Shares in Brazilian mining giant Vale fell 20 percent on the Sao Paulo stock exchange on Monday after the collapse of one its dams killed dozens and left hundreds missing.

Its share price fell 8 percent on the New York Stock Exchange on Friday, when the Brazilian bourse was closed due to a public holiday.

Vale, the world's biggest iron-ore miner, has seen its reputation severely tarnished by the deadly accident, the second involving a company-owned mine in the southeast Brazilian state of Minas Gerais in just over three years.