Marr-Johnson served as CEO for 18 months.
Du Plessis has over 20 years' experience in the finance sector and was most recently executive director at the Canadian Imperial Bank of Commerce.
Prior to this he was a director at Bank of America Merrill Lynch and Lehman Brothers, and a vice president at Morgan Stanley.
Du Plessis has worked on transactions across a variety of commodities and for a number of leading global miners, including AngloGold Ashanti, Rio Tinto and BHP Billiton.
Before moving into investment banking, he worked on corporate finance and business development at Anglo American.
Marr-Johnson was said to have resigned to "pursue other business interests".