Artisanal & small-scale gold mining research field work, Migori County, Kenya

Mitchell, C.J. ORCID:; Palumbo-Roe, B.; Bide, T.. 2020 Artisanal & small-scale gold mining research field work, Migori County, Kenya. Nottingham, UK, British Geological Survey, 39pp. (OR/20/010) (Unpublished)

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Artisanal & Small-scale Gold Mining (ASGM) is a subsistence level livelihood for many rural communities across the world. In Kenya it provides work for an estimated 40,000 people and produces 5 tonnes of gold per year. The impact of ASGM is double-edged with the economic benefits offset by damage to the environment and the health of the mining communities, particularly due to the widespread use of mercury to recover gold. As a signatory to the Minamata Convention on Mercury, Kenya has agreed to eliminate the use of mercury, formalise the ASGM sector, introduce good practice and protect the health of mining communities. Migori County is a major ASGM centre in southwest Kenya where gold is produced from the quartz–carbonate reefs in the Migori greenstone belt. Recovery of gold involves extraction of the ore by mining. The deep mine shafts are unstable and dangerous places to work. There are regular reports of fatalities due to mine collapse. The gold is recovered by manual crushing, ball milling, sluice box concentration and mercury amalgamation. Residual gold in the tailings is recovered by cyanidation. The local ASGM communities are primarily concerned about the safety of the mining, the environmental impact of mercury and poor gold recovery. The extent to which pollution from the mining activities leaches into groundwater and impacts water resources is also unknown. The British Geological Survey (BGS) is working with the University of Nairobi and the Migori County Artisanal Miners Co-operative (MICA) to promote good ASGM practice, reduce mercury use and improve gold recovery using appropriate technology, alongside assessing the potential pressures ASGM poses on water resources. Samples of gold ore, crushed and milled ore, concentrates and tailings were collected from ASGM operations. On average hard rock gold is finer than 100 microns. This makes the use of a sluice box a very inefficient recovery method with expected recoveries as low as 20% for gold of 100 microns or finer. Characterisation of the ore will provide the particle-size distribution of the gold and enable the liberation size to be determined. Size analysis of the milled material is expected to show that the ore has been ‘over-milled’ with a large proportion finer than 50 microns. It is likely that some of the gold has been reduced in size to the point where simple gravity processing methods such as sluice boxes will not work. A total of 30 waters were sampled from shallow wells, boreholes, springs and mine shafts, to represent the different sources from which water is obtained by the public, during the period from the 15th to 20th November 2019, while assessment of surface water quality was carried out in a previous survey in January 2019. Mine processing waters and spoil runoff were also sampled. This work will develop good practice guidance for ASGM. It will include advice from a mining engineer to improve mine safety; the use of retorts to reduce mercury consumption; and the use of longer sluice channels (at least 3 metres), appropriate sluice box gradients, consistent sluice box feed supply, alternatives to manual crushing, modification to the milling and alternative processing methods to improve gold recovery. The analysis of the inorganic chemical status of groundwater in the ASGM areas around Migori will assess the potential pressures posed by ASGM on water resources. This BGS research project is part of the BGS Official Development Assistance (ODA) research project “From source to sink: Quantifying the local and downstream environmental impacts of ASGM”.

Item Type: Publication - Report
Funders/Sponsors: British Geological Survey, BGS Official Development Assistance (ODA) research programme
Additional Information. Not used in RCUK Gateway to Research.: This item has been internally reviewed, but not externally peer-reviewed.
Additional Keywords: Artisanal and Small-scale Gold Mining; ASGM; Kenya; Mercury; Minamata; GroundwaterBGS; Groundwater
NORA Subject Terms: Earth Sciences
Date made live: 10 Nov 2020 17:00 +0 (UTC)

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