RECYCLE, REUSE, REPURPOSE – THE CAPE TOWN TIN MINES

July 27, 2020

Duncan Miller

This month we are going to do all three, recycle an old publication, reuse it with additional photographs, and repurpose it as an article on the club’s website. The article describes Cape Town’s former tin mines, and the website article has a virtual tour of the Vredehoek tin mine on Devil’s Peak, courtesy of Dr Gregor Borg of Halle University in Germany. All of this is available for download from http://ctminsoc.org.za/resources/CAPETOWNSTINMINES.pdf

Few people know that there were two early 20th century tin mines in the vicinity of Cape Town. The deposit first mined at the turn of the century was on the farm Annex Langverwacht 245 near Kuils River. Most of the production was from alluvial deposits, producing over 700 tons of cassiterite (SnO2) until the intermittent working ceased in 1956. The visible remains of mining include various trenches and shafts, sunk into or near the ore lodes. The primary tin-bearing lodes from which the alluvial cassiterite weathered consist of aplite dykes (fine-grained granitic rock) and quartz veins, in a zone about 500 m wide. The quartz veins reportedly contain molybdenite, wolframite, cassiterite, arsenopyrite, tourmaline, mica, and quartz. These veins are similar in mineralogy to others located in the metamorphosed sediments of the Malmesbury Group around Durbanville and in granites in the Helderberg area. Clearly this mineralisation is related directly to the intrusion of the Cape Granites.

In 1909 cassiterite was discovered in a stream above Vredehoek on Devil's Peak. Mining here started in 1911, but had ceased by 1916 after producing an estimated total of about 4 tons of concentrate. The visible remains include the concrete flumes for concentrating the dense cassiterite, as well as beautifully constructed retaining walls, a blocked vertical shaft some 55 m deep, and an accessible horizontal adit penetrating the Malmesbury Group rocks about 100 m. The mineralised zone is about 8 m wide and consists of a series of thin quartz veins with crystals of dark reddish-brown cassiterite near their margins and between the constituent interlocking quartz grains. Here the mineralisation is over a kilometre from the granite-Malmesbury contact, but no doubt also owes its existence to the intrusive Cape Granite.


An adit at the former tin mines on Zevenwacht Farm on the Eerste River Hills (photograph by Amour Venter)


The neatly constructed walling at the former Vredehoek in mine on Devil’s Peak


 

MINERAL OF THE MONTH – OLIVINE

July 27, 2020
Duncan Miller



Olivine is the name given to both a mineral and a mineral group. The mineral olivine is an orthorhombic silicate with the relatively simple chemical formula of (Mg,Fe)2[SiO4]. It has a continuous range of composition between two end members, one magnesium-rich and the other iron-rich. The magnesium end member of the range is called forsterite (Mg2SiO4) and the iron end member is fayalite (Fe2SiO4). These distinct minerals form part of the olivine group, which also includes a mang...

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FABERGE’S BIRDS

July 27, 2020

Lesley Andrews

Peter Carl Fabergé was born in St Petersburg in 1846, and eventually became Goldsmith to the Imperial Court of Russia, and a supplier of wonderful artworks worldwide. He assumed charge of the workshop his father had established in St Petersburg when he was only twenty-four years old, and presented the first Imperial Easter Egg to the Russian royal family in 1884. The workshop survived until 1918, by which time Russia had succumbed to revolution and the royal family was no mor...


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A GEM CUTTER’S JUNK BOX

June 25, 2020

Duncan Miller

After several years, or after many years, a gem cutter lands up with a junk box. Mine contains disappointing stones abandoned in disgust and partly-worked stones that came over the years with various faceting machines and batches of rough. As a lock-down project I decided to see what I could make from the contents of the faceting junk box. (There are another two – one with cabochons and another with broken synthetics. You never know, you know…)

To make it something of a ...


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A MINERAL COLLECTOR’S SHOW BOX

June 25, 2020

Malcolm Jackson

The Blue Lace Agate article by Jo and Duncan in last month’s Mineral Chatter inspired me to make a box and as I had some really nice pieces of Yellowwood around, I got sawing and made the box you see in the picture. I made the box 300 mm × 400 mm × 100 mm deep. I hope to catalogue the specimens and add some artwork. I also want to include Jo and Duncan’s article in a booklet format.



I wanted to house some of my Blue Lace Agate specimens that I had collected over many...

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TIME

June 25, 2020

Duncan Miller

Our individual lives are so short, and geological time so long, that it is difficult to comprehend ‘deep time’, the most awe inspiring aspect of geology. Geologists often seem to work in units of a million years, as though that is the basic unit for the passage of time on Earth. So let’s make some effort to comprehend geological time – after all it is what makes geology tick.

Consider a rare, long-lived human life span of 100 years. There would be 10 000 of those in a ...


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The World of Tourmaline (in brief)

June 25, 2020

Peter Rosewarne

I’ve borrowed the title of a new coffee table book by Gerhard Wagner for this article as it seems appropriate in that the Tourmaline Group encompasses some 14 species currently and it is found in classic localities around the world. The idea for doing this article came from a comment from Jo that EXCO had raised tourmaline as a possible discussion topic. I also have and have had quite a few tourmaline specimens in my collection over the years and have attempted to limit dis...


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COVID-19 eclipses the Amazonite clouds

May 24, 2020

by Jay and JD Haasbroek

I know for sure it was last year some time, but that’s about all I am prepared to admit to any sense of time or space in these times.

As was usual back then Verna Jooste was visiting round the kitchen table with me, and Jay was busy with stones in her adjacent cutting studio and joining the conversation every so often, with a stone in her hand. Verna, a teaching jeweller and artist, is always interested in the stone in the hand. She comes from a family of diamond c...


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BLUE LACE AGATE FROM YSTERPUTS, SOUTHERN NAMIBIA

May 24, 2020

by Jo Wicht and Duncan Miller

For several decades small mines in southern Namibia have produced an attractive banded agate marketed as lapidary material. The major source has been a mine on Ysterputs farm, producing blue lace agate. It was promoted widely by the late George Swanson who owned the mine, so this material with its wavy blue and white lines is quite familiar. What is less well known is that the blue lace agate from Ysterputs is accompanied by several minerals forming aesthetic, c...


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HOW TO KEEP YOURSELF BUSY WHEN UNDER HOUSE ARREST

April 25, 2020

Duncan Miller

When you have finished the chores, tidied the garage, weeded the garden, washed the windows, painted the house and knitted the dog a winter coat, how do you keep yourself busy around the home during lock-down? You could take advantage of the enforced holiday to photograph and catalogue your mineral collection, even if you are not preparing it for sale.


Bonnie, decked out in MinSoc green (courtesy of Jo Wicht)

Photography of minerals need not involve very expensive equipment. My...


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