Giardini, previously Kinross Gold's CFO, had returned to Ivanhoe as president in May 2019 to replace the retiring Lars-Eric Johansson.
Cloete had joined Ivanhoe (TSX: IVN) in 2005, became CFO in 2009 and had "extensive knowledge" of the company's asset base and operational personnel, Ivanhoe said.
"We are truly excited that Marna will lead Ivanhoe Mines at this critical junction as we look to fast-track development at each of our three principal mining projects," co-chairs Robert Friedland and Yufeng "Miles" Sun said.
"Tony took on the task of helping Ivanhoe through a transition following the retirement of its long time CEO, Lars-Eric Johannson, and we personally thank him to help bridge Ivanhoe from its exploration and development phase to its forthcoming operational phase."
Ivanhoe said under Cloete's leadership, it would review the organisational structure and the board had also elevated responsibilities of other executives, including executive VP sustainability and special projects and executive chairperson at its Platreef project, Dr Patricia Makhesha.
It said Matthieu Bos would be promoted to executive VP Africa, and Peter Zhou would be promoted to executive VP China, and together they would have greater roles liaising with joint venture partners in the Democratic Republic of Congo, South Africa, China and key institutional shareholders.
Ivanhoe is considering financing options as it progresses development at its Kamoa-Kakula copper project in the DRC, where first production from Kakula is expected next year, looks to fast-track initial PGM production at Platreef in South Africa and redevelops the Kipushi zinc mine in the DRC.
Ivanhoe shares lost 11.5% yesterday as markets slid, to close around a 52-week low of C$2.39, about half the value of April 2019's high of $4.54.
It is capitalised at $2.86 billion (US$2 billion).