How geologists search for the next mine

Botswana is at present enjoying a welcome mineral boom. Planned Underground mining at Lucara’s Karowe diamomd mine near Letlhakane, the recent opening of an iron ore mine at Ikongwe and Khomecau’s copper- zinc mine on the Kalahari Copperbelt and the likely reopening soon of the Mowana copper mine near Dukwi is encouraging news for our faltering economy. And Sandfire’s Motheo copper mine will come on stream next year. In addition, there are plans to reopen old manganese mines at Otse and Kanye. And no doubt more mines will commence operations in the near future. Platinum, a very valuable metal, has also been found in the Molopo Farms area in the south of the country. Such mining activity is a direct response to stronger base metal prices and the recent upsurge in the demand for our most valuable product – diamonds.

With the welcome expansion of our leading mining sector, readers might like to know how geologists find such valuable mineral deposits, or orebodies, in the first place.

I once worked as an exploration geologist for BCL and it was our task to discover and evaluate new base metal deposits in eastern Botswana. I have personally worked on the copper orebodies that have since been mined at the Mowana mine, near Dukwi, and the Thakadu mine, near Matsitama. I also discovered some small copper orebodies in the Mmadinare area. Although digitisation has made inroads into the work of geologists today, little has really changed over the years in the search for the next mine. The geologist still needs a strong pair of boots and a geological hammer!

When moving into a new unexplored area, geologists may be assisted by the presence of ancient mine workings. These were dug hundreds of years ago. Many such workings occur in eastern Botswana. At the time, the ancients only used simple tools and so were only able to excavate a few metres underground. Examples of such workings occur at the Mowana copper mine near Dukwi and some old gold workings around Francistown. And evidence of smelting may also be seen at such places. Smelting is the heating of ore to remove impurities as was done at the smelter at the Phikwe mine. These impurities form slag.

Geologists may then examine black and white aerial photographs. These photos are taken from a small aircraft that is equipped with special cameras that take photos of the landscape below. The photos may appear in different hues, or shades, of black, grey and white. Such differences may reflect different types of rock or natural vegetation. For example, a blackish colour may be due to the presence of dense vegetation, or forest.

And such vegetation may grow on areas underlain by certain types of rock such as quartzite. Light grey areas may indicate the presence of marble whilst dark areas may indicate the presence of amphibolite, a blackish rock that is a host rock for copper- nickel mineralisation in eastern Botswana, including the Phikwe copper- nickel orebody.

Bands of different colours may indicate the presence of a number of different rock types in an area. If they cross the photo from west to east, this will tell geologists that the rocks run in an east- west direction; hence these rocks have an east- west strike. If the bands are curved, this may indicate that the rocks have been folded. On one side, or limb, of the fold, the rocks may slope, or dip, in one direction, say, to the north. On the other limb of the fold, the rocks will dip in the opposite direction, to the south. Many orebodies occur where rocks have been folded by intense pressure in the earth’s crust.

But aerial photos are limited in the amount of information they can give to geologists. This is because they only show areas of possible geological interest. So, boots on the ground are essential! The next stage is soil sampling. Two straight lines, or baselines, are first cut through the bush.

These lines may be 5- 10 metres wide and enclose an area of special interest that has been identified on aerial photos by the geologists. These lines usually run parallel to the strike of the rocks. Soil sample teams then cut lines through the bush from one baseline to the other and at right angles to the baselines. These lines may be spaced, say, 400 metres apart. Along these lines, the team takes soil samples at intervals of 50 metres. Soil samples are taken at a depth of about 20 centimetres and are then placed in polythene bags. Each soil sample is given a number.

For example, the number MB200/ 150 indicates that the sample was taken from the Mmadinare Block. It was taken along line MB 200 at a distance of 150 metres from one of the baselines.

The samples are then sent to a geochemical laboratory where they may be analysed for a number of base metals, such as copper, nickel, lead, zinc, silver and gold. Since soils are formed by the weathering, or breakdown, of rocks, then if a copper orebody occurs in the rocks, then high amounts of copper might occur in the soil directly overlying the orebody. Geologists may also follow the soil sample team and gather information about the rocks observed along the sample lines – rock type, strike and angle of dip. Back in the office, they will then use such information to draw up a geological map of the area showing the rock types, and their dip and strike.

Mineral content in the soil is usually stated in parts per million [ ppm]. Most soils contain an average of about 20 ppm copper. However, values exceeding 100 ppm are well above average and may show the presence of a copper ‘ anomaly’. And values exceeding 500 ppm would be considered to be very high and could indicate a copper deposit in the rocks below.

Copper values in excess of 1000ppm might lead the directors of the mining company to dream of millions of Pulas filling the company’s coffers in the near future and a happy bank manager! If interesting copper anomalies are indeed identified, then a smaller soil sample grid may be set up.

Now, soil sample lines will be spaced closer together, say, 100 metres apart. And along these lines, samples may be taken at intervals of 25 metres. Again, the samples will be analysed at the lab.

If the results are encouraging, then more work will be needed. Geologists may now carry out pitting. Here pits, about one metre square may be dug into the soil and underlying rock to a depth of one metre, or so. Geologists may then take samples down one side of the pit at intervals of, say, 20 centimetres. Long narrow trenches may also be dug; these are usually deeper than pits.