By Dineo Phoshoko

Geophysical surveys are crucial in mineral exploration activities, as they give detailed information about the mineral potential of a geographical area. Over the years, the way in which geophysical surveys are conducted has improved significantly, making it possible for efficient and accurate results.

In 2019, Reid Geophysics collaborated with Sierra Leone’s Ministry of Mines and Mineral Resources, along with the National Minerals Agency, to commission the nationwide airborne geophysical survey.

Speaking to Inside Mining, Tim Archer, managing director of Reid Geophysics, explains that the main objective of the nationwide survey is to obtain integrated geophysical data that will form the basis for a review of the country’s geology and mineral resources and their distribution. “The ultimate goal is to gain improved knowledge of the quantity and distribution of the country’s mineral resources so as to ensure their sustainable exploitation in future,” he says.

Archer explains that the survey was conducted in Sierra Leone because the country has a long history of mining commodities and is home to various

minerals, including diamonds, gold, iron ore, rutile and bauxite. “The government now wants to attract further international mining investment, and has secured funding from the World Bank for a nationwide airborne geophysical survey.”

The survey was unique in that more than 543 000 line km of magnetic and radiometric data was collected. The data was collected at a flight line spacing of 150 m and a nominal terrain clearance (subject to safety constraints) of 50 m. “As far as we are aware, this is the highest-resolution nationwide dataset ever collected,” Archer said.

Another important aspect of the survey is that a customised aircraft was used to conduct the actual survey. The aircraft had eye-level navigation display and wingtip magnetic sensors. In addition, the aircraft also had high-volume radiometric crystals.

Specialist aircraft are used for geophysical surveys. They fly low and slow to improve data quality, and in this example carry magnetic sensors in wingtip pods.
(Credit: Reid Geophysics)

Survey advantages and limitations

Conducting an airborne geophysics survey has major advantages. Archer explains that airborne geophysics is a rapid and cost-effective means of surveying large areas of ground for mineral exploration. “The survey model adopted in Sierra Leone can be successfully applied to other African countries, to map mineral resources and attract foreign investment,” he adds.

At the 2020 Investing in African Mining Indaba, a case study was presented about Sierra Leone, titled ‘The New Direction for Sierra Leone’s Mining Sector’. Speakers during the presentation were Archer, Sierra Leone’s Minister of Mines and Mineral Resources, Foday Rado Yokie, as well as the country’s director: Geological Survey at the National Minerals Agency, Prince Cuffey. The presentation was also attended by the President of Sierra Leone, His Excellency Julius Maada Bio.

With reference to the geophysical survey, the minister mentioned that geodata was crucial to targeted exploitation activities that produce

high-value resources with minimal expenditure within a reasonable timeframe. “The flagship achievement of the Ministry of Mines and Mineral Resources – which is the completion of the nationwide airborne geophysical survey – has uniquely positioned the government of President Julius Maada Bio to present Sierra Leone as a competitive destination for mining operations by ensuring that we have accurate geodata.”

Although the survey was not conclusive in directly locating mineral deposits, Yokie said it was beneficial because it contributed to the interpretation of resource and geochemical mapping activities that will secure accurate geographical data on Sierra Leone’s mineralisation.

Archer emphasised that the survey collects geophysical data, which needs to be interpreted by experienced geoscientists familiar with the airborne technology as well as the local geology. “It is not X-ray vision,” he highlighted.

“As far as we are aware, this is the highest-resolution nationwide dataset ever collected”

Tim Archer, managing director of Reid Geophysics

Sierra Leone’s mining sector

In his opening address of the presentation at Mining Indaba, Yokie mentioned that the country’s mining sector, as well as its development, was crucial to realising the vision of transformation and development. He also proudly highlighted that President Bio – who was in the audience – was the first sitting president of Sierra Leone to attend Mining Indaba, which took place in February 2020 at the Cape Town International Convention Centre. Yokie asserted that the president’s attendance was a testament of enduring commitment to showcase the competitiveness of the mineral market and attracting investment to the country.

“The government of Sierra Leone, under the leadership of President Bio, is keen to unleash the potential of the mining sector in Sierra Leone as the key driver of the economic boost and poverty reduction,” Yokie said. One of the ways to achieve this would be through deliberate and targeted decision-making processes that consider the mitigation of environmental and social impacts inherent in mineral extraction.

With upstream, downstream and sidestream linkages between the mining sector and other sectors of the economy, the ministry has prioritised the application of prudent mineral sector regulatory management practices. Such practices will create and support an economy that is able to absorb the shock of unpredictable mineral commodity prices,

without negatively impacting on key stakeholders in the industry.

