Geological background:
Goudkoppie has a very interesting geological history. More than 2000 million years ago this area was part of a huge inland sea which stretched from the present Evander Goldfields in the East, through the East Rand, Johannesburg, the West Rand, Carletonville and Klerksdorp to the south of Welkom. Rivers and floods from the hinterland to this inland sea transported a variety of minerals, including gold. Those minerals were deposited on the shores of the sea and sedimentation took place. (Sedimentation means that material settles at the bottom of liquid and also the process of forming sedimentary rocks). Due to the action of the waves of the sea, the minerals were concentrated into mass layers (called conglomerate). These gold –rich layers late became the famous gold reefs of the Witwatersrand Super Group. More sedimentary layers were formed over millions of years. Due to crustal movement of the earth, erosion, violent volcanic action and an enormous meteorite that crashed to the earth near Vredefort in the Free State, this whole area was tilted at an angle. The water flowed away and evaporated over a long period of time and today we can see the exposed Ada May Reef on Goudkoppie. The enormous impact of the meteorite, 10 km. in diameter, caused an explosion deep underground, which pushed some of the earth’s inner crust to the surface, leaving a dome of granite. The impact also threw a trio of crater-like rings covering an area of about 150km². It had the same effect on the crust of the earth as that of a stone being cast into a pool of water: a series of concentric ripples. In the case of water the surface soon becomes smooth again, but the ripples caused by the meteorite remained as a series of hills. Klerksdorp lies well within the influence of the meteorite impact. Therefore Goudkoppie is the direct result of the meteorite: 7 minutes after it crashed to the earth to create the world-famed Vredefort Dome. Much has changed since the time of the inland sea until the arrival of the Early Stone Age 2 million years ago. Even more changes in the appearance and climate of this area took place before people from the Late Stone Age populated the Klerksdorp district about 30 000 years ago. Still it must have had enough water and a lush vegetation to satisfy the needs of those nomadic hunters. Stone tools found on Goudkoppie testify to their presence here and include quartzite tools, which were probably used as weapons, as well as engraving tools on the softer sandstone. A large amount of chippings and implements are to be found on the hill, especially in the valley.
Historical background:
In 1886 Apie Roos took a sample of Witwatersrand conglomerate to the Rand for assaying. The sample was obtained from an exposure at the Commonage Reef conglomerate. News of the discovery of gold led to a gold rush on the Commonage. On 11 July 1887 the farm Rietkuil, west of Klerksdorp was proclaimed an area for gold diggings. In the same month the commonage was made available for prospecting. Eventually the commonage was surveyed and four syndicated companies were formed to mine the area around and to the south of the Adamay Koppie (Goudkoppie). In 1888 all four were in operation, but it didn’t proof to be profitable. In 1891 the company collapsed and the land was abandoned, with the Ada May Reef on the koppie proven barren and the Commonage Reef below the oxidised zone being refractory, making it too costly to extract the gold by amalgamation with mercury. In 1896 after the introduction of the new cyanide process the Ada May/Rose claims were taken up by the Klerksdorp Proprietary Mines Limited, who worked payable sections of the Commonage Reef on a limited scale. No further mining was ever carried out on the koppie itself. All traces of the mining plant and material were removed in 1901. All that remains today are the entrances to the old mine shafts.
What is Conglomerate?
Conglomerate is a clastic rock (fragments of pre-existing minerals and rock sedimentation) that contains large (greater than two millimeters in diameter) rounded clasts. In these sedimentary layers, large quantities of gold were deposited with other rocks. The space between the clasts is generally filled with smaller particles and/or chemical cement that bind the rock together.
How Does Conglomerate Form?
Conglomerate forms where a sediment of rounded clasts at least two millimeters in diameter accumulates. It takes a strong water current to transport and shape particles this large. So the environment of deposition might be along a swiftly flowing stream or a beach with strong waves. There must also be a source of large-size sediment particles somewhere up current. The rounded shape of the clasts reveal that they were tumbled by running water or moving waves.
The EarthCache:
You have to make your way to the co-ordinates which will take you to an abandoned and closed off shaft.
To log the cache you have to email me the answers to the following questions:
- ) The koppie consists of sedimentary rocks which are exposed because of upliftment and weathering. What is the approximate gradient and in which direction is the slope of the layers?
- ) What is the approximate thickness of the conglomerate layers you see at these mines?
- ) Why do you think the mine shafts were made in the direction it were?
- ) What other rock layers can you identify in the area?
- ) Uploading photos to the cache page and to allocate a favourite point are the best way to say thank you to the cache developer and to encourage others to visit the location but is only optional.
Notes
1.) You can sent me your answers in either Afrikaans or English.
