TAKING IN TSUMEB
Exploring a Historically Rich Locality
The mine at Tsumeb, Namibia, now known as the Ongopolo mine, is world-famous for its amazing assortment of common and rare mineral species, some of them the world’s finest. Among the nearly 350 different known species from this location are unknown species first found in the ores of Tsumeb, where the study continues.
The deposit produced a variety of metals, including copper, lead, zinc, silver, gold, cobalt, and germanium, from a huge mineralized nearly vertical pipe that had surfaced and was heavily oxidized to depth. Mining reached nearly a depth of over 4,000 feet before closing.
The variety, high quality and in many cases huge quantity of some species of this mine is so well known that I doubt there are many mineral collectors who haven’t heard of this location in Tsumeb, Namibia. Many general mineral collections and mineral museums have fine examples of minerals from this African source. This is because Tsumeb is an exceptional ore deposit, so much so that it is unique, unlike all other copper-lead deposits. No other major ore deposit is like Tsumeb for one reason alone. It had not one upper oxide zone but three zones of oxidation separated by primary sulfide ore bodies. In addition, each oxide zone has a somewhat different suite of minerals with its own variety and dominant species that developed there.
Most ore bodies exposed to the elements have an upper oxide zone where the primary sulfide ores have been broken down and changed by atmospheric components. Below a deposit’s oxide zone is the primary ores untouched by weathering. At Tsumeb, in some areas, both primary and secondary minerals occur together.
In the desert, oxidation may penetrate a thousand feet. Other regions like Cornwall may only have an oxide zone of tens of feet. At Tsumeb, the upper oxide zone extends much deeper than normal. After mining the oxide zone, miners expected and did encounter sulfide ores but continued mining at depth, opening a second oxide zone that extended down several mine levels before the ores were, once again, primary sulfides. As mining continued at depth, the miners had no idea yet a third oxide zone was waiting for them. It contained another treasure trove of secondary collectible specimens like scorodite and wulfenite.
The obvious question has to be how three oxide zones happened in a single ore body at great depth. The answer is because not only was the ore body fractured by normal faulting, but a major fault, the Great Fault, developed some distance away and extended diagonally deeper and deeper from a distant surface, cutting across the Tsumeb ore body at a great depth. This created a rare avenue, which introduced oxidizing conditions where the ore body was exposed to the earth’s surface. Connected to this Great Fault were the North Break Zone and South Break Zone. Together these faults created a second and then the third suite of oxidized secondary minerals. All this makes the Tsumeb ore body very complex structurally and a unique hydrothermally produced ore body. In addition, the hydrothermal solutions that created the deposits were also very complex, bringing in many trace elements which helped create a wide range of species. Currently, specimens from Tsumeb are still being studied, despite a 1990s closure. Thus far, the deposit has produced about 350 different species, many of them from the three oxide zones.
With three oxide zones discovered over a long period of time, Tsumeb was a prolonged source of choice secondary minerals, starting in about 1906 and continuing into the 1990s. As each oxide zone was breached at depth, various species were produced in quantity, with each oxide zone producing a variety of species in common and each with its own unique suite of species. This development gave stimulus to the hobby as each oxide zone came online with its own suite of secondary species trigging another period of high interest among collectors. This fact is reflected in my book, A Fifty Year History of The Tucson Show, as I labeled some show years “The Year of Cerussite” or “The Year of Dioptase.”
The upper oxide zone at Tsumeb is noted for exceptional azurite and malachite, including the ‘Bird’s Nest’ azurite to be described later. The middle zone is known for its cerussite production and dioptase discoveries. The lowest oxide zone is better known for quantities of smithsonite. Much more rare are the scorodite crystals found in the lowest oxide zone. Only a few specimens were found, and it is one of the very uncommon species found here.
An interesting example of Tsumeb smithsonite was explained to me by Prosper Williams, a Canadian mineral dealer. Each year before the Tucson Show, Prop, as we called him, would stay at my house for a day and show his mineral stock to local collectors we would invite. His stock was always rich in the latest Tsumeb material because he made frequent trips there, especially before the Tucson Show.
On one Tsumeb trip when the lower oxide zone was being mined, Prop told me about when the Tsumeb miners set off a blast, returning to the area after the gases had cleared to mine the broken ore. Exposed by the blast was one wall covered by bright pink smithsonite crystals. The miners collected what they could, but the rest went to the crusher. He had some of the pink smithsoniite specimens whose color is due to a trace of cobalt to sell at Tucson.
