New Tsumeb Update 27 July 2021

27 July 2021

WE return to Tsumeb this week, a mine now closed for over 20 years, but whose mineralogical legacy will live on forever. We will begin with a brief look at the orebody and what made it so special, then look at some of the highlights from this week’s update.

But first a request to all our American customers. Crystal Classics are attending many of the major USA shows this year, two of which are in Denver this September. Next week we will be dispatching specimens from our East Coker showroom destined for the shows, so if there are any specimens you would like to examine in person from either these weekly updates or from our website please let us know at orders@crystalclassics.co.uk by the close of next Wednesday, August 4, and we will ensure they are in Denver for you to see.

So what about the Tsumeb orebody? Well, for a start, it was big and deep. Although complex in detail, it was basically an infilled, near-vertical pipe descending for over 1400 m (4600 ft) below surface and varying significantly in cross sectional area with pinch points as narrow as 26 x 6 m. The pipe cut through a Precambrian sequence of carbonate rocks underlain by the Grootfontein Granite complex. Theories as to its origin still vary, but a strong possibility is that is was an ancient cave system which was subsequently backfilled with sand and rock debris from the surface, finally undergoing various phases of hydrothermal alteration and the deposition of polymetallic mineralisation which included copper, lead, silver, zinc, arsenic, gallium and germanium. Oxidation processes then followed and we will speak about these in Friday’s introduction.

The 21 Tsumeb specimens in this update include many well crystallised examples of some relatively common Tsumeb species and a smattering of some much rarer. We will begin with a Linarite, as this was only ever found in the two near-surface zones (supergene and 1st Oxidation Zone) and it is even possible it originates from the legendary outcrop, The Green Hill. Composed almost entirely of intergrown inky, azure blue Linarite crystals on all surfaces, this miniature is accompanied by a very old label drafted in copperplate script. On it is written ‘D.S.W.A.’ for ‘Deutsch-Südwestafrika’, dating it to between 1884 and 1918. Mining began at Tsumeb around 1906.

Certain species found in Tsumeb, that are ubiquitous in many other mineral deposits, rarely form good crystals here, so the likes of well crystallised Galena, Siderite, Pyrite, Quartz and Gypsum are seldom seen in collections. At the very extreme, Fluorite is almost non-existent with only a handful of specimens known. Almost all Fluorite labelled ‘Tsumeb’ is from somewhere else!

Rare for Tsumeb, skeletal-hopper habit metallic grey Galena crystals form a fascinating intergrown group. Individual hoppered crystals attain almost 2 cm in length and display various complex habits, some like elongated chisels and others, intertwined bent crystals. Although Siderite occurred in all levels throughout Tsumeb, crystallised specimen-grade material was always rare. This lovely specimen is thought to be from the 2nd Oxidation Zone, because of its translucent golden yellow modified rhombohedral crystals richly distributed over a mixed granular Chalcocite / Tennantite ore. The colourless variety of Gypsum, Selenite, forms a splendid intergrown cluster of crystals in this large miniature, the largest measuring 6.9 cm tip to tip. Gypsum is rare at Tsumeb and was found in the supergene, 1st Oxidation Zone and surrounding country rock as a component of the dolostone stratigraphy.

Two other minerals uncommon as good specimens from Tsumeb are a Pyrite with Quartz and a lovely Limonite pseudomorph after Pyrite, the larger crystal measuring up to 3.2 cm. And speaking of pseudomorphs, take a look at a mysterious specimen we have. We are unsure exactly what the pale candy pink Cobaltoan Dolomite is pseudomorphing or epimorphing after but, whatever, it forms a hollow cast of a well-formed crystal on a deep leaf green Mottramite matrix. Very aesthetic.

Rarer minerals include a large thumbnail of the zinc arsenate hydroxide Paradamite, as terminated lustrous acid lemon-yellow crystals; the rare lead-iron arsenite Ludlockite on an iridescent Germanite-Chalcocite-Tennantite matrix and an unusual Willemite with inclusions of Hydrozincite.

I have to give final mention to three little beauties, thumbnails of exquisite form: a near-equant blocky Azurite crystal perched on a tiny matrix of micro-crystalline creamy Dolomite; a 2.4 cm tall plume of deep leafy emerald green Brochantite crystals and a charming Cuprian Smithsonite stalactite adopting a bluish nickel green hue from one direction, graduating to a more yellowish chrome green down its opposite face.

Unbelievably it is almost 40 years since I began collecting Tsumeb material and no matter how many specimens pass through my hands there never cease to be colours, habits and combinations new to me. Tsumeb is a mine which perpetually delivers, even after all these years since the De Wet Shaft produced its very final skip of ore. We hope you enjoy looking through today’s Tsumeb selection, it’s all thumbnails and miniatures, and small is beautiful! PT

                                         

If you'd like to see a video of any of the specimens listed above then please contact us at orders@crystalclassics.co.uk and we'll be happy to assist you. We endeavour to respond to all enquires within 24 hours during weekdays, however at the weekend it might take us a little longer to respond.

 

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Author: JH
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