Into the Skorpion's Pit

Posted by Site Moderator Webmaster on Wednesday, December 14, 2011

The Skorpion zinc mine lies 25 km north of the town of Rosh Pinah, on the main road to Aus. On entering the mine, one is immediately struck that the area is split into two distinct operations. The first is the chemical refining plant, a huge steel structure of conveyors, tanks, etc., that uses 25% of Namibia’s electricity. Tucked away at the back of the plant is the open pit where the ore is mined by huge double-storey tip trucks, bulldozers and front-end loaders.

This deposit was first discovered by Anglo American in the 1970s, but as the ores are not sulphides, at that time there was no known way of recovering the zinc. Only after the discovery of the ‘electro winning’ method of ore extraction, could mining commence in 2001 (for further technical details see References below). The explanation of the origin of the ore follows a supergene enrichment model, sometimes called secondary enrichment.

The primary sulphide deposit was a Sedex deposit that was laid down in a sea bed from smokers (fumaroles) about 750 Ma, as described in the Rosh Pinah article in last month’s Mineral Chatter. It was subjected to faulting, thrusting and folding about 550 Ma, and after that was weathered down and took on the elevation as in the diagram below. There is probably nothing left of the primary ore. Finally from about 21 Ma an overburden of calcrete, desert gravels and sand dunes was deposited on top.

SUPERGENE ELEVATION MODEL DIAGRAM

1	0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0	Weathered zone – Sometimes called the ‘iron hat’ or gossan layer. Gossan is Cornish for blood because of the colour it takes on. It is usually a hydrated iron oxide, i.e. limonite and goethite. This upper surface is exposed to slightly acidic rain water (if any) which migrates downwards. The primary sulphide ore also causes acidity
2	x x x x x x x x x x x x	Leached zone – The ore minerals decompose in acidic solutions and are carried downwards. Minor crystallization takes place.
3	+ + + + + + + + + + + +	Oxidised zone – The metal ions are converted to oxides, carbonates and phosphates, etc. Most of the zinc minerals that are listed at the end of this article are formed at this level. This is what we saw at Skorpion.
4	= = = = = =	Water table
5	^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^	Enrichment zone – The metal ions revert to sulphides due to lack of oxygen. Brunckite, which is a colloidal form of sphalerite, (i.e. ZnS), is formed.
6	V V V V V V	Primary ore – The same sulphides we started with, which have not been altered.

As one can see from the diagram, the oxidation of sulphide ores is very interesting to us as mineral collectors. We were given a list of 28 minerals that have been identified at Skorpion, and it would appear that lovely specimens are mined out daily and crushed up for zinc. These minerals are found in voids and breccias. Photographs of some examples are to be found further below.

When we visited the mine in September, the Chief Geologist Callum Kerr gave us a very detailed and interesting presentation on Skorpion Mine – its history, methods of mining, geology, refinery, projected life, and so on. Perhaps the most important part of the geologist’s work at this mine is to oversee the drilling of core samples ahead of mining to determine the grade of ore in various parts of the pit. It is essential that a mix of ore containing approximately 13% zinc is supplied to the refinery. We unfortunately did not have time to see this side of the operation.

The drilled areas are marked and later, when loading ore, spotters identify its source and send it to the relevant heap whose grade has been carefully indicated. From these heaps, a suitable blend of zinc ore is collected, crushed, and mixed prior to processing. A watch must be kept for excess copper in the ore to prevent problems. It is a dangerous contaminant as it makes hydrogen (which can lead to fires) during the refining process, because of the particular chemicals used. There are also small quantities of lead, manganese and calcite present, the zinc ore itself being found in sauconite, smithsonite, hemimorphite, and tarbuttite. Scorpionite is a newly recognised mineral from this mine.

The processed ore ultimately is made into ingots, which are strapped together, loaded onto pallets and taken by truck for export from Luderitz. The remaining quantities of sludge from the refinery are sent on immensely long conveyor belts to an area 3 km away. If by accident you should slip into this morass, you feel you could disappear forever. During his research on the uses of zinc, Charlie came across a flower that particularly enjoys zinc in excess. It is called a zinc pansy (Viola calaminaria et zinci) and it flourishes on the dumps of old zinc mines – unfortunately these dumps are too new to support the plant.

During the lunch hour Callum took us right into the bottom of the open pit, where the ore is currently being extracted. Very, very, large ore trucks rule the roads here, and stop for no one. At a high grade ore heap we found pieces of pale green botryoidal smithsonite – one contained a piece of newly-noted opaque white barite, unfortunately too embedded in a large rock for us to remove. At the very bottom of the pit, different looking specimens of smithsonite and some tarbuttite were seen. Another exciting find was a turquoise blue zincolibethanite specimen found by Gisela. However, we only had a short while to search here before voices over the two-way radio demanded: “Get that white bakkie out of the area!……….. Get that white bakkie out of the area!” – lunch time was over, and so was our visit.

Our sincere thanks go to Callum Kerr for sparing us so much of his time, to Gisela Hinder for setting up the visit and accompanying us to the mine, and to Albert Johr for detailed help in identifying the minerals afterwards.

Below are examples of mineral specimens that have come from Skorpion over the years:

Smithsonite – ZnCO3 Skorpionite Tarbuttite – Zn2(PO4)(OH)

w. hydrohetaerolite – Zn2Mn4O8·H2O Ca3Zn2(PO4)2CO3(OH)2·H2 w. hydrozincite - Zn5(OH)6CO3)2

Skorpionite and scholzite are both found at the mine, and are of very similar appearance. Skorpionite has fine chisel-like points, whereas scholzite has more evenly pointed sword-shaped tips. Skorpionite is occasionally found as minute crystals on specimens of tarbuttite with hydrozincite as per the example above right.