How geologists search for the next mine: Part II
This week, we will continue on how geologists find orebodies. If results are favourable from pitting and trenching, geologists will then need to investigate the rocks at much greater depths. This can be achieved through drilling. Geologists will first tell the drillers where to drill, the direction in which to drill [ for example, at a compass bearing of 270], and the angle of inclination of the drill rods [ for example, at an angle of 50 degrees to the horizontal]. The drill holes are planned so as to intersect the layer of rock that is likely to contain copper minerals.
The drillers will first drill shallow holes, maybe to a depth of some twenty metres. Drilling breaks up the rock into powder, or small chips, which may be collected, say, from depths of every half a metre. For example, at depths of 0.5m, 1.0m, 1.5m… The drillers then lay out the samples on the ground and these are then examined by geologists. Rock types may, for example, be identifies by the colour of the samples. For example, reddish powder may come from sandstone, white from marble and grey from shale.
The samples are then analysed in the lab. Since the samples come from the rocks themselves, the results will give geologists a clearer and more accurate picture about the orebody, if it exists. For example, the thickness of the orebody and its grade, usually given as a percentage of copper. Although an orebody may contain copper, that does not mean that the orebody contains 100 percent copper! Far from it! For example, a viable copper orebody may contain just 2 percent copper which means that the remaining 98 percent is of no value but still has to be mined! And the Phikwe orebody contains a mere 0.8 percent copper and 0.7 percent nickel; hence, it contains only 1.5 percent copper- nickel. To put things into perspective, orebodies on the Zambian Copperbelt may contain 5 percent copper and these are considered to be of high grade.
If results are encouraging, a diamond drilling programme may follow. A number of hollow rods are connected together; at the end of the bottom rod is attached a bit which is lined with small diamonds. Diamonds, being the hardest substance known, can cut into any rock. But these are not high value gem diamonds, but low value industrial diamonds. As the bit rotates, the hole becomes deeper and cylindrical- shaped sections of rock, or core, are forced into the hollow rod behind. At times, the rods are raised to the surface where they are opened up and the core removed. Seeing the rock intact will provide yet more information about the orebody and the surrounding rocks. For example, the type of copper minerals and if the minerals occur as scattered grains [ disseminated], or in larger crystals or masses [ massive]. Such information may be useful later to mining engineers since some copper minerals may be easier to process than others.
Geologists then ‘ log’ the core by describing it and estimating the percentage of copper in the orebody. Core containing the orebody is then split lengthwise and one half is sent to the lab for analysis. Since diamond drills can penetrate hundreds of metres below ground, the core results will tell geologists whether the grade and thickness of the orebody increases or decreases with depth. If it increases, a more extensive programme of deeper drill holes may be planned.
If results continue to please the company, a shaft may be sunk several metres into the ground. This will enable geologists to see the orebody up close and to gather more information which can then be used to determine as to whether mining the orebody would be a viable economic proposition. But before a final decision can be made, geologists need to work out the total tonnage of ore. This can be done if they know the overall shape of the orebody, its grade, its thickness and length, and the density of the ore minerals. These days, geologists have access to specific software packages to carry out this all important task. When completed, the company bosses will then decide whether to shelve the project, or give the green light to the development of a new mine.
But the decision to open a mine may be risky!
Although the decision may be taken in the light of high metal prices, it may take some years to establish a mine and only then can mining commence. But by that time metal prices might have slumped thus possibly resulting in a mine that is no longer economically viable and which might now operate at a loss. It is an inescapable fact that the mining industry experiences boom and bust cycles and reduced metal prices might be due to external factors. For example, the demand for copper is now high since the metal is used to make electric wires and components for electric cars. Hence the price for copper will rise. But maybe in the near future some scientists will invent cheaper synthetic wire that can serve the same purpose. The demand for copper may then plummet as will its price. So, opening a mine is a gamble!
In the 1960s, a young geology graduate could pitch up at the offices of a mining company in Australia and get hired on the spot. But in the 1970s, metal prices fell and companies began to lay off geologists!
After carrying out much work on the Mowana copper orebody in the 1970s, BCL finally abandoned it. One reason given at the time was that much of the orebody contains copper oxide, or carbonate minerals, such as cuprite, malachite and chyrsocolla, which were then considered difficult to process. Such minerals occur near to the surface in the zone of oxidation. Lower down, below this zone, they are replaced by a mixture of copper sulphide minerals, the most common of which is chalcopyrite which has a distinctive golden yellow colour.
Mining companies may also undertake geophysical surveys, either by air or on foot. Geologists can walk along the soil sample lines with a hand- held magnetometer which measures the magnetic properties of the rocks below ground. Some metals are magnetic – iron, cobalt and nickel – and high magnetic values may indicate the presence of an orebody containing one or more of these metals. An Induced Polarisation [ IP] survey may also be carried out to identify the electrical chargeability of the rocks below ground. This will indicate the presence of IP targets at depth. This may then be followed up by a diamond drilling programmed to ascertain the cause of the magnetic values. But all too often the target may be a rock such as gabbro or dolerite which many contain no valuable minerals at all.
In contrast to base metals, diamond exploration is very different. Diamonds are pure carbon and were formed millions of years ago under conditions of high temperature and pressure several kilometres below ground. They were then carried up to the surface in red hot molten magma which, on reaching the surface, erupted to form volcanoes. On cooling the magma solidified forming a rock known as kimberlite, named after the town of Kimberly in the Northern Cape Province of South Africa where diamonds were discovered in the late 19th century. The kimberlite rock forms cylindrical- shaped pipes that are often covered with great thicknesses of sand in the Kgalagadi. Since no outcrops of kimberlite occur at the surface there, soil sampling is carried out. Now over a long period of time the kimberlite near the surface has broken down by weathering and so its constituent minerals, such as garnet, may now occur in the overlying sand. And it is these minerals that geologists look for in the sand rather than the diamonds themselves which only occur in very small amounts, if at all, in the kimberlite. Once the presence of a kimberlite pipe has been located, a drilling programme may then be carried out to ascertain the grade of the kimberlite. The grade of a diamondiferous pipe is stated in carats per tonne – one carat is equivalent to just 0.2 grammes of diamond!
So, these are exciting times for geologists! Mining companies are expanding their operations and are keen to take on graduates as exploration or mining geologists. But would be geologists must be prepared to work and live in remote areas away from civilization. However, if you love the bush, it will prove to be a rewarding experience!