DID YOU KNOW
The Beaconsfield Library has always been in what is now known as the Gardner Williams Hall. Initially the library committee wished to add a “lean-to” to the Beaconsfield Town Hall, but after protracted negotiations with John Blades Currey, were allowed to utilise certain rooms of the London and South African Exploration Company Beaconsfield offices instead. The L and SA Exploration Company buildings are now the Gardner Williams Hall. The library has been there since 1889.
DID YOU KNOW
Diamonds were formed many millions of years ago in what is known is the mantle of the earth more than 150 kilometres below surface. Carbon present in the mantle was subjected to very high pressure and temperature and crystallized to form diamonds. Much later – in the Kimberley area about 90 million years ago – melted rock or magma formed in the mantle, forced its way through the earth’s crust and burst through to surface in volcanic eruptions. The magma carried diamonds with it – and when it eventually cooled in the pipes the diamonds became entrapped in the rock that was formed. This became known as kimberlite, named after the city of Kimberley.
There are 16 known kimberlite pipes within a 10-kilometre radius of Kimberley. Most of them have been mined with varying degrees of success in the past, but at present only three remain in production after more than a hundred years.
The larger pipes in the Kimberley cluster are oval-shaped and measure between 3 and 11 hectares at present day surface. However, they do decrease regularly in size with depth, so that they measure only between one and five hectares at a depth of 600 metres. Below this depth the pipes become highly irregular and fragmented in shape.
None of the pipes was formed in a single event; multiple stages of intrusion resulted in the presence of separate columns of kimberlite within a single pipe. Each column has different mechanical and/or chemical characteristics; that is, the kimberlites vary from soft, friable and hygroscopic, to hard, water-resistant varieties on a single level.
The quantity of diamonds in each column varies, resulting in a non-uniform grade distribution both horizontally and vertically throughout the ore body. In general, however, there is an overall tendency for grade to decrease with depth.
Initially, the various mines were worked by opencast methods. In the early days at Kimberley, the workings of numerous independent claim holders were crowded into the relatively small area of the pipe. As mining took place at greater depths, operations were hampered by many hundreds of independent operators, each running his own aerial rope haulage way.
With the increasing depth of excavations, work became steadily more difficult and dangerous. Efficient, large-scale operations became possible on these pipes only when the smallholdings were ultimately amalgamated into one company with the formation of De Beers Consolidated Mines Limited in 1888. Opencast work was abandoned finally in 1890.
Chambering is the original underground mining method developed in Kimberley around 1890 and successfully used until the mid-1950s. Chambering was essentially a combination of shrinkage stoping and caving, in which advantage was taken of the pressure exerted by loose material accumulated in the open mines. Production was effected by manual loading of ore into cocopans, which were hand-trammed to ore passes. Chambering was superseded by the introduction of block caving which has since been extensively used at the Kimberley mines.
Block Caving involves production on a single level by the development of concrete-lined tunnels called scraper drift, side by side at 14-metre intervals across the pipe, beneath a solid block of kimberlite about 180 metres high. From draw points set at five-metre intervals in the sidewalls of the drifts, cones are cut into the still solid block of kimberlite above to a height of 9 metres. The block is then completely undercut by stoping out a slice about 3 metres high, immediately above the cones, causing the solid mass of kimberlite to cave and break up under its own weight. Broken ground is drawn through the draw points into the drifts, where scraper winches remove it to the haulage. The ore is then hauled by electric train to ore passes leading to the underground crushing plant, which reduces the material to 150 millimeters in diameter. It is then conveyed to loading flasks which feed the skips for hoisting to the surface. (Info direct from a DBCM promotional booklet).