Geology off the Beaten Track by Nick Norman

Geology off the Beaten Track by Nick Norman

Author:Nick Norman
Language: eng
Format: epub
Publisher: Penguin Random House South Africa
Published: 2013-12-09T16:00:00+00:00


Although most of the hills in these parts are capped by dolerite the sandstone ledges on the slopes of this plateau are conspicuous.

Getting back to the scenery, you will note that over much of the Karoo the high ground is composed mainly of dolerite. This intrusive igneous rock is clearly much harder and more erosion-resistant than the soft shales that predominate in the landscape. After Reddersburg, a very prominent massif capped by dolerite is visible some distance to the left of the road. There is a similar, slightly less imposing feature closer to the road, starting behind the farm ‘Bougainvillea’. Horizontal sandstone ledges are also evident on the lower slopes of the hill. As you approach Smithfield, a popular stopover point for travellers en route between the Highveld and the East London coast, the country becomes more varied and you drive through a number of low cuttings (geosite 1, map 11) (GPS S 30° 11.670’, E 26° 31.376’), mostly dolerite intruded in mudrock.

A hint of uranium

The farm where the first occurrence was investigated was just over the horizon from Reddersburg. The discovery led to Rio Tinto’s mounting a concerted campaign to secure all potentially mineralized ground over an area of hundreds of square kilometres. As a Rio Tinto geologist at the time (in 1978), I clearly remember the rush of adrenalin on walking over the ‘discovery outcrop’, and the sustained excitement that followed. Similar occurrences of uranium had been found by other companies hundreds of kilometres away, near Beaufort West, five years earlier. As at Beaufort West, and later at Laingsburg, the individual bodies were found to be too small to be developed economically, despite occurring in fairly numerous clusters.

Like that of oil, the distribution of uranium in sandstone is controlled first and foremost by the architecture of the sedimentary environment of the time. So it is essential to understand the environment in which the fluvial deposition took place, and the processes that occurred subsequently – first to introduce the uranium-bearing fluids, and then to fix the uranium in the sediment. To evaluate the uranium deposits, mining companies explored the whole of the western outcrop area of the Beaufort Group (at least 100 000 km2) for a decade or more, deploying teams of geologists who compiled a detailed geological data-base of the area, which has significantly enriched our knowledge and understanding of the region’s geological history.

You may wonder how the uranium found its way into the Karoo rocks. During lithification of the Beaufort Group sediments – a process that could have taken millions of years – water trapped between grains of sand and clay was squeezed out of the water-laid sediments and moved up the pressure gradient from deeper to shallower (lower pressure) parts. As they moved, the fluids took with them soluble components from the adjacent rock and hardening sediments through which they travelled, like uranium and, in the southwestern Karoo, molybdenum too. These minerals were then precipitated in permeable sandstone bodies whose chemistry was reducing. The uranium present



Download



Copyright Disclaimer:
This site does not store any files on its server. We only index and link to content provided by other sites. Please contact the content providers to delete copyright contents if any and email us, we'll remove relevant links or contents immediately.