The gold producer Endeavour Mining has fired its chief executive, Sébastien de Montessus, for alleged “serious misconduct” over allegations of an “irregular payment” of $5.9m (£4.7m). London-headquartered Endeavour said it was also investigating whistleblower allegations made against De Montessus “relating to his personal conduct with colleagues”. Shares in the company fell by 12% on Friday, making it the biggest faller on the FTSE 100. The company announced the departure of De Montessus, who was also the president of Endeavour, with “immediate effect” late on Thursday. “This follows an investigation by the board into an irregular payment instruction issued by him in relation to an asset disposal undertaken by the company,” Endeavour said in a statement. “The amount of this irregular payment instruction is $5.9m. The board recently became aware of this in the course of a review of acquisitions and disposals, which is ongoing. “Separately, in October 2023, allegations were made against Sébastien de Montessus through the company’s confidential whistleblowing channel relating to his personal conduct with colleagues. Following a recent external investigation, the board had received the findings and, at the point of termination, was considering the appropriate course of action.” Endeavour, which operates four mines in west Africa, including in Senegal, Ivory Coast and Burkina Faso, said that its investigation into the allegedly irregular payment was continuing. De Montessus said: “In 2021, I instructed a creditor of Endeavour plc to offset an amount owed to the company to pay for essential security equipment to protect our partners and employees in a conflict zone. The decision had no additional cost to the company and did not benefit me personally in any way. I omitted to inform the board that I had arranged for this offset, which I have freely accepted was a lapse in judgment. “This week I was given 48 hours’ notice of the concerns and no proper opportunity to answer them. As to the other investigation: no misconduct of any kind was discovered because none occurred. I am proud of what we have built together at Endeavour over the past eight years. I will take my time to consider my position with my advisers.” In December, BP formally dismissed its chief executive, Bernard Looney, with immediate effect, and said it would deny him more than £32m in pay and share awards, over “serious misconduct” relating to his past relationships with colleagues.