Shares in Endeavour Mining (LSE: EDV) plunged 12% on Friday after the West African gold miner abruptly fired its Chief Executive Sebastien de Montessus for “serious misconduct”.

The company said an internal investigation uncovered an irregular $5.9 million payment instruction issued by de Montessus regarding an asset disposal. While de Montessus claimed the payment was to buy security equipment for staff in a conflict zone and did not benefit him personally, Endeavour’s board said he should have informed them of the arrangement.

De Montessus, who served as CEO for seven years and led Endeavour’s rapid growth into one of the largest gold miners in West Africa, admitted his failure to inform the board “was a lapse in judgement”.

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He has been replaced with immediate effect by Deputy Chairman Ian Cockerill, formerly CEO of Gold Fields and Anglo Coal.

The shocking exit caps a turbulent week for Endeavour, which operates mines in Senegal, Ivory Coast and Burkina Faso. It casts uncertainty over the company’s future direction just two years after transformational deals to acquire Teranga Gold and Semafo.