Yokie highlighted the importance of creating a fair and competitive environment for the country, companies and communities. Furthermore, he added that reforms need to bring predictable operational obligations for all mineral rights holders, as well as enhance the capacity of the state to protect all investments. “[It is important to] ensure that all stakeholders, private investors, government and communities directly impacted by mining operations get a fair deal and a fair share from investment benefits,” Yokie said.

In addition, he mentioned that in order to optimise government’s stake for mining investment, it was necessary for a departure from the current practice of awarding mineral rights on a first come, first serve basis. Instead, it was important for rights to be awarded based on a competitive methodology – which required the availability of accurate geodata.

Yokie concluded by expressing gratitude to the World Bank for its support during the survey and other critical projects in Sierra Leone’s mining sector. “The World Bank continues to be a strong partner for the Ministry of Mines and Mineral Resources. We value this partnership that has proven to be mutually beneficial to all.”

From left: Prince Cuffey (director of the Geological Survey) and Julius Mattai (director general of the National Minerals Agency) representing Sierra Leone at Mining Indaba 2020.
(Credit: National Minerals Agency)
Geological potential

Geophysics helps to provide a better understanding of the geology of a survey area. So, after Yokie’s opening, Cuffey took to the stage to give insight into the country’s geology and mineral potential.

Several minerals have been found in Sierra Leone’s geology. These include diamonds, platinum, bauxite, rutile, gold, iron ore and lignite. The minerals are found across the country in various mineral belts and complexes. Cuffey explained that the Freetown Layered Igneous Complex has potential for platinum and bauxite. “We have reason to believe that it also has nickel, copper and gold associated with sulfides.” The Freetown Complex has an area of 430 km2.

With regard to the bauxite deposits in the Kasila Group, Cuffey referred to two deposits – Port Loko and Gondama. Port Loko is still in development and has approximately 77 million tonnes of bauxite. Gondama is an existing mine operated by Vimetco with 31 million tonnes of bauxite.

With an area of 8 900 km2, the Bullom Group along the coast has coal potential, as well as other minerals including lignite and rutile. The Kasila Group has an area of 6 600 km2. Information on the Sierra Leone Ministry of Mines and Mineral Resources website states that the Kasila Group comprises a high-grade series of granulites, consisting of garnet, hypersthene and hornblende gneisses, quartzites and associated migmatites.

Cuffey emphasised that the Kasila Group was very important, as it is the primary source of rutile in Sierra Leone. “It actually has one of the largest rutile deposits in the world,” he said. It is estimated that the Kasila Group has 867 million tonnes of rutile deposits. “We also have issued a series of small-scale mining licences for rutile lately.”

At 720 km2, the Marampa Group – which forms part of the greenstone belt – could potentially be a good source for iron ore. Cuffey explained that deposits in the Marampa Group in excess of 1.68 billion tonnes had been identified.

Sierra Leone has vast iron ore deposits, with billions of tonnes in cumulative resources. “What is important to know is that most of these on the greenstone belt equally have the potential for iron ore, but we need to put exploration efforts into them,” Cuffey said.

The Rokel River Group consists of an assemblage of sedimentary rocks. With an area of 7 350 km2, this group has significant potential for bauxite.

Most of Sierra Leone’s geological potential is found in the granite-greenstone terrain, which makes up 47 000 km2 of the country’s geological potential (65% to 70%). “It’s important because that’s where our kimberlites are emplaced,” Cuffey said. Koidu, Meya and Tongo-Tonguma kimberlite deposits have been identified and they have approximately 17.54 million carats combined. Cuffey added that the Tongo kimberlite is one of the highest-value kimberlites in the world in terms of US dollars per tonne.

With an area of 430 km2,the greenstone belt is the most diverse, as it has potential for various minerals including gold, iron ore, nickel and copper. According to Cuffey, the greenstone belts are the tried and tested sources of gold in Sierra Leone. There are two gold deposits in the belt, namely Baomahun and Komahun, with 3.6 and 2.6 Moz (million ounces) of gold respectively. According to Cuffey, other deposits on the greenstone belt with similar gold potential have been identified but still need to be explored. Small-scale mining activities exist in some areas of the greenstone belt.

The nationwide airborne geophysical survey could potentially change and improve the way geophysical data is collected in the mining industry. Archer concluded his presentation by saying that an important component of this survey was the transfer of specialist technical knowledge to staff of the National Minerals Agency (NMA). “The NMA has taken a keen interest in this project, and is now well equipped to manage its technical benefits for mineral explorers and the people of Sierra Leone.”

A map showing Sierra Leone’s mineral potential. (Credit: National Minerals Agency)

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