SYDNEY -- Rio Tinto's chief executive is stepping down amid an investor backlash after the mining giant blasted one of the world's oldest heritage sites three months ago -- but his exit is unlikely to contain the questions hanging over Australia's lucrative mining sector. Rio's iron ore boss, Chris Salisbury, and corporate relations head Simone Niven will also step down. The group's handling of the controversy has heightened disquiet within the Australian mining industry ahead of an impending legislative overhaul as investors, media and lawmakers step up pressure on the company and the industry to change.