Remote-sensing mapping (Landsat) of abandoned mining wastes over time by artisanal gold mining in Koma Bangou (Liptako, Niger).

The artisanal mining site of Koma Bangou (Niger) is located belongs to the Diagorou-Darbani greenstone belt in the arid area of Liptako (northeastern edge of the Leo Man craton) with lateritic and dune cover. The gold mineralization of Koma Bangou is of vein-type with native gold, iron, copper and arsenic sulfides paragenesis [1]. The severe drought of 1984, and its socio-economic impacts for people relying on farming and animal husbandry had for consequence the first major rush to artisanal gold mining in Niger, with the opening of the Koma Bangou site. This artisanal exploitation was stopped by the ministry of mines from 1989 to 1999, for prospecting reasons. Later, the site was re-classified as a gold panning site and artisanal mining resumed until present-day. Mining waste produced during this period includes mine tailings, host rocks, and residues, with impacts on the ecosystems. The oriented Principal Component Analysis (PCA) [2 ], Band Ratio (BR) and Minimum Noise Fraction (MNF) methods were applied on multispectral Landsat Satellite Images (TM5, ETM7, OLI8) for the period 1984-2019 in order to highlight the temporal evolutions of extraction, cyanidation, minerals richness of wastes and vegetated areas at Koma Bangou. The surface area associated with extraction has increased from 14 ha in 1984 to 255 ha in 2019 and that associated with cyanidation has increased from 40 ha in 2013 to 107 ha in 2019. Mining wastes are rich in hydroxyl-OH, clay, carbonate and sulfate minerals and poor in iron oxides and ferrous minerals. The NDVI reveals the evolution of the vegetation cover as a consequence of the droughts and anthropic activities. The low vegetal cover in 1984 is likely the consequence of a strong drought. Then, a progressive decline is observed from 2009 until today and is likely the result of the introduction of cyanidation in 2009-2010, which uses significant amount of wood.

[1] Machens, E. (1964) Mission de prospection générale du Liptako, rapport de fin de mission (1958-1964) et inventaire d'indice de minéralisation. BRGM, 84p. [2] Crosta A., Moore J. McM. (1989) Enhancement of Landsat Thematic Mapper imagery for residual soil mapping in SW Minais Gerais State, Brazil: A prospecting case history in Greenstone belt terrain; Int. Proc. of the 7^th Erim Thematic Conference: Remote Sensing for Exploration Geology, 1173-1187.