The Tsumeb ore body is an oval-shaped vertical pipe structure that forced its way into a series of crustal dolomitic and carbonate limestone formations. These host rocks date over 500 million years. The ore body did not appear at that time. It invaded the host rocks less than 300 million years ago. The entire area was later uplifted and subjected to repeated fracturing, faulting and dissolution, so the formations of this region are termed karst formations. Groundwaters and movements have created caves and underground channels by dissolving some of the dolomite and limestone formations. These are subject to surface collapse when large enough, and the resulting pits fill with water to form a lake. There is such a large lake at Tsumeb.
New Mexico’s Carlsbad Caverns is a good example of a cave in a karst region. Subjected to repeated faulting, the rocks of the ore deposit were also brecciated and vuggy, which lends itself to crystal pockets. The limestone formations had also been the site of stromatolite growths. As these oxygen-producing bacterial growths die, they leave behind openings that can also host later forming mineral crystals.
The Tsumeb copper-lead deposit was known and surface mined by natives before Europeans explored Africa. The local tribes were drawn to the area, which was a green stained low hill of rocks. They collected surface minerals and used them for trade to other tribes like the Ovambo peoples, who knew crude smelting methods to recover copper. As for the name “Tsumeb”, there is some question of its origin. It may mean “place of algae,” probably referring to the greenish waters or “place of frogs,” many of whom lived there.
When Europeans did begin to explore Africa, the English and others learned of Tsumeb’s ‘green hill,’ and in the mid-1800s, bought the area from the local tribes in hopes of mining it. They were not successful, so when Germany declared the entire Namibia area a German Protectorate, they took over development. The Protectorate was later named South West Africa, and mining began in about 1906.
During World War I, Germany’s Frederick
Kagel was the mine superintendent, and he was also a collector. They mined the upper oxide zone
and produced some amazing azurite specimens, most of which ended up in Germany.
One German collector, Carl Bosch. a noted industrialist inventor, had a huge collection rich in Tsumeb species. After World War II, the Carl Bosch collection was brought to America and placed at Yale in hopes the family could sell it. Stored under lock and key and not well known publicly, I was lucky enough to be allowed to see it in 1948. Finally, Curator Paul Desautes of the Smithsonian found out about the collection and bought it. It took two huge moving vans to transport it to D.C.
You can also see Frederich Kagel’s Tsumeb pieces in the Los Angeles County Museum as part of the collection of Dr. Mark and Jean Bandy, good friends of mine. Before Jean donated the collection to the L.A. Museum, Mark was commissioned by the Smithsonian to go to Switzerland to pack and ship Kagel’s collection. Mark was allowed a couple of duplicate specimens for his efforts.
If you live in the New York area, you can see an upper oxide zone azurite that many judge as the most beautiful Tsumeb azurite in the world. It is the “Bird’s Nest” azurite, in the New York Natural History Museum. The specimen is about 6 x 5 inches, a more or less oval flat minimum matrix which is crowned by an inter-grown stack of tightly clustered azurite crystals measuring up to five inches set at a jaunty angle leaning against smaller sharp blue crystals of azurite that support the large crystals. There is no damage and the larger crystals have subtle parallel striations on each crystal face. If this is not the top azurite specimen in the world, I’d love to see its competition.
The Bird’s Nest was mined in the 1940s in the upper oxide zone by a miner who was off shift. He kept it for a time, but eventually, his bar bill in the miner’s hotel got so large that he had to use the azurite to pay the bill. There it was displayed for some time, but eventually, the Newmont Mining company took possession of the piece and put it on display in the lobby of their New York office along with a major gold specimen. And there it stayed until the Tucson Show Committee decided to request it for display at the show when azurite was the show theme. A company officer went to the lobby to get the Bird’s Nest and ship it to Tucson. He found that the glass case holding the very valuable gold and equally valuable azurite was not even locked! I’ll bet it is now in a locked case at the New York Museum.
What is fortunate about Tsumeb today, though it is closed, is the number of specimens still seen for sale at shows. Most of what you see will be azurite, cerussite, dioptase and smithsonite. In each case, large quantities of each of these species were found in more than one oxide zone. Cerussite is probably the most often mineral seen today. Smithsonite is less often available and well worth having.
Part Two of “Unique Tsumeb, Namibia” will appear in the October issue of Rock & Gem. We will describe the wide range of minerals found in Tsumeb, which are still on the market today.