they collapse partially or fully into the magma chamber below.
Figure 1: A collection of hornitos in the summit of Ol Doinyo Lengai
In exploring the hornitos, we discovered one (named T57B by scientists who study this volcano) that had partially collapsed, and as such, allowed us to actually view the magma chamber, a very unique opportunity (Fig. 2). The magma within was churning and frothing about wildly, and soon erupted, creating a hole in the side of the hornito and draining out violently. There were three separate lava flows during our visit and lava from the magma chamber was sampled (Fig. 3).
Figure 2: Part of the magma chamber as viewed from hornito T57B. Magma is visible in the lower half of the image.
Figure 3: Eruption of natrocarbonatite lava from hornito T57B. The lava is not hot enough to display incandescence in daylight.
Figure 4: The lava solidifies black, but as we experienced rainfall, the unstable carbonate minerals weather to a grey-white colour.
Ecology
Tanzania hosts two of the world’s most notable game reserves, the Serengeti National Park, and the Ngorongoro Conservation Area (NCA), both of which we explored.
Serengeti National Park
The Serengeti is home to millions of grazing ungulates, and hundreds of lions, cheetah and leopard. The park spans 15,000 km2, and is continuous across the border into Kenya, where it is known as Masai Mara Game Reserve. It also shares a border with the Ngorongoro Conservation Area.
(coming soon)
Figure 5: A herd of zebra on the Serengeti plains.
Ngorongoro Conservation Area
The NCA is a game reserve bordering the western edge of the rift valley, and half its size. It contains the famous Ngorongoro Crater, the largest intact volcanic caldera on the Earth (at 19 km in diameter), which is home to thousands of wildebeest, zebra, gazelles, as well as several lion prides, and a herd of black rhinoceros (Fig. 6 and 7). With an armed ranger, it was possible to study the volcanic rocks of the crater during our descent to the floor. The two million year old Ngorongoro crater has a steeply dipping, 600 m high wall composed of trachytic to trachyandesitic lavas representing pre-collapse eruptions of the volcano. Small post-collapse cones of basaltic composition occur within the caldera.
Figure 6: Ngorongoro Crater.
(coming soon)
Figure 7: A cheetah in Ngorongoro Crater.
Paleontology
Within the NCA is Olduvai Gorge (Fig.8), a geological setting that exposes 2 million years of sediment and ash layers, which was made famous in 1959 when Mary Leakey discovered, a 1.75 million year old human ancestor near the bottom of the gorge. EPS students trekked to the exact location where this discovery was made (Fig. 9). While no human fossils were found by our students, pieces of fossilised bone from various animals were easily located (e.g. hippopotamus).
Figure 8: Laterite horizon in Olduvai Gorge.
Figure 9: Site where a fossilised jawbone of Australopithecus boisei was found.
Economic Geology
On February 23, we visited Tanzanite One, a large, modern tanzanite mining facility. Tanzanite is a brilliant blue to purple gemstone variety of the mineral zoisite that is only found in the Meralani hills of northern Tanzania (Fig. 10). At Tanzanite One, we learned about the geological history of the area, why tanzanite formed here, and had an opportunity to examine some of the tanzanite-bearing rocks.
Figure 10: Tanzanite crystals (left) and EPS students studying diamond drill core from the mining site.
Figure 11: EPS students emerging from the location of the original mine shaft at Tanzanite One.
In the Meralani Hills, zoisite has been sufficiently enriched in vanadium to become triochroic, and thus each crystallographic axis is associated with a characteristic colour (radiant blue, purple or burgundy). The source of the vanadium is the graphitic black metapelite bounding the ore body. Tanzanite mineralization is restricted to fractures within or shadow zones surrounding calc-silicate boudins (football-shaped bodies) formed during deformation of layers with contrasting competencies (hard calc-silicate layers and less competent pelite layers) and is limited to the hinges of tight isoclinal "z" folds. The folds are part of a larger anticline within the Mozambique fold belt, formed during the assembly of Gondwanaland. This belt is characterized by high-grade metamorphism evidenced by the presence of kyanite and biotite-garnet schist around the orebody. Retrograde metasomatism, which facilitated the transport of vanadium from the graphitic metapelites to low pressure zones in the calc-silicate bodies, is believed to have produced the unusual environment responsible for tanzanite formation. The axes of the hinges plunge at approximately 40 degrees, thus mining shafts have been built to follow these hinge lines at depth in the Tanzanite One mine.
Society & Culture
During the trip, we were exposed to the people of Tanzania, who descend from a variety of tribes in the area. The Maasai are the most famous of these people, as they are still practising their tribal traditions (Fig. 12). They believe that their god gave all the cows on the planet to the Maasai, who tend and herd them.
Figure 12: A Maasai warrior who was also our guide up Ol Doinyo Lengai.
We also were exposed to the bustling African city culture, spending time in the busy capitals, Dar Er Salaam, Nairobi (Kenya) and in Stone Town, a labyrinth of residences and shops, on Zanzibar Island.
(coming soon)
Figure 13: A house perched atop another building in Stone Town.
Acknowledgements
We would like to acknowledge our sponsors for helping to provide this educational opportunity. Special thanks to:
- Rosemary Thomas, who taught us Swahili
- the Dean of Science Martin Grant, McGill University
- Newmont Mining Corporation
- Lafarge Group
- Mix 96 and CHOM Radio Station, Montreal
- The Department of Earth and Planetary Sciences, McGill University
- The McGill Chapter of the Society of Economic Geology, McGill University
- The Science Undergraduate Society, McGill University
- The Monteregian Undergraduate Society, McGill University
Trip Participants:
- Karine Benchetrit
- Pénélope Burniaux
- Michelle Campbell
- Louis Cohalan
- Jonathan Davidson
- Michelle Deakin
- Chris DeVries
- John Evangelatos
- Oliver Fu
- Carol Gilbert
- Shoshana Goldstein
- Jonathan Hanson
- Ariel Harlap
- Adam Hodge
- Alexandre Jean
- Kerry Klein
- Kent MacWilliam
- Katherine Rempel
- Kirsten Rempel
- Jake Ross
- Professor "Willy" Williams-Jones
Balmat Mine Field Trip
On April 23rd, 5 members of the McGill Chapter of the Society of Economic Geologists drove off to the New York State to tour the Balmat Zn deposit. The Balmat deposit is owned by HudBay Minerals Inc. and just reopened this year. Resources are estimated at approximately 4 million tonnes at11% Zn. Bill deLorraine, the mine geologist at Balmat, was kind enough to give us a comprehensive overview of the geology, morphology and structural controls on the mineralization before taking us underground.
Field trip participants at the Balmat mine (from left): P. Burniaux, B. deLorraine (mine geologist), S. Goodman, S. Gagne. Not in picture S. Lee.
A new genetic model developed by Mr. deLorraine from his many years working at the mine suggests that the presently known bodies of zinc ore at the Balmat mine lie in a 'parent-daughter' relationship, best exemplified in the Mud Pond zone. The known 'parent' orebody is conformable with bedding and linked by mineralized thrust faults to 'daughter' orebodies which cross-cut bedding. It is proposed that the parent orebodies originated in a sedimentary exhalative environment and were later remobilized during amphibolite-grade metamorphism along fault and ductile shear zones to form daughter orebodies. The Grenville Orogeny (ca. 1.0Ga) is likely the tectonic force responsible for the high metamorphic grade and intense deformation observed at Balmat. This new model for the structural control of the zinc mineralization is currently used by Hudbay Minerals exploration team to expand the mine's resources and find the target parent orebody, locally named 'motherlode.'
After a foggy cage ride down, we had the opportunity to visit the 2500' and 3100' levels of the deposit and view excellent exposures of host lithologies and mineralization at the Fowler and Mud Pond orebodies. Zinc ore is principally hosted by metamorphosed and polydeformed, Middle-Proterozoic dolomitic marbles which distally exhibit stromatolite structure where silicified. The mineralization is conformable to relict bedding within certain stratigraphic horizons for parents, and in cross-cutting, ductile shear zones for dauther ore bodies
While underground, we saw fantastic "durchbewegung" textures (lit. 'joint mobilization'). This texture results when sulphides are milled and streaked during deformation and were localized along thrust faults connecting parent and daughter orebodies.
Coarse sphalerite mineralization (brown) and dolomitized wall rock resembling an intensely folded zebra.
Our Balmat tour ended late in the afternoon, after trekking through the mine and collecting samples we were very pleased with a day learning about such exciting geology. The trip was a memorable experience for all who participated and we wish Bill and HudBay Minerals Inc luck in finding the motherlode for their orphan daughter orebodies.
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